Contents Integrated Optics, 90 Article(s)
Self-configuring universal linear optical component [Invited]
David A. B. Miller

We show how to design an optical device that can perform any linear function or coupling between inputs and outputs. This design method is progressive, requiring no global optimization. We also show how the device can configure itself progressively, avoiding design calculations and allowing the device to stabilize itself against drifts in component properties and to continually adjust itself to changing conditions. This self-configuration operates by training with the desired pairs of orthogonal input and output functions, using sets of detectors and local feedback loops to set individual optical elements within the device, with no global feedback or multiparameter optimization required. Simple mappings, such as spatial mode conversions and polarization control, can be implemented using standard planar integrated optics. In the spirit of a universal machine, we show that other linear operations, including frequency and time mappings, as well as nonreciprocal operation, are possible in principle, even if very challenging in practice, thus proving there is at least one constructive design for any conceivable linear optical component; such a universal device can also be self-configuring. This approach is general for linear waves, and could be applied to microwaves, acoustics, and quantum mechanical superpositions.

Photonics Research
Feb. 27, 2013, Vol. 1 Issue 1 01000001 (2013)
High-order sideband optical properties of a DNA–quantum dot hybrid system [Invited]
Yang Li, and Kadi Zhu

High-order sideband nonlinear optical properties in a DNA–quantum dot coupled system are investigated theoretically here. In this paper, we demonstrate the significant enhancement of the third- and fifth-order optical nonlinear properties of the system by applying the pump-probe technique with pump-exciton detuning tuned to zero. It is shown clearly that these phenomena cannot occur without the DNA–quantum dot coupling, implying some potential applications like DNA detection. We can also obtain and tune the significantly amplified sideband beams at frequencies ωp  2ωD. This research could provide people a deeper insight into the nonlinear optical behaviors in coupled DNA–quantum dot systems.

Photonics Research
Apr. 22, 2013, Vol. 1 Issue 1 01000016 (2013)
High-performance waveguide-integrated germanium PIN photodiodes for optical communication applications [Invited]
Leopold Virot, Laurent Vivien, Jean-Marc Fedeli, Yann Bogumilowicz, Jean-Michel Hartmann, Frederic Boeuf, Paul Crozat, Delphine Marris-Morini, and and Eric Cassan

This paper reports on high-performance waveguide-integrated germanium photodiodes for optical communications applications. 200 mm wafers and production tools were used to fabricate the devices. Yields over 97% were obtained for three different compact photodiodes (10 × 10 μm and intrinsic region width of 0.5, 0.7, and 1 μm) within the same batch of three wafers. Those photodiodes exhibit low dark currents under reverse bias with median values of 74, 62, and 61 nA for intrinsic widths of 0.5, 0.7, and 1 μm, respectively, over a full wafer. Responsivities up to 0.78 A∕W at 1550 nm and zero bias were measured. Zero bias operation is possible for 25 and 40 Gbps with receiver sensitivity estimated to -13.9 and -12.3 dBm, respectively.

Photonics Research
Sep. 15, 2013, Vol. 1 Issue 3 03000140 (2013)
Simultaneous evaluation of two branches of a multifunctional integrated optic chip with an ultra-simple dual-channel configuration
Yonggui Yuan, Chuang Li, Jun Yang, Ai Zhou, Shuai Liang, Zhangjun Yu, Bing Wu, Feng Peng, Yu Zhang, Zhihai Liu, and Libo Yuan

We propose an ultra-simple dual-channel configuration for simultaneously evaluating two branches of a multifunctional integrated optic chip (MFIOC). In the configuration, the MFIOC is employed as a beam splitter to construct the demodulation interferometer together with a 2 × 2 fiber coupler. Interference happens between polarization modes traveling through different channels of the MFIOC. The cross-couplings of each channel are respectively characterized by the interference peaks which distribute on opposite sides of the central interference peak. Temperature responses of the MFIOC are experimentally measured from ?40°C to 80°C. Results show that the proposed configuration can achieve simultaneous dual-channel transient measurements with resolution of ?90 dB and dynamic range of 90 dB. In addition, the two channels of the configuration have consistent measuring performance, and the two branches of the MFIOC have different responses to temperature variation.

Photonics Research
May. 15, 2015, Vol. 3 Issue 4 04000115 (2015)
Design of a barcode-like waveguide nanostructurefor efficient chip–fiber coupling
Xiang Wen, Ke Xu, and Qinghai Song

A barcode-like waveguide nanostructure with discretized multilevel pixel lines is designed and optimized by a nonlinear search algorithm. We obtain the design of a one-dimensional multilevel nanostructure with ?1.04 dB efficiency for surface normal coupling to a standard single-mode fiber. Another design is achieved from the automatic optimization process, which enables polarization-independent coupling to a single-mode fiber. The optimum coupling efficiency is simulated to be ?2.83 dB for TE and ?3.49 for TM polarization centered near the 1550 nm wavelength. Polarization-dependent loss of less than 1 dB over 45.3 nm is achieved.

Photonics Research
Nov. 15, 2016, Vol. 4 Issue 6 06000209 (2016)
InP-based directly modulated monolithic integrated few-mode transmitter
Zhaosong Li, Dan Lu, Yiming He, Fangyuan Meng, Xuliang Zhou, and Jiaoqing Pan

A monolithic integrated few-mode transmitter comprising of two directly modulated distributed feedback lasers and a multimode-interference-coupler-based mode converter-multiplexer with 66% mode conversion efficiency was designed and demonstrated. A fundamental TE0 mode and a first-order TE1 mode were successfully generated from the transmitter, with the output power of 4 and 5.5 mW at a pump current of around 150 mA, respectively, at the common output port. The small signal modulation bandwidth of the TE0 and TE1 channels reached 17.4 and 14.7 GHz, respectively. Error-free 2×10-Gbit/s direct modulation of the two-mode transmitter was demonstrated, with a power penalty of 4.3 dB between the TE0 mode and the TE1 mode at the bit error rate of 1×10 9.

Photonics Research
Apr. 20, 2018, Vol. 6 Issue 5 05000463 (2018)
Ultra-compact and broadband electro-absorption modulator using an epsilon-near-zero conductive oxide
Qian Gao, Erwen Li, and Alan X. Wang

Transparent conductive oxides have emerged as a new type of plasmonic material and demonstrated unique electro-optic (E-O) modulation capabilities for next-generation photonic devices. In this paper, we report an ultra-compact, broadband electro-absorption (EA) modulator using an epsilon-near-zero (ENZ) indium-tin oxide (ITO). The device is fabricated on a standard silicon-on-insulator platform through the integration with a 3 μm long, 300 nm wide gold plasmonic slot waveguide. The active E-O modulation region consists of a metal–HfO2–ITO capacitor that can electrically switch the ITO into ENZ with ultra-high modulation strengths of 2.62 and 1.5 dB/μm in simulation and experiment, respectively. The EA modulator also demonstrated a uniform E-O modulation with 70 nm optical bandwidth from 1530 to 1600 nm wavelength.

Photonics Research
Mar. 22, 2018, Vol. 6 Issue 4 04000277 (2018)
High-efficiency and broadband four-wave mixing in a silicon-graphene strip waveguide with a windowed silica top layer
Yuxing Yang, Zhenzhen Xu, Xinhong Jiang, Yu He, Xuhan Guo, Yong Zhang, Ciyuan Qiu, and Yikai Su

We experimentally demonstrate high-efficiency and broadband four-wave mixing in a silicon-graphene strip waveguide. A four-wave mixing conversion efficiency of 38.7 dB and a 3-dB conversion bandwidth of 35 nm are achieved in the silicon-graphene strip waveguide with an optimized light-graphene interaction length of 60 μm. The interaction length is controlled by a windowed area of silica layer on the silicon waveguide. Numerical simulations and experimental studies are carried out and show a nonlinear parameter γGOS as large as 104 W 1 ·m 1.

Photonics Research
Sep. 18, 2018, Vol. 6 Issue 10 10000965 (2018)
Broadband rhenium disulfide optical modulator for solid-state lasers
Xiancui Su, Baitao Zhang, Yiran Wang, Guanbai He, Guoru Li, Na Lin, Kejian Yang, Jingliang He, and Shande Liu

Rhenium disulfide (ReS2), a member of group VII transition metal dichalcogenides (TMDs), has attracted increasing attention because of its unique distorted 1T structure and electronic and optical properties, which are much different from those of group VI TMDs (MoS2, WS2, MoSe2, WSe2, etc.). It has been proved that bulk ReS2 behaves as a stack of electronically and vibrationally decoupled monolayers, which offers remarkable possibilities to prepare a monolayer ReS2 facilely and offers a novel platform to study photonic properties of TMDs. However, due to the large and layer-independent bandgap, the nonlinear optical properties of ReS2 from the visible to mid-infrared spectral range have not yet been investigated. Here, the band structure of ReS2 with the introduction of defects is simulated by the ab initio method, and the results indicate that the bandgap can be reduced from 1.38 to 0.54 eV with the introduction of defects in a suitable range. In the experiment, using a bulk ReS2 with suitable defects as the raw material, a few-layered broadband ReS2 saturable absorber (SA) is prepared by the liquid phase exfoliation method. Using the as-prepared ReS2 SA, passively Q-switched solid-state lasers at wavelengths of 0.64, 1.064, and 1.991 μm are in

Photonics Research
Apr. 26, 2018, Vol. 6 Issue 6 06000498 (2018)
High-performance AlGaInP light-emitting diodes integrated on silicon through a superior quality germanium-on-insulator
Yue Wang, Bing Wang, Wardhana A. Sasangka, Shuyu Bao, Yiping Zhang, Hilmi Volkan Demir, Jurgen Michel, Kenneth Eng Kian Lee, Soon Fatt Yoon, Eugene A. Fitzgerald, Chuan Seng Tan, and Kwang Hong Lee

High-performance GaInP/AlGaInP multi-quantum well light-emitting diodes (LEDs) grown on a low threading dislocation density (TDD) germanium-on-insulator (GOI) substrate have been demonstrated. The low TDD of the GOI substrate is realized through Ge epitaxial growth, wafer bonding, and layer transfer processes on 200 mm wafers. With O2 annealing, the TDD of the GOI substrate can be reduced to ~1.2×106 cm 2. LEDs fabricated on this GOI substrate exhibit record-high optical output power of 1.3 mW at a 670 nm peak wavelength under 280 mA current injection. This output power level is at least 2 times higher compared to other reports of similar devices on a silicon (Si) substrate without degrading the electrical performance. These results demonstrate great promise for the monolithic integration of visible-band optical sources with Si-based electronic circuitry and realization of high-density RGB (red, green, and blue) micro-LED arrays with control circuitry.

Photonics Research
Mar. 28, 2018, Vol. 6 Issue 4 04000290 (2018)
Analysis of optical coupling behavior in two-dimensional implant-defined coherently coupled vertical-cavity surface-emitting laser arrays
Guanzhong Pan, Yiyang Xie, Chen Xu, Yibo Dong, Jun Deng, Hongda Chen, and Jie Sun

Optical coupling behavior and associated effects in two-dimensional implant-defined coherently coupled vertical-cavity surface-emitting laser (VCSEL) arrays are studied via both experiments and theoretical calculations. Experiments show that optical coupling between array elements can enhance the array’s output power. Additionally, optical coupling via leaky optical fields can provide extra optical gain for the array elements, which can then reduce the thresholds of these elements. Elements can even be pumped without current injection to emit light by receiving a strong leaky optical field from other array elements. Optical coupling can also cause unusual phenomena: the central elements in large-area coherently coupled VCSEL arrays that lase prior to the outer elements when the arrays are biased, or the average injection current required for each element to lase, which is much lower than the threshold for a single VCSEL. Theoretical calculations are performed to explain the experimental results.

Photonics Research
Oct. 16, 2018, Vol. 6 Issue 11 11001048 (2018)
Capacitive actuation and switching of add–drop graphene-silicon micro-ring filters
Tommaso Cassese, Marco Angelo Giambra, Vito Sorianello, Gabriele De Angelis, Michele Midrio, Marianna Pantouvaki, Joris Van Campenhout, Inge Asselberghs, Cedric Huyghebaert, Antonio D’Errico, and Marco Romagnoli

We propose and experimentally demonstrate capacitive actuation of a graphene–silicon micro-ring add/drop filter. The mechanism is based on a silicon–SiO2–graphene capacitor on top of the ring waveguide. We show the capacitive actuation of the add/drop functionality by a voltage-driven change of the graphene optical absorption. The proposed capacitive solution overcomes the need for continuous heating to keep tuned the filter’s in/out resonance and therefore eliminates “in operation” energy consumption.

Photonics Research
Sep. 27, 2017, Vol. 5 Issue 6 06000762 (2017)
Bound states in the continuum and high-Q resonances supported by a dielectric ridge on a slab waveguide
Evgeni A. Bezus, Dmitry A. Bykov, and Leonid L. Doskolovich

We investigate the diffraction of the guided modes of a dielectric slab waveguide on a simple integrated structure consisting of a single dielectric ridge on the surface of the waveguide. Numerical simulations based on aperiodic rigorous coupled-wave analysis demonstrate the existence of sharp resonant features and bound states in the continuum (BICs) in the reflectance and transmittance spectra occurring at the oblique incidence of a transverse-electric (TE)-polarized guided mode on the ridge. Using the effective index method, we explain the resonances by the excitation of cross-polarized modes of the ridge. Formation of the BICs are confirmed using a theoretical model based on coupled-wave theory. The model suggests that the BICs occur due to the coupling of quasi-TE and quasi-transverse-magnetic modes of the structure. Simple analytical expressions for the angle of incidence and the ridge width predicting the location of the BICs are obtained. The existence of high-Q resonances and BICs enables using the considered integrated structure for sensing, transformation of optical signals, and enhancing nonlinear light–matter interactions. Due to the Lorentzian line shape of the resonances near the BICs, the structure is also promising for filtering applications.

Photonics Research
Nov. 01, 2018, Vol. 6 Issue 11 11001084 (2018)
Broadband terahertz rotator with an all-dielectric metasurface
Quanlong Yang, Xieyu Chen, Quan Xu, Chunxiu Tian, Yuehong Xu, Longqing Cong, Xueqian Zhang, Yanfeng Li, Caihong Zhang, Xixiang Zhang, Jiaguang Han, and Weili Zhang

Polarization manipulation is essential in developing cutting-edge photonic devices ranging from optical communication displays to solar energy harvesting. Most previous works for efficient polarization control cannot avoid utilizing metallic components that inevitably suffer from large ohmic loss and thus low operational efficiency. Replacing metallic components with Mie resonance-based dielectric resonators will largely suppress the ohmic loss toward high-efficiency metamaterial devices. Here, we propose an efficient approach for broadband, high-quality polarization rotation operating in transmission mode with all-dielectric metamaterials in the terahertz regime. By separating the orthogonal polarization components in space, we obtain rotated output waves with a conversion efficiency of 67.5%. The proposed polarization manipulation strategy shows impressive robustness and flexibility in designing metadevices of both linear- and circular-polarization incidences.

Photonics Research
Oct. 17, 2018, Vol. 6 Issue 11 11001056 (2018)
Broadband athermal waveguides and resonators for datacom and telecom applications
Liuqing He, Yuhao Guo, Zhaohong Han, Kazumi Wada, Jurgen Michel, Anuradha M. Agarwal, Lionel C. Kimerling, Guifang Li, and Lin Zhang

The high-temperature sensitivity of the silicon material index limits the applications of silicon-based micro-ring resonators in integrated photonics. To realize a low but broadband temperature-dependent-wavelength-shift microring resonator, designing a broadband athermal waveguide becomes a significant task. In this work, we propose a broadband athermal waveguide that shows a low effective thermo-optical coefficient of ±1×10 6/K from 1400 to 1700 nm. The proposed waveguide shows a low-loss performance and stable broadband athermal property when it is applied to ring resonators, and the bending loss of ring resonators with a radius of >30 μm is 0.02 dB/cm.

Photonics Research
Oct. 08, 2018, Vol. 6 Issue 11 11000987 (2018)
Integrated flat-top reflection filters operating near bound states in the continuum
Leonid L. Doskolovich, Evgeni A. Bezus, and Dmitry A. Bykov

We propose and theoretically and numerically investigate narrowband integrated filters consisting of identical resonant dielectric ridges on the surface of a single-mode dielectric slab waveguide. The proposed composite structures operate near a bound state in the continuum (BIC) and enable spectral filtering of transverse-electric-polarized guided modes propagating in the waveguide. We demonstrate that by proper choice of the distances between the ridges, flat-top reflectance profiles with steep slopes and virtually no sidelobes can be obtained using just a few ridges. In particular, the structure consisting of two ridges can optically implement the second-order Butterworth filter, whereas at a larger number of ridges, excellent approximations to higher-order Butterworth filters can be achieved. Owing to the BIC supported by the ridges constituting the composite structure, the flat-top reflection band can be made arbitrarily narrow without increasing structure size. In addition to the filtering properties, the investigated structures support another type of BIC—the Fabry–Perot BIC—arising when the distances between adjacent ridges meet the Fabry–Perot resonance condition. In the vicinity of the Fabry–Perot BIC, an effect similar to electromagnetically induced transparency is observed, namely, sharp transmittance peaks against the background of a wide transmittance dip.

Photonics Research
Nov. 01, 2019, Vol. 7 Issue 11 11001314 (2019)
Time-domain compact macromodeling of linear photonic circuits via complex vector fitting
Yinghao Ye, Domenico Spina, Dirk Deschrijver, Wim Bogaerts, and Tom Dhaene

In this paper, a novel baseband macromodeling framework for linear passive photonic circuits is proposed, which is able to build accurate and compact models while taking into account the nonidealities, such as higher order dispersion and wavelength-dependent losses of the circuits. Compared to a previous modeling method based on the vector fitting algorithm, the proposed modeling approach introduces a novel complex vector fitting technique. It can generate a half-size state-space model for the same applications, thereby achieving a major improvement in efficiency of the time-domain simulations. The proposed modeling framework requires only measured or simulated scattering parameters as input, which are widely used to represent linear and passive systems. Three photonic circuits are studied to demonstrate the accuracy and efficiency of the proposed technique.

Photonics Research
Jun. 24, 2019, Vol. 7 Issue 7 07000771 (2019)
Compact and broadband multimode waveguide bend by shape-optimizing with transformation optics
Shuyi Li, Lifeng Cai, Dingshan Gao, Jianji Dong, Jin Hou, Chunyong Yang, Shaoping Chen, and Xinliang Zhang

Multimode waveguide bend is one of the key components for realizing high-density mode-division multiplexing systems on chip. However, the reported multimode waveguide bends are either large, bandwidth-limited or fabrication-complicated, which hinders their applications in future high-density multimode photonic circuits. Here we propose a compact multimode waveguide bend supporting four TE modes simply by shape-optimizing with transformation optics. The shape of the waveguide is optimized in the virtual space with gradient distribution of the refractive index, so that the scattering loss and intermode cross talk are well suppressed. After conformal mapping back into the physical space, a compact (effective radius of 17 μm) multimode bending waveguide is obtained. Simulations show that the proposed multimode waveguide bend has little loss (0.1 dB) and low cross talk (-20 dB) throughout an ultrabroad wavelength range of 1.16–1.66 μm. We also fabricated the shape-optimized multimode bending waveguide on a silicon-on-insulator wafer. At 1550 nm wavelength, the measured excess losses for the four lowest-order TE modes are less than 0.6 dB, and the intermode cross talks are all below -17 dB. Our study paves the way for realizing high-density and large-scale multimode integrated optical circuits for optical interconnect.

Photonics Research
Spotlight on OpticsNov. 17, 2020, Vol. 8 Issue 12 12001843 (2020)
Surface-illuminated photon-trapping high-speed Ge-on-Si photodiodes with improved efficiency up to 1700  nm
Hilal Cansizoglu, Cesar Bartolo-Perez, Yang Gao, Ekaterina Ponizovskaya Devine, Soroush Ghandiparsi, Kazim G. Polat, Hasina H. Mamtaz, Toshishige Yamada, Aly F. Elrefaie, Shih-Yuan Wang, and M. Saif Islam

In this paper, high-speed surface-illuminated Ge-on-Si pin photodiodes with improved efficiency are demonstrated. With photon-trapping microhole features, the external quantum efficiency (EQE) of the Ge-on-Si pin diode is >80% at 1300 nm and 73% at 1550 nm with an intrinsic Ge layer of only 2 μm thickness, showing much improvement compared to one without microholes. More than threefold EQE improvement is also observed at longer wavelengths beyond 1550 nm. These results make the microhole-enabled Ge-on-Si photodiodes promising to cover both the existing C and L bands, as well as a new data transmission window (1620–1700 nm), which can be used to enhance the capacity of conventional standard single-mode fiber cables. These photodiodes have potential for many applications, such as inter-/intra-datacenters, passive optical networks, metro and long-haul dense wavelength division multiplexing systems, eye-safe lidar systems, and quantum communications. The CMOS and BiCMOS monolithic integration compatibility of this work is also attractive for Ge CMOS, near-infrared sensing, and communication integration.

Photonics Research
On the CoverJun. 21, 2018, Vol. 6 Issue 7 07000734 (2018)
3D integrated photonics platform with deterministic geometry control
Jérôme Michon, Sarah Geiger, Lan Li, Claudia Goncalves, Hongtao Lin, Kathleen Richardson, Xinqiao Jia, and Juejun Hu

3D photonics promises to expand the reach of photonics by enabling the extension of traditional applications to nonplanar geometries and adding novel functionalities that cannot be attained with planar devices. Available material options and device geometries are, however, limited by current fabrication methods. In this work, we pioneer a method that allows for placement of integrated photonic device arrays at arbitrary predefined locations in 3D using a fabrication process that capitalizes on the buckling of a 2D pattern. We present theoretical and experimental validation of the deterministic buckling process, thus demonstrating implementation of the technique to realize what we believe to be the first fully packaged 3D integrated photonics platform. Application of the platform for mechanical strain sensing is further demonstrated.

Photonics Research
Editors' PickJan. 31, 2020, Vol. 8 Issue 2 02000194 (2020)
Ultra-flat dispersion in an integrated waveguide with five and six zero-dispersion wavelengths for mid-infrared photonics
Yuhao Guo, Zeinab Jafari, Lijuan Xu, Changjing Bao, Peicheng Liao, Guifang Li, Anuradha M. Agarwal, Lionel C. Kimerling, Jurgen Michel, Alan E. Willner, and Lin Zhang

We propose a new type of dispersion flattening technology, which can generate an ultra-flat group velocity dispersion profile with five and six zero-dispersion wavelengths (ZDWs). The dispersion value varies from 0.15 to 0.35 ps/(nm·km) from 4 to 8 μm, which to the best of our knowledge is the flattest one reported so far, and the dispersion flatness is improved by more than an order of magnitude. We explain the principle of producing six ZDWs. Mode distribution in this waveguide is made stable over a wide bandwidth. General guidelines to systematically control the dispersion value, sign, and slope are provided, and one can achieve the desired dispersion by properly adjusting the structural parameters. Fabrication tolerance of this waveguide is also examined.

Photonics Research
Editors' PickOct. 29, 2019, Vol. 7 Issue 11 11001279 (2019)
Advances in on-chip photonic devices based on lithium niobate on insulator
Jintian Lin, Fang Bo, Ya Cheng, and Jingjun Xu

Crystalline lithium niobate (LN) is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients. Furthermore, the recent development and commercialization of LN-on-insulator (LNOI) technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss, optical nonlinearity, and electro-optic tunability. This review begins with a brief introduction of the history and current status of LNOI photonics. We then discuss the fabrication techniques of LNOI-based photonic structures and devices. The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI, which enables the production of building block structures, such as optical microresonators and waveguides of unprecedented optical qualities. The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits. Some conclusions and future perspectives are provided.

Photonics Research
Editors' PickNov. 30, 2020, Vol. 8 Issue 12 12001910 (2020)
Numerical modeling of a linear photonic system for accurate and efficient time-domain simulations
Yinghao Ye, Domenico Spina, Yufei Xing, Wim Bogaerts, and Tom Dhaene

In this paper, a novel modeling and simulation method for general linear, time-invariant, passive photonic devices and circuits is proposed. This technique, starting from the scattering parameters of the photonic system under study, builds a baseband equivalent state-space model that splits the optical carrier frequency and operates at baseband, thereby significantly reducing the modeling and simulation complexity without losing accuracy. Indeed, it is possible to analytically reconstruct the port signals of the photonic system under study starting from the time-domain simulation of the corresponding baseband equivalent model. However, such equivalent models are complex-valued systems and, in this scenario, the conventional passivity constraints are not applicable anymore. Hence, the passivity constraints for scattering parameters and state-space models of baseband equivalent systems are presented, which are essential for time-domain simulations. Three suitable examples demonstrate the feasibility, accuracy, and efficiency of the proposed method.

Photonics Research
May. 23, 2018, Vol. 6 Issue 6 06000560 (2018)
Reconfigurable directional coupler in lithium niobate crystal fabricated by three-dimensional femtosecond laser focal field engineering
Qian Zhang, Meng Li, Jian Xu, Zijie Lin, Haofeng Yu, Min Wang, Zhiwei Fang, Ya Cheng, Qihuang Gong, and Yan Li

For crystals, depressed cladding waveguides have advantages such as preservation of the spectroscopic as well as non-linear properties and the capability to guide both horizontal and vertical polarization modes, but fabrication is always quite time consuming. In addition, it is usually difficult to couple modes propagating in different depressed cladding waveguides through evanescent field overlap, so it is often required to dynamically reconfigure photonic waveguide devices using external fields for classical or quantum applications. Here, we experimentally demonstrate the single-scan femtosecond laser transverse writing of depressed cladding waveguides to form a 2×2 directional coupler inside lithium niobate crystal, which is integrated with two deeply embedded microelectrodes on both sides of the interaction region to reconfigure the coupling. By focal field engineering of the femtosecond laser, we specially generate a three-dimensional longitudinally oriented ring-shaped focal intensity profile composed of 16 discrete spots to simultaneously write the entire cladding region. The fabricated waveguides exhibit good single guided modes in two orthogonal polarizations at 1550 nm. By applying voltage to the deeply embedded microelectrodes fabricated with the femtosecond laser ablation followed by selective electroless plating, we successfully facilitate the light coupling from the input arm to the cross arm and thus actively tune the splitting ratio. These results open new important perspectives in the efficient fabrication of reconfigurable complex three-dimensional devices in crystals based on depressed cladding waveguides.

Photonics Research
Apr. 11, 2019, Vol. 7 Issue 5 05000503 (2019)
High-Q germanium optical nanocavity
Ting-Hui Xiao, Ziqiang Zhao, Wen Zhou, Mitsuru Takenaka, Hon Ki Tsang, Zhenzhou Cheng, and Keisuke Goda

Mid-infrared (MIR) integrated photonics has attracted broad interest due to its promising applications in biochemical sensing, environmental monitoring, disease diagnosis, and optical communication. Among MIR integration platforms, germanium-based platforms hold many excellent properties, such as wide transparency windows, high refractive indices, and high nonlinear coefficients; however, the development of MIR germanium photonic devices is still in its infancy. Specifically, MIR high-Q germanium resonators with comparable performance to their silicon counterparts remain unprecedented. Here we experimentally demonstrate an MIR germanium nanocavity with a Q factor of ~18,000, the highest-to-date of reported nanocavities across MIR germanium-based integration platforms. This is achieved through a combination of a feasible theoretical design, Smart-Cut methods for wafer development, and optimized device fabrication processes. Our nanocavity, with its high Q factor and ultrasmall mode volume, opens new avenues for on-chip applications in the MIR spectral range.

Photonics Research
Aug. 29, 2018, Vol. 6 Issue 9 09000925 (2018)
Tunable optical delay line based on integrated grating-assisted contradirectional couplers
Xu Wang, Yuhe Zhao, Yunhong Ding, Sanshui Xiao, and Jianji Dong

Tunable optical delay lines are one of the key building blocks in optical communication and microwave systems. In this work, tunable optical delay lines based on integrated grating-assisted contradirectional couplers are proposed and experimentally demonstrated. The device performance is comprehensively improved in terms of parameter optimization, apodization analysis, and electrode design. Tunable group delay lines of 50 ps at different wavelengths within the bandwidth of 12 nm are realized with a grating length of 1.8 mm. Under thermal tuning mode, the actual delay tuning range is around 20 ps at 7.2 V voltage. At last, a new scheme adopting an ultra-compact reflector for doubling group delay is proposed and verified, achieving a large group delay line of 400 ps and a large dispersion value up to 5.5×106 ps/(nm·km) within bandwidth of 12 nm. Under thermal tuning mode, the actual delay tuning range is around 100 ps at 8 V voltage.

Photonics Research
Aug. 21, 2018, Vol. 6 Issue 9 09000880 (2018)
High-gain waveguide amplifiers in Si3N4 technology via double-layer monolithic integration
Jinfeng Mu, Meindert Dijkstra, Jeroen Korterik, Herman Offerhaus, and Sonia M. García-Blanco

Silicon nitride (Si3N4)-on-SiO2 attracts increasing interest in integrated photonics owing to its low propagation loss and wide transparency window, extending from ~400 nm to 2350 nm. Scalable integration of active devices such as amplifiers and lasers on the Si3N4 platform will enable applications requiring optical gain and a much-needed alternative to hybrid integration, which suffers from high cost and lack of high-volume manufacturability. We demonstrate a high-gain optical amplifier in Al2O3:Er3+ monolithically integrated on the Si3N4 platform using a double photonic layer approach. The device exhibits a net Si3N4-to-Si3N4 gain of 18.1±0.9 dB at 1532 nm, and a broadband gain operation over 70 nm covering wavelengths in the S-, C- and L-bands. This work shows that rare-earth-ion-doped materials and in particular, rare-earth-ion-doped Al2O3, can provide very high net amplification for the Si3N4 platform, paving the way to the development of different active devices monolithically integrated in this passive platform.

Photonics Research
On the CoverSep. 28, 2020, Vol. 8 Issue 10 10001634 (2020)
Acousto-optical modulation of thin film lithium niobate waveguide devices
Lutong Cai, Ashraf Mahmoud, Msi Khan, Mohamed Mahmoud, Tamal Mukherjee, James Bain, and Gianluca Piazza

Due to its strong piezoelectric effect and photo-elastic property, lithium niobate is widely used for acousto-optical applications. However, conventional bulk lithium niobate waveguide devices exhibit a large footprint and limited light–sound interaction resulting from the weak guiding of light. Here, we report the first acousto-optical modulators with surface acoustic wave generation, phononic cavity, and low-loss photonic waveguide devices monolithically integrated on a 500 nm thick film of lithium niobate on an insulator. Modulation efficiency was optimized by properly arranging the propagation directions of surface acoustic waves and optical guided modes. The effective photo-elastic coefficient extracted by comparing the first and third harmonic modulation signals from an on-chip Mach–Zehnder interferometer indicates the excellent acousto-optical properties of lithium niobate are preserved in the thin film implementation. Such material property finding is of crucial importance in designing various types of acousto-optical devices. Much stronger amplitude modulation was achieved in a high Q (>300,000) optical resonator due to the higher optical sensitivity. Our results pave the path for developing novel acousto-optical devices using thin film lithium niobate.

Photonics Research
On the CoverAug. 12, 2019, Vol. 7 Issue 9 09001003 (2019)
All-optical tuning of a diamond micro-disk resonator on silicon
Paul Hill, Charalambos Klitis, Benoit Guilhabert, Marc Sorel, Erdan Gu, Martin D. Dawson, and Michael J. Strain

High-quality integrated diamond photonic devices have previously been demonstrated in applications from non-linear photonics to on-chip quantum optics. However, the small sample sizes of single crystal material available, and the difficulty in tuning its optical properties, are barriers to the scaling of these technologies. Both of these issues can be addressed by integrating micrometer-scale diamond devices onto host photonic integrated circuits using a highly accurate micro-assembly method. In this work a diamond micro-disk resonator is integrated with a standard single-mode silicon-on-insulator waveguide, exhibiting an average loaded Q-factor of 3.1×104 across a range of spatial modes, with a maximum loaded Q-factor of 1.05×105. The micrometer-scale device size and high thermal impedance of the silica interface layer allow for significant thermal loading and continuous resonant wavelength tuning across a 450 pm range using a milliwatt-level optical pump. This diamond-on-demand integration technique paves the way for tunable devices coupled across large-scale photonic circuits.

Photonics Research
Feb. 17, 2020, Vol. 8 Issue 3 03000318 (2020)
Effects of coupling and phase imperfections in programmable photonic hexagonal waveguide meshes
Iman Zand, and Wim Bogaerts

We present a study of the effect of imperfections on the transmission and crosstalk in programmable photonic meshes with feedback loops consisting of tunable couplers and phase shifters. The many elements in such a mesh can generate a multitude of parasitic paths when the couplers and phase shifters deviate even slightly from their nominal value. Performing Monte Carlo simulations, we show that small stochastic imperfections in the phase and coupling (1.0%) can introduce unwanted interferences and resonances and significantly deteriorate the frequency response of the circuit. We also demonstrate that, in the presence of imperfections, the programming strategy of the unused couplers can reduce effects of such parasitics.

Photonics Research
Jan. 31, 2020, Vol. 8 Issue 2 02000211 (2020)
Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors
You Wu, Zhiwen Li, Kah-Wee Ang, Yuping Jia, Zhiming Shi, Zhi Huang, Wenjie Yu, Xiaojuan Sun, Xinke Liu, and Dabing Li

With the increasing demand for high integration and multi-color photodetection for both military and civilian applications, the research of multi-wavelength detectors has become a new research hotspot. However, current research has been mainly in visible dual- or multi-wavelength detectors, while integration of both visible light and ultraviolet (UV) dual-wavelength detectors has rarely been studied. In this work, large-scale and high-quality monolayer MoS2 was grown by the chemical vapor deposition method on transparent free-standing GaN substrate. Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors was demonstrated using common semiconductor fabrication technologies such as photolithography, argon plasma etching, and metal deposition. High performance of a 280 nm and 405 nm dual-wavelength photodetector was realized. The responsivity of the UV detector reached 172.12 A/W, while that of the visible detector reached 17.5 A/W. Meanwhile, both photodetectors achieved high photocurrent gain, high external quantum efficiency, high normalized detection rate, and low noise equivalent power. Our study extends the future application of dual-wavelength detectors for image sensing and optical communication.

Photonics Research
On the CoverSep. 10, 2019, Vol. 7 Issue 10 10001127 (2019)
Ultra-compact broadband polarization beam splitter with strong expansibility
Jie Huang, Junbo Yang, Dingbo Chen, Xin He, Yunxin Han, Jingjing Zhang, and Zhaojian Zhang

Based on the traditional directional coupler, we proposed a scheme to design on-chip polarization beam splitters using an inverse design method. In our scheme, the coupling area of the designed devices are only 0.48 μm×6.4 μm. By manipulating the refractive index of the coupling region, the devices can work in C-band, L-band, O-band, or any other communication band. Different from conventional design methods, which need to adjust the design parameters artificially, if the initial conditions are determined, the proposed scheme can automatically adjust the design parameters of devices according to specific requirements. The simulation results show that the insertion losses of the designed polarization beam splitters can be less than 0.4 dB (0.35 dB) for TE (TM) mode at the wavelengths of 1310, 1550, and 1600 nm, and the extinction ratios are larger than 19.9 dB for the TE and TM modes at all three wavelengths. Besides, the extinction ratios of both polarization states are more than 14.5 dB within the wavelength range of 1286–1364 nm, 1497–1568 nm, and 1553–1634 nm. At the same time, the insertion losses are smaller than 0.46 dB.

Photonics Research
May. 23, 2018, Vol. 6 Issue 6 06000574 (2018)
Bias-drift-free Mach–Zehnder modulators based on a heterogeneous silicon and lithium niobate platform
Shihao Sun, Mingbo He, Mengyue Xu, Shengqian Gao, Ziyan Chen, Xian Zhang, Ziliang Ruan, Xiong Wu, Lidan Zhou, Lin Liu, Chao Lu, Changjian Guo, Liu Liu, Siyuan Yu, and Xinlun Cai

Optical modulators have been and will continue to be essential devices for energy- and cost-efficient optical communication networks. Heterogeneous silicon and lithium niobate modulators have demonstrated promising performances of low optical loss, low drive voltage, and large modulation bandwidth. However, DC bias drift is a major drawback of optical modulators using lithium niobate as the active electro-optic material. Here, we demonstrate high-speed and bias-drift-free Mach–Zehnder modulators based on the heterogeneous silicon and lithium niobate platform. The devices combine stable thermo-optic DC biases in silicon and ultra-fast electro-optic modulation in lithium niobate, and exhibit a low insertion loss of 1.8 dB, a low half-wave voltage of 3 V, an electro-optic modulation bandwidth of at least 70 GHz, and modulation data rates up to 128 Gb/s.

Photonics Research
Editors' PickNov. 30, 2020, Vol. 8 Issue 12 12001958 (2020)
Erbium-doped TeO2-coated Si3N4 waveguide amplifiers with 5 dB net gain
Henry C. Frankis, Hamidu M. Mbonde, Dawson B. Bonneville, Chenglin Zhang, Richard Mateman, Arne Leinse, and Jonathan D. B. Bradley

We demonstrate 5 dB net gain in an erbium-doped tellurium-oxide-coated silicon nitride waveguide. The amplifier design leverages the high refractive index and high gain in erbium-doped tellurite glass as well as the ultra-low losses and mature, reliable, and low-cost fabrication methods of silicon nitride waveguide technology. We show that the waveguide platform demonstrates low background propagation losses of 0.25 dB/cm based on a ring resonator device with a Q factor of 1.3×106 at 1640 nm. We measure 5 dB peak net gain at 1558 nm and >3 dB of net gain across the C band in a 6.7 cm long waveguide for 35 mW of launched 1470 nm pump power. Gain per unit length of 1.7 and 1.4 dB/cm is measured in a 2.2 cm long waveguide for 970 and 1470 nm pump wavelengths, respectively. Amplifier simulations predict that >10 dB gain can be achieved across the C band simply by optimizing waveguide length and fiber-chip coupling. These results demonstrate a promising approach for the monolithic integration of compact erbium-doped waveguide amplifiers on silicon nitride chips and within silicon-based photonic integrated circuits.

Photonics Research
Jan. 22, 2020, Vol. 8 Issue 2 02000127 (2020)
Lithium niobate waveguides with high-index contrast and preserved nonlinearity fabricated by a high vacuum vapor-phase proton exchange
Alicia Petronela Rambu, Alin Marian Apetrei, Florent Doutre, Hervé Tronche, Vasile Tiron, Marc de Micheli, and Sorin Tascu

Highly confining waveguides (Δne>0.1) without a degraded nonlinear coefficient and low propagation losses have been fabricated in lithium niobate (LN) by a new process that we called high vacuum vapor-phase proton exchange (HiVac-VPE). Index contrast, index profile, nonlinearity, and crystallographic phases are carefully investigated. Original analysis of index profiles indicates that the waveguides contain sub-layers whose depths depend on the exchange durations. Propagation behavior, propagation losses, and second-harmonic generation response of HiVac-VPE channel waveguides are investigated at telecom wavelength. The results recommend HiVac-VPE as a very promising technique for fabricating efficient nonlinear photonic integrated circuits in LN crystals.

Photonics Research
Editors' PickJan. 01, 2020, Vol. 8 Issue 1 01000008 (2020)
Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion
Ahmed E. Hassanien, Steffen Link, Yansong Yang, Edmond Chow, Lynford L. Goddard, and Songbin Gong

Microwave photonics, a field that crosscuts microwave/millimeter-wave engineering with optoelectronics, has sparked great interest from research and commercial sectors. This multidisciplinary fusion can achieve ultrawide bandwidth and ultrafast speed that were considered impossible in conventional chip-scale microwave/millimeter-wave systems. Conventional microwave-to-photonic converters, based on resonant acousto-optic modulation, produce highly efficient modulation but sacrifice bandwidth and limit their applicability for most real-world microwave signal-processing applications. In this paper, we build highly efficient and wideband microwave-to-photonic modulators using the acousto-optic effect on suspended lithium niobate thin films. A wideband microwave signal is first piezoelectrically transduced using interdigitated electrodes into Lamb acoustic waves, which directly propagates across an optical waveguide and causes refractive index perturbation through the photoelastic effect. This approach is power-efficient, with phase shifts up to 0.0166 rad/√mW over a 45 μm modulation length and with a bandwidth up to 140 MHz at a center frequency of 1.9 GHz. Compared to the state-of-the-art, a 9× more efficient modulation has been achieved by optimizing the acoustic and optical modes and their interactions.

Photonics Research
Editors' PickJun. 07, 2021, Vol. 9 Issue 7 07001182 (2021)
Towards the integration of nanoemitters by direct laser writing on optical glass waveguides
Xiaolun Xu, Aurélie Broussier, Tiziana Ritacco, Mackrine Nahra, Fabien Geoffray, Ali Issa, Safi Jradi, Renaud Bachelot, Christophe Couteau, and Sylvain Blaize

A major challenge towards nanophotonics is the integration of nanoemitters on optical chips. Combining the optical properties of nanoemitters with the benefits of integration and scalability of integrated optics is still a major issue to overcome. In this work, we demonstrate the integration of nanoemitters positioned in a controlled manner onto a substrate and onto an optical ion-exchanged glass waveguide via direct laser writing based on two-photon polymerization. Our nanoemitters are colloidal CdSe/ZnS quantum dots (QDs) embedded in polymeric nanostructures. By varying the laser parameters during the patterning process, we make size-controlled QD-polymer nanostructures that were systematically characterized using optical and structural methods. Structures as small as 17 nm in height were fabricated. The well-controlled QD-polymer nanostructure systems were then successfully integrated onto a new photonic platform for nanophotonics made of an ion-exchanged waveguide. We show that our QDs maintain their light emitting quality after integration as verified by photoluminescence (PL) measurements. Ultimately, QD emission coupled to our waveguides is detected through a home-built fiber-edge coupling PL measurement setup. Our results show the potential for future integration of nanoemitters onto complex photonic chips.

Photonics Research
Sep. 01, 2020, Vol. 8 Issue 9 09001541 (2020)
Digitally tunable optical delay line based on thin-film lithium niobate featuring high switching speed and low optical loss
Wei Ke, Yanmei Lin, Mingbo He, Mengyue Xu, Jiaxiang Zhang, Zhongjin Lin, Siyuan Yu, and Xinlun Cai

A tunable optical delay line (ODL) featuring high switching speed and low optical loss is highly desirable in many fields. Here, based on the thin-film lithium niobate platform, we demonstrate a digitally tunable on-chip ODL that includes five Mach–Zehnder interferometer optical switches, four flip-chip photodetectors, and four delay-line waveguides. The proposed optical switches can achieve a switching speed of 13 ns and an extinction ratio of 34.9 dB. Using a modified Euler-bend-based spiral structure, the proposed delay-line waveguide can simultaneously achieve a small footprint and low optical propagation loss. The proposed ODL can provide a maximum delay time of 150 ps with a resolution of 10 ps and feature a maximum insertion loss of 3.4 dB.

Photonics Research
Oct. 28, 2022, Vol. 10 Issue 11 2575 (2022)
Ultralow-loss compact silicon photonic waveguide spirals and delay lines
Shihan Hong, Long Zhang, Yi Wang, Ming Zhang, Yiwei Xie, and Daoxin Dai

Low-loss and compact optical waveguides are key for realizing various photonic integrated circuits with long on-chip delay lines, such as tunable optical delay lines, optical coherence tomography, and optical gyroscopes. In this paper, a low-loss and compact silicon photonic waveguide spiral is proposed by introducing broadened Archimedean spiral waveguides with a tapered Euler S-bend. A 100-cm-long waveguide spiral is realized with a minimal bending radius as small as 10 μm by using a standard 220-nm-thick silicon-on-insulator foundry process, and the measured propagation loss is as low as 0.28 dB/cm. Furthermore, the present waveguide spirals are used to realize a 10-bit tunable optical delay line, which has a footprint as small as 2.2 mm×5.9 mm and a dynamic range of 5120 ps with a fine resolution of 10 ps.

Photonics Research
Jan. 01, 2022, Vol. 10 Issue 1 01000001 (2022)
On-chip reconfigurable mode converter based on cross-connected subwavelength Y-junctions
Longhui Lu, Deming Liu, Max Yan, and Minming Zhang

A novel power-efficient reconfigurable mode converter is proposed and experimentally demonstrated based on cross-connected symmetric Y-junctions assisted by thermo-optic phase shifters on a silicon-on-insulator platform. Instead of using conventional Y-junctions, subwavelength symmetric Y-junctions are utilized to enhance the mode splitting ability. The reconfigurable functionality can be realized by controlling the induced phase differences. Benefited from the cross-connected scheme, the number of heating electrodes can be effectively reduced, while the performance of the device is maintained. With only one-step etching, our fabricated device shows the average insertion losses and cross talks are less than 2.45 and -16.6 dB, respectively, measured with conversions between two arbitrary compositions of the first four TE modes over an observable 60 nm bandwidth. The converter is switchable and CMOS-compatible, and could be extended for more modes; hence, it can be potentially deployed for advanced and flexible mode multiplexing optical networks-on-chip.

Photonics Research
Jan. 01, 2021, Vol. 9 Issue 1 01000043 (2021)
Dry-etched ultrahigh-Q silica microdisk resonators on a silicon chip
Jiaxin Gu, Jie Liu, Ziqi Bai, Han Wang, Xinyu Cheng, Guanyu Li, Menghua Zhang, Xinxin Li, Qi Shi, Min Xiao, and Xiaoshun Jiang

We demonstrate the fabrication of ultrahigh quality (Q) factor silica microdisk resonators on a silicon chip by inductively coupled plasma (ICP) etching. We achieve a dry-etched optical microresonator with an intrinsic Q factor as high as 1.94×108 from a 1-mm-diameter silica microdisk with a thickness of 4 μm. Our work provides a chip-based microresonator platform operating in the ultrahigh-Q region that will be useful in nonlinear photonics such as Brillouin lasers and Kerr microcombs.

Photonics Research
Apr. 22, 2021, Vol. 9 Issue 5 05000722 (2021)
Broadband quasi-phase-matching in dispersion-engineered all-optically poled silicon nitride waveguides
Edgars Nitiss, Boris Zabelich, Ozan Yakar, Junqiu Liu, Rui Ning Wang, Tobias J. Kippenberg, and Camille-Sophie Brès

Quasi-phase-matching (QPM) has become one of the most common approaches for increasing the efficiency of nonlinear three-wave mixing processes in integrated photonic circuits. Here, we provide a study of dispersion engineering of QPM second-harmonic (SH) generation in stoichiometric silicon nitride (Si3N4) waveguides. We apply waveguide design and lithographic control in combination with the all-optical poling technique to study the QPM properties and shape the waveguide dispersion for broadband spectral conversion efficiency inside Si3N4 waveguides. By meeting the requirements for maximal bandwidth of the conversion efficiency spectrum, we demonstrate that group-velocity matching of the pump and SH is simultaneously satisfied, resulting in efficient SH generation from ultrashort optical pulses. The latter is employed for retrieving a carrier-envelope-offset frequency of a frequency comb by using an f-2f interferometric technique, where supercontinuum and SH of a femtosecond pulse are generated in Si3N4 waveguides. Finally, we show that the waveguide dispersion determines the QPM wavelength variation magnitude and sign due to the thermo-optic effect.

Photonics Research
Aug. 20, 2020, Vol. 8 Issue 9 09001475 (2020)
Silicon-integrated nonlinear III-V photonics
Weiqiang Xie, Chao Xiang, Lin Chang, Warren Jin, Jonathan Peters, and John E. Bowers

Mainstream silicon photonic integrated circuits are based on compact and low-loss silicon-on-insulator (SOI) waveguide platforms. However, monolithic SOI-based photonics provides only a limited number of functional device types. Here, to extend the on-chip capabilities, we propose a general heterogeneous integration approach to embed highly nonlinear III-V (AlGaAs) photonics into the SOI platform. We develop low-loss AlGaAs-on-SOI photonic circuits with integrated Si waveguides and showcase sub-milliwatt-threshold (∼0.25 mW) Kerr frequency comb generation in ultrahigh-Q AlGaAs microrings (Q over 106) at the telecom bands. Our demonstration complements existing mature Si photonics technology with efficient nonlinear functionalities provided by III-V and propels conventional Si photonics into emerging nonlinear photonic applications towards fully chip-based nonlinear engines.

Photonics Research
On the CoverFeb. 01, 2022, Vol. 10 Issue 2 02000535 (2022)
Polarization-insensitive interferometer based on a hybrid integrated planar light-wave circuit
Guo-Wei Zhang, Yu-Yang Ding, Wei Chen, Fang-Xiang Wang, Peng Ye, Guan-Zhong Huang, Shuang Wang, Zhen-Qiang Yin, Jun-Ming An, Guang-Can Guo, and Zheng-Fu Han

Interferometers are essential elements in classical and quantum optical systems. The strictly required stability when extracting the phase of photons is vulnerable to polarization variation and phase shift induced by environment disturbance. Here, we implement polarization-insensitive interferometers by combining silica planar light-wave circuit chips and Faraday rotator mirrors. Two asymmetric interferometers with temperature controllers are connected in series to evaluate the single-photon interference. Average interference visibility over 12 h is above 99%, and the variations are less than 0.5%, even with active random polarization disturbance. The experiment results verify that the hybrid chip is available for high-demand applications like quantum key distribution and entanglement measurement.

Photonics Research
Oct. 06, 2021, Vol. 9 Issue 11 11002176 (2021)
Phase regimes of parity-time-symmetric coupled-ring systems at exceptional points
Zhuang Ma, Xiaoyan Zhou, and Lin Zhang

The optical coupled resonant system consisting of an integrated resonator with gain and a resonator with loss provides an excellent platform to create exceptional points (EPs) in non-Hermitian systems. Most previous studies have focused on the striking intensity feature of EPs, but its phase response is seldom investigated. In this work, we present a thorough study on the phase response of an EP system. Intriguingly, the phase response exhibits distinct behavior depending on the ordering of the ring resonators: when the input light in a bus waveguide is coupled directly or indirectly to the ring with a gain, the phase response is featured by nonmonotonic transition and 2π monotonic transition, respectively. We also prove that the newly identified phase features are theoretically guaranteed. These phase responses produce unique group delays that have never been found in other coupled resonant systems. The results deepen our understanding on EPs in non-Hermitian systems and are potentially useful for practical applications exploiting phase features.

Photonics Research
Sep. 28, 2022, Vol. 10 Issue 10 2374 (2022)
Blind zone-suppressed hybrid beam steering for solid-state Lidar
Chao Li, Xianyi Cao, Kan Wu, Gaofeng Qiu, Minglu Cai, Guangjin Zhang, Xinwan Li, and Jianping Chen

We demonstrate a blind zone-suppressed and flash-emitting solid-state Lidar based on lens-assisted beam-steering technology. As a proof-of-concept demonstration, with the design of a subwavelength-gap 1D long-emitter array and multiwavelength flash beam emitting, the device was measured to have 5% blind zone suppression, 0.06°/point-deflection step, and 4.2 μs scanning speed. In time-of-flight ranging experiments, Lidar systems have a field of view of 11.3°×8.1° (normal device) or 0.9°×8.1° (blind-zone suppressed device), far-field number of resolved points of 192, and a detection distance of 10 m. This work demonstrates the possibility that a new integrated beam-steering technology can be implemented in a Lidar without sacrificing other performance.

Photonics Research
Sep. 01, 2021, Vol. 9 Issue 9 09001871 (2021)
Ultracompact, polarization-independent, and highly scalable optical power splitting model employing fan-out bending metamaterials
Zhenzhao Guo, Jinbiao Xiao, and Shengbao Wu

Optical power splitters (OPSs) are essential components in the photonic integrated circuits. Considerable power splitting schemes have been reported on the silicon-on-insulator platform. However, the corresponding device lengths are enlarged, and polarization-sensitive operations are usually encountered when the splitting channels are increased from two to five. In this paper, a novel power splitting model is proposed to overcome these limitations. Here, fan-out bending subwavelength grating (FBSWG) metamaterials instead of classical straight SWGs are leveraged to expand the input TE/TM mode in an ultracompact region and further bend its wavefronts. By using N-angled tapers to match bending wavefronts, the light expanded by FBSWGs can be collected and evenly distributed into N output channels. Based on such a model, three OPSs are designed and experimentally demonstrated, which are the shortest polarization-independent 1×3, 1×4, and 1×5 OPSs reported until now to our knowledge. The characterizations show low insertion losses (1.2 dB, 1.35 dB, and 1.65 dB) and uniformities (0.9 dB, 1 dB, and 1 dB) over bandwidths of 54 nm, 49 nm, and 38 nm for the 1×3, 1×4, and 1×5 OPSs, respectively. For the first time, an ultracompact device length of 4.3 μm and a polarization-independent operation can be maintained simultaneously as the output splitting channels are increased.

Photonics Research
Editors' PickOct. 10, 2022, Vol. 10 Issue 11 2448 (2022)
Thermo-optically tunable spectral broadening in a nonlinear ultra-silicon-rich nitride Bragg grating
Yanmei Cao, Ezgi Sahin, Ju Won Choi, Peng Xing, George F. R. Chen, D. K. T. Ng, Benjamin J. Eggleton, and Dawn T. H. Tan

Spectral tunability methods used in optical communications and signal processing leveraging optical, electrical, and acousto-optic effects typically involve spectral truncation that results in energy loss. Here we demonstrate temperature tunable spectral broadening using a nonlinear ultra-silicon-rich nitride device consisting of a 3-mm-long cladding-modulated Bragg grating and a 7-mm-long nonlinear channel waveguide. By operating at frequencies close to the grating band edge, in an apodized Bragg grating, we access strong grating-induced dispersion while maintaining low losses and high transmissivity. We further exploit the redshift in the Bragg grating stopband due to the thermo-optic effect to achieve tunable dispersion, leading to varying degrees of soliton-effect compression and self-phase-modulation-induced spectral broadening. We observe an increase in the bandwidth of the output pulse spectrum from 69 to 106 nm as temperature decreases from 70°C to 25°C, in good agreement with simulated results using the generalized nonlinear Schrödinger equation. The demonstrated approach provides a new avenue to achieve on-chip laser spectral tuning without loss in pulse energy.

Photonics Research
On the CoverMar. 31, 2021, Vol. 9 Issue 4 04000596 (2021)
Terabit FSO communication based on a soliton microcomb
Wen Shao, Yang Wang, Shuaiwei Jia, Zhuang Xie, Duorui Gao, Wei Wang, Dongquan Zhang, Peixuan Liao, Brent E. Little, Sai T. Chu, Wei Zhao, Wenfu Zhang, Weiqiang Wang, and Xiaoping Xie

Free-space optical (FSO) communication technology is a promising approach to establish a secure wireless link, which has the advantages of excellent directionality, large bandwidth, multiple services, low mass and less power requirements, and easy and fast deployments. Increasing the communication capacity is the perennial goal in both scientific and engineer communities. In this paper, we experimentally demonstrate a Tbit/s parallel FSO communication system using a soliton microcomb as a multiple wavelength laser source. Two communication terminals are installed in two buildings with a straight-line distance of ∼1 km. 102 comb lines are modulated by 10 Gbit/s differential phase-shift keying signals and demodulated using a delay-line interferometer. When the transmitted optical power is amplified to 19.8 dBm, 42 optical channels have optical signal-to-noise ratios higher than 27 dB and bit error rates less than 1×10-9. Our experiment shows the feasibility of a wavelength-division multiplexing FSO communication system which suits the ultra-high-speed wireless transmission application scenarios in future satellite-based communications, disaster recovery, defense, last mile problems in networks and remote sensing, and so on.

Photonics Research
On the CoverNov. 22, 2022, Vol. 10 Issue 12 2802 (2022)
Monolithic and single-functional-unit level integration of electronic and photonic elements: FET-LET hybrid 6T SRAM
Antardipan Pal, Yong Zhang, and Dennis D. Yau

A broad range of technologies have been developed for the chip and wafer scale connections and integrations of photonic and electronic circuits, although major challenges remain for achieving the single-functional-unit-level integration of electronic and photonic devices. Here we use field-effect transistor/light-effect transistor (FET–LET) hybrid 6T static random-access memory (SRAM) as an example to illustrate a novel approach that can alleviate three major challenges to the higher-level integration of the photonic and electronic elements: size mismatch, energy data rate, and cascadability. A hybrid 6T SRAM with two access FETs being replaced by LETs and the electrical word lines replaced by optical waveguides is proposed. This hybrid 6T SRAM is analyzed to reveal its potential in improvement of the switching speed and thus total energy consumption over the conventional 6T SRAM. Numerical analyses, for instance, for a prototype 64 kB hybrid SRAM array, show a factor of 4 and 22 reduction in read delay and read energy consumption, and 3 and 4 in write delay and write energy consumption, respectively, when the access FETs are replaced by LETs. The potential impacts on the peripheral and assist circuits due to this hybrid structure and application of the LETs there are also briefly discussed.

Photonics Research
Jul. 01, 2021, Vol. 9 Issue 7 07001369 (2021)
High-sensitivity integrated SiN rib-waveguide long period grating refractometer
Clement Deleau, Han Cheng Seat, Olivier Bernal, and Frederic Surre

In this research, we demonstrate a high-sensitivity integrated silicon nitride long period grating (LPG) refractometer based on a rib waveguide with sinusoidally modulated width. While integrated LPG architectures typically achieve ultrahigh sensitivity only over a narrow optical bandwidth using a phase-matching turning-point optimization technique, our sensor exhibits a very high refractometric sensitivity that was designed to remain constant over a broad operational optical spectral bandwidth. The proposed design method relies on multi-modal dispersion tailoring that consists of homogenizing the spectral behaviors of both group and effective indices of the coupling modes. Experimental results are in agreement with numerical simulations, demonstrating not only a sensitivity reaching 11,500 nm/RIU but, more significantly, also that this sensitivity remains almost constant over a broad spectral range of at least 100 nm around 1550 nm. Additional advantages of the proposed sensor architecture encompass a low temperature sensitivity, down to -0.15 nm/K, and simplicity of the fabrication process. These results demonstrate the feasibility of chip-scale photonic integration to achieve both high sensitivity and large dynamic range of the proposed refractometer.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000564 (2022)
Hybrid integrated low-noise linear chirp frequency-modulated continuous-wave laser source based on self-injection to an external cavity
Liwei Tang, Hongxiang Jia, Shuai Shao, Sigang Yang, Hongwei Chen, and Minghua Chen

A hybrid integrated low-noise linear chirp frequency-modulated continuous-wave (FMCW) laser source with a wide frequency bandwidth is demonstrated. By employing two-dimensional thermal tuning, the laser source shows frequency modulation bandwidth of 10.3 GHz at 100 Hz chirped frequency and 5.6 GHz at 1 kHz chirped frequency. The intrinsic linewidth of 49.9 Hz with 42 GHz continuous frequency tuning bandwidth is measured under static operation. Furthermore, by pre-distortion linearization of the laser source, it can distinguish 3 m length difference at 45 km distance in the fiber length measurement experiment, demonstrating its application potential in ultra-long fiber sensing and FMCW light detection and ranging.

Photonics Research
Sep. 15, 2021, Vol. 9 Issue 10 10001948 (2021)
Ultra-compact titanium dioxide micro-ring resonators with sub-10-μm radius for on-chip photonics
Meicheng Fu, Yi Zheng, Gaoyuan Li, Wenjun Yi, Junli Qi, Shaojie Yin, Xiujian Li, and Xiaowei Guan

Microring resonators (MRRs) with ultracompact footprints are preferred for enhancing the light-matter interactions to benefit various applications. Here, ultracompact titanium dioxide (TiO2) MRRs with sub-10-μm radii are experimentally demonstrated. Thanks to the large refractive index of TiO2, the quality factors up to ∼7.9×104 and ∼4.4×104 are achieved for TiO2 MRRs with radii of 10 μm and 6 μm, respectively, which result in large nonlinear power enhancement factors (>113) and large Purcell factors (>56). The four-wave mixing (FWM) measurements indicate that, compared to the large MRR, the FWM conversion efficiency of the ultracompact TiO2 MRRs can be greatly improved (e.g., -25 dB versus -31 dB), a harbinger of significant superiorities. Demonstrations in this work provide more arguments for the TiO2 waveguides as a promising platform for various on-chip photonic devices.

Photonics Research
Jul. 01, 2021, Vol. 9 Issue 7 07001416 (2021)
Silicon mode-loop Mach-Zehnder modulator with L-shaped PN junction for 0.37 V·cm VπL high-efficiency modulation
Jiacheng Liu, Gangqiang Zhou, Jiangbing Du, Weihong Shen, Linjie Zhou, and Zuyuan He

Optical signaling without a high voltage driver for electric-optic modulation is in high demand to reduce power consumption, packaging complexity, and cost. In this work, we propose and experimentally demonstrate a silicon mode-loop Mach–Zehnder modulator (ML-MZM) with record-high modulation efficiency. We used a mode-loop structure to recycle light twice in the phase shifter. With an L-shaped PN junction, a comparably large overlap between the PN junction and optical modes of both TE0 and TE1 was achieved to lower the driving voltage or decrease the photonic device size. Proof-of-concept high-efficiency modulation with low VπL of 0.37 V·cm was obtained. Subvoltage Vπ can be realized with a millimeter’s length phase shifter by this scheme, which makes the realization of CMOS-compatible driverless modulation highly possible. 40 Gb/s signaling with a bit error rate below the 7% forward-error-correction threshold was then demonstrated with the fabricated ML-MZM, indicating great potential for high-speed optical communication.

Photonics Research
Jan. 01, 2022, Vol. 10 Issue 1 01000214 (2022)
Silicon substrate-integrated hollow waveguide for miniaturized optical gas sensing
Shaonan Zheng, Hong Cai, Linfang Xu, Nanxi Li, Zhonghua Gu, Yao Zhang, Weiguo Chen, Yanyan Zhou, Qingxin Zhang, and Lennon Yao Ting Lee

Gas sensors have a wide variety of applications. Among various existing gas sensing technologies, optical gas sensors have outstanding advantages. The development of the Internet of Things and consumer electronics has put stringent requirements on miniaturized gas sensing technology. Here, we demonstrate a chip-scale silicon substrate-integrated hollow waveguide (Si-iHWG) to serve as an optical channel and gas cell in an optical gas sensor. It is fabricated through silicon wafer etching and wafer bonding. The Si-iHWG chip is further assembled with an off-chip light source and detector to build a fully functional compact nondispersive infrared (NDIR) CO2 sensor. The chip size is 10 mm×9 mm, and the dimension of the sensor excluding the microcontroller board is 50 mm×25 mm×16 mm. This chip solution with compactness, versatility, robustness, and low cost provides a cost-effective platform for miniaturized optical sensing applications ranging from air quality monitoring to consumer electronics.

Photonics Research
Jan. 01, 2022, Vol. 10 Issue 1 01000261 (2022)
Thermally enhanced responsivity in an all-silicon optical power monitor based on defect-mediated absorption
Qikai Huang, Hui Yu, Qiang Zhang, Yan Li, Weiwei Chen, Yuehai Wang, and Jianyi Yang

We demonstrate a high responsivity all-silicon in-line optical power monitor by using the thermal effect to enhance the quantum efficiency of defect-mediated absorption at 1550 nm. The doping compensation technique is utilized to increase the density of lattice defects responsible for the sub-bandgap absorption and suppress the detrimental free carrier absorption. The 200-μm-long device presents a propagation loss as low as 2.9 dB/cm. Its responsivity is enhanced from 12.1 mA/W to 112 mA/W at -9 V bias by heating the optical absorption region. With this device, we build an optical power monitoring system that operates in the sampling mode. The minimal detectable optical power of the system is below -22.8 dBm, while the average power consumption is less than 1 mW at a sampling frequency of 10 Hz. Advantages of this scheme in terms of high responsivity, low insertion loss, and low power consumption lend itself to implement the feedback control of advanced large-scale silicon photonic integrated circuits.

Photonics Research
Oct. 21, 2021, Vol. 9 Issue 11 11002205 (2021)
Gain-enabled optical delay readout unit using CMOS-compatible avalanche photodetectors
Ranjan Das, Yanran Xie, Henry Frankis, Keru Chen, Hermann Rufenacht, Guillaume Lamontagne, Jonathan D. B. Bradley, and Andrew P. Knights

A compact time delay unit is fundamental to integrated photonic circuits with applications in, for example, optical beam-forming networks, photonic equalization, and finite and infinite impulse response optical filtering. In this paper, we report a novel gain-enabled delay readout system using a tunable optical carrier, low-frequency RF signal and CMOS-compatible photodetectors, suitable for silicon photonic integration. The characterization method relies on direct phase measurement of an input RF signal and thereafter extraction of the delay profile. Both integrated silicon and germanium photodetectors coupled with low-bandwidth electronics are used to characterize a microring resonator-based, true-time delay unit under distinct ring–bus coupling formats. The detectors, used in both linear and avalanche mode, are shown to be successful as optical-to-electrical converters and RF amplifiers without introducing significant phase distortion. For a Si–Ge separate-absorption-charge-multiplication avalanche detector, an RF amplification of 10 dB is observed relative to a Ge PIN linear detector. An all-silicon defect-mediated avalanche photodetector is shown to have a 3 dB RF amplification compared to the same PIN detector. All ring delay measurement results are validated by full-wave simulation. Additionally, the impact of photodetector biasing and system linearity is analyzed.

Photonics Research
Sep. 30, 2022, Vol. 10 Issue 10 2422 (2022)
Broadband adiabatic polarization rotator-splitter based on a lithium niobate on insulator platform
Zhaoxi Chen, Jingwei Yang, Wing-Han Wong, Edwin Yue-Bun Pun, and Cheng Wang

Polarization rotator-splitters (PRSs) are crucial components for controlling the polarization states of light in classical and quantum communication systems. We design and experimentally demonstrate a broadband PRS based on the lithium niobate on insulator (LNOI) platform. Both the rotator and splitter sections are based on adiabatically tapered waveguide structures, and the whole device only requires a single etching step. We show efficient PRS operation over an experimentally measured bandwidth of 130 nm at telecom wavelengths, potentially as wide as 500 nm according to simulation prediction, with relatively low polarization crosstalks of ∼-10 dB. Our PRS is highly compatible with the design constraints and fabrication processes of common LNOI photonic devices, and it could become an important element in future LNOI photonic integrated circuits.

Photonics Research
Editors' PickNov. 08, 2021, Vol. 9 Issue 12 12002319 (2021)
Coherent self-control of free-space optical beams with integrated silicon photonic meshes
Maziyar Milanizadeh, Fabio Toso, Giorgio Ferrari, Tigers Jonuzi, David A. B. Miller, Andrea Melloni, and Francesco Morichetti

In technologies operating at light wavelengths for wireless communication, sensor networks, positioning, and ranging, a dynamic coherent control and manipulation of light fields is an enabling element for properly generating and correctly receiving free-space optical (FSO) beams even in the presence of unpredictable objects and turbulence in the light path. In this work, we use a programmable mesh of Mach–Zehnder (MZI) interferometers to automatically control the complex field radiated and captured by an array of optical antennas. The implementation of local feedback control loops in each MZI stage, without global multivariable optimization techniques, enables an unlimited scalability. Several functionalities are demonstrated, including the generation of perfectly shaped beams with nonperfect optical antennas, the imaging of a desired field pattern through an obstacle or a diffusive medium, and the identification of an unknown obstacle inserted in the FSO path. Compared to conventional devices used for the manipulation of FSO beams, such as spatial light modulators, our programmable device can self-configure through automated control strategies and can be integrated with other functionalities implemented onto the same photonic chip.

Photonics Research
Spotlight on OpticsOct. 18, 2021, Vol. 9 Issue 11 11002196 (2021)
Chalcogenide glass photonic integration for improved 2 μm optical interconnection
Weihong Shen, Pingyang Zeng, Zelin Yang, Di Xia, Jiangbing Du, Bin Zhang, Ke Xu, Zuyuan He, and Zhaohui Li

In this work, on-chip chalcogenide glass photonic integrations with several fundamental photonic building blocks are designed and fabricated based on the As2S3 platform for improved 2 μm optical interconnection, achieving a broadened wavelength bandwidth and improved fabrication tolerance. A 600 nm thick As2S3 strip waveguide has low propagation loss of 1.447 dB/cm at 2 μm. Broadband vertical coupling is realized by a grating coupler with 4.3 dB coupling loss. A Bragg grating filter, power splitter, Mach–Zander interferometer, and mode converter for on-chip mode division multiplexing (MDM) are also reported at 2 μm with reliable performances. Finally, a record high MDM optical interconnection capacity of 3×80 Gbps at 2 μm is experimentally demonstrated based on the proposed As2S3 chip, drawing promising prospects for future photonic integration and high-speed interconnection at the 2 μm waveband.

Photonics Research
Editors' PickAug. 21, 2020, Vol. 8 Issue 9 09001484 (2020)
Inverse design of digital nanophotonic devices using the adjoint method
Kaiyuan Wang, Xinshu Ren, Weijie Chang, Longhui Lu, Deming Liu, and Minming Zhang

A high-efficiency inverse design of “digital” subwavelength nanophotonic devices using the adjoint method is proposed. We design a single-mode 3 dB power divider and a dual-mode demultiplexer to demonstrate the efficiency of the proposed inverse design approach, called the digitized adjoint method, for single- and dual-object optimization, respectively. The optimization comprises three stages: 1) continuous variation for an “analog” pattern; 2) forced permittivity biasing for a “quasi-digital” pattern; and 3) a multilevel digital pattern. Compared with the conventional brute-force method, the proposed method can improve design efficiency by about five times, and the performance optimization can reach approximately the same level. The method takes advantages of adjoint sensitivity analysis and digital subwavelength structure and creates a new way for the efficient and high-performance design of compact digital subwavelength nanophotonic devices, which could overcome the efficiency bottleneck of the brute-force method, which is restricted by the number of pixels of a digital pattern, and improve the device performance by extending a conventional binary pattern to a multilevel one.

Photonics Research
On the CoverMar. 24, 2020, Vol. 8 Issue 4 04000528 (2020)
Proposal and demonstration of a controllable Q factor in directly coupled microring resonators for optical buffering applications
Ying Zhang, Qiang Liu, Chenyang Mei, Desheng Zeng, Qingzhong Huang, and Xinliang Zhang

Optical resonators with controllable Q factors are key components in many areas of optical physics and engineering. We propose and investigate a Q-factor controllable system composed of two directly coupled microring resonators, one of which is tunable and coupled to dual waveguides. By shifting the resonance of the controllable microring, the Q factor of the system as well as the other microring changes significantly. We have demonstrated wide-range controllable Q factors based on this structure in silicon-on-insulator, for example. The influences of spectral detuning and coupling strength between two resonators on the variation of Q factors are studied in detail experimentally. Then, we explore its applications in optical buffering. Tunable fast-to-slow/slow-to-fast light has been carried out by switching the system between the high-Q state and low-Q state. Moreover, optical pulse capture and release are also achievable using this structure with dynamic tuning, and the photon storage properties are investigated. The proposed Q-factor tunable system is simple, flexible, and realizable in various integrated photonic platforms, allowing for potential applications in on-chip optical communications and quantum information processing.

Photonics Research
Sep. 16, 2021, Vol. 9 Issue 10 10002006 (2021)
Wide-steering-angle high-resolution optical phased array
Yingzhi Li, Baisong Chen, Quanxin Na, Qijie Xie, Min Tao, Lanxuan Zhang, Zihao Zhi, Yuxuan Li, Xiaobin Liu, Xianshu Luo, Guoqiang Lo, Fengli Gao, Xueyan Li, and Junfeng Song

Optical phased array (OPA) technology is considered a promising solution for solid-state beam steering to supersede the traditional mechanical beam steering. As a key component of the LIDAR system for long-range detection, OPAs featuring a wide steering angle and high resolution without beam aliasing are highly desired. However, a wide steering range requires a waveguide pitch less than half of the wavelength, which is easily subjected to cross talk. Besides, high resolution requires a large aperture, and it is normally achieved by a high count number of waveguides, which complicates the control system. To solve the mentioned issues, we design two high-performance 128-channel OPAs fabricated on a multilayered SiN-on-SOI platform. Attributed to the nonuniform antenna pitch, only 128 waveguides are used to achieve a 4 mm wide aperture. Besides, by virtue of innovative dual-level silicon nitride (Si3N4) waveguide grating antennas, the fishbone antenna OPA achieves a 100°×19.4° field of view (FOV) with divergence of 0.021°×0.029°, and the chain antenna OPA realizes a 140°×19.23° FOV with divergence of 0.021°×0.1°. To our best knowledge, 140° is the widest lateral steering range in two-dimensional OPA, and 0.029° is the smallest longitudinal divergence. Finally, we embed the OPA into a frequency-modulated continuous-wave system to achieve 100 m distance measurement. The reflected signal from 100 m distance is well detected with 26 dBm input transmitter power, which proves that OPA serves as a promising candidate for transceiving optical signal in a LIDAR system.

Photonics Research
Dec. 01, 2021, Vol. 9 Issue 12 12002511 (2021)
Free-spectral-range-free filters with ultrawide tunability across the S + C + L band
Chunlei Sun, Chuyu Zhong, Maoliang Wei, Hui Ma, Ye Luo, Zequn Chen, Renjie Tang, Jialing Jian, Hongtao Lin, and Lan Li

Optical filters are essential parts of advanced optical communication and sensing systems. Among them, the ones with an ultrawide free spectral range (FSR) are especially critical. They are promising to provide access to numerous wavelength channels highly desired for large-capacity optical transmission and multipoint multiparameter sensing. Present schemes for wide-FSR filters either suffer from limited cavity length or poor fabrication tolerance or impose an additional active-tuning control requirement. We theoretically and experimentally demonstrate a filter that features FSR-free operation capability, subnanometer optical bandwidth, and acceptable fabrication tolerance. Only one single deep dip within a record-large waveband (S+C+L band) is observed by appropriately designing a side-coupled Bragg-grating-assisted Fabry–Perot filter, which has been applied as the basic sensing unit for both the refractive index and temperature measurement. Five such basic units are also cascaded in series to demonstrate a multichannel filter. This work provides a new insight to design FSR-free filters and opens up a possibility of flexible large-capacity integration using more wavelength channels, which will greatly advance integrated photonics in optical communication and sensing.

Photonics Research
May. 24, 2021, Vol. 9 Issue 6 06001013 (2021)
Anti-parity-time symmetry enabled on-chip chiral polarizer
Yanxian Wei, Hailong Zhou, Yuntian Chen, Yunhong Ding, Jianji Dong, and Xinliang Zhang

Encircling an exceptional point (EP) in a parity-time (PT) symmetric system has shown great potential for chiral optical devices, such as chiral mode switching for symmetric and antisymmetric modes. However, to our best knowledge, chiral switching for polarization states has never been reported, although chiral polarization manipulation has significant applications in imaging, sensing, communication, etc. Here, inspired by the anti-PT symmetry, we demonstrate, for the first time to our best knowledge, an on-chip chiral polarizer by constructing a polarization-coupled anti-PT symmetric system. The transmission axes of the chiral polarizer are different for forward and backward propagation. A polarization extinction ratio of over 10 dB is achieved for both propagating directions. Moreover, a telecommunication experiment is performed to demonstrate the potential applications in polarization encoding signals. It provides a novel functionality for encircling-an-EP parametric evolution and offers a new approach for on-chip chiral polarization manipulation.

Photonics Research
Spotlight on OpticsJan. 01, 2022, Vol. 10 Issue 1 01000076 (2022)
Metasurface-based subtractive color filter fabricated on a 12-inch glass wafer using a CMOS platform
Zhengji Xu, Nanxi Li, Yuan Dong, Yuan Hsing Fu, Ting Hu, Qize Zhong, Yanyan Zhou, Dongdong Li, Shiyang Zhu, and Navab Singh

Optical color filters are widely applied in many areas including display, imaging, sensing, holography, energy harvest, and measurement. Traditional dye-based color filters have drawbacks such as environmental hazards and instability under high temperature and ultraviolet radiation. With advances in nanotechnology, structural color filters, which are based on the interaction of light with designed nanostructures, are able to overcome the drawbacks. Also, it is possible to fabricate structural color filters using standard complementary metal-oxide-semiconductor (CMOS) fabrication facilities with low cost and high volume. In this work, metasurface-based subtractive color filters (SCFs) are demonstrated on 12-inch (300-mm) glass wafers using a CMOS-compatible fabrication process. In order to make the transmissive-type SCF on a transparent glass wafer, an in-house developed layer transfer process is used to solve the glass wafer handling issue in fabrication tools. Three different heights of embedded silicon nanopillars (110, 170, and 230 nm) are found to support magnetic dipole resonances. With pillar height and pitch variation, SCFs with different displayed colors are achieved. Based on the resonance wavelength, the displayed color of the metasurface is verified within the red-yellow-blue color wheel. The simulation and measurement results are compared and discussed. The work provides an alternative design for high efficiency color filters on a CMOS-compatible platform, and paves the way towards mass-producible large-area metasurfaces.

Photonics Research
On the CoverJan. 01, 2021, Vol. 9 Issue 1 01000013 (2021)
Robust hybrid laser linewidth reduction using Si3N4-based subwavelength hole defect assisted microring reflector
Jiachen Li, Baoyu Zhang, Sigang Yang, Hongwei Chen, and Minghua Chen

We demonstrate a hybrid laser with a low intrinsic linewidth of 34.2 Hz and a high fiber-coupled output power of 11.7 dBm, by coupling a Si3N4-based subwavelength hole defect assisted microring reflector (SHDA-MRR) to a commercially available distributed feedback semiconductor laser. The proposed SHDA-MRR structure features an accurately controlled reflection response, with the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide. With further joint optimization of cavity parameters, this Si3N4 SHDA-MRR structure is expected to reduce the laser intrinsic linewidth to a sub-hertz level. This work explores a low-cost and robust linewidth reduction scheme for the applications of high-speed coherent optical communications systems and high-resolution optical metrology.

Photonics Research
Mar. 26, 2021, Vol. 9 Issue 4 04000558 (2021)
Four-channel CWDM device on a thin-film lithium niobate platform using an angled multimode interferometer structure
Gengxin Chen, Ziliang Ruan, Zong Wang, Pucheng Huang, Changjian Guo, Daoxin Dai, Kaixuan Chen, and Liu Liu

A compact and high-performance coarse wavelength-division multiplexing (CWDM) device is introduced with a footprint of 2.1 mm×0.02 mm using an angled multimode interferometer structure based on a thin-film lithium niobate (TFLN) platform. The demonstrated device built on a 400 nm thick x-cut TFLN shows ultra-low insertion losses of 0.72 dB. Measured 3 dB bandwidths are 12.1 nm for all channels, and cross talks from adjacent channels are better than 18 dB. Its peak wavelength positions comply with the CWDM standard with a channel spacing of 20 nm. The filter bandwidth of the proposed CWDM device can be tuned by adjusting the structural parameters. This demonstrated CWDM device will promote future realization of multi-channel and multi-wavelength transmitter chips on TFLN.

Photonics Research
Jan. 01, 2022, Vol. 10 Issue 1 01000008 (2022)
80 GHz germanium waveguide photodiode enabled by parasitic parameter engineering
Yang Shi, De Zhou, Yu Yu, and Xinliang Zhang

A high-speed germanium (Ge) waveguide photodiode (PD) is one of the key components of an integrated silicon photonics platform for large-capacity data communication applications, but the parasitic parameters limit the increase of its bandwidth. Several studies have been reported to reduce parasitic parameters, at the cost of compromising other performances. Here, we propose and investigate a bandwidth-boosting technique by comprehensively engineering the parasitic parameters. Experimentally, a bandwidth up to 80 GHz is realized for vertical positive-intrinsic-negative (PIN) Ge PDs without decreasing the responsivity and dark current, indicating that parasitic parameter engineering is a promising method to promote high-speed performance of Ge PDs.

Photonics Research
Mar. 31, 2021, Vol. 9 Issue 4 04000605 (2021)
Hybrid-integrated high-performance microwave photonic filter with switchable response
Yuansheng Tao, Haowen Shu, Xingjun Wang, Ming Jin, Zihan Tao, Fenghe Yang, Jingbo Shi, and Jun Qin

The integrated microwave photonic filter (MPF), as a compelling candidate for next-generation radio-frequency (RF) applications, has been widely investigated for decades. However, most integrated MPFs reported thus far have merely incorporated passive photonic components onto a chip-scale platform, while all necessary active devices are still bulk and discrete. Though few attempts to higher photonic integration of MPFs have been executed, the achieved filtering performances are fairly limited, which impedes the pathway to practical deployments. Here, we demonstrate, for the first time to our knowledge, an all-integrated MPF combined with high filtering performances, through hybrid integration of an InP chip-based laser and a monolithic silicon photonic circuit consisting of a dual-drive Mach–Zehnder modulator, a high-Q ring resonator, and a photodetector. This integrated MPF exhibits a high spectral resolution as narrow as 360 MHz, a wide-frequency tunable range covering the S-band to K-band (3 to 25 GHz), and a large rejection ratio of >40 dB. Moreover, the filtering response can be agilely switched between the bandpass and band-stop function with a transient respond time (∼48 μs). Compared with previous MPFs in a similar integration level, the obtained spectral resolution in this work is dramatically improved by nearly one order of magnitude, while the valid frequency tunable range is broadened more than twice, which can satisfy the essential filtering requirements in actual RF systems. As a paradigm demonstration oriented to real-world scenarios, high-resolution RF filtering of realistic microwave signals aiming for interference rejection and channel selection is performed. Our work points out a feasible route to a miniaturized, high-performance, and cost-effective MPF leveraging hybrid integration approach, thus enabling a range of RF applications from wireless communication to radar toward the higher-frequency region, more compact size, and lower power consumption.

Photonics Research
Editors' PickJul. 30, 2021, Vol. 9 Issue 8 08001569 (2021)
Versatile on-chip light coupling and (de)multiplexing from arbitrary polarizations to controlled waveguide modes using an integrated dielectric metasurface
Yuan Meng, Zhoutian Liu, Zhenwei Xie, Ride Wang, Tiancheng Qi, Futai Hu, Hyunseok Kim, Qirong Xiao, Xing Fu, Qiang Wu, Sang-Hoon Bae, Mali Gong, and Xiaocong Yuan

Metasurfaces have found broad applicability in free-space optics, while its potential to tailor guided waves remains barely explored. By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition, we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity, multiple working wavelengths, and high efficiency concurrently. The coupling direction, operation frequency, and excited mode type can be designed at will for arbitrary incident polarizations, outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability. Here, using silicon-nanoantenna-patterned silicon-nitride photonic waveguides, we numerically demonstrate a set of chip-scale optical couplers around 1.55 μm, including mode-selective directional couplers with high coupling efficiency over 57% and directivity about 23 dB. Polarization and wavelength demultiplexer scenarios are also proposed with 67% maximum efficiency and an extinction ratio of 20 dB. Moreover, a chip-integrated twisted light generator, coupling free-space linear polarization into an optical vortex carrying 1? orbital angular momentum (OAM), is also reported to validate the mode-control flexibility. This comprehensive method may motivate compact wavelength/polarization (de)multiplexers, multifunctional mode converters, on-chip OAM generators for photonic integrated circuits, and high-speed optical telecommunications.

Photonics Research
Apr. 01, 2020, Vol. 8 Issue 4 04000564 (2020)
Polymer waveguide tunable transceiver for photonic front-end in the 5G wireless network
Tae-Hyun Park, Sung-Moon Kim, Eun-Su Lee, and Min-Cheol Oh

A photonic front-end in the 5G wireless network based on wavelength-division multiplexing optical communication requires low-cost tunable transceivers. By exploiting polymer waveguide Bragg-grating technology, we propose a tunable transceiver consisting of an external cavity tunable laser and a tilted grating tunable filter. In particular, a double-reflection tunable filter provides narrower reflection bandwidth and suppresses undesired mode coupling, improving the side-mode suppression ratio (SMSR) and reducing adjacent-channel crosstalk. By introducing perfluorinated polymers with low birefringence, polarization independence, which is a prerequisite for wavelength filter elements, is secured, and 20 dB bandwidth of 0.69 nm, wavelength tunability over 40 nm, and SMSR of 42 dB are achieved.

Photonics Research
Editors' PickJan. 26, 2021, Vol. 9 Issue 2 02000181 (2021)
Million-Q integrated Fabry-Perot cavity using ultralow-loss multimode retroreflectors
Hongnan Xu, Yue Qin, Gaolei Hu, and Hon Ki Tsang

The monolithic integration of Fabry-Perot cavities has many applications, such as label-free sensing, high-finesse filters, semiconductor lasers, and frequency comb generation. However, the excess loss of integrated reflectors makes it challenging to realize integrated Fabry-Perot cavities working in the ultrahigh-Q regime (>106). Here, we propose and experimentally demonstrate what we believe is the first silicon integrated million-Q Fabry-Perot cavity. Inspired by free-space optics, a novel monolithically integrated retroreflector is utilized to obtain near-unity reflectance and negligible reflection losses. The corner scattering in the retroreflector is prevented by the use of the TE1 mode, taking advantage of its zero central field intensity. Losses incurred by other mechanisms are also meticulously engineered. The measurement results show resonances with an ultrahigh intrinsic Q factor of ≈3.4×106 spanning an 80-nm bandwidth. The measured loaded Q factor is ≈2.1×106. Ultralow reflection losses (≈0.05 dB) and propagation losses (≈0.18 dB/cm) are experimentally realized.

Photonics Research
Editors' PickOct. 24, 2022, Vol. 10 Issue 11 2549 (2022)
Characterization of field-effect mobility at optical frequency by microring resonators
Wei-Che Hsu, Erwen Li, Bokun Zhou, and Alan X. Wang

A novel characterization method is proposed to extract the optical frequency field-effect mobility (μop,FE) of transparent conductive oxide (TCO) materials by a tunable silicon microring resonator with a heterogeneously integrated titanium-doped indium oxide (ITiO)/SiO2/silicon metal–oxide–semiconductor (MOS) capacitor. By operating the microring in the accumulation mode, the quality factor and resonance wavelength shift are measured and subsequently used to derive the μop,FE in the ultra-thin accumulation layer. Experimental results demonstrate that the μop,FE of ITiO increases from 25.3 to 38.4 cm2?V-1?s-1 with increasing gate voltages, which shows a similar trend as that at the electric frequency.

Photonics Research
Mar. 31, 2021, Vol. 9 Issue 4 04000615 (2021)
Ultra-high-linearity integrated lithium niobate electro-optic modulators
Hanke Feng, Ke Zhang, Wenzhao Sun, Yangming Ren, Yiwen Zhang, Wenfu Zhang, and Cheng Wang

Integrated lithium niobate (LN) photonics is a promising platform for future chip-scale microwave photonics systems owing to its unique electro-optic properties, low optical loss, and excellent scalability. A key enabler for such systems is a highly linear electro-optic modulator that could faithfully convert analog electrical signals into optical signals. In this work, we demonstrate a monolithic integrated LN modulator with an ultra-high spurious-free dynamic range (SFDR) of 120.04 dB·Hz4/5 at 1 GHz, using a ring-assisted Mach–Zehnder interferometer configuration. The excellent synergy between the intrinsically linear electro-optic response of LN and an optimized linearization strategy allows us to fully suppress the cubic terms of third-order intermodulation distortions (IMD3) without active feedback controls, leading to ∼20 dB improvement over previous results in the thin-film LN platform. Our ultra-high-linearity LN modulators could become a core building block for future large-scale functional microwave photonic integrated circuits by further integration with other high-performance components like low-loss delay lines, tunable filters, and phase shifters available on the LN platform.

Photonics Research
Editors' PickSep. 27, 2022, Vol. 10 Issue 10 2366 (2022)
Radio-frequency line-by-line Fourier synthesis based on optical soliton microcombs
Beichen Wang, Zijiao Yang, Shuman Sun, and Xu Yi

Radio-frequency (RF) waveform synthesis has broad applications in ultrawide-bandwidth wireless communications, radar systems, and electronic testing. Photonic-based approaches offer key advantages in bandwidth and phase noise thanks to the ultrahigh optical carrier frequency. In this work, we demonstrate Fourier synthesis arbitrary waveform generation (AWG) with integrated optical microresonator solitons. The RF temporal waveform is synthesized through line-by-line amplitude and phase shaping of an optical soliton microcomb, which is down-converted to the RF domain through dual-comb optical coherent sampling. A variety of RF waveforms with tunable repetition cycles are shown in our demonstration. Our approach provides not only the possibility of precise Fourier synthesis at microwave and millimeter-wave frequencies, but also a viable path to fully integrated photonic-based RF AWG on a chip.

Photonics Research
Editors' PickMar. 11, 2022, Vol. 10 Issue 4 04000932 (2022)
Bridging the gap between resonance and adiabaticity: a compact and highly tolerant vertical coupling structure
Chunhui Yao, Qixiang Cheng, Günther Roelkens, and Richard Penty

We present a compact, highly tolerant vertical coupling structure, which can be a generic design that bridges the gap between conventional resonant couplers and adiabatic couplers for heterogeneously integrated devices. We show insights on relaxing the coupler alignment tolerance and provide a detailed design methodology. By the use of a multisegmented inverse taper structure, our design allows a certain proportion of the odd supermode to be excited during the coupling process, which simultaneously facilitates high tolerance and compactness. With a total length of 87 μm, our coupler is almost threefold shorter than the state-of-the-art alignment-tolerant adiabatic couplers and outperforms them by demonstrating a more than 94% coupling efficiency (for 0.3 dB coupling loss) with ±1 μm misalignment tolerance, which, to our best knowledge, is a new record for III-V-on-silicon vertical couplers. Furthermore, our design has high tolerance to fabrication-induced structural deformation and ultrabroad bandwidth. These features make it particularly suitable for building densely integrated III-V-on-silicon photonic circuits with commercially available microtransfer printing assembly tools. The proposed design can be widely adopted in various integration platforms.

Photonics Research
Aug. 15, 2022, Vol. 10 Issue 9 2081 (2022)
Soliton frequency comb generation in CMOS-compatible silicon nitride microresonators
Yaozu Xie, Jiaqi Li, Yanfeng Zhang, Zeru Wu, Shihao Zeng, Shuqing Lin, Zhaoyang Wu, Wenchao Zhou, Yujie Chen, and Siyuan Yu

The monolithic integration of soliton microcomb devices with active photonic components and high-frequency electronics is highly desirable for practical applications. Among many materials, silicon nitride (SiNx) waveguide layers prepared by low-pressure chemical vapor deposition (LPCVD) have been the main platform for on-chip optical frequency comb generation. However, the high temperatures involved in LPCVD render it incompatible as a back-end process with complementary metal oxide semiconductor (CMOS) or active III-V compound semiconductor fabrication flows. We report the generation of coherent soliton frequency combs in micro-ring resonators fabricated in deuterated silicon nitride (SiNx:D) waveguides with a loss of 0.09 dB/cm. Deposited at 270°C by an inductance-coupled plasma chemical vapor deposition (ICP-CVD) process, the material preparation and fabrication flow are fully CMOS-compatible. These results enable the integration of silicon-nitride-based optical combs and photonic integrated circuits (PICs) on prefabricated CMOS and/or III-V substrates, therefore marking a major step forward in SiNx photonic technologies.

Photonics Research
Apr. 29, 2022, Vol. 10 Issue 5 05001290 (2022)
Terahertz topological photonic waveguide switch for on-chip communication
Xudong Liu, Jialiang Huang, Hao Chen, Zhengfang Qian, Jingwen Ma, Xiankai Sun, Shuting Fan, and Yiwen Sun

Terahertz (THz) topological photonic structures are promising for last-centimeter communication in intra/interchip communication systems because they support bit-error-free THz signal transmission with topological robustness. Active and dynamically tunable THz topological photonic components have not yet been experimentally realized. Here, we experimentally demonstrate a THz topological switch (270–290 GHz) based on a valley Hall photonic crystal structured high-resistivity silicon substrate, in which the THz waves can be dynamically turned on/off by an external 447 nm continuous-wave laser. Our device exhibited an on/off ratio of 19 dB under a pumping light intensity of 240 mW/mm2. The 3 dB switching bandwidth was ∼60 kHz.

Photonics Research
Mar. 29, 2022, Vol. 10 Issue 4 04001090 (2022)
Interferometric method to estimate the eigenvalues of a non-Hermitian two-level optical system
Stefano Biasi, Riccardo Franchi, Filippo Mione, and Lorenzo Pavesi

Non-Hermitian physics has found a fertile ground in optics. Recently, the study of mode coalescence, i.e., exceptional points, has led to the discovery of intriguing and counterintuitive phenomena. Degeneracies are typically modeled through the coupled mode theory to determine the behavior of eigenstates and eigenvalues. However, the complex nature of the eigenvalues makes their characterization from the response spectrum difficult. Here, we demonstrate that a coherent interferometric excitation allows estimation of both the real and imaginary parts of the eigenvalues. We study the clockwise and counter-clockwise modes in optical microresonators both in the case of Hermitian and non-Hermitian intermodal coupling. We show the conditions by which a resonant doublet, due to the dissipative coupling of counter-propagating modes caused by surface roughness backscattering, merges to a single Lorentzian. This permits us to estimate the optimal quality factor of the microresonator in the absence of modal coupling caused by backscattering. Furthermore, we demonstrate that a taiji microresonator working at an exceptional point shows a degeneracy splitting only in one propagation direction and not in the other. This follows from the strongly non-Hermitian intermodal coupling caused by the inner S-shaped waveguide.

Photonics Research
Mar. 29, 2022, Vol. 10 Issue 4 04001134 (2022)
Machine-learning-empowered multispectral metafilm with reduced radar cross section, low infrared emissivity, and visible transparency
Ruichao Zhu, Jiafu Wang, Jinming Jiang, Cuilian Xu, Che Liu, Yuxiang Jia, Sai Sui, Zhongtao Zhang, Tonghao Liu, Zuntian Chu, Jun Wang, Tie Jun Cui, and Shaobo Qu

For camouflage applications, the performance requirements for metamaterials in different electromagnetic spectra are usually contradictory, which makes it difficult to develop satisfactory design schemes with multispectral compatibility. Fortunately, empowered by machine learning, metamaterial design is no longer limited to directly solving Maxwell’s equations. The design schemes and experiences of metamaterials can be analyzed, summarized, and learned by computers, which will significantly improve the design efficiency for the sake of practical engineering applications. Here, we resort to the machine learning to solve the multispectral compatibility problem of metamaterials and demonstrate the design of a new metafilm with multiple mechanisms that can realize small microwave scattering, low infrared emissivity, and visible transparency simultaneously using a multilayer backpropagation neural network. The rapid evolution of structural design is realized by establishing a mapping between spectral curves and structural parameters. By training the network with different materials, the designed network is more adaptable. Through simulations and experimental verifications, the designed architecture has good accuracy and robustness. This paper provides a facile method for fast designs of multispectral metafilms that can find wide applications in satellite solar panels, aircraft windows, and others.

Photonics Research
Apr. 06, 2022, Vol. 10 Issue 5 05001146 (2022)
General strategy for ultrabroadband and wide-angle absorbers via multidimensional design of functional motifs
Qi Yuan, Cuilian Xu, Jinming Jiang, Yongfeng Li, Yang Cheng, He Wang, Mingbao Yan, Jiafu Wang, Hua Ma, and Shaobo Qu

Developing wide-angle, polarization-independent, and effective electromagnetic absorbers that endow devices with versatile characteristics in solar, terahertz, and microwave regimes is highly desired, yet it is still facing a theoretical challenge. Herein, a general and straightforward strategy is proposed to surmount the impedance mismatching in the ultrabroadband and wide-angle absorber design. A vertical atom sticking on N×N horizontal meta-atoms with conductive film is proposed as the functional motif, exhibiting the strong ohmic dissipation along both vertical and horizontal directions. Assisted by the intelligent optimization strategy, the structure dimension, location, and film distribution are designed to maintain absorbing performance under different incident angles. As a demonstration, an absorber was designed and proved in both simulation and experiment. Significantly, the over 10 dB absorption from 5 to 34 GHz is achieved in the range of 0° to 70° for both TE and TM, and even 3 to 40 GHz from 60° to 70° for the TE wave. Meanwhile, the proposed multidimensional design of functional motifs can be attached with optical transparency function at will. That is to say, our effort provides an effective scheme for expanding matching area and may also be made in optical, infrared, and terahertz regimes.

Photonics Research
Sep. 01, 2022, Vol. 10 Issue 9 2202 (2022)
Stimulated generation of deterministic platicon frequency microcombs
Hao Liu, Shu-Wei Huang, Wenting Wang, Jinghui Yang, Mingbin Yu, Dim-Lee Kwong, Pierre Colman, and Chee Wei Wong

Dissipative Kerr soliton generation in chip-scale nonlinear resonators has recently observed remarkable advances, spanning from massively parallel communications, to self-referenced oscillators, and to dual-comb spectroscopy. Often working in the anomalous dispersion regime, unique driving protocols and dispersion in these nonlinear resonators have been examined to achieve the soliton and soliton-like temporal pulse shapes and coherent frequency comb generation. The normal dispersion regime provides a complementary approach to bridge the nonlinear dynamical studies, including the possibility of square pulse formation with flattop plateaus, or platicons. Here we report observations of square pulse formation in chip-scale frequency combs through stimulated pumping at one free spectral range and in silicon nitride rings with +55 fs2/mm normal group velocity dispersion. Tuning of the platicon frequency comb via a varied sideband modulation frequency is examined in both spectral and temporal measurements. Determined by second-harmonic autocorrelation and cross correlation, we observe bright square platicon pulse of 17 ps pulse width on a 19 GHz flat frequency comb. With auxiliary-laser-assisted thermal stabilization, we surpass the thermal bistable dragging and extend the mode-locking access to narrower 2 ps platicon pulse states, supported by nonlinear dynamical modeling and boundary limit discussions.

Photonics Research
Jul. 22, 2022, Vol. 10 Issue 8 1877 (2022)
Topological protection of partially coherent light
Konrad Tschernig, Gabriel Martinez-Niconoff, Kurt Busch, Miguel A. Bandres, and Armando Perez-Leija

Topological physics exploits concepts from geometry and topology to implement systems capable of guiding waves in an unprecedented fashion. These ideas have led to the development of photonic topological insulators, which are optical systems whose eigenspectral topology allows the creation of light states that propagate along the edge of the system without any coupling into the bulk or backscattering even in the presence of disorder. Indeed, topological protection is a fully coherent effect, and it is not clear to what extent topological effects endure when the wavefronts become partially coherent. Here, we study the interplay of topological protection and the degree of spatial coherence of classical light propagating in disordered photonic topological insulators. Our results reveal the existence of a well-defined spectral window in which partially coherent light is topologically protected. This opens up the design space to a wider selection of light sources, possibly yielding smaller, cheaper, and more robust devices based on the topological transport of light.

Photonics Research
Editors' PickApr. 14, 2022, Vol. 10 Issue 5 05001223 (2022)
Improving long-term temperature bias stability of an integrated optical gyroscope employing a Si3N4 resonator
Changkun Feng, Yonggui Zhang, Honghao Ma, Hui Li, and Lishuang Feng

The presence of polarization noise generated by the waveguide resonator limits the performance of a resonant integrated optical gyroscope (RIOG). Using silicon nitride (Si3N4) to fabricate a waveguide with an ultralow-aspect-ratio can result in a resonator that only supports light transmission in a single-polarization state, suppressing polarization noise. We successfully fabricated a Si3N4 resonator with a bending radius of 17.5 mm, a finesse (F) of 150, a quality factor (Q) of 1.54×107, and a propagation loss of 1.2 dB/m. The Si3N4 resonator was used to construct a double closed-loop RIOG that showed long-term bias stability (3600 s) of 13.2°/h at room temperature, 14.8°/h at 40°C, 21.2°/h at 50°C, and 23.6°/h at 60°C. We believe this to be the best performance reported to date for a Si3N4 resonator-based RIOG. This advancement paves the way for the wider application of RIOGs.

Photonics Research
Jun. 30, 2022, Vol. 10 Issue 7 07001661 (2022)
Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform
Lutong Cai, Jingwei Li, Ruixuan Wang, and Qing Li

Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time, to our knowledge. Such broadband operation is enabled by optimized nanofabrication achieving >1 million intrinsic quality factors in a 36-μm-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing f-2f self-referencing. In addition, we also observed a coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth.

Photonics Research
Editors' PickMar. 04, 2022, Vol. 10 Issue 4 04000870 (2022)
Compact electro-optic modulator on lithium niobate
Bingcheng Pan, Hongyuan Cao, Yishu Huang, Zong Wang, Kaixuan Chen, Huan Li, Zejie Yu, and Daoxin Dai

Fast electro-optic modulators with an ultracompact footprint and low power consumption are always highly desired for optical interconnects. Here we propose and demonstrate a high-performance lithium niobate electro-optic modulator based on a new 2×2 Fabry–Perot cavity. In this structure, the input and reflected beams are separated by introducing asymmetric multimode-waveguide gratings, enabling TE0-TE1 mode conversion. The measured results indicate that the fabricated modulator features a low excess loss of ∼0.9 dB, a high extinction ratio of ∼21 dB, a compact footprint of ∼2120 μm2, and high modulation speeds of 40 Gbps OOK and 80 Gbps PAM4 signals. The demonstrated modulator is promising for high-speed data transmission and signal processing.

Photonics Research
Feb. 22, 2022, Vol. 10 Issue 3 03000697 (2022)
On-chip path encoded photonic quantum Toffoli gate
Meng Li, Chu Li, Yang Chen, Lan-Tian Feng, Linyu Yan, Qian Zhang, Jueming Bao, Bi-Heng Liu, Xi-Feng Ren, Jianwei Wang, Shufeng Wang, Yunan Gao, Xiaoyong Hu, Qihuang Gong, and Yan Li

The quantum Toffoli gate is one of the most important three-qubit gates, but it is challenging to construct a chip according to the complicated traditional circuit. Using the optimized 3D configuration with an overpass waveguide to reduce the circuit complexity, we successfully fabricate an on-chip path encoded photonic quantum Toffoli gate enabled by the 3D capability of the femtosecond laser direct writing (FLDW) for the first time to our knowledge, whose truth-table fidelity is higher than 85.5%. Furthermore, a path encoded four-qubit controlled-controlled-controlled NOT gate is written to confirm the scalability of this resource-saving technique. This work paves the way for the FLDW of more complex and powerful photonic quantum computation chips.

Photonics Research
On the CoverJun. 09, 2022, Vol. 10 Issue 7 07001533 (2022)
Heterogeneous integrated phase modulator based on two-dimensional layered materials
Hao Chen, Zexing Zhao, Ziming Zhang, Guoqing Wang, Jiatong Li, Zhenyuan Shang, Mengyu Zhang, Kai Guo, Junbo Yang, and Peiguang Yan

Silicon nitride, with ultralow propagation loss and a wide transparency window, offers an exciting platform to explore integrated photonic devices for various emerging applications. It is appealing to combine the intrinsic optical properties of two-dimensional layered materials with high-quality optical waveguides and resonators to achieve functional devices in a single chip. Here we demonstrate a micro-ring resonator-based phase modulator integrated with few-layer MoS2. The ionic liquid is employed directly on the surface of MoS2 to form a capacitor configuration. The effective index of the composite MoS2–SiN waveguide can be modulated via adjusting bias voltages to achieve different charged doping induced electro-refractive responses in MoS2 film. The maximum effective index modulation of the composite MoS2–SiN waveguide can be achieved to 0.45×10-3. The phase tuning efficiency is measured to be 29.42 pm/V, corresponding to a VπL of 0.69 V·cm. Since the micro-ring resonator is designed near the critical coupling regime, the coupling condition between the bus waveguide and micro-ring resonator can also be engineered from under-coupling to over-coupling regime during the charged doping process. That can be involved as a degree of freedom for the coupling tailoring. The ability to modulate the effective index with two-dimensional materials and the robust nature of the heterostructure integrated phase modulator could be useful for engineering reliable ultra-compact and low-power-consumption integrated photonic devices.

Photonics Research
May. 16, 2022, Vol. 10 Issue 6 06001401 (2022)
Fully integrated hybrid microwave photonic receiver
Jiachen Li, Sigang Yang, Hongwei Chen, Xingjun Wang, Minghua Chen, and Weiwen Zou

Microwave photonic receivers are a promising candidate in breaking the bandwidth limitation of traditional radio-frequency (RF) receivers. To further balance the performance superiority with the requirements regarding size, weight, and power consumption (SWaP), the implementation of integrated microwave photonic microsystems has been considered an upgrade path. However, up to now, to the best of our knowledge, chip-scale fully integrated microwave photonic receivers have not been reported due to the limitation of material platforms. In this paper, we report a fully integrated hybrid microwave photonic receiver (FIH-MWPR) obtained by comprising the indium phosphide (InP) laser chip and the monolithic silicon-on-insulator (SOI) photonic circuit into the same substrate based on the low-coupling-loss micro-optics method. Benefiting from the integration of all optoelectronic components, the packaged FIH-MWPR exhibits a compact volume of 6 cm3 and low power consumption of 1.2 W. The FIH-MWPR supports a wide operation bandwidth from 2 to 18 GHz. Furthermore, its RF-link performance to down-convert the RF signals to the intermediate frequency is experimentally characterized by measuring the link gain, the noise figure, and the spurious-free dynamic range metrics across the whole operation frequency band. Moreover, we have utilized it as a de-chirp receiver to process the broadband linear frequency-modulated (LFM) radar echo signals at different frequency bands (S-, C-, X-, and Ku-bands) and successfully demonstrated its high-resolution-ranging capability. To the best of our knowledge, this is the first realization of a chip-scale broadband fully integrated microwave photonic receiver, which is expected to be an important step in demonstrating the feasibility of all-integrated microwave photonic microsystems oriented to miniaturized application scenarios.

Photonics Research
Editors' PickMay. 26, 2022, Vol. 10 Issue 6 06001472 (2022)
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