Contents Instrumentation and Measurements, 35 Article(s)
Photonics-based radar with balanced I/Q de-chirping for interference-suppressed high-resolution detection and imaging
Xingwei Ye, Fangzheng Zhang, Yue Yang, and Shilong Pan

Photonics-based radar with a photonic de-chirp receiver has the advantages of broadband operation and real-time signal processing, but it suffers from interference from image frequencies and other undesired frequency-mixing components, due to single-channel real-valued photonic frequency mixing. In this paper, we propose a photonics-based radar with a photonic frequency-doubling transmitter and a balanced in-phase and quadrature (I/Q) de-chirp receiver. This radar transmits broadband linearly frequency-modulated signals generated by photonic frequency doubling and performs I/Q de-chirping of the radar echoes based on a balanced photonic I/Q frequency mixer, which is realized by applying a 90° optical hybrid followed by balanced photodetectors. The proposed radar has a high range resolution because of the large operation bandwidth and achieves interference-free detection by suppressing the image frequencies and other undesired frequency-mixing components. In the experiment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated. The balanced I/Q de-chirping receiver achieves an image-rejection ratio of over 30 dB and successfully eliminates the interference due to the baseband envelope and the frequency mixing between radar echoes of different targets. In addition, the desired de-chirped signal power is also enhanced with balanced detection. Based on the established photonics-based radar, inverse synthetic aperture radar imaging is also implemented, through which the advantages of the proposed radar are verified.

Photonics Research
Feb. 11, 2019, Vol. 7 Issue 3 03000265 (2019)
Spectral measurement of coherence Stokes parameters of random broadband light beams
Henri Partanen, Ari T. Friberg, Tero Set?l?, and Jari Turunen

We introduce a spectrally resolved Young’s interferometer based on a digital micromirror device, a grating spectrometer, and a set of polarization-modulation elements to measure the spectral coherence (two-point) Stokes parameters of random light beams. An experimental demonstration is provided with a spatially partially coherent superluminescent diode amounting to a complex structure of spatio-spectral coherence induced by a quartz-wedge depolarizer. We also show that the polarization and spatial coherence of light can vary with wavelength on a subnanometer scale. The technique is simple and robust and applies to light beams with any spectral bandwidth.

Photonics Research
On the CoverMay. 29, 2019, Vol. 7 Issue 6 06000669 (2019)
High-speed and high-efficiency three-dimensional shape measurement based on Gray-coded light
Zhoujie Wu, Wenbo Guo, Yueyang Li, Yihang Liu, and Qican Zhang

Fringe projection profilometry has been increasingly sought and applied in dynamic three-dimensional (3D) shape measurement. In this work, a robust, high-efficiency 3D measurement based on Gray-coded light is proposed. Unlike the traditional method, a tripartite phase unwrapping method is proposed to avoid the jump errors on the boundary of code words, which are mainly caused by the defocusing of the projector and the motion of the tested object. Subsequently, the time-overlapping coding strategy is presented to greatly increase the coding efficiency, decreasing the projected number in each group from seven (i.e., 3+4) to four (i.e., 3+1) for one restored 3D frame. The combination of two proposed techniques allows the reconstruction of a pixel-wise and unambiguous 3D geometry of dynamic scenes with strong noise using every four projected patterns. To the best of our knowledge, the presented techniques for the first time preserve the high anti-noise ability of a method based on the Gray code while overcoming the drawbacks of jump errors and low coding efficiency. Experiments have demonstrated that the proposed method can achieve robust, high-efficiency 3D shape measurement of high-speed dynamic scenes even polluted by strong noise.

Photonics Research
May. 06, 2020, Vol. 8 Issue 6 06000819 (2020)
Distributed Brillouin frequency shift extraction via a convolutional neural network
Yiqing Chang, Hao Wu, Can Zhao, Li Shen, Songnian Fu, and Ming Tang

Distributed optical fiber Brillouin sensors detect the temperature and strain along a fiber according to the local Brillouin frequency shift (BFS), which is usually calculated by the measured Brillouin spectrum using Lorentzian curve fitting. In addition, cross-correlation, principal component analysis, and machine learning methods have been proposed for the more efficient extraction of BFS. However, existing methods only process the Brillouin spectrum individually, ignoring the correlation in the time domain, indicating that there is still room for improvement. Here, we propose and experimentally demonstrate a BFS extraction convolutional neural network (BFSCNN) to retrieve the distributed BFS directly from the measured two-dimensional data. Simulated ideal Brillouin spectra with various parameters are used to train the BFSCNN. Both the simulation and experimental results show that the extraction accuracy of the BFSCNN is better than that of the traditional curve fitting algorithm with a much shorter processing time. The BFSCNN has good universality and robustness and can effectively improve the performances of existing Brillouin sensors.

Photonics Research
Apr. 24, 2020, Vol. 8 Issue 5 05000690 (2020)
Emerging micro-additive manufacturing technologies enabled by novel optical methods
Wei Lin, Dihan Chen, and Shih-Chi Chen

This paper presents a comprehensive review of recent advances in micro-additive manufacturing enabled by novel optical methods with an emphasis on photopolymerization-based printing processes. Additive manufacturing, also known as three-dimensional (3D) printing, has become an important engineering solution to construct customized components or functional devices at low cost. As a green manufacturing technology, 3D printing has the advantages of high energy efficiency, low material consumption, and high precision. The rapid advancement of 3D printing technology has broadened its applications from laboratory research to industrial manufacturing. Generally, 3D objects to be printed are constructed digitally [e.g., via computer-aided design (CAD) programs] by connecting a 3D dot array, where a dot is defined as a voxel through mechanical, electrical, or optical means. The voxel size ranges from a few orders of magnitude of the wavelength of light to the sub-diffraction limit, achieved by material nonlinearity and precise power thresholding. In recent years, extensive research in optical additive manufacturing has led to various breakthroughs in quality, rate, and reproducibility. In this paper, we review various micro-3D printing techniques, including single-photon and two-photon processes, with a focus on innovative optical methods, e.g., ultrafast beam shaping, digital holography, and temporal focusing. We also review and compare recent technological advances in serial and parallel scanning systems from the perspectives of resolution, rate, and repeatability, where the strengths and weaknesses of different methods are discussed for both fundamental and industrial applications.

Photonics Research
Nov. 12, 2020, Vol. 8 Issue 12 12001827 (2020)
Long-distance ranging with high precision using a soliton microcomb
Jindong Wang, Zhizhou Lu, Weiqiang Wang, Fumin Zhang, Jiawei Chen, Yang Wang, Jihui Zheng, Sai T. Chu, Wei Zhao, Brent E. Little, Xinghua Qu, and Wenfu Zhang

Laser-based light detection and ranging (lidar) plays a significant role in both scientific and industrial areas. However, it is difficult for existing lidars to achieve high speed, high precision, and long distance simultaneously. Here, we demonstrate a high-performance lidar based on a chip-scaled soliton microcomb (SMC) that can realize all three specialties simultaneously. Aided by the excellent properties of ultrahigh repetition rate and the smooth envelope of the SMC, traditional optical frequency comb (OFC)-based dispersive interferometry is heavily improved and the measuring dead zone induced by the mismatch between the repetition rate of the OFC and resolution of the optical spectrum analyzer is totally eliminated. Combined with an auxiliary dual-frequency phase-modulated laser range finder, the none-dead-zone measurable range ambiguity is extended up to 1500 m. The proposed SMC lidar is experimentally implemented in both indoor and outdoor environment. In the outdoor baseline field, real-time, high-speed (up to 35 kHz) measurement of a long distance of ~1179 m is achieved with a minimum Allan deviation of 5.6 μm at an average time of 0.2 ms (27 nm at an average time of 1.8 s after high-pass filtering). The present SMC lidar approaches a compact, fast, high-precision, and none-dead zone long-distance ranging system, aimed at emerging applications of frontier basic scientific research and advances in industrial manufacturing.

Photonics Research
On the CoverNov. 30, 2020, Vol. 8 Issue 12 12001964 (2020)
Optical frequency synthesizer referenced to an ytterbium optical clock
Yuan Yao, Bo Li, Guang Yang, Xiaotong Chen, Yaqin Hao, Hongfu Yu, Yanyi Jiang, and Longsheng Ma

Optical clocks with an unprecedented accuracy of 10-18 promise innovations in precision spectroscopy and measurement. To harness the full power of optical clocks, we need optical frequency synthesizers (OFSs) to accurately convert the stabilities and accuracies of optical clocks to other desired frequencies. This work demonstrates such an OFS referenced to an ytterbium optical clock. The OFS is based on an optical frequency comb phase-locked to a commercial rubidium microwave clock; in this way most combs can operate robustly. Despite comb frequency instability at 10-11, the synthesis noise and uncertainty reach 6×10-18 (1 s) and 5×10-21, respectively, facilitating frequency synthesis of the best optical clocks. In the OFS, the coherence of the OFS internal oscillator at 1064 nm is accurately transferred to a 578 nm laser for resolving the hertz-level-linewidth ytterbium clock transition (unaffected by megahertz-linewidth comb lines) and faithfully referencing the OFS to an ytterbium optical clock.

Photonics Research
On the CoverJan. 13, 2021, Vol. 9 Issue 2 02000098 (2021)
Dual-comb spectroscopy resolved three-degree-of-freedom sensing
Siyu Zhou, Vunam Le, Shilin Xiong, Yuetang Yang, Kai Ni, Qian Zhou, and Guanhao Wu

Precise and fast determination of position and orientation, which is normally achieved by distance and angle measurements, has broad applications in academia and industry. We propose a dynamic three-degree-of-freedom measurement technique based on dual-comb interferometry and a self-designed grating-corner-cube (GCC) combined sensor. Benefiting from its unique combination of diffraction and reflection characteristics, the absolute distance, pitch, and yaw of the GCC sensor can be determined simultaneously by resolving the phase spectra of the corresponding diffracted beams. We experimentally demonstrate that the method exhibits a ranging precision (Allan deviation) of 13.7 nm and an angular precision of 0.088 arcsec, alongside a 1 ms reaction time. The proposed technique is capable of precise and fast measurement of distances and two-dimensional angles over long stand-off distances. A system with such an overall performance may be potentially applied to space missions, including in tight formation-flying satellites, for spacecraft rendezvous and docking, and for antenna measurement as well as the precise manufacture of components including lithography machines and aircraft-manufacturing devices.

Photonics Research
Feb. 01, 2021, Vol. 9 Issue 2 02000243 (2021)
Graphene metalens for particle nanotracking
Xueyan Li, Shibiao Wei, Guiyuan Cao, Han Lin, Yuejin Zhao, and Baohua Jia

Particle nanotracking (PNT) is highly desirable in lab-on-a-chip systems for flexible and convenient multiparameter measurement. An ultrathin flat lens is the preferred imaging device in such a system, with the advantage of high focusing performance and compactness. However, PNT using ultrathin flat lenses has not been demonstrated so far because PNT requires the clear knowledge of the relationship between the object and image in the imaging system. Such a relationship still remains elusive in ultrathin flat lens-based imaging systems because they operate based on diffraction rather than refraction. In this paper, we experimentally reveal the imaging relationship of a graphene metalens using nanohole arrays with micrometer spacing. The distance relationship between the object and image as well as the magnification ratio is acquired with nanometer accuracy. The measured imaging relationship agrees well with the theoretical prediction and is expected to be applicable to other ultrathin flat lenses based on the diffraction principle. By analyzing the high-resolution images from the graphene metalens using the imaging relationship, 3D trajectories of particles with high position accuracy in PNT have been achieved. The revealed imaging relationship for metalenses is essential in designing different types of integrated optical systems, including digital cameras, microfluidic devices, virtual reality devices, telescopes, and eyeglasses, and thus will find broad applications.

Photonics Research
Jul. 15, 2020, Vol. 8 Issue 8 08001316 (2020)
Adaptive optical focusing through perturbed scattering media with a dynamic mutation algorithm
Huanhao Li, Chi Man Woo, Tianting Zhong, Zhipeng Yu, Yunqi Luo, Yuanjin Zheng, Xin Yang, Hui Hui, and Puxiang Lai

Optical imaging through or inside scattering media, such as multimode fiber and biological tissues, has a significant impact in biomedicine yet is considered challenging due to the strong scattering nature of light. In the past decade, promising progress has been made in the field, largely benefiting from the invention of iterative optical wavefront shaping, with which deep-tissue high-resolution optical focusing and hence imaging becomes possible. Most of the reported iterative algorithms can overcome small perturbations on the noise level but fail to effectively adapt beyond the noise level, e.g., sudden strong perturbations. Reoptimizations are usually needed for significant decorrelation to the medium since these algorithms heavily rely on the optimization performance in the previous iterations. Such ineffectiveness is probably due to the absence of a metric that can gauge the deviation of the instant wavefront from the optimum compensation based on the concurrently measured optical focusing. In this study, a square rule of binary-amplitude modulation, directly relating the measured focusing performance with the error in the optimized wavefront, is theoretically proved and experimentally validated. With this simple rule, it is feasible to quantify how many pixels on the spatial light modulator incorrectly modulate the wavefront for the instant status of the medium or the whole system. As an example of application, we propose a novel algorithm, the dynamic mutation algorithm, which has high adaptability against perturbations by probing how far the optimization has gone toward the theoretically optimal performance. The diminished focus of scattered light can be effectively recovered when perturbations to the medium cause a significant drop in the focusing performance, which no existing algorithms can achieve due to their inherent strong dependence on previous optimizations. With further improvement, the square rule and the new algorithm may boost or inspire many applications, such as high-resolution optical imaging and stimulation, in instable or dynamic scattering environments.

Photonics Research
Jan. 26, 2021, Vol. 9 Issue 2 02000202 (2021)
Generalizing the Gerchberg–Saxton algorithm for retrieving complex optical transmission matrices
Guoqiang Huang, Daixuan Wu, Jiawei Luo, Liang Lu, Fan Li, Yuecheng Shen, and Zhaohui Li

The Gerchberg–Saxton (GS) algorithm, which retrieves phase information from the measured intensities on two related planes (the source plane and the target plane), has been widely adopted in a variety of applications when holographic methods are challenging to be implemented. In this work, we showed that the GS algorithm can be generalized to retrieve the unknown propagating function that connects these two planes. As a proof-of-concept, we employed the generalized GS (GGS) algorithm to retrieve the optical transmission matrix (TM) of a complex medium through the measured intensity distributions on the target plane. Numerical studies indicate that the GGS algorithm can efficiently retrieve the optical TM while maintaining accuracy. With the same training data set, the computational time cost by the GGS algorithm is orders of magnitude less than that consumed by other non-holographic methods reported in the literature. Besides numerical investigations, we also experimentally demonstrated retrieving the optical TMs of a stack of ground glasses and a 1-m-long multimode fiber using the GGS algorithm. The accuracy of the retrieved TM was evaluated by synthesizing high-quality single foci and multiple foci on the target plane through these complex media. These results indicate that the GGS algorithm can handle a large TM with high efficiency, showing great promise in a variety of applications in optics.

Photonics Research
Jan. 01, 2021, Vol. 9 Issue 1 01000034 (2021)
Photonic smart bandage for wound healing assessment
Arnaldo Leal-Junior, Jingjing Guo, Rui Min, António José Fernandes, Anselmo Frizera, and Carlos Marques

Chronic wounds affect around 2% of the world population with an annual multi-billion dollar cost to the healthcare system. This background pushes the development of new therapies and procedures for wound healing and its assessment. Among them, the potential of hydrogen (pH) assessment is an important indicator of the wound healing stage and condition. This paper presents the development of the first optical fiber-embedded smart wound dressing for pH assessment. An intrinsically pH-sensitive optical fiber is fabricated using a polydimethylsiloxane (PDMS) precursor doped with rhodamine B dye. Raman and Fourier transform infrared (FTIR) spectroscopies are performed in order to verify the presence of rhodamine B and PDMS in the fiber samples. Then, the fiber is embedded in gauze fabric and hydrocolloid wound dressing. In addition, such low Young’s modulus of PDMS fiber enables its use as a highly sensitive pressure sensor, where the results show that the fiber-embedded bandage can measure pressures as low as 0.1 kPa with a high linearity in the range of 0 to 0.3 kPa. The smart bandage is subjected to different pH, which resulted in a wavelength shift of 0.67 nm/pH when the absorption peak at 515 nm was analyzed. Furthermore, pH increase leads to linear decrease of the transmitted optical power (R2 of 0.998), with rise and fall times below 20 s and 30 s, respectively. Therefore, the proposed optical fiber-embedded smart bandage enables the simultaneous assessment of pressure and pH on the wound region.

Photonics Research
Feb. 05, 2021, Vol. 9 Issue 3 03000272 (2021)
Dispersion engineering and measurement in crystalline microresonators using a fiber ring etalon
Xiaobao Zhang, Guoping Lin, Tang Sun, Qinghai Song, Guangzong Xiao, and Hui Luo

Dispersion engineering and measurement are significant for nonlinear photonic applications using whispering gallery mode microresonators. Specifically, the Kerr microresonator frequency comb as an important example has attracted a great amount of interest in research fields due to the potential capability of full integration on a chip. A simple and cost-efficient way for dispersion measurements is thereby in high demand for designing such a microcomb device. Here, we report a dispersion measurement approach using a fiber ring etalon reference. The free spectral range of the etalon is first measured through sideband modulation, and the dispersion of the etalon is inferred by binary function fitting during the dispersion measurement. This method is demonstrated on two MgF2 disk resonators. Experimental results show good agreement with numerical simulations using the finite element method. Dispersion engineering on such resonators is also numerically investigated.

Photonics Research
Oct. 21, 2021, Vol. 9 Issue 11 11002222 (2021)
Synchronous nanoscale topographic and chemical mapping by differential-confocal controlled Raman microscopy
Han Cui, Yun Wang, Lirong Qiu, Shucheng Li, Jonathan M. Cooper, and Weiqian Zhao

Confocal Raman microscopy is currently used for label-free optical sensing and imaging within the biological, engineering, and physical sciences as well as in industry. However, currently these methods have limitations, including their low spatial resolution and poor focus stability, that restrict the breadth of new applications. This paper now introduces differential-confocal controlled Raman microscopy as a technique that fuses differential confocal microscopy and Raman spectroscopy, enabling the point-to-point collection of three-dimensional nanoscale topographic information with the simultaneous reconstruction of corresponding chemical information. The microscope collects the scattered Raman light together with the Rayleigh light, both as Rayleigh scattered and reflected light (these are normally filtered out in conventional confocal Raman systems). Inherent in the design of the instrument is a significant improvement in the axial focusing resolution of topographical features in the image (to ~1 nm), which, when coupled with super-resolution image restoration, gives a lateral resolution of 220 nm. By using differential confocal imaging for controlling the Raman imaging, the system presents a significant enhancement of the focusing and measurement accuracy, precision, and stability (with an antidrift capability), mitigating against both thermal and vibrational artefacts. We also demonstrate an improved scan speed, arising as a consequence of the nonaxial scanning mode.

Photonics Research
Aug. 14, 2020, Vol. 8 Issue 9 09001441 (2020)
Long range dynamic displacement: precision PGC with sub-nanometer resolution in an LWSM interferometer
Yisi Dong, Peng-Cheng Hu, Haijin Fu, Hongxing Yang, Ruitao Yang, and Jiubin Tan

We propose a precision phase-generated-carrier (PGC) demodulation method with sub-nanometer resolution that avoids nonlinear errors in a laser wavelength sinusoidal modulation fiber-optic interferometer for long range dynamic displacement sensing. Using orthogonal detection and an AC-DC component extraction scheme, the PGC carrier phase delay (CPD) and laser intensity modulation phase delay can be obtained simultaneously to eliminate the nonlinear error from accompanied optical intensity modulation and CPD. Further, to realize long range displacement sensing, PGC phase modulation depth (PMD), determined by the laser wavelength modulation amplitude and the working distance of the interferometer, is required to maintain an optimal value during measurement, including initial position and dynamic movement. By combining frequency sweeping interference and modified PGC-arctan demodulation to measure real-time working distance, adaptive PMD technology is realized based on proportion control. We construct a fiber-optic Michelson and SIOS commercial interferometer for comparison and perform experiments to verify the feasibility of the proposed method. Experimental results demonstrate that an interferometer with sub-nanometer resolution and nanometer precision over a large range of 400 mm can be realized.

Photonics Research
Jan. 01, 2022, Vol. 10 Issue 1 01000059 (2022)
High-throughput microfabrication of axially tunable helices
He Cheng, Pooria Golvari, Chun Xia, Mingman Sun, Meng Zhang, Stephen M. Kuebler, and Xiaoming Yu

Helical structures exhibit novel optical and mechanical properties and are commonly used in different fields such as metamaterials and microfluidics. A few methods exist for fabricating helical microstructures, but none of them has the throughput or flexibility required for patterning a large surface area with tunable pitch. In this paper, we report a method for fabricating helical structures with adjustable forms over large areas based on multiphoton polymerization (MPP) using single-exposure, three dimensionally structured, self-accelerating, axially tunable light fields. The light fields are generated as a superposition of high-order Bessel modes and have a closed-form expression relating the design of the phase mask to the rotation rate of the beam. The method is used to fabricate helices with different pitches and handedness in the material SU-8. Compared to point-by-point scanning, the method reported here can be used to reduce fabrication time by two orders of magnitude, paving the way for adopting MPP in many industrial applications.

Photonics Research
Editors' PickJan. 07, 2022, Vol. 10 Issue 2 02000303 (2022)
Measurement of sub-fm/Hz1/2 displacement spectral densities in ultrahigh-Q single-crystal microcavities with hertz-level lasers
Yoon-Soo Jang, Jinkang Lim, Wenting Wang, Seung-Woo Kim, Anatoliy Savchenkov, Andrey B. Matsko, and Chee Wei Wong

Tracing a resonance frequency of a high quality factor (Q) optical cavity facilitates subpicometer displacement measurements of the optical cavity via Pound–Drever–Hall (PDH) locking scheme, tightly synchronizing a laser frequency to the optical cavity. Here we present observations of subfemtometer displacements on a ultrahigh-Q single-crystal MgF2 whispering-gallery-mode microcavity by frequency synchronization between a 1 Hz cavity-stabilized laser and a resonance of the MgF2 cavity using PDH laser-cavity locking. We characterize not only the displacement spectral density of the microcavity with a sensitivity of 1.5×10-16 m/Hz1/2 over the Fourier offset frequency ranging from 15 mHz to 100 kHz but also a 1.77 nm displacement fluctuation of the microcavity over 4500 s. Such measurement capability not only supports the analysis of integrated thermodynamical and technical cavity noise but allows for minute displacement measurements using laser-cavity locking for ultraprecise positioning, metrology, and sensing.

Photonics Research
Apr. 14, 2022, Vol. 10 Issue 5 05001202 (2022)
Regenerated polymer optical fiber Bragg gratings with thermal treatment for high temperature measurements
Dinusha Serandi Gunawardena, Xin Cheng, Jingxian Cui, Geraldi Edbert, Linyue Lu, Yuk Ting Ho, and Hwa-Yaw Tam

We report for the first time, to the best of our knowledge, regenerated polymer optical fiber Bragg gratings (RPOFBGs) in ZEONEX-based polymer optical fibers (POFs). The regeneration temperature can be adjusted using a heat treatment process on the POF before FBG inscription, enabling a scalable improvement of the thermal stability of the RPOFBGs. Thermal sustainability of the RPOFBGs at high temperature conditions was investigated for their prolonged use in diverse environments. Furthermore, these RPOFBGs can withstand strain levels up to 2.8% while maintaining a good linearity, even at temperature of 110°C. The RPOFBGs are capable of short-term operation at elevated temperatures of up to 132°C, which is the standard temperature for steam sterilization with at least a 4 min exposure period. The distinction in the morphologies of the two grades of ZEONEX (E48R and 480R, ZEON Corp.) used to fabricate the optical fiber together with the characteristics of UV irradiated and regenerated gratings is explained using micro-Raman spectroscopy. Collectively, these findings provide new heights for long-term operation of POF Bragg gratings (POFBGs) at elevated temperature environments and would be applicable to a wide range of disciplines.

Photonics Research
Mar. 25, 2022, Vol. 10 Issue 4 04001011 (2022)
Superfast and large-depth-range sinusoidal fringe generation for multi-dimensional information sensing
Sijie Zhu, Zhoujie Wu, Jing Zhang, Qican Zhang, and Yajun Wang

Among many multi-dimensional information sensing methods such as structured-light and single-pixel imaging technologies, sinusoidal fringe generation is general and crucial. Current methods of sinusoidal fringe generation force concessions in either the speed or the depth range. To mitigate this trade-off, we have simultaneously achieved both speed breakthrough and depth range enhancement by improving both the optical projection system and binary coding algorithm based on an off-the-shelf projector. Specifically, we propose a multifocal projection system and oblique projection method, which essentially eliminates the existence of a single focal plane in the conventional axisymmetric system and utilizes its anisotropy characteristics to achieve a superior filtering effect. Furthermore, the optimal pulse width modulation technique is introduced to modulate the square binary pattern for eliminating specific harmonics. To the best of our knowledge, the proposed method, for the first time, simultaneously achieved superfast (9524 frames per second) and large-depth-range (560 mm, about three times that of the conventional method) sinusoidal fringe generation with consistently high accuracy. Experimental results demonstrate the superior performance of the proposed method in multi-dimensional information sensing such as 3D, 4D, and [x, y, z, t; s (strain)].

Photonics Research
Oct. 28, 2022, Vol. 10 Issue 11 2590 (2022)
Influence of light pattern thickness on the manipulation of dielectric microparticles by optoelectronic tweezers
Shuailong Zhang, Mohamed Elsayed, Ran Peng, Yujie Chen, Yanfeng Zhang, Steven L. Neale, and Aaron R. Wheeler

Optoelectronic tweezer (OET) is a useful optical micromanipulation technology that has been demonstrated for various applications in electrical engineering and most notably cell selection for biomedical engineering. In this work, we studied the use of light patterns with different shapes and thicknesses to manipulate dielectric microparticles with OET. It was demonstrated that the maximum velocities of the microparticles increase to a peak and then gradually decrease as the light pattern’s thickness increases. Numerical simulations were run to clarify the underlying physical mechanisms, and it was found that the observed phenomenon is due to the co-influence of horizontal and vertical dielectrophoresis forces related to the light pattern’s thickness. Further experiments were run on light patterns with different shapes and objects with different sizes and structures. The experimental results indicate that the physical mechanism elucidated in this research is an important one that applies to different light pattern shapes and different objects, which is useful for enabling users to optimize OET settings for future micromanipulation applications.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000550 (2022)
Noncoaxial RDE of circular asymmetry optical vortex for rotating axis detection
Song Qiu, Xiangyang Zhu, Ruoyu Tang, Tong Liu, Ruijian Li, and Yuan Ren

We reveal the mechanism of the noncoaxial rotational Doppler effect (RDE) of an optical vortex and report its application in discriminating the orientation of the rotating axis of the rotating body. In most cases of the RDE-based measurement, the beam axis must be aligned with the rotating axis of the rotational body to observe a good signal. Once the beam axis is not coaxial with the rotating axis, the RDE frequency shift would change related to the misalignment distance, which can be called the noncoaxial RDE. Here, we take the advantage of the misaligned RDE augment with precise light-field modulation and successfully realize the discrimination of the orientation of the rotating axis relative to the illuminating beam. We clarify the principle of noncoaxial RDE and explain why the incomplete optical vortex (OV) is sensitive to the position of the rotating axis. We switch the OV field into four quadrants synchronized with sampling by the data acquisition system, and conduct Fourier transformation of the signals. Combined with the fitting algorithm, the orientation of the rotating axis can be recognized directly. This method may find applications for the noncontact detection of rotating bodies in both industrial and astronomical scenarios.

Photonics Research
Oct. 24, 2022, Vol. 10 Issue 11 2541 (2022)
Single-shot three-input phase retrieval for quantitative back focal plane measurement
Mengqi Shen, Qi Zou, Xiaoping Jiang, Fu Feng, and Michael G. Somekh

This paper presents quantitative measurements facilitated with a new optical system that implements a single-shot three-input phase retrieval algorithm. The new system allows simultaneous acquisition of three distinct input patterns, thus eliminating the requirement for mechanical movement and reducing any registration errors and microphonics. We demonstrate the application of the system for measurement and separation of two distinct attenuation measurements of surface waves, namely, absorption and coupling loss. This is achieved by retrieving the phase in the back focal plane and performing a series of virtual optics computations. This overcomes the need to use a complicated series of hardware manipulations with a spatial light modulator. This gives a far more accurate and faster measurement with a simpler optical system. We also demonstrate that phase measurements allow us to implement different measurement methods to acquire the excitation angle for surface plasmons. Depending on the noise statistics different methods have superior performance, so the best method under particular conditions can be selected. Since the measurements are only weakly correlated, they may also be combined for improved noise performance. The results presented here offer a template for a wider class of measurements in the back focal plane including ellipsometry.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000491 (2022)
Intensity diffusion: a concealed cause of fringe distortion in fringe projection profilometry
Zheng Sun, Minghui Duan, Yabing Zheng, Yi Jin, Xin Fan, and Jinjin Zheng

Fringe projection profilometry (FPP) is widely used in optical three-dimensional (3D) measurements because of its high stability. In FPP, fringe distortion is an inevitable and highly complex systematic error that significantly reduces the 3D measurement accuracy. At this point, the existing causes of fringe distortion represented by gamma distortion, high-order harmonics, and image saturation have been effectively analyzed and compensated to restore high-quality fringe images. In this paper, we innovatively reveal a concealed cause of fringe distortion, i.e., intensity diffusion across pixels, which is induced by photocarrier diffusion between photodiodes. To the best of our knowledge, intensity diffusion has not been studied in the field of fringe restoration. Based on the motion of photocarrier diffusion, we theoretically analyze the mechanism of how the intensity diffusion affects FPP. Subsequently, an intensity diffusion model is established for quantifying the diffused intensity in each pixel, and an intensity diffusion correction algorithm is presented to remove the diffused intensity from the fringe images and correct the fringe distortion. Experiments demonstrate the impact of intensity diffusion on FPP, and the 3D measurement results prove the effectiveness of the proposed methods on improving the 3D measurement accuracy by correcting the fringe distortion.

Photonics Research
Apr. 14, 2022, Vol. 10 Issue 5 05001210 (2022)
Ultrasensitive measurement of angular rotations via a Hermite–Gaussian pointer
Binke Xia, Jingzheng Huang, Hongjing Li, Miaomiao Liu, Tailong Xiao, Chen Fang, and Guihua Zeng

corresponding author guidelines for details."?>Exploring high sensitivity on the measurement of angular rotations is an outstanding challenge in optics and metrology. In this work, we employ the mn-order Hermite–Gaussian (HG) beam in the weak measurement scheme with an angular rotation interaction, where the rotation information is taken by another HG mode state completely after the post-selection. By taking a projective measurement on the final light beam, the precision of angular rotation is improved by a factor of 2mn+m+n. For verification, we perform an optical experiment where the minimum detectable angular rotation improves 15-fold with HG55 mode over that of HG11 mode, and achieves a sub-microradian scale of the measurement precision. Our theoretical framework and experimental results not only provide a more practical and convenient scheme for ultrasensitive measurement of angular rotations but also contribute to a wide range of applications in quantum metrology.

Photonics Research
Nov. 28, 2022, Vol. 10 Issue 12 2816 (2022)
Brillouin-scattering-induced transparency enabled reconfigurable sensing of RF signals
Jaffar Kadum, Ranjan Das, Arijit Misra, and Thomas Schneider

Real-time spectrum sensing is essential to enable dynamic and rapid spectrum sharing of unused frequencies to cater the substantial demands of new wireless services deploying the existing RF bands. In this paper, we present a novel, real-time spectrum sensing approach for widely used RF signals based on Brillouin-scattering-induced transparency (BIT). A temporal discrimination of multi-channel input frequencies is achieved through the group delay tuning by BIT. By tuning the pump power and frequency, the proposed technique is fully reconfigurable and viable for a broad range of spectrum sensing. Several experimental illustrations of the time domain sensing are presented for two-tone channels with 0.9, 1.8, and 5 GHz frequencies to detect the unused spectrum within 3G, 4G, and 5G signals.

Photonics Research
Editors' PickJul. 20, 2021, Vol. 9 Issue 8 08001486 (2021)
Navigation-grade resonant fiber-optic gyroscope using ultra-simple white-light multibeam interferometry
Shuangxiang Zhao, Qingwen Liu, Yuanyuan Liu, Huilian Ma, and Zuyuan He

The miniaturization of the gyroscope is critical for spacecrafts, drones, wellbore surveys, etc. The resonant fiber-optic gyroscope (RFOG) is a competitive candidate due to its potential in both miniaturization and high resolution, while its actual performance is well below expectation because of laser-induced noise and complexity. Here we report the first navigation grade RFOG with a bias instability of 0.009°/h and an angle random walk of 0.0093°/h. The results are realized using a fiber resonator with finesse of 63 containing 100-m long fiber. Compared with the traditional RFOGs using narrow-linewidth lasers, the key feature of the proposed RFOG is that it is driven with a broadband light source. A white-light multibeam interference method is proposed to detect the Sagnac effect, representing the simplest scheme of RFOG to date. The complexity caused by multiple feedback loops and coherent noise suppression in traditional RFOG scheme is avoided. The minimal scheme and simple modulation algorithm will also promote the on-chip waveguide gyroscope.

Photonics Research
Feb. 01, 2022, Vol. 10 Issue 2 02000542 (2022)
Time shifting deviation method enhanced laser interferometry: ultrahigh precision localizing of traffic vibration using an urban fiber link
Guan Wang, Zhongwang Pang, Bohan Zhang, Fangmin Wang, Yufeng Chen, Hongfei Dai, Bo Wang, and Lijun Wang

Using a fiber network as a huge sensing system will enrich monitoring methods of public infrastructures and geological disasters. With the traditional cross-correlation method, a laser interferometer has been used to detect and localize the vibration event. However, the random error induced by the cross-correlation method limits the localization accuracy and makes it not suitable for ultrahigh precision localizing applications. We propose a novel time shifting deviation (TSDEV) method, which has advantages over the cross-correlation method in practicability and localization accuracy. Three experiments are carried out to demonstrate the novelty of the TSDEV method. In a lab test, vibration localization accuracy of ∼2.5 m is realized. In field tests, TSDEV method enhanced interferometry is applied to monitor the urban fiber link. Traffic vibration events on the campus road and Beijing ring road have been precisely localized and analyzed, respectively. The proposed technique will extend the function of the existing urban fiber network, and better serve the future smart city.

Photonics Research
Jan. 24, 2022, Vol. 10 Issue 2 02000433 (2022)
Fiber optic strain rate sensor based on a differentiating interferometer
Huicong Li, Wenzhu Huang, Wentao Zhang, and Jianxiang Zhang

Strain rate is an important basic physical parameter in the fields of deformation observation, geodetic measurement, and geophysical monitoring. This paper proposes a novel fiber optic strain rate sensor (FOSRS) that can directly measure the strain rate through a differentiating interferometer that converts the strain rate to the optical phase. The sensing principle, sensitivity, resolution, and dynamic range of the proposed FOSRS are theoretically analyzed and verified by experiment. The experimental results show that the developed FOSRS with a 12.1 m sensing fiber has a flat sensitivity of 69.50 dB, a nanostrain rate (nε/s) resolution, and a dynamic range of better than 95 dB. An ultrahigh static resolution of 17.07 pε/s can be achieved by using a 25.277 km sensing fiber for long baseline measurements. The proposed method significantly outperforms existing indirect measurement methods and has potential applications in geophysical monitoring and crustal deformation observation.

Photonics Research
Oct. 28, 2022, Vol. 10 Issue 11 2599 (2022)
Photonic extreme learning machine by free-space optical propagation
Davide Pierangeli, Giulia Marcucci, and Claudio Conti

Photonic brain-inspired platforms are emerging as novel analog computing devices, enabling fast and energy-efficient operations for machine learning. These artificial neural networks generally require tailored optical elements, such as integrated photonic circuits, engineered diffractive layers, nanophotonic materials, or time-delay schemes, which are challenging to train or stabilize. Here, we present a neuromorphic photonic scheme, i.e., the photonic extreme learning machine, which can be implemented simply by using an optical encoder and coherent wave propagation in free space. We realize the concept through spatial light modulation of a laser beam, with the far field acting as a feature mapping space. We experimentally demonstrate learning from data on various classification and regression tasks, achieving accuracies comparable with digital kernel machines and deep photonic networks. Our findings point out an optical machine learning device that is easy to train, energetically efficient, scalable, and fabrication-constraint free. The scheme can be generalized to a plethora of photonic systems, opening the route to real-time neuromorphic processing of optical data.

Photonics Research
Jul. 08, 2021, Vol. 9 Issue 8 08001446 (2021)
Coherent-detection-based distributed acoustic impedance sensing enabled by a chirped fiber Bragg grating array
Zhou Zheng, Zhengying Li, Xuelei Fu, and Xin Gui

Distributed optical fiber sensing exploring forward stimulated Brillouin scattering (FSBS) has received wide attention, as it indicates a new sensing method to measure the liquid property surrounding an optical fiber. In the existing techniques, backward stimulated Brillouin scattering is adopted for detection of the sensing signal, which requires time-consuming signal acquisition and post-processing. In this work, an approach that distributedly measures FSBS spectra is proposed and demonstrated based on coherent detection. While an excitation pulse with single-frequency amplitude modulation is used to induce a guided acoustic mode in the fiber, a following pulse is adopted to probe the induced phase modulation. Using a chirped fiber Bragg grating array, an enhanced-backward-propagating sensing signal is generated from the probe pulse. Heterodyne coherent-detection-based phase demodulation is then realized by mixing the sensing signal with a local oscillator. The FSBS spectra can then be reconstructed from the beat signals with only one round of frequency sweeping. With significantly accelerated signal acquisition and simplified post-processing, the proposed distributed acoustic sensing system has achieved spatial resolution of 5 m over a 500-m sensing range.

Photonics Research
May. 06, 2022, Vol. 10 Issue 6 06001325 (2022)
Generalized robust training scheme using genetic algorithm for optical neural networks with imprecise components
Rui Shao, Gong Zhang, and Xiao Gong

One of the pressing issues for optical neural networks (ONNs) is the performance degradation introduced by parameter uncertainties in practical optical components. Hereby, we propose a novel two-step ex situ training scheme to configure phase shifts in a Mach–Zehnder-interferometer-based feedforward ONN, where a stochastic gradient descent algorithm followed by a genetic algorithm considering four types of practical imprecisions is employed. By doing so, the learning process features fast convergence and high computational efficiency, and the trained ONN is robust to varying degrees and types of imprecisions. We investigate the effectiveness of our scheme by using practical machine learning tasks including Iris and MNIST classifications, showing more than 23% accuracy improvement after training and accuracy (90.8% in an imprecise ONN with three hidden layers and 224 tunable thermal-optic phase shifters) comparable to the ideal one (92.0%).

Photonics Research
Jul. 22, 2022, Vol. 10 Issue 8 1868 (2022)
Simultaneous multiple time scale imaging for kHz–MHz high-speed accelerometry
Vassily Kornienko, David Andersson, Mehdi Stiti, Jonas Ravelid, Simon Ek, Andreas Ehn, Edouard Berrocal, and Elias Kristensson

Fast transient events, such as the disintegration of liquid bodies or chemical reactions between radical species, involve various processes that may occur at different time scales. Currently, there are two alternatives for monitoring such events: burst- or high-speed imaging. Burst imaging at ultrahigh speeds (∼100 MHz to THz) allows for the capture of nature’s fastest processes but only for a narrowly confined period of time and at a repetition rate of ∼10 Hz. Monitoring long lasting, rapidly evolving transient events requires a significantly higher repetition rate, which is met by existing ∼kHz to 1 MHz high-speed imaging technology. However, the use of such systems eliminates the possibility to observe dynamics occurring on the sub-microsecond time scale. In this paper, we present a solution to this technological gap by combining multiplexed imaging with high-speed sensor technology, resulting in temporally resolved, high-spatial-resolution image series at two simultaneous time scales. We further demonstrate how the collection of such data opens up the tracking of rapidly evolving structures up to MHz burst rates over long durations, allowing, for the first time, to our knowledge, the extraction of acceleration fields acting upon the liquid bodies of an atomizing spray in two dimensions at kHz frame rates.

Photonics Research
Spotlight on OpticsJun. 30, 2022, Vol. 10 Issue 7 1712 (2022)
Comb-mode resolved spectral domain interferometer enabled by a broadband electro-optic frequency comb
Yoon-Soo Jang, Jungjae Park, and Jonghan Jin

The spectral domain interferometer (SDI) has been widely used in dimensional metrology. Depending on the nature of the SDI, both wider spectral bandwidth and narrower linewidth of the light source are paradoxically required to achieve better resolution and longer measurable distances. From this perspective, a broadband frequency comb with a repetition rate high enough to be spectrally resolved can be an ideal light source for SDIs. In this paper, we propose and implement a broadband electro-optic frequency comb to realize a comb-mode resolved SDI. The proposed electro-optic frequency comb was designed with an optically recirculating loop to provide a broadband spectrum, which has a repetition rate of 17.5 GHz and a spectral range of 35 nm. In a preliminary test, we demonstrated absolute distance measurements with sub-100 nm repeatability. Because of these advantages, we believe this electro-optic frequency comb can open up new possibilities for SDIs.

Photonics Research
Jan. 01, 2023, Vol. 11 Issue 1 72 (2023)
Ultrafast miniaturized GaN-based optoelectronic proximity sensor
Xiaoshuai An, Hongying Yang, Yumeng Luo, Zhiqin Chu, and Kwai Hei Li

In this work, a novel ultrafast optoelectronic proximity sensor based on a submillimeter-sized GaN monolithic chip is presented. Fabricated through wafer-scale microfabrication processes, the on-chip units adopting identical InGaN/GaN diode structures can function as emitters and receivers. The optoelectronic properties of the on-chip units are thoroughly investigated, and the ability of the receivers to respond to changes in light intensity from the emitter is verified, revealing that the sensor is suitable for operation in reflection mode. Through a series of dynamic measurements, the sensor is highly sensitive to object movement at subcentimeter distances with high repeatability. The sensor exhibits ultrafast microsecond response, and its real-time monitoring capability is also demonstrated by applying it to detect slight motions of moving objects at different frequencies, including the human heart rate, the vibration of the rotary pump, the oscillation of the speaker diaphragm, and the speed of the rotating disk. The compact and elegant integration scheme presented herein opens a new avenue for realizing a chip-scale proximity sensing device, making it a promising candidate for widespread practical applications.

Photonics Research
Jul. 28, 2022, Vol. 10 Issue 8 1964 (2022)
Single-shot terahertz polarization detection based on terahertz time-domain spectroscopy
Qinggang Lin, Xinming Yuan, Xuanke Zeng, Yatao Yang, Yi Cai, Xiaowei Lu, Maijie Zheng, Congying Wang, Wenhua Cao, and Shixiang Xu

This paper presents a novel design for single-shot terahertz polarization detection based on terahertz time-domain spectroscopy (THz-TDS). Its validity has been confirmed by comparing its detection results with those of the THz common-path spectral interferometer through two separate measurements for the orthogonal components. Our results also show that its detection signal-to-noise ratios (SNRs) are obviously superior to those of the 45° optical bias THz-TDS by electro-optical sampling due to its operation on common-path spectral interference rather than the polarization-sensitive intensity modulation. The setup works without need of any optical scan, which does not only save time, but also efficiently avoids the disturbances from the fluctuations of the system and environment. Its single-shot mode allows it to work well for the applications with poor or no repeatability.

Photonics Research
May. 12, 2022, Vol. 10 Issue 6 06001374 (2022)
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