Photonics Research
Co-Editors-in-Chief
Paul Urbach, Siyuan Yu, Xiaocong Yuan
Vol. 4, Issue 5, 2016
Editor(s): Paul Urbach, Siyuan Yu, Xiaocong Yuan
Year: 2016
Status: Published

This special issue focuses on optical vortices and solicits original papers on all aspects of optical vortices.

Contents 16 article(s)
Influences of atmospheric turbulence effects on the orbital angular momentum spectra of vortex beams
Shiyao Fu, and Chunqing Gao

We investigate the atmospheric turbulence effects on orbital angular momentum (OAM) spectra of different kinds of vortex beams, including Laguerre–Gaussian (LG) beams and Bessel beams, numerically. We generate the holograms of atmospheric turbulence with different structure constants of the refractive index. The OAM spectra of distorted single-mode or multiplexed LG beams and Bessel beams are analyzed. Compared with the OAM spectra of the two kinds of vortex beams, the spectrum of the Bessel beams is more dispersive. The results illustrate that Bessel beams suffer more from turbulent atmosphere than LG beams.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 050000B1 (2016)
Orbital-angular-momentum-based reconfigurable optical switching and routing
Alan E. Willner, Long Li, Guodong Xie, Yongxiong Ren, Hao Huang, Yang Yue, Nisar Ahmed, Moshe J. Willner, Asher J. Willner, Yan Yan, Zhe Zhao, Zhe Wang, Cong Liu, Moshe Tur, and Solyman Ashrafi

Orbital angular momentum (OAM) has gained interest due to its potential to increase capacity in optical communication systems as well as an additional domain for reconfigurable networks. This is due to the following: (i) coaxially propagated OAM beams with different charges are mutually orthogonal, (ii) OAM beams can be efficiently multiplexed and demultiplexed, and (iii) OAM charges can be efficiently manipulated. Therefore, multiple data-carrying OAM beams could have the potential capability for reconfigurable optical switching and routing. In this paper, we discuss work involving reconfigurable OAM-based optical add/drop multiplexing, space switching, polarization switching, channel hopping, and multicasting.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 050000B5 (2016)
Generation and propagation characteristics of electromagnetic vortices in radio frequency
Shilie Zheng, Weite Zhang, Zhuofan Zhang, Xiaofeng Jin, Hao Chi, and Xianmin Zhang

Electromagnetic vortices, which describe the orbital angular momentum (OAM) carrying waves with a helical phase front, have recently attracted much interest in a radio frequency domain due to their potential applications in many diverse areas. In an OAM-based scenario, the antenna for OAM mode multiplexing/demultiplexing plays an essential role in controlling the overall system performance. In this paper, we demonstrated theoretically and experimentally an easily realized OAM antenna based on the traveling-wave circular loop structure for efficiently multiplexing/demultiplexing multiple OAM modes; in addition, its general propagation characteristics including the polarization, divergence, and radiation pattern are mathematically analyzed. Schemes for antenna size reduction and various radiation pattern manipulations have also been discussed to realize a more flexible and compact system.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 050000B9 (2016)
Advances in communications using optical vortices
Jian Wang

An optical vortex having an isolated point singularity is associated with the spatial structure of light waves. A polarization vortex (vector beam) with a polarization singularity has spatially variant polarizations. A phase vortex with phase singularity or screw dislocation has a spiral phase front. The optical vortex has recently gained increasing interest in optical trapping, optical tweezers, laser machining, microscopy, quantum informationprocessing, and optical communications. In this paper, we review recent advances in optical communications using optical vortices. First, basic concepts of polarization/phase vortex modulation and multiplexing in communications and key techniques of polarization/phase vortex generation and (de)multiplexing are introduced. Second, free-space and fiber optical communications using optical vortex modulation and optical vortex multiplexing are presented. Finally, key challenges and perspectives of optical communications using optical vortices are discussed. It is expected that optical vortices exploiting the space physical dimension of light waves might find more interesting applications in optical communications and interconnects.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000B14 (2016)
Unveiling stability of multiple filamentation caused by axial symmetry breaking of polarization
Si-Min Li, Zhi-Cheng Ren, Ling-Jun Kong, Sheng-Xia Qian, Chenghou Tu, Yongnan Li, and Hui-Tian Wang

Femtosecond laser filamentation is generally initialized from unpredictable symmetry breaking caused by random noise, causing it to be barely controlled. However, it is always anticipated for stable and controllable filamentation. We present and demonstrate the idea that hybridly polarized vector fields with axial symmetry broken polarization, associated with a pair of orthogonally linearly polarized vortices carrying the opposite-handed orbital angular momenta, could achieve controllable and robust multiple filamentation. Here, our motivation is to unveil the underlying physics behind such controllable and robust multiple filamentation. The symmetry breaking should first be actively controllable and then be able to effectively inhibit random noise. Robust multiple filamentation is inseparable from the fact that the phases between the multiple filaments are always locked. In contrast, uncontrollable multiple filamentation is always accompanied with loss of phase, i.e., the multiple filaments become incoherent to each other. Our results may offer a suggestion for achieving controllable and robust multiple filamentation in other systems.and Equipment Development Project (2012YQ17004); Collaborative Innovation Center of Extreme Optics.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000B29 (2016)
Creation of radially polarized optical fields with multiple controllable parameters using a vectorial optical field generator
Sichao Zhou, Shiyi Wang, Jian Chen, Guanghao Rui, and Qiwen Zhan

A vectorial optical field generator (VOF-Gen) based on two reflective phase-only liquid crystal spatial light modulators enables the creation of an arbitrary optical complex field. In this work, the capabilities of the VOF-Gen in terms of manipulating the spatial distributions of phase, amplitude, and polarization are experimentally demonstrated by generating a radially polarized optical field consisted of five annular rings, the focusing properties of which are also numerically studied with vectorial diffraction theory. By carefully adjusting the relative amplitude and phase between the adjacent rings, an optical needle field with purely longitudinal polarization can be produced in the focal region of a high numerical aperture lens. The versatile method presented in this work can be easily extended to the generation of a vectorial optical field with any desired complex distributions.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000B35 (2016)
Optical vortices and vector beams
Xiaocong Yuan, Siyuan Yu, and H. Paul Urbach

Amplitude, phase and polarization are essential parameters of an optical field. In the past decade, with the enhancement of techniques in manipulating phase and polarization states in complex optical fields, optical vortices and vector beams are investigated in a closely coupled fashion in terms of efficient beam generation and manipulation, stable transmission and novel detection aimed at various applications. In this special issue 6 invited papers demonstrate a number of typical topics in the field, with emphasis on the vectorial beam generation and optical communication systems utilising orbital angular momentum (OAM).

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 0500OVB1 (2016)
Characteristic analysis of a Smith–Purcell terahertz source
Xian-Zhu Meng, Ming-Hong Wang, Li-Ming Zhang, and Zhong-Min Ren

We investigate a novel Smith–Purcell terahertz source. This device is composed of an electron gun, a cylindrical resonator, a metallic grating, and a collector. The characteristics of the Smith–Purcell terahertz source are discussed with the help of three-dimensional particle-in-cell simulation. In this device, coherent and high-power Smith–Purcell radiation (SPR) at the terahertz frequency range can be produced for the reasonable parameters of charge energy and grating. Our results indicate that coherent SPR at 506.529 GHz with a power around 1000 W can be obtained for a grating of period l = 0.3 mm operating at the beam energy E = 50 keV and beamcurrent I = 10 A.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000162 (2016)
Experimental performance evaluation of quadrature amplitude modulation signal transmission in a silicon microring
Chengcheng Gui, and Jian Wang

We comprehensively characterize the transmission performance of m-ary quadrature amplitude modulation (m-QAM) signals through a silicon microring resonator in the experiment. Using orthogonal frequency-division multiplexing based on offset QAM (OFDM/OQAM) which is modulated with m-QAM modulations, we demonstrate low-penalty data transmission of OFDM/OQAM 64-QAM, 128-QAM, 256-QAM, and 512-QAM signals in a silicon microring resonator. The observed optical signal-to-noise ratio (OSNR) penalties are 1.7 dB for 64-QAM, 1.7 dB for 128-QAM, and 3.1 dB for 256-QAM at a bit-error rate (BER) of 2 × 10?3 and 3.3 dB for 512-QAM at a BER of 2 × 10?2. The performance degradation due to the wavelength detuning from the microring resonance is evaluated, showing a wavelength range of ~0.48 nm with BER below 2 × 10?3. Moreover, we demonstrate data transmission of 191.2-Gbit/s simultaneous eight wavelength channel OFDM/OQAM 256-QAM signals in a silicon microring resonator, achieving OSNR penalties less than 2 dB at a BER of 2 × 10?2.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000168 (2016)
Improved optical enhancement using double-width plasmonic gratings with nanogaps
Ahmad A. Darweesh, Stephen J. Bauman, and Joseph B. Herzog

Plasmonic grating structures have been shown effective at increasing near-field optical enhancement. A doublewidth plasmonic grating design is introduced, where each period has two alternating metal widths separated by a nanogap. With this new design, analysis has shown that plasmonic resonances couple between each metal section, resulting in even greater optical enhancement compared with single-width gratings. The geometry that gives the greatest optical enhancement has been determined with a computational model. This work demonstrates that the increased enhancement is due to hybridized modes that couple between the two grating segments.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000173 (2016)
Multilayer black phosphorus as saturable absorber for an Er:Lu2O3 laser at ~3 μm
Mingqi Fan, Tao Li, Shengzhi Zhao, Guiqiu Li, Xiaochun Gao, Kejian Yang, Dechun Li, and Christian Kr?nke

Multilayer black phosphorus (BP) nanoplatelets of different thicknesses were prepared by the liquid phase exfoliation method and deposited onto yttrium aluminum garnet substrates to form saturable absorbers (SAs). These were characterized with respect to their thickness-dependent saturable absorption properties at 3 μm. The BP-SAs were employed in a passively Q-switched Er:Lu2O3 laser at 2.84 μm. By using BP exfoliated in different solvents, stable pulses as short as 359 ns were generated at an average output power of up to 755 mW. The repetition rate in the experiment was 107 kHz, corresponding to a pulse energy of 7.1 μJ. These results prove that BP-SAs have a great potential for optical modulation in the mid-infrared range.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000181 (2016)
Probing the fractional topological charge of a vortex light beam by using dynamic angular double slits
Jing Zhu, Pei Zhang, Dongzhi Fu, Dongxu Chen, Ruifeng Liu, Yingnan Zhou, Hong Gao, and Fuli Li

Vortex beams with fractional topological charge (FTC) have many special characteristics and novel applications. However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit (ADS), thefar-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000187 (2016)
Extinction ratio and resonant wavelength tuning using three dimensions of silica microresonators
Song Zhu, Yang Liu, Lei Shi, Xinbiao Xu, and Xinliang Zhang

In this paper, a multidimensional tuning method of the silica microcapillary resonator (MCR) is proposed and demonstrated whereby the extinction ratio (ER) as well as the resonant wavelength can be individually controlled. An ER tuning range of up to 17 dB and a maximum tuning sensitivity of 0.3 dB/μm are realized due to the taperedprofile of the silica optical microfiber (MF) when the MF is adjusted along its axial direction. Compared to direct tuning of the coupling gap, this method could lower the requirement for the resolution of displacement stage to micrometers. When the MF is adjusted along the axial direction of the silica microcapillary, a resonance shift of3.06 nm and maximum tuning sensitivity of 0.01 nm/μm are achieved. This method avoids the use of an applied external field to control the silica microresonators. Moreover, when air is replaced by ethanol and water in the core of the silica microcapillary, a maximum resonance shift of 5.22 nm is also achieved to further enlarge the resonancetuning range. Finally, a microbubble resonator with a higher Q factor is also fabricated to achieve an ER tuning range of 8.5 dB. Our method fully takes advantage of the unique structure of the MCR to separately and easily tune its key parameters, and may broaden its applications in optical signal processing and sensing.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000191 (2016)
Ultrasensitive magnetic field sensor based on an in-fiber Mach–Zehnder interferometer with a magnetic fluid component
Zhengyong Li, Changrui Liao, Jun Song, Ying Wang, Feng Zhu, Yiping Wang, and Xiaopeng Dong

An ultrasensitive magnetic field sensor based on a compact in-fiber Mach–Zehnder interferometer (MZI) created in twin-core fiber (TCF) is proposed, and its performance is experimentally demonstrated. A section of TCF was spliced between two sections of standard single-mode fibers, and then a microchannel was drilled throughone core of the TCF by means of femtosecond laser micromachining. The TCF with one microchannel was then immersed in a water-based Fe3O4 magnetic fluid (MF), forming a direct component of the light propagation path, and then sealed in a capillary tube, achieving a magnetic sensing element, which merges the advantages of an MZI with an MF. Experiments were conducted to investigate the magnetic response of the proposed sensor. The developed magnetic field sensor exhibits a linear response within a measurement range from 5 to 9.5 mT and an ultrahigh sensitivity of 20.8 nm/mT, which, to our best knowledge, is 2 orders of magnitude greater than other previously reported magnetic sensors. The proposed sensor is expected to offer significant potential for detecting weak magnetic fields.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000197 (2016)
16 × 16 silicon Mach–Zehnder interferometer switch actuated with waveguide microheaters
Shuoyi Zhao, Liangjun Lu, Linjie Zhou, Dong Li, Zhanzhi Guo, and Jianping Chen

We experimentally demonstrate a 16 × 16 reconfigurably nonblocking optical switch fabric using a Benes architecture. The switch fabric consists of 56 2 × 2 Mach–Zehnder interferometer based elementary switches, with each integrated with a pair of waveguide microheaters. The average on-chip insertion loss is ~5.2 dB for both of the “all-cross” and the “all-bar” states, with a loss variation of 1 dB over all routing paths. The cross talk for all switching states is better than ?30 dB. The switching time of the switch element is about 22 μs. The switching functionality is verified by transmission of a 40 Gb∕s quadrature phase-shift keying optical signal.

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000202 (2016)
Study of photoexcited-carrier dynamics in GaAs photoconductive switches using dynamic terahertz emission microscopy: publisher’s note
Hironaru Murakami, Shogo Fujiwara, Iwao Kawayama, and Masayoshi Tonouchi

This publisher’s note reports corrections to the funding acknowledgment in [Photon. Res. 4, A9–A15 (2016)].

Photonics Research
Jan. 01, 1900, Vol. 4 Issue 5 05000208 (2016)
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