Contents
2017
Volume: 5 Issue 3
20 Article(s)

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Reviews
Fiber Optics and Optical Communications
Polarization-maintaining few mode fiber composed of a central circular-hole and an elliptical-ring core
Jiajia Zhao, Ming Tang, Kyunghwan Oh, Zhenhua Feng, Can Zhao, Ruolin Liao, Songnian Fu, Perry Ping Shum, and Deming Liu
We propose a novel waveguide design of a polarization-maintaining few mode fiber (PM-FMF) supporting ≥10 non-degenerate modes, utilizing a central circular air hole and a circumjacent elliptical-ring core. The structure endows a new degree of freedom to adjust the birefringence of all the guided modes, including the fundamental polarization mode. Numerical simulations demonstrate that, by optimizing the air hole and elliptical-ring core, a PM-FMF supporting 10 distinctive polarization modes has been achieved, and the effective index difference Δneff between the adjacent guided modes could be kept larger than 1.32×10 4 over the whole C+L band. The proposed fiber structure can be flexibly tailored to support an even larger number of modes in PM-FMF (14-mode PM-FMF has been demonstrated as an example), which can be readily applicable to a scalable mode division multiplexing system.
Photonics Research
  • Publication Date: May. 30, 2017
  • Vol. 5, Issue 3, 261 (2017)
Physical Optics
Measurement of the degree of temporal coherence of unpolarized light beams
Lasse-Petteri Leppänen, Kimmo Saastamoinen, Ari T. Friberg, and Tero Setälä
We measure the electromagnetic degree of temporal coherence and the associated coherence time for quasi-monochromatic unpolarized light beams emitted by an LED, a filtered halogen lamp, and a multimode He–Ne laser. The method is based on observing at the output of a Michelson interferometer the visibilities (contrasts) of the intensity and polarization-state modulations expressed in terms of the Stokes parameters. The results are in good agreement with those deduced directly from the source spectra. The measurements are repeated after passing the beams through a linear polarizer so as to elucidate the role of polarization in electromagnetic coherence. While the polarizer varies the equal-time degree of coherence consistently with the theoretical predictions and alters the inner structure of the coherence matrix, the coherence time remains almost unchanged when the light varies from unpolarized to polarized. The results are important in the areas of applications dealing with physical optics and electromagnetic interference.
Photonics Research
  • Publication Date: Apr. 10, 2017
  • Vol. 5, Issue 3, 156 (2017)
Research Articles
Fiber Optic Sensors
Sensing of microparticles based on a broadband ultrasmall microcavity in a freely suspended microfiber
Yang Yu, Ting-Hui Xiao, Hong-Lian Guo, and Zhi-Yuan Li
We theoretically design and experimentally realize a broadband ultrasmall microcavity for sensing a varying number of microparticles whose diameter is 2 μm in a freely suspended microfiber. The performance of the microcavity is predicted by the theory of one-dimensional photonic crystals and verified by the numerical simulation of finite-difference time domain and the experimental characterization of reflection and transmission spectra. A penetrating length into the reflectors as small as about four periods is demonstrated in the numerical simulation, giving rise to an ultrasmall effective mode volume that can increase the sensitivity and spatial resolution of sensing. Moreover, a reflection band as large as 150 nm from the reflectors of the microcavity has been realized in silica optical microfiber in the experiment, which highly expands the wavelength range of sensing. Our proposed microcavity integrated into a freely suspended optical fiber offers a convenient and stable method for long-distance sensing of microparticles without the need for complicated coupling systems and is free from the influence of substrates.
Photonics Research
  • Publication Date: Apr. 10, 2017
  • Vol. 5, Issue 3, 143 (2017)
Lasers and Laser Optics
Wavelength-swept fiber laser based on bidirectional used linear chirped fiber Bragg grating
Lin Wang, Minggui Wan, Zhenkun Shen, Xudong Wang, Yuan Cao, Xinhuan Feng, and Bai-ou Guan
Photonics Research
  • Publication Date: Apr. 18, 2017
  • Vol. 5, Issue 3, 219 (2017)
Methods to improve the performance of the swept source at 1.0  μm based on a polygon scanner
Jing Cao, Pinghe Wang, Yan Zhang, Guohua Shi, Bo Wu, Shangjian Zhang, and Yong Liu
In this work, we investigate the methods to improve the performance of the swept source at 1.0 μm based on a polygon scanner, including in-cavity parameters and booster structures out of the cavity. The three in-cavity parameters are the cavity length, the rotating speed of the polygon scanner, and the in-cavity energy. With the decrease of cavity length, the spectrum bandwidth becomes wider and the duty cycle becomes higher. With the increase of the rotating speed of the polygon, the spectrum bandwidth becomes narrower, and the duty cycle becomes lower but the repetition rate becomes higher. With more energy in-cavity, the spectrum bandwidth becomes wider and the duty cycle becomes higher. The booster structures include the buffered structure, secondary amplifier, and dual-semiconductor optical amplifier configuration, which are used to increase the sweep frequency to 86 kHz, the output power to 18 mW, and the tuning bandwidth to 131 nm, respectively.
Photonics Research
  • Publication Date: May. 23, 2017
  • Vol. 5, Issue 3, 245 (2017)
Fiber laser for on-demand mode generation in 1550  nm band
Chenghui Tian, Song Yu, Shanyong Cai, Mingying Lan, and Wanyi Gu
Transverse mode characteristics of a laser are related to a variety of interesting applications. An on-demand mode solid laser in the 1064 nm band was proposed previously. In this paper, we provide a fiber laser for on-demand modes in the 1550 nm band to prescribe the pure and high-quality emission of a higher-order transverse laser mode, based on a simple construction with one spatial light modulator (SLM) and a single-mode erbium-doped fiber (SM-EDF). The SLM is designated to generate the desired higher-order mode and separate the higher-order mode and the fundamental mode. The fundamental mode oscillates in the fiber ring laser, and therefore the SM-EDF can be pumped with a single-mode 980 nm laser, no matter what higher-order mode is prescribed. In this proof-of-principle experiment, high-quality higher-order modes are observed from LP01 to LP105. Stable emission and real-time switching between modes can be easily realized by altering the phase on the SLM. In addition, the propagation of the LP01, LP11, LP21, and LP02 modes from the fiber laser is also demonstrated in a four-mode few-mode fiber.
Photonics Research
  • Publication Date: May. 30, 2017
  • Vol. 5, Issue 3, 256 (2017)
Medical Optics and Biotechnology
Coherent optical adaptive technique improves the spatial resolution of STED microscopy in thick samples
Wei Yan, Yanlong Yang, Yu Tan, Xun Chen, Yang Li, Junle Qu, and Tong Ye
Stimulated emission depletion (STED) microscopy is one of far-field optical microscopy techniques that can provide sub-diffraction spatial resolution. The spatial resolution of the STED microscopy is determined by the specially engineered beam profile of the depletion beam and its power. However, the beam profile of the depletion beam may be distorted due to aberrations of optical systems and inhomogeneity of a specimen’s optical properties, resulting in a compromised spatial resolution. The situation gets deteriorated when thick samples are imaged. In the worst case, the severe distortion of the depletion beam profile may cause complete loss of the super-resolution effect no matter how much depletion power is applied to specimens. Previously several adaptive optics approaches have been explored to compensate aberrations of systems and specimens. However, it is difficult to correct the complicated high-order optical aberrations of specimens. In this report, we demonstrate that the complicated distorted wavefront from a thick phantom sample can be measured by using the coherent optical adaptive technique. The full correction can effectively maintain and improve spatial resolution in imaging thick samples.
Photonics Research
  • Publication Date: Apr. 12, 2017
  • Vol. 5, Issue 3, 176 (2017)
Mode-locked Lasers
Nonlinear Optics
Complete crossing of Fano resonances in an optical microcavity via nonlinear tuning
Martino Bernard, Fernando Ramiro Manzano, Lorenzo Pavesi, Georg Pucker, Iacopo Carusotto, and Mher Ghulinyan
We report on the modeling, simulation, and experimental demonstration of complete mode crossings of Fano resonances within chip-integrated microresonators. The continuous reshaping of resonant line shapes is achieved via nonlinear thermo-optical tuning when the cavity-coupled optical pump is partially absorbed by the material. The locally generated heat then produces a thermal field, which influences the spatially overlapping optical modes, allowing us to alter the relative spectral separation of resonances. Furthermore, we exploit such tunability to continuously probe the coupling between different families of quasi-degenerate modes that exhibit asymmetric Fano interactions. As a particular case, we demonstrate a complete disappearance of one of the modal features in the transmission spectrum as predicted by Fano [Phys. Rev.124, 1866 (1961)PHRVAO0031-899X10.1103/PhysRev.124.1866]. The phenomenon is modeled as a third-order nonlinearity with a spatial distribution that depends on the stored optical field and thermal diffusion within the resonator. The performed nonlinear numerical simulations are in excellent agreement with the experimental results, which confirm the validity of the developed theory.
Photonics Research
  • Publication Date: Apr. 12, 2017
  • Vol. 5, Issue 3, 168 (2017)
Numerical simulation and temporal characterization of dual-pumped microring-resonator-based optical frequency combs
Xiaohong Hu, Weiqiang Wang, Leiran Wang, Wenfu Zhang, Yishan Wang, and Wei Zhao
Dual-pumped microring-resonator-based optical frequency combs (OFCs) and their temporal characteristics are numerically investigated and experimentally explored. The calculation results obtained by solving the driven and damped nonlinear Schr dinger equation indicate that an ultralow coupled pump power is required to excite the primary comb modes through a non-degenerate four-wave-mixing (FWM) process and, when the pump power is boosted, both the comb mode intensities and spectral bandwidths increase. At low pump powers, the field intensity profile exhibits a cosine variation manner with frequency equal to the separation of the two pumps, while a roll Turing pattern is formed resulting from the increased comb mode intensities and spectral bandwidths at high pump powers. Meanwhile, we found that the power difference between the two pump fields can be transferred to the newly generated comb modes, which are located on both sides of the pump modes, through a cascaded FWM process. Experimentally, the dual-pumped OFCs were realized by coupling two self-oscillating pump fields into a microring resonator. The numerically calculated comb spectrum is verified by generating an OFC with 2.0 THz mode spacing over 160 nm bandwidth. In addition, the formation of a roll Turing pattern at high pump powers is inferred from the measured autocorrelation trace of a 10 free spectral range (FSR) OFC. The experimental observations accord well with the numerical predictions. Due to their large and tunable mode spacing, robustness, and flexibility, the proposed dual-pumped OFCs could find potential applications in a wide range of fields, including arbitrary optical waveform generation, high-capacity optical communications, and signal-processing systems.
Photonics Research
  • Publication Date: Apr. 18, 2017
  • Vol. 5, Issue 3, 207 (2017)
Stimulated Brillouin scattering evolution and suppression in an integrated stimulated thermal Rayleigh scattering-based fiber laser
Jiwen Cui, Hong Dang, Kunpeng Feng, Wenlei Yang, Tao Geng, Yang Hu, Yuanhang Zhang, Dong Jiang, Xudong Chen, and Jiubin Tan
The spectral purity of fiber lasers has become a critical issue in both optical sensing and communication fields. As a result of ultra-narrow intrinsic linewidth, stimulated thermal Rayleigh scattering (STRS) has presented special potential to compress the linewidth of fiber lasers. To suppress stimulated Brillouin scattering (SBS), the most dominant disturbance for STRS in optical fibers, a semi-quantitative estimation has been established to illuminate the mechanism of suppressing SBS in a periodic tapered fiber, and it agrees with experimental results. Finally, a linewidth compression device based on STRS is integrated into a single-longitudinal-mode ring-cavity fiber laser with secondary cavities, and its linewidth is verified to be 200 Hz through a self-heterodyne detecting and Voigt fitting method.
Photonics Research
  • Publication Date: May. 16, 2017
  • Vol. 5, Issue 3, 233 (2017)
Optical Resonators
Effects of the slot width and angular position on the mode splitting in slotted optical microdisk resonator
Lingling Dai, Yiheng Yin, Yanhui Hu, Biyao Yang, and Ming Ding
A novel slotted optical microdisk resonator, which significantly enhances light–matter interaction and provides a promising approach for increasing the sensitivity of sensors, is theoretically and numerically investigated. In this slotted resonator, the mode splitting is generated due to reflection of the slot. Remarkably, effects of the slot width and angular position on the mode splitting are mainly studied. The results reveal that the mode splitting is a second function of the slot width, and the maximum mode splitting induced by the slot deformation is achieved with 2.7853×109 Hz/nm. Therefore, the slotted resonator is an excellent candidate for pressure and force sensing. Besides, the influence of the slot angular position on the mode splitting is a cosine curve with the highest sensitivity of 1.23×1011 Hz/deg ; thus, the optical characteristic demonstrates that the slotted resonator can be used for inertial measurements.
Photonics Research
  • Publication Date: Apr. 17, 2017
  • Vol. 5, Issue 3, 194 (2017)
Optical Vortices
Non-probe compensation of optical vortices carrying orbital angular momentum
Shiyao Fu, Tonglu Wang, Shikun Zhang, Zheyuan Zhang, Yanwang Zhai, and Chunqing Gao
Photonics Research
  • Publication Date: May. 24, 2017
  • Vol. 5, Issue 3, 251 (2017)
Photonic Crystals
Light-driven crystallization of polystyrene micro-spheres
Jing Liu, and Zhi-Yuan Li
We investigate the dynamic crystallization processes of colloidal photonic crystals, which are potentially invaluable for solving a number of existing and emerging technical problems in regards to controlled fabrication of crystals, such as size normalization, stability improvement, and acceleration of synthesis. In this paper, we report systematic high-resolution optical observation of the spontaneous crystallization of monodisperse polystyrene (PS) micro-spheres in aqueous solution into close-packed arrays in a static line optical tweezers. The experiments demonstrate that the crystal structure is mainly affected by the minimum potential energy of the system; however, the crystallization dynamics could be affected by various mechanical, physical, and geometric factors. The complicated dynamic transformation process from 1D crystallization to 2D crystallization and the creation and annihilation of dislocations and defects via crystal relaxation are clearly illustrated. Two major crystal growth modes, the epitaxy growth pattern and the inserted growth pattern, have been identified to play a key role in shaping the dynamics of the 1D and 2D crystallization process. These observations offer invaluable insights for in-depth research about colloidal crystal crystallization.
Photonics Research
  • Publication Date: Apr. 18, 2017
  • Vol. 5, Issue 3, 201 (2017)
Quantum Optics
Entanglement and nonlocality in a coupled-cavity system | On the Cover
Zheyong Zhang, Jianping Ding, and Hui-Tian Wang
Photonics Research
  • Publication Date: Apr. 24, 2017
  • Vol. 5, Issue 3, 224 (2017)
Research Articles
Direct bandgap photoluminescence from n-type indirect GaInP alloys
Cong Wang, Bing Wang, Riko I. Made, Soon-Fatt Yoon, and Jurgen Michel
This work studies Te doping effects on the direct bandgap photoluminescence (PL) of indirect GaxIn1?xP alloys (0.72≤x≤0.74). The temperature-dependent PL shows that the energy difference between direct Γ valley and indirect X valleys is reduced due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. Carrier thermalization has been observed for Te-doped GaxIn1?xP samples, as integrated PL intensity increases with increasing temperature from 175 to 300 K. The activation energy for carrier thermalization is reduced as doping concentration increases. Both BGN effect and carrier thermalization contribute to the carrier injection into the Γ valley. As a result, the direct band transition is enhanced in the Te-doped indirect GaxIn1?xP alloys. Therefore, the PL intensity of the Ga0.74In0.26P sample with active doping concentration of 9×1017 cm?3 is increased by five times compared with that of a nominally undoped sample. It is also found that the PL intensity is degraded significantly when the doping concentration is increased to 5×1018 cm?3. From cross-section transmission electron microscopy, no large dopant clusters or other extended defects were found contributing to this degradation.
Photonics Research
  • Publication Date: May. 22, 2017
  • Vol. 5, Issue 3, 239 (2017)
Silicon Photonics
Broadband on-chip integrator based on silicon photonic phase-shifted Bragg grating
Xu Wang, Feng Zhou, Siqi Yan, Yuan Yu, Jianji Dong, and Xinliang Zhang
Photonics Research
  • Publication Date: Apr. 17, 2017
  • Vol. 5, Issue 3, 182 (2017)
Ultra-compact on-chip slot Bragg grating structure for small electric field detection
Wentao Qiu, Huihui Lu, Fadi Issam Baida, and Maria-Pilar Bernal
In this paper, we present an ultra-compact 1D photonic crystal (PhC) Bragg grating design on a thin film lithium niobate slot waveguide (SWG) via 2D- and 3D-FDTD simulations. 2D-FDTD simulations are employed to tune the photonic bandgap (PBG) size, PBG center, cavity resonance wavelength, and the whole size of PhC. 3D-FDTD simulations are carried out to model the real structure by varying different geometrical parameters such as SWG height and PhC size. A moderate resonance quality factor Q of about 300 is achieved with a PhC size of only 0.5 μm×0.7 μm×6 μm. The proposed slot Bragg grating structure is then exploited as an electric field (E-field) sensor. The sensitivity is analyzed by 3D-FDTD simulations with a minimum detectable E-field as small as 23 mV/m. The possible fabrication process of the proposed structure is also discussed. The compact size of the proposed slot Bragg grating structure may have applications in on-chip E-field sensing, optical filtering, etc.
Photonics Research
  • Publication Date: Apr. 18, 2017
  • Vol. 5, Issue 3, 212 (2017)
Surface Plasmons
Graphene-supported manipulation of surface plasmon polaritons in metallic nanowaveguides
Hua Lu, Xuetao Gan, Dong Mao, and Jianlin Zhao
We investigate the electrically controlled light propagation in the metal–dielectric–metal plasmonic waveguide with a sandwiched graphene monolayer. The theoretical and simulation results show that the propagation loss exhibits an obvious peak when the permittivity of graphene approaches an epsilon-near-zero point when adjusting the gate voltage on graphene. The analog of electromagnetically induced transparency (EIT) can be generated by introducing side-coupled stubs into the waveguide. Based on the EIT-like effect, the hybrid plasmonic waveguide with a length of only 1.5 μm can work as a modulator with an extinction ratio of ~15.8 dB, which is 2.3 times larger than the case without the stubs. The active modulation of surface plasmon polariton propagation can be further improved by tuning the carrier mobility of graphene. The graphene-supported plasmonic waveguide system could find applications for the nanoscale manipulation of light and chip-integrated modulation.
Photonics Research
  • Publication Date: Apr. 12, 2017
  • Vol. 5, Issue 3, 162 (2017)
Spatial evolution of the near-field distribution on planar gold nanoparticles with the excitation wavelength across dipole and quadrupole modes
Jinghuan Yang, Quan Sun, Han Yu, Kosei Ueno, Hiroaki Misawa, and Qihuang Gong
We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally, by using photoemission electron microscopy (PEEM), and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plasmonic hot spots located at the plane in the interface between the Au particles and the substrate in the PEEM experiments.
Photonics Research
  • Publication Date: Apr. 17, 2017
  • Vol. 5, Issue 3, 187 (2017)