Showing papers in "IEEE Photonics Journal in 2019"

Surface Plasmon Resonance Based Titanium Coated Biosensor for Cancer Cell Detection

Abstract: A new optimized bowl-shaped mono-core surface plasmon resonance based cancer sensor is proposed for the rapid detection of different types of cancer affected cell. By considering the refractive index of each individual cancer contaminated cell with respect to their normal cell, some major optical parameters variation are observed. Moreover, the cancerous cell concentration is considered at 80% in liquid form and the detection method is finite element method with 2 100 390 mesh elements. The variation of spectrum shift is obtained by plasmonic band gap between the silica and cancer cell part which is separated by a thin (35 nm) titanium film coating. The proposed sensor depicts a high birefringence of 0.04 with a maximum coupling length of 66 $\mu$ m. However, the proposed structure provides an optimum wavelength sensitivity level between about 10 000 nm/RIU and 17 500 with a resolution of the sensor between 1.5 × 10−2 and 9.33 × 10−3 RIU. Also, the transmittance variance of the cancerous cell ranges from almost 3300 to 6100 dB/RIU and the amplitude sensitivity ranges nearly between −340 and −420 RIU $^{-1}$ for different cancer cells in major polarization mode with the maximum detection limit of 0.025. Besides, the overall sensitivity performance is measured with respect to their normal cells which can be better than any other prior structures that have already proposed.

Physical Layer Security of Hybrid Satellite-FSO Cooperative Systems

Abstract: In this paper, we study the physical layer secrecy performance of a hybrid satellite and free-space optical (FSO) cooperative system. The satellite links are assumed to follow the shadowed-Rician fading distribution, and the channel of the terrestrial link between the relay and destination is assumed to experience the gamma-gamma fading. For the FSO communications, the effects of different types of detection techniques (i.e., heterodyne detection and intensity modulation with direct detection) as well as the pointing error are considered. We derive exact analytical expressions for the average secrecy capacity and secrecy outage probability (SOP) for both cases of amplify-and-forward (AF) and decode-and-forward (DF) relaying. The asymptotic analysis for the SOP is also conducted to provide more insights on the impact of FSO and satellite channels on secrecy performance. It is found that with the AF with fixed gain scheme, the secrecy diversity order of the investigated system is only dependent on the channel characteristics of the FSO link and the FSO detection type, whereas the secrecy diversity is zero when the relay node employs DF or AF with variable-gain schemes.

Ultra-Broadband Mode Converter Using Cascading Chirped Long-Period Fiber Grating

Abstract: We propose and demonstrate an ultra-broadband mode converter based on a cascade chirped long-period fiber grating (CLPFG) written in a two-mode fiber. The mode converter can convert fundamental mode ( ${\rm{HE}}_{11}^{even}$ and ${\rm{HE}}_{11}^{odd}$ modes) into the first order cylindrical vector (CV) modes (TE01, TM01, ${\rm{HE}}_{21}^{even}$ and ${\rm{HE}}_{21}^{{odd}}$ modes). We design and analyze the mode conversion characteristics of this kind of grating in theory. The simulation results show a 10 dB bandwidth of 170 nm and 20 dB bandwidth of 145 nm can be achieved by optimizing the parameters of the CLPFG. In the terms of experiment, we achieve the broadband mode converter with 10 dB bandwidth of 170 nm from 1472 nm to 1642 nm.

L -Band GHz Femtosecond Passively Harmonic Mode-Locked Er-Doped Fiber Laser Based on Nonlinear Polarization Rotation

Abstract: Via using an L -band optimized in-fiber polarizing grating device, a GHz L -band femtosecond passively harmonic mode-locked Er-doped fiber laser based on nonlinear polarization rotation is first demonstrated. In total, 4.22 GHz pulses with the duration of 810 fs and super-mode suppression ratio (SMSR) of 32 dB are obtained under the pump power of 712 mW corresponding to 215th harmonic order. The central wavelength of 4.22 GHz pulses is 1581.7 nm with 10.1 nm 3 dB bandwidth. Furthermore, under this fixed pump power, higher harmonic orders can also be attained by rotating the polarization controllers properly. The highest repetition rate we obtained is 7.41 GHz with the SMSR of 20.7 dB.

Fast Optical Phased Array Calibration Technique for Random Phase Modulation LiDAR

Abstract: Optical phased array (OPA) imaging technique, which uses electro-optic modulation to achieve beam steering rather than mechanical scanning, is a raster scanning imaging method with great potential due to its noninertia and high speed. However, fabrication imperfection of an OPA causes pre-designed phase modulations not yielding desired steering angles, and a time-consuming calibration is usually required before practical use. Alternatively, it is possible to obtain images with a random phase modulation OPA. In this paper, we propose a fast calibration for the random phase modulation OPA LiDAR. Experimental results demonstrate that, to obtain images of the same quality, the proposed calibration is three times faster than the calibration used in raster scanning scheme. In the meantime, the proposed calibration simultaneously retrieves the point spread function of the imaging system during the process, which can be used to remove the image blurring caused by the sidelobes and further improve the image quality.

Numerical Study of a Wide-Angle and Polarization-Insensitive Ultrabroadband Metamaterial Absorber in Visible and Near-Infrared Region

Abstract: An ultrabroadband metamaterial absorber structure based on a periodic array of metallic-dielectric multilayered conical frustums is numerically investigated and proposed. The metamaterial absorber indicated an absorptivity of higher than 90%, which covered the visible and near-infrared region at 480–1480 nm, and a relative absorption bandwidth of 102%. The high absorptivity can be maintained with large incident angles up to 60° under both transverse electric and transverse magnetic polarizations. Furthermore, the proposed absorber exhibits polarization insensitivity owing to its rotational symmetry structure. Compared with the previously reported ultrabroadband metamaterial absorbers, the design in this work indicates high practical feasibility in terms of a compact structure for a large bandwidth, a wide incident angle, and polarization insensitivity, thereby suggesting its promising application, for example, in solar cells and thermal emitters.

Highly Sensitive Dual-Core PCF Based Plasmonic Refractive Index Sensor for Low Refractive Index Detection

Abstract: A highly sensitive surface plasmon resonance (SPR) sensor on a dual-core photonic crystal fiber (PCF) for low refractive index (RI) detection is presented in this paper. Plasmonic material silver (Ag) is deposited outside of the fiber structure to detect changes of the surrounding medium's refractive index. To prevent oxidation a thin layer of titanium dioxide (TiO 2 ) is employed on top of the silver. The sensor shows maximum wavelength sensitivity and amplitude sensitivity of 116,000 nm/RIU and 2452 RIU -1 with corresponding resolutions (R) of 8.62 × 10 -7 and 5.55 × 10 -6 RIU, respectively. A thorough study of the relevant literature yielded that these attained sensitivities in both interrogation methods are the highest among reported PCF-SPR sensors to date. In addition, the sensor possesses a very high figure of merit of 2320 in the sensing range of 1.29 to 1.39. Therefore, it would be a suitable candidate for pharmaceutical inspection, organic chemical sensing, and biosensing and other analytes detection.

Titanium Dioxide Hole-Blocking Layer in Ultra-Thin-Film Crystalline Silicon Solar Cells

Abstract: One of the remaining obstacles to achieving the theoretical efficiency limit of crystalline silicon (c-Si) solar cells is high interface recombination loss for minority carriers at the Ohmic contacts. The contact recombination loss of the ultra-thin-film c-Si solar cells is more severe than that of the state-of-art thick cells due to the smaller volume and higher minority carrier concentration. This paper presents a design of an electron passing (Ohmic) contact for n-type Si that is hole-blocking with significantly reduced hole recombination. By depositing a thin titanium dioxide (TiO2) layer, we form a metal-insulator-semiconductor (MIS) contact for a 2 μm-thick Si cell to achieve an open circuit voltage ( $V_{oc}$ ) of 645 mV, which is 10 mV higher than that of an ultra-thin cell with a traditional metal contact. This TiO2 MIS contact constitutes a step towards high-efficiency ultra-thin-film c-Si solar cells.

Design of Ellipsoid and Spherical Combined Light Source for Uniform Flux and Color Mixing

Abstract: In this letter, an ellipsoid–spherical combined light source structure is presented for the purpose of uniform flux and color mixing, when considering heat dissipation. To allow more rays couple into spherical chamber while keeping illumination uniformity, algebraic equations to estimate light spot size focused by ellipsoidal reflector are derived and then employed to decide input slot size of the chamber. Then Monte Carlo ray tracing simulation suggests that the most preferable output slot size is between 15% and 30% of the chamber radius when considering both uniformity and available test area. A prototype with a 150-W Xenon arc lamp is fabricated to demonstrate heat distribution for the system, and experiment about uniformity indicates that 89.4% of the test circle radius can reach 2% nonuniformity. Color mixing performance for the system is studied by equipped with three ellipsoidal reflectors to collect rays from different color LEDs, and experiment shows that the maximum root-mean-square error of the spectrum across 90% of output circle radius is 1.4%. Results demonstrate it is a structure with simplified design process and having less dependence with light source type.

Tunable Plasmon Induced Transparency in Graphene and Hyperbolic Metamaterial-Based Structure

Abstract: A specially designed tunable hyperbolic metamaterial (HMM) based on plasmon induced transparency (PIT) of fractal in the near-infrared (NIR) regime was proposed. The HMM-layer constitutes the top metasurface, which is comprised of fractal-like nanospheres of silver (Ag) metal. A bilayer of graphene is sandwiched between the top HMM and bottom silicon (Si) substrate. The permittivity of graphene bilayer was deduced corresponding to different chemical potentials (of graphene). PIT of the proposed structure was obtained in the 3000–4000 nm wavelength band by employing the finite difference time domain simulation under the excitation of a fundamental transverse magnetic (TM) mode. The effects of incidence angle and graphene chemical potential on the transmission characteristics were investigated. Furthermore, the PIT windows could be tuned by altering the radii of Ag nanospheres in the HMM layer and chemical potential of bilayer graphene. Such systems would be useful in varieties of applications, e.g., switching, energy harvesting, sensing in environmental, and/or medical diagnostics, particularly in detecting the existing impurities in human blood and urine.

Efficient Recognition of the Propagated Orbital Angular Momentum Modes in Turbulences With the Convolutional Neural Network

Abstract: The vortex beam carrying orbital angular momentum (OAM) has attracted great attentions in optical communication field, which can extend the channel capacity of communication system due to the orthogonality between different OAM modes. Generally, atmospheric turbulence can distort the helical phase fronts of OAM beams, which presents a critical challenge to the effective recognition of OAM modes. Recently, convolutional neural network (CNN), as a model of deep learning, has been widely applied to machine vision. In this paper, based on the CNN theory, we make a tradeoff between the computational complexity of the system and the efficiency of recognition by establishing a specially designed six-layer CNN structure in CPU station to efficiently achieve the recognition of OAM mode in turbulent environment through the feature extraction of the received Laguerre-Gaussian beam's intensity distributions. Furthermore, we examine the performances of our designed CNN with respect to various turbulence levels, transmission distances, mode spacings, and we have also compared the performances of recognizing single OAM mode with multiplexed OAM modes. The numerical simulation shows that basing on CNN method, the coaxial multiplexed OAM modes can obtain higher recognizing accuracy about 96.25% even under long transmission distance with strong turbulence. It is anticipated that the results might be helpful for future implementation of high-capacity OAM-based optical communication technology.

Beam Size Optimization and Adaptation for High-Altitude Airborne Free-Space Optical Communication Systems

Abstract: High-altitude airborne platforms interconnected by free-space optical communications (FSOCs) have recently emerged as a promising solution for establishing wireless networks for rural and remote areas. The performance of FSOC system is severely degraded by the angle-of-arrival (AoA) fluctuation and pointing error. The precise alignment between the optical transmitter and receiver can be achieved by using the pointing, acquisition, and tracking (PAT), but it should work within the tight constraints of airborne platforms on size, weight, and power. It is also highly desirable that the PAT operates rapidly (e.g., without iteration for optimization) since the airborne platforms can be on the fast move. We propose a rapid and computation power-efficient adaptive beam control technique, where the beam sizes are adjusted without iterations at both the transmitter and receiver using nonmechanical variable-focus lenses, to mitigate the deleterious effects of AoA fluctuation and pointing error simultaneously. For this purpose, we provide the closed-form expressions about the optimum beam sizes at the transmitter and receiver for the outage probability. We carry out Monte Carlo simulations to validate the accuracy of our theoretical derivations. We show that the airborne FSOC system using the adaptive beam control technique outperforms the system having fixed beam sizes over wide ranges of AoA fluctuation and pointing error.

Numerical Study of an Efficient Broadband Metamaterial Absorber in Visible Light Region

Abstract: We report a numerical study of a broadband metamaterial absorber in visible light region by utilizing a single layer of metal–dielectric–metal configuration. The absorption bandwidth and absorption performances are tailored by varying the resonator shapes and metal materials. The absorption bandwidth of the proposed metamaterial absorber (MA) structure is enhanced significantly with decreasing the order of rotational symmetry of the resonator shape. Using gold configuration, the twofold symmetry MA structure based on the double-sized axe shaped resonator exhibits the broadband absorption response over the entire visible light and apart of infrared spectrum range from 320 to 982 nm with absorptivity above 90% for both transverse electric and transverse magnetic polarizations. The physical mechanism of broadband absorption is explained by the current, electric, and magnetic distributions, significantly affected by the propagating surface and localized surface plasmon resonances. Furthermore, the high absorber performances of the twofold symmetry MA structure can be obtained over entire visible light region (400–700 nm) for both noble metal of gold and low-cost metal of nickel configurations, indicating the proposed absorber is a promising candidate for low-cost and large-scale fabricate device operated in visible light region.

Multiscale Vascular Enhancement Filter Applied to In Vivo Morphologic and Functional Photoacoustic Imaging of Rat Ocular Vasculature

Abstract: Optical-resolution photoacoustic microscopy (OR-PAM) is used for in vivo imaging of a variety of albino and pigmented eyes taking advantages of requiring no exogenous dye, performing high-resolution imaging, and achieving morphologic and functional imaging at the same time. However, to accurately diagnose the ophthalmic disease in the OR-PAM images, vascular enhancement algorithms are necessary for extracting vessels and quantifying them correctly. Vascular enhancement algorithms developed for other imaging technologies, are not suitable to be used for OR-PAM, because of the underlying differences in the physics of the formation of images. In this study, a new vascular enhancement algorithm called photoacoustic imaging vasculature enhancement filter (PAIVEF) is proposed, which not only enhances vasculature including micro-vessels signals, suppresses noise signals effectively, but also achieves highly sensitive and accurate enhancement of the vasculature within a large depth range in and out of the system's depth of focus (DOF). Using the PAIVEF, the morphologic and functional 3D images of the whole rat's ocular anterior vasculature segment was displayed simultaneously for a depth range of ~0.6 mm, which was ~7 times of the system's DOF. This study paves the way for the application of OR-PAM technology in ophthalmic disease research.

Cascaded Graphene Frequency Selective Surface Integrated Tunable Broadband Terahertz Metamaterial Absorber

Abstract: The quest of novel materials and structures to design an efficient absorber for realizing wave trapping and absorption at terahertz (THz) frequencies is an open topic. But the design of a thin, wideband, and tunable THz absorber is still an arduous job. Hence, in this paper, a hybrid THz metamaterial absorber integrated with a cascaded graphene frequency selective surface (FSS), with ultra-high absorbance over a wide frequency range is designed using an analytical equivalent circuit model. Such an approach provides a feasible way to optimize the device by interrelating the effective electromagnetic and circuit parameters with the unit cell dimensions of FSS. A systematic study and critical analysis over a wide range of device parameters including graphene chemical potential and FSS design variables is demonstrated. A peak dip in reflection coefficient of -30.27 dB is observed at 2.94 THz for an optimal device with a chemical potential (μ c ) of 0.38 eV (μ c1 ), and 0.25 eV (μ c2 ) in the range of 0.1-4.0 THz. The cascaded FSS configuration results in the unique anti-reflection-based absorption phenomena, which is responsible for the achievement of -10 dB absorption bandwidth of 2.34 THz (0.85-3.19 THz). In addition, the frequency-dependent effective permittivity, permeability, and impedance is extracted using reflection data, in order to understand the device physics. Such ultra-thin and broadband absorbing device architecture may confer potential application perspectives in THz sensing, imaging, and detection.

Terahertz Absorber With Reconfigurable Bandwidth Based on Isotropic Vanadium Dioxide Metasurfaces

Abstract: The design of a broadband tunable absorber is proposed based on a thin vanadium dioxide metasurface, which is composed of a simple array of vanadium dioxide and a bottom gold film. When the conductivity of vanadium dioxide is equal to 2000 Ω -1 cm -1 , simulated absorptance exceeds 90% with 71% bandwidth from 0.47 to 0.99 THz and full width at half maximum is 98% from 0.354 to 1.036 THz with center frequency of 0.695 THz. Simulated results show that absorptance peak can be tuned from 5% to 100% when the conductivity changes continually from 10 Ω -1 cm -1 to 2000 Ω -1 cm -1 . The designed absorber may have useful applications in terahertz spectrum such as energy harvesting, thermal emitter, and sensing.

Few-Mode SDM Receivers Exploiting Parallelism of Free Space

Abstract: Few-mode receivers exploiting parallelism of free space for space-division multiplexed (SDM) transmission, in which one free-space 90° optical hybrid can be shared among multiple SDM channels, are proposed and experimentally demonstrated. A $\text{2} \times \text{20 Gb}/\text{s}$ quadrature phase-shift keying two-mode mode-division multiplexed receiver has been demonstrated experimentally.

A Single Layer Neural Network Implemented by a $4\times 4$ MZI-Based Optical Processor

Abstract: Implementing any linear transformation matrix through the optical channels of an on-chip reconfigurable multiport interferometer has been emerging as a promising technique for various fields of study, such as information processing and optical communication systems. Recently, the use of multiport optical interferometric-based linear structures in neural networks has attracted a great deal of attention. Optical neural networks have proven to be promising in terms of computational speed and power efficiency, allowing for the increasingly large neural networks that are being created today. This paper demonstrates the experimental analysis of programming a $4\times 4$ reconfigurable optical processor using a unitary transformation matrix implemented by a single layer neural network. To this end, the Mach-Zehnder interferometers (MZIs) in the structure are first experimentally calibrated to circumvent the random phase errors originating from fabrication process variations. The linear transformation matrix of the given application can be implemented by the successive multiplications of the unitary transformation matrices of the constituent MZIs in the optical structure. The required phase shifts to construct the linear transformation matrix by means of the optical processor are determined theoretically. Using this method, a single layer neural network is trained to classify a synthetic linearly separable multivariate Gaussian dataset on a conventional computer using a stochastic optimization algorithm. Additionally, the effect of the phase errors and uncertainties caused by the experimental equipment inaccuracies and the device components imperfections is also analyzed and simulated. Finally, the optical processor is experimentally programmed by applying the obtained phase shifts from the matrix decomposition process to the corresponding phase shifters in the device. The experimental results show that the optical processor achieves 72 $\%$ classification accuracy compared to the 98.9 $\%$ of the simulated optical neural network on a digital computer.

A Symmetrical Dual-Beam Bowtie Antenna With Gain Enhancement Using Metamaterial for 5G MIMO Applications

Abstract: A symmetrical dual-beam end-fire bowtie antenna with gain enhancement is achieved by integrating three pairs of metamaterial (MTM) arrays for 5G MIMO applications. The first pair of MTM array with high refractive index (HRI) are deployed to form a wide beam antenna. The second pair of HRI MTM array are arranged along the end-fire direction (x-direction) in front of the radiators in order to split the single wide beam into dual beam. Besides, the third pair of anisotropic MTM array with HRI along x-direction and near zero refractive along y-direction are incorporated in front of the second pair of MTM array to improve the gain performance. The proposed technique is verified by both the full-wave electromagnetic simulation and experiment, and the simulated and measured results agree very well with each other. Moreover, the measured results reveal that the main beam directions of the proposed antenna point to ±30° with respect to the end-fire direction (0°) over 24.25-27.5 GHz, with a maximum gain of 7.4 dBi at 26 GHz and a 4.2 dB gain improvement compared to the wide beam antenna.

On OCT Image Classification via Deep Learning

Abstract: Computer-aided diagnosis of retinopathy is a research hotspot in the field of medical image classification. Diabetic macular edema (DME) and age-related macular degeneration (AMD) are two common ocular diseases that can result in partial or complete loss of vision. Optical coherence tomography imaging (OCT) is widely applied to the diagnosis of ocular diseases including DME and AMD. In this paper, an automatic method based on deep learning is proposed to detect AME and AMD lesions, in which two publicly available OCT datasets of retina were adopted and a network model with effective feature of reuse feature was applied to solve the problem of small datasets and enhance the adaptation to the difference of different datasets of the approach. Several network models with effective feature of reusable feature were compared and the transfer learning on networks with pre-trained models was realized. CliqueNet achieves better, classification results compared with other network models with a more than 0.98 accuracy and 0.99 of area under the curve (AUC) value finally.

Precise Performance Analysis of Dual-Hop Mixed RF/Unified-FSO DF Relaying With Heterodyne Detection and Two IM-DD Channel Models

Abstract: This paper provides precise performance analysis of the dual-hop mixed radio frequency (RF)/unified free space optical (FSO) decode-and-forward (DF) relaying system, in which the heterodyne detection and the intensity modulation-direct detection (IM-DD) are taken into account for FSO detection. To this end, we derive closed-form expressions for the outage probability, average bit error rate (BER), and the ergodic channel capacity of this system. In this analysis, we utilize, for the first time as per our knowledge, a precise channel capacity result for the IM-DD channel. Moreover, this is the first time that not only the (IM-DD input-independent) but also the (IM-DD cost-dependent) additive white Gaussian noise (AWGN) channel is considered in such system analyses. Additionally in this study, we assume that the first hop (RF link) follows Nakagami- m fading, and the second hop (FSO link) follows Malaga ( M ) turbulence with pointing errors. These fading and turbulence models contain other models (such as Rayleigh and Gamma-Gamma) as special cases, thus, our analyses can be seen as a generalized one from the RF and FSO fading models point of view. Also, in BER derivation, we assume that the modulation schemes in the two hops can be different, since not all modulation schemes are suitable for IM-DD FSO links. In addition, the system performance is investigated asymptotically at high signal to noise ratios. This investigation leads to new nonreported coding gain and diversity order analyses of such system. Interestingly, we find that in the FSO hop, at high transmitted powers, all the considered FSO detectors result in the same diversity order. Furthermore, we provide simulation results that verify the accuracy of the obtained analytical and asymptotic expressions.

Integrated Silicon Photonics Remote Radio Frontend (RRF) for Single-Sideband (SSB) Millimeter-Wave Radio-Over-Fiber (ROF) Systems

Abstract: Radio-over-fiber (ROF)-based mobile fronthaul network supporting the baseband unit and simple remote radio head is a promising network architecture for future wireless networks. In this paper, we propose and demonstrate a silicon photonics (SiPh)-based remote radio frontend (RRF) for the mm-wave ROF systems. The proposed SiPh-based RRF consisted of an integrated Mach-Zehnder modulator and a micro-ring modulator, producing a single-sideband 40 GHz millimeter-wave orthogonal-frequency-division-multiplexing ROF signal, which is robust against fiber chromatic dispersion in the mobile fronthaul transmission. The RRF is fabricated using SiPh platform and could be potentially low cost. High split-ratios can be potentially achieved. Data rate of 7.813 Gbit/s per wavelength channel is achieved in the experiment. Numerical analysis is also performed; there are good matches of the simulation and experimental results.

Implementation of a Highly-Sensitive and Wide-Range Frequency Measurement Using a Si 3 N 4 MDR-Based Optoelectronic Oscillator

Abstract: Microwave photonic technologies have been introduced for achieving broadband radio-frequency signal measurement. However, few of the proposed schemes mention the low-power radio-frequency signal detection, which stringently limits their practical applications in certain areas. In this paper, we designed and demonstrated a wideband low-power radio-frequency signal measurement system with optoelectronic oscillator. Here, the unknown radio-frequency signal matched the potential oscillation mode is allowable to be detected, amplified and estimated. The key component in the tunable optoelectronic oscillator is a silicon nitride micro-disk resonator with a very high Q-factor, which is utilized to achieve frequency selection as a microwave filter. A frequency measurement system range from 1 ∼ 20 GHz with radio frequency power as low as −105 dBm, measurement errors of ±375 MHz and the maximum gain of 61.7 dB were realized experimentally

2-D Grating Couplers for Vertical Fiber Coupling in Two Polarizations

Abstract: We demonstrate highly efficient couplers in a two-dimensional grating configuration for a vertical fiber-coupling fabricated in the silicon-on-insulator platform. A combination of sub-wavelength reflectors and blazed gratings enables both high coupling efficiency and vertical grating coupling. We experimentally achieve coupling efficiencies of −2.6 dB at 1544 nm and −3.4 dB at 1536 nm for x- and y-polarized LP01 modes, respectively. The experimental results are in good agreement with the simulated −2.4 dB coupling efficiency for both polarizations at a wavelength of 1550 nm. Simulated and measured crosstalk between two polarizations were −19 dB and <−16 dB, respectively.

High-Efficiency Ultra-Broadband Multi-Tip Edge Couplers for Integration of Distributed Feedback Laser With Silicon-on-Insulator Waveguide

Abstract: A high-efficiency ultra-broadband multi-tip edge coupler based on a silicon-on-insulator platform for direct coupling with the elliptic beam of a distributed feedback laser was developed. The device is composed of a multi-tip section and a combiner section with extra offset regions to reduce the mode mismatch caused by the structural discontinuity which results from a limitation of the fabrication process that creates an inevitable gap width at the junction between the two sections. The widths and the spacing of the tips for the multi-tip section and the extra offset region for the combiner section are fine-tuned by using the particle swarm optimization method to reduce the mode mismatch. A high overall coupling efficiency up to 90.68% (0.4249 dB) at 1550 nm can be achieved for the optimized 90-μm-long four-tip edge coupler. The device can be operated over a broad spectral range of 1260–1675 nm with less than 1 dB extra loss. With its high-efficiency ultra-broadband operation and small device footprint, the proposed device is promising for laser-to-chip edge coupling in silicon photonics.

High Strain Sensitivity Temperature Sensor Based on a Secondary Modulated Tapered Long Period Fiber Grating

Abstract: A novel sensor structure has been proposed and experimentally investigated for simultaneous strain and temperature measurement. The structure is fabricated by weak power modulation of CO 2 laser exposure on tapered long period fiber grating (LPFG). Compared with the transmission spectrum of the tapered LPFG, two peaks appear in the transmission spectrum of the novel structure. These resonance peaks exhibit different sensitivity responses; thus, simultaneous measurement of strain and temperature is realized by monitoring the wavelength shift of the two peaks. Experiment results indicate that strain sensitivities of the two peaks are 1.82 pm/μe and 8.17 pm/μe, and temperature sensitivities are 47.9 pm/°C and 65 pm/°C, respectively.

Design of a Broadband Polarization Splitter Based on Anisotropy-Engineered Tilted Subwavelength Gratings

Abstract: Polarization management is of paramount importance in integrated optics, particularly for highly birefringent photonic platforms such as silicon-on-insulator. In this paper, we present a polarization beam splitter based on a multimode interference coupler incorporating tilted subwavelength gratings. The tilt provides accurate control of the structural anisotropy and enables independent selection of the beat length for two orthogonal polarization states. As a result, device length is reduced to less than 100 μm while simultaneously achieving broadband operation through subwavelength grating dispersion engineering. Insertion losses below 1 dB and an extinction ratio higher than 20 dB are demonstrated through three-dimensional FDTD simulation in a 131-nm bandwidth.

1250 Mbit/s OOK Wireless White-Light VLC Transmission Based on Phosphor Laser Diode

Abstract: In this work, for the first time, we demonstrate a 1250 Mbit/s on-off keying white-light visible light communication (VLC) transmission in a free space transmission length of 1 m by utilizing a yellow-phosphor laser diode (LD). Here, the blue optical filter, which is used to enhance the data rate by removing the yellow color component, is not required to. Besides, the wireless transmission lengths, illuminations, and modulation traffic rates of the proposed LD-based VLC system are analyzed and discussed.

Orbital Angular Momentum (OAM) Mode Mixing in a Bent Step Index Fiber in Perturbation Theory

Abstract: Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum (OAM) modes due to a fiber bend in a step-index multimode fiber. Using a scalar wave equation, we develop a complete set of analytic expressions for modemixing, including those for the 2π walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber.

III-Nitride Deep UV LED Without Electron Blocking Layer

Abstract: AlGaN-based deep UV (DUV) LEDs generally employ a p-type electron blocking layer (EBL) to suppress electron overflow. However, Al-rich III-nitride EBL can result in challenging p-doping and large valence band barrier for hole injection as well as epitaxial complexity. As a result, wall plug efficiency (WPE) can be compromised. Our systematic studies of band diagram and carrier concentration reveal that carrier concentrations in the quantum well and electron overflow can be significantly impacted because of the slope variation of the quantum barrier (QB) conduction and valence bands, which in turn influence radiative recombination and optical output power. Remarkably, grading the Al composition from 0.60 to 0.70 for the 12-nm-thick AlGaN QB of the DUV LED without the EBL can lead to 13.5% higher output power and similar level of overflown electron concentration (~1 × 10 15 /cm 3 ) as opposed to the conventional DUV LED with the p-type EBL. This paradigm is significant for the pursuit of higher WPE or shorter emission wavelength for DUV LEDs and lasers, as it provides a new direction for addressing electron overflow and hole injection issues.