Optimum Coupling Efficiency Research Articles

A methodical approach to design adiabatic waveguide couplers for heterogeneous integrated photonics

Abstract We present an elegant and effective approach for the design of adiabatic waveguide couplers tailored for the heterogeneous integration of photonic building blocks. This method empowers users to incorporate the shortest taper(s) in their designs, while upholding optimal coupling efficiency. The technique assesses mode overlap between a minimum of two waveguides within the cross-section of any heterogeneous material stack, determining the necessary waveguide cross-sectional dimension to achieve optimal coupling efficiency. Two illustrative design applications are showcased and compared to a linear, concave, and convex taper for reference: a SiN-to-polymer structure exhibiting a 40% coupling improvement and a Si-to-GeSi structure having a 2.2 up to 5 times shorter length.

Improving fiber coupling efficiency by shaping the transmission trajectory of the vortex beam

Vortex beams with orbital angular momentum (OAM) have the potential to significantly increase the communication capacity in long-distance free-space optical communication systems. In order to obtain optimal coupling efficiency, few-mode fibers (FMFs) are employed to receive spatial light. This paper presents a computer model that describes the process of coupling vortex beams into FMFs after they have been transmitted via free space. We propose to improve the coupling efficiency by controlling the transmission trajectory of the vortex beam in free space. The coupling efficiency of a vortex beam into FMFs can be enhanced at a specific transmission distance by pre-designing the transmission trajectory of the optical field and tweaking the parameters of the coupling lens and fiber. This paper also examines the coupling correlation between higher-order OAM modes vortex beams and FMF modes. The numerical results of this study show that controlling the radial phase distribution to manage the transmission trajectory of the optical field is an effective and flexible method for improving the efficiency of coupling light between space and fiber. The results of this study can be applied in the domain of free-space optical communications.

Design and Experimental Analysis of an Optical Fiber Coupling System for a Ground-based Telescope with Adaptive Optics

Astronomy photonics has opened up a new era for the application of astronomical optical instruments. The fiber coupling system serves as the crucial link between the telescope and photonic devices. This paper explores a beam shaping method that utilizes a coupled lens to enhance the efficiency of coupling light into an optical single mode fiber. Compared to directly coupling the telescope beam into the fiber, this approach offers improved coupling efficiency and greater adjustment tolerance. The laboratory-based optical fiber coupling system described in this study comprises an imaging component with an F/50 ratio and a fiber coupling component. Theoretical analysis indicates that optimal coupling efficiency is achieved when the diameter of the focusing spot, limited by diffraction, matches the fiber core size. Any axial error, position error, or tip-tilt error between the lens and the fiber will reduce the coupling efficiency. Experimental results confirm that the coupling system achieves an efficiency of approximately 70%, which is close to the theoretical limit of 78%. These findings underscore the effectiveness of the fiber coupling method.

Research on the Efficiency of Bridge Crack Detection by Coupling Deep Learning Frameworks with Convolutional Neural Networks.

Bridge crack detection based on deep learning is a research area of great interest and difficulty in the field of bridge health detection. This study aimed to investigate the effectiveness of coupling a deep learning framework (DLF) with a convolutional neural network (CNN) for bridge crack detection. A dataset consisting of 2068 bridge crack images was randomly split into training, verification, and testing sets with a ratio of 8:1:1, respectively. Several CNN models, including Faster R-CNN, Single Shot MultiBox Detector (SSD), You Only Look Once (YOLO)-v5(x), U-Net, and Pyramid Scene Parsing Network (PSPNet), were used to conduct experiments using the PyTorch, TensorFlow2, and Keras frameworks. The experimental results show that the Harmonic Mean (F1) values of the detection results of the Faster R-CNN and SSD models under the Keras framework are relatively large (0.76 and 0.67, respectively, in the object detection model). The YOLO-v5(x) model of the TensorFlow2 framework achieved the highest F1 value of 0.67. In semantic segmentation models, the U-Net model achieved the highest detection result accuracy (AC) value of 98.37% under the PyTorch framework. The PSPNet model achieved the highest AC value of 97.86% under the TensorFlow2 framework. These experimental results provide optimal coupling efficiency parameters of a DLF and CNN for bridge crack detection. A more accurate and efficient DLF and CNN model for bridge crack detection has been obtained, which has significant practical application value.

Biosensing Technologies: A Focus Review on Recent Advancements in Surface Plasmon Coupled Emission.

In the past decade, novel nano-engineering protocols have been actively synergized with fluorescence spectroscopic techniques to yield higher intensity from radiating dipoles, through the process termed plasmon-enhanced fluorescence (PEF). Consequently, the limit of detection of analytes of interest has been dramatically improvised on account of higher sensitivity rendered by augmented fluorescence signals. Recently, metallic thin films sustaining surface plasmon polaritons (SPPs) have been creatively hybridized with such PEF platforms to realize a substantial upsurge in the global collection efficiency in a judicious technology termed surface plasmon-coupled emission (SPCE). While the process parameters and conditions to realize optimum coupling efficiency between the radiating dipoles and the plasmon polaritons in SPCE framework have been extensively discussed, the utility of disruptive nano-engineering over the SPCE platform and analogous interfaces such as 'ferroplasmon-on-mirror (FPoM)' as well as an alternative technology termed 'photonic crystal-coupled emission (PCCE)' have been seldom reviewed. In light of these observations, in this focus review, the myriad nano-engineering protocols developed over the SPCE, FPoM and PCCE platform are succinctly captured, presenting an emphasis on the recently developed cryosoret nano-assembly technology for photo-plasmonic hotspot generation (first to fourth). These technologies and associated sensing platforms are expected to ameliorate the current biosensing modalities with better understanding of the biophysicochemical processes and related outcomes at advanced micro-nano-interfaces. This review is hence envisaged to present a broad overview of the latest developments in SPCE substrate design and development for interdisciplinary applications that are of relevance in environmental as well as biological heath monitoring.

Analysis of Coupling Efficiency in Optic Gyroscope Transceiver Module With Elliptical Core Fiber

Improving the coupling efficiency of the optical transceiver module in fiber optic gyroscopes (FOGs) benefits both the working efficiency of the module and the signal-to-noise ratio of the FOG system. In this work, a single-mode fiber (SMF) with an elliptical mode field distribution is proposed as the coupling fiber for the FOG transceiver module. The coupling efficiency of the transceiver module using fibers with different mode field distributions at the endface is analyzed. Two structure schemes of the transceiver module are compared: one focuses the beam directly onto the endface of the fiber, and the other collimates the beam and then converge it. The optimal coupling efficiency can reach above 91.5% when the horizontal and vertical mode field radii of the fiber endface are approximately 4.05 μm and 3.37 μm respectively. The influence of the axial and angular alignment errors of the pigtail on the coupling efficiency is also investigated. The maximum tolerance of the axial alignment reaches approximately ±40.10 μm and ±30.08 μm for the two structure schemes. The maximum tolerance of the angular alignment reaches approximately ±4.4° and ±5.2° in the horizontal direction, and approximately ±3.5° and ±3.6° in the vertical direction, respectively. These tolerances are favorable for practical applications.

Quantum-Coherent Light-Electron Interaction in a Scanning Electron Microscope.

The last two decades experimentally affirmed the quantum nature of free electron wave packets by the rapid development of transmission electron microscopes into ultrafast, quantum-coherent systems. So far, all experiments were restricted to the bounds of transmission electron microscopes enabling one or two photon-electron interaction sites. We show the quantum coherent coupling between electrons and light in a scanning electron microscope, at unprecedentedly low, subrelativistic energies down to 10.4keV. These microscopes not only afford the yet-unexplored energies from ∼0.5 to 30keV providing the optimum electron-light coupling efficiency, but also offer spacious and easily configurable experimental chambers for extended, cascaded optical set ups, potentially boasting thousands of photon-electron interaction sites. Our results make possible experiments in electron wave packet shaping, quantum computing, and spectral imaging with low-energy electrons.

Coupling effect between semiconductor diodes and polarization-maintaining fibers

An innovative polarization coupler based on a long-range surface plasmon-polariton waveguide, which demonstrates efficient coupling between a semiconductor laser diode and a polarization-maintaining fiber, is proposed and analyzed. The proposed coupler—with different coupling facet sizes—can simultaneously match the mode sizes of the semiconductor laser diode and the polarization-maintaining fiber and can achieve polarization light coupling between different devices according to the features of the long-range surface plasmon-polariton waveguide. The coupling efficiency and alignment tolerance of the proposed coupler are investigated using the numerical-analysis method, and the design is optimized to obtain the best properties of the coupler. The calculated optimal coupling efficiency and the insertion loss are 89.96% and 0.97 dB, respectively. Further, the device exhibits a good alignment tolerance and has widespread potential applicability in the field of optical interconnections.

Entropy-Regularized Optimal Transport on Multivariate Normal and q-normal Distributions.

The distance and divergence of the probability measures play a central role in statistics, machine learning, and many other related fields. The Wasserstein distance has received much attention in recent years because of its distinctions from other distances or divergences. Although computing the Wasserstein distance is costly, entropy-regularized optimal transport was proposed to computationally efficiently approximate the Wasserstein distance. The purpose of this study is to understand the theoretical aspect of entropy-regularized optimal transport. In this paper, we focus on entropy-regularized optimal transport on multivariate normal distributions and q-normal distributions. We obtain the explicit form of the entropy-regularized optimal transport cost on multivariate normal and q-normal distributions; this provides a perspective to understand the effect of entropy regularization, which was previously known only experimentally. Furthermore, we obtain the entropy-regularized Kantorovich estimator for the probability measure that satisfies certain conditions. We also demonstrate how the Wasserstein distance, optimal coupling, geometric structure, and statistical efficiency are affected by entropy regularization in some experiments. In particular, our results about the explicit form of the optimal coupling of the Tsallis entropy-regularized optimal transport on multivariate q-normal distributions and the entropy-regularized Kantorovich estimator are novel and will become the first step towards the understanding of a more general setting.

Single-mode fiber auto-coupling system with wedges

Efficiently coupling Gaussian beams into single-mode fibers (SMF) plays an important role in scientific experiments. However, the optical misalignment will decrease the coupling efficiency dramatically. In this paper, we proposed and investigated a SMF auto-coupling system consisting of two wedges driven by stepper motors. We simulated and analysed the dual-wedge system’s ability of correcting optical misalignment and experimentally demonstrated the feasibility of this proposal. The results show that when the optical path is misaligned within a certain range, by automatically rotating the two wedges, the system can restore the efficiency to near the manually optimal coupling efficiency. The system inherits the stability of the stepper motors and will be beneficial for space applications and other unmanned scientific experiments.

4-by-4 Integrated Waveguide Coupler Based on Bi-Directional Propagation in Two Single-Mode Waveguides

We propose and investigate theoretically the use of bi-directional propagation of light in silica integrated coupled waveguide structures for linear optical processing of fiber-coupled photons. We show that the class of linear operations that can be implemented in such a system is given by symmetric and unitary matrices. We present how an arbitrary 4 × 4 coupler of this form can be realized by a linear sequence of more fundamental 2 × 2 couplers and single-waveguide phase shifters and discuss in detail the implementation in a silica integrated platform utilizing direct UV-written waveguides and long- and short-period grating couplers with tilted gratings for optimized coupling efficiency.

Optimized design for grating-coupled waveguide-enhanced Raman spectroscopy.

We report a new design optimization process for planar photonic waveguides applied to waveguide-enhanced Raman spectroscopy (WERS) that combines the optimization of both the surface intensity performance and the grating coupling efficiency. We consider the impact of film thickness on the grating coupling efficiency of two materials with different refractive indices, namely tantalum pentoxide (Ta2O5) and silicon (Si). We propose a new figure-of-merit (FOM) that takes into account both the coupling efficiency and surface intensity dependence for Raman excitation on the film thickness. Our study shows that the optimum surface-sensitive waveguide thickness is thinner than the optimum coupling efficiency thickness for both material systems. As an example, for a tantalum pentoxide waveguide operating at 785 nm, our optimization strategy proposes a 20% increase in waveguide core thickness relative to the optimum surface-sensitive thickness to achieve the best performance in WERS applications.

Sub-Hertz frequency stabilization of 1.[formula omitted] laser on Higher order HG[formula omitted] mode

We present an ultra-stable laser that is stabilized to a higher order Hermite-Gaussian mode in a 100 mm long optical reference cavity. The fundamental characteristics of the Hermite-Gaussian mode related to the frequency stabilization are discussed. With an optimum coupling efficiency of 11.2% and moderate finesse of 417000, the laser frequency is locked to the HG01 mode instead of the HG00 mode due to nonnegligible losses at the center of cavity mirrors. A comparison of two similar ultra-stable lasers shows that the median of the linewidth distribution is approximately 0.3 Hz, and the fractional frequency instability is less than 1 ×10−15 at 1 s–10 s averaging time. This value approaches the noise limit 8.3 ×10−16 of the HG01 mode caused by the thermal noise and residual amplitude modulation.

Parametric optimization of fiber to waveguide coupler using Bragg gratings

The uniform grating coupler which consists of two pairs of reflector layers in the substrate region has been proposed to couple the optical power from single mode fiber to standard 220 nm silicon-on-insulator waveguide. The efficiency of ≈ 79% in a broadband 3 dB bandwidth of ≈ 150 nm is achieved for uniform Bragg grating coupler devices. Further, by making a sequence of chirped and uniform grating, the power coupling efficiency of ≈ 98% at 1550 nm wavelength and 3 dB bandwidth of 35 nm is achieved. The importance and benefits of reflector layers along with its parametric optimization is thoroughly investigated for the calculation of optimum coupling efficiency. This research mainly focus on fiber-to-waveguide/waveguide-to-fiber coupling design. The major contribution of the research focuses on compact and efficient optical interconnection of coupler devices.

Silicon grating coupler on a lithium niobate thin film waveguide

A silicon (Si) grating coupler on a Si-strip-loaded lithium niobate (LN) thin film waveguide was proposed and realized. The optimized coupling efficiency (CE) by simulation was −5 dB at a wavelength of 1550 nm by the two-dimensional finite-different time-domain method (2D-FDTD), while the measured CE was about −18 dB. The coupler was etched by the focused ion beam etching method (FIB) in the Si strip thin film which was deposited by magnetron sputtering technology, patterned by photolithography and a lift-off process. The realization of the Si grating coupler and tapered waveguide could benefit the research of more advanced and complicated integrated optical devices.

空间光耦合自动对准方法与实现

The sticking point of spatial light-single-mode fiber is to find the best position in the transverse plane of fiber precisely. Based on optical fiber migration's influence on the spatial light-single-mode fiber coupling efficiency, an auto-coupling system was proposed, which mainly consisted of two-dimensional piezoceramics, driver, controller, photoelectric detector which was a closed loop control system and coupling lens. The automatic alignment coupling of the spatial light-fiber was realized by combining with simulated annealing algorithm. The coupling efficiency for free-space laser coupling into the fibers and simulated annealing algorithm were analyzed theoretically. Analysis results of experiment show that the method can make the system search the best position automatically based on the power of the light which is coupled into the single-mode fiber. The optimal coupling efficiency is about 51.4%, which can be obtained in short times by the system and it is improved by 6.5% compared with no automatic alignment before. The scheme is practical and it is of great significance for the study of the automatic coupling of spatial light-single-mode fiber.

High order vector mode coupling mechanism based on mode matching method

The high order vector mode (HOVM) coupling mechanism is investigated based on the mode matching method (MMM). In the case of strong HOVM coupling where the weakly guiding approximation fails, conventional coupling analysis methods become invalid due to the asynchronous coupling feature of the horizontal and vertical polarization components of HOVM. The MMM, which uses the interference of the local eigenmodes instead of the assumptive modes to simulate the light propagation, is adopted as a more efficient analysis method for investigating HOVM coupling processes, especially for strong coupling situations. The rules of the optimal coupling length, coupling efficiency, and mode purity in microfiber directional coupler are firstly quantitatively analyzed and summarized. Different from the specific input modes, some special new modes would be excited at the output through the strong HOVM coupling process. The analysis of HOVM coupling mechanism based on MMM could provide precise and accurate design guidance for HOVM directional coupler and mode converter, which are believed to be fundamental devices for multi-mode communication applications.

Thin-wall tubes for coupling terahertz waves to metal wires.

Bare metal wires are among the most promising waveguides for guiding terahertz (THz) surface plasmon polaritons. In this study, a thin-wall tube is proposed for coupling THz waves to a metal wire with ultrahigh efficiency, which results from three high mode matchings for the two waveguides: field distributions, polarization directions, and wave vectors. According to the mode-overlap calculation, the coupling efficiency can be always between 84% and 94% when the frequency of THz waves is in the range of 0.2-3 THz and the metal wire radius is 0.5 mm. The maximum efficiency is as high as 94% at 0.5 THz, which is much higher than that obtained by the previous methods. We further conclude that the optimal coupling efficiency can be obtained when the outer tube radius is equal to the wire radius and simultaneously the real propagation constants of modes in the two waveguides are the same.

Design of a barcode-like waveguide nanostructure for efficient chip–fiber coupling

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

Focus on lasers and imaging

Focus on lasers and imaging