Misalignment Loss Research Articles

Management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers

Abstract This paper has illustrated the management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers. The beam waist variations are clarified versus the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. Besides, the coupling loss is demonstrated against the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. The optimum beam waist and optimum coupling loss are deeply studied against the fiber coupler core radius variations for the silica/fluoride glass fiber coupler with the optimum incident beam angle of 60° and wavelength of 1,550 nm.

On the performance of all-optical RORIS dual hop UAV based FSO systems

In this work, we investigate the performance of an all-optical dual hop free-space optical (FSO) communication system using an unmanned aerial vehicle (UAV) as a relay and utilizing a reconfigurable optical intelligent reflecting surface (ROIRS) to improve the coverage. We statistically characterize the combined effects of the three impairments: atmospheric turbulence (AT), angle of arrival (AoA) fluctuation of UAV, and the geometric and misalignment loss (GML) caused by the fluctuations of the transmitter, ROIRS, and the position and orientation of the UAV. Incorporating the aforementioned effects and the non-orthogonality of the received beam at the UAV and the receiver, closed-form expressions of the overall outage probability and the average bit error rate for the considered system are derived. Furthermore, the analytically derived expressions are verified by Monte-Carlo simulations.

Interference and noise analysis for hybrid FSO/RF‐based 6G mobile backhaul

Optical wireless communication, or free space optics, is a promising solution for backhauls in sixth-generation mobile systems. However, the susceptibility of optical links to weather conditions has led to FSO links being furnished with radio frequency (RF) backups. These Hybrid FSO/RF systems provide enhanced link availability but lead to RF resource wastage. Cognitive radio technology, in contrast, is well known for its optimal use of RF resources and may be combined with an FSO link to create a Cognitive Hybrid FSO/RF system. This work uses such a system to analyze a configuration for a mobile backhaul in sixth-generation mobile systems. This configuration can seamlessly coexist with established large scale RF cellular networks. The performance of this configuration is analyzed with respect to outage probability and average bit error by considering the impact of optical channel turbulence, misalignment loss, RF interference, and noise. Mathematical closed-form expressions are verified by simulations.

Multi-Cell Multi-Beam Prediction Using Auto-Encoder LSTM for mmWave Systems

Millimeter wave (mmWave) systems rely on communication in narrow beams for directional and spatial multiplexing gains. A key challenge in realizing these systems is beam tracking, particularly in environments with high mobility and blockage. Additionally, in wide-area mmWave cellular systems, user equipment (UE) devices must often simultaneously track signals from multiple cells, since links to individual cells can be unreliable. Models of the channel dynamics across multiple cells and multiple beams are difficult to derive from first principles. In this work, we propose a fully data-driven approach based on a novel auto-encoder integrated long short term memory (LSTM) network, which predicts multiple beams from multiple cells, one time step in the future. The key innovation is to use an auto-encoder pre-processing step, which reduces the dimensionality of the input– the main challenge in multi-cell, multi-beam tracking. The prediction capability of the proposed network is verified and compared to common baseline predictors as well as popular machine learning (ML) based neural network predictors in realistic system-level simulations using a commercial ray-tracer. We observe that predictions from the proposed network, which utilizes auto-encoders for dimensionality reduction, offers significantly better best beam accuracy and lower beam misalignment loss than common baseline approaches. We also discuss outage prediction and proactive beam switching as applications of the multi-cell multi-beam prediction.

Optical Intelligent Reflecting Surface for Mixed Dual-Hop FSO and Beamforming-Based RF System in C-RAN

Optical intelligent reflecting surface (IRS) is an emerging and low-cost technology that can establish a stable communication route in free space optical (FSO) transmission environment with obstacles. In this work, optical IRS-aided dual-hop mixed FSO and RF system is first proposed for cloud radio access network (C-RAN). Specifically, polar codes are introduced to combat turbulence and building sway induced fading in FSO link, and transmit beamforming (TBF) technique is designed to achieve optimal data-rate for RF link. Supposing that the IRS-aided FSO link is subject to exponentiated Weibull distribution with geometric and misalignment loss, whereas the RF link experiences the Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {F}$ </tex-math></inline-formula> composite fading, the analytical closed-form outage probability expression is derived in terms of Meijer G and Lauricella multivariate hypergeometric functions with decode-and-forward strategy. Exact closed-form average bit error rate expressions are obtained when the relay of C-RAN adopts single antenna and multiantenna beamforming techniques. On the basis of moment generating function of end-to-end signal-to-noise ratio, the ergodic capacity is obtained in terms of univariate and multivariate Fox H-functions over independent but not identically distributed Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {F}$ </tex-math></inline-formula> fading channels. The correctness of the analytical results is verified through Monte-Carlo simulations. We further provide an asymptotic analysis and discuss the achievable diversity orders.

Channel Modeling and Analysis of Inter-Small-Satellite Links in Terahertz Band Space Networks

Small satellites have shorter development cycles and lower costs than the traditional satellites in low-earth orbits (LEOs) and geostationary orbits (GEOs). These advantages and the ultra-wide spectrum availability in the Terahertz (THz) band enable profound potentials for inter-satellite links (ISLs) to achieve ultra-high throughput in both near-Earth and deep-space orbits. In this paper, the ISL channels at THz band are characterized and their link capacity is analyzed. First, the propagation channel effects in both Earth’s upper atmosphere and deep space are investigated to demonstrate the feasibility of THz band for ISLs. Second, an orbital perturbation model for satellite formation flying is proposed and analyzed with actual ephemeris information. Finally, a channel model is developed to investigate the diversity in both polarization and frequency domain for THz band inter-small-satellite communications. Our numerical results demonstrate that: 1) the ionospheric plasma does not affect the THz band signal propagation in near-Earth orbits; 2) the beam misalignment loss can be characterized by orbital perturbation factors; and 3) the ISLs in THz band can utilize the diversity techniques to achieve high throughput in both near-Earth and deep-space links.

Outage probability analysis in multi-user FSO/RF and UAV-enabled MIMO communication networks

This paper considers a mixed free-space optical/radio frequency (FSO/RF) system, which facilitates the communication between a ground central unit (CU) and multiple ground users (GUs) via a hovering unmanned aerial vehicle (UAV) acting as a decode-and-forward (DF) aerial relay. It is considered that the CU employs multiple transmit apertures and is interconnected with the aerial relay via an FSO link with transmit aperture selection (TAS), whereas the relay communicates with the GUs over multiple-input multiple-output (MIMO) RF links with orthogonal space–time block coding (OSTBC) transmission. It is assumed that the optical channels are mainly affected by atmospheric attenuation as well as geometric and misalignment loss (GML). Besides, the atmospheric turbulence is assumed weak for short-range links. Also, fully correlated shadowed conditions are considered in the RF links and the MIMO channels are modeled using the Nakagami/Inverse Gamma (IG) composite fading distribution. Opportunistic GU scheduling is applied in the downlink and novel closed-form mathematical formulas for the outage probability (OP) are derived. The results demonstrate the theoretical derivations and the performance benefits of the proposed approach.

Evolving Architectures With Gradient Misalignment Toward Low Adversarial Transferability

Deep neural network image classifiers are known to be susceptible, not only to adversarial examples created for them, but also to those created for others. This phenomenon poses a potential security risk in various black-box systems that rely on image classifiers. One of the observations on networks that have transferability of adversarial examples between them is the similarity of their architectures. Networks with high architectural similarity tend to share high transferability as well. Thus, in this study, we address this problem from a novel perspective by investigating the contribution of network architecture to transferability. Specifically, we propose an architecture searching framework that employs neuroevolution to evolve network architectures and gradient misalignment loss to encourage networks to converge into dissimilar functions after training. Our findings indicate that the proposed framework successfully discovers architectures that reduce transferability from four standard networks, including ResNet and VGG, while maintaining good accuracy on unperturbed images. In addition, the evolved networks trained with gradient misalignment exhibit significantly lower transferability than a standard network trained with gradient misalignment, which indicates that network architecture plays an important role in reducing transferability. We demonstrate that designing or exploring proper network architectures is a promising approach to tackle the transferability issue and train adversarially robust image classifiers.

Fabrication and transmission of optical polymer waveguide backplane for high - performance computers.

In this paper, a high-speed, large-capacity and compact optical backplane architecture for high-performance computers (HPC) is proposed and designed. The MT couplers is designed without additional mirror to divert the light from vertical cavity surface emitting laser (VCSEL) array by 90°. The light is then coupled into the optical waveguide through the MT. A bidirectional 8 channels polymer optical waveguide array with low insertion loss is designed and fabricated. The waveguides are embedded in the printed circuit board. We test the performance of 8 channels for the optical waveguide backplane. In the entire optical backplane, the averaged insertion loss of optical backplane with 50 μm input fiber is 1.62 dB with 850 nm VCSEL. The misalignment loss is 0.5 dB when the misalignment between waveguide and multi-mode fiber is 8.5 μm. Then, the transmission performances of 4 channels with different speed signals is demonstrated by off-line experiment. The optical backplane network can achieve 15 G data error-free transmission in the range of [-2 dBm, -10 dBm] received optical power (ROP). Based on VCSEL arrays, the optical interconnection network system can achieve 8 channels parallel signal transmission. In the optical backplane, the 10 Gbit / s data generated / processed by field programmable gate array (Xilinx Kintex-7) chips can realize error-free transmission. In large-capacity, high-speed parallel HPC, the designed optical backplane system can facilitate the establishment of a large number of parallel transmissions.

Flip-chip optical couplers with scalable I/O count for silicon photonics

A scalable and tolerant optical interfacing method based on flip-chip bonding is developed for silicon photonics packaging. Bidirectional optical couplers between multiple silicon-on-insulator waveguides and single-mode polymer waveguides are designed and fabricated. Successful operation is verified experimentally in the 1530-1570 nm spectral window. At the wavelength of 1570 nm, the coupling loss is as low as 0.8 dB for both polarization states and the planar misalignment loss is less than 0.6 dB for TE and 0.3 dB for TM polarization in a lateral silicon-polymer waveguide offset range of ± 2 µm. The coupling loss does not exhibit any temperature dependence up to the highest measurement point of 70°C.

Monte Carlo simulation for coupling between single-mode fiber and slab waveguide with spherical fiber microlenses

For the butt-joint coupling between a single mode fiber (SMF) and a single mode dielectric slab waveguide (SMDSW) with a spherical fiber microlens (SFML), because the SMF and the rectangular-section SMDSW have different symmetry, it is difficult to calculate the coupling efficiencies using the traditional analytic method. In this paper, a Monte Carlo model of the coupler is proposed. This model incorporates the geometrical surface of both the SFML and the SMDSW, the relative position of the SMF and the SMDSW, and both the aberration and diffraction effects into consideration. The propagation characteristics of the coupler are simulated. And the coupling efficiencies and misalignment loss are investigated.

Effective Cutoff Wavelength Measurement of Bend-Insensitive Fiber by Longitudinal Misalignment Loss Method

We introduced a simple experimental set-up and determined the effective cutoff wavelength of bend-insensitive optical fibers by applying the longitudinal misalignment loss with spectral transmission method.

Using a cat's eye cavity to improve displacement self-sensing laser

A displacement self-sensing laser with a resolution of λ/8 was invented by us a few years ago. This simple laser device mainly consists of a half-intracavity He–Ne laser with a birefringence element in the cavity, outputting a beam with two orthogonally polarized components. One of its cavity mirrors can move along the optical axis of the laser to change the cavity length. When this mirror is pushed (or pulled) by an item to be measured, the polarization of the laser beam varies once every λ/8 variation of the cavity length. Though the displacement self-sensing laser has many outstanding functions, its measuring range was badly limited by inevitable misalignment resulted from sway of the cavity mirror in motion. The limited range prevents the sensor from going into actual application. This paper gives a solution to the problem, using a cat's eye retroreflector to replace the flat cavity mirror of the displacement self-sensing laser to construct an anti-misalignment cavity—cat's eye cavity (CEC). The misalignment loss in the CEC is theoretically analyzed and compared with that in a flat-concave cavity, and then the experiment to test the optical stability of CEC is carried out. Furthermore, the performance of the displacement self-sensing laser based on the CEC is also investigated. Results indicate that the misalignment tolerance of the CEC is much higher than that of the flat-concave cavity, i.e. the cavity stability is greatly improved. The measuring range of the displacement self-sensing laser can be up to 12 mm with a resolution of 79.097 nm and linearity better than 0.005%.

Gain-Flattened Optical Limiting Amplifier Modules for Wavelength Division Multiplexing Transmission

By integration of a bidirectional erbium-doped fiber amplifier and a three-port optical circulator with several fiber Bragg gratings (FBGs), two gain-flattened optical limiting amplifier (OLA) modules are proposed and experimentally demonstrated. They rely on the use of central wavelength misalignment and bending loss methods, respectively. They can effectively cover the whole useful 1.55-mum band. Both modules can provide gain-flattening characteristics over a large input dynamic range. The FBGs integrated OLA configuration has potential application in wavelength division multiplexing lightwave communication systems.

A MATHEMATICAL MODEL DESCRIBING THE EFFECTS OF POINTING AND TRACKING ERRORS DUE TO ‘MECHANICAL NOISE’ IN COHERENT OPTICAL INTERSATELLITE LINKS

In optical intersatellite links random pointing and tracking errors lead to random antenna misalignments against the line of sight, and therefore to a corresponding random fading of the received light power. The objective of this paper is to develop a mathematical model as simple as possible, in order to describe the influence of the fading process on the information transmission. To achieve this goal, the properties of optical antennas are derived, the sensitivity of the received light power on antenna misalignments is calculated and the probability density function of the fading of the received power (called misalignment loss here) is computed. Finally, the quality of the information transmission is calculated and discussed explicitly for the PSK-homodyne system.

The Rayleigh scattering loss of isotropic weakly guiding single-mode waveguides

The relation between the Rayleigh scattering loss and the Laplacian mode-field-half-width (MFHW) is derived. It is shown that not only the Rayleigh scattering loss but also the transverse offset loss, the angular misalignment loss, and the microbending loss will be small if the field distribution is similar to an elliptic-Gaussian distribution. >

Angular tilt misalignment loss at a GRIN rod lens coupler

A geometric optics model for evaluating end separation and transverse offset losses at a graded-index (GRIN) rod lens to lens coupler is extended to the case of angular tilt misalignment loss. Two different fiber–lens combinations are examined, and the model is verified by comparison to experimental measurements, which show close agreement to the theoretical predictions. In addition, invariance of the angular tilt loss to differing optical launch and receive conditions is shown, and a possible explanation for this phenomenon is provided. Finally, the theoretical model is used to predict fiber–lens combinations which minimize possible angular tilt losses.

Microbending and modal noise

The effect of microbending loss on SNRs received from multimode graded-index fibers excited by single-mode laser sources is considered. Analytical expressions are derived which describe the combined effects of microbending loss and detector misalignment loss on the integrated intensity statistics in the fiber. The theoretical predictions for the SNR are experimentally checked and found to be in good agreement with the experimental data.

Waveguide lasers with intracavity electrooptic modulators: misalignment loss

Coupling from the EH(11) fundamental mode of a square, hollow bore waveguide laser to a square electrooptic modulator is treated. The misalignment loss that results from coupling of the laser fundamental mode into higher order modulator modes is calculated. Results are presented for loss from transverse displacement, angular tilt, and axial separation of the two elements. From these results, the alignment tolerances required to minimize loss for an intracavity modulator are found.