Vladimir V. Nikulin

Modeling of an acousto-optic laser beam steering system intended for satellite communication

High data rate of laser communication systems should be matched by wide operational bandwidth of beam positioning. This could be achieved by the application of nonmechanical beam steering technologies utilizing acousto-optic phenomena. Major components of acousto-optic beam steering systems, a Bragg cell, and a quadrant detector are subjected to comprehensive laboratory testing and characterization as components of a control system. A mathematical and computer simulation model of the entire steering system addressing system dynamics, cross coupling of azimuth and elevation channels, and nonlinearity, and facilitating synthesis of advanced control laws is developed.

Adaptive Optimization of a Free Space Laser Communication System Under Dynamic Link Attenuation

Free space laser communication is a potentially attractive technology that can offer intrinsically high data rates and resistance to jamming, and facilitates low probability of interception and low probability of detection (LPI/LPD). However, practical links established in the atmosphere are adversely affected by signal attenuation and dynamic turbulence, which can create spatial and temporal variations in the refractive index. The resulting distortions lead to reduced signal power and increased bit error rate (BER), even over short ranges. To overcome possible signal degradation under adverse conditions, laser communication systems must increase power and reduce the communication bit rate. Under dynamic link attenuation both of these parameters can be tuned to optimize performance. In this paper, we present and compare three methods for optimizing optical link efficiency. The work is based on experiments conducted with a commercially available system, and its scaled-down laboratory prototype. The proposed methods demonstrate different degrees of optimization capabilities under practical operating conditions, but, in general, they maintain the highest possible bit rate at the minimum power consumption, while obtaining an acceptable BER.

Omni-Wrist III - a new generation of pointing devices. Part I. Laser beam steering devices - mathematical modeling

Omni-Wrist III is a new sensor mount developed with U.S. Air Force funding that emulates the kinematics of a human wrist. Driven by two linear motors and computer controlled, it is capable of a full 180/spl deg/ hemisphere of pitch/yaw motion. A comprehensive laboratory testing of one of the few existing devices of this type, installed in the Laser Communications Research Laboratory at Binghamton University, has resulted in the establishment of a complete mathematical model relating pitch/yaw coordinates of the sensor mount to the motor encoder signals. This paper presents the development of the model that incorporates inverse and forward pose kinematics solutions as well as the dynamics of the novel gimbal-like pointing system.

New generation of gimbals systems for laser positioning applications

Omniwrist III is a new sensor mount developed under Air Force funding that emulates the kinematics of a human wrist. Driven by two linear motors and computer controlled, it is capable of a full 180° hemisphere of pitch/yaw motion. A comprehensive laboratory testing of one of few existing devices of this type, installed in the Laser Research Laboratory at Binghamton University, has resulted in the establishment of a complete transfer matrix-type model relating pitch/yaw coordinates of the sensor mount to the voltage signals applied to the motors. Although dynamic characteristics of the device are position-dependent, it has the potential for exceeding bandwidth and positioning accuracy of a traditional gimbals system at least by the factor of ten. The device is suitable for the application of the most advanced control strategies that will result in the further enhancement of its dynamic performance thus extending the scope of its application to various problems of satellite communications, LADAR, laser weapon systems, etc. This study is aimed at the investigation of the best performance characteristics (bandwidth, tracking error, cross-coupling effects, etc.) attainable under advanced control laws. The authors intend to consider implementation of such control laws as optimal control utilizing dynamic programming, gain scheduling, and fuzzy logic control. The results of this research will be incorporated in the future papers. It is shown that Omniwrist III with the appropriate controls could be considered as a new generation of gimbals system.

Performance of a laser communication system with acousto-optic tracking : An experimental study

Laser communication systems hold great promise for broadband applications. This technology uses much higher-than-RF region of the spectrum and allows concentration of the signal within a very small spatial angle, thus offering unsurpassed throughput, information security, reduced weight and size of the components and power savings. Unfortunately, these intrinsic advantages do not come without a price: small beam divergence requires precise positioning, which becomes very critical at high bit rates. Complex motion patterns of the communicating platforms, resident vibrations, and atmospheric effects are known to cause significant signal losses through the mechanisms of the pointing errors, beam wander and other higher-order effects. Mitigation of those effects is achieved through the multiple means of fast tracking and wavefront control. In this paper we focus on the application of a beam steering technology and its effect on the communication performance of the system. We present the results of an experimental study of a laser communication link subjected to pointing distortions. These distortions are generated by a special disturbance element in the optical setup, which recreates specific operation environments with particular spectral characteristics. The acousto-optic technology is used to build an agile tracking system to assure the maximum signal reception in spite of the harsh operational conditions. The received communication signal is recorded and statistically analyzed to calculate the bit-error-rates. This paper presents the synthesis of a tracking system and the experimental results characterizing the communication performance under uncompensated pointing disturbance and with tracking.

Effect of the sampling rate of the tracking system on free-space laser communications

Lasers play an ever-increasing role in aerospace communication systems by providing the most logical connectivity channels. They drive the advancements in modern optoelectronics; however, successful implementation of this technology hinges on having an equally advanced beam-steering system for tracking the communication counterpart in the presence of complex maneuvers and the resident vibration of the airframe. The work presented in this paper concentrates on the development of agile acousto-optic beam-steering systems for laser communication terminals, which use constant-gain controllers augmented with an adaptive Kalman filter. Experimental results are presented to demonstrate communication performance as a function of the sampling rate in the tracking loop.

Performance of laser communication uplinks and downlinks in the presence of pointing errors and atmospheric distortions

Performance of laser communication links between ground terminals, both fixed and mobile, and satellites is generally limited by several factors. Continuous movement of the communicating platforms, complemented by mechanical vibrations, is the main cause of pointing errors. In addition, atmospheric turbulence causes changes of the refractive index along the propagation path, thus creating wavefront distortions of the optical beam resulting in spatio-temporal redistribution of the received energy. The total effect of these phenomena leads to an increased bit-error probability under adverse operation conditions. This paper presents a combined approach to the analysis of a laser link in the presence of pointing errors and turbulence effects, and their contribution to the increased bit-error rates (BER). Analysis of both uplink and downlink communication is performed in the simulation environment. Two distinct approaches to wavefront distortion modeling are used for these scenarios. In uplink propagation the beam is distorted in the initial transition through the atmosphere, and then it travels over a long distance in free space, where even more self-interference occurs. In downlink communication the effects of distortion are only observed during the final transition through the atmosphere, and; therefore, are less severe. Communication performance under different conditions is assessed in terms of the bit-error rate as a function of the pointing error variance and the scintillation index.

Omni-Wrist III - a new generation of pointing devices. Part II. Gimbals systems - control

The Omni-Wrist III robotic manipulator, inspired in design by the kinematics of the human wrist, represents, with the full 180/spl deg/ hemisphere of singularity-free range, a very capable alternative to traditional gimbals systems. Research efforts at the Laser Communications Research Laboratory at Binghamton University have resulted in the establishment of an accurate mathematical model, which was utilized in the development of control systems presented in this paper. A state variable controller and two linear model following controllers are developed and evaluated. The third controller is designed to achieve full decoupling of the highly coupled system. The adaptation and disturbance rejection capabilities of the controllers are demonstrated through friction compensation.

Decentralized control of an OmniWrist laser beam tracking system

Laser communication systems developed for mobile platforms, such as satellites, aircraft, and terrain vehicles require fast wide-range beam steering devices to establish and maintain a communication link. Conventionally, the low-bandwidth, high steering range part of the beam-positioning task is performed by gimbals that inherently constitutes the system bottleneck in terms of reliability, accuracy and dynamic performance. OmniWrist, a novel robotic sensor mount capable of carrying a payload of 5 lbs and providing a full 180° hemisphere of azimuth/declination motion is known to be free of the most of deficiencies of gimbals. Provided with appropriate controls, it has the potential for becoming a new generation of gimbals systems. The approach demonstrated in this paper describes an adaptive controller enabling OmniWrist to be utilized as a part of a laser beam positioning system. It is based on a Lyapunov function that assures global asymptotic stability of the entire system while achieving high tracking accuracy. The proposed scheme is highly robust, does not require the knowledge of complex system dynamics, and facilitates independent control of each channel by full decoupling of the OmniWrist dynamics. The paper summarizes the basic algorithm and demonstrates implementation results.

Hybrid laser beam steerer for laser communications applications

Omniwrist is a new sensor mount developed under the Air Force funding that emulates the kinematics of a human wrist. Driven by two linear motors and equipped with a dedicated computer implementing advanced control laws, it is capable of a full 180° hemisphere of pitch/yaw motion and demonstrates performance characteristics comparable with an electro-mechanical beam steering system. While exceeding the bandwidth requirements for the coarse beam steering task, Omniwrist’s dynamic response is much slower than the one of the acousto-optic device (Bragg cell) that is virtually inertia-free. At the same time, the steering range of a Bragg cell, ± .5°, is too small for many applications. The authors have been successful in the enhancement of the design and development of control laws improving its dynamic characteristics of a Bragg cell. This paper presents the research aimed at the development of a hybrid laser beam steering system comprising Bragg cells installed on the Omniwrist platform. An optimal control strategy facilitating such applications as scanning, search, rapid repositioning, tracking, feedback and feedforward compensation of environmental vibration of the optical platform (satellite-based and airborne) has been developed, implemented and tested. This includes the solution of such underlying problems as mathematical description of the hybrid system, optimal task distribution between the “coarse” and the “fine” positioning tasks, coordination of the operation of the “coarse” and “fine” system controllers. The efficiency of the developed system in various applications will be investigated further and compared against known designs.