Rahul M. Khandekar

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.

Laser communication system with acousto-optic tracking and modulation: experimental study

Laser communication systems are highly preferred for broadband applications. This technology uses higher regions of the spectrum, and offers unsurpassed throughput, information security, reduced weight and size of the components, and power savings. Unfortunately, small beam divergence requires precise positioning, which becomes very critical at high data rates. Complex motion patterns of the communicating platforms, vibrations, and atmospheric effects cause significant signal losses due to the pointing errors, beam wander, and other higher order effects. Mitigation of those effects is achieved by fast tracking, which can be successfully combined with signal modulation. In this work, we focus on the application of acousto-optic technology and its effect on communication performance. We present experimental results for a laser communication link affected by pointing distortions. These distortions are generated to emulate specific operation environments with particular spectral characteristics. The acousto-optic technology is used to build an agile tracking system combined with signal modulation in the same device to assure 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 work presents synthesis of a tracking system and experimental results characterizing the communication performance under uncompensated pointing disturbance and with tracking.

Experimental demonstration of a retro-reflective laser communication link on a mobile platform

Successful pointing, acquisition, and tracking (PAT) are crucial for the implementation of laser communication links between ground and aerial vehicles. This technology has advantages over the traditional radio frequency communication, thus justifying the research efforts presented in this paper. The authors have been successful in the development of a high precision, agile, digitally controlled two-degree-of-freedom electromechanical system for positioning of optical instruments, cameras, telescopes, and communication lasers. The centerpiece of this system is a robotic manipulator capable of singularity-free operation throughout the full hemisphere range of yaw/pitch motion. The availability of efficient two-degree-of-freedom positioning facilitated the development of an optical platform stabilization system capable of rejecting resident vibrations with the angular and frequency range consistent with those caused by a ground vehicle moving on a rough terrain. This technology is being utilized for the development of a duplex mobile PAT system demonstrator that would provide valuable feedback for the development of practical laser communication systems intended for fleets of moving ground, and possibly aerial, vehicles. In this paper, a tracking system providing optical connectivity between stationary and mobile ground platforms is described. It utilizes mechanical manipulator to perform optical platform stabilization and initial beam positioning, and optical tracking for maintaining the line-of-sight communication. Particular system components and the challenges of their integration are described. The results of field testing of the resultant system under practical conditions are presented.

Mitigation of dynamic wavefront distortions using a modified simplex optimization approach

Laser beam propagating through the dynamically changing atmosphere is subjected to severe wavefront distortions caused by the optical turbulence. The resulting spatial and temporal fields of the refractive index lead to performance degradation in the form of reduced signal power and increased BER, even for short link ranges. An electrically addressed liquid crystal spatial light modulator (SLM) is proposed to perform correction of the optical path difference (OPD) pattern resulting from the atmospheric distortions. Controlling every individual pixel of the SLM is a rigorous and time-consuming task that calls for a stable and simple procedure that could be performed in real-time. This could be addressed by approximating the phase profile of the distorted beam using Zernike formalism, which provides efficient mapping between large number of SLM pixels and smaller number of coefficients of Zernike polynomials. A possible solution to the dynamic correction problem is the application of Simplex optimization by Nelder and Mead, which is well known for fast improvement of an optimization metric. As has been shown before, this approach presents a problem of locking up in local minima while correcting dynamic changes. This paper presents experimental results of different approaches to resolve this problem by modifying simplex procedure as well as modification in a previously presented experimental setup.

Mitigation of dynamic wavefront distortions using a nematic liquid crystal spatial light modulator and simplex optimization

Laser beam propagating through the atmosphere is subjected to severe wavefront distortions due to the optical turbulence. This leads to reduction in the received power, ultimately resulting in the BER degradation, even for short ranges. Optical properties of the atmospheric channel change over time; hence, maintaining a reliable link requires dynamic wavefront control to mitigate the effects of the atmospheric turbulence. An electrically addressed programmable nematic liquid crystal spatial light modulator (SLM) is proposed to perform this task. Wavefront correction is achieved by computing a phase shift for each pixel of the SLM, which could be a rigorous and time-consuming procedure. Hence, the goal is to obtain a stable and relatively simple approach to dynamically control the modulator elements. The phase profile of the distorted beam can be approximated using Zernike formalism or another type of wavefront polynomial, which provides efficient mapping between a large number of SLM pixels and a much smaller number of approximation coefficients. Furthermore, wavefront correction needs to be performed in real-time; hence the Simplex method by Nelder and Mead, known for fast improvement of an optimization metric, is used to adjust the approximation coefficients. The phase profile obtained from the optimization procedure is imposed on the received beam by the SLM. This facilitates the reduction of the optical path difference (OPD) present in the distorted wavefront by applying an inverse OPD, and mitigating the effects of the optical turbulence. This paper presents a basic algorithm as well as the experimental results.

Performance of free-space laser communication systems as a function of the sampling rate in the tracking loop

Laser technology plays an ever-increasing role in aerospace and communication systems and is often viewed as a technology that has the potential for providing the material base for high-bandwidth applications. Laser provides the most logical connectivity channel for mobile systems requiring high data rates, low power consumption, covert operation, and high resistance to jamming. While advancements in modern opto-electronics have resulted in small size, reliable and power efficient lasers and modulators, successful operation of any communication technology hinges upon the ability to develop an equally advanced beam steering/positioning system. In many aerospace applications, when the transmitting optical platform is placed on board of an airplane, the ability to track the target is affected by the complex high-speed maneuvers performed by the aircraft and the resident vibration of the airframe. The tracking system must assure that in spite of the relative motion of both the transmitting and receiving stations and adverse environments, such as vibration, mutual alignment of two systems will be maintained to minimize communication errors. The work presented in this paper concentrates on the development of agile beam steering systems for laser communication terminals. Acousto-optic Bragg cells are used as deflectors while feedback information is generated by a quadrant detector. The control system is synthesized using a relatively simple constant-gain controller augmented with an adaptive Kalman filter to mitigate the effects of measurement noise in the tracking system. Laboratory experiments are conducted to investigate communication performance as a function of the sampling rate in the beam position feedback.

Diffraction-Based Optical Sensor Detection System for Capture-Restricted Environments

The use of digital cameras and camcorders in prohibited areas presents a growing problem. Piracy in the movie theaters results in huge revenue loss to the motion picture industry every year, but still image and video capture may present even a bigger threat if performed in high-security locations. While several attempts are being made to address this issue, an effective solution is yet to be found. We propose to approach this problem using a very commonly observed optical phenomenon. Cameras and camcorders use CCD and CMOS sensors, which include a number of photosensitive elements/pixels arranged in a certain fashion. Those are photosites in CCD sensors and semiconductor elements in CMOS sensors. They are known to reflect a small fraction of incident light, but could also act as a diffraction grating, resulting in the optical response that could be utilized to identify the presence of such a sensor. A laser-based detection system is proposed that accounts for the elements in the optical train of the camera, as well as the eye-safety of the people who could be exposed to optical beam radiation. This paper presents preliminary experimental data, as well as the proof-of-concept simulation results.

Mitigation of optical turbulence effects using a modified simplex optimization approach: experimental study

Dynamically changing turbulence in the atmosphere distorts the wavefront of the laser beam propagating through it. The resulting spatial and temporal fields of the refractive index lead to performance degradation in the form of reduced signal power and increased BER, even for short link ranges. An electrically addressed liquid crystal spatial light modulator (SLM) can be used to correct the optical path difference (OPD) pattern resulting from the atmospheric distortions. Approximating the phase profile of the distorted beam using well-known Zernike formalism reduces the complexity of controlling each pixel of the SLM. Real time correction of the wavefront can be achieved using the Simplex optimization procedure by Nelder and Mead. Previously, some modifications have been proposed to overcome the local minima problems as well as the faster convergence. Yet the better and faster performance could be achieved by more accurate prediction of the simplex initialization along with the modifications in the simplex procedure. This paper presents the experimental results of such modifications to the earlier proposed system.