Intensity feedback-based beam wandering mitigation in free-space optical communication using neural control technique

Abstract

Over the last two decades, free-space optical communication (FSOC) has become more and more interesting as an adjacent and/or alternative to radio frequency (RF) and optical fiber communications. The optical wave propagation in the FSOC channel is severely affected by atmospheric parameters, and it leads to the degradation of the data transmission quality and reliability. Among the various atmospheric effects, the beam wandering is the main cause of major power loss and cannot be solved without the incorporation of the beam centroid stabilization system. Therefore, designing a suitable opto-electronic assembly with a beam wandering mitigation system becomes significant to improve the performance of the FSOC system. A FSOC experimental setup is developed for the link range of 0.5 km in the college campus. A neural controller is designed for beam wandering mitigation. The neural controller processed the beam pointing (beam location) information obtained from an opto-electronic position detector (OPD) and then generated the necessary control outputs to the fast steering mirror (FSM) to steer the beam in the counter-direction to mitigate the beam wander at the receiver station. New design approach and architecture development for the implementation of the designed neural controller in the field-programmable gate array (FPGA) to mitigate the beam wandering are presented. The investigations on the performance of the neural controller in aiming a laser beam to be at a particular point on the detector plane are tested under dynamic disturbances generated at the transmitter in addition to the atmospheric effects through which the maximum correction capability of the developed neural controller is examined. Evidence of the suitability and the effectiveness of the proposed neural controller is evaluated in terms of beam centroid displacement, power spectral density (PSD), radial displacement on a 2D plane, effective scintillation index (ESI), Q-factor, and bit error rate (BER). The beam wandering range of −0.13 to +0.16 mm, maximum of 55 dB disturbance band attenuation with a frequency range of 0 Hz through 2 kHz, ESI of 0 to 0.15, Q-factor of 6, and BER of 6.45 × 10 −9 are achieved due to the incorporation of the developed neural controller in different real-world environmental conditions.

Quality Metrics and Reliability Analysis of Laser Communication System

“…In this work, the Q-factor is obtained using the constructed eyediagram and BER is measured using Keysight N4902B serial BER tester. The maximum Q-factor i.e., Q max is computed as a function of T amax and received signal statistics by 3,6,8 1 0…”

“…Telescope captures all optical power and reflects to fall on the FSM. The FSM consists of three terminal piezoelectric actuators based moving platform on which the beam steering mirror is mounted 3,8,14,21 . The incident laser beam of FSM gets reflected to the beam splitter, through NBIOF, which splits the incident beam into reflecting and propagating beams.…”

“…The propagating beam (passed through the beam splitter) is made to fall on the OPD. The OPD and MPAC are the key devices involving for the measurements of beam wandering i.e., beam centroid position information 3,9,16 . The OPD consists of four separate identical silicon photodiodes arranged in quadrant geometry 16 .…”

“…These are all the general problems that require attention to be dealt with. The major impact on the laser communication data link due to the random fluctuations of the atmospheric turbulence is as follows: (i) optical scattering and observation (characterised by A att ), (ii) random optical power fluctuation (characterised by scintillation index), (iii) beam wandering i.e., beam surface global (temporal) tilt due to beam centroid displacement on the detector plane (characterised by effective scintillation index ) and (iv) wavefront distortions i.e., beam surface (spatial) local tiptilt (characterised by the Zernike polynomials) 3,13,14,17 . The conventional data coding and/or modulation techniques can effectively be used to equalise/compromise the first two effects with respect to the weather condition at a given instant of time.…”

“…Resolving these problems is mandatory for the successful installation of the laser communication system 8,9,14,15 . One of the main reasons for the failure of laser communication is the absence of beam pointing, acquisition and tracking (PAT) system and therefore, a perfect and continuous adaptive alignment is important to reduce the pointing loss 3,4,14,15 . To demonstrate this, a mandatory requirement for the successful installation of the laser communication system, a real-world open atmospheric optical data transmission system, as in Fig.…”