| MS TSA Meeting


Name of the Speaker: Mr. M Venkata Akhil (EE20S132)
Guide: Prof. Anil Prabhakar
Venue: ESB-244 (Seminar Hall)
Date/Time: 6th August 2024 (Tuesday), 4:00 PM
Title: Lock-in amplifier based alignment system for free space optics link.

Abstract :

Free space optical (FSO) communication is a viable candidate for fulfilling the ever-increasing demand for high-speed internet. It involves sending data modulated over a laser, which travels in free space. It requires a line of sight (LOS) between the transmitter and receiver. This LOS tend to get misaligned due to various atmospheric effects, effectively due to random changes in the refractive index. FSO requires an alignment system to maintain this LOS.

This work presents a lock-in amplifier (LIA) based alignment system. To compensate for the LOS misalignments, the laser beam is steered at the transmitter side based on the feedback from the receiver side, which is sent via a wireless RF link. The receiver decides the direction of steering using a steepest gradient descent (SGD) based algorithm. An in-house built LIA is used at the receiver to detect the weak optical signals affected by the atmosphere. The LIA increased the receiver’s sensitivity by 11 dB. The alignment system is tested to recover a misaligned link indoors at distances of up to 80 m and outdoors at a 150 m link distance. The tests were done with manually misaligned links, simulating beam displacement, and on atmosphere-induced misaligned links.

In the outdoor setup, a bit error rate of 1E-11 was obtained after recovering a manually misaligned link. In the indoor setup at 80 m, the FSO link without our alignment system experienced a power loss of 19 dB over a period of 1 hour due to atmospheric misalignments. When our system was employed, it partially corrected these misalignments and reduced the power loss to 8 dB.