Improved measurement of beacon light angle deviation based on a quadrant detector

Abstract

Free space optical (FSO) communication need to keep aiming and tracking each other to keep the communication link unimpeded. In this article, quadrant detector (QD) is used to measure beacon angle deviation. But due to the background light and the dark current, the precision of the direct method cannot match the requirement of the angle measurement. To solve this problem, a proposed method based on intensity modulation and Fourier-Transform is adopted to detect the beacon position accurately. We could consider use a cosine signal to modulate the intensity of beacon light at transmitting end. So the QD output photocurrents are also cosine signals with the same frequency. Then, the energy of QD output signal is concentrated at a certain frequency point, so that the signal-to-noise ratio (SNR) at the frequency is enhanced. Finally Fourier-Transform operation can detect the amplitudes of the photocurrents at the certain frequency point which correspond to the energy of the beacon light illuminated to each quadrant respectively. Therefore, we can calculate the beacon spot position accurately. Experiment results show that even when QD output SNR is only -7.66dB, the max absolute error of the proposed method is 0.016mm and is a quarter of the direct method.