High-Precision Light Spot Position Detection in Low SNR Condition Based on Quadrant Detector

Abstract

In free space optical communications, long-distance transmission leads to the attenuation of beacon light, where we adopt a quadrant detector (QD) to receive the weak signal. However, the background light interferes so strongly that the output signal-to-noise ratio (SNR) of QD is at a low level, which causes a decrease in accuracy of the direct detection method. This requires finding a new light spot detection method, so an improved detection method is proposed. Because the dark current noise and the background light noise are both white noise, we adopt a Kalman filter to estimate the real output of four electric signals of QD. Unfortunately, running these through an amplifier introduces some direct current (DC) offsets into the signals. In order to balance the effect of the DC offsets, we consider using the modulation method, where we employ a sine signal to modulate the intensity of the beacon light at the transmitting end, after which we can give an inverse gain to move the center of signals to near zero to eliminate the DC offsets when we calculate the data. In Kalman filtering, we use the peak values of the signals in every period after the analog to digital converter (ADC) as the elements of the measurement matrix. Experimental results show that even when QD output SNR is about −10 dB, the detection root-mean-square errors decrease by 51.5% using the improved detection method compared with the direct detection method. Moreover, Kalman filtering does not require a large amount of data, which means it works efficiently, can reduce the cost of hardware resources, and is available for the real-time calculation of spot position.

A Robust Real-Time Automatic Recognition Prototype for Maritime Optical Morse-Based Communication Employing Modified Clustering Algorithm

“…It has been demonstrated as a promising technology as a complementary solution to conventional radio frequency (RF), fiber optics, and microwave communications, particularly for applications with size, weight, and power restrictions. The potential advantages of FSO communication including eye safety, no spectrum licensing issues, smaller and lighter payloads, low probability of intercept, and immunity from jamming make it a natural candidate for maritime long-range communication scenarios [1][2][3][4].…”

A Method for Improving the Detection Accuracy of the Spot Position of the Four-Quadrant Detector in a Free Space Optical Communication System

“…In addition, the relationship between the output signal of the 4QD and the true position of the light spot on the photosensitive surface is complicated, and there is an inevitable gap between each quadrant on the photosensitive surface of the detector. Therefore, some researchers have developed detection algorithms that can improve the spot position detection accuracy of the 4QD [ 12 , 13 , 14 , 15 , 16 ].…”

TRC-Based High-Precision Spot Position Detection in Inter-Satellite Laser Communication

“…However, this method requires high synchronization between the two signals. Yu Jiawei [ 14 ] used the Kalman filter to process the photocurrent signal output by the QD to improve the accuracy of spot position detection. Gao Siyuan [ 15 ] established the beam detection range model of the laser guidance system based on QD, calibrated the influence of temperature parameters on the beam detection range, and performed temperature compensation for this, which greatly improves the beam detection performance of QD.…”

“…In the moving target tracking system based on QD, Zhang Wugang [ 16 ] used low-order least squares fitting and Kalman filter to improve the detection accuracy of the spot position under strong noise environment. However, the methods discussed in the literatures [ 14 , 16 ] cannot provide sufficient detection accuracy when the SNR is extremely low (such as −17 dB).…”