Underwater Wireless Optical Communication System With Adaptive Receiver Based on Motorized Aperture

In this paper, an adaptive receiver is proposed to achieve optimum data rates at different underwater transmission distances and to extend the operational range without changing the transmitted optical power. The proposed receiver comprises a motorized aperture and a large-area positive-intrinsic-negative photodiode (PIN PD) with a strip-shaped anode located at the edge. Theoretical analysis and experimental results indicate that the bandwidth of this PIN PD depends not only on the reverse bias voltage but also on the incident light's position. On this basis, the motorized aperture is designed to adaptively adjust the position and the active area in the detector to balance the bandwidth and signal-to-noise ratio (SNR) in various transmission distances. By adjusting the aperture, the 3-dB system bandwidth is finally extended from 4 MHz to 16.3 MHz with a reverse bias voltage of 25 V. Experimental results in air show that using the adaptive receiver can increase the data rate by up to 89.5% without any digital signal processing and avoid detector saturation even when the incident optical power is excessive (up to 22 dBm). The underwater experiments verify the adaptability of the proposed system at varying distances. The proposed adaptive receiver in this paper can increase data rate and reduce the risk of detector saturation in scenarios with varying transmission distances, such as the communication between underwater vehicles and base stations.