Design and Implementation of Automatic Beam Alignment System for LD-PMT Based UOWC

This paper demonstrates an automatic beam alignment system for a laser diode (LD)-photomultiplier tube (PMT) based underwater optical wireless communication (UOWC) system. Initially, the imaging characteristics of the underwater laser spot are described. Subsequently, a laser outlet estimation algorithm based on principal component analysis (PCA) is proposed. The direction of the emitted light beam is rapidly determined by reflecting it with a micro-electro-mechanical system (MEMS) mirror to align with the receiver. Experimental results validate the feasibility of the proposed beam alignment method, indicating an acceptable alignment error. Moreover, the proposed system effectively enhances the bit error rate (BER) performance, meeting the BER requirements of the forward error correction (FEC) limit when the receiver moving speed is below 0.45 m/s.