Underwater Optical Communications: From Photodiodes to Single-Photon Detectors

This paper reviews key hardware components of underwater wireless optical communica-tions (UWOC) , with particular emphasis on photodetectors (PD). The growing demand for high-speed, low-latency underwater data links, driven by applications in autonomous underwater vehicle (AUV) networks, environmental monitoring, offshore exploration, and tactical surveillance, motivates a systematic comparison of available device technologies and their suitability across different deployment scenarios. The paper first outlines the fundamental constraints imposed by the aquatic optical channel, including wave-length-dependent absorption and scattering, turbulence-induced intensity fluctuations, and their combined impact on achievable link range and data rate. The spectral transmis-sion window of water in the 450–550 nm range is identified as the primary wavelength constraint for selecting the light sources and photodetectors. It also includes a brief over-view of radiation sources used in UWOC systems. The operating principles, modulation bandwidths, and beam characteristics of light-emitting diodes (LEDs) and laser diodes (LDs), have been compared. In the main part, the review focuses on analyzing the UWOC receiver's construction across five PD’s technologies: photomultiplier tubes (PMTs), p-i-n photodiodes (PINs), avalanche photodiodes (APDs), single-photon avalanche diodes (SPADs), and silicon photomultipliers (SiPMs/MPPCs). For each technology, the operating principle, gain mechanism, and key parameters, including responsivity, noise-equivalent power, bandwidth, and bias requirements, are described and linked to reported experi-mental UWOC results. The analysis covers both analog and photon-counting reception modes, as well as advanced modulation and signal-processing techniques that extend link reach and throughput. The analysis of PD technologies for their suitability for underwater optical communication systems constitutes a unique data source for technologists and de-signers. It highlights experimental results from many researchers across various PD types and modulation formats. A fundamental trade-off between receiver sensitivity and data rate is observed, e.g., PINs achieve the highest data rates (up to ~25 Gbps over short dis-tances) but have the lowest sensitivity, whereas single SPADs and PMTs offer the highest sensitivity (down to approximately −85 dBm) at the cost of reduced bandwidth. SiPM/MPPC arrays are identified as a promising intermediate technology, combining high single-photon sensitivity, spectrally efficient modulation formats, and low-bias-voltage operation. Emerging photodetector technologies, including perovskite-based photodetec-tors, SiC photoelectrochemical devices, and large-area scintillating-fiber receivers, are also reviewed as promising candidates for next-generation UWOC and Internet of Underwater Things deployments.