Impact of Laser Linewidth on the Performance of Modulating Retro-Reflector Wireless Coherent Optical Communication Systems in Oceanic Turbulence

Abstract

The combination of modulating retro-reflector (MRR) architecture and coherent detection constitutes a highsensitivity, lowpower asymmetric link solution for underwater wireless optical communication (UWOC). Because the transmitter and receiver share the same optical source and the signal undergoes round-trip propagation, coherent MRR-UWOC systems give rise to a unique phase noise accumulation mechanism, rendering the system markedly more sensitive to laser-noise-turbulence coupling. In this paper, an analytical expression for the average symbol error probability (SEP) of a balanced detection coherent MRR-UWOC system using binary phase-shift keying (BPSK) is derived, based on the generalized oceanic turbulence optical power spectrum (OTOPS) and the Wiener model for laser phase noise. Based on this expression, the performance of the MRR coherent communication system under turbulent conditions is analyzed for linewidths ranging from 100 kHz to 4.9 MHz. The results show that system performance is sensitive to both transmission distance and laser linewidth: at 10 m, a 5 MHz linewidth is acceptable, whereas at 40 m, the linewidth must be below 2 MHz to maintain an acceptable SEP. Water temperature amplifies linewidth sensitivity much more than salinity does. The rate of dissipation of mean-squared temperature markedly worsens linewidth-induced performance degradation and is the most sensitive turbulence parameter. A higher kinetic energy dissipation rate corresponds to a lower SEP floor and greater linewidth tolerance. For linewidths in the MHz regime, increasing oceanic turbulence not only significantly raises the baseline SEP of the system, but also leads to a systematic increase in its sensitivity to linewidth variations. In addition, the system linewidth tolerance improves with increasing wavelength, indicating stronger robustness to linewidth at longer wavelengths. This research offers critical insights and practical guidelines for laser selection and engineering design, facilitating the optimization of coherent MRR-UWOC systems in varying turbulence regimes.