Doped fiber amplifiers assisted hybrid UWOC-fiber-FSO communication network for underwater optical sensing

In this work, a hybrid optical communication link is presented that seamlessly integrates underwater wireless optical communication (UWOC), fiber, and free-space optics (FSO) to support high-speed underwater sensing. Ten underwater sensors form a \(10× 10\) Gb/s underwater wireless sensor network (UWSN) for continuous data collection. Communication is established through line-of-sight (LOS) UWOC and FSO channels, interconnected via a 1 km single-mode fiber (SMF) between an underwater drone (UD) and a surface ship. The UD employs a detect-remodulate-forward (DRF) relay to process received signals under varying water conditions, followed by wavelength translation from 532–539.2 nm into the O-band (1300–1307.2 nm), S-band (1470–1477.2 nm), and C-band (1550–1557.2 nm). At the ship, amplification is provided using rare-earth-doped optical fiber amplifiers (REDOFAs), including erbium-doped fiber amplifier (EDFA), thulium-doped fiber amplifier (TDFA), praseodymium-doped fiber amplifier (PDFA) and their hybrid combinations. Performance is evaluated in terms of bit error rate (BER) and quality factor (Q-Factor) across underwater environments (pure sea, clear ocean, coastal ocean, harbor-I, harbor-II), atmospheric attenuation (light haze: 2 dB/km, dense fog: 12 dB/km, heavy rain: 22 dB/km), and turbulence regimes ( \({C_n}^2=5× 10⁻¹⁶\) to \(5× 10⁻¹⁴\) m \(-2/3\) ). Results show that turbidity reduces the UWOC transmission distance from 29 m in pure sea to 4.1 m in harbor-II, while maintaining BER below \(9× 10⁻⁹\) and Q-Factor above 5.63. Atmospheric effects degrade the FSO transmission range; however, shipboard amplification compensates impairments and extends the overall achievable range of the hybrid system.