High-Speed 645-nm VCSELs for Low-Scattering-Loss Gb/s Underwater Wireless Optical Communications

With the evolution of underwater wireless optical communications, semiconductor lasers, particularly blue and green diode lasers, have exhibited high performance as optical transmission sources. They have a wide modulation bandwidth and can offer data rates in the Gb/s range in pure water. However, blue and green wavelengths experience higher attenuation coefficients and more considerable scattering losses in the turbid harbor and turbulent water compared to red light. In this study, we investigated the capability of red vertical-cavity surface-emitting lasers (VCSELs) for data transmission in an underwater wireless optical communication (UWOC) system. The wavelength of our VCSEL is 645 nm, and the output power is 2 mW. A 2-Gb/s data rate was achieved underwater by direct-current-biased orthogonal frequency-division multiplexing (DC-OFDM) modulation. The bit error ratio (BER) was 3.5× 10-3, which is below the hard-decision forward error correction (HD-FEC) limit of 3.8× 10-3. The availability of 650-nm transmission window of plastic optical fibers (POFs) makes this VCSEL wavelength an attractive solution for POF communication and UWOC convergent systems. Moreover, the use of this wavelength will enable wavelength division multiplexing (WDM) in highly turbid water with other red wavelengths (e.g., 680 nm) to improve the data throughput in these harsh environments.