Laser Transmitter for Undersea Communications Using Third-Harmonic Generation of Fiber-Laser System at 1.5 $\mu$ m

We report a viable laser transmitter for free-space undersea communications. An all-fiber, picosecond, Watt-level master-oscillator-power-amplifier (MOPA) system at 1.5 mum based on rapid amplification of mode-locked pulses in heavily Er:Yb codoped phosphate fiber is combined with fiber pigtailed lithium niobate intensity modulator (pulse picker), to construct a fully integrated eye-safe transmitter operating at 65-Mb/s data rate, that can be used in intermediate-range (few kilometers) atmospheric communication links. For undersea use, the output of the MOPA system is frequency-tripled into the blue-green transparency window of ocean water. The wavelength conversion occurs in a simple single-pass setup utilizing a sequence of two periodically poled lithium niobate crystals, both of which are operated at room temperature. The conversion efficiency from fundamental to third harmonic reached 14% and resulted in generation of 140 mW of average power at 518 nm. The conversion efficiency can be straightforwardly increased threefold using properly antireflection-coated optics in the free-space part of the setup, and the data rate can be scaled up into the gigabit-per-second range by using a faster mode-locked oscillator in the MOPA system.