Seawater has a transmission window between 350 and 580 nm, and a collaboration among scientists at the University of Arizona in Tucson and at Stanford University in California has resulted in what they believe is the first high-average-power, high-repetition-rate picosecond laser transmitter that can take advantage of that window. It is a frequency-tripled master-oscillator-power-amplifier fiber laser that, with the frequency-tripling components removed, also can function as an eye-safe transmitter at 1.5 μm.The system begins with a fiber oscillator passively mode-locked at 65 MHz (Figure 1). The heavily doped Er:Yb fibers in both the oscillator and amplifier were side-pumped, and the oscillator generated up to 100 mW of power whose wavelength the scientists tuned between 1554.5 and 1557.0 nm by adjusting the intracavity polarization. For reasons they did not fully understand, they saw the oscillator’s bandwidth increase from 0.7 to 2.3 nm as they tuned from the short- to the long-wavelength end of that range. The duration of the hyperbolic secant pulses sharpened from 3.8 to 1.1 ps at the same time, so the time-bandwidth product remained relatively constant at 0.32 to 0.35. Because the oscillator used a single-mode polarization-maintaining fiber, its 100-mW output was diffraction-limited and polarized.Figure 1. The fiber oscillator was mode-locked at 65 MHz with a semiconductor saturable absorber mirror (SESAM) (PC = polarization controller, PMF = polarization-maintaining fiber, EOM = electro-optic modulator, FBG = fiber Bragg grating). Images reprinted with permission of IEEE Photonics Technology Letters.One key to the system’s success was the short length — less than 15 cm — of the amplifier fiber, which eliminated deleterious nonlinear effects, including self-phase modulation and stimulated Raman scattering. As a result, the scientists avoided complex schemes such as chirped-pulse amplification, and the pulses emerging from the amplifier were nearly transform limited. The 100-mW output from the mode-locked oscillator was boosted to 1 W in the amplifier, and there was no distortion of the oscillator’s beam quality or of its linear polarization.Between the oscillator and amplifier, they spliced a fiber-pigtailed lithium-niobate intensity modulator that encoded digital information onto the signal. The modulator’s insertion loss was less than 2 dB, so it easily could handle the 100-mW beam passing through it. The 65-MHz fiber system alone was well-suited as an eye-safe transmitter for a free-space atmospheric communications system. For underwater transmission, the scientists added a pair of periodically poled, MgO-doped lithium-niobate crystals. The first crystal doubled the 1.55-μm output of the fiber amplifier, and the second one mixed the 1.55-μm fundamental with the 778-nm second harmonic to produce a green output at 518 nm (Figure 2). The overall conversion efficiency, from infrared to green, was 14 percent, with a green power of 140 mW. The green pulses had a duration of 1.5 ps and an energy of 2.15 nJ, giving them peak power of 1.3 kW. Figure 2. The green output at 518 nm was generated by frequency doubling the 1.5-μm output of the fiber master oscillator power amplifier (MOPA) and mixing the 778-nm second harmonic with the residual 1.5-μm fundamental wavelength (PPLN = periodically poled lithium niobate).As is frequently the case in experimental science, the scientists used components that were available and not necessarily those best suited for the experiment. None of the optics in the wavelength conversion setup was antireflection-coated. They estimate that, by using appropriately coated optics, they could achieve two- to threefold improvement of the overall conversion efficiency. IEEE Photonics Technology Letters, Sept. 1, 2007, pp. 1328-1330.