Design trades for optimized second harmonic conversion efficiency of high peak power Enfiber source

摘要

Second harmonic generation efficiency of a pulsed Er fiber laser with periodically poled lithium niobate (PPLN) is optimized by varying input pulsewidth. The Er-doped fiber amplifier was a 3-stage, 1550 nm amplifier with 1-10 ns variable pulsewidth, 180 kHz pulse repetition frequency, and 5.5 W of output power. The laser was focused into a single, 10 mm long piece of PPLN to convert to 775 nm through second harmonic generation. The pulsewidth was varied and we observed a correlation between pulsewidth and conversion efficiency. At the minimum pulsewidth, τ = 2 ns, we achieved 31% conversion efficiency, and as we increased the pulsewidth we saw an increase in second harmonic generation conversion efficiency. At τ= 10 ns, our maximum pulsewidth, we saw a conversion efficiency of 68%, which was the highest conversion efficiency achieved in this experiment. The increase of efficiency with reduced pump intensity is attributed to the decrease in spectral width of the laser at longer pulsewidths. Measuring the spectrum of the laser verified the presence of self-phase modulation at the shorter pulsewidths.