Fourier transform-limited, nanosecond, CW-seeded optical parametric amplifier with high spectral purity

We report on a narrow linewidth nanosecond pulse optical parametric amplifier (OPA) seeded by a tunable continuous wave (CW) laser diode emitting around 1647 nm with a high spectral contrast. The system is pumped by a 1030 nm fiber laser delivering an average power of 10 W, pulses with a duration of 10 ns at a repetition rate of 100 kHz. A maximum signal conversion efficiency of 21% is achieved, corresponding to 2.16 W of average power and 20 µJ pulse energy with a beam quality factor of M² < 1.25. Optical spectrum analysis shows a spectral purity > 99.97%, and optical heterodyne measurements demonstrate near Fourier transform-limited operation with a spectral linewidth < 110 MHz for 10 ns pulses. Methane absorption spectroscopy is demonstrated using a gas cell, confirming the narrow linewidth and spectral stability of the source. These results highlight the potential of CW-seeded OPAs as laser sources for gas-sensing LIDAR systems.