Alexandrite laser in Q-switched, single longitudinal mode operation pumped by a fiber-coupled diode module

In this work a diode-pumped Q-switched Alexandrite laser operating in single longitudinal mode (SLM) at the potassium resonance line is presented. The self-developed laser diode pump device is fiber-coupled ( = 400μm, NA=0.22) and delivers a pump energy of 18 mJ at 636 nm with a pulse duration of 120 μs and a repetition rate of 500 Hz. Pump light not absorbed in single-pass through the 7 mm long crystal is recollimated, polarization adjusted and refocused into the crystal. The Alexandrite laser yields a pulse energy of 1.7 mJ at a repetition rate of 500 Hz with a high pulse-to-pulse stability of 0.2 % (rms) and a beam quality of M² < 1.1 in both spatial directions. The output beam is round and stigmatic without further beam shaping. The electro-optical efficiency of the laser system is 2 % which is approximately two magnitudes higher than of comparable flashlamp-pumped Alexandrite laser systems. By seeding the resonator with a SLM diode laser and actively stabilizing the cavity length, SLM-operation at the resonance line of potassium at 769.898 nm with a linewidth of approximately 10 MHz is achieved. Thereby the laser fulfills all the requirements for a resonance-lidar system. The investigations presented in this publication show the feasibility for pumping a complex ring resonator with a fibercoupled pump module in the red spectral region. This presents an important step to compact lidar systems for autonomous measurements under rough environmental conditions.