Compensation of thermal lensing in an eye-safe cascade Raman laser with Ba(NO₃)₂ crystal

Recently, lasers operating in the eye-safe wavelength around 1.5 μm have found increasing demand in laser radar (LIDAR) applications. A high power laser output in the eye-safe wavelength region is obtainable with a Ba(NO₃)₂ Raman conversion crystal in a third Stokes line pumped by 1.06 μm Nd:YAG laser. During our previous experiments, we obtained 28 mJ, 27 mJ, and 8.5 mJ output energy for first, second, and third Stokes pulses, respectively, with a fixed pump energy of 140 mJ at 20 Hz repetition rate. These values are appropriate to LIDAR applications. Especially, the 20 Hz operation can significantly reduce data accumulation time in LIDAR systems compared with Ho:YAG lasers that are typically operated at around 5-10 Hz. However, we observed substantial decrease in the output energy during the first 50 sec of the continuous operation at 20 Hz. The energy drop is -7%, -46%, and -76% of the initial output for the first, second, and third Stokes, respectively. We supposed that the decrease of the output energy was caused by thermal loading associated with phonon relaxation in the Raman process. Hence, we experimentally studied the cause of the degradation in the output energy and tried to improve the performance.