Pump–Probe Measurements of the Raman Gain Coefficient in Crystals Using Multi-Longitudinal-Mode Beams

The effect of multi-longitudinal-mode interference on pump-probe Raman gain measurements is studied as the functions of laser linewidth and correlation between the pump and probe temporal structures. For Raman linewidths typical of crystals, correlations that develop between the pump and the probe increase the Raman amplification beyond the ideal single longitudinal mode case, leading to inflated Raman gain coefficient values. We show that the linewidth correction factors used previously are valid only for uncorrelated beams and small gain factors. Taking into account these considerations, a Raman gain coefficient for a diamond of 3.80 ± 0.35 cm/GW is reported at a pump wavelength of 1.864 μm (polarizations parallel to a 〈110〉 crystal axis).