Effect of pump spectra and axial mode separation on the single longitudinal mode performance in end pumped solid-state lasers with semi-monolithic gain medium

We present a theoretical model to study the effect of the pump spectrum and axial mode separation on the single longitudinal mode operation of a laser with a homogeneously broadened semi-monolithic gain medium of short absorption depth. The characterizing parameter is the ratio of the pump power up to which single longitudinal mode is possible to the pump power at the lasing threshold and is denoted by rmax. A numerical study using a Gaussian-shaped pump spectrum reveals that, for a small value of the axial mode separation and for a crystal with short absorption depth at the pump wavelength, the value of rmax reduces significantly with increase in the FWHM of the pump spectrum and thereby results in degradation of the single longitudinal mode performance of the system. However, for large value of the axial mode separation, the SLM performance was found to be nearly independent of the variations in the spectral bandwidth of the pump beam, location of its peak emission wavelength and its polarization.