Pumping wavelength related population inversion in Nd:doped laser

Pumping wavelength related population inversion in an Nd:doped laser has been investigated, where the laser is regarded as two thermodynamic equilibrium systems: the upper state and the lower state. The population inversion limits at various pumping wavelengths and temperatures are compared. The effective metastable lifetime has been derived, where both pump and laser fields within the Nd:doped laser medium reduce the lifetime. The saturation of the lifetime is discussed. As a result, in-band pumping of the Nd:doped laser is good for improving the efficiency and achieving the narrow-linewidth of the laser wavelength due to less population inversion; off-band pumping is beneficial for extracting high pulse energy, thanks to large population inversion.