Homogeneous And Inhomogeneous Gain Saturation

If all active ions (or atoms) exhibit the same emission spectrum and absorption can be neglected (such as in quasi-three-level gain media), gain saturation only reduces the magnitude of the gain without altering the spectral shape. This phenomenon is known as homogeneous gain saturation, which commonly occurs in laser crystals because all active ions occupy equivalent lattice sites.

When different ions exhibit different emission spectra—due to variations in lattice sites within solid-state media—or when gas particles possess different velocities, inhomogeneous saturation occurs. In this case, a narrow-linewidth laser beam primarily saturates the ions (or atoms) that emit most strongly at a specific wavelength, thereby changing the balance among different gain contributions and modifying the gain spectrum. As shown in the figure below, a narrow-linewidth laser beam at 1064 nm produces this effect, with the most significant gain reduction occurring in the spectral region around 1064 nm.

Obviously, the calculation of saturation effects becomes much more complicated under inhomogeneous saturation conditions. Gain saturation has a significant influence on the operation of tunable lasers and single-frequency lasers.