The experiments with the use of a nanosecond laser system for investigation into the OF transmittance variations in the case of nanosecond laser pulses are reported. In this report, the model of numerical aperture photostimulated changes and absorption saturation is developed, which is meant for elucidation of the nonmonotonic dependence of losses on the light intensity. The experiments have been carried out using a YAG:Nd3+ nanosecond pulse wavelength laser, with Q-factor control at high intensities of (3 ÷125)⋅106 W/cm2 (the intensity range was dictated by the laser’s capabilities) and the first harmonic λ1 = 1064 nm.