Influence of superradiance in active media with large gains on the energy characteristics of giant-pulse lasers

Abstract

We carried out experiments which made it possible to substantiate the approximate calculation. An active element of cylindrical shape made from YAG with neodymium (with a 5 • mm diameter) was used to measure the dependence of the value of the population inversion A = Nm/No (where N m is the population of the metastable level in the four-level lasing scheme and No is the concentration of active ions) on the pumping energy, which we shall characterize in the following by the dimensionless parameter K =W:4r32 (where W14 is the probability of a transition from level 1 to level 4 under the action of the pumping radiation, and i/r32 is the probability of a spontaneous transition between the laser levels). For this purpose, the pumping energy corresponding to the threshold for the appearance of stimulated emission was determined in a plane-parallel cavity with various values of the losses ~, and the quantity A =B/o32No (where os2 is the cross section of the lasing transition) was determined. The dependence obtained is presented in Fig. la. From the figure it is seen that the value of the population inversion A has a tendency to reach saturation as K is increased. The restriction of A with increasing pumping energy owing to the influence of superradiance is also supported by the investigation of plots of the dependence of the time-integrated luminescence intensity I in the direction of the longitudinal axis of the sample on the pumping energy obtained by integrating the signal from the light detector. Figure 1 presents plots of the dependence of I on K for crystals of YAG with neodymium having a diameter of 5 x 50 mm for luminescence at the output wavelength Iz and at a wavelength of ~0.9 ~m I2 (the 4F3/2-+4/9/2 transition). As we see from Fig. I, with the same values of K at which the restriction of A (see Fig. la) and the value of I2, which reflect the population of the metastable level, is observed, the value of II increases sharply owing to the significant enhancement of the luminescence at the output wavelength along the sample. (Similar investigations for crystals of YAL with neodymium were carried out in [2].) Thus, the restriction of A with increasing pumping energy is due to the influence of the superradiance.