## 5 Conclusions  
The digital holography method revealed defects with a characteristic size of \(\approx\)300 \(\upmu\)m in the volume of In-doped GaSe. LIDT was determined from the fluence and the peak  
Figure 7: (**a**) Dependences of the sample no. 1 (GaSe) damage probability on the damage peak power (exposure time—1 s): 1—at a pulse repetition rate of 10 kHz; 2—at a pulse repetition rate of 12 kHz; 3—at a pulse repetition rate of 14 kHz; 4—at a pulse repetition rate of 20 kHz. (**b**) Dependences of the sample no. 2 (GaSe:In) damage probability on the damage peak power (exposure time—1 s): 1—at a pulse repetition rate of 10 kHz; 2—at a pulse repetition rate of 12 kHz; 3—at a pulse repetition rate of 14 kHz; 4—at a pulse repetition rate of 20 kHz. power of GaSe (sample no. 1) and GaSe:In (sample no. 2) crystals with the parameters of the incident radiation close to the pump radiation parameters of promising dual-wavelength optical parametric oscillators (effective pump sources for THz difference frequency oscillators): wavelength was \(\approx\)2.1 \(\upmu\)m, pulse repetition rate was 10-20 kHz, pulse duration was 15-22 ns. The damage threshold of GaSe crystal (sample no. 1) at a pulse repetition rate of 10 kHz was \(W_{\mathrm{d}}^{E}=2.4\) J/cm\({}^{2}\) in terms of the fluence of the acting laser radiation, and \(W_{\mathrm{d}}^{P}=95\) MW/cm\({}^{2}\) in terms of the peak power at a pulse repetition rate of 20 kHz \(W_{\mathrm{d}}^{E}=3.01\) J/cm\({}^{2}\), \(W_{\mathrm{d}}^{P}=50\) MW/cm\({}^{2}\). The damage threshold of GaSe:In crystal (sample no. 2) at a pulse repetition rate of 10 kHz was \(W_{\mathrm{d}}^{E}=4.1\) J/cm\({}^{2}\) in terms of the fluence of the acting laser radiation, and \(W_{\mathrm{d}}^{P}=157\) MW/cm\({}^{2}\) in terms of the peak power at a pulse repetition rate of 20 kHz \(W_{\mathrm{d}}^{E}=2.2\) J/cm\({}^{2}\), \(W_{\mathrm{d}}^{P}=42\) MW/cm\({}^{2}\). The obtained results allow for the conclusion that the value of GaSe damage threshold at a wavelength of 2.091 \(\upmu\)m of the incident radiation was influenced by the accumulation effects (the damage threshold decreased as the pulse repetition rate increased). The accumulation effects were more significant in the case of the In-doped sample, since a more significant decrease in the damage threshold was observed with increasing frequency in terms of the peak power and the fluence. The presence of volumetric defects reaching geometric dimensions of \(\approx\)300 \(\upmu\)m in the GaSe:In sample seemed to enhance the accumulative effect. The results of this work confirm the conclusions obtained in [12] on the significant influence of thermal effects on GaSe LIDT when exposed to pulsed nanosecond laser radiation, the role of which increased with the increase of pulse duration and laser pulse repetition rate. The influence of thermal effects was enhanced by the presence of volumetric defects in the samples. The obtained results will be useful in the design and development of THz radiation oscillators based on GaSe crystals with dual-wavelength pumping by radiation in the region of \(\approx\)2.1 \(\upmu\)m. The obtained results provide an analytical basis for determining the energy parameters of the pump radiation in order to improve energy characteristics of the THz radiation generated in GaSe and the stability of the system in the continuous mode. Conceptualization, G.V., O.A., N.Y., C.Y. ; methodology, O.A., N.Y., M.Z., V.D. ; software, Y.Z., Y.S. ; validation, C.Z., C.Y. ; formal analysis, C.Z., S.P. ; investigation, N.Y., C.Z., O.A., I.E. ; resources, C.Y., O.A., V.D. ; data curation, V.D., O.A. ; writing--original draft preparation, N.Y., M.Z., V.D., O.A. ; writing--review and editing, O.A., V.D. ; visualization, G.V., I.E. ; supervision, O.A., V.D. ; project administration, V.D., O.A., C.Y. ; funding acquisition, V.D., O.A., C.Y. All authors have read and agreed to the published version of the manuscript. This research was supported by Ministry of Science and Higher Education of the Russian Federation, project no.