Significance:  All-solid-state single-frequency continuous-wave (CW) laser have found extensive applications in diverse domains such as the generation of non-classical light fields, cold atom physics, detection of gravitational waves, and so on, which primarily attributed to their merits of low noise, narrow bandwidth, excellent beam quality, and high power stability. In line with the advancement in science and technology, the output power of the traditional all-solid-state laser (ASSL) cannot satisfy the application requirements of many frontier research fields, so it is necessary to further scale the ASSL power and simultaneously maintain other excellent performance. For the purpose of improving the output power of the ASSL, its pump power has to be primarily elevated. However, with the increasement of the pump power, the laser gain is enhanced, and the non-oscillating laser modes of the ASSL start to oscillate, which results in the mode-hopping or the multi-mode oscillating operation of the ASSL. Moreover, the severe thermal effect of the laser gain medium and its relatively lower damage threshold also further restrict the improvement of the ASSL power. In this paper, an effective method of improving the all-solid-state single-frequency CW laser power via deliberately introducing a nonlinear loss into the resonator was presented. When the nonlinear loss was introduced, the nonlinear loss of the lasing mode was half of that of non-lasing mode, and the non-lasing mode was effectively inhibited, under the mode competition of the laser. As a consequence, the stable single-longitudinal mode operation of the laser can be guaranteed at higher laser gain. In addition, the design of multi-laser-crystal resonator can be adapted to efficiently mitigate the negative impact of the thermal effects of the laser crystal. By combining the nonlinear loss technique and the multi-laser-crystal resonator scheme, the output power of the all-solid-state single-frequency CW laser had been scaled up to 100-watt level and continuously increased.

  Progress:   First, the fundamental principle of mode selection implemented by intra-cavity nonlinear loss is presented. When the nonlinear loss is introduced into the resonator, the nonlinear loss of the lasing mode is half of that of the non-lasing mode, and the non-lasing modes are suppressed effectively under the mechanics of mode competition. Based on the principle above, the physical condition of stable SLM operation for ASSL is proposed. The condition depends on the intra-cavity linear and nonlinear losses, which is experimentally validated by changing the transmission of the output coupler. In the experiment, when the output coupler transmission is 19%, and the temperature of the type-I phase matched nonlinear crystal LBO is 149 ℃, the maximal output power of 33.7 W for the stable single-frequency 1064 nm laser is realized. On this basis, the intra-cavity round-trip loss of an ASSL is measured precisely by simply changing the temperature of the nonlinear LBO crystal to manipulate the nonlinear loss within the SLM region of the laser. According to the measured results and the oscillating condition of the ASSL, the output coupler transmission of the designed laser as well as its pump power is further optimized and the maximal output power of 50.3 W for the single-frequency 1064 nm laser is obtained.　　To further increase the output power of the single-frequency laser, the pump power of the laser has to be raised. However, the sever thermal effect of the laser gain medium and its lower damage threshold restrict the continuously increasing of the single-frequency laser power. For the purpose of breaking aforementioned restriction and attaining higher power single-frequency laser, a laser resonator with two identical laser crystals was designed, where the precise mode-reproduction of the two crystals was implemented by a pair of lenses with identical focal length of 100 mm. When the total pump power was 240 W, a single frequency 1064 nm laser with maximal output power of 101 W was realized. In this laser, the focal lengths of the two lenses were fixed, so the laser only would be operated at a given incident pump power, and simultaneously the optical length between the imaging lenses had to be precisely adjusted. To this end, a self-mode-matching laser with four laser crystals in a single resonator was further designed. The total four laser crystals were used for both laser gain media and mode-matching elements. Under an appropriate combination of pump powers on four crystals, a stable CW single-frequency 1064 nm laser with 140 W power was obtained.

  Conclusions and Prospects:   Introducing nonlinear losses within the resonator is a robust way to realize SLM laser output, which has been experimentally proved. With multiple gaining crystals inserted in one cavity, the heat load on each crystal is effectively shared, so more total power is tolerable. With suitable mode matching and mode reproducing in the resonant, a high-power single-frequency CW ASSL has been designed and built which can deliver 140 W single-frequency CW laser, this is to our knowledge the highest SLM ASSL power. The progress in high-power single-frequency ASSL has significantly broadened its potential applications and made substantial contributions to the advancement of related disciplines.