Pulsed single-longitudinal-mode operation based on modal-gain difference in repetitively passively <i>Q</i>-switched lasers

Abstract The pulsed single-longitudinal-mode (SLM) operation caused by the modal-gain difference in a repetitively passively Q -switched (PQS) laser is studied in detail. Firstly, the analytical expressions for the pulse buildup-time difference of repetitively PQS four-level and quasi-three-level lasers have been developed respectively. Then, according to the temporal criterion, the required conditions for repetitively PQS four-level and quasi-three-level lasers to achieve SLM operation are analyzed. The analysis results show that in addition to the short cavity is conducive to obtaining the pulsed SLM laser, the use of a lower pump power (compared to the threshold power) will help to obtain a longer pulse buildup-time difference and thus enabling the SLM operation. Moreover, it is worth noting that for the quasi-three-level lasers, the pulse buildup-time difference also depends on the initial population inversion density. The results also reveal that setting resonator parameters that can obtain large initial population inversion density will be helpful to the SLM operation in both four-level and quasi-three-level regimes. In addition, the use of saturable absorber with a low absorption cross-section ratio between the excited state and ground state also contributes to the realization of the SLM. Finally, the optimization model of passively Q -switched single-longitudinal-mode laser is established. In addition to predicting the output performance of the laser, this model can also be used to obtain the optimal resonator parameters and the upper limit of pump power for SLM operation.