Effect of cavity length on low-energy single longitudinal mode pre-lase Q-switched laser

•The effect of cavity length on pre-lase and normal Q-switched is different.•The cavity length determines whether the SLM can be achieved availably.•The cavity length determines the value of injected energy.•The quality of output beam is influenced greatly by cavity length. In this paper, the effect of cavity length on a low-energy single longitudinal mode (SLM) pre-lase Q-switched laser is analyzed and demonstrated. Taking a Pr:YLF laser as an example, the basic output characteristics under pre-lase technology are shown. The SLM is degraded when the cavity length is as large as 25mm. Further, for cavity lengths of 15 or 20mm, SLM is achieved with different output characteristics. Compared with a long cavity (20mm), the short-cavity case (15mm) is indeed helpful for obtaining an SLM laser; however, the single-pulse energy, pulse width, and energy extraction efficiency are decreased by 4.7, 48, and 6.7%, respectively. The results of this analysis show that the cavity length influences the output characteristics and determines the realization of SLM in a pre-lase Q-switched laser. This is because the short cavity induces a relatively strong gain identification for the seed signal. Then, the time cost of the mode competition decreases and SLM can be achieved easily. However, a long cavity is conducive to mode competition, which generates superior output characteristics.