## 4 Experimental Results and Discussion
### Output Characteristics of Q-Switched Laser Based on PSM-SA  
In the experiment described in this paper, the prepared PSM-SA was integrated into a ring cavity system [41], providing insights into its behavior within the laser setup as  
Figure 3: Characterization of PSM-SA device: (**a**) Schematic diagram of a nonlinear test system using two-arm method; (**b**) Nonlinear optical properties of PSM-SA. depicted in Figure 4. In the experimental test system, the 976 nm pump laser is coupled into the ring cavity through a WDM. The 4 m long EDF gain medium, a key component, played a pivotal role. It enabled the excitation of erbium-doped ions to a higher energy state upon interaction with the pump source. Subsequently, these excited ions rapidly relaxed to a metastable state, initiating the emission of C-band laser radiation through a well-established process [42]. To ensure the stability and efficiency of our laser system, we incorporated a 1550 nm ISO for unidirectional transmission. This essential component ensured that the optical signal traveled in a single direction within the setup, preventing damage to the pump source. Once the continuous wave light from the gain medium passes through the CIR, it irradiates the PSM-SA. This interaction led to the modulation of light into a pulsed form, a pivotal step in our laser operation. Subsequently, these modulated pulsed light signals were recoupled back into the ring cavity. To further manipulate the characteristics of the output pulses, we utilized a polarization controller (PC) to control the polarization state of the pulsed laser. This precise control allowed us to tailor the output pulses to meet our specific requirements. After passing through the OC, an ingeniously designed optical component, 20% of the optical signal was extracted for monitoring and meticulous measurement purposes. This extracted signal allowed us to continuously monitor the laser's performance characteristics, including output power, pulse duration, and spectral properties. The remaining 80% of the optical signal was cleverly recoupled back into the ring cavity using the WDM, thus ensuring that the laser reaches the gain threshold. The output characteristics of the fiber laser are monitored and analyzed using a photodetector, a digital oscilloscope (Keysight DSOS054A, Keysight Technologies, Santa Rosa, CA, USA), and a spectral analyzer (Anritsu MS9740A, Anritsu Corporation, Atsugi, Japan). These instruments provide detailed measurements, such as output power, pulse duration, and spectral properties, allowing for a thorough understanding and analysis of the performance. Through this designed experimental setup and precise measurement apparatus, we gained a profound understanding of the capabilities and performance of laser, further advancing our research and insights into photonic crystal applications in nonlinear optical devices and the high performance of PSM-SA. Within the confines of our experimental setup, we made a remarkable observation regarding the PSM-SA's behavior. It has been observed that the PSM-SA in this setup exhibits the ability to self-initiate Q-switched output characteristics when the output power is increased. In Figure 5a, we present the compelling time-domain pulse sequence captured from the laser system's output via the OC. This pulse sequence illustrates the dynamic evolution of the Q-switched pulses as the pumping power gradually escalates. On the horizontal axis we traverse through time, while the vertical axis portrays the pulse intensity. Figure 4: Schematic diagram of fiber laser system. As the pumping power progressively rises and reaches the threshold of 80 mW, the laser system seamlessly transitions into a Q-switched regime. In this regime, the laser generates distinct short, high-energy pulses with precisely defined time intervals between them. The phenomenon is indicative of the laser's newfound capability to produce pulsed output with remarkable temporal precision.