## 3 Results and Discussions  
The grating was first tuned to maximize the power of the laser output. The Q-switched 2.8 \(\upmu\)m laser was demonstrated, and the pulse was measured and shown in Figures 2 and 3. Figure 2 shows the output power as a function of pump power. At a pump power of 0.13 W, a 2.8 \(\upmu\)m laser is emitted and operated under the CW regime. As the pump power increases beyond 0.98 W, a stable Q-switching regime is achieved. Additionally, as the pump power further increases to 9.6 W, mode locking is realized. A pump power higher than 14.5 W brings about thermal damage to the SAM, and the maximum output power is measured to be 295 mW. The output power increases almost linearly with the pump power, and the slope efficiency is fitted to be 2.0%. At a pump power of 0.98 W, the Q-switched pulse appears first, with a repetition rate of 10.92 kHz and a pulse width of 4.6 \(\upmu\)s, as shown in Figure 3a. As the pump power increases, the Q-switching regime is stable until 9.6 W of pump power, corresponding to a repetition rate of 115.8 kHz and a pulse width of 1.3 \(\upmu\)s, shown in Figure 3b. At higher pump power, the intensity of pulse trains is more stable, and the single pulse envelope is cleaner. This change suggests the Q-switching is more stable, and the self-mode locking is restrained at higher pump power. At 9.6 W of pump power, the optical and RF spectrum of the laser is measured and shown in Figure 3c,d. The FWHM of the laser spectrum is 0.1 nm. A free-running 2.8 \(\upmu\)m CW laser spectrum is measured at about the same output power, which is 188 mW, and the FWHM is 0.45 nm. The compression of the spectrum is caused by the spectrum sensitively grating in the laser cavity. The signal-to-noise ratio (SNR) of  
Figure 1: Experimental setup for Q-switched wavelength-tunable fiber laser. L1, aspheric lens; L2, L3, L4, plano-convex lenses; DM, dichroic mirror; Ge, germanium plate. Figure 2: Output power as a function of pump power. CW, continuous wave; QS, Q-switching; ML, mode locking. the laser is about 35 dB, which is typical for stable passive Q-switching [16; 18; 26]. The laser cavity has SAM in one end and grating in another; thus, the reflectivity of the cavity is high. A high inter-cavity laser energy causes obvious side noise in the RF spectrum. Figure 4 shows the evolution of the output pulses as a function of the pump power, corresponding to the QS area of Figure 2. Figure 4a shows that as the pump power increases from 0.98 to 9.6 W, the repetition rate increases from 10.92 to 115.8 kHz, and the pulse width of the laser pulse decreases from 3.7 to 1.3 \(\upmu\)s. Figure 4b shows that within the same pump power range, the peak power increases from 0.27 to 1.27 W, and the pulse energy increases from 1.01 to 1.68 \(\upmu\). Continuous increases in the pump power lead to the mode-locking regime of the laser, as shown in Figure 5. The repetition rate of the mode-locking pulse is 32.77 MHz, which is determined by the effective cavity length. This configuration contains a 3-m-long fiber and a 15-cm-long external cavity. The refractive index of the fiber core is 1.483, according to the fiber manufacturer. Thus, the effective length is 4.6 m, corresponding  
Figure 3: Q-switched laser pulse trains and single pulse envelope at a pump power of (**a**) 0.98 W and (**b**) 9.6 W. (**c**) Optical spectrum of Q-switched laser and free running laser in linear scale at a pump power of 9.6 W. (**d**) RF spectrum of Q-switched laser at a pump power of 9.6 W, measured with a resolution bandwidth (RBW) of 30 Hz. to a theoretical repetition rate of 32.6 MHz given by \(c/2L_{\textit{eff}}\), where \(c\) is the speed of light and \(L_{\textit{eff}}\) is the effective length of the fiber cavity. The insert shows that the mode-locked pulses have a shorter rising edge than the falling edge. Before the pump power increases higher than 14.5 W, the Q-switched state can be brought back by decreasing the pump power, suggesting that the SAM is not damaged. The RF spectrum is measured and shown in Figure 5b.