# Widely Wavelength-Tunable High Power Single-Longitudinal-Mode Fiber Laser in Mid-Infrared Waveband
## 3 Results  
In Case 1, most of the energy was transmitted out of the cavity due to the low reflection of the right fiber end-face, and consequently the cavity had a large loss and the lasing threshold for the pump light was as high as 2.7 W (measured from the pigtail fiber of the laser diode, the same as all of the pump power mentioned below). Once the pump power exceeded the threshold, the laser worked in the state of free operation. The spectrum of the output laser is shown in Figure 2 and the central wavelength could be tuned from 2766 nm towards a longer wavelength (2789 nm) when the pump power was increased to 3.6 W, which is mainly caused by the gain competition in the gain spectrum of the Er\({}^{3+}\) ZBLAN fiber. Then, the central wavelength remained basically constant at 2789 nm when the pump power was further increased. Figure 1: Experimental setup of the laser cavity. The measured result of the output power in Case 1 is plotted in Figure 3 (gray line). In Case 2, the obvious improvement on the output power shown in Figure 3 (red line) is easy to understand. The laser transmitted out of the cavity through the right fiber end-face was collimated by L3, then incident on the grating at the blazed angle and finally reflected back into the fiber by the grating at the first order diffraction. As a result, the total output power was greatly improved and could reach 450 mW at the pump power of 5 W.  
When the pump power was fixed at 3.6 W, the beating RF spectra in both cases were determined and are shown in Figure 4. If the laser is operated in a multi-longitudinal-mode (MLM) state, different longitudinal modes will interfere with each other and generate beat frequency signals in the RF domain. It is worthwhile noting that in Case 1 the laser operates in an MLM state and the longitudinal mode interval is 69.3 MHz, which is matched well with the cavity length as shown in Figure 4a. In contrast, no beat frequency signal was observed in Case 2 as shown in Figure 4b. When a beam blocker (BT620, Thorlabs, NJ, USA) was inserted between L3 and the grating, beat frequency signals were observed again in the RF spectrum. However, once the beam blocker was removed, the beat frequency signals became undetectable immediately, indicating that the operation regime in Figure 4b could be rebuilt very quickly. Figure 4c shows the temporal dependence of the RF spectrum in Case 2 every 5 min. No beat frequency signal was observed during 30 min and the RF spectrum maintained a strong stability. Figure 5a shows the detailed output spectra at the central wavelength of about 2784.5 nm in both cases when the pump power was fixed at 3.6 W. The bandwidth of the spectrum in Case 2 was obviously narrower than that in Case 1. Consequently, based on the variations in the beat frequency signal and the bandwidth of the spectrum, SLM  
Figure 3: Output power in the two cases. Pump power is measured from the pigtail fiber of the laser diode. Figure 2: Output spectrum at different pump powers in Case 1.  
operation was achieved in Case 2. By rotating the grating, the lasing wavelength could be continuously tuned over 100 nm from 2705 nm to 2806 nm with SLM operation maintained, as shown in Figure 5b. It is worth noting that the SLM operation occurs only when the distance between the grating and the right fiber end-face is larger than 19 cm as shown in Figure 6. This is because when the distance is small, many different wavelength components reflected by the grating will be collected by the fiber and consequently beating frequency signals will be generated at the RF spectrum. When the distance is large, the fiber will collect only the reflected light of narrow bandwidth around one specific wavelength, which is crucial for the SLM operation in this laser. Figure 4: Beating spectra at RF with a 3.6 W pump. (**a**) Case 1 and (**b**) Case 2. (**c**) Stability of the RF spectrum in Case 2 during 30 min. Figure 5: (**a**) The output spectra around 2784.5 nm in Case 1 and Case 2.