# Wavelength-Switchable Ytterbium-Doped Fiber Laser Based on All-Fiber Lyot Interferometer Filter
## 3 Experimental Results
Secondly, for the proposed YDFL, a triple-wavelength laser output was obtained in the experiment through changing the intra-cavity gain and loss. In Figure 5a, three different sets of triple-wavelength lasers  
Figure 3: Wavelength-switchable single-wavelength generation: (**a**) single-wavelength laser spectrum; (**b**) power fluctuations for different single-wavelength lasers. Figure 2: Laser generation and comb filter spectrum: (**a**) laser output without filter; (**b**) Lyot comb spectrum with 2.5 m PMF; (**c**) Lyot comb spectrum with 1 m PMF; (**d**) frequency spectrum. are illustrated under the Lyot comb filter action, and there are no other obvious laser modes. The triple-wavelength laser output can be generated from 1068.31 to 1080.23 nm, and the SNR is more than 33.801 dB, as shown in Figure 5b. Ultimately, a quadruple-wavelength laser could be realized in the experiment, as shown in Figure 6a. The center wavelengths are 1071.06, 1073.65, 1076.32, and 1078.92 nm, respectively; the SNR is more than 33.031 dB; and the laser power difference is less than 3.067 dB. Three different sets of quadruple-wavelength lasers could be obtained by adjusting the PC. As shown in Figure 6b, samples 1, 2, and 3 demonstrated that quadruple-wavelength-switchable laser output was achieved, and the SNRs were more than 33.031, 26.233, and 29.014 dB, respectively. In addition, a quadruple wavelength from 1068.95 to 1081.19 nm was generated with a minimum wavelength interval of 2.23 nm and an SNR exceeding 26.233 dB. It can be seen that the SNR decreases gradually with the increase in the number of wavelengths by comparing the double-, triple-, and quadruple-wavelength laser SNRs. With the increase in laser generation wavelength, the laser signals of different modes are superimposed, and the corresponding background noise is gradually enhanced. In addition, the nonuniformity of the laser power corresponding to different wavelengths leads to the suppression of other modes with weaker power wavelengths, thus affecting the overall signal-to-noise ratio. In the experiment, the tunable range of laser generation position and wavelength intervals for the dual-, triple-, and quadruple-wavelength lasers was not random because the designed laser generates the interference spectrum through the Lyot filtering structure and then realizes the wavelength-tunable laser output. The generated laser wavelength is located in the place where the interference spectrum gain is strong, and lasers cannot be generated at locations with low-gain interference spectra. Through the above experiments and analysis, the switching of the multi-wavelength laser output confirmed that the designed filter is crucial for tuning the YDFL laser wavelength. Figure 4: Wavelength-switchable double-wavelength generation: (**a**) double-wavelength laser generation; (**b**) double-wavelength laser spectrum analysis. Finally, the output power stability of the proposed YDFL was tested in the experiment. Since it was difficult to obtain a stable quadruple-wavelength laser output, the spectral stability of the single-, double-, and triple-wavelength lasers was tested. For the 1078 nm single-wavelength laser generation, when the pump power was 150 mW, its stability was monitored every 5 min over a 25-min period. As shown in Figure 7a, the spectrum remained stable with no significant wavelength drift or generation of strong laser mode jumping, and its power fluctuation was less than 0.574 dB, as illustrated in Figure 7b. Under the same experimental conditions, the wavelength stability of the 1071 and 1074 nm double-wavelength laser outputs was tested. As shown in Figure 7c, there was no substantial wavelength shift or extra laser mode observed, and the power variations were less than 0.631 and 0.37 dB, as shown in Figure 7d. Similarly, the 1071, 1073, and 1076 nm triple-wavelength laser spectrum stability was tested in the experiment at the same pump power. As shown in Figure 7e, during the monitoring time, no significant wavelength fluctuation was detected.