# Wavelength-Switchable Ytterbium-Doped Fiber Laser Based on All-Fiber Lyot Interferometer Filter
## 3 Experimental Results  
After completing the laser structure design, we set up the YDFL experimental system. In the experiment, the gain-medium YDF length was 4 m, and we tested the laser spectra of the ring-cavity YDFL without the proposed Lyot filter first, as shown in Figure 2a. It can be observed that the laser generated by self-excitation is around 1036 nm, and it is difficult to achieve certain ranges of wavelength-switchable laser generation due to the large number of modes in the resonator. The Lyot filter was manufactured and tested in the experiment. The 2.5 m long PMF and one PC with a single-mode pigtail fiber were connected, and the Lyot transmission spectrum is shown in Figure 2b. The mode interferometer was successfully realized in the YDF gain spectrum range, and the local spectrum details are magnified in the embedded picture, showing an FSR of 2.2 nm. In the experiment, the interference spectra of PMFs with different lengths were measured and analyzed. When 1 m long PMFs were used in the Lyot filter, the FSR was around 6.4 nm. (The interference spectrum is shown in Figure 2c). It is obvious that the FSR of the interference lines increases with the decrease in PMF length. In order to obtain as dense a filtering effect as possible, the PMF length was finally selected to be 2.5 m. Fourier transformation was carried out on the interference spectrum to analyze its frequency component information, and the low-frequency noise influence could be reduced by extracting the high-frequency component. The interference spectrum signals of the 2.5 and 1 m long PMFs were determined through Fourier analysis to obtain the respective frequency diagrams. As shown in Figure 2d, the main frequencies corresponding to PMF lengths of 2.5 m and 1 m were 0.42 and 0.19, respectively; obviously, the comb spectrum of the 2.5 m long PMF has a larger high-frequency component, and there are no obvious components in other positions. Subsequently, a YDFL with a Lyot filter was constructed and tested in the experiment; it had a gain medium length of 4 m, and the laser working threshold was 54 mW. As the pump power was increased to 150 mW, the single-wavelength laser switching capability was examined and analyzed. Through adjusting the PC, different single-wavelength lasers were realized, as illustrated in Figure 3a. The single-wavelength laser could be tuned in the range from 1073.76 to 1086.78 nm by adjusting the PC with a signal-to-noise ratio (SNR) of 41.283 dB and a 3 dB linewidth of 0.3 nm. As shown in Figure 3a, there were no excess longitudinal modes near the laser; moreover, the comb filtering spectrum can be observed also, and the laser was generated at the top position of the comb spectrum, thereby validating the effectiveness of the designed Lyot filtering structure. Figure 3a illustrates that the proposed YDFL has a 13.02 nm single-wavelength tuning range with a 2.23 nm minimum tuning interval. During the single-wavelength switching procedure, the maximum power drift was less than 1.959 dB; its fluctuation curve is shown in Figure 3b. Next, the laser's ability to generate a multi-wavelength laser under the same pumping light was tested and analyzed. The gain and loss in the laser cavity can be changed by adjusting the polarization controller; as the gain intensity changes, certain modes that achieve the gain condition will be excited, enabling multi-wavelength laser output. In the experiment, the YDFL also achieved double-, triple-, and quadruple-wavelength laser generation with adjustment to the PC. Firstly, double-wavelength laser outputs with four different wavelength groups could be realized, as shown in Figure 4a. Other wavelengths did not appear during the switching process, and the filtering effect of the comb spectra can be observed. The double-wavelength laser could be tuned from 1071.64 to 1081.65 nm, as shown in Figure 4b, with minimum and maximum wavelength intervals of 3.02 and 5.73 nm, respectively, and the SNR for these outputs was above 41.157 dB.