# Wavelength-Switchable Ytterbium-Doped Fiber Laser Based on All-Fiber Lyot Interferometer Filter
## 2 Experimental Setup and Procedures  
Figure 1 presents the proposed wavelength-switchable YDFL setup, which features a ring-cavity configuration. The laser system includes a laser-diode (LD) pump source with a 915 nm center wavelength, one segment of ytterbium-doped fiber (YDF; SM-YSF, Nufern Co. East Granby, CT, USA) used as the YDFL gain medium, one 915/1060 wavelength-division multiplexer (WDM), two optical couplers (OCs), two polarization controllers (PCs), and polarization-maintaining fiber (PMF). For the proposed YDFL, a central wavelength of 915 nm LD (JDSU Co. Milpitas, CA, USA) was selected for the system. The OC1 has a splitting ratio of 50:50, and two ends of the coupler are fused together to constitute a laser sub-cavity structure. A sub-cavity is used as a resonator that allows a specific wavelength to resonate inside it; both the sub-cavity and the main cavity affect the laser wavelength, and some light waves that meet the specific matching conditions can achieve a stable output. The PC1 and PMF (PM1060, Panda-Type, Nufern Co.) constitute the Lyot filter, and the PMF birefringence effect causes light on the fast and slow axes to experience different phase delays during transmission. This difference in phase delay causes the two beams to have an interference effect when they are recombined. When two beams recombine, because they experience different phase delays, there will be constructive interference at some wavelengths and destructive interference at others. As a result, specific light wavelengths are enhanced while other wavelengths are weakened. Therefore, the Lyot filter is designed in the sub-cavity, such that a better laser filtering effect can be achieved under the hybrid action. The OC2 is used to connect an optical spectrum analyzer (OSA; AQ6370, Yokogawa Co. Tokyo, Japan). The PMF, shown in Figure 1, is a single-mode fiber with an 8.5/125 \(\upmu\)m core/cladding diameter.  
The Lyot filter transmission characteristics can be analyzed by the Jones matrix. The output transmission function can be expressed as follows:  
\[T=\frac{1}{2}\cos^{2}(\frac{\pi\Delta n}{\lambda}L)(1+\sin 2\theta) \tag{1}\]  
Figure 1: Schematic diagram of proposed YDFL.  
where \(\theta\) is the angle between the input light polarization direction and the PMF fast axis, and \(\theta\) can be tuned through adjusting the PCs; \(\Delta n\) is the refractive index difference between the fast axis and the slow axis of the PMF; \(\lambda\) is the central wavelength; and \(L\) is the length of the PMF. When \(\lambda\) and \(L\) are fixed, the transmission changes depend on \(\theta\). Therefore, the transmission spectrum of the Lyot filter can be adjusted by the PCs. The free spectrum range (\(FSR\)) of the Lyot filter is determined by Equation (2); the FSR of the Lyot filter is inversely proportional to \(\Delta n\) and the length of the PMF.  
\[FSR=\lambda^{2}/\Delta nL \tag{2}\]  
The designed Lyot filter can produce a comb spectrum, which is inserted into the laser cavity to adjust the YDFL intra-cavity gain and loss. The Lyot filter is used to select the specific wavelength of the laser output. When the wavelength selectivity of the filter changes, the feedback characteristics of the entire laser cavity also change, resulting in a change in the wavelength of the laser output. Therefore, the output wavelength of the fiber laser can be precisely controlled and tuned by adjusting the interference spectrum intensity of the designed interference filter.