设计了一种基于多模干涉滤波器和双折射滤波器的四波长可开关光纤激光器，理论分析了多模干涉滤波器和双折射滤波器及其级联结构的滤波特性.双折射滤波器能有效地抑制多模干涉滤波器的不规则边模.保偏掺铒光纤同时作为增益介质和滤波元件，优化多模光纤长度和保偏掺铒光纤长度保证了波长间隔的最佳匹配，级联滤波器的滤波周期为4.89 nm.调节偏振控制器，光纤激光器实现了单波长、双波长、三波长以及四波长可开关激光输出，输出激光的边模抑制比均大于40 dB，1 h内波长波动均小于0.1 nm.

In order to realize the switchable multiwavelength oscillation, a fiber ring laser based on a multimode interference filter and a birefringent filter was proposed and demonstrated. The filtering characteristic of the cascaded fiber filters was analyzed theoretically, and the irregular side modes of the multimode interference filter can be effectively restrained by cascading birefringent filter. The Polarization Maintaining Erbium Doped Fiber (PMEDF) is utilized to serve as gain medium and a part of birefringent filter simultaneously, the length of the multimode fiber and PMEDF are optimized to match the same wavelength spacing, the filtering period of the cascaded fiber filter is 4.89 nm. The laser could switch to output single, dual, triple and quadruplewavelength by finely adjusting polarization controller in the experiment, sidemode suppression ratios of lasing wavelengths are more than 40dB, and wavelength fluctuations are less than 0.1 nm during a period of 1h.