窄线宽全光纤环形滤波器特性研究

Narrow Linewidth All-Fiber Ring Filters

设计了两种用于单纵模光纤激光器的窄线宽全光纤环滤波器,其由超低反光纤光栅和1个(或2个)2×2光纤耦合器构成。从理论上推导出两种滤波器的反射率表达式,并且对其反射率、窄线宽和群时延特性进行了详细的数值分析和实验验证。研究结果表明:由超低反光纤光栅和光纤耦合器构成的单环滤波器具有高反射率、超窄线宽和大群时延特性,表现出良好的慢光效应。双环滤波器除了保持了以上良好特性,还因游标效应,共振幅度可调,相对单环滤波器,具有更大的有效自由光谱范围(FSR)、更大的边模抑制比(SMSR)以及不易跳模等特点。所研究的光纤环形滤波器具有窄线宽、结构紧凑和低成本等优点,可用作光纤激光器的反射腔镜来进一步压窄激光器线宽,尤其是当双环滤波器用作腔镜时,激光器的线宽更窄,模式更稳定。

Two narrow-linewidth filters,consisting of an ultra-low-reflectivity fiber Bragg grating（LRFBG）and one（or two）2×2 fiber couplers,are designed for single-longitudinal-mode fiber lasers.The reflectivity expressions of the two filters are deduced theoretically.The characteristics of the two filters,such as reflectivity,narrow linewidth and group delay,are numerically analyzed and experimentally verified.The two fiber filters have the characteristics of high reflectivity,ultra-narrow linewidth,large group delay and good slow-light effect.What＇s more,in the case of the double-ring filter,the amplitudes of resonant peaks are modulated because of the Vernier effect.Therefore,the effective free spectral range（FSR）and side mode suppression ratio（SMSR）of the double-ring filter are larger than those of the single-ring filter,which means the double-ring filter is hardly mode hopping.Because of the advantages of narrow linewidth,compact structure and low cost,both of the designed filters can be used for fiber laser as the cavity mirror to narrow the linewidth of laser further.And the results show the linewidth of the laser with double-ring filter is narrower than that of the laser with single-ring filter.