Narrow linewidth single frequency fiber lasers have potential application i wavelength division multiplexing (DWDM) communications, coherent laser radar, high resolution spectrum analysis and various fiber optical sensors. In recent years, the importance of the laser’s Coherent length in some application such as rangefinder has attracted much attention to narrow linewidth fiber lasers. By decreasing the laser output linewidth, the coherent length of the laser increases. In many applications such as rangefinder a laser with linewidth in the order of kHz is needed. The simplest way to restrict the number of modes is to include a bandwidth-limiting element in the cavity. One convenient element is using fiber bragg grating, however the price of the fiber bragg grating is too expensive. Recently Fabry-Perot fiber laser with short cavity length is demonstrated as narrow linewidth fiber laser. Due to their short length, these lasers suffer from a low pump absorption and hence low slope efficiency. Alternatively narrow linewidth fiber laser can be obtained using travelling wave rings. They have potentially high slope efficiencies. Owing to the long cavity lengths, they also give narrow linewidths. Unfortunately they are susceptible to mode hopping. To suppress mode hopping, narrow linewidth filters were used. It has been shown that standing wave saturation effects in un-pumped erbium doped fiber can be used to establish narrow band reflection filters. More recently an un-pumped PM erbium doped fiber was incorporated travelling wave ring as a saturable absorber to establish linewidth narrowing. In this research, we investigate different method and based on the simulation results and the available components a proper method is selected to implement a narrow linewidth fiber laser. In this method Compound sub-rings because of simple design and high efficiency as a filter for decreasing the linewidth is used. The effect of different parameters such as the length of active medium, pumped power, doping concentration,..on the linewidth and operation of the laser analyzed. Then the measurement results is presented and compared with simulation results. Finally our implemented fiber laser with sub-rings has a linewidth of less than 1kHz. Keywords: Fiber Laser, Narrow Linewidth, Compound Ring, Travelling Wave
