Design Of Single Longitudinal Mode Erbium-doped Fiber Laser Based On Saturable Absorber

The central wavelength of erbium-doped fiber lasers is safe for the human eye and has a wide bandwidth.It is also in the low-loss window band of quartz glass.It is widely used in optical fiber communication,atmospheric measurement,optical fiber sensing and other fields.At the same time,with the increasing requirements for measurement accuracy and radar resolution,narrow linewidth single longitudinal mode erbium-doped fiber lasers have received extensive attention.The survey found that a variety of narrow linewidth fiber lasers have been obtained at home and abroad,but there are still many problems.For example,in the process of designing and building a single longitudinal mode fiber laser,there are few studies on the influence of the saturable absorber fiber parameters on the reflectivity of the main longitudinal mode of the saturable absorber.In addition,most of the linewidth optimization structures commonly used in fiber lasers are complex and technically difficult.Therefore,this paper explores the influence of the saturable absorber on the reflectivity of its main longitudinal mode through numerical simulation,so as to guide the design of the single longitudinal mode fiber laser,and find a simple and easy-to-implement linewidth optimization structure.Firstly,by comparing the advantages and disadvantages of different resonator structures,the composite ring cavity is determined as the basic resonator structure of the designed fiber laser.By comparing different mode selection methods,it is determined that the mode selection package is composed of the saturable absorber followed by the fiber Bragg grating.Second,the laser mode field radius is defined by introducing the second type of Petermann mode field radius.The effects of device parameters such as filter reflectivity,output loss,erbium-doped fiber length,and erbium-doped concentration on the output characteristics of the fiber laser and the reflectivity of the main longitudinal mode of the saturable absorber are simulated and analyzed.The simulation results show that when the pump power is constant,the higher the overall reflectivity of the filter,the smaller the output loss,and the greater the output optical power.At the same time,the shorter the length of the erbium-doped fiber as the saturable absorber and the lower the erbium-doped concentration,the greater the reflectivity of its main longitudinal mode.Then,a single longitudinal mode composite ring erbium-doped fiber laser with a center wavelength of 1550±5nm,a wavelength drift of less than ±0.01 nm,and a linewidth of less than3 k Hz can be built stably.The single-longitudinal model nature of the operation was verified by the optical heterodyne measurement system,and the causes of the output optical power loss were analyzed.Through the analysis,it can be seen that the bending loss of the optical fiber has a greater impact on the output power of the fiber laser,and the smaller the radius of curvature of the fiber,the higher the loss caused by the bending of the fiber.Finally,the linewidth of the fiber laser was measured and optimized.A delay self-heterodyne linewidth measurement system is constructed by using acousto-optic frequency shifter,delay optical fiber,balance detector and spectrum analyzer.The linewidth of the fiber laser in single longitudinal mode is 1.782 k Hz measured by the spectrum analyzer.An optimized structure of optical self-injection dual-cavity feedback linewidth was built by a coupler and two circulators of the same type,and the optimized linewidth was measured to be 1.319 k Hz.The experimental results show that the optical self-injection dual-cavity feedback linewidth optimization structure can effectively narrow the linewidth,the optimization efficiency can attained to 26%.