Switchable Single-longitudinal-mode Thulium-doped Fiber Laser Based On FP Fiber Grating Filter

The thulium-doped fiber laser(TDFL)has excellent stability and flexible structure,and works at the 2μm band that is safe for the human eyes,and the single longitudinal mode(SLM)TDFL can be used in military,medicine,radar optical communications,sensor systems and many other fields,wavelength-switchable fiber lasers are widely used in wave division multiplex systems,sensor networks and optical communication networks due to their tunability and flexibility of output laser wavelength.TDFL based on multi-channel Fabry-Perot Bragg fiber grating(FP-FBG)filter has the advantages of low loss,good beam quality,and easy connection with optical fiber systems.This paper conducts experimental research and results analysis on wavelength-switchable SLM TDFLs with different structures based on the multi-channel FP-FBG filter.The main innovations and results of this paper are as follows:(1)The characteristics of fiber bragg grating(FBG)and FP-FBG were analyzed and simulated theoretically by the coupled mode theory and the transmission matrix method.The transmission characteristics of fiber grating with different parameters were discussed.The simulation showed that the transmission spectrum of the FP-FBG filter had three narrow-band filtering channels with center wavelengths of 1939.91 nm,1940nm and 1940.9nm,and corresponding 3d B bandwidths of about 0.015 nm,0.002 nm and0.015 nm.According to the simulation,the FBG and FP-FBG filters were fabricated,The center wavelength of the FBG obtained in the experiment was 1941.45 nm,the 3d B bandwidth was 0.11 nm,and the grating reflectivity was about 97%;the transmission spectrum of the FP-FBG filter had three center wavelengths of 1941.48 nm,1941.57 nm and 1941.65 nm.The 3d B bandwidths were 0.06 nm,0.054 nm and 0.066 nm,respectively.It laid the foundation for the subsequent design of multi-wavelength,singlelongitudinal-mode thulium-doped fiber laser.(2)A three-wavelength-switchable SLM TDFL based on the saturable absorber,FBG and FP-FBG filter was proposed.A section of passive thulium-doped fiber(TDF)was used as a saturable absorber to reduce the gain bandwidth,a three-channel narrow-band FP-FBG filter ensured the operation of the SLM laser in the cavity,and a FBG was used to switch the output laser wavelength.When the pump source power was2.7W at room temperature,the laser wavelengths were 1941.48 nm,1941.57 nm and1941.65 nm,with 61 d B,62.d B and 62 d B optical signal to noise ratio(OSNR),respectively.The threshold power was 2.56 W.In the continuous 50 min stability measurement,the fluctuations of the TDFL output power were less than 0.87 d B,0.91 d B and 0.88 d B,respectively,and the wavelength fluctuations were less than 0.01 nm,respectively,which were less than the minimum resolution of the spectrometer 0.05 nm.At the same time,the measurement verified that the output laser worked at single longitudinal mode operation.(3)A three-wavelength-switchable SLM TDFL based on FBG,FP-FBG filter and8-shape sub-cavity was proposed.The longitudinal mode spacing was enlarged by the sub-cavity.The FP-FBG filter can select and narrow the optical signal in the cavity,which can ensure the single-frequency operation.The FBG was used to select the output wavelength of the TDFL.When the pump source power was 2.34 W at room temperature,the laser wavelengths were 1941.48 nm,1941.57 nm and 1941.65 nm,with61d B,61 d B and 60 d B OSNR respectively.The threshold power was 1.68 W.In the continuous 50 min stability measurement,the fluctuations of the TDFL output power were less than 0.39 d B,0.61 d B and 0.55 d B,respectively,and the wavelength fluctuations were less than 0.01 nm,respectively,which were less than the minimum resolution of the spectrometer 0.05 nm.At the same time,the measurement verified that the output laser worked at single longitudinal mode operation.