Research Of 2?m Tunable Fiber Laser Based On Biconical Taper Filter

Tunable fiber laser can replace multiple single-wavelength fiber lasers and become an ideal light source for dense wavelength division multiplexing system due to its simple structure and flexible wavelength.Recent years,tunable lasers at 2?m have become a research hotspot for their unique advantages in laser medicine,radar,environmental monitoring,non-metal processing and human eye safety compared with short wavelength tunable laser sources?1?m,1.5?m?.However,traditional filters have many disadvantages such as high cost,high insertion loss and are difficult to achieve all-fiber laser.Considering the above problems,the filter and gain fiber length of 2?m tunable fiber laser are optimized,and the tunable characteristics are studied.Firstly,the research status of the biconical filter in fiber lasers and the tuning methods of 2?m fiber lasers are presented,then the tuning principle and wavelength range are summarized.On this basis,a continuous and pulsed fiber laser system with a1550 nm wavelength laser as the pump source,a thulium/holmium co-doped fiber as the gain medium and a self-made biconical fiber filter as the tunable filter is established.Secondly,according to the design scheme,the pump scheme and length of thulium/holmium co-doped fiber are optimized.Based on the ³H₆-³F₄ pump scheme,the rate equation model of the thulium/holmium co-doped fiber laser is established.In different doping concentrations,the transmission characteristics of pump light and signal light along the thulium/holmium co-doped fiber are studied under this model.The results show that with a certain doping concentration,the optimal gain length is 2.5 m when the theoretical analysis results and intracavity loss are considered.Subsequently,a biconical Mach-Zehnder tunable fiber filter is designed and fabricated.Rsoft software is applied to simulate the field distribution of taper fiber.The results show that with the decrease of beam waist diameter,the excited high-order mode increases,but the loss of taper fiber increases.In order to obtain the biconical filter with low loss and more modes,the motor speed and discharge intensity of the taper are determined to be 250 and 120 after repeated experiments.Temperature tuning and strain tuning methods are designed according to the thermo-optic effect and elastic-optic effect of fiber.Finally,a 2?m tunable thulium/holmium co-doped fiber laser based on biconical filter is realized.In order to optimize the output characteristics of 2?m tunable laser,couplers with splitting ratio of 50:50,70:30 and 90:10 are selected to obtain continuous thulium/holmium co-doped fiber lasers with threshold pump power of 450 mW,365mW and 330 mW and slope efficiency of 3.2%,2.6%and 1.2%.The optimal splitting ratio is selected as 90:10,and biconical filters with different waist diameter are introduced into the resonator cavity,under the temperature tuning method,the wavelength tuning range of the laser is obtained as 2.6 nm,2.92 nm,5.14 nm and 3.58nm.When the strain increases,the output wavelength moves towards the short-wavelength direction and the tuning effect is better.The tuning range of the laser is 18.08 nm,20.86 nm,21.64 nm,25.46 nm and 14.64 nm,respectively.The waist diameter of the fiber beam corresponding to the maximum tuning range is 60?m.A biconical filter with a beam waist diameter of 60?m is introduced into a figure-9 cavity structure,and a tunable mode-locked thulium/holmium co-doped fiber laser is realized by using a strain tuning method.The wavelength tuning range and full width at half maximum of the spectrum are 1882.02 nm-1905.96 nm?23.94 nm?and 1.24 nm-1.34 nm,respectively.Pulse period,repetition frequency,maximum output power,pulse energy and signal-to-noise ratio are 2.658?s,372 kHz,36.3 mW,97.4 nJ and⁴0 dB,respectively.Compared with the current 2?m tunable fiber laser based on biconical filter,the results show that the wavelength tuning range is improved in both pulse and continuous operation,and the experimental parameters are analyzed in detail,which is helpful to the wide application of 2?m tunable fiber laser.