Research Of 2μm Mode-Locked Fiber Laser Based On 45° Grating And Saturable Absorber

Pulsed fiber laser sources operating at 2 μm have attracted more and more attention owing to their wide applications in laser ranging, laser radar, industrial production, laser snuggery etc. Fiber lasers have many advantages such as simple structure, convenient integration, and high beam quality compared with traditional lasers. Especially, the passively mode-locked fiber laser capable of generating ultra-short pulses has been the important developing direction of 2 μm fiber laser. In this dissertation, we achieved a real all-fiber passively mode-locked fiber laser operating at 2 μm based on 45° tilted fiber Bragg grating as a polarizer and a carbon nanotubes as saturable absorber(SA), which can be divided into the following three sections:(1) The pulse transmission equation in fiber was derivated based on the nonlinear Schrodinger equation. Transfer function of the mode-locked fiber laser based on NPR structure was solved by using Jones Transfer Matrix method. The numerical model of passively mode-locked fiber laser based on a hybrid structure was constructed, and the pulse evolution was numerically simulated.(2) First, a 45° titled fiber Bragg grating(TFG) was UV inscribed with a polarization dependent loss of >12 d B at 1850-2150 nm. An all-fiber 2 μm passively mode-locked fiber laser with a 45° TFG based NPR structure was designed and built. Stable soliton pulses centered at 1992.7 nm were produced at a repetition rate of 1.902 MHz with pulse duration of 2.2 ps and signal noise ratio of 60.5 d B. Moreover, the laser can also switch to noise-like regime by increasing the pump power. Stable noise-like pulses centered at 1994.2 nm were obtained with a signal noise ratio of 47.3 d B. In order to show the broad bandwidth advantage of 45° TFG, the Tm3+-doped fiber in our experiment was used with different lengths. Mode-locked pulses centered at the range of 1971.8-2051.3 nm were achieved. The characteristic parameters of carbon nanotubes were tested. Then, a hybrid structure that combines a 45° TFG based NPR structure and a carbon nanotubes was used to achieve 2 um passively mode-locked fiber laser. Stable self-starting soliton pulses centered at 1983.7 nm were produced at a repetition rate of 22.5 MHz with pulse duration of 1.4 ps and signal noise ratio of 65 d B. Compared to the NPR structure, the hybrid structure has the advantages of high repetition rate and the soliton pulses can be self-started. Lastly, a wavelength switchable mode-locked fiber laser using an array of fiber Bragg gratings(FBGs) and the above hybrid structure was designed and built. The output pulses were switchable at three central wavelengths of 1969.75 nm, 1956.6 nm and 2022.15 nm.(3) Based on the transmission equation, the numerical model of 2 μm dark soliton fiber laser was constructed, and the pulse evolution was numerically simulated. Experimentally,the performance of a hybrid structure mode-locked fiber laser was investigated at normal dispersion regime by using a 10.0 m Er3+-doped fiber as a dispersion compensated fiber. However, different from the theoretical simulation, a stable dissipative soliton pulses instead of dark soliton pulses centered were obtained with a repetition rate of 10.9 MHz and a signal noise ratio of 61 d B. The measured center wavelength and FWHM were 1887.5 nm and 5.77 nm, respectively.