Theoretical And Experimental Study Of Thulium-doped Pulsed Fiber Lasers At2μm Based On SESAMs

Compared with conventional solid-state lasers, fiber lasers have many advantagessuch as convenient thermal management, high single-pass gain, and excellent beamquality. In the past decades, intensive research works on mode-locked fiber lasers hadbeen focused on the wavelength of1um and1.55um, which are generated in Yb3+dopedfiber lasers and Er3+doped fiber lasers, respectively. Due to the important applicationsin national defense, medical, atmosphere monitoring, range finding and remote sensing,thulium-doped fiber lasers (TDFL) have attracted much attention recently. Firstly, given2μm laser as the pump source, through nonlinear frequency conversion, it can beconverted to3-5um, which is crucial and valuable in the national defense. What’s more,it is another window for atmosphere, and it is in accordance with the wheeling andvibrating energy level and the spectrum of characteristic fingerprint of many molecules,and has many potential application such as in physics, chemical, and materials.Secondly, the gain bandwidth of thulium ions in the silica fiber exceeds300nm(~1.7-2.1um), which is appropriate for generation of high-peak-power ultra-short lasersbased on the mode-locking technique. Finally, lasers operating at2μm can be used inlidar. Due to the advantages such as high stability and the ability to achieve self-startingpulses, mode-locked fiber lasers based on SESAMs are intensively investigated. In thisthesis, we investigate the characteristics of Q-switched and mode-locked thulium dopedfiber lasers both theoretically and experimentally.1. Theoretically, we introduce two equations that are often used in mode-lockedfiber lasers, and show some simulation results in thulium doped fiber lasers throughsolving these equations using the split step Fourier method. It is shown in the thesis thatsingle pulse、multiple pulses、dissipative solitons in normal dispersion regime andvarious bound-state solitons can be achieved in TDFL. Furthermore, it is proved in thesimulation that the multiple pulses are generated either from pulse splitting or dispersivewave shaping. In order to decrease the influence of spectrum sideband, the cavity lengthshould be shortened.2. For experimental study, first, a linear cavity configuration is built up. A LD at 792nm with a pigtail as the output coupler is used as the pump source, and asemiconductor saturable absorber is used as the main initiation equipment,characteristics of Q-switched TDFL is investigated. It is revealed that, with the increaseof the pump power, pulse duration is decreasing, the repetition, as the same as the pulseenergy, is increased.3. Without any dispersion compensation element in the cavity, equivalent tooperating in the all-anomalous regime, by using a SESAMs with modulation depth of26%, a pulse train with a repetition rate of11.5MHz, and the pulse width of800ps ismeasured by an autocorrelator, pulse energy of54nJ, and central wavelength of2025nmwith a full width of half maximum (FWHM) of5nm, and average power of800mW isobtained. By shortening the cavity length, only Q-switched mode-locking pulses areobtained, so the selection of cavity length is decisive for pulse shape. The soliton weobtained has four characteristics, high pulse energy, narrow spectrum width, broad pulsewidth, and large chirp. To the best of our knowledge, this is the largest single pulseenergy obtained in passively mode-locked thulium-doped fiber oscillators withoutdispersion management.4. In order to optimize the experimental results, a segment of dispersioncompensation fiber is added in the cavity, and in this circumstances, we obtained a pulsetrain of repetition of7.85MHz, and average power of1.1W, corresponding to a singlepulse energy of140nJ, which theoretically support a transform-limited pulse of580fs inpulse width. To the best of our knowledge, the pulse we obtained has the largest singlepulse energy in passively mode-locked thulium-doped fiber oscillators.