Study On Injection-Locked Thulium-Doped Laser

2μm solid-state laser has been an important choice for the light source of Coherent Doppler Lidar and Differential Absorption Lidar, due to their characteristics of eye-safe, low atmospheric extinction ratio, small size and so on. With the application of injection locking technology to solid-state laser, the laser can be operated with single longitudinal mode(SLM), narrow linewidth, high frequency stability, sufficient pulse energy and long pulse width output simultaneously, which could meet the requirements for lidar system. Injection locked laser contains master laser, slave laser and servo system. View of this condition and background, this dissertation intends to make efforts on the research of Tm³⁺-doped injection locked solid-state laser theoretically and experimentally, and hopefully we can make some contribution on the development and the application of pulsed SLM 2μm solid-state lasers.In theory, the Tm³⁺-doped quasi-three-level rate equation is established using the transition energy level structure of Tm³⁺ laser medium and the Boltzmann distribution of particles. Based on the continuous wave operation rate equation, optimum length and doping concentration of the laser medium and the best cavity output mirror transmissivity are calculated, and the effect of doping concentration of particles to the laser output performance is discussed. The laser relaxation oscillation frequency is also calculated under continuous and long pulse pumping. The results show that Tm:YAG crystal is more suitable for Q-switched operation under kHz repetition rate. According to the Q-switched operation rate equation using pulsed laser-diode(LD) pumping, the build-up time of laser pulse, the output pulse width and the energy extraction efficiency are discussed. Using the classical theory model of injection locking, injection field, noise field and locking bandwidth are analyzed. The minimum injection locking power is calculated. The relationship between the injected laser intensity and the maximum detuning frequency of the injection locking laser is discussed. The disadvantageous factors for extracted signal about injection locking are analyzed. Good matches between horizontal mode and vertical mode are helpful to achieve extracted locking signal effectively and to avoid the generation of false signals.In experiment, the absorption spectra, fluoresce spectra and fluorescence lifetime of Tm:YAG and Tm:LuAG are measured. That is helpful to select the laser medium and pump source, to explain the experimental phenomena and to provide a premise analysis for the particle level transition. For the master laser, by inserting two F-P etalons to the cavity, stable, single longitudinal mode and tunable laser output was realized. Furthermore, experiments are carried out to improve the output power of SLM linear-polarized laser laser. For the slave laser, the output laser characteristics are optimized, such as pulse width, pulse energy and the beam quality. The thermal effect of the pulsed LD pumping laser is discussed. The thermal focal length of Tm: YAG laser is measured experimentally. Then,'8'ring laser cavity is designed and optimized by using the ABCD matrix theory, which is thermal insensitive, and it can be adapted to a wide change of thermal focal length and operated stably. After optimization, the single pulse energy, pulse width and beam quality of the Tm:YAG and Tm:LuAG slave laser can meet the requirements. Besides master laser and slave laser, the extract of the injection locking signal is an important part. Injection locking signal extraction is directly infected by the mode matching between the master laser and the slave laser. The matching of transverse mode and longitudinal mode were designed. And effective injection locking signal was extracted in experiment. Finally, Tm³⁺-doped injection locked lasers are achieved experimentally. The characteristics of the injection locking laser are discussed. The linewidth of the laser is measured. The influence of the detuning, the injected power and the trigger time of the Q-switch to the injection locking laser are analyzed. As a result, the single pulse energy of 2mJ and pulse width of 356.2ns for Tm:YAG injection locked laser is obtained at a repetition rate of 15Hz. Using a composite Tm:YAG laser crystal, the single pulse energy of 2.4mJ and pulse width of 347.7ns pulsed SLM laser is achieved at 100Hz. In addition, the single pulse energy of 1.8mJ Tm:LuAG injection locked laser is realized at 50Hz, with the pulse width of 293ns.