Study On Acousto-optical Cavitydumping Technology At Thuliumsingle-doped Laser

Acousto-optical cavity dumping is a laser technology to obtain high-repetition-rate and short-pulse-width laser pulse.2μm high-repetition-rate and short-pulse-width laser obtained by Thulium single-doped laser medium can be used to lidar and atmospheric remote sensing. But there is no research on acousto-optical cavity dumping technology at thulium single-doped laser, therefore it has a certain research value. In this paper, the acousto-optical cavity dumping technology is studied in theoretical and experimental aspects.The working principle of the acousto-optic modulator (AOM) is analyzed theoretically, the factors that influence the diffraction efficiency are analyzed, and the space sound field distribution is analyzed. Based on the energy level transition and rate equation of Tm3+, the establishment stage of light field of acousto-optical cavity dumping is analyzed in theory. The changing trends of the intracavity photon number and the upper level ions number are analyzed. Based on the diffraction process of the acousto-optical modulator, the production process of the nanosecond acousto-optical cavity dumping pulse is analyzed, the theory model of dumping stage of the light field in acousto-optical cavity dumping is established. The relationship of the pulse width and the diffraction efficiency of the AOM, the laser beam size in the AOM, the distribution of photons in the AOM, the rising time of the ultrasonic wave, and the cavity length are studied theoretically.The stability of the acousto-optical cavity dumping is researched, and the effects of repetition rate, pumping rate, single loss, and cavity length on the stability of cavity dumping are analyzed.Tm: YAG crystal acousto-optical cavity dumping experiment is carried firstly, with a crystal volume of771.5mm3and a doped concentration of3.5%. When the pump power is19.47W, cavity dumping pulse with a repetition rate of200KHz, a pulse width of54ns, a wavelength of2013.3nm, and an average power of595mW is obtained. Tm: YAG ceramics acousto-optical cavity dumping experiment is carried secondly, with a crystal volume of391.5mm3and a doped concentration of4%. When the pump power is9.77W, cavity dumping pulse with a repetition rate of200KHz, a pulse width of31ns, a wavelength of2012.6nm, and an average power of180mW is obtained. But the pulse shape is aberrant.Finally Tm: YAG crystal acousto-optical cavity dumping experiment is carried, with a crystal volume of3316mm3and a doped concentration of3.5%. The maximum pump power is40W. When the pump beam diameter is720μm, the pulse shape distortion always exists. When the pump beam diameter is800μm, there is not distortion if the pump power is low, and conversely at high pump power. When the pump beam diameter is1200μm, there is almost not distortion. Cavity dumping pulse with a repetition rate of200KHz, a pulse width of50ns, a wavelength of1989.8nm, and an average power of1.28W is obtained when the pump beam diameter is1200μm.By three groups of experimental study, the difference of the average power under different repetition rates are compared, the differences of the average power under different width of the radio frequency pulse are compared, and the unstable phenomena of the acousto-optical cavity dumping are observed. These pulses are compared to the pulses of the active acousto-optical Q switching laser. The theoretical calculation results and the experimental results are compared as discussion. The reason of aberrant pulse is analyzed.