Research On High Power Two Micro-meter Single-doped Ho:YAG Solid-state Laser

Due to eye safe and atmospheric weak absorption, 2μm lasers have various important applications, such as, atmospheric observation, free-space optical communication, laser medicine, laser radar and optical spectroscopy, and so on. Especially, 2μm pulsed laser was the efficient pumping source of the mid-IR(3~5μm) and far-IR(8~12μm) optical parametric oscillator(OPO) and optical parametric amplifier(OPA). The mid-IR and far-IR lasers have various significance applications in the fields of laser radar, optoelectronic countermeasure, laser ranging and infrared guidance technology, and so on. To meet the needs of these military, industry and agriculture application, the most significant step is to investigate a high performance 2μm laser. For the past few years, Ho single-doped lasers based on YAG, YLF and YAP were in large amounts of investigated. Comparative analysis of the various technological approaches research progress of Ho single-doped lasers, we pointed out the merit and demerit of them and demonstrated the study necessity of Ho:YAG laser pumped by Tm doped lasers. Thus, in order to make a certain effort on the high power, high efficiency and high beam quality 2μm laser, the main subject of this project was the high power continuous wave and Q-switched Ho:YAG laser.By analysising of apsorption spectum and emission spectrum of Ho:YAG crystal, the Ho3+ ion has a larger absorption peak around 1908 nm, and the full width at half maximum(FWHM) of the absorption peak is very narrow. For the sake of larger absorption in Ho:YAG crystal, the output wavelength of the pump must be stable and the linewidth must be narrow. Consider with energy upconversion and ground state depletion, we establish the quasi-four-level rate equation of dual-end-pumped continuous wave Ho:YAG laser by energy level transition theory and laser machnism. Take into account Boltzmann occupy factors affected by operating temperature, we numerical analyzed various parameters influenced the output performance of Ho:YAG laser by rate equation, such as ground state depletion, energy level transition, crystal operating temperature, pump beam radius, output coupler transmission, pump beam quality and beam waist position, and doping consentration and length of crystal, and so on. We established the rate equation of pulsed Ho:YAG laser, and theoretically analyzed the main parameters which influenced the energy per pulse, energy extraction efficiency and pulse width of the output laser.In order to provide pumping source for Ho:YAG laser, the Tm:YLF laser whose emission lasing was well matched the absorption peak of Ho3+ ion was theoretical and experimental studied by using volume Bragg grating(VBG) and F-P etalon. With multicrystal series technique and slab crystals, the total pump power and output power of the Tm:YLF laser was improved. We theoretical analyzed the thermal effect of two different form Tm:YLF crystals and numerical analyzed and discussed crystal operating temperature and thermal focal lens influenced by beam radius in different pump powers. In the rod Tm:YLF laser experiment, the maximum output power was 60.1W, corresponding to a slope efficiency of 35.1%. For the Tm:YLF laser with two slab crystals, the output performance was comparatively studied with different crystal dimensions. The parameters of pumping and resonator which affected the laser output performance were experimental investigated. For the laser with slab crystals, the maximum output power was about 108 W at the total pump power of 304.5W, corresponding to a slope efficiency and conversion efficiency of 39.0% and 35.5% respectively. From threshold to the highest output power, the output wavelength of the Tm:YLF laser was shifted only 0.3nm, and the FWHM was less than 1nm. The beam quality factor was less than 3. Based on the above investigation, we achieved a high power pumping source with high beam quality and narrow width.The stability of Ho:YAG laser resonator with single crystal and the thermal lens effect of Ho:YAG crystal were theoretical analyzed. By using the resonator stability condition, the Ho:YAG thermal focal length was measured with differernt pump powers. In additon, theoretical analysis of the Ho:YAG crystal thermal focal length influence on the resonator stability was achieved. By using the designed Tm:YLF solid-state laser as pumping source, the output performance of CW and Q-switched Ho:YAG laser with three different doping concentrations and lengths was comparative investigated. Under the same parameters of pumping and resonator, the crystal with doping concentration of 0.8at.% and length of 45 mm shows the highest slope efficiency from the three less than 50 mm Ho:YAG crystals.The maximum output power of CW Ho laser was about 58.7W under the incident pump power of 83.2W. Under Q-switched mode, up to 2.9m J energy per pulse was achieved, corresponding to a pulse width of 31 ns and a peak power of 94.7k W.Based on the experimental investigation of Ho:YAG laser with one crystal, the output power of Ho:YAG laser pumped by the designed Tm:YLF solid-state lasers was improved by using the multicrystal series technique. In the double pass pumping, a thin film polarizer combined with a quarter wave plate was to avoid the feedback of the pump. In the same pumping conditions, the slope efficiency and conversion efficiency of the laser with double pass pumping was 4.8 percent higher than that of the laser with single end pumping. Moreover, the pump threshold of the laser with double pumping was about 2.6W lower than that of the laser with single end pumping. In the dual end pumping, several thin film polarizers were employed in the pumping to avoid the Tm:YLF laser influenced by each other. Under the CW mode, the maximum output CW power of 119.0W was achieved at the incident pump power of 174.6W, corresponding to a slope efficiency of 74.6%. Under the Q-switched mode, several certain thickness F-P etalons were inserted in the resonator to make the laser to emit at a single wavelength. At the pulse repetition frequency of 20 k Hz, up to 118.0W average output power of Q-switched laser at the total incident pump power of 209.5W was obtained, corresponding to a slope efficiency of 61.7%. The maximum energy per pulse of 5.9m J was achieved, which corresponded to a pulsed width of 24 ns. The beam quality factor of M2 was less than 2 at the output power around 100.0W.