Research On Direct-liquid-cooled Thin Disk Laser

In recent years, direct-liquid-cooled thin disk lasers (DLCTDLs) have attracted growing interests because of their excellent heat dissipation ability and compact structure. This dissertation is mainly composed of three parts:the design and realization of the kilowatt-level direct-’refractive index matching liquid’-cooled Nd:YLF thin disk laser resonator; investigation on the wavefront distortion of the DLCTDL; a few joule level and high beam quality laser is achieved by both theoretical and experimental study on the wavefront distortion and unstable cavity in DLCTDL.Firstly, the design points of DLCTDL are analyzed. A suitable refractive index matching liquid is adopted as coolant, whose refractive index equals that of the gain medium and absorption coefficient is as low as possible at pump wavelength and laser wavelength. The design of an individual flow channel based on the existing low speed wind tunnel designs is used to achieve the laminar flow state as well as the high strong convective heat transfer capability (the convective heat transfer coefficient is more than 1000 W/(m2K)) in the laser zone. The crystals with different doping levels from 0.36 at.% to 0.76 at.% are used to make the thermal loading in each crystal nearly the same. Furthermore, they are elastically held by the steels. The maximum pump power of this mounting method that a crystal can sustain is about 8 times higher than that of the method of the rigid fixation. The end-pump structure is used in DLCTDL to simplify the complex design. The flow rate and the thickness of the channel which largely determine the maximum pump power the crystal can sustain are also well-designed. With the flow rate of 5 m/s, the thickness of 0.5 mm and ten pieces of a-cut Nd:YLF thin disks of different doping levels, a linear polarized laser with an average output power of 1120 W is achieved at the pump power of 5202 W, corresponding to an optical-optical efficiency of 21.5%, and a slope efficiency of 30.8%. To the best of our knowledge, this is the first demonstration of kilowatt-level direct-’refractive index matching liquid’-cooled Nd:YLF thin disk laser resonator.The wavefront distortion of gain module induced by thermal determines the beam quality of the unstable optical resonator. An excellent gain module should feature lower wavefront aberration especially when it is intensively pumped by high power laser diodes. Therefore, the wavefront aberration induced by thermal is investigated numerically and experimentally. We find that the liquid layer plays the major role in causing of the wavefront aberration in the gain module. The wavefront aberration is quantitatively evaluated by optical path difference (OPD) and can be fitted by Legendre polynomials. Besides the main aberrations of tilt and defocus, the higher order aberrations can also be well described by the Legendre polynomial of some certain orders. They are mainly caused by the non-uniform cooling and the un-pumped zone of the disk at top and bottom. The wavefront distributions are compared by varying the pumping and flowing states. It is found that with highly uniform pumping, narrow channel and high velocity flow, the wavefront distortion can be greatly released. Finally, a direct-liquid-cooled Nd:YLF thin disk laser gain module is developed and the results have been carried out to validate the correctness and accuracy of numerical results.Based on the study of the wavefront distortion, a theoretical model of unstable resonator is developed, considering both of the thermal effects and gain effects. The self-reproduction mode is achieved in the traditional unstable resonator with magnification of 1.3 as well as stepped reflectivity unstable resonator with geometric magnification of 1.2 and effective magnification of 1.3. The output power and beam quality factor β of unstable resonator are defined. By using of the stepped reflectivity unstable resonator, we predict that the pulse energy of 4.6 J can be achieved with the beam quality factor β of 7 and 4 both in x-axis and y-axis direction, respectively. It is pointed out that the step reflectivity unstable resonator can obtain higher output power than that of the traditional unstable resonator. In the experiment, a maximum pulse output energy of 4 J with a pulse width of 450μs is realized at the pump energy of 20 J with a pulse width of 500μs, corresponding to the optical-optical efficiency of 20%。...