Kilowatt-level direct-‘refractive index matching liquid’-cooled Nd:YLF thin disk laser resonator

Abstract

A direct-liquid-cooled Nd:YLF thin disk laser resonator is presented, which features the use of refractive index matching liquid (RIML) as coolant. Highly uniform pump intensity distribution with rectangular shape is realized by using metallic planar waveguides. Much attention has been paid on the design of the gain module, including how to achieve excellent cooling ability with multi-channel coolers and how to choose the doping levels of the crystals for realizing well-distributed pump absorption. The flow velocity of the coolant is found to be a key parameter for laser performance and optimized to keep it in laminar flow status for dissipating unwanted heat load. A single channel device is used to measure the convective heat transfer coefficient (CHTC) at different flow velocities. Accordingly, the thermal stress in the disk is analyzed numerically and the maximum permissible thermal load is estimated. Experimentally, with 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%. Furthermore, the wavefront aberration of the gain module is measured to be quite weak, with a peak to valley (PV) value of 4.0 μm when it is pumped at 5202 W, which enables the feasibility of its application in an unstable resonator. 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.

7kW direct-liquid-cooled side-pumped Nd:YAG multi-disk laser resonator

“…3(a) is used to achieve a fully developed pipe flow. Its structure is with reference to our previous work [14]. The entrances of the homogenizer are sharp so that the impact of incoming flow and the turbulence intensity are weakened.…”

“…Based on this work, a design and simulation of 30-kW class laser resonator was presented in 2015 [13]. Z. Cai et al had developed an end-pumped direct-liquid-cooled Nd:YLF thin disk laser resonator working at kW-level (CW) using the refractive-index-matching liquid as coolant to restrain loss at the interface between gain medium and the coolant [14]. The reports [12][13][14] are all based on the end-pumped structure, in which a uniform pumping distribution is relatively easy to obtain, thereby the wavefront distortion is restrained.…”

“…Z. Cai et al had developed an end-pumped direct-liquid-cooled Nd:YLF thin disk laser resonator working at kW-level (CW) using the refractive-index-matching liquid as coolant to restrain loss at the interface between gain medium and the coolant [14]. The reports [12][13][14] are all based on the end-pumped structure, in which a uniform pumping distribution is relatively easy to obtain, thereby the wavefront distortion is restrained. However, there is an overlap of the laser beam with the pumping beam outside the gain module in the end-pumped configuration, which is hard to be avoided.…”

“…However, there is an overlap of the laser beam with the pumping beam outside the gain module in the end-pumped configuration, which is hard to be avoided. It increases the complexity of the system [12,13], and even induce additional loss due to the insertion of dichroic mirrors [14]. Moreover, thin disks at different doping levels should be adopted to realize an equal gain in every disk [9,14].…”

“…It increases the complexity of the system [12,13], and even induce additional loss due to the insertion of dichroic mirrors [14]. Moreover, thin disks at different doping levels should be adopted to realize an equal gain in every disk [9,14]. With the increase of laser power scaling, the difficulty of fabrication increases with the increasing disk quantity as a result of an improving doping requirement.…”