In diode-pumped solid-state laser system, the performance of pumped coupling system directly determines the output power and the beam quality of laser. we report a pump coupling system for sandwich structured slab gain medium. Image relaying along the slow axis is used to obtain the uniform distribution of pumping light in the slab width direction and the methods of spatial multiplexing and polarization multiplexing are used to increase the density of pump power. When the pump power is 11.2kW, the continuous-wave output power of 4.55kW is obtained through the plane-parallel resonator. Meanwhile, the influence of the methods of spatial/polarized composite splicing on the pump efficiency of the pump coupling system is analyzed, and the experiment results provide the basis for optimization design to high power slab laser.
To theoretically study the feasibility of antifreeze coolants applied as cooling fluids for high power LD heat sink, detailed Computational Fluid Dynamics (CFD) analysis of liquid cooled micro-channels heat sinks is presented. The performance operated with antifreeze coolant (ethylene glycol aqueous solution) compared with pure water are numerical calculated for the heat sinks with the same micro-channels structures. The maximum thermal resistance, total pressure loss (flow resistance), thermal resistance vs. flow-rate, and pressure loss vs. flow-rate etc. characteristics are numerical calculated.
The results indicate that the type and temperature of coolants plays an important role on the performance of heat sinks. The whole thermal resistance and pressure loss of heat sinks increase significantly with antifreeze coolants compared with pure water mainly due to its relatively lower thermal conductivity and higher fluid viscosity. The thermal resistance and pressure loss are functions of the flow rate and operation temperature.
Increasing of the coolant flow rate can reduce the thermal resistance of heat sinks; meanwhile increase the pressure loss significantly. The thermal resistance tends to a limit with increasing flow rate, while the pressure loss tends to increase exponentially with increasing flow rate. Low operation temperature chiefly increases the pressure loss rather than thermal resistance due to the remarkable increasing of fluid viscosity.
The actual working point of the cooling circulation system can be determined on the basis of the pressure drop vs. flow rate curve for the micro-channel heat sink and that for the circulation system. In the same system, if the type or/and temperature of the coolant is changed, the working point is accordingly influenced, that is, working flow rate and pressure is changed simultaneously, due to which the heat sink performance is influenced. According to the numerical simulation results, if ethylene glycol aqueous solution is applied instead of pure water as the coolant under the same or a higher working temperature, the available output of optical power will decrease due to the worse heat sink performance; if applied under a lower working temperature(0 °C, －20 °C), although the heat sink performance become worse, however the temperature difference of heat transfer rises more significantly, the available output of optical power will increase on the contrary.
A compact, high power and high beam quality laser diode stacks partially end-pumped
quasi-continuous wave slab laser with hybrid resonator is demonstrated. Using this
configuration, with Nd:YVO<sub>4</sub> as the slab gain media, 101 W output power is obtained
when the pumping power is 216.5W with the repetition of 1kHz, the optical-to-optical
efficiency and slope efficiency are 46.7% and 51.14%, respectively. The beam quality
M<sup>2 </sup>factors in the unstable direction and the stable direction are 1.36 and 1.56
respectively at the output power of 101 W.
Coherent combination of multiple laser amplifier is an important technique for high power and high beam quality laser.
Laser amplifier uses master oscillator power amplifier (MOPA) configuration for narrow bandwidth and high beam
quality laser. Then active phase control is used to make multiple laser amplifier phase synchronization for coherent spot
in the far field. The center spot is N times brighter than the one of incoherent combination. At present, researchers have
used this method to achieve 1.56kW coherent combination of nine fiber lasers and 105kW of seven slab lasers.
The phase noise of the laser amplifier is an important factor affecting the coherent combination. There are two key
technologies in the coherent combination. The one is fill factor of multiple laser spatial distribution, which determines
main lobe energy of coherent spot. And the other is the phase noise of the laser amplifier, which decides stability of the
coherent combination. The phase noise of laser amplifier is caused by many factors, mainly thermal disturbance and
mechanical vibration. Due to the complexity of the phase noise generation, the research on phase noise is generally
concentrated in the qualitative analysis. The phase noise is generally considered the time phase noise. It is phase
dithering from variation refractive index by external disturbance. A variety of factors, such as temperature changes,
mechanical vibrations, the pump power, affect phase noise. We establish an externally imposed discrete disturbance
source on fiber laser amplifier and then analyze above-mentioned factors independently and quantitatively by the
method. The experiments demonstrate space phase noise, dithering of beam direction, is simultaneously induced by
either mechanical vibration or thermal disturbance. This experiment is significant for understanding mechanism of the
phase noise and suppression of phase noise.
A new design of water cooled heat sink with micro-channels and diamond heat spreader for high power diode laser (LD)
is presented. The design mainly discusses heat sink made of pure copper plates with micro-channels of three sizes (75
μm, 150 μm and 300 μm). And the design is simulated with Computational Fluid Dynamics (CFD) and Numerical Heat
Transfer (NHT) methods, the influences of the size, numbers of the cooling channels and the flow rate of the cooling
water to the temperature and the flow resistance characteristics of the heat sink are investigated. In general, decreasing of
characteristic size, or, increasing of channel numbers, or, increasing of the flow rate of the cooling water can reduce
thermal resistance of heat sink; meanwhile increase the pressure drop significantly. The performance with a diamond heat spreader is numerical calculated for all three sizes of micro-channels as well. The results indicate that the diamond heat spreader can play an important role on decreasing the maximum temperature of the heat sink. The whole thermal resistance of heat sink can be reduced 10 % to 20 % with a 100 μm thickness diamond heat spreader compared with a heat sink without it.