A growing number of applications are calling for the compact high energy laser sources. In the last decade, significant progress has been made in the area of solid state lasers especially fiber lasers. The solid state laser is widely used in the processing industry, telecommunication systems at present. However, thermal effect, nonlinear effect and the damage of optical components limits the output power. We present a laser coherent combining technique based on heterodyne method in all-fiber feedback format. In this technique the feedback signals are coupled and transferred by fiber to simplify the system, while all factors of the signals such as the wavefront distribution, polarization state, power ratio of the sub-beams and the reference beam need to be considered and strictly controlled. Compared with the previous system, this technique brings another important advantage: Only the coupling side should meet stringent tolerance toward collimation. Phase detection for laser interferometry, phase control of sub-lasers is theoretically analysis and simulated to reveal the system control bandwidth, phase precision. A high speed phase detection circuit and a phase control circuit are developed. Proof of concept of this technique is experimentally demonstrated at 1.06μm. The experiment setup is shown. Stable results are obtained. The peak power rises up while the theoretical result is 2. Experiments reveal the validity of the technique in nanosecond pulse laser coherent combining.
We demonstrated a large face pumped double-sided liquid cooling Nd:YAG slab laser. The pump light incident from the large surface of the crystal, which are cooled by high-speed flowing cooling water, while the laser beam incident on to the end face, and travels in ZigZag path along the long direction in the crystal. The flat-flat resonant cavity was built, and the output coupler transmission was 30%. The Nd:YAG slab crystal with trapezoidal shape was used as the gain medium, the size of which was 190mm×12mm×4mm, one of the surfaces of 190mm×12mm was coated with antireflection film for 808nm and another was coated with reflection film for 808nm, and the end faces of 4mm×12mm were coated with antireflection film for 1064nm, the doping concentration of Nd<sup>3+</sup> ion was 1.0at.%. The CW LD array and QCW LD array were used as the pump source to pump the slab crystal, the light emitting surfaces of which have the same size, and the pumping light passed through the pump windows made off used silica and incident into the crystal. Under CW LD pump, the maximum of 420W laser output was gotten, and under QCW LD pump, the maximum of 502W laser out put was gotten. Due to the much higher peak power of QCW laser diode, the small-signal gain was much higher, and induced the optical efficiency of QCW pumping system was much higher, and its thermal effect was relatively smaller because of the high extraction efficiency.
Thermal effect is a plateau that limits the output of high-power, high beam quality laser, and thermal effects become worse with the increase of pump power. We can reduce the effects caused by thermal effects from pumping, laser medium shape, cooling method and other aspects. In this article, by using finite element analysis software, the thermal effects between Nd:Glass and Nd:YAG laser crystal was analyzed and compared. The causes of generation for thermal effects, and factors that influence the distribution in laser medium were analyzed, including the light source, the laser medium shape and the working mode. Nd:Glass is more suitable for low repetition frequency, high energy pulsed laser output, due to its large size, line width and so on, and Nd:YAG is more suitable for continue or high repetition rate laser output, due to its higher thermal conductivity.