Special Issue Article
Study on the efficiency improvement of Diode Side Pumped Solid State Lasers
Improving the efficiency of Diode Pumped Solid State Laser (DPSSLs) is a challenging job ever since the discovery of DPSSLs. End pumped DPSSL has an inherent advantage of higher coupling efficiency compared to side pumped DPSSLs due to higher absorption length for the input diode power. However this technique is not applicable to high power DPSSL especially above 100 W CW lasers as the laser crystal cannot withstand the very high pump power density and the get fractured under thermal stress. Therefore, obviously side pumping is the only option for high power DPSSLs typically above 100 W average power. The limitation of side pumping is the lower absorption length and multiple reflection of diode power is required for efficient use of pump power. This is partially solved by increasing the doping concentration but the lower beam quality is the price to be paid for it. Further doping concentration cannot be increased beyond certain level as the crystal breaks under strain developed by the dopant. This introduces stringent design constraints for high power DPSSL and also limitations to the power coupling. Furthermore bare diodes have typical divergence of 40 to 50 deg and very close coupling becomes essential for efficient utilization of diode power. Use of micro-lens array is the usual solution, but this reduces the available diode power to be coupled to the laser crystal due to Fresnel reflections leading to reduction in overall efficiency of the system. Thus the laser head design becomes a real challenge for any designer working with high average power DPSSLs. In this paper we describe various techniques employed to improve the overall efficiency of diode side pumped DPSSL.