Compact, High Brightness and High Repetition Rate Side-Diode-Pumped Yb:YAG Laser

Abstract

Efficient, compact, high average power (100 W and higher) and brightness Q-switched solidstate lasers capable of operating at high pulse repetition frequencies (PRF) of 10 kHz and higher are required for many applications such as material processing, frequency conversion, remote sensing, etc. These lasers represent a scaling up of nearly an order of magnitude over the current generation of diode-pumped solid state lasers. To achieve this level of performance, it is essential to provide a high pump power density in the laser medium, to reduce thermal loads and gradients in active medium and to obtain a good laser beam quality and brightness. Thermal effects in a laser gain medium are generally the main limiting factors for power scaling of diode-pumped solid-state lasers when near diffraction limited output beam is required. Diode-pumped Yb:YAG lasers are a very attractive alternative to the lasers utilizing classical laser material such as Nd:YAG for reducing thermal effects and for scaling the Q-switched output power to the desired level. Yb:YAG has nearly four times less heat generation during lasing than comparable Nd:YAG laser systems [Bibeau et al., 1998, Honea et al., 2000, Rutherford et al., 2001, Goodno et al., 2001] due to a much smaller quantum defect in Yb3+. However, there are two shortcomings with Yb:YAG crystals related to a quasi-three-level nature of its laser transition. The first shortcoming is significant reabsorption at the laser wavelength preventing many laser configurations from being effective. However, recent advances in the development of diffused bonded composite YAG crystals have made it possible to diminish reabsorption losses and achieve a high brightness output. Another drawback is the relatively high level of the laser threshold pump power, which is noticeably, higher than in Nd:YAG lasers. But the last disadvantage is not too important for high average output power lasers, which usually are pumped significantly above threshold. Along with the choice of the gain medium, the important parameters to consider in the design of high power and brightness solid state lasers are the architectures chosen for the diode pumping scheme, laser resonator layout for thermal lensing compensation, energy extraction and cooling of the laser crystal. All of these play critical roles in average power scaling especially when a good quality laser beam is needed. A number of different approaches have been tried by other investigators for developing high power Yb:YAG lasers. Conventional rod lasers allow scaling to high average powers [Bibeau et al., 1998, Honea et al., 2000]. But obtaining a good beam quality at high average power is a difficult task due to considerable stress-induced birefringent and