Resonator Design Considerations for Efficient Operation of Solid-State Lasers End-Pumped by High-Power Diode-Bars

Abstract

Advances in semiconductor laser technology over the last decade have been dramatic, leading to a very significant increase in available output power from diode-bar pump sources. As a result, in combination with the use of novel beam shaping techniques to allow focusing their output beam into a form more suitable for end-pumping (e.g. [1]), it is possible to achieve very intense pumping of solid-state laser media. This regime of operation, so far little explored, offers the attractive prospect of efficient operation on low gain and/or quasi-three-level transitions at high power levels. However, progress in the development of such lasers has been hindered by strong thermal effects, such as thermal lensing and stress-induced birefringence, which are particularly pronounced owing to the increased thermal loading density. One of the key problems encountered in end-pumped lasers is the highly aberrated nature of the thermal lens which makes it difficult to simultaneously achieve high efficiency and good beam quality. To overcome this problem a number of approaches have been reported (e.g. [2]) including the use of aspheric lenses [3] as compensators. In this paper we describe an alternative strategy for resonator design which aims to reduce the beam distortion resulting from strong thermally-induced aberrations without using compensating components. The underlying basis of this approach is that the aberrations which result from a Gaussian-like pump beam are most pronounced toward the wings of the inversion distribution. Hence by using a resonator with a TEM00 mode size which is smaller than the pump beam size it is possible to achieve a near-diffraction-limited output beam. This approach is in contrast to the situation at low pump powers where it is generally accepted that the laser mode size should be at least as large as the pump beam size. However, a consequence of the smaller mode size is that it is more difficult to achieve efficient extraction of the gain stored in the wings of the inversion distribution. Experimental results for various diode-bar pumped Nd:YAG lasers which confirm the benefits of our approach will be presented together with a detailed discussion of thermal effects in Nd:YAG which indicate how the various factors, such as end-face curvature and the temperature and stress dependence of refractive index influence the thermal lensing behaviour. The merits of Nd:YLF with its weaker thermal lensing on the 1.053µm polarisation compared to Nd:YAG for use in high-power end-pumped lasers will also be considered.