Dawei Liang and Pedro Bernardes

CEFITEC, Departamento de Fisica, FCT, Universidade Nova de Lisboa,
2825,Campus de Caparica, Portugal

In Sun-pumped lasers, a laser crystal is usually pumped by concentrated solar light through a secondary compound parabolic concentrator (2D-CPC). Maximum concentration can be obtained through this pumping configuration. The output beam quality presents some non-uniformity problems. Aiming at a better laser beam quality, a new light guide pumping scheme is introduced. Nine fused silica light guides of 5x5mm square cross sections were used to form a novel light guide assembly. Except a central straight guide for end pumping, all the other eight guides were curved, four of which were then twisted, so as to pump a laser crystal symmetrically from eight sides through a quartz flow tube. For an efficient side pumping, the output ends of these light guides were slightly polished to a cylindrical lens shape. The output end of the central guide was also spherically polished. A multiplayer HR (for 1.06pm) coating was deposited at one end of a laser crystal. A semi-spherical optical resonator was formed by using an output coupler of 94% reflectivity. A Nd. YAG laser crystal of 5mm in diameter and 20mm in length was mounted. In order to further concentrate the solar light from the straight light guide to the laser crystal, a convex-lens-window was added to one end of the flow tube. In outdoors experiments, 880W of solar power was measured from the output end of nine fused silica guides. About 700W solar power was absorbed by the laser crystal, resulting in 8.6W of laser power with good beam quality.

INTRODUCTION

The idea of directly converting broadband solar radiation into coherent and narrow-band laser radiation is almost as old as the laser itself. The solar laser is much simpler and more reliable due to the complete elimination of the electrical power generation and power conditioning equipments. If lasers are needed in remote locations where sunlight is abundant and other forms of energy are scarce (spaces, for example), a solar laser would seem to be a natural choice. The solar laser power of 1W was firstly produced by Young (1) nearly forty years ago and the solar laser output power was boosted to 18W by Harou Arashi et. al,(2) Weksler et. al. (3) increased the solar laser output to 60W by using a 2-D CPC for side-pumping the laser crystal. End-pumped solar laser was also reported by Cooke (4). Solar laser of different active materials was also produced by A. Yogev et. al. of Weizmann Institute of Science(5).

To maximize the solar radiation that impinges on the laser crystal, it is necessary to use a secondary concentrator. 2-D CPC (Compound Parabolic Concentrator) are usually used in the Sun-pumped lasers because it wraps the solar radiation incident on its aperture around the laser rod and gives an additional concentration. In the past, most efforts in the development of solar pumped lasers were devoted to the achievement of maximum power and high efficiency while the beam quality (6) was neglected. However the beam quality is important for power transmission, satellite communication, frequency doubling and other applications of solar lasers.

Laser beam quality can be improved by pumping uniformly the laser crystal. It is with this aim that a new fused silica light guide assembly-pumping scheme was put forward and tested. The light coupling assembly composed of a straight, curved and a twisted light guides will be introduced firstly. The end-side pumping scheme of the Nd:YAG laser crystal will than be explained. The final result of sun-pumped laser by light guides will finally be discussed.