~/vK.\ & Luwr Twhrrolo~~~. Vol. 2’). No. 3. pp. 145 144. 1997 1’ 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain ELSEVIER PII: 0030-3992(96)00064-3 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA on30-3YY?.Y3 $17.00 +n.nn Investigation of a Cr4+:YAG passive Q-switch in CW pumped Nd : YAG lasers T. DASCALU, G. PHILIPPS, H. WEBER This work describes the experimental results obtained on the Cr4+: YAG crystal used as a passive Q-switch in Nd:YAG lasers. In addition, the combination of a Cr 4+ : YAG passive Q-switch with an acousto-optical Q-switch operating simultaneously in one cavity was investigated. More than 180 W average output in the passive Q-switched mode was obtained with the Cr4+ : YAG crystal. The pulse- to-pulse stability was better than 1%. A mode selection occurred when the laser system operated near threshold. @ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1997 Elsevier Science Ltd. KEYWORDS: Cr4+ : YAG passive Q-switches, lasers (Nd : YAG), acousto-optical Q-switches Introduction Q-switched continuous wave (CW) pumped solid-state lasers with high average output powers are desired for research and applications. Pulses with high repetition rate (kHz range), high average and high peak power are very attractive for various applications in frequency doubling and materials processing. In these applications the beam quality and thus the brightness of the laser radiation is an important parameter. Active Q-switching methods (mechanical, electro-optic and acousto-optic) are usually used in Nd : YAG systems to obtain pulses with high repetition rate and large average output power’,2. The technique of an active Q-switch is more complicated but the advantage is that the repetition rate is independent of pumping power. The CW pumped Nd : YAG laser is usually Q-switched by acousto-optical (AO) devices. This kind of Q-switch has negligible insertion losses. The switching speed is a function of the beam diameter and the depth of modulation. It depends on the beam divergence inside the Q-switching device. For high average power lasers it is more difficult to have 100% modulation because of the decreasing beam quality. In resonators with laser active media acting as a variable thermal lens, the divergence and radius of the laser beam depend on the electrical input power. Therefore, the performances are not constant over the entire range of operation. TD is at the Institute of Atomic Physics, PO Box MG6, Magurele, 76900, Bucharest, Romania and is a guest scientist at the Optisches Institut, Technical University Berlin. GP is at the Laser Medizin und Technologie lnstitut Berlin, Strasse des 17 Juni 135, W-l 000 Berlin 12, Berlin, Germany. HW is at the Optisches lnstitut Technical University Berlin, Strasse des 17 Juni 135, W-l 000 Berlin 12, Berlin, Germany. Received 11 December 1995. Revised 30 September 1996. Accepted 5 November 1996. 145 The passive Q-switching of a CW pumped Nd : YAG laser is possible by using LiF : F? colour centre crystals. Stable laser operation with periodic pulses at a high repetition rate was obtained at low average output power by using colour centre crystals3-‘. New potential saturable absorbers are crystals with impurity centres, characterized by a long time stability, high damage threshold, good thermal conductivity, and good chemical stability. Gadolinium scandium gallium, gadolinium scandium aluminium, and yttrium aluminium garnets with phototropic Cr-ion centres have been used as saturable absorbers in the Q-switching of neodymium lasers ’ 9 Cr4+ ions in garnet seem to be stable whereas a LiF’: F; passive Q-switch at high average output power shows degradation with timei”. With a Cr4+ : YSGG passive Q-switch in pulsed Nd : YAG lasers, an average laser output power of 17.5 W in transverse multimode operation was reported”. Eichler et al.” obtained 13 W average output power when Cr4’ : YAG was used as a passive Q-switch in Nd : YALO pulsed lasers. There are some advantages and disadvantages for each type of Q-switch-passive and active. By using an active Q-switch, the repetition rate can be controlled by an external driver, whereas for a passive Q-switch the repetition rate is given by the internal parameters of the laser. The passive Q-switch is initiated by the laser intensity inside the resonator itself. Therefore, this Q-switch is simple because there is no need for drivers but, at the same time, there is reduced flexibility in choosing laser parameters. That means that it is not possible to change the repetition rate for a given input pumping power and absorber transmission. At high depth of modulation the residual losses are also high and therefore the efficiency is low. Some other disadvantages of a passive Q-switch are the large