IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 8, AUGUST 2005 1097 High-Brightness CuBr MOPA Laser With Diffraction-Limited Throughout-Pulse Emission Dimo N. Astadjov, Lyubomir I. Stoychev, Sudhir K. Dixit, Shankar V. Nakhe, and Nikola V. Sabotinov, Member, IEEE Abstract—CuBr master oscillator-power amplifier (MOPA) laser system fitted with a generalized diffraction filtered res- onator (GDFR) is reported to produce high-quality beam of throughout-pulse diffraction-limited laser emission at the repe- tition rate of 19 kHz. The comparison with typical (stable and unstable) resonator configurations for CuBr lasers has featured out its high brightness accompanied by a very low beam diver- gence permanent within laser pulse. This intrinsic characteristic of GDFR-MOPA laser radiation makes it a perfect light source for fine laser applications. Index Terms—Gas lasers, laser amplifiers, laser beams, laser beam focusing, laser resonators. I. INTRODUCTION C OPPER vapor lasers (CVLs), their variants and the asso- ciated master oscillator-power amplifier (MOPA) systems have quite long ago come to age which can be regarded as their maturity [1], [2]. Nevertheless, there are still important issues worth paying attention that keep the stand of CVLs in the frontline of visible laser’s research while gaining in power, beam quality and utility for many of the applications. One such issue is the evolving characteristics within a laser pulse namely beam divergence, spatial coherence, focal spot size, brightness etc, from a CVL oscillator with commonly used stable and unstable resonators [3]–[8]. This is due to the high Fresnel number resonator optics used coupled with limited inversion time available. For such a master oscillator (MO) based high power MOPA scheme, the MOPA output also suffer from same deficiencies of nonconstancy of output beam param- eters [9]. More importantly, the MOPA beam characteristics become highly prone to circuit jitters inherently present in high repetition rate pulsed electronics [10]. This limits the applica- tion potential of such beams. However, a good exception is a CVL MO based on a filtering resonator [11]–[14]. It had been demonstrated that a CVL with generalized diffraction filtering resonator (GDFR) [13], an output beam with diffraction limited divergence constant throughout the pulse [8], [9] is produced. These constant spatial coherence MOPA features were inde- pendent of delay between the oscillator and amplifier [9], [15]. Manuscript received February 10, 2005; revised April 6, 2005. This work was supported in part by the Project “High Power Copper and Copper Halide Lasers of High Beam Quality for Industrial Applications” under the Indo-Bulgarian Inter-Governmental Programme of Cooperation in Science and Technology. D. N. Astadjov, L. I. Stoychev, and N. V. Sabotinov are with the Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria (e-mail: asta55@issp.bas.bg; list@issp.bas.bg; n.sabotinov@issp.bas.bg). S. K. Dixit and S. V. Nakhe are with the Centre for Advanced Technology, Indore 452013, India (e-mail: skdixit@cat.ernet.in; nakhe@cat.ernet.in). Digital Object Identifier 10.1109/JQE.2005.850701 The GDFR CVL MOPA had also produced diffraction-limited (DL) high-power constant characteristics (divergence, spatial coherence, flux) pulses [15]. This is the perfect pulsed laser light source to be utilized. In this paper, for first time we report the performance of a GDFR CuBr MOPA laser system. A CuBr laser (with added H ) is a lower temperature ( 500 C), high repetition rate, compact and sealed off version of CVL with axially peaked gains and low thermally induced wave-front distortion [16]. It is expected for comparable dimensions of CuBr laser and higher tempera- ture CVLs, the beam quality performance of the former will be much better. The present study focuses on time resolved as well as time averaged beam divergence and brightness characteristics of GDFR CuBr master oscillator and one stage MOPA system. This performance is also compared with typical unstable (UR) and plane-plane resonator (PPR) CuBr MO. The CuBr MOPA laser system used is of low-power variety since our attention was mainly concentrated to comparative beam quality features rather than high energy/power output. It is established GDFR CuBr MOPA system considerably outperforms PPR/UR MOPA in terms of divergence and brightness features. Constant diver- gence as well as high brightness also almost constant throughout the pulse was the special features of GDFR CuBr MOPA not available from other resonator geometries. II. EXPERIMENTAL SETUP The basic CuBr MOPA setup is given in Fig. 1. The laser system comprised a master oscillator (MO) and a power ampli- fier (PA) having respectively bore diameter of 14 and 20 mm, and electrode separation of 60 and 55 cm. The lasing medium was formed from CuBr, Ne of 18 torr, and hydrogen of 0.3 torr. Two separate electric power supplies were capable to deliver av- erage power not higher than 1 kW each at repetition frequency of 18 910 Hz. That moderate power input was appropriate just for the basic preliminary research we aimed to carry out. An electronic delay unit was used for managing the delay between the triggering pulses for the MO and PA. The optical set up consists of three types of resonators for the MO, namely a stable plane-plane resonator (PPR: m), a confocal unstable resonator of positive branch (PBUR: m, cm, m) and a filtering resonator (GDFR: cm, , di- ameter of diffracting aperture A1 at plane mirror mm, m) and also the beam guiding, spatial filtering and measuring optics in between oscillator-amplifier/after the am- plifier, PA (Fig. 1). The amplified spontaneous emission (ASE) filtered MO beam (by L1-A2-L2 spatial filter) was sent through PA. The optical MOPA/measurement setup was being changed 0018-9197/$20.00 © 2005 IEEE