112 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 50, NO. 2, FEBRUARY 2014 Studies on Spatial, Spectral, and Energy Characteristics of Copper-HBr Laser Radiations Ramakanta Biswal, Praveen K. Agrawal, Om Prakash, Govind K. Mishra, Sudhir K. Dixit, and Shankar V. Nakhe Abstract—This paper presents, for the first time, a study on the pulse-to-pulse variation in the spatial, spectral, and energy characteristics (pointing instability, divergence, intensity, as well as the emission line-width and their fluctuations) of both green and yellow radiations of a copper-HBr laser (Cu-HBrL). The experimental study was performed in a high power (specific power: 70 W/L, total power: 72 W), 18 kHz repetition rate Cu-HBrL for different wall plug electrical input powers (6–9.5 kW) and HBr concentrations (5%–9%). These observa- tions are supplemented with studies on near field spatial intensity and temporal profiles under all working conditions. It is observed that the characteristics are widely different for the two radiation components. The experimental observations are supported by comprehensive analysis in terms of input power coupling, laser gain and its radial distribution, thermally induced wave-front aberrations, optical noise contribution, and laser kinetics. Index Terms—Laser beam pointing stability, laser divergence and amplitude fluctuation, laser line-width and fluctuation, copper-HBr laser. I. I NTRODUCTION C OPPER-HYDROGEN Bromide laser (Cu-HBrL) that produces coherent radiations in green (G: 510.6 nm) and yellow (Y: 578.2 nm) wavelength, is a highly performing variant of atomic copper laser in important aspects such as specific average output power, beam quality and efficiency [1], [2]. Often this laser is an alternative to frequency doubled solid-state lasers with some advantages for applications that require high pulse repetition rate (PRR: 5–25 kHz), high aver- age spectral brightness as well as stability in the visible and its frequency converted ultraviolet (UV) region. For example, in non-linear UV conversion of the copper laser radiations and their subsequent use in writing fiber Bragg gratings (FBGs)/visible-UV precision material processing etc., pulse to pulse variation of divergence, far-field pointing, spectral purity and amplitude fluctuation of the fundamental beam play important roles in deciding the conversion efficiency as well as the quality of the FBGs-writing/material-processing [3]–[6]. Some of these characteristics of the copper laser are also important while using it as a pump source for generation Manuscript received August 6, 2013; revised November 19, 2013; accepted December 15, 2013. Date of publication December 23, 2013; date of current version January 6, 2014. The authors are with the Department of Atomic Energy, Laser Systems Engineering Section, Raja Ramanna Center for Advanced Technology, Indore 452013, India (e-mail: rbiswal@rrcat.gov.in; praveen@rrcat.gov.in; oprakash@rrcat.gov.in; gkmishra@rrcat.gov.in; skdixit@rrcat.gov.in; nakhe@ rrcat.gov.in). Digital Object Identifier 10.1109/JQE.2013.2295880 of high average power, narrow line-width, stable operation of tunable dye and Ti: Sapphire lasers [1], [7].These issues are more relevant for a Cu-HBrL due to its operation at high PRR, high specific power conditions that lead to increased power loading and thermally induced phenomena affecting the spatio- temporal laser beam characteristics. In this perspective, it is essential as well as interesting to carry out studies on these aspects in any laser in general and in a Cu-HBrL in particular. However, there is no reported study on this aspect in a Cu- HBrL in general and high specific output power (>10’s of W/liter) in particular, It is expected that these characteristics of the Cu-HBrL radiations would be very much different from that of the conventional elemental copper laser owing to their different gas composition (Cu + HBr + Ne vs. Cu + Ne), operating temperature (∼900 K vs. ∼1800 K), pulse repetition rate (15–20 kHz vs. 5–6 kHz), input power loading and spatio- temporal laser gain characteristics etc. In this paper, for the first time, we report a comprehen- sive study on the spatial and energy characteristics (i.e., the pulse to pulse variation in far-field divergence, pointing & amplitude) as well as the spectral characteristics (i.e., the emission line-width as well as its fluctuations) of both the radiation components (G & Y) of a solid state pulser based high specific output power (∼70 W/liter)/ total average output power (72 W) Cu-HBrL with respect to variation in different electrical input power and HBr concentration/active medium composition. It was observed that these characteristics were widely different for the two radiation components and were strongly dependent on laser gain characterizing parameters such as electrical input power and HBr concentration. These observations are supplemented with studies on near field (beam diameter) and temporal (intensity) profiles under all working conditions. The experimental observations are suitably sup- ported by comprehensive analysis in terms of laser gain and its radial distribution, thermally induced wave-front aberrations, optical noise contribution and laser kinetics. II. EXPERIMENTAL ARRANGEMENT Fig. 1 shows the schematic of the experimental set up for studying the spatial, spectral and energy characteristics of the Cu-HBrL. The laser system, developed in-house, was based on an alumina discharge tube (4.7 cm bore dia. × 150 cm long) and was excited using IGBT-solid state switch based pulser at 18 kHz repetition rate [8]. A precision HBr-Ne gas mixing set up, consisting of a fine metering valve and a thermal mass flow controller, was used to change the gas composition. 0018-9197 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.