Optics Communications88 (1992) 397-402 North-Holland OPTICS COMMUNICATIONS Asymmetry in the characteristics of collimated and uncollimated outputs from a copper vapor laser with a confocal positive branch unstable resonator S.K. Dixit, J.K. Mittal, B. Singh, P. Saxena, H. Vora and R. Bhatnagar Centerfor Advanced Technology, Indore 452013, India Received 4 September 1991 In a confocalpositive branch unstable resonator, collimated output is extracted by placing a scraper mirror near the shorter focal length mirror. The output from the other end is generallynot preferred because it has geometricaldivergencein addition to the natural divergence. Recent experiments with noncavitylasers have indicated that asymmetryin the emission characteristics in the two counterdirections could exist becauseof the fact that the radiation sees different radii of curvature mirrors duringthe first transit. Such behavior is particularly expected in high gain short inversion lifetime lasers like copper vapor lasers. In this paper, spatial and temporal characteristicsof the output from the two ends ofa PBURCVL are studied. The results establish that significant asymmetryin the divergenceof the two outputs exists. The temporal evolution of the divergence of the two outputs are different. The uncollimatedoutput has better divergencecomparedto the collimatedoutput. 1. Introduction Confocal unstable resonators are an excellent choice for large aperture high gain lasers. In these resonators the output is generally taken from the shorter focal length mirror side in order to obtain a collimated output beam. It is believed that the char- acteristics of the output beam do not depend on the method used for coupling the radiation out of the resonator. In general this is true for lasers with round trip transit time orders of magnitude smaller than the time for which the gain lasers. However this may not be true for high gain pulsed lasers like excimer, nitrogen and copper vapor where the time for which the inversion lasts is only a couple of transit times. Positive branch unstable resonators (PBUR) are ex- tensively used with copper vapor lasers (CVL) to obtain diffraction limited beams [ 1-4]. Time re- solved divergence measurements have indicated that the divergence of the pulse is high during its first transit through the resonator and reduces in succes- sive transits, ultimately reaching the diffraction limit towards the end of the pulses [ I ]. Due to the self- terminating nature of CVL, the pulse, during the first transit, sees higher gain compared to the subsequent transits. The average divergence therefore is gov- erned by the divergence of the first transit. Experi- ments with noncavity CVL, with mirrors of different focal lengths placed at one end, indicate that the di- vergence obtained with a smaller focal length convex mirror is much less then that with a larger focal length concave mirror [ 5]. In PBUR CVL, where a large focal length concave mirror is used in conjunction with a small focal length convex mirror, an asym- metry in the divergence of the first transit of the two- counter-propagating pulses is expected. This should be reflected in the average divergence, after the geo- metrical divergence due to uncollimated nature of the output from the other end is taken into account. In unstable resonator a scraper mirror is usually placed, on the resonator axis, close to the smaller fo- cal length mirror to obtain collimated output beam (case l ). If by some means output is taken from the other side, the radiation will, in addition to natural divergence, also have geometrical divergence (case 2). In this paper the characteristics of the collimated (case 1 ) and uncollimated (case 2) outputs from a PBUR CVL are studied. The results indicate signif- icant asymmetry in the spatial and temporal char- acteristics in the two cases. 0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved. 397