Volume 78, number 1 OPTICS COMMUNICATIONS I August 1990 Mode stabilization of a high power laser via computer vision C.V. Sellathamby, H.J.J. Seguin and S.K. Nikumb Department of Electrical Engineering, University ofAlberta, Edmonton, Alberta T6G 2G7, Canada Received 5 December 1989; revised manuscript received 26 March 1990 A computer vision system which performs real-time mode control of a high powered CO* laser has been developed. The device continuously monitors the output beam to derive alignment data for the primary resonator optic. The technique ensures that the laser’s intensity distribution is as uniform as possible. Valuable profile data and beam diagnostics are also generated by the system. 1. Introduction Multikilowatt CO1 lasers are being increasingly used for industrial processes, such as welding, cut- ting, heat treating, and surface alloying [ 1,2]. The principle reason is that carbon dioxide systems are capable of generating high average powers, either pulsed or CW, with relatively high efficiency [ 3,4]. Experience has shown, however, that implemen- tation of such devices in a production environment, to acquire maximum quality and efficiency, neces- sitates that their output parameters be well stabi- lized. This stems from the fact that most materials interactions are heavily dependent upon the optical properties of the source. Thus, to avoid substandard production, a laser must deliver a high quality beam with uniform power distribution at a specified power level, for the duration of the processing sequence zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE [WI. Preserving mode quality for extended periods, at elevated power levels, has proven to be a non-trivial task. In particular, non-uniform heating can cause physical distortion of optical surfaces, contributing to beam degradation. Thermal gradients may also in- duce excessive strain in optical mounts, resulting in misalignment of the resonator. Although such prob- lems can be minimized on short runs with adequate cooling and incorporation of thermally stabilized op- tical benches, they remain serious impediments for long term operation [ 7,8]. Consequently, continu- ous mode monitoring, with frequent adjustment of the resonator optics, have usually been required to generate optimum results. A number of external beam parameters may be analyzed to derive a measure of mode quality. These include power distribution, divergence, coherence and polarization. Of these, power distribution mon- itoring has proven most helpful in maintaining a symmetric and uniform mode of maximum inten- sity. Several instruments have been developed to measure beam uniformity [ 9,111. Unfortunately, the data from these devices must first be interpreted by a human operator, who then administers the nec- essary adjustments to the optics. A more desirable approach would substitute an entirely electronic system to automatically control the resonator. In particular, a computer-based con- trol system possessing a vision capability could con- tinually analyze a sample of the beam, while simul- taneously instigating appropriate corrections to the alignment apparatus. Such an on-line feedback con- trol strategy should foster materials processing with increased efficiency and repeatability. This paper re- ports on such a computer vision resonator alignment system, developed for a high powered CO* laser. 2. System apparatus The approach selected for automatic resonator alignment is illustrated in fig. 1. The test laser con- sisted of a Photo-initiated Impulse-enhanced Elec- trically-excited (PIE) COZ device capable of output 0030-4018/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland ) 47