Extremely high-power CO 2 laser beam correction ALEXIS KUDRYASHOV, 1, *ALEXANDER ALEXANDROV, 1 ALEXEY RUKOSUEV, 1 VADIM SAMARKIN, 1 PIERRE GALARNEAU, 2 SIMON TURBIDE, 2 AND FRANÇOIS CHÂTEAUNEUF 2 1 Adaptive Optics Lab, Moscow State Technical University (MAMI), Sudostroitel’naya 18, bld. 5, Moscow 115407, Russia 2 INO, 2740 Einstein St., Quebec, Quebec G1P 4S4 Canada *Corresponding author: kud@activeoptics.ru Received 28 January 2015; revised 6 April 2015; accepted 11 April 2015; posted 13 April 2015 (Doc. ID 232659); published 5 May 2015 This paper presents the results of high-power CO 2 laser-aberration correction and jitter stabilization. A bimorph deformable mirror and two tip-tilt piezo correctors were used as executive elements. Two types of wavefront sensors, one Hartmann to measure higher-order aberrations (defocus, astigmatism etc.) based on an uncooled microbolometer long-wave infrared camera and the other a tip-tilt one based on the technology of obliquely sputtered, thin chromium films on Si substrates, were applied to measure wavefront aberrations. We discuss both positive and negative attributes of suggested wavefront sensors. The adaptive system is allowed to reduce aberra- tions of incoming laser radiation by seven times peak-to-valley and to stabilize the jitter of incoming beams up to 25 μrad at a speed of 100 Hz. The adaptive system frequency range for high-order aberration correction was 50 Hz. © 2015 Optical Society of America OCIS codes: (220.1080) Active or adaptive optics; (140.3470) Lasers, carbon dioxide; (040.6808) Thermal (uncooled) IR detectors, arrays and imaging; (120.5050) Phase measurement. http://dx.doi.org/10.1364/AO.54.004352 1. INTRODUCTION It is very well known that high-power laser wavefronts are highly aberrated [ 1]. This does not allow for obtaining a good focus and high concentration of laser beam energy. The reasons for the wavefront distortions are, first of all, thermally induced aberrations in laser active elements (laser active medium) and also some residual aberrations of various optical elements. In general, the initial quality of each optical element is high enough [peak-to-valley (P-V) about λ∕10] but the whole opti- cal setup consists sometimes of tens or hundreds of such ele- ments that altogether introduce sufficiently large aberrations. Furthermore, it is important to take into account errors that occur due to misalignment of the whole setup. So, in order to improve the quality of a laser beam it is initially necessary to be able to measure and correct for existing aberrations. For some solid-state lasers that generate light in a range from 400 to 1100 nm, there are huge varieties of wavefront sensors or inter- ferometers that could be used for this application. Also the coat- ing technique for mirrors in this spectral range is very well developed; one can get up to 99.98% of reflectivity of the mir- ror. But as soon as we move to the far infrared spectrum (10 μm radiation) the situation changes. There are almost no commer- cially available wavefront sensors that could be used in this spectral range. In fact, there is a problem with the reliability of infrared cameras (arrays of independent sensors) or single sensors. At the same time, most industrial lasers and laser complexes are still based on high-power CO 2 lasers with 10.6 μm output wavelength. More problems appear for high- power gas dynamic CO 2 lasers (GDL) where initial radiation is poorly controlled due to a rather unpredicted pump [ 2]. For these lasers one can expect huge beam jitter, sometimes close to 1 mrad, and higher-order aberrations in the range of a tenth of microns. This kind of radiation needs to be evaluated in real time for an optimal correction. For CO 2 laser-beam correction, a wavefront sensor becomes the key element of the whole adaptive optical system. From the variety of wavefront sensors, the most suited for this type of application is, for sure, the Hartmann one since it could be easily adjusted to measure any kind of laser beam. This paper presents the results of aberration correction and jitter stabiliza- tion of high-power CO 2 laser radiation using a bimorph de- formable mirror, two tip-tilt correctors, and two types of wavefront sensors: a Hartmann sensor to measure higher-order aberrations (defocus, astigmatism, etc.) based on a long-wave infrared (LWIR) uncooled microbolometer camera and a ther- mal anisotropic tip-tilt sensor based on the technique of thin films obliquely deposited on silicon substrate. The whole adap- tive system was aimed to correct various types of high-power CO 2 lasers (including GDLs). But all experiments to test the parameters of an adaptive system were realized with the help of an industrial 1 kW continuous-wave (CW) CO 2 laser beam. This was done in order to get reliable results. 4352 Vol. 54, No. 14 / May 10 2015 / Applied Optics Research Article 1559-128X/15/144352-07$15/0$15.00 © 2015 Optical Society of America