Intracavity Laser Beam Control and Formation




4. Intracavity copper vapor laser beam correction

Copper vapor lasers are used increasingly in microtechnology and medicine. Investigations of their characteristics are reported in, for example, 42,43. In practical applications of these lasers there are special requirements in respect of the distribution of the intensity of the output beam and in rapid changes of its distributions.

In the view of high gain of the laser, it is quite easy to construct an unstable telescopic resonator with a high value of the gain. In our experiments we used ILGI-101 laser, produced in Russia. This laser emitted pulses of 40 ns duration at repetition frequency of 10 kHz. The experimental setup is shown in Fig. 24.

Copper vapor laser with intracavity active mirror
Fig.24. Copper vapor laser with intracavity active mirror: 1 - active bimorph corrector; 2 - resonator mirror; 3 - active media; 4 - telescope; 5 - semireflecting plate; 6 - focusing lens; 7 - plate with calibrating scale; 8 - imaging lens; 9 - screen

Active 13-electrode bimorph mirror17 1, and a nontransmitting spherical mirror 2 with a radius of curvature 50 mm formed an unstable telescopic resonator with a magnification of about 100 and of 2 m length. The aperture of the laser active element (tube) was 15 mm. So, a telescopic expander 4, with a threefold magnification, consisted of two lenses was used to expand the beam incident on the mirror 1. Misalignment of this telescopic expander could be used to vary the magnification in a certain range. The radiation was coupled out of the resonator using a plan-parallel glass plate 5. A thin plate 7 with a reading scale (calibrated in scale of 0.1 mm) was placed in the focal plane of a lens 6 (focal length 250 mm). The images of the focal spot and the reading scale were projected by a lens onto screen 9.

The divergence of the output radiation was 0.25 mrad for the optimal tuning of the resonator, but variation of the shape of the adaptive mirror surface made it possible to relimit the divergence to 0,08 mrad. The diffraction limit of the divergence was 0.04 mrad. The output radiation divergence was measured near the lasing threshold.

Placing of the adaptive mirror inside the laser cavity made it possible to correct the phase distortions caused by change in the refractive index of the active medium and by aberrations of the optical components44. Changes in the phase of the output beam resulted in a redistribution of the radiation intensity in the far field zone. We determined the distribution of the intensity in the focal plane of the lens when the minimum divergence was reached (Fig. 25b).

Different intensity distributions in the focal plane
Fig.25. Different intensity distributions in the focal plane

When the mirror sag was 4 μ the divergence increased by a factor of 10 (Fig. 25c). A deliberately induced astigmatic aberration converted the focal spot to a rectangular segment of 0.6x0.02 mm dimensions (Fig. 25c). Application of various combinations of the control voltage to the adaptive mirror electrodes produced different distributions of the radiation intensity in the focal plane.



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