The parameters of an excimer laser beam depend largely on various distortions inside the resonator. In most cases these distortions are manifested in the pure phase form such as fluctuations of the refractive index of the active medium, thermal deformations of the cavity mirrors, etc. This in turn may result in an uneven distribution of the intensity of the radiation over the beam cross section and increase the divergence. If an electrically controlled mirror is placed inside the resonator, it is possible to reduce these undesirable effects, influence the geometry of the output modes, stabilise the output energy, and suppress fluctuations of the output radiation power.

A typical amplitude w of deformation of the surface of an active mirror usually does not exceed 5 - 7 μ, which represents ~0.5λ for infrared CO₂ lasers and ~20λ for excimer lasers. Such strong modulation of the phase in the ultraviolet part of the spectrum makes it possible to influence significantly the parameters of the output radiation even when the beam is controlled outside the resonator. In the case of the intracavity control of the radiation inside the resonator with a plane mirrors, when the pulses are short, so that the mode structure does not form in available time, the range of correction w_c can be estimated from the following relationship

w_c=Nw,

where N= τ c/L, τ is the pulse duration, L is the length of the resonator, c is the velocity of light. The factor N determined the number of passes of the radiation across the resonator during one pulse. In the case of our excimer laser (L=1 m, τ=30 ns) N is equal to 10, and consequently, the depth of modulation of the phase was w_c~170λ. This estimation demonstrates the exceptionally great opportunities for intracavity control of excimer laser radiation by active corrector.

An excimer laser (model ELI-91, made in USSR) utilizing a mixture of HCl:Xe:Ne=1:10:1000 composition at a total pressure of 4 atm generated 308 nm pulses of 30 ns duration at midamplitude with an energy 80 mJ (in the absence of an intracavity telescopic expander). The laser resonator was formed by a plane-parallel quartz plate and a nontransmitting 13-electrode controlled bimorph mirror with the aluminum reflecting coating (Fig. 26).

Fig.26. Excimer laser with intracavity bimorph corrector: 1 - bimorph mirror; 2 - 3^x telescope; 3 - output coupler; 4 - focusing lens; 5 - photocamera; 6 - block of manual control

The maximum filling of the adaptive mirror surface with the radiation due to the discharge in the XeCl laser was ensured by a quartz telescopic expander (with a magnification 3^x). The distribution of the intensity over the beam cross section was photographed at a distance of 1 m from exit window of the laser and in the focal plane of a lens (f=15 cm).

The results of our experiments are presented in Fig. 27.

Fig.27. Distribution of the intensity of the radiation across a beam at the exit from XeCl laser (a, c) and in the far-field zone, obtained for different control voltages applied to an intracavity adaptive mirror (b, d, e, f)

The structure of the radiation at the resonator exit and in the far-field zone (i.e., in the focal plane of the lens), obtained in the absence of control voltages on the mirror electrodes, is shown on Figs.24a,b⁴⁵. The capabilities of intracavity control are illustrated by photographs in Figs.27c,d. The constant voltages applied to the electrodes were selected to form a rectangular spot on the resonator exit. Deformation of the surface of this controlled mirror had the strongest influence on the phase of the output radiation. On the other hand, the experimental results presented on Figs.27c,d demonstrate that intracavity phase correction could have a direct influence on the distribution of the intensity of the output radiation obtained from an excimer.

The patterns shown on Figs.27e,f provide a clear demonstration of the feasibility of control of the spatial structure of the radiation in the focal plane of the lens when various voltages were applied to the mirror electrodes. The energy of the pulses was then 30-50 mJ.

These experiments demonstrated that the use of an intracavity adaptive optical components in excimer lasers provides an effective means for controlling the characteristics of the output radiation and compensating for the aberrations of a laser cavity as well as laser active media.