UV​ GAIN  AND  OPTICS

     Like nitrogen lasers, the gain of excimer lasers is extremely high, so the output is super radiant. A single rear mirror is employed and an output coupler of 4 to 8% which transmits in the region of interest (which is in the UV or vacuum UV region, depending on the gas mixture used). Divergence of the beam is reduced when a full optical cavity is used, and alignment is easy since the laser operates even when cavity mirrors are completely misaligned. The beam is rectangular in profile. A stable resonator consisting of a totally reflecting rear mirror and an output window yields the highest output pulse energies and uniform energy distribution throughout the beam, although divergence is somewhat high (usually, a few mrad). Unstable resonators are often used with excimer lasers to improve the high divergence. This type of arrangement also increases the brightness at the center of the beam, making the beam more focusable for cutting applications. For use as an illumination source for photolithography (a primary use for excimer lasers), the effects of laser speckle must be eliminated, so it is desirable that the output beam be multimode, a simple matter since the beam profile is quite large compared to most lasers. Unlike nitrogen lasers, quartz cannot be used for most excimers since fluorine attacks the material. Output couplers are hence made primarily of magnesium fluoride (which absorbs less UV at most wavelengths than quartz does, making it a better choice regardless).