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What is an excimer laser?


Excimer is a term used today to describe a family of lasers with similar output characteristics, in that they all emit powerful pulses lasting nanoseconds or tens of nanoseconds, at wavelengths in or near the ultraviolet, and the lasing medium is a diatomic molecule, or dimer, in which the component atoms are bound in the excited state but not in the ground state. Thus, excimer stands for excited di mer. The most important excimer molecules are rare gas halides such as ArF, KrF, XeF and XeCl, which do not exist in nature but can be produced by passing an electrical discharge through a suitable gas mixture. In excimer lasers the rare gas corresponds to about 1-9% of the mixture, with a halogen donor concentration of about 0.1 to 0.2%.The rest of the gas is usually helium or neon and is used to promote energy transfer.

Due to the corrosive nature of the gases used, the structure of excimer lasers is currently engineered from stainless steel, polyvinyl and teflon components. The discharge is usually transverse, see Fig.1, and the electrodes are long, flat and made from nickel or brass. In a similar manner to the pulsed TEA (transversely excited atmospheric) CO 2 laser, a pre-ionisation pulse is required to establish uniform excitation. The laser cavity can be sealed and repeatedly refilled as the laser gas degrades during use. Fan type pumps are used to circulate the lasing gases. As with most gas lasers, the rear cavity mirror is highly reflective.

However, the high gain available eliminates the need for a reflective coating on the output mirror. The most common optical material used in these lasers is magnesium fluoride, which is more transparent than quartz or fused silicon (which is also used) at short wavelength. Sapphire is also used as a window material in ArF lasers.

Fig. 1. Excimer laser schematic
Fig. 1. Excimer laser schematic

Standard stable resonator configurations generate fairly large (20 x 10mm), divergent beams because of the high gain available. Typical wall plug efficiency for a discharge driven commercial KrF laser is about 2%, this gas mixture being one of the most efficient.

As a repetitively pulsed laser, the excimer laser has an average output (in watts) that is the product of the pulse energy (in joules) multiplied by the number of pulses per second (repitition rate). Typical average powers range from under a watt to over 100W. However, lasers with similar average power may have quite different output characteristics. A 10W average power laser may be produced by generating 100 pulses of 100mJ each, or 10 1J pulses. Many laser manufacturers list maximum values for average power, pulse energy, and repetition rate, but the average power is often not the product mentioned above because less energy per pulse is produced once the repetition rate exceeds a certain value.

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