TEA laser

A TEA laser is a gas laser energized by a high voltage electrical discharge in a gas mixture generally at or above atmospheric pressure. The most common types are CO₂ lasers and Excimer lasers, both used extensively in industry and research. The acronym "TEA" stands for Transversely Excited Atmospheric.

The CO₂ TEA laser was invented in the late 1960s by Dr Jacques Beaulieu working at the Defence Research Establishment, Valcartier, in Quebec, Canada. The development was kept secret until the year 1970 when brief details were published.

C K N Patel, working at the Bell Telephone Laboratories in 1963, first demonstrated laser output at 10.6 µm from a low pressure RF excited CO₂ gas discharge. With the addition of nitrogen and helium and using a DC electrical discharge, CW powers of around 100 W were achieved. By pulsing the discharge using higher voltages, or Q-switching using a rotating mirror, pulse powers of a few kilowatts could be obtained but this was the practical limit.

Higher peak powers could only be achieved by increasing the density of excited CO₂ molecules. The capacity for stored energy per unit volume of gas increases linearly with density and thus gas pressure, but the voltage needed to achieve gas breakdown and couple energy into the upper laser levels increases at the same rate. The practical solution avoiding very high voltages was to pulse the voltage transversely to the optical axis (rather than longitudinally as was the case for low pressure lasers), limiting the breakdown distance to a few centimetres. This allowed the use of manageable voltages of a few tens of kV. The problem was how to initiate and stabilize a glow discharge at these much higher gas pressures, without the discharge degenerating into a bright high-current arc, and how to achieve this over a useful volume of gas.

Beaulieu (in 1970) reported a transversely-excited atmospheric-pressure CO₂ laser. His solution to the problem of arc formation was to have a conducting bar facing a linear array of pins with a separation of a few centimetres. The pins were individually loaded with resistors forcing the discharge from each pin into a low current brush or glow discharge which fanned out towards the bar. The laser cavity probed 100-200 of these discharges in series providing the laser gain. A fast discharge capacitor rapidly switched across the laser electrodes using a spark gap or thyratron provided the high voltage pulses.

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