Microwave excited copper halide and strontium-ion recombination lasers

Abstract

Microwave excitation of pulsed metal vapour lasers (copper halide and strontium-ion recombination lasers) is investigated. Two waveguide coupling structures were designed and built, one based on a ridge waveguide; and the other based on a rectangular waveguide with a tapered narrow wall which was designed to produce a uniform, travelling microwave field. These coupling structures were designed to produce a transverse electric field at high pressure, with high electron densities, using pulsed microwaves with peak powers of up to 2.5 MW, compatible with the necessary requirements for copper and strontium based systems. The magnetron power supply was modified to produce double pulses of variable spacing (between 15 and 500 mus). Laser oscillation was observed on the cyclic transitions of neutral copper (=510.6 and 578.2 nm) and on the recombination transition of singly ionised strontium (=430.5 nm) for the first time in microwave excited systems. The performance of the tapered waveguide coupling structure was found to be superior to the ridge waveguide coupling structure. The efficiency of coupling of microwave power into the discharge was mainly dependent the buffer gas type and pressure and the electron density. At higher pressures, higher coupling efficiencies are observed (over 70 percent for pressures of over 500 mbar of helium). The performance of the lasers using both the coupling stractures was poor when compared to conventional copper based lasers. This was attributed to the interaction of the copper halide with the discharge and a nonuniform temperature distribution along the axis of the quartz tube. An average output power of 18 mW was achieved for both lines in a copper bromide laser. The strontium-ion recombination laser was operated at threshold average microwave input powers, no output powers were measured. The electric field and the electron density of the discharges in both laser systems were estimated and compared with those occurring in the respective conventional laser systems.