The conventional high power $CO_2$ laser systems make use of high voltage DC for the excitation of the medium, and the light propagates in the free space between the two mirrors of the resonator. The light can travel back and forth along a hollow narrow tube, however, with greater frequency differences between neighboring transverse modes, which gives inherent stability of the resonator with permanent mirror alignments.
One of the possible excitation of $CO_2$ laser medium is by radio frequencies (RF). With the advent of high frequency power semi-conductor devices it is possible to make light weight and small RF source for laser pumping. The higher frequency produces the higher efficiency and the lower sustaining voltage. As the frequency increases, the RF standing wave voltage variation along the optical waveguide rises, though.
This paper analyzes the incident and reflected RF wave on the waveguide and derives the value of the optimal inductances of the shunt reactors pending in the laser channel to give the smallest RF voltage variation along the transmission line.
A sealed-off waveguide $CO_2$ laser with a length of 30 cm and a cross-sectional area of 2mm square was designed and constructed with brass electrodes and beryllia dielectric walls. The laser is transversely excited by radio frequency between 60 and 120 MHz. The output characteristics of the laser is investigated by performing the variation of the total gas pressure, RF input power, and the mixing ratios of the gases.
The RF characteristics of the laser channel were measured by a RF probe coupled with a standing wave ratio (SWR) meter and a network analyzer system. With pertinent inductors at the ends of the channel the optical output power produces 40 % more power than that without them. Also the waveguide channel conductances of both the cold and hot states were estimated by the computer analysis of the measured RF reflection coefficients. The estimated shunt resistances of 2,000 Ω and 700 Ω for cold and hot states, each other, make the best computer response to the measured reflection curve.
Maximum output power of 3.5 watts is measured under the conditions of the gas mixture of 3:1:1 of He:$CO_2$:$N_2$, the total gas pressure of 50 Torr, input RF power of 200 W. The total reflecting mirror has 1 inch diameter, 2 m radius, and enhanced dielectric coating on the silver layer on the silicon substrate of 0.120 inch (3 mm) thickness, which gives more than 99.5 % reflectivity at 10.6 μm. The partial mirror has the same geometric dimensions, but has 95.7 % reflectivity at the resonator side and antireflection coating on the flat ZnSe substrate at the output side. It is expected to give more optical output power by the optimal mirror reflectance and also by th addition of small amount of xenon gas.
