The optogalvanic effect in molecular discharges and the stabilization of CO₂ lasers

Abstract

The optical perturbation of discharge current voltage characteristics (optogalvanic effect or OGE) has been investigated for CO2 and CO laser discharges. A quantitative power perturbation model is constructed and a series of experiments show, a close agreement for changes of all the major operating parameters of the CO2 laser. The theory consists of an evaluation of the microscopic kinetic relaxation processes leading to the changes in thermal balance of the discharge that occur due to the absorption and amplification of the resonant laser radiation. A generation of compact and efficient cw CO2 lasers has been developed which produces higher stable output powers per unit length than previously reported, and these have been actively stabilized by OGE to provide a high degree of frequency stability (<50 kHz) and amplitude stability (< 0.5%) which is a six orders and two orders of magnitude improvement over the passive resonator capability, respectively. New optogalvanic effects have been discovered both at high frequencies (up to 100 kHz) and for sequence (00&deg;2) laser transitions in CO2 and also in the cw CO laser. Preliminary investigation of optogalvanic detection of standing wave saturation resonances in low pressure discharged CO2 have been carried out and some analysis of discharge noise has been necessary to evaluate the ability of such a narrow band detector for laser stabilization. The empirical evidence provided by the temporal response of OGE combined with the gas composition dependence (including N2 free mixtures) proves conclusively that no major ionization mechanism described so far can be responsible for the effect. A thermal explanation of the effect due to modified kinetic cooling of the laser gas has been developed from existing qualitative explanations. This proposed "gas temperature" power perturbation model provides for the first time an accurate (~20%) prediction of perturbations.(amplitude, phase, and frequency) due to the resonant interaction of a CO2 laser beam over a wide range (up to 4 orders of magnitude) of detailed parametric changes, with a CO2 or laser mixture discharge.