A simple far-infrared laser interferometer for measuring electron densities in reactive low-temperature plasmas

A sensitive far-infrared (fir) interferometer for electron density measurements in reactive low-temperature plasmas is described. The instrument is based on an optically pumped fir laser (wavelength range 50-600 μm depending on the working gas) and makes use of the nonlinear relation between output power and cavity loss. The fir beam, which leaves the resonator through a coupling hole in the end mirror, is reflected back into the cavity, such that the coupling hole behaves like a variable 'leak' with a loss rate depending on the phase of the reentering wave relative to the standing wave within the resonator. As a result of the feedback, the output intensity undergoes strong nonlinear variations if the optical distance of the external mirror is changed by small amounts, Δz. The power variation is monitored through a small opening in the external mirror. Test experiments using a wavelength of 432.6 μm and a Schottky-diode detector have yielded a minimum detectable pathlength variation of Δz=0.4 μm, corresponding to a change of the line-integrated electron density nexL of about 5x1015 m-2. A first application to argon plasmas in inductively coupled rf discharges has been made, and the results have been compared to concomitant Langmuir probe measurements