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 $50–600μm$ 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 $432.6μm$ and a Schottky-diode detector have yielded a minimum detectable pathlength variation of $Δz=0.4μm$⁠, corresponding to a change of the line-integrated electron density $n¯e×L$ of about $5×1015m−2$⁠. 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.