A 337-μ HCN laser interferometer for plasma diagnostics

A self-modulating laser interferometer incorporating a 337-μ HCN laser is described and a simple theory of its operation is developed. This theory shows that the long lifetimes of levels involved in molecular laser transitions may cause a peaked frequency response in such an instrument. Experimental observations indicate a flat frequency response up to the maximum attained fringing rate of 10 kHz. The interferometer was used to make electron density measurements in the range 2 × 1012 cm−3−1014 cm−3 for an effective path length of 150 cm in a decaying helium plasma. The interferograms obtained show a marked time variation in fringe amplitude which is attributed to the refractive effects of electron density gradients in the plasma.