Performance of the He–Ne Gas Laser as an Interferometer for Measuring Plasma Density

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Abstract
The characteristics of the He–Ne gas laser used in a new simple interferometric technique have been studied experimentally and theoretically. The interferometer has two novel features: first, the intensity of the laser itself is used to detect the fringes and second, because the intensities of the 0.63‐μ (red) and 3.39‐μ (infrared) laser beams are coupled, interference in the infrared can be detected by a simple photomultiplier monitoring the red beam. The system does not respond instantaneously to changes in the optical path length; experimental measurements show that when the red beam is used to follow interference in the infrared, the maximum detectable response is limited to about 3×106 fringes per second. Discussion of the frequency response and the cross‐coupling between the two wavelengths leads to the conclusion that the frequency response is limited by the red channel only. Experimental details of the interferometer are described, including the application of a multipass system which, with some loss in spatial resolution, increases the sensitivity of the interferometer by at least a factor of 20.