A sensitive He-Ne interferometer with passive beam power stabilization for low-density pulsed-plasma measurements

A sensitive interferometer based on a Michelson scheme was constructed and applied to low-density pulsed-plasma measurements. Using a moderate power He-Ne laser and common photodetectors, a sensitivity as high as 1/1000 FWHM of an interference fringe was reached with a minimum line-integrated plasma density as low as 〈N_eL〉_min=8×10¹³ cm ^-2. This sensitivity was obtained in a real plasma experiment with space and time resolution of 1-2 mm and 120 ns, respectively. The time resolution for this technique is restricted mainly by the oscilloscope finite input capacitance and could be made higher but with a loss of sensitivity. In order to obtain such high a sensitivity, we used loads for the photocurrent registration higher than the 50-Ω matched load. The initial phase is not assumed to be optimized and is recorded separately during the operation of the interferometer. Suppression of electrical noise and of instabilities associated with the laser power requires a supplementary detector for background subtraction to improve the signal-to-noise ratio. A description of the high sensitivity interferometer with the passive beam power stabilization, analysis of the data treatment procedure, as well as measurements for an electron density distribution, are presented.