Bragg cell heterodyne interferometry of fast plasma events

Abstract

Plasma interferometry using the fringe shift technique has been extended to the 100-MHz range by the use of Bragg diffraction in a lead molybdate acousto-optic cell. The cell splits a 633-nm laser beam into zero and first-order beams, which are separated in frequency by 80 MHz. These beams are used in the scene and reference arms of a modified Michelson interferometer and are then recombined on a silicon photodetector. Phase changes are deduced from the variations in time separation of consecutive peaks in the beat signal. The technique has been used to make electron density measurements in a 1-m-long, shock heated theta pinch with 6-nsec time resolution. Density variations during the implosion produce a total phase shift of nearly 1 fringe in 150 nsec. Phase noise in vacuum shots (without plasma) is 1/40 fringe rms; the peak-to-peak noise is 1/10 fringe, thus corresponding to a maximum area density (ℱn dl) uncertainty of 2×1016 cm−2.