Heterodyne interferometer using coaxial CO/sub 2/ and He-Ne lasers for plasma diagnostics

A wide variety of lasers have been utilized successfully for the diagnostics of the magnetically confined fusion. The authors have developed a compact CO/sub 2/ laser interferometer[3] for the electron density measurement of the reversed field pinch plasma in the REPUTE of the University of Tokyo, of which plasma minor radius is 40 cm. A common-path He-Ne laser interferometer was used in order to compensate the mechanical vibrations of the interferometer. The schematic of the interferometer is shown in the figure. The light source is a waveguide CO/sub 2/ laser. An acousto-optical modulator(AOM) generated a diffracted beam which is frequency-shifted by 60 MHz. The diffracted beam is used as a reference one, while the transmitted beam passes through the plasma twice and is used as a signal beam. A HgCdTe detector is used to generate the beat signal between the two beams