Two color far infrared laser interferometer

Abstract

Two color interferometer using a short wavelength far infrared laser has been developed for high performance plasmas on large helical device and for future fusion devices such as ITER. High power laser lines simultaneously oscillating at 57.2 and $47.6\mu \mathrm{m}$ were achieved in a ${\mathrm{CO}}_2$ -laser-pumped $\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{D}$ laser. By introducing Ge:Ga photoconductive detectors operating at liquid He temperature, we have successfully detected two color beat signals (0.55 and $1.2\mathrm{MHz}$ ) with excellent signal-to-noise ratio $(\sim 40\mathrm{dB})$ . These beat signals were fed into phase comparators for phase measurement after passing through intermediate frequency bandpass filters. Two color far infraned laser interferometer work was successful in the demonstration of mechanical vibration compensation.