Development of a new two color far infrared laser interferometer for future fusion devices

A new two color far infrared (FIR) laser interferometer under development for future fusion devices will be presented. The laser wavelength is optimized from the consideration of the beam refraction effect due to plasma density gradient and signal-to-noise ratio for an expected phase shift due to plasmas. Laser lines of 57.2 and 47.6 ?m are found to be suitable for the applications to high performance plasmas of Large Helical Device and future fusion devices such as the International Thermonuclear Experimental Reactor. The output power of 57.2 ?m CH_3OD laser is estimated to be ~1.6 W, which is the highest laser power in the FIR wavelength regime. The optical configuration of a new interferometer system using two colors will be proposed. In the system, one detector simultaneously detects the beat signals of the 57.2 and 47.6 ?m laser lines, and each interference signal can be separated electronically (1 MHz for 57.2 ?m and 0.84 MHz expected for 47.6 ?m). Mechanical vibration can be compensated by using the two color interferometer. The present status of the development of the system is also presented