Development of an HCN dual laser for the interferometer on EAST

A two-color continuous wave (CW) discharge-pumped far-infrared (FIR) hydrogen cyanide (HCN) laser was developed as the source of an interferometer for measuring the line-averaged electron density in the Experimental Advanced Superconducting Tokamak (EAST). The output power of the dual laser system was about 120mW from each laser on the 337 μm (0.89 THz) line. The polarization of each output beam was fixed using thin tungsten filaments and oscillated
in the EH11 mode. Different megahertz intermediate frequencies (IF) and a slight frequency offset (∼1 MHz) were generated in this system to replace the traditional rotating grating with ∼10 kHz IF, and this can improve the time resolution of the interferometer significantly. The experimental result showed that different IF signals were obtained by successfully adjusting the cavity length. In particular, the beat frequency was captured at ∼1.3 MHz by a Schottky mixer when the length of the resonant cavities was changed by 5 μm by an automatic adjustment system. In order to study the character of IF, a long time record of the IF signal was carried out, and the IF signal could be stabilized for a few minutes in the range of 2 MHz to 3 MHz. A realtime IF stability control system was initially designed for long pulse discharge experiments on the EAST. The ∼MHz frequency response and good phase sensitivity of the dual laser HCN interferometer will allow the system to track fast density profiles and resolve fast MHD events, such as tearing/neoclassical tearing, disruptions, etc.