Improved resolution for a multi-fringe phase detection circuit of a far infrared laser interferometer on LHD

A multi-fringe phase detection circuit for a far infrared (FIR) laser interferometer of a fusion plasma experiment has been designed. The specification of this circuit is a follows, when the intermediate beat signal is 1 MHz, the phase detection range is 31 fringes with a phase resolution of 1/80 fringe, and the response speed is 10 μs. Because all phase comparison parts of this circuit are constructed of TTL ICs, the manufacture of this circuit is easy. It has been used to measure the electron density profile in the Large Helical Device (LHD).