Two-color CO2 interferometer with single Bragg cell and improved vibration compensation at 3.39 μm

A single-chord CO2 (10.6-μm) He–Ne (3.39-μm) two-color laser interferometer which has been developed for the ETA-BETA II RFP machine is presented. Compared to the usual 0.633-μm vibration compensation systems, the 3.39-μm interferometer allows faster optical path-length variations to be measured without losing fringes, can stand larger angular deflections of the beams (e.g., due to vibrations of the optic mounts) without losing alignment, and uses the same acousto-optic modulator as the CO2 system, thus minimizing the differences in the optical path of the two interferometers. The instrument has been extensively utilized on ETA-BETA II, where line density variations larger than 1020 m−2 and occurring in less than 4 μs can be routinely measured and the residual error after vibration compensation is ≊2×1018 m−2. Some further developments of the diagnostic are presented, which are presently underway or planned.