Frequency-stabilized single-mode cw 118.8-μm CH3OH waveguide laser for large tokamak diagnostics

A frequency-stabilized single-mode cw 118.8-μm CH3OH waveguide laser was newly developed for polari-interferometry on the JT-60 tokamak. Interferometric plasma diagnostics on large fusion devices requires a mode purity of a source laser for the long-distance propagation and frequency stability to obtain an adequate fringe resolution. The accomplished performances are Δf/f<1×10−8, power stability <0.03%, and single EH11 mode to yield a Gaussian-shaped profile in the far field. Homogeneous pumping of laser media with a Ge étalon beam coupler and feedback stability control are the keys to the source far-infrared laser performances required for large tokamak diagnostics. The developed laser was totally adjustment free for more than 2 weeks and reliably in operation for 2 years, including the real-time electron density feed-back control on the JT-60 tokamak.