Upgrade of far-infrared laser-based Faraday rotation measurement on MST.

Recently, the far-infrared (FIR) laser (λ(0)=432 μm) Faraday rotation measurement system on MST has been upgraded. The dc flowing-gas discharge CO(2) pump laser is replaced by a rf-excited, sealed CO(2) laser at 9.27 μm (GEM select 100, Coherent Inc., Santa Clara, CA), which is subdivided equally into three parts to simultaneously pump three FIR cavities. The total infrared pump power is approximately 80 W on the 9R(20) line required to pump the formic acid molecule. Each FIR cavity produces ∼12 mW, sufficient for 11 simultaneous chord interferometry-polarimetry operations. Three key issues [(1) conservation of circularly polarized wave, (2) colinearity of two probe waves, and (3) stability of intermediate frequencies between lasers] affecting the Faraday rotation measurement have been resolved experimentally.