Fourier spectroscopy of water vapor absorption in 40 m optical transport channel of the NovoFEL

Radiation of the Novosibirsk terahertz free electron laser (NovoFEL) is transported to user stations by a 40 m long optical channel. A high vacuum volume of the NovoFEL and the optical channel are separated by a CVD-diamond window. Because of a sufficiently large size of submillimeter beams and the absence of chip and firm vacuum windows of an adequate size, we use a nitrogen-filled channel at normal atmospheric pressure and thin polypropylene windows for radiation coupling on the user stations. The main problem of the long channel is water vapor absorption. A special system of drying with round circulation was made on the basis of a zeolite absorber. A combination of Fourier and free electron laser spectroscopy of the system is presented in this paper.