Development of the liquid nitrogen cooled hollow core waveguide fiber for the mid-infrared region

We report on the development of the liquid nitrogen cooled hollow core waveguide (HCW) applicable for guiding mid-infrared light from telescopes to the cryogenically cooled instruments with high transmittance. We employed a commercially available HCW, 2-m in length, transmitting 8 < lambda < 13 mu m in wavelength (astronomical N-band), made of polyimide tube with inner diameter of 0.8mm9 coated with a thin glass clad with Ag coated with AgI. In the present study, we designed and fabricated a flexible triple tube capable of cooling the HCW with liquid nitrogen flow, preventing ambient water vapor condensation on both inner and outer surface. One end of the HCW was sealed with 2mm phi and 1mm thick AR-coated Ge window employed with a vacuum adhesive, while another end left open for evacuation inside the HCW. The transmittance of the liquid nitrogen cooled HCW was measured by using the light source: quantum cascade laser continuously oscillating at 10.4 mu m in wavelength with similar to 50 mW, and a thermopile detector. We have found similar to 10% improvement of the transmittance at 77 K compared with the room temperature condition. The observed increase of HCW transmittance is in reasonable accordance with a theoretical prediction based on the Drude model and the experimental data for electrical conductivity of thin Ag film at low temperature.