Multilayer thin film coatings for reduced infrared loss in hollow glass waveguides

Hollow glass waveguides (HGWs) are an attractive alternative to traditional solid-core and 2D photonic crystal, infrared transmissive fibers. Applications for HGWs at wavelengths longer than 2 microns include use of the guides for the delivery of laser power and for use as chemical and thermal sensors. To date, the most common HGW is one with an inner coating of Ag followed by a single-dielectric layer of AgI. These single-layer dielectric coated HGWs have losses for a 700-micron bore guide as low as 0.2 dB/m at 10.6 microns. However, if a multilayer stack of alternating high/low index thin films is deposited instead of a single dielectric layer then the loss can be reduced substantially. In the present study, multilayer dielectric thin films have been deposited inside silica tubing using a liquid-phase deposition method. High index coating materials used include metal sulfides such as PbS while the low index materials include polystyrene (PS) and some sulfides. To date it has been possible to deposit two-layer coatings using, for example, CdS and PS but a lower loss is possible if the coating stack is composed of three dielectric layers. In past work CdS/PbS/CdS coatings were deposited and found to have a measured a loss at λ = 10.6 microns that is approximately two times lower than that for a single dielectric layer. In this paper the theory of multilayer coatings will be presented along with the optical loss measurements from λ = 2 to 12 microns for the multilayer dielectric coatings.