New dielectric coatings for low-loss hollow glass waveguides and bundles

The dielectric coated metallic hollow glass waveguides (HGWs) are used for transmitting radiation of different wavelengths for a number of applications including laser power delivery and thermal imaging. HGWs with very small bore size, 320 μm down to 50 μm, are desirable to obtain good imaging resolution.; In this study, we have succeeded in optimizing the deposition of silver (Ag) and silver iodide (AgI) thin films for these very small bore size HGWs. Rigid HGW bundles having 900 holes of bore size 50 μm have been made by coating the capillary arrays from Collimated Holes®. Ag and AgI films are characterized to correlate their morphology with their processing conditions. The spectral attenuation for these bundles is relatively low in the 8 to 12 μm region. The results of IR imaging using the rigid HGW bundles, having an active core area of 30%, are very promising. The loss measurements for a 100 μm HGW at 10.6 μm, using a CO₂ laser, yield values around 33 dB/m. Therefore, these very small bore size HGW bundles are useful for short length imaging applications (<15 cm) such as image transfer faceplates.; HGW theory shows that the loss of HGWs can be reduced by a factor of five by using a 3-layer high index contrast dielectric coated Ag HGW. We have subsequently studied new additive dielectric coatings such as polymers and metal sulfides such as cadmium sulfide (CdS), and lead sulfide (PbS).; Optical polymers such as polystyrene (PS), cyclic olefin copolymer (COP), and Teflon AF are good low index dielectric materials for HGWs and have excellent optical properties in the visible and IR. We have found that the morphology of these polymer films that are solution processed depends on the solvent, the deposition environment, and the curing process.; CdS and PbS have excellent optical properties in the IR with a refractive index of 2.3 and 4, respectively at 10.6 μm. For the first time, dynamic liquid phase deposition has been used to deposit single and multilayers of good optical quality US and PbS thin films inside hollow glass waveguides. A linear growth rate regime is observed for the deposition of CdS and PbS films on Ag with growth rates of 2.3 and 6.9 nm/min respectively. The rms roughness of 150 nm thick US and PbS thin films on Ag is around 18 nm. The FESEM analysis of multilayer CdS/PbS coated Ag HGWs indicates that the growth kinetics of PbS is very similar on US and Ag. Also both US and PbS are chemically compatible due to their very similar processing chemistry and solubility products. The loss measurements at 1.55 μm for a 1000 μm, 3-layer CdS/PbS coated Ag HGW is 0.06 x 0.04 dB/m while that of a single dielectric US coated Ag HGW is 0.2 x 0.04 dB/m. This indicates a reduction in the attenuation by a factor of three for the 3-layer dielectric coated Ag HGW.