Hollow Glass Waveguides
One of the most popular hollow waveguides today is the hollow glass waveguide (HGW) developed by Harrington’s group at Rutgers University. This hollow glass structure has the advantage over other hollow structures because it is simple in design, extremely flexible, and, most important, has a very smooth inner surface. HGWs have a metallic layer of Ag on the inside of silica glass tubing and then a dielectric layer of AgI over the metal film identical to that used to make the hollow plastic guides. Figure 1 shows a cross-section of the structure of the HGWs. The fabrication of HGWs begins with silica tubing which has a polymer (UV acrylate or polyimide) coating on the outside surface. A wet-chemistry technique (see Figure 2), similar to that used by Croitoru and his co-workers to deposit metal and dielectric layers on the inside of plastic tubing, is employed to first deposit a silver film using standard Ag plating technology. Next, a very uniform dielectric layer of AgI is formed through an iodization process in which some of the Ag is converted to AgI. Using these methods, HGWs with bore sizes ranging from 250 to 1000 µm and lengths as long as 13 m have been made.
Figure 1 - Structure of the HGWs showing the metallic and dielectric films deposited
inside silica glass tubing
Figure 2 - Schematic of the experimental set-up for depositing the Ag metallic and AgI
dielectric films inside silica tubing to form the HGWs
The spectral response for HGWs depends critically on the thickness of the dielectric film. Generally, for the AgI films, the film thickness ranges from 0.2 to 0.8 µm. In Figure 3, we show the spectral response of two waveguides which have different thickness films deposited on the inside of a 700 µm bore silica tube, 1 m in length. The thickest film gives a minimum loss at 10.6 µm while the thin film was selected for minimum loss near 3 µm. The latter guide has a fairly flat response beyond 3 µm and, therefore, this guide would be useful in broadband applications. The structure observed in the spectra is due to thin-film interference effects similar to that commonly observed in thin-film coatings on optical components. These effects have been observed and extensively discussed in the work of Matsuura, et al.
Figure 3 - Spectral response of two HGWs; one designed for low loss at the CO2 laser
wavelength of 10.6 µm and the other for low loss at the Er:YAG laser
wavelength of 2.94 µm
The strong bore-size dependent loss for straight HGWs is shown for two guides in Figure 4. These data were taken using CO2 and Er:YAG lasers and the guides were optimized for minimal loss at 10 and 3 µm, respectively. The solid curves are theoretical calculations of the losses for the lowest order HE11 mode. At the CO2 laser wavelengths we see not only the strong 1/a3 dependence predicted by MS theory but also that there is good agreement with the experimental results. However, at 3 µm the calculated losses are much lower than the measured values. This is a result of increased scattering losses at the shorter wavelengths and the multimode character of the Er:YAG laser.
Figure 4 - Measured losses for straight HGWs using CO2 and Er:YAG lasers. Note
that the predicted losses are well below the measured ones at 2.94 µm
Bending increases the loss in hollow waveguides beyond the straight loss of a HGW. The additional bending loss varies as 1/R as reflected in the data for two 530 µm bore guides in Figure 5. These data show the total loss for guides with a constant length of fiber under bending. A curvature of 20 cm-1 represents a bend diameter of only 10 cm. This is sufficiently small for most applications.
Figure 5 - Bending losses for two 530-µm-bore HGWs measured at 10.6 and 2.94 µm