Hollow Glass Waveguides
Applications of hollow waveguides fall into two broad categories: laser power delivery and fiber sensors. As fiber sensors, hollow waveguides are generally used either to transmit blackbody radiation for temperature measurements or as an active or passive link for chemical sensing. Saito and Kikuchi give a good review of the use of hollow guides as IR fiber sensors. The use of hollow glass or metallic waveguides to deliver laser power has largely been relegated to laser surgery where the required power is less than 100 W. Furthermore, most of the surgical applications to date involve the CO2 laser, as this laser is one of the most commonly used medical lasers. As mentioned above, the HGWs are capable of delivering over a kilowatt of CO2 laser power yet they have not been accepted as flexible delivery systems for industrial lasers. There are two most likely explanations for this: 1. hollow guides have a somewhat higher loss when compared to current articulated arm technology; and 2. industrial applications generally require a high quality (low M2) laser output mode whereas hollow waveguides can distort the TEM00 input beam of the CO2 laser and this can lead to unacceptable kerfs and welds in cutting and welding applications.
Hollow waveguides are an ideal means of transmitting blackbody radiation for thermometric measurements. In particular, the peak of blackbody radiation near room temperature is around 10 µm where these guides transmit very well. They have also been used to transmit radiation above 1000°C for the measurement of jet engine blade temperatures. As a delivery system in chemical sensing applications, hollow guides may be used merely as a passive fiber link from the chemical processing area to a remote detector, or they may play a more active role in which the guide is filled with the gas to be sensed. The latter application involves using the hollow guide itself as both the container for the gas sample and as a waveguide. That is, a coiled hollow guide can replace a standard White cell to give a long pathlength and a small volume cell. Several researchers have used this method to measure small quantities of benign gases.