IR Fiber Optics-An Introduction

During the past many years of the development of IR fibers there has been a great deal of fundamental research designed to produce a fiber with optical and mechanical properties close to that of silica. We can see that today we are still far from that Holy Grail but some viable IR fibers have emerged which as a class can be used to address some of the needs for a fiber which can transmit greater than 2 μm. Yet we are still limited with the current IR fiber technology by high loss and low strength. Nevertheless, more applications are being found for IR fibers as users become aware of their limitations and, more importantly, how to design around their properties. There are two near-term applications of IR fibers; laser power delivery and sensors. An important future application for these fibers, however, may be more in active fiber systems like the Er and Pr doped fluoride fibers and emerging doped chalcogenide fibers. As power delivery fibers, the best choice seems to be hollow waveguides for CO2 lasers and either SC sapphire, germanate glass, or HGWs for Er: YAG laser delivery. Chemical, temperature, and imaging bundles make use mostly of solid -core fibers. Evanescent wave spectroscopy (EWS) using chalcogenide and fluoride fibers are quite successful. A distinct advantage of an IR fiber EWS sensor is that the signature of the analyte is often very strong in the infrared or fingerprint region of the spectrum. Temperature sensing generally involves the transmission of blackbody radiation. IR fibers can be very advantageous at low temperatures especially near room temperature where the peak in the blackbody radiation is near 10 m. Finally, there is an emerging interest in IR imaging using coherent bundles of IR fibers. Several thousand chalcogenide fibers have been bundled by Amorphous Materials (Garland, TX) to make an image bundle for the 3 to 10 m region.