There has been much recent interest in infrared-transmitting materials because of their potential of becoming the next generation of optical communication fibers. One of these polycrystalline fibers, thallium bromo-iodide (commonly known as KRS-5), is considered to be a good candidate because it is both transparent down to 25 μm and, unlike fluoride glasses, is not hygroscopic. Moreover, it has been found that the long 10.6-μm wavelength is strongly absorbed by animal tissues and thus the combination of a CO<sub>2</sub> laser with a KRS-5 fiber would be good choice for medical surgery. Usually, fibers of KRS-5 are extruded continuously from a high-pressure ram extrusion apparatus; thus it is important to understand the effect of pressure on the optical properties of KRS-5 fibers, particularly as pressure has been shown to induce a large red shift in the optical absorption edges of thallium halide salts. Furthermore, it is not clear whether the frequency shift is a reversible process in KRS-5 and thus whether the pressure used in the manufacturing process will have an impact on the transmitting characteristics of the final products.
P. W. Yang, P. T. T. Wong, F. Baudais, and H. H. Mantsch, "Pressure Dependence of the Optical Absorption Edge of KRS-5: An Interferometric Study," Appl. Spectrosc. 40, 1240-1242 (1986)