The use of fiber optics to transmit laser energy has important applications in areas as diverse as laser surgery, printing and marking, and industrial welding. For example, industrial welding, cutting, and heat treating require delivering high laser power to remote locations. In these and other applications fiber optics has some clear advantages over bulky and expensive articulated arms, i.e., moving tube-and-mirror systems. Fiber delivery systems are small, much more flexible than articulated arms, and less expensive. Large-core, MM silica fiber optics have been used to deliver several kilowatts of Nd:YAG laser power over tens of meters. In surgical laser systems, surgeons prefer delivery via fiber optics because the distal (output) end of a fiber gives the surgeon the tactile feel of a surgical instrument, and fibers can be inserted directly into the body through endoscopes for least-invasive procedures. From the earliest beginnings of IR fiber technology, it has been the dream of many to reliably transmit IR laser power for applications analogous to those employing silica fibers. As will be seen, the goal of a reliable IR laser power fiber optic has not been fully realized, but great progress has been made, largely through the use of hollow waveguides.
Back in the mid-1970s when IR fibers were first being seriously studied, it was felt by many that one of the most important and exciting uses for non-glass IR fibers was in the transmission of CO2 laser power. At that time the CO2 laser was already an important medical and industrial laser, yet the only beam delivery systems were articulated arms. It was generally felt that the new IR fibers being developed would find an immediate home as power fibers for 10.6 Î¼m, just as silica fibers were being used to transmit Nd:YAG laser power. When the first PC, KRS-5 fiber was developed at HRL, it was felt that this fiber would revolutionize CO2 laser surgery. Unfortunately, some serious problems developed within only a few years after the first extrusion of this fiber. The major problem was that the melting point of the thallium as well as the silver halide fibers is quite low compared to silica fiber (see Table 6.1). During transmission of laser power, the KRS-5 fiber heats up due to absorption. Heating at the output end is greatest because of the high reflection (~17%) at the end face, which leads to large scattering losses.