Single-crystal (SC) fiber optics have been grown for many years using some variation on crystal pulling. There are currently two techniques for the growth of SC fibers. One is the edge-defined, film-fed growth (EFG) method and the other is the laser-heated pedestal growth (LHPG) method. The most common method used to grow SC fibers is the LHPG technique. In this method a CO2 laser is used to melt the tip of a crystalline source rod and a fiber is pulled upward from the molten tip. To date the majority of passive SC fibers grown have been made from aluminum oxide (Al2O3) or sapphire. Sapphire is a uniaxial crystal in the trigonal crystal system, hexagonal class. Sapphire is inert and insoluble with a melting point of 2053°C. It is an extremely robust material with a usable fiber transmission from about 0.5 to 3.2 µm. The key physical properties of sapphire as well as other oxide crystal fiber candidates are given in Table 1. The properties of these oxide crystals clearly indicate the outstanding physical characteristics of these materials and how well they rival even those of silica. For example, sapphire has a Young’s modulus approximately 6 times greater than silica, the melting point is over 2000°C, and it is extremely hard. These properties make oxide crystal fibers almost ideal for applications not exceeding 5 µm.
Table 1 - Properties of some oxide crystals used for fabrication of SC fiber optics
Other infrared transmitting fibers that are good candidates for sensor and power delivery applications include the heavy-metal fluoride and chalcogenide glasses, polycrystalline silver halides, and hollow waveguides. Fluoride glass fibers transmit to ~4.5 mm with losses below 0.05 dB/m at 2.94 mm. In addition to their excellent transparency, they are also quite flexible and have been used to deliver Er: YAG laser fluences as high as 200 J/cm2. However, fluoride glass fibers have major drawbacks: limited mechanical strength, low glass transition temperature (~150 °C), and chemical reactivity with water. Chalcogenide glass fibers, made from the chalcogen elements such as As, Ge, Te, S, and Se, transmit beyond 10 mm but they also have low glass transition temperatures. Like the fluoride glasses, they are weak in shear strength which makes them more fragile than sapphire. Silver halide fibers have a large transparency window as high as 20 mm but again they are weak, sensitive to light, and have low melting points.