The TIBr-TIl (thallium bromoiodide) crystal and its fiber preparation method were studied in order to develop a high power COzlaser beam fiber. As a result of this study, the most preferable composition for the fiber was determined to be 42 wt. % TIBr. The fiber (0.5 mm in diameter and 1.5 m in length) at that composition had the following excellent characteristics:(1) a high output power of 138 W (70 kW /cm 2 ), (2) a high total transmissivity of more than 93%/1.5 m, and (3) a bending radius of ultimate 3.5 cm.

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Pressure‐extruded infrared optical fiber is a polycrystalline structure with grain sizes of 30 to 70 μm, and it exhibits a large amount of attenuation. the attenuation due to scattering is more than one‐half the total attenuation and is induced by the residual strain, surface roughness, and grain boundary defects. In the investigation of loss mechanisms, it is indispensable to measure separately the different losses such as absorption loss and scattering loss. Since the scattering loss can be measured by an integrating sphere sliding along the fiber axis, the absorption loss is obtained by the subtraction of the scattering loss from the total loss. This paper proposes a new method for directly measuring the absorption loss, and experiments using this method were carried out. the method is based on the thermal expansion associated with temperature rise due to the absorption. It is shown that the absorption loss can be evaluated by measuring the time response of the elongation of the fiber in which the laser power is absorbed. In addition a new measuring method for the heat transfer coefficient of the fiber also is proposed. Using these methods, the absorption and scattering losses of KRS‐5 optical fiber were measured. the results indicate that the scattering loss is about 1.5 times larger than the absorption loss.

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