A NEW METHOD FOR MEASURING ABSORPTION LOSSES IN AN INFRARED OPTICAL-FIBER

Pressure-extruded infrared optical fiber is a polycrystalline structur e with grain sizes of 30 to 70 mu m, 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 r oughness, and grain boundary defects. In the investigation of loss mec hanisms, it is indispensable to measure separately the different losse s such as absorption loss and scattering loss. Since the scattering lo ss can be measured by an integrating sphere sliding along the fiber ax is, the absorption loss is obtained by the subtraction of the scatteri ng loss from the total loss. This paper proposes a new method for dire ctly measuring the absorption loss, and experiments using this method were carried out. The method is based on the thermal expansion associa ted 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 addi tion, a new measuring method for the heat transfer coefficient of the fiber also is proposed. Using these methods, the absorption and scatte ring losses of KRS-5 optical fiber were measured. The results indicate that the scattering loss is about 1.5 times larger than the absorptio n loss.