Methods for reducing the fresnel reflection in an optical‐fiber connector with index matching material

If there is a discontinuity of the refractive index in optical fibers connected with an optical connector, the light is reflected from this part to the light source, creating a problem in its light transmission characteristic. The main causes of the discontinuity are: (1) the gap between the endfaces of the optical fibers; and (2) the change of the refractive index at the end‐face of the fibers due to their polishing. Solutions to these problems are investigated in this paper, using silica‐based graded index multimode optical fibers. For (l), a refractive‐index matching material is employed, and the method for selecting the central value of the refractive index and the temperature coefficient from the reflected light in the visual‐light wavelength region is investigated. It has been confirmed that the refractive‐index matching material made of silicone resin and chosen by this method can obtain the required reflection loss greater than 40 dB, in each of the transmission wavelengths of 0.85 pm and 1.3 μm, and in the temperature range of −30°C to +60°C. For (2), the relationship between the polishing conditions and the reflected light power was investigated. It has been found that the refractive index of the endface of an optical fiber changes slightly when polished. The conditions of polishing for suppressing this change have been determined, and the property of the polished endface has been clarified.