In [[telecommunication]], an '''index-matching material''' is a substance, usually a liquid, cement (adhesive), or [[gel]], which has an index of [[refraction]] that closely approximates that of an [[optical fiber]], and is used to reduce [[Fresnel reflection]] at the fiber endface. (FAA)
<i>''Note 1:</i>'' An index-matching material may be used in conjunction with pairs of mated connectors, with mechanical splices, or at the ends of fibers.
<i>''Note 2:</i>'' Without the use of an index-matching material, Fresnel reflections will occur at the smooth endfaces of a fiber. These reflections may be as high as -14 [[Bel|dB]] (<i>''i.e.,</i>'' 14 dB below the level of the incident [[signal]]). When the reflected signal returns to the transmitting end, it is reflected again and returns to the receiving end at a level that is (28 plus twice the fiber [[loss]]) dB below the direct signal. The reflected signal will also be delayed by twice the [[delay]] [[time]] introduced by the fiber. The reflected [[signal]] will have no practical effect on [[digital]] systems because of its low level relative to the direct signal; <i>''i.e.</i>'' , it will have no practical effect on the detected signal seen at the decision point of the digital [[optical receiver]]. It may be noticeable in an analog [[baseband]] [[intensity]]-modulated [[video]] signal.
