In optics and fiber optics, 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 element or fiber, and is used to reduce Fresnel reflection at the surface of the element.
In fiber optics and telecommunications, an index-matching material may be used in conjunction with pairs of mated connectors, with mechanical splices, or at the ends of fibers (see: Optical fiber connector). Without the use of an index-matching material, Fresnel reflections will occur at the smooth end faces of a fiber. These reflections may be as high as -14 dB (i.e., 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.e., 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.
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“Science has done great things for us; it has also pushed us hopelessly back. For, not content with filling its own place, it has tried to supersede everything else. It has challenged the super-eminence of religion; it has turned all philosophy out of doors except that which clings to its skirts; it has thrown contempt on all learning that does not depend on it; and it has bribed the skeptics by giving us immense material comforts.”
—Katharine Fullerton Gerould (1879–1944)