Index-matching material: Difference between revisions

Revision as of 05:48, 21 April 2006

Revision as of 01:18, 24 November 2006

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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 [[optical fiber|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. 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 [[decibel|dB]] (''i.e.,'' 14 dB below the level of the incident [[Signalling (telecommunication)|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.

==References==