Clarification regarding 'no practical effect' on digital systems including potential degradation for DFB lasers. Mention of Angle Polished Connectors as alternative solution to the same problem.
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, or with mechanical splices,orto atreduce signal reflected in the endsguided ofmode fibers(known as return loss) (see: [[Optical fiber connector]]). Without the use of an index-matching material, Fresnel reflections will occur at the smooth end faces of a fiber unless there is no fiber-air interface or other significant mismatch in refractive index. These reflections may be as high as -14 [[decibel|dB]] (''i.e.,'' 14 dB below the leveloptical power of the incident [[Signaling (telecommunication)|signal]]). When the reflected signal returns to the transmitting end, it ismay be reflected again and returnsreturn 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 twice-reflected, delayed signal willsuperimposed haveon nothe practicaldirect effectsignal onmay noticeably degrade an analog [[digitalbaseband]] systems[[intensity]]-modulated because[[video]] ofsignal. itsConversely, lowfor leveldigital relative totransmission, the directreflected signal;i.e.,will it willoften have no practical effect on the detected signal seen at the decision point of the digital [[optical receiver]] except in marginal cases where bit-error ratio is significant. ItHowever, maycertain bedigital noticeabletransmitters insuch anas analogthose [[baseband]]employing a [[intensityDistributed Feedback Laser]] may be affected by back reflection and then fall outside specifications such as Side Mode Suppression Ratio, potentially degrading system bit error ratio, so networking standards intended for DFB lasers may specify a back-{{Disambiguationreflection needed|date=Junetolerance 2011}}modulatedsuch [[video]]as signal-10dB for transmitters so that they remain within specification even without index matching. This back-reflection tolerance might be achieved using an optical isolator or by way of reduced coupling efficiency.
For some applications, instead of standard polished connectors (e.g. FC/PC), angle polished connectors (e.g. FC/APC) may be used, whereby the non-perpendicular polish angle greatly reduces the ratio of reflected signal launched into the guided mode even in the case of a fiber-air interface.
