Optical Return Loss
Optical Return Loss (ORL) is a reflection that travels down the fiber back to the source. In high speed systems this is undesirable because it can interfere with the transmission. In an analog transmission, such as cable television, this can result in "snow". In a digital system it can result in an excessive bit error rate. In either case, it results in a garbled transmission. In the high power systems used by Telecoms and CATV companies, it can also significantly shorten the life of the expensive lasers.
To decrease reflections, special polishes are used. A standard PC polish will have a back reflection of about -30 dB. A UPC polish, which can be done on the same connectors, will have a back reflection of less than -50 dB or -55 dB, depending on the polish procedure used. This is done by decreasing the "radius of curvature" of the end-face. The tighter the radius, the more curved the end-face is. The curve of the end-face causes reflection to be deflected into the cladding, rather than back into the core of the fiber. The light is still reflected; it just can't travel back to the source.
For high-end applications, an APC connector is used. The end of the connector is polished to an 8° angle. This angle causes almost all of the reflected light to be deflected into the cladding of the fiber and will deliver a back reflection of less than -65 dB or less than -70 dB, depending on the polishing procedure used.
A PC or UPC connector that is not mated to another connector will have a back reflection of about -14.5 dB. For unused ports, which may be present when a splitter is installed with more splits than is required for the current requirements (leaving room for future expansion), a Low Reflection Termination, or Black Hole Connector, is used. These are very short fibers that are connectorized with a UPC or APC connector on one end and an angled cleave on the other. The fiber is so short that it actually ends inside the connector boot. The angled cleave reduces the reflection the same way that an angled connector does, deflecting the light into the cladding where it leaks out of the fiber and is lost.
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