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If you’ve ever looked out a window at night from a well-lit room, you’ve observed Fresnel Reflection. Since it’s dark outside, you can very clearly see your reflection in the window. This is because whenever light travels from one medium to another, a portion of light striking the boundary of the medium is reflected. The amount of light reflected depends on the difference in index of refraction of the two materials. For light traveling from air to glass (or from glass to air), about 4 percent of the light is reflected. At night, the 4 percent reflection of the indoor lights may be brighter than the light from outside; therefore, you see your reflection. Fresnel Reflection isn’t as noticeable during the day since the reflected light is much less than the sunlight from outside.
A similar occurrence happens with fiber optics and connectors. There are two common types of fiber connectors: Ultra Physical Contact (UPC) Connectors and Angled Physical Contact (APC) Connectors. Both connector types will bring two optical fibers in physical contact with one another to allow for a low loss connection. Fresnel reflections have implications for which one you choose to meet your system requirements.
UPC connectors typically are lower-cost, lower-loss, and are commonly used for connecting transceivers. That said, some transmitters and receivers are designed to use an APC connector. Why? In some systems, light reflected back towards the transmitter can degrade the optical signal and lower the overall system performance. For example, light reflected into the transmitter laser cavity can cause the laser to become unstable and distort the data signal. In addition, reflected light can introduce multi-path interference which is indistinguishable from the original signal, adding noise and degrading system performance.
The 8 degree angled connector interface in APC connectors re-directs reflected light out of the fiber instead of back towards the transmitter. Some systems can benefit from reduced reflections such as those using analog signals or high-power amplifiers. Many other LAN systems are much less sensitive to reflected power and can use UPC connectors.
The choice between UPC and APC connectors is an important engineering decision to ensure the best performance of your optical network while matching the application and system requirements.