UPC_fiber_imageIf 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. 

CLICK TO TWEET:  CommScope's Earl Parsons explains the differences between UPC and APC connectors. 

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.

About the Author

Earl Parsons

Dr. Earl Parsons is a Principal Optical Engineer in the Fiber Systems and Standards group with CommScope. He is responsible for the end-to-end optical performance of CommScope fiber products and actively participates in IEEE 802.3 Ethernet Standards.


Dr. Parsons received a BS in electrical engineering from Brigham Young University, Provo, Utah, USA in 2006 and a MS and PhD in optical sciences from the University of Arizona, Tucson, Arizona, USA in 2008 and 2010. From 2010 to 2013 he was a Senior Member of Technical Staff at TE SubCom, formerly known as Tyco Telecommunications. Since 2014 he has worked as a Principal Optical Engineer at CommScope in Richardson, Texas, USA. Dr. Parsons was a recipient of the IEEE Dallas Photonics Society Volunteer of the Year award in 2017.

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2 comments for "Choose your connectors wisely"
Richard Suess Thursday, July 04, 2019 4:05 PM

Earl, Thank you for your 2/2019 article on choosing your connectors wisely. I am a DHS contractor supporting fiber transmission systems. Our DHS facility can be classified as an isolated facility with a mix of existing multimode for short runs, and a growing OS2 single mode cable plant for all new activity. We have begun transition to BiDi single fiber transceivers so WDM rules and related concerns with many patch points apply. We are also considering the use of free space, single fiber CWDM mux units. No RFoG yet, but could happen. With this said, can you see a reason why we should NOT use APC connectors in our OSx single mode cable plant for adds, moves or changes? After reviewing many white papers I am miffed as to why the industry stops short of suggesting that new builds include APC as the preferred connector standard? I am receiving pushback form multimodal vintage types that cherish COTS ST connectors that do not want change, so your input is appreciated. Richard Suess (FOA corp member/ Tellabs OLAN certified).

Earl Parsons Monday, July 08, 2019 9:41 PM

Hi Richard,
Thank you for your comment.

For your application I would recommend APC connectors for your single mode fiber, especially if you think RFoG is a potential application. I can’t think of a reason not to use APC.

For multimode links you would use UPC since there are currently no standards for multimode APC connectors.

Thank you.
- Earl

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