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