Connector Loss, Return Loss, and Reflectance – “Highs and Lows”
The condition and characteristics of fiber optic connectors greatly affects the performance of an installed fiber optic link. High connector loss (e.g., insertion loss), low return loss, or high reflectance will impair an application (i.e. 10GBASE-LRM) from running on a network. A high return loss is a good thing and usually results in low insertion loss. Let’s examine the differences between these three terms because they can be confusing.
Optical loss (for connectors), sometimes called attenuation, is simply the reduction of optical power induced by transmission through a medium such as a pair of fiber optic connectors. Return loss is the amount of light reflected from a single discontinuity in an optical fiber link such as a connector pair. Return loss is also called reflectance. For perfect transmission, optical loss and reflected power should be zero.
The equations for optical loss, return loss and reflectance are as follows:
|Optical Loss = 10*log(input power/output power) in +dB|
|Return Loss = 10*log(incident power/reflected power) in +dB|
|Reflectance = 10*log(reflected power/incident power) in -dB|
Return loss and reflectance are both used to describe back reflection at a connector pair. However, one has a negative dB sign and one has a positive dB sign. Perhaps reflectance has its origins in the electrical world where return loss having a negative sign is more properly called the reflection coefficient.
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Last May of this year, a Technical Report (IEC/TR 61282-15) on testing MPO terminated optical fiber cabling was published. More recently, the subcommittee within TIA (TR42.11) decided to reference the Technical Report in a structured cabling standard undergoing updates (TIA 568.3-D). While there was agreement to reference the Technical Report, one error was pointed out that deserves a quick review - return loss.
In IEC/TR 61282-15, the clause on optical return loss and reflectance states: “However, all single-mode MPO plugs have APC end faces specified with a typically low return loss (e.g., > 50 dB).” And, “Multimode MPO plugs terminated with physical contact (PC) end faces typically have high return loss (e.g., 20 dB).”
The point of contention was that > 50 dB return loss should be considered a high value not a low value. Likewise, 20 dB return loss should be considered a low return loss not a high return loss. The confusion may arise because we tend to think of a 20 dB return loss or -20 dB reflectance as having a large percentage of reflected power, so we use the word high in this case. That is technically incorrect and the connector manufacturers would be quick to point that out.
What confuses the issue is that reflectance, a negative value (e.g., -20 dB), uses different language. For example, -20 dB is considered high reflectance and -50 is considered low reflectance. Return loss and reflectance are the same thing except the signs are different. A positive value, is normally used to define the return loss of a connection (two mated connectors). A negative value is used to define a reflectance of a connection. OTDR’s typically use a negative value for connection reflectance.
By example, an open connector end face may have a reflectance of -14 dB or return loss of +14 dB. If there is a lot of reflected power, say a 4% reflectance from an open-ended connector, the calculation for reflectance would be [10 * log (0.04)] and depending on the arrangement of the ratio that produced the 4 % (e.g., 0.04) reflectance, the calculated return loss would be +14 dB return loss or -14 dB reflectance.
Regarding connections in installed optical fiber cabling, all you have to remember is that low insertion loss is best, a 50 dB return loss is better than a 20 dB return loss and -50 dB reflectance is better than -20 dB reflectance. If you want good performance pick connectors rated with 50 dB or -50 dB values that have insertion loss as close to 0 dB as you can find.