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Return Loss and Reflectance
Reflections occur at optical fiber connections. Optical power may be reflected back
into the source fiber when connecting two optical fibers. In laser-based systems,
reflected power reaching the optical source can reduce system performance by affecting the
stability (operation) of the source. In addition, multiple reflections occur in fiber
optic data links containing more than one connection. Multiple reflections can reduce data
link performance by increasing the signal noise present at the optical detector.
Reflectance is a measure of the portion of incident light that is reflected back
into the source fiber at the point of connection. Reflectance is given as a ratio (R) of
the reflected light intensity to the incident light intensity. The reflectance ratio (R)
for Fresnel reflection is discussed in chapter 4.
Return loss and reflectance are measured using EIA/TIA-455-107. They are measured using
an optical source connected to one input of a 2 X 2 fiber optic coupler. Light is launched
into the component under test through the fiber optic coupler. The light reflected from
the component under test is transmitted back through the fiber optic coupler to a detector
connected to the other input port. The optical power is measured at the output of the
device under test (Po) and at the input port of the coupler where the detector
is located (Pr). Po is corrected to account for the loss in power
through the device under test. Pr is corrected to account for the loss in power
through the coupler and any other connection losses in the path. The reflectance is then
given by the ratio Pr/Po.
Return loss is the amount of loss of the reflected light compared with the power of
the incident beam at the interface. The optical return loss at the fiber interface is
defined as
Return loss = -10 Log R
Return loss is only the amount of optical power reflected and does not include power
that is transmitted, absorbed, or scattered.
Q.22 List two effects that reflections can have on a fiber optic data link.
Q.23 Reflectance is given as what ratio?
Q.24 Does return loss include power that is transmitted, absorbed, and/or scattered?
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