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 (P_o) and at the input port of the coupler where the detector is located (P_r). P_o is corrected to account for the loss in power through the device under test. P_r 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 P_r/P_o.

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?