Fiber Return Loss and Reflectance

Optical Return Loss and Reflectance Optical Return Loss or commonly called as ORL is the ratio of optical power reflected by a component or an assembly to the optical power incident on a component port when that component or assembly is introduced into a link or system. Optical return loss is expressed in decibels, dB.

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, scattered or absorbed inside the fiber.

Return loss and reflectance are important for fiber optic patch chords and connectors. Reflections occur at optical fiber connections. At the meeting point of two optical fibers, optical power will be reflected back into the medium, i.e., fiber. For a laser based fiber optic system, back reflected power reaching the optical source will reduce system performance. This happens and it will affect the stability of the laser source. Multiple reflections occur in fiber optic data links that consists of more than one connection. Signal noise level will be high in such systems. Multiple reflections will reduce fiber data link performance by increasing the signal noise that is 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 expressed as a ratio of the intensity of light reflected to the incident light intensity.

Return loss and reflectance are measured as per the test procedure mentioned in FOTP-107 or EIA/TIA-455-107. Optical return loss and reflectance are measured using an optical source connected to one input of a 2 X 2 fiber optic coupler.

Through a fiber optic coupler, light is launched into the component under test. The light reflected from the component 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 and denoted as(OPo) and at the input port of the coupler where the detector is located (OPr). OPo is corrected to account for the loss in power through the device under test. OPr is corrected to account for the loss in power through the coupler and any other connection losses in the path. The ratio OPr/OPo is the reflectance.

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