Optical Reflectance, Optical Return Loss and Bit Error Rate

Most installers and operators focus on optical attenuation and dispersion to handle increasing data rates. However, in single-mode systems, reflectance is just as critical to achieve the desired bit error rate (BER).

Lasers used with single-mode transmitters include the Fabry-Perot (FP) and the Distributed Feedback (DFB) types, both of which are susceptible to optical reflectance. The reflected energy disturbs the standing optical wave (oscillation) in the laser’s cavity, increasing its noise floor. This in turn affects the BER.

There are two types of reflections. Rayleigh backscatter results when impurities introduced during fiber manufacturing cause light to be reflected in all directions, including back to the optical source. Fresnel reflections are caused by abrupt changes between two media with different refractive indexes, such as a glass to air interface. These commonly occur at mechanical splices, lensed components, or at mated, contaminated, or open connector ports.

When we speak of reflection, there are two terms often used. “Back reflection” refers to a single reflective event such as a connection or mechanical splice. ”Optical return loss” (ORL) represents the combined amount of reflectance from all the link components plus the Rayleigh backscatter from the fiber itself.  System manufacturers specify maximum ORL values to ensure quality of signal performance. Passive networks will often have back reflection specifications for the connector components to be installed in each span.

When testing with an OTDR or loss test set with ORL capability, the operator can measure the ORL of the span. If it is out of spec, it is most often caused by one or more Fresnel reflections at a connection point in that span. This is why it is critical to define the proper connector specifications for both loss and back reflection. For single-mode networks where ORL is critical, angled physical contact (APC) connectors should be specified. APC connectors are typically specified at –65 dB. At a minimum, a –55 dB back reflection specification should be placed on UPC connectors for today’s high speed networks.

EXFO-ORL-Trace.jpg    An OTDR displaying optical return loss

When using an OTDR to measure reflections, a dead-zone box or launch cord must be used to connect to the span under test. The ideal test set up adds a second receive cord to the far end. This will show the results of loss and reflections on the entire link without adding a falsely high reflection from the far end.