Backreflection vs Return Loss – What Difference Does It Make?
When light is emitted into a fiber-optic component (ex: connector, fiber, coupler, etc.) some of the energy is transmitted, some is absorbed and some is reflected. In fiber-optic systems, reflected light is due to Fresnel reflections and Rayleigh scattering. Fresnel reflections occur at discrete components and are a result of air gaps, misalignment, and non-matching refractive indexes. Rayleigh scattering is a result of the variations in material density and compositional fluctuations which occur during fiber manufacturing.
Backreflection and Return Loss have been measured on components, connectors, patchcords and cables for years and remain to be two of the key parameters to define the reflections created by the device under test. However, some confusion remains in the industry between Backreflection and Return Loss, mostly due to misunderstanding the differences between the two.
Backreflection, also called Optical Return Loss (ORL), refers to the total effect of multiple reflections and scattering events in a fiber-optic system.
Backreflection caused by Rayleigh backscattering varies with the length and type of fiber. As can be seen in Figure 1 below, the longer the cable under test, the higher the backscattering. For example, backscattering of a typical 10m fiber is almost -60 dB. Backscattering is therefore limiting the possibility to measure Backreflection for long cables or components with long pigtails.
Return Loss, also called Reflectance, measures the amount of light reflected by a single discontinuity in a transmission line or optical fiber.
Return Loss can measure the discrete reflectance of the input and output connectors without the impact of fiber backscattering. This accomplished through an approach based on optical time-domain reflectometry (OTDR).
Credits: JGR Optics - Application Notes