Return loss happens when transmitted energy reflects back to its source.
Designers must pay close attention to optical return loss (ORL), an important metric in an optical network. ORL is defined as return loss for the entire optical fiber under test, which includes fiber backscatter and reflections relative to the source pulse. This ORL metric is a critical system parameter that greatly affects the amount of power delivered to a load and how much of that power reflects back to the transmitter.
ORL is the measurement of the reflected signal loss value when the optical link encounters component access. You can calculate ORL through the following equation:P0 is the reflected optical power signal, and P1 is the input/incident optical signal power. The return loss value is typically expressed in dB, where a higher magnitude of the ORL value indicates less return loss.
Some main sources of ORL include:
- Patch cords
- Glass/air terminations
- Mechanical splices
What is an APC vs. UPC connector?
Industry standards suggest that ultra-physical contact (UPC) polished fiber optic connectors should have a return loss greater than -50 dB. Angled physical contact (APC) polished fiber optic connectors usually have a return loss of better than -65 dB. Physical Contact (PC) type polished connectors must have a return loss greater than -40 dB. For multi-mode fibers, the typical RL value is between -20 dB and -40 dB.
The optical signal will travel through several components (Figure 1).
In Figure 1, the optical signal pulse at the source or transmitter (Tx) begins its journey into the optical fiber with PR being some reflected power received back at the source. Next, the optical signal pulse has its first encounter at an angled physical contact (APC), PAPC, connector pair. Here, the signal reflects into the cladding of the optical fiber instead of continuing its travel back through the fiber core. This action lets the APC connector experience a low reflectance with less of an impact on the ORL.
Next, we see that PBS is the backscatter from the fiber impurities. Optical fiber has far fewer impurities than windowpane glass.
PPC, is the power reflected back from an ultra-physical contact (UPC) connector pair. PELEMENT represents elements that are other than standard connector pairs, such as splitters (a type of optical passive device that decomposes an optical fiber signal into multiple optical signal outputs).
The optical fiber splitter
An Optical Fiber Splitter (Figure 2) is typically employed in passive optical networks (PONs) for the distribution of fiber to various locations such as businesses and homes.
The optical fiber
The optical fiber will have a natural loss, measured in a negative dB value, due to Rayleigh scattering, which is defined as a linear scattering of light that has a center of scattering far smaller than the wavelength of that light. Rayleigh scattering is caused by non-homogeneities, on a small scale, that is typically formed during the fiber construction process.
Splices and connectors will contribute to that optical fiber return loss as well.
Maximizing Optical performance to reduce ORL
- Use ultra-polish connectors, which have low reflectance, such as UPC type. APC-type connectors have even better reflectance values but are not compatible with other non-APC connectors.
- Use fusion splices instead of mechanical connectors or mechanical splices where possible.
- Re-do fusion splices that are shown to have reflectance.
- Install optical isolators at the laser to reduce back reflectance.
The return loss for an entire optical fiber under test, which includes fiber backscatter and reflections relative to a source pulse, is known as Optical Return Loss (ORL). Units of dB are typical and always will have a positive value.