Optical return loss
The ratio of power launched into a component or link to the power reflected back, in dB. Larger is better. This budget keeps reflections from destabilizing lasers and corrupting signals.
Return loss quantifies how much light comes back:
By this (dominant) convention RL is a positive number and bigger means quieter: 50 dB RL returns one part in . The same physics quoted per-component sometimes appears as negative "reflectance" (−50 dB); read signs carefully across datasheets. For an entire link, ORL aggregates every reflection: connectors, splices, components, plus the distributed Rayleigh backscatter of the fiber itself, which sets a ceiling of roughly 30-something dB on long spans no matter how perfect the joints.
Where reflections originate: any index discontinuity. An open PC connector or cleaved fiber end in air reflects ~4 % (−14 dB). That is the loudest thing in most systems and the first suspect in any feedback mystery. Mated connectors range from ~50 dB (UPC) to ≥60 dB (APC); fusion splices are effectively silent; chip facets, filter edges, and poorly matched components contribute their own terms.
Why the budget exists:
Laser stability. Semiconductor lasers are exquisitely feedback-sensitive: returns as small as −40 to −30 dB relative can push a DFB from clean single-mode operation into linewidth broadening, mode hopping, or coherence collapse. This is the entire justification for optical isolators in transmitter packages and for angled connectors in analog and sensing systems.
Signal integrity. Pairs of reflections create interferometric paths. Multipath interference converts laser phase noise into intensity noise on the signal, degrading analog links (CATV historically demanded the strictest ORL specs) and high-order-modulation digital links alike.
Measurement corruption. Reflections inside a test setup ripple through spectra and skew loss readings.
Measurement: OCWR (optical continuous-wave reflectometry) for total ORL, OTDR for locating discrete reflectors along a link. System specs commonly demand link ORL better than 27–32 dB, tightening for analog and coherent applications.