How much light a connection reflects back toward the source — quoted as a positive number you want to be large.
What Is Return Loss?
Return loss compares reflected power against incident power at a discontinuity. Sixty decibels means one millionth of the light comes back. Unlike insertion loss, a larger number is better: it is quoted as a positive value describing how far the reflection has been suppressed.
Reflectance is the same measurement with the sign reversed, so −60 dB reflectance and 60 dB return loss are identical statements. Both appear on test reports, often on the same page, and they are easy to confuse. Optical return loss, ORL, is the aggregate figure for a whole link rather than one joint.
Where Reflections Come From
Light reflects wherever the refractive index changes abruptly. A cleaved fibre end facing air reflects about 3.6% of the incident power — roughly 14.4 dB of return loss, which is catastrophic by modern standards. That is precisely what physical contact eliminates: dome-polished ferrules pressed together leave no air, and a good mate reaches 50 dB.
The other lever is geometry. An APC ferrule tilts the face eight degrees so the reflection leaves the core entirely, buying another 10 dB. Fusion splices, having no interface at all, are usually better than 60 dB; mechanical splices and damaged connectors are the reflective events an OTDR trace picks out as spikes.
Why It Matters
Reflected light returning into a laser cavity perturbs the oscillation, raising relative intensity noise and jitter and, in bad cases, causing mode hopping. Direct-detect short-reach links tolerate a surprising amount of this. Coherent systems do not, because the receiver recovers phase and a reflected copy of the signal is indistinguishable from the signal itself.
As a pure loss term reflection is almost irrelevant — at 40 dB return loss the reflected power is 0.01% and contributes nothing measurable to the budget. Return loss is a stability specification, not a loss specification, and reading it as the latter is the usual misunderstanding.
Where You Meet It
On assembly test reports beside insertion loss, in Tier 2 OTDR traces, and in the specification of anything that terminates a fibre. A connector whose return loss has degraded is nearly always a connector with a contaminated or chipped end face, which makes end-face inspection the first check rather than the last. Our attenuators and passive components are specified for return loss as well as attenuation, because a reflective attenuator solves one problem and creates another.
Return loss reference
| Definition | 10 log10 (incident power / reflected power), in dB |
|---|---|
| Reflectance | the same quantity with the opposite sign |
| Single-mode UPC | ≥ 50 dB |
| Single-mode APC | ≥ 60 dB |
| Multimode PC | ≥ 30 dB |
| Fusion splice | typically better than 60 dB |
| Bare glass-to-air end face | about 14.4 dB — 3.6% reflected |
| IEC 61753-1 grades | 1 = ≥60 dB, 2 = ≥45 dB, 3 = ≥35 dB, 4 = ≥26 dB |
| Measured by | IEC 61300-3-6; visible on an OTDR as a reflective spike |
Related Terms
APC vs UPC, insertion loss, OTDR, coherent, end-face inspection.