Connector Return Loss
Calculate the return loss of a flat or angle-polished fiber end face from the index step, and compare with IEC 61753-1 grades.
Return Loss (this face)
14.8 dB
Flat-face Fresnel RL
14.8 dB
Reflectance into the Mode
3.34e-2
Reflected Power
3.34%
Flat face: R = ((n₁ − n₂)/(n₁ + n₂))². Angled face at α: unpolarized Fresnel R(α) times the coupling of the reflected beam, tilted by 2α, back into the mode: exp(−(2π n₁ w α/λ)²), w = MFD/2. For angled faces this is an optimistic bound: the true LP01 mode has wider far-field tails than a Gaussian, and surface scatter adds to it, so APC connectors are specified at ≥ 55 dB unmated and ≥ 60 dB mated (IEC grade 1), well below the ≈ 94 dB computed here for 8°.
This is the open (unmated) end face against a medium of index n₂. A mated physical-contact connector has no gap; its return loss is set by polish quality and is specified by grade (IEC 61753-1, random mate): grade 1 ≥ 60 dB (APC), grade 2 ≥ 45 dB, grade 3 ≥ 35 dB, grade 4 ≥ 26 dB.
Model and references
Textbook approximation. A standard approximation from the literature (for example paraxial, plane-wave or Gaussian-beam), valid within the range the page states. About the tiers
Fresnel reflection of an open end face (phase index); an angled face adds Gaussian back-coupling at 2α, an optimistic bound (APC is specified at ≥ 55 dB unmated). Mated values: IEC 61753-1 grades.
- Born M., Wolf E. (1999). Principles of Optics, 7th ed., §1.5.2. Cambridge University Press. Fresnel reflection coefficients. doi:10.1017/CBO9781139644181
- Marcuse D. (1989). Reflection loss of laser mode from tilted end mirror. J. Lightwave Technol. 7, 336. Back-coupling of a beam reflected at an angled face. doi:10.1109/50.17776
- Kolesar P. (2013). IEC single-mode connection performance grades (IEC 61753-1), slide 6. IEEE 802.3bm SMF ad hoc, 30 April 2013. grouper.ieee.org