Output interface with fibre-optic ports (the SC adapter panel on a rack signal processor, and matching its end faces and colour codes)
The fibre ports where a rack signal processor sends its output · adapter colours for singlemode, singlemode APC and each multimode grade, UPC against APC end faces at 55 dB and 65 dB return loss, the 8° angle and its key, apex offset within 50 μm, and key error
When a signal processor sends its output over fibre, the rear or front panel carries mating adapters, and patch cords from the far end plug into them. Each port has to match the fibre type and the end-face polish of the cords that will be connected; a mismatch costs loss and back-reflection. The adapter colour is the first check, and the connector end-face geometry is the second.
Fibre-optic output ports on signal processing equipment: panel-mount mating adapters, their colour codes and the end-face requirements of the connectors that mate in them. Serial (RS-232/422/485) interfaces are covered on the Serial communication interface page; pluggable optics on the Optical transceiver module (SFP, SFP+, QSFP) page; ferrules on the Precision fibre optic ferrule page; panel-mount electrical receptacles on the Connector assembly for an interface panel: receptacles mounted on a bulkhead plate and wired to the module page; camera interface boards on the Interface board for a vision processing unit: multi-port GigE Vision PoE+ camera interface page.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
The Fiber Optic Association (FOA) notes that metallic connectors such as FC and ST made colour coding difficult, so coloured strain-relief boots were used. Its table of connector body, strain relief and mating adapter colours is reproduced below. [1]
The FOA's connector identifier page gives a shorter rule: "The TIA 568 color code for connector bodies and/or boots is Beige for multimode fiber, Blue for singlemode fiber, and Green for singlemode APC (angled) connectors." That rule calls all multimode beige, while the FOA colour code table separates 62.5/125 (beige), 50/125 OM2 (black), OM3/OM4 (aqua) and OM5 (lime). [2][1]
The SC connector, a snap-in connector with a 2.5 mm ferrule, is covered by the TIA intermateability standard FOCIS-3 (TIA-604-3), according to the FOA. [2]
List the fibre type and polish for every output port, and if a panel uses an adapter colour outside this table, get its meaning from the supplier in writing.
02
UPC or APC end faces: return loss, angle and key
A SENKO application note (Rev. 01, February 2022) says that reflected signals can make laser diode output unstable and increase noise and bit errors, that "Ultra Physical Contact (UPC) connectors with RLs of up to 55 dB have been good enough", and that Angled Physical Contact (APC) connectors require more precise end-face geometry "with RLs of 65 dB or better". [3]
Per the note, Ø2.5 mm PC or UPC ferrules have a spherical end face with a radius of curvature of 10 – 25 mm; Ø2.5 mm APC connectors have an angled end face "(typically at 8°)" with a radius of 5 – 12 mm. Both carry an apex offset of ≤50 μm under Telcordia GR-326, IEC and TIA/EIA specifications. [3]
For SC/APC and FC/APC, the angle is oriented relative to the connector key, "otherwise the two angled connectors would never mate properly upon physical contact". SENKO adds that any rotation of the ferrule against the key introduces an air gap, and that poorly designed connectors can shift apex offset by more than 30 μm during mating, so a connector at 30 μm could reach 60 μm. [3]
Ask for return loss and insertion loss per port and polish, and for interferometer data (radius of curvature, apex offset, and key error for APC) on the connectors supplied with the unit.
03
Fibre output port details
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What colour is a singlemode APC adapter?
Green, in the FOA's colour code table; plain singlemode is blue. [1]
Which multimode grade is aqua?
50/125 laser optimized fibre, OM3 and OM4, per the FOA table. [1]
How much return loss do UPC and APC connectors give?
SENKO cites up to 55 dB for UPC and 65 dB or better for APC. [3]
Why does an APC connector need a key?
The 8° angle has to face the same way on both ferrules for physical contact; rotation leaves an air gap. [3]
Sources and what still needs a direct check
Sources checked 2026-10-13. The Fiber Optic Association publishes its colour code and connector identifier pages; SENKO publishes an application note on key error and apex offset in APC connectors.
