A single-mode MPO APC trunk cable is a fixed OS2 cable with 12 or 24 fibres and an 8 degree angled MPO connector at each end. The angle sends reflected light into the cladding, which single-mode parallel optics such as 400G DR4 need. Order APC at both ends, because angled and flat MPO connectors must never be mated.
What a single mode MPO APC trunk cable is
A single mode MPO APC trunk cable is the fixed run of a single-mode parallel-optic channel. It is built on OS2 fibre, usually ITU-T G.652.D or the bend-insensitive G.657 grades, and terminated at each end in an MPO or MTP connector with an angle-polished (APC) ferrule. It runs between patch fields, and the cassettes, adapters and cords at each end connect it to equipment.
The cable jacket is normally yellow for OS2. The connector body or boot is normally green for APC. The two colours describe different things, so a yellow cable with green connectors is the expected combination, and the MPO trunk cables page covers the general construction options.
Why single-mode parallel optics use APC polish
A single-mode laser launches light into a core only about nine microns across, and it is sensitive to light that returns to it. At every flat connector joint a small fraction of the signal reflects straight back down the core. Over a channel with several mated pairs, those reflections can add noise or destabilise the laser.
An APC ferrule is angled at 8 degrees. Light that reflects off the joint meets the angled surface and is deflected into the cladding, where it is lost, instead of travelling back to the transmitter. Widely used industry figures put the return loss of a good APC connector at 60 dB or better, against 50 dB or better for UPC. The UPC vs APC guide explains the polish in more detail.
Single-mode parallel optics are therefore built with APC MPO ports. Multimode parallel optics are different: they use flat polish.
Optics that expect an MPO APC trunk
| Optic | Fibres used | MPO interface | Note |
|---|---|---|---|
| 400GBASE-DR4 (IEEE 802.3bs) | 8 | MPO-12 APC | Four lanes each way over OS2, 500 m reach |
| 100G PSM4 (multi-source agreement) | 8 | MPO-12 APC | Four lanes each way over OS2, 500 m reach |
| 800G DR8 | 16 | MPO-16 APC, or two MPO-12 APC | Check the module, since the port style varies |
Reach figures belong to the whole channel. A single-mode link has a tight loss budget, so every mated pair counts, and the budget itself is set by the standard or datasheet for the optic. Work through it with the fibre loss budget calculator before fixing the trunk length and the number of cassettes. Place these optics in a wider design with the parallel optics guide listed under related pages.
UPC and APC must not be mixed
An angled MPO plug and a flat MPO plug will both go into the same adapter, so the error is easy to make and hard to see. Once seated, the two ferrule faces touch along one edge and the rest of the fibre array sits apart. Loss climbs well above any specified value, return loss collapses, and the contact pressure falls on a small area that can mark both ferrules. The rule covers every mated pair in the channel, including the interface to the transceiver.
Rules that keep the channel clean:
- Specify APC at both ends of the trunk, and APC on every cord, cassette and adapter in the same channel.
- Read green as a strong hint of APC, not as proof, because housing colours differ between makers. The label and the drawing carry the polish.
- Do not infer a flat polish from a non-green housing.
- APC plugs only mate key-up to key-down, so the adapters in the channel must be of that type. The MPO polarity guide shows how this combines with the trunk type.
When equipment with a flat port has to join an APC channel, ask for a conversion cord with the right polish at each end rather than forcing a mate.
12-fibre and 24-fibre OS2 trunks
| Fibre count | Connector | Typical use | Ordering note |
|---|---|---|---|
| 8 | MPO-8 or MPO-12 with 8 fibres fitted | One DR4 or PSM4 link with no idle fibres | Used in Base-8 cabling |
| 12 | MPO-12 | One DR4 or PSM4 link on 8 fibres (the middle four idle), or six duplex channels through cassettes | The standard Base-12 trunk |
| 24 | MPO-24, or two MPO-12 on a multi-unit cable | Doubling fibre density in the same pathway, usually ending on cassettes or a conversion harness | State whether the ends are one MPO-24 or two MPO-12 |
Equipment ports are MPO-12 or MPO-16, so a 24-fibre trunk usually ends on cassettes or a conversion harness rather than on an optic. An MPO-16 connector has an offset key and does not mate with a 12-fibre or 24-fibre connector. Build 16-fibre channels on MPO-16 throughout.
Construction, loss and testing
Single-mode MPO trunks are pulled through the same pathways as multimode ones, so the construction choices are alike: LSZH, riser or plenum jackets, armour where the route needs it, and a pulling grip that carries the load away from the connectors. Bend-insensitive G.657 fibre gives margin where the route is tight, but it does not protect a kinked cable.
Ask for single-mode loss to be reported at 1310 and 1550 nm, with insertion loss and return loss on every fibre and end-face inspection to the IEC 61300-3-35 criteria. State the maximum insertion loss you need per mated pair. Return loss matters more on a single-mode APC channel than on multimode, so request it in the report rather than assuming it. See the single-mode fibre optic cable guide for the fibre grades.
What to send us for a quote
Send one line per configuration with:
- Fibre: OS2, with G.652.D or bend-insensitive G.657 stated.
- Fibre count, and connector at each end: MPO or MTP, MPO-12 or MPO-24.
- Polish: APC at both ends, written out on the line.
- Gender at each end, matched to the cassette or adapter.
- Polarity type, A, B or C, and the key orientation it implies.
- Measured route length plus slack, with the tolerance.
- Jacket rating and colour, armour, and any pulling grip.
- Labels, test wavelengths and the maximum insertion loss per mated pair.
- Quantity per line.
List the transceiver or cassette part numbers at each end if you have them, so the polish, fibre count and gender can be checked against the hardware. Send the list through the request a quote page.
Questions buyers ask
Why do single-mode MPO cables use APC and multimode cables use flat polish?
Single-mode lasers are sensitive to reflected light, and an 8 degree angle deflects reflections into the cladding. Multimode shortwave optics tolerate the reflection of flat connectors, so they use flat polish.
What colour is an MPO APC connector?
Green is the usual colour for the housing or boot of an APC connector, following the convention used for SC/APC and LC/APC. It is a convention, not a guarantee, so check the label and the drawing as well.
Can I mate an MPO APC cable to an MPO UPC cable?
No. The angled face meets the flat face along one edge only and leaves a gap over the fibres. Loss rises sharply and the ferrules can be damaged. Use APC on every part of the channel.
Do I need a 12-fibre or a 24-fibre OS2 trunk?
Use 12 fibres for one DR4 or PSM4 link or for six duplex channels. Use 24 fibres where you need double the density in the same pathway, and plan how the ends will break out, since optics have 12-fibre or 16-fibre ports.
Does a 400G DR4 link use all 12 fibres?
No. DR4 uses 8 of the 12 fibres, with transmit on positions 1 to 4 and receive on positions 9 to 12. The four middle positions are idle in a Base-12 trunk.
Which MPO APC details should I check against the datasheet?
Check the fibre count, the polish, the gender of the port and the key orientation. Confirm each against the optic and the cassette datasheets before you place the order.