An OS2 LC/APC patch cord is a single-mode cord whose LC ferrule is polished at 8 degrees, so reflected light is thrown into the cladding instead of back toward the laser. The connector body is green. It is used on PON, RF video and other reflection-sensitive links, and must never be mated with LC/UPC. Radix Fibrotech makes them to order in Pune.
LC/APC single-mode cord specification
An LC/APC patch cord is a single-mode cord with an angled physical contact end face on a 1.25 mm LC ferrule. Radix makes each cord to order in Pune, with the fibre, cable and length set per order line. The UPC vs APC guide covers the polish in general; this page covers the LC cord.
| Parameter | LC/APC cord | Note |
|---|---|---|
| Fibre | OS2 9/125 µm single-mode (ITU-T G.652.D), or bend-insensitive G.657.A1 or A2 | Cabling standards allow up to 0.4 dB/km for OS2 at 1310 and 1550 nm |
| End face | APC, polished at 8 degrees to the fibre axis | Single-mode only |
| Connector colour | Green body and boot | The industry marking for APC; UPC single-mode is blue |
| Cable jacket | Yellow is the usual single-mode jacket colour | A green jacket is sometimes requested to flag APC; say so on the order |
| Return loss | 60 dB or better is the widely used figure for APC | Ask for the measured value in the report |
| Format | Simplex, duplex zip, or uniboot duplex | 1.6, 2.0 or 3.0 mm cable |
| Ends | LC/APC to LC/APC, or hybrid to another connector | Hybrid ends are made APC at one end and UPC or another type at the other |
Why the end face is angled and what return loss measures
On a flat or domed (UPC) end face, a small part of the light is reflected at the glass-to-air boundary and travels back along the core toward the source. Return loss is the ratio of the light sent to the light that comes back, written in decibels. A higher number is better, because less light is returning.
An APC ferrule is polished at 8 degrees. Light that reflects off the angled face meets the core at a steep angle, and the core cannot guide it, so it leaks into the cladding and is lost. The reflection does not reach the transmitter. Commonly specified return loss is 50 dB or better for UPC and 60 dB or better for APC.
Many data links do not mind a UPC reflection. Analogue and amplified systems do. A distributed-feedback laser, an optical amplifier or an analogue RF signal can show noise, distortion or instability when reflections return, and the more connectors in the path, the more reflections there are to control.
Where LC/APC is required
APC is specified by the equipment, not chosen for preference. Read the port label or datasheet before ordering.
| Application | Why APC is used | What to check |
|---|---|---|
| PON and FTTH access networks | Low reflection through splitters and drop links; APC is standard through the optical distribution network | SC/APC is the usual connector. LC/APC appears on some compact equipment, so check the port |
| RF video overlay at 1550 nm | Analogue video carried on the same fibre is sensitive to reflections | Every connector in the chain should be APC |
| Radio-over-fibre and analogue RF links | Reflections add noise and distortion to the analogue signal | Follow the equipment maker's polish call-out |
| Links with optical amplifiers | Reflections can disturb gain and add noise | Check the amplifier and line-system datasheet |
| Test and reference leads | APC ports on test sets need APC leads to give valid readings | Match the test port polish |
A standard data-centre transceiver, such as 10GBASE-LR or 100GBASE-LR4, expects UPC. An APC cord there is not an upgrade, and it creates a mismatch at the port. Use the single-mode patch cord range for UPC links.
Do not mate APC with UPC
LC/APC and LC/UPC connectors have the same body, the same latch and the same adapter. Nothing in the shape stops one being pushed into the other. When they meet, the angled face and the domed face leave an air gap between the cores, which adds several decibels of loss and removes the return-loss benefit that APC was chosen for. The mismatched faces can also be marked by the contact pressure.
Colour and labelling are the only protection, so use them:
- Use green LC/APC adapters and panels where the plant is APC, and blue where it is UPC.
- Keep APC and UPC cords in separate, labelled bags and bins.
- Mark the polish in the cord label text, as well as by colour.
- Order a hybrid cord (APC at one end, UPC at the other) where two ports differ. Each end then mates with its own kind, and the two ends of the cord are never mated together.
Colour conventions are summarised in the fibre colour codes guide.
Simplex, duplex and uniboot LC/APC
PON equipment uses one fibre for both directions, so PON patching with LC/APC is simplex. Duplex LC/APC suits two-fibre equipment that specifies angled polish, with the transmit fibre at one end meeting the receive fibre at the other. The default is an A-to-B cord, and an A-to-A cord is made when the channel design needs it. The LC to LC patch cord page sets out polarity and uniboot handling in full.
A duplex cord has two APC connectors at each end, so label the cord as APC at both ends to avoid a UPC cord being fitted in its place during a move.
Testing and inspecting APC cords
Insertion loss and return loss on single-mode cords are measured at 1310 nm and 1550 nm. APC cords are tested against APC reference leads, because a UPC reference would itself create a mismatch. An interferometer measures end-face geometry for angle, radius of curvature and apex offset; ask for those figures in the quote if the project requires them.
End faces are inspected against IEC 61300-3-35 criteria. When inspecting in the field, use a scope tip made for APC adapters, so the angled face sits square in the view. Clean before each mating. Dirt on an APC face raises insertion loss and can undo the reflection control that justified the polish.
What to send us for a quote
Give one line per configuration, with:
- fibre: OS2, and G.657 bend-insensitive if routing is tight
- simplex, duplex or uniboot
- polish at each end: APC, UPC or hybrid, and the other connector type if hybrid
- polarity: A-to-B unless stated
- jacket colour, rating and diameter
- length and tolerance
- label text, including the polish
- return-loss and insertion-loss report format, and quantity
Send the list through the request a quote page. Radix quotes against the specification and does not publish a price list.
Questions buyers ask
What colour is an LC/APC connector?
Green. The connector body and boot are green for APC, and blue for single-mode UPC. Some buyers also ask for a green cable jacket on APC cords, which is a request for the order line, not a standard rule.
Can I plug an LC/APC cord into a UPC transceiver?
It will physically latch, but it is a mismatch. The angled and domed faces do not meet properly, loss rises and the reflection control is lost. Use an LC/UPC cord for a UPC port, or a hybrid cord if the other end must be APC.
What does the 8 degree angle do?
It tilts the end face so light reflected from the glass-to-air boundary is thrown into the cladding instead of back along the core. The source sees far less reflected light, which is why APC is used for PON, RF and amplified links.
Do data centre links need LC/APC cords?
Usually no. Standard LC transceivers for 10G to 100G single-mode links expect UPC. Use APC only where the equipment datasheet or port marking calls for it.
Is multimode LC available in APC?
Standard multimode LC cords are PC or UPC. APC is used on single-mode fibre, where reflection is a concern for laser and analogue systems, so this page is about OS2 only.
Should an LC/APC cord be simplex or duplex?
Simplex for PON and single-fibre bidirectional equipment, duplex for two-fibre equipment that specifies APC. Check the port. Order the format the equipment uses, because an unused second fibre only adds clutter.