An FC to FC patch cord is a fibre cable with a threaded, keyed FC connector on a 2.5 mm ferrule at each end. It is used on test instruments, laser sources, sensing systems and some single-mode telecom equipment, where the screw coupling holds firm under vibration. Polish (APC or UPC) and key width must match each port. Radix Fibrotech makes FC to FC cords to order in Pune.
FC to FC patch cord options
An FC to FC patch cord has an FC connector at both ends. The connector has a round metal body, a key that sets the ferrule rotation, and a knurled nut that screws onto the adapter. Radix makes FC to FC cords to order in Pune, with each option below chosen per order line. The ST and FC connector guide explains how FC compares with the other threaded and bayonet connectors.
| Option | Choices | Notes |
|---|---|---|
| Fibre type | OS2 single-mode (ITU-T G.652.D, or bend-insensitive G.657.A1 or A2); OM2, OM3 or OM4 50/125 µm or OM1 62.5/125 µm multimode | Most FC work is single-mode; multimode FC is found on older instruments and sensing setups |
| Fibre count | Simplex or duplex | Simplex for instruments and lasers; duplex for two-fibre equipment |
| End faces | FC/APC to FC/APC, FC/UPC to FC/UPC, or FC/UPC to FC/APC | Multimode FC is PC or UPC polished only |
| Key width | Narrow key or wide key, stated for each end | FC/APC plugs and adapters come in both; check the port |
| Connector colour | Green boot or body for APC, blue for single-mode UPC | Colour is a guide only, so confirm the polish on the order line |
| Jacket | LSZH, PVC or armoured | Armoured where the cord crosses a floor or a test bench with heavy traffic |
| Diameter | 2.0 or 3.0 mm simplex; 2.0 or 3.0 mm duplex zip | 3.0 mm is firmer and survives repeated handling better |
| Length | Any length to order | Reference and launch cords are often cut to a set length |
Where FC to FC cords are used
Test and measurement
Optical power meters, light sources, optical spectrum analysers and many OTDRs have FC ports, often with a changeable adapter. Reference cords and launch cords for these instruments are the commonest FC to FC order. They are mated and unmated often, so a firm threaded coupling and a durable cable matter more than size. Send the instrument model or the port type, because some instruments take an FC/APC port while others use FC/UPC or an SC interface.
Laser sources and sensing
Lasers, fibre amplifiers, interferometers and distributed sensing units often use FC/APC so that back-reflection does not return into the source. The key lets the ferrule return to the same rotation each time it is mated, which keeps results repeatable. Cords for polarisation-maintaining fibre are normally built with the slow axis aligned to the key, so state that on the order line if you need it.
Telecom equipment and laboratories
FC was once a standard single-mode connector in telecom plant, and it is still found on older transmission equipment, test racks and university or research laboratories. Where new LC or SC equipment is connected to an FC port, a hybrid cord is the right part: see LC to LC patch cords and SC to SC patch cords for those connectors, and send the pair you need through the quote page.
FC/APC and FC/UPC: polish and key width
An FC/UPC end face has a slightly domed polish at right angles to the fibre. An FC/APC end face is polished at 8 degrees. Mating the two leaves an air gap and an angle between the cores, so loss is high, reflection is poorly controlled and both faces can be damaged by the spring force. Check the colour first (green is APC, blue is single-mode UPC), then confirm the polish against the instrument manual. The UPC vs APC guide sets out the difference in full.
FC/APC also comes in two key widths, narrow and wide. A narrow-key plug does not fit a wide-key adapter properly, and forcing it can damage the key or the ferrule. Many instruments ship with an adapter that accepts both, but a fixed bulkhead usually does not. Write the key width for each end on the order line, and if you are unsure, send a photo or the part number of the port.
Choosing the length of an FC to FC patch cord
Measure the path the cord will follow between the two ports, add slack for a gentle loop at each end, and round up. Do not coil the cord tightly: single-mode fibre loses light when bent too tightly, and G.657 bend-insensitive fibre is available where space is short. Radix cuts FC to FC cords to any length to order, so the length can follow the route rather than a standard size.
| Typical use | Lengths commonly specified | Notes |
|---|---|---|
| Bench cord between a source and a meter | 1 to 3 m | Keep a gentle loop and avoid pulling on the connector |
| Launch or reference cord for an OTDR | As the test method calls for | State the length and tolerance, and the fibre type |
| Rack to rack in a laboratory or test room | 3 to 10 m | 3.0 mm cable handles better on longer runs |
| Sensor head to interrogator | Per installation drawing | Armoured jacket where the route is exposed |
State the length tolerance on each quote line, particularly where a cord sets a measurement distance.
Mating and caring for FC connectors
Align the key with the slot in the adapter, push the plug in, then turn the nut by hand until it stops. Finger-tight is enough. A tool, or a hard final turn, can crush the ferrule against the adapter sleeve and damage the end face. Never twist the cable while tightening, because the nut turns independently of the ferrule.
Keep dust caps on whenever a cord is not mated. Inspect the end face under magnification before every mating, and clean it with a method suited to the connector. Contamination is the commonest cause of high loss and unstable readings on FC ports, which are handled often on test benches.
What to put in an FC to FC quote
List each configuration on its own line, with:
- fibre type, and G.657 bend-insensitive or polarisation-maintaining fibre if needed
- simplex or duplex
- polish at each end: APC, UPC or hybrid
- key width at each end: narrow or wide
- jacket and outer diameter
- length and tolerance
- labels, such as instrument or port references
- test report format, and the insertion loss and return loss limits you need
- quantity for that line
Send the list through the request a quote page. Other connector pairs are on the fibre optic patch cords page, and cords for harsh sites are covered under armoured fibre optic cables.
Questions buyers ask
What is an FC to FC patch cord used for?
It connects two FC ports, most often on test instruments, laser sources, sensing systems and older single-mode transmission equipment. The threaded coupling holds the connector firmly under vibration and handling, and the key keeps the ferrule rotation the same at each mating.
Is FC/APC the same as FC/UPC?
No. The connector body is the same, but the end face is polished at 8 degrees for APC and with a slight dome at right angles for UPC. They must not be mated with each other. Check the port label or manual, and order the polish at each end separately.
What is the difference between narrow key and wide key FC?
They are two key sizes on FC/APC plugs and adapters. A narrow-key plug does not seat properly in a wide-key adapter. State the key width for each end on the order line, or send the part number of the port if you are not sure.
Can I use an FC cord with an LC or SC port?
Only through a hybrid cord, with FC at one end and LC or SC at the other. Do not use a mating sleeve to join different connector types unless it is made for that purpose. The polish and fibre type must still match at every connection.
Should an FC to FC cord be simplex or duplex?
Simplex suits most instruments, lasers and sensors, which use one fibre per port. Duplex suits two-fibre equipment where transmit and receive are separate ports. Order the number of fibres that the equipment actually uses.
What length of FC to FC patch cord should I order?
Measure the route between the two ports, add slack for a gentle loop at each end, and round up. Radix cuts FC to FC cords to any length to order. State the length tolerance on the quote line, especially for reference and launch cords.