Single-mode fibre optic cable

What single-mode fibre is

A single mode fiber optic cable guides light along one path through a glass core about 9 µm in diameter, inside 125 µm cladding and a 250 µm protective coating. The notation 9/125 gives core and cladding diameters. With only one mode there is no modal dispersion, so reach is limited mainly by attenuation and chromatic dispersion.

The light travels in a mode field slightly wider than the core, about 9.2 µm at 1310 nm in standard fibre and wider at 1550 nm. A mismatch in mode field diameter between two fibres adds splice and connector loss.

A fibre is only single-mode above its cut-off wavelength. For G.652 fibre the cabled cut-off is at or below 1260 nm, so at 850 nm it is no longer single-mode and does not suit multimode optics. For how the two families compare on reach and cost, see single-mode vs multimode fibre.

Operating wavelengths: 1310 and 1550 nm

Single-mode systems run in the near infrared, in wavelength bands named by ITU-T.

At 1310 nm chromatic dispersion in standard fibre is close to zero, and typical fibre attenuation is about 0.3 to 0.35 dB/km. At 1550 nm attenuation falls to about 0.2 dB/km, but dispersion rises to about 17 ps/(nm·km), so long high-speed links there use dispersion-tolerant or coherent optics. These are typical industry values for the fibre itself.

Fibre standards: G.652.D and bend-insensitive G.657

ITU-T recommendations describe the fibre before it is cabled.

• G.652.D: standard single-mode fibre with low water peak, so the region around 1383 nm is usable and all 18 CWDM channels from 1271 to 1611 nm can run on one fibre. Most single-mode cable made today uses it.
• G.657.A1: bend-insensitive fibre specified down to a 10 mm bend radius, compatible with G.652.D in splicing and connection.
• G.657.A2: specified down to 7.5 mm radius, common in patch cords, FTTH drop cable and crowded panels.
• G.657.B3: specified down to 5 mm for very tight routing in homes and small enclosures. Check compatibility with the installed plant before mixing it in.

Bend-insensitive fibre still has limits. The cable's own minimum bend radius, set by its jacket and strength members, usually governs the installation.

OS1a vs OS2 cable

OS1a and OS2 are cabled single-mode categories in ISO/IEC 11801. Both use low water peak fibre such as G.652.D. They differ in maximum cabled attenuation, which reflects the cable construction more than the glass.

Tight-buffered cable tends to measure higher loss per kilometre. Over the few metres of a patch cord the difference does not matter, which is why suppliers often label patch cords OS2 when they mean the cord uses G.652.D or G.657 fibre. The older OS1 category, which did not require low water peak fibre, was replaced by OS1a. TIA cabling standards set single-mode attenuation by cable type (inside plant, indoor-outdoor and outside plant), but North American datasheets also use the OS names.

Jacket colour, connectors and polish

TIA-598 assigns yellow to single-mode premises cable, so most indoor single-mode cords and cables are yellow. Outdoor cable is usually black whatever the fibre, so read the printed legend.

Single-mode cable takes all the common connector families: LC for duplex transceivers and dense panels, SC for FTTH and telecom, FC for test and laser equipment, MPO for parallel optics and trunks, and E2000 in European carrier networks.

• UPC (blue body): domed end face, return loss typically 50 dB or better. The standard for Ethernet transceivers.
• APC (green body): end face polished at 8 degrees, return loss typically 60 dB or better. Used for PON, CATV and RF over fibre, and normal for single-mode MPO.

Never mate APC to UPC. The UPC vs APC guide explains where each belongs. Single-mode ferrules and adaptor sleeves are held to tighter tolerances than multimode parts, because a small offset is large against a 9 µm core, so use single-mode adaptors on single-mode links.

Typical single-mode cable constructions

• Patch cords: simplex or duplex, commonly 2.0 mm or 3.0 mm, with G.657.A1 or A2 fibre and an LSZH or PVC jacket. See fibre optic patch cords.
• Indoor distribution and breakout: tight-buffered fibres for risers and equipment rooms, LSZH or listed OFNR or OFNP for US buildings.
• Outdoor loose tube: 250 µm fibres in gel-filled or dry tubes under a polyethylene jacket, for duct, aerial and burial routes, terminated by splicing pigtails.
• Armoured: steel tape, steel wire or stainless steel tube over the core for rodents, crush loads or direct burial.
• FTTH drop cable: low fibre count, often flat, with bend-insensitive fibre for the final span to the premises.
• Ribbon and MPO trunks: high fibre counts for data centre backbones, terminated in MPO/APC.

The guide to types of fibre optic cable compares these constructions and their fire ratings.

Where single-mode fibre is used

• Long links: campus, metro and long-haul routes. IEEE 802.3 gives 10GBASE-LR a reach of 10 km and 10GBASE-ER 40 km; amplified DWDM systems run much further.
• FTTx and PON: GPON sends downstream at 1490 nm and upstream at 1310 nm over one fibre, split passively to many premises, usually on SC/APC connectors.
• Data centres: 400G DR4 and 800G DR8 optics run 500 m over parallel single-mode fibre on MPO connectors, and FR optics such as 400G FR4 run 2 km over a duplex LC pair. Many operators now install single-mode even for short links in the hall.
• Broadcast: SMPTE hybrid camera cable carries two single-mode fibres with copper power conductors, and fibre-SDI links carry video between trucks, studios and venues.

What to specify when ordering single-mode assemblies

1. Fibre standard (G.652.D, G.657.A1 or G.657.A2), and the cabled category if the project calls for OS2.
2. Connector and polish at each end, for example LC/UPC to SC/APC, or MPO/APC with pin gender and fibre count.
3. Polarity for duplex and MPO assemblies.
4. Length, length tolerance and breakout leg lengths.
5. Cable construction, outer diameter, jacket material and the fire rating for the country of installation.
6. Maximum insertion loss and minimum return loss per connector, and the test report needed, such as insertion and return loss at 1310 and 1550 nm.
7. Labelling and jacket marking.

Radix Fibrotech builds single-mode and multimode assemblies to drawing in Pune. Send the list above with a request for a quote and ask for the loss values that apply to your build.