Single-mode vs multimode fibre: differences, physics and selection guide
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You need to cable a data centre, an FTTH network or a corporate backbone — and you are hesitating between single-mode and multimode fibre. It is not just a question of jacket colour: the two technologies rely on different physical principles, involve incompatible active equipment, and cover distinctly different use cases. This guide explains the propagation mechanisms, the current standards and how to choose without making a mistake.
What is a propagation mode?
In an optical fibre, light propagates by total internal reflection: a silica core with a high refractive index is surrounded by a cladding with a lower index. The index difference confines the light within the core.
A mode corresponds to a distinct propagation path inside the core. Light can follow this path at different angles — each angle defines a mode. The number of guided modes depends on the core diameter and the index difference between core and cladding, characterised by the V parameter (V number or normalised frequency):
V = (π · d · NA) / λ — with d = core diameter, NA = numerical aperture, λ = wavelength. If V < 2.405, the fibre is single-mode.
In multimode fibre, the large core diameter (50 or 62.5 µm) allows several hundred modes to propagate simultaneously. These modes do not all arrive at the same time at the end of the fibre — this is modal dispersion, which broadens the light pulses and limits the bandwidth-distance product.
In single-mode fibre, the very narrow core (9 µm) guides only a single mode. There is no modal dispersion. The only limitation is chromatic dispersion (the different wavelengths travel along the fibre at slightly different speeds), which remains manageable over hundreds of kilometres with appropriate compensation.
Single-mode fibre: characteristics and standards
Single-mode fibre uses a 9 µm diameter core (125 µm outer cladding). It is defined by several ITU-T standards depending on the application:
- G.652D (OS2): FTTH and telecoms standard. Attenuation ≤ 0.4 dB/km at 1310 nm, ≤ 0.3 dB/km at 1550 nm. Unlimited bandwidth over thousands of km.
- G.657A1 / A2: "bend-insensitive" fibre for in-building installation. Minimum bend radius 10 mm (A1) or 7.5 mm (A2). Compatible with G.652D.
- G.657B3: ultra-flexible fibre for subscriber drop cables. Minimum bend radius 5 mm. Used in transparent cables and decorative patch cords.
The light source is always a laser (Fabry-Perot or DFB laser diode) emitting at 1310 nm (short-distance transmission / bidirectional GPON upstream) or 1550 nm (long distance, DWDM). The laser emits a highly collimated beam suited to the small 9 µm core.
The protective jacket is generally yellow (OS2 outdoor/indoor), although some single-mode patch cords are available in white, dark blue or red depending on the wavelength or use.
Elfcam OS2 single-mode patch cords
- SC/APC–SC/APC OS2 simplex — residential FTTH standard (Orange, SFR, Bouygues, Free)
- SC/APC–SC/APC steel-armoured — installation in mechanically demanding environments
- LC/UPC–LC/UPC OS2 duplex — network racks, optical switches
Multimode fibre: OM1, OM2, OM3, OM4, OM5
Multimode fibre comes in several generations, defined by the ISO/IEC 11801 and ANSI/TIA-568 standards:
- OM1: 62.5 µm core, modal bandwidth 200 MHz·km at 850 nm. Orange jacket. Obsolete — 1 Gbit/s over 275 m max.
- OM2: 50 µm core, bandwidth 500 MHz·km. Orange jacket. Limited — 10 Gbit/s over 82 m only.
- OM3: 50 µm core, bandwidth ≥ 2,000 MHz·km (VCSEL-optimized). Light blue / aqua jacket. Data centre standard — 10 Gbit/s over 300 m, 40 Gbit/s over 100 m.
- OM4: 50 µm core, bandwidth ≥ 4,700 MHz·km. Violet / magenta jacket. 10 Gbit/s over 400 m, 100 Gbit/s over 150 m (MPO).
- OM5: 50 µm core, designed for SWDM (Short Wavelength Division Multiplexing) over 4 wavelengths. Lime green jacket. 400 Gbit/s over 150 m via SWDM4.
The light source for OM3/OM4 multimode fibres is always a VCSEL (Vertical-Cavity Surface-Emitting Laser) at 850 nm. VCSELs are less expensive than single-mode DFB lasers, which explains the lower cost of multimode transceivers for short distances.
OM3 or OM4 for a new data centre?
If you are deploying a data centre network today, go straight for OM4. The extra cost compared to OM3 is marginal, but OM4 lets you reach 100 Gbit/s over 150 m with an existing MPO-12 cable. Avoid OM1 and OM2: they do not support 10G/25G data rates at useful distances.
Single-mode vs multimode comparison table
| Criterion | Single-mode (SM / OS2) | Multimode OM3 | Multimode OM4 |
|---|---|---|---|
| Core diameter | 9 µm | 50 µm | 50 µm |
| Outer cladding | 125 µm | 125 µm | 125 µm |
| Light source | DFB / Fabry-Perot laser | VCSEL 850 nm | VCSEL 850 nm |
| Wavelength | 1310 nm / 1550 nm | 850 nm / 1300 nm | 850 nm / 1300 nm |
| Typical attenuation | 0.35 dB/km at 1310 nm | 3.5 dB/km at 850 nm | 3.5 dB/km at 850 nm |
| Max distance 1 Gbit/s | > 100 km | 1,000 m | 1,000 m |
| Max distance 10 Gbit/s | > 40 km (LR) | 300 m | 400 m |
| Max distance 100 Gbit/s | 10–40 km | 100 m (MPO) | 150 m (MPO) |
| Jacket colour | Yellow (OS2) | Light blue / Aqua | Violet / Magenta |
| Cable cost | Low | Low to moderate | Moderate |
| SFP/SFP+ module cost | Higher (precise laser) | Lower (VCSEL) | Lower (VCSEL) |
| Main application | FTTH, telecoms, long-distance campus | Data centre, SAN, 10G building backbone | High-density data centre, 40/100G |
| Standard | ITU G.652D / G.657 | ISO 11801 OM3 / TIA-568 | ISO 11801 OM4 / TIA-568 |
Connectors and colour codes
The connector colour helps identify the fibre type and the polish type:
- Green SC/APC: single-mode, 8° angled polish. Standard FTTH PTO connector in France. Return loss ≥ 65 dB.
- Blue SC/UPC: single-mode, flat polish. Used in active equipment (SFP, OLT). Return loss ≥ 50 dB.
- Blue LC/UPC: single-mode or multimode depending on the associated fibre. Compact format, data centre and SFP switch standard.
- Green LC/APC: single-mode, 8° angle. Used in some telecoms equipment and FTTH distribution cables.
- Beige/cream connectors: multimode OM1 (62.5 µm) — obsolete.
- Light blue / aqua connectors: multimode OM3 in LC format.
Do not mix single-mode and multimode
A single-mode cable connected to a multimode transmitter (or vice versa) generates major insertion losses (often 10–20 dB) and can make the link completely non-functional. The incompatible core diameters (9 µm vs 50 µm) prevent efficient optical coupling. Always check the fibre/module consistency before installation.
SFP modules and compatibility
The choice of fibre and SFP module are inseparable. SFP/SFP+ modules come in variants according to the fibre type:
- SFP+ SR (Short Range): multimode OM3/OM4, 850 nm VCSEL, LC/UPC, 10 Gbit/s over 300/400 m — e.g. ref 477, 2583
- SFP+ LR (Long Range): single-mode OS2, 1310 nm laser, LC/UPC, 10 Gbit/s over 10 km
- SFP 1.25G SM: single-mode OS2, SC/UPC, 1310 nm, 1 Gbit/s over 10–20 km — e.g. ref 6603
- SFP 1.25G MM: multimode, LC/UPC, 850 nm, 1 Gbit/s over 550 m — e.g. ref 6598
- GPON SFP OLT: single-mode SC/UPC, TX 1490 nm / RX 1310 nm, specific to PON networks — e.g. ref 6618, 6631
Most modern switches and routers accept both types of module in their SFP ports. Check the specifications of your active equipment to confirm DDM (Digital Diagnostic Monitoring) compatibility and the list of approved modules.
Selection guide by application
Choose single-mode OS2 fibre if:
- FTTH / FTTP connection (subscriber ↔ PTO ↔ box)
- Inter-building link on a campus (distance > 550 m)
- WAN or metro backbone (several kilometres)
- Telecoms network, DWDM/CWDM transport
- PON infrastructure (GPON, XGS-PON, EPON)
- Industrial link in a disturbed environment (full immunity to interference)
Choose multimode OM3 or OM4 fibre if:
- Horizontal cabling in a data centre (servers ↔ ToR switch, < 100 m)
- SAN (Storage Area Network) fibre channel
- Vertical building backbone over short distance (< 300 m)
- Tight budget on many short links (less expensive VCSEL modules)
- High-density connections with MPO-12/24 cables (40G, 100G in rack)
The case of corporate campuses
For a campus with buildings less than 300 m apart, OM4 may be sufficient at 10G. Beyond 300 m or for 25/100 Gbit/s links over duplex cable, switch to single-mode OS2 fibre + SFP LR modules. The module cost gap (SM vs MM) is offset from the very first active equipment replacement if distances exceed multimode limits.
1Can single-mode and multimode fibre be mixed on the same link?
No. Single-mode (9 µm) and multimode (50/62.5 µm) fibres have incompatible core diameters. Connecting a single-mode cable to a multimode module — or vice versa — generates insertion losses of 10 to 20 dB, generally rendering the link non-functional. Every link must be homogeneous: the same fibre type from module A to module B, with connectors of the same polish (UPC or APC).
2Is single-mode fibre always better than multimode?
No — it is a question of matching use/distance/budget. Single-mode fibre excels over long distances (kilometres to thousands of km) and at high data rates. But for short data centre links (< 300 m), multimode VCSEL modules are significantly less expensive than equivalent single-mode laser modules. At 10 Gbit/s over 50 m, an SFP+ SR module (OM3) costs 2 to 3× less than an SFP+ LR (OS2).
3How can you identify single-mode or multimode fibre with the naked eye?
By the jacket colour: yellow = single-mode OS2; aqua/light blue = multimode OM3; violet/magenta = multimode OM4; orange = multimode OM1/OM2 (obsolete); lime green = OM5. The connector colour also helps: green SC/APC = always single-mode with angled polish. If in doubt, an attenuation measurement at 850 nm then at 1310 nm allows them to be distinguished: a multimode fibre shows normal attenuation at 850 nm, but excessive attenuation at 1310 nm if injected with a single-mode source.
4Which fibre should be used for a home FTTH installation?
Always single-mode OS2 fibre with an SC/APC connector (green 8° bevel). It is the standard for all residential PTOs in France (Orange, SFR, Bouygues, Free). The patch cord between the PTO and your box must be of the SC/APC–SC/APC OS2 simplex type. A multimode fibre would not work with the operator's GPON OLT, which uses 1490/1310 nm single-mode lasers.
5What is the minimum bend radius of a single-mode fibre?
It depends on the G.657 standard: G.652D standard: min radius 30 mm (in operation), 15 mm (installation). G.657A1: 10 mm. G.657A2: 7.5 mm. G.657B3 (transparent, decorative cables): 5 mm. OM3/OM4 multimode fibre supports bend radii similar to G.657A1, i.e. around 10 mm. Below the minimum radius, bend losses increase exponentially and the link can become non-compliant.
6Can an OM4 cable be used to replace an existing OM3?
Yes, OM4 is backward-compatible with OM3. Both use a 50 µm core, a 125 µm cladding, and the same LC/UPC or SC/UPC connectors. An SFP+ SR module designed for OM3 will work on an OM4 cable — with better noise margins. The reverse (OM3 on an OM4-sized link) is possible provided you stay within the OM3 distance limits.
7What wavelength does a single-mode SFP module use?
Single-mode SFP modules mainly use 1310 nm (short to medium-distance transmission, up to 40 km) and 1550 nm (long distance, up to 120 km, or DWDM/CWDM systems). BiDi modules (bidirectional over a single fibre) use two distinct wavelengths: for example TX 1310 nm / RX 1550 nm in one direction, TX 1550 nm / RX 1310 nm in the other. GPON OLT modules use TX 1490 nm / RX 1310 nm.
8What are the delivery times for Elfcam fibre cables and modules?
Elfcam OS2 single-mode patch cords, OM3/OM4 multimode cables and SFP/SFP+ modules are in stock in France with dispatch within 24 working hours. Orders placed before 14:00 are generally shipped the same day. For structured cabling projects with large quantities, custom quotes are available with guaranteed delivery times.
