The Two Fundamental Types: Single-Mode vs Multimode
Every fiber optic cable belongs to one of two categories based on the diameter of its glass core and the way light travels through it. This distinction determines which transceivers you can use with the cable, how far the signal can travel, and what the cable costs. Mixing single-mode cable with multimode transceivers, or vice versa, results in a link that will not come up or will have extremely high losses — the light simply cannot couple efficiently between the mismatched core sizes.
Single-mode fiber (SMF) has a very narrow core (typically 8–10 µm), which forces light to travel in a single path (mode) down the fiber with minimal dispersion. This allows signal transmission over distances of kilometers to hundreds of kilometers depending on the transceiver. Single-mode transceivers use laser light sources. SMF is the cable used by ISPs for long-haul runs, building-to-building links, and anywhere distance matters more than short-run cost.
Multimode fiber (MMF) has a wider core (50 or 62.5 µm), allowing multiple light paths to travel simultaneously. This causes modal dispersion that limits the effective distance to typically 100–550 meters depending on cable grade and data rate. Multimode transceivers use cheaper VCSEL (Vertical Cavity Surface Emitting Laser) light sources. MMF is the standard for within-building and within-datacenter runs where the short distances make multimode practical and cost-effective.
Multimode Fiber Grades
| Grade | Core Diameter | Color (jacket) | Max Distance at 10G | Max Distance at 100G |
|---|---|---|---|---|
| OM1 | 62.5 µm | Orange | 33 m | Not supported |
| OM2 | 50 µm | Orange | 82 m | Not supported |
| OM3 | 50 µm | Aqua | 300 m | 70 m |
| OM4 | 50 µm | Aqua or Erika Violet | 400 m | 150 m |
| OM5 | 50 µm | Lime green | 400 m | 150 m (400G with SWDM) |
OM1 and OM2 are legacy grades — they should not appear in new installations. OM3 is the current minimum for new work; OM4 is preferred if the runs will support 40G/100G now or in the future. OM5 adds support for short-wavelength division multiplexing (SWDM), which enables 400G over the same fiber pair that would otherwise support only 100G — relevant for high-density data center upgrades.
Single-Mode Fiber Grades
| Grade | ITU Standard | Color | Use Case |
|---|---|---|---|
| OS1 | G.652 | Yellow | Indoor single-mode; tight buffer construction; conduit and cable tray |
| OS2 | G.652D | Yellow | Outdoor and long-haul; loose tube; lower attenuation specification than OS1 |
OS1 and OS2 use the same 8–10 µm core and are optically compatible — transceivers designed for one work with the other. The difference is the cable's physical construction and attenuation specification. OS2 has lower attenuation (maximum 0.4 dB/km vs 1.0 dB/km for OS1), making it the preferred choice for long runs. For short indoor patch cords, OS1 is fine.
Jacket Types and Outdoor Ratings
Fiber optic cable jackets are rated for different installation environments:
- OFNR (Riser-rated): safe for vertical runs between floors inside a building; resists fire from spreading between floors.
- OFNP (Plenum-rated): required for runs in air-handling spaces (raised floors, ceiling plenums in HVAC systems); uses low-smoke, low-toxicity jacket material.
- Outdoor/Direct Burial: UV-resistant jacket and optional armoring for direct burial without conduit; contains a water-blocking gel or dry tape to prevent moisture ingress.
- Armored: steel or aluminum interlocking armor provides crush and rodent resistance for outdoor or industrial environments.
Frequently Asked Questions
Can I use single-mode fiber with multimode transceivers?
No, not reliably. The core diameter mismatch between single-mode fiber (8–10 µm) and multimode transceivers designed for 50 µm fiber means the VCSEL light source from a multimode transceiver cannot efficiently couple into the single-mode core. The link may not come up at all, or it may come up but with extremely high loss that causes errors. The reverse (single-mode transceiver into multimode fiber) has slightly different behavior but also does not work correctly. Always match fiber type to transceiver type.
How do I identify what fiber I already have installed?
Check the jacket color: orange typically indicates OM1/OM2 multimode (legacy), aqua indicates OM3/OM4, lime green indicates OM5, and yellow indicates single-mode OS1/OS2. However, color coding is not universally standardized — some manufacturers use different colors, particularly for non-standard grades. The definitive identification is the print on the cable jacket, which should include the fiber type (e.g. "50/125 OM4" or "9/125 OS2"). If the cable is already installed and unlabeled, an optical power meter and light source can measure attenuation to help determine the grade.
Which multimode grade should I install for a new data center or office?
Install OM4 as the minimum for any new structured cabling project. OM3 supports 10G and 40G at typical building distances, but OM4 is only marginally more expensive and supports 100G at up to 150 meters — enough for most building-scale networks. If the infrastructure will be expected to handle 400G in the foreseeable future, OM5 with SWDM-capable transceivers is worth the premium for backbone runs. For desktop-to-switch copper runs, standard Cat6A Ethernet remains more cost-effective than fiber for distances under 100 meters.