3.2 Singlemode vs Multimode Fiber
Key Takeaways
- Singlemode fiber has a ~9 µm core that supports only one mode; multimode has a 50 µm or 62.5 µm core that supports many modes and suffers modal dispersion.
- OS1 is indoor tight-buffered singlemode; OS2 is outside-plant loose-tube singlemode with lower attenuation (about 0.4 dB/km at 1310 nm).
- Singlemode uses laser sources at 1310 nm and 1550 nm and can span many kilometers; multimode uses VCSEL or LED sources at 850 nm and 1300 nm for short premises links.
- Modal dispersion—not attenuation—is the dominant distance limiter for multimode at 1 Gb/s and higher.
- Never mate a singlemode launch cable to a multimode test source or vice versa; the resulting center-launch vs overfilled-launch conditions produce invalid measurements.
The Core Size Difference
The single most important number distinguishing the two fiber families is core diameter. Singlemode cores are around 8–10 µm, conventionally labeled 9 µm, while multimode cores are either 50 µm (laser-optimized OM2/OM3/OM4/OM5) or 62.5 µm (legacy OM1). Because the cladding is always 125 µm, the ratio alone identifies the fiber: 9/125 is singlemode, 50/125 and 62.5/125 are multimode.
A 9 µm core is so small that it supports only one guided mode—hence "singlemode." A 50 µm or 62.5 µm core supports hundreds to thousands of modes, which is why it is "multimode." This single geometric difference drives every other performance difference.
OS1 vs OS2 Singlemode
Singlemode fiber used in ICT cabling is split into two cable constructions:
| Type | Construction | Attenuation (typical) | Use |
|---|---|---|---|
| OS1 | Tight-buffered, indoor | ~1.0 dB/km at 1310/1550 nm (older spec) | Short indoor runs, patch cords |
| OS2 | Loose-tube, outside plant | ~0.4 dB/km at 1310 nm, ~0.3 dB/km at 1550 nm | Campus, outside plant, long premises runs |
Both use the same 9/125 glass. The difference is in the cable packaging and the resulting attenuation. Modern premises singlemode is increasingly specified as OS2 even indoors because of its lower loss. The jacket is yellow for both OS1 and OS2.
Singlemode uses narrow-spectral-width laser sources at 1310 nm (zero-dispersion wavelength, used for campus links) and 1550 nm (lowest attenuation, used for long-haul and bend-sensitive OTDR testing). Because only one mode propagates, modal dispersion does not exist. The distance limit is set by chromatic dispersion (different wavelengths travel at different speeds) and by attenuation, which on OS2 is so low that singlemode links routinely span tens of kilometers before amplification.
Multimode: 50 µm vs 62.5 µm
Multimode fiber comes in two core sizes that evolved historically:
- 62.5/125 (OM1) — early LED-based fiber, optimized for short premises links at 100 Mb/s and 1 Gb/s. Its larger core is easy to couple to LEDs, but modal dispersion is severe.
- 50/125 (OM2, OM3, OM4, OM5) — smaller core with lower modal dispersion. OM3 and above are "laser-optimized" and use VCSEL sources at 850 nm for gigabit and faster links.
Multimode uses 850 nm (VCSEL, data center) and 1300 nm (LED or laser, longer premises) wavelengths. The modal dispersion problem grows sharply with data rate: a link that runs 1 Gb/s for hundreds of meters may only run 10 Gb/s for a few hundred meters and 40/100 Gb/s for under 150 meters.
Modal Dispersion Explained
In a multimode fiber, different modes travel different path lengths. The axial mode goes straight down the center; higher-order modes bounce back and forth and travel farther. They arrive at the receiver at slightly different times, smearing the pulse. The wider the pulse gets, the harder it is for the receiver to distinguish individual bits. The effect scales roughly with the inverse square of the bandwidth-distance product, which is why manufacturers specify multimode by its modal bandwidth (MHz·km) rather than just attenuation.
Laser-optimized 50 µm fiber (OM3/OM4/OM5) controls modal dispersion by manufacturing the core to have a near-parabolic graded-index profile that equalizes mode travel times. That is the only reason 10 Gb/s over 300–550 m is possible on multimode at all.
Source Matching
Singlemode and multimode use different light sources, and they cannot be swapped casually:
- VCSEL (vertical-cavity surface-emitting laser) — multimode at 850 nm; low cost, high coupling efficiency to 50 µm core.
- Fabry-Pérot or DFB laser — singlemode at 1310/1550 nm; narrow spectral width to limit chromatic dispersion.
- LED — legacy, multimode only; overfills the NA and is used for low-speed or test sources.
Coupling a singlemode laser into multimode creates an "underfilled launch" that underestimates modal dispersion; coupling a multimode LED into singlemode loses almost all power because the 9 µm core is tiny compared to the source spot. Test equipment must use a launch condition matching the fiber under test—this is why OLTS kits come with interchangeable source modules.
Distance Comparison
Practical maximum distances at common data rates (representative, not guarantees):
| Fiber | 1 Gb/s | 10 Gb/s | 40/100 Gb/s |
|---|---|---|---|
| OM1 (62.5) | ~300 m | ~33 m | not supported |
| OM2 (50) | ~500 m | ~82 m | not supported |
| OM3 (50) | ~700 m | 300 m | 100 m |
| OM4 (50) | ~1,000 m | 550 m | 150 m |
| OS2 (9) | tens of km | tens of km | many km (with proper optics) |
These numbers are why the data center industry moved to OM3/OM4 for short high-speed links and to singlemode for anything longer.
Common TECH-Exam Traps
- Mating OS2 to OM4 at a patch panel because both have the same SC/APC connector. The physical adapter fits, but the source mismatch and core mismatch produce huge loss and reflection.
- Assuming yellow jacket means "faster" fiber; the color only identifies singlemode, not a speed rating.
- Reporting a multimode link as "failing" when the technician used a 1310 nm singlemode OTDR module; the trace is meaningless because the wavelength and source type do not match the fiber.
What is the principal distance-limiting mechanism in multimode fiber at 10 Gb/s and higher?
Which combination correctly describes OS2 singlemode cable?