The standard single-mode fibre — nine-micron core, zero water peak, usable from 1260 to 1625 nm.
What Is OS2 Fiber?
OS2 is the cabled performance class for standard single-mode fibre built to ITU-T G.652.D. The guiding region is about nine microns across — small enough that only one mode propagates at the wavelengths it is used at, which removes modal dispersion completely and makes reach a question of loss and chromatic dispersion instead.
The distinction between OS1 and OS2 is a cabling one rather than a glass one. OS1 is a tight-buffered indoor construction allowed 1.0 dB/km; OS2 is loose-tube or ribbon and held to 0.4 dB/km at 1310, 1383 and 1550 nm.
The nine microns is a mode field diameter rather than a physical boundary: it describes where the optical power actually sits, which is a little wider than the doped region, and it is the number that governs splice alignment and connector tolerance. The cladding is 125 µm to ±0.7 µm — the same as multimode — so ferrules, adapters and cleavers are common across both.
What the .D Suffix Means
G.652.D is the low-water-peak variant. Older G.652.B fibre carries an attenuation spike near 1383 nm caused by residual hydroxyl ions left in the glass during manufacture, and that spike makes the E-band unusable. Removing it opens the whole 1260 to 1625 nm range.
That is what makes the full 18-channel CWDM grid practical, because six of those channels — 1351 through 1451 nm — sit in or beside the old water peak. If a site is running CWDM above 1360 nm and seeing unexplained loss on the lower channels, the age of the fibre is the first thing to check.
Loss Against Dispersion
The two windows trade against each other. At 1310 nm dispersion is near zero but attenuation is around 0.32 dB/km; at 1550 nm attenuation falls to roughly 0.19 dB/km but dispersion rises to about 17 ps/nm·km. Long-haul lives at 1550 because 0.19 dB/km is what makes hundreds of kilometres affordable, and it pays for that with dispersion compensation.
Short-reach data-centre single-mode does the opposite. DR4, FR4 and PSM4 all sit in the O-band, where an uncompensated 100G lane can run kilometres without a dispersion penalty. Work a specific case in the dispersion calculator.
Where You Meet It
Effectively all single-mode plant laid in the last twenty years, indoor and outdoor. It is the glass inside our single-mode patch cords and the whole outdoor cable range. Propagation is about 4.9 µs per kilometre, which the latency calculator will turn into a round-trip number.
OS2 / G.652.D reference
| Mode field diameter | 8.6 to 9.5 µm nominal at 1310 nm, ±0.4 µm |
|---|---|
| Cladding | 125 µm, tolerance ±0.7 µm |
| Cable cut-off wavelength | ≤ 1260 nm |
| OS2 cabled attenuation class | ≤ 0.4 dB/km at 1310, 1383 and 1550 nm |
| Typical fibre attenuation | 0.32 dB/km at 1310 nm, 0.19 dB/km at 1550 nm |
| Zero-dispersion wavelength | 1300 to 1324 nm |
| Chromatic dispersion at 1550 nm | about 17 ps/nm·km |
| PMD link design value | ≤ 0.20 ps/√km |
| Usable bands | O, E, S, C and L — 1260 to 1625 nm |
Related Terms
G.657, chromatic dispersion, PMD, CWDM, OM4.