| Full title | Characteristics of a bending-loss insensitive single-mode optical fibre and cable |
| Publisher | ITU-T Study Group 15 |
| Current edition | Edition 5.0, approved 29 August 2024 (G.657 08/2024) |
| Status | In force. Replaces G.657 (11/2016) |
| Categories | A1, A2 and B3. B2 was merged into A2 in 2024 |
| Related standards | ITU-T G.652, G.650.1, L.103; IEC 60793-2-50 (B-657); IEC 60793-1-47 |
What it covers
ITU-T G.657 describes single-mode fibre and cable with much lower macrobending loss than standard ITU-T G.652 fibre. It was first published in 2006 for access networks, where fibre is coiled in small enclosures, routed round tight corners and handled many times.
The 2016 edition widened category A to all networks where G.652.D is used. The 2024 edition added datacentres to the application space, merged category B2 into A2 and updated the cut-off wavelength note.
The categories
| Subcategory | Minimum design radius | G.652.D status | Intended use |
|---|---|---|---|
| G.657.A1 | 10 mm | Compliant | All networks where G.652.D is specified |
| G.657.A2 | 7.5 mm | Compliant | All networks where G.652.D is specified |
| G.657.B3 | 5 mm | System compatible only | Short reach, under 1 000 m, in or near buildings and datacentres |
Category A fibres are a subset of G.652.D. They meet or exceed every G.652.D attribute and have the same transmission and interconnection properties, so they can be used anywhere G.652.D is specified. They work across 1260 nm to 1625 nm.
Category B fibres are optimised for even lower bend loss. They are not necessarily compliant with G.652.D for chromatic dispersion and PMD, but are system compatible with G.657.A and G.652.D in access networks. The previous B2 subcategory had the same macrobending limits as A2, and the 2024 edition merged it into A2. B2 fibre was specified with looser dispersion and PMD limits, so it is not automatically G.652.D or A2 compliant.
Macrobending loss limits (uncabled fibre)
| Subcategory | Radius | Turns | Max at 1550 nm | Max at 1625 nm |
|---|---|---|---|---|
| G.657.A1 | 15 mm | 10 | 0.25 dB | 1.0 dB |
| G.657.A1 | 10 mm | 1 | 0.75 dB | 1.5 dB |
| G.657.A2 | 15 mm | 10 | 0.03 dB | 0.1 dB |
| G.657.A2 | 10 mm | 1 | 0.1 dB | 0.2 dB |
| G.657.A2 | 7.5 mm | 1 | 0.5 dB | 1.0 dB |
| G.657.B3 | 10 mm | 1 | 0.03 dB | 0.1 dB |
| G.657.B3 | 7.5 mm | 1 | 0.08 dB | 0.25 dB |
| G.657.B3 | 5 mm | 1 | 0.15 dB | 0.45 dB |
For comparison, G.652.D is specified at 30 mm radius over 100 turns (≤ 0.1 dB at 1625 nm). The Recommendation notes that G.652 fibre at 15 mm radius can lose several dB per 10 turns at 1625 nm.
Loss can be measured by the mandrel winding method (method A) of IEC 60793-1-47. These are fibre limits. The cable construction, and how the cable is installed, also affect bend loss in the field.
Other key values
| Attribute | G.657.A1 / A2 | G.657.B3 |
|---|---|---|
| Mode field diameter at 1310 nm | 8.6 to 9.2 µm, ±0.4 µm | 8.6 to 9.2 µm, ±0.4 µm |
| Cladding diameter | 125.0 µm ± 0.7 µm | 125.0 µm ± 0.7 µm |
| Core concentricity error | ≤ 0.5 µm | ≤ 0.5 µm |
| Cable cut-off wavelength | ≤ 1260 nm | ≤ 1260 nm |
| Proof stress | ≥ 0.69 GPa | ≥ 0.69 GPa |
| Zero-dispersion wavelength | 1300 to 1324 nm | 1250 to 1350 nm |
| Attenuation, 1310 to 1625 nm | ≤ 0.40 dB/km | ≤ 0.40 dB/km |
| Attenuation, 1530 to 1565 nm | ≤ 0.30 dB/km | ≤ 0.30 dB/km |
| PMDQ (M = 20, Q = 0.01%) | ≤ 0.20 ps/√km | ≤ 0.50 ps/√km |
The category A chromatic dispersion limits are the same as G.652.D. The Recommendation notes that, because the attenuation of a short cable cannot be measured accurately, its value can be taken from the longer donor cable it was cut from.
Bend radius and fibre lifetime
- The design radius is an optical rating. Mechanical failure probability rises as the bend radius falls, so small bends can shorten fibre life even when the optical loss is acceptable.
- The Recommendation advises that proof stress and required reliability are agreed between supplier and customer. Guidance on lifetime under small radius bending has moved to ITU-T G-Sup.59.
- ITU-T L.103 recommends that any bend left after installation is large enough to limit loss and long-term strain. It advises against demanding methods such as stapling indoor cable with flat staples.
How it relates to other standards
| Standard | Relationship |
|---|---|
| ITU-T G.652 | G.657.A fibres comply with G.652.D. G.657.B3 fibres are system compatible only |
| IEC 60793-2-50 | IEC product specification covering the same fibres under category B-657 |
| IEC 60793-1-47 | Measurement method for macrobending loss |
| ITU-T L.103 | Optical fibre cables for indoor applications, including installation advice |
What to check on a datasheet
- The subcategory (A1, A2 or B3), not just “G.657”.
- Whether the fibre is also stated as G.652.D compliant. This holds for A1 and A2 and is what allows mixing with existing G.652.D plant.
- The cable minimum bend radius, which is set by the cable construction and is usually larger than the fibre design radius.
- For older stock labelled G.657.B2, check whether it is also stated as G.652.D compliant. B2 had the same bend limits as A2 but looser dispersion and PMD limits.
Related reading: Single-mode vs multimode fibre: OM3, OM4 and OS2 explained · Fibre optic cable construction: tight buffered vs loose tube.