Single-mode fibre engineered to tolerate tight bends without leaking light — what patch cords and FTTH drops are made of.
What Is G.657 Fiber?
Standard single-mode fibre leaks light when it is bent tightly. At the outside of a curve the guided mode can no longer be supported by total internal reflection and part of it radiates into the cladding. In a patch panel, a riser or a wall box, tight bends are unavoidable, and the resulting macrobend loss is a common and frequently misdiagnosed cause of link failure.
G.657 changes the refractive index profile — usually by adding a depressed trench around the core — so the mode is held more tightly and survives radii that would badly degrade ordinary G.652.D fibre.
The Sub-classes, and What They Trade
Category A is defined to be fully compliant with G.652.D, so A1 and A2 splice and mate into existing plant with no penalty worth measuring. A1 is specified to a 10 mm minimum radius, where a single turn costs no more than 0.75 dB at 1550 nm; A2 goes to 7.5 mm at 0.5 dB per turn, and at 10 mm is an order of magnitude better than A1 at 0.1 dB.
Category B is a different bargain. B2 and B3 are described as compatible with G.652.D rather than compliant — they will not cause system trouble, but their dispersion and PMD attributes are allowed to differ. B3 is rated to a 5 mm radius, which is what makes fibre that can be routed round a corner of a wall box at all.
How to Choose
Use A1 or A2 anywhere the fibre joins the wider plant, which in practice means patch cords, pigtails and distribution cable. Reserve B-class for tightly confined terminations where nothing else physically fits — drop cable stapled round architrave, compact wall outlets, small closures.
Note that bend-insensitive fibre has a diagnostic cost as well as a benefit: because it does not leak, a tight bend that would once have shown up as a clear OTDR event may now be nearly invisible, while still stressing the glass mechanically.
Where You Meet It
Almost every patch cord made today, all FTTH drop cable, and anything routed inside a cabinet. Most of our patch cords are built on G.657.A1 or A2 as standard, and the FTTH drop cable uses bend-insensitive glass throughout — see the drop cable installation guide. In a dense panel the bend radius is the real constraint, not the fibre grade.
G.657 reference
| Categories | A1, A2 (G.652.D compliant); B2, B3 (compatible, not compliant) |
|---|---|
| A1 minimum bend radius | 10 mm |
| A1 at 10 mm, one turn | ≤ 0.75 dB at 1550 nm, ≤ 1.5 dB at 1625 nm |
| A2 minimum bend radius | 7.5 mm |
| A2 at 7.5 mm, one turn | ≤ 0.5 dB at 1550 nm, ≤ 1.0 dB at 1625 nm |
| A2 at 10 mm, one turn | ≤ 0.1 dB at 1550 nm |
| B2 / B3 minimum bend radius | 7.5 mm / 5 mm |
| Mode field diameter, category A | 8.6 to 9.5 µm at 1310 nm |
| For contrast, G.652.D | specified at 30 mm radius over 100 turns |