In singlemode fiber the light is not confined strictly to the core: part of the field spreads into the cladding, so the spot that matters for coupling is wider than the glass core. A standard G.652 fiber has an MFD of about 9.2 µm at 1310 nm and 10.4 µm at 1550 nm. Joining fibers with different MFDs adds loss and makes OTDR readings lopsided, because the two fibers also scatter different amounts of light back.
That splice reads 0.25 one way and minus 0.15 the other, classic MFD mismatch between the G.652 feeder and the bend-insensitive drop.
When a splice looks bad from one end and like a gainer from the other, think MFD mismatch before you think bad splice.
Also heard as
- MFD
Related terms
Singlemode fiber
Optical fiber with a core so small that light travels in a single path, used for long-haul, metro, FTTH and campus backbones.
Gainer
A splice that shows an apparent gain of light on an OTDR trace, an artifact of mismatched backscatter.
Bidirectional averaging
Testing a fiber with an OTDR from both ends and averaging the results to get true splice losses.
Core
The central glass region of an optical fiber where the light actually travels.