Join two fibers with different mode-field diameters and there's a loss floor you can't align away — set only by the MFD ratio.
| MFD ratio | Loss | Example |
|---|---|---|
| 1 : 1 | 0 dB | Same fiber (ideal alignment) |
| 1.2 : 1 | 0.14 dB | PM1550 ↔ SMF-28 class |
| 1.5 : 1 | 0.7 dB | — |
| 1.65 : 1 | 1.05 dB | SMF-28 ↔ Hi1060 |
| 2 : 1 | 1.9 dB | Large mismatch — consider transition |
This formula gives the loss floor from mode-field diameter difference alone, at perfect alignment; real joints add lateral offset, angular error and end-gap losses on top, and your splicer's core alignment decides how close you get to the floor. Beyond a 2:1 mismatch, thermally-expanded cores (TEC) or a bridge fiber taper the mode adiabatically and can bring the loss back to the 0.2 dB class.
In the Gaussian approximation MFD ≈ 2λ/(π·NA): higher numerical aperture means a smaller mode field — why "high-NA" fibers such as Hi1060 (NA≈0.14) have visibly smaller MFDs than standard single-mode fiber. Pigtail MFD/NA must therefore be chosen together with the downstream optics; our packaging line accepts each unit on measured coupling efficiency.
Related tools: Collimation & C-LENS · dB↔linear ratio
※ Formulas on this page assume ideal models; all device parameters shown are typical values — refer to the datasheet and the serialized factory test report shipped with each unit. For selection support, contact sales@lncetek.com.