Insertion loss (IL)
The optical power lost by inserting a component into an otherwise-lossless signal path, in dB. Positive value indicates power reduction.
Insertion loss in decibels is
By convention, IL is reported as a positive number indicating power loss. IL aggregates all mechanisms by which a component reduces transmitted power: absorption, scattering, reflection, mode mismatch, polarization mismatch, and coupling loss.
Typical IL for common components at 1550 nm:
| Component | Typical IL |
|---|---|
| Mated FC/UPC connector | 0.2 – 0.5 dB |
| FC/APC fusion splice | 0.05 dB |
| 50:50 fiber splitter (one output) | 3.0 – 3.5 dB |
| Optical isolator | 0.4 – 1.0 dB |
| Tunable optical filter | 1 – 5 dB |
| Mach–Zehnder modulator | 3 – 7 dB |
| Edge coupler (SOI, lensed fiber) | 2 – 4 dB |
| Grating coupler (SOI, apodized) | 1 – 3 dB |
| Grating coupler (SOI, uniform) | 3 – 6 dB |
| Inverse-taper edge coupler to SMF-28 | 8 – 12 dB (3 – 5 dB to lensed fiber) |
| Polarization controller | 0.5 – 2 dB |
| EDFA pass-through (gain block + isolators) | 1 dB additional |
IL is typically wavelength-dependent and polarization-dependent. Published values usually specify the wavelength range and polarization state of the measurement.
For PIC components, IL extracted from a single device measurement combines per-device loss with the insertion loss of input/output couplers. Isolating the per-device loss requires either calibration against a loopback structure on the same chip or use of multiple devices of varying length (see Waveguide Propagation Loss by the Cutback Method).
For fiber-to-fiber connections with dissimilar fiber types or core sizes, IL is dominated by mode mismatch loss.