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 | 0.1 – 0.5 dB |
| SMF–SMF 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, with a mode converter) | 1 – 3 dB; under 0.5 dB at best with lensed or high-NA fiber |
| Grating coupler (SOI, apodized) | 1 – 3 dB |
| Grating coupler (SOI, uniform) | 3 – 6 dB |
| 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. The bench procedure for fiber components, including the reference methods that decide which connections are counted, is in Measuring Insertion Loss and Return Loss.
References: IEC 61300-3-4, Fibre optic interconnecting devices and passive components: Basic test and measurement procedures, Part 3-4: Examinations and measurements, Attenuation. D. Derickson (ed.), Fiber Optic Test and Measurement (Prentice Hall, 1998). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010).