Glossary entry

Propagation loss

The optical power attenuation per unit length in a waveguide, expressed in dB/cm or cm⁻¹. The standard figure of merit for waveguide quality in photonic integrated circuits.

Propagation loss $\alpha$ quantifies the attenuation of optical power per unit length as it propagates through a waveguide. Power decays exponentially:

P(L) \;=\; P(0) \cdot 10^{-\alpha L / 10} \quad (\alpha \text{ in dB/length}),

or equivalently

P(L) \;=\; P(0) \cdot \exp(-\alpha_n L) \quad (\alpha_n \text{ in nepers/length}),

with the conversion $\alpha \text{ [dB/cm]} = 4.343 \, \alpha_n \text{ [cm}^{-1}\text{]}$ .

Loss originates from several physical mechanisms. Sidewall scattering arises from the lithographic roughness of waveguide sidewalls and dominates in high-index-contrast platforms such as SOI. Bulk absorption sets a wavelength-dependent floor — silicon absorbs at wavelengths shorter than $\sim 1.1$ μm; silicon nitride absorbs only above $\sim 4$ μm. Bend losses appear where waveguides curve and scale strongly with bend radius. Substrate leakage occurs when the lower cladding is thin enough that the evanescent tail of the mode couples into the substrate.

Typical propagation loss values:

Platform	Wavelength	Typical $\alpha$
SOI 220 nm strip	1550 nm	2–4 dB/cm
SOI 220 nm ridge	1550 nm	1–3 dB/cm
Silicon nitride, 200 nm	1550 nm	0.05–0.2 dB/cm
Silicon nitride, 800 nm thick	1550 nm	0.001–0.05 dB/cm
InP shallow ridge	1550 nm	1.5–4 dB/cm
Optical fiber (SMF-28)	1550 nm	0.2 dB/km

Measurement methods include the cutback method (multi-length transmission measurements), Fabry–Pérot fringe analysis (transmission ripple from facet reflections), and ring-resonator linewidth measurements (Q factor extraction). The cutback method is the most direct and is treated in Waveguide Propagation Loss by the Cutback Method.