Silicon photonics
Which correctly describes the cutback method for measuring the propagation loss of silicon waveguides?
- AStrong light is launched into a single waveguide, and propagation loss is derived from the change in color (wavelength) of the output light
- BThe losses of waveguides of different lengths are measured; propagation loss is obtained from the slope versus length, and coupling loss from extrapolation to zero length
- CThe waveguide is cut and its cross-section observed with an electron microscope, and the loss is calculated solely from the size of the sidewall roughness
Show answer and explanation
Answer: B. The losses of waveguides of different lengths are measured; propagation loss is obtained from the slope versus length, and coupling loss from extrapolation to zero length
Loss also arises from fiber coupling at the waveguide's input and output, so measuring the transmission of a single waveguide cannot separate propagation loss from coupling loss. In the cutback method, waveguides of identical structure differing only in length (e.g., 1, 2 and 4 cm) are prepared, and the total loss is measured in dB and plotted against length. The slope of the line gives the propagation loss per cm (dB/cm), and the value at zero length (the intercept) gives the combined input and output coupling loss. Keeping coupling conditions consistent is the key to accuracy, and measurements are often made on the wafer using identical grating couplers.