Mode-crossing spectroscopy for photonic waveguide characterization

We report a novel spectroscopic technique to characterize photonic waveguides over more than an octave of wavelength. The technique, called mode-crossing spectroscopy, uses a broadband source and a spectrometer that are coupled to the input and output ends of a series of straight dielectric waveguides via single-mode optical fibers. Measurements of the wavelengths at which multiple modes are degenerate in a single waveguide enable the determination of the refractive index of the core, as well as the waveguide width variation. In addition, mode-dependent losses are reported from 700 nm to 1550 nm using waveguides of varying lengths. The core refractive indices are measured within an error of ±0.5%, waveguide width variation to an accuracy of less than 5 nm, and waveguide propagation losses within an error of ±0.5 dB/cm. Ultrabroadband in situ measurements of loss and index over an octave of bandwidth are crucial for the accurate characterization of photonic integrated circuits and devices, especially those based on precision broadband couplers, interferometers, and resonant cavities.