We report Bragg grating structures fabricated by focused ion beam milling in optical
waveguides, and demonstrate that they can be used as a powerful diagnostic of optical
modes in very high index waveguides. We show that higher-order lossy modes, which can
be present in large numbers even in single-moded silicon-on-insulator waveguides, can
dramatically affect the optical transmission spectra of Bragg gratings in these waveguides,
even though these modes are normally not observable. Our results not only illuminate …