Understanding fiber Bragg gratings at harmonics of the Bragg wavelength through spectra and images

Fiber Bragg gratings written using a phase-mask exhibit a complex refractive index pattern in the fiber core, as observed in differential interference contrast (DIC) microscopy. This results in dual periodicities along the fiber core and gratings at both the Bragg wavelength, twice the Bragg wavelength, and the harmonics of these. Features in grating spectra arise from one or both of these periodicities and spectra for twice the Bragg wavelength (or its odd harmonics) are very sensitive to the details of how the UV complex diffraction field of the phase mask becomes imprinted as refractive index changes in the fiber core.