Bragg gratings have been used relatively extensively in recent years due to their highly dispersive and wavelength selective nature. Typically used as a reflective structure, the gratings reflect specific wavelengths at specific locations along the structure based on the grating periodicity to spatially shape an incident pulse of light according to its spectral components. Usually the purpose is to either compress or stretch the pulse.
Unfortunately, fabrication tolerances severely limit the amount of chirp per unit of waveguide length that can be placed on a Bragg grating. For some applications, a few nanometers of chirp over a meter or more of waveguide would be ideal, yet placement accuracy of individual features is usually far less than is needed for such a task. We propose an alternative fabrication method which would provide a long grating with substantially increased placement accuracy. Instead of fashioning the grating in the typical linear manner, a waveguide is fabricated in a spiral shape. This has been done for delay lines and amplifier structures in the past. However, we propose to incorporate a radial grating underneath it. This provides us an additional degree of freedom, since the period of the grating changes very linearly with its radius, and a waveguide can be accurately positioned on top of it so as to gradually spiral inwards (or outwards) and change radius (and, hence, grating period) very slowly along its length. We present fabrication results, optical comparisons between similar linear and spiral structures, and preliminary theoretical modeling of the structures.