Design of superposed fiber Bragg grating for high-resolution time-delay signal processing

摘要

Superposed WDM fiber Bragg gratings (FBGs) provide a novel method to realize a large number of very short time-delay steps in applications such as photonics-based beam-forming for multi-beam phased arrays. In this paper, a systematic design method for superposed FBGs is presented. This provides information for the key requirements of grating length determination, grating apodization function selection and channel spacing control, and the selection of a suitable method to obtain a linear time-delay profile of the designed superposed FBG. Grating length versus maximum number of superposed gratings for ps time-delays under different apodizations is presented. The effects of grating apodization on wavelength channel spacing of the superposed gratings is also given, and an optimization is described to minimize the wavelength step so as to maximize the number of channels in a given optical spectral width. Finally, a technique for time-delay profile linearization of WDM superposed FBGs with a large number of ps delay steps is presented. Using these design procedures, results are given for a 32-superposed WDM FBG unit which shows 1ps delay step and 1.7% standard deviation from linearity on the time-delay profile, while maintaining >99% reflectivity for all grating channels and keeping the total refractive index change in the fiber below 0.01.