Phase-shifted Bragg-Grating filters with improved transmission characteristics

Standard Bragg filters (dielectric waveguides with a grating overlay) show a stopband only in reflection. Filters with quarter-wave-shifted gratings have an extremely narrow transmission peak in the center of the stopband but its shape (a sharp triangle with a broad bottom) is not suitable for system design. Using standard coupled-mode theory together with transfer-matrix formalism, we show that by introducing several phase-shift regions and properly choosing their locations and magnitudes the transmission spectrum can be tailored into a nearly rectangular shape. For InP/InGaAsP-based Bragg-grating filters estimates are made of achievable channel spacings in wavelength-division multiplexing systems.