Bandpass spectra of evanescent couplers with long period gratings

The authors propose a novel optical bandpass filter with strong sidelobe extinction but without the reflected light associated with Bragg grating filters. Long period index gratings written in the cores of an evanescent singlemode coupler suppress cross-coupling at the desired transmission wavelength. While strong optical loss in the secondary core rejects out-of-band wavelengths. Transmission spectra are calculated using coupled mode theory.