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In this analysis, we show that, using a Bragg grating in transmission mode, an optical filter based on an asymmetric twin-core fibre can operate with excellent narrow bandpass characteristics (e.g., pass bandwidths less than 1 nm) for optical wavelength division multiplexing systems. The grating in this scheme operates outside the band-gap. This ensures that the grating functions are in ‘transmission mode', i.e., that most of the light is transmitted through the device. More importantly, the grating introduces a periodic modulation in propagation constant which produces resonantly enhanced dispersion in coupling, and this leads to a significant reduction in pass bandwidth.