Dual transmission band Bragg grating assisted asymmetric directional couplers.

The use of artificial dispersion by material structuring is investigated for the design of highly wavelength selective directional couplers. Systems of two highly asymmetric coupled waveguides are considered with the artificial dispersion created by distributed Bragg gratings (BGs) operated near photonic band gap. It is shown that even in the case of an asymmetrical directional coupler with initially phase matched waveguides, the achievement of high wavelength selectivity requires the fulfillment of a threshold condition on the BG coupling coefficient. The presence of BG(s) leads in turn to the appearance of two transmission bands instead of one. The wavelength selectivity associated to one of these bands is much higher than that obtained in the absence of BG(s). It is also shown that under particular circumstances, dual band operation can be achieved without threshold condition. The directional coupler then exhibits two transmission bands with approximately the same width and a very low level of insertion losses. Such a dual band transmission coupler is expected to offer new functionalities for wavelength demultiplexing applications.

Proposal and analysis of narrow band transmission asymmetric directional couplers with Bragg grating induced phase matching.

This paper addresses the design of narrow band transmission co-directional couplers suitable for wavelength division multiplexing applications. The originality of the proposed asymmetric two-waveguide configuration stems from the use of Bragg gratings operated near band gap to simultaneously achieve high wavelength dispersion and selectivity as well as co-directional phase matching between guides which would be mismatched otherwise. Our theoretical analysis reveals the existence of a minimum Bragg grating coupling strength for co-directional phase matching. The threshold condition is analytically determined, and a coupled mode theory (CMT) four-wave model is successfully applied to describe the behavior of the investigated device. A numerical validation of CMT results is reported in the case of slab waveguides with Bragg grating assisted coupling. The proposed design is shown to be compatible with existing micro-nano-fabrication technology.