Etched diffraction grating with one-dimensional photonic crystal bandgap theory

Abstract

Etched diffraction grating (EDG) with Bragg reflectors based demultiplexer offers advantages of low insertion loss, compact size and tolerant process, which has been the focus of a great deal of research. The Bragg reflector, consisting of alternating dielectric layers coupled to a homogeneous medium, is a kind of one-dimensional photonic crystal (1-D PC). This paper proposed a design method based on 1-D PC theory to design Bragg-EDG. With this design method, the reflection condition calculated by the 1-D PC theory can be matched perfectly with the diffraction condition. As a result, the shift of central wavelength of diffraction spectra can be improved, while keeping high diffraction efficiency. With a small number of dielectric layers, a Bragg-EDG with broad bandwidth, good channel uniformity and low next-channel crosstalk level can be obtained. Performances of Bragg-EDG for TE and TM-mode are also investigated, and the result shows that the grating by this method has a small polarization dependent loss. The reflection bandwidth of the TE-mode is greater than that of the TM-mode; while for the diffraction efficiency, TM-mode performs better. An analysis of etching deviation was also carried on to study the effect of fabrication errors on the performance of the grating. Simulation results indicate that the diffraction grating designed by this approach is very tolerant concerning fabrication imperfections, and a 10% deviation of the width of the etched trench does not lead to a significant decrease in reflection and diffraction efficiencies of the grating at designed work band.