Silicon Photonics: Fundamentals and Devices

12.7 Echelle Grating

Diffractions gratings have been in use over decades to separate light into its different component wavelengths. In principle, gratings can be used as demultiplexers in WDM systems. A reflection grating has been used for demultiplexing WDM signals.

Due to difficulty in fabricating planar diffraction gratings, AWGs have so far enjoyed a monopoly in the WDM demultiplexing scenario. Three fundamental challenges in the fabrication of planar diffraction gratings must be overcome before these devices could be employed on a large scale in present DWDM communications systems. These are (1) fabrication of vertical grating facets in a waveguide structure, (2) reduction of polarization dependence of the grating diffraction efficiency, and (3) elimination or compensation of the birefringence of the device.

Blaze is a tilt on the grooves of a reflection grating to concentrate the light at a higher angle, and therefore in orders M > 0. Thus light is concentrated in the dispersed orders. An echelle grating is an extreme example of a blazed grating. The blaze angle is set very high (to the order of 65̊), where i and d are, respectively, the angles of incidence and diffraction.

In recent years, echelle gratings (EGs) have proven to be promising devices for demultiplexing DWDM light and potential competitors to the more advanced AWG devices. The EGs have some inherent advantages ...