Planar grating demultiplexers using silicon nanowire technology with flat passband and low crosstalk

Abstract

A design and fabrication for planar grating demultiplexers based on a nano silicon platform are presented to obtain both a flat passband and a low crosstalk using the optimal size and structure for each grating facet. Measured results show a 230 dB crosstalk and a 70 GHz 3 dB bandwidth can be obtained with acceptable additional loss.Introduction: Among various planar lightwave circuits for multiplexing/demultiplexing in an optical communication system, planar grating demultiplexers have shown great potential owing to their compactness and high spectral finesse [1]. Different technologies based on different materials, e.g. SiO 2 , a III-V semiconductor, silicon-on-insulator (SOI), have been introduced to support planar grating demultiplexers. Recently, more compact devices based on silicon nanowire technology have been studied [2,3]. We have designed and fabricated grating demultiplexers based on both echelle [4] and total internal reflection (TIR) facets [5] using silicon nanowire technology. However, all fabricated grating demultiplexers using silicon nanowire technology [2 -5] have a sidelobe level from 210 to 215 dB owing to their sub-wavelength facet size, which leads to too high crosstalk to satisfy practical wavelength division multiplexing (WDM) requirements. To overcome this drawback, we have reshaped the transfer function of the demultiplexers with TIR facets into a Gaussian distribution to reduce the noise floor by slightly adjusting each grating facet's size and structure [6]. Although the analytical method is simple and effective, the Gaussian transfer function is not the best function to achieve crosstalk suppression. In the Letter, we optimise the size and structure of each facet using a global optimisation (i.e. simulated annealing) algorithm to get the best distribution. Moreover, the optimised device ha been fabricated to verify the design.