Study On Silicon Nanowire Arrayed Waveguide Grating

Wavelength Division Multiplexing (WDM) can simultaneously transfer different kinds of signals with different wavelengths in a single fiber, which greatly increases the speed and capacity of optical fiber communications. Arrayed waveguide grating (AWG) is considered to be the best choice as the multiplexer/demultiplexer devices due to its low transmission loss, small size, easy to be integrated with other devices, etc. Recently, great interest has been focused on silicon nanowire based AWGs, which can be made very compact because of the high index contrast and small bending radius of the silicon nanowire waveguide. However the effective index of the silicon nanowire waveguide is very sensitive to the width variation of the waveguide. And the arrayed waveguides are normally rather long, so even very small waveguide width variation can result in large phase errors in AWG and bring big side lobes and crosstalk in its filtering spectra.This seriously deteriorates the crosstalk performance of AWG. In this thesis, the design and fabrication of silicon nanowire AWG devices are studied and especially show how to reduce the crosstalk caused by fabrication errors. The main works include:(1) Different approaches are introduced to accurately calculate the effective index of the basic waveguide structures in AWG, such as slab waveguide, rectangular waveguide and ridge waveguide. And single mode condition for ridge waveguide is also involved.(2) Cascaded grating filters are firstly proposed to cut off the side lobes in transmission spectrum of silicon nanowire arrayed waveguide grating (AWG), which are introduced by fabrication errors. This new method needs no extra fabrication procedure and can maintain the AWG compact. The numerical simulation results shows that, when cascaded with only 54μm length of filters, the crosstalk of AWG can be effectively reduced by 15dB.(3) Several modeling methods for AWG and grating filter are compared to show their advantages and disadvantages. And simulation results by these methods are presented and discussed.(4) Detailed rules are given for determining structure parameters of AWG and grating filters. In order to get better crosstalk performance, global optimization of these parameters is also discussed with examples.(5) 8×8 AWG is fabricated and tested. The measured channel spacing is 3.2nm, which is consistent with the design. But the crosstalk performance is just -4dB. Fabrication error introduced crosstalk is also evaluated. According to the simulation results, even waveguide width variation of only several nanometers will bring more than 10dB crosstalk in its filtering spectra. For the grating filter, the width variation will bring larger loss and central wavelength shift.