Silicon-based tunable single passband optical filter
A tunable optical filter includes a tunable Fabry-Perot (FP) filter, two tunable waveguide Bragg gratings (WBGs) and a 2×2 3-dB coupler. In one embodiment, the WBGs are implemented in a silicon substrate using polysilicon filled trenches. The FP filter is implemented with two silicon nitride trench WBGs with a gap region between them. The FP filter and the WBGs are respectively tuned to transmit and reflect a selected wavelength. A broadband optical signal is propagated into a first port of the coupler. The coupler propagates half of the beam to one WBG and the other half to the other WBG. The WBGs reflect these portions back to the coupler, which then propagates the reflected portions to the FP filter.
1. A method, comprising:
propagating a multiple wavelength optical signal into a waveguide formed in a semiconductor substrate;
splitting the optical signal into first and second substantially equal portions to respectively propagated in second and third planar waveguides formed in the semiconductor substrate;
reflecting a selected wavelength of the first portion with a first plurality of silicon and polysilicon interfaces disposed in the second planar waveguide to propagate the selected wavelength in the second planar waveguide;
reflecting the selected wavelength of the second portion with a second plurality of silicon and polysilicon interfaces disposed in the third planar waveguide to propagate the selected wavelength in the third planar waveguide;
combining the reflected wavelengths from the first and second portions to propagate in a fourth planar waveguide formed in the semiconductor substrate; and
filtering the combined reflected wavelengths to transmit the selected wavelength.
2. The method of claim 1 wherein reflecting a selected wavelength comprises introducing a ½ pi phase shift between the first and second portions with a three-dB coupler.
3. The method of claim 2 wherein a silicon-based Fabry-Perot filter is used to filter the combined reflected wavelengths.
4. The method of claim 2 wherein silicon-based Bragg gratings are used to reflect the selected wavelength from the first and second portions.