Grating apodization technique for diffused optical waveguides
View Patent ↗An optical channel waveguide having a reflection grating and a related method for its fabrication. The grating is apodized to provide a desired reflection or transmission spectral characteristic, by varying the grating width along the length of the grating in the direction of light propagation. The grating has multiple parallel elements extending across the waveguide channel width, and apodization is effected by appropriate selection of the width of each element relative to the width of the channel, without varying the grating duty cycle or other parameters.
1. An optical channel waveguide and grating, comprising:
a substrate;
a waveguide channel formed in the substrate, the channel having a length dimension along which light may be propagated and a width dimension; and
a periodic grating formed in close association with the waveguide channel, to provide reflection of light at selected frequencies and transmission through the waveguide at other frequencies;
wherein the periodic grating has multiple parallel elements extending at least part-way across the width dimension of the channel, at least two of the multiple parallel elements having different lengths;
and wherein the length of each grating element, extending in the width dimension across the channel, is selected to provide an apodized grating with a desired reflection spectral characteristic.
2. An optical channel waveguide as defined in claim 1 , wherein:
the lengths of the grating elements are selected to provide Gaussian apodization.
3. An optical channel waveguide as defined in claim 1 , wherein:
the substrate is a dielectric material;
the waveguide channel is formed in the substrate by diffusing a metal into the dielectric material.
4. An optical channel waveguide as defined in claim 3 , wherein:
the grating elements are formed as parallel ridges of a material overlying the waveguide channel in a grating region, separated by grooves filled with a material with a refractive index that is high relative to the material overlying the waveguide channel in the grating region.
5. An optical channel waveguide as defined in claim 4 , wherein:
the substrate material is lithium niobate;
the metal diffused into the substrate to form the channel is titanium;
the material overlying the waveguide channel in the grating region is silicon dioxide; and
the material filling the grooves is niobium pentoxide.
6. A method for fabrication of an optical channel waveguide and grating, comprising the steps of:
forming a surface channel having an elevated refractive index, in a dielectric substrate, the channel having a length dimension along which light may be propagated and a width dimension perpendicular to the length dimension; and
forming a plurality of parallel grating elements over the substrate and extending in the channel width direction at least part-way across the channel;
wherein the grating elements are selected in length such that at least two of the grating elements have different lengths to provide apodization of the grating, resulting in a desired spectral characteristic of the waveguide.
7. A method as defined in claim 6 , wherein:
the lengths of the grating elements are selected to provide Gaussian apodization.
8. A method as defined in claim 6 , wherein:
the step of forming the surface channel comprises diffusing a metal into the substrate, which is of a dielectric material.
9. A method as defined in claim 8 , wherein the step of forming the grating elements comprises:
overlaying the waveguide channel, in a grating region, with a layer of silicon dioxide;
etching away parallel grooves of the silicon dioxide to leave a set of parallel ridges separated by grooves; and
filling the grooves with a material having a refractive index that is high relative to the layer of silicon dioxide.
10. A method as defined in claim 9 , wherein the step of forming the grating further comprises:
forming an overlying layer of material with a refractive index that is high relative to the layer of silicon dioxide over the grating ridges and filled grooves.
11. A method as defined in claim 10 , wherein:
the substrate material is lithium niobate; and
the diffusing step comprises diffusing titanium into the substrate.