Methods and apparatus for optical coupler
There is described an adiabatic edge coupler comprising two coupled waveguides. A first waveguide extends from a tip width to a terminal width. A second waveguide comprises a first tapered portion that extends from a tip width to an intermediate width, and a second tapered portion that extends from the intermediate width to a terminal width. Upon receipt of an input optical signal, a coupled mode is formed between the two coupled waveguides. Over the first tapered portion, the coupled mode is adiabatically transformed onto the first waveguide. The coupled mode then decouples from the second waveguide over the second tapered portion. The adiabatic transformer enables low loss, small footprint, and fabrication-tolerant in-chip propagation of the coupled mode with relatively simplified fabrication processes and small critical dimensions.
1 . An optical coupler having at most two optical waveguides, the optical coupler comprising:
a substrate;
a cladding layer supported above the substrate, the cladding layer and the substrate defining a facet;
a first optical waveguide arranged within the cladding layer, the first optical waveguide configured with a first tapering angle; and
a second optical waveguide arranged substantially parallel to the first optical waveguide and spaced apart from the first optical waveguide by a transverse distance at the facet, the second optical waveguide comprising a first tapered portion with an increasing tapering angle, the increasing angle is less than the first tapering angle, and a second tapered portion with a decreasing tapering angle, thereby defining a first operating region and a second operating region,
wherein a coupled mode formed between the first optical waveguide and the second optical waveguide from an incoming optical signal is transmitted along the first and the second optical waveguides, the confinement of the coupled mode increases in the first operating region, and the coupled mode is transformed onto the first optical waveguide, and gradually decoupled from the second optical waveguide over the second operating region.
2 . The optical coupler of claim 1 , wherein the first optical waveguide widens from a first tip width at the facet to a first terminating width at a first terminal end.
3 . The optical coupler of claim 2 , wherein the first tapered portion widens from a second tip width to an intermediate width; and
the second tapered portion narrows from the intermediate width to a second terminating width at a second terminal end.
4 . The optical coupler of claim 3 , wherein the first tip width is substantially equal to the second tip width.
5 . The optical coupler of claim 1 , wherein each of the first optical waveguide and the second optical waveguide further comprises an extension portion, respectively.
6 . The optical coupler of claim 5 , wherein the extension portion of each of the first and second optical waveguide is of uniform width.
7 . The optical coupler of claim 3 , wherein one or more of the transverse distance, the first tip width, and the second tip width are configured to mode match with the incoming optical signal at the facet.
8 . The optical coupler of claim 1 , wherein the first optical waveguide and the second optical waveguide are ribbed waveguides.
9 . The optical coupler of claim 1 , wherein the second operating region has a length of at least 4% of a total length of the second optical waveguide.
10 . The optical coupler of claim 1 , wherein the second optical waveguide further comprises a radiation tip extending from the second tapered portion, the radiation tip configured to reduce optical reflections within the second optical waveguide.
11 . The optical coupler of claim 10 , wherein the radiation tip is configured with one or more of a curved profile and a narrowing tip.
12 . The optical coupler of claim 1 , wherein the first tapering angle, the increasing tapering angle, and the decreasing tapering angle is one of a constant tapering angle, a polynomial tapering angle, and a discrete step tapering angle.
13 . A method of optically coupling a light source and an optical circuit, the method comprising:
receiving an input optical mode from the light source at a chip facet of an optical coupler by a first optical waveguide and a second optical waveguide, the optical coupler having at most two optical waveguides;
causing an optical mode to be optically coupled between the first optical waveguide having a first tapering angle and the second optical waveguide;
optically confining the coupled mode between the first and second waveguides;
transforming the coupled mode onto the first optical waveguide over a first operating region defined by a first tapered portion of the second optical waveguide having an increasing tapering angle; and
decoupling the coupled mode from the second optical waveguide over a second operating region defined by a second tapered portion of the second optical waveguide having a decreasing tapering angle.
14 . The method of claim 13 further comprising reducing reflections in the second optical waveguide with a radiation tip that extends from the second tapered portion.
15 . The method of claim 13 further comprising configuring one of a gap distance between the first optical waveguide and the second optical waveguide at the chip facet, a first width of the first optical waveguide at the chip facet, and a second width of the second optical waveguide at the chip facet for mode matching with the input optical mode.
16 . The method of claim 13 , wherein
the first optical waveguide is configured with a first tapering angle greater than the increasing tapering angle of the first tapered portion of the second optical waveguide.
17 . The method of claim 13 further comprising
configuring the first tapered portion to extend from a first width to a second width;
configuring the second tapered portion to extend from the second width to a third width; and
configuring the second width based on one or more of a total length of the second optical waveguide, a gap distance between the first and the second optical waveguides at the chip facet, and tip widths of the first and second optical waveguides at the chip facet.
18 . The method of claim 16 , wherein any one of the first tapering angle, the increasing tapering angle, and the decreasing tapering angle is configured with one of a constant tapering angle, a polynomial tapering angle, and a discrete step tapering angle.