Optical device with vertical waveguide structure
An optical device includes a semiconductor substrate having a major surface, and an optical waveguide structure on the major surface. The optical waveguide structure comprises a lower optical core, an upper optical core, and an intermediate optical cladding, where the upper optical core is vertically above the lower optical core and the intermediate optical cladding separates and is in contact with the optical cores. The optical cores are optically coupled in first and second sections of the optical waveguide structure near the respective first and second ends, and the optical waveguide structure has a parallel pair of optical waveguides extending from the first section to the second section with each of the optical waveguides including one of the optical cores.
1 . An optical device, comprising:
a semiconductor substrate having a major surface; and
an optical waveguide structure formed vertically on the major surface, the optical waveguide structure comprising a lower optical core, an upper optical core, and an intermediate optical cladding;
wherein at least one of the lower and upper optical cores extends from a first end of the optical waveguide structure to a second end thereof;
wherein the upper optical core is vertically above the lower optical core in a vertical direction perpendicular to the major surface and the intermediate optical cladding separates and is in contact with the optical cores;
wherein a cladding thickness of the intermediate optical cladding in the vertical direction varies along a length of the optical waveguide structure so that the optical cores are optically coupled in first and second sections of the optical waveguide structure near the respective first and second ends, and are optically uncoupled in a third section between the first and second sections;
wherein the intermediate optical cladding is formed with a growth process that selectively grows semiconductor material of the intermediate optical cladding on the lower optical core to vary the cladding thickness from a greater thickness in the third section to a lesser thickness in the first and second sections;
wherein the optical waveguide structure has a parallel pair of optical waveguides extending from the first section to the second section, a lower optical waveguide of the parallel pair includes the lower optical core and an upper optical waveguide of the parallel pair includes the upper optical core;
wherein the lower optical waveguide and the upper optical waveguide are vertically stacked with the upper optical waveguide disposed above the lower optical waveguide in the vertical direction such that a vertical plane intersects both the lower optical core and the upper optical core in the first, second, and third sections.
2 . The optical device of claim 1 , wherein a separation of the optical cores is less in the first and second sections than between the first and second sections.
3 . The optical device of claim 1 , wherein the lower optical core, the intermediate optical cladding, and the upper optical core are semiconductor layers.
4 . The optical device of claim 3 , wherein the lower optical core, the intermediate optical cladding, and the upper optical core are III-V semiconductors.
5 . The optical device of claim 3 , wherein the optical waveguide structure includes a p-n-p or an n-p-n semiconductor-junction structure along a direction perpendicular to the major surface.
6 . The optical device of claim 1 , wherein the optical waveguide structure further comprises a lower optical cladding on the semiconductor substrate, the lower optical core being above a part of the lower optical cladding in the vertical direction and in contact with the lower optical cladding.
7 . The optical device of claim 1 , wherein the optical waveguide structure further comprises:
a lower optical cladding separating the lower optical core from the semiconductor substrate; and
an upper optical cladding formed on the upper optical core.
8 . The optical device of claim 1 , wherein the optical device comprises a Mach-Zehnder optical modulator, the optical waveguides of the parallel pair being parallel optical arms of the Mach-Zehnder optical modulator, at least one of the optical arms having a segment configured as an electro-optical phase shifter.
9 . The optical device of claim 8 , wherein at least one of the optical waveguides has a thermo-optical phase shifter along a segment thereof.
10 . The optical device of claim 8 , wherein the Mach-Zehnder optical modulator comprises one or more radio frequency traveling wave electrodes along the segment of the at least one of the optical arms.
11 . The optical device of claim 8 , wherein the segment of the at least one of the optical arms includes a semiconductor junction therealong, the semiconductor junction being in the optical waveguide structure.
12 . A method of fabricating an optical device having an optical waveguide structure, the method comprising:
acquiring a semiconductor substrate; and
forming the optical waveguide structure vertically on a major surface of the semiconductor substrate;
wherein the forming the optical waveguide structure comprises:
forming a lower optical core;
forming an intermediate optical cladding on the lower optical core; and
forming an upper optical core on the intermediate optical cladding;
wherein at least one of the lower and upper optical cores extends from a first end of the optical waveguide structure to a second end thereof;
wherein the upper optical core is vertically above the lower optical core in a vertical direction perpendicular to the major surface and the intermediate optical cladding separates and is in contact with the optical cores;
wherein a cladding thickness of the intermediate optical cladding in the vertical direction varies along a length of the optical waveguide structure so that the optical cores are optically coupled in first and second sections of the optical waveguide structure near the respective first and second ends, and are optically uncoupled in a third section between the first and second sections;
wherein the forming of the intermediate optical cladding comprises forming the intermediate optical cladding with a growth process that selectively grows semiconductor material of the intermediate optical cladding on the lower optical core to vary the cladding thickness from a greater thickness in the third section to a lesser thickness in the first and second sections;
wherein the optical waveguide structure has a parallel pair of optical waveguides extending from the first section to the second section, a lower optical waveguide of the parallel pair includes the lower optical core and an upper optical waveguide of the parallel pair includes the upper optical core;
wherein the lower optical waveguide and the upper optical waveguide are vertically stacked with the upper optical waveguide disposed above the lower optical waveguide in the vertical direction such that a vertical plane intersects both the lower optical core and the upper optical core in the first, second, and third sections.
13 . The method of claim 12 , wherein:
the optical device comprises a Mach-Zehnder optical modulator, the optical waveguides of the parallel pair being parallel optical arms of the Mach-Zehnder optical modulator.
14 . The optical device of claim 1 , wherein:
the lower optical core is straight and the upper optical core is non-straight along the length of the optical waveguide structure; and
the cladding thickness of the intermediate optical cladding varies from a greater thickness where the lower optical core and the upper optical core are optically uncoupled in the third section, to a lesser thickness where the lower optical core and the upper optical core are optically coupled in the first and second sections.
15 . The optical device of claim 1 , wherein:
the upper optical core is straight and the lower optical core is non-straight along the length of the optical waveguide structure; and
the cladding thickness of the intermediate optical cladding varies from a greater thickness where the lower optical core and the upper optical core are optically uncoupled in the third section, to a lesser thickness where the lower optical core and the upper optical core are optically coupled in the first and second sections.
16 . The optical device of claim 1 , wherein:
the upper optical core is non-straight and the lower optical core is non-straight along the length of the optical waveguide structure; and
the cladding thickness of the intermediate optical cladding varies from a greater thickness where the lower optical core and the upper optical core are optically uncoupled in the third section, to a lesser thickness where the lower optical core and the upper optical core are optically coupled in the first and second sections.
17 . The optical device of claim 1 , wherein:
thicknesses of the lower optical core and the upper optical core are uniform along the length of the optical waveguide structure.
18 . The optical device of claim 8 , wherein:
the first section operates as an optical power splitter of the Mach-Zehnder optical modulator; and
the second section operates as an optical power combiner of the Mach-Zehnder optical modulator.