Optical modulator
To prevent deterioration in an extinction ratio due to asymmetry between a pair of optical waveguides. An optical modulator has a Mach-Zehnder optical waveguide including mutually parallel first and second waveguides and a signal electrode for controlling the phase of light propagating in the Mach-Zehnder optical waveguide. The first and second waveguides have a first section in which the second waveguide has a line width smaller than that of the first waveguide and a second section in which the first waveguide has a line width smaller than that of the second waveguide. The first section and the second section are replaced with each other in curved parts.
1 . An optical modulator comprising:
a Mach-Zehnder optical waveguide having an input waveguide, a demultiplexer demultiplexing light propagating in the input waveguide, first and second waveguides extending in parallel from the demultiplexer, a multiplexer multiplexing lights propagating in the first and second waveguides, and an output waveguide propagating light output from the multiplexer; and
a signal electrode controlling a phase of light propagating in the Mach-Zehnder optical waveguide,
wherein the first and second waveguides each include:
a first straight section in which a line width of the second waveguide is smaller than a line width of the first waveguide;
a second straight section provided parallel to the first straight section and in which a line width of the first waveguide is smaller than a line width of the second waveguide so as to balance optical paths of the first waveguide and the second waveguide; and
a first curved section connecting the first straight section and the second straight section,
wherein the first straight section is disposed closer to the input waveguide than the second straight section,
wherein the line width of the first waveguide in the first straight section is smaller than the line width of the first waveguide in the second straight section while the line width of the second waveguide in the first straight section is smaller than the line width of the second waveguide in the second straight section, and
wherein a line width of the first waveguide in the first curved section continuously increases from the line width of the first waveguide in the first straight section to the line width of the first waveguide in the second straight section through an entirety of the first curved section, while a line width of the second waveguide in the first curved section continuously increases from the line width of the second waveguide in the first straight section to the line width of the second waveguide in the second straight section through the entirety of the first curved section.
2 . The optical modulator as claimed in claim 1 , wherein the first and second waveguides include:
a third straight section provided parallel to the second straight section in which the line width of the second waveguide is smaller than the line width of the first waveguide; and
a second curved section connecting the second straight section and the third straight section,
wherein the second straight section is disposed closer to the input waveguide than the third straight section,
the line width of the second waveguide in the second straight section is smaller than the line width of the second waveguide in the third straight section, and
each line width of the first and second waveguides in the second curved section continuously transitions from a line width in the second straight section to a line width in the third straight section in through an entirety of the second curved section.
3 . The optical modulator as claimed in claim 2 , wherein the Mach-Zehnder optical waveguide is a ridge waveguide wherein a lithium niobate film formed on a substrate is shaped into a ridge.
4 . The optical modulator as claimed in claim 2 , wherein a total length of the first straight section and the third straight section is equal to a length of the second straight section.
5 . The optical modulator as claimed in claim 1 , wherein the Mach-Zehnder optical waveguide is a ridge waveguide wherein a lithium niobate film formed on a substrate is shaped into a ridge.