90-degree optical hybrid

1. A 90-degree optical hybrid comprising:

two optical splitters configured to respectively split inputted light into two beams;

two optical combiners configured to respectively combine two beams of inputted light and thereby output two beams of interfering light respectively; and

four arm waveguides that are optical waveguides separated from each other, each of the four arm waveguides being configured to input light splitted by any of the two optical splitters into any of the two optical combiners, wherein

each of the four arm waveguides has a bend waveguide arranged at center thereof and a plurality of optical waveguides including a tapered waveguide having a width that decreases toward the bend waveguide,

both ends of each of the plurality of optical waveguides are respectively in contact with a first end surface of any one of the two optical splitter, the two optical combiners, the bend waveguide and the plurality of optical waveguides, and

each of the plurality of optical waveguides is the tapered waveguide or a linear waveguide having a constant width.

2. The 90-degree optical hybrid according to claim 1 , wherein

one of the two optical splitters is configured to split first light inputted therein into first split light and second split light,

the other of the two optical splitters is configured to split second light inputted therein into third split light and fourth split light,

the four arm waveguides include a first arm waveguide configured to input the first split light into a first optical combiner, which is one of the two optical combiners, a second arm waveguide configured to input the second split light into a second optical combiner, which is the other of the two optical combiners, a third arm waveguide configured to input the third split light into the second optical combiner, and a fourth arm waveguide configured to input the fourth split light into the first optical combiner, and

when ϕ1 denotes a phase given to an electric field of the first split light by the first arm waveguide, ϕ2 denotes a phase given to an electric field of the second split light by the second arm waveguide, ϕ3 denotes a phase given to an electric field of the third split light by the third arm waveguide, ϕ4 denotes a phase given to an electric field of the fourth split light by the fourth arm waveguide, Δ denotes an allowable error, and m denotes an integer equal to or greater than zero, then in a case where respective wavelengths of the first light and the second light are a specific first wavelength,

a phase difference θ satisfies

θ=(ϕ2−ϕ1)+(ϕ4−ϕ3), and

90°−Δ+360°× m≤|θ|≤ 90°+Δ+360°× m.

3. The 90-degree optical hybrid according to claim 2 , wherein the allowable error is 0° or more and 5° or less.

4. The 90-degree optical hybrid according to claim 2 , wherein

the first arm waveguide has a first phase-shift waveguide that is the linear waveguide and that is configured to give a value other than zero to the phase difference θ,

the second arm waveguide has a second phase-shift waveguide that is the linear waveguide and that is configured to give a value other than zero to the phase difference θ, and

respective lengths and respective widths of the first phase-shift waveguide and the second phase-shift waveguide are set so that, when a wavelength of each of the first to fourth split light is a second wavelength, an absolute value of an average change rate of the phase difference θ with respect to the second wavelength is equal to or lower than an allowable value within a specific range of the second wavelength including the specific first wavelength.

5. The 90-degree optical hybrid according to claim 4 , wherein the allowable value is 0 degrees/nm or more and 0.14 degrees/nm or less.

6. The 90-degree optical hybrid according to claim 2 , wherein

the first arm waveguide has a first phase-shift waveguide that is the linear waveguide and that is configured to give a value other than zero to the phase difference θ,

the second arm waveguide has a second phase-shift waveguide that is the linear waveguide and that is configured to give a value other than zero to the phase difference θ, and

respective lengths and respective widths of the first phase-shift waveguide and the second phase-shift waveguide are set so that, when a first error of a width of the first phase-shift waveguide from a target value is a second error of a width of the second phase-shift waveguide from a target value, an absolute value of an average change rate of the phase difference θ with respect to the first error is equal to or lower than an allowable value within a specific range of the first error.

7. The 90-degree optical hybrid according to claim 6 , wherein the allowable value is 0 degrees/nm or more and 0.2 degrees/nm or less.

8. The 90-degree optical hybrid according to claim 1 , wherein

a curvature radius of a first side surface of the bend waveguide is greater than a curvature radius of a second side surface of the bend waveguide,

the first side surface is in contact with a second end surface of any one of the plurality of optical waveguides, and

the second side surface is separated from the second end surface.

9. The 90-degree optical hybrid according to claim 1 , wherein

curvature radiuses of each side surface of the bend waveguide continuously decrease from one end thereof toward a central part thereof and, further, continuously decrease from another end thereof toward the central part.