Optical circuit

An optical circuit that receives a signal beam and a local beam, the signal beam being modulated by a configuration of a dual-polarization quadrature phase shift keying (DP-QPSK), the signal beam and the local beam having a common wavelength λ, the signal beam including a first signal beam attributed to a first polarization and a second signal beam originating to a second polarization orthogonal to the first polarization, the optical circuit comprising:

• a substrate;
• a splitter formed in the substrate, the splitter splitting the local beam into a first local beam and a second local beam;
• a first optical 90° hybrid formed in the substrate, the first optical 90° hybrid complementarily extracting in-phase components and quadrature components in the first signal beam, the in-phase components of the first signal beam having phases matching with phases of the first local beam, the quadrature components of the first signal beam having phases shifted by 90° with respect to the first local beam;
• a second optical 90° hybrid formed in the substrate, the second optical 90° hybrid complementarily extracting in-phase components and quadrature components in the second signal beam, the in-phase components of the second signal beam having phases matching with phases of the second local beam, the quadrature components of the second signal beam having phases shifted by 90° with respect to the second local beam;
• and
• first to fourth waveguides formed in the substrate, the first and second waveguides providing the first signal beam to the first optical 90° hybrid and the second signal beam to the second optical 90° hybrid, respectively, the third and fourth waveguides providing the first local beam to the first optical 90° hybrid and the second local beam to the second optical 90° hybrid, respectively, without intersecting with the first and second waveguides on the substrate,
• wherein the quadrature components and the in-phase components each extracted in the first optical 90° hybrid have phase statuses identical with phase statuses between the quadrature components and the in-phase components each extracted in the second optical 90° hybrid,
• wherein the first optical 90° hybrid and the second optical 90° hybrid each includes a 2×4 multi-mode interference (MMI) element, a 2×2 MMI element, and a phase shifter,
• wherein the 2×4 MMI element includes a signal input port and a local input port, and first to fourth output ports, the first output port causing phase shifts of 135° and −45° for beams inputting to the signal input port and the local input port, respectively, the second and third output ports each causing phase shifts of 0° and 0° for beams inputting to the signal input port and the local input port, respectively, and the fourth output port causing phase shifts of −45° and 135° for beams inputting to the signal input port and the local input port respectively, where angles of 135°, 0°, and −45° correspond to the wavelength λ and include equivalent angles thereof, and
• wherein the 2×2 MMI element includes first and second input ports and first and second output ports, the first output port causing phase shifts of 0° and −90° for the first input port and the second input, respectively, the second output port causing phase shifts of −90° and 0° for the first input port and the second input port, respectively, where angles of 0°, and −90° correspond to the wavelength λ and include equivalent angles thereof.