Optical semiconductor device

1. An optical semiconductor device, comprising:

a first waveguide that an input light is input into;

a ring modulator arranged to optically couple with the first waveguide;

a first ring resonator arranged to optically couple with the first waveguide and having an optical path length smaller than an optical path length of the ring modulator;

a second ring resonator arranged to optically couple with the first waveguide and having an optical path length larger than the optical path length of the ring modulator;

a heater arranged adjacent to the ring modulator, the first ring resonator and the second ring resonator;

a first photodetector configured to monitor a light power in the first ring resonator;

a second photodetector configured to monitor a light power in the second ring resonator;

a controller controlling the heater so that a resonance wavelength of the ring modulator agrees with a wavelength of the input light, based on signals detected by the first photodetector the second photodetector;

a third ring resonator arranged adjacent the heater to optically couple with the first waveguide, and having an optical path length which is larger than the optical path length of the first ring resonator and smaller than the optical path length of the ring modulator;

a fourth ring resonator arranged adjacent the heater to optically couple with the first waveguide, and having an optical path length which is larger than the optical path length of the ring modulator and smaller than the optical path length of the second ring resonator;

a third photodetector configured to monitor a light power in the third ring resonator; and

a fourth photodetector configured to monitor a light power in the fourth ring resonator.

2. The optical semiconductor device according to claim 1 , wherein

a difference between the optical path length of the ring modulator and the optical path length of the first ring resonator is equal to a difference between the optical path length of the ring modulator and the optical path length of the second ring resonator.

3. The optical semiconductor device according to claim 1 , wherein

an interval between the first waveguide and the first ring resonator and an interval between the first waveguide and the second ring resonator are smaller than an interval between the first waveguide and the ring modulator.

4. The optical semiconductor device according to claim 1 , wherein

the first ring resonator and the second ring resonator are arranged nearer on a input terminal side of the first waveguide than the ring modulator.

5. The optical semiconductor device according to claim 1 , wherein

the first waveguide includes a first branch waveguide and a second branch waveguide,

the first ring resonator is arranged to optically couple with the first branch waveguide, and

the second ring waveguide is arranged to optically couple with the second branch waveguide.

6. The optical semiconductor device according to claim 1 , wherein

the first ring resonator and the second ring resonator are arranged to optically couple with each other at a same position of the first waveguide.

7. The optical semiconductor device according to claim 1 ,

further comprising: a second waveguide arranged to optically couple with the ring modulator,

wherein an output light is outputted from the second waveguide.

8. The optical semiconductor device according to claim 1 , wherein

the first photodetector is formed in a part of the first ring resonator, and

the second photodetector is formed in a part of the second ring resonator.

9. The optical semiconductor device according to claim 1 , further comprising:

a thermal insulation trench formed around the ring modulator, the first ring resonator and the second ring resonator.

10. The optical semiconductor device according to claim 1 , wherein

the controller includes

a current subtraction circuit into which an output signal of the first photodetector and an output signal of the second photodetector are inputted; and

a PID control circuit controlling the heater, based on an output signal outputted from the current subtraction circuit.

11. The optical semiconductor device according to claim 10 , wherein

the PID control circuit so controls the heater that the output signal to be outputted from the current subtraction circuit becomes zero.

12. The optical semiconductor device according to claim 1 , wherein

a difference between the optical path length of the ring modulator and the optical path length of the third ring resonator is equal to a difference between the optical path length of the ring modulator and the optical path length of the fourth ring resonator.

13. The optical semiconductor device according to claim 1 , wherein

an interval between the first waveguide and the third ring resonator, and an interval between the first waveguide and the fourth ring resonator are smaller than an interval between the first waveguide and the ring modulator and is larger than an interval between the first waveguide and the first ring resonator and an interval between the first waveguide and the second ring resonator.

14. The optical semiconductor device according to claim 13 , wherein

the third ring resonator and the fourth ring resonator are arranged nearer on an input terminal side of the first waveguide than the ring modulator.