Optical transmission apparatus and optical element

1. An optical transmission apparatus, comprising:

a first optical waveguide and a second optical waveguide configured to transmit light having a plurality of types of wavelengths;

a plurality of optical couplers provided on the first optical waveguide and the second optical waveguide, and configured to couple the light transmitted through the first optical waveguide with the light transmitted through the second optical waveguide, so as to output the coupled light to the first optical waveguide and the second optical waveguide;

a plurality of phase shifters provided at preceding stages of part of the plurality of optical couplers, and configured to change a phase shift amount of the light transmitted through the first optical waveguide, transmitted through the second optical waveguide, or transmitted through both the first optical waveguide and the second optical waveguide, wherein a number of optical couplers in the part is greater than or equal to a number of the plurality of types of wavelengths;

a monitor configured to monitor an intensity of the light output to the second optical waveguide via one of the plurality of optical couplers at a last stage; and

a controller configured to control the plurality of phase shifters,

wherein the controller increases or decreases the phase shift amount for each of the plurality of phase shifters to determine a direction of change to decrease an output of the monitor for said each of the plurality of phase shifters, and changes the phase shift amount in the direction in said each of the plurality of phase shifters.

2. The optical transmission apparatus as claimed in claim 1 , wherein a polarization direction of the light transmitted through the first optical waveguide matches a polarization direction of the light transmitted through the second optical waveguide.

3. The optical transmission apparatus as claimed in claim 1 , wherein the controller repeatedly executes a process of changing the phase shift amount in the direction in said each of the plurality of phase shifters, among the plurality of phase shifters.

4. The optical transmission apparatus as claimed in claim 1 , wherein at each portion where said each of the plurality of phase shifter is provided, a waveguide length difference between the first optical waveguide and the second optical waveguide is a fraction of c/(n G ·Δf) obtained by dividing c/(n G ·Δf) by an integer, where c represents a velocity of light, n G represents a group refractive index, and Δf represents a frequency spacing among the plurality of types of wavelengths.

5. The optical transmission apparatus as claimed in claim 1 , wherein at a preceding stage of one of the plurality of optical couplers at a first stage, the first optical waveguide and the second optical waveguide are connected to two outputs from a two-dimensional grating coupler.

6. The optical transmission apparatus as claimed in claim 1 , further comprising:

a wavelength demultiplexing element connected to the first optical waveguide via one of the plurality of optical couplers at a last stage, and configured to demultiplex the plurality of types of wavelengths.

7. The optical transmission apparatus as claimed in claim 6 , wherein the wavelength demultiplexing element is a demultiplexer.

8. The optical transmission apparatus as claimed in claim 6 , further comprising:

a same number of light-receiving elements as the number of the plurality of types of wavelengths, configured to receive output of the wavelength demultiplexing element.

9. The optical transmission apparatus as claimed in claim 8 , further comprising:

an electrical receiver circuit configured to apply signal processing to an electrical signal output from the light-receiving element.

10. An optical element, comprising:

a first optical waveguide and a second optical waveguide configured to transmit light having a plurality of types of wavelengths;

a plurality of optical couplers provided on the first optical waveguide and the second optical waveguide, and configured to couple the light transmitted through the first optical waveguide with the light transmitted through the second optical waveguide, so as to output the coupled light to the first optical waveguide and the second optical waveguide;

a plurality of phase shifters provided at preceding stages of part of the plurality of optical couplers, and configured to change a phase shift amount of the light transmitted through the first optical waveguide, transmitted through the second optical waveguide, or transmitted through both the first optical waveguide and the second optical waveguide, wherein a number of optical couplers in the part is greater than or equal to a number of the plurality of types of wavelengths;

a monitor configured to monitor an intensity of the light output to the second optical waveguide via one of the plurality of optical couplers at a last stage; and

a controller configured to control the plurality of phase shifters,

wherein the controller increases or decreases the phase shift amount for said each of the plurality of phase shifters to determine a direction of change to decrease an output of the monitor for said each of the plurality of phase shifters, and changes the phase shift amount in the direction in said each of the plurality of phase shifters.