Optical demultiplexer, optical transport apparatus, and method of controlling optical demultiplexing

1. An optical demultiplexer that separates light including a plurality of wavelengths into light of respective wavelengths, comprising:

optical demultiplexer components each including, N 2×2 couplers where N is 2 or more, and N−1 Asymmetric Mach-Zehnder interferometers each provided with phase shifters, the N−1 Asymmetric Mach-Zehnder interferometers being respectively arranged among the N 2×2 couplers;

unit circuits each including three of the optical demultiplexer components having a same structure and being cascaded in a tree structure,

wherein the unit circuits are cascaded in the tree structure,

in the optical demultiplexer components having the same structure, a combination of arm length differences in waveguide pairs is the same with respect to the N−1 Asymmetric Mach-Zehnder interferometers in which the phase shifters are arranged, and

in three of the optical demultiplexer components in at least one of the unit circuits, N is three or more; and

control circuits each of which controls the phase shifters arranged in a corresponding optical demultiplexer component of a corresponding unit circuit in order to increase or decrease a value of a function having, as an argument, a power value acquired by a monitor from among monitors arranged at four optical waveguides at an output side of the corresponding unit circuit.

2. The optical demultiplexer as claimed in claim 1 , wherein ΔL m =ΔL m-1 /2 is satisfied, where

ΔL m denotes a minimum arm length difference in one or more Asymmetric Mach-Zehnder interferometers at a m-th unit circuit among the N−1 Asymmetric Mach-Zehnder interferometers, and

ΔL m-1 denotes a minimum arm length difference in one or more Asymmetric Mach-Zehnder interferometers at an (m−1)-th unit circuit among the N−1 Asymmetric Mach-Zehnder interferometers.

3. The optical demultiplexer as claimed in claim 1 , wherein the phase shifters include one or more heaters.

4. The optical demultiplexer as claimed in claim 1 , wherein each of the control circuits repeats

determining a direction, in which a phase representing an increase or a decrease of a power value acquired by a corresponding monitor of the monitors changes of the monitors changes, by detecting the increase or the decrease of the power value acquired by the corresponding monitor in order to change a phase of corresponding phase shifters of the phase shifters in a direction for the power value acquired by the corresponding monitor to increase or decrease, the power value being from among the power values, and

changing the phase of the corresponding phase shifters based on the determined direction.

5. An optical transport apparatus used in a wavelength division multiplexing communication, comprising an optical demultiplexer as claimed in claim 1 .

6. A method of controlling optical demultiplexing, comprising:

inputting light of one of the plurality of wavelengths to an input port of the optical demultiplexer as claimed in claim 1 ;

adjusting a transmission characteristic of each of one or more Asymmetric Mach-Zehnder interferometers positioned between the input port and an output port for outputting the light of the one of the plurality of wavelengths, among the N−1 Asymmetric Mach-Zehnder interferometers,

repeating the adjusting of the transmission characteristic and a stabilization by switching a wavelength to be input to the input port, after the stabilization of the one wavelength, and

controlling the N−1 Asymmetric Mach-Zehnder interferometers of the optical demultiplexer at one time by injecting light including at least the plurality of wavelengths in response to the adjusting with respect to each of the plurality of wavelengths.