Optical communication component, optical transmitter, and control method

An optical communication component includes three or more couplers, a pair of waveguides, a phase shifter, a detector, and a controller. Each of the couplers multiplexes two input optical signals and two-branch outputs the multiplexed optical signals. Each of the pair of waveguides connects between the couplers and outputs each of the optical signals two-branch output from one of the couplers to another one of the couplers. The phase shifter, included in each of the waveguides, adjusts a phase amount of each of the optical signals passing through the waveguides. The detector detects an amount of power of the optical signal that has been subjected to phase adjustment and that is two-branch output from a most downstream coupler, from among the couplers, located in the traveling direction of the optical signal. The controller controls, based on the detected amount of power, each of the phase shifters.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2020-052257, filed on Mar. 24, 2020, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical communication component, an optical transmitter, and a control method.

BACKGROUND

In recent years, an optical communication component can be reduced its size by mounting, for example, optical modulating units, light receiving elements, and the like on a single silicon integrated optical circuit.

In a silicon waveguide included in the silicon integrated optical circuit, a core formed of a high refractive index material and a clad formed of a low refractive index material are included. In the silicon waveguide, because the contrast of refractive index between the core and the clad is large, a change in equivalent refractive index that is average refractive index of light propagated in the waveguide greatly affects fluctuations in core in the waveguide. Due to fluctuations in core in the waveguide, the characteristic of an interferometer in a wavelength division multiplexing (WDM) unit varies. As a result, phase variations are generated in a signal for each waveguide in the WDM unit. Consequently, it is difficult to implement the WDM unit in the silicon integrated optical circuit.

FIG. 23Ais a diagram illustrating an example of the ideal multiplexing characteristics exhibited in a conventional WDM unit, andFIG. 23Bis a diagram illustrating an example of the actual multiplexing characteristics exhibited in the conventional WDM unit. The characteristics of λ1P to λ4P illustrated inFIG. 23Aare the ideal output characteristics of, at the time of design, output power of optical signals at λ1to λ4that are outputs of the WDM unit, respectively. The symbols λ1P to λ4P illustrated inFIG. 23Aare output power λ1P of an optical signal at λ1, output power λ2P of an optical signal at λ2, output power λ3P of an optical signal at λ3, and output power λ4P of an optical signal at λ4, respectively. In contrast, the characteristics of λ1P to λ4P illustrated inFIG. 23Bare the actual output characteristics of output power of optical signals at λ1to λ4that are outputs of the WDM unit, respectively. The symbols λ1P to λ4P illustrated inFIG. 23Bare the output power λ1P of an optical signal at λ1, the output power λ2P of an optical signal at λ2, the output power λ3P of the optical signal at λ3, and the output power λ4P of an optical signal at λ4, respectively. When comparing the ideal characteristics illustrated inFIG. 23Awith the actual characteristics illustrated inFIG. 23B, phase variations are generated in the signal for each waveguide in the WDM unit.

Furthermore, not limited to the silicon integrated optical circuit, in the WDM unit, phase variations are also generated in a signal for each waveguide; however, in the WDM unit implemented in the silicon integrated optical circuit, phase variations in a signal for each waveguide are noticeably represented.

SUMMARY

According to an aspect of an embodiment, an optical communication component includes at least three or more couplers, a pair of waveguides, a phase shifter, a detector and a controller. The three or more couplers multiplex two input optical signals and two-branch output the multiplexed optical signal. The pair of waveguides connect between the couplers and output each of the optical signals two-branch output from one of the couplers to another one of the couplers. The phase shifter adjusts a phase amount of each of the optical signals passing through the waveguides and is included in each of the waveguides. The detector detects an amount of power of the optical signal that has been subjected to phase adjustment and is two-branch output from a most downstream coupler, from among the three or more couplers, located in the traveling direction of the optical signal. The controller controls, based on the amount of power detected by the detector, each of the phase shifters included in the pair of waveguides.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The present invention is not limited to the embodiments. Furthermore, the embodiments described below may also be used in any appropriate combination as long as the embodiments do not conflict with each other.

[a] First Embodiment

FIG. 1is a block diagram illustrating an example of an optical transmitter1according to an embodiment. The optical transmitter1illustrated inFIG. 1includes four light sources2, four optical modulating units3as modulators, two wavelength division multiplexing (WDM) units4, and a polarization beam combiner (PBC)5. The four light sources2are, for example, a first light source2A, a second light source2B, a third light source2C, and a fourth light source2D. The first light source2A emits light of, for example, an optical signal at λ1. The second light source2B emits light of, for example, an optical signal at λ2. The third light source2C emits light of, for example, an optical signal at λ3. The fourth light source2D emits light of, for example, an optical signal at λ4. Furthermore, the optical signals λ1to λ4are optical signals each having different wavelengths. The four optical modulating units3are, for example, a first optical modulating unit3A, a second optical modulating unit3B, a third optical modulating unit3C, and a fourth optical modulating unit3D. The first optical modulating unit3A performs optical modulation on the optical signal at λ1output from the first light source2A by a data signal. The second optical modulating unit3B performs optical modulation on the optical signal at λ2output from the second light source2B by a data signal. The third optical modulating unit3C performs optical modulation on the optical signal at λ3output from the third light source2C by a data signal. The fourth optical modulating unit3D performs optical modulation on the optical signal at λ4output from the fourth light source2D by a data signal.

The first optical modulating unit3A includes a data modulating unit11, DAC12, and two modulators13. Furthermore, for convenience of description, an internal configuration of the first optical modulating unit3A is illustrated as an example; however, the internal configuration of each of the second optical modulating unit3B, the third optical modulating unit3C, and the fourth optical modulating unit3D is the same as that of the first optical modulating unit3A. Thus, by assigning the same reference numerals to components having the same configuration, overlapped descriptions of the configuration and the operation thereof will be omitted. The data modulating unit11modulates a data signal. The digital-to-analogue convertor (DAC)12performs analog conversion on the data signal modulated by the data modulating unit11.

The two modulators13is, for example, a Mach-Zehnder (MZ) interferometer that includes, for example, a first modulator13A and a second modulator13B. The first modulator13A performs optical modulation on the optical signal at λ1output from the first light source2A by a data signal that has been subjected to analog conversion, and then, outputs a horizontal polarization optical signal at λ1that has been subjected to optical modulation. The second modulator13B performs optical modulation on the optical signal at λ1output from the first light source2A by a data signal that has been subjected to analog conversion, and then, outputs a vertical polarization optical signal at λ1that has been subjected to optical modulation.

The two WDM units4are multiplexers of multi-stage-connection asymmetric MZ interferometric type. The two WDM units4are, for example, optical communication components, such as a first WDM unit4A and a second WDM unit4B. The first WDM unit4A is constituted by connecting a plurality of MZ interferometers with each other. The first WDM unit4A multiplexes the horizontal polarization optical signal at λ1that has been subjected to optical modulation, the horizontal polarization optical signal at λ2that has been subjected to optical modulation, the horizontal polarization optical signal at λ3that has been subjected to optical modulation, and the horizontal polarization optical signal at λ4that has been subjected to optical modulation.

The second WDM unit4B is constituted by connecting a plurality of MZ interferometers with each other. The second WDM unit4B multiplexes the vertical polarization optical signal at λ1that has been subjected to optical modulation, the vertical polarization optical signal at λ2that has been subjected to optical modulation, the vertical polarization optical signal at λ3that has been subjected to optical modulation, and the vertical polarization optical signal at λ4that has been subjected to optical modulation.

The PBC5multiplexes and outputs the horizontal polarization optical signal λ1+λ2+λ3+λ4received from the first WDM unit4A and the vertical polarization optical signal λ1+λ2+λ3+λ4received from the second WDM unit4B.

FIG. 2is a diagram illustrating an example of the first WDM unit4A. Furthermore, for convenience of description, an internal configuration of the first WDM unit4A is illustrated as an example; however, because the second WDM unit4B has also the same configuration as that of the first WDM unit4A, by assigning the same reference numerals to components having the same configuration, overlapped descriptions of the configuration and the operation thereof will be omitted.

The first WDM unit4A illustrated inFIG. 2includes three variable couplers21, three detecting units22as detectors, and a control unit23as a controller. The three variable couplers21are, for example, a first variable coupler21A, a second variable coupler21B, and a third variable coupler21C. The first variable coupler21A multiplexes the optical signal at λ1and the optical signal at λ2and branch outputs the multiplexed optical signal λ1+λ2to a first output port (an output forward-direction port) and a second output port (an output reverse-direction port). The first output port included in the first variable coupler21A is connected to the third variable coupler21C, whereas the second output port is connected to a first detecting unit22A that will be described later. The first output port is one of the branch output ports connected to an input port in the third variable coupler21C as succeeding stage. The second output port is one of the branch output ports not connected to the input port in the succeeding stage. The second variable coupler21B multiplexes the optical signal at λ3and the optical signal at λ4and branch outputs the multiplexed optical signal at λ3+λ4to a first output port and a second output port. The first output port included in the second variable coupler21B is connected to the third variable coupler21C, whereas the second output port is connected to a second detecting unit22B that will be described later. The first output port is one of the branch output ports connected to an input port in the third variable coupler21C as the succeeding stage. The second output port is one of the branch output ports not connected to the input port in the succeeding stage. The third variable coupler21C multiplexes the multiplexed optical signal at λ1+λ2received from the first variable coupler21A and the multiplexed optical signal at λ3+λ4received from the second variable coupler21B, and then, branch outputs the multiplexed optical signal at λ1+λ2+λ3+λ4to a first output port or a second output port. The first output port in the third variable coupler21C is connected to an output, whereas the second output port is connected to a third detecting unit22C that will be described later. The first output port is one of the branch output ports connected to the output of the first WDM unit4A as the succeeding stage. The second output port is one of the branch output ports not connected to the output of the first WDM unit4A as the succeeding stage.

The three detecting units22are, for example, the first detecting unit22A, the second detecting unit22B, and the third detecting unit22C. The detecting unit22are, for example, Ge photodiodes. The first detecting unit22A detects an amount of power of the multiplexed optical signal at λ1+λ2that is received from the second output port and that is branch output from the first variable coupler21A. The second detecting unit22B detects an amount of power of the multiplexed optical signal at λ3+λ4that is received from the second output port and that is branch output from the second variable coupler21B. The third detecting unit22C detects an amount of power of the multiplexed optical signal at λ1+λ2+λ3+λ4that is received from the third variable coupler21C and that is branch output from the second output port.

The first variable coupler21A includes at least three or more 2×2 type fixed couplers31that multiplex the two input optical signals and that two-branch outputs the multiplexed optical signal. The first variable coupler21A includes, for example, four fixed couplers31, three waveguide pairs32, and three heater control units33. The four fixed couplers31are, for example, a first fixed coupler31A, a second fixed coupler31B, a third fixed coupler31C, and a fourth fixed coupler31D. The first fixed coupler31A is the most upstream fixed coupler31located in the optical transmission direction from among the plurality of the fixed couplers31included in the first variable coupler21A. The second fixed coupler31B is the second upstream fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A. The third fixed coupler31C is the third upstream fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A. The fourth fixed coupler31D is the fourth upstream (most downstream) fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A. The fourth fixed coupler31D branch outputs the optical signal at λ1+λ2to the first fixed coupler31A included in the third variable coupler21C as an output of each of the first detecting unit22A and the first variable coupler21A.

The three waveguide pairs32are, for example, a first waveguide pair32A, a second waveguide pair32B, and a third waveguide pair32C.FIG. 3is a diagram illustrating an example of the first waveguide pair32A. The first waveguide pair32A illustrated inFIG. 3includes a pair of waveguides35that connect the first fixed coupler31A and the second fixed coupler31B and is the most upstream waveguide pair32located in the optical transmission direction from among the plurality of the waveguide pairs32included in the first variable coupler21A. Furthermore, the pair of the waveguides35are, for example, silicon waveguides. The second waveguide pair32B includes the pair of the waveguides35that connect the second fixed coupler31B and the third fixed coupler31C and is the second upstream waveguide pair32from among the plurality of the waveguide pairs32included in the first variable coupler21A. The third waveguide pair32C includes the pair of the waveguides35that connect the third fixed coupler31C and the fourth fixed coupler31D and is the third upstream (most downstream) waveguide pair32from among the plurality of the waveguide pairs32included in the first variable coupler21A.

The pair of the waveguides35are, for example, a first waveguide35A and a second waveguide35B. The first waveguide35A includes a first heating unit34A that is a first phase shifter. The second waveguide35B includes a second heating unit34B that is a second phase shifter.

The first variable coupler21A is constituted by at least two or more, for example, three MZ interferometers, and the most upstream MZ interferometer is constituted by, for example, the first fixed coupler31A, the first waveguide pair32A, and the second fixed coupler31B. Furthermore, the second upstream MZ interferometer is constituted by, for example, the second fixed coupler31B, the second waveguide pair32B, and the third fixed coupler31C. Furthermore, the most downstream MZ interferometer is constituted by, for example, the third fixed coupler31C, the third waveguide pair32C, and the fourth fixed coupler31D.

The three heater control units33are, for example, a first heater control unit33A, a second heater control unit33B, and a third heater control unit33C. The heater control units33are, for example, CMOS electronic circuits or the like. The first heater control unit33A controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the first variable coupler21A. By adjusting the heater amount, a phase amount of a transmission spectrum is changed. A heater amount of the first heating unit34A is an amount of heater electrical power PUpper of the first heating unit34A calculated from VUpper2/RHeater by using a heater resistance value RHeater of the first heating unit34A and a voltage VUpper to the first heating unit34A. A heater amount of the second heating unit34B is an amount of heater electrical power PLower of the second heating unit34B calculated from VLower2/RHeater by using a heater resistance value RHeater of the second heating unit34B and a voltage VLower to the second heating unit34B. Furthermore, in the first heating unit34A and the second heating unit34B, a phase difference is generated between the optical signal passing through the first waveguide35A and the optical signal passing through the second waveguide35B by adjusting at least one of the heater amounts, and thus, the light intensity of output light is changed in accordance with the phase difference. Consequently, the output light is dispersed at an arbitrary ratio in accordance with the light intensity of the output light.

The second heater control unit33B controls a heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the first variable coupler21A. The third heater control unit33C controls a heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the first variable coupler21A. The first detecting unit22A detects an amount of power of the optical signal at λ1+λ2, at the second output port, that is branch output to the first detecting unit22A side at the fourth fixed coupler31D located in the fourth upstream corresponding to the most downstream in the first variable coupler21A.

The first heater control unit33A in the first variable coupler21A sets, based on the amount of power of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power is minimized. Then, the first heater control unit33A shifts the phase of the transmission spectrum to the increasing direction by increasing the heater amount. consequently, when the amount of power of the optical signal to the first detecting unit22A that is one of branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of branch outputs of the fourth fixed coupler31D increases. The one of the branch outputs of the fourth fixed coupler31D is a second output port and the other one of the branch outputs is a first output port. The first heater control unit33A sets, based on the amount of power of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power is minimized. Then, the first heater control unit33A shifts the phase of the transmission spectrum to the decreasing direction by decreasing the heater amount. Consequently, when the amount of power of the optical signal to the first detecting unit22A that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The second heater control unit33B in the first variable coupler21A sets, based on the amount of power of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power is minimized. Then, the second heater control unit33B shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the first detecting unit22A that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of branch outputs of the fourth fixed coupler31D increases. The one of the branch outputs of the fourth fixed coupler31D is a second output port and the other one of the branch outputs is a first output port. The second heater control unit33B sets, based on an amount of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the second heating unit34B in the second waveguide pair32B to the decreasing direction such that the amount of power is minimized. Then, the second heater control unit33B shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the first detecting unit22A that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of the branch outputs in the fourth fixed coupler31D increases.

The third heater control unit33C in the first variable coupler21A sets, based on the amount of power of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the first detecting unit22A that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of the branch outputs of the fourth fixed coupler31D increases. The one of the branch outputs of the fourth fixed coupler31D is a second output port and the other one of the branch outputs is a first output port. The third heater control unit33C sets, based on the amount of power of the optical signal at λ1+λ2detected by the first detecting unit22A, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the first detecting unit22A that is the one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the first variable coupler21A that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The control unit23sequentially performs an individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the first heater control unit33A→the second heater control unit33B→the third heater control unit33C included in the first variable coupler21A. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the first variable coupler21A by improving the phase variation in signal for each waveguide included in the first variable coupler21A.

The second variable coupler21B includes at least three or more 2×2 type fixed couplers31that multiplex the two input optical signals and that two-branch outputs the multiplexed optical signal. The second variable coupler21B includes, for example, the four fixed couplers31, the three waveguide pairs32, and the three heater control units33. The four fixed couplers31are, for example, the first fixed coupler31A, the second fixed coupler31B, the third fixed coupler31C, and the fourth fixed coupler31D. The first fixed coupler31A is the most upstream fixed coupler31located in the optical transmission direction from among the plurality of the fixed couplers31included in the second variable coupler21B. The second fixed coupler31B is the second upstream fixed coupler31from among the plurality of the fixed couplers31included in the second variable coupler21B. The third fixed coupler31C is the third upstream fixed coupler31from among the plurality of the fixed couplers31included in the second variable coupler21B. The fourth fixed coupler31D is the fourth upstream (most downstream) fixed coupler31from among the plurality of the fixed couplers31included in the second variable coupler21B. The fourth fixed coupler31D branch outputs the optical signal at λ3+λ4to the first fixed coupler31A included in the third variable coupler21C as an output of each of the second detecting unit22B and the second variable coupler21B.

The three waveguide pairs32are, for example, the first waveguide pair32A, the second waveguide pair32B, and the third waveguide pair32C. The first waveguide pair32A includes the pair of the waveguides35that connect the first fixed coupler31A and the second fixed coupler31B and is the most upstream waveguide pair32located in the optical transmission direction from among the plurality of the waveguide pairs32included in the second variable coupler21B. The second waveguide pair32B includes the pair of the waveguides35that connect the second fixed coupler31B and the third fixed coupler31C and is the second upstream waveguide pair32from among the plurality of the waveguide pairs32included in the second variable coupler21B. The third waveguide pair32C is the third upstream (most downstream) waveguide pair32from among the plurality of the waveguide pairs32included in the second variable coupler21B.

The pair of the waveguides35are, for example, the first waveguide35A and the second waveguide35B. The first waveguide35A includes the first heating unit34A that is the first phase shifter. The second waveguide35B includes the second heating unit34B that is the second phase shifter.

The second variable coupler21B is constituted by at least two or more, for example, three MZ interferometers and the most upstream MZ interferometer is constituted by, for example, the first fixed coupler31A, the first waveguide pair32A, and the second fixed coupler31B. Furthermore, the second upstream MZ interferometer is constituted by, for example, the second fixed coupler31B, the second waveguide pair32B, and the third fixed coupler31C. Furthermore, the most downstream MZ interferometer is constituted by, for example, the third fixed coupler31C, the third waveguide pair32C, and the fourth fixed coupler31D.

The three heater control units33are, for example, the first heater control unit33A, the second heater control unit33B, and the third heater control unit33C. The first heater control unit33A controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the second variable coupler21B. The second heater control unit33B controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the second variable coupler21B. The third heater control unit33C controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the second variable coupler21B. The second detecting unit22B detects an amount of power of the optical signal at λ3+λ4, at the second output port, that is branch output to the second detecting unit22B side at the fourth fixed coupler31D located in the fourth upstream corresponding to the most downstream in the second variable coupler21B.

The first heater control unit33A in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power is minimized. Then, the first heater control unit33A shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases. The first heater control unit33A sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power is minimized. Then, the first heater control unit33A shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The second heater control unit33B in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power is minimized. Then, the second heater control unit33B shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases. The second heater control unit33B sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the second heating unit34B of the second waveguide pair32B to the decreasing direction such that the amount of power is minimized. Then, the second heater control unit33B shifts the phase of the transmission spectrum to the decreasing direction in order to change the heater amount. Consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The third heater control unit33C in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases. The third heater control unit33C sets, based on the amount of power of the optical signal at λ3+λ4detected by the second detecting unit22B, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the decreasing direction in order to change the heater amount. Consequently, when the amount of power of the optical signal to the second detecting unit22B that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the second variable coupler21B that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The control unit23sequentially performs the individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the first heater control unit33A→the second heater control unit33B→the third heater control unit33C included in the second variable coupler21B. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the second variable coupler21B by improving the phase variation in signal for each waveguide included in the second variable coupler21B.

The third variable coupler21C includes at least three or more 2×2 type fixed couplers31that multiplex the two input optical signals and that two-branch outputs the multiplexed optical signal. The third variable coupler21C includes, for example, the four fixed couplers31, the three waveguide pairs32, and the three heater control units33. The four fixed couplers31are, for example, the first fixed coupler31A, the second fixed coupler31B, the third fixed coupler31C, and the fourth fixed coupler31D. The first fixed coupler31A is the most upstream fixed coupler31located in the optical transmission direction from among the plurality of the fixed couplers31in the third variable coupler21C. The second fixed coupler31B is the second upstream fixed coupler31from among the plurality of the fixed couplers31in the third variable coupler21C. The third fixed coupler31C is the third upstream fixed coupler31from among the plurality of the fixed couplers31in the third variable coupler21C. The fourth fixed coupler31D is the fourth upstream (most downstream) fixed coupler31from among the plurality of the fixed couplers31included in the third variable coupler21C. The fourth fixed coupler31D branch outputs the optical signal at λ1+λ2+λ3+λ4to the output stage of each of the third detecting unit22C and the third variable coupler21C.

The tree waveguide pairs32are, for example, the first waveguide pair32A, the second waveguide pair32B, and the third waveguide pair32C. The first waveguide pair32A includes the pair of the waveguides35that connect the first fixed coupler31A and the second fixed coupler31B and is the most upstream waveguide pair32in the optical transmission direction from among the plurality of the waveguide pairs32included in the third variable coupler21C. The second waveguide pair32B includes the pair of the waveguides35that connect the second fixed coupler31B and the third fixed coupler31C and is the second upstream waveguide pair32from among the plurality of the waveguide pairs32included in the third variable coupler21C. The third waveguide pair32C is the third upstream (most downstream) waveguide pair32from among the plurality of the waveguide pairs32included in the third variable coupler21C.

The pair of the waveguides35are, for example, the first waveguide35A and the second waveguide35B. The first waveguide35A includes the first heating unit34A that is the first phase shifter. The second waveguide35B includes the second heating unit34B that is the second phase shifter.

The third variable coupler21C is constituted by at least two or more, for example, three MZ interferometers and the most upstream MZ interferometer is constituted by, for example, the first fixed coupler31A, the first waveguide pair32A, and the second fixed coupler31B. Furthermore, the second upstream MZ interferometer is constituted by, for example, the second fixed coupler31B, the second waveguide pair32B, and the third fixed coupler31C. Furthermore, the most downstream MZ interferometer is constituted by, for example, the third fixed coupler31C, the third waveguide pair32C, and the fourth fixed coupler31D.

The three heater control units33are, for example, the first heater control unit33A, the second heater control unit33B, and the third heater control unit33C. The first heater control unit33A controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the third variable coupler21C. The second heater control unit33B controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the third variable coupler21C. The third heater control unit33C controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the third variable coupler21C. The third detecting unit22C detects an amount of power of the optical signal at λ1+λ2+λ3+λ4, at the second output port, that is branch output to the third detecting unit22C side at the fourth fixed coupler31D located in the fourth upstream corresponding to the most downstream in the third variable coupler21C.

The first heater control unit33A in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power is minimized. Then, the first heater control unit33A shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is the one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases. The first heater control unit33A sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power is minimized. The first heater control unit33A shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The second heater control unit33B in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power is minimized. Then, the second heater control unit33B shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases. The second heater control unit33B sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the second heating unit34B in the second waveguide pair32B to the decreasing direction such that the amount of power is minimized. The second heater control unit33B shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The third heater control unit33C in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage in the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases. The third heater control unit33C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the third detecting unit22C, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power is minimized. Then, the third heater control unit33C shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, when the amount of power of the optical signal to the third detecting unit22C that is one of the branch outputs of the fourth fixed coupler31D decreases, the amount of power of the optical signal at the output stage of the third variable coupler21C that is the other one of the branch outputs of the fourth fixed coupler31D increases.

The control unit23sequentially performs the individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the first heater control unit33A→the second heater control unit33B→the third heater control unit33C included in the third variable coupler21C. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the third variable coupler21C by improving the phase variation in signal for each waveguide included in the third variable coupler21C. Then, the control unit23sequentially performs the individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the first variable coupler21A→the second variable coupler21B→the third variable coupler21C. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the WDM unit4by improving the phase variation in signal for each waveguide included in the WDM unit4.

FIG. 4is a diagram illustrating an example of processing timings of the individual phase adjustment process. The processing order of the individual phase adjustment process is in the order of a process A performed in the first heater control unit33A included in the first variable coupler21A, a process B performed in the second heater control unit33B included in the first variable coupler21A, and a process C performed in the third heater control unit33C included in the first variable coupler21A. Subsequently, the processing order is in the order of a process D performed in the first heater control unit33A included in the second variable coupler21B, a process E performed in the second heater control unit33B included in the second variable coupler21B, and a process F performed in the third heater control unit33C included in the second variable coupler21B. Subsequently, the processing order is in the order of a process G performed in the first heater control unit33A included in the third variable coupler21C, a process H performed in the second heater control unit33B included in the third variable coupler21C, and a process I performed in the third heater control unit33C included in the third variable coupler21C. Furthermore, the control unit23performs the individual phase adjustment process in each of the heater control units33for each of the variable couplers21, and then, sets an executed flag of the phase adjustment process associated with the waveguide pair32included in the heater control unit33in the variable coupler21.

Namely, when the control unit23performs the process A for the first heater control unit33A included in the first variable coupler21A, the control unit23sets the executed flag of the first waveguide pair32A included in the first variable coupler21A. Furthermore, when the control unit23performs the process B for the second heater control unit33B included in the first variable coupler21A, the control unit23sets the executed flag of the second waveguide pair32B included in the first variable coupler21A. Furthermore, when the control unit23performs the process C for the third heater control unit33C included in the first variable coupler21A, the control unit23sets the executed flag of the third waveguide pair32C included in the first variable coupler21A. Namely, the control unit23refers to the setting content of the executed flag of all of the waveguide pairs32included in the first variable coupler21A and recognizes that the phase adjustment process for all of the waveguide pairs32included in the first variable coupler21A has been performed.

Furthermore, when the control unit23performs the process D for the first heater control unit33A included in the second variable coupler21B, the control unit23sets the executed flag of the first waveguide pair32A included in the second variable coupler21B. Furthermore, when the control unit23performs the process E for the second heater control unit33B included in the second variable coupler21B, the control unit23sets the executed flag of the second waveguide pair32B included in the second variable coupler21B. Furthermore, when the control unit23performs the process F for the third heater control unit33C included in the second variable coupler21B, the control unit23sets the executed flag of the third waveguide pair32C included in the second variable coupler21B. Namely, the control unit23refers to the setting content of the executed flag of all of the waveguide pairs32included in the second variable coupler21B and recognizes that the phase adjustment process for all of the waveguide pairs32included in the second variable coupler21B has been performed.

Furthermore, when the control unit23performs the process G for the first heater control unit33A included in the third variable coupler21C, the control unit23sets an executed flag of the first waveguide pair32A included in the third variable coupler21C. Furthermore, when the control unit23performs the process H for the second heater control unit33B included in the third variable coupler21C, the control unit23sets an executed flag of the second waveguide pair32B included in the third variable coupler21C. Furthermore, when the control unit23performs the process I for the third heater control unit33C included in the third variable coupler21C, the control unit23sets an executed flag of the third waveguide pair32C included in the third variable coupler21C. Namely, the control unit23refers to the setting content of the executed flag of all of the waveguide pairs32included in the third variable coupler21C and recognizes that the phase adjustment process for all of the waveguide pairs32included in the third variable coupler21C has been performed. Then, when the control unit23sets the executed flag of the third waveguide pair32C included in the third variable coupler21C, the control unit23resets all of the executed flags.

FIG. 5is a flowchart illustrating an example of a processing operation of the control unit23related to the overall phase adjustment process. Furthermore, for convenience of description, it is assumed that the processing timing of the phase adjustment process is in the order of the first variable coupler21A→the second variable coupler21B→the third variable coupler21C. InFIG. 5, the control unit23determines whether the control unit23detects a phase adjustment request (Step S11). Furthermore, the phase adjustment request is a request issued by a controller (not illustrated) included in the optical transmitter1at the timing in which, for example, an optical signal is input to the WDM unit4. When the control unit23detects a phase adjustment request (Yes at Step S11), the control unit23determines whether the phase adjustment of the first variable coupler21A has been completed (Step S12). Furthermore, the process of determining whether the phase adjustment of the first variable coupler21A has been completed is determined whether the executed flags of all of the waveguide pairs32included in the first variable coupler21A have been set or is determined whether the executed flag of the most downstream third waveguide pair32C has been set.

When the phase adjustment of the first variable coupler21A has been completed (Yes at Step S12), the control unit23determines whether the phase adjustment of the second variable coupler21B has been completed (Step S13). Furthermore, the process of determining whether the phase adjustment of the second variable coupler21B has been completed is determined whether the executed flags of all of the waveguide pairs32included in the second variable coupler21B have been set or is determined whether the executed flag of the most downstream third waveguide pair32C has been set. When the phase adjustment of the second variable coupler21B has been completed (Yes at Step S13), the control unit23determines whether the phase adjustment of the third variable coupler21C has been completed (Step S14). Furthermore, the process of determining whether the phase adjustment of the third variable coupler21C has been completed is determined whether the executed flags of all of the waveguide pairs32included in the third variable coupler21C have been set or is determined whether the executed flag of the most downstream third waveguide pair32C has been set.

When the phase adjustment of the third variable coupler21C has been completed (Yes at Step S14), the control unit23determines that all of the phase adjustment processes have been completed, resets the executed flags (Step S15), and ends the processing operation illustrated inFIG. 5.

When the control unit23does not detect the phase adjustment request (No at Step S11), the control unit23ends the processing operation illustrated inFIG. 5. When the phase adjustment of the first variable coupler21A has not been completed (No at Step S12), the control unit23performs the individual phase adjustment process (Step S16). The individual phase adjustment process is a process of adjusting the phase of each of the waveguide pairs32. After the control unit23performs the individual phase adjustment process, the control unit23moves to Step S12in order to determine whether the phase adjustment of the first variable coupler21A has been completed. Furthermore, when the phase adjustment of the second variable coupler21B has not been completed (No at Step S13) or when the phase adjustment of the third variable coupler21C has not been completed (No at Step S14), the control unit23moves to Step S16in order to perform the individual phase adjustment process.

FIG. 6is a flowchart illustrating an example of a processing operation of the heater control unit33related to the first individual phase adjustment process. The first individual phase adjustment process is a process of performing phase adjustment by each of the heater control units33included in the variable couplers21. InFIG. 6, the heater control unit33determines whether the undesignated waveguide pair32is present in the variable coupler21(Step S21). Furthermore, the undesignated waveguide pair32is the waveguide pair32that has not been subjected to the phase adjustment process. When the undesignated waveguide pair32is present in the variable coupler21(Yes at Step S21), the heater control unit33designates the most upstream waveguide pair32located in the optical transmission direction included in the undesignated waveguide pair32(Step S22).

Then, the heater control unit33sets the current heater amount to the reference heater amount (Step S23), and sets the first heater amount corresponding to an amount in which a predetermined amount is increased from the reference heater amount into the first heating unit34A (Step S24). The heater control unit33determines whether a certain period of time has elapsed (Step S25). Furthermore, a certain period of time is the time needed for the heater amount to increase to a predetermined amount. When a certain period of time has elapsed (Yes at Step S25), the heater control unit33starts a measurement operation of an amount of power of the most downstream optical signal in the detecting unit22(Step S26). Furthermore, the amount of power of the most downstream optical signal is the amount of power of the optical signal that has been branch output from the most downstream fixed coupler31located in the optical transmission direction included in the same variable coupler21.

After the heater control unit33started the measurement operation of the amount of power of the detecting unit22, the heater control unit33determines whether the measurement of the amount of power has been completed (Step S27). The heater control unit33stores the first amount of power that is the measurement result (Step S28).

Furthermore, the heater control unit33sets the second heater amount corresponding to an amount in which a predetermined amount is decreased from the reference heater amount into the second heating unit34B (Step S29). The heater control unit33determines whether a certain period of time has elapsed (Step S30). Furthermore, a certain period of time is the time needed for the heater amount to decrease to a predetermined amount. When a certain period of time has elapsed (Yes at Step S30), the heater control unit33starts a measurement operation of an amount of power of the most downstream optical signal in the detecting unit22(Step S31). Furthermore, the amount of power of the most downstream optical signal is the amount of power of the optical signal that has been branch output from the most downstream fixed coupler31in the optical transmission direction included in the same variable coupler21.

After the heater control unit33started the measurement operation of the amount of power of the detecting unit22, the heater control unit33determines whether the measurement of the amount of power has been completed (Step S32). The heater control unit33stores the second amount of power that is the measurement result (Step S33).

The heater control unit33determines whether the first amount of power is less than the second amount of power (Step S34). When the first amount of power is less than the second amount of power (Yes at Step S34), the heater control unit33sets the first heater amount in the first heating unit34A in order to shift the phase of the transmission spectrum to the increasing direction in which the amount of power decreases (Step S35). Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the increasing direction, the amount of power of the optical signal that has been branch output from the variable coupler21and that is input to the detecting unit22is decreased and the amount of power of the optical signal at the output stage of the variable coupler21is increased.

Then, the heater control unit33sets the executed flag of the waveguide pair32corresponding to the designated control target (Step S36), and moves to Step S21in order to determine whether the waveguide pair32that has not been is present in the variable coupler21.

Furthermore, when the first amount of power is not less than the second amount of power (No at Step S34), the heater control unit33sets the second heater amount in the second heating unit34B in order to shift the phase of the transmission spectrum to the decreasing direction in which the amount of power is decreased (Step S37). Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the decreasing direction, the amount of power of the optical signal that has been branch output from the variable coupler21and that is input to the detecting unit22decreases, and the amount of power of the optical signal in the output stage of the variable coupler21increases. Then, after the heater control unit33sets the second heater amount in the second heating unit34B, the heater control unit33moves to Step S36in order to set the executed flag of the waveguide pair32corresponding to the designated control target.

Furthermore, when the undesignated waveguide pair32is not present in the variable coupler21(No at Step S21), the heater control unit33ends the processing operation illustrated inFIG. 6. Furthermore, when a certain period of time has not elapsed at Step S25, the heater control unit33moves to Step S25in order to determine whether a certain period of time has elapsed. When the measurement of the amount of power has not been completed (No at Step S27), the heater control unit33moves to Step S27in order to determine whether the measurement of the amount of power is completed. Furthermore, when a certain period of time has not elapsed at Step S30, the heater control unit33moves to Step S30in order to determine whether a certain period of time has elapsed. When the measurement of the amount of power has not been completed (No at Step S32), the heater control unit33moves to Step S32in order to determine whether the measurement of the amount of power is completed.

FIG. 7Ais a diagram illustrating an example of the multiplexing characteristics of the WDM unit4before phase adjustment. The characteristics of λ1P to λ4P illustrated inFIG. 7Aindicate an input power λ1P of the optical signal at λ1, an input power λ2P of the optical signal at λ2, an input power λ3P of optical signal at λ3, and an input power λ4P of the optical signal at λ4. In contrast, the characteristics of λ1S to λ4S indicate a transmission spectrum λ1S of the optical signal at λ1, a transmission spectrum λ2S of the optical signal at λ2, a transmission spectrum λ3S of the optical signal at λ3, and a transmission spectrum λ4S of the optical signal at λ4. The characteristics before phase adjustment illustrated inFIG. 7Ahave a state in which a large phase difference is generated between the input power λ1P of the optical signal at λ1and the transmission spectrum λ1S of the optical signal at λ1, and between the input power λ2P of the optical signal at λ2and the transmission spectrum λ2S of the optical signal at λ2. Furthermore, the characteristics before phase adjustment have a state in which a large phase difference is generated between the input power λ3P of the optical signal at λ3and the transmission spectrum λ3S of the optical signal at λ3, and between the input power λ4P of the optical signal at λ4and the transmission spectrum λ4S of the optical signal at λ4. Namely, there is a state in which the phase variation in optical signal is generated for each waveguide.

FIG. 7Bis a diagram illustrating an example of the multiplexing characteristics of the WDM unit4after phase adjustment. The characteristics after phase adjustment illustrated inFIG. 7Bhave a state in which the phase of each of the transmission spectra is shifted such that the input power λ1P of the optical signal at λ1passes through the transmission spectrum λ1S of the optical signal at λ1and the input power λ2P of the optical signal at λ2passes through the transmission spectrum λ2S of the optical signal at λ2. Furthermore, the characteristics after phase adjustment have a state in which the phase of each of the transmission spectra is shifted such that the input power λ3P of the optical signal at λ3passes through the transmission spectrum λ3S of the optical signal at λ3and the input power λ4P of the optical signal at λ4passes through the transmission spectrum λ4S passes through the optical signal at λ4. Namely, this corresponds to the state in which the phase variation in optical signal for each waveguide has been improved.

Consequently, it is possible to improve the phase variation in optical signal for each waveguide, such as the first variable coupler21A, the second variable coupler21B, and the third variable coupler21C included in the WDM unit4.

The heater control unit33according to the first embodiment detects the amount of power of the optical signal in the detecting unit22that is optically branched from the most downstream fourth fixed coupler31D included in the variable coupler21. Furthermore, the heater control unit33sets the heater amount of the first heating unit34A to the increasing direction in order to shift the phase of the transmission spectrum to the increasing direction so as to change the heater amount such that the amount of power of the optical signal detected by the detecting unit22is minimized. Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the increasing direction, the amount of power of the optical signal detected by the detecting unit22decreases and the amount of power of the optical signal that is output at the output stage of the variable coupler21increases. Namely, it is possible to improve the phase variation in optical signal for each waveguide.

The heater control unit33detects the amount of power of the optical signal in the detecting unit22that is optically branched from the most downstream fourth fixed coupler31D included in the variable coupler21. Furthermore, the heater control unit33sets the heater amount of the second heating unit34B to the decreasing direction in order to shift the phase of the transmission spectrum to the decreasing direction so as to change the heater amount such that the amount of power of the optical signal detected in the detecting unit22is minimized. Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the decreasing direction, the amount of power of the optical signal that is branch output from the variable coupler21decreases and it is thus possible to improve the phase variation in optical signal for each waveguide.

The heater control unit33controls the first heating unit34A and the second heating unit34B for each waveguide at different timings for each of the waveguide pair32, for example, the heater control unit33performs the individual phase adjustment process at different timings for each of the waveguide pair32. Consequently, it is possible to avoid simultaneous occurrences of the influence of the individual phase adjustment process for each waveguide pair.

The heater control unit33controls, at different timings, the first heating unit34A and the second heating unit34B included in the waveguide pairs32starting from the upstream side waveguide pair32located in the traveling direction of the optical signal from among the waveguide pairs32. The influence of the phase adjustment of the waveguide pair32performed on the upstream side greatly affects the downstream waveguide pair in the optical transmission direction. Thus, it is possible to efficiently perform phase adjustment by sequentially performing phase adjustment starting from the upstream side waveguide pair32toward the downstream waveguide pair32.

The heater control unit33controls, at different timing for each of the waveguide pairs32, the first heating unit34A and the second heating unit34B included in the waveguide pair32based on the amount of power of the optical signal that has been subjected to phase adjustment and that is branch output from the most downstream fourth fixed coupler31D included in the variable coupler21. Consequently, it is possible to improve the phase variation in optical signal in units of the variable couplers21.

The WDM unit4corresponding to the optical communication component has a tree structure in which the first variable coupler21A and the third variable coupler21C are connected and the second variable coupler21B and the third variable coupler21C are connected. Furthermore, the WDM unit4sequentially controls the first heating unit34A and the second heating unit34B starting from the upstream side waveguide pair32from among the plurality of the waveguide pairs32included in the variable coupler21located on the upstream side in the traveling direction of the optical signal. Consequently, even in a case of the tree structure, it is possible to perform phase adjustment in units of variable couplers, and it is thus possible to efficiently perform phase adjustment by sequentially performing phase adjustment starting from the upstream side variable coupler21toward the downstream variable coupler21.

The WDM unit4is constituted by a silicon integrated optical circuit. Consequently, even in a case of constituting the silicon integrated optical circuit, it is also possible to improve the phase variation in optical signal for each optical waveguide. In a silicon waveguide included in the WDM unit4, a contrast of the refractive index between the core and the clad is large; however, the heater amount of the first heating unit34A or the second heating unit34B has been set to the decreasing direction. Consequently, it is possible to improve the phase variation in optical signal for each waveguide in the WDM unit4constituted by the silicon integrated optical circuit.

Furthermore, the order of the processing timings of the individual phase adjustment process illustrated inFIG. 4is indicated by a case, as an example, of performing the individual phase adjustment process in the order of the first variable coupler21A→the second variable coupler21B→the third variable coupler21C→the first variable coupler21A→ . . . , and the like. However, the order is not limited to this as long as the process may also be sequentially performed from the upstream variable coupler21in the optical transmission direction, and furthermore, modifications are possible as needed.

FIG. 8is diagram illustrating an example of another processing timing of the individual phase adjustment process. In the WDM unit4, it is assumed that the order for determining whether the first variable coupler21A and the second variable coupler21B are located at the upstream of the optical transmission direction is the same, i.e., for example, it is assumed that the first waveguide pair32A included in the first variable coupler21A and the first waveguide pair32A included in the second variable coupler21B have the same order. Furthermore, it is assumed that the second waveguide pair32B included in the first variable coupler21A and the second waveguide pair32B included in the second variable coupler21B have the same order, and it is assumed that the third waveguide pair32C included in the first variable coupler21A and the third waveguide pair32C included in the second variable coupler21B have the same order. The processing order may also be in the order of the process A performed in the first heater control unit33A included in the first variable coupler21A and the process D performed in the first heater control unit33A included in the second variable coupler21B→the process B performed in the second heater control unit33B included in the first variable coupler21A and the process E performed in the second heater control unit33B included in the second variable coupler21B→the process C performed in the third heater control unit33C included in the first variable coupler21A and the process F performed in the third heater control unit33C included in the second variable coupler21B→the process G performed in the first heater control unit33A included in the third variable coupler21C→the process H performed in the second heater control unit33B included in the third variable coupler21C→the process I performed in the third heater control unit33C included in the third variable coupler21C→ . . . , and the like, and furthermore, modifications are possible as needed.

Furthermore, for convenience of description, a description has been given as an example of a case in which the heating unit, such as the first heating unit34A and the second heating unit34B, is used as a phase shifter; however, any device may also be used as long as a device has a function for adjusting the phase of the optical signal in the waveguide, and furthermore, modifications are possible as needed.

Furthermore, a description has been given as an example of a case in which the phase of the transmission spectrum is adjusted by adjusting the heater amount (phase amount) of the heating unit; however, the embodiment is not limited to this. The phase on the optical signal may also be adjusted, and furthermore, modifications are possible as needed. Furthermore, a description has been given as an example of a case in which the phase amount is set by adding the first phase amount to the current phase amount and a case in which the phase amount is set by subtracting the second phase amount from the current phase amount; however, the first phase amount and the second phase amount may also be the same amount or may also be a different amount, and furthermore, modifications are possible as needed.

A description has been given as an example of a case in which the heater control unit33according to the first embodiment adjusts the heater amount (phase amount) in order to adjust the phase of the transmission spectrum so as to decrease the amount of power of the optical signal that is detected in the detecting unit22and that is branch output at the most downstream fourth fixed coupler31D included in the variable coupler21.

However, in addition to adjusting the phase of the transmission spectrum in order to change the heater amount such that the amount of power decreases, it may also be possible to adjust the phase of the transmission spectrum so as to increase the amount of power and the embodiment thereof will be described below as a second embodiment.

[b] Second Embodiment

FIG. 9is a diagram illustrating an example of the WDM unit4according to the second embodiment. Furthermore, for convenience of description, the internal configuration of the first WDM unit4A has been exemplified. However, because the second WDM unit4B has also the same configuration, by assigning the same reference numerals to components having the same configuration as those in the first WDM unit4A, overlapped descriptions of the configuration and the operation thereof will be omitted.

The first WDM unit4A illustrated inFIG. 9includes the three variable couplers21, the three detecting units22, and the control unit23. The three variable couplers21are, for example, the first variable coupler21A, the second variable coupler21B, and the third variable coupler21C. The first variable coupler21A multiplexes the optical signal at λ1and the optical signal at λ2and branch outputs the multiplexed optical signal at λ1+λ2. The second variable coupler21B multiplexes the optical signal at λ3and the optical signal at λ4and branch outputs of the multiplexed optical signal at λ3+λ4. The third variable coupler21C multiplexes multiplexed optical signal at λ1+λ2received from the first variable coupler21A and the multiplexed optical signal at λ3+λ4received from the second variable coupler21B and branch outputs the multiplexed optical signal at λ1+λ2+λ3+λ4.

The three detecting units22are, for example, a fourth detecting unit22D, a fifth detecting unit22E, and a sixth detecting unit22F. The fourth detecting unit22D detects, in an optical tap, part of the amount of power of the multiplexed optical signal at λ1+λ2, at the output stage (first output port) of the first variable coupler21A, that is branch output from the fourth fixed coupler31D included in the first variable coupler21A. The fifth detecting unit22E detects, in an optical tap, part of the amount of power of the multiplexed optical signal at λ3+λ4, at the output stage (first output port) of the second variable coupler21B, that is branch output from the fourth fixed coupler31D included in the second variable coupler21B. The sixth detecting unit22F detects, in an optical tap, part of the amount of power of the multiplexed optical signal at λ1+λ2+λ3+λ4, at the output stage (first output port) of the third variable coupler21C, that is branch output from the fourth fixed coupler31D included in the third variable coupler21C.

The first variable coupler21A is a 2×2 type coupler. The first variable coupler21A includes, for example, the four fixed couplers31, the three waveguide pairs32, and the three heater control units33. The four fixed couplers31are, for example, the first fixed coupler31A, the second fixed coupler31B, the third fixed coupler31C, and the fourth fixed coupler31D. The first fixed coupler31A is the most upstream fixed coupler31located in the optical transmission direction from among the plurality of the fixed couplers31included in the first variable coupler21A. The second fixed coupler31B is the second upstream fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A. The third fixed coupler31C is the third upstream fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A. The fourth fixed coupler31D is the fourth upstream (most downstream) fixed coupler31from among the plurality of the fixed couplers31included in the first variable coupler21A.

The three waveguide pairs32are, for example, the first waveguide pair32A, the second waveguide pair32B, and the third waveguide pair32C. The first waveguide pair32A includes the pair of the waveguides35that connect the first fixed coupler31A and the second fixed coupler31B and is the most upstream waveguide pair32from among the plurality of the waveguide pairs32included in the first variable coupler21A. The second waveguide pair32B includes the pair of the waveguides35that connect the second fixed coupler31B and the third fixed coupler31C and is the second upstream waveguide pair32from among the plurality of the waveguide pairs32included in the first variable coupler21A. The third waveguide pair32C includes the pair of the waveguides35that connect the third fixed coupler31C and the fourth fixed coupler31D and is the third upstream (most downstream) waveguide pair32from among the plurality of the waveguide pairs32included in the first variable coupler21A.

The pair of the waveguides35are, for example, the first waveguide35A and the second waveguide35B. The first waveguide35A includes the first heating unit34A that is the first phase shifter. The second waveguide35B includes the second heating unit34B that is the second phase shifter.

The three heater control unit33are, for example, a fourth heater control unit33D, a fifth heater control unit33E, and a sixth heater control unit33F. The fourth heater control unit33D controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the first variable coupler21A. The fifth heater control unit33E controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the first variable coupler21A. The sixth heater control unit33F controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the first variable coupler21A. The fourth detecting unit22D detects, in the optical tap, part of the amount of power of the optical signal at λ1+λ2that is at the output stage (first output port) of the first variable coupler21A and that is branch output the fourth fixed coupler31D located at the fourth upstream, i.e., the most downstream, included in the first variable coupler21A.

The fourth heater control unit33D in the first variable coupler21A sets, based on the amount of power of the optical signal at λ1+λ2detected by the fourth detecting unit22D, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the first variable coupler21A and that is branch output from the fourth fixed coupler31D increases. The fourth heater control unit33D sets, based on the amount of power of the optical signal at λ1+λ2detected by the fourth detecting unit22D, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the first variable coupler21A and that is branch output from the fourth fixed coupler31D increases.

The fifth heater control unit33E in the first variable coupler21A sets, based on the amount of power of the optical signal at λ3+λ4detected by the fourth detecting unit22D, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D. The fifth heater control unit33E sets, based on the amount of power of the optical signal at λ3+λ4detected by the fourth detecting unit22D, the heater amount of the second heating unit34B in the second waveguide pair32B to the decreasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases.

The sixth heater control unit33F in the first variable coupler21A sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the fourth detecting unit22D, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases. The sixth heater control unit33F sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the fourth detecting unit22D, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases.

The second variable coupler21B is a 2×2 type coupler. The second variable coupler21B includes, for example, the four fixed couplers31, the three waveguide pairs32, and the three heater control units33. The four fixed couplers31are, for example, the first fixed coupler31A, the second fixed coupler31B, the third fixed coupler31C, and the fourth fixed coupler31D. The three waveguide pairs32are, for example, the first waveguide pair32A, the second waveguide pair32B, and the third waveguide pair32C.

The three heater control units33are, for example, the fourth heater control unit33D, the fifth heater control unit33E, and the sixth heater control unit33F. The fourth heater control unit33D controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the second variable coupler21B. The fifth heater control unit33E controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the first variable coupler21A. The sixth heater control unit33F controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the second variable coupler21B. The fifth detecting unit22E detects, at an optical tap, part of the amount of power of the optical signal at λ3+λ4that is at the output stage (first output port) of the second variable coupler21B and that is branch output from the fourth fixed coupler31D located in the fourth upstream, i.e., the most downstream, included in the second variable coupler21B.

The fourth heater control unit33D in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases. The fourth heater control unit33D sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases.

The fifth heater control unit33E in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases. The fifth heater control unit33E sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the second heating unit34B in the second waveguide pair32B to the decreasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases.

The sixth heater control unit33F in the second variable coupler21B sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases. The sixth heater control unit33F sets, based on the amount of power of the optical signal at λ3+λ4detected by the fifth detecting unit22E, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the second variable coupler21B and that is branch output from the fourth fixed coupler31D increases.

The third variable coupler21C is a 2×2 type coupler. The third variable coupler21C includes, for example, the four fixed couplers31, the three waveguide pairs32, and the three heater control units33. The four fixed couplers31are, for example, the first fixed coupler31A, the second fixed coupler31B, the third fixed coupler31C, and the fourth fixed coupler31D. The three waveguide pairs32are, for example, the first waveguide pair32A, the second waveguide pair32B, and the third waveguide pair32C.

The three heater control units33are, for example, the fourth heater control unit33D, the fifth heater control unit33E, and the sixth heater control unit33F. The fourth heater control unit33D controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the first waveguide pair32A in the third variable coupler21C. The fifth heater control unit33E controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the second waveguide pair32B in the third variable coupler21C. The sixth heater control unit33F controls the heater amount of each of the first heating unit34A and the second heating unit34B included in the third waveguide pair32C in the third variable coupler21C. The sixth detecting unit22F detects, at an optical tap, part of the amount of power of the optical signal at λ1+λ2+λ3+λ4that is at the output stage (first output port) of the third variable coupler21C and that is branch output from the fourth fixed coupler31D located in the fourth upstream that is the most downstream coupler included in the third variable coupler21C.

The fourth heater control unit33D in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the first heating unit34A in the first waveguide pair32A to the increasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases. The fourth heater control unit33D sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the second heating unit34B in the first waveguide pair32A to the decreasing direction such that the amount of power increases. Then, the fourth heater control unit33D shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases.

The fifth heater control unit33E in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the first heating unit34A in the second waveguide pair32B to the increasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C that is branch output from the fourth fixed coupler31D increases. The fifth heater control unit33E sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the second heating unit34B in the second waveguide pair32B to the decreasing direction such that the amount of power increases. Then, the fifth heater control unit33E shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases.

the sixth heater control unit33F in the third variable coupler21C sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the first heating unit34A in the third waveguide pair32C to the increasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of the transmission spectrum to the increasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases. The sixth heater control unit33F sets, based on the amount of power of the optical signal at λ1+λ2+λ3+λ4detected by the sixth detecting unit22F, the heater amount of the second heating unit34B in the third waveguide pair32C to the decreasing direction such that the amount of power increases. Then, the sixth heater control unit33F shifts the phase of the transmission spectrum to the decreasing direction by changing the heater amount. Consequently, the amount of power of the optical signal that is at the output stage of the third variable coupler21C and that is branch output from the fourth fixed coupler31D increases.

The control unit23sequentially performs the individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the fourth heater control unit33D→the fifth heater control unit33E→the sixth heater control unit33F included in the third variable coupler21C. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the third variable coupler21C by improving the phase variations in optical signal for each waveguide included in the third variable coupler21C. Then, the control unit23sequentially performs the individual phase adjustment process starting from the upstream of the optical transmission direction in the order of the first variable coupler21A→the second variable coupler21B→the third variable coupler21C. Consequently, it is possible to increase the amount of power of the optical signal at the output stage of the WDM unit4by improving the phase variations in optical signal for each waveguide included in the WDM unit4.

FIG. 10is a flowchart illustrating a processing operation of the heater control unit33related to the second individual phase adjustment process. Furthermore, it is assumed that the fourth detecting unit22D, the fifth detecting unit22E, and the sixth detecting unit22F detects, at an optical tap, the amount of power of the optical signal that is at the output stage (first output port) of the variable coupler21and that is branch output at the most downstream fourth fixed coupler31D included in the variable coupler21. InFIG. 10, the heater control unit33determines whether the first amount of power is greater than or equal to the second amount of power (Step S34A). When the first amount of power is greater than or equal to the second amount of power (Yes at Step S34A), the heater control unit33sets the first heater amount into the first heating unit34A in order to shift the phase of the transmission spectrum to the increasing direction in which the amount of power increases (Step S35A). Consequently, in the WDM unit4, the amount of power of the optical signal that is branch output from the variable coupler21increases because the phase of the transmission spectrum is shifted to the increasing direction.

After the heater control unit33sets the second heater amount in the second heating unit34B, the heater control unit33sets the executed flag of the designated waveguide pair32corresponding to the control target (Step S36A), and moves to Step S21in order to determine whether the undesignated waveguide pair32is present in the variable coupler21.

Furthermore, when the first amount of power is not greater than or equal to the second amount of power (No at Step S34A), the heater control unit33sets the second heater amount in the second heating unit34B in order to the phase of the transmission spectrum to the decreasing direction in which the amount of power increases (Step S37A). Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the decreasing direction, the amount of power of the optical signal that is branch output from the variable coupler21increases. Then, after having set the second heater amount in the second heating unit34B, the heater control unit33moves to Step S36A in order to set the executed flag of the designated waveguide pair32.

The heater control unit33according to the second embodiment sets, based on the amount of power of the optical signal, the heater amount of the first heating unit34A to the increasing direction in order to shift the phase of the transmission spectrum to the increasing direction so as to change the heater amount such that the amount of power increases. Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the increasing direction, the amount of power of the optical signal that is branch output from the variable coupler21increases and it is thus possible to improve the phase variations in optical signal for each waveguide.

The heater control unit33sets, based on the amount of power of the optical signal, the heater amount of the second heating unit34B to the decreasing direction in order to shift the phase of the transmission spectrum to the decreasing direction so as to change the heater amount such that the amount of power increases. Consequently, in the WDM unit4, because the phase of the transmission spectrum is shifted to the decreasing direction, the amount of power of the optical signal that is branch output from the variable coupler21increases and it is thus possible to improve the phase variations in optical signal for each waveguide.

It may also be possible to use an optical transmitter1A according to a third embodiment instead of the optical transmitter1according to the first embodiment, and the embodiment thereof will be described below as a third embodiment.FIG. 11is a block diagram illustrating an example of the optical transmitter1A according to the third embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first embodiment, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four light sources2, the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1A illustrated inFIG. 11includes four semiconductor optical amplifiers (SOAs)101arranged between the light sources2and the optical modulating units3. The four SOAs101are, for example, a first SOA101A, a second SOA101B, a third SOA101C, and a fourth SOA101D.

The first SOA101A performs optical amplification on the optical signal at λ1received from the first light source2A and outputs the optical signal at λ1that has been subjected to optical amplification to the first optical modulating unit3A. The second SOA101B performs optical amplification on the optical signal at λ2received from the second light source2B and outputs the optical signal at λ2that has been subjected to optical amplification to the second optical modulating unit3B. The third SOA101C performs optical amplification on the optical signal at λ3received from the third light source2C and outputs the optical signal at λ3that has been subjected to optical amplification to the third optical modulating unit3C. The fourth SOA101D performs optical amplification on the optical signal at λ4received from the fourth light source2D and outputs the optical signal at λ4that has been subjected to optical amplification to the fourth optical modulating unit3D.

In the optical transmitter1A according to the third embodiment, it is possible to ensure a high OSNR while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B even when output power of each of the light sources2is small.

It may also be possible to use an optical transmitter1B according to a fourth embodiment instead of the optical transmitter1according to the first embodiment, and the embodiment thereof will be described below as a fourth embodiment.FIG. 12is a block diagram illustrating an example of the optical transmitter1B according to the fourth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter according to the first to the third embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four light sources2, the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1B illustrated inFIG. 12includes eight SOAs102arranged between the optical modulating units3and the WDM units4. The eight SOAs102are, for example, a pair of fifth SOAs102A, a pair of sixth SOAs102B, a pair of seventh SOAs102C, and a pair of eighth SOAs102D.

The pair of the fifth SOAs102A includes a fifth SOA102A1connected between the first modulator13A included in the first optical modulating unit3A and the first WDM unit4A and a fifth SOA102A2connected between the second modulator13B and the second WDM unit4B. The fifth SOA102A1performs optical amplification on a horizontal polarization optical signal at λ1received from the first modulator13A and outputs the horizontal polarization optical signal at λ1that has been subjected to optical amplification to the first WDM unit4A. The fifth SOA102A2performs optical amplification on a vertical polarization optical signal at λ1received from the second modulator13B and outputs the vertical polarization optical signal at λ1that has been subjected to optical amplification to the second WDM unit4B.

The pair of the sixth SOA102B includes a sixth SOA102B1connected between the first modulator13A included in the second optical modulating unit3B and the first WDM unit4A and a sixth SOA102B2connected between the second modulator13B and the second WDM unit4B. The sixth SOA102B1performs optical amplification on a horizontal polarization optical signal at λ2received from the first modulator13A and outputs the horizontal polarization optical signal at λ2that has been subjected to optical amplification to the first WDM unit4A. The sixth SOA102B2performs optical amplification on a vertical polarization optical signal at λ2received from the second modulator13B and outputs the vertical polarization optical signal at λ2that has been subjected to optical amplification to the second WDM unit4B.

The pair of the seventh SOA102C includes a seventh SOA102C1connected between the first modulator13A included in the third optical modulating unit3C and the first WDM unit4A and a seventh SOA102C2connected between the second modulator13B and the second WDM unit4B. The seventh SOA102C1performs optical amplification on a horizontal polarization optical signal at λ3received from the first modulator13A and outputs the horizontal polarization optical signal at λ3that has been subjected to optical amplification to the first WDM unit4A. The seventh SOA102C2performs optical amplification on a vertical polarization optical signal at λ3received from the second modulator13B and outputs the vertical polarization optical signal at λ3that has been subjected to optical amplification to the second WDM unit4B.

The pair of the eighth SOA102D includes an eighth SOA102D1connected between the first modulator13A included in the fourth optical modulating unit3D and the first WDM unit4A and an eighth SOA102D2connected between the second modulator13B and the second WDM unit4B. The eighth SOA102D1performs optical amplification on a horizontal polarization optical signal at λ4received from the first modulator13A and outputs the horizontal polarization optical signal at λ4that has been subjected to optical amplification to the first WDM unit4A. The eighth SOA102D2performs optical amplification on a vertical polarization optical signal at λ4received from the second modulator13B and outputs the vertical polarization optical signal λ4that has been subjected to optical amplification to the second WDM unit4B.

In the optical transmitter1B according to the fourth embodiment, it is possible to ensure a high OSNR while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B by compensating a loss due to the optical modulating unit3.

It may also be possible to use an optical transmitter1C according to a fifth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the fifth embodiment.FIG. 13is a block diagram illustrating an example of the optical transmitter1C according to the fifth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the fourth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four light sources2, the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1C illustrated inFIG. 13includes two SOAs103arranged between the WDM unit4and the PBC5. The two SOAs103are, for example, a tenth SOA103A and an eleventh SOA103B. The tenth SOA103A performs optical amplification on a horizontal polarization optical signal at λ1+λ2+λ3+λ4received from the first WDM unit4A and outputs the horizontal polarization optical signal that has been subjected to optical amplification to the PBC5. The eleventh SOA103B performs optical amplification on a vertical polarization optical signal at λ1+λ2+λ3+λ4received from the second WDM unit4B and outputs the vertical polarization optical signal that has been subjected to optical amplification to the PBC5.

In the optical transmitter1C according to the fifth embodiment, it is possible to ensure a high OSNR while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B by compensating a loss due to the WDM unit4.

It may also be possible to use an optical transmitter1D according to a sixth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the sixth embodiment.FIG. 14is a block diagram illustrating an example of the optical transmitter1D according to the sixth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the fifth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four light sources2, the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1D illustrated inFIG. 14includes the four SOAs101arranged between the light sources2and the optical modulating units3, and the eight SOAs102arranged between the optical modulating unit3and the WDM unit4. The four SOAs101are, for example, the first SOA101A, the second SOA101B, the third SOA101C, and the fourth SOA101D. The four SOAs101are amplifiers that amplify an output of the optical signal of each of the light sources2. The eight SOAs102are, for example, the pair of the fifth SOA102A, the pair of the sixth SOA102B, the pair of the seventh SOA102C, and the pair of the eighth SOA102D. The eight SOAs102are amplifiers that compensate a loss due to each of the optical modulating units3.

In the optical transmitter1D according to the sixth embodiment, it is possible to amplify an output of each of the light sources2while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B and ensure a high OSNR by compensating a loss due to the optical modulating unit3.

It may also be possible to use an optical transmitter1E according to a seventh embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the seventh embodiment.FIG. 15is a block diagram illustrating an example of the optical transmitter1E according to the seventh embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the sixth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1E illustrated inFIG. 15includes a multiple-wavelength light source106, and a third WDM unit107that is arranged between the multiple-wavelength light source106and the optical modulating unit3. The multiple-wavelength light source106emits light of optical signals having a plurality of wavelengths, for example, the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4. The third WDM unit107is a demultiplexer of a multi-stage-connection asymmetric MZ interferometric type. The third WDM unit107demultiplexes and outputs the optical signals having the plurality of wavelengths received from the multiple-wavelength light source106to the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first optical modulating unit3A. The third WDM unit107outputs the demultiplexed optical signal at λ2to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third optical modulating unit3C. Furthermore, the third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth optical modulating unit3D.

In the optical transmitter1E according to the seventh embodiment, it is possible to ensure a high OSNR by reducing the mounting area by using the single multiple-wavelength light source106while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B.

It may also be possible to use an optical transmitter1F according to an eighth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the eighth embodiment.FIG. 16is a block diagram illustrated an example of the optical transmitter1F according to the eighth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the seventh embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1F illustrated inFIG. 16includes the multiple-wavelength light source106, the third WDM unit107, and a single piece of twelfth SOA108arranged between the multiple-wavelength light source106and the third WDM unit107.

The twelfth SOA108performs optical amplification on the optical signals having a plurality of different wavelengths, for example, the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4, that are input from the multiple-wavelength light source106, and then, outputs the optical signals that have been subjected to optical amplification to the third WDM unit107.

The third WDM unit107demultiplexes and outputs the optical signal at λ1that has been subjected to optical amplification, the optical signal at λ2that has been subjected to optical amplification, the optical signal at λ3that has been subjected to optical amplification, and the optical signal at λ4that has been subjected to optical amplification. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first optical modulating unit3A. The third WDM unit107outputs the demultiplexed optical signal at λ2to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third optical modulating unit3C. Furthermore, the third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth optical modulating unit3D.

In the optical transmitter1F according to the eighth embodiment, it is possible to ensure a high OSNR while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B even when the multiple-wavelength light source106having small output power is used.

It may also be possible to an optical transmitter1G according to a ninth embodiment instead of using the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the ninth embodiment.FIG. 17is a block diagram illustrating an example of the optical transmitter1G according to the ninth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the eighth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1G illustrated inFIG. 17includes the multiple-wavelength light source106, the third WDM unit107, and four SOAs101arranged between the third WDM unit107and the optical modulating unit3. The four SOAs101are, for example, the first SOA101A, the second SOA101B, the third SOA101C, and the fourth SOA101D.

The third WDM unit107demultiplexes the optical signals received from the multiple-wavelength light source106into the optical signals at λ1to λ4. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first SOA101A. The first SOA101A performs optical amplification on the optical signal at λ1and outputs the optical signal at λ1that has been subjected to optical amplification to the first optical modulating unit3A.

The third WDM unit107outputs the demultiplexed optical signal at λ2to the second SOA101B. The second SOA101B performs optical amplification on the optical signal at λ2and outputs the optical signal at λ2that has been subjected to optical amplification to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third SOA101C. The third SOA101C performs optical amplification on the optical signal at λ3and outputs the optical signal at λ3that has been subjected to optical amplification to the third optical modulating unit3C. The third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth SOA101D. The fourth SOA101D performs optical amplification on the optical signal at λ4and outputs the optical signal at λ4that has been subjected to optical amplification to the fourth optical modulating unit3D.

The optical transmitter1G according to the ninth embodiment demultiplexes multiple wavelength light by using the third WDM unit107while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B, and compensates a loss due to demultiplexing into single wavelength light in which output power is relatively small. Consequently, it is possible to ensure a high OSNR.

It may also be possible to use an optical transmitter1H according to a tenth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the tenth embodiment.FIG. 18is a block diagram illustrating an example of the optical transmitter1H according to the tenth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the ninth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1H illustrated inFIG. 18includes the multiple-wavelength light source106, the third WDM unit107, the four SOAs101, and eight SOAs102. Each of the four SOAs101connects between the third WDM unit107and the optical modulating units3. Each of the eight SOAs102connects between the optical modulating units3and the two WDM units4.

The four SOAs101are, for example, the first SOA101A, the second SOA101B, the third SOA101C, and the fourth SOA101D. The eight SOAs102are, for example, the pair of fifth SOAs102A, the pair of sixth SOAs102B, the pair of the seventh SOAs102C, and the pair of the eighth SOAs102D.

The third WDM unit107demultiplexes the optical signals received from the multiple-wavelength light source106into the optical signals at λ1to λ4. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first SOA101A. The first SOA101A performs optical amplification on the optical signal at λ1and outputs the optical signal at λ1that has been subjected to optical amplification to the first optical modulating unit3A.

The third WDM unit107outputs the demultiplexed optical signal at λ2to the second SOA101B. The second SOA101B performs optical amplification on the optical signal at λ2and outputs the optical signal at λ2that has been subjected to optical amplification to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third SOA101C. The third SOA101C performs optical amplification on the optical signal at λ3and outputs the optical signal at λ3that has been subjected to optical amplification to the third optical modulating unit3C. The third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth SOA101D. The fourth SOA101D performs optical amplification on the optical signal at λ4and outputs the optical signal at λ4that has been subjected to optical amplification to the fourth optical modulating unit3D.

The pair of the fifth SOAs102A includes the fifth SOA102A1connected between the first modulator13A included in the first optical modulating unit3A and the first WDM unit4A and the fifth SOA102A2connected between the second modulator13B and the second WDM unit4B. The fifth SOA102A1performs optical amplification on the horizontal polarization optical signal at λ1received from the first modulator13A and outputs the horizontal polarization optical signal at λ1that has been subjected to optical amplification to the first WDM unit4A. The fifth SOA102A2performs optical amplification on the vertical polarization optical signal at λ1received from the second modulator13B and outputs the vertical polarization optical signal at λ1that has been subjected to optical amplification to the second WDM unit4B.

The pair of the sixth SOAs102B includes the sixth SOA102B1connected between the first modulator13A included in the second optical modulating unit3B and the first WDM unit4A and the sixth SOA102B2connected between the second modulator13B and the second WDM unit4B. The sixth SOA102B1performs optical amplification on the horizontal polarization optical signal at λ2received from the first modulator13A and outputs the horizontal polarization optical signal at λ2that has been subjected to optical amplification to the first WDM unit4A. The sixth SOA102B2performs optical amplification on the vertical polarization optical signal at λ2received from the second modulator13B and outputs the vertical polarization optical signal at λ2that has been subjected to optical amplification to the second WDM unit4B.

The pair of the seventh SOA102C includes the seventh SOA102C1connected between the first modulator13A included in the third optical modulating unit3C and the first WDM unit4A and the seventh SOA102C2connected between the second modulator13B and the second WDM unit4B. The seventh SOA102C1performs optical amplification on the horizontal polarization optical signal at λ3received from the first modulator13A and outputs the optical signal at λ3that has been subjected to optical amplification to the first WDM unit4A. The seventh SOA102C2performs optical amplification on the vertical polarization optical signal at λ3received from the second modulator13B and outputs the optical signal at λ3that has been subjected to optical amplification to the second WDM unit4B.

The pair of the eighth SOA102D includes the eighth SOA102D1connected between the first modulator13A included in the fourth optical modulating unit3D and the first WDM unit4A and the eighth SOA102D2connected between the second modulator13B and the second WDM unit4B. The eighth SOA102D1performs optical amplification on the horizontal polarization optical signal at λ4received from the first modulator13A and outputs the horizontal polarization optical signal at λ4that has been subjected to optical amplification to the first WDM unit4A. The eighth SOA102D performs optical amplification on the vertical polarization optical signal at λ4received from the second modulator13B and outputs the vertical polarization optical signal at λ4that has been subjected to optical amplification to the second WDM unit4B.

In the optical transmitter1H according to the tenth embodiment, it is possible to ensure a high OSNR by compensating a loss due to demultiplexing into each of single wavelengths performed by the third WDM unit107and a loss due to the optical modulating unit3while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B.

It may also be possible to use an optical transmitter1J according to an eleventh embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the eleventh embodiment.FIG. 19is a block diagram illustrating an example of the optical transmitter1J according to the eleventh embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the tenth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1J illustrated inFIG. 19includes a multiple-wavelength light source110, and the third WDM unit107that is arranged between the multiple-wavelength light source110and optical modulating unit3. The multiple-wavelength light source110includes a single-wavelength light source111and a phase modulator112. The single-wavelength light source111emits light of an optical signal having a single wavelength. The phase modulator112performs phase modulation on the single wavelength optical signal output from the single-wavelength light source111, thereby outputting, for example, the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4to the third WDM unit107. The third WDM unit107demultiplexes and outputs the optical signals having a plurality of different wavelengths received from the phase modulator112into the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first optical modulating unit3A. The third WDM unit107outputs the demultiplexed optical signal at λ2to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third optical modulating unit3C. Furthermore, the third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth optical modulating unit3D.

In the optical transmitter1J according to the eleventh embodiment, it is possible to ensure a high OSNR by reducing the mounting area by using the single piece of the multiple-wavelength light source110while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B.

It may also be possible to use an optical transmitter1K according to a twelfth embodiment instead of using the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the twelfth embodiment.FIG. 20is a block diagram illustrating an example of the optical transmitter1K according to the twelfth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the eleventh embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

In addition to the four optical modulating units3, the two WDM units4, and the single piece of the PBC5, the optical transmitter1K illustrated inFIG. 20includes a multiple-wavelength light source110A, and the third WDM unit107that is arranged between the multiple-wavelength light source110A and the optical modulating unit3. The multiple-wavelength light source110A includes the single-wavelength light source111and a resonant type phase modulator112A. The single-wavelength light source111outputs a single wavelength optical signal. The resonant type phase modulator112A performs phase modulation on the single wavelength optical signal output from the single-wavelength light source111, thereby outputting, for example, the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4to the third WDM unit107. Furthermore, the resonant type phase modulator112A can output the wavelength of the optical signal at a lower driving voltage compared with the phase modulator112. The third WDM unit107demultiplexes and outputs the optical signals at a plurality of different wavelengths received from the resonant type phase modulator112A into the optical signal at λ1, the optical signal at λ2, the optical signal at λ3, and the optical signal at λ4. The third WDM unit107outputs the demultiplexed optical signal at λ1to the first optical modulating unit3A. The third WDM unit107outputs the demultiplexed optical signal at λ2to the second optical modulating unit3B. The third WDM unit107outputs the demultiplexed optical signal at λ3to the third optical modulating unit3C. Furthermore, the third WDM unit107outputs the demultiplexed optical signal at λ4to the fourth optical modulating unit3D.

In the optical transmitter1K according to the twelfth embodiment, it is possible to ensure a high OSNR by reducing the mounting area by using the single piece of the multiple-wavelength light source110A while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B.

It may also be possible to use an optical transmitter/receiver1L according to a thirteenth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the thirteenth embodiment.FIG. 21is a block diagram illustrating an example of the optical transmitter/receiver1L according to the thirteenth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter1according to the first to the twelfth embodiments, overlapped descriptions of the configuration and the operation thereof will be omitted.

The optical transmitter/receiver1L illustrated inFIG. 21includes the multiple-wavelength light source106, an optical transmitting unit201, and an optical receiving unit202. The optical signals at a plurality of different wavelengths received from the multiple-wavelength light source106are used for a transmission light source of the optical transmitting unit201and a local light source of the optical receiving unit202.

The multiple-wavelength light source106emits light of optical signals at a plurality of different wavelengths, for example, the optical signals at λ1to λ4. The optical transmitting unit201includes a fourth WDM unit107A, the four optical modulating units3, the two WDM units4, and the single piece of the PBCs5. The fourth WDM unit107A is a demultiplexer of a multi-stage-connection asymmetric MZ interferometric type. The fourth WDM unit107A demultiplexes and outputs the optical signals received from the multiple-wavelength light source106into the optical signals at λ1to λ4. The four optical modulating units3are, for example, the first optical modulating unit3A, the second optical modulating unit3B, the third optical modulating unit3C, and the fourth optical modulating unit3D. The first optical modulating unit3A performs optical modulation on the optical signal at λ1by a data signal and outputs the horizontal polarization optical signal at λ1that has been subjected to optical modulation to the first WDM unit4A. The first optical modulating unit3A performs optical modulation on the optical signal at λ1by a data signal and outputs the vertical polarization optical signal at λ1that has been subjected to optical modulation to the second WDM unit4B. The second optical modulating unit3B performs optical modulation on the optical signal at λ2by a data signal and outputs the horizontal polarization optical signal at λ2that has been subjected to optical modulation to the first WDM unit4A. The second optical modulating unit3B performs optical modulation on the optical signal at λ2by a data signal and outputs the vertical polarization optical signal at λ2that has been subjected to optical modulation to the second WDM unit4B. The third optical modulating unit3C performs optical modulation on the optical signal at λ3by a data signal and outputs the horizontal polarization optical signal at λ3that has been subjected to optical modulation to the first WDM unit4A. The third optical modulating unit3C performs optical modulation on the optical signal at λ3by a data signal and outputs the vertical polarization optical signal at λ3that has been subjected to optical modulation to the second WDM unit4B. The fourth optical modulating unit3D performs optical modulation on the optical signal at λ4by a data signal and outputs the horizontal polarization optical signal at λ4that has been subjected to optical modulation to the first WDM unit4A. The fourth optical modulating unit3D performs optical modulation on the optical signal at λ4by a data signal and outputs the vertical polarization optical signal at λ4that has been subjected to optical modulation to the second WDM unit4B.

The optical receiving unit202includes a fifth WDM unit107B and four optical demodulating units120as demodulators. The fifth WDM unit107B is a demultiplexer of the multi-stage-connection asymmetric MZ interferometric type. The fifth WDM unit107B demultiplexes and outputs the optical signals received from the multiple-wavelength light source106into the optical signals (local emission optical signals) at λ1to λ4. The four optical demodulating units120are, for example, a first optical demodulating unit120A, a second optical demodulating unit120B, a third optical demodulating unit120C, and a fourth optical demodulating unit120D.

Each of the optical demodulating units120includes a coherent front end121, an analog-to-digital convertor (ADC)122, and a data demodulating unit123. The coherent front end121interferes the received light with local emission light and generates an optical electric field information signal extracted from the received light. The ADC122performs digital conversion on the electric field information signal output from the coherent front end121. The data demodulating unit123demodulates the data signal from the electric field information signal that has been subjected to digital conversion.

In the optical transmitter/receiver1L according to the thirteenth embodiment, it is possible to implement transmission and reception of optical signals while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B. Furthermore, the number of parts is reduced by sharing the transmission light source and the local light source.

It may also be possible to use an optical transmitter/receiver1M according to a fourteenth embodiment instead of the optical transmitter1according to the first embodiment and the embodiment thereof will be described below as the fourteenth embodiment.FIG. 22is a block diagram illustrating an example of the optical transmitter/receiver1M according to the fourteenth embodiment. Furthermore, by assigning the same reference numerals to components having the same configuration as those in the optical transmitter/receiver1L according to the thirteenth embodiment, overlapped descriptions of the configuration and the operation thereof will be omitted.

The optical transmitter/receiver1M according to the fourteenth embodiment differs from the optical transmitter/receiver1L according to the thirteenth embodiment in that a sixth WDM unit107C is arranged instead of the fourth WDM unit107A and the fifth WDM unit107B. The sixth WDM unit107C is a demultiplexer of the multi-stage-connection asymmetric MZ interferometric type. The sixth WDM unit107C demultiplexes and outputs the optical signals at λ1to λ4received from the multiple-wavelength light source106. The sixth WDM unit107C outputs the optical signal at λ1that is demultiplexed from the multiple-wavelength light source106to each of the first optical modulating unit3A on an optical transmitting unit201A side and the first optical demodulating unit120A on an optical receiving unit202A side. The first optical modulating unit3A modulates the optical signal at λ1by a data signal and outputs the modulated horizontal polarization and vertical polarization optical signals at λ1to the WDM unit4. Furthermore, the first optical demodulating unit120A interferes the received light with the local emission light at λ1and demodulates the data signals from the received light.

Furthermore, the sixth WDM unit107C outputs the optical signal at λ2that is demultiplexed from the multiple-wavelength light source106to each of the second optical modulating unit3B on the optical transmitting unit201A side and the second optical demodulating unit120B on the optical receiving unit202A. The second optical modulating unit3B modulates the optical signals at λ2by data signals and outputs the modulated horizontal polarization and vertical polarization optical signals λ2to the WDM unit4. Furthermore, the second optical demodulating unit120B interferes the received light with the local emission light at λ2and demodulates the data signals from the received light.

Furthermore, the sixth WDM unit107C outputs the optical signal at λ3that is demultiplexed from the multiple-wavelength light source106to each of the third optical modulating unit3C on the optical transmitting unit201A side and the third optical demodulating unit120C on the optical receiving unit202A side. The third optical modulating unit3C modulates the optical signals at λ3by data signals and outputs the modulated horizontal polarization and vertical polarization optical signals at λ3to the WDM unit4. Furthermore, the third optical demodulating unit120C interferes the received light with the local emission light at λ3and demodulates the data signals from the received light.

Furthermore, the sixth WDM unit107C outputs the optical signal at λ4that is demultiplexed from the multiple-wavelength light source106to each of the fourth optical modulating unit3D on the optical transmitting unit201A side and the fourth optical demodulating unit120D on the optical receiving unit202A side. The fourth optical modulating unit3D modulates the optical signals at λ4by data signals and outputs the modulated horizontal polarization and vertical polarization optical signals at λ4to the WDM unit4. Furthermore, the fourth optical demodulating unit120D interferes the received light with the local emission light at λ4and demodulates the data signals from the received light.

In the optical transmitter/receiver1M according to the fourteenth embodiment, it is possible to implement transmission and reception of optical signals while suppressing the phase variations in optical signal for each waveguide in the first WDM unit4A and the second WDM unit4B. Furthermore, the number of parts is reduced by sharing the sixth WDM unit107C that demultiplexes transmission light and local light.

Furthermore, the first heating unit34A and the second heating unit34B each of which adjusts the phase of the optical signal by adjusting a heater amount according to the embodiment are exemplified as phase shifters; however, the phase shifters are not limited to the heating units and modifications are possible as needed.

The optical multiplexers, such as the first WDM unit4A and the second WDM unit4B, are exemplified as optical communication components according to the embodiment; however, the optical multiplexers may also be used for optical demultiplexers, such as the third WDM unit107, the fourth WDM unit107A, the fifth WDM unit107B, and the sixth WDM unit107C, and furthermore, modifications are possible as needed. In this case, the processing order may also be set in the order of, based on the third waveguide pair32C in the third variable coupler21C as the most upstream, the third waveguide pair32C in the third variable coupler21C→the second waveguide pair32B in the third variable coupler21C→the first waveguide pair32A in the third variable coupler21C→the third waveguide pair32C in the second variable coupler21B→the second waveguide pair32B in the second variable coupler21B→the first waveguide pair32A in the second variable coupler21B→the third waveguide pair32C in the first variable coupler21A→the second waveguide pair32B in the first variable coupler21A→the first waveguide pair32A in the first variable coupler21A. Furthermore, in this case, the control unit23detects an amount of power of the optical signal branched from the most downstream waveguide pair32included in the third variable coupler21C and performs, based on the detected amount of power, the phase adjustment process for each of the waveguide pairs32included in the third variable coupler21C. The control unit23detects an amount of power of the optical signal branched from the most downstream waveguide pair32included in the second variable coupler21B and performs, based on the detected amount of power, the phase adjustment process for each of the waveguide pairs32included in the second variable coupler21B. The control unit23detects an amount of power of the optical signal branched from the most downstream waveguide pair32included in the first variable coupler21A and performs, based on the detected amount of power, the phase adjustment process for each of the waveguide pairs32included in the first variable coupler21A.

Each of the components in the units illustrated in the drawings is not always physically configured as illustrated in the drawings. In other words, the specific shape of a separate or integrated unit is not limited to the drawings; however, all or part of the unit can be configured by functionally or physically separating or integrating any of the units depending on various kinds of loads or use conditions.

Furthermore, all or any part of various processing functions performed by each unit may also be executed by a central processing unit (CPU) (or a microcomputer, such as a micro processing unit (MPU), a micro controller unit (MCU), or the like). Furthermore, all or any part of various processing functions may also be, of course, executed by programs analyzed and executed by the CPU (or the microcomputer, such as the MPU or the MCU), or executed by hardware by wired logic.

According to an aspect of an embodiment, it is possible to improve phase variations in signal for each waveguide.