Optical Transmitter

Abstract

This reduces the cost of optical transmission equipment that transmits multiple optical signals by combining multiple different subcarriers. [Solution] Multiple modulated optical signals are generated using a reference light source and multiple tunable light sources. The optical circuit that combines the multiple modulated optical signals includes multiple sub-optical circuits that constitute a binary tree circuit. The reference optical signal generated using the reference light source is led to the output port of one or more of the multiple sub-optical circuits. The transmission characteristics of each sub-optical circuit and the oscillation frequency of each tunable light source are controlled based on a monitor value representing the optical power of the output port of each sub-optical circuit and a monitor value representing the optical power of the input port of the sub-optical circuit to which the reference optical signal is input.

Description

Technical Field

[0001] The present invention relates to an optical transmission device and an optical transceiver device.

Background Art

[0002] An optical communication system that transmits a plurality of optical signals in parallel by multiplexing a plurality of different subcarriers (or wavelengths) has been put into practical use. In such an optical communication system, since the baud rate of each subcarrier can be lowered, the power consumption of the optical transmission device (or the optical transceiver device) can be reduced. Note that techniques for multiplexing a plurality of different wavelengths to transmit a plurality of optical signals are described in, for example, Patent Documents 1 and 2.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] An optical transmission device (or an optical transceiver device) for multiplexing a plurality of different subcarriers (or wavelengths) to transmit a plurality of optical signals includes a plurality of light sources. Here, in order to realize high-quality optical communication, each light source needs to accurately generate light of a specified frequency. However, a light source that can accurately generate light of a specified frequency is expensive. Therefore, in order to realize high-quality optical communication, the cost of the optical transmission device (or the optical transceiver device) becomes high.

[0005] An object related to one aspect of the present invention is to reduce the cost of an optical transmission device or an optical transceiver device that multiplexes a plurality of different subcarriers to transmit a plurality of optical signals.

Means for Solving the Problems

[0006] An optical transmitting device according to one aspect of the present invention comprises a plurality of light sources, a plurality of optical modulators that generate a plurality of modulated optical signals using the output light of the plurality of light sources, and an optical circuit that combines the plurality of modulated optical signals. One of the plurality of light sources is a reference light source that outputs reference light. The other light sources of the plurality of light sources are each tunable light sources. The optical circuit includes a plurality of sub-optical circuits. Each of the plurality of sub-optical circuits comprises a first port, a second port, a third port, and a fourth port. Each of the plurality of sub-optical circuits has a periodic transmission characteristic. The transmission characteristic between the first port and the third port is substantially the same as the transmission characteristic between the second port and the fourth port. The transmission characteristic between the first port and the third port is substantially the same as the transmission characteristic between the second port and the third port. The transmission characteristic between the first port and the third port is complementary to the transmission characteristic between the first port and the fourth port. The transmission characteristics between the second port and the third port are complementary to those between the second port and the fourth port. Each of the plurality of sub-optical circuits includes a phase shifter for adjusting the transmission characteristics. Each of the plurality of sub-optical circuits is configured, by the phase shifter, to combine the input light of the first port and the input light of the second port and output it through the third port. The plurality of sub-optical circuits are optically coupled to each other to form a binary tree circuit having N stages. The third port of a pair of sub-optical circuits located in the (i+1)th stage of the binary tree circuit is optically coupled to the first and second ports of a sub-optical circuit located in the i-th stage of the binary tree circuit with respect to the output port of the optical transmitter. From the plurality of optical modulators, corresponding modulated optical signals from the plurality of modulated optical signals are led to the first and second ports of each sub-optical circuit located in the N-th stage of the binary tree circuit, respectively.A reference optical signal generated using the reference light by one of the plurality of optical modulators is led to the fourth port of the output stage sub-optical circuit connected to the output port of the optical transmitter. [Effects of the Invention]

[0007] According to the above-described embodiment, the cost of an optical transmitting device or optical transceiver that transmits multiple optical signals by combining multiple different subcarriers can be reduced. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows an example of an optical transmission device that transmits WDM signals. [Figure 2] This figure shows an example of the power consumption and cost of an optical transceiver in relation to the number of subcarriers. [Figure 3] This figure shows an example of an optical transmission device according to an embodiment of the present invention. [Figure 4] This figure shows an example of the arrangement of light emitted from multiple light sources. [Figure 5] This is a diagram showing an example of a sub-optical circuit configuration. [Figure 6] This diagram illustrates the transmission characteristics of the sub-optical circuit. [Figure 7] This figure shows an example of the filter characteristics of a sub-optical circuit. [Figure 8] This diagram schematically illustrates a binary tree circuit composed of multiple sub-optical circuits. [Figure 9] This is a diagram (part 1) showing an example of the configuration of a sub-optical circuit. [Figure 10] Figure 9 shows an example of the operation of the sub-optical circuit. [Figure 11] This is a diagram (part 2) showing an example of the configuration of a sub-optical circuit. [Figure 12] Figure 11 shows an example of the operation of the sub-optical circuit. [Figure 13] This is a diagram (part 3) showing an example of the configuration of a sub-optical circuit. [Figure 14]This is a diagram showing an example of the operation of a sub-optical circuit for shaping a spectrum. [Figure 15] This is a diagram showing an example of the operation of a sub-optical circuit provided in the output stage of an optical circuit. [Figure 16] This is (Part 1) of a diagram showing an example of the operation of a sub-optical circuit with respect to a reference optical signal. [Figure 17] This is (Part 2) of a diagram showing an example of the operation of a sub-optical circuit with respect to a reference optical signal. [Figure 18] This is (Part 3) of a diagram showing an example of the operation of a sub-optical circuit with respect to a reference optical signal. [Figure 19] This is a diagram showing an example of the effect related to cost reduction according to an embodiment of the present invention. [Figure 20] This is a flowchart showing an example of a control method used in an optical transmission device. [Figure 21] This is a diagram showing the result of a simulation regarding the process of correcting the frequency of each sub-carrier. [Figure 22] This is a diagram showing the result of a simulation regarding the transmittance of an optical transmission device. [Figure 23] This is a diagram showing an example of an optical transceiver according to an embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0009] FIG. 1 shows an example of an optical transmission device for transmitting a WDM signal. The optical transmission device 100 shown in FIG. 1 includes a plurality of light sources LD and a plurality of optical modulators Mod, and generates a plurality of modulated optical signals. The plurality of light sources LD output lights having different frequencies (or wavelengths) from each other. Each optical modulator Mod generates a modulated optical signal by modulating the continuous light output from the corresponding light source LD with a data signal. In this example, the optical transmission device 100 includes 8 light sources LD and 8 optical modulators Mod, and generates 8 modulated optical signals. The 8 light sources LD output lights having frequencies f1 to f8 (or wavelengths λ1 to λ8). In the following description, the modulated optical signal having the frequency fi (i = 1 to 8) may be referred to as "optical signal fi".

[0010] The optical transmitter 100 includes an optical circuit that combines multiple modulated optical signals. This optical circuit includes sub-optical circuits F1 to F9.

[0011] Sub-optical circuit F1 combines optical signals f1 and f2. In the following description, the optical signal obtained by combining optical signals fi and fj may be referred to as "optical signal fi + fj". Sub-optical circuit F2 combines optical signals f3 and f4 to generate optical signal f3 + f4, sub-optical circuit F3 combines optical signals f5 and f6 to generate optical signal f5 + f6, and sub-optical circuit F4 combines optical signals f7 and f8 to generate optical signal f7 + f8. Sub-optical circuit F5 combines optical signals f1 + f2 and f3 + f4 to generate optical signal f1 + f2 + f3 + f4, and sub-optical circuit F6 combines optical signals f5 + f6 and f7 + f8 to generate optical signal f5 + f6 + f7 + f8.

[0012] Sub-optical circuit F7 shapes the spectra of each optical signal (f1, f2, f3, f4) that make up the optical signal f1+f2+f3+f4, and sub-optical circuit F8 shapes the spectra of each optical signal (f5, f6, f7, f8) that make up the optical signal f5+f6+f7+f8. Then, sub-optical circuit F9 combines the optical signals f1+f2+f3+f4 and f5+f6+f7+f8 to generate the optical signal f1+f2+f3+f4+f5+f6+f7+f8.

[0013] In the optical transmitting device 100 with the above configuration, it is preferable that the optical signals f1 to f8 are set on a predetermined frequency grid. That is, it is preferable that the optical signals f1 to f8 are arranged at predetermined frequency intervals. Here, if the optical signals f1 to f8 are set on the predetermined frequency grid with high precision, the quality of the optical signals f1 to f8 will be improved. Therefore, it is preferable that each light source LD is a light source that can accurately generate light of a specified frequency (hereinafter referred to as a high-precision light source).

[0014] However, high-precision light sources are expensive. Therefore, in configurations where a large number of optical signals are multiplexed in a WDM signal, the cost of the optical transmitter increases. For example, by increasing the number of subcarriers (or wavelength channels), the baud rate per subcarrier can be lowered, which reduces the power consumption of the optical transmitter, as shown in Figure 2A. In Figure 2A, the shaded area represents the power consumption of the light source (and optical modulator). However, an optical transmitter that transmits a WDM signal needs to have the same number of light sources as the number of subcarriers. Therefore, increasing the number of subcarriers increases the cost of the optical transmitter, as shown in Figure 2B. In Figure 2B, the shaded area represents the cost of the light source. The cost of the light source is proportional to the number of light sources provided in the optical transmitter.

[0015] <Embodiment> Figure 3 shows an example of an optical transmitting device according to an embodiment of the present invention. The optical transmitting device 200 according to an embodiment of the present invention can generate a WDM signal in which eight subcarriers (f1 to f7, fref) are combined or multiplexed. That is, the optical transmitting device 200 can generate a WDM signal in which eight modulated optical signals are multiplexed.

[0016] The optical transmitter 200 includes wavelength-tunable light sources LD1 to LD7, a high-precision light source LD8, optical modulators Mod1 to Mod8, and sub-optical circuits F11 to F14, F21 to F22, F31 to F32, and F41. In addition, the optical transmitter 200 includes optical monitors M11 to M14, M21 to M22, M31 to M34, M41 to M42, and controllers C11 to 14, C21 to C22, C31 to C32, and C41.

[0017] The high-precision light source LD8 is configured to output continuous light at a predetermined reference frequency fref. The reference frequency fref is, for example, one of several frequencies defined as a frequency grid for WDM transmission. The high-precision light source LD8 may include, for example, an optical bandpass filter that transmits the reference frequency fref. The high-precision light source LD8 may also be configured to allow adjustment of frequency (or wavelength).

[0018] Each of the tunable light sources LD1 to LD7 outputs continuous light. The frequency of the continuous light output by each of the tunable light sources LD1 to LD7 is controlled by a controller. In this embodiment, the frequencies of the tunable light sources LD1 to LD7 are controlled to f1 to f7, respectively. In this embodiment, the frequencies f1 to f7 are controlled to be positioned on a frequency grid for WDM transmission. Unlike the high-precision light source LD8, each of the tunable light sources LD1 to LD7 does not have an optical bandpass filter that transmits a predetermined frequency. Therefore, each of the tunable light sources LD1 to LD7 is less expensive than the high-precision light source LD8.

[0019] Figure 4 shows an example of the arrangement of oscillation frequencies for the tunable light sources LD1-LD7 and the high-precision light source LD8. In this embodiment, the continuous light generated by the tunable light sources LD1-LD7 and the continuous light generated by the high-precision light source LD8 are arranged at a constant frequency interval Δf. Δf corresponds to the frequency interval defined as the frequency grid for WDM transmission.

[0020] Each optical modulator Mod1 to Mod8 modulates the continuous light output from its corresponding light source to generate a modulated optical signal. Specifically, optical modulators Mod1 to Mod7 each modulate the continuous light output from the tunable light sources LD1 to LD7 to generate a modulated optical signal. Optical modulator Mod8 modulates the continuous light output from the high-precision light source LD8 to generate a modulated optical signal. In the following description, the modulated optical signal generated by optical modulator Modi using the continuous light output from the tunable light source LDi (i=1 to 7) may be referred to as "optical signal fi". Also, the modulated optical signal generated by optical modulator Mod8 using the continuous light output from optical modulator Mod8 may be referred to as "reference optical signal fref". The reference optical signal fref is used to adjust the optical transmitter 200, but like the other optical signals f1 to f7, it can transmit data.

[0021] Each of the sub-optical circuits F11-F14, F21-F22, F31-F32, and F41 is an optical device equipped with two input ports (P1, P2) and two output ports (P3, P4). Hereafter, each of the sub-optical circuits F11-F14, F21-F22, F31-F32, and F41 will be collectively referred to as "sub-optical circuit F".

[0022] Each sub-optical circuit F is equipped with multiple 2x2 couplers and phase shifters placed between the 2x2 couplers, as shown in Figure 5. Here, when the sub-optical circuit F is equipped with N 2x2 couplers, the number of phase shifters is N-1.

[0023] In the example shown in Figure 5A, the sub-optical circuit F is equipped with two 2x2 couplers (51, 52). In this case, the sub-optical circuit F is equipped with one phase shifter (61). The 2x2 coupler provided at the input terminal of the sub-optical circuit F is sometimes called the "input coupler." The 2x2 coupler provided at the output terminal of the sub-optical circuit F is sometimes called the "output coupler." In the example shown in Figure 5A, the 2x2 coupler 51 is used as the "input coupler," and the 2x2 coupler 52 is used as the "output coupler."

[0024] The phase shifter 61 includes a pair of optical waveguides (an upper arm waveguide and a lower arm waveguide). The phase shifter 61 provides a predetermined phase difference between the upper arm waveguide and the lower arm waveguide. That is, when the input light to the phase shifter 61 is branched and guided to the upper arm waveguide and the lower arm waveguide, the phase shifter 61 is configured such that the difference between the phase of the light propagating through the upper arm waveguide and arriving at the output terminal and the phase of the light propagating through the lower arm waveguide and arriving at the output terminal is a predetermined value. Therefore, this phase difference corresponds to the difference between the optical path length of the upper arm waveguide and the optical path length of the lower arm waveguide. In the following description, this difference may be referred to as the "path length difference".

[0025] The path length difference of the phase shifter 61 is adjusted, for example, by controlling the temperature of the optical waveguides that make up the phase shifter 61. In this embodiment, a heater 71 is provided near the optical waveguides that make up the phase shifter 61. The heater 71 is implemented, for example, by an electrical resistor.

[0026] The control unit 81 controls the path length difference of the phase shifter 61 based on the optical power monitor value, which represents the output power of the sub-optical circuit F. The optical power monitor value is detected by an optical power monitor (not shown). The control unit 81 then controls the current of the heater 71 to increase or decrease the optical power monitor value. When the current of the heater 71 changes, the refractive index of the optical waveguide constituting the phase shifter 61 changes, and the path length difference of the phase shifter 61 is adjusted.

[0027] In the example shown in Figure 5B, the sub-optical circuit F comprises four 2x2 couplers (51-54) and three phase shifters (61-63). Each phase shifter 61-63 is located between two 2x2 couplers. Specifically, phase shifter 61 is located between 2x2 couplers 51 and 52, phase shifter 62 is located between 2x2 couplers 52 and 53, and phase shifter 63 is located between 2x2 couplers 53 and 54. In this case, heaters 71-73 are located near each of the phase shifters 61-63. The control unit 81 adjusts the path length difference between the phase shifters 61-63 by controlling the current of the heaters 71-73. The path length differences between the phase shifters 61-63 may be the same or they may not be the same. For example, when the path length difference of the phase shifter located closest to the input (phase shifter 61 in Figure 5B) is ΔL, the path length differences of the other phase shifters are set to 2ΔL or 4ΔL.

[0028] Each sub-optical circuit F is equipped with two input ports (P1, P2) and two output ports (P3, P4), as shown in Figure 5. Input ports P1 and P2 correspond to the input ports of a 2x2 coupler (i.e., input coupler) provided at the input end of the sub-optical circuit. Output ports P3 and P4 correspond to the output ports of a 2x2 coupler (i.e., output coupler) provided at the output end of the sub-optical circuit.

[0029] Figure 6 illustrates the transmission characteristics of the sub-optical circuit F. The sub-optical circuit F has periodic transmission characteristics. The period of the transmission characteristics is determined by the path length difference generated by the phase shifter, as explained with reference to Figure 5.

[0030] In sub-optical circuit F, the transmission spectra of the optical paths between port P1 and port P3 and between port P2 and port P4 are substantially the same. Furthermore, the transmission spectra of the optical paths between port P1 and port P4 and between port P2 and port P3 are substantially the same. In contrast, the transmission spectra of the optical paths between port P1 and port P3 and between port P1 and port P4 are complementary. Also, the transmission spectra of the optical paths between port P2 and port P3 and between port P2 and port P4 are complementary.

[0031] For example, optical signal f1 input via port P1 is directed to port P3 but not to port P4. Optical signal f2 input via port P1 is directed to port P4 but not to port P3. Optical signal f1 input via port P2 is directed to port P4 but not to port P3. Optical signal f2 input via port P2 is directed to port P3 but not to port P4.

[0032] It is also possible to input light from port P3 or port P4. In this case, ports P1 and P2 are used as output ports. Furthermore, the transmission characteristics shown in Figure 6 are substantially the same whether ports P1 and P2 are used as input ports or ports P3 and P4 are used as input ports.

[0033] Figure 7 shows an example of the filter characteristics of the sub-optical circuit F. In this embodiment, as shown in Figure 7A, the subcarriers (or wavelength channels) of the WDM signal are assumed to be arranged at a constant frequency interval Δf.

[0034] The phase shifter constituting the sub-optical circuit F can function as a frequency filter (or wavelength filter). For example, the phase shifter 61 shown in Figure 5A or Figure 5B has filter characteristics that depend on the difference (i.e., path length difference) between the optical path length of the upper arm waveguide and the optical path length of the lower arm waveguide. Specifically, the transmission characteristics of the phase shifter change periodically with respect to wavelength. This period depends on the path length difference of the phase shifter. In the following description, the path length difference of the phase shifter when the period of the transmission characteristics of the phase shifter is 8Δf is represented as "ΔL". In this case, when the path length difference of the phase shifter 41 is controlled to ΔL, the period of the transmission characteristics of the phase shifter becomes 8Δf, as shown in Figure 7B. Δf represents, for example, the spacing of the frequency grid of the WDM.

[0035] As the path length difference of the phase shifter increases, the period of the transmission characteristic of the phase shifter decreases, and as the path length difference of the phase shifter decreases, the period of the transmission characteristic of the phase shifter increases. Here, the period of the transmission characteristic of the phase shifter is inversely proportional to the path length difference of the phase shifter. Therefore, when the path length difference of the phase shifter is controlled to 2ΔL, the period of the transmission characteristic of the phase shifter becomes 4Δf, as shown in Figure 7C. Similarly, when the path length difference of the phase shifter is controlled to 4ΔL, the period of the transmission characteristic of the phase shifter becomes 2Δf, as shown in Figure 7D.

[0036] Figure 8 schematically shows a binary tree circuit composed of multiple sub-optical circuits. In this embodiment, the binary tree circuit is composed of sub-optical circuits F11-F14, F21-F22, F31-F32, and F41. That is, the optical circuit with a binary tree structure is composed of sub-optical circuits F11-F14, F21-F22, F31-F32, and F41. As will be explained later, sub-optical circuits F31-F32 do not operate as combiners that combine modulated optical signals. Therefore, in the explanation of the binary tree circuit shown in Figure 8, sub-optical circuits F31-F32 are considered nonexistent. Furthermore, the optical circuit constitutes a binary tree including three stages. Specifically, sub-optical circuits F11-F14 constitute the first stage, sub-optical circuits F21-F22 constitute the second stage, and sub-optical circuits F31, F32, and F41 constitute the third stage. Alternatively, sub-optical circuits F11-F14 constitute the first stage, sub-optical circuits F21-F22 constitute the second stage, and sub-optical circuit F41 constitutes the third stage.

[0037] The "trunk" of the binary tree circuit is the output port Pout of the optical transmitter 200. That is, the output port Pout connected to the output side of the sub-optical circuit F41 is the trunk of the binary tree circuit. In the following description, the sub-optical circuit connected to the output port Pout of the optical transmitter 200 may be referred to as the "output stage sub-optical circuit." In this embodiment, the output stage sub-optical circuit is sub-optical circuit F41. The "branches and leaves" of the binary tree circuit are the input ports of the modulated optical signal. That is, ports P1 and P2 of each sub-optical circuit F11 to F14 correspond to branches and leaves, respectively.

[0038] In the path from the trunk to the branches and leaves of the binary tree circuit, an output stage sub-optical circuit (i.e., sub-optical circuit F41) is provided in the first stage. In this case, sub-optical circuits F21 and F22 are provided in the second stage, and sub-optical circuits F11 to F14 are provided in the third stage. The transmission characteristics of the sub-optical circuits provided in each stage have the same period. Specifically, the transmission characteristics of each sub-optical circuit F21 and F22 have a period of 4Δf, and the transmission characteristics of each sub-optical circuit F11 to F14 have a period of 8Δf. Furthermore, the transmission characteristics of the sub-optical circuit provided in the (i+1)th stage are twice the transmission characteristics of the sub-optical circuit provided in the i-th stage. "i" is an integer greater than or equal to 1.

[0039] A binary tree circuit is composed of multiple sub-optical circuit groups. One sub-optical circuit group consists of one sub-optical circuit F located in the i-th stage and two sub-optical circuits F located in the (i+1)th stage. In Figure 8, the sub-optical circuit group enclosed by the dashed frame consists of sub-optical circuit F21 located in the second stage and sub-optical circuits F11 and F12 located in the third stage. Sub-optical circuits F22, F13, and F14 constitute one sub-optical circuit group, and sub-optical circuits F41, F21, and F22 also constitute one sub-optical circuit group.

[0040] In each sub-optical circuit group, ports P1 and P2 of sub-optical circuit F located in the i-th stage are connected to ports P3 of one sub-optical circuit F located in the (i+1)th stage, and ports P3 of the other sub-optical circuit F located in the (i+1)th stage, respectively. For example, ports P1 and P2 of sub-optical circuit F21 are connected to ports P3 of sub-optical circuit F11, and ports P3 of sub-optical circuit F12, respectively. Note that ports P4 of each sub-optical circuit F are not used to form a binary tree circuit.

[0041] Modulated optical signals are input from each branch and leaf of the binary tree circuit configured as described above. These multiple modulated optical signals are then combined to generate a WDM signal.

[0042] Returning to the explanation of Figure 3, the modulated optical signal f1 and the modulated optical signal f5 are input to ports P1 and P2 of sub-optical circuit F11, respectively. The modulated optical signal f3 and the modulated optical signal f7 are input to ports P1 and P2 of sub-optical circuit F12, respectively. The modulated optical signal f2 and the modulated optical signal f6 are input to ports P1 and P2 of sub-optical circuit F13, respectively. The modulated optical signal f4 and the reference optical signal fref are input to ports P1 and P2 of sub-optical circuit F14, respectively.

[0043] Port P3 of sub-optical circuit F11 is optically coupled to port P1 of sub-optical circuit F21, port P3 of sub-optical circuit F12 is optically coupled to port P2 of sub-optical circuit F21, port P3 of sub-optical circuit F13 is optically coupled to port P1 of sub-optical circuit F22, and port P3 of sub-optical circuit F14 is optically coupled to port P2 of sub-optical circuit F22. Optical monitor M11 is connected to port P4 of sub-optical circuit F11, optical monitor M12 is connected to port P4 of sub-optical circuit F12, optical monitor M13 is connected to port P4 of sub-optical circuit F13, and optical monitor M14 is connected to port P4 of sub-optical circuit F14.

[0044] Port P3 of sub-optical circuit F21 is optically coupled to port P1 of sub-optical circuit F31, and port P3 of sub-optical circuit F22 is optically coupled to port P1 of sub-optical circuit F32. In addition, optical monitor M21 is connected to port P4 of sub-optical circuit F21, and optical monitor M22 is connected to port P4 of sub-optical circuit F22.

[0045] Port P3 of sub-optical circuit F31 is optically coupled to port P1 of sub-optical circuit F41, and port P3 of sub-optical circuit F32 is optically coupled to port P2 of sub-optical circuit F41. In addition, optical monitor M31 is connected to port P3 of sub-optical circuit F31, and optical monitor M32 is connected to port P3 of sub-optical circuit F32. Furthermore, optical monitor M33 is connected to port P1 of sub-optical circuit F31, and optical monitor M34 is connected to port P1 of sub-optical circuit F32.

[0046] Port P3 of the sub-optical circuit F41 is optically coupled to the output port Pout of the optical transmitter 200. Additionally, an optical monitor M41 is connected to port P3 of the sub-optical circuit F41. Furthermore, an optical monitor M42 is connected to port P1 of the sub-optical circuit F41.

[0047] The reference optical signal fref, generated by the high-precision light source LD8 and the optical modulator Mod8, is led to port P4 of sub-optical circuit F41. Furthermore, in this embodiment, the reference optical signal fref is also led to port P4 of sub-optical circuit F31 and port P4 of sub-optical circuit F32.

[0048] The optical paths between the optical modulator Mod and the sub-optical circuit F, and the optical paths between the sub-optical circuits F, are each realized by, for example, optical waveguides. In this case, the optical waveguides are composed of, for example, a Si nanowire waveguide core and an SiO2 waveguide cladding.

[0049] Figure 9 shows an example of the configuration of the sub-optical circuit F11. In this example, the sub-optical circuit F11 is equipped with one phase shifter (PS1). This phase shifter is composed of a pair of waveguides (upper arm waveguide and lower arm waveguide). The difference (i.e., path length difference) between the optical path length of the upper arm waveguide and the optical path length of the lower arm waveguide is approximately ΔL. When the path length difference is ΔL, the period of the transmission characteristic of the sub-optical circuit F11 is 8Δf, as explained with reference to Figure 7B.

[0050] A heater (H1) is provided near at least one of the upper arm waveguide or the lower arm waveguide. The current supplied to the heater is controlled by controller C11. Here, the refractive index or optical path length of the optical waveguide depends on the temperature. Therefore, by controlling the heater, the path length difference of the sub-optical circuit F11 can be adjusted, and the transmission spectrum can be set to the target state.

[0051] Figure 10 shows an example of the operation of the sub-optical circuit F11. Optical signals f1 and f5 are input to ports P1 and P2 of the sub-optical circuit F11, respectively. An optical monitor M11 is connected to port P4. Controller C11 controls the filter characteristics (i.e., transmission spectrum) of the sub-optical circuit F11.

[0052] The sub-optical circuit F11 is configured such that the path length difference is approximately ΔL. Therefore, the period of the transmission characteristics of the sub-optical circuit F11 is approximately 8Δf. In other words, the sub-optical circuit F11 has the transmission spectrum shown in Figure 10.

[0053] An optical signal f1 is input to port P1. When the transmission spectrum of the sub-optical circuit F11 is set to the target state, the optical path between port P1 and port P3 allows light of frequency f1 to pass through. On the other hand, the optical path between port P1 and port P4 does not allow light of frequency f1 to pass through. Therefore, the optical signal f1 input via port P1 is output from port P3.

[0054] An optical signal f5 is input to port P2. When the transmission spectrum of the sub-optical circuit F11 is set to the target state, the optical path between port P2 and port P3 allows light of frequency f5 to pass through. On the other hand, the optical path between port P2 and port P4 does not allow light of frequency f5 to pass through. Therefore, the optical signal f5 input via port P2 is output from port P3.

[0055] Thus, the sub-optical circuit F11 combines optical signals f1 and f5 and outputs them via port P3. In the following description, the optical signal obtained by combining optical signals fi and fj may be referred to as "optical signal fi + fj". In other words, optical signals f1 + f5 are output via port P3.

[0056] At this time, if the transmission spectrum of the sub-optical circuit F11 is adjusted to the target state, no optical signal will be output through port P4. In other words, if the sub-optical circuit F11 is controlled so that the output optical power of port P4 is reduced, the transmission spectrum of the sub-optical circuit F11 will approach the target state. Therefore, the optical monitor M11 monitors the output optical power of port P4 of the sub-optical circuit F11. The controller C11 then controls the phase shifter of the sub-optical circuit F11 to reduce the optical power monitor value obtained by the optical monitor M11. Specifically, the controller C11 adjusts the path length difference of the sub-optical circuit F11 by controlling the current supplied to a heater (heater 71 in Figure 5) located near the sub-optical circuit F11. As a result, the transmission spectrum of the sub-optical circuit F11 is set to the target state, and the loss to optical signals f1 and f5 is reduced.

[0057] In the above-described embodiment, the phase shifter is controlled based on the output power of port P4, but the embodiments of the present invention are not limited to this configuration. For example, the controller C11 may control the phase shifter based on the output power of port P3. However, in this case, the controller C11 controls the phase shifter so that the output power of port P3 increases.

[0058] The configuration and operation of sub-optical circuits F12 to F14 are substantially the same as those of sub-optical circuit F11. Specifically, sub-optical circuit F12 combines optical signals f3 and f7 to output optical signal f3+f7. Sub-optical circuit F13 combines optical signals f2 and f6 to output optical signal f2+f6. Sub-optical circuit F14 combines optical signal f4 and reference optical signal fref to output optical signal f4+fref.

[0059] Figure 11 shows an example of the configuration of the sub-optical circuit F21. In this embodiment, the sub-optical circuit F21 is equipped with two phase shifters (PS1 and PS2). The path length difference of phase shifter PS1 is approximately ΔL, and the period of its transmission characteristic is 8Δf. The path length difference of phase shifter PS2 is approximately 2ΔL, and the period of its transmission characteristic is 4Δf. Therefore, when the frequency at which the peak of the transmission characteristic due to phase shifter PS1 appears coincides with the frequency at which the peak of the transmission characteristic due to phase shifter PS2 appears, the period of the transmission characteristic of the sub-optical circuit F21 is 4Δf.

[0060] Heaters H1 and H2 are provided for the phase shifters PS1 and PS2, respectively. Controller C21 controls heaters H1 and H2 individually. This adjusts the path length difference of the sub-optical circuit F21, setting the transmitted spectrum to the target state.

[0061] Figure 12 shows an example of the operation of the sub-optical circuit F21. The optical signals f1+f5 are input to port P1, and the optical signals f3+f7 are input to port P2. The optical monitor M21 is connected to port P4. Controller C21 controls the filter characteristics of the sub-optical circuit F21.

[0062] The sub-optical circuit F21 includes a phase shifter with a path length difference of approximately 2ΔL. Therefore, the period of the transmission characteristics of the sub-optical circuit F21 is approximately 4Δf. That is, the sub-optical circuit F21 has the transmission spectrum shown in Figure 12.

[0063] The optical signal f1+f5 is input to port P1. When the transmission spectrum of the sub-optical circuit F21 is set to the target state, the optical path between port P1 and port P3 allows light of frequencies f1 and f5 to pass through. On the other hand, the optical path between port P1 and port P4 does not allow light of frequencies f1 and f5 to pass through. Therefore, the optical signal f1+f5 input via port P1 is output from port P3.

[0064] The optical signal f3+f7 is input to port P2. When the transmission spectrum of the sub-optical circuit F21 is set to the target state, the optical path between port P2 and port P3 allows light of frequencies f3 and f7 to pass through. On the other hand, the optical path between port P2 and port P4 does not allow light of frequencies f3 and f7 to pass through. Therefore, the optical signal f3+f7 input via port P2 is output from port P3.

[0065] Thus, the sub-optical circuit F21 combines the optical signals f1+f5 and f3+f7 and outputs them via port P3. In other words, the optical signals f1+f3+f5+f7 are output via port P3.

[0066] At this time, if the transmission spectrum of the sub-optical circuit F21 is adjusted to the target state, no optical signal will be output through port P4. In other words, if the sub-optical circuit F21 is controlled so that the output optical power of port P4 is reduced, the transmission spectrum of the sub-optical circuit F21 will approach the target state. Therefore, the optical monitor M21 monitors the output optical power of port P4 of the sub-optical circuit F21. The controller C21 then controls the phase shifter of the sub-optical circuit F21 to reduce the optical power monitor value obtained by the optical monitor M21. As a result, the transmission spectrum of the sub-optical circuit F21 is set to the target state, and the loss to the optical signals f1, f3, f5, and f7 is reduced.

[0067] In the above-described embodiment, the phase shifter is controlled based on the output power of port P4, but the embodiments of the present invention are not limited to this configuration. For example, the controller C21 may control the phase shifter based on the output power of port P3. However, in this case, the controller C21 controls the phase shifter so that the output power of port P3 increases.

[0068] The configuration and operation of sub-optical circuit F22 are substantially the same as those of sub-optical circuit F21. That is, sub-optical circuit F22 combines optical signals f2+f6 and f4+fref to output optical signal f2+f4+f6+fref.

[0069] Figure 13 shows an example of the configuration of the sub-optical circuit F31. In this embodiment, the sub-optical circuit F31 comprises five phase shifters (PS1 to PS5). The path length difference of phase shifter PS1 is approximately 2ΔL, and the period of its transmission characteristic is 4Δf. The path length difference of each phase shifter PS2 to PS5 is approximately 4ΔL, and the period of their transmission characteristics is 2Δf. Therefore, when the frequency at which the peak of the transmission characteristic due to phase shifter PS1 appears coincides with the frequency at which the peak of the transmission characteristic due to phase shifters PS2 to PS5 appears, the period of the transmission characteristic of the sub-optical circuit F31 is 2Δf.

[0070] Heaters H1 to H5 are provided for phase shifters PS1 to PS5, respectively. Controller C31 controls heaters H1 to H5 individually. This adjusts the path length difference of the sub-optical circuit F31, setting the transmitted spectrum to the target state.

[0071] Figure 14 shows an example of the operation of the sub-optical circuit F31. The optical signals f1+f3+f5+f7 are input to port P1. The optical monitor M31 is connected to port P3. Controller C31 controls the filter characteristics of the sub-optical circuit F31.

[0072] The sub-optical circuit F31 includes a phase shifter with a path length difference of approximately 4ΔL. Therefore, the period of the transmission characteristics of the sub-optical circuit F31 is approximately 2Δf. That is, the sub-optical circuit F31 has the transmission spectrum shown in Figure 14.

[0073] The optical signal f1+f3+f5+f7 is input to port P1. When the transmission spectrum of the sub-optical circuit F31 is set to the target state, the optical path between port P1 and port P3 allows light of frequencies f1, f3, f5, and f7 to pass through. On the other hand, the optical path between port P1 and port P4 does not allow light of frequencies f1, f3, f5, and f7 to pass through. Therefore, the optical signal f1+f3+f5+f7 input via port P1 is output from port P3.

[0074] Thus, the optical signals f1+f3+f5+f7 pass through the sub-optical circuit F31. At this time, the spectra of each optical signal f1, f3, f5, and f7 are shaped by passing through the sub-optical circuit F31.

[0075] The reference optical signal fref is input to port P4 of the sub-optical circuit F31. Therefore, the optical monitor M31, which monitors the output power of the sub-optical circuit F31, is connected to port P3, not port P4.

[0076] If the transmission spectrum of the sub-optical circuit F31 is adjusted to the target state, the optical signals f1+f3+f5+f7 are guided from port P1 to port P3. In other words, if the sub-optical circuit F31 is controlled so that the output optical power of port P3 is increased, the transmission spectrum of the sub-optical circuit F31 approaches the target state. Therefore, the optical monitor M31 monitors the output optical power of port P3 of the sub-optical circuit F31. The controller C31 then controls the phase shifter of the sub-optical circuit F31 to increase the optical power monitor value obtained by the optical monitor M31. As a result, the transmission spectrum of the sub-optical circuit F31 is set to the target state, and the loss to the optical signals f1, f3, f5, and f7 is reduced.

[0077] The configuration and operation of sub-optical circuit F32 are substantially the same as those of sub-optical circuit F31. That is, the optical signals f2 + f4 + f6 + fref pass through sub-optical circuit F32. In this process, the spectra of each optical signal f2, f4, f6, and fref are shaped.

[0078] Figure 15 shows an example of the operation of the sub-optical circuit F41. The optical signals f1+f3+f5+f7 are input to port P1, and the optical signals f2+f4+f6+fref are input to port P2. The optical monitor M41 is connected to port P3. Controller C41 controls the filter characteristics of the sub-optical circuit F41.

[0079] The sub-optical circuit F41 includes a phase shifter with a path length difference of approximately 4ΔL. Therefore, the period of the transmission characteristics of the sub-optical circuit F41 is approximately 2Δf. In other words, the sub-optical circuit F41 has the transmission spectrum shown in Figure 15.

[0080] The optical signal f1+f3+f5+f7 is input to port P1. When the transmission spectrum of the sub-optical circuit F41 is set to the target state, the optical path between port P1 and port P3 allows light of frequencies f1, f3, f5, and f7 to pass through. On the other hand, the optical path between port P1 and port P4 does not allow light of frequencies f1, f3, f5, and f7 to pass through. Therefore, the optical signal f1+f3+f5+f7 input via port P1 is output from port P3.

[0081] The optical signal f2+f4+f6+fref is input to port P2. When the transmission spectrum of the sub-optical circuit F41 is set to the target state, the optical path between port P2 and port P3 allows light of frequencies f2, f4, f6, and fref to pass through. On the other hand, the optical path between port P2 and port P4 does not allow light of frequencies f2, f4, f6, and fref to pass through. Therefore, the optical signal f2+f4+f6+fref input via port P2 is output from port P3.

[0082] Thus, the sub-optical circuit F41 combines the optical signals f1+f3+f5+f7 and f2+f4+f6+fref and outputs them via port P3. In other words, the optical signals f1+f2+f3+f4+f5+f6+f7+fref are output via port P3.

[0083] The reference optical signal fref is input to port P4 of the sub-optical circuit F41. Therefore, the optical monitor M41, which monitors the output power of the sub-optical circuit F41, is connected to port P3, not port P4.

[0084] If the transmission spectrum of the sub-optical circuit F41 is adjusted to the target state, all optical signals f1~f7 and fref will be output via port P3. In other words, by controlling the phase shifter of the sub-optical circuit F41 so that the output optical power of port P3 increases, the transmission spectrum of the sub-optical circuit F41 will approach the target state. Therefore, the optical monitor M41 monitors the output optical power of port P3 of the sub-optical circuit F41. The controller C41 then controls the phase shifter of the sub-optical circuit F41 so that the optical power monitor value obtained by the optical monitor M41 increases. As a result, the transmission spectrum of the sub-optical circuit F41 is set to the target state, and the loss for each optical signal f1~f7 and fref is reduced.

[0085] In this embodiment, the configuration of the sub-optical circuit F41 is substantially the same as that of the sub-optical circuit 31. That is, as shown in Figure 13, the sub-optical circuit F41 comprises phase shifters PS1 to PS5 and heaters H1 to H5. The controller C41 individually controls the heaters H1 to H5 to adjust the path length difference of the phase shifters PS1 to PS5 individually.

[0086] In the optical transmitter 200 with the above configuration, the reference optical signal fref generated by the high-precision light source LD8 and the optical modulator Mod8 is led to port P2 of sub-optical circuit F14, as well as to port P4 of sub-optical circuit F41, port P4 of sub-optical circuit F31, and port P4 of sub-optical circuit F32. Therefore, the operation of sub-optical circuits F41, F31, and F32 in relation to the reference optical signal fref will be explained below.

[0087] Figure 16 shows an example of the operation of the sub-optical circuit F41 in response to the reference optical signal fref. The reference optical signal fref is input via port P4. An optical monitor M42 is connected to port P1. Controller C41 controls the filter characteristics of the sub-optical circuit F41.

[0088] In the sub-optical circuit F41, the transmission characteristics for light traveling from the input ports (P1, P2) to the output ports (P3, P4) and the transmission characteristics for light traveling from the output ports (P3, P4) to the input ports (P1, P2) are the same. That is, the sub-optical circuit F41 has the transmission spectrum shown in Figure 16 with respect to the reference optical signal fref.

[0089] The reference optical signal fref is input to port P4. When the transmission spectrum of the sub-optical circuit F41 is set to the target state, the optical path between port P4 and port P1 allows light of frequency fref to pass through. On the other hand, the optical path between port P4 and port P2 does not allow light of frequency fref to pass through. Therefore, the reference optical signal fref input via port P4 is output from port P1.

[0090] Therefore, the optical monitor M42 monitors the output optical power of port P1 of the sub-optical circuit F41. The controller C41 then controls the phase shifter of the sub-optical circuit F41 to increase the optical power monitor value obtained by the optical monitor M42. As a result, the transmission spectrum of the sub-optical circuit F41 is calibrated based on the reference optical signal fref.

[0091] Figure 17 shows an example of the operation of the sub-optical circuit F31 in response to the reference optical signal fref. The reference optical signal fref is input via port P4. An optical monitor M33 is connected to port P1. Controller C31 controls the filter characteristics of the sub-optical circuit F31.

[0092] The operation of sub-optical circuit F31 with respect to the reference optical signal fref is substantially the same as the operation of sub-optical circuit F41, as described with reference to Figure 16. That is, the optical monitor M33 monitors the output optical power of port P1 of sub-optical circuit F31. The controller C31 then controls the phase shifter of sub-optical circuit F31 to increase the optical power monitor value obtained by the optical monitor M33. As a result, the transmission spectrum of sub-optical circuit F31 is calibrated based on the reference optical signal fref.

[0093] Figure 18 shows an example of the operation of the sub-optical circuit F32 in response to the reference optical signal fref. The reference optical signal fref is input via port P4. An optical monitor M34 is connected to port P1. Controller C32 controls the filter characteristics of the sub-optical circuit F32.

[0094] The operation of sub-optical circuit F32 with respect to the reference optical signal fref is substantially the same as the operation of sub-optical circuit F41, as described with reference to Figure 16. However, sub-optical circuit F32 is configured such that frequencies f2, f4, f6, and fref pass through between ports P1 and P3, and between ports P2 and P4, while frequencies f2, f4, f6, and fref are blocked between ports P1 and P4, and between ports P2 and P3. In other words, the transmission characteristics of sub-optical circuit F32 are shifted by Δf, as shown in Figure 7, with respect to the transmission characteristics of sub-optical circuit F31 or sub-optical circuit F41. Therefore, sub-optical circuit F32 has the transmission spectrum shown in Figure 18 with respect to the reference optical signal fref.

[0095] The reference optical signal fref is input to port P4. When the transmission spectrum of the sub-optical circuit F32 is set to the target state, the optical path between port P4 and port P2 allows light of frequency fref to pass through. On the other hand, the optical path between port P4 and port P1 does not allow light of frequency fref to pass through. Therefore, the reference optical signal fref input via port P4 is output from port P2.

[0096] Therefore, the optical monitor M34 monitors the output optical power of port P2 of the sub-optical circuit F32. The controller C32 then controls the phase shifter of the sub-optical circuit F32 to increase the monitored value obtained by the optical monitor M34. As a result, the transmission spectrum of the sub-optical circuit F32 is calibrated based on the reference optical signal fref.

[0097] In this way, the phase shifters of each sub-optical circuit F are adjusted. This allows each modulated optical signal to be combined with minimal loss, resulting in the generation of a high-quality WDM signal.

[0098] In addition, the optical transmitter 200 controls the oscillation frequency of each tunable light source LD1 to LD7. Furthermore, if the high-precision light source LD8 is a tunable light source, the optical transmitter 200 may control the high-precision light source LD8 in addition to the tunable light sources LD1 to LD7.

[0099] The tunable light source is controlled based on the optical power monitored in any one or more sub-optical circuits F from sub-optical circuits F11-F14, F21-F22, F31-F32, and F41. In the embodiment shown in Figure 3, the tunable light source is controlled based on the optical power monitored in sub-optical circuits F31, F32, and F41.

[0100] In the sub-optical circuit F31, the optical signals f1+f3+f5+f7 are output via port P3. The optical monitor M31 monitors the output power of port P3. The controller C31b then controls the tunable light sources LD1, LD3, LD5, and LD7 based on the optical power monitor value obtained by the optical monitor M31. Here, the optical power monitor value is considered to be maximum when the frequency interval of the optical signals f1, f3, f5, and f7 is 2Δf. Therefore, the controller C31b controls the tunable light sources LD1, LD3, LD5, and LD7 so that the optical power monitor value obtained by the optical monitor M31 is large.

[0101] In the sub-optical circuit F32, the optical signal f2+f4+f6+fref is output via port P3. The optical monitor M32 monitors the output power of port P3. The controller C32b then controls the tunable light sources LD2, LD4, and LD6 based on the optical power monitor value obtained by the optical monitor M32. Here, the optical power monitor value is considered to be maximum when the frequency interval of the optical signals f1, f3, f5, and fref is 2Δf. Therefore, the controller C32b controls the tunable light sources LD2, LD4, and LD6 so that the optical power monitor value obtained by the optical monitor M32 is large.

[0102] In the sub-optical circuit F41, the optical signals f1+f2+f3+f4+f5+f6+f7+fref are output via port P3. The optical monitor M41 monitors the output power of port P3. The controller C41b then controls the tunable light sources LD1~LD7 based on the optical power monitor value obtained by the optical monitor M41. Here, the optical power monitor value is considered to be maximum when the frequency interval of the optical signals f1~f7 and fref is Δf. Therefore, the controller C41b controls the tunable light sources LD1~LD7 so that the optical power monitor value obtained by the optical monitor M41 is large.

[0103] As described above, when the tunable light sources LD1 to LD7 are controlled, the optical signals f1 to f7 and fref are arranged at frequency intervals Δf. Here, the reference optical signal fref is generated using the high-precision light source LD8. The high-precision light source LD8 is configured to output continuous light at a predetermined reference frequency (for example, one of several frequencies defined as the frequency grid for WDM transmission). Therefore, each optical signal f1 to f7 is also placed on the frequency grid for WDM transmission.

[0104] As a result, high-quality WDM transmission is achieved. Here, only one of the multiple light sources in the optical transmitter 200 is a high-precision light source, while the other light sources are inexpensive tunable light sources. Therefore, high-quality WDM transmission can be achieved while keeping the cost of the optical transmitter 200 down.

[0105] Figure 19 shows an example of the cost reduction effect according to an embodiment of the present invention. In Figure 19, the shaded area represents the cost of the light source (tunable light source and high-precision light source) in the configuration according to the embodiment of the present invention. The dashed line represents the cost when all light sources are high-precision light sources. Thus, according to the embodiment of the present invention, the cost of an optical transmission device or optical transceiver can be reduced. This effect is particularly noticeable when the number of subcarriers (or wavelength channels) is large.

[0106] Figure 20 is a flowchart illustrating an example of a control method used in the optical transmitter 200. The processes in this flowchart are performed, for example, before the optical transmitter 200 starts transmitting data. These processes may also be performed when the ambient temperature around the optical transmitter 200 changes. Furthermore, these processes may be performed periodically to account for the aging degradation of the optical device.

[0107] In S1, the controller C11 adjusts the transmission characteristics of the sub-optical circuit F11 based on the optical power monitor value obtained by the optical monitor M11. Here, the sub-optical circuit F11 is equipped with a heater H1 as shown in Figure 9. In this case, the controller C11 controls the heater H1 so that the optical power monitor value becomes small. In addition, the controller C11 may control the oscillation frequency of the tunable light source corresponding to the modulated optical signal input to the sub-optical circuit F11 so that the optical power monitor value becomes large. In this embodiment, the controller C11 may adjust the oscillation frequencies of the tunable light sources LD1 and LD5.

[0108] In S2, controller C12 adjusts the transmission characteristics of the sub-optical circuit F12 based on the optical power monitor value obtained by the optical monitor M12. The operation of controller C12 is substantially the same as that of controller C11. That is, controller C12 controls heater H1 so that the optical power monitor value decreases. In addition, controller C12 may adjust the oscillation frequencies of the tunable light sources LD3 and LD7 so that the optical power monitor value increases.

[0109] In S3, controller C13 adjusts the transmission characteristics of the sub-optical circuit F13 based on the optical power monitor value obtained by the optical monitor M13. The operation of controller C13 is substantially the same as that of controller C11. That is, controller C13 controls heater H1 so that the optical power monitor value decreases. In addition, controller C13 may adjust the oscillation frequencies of the tunable light sources LD2 and LD6 so that the optical power monitor value increases.

[0110] In S4, controller C14 adjusts the transmission characteristics of the sub-optical circuit F14 based on the optical power monitor value obtained by the optical monitor M14. The operation of controller C14 is substantially the same as that of controller C11. That is, controller C14 controls heater H1 so that the optical power monitor value decreases. In addition, controller C14 may adjust the oscillation frequency of the tunable light source LD4 so that the optical power monitor value increases.

[0111] In S5, the controller C21 adjusts the transmission characteristics of the sub-optical circuit F21 based on the optical power monitor value obtained by the optical monitor M21. Here, the sub-optical circuit F21 is equipped with heaters H1 and H2 as shown in Figure 11. In this case, the controller C21 controls heaters H1 and H2 so that the optical power monitor value becomes small. In addition, the controller C21 may adjust the oscillation frequencies of the tunable light sources LD1, LD3, LD5, and LD7 so that the optical power monitor value becomes large.

[0112] In S6, controller C22 adjusts the transmission characteristics of the sub-optical circuit F22 based on the optical power monitor value obtained by the optical monitor M22. The operation of controller C22 is substantially the same as that of controller C21. That is, controller C22 controls heaters H1 and H2 so that the optical power monitor value increases. In addition, controller C22 may adjust the oscillation frequencies of the tunable light sources LD2, LD4, and LD6 so that the optical power monitor value decreases.

[0113] In S7, controller C31 adjusts the transmission characteristics of sub-optical circuit F31 based on the optical power monitor values ​​obtained by optical monitor M31 and optical power monitor values ​​obtained by optical monitor M33. Here, sub-optical circuit F31 is equipped with heaters H1 to H5 as shown in Figure 13. In this case, controller C31 controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M31 increases. Also, controller C31 controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M33 increases.

[0114] In S8, controller C32 adjusts the transmission characteristics of the sub-optical circuit F32 based on the optical power monitor values ​​obtained by optical monitor M32 and optical power monitor values ​​obtained by optical monitor M34. The operation of controller C32 is substantially the same as that of controller C31. That is, controller C32 controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M32 increases, and controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M34 increases.

[0115] In S9, controller C41 adjusts the transmission characteristics of sub-optical circuit F41 based on the optical power monitor values ​​obtained by optical monitor M41 and optical power monitor values ​​obtained by optical monitor M42. Here, sub-optical circuit F41 is equipped with heaters H1 to H5 as shown in Figure 13. In this case, controller C41 controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M41 increases. Controller C31 also controls heaters H1 to H5 so that the optical power monitor value obtained by optical monitor M42 increases.

[0116] In S10, the controller 31b controls the tunable light source based on the optical power monitor value obtained by the optical monitor M31. Specifically, the controller 31b adjusts the oscillation frequencies of the tunable light sources LD1, LD3, LD5, and LD7 so that the optical power monitor value obtained by the optical monitor M31 increases.

[0117] In S11, the controller 32b controls the tunable light source based on the optical power monitor value obtained by the optical monitor M32. Specifically, the controller 32b adjusts the oscillation frequencies of the tunable light sources LD2, LD4, and LD6 so that the optical power monitor value obtained by the optical monitor M32 increases.

[0118] In S12, the controller 41b controls the tunable light source based on the optical power monitor value obtained by the optical monitor M41. Specifically, the controller 41b adjusts the oscillation frequencies of the tunable light sources LD1 to LD7 so that the optical power monitor value obtained by the optical monitor M41 increases.

[0119] The processes S1 to S12 are repeatedly executed until predetermined convergence conditions are met. The convergence conditions may be, for example, the quality of each subcarrier. In this case, the processes S1 to S12 are repeatedly executed until the optical signal-to-noise ratio or error rate measured at the receiving node receiving the WDM signal falls below a predetermined level. Alternatively, the processes S1 to S12 may be executed a predetermined number of times. In this case, the number of repetitions is determined by simulation or the like.

[0120] The order in which S1 to S12 are executed is not limited to the embodiment shown in Figure 20, and the optical transmitter 200 can execute S1 to S12 in any order. Furthermore, two or more processes may be executed in parallel. For example, the process of adjusting the characteristics of the sub-optical circuit F and the process of adjusting the oscillation frequency of the tunable light source may be executed in parallel or in a time-division method.

[0121] The controller (C11-14, C21-C22, C31, C31b, C32, C32b, C41, C41b) may be implemented by a single processor or by multiple processors. Furthermore, the controller may be implemented as a hardware circuit.

[0122] Figure 21 shows the simulation results for the process of correcting the frequency of each subcarrier. The horizontal axis of this graph represents time (or the number of iterations of the process in the flowchart shown in Figure 20). The vertical axis represents the error of each subcarrier relative to the target frequency. The target frequency is, for example, the frequency grid of the WDM. It is assumed that the frequency error of the high-precision light source L8 is zero. According to this simulation, the frequencies f1 to f7 of each subcarrier converge to the target frequency.

[0123] Figure 22 shows the simulation results for the transmittance of the optical transmitter 200. Figure 22A shows the state before frequency correction is performed according to the embodiment of the present invention. This state corresponds to the spectrum at time T0 shown in Figure 21. Figure 22B shows the state after frequency correction is performed according to the embodiment of the present invention. As shown above, when frequency correction is performed according to the embodiment of the present invention, the transmission characteristics of each subcarrier f1 to f7 and fref are improved, so that a high-quality WDM signal is transmitted.

[0124] Figure 23 shows an example of an optical transceiver according to an embodiment of the present invention. The optical transceiver according to an embodiment of the present invention comprises an optical transmitter 300 and an optical receiver 400. The optical transmitter 300 corresponds to the optical transmitter 200 shown in Figure 3. Therefore, one of the light sources LD1 to LD8 shown in Figure 23 corresponds to the high-precision light source LD8 shown in Figure 3. In addition, the optical circuit 301 shown in Figure 23 corresponds to the sub-optical circuits F11 to F14, F21 to F22, F31 to F32, F41, optical monitors M11 to M14, M21 to M22, M31 to M34, M41 to M42, and controllers C11 to 14, C21 to C22, C31, C31b, C32, C32b, and C41 shown in Figure 3. Therefore, the continuous light f1 to f8 output from the light sources LD1 to LD8 are each arranged on a WDM grid.

[0125] The optical receiver 400 includes an optical inverse multiplexer 401 and coherent receivers 402#1 to 402#8. The optical inverse multiplexer 401 separates the received WDM signal for each wavelength channel and directs it to the corresponding coherent receivers 402#1 to 402#8. Each coherent receiver 402#1 to 402#8 uses continuous light generated by the corresponding local light source to generate an electric field information signal representing the electric field of the modulated optical signal for the corresponding wavelength channel. Here, the local light source is the light source LD1 to LD8 implemented in the optical transmitter 300. Therefore, each coherent receiver 402#1 to 402#8 uses the light source LD1 to LD8 to generate an electric field information signal of the received optical signal. The electric field information signals generated by the coherent receivers 402#1 to 402#8 are processed by a DSP (Digital Signal Processor) (not shown). [Explanation of symbols]

[0126] LD1~LD7 Wavelength Tunable Light Source LD8 High precision light source Mod1~Mod8 Optical Modulator F11-F14, F21-F22, F31-F32, F41 Sub-optical circuits M11-M14, M21-M22, M31-M34, M41-M42 Optical Monitor Controllers C11-14, C21-C22, C31, C31b, C32, C32b, and C41 51-54 2x2 coupler 61~63 Phase shifter 71-73 Heater 81 Control Unit

Claims

1. An optical transmission device that combines and outputs multiple optical signals, Multiple light sources, Multiple optical modulators that generate multiple modulated optical signals using the output light of the multiple light sources, The system comprises an optical circuit that combines the plurality of modulated optical signals, One of the aforementioned multiple light sources is a reference light source that outputs reference light, Among the aforementioned multiple light sources, the other light sources are each tunable light sources. The optical circuit includes a plurality of sub-optical circuits, Each of the aforementioned sub-optical circuits comprises a first port, a second port, a third port, and a fourth port. Each of the aforementioned sub-optical circuits has a periodic transmission characteristic, The permeability characteristics between the first port and the third port and the permeability characteristics between the second port and the fourth port are substantially the same. The permeability characteristics between the first port and the fourth port and the permeability characteristics between the second port and the third port are substantially the same. The permeability characteristics between the first port and the third port and the permeability characteristics between the first port and the fourth port are complementary to each other. The permeability characteristics between the second port and the third port and the permeability characteristics between the second port and the fourth port are complementary to each other. Each of the aforementioned sub-optical circuits includes a phase shifter for adjusting the transmission characteristics. Each of the aforementioned sub-optical circuits is configured to combine the input light from the first port and the input light from the second port using the phase shifter and output it through the third port. The plurality of sub-optical circuits are optically coupled to each other to form a binary tree circuit including N stages. With reference to the output port of the optical transmitter, the third port of a pair of sub-optical circuits located at the (i+1)th stage of the binary tree circuit is optically coupled to the first port and the second port of a sub-optical circuit located at the ith stage of the binary tree circuit, respectively. From the plurality of optical modulators, the corresponding modulated optical signals from the plurality of modulated optical signals are led to the first port and the second port of each sub-optical circuit provided in the Nth stage of the binary tree circuit, respectively. A reference optical signal generated using the reference light by one of the multiple optical modulators is led to the fourth port of the output stage sub-optical circuit connected to the output port of the optical transmitter. An optical transmission device characterized by the following features.

2. The system further includes a controller that controls the oscillation frequency of a tunable light source corresponding to the modulated optical signal input to the first sub-optical circuit, based on the output power of the third or fourth port of the first sub-optical circuit among the plurality of sub-optical circuits. The optical transmitting device according to feature 1.

3. The controller controls the oscillation frequency of the tunable light source corresponding to the modulated optical signal input to the first sub-optical circuit so as to increase the output power of the third port or decrease the output power of the fourth port. The optical transmitting device according to feature 2.

4. The system further comprises a second controller that controls the phase shifter of the second sub-optical circuit based on the output power of the third or fourth port of the second sub-optical circuit among the plurality of sub-optical circuits. The optical transmitting device according to feature 1.

5. The second controller controls the phase shifter in the second sub-optical circuit so that the output power of the third port increases or the output power of the fourth port decreases. The optical transmitting device according to feature 4.

6. The system further comprises a third controller that controls the phase shifter of the output stage sub-optical circuit based on the output power of the first or second port of the output stage sub-optical circuit. The optical transmitting device according to feature 1.

7. The period of the transmission characteristics of the sub-optical circuit provided in the (i+1)th stage of the binary tree circuit is twice the period of the transmission characteristics of the sub-optical circuit provided in the ith stage of the binary tree circuit. The optical transmitting device according to feature 1.

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