Optical spectrum control device

Abstract

An exemplary optical spectrum control device disclosed herein has an optical spectrum variable circuit and a control circuit. The optical spectrum variable circuit receives output light of a gain filter for shaping a gain spectrum of an optical amplifier by means of a predetermined filter characteristic, and varies the optical spectrum of the output light. The control circuit controls the optical spectrum variable circuit so that the output light spectrum of the optical spectrum variable circuit approaches a desired optical spectrum.

Description

Optical Spectrum Control Device

[0001] The present disclosure relates to an optical spectrum control device.

[0002] With the expansion of data communication networks such as the Internet, there is a demand for ever-increasing capacity in optical communication networks. To meet this expanding network demand, multi-band transmission methods that utilize multiple wavelength bands are being considered and adopted.

[0003] The multi-band transmission system is expected to utilize O-band (original band), E-band (extended band), S-band (short-wavelength band), C-band (conventional band), L-band (long-wavelength band), and U-band (ultra long-wavelength band). Of these bands, the multi-band system using C-band and L-band is beginning to be applied to practical systems.

[0004] Tadashi Sakamoto et al., "Broadband Rare Earth Doped Fiber Amplification Technology - Wideband C and L Bands," NTT Technical Journal, p. 11, October 2004.

[0005] An exemplary objective of the present disclosure is to enable control (eg, flattening) of the optical spectrum with a simple configuration without increasing cost and / or optical loss.

[0006] Therefore, an optical spectrum control device according to one aspect of the present disclosure includes an optical spectrum tuning circuit that receives output light from a gain filter that shapes the gain spectrum of an optical amplifier according to predetermined filter characteristics and tunes the optical spectrum of the output light, and a control circuit that controls the optical spectrum tuning circuit so that the output optical spectrum of the optical spectrum tuning circuit approaches a desired optical spectrum.

[0007] 1 is a diagram illustrating an example of a multi-band transmission system. It is a diagram illustrating an example of the configuration of an erbium-doped optical fiber amplifier (EDFA). It is a block diagram illustrating an example of the configuration of an optical node including an optical spectrum control device according to a first embodiment. It is a block diagram illustrating an example of the configuration of an optical node including an optical spectrum control device according to a modification of the first embodiment. It is a block diagram illustrating an example of the configuration of an optical node including an optical spectrum control device according to a second embodiment. It is a block diagram illustrating an example of the configuration of an optical node including an optical spectrum control device according to a third embodiment. It is a diagram illustrating an example of the configuration of a dynamic gain equalizer (DGE) to which a lattice filter circuit is applied according to a fourth embodiment. It is a diagram illustrating an example of the configuration of an optical monitor using an arrayed waveguide grating (AWG) according to a fifth embodiment. It is a diagram illustrating an example of an optical monitor using a plurality of Mach-Zehnder interferometers (MZIs) according to a sixth embodiment. It is a diagram schematically illustrating an example of the output optical spectrum of the optical monitor illustrated in FIG. 9. It is a diagram illustrating an example of the configuration of an optical spectrum control device according to a seventh embodiment. It is a block diagram illustrating an example of the configuration of a control circuit according to an eighth embodiment. It is a diagram illustrating an example of a look-up table (LUT) stored in the storage device illustrated in FIG. 12. It is a flowchart illustrating an example of the operation of a control circuit according to a ninth embodiment.

[0008] Hereinafter, embodiments will be described in detail with reference to the drawings. However, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims. Furthermore, more detailed descriptions than necessary may be omitted. For example, detailed descriptions of well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0009] Furthermore, in the drawings, identical or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships or ratios of elements may differ from reality. The drawings may also include portions in which the dimensional relationships or ratios differ. When numerical values ​​are used in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0010] <Summary> In multiband transmission systems, the flatness of the signal light spectrum can be impaired during transmission due to wavelength-dependent loss in the optical fiber, which is the transmission medium, or an optical nonlinear phenomenon called stimulated Raman scattering (SRS). When the flatness of the signal light spectrum is impaired, it can become difficult to achieve good transmission performance across the entire transmission wavelength band. SRS is a phenomenon in which the power of a short-wavelength signal light is transferred to a long-wavelength signal light.

[0011] Fig. 1 shows an example of a multiband transmission system, in which wavelength division multiplexing (WDM) is applied to multiband transmission. In Fig. 1, signals output from two optical transmitters 101a and 101b of different bands (wavelength bands) are wavelength-multiplexed by an optical multiplexer 102 and propagated through a first transmission fiber 103.

[0012] In the following description, it is assumed that the optical transmitter 101a transmits, for example, a C-band optical signal, and the optical transmitter 101b transmits, for example, an L-band optical signal. The number of each of the optical transmitters 101a and 101b is one or more, and in FIG. 1, for example, two each of the optical transmitters 101a and 101b are provided.

[0013] The transmission wavelengths of the two optical transmitters 101a are different from each other in, for example, the C-band, and the transmission wavelengths of the two optical transmitters 101b are different from each other in, for example, the L-band. In other words, in the example of Figure 1, the number of wavelengths multiplexed is "2" for each of the C-band and the L-band.

[0014] The signal light propagating through the first transmission fiber 103 is subject to wavelength-dependent loss in the transmission fiber 103, causing its intensity to attenuate, and deviations in the signal light intensity may occur between wavelengths in the C band and between wavelengths in the L band.

[0015] The signal light propagating through the first transmission fiber 103 is then separated into a C-band component and an L-band component by an optical filter 104 that separates signal light of different wavelength bands. The separated C-band signal light is amplified in intensity by a C-band optical amplifier 105a. The L-band signal light is amplified in intensity by an L-band optical amplifier 105b. When the optical amplifiers 105a and 105b are not to be distinguished from each other, they may be abbreviated as "optical amplifier 105."

[0016] The C-band and L-band signal lights amplified by the optical amplifiers 105a and 105b, respectively, are multiplexed by an optical filter 106 that multiplexes signal lights of different wavelength bands, propagates through a second transmission fiber 107, and is input to an optical demultiplexer 108.

[0017] The optical demultiplexer 108 separates the input signal light from the second transmission fiber 107 into signal lights of different wavelengths. Of the separated signal lights of different wavelengths, the signal light of the C-band wavelength is input to the C-band optical receiver 109a, and the signal light of the L-band wavelength is input to the L-band optical receiver 109b.

[0018] 1 exemplarily shows two optical receivers 109a and two optical receivers 109b provided corresponding to the number of optical transmitters 101a and 101b. For example, the reception wavelengths of the two optical receivers 109a are different from each other in the C-band, and the reception wavelengths of the two optical receivers 109b are different from each other in the L-band.

[0019] In the above-described example, the optical amplifier 105 may be, but is not limited to, an optical fiber amplifier in which rare earth ions such as praseodymium (Pr), erbium (Er), thulium (Tm), or ytterbium (Yb) are added to an optical fiber serving as an amplification medium.

[0020] The following describes an example in which an erbium-doped optical fiber amplifier (EDFA) is applied to the optical amplifier 105. The EDFA amplifies signal light by generating stimulated emission from a higher level to a lower level in the excited population inversion, but the gain spectrum is not necessarily flat with respect to the signal light wavelength.

[0021] Therefore, the EDFA may be provided with a gain flattening filter (GFF) for shaping (e.g., flattening) the gain spectrum. The GFF has fixed filter characteristics (or transmission characteristics) optimized to match the amplification characteristics of the EDFA, for example.

[0022] 2 is a diagram showing an example of the configuration of the EDFA 105. Signal light input to an input port 201 of the EDFA 105 is multiplexed in a WDM coupler 203 with pumping light from a pumping light source 202 of the EDFA 105, and the multiplexed signal is input to an EDF 204.

[0023] In the EDF 204, the energy of the pump light is transferred to the signal light, thereby amplifying the intensity of the signal light. The signal light output from the EDF 204 has the remaining pump light component 207 removed in the WDM coupler 206, and then propagates to the output port 210 via the isolator 208 and the GFF 209. The order of the isolator 208 and the GFF 209 may be interchanged in the light propagation direction.

[0024] The GFF 209 is designed and installed to have a loss spectrum that compensates for the gain deviation of the output signal light that may occur due to the gain spectrum of stimulated emission in the EDF 204. For example, the GFF 209 is designed to compensate for a typical profile of the gain spectrum of the EDF 204, and is formed by a dielectric multilayer film deposited on a substrate.

[0025] In an optical communication system using the EDFA 105, when long-distance transmission is performed, a plurality of optical amplifiers 105 are installed in multiple stages in the transmission direction of the signal light. In order to compensate for the transmission fiber loss according to the spacing between the optical amplifiers, the output signal light level of each optical amplifier 105 can be adjusted.

[0026] For example, in FIG. 1, the C-band optical amplifier 105a and the L-band optical amplifier 105b amplify the signal light to an intensity capable of compensating for the losses occurring in the second transmission fiber 107 for the C-band and the L-band, respectively.

[0027] Therefore, gains can be set in each optical amplifier 105 to compensate for transmission fiber losses at each of the multiple stages, where the distance of the transmission fiber may vary depending on, for example, the location where the optical amplifier 105 (or an optical node including the optical amplifier 105) can be installed.

[0028] The location where the optical amplifier 105 can be installed may vary depending on, for example, the location of a telecommunications carrier's central office, and therefore the distance of the transmission fiber may also vary depending on the location of the central office. Therefore, the transmission fiber loss compensated by the optical amplifier 105 may also vary for each optical amplifier 105.

[0029] Losses that may differ for each transmission fiber section can be appropriately compensated for by, for example, adjusting the pumping light intensity of the optical amplifier 105 for each node. However, since the pumping state of the optical amplifier 105 may change depending on the adjustment of the pumping light intensity, the gain spectrum may also change depending on the change in the pumping state. Therefore, the GFF 209, which flattens the gain spectrum of the EDF 204, has limitations in flattening the signal light spectrum.

[0030] Furthermore, in a transmission system transmitting multi-band signal light such as C-band and L-band, the wavelength-dependent loss spectrum of the transmission fiber may differ for each wavelength band. Furthermore, the influence of SRS causes optical energy to shift from the short wavelength band to the long wavelength band, which may complicate pump light control for the EDFA 105. This may impair the flatness of the signal light spectrum output from the EDFA 105. Furthermore, the pumping state may fluctuate depending on the environmental temperature in which the EDF 204 is installed, which may cause the gain spectrum to fluctuate.

[0031] Furthermore, a wavelength selective switch (WSS) may be implemented in a reconfigurable optical add / drop multiplexing (ROADM) system. The WSS has a wavelength-dependent attenuation function, and by utilizing this attenuation function, flatness of the signal light spectrum can be achieved in the ROADM.

[0032] However, in optical transmission between two points, such as point-to-point transmission, implementing a WSS whose primary function is switching is redundant and undesirable from the standpoint of cost or optical loss.

[0033] Therefore, the following describes several exemplary embodiments of an apparatus or method that can control (for example, flatten) an optical spectrum with a simple configuration without increasing cost and / or optical loss.

[0034] 3 is a block diagram showing an exemplary configuration of an optical node including an optical spectrum control device 30 according to embodiment 1. In the following description, the term "optical spectrum control device" may be interchangeably read as other terms such as "optical signal processing device" or "optical intensity control device."

[0035] The optical node illustrated in FIG. 3 illustratively includes an input port 301 , a pumping light source 302 , a WDM coupler 303 , an EDF 304 , another WDM coupler 306 , an isolator 308 , and a GFF 309 .

[0036] The pumping light source 302, the WDM coupler 303, the EDF 304, the WDM coupler 306, the isolator 308, and the GFF 309 may constitute, for example, the above-mentioned optical amplifier (for example, an EDFA) 105. The EDFA 105 may correspond to an optical amplifier for any of multiple bands, such as the C-band and the L-band.

[0037] 3 also includes a dynamic gain equalizer (DGE) 310, an optical tap 311, an optical monitor 312, a control circuit 313, and an output port 314 after the GFF 309. The DGE 310, the optical tap 311, the optical monitor 312, and the control circuit 313 exemplarily constitute an optical spectrum control device 30.

[0038] 3, signal light input to an input port 301 of the EDFA 105 is multiplexed with pump light from a pump light source 302 of the EDFA 105 in a WDM coupler 303, and then input to an EDF 304. In the EDF 304, the energy of the pump light is transferred to the signal light, thereby amplifying the intensity of the signal light.

[0039] The signal light output from the EDF 304 has residual pump light components 307 removed by the WDM coupler 306 , and is then output to the output port 314 via the isolator 308 , the GFF 309 and the DGE 310 .

[0040] The GFF 309 compensates for the optical spectrum profile (e.g., non-flatness) that may occur due to the EDF 304 having a steep gain peak near the signal light wavelength (e.g., 1530 nm). The GFF 309 is an example of an optical filter that shapes the gain spectrum of the optical amplifier 105 with predetermined filter characteristics.

[0041] In contrast, the DGE 310 compensates for (or equalizes) the non-flatness of the optical spectrum, which changes more slowly with respect to wavelength than the compensation of the optical spectrum by the GFF 309. Note that the DGE 310 is a non-limiting example of an optical spectrum tunable circuit.

[0042] For example, the non-flatness of the optical spectrum, which may be caused by a change in the gain spectrum depending on the pump light intensity, the wavelength-dependent loss spectrum of the transmission fiber, and / or the transition of optical energy from short wavelengths to long wavelengths due to SRS, varies slowly with wavelength.

[0043] Compensating for non-flatness of the optical spectrum may be understood to correspond to an example of optical spectrum control, for example, by controlling or adjusting the optical spectrum in a specific wavelength band, thereby minimizing deviation from the optical spectrum expected in that wavelength band.

[0044] The DGE 310 operates to compensate for non-flatness of the output optical spectrum of the GFF 309, for example, by a control signal generated in the control circuit 313 based on the output light of the DGE 310.

[0045] For example, a portion of the output light from the DGE 310 is power-branched in an optical tap 311 whose wavelength dependency is so small that it can be ignored, and input as monitor light to an optical monitor 312. Note that the output light from the DGE 310 that is not tapped in the optical tap 311 propagates to an output port 314.

[0046] Since the optical tap 311 is used to know the components for measuring the optical spectrum, it is sufficient if it can tap optical power with a sensitivity that can be received by the optical monitor 312. As a non-limiting example, about 1% to 5% of the output light from the DGE 310 may be tapped to the optical monitor 312 as monitor light.

[0047] The optical monitor 312 measures the spectrum of the monitor light input from the optical tap 311 and outputs an electrical signal indicating the measured spectrum to the control circuit 313. The control circuit 313 generates a control signal for the DGE 310 that can compensate for non-flatness of the measured optical spectrum according to a control method described below, and provides the control signal to the DGE 310. Alternatively, the control circuit 313 may generate a control signal for the DGE 310 so as to obtain a desired output optical spectrum, and provide the control signal to the DGE 310. For example, in cases where loss that may occur in the transmission fiber 107 needs to be compensated for in advance, the control circuit 313 may generate a control signal for the DGE 310 based on a control input.

[0048] In other words, the control circuit 313 controls the DGE 310 based on the measurement results of the optical monitor 312 or based on the control input so that the output optical spectrum of the DGE 310 approaches a desired optical spectrum. The DGE 310 compensates for the non-flatness of the optical spectrum by varying (or shaping) the output optical spectrum of the GFF 309 in accordance with the control signal from the control circuit 313. Note that if the control circuit 313 generates the control signal for the DGE 310 based on the control input, the optical monitor 312 may be unnecessary.

[0049] Thus, according to the first embodiment, the non-flatness of the optical spectrum, which has a relatively large change with respect to wavelength, is compensated for by the GFF 309, and the non-flatness of the optical spectrum, in which the steep change with respect to wavelength has been suppressed or alleviated by this compensation, is further compensated for by the DGE 310.

[0050] Therefore, the DGE 310 does not need to have the ability to compensate for, for example, abrupt changes in the optical spectrum, and the DGE 310 can be realized with a simple optical circuit configuration, and increases in costs can also be suppressed.

[0051] <Modification of First Embodiment> In the first embodiment, an example has been described in which the output optical spectrum of the DGE 310 is measured by the optical monitor 312. In the first embodiment, the optical spectrum obtained after the output optical spectrum of the optical amplifier 105 is shaped by the DGE 310 is measured by the optical monitor 312, so that the spectrum of the signal light that is finally output to the output port 314 can be known.

[0052] However, the optical monitor 312 may monitor the output light of the GFF 309. By monitoring the output light of the GFF 309 in the optical monitor 312, the optical spectrum to be compensated by the DGE 310 can be grasped more accurately than when the output light of the DGE 310 is monitored.

[0053] 4 , the optical tap 311 may be disposed between the GFF 309 and the DGE 310. The optical tap 311 measures the spectrum of the monitor light (output light of the GFF 309) input from the optical tap 311, and outputs an electrical signal indicating the measured spectrum to the control circuit 313.

[0054] The control circuit 313 generates a control signal for the DGE 310 that can compensate for the non-flatness of the measured optical spectrum according to a control method described below, and provides the control signal to the DGE 310. The DGE 310 compensates for the non-flatness of the optical spectrum by varying (or shaping) the output optical spectrum of the GFF 309 according to the control signal from the control circuit 313.

[0055] According to this modification, optical spectrum compensation by the DGE 310 is controlled based on the output light of the GFF 309 (in other words, the input light to the DGE 310), so the accuracy of optical spectrum compensation can be improved compared to the configuration illustrated in FIG. 3 .

[0056] Second Embodiment FIG. 5 is a block diagram showing an exemplary configuration of an optical node including an optical spectrum control device 50 according to a second embodiment. The optical node shown in FIG. 5 is a node that supports multi-band transmission (hereinafter, may also be referred to as a "multi-band node").

[0057] As a non-limiting example, one of the multibands is the C band and the other of the multibands is the L band. However, the second embodiment is also applicable to multiband transmission combining any two or more bands selected from the O band, the E band, the S band, the C band, the L band, the U band, and other bands.

[0058] As shown in FIG. 5, the multi-band node illustratively includes an optical filter 104 that separates signal lights of different wavelength bands, a C-band optical amplifier (for example, an EDFA) 105a, and an L-band optical amplifier (for example, an EDFA) 105b.

[0059] An optical spectrum control device 50 is provided after the EDFA 105. The optical spectrum control device 50 includes a set of a DGE 310a, an optical monitor 312a, and a control circuit 313a corresponding to the C band, and a set of a DGE 310b, an optical monitor 312b, and a control circuit 313b corresponding to the L band.

[0060] In other words, the configuration illustrated in FIG. 5 corresponds to a configuration in which optical spectrum control device 30 having the configuration illustrated in FIG. 3 is provided corresponding to each of multiple bands (e.g., C-band and L-band).

[0061] In the following description, when components are not distinguished from one another between different bands, the letters "a" and "b" attached to the reference numerals may be omitted, such as the optical amplifier 105, the DGE 310, the optical monitor 312, and the control circuit 313.

[0062] In FIG. 5, multi-band signal light containing signal light components of a plurality of different wavelength bands is input to the optical filter 104, which separates the signal light into each wavelength band (for example, into the C-band and the L-band).

[0063] The C-band signal light separated by the optical filter 104 is amplified in the C-band EDFA 105a, and the L-band signal light separated by the optical filter 104 is amplified in the L-band EDFA 105b.

[0064] Steep non-flatness of the gain spectrum that may occur due to the gain spectrum of the EDF 204 (not shown in FIG. 5) that constitutes each of the EDFAs 105 is compensated for by the GFF 309 (not shown in FIG. 5) provided in each of the EDFAs 105 for each band.

[0065] The signal light that has been amplified by the C-band EDFA 105a and whose gain spectrum non-flatness has been compensated for by the GFF 309 provided in the EDFA 105a is input to the C-band DGE 310a.

[0066] Similarly, the signal light that has been amplified by the L-band EDFA 105b and whose gain spectrum non-flatness has been compensated for by the GFF 309 provided in the EDFA 105b is input to the L-band DGE 310b.

[0067] The non-flatness that may be induced in the signal light spectrum of each band after compensation by the GFF 309 is dominated by wavelength-dependent loss or SRS of the optical fiber, and is milder than the non-flatness of the optical spectrum caused by the EDF 304.

[0068] The DGEs 310a and 310b compensate for the non-flatness of the C-band and L-band, which is gradually suppressed with respect to wavelength. When compensating for the optical spectrum, the spectra of the C-band signal light and the L-band signal light are individually monitored by the optical taps 311a and 311b corresponding to the C-band and the L-band, and the optical monitors 312a and 312b.

[0069] Based on the monitoring results for each of the C band and the L band, control circuits 313a and 313b generate control signals for compensating for the non-flatness of the optical spectrum for each of the C band and the L band. The control signal for the C band is provided to DGE 310a, and the control signal for the L band is provided to DGE 310b.

[0070] The DGEs 310a and 310b compensate for the non-flatness of the optical spectrum of each band by varying (or shaping) the optical spectrum of each of the C and L bands in accordance with control signals from the control circuits 313a and 313b, respectively.

[0071] The C-band and L-band signal lights, the non-flatness of which has been compensated for by the DGEs 310a and 310b, respectively, are multiplexed by the optical filter 106 that multiplexes wavelength bands and then output.

[0072] As will be described later, the C-band DGE 310a and optical monitor 312a may be integrated into a single package, and the L-band DGE 310b and optical monitor 312b may also be integrated into a single package.

[0073] When a set of DGE 310 and optical monitor 312 for each different band is integrated into a package, for example, a package can be provided for each band. Therefore, for both single-band optical transmission and multi-band optical transmission, optical spectrum compensation for each band can be achieved by providing an appropriate number of packages according to the number of transmission bands. This reduces the number of package types that need to be prepared, which is effective from an economical standpoint.

[0074] On the other hand, for a multi-band transmission system, the DGE 310 and the optical monitor 312 for multiple bands may be integrated into a single package, which may further improve economy.

[0075] In the configuration illustrated in FIG. 5, the output light from DGE 310 is monitored by optical monitor 312 for each different band. However, the configuration may also be such that the input light to DGE 310 is monitored by optical monitor 312, as in the modified example of embodiment 1 (FIG. 4).

[0076] <Embodiment 3> Fig. 6 is a block diagram showing an exemplary configuration of an optical node including an optical spectrum control device 60 according to embodiment 3. The optical node shown in Fig. 6 is a multi-band node like the optical node shown in Fig. 5, but differs from the configuration shown in Fig. 5 in that an optical monitor 312 and a control circuit 313 are common to the C-band and the L-band.

[0077] In the third embodiment, the multiband may be the C band and the L band, as in the second embodiment. However, the second and third embodiments are also applicable to multiband transmission in which any two or more bands selected from the O band, the E band, the S band, the C band, the L band, the U band, and other bands are combined.

[0078] In FIG. 6, multi-band signal light containing signal light components of a plurality of different wavelength bands is input to the optical filter 104, which separates the signal light into each wavelength band (for example, into the C-band and the L-band).

[0079] The C-band signal light separated by the optical filter 104 is amplified in the C-band EDFA 105a, and the L-band signal light separated by the optical filter 104 is amplified in the L-band EDFA 105b.

[0080] Steep non-flatness of the gain spectrum that may occur due to the gain spectrum of the EDF 204 (not shown in FIG. 6) that constitutes each of the EDFAs 105 is compensated for by the GFF 309 (not shown in FIG. 6) provided in each of the EDFAs 105 for each band.

[0081] The signal light that has been amplified by the C-band EDFA 105a and whose gain spectrum non-flatness has been compensated for by the GFF 309 provided in the EDFA 105a is input to the C-band DGE 310a.

[0082] Similarly, the signal light that has been amplified by the L-band EDFA 105b and whose gain spectrum non-flatness has been compensated for by the GFF 309 provided in the EDFA 105b is input to the L-band DGE 310b.

[0083] The non-flatness that may be induced in the signal light spectrum of each band after compensation by the GFF 309 is dominated by wavelength-dependent loss or SRS of the optical fiber, and is milder than the non-flatness of the optical spectrum caused by the EDF 304.

[0084] The DGEs 310a and 310b compensate for the non-flatness of the optical spectra of the C-band and L-band, which are gradually suppressed with respect to wavelength. The compensated signal light of each band is input to the optical filter 106 that multiplexes wavelength bands and is multiplexed.

[0085] The signal optical spectrum multiplexed by the optical filter 106 is monitored by an optical tap 311 and an optical monitor 312 that are common to each band. Based on the monitoring results, a control circuit 313 generates a control signal that can compensate for the non-flatness of the optical spectrum of each of the C-band and the L-band. By providing this control signal to the DGEs 310a and 310b, the non-flatness of the optical spectrum of each of the C-band and the L-band is compensated for.

[0086] As in the second embodiment, by integrating the DGEs 310a and 310b for multiple bands used in a transmission system into a single package, economic efficiency can be ensured.

[0087] As in the third embodiment, measuring the non-flatness of optical spectra of multiple bands using an optical monitor 312 common to each band can reduce the cost and / or size of the optical spectrum control device 60. Therefore, it is possible to reduce the cost and / or size of the multi-band node.

[0088] As described above, the DGE 310 does not need to be capable of compensating for, for example, abrupt changes in the optical spectrum, but only needs to be capable of compensating for optical spectra that change gradually with wavelength. Therefore, for example, a lattice filter circuit formed on a substrate, a circuit in which Mach-Zehnder interferometers are connected in multiple stages, or a transversal filter circuit can be applied to the DGE 310.

[0089] 7 shows an example of the configuration of the DGE 310 to which a lattice filter circuit 701 is applied. The lattice filter circuit 701 can be formed using, for example, a quartz-based planar lightwave circuit (PLC) or silicon photonics.

[0090] The lattice filter circuit 701 illustratively includes two parallel optical waveguides 702 and 704, and has a configuration in which a plurality of optical interferometers 705 are connected in cascade, with each optical interferometer (e.g., MZI) 705 being a unit made up of an optical coupler 706 and an interference arm 707. Note that the "interference arm" may also be referred to as an "arm waveguide."

[0091] For example, a phase shifter 703 may be loaded into both (or either) of the optical waveguides 702 and 704 that constitute each optical interferometer 705. For example, a phase shifter based on the thermo-optic effect or the electro-optic effect (e.g., a heater in the case of the thermo-optic effect) may be applied to the phase shifter 703. A control signal generated in the control circuit 313, for example, is applied to the phase shifter 703.

[0092] Here, the order of the optical circuit constituting the DGE 310 may be an order that can compensate for a change in the gain spectrum depending on the pump light intensity, a wavelength-dependent loss spectrum of the optical fiber, and / or a non-flatness that is gentler than the non-flatness of the optical spectrum caused by SRS. For example, in the case of the lattice filter circuit 701, the order of the spectral shape of the filter is determined depending on the number of stages of the multiple optical interferometers 705 connected in cascade. Therefore, the lattice filter circuit 701 may be configured using optical interferometers 705 with a number of stages that can compensate for a non-flatness that is gentler than the non-flatness of the optical spectrum caused by the above-mentioned phenomena.

[0093] For example, the lattice filter circuit 701 may be provided with an optical interferometer 705 having a sufficient number of stages to compensate for changes in the gain spectrum depending on the pump light intensity, the wavelength-dependent loss spectrum of the optical fiber, and / or non-flatness of the spectrum that may occur due to SRS.

[0094] Below is a numerical example of the case where the DGE 310 is configured by a lattice filter circuit 701 using a quartz-based PLC. The lattice filter circuit 701 has a free spectral range (FSR) that is at least twice the signal light band.

[0095] For example, since the signal light bandwidth of the C-band is about 4.8 terahertz (THz), the lattice filter circuit 701 only needs to have an FSR of about 20 THz, which is twice that bandwidth. If the equivalent refractive index of the optical waveguides 702 and 704 is expressed as n, the representative wavelength of the signal light (e.g., the central wavelength of the C-band), and the speed of light in a vacuum as C0, then the path length difference L of the optical interferometer 705 can be expressed as L=nC0 / λFSR.

[0096] Therefore, when an FSR of, for example, about 20 THz is imparted to the lattice filter circuit 701, the optical interferometer 705 may be set to have an optical path length difference of about L = 5 micrometers (μm) in the case of a lattice filter made of, for example, a quartz-based planar lightwave circuit (PLC).

[0097] Furthermore, the change in gain spectrum depending on the pump light intensity, the wavelength-dependent loss spectrum of the optical fiber, and / or the non-flatness of the optical spectrum that may be caused by SRS have linear properties such as periodically repeating monotonic increases or decreases with wavelength.

[0098] Therefore, the non-flatness can be expressed as a loss spectrum with a sawtooth or triangular wave shape relative to the wavelength. Therefore, by adding higher-order components, for example, third order or higher, to the frequency periodicity of the lattice filter circuit 701, it is possible to improve the reproducibility of the optical spectrum that can compensate for the loss spectrum.

[0099] For example, in the case of a lattice filter circuit 701 using a quartz-based planar lightwave circuit (PLC), adding an optical interferometer 705 with an optical path length difference L of approximately 10 μm can provide an FSR (or spectral component) with a period of 10 THz.

[0100] Here, by adding the optical interferometer 705 with an optical path length difference L=10 μm, an FSR (or spectral component) corresponding to an optical path length difference of L=5+10=15 μm is also imparted. Therefore, for example, in addition to the spectral components with a 20 THz period and a 10 THz period, a spectral component with a 7 THz period is also imparted in the lattice filter circuit 701.

[0101] The two types of optical interferometers 705 having different optical path length differences L described above can be arranged, for example, one between three optical interferometers 705 (e.g., optical interferometers with L = 0) having path length differences L such that wavelength dependency is negligible in the signal light wavelength band.

[0102] As a non-limiting example, the first optical interferometer 705 has L = 0, the second optical interferometer 705 has L = 5 μm, the third optical interferometer 705 has L = 0, the fourth optical interferometer 705 has L = 10 μm, and the fifth optical interferometer 705 has L = 0. Note that the numerical example regarding the optical path length difference L is merely an example, and other numerical example may also be applied.

[0103] Such an arrangement makes it possible to adjust the contribution ratios of spectral components of a plurality of different periods (for example, 20 THz, 10 THz, 7 THz) in the lattice filter circuit 701.

[0104] Fifth Embodiment Fig. 8 is a diagram showing an exemplary configuration of the aforementioned optical monitor 312. As shown in Fig. 8, the optical monitor 312 illustratively includes an arrayed waveguide grating (AWG) 801 formed on a substrate and a plurality of optical receivers 807.

[0105] The AWG 801 includes, for example, an input waveguide 802, a first slab waveguide 803, an arrayed waveguide 804, a second slab waveguide 805, and a plurality of output waveguides 806. The signal light output from each of the output waveguides 806 is received by an optical receiver 807.

[0106] Signal light (monitor light) partially branched by the optical tap 311 described above is input to the input waveguide 802. The monitor light input to the input waveguide 802 is guided to the first slab waveguide 803 and distributed to the arrayed waveguides 804 by the first slab waveguide 803.

[0107] Each monitor light wave guided through the arrayed waveguide 804 is wavelength-demultiplexed into a plurality of output waveguides 806 via a second slab waveguide 805. The wavelength-demultiplexed monitor light waves are coupled to photodetectors 807 provided corresponding to the output waveguides 806.

[0108] Each of the optical receivers 807 measures the optical intensity for each different wavelength according to the demultiplexing performance of the AWG 801. This allows the change in optical intensity with respect to wavelength in the wavelength band of the signal light, that is, the optical spectrum, to be measured.

[0109] Here, the optical monitor 312 provides a spectrum measurement function for the signal light, and the number of wavelengths to be measured (hereinafter also referred to as the “wavelength resolution”) may be sufficient to measure, for example, optical spectrum changes that are more gradual than the steep optical spectrum changes caused by the EDF 304.

[0110] Therefore, the number of output waveguides 806 and photodetectors 807 may be sufficient to measure, for example, the change in gain spectrum depending on the pump light intensity, the wavelength-dependent loss spectrum of the optical fiber, and / or the non-flatness of the spectrum that may occur due to SRS.

[0111] Furthermore, the number of output waveguides 806 and photodetectors 807 may be determined in relation to the order of the lattice filter circuit 701. For example, the optical monitor 312 does not need a wavelength resolution that exceeds the wavelength resolution of the loss spectrum that can be generated by the DGE 310 using the lattice filter circuit 701.

[0112] For example, the wavelength resolution of the optical monitor 312 is sufficient as long as it can fully reproduce the order of the lattice filter circuit 701 (for example, in the above-mentioned example, the third order of 20 THz, 10 THz, and 7 THz), and a wavelength resolution exceeding this order is not necessary. For example, if the order of the lattice filter circuit 701 is represented by N (N is an integer equal to or greater than 1), the wavelength resolution M (M is an integer equal to or greater than 1) of the optical monitor 312 is sufficient as long as it satisfies the relationship M≧N.

[0113] Furthermore, the configuration of the optical monitor 312 is not limited to the above-described AWG 801. For example, a combination of multiple MZIs with different interference arm lengths, or a transversal wavelength separation configuration may alternatively be applied to the optical monitor 312. When the AWG 801 is used for the optical monitor 312, it is easier to increase the number of wavelengths resolved compared to a configuration using multiple MZIs, which will be described later, and it is also superior in the ability to remove optical components of wavelengths different from the wavelength of the measurement target.

[0114] Sixth Embodiment Fig. 9 is a diagram showing an example of an optical monitor 312 configured by combining a plurality of MZIs in multiple stages. The optical monitor 312 shown in Fig. 9 is, for example, a multi-stage MZI circuit 911 formed on a substrate. Note that, in the sixth embodiment, two-stage wavelength separation will be described, but the number of stages may be more than two as long as the above-mentioned M is satisfied. Also, M may be 1.

[0115] The multi-stage MZI circuit 911 includes, for example, a first-stage MZI 912 and a second-stage MZI 913. The first-stage MZI 912 illustratively has an input waveguide 914, two optical directional couplers 915 and 917, and an arm waveguide 916 located between the optical directional couplers 915 and 917.

[0116] The signal light input to the input waveguide 914 is input to the first-stage MZI 912. In the first-stage MZI 912, the two waveguides that make up the arm waveguide 916 have different path lengths, and demultiplexed signal light is output to two outputs 920a and 920b of the MZI 912 in accordance with the path lengths.

[0117] For example, one output 920a outputs the light intensity of the shaded portion of the optical spectrum 1012a illustrated in FIG. 10, and the other output 920b outputs the light intensity of the unshaded white portion of the optical spectrum 1012a.

[0118] 9, the second-stage MZI 913 is connected to each of two outputs 920a and 920b of the first-stage MZI 912. Each of the second-stage MZIs 913 is different from the first-stage MZI 912 in the path length difference between the two waveguides that make up the arm waveguide.

[0119] For example, in each of the second-stage MZIs 913, the path length difference between the two waveguides that constitute the arm waveguide may be set to about half the path length difference between the two waveguides that constitute the arm waveguide 916 of the first-stage MZI 912. By setting the path length difference in this way, the output of the second-stage MZI 913 can be, for example, the output shown in optical spectrum 1013a in FIG.

[0120] A total of four outputs from the two second-stage MZIs 913 are received by four photodetectors 918a to 918d in Fig. 9, for example, and the intensity for each wavelength is measured. Here, for example, the photodetector 918a receives the signal light intensity of an optical spectrum 1019 that corresponds to the product of the shaded portion of the optical spectrum 1013a and the shaded portion of the optical spectrum 1012a in Fig. 10.

[0121] The optical monitor 312 configuration using the above-described multistage MZI circuit 911 can be easily integrated onto a single optical circuit chip together with the lattice filter circuit 701 described in the fourth embodiment (FIG. 7), for example, in terms of layout, which can contribute to the miniaturization of the optical spectrum control device.

[0122] Seventh Embodiment For example, as shown in FIG. 11 , it is possible to form (or integrate) a DGE 310 using the lattice filter circuit 701 illustrated in FIG. 7 and an optical monitor 312 using the multistage MZI circuit 911 illustrated in FIG. 9 on a single substrate 1150 constituting an optical circuit chip.

[0123] 11, an optical switch 1151 that selectively inputs a portion of the input and a portion of the output of DGE 310 to DGE 312 (lattice filter circuit 701) may be provided on substrate 1150. A portion of the input to DGE 310 is branched, for example, by optical tap 1111a, and a portion of the output of DGE 310 is branched, for example, by optical tap 1111b.

[0124] With this configuration, the optical switch 1151 selectively inputs the optical spectrum of the input or output of the DGE 310 to the optical monitor 312. Therefore, as described in the first embodiment (FIG. 3) or the modification of the first embodiment (FIG. 4), optical spectrum monitoring at different positions, i.e., the input or output of the DGE 310, can be realized by the common optical monitor 312. This allows, for example, downsizing and / or efficient control of the optical spectrum control device.

[0125] Whether the input or output of the DGE 310 is monitored may be set statically, or may be switched between semi-statically and dynamically (e.g., time-divisionally). For example, in a first time interval, the optical spectrum control by the DGE 310 may be performed based on the input light monitoring, and in a second time interval, it may be confirmed based on the output light monitoring whether the output optical spectrum of the DGE 310 matches the desired optical spectrum.

[0126] Eighth Embodiment In an eighth embodiment, a configuration example of the control circuit 313 will be described. Fig. 12 is a block diagram showing an exemplary configuration of the control circuit 313 according to the eighth embodiment. As shown in Fig. 12, the control circuit 313 illustratively includes a processor 3131, a storage device 3132, and a driver 3133. Note that the DGE 310 is provided with a temperature sensor 315, and temperature information detected by the temperature sensor 315 is provided to the processor 3131.

[0127] The received light intensity information for each wavelength measured by the optical monitor 312 is input as a digital signal to the processor 3131 via an analog-to-digital converter (ADC) or the like (not shown). The processor 3131 may be a central processing unit (CPU), an MPU (micro processing unit), or any other processing device having computing capabilities.

[0128] The processor 3131 generates a control signal from the measured spectrum using a method described below. The generated control signal is supplied to the DGE 310 via a driver 3133 for controlling the DGE 310.

[0129] The driver 3133 may have any configuration suitable for driving the DGE 310. For example, if the DGE 310 is a lattice filter circuit 701 that utilizes the thermo-optical effect, the driver 3133 may be a current driver that drives a phase shifter (heater) using the thermo-optical effect with a current.

[0130] When generating a control signal based on the input measured spectrum, the processor 3131 references the storage device 3132 in which control information for the DGE 310 is stored to generate the control signal.

[0131] An example of generating the control signal will now be described. A plurality of sets of control signals that provide a predetermined typical spectrum are stored in the storage device 3132, for example, as a look-up table (LUT). In the case of the lattice filter circuit 701 described above, the optical interferometer 705 has five stages, so one control signal can include, for example, five numerical values ​​(control values).

[0132] Furthermore, since optical interferometers using silica-based planar lightwave circuits (PLCs) or silicon photonics have temperature dependence due to the thermo-optic constants of the materials, the control signals stored in memory device 3132 may include sets of control values ​​for each temperature.

[0133] The processor 3131 searches the memory device 3132 for a set of control values ​​that are close to the temperature of the DGE 310 indicated by the temperature sensor 315 and that give an optical spectrum that is closest to the measured optical spectrum, and determines the control signal to be given to the DGE 310.

[0134] Alternatively, the processor 3131 may search the memory device 3132 for multiple sets of control signals that are close to the temperature of the DGE 310 indicated by the temperature sensor 315 and that give an optical spectrum close to the measured optical spectrum, and determine the control signal by calculating them.

[0135] Fig. 13 is a diagram showing an example of an LUT stored in the storage device 3132 shown in Fig. 12. Fig. 13 shows an example in which an LUT for each of K (#1 to #K) optical interferometers 705 (see Fig. 7) is stored in the storage device 3132.

[0136] For example, each LUT may store measurement values ​​for each of multiple wavelengths (monitored wavelengths) that the optical monitor 312 can indicate and control values ​​for each of multiple temperatures that the temperature sensor 315 can indicate, in correspondence (or association).

[0137] Note that the numerical values ​​in each LUT illustrated in FIG. 13 are not limiting and are merely examples. Furthermore, it is not necessary to prepare an infinite number of LUTs. It is preferable to determine a typical optical spectrum according to the optical transmission system, and considering manufacturing errors, it is preferable to determine the value of the control signal for each individual DGE 310. Additionally, the optical spectrum change due to wavelength-dependent loss and / or SRS of the optical fiber, or its tendency, is known. Therefore, it is sufficient to prepare a certain number of LUTs that are appropriate for the tendency of the optical spectrum change, for example.

[0138] The method for determining (e.g., calculating) the control signal is described below, assuming that DGE 310 is a K-stage lattice filter circuit 701 and optical monitor 312 outputs (or indicates) the measurement results for the spectrum of four wavelengths.

[0139] (1) First, the control circuit 313 (e.g., the processor 3131) extracts, from the LUT shown in FIG. 13, at least one row of control values ​​at temperatures on either side of the temperature T of the DGE 310 detected by the temperature sensor 315 for each optical interferometer 705. For example, when T=45° C., two rows of 40° C. and 50° C., shown shaded in FIG. 13, are extracted. These two temperature values ​​are designated T1 and T2, respectively.

[0140] (2) Next, the control circuit 313 extracts, for example, at least one column from the LUT of each optical interferometer that has a label closest to the measurement value at each wavelength indicated by the optical monitor 312. The unit of the measurement value by the optical monitor 312 may be, for example, optical power (watts, W) or the output current value of the optical receiver (amperes, A).

[0141] For example, assume that the measured values ​​indicated by the optical monitor 312 are "-2.5" at "wavelength 1," "-1.5" at "wavelength 2," "-0.75" at "wavelength 3," and "0" at "wavelength 4." In this case, the values ​​of the columns in the LUT that sandwich these four values, for example, the columns shown with bold frames in Figure 13, are extracted. These two columns are labeled C1 and C2, respectively.

[0142] (3) Then, the control circuit 313 derives a control value for the DGE 310 based on the value extracted from the LUT. For example, the control circuit 313 determines the control value Vi to be provided to the i-th optical interferometer 705-i using the interpolation formula shown in the following formula (1), where f(T, C) is the value of the LUT at temperature T and label C.

[0143] Although Equation (1) indicates that the control value V is calculated as the intermediate value of the values ​​constituting the LUT, the present disclosure is not limited thereto. For example, the control value V may be calculated as a weighted ratio based on the temperature T and / or the reading of the light monitor 312, or may be calculated using a higher-order interpolation formula by extracting temperatures from three or more rows or labels from three or more columns of the LUT.

[0144] (4) Finally, the control circuit 313 sequentially supplies the obtained control value Vi to each of the optical interferometers 705-i via the driver 3133.

[0145] The control circuit 313 repeats the above steps (1) to (4) at a control period Tc appropriate for the system in which the DGE 310 is installed. Considering that the probability of a sudden change in temperature in a general temperature environment is low, the control period Tc may be, for example, once every several tens of seconds to several minutes.

[0146] In the above example, the indicated value of the optical monitor 312 is used as the input to the control circuit 313, but other control inputs may also be used. For example, an input from a user may be given as a control input to the control circuit 313. This also applies to the following ninth embodiment.

[0147] Ninth Embodiment In a ninth embodiment, a method for determining a control value Vi without using an LUT based on circuit parameters stored in the storage device 3132 will be described using the lattice filter circuit 701 described in the fourth embodiment as an example, with reference to the flowchart shown in Fig. 14. Note that "circuit parameters" may be interchangeably read as "device parameters" or "device model," for example.

[0148] As in the eighth embodiment, the control circuit 313 derives the control value Vi by referring to the temperature T indicated by the temperature sensor 315 installed in the DGE 310 and the measurement value for each wavelength from the optical monitor 312 .

[0149] The circuit parameters may include, by way of non-limiting example, the group refractive index, refractive index dispersion, and temperature dependence of the equivalent refractive index of the interference arm 707 in the lattice filter circuit 701, and / or the wavelength dependence of the coupling rate of the optical coupler 706 that constitutes the interferometer. The circuit parameters may be stored in the storage device 3132 as pre-acquired data.

[0150] Based on the circuit parameters, the control circuit 313 can derive, for example, the transmission characteristic Ic of the lattice filter circuit 701 when a control value Vi is applied at a temperature T. For example, a vector E representing the electric field of "output 1" and "output 2" of the lattice filter circuit 701 illustrated in FIG. 7 is expressed by the following equation (2):

[0151] In this equation (2), “Ci” (i is an integer equal to or greater than 0) is a 2×2 transfer matrix representing the directional coupler (optical coupler) 706 located at the i-th position from the input side of the lattice filter circuit 701, and is expressed, for example, by the following equation (3).

[0152] In addition, in equation (2), "Ai" is a 2x2 transfer matrix representing the optical propagation in the interference arm 707 located at the i-th position from the input side of the lattice filter circuit 701, and is expressed, for example, by the following equation (4).

[0153] In equation (3), “φ” represents the phase angle that changes depending on the coupling length of the directional coupler 706, and in equation (4), “β” represents the propagation constant of the interference arm 707, and “L” represents the path length difference of the interference arm 707.

[0154] The transmission characteristic Ic is derived from the formula (2) by the following formula (4.1), for example: where (1) in the formula (4.1) represents the first component of the vector indicated by E in the formula (2), * represents the complex conjugate.

[0155] Next, the control circuit 313 derives a target spectrum (which may also be referred to as a "loss spectrum") It of the DGE 310 that can compensate for the current optical spectrum based on the measured values ​​for the wavelengths measured by the optical monitor 312 (S141 in FIG. 14).

[0156] For example, the control circuit 313 calculates the evaluation function F expressed by the following equation (5) using variables Vi (i = 1 to K), and performs calculations sequentially so that the evaluation function F is minimized or the error ε is within an acceptable range (S142 in Figure 14).

[0157] For example, optimization methods such as the steepest descent method and / or the annealing method can be applied to this optimization calculation. Furthermore, without being limited to this example, the evaluation function F may be, for example, the following equation (6), which weights wavelengths at which the optical signal-to-noise ratio (OSNR) is likely to deteriorate (e.g., wavelengths at which the input optical power is relatively small). In equation (6), W = Σw(i).

[0158] 14, the control circuit 313 repeats the optimization calculation until the evaluation function F is minimized or the error ε is within the allowable range (S143; NO). If the evaluation function F is minimized or the error ε is within the allowable range (S143; YES), the control circuit 313 provides the finally obtained set of control values ​​Vi to the DGE 310 as a control signal (S144).

[0159] A first advantage of the ninth embodiment is that, unlike the eighth embodiment, it is possible to generate a target spectrum that is not included in the LUT shown in Fig. 13 and provide it to the DGE 310. A second advantage of the ninth embodiment is that it is possible to further reduce errors with respect to the target spectrum that may occur by the interpolation method shown in the eighth embodiment.

[0160] Furthermore, in the ninth embodiment, instead of actually finding a target spectrum by changing the control values ​​of the actual DGE 310 and conducting trials, the control values ​​are found in the processor 3131 based on the circuit parameters of the DGE 310 stored in advance in the storage device 3132 and are provided to the DGE 310. Therefore, for example, since it is not necessary to actually control the optical communication system in the control value search, a third advantage is obtained in that no effect is caused on the actual signal during optical communication operation. In other words, it is possible to avoid the occurrence of an unexpected spectrum when the optical communication system is actually controlled.

[0161] Tenth Embodiment In a tenth embodiment, the above-mentioned method of deriving (or identifying) circuit parameters will be described. In the control method described in the ninth embodiment, it is expected that the circuit parameters of the lattice filter circuit 701 can be derived with high accuracy and used to derive control values.

[0162] In deriving the circuit parameters, for example, an appropriate control signal is applied to at least one of all control terminals of the manufactured DGE 310, and the output optical spectrum is measured. In the case of the lattice filter circuit 701, power is applied as an appropriate control signal to at least one of the phase shifters 703 corresponding to the control terminals, and the output optical spectrum is measured.

[0163] Here, in order to identify multiple circuit parameters such as the thermo-optic constant, group refractive index, refractive index dispersion of the optical waveguide, and wavelength characteristics of the directional coupler, it is preferable to evaluate by applying multiple control signals. This set of control signals is defined as a set of vectors c, {c j} (j=0 to p).

[0164] c j In the example of FIG. 7, there are 10 phase shifters 703, so c is expressed as a vector with 10 elements. j The evaluation of the optical spectrum by applying the above equation is hereinafter referred to as a trial.

[0165] From the viewpoint of manufacturability, it is preferable to perform as few trials as possible. Furthermore, when the lattice filter circuit 701 is configured using a silica-based PLC or silicon photonics, it is preferable to perform trials at a plurality of environmental temperatures, because the equivalent refractive index of the optical waveguide may change depending on the environmental temperature via the thermo-optic effect.

[0166] Therefore, the set with fewer elements {c j} is expected to enable accurate parameter identification (or parameter fitting). To accurately identify circuit parameters, it is preferable that the functions representing the optical spectrum in each trial are nearly independent of each other. An example of a method for expressing this nearly independent state is shown below.

[0167] A vector whose elements are the circuit parameters to be identified is represented by n (the number of elements is r, i.e., the number of parameters to be identified is r), and the control value c j The spectrum obtained in the trial when l ,c j ) is expressed as

[0168] S(λ l ,c j ) are wavelengths λ0, λ1, λ2, ..., λ q For each of the control values ​​c0, c1, c2, ..., c p represents a vector whose elements are the light intensities measured (sampled) by the light monitor 312 when a given value is given. Note that q elements are obtained in one trial (spectrum scan).

[0169] Here, for r elements of vector n, S(λ l ,c j ) is partially differentiated, and the Jacobian J is defined as shown in the following equation (7).

[0170] The Jacobian J is a matrix with q×p rows and r columns. Using this matrix, the condition number κ is defined by the following equation (8).

[0171] In addition, in the formula (8), represents the norm of the vector. S is the sum of S(λ l ,c j ) arranged in the column direction.

[0172] Here, the smaller the condition number κ is (in other words, the smaller the error), the higher the accuracy of the solution (in other words, the smaller the error). Therefore, for example, a set of control values ​​{c j} is preferably used to identify the circuit parameters.

[0173] For example, the control circuit 313 determines a set of control values ​​{c j}, and then find the set {c j} based on each c j Then, a list of circuit parameters that reproduce the measured optical spectrum is derived by optimization using a method similar to that described in the ninth embodiment. Note that the circuit that derives the circuit parameters is not limited to the control circuit 313. For example, the circuit parameters may be derived by a computing device, such as a personal computer, separate from the control circuit 313, and provided to the control circuit 313 (for example, stored in the storage device 3132).

[0174] In the above-described embodiments, examples have been described in which the target of optical spectrum control by the DGE 310 is the output light of the optical amplifier 105 (e.g., the output light of the GFF 309), but the present disclosure is not limited to this. For example, the target of optical spectrum control by the DGE 310 may be the input light to the optical amplifier 105.

[0175] Furthermore, the optical spectrum control of the present disclosure is not limited to control that compensates for or equalizes the non-flatness of the optical spectrum, but may be control that minimizes deviation from the optical spectrum expected in a specific wavelength band, for example.

[0176] The term "connect" used in this disclosure may be read as "coupled." "Connected" or "coupled" may be understood to mean any direct or indirect "connection" or "coupling" between two or more elements. For example, the term may also be understood to include an indirect "connection" or "coupling" where one or more intermediate elements are interposed between two elements that are "connected" or "coupled" to each other.

[0177] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0178] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0179] The present disclosure is useful, for example, in optical communication technology.

Claims

1. An optical spectrum control device comprising: an optical spectrum tuning circuit that receives output light from a gain filter that shapes the gain spectrum of an optical amplifier according to predetermined filter characteristics and tunes the optical spectrum of the output light; and a control circuit that controls the optical spectrum tuning circuit so that the output optical spectrum of the optical spectrum tuning circuit approaches a desired optical spectrum.

2. The optical spectrum control device according to claim 1, further comprising an optical monitor that monitors the input light or output light of said optical spectrum tunable circuit, wherein control of said optical spectrum tunable circuit by said control circuit is based on the measurement results of said optical monitor.

3. The optical spectrum control device according to claim 2, wherein the optical spectrum tunable circuit is a lattice filter circuit having frequency periodicity of order N (N is an integer of 1 or greater), and the wavelength resolution M (M is an integer of 1 or greater) of the object to be measured by the optical monitor satisfies M≧N.

4. The optical spectrum control device of claim 1, wherein the optical spectrum tuning circuit includes a temperature sensor, and the control circuit includes a processor, a memory device storing control values ​​for the optical spectrum tuning circuit for each of a plurality of wavelengths and a plurality of temperatures associated with the desired optical spectrum, and a driver for driving the optical spectrum tuning circuit, and the processor is configured to determine the control value to be provided to the optical spectrum tuning circuit from the driver based on the temperature indicated by the temperature sensor, a measurement result of the optical spectrum measured for input light or output light of the optical spectrum tuning circuit or a control input, and the control value stored in the memory device.

5. An optical spectrum control device according to claim 1, wherein the optical spectrum tuning circuit includes a temperature sensor; the control circuit includes a processor, a storage device storing one or more circuit parameters related to the optical spectrum tuning circuit, and a driver for driving the optical spectrum tuning circuit; the processor determines the transmission characteristics of the optical spectrum tuning circuit based on the temperature indicated by the temperature sensor and the circuit parameters, and determines a control value to be provided to the optical spectrum tuning circuit from the driver that minimizes the difference between the desired spectrum corresponding to the transmission characteristics and the measurement result of the optical spectrum measured for input light or output light of the optical spectrum tuning circuit or a control input.

6. An optical spectrum control device according to claim 5, wherein the circuit parameters are identified from an optical spectrum measured when the optical spectrum tuning circuit is controlled using a set of control values ​​based on the circuit parameters, and the set of control values ​​is a set such that a condition number related to the transmission characteristics when the set of control values ​​is applied to the optical spectrum tuning circuit is small enough to satisfy the reproducibility accuracy of the measured optical spectrum.

7. An optical spectrum control device according to claim 1, wherein said optical spectrum tunable circuit has a free spectral range at least twice as large as the optical amplification band of said optical amplifier.

8. An optical spectrum control device according to claim 1, wherein a plurality of said optical spectrum tunable circuits are provided corresponding to a plurality of said optical amplifiers provided for a plurality of wavelength bands, respectively.

9. The optical spectrum control device according to claim 2, wherein the optical spectrum tunable circuit is a dynamic gain equalizer using a lattice filter circuit, the optical monitor has a configuration in which a plurality of Mach-Zehnder interferometers are connected in multiple stages, and the lattice filter circuit and the plurality of Mach-Zehnder interferometers are integrated on the same substrate.

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