Optical spectrum control device

Abstract

An exemplary optical spectrum control device according to the present disclosure comprises: an optical spectrally tunable circuit that tunes the optical spectrum of input light; an optical switch that selectively outputs the input light or output light of the optical spectrally tunable circuit; an optical monitor that monitors the light selected by the optical switch; and a control circuit that controls the optical spectrally tunable circuit on the basis of a measurement result of the optical monitor. The optical monitor includes: a wavelength filter that selectively outputs the light selected by the optical switch in units of wavelengths; and a light receiver that receives the light of each wavelength selected by the wavelength filter and detects the light intensity. The control circuit controls the selective outputs by the optical switch and the wavelength filter to cause the light receiver to receive light of each wavelength individually, thereby acquiring information about the wavelength distribution of light intensity.

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, optical communication networks are expected to have even higher capacity. 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 - Widening of C-Band and L-Band Amplification," NTT Technical Journal, p. 11, October 2004. K. Suzuki, et al., "Ultra Wide Range Dynamic Gain Equalizer with High Contrast Silica Planar Lightwave Circuit," in Integrated Photonics Research, A. Sawchuk, ed., Vol. 78 of OSA Trends in Optics and Photonics (Optica Publishing Group, 2002), paper IThG2.

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

[0006] According to one aspect of the present disclosure, an optical spectrum control device includes an optical spectrum tuning circuit that tunes the optical spectrum of input light, an optical switch that selectively outputs the input light or output light of the optical spectrum tuning circuit, an optical monitor that monitors the light selected by the optical switch, and a control circuit that controls the optical spectrum tuning circuit based on measurement results of the optical monitor. The optical monitor includes a wavelength filter that selectively outputs the light selected by the optical switch on a wavelength-by-wavelength basis, and a photoreceiver that receives the light of the wavelength selected by the wavelength filter and detects its optical intensity. The control circuit controls the selective outputs of the optical switch and the wavelength filter to cause the photoreceiver to receive light wavelength by wavelength, thereby obtaining wavelength distribution information of the optical intensity in the wavelength band.

[0007] 1 is a diagram illustrating an example of a multi-band transmission system. 2 is a diagram illustrating an example of the configuration of an erbium-doped optical fiber amplifier (EDFA). 3 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. 4 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. 5 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. 6 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. 7 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. 8 is a diagram illustrating an example of an optical monitor using an arrayed waveguide grating (AWG) according to a fifth embodiment. 9 is a diagram illustrating an example of an optical monitor using a plurality of Mach-Zehnder interferometers (MZIs) according to a sixth embodiment. 10 is a diagram illustrating an example of the output optical spectrum of the optical monitor illustrated in FIG. 9. 11 is a diagram illustrating an example of the configuration of an optical spectrum control device according to a seventh embodiment. 12 is a diagram illustrating an example of an optical monitor using an AWG according to an eighth embodiment. 13 is a diagram illustrating an example of an optical spectrum control device including an optical monitor using an MZI according to a ninth embodiment. 10 is a diagram showing an exemplary configuration of an optical spectrum control device including an optical monitor using an MZI according to embodiment 10. FIG. 11 is a diagram showing an exemplary configuration of an optical spectrum control device including an optical monitor using a wavelength tunable filter according to embodiment 11.

[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, for example, in the C-band, and the transmission wavelengths of the two optical transmitters 101b are different from each other, for example, in 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 signal light in multiple bands, such as the C-band and the 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. As a result, the flatness of the signal light spectrum output from the EDFA 105 may be impaired.

[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 non-flatness of the optical spectrum 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 using 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 negligible, 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 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, 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 in accordance with 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 of the optical spectrum 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 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 constituted by 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 an optical path length difference of, for example, L=about 5 micrometers (μm).

[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, by adding an optical interferometer 705 with an optical path length difference L of about 10 μm to the lattice filter circuit 701, an FSR (or spectral component) with a period of 10 THz can be imparted.

[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 Furthermore, when the change in the spectrum to be compensated for is gradual over time, it is possible to reduce the number of photodetectors for optical monitoring by combining, for example, an optical switch and a wavelength filter. An example of this will be described below.

[0127] The wavelength multiplexer also functions as a wavelength demultiplexer (or wavelength filter) that can extract different wavelengths from each port by inputting light in the opposite direction. For example, as shown in Figure 12, the AWG 801 illustrated in Figure 8 is used with the input and output interchanged. In this case, each of the multiple waveguides 806 functions as an input waveguide (or input port), and the waveguide 802 functions as an output waveguide (or output port).

[0128] An optical switch 1201 that selects which of the multiple input ports 806 to input light to is provided, for example, in the upstream stage of the AWG 801. By selectively switching the input port 806 to which light is input using the optical switch 1201, it becomes possible to change the wavelength of light received (or measured or detected) by the photodetector 807.

[0129] Therefore, with a minimum of one photodetector 807, the control circuit 313 can acquire light intensity information for each wavelength in the wavelength band to be measured, in other words, optical spectrum information, which is wavelength distribution information of light intensity.

[0130] Ninth Embodiment Instead of the AWG 801 illustrated in FIG. 12, the optical monitor 312 may be configured to use, for example, a combination of a plurality of MZIs with different interference arm lengths, or a transversal wavelength separation configuration.

[0131] FIG. 13 shows an example in which an optical monitor 312 is configured by a multistage MZI circuit 911 in which a plurality of MZIs are combined in multiple stages (for example, connected in cascade), similar to the configuration exemplified in the sixth embodiment (FIG. 9) or the seventh embodiment (FIG. 11).

[0132] The optical monitor 312 shown in Fig. 13 corresponds to a configuration in which the input and output are interchanged and light is input in the opposite direction in the configurations illustrated in Fig. 9 and Fig. 11. As shown in Fig. 13, a DGE 310 using a lattice filter circuit 701, an optical monitor 312 using a multistage MZI circuit 911, optical taps 1311a and 1311b, and optical switches 1351 and 1352 can be formed on a single substrate 1350 that constitutes an optical circuit chip.

[0133] The optical switch 1351 is, for example, a two-input, one-output (2x1) optical switch for selecting which of the two monitor lights branched by the optical taps 1311a and 1311b provided at the input and output of the DGE 310 is to be input to the optical monitor 312.

[0134] The optical switch 1352 is, for example, a 1-input K-output (1×K) optical switch for selecting, in wavelength units, to which input port of the optical monitor 312 the monitor light input from the optical switch 1351 is to be coupled, where K is an integer of 2 or more and corresponds to the number of wavelengths.

[0135] Each of the optical switches 1351 and 1352 can be configured by a combination of MZIs. A phase shifter 1303 using, for example, the thermo-optic effect or the electro-optic effect is loaded on both (or one) of the interference arms configuring each MZI, similar to the lattice filter circuit 701. A control signal for selecting the light or wavelength to be input to the optical monitor 312 is generated by, for example, the control circuit 313 described above and applied to the phase shifter 1303.

[0136] With this configuration, either one of the monitor lights input from the optical taps 1311a and 1311b is selected by the optical switch 1351, and the input of the optical monitor 312 corresponding to the wavelength to which the selected monitor light is to be coupled is selected by the optical switch 1352. Therefore, it is possible to selectively measure the monitor light intensity of any wavelength using the two monitor lights and the photodetector 1307 common to multiple wavelengths.

[0137] For example, the control circuit 313 switches the optical switch 1352 to cause the optical receiver 1307 to receive the input or output light of the DGE 310 as monitor light for each wavelength, thereby measuring the received light intensity. The control circuit 313 stores the measurement result, that is, received light intensity information, in, for example, a storage device (not shown).

[0138] The control circuit 313 repeatedly controls the switching of the optical switch 1352 for each wavelength until finally obtaining received light intensity information for the multiple wavelengths to be monitored. This allows the optical receiver 1307, which is common to each wavelength, to obtain wavelength distribution information of the light intensity in the band to be monitored, i.e., optical spectrum information. Based on the optical spectrum information, the control circuit 313 controls, for example, compensation of spectral non-flatness by the DGE 310.

[0139] As described above, according to the ninth embodiment, unlike the case where multiple photodetectors are used corresponding to the number of wavelengths to be monitored, it is not necessary to process the received light intensity information all at once. Also, the electrical circuit for converting the received light intensity of the photodetector 1307 into an electrical signal may be common to each wavelength. Therefore, the processing load, size, and / or cost of the control circuit 313 can be reduced.

[0140] Even when using an optical monitor 312 using an AWG 801 as shown in FIG. 12, it is possible to integrate each component into a single optical circuit chip by forming it using an optical waveguide.

[0141] However, as in the present embodiment 9, by configuring the DGE 310, the optical monitor 312, and the optical switches 1351 and 1352 by combining MZIs, it is possible to unify the requirements in the manufacturing process of each element, which has the advantage of making it easier to integrate them into one optical circuit chip.

[0142] 14, the optical tap 1311b may be omitted in the configuration illustrated in FIG. 13. For example, by monitoring the light of an unused output of the two outputs (ports) of the DGE 310 by the lattice filter circuit 701, the optical receiver 1307 can selectively measure the monitor light intensity of an arbitrary wavelength.

[0143] For example, light other than the loss generated in the DGE 310 is output to either "output 1" or "output 2" in Fig. 14. Therefore, when the loss in the DGE 310 is negligibly small or known, the value obtained by subtracting the intensity of unused light different from the main signal light ("output 2" in Fig. 14) and the loss in the DGE 310 from the intensity of the input light corresponds to the intensity of the output of the main signal light that is originally used ("output 1" in Fig. 14).

[0144] By performing such subtraction in, for example, the control circuit 313, the optical tap 1311b illustrated in Fig. 13 can be eliminated, and the number of taps for the output light (main signal light) that is originally used can be reduced. Therefore, according to the tenth embodiment, it is possible to suppress an increase in optical loss.

[0145] Eleventh Embodiment The optical monitor 312 described above may be configured using, for example, a tunable filter. By using a tunable filter, it is possible to extract a specific wavelength from light containing multiple wavelengths or to block a specific wavelength, and receive the extracted wavelength or blocked wavelength at the optical receiver 1307. Then, by switching (or sweeping) the wavelength to be extracted or blocked from the light containing multiple wavelengths, it is possible to acquire wavelength distribution information of light intensity, that is, spectrum information.

[0146] 15 shows a configuration example in which a wavelength-tunable filter 1501 is used in the optical monitor 312. Since the wavelength-tunable filter 1501 is used in the optical monitor 312, the 1×N optical switch 1352 shown in FIG. 14 can be omitted in FIG. 15, and the output of the 2×1 optical switch 1351 is coupled to the input of the wavelength-tunable filter 1501.

[0147] The wavelength tunable filter 1501 is illustratively configured by combining an MZI 1502 and a phase shifter 1503 loaded on one or both of the interference arms of the MZI 1502. A control signal for controlling the interference state of the MZI 1502 and selecting a wavelength is generated by the control circuit 313 and applied to the phase shifter 1503.

[0148] When measuring the received light intensity of multiple wavelengths by the photodetector 1307, the control circuit 313 controls the wavelength-tunable filter 1501 to cause the photodetector 1307 to receive monitor light one wavelength at a time. The control circuit 313 stores the received light intensity information, which is the measurement result, in, for example, a storage device (not shown).

[0149] The control circuit 313 repeatedly controls the switching of the optical switch 1352 for each wavelength until finally obtaining received light intensity information for the multiple wavelengths to be monitored. As a result, similar to the ninth embodiment, the optical receiver 1307 common to each wavelength can obtain wavelength distribution information of the light intensity in the band to be monitored, that is, optical spectrum information. Based on this optical spectrum information, the control circuit 313 controls, for example, compensation of the spectral non-flatness by the DGE 310.

[0150] According to the eleventh embodiment, by using a wavelength-tunable filter 1501 using an MZI in the optical monitor 312 instead of the optical switch 1352 for wavelength selection, it is possible to obtain the same effects as those of the ninth embodiment, and further reduce the size and / or cost of the optical monitor 312.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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 tunes the optical spectrum of input light; an optical switch that selectively outputs the input light or output light of said optical spectrum tuning circuit; an optical monitor that monitors the light selected by said optical switch; and a control circuit that controls said optical spectrum tuning circuit based on the measurement results of said optical monitor, wherein said optical monitor includes: a wavelength filter that selectively outputs the light selected by said optical switch on a wavelength-by-wavelength basis; and a photoreceiver that receives light of the wavelength selected by said wavelength filter and detects the light intensity, and said control circuit controls the selective output by each of said optical switch and said wavelength filter to cause said photoreceiver to receive light one wavelength at a time, thereby obtaining wavelength distribution information of said light intensity.

2. The optical spectrum control device according to claim 1, wherein said wavelength filter has a configuration using an arrayed waveguide grating or a configuration in which a plurality of Mach-Zehnder interferometers are connected in cascade.

3. The optical spectrum control device according to claim 1, wherein the optical spectrum tunable circuit is a lattice filter circuit including a plurality of Mach-Zehnder interferometers, the optical switch and the wavelength filter each include one or more Mach-Zehnder interferometers, and the lattice filter circuit, the optical switch, and the wavelength filter are integrated on a single substrate.

4. The optical spectrum control device according to claim 3, wherein said optical monitor monitors output light from a second output port different from a first output port from which signal light is output from said lattice filter circuit.

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