Method for driving optical transmission nodes and wavelength converters

Abstract

To provide an optical transmission node, a wavelength converter, and a method for driving a wavelength converter, which suppress the accumulation of phase noise accompanying wavelength conversion.SOLUTION: An optical transmission node 10 includes: a first wavelength converter 20-1 that converts an optical signal of a first wavelength band to a second wavelength band; a second wavelength converter 20-2 that reconverts an optical signal of the second wavelength band to the first wavelength band; an excitation light source 11 that is used in common by the first wavelength converter 20-1 and the second wavelength converter 20-2; and a coupler 13 that distributes light output from the excitation light source 11 to the first wavelength converter 20-1 and the second wavelength converter 20-2.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This disclosure pertains to optical transmission nodes and This relates to a method for driving wavelength converters. [Background technology]

[0002] To expand the transmission capacity of optical communication networks, it is effective to extend the communication bandwidth through multiband transmission using S-band and E-band (shorter wavelengths than the C-band) and U-band (longer wavelengths than the L-band) in addition to the currently used C-band and L-band. While transponders or transceivers capable of supporting the C-band and L-band are in practical use, it is technically difficult to prepare transponders capable of supporting newly considered wavelength bands. It is more efficient to use currently used C-band or L-band devices for transponders and perform wavelength conversion to other bands at optical transmission nodes on the network.

[0003] Configurations have been proposed for reducing optical phase fluctuations, i.e., optical phase noise, in differential phase shift keying (DPSK) systems (see, for example, Patent Document 1). Configurations are known for using local light emission (LO) to cancel out the optical phase noise of a modulated optical signal in self-homodyne detection optical transceiver nodes (see, for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2011-182198 [Patent Document 2] U.S. Patent No. 9654219 [Non-patent literature]

[0005] [Non-Patent Document 1] W. Shieh et al., Opt. Express, 16, 15718-15727b (2008) [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In systems with high symbol rates and large wavelength dispersion, the requirements for laser phase noise are stringent, demanding lasers with narrow spectral linewidths. State-of-the-art transponders have symbol rates exceeding 100 Gbaud, and considering margins, a laser linewidth of 100 kHz or less is desirable. However, realizing lasers with spectral linewidths of 100 kHz or less presents significant technical challenges, and state-of-the-art transponders currently operate without sufficient linewidth margin.

[0007] When wavelength conversion is performed in an optical transmission node such as an optical add-drop multiplexer (OADM), phase noise contained in the excitation light source used for wavelength conversion is added to the optical signal, degrading the signal. The amount of added phase noise is equivalent to using a laser with a wider linewidth in the transponder, and signal distortion increases at high symbol rate transmissions. The more locations where wavelength conversion is performed, the more phase noise accumulates. One objective of this disclosure is to provide an optical transmission node that suppresses the accumulation of phase noise associated with wavelength conversion, a wavelength converter, and a method for driving the wavelength converter. [Means for solving the problem]

[0008] In one embodiment, the optical transmission node is A first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, A second wavelength converter that converts the optical signal of the second wavelength band back to the first wavelength band, An excitation light source used in common with the first wavelength converter and the second wavelength converter, A coupler that distributes the light output from the excitation light source to the first wavelength converter and the second wavelength converter, It is equipped with. [Effects of the Invention]

[0009] This enables the realization of an optical transmission node that suppresses the accumulation of phase noise due to wavelength conversion, and a method for driving the wavelength converter. [Brief explanation of the drawing]

[0010] [Figure 1] This diagram shows the technical challenges that can arise when performing wavelength conversion in optical transmission nodes for multiband transmission. [Figure 2] This figure shows the technical challenges in Figure 1 expressed mathematically. [Figure 3] This is a schematic diagram of an optical transmission node in an embodiment. [Figure 4] This figure shows an example of a wavelength converter configuration. [Figure 5] This figure shows the relationship between the symbol rate and the required linewidth. [Figure 6] This figure shows the relationship between the percentage difference between LA+LB and LC relative to the coherence length and the increase in the SNR penalty. [Figure 7] This figure shows the relationship between the number of passing nodes and the percentage difference (%) between LA+LB and LC relative to the coherence length, under different SNR penalty increases. [Figure 8] This figure shows the relationship between the symbol rate and the difference (m) between LA+LB and LC. [Figure 9] This figure shows an example configuration of a wavelength converter for polarization diversity. [Figure 10] This is a schematic diagram of a four-way optical transmission node. [Figure 11] Figure 10 shows the path lengths LA, LB, and LC of each wavelength converter used in the diagram. [Modes for carrying out the invention]

[0011] Figure 1 illustrates the technical challenges that may arise when performing wavelength conversion at a multiband optical transmission node 1000. The optical transmission node 1000 is used in a multiband WDM system that transmits optical signals by wavelength division multiplexing (WDM) across multiple wavelength bands, for example, an OADM node. In this example, the multiple wavelength bands used are the S band (1460-1530 nm), the C band (1530-1565 nm), the L band (1565-1625 nm), and the U band (1625-1675 nm). Furthermore, the bands in a multiband WDM system do not need to strictly adhere to these divisions; for example, a region spanning two bands may be used. In this disclosure, such cases are also referred to as bands.

[0012] Optical signals entering the optical transmission node 1000 from the transmission path are separated into their respective wavelength bands by the WDM filter 16. One or more transponders 31 are connected to the optical transmission node 1000 via a multicast switch (MCS). Each transponder 31 has a transmitter (TX) and a receiver (RX), and is also called an optical transceiver. At the optical transmission node 1000, signals transmitted from the transponders 31 are added and transmitted to the intended path. Signals dropped at the optical transmission node 1000 are received by the destination transponder 31.

[0013] The transponder 31 can process C-band and L-band signals, but it does not have the capability to process signals in other wavelength bands. Therefore, at the optical transmission node 1000, wavelength conversion is performed between the C-band or L-band signals that can be processed by the transponder 31 and wavelength bands other than the C-band and L-band. In the example in Figure 1, wavelength conversion is performed between the S-band and the C-band, but wavelength conversion may also be performed between the S-band and the L-band, between the C-band and the U-band, or between the L-band and the U-band.

[0014] The S-band signal separated by the WDM filter 16 is converted to a C-band signal by the wavelength converter 120-1. A portion of this signal is dropped to the transponder 31 by the wavelength selective switch (WSS) 23, while the remaining portion passes through the optical transmission node 100 or is routed to another path. The C-band signal added at the optical transmission node 1000 by the WSS 27 is converted from C-band to S-band by the wavelength converter 20-2, combined with signals of other wavelength bands by the WDM filter 19, and output to the transmission path.

[0015] The S-band to C-band wavelength converter 120-1 and the C-band to S-band wavelength converter 120-2 are configured to operate in reverse. The excitation light generated from the output light of the excitation light source 101 (denoted as "LD1") is input to the wavelength converter 120-1 along with the S-band signal light. The excitation light generated from the output light of the excitation light source 102 (denoted as "LD2") is input to the wavelength converter 20-2 along with the C-band signal light. In this configuration, an excitation light source is provided for each wavelength converter.

[0016] When performing wavelength conversion between the S-band and C-band, excitation light with a wavelength of approximately 764 nm is required. Since it is difficult to obtain high-power LDs in this wavelength range, the output light from the excitation light source 101 is amplified by the optical amplifier 103, and light with a wavelength of 764 nm is generated by the second harmonic generator (indicated as "SHG" in the figure) 104 and used as excitation light. Similarly, on the output side to the transmission line, the output light from the excitation light source 102 is amplified by the optical amplifier 105, and light with a wavelength of 764 nm is generated by the second harmonic generator 106 and used as excitation light for the wavelength converter 20-2.

[0017] Each of the pump light sources 101 and 102 has phase noise. The phase noise of the pump light incident on the wavelength converter 120-1 is added to the signal light, and the phase noise of the pump light incident on the wavelength converter 120-2 is added to the reconverted light into the S-band. Each transponder 31 operates at a high symbol rate, and there is insufficient margin in the light source (LD) used in the transponder 31. When phase noise is added for each pump light source, this is equivalent to using an LD with a wider linewidth in the transponder 31 by an amount corresponding to the added phase noise, which results in signal degradation.

[0018] Further, there are variations in the wavelength accuracy of the pump light sources 101 and 102. When variations in wavelength accuracy accumulate, there is a risk that the wavelength of the signal light itself will shift. In this case, there is a possibility that the signal wavelength of the S-band reconverted by the wavelength converter 120-2 deviates from the signal wavelength of the S-band incident on the optical transmission node 1000.

[0019] FIG. 2 is a diagram expressing the technical problem of FIG. 1 in mathematical expressions. The signal light E incident on the wavelength converter 120-1 S (t) and the pump light E P (t), the converted light E output from the wavelength converter 120-1 I (t), and the reconverted light E reconverted by the wavelength converter 120-2 S' (t) are expressed as follows. [Num.]

[0020] The reconverted light E output from the wavelength converter 120-2 S' (t) includes the angular frequency ω of the first pump light P1 and the angular frequency ω of the second pump light P2 difference (ω P2 -ω P1 ) and the phase difference between the first pump light and the second pump light (φ P2 (t)-φ P1 (t)). The term of "ω P2 -ω P1 " represents the frequency variation (i.e., wavelength variation) of the pump light source, and "φ P2 (t)-φP1 The term "(t)" represents the phase noise.

[0021] Phase noise and frequency variation increase with the number of excitation sources used for conversion between specific wavelength bands. For example, if conversion between the S-band and C-band is performed for multiple paths and a separate excitation source is used for each wavelength converter, phase noise and frequency variation accumulate, leading to significant signal degradation within the optical transmission node. Furthermore, phase noise accumulates each time the signal passes through multiple optical transmission nodes, again resulting in significant signal degradation.

[0022] The configuration of the embodiment was conceived and implemented to solve the problem of phase noise accumulation. The configuration of the embodiment that suppresses phase noise accumulation can also solve the problem of wavelength accuracy variation. The specific configuration and method of suppressing phase noise accumulation in the embodiment will be described below with reference to the drawings. The following forms are examples for realizing the technical concept of this disclosure and do not limit the scope of the disclosure. The size, positional relationships, etc., of the components shown in each drawing may be exaggerated to facilitate understanding of the invention. The same components or functions may be given the same name or reference numerals, and redundant explanations may be omitted.

[0023] <Optical transmission node of the embodiment> Figure 3 is a schematic diagram of an optical transmission node 10 of an embodiment. The optical transmission node 10 includes a wavelength converter 20-1 that converts an optical signal of a first wavelength band to a second wavelength band, a wavelength converter 20-2 that converts an optical signal of the second wavelength band back to the first wavelength band, and an excitation light source 11 (indicated as "LD" in the figure) that is used in common between wavelength converters 20-1 and 20-2. The optical transmission node 10 also includes a coupler 13 that distributes the light output from the excitation light source 11 to the wavelength converters 20-1 and 20-2.

[0024] The first wavelength band is used for optical transmission over the network, but is a wavelength band that cannot be handled by the transponder 31 connected to the optical transmission node 10 via MCS 24 and 26. The second wavelength band is a wavelength band that can be handled by the transponder 31. In the example in Figure 3, the first wavelength band is the S band and the second wavelength band is the C band. In wavelength conversion between the C band and the U band, or between the L band and the U band, the first wavelength band is the U band and the second wavelength band is the C band or L band. The add and drop configurations via WSS 23 and 27 are as described with reference to Figure 1. If necessary, an optical amplifier 22 for the C band may be provided before WSS 23 and an optical amplifier 28 for the C band may be provided after WSS 27.

[0025] The optical signal incident from the transmission line to the optical transmission node 10 is separated into multiple wavelength bands by the WDM filter 16. The signals of each wavelength band separated by the WDM filter 16 are amplified by preamplifiers 17S, 17C, 17L, and 17U for their respective wavelength bands. Focusing on the S band, the S band signal light amplified by preamplifier 17S is input to the wavelength converter 20-1 along with the excitation light and converted into a C band signal.

[0026] The excitation light input to the wavelength converter 20-1 is generated using light distributed by the coupler 13. The coupler 13 distributes the light output from the excitation light source 11 and amplified by the optical amplifier 12 to the wavelength converters 20-1 and 20-2. In Figure 3, for the sake of illustration simplicity, signal transmission to a single path is shown, but a signal converted from the S-band to the C-band, or a signal converted from the C-band to the S-band, can be sent to multiple paths. In this case, the coupler 13 distributes the excitation light to all the wavelength converters between the C and S bands provided for multiple paths within the optical transmission node 10.

[0027] Since it is difficult to prepare a high-power laser light source suitable for the wavelength of the excitation light used for wavelength conversion between the S-band and C-band, a laser light source with a wavelength of 1528 nm is used as the excitation light source 11. The light for the wavelength converter 20-1, distributed by the coupler 13, is amplified by the optical amplifier 14-1, and excitation light with a wavelength of 764 nm is generated by the second harmonic generator 15-1. The excitation light is combined with the S-band signal light by the optical filter 201 and incident on the wavelength converter 20-1. The second harmonic generator 15-1 is a crystal with a nonlinear optical effect, and a ferroelectric crystal such as periodically-poled lithium niobate (PPLN) can be used.

[0028] The light for the wavelength converter 20-2, distributed by coupler 13, is amplified by optical amplifier 14-2 and incident on second harmonic generator 15-2 to generate excitation light with a wavelength of 764 nm. The second harmonic generator 15-2 is an optical crystal with high nonlinear optical effects, such as PPLN. The excitation light output from the second harmonic generator 15-2 is combined with C-band signal light that passes through or is added to optical transmission node 10 by optical filter 203 and incident on wavelength converter 20-2. The re-converted light, wavelength-converted to the S-band by wavelength converter 20-2, is amplified by post-amplifier 18S. The S-band signal light is combined with light of other wavelength bands amplified by post-amplifiers 18C, 18L, and 18U by WDM filter 19 and output to the transmission line.

[0029] Figure 4 shows an example configuration of a wavelength converter 20 used in the optical transmission node 10. The wavelength converter 20 includes an optical filter 201 that combines excitation light with S-band signal light, a nonlinear optical medium 200, and an optical filter 202 that extracts C-band converted light from the output light of the nonlinear optical medium 200.

[0030] The S-band signal is a DWDM (Dense WDM) signal in which signals of many wavelengths are densely arranged. The light distributed to the wavelength converter 20 by the coupler 13 is amplified by the optical amplifier 14 to the power required for wavelength conversion, and the second harmonic generator 15 generates excitation light of the desired wavelength. When the high-power excitation light is incident on the nonlinear optical medium 200 along with the S-band signal light, a new idler light of a new wavelength band (in this case, the C-band) is generated by second-order nonlinear effects such as difference frequency generation (DFG). The idler light, or converted light, has a wavelength corresponding to the angular frequency difference or energy difference between the signal light and the excitation light.

[0031] A PPLN with high conversion efficiency can be used as the nonlinear optical medium 200. Of the light output from the nonlinear optical medium 200, the pump and signal components are removed by the optical filter 202, and the C-band idler light is extracted as converted light from the wavelength converter 20. As a result, the numerous DWDM signals contained in the S-band are converted collectively into C-band DWDM signals. At the wavelength converter 20-2 on the output side to the transmission line, in the opposite operation to that in Figure 4, the C-band signal light and the pump light generated from the distribution light are input to the nonlinear optical medium 200. The numerous DWDM signals contained in the C-band are converted collectively back into S-band DWDM signals, and the re-converted light is output from the wavelength converter 20-2.

[0032] Returning to Figure 3, let L be the path length between coupler 13 and wavelength converter 20-1, more specifically, the fiber length from coupler 13 to the intersection point of the S-band signal light and the excitation light (optical filter 201 in Figure 4). A Let L be the path length between the coupler 13 and the wavelength converter 20-2, more specifically, the fiber length from the coupler 13 to the point where the C-band signal light and the excitation light combine. C Let L be the path length between wavelength converters 20-1 and 20-2, more specifically, the fiber length from the intersection point of the S-band signal light and the excitation light to the intersection point of the C-band signal light and the excitation light. B Let's assume that.

[0033] Path length L A , LB , and L A The relationship is adjusted so that the phase of the phase noise generated in the converted light output from wavelength converter 20-1 by a portion of the excitation light from the excitation light source cancels out the phase of the phase noise generated in the re-converted light output from wavelength converter 20-2 by another portion of the excitation light. Also, if the tolerance of the path length inside the optical transmission node is ±Δ, L C =L A +L B ±Δ The optical wiring is designed to achieve this.

[0034] The fiber length of each path also includes the length of the optical wiring inside the devices included in that path. For example, when erbium-doped fiber amplifiers (EDFA) or Raman amplifiers are used as optical amplifiers 14-1, 14-2, 22, and 28, tens of meters to several kilometers of fiber are stored inside the amplifier. The length of these fibers is also included in the path length. A +L B and L C A certain range of error is acceptable in the isometization.

[0035] At the optical transmission node 10, by using a common excitation light source 11 for multiple wavelength converters used for conversion between specific wavelength bands, the accumulation of phase noise from the excitation light source is suppressed, thereby suppressing signal distortion. Furthermore, wavelength shifts caused by variations in wavelength accuracy between excitation light sources are suppressed. This can be expressed mathematically as follows:

number

[0036] Wavelength conversion by DFG in wavelength converter 20-1 is performed as follows: [Converted light] = [Excitation light] - [Signal light] It is represented as follows: The converted light E output from the wavelength converter 20-1 I (t) contains the difference between the excitation light and the signal light, and "ω P1 -ω S The ingredients of " and "φ P1 (t)-φ SThe component (t) is included. The reconverted light E output from the wavelength converter 20-2 S' (t) is given the component of excitation light generated from the distributed light, ω P2 -(ω P1 -ω S ) and its components, P2 (t)-(φ P1 (t)-φ S (t)) contains the component (t).

[0037] When using the same excitation light source 11, ω P2 =ω P1 Therefore, "ω P2 -ω P1 " can be made zero. Also, if time t is the same, φ P2 (t) = φ P1 (t) is "φ P2 (t)-φ P1 (t) can be set to zero or to the minimum, and the incident signal light E S The state is almost the same as (t). That is, by converting from one wavelength band to another and then back to the original wavelength band, the frequency shift and phase noise cancel each other out. In particular, Lc=L A +L B By satisfying ±Δ, simultaneity is ensured and the inclusion of phase noise is minimized. The allowable error ±Δ of the path length within the optical transmission node 10 is an error within a range that maintains simultaneity to the extent that the effects of phase noise can be suppressed.

[0038] <Tolerance range> Figure 5 shows the relationship between symbol rate and required linewidth when the transmission distance is 5000 km. The optical transmission node 10 of this embodiment is applicable to long-distance core networks. As the symbol rate increases, the required linewidth of the laser light source used in the transponder 31 becomes narrower. When the symbol rate of the transponder 31 is 100 Gbaud, it operates with a certain margin provided for a spectral linewidth of 100 kHz. When wavelength conversion between specific wavelength bands is performed in the optical transmission node 10, the output light from the same excitation light source 11 is set to Lc=L A +L BBy distributing the signal to multiple wavelength converters 20 while satisfying ±Δ, phase noise added to the signal light is suppressed. Signal distortion can be suppressed by suppressing the inclusion of phase noise associated with wavelength conversion while each transponder 31 operates within the margin of the laser linewidth.

[0039] Laser line width (f) in frequency display 3dB ) and coherence length (L Coh The relationship given by equation (1) exists between them.

number

[0040] In wavelength conversion by difference frequency generation (DFG), the phase noise (f) of the signal light for frequency display is Signal ) and the phase noise of the excitation light source 11 (f Pump ) and the phase noise of the converted light (f Idler The relationship between ) is expressed by equation (2).

number

[0041] In order to reduce the phase noise of the converted light and output a converted light with a narrow spectral linewidth, it is necessary to reduce the phase noise of both the signal light and the excitation light. Of these, the signal light noise is determined by the laser linewidth of the transponder 31. In the configuration shown in Figure 3, the phase noise originating from the excitation light (f) is added to the phase noise of the signal light. Pump By canceling or minimizing the signal, it is possible to output converted light with a line width that is almost the same as the line width of the signal light.

[0042] Figure 6 shows the coherence length (L Coh ) for L A +L B and L CThis shows the relationship between the percentage difference and the increase in the SNR penalty. According to Non-Patent Literature 1, the SNR penalty ΔP (dB) is ΔP(dB)≈4.343×α(1+γ0) It is approximated as follows: γ0 is the effective SNR in a system including phase noise, and α is, α ≈ πc(2f0 2 ) -1 D t Bf 3dB This is approximated as follows: Here, f0 is the central wavelength of the laser of transponder 31, f 3dB is the laser line width, D t is the cumulative wavelength dispersion, and B is the symbol rate. In Figure 6, the required linewidth of the transponder 31 and the linewidth of the excitation light source 11 are set to be the same, and f 3dB The SNR penalty ΔP is calculated using a frequency of 100kHz and a symbol rate B of 100Gbaud.

[0043] The error in path length, i.e., L A +L B and L C The difference is the coherence length L Coh It is judged as a ratio to . The error is zero, i.e., L A +L B =L C Around this point, the larger the absolute value of the error, the larger the SNR penalty. For example, let's assume an acceptable increase in the SNR penalty per node of 0.1 dB. An increase in the SNR penalty of 0.1 dB corresponds to a ±24% error in fiber length relative to coherence length. A +L B and L C In between, L C An error of approximately 1 / 4 of the length is acceptable. Of course, the allowable path length error can be further reduced by setting the increase in the permissible per-node SNR penalty to less than 0.1 dB, for example, 0.05 dB.

[0044] Figure 7 shows the relationship between the number of traversed nodes and coherence length for different SNR penalty increases. A +L B and L Cshows the relationship with the percentage (%) of the difference. In a network, a signal passes through a plurality of nodes and undergoes wavelength conversion at each node. When the increase in SNR penalty per node is 0.1 dB, the absolute value of the allowable error of the path length at the first node is 24%. The reason why the allowable error gradually increases as the number of passing nodes increases is that phase noise is accumulated in the signal light as the signal passes through the nodes, and the phase noise of the pump light becomes relatively small.

[0045] When the allowable increase in SNR penalty per node is 0.05 dB, a path length error of ±12% with respect to the coherence length is allowed. Compared with the case where the increase in SNR penalty per node is 0.1 dB, the slope of the change is smaller. As the increase in SNR penalty is reduced to 0.02 dB and 0.01 dB, the allowable error of the path length becomes smaller, and thus L with respect to the coherence length is reduced regardless of the number of passing nodes A +L B and L C the percentage difference is substantially constant.

[0046] FIG. 8 is a diagram illustrating the relationship between the symbol rate when the increase in SNR penalty per node is 0.1 dB and L A +L B and L C difference in meters. The difference between L A +L B and L C is converted as an actual distance (physical fiber length), not as a percentage relative to the coherence length. When the symbol rate is 100 Gbaud, the difference between L A +L B and L C is 252 m. The path length, that is, the transmission path L in the optical transmission node 10 A , L B , and L CEven if the optical fiber used has an error of 250m or less, it can handle symbol rates of 100 Gbaud or more. Since wavelength conversion is performed mainly to convert signals to C-band and L-band, EDFA can be used as the optical amplifier. Because the internal fiber length of EDFA is several tens of meters, the error in fiber length between actual amplifiers is within several tens of meters. The allowable per-node SNR penalty increase may be set to a value smaller than 0.1dB, for example, 0.05dB.

[0047] <Variations of wavelength converters> Figure 9 shows an example configuration of a wavelength converter 30 for polarization diversity. The wavelength converter 30 includes a polarization beam splitter 324, a polarization beam combiner 325, a nonlinear optical medium 200X for X polarization, and a nonlinear optical medium 200Y for Y polarization.

[0048] The signal light (S) input to the wavelength converter 30 is separated into two mutually orthogonal polarizations by the polarization beam splitter 324. Here, the orthogonal polarizations are called the X polarization and the Y polarization. The wavelength converter 30 receives light output from an excitation light source 11, which is used in common among multiple wavelength converters, and which is distributed by the coupler 13 (see Figure 3). Assuming wavelength conversion between the S band and the C band, the wavelength of the distributed light is set to 1528 nm.

[0049] The distributed light is amplified by the optical amplifier 311 and split by the beam splitter 312. The split light is power-adjusted by variable optical attenuators (VOAs) 313 and 314, respectively, and input to the second harmonic generators 315 and 316. The excitation light (Lpump) generated by the second harmonic generator 315 is combined to X polarization by the multiplexer 317X. The excitation light (Lpump) generated by the second harmonic generator 316 is combined to Y polarization by the multiplexer 317Y.

[0050] The X-polarized beam and excitation light are incident on a nonlinear optical medium 200X for the X-polarization, and a C-band X-polarized beam is generated by difference frequency generation (DFG). Unwanted signal light and excitation light are removed by optical filter 321, and the C-band X-polarized beam is extracted. The Y-polarized beam and excitation light are incident on a nonlinear optical medium 200Y for the Y-polarization, and a C-band Y-polarized beam is generated by difference frequency generation (DFG). Unwanted signal light and excitation light are removed by optical filter 322, and the C-band Y-polarized beam is extracted. The C-band X-polarized beam and the C-band Y-polarized beam are delayed and adjusted by an optical delay line (ODL) 323, combined by a polarization beam combiner 325, and output from a wavelength converter 30.

[0051] The same configuration as in Figure 9 can be used when converting a C-band signal to an S-band signal at the output side to the transmission line. In this case as well, two excitation beams are generated from the light output from a common excitation light source and distributed by the coupler 13, and these are combined into a C-band X-polarized beam and a C-band Y-polarized beam, respectively. Using the same excitation light source, L is used between the SC conversion wavelength converter and the CS conversion wavelength converter. C =L A +L B By satisfying ±Δ, the accumulation of phase noise can be suppressed, and signal distortion can be reduced.

[0052] <Expansion to 4-way optical transmission nodes> Figure 10 is a schematic diagram of a four-way optical transmission node 10A. Signal light entering optical transmission node 10A from each of the paths A, B, C, and D is output to the destination path. In Figure 10, the connection relationships are depicted with a focus on wavelength conversion between the S-band and C-band.

[0053] Of the wavelength converters 30, the one located on the incident side from the transmission line will be denoted as 30-1, and the one located on the output side to the transmission line will be denoted as 30-2. Furthermore, corresponding to paths A, B, C, and D, the wavelength converter on the incident side from path A will be denoted as 30-1A, and the wavelength converter on the output side to path A will be denoted as 30-2A. The wavelength conversion from the S band to the C band performed by wavelength converter 30-1A will be referred to as "wavelength conversion 1A," and the wavelength conversion from the C band to the S band performed by wavelength converter 30-2A will be referred to as "wavelength conversion 2A." The same applies to paths B, C, and D.

[0054] At optical transmission node 10A, the excitation light source 11 is used in common by wavelength converters 30-1A to 30-2D, which perform wavelength conversion between the S-band and the C-band. Wavelength converters 30-1A, 30-1B, 30-1C, and 30-1D each have the polarization diversity configuration shown in Figure 9, but are not limited to this example. Wavelength converters 30-2A, 20-2B, 30-2C, and 30-2D operate in the reverse direction of Figure 9, polarizing the C-band signal light, generating X-polarization and Y-polarization of the S-band by incident excitation light, combining them, and outputting converted light in the S-band.

[0055] The light output from the excitation light source 11 is distributed by the coupler 13 from wavelength converter 30-1A to 30-2D. The light may be amplified by the optical amplifier 12 before input to the coupler 13. The path length L from the coupler 13 to the wavelength converter 30-1A is also specified. A And the path length L from wavelength converter 30-1A to, for example, wavelength converter 20-2C. B The sum of these is the path length L from coupler 13 to wavelength converter 30-2C. C And they are the same within the tolerance range of ±Δ (L C =L A +L B (±Δ).

[0056] In each of the multiple combinations of paths, L C =L A +L BThe conditions ±Δ are satisfied. Due to this isometality, or simultaneity, phase noise originating from the excitation light is canceled out through wavelength conversion and reconversion, and signal distortion is suppressed. Furthermore, since the same excitation light source 11 is used, the problem of wavelength variation between excitation light sources does not occur.

[0057] Figure 11 shows the respective path lengths L of the wavelength converters 30-1A to 30-2D used in Figure 10. A , L B , and L C This shows that the path length from coupler 13 to the first wavelength converter, more specifically, the fiber length from coupler 13 to the X-polarization multiplexer 317X of the first wavelength converter (see Figure 10), is L. A Let L be the fiber length from the X-polarization multiplexer 317X of the first wavelength converter to the X-polarization multiplexer 317X of the second wavelength converter. B Let's assume the fiber length from coupler 13 to the X-polarization multiplexer 317X of the second wavelength converter is L. C Let's assume that.

[0058] Similarly, for Y polarization, the fiber length from coupler 13 to the Y-polarization multiplexer 317Y of the first wavelength converter (see Figure 10) is L. A Let's assume that the fiber length from the Y-polarization multiplexer 317Y of the first wavelength converter to the Y-polarization multiplexer 317Y of the second wavelength converter is L. B Let's assume the fiber length from coupler 13 to the Y-polarization multiplexer 317Y of the second wavelength converter is L. C Let's assume that.

[0059] Path length L A The total length of the column is L C =L A +L B Keep them the same within the range where ±Δ holds true. Path length L B The total length of the column and L C The total length of the column is also L C =L A +L BThe values ​​should be made identical within the range where ±Δ holds true. If the fiber lengths for X-polarization and Y-polarization are set to be equal in each of the wavelength converters 30-1A to 30-2D, it is not necessary to consider the polarization separately. By using distributed light from the same excitation light source 11 and ensuring simultaneity or equal length of propagation, the accumulation of phase noise originating from the excitation light can be suppressed.

[0060] The above describes wavelength conversion performed at the optical transmission node 10 (or 10A) based on a specific configuration example, but this disclosure is not limited to the above configuration. A configuration in which light emitted from a single excitation light source is distributed to multiple wavelength converters can also be applied to wavelength conversion between the C-band and U-band, and between the L-band and U-band. Optical transmission node 10 (or 10A) The method for driving the wavelength converter in is: A first wavelength converter that converts an optical signal in a first wavelength band (e.g., the S-band) to a second wavelength band (e.g., the C-band), and a second wavelength converter that converts the optical signal in the second wavelength band back to the first wavelength band are arranged. Light output from a single excitation light source is distributed to the first wavelength converter and the second wavelength converter, and the first wavelength converter and the second wavelength converter are driven independently.

[0061] In a preferred drive configuration, L A And, L B And, L C The relationship is adjusted so that the phase noise contained in the output light of the first wavelength converter and the phase noise contained in the output light of the second wavelength converter cancel each other out. Alternatively, L C =L A +L B By designing the optical wiring so that ±Δ is satisfied, the accumulation of phase noise originating from the excitation light is suppressed. This makes it possible to suppress signal distortion caused by the addition of phase noise during high symbol rate transmission.

[0062] In addition to PPLN, other wavelength conversion elements such as Highly Nonlinear Fiber (HNLF) can also be used. Wavelength conversion with HNLF is not DGF, but rather a nonlinear effect of Four-Wave Mixing (FWM). Wavelength conversion by FWM has differences from equations 1 and 2, such as the fact that the excitation light wavelength is different and SHG is not required, but the signal distortion suppression effect is the same. Instead of MCS, the connection and switching of transponder 31 may be done using wavelength selection switches with multiple inputs and outputs, or wavelength multiplexing / demultiplexing elements such as arrayed waveguide gratings (AWG).

[0063] In response to the above disclosure, the following additional notes are provided. (Note 1) A first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, A second wavelength converter that converts the optical signal of the second wavelength band back to the first wavelength band, An excitation light source used in common with the first wavelength converter and the second wavelength converter, A coupler that distributes the light output from the excitation light source to the first wavelength converter and the second wavelength converter, Optical transmission node. (Note 2) The relationship between the first path length between the coupler and the first wavelength converter, the second path length between the first wavelength converter and the second wavelength converter, and the third path length between the coupler and the second wavelength converter is adjusted so that the phase of the phase noise generated in the converted light output from the first wavelength converter by the excitation light of the excitation light source cancels out the phase of the phase noise generated in the re-converted light output from the second wavelength converter by the excitation light. The optical transmission node described in Appendix 1. (Note 3) The first path length between the coupler and the first wavelength converter is L. A The second path length between the first wavelength converter and the second wavelength converter is L. BThe third path length between the coupler and the second wavelength converter is L. C If the tolerance for the path length within the optical transmission node is ±Δ, L C =L A +L B ±Δ The optical transmission node described in Appendix 1. (Note 4) The aforementioned tolerance is a range that keeps the increase in the signal-to-noise ratio penalty for each node due to passing through the optical transmission node to 0.1 dB or less. The optical transmission node described in Appendix 3. (Note 5) The aforementioned tolerance is a range that keeps the increase in the signal-to-noise ratio penalty for each node due to passing through the optical transmission node to 0.05 dB or less. The optical transmission node described in Appendix 4. (Note 6) A first filter is provided in front of the first wavelength converter and combines the first excitation light generated from the first distributed light distributed by the coupler with the optical signal of the first wavelength band, A second filter is provided prior to the second wavelength converter and combines the second excitation light generated from the second distributed light distributed by the coupler with the optical signal of the second wavelength band, An optical transmission node having any of the descriptions in Appendix 1 to 5. (Note 7) The first excitation light and the second excitation light are the second harmonics of the light output from the excitation light source. The optical transmission node described in Appendix 6. (Note 8) A transponder connected to the optical transmission node, The transponder includes the following: the transponder operates in the second wavelength band and does not operate in the first wavelength band. An optical transmission node as described in any of the appendices 1 to 7. (Note 9) A first filter combines excitation light, generated from a portion of the emitted light of an excitation light source commonly used for wavelength conversion between the first and second wavelength bands, with signal light. A nonlinear optical medium connected to the output of the first filter, which generates converted light of a different wavelength from the excitation light and the signal light based on the excitation light and the signal light, A second filter that extracts the converted light from the light emitted from the nonlinear optical medium, A wavelength converter having the following features. (Note 10) A polarization beam splitter that separates the signal light into a first polarization and a second polarization, A polarization beam combiner that combines the first polarization and the second polarization, It has, The first filter includes a 3-filter that combines the excitation light to the first polarization and a 4-filter that combines the excitation light to the second polarization. The nonlinear optical medium includes a first nonlinear optical medium that generates a first converted light of the first polarization from the first polarization, and a second nonlinear optical medium that generates a second converted light of the second polarization from the second polarization. The polarization beam combiner outputs the first converted light and the second converted light. Wavelength converter as described in Appendix 8. (Note 11) An optical transmission node is provided with a first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, and a second wavelength converter that converts an optical signal in the second wavelength band back to the first wavelength band. The light output from a single excitation light source is distributed to the first wavelength converter and the second wavelength converter, and the first wavelength converter and the second wavelength converter are driven independently. A method for driving a wavelength converter. (Note 12) A coupler is provided to distribute the light emitted from the single excitation light source, The relationship between the first path length between the coupler and the first wavelength converter, the second path length between the first wavelength converter and the second wavelength converter, and the third path length between the coupler and the second wavelength converter is adjusted such that the phase of the phase noise generated in the converted light output from the first wavelength converter by the excitation light distributed by the coupler cancels out the phase of the phase noise generated in the re-converted light output from the second wavelength converter by the excitation light. The method for driving the wavelength converter described in Appendix 11. (Note 13) A coupler is provided to distribute the light emitted from the single excitation light source, The path length between the coupler and the first wavelength converter is L. A The path length between the first wavelength converter and the second wavelength converter is L. B The path length from the coupler to the second wavelength converter is L. C The tolerance for the path length within the optical transmission node is ±Δ, L C =L A +L B ±Δ Design the optical wiring within the optical transmission node to satisfy the following conditions: The method for driving the wavelength converter described in Appendix 11. (Note 14) The tolerance is set such that the penalty increase in the signal-to-noise ratio per node due to passing through the optical transmission node is 0.1 dB or less. The method for driving the wavelength converter described in Appendix 11. [Explanation of Symbols]

[0064] 10 Optical transmission nodes 11 Excitation light source 13 Couplers 15, 15-1, 15-2 Second harmonic generator Wavelength converters for 20, 20-1, 20-2, 30, 30-1A~30-2D 31 Transponder 200 Nonlinear optical medium 201 Optical filter (first filter) 202 Optical Filters 203 Optical filter (second filter) L A Path length (first path length) L B Path length (second path length) L C Path length (third path length)

Claims

1. A first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, A second wavelength converter that converts the optical signal of the second wavelength band back to the first wavelength band, An excitation light source used in common with the first wavelength converter and the second wavelength converter, A coupler that distributes the light output from the excitation light source to the first wavelength converter and the second wavelength converter, It has, The relationship between the first path length between the coupler and the first wavelength converter, the second path length between the first wavelength converter and the second wavelength converter, and the third path length between the coupler and the second wavelength converter is adjusted so that the phase of the phase noise generated in the converted light output from the first wavelength converter by the excitation light of the excitation light source cancels out the phase of the phase noise generated in the re-converted light output from the second wavelength converter by the excitation light. Optical transmission node.

2. A first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, A second wavelength converter that converts the optical signal of the second wavelength band back to the first wavelength band, An excitation light source used in common with the first wavelength converter and the second wavelength converter, A coupler that distributes the light output from the excitation light source to the first wavelength converter and the second wavelength converter, It has, The first path length between the coupler and the first wavelength converter is L. Ａ The second path length between the first wavelength converter and the second wavelength converter is L. Ｂ The third path length between the coupler and the second wavelength converter is L. Ｃ If the tolerance for path length within the optical transmission node is ±Δ, L Ｃ =L Ａ +7 Ｂ ±Δ This is an optical transmission node.

3. The aforementioned tolerance is a range that keeps the increase in the signal-to-noise ratio penalty for each node due to passing through the optical transmission node to 0.1 dB or less. The optical transmission node according to claim 2.

4. A first filter is provided prior to the first wavelength converter, which combines the first excitation light generated from the first distributed light distributed by the coupler with the optical signal of the first wavelength band, A second filter is provided prior to the second wavelength converter, which combines the second excitation light generated from the second distributed light distributed by the coupler with the optical signal of the second wavelength band, An optical transmission node according to claim 1 or 2, having the following features.

5. The first excitation light and the second excitation light are the second harmonics of the light output from the excitation light source. The optical transmission node according to claim 4.

6. The first wavelength converter and the second wavelength converter are A first filter that combines the excitation light with the optical signal, A nonlinear optical medium connected to the output of the first filter, which generates converted light of a different wavelength from the excitation light and the optical signal based on the excitation light and the optical signal, A second filter that extracts the converted light from the light emitted from the nonlinear optical medium, An optical transmission node according to claim 1, having the above characteristics.

7. A polarization beam splitter that separates the optical signal into a first polarization and a second polarization, A polarization beam combiner that combines the first polarization and the second polarization, It has, The first filter includes a third filter that combines the excitation light with the first polarization and a fourth filter that combines the excitation light with the second polarization. The nonlinear optical medium includes a first nonlinear optical medium that generates a first converted light of the first polarization from the first polarization, and a second nonlinear optical medium that generates a second converted light of the second polarization from the second polarization. The polarization beam combiner outputs the first converted light and the second converted light. The optical transmission node according to claim 6.

8. An optical transmission node is provided with a first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, and a second wavelength converter that converts the optical signal in the second wavelength band back to the first wavelength band. The light output from a single excitation light source is distributed to the first wavelength converter and the second wavelength converter, and the first wavelength converter and the second wavelength converter are driven independently. The relationship between the first path length between the point where the light output from the single excitation light source is distributed to the first and second wavelength converters and the first wavelength converter, the second path length between the first and second wavelength converters, and the third path length between the point and the second wavelength converter is adjusted so that the phase of the phase noise generated in the converted light output from the first wavelength converter by the excitation light from the excitation light source cancels out the phase of the phase noise generated in the re-converted light output from the second wavelength converter by the excitation light. A method for driving a wavelength converter.

9. An optical transmission node is provided with a first wavelength converter that converts an optical signal in a first wavelength band to a second wavelength band, and a second wavelength converter that converts the optical signal in the second wavelength band back to the first wavelength band. The light output from a single excitation light source is distributed to the first wavelength converter and the second wavelength converter, and the first wavelength converter and the second wavelength converter are driven independently. Let L be the first path length between the location where light output from the single excitation light source is distributed to the first wavelength converter and the second wavelength converter, and the first wavelength converter Ａ , let L be the second path length between the first wavelength converter and the second wavelength converter Ｂ , let L be the third path length between said location and the second wavelength converter Ｃ , and if the allowable error of the path length in the optical transmission node is ±Δ, then L Ｃ =L Ａ +7 Ｂ ±Δ A method for driving a wavelength converter.

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