Optical transmission system, optical transmission method, and program

Abstract

Provided is an optical transmission system comprising: a transmission unit for maximally increasing the frequency interval between a plurality of channel components within a transmission band and generating a first optical signal that is an optical signal in which a plurality of channel components have been wavelength-division-multiplexed; a first transmission path for transmitting the first optical signal; a phase conjugation conversion unit for inverting the spectrum of the first optical signal to generate a second optical signal; and a second transmission path for transmitting the second optical signal. The first transmission path and the second transmission path cause wavelength dispersion of the plurality of channel components of the first optical signal and of the second optical signal, respectively. The first transmission path may comprise one or more first optical relay units for amplifying and relaying the first optical signal. The second transmission path may comprise one or more second optical relay units for amplifying and relaying the second optical signal.

Description

Technical Field

[0001] The present invention relates to an optical transmission system, an optical transmission method, and a program.

Background Art

[0002] When an optical signal is transmitted over a long distance using an optical fiber, an optical amplification relay transmission method may be adopted for the purpose of compensating for optical loss occurring in the optical fiber. In the optical amplification relay transmission method, an optical amplifier amplifies the optical signal. Therefore, the transmission band of the optical signal in the optical transmission system is limited to the amplification band of the optical amplifier.

[0003] For the optical amplifier, an optical fiber doped with a rare earth element is used. An erbium-doped fiber amplifier (EDFA) is one of the typical rare earth-doped optical amplifiers. The amplification band of the erbium-doped fiber amplifier is approximately 4 THz within a band called the "C-band" or "L-band". Therefore, the amplification band of the optical signal in the optical transmission system is designed to be approximately 4 THz.

[0004] In the centralized amplification relay method, erbium-doped fiber amplifiers and the like arranged at regular intervals in the transmission line amplify the optical signal. On the other hand, in the distributed amplification relay method, the optical signal being transmitted in the transmission line (optical fiber) is amplified using Raman optical amplification or the like. The transmission power of the optical signal transmitted in the distributed amplification relay method is kept higher compared to the transmission power of the optical signal transmitted in the centralized amplification relay method. For this reason, in the distributed amplification relay method, the optical signal-to-noise ratio (OSNR) in the optical signal after transmission is maintained at a high level.

[0005] In the distributed amplification relay method, in order to sufficiently compensate for the transmission loss in the transmission line, it is necessary to input an excitation light with a very strong light intensity into the optical fiber. Therefore, from the perspective of ensuring safety, the applicable area is limited. Therefore, the light intensity of the excitation light in the distributed amplification relay method is suppressed, and the losses that were not compensated by the distributed amplification relay method are compensated by the concentrated amplification relay method. Such a hybrid amplification relay method may be used.

[0006] In the amplification relay transmission method, the transmission distance and relay interval of the optical signal are limited by the amplified spontaneous emission (ASE) noise output from the optical amplifier. When the optical signal-to-noise ratio deteriorates due to the amplified spontaneous emission noise, optical signal regeneration relay is required. In regeneration relay, the optical signal is converted into an electrical signal, the electrical signal is reconverted into an optical signal, and the reconverted optical signal is retransmitted.

[0007] On the other hand, in order to construct an economical optical network, it is important to extend the intervals of amplification relay and regeneration relay, respectively. In order to suppress the deterioration of the optical signal-to-noise ratio due to the increased transmission loss caused by the extension and the amplified spontaneous emission noise that increases according to the number of amplification relays, it is necessary to increase the transmission power (light intensity) of the optical signal.

[0008] However, the higher the transmission power, the more prominent the nonlinear optical effect in the optical fiber. When the refractive index of the optical fiber, which is the transmission medium, changes due to the nonlinear optical effect, waveform distortion occurs in the optical signal. Therefore, the maximum transmission capacity and transmission distance of the optical signal are determined according to the trade-off between the improvement of the optical signal-to-noise ratio by the transmission power and the suppression of the waveform distortion by the nonlinear optical effect. Hereinafter, the signal-to-noise ratio is a quantitative index of the signal quality including the noise due to the nonlinear optical effect and the optical signal-to-noise ratio.

[0009] Nonlinear noise is distinguished based on the component on which the nonlinear noise acts. Phase noise (nonlinear phase noise) due to the nonlinear optical effect of self-phase modulation (SPM) is generated by the optical power of the transmission channel itself that undergoes waveform distortion. Also, phase noise (nonlinear phase noise) due to the nonlinear optical effect of cross-phase modulation (XPM) is generated by the optical power of other transmission channels that are wavelength-division multiplexed (WDM).

[0010] The transmission performance limit due to the nonlinear optical effect is called the nonlinear Shannon limit. In improving the frequency utilization efficiency and extending the transmission distance of optical signals in an optical transmission system, the nonlinear Shannon limit is a major issue.

[0011] On the other hand, the waveform of the optical signal during transmission changes due to chromatic dispersion, and thus the nonlinear phase noise is averaged. Such an effect is called walk-off. For channel components with widely separated frequencies, the waveform change due to chromatic dispersion is larger compared to channel components with close frequencies. Therefore, in optical fiber transmission, it is common to use an optical fiber without a zero-dispersion wavelength within the transmission band of the optical signal so that the generation of nonlinear distortion due to cross-phase modulation is suppressed by walk-off. The zero-dispersion wavelength is the wavelength at which the chromatic dispersion becomes 0.

[0012] Wavelength dispersion induces the broadening of optical signal pulses, and the pulse broadening may induce inter-symbol interference. In order to suppress inter-symbol interference, it is necessary to compensate for wavelength dispersion during the demodulation of the optical signal. As a method for compensating wavelength dispersion, dispersion-managed transmission is widely used. In dispersion-managed transmission, an optical fiber having characteristics in a main transmission path is combined with an optical fiber (optical fiber for dispersion compensation) having characteristics opposite to those (wavelength dispersion) of the former. However, in dispersion-managed transmission, the optical transmission system transmits the optical signal while compensating for the wavelength dispersion of the optical signal. For this reason, the influence of walk-off is reduced, and non-linear noise increases.

[0013] In recent years, the practical application of digital coherent optical transmission has been progressing. In digital coherent optical transmission, a communication device on the receiving side executes digital signal processing. As a result, the wavelength dispersion accumulated in the optical signal is compensated all at once. Since the communication device on the receiving side compensates for the wavelength dispersion all at once by digital signal processing, it is not necessary to perform dispersion management in the transmission path, and the noise resulting from the cross-phase modulation occurring in the optical signal during transmission is suppressed by a large walk-off. Such a transmission method is called a non-dispersion-managed transmission method.

[0014] Also, as one type of optical amplifier, there is an optical parametric amplifier (OPA: Optical parametric amplifier). The optical parametric amplifier amplifies an input optical signal by utilizing a non-linear optical effect in a non-linear optical medium. The non-linear optical medium is, for example, lithium niobate which is a second-order non-linear medium, or an optical fiber which is a third-order non-linear medium.

[0015] Non-Patent Document 1 discloses an optical parametric amplifier using periodically poled lithium niobate (PPLN: Periodically poled lithium niobate) as an amplification medium (see Non-Patent Document 1). Such an optical parametric amplifier achieves both wide bandwidth and gain. For example, amplification relay transmission that achieves both a wide bandwidth of "more than 10 THz" and an amplification gain of "15 dB" has been demonstrated.

Prior Art Documents

Non-Patent Documents

[0016]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0017] In an optical parametric amplifier, when an optical signal is amplified, phase conjugate light is generated at a frequency determined by the frequency relationship between the optical signal and the pump light. This phase conjugate light is called idler light. The phase conjugate light is a complete replica of the input optical signal except for the point of being phase conjugate. That is, the phase conjugate light has the data and noise components of the input optical signal (original optical signal).

[0018] Therefore, one of the optical signal (original optical signal) input to the optical parametric amplifier and the phase conjugate light is selected by, for example, a band-pass filter. The selected optical signal or phase conjugate light is transmitted to a stage subsequent to the optical parametric amplifier. When the phase conjugate light is selected, it means that a phase conjugate conversion called optical phase conjugation (OPC) has been performed on the optical signal input to the optical parametric amplifier.

[0019] In this way, the phase conjugate conversion unit (amplification relay unit) having an optical parametric amplifier performs phase conjugate conversion and extraction of phase conjugate light. As a result, the extracted phase conjugate light is transmitted to the subsequent stage of the phase conjugate conversion unit. The distortion in the phase direction that occurred in the optical signal input to the optical parametric amplifier is compensated through, for example, the processes from (A1) to (A4) below. Hereinafter, the wavelength-division multiplexed optical signal is referred to as a "wavelength multiplexed signal".

[0020] (A1) In the transmission path of the wavelength multiplexed signal (WDM signal), phase rotation occurs in the wavelength multiplexed signal due to the non-linear optical effect and wavelength dispersion of the transmission medium (optical fiber). (A2) The phase conjugate conversion unit converts (phase conjugate conversion) the wavelength multiplexed signal input from the transmission path in the previous stage of the phase conjugate conversion unit into phase conjugate light. The sign (positive or negative) of the phase rotation of the input wavelength multiplexed signal is different from the sign of the phase rotation of the converted phase conjugate light. (A3) The transmission path in the subsequent stage of the phase conjugate conversion unit transmits the phase conjugate light. In the transmission path of the phase conjugate light, similar to the transmission path of the wavelength multiplexed signal (original optical signal) input to the phase conjugate conversion unit, phase rotation occurs in the phase conjugate light due to the non-linear optical effect and wavelength dispersion. (A4) The sign itself of the phase rotation that occurs in the optical signal is the same in each transmission path. The phase rotation of the wavelength multiplexed signal (original optical signal) in the transmission path in the previous stage of the phase conjugate conversion unit is inverted by the phase conjugate conversion. As a result, in the transmission path in the subsequent stage of the phase conjugate conversion unit, the phase rotation of the phase conjugate light is canceled out.

[0021] By performing phase conjugate conversion through the processes from (A1) to (A4) above, it is possible to compensate for the phase rotation resulting from the non-linear optical effect and wavelength dispersion. For this reason, phase conjugate conversion has attracted attention as a technology that breaks through the conventional non-linear Shannon limit.

[0022] In order to completely compensate for the non-linear phase noise, the amount of phase rotation generated in the front stage of the phase conjugate conversion unit and the amount of phase rotation generated in the rear stage of the phase conjugate conversion unit need to be the same. The non-linear phase noise includes noise caused by the interaction between signals that depends on the modulated data, and noise with random fluctuations generated by the interaction between the optical signal and the noise.

[0023] The non-linear phase noise caused by the interaction between signals is determined according to the change in the optical intensity (optical power) during transmission (hereinafter referred to as the "power map") and the change in the wavelength dispersion (hereinafter referred to as the "dispersion map"). In order for the amount of phase rotation generated in the front stage of the phase conjugate conversion unit to be the same as the amount of phase rotation generated in the rear stage of the phase conjugate conversion unit, it is sufficient that the power map is symmetric and the dispersion map is symmetric with the position of the phase conjugate conversion unit as the target axis (boundary).

[0024] If the wavelength dispersion coefficient in the front stage of the phase conjugate conversion unit is the same as the wavelength dispersion coefficient in the rear stage of the phase conjugate conversion unit, the dispersion map will be symmetric with the position of the phase conjugate conversion unit as the target axis. Therefore, it is important that the power map is symmetric with the position of the phase conjugate conversion unit as the target axis.

[0025] However, in an optical transmission system using the concentrated amplification relay method, since the power map becomes sawtooth-shaped according to the transmission distance of the optical signal, it is difficult to ensure the symmetry of the power map with the position of the phase conjugate conversion unit as the target axis. On the other hand, in the distributed amplification relay method, the symmetry of the power map can be ensured to a certain extent. For this reason, the effect of non-linear noise compensation by phase conjugate conversion in the distributed amplification relay method is higher than the effect of non-linear noise compensation by phase conjugate conversion in the concentrated amplification relay method.

[0026] Thus, particularly in an optical transmission system adopting the centralized amplification relay method, it is difficult to ensure the symmetry of the power map, so the compensation of non-linear noise by phase conjugate conversion is incomplete. Usually, phase noise caused by cross-phase modulation is suppressed to some extent by walk-off due to wavelength dispersion. However, in an optical transmission system having a phase conjugate conversion section, since the optical signal is transmitted while wavelength dispersion is compensated, the influence of walk-off is small as in dispersion-managed transmission.

[0027] On the other hand, since the sign of the phase rotation of the optical signal in the front stage of the phase conjugate conversion section is different from the sign of the phase rotation of the optical signal in the rear stage of the phase conjugate conversion section, the phase rotations of the optical signals cancel each other out. Therefore, the amount of non-linear phase noise in the communication device on the receiving side is determined by the balance between the increased amount of phase noise due to the reduced influence of walk-off and the amount that cancels out.

[0028] Also, most of the non-linear noise derived from self-phase modulation, which has little dependence on the power map and the dispersion map, is compensated by phase conjugate conversion even when the power map and the dispersion map are asymmetric. For these reasons, in an optical transmission system (asymmetric system) adopting the centralized amplification relay method, non-linear noise derived from cross-phase modulation becomes dominant when phase conjugate conversion is applied. Thus, there is a problem that the transmission distance of the optical signal cannot be improved.

[0029] In view of the above circumstances, an object of the present invention is to provide an optical transmission system, an optical transmission method, and a program capable of improving the transmission distance of an optical signal.

Means for Solving the Problem

[0030] One aspect of the present invention is a optical transmission system comprising a transmitter that generates a first optical signal in which a frequency interval between a plurality of channel components is maximally widened within a transmission band and the plurality of channel components are wavelength-division multiplexed optical signals, a first transmission line that transmits the first optical signal, a phase conjugate conversion unit that generates a second optical signal by inverting the spectrum of the first optical signal, and a second transmission line that transmits the second optical signal.

[0031] One aspect of the present invention is an optical transmission method executed by an optical transmission system, the method including a transmitter that maximally widens a frequency interval between a plurality of channel components within a transmission band and generates a first optical signal in which the plurality of channel components are wavelength-division multiplexed optical signals, a first transmission step of transmitting the first optical signal, a phase conjugate conversion step of generating a second optical signal by inverting the spectrum of the first optical signal, and a second transmission step of transmitting the second optical signal.

[0032] One aspect of the present invention is a program for causing a computer to function as the above optical transmission system.

Advantages of the Invention

[0033] According to the present invention, it is possible to improve the transmission distance of an optical signal.

Brief Description of the Drawings

[0034]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0035] (Overview) The generation efficiency of non-linear noise derived from cross-phase modulation depends on the interval (arrangement) of the channel components of the wavelength multiplexed signal on the frequency axis. Therefore, in an optical transmission system that performs phase conjugate conversion, it is strongly affected by the interval of the channel components as compared with an optical transmission system that does not perform phase conjugate conversion.

[0036] In an optical transmission system that performs phase conjugate conversion, the wider the interval of the channel components, the lower the influence of cross-phase modulation. In the following, regarding the noise of the non-linear phase, as the influence of cross-phase modulation decreases, the noise derived from self-phase modulation becomes more dominant.

[0037] By doing so, the performance of compensating for the non-linear phase noise of phase conjugate conversion is improved. Also, long-distance transmission exceeding the non-linear Shannon limit in an optical transmission system that does not perform phase conjugate conversion is realized.

[0038] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a diagram showing an example of the configuration of an optical transmission system 1. The optical transmission system 1 is a system that transmits optical signals (wavelength multiplexed signals). The optical transmission system 1 includes a transmitter 2, a plurality of optical repeaters 3, a plurality of optical transmission lines 4, one or more phase conjugate converters 5a, and a receiver 6. The optical repeaters 3 and the phase conjugate converters 5a are cascade-connected using the optical transmission lines 4.

[0039] For example, the optical transmission line 4-1, the phase conjugate conversion unit 5a-1, and the optical transmission line 4-2 form one set. The optical transmission system 1 may include a plurality of such sets. For example, the optical transmission line 4-3, the phase conjugate conversion unit 5a-2, and the optical transmission line 4-4 form another set. Such a plurality of sets are cascade-connected in the optical transmission system 1.

[0040] 1, an optical transmission line 4-1 (first transmission line) includes a transmitter 2 that transmits a wavelength-multiplexed signal (first optical signal). An optical transmission line 4-2 (second transmission line) includes one or more optical repeaters 3-1 that amplify and repeat a new wavelength-multiplexed signal (second optical signal) that is phase conjugate light.

[0041] The transmitter 2 is a communication device on the transmitting side. The transmitter 2 generates a wavelength multiplexed signal. In the wavelength multiplexed signal, channel components of multiple wavelengths are multiplexed (wavelength division multiplexed). The transmitter 2 transmits the wavelength multiplexed signal to the optical phase conjugate converter 5a-1.

[0042] It is desirable that the spacing between the channel components of the wavelength-multiplexed signal within the transmission band be maximized while the required number of channel components is ensured in the optical transmission system 1. This transmission band is predetermined according to the bandwidth of the amplification repeater (optical repeater unit 3 and phase conjugate conversion unit 5a). The transmitter 2 transmits the wavelength-multiplexed signal, in which the spacing between the channel components within the predetermined transmission band has been widened to a predetermined threshold or more, to the phase conjugate conversion unit 5a-1. The maximum spacing between the channel components of the wavelength-multiplexed signal on the frequency axis, "Δf max " is defined as in equation (1).

[0043] Δf max =W / N …(1)

[0044] Here, "N" represents the number of channel components required for the optical transmission system. "W" represents the transmission band. The transmission band is predetermined according to the band of the optical amplification repeater (optical relay unit 3 and phase conjugate conversion unit 5a) in the optical transmission system 1 and the like.

[0045] The transmitter 2 adjusts the interval between the channel components of the wavelength-division multiplexed signal by adjusting the output wavelength of the light source that generates the wavelength-division multiplexed signal for each channel component. The transmitter 2 may adjust the interval (oscillation frequency) between the channel components of the wavelength-division multiplexed signal by using light sources having different oscillation frequencies (oscillation wavelengths) for each channel component for generating the wavelength-division multiplexed signal.

[0046] When the intervals between the channel components of the wavelength-division multiplexed signal are arranged on the frequency axis at an interval of the maximum value "Δf" max ", the effect of phase conjugate conversion is maximized. Also, the transmission performance is significantly improved.

[0047] To improve the signal-to-noise ratio, optical signal amplification and relay may be performed a predetermined number of times in each transmission line before and after the phase conjugate conversion unit 5a. For example, the number of optical relay units 3-1 in the optical transmission line 4-3 before the phase conjugate conversion unit 5a-2 may be equal to the number of optical relay units 3-2 in the optical transmission line 4-4 after the phase conjugate conversion unit 5a-2.

[0048] In FIG. 1, as an example, the process of generating the phase conjugate light of the wavelength-division multiplexed signal is executed every two spans (for example, two optical transmission lines 4).

[0049] In Fig. 1, the processes from (A1) to (A4) (one set) are executed in the section from the optical transmission path 4 in the front stage of the phase conjugate conversion unit 5a to the optical transmission path 4 in the rear stage of the phase conjugate conversion unit 5a. That is, in the processes from (A1) to (A4), as an example, one phase conjugate conversion unit 5a and one optical relay unit 3 are used. The number of optical relay units 3 does not have to be limited to a specific number, and more optical relay units 3 may be used in the processes from (A1) to (A4). In Fig. 1, as an example, in a 6-span including two amplification relays by the optical relay unit 3 and three amplification relays by the phase conjugate conversion unit 5a, the wavelength division multiplexed signal is transmitted to the receiving unit 6.

[0050] In addition, since the gain of the optical parametric amplifier that performs phase conjugate conversion is not always sufficient for amplification relay, an amplification relay may be required separately from the optical relay unit 3. That is, for the purpose of obtaining the relay gain of the optical signal, the phase conjugate conversion unit 5a may be provided with an amplification relay (for example, an erbium-doped optical fiber amplifier). In this case, the amplification relay provided in the phase conjugate conversion unit 5a performs amplification relay in the same manner as the optical relay unit 3, and the optical parametric amplifier provided in the phase conjugate conversion unit 5a performs phase conjugate conversion. When the gain of the optical parametric amplifier is sufficient for amplification relay, another amplification relay is not required, and the phase conjugate conversion unit 5a can be configured only by the optical parametric amplifier that performs phase conjugate conversion.

[0051] Fig. 2 is a diagram (wavelength dispersion map) showing an example of the change in the amount of wavelength dispersion in the first embodiment. The transmitting unit 2 generates a wavelength division multiplexed signal. "L0" represents the position of the transmitting unit 2. "L1" represents the distance from the transmitting unit 2 to the phase conjugate conversion unit 5a-1. In other words, "L1" represents the position of the phase conjugate conversion 5a-1 from the transmitting unit 2.

[0052] "L2" represents the distance from the transmission unit 2 to the optical relay unit 3-1. In other words, "L2" represents the position of the optical relay unit 3-1. "L3" represents the distance from the transmission unit 2 to the phase conjugate conversion unit 5a-2. In other words, "L3" represents the position of the phase conjugate conversion unit 5a-2. "L4" represents the distance from the transmission unit 2 to the optical relay unit 3-2. In other words, "L4" represents the position of the optical relay unit 3-2.

[0053] "L5" represents the distance from the transmission unit 2 to the phase conjugate conversion unit 5a-3. In other words, "L5" represents the position of the phase conjugate conversion unit 5a-3. "L6" represents the distance from the transmission unit 2 to the reception unit 6. In other words, "L6" represents the position of the reception unit 6.

[0054] Optical transmission lines 4 with the same length and the same characteristics are respectively arranged in the front stage and the rear stage of the phase conjugate conversion unit 5a that performs optical parametric amplification. That is, the difference between the distance "L1" and the distance "L0" is equal to the difference between the distance "L2" and the distance "L1". The difference between the distance "L3" and the distance "L2" is equal to the difference between the distance "L4" and the distance "L3". Also, the difference between the distance "L5" and the distance "L4" is equal to the difference between the distance "L6" and the distance "L5". Note that the criteria for determining whether they have the same length are predefined. Also, the criteria for determining whether they have the same characteristics are predefined.

[0055] In the optical transmission system 1 where the phase conjugate conversion process is executed, for example, every two spans, it is sufficient that the span length before the phase conjugate conversion process is equal to the span length after the phase conjugate conversion process, and the number of amplification relays before the phase conjugate conversion process is equal to the number of amplification relays after the phase conjugate conversion process. In the optical transmission system 1, the number of times the phase conjugate conversion process (the process of generating phase conjugate light) is executed is not limited to a specific number. For example, in the optical transmission system 1 that amplifies and relays an optical signal over six spans, even when the phase conjugate conversion is executed every three spans, the wavelength dispersion map is symmetric with the position of the phase conjugate conversion unit 5a as the axis of symmetry.

[0056] Returning to Fig. 1, the description of the configuration example of the optical transmission system 1 will be continued. The transmission unit 2 transmits the wavelength-division multiplexed signal to the phase conjugate conversion unit 5a-1. The optical relay unit 3 compensates for the loss generated in the wavelength-division multiplexed signal in the optical transmission line 4. Further, the optical relay unit 3 compensates for the loss generated in the phase conjugate light in the optical transmission line 4.

[0057] The optical transmission line 4 has a transmission line such as an optical fiber. In the optical transmission line 4, signal distortion due to the non-linear optical effect occurs in the wavelength-division multiplexed signal. The optical transmission line 4 has wavelength dispersion for all channel components of the wavelength-division multiplexed optical signal so that none of the channel components of the wavelength-division multiplexed optical signal becomes zero dispersion. Further, the optical transmission line 4 has wavelength dispersion for all channel components of the phase conjugate light so that none of the channel components of the phase conjugate light becomes zero dispersion.

[0058] The phase conjugate conversion unit 5a collectively converts the wavelength-division multiplexed signal into phase conjugate light by phase conjugate conversion called optical phase conjugation. Due to the effect of the phase conjugate conversion, the wavelength-division multiplexed signal is transmitted while compensating for the wavelength dispersion of the wavelength-division multiplexed signal.

[0059] The receiving unit 6 is a communication device on the receiving side. The receiving unit 6 receives the phase conjugate light of the wavelength-division multiplexed signal from the phase conjugate conversion unit 5a-3. The receiving unit 6 executes a predetermined receiving process on the phase conjugate light of the wavelength-division multiplexed signal. For example, the receiving unit 6 demodulates the modulated data in the phase conjugate light.

[0060] Fig. 3 is a diagram showing a configuration example of the phase conjugate conversion unit 5a in the first embodiment. Generally, the process of optical parametric amplification has polarization dependence. For this reason, the phase conjugate conversion unit 5a has a polarization diversity configuration. That is, the phase conjugate conversion unit 5a includes a polarization beam splitter 51, two optical amplifying units 52, a polarization combiner 53, and a band-pass filter 54. The optical amplifying unit 52 has a non-linear medium (non-linear optical medium).

[0061] Note that the configuration of the polarization diversity in the phase conjugate conversion unit 5a may be, for example, the configuration of the polarization diversity shown in Reference 1 (T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto and H. Takenouchi., “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier.).

[0062] The polarization beam splitter 51 splits the input wavelength-division multiplexed signal into two orthogonal polarization components. Each optical amplifier 52 receives the pump light. Each optical amplifier 52 multiplexes the polarization component and the pump light using a wavelength-division multiplexing coupler, a dichroic mirror, etc. In each optical amplifier 52, the multiplexed polarization component and pump light are input to the nonlinear medium. The optical amplifier 52 (nonlinear medium) performs optical parametric amplification for each polarization component using the pump light. The nonlinear medium may be a third-order nonlinear medium such as an optical fiber, or a second-order nonlinear medium such as lithium niobate. At the output end of the nonlinear medium, the pump light is separated from the polarization component using a wavelength-division multiplexing coupler, a dichroic mirror, etc.

[0063] The polarization multiplexer 53 recombines the two polarization components. The band-pass filter 54 extracts the phase conjugate light (idler light) generated by optical parametric amplification from the recombined two polarization components.

[0064] Next, an operation example of the optical transmission system 1 will be described. FIG. 4 is a flowchart showing an operation example of the optical transmission system 1 in the first embodiment. The transmitter 2 generates a first optical signal, which is an optical signal in which a plurality of channel components are wavelength-division multiplexed, by spreading the frequency interval between the plurality of channel components to the maximum within the transmission band (step S101). The optical transmission line 4 (first transmission line) in the front stage of the phase conjugate conversion unit 5a transmits the first optical signal (step S102).

[0065] The phase conjugate conversion unit 5a generates a second optical signal (phase conjugate light) by inverting the spectrum of the first optical signal (step S103). The optical transmission path 4 (second transmission path) in the subsequent stage of the phase conjugate conversion unit 5a transmits the second optical signal (step S104). The optical relay unit 3 may amplify and relay the second optical signal (step S105). The receiving unit 6 receives the phase conjugate light from the optical transmission path 4 (second transmission path) (step S106).

[0066] As described above, the transmitting unit 2 maximally expands the frequency interval between the plurality of channel components within the transmission band. The transmitting unit 2 generates a first optical signal (wavelength multiplexed signal) which is an optical signal in which a plurality of channel components are wavelength division multiplexed. The first transmission path (for example, the optical transmission path 4-1) transmits the first optical signal. The phase conjugate conversion unit 5a generates a second optical signal (phase conjugate light) by inverting the spectrum of the first optical signal. The second transmission path (for example, the optical transmission path 4-2) transmits the second optical signal. The first transmission path wavelength disperses the plurality of channel components of the first optical signal. The second transmission path wavelength disperses the plurality of channel components of the second optical signal.

[0067] The first transmission path (for example, the optical transmission path 4-3) may include one or more first optical relay units (for example, the optical relay unit 3-1) that amplify and relay the first optical signal. The second transmission path (for example, the optical transmission path 4-4) may include one or more second optical relay units (for example, the optical relay unit 3-2) that amplify and relay the second optical signal. The number of the first optical relay units in the previous stage of the phase conjugate conversion unit 5a and the number of the second optical relay units in the subsequent stage of the phase conjugate conversion unit 5a may be equal.

[0068] In this way, the influence of the cross-phase modulation on the non-linear phase noise is reduced, and the noise derived from the self-phase modulation becomes more dominant. Therefore, the performance of compensating for the non-linear phase noise by the phase conjugate conversion is maximized. As a result, it is possible to improve the transmission distance of the optical signal.

[0069] (Second Embodiment) In the second embodiment, as an example, the difference from the first embodiment is that the optical transmission system 1 includes a 12-span optical transmission line 4. In the second embodiment, the description will focus on the differences from the first embodiment.

[0070] The optical transmission system 1 in the second embodiment includes, as an example, one transmission unit 2, five optical relay units 3, a 12-span optical transmission line 4, six phase conjugate conversion units 5a, and one reception unit 6.

[0071] In the optical transmission system 1 of the second embodiment, a total of 12 (= 5 + 6 + 1) times of amplified relaying is performed by the five optical relay units 3, the six phase conjugate conversion units 5a, and the one reception unit 6. Also, similar to the phase conjugate conversion unit 5a illustrated in FIG. 1, the phase conjugate conversion unit 5a performs phase conjugate conversion every two spans.

[0072] FIG. 5 is a diagram showing an example of the frequency arrangement (frequency dependence) of the optical signal before phase conjugate conversion and the optical signal after phase conjugate conversion in the second embodiment. The transmission unit 2 transmits a wavelength-division multiplexed signal in which five-channel components are multiplexed on the low-frequency side of the central frequency "f0" of optical parametric amplification to the phase conjugate conversion unit 5a. The central frequency "f0 - f1" of the wavelength-division multiplexed signal is 192.5 THz. The central frequency "f0" of the optical parametric amplification used for phase conjugate conversion is 194 THz. The frequency "f0 + f1" of the phase conjugate light generated by phase conjugate conversion is 195.5 THz. The nonlinear medium is, as an example, a second-order nonlinear medium. Therefore, the frequency of the pump light is the second harmonic "2f0" (= 388.0 THz) of the central frequency.

[0073] The channel component 10 is the channel component of the wavelength-division multiplexed signal input to the phase conjugate conversion unit 5a. The channel component 11 is the channel component of the wavelength-division multiplexed signal output from the phase conjugate conversion unit 5a.

[0074] FIG. 6 is a diagram showing an example of the relationship between wavelength dispersion and signal-to-noise ratio (numerical analysis result) in the second embodiment. The vertical axis represents the signal-to-noise ratio (quality of the received wavelength-division multiplexed signal). The horizontal axis represents the wavelength dispersion of the wavelength-division multiplexed signal at the local wavelength of 1550 nm.

[0075] In FIG. 6, the length of the optical transmission line 4 (optical fiber) is 80 km. Long-distance transmission of 960 km is performed by a total of 12 amplification relays. The dispersion slope of the optical transmission line 4 as the transmission medium is, as an example, 0.07 ps / nm 2 / km. The dispersion slope is the wavelength derivative of the wavelength dispersion. The nonlinear optical constant is, as an example, 2.3 / W / km. The propagation loss is, as an example, 0.23 dB / km.

[0076] The noise figure of the amplification relay (optical relay unit 3 and phase conjugate conversion unit 5a) is, as an example, 4.5 dB. The modulation format of the wavelength-division multiplexed signal is, as an example, 32 Gbaud DP-QPSK (Dual Polarization Differential Quadrature Phase Shift Keying). The power (optical intensity) of the wavelength-division multiplexed signal input to the optical transmission line 4 is determined to be the power value at which the signal-to-noise ratio is the highest among the respective dispersion conditions.

[0077] The intervals of the channel components 10 are three types: 100 GHz, 200 GHz, and 400 GHz. Similarly, the intervals of the channel components 11 are three types: 100 GHz, 200 GHz, and 400 GHz.

[0078] When phase conjugate conversion is not performed (when the phase conjugate conversion unit 5a only performs amplification and does not perform phase conjugate conversion), it was confirmed that the transmission performance deteriorates the most when the wavelength dispersion is "-0.5 ps / nm / km". Here, the local wavelength dispersion at the center frequency (center wavelength) of the input wavelength-division multiplexed signal becomes almost 0 according to the dispersion slope of the optical transmission line 4.

[0079] Since the wavelength dispersion is small, the influence of walk-off is small. Since the influence of walk-off is small, the generation efficiency of phase noise due to cross-phase modulation is high. For this reason, the transmission performance deteriorates. The amount of deterioration of the transmission performance is smaller when the interval of the channel components 11 is 400 GHz than when the interval of the channel components 11 is 100 GHz. This is because the influence of cross-phase modulation is reduced by expanding the interval of the channel components.

[0080] When phase conjugate conversion is performed (when the phase conjugate conversion unit 5a performs optical parametric amplification used as phase conjugate conversion), the transmission performance deteriorates between the wavelength dispersion of "-0.5 ps / nm / km" and the wavelength dispersion of "1.0 ps / nm / km".

[0081] The reason why the transmission performance deteriorates when the wavelength dispersion is "-0.5 ps / nm / km" is the same as the reason when phase conjugate conversion is not performed. The reason why the transmission performance also deteriorates when the wavelength dispersion is "1.0 ps / nm / km" is that the local wavelength dispersion becomes 0 in the band of the phase conjugate light generated by phase conjugate conversion.

[0082] Therefore, in the optical transmission system 1 that performs phase conjugate conversion, in order to reduce the influence of cross-phase modulation, it is necessary to select a transmission medium so that the wavelength dispersion of the optical transmission line 4 is sufficiently large in both the band of the wavelength multiplexed signal and the band of the phase conjugate light.

[0083] Specifically, the absolute value of the wavelength dispersion is, for example, "2 ps / nm / km" or more in both the band of the wavelength multiplexed signal and the band of the phase conjugate light. The zero-dispersion wavelength of the standard single-mode fiber often used for long-distance transmission is about 1.30 μm. Since the band of the zero-dispersion wavelength of "1.30 μm" is far from the generally used band of "C-band", this requirement is satisfied.

[0084] The zero-dispersion wavelength of a non-zero dispersion shifted fiber exists near 1.50 μm. Non-zero dispersion shifted fibers are also often used as transmission media. However, when a non-zero dispersion shifted fiber is used as a transmission medium, the wavelength of the phase-conjugated light generated by phase-conjugate conversion (the wavelength of the optical signal after phase-conjugate conversion) is close to the zero-dispersion wavelength. Therefore, it is necessary to determine the wavelength of the phase-conjugated light so that the absolute value of the wavelength dispersion does not become less than "2 ps / nm / km".

[0085] The wavelength of the phase-conjugated light is determined according to the phase matching condition of the optical parametric amplification medium used as the phase-conjugate conversion. Also, when comparing the results in the case where the channel component interval is 400 GHz, in the optical transmission system 1 that performs phase-conjugate conversion, compared with the optical transmission system that does not perform phase-conjugate conversion, the amount of degradation in the case where the wavelength dispersion is "-0.5 ps / nm / km" becomes larger. The fact that the amount of degradation becomes larger in this way indicates that the influence of the phase noise derived from the cross-phase modulation becomes greater due to the use of phase-conjugate conversion.

[0086] FIG. 7 is a diagram showing an example of the relationship between the transmission path input power and the signal-to-noise ratio in the second embodiment. Specifically, the characteristics of the optical intensity (transmission path input power) of the wavelength division multiplexed signal input to the transmission path in the case where the wavelength dispersion of the transmission medium at the wavelength "1550 nm" is "3 ps / nm / km" are shown.

[0087] When phase-conjugate conversion is not performed (when there is no OPC), the improvement amount of the transmission performance due to the expansion of the channel component interval is not very large. This is because the walk-off of the non-linear phase noise due to the cross-phase modulation is caused by the wavelength dispersion, so the non-linear phase noise derived from the self-phase modulation becomes relatively dominant.

[0088] When phase conjugation conversion is performed (when "OPC is present"), the walk-off of the non-linear phase noise due to cross-phase modulation is small. Therefore, in the optical transmission system 1 that performs phase conjugation conversion, the transmission performance varies greatly depending on the interval between channel components. This indicates that the phase noise originating from self-phase modulation has been greatly reduced by performing phase conjugation conversion, and that the phase noise originating from cross-phase modulation has become dominant due to the reduced walk-off.

[0089] Therefore, in order to maximize the transmission performance of the optical transmission system 1 that performs phase conjugation conversion, it is important that the interval between channel components be set as large as possible. In practice, the longer the interval between channel components, the fewer the number of channel components that need to be reduced. For this reason, it is necessary to widen the interval between channel components to the maximum extent in consideration of the required number of channels.

[0090] As described above, the transmission unit 2 ensures the required number of channels in the optical transmission system 1 and widens the interval between a plurality of channel components to the maximum extent within the transmission band. Thereby, it is possible to improve the transmission distance of the optical signal.

[0091] (Third Embodiment) In the third embodiment, the difference from the first embodiment is that the optical transmission system 1 includes a complementary spectral inversion (CSI) type phase conjugation conversion unit. In the third embodiment, the description will be centered on the difference from the first embodiment.

[0092] The optical transmission system 1 in the third embodiment includes a transmission unit 2, a plurality of optical relay units 3, a plurality of optical transmission paths 4, one or more phase conjugation conversion units 5b, and a reception unit 6.

[0093] It is necessary to leave a band for the phase conjugate light. For this reason, on the frequency axis, the wavelength multiplexed signal can be arranged only on either the low frequency side or the high frequency side with respect to the center frequency “f0” of the band of the optical parametric amplification as a reference. The band that can be used for signal transmission is half of the band of the optical parametric amplification used as the phase conjugate conversion.

[0094] Therefore, the bandwidth in which the necessary number “N” of channel components can be arranged becomes “B / 2” with respect to the band “B” of the optical parametric amplification. Also, the interval in which the channel components can be secured also becomes “B / 2N”. To avoid this, the optical transmission system 1 in the third embodiment includes a phase conjugate conversion unit 5b as a complementary spectrum inversion type phase conjugate conversion unit.

[0095] Note that the phase conjugate conversion unit 5b may have a configuration similar to the configuration of the complementary type spectrum inversion unit shown in Reference 2 (Japanese Patent Application Laid-Open No. 2016-218173).

[0096] FIG. 8 is a diagram showing a configuration example of the phase conjugate conversion unit (complementary spectrum inversion type phase conjugate conversion unit) in the third embodiment. The optical transmission system 1 includes two polarization beam splitters 51, four optical amplifiers 52, two polarization beam combiners 53, two band pass filters 54, a band demultiplexer 55, and a band multiplexer 56.

[0097] A single wavelength channel component is input to the band demultiplexer 55. The band demultiplexer 55 divides the band of the single wavelength channel component into a first band and a second band with the center frequency “f0” of the optical parametric amplification by the optical amplifier 52 as a boundary. For example, the channel component of the first band is input to the polarization beam splitter 51-1. The channel component of the second band is input to the polarization beam splitter 51-2.

[0098] Nonlinear optical effects including the process of optical parametric amplification have polarization dependence. Therefore, the polarization beam splitter 51 splits the input channel components into a first polarization component and a second polarization component. Here, the first polarization component and the second polarization component are orthogonal to each other. The polarization beam splitter 51 splits the optical signal into a first polarization component and a second polarization component using, for example, a polarization beam splitter.

[0099] Excitation light is input to the nonlinear medium of the optical amplification unit 52. In the phase conjugate conversion unit 5a, the optical amplification unit 52 performs spectral inversion for each of the split polarization components with the center frequency “f0” of the optical parametric amplification as the axis of symmetry (boundary).

[0100] The first polarization component is input from the polarization beam splitter 51 to the nonlinear medium of the optical amplification unit 52-1-n (n is an integer of 1 or more). The second polarization component is input from the polarization beam splitter 51 to the nonlinear medium of the optical amplification unit 52-1-(n + 1). The same applies to the optical amplification unit 52-2.

[0101] The optical amplification unit 52 combines the input polarization component and the excitation light using, for example, a dichroic mirror. The optical amplification unit 52 may combine the input polarization component and the excitation light using, for example, a wavelength division multiplexing coupler. By optical parametric amplification by the nonlinear medium of the optical amplification unit 52, each polarization component is amplified. In this case, phase conjugate light is generated in a symmetric band with the center frequency “f0” of the optical parametric amplification as the axis of symmetry (boundary).

[0102] The polarization combiner 53 combines each polarization component using, for example, a polarization beam combiner or the like. The band-pass filter 54 allows the optical signal in the spectrally inverted band to pass through among the optical signals of the combined polarization components. In this way, the band-pass filter 54 removes the optical signal in the band that is not spectrally inverted (channel components other than the phase conjugate light) from the combined polarization components. That is, the band-pass filter 54 extracts the optical signal in the band whose spectrum is inverted (phase conjugate light) from the combined polarization components.

[0103] The wavelength multiplexing unit 56 multiplexes a single-wavelength channel component in the first band and a single-wavelength channel in the second band. As a result, phase-conjugated light is obtained. This phase-conjugated light is light in which the input wavelength-division multiplexed signal (original optical signal) is spectrally inverted with the center frequency of the band of optical parametric amplification as the axis of symmetry.

[0104] FIG. 9 is a diagram showing an example of the frequency arrangement of the optical signal before phase-conjugate conversion and the optical signal after phase-conjugate conversion in the third embodiment. In the upper part of FIG. 9, a total of 10-wavelength wavelength-division multiplexed signals before phase-conjugate conversion are arranged on the frequency axis in groups of 5 with the center frequency "f0" of the band of optical parametric amplification as the axis of symmetry (boundary). In the lower part of FIG. 9, a total of 10-wavelength wavelength-division multiplexed signals after phase-conjugate conversion are arranged on the frequency axis in groups of 5 with the center frequency "f0" of the band of optical parametric amplification as the axis of symmetry (boundary).

[0105] As described above, the phase-conjugate conversion unit 5b is a complementary spectral inversion type phase-conjugate conversion unit. As a result, since the wavelength-division multiplexed signals are arranged on both the low-frequency side and the high-frequency side with the center frequency of the band of optical parametric amplification as a reference, it is possible to further improve the transmission distance of the optical signal. Since the interval between channel components is maximized over the entire band of optical parametric amplification, it is possible to further improve the transmission distance of the optical signal.

[0106] (Example of Hardware Configuration) FIG. 10 is a diagram showing a hardware configuration example of a communication device (transmission unit) (reception unit) in each embodiment. The communication device 100 corresponds to at least one of the transmission unit and the reception unit in each embodiment. The communication device 100 generates or processes data transmitted using an optical signal. Some or all of the functional units of the communication device 100 are realized as software by a processor 101 such as a CPU (Central Processing Unit) executing a program stored in a storage device 102 having a non-volatile recording medium (non-temporary recording medium) and a memory 103. The program may be recorded on a computer-readable non-temporary recording medium. A computer-readable non-temporary recording medium is, for example, a non-temporary recording medium such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a CD-ROM (Compact Disc Read Only Memory) and other portable media, or a storage device such as a hard disk built into a computer system. The communication unit 104 executes predetermined communication processing. The communication unit 104 may acquire data and programs.

[0107] Some or all of the functional units of the communication device 100 may be realized using hardware including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0108] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Industrial Applicability

[0109] The present invention is applicable to an optical transmission system (optical communication system).

Description of Signs

[0110] 1... optical transmission system, 2... transmission unit, 3... optical relay unit, 4... optical transmission path, 5a, 5b... phase conjugate conversion unit, 6... reception unit, 10... channel component, 11... channel component, 51... polarization beam splitter, 52... optical amplifier, 53... polarization combiner, 54... band-pass filter, 55... band demultiplexer, 56... band multiplexer, 100... communication device, 101... processor, 102... storage device, 103... memory, 104... communication unit

Claims

1. A transmitter that maximally widens the frequency intervals between a plurality of channel components within a transmission band and generates a first optical signal that is an optical signal in which the plurality of channel components are wavelength-division multiplexed; A first transmission path that transmits the first optical signal; A phase conjugate conversion unit that generates a second optical signal by inverting the spectrum of the first optical signal; A second transmission path that transmits the second optical signal and comprising: The phase conjugate conversion unit arranges the first optical signal on both the low-frequency side and the high-frequency side with reference to the center frequency of the band of optical parametric amplification. An optical transmission system.

2. The first transmission path disperses the plurality of channel components of the first optical signal, The second transmission path disperses the plurality of channel components of the second optical signal. The optical transmission system according to Claim 1.

3. The first transmission path includes one or more first optical relay units that amplify and relay the first optical signal, The second transmission path includes one or more second optical relay units that amplify and relay the second optical signal, The number of the first optical relay units is equal to the number of the second optical relay units. The optical transmission system according to Claim 1 or Claim 2.

4. An optical transmission method executed by an optical transmission system, comprising: A transmitter that maximally widens the frequency intervals between a plurality of channel components within a transmission band and generates a first optical signal that is an optical signal in which the plurality of channel components are wavelength-division multiplexed; A first transmission step of transmitting the first optical signal; A phase conjugate conversion step of generating a second optical signal by inverting the spectrum of the first optical signal; A second transmission step of transmitting the second optical signal and including: The phase conjugate conversion step includes arranging the first optical signal on both the low-frequency side and the high-frequency side with reference to the center frequency of the band of optical parametric amplification. An optical transmission method.

5. A program for causing a computer to function as the optical transmission system according to any one of Claims 1 to 3.

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