JP2015035719A - System and method for optical transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmission system capable of compensating signal quality deterioration in optical transmission without excessively enlarging a transmission/reception device scale, by using optical compensation and digital signal processing together.SOLUTION: The optical transmission system includes an optical dispersion compensation function unit for performing optical dispersion compensation on a span-by-span basis. There is performed one or both of the compensation of phase rotation, using waveform information in the middle of propagation producing maximum peak power, and the compensation of phase rotation by mutual phase modulation based on adjacent channel intensity.

Description

  The present invention relates to an optical transmission system and an optical transmission method having a function of compensating for signal quality degradation in a WDM (Wavelength Division Multiplex) digital coherent optical transmission system.

  In recent years, research and development of a digital coherent fiber transmission system in which a digital signal processing technique is applied to a coherent transmission method has progressed, and a part of the system has been introduced. In digital coherent transmission, chromatic dispersion can be compensated at once by digital signal processing at the receiving end, so that a dispersion compensating fiber installed for each repeater of a conventional optical transmission system is no longer indispensable (for example, Patent Documents). 1).

  Also in digital coherent transmission, studies are also underway on the influence of fiber nonlinear optical effects such as self phase modulation (SPM) and cross phase modulation (XPM) on transmission quality. These are phenomena in which the phase is modulated according to the light intensity of the target channel and other channels, and when complex with elements that cause waveform distortion by changing the intensity waveform of the optical signal, such as chromatic dispersion, a complex composite Effect. In order to compensate all of these efficiently, it is necessary to compensate for the nonlinear effect and the linear effect simultaneously. In order to perform this approximately, a compensation method has been proposed in which a configuration in which the nonlinear effect is slightly compensated after compensating for the wavelength dispersion a little by the receiving end digital signal processing in the digital coherent transmission is repeated a plurality of times. For example, Patent Document 2 shows that SPM compensation is possible.

  In such a circuit, it has been reported that it is desirable that the number of repetitions is equal to or greater than the number of optical amplification repeater spans from the viewpoint of performance. However, if the waveform distortion compensation and the phase rotation compensation are performed by the receiver a plurality of times, there arises a problem that the receiving circuit scale becomes large. Dispersion compensation by digital signal processing requires a procedure in which a signal is converted to the frequency domain by Fourier transform, multiplied by a transfer function for dispersion compensation, and then returned to the time domain by inverse Fourier transform. In order to perform dispersion compensation and non-linear compensation of a transmission line with M (M is a natural number) span, 2M or more real-time fast Fourier transform / inverse transform (FFT / IFFT) circuits are required, so the circuit is unrealistic. The problem of becoming a large scale arises. FIG. 58 shows a schematic diagram of a method for compensating for the nonlinear optical effect in a system in which dispersion compensation is not performed by the relay device. As shown in FIG. 58, in the case of an M span transmission line, it is necessary to perform linear equalization and nonlinear equalization processing of M steps. This corresponds to performing M times of real-time fast Fourier transform / inverse transform (FFT / IFFT). Further, Non-Patent Document 1 reports that a system that does not perform dispersion compensation with relays is more advantageous in order to suppress the occurrence of nonlinear effects.

  A method of compensating XPM by digital signal processing at the receiving end is also being studied, but because it is due to the influence of an uncorrelated adjacent channel with the received signal, it is completely possible to receive only the measurement target channel by digital signal processing. It is difficult to compensate. Although a method of simultaneously receiving adjacent channels and compensating them by digital signal processing has been studied, there is a report that the amount of calculation of waveform distortion compensation and phase rotation compensation increases compared to the case where only SPM is compensated. It is difficult to keep the scale within a realistic scale (see Non-Patent Document 2, for example). On the other hand, a method of suppressing XPM generation by optical dispersion compensation for each span and provision of a time difference between channels has been proposed. (For example, see Patent Document 3)

Japanese Patent No. 4872003 Japanese Patent No. 4759625 Japanese Patent No. 4879155

Journal of Lightwave Technology vol.26, no.20, 2008, pp.3416-3425, E.Ip et al., "Compensation of dispersion and nonlinear impairments using digital backpropagation" Optics Express vol.19, no.2, 2011, pp.570-583, E.Mateo et al., "Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems"

  As described above, if only chromatic dispersion and compensation for all nonlinear optical effects are performed with only digital signal processing, there arises a problem that the apparatus scale becomes large. In addition, when there is XPM in WDM transmission, it is difficult to separate the contribution of phase rotation due to SPM and phase rotation due to XPM from adjacent channels, and XPM is completely compensated only by digital signal processing of the received signal. There is a problem that it is difficult.

  The present invention has been made in view of such circumstances, and by combining optical compensation and digital signal processing, it is possible to compensate for signal quality degradation in optical transmission without excessively increasing the size of a transmission / reception device. An object of the present invention is to provide an optical transmission system and an optical transmission method capable of performing the above.

  The present invention includes an optical dispersion compensation function unit that performs optical dispersion compensation for each span, and compensates for phase rotation using waveform information during propagation in which peak power is maximized, or based on adjacent channel strength. One or both of compensation for phase rotation due to phase modulation is performed.

  The present invention relates to a plurality of optical nodes, a plurality of optical transmitters, and a plurality of optical receivers each including an optical amplifier that compensates for a loss of each span of an optical transmission signal, and a dispersion compensation function unit that compensates dispersion for each span. A digital coherent optical transmission system comprising: an optical transmitter, wherein the optical transmitter and the optical receiver simultaneously process time-synchronized signals of a plurality of wavelength channels to compensate for a nonlinear optical effect in a transmission path It is characterized by.

  According to the present invention, the optical node includes at least one of a functional unit that gives a time difference between wavelength channels, a functional unit that gives a time difference between polarized waves, and a functional unit that gives a time difference between both wavelength channels and between polarized waves. It is characterized by providing one.

  The present invention includes a skew adjustment unit that adjusts a skew and a phase rotation unit that rotates a phase, and the compensation of the nonlinear optical effect is performed by skew adjustment and phase rotation.

  The present invention further includes a chromatic dispersion adjusting unit that adjusts chromatic dispersion before and after the phase rotating unit.

  The present invention is characterized in that the nonlinear optical effect is compensated by one phase rotation.

  The present invention relates to an optical transmission method performed by an optical transmission system including one or both of a pre-compensation function unit that compensates chromatic dispersion and an optical dispersion compensation function unit that performs optical dispersion compensation. One or both of compensating phase rotation using waveform information during propagation in which peak power is maximized due to the chromatic dispersion, or compensating phase rotation due to cross-phase modulation based on adjacent channel strength It is characterized by performing.

  The present invention relates to a plurality of optical nodes, a plurality of optical transmitters, and a plurality of optical receivers each including an optical amplifier that compensates for a loss of each span of an optical transmission signal, and a dispersion compensation function unit that compensates dispersion for each span. A digital coherent optical transmission system comprising: an optical transmitter, wherein the optical transmitter and the optical receiver simultaneously process time-synchronized signals of a plurality of wavelength channels, thereby causing nonlinearity in a transmission path It is characterized by compensating for optical effects.

  According to the present invention, it is possible to improve the signal quality by SPM and XPM compensation with a simple configuration by using optical compensation together with the nonlinear optical effect compensation only by digital signal processing, and the apparatus. The effect that the scale can be reduced is obtained.

1 is a block diagram showing a configuration of an optical transmission system according to a first embodiment of the present invention. It is explanatory drawing which shows operation | movement of the optical transmission system shown in FIG. It is a block diagram which shows the structure of the optical receiver 61 shown in FIG. It is a figure which shows the structure of the nonlinear equalization part 614 shown in FIG. It is a figure which shows the modification of the nonlinear equalization part 614 shown in FIG. It is explanatory drawing which shows the intention of the deformation | transformation structure of a nonlinear equalization part. It is explanatory drawing which shows the intention of the deformation | transformation structure of a nonlinear equalization part. It is a figure which shows the structure of an experimental system. It is a figure which shows the calculation procedure of nonlinear effect compensation. It is a figure which shows the experimental result which shows the effect of this embodiment obtained by the experimental system shown in FIG. It is a block diagram which shows the structure of the optical transmission system by 2nd Embodiment of this invention. It is a block diagram which shows the structure of the optical transmission system by 2nd Embodiment of this invention. It is a block diagram which shows the structure of the optical transmission system by 3rd Embodiment of this invention. It is a block diagram which shows the structure of the optical transmission system by 4th Embodiment of this invention. It is explanatory drawing which shows operation | movement of the optical transmission system shown in FIG. It is a block diagram which shows the structure of the optical transmission system by 5th Embodiment of this invention. It is explanatory drawing which shows operation | movement of the optical transmission system shown in FIG. It is a block diagram which shows the structure of the optical transmission system by 6th Embodiment of this invention. It is explanatory drawing which shows operation | movement of the optical transmission system shown in FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 2 is a block diagram showing a configuration of an optical node 4. FIG. 4 is a diagram illustrating a configuration of an inter-polarization delay providing function unit 43. FIG. 4 is a diagram illustrating a configuration of an inter-polarization delay providing function unit 43. FIG. 4 is a diagram illustrating a configuration of an inter-polarization delay providing function unit 43. FIG. 3 is a diagram illustrating a configuration of an inter-wavelength delay providing function unit 44. FIG. 3 is a diagram illustrating a configuration of an inter-wavelength delay providing function unit 44. FIG. 3 is a diagram illustrating a configuration of an inter-wavelength delay providing function unit 44. FIG. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a figure which shows the structure of the delay provision + polarization delay provision function part 45 between polarization waves. It is a schematic diagram which shows the method of compensating the nonlinear optical effect in the system which does not perform dispersion compensation with a relay apparatus.

<First Embodiment>
Hereinafter, an optical transmission system according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the embodiment. In this figure, reference numeral 11 denotes an optical transmitter that uses the wavelength 1. Reference numeral 12 denotes an optical transmitter that uses the wavelength 2. Reference numeral 13 denotes an optical transmitter that uses a wavelength N (N is a natural number). Reference numeral 2 denotes a multiplexing unit that multiplexes optical signals output from the optical transmitters 11, 12, and 13. Reference numeral 3 denotes an optical fiber. Reference numeral 4 denotes an optical node having an optical dispersion compensation function for relaying an optical signal transmitted through the optical fiber 3. Reference numeral 5 denotes a demultiplexing unit that demultiplexes the optical signal.

  Reference numerals 61, 62, and 63 denote optical receivers having a non-linear degradation compensation function that simultaneously processes a plurality of wavelength channels to compensate for non-linear degradation. The optical transmission system shown in FIG. 1 is a digital coherent WDM transmission system including a plurality of transceivers and a plurality of repeaters (optical nodes). Multiple transceivers have a function to compensate for nonlinear optical effects (SPM and XPM) by digital signal processing, and optical compensation for signal distortion is achieved by dispersion compensation in each repeater (optical node). However, the waveform at the fiber input end where the optical power is strong can be made constant in any span. Therefore, for example, nonlinear optical compensation for M spans can be realized by performing phase rotation once, which is M times the phase rotation for one span. “Simultaneous processing” means that at least one of a plurality of optical transmitters or a plurality of optical receivers simultaneously processes the optical power of a plurality of wavelength channels (including self-wavelength channels and other wavelength channels). It is.

  FIG. 2 is an explanatory diagram showing the operation of the optical transmission system shown in FIG. In FIG. 2, a graph of changes in light intensity and dispersion, and the progress of the waveform over the transmission distance are schematically shown by taking two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span. As a result, nonlinear compensation can be realized with one phase rotation in the optical receiver.

  Next, the configuration of the optical receiver 61 shown in FIG. 1 will be described with reference to FIG. FIG. 3 is a block diagram showing a configuration of the optical receiver 61 shown in FIG. Since the optical receivers 61, 62, and 63 have the same configuration, the optical receiver 61 will be described as an example here. The configuration shown in FIG. 3 is a configuration of the optical receiver 61 that realizes nonlinear compensation by one phase rotation. The optical receiver 61 includes an O / E converter 611 that converts an optical signal into an electrical signal, an A / D converter 612 that converts an analog signal into a digital signal, and a trigger signal generator that matches the sampling time of A / D conversion. 613, a non-linear equalization unit 614 and a demodulation unit 615. The received WDM optical signal (including wavelength 1, wavelength 2,..., Wavelength N) passes through the O / E conversion unit 611 and the A / D conversion unit 612 and is converted into an electric digital signal, respectively, and a nonlinear equalization unit In 614, one phase rotation is performed to compensate for the nonlinear effect, and signal data identification is performed in the demodulator 615.

  Next, the configuration of the nonlinear equalizer 614 shown in FIG. 3 will be described with reference to FIG. FIG. 4 is a diagram showing the configuration of the nonlinear equalization unit 614 shown in FIG. The nonlinear equalization unit 614 includes a skew adjustment unit 6141 and a phase rotation unit 6142. The skew adjustment unit 6141 is for compensating for the conversion delay inside the A / D conversion unit 612 in FIG. The A / D conversion unit 612 shown in FIG. 3 uses a common trigger signal as a reference in order to match the time at which A / D conversion is started for each WDM optical signal. On the other hand, the time required for conversion in the A / D conversion unit 612 differs due to individual manufacturing differences of the A / D converters used, and a time difference occurs between each converted WDM digital signal. For example, a time difference of about several hundred picoseconds may occur. The skew adjustment unit 6141 is for eliminating these time differences.

  In the XPM of the nonlinear effect, the optical signal powers of the respective wavelengths having the same time among the optical signals in the optical fiber influence each other. Therefore, when optical signals at different times are received, calculation is performed using optical signal powers at different times, which may reduce the XPM compensation effect. In order to enhance the XPM compensation effect, it is necessary to align the time, and this is performed by the skew adjustment unit 6141. Conventional methods (eg, Reference: Photonics Journal vol.2, no.5, 2010, pp.816-832, "Nonlinear impairment compensation for polarization-division multiplexed WDM transmission using digital backward propagation") The frequency component of the received signal is divided using a filter, and each signal is processed as a separate wavelength channel. Therefore, skew adjustment is not necessary.

  However, in the example of the reference document, the receivable wavelength band is limited by the band of the optical receiver, and there is only a band corresponding to one wavelength channel in wavelength division multiplex transmission at intervals of 50 GHz, and a plurality of wavelength channels cannot be handled. Therefore, in order to handle a plurality of wavelength channels, it is necessary to perform time-synchronized processing using a plurality of optical receivers as in this embodiment. “Synchronous” means that when the sampling frequency is the same and there is a skew (time difference) in the data between wavelength channels, multiple optical transmitters or multiple optical receivers are used to eliminate the skew in signal processing. At least one of these is “time synchronization”.

  It is to be noted that the same effect can be obtained by adjusting the time for inputting the trigger signal for matching the sampling time of the A / D conversion in FIG. 3 instead of the skew adjustment unit 6141. The phase rotation unit 6142 processes signals of a plurality of wavelengths and compensates for nonlinear effects. This process will be described later. As described above, the digital signal of each wavelength passes through the skew adjustment unit 6141 and the phase rotation unit 6142, and becomes a signal in which the nonlinear effect is compensated.

  Next, a modification of the nonlinear equalization unit 614 shown in FIG. 3 will be described with reference to FIG. FIG. 5 is a diagram showing a modification of the nonlinear equalization unit 614 shown in FIG. In the configuration shown in FIG. 5, a chromatic dispersion adjustment unit 6143 is added after the skew adjustment unit 6141 and the phase rotation unit 6142, respectively. The intention to use this modified configuration will be described with reference to FIGS. In the case of a fiber having a relatively large dispersion coefficient (for example, a single mode fiber), waveform distortion during propagation increases due to chromatic dispersion, and the peak power at the bit boundary may be larger than the power at the fiber incident end. Since SPM and XPM are proportional to the peak power of the transmission signal, SPM / XPM compensation does not use the waveform information at the fiber incident end, but compensates more by using the waveform information during propagation at which the peak power is maximum. An effect will be obtained.

  FIG. 7 is a diagram showing an operation flow of the nonlinear equalization processing based on FIG. First, dispersion compensation (chromatic dispersion adjustment) is performed so that the peak power during propagation becomes a maximum waveform (step S1). Then, one phase rotation is performed using the information at the time of maximum peak power (step S2). However, the signal power is not the power at the fiber incident end, but the peak power of the waveform obtained in step S1. Subsequently, chromatic dispersion adjustment is performed to cancel the dispersion compensation in step S1 (step S3).

  Because dispersion compensation is performed, the load of digital signal processing increases. However, since the entire system is dispersion-compensated, there is no need to perform dispersion compensation for the number of spans, and dispersion compensation should be performed once before and after phase rotation. Therefore, the digital signal processing circuit scale is not so large. As described above, when the dispersion coefficient of the fiber is large, it is possible to further improve the compensation effect by performing SPM / XPM compensation based on the peak power during propagation. The optimum value of the chromatic dispersion amount to be adjusted can be estimated by repeating measurement several times in advance using, for example, a local search method between the minimum and maximum integrated dispersion amounts of the system.

  An experimental result for confirming the effect of the first embodiment will be described. FIG. 8 is a diagram showing the configuration of the experimental system. By passing a laser light source (Laser Diode: LD) of 188.5 THz and 188.55 THz through an optical modulator (I / Q Modulator: IQ Mod.), Respectively, two channels 32 Gbaud (11-bit pseudorandom pattern (11-bit pseudo random pattern ( PN11)) is modulated by a polarization division multiplexing (PDM) QPSK signal. After passing through an attenuator (Attenuator: Att.), They are combined by a 50/100 GHz interleave filter (ILF). Amplification is performed by an erbium doped fiber amplifier (EDFA), and an acousto-optic modulator (AOM) is inserted into a circular transmission system as a switch for transmission.

  The signal light source and local light (LD) are external resonator type lasers having a line width <100 kHz or less. In the polarization multiplexing unit (PDM), after the signal is branched into two by the coupler, a delay of 320 symbols is added to only one of them, and the polarization multiplexed signal is multiplexed by the polarization beam coupler to obtain a polarization multiplexed signal. The transmission line is composed of a dispersion shifted fiber (DSF) 20 km and an optical node unit. The zero dispersion wavelength of the DSF used in the experiment was 1573 nm. The optical node unit includes a wavelength selective switch (Wavelength) using a liquid crystal on silicon (LCOS) technology in which an optical amplifier that compensates for a loss in a transmission path and a multi-channel variable optical dispersion compensator (TODC) are integrated. selective switch: WSS, using Finical Corporation WaveShaper).

  The chromatic dispersion in the DSF is completely compensated at the optical node unit. Gain equalization is also performed in the WSS portion. The ASE noise adding unit was adjusted so that the optical SN (Optical signal-to-noise ratio: OSNR) of the received signal after 16 rounds was constant at 18 dB. Each signal having a frequency of 188.5 THz and 188.55 THz is converted into an electrical signal by an optical front end, synchronously received by a sampling oscilloscope, subjected to nonlinear effect compensation offline, and then demodulated by signal processing. In this experiment, the nonlinear compensation effect described later was obtained more clearly by performing synchronous reception within a time difference of about 1/10 of one symbol time.

FIG. 9 is a diagram illustrating a calculation procedure for nonlinear effect compensation. For each signal having a wavelength of 188.5 THz (Ch.1) and 188.55 THz (Ch.2), skew adjustment corresponding to delay times τ _1x , τ _1y , τ _2x , and τ _2y is performed for each polarization, and D1 _The amount of chromatic dispersion represented by _ch1 and D1 _ch2 is adjusted (F represents Fourier transform and F- ¹ represents inverse Fourier transform). Next, phase rotation represented by the following equation (1) is performed once for each polarization component with a complex exponential function.
Δθ = (α _SPM P _ch1 + α _XPM P _ch2 ) γ _m L _eff M (1)
However, α _SPM and α _XPM are an SPM compensation coefficient and an XPM compensation coefficient, respectively, and P _ch1 and P _ch2 are Ch. 1 and Ch. 2 is the fiber input power obtained by calculating the square of the absolute value of each polarization component, and γ _m , L _eff , and M are the nonlinear coefficient, effective length, and number of spans, respectively.

Thereafter, D1 ⁻¹ _ch1 is applied to return to the original chromatic dispersion amount, and demodulation processing is performed. Since the measurement target wavelength in this experiment is 188.5 THz (Ch. 1), only this wavelength is demodulated, but the other wavelength can also be demodulated by the same processing. The specific values of the delay times τ _1x , τ _1y , τ _2x , τ _2y are obtained by measuring the difference in processing time between a plurality of receivers in advance by means such as receiving a signal of a known information pattern, Set to compensate. In the demodulation processing, signal data is identified by performing polarization separation, frequency offset compensation, phase estimation, and hard decision processing normally used in a digital coherent system. In the example shown in FIG. 9, only the example of two channels is shown, but when the number of channels increases, the calculation can be performed by increasing the second term of the equation (1) to the number of channels.

  FIG. 10 shows experimental results showing the effects of the present embodiment obtained by the experimental system shown in FIG. FIG. 10A shows the received signal quality (Q value) due to the nonlinear effect by changing the fiber input power. Non-linear effects are not compensated (Legend ● No comp.), SPM compensation (Legend □ SPM comp.), XPM compensation (Legend ◇ XPM comp.), Both SPM and XPM are compensated (Legend) ΔSPM + XPM) is also shown. It can be seen that the received signal quality decreases as the fiber input power increases, and the received signal quality increases as the nonlinear effect is compensated. FIG. 10B shows an improvement in received signal quality in FIG. 10A obtained by nonlinear effect compensation. When the fiber input power is 1 dBm / ch, when both SPM and XPM are compensated, the received signal quality is improved by about 2.4 dB in terms of Q value. FIG. 10B also shows the phase arrangement constellation of the received signal, and it can be seen that the phase distribution is converged by compensation of the nonlinear effect, and the received signal quality is improved.

  Therefore, it can be seen that the nonlinear optical effect can be compensated by combining the optical dispersion compensation of the present embodiment with the SPM and XPM compensation by digital signal processing. Further, since compensation for SPM and XPM need only be performed once for simple phase rotation, signal processing at the receiving end is reduced, and the receiving circuit scale can be reduced. For example, when converted from Non-Patent Document 1, the number of calculations in the conventional method is about 160, whereas in the present embodiment, it can be reduced to one.

Second Embodiment
Next, an optical transmission system according to a second embodiment of the present invention will be described. FIG. 11 is a diagram showing a configuration of the second exemplary embodiment of the present invention. The first embodiment shown in FIG. 1 is a point-to-point transmission system, but FIG. 11 shows an optical cross connect (OXC) or multi-degree ROADM in which an optical node has multiple paths. (Reconfigurable Optical Add / Drop Multiplexing) system. In FIG. 11, the same components as those in the configuration shown in FIG. 1 (first embodiment) are denoted by the same reference numerals, and the description thereof is omitted. In FIG. 11, reference numerals 11 to 14 denote optical transmitters. Reference numeral 2 denotes a multiplexing unit. Reference numeral 3 denotes an optical fiber. Reference numeral 4 denotes an optical node. Reference numeral 5 denotes a demultiplexing unit. Reference numerals 61 to 65 denote optical receivers.

  As in the first embodiment, optical dispersion compensation for each span is performed at each optical node, and SPM and XPM compensation by digital signal processing is performed once at the receiver. Since the wavelength 1 and the wavelength 2 pass through the same path from the optical transmitters 11 and 12 to the optical receivers 61 and 62, SPM and XPM can be compensated once by digital signal processing. In an optical node to which signals are input from a plurality of routes, it is necessary to change the dispersion compensation amount according to the route. For this reason, in order to perform dispersion compensation for each span, the optical node 4 performs dispersion compensation according to the transmission path when the path differs depending on the wavelength channel. In the present embodiment, the XPM compensation effect may be reduced when the number of wavelength channels increases or decreases along the path in the optical transmission system.

  For example, referring to FIG. 12, the number of passing nodes, that is, the number of spans, differs between the original wavelength 1 and wavelength 2 that pass through the same path from the transmitter to the receiver and wavelength 3 that joins in the middle. At this time, if the amount of phase rotation is calculated from the number of spans of wavelength 1 and wavelength 2 as in equation (1), the compensation effect of XPM received from wavelength 3 is reduced because it differs from the number of spans for wavelength 3. . On the other hand, in the super channel transmission system that considers a plurality of wavelengths as one channel and expands the transmission capacity, the plurality of wavelengths constituting one channel always pass the same path from the transmitter to the receiver. The system also has an XPM compensation effect.

<Third Embodiment>
Next, an optical transmission system according to a third embodiment of the present invention will be described. FIG. 13 is a diagram illustrating a configuration of an optical transmission system according to the third embodiment. In FIG. 13, the same components as those in the configuration shown in FIG. 1 (first embodiment) are denoted by the same reference numerals, and the description thereof is omitted. In the first embodiment, the SPM and XPM compensation performed at the receiving end is different from the third embodiment in that pre-equalization is performed by the optical transmitters 11 to 13. If a vector modulator capable of controlling the phase component of the signal is used, it is possible to perform modulation so as to cancel the nonlinear effect caused by SPM / XPM during transmission. When SPM / XPM compensation is performed by digital signal processing using a received signal, the received signal includes ASE noise, and therefore, it is affected by other than SPM / XPM. On the other hand, when pre-equalization is performed by a transmitter, a distortion-free and noise-free signal is processed, so that the effect of SPM / XPM compensation can be expected. This embodiment is suitable for cases where the influence of chromatic dispersion and nonlinear effects occurring in the transmission path is known, or when the nonlinear effects measured at the receiving end can be fed back to the transmitting end even if unknown.

  Furthermore, SPM / XPM compensation by digital signal processing should be performed in both the pre-equalization in the transmitter and the processing in the receiver if the sum of the phase rotation amounts in transmission / reception satisfies equation (1). It doesn't matter. At this time, time synchronization is necessary between a plurality of transmitters or between a plurality of receivers, and synchronization between a transmitter and a receiver is not necessary.

  Although the description of the configuration itself that feeds back the influence measured at the receiving end to the transmitting end is omitted here, the feedback may be performed using a reverse communication path of the same path or other communication paths.

<Fourth embodiment>
Next, an optical transmission system according to a fourth embodiment of the present invention will be described. FIG. 14 is a diagram showing a configuration of an optical transmission system according to the fourth embodiment of the present invention. In FIG. 14, the same components as those in the configuration shown in FIG. 1 (first embodiment) are denoted by the same reference numerals, and the description thereof is omitted. The difference between the optical transmission system according to the fourth embodiment and the optical transmission system according to the first embodiment is that, in addition to optical dispersion compensation for each span in each optical node 4, an optical delay is provided between the polarizations. is there. As a result, it is possible to suppress the occurrence of cross polarization modulation (XPolM), which is one of the nonlinear effects between channels. Examples of means for optically providing a delay between polarizations include a polarization maintaining optical fiber, a combination of a polarization multiplexer / demultiplexer and a delay line, and a WSS device using LCOS technology. In the present embodiment, in addition to SPM / XPM compensation by digital signal processing, XPolM can be optically suppressed. Therefore, the influence of nonlinear effects is further reduced, and further improvement of received signal quality can be realized. This embodiment is suitable for a transmission line in which the influence of XPolM is large among nonlinear effects.

  FIG. 15 is an explanatory diagram showing the operation of the optical transmission system shown in FIG. In FIG. 15, a graph of changes in light intensity and dispersion, and the progress of the waveform by passing through the transmission distance are schematically represented by taking two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span. As a result, nonlinear compensation can be realized with one phase rotation in the optical receiver.

<Fifth Embodiment>
Next, an optical transmission system according to a fifth embodiment of the present invention will be described. FIG. 16 is a diagram illustrating a configuration of an optical transmission system according to the fifth embodiment. In FIG. 16, the same parts as those in the configuration shown in FIG. 1 (first embodiment) are denoted by the same reference numerals, and the description thereof is omitted. The difference between the optical transmission system according to the fifth embodiment and the optical transmission system according to the first embodiment is that, in addition to optical dispersion compensation for each span in each optical node 4, optical delay is optically added. is there. As a result, the occurrence of XPM can be suppressed. Means for optically providing a delay between wavelengths includes a combination of a wavelength multiplexer / demultiplexer and a delay line, a WSS device using LCOS technology, and the like. In this embodiment, in addition to suppressing the occurrence of XPM at each optical node, a XPM compensation effect can be further expected by combining compensation at the receiver. Further, if the occurrence of XPM is sufficiently suppressed in each optical node, there is no need for XPM compensation in the receiver, and digital signal processing reduction and circuit scale reduction can be realized.

  FIG. 17 is an explanatory diagram showing the operation of the optical transmission system shown in FIG. In FIG. 17, a graph of changes in light intensity and dispersion, and the progress of the waveform through the transmission distance are schematically shown by taking two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span. As a result, nonlinear compensation can be realized with one phase rotation in the optical receiver.

<Sixth Embodiment>
Next, an optical transmission system according to a sixth embodiment of the present invention will be described. FIG. 18 is a diagram showing a configuration of an optical transmission system according to the sixth embodiment of the present invention. In FIG. 18, the same parts as those in the configuration shown in FIG. 1 (first embodiment) are denoted by the same reference numerals, and the description thereof is omitted. An optical node 4 shown in FIG. 18 is a combination of the optical node 4 according to the fourth embodiment and the optical node 4 in the fifth embodiment. Thereby, in addition to optical dispersion compensation for each span in each optical node 4, it is possible to suppress the occurrence of XPolM and XPM by optically providing a delay between polarizations and wavelengths.

  FIG. 19 is an explanatory diagram showing the operation of the optical transmission system shown in FIG. In FIG. 19, a graph of changes in light intensity and dispersion, and the progress of the waveform due to passing through the transmission distance are schematically shown by taking two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span. As a result, nonlinear compensation can be realized with one phase rotation in the optical receiver.

  Note that the fourth, fifth, and sixth embodiments can also be applied to OXC and ROADM systems in which optical nodes have multiple paths as in the second embodiment. Furthermore, the present invention can also be applied to an embodiment in which SPM / XPM compensation is performed by digital signal processing in a transmitter and digital signal processing in both a transmitter and a receiver as in the third embodiment.

  Next, the configuration of the optical node 4 described above will be described. 20 to 45 are diagrams showing the configuration of the optical node 4 described above. In the first to sixth embodiments described above, the essential functions of the optical node 4 are the dispersion compensation function unit 41 that performs dispersion compensation for each span and the optical amplifier 42 that compensates for the loss for each span. These two functions can be in any order. FIG. 20 shows an example in which the dispersion compensation function unit 41 and the optical amplifier 42 are configured in this order. FIG. 21 shows an example in which the optical amplifier 42 and the dispersion compensation function unit 41 are configured in this order.

  22 to 25 are diagrams showing the configuration of the optical node 4 according to the fourth embodiment. In the optical node 4 according to the fourth embodiment, an inter-polarization delay providing function unit 43 is added in addition to the essential dispersion compensation function unit 41 and the optical amplifier 42. FIG. 22 shows an example in which the dispersion compensation function unit 41, the inter-polarization delay providing function unit 43, and the optical amplifier 42 are configured in this order. FIG. 23 shows an example in which the interpolarization delay imparting function unit 43, the dispersion compensation function unit 41, and the optical amplifier 42 are configured in this order. FIG. 24 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, and the inter-polarization delay providing function unit 43 are configured in this order. FIG. 25 shows an example in which the optical amplifier 42, the inter-polarization delay imparting function unit 43, and the dispersion compensation function unit 41 are configured in this order.

  26 to 29 are diagrams showing the configuration of the optical node 4 according to the fifth embodiment. In the optical node 4 according to the fifth embodiment, an inter-wavelength delay providing function unit 44 is added in addition to the essential dispersion compensation function unit 41 and the optical amplifier 42. FIG. 26 shows an example in which the dispersion compensation function unit 41, the inter-wavelength delay imparting function unit 44, and the optical amplifier 42 are configured in this order. FIG. 27 shows an example in which an inter-wavelength delay adding function unit 44, a dispersion compensation function unit 41, and an optical amplifier 42 are configured in this order. FIG. 28 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, and the inter-wavelength delay addition function unit 44 are configured in this order. FIG. 29 shows an example in which an optical amplifier 42, an inter-wavelength delay adding function unit 44, and a dispersion compensation function unit 41 are configured in this order.

  30 to 41 are diagrams showing the configuration of the optical node 4 according to the sixth embodiment. In the optical node 4 according to the sixth embodiment, in addition to the essential dispersion compensation function unit 41 and the optical amplifier 42, an inter-polarization delay imparting function unit 43 and an inter-wavelength delay imparting function unit 44 are added. FIG. 30 shows an example in which a dispersion compensation function unit 41, an inter-wavelength delay imparting function unit 44, an inter-polarization delay imparting function unit 43, and an optical amplifier 42 are configured in this order. FIG. 31 shows an example in which a dispersion compensation function unit 41, an inter-polarization delay imparting function unit 43, an inter-wavelength delay imparting function unit 44, and an optical amplifier 42 are configured in this order. FIG. 32 is an example in which an inter-wavelength delay imparting function unit 44, a dispersion compensation function unit 41, an inter-polarization delay imparting function unit 43, and an inter-polarization delay imparting function unit 43 are configured in this order. FIG. 33 shows an example in which an inter-polarization delay imparting function unit 43, a dispersion compensation function unit 41, an inter-wavelength delay imparting function unit 44, and an optical amplifier 42 are configured in this order. FIG. 34 shows an example in which an inter-wavelength delay imparting function unit 44, an inter-polarization delay imparting function unit 43, a dispersion compensation function unit 41, and an optical amplifier 42 are configured in this order. FIG. 35 shows an example in which an inter-polarization delay imparting function unit 43, an inter-wavelength delay imparting function unit 44, a dispersion compensation function unit 41, and an optical amplifier 42 are configured in this order.

  FIG. 36 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, the dispersion compensation function unit 41, and the inter-polarization delay providing function unit 43 are configured in this order. FIG. 37 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, the inter-polarization delay imparting function unit 43, and the inter-wavelength delay imparting function unit 44 are configured in this order. FIG. 38 shows an example in which an optical amplifier 42, an inter-wavelength delay imparting function unit 44, a dispersion compensation function unit 41, and an inter-polarization delay imparting function unit 43 are configured in this order. FIG. 39 shows an example in which an optical amplifier 42, an inter-polarization delay imparting function unit 43, a dispersion compensation function unit 41, and an inter-wavelength delay imparting function unit 44 are configured in this order. FIG. 40 shows an example in which an optical amplifier 42, an inter-wavelength delay imparting function unit 44, an inter-polarization delay imparting function unit 43, and a dispersion compensation function unit 41 are configured in this order. FIG. 41 shows an example in which an optical amplifier 42, an inter-polarization delay imparting function unit 43, an inter-wavelength delay imparting function unit 44, and a dispersion compensation function unit 41 are configured in this order.

  42 to 45 show an optical node provided with an interpolarization delay granting / interwavelength delay granting function unit 45 for providing an interpolarization delay and an interwavelength delay in addition to the essential dispersion compensation function unit 41 and the optical amplifier 42. FIG. FIG. 42 shows an example in which the dispersion compensation function unit 41, the inter-polarization delay provision + inter-wavelength delay provision function unit 45, and the optical amplifier 42 are configured in this order. FIG. 43 shows an example in which an interpolarization delay grant + interwavelength delay grant function unit 45, a dispersion compensation function unit 41, and an optical amplifier 42 are configured in this order. FIG. 44 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, and the interpolarization delay grant + interwavelength delay grant function unit 45 are configured in this order. FIG. 45 shows an example in which an optical amplifier 42, an interpolarization delay grant + interwavelength delay grant function unit 45, and a dispersion compensation function unit 41 are configured in this order.

  When the dispersion compensation function unit 41 is performed using a dispersion compensation fiber (DCF) in the optical node 4 shown in FIGS. 20 to 45, the core diameter of the DCF is smaller than that of a normal transmission line fiber. Since generation of nonlinearity in the DCF becomes a problem, it is usually used where the optical signal power is small. For this reason, the DCF is preferably used before the optical amplification function. On the other hand, when the dispersion compensation function unit 41 is realized by the TODC function of the LCOS WSS shown in the first embodiment, it does not depend on the optical signal power, and any configuration may be used.

  Next, the configuration of the inter-polarization delay providing function unit 43 will be described. 46 to 48 are diagrams illustrating the configuration of the above-described inter-polarization delay imparting function unit 43. FIG. 46, the wavelength demultiplexing function unit 431 demultiplexes the signals into three wavelengths, the interpolarization delay adding unit 432 adds different interpolarization delay amounts for each wavelength, and then the wavelength multiplexing function unit 433 multiplexes them. The configuration is shown. Here, when an AWG (Arrayed Waveguide Grating) having circularity is used for the wavelength demultiplexing function unit 431, signals are routed to the same port in three wavelength periods, so that an operation in a wide wavelength range is possible.

  FIG. 47 shows a configuration when an ILF is used for the wavelength demultiplexing function unit 431 and the wavelength multiplexing function unit 433. The odd-numbered wavelength channel and the even-numbered wavelength channel are provided with different inter-polarization delay amounts from the inter-polarization delay applying unit 432 through different paths. FIG. 48 illustrates the configuration of the inter-wavelength delay applying section 434 using the LCOS WSS (for example, Finical Corporation WaveShaper). Since it is possible to give different amounts of inter-polarization delay for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device. 46 and 47, the delay amount providing unit 432 between polarizations does not need to be variable, and may be a fixed delay amount that differs for each wavelength.

  Next, the configuration of the inter-wavelength delay imparting function unit 44 will be described. 49 to 51 are diagrams showing the configuration of the inter-wavelength delay adding function unit 44. FIG. 49 shows a configuration in which the wavelength demultiplexing function unit 441 demultiplexes into three wavelengths, the delay applying unit 442 adds a different delay amount for each wavelength, and then combines by the wavelength combining function unit 443. Yes. Here, when an AWG (Arrayed Waveguide Grating) having circularity is used for the wavelength demultiplexing function unit 441, signals are routed to the same port in three wavelength periods, so that it is possible to operate in a wide wavelength range.

  FIG. 50 shows a configuration when ILF is used for the wavelength demultiplexing function unit 441 and the wavelength multiplexing function unit 443. The odd and even wavelength channels are assigned different delay amounts from the delay applying unit 442 on different paths. FIG. 51 shows the configuration of the delay-by-wavelength imparting unit 444 using LCOS WSS (for example, Finisar WaveShaper). Since it is possible to give different amounts of inter-polarization delay for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device. Note that the delay adding unit 442 in FIGS. 49 and 50 does not have to be variable in delay amount, and may be a fixed delay amount that differs for each wavelength.

  Next, the configuration of the interpolarization delay addition + interwavelength delay addition function unit 45 will be described. 52 to 57 are diagrams showing the configuration of the inter-polarization delay grant + inter-wavelength delay grant function unit 45. In FIG. 52, the wavelength demultiplexing function unit 451 demultiplexes the signals into three wavelengths, and the inter-polarization delay applying unit 452 and the delay applying unit 453 provide different delay amounts for each wavelength, and then the wavelength multiplexing function unit 454 A configuration for multiplexing is shown. In FIG. 53, the wavelength demultiplexing function unit 451 demultiplexes the signals into three wavelengths, the delay adding unit 453 and the interpolarization delay adding unit 452 add different delay amounts for each wavelength, and then the wavelength multiplexing function unit 454 A configuration for multiplexing is shown. Here, when an AWG (Arrayed Waveguide Grating) having circularity is used for the wavelength demultiplexing function unit 451, signals are routed to the same port in three wavelength periods, so that an operation in a wide wavelength range is possible.

  FIG. 54 shows a configuration when an ILF is used for the wavelength demultiplexing function unit 451 and the wavelength multiplexing function unit 454. The odd-numbered wavelength channel and the even-numbered wavelength channel are provided on different paths, and different delay amounts are provided by the inter-polarization delay applying unit 452 and the delay applying unit 453. FIG. 55 shows a configuration when an ILF is used for the wavelength demultiplexing function unit 451 and the wavelength multiplexing function unit 454. The odd-numbered wavelength channel and the even-numbered wavelength channel are provided on different paths, and different delay amounts are provided by the delay applying unit 453 and the inter-polarization delay applying unit 452.

  FIG. 56 shows the configuration of the inter-wavelength polarization delay applying unit 455 and the per-wavelength delay applying unit 456 using LCOS WSS (for example, Finsar WaveShaper). FIG. 57 shows the configuration of a delay delay unit for each wavelength 456 and an inter-polarization delay grant unit for each wavelength 455 using LCOS WSS (for example, Finical Corporation WaveShaper). Since it is possible to give different amounts of inter-polarization delay for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device.

  In the above description, an example in which nonlinear compensation is performed by performing phase rotation of N times the phase rotation for one span once has been described. However, the phase rotation may be divided into a plurality of times and may be divided for one span. Nonlinear compensation may be performed by multiplying the phase rotation by the propagation distance and rotating the phase by dividing the distance of one span.

  As described above, by providing an optical dispersion compensation function, it is possible to provide an optical transmission system that can reduce equalization processing in a DSP (Digital Signal Processor). In particular, at least one of compensating phase rotation using waveform information during propagation in which peak power is maximized due to chromatic dispersion, or compensating for phase rotation due to XPM due to adjacent channel strength as well as SPM due to self channel strength. I tried to do one. As a result, it is possible to improve the signal quality by SPM and XPM compensation with a simple configuration by using optical compensation together with the nonlinear optical effect compensation only by digital signal processing, and the apparatus scale is also small. can do.

  As mentioned above, although embodiment of this invention has been described with reference to drawings, the said embodiment is only the illustration of this invention, and it is clear that this invention is not limited to the said embodiment. is there. Accordingly, additions, omissions, substitutions, and other changes of the components may be made without departing from the technical idea and scope of the present invention.

  In a WDM digital coherent transmission system, it is useful for improving the efficiency of nonlinear optical effect compensation and reducing the load on the digital signal processing unit.

  11-14 ... Optical transmitter, 2 ... Multiplexing unit, 3 ... Optical fiber, 4 ... Optical node, 5 ... Demultiplexing unit, 61-64 ... Optical receiver, 41: Dispersion compensation function unit, 42: Optical amplifier, 43: Inter-polarization delay imparting function unit, 44: Inter-wavelength delay imparting function unit, 45 ... Inter-polarization delay provision + inter-wavelength delay Granting function

Claims (8)

It has an optical dispersion compensation function unit that performs optical dispersion compensation for each span.
Compensate for phase rotation using waveform information during propagation that maximizes peak power, or compensate for phase rotation due to cross-phase modulation based on adjacent channel strength, or both And optical transmission system. An optical amplifier that compensates for loss per span of an optical transmission signal, a plurality of optical nodes that include a dispersion compensation function unit that compensates dispersion for each span, a plurality of optical transmitters, and a plurality of optical receivers A digital coherent optical transmission system,
The optical transmitter and the optical receiver compensate for nonlinear optical effects in a transmission line by simultaneously processing signals of a plurality of wavelength channels synchronized in time.   The optical node includes at least one of a functional unit that gives a time difference between wavelength channels, a functional unit that gives a time difference between polarized waves, and a functional unit that gives a time difference between both wavelength channels and between polarized waves. The optical transmission system according to claim 2.   4. The optical transmission according to claim 2, further comprising: a skew adjustment unit that adjusts a skew; and a phase rotation unit that rotates a phase, wherein the compensation of the nonlinear optical effect is performed by skew adjustment and phase rotation. system.   The optical transmission system according to claim 4, further comprising a chromatic dispersion adjusting unit that adjusts chromatic dispersion before and after the phase rotation unit.   The optical transmission system according to claim 4, wherein the nonlinear optical effect is compensated by one phase rotation. An optical transmission method performed by an optical transmission system including any one or both of a pre-compensation function unit that compensates chromatic dispersion and an optical dispersion compensation function unit that performs optical dispersion compensation,
Either or both of compensating for phase rotation using waveform information during propagation in which peak power is maximized due to chromatic dispersion, or compensating for phase rotation due to cross-phase modulation based on adjacent channel strength, An optical transmission method characterized by performing. An optical amplifier that compensates for loss per span of an optical transmission signal, a plurality of optical nodes that include a dispersion compensation function unit that compensates dispersion for each span, a plurality of optical transmitters, and a plurality of optical receivers An optical transmission method performed by a digital coherent optical transmission system,
An optical transmission method, wherein the optical transmitter and the optical receiver simultaneously process signals of a plurality of wavelength channels synchronized in time to compensate for a nonlinear optical effect in a transmission path.

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