JP6426677B2 - Optical transmission system and optical transmission method - Google Patents

Description

  The present invention relates to an optical transmission system and an optical transmission method having a function of compensating for signal quality deterioration 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 digital signal processing technology is applied to a coherent transmission method has progressed, and some introductions have been started. In digital coherent transmission, since chromatic dispersion can be collectively compensated by digital signal processing at the receiving end, the dispersion compensating fiber provided for each repeater of the conventional optical transmission system is no longer required (for example, Patent Literature 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 they are complex complex when they coexist with elements that change the intensity waveform of the optical signal and cause waveform distortion like wavelength dispersion. Have a positive effect. In order to compensate all these efficiently, it is necessary to simultaneously compensate for nonlinear effects and linear effects. In order to approximate this, a compensation method has been proposed in which a configuration in which the wavelength dispersion is slightly compensated by the digital signal processing at the receiving end in digital coherent transmission and the nonlinear effect is slightly compensated is repeated several times. For example, in Patent Document 2, it is known that SPM compensation is possible.

  In such a circuit, it is reported that it is desirable to set the number of repetitions equal to or more than the number of optical amplification relay spans from the viewpoint of performance. However, performing the waveform distortion compensation and the phase rotation compensation a plurality of times in the receiver causes a problem that the size of the receiving circuit becomes large. Dispersion compensation by digital signal processing requires a procedure of transforming a signal into a frequency domain by Fourier transform, multiplying it by a transfer function for dispersion compensation, and returning it to the time domain by inverse Fourier transform. In order to perform dispersion compensation and nonlinear compensation of a transmission line of M (M is a natural number) span, 2M or more real-time fast Fourier transform / inverse transform (FFT / IFFT) circuits are required, which makes the circuit impractical. There is a problem of becoming A schematic diagram of a method of compensating for non-linear optical effects in a system in which the relay device does not perform dispersion compensation is shown in FIG. As shown in FIG. 58, in the case of an M span transmission line, it is necessary to perform M step linear equalization and non-linear equalization processing. This corresponds to performing M real time fast Fourier transform / inverse transform (FFT / IFFT). In addition, in Non-Patent Document 1, there is a report that a system in which dispersion compensation is not performed in relay equipment is advantageous in order to suppress the occurrence of nonlinear effects.

Although a method to compensate XPM by digital signal processing at the receiving end is also considered, it is due to the influence from the adjacent channel uncorrelated with the received signal, so digital signal processing is perfect only by receiving only the channel to be measured. It is difficult to compensate for Although methods to simultaneously receive adjacent channels and compensate by digital signal processing are being studied, there is a report that the amount of calculation of waveform distortion compensation and phase rotation compensation increases compared to the case of compensating only for SPM. It is difficult to reduce the scale to a realistic scale (see, for example, Non-Patent Document 2).
On the other hand, there has been proposed a method of suppressing the occurrence of XPM by optical dispersion compensation for each span and time difference between channels (see, for example, Patent Document 3).

Patent No. 4872003 Patent No. 4759625 gazette 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 wavelength dispersion and all nonlinear optical effect compensation are performed only by digital signal processing, there arises a problem that the scale of the apparatus becomes large. In addition, in the case of WDM transmission, when there is XPM, it is difficult to separate the contribution of phase rotation due to SPM and phase rotation due to XPM from an adjacent channel, and XPM is completely compensated only by digital signal processing of the received signal. There is a problem that is difficult.

  The present invention has been made in view of such circumstances, and by using optical compensation and digital signal processing in combination, it is possible to compensate for signal quality deterioration in optical transmission without excessively increasing the size of the transmission / reception apparatus. It is an object of the present invention to provide an optical transmission system and an optical transmission method capable of

One aspect of the present invention is a plurality of optical nodes including an optical amplifier that compensates for the loss of each optical transmission signal span, a dispersion compensation function unit that compensates for the dispersion of each span, a plurality of optical transmitters, Digital coherent optical transmission system comprising: a plurality of optical receivers, wherein the optical transmitters cancel a plurality of nonlinear optical effects in a transmission path by using a vector modulator capable of controlling the phase component of a signal; A digital coherent optical transmission system that compensates for nonlinear optical effects in a transmission path by modulating a wavelength channel . Further, according to one aspect of the present invention, there are provided an optical amplifier for compensating for a loss in each span of an optical transmission signal, a plurality of optical nodes including a dispersion compensation function unit for compensating for a dispersion for each span, and a plurality of optical transmitters. A digital coherent optical transmission system comprising a plurality of optical receivers, wherein the optical receivers are time-synchronized between the plurality of optical receivers and adjusted so as to eliminate skew occurring in signals of a plurality of wavelength channels, This is an optical transmission system that compensates for the non-linear optical effect of M spans by performing one phase rotation of M (M is a natural number) times the phase rotation of one span for the signal after adjustment.

  One embodiment of the present invention is the digital coherent optical transmission system, wherein the optical node includes a functional unit for giving a time difference between wavelength channels, a functional unit for giving a time difference between polarizations, between wavelength channels and between polarizations At least one of the functional units for providing a time difference to both is provided.

  One aspect of the present invention is the digital coherent optical transmission system, further comprising: 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 includes skew adjustment and a phase It does by rotation.

  One aspect of the present invention is the digital coherent optical transmission system, further including a wavelength dispersion adjustment unit that adjusts wavelength dispersion before and after the phase rotation unit.

One aspect of the present invention is a plurality of optical nodes including an optical amplifier that compensates for the loss of each optical transmission signal span, a dispersion compensation function unit that compensates for the dispersion of each span, a plurality of optical transmitters, And an optical receiver, wherein the optical transmitter cancels nonlinear optical effects in the transmission path by using a vector modulator capable of controlling the phase component of the signal. As described above, the optical transmission method compensates for non-linear optical effects in a transmission path by modulating a plurality of wavelength channels . Further, according to one aspect of the present invention, there are provided an optical amplifier for compensating for a loss in each span of an optical transmission signal, a plurality of optical nodes including a dispersion compensation function unit for compensating for a dispersion for each span, and a plurality of optical transmitters. An optical transmission method performed by a digital coherent optical transmission system including a plurality of optical receivers, wherein the optical receivers synchronize time with a plurality of optical receivers and eliminate skews generated in signals of a plurality of wavelength channels It is an optical transmission method that compensates for non-linear optical effects of M span by performing one adjustment of phase rotation that is adjusted and M phase (M is a natural number) times phase rotation of one span to the adjusted signal. .

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

FIG. 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 intent of the deformation | transformation structure of a nonlinear equalization part. It is explanatory drawing which shows the intent 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. FIG. 17 is an explanatory view showing an operation of the optical transmission system shown in FIG. 16; It is a block diagram which shows the structure of the optical transmission system by 6th Embodiment of this invention. FIG. 18 is an explanatory view showing an operation of the optical transmission system shown in FIG. 17; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 2 is a block diagram showing the configuration of an optical node 4; FIG. 7 is a diagram showing a configuration of an inter-polarization delay application unit 43. FIG. 7 is a diagram showing a configuration of an inter-polarization delay application unit 43. FIG. 7 is a diagram showing a configuration of an inter-polarization delay application unit 43. FIG. 7 is a diagram showing the configuration of an inter-wavelength delay application unit 44. FIG. 7 is a diagram showing the configuration of an inter-wavelength delay application unit 44. FIG. 7 is a diagram showing the configuration of an inter-wavelength delay application unit 44. FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; FIG. 7 is a diagram showing a configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45; It is a schematic diagram which shows the method to compensate the nonlinear optical effect in the system which does not perform dispersion compensation in 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 is an optical transmitter using wavelength 1. Reference numeral 12 denotes an optical transmitter that uses wavelength 2. The code | symbol 13 is an optical transmitter which uses wavelength N (N is a natural number). The code | symbol 2 is a combining part which combines the optical signal which the optical transmitters 11, 12, 13 output. Reference numeral 3 is an optical fiber. Reference numeral 4 is an optical node having an optical dispersion compensation function to relay an optical signal transmitted through the optical fiber 3. Reference numeral 5 is a demultiplexing unit that demultiplexes the optical signal.

  Reference numerals 61, 62 and 63 are optical receivers having a non-linear deterioration compensation function of simultaneously processing a plurality of wavelength channels to compensate non-linear deterioration. 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). The plurality of transceivers have a function of compensating for nonlinear optical effects (SPM and XPM) by digital signal processing, and in the repeater (optical node), signal distortion is optically compensated by dispersion compensation for each span. The waveform at the fiber input end where the optical power is strong can be made constant at any span. Therefore, for example, non-linear optical compensation for M spans can be realized by performing one phase rotation M times 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 process the optical power of a plurality of wavelength channels (including the own wavelength channel and the other wavelength channel). It is.

  FIG. 2 is an explanatory view showing the operation of the optical transmission system shown in FIG. In FIG. 2, graphs of changes in light intensity and dispersion, and development of a waveform by passing through a transmission distance are schematically represented using two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span, and as a result, non-linear 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 the configuration of the optical receiver 61 shown in FIG. The configurations of the optical receivers 61, 62, 63 are the same, and thus, the optical receiver 61 will be described as an example. The configuration shown in FIG. 3 is a configuration of the optical receiver 61 that realizes non-linear compensation with 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 synchronizes sampling times of A / D conversion. 613, comprising 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 converter 611 and the A / D converter 612 and is converted into an electric digital signal, respectively, and the nonlinear equalizer One phase rotation is performed at 614 to compensate for the non-linear effect, and the demodulator 615 performs signal data identification.

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

  Among the optical signals in the optical fiber, the optical signal power of each wavelength at the same time among the optical signals in the optical fiber affects each other in the nonlinear effect XPM. 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 make the time equal, and this is performed by the skew adjustment unit 6141. In the conventional method (for example, Photonics Journal vol. 2, no. 5, 2010, pp. 816-832, "Nonlinear impairment compensation for polarization-division multiplexed transmission using digital backward propagation"), reception by one receiver The frequency components of the resulting signal are divided using a filter, and each is regarded as another wavelength channel and processed. Therefore, skew adjustment was unnecessary.

  However, in the example of the reference, the wavelength band that can be received is limited by the band of the optical receiver, and there is only a band corresponding to one wavelength channel in wavelength division multiplexing transmission at 50 GHz intervals, and multiple wavelength channels can not be handled. Therefore, in order to handle a plurality of wavelength channels, it is necessary to perform time-synchronization processing using a plurality of optical receivers as in the present embodiment. “Synchronization” means that if the sampling frequency is the same and there is a skew (time difference) in the data between the wavelength channels, a plurality of optical transmitters or a plurality of optical receivers are mutually eliminated so as to eliminate the skew in signal processing. Or at least one of them is "time synchronization".

  The same effect can be obtained by adjusting the time for inputting the trigger signal for adjusting the sampling time of 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 to compensate for non-linear 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 non-linear effect is compensated.

  Next, a modification of the non-linear equalization unit 614 shown in FIG. 3 will be described. FIG. 5 is a diagram showing a modification of the non-linear equalization unit 614 shown in FIG. In the configuration shown in FIG. 5, a wavelength dispersion adjustment unit 6143 is added after the skew adjustment unit 6141 and the phase rotation unit 6142 respectively. The intention of using this modified configuration will be described with reference to FIG. 6 and FIG. In the case of a fiber having a relatively large dispersion coefficient (for example, a single mode fiber), waveform distortion during propagation may increase due to wavelength dispersion, and the peak power at the bit boundary may be larger than the power at the fiber input end. Since SPM and XPM are proportional to the peak power of the transmission signal, SPM / XPM compensation is not compensated by using the waveform information at the fiber incident end but by using the waveform information during propagation where the peak power is maximized. The effect will be obtained.

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

  Although the load of digital signal processing increases because dispersion compensation is performed, it is not necessary to perform dispersion compensation for the number of spans because the entire system is dispersion compensated, and dispersion compensation is performed once before and after phase rotation. Since it is only necessary, the scale of the digital signal processing circuit does not increase very much. As described above, when the dispersion coefficient of the fiber is large, the compensation effect can be further improved by performing the SPM · XPM compensation based on the peak power during propagation. The optimum value of the chromatic dispersion to be adjusted can be estimated between the minimum and maximum integrated dispersions of the system, for example, by repeating measurement several times in advance using a local search method.

  The experimental result which confirms the effect of 1st Embodiment is demonstrated. FIG. 8 is a diagram showing the configuration of an experimental system. Two channels 32 Gbaud (11-bit pseudo-random pattern (50-GHz interval) by passing 188.5THz and 188.55THz laser light source (Laser Diode: LD) through an optical modulator (I / Q Modulator: IQ Mod.) It modulates with the polarization division multiplexing (PDM) QPSK signal of PN11)). After passing through an attenuator (Attenuator: Att.), These are combined by an interleave filter (ILF) of 50/100 GHz. The signal is amplified by an erbium doped fiber amplifier (EDFA), and an acousto-optic modulator (AOM) is inserted as a switch into the loop transmission system and transmitted.

The signal light source and the local light (LD) are external resonator type lasers having a line width of <100 kHz or less. In the polarization multiplexing unit (PDM), after the signal is branched into two by the coupler, only one of the signals is delayed for 320 symbols and multiplexed by the polarization beam coupler to obtain a polarization multiplexed signal.
The transmission line consists of 20 km of dispersion shifted fiber (DSF) and an optical node part. The zero dispersion wavelength of the DSF used in the experiment was 1573 nm. The optical node unit is a wavelength selective switch (Wavelength) using Liquid Crystal on Silicon (LCOS) technology, which integrates an optical amplifier and a multi-channel variable optical dispersion compensator (TODC) that compensates for transmission line loss. selective switch: WSS (using Finisar's WaveShaper)

  The chromatic dispersion at DSF is completely compensated at the optical node. It also performs gain equalization in the WSS section. The ASE noise addition unit was adjusted so that the optical SN (Optical signal-to-noise ratio: OSNR) of the received signal after 16 rounds became constant by 18 dB. Each signal of frequency 188.5 THz and 188.55 THz is converted to an electric signal at the optical front end, received synchronously by the sampling oscilloscope, subjected to nonlinear effect compensation off-line, and then demodulated by signal processing. In this experiment, the non-linear compensation effect described later is more clearly obtained by performing synchronous reception within a time difference of about one tenth of one symbol time.

FIG. 9 is a diagram showing a calculation procedure of non-linear effect compensation. Wavelength 188.5THz (Ch.1), for each signal 188.55THz (Ch.2), the delay time for each polarization _{τ 1x, τ 1y, τ 2x} , performs skew adjustment corresponding to tau _2y, D1 _The chromatic dispersion amount represented by _ch1 and D1 _ch2 is adjusted (F represents a Fourier transform and F- ¹ represents an inverse Fourier transform). Next, phase rotation represented by the following equation (1) is applied once for each polarization component with a complex exponential function.
Δθ = (α _SPM P _{ch 1} + α _XP M P _{ch 2} ) γ _m L _eff M (1)
Here, α _SPM and α _XPM are the SPM compensation coefficient and the XPM compensation coefficient, respectively, and P _ch1 and P _ch2 are Ch. 1 and Ch. It is the fiber input power obtained by calculating the square of the absolute value of each polarization component of 2, and γ _m , L _eff , and M are the nonlinear coefficient, the effective length, and the number of spans, respectively.

Thereafter, D1 ^-1 _ch1 is applied to restore the original amount of chromatic dispersion, and demodulation processing is performed. Since the measurement target wavelength of 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 and τ _2y are measured in advance by means such as receiving a signal of a known information pattern and the difference in processing time among a plurality of receivers. Set to compensate. In demodulation processing, polarization separation, frequency offset compensation, phase estimation and hard decision processing usually used in digital coherent systems are performed to identify signal data. Although only two channels are shown in the example shown in FIG. 9, if the number of channels is increased, calculation can be made by increasing the second term of equation (1) to the number of channels.

  FIG. 10 is an experimental result showing the effect of the present embodiment obtained by the experimental system shown in FIG. FIG. 10A shows the received signal quality (Q value) due to the non-linear effect by changing the fiber input power. No compensation for non-linear effects (Legend: No comp.), SPM compensation (Legend: SPM comp.), XPM compensation (Legend: XPM comp.), SPM and XPM for compensation (Legend SPM + XPM) is shown together. It can be seen that the higher the fiber input power, the lower the received signal quality, and the higher the compensated non-linear effects, the higher the received signal quality. FIG. 10 (b) shows the improvement amount of the received signal quality in FIG. 10 (a) obtained by the non-linear effect compensation. When the fiber input power is 1 dBm / ch, the received signal quality is improved by about 2.4 dB in Q value when both SPM and XPM are compensated. Further, FIG. 10 (b) also shows the phase arrangement constellation of the received signal, and it can be seen that the phase distribution converges by the compensation of the non-linear effect, and the received signal quality is improved.

  Therefore, it is understood 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. Furthermore, since the SPM and XPM compensations need only be performed once with a simple phase rotation, the signal processing at the receiving end is reduced, and the size of the receiving circuit can be reduced. For example, when converted from Non-Patent Document 1, the number of calculations in the conventional method is about 160 times, but 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 the configuration of the second 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 the optical node has multiple routes. It corresponds to a (reconfigurable Optical Add / Drop Multiplexing) system. In FIG. 11, the same parts as those in the configuration (first embodiment) shown in FIG. 1 are given the same reference numerals, and the description thereof will be omitted. In FIG. 11, reference numerals 11 to 14 denote optical transmitters. The code | symbol 2 is a combining part. Reference numeral 3 is an optical fiber. Reference numeral 4 is an optical node. The code | symbol 5 is a splitter. Reference numerals 61 to 65 denote optical receivers.

  As in the first embodiment, optical dispersion compensation for each span at each optical node and SPM and XPM compensation by digital signal processing are performed once in the receiver. 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, so SPM and XPM can be compensated at one time by digital signal processing. In an optical node to which signals are input from a plurality of routes, it is necessary to change the amount of dispersion compensation according to the route. Therefore, 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, when the number of wavelength channels increases or decreases in the middle of the optical transmission system, the XPM compensation effect may be reduced.

  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 passing along the same route from the transmitter to the receiver and wavelength 3 joining halfway. At this time, when the phase rotation amount is calculated from the number of spans of wavelength 1 and wavelength 2 as in equation (1), since the number of spans for wavelength 3 is different, the compensation effect of XPM received from wavelength 3 is reduced. . On the other hand, in the super channel transmission method for expanding the transmission capacity by regarding multiple wavelengths as one channel, the multiple wavelengths constituting one channel always pass the same path from the transmitter to the receiver, so this optical transmission The system also has the 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 showing the configuration of the optical transmission system according to the third embodiment. In FIG. 13, the same parts as those in the configuration (first embodiment) shown in FIG. The second embodiment differs in that the SPM and XPM compensation is performed at the receiving end in the first embodiment, and the optical transmitters 11 to 13 are pre-equalized in the third embodiment. 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 non-linear effect due to SPM and XPM received during transmission. When SPM / XPM compensation is performed by digital signal processing using a received signal, ASE noise is included in the received signal, which means that the signal other than SPM / XPM is affected. On the other hand, in the case of pre-equalization in the transmitter, since the signal without distortion and noise is processed, the effect of SPM / XPM compensation can be expected more. The present embodiment is suitable for the case where the effects of wavelength dispersion and nonlinear effects occurring in the transmission path are known, or where the nonlinear effects measured at the receiving end can be fed back to the transmitting end.

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

  Here, although the description of the configuration itself for feeding back the influence measured at the receiving end to the transmitting end is omitted, feedback may be performed using communication paths in the reverse direction 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 view showing the configuration of the optical transmission system according to the fourth embodiment of the present invention. In FIG. 14, the same parts as those in the configuration (first embodiment) shown in FIG. 1 are given 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 between polarizations is provided. is there. As a result, it is possible to suppress the occurrence of cross polarization modulation (XPolM), which is one of the interchannel nonlinear effects. Means for optically applying 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 this embodiment, since XPolM can be optically suppressed in addition to SPM / XPM compensation by digital signal processing, the influence of non-linear effects can be further reduced, and further improvement of received signal quality can be realized. The present embodiment is suitable for a transmission line in which the influence of XPolM is large among non-linear effects.

  FIG. 15 is an explanatory view showing an operation of the optical transmission system shown in FIG. In FIG. 15, graphs of changes in light intensity and dispersion, and development of waveforms by passing through transmission distances are schematically represented using two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span, and as a result, non-linear 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 showing the configuration of the optical transmission system according to the fifth embodiment. In FIG. 16, the same parts as those in the configuration (first embodiment) shown in FIG. 1 are given the same reference numerals, and the description thereof will be 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 at each optical node 4, an optical delay between wavelengths is provided. is there.
As a result, the occurrence of XPM can be suppressed. As means for optically applying the delay between wavelengths, there are 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 in each optical node, the XPM compensation effect can be expected more by combining the compensation in the receiver. In addition, if the occurrence of XPM is sufficiently suppressed at each optical node, the need for XPM compensation in the receiver is eliminated, and processing reduction of digital signal processing and reduction of circuit scale can be realized.

  FIG. 17 is an explanatory view showing an operation of the optical transmission system shown in FIG. In FIG. 17, graphs of changes in light intensity and dispersion, and development of a waveform by passing a transmission distance are schematically represented using two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span, and as a result, non-linear 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 the configuration of the optical transmission system according to the sixth embodiment of the present invention. In FIG. 18, the same parts as those in the configuration (first embodiment) shown in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted. The 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. As a result, in addition to optical dispersion compensation for each span in each optical node 4, generation of XPolM and XPM can be suppressed by optically providing delays between polarizations and wavelengths.

  FIG. 19 is an explanatory view showing an operation of the optical transmission system shown in FIG. In FIG. 19, graphs of changes in light intensity and dispersion, and development of waveforms by passing through transmission distances are schematically represented using two wavelengths as an example. Since dispersion compensation is performed for each span, the fiber input waveform is constant for each span, and as a result, non-linear compensation can be realized with one phase rotation in the optical receiver.

  The fourth, fifth, and sixth embodiments can be applied to OXC and ROADM systems in which the optical node has multiple routes as in the second embodiment. Furthermore, as in the third embodiment, the present invention can also be applied to digital signal processing in the transmitter, and digital signal processing in both the transmitter and the receiver in the SPM · XPM compensation.

  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 may 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 arranged in this order.

  22 to 25 show the configuration of the optical node 4 according to the fourth embodiment. The optical node 4 according to the fourth embodiment has an inter-polarization delay addition function unit 43 in addition to the dispersion compensation function unit 41 and the optical amplifier 42 which are essential. FIG. 22 shows an example in which the dispersion compensation function unit 41, the inter-polarization delay addition function unit 43, and the optical amplifier 42 are configured in this order. FIG. 23 shows an example in which the inter-polarization delay application unit 43, the dispersion compensation function unit 41, and the optical amplifier 42 are arranged 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 addition function unit 43 are arranged in this order. FIG. 25 shows an example in which the optical amplifier 42, the inter-polarization delay adding function unit 43, and the dispersion compensation function unit 41 are arranged in this order.

  26 to 29 show the configuration of the optical node 4 according to the fifth embodiment. The optical node 4 according to the fifth embodiment has an inter-wavelength delay addition function unit 44 in addition to the dispersion compensation function unit 41 and the optical amplifier 42 which are essential. FIG. 26 shows an example in which the dispersion compensation function unit 41, the inter-wavelength delay addition function unit 44, and the optical amplifier 42 are configured in this order. FIG. 27 shows an example in which the inter-wavelength delay application unit 44, the dispersion compensation function unit 41, and the optical amplifier 42 are arranged 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 arranged in this order. FIG. 29 shows an example in which the optical amplifier 42, the inter-wavelength delay adding function unit 44, and the dispersion compensation function unit 41 are arranged in this order.

  30 to 41 illustrate 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 dispersion compensation function unit 41 and the optical amplifier 42 which are essential, an inter-polarization delay addition function unit 43 and an inter-wavelength delay addition function unit 44 are added. FIG. 30 shows an example in which the dispersion compensation function unit 41, the inter-wavelength delay addition function unit 44, the inter-polarization delay addition function unit 43, and the optical amplifier 42 are configured in this order. FIG. 31 shows an example in which the dispersion compensation function unit 41, the inter-polarization delay addition function unit 43, the inter-wavelength delay addition function unit 44, and the optical amplifier 42 are configured in this order. FIG. 32 shows an example in which the inter-wavelength delay application unit 44, the dispersion compensation function unit 41, the inter-polarization delay application unit 43, and the inter-polarization delay application unit 43 are arranged in this order. FIG. 33 shows an example in which the inter-polarization delay application unit 43, the dispersion compensation function unit 41, the inter-wavelength delay application unit 44, and the optical amplifier 42 are arranged in this order. FIG. 34 shows an example in which the inter-wavelength delay application unit 44, the inter-polarization delay application unit 43, the dispersion compensation function unit 41, and the optical amplifier 42 are arranged in this order. FIG. 35 shows an example in which the inter-polarization delay application unit 43, the inter-wavelength delay application unit 44, the dispersion compensation function unit 41, and the optical amplifier 42 are arranged 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 addition function unit 43 are arranged in this order. FIG. 37 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, the inter-polarization delay addition function unit 43, and the inter-wavelength delay addition function unit 44 are arranged in this order. FIG. 38 shows an example in which the optical amplifier 42, the inter-wavelength delay addition function unit 44, the dispersion compensation function unit 41, and the inter-polarization delay addition function unit 43 are arranged in this order. FIG. 39 shows an example in which the optical amplifier 42, the inter-polarization delay addition function unit 43, the dispersion compensation function unit 41, and the inter-wavelength delay addition function unit 44 are arranged in this order. FIG. 40 shows an example in which the optical amplifier 42, the inter-wavelength delay application unit 44, the inter-polarization delay application unit 43, and the dispersion compensation function unit 41 are arranged in this order. FIG. 41 shows an example in which the optical amplifier 42, the inter-polarization delay adding function unit 43, the inter-wavelength delay adding function unit 44, and the dispersion compensation function unit 41 are arranged in this order.

  42 to 45 show an optical node provided with an inter-polarization delay addition + inter-wavelength delay addition function unit 45 for adding inter-polarization delay and inter-wavelength delay in addition to the dispersion compensation function unit 41 and the optical amplifier 42 which are essential. FIG. 6 is a diagram showing a configuration of 4. FIG. 42 shows an example in which the dispersion compensation function unit 41, inter-polarization delay addition + inter-wavelength delay addition function unit 45, and the optical amplifier 42 are configured in this order. FIG. 43 shows an example in which the inter-polarization delay addition + inter-wavelength delay addition function unit 45, the dispersion compensation function unit 41, and the optical amplifier 42 are arranged in this order. FIG. 44 shows an example in which the optical amplifier 42, the dispersion compensation function unit 41, and the inter-polarization delay addition + inter-wavelength delay addition function unit 45 are arranged in this order. FIG. 45 shows an example in which the optical amplifier 42, the inter-polarization delay addition + inter-wavelength delay addition function unit 45, and the dispersion compensation function unit 41 are arranged in this order.

  When the dispersion compensation function unit 41 is performed using a dispersion compensation fiber (DCF) at the optical node 4 shown in FIGS. 20 to 45, the core diameter of DCF is smaller than that of a normal transmission path fiber, Since non-linear generation in the DCF is a problem, it is usually used where the optical signal power is small. Therefore, it is preferable to use DCF in front of the light 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, any configuration may be used because it does not depend on the optical signal power.

  Next, the configuration of the inter-polarization delay application unit 43 will be described. 46 to 48 are diagrams showing the configuration of the inter-polarization delay adding function unit 43 described above. In FIG. 46, the wavelength demultiplexing function unit 431 demultiplexes into three wavelengths, and the inter-polarization delay applying unit 432 adds different inter-polarization delay amounts for each wavelength, and then combines by the wavelength multiplexing function unit 433. It shows the configuration. Here, when an AWG (Arrayed Waveguide Grating) having cyclicity is used for the wavelength demultiplexing function unit 431, signals are routed to the same port in three wavelength cycles, and hence operation in a wide wavelength range becomes possible.

  FIG. 47 shows the configuration in the case where an ILF is used for the wavelength demultiplexing function unit 431 and the wavelength multiplexing function unit 433. The odd-numbered wavelength channels and the even-numbered wavelength channels are provided with different inter-polarization delay amounts from the inter-polarization delay application unit 432 in separate paths. FIG. 48 shows the configuration of the inter-wavelength delay addition unit 434 using LCOS WSS (for example, WaveShaper manufactured by Finisar). Since it is possible to provide different inter-polarization delay amounts for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device. The inter-polarization delay application unit 432 in FIGS. 46 and 47 does not have to have variable delay amounts, and may have fixed delay amounts that differ depending on the wavelength.

  Next, the configuration of the inter-wavelength delay application unit 44 will be described. 49 to 51 show the configuration of the inter-wavelength delay application unit 44. FIG. FIG. 49 shows a configuration in which the wavelength demultiplexing function unit 441 demultiplexes into three wavelengths, the delay applying unit 442 applies different delay amounts for each wavelength, and then the wavelength multiplexing function unit 443 multiplexes. There is. Here, when an AWG (Arrayed Waveguide Grating) having cyclicity is used for the wavelength demultiplexing function unit 441, signals are routed to the same port in three wavelength cycles, so operation in a wide wavelength range becomes possible.

FIG. 50 shows the configuration in the case where an ILF is used for the wavelength demultiplexing function unit 441 and the wavelength multiplexing function unit 443. The odd-numbered and even-numbered wavelength channels are provided with different amounts of delay by the delay application unit 442 in different paths. FIG. 51 shows the configuration of the wavelength-specific delay application unit 444 using LCOS WSS (for example, WaveShaper manufactured by Finisar).
Since it is possible to provide different inter-polarization delay amounts for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device. The delay amount of the delay application unit 442 in FIG. 49 and FIG. 50 does not have to be variable, and may be a fixed delay amount different for each wavelength.

  Next, the configuration of the inter-polarization delay application + inter-wavelength delay application unit 45 will be described. 52 to 57 are diagrams showing the configuration of inter-polarization delay addition + inter-wavelength delay addition function unit 45. FIG. In FIG. 52, the wavelength demultiplexing function unit 451 demultiplexes into three wavelengths and the inter-polarization delay applying unit 452 and the delay applying unit 453 apply different delay amounts for each wavelength, and then the wavelength multiplexing function unit 454 The configuration to combine is shown. In FIG. 53, after the wavelength demultiplexing function unit 451 demultiplexes into three wavelengths and the delay imparting unit 453 and the inter-polarization delay imparting unit 452 impart different delay amounts for each wavelength, the wavelength multiplexing function unit 454 The configuration to combine is shown. Here, when an AWG (Arrayed Waveguide Grating) having cyclicity is used for the wavelength demultiplexing function unit 451, signals are routed to the same port in three wavelength cycles, and therefore, operation in a wide wavelength range becomes possible.

  FIG. 54 shows the configuration in the case where an ILF is used for the wavelength demultiplexing function unit 451 and the wavelength multiplexing function unit 454. The odd-numbered wavelength channels and the even-numbered wavelength channels are provided with different delay amounts by the inter-polarization delay application unit 452 and the delay application unit 453 in different paths. FIG. 55 shows the configuration in the case where an ILF is used for the wavelength demultiplexing function unit 451 and the wavelength multiplexing function unit 454. The odd-numbered and even-numbered wavelength channels are provided with different delay amounts by the delay applying unit 453 and the inter-polarization delay applying unit 452 in different paths.

  FIG. 56 shows the configurations of the inter-wavelength delay addition unit 455 and the per-wavelength delay addition unit 456 using the LCOS WSS (for example, WaveShaper manufactured by Finisar). FIG. 57 shows the configurations of the wavelength-specific delay application unit 456 and the wavelength-specific inter-polarization delay application unit 455 using LCOS WSS (for example, WaveShaper manufactured by Finisar). Since it is possible to provide different inter-polarization delay amounts for each wavelength channel inside the LCOS WSS, a simple configuration is possible with one device.

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

  As described above, by providing the optical dispersion compensation function, it is possible to provide an optical transmission system capable of reducing equalization processing in a DSP (Digital Signal Processor). In particular, at least one of compensating for phase rotation using waveform information in the middle of propagation where peak power is maximized by chromatic dispersion, or compensating for phase rotation due to XPM due to adjacent channel strength as well as SPM due to own channel strength. I tried to do one. As a result, it is possible to improve signal quality with SPM and XPM compensation with a simple configuration by using optical compensation in combination with non-linear optical effect compensation only by digital signal processing, and the device scale is also smaller. can do.

  Although the embodiments of the present invention have been described above with reference to the drawings, it is apparent that the above embodiments are merely examples of the present invention, and the present invention is not limited to the above embodiments. is there. Therefore, additions, omissions, replacements, and other changes of components may be made without departing from the technical concept and scope of the present invention.

  The present invention is useful for improving the efficiency of nonlinear optical effect compensation and reducing the load on the digital signal processor in a WDM digital coherent transmission system.

  11-14 ... optical transmitter, 2 ... multiplexing part, 3 ... optical fiber, 4 ... optical node, 5 ... demultiplexing part, 61-64 ... optical receiver, 41: dispersion compensation functional unit 42: optical amplifier 43: inter-polarization delay application unit 44: inter-wavelength delay application unit 45: inter-polarization delay application + inter-wavelength delay Functioning unit

Claims (1)

A plurality of optical nodes including an optical amplifier that compensates for the loss of each span of the optical transmission signal, and a dispersion compensation function unit that compensates for the dispersion for each span, a plurality of optical transmitters, and a plurality of optical receivers A digital coherent optical transmission system,
The optical transmitter compensates for nonlinear optical effects in the transmission path by modulating the multiple wavelength channel so as to cancel nonlinear optical effects in the transmission path by using a vector modulator that can control the phase component of the signal. And
The optical node has a function unit for controlling the time difference between chromatic dispersion and the wavelength channel for each of the spans independently functional unit to independently control the time difference between the chromatic dispersion and polarization of each of the spans, the span an optical transmission system Ru comprising of the functional unit, at least one to independently control the time difference between both chromatic dispersion and the wavelength channel and between polarizations for each.

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