JP2004254333A - Method for controlling wavelength dispersion and method for detecting amount of dispersion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To supervise and control a wavelength dispersion of a transmission line which transmits an optical signal essentially not including a clock component such as a RZ signal and an OTDM signal, and control a dispersion of a transmission line without interrupting a system operation. <P>SOLUTION: When a bit rate of an optical signal is Bb/s, a band pass filter 42 extracts BHz component from a base band signal obtained by a receiver side light receiving device 40 and a variable dispersion compensation device 34 is controlled so as to minimize an intensity of the extracted signal. Since the control is performed under system operation, a low-frequency signal from an oscillator 52 is superimposed on a control signal of the variable dispersion compensation device 34. A band pass filter 60 extracts a low frequency component from a detection output of the BHz component and a phase of both its extracted signal and a low-frequency signal from an oscillator 52 is compared and an amount of a dispersion compensation is automatically controlled depending on the result. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

The present invention relates to a method and an apparatus for controlling chromatic dispersion of a transmission line, and a dispersion amount detection method.
At present, a 10 Gb / s optical transmission system is being put to practical use in backbone optical communication. With the rapid increase in the amount of information, a further increase in the capacity of the optical communication system is desired. Time division multiplexing (including optical time division multiplexing) and wavelength multiplexing have been considered as candidates, and studies on the 40 Gb / s system in the time division multiplexing method have been active in Japan and overseas.

  The present invention refers to a technique for monitoring and controlling chromatic dispersion of a transmission line for realizing a large-capacity optical transmission system using a time division multiplexing method.

  One of the factors that limit the transmission distance in a 40 Gb / s system is chromatic dispersion (group velocity dispersion: GVD). Since the dispersion proof stress is inversely proportional to the square of the bit rate, the dispersion proof strength is about 800 ps / nm at 10 Gb / s, but becomes as severe as about 16 ps / nm at 40 Gb / s.

  According to the measurement results, an optical time-division multiplexing (OTDM) signal having a signal light wavelength of 1.55 μm (the wavelength at which the transmission loss in the silica-based fiber is minimum), an input signal light power of +3 dBm, and a bit rate of 40 Gb / s (described later) ) Was transmitted over a distance of 50 km over a single mode fiber (SMF) having a zero-dispersion wavelength of 1.3 μm (which is currently the most widely laid in the world), and dispersion compensation was performed using a dispersion compensation fiber (DCF). At this time, the width (dispersion compensation tolerance) of the range of the dispersion compensation value allowed to suppress the power penalty (reception sensitivity deterioration of the optical signal due to transmission) to 1 dB or less was 30 ps / nm. Since the dispersion compensation value required at this time is 930 ps / nm (18.6 ps / nm / km × 50 km), it is necessary to perform dispersion compensation with an accuracy close to 100%, which is 930 ± 15 ps / nm. Understand.

On the other hand, the dispersion of the transmission line changes due to a temporal change such as a temperature change. For example, in SMF 50 km transmission, the amount of change in transmission line dispersion when there is a temperature change of −50 to 100 ° C. is estimated as follows.
(Temperature dependence of transmission line zero dispersion wavelength) x (temperature change) x (dispersion slope) x (transmission distance)
= 0.03nm / ℃ × 150 ℃ × 0.07ps / nm 2 / km × 50km
= 16ps / nm
This is a value that cannot be ignored even when compared with the dispersion compensation tolerance. Therefore, in large-capacity transmission of 40 Gb / s or more, it is necessary to constantly monitor the transmission line dispersion and adjust the total dispersion to zero. The same applies to the case where a dispersion-shifted fiber (DSF) having a small chromatic dispersion in the 1.55 μm band is used.

When developing an automatic dispersion equalization system (a system that automatically performs feedback control of the total dispersion to zero), the following points are problematic.
(I) Realization of tunable dispersion compensator (ii) Detection method of transmission line dispersion (or total dispersion amount after dispersion compensation) (iii) Feedback control method of dispersion compensation amount It is conceivable to change the dispersion compensation amount discontinuously by switching DCFs having different compensation amounts with an optical switch. As a method of continuously changing, a stress (MM Ohm et al., Tunable grating dispersion using a piezoelectric stack, OFC '97 Technical Digest, WJ3, pp.155-156) and a temperature gradient (Sergio Barcelos et al., Characteristics of chirped fiber gratings for dispersion compensation, OFC '96 Technical Digest, WK12, pp. 161-262), method of giving phase change due to temperature change to PLC (Planer Lightwave Circuit) (K. Takiguchi, et al., Variable Group-Delay Dispersion Equalizer Using Lattice-Form Programmable Optical Filter on Planar Lightwave Circuit, IEEE J. Selected Topics in Quantum Electronics, 2 , 1996, pp.270-276) Have been tried. Instead of using a tunable dispersion compensator, a method of changing the transmission line dispersion using a tunable light source is also conceivable. In this case, it is necessary to simultaneously change the center wavelength of the optical filter.

  Regarding (ii), a pulse method or a phase method of inputting a plurality of lights of different wavelengths into a fiber and measuring a group delay difference or a phase difference between output lights has been conventionally used. However, when these methods are performed during system operation, it is necessary to interrupt the system operation when measuring the amount of dispersion or to perform wavelength multiplexing using measurement light having a wavelength different from the signal light. In the latter case, there is a problem that it is necessary to estimate the amount of dispersion in the signal light from the measurement result of the amount of dispersion in the measurement light because the transmission line dispersion differs depending on the wavelength. “A. Sano et al., Automatic dispersion equalization by monitoring extracted-clock power level in a 40-Gbit / s, 200-km Transmission line, ECOC '96, TuD. 3.5, 1996, pp.207-210” Discloses that a power of a clock component (a BHz component when the bit rate of a data signal is Bb / s) is detected from a received optical signal, and the amount of dispersion compensation is controlled so as to maximize the detected power. However, although a clock component is included in the case of a return-to-zero (RZ) signal, a non-return-to-zero (NRZ) signal or an OTDM signal obtained by time-division multiplexing a plurality of RZ signals so that their bottoms overlap each other is provided. As described above, when the clock component intensity does not become maximum at the time of zero dispersion, this method cannot be applied.

  Regarding (iii), a method of suspending the system operation, sweeping the total dispersion over a wide range by using a tunable dispersion compensator or a tunable light source, and detecting a set point at which the total dispersion becomes zero, and then setting the value to that value. Can be considered. However, a method that can always be controlled without interrupting the system operation is desirable.

  Accordingly, an object of the present invention is to control transmission line dispersion for an optical signal whose clock component intensity does not become maximum at the time of zero dispersion, such as an OTDM signal obtained by multiplexing an NRZ signal and a plurality of RZ signals so that the skirts overlap each other. To provide a method and apparatus for the same.

  It is another object of the present invention to provide a method and apparatus for controlling transmission line dispersion without interrupting system operation, and a dispersion amount detection method.

  According to the present invention, there is provided a dispersion control method for controlling chromatic dispersion of a transmission line for transmitting an optical signal modulated with a data signal, wherein the intensity of a component of a specific frequency is determined from the optical signal transmitted on the transmission line. There is provided a dispersion control method including a step of detecting and controlling a total dispersion amount of a transmission line such that the intensity of the detected specific frequency component is minimized.

  According to the present invention, there is provided a dispersion control method for controlling chromatic dispersion of a transmission line for transmitting an optical signal modulated with a data signal, wherein a low-frequency signal is superimposed on a control signal of a total dispersion amount, and the low-frequency signal is The total dispersion of the transmission line is changed in accordance with the superimposed control signal, the intensity of a specific frequency component is detected from the optical signal transmitted through the transmission line, and the same frequency as the low frequency signal is detected from the detection signal of the specific frequency component. A dispersion control method comprising the steps of: extracting a component of the frequency component; comparing the phase of the extracted frequency component with the phase of the low-frequency signal; and generating a control signal of the total variance based on the phase comparison result. Provided.

According to the present invention, there is provided a method for detecting the amount of dispersion of a transmission line for transmitting an optical signal modulated with a data signal, wherein the intensity of a component of a specific frequency is detected from the optical signal transmitted on the transmission line. And determining the total dispersion amount of the transmission path from the detected intensity of the specific frequency component.
According to the present invention, there is provided a dispersion control device for controlling chromatic dispersion of a transmission line for transmitting an optical signal modulated with a data signal, wherein the intensity of a component of a specific frequency is determined from the optical signal transmitted on the transmission line. There is also provided a dispersion control device including a photodetector to be detected and dispersion control means for controlling the total dispersion amount of the transmission line so that the intensity of the detected specific frequency component is minimized.

  According to the present invention, there is provided a dispersion control device that controls chromatic dispersion of a transmission line that transmits an optical signal modulated with a data signal, and a low-frequency superimposing circuit that superimposes a low-frequency signal on a control signal of a total dispersion amount. A variable dispersion means for changing the total amount of dispersion of a transmission line according to a control signal on which a low-frequency signal is superimposed; a photodetector for detecting the intensity of a specific frequency component from an optical signal transmitted on the transmission line; A signal extraction circuit that extracts a component having the same frequency as the low-frequency signal from the detection signal of the frequency component; a phase comparison circuit that compares the phase of the extracted frequency component with the phase of the low-frequency signal; And a control signal generation circuit for generating a control signal of the total amount of dispersion based on an output.

  According to the present invention, a time-division multiplexed optical signal modulated by an nm bit / second data signal obtained by time-division multiplexing of n optical signals amplitude-modulated by an m-bit / second data signal. A signal is transmitted to an optical fiber transmission line, and a frequency component of nmhertz or mhertz is extracted from the time-division multiplexed optical signal received from the optical fiber transmission line, and the extracted nmhertz or mhertz is extracted. A dispersion control method for an optical fiber transmission line, characterized in that the dispersion of the optical fiber transmission line is made variable so that the frequency components of the optical fiber transmission line indicate a minimum value or a maximum value, respectively.

  According to the present invention, means for receiving, from an optical fiber transmission line, n optical signals time-division multiplexed and amplitude-modulated with an m-bit / second data signal, and the time-division multiplexed signal received A means for extracting a frequency component of nmhertz or mhertz from the optical signal; and transmitting the optical fiber so that the extracted frequency component of nmhertz or mhertz indicates a minimum value or a maximum value, respectively. Means for controlling the amount of dispersion of the optical fiber transmission line.

  Computer simulation of total dispersion dependence of 40 GHz component intensity in baseband spectrum of OTDM signal, NRZ optical signal, RZ optical signal (duty 50%), and RZ optical signal (duty 25%) with data signal bit rate 40 GHz 1 to 4 show the results. 1 to 4 also show the eye opening degree in the amplitude direction. The power of the input light was -5 dBm on average, and the SMF length was 50 km. The total dispersion was changed by changing the dispersion of the DCF connected in series to the SMF.

The OTDM signal is an optical signal output from the optical modulator 10 as shown in FIG. In FIG. 5, Ti is thermally diffused into a LiNbO ₃ substrate 12 to form an optical waveguide 14 as shown in FIG. 5, on which an electrode pattern 16 shown by hatching in FIG. A two-output optical switch 18, a data modulator 20 having two independent optical modulators, a phase controller 22, and an optical multiplexer 24 are formed. When continuous light is input to the optical waveguide of the one-input two-output optical switch 18 and a 20 GHz clock having a phase difference of 180 ° is applied to the two electrodes, two opposite phases shown in columns (a) and (b) of FIG. The 20 GHz optical clock of the system is output from the optical switch 18 and input to the two optical modulators of the data modulator 20. A data signal of 20 Gb / s is applied to each of the two optical modulators, and two series of RZ signals shown in columns (c) and (d) of FIG. The phase control unit 22 adjusts the phases of the light waves so that the phase difference between the two light waves becomes 180 °, and the light waves are combined by the optical multiplexing unit 24. Since the phase difference between the two light waves is 180 °, as shown in the column (e) of FIG. 6, where 1s continue, the skirt portions cancel each other out, resulting in a waveform close to the RZ signal. When at least one becomes zero, the waveform becomes close to the waveform of the NRZ signal.

As for the RZ signals of FIGS. 3 and 4, it can be seen that the intensity of the 40 GHz component becomes maximum when the total amount of dispersion becomes zero. On the other hand, in the OTDM signal of FIG. 1 and the NRZ signal of FIG. 2, when the total dispersion amount is zero, the 40 GHz component intensity is minimal.
FIGS. 7 and 8 show optical modulation signal base hand spectra for OTDM and NRZ, respectively, for reference. It is qualitatively considered that the NRZ has no 40 GHz component and that after receiving chromatic dispersion, a 40 GHz component is generated due to spectrum spreading. 9 and 10 show waveforms (equalized waveforms) after receiving dispersion of −40, 0, and +40 ps / nm for each of OTDM and NRZ. Since the 1 "level rises after receiving the variance (positive / negative) and the cross point position decreases, the intensity fluctuation of the same cycle as the length of one time slot occurs, thereby generating a 40 GHz component. I understand.

  Therefore, with respect to the point (ii), when transmitting an optical signal whose bit rate is generally Bb / s and the B hertz component is minimized at zero dispersion, the control point of the variable dispersion device such as the amount of dispersion compensation and signal light wavelength is transmitted. If the control point at which the intensity of the BHz component in the received light signal is minimized can be detected, the total dispersion can be set to zero. It is also conceivable to perform similar control using other frequency components such as a harmonic component of BHz other than the BHz component. In addition, in the OTDM and NRZ waveforms, since two maximum points exist symmetrically on both sides of the minimum point as is clear from FIGS. 1 and 2, when it is difficult to detect the minimum point, a variable giving two maximum points is used. By detecting the control point of the dispersion compensating device and taking the midpoint, the total dispersion amount can be set to zero.

  Further, in the case of an OTDM signal modulated by an nm bit / second data signal obtained by time division multiplexing n RZ signals amplitude-modulated with an m bit / second data signal, as described above. Instead of extracting the nm hertz component and controlling the total dispersion of the transmission line so that it becomes a minimum, instead of extracting the m Hertz component and controlling the total dispersion of the transmission line so that it becomes a maximum, May be. The RZ signal having the rate of m bits / sec, which constitutes the OTDM signal, includes an m hertz component, and as is apparent from FIGS. 3 and 4, this is because when the total variance is 0, it becomes a maximum. That is, in this case, the nm dispersion component or the m Hz component is extracted, and the total dispersion amount of the transmission path is controlled such that the component becomes minimum or maximum, respectively.

Regarding the point (iii), in order to always detect the point where the BHz component intensity is minimal (or maximal), the total variance around the minimal point (or maximal point) is slightly changed at a low frequency f _0. There is a way to do this. The principle of the method is shown in FIGS. When the dispersion compensation amount is at the minimum point (or the maximum point) as shown in FIG. 11, the BHz component intensity changes with time at the frequency 2 × f ₀ , and does not include the frequency f ₀ component. If the amount of dispersion compensation deviates from this state as shown in FIGS. 12B and 12C, the frequency f ₀ component appears in the time change of the BHz component intensity as shown in FIG. And (c), the signs of the components are reversed. Therefore, consider the case where the component of the frequency f ₀ is detected from the BHz component intensity, and feedback is performed so as to change the total variance so as to eliminate the component. Direction of the change can be determined from the phase component of the frequency f _0.

  Further, the total dispersion amount can be detected by using the characteristics shown in FIGS. That is, the intensity of a specific frequency component is detected, and the total variance can be obtained from the magnitude and the correspondence between FIGS. However, since the specific frequency component intensity and the total dispersion amount do not have a one-to-one relationship, the characteristic measurement is performed by sweeping the control points of the variable dispersion device within a certain range as necessary.

The above dispersion equalization method and dispersion detection method are applicable not only to a time division multiplexing system but also to a wavelength division multiplexing (WDM) system. That is, after separating the different wavelength components, the dispersion equalization method and the dispersion detection method according to the present invention can be applied to each component.
FIG. 13 shows an embodiment of the automatic distributed equalization system according to the present invention. The optical signal of the bit rate Bb / s from the optical transmitter 30 is transmitted through an optical transmission line (SMF) 32 and then input to an optical receiver 36 via a variable dispersion compensator 34. A part of the optical signal input to the optical receiver 36 is branched by the optical coupler 38 and converted into an electric signal by the optical receiver 40. A BHz component is extracted from the output of the light receiver 40 by a band-pass filter 42 having a center frequency of BHz, and the intensity is detected by an intensity detector 44. The compensation amount controller 46 adjusts the dispersion compensation amount of the tunable dispersion compensator 34 in a direction in which the BHz component is maximized for the RZ signal, and in a direction in which the BHz component is minimized for the OTDM or NRZ waveform. Control. Although the variable dispersion compensator 34 is arranged at the receiving end here, the same control can be performed even if the variable dispersion compensator 34 is arranged at another position such as on the transmitting side or in a linear repeater. Also, in the case of an OTDM signal in which n mb / s RZ signals are multiplexed, the m hertz component may be maximized instead of minimizing the mn hertz component.

14 and 15 show specific examples of the optical transmitter 30 and the optical receiver 36 in FIG. In the optical transmitter 30 of FIG. 14, the OTDM modulator 10 shown in FIG. 5 is used as an optical modulator for generating an optical signal. In FIG. 14, the OTDM modulator 10 of FIG. 5 is functionally represented by using the same reference numerals for the same components as those of FIG.
In this example, two parallel 10 Gb / s data signals are converted by the parallel / serial converter 70 to obtain one 20 Gb / s NRZ signal. The 20 Gb / s NRZ signal is input to the driver 72 to obtain a 20 Gb / s drive signal for driving the optical modulator 20. The output (RZ optical signal of 20 Gb / s) of each optical modulator 20 is phase-adjusted by the phase adjuster 22 (the phase is shifted so that the phase difference of the light becomes 180 °), and then the light is converted to light. The multiplexed signal is multiplexed by the multiplexing unit 24 (optical coupler) to obtain one NRZ type 40 Gb / s optical signal, which is transmitted to the transmission line via the optical post-amplifier 74. FIG. 16 shows a detailed circuit diagram of such an optical transmitter.

In FIG. 15, a 40 Gb / s optical signal is input to an optical DEMUX 78 via a tunable dispersion compensator 34, an optical preamplifier 76, and a beam splitter 38. As the light DEMUX 78, a polarization-independent light DEMUX shown in FIG. 17 can be used.
FIG. 17 shows a structure diagram of the polarization independent light DEMUX78. The optical DEMUX arranged on the receiving side is required to be polarization independent. Therefore, first, the 40 Gb / s OTDM signal input after the fiber transmission is polarized and separated into a TE component and a TM component by the first-stage cross waveguide polarization splitter 80. Here, the intersection length is optimized so that a polarization extinction ratio of 20 dB or more can be secured. Next, for each mode, optical time division separation into a 20 Gb / s optical RZ signal is performed using a 1 × 2 switch 84 driven by a 20 GHz sine wave signal. At this time, the two outputs of each 1 × 2 switch are in a complementary relationship. However, in general, in the LN switch (modulator), the TM mode has higher modulation efficiency than the TE mode. Therefore, in this device, the TE mode light after polarization separation is converted into the TM mode light by the half-wave plate 82. Light separation. In the final stage, two polarization beam combiners combine the same bit sequence. At this time, if TM mode light is multiplexed, a problem of optical interference occurs as in the case of the above-mentioned OTDM modulator. Therefore, the latter stage of the 1 × 2 switch 84 of the port which did not perform the TE / TM mode conversion first. After the TM / TE mode conversion is performed by the half-wave plate 88 in the above, the orthogonal polarization components are power-multiplexed.

  The two 20 Gb / s optical RZ signals obtained by the optical DEMUX 78 are respectively input to the photodiodes 90, converted into electric signals, amplified by the preamplifier 92, and then shaped by the equalizing amplifier 94. . Then, the serial / parallel converter 96 reproduces the original 10 Gb / s NRZ data. Although not shown, the data is thereafter reproduced by the 10 Gb / s identification unit. FIG. 18 shows a detailed circuit diagram of the portion of the optical receiver 36 up to the optical separation.

Next, an example of a variable dispersion compensator (MM Ohm et al., "Tunable fiber grating dispersion using a piesoelectric stack", OFC '97 Technical Digest, WJ3, pp.155-156) will be described.
As shown in FIG. 19, a piezoelectric element 92 is separately attached to each of the 21 segments of the chirped fiber grating 90. When a voltage is applied with an inclination as shown in FIG. 20 as applied voltages V _{1 to} V ₂₁ to the respective piezoelectric elements, the pressure applied in the longitudinal direction of the crating 90 changes, and the voltage patterns A to D in FIG. In contrast, the variance value (the slope of the line) changes as shown in FIG.

FIG. 22 is a diagram illustrating an example of the compensation amount control unit. The intensity value of the 40 Gb / s frequency component is A / D converted by an A / D converter 94 and input to the MPU 96 as a digital signal. The MPU 96 compares the previously received intensity value Ip stored in the memory 98 with the current intensity value Ic, and determines the relationship between the current dispersion amount and the intensity of 40 Gb / s to the slope of X in FIG. Check if it is on the Y slope. That is, if the dispersion amount of the variable dispersion compensator 34 is reduced if the slope is X, the dispersion amount converges to 0 (Z point). If the slope is Y, the dispersion amount converges to 0 if the dispersion amount of the variable dispersion compensator 34 is increased. Therefore, when Ic> Ip, it is regarded as being on the X slope, and in order to control the voltage applied to the variable dispersion compensator 34 in FIG. 19, the values of V _{1 to} V ₂₁ that reduce the amount of dispersion are obtained. A voltage to be applied to each piezoelectric element is output via the / A converter 100. Conversely, if Ic <Ip, it is regarded as being on the Y slope, and values of V _{1 to} V ₂₁ that increase the amount of dispersion are obtained to control the voltage applied to the variable dispersion compensator 34 in FIG.

In order to obtain the values of V _{1 to} V _21, the data shown in FIGS. 20 and 21 (data indicating the relationship between the variance and V _{1 to} V ₂₁ ) and the data shown in FIG. Data indicating the relationship between the intensity and the amount of dispersion) is stored in the memory in advance. Then, it is first determined which slope of the X or Y slope in FIG. 2 is present, and the current dispersion amount Ic is determined from the data shown in FIG. From the current dispersion amount Ic, a dispersion amount Ic 'to be compensated by the variable dispersion compensator 34 to converge to the Z point of the dispersion amount 0 is obtained. That is, Ic 'is determined so that Ic + Ic' = 0.

'If is determined, based on the data shown in FIGS. 20 and 21, Ic' this way Ic Request V ₁ ~V ₂₁ applied to the variable dispersion compensator 34 in order to obtain.
FIG. 23 shows a variation of the system of FIG. In the drawings following FIG. 23, the same components are denoted by the same reference numerals and description thereof will be omitted. In the system shown in FIG. 23, a tunable light source 48 is used for the optical transmitter 30 instead of the tunable dispersion compensator 34 in the system shown in FIG. The chromatic dispersion amount of the path 32 is controlled.

FIG. 24 shows another example of a more detailed configuration of the automatic distributed equalization system of FIG. Oscillator 52 generates a sine wave of low frequency f _0. The low-frequency signal generated by the oscillator 52 is superimposed on the compensation amount setting signal from the dispersion compensation amount setting circuit 56 in the low-frequency superimposing circuit 54 and is supplied to the variable dispersion compensator 34. BHz component output from the band-pass filter 42 whose intensity is detected by the intensity detector 58 (for example, square detector), f ₀ component is extracted from the detection output in the band-pass filter 60. The phase of the f ₀ component extracted by the band-pass filter 60 and the phase of the low-frequency signal output by the oscillator 52 are compared in the phase comparison circuit 62. The dispersion compensation amount setting circuit 56 generates and outputs a compensation amount setting signal based on the comparison result in the phase comparison circuit 62. The intensity detector 58 is realized by a multiplier, a mixer, a general power detector, or the like, and the phase comparison circuit 62 is realized by a multiplier, a mixer, a full-wave rectifier, or the like, and a low-pass filter.

The signal waveforms at the points indicated by (a) to (g) in FIG. 24 are shown in columns (a) to (g) of FIG.
When the dispersion compensation amount deviates from the optimum value to the positive side as shown in FIG. 25A, the output of the low frequency superposition circuit 54 becomes as shown in FIG. If the amount of dispersion compensation deviates from the optimum value to the positive side, the BHz component intensity increases in the case of NRZ or OTDM if the amount of dispersion compensation increases (see FIGS. 1, 2 and 12). The amplitude of the extracted BHz component changes at the frequency f ₀ as shown in the column (c), the intensity is detected ((d)), and the result of extracting the f ₀ component (the column (e)) is the oscillator 52. Are in phase with the low-frequency signal output (column (f)), and a positive voltage is output from the phase comparison circuit 62 (column (g)). By performing feedback control on the positive output signal so that the dispersion compensation amount control signal (column (a)) output from the dispersion compensation amount setting circuit 56 decreases, the dispersion compensation amount approaches an optimum value. When the dispersion compensation amount is lower than the optimum value, the output of the band-pass filter 60 is in the opposite phase to the output of the oscillator 52, and a negative voltage is output from the phase comparison circuit 62. By performing feedback control so that the dispersion compensation amount signal increases, the dispersion compensation amount changes toward the optimum value. In the case of the RZ signal, the direction of the change in the amount of dispersion compensation may be reversed.

  FIG. 26 shows a variation of the system of FIG. As in the modification from FIG. 13 to FIG. 23, the control is the same as that in FIG. 24 except that the control of the dispersion compensation amount by the variable dispersion compensator 34 is replaced by the control of the wavelength by the wavelength variable light source 48. In the case of this method, it is necessary to transmit the detection signal obtained by the phase comparison on the receiving side to the transmitting side. For this purpose, a method of preparing another low-speed line or a method of putting information on a transmission signal in the opposite direction can be considered.

  FIG. 27 shows an example of a distributed equalization system according to another embodiment of the present invention. In the previous examples, dispersion value control is assumed while the system is operating.However, here, when the system is started, and when the automatic decentralization control is significantly deviated from the optimal point, it is restarted. It is assumed to be applied to the case where the system operation is intentionally interrupted and the amount of dispersion is optimized. The dispersion compensation amount of the variable dispersion compensator 34 is swept over a wide range, and a change in the BHz component intensity is detected from the output of the intensity detection unit 44 at that time. As described above, the total dispersion amount can be detected by comparing the BHz component intensity characteristics with the characteristics in FIGS. For the RZ signal, the dispersion compensation amount at which the BHz component is maximized is recorded, and after the compensation amount is swept, the dispersion compensation amount is set, and then the system operation is started. For the OTDM or NRZ waveform, for example, two dispersion compensation amounts at which the BHz component intensity is maximum may be recorded, and after the compensation amount sweep, the dispersion compensation amount may be set between the two values.

  FIG. 28 shows a variation of the system of FIG. 27 is the same as FIG. 27 except that the sweep and setting of the tunable dispersion compensator 34 in FIG. 27 are replaced by the sweep and setting of the wavelength by the wavelength variable light source 48.

  As described above, according to the present invention, it is possible to monitor and control transmission line dispersion for an optical signal whose clock component is minimized when the dispersion is zero, such as an NRZ waveform and an OTDM waveform, and interrupt system operation. It is possible to control transmission line dispersion without performing.

FIG. 10 is a graph showing the results of computer simulation of the total dispersion amount dependence of the 40 GHz clock component intensity for a 40 Gb / s OTDM signal. FIG. FIG. 11 is a graph showing the results of computer simulation of the total variance dependence of the 40 GHz clock component strength for a 40 Gb / s NRZ signal. FIG. 10 is a graph showing the results of computer simulation of the total dispersion amount dependence of the 40 GHz clock component intensity for a 40 Gb / s RZ signal (duty 50%). 10 is a graph showing the results of computer simulation of the total dispersion amount dependence of the 40 GHz clock component intensity for a 40 Gb / s RZ signal (duty 25%). FIG. 3 is a plan view of an optical modulator that generates a 40 Gb / s OTDM signal. FIG. 6 is a waveform chart illustrating an operation of the optical modulator of FIG. 5. It is a baseband spectrum of an OTDM signal. It is a baseband spectrum of an NRZ signal. FIG. 4 is a waveform diagram of an OTDM signal after receiving chromatic dispersion. FIG. 4 is a waveform diagram of an NRZ signal after receiving chromatic dispersion. FIG. 9 is a diagram illustrating a case where the dispersion compensation amount is at a minimum point in the method of slightly varying the total dispersion amount at a low frequency f ₀ . FIG. 8 is a diagram illustrating a case where the dispersion compensation amount deviates from a minimum point in the method of slightly varying the total dispersion amount at a low frequency f ₀ . 1 is a block diagram illustrating an embodiment of an automatic distributed equalization system according to the present invention. FIG. 14 is a block diagram illustrating a specific example of the optical transmitter 30 in FIG. 13. FIG. 14 is a block diagram illustrating a specific example of the optical receiver 36 in FIG. 13. It is a detailed circuit diagram of an optical transmitter. It is a figure showing polarization independent light DEMUX. FIG. 3 is a detailed circuit diagram of a part of the optical receiver. FIG. 3 is a diagram illustrating an example of a tunable dispersion compensator. 20 is a graph showing patterns A to D of voltages V _{1 to} V ₂₁ applied to each segment of the variable dispersion compensator in FIG. 19. It is a graph which shows the dispersion value in each voltage pattern AD. It is a block diagram showing an example of composition of a compensation amount control part. FIG. 14 is a block diagram illustrating a modification of the system of FIG. 13. FIG. 14 is a block diagram illustrating an example of a more detailed configuration of the system in FIG. 13. FIG. 25 is a waveform chart for explaining the operation of the system in FIG. 24. FIG. 25 is a block diagram showing a modification of the system in FIG. 24. FIG. 11 is a block diagram illustrating an example of a distributed equalization system according to another embodiment of the present invention. FIG. 28 is a block diagram illustrating a modification of the system of FIG. 27.

Explanation of reference numerals

10 ... optical modulator 12 ... LiNbO ₃ substrate 14 ... optical waveguide 16 ... electrode pattern 18 ... 1 Input 2 Output optical switch 20 ... data modulation unit 22 ... phase controller 24 ... optical multiplexing unit

Claims (23)

A dispersion control method for controlling chromatic dispersion of a transmission line that transmits an optical signal modulated with a data signal,
The optical signal is an optical time-division multiplexed signal obtained by multiplexing a plurality of return-to-zero signals having different phases of light waves, or a non-return-to-zero signal,
(A) detecting, from the optical signal transmitted through the transmission line, the intensity of a frequency component which is a component of a specific frequency and whose intensity is maximized on both sides of zero dispersion with respect to the amount of dispersion;
(B) a dispersion control method comprising the step of controlling the total dispersion amount of the transmission line so that the detected intensity of the specific frequency component is minimized. A bit rate of the data signal is B bits / sec;
The method according to claim 1, wherein in step (a), the intensity of a component whose frequency is a frequency obtained by multiplying B hertz or a frequency obtained by dividing B hertz is detected. A bit rate of the data signal is B bits / sec;
2. The method according to claim 1, wherein in step (a) the intensity of the component whose frequency is B hertz is detected. The transmission path includes a variable dispersion compensator having a variable dispersion value,
4. The method according to claim 1, wherein step (b) includes controlling a dispersion value of the variable dispersion compensator to control a total dispersion amount of the transmission line. The transmission path includes a wavelength variable light source that can change the wavelength of the optical signal,
4. The method according to claim 1, wherein step (b) includes controlling a wavelength of the tunable light source to control a total dispersion amount of the transmission line.   The method according to any one of claims 1 to 5, wherein in step (b), the total amount of dispersion is continuously controlled during operation based on the detected intensity of the specific frequency component. Step (b)
(I) superimposing a low-frequency signal on the control signal of the total variance,
(Ii) extracting a component having the same frequency as the low-frequency signal from the detection signal of the specific frequency component,
(Iii) comparing the phase of the extracted frequency component with the phase of the low-frequency signal,
7. The method according to claim 6, further comprising the step of: (iv) generating a control signal for controlling the total variance based on the phase comparison result. Step (b)
(I) finding two control points of the total variance at which the intensity of the detected specific frequency component is maximized,
6. The method according to claim 1, further comprising the step of: (ii) setting a middle point between the two control points as a control point at which the intensity of the specific frequency component is minimized. Step (b)
(I) sweeping the total variance before or during operation interruption,
The method according to any one of claims 1 to 5, further comprising the substep of (ii) finding a control point of the total variance from the intensity of the specific frequency component being swept. A method for detecting the amount of dispersion of a transmission line that transmits an optical signal modulated with a data signal,
The optical signal is an optical time-division multiplexed signal obtained by multiplexing a plurality of return-to-zero signals having different phases of light waves, or a non-return-to-zero signal,
(A) detecting, from the optical signal transmitted through the transmission line, the intensity of a frequency component which is a component of a specific frequency and whose intensity is maximized on both sides of zero dispersion with respect to the amount of dispersion;
(B) a dispersion detection method comprising a step of determining a total dispersion amount of the transmission line from the detected intensity of the specific frequency component. A bit rate of the data signal is B bits / sec;
The method according to claim 10, wherein in step (a), the intensity of a component whose frequency is a frequency obtained by multiplying B hertz or a frequency obtained by dividing B hertz is detected. A bit rate of the data signal is B bits / sec;
11. The method according to claim 10, wherein in step (a) the intensity of the component whose frequency is B hertz is detected. A dispersion control device that controls chromatic dispersion of a transmission line that transmits an optical signal modulated with a data signal,
The optical signal is an optical time-division multiplexed signal obtained by multiplexing a plurality of return-to-zero signals having different optical wave phases, or a non-return-to-zero signal,
From the optical signal transmitted through the transmission line, a photodetector that detects the intensity of a frequency component that is a component of a specific frequency and whose intensity is maximized on both sides of zero dispersion with respect to the amount of dispersion,
A dispersion control device comprising: a dispersion control unit that controls a total dispersion amount of the transmission line so that the intensity of the detected specific frequency component is minimized. A bit rate of the data signal is B bits / sec;
14. The apparatus according to claim 13, wherein the photodetector detects the intensity of a component whose frequency is a frequency obtained by multiplying B hertz or a frequency obtained by dividing B hertz. A bit rate of the data signal is B bits / sec;
14. The apparatus of claim 13, wherein the photodetector detects the intensity of a component having a frequency of B hertz. The transmission path includes a variable dispersion compensator having a variable dispersion value,
The apparatus according to any one of claims 13 to 15, wherein the dispersion control unit controls a dispersion value of the variable dispersion compensator to control a total dispersion amount of the transmission line. The optical transmitter that generates the optical signal includes a wavelength variable light source that can change the wavelength of the optical signal,
The apparatus according to any one of claims 13 to 15, wherein the dispersion control means controls a total amount of dispersion of the transmission line by controlling a wavelength of the variable wavelength light source.   The apparatus according to any one of claims 13 to 17, wherein the dispersion control means continuously controls the total dispersion amount during operation based on the detected intensity of the specific frequency component. The distribution control means,
A low-frequency superimposing circuit that superimposes a low-frequency signal on the control signal of the total dispersion amount,
A signal extraction circuit for extracting a component having the same frequency as the low-frequency signal from the detection signal of the specific frequency component,
A phase comparison circuit that compares the phase of the extracted frequency component with the phase of the low-frequency signal,
19. The apparatus according to claim 18, further comprising: a control signal generation circuit that generates a control signal for controlling the total dispersion amount based on an output of the phase comparison circuit. Time division modulated with an nm bit / second data signal obtained by time division multiplexing n time reset multiplexing of n pieces of return-to-zero signals having different phases of light waves, which are amplitude modulated with an m bit / second data signal. Transmitting the multiplexed optical signal to the optical fiber transmission line,
From the time-division multiplexed optical signal received from the optical fiber transmission line, extract nm · m hertz or m hertz frequency components,
An optical fiber transmission line, wherein the dispersion of the optical fiber transmission line is made variable so that the extracted nm-hertz frequency component shows a local minimum value or the m-hertz frequency component shows a local maximum value. Distributed control method. Means for receiving, from an optical fiber transmission line, n time-division multiplexed n return-to-zero signals whose amplitudes are modulated by an m-bit / second data signal and whose light waves have different phases from each other;
Means for extracting nm frequency components or m hertz frequency components from the received time-division multiplexed optical signal;
Means for controlling the amount of dispersion of the optical fiber transmission line so that the extracted nm-hertz frequency component shows a local minimum value or the m-hertz frequency component shows a local maximum value. Optical fiber transmission line dispersion controller. The distribution control means,
Means for finding two control points of the total variance at which the detected specific frequency component has a maximum intensity;
Means for setting a midpoint of the two control points as a control point at which the intensity of the specific frequency component is minimized. The distribution control means,
Means for sweeping the total variance prior to or during operation, and
Means for finding a control point of the total variance from the intensity of the specific frequency component being swept.

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