JP4675796B2 - Automatic dispersion compensating optical transmission system - Google Patents

Description

  The present invention is used in an optical transmission system to which a differential phase shift keying-direct detection (DPSK-DD) system and a frequency shift keying (FSK) -differential detection system are applied.

  Optical transmission systems are increasingly required to have a large capacity due to the arrival of the broadband era. Although it is becoming possible to increase the capacity relatively easily by wavelength multiplexing (WDM) technology, increasing the bit rate per wavelength is also being actively studied. The reason is that by increasing the bit rate per wavelength, the apparatus cost can be reduced, and the apparatus can be reduced in size and power consumption, so that the total initial cost and running cost of the system can be reduced.

  Already, 40 Gbit / s / Ch. An electric circuit that realizes the above is in a practical stage (for example, see Non-Patent Document 1).

  When such a high-speed optical signal is transmitted by WDM, there are problems such as transmission distance limitation due to chromatic dispersion of the fiber and input power limitation to the fiber due to nonlinearity.

  Degradation due to chromatic dispersion of an optical fiber occurs when the optical pulse is deformed by acting on the bandwidth of the optical signal, and interference is received from the optical pulse in an adjacent time slot or the like. In addition, the chromatic dispersion of an optical fiber or the like varies with the environmental temperature. Furthermore, the same problem also occurs due to a change in chromatic dispersion of the transmission line due to the switching of the path when a failure such as disconnection occurs in a certain transmission section. In order to solve these problems, optical transmission systems combining various chromatic dispersion detection methods and dispersion compensation techniques have been proposed.

  In order to eliminate such chromatic dispersion fluctuation factors, it is important to adopt a method that can be applied not only at the time of system introduction but also at the time of in-service. As a requirement, not only the absolute value of chromatic dispersion but also its sign It is essential to detect. This is because in a method in which a code cannot be detected, it is necessary to deliberately shift the chromatic dispersion value in order to discriminate it, and if applied in-service, the transmission quality deteriorates due to eye opening deterioration, resulting in a problem.

  As an example of a monitoring method for simultaneously detecting the absolute value and the sign of chromatic dispersion, FIG. 8 shows a configuration of a chromatic dispersion detection method using optical frequency modulation (see, for example, Non-Patent Document 2).

  This method measures chromatic dispersion using the fact that the optical frequency modulation component appears as a change in the phase of the intensity modulation component. It has a very wide dynamic range and can measure with high accuracy near zero dispersion.

  On the other hand, as a modulation code capable of high-sensitivity reception, in recent years, WDM transmission technology using a differential phase shift keying-direct detection (DPSK-DD) system, a more nonlinear tolerance RZ-DPSK system, or CSRZ-DPSK system has been used. It has been actively studied (for example, see Non-Patent Document 3).

  Further, as another modulation method that enables high-sensitivity reception, a frequency shift keying (FSK) -differential detection method has been proposed (see, for example, Non-Patent Document 4).

  In the DPSK-DD system (including RZ-based DPSK-DD systems such as RZ-DPSK and CSRZ-DPSK), the phase modulation signal is converted into an intensity modulation signal using a demodulator such as a Mach-Zehnder interferometer (MZI), Direct detection with a receiver. At this time, a differential receiver is used, and a balanced receiver is used for the light receiver because of the advantage that the identification sensitivity is improved by 3 dB compared to the case where the direct detection is performed by one light receiver.

  In order to realize this balanced receiver, (1) each input power to the balanced receiver is made constant, (2) the transmission center wavelength of the MZI is matched with the signal light center wavelength, (3) It is necessary to match each input signal delay to the balanced receiver. In particular, in the control for making the transmission center wavelength of MZI coincide with the center wavelength of the signal light, control of ± 1 GHz or less is required (for example, see Non-Patent Document 5). There is a similar problem with the FSK-differential detection method.

  In addition, these dispersion proof strengths are comparable or less than those of the respective on-off keying (OOK) systems (see Non-Patent Document 6, for example). FIG. 9 shows the results of measuring the dispersion tolerance of the DPSK method and the OOK method after CSRZ encoding at 43 Gbit / s. The dispersion tolerance of the OOK system is 58 ps / nm, while that of the DPSK-DD system is 40 ps / nm. As described above, the chromatic dispersion detection method and the dispersion compensation technique are indispensable when realizing a high speed of 40 Gbit / s or more per wavelength even in these methods.

M.M. Yoneyama, et al. "A 40-Gbit / s Optical Repeater Circuits using InAlAs / InGaAs HEMT Digital IC Chip Set", IEEE EMTTT-S Digest, WE1D-2, 1997. Y. Takashima et al. , “In-Service Dispersion Monitoring in 32 10.7 Gbps WDM Transmission System Over Transient Distance Distinguishing Optical Frequencies-Modulation Method”. 22, NO. 1, 2004, pp. 257-265 Y. Miyamoto et al. , “Novel Modulation and Detection for Bandwidth-Reduced RZ formats Usage Dubinary-Mode Splitting in Wideband PSK / ASK Conquest Liv. 20, NO. 12, 2002, pp. 2067-2078 A. Klekamp et al. , "Comparison of FSK by directly modulated DFB laser with DPSK, NRZ and RZ modulation formats", Proc. OfECOC-IOOC2003, paper Jeffrey H. Sinsky et al., “A 40-Gb / s Integrated Balanced Optical Front End and RZ-DPSK Performance Letters”, Photonics Technology Letters, Vol. 15, No8, 2003, pp. 1135-1137 P. Pecci et al. , “Tolerance to dispersion compensation parameters of six modulation formats in systems operating at 43 Gbit / s”, Electronics Letters, Vol. 39, no. 25, 2003 A. Hirano, “Spectral Mode Splitting-Concepts and Applications”, IEEE / LEOS workshop 2004, paper FB3. S. Kuwahara et al. , "WDM field demonstration of path provisioning by automatic dispersal compensation using tone modulated CS-RZsignal," Electronics Letters, Vol. 39, Issue 22, 2003, pp. 1601-1603

  When the above-described chromatic dispersion detection method using optical frequency modulation is applied to these methods (DPSK-DD method, FSK-differential detection method, etc.), the optical carrier frequency fluctuates, causing transmission deterioration. . In the above-mentioned report, the optical carrier frequency fluctuation of 2 GHz is observed, the requirements for the DPSK-DD balanced receiver cannot be satisfied, and the optical transmission system using the DPSK-DD scheme and the FSK-differential detection scheme is applied. It cannot be applied at all to automatic dispersion compensation technology.

  The present invention has been made under such a background. In an optical transmission system to which the DPSK-DD system and the FSK-differential detection system are applied, the chromatic dispersion in the optical transmission path is introduced at the time of system introduction and in-service. Measures with high accuracy without affecting the main signal, calculates the amount of chromatic dispersion to compensate for this, and combines it with a tunable dispersion compensator to automatically detect chromatic dispersion and fluctuations in the optical transmission line. The purpose is to compensate and improve the stability and reliability of the optical transmission system.

  In order to solve the above problems in an optical transmission system, the chromatic dispersion of an optical fiber as a transmission path is measured with high accuracy and without affecting the main signal at the time of system introduction and in-service. An automatic dispersion compensation type optical transmission system using the DPSK-DD method and FSK-differential detection method for controlling the dispersion compensator so as to compensate for chromatic dispersion was constructed.

  FIG. 10 shows the chromatic dispersion detection principle of the present invention. In the present invention, an intensity modulation signal for monitoring the wavelength dispersion of the transmission path is superimposed on DPSK (including RZ systems such as RZ-DPSK and CSRZ-DPSK) and FSK signals. In the example of FIG. 10, the intensity modulation signal is a tone signal. The tone-modulated DPSK signal and FSK signal after transmission are divided into bands (also referred to as mode division in this specification), and each of the divided signals (Upper Side-Band (USB) and Lower Side-Band ( LSB)) is received.

  Baseband frequency components monitored by the received USB and LSB are extracted, and a phase difference is detected. In the band division, the optical frequency at which the transmittance of the band dividing means is maximized is matched with the frequency at which the optical frequency difference from the center carrier frequency of the DPSK signal and the FSK signal is equal (see, for example, Non-Patent Document 7). Since the divided bands have different optical frequencies (wavelengths), they receive different group delays due to dispersion. By monitoring the delay (phase) difference Δτ between two tone signals after transmission, the absolute value and sign of chromatic dispersion can be monitored.

FIG. 11 to FIG. 15 show the results of the principle confirmation experiment of the present invention. FIG. 11 shows spectra before and after band division. Band division was performed on the bit rate 43 Gbit / s CSRZ-DPSK using MZI of FSR = 100 GHz. 12 to 14 show baseband spectra before and after band division. FIG. 12 shows a baseband spectrum before band division, and FIG. 13 shows LSBwith after band division.
FIG. 14 shows a baseband spectrum of USB with 21% tone modulation after band division. The modulation frequency of the superimposed tone signal is 1 GHz. It was confirmed that the tone modulation frequency component can be detected even after the band division. FIG. 15 shows the results of measuring the phase difference of the tone modulation frequency components detected from each band and measuring the change in the phase difference output with respect to the dispersion. As shown in FIG. 15, it was demonstrated that the phase difference output changes with respect to dispersion, and the absolute value of chromatic dispersion and its sign can be monitored from the output result. By changing the tone modulation frequency, the measurement accuracy and measurement range required for the system can be adjusted.

  The chromatic dispersion monitoring system for monitoring the phase difference of the tone signal after dividing the modulation band is applicable to the OOK system having two bands such as CS-RZ and DCS-RZ, and the absolute value of the chromatic dispersion is its sign. Is an effective method for simultaneously monitoring (see Non-Patent Document 8, for example).

  However, when applied to the OOK system, since the light source is directly modulated by the tone signal, there is a problem that the intensity noise of the tone modulation frequency component becomes a reception penalty.

  The present invention is applied to the DPSK-DD system and the FSK-differential detection system, and differentially receives light using a balanced receiver as a light receiver. Therefore, as shown in FIGS. 16 to 20, the tone modulation frequency component is canceled in the baseband signal after differential light reception, and there is an advantage that deterioration due to superposition of the tone signal is less likely to occur compared to the OOK method. FIG. 16 shows a configuration example of a balanced receiver using CSRZ-DPSK. FIG. 17 shows the MZI input with tone signal for demodulation. FIG. 18 shows a baseband spectrum when light is received only by the constructive port. FIG. 19 shows a baseband spectrum when light is received only by the Destructive port. FIG. 20 shows a baseband spectrum when receiving balanced light.

  FIG. 21 shows the result of evaluating the reception penalty with respect to the modulation degree of the tone signal in CSRZ-DPSK and CSRZ-OOK. In the OOK system, in order to reduce the penalty to 1 dB or less, the modulation degree needs to be 16% or less. On the other hand, with the DPSK-DD system, a penalty of 0.4 dB can be realized even when the modulation degree is 21%.

  As described above, in the present invention, in the optical transmission system to which the DPSK-DD method and the FSK-differential detection method are applied, the chromatic dispersion in the optical transmission line is measured with high accuracy without affecting the main signal, and this is compensated. An automatic dispersion compensation type optical transmission system that calculates the amount of chromatic dispersion for the purpose and further automatically compensates the chromatic dispersion of the optical transmission line in combination with a variable dispersion compensator is provided.

  That is, the first aspect of the present invention is that an optical transmitter that outputs phase-modulated signal light and the received phase-modulated signal light are branched into two, and after delaying one of the signal lights, the two signal lights are mutually connected. A Mach-Zehnder interferometer that causes interference to be converted into intensity-modulated signal light, and a balance-type light receiver that photoelectrically converts signal light from the two output ports of the Mach-Zehnder interferometer and outputs a difference between the converted electrical signals. An optical transmission system including an optical receiver provided.

  Here, a feature of the present invention is that the optical transmitter includes a tone signal superimposing unit that superimposes a tone signal on a phase modulation signal, and the optical receiver compensates for chromatic dispersion of the optical transmission line. A variable chromatic dispersion compensator having a variable compensation value and a phase-modulated signal light from the optical transmitter output from the variable chromatic dispersion compensator and branching one of them to the Mach-Zehnder interferometer An optical branching unit, a band dividing unit for dividing the modulation band of the other phase modulation signal branched by the optical branching unit, and a light receiver for receiving the phase modulation signal band-divided by the band dividing unit, Tone phase detection means for detecting the phase state of the tone modulation frequency component in the two baseband signals respectively received by the light receiver, and detection by the tone phase detection means A phase difference calculating means for calculating chromatic dispersion based on the phase state thus obtained, wherein the variable chromatic dispersion compensation means receives the phase modulation received from the optical transmitter based on the chromatic dispersion value calculated by the phase difference calculating means. A means for compensating for dispersion of the signal light is provided. Thereby, an automatic dispersion compensation type optical transmission system can be realized.

  Alternatively, the automatic dispersion compensation type optical transmission system of the present invention includes a variable chromatic dispersion compensation means in which the dispersion value for compensating the chromatic dispersion of the optical transmission line is variable in the optical receiver, and the variable chromatic dispersion compensation. Optical branching means for branching the phase modulated signal light from the optical transmitter output from the means and outputting one of them to the Mach-Zehnder interferometer, and the modulation band of the other phase modulated signal branched by the optical branching means Band dividing means for dividing the signal, a light receiver for receiving the two phase-modulated signals divided by the band dividing means, and predetermined frequency components of the two baseband signals received by the light receiver are respectively extracted. A band pass filter that detects the phase state of the frequency components of the two baseband signals extracted by the band pass filter; Phase difference calculating means for calculating chromatic dispersion based on the phase state detected by the phase detecting means, and the variable chromatic dispersion compensating means is configured to transmit the optical transmission based on the chromatic dispersion value calculated by the phase difference calculating means. And means for compensating for dispersion of the phase-modulated signal light received from the detector.

  Alternatively, the present invention provides an optical transmitter that generates frequency-modulated signal light, band-dividing means that divides the modulation band of the received frequency-modulated signal light, and signal light from two output ports of the band-dividing means. An optical transmission system including an optical receiver including a balance type light receiver that performs photoelectric conversion and outputs a difference between converted electric signals.

  Here, a feature of the present invention is that the optical transmitter includes a tone signal superimposing unit that superimposes a tone signal on the frequency-modulated signal light, and the optical receiver receives the signal from the optical transmitter. The frequency-modulated signal light is output to the band dividing means and output from two output ports of the variable wavelength dispersion compensating means where the dispersion value for compensating the chromatic dispersion of the optical transmission line is variable and the band dividing means. An optical branching means for branching the signal light and outputting one of the two signal lights to the balanced light receiver; a light receiver for receiving the other two signal lights branched by the optical branching means; Tone phase detection means for detecting the phase state of the tone modulation frequency component in the two baseband signals of the signal light received by the light receiver, and the phase detected by the tone phase detection means A phase difference calculating means for calculating chromatic dispersion based on the state, and the variable chromatic dispersion compensating means is configured to apply frequency modulated signal light received from the optical transmitter based on the chromatic dispersion value calculated by the phase difference calculating means. On the other hand, a means for compensating for dispersion is provided. Thereby, an automatic dispersion compensation type optical transmission system can be realized.

  Alternatively, in the automatic dispersion compensating optical transmission system of the present invention, the optical receiver outputs the frequency-modulated signal light received from the optical transmitter to the band dividing unit to compensate for the chromatic dispersion of the optical transmission line. The chromatic dispersion compensation means having a variable dispersion value and the signal light output from the two output ports of the band dividing means are respectively branched and one of the two signal lights is output to the balanced light receiver. And a light receiver for receiving the other two signal lights branched by the light branching means, and extracting predetermined frequency components from the two baseband signals of the signal light received by the light receiver. A band-pass filter that detects the phase state of the frequency component of the baseband signal extracted by the band-pass filter, and the phase detection unit A phase difference calculation unit that calculates chromatic dispersion based on the phase state that is output, and the variable chromatic dispersion compensation unit receives a frequency received from the optical transmitter based on the chromatic dispersion value calculated by the phase difference calculation unit. Means is provided for performing dispersion compensation on the modulated signal light.

  Further, a variable PMD compensation unit having a variable PMD (Polarization mode dispersion) is provided between the variable wavelength dispersion compensation unit and the optical branching unit, and is output from the light receiver. Intensity detecting means for monitoring the intensity of the frequency component in the baseband signal, and PMD control means for controlling the variable PMD compensating means so that the intensity detected by the intensity detecting means is maximized.

  Thereby, the optical receiver receives the signal light dispersion-compensated by the dispersion compensator having a fixed chromatic dispersion value for each linear repeater in the linear repeater section, and compensates the residual dispersion for each channel after transmission. Can be provided.

  The phase-modulated light is, for example, one of a DPSK signal, an RZ-DPSK signal, and a CSRZ-DPSK signal. Alternatively, the phase-modulated light is, for example, one of a DQPSK signal, an RZ-DQPSK signal, and a CSRZ-DQPSK signal.

  According to a second aspect of the present invention, an optical transmitter outputs phase modulated signal light, and an optical receiver branches the received phase modulated signal light into two by a Mach-Zehnder interferometer, and outputs one of the signal lights. After delaying, both signal lights interfere with each other to convert them into intensity-modulated signal lights. The balanced light receiver photoelectrically converts the signal lights from the two output ports of the Mach-Zehnder interferometer, respectively. This is an optical transmission method that outputs the difference between the two.

  Here, a feature of the present invention is that the optical transmitter superimposes a tone signal on a phase modulation signal by a tone signal superimposing unit, and the optical receiver is separated from the optical transmitter by an optical branching unit. The phase-modulated signal light is branched, the band-dividing means divides the modulation band of the branched phase-modulated signal, and the optical receiver receives the two phase-modulated signals divided by the band, respectively. The detecting means detects the phase state of the tone modulation frequency component in the two baseband signals respectively received by the light receiver, and the phase difference calculating means performs chromatic dispersion based on the phase state detected by the tone phase detecting means. The wave calculated by the phase difference calculating means is calculated by the variable chromatic dispersion compensating means where the dispersion value for calculating and compensating the chromatic dispersion of the optical transmission line is variable. There is to be dispersion compensation on the phase-modulated signal received from the optical transmitter on the basis of the variance value. Thus, an optical transmission method that performs automatic dispersion compensation can be realized.

  Alternatively, in the optical transmission system that performs automatic dispersion compensation according to the present invention, the optical receiver branches the phase-modulated signal light from the optical transmitter by the optical branching unit, and the phase modulation branched by the band dividing unit. The modulation band of the signal is divided into bands, the optical receiver receives the two phase-modulated signals divided by the band, and the band-pass filter receives the predetermined frequency components of the two baseband signals received by the optical receiver. Each phase is extracted, the phase detection means detects the phase state of the frequency components of the two baseband signals extracted by the bandpass filter, and the phase difference calculation means detects the phase state based on the phase state detected by the phase detection means. By calculating the chromatic dispersion and compensating for the chromatic dispersion of the optical transmission line, the variable chromatic dispersion compensation means has a variable dispersion value. Characterized by dispersion compensation on the phase-modulated signal received from the optical transmitter on the basis of the chromatic dispersion value calculated by the means.

  Alternatively, in the present invention, the optical transmitter generates the frequency modulated signal light, the optical receiver divides the modulation band of the received frequency modulated signal light by the band dividing means, and the balanced light receiver This is an optical transmission method in which signal light from two output ports of the band dividing means is photoelectrically converted and a difference between the converted electric signals is output.

  Here, the present invention is characterized in that the optical transmitter superimposes a tone signal on the frequency-modulated signal light by a tone signal superimposing unit, and the optical receiver transmits the band dividing unit by an optical branching unit. The signal light output from each of the two output ports is branched and one of the two signal lights is output to the balance type light receiver. The other two signals branched by the light branching means by the light receiver. Receiving each of the lights, the tone phase detection means detects the phase state of the tone modulation frequency component in the baseband signal of the two signal lights received by the light receiver, and the tone phase detection means detects the tone phase detection means. The chromatic dispersion is calculated based on the phase state detected by the chromatic dispersion compensation means for compensating the chromatic dispersion of the optical transmission line, and the chromatic dispersion compensation means is variable. There is to be a dispersion compensation for the frequency-modulated optical signal received from the optical transmitter on the basis of the chromatic dispersion value calculated by the phase difference calculating means. Thus, an optical transmission method that performs automatic dispersion compensation can be realized.

  Alternatively, in the optical transmission system that performs automatic dispersion compensation according to the present invention, the optical receiver splits the signal light output from the two output ports of the band splitting means by the optical branching means, respectively. Two signal lights are output to the balance type light receiver, and the other two signal lights branched by the light branching means are respectively received by the light receiver, and the two light beams received by the light receiver are received by the band pass filter. Predetermined frequency components in the baseband signal of the signal light are respectively extracted, the phase detection means detects the phase state of the frequency components of the two baseband signals extracted by the bandpass filter, and the phase difference calculation means The chromatic dispersion is calculated based on the phase state detected by the phase detection means, and the dispersion value for compensating the chromatic dispersion of the optical transmission line is variable. The Lono tunable dispersion compensator, characterized in that dispersion compensation for the frequency-modulated optical signal received from the optical transmitter on the basis of the chromatic dispersion value calculated by the phase difference calculation means.

  Further, a variable PMD compensation means where the PMD is variable is provided between the variable wavelength dispersion compensation means and the optical branching means, and in the baseband signal output from the light receiver by the intensity detection means. The intensity of the frequency component can be monitored, and the variable PMD compensation means can be controlled by the PMD control means so that the intensity detected by the intensity detection means is maximized.

  As a result, dispersion compensation is performed by a dispersion compensator having a fixed chromatic dispersion value for each linear repeater in the linear repeater section, and residual dispersion for each channel after transmission can be compensated by the optical receiver.

  The phase-modulated light is, for example, one of a DPSK signal, an RZ-DPSK signal, and a CSRZ-DPSK signal. Alternatively, the phase-modulated light is, for example, one of a DQPSK signal, an RZ-DQPSK signal, and a CSRZ-DQPSK signal.

  As described above, according to the present invention, in the optical transmission system to which the DPSK-DD system and the FSK-differential detection system are applied, the chromatic dispersion value of the optical transmission line is affected to the main signal at the time of system introduction and in-service. It can be measured without giving.

  In addition, the present invention has sufficient detection sensitivity and accuracy as compared with the dispersion tolerance at the bit rate of the main signal, and it is possible to detect and compensate for deterioration in transmission quality due to the influence of chromatic dispersion.

  Furthermore, because detection sensitivity and accuracy can be changed flexibly, it can detect chromatic dispersion deviations when switching routes due to optical transmission line failures, etc., and immediately calculate an appropriate dispersion compensation amount to compensate for this. It becomes possible to do. This greatly contributes to the stable operation and improvement of reliability of the optical transmission system using this method.

(First embodiment)
A first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing a configuration of an automatic dispersion compensating optical transmission system according to a first embodiment of the present invention. As shown in FIG. 1, the automatic dispersion compensating optical transmission system according to the first embodiment includes an encoder 1 that generates a DPSK signal by converting an NRZ code input signal into an NRZ-I code signal, and a stable continuous signal. A light source 2 that generates light, a phase modulator 3 that outputs phase modulated signal light in which a phase amplitude Δφ is given in a range of 0 <Δφ <π with respect to a mark and a space encoded by the encoder 1, and a tone An optical transmitter 5 comprising tone modulation superimposing means 4 for superimposing a signal, variable chromatic dispersion compensating means 6 for compensating for group velocity dispersion in the optical transmission line, optical branching means 7 for branching the optical signal, and DPSK code demodulation. Mach-Zehnder interferometer 8, balanced light receiver 9, band dividing means 10 for dividing the phase modulation band into USB and LSB, and receiver for receiving USB and LSB respectively. From the optical devices 11-U and 11-L, the tone phase detecting means 12 for detecting the phase state of the tone modulation frequency component in the received baseband signal, and the phase difference calculating means 13 for calculating the chromatic dispersion from this phase state. And the optical receiver 14.

  The light source 2 is provided with a terminal that can superimpose minute intensity modulation by a tone signal supplied from the outside, or can generate continuous light subjected to minute intensity modulation by directly changing its drive current. .

  In the first embodiment, the light source 2 is directly modulated as a method of superimposing the tone signal. However, the tone signal may be superimposed by inputting the data-modulated phase modulation to the intensity modulator. Further, the phase modulator 3 is used as means for generating the DPSK signal. The phase modulator 3 is provided with a linear waveguide on the surface of LiNbO3 (LN), and changes the phase of light by utilizing the refractive index change of the waveguide due to the electro-optic effect.

  As another means, there is a method using a Push-Pull intensity modulator. The Push-Pull intensity modulator is a Mach-Zehnder type optical interferometer type intensity modulator generated on an LN substrate or semiconductor, and electrodes for modulation are formed on both of its two arms, and both are made complementary. It can be modulated chirp-free by driving.

  A DPSK signal can be generated by applying a half-wave voltage to each arm using this Push-Pull intensity modulator, and giving a phase difference equivalent to twice the half-wave voltage. Needless to say, if an X-cut structure is used, it can be realized with a single electrode. Furthermore, although basic DPSK signals are shown, RZ-based DPSK signals such as RZ-DPSK and CZRZ-DPSK may be generated using an RZ pulse carver. The RZ pulse carver is realized by an intensity modulator. Generally, a Push-Pull intensity modulator or an X-cut intensity modulator is used.

  The tone modulation superimposing means 4 generates a signal having a single frequency as a tone signal. The variable chromatic dispersion compensation means 6 changes the dispersion amount based on the dispersion value information obtained by the chromatic dispersion monitoring method of the present invention, and the total dispersion amount combined with the chromatic dispersion value possessed by the optical transmission path is determined at the time of reception. The dispersion amount is adjusted so as to be an optimum value, and there are a dispersion compensation circuit using AWG, a Virtual Imaged Phased Array (VIPA), a variable fiber grating (FBG), and the like. The optical branching means 7 branches the optical signal into two, and is a fiber fusion type optical coupler or a half mirror. The band dividing means 10 divides the phase modulation signal into USB and LSB bands, and includes an MZI type filter, a dielectric multilayer filter, an FBG filter, and the like.

  The light receivers 11-U and 11-L receive the respective bands that are divided. The optical receivers 11-U and 11-L need only have a sufficient band for detecting the frequency component of the tone signal.

  The tone phase detector 12 detects the relative phase difference between the two tone signals from the light receivers 11-U and 11-L. In this example, the tone phase detector 12 corresponds to the phase difference between the two tone signals. A mixer that outputs the measured voltage is used. A circuit that samples a waveform via an A / D converter or the like and obtains the phase difference can be used.

(Second embodiment)
A second embodiment of the present invention will be described with reference to FIG. FIG. 2 is a block diagram showing the configuration of an automatic dispersion compensating optical transmission system according to the second embodiment of the present invention. The feature of this embodiment is that the DQPSK modulation method is applied to the modulation method.

  In order to realize the DQPSK modulation method, on the optical transmitter 5 ′ side, instead of the encoder 1 and the phase modulator 3 in the first embodiment, an in-phase component and a quadreture component are input from two input binary data. DQPSK precoding circuit 1 ′ for generating a modulated signal of D, and a DQPSK signal is generated by adding phase modulation to each of the in-phase component and quadreture component and superimposing it after giving a phase difference of π / 2 The quadrature phase modulator 3 ′ is used.

  On the optical receiver 14 'side, a dual Mach-Zehnder interferometer 8' is used in place of the Mach-Zehnder interferometer 8 of the first embodiment, and interference is performed by adding a phase difference ± π / 4 between arms in each interferometer. By doing so, the original binary data # 1 and binary data # 2 are demodulated by the balanced photoreceivers 9′-1 and 9′-2.

  As in this embodiment, even when a DQPSK signal is used as a transmission signal, tone modulation is superimposed on the light source output intensity, and after the band is divided on the receiving side, the phase difference between USB / LSB is measured to measure the wavelength dispersion. Measurements can be made. The transmission signal can be measured in the same manner even when an RZ pulse carver that applies RZ to the NRZ-DQPSK signal of the present embodiment is used, and the RZ-DQPSK and CSRZ-DQPSK signals are used as the transmission signal. .

(Third embodiment)
A third embodiment of the present invention will be described with reference to FIG. FIG. 3 is a block diagram showing a configuration of an automatic dispersion compensating optical transmission system according to the third embodiment of the present invention. As shown in FIG. 3, the automatic dispersion compensating optical transmission system according to the third embodiment includes an optical transmitter 25 having a configuration in which the tone modulation superimposing means 4 is omitted from the optical transmitter 5 shown in the first embodiment. Prepare. That is, the encoder 1 of the optical transmitter 5 corresponds to the encoders 21-1 and 21-2 of the optical transmitter 25. The phase modulator 3 of the optical transmitter 5 corresponds to the Push-Pull intensity modulator 23 and the RZ pulse carver 24 of the optical transmitter 25. The light source 2 and the light source 22 are the same.

Further, as shown in FIG. 3, the optical receiver 35 includes a variable chromatic dispersion compensating means 26 for compensating for group velocity dispersion in the optical transmission line, an optical branching means 27 for branching an optical signal, and a Mach-Zehnder for DPSK code demodulation. Interferometer 28, balanced light receiver 29, band dividing means 30 for dividing the phase modulation band into USB and LSB, light receivers 31-U and 31-L for receiving USB and LSB, respectively, and the received base Band pass filters 32-U and 32-L for extracting an arbitrary modulation frequency component of the band signal, phase detection means 33 for detecting the phase state of the extracted frequency component, and a position for calculating chromatic dispersion from this phase state. And a phase difference calculation means 34.

  A feature of this embodiment is that, instead of superimposing a tone signal and comparing the phase difference of the tone modulation frequency component, an arbitrary baseband frequency component of the data signal is extracted, and the phase difference of the frequency component is compared. . A band-pass filter that extracts an arbitrary baseband frequency component includes a resonator such as a dielectric or a waveguide. In the present embodiment, a stable circuit can be realized without considering transmission degradation due to the superimposed signal.

(Fourth embodiment)
A fourth embodiment of the present invention will be described with reference to FIG. FIG. 4 is a block diagram showing the configuration of an automatic dispersion compensating optical transmission system according to the fourth embodiment of the present invention. As shown in FIG. 4, the automatic dispersion compensating optical transmission system according to the fourth embodiment includes an optical transmitter 43 capable of generating FSK including tone modulation superimposing means 41 for superimposing a tone signal on an optical carrier, and an optical transmission line. Variable chromatic dispersion compensation means 44 for compensating for group velocity dispersion, band division means 45 for dividing the modulation band of FSK, optical branching means 46-U, 46-L for optically dividing the band-divided bands, Balanced light receiver 47 that differentially detects each optically branched signal, type light receivers 48-U and 48-L that receive the other optically branched signal, and a tone in the received baseband signal The optical receiver 51 comprises a tone phase detecting means 49 for detecting the phase state of the modulation frequency component and a phase difference calculating means 50 for calculating the chromatic dispersion from the phase state. That.

  In the fourth embodiment, the light source 42 is directly modulated as means for generating FSK. The light source has a modulation band that can generate FSK at a data rate. As another means, a structure for generating two wavelengths having a certain optical frequency interval and a configuration for selecting one wavelength using asymmetric MZI may be used. As a means for generating two wavelengths having a certain optical frequency interval, for example, a two-band beat pulse using a Push-Pull modulator is generated.

  Further, the tone modulation superimposing means 41 may be provided with a terminal capable of superimposing a slight intensity modulation by a tone signal supplied from the outside in order to directly modulate the light source 42, or directly changing the drive current. It is also possible to generate continuous light that has undergone minute intensity modulation. Further, the data-modulated phase modulation may be input to the intensity modulator and the tone signal may be superimposed.

  The tunable dispersion compensator 44 is the same as that described in the first embodiment. The band dividing means 45 is one in which the optical frequency at which the transmittance of the mode dividing means is maximized is adjusted to the frequency at which the optical frequency difference from the center carrier frequency becomes equal, and the MZI type filter, dielectric multilayer filter, FBG filter Etc.

  The optical branching units 46-U and 46-L branch the optical signal into two, and are a fiber fusion type optical coupler or a half mirror.

  The light receivers 48-U and 48-L receive each of the divided bands. The bands of the light receivers 48-U and 48-L need only have a band sufficient for detecting the frequency component of the tone signal.

  The tone phase detector 49 detects the relative phase difference between the two tone signals from the light receivers 48-U and 48-L. In this example, the tone phase detector 49 corresponds to the phase difference between the two tone signals. A mixer that outputs the measured voltage is used. A circuit that samples a waveform via an A / D converter or the like and obtains its phase difference can also be used.

  The phase difference calculation means 50 is the same as in the first and second embodiments.

(Fifth embodiment)
A fifth embodiment of the present invention will be described with reference to FIG. FIG. 5 is a block diagram showing a configuration of an automatic dispersion compensating optical transmission system according to the fifth embodiment of the present invention. As shown in FIG. 5, the optical transmitter 54 in the automatic dispersion compensating optical transmission system according to the fifth embodiment has a configuration in which the tone modulation superimposing means 4 is omitted from the optical transmitter 5 shown in the fourth embodiment. A light source 53 and a frequency modulator 52.

  Further, as shown in FIG. 5, the optical receiver 63 is band-divided by a variable chromatic dispersion compensation unit 55 for compensating for group velocity dispersion in the optical transmission line, a band division unit 56 for dividing the modulation band of the FSK. Optical branching means 57-U, 57-L for optically splitting the respective optical bands, a balanced light receiver 58 for differentially detecting the optically branched signals, and receiving the other optically branched signals. Receiving devices 59-U and 59-L, bandpass filters (BPF) 60-U and 60-L for extracting arbitrary modulation frequency components of the received baseband signal, and detecting the phase state of the extracted frequency components And a phase difference calculating means 62 for calculating chromatic dispersion from the phase state.

  A feature of this embodiment is that, instead of superimposing a tone signal and comparing the phase difference of the tone modulation frequency component, an arbitrary baseband frequency component of the data signal is extracted, and the phase difference of the frequency component is compared. . The bandpass filters 60-U and 60-L for extracting an arbitrary baseband frequency component are constituted by a resonator such as a dielectric or a waveguide. In the present embodiment, a stable circuit can be realized without considering transmission degradation due to the superimposed signal.

(Sixth embodiment)
A sixth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a block diagram showing the configuration of an automatic dispersion compensating optical transmission system according to the sixth embodiment of the present invention. Although FIG. 6 illustrates the optical transmitter 5 of the first embodiment, the optical transmitter may be any of the first to fifth embodiments.

  As shown in FIG. 6, the optical receiver 81 divides an optical signal by a variable chromatic dispersion compensation means 70 for compensating for group velocity dispersion in the optical transmission line, a variable PMD compensation means 71 for compensating PMD in the optical transmission line, and the optical signal. Optical branching means 72, a DPSK code demodulating Mach-Zehnder interferometer 73, a balanced light receiver 74, a band dividing means 75 for dividing the phase modulation band into USB and LSB, and a light receiving device for receiving USB and LSB respectively. Units 76-U and 76-L, a tone phase detecting means 77 for detecting the phase state of the tone modulation frequency component in the received baseband signal, a phase difference calculating means 78 for calculating chromatic dispersion based on this phase state, and a tone Intensity detection means 79 for monitoring the intensity of the modulation frequency component, and variable PMD compensation means 71 so that the intensity is maximized from the intensity information. A control for PMD control means 80..

  The feature of this embodiment is that the chromatic dispersion value is measured in the phase state of the extracted tone signal, and automatic compensation for PMD is performed with the intensity information. Therefore, each factor can be monitored and compensated independently. As the variable PMD compensation means 71, a circuit constituted by a combination of a PMD medium and a polarization controller (PC), a circuit in which a polarization beam splitter (PBS) is combined with the circuit, or the like is used.

  In the example of FIG. 6, the configuration example in which the intensity detection unit 79, the variable PMD compensation unit 71, and the PMD control unit 80 are applied to the configuration example of the optical receiver 14 of the first embodiment illustrated in FIG. In each of the configuration examples of the optical receivers 35, 51, and 62 of the third, fourth, and fifth embodiments shown in FIGS. 3, 4, and 5, the frequency component intensity extracted from the data signal is monitored. Therefore, the PMD fluctuation can be monitored, so that the intensity detection means 79, the variable PMD compensation means 71, and the PMD control means 80 in the present embodiment can be similarly applied. In this case, the operations relating to the intensity detection means 79, the variable PMD compensation means 71, and the PMD control means 80 are the same as described above.

(Seventh embodiment)
A seventh embodiment of the present invention will be described with reference to FIG. FIG. 7 is a block diagram showing a configuration of an automatic dispersion compensating optical transmission system according to the seventh embodiment of the present invention. As shown in FIG. 7, the automatic dispersion compensation type optical transmission system according to the seventh embodiment is provided for each linear repeater section when the automatic dispersion compensation type optical transmission system of the first to sixth embodiments is wavelength division multiplexed and multi-relay transmission is performed. And fixed chromatic dispersion compensators 90-1 to 90-3 are installed, and the residual dispersion is automatically compensated for each channel at the receiving end.

  The optical multiplexer 91 uses an arrayed waveguide (AWG) filter or the like. For the chromatic dispersion compensators 90-1 to 90-3, a fiber, a fiber grating, an etalon filter, or the like is used.

  In multi-relay transmission, in consideration of the nonlinear optical effect, a fixed chromatic dispersion compensator is installed for each linear repeat section to perform dispersion compensation. On the other hand, the fiber dispersion value of the transmission line is different for each linear repeater section and has a wavelength dependency of dispersion (dispersion slope). Therefore, in dispersion compensation using a fixed dispersion compensator, the dispersion value accumulated for each channel (residual value) Dispersion) is different. Furthermore, the dispersion compensation value designed at the time of system introduction changes due to a path change or a temperature change due to trouble transfer after the system introduction.

  The feature of this embodiment is that the residual dispersion for each channel is automatically compensated at the time of introduction, and the dispersion value at the time of system operation is also automatically compensated for each channel.

  As described above, according to the present invention, in the optical transmission system to which the DPSK-DD system and the FSK-differential detection system are applied, the chromatic dispersion value of the optical transmission line is affected to the main signal at the time of system introduction and in-service. It can be measured without giving.

  In addition, the present invention has sufficient detection sensitivity and accuracy as compared with the dispersion tolerance at the bit rate of the main signal, and it is possible to detect and compensate for deterioration in transmission quality due to the influence of chromatic dispersion.

  Furthermore, because detection sensitivity and accuracy can be changed flexibly, it can detect chromatic dispersion deviations when switching routes due to optical transmission line failures, etc., and immediately calculate an appropriate dispersion compensation amount to compensate for this. It becomes possible to do. This greatly contributes to the stable operation and improvement of reliability of the optical transmission system using this method.

The block diagram which shows 1st embodiment of this invention. The block diagram which shows 2nd embodiment of this invention. The block diagram which shows 2nd embodiment of this invention. The block diagram which shows 3rd embodiment of this invention. The block diagram which shows 4th embodiment of this invention. The block diagram which shows 5th embodiment of this invention. The block diagram which shows 6th embodiment of this invention. The block diagram of the dispersion | distribution monitor using the conventional optical frequency. The figure for demonstrating the comparison of the dispersion tolerance of DPSK and an OOK system. The figure for demonstrating the wavelength dispersion detection principle of this invention. The figure which shows the principle confirmation experiment result of this invention using CSRZ-DPSK (spectrum before and behind band division). The figure which shows the principle confirmation experiment result of this invention using CSRZ-DPSK (baseband spectrum before band division). The figure which shows the principle confirmation experiment result of this invention using CSRZ-DPSK (LSBwith 21% tone modulation after band division). The figure which shows the principle confirmation experiment result of this invention using CSRZ-DPSK (USBwith 21% tone modulation after band division). The figure for demonstrating the phase difference change of the tone modulation frequency component with respect to dispersion | distribution using CSRZ-DPSK. The figure which shows the structural example of the balance type receiver using CSRZ-DPSK. The figure which shows the baseband spectrum in each port using CSRZ-DPSK (MZI input with tone signal for demodulation). The figure which shows the baseband spectrum in each port using CSRZ-DPSK (when light is received only by the Constructive port). The figure which shows the baseband spectrum in each port using CSRZ-DPSK (when light is received only at the Destructive port). The figure which shows the baseband spectrum in each port using CSRZ-DPSK (when balance light reception is carried out). The figure which shows the result of having evaluated the reception penalty with respect to the modulation degree of the tone signal in CSRZ-DPSK and CSRZ-OOK.

Explanation of symbols

1, 21-1, 21-2 Encoder 1 'DQPSK precoding circuit 2, 22, 42, 53 Light source 3 Phase modulator 3' Quadrature phase modulator 4, 41 Tone modulation superimposing means 5, 5 ', 25, 43, 54 Optical transmitters 6, 26, 44, 55, 70 Variable chromatic dispersion compensation means 7, 27, 46-U, 46-L, 57-U, 57-L, 72 Optical branching means 8, 28, 73 Mach-Zehnder interferometer 8 'dual Mach-Zehnder interferometer 9, 9'-1, 9'-2, 29, 47, 58, 74 Balanced light receiver 10, 30, 45, 56, 75 Band dividing means 11-U, 11-L, 31 -U, 31-L, 48-U, 48-L, 59-U, 59-L, 76-U, 76-L Receiver 12, 49, 77 Tone phase detecting means 13, 34, 50, 62, 78 Phase difference calculation means 14, 14 ', 35, 51, 3, 81 Optical receiver 23 Push-Pull intensity modulator 24 RZ pulse carvers 32-U, 32-L, 60-U, 60-L Bandpass filters 33, 61 Phase detection means 52 Frequency modulator 71 Variable PMD compensation means 79 Intensity detection means 80 PMD control means 90-1 to chromatic dispersion compensator 91 optical multiplexer

Claims (8)

An optical transmitter for generating frequency-modulated signal light;
Band division means for dividing the modulation band of the received frequency modulated signal light into two different frequency bands so that the frequency difference from the center frequency of the signal light carrier is equal, and from two output ports of this band division means An optical receiver comprising: a balanced receiver that photoelectrically converts signal light and outputs a difference between the converted electric signals;
The optical transmitter includes a tone signal superimposing unit that superimposes a tone signal on the frequency-modulated signal light,
In the optical receiver,
A variable chromatic dispersion compensating means for outputting a frequency modulated signal light received from the optical transmitter to the band dividing means, and having a variable dispersion value for compensating the chromatic dispersion of the optical transmission line;
Optical branching means for branching the signal light output from the two output ports of the band dividing means and outputting one of the two signal lights to the balanced light receiver;
A light receiver for receiving the other two signal lights branched by the light branching means;
Tone phase detection means for detecting the phase state of the tone modulation frequency component in the two baseband signals of the signal light received by the light receiver;
Phase difference calculating means for calculating chromatic dispersion based on the phase state detected by the tone phase detecting means,
The variable chromatic dispersion compensation means includes means for compensating for dispersion of the frequency-modulated signal light received from the optical transmitter based on the chromatic dispersion value calculated by the phase difference calculation means. Compensated optical transmission system. An optical transmitter for generating frequency-modulated signal light;
Band division means for dividing the modulation band of the received frequency modulated signal light into two different frequency bands so that the frequency difference from the center frequency of the signal light carrier is equal, and from two output ports of this band division means In an optical transmission system including an optical receiver including a balanced light receiver that photoelectrically converts signal light and outputs a difference between the converted electrical signals,
In the optical receiver,
A variable chromatic dispersion compensating means for outputting a frequency modulated signal light received from the optical transmitter to the band dividing means, and having a variable dispersion value for compensating the chromatic dispersion of the optical transmission line;
Optical branching means for branching the signal light output from the two output ports of the band dividing means and outputting one of the two signal lights to the balanced light receiver;
A light receiver for receiving the other two signal lights branched by the light branching means;
A bandpass filter for extracting a predetermined frequency component in the two baseband signals of the signal light received by the light receiver;
Phase detection means for detecting the phase state of the frequency component of the baseband signal extracted by the bandpass filter;
Phase difference calculation means for calculating chromatic dispersion based on the phase state detected by the phase detection means, and
The variable chromatic dispersion compensation means includes means for compensating for dispersion of the frequency-modulated signal light received from the optical transmitter based on the chromatic dispersion value calculated by the phase difference calculation means. Compensated optical transmission system. Between the variable chromatic dispersion compensation means and the optical branching means, there is provided a variable PMD compensation means where PMD (Polarisation mode dispersion) is variable,
Intensity detecting means for monitoring the intensity of the frequency component in the baseband signal output from the light receiver;
The automatic dispersion compensation type optical transmission system according to claim 1 or 2, further comprising: PMD control means for controlling the variable PMD compensation means so that the intensity detected by the intensity detection means is maximized. The optical receiver includes means for receiving signal light dispersion-compensated by a dispersion compensator having a fixed chromatic dispersion value for each linear repeater in a linear repeater section and compensating for residual dispersion for each channel after transmission. The automatic dispersion compensating optical transmission system according to any one of claims 1 to 3 . The optical transmitter generates frequency modulated signal light,
The optical receiver divides the modulation band of the received frequency-modulated signal light into two different frequency bands so that the frequency difference from the center frequency of the signal optical carrier is equal by the band dividing means, In the optical transmission method for photoelectrically converting the signal light from the two output ports of the band dividing means and outputting the difference between the converted electrical signals,
The optical transmitter superimposes the tone signal on the frequency modulation signal light by the tone signal superimposing means,
The optical receiver is:
The optical branching means branches the signal lights output from the two output ports of the band dividing means, respectively, and outputs one of the two signal lights to the balanced light receiver,
The other two signal lights branched by the light branching means are respectively received by the light receiver,
The tone phase detection means detects the phase state of the tone modulation frequency component in the baseband signal of the two signal lights received by the light receiver,
The phase difference calculating means calculates chromatic dispersion based on the phase state detected by the tone phase detecting means,
The optical transmission based on the chromatic dispersion value calculated by the phase difference calculation means by the variable chromatic dispersion compensation means provided in the preceding stage of the optical branching means and having a variable dispersion value for compensating the chromatic dispersion of the optical transmission line. An optical transmission method for performing automatic dispersion compensation, comprising performing dispersion compensation on frequency-modulated signal light received from an optical device. The optical transmitter generates frequency modulated signal light,
The optical receiver divides the modulation band of the received frequency-modulated signal light into two different frequency bands so that the frequency difference from the center frequency of the signal optical carrier is equal by the band dividing means, In the optical transmission method for photoelectrically converting the signal light from the two output ports of the band dividing means and outputting the difference between the converted electrical signals,
The optical receiver is:
The optical branching means branches the signal lights output from the two output ports of the band dividing means, respectively, and outputs one of the two signal lights to the balanced light receiver,
The other two signal lights branched by the light branching means are respectively received by the light receiver,
The band-pass filter extracts predetermined frequency components in the baseband signals of the two signal lights received by the light receiver,
The phase detection means detects the phase state of the frequency components of the two baseband signals extracted by this bandpass filter,
The phase difference calculating means calculates chromatic dispersion based on the phase state detected by the phase detecting means,
The optical transmission based on the chromatic dispersion value calculated by the phase difference calculation means by the variable chromatic dispersion compensation means provided in the preceding stage of the optical branching means and having a variable dispersion value for compensating the chromatic dispersion of the optical transmission line. An optical transmission method for performing automatic dispersion compensation, comprising performing dispersion compensation on frequency-modulated signal light received from an optical device. Between the variable chromatic dispersion compensation means and the optical branching means, there is provided a variable PMD compensation means where PMD is variable,
The intensity detection means monitors the intensity of the frequency component in the baseband signal output from the light receiver,
The optical transmission method for performing automatic dispersion compensation according to claim 5 or 6 , wherein the variable PMD compensation means is controlled by a PMD control means so that the intensity detected by the intensity detection means is maximized. Wavelength dispersion value for each linear repeater performs dispersion compensation by the dispersion compensator fixed in a linear repeater section by the optical receiver, to any one of claims 5 to 7 for compensating the residual dispersion for each channel after transmission An optical transmission method for performing automatic dispersion compensation as described.

Images