JP2005252369A - Wdm transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a WDM transmission system increasing transmission distance and usable wavelength band by improving durability against residual dispersion deviation between wavelengths by a residual dispersion slope. <P>SOLUTION: In a WDM transmission system, a transmission transponder 1 has a plurality of chirp variable type transmission transponders 10-1 to 10-n provided so as to correspond to each of a plurality of input optical signals 1-n, configured so that the chirp quantity varies and outputting an optical signal modulated by superimposing the set chirp quantity; and a chirp quantity control section 11 for setting a chirp quantity for each of the transponders 10-1 to 10-n, corresponding to an output wavelength of each of the transponders 10-1 to 10-n. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention, in a large-capacity wavelength-division multiplexing (WDM) transmission system, relaxes transmission distance limitation and wavelength band limitation due to deviation of residual dispersion amount between wavelengths to increase the distance and increase the wavelength. The present invention relates to a possible WDM transmission technology.

In a long-distance optical transmission system, a WDM transmission technology that multiplexes and transmits a plurality of wavelengths in one optical fiber is applied, and economical and large-capacity information transmission is realized.
In a WDM transmission system, increasing the transmission rate per wavelength is being studied for further reduction in transmission device cost. At present, a 10 Gbit / s system has already been put into practical use, and a 40 Gbit / s transmission system is being studied for realization (for example, see Non-Patent Document 1).

  One of the factors that limit the transmittable distance of a long-distance WDM transmission system is waveform distortion due to wavelength dispersion of an optical fiber (transmission fiber) that forms a transmission path. Chromatic dispersion is the dependence of the group velocity of optical signals in an optical fiber on the wavelength. In the IM-DD (Intensity Modulation-Direct Detection) method, which modulates the intensity of light and transmits information, the time waveform of an optical pulse spreads through the optical fiber due to this chromatic dispersion and overlaps with adjacent bits. This causes a penalty in reception sensitivity to cause intersymbol interference.

Since the allowable dispersion amount is inversely proportional to the square of the transmission speed, in a low-speed system of about 2.5 Gbit / s, the influence on chromatic dispersion is negligible at a transmission distance of about several hundred km. When the transmission speed is about 10 Gbit / s, the transmission distance is limited due to the influence of waveform distortion due to the wavelength dispersion of the transmission fiber. For this reason, on the transmission side, in the transmission path, or on the reception side, a dispersion compensator having a dispersion characteristic opposite to the chromatic dispersion of the transmission fiber is inserted to reduce the total dispersion amount, thereby relaxing the transmission distance limitation. It is possible.
1: S. Kuwahara et al., “WDM field demonstration of 43 Gbit / s / channel path provisioning by automatic dispersion compensation using tone modulated CS-RZ signal”, Paper Tul. 6.2, ECOC2003, (2003)

However, in order to improve the transmission speed per wavelength to about 40 Gbit / s in the WDM transmission system, there are the following problems.
In general, the wavelength dispersion characteristic of an optical fiber has wavelength dependency. In the C-band and L-band used for long-distance transmission, the dispersion value increases toward the longer wavelength side, and has a dispersion slope of about 0.05 to 0.07 ps / nm ² / km. For this reason, when compensating for the dispersion of the transmission fiber, if a dispersion compensator common to each wavelength is used, there is a problem that a deviation occurs in the residual dispersion, which causes a transmission distance restriction and a wavelength band restriction.

In order to avoid this, it is necessary to perform dispersion compensation for each wavelength, which causes an increase in cost and an increase in apparatus size.
In recent years, reverse slope type dispersion compensation fibers have been developed that have characteristics opposite to those of transmission fibers in addition to dispersion values. However, in the L-band band of dispersion shifted fibers (DSF), even slopes have been developed. It is difficult to realize a sufficiently flat dispersion compensation characteristic including the current situation, and dispersion compensation must be performed for each wavelength.

  The present invention has been made in view of such circumstances, and by setting the chirp characteristics of the transmission signal for each wavelength, the resistance to residual dispersion deviation between wavelengths due to the residual dispersion slope is improved, and the transmission possible distance An object of the present invention is to provide a WDM transmission system in which the extension of the usable wavelength band is extended.

  In order to achieve the above object, the invention described in claim 1 is directed to a transmission transponder unit that receives an optical signal transmitted from a client device, converts a wavelength, and outputs the optical signal, and a plurality of transmission transponder units received from the transmission transponder unit. Wavelength multiplexing unit that wavelength-multiplexes an optical signal and sends it to a transmission line; a dispersion compensation unit that compensates for chromatic dispersion of the transmission line; and a wavelength that separates and outputs the wavelength multiplexed signal received from the transmission line for each wavelength A WDM transmission system including a separation unit and a reception transponder unit that receives an optical signal from the wavelength separation unit, performs identification reproduction, and transmits the optical signal to the opposite client device, wherein the transmission transponder unit includes the plurality of transmission transponders Multiple chirps are provided corresponding to each input signal, the chirp amount is variable, and a modulated optical signal is output by superimposing the set chirp amount. And a chirp amount control unit that sets a chirp amount in each of the plurality of chirp variable transmission transponders in accordance with an output wavelength of each of the plurality of chirp variable transmission transponders. .

  The invention according to claim 2 is the WDM transmission system according to claim 1, wherein the chirp amount control unit counts from the short wavelength side with respect to the maximum number of wavelengths input to the wavelength multiplexing unit. The chirp amount of a wavelength of at least 40% or more is set to red chirp, and the chirp amount of a wavelength of at least 40% or more counted from the long wavelength side is set to blue chirp.

  According to a third aspect of the present invention, there is provided a transmission transponder unit that receives an optical signal transmitted from a client device, converts the wavelength and outputs the optical signal, and wavelength-multiplexes a plurality of optical signals received from the transmission transponder unit. A wavelength multiplexing unit that transmits to the transmission line, a dispersion compensation unit that compensates for chromatic dispersion of the transmission line, a wavelength separation unit that separates and outputs the wavelength multiplexed signal received from the transmission line, and the wavelength A WDM transmission system including a reception transponder that receives an optical signal from a separation unit, performs identification reproduction, and transmits the optical signal to an opposite client device, wherein the transmission transponder corresponds to each of the plurality of input signals A plurality of chirp variable transmission transponders configured to output an optical signal modulated by superimposing the set chirp amount. A dispersion measuring unit that measures the total dispersion amount of the transmission fiber forming the transmission path and the dispersion compensating unit, and an output wavelength and dispersion measuring unit of each of the plurality of chirp variable transmission transponders. And a chirp amount control unit that sets a chirp amount in each of the plurality of chirp variable transmission transponders in accordance with the measured total dispersion amount.

  According to a fourth aspect of the present invention, in the WDM transmission system according to the third aspect, the chirp amount control unit has a wavelength shorter than the zero dispersion wavelength with respect to the total dispersion amount of the transmission fiber and the dispersion compensation unit. The chirp amount is set to red chirp, and the chirp amount of a channel longer than the zero dispersion wavelength is set to blue chirp.

  According to a fifth aspect of the present invention, there is provided a wavelength multiplexing unit that wavelength-multiplexes a plurality of client optical signals transmitted from a client device and transmits the optical signals to a transmission line, and a dispersion compensation unit that compensates for chromatic dispersion of the transmission line. A wavelength demultiplexing unit that separates and outputs the wavelength division multiplexed signal received from the transmission path for each wavelength, and a reception transponder unit that receives the optical signal from the wavelength demultiplexing unit, performs identification reproduction, and sends it to the opposite client device A plurality of bit-synchronized phase modulations that are provided according to each of the plurality of client optical signals and superimpose a set phase modulation in synchronization with the bits of each client optical signal And a phase modulation amount for setting a phase modulation amount of each of the plurality of bit synchronous phase modulation units according to each wavelength of the plurality of inputted client optical signals And having a control unit.

  According to a sixth aspect of the present invention, there is provided a wavelength multiplexing unit that wavelength-multiplexes a plurality of client optical signals transmitted from a client device and transmits the signals to a transmission line, and a dispersion compensation unit that compensates chromatic dispersion of the transmission line. A wavelength demultiplexing unit that separates wavelength-multiplexed signals received from the transmission path for each wavelength and transmits the signals to the receiving transponder unit; receives an optical signal from the wavelength demultiplexing unit; performs identification reproduction; and transmits to the opposite client device A WDM transmission system including a receiving transponder unit, a dispersion measuring unit for measuring a total dispersion amount of a transmission fiber and a dispersion compensating unit forming the transmission path, and a plurality of client optical signals A bit-synchronized phase modulation unit that superimposes the set phase modulation in synchronization with the bits of each client optical signal, and the plurality of inputted clients And having a phase modulation amount controller for setting the phase modulation amount of each of the plurality of bit synchronous phase modulation unit according to the total amount of dispersion was measured at a wavelength and dispersion measuring unit each of the signals.

  As described above, according to the WDM transmission system according to the present invention, by setting the chirp characteristics and phase modulation characteristics of the transmission signal for each wavelength, the resistance to residual dispersion deviation between wavelengths due to the residual dispersion slope can be improved. The transmission distance can be extended and the usable wavelength band can be expanded.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First embodiment]
A WDM transmission system according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows the configuration of a WDM transmission system according to the first embodiment of the present invention. In the WDM transmission system according to the first embodiment of the present invention, an input optical signal is provided by providing a chirp amount control unit in the transmission transponder unit 1 and using a chirp variable transponder capable of changing the chirp characteristics of each transponder unit. The chirp is set according to the wavelength of.

1, the WDM transmission system according to the first embodiment of the present invention includes a transmission transponder unit 1, a wavelength multiplexing unit 2, optical amplification units 3 and 5, a dispersion compensation unit 6, a wavelength separation unit 7, A reception transponder unit 8. Reference numeral 4 denotes a transmission fiber.
The transmission transponder unit 1 has a function of receiving an optical signal transmitted from a client device (not shown), converting the wavelength, and outputting the optical signal.

The transmission transponder unit 1 includes a plurality of chirp variable transmission transponders 10-1 to 10-n provided corresponding to each of a plurality of input signals, and a plurality of chirp variable transmission transponders 10-1 to 10-n. The chirp amount control unit 11 sets the chirp amount.
Each of the chirp variable transmission transponders 10-1 to 10-n is configured such that the chirp amount is variable, and has a function of outputting an optical signal modulated by superimposing the set chirp amount.

The chirp amount control unit 11 sets a chirp amount for each of the plurality of chirp variable transmission transponders 10-1 to 10-n according to the output wavelength of each of the chirp variable transmission transponders 10-1 to 10-n. have.
The wavelength multiplexing unit 2 wavelength-multiplexes a plurality of optical signals received from the transmission transponder unit 1 and transmits the multiplexed signals to the transmission fiber 4 serving as a transmission path via the optical amplification unit 3. The dispersion compensation unit 6 compensates the chromatic dispersion of the transmission fiber 4.

The wavelength separation unit 7 separates and outputs the wavelength multiplexed signal received from the transmission fiber 4 through the optical amplification unit 5 for each wavelength.
The reception transponder unit 8 receives the optical signal from the wavelength separation unit 7, performs identification reproduction, and sends it to the opposite client device.
The reception transponder unit 8 includes reception transponders 80-1 to 80-n, and the reception transponders 80-1 to 80-n transmit the output optical signals 1 to n to the opposite client apparatus.

  Next, the operation of the WDM transmission system according to the first embodiment of the present invention will be described with reference to FIG. The input optical signals 1 to n received from the client apparatus are input to the chirp variable transmission transponders 10-1 to 10-n in the transmission transponder 1, and in the chirp variable transmission transponders 10-1 to 10-n, respectively. The signal is modulated by an intensity modulation signal having a chirp characteristic determined in advance based on the chirp amount set by the chirp amount control unit 11 according to the output wavelength, and is output to the wavelength multiplexing unit 2.

  In the WDM transmission system according to the first embodiment, a case where the transmission rate of each channel is 40 Gbit / s and each channel is modulated by an intensity modulation signal of an NRZ code will be described as an example. The optical signals output from the chirp variable transmission transponders 10-1 to 10-n corresponding to the respective wavelengths are multiplexed in the wavelength region in the wavelength multiplexing unit 2, and are transmitted through one optical fiber 4 via the optical amplification unit 3. Is transmitted.

  After transmission, the signal is separated for each wavelength in the wavelength separation unit 7 via the optical amplification unit 5 and the dispersion compensation unit 6, and is identified and reproduced in the reception transponders 80-1 to 80-n in the reception transponder unit 8. After that, it is sent to the client device on the opposite side.

  Next, since the chirp variable transmission transponders 10-1 to 10-n shown in FIG. 2 have the same configuration as the configuration example of the chirp variable transmission transponder, only the chirp variable transmission transponder 10-1 will be described. In the figure, a chirp variable transmission transponder 10-1 includes a photodiode 100, an amplifier 101, a CDR (Clock Data Recovery) circuit 102 for identifying data from an input signal, and an RF switch for switching the output of the CDR circuit 102 ( SW) 103, a CW LD (laser diode) 104 that oscillates CW light, an MZ (Mach-Zehnder, MZ) type intensity modulator 105 that modulates the intensity of input light from the CW LD 104, and an MZ type intensity modulator 105 And a bias control circuit 106 for controlling the bias voltage applied to the.

  In the above configuration, the input optical signal is OE converted by the photodiode 100, and data is identified and reproduced in the CDR circuit 102. Further, the MW type intensity modulator 105 driven by the data signal after data identification modulates the CW light emitted from the CW LD 104 to be EO-converted and output as an output optical signal.

  FIG. 3 shows the relationship between the bias voltage applied to the MZ-type intensity modulator 105 and the output power, and the relationship between the drive signal to be selected when the bias point is shifted by the half-wave voltage Vπ. In the configuration example of FIG. 2, the chirp adjustment of the output signal is performed by the chirp amount control unit 11 changing the bias point of the MZ type intensity modulator 105 by Vπ as shown in FIG. 3 via the bias control circuit 106. It is realized by.

  However, since the logic of the output signal is inverted when the bias point of the MZ type intensity modulator 105 is changed, the chirp amount control unit 11 also causes the RF switch 103 to invert the logic of the modulator drive signal at the same time. Control. With such a configuration, it is possible to invert the chirp sign of the output signal. In the first embodiment, as shown in FIG. 4, consider a case where the shorter wavelength side than the center of the wavelength band is set to red chirp and the longer wavelength side is set to blue chirp.

Here, the red chirp is a state where the rising edge of the light pulse is shifted to the short wavelength side and the falling edge is shifted to the long wavelength side, and the blue chirp is the opposite.
Further, the chirp characteristic of the MZ type intensity modulator 105 is expressed using an α parameter. α> 0 is red chirp and α <0 is blue chirp. The value of the α parameter depends on the electrode shape of the modulator and the like. Here, a case where α = ± 1, which is a typical value in the case of one-side electrode driving, is considered.

Hereinafter, a mechanism for improving the resistance to residual dispersion deviation between wavelengths caused by the residual dispersion slope by setting the chirp characteristics for each wavelength will be described. FIG. 5 shows a simulation result of the dependency of the eye opening deterioration on the residual dispersion.
Here, in this simulation, the penalty due to chromatic dispersion in linear transmission was calculated based on the eye aperture when the code of the transmission signal is a 43.1 Gbit / s NRZ code and the dispersion value is zero.

As can be seen from FIG. 5, when red chirp is applied to the transmission signal (α = 1), pulse compression occurs in the region where the residual dispersion is negative dispersion (normal dispersion), so the increase in eye opening penalty is small. It has become.
Conversely, when the transmitted signal is blue chirped (α = −1), it can be seen that pulse compression occurs in the region where the residual dispersion is normal dispersion (abnormal dispersion), and the eye opening penalty is small.

On the other hand, the chromatic dispersion of the transmission fiber 4 has a characteristic that the longer the wavelength, the larger the chromatic dispersion in the C-band and L-band used for long-distance transmission. For example, it is known that the dispersion slope has a positive value of about 0.07 ps / nm ² / km in the L band of DSF.
Also in a dispersion compensating fiber widely used as a dispersion compensator, it is usually a positive value.

  In recent years, an inverse slope type dispersion compensating fiber having a negative dispersion slope characteristic has been developed. However, it is difficult to perfectly match the slope characteristic of the transmission line fiber, and a positive residual dispersion slope remains. It will be. Therefore, when the dispersion of the transmission line is compensated by the dispersion compensating fiber, it can be seen that the residual dispersion increases as the wavelength becomes longer.

  From the above, dispersion compensation is made so that zero dispersion is near the center of the wavelength band used, and the residual dispersion deviation is set by setting the transmission chirp of the short wavelength channel to red chirp and the long wavelength side to blue chirp. It is possible to suppress an increase in eye opening deterioration (corresponding to a power penalty of reception sensitivity) due to. When the chirp characteristics of each wavelength are the same as in the prior art, the power penalty increases on the long wavelength side when α = 1 is set, and on the short wavelength side when α = −1 is set. Needless to explain.

In the above embodiment, the short wavelength side half of the total number of wavelengths is set to α = 1, and the long wavelength side half is set to a = -1. However, if the eye opening penalty due to residual dispersion is allowed up to 1 dB, as can be seen from FIG.
-91ps / nm≤residual dispersion≤-18ps / nm: α = 1
--18 ps / nm ≤ residual dispersion ≤ 18 ps / nm: α = ± 1 is acceptable-18 ps / nm ≤ residual dispersion ≤ 91 ps / nm: α = -1
Even if a chirp is set as described above, transmission is possible, and a more flexible chirp setting is possible.

  Considering that the residual dispersion is usually linearly dependent on the wavelength, the above results show that 40% on the short wavelength side when dispersion compensation is performed so that zero dispersion is obtained at the center of the wavelength band. Red chirp (α = 1) for the wavelength of the minute, blue chirp (α = -1) for the 40% wavelength on the long wavelength side, and α = ± 1 for other channels near the center of the wavelength band It can be seen that good transmission characteristics can be obtained over the entire wavelength band even when the chirp is set.

  As described above, according to the WDM transmission system according to the first embodiment of the present invention, the transmission distance is expanded by controlling the chirp of the transponder unit of each wavelength according to the output wavelength of the transmission transponder unit. Or it becomes possible to aim at expansion of the number of waves.

[Second Embodiment]
Next, a WDM transmission system according to the second embodiment of the present invention will be described.
In the WDM transmission system according to the first embodiment, an example in which the chirp characteristic of the transmission transponder unit is set according to the number of wavelengths has been shown. When the chromatic dispersion of the transmission line is compensated and the zero-dispersion wavelength is almost in the center of the transmission wavelength band, the short wavelength side is set to red chirp and the long wavelength side is set to blue chirp as described in the above embodiment. As a result, good reception characteristics can be obtained.

  However, the dispersion value of the transmission line is not always constant, and may change due to a route change or a fiber core wire change after the start of operation. The second embodiment of the present invention provides a WDM transmission system that can cope with such dispersion variation in the transmission path.

  FIG. 6 shows the configuration of a WDM transmission system according to the second embodiment of the present invention. The WDM transmission system according to the second embodiment is characterized in that a dispersion measurement transmission unit 9-1 is provided on the transmission side, and a dispersion measurement unit reception unit 9-2 is provided after the dispersion compensation unit 6 on the reception side for transmission. The dispersion amount of the fiber 4 and the dispersion compensator 6 can be measured, and the result of dispersion measurement is notified to the chirp amount control unit 11A. In the chirp amount control unit 11A, the output wavelength of each transponder unit and the result of dispersion measurement The function of determining the chirp amount in the channel is provided, and the other configuration is the same as that of the WDM transmission system according to the first embodiment.

  6, the WDM transmission system according to the second embodiment includes a transmission transponder unit 1, a wavelength multiplexing unit 2, optical amplification units 3, 5, a dispersion compensation unit 6, a wavelength separation unit 7, and a reception transponder unit. 8, a dispersion measurement transmission unit 9-1 provided on the transmission side in the transmission fiber 4, and a dispersion measurement reception unit 9-2 provided on the reception side.

The transmission transponder unit 1 has a function of receiving an optical signal transmitted from a client device (not shown), converting the wavelength, and outputting the optical signal.
The transmission transponder unit 1 includes a plurality of chirp variable transmission transponders 10-1 to 10-n provided corresponding to each of a plurality of input signals, and a plurality of chirp variable transmission transponders 10-1 to 10-n. The chirp amount control unit 11 sets the chirp amount.
Each of the chirp variable transmission transponders 10-1 to 10-n is configured such that the chirp amount is variable, and has a function of outputting an optical signal modulated by superimposing the set chirp amount.

In the dispersion measurement transmission unit 9-1 and the dispersion measurement reception unit 9-2, for example, a measurement method based on a phase difference method used in a normal dispersion measurement device is applied, and the wavelength interval is known in the vicinity of the long wavelength end and short wavelength end. By measuring the phase difference between the two wavelengths, the chromatic dispersion at both wavelength ends can be measured, and the dispersion of each wavelength can be obtained by linear approximation in the intermediate wavelength band.
The dispersion measurement receiver 9-2 has a function of notifying the chirp amount controller 11A of the dispersion of each wavelength measured by the above method.

The chirp amount control unit 11A includes a plurality of chirp variable types according to the output wavelength of each of the chirp variable transmission transponders 10-1 to 10-n and the result of dispersion measurement received from the dispersion measurement reception unit 9-2. Each of the transmission transponders 10-1 to 10-n has a function of setting a chirp amount.
The wavelength multiplexing unit 2 wavelength-multiplexes a plurality of optical signals received from the transmission transponder unit 1 and transmits the signals to the transmission fiber 4 serving as a transmission path via the optical amplification unit 3. The dispersion compensation unit 6 compensates the chromatic dispersion of the transmission fiber 4.

The wavelength separation unit 7 separates and outputs the wavelength multiplexed signal received from the transmission fiber 4 through the optical amplification unit 5 for each wavelength.
The reception transponder unit 8 receives the optical signal from the wavelength separation unit 7, performs identification reproduction, and sends it to the opposite client device.
The reception transponder unit 8 includes reception transponders 80-1 to 80-n, and the reception transponders 80-1 to 80-n transmit the output optical signals 1 to n to the opposite client apparatus.

Next, the operation of the WDM transmission system according to the second embodiment will be described with reference to FIG. FIG. 7A shows the wavelength characteristics of the total dispersion amount of the transmission fiber 4 and the dispersion compensator 6 in the initial state (indicated by a one-dot chain line) and the state of chirp setting at each wavelength (as shown in this figure). In addition, the wavelength dependence of chromatic dispersion can be approximated by a straight line).
The chirp amount control unit 11A sets the chirp amount at each wavelength such that the channel on the shorter wavelength side than the zero dispersion wavelength λ _{01 with} respect to the total dispersion amount of the transmission fiber and the dispersion compensator in the initial state is α = 1 and the wavelength λ _It is assumed that the channel on the longer wavelength side than ₀₁ is set to α = −1.

In this state, consider a case where the total dispersion amount changes as indicated by a solid line due to a change in the route of the transmission fiber 4. At this time, as shown in FIG. 7A, in the region on the long wavelength side of the wavelength λ ₀₁ , the allowable dispersion amount in the case of α = −1 is deviated and a channel that cannot be transmitted is generated. I understand. To avoid this, after the dispersion change, the total amount of dispersion was measured to obtain the zero-dispersion wavelength lambda ₀₂ after the change, as shown in FIG. 7 (b), λ ₀₂ wavelengths of shorter wavelength side Is set to α = 1, and the channel on the long wavelength side is set to α = −1.

  As described above, according to the WDM transmission system according to the second embodiment of the present invention, the dispersion measurement unit is provided, and when the amount of distribution of the transmission fiber changes, the dispersion measurement is performed. Zero-dispersion wavelength, red chirp on the shorter wavelength side than the zero-dispersion wavelength after the change, and blue chirp on the longer wavelength side, avoiding the generation of wavelengths that cannot be transmitted, and good reception over a wide wavelength band It becomes possible to realize the characteristics.

[Third embodiment]
Next, a WDM transmission system according to the third embodiment of the present invention will be described.
In the WDM transmission system according to the first embodiment and the second embodiment, a configuration in which a client optical signal received from a client device is once subjected to OE conversion in a transponder unit, and a chirp is added to a transmission signal by a chirp characteristic of a modulator. Was used. In the third embodiment of the present invention, the means for adding a chirp is realized with a simpler configuration.

FIG. 8 shows the configuration of a WDM transmission system according to the third embodiment of the present invention.
A feature of the WDM transmission system according to the third embodiment is that a bit synchronization phase in which phase modulation is superimposed on each signal light pulse in synchronization with the transmission speed of the input signal with respect to the optical signal input from the client device. Modulation units 20-1 to 20-n are provided, and the phase modulation amount can be set according to the wavelength of the input optical signal. Other configurations are related to the first embodiment. This is the same as the WDM transmission system.

  In FIG. 8, a WDM transmission system according to the third embodiment of the present invention includes bit synchronous phase modulation units 20-1 to 20-n provided corresponding to input optical signals 1 to n from a client device, A phase modulation amount control unit 30 that controls the phase modulation amounts of the synchronous phase modulation units 20-1 to 20-n, a wavelength multiplexing unit 2, optical amplification units 3 and 5, a dispersion compensation unit 6, and a wavelength separation unit 7 And a reception transponder unit 8. Reference numeral 4 denotes a transmission fiber.

The bit synchronization phase modulation units 20-1 to 20-n have a function of superimposing the phase modulation set by the phase modulation amount control unit 30 in synchronization with the bits of each client optical signal.
The phase modulation amount control unit 30 has a function of setting the phase modulation amount of each of the bit synchronization phase modulation units 20-1 to 20-n according to the wavelength of each of the plurality of inputted client optical signals. .

The wavelength multiplexing unit 2 wavelength-multiplexes a plurality of client optical signals (input optical signals) 1 to n, which are transmitted from the client device and on which phase modulation is superimposed, by the bit synchronous phase modulation units 20-1 to 20-n. The data is sent to the transmission optical fiber 4 through the optical amplification unit 3 which is a transmission path.
The dispersion compensation unit 6 compensates the chromatic dispersion of the transmission fiber 4.
The wavelength separation unit 7 separates and outputs the wavelength multiplexed signal received from the transmission fiber 4 through the optical amplification unit 5 for each wavelength.

The reception transponder unit 8 receives the optical signal from the wavelength separation unit 7, performs identification reproduction, and sends it to the opposite client device.
The reception transponder unit 8 includes reception transponders 80-1 to 80-n, and the reception transponders 80-1 to 80-n transmit the output optical signals 1 to n to the opposite client apparatus.

  Next, a specific configuration of the bit synchronous phase modulation unit is shown in FIG. Since the bit synchronization phase modulation units 20-1 to 20-n have the same configuration, FIG. 9 illustrates the bit synchronization phase modulation unit 20-1. In the figure, a bit-synchronized phase modulation unit 20-1 includes an optical coupler 200 that branches an input optical signal 1, a phase modulator 201 that phase-modulates one optical signal branched by the optical coupler 200, and an optical coupler 200. An OE conversion unit 202 that performs OE conversion (optical / electrical conversion) on the other optical signal branched by the OE conversion unit, a clock extraction circuit 203 that extracts a clock from the electric signal converted by the OE conversion unit 202, a phase shifter 204, And an amplifier 205.

In the above configuration, the input optical signal 1 input to the bit synchronization phase modulation unit 20-1 is branched by the optical coupler 200, one is input to the phase modulator 201, and the other signal light is input to the OE conversion unit 202. Entered.
The signal light input to the OE conversion unit 202 is subjected to OE conversion, and then the clock signal is extracted by the clock extraction circuit 203. The extracted clock signal is input to the phase shifter 204.

  The phase shift amount of the clock signal input to the phase shifter 204 is adjusted under the control of the phase modulation amount control unit 30, and the phase modulator 201 is driven while being amplified by the amplifier 205. As a result, the signal light input to the phase modulator 201 undergoes phase modulation with a period equal to the pulse repetition period.

  FIG. 10 is a diagram for explaining the phase change and frequency change of the optical signal at the output of the bit synchronous phase modulation unit. As shown in FIG. 10, the input intensity-modulated light (FIG. 10 (a)) is converted into a sine wave signal synchronized with the pulse repetition period so that the phase change becomes maximum at the position where the intensity of the optical pulse becomes maximum. In the case of phase modulation (FIG. 10B), since the frequency shift is given by time differentiation of the phase change amount, the rising edge of the optical pulse is shifted to the high frequency side (short wavelength side), and the falling edge is low. It can be seen that the shifted red chirp is superimposed on the frequency side (long wavelength side) (FIG. 10C).

Further, a blue chirp can be obtained by shifting the phase of the phase modulation signal by π, and the magnitude of the chirp amount can be adjusted by changing the amplitude of the phase modulation signal.
As shown in FIG. 8, in the third embodiment, phase modulation corresponding to each wavelength is superimposed on the input optical signal of each wavelength. This is apparent from the above description. Furthermore, it is equivalent to controlling the chirp amount of each wavelength.

Therefore, as described in the first embodiment of the present invention, phase modulation is performed so that red chirp is applied to a short wavelength optical signal and blue chirp is applied to a long wavelength optical signal. It is possible to improve the resistance to dispersion dispersion between wavelengths.
Further, as described in the second embodiment of the present invention, the total dispersion amount of the transmission fiber and the dispersion compensator is measured, and the phase modulation amount is determined according to the output wavelength of each optical signal and the measured total dispersion amount. Needless to say, it is effective to obtain good reception characteristics even when the dispersion of the transmission fiber is changed.

1 is a block diagram showing a configuration of a WDM transmission system according to a first embodiment of the present invention. The block diagram which shows the structural example of the chirp variable type | mold transmission transponder in the WDM transmission system which concerns on 1st Embodiment of this invention shown in FIG. FIG. 3 is an explanatory diagram showing a relationship between a bias voltage and output power applied to the MZ type intensity modulator shown in FIG. 2 and a relationship with a drive signal to be selected when the bias point is shifted by a half-wave voltage Vπ. Explanatory drawing which shows the example of a setting of the chirp amount by a chirp amount control part. The figure which shows the simulation result of the dependence of the eye opening penalty with respect to chromatic dispersion. The block diagram which shows the structure of the WDM transmission system which concerns on 2nd Embodiment of this invention. Explanatory drawing which shows the change state of the total amount of dispersion | distribution in the WDM transmission system which concerns on 2nd Embodiment of this invention, and the example of a setting of a chirp amount. The block diagram which shows the structure of the WDM transmission system which concerns on 3rd Embodiment of this invention. The block diagram which shows the structural example of the bit synchronous phase modulation part in the WDM transmission system which concerns on 3rd Embodiment of this invention. Explanatory drawing which shows the mode of the phase and frequency change of the optical signal by the bit synchronous phase modulation in the WDM transmission system concerning 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Transmission transponder part 2 ... Wavelength multiplexing part 3, 5 ... Optical amplification part 4 ... Transmission fiber 6 ... Dispersion compensation part 7 ... Wavelength separation part 8 ... Reception transponder part 9-1 ... Dispersion measurement transmission part 9-2 ... Dispersion measurement Receiving units 10-1 to 10-n ... chirp variable transmission transponders 11, 11A ... chirp amount control units 20-1 to 20-n ... bit synchronization phase modulation units 80-1 to 80-n ... receiving transponders

Claims (6)

A transmission transponder that receives an optical signal transmitted from a client device, converts the wavelength, and outputs the wavelength; a wavelength multiplexing unit that multiplexes a plurality of optical signals received from the transmission transponder and transmits the optical signal to a transmission path; A dispersion compensator that compensates for chromatic dispersion of the transmission line, a wavelength separation part that separates and outputs wavelength multiplexed signals received from the transmission line for each wavelength, receives an optical signal from the wavelength separation part, and performs identification reproduction A WDM transmission system comprising a reception transponder unit that performs transmission to the opposite client device,
The transmission transponder unit
A plurality of chirp variable transmission transponders that are provided corresponding to each of the plurality of input signals, the chirp amount is variably configured, and outputs a modulated optical signal by superimposing the set chirp amount;
A WDM transmission system comprising: a chirp amount control unit that sets a chirp amount in each of the plurality of chirp variable transmission transponders in accordance with an output wavelength of each of the plurality of chirp variable transmission transponders.   The chirp amount control unit sets a chirp amount of at least 40% or more of the wavelength counted from the short wavelength side to the maximum number of wavelengths input to the wavelength multiplexing unit, and counts from the long wavelength side. 2. The WDM transmission system according to claim 1, wherein a chirp amount of a wavelength of at least 40% or more is set to blue chirp. A transmission transponder that receives an optical signal transmitted from a client device, converts the wavelength, and outputs the wavelength; a wavelength multiplexing unit that multiplexes a plurality of optical signals received from the transmission transponder and transmits the optical signal to a transmission path; A dispersion compensator that compensates for chromatic dispersion of the transmission line, a wavelength separation part that separates and outputs wavelength multiplexed signals received from the transmission line for each wavelength, receives an optical signal from the wavelength separation part, and performs identification reproduction A WDM transmission system comprising a reception transponder unit that performs transmission to the opposite client device,
The transmission transponder unit is provided corresponding to each of the plurality of input signals, is configured to have a variable chirp amount, and outputs a plurality of chirp variable types that output a modulated optical signal by superimposing the set chirp amount Comprising a transmission transponder,
A dispersion measuring unit for measuring a total dispersion amount of the transmission fiber forming the transmission line and the dispersion compensating unit;
A chirp amount controller configured to set a chirp amount in each of the plurality of chirp variable transmission transponders according to an output wavelength of each of the plurality of chirp variable transmission transponders and a total dispersion measured by a dispersion measuring unit;
A WDM transmission system.   The chirp amount control unit sets the chirp amount on the shorter wavelength side than the zero dispersion wavelength with respect to the total dispersion amount of the transmission fiber and the dispersion compensation unit to red chirp, and chirps the channel on the longer wavelength side than the zero dispersion wavelength. The WDM transmission system according to claim 3, wherein the amount is set to blue chirp. A wavelength multiplexing unit that wavelength-multiplexes a plurality of client optical signals transmitted from a client device and transmits the optical signals to a transmission line, a dispersion compensation unit that compensates for chromatic dispersion of the transmission line, and a wavelength multiplexed signal received from the transmission line. A WDM transmission system comprising: a wavelength separation unit that separates and outputs each wavelength; and a reception transponder unit that receives an optical signal from the wavelength separation unit, performs identification reproduction, and transmits the signal to the opposite client device,
A plurality of bit-synchronized phase modulation units that are provided according to each of the plurality of client optical signals and superimpose a set phase modulation in synchronization with the bits of each client optical signal;
A phase modulation amount control unit that sets a phase modulation amount of each of the plurality of bit synchronization phase modulation units according to each wavelength of the plurality of input client optical signals;
A WDM transmission system comprising: A wavelength multiplexing unit that wavelength-multiplexes a plurality of client optical signals transmitted from a client device and transmits the optical signals to a transmission line, a dispersion compensation unit that compensates for chromatic dispersion of the transmission line, and a wavelength multiplexed signal received from the transmission line. A WDM transmission system comprising a wavelength demultiplexer that separates each wavelength and sends it to a receiving transponder, and a receiving transponder that receives an optical signal from the wavelength demultiplexer, performs identification reproduction, and sends it to the opposite client device There,
A dispersion measuring unit for measuring a total dispersion amount of the transmission fiber and the dispersion compensating unit forming the transmission path;
A bit-synchronized phase modulation unit that is provided according to each of the plurality of client optical signals and superimposes a set phase modulation in synchronization with the bits of each client optical signal;
A phase modulation amount controller configured to set a phase modulation amount of each of the plurality of bit-synchronized phase modulators according to the wavelength of each of the plurality of inputted client optical signals and the total dispersion measured by the dispersion measuring unit;
A WDM transmission system comprising:

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