JP4842100B2 - Distributed pre-equalized optical transmitter and optical communication system - Google Patents

Description

  The present invention relates to a distributed pre-equalized optical transmitter and an optical communication system.

  In a long-distance optical communication system, a technique for more efficiently compensating for the chromatic dispersion of a transmission line is important in order to reduce the cost of the system and easily perform system upgrade by increasing the transmission rate. For example, it is possible to reduce the cost of the system by reducing the dispersion compensating fiber and the optical amplifier used for the loss compensation. Also, although the chromatic dispersion tolerance varies depending on the transmission rate and modulation format, the transmission rate and modulation format can be reduced by using no dispersion compensation fiber or using only a few types of dispersion compensation fiber in any transmission system. It is possible to adopt a transmission line configuration that does not depend on the system, and as a result, it is easy to upgrade an existing system and to realize a system in which a plurality of modulation methods are mixed.

  As one method for obtaining the above effects, as a method for performing very large dispersion pre-equalization such as 20000 to 40,000 ps / nm for 10 Gb / s, the absolute value is the same for the chromatic dispersion of the transmission line. In addition, a dispersion pre-equalization transmission method for transmitting an optical signal to which an effect of chromatic dispersion having a reverse sign is added in advance has been actively studied (see, for example, Non-Patent Documents 1 and 2).

  FIG. 8 is a block diagram showing the configuration of this type of conventional dispersion pre-equalization optical transmitter. As shown in FIG. 8, the conventional dispersion pre-equalization optical transmitter includes a light source 10 and an optical modulation unit 20 connected between the light source 10 and the optical fiber transmission line 15. Further, an optical phase control unit 16 and a dispersion pre-equalization data generation unit 17 connected to the optical modulation unit 20 are provided. Further, an equalization parameter setting unit 19 is connected to the distributed pre-equalization data generation unit 17.

  Further, in the light modulation means 20, connected to the light source 10, an optical demultiplexing means 11 for demultiplexing the light from the light source 10, and connected in parallel to the optical demultiplexing means 11, and divided into two There are provided two Mach-Zehnder (MZ-Zehnder) type light modulators 12a and 12b to which the inputted light is respectively input. Furthermore, an optical phase adjustment unit 13 connected to one Mach-Zehnder (MZ) type optical modulation unit 12b is provided. The optical phase adjustment unit 13 receives a control signal from the optical phase control unit 16 described above, and is generated by two Mach-Zehnder (MZ) optical modulation units 12a and 12b according to the control signal. The relative phase difference of the optical electric field is controlled (π / 2 phase shift is performed). Further, in the optical modulation means 20, an optical multiplexing means 14 connected to a Mach-Zehnder (MZ) type optical modulation section 12 a and an optical phase adjustment section 13 is provided. The other end of the optical multiplexing means 14 is connected to the optical fiber transmission line 15.

  Further, in the distributed pre-equalization data generation means 17, a distributed inverse function calculation means 30 to which transmission source data is input from the outside and equalization parameters are input from the equalization parameter setting means 19 is provided. . The distributed inverse function calculation means 30 is connected in parallel with two modulator inverse function calculation means 31a and 31b. The modulator inverse function calculation means 31a and 31b are further provided with D / A converters 32a and 32b and amplifiers 33a and 33b connected in series, respectively. The output from the amplifier 33a is input to a Mach-Zehnder (MZ) optical modulation unit 12b of the light modulation means 20, and the output from the amplifier 33b is a Mach-Zehnder (MZ: Mach) of the light modulation means 20. -Zehnder) type light modulator 12a.

  The operation of the conventional dispersion pre-equalized optical transmitter will be described below with reference to FIG. In the conventional example, the inverse function calculation of the chromatic dispersion of the transmission line and the inverse function calculation of the optical modulator transfer function are performed on the transmission source data series by the dispersion inverse function calculation means 30, and two series of data (I The optical signal subjected to dispersion pre-equalization is transmitted by driving the I / Q modulator using (channel, Q-channel). At this time, as an inverse function calculation of the chromatic dispersion of the transmission line, a chromatic dispersion transfer function having the same absolute value as the chromatic dispersion of the transmission line and an opposite sign, and a multiplication operation of the transmission data series, or the same transfer function, Complex data by convolution calculation of the impulse response obtained and the transmission source data series (that is, a convolution calculation of the inverse function of the real part and the imaginary part of the transfer function of transmission line chromatic dispersion and the input two series data of the transmission source) The method of obtaining a sequence is common. At this time, since the transfer function or impulse response used for the calculation varies depending on the pre-equalization amount, the related data held in the equalization parameter setting unit 19 is reset in the distributed inverse function calculation unit 30 in accordance with the pre-equalization amount. In general, since the response characteristics of electrical / optical conversion are not linear, in order to generate an ideal optical signal, a modulator for performing an operation for correcting the response characteristics of electrical / optical conversion is usually used. Inverse function calculation means 31a and 31b are added. Further, when each of the above operations is performed by digital operation processing, an analog signal for driving the optical modulator is obtained through the D / A converters 32a and 32b and the amplifiers 33a and 33b.

  The optical I / Q modulation is performed, for example, by using each of the Mach-Zehnder (MZ) optical modulators 12a and 12b with a nested optical modulator (optical modulator 20) having the configuration shown in FIG. -It carries out by driving by a channel and Q-channel (for example, refer patent document 1). The modulator disclosed in Patent Document 1 is a nested optical modulator used for differential quadrature phase shift keying (DQPSK: Differential Quadrature Phase Shift Keying, hereinafter referred to as DQPSK). The modulator performs the π / 2 phase shift by the optical phase adjusting unit 13 according to the control of the optical phase control means 16, thereby converting the modulated optical signal in the I-channel and the Q from the two I / Q series data. Since the modulated optical signal in the channel can generate a complex optical electric field having an orthogonal relationship as shown in FIG. 8, it can also be used as an optical modulator for dispersion pre-equalization transmission. In this way, the obtained two optical electric fields that are orthogonal to each other are combined by the optical combining means 14 and output to the optical fiber transmission line 15.

JP-T-2004-516743 D. McGhan, et al., “5120-km RZ-DPSK Transmission Over G.652 Fiber at 10 Gb / s Without Optical Dispersion Compensation”, IEEE Photon. Technol. Lett., Vol. 18, no. 2, Jan. 15 , 2006 P. J. Winzer, et al., “Electronic pre-distortion for advanced modulation formats”, ECOC2005, Tu4.2.2, Glasgow, UK, Sep. 2005

  Here, consider the case where dispersion compensation across positive and negative is required as the chromatic dispersion of the transmission line. For example, when applying a dispersion pre-equalization optical transmitter to an existing optical communication system network, in consideration of a compensation device such as a dispersion compensation fiber that has already been introduced and an installed optical fiber transmission line type, It is assumed that the sign of the residual dispersion to be compensated is not always positive or negative.

  Thus, when dispersion pre-equalization is performed for positive / negative transmission line chromatic dispersion, the data corresponding to all dispersion values including the sign is set as the drop parameter with the conventional configuration of FIG. It is necessary to hold it in a means or an external storage element (not shown).

  As described above, in the conventional dispersion pre-equalization optical transmitter, when positive / negative dispersion pre-equalization is performed, the amount of data to be held in the dispersion pre-equalization optical transmitter increases. There was a problem of end.

  The present invention has been made in order to solve such a problem. By switching the control point of the optical phase adjustment according to the sign of the pre-equalization dispersion amount, a large amount of data is not retained. An object of the present invention is to obtain a dispersion pre-equalization optical transmitter and an optical communication system capable of performing positive / negative dispersion pre-equalization.

According to the present invention, a transmission source data sequence is input, and for the transmission source data sequence, an inverse function calculation of a chromatic dispersion of a transmission path through which the transmission source data sequence is transmitted and an inverse function calculation of a transfer function of an optical modulation means are performed. performed, whereby a force distributing pre-equalization data generating means out of the data of the two series obtained, and two Mach-Zehnder type optical modulator section provided in parallel, connected to one of the Mach-Zehnder optical modulator unit The two phase data from the dispersion pre-equalization data generating means are respectively input to the two Mach-Zehnder optical modulators one by one, and thereby the two Mach-Zehnder optical modulators are used. each Zehnder light modulator is driven, the two Mach-Zehnder type optical modulator unit modulates the demultiplexed light input from a light source through an optical demultiplexing means, orthogonal to each other And it generates comprising first and second light field, respectively, the optical phase adjusting section adjusts the relative phase difference between the first and second light field by set value, and the light modulation means, Control means for performing switching control of a set value for the relative phase difference used in the adjustment by the light modulation means, and thereby changing the sign of the dispersion pre-equalization amount; Is an optical transmitter.

According to the present invention, a transmission source data sequence is input, and for the transmission source data sequence, an inverse function calculation of a chromatic dispersion of a transmission path through which the transmission source data sequence is transmitted and an inverse function calculation of a transfer function of an optical modulation means are performed. performed, whereby a force distributing pre-equalization data generating means out of the data of the two series obtained, and two Mach-Zehnder type optical modulator section provided in parallel, connected to one of the Mach-Zehnder optical modulator unit The two phase data from the dispersion pre-equalization data generating means are respectively input to the two Mach-Zehnder optical modulators one by one, and thereby the two Mach-Zehnder optical modulators are used. each Zehnder light modulator is driven, the two Mach-Zehnder type optical modulator unit modulates the demultiplexed light input from a light source through an optical demultiplexing means, orthogonal to each other And it generates comprising first and second light field, respectively, the optical phase adjusting section adjusts the relative phase difference between the first and second light field by set value, and the light modulation means, Control means for performing switching control of a set value for the relative phase difference used in the adjustment by the light modulation means, and thereby changing the sign of the dispersion pre-equalization amount; Because it is an optical transmitter, switching the control point of optical phase adjustment according to the sign of the pre-equalization dispersion amount enables positive / negative dispersion pre-equalization even if a large amount of data is not retained. Is possible.

  Embodiments of a distributed pre-equalized optical transmitter and an optical communication system according to the present invention will be described below.

Embodiment 1 FIG.
1 is a configuration diagram relating to a dispersion pre-equalized optical transmitter according to Embodiment 1 of the present invention. As shown in FIG. 1, the dispersion pre-equalization optical transmitter according to the first embodiment performs an inverse function calculation of transmission line chromatic dispersion by inputting a transmission source data series, and obtains two series of data obtained thereby. Is distributed between the light source 10 and the optical fiber transmission line 15, and two series of output data obtained from the dispersion pre-equalization data generation unit 17 is output as the drive signal. And optical modulation means 20 for generating first and second optical electric fields that are orthogonal to each other from the demultiplexed light input from the light source 10 through the optical demultiplexing means 11. The dispersion pre-equalization optical transmitter according to the first embodiment changes the sign of the dispersion pre-equalization amount by controlling the relative phase difference between the first and second optical electric fields. Further, as shown in FIG. 1, the equalization parameter setting means 19 for inputting equalization parameters to the distributed pre-equalization data generation means 17 and the control signal to the optical modulation means 20 are input to control the setting of the optical phase. An optical phase control means 16 and an optical phase setting switching means 18 connected between the optical phase control means 16 and the equalization parameter setting means 19 are provided.

  In the light modulation means 20, the light demultiplexing means 11 to which light from the light source 10 is input, and the demultiplexed light that is connected in parallel to the light demultiplexing means 11 are input and the first and second photoelectric elements are input. Two Mach-Zehnder (MZ) light modulators 12a and 12b that generate a field are provided, and one Mach-Zehnder (MZ) light modulator 12b includes a first MZ-Mehnder (MZ) light modulator 12b. And the optical phase adjustment part 13 for controlling the phase difference of the 2nd optical electric field is connected. The optical phase adjustment unit 13 receives a control signal from the optical phase control unit 16 described above. In the optical modulation means 20, the optical electric fields output from the Mach-Zehnder (MZ) type optical modulation section 12 a and the optical phase adjustment section 13 are multiplexed by the optical multiplexing means 14, and the optical fiber transmission line 15. Is output. The light modulation means 20 is composed of a nested light modulator configured as described above.

  Next, the operation of the distributed pre-equalization optical transmitter according to the first embodiment configured as described above will be described. Compared with the conventional example of FIG. 8, the first embodiment shown in FIG. 1 is different in that an optical phase setting switching means 18 is provided, and the optical modulation means 20 and the dispersion pre-equalization data generation means 17 are different. Since the configuration and operation itself are the same as those in FIG. 8, they will be referred to and description thereof will be omitted here. In the optical phase setting switching unit 18, the setting phase (relative phase difference between the I-channel and the Q-channel) of the optical phase adjustment unit 13 of the optical modulation unit 20 is set at two points, for example, π / 2 and −π / 2. A control signal is sent to the optical phase control means 16 to switch. Accordingly, the optical phase control means 16 outputs a control signal, performs switching control of the value of the setting phase (setting value) used in the control in the optical phase adjustment unit 13 of the light modulation means 20, and sets the setting phase. Causes the value to be set to the specified value. When the optical modulation unit 20 is configured as a LiNbO 3 optical modulator, the optical phase control unit 16 usually controls the DC bias applied to the optical phase adjustment unit 13. Similarly, DC bias control is also performed in Mach-Zehnder (MZ: Mach-Zehnder) type optical modulators 12a and 12b, but the description is omitted in FIG.

  In the following, in the configuration of FIG. 1, the operation when the setting phase of the optical phase adjusting unit 13 is changed from π / 2 to −π / 2 by the optical phase setting switching unit 18 via the optical phase control unit 16. Will be described.

  When an optical electric field that has been subjected to dispersion pre-equalization processing by the distributed pre-equalization data generation means 17 is output from the optical modulation means 20 composed of a nested optical modulator, the actual output optical electric field (complex electric field) is usually obtained. The imaginary part and the imaginary part are generated by Mach-Zehnder (MZ) type optical modulation units 12a and 12b, respectively. The electrical data for driving each Mach-Zehnder (MZ-Mehnder) type optical modulator 12a, 12b is, for example, based on the original data series by the configuration described in the distributed pre-equalization data generation means 17 in FIG. It is obtained by convolution with the inverse transfer function of transmission line chromatic dispersion. Here, when the transmission source data is d (t) and the time axis expression of the inverse transfer function of the transmission line chromatic dispersion is h (t), it corresponds to a desired complex electric field by the calculation shown in the following equation (1). An electric signal can be obtained. When an output optical electric field proportional to the drive electric signal is obtained as an ideal optical modulator, the first term of the formula (1) is changed to the I-channel optical modulation unit (of the optical modulation means 20 of the nested modulator). Mach-Zehnder (MZ: Mach-Zehnder) type optical modulation unit 12a), the second term is obtained by a Q-channel optical modulation unit (Mach-Zehnder (MZ: Mach-Zehnder) type optical modulation unit 12b). It can be seen that it is sufficient to combine the signals with a phase difference of π / 2, assuming that the electric field is generated. In practice, additional calculations such as correction of response characteristics at the time of electro-optical conversion of the optical modulator are necessary, but they are omitted in this description.

  When the phase difference at the time of combining the real part (I-channel drive signal) and the imaginary part (Q-channel drive signal) is −π / 2, the expression (1) becomes the following expression (2), The complex optical electric field obtained as an output is obtained by performing a convolution operation with the complex conjugate of the inverse transfer function of the transmission line chromatic dispersion with respect to the original data series.

  Here, the time axis expression h (t) and the frequency axis expression H (ω) of the transfer function of the chromatic dispersion of the transmission line are in a Fourier transform relationship with each other, and are expressed as the following expressions (3) and (4), respectively. expressed.

Here, L is the propagation distance of the optical fiber, and β ₂ is a second-order propagation constant related to the chromatic dispersion D of the optical fiber. Note that there is a relationship of the following equation (5) between β ₂ [ps ² / km] and chromatic dispersion D [ps / nm / km]. Here, λ is the transmission line wavelength, and c is the speed of light.

Since the frequency axis expression H (ω) can be expressed as an even function of ω as shown in Expression (4), the complex conjugate {h (t)} ^{* of} the time axis expression h (t) and the frequency axis expression H (ω ) Complex conjugate {H (ω)} ^* is expressed by the following equations (6) and (7), respectively, and the time axis representation and the frequency of the transfer function corresponding to the transmission line chromatic dispersion with the sign inverted. Equivalent to axis representation.

  Therefore, if the π / 2 phase shift in the optical phase adjustment unit of the nested optical modulator is changed to −π / 2, the drive electricity of the Mach-Zehnder (MZ) type optical modulation units 12a and 12b of each sub is changed. Even if the signals are in the same state, the sign of the distributed pre-equalization amount is inverted.

  As described above, in the first embodiment, by switching the setting of the optical phase adjustment unit 13 between π / 2 and −π / 2, the absolute value is the same and the dispersion value is the same without changing other settings. It is possible to obtain a dispersion pre-equalized optical transmitter corresponding to transmission line chromatic dispersion having different codes. Thus, it is possible to perform positive / negative distributed pre-equalization without holding a large amount of data as conventionally required.

  At this time, the equalization parameter setting means 19 only needs to hold only data necessary for compensation of positive dispersion values, for example, compared to holding all the individual data corresponding to positive and negative dispersion values. Thus, the necessary distributed pre-equalization operation can be performed with a data amount of approximately ½.

  Also, if you want to vary the dispersion pre-equalization amount at the dispersion pre-equalization optical transmitter, keep only the data corresponding to the absolute value of the dispersion pre-equalization amount as individual data, and perform the necessary pre-equalization It can be said that dispersion pre-equalization of arbitrary positive and negative values can be realized by interlocking the setting of the pre-equalization calculation data corresponding to the absolute value according to the amount and the setting of the optical phase. Not too long.

Embodiment 2. FIG.
FIG. 2 is a configuration diagram relating to the dispersion pre-equalized optical transmitter according to the second embodiment of the present invention. FIG. 2 is a diagram in which the dispersion pre-equalized optical transmitter in FIG. 1 is adapted to DQPSK modulation. The difference between the configuration in FIG. 1 and the configuration in FIG. Data switching means 21 for switching, DQPSK encoding means 22 for generating a DQPSK modulation data sequence from two series of data, and data switching control means 23 for controlling switching processing by the data switching means 21 are added. is there. The other configuration is the same as the configuration of FIG. In the present embodiment, as shown in FIG. 2, the distributed pre-equalization data generator 17, the DQPSK encoding unit 22, and the data switching unit 21 constitute a transmission data generation unit.

  Next, the operation of the dispersion pre-equalized optical transmitter according to the second embodiment configured as described above will be described. Compared with the first embodiment shown in the configuration of FIG. 1, the operations of the optical modulation means 20, the dispersion pre-equalization data generation means 17, and the optical phase control means 16 are the same. Incidentally, as the internal configuration of the distributed pre-equalized data generating means 17, it is possible to realize the distributed pre-equalized data generation by performing complex operations on the two output series signals of the DQPSK encoding means 22, respectively.

  In the following, in explaining the operation principle of FIG. 2, first, the relationship between two series of tributaries of transmission source data to be input to the DQPSK encoding means and the optical phase will be explained.

  When the dispersion pre-equalization amount is zero in the nest type optical modulator (optical modulation means 20) represented by FIG. 2, the configuration is the same as that of a normal DQPSK optical transmitter (for example, Japanese translations of PCT publication No. 2004-516743). Can be considered. Here, the DQPSK encoding operation for two series of input data can be expressed by the following logical expressions (8) and (9) (for example, Y. Konishi, et al., “Electrical domain compensation of optical dispersion”). ”, OFC / NFOEC2006, OThR3, Anaheim, CA, Mar. 2006).

Here, (b _I , b _Q ) is the original two-sequence data for transmission, and (d _I , d _Q ) is the two-sequence data after encoding for DQPSK (I-channel, Q-channel). This is drive signal data for the light modulation means 20. Here, in the equation (8), when b _I and b _Q are interchanged, the following equation (10) is obtained. From a comparison with Expression (9) and Expression (10), it can be seen that the logical expression when b _I and b _Q are interchanged is equivalent to the expression where d _I and d _Q are interchanged.

Here, when the output optical electric field of the nest type optical modulator is considered, assuming that the basic coordinate axis of the output optical signal is the one shown in FIG. 2 phase advance state), the output optical electric field obtained as a result of optical modulation using the d _I and d _Q signals generated by the exchange of b _I and b _Q data is d _I and d _Q with respect to the original axis. Therefore, it is equivalent to the coordinate conversion shown in FIG. 3B, and the I axis is in a state in which the π / 2 phase is advanced with respect to the Q axis. This is equivalent to shifting the π / 2 phase shift by the optical phase adjustment unit 13 of the nest type optical modulator (light modulation means 20) to −π / 2 phase shift. In this way, the original two-sequence data related to DQPSK modulation is exchanged, or the drive signals of the Mach-Zehnder (MZ: Mach-Zehnder) type optical modulation units 12a and 12b are exchanged to exchange the I axis and the Q axis themselves, It can be seen that the operations of changing the phase shift amount from π / 2 to −π / 2 in the optical phase adjustment unit 13 of the nested optical modulator (optical modulation means 20) are equivalent to each other. Therefore, when the π / 2 shift in the optical phase adjustment unit 13 of the nested optical modulator (optical modulation means 20) is changed to −π / 2 shift, the Mach-Zehnder (MZ) optical modulation of each sub is performed. Even if the drive electric signals of the units 12a and 12b are the same, the combination of the two series data signals at the time of reception is switched from (b _I , b _Q ) to (b _Q , b _I ).

  Next, considering the case where dispersion pre-equalization is performed in FIG. 2, as in FIG. 1, changing the setting of the optical phase adjustment unit 13 from π / 2 to −π / 2 is the amount of pre-equalization dispersion. The effect of inverting the sign also occurs. In the configuration of FIG. 2, the setting of the optical phase adjustment unit 13 is switched by the optical phase setting switching unit 18, and two series of data are switched in conjunction with the data switching control unit 23 and the data switching unit 21. Even when DQPSK modulation is used, the sign of the distributed pre-equalization amount can be inverted without changing the two series of tributaries at the time of transmission.

  For example, in a state where dispersion pre-equalization is performed with respect to a positive chromatic dispersion value in a state of (tributary: normal) + (optical phase: + π / 2), (tributary: inversion) + (optical phase: -Π / 2) switching is performed without changing the operation contents of the DQPSK encoding unit 22 and the distributed pre-equalized data generation unit 17 while maintaining the tributary relationship of the two series data. Only the sign of the pre-equalization amount can be inverted (similarly, only the tributary can be inverted without the variation of the sign of the distributed pre-equalization amount). As a result, dispersion pre-equalization for positive and negative dispersion values can be performed without performing calculation for each sign of each dispersion value, and without causing inversion of tributary on the receiving side, and the data switching control means 23 and the light. This can be realized only by controlling the phase setting switching means 18. As described above, also in the second embodiment, as in the first embodiment, other settings are changed by switching the setting of the optical phase adjustment unit 13 to two points of π / 2 and −π / 2. Therefore, it is possible to obtain a dispersion pre-equalization optical transmitter that supports transmission line chromatic dispersion having the same absolute value and different signs of dispersion values. Thus, it is possible to perform positive / negative distributed pre-equalization without holding a large amount of data as conventionally required.

  In FIG. 2, the configuration in which the two-sequence data is switched in the preceding stage of the DQPSK encoding unit 22 has been described. However, the two-sequence data is switched in the final Mach-Zehnder (MZ) optical modulation. Considering that this corresponds to switching of the I-channel and Q-channel of the drive signals of the units 12a and 12b, for example, as shown in FIG. 4, the interstage between the DQPSK encoding means 22 and the distributed pre-equalized data generation means 17 The data switching unit 21 may perform data switching, or the data switching unit 17 may perform data switching after the distributed pre-equalization data generation unit 17 as shown in FIG. Needless to say. Furthermore, it goes without saying that an arbitrary transmitted optical field can be generated by independently switching the tributary and the optical phase setting in FIG. 2 for the purpose of test adjustment and the like.

  The optical phase control means 16 uses the switching signal from the optical phase setting switching means 18 to change the relative positions of the optical electric fields output from the two Mach-Zehnder (MZ) modulators 12a and 12b. The shift amount of the phase difference is set to any one of mπ ± (π / 2) (where m is an integer), and the setting in the optical phase setting switching unit 18 is linked with the setting in the data switching control unit 23. It may be performed or may be performed independently.

  The switching by the data switching control means 23 and the setting in the optical phase adjustment unit 13 may be switched based on a signal from the receiving end.

Embodiment 3 FIG.
FIG. 6 is a configuration diagram of the dispersion pre-equalized optical transmitter according to the third embodiment. In FIG. 6, 40a and 40b are dispersion pre-equalization optical transmitters, 41a and 41b are optical receivers, 42a and 42b are optical transceivers, 43 is a control line, and 15a and 15b are optical fiber transmission lines.

  The operation of the dispersion pre-equalized optical transmitter of the third embodiment configured as described above will be described. The dispersion pre-equalization optical transmitters 40a and 40b in FIG. 6 are the optical modulation means 20, the transmission data generation means 24, the optical phase control means 16, and the light in the dispersion pre-equalization optical transmitter shown in FIGS. This is an optical transmission unit including phase setting switching means 18, data switching control means 23, and the like. FIG. 7 is a diagram illustrating an example of the configuration of the optical receivers 41a and 41b including, for example, a DQPSK receiving circuit. In FIG. 7, 50a and 50b are 1-bit delay interferometers, 51a and 51b are differential optical receivers, and 52 is a data identification means.

  The optical receiver circuit shown in FIG. 7 performs the same operation as the optical receiver for DQPSK disclosed in, for example, JP-T-2004-516743. However, in general, whether the output of the data identification means 52 is two series is the set phase of the 1-bit delay interferometers 50a and 50b (setting of ± π / 4 phase shift in FIG. 7) or the above-mentioned Since it changes depending on the relative phase of the I-channel and Q-channel at the time of transmission, the receiving circuit alone is undefined. Therefore, a function for performing tributary switching of two series of data by the data switching means 21 on the receiving side is necessary.

Here, for the optical signal transmitted from the dispersion pre-equalization optical transmitter in FIG. 6, the tributary switching of the post-identification data by the optical receiver and the phase adjustment by the 1-bit delay interferometers 50a and 50b are performed. Assume that the two series of (b _I , b _Q ) are not fixed even after all combinations of the two series data have been tried by Tari switching. In this case, as a factor that the tributary is not determined only by the combination of reception, for example, the optical waveform after transmission through the optical fiber because the tributary is correct due to an error in setting the optical phase of transmission but the sign of the dispersion pre-equalization amount is reversed. It can be considered that the device itself is greatly deteriorated. In such a case, an instruction to change each setting of the optical phase setting switching means 18 and the data switching control means 23 is issued from the receiving side to the transmitting end through the control line 43, and the tributary setting and distributed pre-equalization on the transmitting side are performed. By trying combinations of codes, optical signal communication between transmission and reception can be more reliably established even when distributed pre-equalization is performed.

  In addition, for the purpose of performing an optimal amount of dispersion pre-equalization with respect to the chromatic dispersion of the transmission path, even when searching for the received signal quality to the optimum state while varying the dispersion pre-equalization amount on the transmission side. By feeding back the received signal quality state to the transmission side through the control line 43, it is possible to perform optimum transceiver settings as an optical communication system.

  Even when the variance pre-equalization amount is variable, as described above, since it is not necessary to hold all the parameters corresponding to the positive / negative distributed pre-equalization amounts, the amount of retained data is small. A wider range of dispersion pre-equalization amounts can be searched.

  If the optical communication system using the dispersion pre-equalization transmitters according to the first to third embodiments described above is constructed, a large amount can be obtained by switching the control point of the optical phase adjustment according to the sign of the pre-equalization dispersion amount. Needless to say, it is possible to perform positive / negative distributed pre-equalization even if the data is not held.

It is the block diagram which showed the structure of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 1 of this invention. It is the block diagram which showed the structure of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 2 of this invention. It is operation | movement explanatory drawing of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 2 of this invention. It is a modification in the structure of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 2 of this invention. It is a modification in the structure of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 2 of this invention. It is the block diagram which showed the structure of the dispersion | distribution pre-equalization optical transmitter based on Embodiment 3 of this invention. It is the block diagram which showed an example of the structure of the optical receiver for DQPSK in Embodiment 3 of this invention. It is the block diagram which showed the structure of the conventional dispersion | distribution pre-equalization optical transmitter.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Light source, 11 Optical demultiplexing means, 12a, 12b MZ type | mold optical modulation part, 13 Optical phase adjustment part, 14 Optical multiplexing means, 15, 15a, 15b Optical fiber transmission line, 16 Optical phase control means, 17 Dispersion pre-equalization Data generation means, 18 optical phase setting switching means, 19 equalization parameter setting means, 20 optical modulation means, 21 data switching means, 22 DQPSK encoding means, 23 data switching control means, 24 transmission data generation means, 30 distributed inverse function Calculation means, 31a, 31b Modulator inverse function calculation means, 32a, 32b D / A converter, 33a, 33b amplifier, 40a, 40b Distributed pre-equalization optical transmitter, 41a, 41b optical receiver, 42a, 42b optical transceiver 43 Control line, 50a, 50b 1-bit delay interferometer, 51a, 51b Differential receiver, 52 Data identifier Step.

Claims (13)

Is input source data sequence, for the source data sequence performs inverse function calculation of the transfer function of the inverse function operation and the light modulating means having a wavelength dispersion of the transmission path the source data sequence is transmitted, thereby a force distributing pre-equalization data generating means out of the data of the obtained two series,
Two Mach-Zehnder type optical modulation units provided in parallel; and an optical phase adjustment unit connected to one of the Mach-Zehnder type optical modulation units. Two series of data from the equalized data generating means are input one by one, respectively, thereby driving each of the two Mach-Zehnder light modulators, and the two Mach-Zehnder light modulators are light sources The first and second optical electric fields that are orthogonal to each other are generated by modulating the demultiplexed light input from the optical demultiplexing means from the optical demultiplexing means, and the optical phase adjusting unit is set according to the set value adjusting the relative phase difference between the first and second light field, and the light modulation means,
Control means for performing switching control of a set value for the relative phase difference used in the adjustment by the light modulation means, and thereby changing the sign of the dispersion pre-equalization amount. Optical transmitter. The transmission source data series is composed of two series of data,
Coding means for generating a data sequence for DQPSK modulation from the two series of data is provided in a preceding stage of the distributed pre-equalization data generating means,
The distributed pre-equalization optical transmitter according to claim 1, wherein tributary switching is performed by an encoding operation by the encoding unit.   The tributary switching by the encoding means and the switching of the relative phase difference between the first and second optical electric fields by the control means are such that there is no change in tributary and the sign of the dispersion pre-equalization amount is inverted. The dispersion pre-equalized optical transmitter according to claim 2, wherein the dispersion pre-equalized optical transmitter is controlled.   The tributary switching by the encoding means and the switching of the relative phase difference between the first and second optical electric fields by the control means are such that there is no change in the sign of the dispersion pre-equalization amount and the tributary is inverted. The dispersion pre-equalized optical transmitter according to claim 2, wherein the dispersion pre-equalized optical transmitter is controlled. Data switching means for switching the two series of data output from the distributed pre-equalized data generating means;
Data switching control means for controlling switching by the data switching means, and
The optical modulation means is composed of a nested optical modulator having the two Mach-Zehnder optical modulators and the optical phase adjuster.
The control means includes an optical phase setting switching means for outputting a switching signal for switching the set value of the optical phase adjusting section, and a shift amount of the optical phase adjusting section according to the switching signal from the optical phase setting switching means. It possesses an optical phase control means for switching,
5. The distribution according to claim 2 , wherein the data switching unit is arranged in a preceding stage of the encoding unit, and switches the two series of data input to the encoding unit. Pre-equalized optical transmitter. Data switching means for switching the two series of data output from the distributed pre-equalized data generating means;
Data switching control means for controlling switching by the data switching means;
Further comprising
The optical modulation means is composed of a nested optical modulator having the two Mach-Zehnder optical modulators and the optical phase adjuster.
The control means includes an optical phase setting switching means for outputting a switching signal for switching the set value of the optical phase adjusting section, and a shift amount of the optical phase adjusting section according to the switching signal from the optical phase setting switching means. Optical phase control means for switching,
The data switching means is arranged after the encoding means and before the distributed pre-equalized data generation means, and switches the data series for DQPSK modulation output from the encoding means. The dispersion pre-equalized optical transmitter according to any one of claims 2 to 4 . Data switching means for switching the two series of data output from the distributed pre-equalized data generating means;
Data switching control means for controlling switching by the data switching means;
Further comprising
The optical modulation means is composed of a nested optical modulator having the two Mach-Zehnder optical modulators and the optical phase adjuster.
The control means includes an optical phase setting switching means for outputting a switching signal for switching the set value of the optical phase adjusting section, and a shift amount of the optical phase adjusting section according to the switching signal from the optical phase setting switching means. Optical phase control means for switching,
The data switching unit is arranged at a subsequent stage of the distributed pre-equalization data generation unit, and switches the drive signal to the optical modulation unit output from the distributed pre-equalization data generation unit. 5. The dispersion pre-equalized optical transmitter according to any one of 2 to 4 . The optical phase control means is a phase of an optical electric field that is a set value with respect to a relative phase difference between the optical electric fields output from the two Mach-Zehnder optical modulators in response to the switching signal from the optical phase setting switching means. The shift amount can be set to any one of mπ ± (π / 2) (where m is an integer), and the setting in the optical phase setting switching unit is performed in conjunction with the setting of the data switching control unit. The dispersion pre-equalized optical transmitter according to any one of claims 5 to 7 , characterized in that: The optical phase control means is a phase of an optical electric field that is a set value with respect to a relative phase difference between the optical electric fields output from the two Mach-Zehnder optical modulators in response to the switching signal from the optical phase setting switching means. The shift amount can be set to any one of mπ ± (π / 2) (where m is an integer), and the setting in the optical phase setting switching unit is performed independently of the setting of the data switching control unit The dispersion pre-equalized optical transmitter according to any one of claims 5 to 7 . The dispersion pre-equalization data generation means is a dispersion inverse function calculation means for performing an inverse function calculation by a convolution calculation of the inverse function of the real part and the imaginary part of the transfer function of transmission line chromatic dispersion and the input two series of data. distributed pre-equalization optical transmitter according to any one of claims 1 to 9, characterized in that it has a. Distributed pre-equalization light according to any one of claims 5 to 10, characterized in that switch based on a signal from the receiving end a setting in the data switching control switching between the optical phase adjusting section by means Transmitter. In order to set an optimum dispersion pre-equalization amount for the transmission line chromatic dispersion, when the dispersion pre-equalization amount is varied and the sign of the dispersion value is inverted, the data switching control means distributed pre-equalization optical transmitter according to any one of claims 5 to 11, characterized in that the corresponding by switching the setting in the switching and the optical phase adjusting section. An optical communication system using the dispersion pre-equalized optical transmitter according to any one of claims 1 to 12 .

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