JP2008211493A - Distributed pre-equalization optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a distributed pre-equalization optical communication system capable of more efficiently compensating the distortion of a wavelength distribution in a transmission line after suppressing the increase in an operational amount for generating distributed pre-equalization data. <P>SOLUTION: The distributed pre-equalization optical communication system includes: a distributed pre-equalization data generating means 12 for generating the reverse function operational result of the wavelength distribution in the transmission line as the distributed pre-equalization data, on the basis of transmission source data series; and an optical modulation means 11 for performing data modulation in an optical signal which is transmitted from an optical transmission means 10, and for transmitting the modulated optical signal to the transmission line 13. The distributed pre-equalization data generating means 12 generates the distributed pre-equalization data by calculation without considering RZ conversion. The optical modulation means 11 generates the modulated optical signal, thereby performing RZ pulse conversion. The system also includes an optical filter means 15 arranged at the reception side of the modulated optical signal in the transmission line 13 and extracting the optical signal close to the carrier frequency of signal light spectrum from the modulated optical signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical communication system, and more particularly, to a dispersion pre-equalization optical communication system that more efficiently compensates for chromatic dispersion distortion of a transmission line.

  In a long-distance optical communication system, a technique for more efficiently compensating for chromatic dispersion of a transmission line is important in order to easily reduce the cost of the system and easily upgrade the system 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.

  In addition, although the chromatic dispersion tolerance varies depending on the transmission rate and modulation format, in either transmission system, the transmission rate can be reduced by using no dispersion compensation fiber or using only a few types of dispersion compensation fiber. It is possible to adopt a transmission path configuration that does not depend on the modulation format. As a result, it is easy to upgrade an existing system and realize a system in which a plurality of modulation methods are mixed.

  As one measure for obtaining the above effects, dispersion pre-equalization transmission that transmits an optical signal in which the effect of chromatic dispersion having the same absolute value and the opposite sign is added in advance with respect to the chromatic dispersion of the transmission line The system is actively studied (for example, see Non-Patent Documents 1 and 2 and Patent Document 1).

  FIG. 2 is a schematic block diagram of a conventional distributed pre-equalization optical communication system. The dispersion pre-equalization optical transmitter of FIG. 2 includes a CW light source 20, an optical modulation unit 21, a dispersion pre-equalization data generation unit 22, an optical fiber transmission line 23, and an optical receiver 24 (for example, non-patent). Reference 1).

  In addition, graphs (a) to (e) shown in FIG. 2 show optical signals or optical spectra of respective parts when the distributed pre-equalization transmission (symbol rate fs) for the RZ pulse is configured. More specifically, (a) is an output signal of the CW light source 20, (b) is an output signal of the light modulation means 21, (c) is an optical spectrum of the output signal of the light modulation means 21, and (d) is an optical fiber. The received optical signal received by the optical receiver 24 via the transmission line 23, and (e) show schematic images of the optical spectrum of the received optical signal.

  Next, a conventional distributed pre-equalization optical communication system will be described with reference to FIG. The dispersion pre-equalization data generation means 22 in the conventional dispersion pre-equalization optical communication system performs inverse function calculation of the chromatic dispersion of the optical fiber transmission line 23 and inverse function calculation of the optical modulator transfer function for the transmission source data series. I do. Then, the light modulation means 21 uses the two series of data (I-ch, Q-ch) obtained from the dispersion pre-equalization data generation means 22 to perform dispersion pre-equalization on the output CW light of the CW light source 20. Modulate for

  As the light modulation means 21, an I / Q modulator is generally used as described in Non-Patent Document 1. At the input of the optical receiver 24, the dispersion pre-equalization and the chromatic dispersion of the optical fiber transmission line 23 cancel each other, thereby obtaining an optical signal in which distortion due to chromatic dispersion is suppressed.

  At this time, as the inverse function calculation of the chromatic dispersion of the optical fiber transmission line 23, the chromatic dispersion of the transmission line has the same absolute value as that of the transmission line, and the chromatic dispersion transfer function having the opposite sign is multiplied by the transmission data series, or In general, a method of obtaining a complex data sequence by convolution of an impulse response obtained from the transfer function and a transmission data sequence is common.

  Although not shown in FIG. 2, generally, the response characteristic of electrical / optical conversion is not linear, and therefore, the response characteristic of electrical / optical conversion is used to generate an ideal optical signal. An operation for correcting is added. Further, when various operations as described above are performed by digital operation processing, it is general to obtain an analog signal for driving the light modulation means 21 through a D / A converter and an amplifier.

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, 400, 2006 . P. J. Winzer, et al., “Electronic pre-distortion for advanced modulation formats,” ECOC2005, Tu4.2.2, Glasgow, UK, Sep. 2005. JP 2006-522508 A

However, the prior art has the following problems.
In Non-Patent Document 1, dispersion pre-equalized optical transmission is performed based on a modulated signal (RZ-DPSK) based on an RZ pulse. Therefore, the distributed pre-equalization data generation means 22 performs an operation in consideration of RZ on the NRZ data sequence that is a general transmission source data sequence to thereby obtain two sequences of data (I-ch, Q-ch). ) To drive the light modulation means 21.

  In general, when an optical signal is converted to RZ, the optical spectrum width is expanded as compared with the NRZ optical signal, and therefore the chromatic dispersion tolerance is deteriorated. As a result, the RZ dispersion pre-equalization data generation means 22 has a larger calculation scale than the NRZ optical signal pre-equalization for the same chromatic dispersion value (see, for example, Non-Patent Document 2).

  That is, in the case of performing dispersion pre-equalization on the modulation signal based on the RZ pulse, the dispersion pre-equalization data generation means 22 is compared with the case of performing dispersion pre-equalization on the modulation signal based on the NRZ data. There is a problem that the amount of computation increases.

  The present invention has been made to solve the above-described problems, and more effectively reduces distortion of chromatic dispersion in a transmission line while suppressing an increase in the amount of calculation for generating dispersion pre-equalization data. An object of the present invention is to obtain a dispersion pre-equalized optical communication system that can compensate for the above.

  The dispersion pre-equalization optical communication system according to the present invention includes a dispersion pre-equalization data generation means for generating an inverse function calculation result of chromatic dispersion of a transmission line as dispersion pre-equalization data based on a transmission source data sequence, Based on the optical transmission means that transmits the base optical signal and the generated dispersion pre-equalization data, the optical signal transmitted from the optical transmission means is subjected to data modulation, and the modulated optical signal is transmitted to the transmission line. In the dispersion pre-equalization optical communication system including the optical modulation means, the dispersion pre-equalization data generation means generates dispersion pre-equalization data by an operation not considering RZ conversion, and the optical modulation means is in the transmission path An optical signal after modulation is generated so that the signal to be transmitted to RZ is converted into an RZ pulse, provided on the receiving side of the optical signal after modulation on the transmission path, and the carrier frequency (f0 Hz) of the signal light spectrum from the optical signal after modulation ) Neighborhood In which further comprising an optical filter for extracting an optical signal.

  According to the present invention, based on the dispersion pre-equalization data generated for NRZ, modulation processing is performed so that the modulated optical signal transmitted to the transmission line becomes the optical signal of the RZ pulse train, and the transmission line is Dispersion pre-equalization data generation by extracting only optical signals of desired frequency components with reduced distortion of chromatic dispersion in the transmission line from the optical signals after modulation using optical filter means Therefore, it is possible to obtain a dispersion pre-equalized optical communication system that can more efficiently compensate for distortion of chromatic dispersion of a transmission line while suppressing an increase in the amount of computation for the purpose.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of a distributed pre-equalization optical communication system of the invention will be described with reference to the drawings.
The dispersion pre-equalization optical communication system according to the present invention reduces the amount of calculation by calculating dispersion pre-equalization data by performing arithmetic processing on a narrow optical spectrum width without considering RZ. In addition to making the optical signal after modulation transmitted to the transmission line into a pulse shape and extracting the optical signal in the vicinity of the carrier frequency of the signal light spectrum with an optical filter, the transmission path is resistant to interference. As a technical feature, it is possible to more efficiently compensate for distortion of chromatic dispersion possessed by.

Embodiment 1 FIG.
FIG. 1 is a configuration diagram of a distributed pre-equalization optical communication system according to Embodiment 1 of the present invention. The dispersion pre-equalization optical communication system shown in FIG. 1 includes a pulse light transmission unit 10, an optical modulation unit 11, a dispersion pre-equalization data generation unit 12, an optical fiber transmission line 13, an optical receiver 14, and an optical filter unit 15. Is done.

  Also, the graphs (a) to (e) shown in FIG. 1 show the light of each part when configuring dispersion pre-equalization transmission (symbol rate fs, optical pulse repetition frequency 2 fs) using the pulsed light transmission means 10. Signal. More specifically, (a) is the output signal of the pulsed light transmission means, (b) is the output signal of the optical modulation means, (c) is the optical spectrum of the output signal of the optical modulation means, (d) is the received optical signal, (e ) Shows a schematic image of the optical spectrum of the received optical signal.

  Next, the operation of the distributed pre-equalization optical communication system according to the first embodiment having such a configuration will be described. Compared with the conventional dispersion pre-equalization optical communication system in FIG. 2, the dispersion pre-equalization optical communication system in FIG. 1 uses a pulsed light transmitting means 10 instead of the CW light source 20 and is newly added. The difference is that an optical filter means 15 is provided.

  Further, the distributed pre-equalization data generation means 12 in the distributed pre-equalization optical communication system of the first embodiment is different in operation from the distributed pre-equalization data generation means 22 in the conventional distributed pre-equalization optical communication system. . Specifically, the distributed pre-equalization data generation means 22 in FIG. 2 performs a distributed pre-equalization process corresponding to the RZ optical pulse on the transmission source data series, whereas the distributed pre-equalization process in FIG. The difference data generation means 12 is different in that the dispersion pre-equalization processing corresponding to the NRZ optical pulse is performed without considering the RZ conversion.

  The optical modulation means 11 in FIG. 1 and the optical modulation means 21 in FIG. 2 have the same function, and the optical receiver 14 in FIG. 1 and the optical receiver 24 in FIG. 2 have the same function. Have. The pulsed light transmission unit 10 corresponds to an optical transmission unit, and the optical fiber transmission line 13 corresponds to a transmission line.

  The optical modulation means 11 in FIG. 1 is based on the two-series data (I-ch, Q-ch) subjected to the dispersion pre-equalization process for the NRZ optical pulse by the dispersion pre-equalization data generation means 12. The input optical signal from the transmission means 10 is modulated. As described above, the distributed pre-equalization data generation unit 12 suppresses an increase in the amount of calculation due to the RZ conversion by not performing the calculation considering the RZ conversion.

  For this reason, when the input optical signal is the output signal of the CW light source 20 as shown in the graph (a) of FIG. 2, the output of the optical modulation means 11 is used as the dispersion pre-equalized optical signal for NRZ. Will be output.

  On the other hand, in the example of FIG. 1 in the first embodiment, as shown in the graph (a) in FIG. 1, a signal having a pulse train output by the pulsed light transmitting means 10 is used as an input optical signal. Input to the light modulation means 11. As a result, the output of the light modulation means 11 becomes an RZ pulse train as shown in the graph (b) in FIG. 1 and is transmitted as an RZ pulse in the optical fiber transmission line 13, which increases the intersymbol interference tolerance. Strong signal.

  At this time, the distributed pre-equalization by the distributed pre-equalization data generation means 12 in FIG. 1 is calculated and optimized for an NRZ data sequence that is a general transmission source data sequence. For this reason, the effect of the dispersion pre-equalization after the transmission through the optical fiber transmission line 13 is that of the spectral component shown by the graph (c) in FIG. 1 near the center wavelength (f0) as the carrier. Only the baseband components within the approximate f0-fs to f0 + fs.

  Therefore, on the optical fiber transmission line 13 connecting the optical modulation means 11 and the optical receiver 14, the optical filter means 15 is provided upstream of the optical receiver 14 or downstream of the optical fiber transmission line 13 subject to dispersion pre-equalization. By arranging, it is possible to extract only signal components in which dispersion pre-equalization and chromatic dispersion of the optical fiber transmission line 13 cancel each other in the vicinity of the carrier wavelength. As a result, it is possible to obtain an optical signal (see graphs (d) and (e) in FIG. 1) in which waveform distortion due to wavelength dispersion in the optical fiber transmission line 13 is suppressed.

  As described above, according to the first embodiment, by providing the pulse light transmission unit and the optical filter unit, the dispersion pre-equalization data generation unit transmits the optical fiber only by performing the calculation for NRZ. As an optical signal, an RZ pulse can be transmitted. As a result, it is possible to easily obtain a feature that the RZ pulse transmission has excellent intersymbol interference tolerance.

  Furthermore, since the distributed pre-equalization data generation means only needs to perform the calculation for NRZ, it can easily realize pulsed transmission without increasing the calculation scale due to RZ pulsing. Furthermore, the optical filter means can extract only signal components in which dispersion pre-equalization and chromatic dispersion of the optical fiber transmission line 13 cancel each other in the vicinity of the carrier wavelength, and an optical signal in which distortion due to chromatic dispersion is suppressed. Can be obtained.

  In FIG. 1, the output light pulse from the pulse light transmitting means 10 has a repetition frequency that is twice the symbol rate, but it goes without saying that the same effect can be obtained if the frequency is twice or more.

  Further, the output optical pulse from the pulsed light transmitting means 10 is at least twice the symbol rate and an integral multiple of the symbol rate, so that the electric drive circuit such as the reference clock signal of the electric pulse can be shared. Can do.

  Furthermore, by setting a higher symbol rate, the interval between the carrier and the sideband can be widened, and the optical spectrum of FIG. 1E can be easily converted from the optical spectrum of FIG. Can be extracted.

  In the above-described embodiment, the optical spectrum extraction range in the optical filter unit 15 has been described as f0−fs to f0 + fs. However, when adjacent sidebands are sufficiently separated from each other, they are affected by the sidebands. Filtering over a wider frequency range may be performed to such an extent as possible.

  Furthermore, in the extraction of the data signal component to be transmitted, for example, the NRZ code only needs to cover the range of f0−fs / 2 to f0 + fs / 2. There is no problem even if the light spectrum is extracted in a narrow range.

  As the pulsed light transmitting means 10, a normal CW light source may be driven by an electric pulse signal corresponding to the output pulsed light signal (corresponding to the graph (a) in FIG. 1), or CW light source + external A system in which the external modulator is driven by an electric pulse signal may be adopted in the configuration of the modulator.

  In addition, in the above description, the NRZ code is assumed as the data modulation. However, even in the case of DPSK (Differential Phase Shift Keying) modulation or DQPSK (Differential Quadrature Phase Keying) modulation based on phase modulation. Needless to say, the distributed pre-equalization processing of the present embodiment can be similarly applied.

It is a block diagram of the dispersion | distribution pre-equalization optical communication system in Embodiment 1 of this invention. It is a schematic block diagram of the conventional dispersion | distribution pre-equalization optical communication system.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Pulse light transmission means (light transmission means), 11 Optical modulation means, 12 Dispersion pre-equalization data generation means, 13 Optical fiber transmission line (transmission path), 14 Optical receiver, 15 Optical filter means

Claims (11)

Dispersion pre-equalization data generation means for generating an inverse function calculation result of chromatic dispersion of the transmission line as dispersion pre-equalization data based on the transmission source data series;
An optical transmission means for transmitting an optical signal as a base of modulation;
Dispersion means comprising: optical modulation means for performing data modulation on the optical signal transmitted from the optical transmission means based on the generated dispersion pre-equalization data and transmitting the modulated optical signal to the transmission path In the pre-equalized optical communication system,
The distributed pre-equalization data generating means generates the distributed pre-equalization data by an operation not considering RZ conversion,
The optical modulation means generates the modulated optical signal so that the signal to be transmitted in the transmission path is RZ-pulsed,
An optical filter means provided on the receiving side of the modulated optical signal on the transmission line, further extracting an optical signal in the vicinity of the carrier frequency (f0 Hz) of the signal light spectrum from the modulated optical signal. A distributed pre-equalization optical communication system. The distributed pre-equalized optical communication system according to claim 1,
The distributed pre-equalization optical communication system, wherein the distributed pre-equalization data generation means uses an NRZ data sequence as the transmission source data sequence. The distributed pre-equalized optical communication system according to claim 1 or 2,
The optical transmission means repeatedly generates an optical pulse signal as an optical signal to be transmitted,
The optical modulation unit performs data modulation on the repetitive optical pulse signal transmitted from the optical transmission unit based on the dispersion pre-equalization data, and generates an optical signal after modulation that is RZ-pulsed. A distributed pre-equalized optical communication system. The distributed pre-equalized optical communication system according to claim 3,
The optical transmission means sets the repetition period of the repetitive optical pulse signal to at least twice the symbol rate (fsHz) of the modulated optical signal transmitted through the transmission path. Optical communication system. The distributed pre-equalized optical communication system according to claim 3,
The optical transmission means sets the repetition period of the repetitive optical pulse signal to be not less than twice the symbol rate (fsHz) of the modulated optical signal transmitted through the transmission path and an integral multiple of the symbol rate. A distributed pre-equalized optical communication system. The dispersion pre-equalized optical communication system according to any one of claims 3 to 5,
The optical transmission means has an unmodulated CW light source and an electric pulse signal source, and drives the CW light from the unmodulated CW light source with an electric pulse signal from the electric pulse signal source, thereby repeating the repetitive optical pulse. A distributed pre-equalized optical communication system characterized by generating a signal. The dispersion pre-equalized optical communication system according to any one of claims 1 to 6,
The optical filter means is within a range in which the filter pass band is not affected by the adjacent sidebands but does not exceed approximately f0−m × fs (Hz) to f0 + m × fs (Hz) (where m is an integer of 1 or more). And only a signal band in which the effect of dispersion pre-equalization and chromatic dispersion of the transmission line cancel each other is extracted as a main component. The dispersion pre-equalized optical communication system according to any one of claims 1 to 6,
The optical filter means has a pre-dispersion with a filter pass band in a range of approximately f0-fs (Hz) to f0 + fs (Hz) and in a range of approximately f0-fs / 2 (Hz) to f0 + fs / 2 (Hz) or more. A dispersion pre-equalization optical communication system characterized in that only a signal band in which the effect of equalization and chromatic dispersion of the transmission line cancel each other is extracted as a main component The dispersion pre-equalized optical communication system according to any one of claims 1 to 8,
The dispersion pre-equalization optical communication system, wherein the light modulation means performs the data modulation by a modulation method that gives information to light intensity. The dispersion pre-equalized optical communication system according to any one of claims 1 to 8,
The dispersion pre-equalization optical communication system, wherein the optical modulation means performs the data modulation by a modulation scheme that gives information to the phase of light. The dispersion pre-equalized optical communication system according to any one of claims 1 to 8,
The distributed pre-equalization optical communication system, wherein the optical modulation means performs the data modulation by NRZ modulation, DPSK modulation, or DQPSK modulation.

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