JP2010148007A - Optical communication system and method of generating modulation optical signal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical communication system and method of generating a modulation optical signal, wherein polarization-independent coherent detection can be performed at an optical receiver. <P>SOLUTION: An optical transmitter comprises: a wavelength sweep light source 11 which outputs an optical signal sweeping a predetermined wavelength range at a predetermined repeat frequency; a modulation signal generator 17 which outputs a modulation signal obtained by multiplexing data of wavelength channels on a time base; an optical modulator 13 for modulating the output optical signal of the wavelength sweep light source 11 with the modulation signal; an optical demultiplexer 13 for demultiplexing the modulated optical signal into two optical signals; an optical retarder 14 for giving time delay to one of the demultiplexed optical signals in such a way that a time difference of the two demultiplexed optical signals becomes a half value of an inverse of the predetermined repeat frequency; a polarization adjuster 15 for controlling a polarization state of the other demultiplexed optical signal in such a way that polarization directions of the two demultiplexed optical signals become orthogonal to each other; and an optical multiplexer 16 for multiplexing and sending out the two demultiplexed optical signals. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical communication system for transmitting a modulated optical signal of a plurality of wavelength channels and a method for generating a modulated optical signal of a plurality of wavelength channels.

  A method for generating modulated optical signals of a plurality of wavelengths using the conventional technique will be described with reference to FIG. This method mainly multiplexes and transmits optical signals of a plurality of wavelengths on a single optical transmission line, and realizes so-called wavelength multiplexing (expansion of transmission capacity, service lineup, and increase in the number of subscribers). It is assumed to be applied to an optical communication system called a WDM (Wavelength Division Multiplexing) system or a Frequency Division Multiplexing (FDM) system.

  The optical transmitter generates wavelength sweep light in which a predetermined wavelength range (N × B) is swept at a constant repetition period (1 / B). As a generation method, for example, as shown in FIG. 7, the signal current of the ramp waveform generated by the wavelength sweep signal generator 23 is appropriately adjusted by the wavelength sweep signal amplitude adjuster 24, and then the semiconductor laser light source 21. A configuration in which the voltage is directly applied to the electrodes can be considered. Here, the period 1 / B is set to match the symbol length of each wavelength channel. The wavelength sweep light is modulated with a signal obtained by time-multiplexing the transmission data of each wavelength channel of the modulation signal generator 25 in the optical modulator 22 with the transmission data corresponding to the wavelength bands of all the channels. Modulated. As a result, modulated optical signals having a plurality of wavelengths are collectively generated. The wavelength sweep light is modulated by the output signal of the modulation signal generator 17 in the optical modulator 12. In the optical receiver, the wavelength of the local light used for optical heterodyne detection is set in the vicinity of the wavelength channel (channel 1 in FIG. 7) to be selectively received, and the intermediate frequency signal output from the light receiver 28 is demodulated, The data of this wavelength channel is reproduced (see Non-Patent Document 1).

Usually, in a wavelength division multiplexing (WDM) system and a frequency division multiplexing (FDM) system, a light source and an optical modulator corresponding to the number of wavelengths are prepared, and the wavelength of each light source is determined in advance. Therefore, the configuration of the optical transmitter becomes complicated. On the other hand, according to the conventional technique, as described above, modulated optical signals of a plurality of wavelengths can be generated at once by using a single light source and an optical modulator, so that the configuration of the optical transmitter can be simplified.
Tomohiro Taniguchi, Kiyoshi Narukawa, Naoya Sakurai, Hideaki Kimura, Shin Tsubokawa, “Proposal of Shared Optical Transmitter Using Wavelength Sweep Light in Optical FDM Access System”, Proceedings of 2007 Society Conference of IEICE, 2007 August 29, B-10-64, p. 258 Tomohiro Taniguchi, Naoya Sakurai, Hideaki Kimura, Shin Tsubokawa, “Transmission Optical Spectrum Shaping for Suppression of Adjacent Channel Interference in Wavelength Sweep Optical FDM System”, Proceedings of 2008 IEICE General Conference, The Institute of Electronics, Information and Communication Engineers Published March 5, 2008, B-10-57, p.340 Shinji Matsuo, Takaaki Sakurazuka, Toru Segawa, Naoki Fujiwara, Yasuo Shibata, Hiroshi Yasaka, Hiroyuki Suzuki, “Transmitting Elements Using Frequency Modulated SSG-DBR Lasers and Wavelength Filters”, Proceedings of the 2007 IEICE Society Conference , The Institute of Electronics, Information and Communication Engineers August 29, 2007, C-4-14, p. 220 L.-S.Yan, C.Yeh, G.Yang, L.Lin, Z.Chen, YQShi, and X.Steve Yao, `` Fast digitally variable differential group delay module using polarization switching, '' Optical Fiber Communication Conference and Exhibit (OFC), 2002, FA5

  In the conventional technique, when coherent detection is performed by an optical receiver, the polarization state of the signal light received via the optical transmission path and the output light from the local light source installed in the optical receiver are matched. There is a need. For this purpose, a polarization tracking function is usually provided on the receiving side, and the polarization state of either the signal light or the local light is controlled so that the polarization states of the two optical signals always coincide with each other. However, the configuration of the optical receiver is complicated regardless of which method is used.

  The present invention has been made in such a background, and an optical communication system and modulated light that can solve the above-described problem of polarization dependence when applying coherent detection with an optical receiver having a simple configuration. An object of the present invention is to provide a signal generation method.

  To achieve the above object, an optical communication system according to the present invention includes an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and a predetermined number of optical signals transmitted from the optical transmitter via an optical transmission line. An optical communication system comprising an optical receiver for extracting and receiving an optical signal of a wavelength channel, wherein the optical transmitter outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency. A modulation signal generator that outputs a modulation signal obtained by multiplexing the data of each wavelength channel on the time axis, an optical modulator that modulates the optical signal output from the wavelength sweep light source with the modulation signal, and One of the optical branching devices that branches the modulated optical signal into two, and the time difference between the two optical signals branched by the optical branching device is half the reciprocal of a predetermined repetition frequency. For optical signal An optical delay device that gives a time delay, and a polarization adjuster that controls the polarization state of the other branched optical signal so that the polarization directions of the two optical signals branched by the optical splitter are orthogonal to each other And an optical multiplexer that combines the two optical signals that are given the time delay and whose polarization state is controlled, and sends the optical signal to the optical transmission line.

  The optical communication system of the present invention includes an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and an optical signal of a predetermined wavelength channel among optical signals transmitted from the optical transmitter via an optical transmission line. An optical communication system comprising an optical receiver that extracts and receives a wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency, and each wavelength channel A modulation signal generator that outputs a modulation signal obtained by multiplexing data on a time axis, an optical modulator that modulates an optical signal output from the wavelength sweep light source with the modulation signal, and the modulated optical signal And a polarization controller for controlling the polarization state to a linear polarization state having an angle difference of 45 degrees with respect to predetermined orthogonal planes of polarization, and a linear polarization state and orthogonal to each other Has polarization component With respect to the signal, causing the differential group delay of half the reciprocal of the predetermined repetition frequency, characterized in that it comprises a polarization dispersion generator for delivering the optical transmission path.

  The optical signal sweeping a predetermined wavelength range at the predetermined repetition frequency is preferably generated by applying a ramp waveform current having a predetermined repetition period to a direct modulation type semiconductor laser. The optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is preferably generated by applying a ramp-shaped signal having a predetermined repetition period to the phase adjustment region of the distributed reflection laser. . An optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is obtained by connecting a single-spectrum laser output to a phase modulator, and applying a parabolic waveform voltage having a predetermined repetition period to the phase modulator. It is preferably generated by inputting. An optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is connected to an external resonator at one reflection end of the laser, and a control signal is input to the external resonator to input an optical signal in the external resonator. It is preferably generated by changing the refractive index of the waveguide or changing the angle between the external resonator and the optical path by mechanical rotation. Further, the optical signal sweeping a predetermined wavelength range at the predetermined repetition frequency generates a pulsed light having a predetermined repetition period and a predetermined wavelength band, and has a group delay characteristic that gives a delay time proportional to the wavelength. It is preferable that it is produced | generated by letting pass.

  The modulated optical signal generation method of the present invention includes a step of generating an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency, and a modulated signal obtained by multiplexing the data of each wavelength channel on the time axis. A step of modulating the output optical signal with the modulation signal, a step of branching the modulated optical signal into two, and a time difference between the two branched optical signals is a predetermined repetition. The step of giving a time delay to one of the branched optical signals so that the half value of the reciprocal of the frequency is half, and the other of the branched optical signals so that the polarization directions of the two branched optical signals are orthogonal to each other The method includes a step of controlling a polarization state of an optical signal and a step of combining two optical signals which are given the time delay and whose polarization state is controlled.

  The modulated optical signal generation method of the present invention includes a step of generating an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency, and a modulated signal obtained by multiplexing the data of each wavelength channel on the time axis. A step of modulating the output optical signal with the modulation signal, and a polarization state of the modulated optical signal with an angle difference of 45 degrees with respect to a predetermined plane of polarization orthogonal to each other A step of controlling to have a linear polarization state, and a group delay time difference of half the reciprocal of the predetermined repetition frequency for an optical signal having polarization components orthogonal to each other in a linear polarization state. Including the step of generating.

  As described above, the optical communication system of the present invention uses a single light source and an optical modulator, applies a wavelength sweep signal to the light source, and modulates the frequency to provide a plurality of optical transmitters with a simple optical transmitter configuration. It is possible to generate an optical signal with a wavelength, and furthermore, in an optical receiver using coherent detection with a simple configuration in which the generated multi-wavelength optical signal is given a predetermined time difference and is orthogonally polarized and combined, polarization fluctuations Therefore, a large loss budget can be secured with an economical system configuration.

Embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
The optical communication system of the present invention extracts an optical signal of a predetermined wavelength channel from an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels and an optical signal transmitted from the optical transmitter via an optical transmission line. It is comprised by the optical receiver which receives. A first embodiment of an optical communication system of the present invention will be described with reference to FIGS. FIG. 1 shows the configuration of an optical transmitter according to the first embodiment of the present invention. The optical transmitter includes a wavelength sweep light source 11, an optical modulator 12, an optical branching device 13, an optical delay device 14, a polarization adjuster 15, and an optical multiplexer 16 as optical components. The wavelength sweep light 11 generates a wavelength sweep light that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition period (1 / B: equal to the symbol length of each wavelength channel). The optical modulator 12 modulates the modulated signal output from the modulation signal generator 17. As shown in FIG. 2, wavelength sweep light is easily obtained by applying a current having a ramp waveform with a period of 1 / B to a direct modulation type semiconductor laser that controls the output wavelength by changing the injection current of the laser. Can be generated. For example, a distributed feedback (DFB) laser that generates a single-spectrum optical signal is used as the laser light source, and wavelength sweep light is generated using a chirp phenomenon in which the oscillation wavelength of the laser changes according to the applied current. A way to do this is conceivable. Thereby, a wavelength sweep range of several tens GHz can be obtained with a repetition period of about several GHz. In this case, since the intensity fluctuation occurs simultaneously with the wavelength sweep, the semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier) operated in the gain saturation region in the subsequent stage of the laser light source is used as necessary. It is also useful to suppress the intensity fluctuation component (see Non-Patent Document 2).

  In addition, in order to generate a wavelength sweep light with a higher speed and a wider band, a predetermined repetition period (1 / B: equal to the symbol length of each wavelength channel) in a phase adjustment region of a distributed reflection (DBR) laser A method of applying a ramp-wave shape signal (wavelength sweep signal) having the above is also conceivable (see Non-Patent Document 3).

  In addition to the above method, wavelength sweep light can be generated by connecting the output of a single spectrum laser light source to a phase modulator and inputting a signal voltage having a parabolic waveform with a period of 1 / B to the phase modulator. . Further, an external resonator is connected to one reflection end of the laser, and a control signal is input to the external resonator to change the refractive index of the optical waveguide in the external resonator, or the external resonator is mechanically rotated. The wavelength of the output optical signal can be swept by changing the angle between the optical path and the optical path. A configuration using a pulse light source as shown in FIG. 3 is also conceivable. In this configuration, pulse light having a repetition period of 1 / B and a wavelength band N × B is generated and passed through a dispersion medium having a group delay characteristic that gives a delay time proportional to the wavelength as shown in the figure, thereby allowing the above wavelength to pass. Sweep light can be generated.

  Further, as shown in FIG. 4, the modulation signal used here is a data signal (B × N [b / s] obtained by multiplexing transmission data (B [b / s]) of each wavelength channel on the time axis. ]). The modulated optical signal generated in this way is branched into two outputs by the optical branching device 13 and input to the optical delay device 14 and the polarization adjuster 15, respectively. The time difference between these two optical signals is 1. After being adjusted so that the polarizations are orthogonal to each other at / 2B, they are multiplexed by the optical multiplexer 16 and sent to the optical transmission line. With this configuration, as shown in FIG. 1, it is an optical signal of a plurality of wavelengths, and is transmitted in a state including the same bit information in which the polarization is orthogonal within one symbol (symbol length 1 / B) in each wavelength channel. be able to. In FIG. 1, the polarization adjuster 15 is installed on a different side from the optical delay device 14, but may be installed on the optical delay device 14 side. Alternatively, the polarization adjuster 15 may be installed on both sides so that the polarizations of the two optical signals are orthogonal to each other.

In the optical receiver, the wavelength of the local light of the local light source 26 is set in the vicinity of a desired wavelength channel by the configuration shown in FIG. 7, and the optical multiplexer 27 combines the local light and the signal light, thereby The data of the desired wavelength channel is reproduced. At this time, the signal light is received in a state where the same bits are orthogonally polarized as described above without controlling the polarization of the local light. The output power of the intermediate frequency signal becomes a constant value as a time average at the symbol length 1 / B. Therefore, by appropriately setting the pass characteristic of the low-pass filter 30 installed at the output of the demodulator 29 and suppressing the power fluctuation within the symbol length 1 / B, the data can be obtained without depending on the polarization state of the signal light. Can be played.
In this way, in the present invention, by performing polarization diversity processing on the transmission side, that is, on the time axis, reception by polarization-independent coherent detection is performed without using particularly complicated components in the optical receiver. be able to.

(Second Embodiment)
A second embodiment of the optical communication system of the present invention will be described with reference to FIG. In the second embodiment, the configurations of the optical transmitter and the optical receiver are the same as those in the first embodiment, but as shown in FIG. 5, the optical branching device 13, the optical delay device 14, and the polarization It is characterized in that instead of the portion composed of the adjuster 15 and the optical multiplexer 16, a polarization adjuster 18 and a polarization dispersion generator 19 connected in cascade are used.

  The polarization dispersion generator 19 has a dispersion characteristic that causes a group delay time difference of 1/2 B with respect to optical signals having polarization components orthogonal to each other. Examples of components having such characteristics include a polarization maintaining fiber (PANDA) and a DGD (Differential Group Delay) generator (see Non-Patent Document 4).

  The polarization adjuster 18 changes the polarization state of the output optical signal of the optical modulator 12 at an angle of 45 degrees with respect to mutually orthogonal polarization planes in which a 1/2 B group delay time difference is generated in the polarization dispersion generator 19. Control is performed so that a linearly polarized state having a difference is obtained. As a result, as shown in FIG. 5, at the output of the polarization dispersion generator 19, as in the first embodiment, in each wavelength channel, the same polarized wave is orthogonal within one symbol (symbol length 1 / B). Since optical signals of a plurality of wavelengths including bit information can be transmitted, it is possible to perform reception by polarization-independent coherent detection without using particularly complicated components in the optical receiver.

  As a configuration example of an optical communication system to which the above-described embodiment is applied, for example, as shown in FIG. 6, an optical signal from an optical transmitter is received by a plurality of optical receivers via a single-core optical transmission path. Passive optical network in which a plurality of optical receivers are connected while sharing a part of the optical transmission line through a single star (SS) system connected to the optical device or an optical branching device installed in the middle of the optical transmission line A (PON) system is conceivable. Further, a WDM-PON system in which a WDM filter is installed in the middle of an optical transmission line and an optical signal is transmitted through a different path depending on the wavelength can be considered. In this case, an optical filter may be one that demultiplexes one wave for each output port, or one that demultiplexes in units of a plurality of wavelengths (wavelength groups).

It is a figure which shows the structure of the optical transmitter in the 1st Embodiment of this invention. It is a figure which shows the structural example of a wavelength sweep light source. It is a figure which shows the example of a production | generation of the wavelength sweep light using a pulse light source. It is a figure which shows the modulation signal in this invention. It is a figure which shows the structure of the optical receiver in the 2nd Embodiment of this invention. It is a figure which shows the structural example of an optical communication system. It is a figure which shows embodiment of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Wavelength sweep light source 12,22 Optical modulator 13 Optical branching device 14 Optical delay device 15,18 Polarization regulator 16 Optical multiplexer 17,25 Modulation signal generator 19 Polarization dispersion generator 21 Laser light source 23 Wavelength sweep signal generation 24 Wavelength sweep signal amplitude adjuster 26 Local light source 27 Optical multiplexer 28 Light receiver 29 Demodulator 30 Low pass filter

Claims (9)

From an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and an optical receiver that extracts and receives an optical signal of a predetermined wavelength channel from optical signals transmitted from the optical transmitter via an optical transmission line An optical communication system
The optical transmitter is
A wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate; and
A modulation signal generator that outputs a modulation signal obtained by multiplexing the data of each wavelength channel on the time axis;
An optical modulator that modulates an optical signal output from the wavelength sweep light source with the modulation signal;
An optical branching device for branching the modulated optical signal into two;
An optical delay device that gives a time delay to one of the branched optical signals so that the time difference between the two optical signals branched by the optical branching device is half the reciprocal of a predetermined repetition frequency;
A polarization controller for controlling the polarization state of the other branched optical signal so that the polarization directions of the two optical signals branched by the optical splitter are orthogonal to each other;
An optical multiplexer that multiplexes two optical signals that are given the time delay and whose polarization state is controlled, and sends the optical signal to an optical transmission line;
An optical communication system. From an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and an optical receiver that extracts and receives an optical signal of a predetermined wavelength channel from optical signals transmitted from the optical transmitter via an optical transmission line An optical communication system
The optical transmitter is
A wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate; and
A modulation signal generator that outputs a modulation signal obtained by multiplexing the data of each wavelength channel on the time axis;
An optical modulator that modulates an optical signal output from the wavelength sweep light source with the modulation signal;
A polarization controller for controlling the polarization state of the modulated optical signal so as to be a linear polarization state having an angle difference of 45 degrees with respect to predetermined orthogonal planes of polarization;
Polarization dispersion generator that generates a group delay time difference of half the reciprocal of the predetermined repetition frequency for an optical signal having polarization components orthogonal to each other in a linear polarization state, and sends the difference to the optical transmission line Comprising
An optical communication system.   The optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is generated by applying a ramp waveform current having a predetermined repetition period to a direct modulation semiconductor laser. The optical communication system according to 1 or 2.   The optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is generated by applying a ramp-shaped signal having a predetermined repetition period to the phase adjustment region of the distributed reflection type laser. The optical communication system according to claim 1 or 2.   For an optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency, the output of a single spectrum laser is connected to a phase modulator, and a parabolic waveform voltage having a predetermined repetition period is input to the phase modulator. The optical communication system according to claim 1, wherein the optical communication system is generated by:   An optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency is connected to an external resonator at one reflection end of a laser, and a control signal is input to the external resonator to input an optical waveguide in the external resonator. The optical communication system according to claim 1 or 2, wherein the optical communication system is generated by changing a refractive index or changing an angle between an external resonator and an optical path by mechanical rotation.   The optical signal that sweeps a predetermined wavelength range at the predetermined repetition frequency generates pulse light of a predetermined wavelength band with a predetermined repetition period and passes through a dispersion medium having a group delay characteristic that gives a delay time proportional to the wavelength. The optical communication system according to claim 1, wherein the optical communication system is generated. Generating an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate;
Generating a modulated signal obtained by multiplexing the data of each wavelength channel on the time axis;
Modulating the output optical signal with the modulation signal;
Branching the modulated optical signal into two;
Applying a time delay to one of the branched optical signals so that the time difference between the two branched optical signals is half the reciprocal of a predetermined repetition frequency;
Controlling the polarization state of the other branched optical signal so that the polarization directions of the two branched optical signals are orthogonal to each other;
Combining two optical signals given the time delay and whose polarization state is controlled,
A method of generating a modulated optical signal. Generating an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate;
Generating a modulated signal obtained by multiplexing the data of each wavelength channel on the time axis;
Modulating the output optical signal with the modulation signal;
Controlling the polarization state of the modulated optical signal so as to be a linear polarization state having an angle difference of 45 degrees with respect to predetermined orthogonal planes of polarization;
Including a group delay time difference of half the inverse of the predetermined repetition frequency for an optical signal having polarization components orthogonal to each other in a linear polarization state,
A method of generating a modulated optical signal.

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