JP2005260696A - Optical transmitter, optical transmission system and signal light modulating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter for reducing waveform deterioration by an SPM (self phase modulation)-GVD (group velocity dispersion) effect in a DPSK (differential phase shift keying) modulation system. <P>SOLUTION: This optical transmitter has a light source (LD), a DPSK generating device (100) for modulating signal light inputted from the light source (LD) into a DPSK signal on the basis of transmission data (electrical signal (90)), and a polarization modulator (200) for modulating the DPSK signal modulated by the DPSK generating device (100) so as to make polarization states cross orthogonally between bits adjacent to the DPSK signal. In a transmission part of the DPSK modulation system, Polarization is made to cross orthogonally between adjacent bits (inter-adjacent signal). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical wavelength division multiplexing (WDM) transmission system, and in particular, an optical transmission device, an optical transmission system, and an optical transmission device that improve transmission characteristics by suppressing the influence of nonlinear effects that occur in a transmission line optical fiber. The present invention relates to a signal light modulation method.

  As a modulation method of a communication method using an optical fiber, a DPSK (Differential Phase Shift Keying) modulation method has recently been actively studied as a method of further expanding transmission characteristics (transmission distance, transmission capacity) (for example, Non-patent document 1).

  This DPSK modulation method differs from the conventional On-Off Keying (OOK) method in that it gives information to the phase portion of the pulse (more precisely, the phase change between one information pulse and the next information pulse) Since there is a pulse in every time slot, the influence of XPM (Cross Phase Modulation), which is one of nonlinear degradation due to bit pattern dependency, can be reduced compared to the OOK method. It is known that there is a merit (for example, refer nonpatent literature 2).

  Furthermore, since the DPSK modulation method can improve the reception sensitivity by about 3 dB compared to the OOK method, the transmission power to the transmission line can be reduced by 3 dB compared to the OOK method. For this reason, the DPSK modulation method is known to reduce the influence of the self-phase modulation effect (SPM (Self Phase Modulation)), which is one factor of nonlinear degradation, compared to the OOK method (for example, non-patented). Reference 3).

  However, paying attention to the main factors that limit the transmission characteristics of the DPSK modulation method, although the amount of waveform deterioration is smaller than that of the OOK method, self-phase modulation effect (SPM) and group velocity dispersion (GVD (Group Velocity Dispersion)) It is known that the amount of waveform deterioration due to the combined effect (SPM-GVD effect) with the above is the largest among the proportion of the total amount of waveform deterioration.

  As a technical document filed prior to the present invention, there is an optical transmitter that can sufficiently suppress signal waveform deterioration due to the SPM-GVD effect (see, for example, Patent Document 1).

  The above Patent Document 1 is based on a light source, an intensity modulator that performs intensity modulation on signal light input from the light source based on transmission data, and signal light input from the intensity modulator, respectively. This is an optical transmitter having a polarization modulator that performs polarization modulation so that the polarization states in temporally adjacent bits are orthogonal.

  In addition, a dispersion-shifted fiber that has already been widely laid as an optical transmission line in an optical wavelength division multiplexing (WDM) transmission system can be used, and even if a large optical power is incident on it, transmission characteristics are deteriorated by four-wave mixing. There is an optical wavelength division multiplexing transmission apparatus that can be minimized (see, for example, Patent Document 2).

Patent Document 2 discloses an optical transmitter that multiplexes and transmits optical digital signals having a plurality of wavelengths arranged at unequal intervals on the optical frequency axis, and separates the wavelength-multiplexed optical digital signals into wavelengths. And an optical receiver having a wavelength dispersion value at which four-wave mixing occurs with respect to a predetermined input optical power, and an optical transmission path for coupling the optical transmitter and the optical receiver. In the wavelength division multiplexing transmission apparatus, the optical digital signal of each wavelength is set to an optical power that causes four-wave mixing in the optical transmission line, and the optical transmitter converts the optical digital signal of each wavelength into an optical angle modulation signal. An optical wavelength division multiplexing transmission apparatus comprising means for transmitting as An optical differential phase shift keying signal (DPSK signal) is used as one of the optical angle modulation signals in this apparatus.
JP-A-11-355215 Japanese Patent Laid-Open No. 10-13342 B. Zhu et al. , “Transmission of 3.2 Tb / s (80 × 42.7 Gb / s) over 5200 km of UltraWave ™ fiber with 100-km dispersion-managed spanning RZ-DPSK format”. ECOC 2002, PD4 A. Gnauck et al. , “40-Gb / s RZ-Differential Phase Shift Keyed Transmission”, OFC 2003, ThE1 Takashi Mizuchi et al. , “A Comparable Study of DPSK and OOK WDM Transmission Over Transient Distances and Theratio Degradations Due to Non-Nelinear Ph.D.” LIGHTWAVE TECHNOLOGY, VOL.21, NO.9, SEPTEMBER 2003 pp. 1933-1943

  The above-mentioned Patent Document 1 aims to sufficiently suppress signal waveform deterioration due to the SPM-GVD effect as in the present invention, but no consideration is given to waveform deterioration due to the SPM-GVD effect in the DPSK modulation method. It has not been. Moreover, although the said patent document 2 is disclosed about the optical wavelength division multiplexing (WDM) transmission system using a DPSK signal, like the said patent document 1, about the waveform degradation by the SPM-GVD effect in a DPSK modulation system Is not considered at all.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical transmission device, an optical transmission system, and a signal light modulation method for reducing waveform deterioration due to the SPM-GVD effect in the DPSK modulation method. It is.

  In order to achieve this object, the present invention has the following features.

  An optical transmitter according to the present invention includes a light source, a DPSK modulation unit that modulates signal light input from the light source based on transmission data into a DPSK (Differential Phase Shift Keying) signal, and a DPSK signal modulated by the DPSK modulation unit. And polarization modulation means for modulating the polarization state between adjacent bits of the DPSK signal so as to be orthogonal to each other.

  In the optical transmitter according to the present invention, the polarization modulator modulates the DPSK signal based on the alternating signal synchronized with the transmission data so that the polarization state is orthogonal between adjacent bits of the DPSK signal. It is characterized by doing.

  The optical transmission apparatus according to the present invention includes a control unit that generates an alternating signal synchronized with the transmission data based on the transmission data, and controls the polarization modulation unit based on the generated alternating signal. It is what.

  In the optical transmission apparatus according to the present invention, the control means generates a clock signal having the same time period as the DPSK signal modulated by the DPSK modulation means based on the transmission data, and generated by the clock generation means. The clock signal is adjusted so that the time difference between the clock signals between adjacent bits of the clock signal is the same, an alternating signal synchronized with the transmission data is generated, and the generated alternating signal is transmitted to the polarization modulation means. Adjusting means for outputting, and controlling the polarization modulation means.

  The optical transmitter according to the present invention generates an alternating signal synchronized with transmission data based on the DPSK signal modulated by the DPSK modulating means, and controls the polarization modulating means based on the generated alternating signal. It has a 2nd control means, It is characterized by the above-mentioned.

  In the optical transmission apparatus according to the present invention, the second control means receives a DPSK signal modulated by the DPSK modulation means, and generates a electrical signal based on the received DPSK signal, and a light receiving means. The clock generation means for generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulation means on the basis of the electrical signal generated by the above-mentioned method, and the clock signal generated by the clock generation means for the adjacent bits of the clock signal Adjusting the time difference between the clock signals between them to generate an alternating signal synchronized with the transmission data, and outputting the generated alternating signal to the polarization modulation means, The modulation means is controlled.

  The optical transmission apparatus according to the present invention is characterized in that a polarization controller for controlling the DPSK signal modulated by the DPSK modulation means is mounted between the DPSK modulation means and the polarization modulation means. It is.

  In the optical transmitter according to the present invention, the polarization controller and the DPSK modulation unit are connected via a fiber that is not polarization-maintained, and the polarization controller and the polarization modulation unit are configured to connect a polarization-maintained fiber. And the polarization controller controls so that the polarization plane of the DPSK signal modulated by the DPSK modulation means matches the polarization plane of the DPSK signal input to the polarization-maintained fiber. It is what.

  The optical transmission device according to the present invention is modulated by a light source, a DPSK modulation unit that modulates signal light input from the light source into a DPSK (Differential Phase Shift Keying) signal based on transmission data, and a DPSK modulation unit. An output waveform in which the DPSK signal is alternately separated in the X-axis direction and the Y-axis direction between adjacent bits of the DPSK signal is generated, and the generated X-axis direction and the Y-axis direction are separated. And orthogonal means for integrating the output waveforms and orthogonalizing the polarization between adjacent bits.

  In the optical transmission apparatus according to the present invention, the orthogonal means includes a first optical coupler connected to the DPSK modulation means via a polarization maintaining fiber, and a first intensity modulation connected to the first optical coupler. Means, a second optical coupler, a Faraday mirror connected to the second optical coupler, a second intensity modulation means, a 1-bit Delay circuit connected to the second intensity modulation means, and a first intensity modulation And a polarization beam splitter connected to the second intensity modulating means.

  In the optical transmission apparatus according to the present invention, the DPSK signal modulated by the DPSK modulation means is input to the first optical coupler, and the DPSK signal input to the first optical coupler is the first intensity modulation means. And the second optical coupler, the DPSK signal input to the second optical coupler is input to the Faraday mirror, and the DPSK signal is rotated by 90 degrees by the Faraday mirror. The first intensity modulating means modulates the DPSK signal based on the alternating signal synchronized with the transmission data, and inputs the modulated DPSK signal to the polarization beam splitter. The second intensity modulating means modulates the DPSK signal rotated 90 degrees based on the alternating signal obtained by delaying the alternating signal by 1 bit by the 1-bit Delay circuit, The adjusted DPSK signal is input to the polarization beam splitter, and the polarization beam splitter integrates the DPSK signals input from the first intensity modulation means and the second intensity modulation means, and performs polarization between adjacent bits. Are orthogonal to each other.

  An optical transmission system according to the present invention includes an optical transmitter described above, an optical receiver that receives and decodes a DPSK signal transmitted from the optical transmitter, and an optical transmitter between the optical transmitter and the optical receiver. And an optical fiber for connecting the two.

  A signal light modulation method according to the present invention is a signal light modulation method performed by an optical transmission device that transmits signal light input from a light source. The signal light input from the light source based on transmission data is converted into DPSK ( A DPSK modulation step that modulates a differential phase shift keying signal, a polarization modulation step that modulates the DPSK signal modulated by the DPSK modulation step so that the polarization state is orthogonal between adjacent bits of the DPSK signal, and Is performed by an optical transmission device.

  Further, in the signal light modulation method according to the present invention, the polarization modulation step is configured such that the polarization state of the DPSK signal is orthogonal between adjacent bits of the DPSK signal based on the alternating signal synchronized with the transmission data. It is characterized by modulation.

  The signal light modulation method according to the present invention generates an alternating signal that is synchronized with transmission data based on transmission data, and based on the generated alternating signal, a DPSK signal is transmitted between adjacent bits of the DPSK signal. The optical transmission apparatus performs the step of modulating the polarization states so as to be orthogonal.

  The signal light modulation method according to the present invention includes a clock generation step for generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulation step based on transmission data, and a clock signal generated by the clock generation step. Is adjusted so that the time difference of the clock signal between adjacent bits of the clock signal matches, and the alternating signal is generated by the optical transmission device performing the step of generating the alternating signal synchronized with the transmission data. Then, based on the generated alternating signal, the DPSK signal is modulated so that the polarization state is orthogonal between adjacent bits of the DPSK signal.

  The signal light modulation method according to the present invention generates an alternating signal synchronized with transmission data based on the DPSK signal modulated by the DPSK modulation step, and converts the DPSK signal into the DPSK signal based on the generated alternating signal. The optical transmission apparatus performs the step of modulating the polarization state between adjacent bits of the signal so that they are orthogonal to each other.

  In the signal light modulation method according to the present invention, the DPSK signal modulated by the DPSK modulation process is received, an electric signal is generated based on the received DPSK signal, and the electric signal generated by the light reception process is received. Based on the clock generation process for generating the clock signal having the same time period as the DPSK signal modulated by the DPSK modulation process, the clock signal generated by the clock generation process is converted into the clock signal between adjacent bits of the clock signal. The optical transmission device performs the step of adjusting the time difference so as to match and generating the alternating signal synchronized with the transmission data, thereby generating the alternating signal, and based on the generated alternating signal, the DPSK signal is Modulation is performed so that the polarization state is orthogonal between adjacent bits of the DPSK signal.

  The signal light modulation method according to the present invention controls the DPSK signal modulated by the DPSK modulation process, and the polarization plane of the DPSK signal modulated by the DPSK modulation process is directly input to the polarization maintaining fiber. The optical transmission apparatus performs the step of adjusting so as to coincide with the polarization plane of the DPSK signal.

  A signal light modulation method according to the present invention is a signal light modulation method performed by an optical transmission device that transmits signal light input from a light source. The signal light input from the light source based on transmission data is converted into DPSK ( A DPSK modulation step that modulates a differential phase shift keying signal, and a DPSK signal modulated by the DPSK modulation step are alternately separated in the X-axis direction and the Y-axis direction between adjacent bits of the DPSK signal. An optical transmission device performs an orthogonal process of generating an output waveform, integrating the generated output waveforms separated in the X-axis direction and the Y-axis direction, and orthogonalizing polarization between adjacent bits. It is what.

  An optical transmission apparatus, an optical transmission system, and a signal light modulation method according to the present invention modulate signal light input from a light source into a DPSK (Differential Phase Shift Keying) signal based on optical transmission data, and the modulated DPSK signal. By modulating the adjacent bits of the DPSK signal so that the polarization states are orthogonal, it is possible to reduce waveform deterioration due to the SPM-GVD effect in the DPSK modulation method. The optical transmitter, the optical transmission system, and the signal light modulation method according to the present invention modulate signal light input from a light source into a DPSK (Differential Phase Shift Keying) signal based on transmission data, and the modulated DPSK. An output waveform in which the signal is alternately separated in the X-axis direction and the Y-axis direction between adjacent bits of the DPSK signal is generated, and the generated output is separated in the X-axis direction and the Y-axis direction. By integrating the waveforms and orthogonalizing the polarization between adjacent bits, it is possible to reduce waveform deterioration due to the SPM-GVD effect in the DPSK modulation method.

  First, the characteristics of the optical transmitter, the optical transmission system, and the signal light modulation method according to the present invention will be described with reference to FIG.

  An optical transmitter, an optical transmission system, and a signal light modulation method according to the present invention include a signal light input from a light source (LD) based on a light source (LD) and transmission data (electrical signal (90)). DPSK modulation means (100) that modulates the signal into a DPSK (Differential Phase Shift Keying) signal, and the DPSK signal modulated by the DPSK modulation means (100) so that the polarization state is orthogonal between adjacent bits of the DPSK signal And a polarization modulation means (200) for modulating the light into the light. In this way, by making the polarization orthogonal between adjacent bits (between adjacent signals) in the transmission part of the DPSK modulation method, waveform deterioration due to the SPM-GVD effect, which is one of nonlinear deteriorations, is reduced.

  Hereinafter, an optical transmission device, an optical transmission system, and a signal light modulation method according to the present invention will be described with reference to the accompanying drawings.

  First, the configuration of an optical transmission apparatus applied to an optical transmission system according to the present invention will be described with reference to FIG.

  The optical transmitter according to the present invention includes a light source (LD), a DPSK generator (100), a polarization maintaining fiber (500), and a polarization modulator (200).

[DPSK generator (100)]
The DPSK generator (100) modulates continuous wave (CW) light emitted from a light source (LD) into a DPSK signal using an electric signal (90), and outputs the modulated DPSK signal to a polarization maintaining fiber (500). To do. For example, when the electrical signal (90) output to the DPSK generator (100) is (1, 0, 1, 1, 1, 0, 0, 1, 0), the DPSK generator (100) The continuous wave (CW) light emitted from (LD) is modulated into a DPSK signal of (0, π, π, π, π, 0, π, π, 0) and output to the polarization maintaining fiber (500). It will be.

  Note that the above example is a case where the differential is performed at 0 with a delay of 1 bit. For this reason, when a 0 level signal comes in time, the phase is inverted by π, and when a 1 level signal comes, the phase is not changed. In the DPSK generator (100), it is also possible to perform differentiation using a signal of one level. In this case, when a signal of level 1 comes, the phase is inverted by π, and when a signal of level 0 comes, the phase is not changed.

  An example of the DPSK signal output from the DPSK generator (100) is suggested in FIGS. 2 (a) and 3 (a). 2 is a diagram in which continuous wave (CW) light emitted from the light source (LD) is modulated by NRZ (NON Return to Zero) which is transmission data (electrical signal (90)) and output. 3 is a diagram in which continuous wave (CW) light emitted from a light source (LD) is modulated and output by RZ (Return to Zero) which is transmission data (electrical signal (90)).

[Polarization-maintaining fiber (500)]
The polarization maintaining fiber (500) stores the polarization plane of the DPSK signal output from the DPSK generator (100).

[Polarization modulator (200)]
The polarization modulator (200) outputs an optical signal whose polarization plane is orthogonal between adjacent bits (between adjacent signals) by using an alternating signal (91) from the DPSK signal output from the DPSK generator (100). is there.

  The alternating signal (91) is an alternating signal of “1, 0, 1, 0,...” Synchronized with the electric signal (90) input to the DPSK generator (100). When X is 1, it becomes X-axis polarization, and when the alternating signal is 0, it becomes Y-axis polarization. Of course, when the alternating signal is 0, the X-axis polarization may be used, and when the alternating signal is 1, the Y-axis polarization may be used.

  Examples of signals output from the polarization modulator (200) are suggested in FIGS. 2 (b) and 3 (b). 2 is a diagram suggesting a signal output by the NRZ (NON Return to Zero) modulation scheme, and FIG. 3 is a diagram suggesting a signal output by the RZ (Return to Zero) modulation scheme.

  Next, the processing operation in the optical transmission apparatus according to the present invention will be described with reference to FIGS.

  First, continuous wave (CW) light from a light source (LD) is modulated into a DPSK signal by an electric signal (90) in a DPSK generator (100). For example, it is modulated into a DPSK signal as shown in FIGS. 2 (a) and 3 (a). 2 and 3, the DPSK signal output from the DPSK generator (100) is an X-axis polarization signal, but the DPSK signal output from the DPSK generator (100) is the Y-axis polarization signal. It can also be a signal.

  The DPSK modulation method performed by the DPSK generator (100) is a method for giving information to the phase portion of the pulse. In FIGS. 2 and 3, as an example, the DPSK modulated to information of π, 0, π, π. A signal is being output. It should be noted that the DPSK signals output by performing DPSK modulation in the DPSK generator (100) all have the same intensity level.

  Next, the DPSK signal output from the DPSK generator (100) is polarization-maintained by the polarization-maintaining fiber (500) and input to the polarization modulator (200). The polarization modulator (200) uses the alternating signal (91) to make the DPSK signal input to the polarization modulator (200) polarization orthogonal between adjacent bits of the DPSK signal (between adjacent signals). Will be modulated. The alternating signal (91) input to the polarization modulator (200) is a signal synchronized with the electric signal (90) input to the DPSK generator (100), and all adjacent bits are different. “1, 0, 1, 0, 1, 0,...”.

  By inputting the alternating signal (91) of “1, 0, 1, 0, 1, 0,...” In which all the adjacent bits are different from each other to the polarization modulator (200), for example, polarization The modulator (200) outputs the DPSK signal output from the DPSK generator (100) suggested in FIGS. 2 (a) and 3 (a) to the signal suggested in FIGS. 2 (b) and 3 (b). Output in the direction orthogonal to the polarization. As a result, the output signal output from the polarization modulator (200) becomes a signal in which the polarization is orthogonal between adjacent bits (between adjacent signals), and the combined effect of SPM and group velocity dispersion (SPM-GVD effect). ) Intersymbol interference can be reduced.

(Effect of the optical transmitter in the first embodiment)
From the above, the optical transmitter according to the present invention has the following effects.

  The first effect is that the polarization is orthogonalized between adjacent bits (between adjacent signals) of the DPSK signal output from the DPSK generator (100), thereby combining the SPM and the group velocity dispersion (SPM-GVD effect). It is possible to reduce the waveform deterioration due to. That is, by making the pulses orthogonal between adjacent bits (between adjacent signals), it is possible to reduce the influence of intersymbol interference between adjacent bits (between adjacent signals). For example, as suggested in FIG. 4A, the DPSK signal output from the DPSK generator (100) generates intersymbol interference between adjacent bits (between adjacent signals). In the modulator (200), as indicated in FIG. 4 (b), by modulating the signals with the orthogonal polarization between adjacent bits (between adjacent signals), between adjacent bits (between adjacent signals) Intersymbol interference does not occur.

  The second effect is that the influence of deterioration due to PDL (Polarization Dependent Loss) can be reduced by the optical receiving apparatus on the receiving side that receives the signal light transmitted from the optical transmitting apparatus according to the present invention. Hereinafter, it will be described in detail with reference to FIG.

  In general, as suggested in FIG. 5A, a DPSK modulation type optical receiving apparatus interferes with a received signal (A) and a signal (B) obtained by delaying the received signal by 1 bit. Thus, the signal (C) decoded in step S3 is generated. Since the DPSK signals generated by the DPSK modulation method have the same level of polarization, the DPSK signals generated by the DPSK modulation method are all affected by the same amount of degradation due to PDL. Therefore, as suggested in FIG. 5B, both the received signal (A) and the received signal (B) obtained by delaying the received signal by 1 bit are affected by PDL. When the loss of the DPSK signal increases due to PDL, the waveform deterioration of the transmission characteristics due to PDL appears greatly in the signal (C) decoded by causing both received signals affected by the PDL to interfere with each other. become. However, since the optical transmission apparatus of the present invention generates a DPSK signal in which the polarization is orthogonal between adjacent pulses, as shown in FIG. 5C, one received signal between adjacent bits is Even if power deteriorates due to the influence of PDL, the power of the other received signal between adjacent bits does not deteriorate. Therefore, since the received signal affected by the PDL interferes with the received signal not affected by the PDL, it is possible to reduce the waveform deterioration due to the PDL as compared with FIG. .

Next, a second embodiment will be described.
As suggested in FIG. 6, the optical transmission apparatus of the second embodiment has a polarization modulator control circuit composed of a CDR (Clock Data Recovery) (300) and a Delay circuit (301). 1 to generate an alternating signal (91) to be input to the polarization modulator (200) constituting the optical transmission apparatus of the first embodiment, and to control the polarization modulator (200). To do. Hereinafter, the configuration of the optical transmission apparatus in the second embodiment will be described with reference to FIG.

  In the optical transmitter in the second embodiment suggested in FIG. 6, the electric signal (90) input to the DPSK generator (100) is also input to the polarization modulator control circuit, and the polarization The CDR (300) constituting the modulator control circuit generates a clock signal having the same time period as the DPSK signal output from the DPSK generator (100) based on the input electric signal (90). The generated clock signal is output to the delay circuit (301). The delay circuit (301) converts the clock signal input to the delay circuit (301) between adjacent bits of the clock signal so as to be synchronized with the DPSK signal output from the DPSK generator (100). The polarization modulator (200) is controlled by adjusting the time difference of the clock signals so as to coincide with each other and outputting the adjusted clock signal to the polarization modulator (200). As a result, the polarization modulator (200) outputs the DPSK signal output from the DPSK generator (100) with the orthogonal polarization as in the first embodiment.

Next, a third embodiment will be described.
As suggested in FIG. 7, the optical transmitter of the third embodiment receives a DPSK signal output from the DPSK generator (100) by a PD (Photo Diode) (302), and receives a PD (Photo Diode). (302) converts the received DPSK signal into an electrical signal, and outputs the converted electrical signal to the CDR (300). The CDR (300) generates a clock signal having the same time period as the DPSK signal output from the DPSK generator (100) based on the input electrical signal, and sends the generated clock signal to the delay circuit (301). Will be output. Then, the delay circuit (301) adjusts the clock signal input to the delay circuit (301) so as to be synchronized with the DPSK signal output from the DPSK generator (100), and polarizes the adjusted clock signal. By outputting to the modulator (200), the polarization modulator (200) is controlled. As a result, the polarization modulator (200) outputs the DPSK signal output from the DPSK generator (100) with the orthogonal polarization as in the first embodiment. Therefore, the optical transmitter in the third embodiment is retrofitted with a configuration for making the DPSK signal output from the DPSK generator (100) orthogonal to the existing DPSK generator (100). It becomes possible to add.

Next, a fourth embodiment will be described.
As suggested in FIG. 8, the optical transmitter of the fourth embodiment includes a DPSK generator (100) and a configuration for orthogonally polarizing a DPSK signal output from the DPSK generator (100). A polarization controller (600) is inserted between them, and the polarization output from the DPSK generator (100) and the polarization input to the polarization-maintaining fiber (503) are matched by the polarization controller (600). It adjusts so that it may do.

  When the DPSK signal output from the DPSK generator (100) is input to the fiber (502) that is not polarization-maintained, the state suggested in FIG. The DPSK signal is controlled by the polarization controller (600), and the DPSK signal output from the DPSK generator (100) is directly applied to the polarization-maintaining fiber (503), as suggested in FIG. 9B. Adjustment is made so that the DPSK signal in the input state is obtained.

  Thus, even if the DPSK signal output from the existing DPSK generator (100) is input to the non-polarization-maintaining fiber (502), the polarization controller (600) does not maintain the polarization-maintaining fiber (502). The polarization controller (600) is mounted after the DPSK generator (100) even when the DPSK signal output from the existing DPSK generator (100) is not preserved by controlling the polarization input to By doing so, it becomes possible to add a configuration for making the DPSK signal output from the DPSK generator (100) orthogonal to the polarization orthogonally.

Next, a fifth embodiment will be described.
In the optical transmitter of the fifth embodiment, the DPSK signal output from the DPSK generator (100) is transmitted between adjacent bits of the DPSK signal (between adjacent signals) in the X-axis direction and the Y-axis direction. It is characterized by generating alternately output waveforms, integrating the generated output waveforms in the X-axis direction and Y-axis direction, and orthogonalizing the polarization between adjacent bits It is. Hereinafter, the optical transmission apparatus in the fifth embodiment will be described with reference to FIGS.

  First, the configuration of the optical transmission apparatus in the fifth embodiment will be described with reference to FIG.

  In the optical transmitter of the fifth embodiment, the optical coupler (1) is connected to the DPSK generator (100) via the polarization maintaining fiber (500). The optical coupler (1) is connected to the intensity modulator (202) and the optical coupler (2). The optical coupler (2) is connected to the Faraday mirror (40) and the intensity modulator (203), and the intensity modulator (203) is connected to the 1-bit Delay (800). Further, the intensity modulator (202) and the intensity modulator (203) are connected to a polarization beam splitter PBS (Polarized Beam Splitter) (41).

  Next, the processing operation in the optical transmission apparatus of the fifth embodiment will be described with reference to FIGS.

  The DPSK signal output from the DPSK generator (100) becomes an output signal suggested in FIG. 11 (a) by the electrical signal (90) input to the DPSK generator (100). Note that the output signal suggested in FIG. 11A suggests an X-axis polarized signal, but it is also possible to output a Y-axis polarized signal.

  The output signal suggested in FIG. 11A output from the DPSK generator (100) is input to the optical coupler (1), and the output signal input to the optical coupler (1) is the intensity modulator (202). To the optical coupler (2).

  The output signal input to the optical coupler (2) is output to the Faraday mirror (40), and the Faraday mirror (40) rotates the output signal input from the optical coupler (2) by 90 degrees. (The polarization input from the optical coupler (2) is converted from the X axis to the Y axis, or from the Y axis to the X axis). Then, the received signal rotated 90 degrees by the Faraday mirror (40) is input to the intensity modulator (203) via the optical coupler (2).

  The intensity modulator (202) modulates the input output signal with an alternating signal (93) of “1, 0, 1, 0,...”, And the intensity modulator (202) is shown in FIG. The output signal suggested in FIG. 11 is modulated only to the output signal suggested in FIG. 11B, which is output only when the alternating signal (93) is “1”, and output to the polarization beam splitter PBS (Polarized Beam Splitter) (41). Will do. Further, the intensity modulator (203) converts the input output signal into an alternating signal (1, 0, 1, 0,...) By a 1-bit delay circuit (800) added to the intensity modulation (203). 93) is modulated by the alternating signal “0, 1, 0, 1,...” Delayed by 1 bit, and the output signal input to the intensity modulator (203) is shown in FIG. The output signal is modulated to a suggested output signal and output to a polarized beam splitter PBS (Polarized Beam Splitter) (41). The polarized beam splitter PBS (Polarized Beam Splitter) (41) is an output signal suggested in FIG. 11 (b) input from the intensity modulator (202) and FIG. 11 (c) input from the intensity modulator (203). And the output signal suggested in FIG. 11 (d) are combined to output the output signal suggested in FIG. 11 (d).

  The alternating signal (93) suggested in FIG. 10 is an alternating signal synchronized with the electrical signal (90) input to the DPSK generator (100), and adjacent bits are all different from “1, 0”. , 1, 0, 1, 0,...

(Verification of the effect of the optical transmission apparatus of the present invention)
Next, the effect of the optical transmission apparatus of the present invention will be verified.
As a system configuration for verifying the configuration of the optical transmission apparatus of the present invention (10G, span length 80 km, transmission distance 4000 km), the configuration of a conventional optical transmission apparatus (DPSK generator (100) suggested in FIG. 1 only) The waveform deterioration due to the SPM-GVD effect is compared between the optical transmission apparatus of the above embodiment and the optical transmission apparatus of the above embodiment, and the validity of the optical transmission apparatus of the present invention is verified. In addition, the model of the transmission line at the time of performing simulation in FIG. 12 is suggested.

  The simulation model suggested in FIG. 12 includes a transmitter (Tx), a first dispersion compensating fiber (Pre-DCF), a Demux Band Pass Filter (DMUX BPF), and a second dispersion fiber (Post-DCF). It is comprised. As suggested in FIG. 12, between the first dispersion compensating fiber (Pre-DCF) and the Demux Band Pass Filter (DMUX BPF), there are two optical amplifiers, 80 km SMF, and DCF. Are arranged in series. The optical signal that has passed through the SMF and DCF is supplemented by an optical amplifier because the power of the optical signal decreases. The Demux Band Pass Filter (DMUX BPF) is a filter for cutting out only the spectrum near the signal, and removes noise. The 100 G-AWG and 50 GHz Interleaver suggested in FIG. 12 suggest the type of filter. In the simulation, these two filters are used.

  In general, when a signal waveform passes through an optical fiber (Single Mode Fiber (SMF)), the shape of the signal waveform spreads due to a phenomenon of dispersion. For this reason, basically, if dispersion is accumulated to some extent, the dispersion accumulated using a dispersion compensating fiber (DCF) is returned. As a result, the signal waveform whose shape has changed returns to the original transmission waveform.

  In the simulation model transmission path suggested in FIG. 12, when a signal is transmitted to an 80 km optical fiber (80 km SMF), dispersion of 20 ps / nm × 80 km = 160 ps / nm is accumulated. (DCF) is configured to give a reverse dispersion of −160 ps / nm × 1 km to return to zero. In addition, since the dispersion compensating fiber (DCF) is built in the repeater in the terrestrial transmission line, the transmission line has a configuration of only the optical fiber (SMF).

  Note that, as suggested in FIG. 12, the dispersion compensating fiber (Pre-DCF, Post-DCF) gives some dispersion to the signal waveform before and after the transmission path. In this way, it is possible to improve the transmission characteristics by mounting dispersion compensating fibers (Pre-DCF, Post-DCF) before and after the transmission line and making the waveform inferior to a certain extent to the signal waveform of transmission. The Eye-Area indicates the area of the signal waveform, and if the Eye-Area is large, it indicates that the transmission characteristics are good.

  The simulation model suggested in FIG. 12 is compensated for dispersion as suggested in FIG. Further, FIG. 14 suggests parameters of the optical fiber (SMF) suggested in FIG.

  FIG. 15 shows waveforms after transmission of the conventional optical transmission apparatus and the optical transmission apparatus of the present invention. FIG. 15 is a diagram suggesting a waveform after transmission when the DPSK-NRZ modulation method and the DPSK-RZ modulation method are applied to the simulation model of FIG.

  From the graph suggested in FIG. 15, it has been found that the optical transmission device of the present invention can provide a better eye opening than the conventional optical transmission device. A good eye indicates that the degree of variation between the 0 level and the 1 level in the graph suggested in FIG. 15 is small.

  In order to compare the penalty after transmission, FIG. 16 suggests the amount of deterioration of the eye opening area after transmission between the conventional optical transmission apparatus and the optical transmission apparatus of the present invention. From this, it was found that the amount of deterioration of the eye opening area is smaller in the optical transmission device of the present invention than in the conventional optical transmission device. As described above, the validity of the optical transmission apparatus of the present invention can be verified by simulation.

  The above-described embodiment is a preferred embodiment of the present invention, and the scope of the present invention is not limited only to the above-described embodiment, and various modifications are made without departing from the gist of the present invention. Implementation is possible. For example, the optical transmission apparatus according to the present invention can be applied to an optical wavelength division multiplexing (WDM) transmission system, and can be applied only to a specific channel. The system configuration of the WDM transmission system includes the optical transmitter in each of the above embodiments, the optical receiver that receives and decodes the DPSK signal transmitted from the optical transmitter, the optical transmitter, The receiving apparatus is configured to include an optical fiber for connecting between the receiving apparatus and the receiving apparatus.

It is a block diagram which suggests the structure of the optical transmission apparatus applied to the optical transmission system concerning this invention, and the continuous line of a figure suggests an optical signal and a dotted line suggests an electrical signal. It is a figure which suggests the waveform output by NRZ (NON Return to Zero) modulation in the optical transmission apparatus concerning this invention, (a) suggests the output waveform of a DPSK generator (100), (b) It is a figure which suggests the output waveform of a polarization modulator (200). It is a figure which suggests the waveform output by RZ (Return to Zero) modulation in the optical transmission device concerning the present invention, (a) suggests the output waveform of a DPSK generator (100), (b), It is a figure which suggests the output waveform of a polarization modulator (200). It is a graph which suggests the difference of the influence of the intersymbol interference between adjacent bits (between adjacent signals), (a) suggests the waveform of the DPSK signal output from the DPSK generator (100), (b) These are figures which suggest the waveform of the DPSK signal output from the polarization modulator (200). It is a figure which suggests an example of the influence of degradation by PDL, (a) is a figure for demonstrating the decoding of a DPSK signal, (b) is a DPSK signal between adjacent bits, and is an influence of PDL. (C) is a diagram suggesting decoding when one DPSK signal between adjacent bits is affected by PDL. It is a figure which suggests the structure of the optical transmitter in a 2nd Example. It is a figure which suggests the structure of the optical transmitter in a 3rd Example. It is a figure which suggests the structure of the optical transmitter in a 4th Example. It is a figure which suggests the output waveform in the optical transmission apparatus in a 4th Example, (a) is the state by which the DPSK signal output from a DPSK generator (100) was input into the fiber (502) which is not polarization-maintaining (B) suggests a waveform in a state after the polarization controller (600) has adjusted the waveform of (a). It is a figure which suggests the structure of the optical transmitter in a 5th Example. It is a figure which suggests the output waveform in the optical transmitter in a 5th Example, (a) suggests the output waveform of a DPSK generator (100), (b) is the output of an intensity modulator (202). (C) is a diagram suggesting an output waveform of the intensity modulator (203), and (d) is a diagram suggesting an output waveform of the PBS (41). It is a figure which suggests the simulation model of the optical transmission system using the optical transmitter concerning this invention. FIG. 13 is a diagram suggesting dispersion compensation (dispersion map) of the simulation model suggested in FIG. 12. It is a figure which suggests the parameter of the optical fiber (SMF) suggested in FIG. The figure which suggests the waveform after the transmission of the conventional optical transmitter (conventional) and the optical transmitter of the present invention (the present invention) when the DPSK-NRZ modulation method and the DPSK-RZ modulation method are used. is there. It is a figure which suggests the deterioration amount of the eye opening area after transmission with the conventional optical transmission apparatus (conventional) and the optical transmission apparatus of the present invention (the present invention).

Explanation of symbols

1, 2 Optical coupler 40 Faraday mirror 41 Polarized beam splitter PBS (Polarized Beam Splitter)
DESCRIPTION OF SYMBOLS 100 DPSK generator 200 Polarization modulator 202, 203 Intensity modulator 300 CDR (Clock Data Recovery)
301 Delay circuit 302 PD (Photo Diode)
500, 503 Polarization-maintaining fiber 502 Non-polarization-maintaining fiber 600 Polarization controller 800 1-bit delay circuit 90 Electrical signal 91, 93 Alternating signal LD Light source Tx Transmitter
Pre-DCF, Post-DCF, DCF Dispersion compensating fiber DMUX BPF Demux Band Pass Filter
SMF Single Mode Fiber

Claims (20)

A light source;
DPSK modulation means for modulating signal light input from the light source based on transmission data into a DPSK (Differential Phase Shift Keying) signal;
Polarization modulation means for modulating the DPSK signal modulated by the DPSK modulation means so that the polarization state is orthogonal between adjacent bits of the DPSK signal;
An optical transmitter characterized by comprising:   The polarization modulation means modulates the DPSK signal based on an alternating signal synchronized with the transmission data so that the polarization state is orthogonal between adjacent bits of the DPSK signal. The optical transmission device according to 1.   3. The light according to claim 2, further comprising: a control unit that generates an alternating signal synchronized with the transmission data based on the transmission data, and controls the polarization modulation unit based on the generated alternating signal. Transmitter device. The control means includes
Clock generating means for generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulating means based on the transmission data;
The clock signal generated by the clock generating means is adjusted so that the time difference between the clock signals between adjacent bits of the clock signal is the same, and an alternating signal synchronized with the transmission data is generated, and the generated alternating signal And adjustment means for outputting to the polarization modulation means,
4. The optical transmitter according to claim 3, wherein the polarization modulator is controlled.   A second control unit configured to generate an alternating signal synchronized with the transmission data based on the DPSK signal modulated by the DPSK modulating unit, and to control the polarization modulating unit based on the generated alternating signal; The optical transmission device according to claim 2. The second control means includes
A light receiving means for receiving a DPSK signal modulated by the DPSK modulating means and generating an electrical signal based on the received DPSK signal;
Clock generating means for generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulating means based on the electrical signal generated by the light receiving means;
The clock signal generated by the clock generating means is adjusted so that the time difference between the clock signals between adjacent bits of the clock signal is the same, and an alternating signal synchronized with the transmission data is generated, and the generated alternating signal And adjustment means for outputting to the polarization modulation means,
6. The optical transmitter according to claim 5, wherein the polarization modulator is controlled.   The polarization controller for controlling the DPSK signal modulated by the DPSK modulation means is mounted between the DPSK modulation means and the polarization modulation means. The optical transmitter according to the item. The polarization controller and the DPSK modulation means are connected via a fiber that does not preserve polarization, and the polarization controller and the polarization modulation means are connected via a fiber that preserves polarization,
The polarization controller performs control so that the polarization plane of the DPSK signal modulated by the DPSK modulation means and the polarization plane of the DPSK signal input to the polarization-maintained fiber coincide with each other. The optical transmission device according to claim 7. A light source;
DPSK modulation means for modulating signal light input from the light source based on transmission data into a DPSK (Differential Phase Shift Keying) signal;
An output waveform obtained by alternately separating the DPSK signal modulated by the DPSK modulation means in the X-axis direction and the Y-axis direction between adjacent bits of the DPSK signal is generated, and the generated X-axis direction An orthogonal means for integrating the output waveforms separated in the Y-axis direction and orthogonalizing the polarization between the adjacent bits;
An optical transmitter characterized by comprising: The orthogonal means is
A first optical coupler connected to the DPSK modulation means via a polarization-maintaining fiber; a first intensity modulation means connected to the first optical coupler; a second optical coupler; and the second optical coupler. A Faraday mirror connected to the optical coupler, a second intensity modulating means, a 1-bit Delay circuit connected to the second intensity modulating means, the first intensity modulating means, and the second intensity modulating means; The optical transmission device according to claim 9, further comprising a polarization beam splitter connected to the optical transmission device.   The DPSK signal modulated by the DPSK modulating means is input to the first optical coupler, and the DPSK signal input to the first optical coupler is the first intensity modulating means and the second optical coupler. The DPSK signal input to the coupler and input to the second optical coupler is input to the Faraday mirror, and the DPSK signal is rotated by 90 degrees by the Faraday mirror and passes through the second optical coupler. The first intensity modulation means is input to the second intensity modulation means, and the first intensity modulation means modulates the DPSK signal based on an alternating signal synchronized with the transmission data, and sends the modulated DPSK signal to the polarization beam splitter. And the second intensity modulation means rotates the alternating signal by 90 degrees based on the alternating signal delayed by 1 bit by the 1-bit Delay circuit. The DPSK signal is modulated, and the modulated DPSK signal is input to the polarization beam splitter. The polarization beam splitter receives the DPSK signal input from the first intensity modulation unit and the second intensity modulation unit. The optical transmission apparatus according to claim 10, wherein signals are integrated and polarization is orthogonalized between the adjacent bits. An optical transmitter according to any one of claims 1 to 11,
An optical receiver that receives and decodes the DPSK signal transmitted from the optical transmitter;
An optical fiber for connecting between the optical transmitter and the optical receiver;
An optical transmission system comprising: A signal light modulation method performed by an optical transmission device that transmits signal light input from a light source,
A DPSK modulation step of modulating signal light input from the light source into a DPSK (Differential Phase Shift Keying) signal based on transmission data;
A polarization modulation step of modulating the DPSK signal modulated by the DPSK modulation step so that the polarization state is orthogonal between adjacent bits of the DPSK signal;
The optical transmission apparatus performs the signal light modulation method. The polarization modulation step includes
14. The signal light modulation method according to claim 13, wherein the DPSK signal is modulated based on an alternating signal synchronized with the transmission data so that polarization states are orthogonal between adjacent bits of the DPSK signal. .   An alternating signal synchronized with the transmission data is generated based on the transmission data, and the DPSK signal is modulated based on the generated alternating signal so that the polarization state is orthogonal between adjacent bits of the DPSK signal. The signal light modulation method according to claim 14, wherein the optical transmission apparatus performs the step of performing A clock generation step of generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulation step based on the transmission data;
Adjusting the clock signal generated by the clock generation step so that the time difference between the clock signals between adjacent bits of the clock signal matches, and generating an alternating signal synchronized with the transmission data. The transmission device generates the alternating signal, and modulates the DPSK signal based on the generated alternating signal so that the polarization state is orthogonal between adjacent bits of the DPSK signal. The signal light modulation method according to claim 15.   An alternating signal synchronized with the transmission data is generated based on the DPSK signal modulated by the DPSK modulation step, and the DPSK signal is polarized between adjacent bits of the DPSK signal based on the generated alternating signal. 15. The signal light modulation method according to claim 14, wherein the optical transmission device performs the step of modulating the states so as to be orthogonal. Receiving a DPSK signal modulated by the DPSK modulation step, and generating an electrical signal based on the received DPSK signal;
A clock generation step for generating a clock signal having the same time period as the DPSK signal modulated by the DPSK modulation step based on the electrical signal generated by the light receiving step;
Adjusting the clock signal generated by the clock generation step so that the time difference between the clock signals between adjacent bits of the clock signal matches, and generating an alternating signal synchronized with the transmission data. The transmission device generates the alternating signal, and modulates the DPSK signal based on the generated alternating signal so that the polarization state is orthogonal between adjacent bits of the DPSK signal. The signal light modulation method according to claim 17.   The DPSK signal modulated by the DPSK modulation step is controlled so that the polarization plane of the DPSK signal modulated by the DPSK modulation step coincides with the polarization plane of the DPSK signal when directly input to the polarization-maintaining fiber. The signal light modulation method according to claim 13, wherein the optical transmission apparatus performs the step of adjusting to the following. A signal light modulation method performed by an optical transmission device that transmits signal light input from a light source,
A DPSK modulation step of modulating signal light input from the light source into a DPSK (Differential Phase Shift Keying) signal based on transmission data;
An output waveform is generated by alternately separating the DPSK signal modulated by the DPSK modulation step in the X-axis direction and the Y-axis direction between adjacent bits of the DPSK signal, and the generated X-axis direction An orthogonal step of integrating the output waveforms separated in the Y-axis direction and orthogonalizing the polarization between the adjacent bits;
The optical transmission apparatus performs the signal light modulation method.

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