JP4709799B2 - Optical SSB modulator - Google Patents

Description

  The present invention relates to an optical modulator that obtains an optical signal modulated by an electrical signal. In particular, the present invention relates to an optical SSB (Single Sideband) modulation technique for suppressing and outputting one sideband.

  As the optical modulator, an intensity modulation method for modulating the intensity of light is widely known. This intensity modulation method includes a so-called optical DSB (Double Sideband) modulation method that outputs sidebands on both sides, and an optical SSB modulation method that suppresses and outputs one sideband (non-patent document). 1 and 2).

  In the optical DSB modulation system, when optical signal transmission is performed, there is a problem that the waveform deteriorates due to the dispersion of the optical fiber (Chromatic Dispersion), the amplitude increases or decreases, and the transmission distance is limited. It was.

  On the other hand, the optical SSB modulation method has an advantage that even if optical signal transmission is performed, the waveform does not deteriorate depending on the dispersion of the optical fiber, the amplitude is constant, and the transmission distance is not limited. Until now, in order to obtain optical SSB modulation, a method using an LN-MZ (Lithium Niobate Mach-Zehndar) modulator and a method using an optical wavelength filter are known (see Patent Document 1 and Non-Patent Document 2). ).

  FIG. 6 is a schematic configuration diagram illustrating an example of a conventional LN-MZ modulator. Carrier light having a wavelength of 1559 nm from a DFB-LD (Distributed feedback laser diode) 31 is input to the LN-MZ modulator 35. A multicarrier electric signal (fs = 11.727 GHz to 11.996 GHz) from the multicarrier signal generator 32 is branched by the 90 ° coupler 33 with a phase difference of 90 ° between the two signals. Here, the multi-carrier electrical signal is called a satellite (BS) signal carrier. The multicarrier electric signal is branched by the 90 ° coupler 33 with a phase difference of 90 ° between the two signals. The two branched signals are applied to the two electrodes of the previous LN-MZ modulator 35, the carrier light is optically SSB modulated, and the modulated signal light is output. The optical spectrum of the output light is shown in FIG. Sidebands 52a and 52b appear on both sides of the optical carrier 50 appearing at a wavelength of 1559 nm of the carrier light, and the sideband 52b is suppressed. Phase shifters 34a and 34b in FIG. 6 adjust the phase of the electrical signal input to the LN-MZ modulator 35 to obtain the maximum degree of sideband suppression.

  FIG. 8 is a schematic configuration diagram illustrating an example of an optical SSB modulator using a conventional optical wavelength filter. The carrier light having a wavelength of 1559 nm from the DFB-LD 31 is intensity-modulated by the multi-carrier electric signal (fs = 11.727 GHz to 11.996 GHz) from the multi-carrier signal generator 32 using the external modulator 36. Then, optical DSB modulation is applied. The optical DSB modulated light is input to the optical wavelength filter 37. The transmission characteristics of the optical wavelength filter 37 used here are shown in FIG. Due to the transmission characteristics of the optical wavelength filter 37, one sideband is suppressed and optical SSB modulated signal light is output.

The optical spectra at points A and B in FIG. 8 are shown in FIGS. 10 and 11, respectively. At point A, optical DSB modulation is applied, but an optical signal is optically SSB modulated by suppressing the sideband on one side by the transmission characteristics of the optical wavelength filter.
JP-A-2005-274806 D. Fonseca, A.M. V. T.A. Cartaxo and P.M. Monteiro, “Optical single-sideband transmitter for various electrical signaling formats,” J. et al. Lightwave Tech. , Vol. 24, no. 5, pp. 2059-2069, May. 2006 J. et al. Park, W.M. V. Sorin and K.K. Y. Lau, “Elimination of the fiber chromatic dispersal penalty 1550 nm mimetre-wave optical transmission,” Electron. Lett. , Vol. 33, no. 6, pp. 512-513, Mar. 1997

  In the conventional LN-MZ modulator, the degree of the sideband suppression depends on the accuracy of the amplitude and phase of the electric signal input to the two electrodes, and it is difficult to obtain a high sideband suppression. Also, the optical SSB modulator using the optical wavelength filter can obtain a high degree of sideband suppression when the modulation frequency is high, but cannot obtain a high degree of sideband suppression when the modulation frequency is low. If a high degree of sideband suppression can be obtained by an optical modulator, even if optical signal transmission is performed, the waveform does not deteriorate due to the dispersion of the optical fiber, and the amplitude becomes constant, and the transmission distance is limited. Ideal transmission is possible. However, if a high degree of sideband suppression cannot be obtained by an optical modulator, when optical signal transmission is performed, the waveform will be slightly degraded depending on the dispersion of the optical fiber, and the amplitude will not increase and decrease. However, the transmission distance is limited depending on the degree of increase / decrease in the amplitude.

  An object of the present invention is to provide an optical SSB modulator capable of obtaining a high degree of sideband suppression.

  In order to achieve the above object, an optical SSB modulator according to the present invention is an optical SSB modulator that suppresses one of the sidebands using a filter. A single carrier electric signal having a constant frequency different from the carrier electric signal is multiplexed. Then, by performing optical intensity modulation with a wavelength multiplexed signal obtained by multiplexing the multicarrier electrical signal and the single carrier electrical signal, one of the sidebands based on the single carrier electrical signal can be used as a new optical carrier.

  Specifically, an optical SSB modulator according to the present invention includes a multicarrier signal generator for generating a multicarrier electrical signal in a predetermined frequency band, and a single carrier having a predetermined frequency different from the frequency band. A single carrier signal generator for generating an electric signal, a multicarrier electric signal generated by the multicarrier signal generator and a single carrier electric signal generated by the single carrier signal generator are combined to generate a frequency multiplexed signal. An output multiplexer; an optical intensity modulator that modulates the intensity of carrier light having a constant wavelength with a frequency multiplexed signal output from the multiplexer; and the frequency generated by the modulation of the optical intensity modulator One wavelength component of the sideband due to the multiplexed signal and the wavelength component of the carrier light are blocked, and the light intensity modulator generates the light Characterized in that it comprises and an optical wavelength filter for passing the other wavelength component sideband by the frequency-multiplexed signal, the.

  The optical wavelength filter leaves one of the sidebands of the multicarrier electrical signal and the single carrier electrical signal. The remaining sideband of the single carrier electric signal can be regarded as a new optical carrier, and the wavelength of the new optical carrier is shifted from the original optical carrier. Since only the other sideband of the multicarrier electric signal exists in the frequency band of the satellite signal carrier centering on the new optical carrier, a high degree of sideband suppression can be obtained. When transmitting a new optical carrier and satellite signal carrier, the influence of one sideband due to the multicarrier electrical signal can be eliminated.

  An optical SSB modulator according to the present invention generates a multicarrier signal having a predetermined frequency band and generates a single carrier electric signal having a predetermined frequency different from that of the frequency band. A single carrier signal generator, a multiplexer for combining the multicarrier electrical signal generated by the multicarrier signal generator and the single carrier electrical signal generated by the single carrier signal generator, and a carrier having a constant wavelength A light generator that directly modulates light using the multi-carrier electric signal and the single carrier electric signal combined by the multiplexer, and the single carrier electric signal and the multi-carrier electric signal generated by the modulation of the light generator. One of the sidebands and the carrier light are blocked, and the light generator is modulated. Characterized in that it and a light wavelength filter which passes the other sideband by the single-carrier electric signal and the multi-carrier electric signal generated I.

  The optical wavelength filter leaves one of the sidebands of the multicarrier electrical signal and the single carrier electrical signal. The remaining single carrier electrical signal sideband can be seen as a new optical carrier, and the wavelength of the new optical carrier is shifted from the original optical carrier. Since only the other sideband of the multicarrier electric signal exists in the frequency band of the satellite signal carrier centering on the new optical carrier, a high degree of sideband suppression can be obtained. When transmitting a new optical carrier and satellite signal carrier, the influence of one sideband due to the multicarrier electrical signal can be eliminated.

  In the optical SSB modulator according to the present invention, the single carrier electric signal preferably has a constant amplitude. Since the amplitude of the new carrier light becomes constant, a new optical carrier and satellite signal carrier can be transmitted without being limited by the transmission distance.

  In the optical SSB modulator according to the present invention, a frequency converter that converts the frequency of the multicarrier electrical signal generated by the multicarrier signal generator so as to be higher by the frequency of the single carrier electrical signal, It is further provided between the carrier signal generator and the multiplexer, and the multiplexer multiplexes the signal to be converted by the frequency converter and the single carrier electric signal generated by the single carrier signal generator. preferable. The frequency difference between the new optical carrier and the satellite signal carrier is equal to the frequency difference between the original optical carrier and the satellite signal carrier. Therefore, an optical signal that is optically SSB modulated by a satellite signal carrier is obtained by regarding the sideband on one side modulated by the remaining single carrier electrical signal as a new optical carrier.

  According to the present invention, since the optical SSB modulator according to the present invention can obtain a high degree of sideband suppression, it is possible to obtain an effect that, when optical signal transmission is performed, the waveform hardly deteriorates due to dispersion of the optical fiber.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
(Embodiment 1)
FIG. 1 is a schematic configuration diagram of an optical SSB modulator according to the present embodiment. The optical SSB modulator according to this embodiment includes a multicarrier signal generator 11, a single carrier signal generator 12, a multiplexer 13, an optical intensity modulator 14, and an optical wavelength filter 15. In this embodiment, the example provided with the light source 16 was shown further. In the present embodiment, it is preferable that a frequency converter 17 is further provided between the multicarrier signal generator 11 and the multiplexer 13.

  The optical SSB modulator according to the present embodiment uses the light intensity modulator 14 to convert the semiconductor laser light output from the light source 16 into a high-frequency signal having a constant amplitude having a frequency fo and a high-frequency signal having a frequency band fs. Apply intensity modulation directly with multiple signals. Then, the output light of the light intensity modulator 14 is input to the optical wavelength filter 15 and only one sideband is output.

  The multicarrier signal generator 11 generates a multicarrier electric signal in a predetermined frequency band. The frequency band of the multicarrier electrical signal is, for example, the frequency band of the satellite signal carrier. The frequency band of the satellite signal carrier is, for example, 11.727 GHz to 11.996 GHz. The number of carriers of the multi-carrier electric signal is a number capable of transmitting information, and is 8 or 64, for example. In this embodiment, since the frequency of the optical carrier is shifted by the frequency fo of the single carrier electric signal, the frequency of the multicarrier electric signal generated by the multicarrier signal generator 11 is increased so as to be higher by the frequency of the single carrier electric signal. It is preferable to provide a frequency converter 17 for conversion. The frequency converter 17 can be omitted if the frequency band of the multicarrier electrical signal is set in advance to a frequency band that is increased or decreased from the original frequency band of the satellite signal carrier by the frequency fo of the single carrier electrical signal. .

  The single carrier signal generator 12 generates a single carrier electric signal having a predetermined frequency fo different from the frequency band. The constant frequency fo is a frequency different from the frequency band fs of the multicarrier electric signal, and may be higher or lower than the frequency band fs. The single carrier electrical signal preferably has a frequency lower than the frequency band of the multicarrier electrical signal. Sidebands based on single carrier electric signals and sidebands based on multicarrier electric signals can be generated in order from the original optical carrier side in the sidebands on the higher frequency side than the optical carrier. Since a new optical carrier is generated on the long wavelength side of the upper sideband and a satellite signal carrier can be generated on the short wavelength side, matching with a conventional demodulator is good.

  The constant frequency fo is a frequency at which the optical wavelength filter 15 can suppress the original optical carrier and extract a sideband based on the single carrier electric signal. For example, when the constant frequency fo is lower than the frequency band fs, it is preferable that the frequency fo has a frequency comparable to the difference between the frequency of the optical carrier and the frequency band of the satellite signal carrier. For example, if the low frequency side of the frequency band of the satellite signal carrier is 11.727 GHz, the frequency fo is preferably about 10 GHz. By making it easy to suppress the original optical carrier, it is possible to improve the signal quality of a new optical carrier generated by modulation with a single carrier electric signal.

  Moreover, it is preferable that the single carrier electric signal has a constant amplitude. For example, the single carrier signal generator 12 generates a single carrier electric signal having a constant amplitude. Further, an amplifier that makes the amplitude constant may be provided. Since the amplitude of the single carrier electric signal is constant, the amplitude of a new optical carrier generated by the single carrier electric signal is constant. Since the amplitude of the optical carrier is constant, a high sideband suppression degree of a new optical carrier and signal carrier can be obtained, so that an optical signal can be transmitted without being limited by the transmission distance.

  The multiplexer 13 multiplexes the multicarrier electric signal generated by the multicarrier signal generator 11 and the single carrier electric signal generated by the single carrier signal generator 12. Then, a wavelength multiplexed signal in which the multicarrier electric signal and the single carrier electric signal are combined is output. When the frequency converter 17 is further provided between the multicarrier signal generator 11 and the multiplexer 13, the multicarrier electric signal converted by the frequency converter 17 and the single carrier generated by the single carrier signal generator 12 are used. Combines electrical signals. The multiplexer 13 multiplexes the multicarrier electric signal and the single carrier electric signal, so that a new optical carrier and signal carrier having a constant frequency difference can be stably generated.

  The light source 16 generates carrier light having a constant wavelength. The light source 16 is, for example, a semiconductor laser such as DFB-LD.

  The light intensity modulator 14 modulates the intensity of the carrier light having a constant wavelength with the wavelength multiplexed signal combined by the multiplexer 13 and outputs the result. The carrier light is light output from the light source 16. The light intensity modulator 14 is, for example, an EA modulator (Electro-Absorption Modulator) or an LN-MZ modulator.

  FIG. 3 is a spectrum diagram showing an example of modulated signal light at point C. Sidebands 51a and 51b modulated by a single carrier electrical signal and sidebands 52a and 52b modulated by a multicarrier electrical signal appear on the long wavelength side and the short wavelength side of an optical carrier having a wavelength of 1559 nm. . In this embodiment, since the constant frequency fo of the single carrier electric signal combined by the multiplexer 13 is 10 GHz and the frequency band fs of the multicarrier electric signal is 21.727 GHz to 21.996 GHz, the single carrier electric signal having a constant 10 GHz is used. Optical DSB modulation is applied to the signal and eight multicarrier electric signals having a frequency band of 21.727 GHz to 21.996 GHz.

  The optical wavelength filter 15 blocks one of the sidebands and the carrier light generated by the modulation of the optical intensity modulator 14 and the single carrier electric signal generated by the modulation of the optical intensity modulator. The other sideband of the signal and the multicarrier electrical signal is passed. The optical wavelength filter 15 preferably has a transmission characteristic capable of separating the original optical carrier and the sideband. An example of the transmission characteristics of the optical wavelength filter 15 is shown in FIG. In this embodiment, since the wavelength of the original optical carrier, 1995 nm, is suppressed and the upper sideband is transmitted, the wavelength longer than the wavelength of 1995 by 10 GHz corresponding to the constant frequency fo of the single carrier electrical signal is transmitted. A high-pass filter that transmits the wavelength can be used. In order to reduce noise at point D in FIG. 1, the optical wavelength filter 15 is preferably a bandpass filter that transmits only one sideband of the single carrier electric signal and the multicarrier electric signal.

  FIG. 4 is a spectrum diagram showing an example of modulated signal light at point D. The lower sidebands 51b and 52b are suppressed by the transmission characteristics of the optical wavelength filter, and the original optical carrier 50 having a wavelength of 1559 nm is also suppressed. The upper sideband 51a modulated by the remaining 10 GHz constant single carrier electric signal and the upper sideband 52a modulated by the multicarrier electric signal are output. Accordingly, the upper sideband 51a can be used as a new optical carrier, and the upper sideband 52a can be used as a new satellite signal carrier.

  The operation of the optical SSB modulator according to this embodiment will be described with reference to FIG. The multicarrier signal generator 11 generates a multicarrier electric signal in a frequency band fs (11.727 GHz to 11.996 GHz in this embodiment) corresponding to the satellite signal carrier. In the present embodiment, the frequency band fs is eight. The frequency converter 17 collectively obtains a multicarrier electric signal having a frequency band of 21.727 GHz to 21.996 GHz.

  A single carrier electric signal having a constant frequency fo (10 GHz in this embodiment) and eight satellite signal carriers having fs = 11.727 GHz to 11.996 GHz in the frequency band obtained by the multiplexer are obtained. A frequency multiplexed signal is obtained by multiplexing. The frequency-multiplexed electrical signal is applied to the light intensity modulator 14 to modulate the intensity of the carrier light output from the light source 16. Then, the optical DSB modulated light is output. The optical DSB modulated light is input to the optical wavelength filter 15.

  The optical wavelength filter 15 obtains an optical signal that is optically SSB modulated by suppressing not only the sideband on one side but also the original optical carrier due to its transmission characteristics. The optical SSB modulated optical signal output from the optical wavelength filter 15 has a frequency obtained by subtracting fo = 10 GHz from fs = 21.727 GHz-21.996 GHz, 11.727 GHz-11. 996 GHz, which is the frequency of the desired input satellite signal carrier.

  The optical spectra at points C and D in FIG. 1 are shown in FIGS. 3 and 4, respectively. At point C, optical DSB modulation is applied with a frequency signal constant at 10 GHz and eight multicarrier electric signals having a frequency of fs = 21.727 GHz to 21.996 GHz. The lower sideband is suppressed by the transmission characteristics of the optical wavelength filter, and the original optical carrier having a wavelength of 1559 nm is further suppressed. The upper sidebands modulated by the remaining 10 GHz constant single carrier electric signal and the upper sidebands modulated by eight multicarrier electric signals are output.

  The frequency difference between one sideband modulated with eight multicarrier electrical signals and one sideband modulated with 10 GHz constant single carrier electrical signal is 11.727 GHz to 11.996 GHz. Yes, this is equal to the frequency of the original satellite signal carrier. If one sideband modulated by the remaining single carrier electrical signal is considered as a new optical carrier, one sideband modulated by 8 satellite signal carriers modulated by multicarrier electrical signals The obtained optical signal is optical SSB modulated. Centering on the new optical carrier, there will be no modulated signal at the opposite side of the sideband on one side modulated by the remaining 8 satellite signal carriers. From this, an infinite sideband suppression degree is obtained.

(Embodiment 2)
FIG. 5 is a schematic configuration diagram of the optical SSB modulator according to the present embodiment. The optical SSB modulator according to the present embodiment is different from the first embodiment in that the light generator is directly intensity-modulated in order to obtain optical DSB-modulated light. The optical SSB modulator shown in FIG. 5 includes a multicarrier signal generator 11, a single carrier signal generator 12, a multiplexer 13, a light generator 18, and an optical wavelength filter 15. In this embodiment, the example provided with the frequency converter 17 was shown further. The functions and operations of the multicarrier signal generator 11, the single carrier signal generator 12, the multiplexer 13, and the optical wavelength filter 15 are the same as those of the optical SSB modulator according to the first embodiment. . In the present embodiment, a light generator 18 is provided instead of the light source 16 and the light intensity modulator 14 described in the optical SSB modulator according to the first embodiment.

  The light generator 18 directly modulates the carrier light having a constant wavelength by the multicarrier electric signal and the single carrier electric signal combined by the multiplexer 13. The light generator 18 is, for example, a DFB-LD. In direct intensity modulation of DFB-LD, the amount of optical frequency fluctuation (chirp) of laser light is generally larger than that of external modulation. If this amount of chirp is ignored, the optical spectra at points E and F in FIG. 5 correspond to the optical spectra at points C and D in FIG. 1, respectively, and the same optical spectrum is obtained. Is shown in FIGS. In this way, light that is optically SSB modulated can be obtained at point F.

  The present invention can be used for optical signal transmission.

1 is a schematic configuration diagram of an optical SSB modulator according to Embodiment 1. FIG. 2 is an example of transmission characteristics of an optical wavelength filter 15; It is a spectrum figure which shows an example of the modulation | alteration signal light in C point. It is a spectrum figure which shows an example of the modulation | alteration signal light in D point. 6 is a schematic configuration diagram of an optical SSB modulator according to Embodiment 2. FIG. It is a schematic block diagram which shows an example of the conventional LN-MZ modulator. It is a spectrum figure which shows an example of the modulation signal light by the conventional LN-MZ modulator. It is a schematic block diagram which shows an example of the optical SSB modulator by the conventional optical wavelength filter. It is an example of the transmission characteristic of the conventional optical wavelength filter. It is a spectrum figure which shows an example of the modulation | alteration signal light in A point. It is a spectrum figure which shows an example of the modulation | alteration signal light in B point.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Multicarrier signal generator 12 Single carrier signal generator 13 Multiplexer 14 Optical intensity modulator 15 Optical wavelength filter 16 Light source 17 Frequency converter 18 Optical generator 31 DFB-LD
32 Multicarrier signal generator 33 90 ° coupler 34 Phase shifter 35 LN-MZ modulator 36 External modulator 37 Optical wavelength filter 50 Optical carrier 51a Upper sideband generated by single carrier electric signal 51b Generated by single carrier electric signal Lower sideband 52a Upper sideband generated by multicarrier electrical signal 52b Lower sideband generated by multicarrier electrical signal

Claims (4)

A multicarrier signal generator for generating a multicarrier electrical signal in a predetermined frequency band;
A single carrier signal generator for generating a single carrier electric signal having a predetermined frequency different from the frequency band;
A multiplexer that combines the multicarrier electrical signal generated by the multicarrier signal generator and the single carrier electrical signal generated by the single carrier signal generator to output a frequency multiplexed signal;
An optical intensity modulator that modulates the intensity of carrier light having a constant wavelength with the frequency multiplexed signal output from the multiplexer,
One side component of the sideband by the frequency multiplexed signal generated by the modulation of the light intensity modulator and the wavelength component of the carrier light are cut off, and the side by the frequency multiplexed signal generated by the modulation of the light intensity modulator An optical SSB modulator comprising: an optical wavelength filter that passes the other wavelength component of the waveband. A multicarrier signal generator for generating a multicarrier electrical signal in a predetermined frequency band;
A single carrier signal generator for generating a single carrier electric signal having a predetermined frequency different from the frequency band;
A multiplexer that multiplexes the multicarrier electric signal generated by the multicarrier signal generator and the single carrier electric signal generated by the single carrier signal generator;
A light generator for directly modulating carrier light having a constant wavelength by a multi-carrier electric signal and a single carrier electric signal combined by the multiplexer;
One of the sidebands by the single carrier electric signal and the multicarrier electric signal generated by the modulation of the light generator and the carrier light are cut off, and the single carrier electric signal generated by the modulation of the light generator and An optical SSB modulator comprising: an optical wavelength filter that passes the other sideband of the multicarrier electric signal.   The optical SSB modulator according to claim 1 or 2, wherein the single carrier electric signal has a constant amplitude. A frequency converter that converts the frequency of the multicarrier electric signal generated by the multicarrier signal generator so as to be higher by the frequency of the single carrier electric signal, the frequency converter of the multicarrier signal generator and the multiplexer More in between,
4. The light according to claim 1, wherein the multiplexer multiplexes the signal converted by the frequency converter and the single carrier electric signal generated by the single carrier signal generator. 5. SSB modulator.

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