Patent Application
20100142959
The embodiments relate to a method and an apparatus, for waveform shaping a plurality of signal lights of different wavelengths contained in a wavelength division multiplexed (WDM) signal light.
In a commercially operated WDM optical fiber communication system which compensates for an optical loss of an optical fiber transmission path by using rare-earth element doped optical amplifiers including an erbium doped fiber amplifier (EDFA), which are characterized of performing collectively direct-amplification on a multiple wavelength signal light, a binary intensity modulation signal is mainly used. This is because the WDM optical fiber communication system can be realized with a simple configuration in which the light is directly modulated with an electric signal in a transmitter and the intensity of signal light is detected in a receiver, and accordingly, because of such simplicity, high reliability can be ensured.
On the other hand, focusing on transmission characteristics, in the WDM system, the signal light quality is deteriorated as the WDM signal light is propagated through the optical fiber transmission path, due to cross-talk among different wavelengths, a nonlinear effect in the optical fiber, chromatic dispersion, accumulation of amplified spontaneous emissions (ASE) noise, and an interaction thereof, so that a transmission distance of the WDM signal light is restricted. As technologies for overcoming such restriction, there are a regeneration repeating technology and a multi-value modulating technology, for example.
The regeneration repeating technology is realized by utilizing a signal regenerator having a configuration in which the receiver and the transmitter is connected to each other. To be specific, the WDM signal light is separated for each one wavelength, and thereafter, the signal light of each wavelength is converted into an electric signal in the receiver of the signal regenerator so that a signal waveform is electrically regenerated. Then, in accordance with the regenerated signal waveform, an optical signal is generated in the transmitter, and a WDM signal light obtained by wavelength multiplexing the signal light of each wavelength is output. Such a regeneration repeating technology can largely improve the signal quality, but has disadvantages in that the signal regenerator needs to be used for each wavelength. Therefore, in the WDM system, such a regeneration repeating technology as described above is not practically used, due to the cost, size, power consumption and the like of the signal regenerator.
On the other hand, the multi-value modulating technology is a technology which applies a multi-value modulating format inclusive of phase modulation to a signal light, and is now actively researched. In such a multi-value modulating technology, comparing with the signal light in the conventional binary intensity modulating format, although a propagation distance of the signal light can be extended, there is a problem in that configurations of transmitter and receiver are complicated, sizes thereof are enlarged and the like.
Based on the above technologies, nowadays, the advent of a signal regenerating technology capable of collectively processing a WDM signal light without separating the WDM signal light for each wavelength is largely expected. Particularly, in view of practical application, as one of requirements for an optical waveform shaping technology, it is demanded that the wavelength of each signal light is not changed before or after optical waveform shaped. This is because, if the wavelength of each signal light is changed, the chromatic dispersion to which each signal light is subjected in the optical fiber transmission path is different to each other, and accordingly, designing of WDM system becomes significantly hard.
Up to the present, there has been proposed some technologies each of which waveform shapes the WDM signal light while maintaining the WDM signal light in an optical state. Patent Document 1 (Japanese Laid-open Patent Publication No. 2002-372730) described below discloses a technology for realizing the waveform shaping of the WDM signal light by utilizing a soliton effect. This conventional technology is a waveform shaping method which focuses on a feature that a signal light waveform self-converges into a soliton type (sech2 function) in a nonlinear fiber. As the nonlinear fiber, there is utilized the one having dispersion characteristics in that the signal light of each wavelength is subjected to self-phase modulation (SPM) for soliton forming but four-wave mixing (FWM) between the signal light of each wavelength is not generated. For the signal light of each channel input to such a nonlinear fiber, intensity fluctuation thereof is converted into fluctuation of spectrum width by the SPM. However, since an incident peak power thereof is near a fundamental soliton power, a center portion of spectrum converges into a shape near a fundamental soliton. Then, the WDM signal light output from the nonlinear fiber is fed to a demultixing filter and a multiplexing filter each of which transmission band is matched with the center wavelength of the signal light of each channel, and the fluctuation of spectrum width of each channel is cut off so that the original intensity fluctuation of each signal light is removed.
Further, Patent Document 2 (Japanese Patent No. 3662463) described below discloses a technology for realizing the waveform shaping of the WDM signal light by utilizing chirping due to the SPM. To be specific, the WDM signal light which has been amplified to have a required power is fed to a normal dispersion fiber so that the chirping due to the SPM is generated, and a component of small chirping is removed by an optical band-stop filter of which blocking band is matched with the center wavelength of each signal light so that the intensity fluctuation in the vicinity of a peak and a bottom of signal light pulse or the accumulated noise is removed.
However, in the above described conventional waveform shaping method utilizing the soliton effect, the noise of “1” level of the signal light can be suppressed in principle, but there is a problem in that the noise of “0” level may not be suppressed. Further, in the above described conventional waveform shaping method utilizing the chirping due to the SPM, the noises of both “0” and “1” levels can be suppressed, but there is a problem in that, since the chirping remains in the signal light of each channel after waveform shaped, unless the chirping is compensated for, it is difficult to maintain the high signal quality of each channel in the transmission of WDM signal light after waveform shaped.
According to an aspect of the invention, an optical waveform shaping method of collectively waveform shaping a plurality of signal lights of different wavelengths contained in a WDM signal light includes: a spectrum expanding process of inputting the WDM signal light to a first nonlinear medium to collectively expand a spectrum of signal light of each wavelength, and separating a signal component and a noise component which are contained in each signal light, into different spectrum regions; an optical filtering process of collectively extracting a signal component in a spectrum region on a shorter wavelength side of a center wavelength of each signal light for when each signal light contained in the WDM signal light has been input to the first nonlinear medium, for each signal light of which spectrum is expanded in the spectrum expanding process, and collectively removing the signal component and the noise component in the other spectrum region; and a wavelength shifting process of inputting the signal component of each signal light, which has been extracted in the optical filtering process, to a second nonlinear medium, collectively shifting, to a longer wavelength side, a wavelength of the signal component of each signal light by self-frequency shifting generated in the second nonlinear medium, and substantially matching a center wavelength of each signal component with the center wavelength of each signal light for when each signal light contained in the WDM signal light has been input to the first nonlinear medium.
Further, according to an aspect of the invention, an optical waveform shaping apparatus for collectively waveform shaping a plurality of signal lights of different wavelengths contained in a WDM signal light includes: a spectrum expanding section configured to have a first nonlinear medium to which the WDM signal light is input, and to collectively expand a spectrum of signal light of each wavelength contained in the WDM signal light in the first nonlinear medium, and to separate a signal component and a noise component which are contained in each signal light, into different spectrum regions; a first optical filter adapted to collectively extract a signal component in a spectrum region on a shorter wavelength side of a center wavelength of each signal light for when each signal light contained in the WDM signal light has been input to the first nonlinear medium, for each signal light of which spectrum is expanded by the spectrum expanding section, and to collectively remove the signal component and the noise component in the other spectrum region; and a wavelength shifting section configured to have a second nonlinear medium to which is input the signal component of each signal light, which has been extracted by the first optical filter, and to collectively shift, to a longer wavelength side, a wavelength of the signal component of each signal light by self-frequency shifting generated in the second nonlinear medium, and to substantially match a center wavelength of each signal component with the center wavelength of each signal light for when each signal light contained in the WDM signal light has been input to the first nonlinear medium.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
Hereunder, there will be described embodiments for implementing the present invention, with reference to accompanying drawings. The same reference numerals denote the same or equivalent parts in all drawings.
In
The first optical amplifier 10 collectively amplifies the WDM signal light input to the input port to output the amplified WDM signal light to the first nonlinear medium. As a specific example of the first optical amplifier 10, there is an erbium-doped optical amplifier (EDFA), a Raman amplifier or the like. However, the first optical amplifier in the present invention is not limited to the above specific example. The WDM signal light input to the input port contains a plurality of signal lights of different wavelengths, and herein, three waves signal lights of wavelengths λ1, λ2, λ3 are input to the input port. As described later, an optical output power of the first optical amplifier 10 (an optical input power to the first nonlinear medium 11) is controlled in accordance with a control signal from the first control circuit 14, so that self-phase modulation (SPM) being a nonlinear optical effect is generated in the first nonlinear medium 11 and spectrums of the signal lights of respective wavelengths λ1 to λ3 are expanded.
The first nonlinear medium 11 receives the signal lights of respective wavelengths λ1 to λ3 at one end thereof, and outputs, from the other end thereof, the signal lights of which spectrums are expanded due to the SPM. It is desirable that the nonlinear effects of same amount are generated for the signal lights of respective wavelengths λ1 to λ3. As a specific example of the nonlinear medium 11, there is a nonlinear optical fiber which is obtained by doping germanium (Ge) to a conventional silica-based optical fiber to enhance a nonlinear refractive index, and further by decreasing a core sectional area to enhance the optical intensity, a photonic crystal fiber in which a hollow is disposed in a core portion to minimize an effective core sectional area, or the like. However, the nonlinear medium applicable to the present invention is not limited to the above specific example. The nonlinear optical fiber or the photonic crystal fiber described above has a nonlinear optical coefficient of fixed value in a wavelength band called C-band (1530 nm to 1365 nm) which is widely used in optical fiber communications, but may have chromatic dispersion of different values. In order to generate the nonlinear effects of same amount for the WDM signal light in the nonlinear medium 11, it is better to use a dispersion flat fiber of which chromatic dispersion value has a fixed value relative to wavelengths. Otherwise, in order to generate the nonlinear effects of same amount, it is also possible to use the one which is designed so that the chromatic dispersion in the entire nonlinear medium is fixed although the chromatic dispersion is locally different. Further, in the present invention, it is desirable to avoid the nonlinear effect, such as four-wave mixing (FWM), among the signal lights of respective wavelengths, and for this purpose, it is preferable to use a short nonlinear medium. Otherwise, it is also possible to use a nonlinear medium, in which chromatic dispersion is locally large, including a portion having an anomalous dispersion value and a portion having a normal dispersion value, in a longitudinal direction.
The first optical coupler 12 branches a part of the WDM signal light which is output from the first nonlinear medium 11 to be sent to the optical filter 20, and outputs a branched light to the first monitor circuit 13. The first monitor circuit 13 monitors an optical spectrum of the WDM signal light branched by the optical coupler 12, and transmits the monitor result to the first control circuit 14.
The first control circuit 14 controls the optical output power of the first optical amplifier 10 according to the monitor result of the first monitor circuit 13, so that optical spectrum shapes of the signal lights of respective wavelengths λ1 to λ3 output from the first nonlinear medium 11 become diphasic, and also, optical spectrum components of adjacent signal light wavelengths do not overlap with each other.
The first optical filter 20 has transmission bands respectively corresponding to the signal lights of wavelengths λ1 to λ3. Center wavelengths of respective transmission bands are shifted by Δλ to a shorter wavelength side relative to the corresponding signal light wavelengths λ1 to λ3. This wavelength shift amount Δλ to the shorter wavelength side is desirable to be set at ¼ times (Δλ=S/4) the wavelength spacing S between adjacent signal lights. Further, the width of each transmission band is set to be narrower than half of the wavelength spacing S. Each signal light which has passed through the nonlinear medium 11 and the optical spectrum of which has been made diphasic, is fed to such an optical filter 20 so that the signal component on the shorter wavelength side is extracted from each diphasic spectrum, and then, is output to the second optical amplifier 30. As a specific configuration of the optical filter 20 described above, there is a Fabry-Perot type optical filter, an arrayed waveguide grating type optical filter or the like. The Fabry-Perot type optical filter has the feature capable of coping with each signal light with a simple configuration, since the spacing between transmission center wavelengths is equal. In particular, the Fabry-Perot type optical filter using an etalon technology has the feature in that an insertion loss is small, wavelength regulation can be simply performed and the like, and accordingly, is suitable to be used as the optical filter 20. However, the optical filter applied to the present invention is not limited to the above specific example.
The second optical amplifier 30 collectively amplifies each signal light of which signal component on the shorter wavelength side is extracted by the optical filter 20, to output each amplified signal light to the second nonlinear medium 31. As the second optical amplifier 30, it is possible to use the one having a configuration same as that of the first optical amplifier 10. As described later, an optical output power of the second optical amplifier 30 (an optical input power to the second nonlinear medium 31) is controlled in accordance with a control signal from the second control circuit 34, so that soliton self-frequency shifting for shifting the wavelength of each signal by Δλ to a longer wavelength side is generated in second first nonlinear medium 31.
The second nonlinear medium 31 receives each signal light amplified by the second optical amplifier 30 at one end thereof, and outputs each signal light subjected to the soliton self-frequency shifting from the other end thereof. The soliton self-frequency shifting is a phenomenon that the wavelength of signal light which satisfies soliton conditions is shifted to the longer wavelength side in the second nonlinear medium 31, and an amount of the wavelength shifting is determined by a parameter of the second nonlinear medium 31 and a parameter of the signal light. As a specific example of the second nonlinear medium 31, there is a nonlinear fiber, a photonic crystal fiber or the like, similar to the first nonlinear medium 11. For the second nonlinear medium 31, it is also desirable to avoid the nonlinear effect, such as four-wave mixing (FWM), among each signal light, and for this purpose, it is preferable to use a short nonlinear medium or to use a nonlinear medium, in which chromatic dispersion is locally large, including a portion having an anomalous dispersion value and a portion having a normal dispersion value, in a longitudinal direction.
The second optical coupler 32 branches a part of the WDM signal light which is output from the second nonlinear medium 31 to be sent to the output port, and outputs a branched light to the second monitor circuit 33. The second monitor circuit 33 monitors an optical spectrum of the WDM signal light branched by the optical coupler 32, and transmits the monitor result to the second control circuit 34.
The second control circuit 34 controls the optical output power of the second optical amplifier 30 according to the monitor result of the second monitor circuit 33, so that peak wavelengths of respective signal lights output from the second nonlinear medium 31 are substantially matched with the wavelengths 801 to λ3 of respective signal lights for when the respective signal lights contained in the WDM signal light have been input to the first nonlinear medium 11.
Next, there will be described an operation of the present embodiment.
In the optical waveform shaping apparatus 1 of the above configuration, when the WDM signal light as illustrated in (A) of
In the first nonlinear medium 11, the spectrum of each signal light is expanded due to the generation of SPM, so that each signal light waveform is expanded. At the time, a noise component of “0” level (intensity fluctuation) has the small optical intensity and is hardly affected by the nonlinear effect, and therefore, the wavelength of the spectrum component of each signal light is not changed. As a result, as illustrated in the lower stage of
Here, considering that there is no noise component of “0” level or “1” level as described above, the optical spectrum components of the signal lights of respective wavelengths λ1 to λ3 become respectively optical spectrums of diphasic. Namely, each optical spectrum is depressed in the center portion thereof, and the spectrum components on both sides of the center portion become large. That is to say, by generating the SPM in the first nonlinear medium 11 to expand the spectrum of each of the signal lights of respective wavelengths λ1 to λ3, it becomes possible to separate the noise components of “0” level and “1” level to the center portion of the optical spectrum of each signal light and to the both end portions of the optical spectrum of each signal light, and to separate the signal component to a peak portion on the shorter wavelength side of the center of the optical spectrum of each signal light and to a peak portion on the longer wavelength side of the center of the optical spectrum of each signal light.
An expansion amount of the optical spectrum due to the SPM in the first nonlinear medium 11 described above depends on the power of each signal light output from the first optical amplifier 10. Therefore, in the present embodiment, the spectrum of each signal light output from the first nonlinear medium 11 is monitored by using the first optical coupler 12 and the first monitor circuit 13, and the first optical amplifier 10 is feedback controlled by the first control circuit 14 according to the monitor result. To be specific, in the first control circuit 14, an amplification operation of the first optical amplifier 10 is controlled so that the optical spectrum shape of each signal light monitored by the first monitor circuit 13 becomes diphasic, and also, the optical spectrum components of adjacent signal light wavelengths do not overlap with each other, and thus, the optical output power of the first optical amplifier 10 is optimized. As a condition that the optical spectrum components of the adjacent signal light wavelengths do not overlap with each other, a state where the shift amount Δλ of the peak wavelength on the shorter wavelength side (or the peak wavelength on the longer wavelength side) of the diphasic spectrum relative to the center wavelengths Δλ1 to Δλ3 of the respective signal lights before the SPM generation is ¼ times (Δλ=S/4) the wavelength spacing S between the adjacent signal lights, is most preferable.
Incidentally, as described in the above, it is desirable that the nonlinear effects (SPM) of same amount are generated for the signal lights of respective wavelengths λ1 to λ3 in the first nonlinear medium 11. For this purpose, the nonlinear optical coefficient of the first nonlinear medium 11 and the value of chromatic dispersion thereof may be made to be substantially fixed in the signal light wavelength band, and also, for the first optical amplifier 10, output wavelength characteristics in the signal light wavelength band may be made flat by utilizing a gain equalizer or the like. Further, in the case where wavelength dependence occurs in the nonlinear optical coefficient of the first nonlinear medium 11 or in the chromatic dispersion value thereof due to the restriction of practically usable optical fiber or the like, the output characteristics of the first optical amplifier 10 may have wavelength dependence so as to deny the wavelength dependence in the nonlinear optical coefficient or in the chromatic dispersion value.
Each signal light output from the first nonlinear medium 11 is next input to the optical filter 20. In the optical filter 20, for each signal light, the shorter wavelength side component is extracted from the diphasic spectrum component (signal component) from which the noise components of “0” level and “1” level are separated. As a result, as illustrated in (C) of
As described in the above, in view of the design of optical fiber communication system, in the optical waveform shaping apparatus, it is strongly demanded that the signal light wavelength is not changed after the processing of the signal light. Therefore, in the present invention, each output light from the optical filter 20 is sequentially fed to the second optical amplifier 30 and the second nonlinear medium 31, so that the wavelength of each signal light is shifted to the longer wavelength side by Δλ.
To be specific, in the second optical amplifier 30, each signal light output from the optical filter 20 is collectively amplified so as to have the power at which the soliton self-frequency shifting is generated in the second nonlinear medium 31, and each amplified signal light is fed to the second nonlinear medium 31. Incidentally, each signal light may be fed to a dispersion medium (not shown in the figure) before being input to the second nonlinear medium 31, to thereby perform chirping compensation of each signal light.
In the second nonlinear medium 31, the wavelength of each signal light which satisfies the soliton conditions is shifted to the longer wavelength side. As described in the above, the wavelength shift amount to the longer wavelength side due to the soliton self-frequency shifting is determined by the parameter of the second nonlinear medium 31 and the parameter of the signal light. Therefore, in the present embodiment, the optical spectrum of each signal light output from the second nonlinear medium 31 is monitored by utilizing the second optical coupler 32 and the second monitor circuit 33, and the second optical amplifier 30 is feedback controlled by the second control circuit 34 according to the monitor result. To be specific, in the second control circuit 34, an amplification operation of the second optical amplifier 30 is controlled so that the peak wavelengths of the respective signal lights monitored by the second monitor circuit 13 are substantially matched with the original wavelengths λ1 to λ3, and thus, the optical output power of the second optical amplifier 30 is optimized. A control speed by the second control circuit 34 can be relatively low, since the parameter of the second nonlinear medium 30 is not dynamically and largely changed.
Incidentally, the wavelength shift amount to the longer wavelength side in the second nonlinear medium 31 needs to be substantially the same value for each signal light. For this purpose, as the second optical amplifier 30, it is preferable to use the one of which output power has not wavelength dependence. Such an optical amplifier can be realized by an EDFA utilizing a gain equalizer or the like. Further, by applying the above described nonlinear fiber, photonic crystal fiber or the like as the second nonlinear medium 31, the wavelength shift amount of same value for each signal light can be realized.
Thus, each signal light of which wavelength is returned, in the second nonlinear medium 31, to the wavelength for when each signal light contained in the WDM signal light has been input to the first nonlinear medium 11, satisfies the soliton conditions, and therefore, becomes the high quality signal without chirping. Consequently, as illustrated in
As described above, according to the optical waveform shaping apparatus 1 in the present embodiment, the waveforms of the signal lights of respective wavelengths contained in the WDM signal light can be collectively shaped, and also, it becomes possible to precisely hold the wavelengths of the respective signal lights before or after waveform shaped.
Incidentally, in the above first embodiment, there has been described the configuration example in which the first optical coupler 12 is disposed between the first nonlinear medium 11 and the optical filter 20 so that the optical spectrum of each signal light output from the first nonlinear medium 11 is monitored by the first monitor circuit 13. However, as illustrated in
Next, there will be described a second embodiment of the optical waveform shaping apparatus.
In
The second optical filter 40 has transmission bands of which center wavelengths are respectively matched with the respective signal light wavelengths λ1 to λ3, and the width of each transmission band is set to be narrower than half of the wavelength spacing S between each signal light, similarly to the first optical filter 20. As a specific configuration of the second optical filter 40, there is a Fabry-Perot type optical filter, an arrayed waveguide grating type optical filter or the like, similarly to the first optical filter 20.
In the optical waveform shaping apparatus 1 of the above configuration, similarly to the above described first embodiment, the optical spectrum of each signal light is expanded due to the SPM in the first nonlinear medium 11, and thereafter, the shorter wavelength side component of the diphasic spectrum is extracted by the first optical filter 20. Further, the output lights from the first optical filter 20 are collectively amplified by the second amplifier 30 to be fed to the second nonlinear medium 31, and then, the wavelengths of the respective signal lights are returned to λ1 to λ3. At the time, if a light which does not satisfy the soliton conditions is contained in the signal lights to be fed to the second nonlinear medium 31, this light does not perform the wavelength shifting to the longer wavelength side, and therefore, becomes a new noise component. Therefore, in the present embodiment, the second optical filter 40 is disposed on the output side of the second nonlinear medium 31, so that the non-soliton component which has not performed the wavelength shifting to the longer wavelength side is removed by the second optical filter 40 whereas the soliton component which has performed the wavelength shifting to the longer wavelength side is transmitted through the second optical filter 40. Further, herein, the power of each signal light transmitted through the second optical filter 40 is monitored by the second optical coupler 32′ and the second monitor circuit 33′, and the second optical amplifier 30 is feedback controlled by the second control circuit 34 so that the monitored power becomes maximum.
As described above, according to the optical waveform shaping apparatus 1 in the second embodiment, in addition to the effects of the above first embodiment, by disposing the second optical filter 40 on the latter stage of the second nonlinear medium 31, it becomes possible to collectively waveform shape, with higher precision, the signal lights of wavelengths λ1 to λ3 contained in the WDM signal light. Further, since the power of each signal light transmitted through the second optical filter 40 is monitored, the monitor circuit can be simplified.
Incidentally, in the second embodiment, differently from the configuration of the first embodiment illustrated in
Next, there will be described an application example of the optical waveform shaping apparatus in each embodiment described above to a WDM optical fiber communication system.
In the WDM optical fiber communication system illustrated in
According to the above described WDM optical fiber communication system, by collectively waveform shaping the WDM signal light by the optical waveform shaping apparatus 1, it becomes possible to suppress the noise occurred in the signal light of each wavelength during the repeating-transmission of the WDM signal light by the transmission path 51 and the optical amplifiers 52, and therefore, reception sensitivity in each receiver 53C in the reception terminal station 53 is improved and the WDM communication of high reliability can be realized. Further, by the waveform shaping of the WDM signal light by the optical waveform shaping apparatus 1, it becomes also possible to extend a distance capable of transmitting the WDM signal light.
Incidentally, in the above WDM optical fiber communication system, there has been described the configuration example in which the optical waveform shaping apparatus 1 of the present invention is arranged on the input end of the reception terminal station 53. However, as illustrated in
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
This application is based upon and claims the benefit of priority of the prior International Application No. PCT/JP2007/065764, filed on Aug. 10, 2007, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2007/065764 | Aug 2007 | US |
| Child | 12656510 | US |
