JP2016218173A - Phase conjugate light converter and optical transmission system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a frequency shift in the entire signal band, reduce the occurrence of excessive noise attributable to phase conjugate light conversion, and compensate for nonlinear noise every span of a transmission path due to inverse transmission characteristic.SOLUTION: A wavelength separation filter 101, a wavelength multiplex filter 102, a first nonlinear optical device 103, and a second nonlinear optical device 104 are included. The wavelength separation filter 101 separates a wavelength division multiplexed signal (WDM signal) into a short wave-side component and a long wave-side component. The nonlinear optical device 103 (104) multiplexes the short wave-side (long wave-side) component of the WDM signal with excitation light and then generates the phase conjugate light of the short wave-side (long wave-side) component by difference frequency generation and also separates the excitation light. The wavelength multiplex filter 102 multiplexes only the phase conjugate light of the short wave-side) component and the phase conjugate light of the long wave-side component of the WDM signal among input lights from the nonlinear optical devices 103, 104, to obtain an output light.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a phase conjugate light converter that collectively converts optical signals having a plurality of wavelengths into phase conjugate light, and an optical transmission system using the same.

  As ICT (Information Communication Technology) continues to expand the industry on a global scale, the use of ICT is expanding its role as a means of solving social issues, along with the social background of the arrival of an aging society. Optical communication systems that support these communication infrastructures are expected to continue to play an important role as social infrastructures.

  Up to now, the capacity of optical communication systems has been increased by various technological innovations. In recent years, the increase in capacity by the improvement of frequency utilization efficiency by digital coherent technology is progressing rapidly. For further increase in capacity, further improvement in frequency utilization efficiency is required.

  However, it is known from Shannon's theory that a higher signal-to-noise ratio (S / N ratio) is required in the system to improve frequency utilization efficiency. Up to now, the S / N ratio has been secured by increasing the signal (S), but the limit of the communication capacity per fiber is beginning to be pointed out.

  As the main factor, noise generated by the nonlinear optical effect of the optical fiber itself as a transmission path is a major limitation on the increase in capacity. Noise generated by this nonlinear optical effect is called nonlinear noise.

  For this reason, in recent years, proposals and examinations of methods for actively using phase conjugate light as a means for compensating for nonlinear noise have begun to progress rapidly. The phase conjugate light is a phase reversal operation, and can produce time reversal for the light wave except for propagation loss and optical amplification process.

  One method utilizing this phase conjugate light is nonlinear noise compression by optical signal processing using a phase sensitive optical amplifier (PSA). It has also been experimentally shown that phase noise can be compressed by allowing phase conjugate light and signal light to interact in a phase sensitive optical amplifier (see, for example, Non-Patent Document 1). In this method, noise compression is possible for QAM signals that are multi-valued in the phase and intensity directions. By applying this phase-sensitive optical amplifier as a repeater optical amplifier for each span of the transmission line, nonlinear noise is greatly reduced. May be reduced.

  As another method, signal light and phase conjugate light are sent to a transmission line, both are received by a receiver, subjected to photoelectric conversion, and then subjected to signal processing in the electrical domain to compensate for nonlinear noise. A method called boosting reception (Boosting reception) has been proposed (for example, see Non-Patent Document 2).

  The two methods described above are promising technologies for the realization of future large-capacity optical transmission systems, but each has its own problems.

  By using the phase sensitive optical amplifier as a repeater optical amplifier, nonlinear noise can be compressed for each transmission span, so that a great effect can be obtained in the entire optical transmission system. In order to effectively cause the interaction between the signal light and the phase conjugate light by the phase sensitive light amplification, it is necessary to accurately match the phase relationship among the signal light, the phase conjugate light, and the excitation light.

  However, generally used optical transmission fibers have chromatic dispersion, which causes a shift in the relative phase between the signal light and the phase conjugate light. There was a problem that it was difficult to effectively use in an optical transmission system. Although it is possible to apply a phase-sensitive optical amplifier by using dispersion compensation means using a fiber having characteristics opposite to the chromatic dispersion of the optical transmission fiber, it is introduced for each span in the entire optical transmission system. There is a problem that the signal light noise ratio (S / N ratio) deteriorates due to the optical loss caused by the dispersion compensation means.

  Boosting reception is a configuration that generates phase conjugate light paired with signal light at the transmission stage and performs signal processing in the electrical domain after receiving both at the reception stage, so it can be applied directly to existing optical transmission lines Has the great advantage of being.

  On the other hand, since Boosting reception is a single signal processing after reception, there is a limit as noise reduction means for nonlinear noise generated by accumulation in the transmission path. Further, since the signal processing needs to be performed after photoelectric conversion and further digitizing the signal at a constant sampling rate by A / D conversion, the operating band of a light receiver such as a photodiode used for photoelectric conversion, A / D There is a limit to the amount of noise that can be reduced by the sampling rate of the D converter. That is, there is a problem that it is difficult to compensate for nonlinear noise beyond the operating band and resolution of the receiver. This becomes more significant as the baud rate (baud rate) of the signal light is increased in order to increase the capacity, and thus there is a limit to the reduction of nonlinear noise.

  As a method different from the above-mentioned two local signal processings, there is a method of canceling the noise generated by the optical transmission fiber in the optical transmission system by using the optical transmission fiber itself. As this method, there is known an intermediate point phase conjugate conversion in which signal light is converted into phase conjugate light at almost the intermediate point of the optical transmission line, and only phase conjugate light is transmitted using the phase conjugate light as new signal light (for example, Non-Patent Document 3).

  FIG. 9A shows an example of this intermediate point phase conjugate transformation. In the figure, 1 is an optical transmitter, 2 is an optical transmission fiber, 3 is an optical amplifier, 4 is an optical receiver, and 5 is a phase conjugate optical converter. The phase conjugate light converter 5 is provided at a substantially intermediate point on the transmission path connecting the optical transmitter 1 and the optical receiver 4, converts the signal light from the optical transmitter 1 into phase conjugate light, and provides phase conjugate. Only phase conjugate light is transmitted using light as new signal light. FIG. 9B shows the wavelength band of the signal light from the optical transmitter 1 (wavelength band in the first half transmission path) and the wavelength band of the new signal light (phase conjugate light) to the optical receiver 4 (second half transmission path). Wavelength band).

  In this method, the signal light is converted to phase conjugate light at the intermediate point of the entire transmission line, and the nonlinearity cancellation is achieved by effectively backpropagating in the fiber using the time reversibility of the phase conjugate light. Is done. That is, the nonlinear noise generated in the optical transmission fiber 2 between the optical transmitter 1 and the intermediate point is compensated by effectively backpropagating in the optical transmission fiber 2 between the intermediate point and the receiver 4. .

T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto and H. Takenouchi., “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier,” Optics Express, February 2014, Vol. 22, No.3, p.2473-2482 Xiang Liu, A. R. Chraplyvy, P. J. Winzer, R. W. Tkach and S. Chandrasekhar., “Phase-conjugated twin waves for communication beyond the Kerr nonlinearity limit” Nature Photonics, Vol7, pp. 560-568 (2013) S. Watanabe, G. Ishikawa, T. Naito, and T. Chikama, “Genaration of optical phase conjugate waves and compensation for pulse shape distortion in a single-mode fiber,” J. Lightwave Technol., Vol.12. Pp. 2139-2146,1994.

  However, there is a problem with the conventional method of intermediate point phase conjugate conversion. For example, a wavelength converter that uses the nonlinear optical effect in a highly nonlinear fiber is used to convert the signal light to the phase conjugate light at the intermediate point, so the frequency shifts after the signal light is converted to the phase conjugate light. End up. For this reason, there is a problem that the design and construction of the optical transmission system becomes very difficult.

  Specifically, for example, a wavelength multiplexed signal (WDM signal) of 30 nm as a wavelength band is transmitted using the C band, and phase conjugate light is generated by wavelength conversion to the L band at an intermediate point of the transmission path. It is assumed that the light converted into is transmitted as signal light after the intermediate point. In this case, since the wavelength band becomes the L band after the intermediate point, the same optical amplifier as before the intermediate point cannot be used.

  In other words, in order to apply this method, it is actually necessary not only to install a converter to phase conjugate light at the intermediate point, but also to make all the optical amplifiers after the intermediate point capable of amplifying the L band. There is a restriction of being. In particular, in order to apply to an existing optical transmission system, it becomes necessary to replace all optical amplifiers after the intermediate point.

  In addition, since it is converted into phase conjugate light only at an intermediate point, there is a limit to the amount of reduction of nonlinear noise. A typical long-distance optical transmission system has a total length of several thousand km. Conversion to phase conjugate light at this intermediate point compensates by transmitting signal light transmitted several thousand km through another optical transmission fiber of several thousand km. In the reverse propagation characteristics using phase conjugate light, the cancellation effect is maximized as the fiber characteristics of the first and second transmission lines are the same, but there is a distance of several thousand km at the ends. In an actual optical transmission system including a plurality of optical amplifiers between them, nonlinear noise cannot be completely canceled because transmission characteristics are different before and after the intermediate point.

  Ideally, it is desirable that non-linear noise can be reduced for each transmission line span (relay span) in the optical transmission system, but not in the entire optical transmission system. Each time the converter is arranged, it is necessary to review the optical amplifier configuration and the like thereafter, and the flexibility of the transmission path is poor.

  In addition, in order to introduce a phase conjugate light converter for each span, there is a problem that the conversion efficiency is low in the prior art. When the conversion efficiency is low, an excessive loss of light occurs every time the light is converted into phase conjugate light. In this case, a separate optical amplifier is required to compensate for this excess loss, which means that excessive noise is generated in the entire system. In other words, new excessive noise is mixed in to cancel the nonlinear noise, so that a plurality of phase conjugate light conversions such as each span cannot be introduced into the optical transmission system.

  In addition, the conventional intermediate point phase conjugate conversion method converts signal light into phase conjugate light, and does not transmit the signal to the original signal wavelength band after the conversion, so the entire system uses twice as much bandwidth. There was also a problem. By reducing the non-linear noise, the signal-to-noise ratio (S / N ratio) of the entire system can be improved. However, since the frequency band is effectively doubled, the frequency utilization efficiency is reduced accordingly. There was a problem that it would end up.

  Because of these problems, it has been difficult to apply the conventional method of intermediate point phase conjugate conversion to an existing optical transmission system. In addition, the conventional method of intermediate point phase conjugate conversion is a local signal that positively utilizes phase conjugate light such as a phase sensitive optical amplifier (PSA) or Boosting reception, which is expected as a new technology described above. There is also a problem in terms of integration with processing technology, and so far there has been no configuration that can use these methods in a complementary manner.

  In other words, it is possible to cooperate with nonlinear noise reduction technology that actively utilizes phase conjugate light such as phase sensitive optical amplifier (PSA), etc., and noise can be compressed for each span, and the frequency shift in the entire signal band In addition, non-linear noise compensation based on reverse transmission characteristics can be applied to the optical transmission path for each relay instead of the entire optical transmission system, and the generation of excess noise associated with phase conjugate light conversion is small. Never before.

  The present invention has been made in order to solve such problems. The object of the present invention is to eliminate the frequency shift in the entire signal band and to reduce the generation of excess noise associated with phase conjugate light conversion. Another object of the present invention is to provide a phase conjugate optical converter capable of performing non-linear noise compensation by reverse transmission characteristics for each span and an optical transmission system using the same.

  In order to achieve such an object, the present invention provides a phase conjugate light converter that collectively converts an optical signal having a plurality of wavelengths into a phase conjugate light, wherein the optical signal having a plurality of wavelengths is converted into a first wavelength component. And the second wavelength component, and the first wavelength component separated by the wavelength separation filter is combined with the excitation light, and then the phase conjugate light of the first wavelength component is generated by the difference frequency generation. The first nonlinear optical device that generates and separates the excitation light and the second wavelength component separated by the wavelength separation filter are combined with the excitation light, and then the phase conjugate light of the second wavelength component is generated by the difference frequency generation. And a first wavelength component output from the first nonlinear optical device and its phase conjugate light and a second nonlinear optical device output from the second nonlinear optical device. The wavelength component and its phase conjugate light are input, and the wavelength combination of the first wavelength component phase conjugate light and the second wavelength component phase conjugate light of the input light is used as the output light. And a wave filter.

  According to the present invention, an optical signal composed of a plurality of wavelengths is separated into a first wavelength component and a second wavelength component by the wavelength separation filter, and the first wavelength component is input to the first nonlinear optical device, Two wavelength components are input to the second nonlinear optical device. The first nonlinear optical device, after combining the excitation light with the first wavelength component from the wavelength separation filter, generates phase conjugate light of the first wavelength component by difference frequency generation and separates the excitation light, The first wavelength component and its phase conjugate light are output. The second nonlinear optical device combines the excitation light with the second wavelength component from the wavelength separation filter, then generates phase conjugate light of the second wavelength component by difference frequency generation and separates the excitation light, The second wavelength component and its phase conjugate light are output. The wavelength multiplexing filter inputs the first wavelength component output from the first nonlinear optical device and the phase conjugate light thereof, and the second wavelength component output from the second nonlinear optical device and the phase conjugate light thereof. In this input light, the phase conjugate light of the first wavelength component and the phase conjugate light of the second wavelength component are combined to obtain output light.

  In the present invention, the first wavelength component and the second wavelength component of an optical signal composed of a plurality of wavelengths are converted into phase conjugate light in such a manner that they are interchanged within the same band. In this case, optical signals composed of a plurality of wavelengths are collectively converted into phase conjugate light, and the frequency is not shifted as the wavelength band of the entire optical signal composed of a plurality of wavelengths. In the present invention, if a direct junction type waveguide is used for the first and second nonlinear optical devices, an increase in excess loss due to low conversion efficiency can be avoided, and excess noise associated with phase conjugate light conversion can be avoided. Can be reduced. Further, by using the phase conjugate light converter of the present invention in an optical transmission system, it is possible to perform nonlinear noise compensation by reverse transmission characteristics for each span of the transmission path.

  In the present invention, an optical signal having a plurality of wavelengths is separated into a first wavelength component and a second wavelength component. As an example, the first wavelength component is a short-wave component, and the second wavelength component is used. It is conceivable that the component is a component on the long wave side. Alternatively, the first wavelength component may be an even channel component, and the second wavelength component may be an odd channel component.

  According to the phase conjugate light converter of the present invention, an optical signal having a plurality of wavelengths is separated into a first wavelength component and a second wavelength component, and each of the separated first wavelength component and second wavelength component is separated. After the excitation light is multiplexed, phase conjugate light is generated by difference frequency generation and the excitation light is separated, and the generated phase conjugate light of the first wavelength component and phase conjugate light of the second wavelength component are generated. Since the output light is multiplexed, the first wavelength component and the second wavelength component of the optical signal composed of a plurality of wavelengths are converted into phase conjugate light in the form of replacement within the same band, There is no frequency shift in the entire signal band, and it is possible to reduce the occurrence of excess noise due to phase conjugate light conversion. Further, by using the phase converter of the present invention in an optical transmission system, it is possible to perform nonlinear noise compensation by reverse transmission characteristics for each span of the transmission path.

It is a figure which shows the structure of 1st Embodiment (Embodiment 1) of the phase conjugate light converter which concerns on this invention. FIG. 6 is a diagram for explaining the operation of the phase conjugate light converter according to the first embodiment. It is a figure explaining operation | movement of the phase conjugate light converter of Embodiment 1 at the time of using an interleave type filter (interleaver) as a wavelength separation filter and a wavelength multiplexing filter. It is a figure which shows the structure of 2nd Embodiment (Embodiment 2) of the phase conjugate light converter which concerns on this invention. It is a figure which shows the structure of 3rd Embodiment (Embodiment 3) of the phase conjugate light converter which concerns on this invention. It is a figure which shows 1st Embodiment (Embodiment 4) of the optical transmission system using the phase conjugate light converter which concerns on this invention. It is a figure which shows the structure of 4th Embodiment (Embodiment 5) of the phase conjugate light converter which concerns on this invention. It is a figure which shows 2nd Embodiment (Embodiment 6) of the optical transmission system using the phase conjugate light converter which concerns on this invention. It is a figure which shows the structure of the optical transmission system using the conventional phase conjugate light converter.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Embodiment 1]
FIG. 1 shows a configuration of a first embodiment (Embodiment 1) of a phase conjugate light converter according to the present invention. The phase conjugate light converter 100 according to the first embodiment includes a wavelength separation filter 101, a wavelength multiplexing filter 102, a first nonlinear optical device 103, and a second nonlinear optical device 104.

  In the phase conjugate light converter 100, the first nonlinear optical device 103 includes a lithium niobate waveguide (hereinafter referred to as PPLN) having a periodically poled structure that satisfies quasi-phase matching between an input optical signal and pumping light. 103a, a dichroic mirror type multiplexer 103b that multiplexes the excitation light, and a dichroic mirror type demultiplexer 103c that divides the excitation light.

  Similarly to the first nonlinear optical device 103, the second nonlinear optical device 104 also includes a PPLN waveguide 104a having a periodically poled structure, a dichroic mirror type multiplexer 104b that combines excitation light, and excitation light. And a dichroic mirror type demultiplexer 104c.

  The wavelength separation filter 101 includes dichroic mirror type multiplexers / demultiplexers 101a, 101b, and 101c, and the wavelength multiplexing filter 102 includes dichroic mirror type multiplexers / demultiplexers 102a, 102b, and 102c.

  Next, the operation of the phase conjugate light converter 100 of the first embodiment will be described with the functions of the respective parts.

  The phase conjugate light converter 100 receives an optical signal having a plurality of wavelengths as input light. In this example, a wavelength multiplexed signal (WDM signal) is input (see FIG. 2A). The input WDM signal is separated into a short wave component (short wavelength signal) and a long wave component (long wavelength signal) by the wavelength separation filter 101 (see FIG. 2B).

  In this embodiment, the WDM signal is separated into a short-wave component and a long-wave component using the center wavelength in the entire WDM signal band as a reference wavelength. However, the reference wavelength is not necessarily the center wavelength.

  The short-wave component of the WDM signal separated by the wavelength separation filter 101 is input to the first nonlinear optical device 103. The long wave component of the WDM signal separated by the wavelength separation filter 101 is input to the second nonlinear optical device 104.

  In the first nonlinear optical device 103, the dichroic mirror type multiplexer 103 b multiplexes the excitation light with the short-wave component of the WDM signal from the wavelength separation filter 101. The short-wave component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 103a. The PPLN waveguide 103a generates phase conjugate light of the short-wave component of the WDM signal by the difference frequency generation.

  The excitation light combined by the dichroic mirror type multiplexer 103b has a wavelength that is half the reference wavelength from which the WDM signal is separated, in this embodiment, half the center wavelength in the entire WDM signal band. If the frequency of the center wavelength is ω0, the frequency of the excitation light is 2ω0. If the frequency of one wavelength of the short-wave component of the WDM signal is ωs, converted light having a frequency of 2ω0−ωs is generated by the difference frequency generation in the PPLN waveguide 103a. Further, as the optical phase, if the phase of the pumping light is Φp and the phase of the signal light is Φs, it becomes Φp−Φs due to the difference frequency generation, and the phase conjugate light of the signal light is generated based on the phase of the pumping light. . That is, the phase conjugate light of the signal light is generated on the long wave side in a form folded on the wavelength axis with the center wavelength as a reference.

  The phase conjugate light of the short-wave component of the WDM signal generated by the PPLN waveguide 103a is sent to the dichroic mirror type demultiplexer 103c together with the short-wave component of the WDM signal combined with the excitation light. The dichroic mirror type demultiplexer 103c separates the excitation light from the light transmitted through the PPLN waveguide 103a, and separates the excitation light (the phase of the short wavelength side component of the WDM signal + the short wavelength side component of the WDM signal). Conjugate light: see FIG. 2 (c), upper diagram) is sent to the wavelength multiplexing filter 102.

  In the second nonlinear optical device 104, the dichroic mirror type multiplexer 104 b multiplexes the excitation light with the component on the long wave side of the WDM signal from the wavelength separation filter 101. The long-wave component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 104a. The PPLN waveguide 104a generates phase conjugate light of the component on the long wave side of the WDM signal by the difference frequency generation in the same manner as the PPLN waveguide 103a in the first nonlinear optical device 103. In this case, phase conjugate light of the component on the long wave side of the WDM signal is generated on the short wave side in a form folded on the wavelength axis with the center wavelength as a reference.

  The phase conjugate light of the long wave side component of the WDM signal generated by the PPLN waveguide 104a is sent to the dichroic mirror type demultiplexer 104c together with the long wave side component of the WDM signal combined with the pump light. The dichroic mirror type demultiplexer 104c separates the excitation light from the light transmitted through the PPLN waveguide 104a, and separates the excitation light (the phase of the component on the long wave side of the WDM signal + the component on the long wave side of the WDM signal) Conjugate light: see FIG. 2 (c) lower diagram) is sent to the wavelength multiplexing filter 102.

  The wavelength multiplexing filter 102 includes the phase conjugate light of the short-wave component and the phase conjugate light of the long-wave component of the WDM signal out of the light input from the first nonlinear optical device 103 and the second nonlinear optical device 104. Are combined into an output light (see FIG. 2D) and sent out as a new WDM signal.

  In this case, the wavelength multiplexing filter 102 includes the original signal light (short-wave component of the WDM signal + long-wave side of the WDM signal) out of the light input from the first nonlinear optical device 103 and the second nonlinear optical device 104. Are quenched so that they are not extracted as output light.

  As described above, by adopting a configuration such as the phase conjugate light converter 100 of the present embodiment, the phase conjugate light is exchanged in the same band so that the short wave component and the long wave component of the WDM signal are interchanged. Can be converted to In other words, the short-wave component of the WDM signal is converted to the long-wave component with the center wavelength folded, and the long-wave component of the WDM signal is converted to the short-wave component with the center wavelength folded. Is done. Here, as the wavelength band of the entire WDM signal (entire signal band), the WDM signals can be collectively converted into phase conjugate light without shifting the frequency. Furthermore, if the relationship between the pumping light wavelength and the signal light wavelength is set appropriately, it can be converted into phase conjugate light in the same state up to the grid position of each signal in the WDM signal.

  When converting to the phase conjugate light by switching the short wave side component and the long wave side component of the WDM signal, between the short wave side called the guard band and the long wave side between the center wavelength to be turned back and the signal light. In order to suppress crosstalk, it is necessary to provide a certain band in which no signal is transmitted. However, in the configuration of the phase conjugate light converter 100 of the present embodiment, the guard band can be made sufficiently small. FIG. 2A shows a guard band as GB in the phase conjugate light converter 100 of the present embodiment.

  As an example, an attempt was made to convert a WDM signal having a wavelength of 1535 nm to 1565 nm in a 30 nm band into phase conjugate light. As the excitation light, light having a wavelength of 775 nm, which is a half wavelength of the center wavelength 1550 nm of the WDM signal, was used. In the phase conjugate light converter 100 of the present embodiment, since the nonlinear optical devices 103 and 104 based on the PPLN waveguide are used, the wavelength of the excitation light is greatly different from the signal wavelength. As a result, it is possible to multiplex / demultiplex with low loss using a dielectric dichroic mirror. In addition, since the excitation light is not in the 1550 nm band that is the same as the signal light wavelength, in principle, a guard band that is about the band for the excitation light of the PPLN waveguide may be provided. The band for the excitation light of the PPLN waveguide depends on the element length, but if a 50 mm long PPLN waveguide is used, a guard band of 0.2 nm (about 25 GHz) may be provided. Become. This is about 1% of the total bandwidth of 30 nm of the WDM signal, which can be said to be very small from the viewpoint of band loss.

  Another limiting factor of the guard band is the extinction characteristic of the wavelength multiplexing filter 102. If the wavelength multiplexing filter 102 does not have a sufficient extinction characteristic, the original signal light is superimposed on the phase conjugate light, so that the signal quality is deteriorated. In order for the wavelength multiplexing filter 102 to have sufficient extinction characteristics, a finite guard band is required.

  In the phase conjugate light converter 100 of the present embodiment, a high extinction ratio is realized by a multistage configuration in which two dichroic mirrors each using a dielectric multilayer film are used for reflection and transmission. Thereby, if a 3 nm guard band was provided, sufficient extinction characteristics of 30 dB or more were obtained. The guard band amount of 3 nm is about 10% of the total bandwidth of 30 nm of the WDM signal, and it can be said that it is small compared to the conventional method that needs to use twice the band from the viewpoint of band loss. In this embodiment, a dichroic mirror using a dielectric multilayer film is used for the wavelength multiplexing filter 102. However, in order to form a filter having a sharper and higher extinction, an optical filter such as LCOS or fiber black grating is used. May be used.

  Furthermore, in the phase conjugate light converter 100 of the present embodiment, in order to avoid an increase in excess loss due to low conversion efficiency, which is a problem of the conventional method, the PPLN waveguides 103a and 104a in the nonlinear optical devices 103 and 104 are used. As described above, a direct junction type waveguide is used. Here, a method for manufacturing the PPLN waveguides 103a and 104a used in this embodiment will be exemplified below.

First, a periodic electrode having a period of about 17 μm was formed on LiNbO ₃ to which Zn was added. Next, a polarization inversion grating corresponding to the above electrode pattern was formed in Zn: LiNbO ₃ by an electric field application method. Next, the Zn: LiNbO ₃ substrate having this periodic domain-inverted structure was directly bonded onto the LiTaO _{3 serving} as the cladding, and heat treatment was performed at 500 ° C. to firmly bond both substrates. Next, the core layer was thinned to about 5 μm by polishing, and a ridge type optical waveguide was formed using a dry etching process. This waveguide can be temperature-controlled by a Peltier element, and the length of the waveguide is 50 mm. The secondary nonlinear optical element having the PPLN waveguide formed as described above is a module capable of inputting and outputting light with a polarization maintaining fiber in a 1.5 μm band.

In this example, LiNbO ₃ to which Zn is added is used, but other nonlinear materials such as KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ , Alternatively, a nonlinear optical material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive may be used.

  An attempt was made to generate phase conjugate light by using the PPLN waveguides 103a and 104a thus manufactured. Excitation light having a wavelength of 775 nm was input to the first nonlinear optical device 103 and the second nonlinear optical device 104. Optical power of 300 mW was input to each of the nonlinear optical devices 103 and 104, respectively. Since the direct junction type PPLN waveguides 103a and 104a were used, stable operation was achieved even at a high power of 100 mW or more, and higher conversion efficiency was obtained as the converted light had a gain. A conversion gain of 4 dB is obtained, which is a value that can compensate for the total loss of 3 dB of the wavelength separation filter 101 and the wavelength multiplexing filter 102, and the entire phase conjugate optical converter 100 operates without loss. Was possible.

  As described above, by adopting the configuration like the phase conjugate light converter 100 of the present embodiment, the guard band is small, lossless, and there is no frequency shift in the entire signal band, and phase conjugate light conversion is performed. A phase-conjugate optical converter with little excess noise was obtained.

  In the present embodiment, a filter having a characteristic of separating a WDM signal into a short-wave component and a long-wave component is used as the wavelength separation filter 101, but the short-wave component of the short-wave component is used as the wavelength multiplexing filter 102. Although the filter having the characteristic of combining the phase conjugate light and the phase conjugate light of the component on the long wave side is used, an interleave type filter may be used.

  The wavelength separation filter 101 is an interleave type filter having the characteristic of separating the WDM signal into even-numbered channel components and odd-numbered channel components, and the wavelength multiplexing filter 102 is phase-conjugate light of even-numbered channel components and odd-numbered channel components. FIG. 3 shows the operation when an interleave type filter having the characteristic of combining the phase conjugate light is used.

  Further, in FIG. 1, the phase conjugate light converter when an interleave type filter (interleaver) is used is 100 ′, and the wavelength separation filter (interleaver) in this phase conjugate light converter 100 ′ is 101 ′, the wavelength The multiplexing filter (interleaver) is 102 ′.

  The phase conjugate light converter 100 ′ receives a WDM signal as input light (see FIG. 3A). The input WDM signal is separated into an even channel component and an odd channel component by a wavelength separation filter (interleaver) 101 '(see FIG. 3B).

  In this embodiment, the WDM signal is separated into the even-numbered channel component and the odd-numbered channel component using the center wavelength in the entire WDM signal band as the reference wavelength. However, the reference wavelength is not necessarily the center wavelength.

  The even channel components of the WDM signal separated by the wavelength separation filter (interleaver) 101 ′ are input to the first nonlinear optical device 103. The components of the odd channel of the WDM signal separated by the wavelength separation filter (interleaver) 101 ′ are input to the second nonlinear optical device 104.

  In the first nonlinear optical device 103, the dichroic mirror type multiplexer 103b multiplexes the excitation light to the even channel component of the WDM signal from the wavelength separation filter (interleaver) 101 '. The even channel component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 103a. The PPLN waveguide 103a generates the phase conjugate light of the even channel component of the WDM signal by the difference frequency generation. At this time, the pumping light wavelength is set so that the phase conjugate light of the even channel component is generated in the odd channel. Specifically, the frequency may be set so that twice the frequency of the excitation light is between the channel intervals of the WDM signal.

  The phase conjugate light of the even channel component of the WDM signal generated by the PPLN waveguide 103a is sent to the dichroic mirror type demultiplexer 103c together with the even channel component of the WDM signal combined with the excitation light. The dichroic mirror type demultiplexer 103c separates the excitation light from the light transmitted through the PPLN waveguide 103a, and separates the excitation light (the phase of the even channel component of the WDM signal + the even channel component of the WDM signal). Conjugate light: see FIG. 3 (c) upper diagram) is sent to the wavelength multiplexing filter (interleaver) 102 ′.

  In the second nonlinear optical device 104, the dichroic mirror type multiplexer 104b multiplexes the excitation light to the odd channel component of the WDM signal from the wavelength separation filter (interleaver) 101 '. The odd channel component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 104a. The PPLN waveguide 104a generates phase conjugate light of the odd-numbered channel component of the WDM signal by the difference frequency generation in the same manner as the PPLN waveguide 103a in the first nonlinear optical device 103. At this time, the phase conjugate light of the odd channel component is generated in the even channel.

  The phase conjugate light of the odd channel component of the WDM signal generated by the PPLN waveguide 104a is sent to the dichroic mirror type demultiplexer 104c together with the odd channel component of the WDM signal combined with the pump light. The dichroic mirror type demultiplexer 104c separates the excitation light from the light transmitted through the PPLN waveguide 104a, and separates the excitation light (the phase of the odd channel component of the WDM signal + the odd channel component of the WDM signal). Conjugate light: see FIG. 3 (c) lower diagram) is sent to the wavelength multiplexing filter (interleaver) 102 ′.

  The wavelength multiplexing filter (interleaver) 102 ′ includes the phase conjugate light of the even channel component of the WDM signal and the odd channel of the light input from the first nonlinear optical device 103 and the second nonlinear optical device 104. Only the component phase conjugate light is combined to be output light (see FIG. 3D) and sent out as a new WDM signal.

  In this case, the wavelength multiplexing filter (interleaver) 102 ′ includes the original signal light (the component of the even channel of the WDM signal) among the light input from the first nonlinear optical device 103 and the second nonlinear optical device 104. + The odd-numbered channel component of the WDM signal) is separated and extinguished so that it is not extracted as output light.

  As a result, even-numbered channel components and odd-numbered channel components of the WDM signal can be converted into phase conjugate light in a manner that they are interchanged within the same band. In this configuration, operation without a card band is possible in principle.

[Embodiment 2]
Next, a second embodiment (Embodiment 2) of the phase conjugate light converter according to the present invention will be described.

  In the phase conjugate light converter according to the present invention, since two nonlinear optical devices are used, if there is a delay difference between the two optical paths, a time shift of the signal bit occurs. Also in the phase conjugate light converter 100 shown in the first embodiment, the delay difference can be minimized if the optical path lengths of the nonlinear optical devices 103 and 104 are appropriately set. In the second embodiment, the loop type is used. By using this phase conjugate light converter, the delay difference can be minimized more easily.

  FIG. 4 shows the configuration of the phase conjugate light converter according to the second embodiment. The phase conjugate light converter 200 according to the second embodiment includes a circulator 201, a wavelength combining / separating filter 202, a first nonlinear optical device 203, a second nonlinear optical device 204, and a polarization rotation element 205. I have.

  In the phase conjugate light converter 200, the first nonlinear optical device 203 includes a PPLN waveguide 203a having a periodically poled structure, a dichroic mirror type multiplexer 203b that multiplexes excitation light, and demultiplexing the excitation light. And a dichroic mirror type duplexer 203c.

  Similarly to the first nonlinear optical device 203, the second nonlinear optical device 204 also includes a PPLN waveguide 204a having a periodically poled structure, a dichroic mirror type multiplexer 204b that combines excitation light, and excitation light. And a dichroic mirror type demultiplexer 204c.

  Next, the operation of the phase conjugate light converter 200 of the second embodiment will be described with the functions of the respective parts. Similarly to the phase conjugate light converter 100 of the first embodiment, a WDM signal is input to the phase conjugate light converter 200 as input light.

  The input WDM signal is received by the circulator 201 and sent to the wavelength multiplexing / separating filter 202. The wavelength combining / separating filter 202 separates the WDM signal received by the circulator 201 into a short-wave component (short wavelength signal) and a long-wave component (long wavelength signal).

  The short-wave component of the WDM signal separated by the wavelength combining / separating filter 202 is input to the first nonlinear optical device 203. The long-wave component of the WDM signal separated by the wavelength multiplexing / separation filter 202 is input to the second nonlinear optical device 204.

  In the first nonlinear optical device 203, the dichroic mirror type multiplexer 203 b multiplexes the excitation light with the short-wave component of the WDM signal from the wavelength multiplexing / separation filter 202. The short-wave component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 203a. The PPLN waveguide 203a generates phase conjugate light of the short-wave component of the WDM signal by difference frequency generation.

  The phase conjugate light of the short-wave component of the WDM signal generated by the PPLN waveguide 203a is sent to the dichroic mirror type demultiplexer 203c together with the short-wave component of the WDM signal combined with the excitation light. The dichroic mirror type demultiplexer 203c separates the pumping light from the light transmitted through the PPLN waveguide 203a, and separates the pumping light (the phase of the component on the short wave side of the WDM signal + the component on the short wave side of the WDM signal). Conjugate light) is sent to the polarization rotation element 205.

  In the second nonlinear optical device 204, the dichroic mirror type multiplexer 204 b multiplexes the excitation light with the component on the long wave side of the WDM signal from the wavelength multiplexing / separation filter 202. The long-wave component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 204a. The PPLN waveguide 204a generates phase conjugate light of a component on the long wave side of the WDM signal by difference frequency generation.

  The phase conjugate light of the long wave component of the WDM signal generated by the PPLN waveguide 204a is sent to the dichroic mirror type demultiplexer 204c together with the long wave component of the WDM signal combined with the excitation light. The dichroic mirror type demultiplexer 204c separates the pumping light from the light transmitted through the PPLN waveguide 204a and separates the pumping light (the phase of the component on the long wave side of the WDM signal + the component on the long wave side of the WDM signal) Conjugate light) is sent to the polarization rotation element 205.

  The polarization rotation element 205 rotates the polarization of the light from the first nonlinear optical device 203 (short-wave component of the WDM signal + phase-conjugate light of the short-wave component of the WDM signal) to generate second nonlinear optics. Send to device 204. The polarization rotation element 205 rotates the polarization of the light from the second nonlinear optical device 204 (the long-wave component of the WDM signal + the phase-conjugate light of the long-wave component of the WDM signal). Send to the nonlinear optical device 203.

  In the present embodiment, the PPLN waveguides 203a and 204a are Z-cut. Therefore, the PPLN waveguides 203a and 204a have polarization dependency, and generate phase conjugate light with respect to TM polarized light. By actively utilizing this polarization dependence, the independence of both the first nonlinear optical device 203 and the second nonlinear optical device 204 can be ensured.

  Specifically, the component on the short wave side of the WDM signal output from the first nonlinear optical device 203 and the phase conjugate light thereof are rotated by the polarization rotating element 205 and then the second nonlinear optical device 204. Therefore, in the second nonlinear optical device 204, even if reflected light of excitation light exists, a nonlinear process does not occur. For this reason, in the state where the influence of the reflected light is reduced, the light passes through the second nonlinear optical device 204 and is returned to the wavelength combining / separating filter 202.

  The long-wave component and the phase conjugate light of the WDM signal output from the second nonlinear optical device 204 are input to the first nonlinear optical device 203 after the polarization is rotated by the polarization rotating element 205. Therefore, a nonlinear process does not occur in the first nonlinear optical device 203 even if reflected light of excitation light exists. For this reason, in the state where the influence of the reflected light is reduced, the light passes through the first nonlinear optical device 204 and is returned to the wavelength combining / separating filter 202.

  Thereby, even if the 1st nonlinear optical device 203 and the 2nd nonlinear optical device 204 are arrange | positioned in the same loop, it can reduce affecting each other.

  The wavelength multiplexing / separating filter 202 includes a component on the long wave side of the WDM signal returned through the first nonlinear optical device 203 and its phase conjugate light and the component on the short wave side of the WDM signal returned through the second nonlinear optical device 204. The component and its phase conjugate light are input, and among the input light, only the phase conjugate light of the short-wave component and the phase conjugate light of the long-wave component are combined and sent to the circulator 201 as output light. The circulator 201 receives the output light sent from the wavelength multiplexing / separation filter 202 and sends it out as a new WDM signal.

  In this case, the wavelength combining / separating filter 202 includes the original signal light (the short-wave component of the WDM signal + the long wave of the WDM signal) out of the light returned through the first nonlinear optical device 203 and the second nonlinear optical device 204. Component is separated and extinguished so that it is not extracted as output light.

  In the phase conjugate light converter 200 according to the second embodiment, the short-wave component and the long-wave component of the WDM signal separated by the wavelength multiplexing / separation filter 202 are strictly matched by adopting a loop configuration. Can be

  In this embodiment, the two nonlinear optical devices 203 and 204 are arranged in the loop, but one nonlinear optical device is bidirectionally excited by making the reflectance of each element in the loop configuration very small. By doing so, it is possible to generate the phase conjugate light of the short-wave component and the long-wave component of the WDM signal with one nonlinear optical device.

  Further, in this embodiment, the wavelength multiplexing / separation filter 202 separates the short wave component and the long wave component of the WDM signal, and the phase conjugate light of the short wave component and the phase conjugate light of the long wave component However, the even channel component and the odd channel component of the WDM signal are separated, and the phase conjugate light of the even channel component and the phase conjugate light of the odd channel component are combined. An interleave type filter having a wave characteristic may be used.

[Embodiment 3]
Next, a third embodiment (Embodiment 3) of the phase conjugate light converter according to the present invention will be described.

  In the phase conjugate light converter according to the present invention, when a PPLN waveguide is used as a nonlinear optical device, the configurations of the first and second embodiments depend on the polarization. In order to be applied to a recent optical communication system, a configuration capable of operating independently of polarization is required. For this reason, Embodiment 3 shows a configuration for operating the phase conjugate light converter according to the present invention independent of polarization.

  FIG. 5 shows the configuration of the phase conjugate light converter according to the third embodiment. The phase conjugate light converter 300 according to the third embodiment includes a wavelength separation filter 301, a wavelength multiplexing filter 302, first to fourth nonlinear optical devices 303 to 306, and first to fourth polarization multiplexing. Demultiplexing elements (polarization beam splitter: PBS) 307 to 310 are provided.

  In the phase conjugate light converter 300, the first nonlinear optical device 303 includes a PPLN waveguide 303a having a periodically poled structure, a dichroic mirror type multiplexer 303b that multiplexes excitation light, and demultiplexing the excitation light. And a dichroic mirror type duplexer 303c.

  Similarly to the first nonlinear optical device 303, the second to fourth nonlinear optical devices 304 to 306 are also combined with the PPLN waveguides 304a to 306a having the periodically poled structure and the dichroic mirror type coupling that combines the excitation light. Demultiplexers 304b to 306c and dichroic mirror demultiplexers 304c to 306c that demultiplex excitation light are provided.

  The wavelength separation filter 301 includes dichroic mirror type multiplexers / demultiplexers 301a, 301b, and 301c, and the wavelength multiplexing filter 302 includes dichroic mirror type multiplexers / demultiplexers 302a, 302b, and 302c.

  Next, the operation of the phase conjugate light converter 300 according to the third embodiment will be described with the functions of the respective parts.

  The phase conjugate light converter 300 receives a WDM signal as input light (an optical signal having a plurality of wavelengths). The input WDM signal is separated into a short wave component (short wavelength signal) and a long wave component (long wavelength signal) by the wavelength separation filter 301.

  In this embodiment, the WDM signal is separated into a short-wave component and a long-wave component using the center wavelength in the entire WDM signal band as a reference wavelength. However, the reference wavelength is not necessarily the center wavelength.

  The short wavelength component of the WDM signal separated by the wavelength separation filter 301 is input to the first polarization multiplexing / demultiplexing element 307 and separated into an X polarization component and a Y polarization component. The long-wave component of the WDM signal separated by the wavelength separation filter 301 is input to the second polarization multiplexing / demultiplexing element 308 and separated into an X polarization component and a Y polarization component.

  The X polarization component of the short-wave component of the WDM signal separated by the first polarization multiplexing / demultiplexing element 307 is input to the first nonlinear optical device 303. The Y polarization component of the short-wave component of the WDM signal separated by the first polarization multiplexing / demultiplexing element 307 is input to the second nonlinear optical device 304.

  The X polarization component of the long wave component of the WDM signal separated by the second polarization multiplexing / demultiplexing element 308 is input to the third nonlinear optical device 305. The Y polarization component of the long-wave component of the WDM signal separated by the second polarization multiplexing / demultiplexing element 308 is input to the fourth nonlinear optical device 306.

  In the first nonlinear optical device 303, the dichroic mirror type multiplexer 303 b multiplexes the excitation light with the X polarization component of the short wavelength side component of the WDM signal from the first polarization multiplexing / demultiplexing element 307. The X polarization component of the short wavelength component of the WDM signal combined with the excitation light is incident on the PPLN waveguide 303a. The PPLN waveguide 303a generates phase conjugate light of the X polarization component of the short-wave component of the WDM signal by difference frequency generation.

  The phase conjugate light of the X-polarized component of the short-wave component of the WDM signal generated by the PPLN waveguide 303a is combined with the X-polarized component of the short-wave component of the WDM signal combined with the excitation light together with the dichroic mirror type component. It is sent to the waver 303c. The dichroic mirror type demultiplexer 303c separates the excitation light from the light transmitted through the PPLN waveguide 303a, and separates the excitation light (the X-polarized component of the component on the short wavelength side of the WDM signal + the short wave of the WDM signal). Phase-conjugate light of the X-polarized component of the side component) is sent to the third polarization multiplexing / demultiplexing element 309.

  In the second nonlinear optical device 304, the dichroic mirror type multiplexer 304 b multiplexes the excitation light with the Y polarization component of the short wavelength component of the WDM signal from the first polarization multiplexing / demultiplexing element 307. The Y polarization component of the short-wave component of the WDM signal combined with this excitation light is incident on the PPLN waveguide 304a. The PPLN waveguide 304a generates the phase conjugate light of the Y polarization component of the short wavelength component of the WDM signal by the difference frequency generation.

  The phase conjugate light of the Y-polarized component of the short-wave component of the WDM signal generated by the PPLN waveguide 304a is dichroic mirror type together with the Y-polarized component of the short-wave component of the WDM signal combined with the excitation light. It is sent to the waver 304c. The dichroic mirror type demultiplexer 304c separates the excitation light from the light transmitted through the PPLN waveguide 304a, and separates the excitation light (the Y-polarized component of the component on the short wavelength side of the WDM signal + the short wave of the WDM signal). Phase-conjugate light of the Y polarization component of the side component) is sent to the third polarization multiplexing / demultiplexing element 309.

  The third polarization multiplexing / demultiplexing element 309 is a beam obtained by separating the pumping light from the first nonlinear optical device 303 (X polarization component of the short-wave component of the WDM signal + X of the short-wave component of the WDM signal) Light obtained by separating the pump light from the second nonlinear optical device 304 (Y-polarization component of the short-wave component of the WDM signal + Y-polarization of the short-wave component of the WDM signal) The component phase conjugate light) is combined and sent to the wavelength combining filter 302.

  In the third nonlinear optical device 305, the dichroic mirror type multiplexer 305 b multiplexes the excitation light to the X polarization component of the long wave side component of the WDM signal from the second polarization multiplexing / demultiplexing element 308. The X polarization component of the component on the long wave side of the WDM signal combined with the excitation light is incident on the PPLN waveguide 305a. The PPLN waveguide 305a generates phase conjugate light of the X polarization component of the long wave side component of the WDM signal by the difference frequency generation.

  The phase conjugate light of the X-polarized component of the long-wave component of the WDM signal generated by the PPLN waveguide 305a is combined with the X-polarized component of the long-wave component of the WDM signal combined with the excitation light together with the dichroic mirror type component. It is sent to the waver 305c. The dichroic mirror type demultiplexer 305c separates the excitation light from the light transmitted through the PPLN waveguide 305a, and separates the excitation light (the long wave of the WDM signal, the X polarization component + the WDM signal long wave) Phase-conjugate light of the X-polarized component of the side component) is sent to the fourth polarization multiplexing / demultiplexing element 310.

  In the fourth nonlinear optical device 306, the dichroic mirror type multiplexer 306b multiplexes the excitation light with the Y polarization component of the long wave side component of the WDM signal from the second polarization multiplexing / demultiplexing element 308. The Y polarization component of the long wave component of the WDM signal combined with this excitation light is incident on the PPLN waveguide 306a. The PPLN waveguide 306a generates phase conjugate light of the Y polarization component of the long wave side component of the WDM signal by the difference frequency generation.

  The phase conjugate light of the Y-polarized component of the long-wave component of the WDM signal generated by the PPLN waveguide 306a, together with the Y-polarized component of the long-wave component of the WDM signal combined with the excitation light, is separated from the dichroic mirror type. It is sent to the correlator 306c. The dichroic mirror type demultiplexer 306c separates the excitation light from the light transmitted through the PPLN waveguide 306a, and separates the excitation light (the Y-polarized component of the component on the long wave side of the WDM signal + the long wave of the WDM signal). Phase-conjugate light of the Y-polarized component of the side component) is sent to the fourth polarization multiplexing / demultiplexing element 310.

  The fourth polarization multiplexing / demultiplexing element 310 is light from which the excitation light from the third nonlinear optical device 305 is separated (X polarization component of the long wave side component of the WDM signal + X of the long wave side component of the WDM signal) Light obtained by separating the pump light from the fourth nonlinear optical device 306 (Y polarization component of the long wave side component of the WDM signal + Y polarization wave of the long wave side component of the WDM signal) The component phase conjugate light) is combined and sent to the wavelength combining filter 302.

  The wavelength multiplexing filter 302 is a component on the short wavelength side of the WDM signal (X polarization component + Y polarization) of the light input from the third polarization multiplexing / demultiplexing element 309 and the fourth polarization multiplexing / demultiplexing element 310. Only the phase conjugate light of the wave component) and the phase conjugate light of the long wave side component (X polarization component + Y polarization component) are combined into an output light and sent out as a new WDM signal.

  In this case, the wavelength multiplexing filter 302 includes the original signal light (on the short wavelength side of the WDM signal) of the light input from the third polarization multiplexing / demultiplexing element 309 and the fourth polarization multiplexing / demultiplexing element 310. The component (X polarization component + Y polarization component) + the component on the long wave side of the WDM signal (X polarization component + Y polarization component) is separated and extinguished so that it is not extracted as output light.

  The phase conjugate light converter 300 according to the third embodiment forms a polarization diversity configuration using two basic configurations of the phase conjugate light converter 100 according to the first embodiment. In this embodiment, the polarization is separated after wavelength demultiplexing. Conversely, the wavelength separation may be performed after polarization separation is performed.

  In this embodiment, the wavelength separation filter 301 is a filter having a characteristic of separating the short wave side component and the long wave side component of the WDM signal, but the short wavelength side component is also used as the wavelength multiplexing filter 302. A filter having the characteristic of combining the phase conjugate light of the wavelength and the phase conjugate light of the component on the long wave side is used, but as the wavelength separation filter 301, the characteristic of separating the even channel component and the odd channel component of the WDM signal Alternatively, an interleave type filter having a characteristic of combining the phase conjugate light of the even channel component and the phase conjugate light of the odd channel component may be used as the wavelength multiplexing filter 302.

[Embodiment 4]
The phase conjugate light converter according to the present invention can collectively convert optical signals composed of a plurality of wavelengths into phase conjugate light as shown in the first to third embodiments. Since the loss due to the wavelength separation filter and the polarization multiplexing / demultiplexing element (PBS) is compensated for in the phase conjugate optical converter by the optical parametric process in the PPLN waveguide, the signal quality is somewhat deteriorated. Depending on how this phase conjugate light converter is used in an optical communication system, the S / N ratio of the entire system varies greatly.

  FIG. 6 shows a first embodiment (Embodiment 4) of an optical transmission system using a phase conjugate light converter according to the present invention. The optical transmission system 400 according to the fourth embodiment includes an optical transmitter 401 that generates a WDM signal, a plurality of optical transmission fibers 402, a plurality of relay optical amplifiers 403, a plurality of phase conjugate optical converters 404, And a receiver 405.

  In the present embodiment, as the phase conjugate light converter 404, the phase conjugate light converter 300 having the polarization-independent configuration shown in the third embodiment is used. The relay optical amplifier 403 uses an EDFA (erbium doped fiber amplifier). As the phase conjugate light converter 404, the phase conjugate light converter 100 shown in the first embodiment or the phase conjugate light converter 200 shown in the second embodiment may be used.

  In this optical transmission system 400, the WDM signal from the optical transmitter 401 is transmitted to the optical receiver 405 using the optical transmission fiber 402 connected in multiple stages between the optical transmitter 401 and the optical receiver 405 as a transmission line 406. Sent.

  In this optical transmission system 400, one section divided by the optical transmission fiber 402 in the transmission path 406 is defined as one span, and the transmission loss in each span is compensated by the repeater optical amplifier 403 provided for each span.

  In this optical transmission system 400, an 80 km single mode fiber is used as the optical transmission fiber 402. Since there is a loss of about 24 dB propagation loss, an EDFA is used as the repeater optical amplifier 403 to compensate for this loss, and the optical transmission fibers 402 are connected in multiple stages. A phase conjugate light converter 404 is arranged immediately after the repeater optical amplifier 403. In general, when optical amplifiers are connected in multiple stages, it is known that the noise figure (NF) at the subsequent stage is divided by the amplification gain (G) at the preceding stage, but the phase conjugate light in this optical transmission system 400 The converter 404 has the same effect.

  By disposing the phase conjugate light converter 404 immediately after the repeater optical amplifier 403, it is possible to reduce the influence of the signal quality degradation of the phase conjugate light converter 404 by the amplification gain of 24 dB. That is, even if the original noise figure (NF) of the phase conjugate light converter 404 is about 4 dB, the entire optical transmission system 400 can be reduced to 1/100 or less. Thereby, it is possible to minimize the degradation of signal quality due to the phase conjugate light converter 404 itself and to maximize the nonlinear noise compensation characteristic of the phase conjugate light converter 404.

  A phase conjugate light converter 404 is arranged immediately after the repeater optical amplifier 403 in each span, and the WDM signal is converted into phase conjugate light by the phase conjugate light converter 404, thereby compensating for nonlinear noise for each span. Can do. Specifically, nonlinear noise generated in the first optical transmission fiber 402 (first span) of the WDM signal sent from the optical transmitter 401 propagates in the next optical transmission fiber 402 (second span). In the meantime, it is compensated by the reverse propagation characteristic of the phase conjugate light. Similarly, after the third and fourth spans, the nonlinear noise can be transmitted with immediate compensation.

  As a result, the entire optical transmission system 400 can improve the S / N ratio by 10 dB or more.

[Embodiment 5]
Next, a fourth embodiment (Embodiment 5) of the phase conjugate light converter according to the present invention will be described. The phase conjugate light converter according to the present invention can also cooperate with noise reduction means that transmits phase conjugate light such as a phase sensitive optical amplifier and Boosting reception, thereby realizing a significant improvement in system characteristics.

  In FIG. 7, as a phase conjugate light converter of the fifth embodiment, a phase sensitive optical amplifier can be applied as a repeater optical amplifier for a multicarrier signal that simultaneously transmits signal light and phase conjugate light of the signal light. The structure of the phase conjugate light converter for this is shown.

  The phase conjugate light converter 500 according to the fifth embodiment includes a wavelength separation filter 501, a wavelength multiplexing filter 502, a first nonlinear optical device 503, a second nonlinear optical device 504, and a first optical branch. A circuit 505, bandpass filters (BPF) 506, 512 that transmit only a specific wavelength, a variable optical attenuator (VOA) 507, a circulator 508, a semiconductor laser 509, a second optical branching circuit 510, An optical amplifier (EDFA) 511, a nonlinear optical device 513 for generating pumping light, a pumping light demultiplexer 514, and a third optical branching circuit 515 are provided.

  In the phase conjugate light converter 500, the first nonlinear optical device 503 includes a PPLN waveguide 503a having a periodically poled structure, a dichroic mirror type multiplexer 503b that multiplexes excitation light, and demultiplexing the excitation light. And a dichroic mirror type duplexer 503c.

  Similarly to the first nonlinear optical device 503, the second nonlinear optical device 504 also includes a PPLN waveguide 504a having a periodically poled structure, a dichroic mirror type multiplexer 504b that combines excitation light, and excitation light. And a dichroic mirror type duplexer 504c.

  Next, the operation of the phase conjugate light converter 500 of the fifth embodiment will be described with the functions of the respective parts.

  The phase conjugate light converter 500 receives as input light (an optical signal composed of a plurality of wavelengths) a multicarrier signal composed of a plurality of pairs of signal light and phase conjugate light and pilot tone signal light.

  Here, the pilot tone signal light is continuous wave light (CW light), and is set to the center wavelength of the band of the multicarrier signal. The pair of the signal light and the phase conjugate light is symmetrically detuned to the short wave side and the long wave side around the wavelength of the pilot tone signal light. The multicarrier signal has a plurality of pairs of the signal light and the phase conjugate light.

  A part of the input multicarrier signal is branched by the first optical branching circuit 505 and sent to a bandpass filter (BPF) 506. A band pass filter (BPF) 506 separates only the pilot tone signal light from the multicarrier signal branched by the first optical branching circuit 505 and sends it to the variable optical attenuator 507.

  The variable optical attenuator 507 adjusts the power of the pilot tone signal light separated by the band pass filter (BPF) 506 so as to be several hundred to several tens of microwatts. The pilot tone signal light whose power is adjusted is sent to the semiconductor laser 509 through the circulator 508.

  As a result, the semiconductor laser 509 is light-injected and synchronized with the pilot tone signal light, and sends light having the same carrier phase as that of the pilot tone signal light to the circulator 508. The circulator 508 sends light having the same carrier phase as the pilot tone signal light from the semiconductor laser 509 to the second optical branching circuit 510.

  The second optical branching circuit 510 splits the light output from the circulator 508 into two, and sends one to the optical amplifier 511 as the fundamental wave light. The optical amplifier 511 amplifies the fundamental light from the second optical branch circuit 510 to 2 W, and then inputs the amplified light to the nonlinear optical device 513 for generating pumping light through the band pass filter (BPF) 512.

  The nonlinear optical device 513 for generating excitation light has the same configuration as that of the first nonlinear optical device 503 and the second nonlinear optical device 504, and includes a PPLN waveguide 513a having a periodically poled structure and a dichroic mirror type multiplexing. 513b and a dichroic mirror type demultiplexer 513c.

  The nonlinear optical device 513 for generating excitation light generates excitation light having a half wavelength of the fundamental light by the second harmonic generation (SHG) process in the PPLN waveguide 513a therein. In this example, 1 W of excitation light is output from the nonlinear optical device 513 for generating excitation light. This excitation light is sent to the excitation light demultiplexer 514 to be branched into two, and the dichroic mirror type multiplexer 503b of the first nonlinear optical device 503 and the dichroic mirror type multiplexer 504b of the second nonlinear optical device 504. Sent to.

  On the other hand, the multicarrier signal transmitted through the first optical branch circuit 505 is separated into a short-wave component (signal light) and a long-wave component (phase conjugate light) by the wavelength separation filter 501. The short-wave component (signal light) of the multicarrier signal separated by the wavelength separation filter 501 is input to the first nonlinear optical device 503. The long-wave component (phase conjugate light) of the multicarrier signal separated by the wavelength separation filter 501 is input to the second nonlinear optical device 504.

  In the first nonlinear optical device 503, the dichroic mirror type multiplexer 503 b is an excitation light transmitted from the excitation light demultiplexer 514 to a short-wave component (signal light) of the multicarrier signal from the wavelength separation filter 501. Are combined. The short-wave component (signal light) of the multicarrier signal combined with the excitation light is incident on the PPLN waveguide 503a. The PPLN waveguide 503a generates phase conjugate light of a short-wave component (signal light) of a multicarrier signal by generating a difference frequency.

  The phase conjugate light of the short-wave component (signal light) of the multi-carrier signal generated by the PPLN waveguide 503a is combined with the short-wave component (signal light) of the multi-carrier signal combined with the excitation light. It is sent to the correlator 503c. The dichroic mirror type demultiplexer 503c separates the excitation light from the light transmitted through the PPLN waveguide 503a and separates the excitation light (short-wave component (signal light) of the multicarrier signal + multicarrier signal). Of the component (signal light) of the short-wave side of the signal) to the wavelength multiplexing filter 502.

  In the second nonlinear optical device 504, the dichroic mirror type multiplexer 504 b is an excitation transmitted from the excitation light demultiplexer 514 to the long-wave component (phase conjugate light) of the multicarrier signal from the wavelength separation filter 501. Combine light. The long-wave component (phase conjugate light) of the multicarrier signal combined with the excitation light is incident on the PPLN waveguide 504a. The PPLN waveguide 504a generates phase conjugate light of a component (phase conjugate light) on the long wave side of the multicarrier signal by difference frequency generation.

  The phase conjugate light of the long-wave component (phase conjugate light) of the multicarrier signal generated by the PPLN waveguide 504a, together with the long-wave component (phase conjugate light) of the multicarrier signal combined with the excitation light, is a dichroic mirror. It is sent to the type duplexer 504c. The dichroic mirror type demultiplexer 504c separates the excitation light from the light transmitted through the PPLN waveguide 504a, and separates the excitation light (long wave side component of the multicarrier signal (phase conjugate light) + multicarrier. The component on the long wave side of the signal (phase conjugate light of the phase conjugate light) is sent to the wavelength multiplexing filter 502.

  The wavelength multiplexing filter 502 includes the phase conjugate light and the long wave side component of the short wave side component (signal light) of the multicarrier signal among the lights input from the first nonlinear optical device 503 and the second nonlinear optical device 504. Only the phase conjugate light of the component (phase conjugate light) is combined and sent to the third optical branch circuit 515.

  In this case, the wavelength multiplexing filter 502 includes the original signal light (short-wave component (signal light) of the multicarrier signal) among the light input from the first nonlinear optical device 503 and the second nonlinear optical device 504. + The long-wave component (phase conjugate light) of the multicarrier signal is separated and extinguished so as not to be extracted as light to be sent to the third optical branch circuit 515.

  The third optical branch circuit 515 uses the other of the two lights branched by the second optical branch circuit 510 as a new pilot tone signal light, and combines the wavelength of this new pilot tone signal light with the new pilot tone signal light. The light transmitted from the wave filter 502 is combined into output light. That is, a pair of a plurality of signal lights and phase conjugate light converted to phase conjugate light output from the second wavelength multiplexing filter 502 and a new pilot tone signal light are passed through the third optical branch circuit 515. The resulting light is combined and used as output light, and is sent out as a new multicarrier signal.

  Since the new pilot tone signal light is sufficiently strong with respect to the converted light intensity, the pilot tone signal light is multiplexed by using an optical branch circuit configured to branch light 14 to 1 as the third optical branch circuit 515. Thus, the loss of converted light could be reduced to 10% or less.

  As a result, the pair of the signal light and the phase conjugate light can be integrally converted into each phase conjugate light while maintaining the phase relationship with the pilot tone signal light. In the present embodiment, the first optical branch circuit 505 and the third optical branch circuit 515 are configured to tap a constant light intensity in the entire wavelength band, but the pilot tone signal light of the center wavelength is used. A band rejection filter that separates only the light and transmits light other than the center wavelength may be used.

  Also, the configuration between the wavelength separation filter 501 and the wavelength multiplexing filter 502 can be a polarization-independent configuration by adopting the same configuration as that of the third embodiment (FIG. 5). Further, a similar configuration may be adopted for the phase conjugate light converter 200 shown in the second embodiment.

[Embodiment 6]
As shown in the fifth embodiment, the phase conjugate light converter according to the present invention generates phase conjugate light with respect to each pair of signal light and phase conjugate light in such a manner that the wavelength relationship is switched. By doing so, it is possible to collectively convert to the phase conjugate light while maintaining the band of the multicarrier signal composed of a pair of the plurality of signal lights and the phase conjugate light. By realizing this phase conjugate light converter, it is possible to construct a non-linear noise compensating transmission line using both a phase sensitive optical amplifier and Boosting reception.

  FIG. 8 shows a second embodiment (sixth embodiment) of an optical transmission system using the phase conjugate light converter according to the present invention. The optical transmission system 600 according to the sixth embodiment includes an optical transmitter 601 that generates a multicarrier signal composed of a pair of signal light and phase conjugate light and pilot tone signal light, and a plurality of optical transmission fibers 602. , A plurality of relay optical amplifiers 603, a plurality of phase conjugate light converters 604, and an optical receiver (Boosting receiver) 605.

  In the present embodiment, as the phase conjugate light converter 604, the phase conjugate light converter 500 shown in the third embodiment, that is, a configuration capable of converting into phase conjugate light while maintaining the phase relationship with the pilot tone signal light. The phase conjugate light converter 500 is used. The repeater optical amplifier 603 uses a Raman optical amplifier 606 and a phase sensitive optical amplifier 607 in combination. The optical receiver 605 includes coherent receivers 605-1 and 605-2, and is configured to be able to receive a pair of signal light and phase conjugate light separately.

  In this optical transmission system 600, the multicarrier signal from the optical transmitter 601 is received by the optical receiver 605 using the optical transmission fibers 602 connected in multiple stages between the optical transmitter 601 and the optical receiver 605 as the transmission path 608. Sent to.

  In this optical transmission system 600, two sections divided by the optical transmission fiber 602 in the transmission path 608 are defined as one span, and transmission loss in each span is compensated by the repeater optical amplifier 603 provided for each span.

  In this optical transmission system 600, one span in the transmission line 608 includes two optical transmission fibers 602 and a phase conjugate light converter 604. Hereinafter, of the two optical transmission fibers 602 constituting one span, the optical transmission fiber 602 (602F) on the optical transmitter 601 side is referred to as a first optical transmission fiber, and the optical transmission fiber 602 on the optical receiver 605 side ( 602R) is referred to as a first optical transmission fiber. The phase conjugate light converter 604 is disposed between the first optical transmission fiber 602F and the second optical transmission fiber 602R.

  In this optical transmission system 600, an 80 km single mode fiber is used as the optical transmission fiber 602. Further, since there is a loss of about 24 dB propagation loss, a Raman optical amplifier 606 is used for each optical transmission fiber 602 to compensate a part of this loss. 14 dB of the loss of 24 dB of the optical transmission fiber 602 of 80 km was compensated by the Raman amplification gain of 14 dB, and the effective loss was set to 10 dB.

  In this optical transmission system 600, the multicarrier signal is transmitted 80 km through the first optical transmission fiber 602F, and then converted into phase conjugate light in a batch by the phase conjugate light converter 604. Thereafter, the converted multicarrier signal is transmitted through the second optical transmission fiber 602R configured using the same 80 km single mode fiber as the first optical transmission fiber 602F.

  The phase conjugate light converter 604 can perform lossless conversion, and the second optical transmission fiber 602R is compensated for part of the propagation loss by the Raman optical amplifier 606 in the same manner as the first optical transmission fiber 602F. Therefore, the loss was 10 dB. That is, the transmission path distance of one span constituted by the first optical transmission fiber 602F, the phase conjugate optical converter 604, and the second optical transmission fiber 602R is 160 km, and the propagation loss is 20 dB.

  The first optical transmission fiber 602F and the second optical transmission fiber 602R are fibers manufactured with the same specifications, and have the same manufacturing error range. As a result, the non-linear noise generated in the first 80 km within one span can be canceled by propagating through the latter 80 km transmission line itself. In addition, by using phase conjugate light, not only non-linear noise can be canceled, but also chromatic dispersion of the single mode fiber can be compensated, and a transmission path with dispersion compensation in one span can be constructed. .

  The multicarrier signal transmitted through the second optical transmission fiber 602R is amplified by the phase sensitive optical amplifier 607. Since the dispersion is compensated within one span, the phase sensitive optical amplifier 607 can be operated without the phase relationship between the pair of the signal light and the phase conjugate light being shifted. This enables low-noise optical amplification and improves the S / N ratio in the entire system. In addition, the signal quality can be further improved by compressing the components in which nonlinear noise and chromatic dispersion remain due to the interaction between the signal light and the phase conjugate light in the phase sensitive optical amplifier 607.

  A long-distance transmission path can be constructed by connecting the transmission path of 1 span 160 km and the relay amplification by the phase sensitive optical amplifier 607 in multiple stages. A transmission line of 16,000 km can be constructed by connecting 100 stages.

  Furthermore, in the configuration of the present embodiment, it is not necessary to use the same optical transmission fiber 602 for each span. That is, the first fiber (first optical transmission fiber 602F) and the latter fiber (second optical transmission fiber 602R) constituting one span need to use the same fiber, but different fibers are used for each span. Also good.

  For example, in the example described above, single mode fibers are used as the first optical transmission fiber 602F and the second optical transmission fiber 602R in the first span, but different fibers such as dispersion shifted fibers are used in the other spans. Alternatively, a fiber having a length other than 80 km may be used.

  Thereby, in constructing the optical transmission system 600, flexibility is improved. Since it is only necessary to use the same fiber in the first and second half for each individual span, even if a part of the system is changed, the whole is not affected.

  In this way, the multicarrier signal from the optical transmitter 601 is transmitted through a plurality of optical transmission fibers 602 connected in multiple stages, and is received by the optical receiver 605. In the optical receiver 605, the pair of the signal light and the phase conjugate light is separated, and the intensity and phase information of the light is converted into an electric signal using separate coherent receivers 605-1 and 605-2, respectively. Thereafter, by taking the sum of the individual data, nonlinear noise is compensated for by electrical signal processing.

  Non-linear noise canceling and dispersion compensation for each span, low noise in the repeater optical amplifier 603 and non-linear noise compression by optical signal processing, and non-linear noise compensation in the optical receiver 605, the entire optical transmission system 600 is 20 dB. The above S / N ratio improvement was able to be brought about.

[Extension of the embodiment]
The present invention has been described above with reference to the embodiment. However, the present invention is not limited to the above embodiment. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the technical idea of the present invention.

  DESCRIPTION OF SYMBOLS 100 (100 ') ... Phase conjugate light converter, 101 (101') ... Wavelength separation filter, 102 (102 ') ... Wavelength combining filter, 103, 104 ... 1st, 2nd nonlinear optical device, 103a, 104a ... PPLN waveguide, 103b, 104b ... Dichroic mirror type multiplexer, 103c, 104c ... Dichroic mirror type duplexer, 200 ... Phase conjugate light converter, 201 ... Circulator, 202 ... Wavelength multiplexing / separation filter, 203, 204 ... First and second nonlinear optical devices, 203a, 204a, PPLN waveguides, 203b, 204b, dichroic mirror type multiplexers, 203c, 204c, dichroic mirror type duplexers, 205, polarization rotation element, 300, phase Conjugate light converter, 301 ... wavelength separation filter, 302 ... wavelength multiplexing filter, 303 306... First to fourth nonlinear optical devices 303a to 306a PPPN waveguide 303b to 306b Dichroic mirror type multiplexer 303c to 306c Dichroic mirror type demultiplexer 307 to 310 First to first 4 polarization multiplexing / demultiplexing elements, 400 ... optical transmission system, 401 ... optical transmitter, 402 ... optical transmission fiber, 403 ... relay optical amplifier, 404 ... phase conjugate optical converter, 405 ... optical receiver, 406 ... transmission 500, phase conjugate light converter, 501 ... wavelength separation filter, 502 ... wavelength multiplexing filter, 503, 504 ... first and second nonlinear optical devices, 503a, 504a ... PPLN waveguide, 503b, 504b ... dichroic Mirror type multiplexers, 503c, 504c ... Dichroic mirror type duplexers, 505 ... First optical branching circuit, 506 ... 507... Variable optical attenuator (VOA), 508. Circulator, 509. Semiconductor laser, 510. Second optical branch circuit, 511. Optical amplifier (EDFA), 512. BPF), 513: nonlinear optical device for generating pump light, 514: pump light splitter, 515: third optical branch circuit, 600: optical transmission system, 601: optical transmitter, 602: optical transmission fiber, 602F DESCRIPTION OF SYMBOLS 1st optical transmission fiber, 602R ... 2nd optical transmission fiber, 603 ... Relay optical amplifier, 604 ... Phase conjugate optical converter, 605 ... Optical receiver, 606 ... Raman optical amplifier, 607 ... Phase sensitive optical amplifier, 608: Transmission path.

In this method, the signal light is converted to phase conjugate light at the intermediate point of the entire transmission line, and the nonlinearity cancellation is achieved by effectively backpropagating in the fiber using the time reversibility of the phase conjugate light. Is done. In other words, non-linear noise generated in the optical transmission fiber within 2 between waypoints from the optical transmitter 1, is compensated optical transmission fiber 2 between the intermediate point of the optical receiver 4 by causing effectively backpropagation The

In order to achieve such an object, the present invention provides a phase conjugate light converter that collectively converts optical signals composed of a plurality of wavelengths into phase conjugate light, and converts the optical signals composed of a plurality of wavelengths to all of the optical signals. A wavelength separation filter that separates the wavelength component on the short wave side and the wavelength component on the long wave side using the center wavelength in the band as a reference wavelength, and an excitation that has a half wavelength of the center wavelength in the wavelength component on the short wave side separated by the wavelength separation filter After the light is multiplexed, the first nonlinear optical device that generates the phase conjugate light of the short wavelength side component by the difference frequency generation and separates the excitation light, and the long wavelength side wavelength component separated by the wavelength separation filter second nonlinear optical device for separating the excitation light together after the excitation light having a wavelength of half the center wavelength of the multiplexed to generate a phase conjugate light of the wavelength component of the long wave side by the difference frequency generation , Wavelength components of the long-wave output from the first wavelength component and its phase conjugate light and the second nonlinear optical device of a short side outputted from the nonlinear optical device and its phase conjugate light is input, is the input And a wavelength multiplexing filter that combines the phase conjugate light of the wavelength component on the short wave side and the phase conjugate light of the wavelength component on the long wave side out of the received light to output light.

According to the present invention, an optical signal consisting of a plurality of wavelengths are separated into wavelength components of the wavelength component and the long wave side of the short side by the wavelength separation filter, wavelength components of the short side is input to the first nonlinear optical device, longwave The wavelength component on the side is input to the second nonlinear optical device. The first nonlinear optical device multiplexes the excitation light having a half wavelength of the center wavelength with the wavelength component on the short wavelength side from the wavelength separation filter, and then generates phase conjugate light of the wavelength component on the short wavelength side by difference frequency generation At the same time, the excitation light is separated and the wavelength component on the short wave side and its phase conjugate light are output. The second nonlinear optical device combines the long wavelength wavelength component from the wavelength separation filter with the excitation light having half the center wavelength, and then generates phase conjugate light of the long wavelength wavelength component by difference frequency generation. At the same time, the excitation light is separated, and the wavelength component on the long wave side and its phase conjugate light are output. The wavelength multiplexing filter inputs the short wavelength side wavelength component output from the first nonlinear optical device and its phase conjugate light and the long wavelength side wavelength component output from the second nonlinear optical device and its phase conjugate light. Of the input light, the phase conjugate light of the wavelength component on the short wave side and the phase conjugate light of the wavelength component on the long wave side are combined to be output light.

In the present invention, the wavelength component on the short wave side and the wavelength component on the long wave side of the optical signal composed of a plurality of wavelengths are converted into phase conjugate light in such a manner that they are interchanged within the same band. In this case, optical signals composed of a plurality of wavelengths are collectively converted into phase conjugate light, and the frequency is not shifted as the wavelength band of the entire optical signal composed of a plurality of wavelengths. In the present invention, if a direct junction type waveguide is used for the first and second nonlinear optical devices, an increase in excess loss due to low conversion efficiency can be avoided, and excess noise associated with phase conjugate light conversion can be avoided. Can be reduced. Further, by using the phase conjugate light converter of the present invention in an optical transmission system, it is possible to perform nonlinear noise compensation by reverse transmission characteristics for each span of the transmission path.

According to the phase conjugate optical converter of the present invention, an optical signal having a plurality of wavelengths is separated into a wavelength component on the short wave side and a wavelength component on the long wave side using the center wavelength in the entire band of the optical signal as a reference wavelength , and separated. After combining excitation light having half the center wavelength with each of the wavelength component on the short wave side and the wavelength component on the long wave side, the phase conjugate light is generated by the difference frequency generation and the excitation light is separated and generated. Since the phase conjugate light of the wavelength component on the short wave side and the phase conjugate light of the wavelength component on the long wave side are combined into an output light, the wavelength component on the short wave side and the long wave side of the optical signal consisting of multiple wavelengths are combined . The wavelength component is converted into phase conjugate light in such a manner that it is exchanged within the same band, and there is no frequency shift in the entire signal band, and it is possible to reduce the occurrence of excess noise due to phase conjugate light conversion. Further, by using the phase converter of the present invention in an optical transmission system, it is possible to perform nonlinear noise compensation by reverse transmission characteristics for each span of the transmission path.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in the following description, what is not included in the scope of the right of the present invention is described as an embodiment, but here, it will be described as an embodiment.

In this optical transmission system 600, one span in the transmission line 608 includes two optical transmission fibers 602 and a phase conjugate light converter 604. Hereinafter, of the two optical transmission fibers 602 constituting one span, the optical transmission fiber 602 (602F) on the optical transmitter 601 side is referred to as a first optical transmission fiber, and the optical transmission fiber 602 on the optical receiver 605 side ( 602R) is referred to as a second optical transmission fiber. The phase conjugate optical converter 604 is disposed between the first optical transmission fiber 602F and the second optical transmission fiber 602R.

Claims (8)

A phase conjugate light converter that collectively converts optical signals composed of a plurality of wavelengths into phase conjugate light,
A wavelength separation filter that separates an optical signal composed of the plurality of wavelengths into a first wavelength component and a second wavelength component;
A first nonlinear component that combines the excitation light with the first wavelength component separated by the wavelength separation filter and then generates phase conjugate light of the first wavelength component by difference frequency generation and separates the excitation light. An optical device;
A second nonlinear component that combines the excitation light with the second wavelength component separated by the wavelength separation filter and then generates phase conjugate light of the second wavelength component by difference frequency generation and separates the excitation light; An optical device;
The first wavelength component output from the first nonlinear optical device and the phase conjugate light thereof and the second wavelength component output from the second nonlinear optical device and the phase conjugate light thereof are input, A phase conjugation filter that combines the phase conjugate light of the first wavelength component and the phase conjugate light of the second wavelength component of the input light to produce output light. Light converter. A phase conjugate light converter that collectively converts optical signals composed of a plurality of wavelengths into phase conjugate light,
A circulator for receiving an optical signal comprising the plurality of wavelengths;
A wavelength combining / separating filter for separating an optical signal having a plurality of wavelengths received by the circulator into a first wavelength component and a second wavelength component;
After combining the excitation light with the first wavelength component separated by the wavelength combining / separating filter, a first phase component light of the first wavelength component is generated by the difference frequency generation and the excitation light is separated. A nonlinear optical device;
A second nonlinear component that combines the excitation light with the second wavelength component separated by the wavelength separation filter and then generates phase conjugate light of the second wavelength component by difference frequency generation and separates the excitation light; An optical device;
The first wavelength component output from the first nonlinear optical device and the polarization of the phase conjugate light thereof are rotated and sent to the second nonlinear optical device, and output from the second nonlinear optical device. A polarization rotation element that rotates the polarization of the second wavelength component and its phase conjugate light and sends it to the first nonlinear optical device,
The wavelength multiplexing filter is
The second wavelength component and its phase conjugate light returned through the first nonlinear optical device and the first wavelength component and its phase conjugate light returned through the second nonlinear optical device are input. And combining the phase conjugate light of the first wavelength component and the phase conjugate light of the second wavelength component of the input light and sending it to the circulator as output light,
The circulator is
A phase conjugate light converter characterized by receiving the output light sent from the wavelength multiplexing / separating filter and sending it out as a new optical signal. A phase conjugate light converter that collectively converts optical signals composed of a plurality of wavelengths into phase conjugate light,
A wavelength separation filter that separates an optical signal composed of the plurality of wavelengths into a first wavelength component and a second wavelength component;
A first polarization multiplexing / demultiplexing element that separates the first wavelength component separated by the wavelength separation filter into a first polarization component and a second polarization component;
A second polarization multiplexing / demultiplexing element that separates the second wavelength component separated by the wavelength separation filter into a first polarization component and a second polarization component;
After combining excitation light with the first polarization component of the first wavelength component separated by the first polarization multiplexing / demultiplexing element, the first wavelength component of the first wavelength component is generated by difference frequency generation. A first nonlinear optical device that generates phase conjugate light of a polarization component and separates the excitation light;
After combining excitation light with the second polarization component of the first wavelength component separated by the first polarization multiplexing / demultiplexing element, the second of the first wavelength component is generated by difference frequency generation. A second nonlinear optical device that generates phase conjugate light of a polarization component and separates the excitation light;
After combining the excitation light with the first polarization component of the second wavelength component separated by the second polarization multiplexing / demultiplexing element, the first wavelength component of the first wavelength component is generated by the difference frequency generation. A third nonlinear optical device that generates phase conjugate light of a polarization component and separates the excitation light;
After combining the excitation light with the second polarization component of the second wavelength component separated by the second polarization multiplexing / demultiplexing element, the second wavelength component of the second wavelength component is generated by the difference frequency generation. A fourth nonlinear optical device that generates phase conjugate light of a polarization component and separates the excitation light;
The first polarization component of the first wavelength component output from the first nonlinear optical device, the phase conjugate light thereof, and the second of the first wavelength component output from the second nonlinear optical device. A third polarization multiplexing / demultiplexing element that combines the first polarization component and the phase conjugate light thereof, and outputs the first wavelength component and the phase conjugate light thereof;
The first polarization component of the second wavelength component output from the third nonlinear optical device, the phase conjugate light thereof, and the second of the second wavelength component output from the fourth nonlinear optical device. A fourth polarization multiplexing / demultiplexing element that combines the polarization component and the phase conjugate light thereof, and outputs the second wavelength component and the phase conjugate light thereof;
The first wavelength component output from the third polarization multiplexing / demultiplexing element and the phase conjugate light thereof, and the second wavelength component output from the fourth polarization multiplexing / demultiplexing element and the phase conjugate thereof. And a wavelength combining filter that combines the phase conjugate light of the first wavelength component and the phase conjugate light of the second wavelength component of the input light to output light. A phase conjugate light converter characterized by The phase conjugate light converter according to any one of claims 1 to 3,
The first wavelength component is a component on the short wave side of the optical signal composed of the plurality of wavelengths,
The phase conjugate light converter, wherein the second wavelength component is a component on the long wave side of the optical signal composed of the plurality of wavelengths. The phase conjugate light converter according to any one of claims 1 to 3,
The first wavelength component is an even channel component of an optical signal composed of the plurality of wavelengths,
The second wavelength component is a component of an odd channel of an optical signal composed of the plurality of wavelengths. In the phase conjugate light converter according to any one of claims 1 to 5,
At least a first optical branch circuit, a semiconductor laser, a second optical branch circuit, an optical amplifier, a nonlinear optical device for generating pumping light, and a third optical branch circuit;
The first optical branch circuit includes:
Branching an optical signal including a pilot tone signal light having a central wavelength as an optical signal composed of a plurality of wavelengths,
The semiconductor laser is
The light having the same carrier phase as the pilot tone signal light is generated by the light injection synchronization of the pilot tone signal light,
The second optical branch circuit includes:
The light generated by the semiconductor laser is branched into two, one being a fundamental wave light and the other being a new pilot tone signal light,
The optical amplifier is
Amplifying the fundamental light,
The nonlinear optical device for generating the excitation light is:
Using the fundamental wave light amplified by the optical amplifier as input and generating pump light by second harmonic generation;
The third optical branch circuit includes:
The phase conjugate light converter, wherein the new pilot tone signal light is combined with the output light from the phase conjugate light converter. An optical transmitter that generates signal light having a plurality of wavelengths as an optical signal, a transmission path that transmits the optical signal generated by the optical transmitter, and an optical receiver that receives the optical signal transmitted through the transmission path; With
The transmission path is
An optical transmission fiber connected in multiple stages between the optical transmitter and the optical receiver;
A repeater optical amplifier provided for each span that compensates for transmission loss in each span, with one section defined by the optical transmission fiber in the transmission line as one span;
An optical transmission system comprising: the phase conjugate light converter according to any one of claims 1 to 5 disposed immediately after each of the relay optical amplifiers. An optical transmitter that generates, as an optical signal, a multicarrier signal including a plurality of pairs of signal light and phase conjugate light and pilot tone signal light; a transmission path that transmits the optical signal generated by the optical transmitter; and An optical receiver for receiving an optical signal sent through the transmission line,
The transmission path is
An optical transmission fiber connected in multiple stages between the optical transmitter and the optical receiver;
A repeater optical amplifier provided for each span that compensates for transmission loss in each span with two sections defined by the optical transmission fiber in the transmission line as one span;
An optical transmission comprising: the phase conjugate optical converter according to claim 6 disposed for each span between a first optical transmission fiber and a second optical transmission fiber in the span. system.

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