JP2006133714A - Spectrum-inverting apparatus for compensating distortion of light signal, and its method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spectrum-inverting apparatus for compensating the distortions of a light signal, and to provide its method. <P>SOLUTION: The spectrum-inverting apparatus for compensating color dispersion and nonlinear phenomenon in an optical communications system includes a first amplifying optical fiber for amplifying the light signal and a pump signal transmitted from a transmission end, a light-isolating means for preventing reflection of the light signal and the pump signal outputted from the first amplifying optical fiber to the transmission end, a nonlinear medium for generating a spectrum inversion signal by four-wave mixing to the light signal and the pump signal inputted via the light isolating means and an amplification filtering means for amplifying the light signal, the pump signal and the spectrum inversion signal outputted from the nonlinear medium and filtering only the spectrum inversion signal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a spectrum inversion apparatus and method, and more particularly, to a spectrum inversion apparatus and method for compensating for signal distortion caused by dispersion and nonlinear phenomena occurring in a high-speed optical transmission system.

  In general, as the transmission speed increases, the influence of dispersion increases rapidly and becomes the main factor limiting the transmission distance. If the transmission rate increases by a factor of 4, the transmission distance decreases to about 1/16 due to dispersion. Therefore, a dispersion compensator is always required in a high-speed optical transmission system.

  In order to compensate for such dispersion, a dispersion compensating optical fiber is used. The dispersion compensating optical fiber has a low connection loss between the transmission optical fibers and is used in a wide band. Therefore, the dispersion compensating optical fiber is suitable for dispersion compensation of Wavelength Division Multiplexing (Wavelength Division Multiplexing) signals.

  However, the dispersion compensating optical fiber has a problem that it is vulnerable to non-linear phenomena because it has a large insertion loss and a small effective area compared to a general optical fiber. For example, in order to compensate for the dispersion generated when transmitting an optical signal through a single mode optical fiber for 80 km, the insertion loss is about 8 dB when using a dispersion compensating optical fiber.

  When a dispersion compensation optical fiber is used in spite of such an insertion loss, an optical amplifier having a two-step structure in which a dispersion compensation optical fiber is inserted in an intermediate step is used to compensate for the insertion loss. ).

  However, such a two-step optical amplifier has a problem that the structure is more complicated and the cost is higher than that of the one-step optical amplifier. In addition, the insertion loss generated in the intermediate step of the optical amplifier lowers the noise figure, thereby reducing the optical signal-to-noise ratio of the entire system.

  In addition, when a low-speed system that does not require dispersion compensation that is already in operation is upgraded at high speed, dispersion compensation is required. At this time, all low-price optical amplifiers having a one-step structure installed on an optical line are expensive. Since the optical amplifier must be replaced with a two-step structure, there is a problem that a lot of cost is added.

  As an alternative to such a problem, a spectrum inversion method using a four-wave mixing phenomenon, which is a kind of third-order nonlinear phenomenon, has been proposed. The spectrum inversion method generates a spectrum inversion signal (or phase conjugation wave) from the original signal at an intermediate point in the optical line, and compensates for dispersion of the optical line by transmitting the spectrum inversion signal in the remaining section. To do.

  Such a spectrum inversion method is disclosed in Non-Patent Document 1 and Non-Patent Document 2.

  The spectrum inversion method has the disadvantage that it is difficult to satisfy the phase matching condition for spectrum inversion. However, if a spectrum inversion signal is generated only once from the midpoint of the optical line, dispersion and nonlinear phenomena are simultaneously compensated. There is an advantage of being.

  Therefore, the spectrum inversion method can prevent a decrease in the optical signal-to-noise ratio due to an insertion loss generated in an intermediate step of the optical amplifier having a two-step structure, and the conventional link is used when a low speed system is upgraded to a high speed system. The one-step optical amplifier used in the above can be used as it is. FIG. 1 is a configuration diagram of an embodiment of an optical link using a conventional spectrum inversion device, and FIG. 2 is a diagram of an embodiment for explaining the characteristics of a spectrum inversion signal by optical wave mixing.

  As shown in FIG. 1, the conventional spectrum inversion apparatus 110 includes a pump generator 111, an optical signal transmitted from the transmitter 130 through the first optical fiber 120, and a pump input from the pump generator 111. The spectrum inversion device 110 includes a coupler 112 for combining the signal and a nonlinear medium 113 for generating a spectrum inversion signal, and the spectrum inversion device 110 outputs a spectrum inversion signal of the optical signal transmitted from the transmitter 130 via the first optical fiber 120. To do.

  At this time, if the distance from the transmitter 130 to the spectrum inverter 110 and the distance from the spectrum inverter 110 to the receiver 140 are the same, the dispersion of the signal is completely compensated.

If strong light having different frequencies f 1 and f 2 is input to the nonlinear medium 113, third-order nonlinear polarization is induced in the nonlinear medium 113. At this time, a new signal component having a frequency of f3 is generated by a four-wave mixing phenomenon, which is a kind of third-order nonlinearity. Here, the frequency relationship between the three light waves is as shown in Equation 1.
f3 = 2f1-f2 Formula 1
As shown in FIG. 2, an original optical signal having a blue transition in the front part of the spectrum and a red transition in the rear part is a spectrum having a red transition in the front part and a blue transition in the rear part. Inverted to component. If such spectral reversal occurs from the intermediate point to the light beam and the dispersion characteristics of the optical line are the same, the signal that is widened by dispersion while passing through the front of the line is spectrally reversed from the intermediate point, and the rest The width of the line at the receiving end becomes the same as the width of the line at the transmitting end.

  On the other hand, as the nonlinear medium 113, a semiconductor laser amplifier, a dispersion transition optical fiber, a nonlinear optical fiber, or PPLN (Periodically Poled Lithium Niobate) is generally used.

  When a semiconductor laser amplifier is used as the nonlinear medium 113, there is an advantage that the phase matching condition is satisfied in a relatively wide wavelength band and the size can be reduced. However, the light of the spectrum inversion signal is generated by a spontaneous emission component. There is a problem in that the optical signal to noise ratio is lowered, and a power supply for driving the semiconductor laser amplifier and a system for monitoring the driving state and performance thereof are required.

  In addition, when PPLN is used as the nonlinear medium 113, the insertion loss is large, and the phase matching period and the temperature must be adjusted for similar phase matching, so that it is independent from a remote point. There is a problem that it is difficult to perform the function.

  Further, when a dispersion transition optical fiber or a nonlinear optical fiber is used as the nonlinear medium 113, since the spontaneous emission component cannot be emitted when the spectrum inversion signal is generated, the characteristics of the optical signal to noise ratio are excellent, and the dispersion transition optical fiber Alternatively, non-linear optical fiber is a passive element and has the advantage that no separate power supply is required.

A collection of articles published in February 1979, Optics Letters Vol. 4, no. 2, pp. “Compensation for channel dispersion by non-linear optical phase conjugation” such as Yariv recorded in 52-54 A collection of articles published in February 1980, Optics Letters Vol. 5, no. 2, pp. 59 to 60. M Pepper etc. “Compensation for phase distortion in non-linear media by phase conjugation”

  However, when a dispersion transition optical fiber or a nonlinear optical fiber is used, it is difficult to satisfy the phase matching condition, so that the inversion efficiency at which the signal is converted into a spectrum inversion signal is lower than −20 dB. . Therefore, there is a difficulty in applying the optical transmission system. Also, it is necessary to install a spectrum inversion device remotely in the middle of the optical line, and for the operation of the spectrum inversion device, a means for providing pump signals and amplification is necessary. There is a problem that must be done.

  Therefore, there is an urgent need for a technology that realizes a spectrum inversion device that has high inversion efficiency and is composed of only passive elements.

  The present invention has been made to solve the problem, and an object of the present invention is to receive an optical signal from a transmission optical fiber provided with a Raman gain, and to use the remaining residual Raman pump signal to make a nonlinear medium. The present invention provides a spectrum inversion apparatus and method for increasing the efficiency of a spectrum inversion signal that compensates for signal distortion by providing Raman gain.

  To achieve the above object, the apparatus of the present invention is a spectrum inversion apparatus for compensating for chromatic dispersion and nonlinear phenomena in an optical communication system, and a first apparatus for amplifying an optical signal and a pump signal transmitted from a transmission end. Amplifying optical fiber, light isolating means for preventing the optical signal and pump signal output from the first amplifying optical fiber from being reflected by the transmitting end, and input via the light isolating means A nonlinear medium for generating a spectrum inversion signal by four-wave mixing for the optical signal and the pump signal, and amplifying the optical signal, pump signal and spectrum inversion signal output from the nonlinear medium, and filtering only the spectrum inversion signal And an amplification filtering means.

  Also, the method of the present invention is a spectrum inversion method for compensating for chromatic dispersion and nonlinear phenomena in an optical communication system, wherein an optical signal and a pump signal input from a transmission end are amplified using a Raman pump signal. 1 Raman amplification step, a first erbium amplification step for amplifying the Raman amplified optical signal and the pump signal using an erbium-doped optical fiber, and four-wave mixing for the erbium amplified optical signal and the pump signal. A spectrum inversion signal generating step for generating a spectrum inversion signal using such a non-linear phenomenon, and an amplification filtering step for amplifying and filtering the generated spectrum inversion signal are included.

  On the other hand, the present invention is based on the following theoretical background.

Pump signal _{E P,} only the optical signal _{E S} were partially degenerated to be related if (partially degenerated) 4 phase matching conditions for wave mixing is satisfied, the intensity _{E SI} of spectrum inversion signal of Formula 2 Is defined as

Here, n is the refractive index of the optical fiber, X ⁽³⁾ is the third-order nonlinear reduction factor of the optical fiber, E _P and E _S are the intensity of the pump signal and the optical signal, respectively, and A _eff is the effective cutoff of the optical fiber. The area, L _eff is the effective length of the optical fiber, L is the length of the optical fiber, and α is the loss factor. Therefore, if the intensity of the input optical signal and the pump signal are large and the loss of the optical fiber is reduced, the intensity of the spectrum inversion signal is maximized.

  In the present invention, a Raman gain is provided to a transmission optical fiber, and a remote optical amplifier is operated using a residual Raman pump signal remaining at this time, thereby increasing the intensity of light input to the nonlinear medium, By providing a Raman gain to the optical fiber used in the medium, the effective loss of the nonlinear medium is reduced and the strength of the spectrum inversion signal is increased.

  The present invention as described above has an effect that the spectrum inversion efficiency can be increased to more effectively compensate for signal distortion caused by dispersion and nonlinear phenomenon generated from the optical transmission line.

  In addition, since the present invention does not use an active element necessary for a conventional spectrum inversion apparatus, it is possible to obtain an effect that the disadvantages due to the characteristics that must be located at the midpoint of the optical line can be eliminated.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 3 is a block diagram of an embodiment of an optical link using the spectrum inversion apparatus according to the present invention, and FIG. 4 is a block diagram of a detailed embodiment of the transmitting end of FIG.

  As shown in FIG. 3, the optical link includes a transmission end 310, a first optical line 320, a spectrum inversion device 350, a second optical line 370, and a reception end 380.

  The transmitting end 310 includes a transmitter 311 for outputting a modulated optical signal, a pump generator 312 for outputting a pump signal for generating a spectrum inversion signal, and a Raman for the modulated optical signal and the pump signal. 4 includes a first Raman pump generator 313 for outputting a Raman pump signal to provide a gain, and a multiplexer 314 for multiplexing the optical signal, the pump signal, and the Raman pump signal. As described above, the transmitting end 310 includes an optical rotator 315 and a photodetector 316 to adjust the strength of the Raman pump signal of the first Raman pump generator 313.

  As defined in Equation 2, since the spectral inversion efficiency is proportional to the magnitude of the pump signal and the input optical signal, the intensity of the pump signal and the optical signal is increased and input to the first optical line 320. Otherwise, high inversion efficiency cannot be obtained.

  However, when increasing the signal magnitude, the first optical line 320 generates stimulated Brillouin scattering (SBS), which is a kind of nonlinear current state. Therefore, to suppress this, frequency dithering, Must be modulated using another phase modulator. However, this is not only structurally complex, but can also induce distortion in the phase reversal signal.

  Therefore, in the present invention, by providing a Raman gain from the first Raman pump generator 313 at the transmission end 310 to the first optical line 320, the light output from the transmitter 311 and the pump generator 312 at the transmission end 310 is provided. Signal and pump signal strength can be kept low.

The optical signal output from the transmitting end 310 is E _S , the pump signal whose amplitude is E _P , and the Raman pump signal whose amplitude is E _R are multiplexed by the multiplexing unit 314 and then the first light beam. The signal is transmitted to the spectrum inversion device 350 via the path 320.

At this time, a distributed Raman gain (G _R1 ) is generated in the first optical line 320 by the Raman pump signal output from the first Raman pump generator 313. Therefore, the optical signal and the pump signal input to the first optical line 320 together with the Raman pump signal are transmitted to the spectrum inversion device 350 without loss due to the Raman gain.

  The optical rotating machine 315 at the transmitting end 310 detects a component scattered in the reverse direction while the pump signal output from the pump generator 312 travels along the optical line 320 and outputs the detected component to the photodetector 316. Then, the photodetector 316 detects the scattered power of the extracted pump signal and outputs it to the first Raman pump generator 313. The first Raman pump generator 313 responds to the detected scattered power of the pump signal. Adjust the strength of the Raman pump signal. By doing so, the Raman gain in the first optical line 320 is kept constant, and the strengths of the optical signal and the pump signal are kept constant. The Raman gain uses a gain that is provided in the reverse direction with respect to scattered light that is not related to signal transmission in order to provide the gain in the forward direction and the reverse direction.

  Meanwhile, as shown in FIG. 3, the spectrum inversion device 350 includes a first amplification optical fiber 351 for amplifying an optical signal and a pump signal transmitted via the first optical line 320, an amplified optical signal. And an optical isolation unit 352 for preventing the pump signal from being reflected by the transmitting end 310, a nonlinear medium 353 for generating a spectrum inversion signal, and a signal output from the nonlinear medium 353 by amplifying the spectrum inversion signal. An amplification filtering unit 354 for filtering only the signal.

The first amplification optical fiber 351 is pumped by the residual Raman pump signal E _{R1 ′} remaining after being provided to the first optical line 320 from the first Raman pump generator 313 at the transmission end 310, and the first optical line 320 is _transmitted through the first optical line 320. The transmitted optical signal and pump signal are amplified. At this time, the first amplification optical fiber 351 may be an erbium-doped optical fiber or an equivalent optical fiber, and provides a gain G _E1 for the optical signal and the pump signal. Therefore, the spectrum inversion apparatus 350 according to the present invention can provide sufficient light intensity necessary for spectrum inversion without generating another amplification apparatus for generating the spectrum inversion signal.

Light isolation unit 352, the optical signal E _S and the pump signal E _P output from the first amplifying optical fiber 351 is prevented from being input to the transmission terminal 310 is reflected, the optical signal E _S and pump signal E _P is output to the nonlinear medium 353.

Nonlinear medium 353 generates a spectrum inversion signal _{E SI} by four wave mixing phenomenon on the optical signal _{E S} and the pump signal _{E P} output from the optical isolation unit 352, the optical signal _{E S,} pump signal _{E P} and The spectrum inversion signal _ESI is output.

At this time, the nonlinear medium 353 has a Raman gain G _RNL due to a residual Raman pump signal E _{R2 ″} transmitted from the Raman pump generator 382 of the receiving end 380, which will be described later, which is transmitted in the opposite direction, and thus the optical signal E _S, and outputs a pump signal _{E P} and spectral inversion signal _{E SI} amplifies higher the Raman gain _{G RNL.} This reduces the effective loss of the optical fiber and can reduce the critical intensity at which the spectrally inverted signal is generated, and in a nonlinear medium to provide a predetermined amplification function for the spectrally inverted signal. Even when the phase matching condition cannot be satisfied and the spectral conversion efficiency is low, a high spectral inversion signal can be obtained.

  The amplification filtering unit 354 amplifies the signal output from the nonlinear medium 353 and filters only the spectrum inversion signal. However, as illustrated in FIG. 3, the amplification filtering unit 354 includes the optical signal output from the nonlinear medium 353. The second amplification optical fiber 355 for amplifying the pump signal and the spectrum inversion signal, and the filtering unit 356 for filtering only the spectrum inversion signal among the signals output from the second amplification optical fiber 355 are included.

At this time, the second amplification optical fiber 355 is pumped by the residual Raman pump signal _{ER2 ′} remaining after being provided from the second Raman pump generator 382 of the receiving end 380 to the second optical line 370, and gain G _E2 is obtained. And amplifies the signal transmitted from the nonlinear medium 353. That is, the spectrum inversion signal is also amplified as a gain G _E2 while through the second amplifying optical fiber 355. Here, the second amplification optical fiber 355 may be an erbium-doped optical fiber or an equivalent optical fiber, and the positions of the second amplification optical fiber 355 and the filtering unit 356 can be changed from each other. .

FIG. 5 is a diagram illustrating an embodiment of the filtering process of the amplification filtering unit shown in FIG. 3. In the forward direction, the first filter 510 includes the optical signal E _S and the pump signal E _P input from the nonlinear medium 353. and filtering the pump signal _{E P} and the spectral inversion signal _{E SI} of spectrum inversion signal _{E SI} and outputs, the second filter 520, the pump signal _{E P} and the spectral inversion signal _{E SI} output from the first filter 510 Among these, the spectrum inversion signal _ESI is filtered and output to the second optical line 370. On the other hand, the first filter 510 and the second filter 520 transmit the residual Raman pump signal from the second Raman pump generator 382 of the receiving end 380 described later to the second amplification optical fiber 355 in the reverse direction. Further, the optical signal extracted from the first filter 510 _{E S,} pump signal _{E P} extracted from the second filter 520 are respectively vertical by the terminating unit 530 and 540 having a high reverse losses.

  On the other hand, as shown in FIG. 3, the receiving end 380 passes through a second Raman pump generator 382 and a second optical line 370 that output a Raman pump signal for providing a Raman gain to the second optical line 370. A receiver 381 for receiving the transmitted spectrum inversion signal.

A distributed Raman gain (Distributed Raman Gain, G _R2 ) is generated in the second optical line 370 by the Raman pump signal output from the second Raman pump generator 382 to reduce the attenuation of the spectrum inversion signal to a minimum. The residual Raman pump signal remaining after being provided to the optical line 370 is transmitted to the second amplification optical fiber 355 and the nonlinear medium 353 through the filtering unit 356 of the spectrum inversion device 350. At this time, as described above, the second amplification optical fiber 355 is pumped by the residual Raman pump signal, and the nonlinear medium 353 has a Raman gain by the residual Raman pump signal.

  FIGS. 6 and 7 are configuration diagrams of an embodiment of a single channel long distance link and a multi-channel long distance link, respectively, using the spectrum inversion apparatus according to the present invention.

  As shown in FIGS. 6 and 7, a single channel long distance link includes one transmitter and receiver, and a multi-channel long distance link includes multiple transmitters and receivers.

  The long distance link includes N optical lines composed of an optical fiber 610 and an optical amplifier 620, and the spectrum inversion apparatus according to the present invention is installed at an intermediate point of the long distance link transmission path. That is, when the total transmission distance is D, a spectrum inversion device is installed at a point where D / 2.

  In general, the point that becomes D / 2 is located in the base station when used together with the repeater, but is located at an arbitrary point on the optical line otherwise. When the D / 2 point is located at an arbitrary point on the optical line, the pump generator 312 at the transmission end and the first Raman pump generator 313 are installed at the optical repeater closest to the transmission end from the spectrum inversion device, and receive signals. The second Raman pump generator 382 at the end is installed in the optical repeater closest to the receiving end side from the spectrum inversion device. At this time, an optical amplifier that does not have a dispersion compensation function is used as the remaining optical repeater.

  By doing so, a system having a 2.5 Gb / s class transmission rate that does not require dispersion compensation can be increased to 10 Gb / s or 40 Gb / s without changing the optical amplifier.

  FIG. 8 is a flowchart of one embodiment for explaining the spectrum inversion method according to the present invention.

As shown in the drawing, the optical signal and the pump signal input from the transmitting end are amplified using the first Raman pump signal (810), and erbium addition is performed using the remaining first Raman pump signal. The optical signal and the pump signal are amplified using an optical fiber amplifier (820).
Thereafter, a spectrum inversion signal is generated for the optical signal and the pump signal amplified while passing through the erbium-doped optical fiber with the help of the residual second Raman pump signal (830). At this time, the residual second Raman pump signal is a residual Raman pump signal from the receiving end.
Next, an optical signal, a pump signal, and a spectrum inversion signal are amplified using an erbium-doped fiber amplifier that is operated using the residual second Raman pump signal (840), and the amplified optical signal, the pump signal is amplified. Then, only the spectrum inversion signal among the spectrum inversion signals is filtered (850).

  Finally, the filtered spectrum inversion signal is amplified using the second Raman pump signal provided from the receiving end (860).

It is a block diagram of one Embodiment of the optical link using the conventional spectrum inversion apparatus. It is drawing of one Embodiment explaining the characteristic of the spectrum inversion signal by light wave mixing. It is a block diagram of one Embodiment of the optical link using the spectrum inversion apparatus which concerns on this invention. It is a block diagram of detailed embodiment of the transmission end of FIG. 4 is a diagram illustrating a filtering process of an amplification filtering unit in FIG. 3 according to an embodiment. 1 is a configuration diagram of an embodiment of a single channel long distance link using a spectrum inversion apparatus according to the present invention. FIG. 1 is a configuration diagram of an embodiment of a multi-channel long distance link using a spectrum inversion device according to the present invention. FIG. It is a flowchart of one Embodiment explaining the spectrum inversion method which concerns on this invention.

Explanation of symbols

311 Transmitter 312 Pump generator 313 First Raman pump generator 314 Multiplexer 320 First optical line 351 First amplification optical fiber 352 Optical isolation unit 353 Nonlinear medium 355 Second amplification optical fiber 356 Filtering unit 370 Second optical line 381 Receiver 382 Second Raman pump generator

Claims (10)

In a spectral inversion device for compensating for chromatic dispersion and nonlinear phenomena in an optical communication system,
A first amplification optical fiber for amplifying the optical signal and the pump signal transmitted from the transmission end;
An optical isolation means for preventing the optical signal and the pump signal output from the first amplification optical fiber from being reflected by the transmission end;
A non-linear medium for generating a spectrum inversion signal by four-wave mixing with respect to the optical signal and the pump signal input through the optical isolation means;
A spectrum inversion apparatus comprising: amplification filtering means for amplifying the optical signal, pump signal, and spectrum inversion signal output from the nonlinear medium, and filtering only the spectrum inversion signal. The amplification filtering means includes
A second amplification optical fiber doped with erbium for amplifying the signal;
Filtering means for filtering only the spectrum inversion signal among the optical signal, the pump signal and the spectrum inversion signal, and transmitting the Raman pump signal provided from the Raman pump generator at the receiving end in the reverse direction to the nonlinear medium; The spectrum inversion device according to claim 1, comprising: The spectrum inversion signal is
Before being input from the receiving end, it is amplified via the Raman pump signal provided from the receiving end Raman pump generator,
The second amplification optical fiber and the nonlinear medium are:
The spectrum inversion apparatus according to claim 2, wherein amplification is performed using a residual Raman pump signal. The optical signal and pump signal are:
Before being input to the first amplification optical fiber, it is amplified via a Raman pump signal provided from a Raman pumps generator at the transmission end,
The first amplification optical fiber is:
The spectrum inversion device according to any one of claims 1 to 3, wherein the spectrum inversion device is an optical fiber to which erbium is added, and amplifies by using a residual Raman pump signal. The Raman pump generator at the transmitting end is
5. The spectrum inversion apparatus according to claim 4, wherein the Raman pump signal is adjusted by detecting the scattered power of the pump signal. In a spectral inversion method for compensating for chromatic dispersion and nonlinear phenomena in an optical communication system,
A first Raman amplification step for amplifying an optical signal and a pump signal input from the transmission end using a Raman pump signal;
A first erbium amplification step of amplifying the Raman amplified optical signal and the pump signal using an erbium-doped optical fiber;
A spectrally inverted signal generating step of generating a spectrally inverted signal using a nonlinear phenomenon such as four-wave mixing for the erbium amplified optical signal and the pump signal;
An amplification filtering step for amplifying and filtering the generated spectrum inversion signal. The amplification filtering step includes:
A second erbium amplification step for amplifying the optical signal, the pump signal and the spectrally inverted signal using an erbium-doped optical fiber;
The method of claim 6, further comprising: filtering only the spectrum inversion signal among the amplified optical signal, the pump signal, and the spectrum inversion signal. The spectrum inversion method according to claim 7, further comprising: a second Raman amplification step of amplifying and filtering the spectrum inversion signal provided from the receiving end and amplifying using the Raman pump signal. The erbium-doped optical fiber of the first erbium amplification step is
9. The spectrum inversion method according to claim 8, wherein pumping is performed by a residual Raman pump signal from a transmitting end.   The erbium-doped optical fiber in the second erbium amplification step is pumped by a residual Raman pump signal from the receiving end, and the spectrum inversion signal generating step amplifies using the residual Raman pump signal from the receiving end. The spectrum inversion method according to claim 9.

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