JP5466520B2 - Wavelength division multiplexing optical fiber transmission system - Google Patents

Description

  The present invention reduces the influence of four-wave mixed light that interferes with signal light on the frequency grid when wavelength-division multiplex transmission is performed on a plurality of signal lights each modulated by a plurality of optical carriers arranged on a predetermined frequency grid. The present invention relates to a wavelength division multiplexing optical fiber transmission system.

  In an optical fiber transmission system, wavelength division multiplexing (WDM) communication is employed to increase capacity. Wavelength division multiplexing can increase the wavelength spacing of WDM signals, increase the optical signal-to-noise ratio, increase the transmission capacity per unit frequency, that is, the frequency utilization efficiency, and increase the transmission capacity per optical fiber. This is important for economically constructing an optical fiber transmission system.

  The conventional WDM optical fiber transmission system is realized by mounting a required number of individual semiconductor lasers having optical frequencies that match a standardized frequency grid.

  In this case, transmission performance is limited due to various nonlinear phenomena that occur during optical fiber transmission. Among them, the four-wave mixed light is noticeably affected by relatively weak input light intensity in optical fiber transmission with small wavelength dispersion. In WDM transmission using optical carriers arranged at equal intervals in a frequency grid, a plurality of four-wave mixed light leaks as crosstalk on the frequency grid and interferes with signal light. The interference behaves as noise when the signal light phase is uncorrelated. At this time, the bit error rate starts to deteriorate from the crosstalk amount of about −32 dB.

  Until now, in order to reduce the influence of beat noise, many measures have been taken to arrange the optical carrier frequencies at unequal intervals or to limit the optical input intensity to the optical fiber.

JP 2007-094143 A

  By the way, if the intensity of the four-wave mixed light that interferes with the signal light can be suppressed by canceling the four-wave mixed light generated in the frequency grid and the amount of crosstalk with respect to the signal light intensity can be reduced, an optical fiber can be used. The light input intensity to the can be increased, and the SNR can be improved. If the SNR is improved, the number of regenerative repeaters can be reduced, and an economical wavelength division multiplexing optical fiber transmission system can be constructed.

  An object of the present invention is to provide a wavelength division multiplexing optical fiber transmission system capable of reducing the influence of four-wave mixed light generated in a predetermined frequency grid and reducing the amount of crosstalk to signal light.

The first invention generates a plurality of signal lights obtained by modulating a plurality of optical carriers arranged in a predetermined frequency grid, multiplexes the plurality of signal lights, and sends them to a transmission optical fiber for wavelength division. In a wavelength division multiplexing optical fiber transmission system that demultiplexes and receives a plurality of multiplexed signal lights, a plurality of optical carriers are compared with a reference optical frequency comb having a predetermined optical frequency interval and synchronized in phase with each other. a first optical phase synchronization means Ru synchronize the optical phase with respect to the reference optical frequency comb, and a second optical phase synchronization means for synchronizing a plurality of signal light of the optical phase, the second optical phase synchronization means By controlling the electric field phase of a plurality of signal lights that are optically phase-locked to set a phase difference of a predetermined pattern between each signal light, and by using four-wave mixed light generated in a predetermined frequency grid during propagation through the transmission optical fiber to reduce the leakage strength And a phase shifting means.

The second invention generates a plurality of signal lights obtained by modulating a plurality of optical carriers arranged in a predetermined frequency grid, multiplexes the plurality of signal lights, and sends them to an optical fiber for transmission. In a wavelength division multiplexing optical fiber transmission system that demultiplexes and receives a plurality of multiplexed signal lights, a plurality of optical carriers are compared with a reference optical frequency comb having a predetermined optical frequency interval and synchronized in phase with each other. Optical phase synchronization means that synchronizes the optical phase and synchronizes the optical phase of a plurality of signal lights, and four light waves that are generated in a predetermined frequency grid while propagating through a transmission optical fiber for signal light in a predetermined frequency grid An optical phase shift means for controlling the electric field phase before multiplexing the plurality of signal lights so that the leakage intensity due to the mixed light is reduced. Phase shift keying (D When generated by the PSK method, the optical phase of at least one of the plurality of signal lights is set to be different by π / 4 [rad] relative to the optical phase of the other signal lights. Then, the four-wave mixed light is generated in which the axis is shifted by π / 4 with respect to the signal light on the predetermined frequency grid to reduce the interference with the signal light. That is, the number of coaxial four-wave mixed light components having a large interference with signal light on a predetermined frequency grid can be reduced, and the amount of crosstalk with respect to signal light intensity can be reduced as a whole.

  In the present invention, when a plurality of signal lights arranged in a predetermined frequency grid and optically phase-synchronized are wavelength division multiplexed, the electric field phase of each signal light is appropriately controlled to be generated in the predetermined frequency grid. It is possible to cancel the four-wave mixed light and to reduce the number of four-wave mixed light that interferes with the signal light. As a result, the amount of crosstalk with respect to the signal light intensity can be reduced, and the SNR can be improved by increasing the light input intensity to the optical fiber.

It is a figure which shows the basic composition of the wavelength division multiplexing optical fiber transmission system of this invention. It is a figure which shows the relationship of the four-wave mixing light intensity with respect to the phase difference ((theta) a- (theta) b) of four-wave mixing light. It is a figure which shows the Example structure of the wavelength division multiplexing optical fiber transmission system of this invention. It is a figure which shows the received waveform in an intensity | strength modulation system. It is a figure which shows the received waveform in a DPSK system. It is a figure which shows the received waveform in a DQPSK system.

(Basic configuration)
FIG. 1 shows a basic configuration of a wavelength division multiplexing optical fiber transmission system according to the present invention.
In FIG. 1, in this basic configuration, four signal lights arranged in # -2, # -1, # 1, and # 2 in five frequency grids # -2 to # 2 are optical phase shifters 11. Input to the optical multiplexer 12 through -1 to 11-4. The four signal lights are optically phase-synchronized with each other, and a phase difference π is set in a predetermined pattern between the signal lights by the optical phase shifters 11-1 to 11-4. The signal light wavelength-multiplexed by the optical multiplexer 12 is transmitted through a transmission optical fiber (for example, dispersion shifted fiber (DSF)) 13, and is demultiplexed into signal light of each wavelength by an opposing optical demultiplexer 14. The

The optical carrier frequencies and phases of the four signal lights are (f ₋₂ , θ ₋₂ ), (f ₋₁ , θ ₋₁ ), (f ₁ , θ ₁ ), and (f ₂ , θ ₂ ). The optical carrier frequency interval is
| F ₋₂ −f ₋₁ | = | f ₁ −f ₂ | = Δf
| F ₋₁ −f ₁ | = 2Δf
It becomes.

Here, the four-wave mixed light generated in the frequency grid # 0 at the optical fiber propagation distance z = 1 when the four signal lights are transmitted through the transmission optical fiber 13 will be considered. For this purpose, only the four-wave mixed light is visible without arranging the signal light in the frequency grid # 0. However, the signal light may be present in the frequency grid # 0 as in an embodiment described later. Let the frequency and phase of this four-wave mixed light be (f _FWM , θ _FWM ).

  Of the signal lights in the frequency grids # -2, # -1, # 1, and # 2, i + j is a frequency grid of three signal lights that generate four-wave mixed light in the frequency grid # 0. Table 1 shows combinations that satisfy -k = 0. In addition, what replaced i and j is the same four-wave mixing light.

The electric field of the signal light having the phase θ _q at the optical fiber propagation distance z = 1 is generally E (ω _q , l) = (1/2) E _q (l) exp (−j (ω _q t−β _q l + θ _q )) + c.c. (1)
It becomes. Here, q = i, j, k, ω _q = 2πf _q is an optical carrier angular frequency, and β _q is a propagation constant.

At this time, the electric field of the four-wave mixed light generated in the frequency grid # 0 is
E _FWM (ω _FWM , l) = (1/2) jlA ₀ E _i (0) E _j (0) E _k ^* (0) exp (−αl / 2)
Exp (−j (ω _FWM t−β _FWM l + θ _FWM )) + c.c. (2)
It becomes. Here, A ₀ is a constant representing the generation efficiency of four-wave mixed light, α is a loss factor of the optical fiber, and θ _FWM = θ _i + θ _j −θ _k . E _k ^* (0) is a complex conjugate. Now
E _FWM (l) = lA ₀ E _i (0) E _j (0) E _k ^* (0) exp (−αl / 2)
If we organize equation (2) as
E _FWM (ω _FWM , l)
= (1/2) jE _FWM (l) exp (−j (θ _FWM −β _FWM l)) exp (−jω _FWM t) + c.c. (3)
It becomes.

Here, the four-wave mixed light a and d generated in the frequency grid # 0
[i, j, k] = a [1, 1, 2], b [-1, -1, -2]
Pay attention to. Each phase θ _{FWM is set} to θa = θ ₁ + θ ₁ −θ ₂
θd = θ _-1 + θ _-1 -θ _-2
Then, the electric field of the two four-wave mixed lights a and d superimposed on the frequency grid # 0 is
E _FWM (ω _FWM , l)
= (1/2) jE _FWM (l). [Exp (−j (θa−β _FWM l)) + exp (−j (θd−β _FWM l))]] exp (−jω _FWM t) + c.c. …(Four)
It becomes.

Therefore, the intensity of the two four-wave mixed lights a and d superimposed on the frequency grid # 0 at the optical fiber propagation distance z = 1 is
P _FWM (ω _FWM , l) = <E _FWM (ω _FWM , l) · E ^* _FWM (ω _FWM , l)>
= (E _FWM (l)) ² (1 + cos (θa−θd)) (5)
It becomes. Here, <•> represents a time average.

  Equation (5) shows that when the two four-wave mixed lights a and d satisfy the relationship of phase difference θa−θd = (2n + 1) π (n is an integer), the two four-wave mixed lights a and d are canceled. Indicates that they will cancel each other.

  Further, if the phase difference of an arbitrary pair of four four-wave mixed lights a, b, c and d generated in the frequency grid # 0 shown in Table 1 is (2n + 1) π, the four-wave mixed light of the pair is Will be cancelled. The same applies to the four-wave mixed light generated in other frequency grids.

  Here, the phase of the four-wave mixed light generated in an arbitrary frequency grid is determined from the phases of the three signal lights that generate the four-wave mixed light. When the signal light is based on the intensity modulation method, the phase of the four-wave mixed light is (2n + 1) π when there is one signal light having the phase π among the three signal lights, and zero signal light having the phase π. Alternatively, when there are two, the phase of the four-wave mixed light becomes 2nπ.

  Table 2 shows the phases and frequencies set for the four signal lights arranged in the frequency grids # -2, # -1, # 1, and # 2 in the optical phase shifters 11-1 to 11-4 in FIG. The phase relationship of the four-wave mixed light a to d generated in the grid # 0 is shown. The phase pattern of the signal light is indicated by 0 and π (rad, the same shall apply hereinafter), which are relative.

  In Table 2, the phase patterns set for the signal lights of the frequency grids # -2, # -1, # 1, and # 2 are all 0, 1, 2, 3, and 4 with the number of signal lights having the phase π. With 16 patterns. For example, the phase pattern [0, 0, 0, π] indicates that the phase of the signal light of the frequency grid # 2 among other signal lights of the frequency grids # -2, # -1, # 1, and # 2 is other frequencies. It shows that π is different from the signal light of the grid. The phase of the four-wave mixed light is (2n + 1) π or 2nπ depending on the phase pattern of the signal light that generates the four-wave mixed light. However, in Table 2, for simplicity, they are expressed as π and 0, respectively. .

  Of the four-wave mixing lights a to d generated in the frequency grid # 0, if the number of four-wave mixing lights having a phase of (2n + 1) π and the number of four-wave mixing lights having a phase of 2nπ is the same, any four pairs The light wave mixing light is canceled. In the example of Table 2, [0, 0, 0, 0], [0, π, π, 0], [π, 0, 0, π], [π, π, π, π] The mixed light becomes the same phase and cannot be canceled. However, in the other 12 patterns, there are two four-wave mixed lights having a phase of (2n + 1) π and two four-wave mixed lights having a phase of 2nπ. it can.

FIG. 2 shows the relationship of the four-wave mixing light intensity with respect to the phase difference (θa−θd) of the four-wave mixing light.
The data indicated by the solid line is the result of calculation using Equation (5), and the data indicated by the dot is the experimental result. It can be seen that by changing the phase using the optical phase shifter, the intensity of the four-wave mixing light changes according to the phase difference (θa−θd) of the four-wave mixing light.

(Example configuration)
FIG. 3 shows an embodiment of the wavelength division multiplexing optical fiber transmission system of the present invention.
In FIG. 3, the reference optical frequency comb generator 22 generates reference optical frequency combs that are phase-synchronized with each other at optical frequency intervals corresponding to the frequency given from the reference clock 21. The optical demultiplexer 23 divides the reference optical frequency comb into n branches (n is an integer equal to or greater than 2), and inputs it to n optical phase-locked CW light sources 24-1,..., 24-n. The optical phase-locked CW light sources 24-1,..., 24-n output optical carriers (CW light) whose optical frequencies and phases are synchronized with optical frequency components (ω _{1 to} ω _n ) having different reference optical frequency combs, respectively. .

  The optical carriers output from the optical phase-locked CW light sources 24-1,..., 24-n are respectively modulated by the modulation signals 1 to n by the modulators 25-1,. 1,..., 11-n are combined by the optical multiplexer 12 and sent to the transmission optical fiber 13. The wavelength multiplexed signal light transmitted through the transmission optical fiber 13 is demultiplexed into signal light of each wavelength by the optical demultiplexer 14, and is received by the optical receivers 26-1, ..., 26-n.

  On the other hand, the wavelength multiplexed signal light multiplexed by the optical multiplexer 12 is branched and input to the optical phase detector 27. The optical phase detector 27 detects the light combined with the wavelength multiplexed signal light and the reference optical frequency comb, and cancels a plurality of four-wave mixed light that causes crosstalk with an arbitrary signal light, or cancels an arbitrary one. In this configuration, the electric field phase of the signal light of each wavelength is feedback-controlled by the optical phase shifters 11-1,..., 11-n so that the number of four-wave mixed light that interferes with the signal light is reduced. The optical phase detector 27 is configured to optically synchronize the signal light with respect to the reference optical frequency comb using the optical PLL when a plurality of signal lights are generated by the intensity modulation method. The optical phase detector 27 uses a Costas loop when a plurality of signal lights are generated by a differential phase shift keying (DPSK) method or a differential quadrature phase shift keying (DQPSK) method. In this configuration, the signal light is optically phase-synchronized with the reference optical frequency comb.

(In the case of wavelength division multiplex transmission using intensity modulation)
Wavelength division multiplex transmission of signal light generated by intensity modulation from 8 wave optical carriers of frequency grids # -3, # -2, # -1, # 0, # 1, # 2, # 3, # 4 In this case, an example of four-wave mixed light generated in the frequency grid # 0 is shown.

  The phase of the four-wave mixed light is determined from the phases of the three signal lights that generate the four-wave mixed light in the frequency grid # 0. Of the three signal lights, the phase of the four-wave mixed light is (2n + 1) π when the number of signal lights having a phase of π is one, and the number of four light waves when the number of signal lights having the phase of π is zero or two. The phase of the mixed light is 2nπ.

As an example of setting the phase of at least one signal light among the eight signal lights arranged in the frequency grids # -3 to # 4 to be different by π, the signal light of each frequency grid is changed by π Is one example: [0,0,0,0, π, 0,0,0]
Two examples of signal light different from π: [0, 0, π, 0, 0, π, 0, 0]
Three examples of signal light different from π: [0, π, 0, 0, π, 0, 0, π]
Table 3 shows the phases of 18 four-wave mixed lights a to r generated in the frequency grid # 0.

  For reference, although the case where the phase of the signal light of 8 waves is set to [0, 0, 0, 0, 0, 0, 0, 0] is shown, all four-wave mixed lights a to r are shown. It has a phase of 2nπ.

  The phase of the signal light of 8 waves is [0, 0, 0, 0, π, 0, 0, 0], [0, 0, π, 0, 0, π, 0, 0], [0, π, 0 , 0, π, 0, 0, π], there are 10 four-wave mixed lights each having a phase of 2nπ and eight four-wave mixed lights having a phase of (2n + 1) π. . Accordingly, the eight sets (16 pieces) of the four-wave mixed light are canceled by the phase difference of π. Note that although two four-wave mixed light remains, the four-wave mixed light that interferes with the signal light can be significantly reduced. In addition, there is a pattern in which all nine sets (18) of four-wave mixed light can be canceled with a phase difference of π by changing the position of signal light having a phase different by π.

FIG. 4 shows a received waveform in the intensity modulation method.
The crosstalk amount of the four-wave mixed light with respect to the signal light of the frequency grid # 0 is set to [0, 0, 0, 0, 0, 0, 0, 0] for the 8-wave phase pattern shown in Fig. 4 (1). Is -8.0 dB, and the 8-wave phase pattern shown in Fig. 4 (2) is set to [0, 0, π, 0, 0, π, 0, 0]. It can be seen that the amount is reduced by about 10 dB, and the influence of four-wave mixing light can be reduced.

  When n> 8, the number of four-wave mixed light patterns generated on an arbitrary frequency grid increases. However, by setting the phase of at least one signal light to be different by π, a four-wave mixed light having a phase of 2πn and a four-wave mixed light having a phase of (2n + 1) π from a combination of three signal lights. Each occurs. Since the phase difference thereof is π, it is canceled and the four-wave mixed light that interferes with the signal light can be reduced.

  The above description is about the four-wave mixed light generated in the frequency grid # 0, but the same applies to the four-wave mixed light generated in other frequency grids. Table 4 shows the number of four-wave mixed light generated in the frequency grids # -3 to # 4 and the number canceled.

  By setting the phase of at least one of the eight signal lights arranged in the frequency grids # -3 to # 4 to be different by π, the four-wave mixed light generated in each frequency grid is canceled. It can be seen that although there are variations in the number, the four-wave mixed light that interferes with the signal light can be reduced.

(For wavelength division multiplexing transmission using the DPSK method)
When wavelength division multiplex transmission is performed on signal light generated by the DPSK system from eight optical carriers of frequency grids # -3, # -2, # -1, # 0, # 1, # 2, # 3, and # 4 Shows an example of the four-wave mixed light generated in the frequency grid # 0.

  The phase of the four-wave mixed light is determined from the phases of the three signal lights that generate the four-wave mixed light in the frequency grid # 0. The four-wave mixed light has a quadrature component when there is one signal light having a phase of π / 2 among the three signal lights, and four when the number of signal lights having a phase of π / 2 is zero or two. The light wave mixing light interferes with the signal light with an in-phase component.

As an example of setting the phase of at least one signal light out of eight signal lights arranged in the frequency grids # -3 to # 4 to be different by π / 2, the phase of the signal light in each frequency grid is set as follows.
[0,0, π / 2,0,0, π / 2,0,0]
Table 5 shows the phases of the 18 four-wave mixed lights a to r generated in the frequency grid # 0.

  For reference, although the case where the phase of the signal light of 8 waves is set to [0, 0, 0, 0, 0, 0, 0, 0] is shown, all four-wave mixed lights a to r are shown. Has in-phase components.

  When the phase of the signal light of 8 waves is set to [0, 0, π / 2, 0, 0, π / 2, 0, 0], 10 four-wave mixed lights with in-phase components and quadrature components Eight four-wave mixed light having Since the four-wave mixed light having the orthogonal component does not interfere with the signal, the four-wave mixed light that interferes with the signal light can be reduced from 18 to 10.

FIG. 5 shows a received waveform in the DPSK system.
The crosstalk amount of the four-wave mixed light with respect to the signal light of the frequency grid # 0 is set to [0, 0, 0, 0, 0, 0, 0, 0] for the 8-wave phase pattern shown in Fig. 5 (1). Is -11.7dB, and when the phase pattern of 8 waves shown in Fig. 5 (2) is set to [0, 0, π / 2, 0, 0, π / 2, 0, 0], it is -15.7dB. It can be seen that the crosstalk amount is reduced by 4 dB, and the influence of the four-wave mixed light can be reduced.

  When n> 8, the number of four-wave mixed light patterns generated on an arbitrary frequency grid increases. However, by setting the phase of at least one signal light to be different by π / 2, a four-wave mixed light having an in-phase component and a four-wave mixed light having a quadrature component can be obtained from a combination of three signal lights. Occur. Since the four-wave mixed light having an orthogonal component does not interfere, the four-wave mixed light that interferes with the signal light can be reduced.

(In case of wavelength division multiplex transmission by DQPSK system)
When wavelength-division multiplex transmission of signal light generated by the DQPSK system from eight optical carriers of frequency grids # -3, # -2, # -1, # 0, # 1, # 2, # 3, and # 4 Shows an example of the four-wave mixed light generated in the frequency grid # 0.

  The phase of the four-wave mixed light is determined from the phases of the three signal lights that generate the four-wave mixed light in the frequency grid # 0. When there is one signal light having a phase of π / 4 among the three signal lights, the axis of the four-wave mixed light is shifted by π / 4 with respect to the signal light, and the signal light having a phase of π / 4 is When there are zero or two, the four-wave mixed light is coaxial with the signal light.

As an example of setting the phase of at least one signal light among the eight signal lights arranged in the frequency grids # -3 to # 4 to be different by π / 4, the phase of the signal light in each frequency grid is set as follows.
[0,0, π / 4,0,0, π / 4,0,0]
Table 6 shows the phases of the 18 four-wave mixed lights a to r generated in the frequency grid # 0.

  For reference, although the case where the phase of the signal light of 8 waves is set to [0, 0, 0, 0, 0, 0, 0, 0] is shown, all four-wave mixed lights a to r are shown. It has a coaxial component with signal light.

  When the phase of the 8 wave signal light is set to [0, 0, π / 4, 0, 0, π / 4, 0, 0], there are 10 four-wave mixed lights that have a coaxial component with the signal light. The number of the four-wave mixed lights whose axes are shifted by π / 4 is 8 with the signal light. The four-wave mixed light whose axis is shifted by π / 4 from the signal light interferes with the signal with an electric field intensity of 1 / √2, and therefore, eighteen four-wave mixed light that interferes with the signal light. Among them, interference from eight four-wave mixed lights can be reduced.

FIG. 6 shows a received waveform in the DQPSK system.
The crosstalk amount of the four-wave mixed light with respect to the signal light on the frequency grid # 0 is set to [0, 0, 0, 0, 0, 0, 0, 0] for the 8-wave phase pattern shown in Fig. 6 (1). Is -16.0dB, and when the phase pattern of 8 waves shown in Fig. 6 (2) is set to [0, 0, π / 4, 0, 0, π / 4, 0, 0], it is -17.5dB. It can be seen that the amount of crosstalk is reduced by about 1.5 dB, and the influence of four-wave mixing light can be reduced.

  When n> 8, the number of four-wave mixed light patterns generated on an arbitrary frequency grid increases. However, by setting the phase of at least one signal light to be different by π / 4, the four-wave mixed light having a coaxial component with the signal light and the signal light and the axis by π / 4 are combined from the combination of the three signal lights. Each of the four-wave mixed light is shifted. The four-wave mixed light whose axis is shifted by π / 4 with respect to the signal light interferes with the signal light with an electric field intensity of 1 / √2, so that the influence on the signal light can be reduced.

11 Optical phase shifter 12 Optical multiplexer 13 Dispersion shifted fiber (DSF)
14 Optical demultiplexer 21 Reference clock 22 Reference optical frequency comb generator 23 Optical demultiplexer 24 Optical phase-locked CW light source 25 Modulator 26 Optical receiver 27 Optical phase detector

Claims (2)

A plurality of signal lights each modulated by a plurality of optical carriers arranged in a predetermined frequency grid are generated, the plurality of signal lights are combined and transmitted to a transmission optical fiber, and a plurality of wavelength division multiplexed transmissions are transmitted. In a wavelength division multiplexing optical fiber transmission system that demultiplexes and receives signal light,
With respect to the reference optical frequency comb phase with each other synchronized a predetermined optical frequency interval, a first optical phase synchronization means Ru synchronize the plurality of optical carriers optical phase,
Second optical phase synchronization means for synchronizing optical phases of the plurality of signal lights with respect to the reference optical frequency comb, and electric field phases of the plurality of signal lights optically synchronized by the second optical phase synchronization means. Optical phase shift means for controlling and setting a phase difference of a predetermined pattern between each signal light, and reducing leakage intensity due to four-wave mixed light generated in the predetermined frequency grid during propagation through the transmission optical fiber; A wavelength division multiplexing optical fiber transmission system comprising: A plurality of signal lights each modulated by a plurality of optical carriers arranged in a predetermined frequency grid are generated, the plurality of signal lights are combined and transmitted to a transmission optical fiber, and a plurality of wavelength division multiplexed transmissions are transmitted. In a wavelength division multiplexing optical fiber transmission system that demultiplexes and receives signal light,
Optical phase synchronization means for synchronizing optical phases of the plurality of optical carriers and synchronizing optical phases of the plurality of signal lights with respect to a reference optical frequency comb having a predetermined optical frequency interval and phases synchronized with each other,
The signal light of the predetermined frequency grid is combined with the plurality of signal lights so that leakage intensity due to four-wave mixed light generated in the predetermined frequency grid during propagation through the transmission optical fiber is reduced. Optical phase shift means to control the electric field phase before
With
When the plurality of signal lights are generated by a differential quadrature phase shift keying (DQPSK) method, the optical phase shifting means has an optical phase of at least one of the plurality of signal lights, Is set to be different by π / 4 [rad] relative to the optical phase of the signal light, and the axis of the signal light on the predetermined frequency grid is shifted by π / 4 to cause interference with the signal light. A wavelength division multiplexing optical fiber transmission system, characterized in that it is configured to generate four-wave mixing light that becomes smaller.

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