JPH05224252A - Suppressing circuit for four-light-wave mixed light - Google Patents

Abstract

PURPOSE:To provide the suppressing circuit for four-light-wave mixed light which can suppress the four-light-wave mixed light in an optical fiber without increasing frequency intervals nor reducing input light power. CONSTITUTION:This circuit is equipped with a Mach-Zehnder type optical multiplxer demultiplexer 7 which demultiplexes the input signal light into plural signal light beams, a Mach-Zehnder type optical multiplexer demultiplexer 7 which multiplexes the demultiplexed signal light beams, and an optical length adjustment device 9 which adjusts the optical length of the optical path between those Mach-Zehnder type optical multiplexer demultiplexer 7 and Mach-Zehnder type optical multiplexer demultiplexer 8; and those Mach-Zehnder type optical multiplexer demultiplexers 7 and 8 are connected by a delay fiber which is longer than the coherence length of the input signal light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for suppressing four-wave mixing light in an optical fiber in an optical frequency multiplex transmission system.

[0002]

2. Description of the Related Art Conventionally, in an optical frequency multiplex transmission system in which signal lights having a plurality of frequencies are multiplexed and transmitted in a single transmission line made of an optical fiber, suppression of four-wave mixing light generated in the optical fiber is a problem. It was.

First, four-wave mixing in an optical fiber will be described.

It is assumed that three lights having different frequencies are input to the optical fiber. In the optical fiber medium, polarization proportional to each electric field mainly occurs, but minute nonlinear polarization components also occur. For example, if the frequencies of the input light are f ₁ , f ₂ , f ₃ ,
If the corresponding optical fields are E1, E2, and E3, a non-linear polarization represented by χ ₃ (E1 + E1 ^* ) (E2 + E2 ^* ) (E3 + E3 ^* ) occurs. However, χ ₃ is a constant.

This non-linear polarization is caused by the original f ₁ , f ₂ , f ₃
, And another frequency component (± f ₁ ± f ₂ ± f ₃ ) generated from the combination thereof. Light of a new frequency is generated from these frequency components. The phenomenon that the fourth light is generated from these three input lights is called a four-wave mixing phenomenon, and the light newly generated by this is called a four-wave mixing light.

Attention is now focused on four-wave mixing light having a frequency of f ₁ + f ₂ −f ₃ = f ₄ . A four-wave mixing optical field E ^NL generated in an optical fiber having a length L is E ^NL = KE1 ⁽⁰⁾ E2 ⁽⁰⁾ E3 ^{(0) *} e ^{{-α / 2 + iβ (f} 4 ^{)} L} · {(e It is known that ^{(-α + iΔβ) L} −1) / (iΔβ−α)} (1). However, E1 ⁽⁰⁾ , E2
⁽⁰⁾ and E3 ⁽⁰⁾ are the optical fields at the frequencies f ₁ , f ₂ and f ₃ at the input end of the optical fiber, K is a constant, β is a propagation constant,
α is a loss factor of the optical fiber. The derivation process of equation (1) is based on KOHill, DCJohnson, BSKawasaki, and
IRMacDonald “CW three-wave mixing in single-mod
e optical fibers ”Journal of Applied Physics, Vol.
49, p.5098 (1978).

From this, the four-wave mixing optical power P ^NL ∝ | E
^NL | ² is P ^NL = K′P1 ⁽⁰⁾ P2 ⁽⁰⁾ P3 ⁽⁰⁾ e ^−αL · [{(1-e ^−αL ) ² + 4e ^−αL sin ² (ΔβL / 2)} / (Δβ ² + α ² )] …… (2). Where K'is a constant, α is the loss factor of the optical fiber, P1 ⁽⁰⁾ , P2 ⁽⁰⁾ , P3 ⁽⁰⁾ are the optical powers at the frequencies f ₁ , f ₂ , and f ₃ at the optical fiber input end, respectively. is there. Further, Δβ is a parameter called a phase mismatch amount, and specifically, Δβ = β (f ₁ ) + β (f ₂ ) −β (f ₃ ) −β (f ₄ using the chromatic dispersion D of the optical fiber. ) = (2πλ ² / c) D (f ₁ −f ₃ ) (f ₂ −f ₃ ) ... (3) Here, β is the propagation constant of light, λ is the wavelength, and c is the speed of light.

The above-mentioned four-wave mixing light causes crosstalk deterioration in the optical frequency multiplex transmission system. In the optical frequency multiplex transmission system, usually, a plurality of signal lights arranged at equal frequency intervals are transmitted through one transmission line made of an optical fiber. Then, due to the above phenomenon, four-wave mixing light is generated at the frequency position of the signal light, and this light becomes crosstalk light and causes signal deterioration. In order to transmit a signal without deterioration, it is desirable to suppress four-wave mixing light as much as possible.
According to the equation (2), it can be seen that when Δβ is large, the four-wave mixing light power is small, and from equation (3), when the frequency difference of the input light is large, Δβ is small. Also, equation (2)
From this, it can be seen that if the input light power is small, the four-wave mixing light power is also small.

Therefore, conventionally, as methods for suppressing the four-wave mixing light in the optical fiber, there have been considered methods such as (a) expanding the frequency interval of the signal light, and (b) setting the input light power to be small.

In the above description, one transmission line is one.
The case where the optical fiber is composed of two optical fibers has been described, but basically the same applies to the case where one transmission line is composed of an optical amplifier as a repeater and a plurality of optical fibers are connected in multiple stages. ..

[0011]

However, widening the frequency interval is not preferable from the standpoint of increasing the transmission capacity by multiplexing light in a high density, and setting the input optical power to a small value is not recommended. There was a problem that it was not desirable from the standpoint of extending the distance.

The present invention has been made in view of the above circumstances, and it is possible to suppress the four-wave mixed light in the optical fiber without widening the frequency interval or reducing the input light power. The purpose is to provide a circuit.

[0013]

In order to achieve the above object in the present invention, in claim 1, an optical demultiplexing means for demultiplexing an input signal light into a plurality of signal lights and a plurality of the demultiplexed signals are provided. Proposed a four-wave mixing light suppression circuit comprising an optical combining means for combining light, and an optical length adjusting means for adjusting an optical length of an optical path between the optical demultiplexing means and the optical combining means, In addition, claim 2
In the above, an optical demultiplexing unit that demultiplexes the input signal light into a plurality of signal lights and an optical multiplexing unit that multiplexes the demultiplexed signal lights are provided, and the optical demultiplexing unit and the optical multiplexing unit are provided. We propose a four-wave mixing light suppression circuit that connects the two with a delay fiber longer than the coherence length of the input signal light.

[0014]

According to claim 1 of the present invention, the light demultiplexed by the optical demultiplexing means is added with a phase by the optical length adjusting means,
Further, the four-wave mixing light generated in the transmission path is canceled by the light combining means. Further, according to claim 2, the light demultiplexed by the optical demultiplexing means is added with a time difference by the delay fiber and further combined by the optical combining means, thereby canceling the four-wave mixing light generated in the transmission path. It

[0015]

The present invention is used in a repeater optical frequency multiplex transmission system using an optical amplifier as a repeater.

FIG. 1 shows a first embodiment of a four-wave mixing light suppressing circuit according to the present invention, but here, for simplification of description, between two optical fibers constituting a transmission line. An example of application in a single repeater system in which one optical amplifier is inserted in will be described. In the figure, 1 is a transmitter, 2 is a first optical fiber that forms a transmission path, 3 is an optical amplifier, 4 is a suppression circuit for four-wave mixing light, 5 is a second optical fiber that forms a transmission path, and 6 Is a receiving device.

The transmitting device 1, the optical fiber 2, the optical amplifier 3, the optical fiber 5 and the receiving device 6 constitute an ordinary one-relay transmission system. Immediately after the optical amplifier 3, the four-wave mixing light suppressing circuit of the present invention is provided. 4 is inserted.

The four-wave mixed light suppression circuit 4 comprises Mach-Zehnder type optical multiplexers / demultiplexers 7, 8 and an optical length adjusting device 9 for adjusting the optical length of an optical path connecting these two optical multiplexers / demultiplexers. There is. The Mach-Zehnder type optical multiplexer / demultiplexers 7 and 8 each have 2 × 2 input / output terminals. One input terminal of the Mach-Zehnder type optical multiplexer / demultiplexer 7 is connected to the output of the optical amplifier 3, and Output terminal is directly connected to one input terminal of the Mach-Zehnder optical multiplexer / demultiplexer 8, and the other output terminal is connected to another input terminal of the Mach-Zehnder optical multiplexer / demultiplexer 8 via the optical length adjusting device 9. Further, one output terminal of the Mach-Zehnder type optical multiplexer / demultiplexer 8 is connected to the optical fiber 5.

As a means for adjusting the optical length of a specific optical path in the optical length adjusting device 9, a method of changing the refractive index of the medium by a thermo-optical effect or an electro-optical effect is effective. When is formed of a glass waveguide, a method of applying heat by an electrode heater can be considered.

FIG. 2 shows the transmission characteristics of the Mach-Zehnder type optical multiplexer / demultiplexer. When the two input terminals are a and b and the two output terminals are c and d, the terminals a-c with respect to the optical frequency are shown.
And b-d are indicated by broken lines, and terminals a-d
And b-c show periodic characteristics as shown by the solid line, and the frequency interval and center frequency can be set arbitrarily.

In order to operate the suppression circuit 4, the characteristics of the Mach-Zehnder type optical multiplexer / demultiplexer 7 are applied to multiplexed signal light of channels 1, 2, ..., K, ... N arranged at equal frequency intervals. As shown in FIG. 2, half of the signal light is split into the output terminal c and the remaining half of the signal light is split into the output terminal d. Here, when the transmission characteristics of the Mach-Zehnder optical multiplexer / demultiplexer 8 are set to be reversible with the Mach-Zehnder optical multiplexer / demultiplexer 7, the Mach-Zehnder optical multiplexer / demultiplexer 8 outputs the multiplexed signal light once demultiplexed. It is multiplexed again and output to the optical fiber 5.

In the optical path from the output terminal d of the Mach-Zehnder type optical multiplexer / demultiplexer 7, as described above, the optical length adjusting device 9 is used.
Is provided. Thereby, the propagation delay phase of the light passing through this path can be adjusted. Therefore, the overall operation of the suppression circuit 4 is one of N-channel multiplexed signal light.
An arbitrary phase can be added to half of the signal light for each channel. In the case of the configuration described above, an arbitrary phase can be added to the light demultiplexed at the output terminal d of the Mach-Zehnder optical multiplexer / demultiplexer 7. When this phase value is set appropriately, it becomes possible to suppress the four-wave mixing light. The specific suppression principle will be described below.

In the case of the system as shown in FIG. 1, the four-wave mixed light input to the receiver 6 is generated in the optical fiber 2 and transmitted through the optical amplifier 3, the suppression circuit 4 and the optical fiber 5. And the four-wave mixing light generated in the optical fiber 5. Four-wave mixing light E generated in the optical fiber 5
^NL2 is ^ENL2 = KEi ⁽²⁾ Ej ⁽²⁾ Ek ^{(2) *} e ^{{-α / 2 + iβ2 (fF )} L2} · {(e ^{(-α + iΔβ2)} L2-1) / (iΔβ2-α)} It is expressed as (4). However, Ei ⁽²⁾ , Ej ⁽²⁾ , and Ek ⁽²⁾ are signal optical fields of frequencies f _i , f _j , and f _k at the input end of the optical fiber 5, f _F is the frequency of four-wave mixing light, and L 2 Is the length of the optical fiber 5, β 2 is the propagation constant in the optical fiber 5, Δβ 2
_{(= Β2 (f i) +} β2 (f j) -β2 (f k) -β2
(F _F )) is the amount of phase mismatch in the optical fiber 5.

For example, frequencies f _i and f as shown in FIG.
Frequency f _F = f _i + f _j − generated from signal lights of _j and f _k
Consider a four wave mixed light of f _k . In the case of this combination, the signal lights of the frequencies f _i and f _j are output to the output terminal d of the Mach-Zehnder optical multiplexer / demultiplexer 7, and the signal lights of the frequencies f _k and f _F are the output terminals of the Mach-Zehnder optical multiplexer / demultiplexer 7. It is demultiplexed to c. Therefore, when each signal light passes through the suppression circuit 4, due to the propagation delay, the signal light of frequencies f _k and f _F becomes φ.
A phase of φ _d is added to the signal lights of _c and the frequencies f _i and f _j . Here, φ _d can be controlled by the optical length adjusting device 9.

Considering the action of the suppression circuit 4 and the fact that the signal light at the input end of the optical fiber 5 is the light that has passed through the optical fiber 2, the optical amplifier 3 and the suppression circuit 4, the input of the optical fiber 5 is considered. The signal optical field at the end is calculated by using the signal optical fields Ei ⁽¹⁾ , Ej ⁽¹⁾ , and Ek ⁽¹⁾ at the input end of the optical fiber 2 as follows: Ei ⁽²⁾ = G ^1/2 Ei ⁽¹⁾ e ^iφ d ^{( fi )} · e ^{{-α / 2 + iβ1 (fi )} L1} Ej ⁽²⁾ = G ^1/2 Ej ⁽¹⁾ e ^iφ d ^(f j ⁾ · e ^{{-α / 2 + iβ1 (f} j ^{)} L1} Ek ⁽²⁾ = G1 ^{/ 2} Ek ⁽¹⁾ e ^iφ c ^(f k ⁾ · e ^{{-α / 2 + i β1 (f} k ^{)} L1} . However, L1 is the length of the optical fiber 2, G is the gain of the optical amplifier 3, and β1 is the propagation constant in the optical fiber 2.

When these are used, the four-wave mixed light generated in the optical fiber 5 is E ^NL2 = KGG ^1/2 Ei ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} · e ^{-3α from the equation (4). ^{/ 2 + iβ1 (f} F ^{) + iΔβ1} L1} · e ^{i {φ} d ^{( fi ) + φ} d ^(f j ^{) −φ} c ^(f k ^)} · e ^{{−α / 2 + iβ2 (f} F ^{)} L2} · {(e ^{(-Α + iΔβ2) L2-1)} / (iΔβ2-α)} (5) _{However, Δβ1 (= β1 (f i} ) + β1
(F _j ) −β1 (f _k ) −β1 (f _F )) is the amount of phase mismatch in the optical fiber 2.

On the other hand, the four-wave mixing light generated in the optical fiber 2 is E ^NL1 = KEi ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} e ^{{-α / 2 + iβ1 (f} F ⁾ at the output end of the optical fiber 2. ^{} L1} · {(e ^{(-α + iΔβ1)} L1-1) / (iΔβ1-α)} (6)

When this light passes through the optical amplifier 3, the suppression circuit 4, and the optical fiber 5, E ^NL1 = KG ^1/2 Ei ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} · e ^{{-α (L1 + L2) / 2 + iφ} c ^(f F ^)} · e ^{i {β1 (f} F ^{) L1 + β2 (f} F ^{) L2}} · {(e ^{(−α + iΔβ1) L1} −1) / (iΔβ1 −α)} …… (7) and Become.

From the equations (5) and (7), the total four-wave mixing light in the receiver 6 is E ^NL1 + E ^NL2 = KG ^1/2 Ei ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} e ^{−α (L1 + L2) / 2} · e ^{[i {β1 ( fF ) L1 + β2 ( fF) L2} + iφc ( fF )]}・ [{(e ^{(-α + iΔβ1)} L1-1) / (iΔβ1-α) } + Ge- ^αL1 e ^{i (Δφ + Δβ1 L1)} · {(e ^{(-α + iΔβ2)} L2-1) / (iΔβ2-α)}] ... (8). _{However, Δφ = φ d (f i} ) + φ d (f j)
−φ _c (f _k ) −φ _c (f _F ).

In the equation (8), the first term and the second term represent the four-wave mixing light generated in the optical fibers 2 and 5, respectively. From this equation, it can be seen that the magnitude of the entire four-wave mixing light depends on the phase relationship between the first term and the second term. If the phases of the first term and the second term deviate by just π, the four-wave mixing light becomes the minimum. By the way, the phase Δφ of the second term can be arbitrarily set by the optical length adjusting device 9 in the suppression circuit 4. Therefore, if the phase difference between the first term and the second term is set to π by the suppression circuit 4, it becomes possible to suppress the entire four-wave mixed light.

For example, it is assumed that the optical fibers 2 and 5 have the same length and the dispersion value is zero. In this case, L1 = L
2, β 1 (f _F ) = β 2 (f _F ), Δβ 1 = Δβ 2 = 0
And the equation (8) is E ^NL1 + E ^NL2 = KG ^1/2 Ei ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} · e ^{{-αL1 + 2iβ1 (f} F ^{) L1}} e ^iφ c ^(f F ⁾ -((E ^{(-α + iΔβ1)} L1-1) / (iΔβ1-α)}-{1 + Ge ^{(-αL1 + iΔφ)} } (9)

If the gain of the optical amplifier 3 is set so as to compensate for the transmission loss of the optical fiber 2, Ge
^−αL1 = 1 and the formula (9) is further expressed by E ^NL1 + E ^NL2 = KG ^1/2 Ei ⁽¹⁾ Ej ⁽¹⁾ Ek ^{(1) *} · e ^{{-αL1 + 2iβ1 (f} F ^{) L1}} e ^iφ c ^(f F ⁾ · {(e ^{(−α + iΔβ1)} L1−1) / (iΔβ1 −α)} · (1 + e ^iΔφ ) (10) Here, when Δφ = π is set, the equation (10) becomes E ^NL1 + E ^NL2 = 0, and it can be seen that the four-wave mixed light is completely suppressed.

In this embodiment, one Mach-Zehnder type optical multiplexer / demultiplexer splits the multiplexed signal light into two optical paths,
A suppression circuit for adding a phase difference between the two signal lights has been shown, but in this case, it is not possible to suppress the four-wave mixed light generated from the signal lights passing through the same path. To improve this, increase the number of stages of the Mach-Zehnder type optical multiplexer / demultiplexer,
It suffices to increase the number of optical paths to be demultiplexed and add an appropriate phase to each path.

Although the Mach-Zehnder type optical multiplexer / demultiplexer is used for demultiplexing the multiplexed signal light in the above embodiment, the invention is not limited to this, and the function of demultiplexing the multiplexed signal light to a plurality of output terminals is provided. If it has, it is possible to construct a four-wave mixing light suppression circuit by the same principle.

FIG. 4 shows a second embodiment of the four-wave mixing light suppressing circuit of the present invention. Here, four-wave mixing light is suppressed without adjusting the optical length as in the first embodiment. An example of a circuit to be used is shown below. That is, in the figure, 10 is a suppression circuit for four-wave mixing light of the present embodiment, the basic configuration is the same as that of the suppression circuit 4, but there is no optical length adjusting device, and instead 2
One of the optical paths connecting the two Mach-Zehnder type optical multiplexers / demultiplexers 7 and 8 has a delay fiber 11. The length of the delay fiber 11 is longer than the coherence length of the input signal light. The other configurations of the transmitter 1, the optical fibers 2, 5, the optical amplifier 3, and the receiver 6 are the same as those in the first embodiment, and the Mach-Zehnder optical multiplexers / demultiplexers 7, 8 are multiplexed. The first is the transmission characteristic for signal light.
Similar to the case of the embodiment, that is, set as shown in FIG.

The four-wave mixed light generated in the system shown in FIG. 4 is as follows.

For example, considering the four-wave mixing light generated from the signal light of the combination shown in FIG. 3, the four-wave mixing light E ^NL2 generated in the optical fiber 5 is E ^NL2 (t) = KEi ⁽²⁾ (tt ₂ ) Ej ⁽²⁾ (tt ₂ ) Ek ⁽²⁾ (tt ₂ ) ^*・ e ^{{-α / 2 + iβ 2 (f} F ^{)} L 2}・ {(e ^{(-α + iΔβ 2) L 2} -1) / (IΔβ 2 −α)} (11) The meaning of each symbol is the same as in the case of formula (4), but for the photoelectric field, the symbol (t) or (tt ₂ ) is added to clarify the time relationship. According to the equation (11), the four-wave mixing light at the output end of the optical fiber 5 at the time t is generated by the interaction between the signal lights leaving the input end of the optical fiber 5 at the time (t−t ₂ ). (However, t ₂ is the propagation time of the optical fiber 5).

Next, the signal light at the input end of the optical fiber 5 is rewritten by the signal optical field at the input end of the optical fiber 2.
The signal light of the frequency f _k reaching the input end of the optical fiber 5 at the time (t−t ₂ ) has a propagation time of the optical fiber 2 of t.
_{If it is 1} , it is light that has left the input end of the optical fiber 2 at time (t−t ₂ −t ₁ ). On the other hand, since the signal lights having the frequencies f _i and f _j have passed through the delay fiber 11, those that have left the input end of the optical fiber 2 at time (t−t ₂ −t ₁ −τ) are Optical fiber 5 at (t−t ₂ ).
Has reached the input end (where τ is the delay fiber 11
Is the propagation time of. ).

Considering these, each signal light at the time (t−t ₂ ) is Ei ⁽²⁾ (tt ₂ ) = G ^1/2 Ei ⁽¹⁾ (t−t ₂ −t ₁ −τ) · e ^{{-α / 2 + iβ1 (f} i)} L1 Ej (2) (tt 2) = G 1/2 Ej (1) (tt 2 -t 1 -τ) · e {-α / 2 + iβ1 (f j) ^{} L1 Ek (2) (tt} 2) = G 1/2 Ek (1) expressed as ^{_{(tt 2 -t 1) · e}} {-α / 2 + iβ1 (f k)} L1. The meaning of each symbol is the same as in the case of the first embodiment.

Substituting these into the equation (11), E ^NL2 (t) = KGG ^1/2 Ei ⁽¹⁾ (t-t ₂ -t ₁ -τ) Ej ⁽¹⁾ (t-t ₂ -t) ₁₋ τ) · Ek ⁽¹⁾ (t−t ₂ −t ₁ ) ^* · e ^{{−3α / 2 + iβ1 (f} F ^{) + iΔβ1} L1} · e ^{{−α / 2 + iβ2 (f} F ^{)} L2} · {(e ^{(-Α + iΔβ2) L2-1)} / (iΔβ2-α)} (12).

Next, the four-wave mixing light E ^NL1 (t) generated in the optical fiber 2 and reaching the receiving device 6 at time t will be considered. Since this light (frequency f _F ) has not passed through the delay fiber 11, it is the light generated at the output end of the optical fiber 2 at time (t−t ₂ ). By the way, time (t-t ₂ )
Four-wave mixing light generated in the output end of the optical fiber 2 in is caused by the interaction between starting the signal light input end of the optical fiber 2 at time _{(t-t 2 -t 1)} . Since the generation process and other propagation effects are the same as those in the first embodiment, E
Regarding ^NL1 (t), E ^NL1 (t) = KG ^1/2 Ei ⁽¹⁾ (t−t ₂ −t ₁ ) · Ej ⁽¹⁾ (t−t ₂ −t ₁ ) similar to the equation (7). ^{) · Ek (1) (t} -t 2 -t 1) * · e -α (L1 + L2) / 2 · e i {β1 (f F) L1 + β2 (f F) L2} · {(e (-α + iΔβ1 ⁾ L1-1) / (iΔβ1-α)} (13) holds.

From the equations (12) and (13), the total four-wave mixing light at the receiving device 6 at the time t is E ^NL1 (t) + E ^NL2 (t) = KG ^1/2 Ek ⁽¹⁾ (t _{^{-t 2 -t 1) * e -α}} (L1 + L2) / 2 · e i {β1 (f F) L1 + β2 (f F) L2} · ^{[Ei (1) (t-t} 2 -t 1) Ej _{^{(1) (t-t 2}} -t 1) · {(e (-α + iΔβ1) L1 -1) / (iΔβ1 -α)} + Ge (-α + iΔβ1) L1 Ei (1) (t-t 2 -t 1 - τ) · Ej ⁽¹⁾ (t−t ₂ −t ₁ −τ) · {(e ^{(−α + iΔβ 2) L 2} −1) / (iΔβ 2 −α)}] ... (14)

Looking at the equation (14), the signal lights of the frequencies f _i and f _j , which are the generation sources of the four-wave mixing light, are transmitted from the transmitter at the time when the first term and the second term differ by the time difference τ. It can be seen that the light emitted is 1. By the way, since the length of the delay fiber 11 in the suppression circuit 10 is set longer than the coherence length of the signal light, the time difference τ is longer than the coherence time of the signal light. Therefore, the first term and the second term are different from each other. The light is completely uncorrelated. The total light power obtained by adding lights having no correlation is a value obtained by adding the respective light powers.
^{Expressed by the} formula, P ^NL ∝ ｜ E ^NL1 (t) + E ^NL2 (t) ｜ ² = | E ^NL1 (t) ｜ ² + | E ^NL2 (t) ｜ ² = | K | ² G ｜ Ek ⁽¹⁾ (T−t ₂ −t ₁ ) | ² e ^{−α (L 1 + L 2)} · {| Ei ⁽¹⁾ (t−t ₂ −t ₁ ) | ² · | Ej ⁽¹⁾ (t−t ₂ −t ₁ ^{) | 2 · | (e (} -α + iΔβ1) L1 -1) / (iΔβ1 -α) | 2 + G 2 e -2αL1 | Ei (1) (t-t 2 -t 1 -τ) | 2 · | Ej ( _{^{1) (t-t 2 -t}} 1 -τ) | 2 · | (e (-α + iΔβ2) L2 -1) / (iΔβ2 -α) | 2} becomes ... (15).

Here, assuming that the optical fibers 2 and 5 are zero-dispersion fibers having the same length, the above equation (15) is given by: | E ^NL1 (t) + E ^NL2 (t) | ² = | E ^NL1 ( ^{t) | 2 + | E NL2} (t) | 2 = | K | 2 G | Ek (1) (t-t 2 -t 1) | 2 e -α (L1 + L2) · {(1-e -αL ) ² / α ² } ・ {| Ei ⁽¹⁾ (t−t ₂ −t ₁ ) | ² · | Ej ⁽¹⁾ (t−t ₂ −t ₁ ) | ² + G ² e ^{−2αL 1} | Ei ^{(1 )} (T−t ₂ −t ₁ −τ) | ² · | Ej ⁽¹⁾ (t−t ₂ −t ₁ −τ) | ² } ...... (16) is rewritten.

When the signal light power is constant in time like a frequency modulation signal or a phase modulation signal, ^{│E NL1} (t) + E ^NL2 (t) │ ² = ^{│E NL1} (t) │ ² + │ E ^NL2 (t) | ² = | K | ² G | Ek ⁽¹⁾ | ² | Ei ⁽¹⁾ | ² | Ej ⁽¹⁾ | ² · e ^{−α (L1 + L2)} (1 + G ² e ^−2αL1 ) ・{(1-e- ^αL ) ² / α ² } (17)

On the other hand, the suppression circuit 10 is operated under the same condition as the expression (17).
If there is no ^{│E NL1} (t) + E ^NL2 (t) │ ² = │K│ ² G ｜ Ek ⁽¹⁾ ｜ ² ｜ Ei ⁽¹⁾ ｜ ² ｜ Ej ⁽¹⁾ ｜ ²・ e ^{−α (L1 + L2)} (1 + Ge- ^αL1 ) ² · {(1-e- ^αL ) ² / α ² } (18).

Comparing the above equations (17) and (18),
They are different by (1 + G ² e ^-2αL 1) and (1 + Ge ^-αL 1) ² . For example, if Ge- ^αL1 = 1 (this corresponds to the case where the gain of the optical amplifier 3 is set to compensate for the loss due to the optical fiber).
The expression (17) is half the value of the expression (18). That is, the suppression circuit 1
With 0, the four-wave mixed light is suppressed.

The length of the delay fiber 11 required for the functioning of this embodiment depends on the frequency line width of the signal light. For example, when the line width of the signal light is 1 MHz, the coherence length of this light is several meters, so the required length of the delay fiber 11 is several meters or more.

In this embodiment, the suppression circuit for dividing the multiplexed signal light into two optical paths by one Mach-Zehnder type optical multiplexer / demultiplexer has been shown. However, the number of stages of the Mach-Zehnder type optical multiplexer / demultiplexer is increased to demultiplex the optical signal. As in the case of the first embodiment, the suppression effect is improved by increasing the number of routes. Furthermore, the means for demultiplexing the multiplexed signal light is not limited to the Mach-Zehnder type optical multiplexer / demultiplexer, and another optical multiplexer / demultiplexer may be used as in the case of the first embodiment.

In the above-mentioned first and second embodiments, the application example to the one-relay system has been described, but the present invention can be similarly applied to the multi-relay system.

FIG. 5 shows an example in which the present invention is applied to a multi-relay system, in which 12 is a large number of optical fibers forming a transmission line, 13 is a large number of optical amplifiers, and 14 is a suppression circuit. is there. If the suppression circuit 14 is inserted at the center of the transmission line as shown in the figure, the phase of the four-wave mixed light generated in the first half and the second half can be inverted, and the entire four-wave mixed light can be suppressed.

Also, there are various combinations of channels that generate four-wave mixing light, and it is actually impossible to set the phase inversion for all combinations in a single-stage suppression circuit. In the case of a system, by appropriately arranging the suppression circuit, it is possible to suppress the four-wave mixed light that cannot be suppressed by the single-stage suppression circuit.

FIG. 6 shows another example in which the present invention is applied to a multi-relay system. In the figure, 15-1, 15-
2, 15-3, 15-4 are optical fibers forming a transmission line, 16-1, 16-2, 16-3 are optical amplifiers, 17-
1, 17-2 and 17-3 are suppression circuits. Here, the suppression circuits 17-1 to 17-3 are arranged between the optical fibers 15-1 to 15-4, that is, at all the relay points. As a result, the suppression circuit 17-1 causes the optical fibers 15-1 and 1
The phase of the four-wave mixed light generated in 5-2 is inverted, the phase of the four-wave mixed light generated in the optical fibers 15-3 and 15-4 is inverted by the suppression circuit 17-2, and the suppression circuit 17-3 is further generated. With respect to the four-wave mixed light that could not be suppressed by the suppression circuits 17-1 and 17-2, the phases of the four-wave mixed light generated in the optical fibers 15-1 and 15-2 and the optical fibers 15-3 and 15-4 are If it is reversed, the degree of suppression can be further increased.

[0054]

As described above, according to the first aspect of the present invention, the optical demultiplexing means for demultiplexing the input signal light into a plurality of signal lights and the demultiplexed signal lights are combined. Optical mixing means and optical length adjusting means for adjusting the optical length of the optical path between the optical demultiplexing means and the optical multiplexing means. The influence of light can be suppressed, and an optical frequency multiplex transmission system can be configured without causing deterioration due to four-wave mixing.

According to a second aspect of the present invention, an optical demultiplexing unit that demultiplexes the input signal light into a plurality of signal lights, and an optical demultiplexing unit that multiplexes the demultiplexed signal lights. Since the optical demultiplexing means and the optical multiplexing means are connected by a delay fiber longer than the coherence length of the input signal light, it is possible to suppress the influence of the four-wave mixing light generated in the optical fiber forming the transmission line. Therefore, an optical frequency multiplex transmission system can be configured without causing deterioration due to four-wave mixing.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a four-wave mixing light suppression circuit of the present invention.

FIG. 2 is a transmission characteristic diagram of a Mach-Zehnder type optical multiplexer / demultiplexer.

FIG. 3 is an explanatory diagram showing the relationship between the multiplexed signal optical frequency and the transmission characteristics of a Mach-Zehnder optical multiplexer / demultiplexer.

FIG. 4 is a configuration diagram showing a second embodiment of a four-wave mixing light suppression circuit of the present invention.

FIG. 5 is a configuration diagram showing an example in which the present invention is applied to a multi-relay system.

FIG. 6 is a configuration diagram showing another example in which the present invention is applied to a multi-relay system.

[Explanation of symbols]

1 ... Transmitter, 2, 5 ... Optical fiber, 3 ... Optical amplifier,
4, 10 ... Suppression circuit for four-wave mixed light, 6 ... Receiving device,
7, 8 ... Mach-Zehnder type optical multiplexer / demultiplexer, 9 ... Optical length adjusting device, 11 ... Delay fiber.

Claims (2)

[Claims] 1. An optical demultiplexing unit that demultiplexes an input signal light into a plurality of signal lights, an optical demultiplexing unit that demultiplexes the demultiplexed signal lights, the optical demultiplexing unit and an optical demultiplexing unit. And an optical length adjusting means for adjusting the optical length of an optical path between the optical path and the optical path. 2. An optical demultiplexing unit for demultiplexing the input signal light into a plurality of signal lights, and an optical demultiplexing unit for demultiplexing the demultiplexed signal lights, the optical demultiplexing unit and the optical demultiplexing unit. A suppression circuit for four-wave mixing light, characterized in that the wave means is connected by a delay fiber longer than the coherence length of the input signal light.