JPH08163094A - Four optical wave mixing light suppression optical transmission path - Google Patents

Abstract

PURPOSE: To suppress a four optical wave mixing light without use of an interference system and a control system by inserting a specific optical element on the way of an optical transmission line using an optical fiber whose dispersion at an operating wavelength band is zero for a transmission medium. CONSTITUTION: A single mode fiber (optical element) 13 whose length Ls and dispersion D satisfy a relation of 2πl<2> (DLs (fk-fi)(fk-fj)/c=(2n-1)π is arranged between dispersion shift fibers 11-1 and 11-2, where c is the velocity of light, λis a wavelength, fi, fj, fk are frequencies at three points in a frequency multiplex signal and n is an integer. Phase relation of the frequency multiplex signal changes before and after the element 13 by the dispersion of the optical element 13. Thus, a difference is produced between a phase of a four optical wave mixture light generated in the dispersion shift fiber 11-2 before the optical element 13 and a phase of a four optical wave mixture light generated in the dispersion shift fiber 11-1 after the optical element 13. Since the phase difference is set to (2n-1)π in the optical element 13, the four optical wave mixture light generated before and after the optical element 13 is cancelled out.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-wave mixing light suppressing optical transmission line for suppressing four-wave mixing light generated in an optical fiber transmission line by transmitting frequency-multiplexed signal light.

[0002]

2. Description of the Related Art In an optical frequency division multiplexing transmission system in which a plurality of signal lights of different frequencies are multiplexed and transmitted in one optical fiber, it is necessary to suppress the four-wave mixing light generated in the optical fiber. . In particular, in a system for transmitting at a zero-dispersion wavelength of an optical fiber (for example, a dispersion-shifted fiber having a zero-dispersion wavelength of 1.55 μm), the generation efficiency of four-wave mixing light is very high, and suppression thereof is an important issue.

One conventionally proposed method is a four-wave mixing optical suppression device (Japanese Patent Laid-Open No. 5-224252) using a Mach-Zehnder interferometer and an optical delay line. The configuration and the operating principle will be described below with reference to FIG. The four-wave mixing optical suppression device includes two Mach-Zehnder interferometers 41-1 and 41-2, two optical paths 42-1 and 42-2 connecting them, and a second optical path 42-.
2 and an optical delay line 43 inserted in the optical delay line 43.

The frequency-multiplexed signal light (frequency f1, f2, f3, ...) Propagated in the dispersion shift fiber 11-1 is input to the first Mach-Zehnder interferometer 41-1 and the frequencies f1, f3, f5,
The signal light of ... Is branched to the optical path 42-1 and the signal light of frequencies f2, f4, f6 ,. The signal lights of the respective optical paths are combined by the second Mach-Zehnder interferometer 41-2 and emitted to the dispersion shift fiber 11-2. At this time, the optical delay line 43 inserted in the second optical path 42-2 sets a phase difference between both signal lights passing through the optical paths 42-1 and 42-2. Thereby, the phases of the four-wave mixed light generated in the dispersion shift fibers 11-1 and 11-2 before and after the four-wave mixed light suppressing device can be made different. Here, by appropriately adjusting the length of the optical delay line 43, the phase difference between the two can be set to π, and as a result, the four-wave mixing light generated before and after the four-wave mixing light suppressing device is canceled and The mixed light wave disappears.

[0005]

In the conventional four-wave mixing optical suppressor, it is necessary to control the temperature of the Mach-Zehnder interferometer with high precision in order to match the combined wavelength of the Mach-Zehnder interferometer with the frequency of the signal light. there were. Moreover, in order to set the phase difference to π, a delay line control system with an accuracy of 0.1 μm was required. Furthermore, loss in the Mach-Zehnder interferometer is unavoidable, which has been a factor of reducing the intensity of signal light.

An object of the present invention is to provide a four-wave mixing light suppression optical transmission line capable of suppressing four-wave mixing light without using an interference system or a control system.

[0007]

A four-wave mixing optical suppression optical transmission line of the present invention is provided in the middle of an optical transmission line (for example, a dispersion shift fiber) using an optical fiber having a dispersion value of zero in a used wavelength band as a transmission medium. , An optical element (for example, a single mode fiber) having a predetermined length L _S and a dispersion value D is inserted.

[0008]

The phase relationship of the frequency-multiplexed signal light changes before and after the dispersion due to the dispersion of the optical element (single mode fiber) inserted in the middle of the dispersion-free optical transmission line (dispersion shift fiber). Therefore, there is a difference in phase between the four-wave mixing light generated in the optical transmission line before the optical element and the four-wave mixing light generated in the optical transmission line after the optical element. Since the length L _S of the optical element and the dispersion value D are set so that this phase difference becomes π (3π, 5π, ...), the four-wave mixed lights generated before and after the optical element cancel each other and disappear. To do.

As described above, according to the present invention, the phase between the frequency-multiplexed signal lights is controlled by utilizing the dispersion of the optical element. Therefore, the Mach-Zehnder interferometer is used to demultiplex the frequency-multiplexed signal lights and shift the phases. The problems of the prior art of going through the wave procedure are eliminated.

[0010]

【Example】

(First Embodiment) FIG. 1 shows the configuration of an optical transmission line according to the first embodiment of the present invention. In the figure, transmission media are dispersion shift fibers 11-1 and 11-2. Dispersion shift fiber 11
Since the optical amplifiers 12 are arranged at -1 and 11-2 to compensate for the transmission loss, it can be assumed that the optical transmission line has no loss as a whole. Dispersion shift fiber 11-
A single mode fiber 13, which is a feature of the present invention, is arranged between 1 and the dispersion shift fiber 11-2. In addition,
It is assumed that the left end of the drawing is the start point of the optical transmission line and the right end is the end point, and the single mode fiber 13 is arranged at the intermediate point of the optical transmission line. The propagation direction of the signal light is the z axis, the starting point is z = 0,
Dispersion shift fiber 11-1 and single mode fiber 13
Z = z1, the connection point between the single mode fiber 13 and the dispersion shift fiber 11-2 is z = z2, and the end point is z =
Let z3.

When the frequency-multiplexed signal light is transmitted from the starting point, arbitrary three waves (frequency f _i , f _j , f _k ) therein are transmitted.
Suppression of the four-wave mixed light (f _F = f _i + f _j −f _k ) induced by will be described. First, the definitions of main variables used in the following description are listed. f _i , f _j , f _k , f _F : frequencies of signal light and four-wave mixing light β _Di , β _Dj , β _Dk , β _DF : propagation constant β _{Si of} the signal light and four-wave mixing light in a dispersion shift fiber, β _Sj , β _Sk , β _SF : Propagation constants of signal light and four-wave mixing light in a single mode fiber E _i (z), E _j (z), E _k (z), E _F (z): Signal light And electric fields L _D and L _{S at} the distance z between the four-wave mixed light: the lengths of the dispersion-shifted fiber and the single-mode fiber.

First, the four-wave mixing light generated in the dispersion shift fiber 11-1 will be analyzed. The electric field of the four-wave mixing light generated in the dispersion shift fiber 11-1 is z =
At z1, it is expressed as E _F (z1) = iAE _i (0) E _j (0) E _k (0) ^* exp [iβ _DF L _D ] ... (1). A is a constant representing the generation efficiency of four-wave mixing. In the derivation of this equation, f _i , f _j , f _k , f _F
Is in the zero dispersion region of the dispersion shift fiber 11-1, and it is assumed that the phase matching condition β _Di + β _Dj = β _Dk + β _DF is satisfied. This four-wave mixing light is generated by the single mode fiber 1
3 and the dispersion shift fiber 11-2, z =
In z3, it is expressed as _{E F (z3) = iAE i} (0) E j (0) E k (0) * exp [i (β DF L D + β SF L S + β DF L D)] ... (2) It

Meanwhile, the four-wave mixing light generated newly in the dispersion-shifted fiber _{11-2, E F '(z3) =} iAE i (z2) E j (z2) E k (z2) * exp [iβ DF L _D ] = iAE _i (0) exp [i (β _Di L _D + β _Si L _S )] ・ E _j (0) exp [i (β _Dj L _D + β _Sj L _S )] ・ E _k (0) exp [-I (β _Dk L _D + β _Sk L _S )] exp [iβ _DF L _D ] = iAE _i (0) E _j (0) E _k (0) ^* exp [i (2β _DF L _D + (β _Si + β _Sj −β _Sk ) L _S )] (3). In the derivation of the equation (3), the phase matching condition β _Di + β _Dj in the dispersion shift fibers 11-1 and 11-2 is obtained.
= Β _Dk + β _DF was used. (2) and (3), the power of four-wave mixed light in the z = z3 _{is, P F (z3) α [} E F (z3) + E F '(z3)] [E F (z3) + E F' (z3)] ^* = 2A ² | E _i (0) | ² | E _j (0) | ² | E _k (0) | ² (1 + cos Δφ) (4) Δφ = (β _SF + β _Sk −β _Si − β _Sj ) L _S (5)

Here, Δφ is the dispersion shift fiber 11-
Four-wave mixing light generated in 1 and dispersion shift fiber 11
It is an amount showing the phase difference at z = z3 of the four-wave mixing light generated at -2. As can be seen from the equation (4), when | Δφ | becomes π, the power of the four-wave mixing light is zero, that is, the four-wave mixing is suppressed. Taylor expansion of each β _{S up} to the second order,

[0015]

[Equation 1]

[0016] Here, λ is the wavelength, D is the dispersion value of the single mode fiber 13, and c is the speed of light. If the single mode fiber 13 having a length such that Δφ represented by the equation (6) is π is inserted between the dispersion shift fibers 11-1 and 11-2, four-wave mixing can be suppressed. The length may be such that Δφ is an odd multiple of π. Above, any frequency f
The suppression of the four-wave mixed light generated by the lights of _i , f _j , and f _k has been described.

In the optical frequency multiplex transmission system to which the present invention is applied, the optical frequency channels (f ₁ , f ₂ , f ₃ , f ₄ , ...) Are arranged at intervals Δf on the frequency axis. In this system,
Arbitrary frequencies f _i , f _j , and f _k are discrete values (f ₁ , f ₂ ,
f ₃ , f ₄ , ...), and many four-wave mixed lights are generated by the combination thereof. As shown in equation (6), the value of Δφ is
Since it depends on the value of (f _k −f _i ) (f _k −f _j ), not all four-wave mixing can be suppressed by one single mode fiber. Therefore, the relationship between the combination of f _i , f _j , and f _k and Δφ is classified into FIGS. 2 and 3.

[0018] In the figure, the horizontal axis represents the frequency, showing long arrow signal light _{(f i, f j, f} k), short arrows four-wave mixing light _{(f F = f i + f} j -f k) . Here, each signal light and four-wave mixing light are described as i, j, k, and F. In addition,
If the frequency spacing becomes large, phase matching cannot be achieved in the dispersion-shifted fiber, the generation efficiency of the four-wave mixing light becomes low, and it is not necessary to suppress it from the beginning. Therefore, | f _k −f _i || f _k −
The values up to f _j | = 6Δf ² are described. Also, since two combinations that are bilaterally symmetric on the frequency axis have the same value of | Δφ |, only one of them is described. In the following description, | f _k
Classification is performed using the value of −f _i || f _k −f _j |.

In the Δf ² type, | Δφ | is 2πλ ² DL _S Δf ²
/ C. Here, in the case of optical frequency division multiplexing transmission with a channel spacing of 100 GHz, D = 17 ps / km / nm, L _S
= 365 m, | Δφ | becomes π, and Δf ² type four-wave mixing is suppressed. At this time, in the 3Δf ² type, | Δφ | =
In the 3π, 5Δf ² type, | Δφ | = 5π, and these four-wave mixing is also suppressed at the same time. On the other hand, in the 2Δf ² type, | Δ
φ | = 2π, the type ² 4Δf | Δφ | = 4π, the type ² 6Δf | Δφ | = since the 6?, four-wave mixing is not suppressed.

Further, if L _S = 183 m, the type ² 2Δf | Δφ | = π, in type ² 6Δf | Δφ | = 3π next,
Four-wave mixing is suppressed. At this time, in Δf ² type, | Δφ
| = Π / ² , 3Δf ² type | Δφ | = 3π / ^2, 5Δf ²
In the type, | Δφ | = 5π / 2, and four-wave mixing is not completely suppressed, but the power is halved. Also, L _S = 91
If m, it is possible to completely suppress four-wave mixing of 4Δf ² type.

Further, L _S = 183 m for suppressing the 2Δf ² type
And L _S = 122 m which suppresses the 3Δf ² type and L _S =
Assuming 146 m, in the 2Δf ² type, | Δφ | = 0.8π, 3Δ
In the f ² type, | Δφ | = 1.2π, so four-wave mixing of both types can be suppressed to about 1/10. (Second Embodiment) FIG. 4 shows the configuration of an optical transmission line according to the second embodiment of the present invention.

In this embodiment, the dispersion shifted fiber 11-1, the single mode fiber 13 and the dispersion shifted fiber 11-2 shown in the first embodiment are used as a unit transmission line 21, and the entire optical transmission line is made up of a plurality of unit transmission lines. 21. Unit transmission line 2
The optical amplifier 12 placed in 1 has a gain that compensates for fiber losses. The feature of this embodiment is that the unit transmission line 2
It is about suppressing four-wave mixing for each one. The advantages will be described below.

As in the first embodiment, it is possible to suppress the four-wave mixing by disposing the single mode fiber 13 satisfying the expression (6) at the intermediate point of the entire optical transmission line. Only if the conditions are fully met. Even if the zero-dispersion wavelength of the dispersion-shifted fiber is slightly deviated due to manufacturing deviation, the phase matching between the four-wave mixing light and the signal light is deviated. When the transmission path becomes long, this deviation accumulates and the effect of the present invention cannot be obtained. Therefore, FIG.
This problem is solved by dividing into the unit transmission lines 21 and suppressing the four-wave mixing individually as shown in FIG.

Although the length of the unit transmission line 21 is such that the two optical amplifiers can compensate for the loss, the length is not limited to this. Further, the unit transmission line 21 can be lengthened by increasing the number of optical amplifiers within the range where the phase matching condition is satisfied. In addition, by measuring the phase matching shift in advance and determining Δφ in consideration of the shift, it is possible to correspond to the case where a long-distance optical transmission line is formed by connecting dispersion-shifted fibers with shifted zero-dispersion wavelengths. .

(Third Embodiment) FIG. 5 shows the configuration of an optical transmission line according to the third embodiment of the present invention. In this embodiment, four dispersion shift fibers 11-1 to 11-4, three single mode fibers 13-1 to 13-3, and four optical amplifiers 12-
1 to 12-4 are unit transmission lines 31, and the entire optical transmission line is composed of a plurality of unit transmission lines 31. The optical amplifier 12 arranged in the unit transmission line 31 has a gain that compensates for the loss of the fiber. The reason for the division into the unit transmission lines 31 is the same as in the second embodiment.

The dispersion of the single mode fibers 13-1 to 13-3 is D = 17 ps / km / nm, and the length is L _S = 183 m. This is to suppress four-wave mixing of 2Δf ² type, 6Δf ² type, 10Δf ² type, ... In optical frequency division multiplexing transmission with a channel spacing of 100 GHz. This type of four-wave mixing light is generated in the dispersion shift fiber 11-1 and is canceled by the dispersion shift fiber 11-2.
-3 and is canceled by the dispersion shift fiber 11-4.

Further, the single mode fibers 13-1, 13
-2 can be regarded equivalently as a single-mode fiber with L _S = 365 m, so the four-wave mixing light of Δf ² type, 3Δf ² type, 5Δf ² type, ... The dispersion shift fiber 11-3 can cancel and suppress. Similarly, due to the action of the single mode fibers 13-2 and 13-3, the same type of four-wave mixing light generated in the dispersion shift fiber 11-2 can be canceled and suppressed in the dispersion shift fiber 11-4. When the unit transmission line 31 of the present embodiment is configured in this way, four-wave mixed light of Δf ² type, 3Δf ² type, 5Δf ² type, ... And 2Δf ² type, 6Δf ² type, 10Δf ² type ,. The four-wave mixed light can be suppressed at the same time.

Further, by applying this, L _{S of} half the length
Based on = 91m single mode fiber, Δf ² type,
3Derutaf ² type, 5Derutaf ² type, ... four-wave mixing light, 2.DELTA.f ² type,
Four-wave mixing light of 6Δf ² type, 10Δf ² type, and 4Δf ² type,
It is possible to simultaneously suppress four-wave mixing light of 12Δf ² type, 20Δf ² type, .... (Additional items common to each example) The single-mode fiber shown in each example has a single propagation mode and has a configuration in which the zero dispersion wavelength is shifted from the 1.3 μm band to the 1.55 μm band. It is a non-existent fiber having a dispersion of about 17 ps / km / nm in the 1.55 μm band. The dispersion-shifted fiber shown in each example has a single propagation mode and has a configuration in which the zero dispersion wavelength is shifted from the 1.3 μm band to the 1.55 μm band, and the dispersion is zero in the 1.55 μm band. Of fiber.

In the present invention, the dispersion of the single mode fiber is used to obtain the phase difference Δφ, but other optical fibers or optical components having dispersion may be used, for example. That is, the feature of the four-wave mixing light suppression optical transmission line of the present invention is that an optical element having dispersion is arranged in the midway of the dispersion-free optical transmission line in the used wavelength band. (Additional remark common to each embodiment) When a single mode fiber is used for the wiring inside the optical amplifier, etc., it is necessary to shorten the length of the single mode fiber used in the present invention. There is. Further, when the optical amplifier has dispersion, it is necessary to determine the length of the single mode fiber in consideration of the phase difference Δφ generated therein. For example, in an erbium-doped fiber amplifier, if the fiber used is 30 m and the dispersion value is 10 ps / km / nm, the single-mode fiber actually required for the L _S = 183 m required by the design. Will be 165m long.

(Additional Remarks Common to Each Embodiment) In each embodiment, it is assumed that the lengths of the sections of the two dispersion shift fibers that cancel the four-wave mixing light are equal. However, if the gains of the optical amplifiers are adjusted so that the powers of the four-wave mixing light generated in the two dispersion shift fibers become equal, the lengths of the two dispersion shift fibers do not necessarily have to be equal.

[0031]

As described above, according to the present invention, an optical element (single-mode fiber) having dispersion is inserted in the middle of a dispersion-free optical transmission line (dispersion shift fiber), so that frequency-multiplexed signal light is obtained. It is possible to suppress the generation of four-wave mixing light accompanying the transmission of. When using single mode fiber as the optical element, the required length is 100
Since it can be stored in a small bobbin or box having a diameter of about 10 cm and a thickness of about 1 cm, the size and weight can be extremely reduced. Further, the suppression characteristic of the four-wave mixed light can be easily adjusted by the length of the single mode fiber.
Moreover, since the dispersion value and the length of the single mode fiber are stable with respect to the temperature change, the temperature control is unnecessary. Furthermore, it does not break down due to external factors such as vibration. This makes it possible to significantly reduce costs. Further, the loss caused by the insertion of the single mode fiber is only the connection loss between the single mode fiber and the dispersion shift fiber. The value is 0.4 dB per connection point and can be ignored.

Therefore, by using the present invention,
An optical transmission line in which generation of four-wave mixed light is suppressed can be provided inexpensively, easily, and stably, and the performance of optical frequency division multiplexing transmission can be dramatically improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical transmission line according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship (part 1) between a combination of three waves and Δφ.

FIG. 3 is a diagram showing a relationship (part 2) between a combination of three waves and Δφ.

FIG. 4 is a diagram showing a configuration of an optical transmission line according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of an optical transmission line according to a third embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of a conventional four-wave mixing light suppression device.

[Explanation of symbols]

 11 dispersion-shifted fiber 12 optical amplifier 13 single-mode fiber 21, 31 unit transmission line 41 Mach-Zehnder interferometer 42 optical path 43 optical delay line

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 10/02 10/18

Claims (2)

[Claims] 1. An optical transmission line for transmitting frequency-multiplexed signal light using an optical fiber having a dispersion value of zero in a used wavelength band as a transmission medium, at a connection point for dividing the optical transmission line into a plurality of light speeds,
When the wavelength is λ, the frequencies of three waves in the frequency-multiplexed signal light are f _i , f _j , f _k , and n is an integer of 1 or more, the length L _S and the dispersion value D are 2πλ ² DL _S ( f _k −f _i ) (f _k −f _j ) / c = (2n−
1) A four-wave mixing optical suppression optical transmission line characterized in that an optical element satisfying π is inserted. 2. The four-wave mixing light suppressing optical transmission line according to claim 1, wherein the optical element is a single-mode fiber having a length L _S and a dispersion value D. Suppressed optical transmission line.