JPH0897771A - Optical wavelength multiplex transmission system - Google Patents

Abstract

PURPOSE: To suppress the effect due to 4-light wave mixture (FWM) crosstalk being a nonlinear energy drop in an optical fiber on the condition that each wavelength interval is equal by providing a wavelength converter and a synthesizer synthesizing signals of each wavelength outputted from the wavelength converter and providing an output. CONSTITUTION: The system is provided with a wavelength selection filter 1 branching an optical signal subjected to wavelength multiplex for each wavelength, a wavelength converter 2 setting a crosstalk light level to be a prescribed level or below through replacement of wavelengths by applying wavelength conversion to a signal of each wavelength branched by the filter 1 according to a control signal, and a synthesizer 3 synthesizing signals of each wavelength outputted from the wavelength converter 2 and providing an output. Then the wavelength filter 1 separates a signal light on which wavelengths λ1 , λ2 ,...λ3 , λ4 are multiplexed for each wavelength and gives the result to the converter Z. The converter Z rearranges the wavelengths λ1 and λ2 each other and sends them to the sythesizer 3. the synthesizer 3 synthesizes the wavelengths and provides an output, then FWM crosstalk amt. with a reference level or below is obtained from all channels.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength multiplex transmission system, and more particularly to an optical wavelength (optical frequency) multiplex transmission system using a band around the zero dispersion wavelength of an optical fiber.

In recent years, a large-capacity communication system has become necessary with the rapid increase in the amount of information, and the optical communication system is the most promising among them. Currently, an optical wavelength division multiplexing transmission system with a transmission speed of 10 Gb / s using an erbium-doped optical fiber amplifier (hereinafter abbreviated as EDFA) and a 1.55 μm band dispersion shift fiber (DSF) transmission line from the research and development stage to the practical stage. Making progress.

In the future, in order to further increase the capacity of an optical communication system, the WDM system (=
The FDM method for achieving multiplexing on the optical frequency axis is promising, and a large capacity of several tens to several hundreds Gb / s is expected.

In this case, in the optical fiber, self-phase modulation (SPM) effect and group velocity dispersion (Grou)
Since the transmission waveform deteriorates due to the interaction with the p-velocity dispersion (GVD) (SPM-GVD effect), the propagation time of the signal light of different wavelength in the optical fiber must be different in order to secure the space between the regenerators. The dispersion (group velocity dispersion) value caused by must be set as small as possible.
The application of 55 μm band dispersion-shifted fiber transmission line is effective.

However, as the wavelength of each signal light approaches the zero-dispersion wavelength of the transmission line, crosstalk between the signal lights occurs and the transmission characteristics deteriorate.

That is, in consideration of chromatic dispersion, when n-wave signal lights having wavelengths λ ₁ to λ _n are arranged near the zero-dispersion wavelength of the optical fiber and input to the optical fiber, arbitrary 3 waves (3 channels) among them ), Λ _i , λ _j , λ _k
Through the following nonlinear susceptibility χ ₁₁₁₁ , the wavelength λ _ijk (i ≠
A fourth light of k, j ≠ k is generated, which is called Four Wave Mixing (FWM).

This FWM wavelength λ _ijk is λ _ijk = λ _i + λ
_When the relationship of _j- λ _k is satisfied and there is signal light at the position of wavelength λ _{ij k} , crosstalk (FWM crosstalk) occurs and the transmission characteristics deteriorate.

In particular, when the channel intervals are equal and the number of channels is large, a plurality of FWM lights are superposed on the position of wavelength λ _{ijk by} the combination of i, j, k, and the amount of crosstalk increases.

The generation efficiency η _ijk of the wavelength λ _ijk changes depending on the phase relationship of λ _i , λ _j , λ _k , and λ _ijk , and becomes large in the vicinity of the zero dispersion wavelength λ ₀ of the optical fiber.

Therefore, it is necessary to take measures to secure the space between the regenerators and suppress the FWM crosstalk.

[0011]

2. Description of the Related Art The optical power of FWM crosstalk is the relationship between the zero dispersion wavelength of the transmission line and the signal light wavelength, the wavelength interval,
It is known that it depends on the number of wavelengths and the signal light power.

In designing an optical wavelength division multiplexing transmission system, first, reproduction is performed based on two factors, optical SNR deterioration and transmission waveform deterioration due to the SPM-GVD effect, using the in-line repeater interval and the allowable dispersion wavelength of signal light as parameters. The repeater interval (the number of repeaters) and the repeater optical output power are determined. Furthermore, WD
The number N of wavelengths is determined from the M transmission wavelength band and the wavelength interval Δλ _S.

In the multi-stage connection of EDFAs, due to the self-filtering effect, the flat gain wavelength band becomes narrower as the number of relays increases, and the WDM transmission wavelength band is limited.

On the other hand, the wavelength interval is determined by the performance of the wavelength selection filter (break of pass band characteristics). For example, 4
In the case of the relay system (see FIG. 15), the flat EDFA gain region is in the range of 1550 to 1560 nm, and when Δλ _S = 3 nm is considered as the characteristic of the dielectric multilayer filter which is currently at a practical level, the number of wavelengths is 4 It will be the limit.

Although the above discussion is for equal wavelength spacing,
There is also a method in which the wavelength intervals are not equal to each other in order to reduce the influence of crosstalk without causing the FWM generation wavelength to coincide with the signal light wavelength.

However, in this case, the WDM transmission wavelength band is expanded, and it is difficult to completely prevent the FWM crosstalk wavelength from falling within the band of the wavelength selection filter in the receiver.

Further, precisely controlling the wavelengths of the laser diodes on the transmitting side at unequal intervals is more complicated than controlling them at equal intervals. That is, in order to secure the number of wavelengths in the limited wavelength band, it is desirable that the wavelength intervals be equal, and under such conditions, a method for suppressing the influence of FWM crosstalk is required.

From the above, the wavelength interval (Δλ _S )
In order to suppress the influence of FWM crosstalk by equalizing the above, the present applicant discloses in Japanese Patent Application No. 5242564 that the FWM light having a predetermined bandwidth including the zero dispersion wavelength λ ₀ of the optical fiber, as shown in FIG. Proposed optical wavelength division multiplexing transmission system in which a guard band Δλ _g for suppression is set and signal lights of a plurality of channels (CH1 to CH4) to be multiplexed are arranged on the short wavelength side or the long wavelength side outside the guard band Δλ _g. are doing.

[0019]

However, in the conventional technique according to Japanese Patent Application No. 5-242564, such a guard band Δλ _g is provided which is apart from the zero-dispersion wavelength λ ₀ by a certain wavelength. The wavelength of itself also deviates from the zero-dispersion wavelength λ ₀ , and the dispersion increases accordingly and the SP above
There was a problem that the M-GVD effect became large.

Therefore, in the present invention, in the optical wavelength division multiplexing transmission system using the band around the zero dispersion wavelength of the optical fiber, the influence of the crosstalk due to the FWM which is the nonlinear effect of the optical fiber is reduced provided that the wavelength intervals are equal. The purpose is to do.

[0021]

[Means and Actions for Solving the Problems]

[Principle] Generally, when the polarizations of the three signal lights causing the FWM crosstalk and the phases at the input ends of the optical fibers match, the FWM optical power P _ijk and the generation efficiency η _ijk.
Are respectively expressed by the following equations.

[0022]

[Equation 1]

[0023]

[Equation 2]

However, d: degeneration coefficient (d when i ≠ j ≠ k, d
= 6, i = j ≠ k d = 3) n: core refractive index of optical fiber c: speed of light α: attenuation coefficient of optical fiber L: optical fiber length L _eff = (1-exp (-αL)) / α: effective fiber length A _eff : effective area (= πW ² , W: mode field diameter)

Here, Δβ is called a phase mismatch amount, and is represented by the following equation when the chromatic dispersion slope dD / dλ of the optical fiber is constant.

[0026]

[Equation 3]

[0027]

[Equation 4]

For example, in the case of a four-wavelength optical WDM transmission system consisting of four repeater sections (see FIG. 15), the wavelength interval Δλ _S = 3
FIG. 17 shows a calculation example of the crosstalk (= the ratio of the FWM optical power at the optical fiber output end to the signal optical power) to each channel at each repeater stage number when they are arranged at equal intervals of nm (see FIG. 16). Shown in.

The parameters used in this calculation are as follows:
Is. λ = 1.55 μm, χ₁₁₁₁= 5.0 x 10^-15esu, A_eff= 4.6 ×
Ten^-11m², Α = 5.4 × 10 ^-Five m^-1(0.24 dB / km), dD / dλ =
0.065 ps / nm²/ km, L = 70 km, P_in= 10 dBm / ch, Δλ_g=
 λ₀−λ₁= 2 nm, D_CH1= 0.13 ps / nm / km

From this, the FW superposed on each channel
From the number of combinations of M lights and the dispersion value of each channel, it can be seen that the crosstalk amount of the channel CH2 is the maximum.

On the other hand, considering the worst case in which the phases of the FWM lights generated at the repeater stages are the same, the electric fields of the crosstalk lights are summed up. Therefore, the FWM optical power P _FWM after transmission in the repeater section of M stages is It is M ² times the FWM optical power generated in one repeater section.

Therefore, if the required crosstalk amount for securing the reception sensitivity after four relays is -18 dB, the required crosstalk in one relay section is -30 dB, as shown in FIG.
Moreover, only channel CH2 does not satisfy this condition.

Therefore, after relaying for two sections and before the EDFA3,
Alternatively, in the EDFA 3, if the wavelengths of the channels CH1 and CH2 are exchanged and the remaining two sections are relayed as shown in FIG. 1, the worst case (when the polarization and phase of the FWM crosstalk light are the same) However, as shown in FIG. 2, it can be seen that all channels have a required crosstalk amount of −18 dB or less after four relays.

[Structure and Operation] (1) Expanding the concept of the invention based on the above case,
In the optical wavelength division multiplexing transmission system according to the present invention, as shown in FIG. 3, a wavelength selection filter 1 for demultiplexing an input wavelength-division-multiplexed optical signal for each wavelength and each wavelength demultiplexed by the filter 1 are provided. The wavelength converter 2 (collectively shown) for converting the signals of the signals in accordance with the control signal to switch between the wavelengths so that the crosstalk light level is equal to or less than a predetermined value, and each of the wavelength converters 2 output the wavelength converters 2. It is characterized by including a wavelength converter provided with a multiplexer 3 which multiplexes and outputs a signal of a wavelength.

That is, in the wavelength conversion device shown in FIG.
Wavelength _{λ 1, λ 2, ... λ} 3, λ 4 wavelength selection filter 1 receives the signal light multiplexed, each of these wavelengths _{λ 1, λ 2, ... λ} 3,
The signal light of λ ₄ is separated and given to the wavelength converter 2.

In the wavelength converter 2, when the wavelengths λ ₁ and λ ₂ are exchanged and sent to the multiplexer 3, as shown in FIG.
Then, since these wavelengths λ _2, λ _1, ... λ _3, λ ₄ are multiplexed and output, for example, when the number of wavelength multiplexes is “4” and the number of optical repeaters is “4”, FIG. As shown, F below the standard value
The amount of WM crosstalk is obtained for all channels.

(2) Further, in the above-mentioned optical wavelength division multiplex transmission system, at least one wavelength conversion device to be wavelength-converted can be designated by the control signal.

That is, when a plurality of the wavelength conversion devices described above are inserted in the optical transmission line, for example, a control signal from a central control unit (not shown) is used for a predetermined wavelength conversion device or a plurality of wavelength conversion devices. By designating a wavelength conversion device that should perform wavelength conversion, the desired wavelength replacement can be performed.

(3) Further, in this case, the wavelengths can be exchanged among a plurality of wavelength conversion devices in accordance with a predetermined rotation. In this case, the FWM crosstalk amount is made uniform, and the waveform is deteriorated by the SPM-GVD effect. The difference between is also small.

(4) Further, in the above-mentioned optical wavelength division multiplexing transmission system, as shown in FIG.
A wavelength demultiplexer that demultiplexes the input wavelength-multiplexed optical signal by the number of the wavelengths that have been multiplexed, and a wavelength that selects different wavelengths from each output optical signal of the demultiplexer 1-1. It can be configured with the selection filter 1-2.

In this case, the signal light in which the wavelengths λ _1, λ _2, ... λ _3, λ ₄ are multiplexed is similarly output for all channels by the demultiplexer 1-1 and is output to the wavelength selection filter 1-2. In the transmission and wavelength selection filters 1-2, the wavelengths λ _1, λ _2, ... λ _3, λ
₄ are separated and sent to the wavelength converter 2. After that, the same operation as in the case of FIG. 3 is performed.

(5) Further, in the above-mentioned optical wavelength division multiplexing transmission system, as shown in FIG. 5, an optical amplifier 4 for compensating for the loss due to the wavelength converter 2 is provided after the multiplexer 3. Is preferred.

When an optical loss occurs in the wavelength converter 2 described above, this loss is collectively compensated by the optical amplifier 4 in the subsequent stage of the multiplexer 3.

(6) Furthermore, in the above optical wavelength division multiplexing transmission system, as shown in FIG. 6 or 7, each wavelength converter 2
An optical amplifier 5 may be provided between the multiplexer 3 and the multiplexer 3 for performing optical amplification at a uniform level in accordance with the conversion efficiency of each wavelength converter 2.

In this case, as shown in FIG. 6, an optical amplifier 5 is inserted for each individual wavelength in the subsequent stage of each wavelength converter 2 so that the gain can be adjusted according to the wavelength conversion efficiency of each wavelength. To compensate for light loss.

Further, as shown in FIG. 7, both the transmission line loss (relay gain) and the wavelength conversion loss may be compensated by combining the configuration example of FIG. 5 and the configuration example of FIG.

The above-mentioned wavelength conversion device can be installed in a conventional repeater, but it is not limited to the repeater and may be inserted anywhere in the transmission line.

As described above, according to the present invention, the influence of FWM crosstalk can be reduced and the regenerator intervals can be secured in the optical wavelength division multiplexing transmission system using the band around the zero dispersion wavelength of the optical fiber.

[0049]

FIG. 8 shows an embodiment (1) of the wavelength converter 2 shown in FIGS. 3 to 7, which can be operated by a control signal and changes the wavelength λ _i of the input optical signal to another wavelength λ. A light-light conversion type semiconductor laser 20 for converting into _j is used.

This semiconductor laser 20 is capable of wavelength conversion in the optical region by entering light and changing the oscillation wavelength by changing the refractive index.

FIG. 9 shows an embodiment (2) of the wavelength converter. In this embodiment, an optical element 21 and a semiconductor laser 22 are used, and the optical element 21 has a wavelength of λ _i . The input optical signal is converted into an electric signal, and the electric signal is converted into another wavelength λ _j by the electro-optical conversion type semiconductor laser 22 which is also operable by the control signal.

In the above description, "4" is used as the design example of the wavelength multiplexing number, but the present invention can be applied even when the wavelength multiplexing number exceeds 4.

FIG. 10 shows an example of wavelength arrangement when wavelengths are switched when the number of wavelength division multiplexes is "10" and there are four repeating sections. In this example, the wavelength interval is 1 nm and P _in = 7 dBm / ch.
_, Δλ _g = 20nm _, D _CH1 = 0.13 ps / nm / km
Channel CH1 and CH3, and channel CH in the stage repeater
The wavelengths of 4 and CH10 and the channels of CH5 and CH9 are exchanged.

As a result, the FWM crosstalk amount is shown in FIG.
As shown in FIG. 12, the final stage repeater has a value lower than the reference value of -18 dB as compared with the case without wavelength replacement shown in FIG.

In this embodiment, wavelength switching is performed only once in the transmission line, but it may be performed multiple times.

FIG. 13 shows an example in which the wavelengths are switched according to a predetermined rotation for each of the relay sections of EDFA1 to EDFA4 in the case of the above-mentioned 4-wave 4-relay section optical wavelength division multiplexing transmission system. In FIG. The FWM crosstalk amount in each channel is shown.

In this case, since the FWM crosstalk amount below the reference and the same crosstalk amount are all present in the fourth-stage repeater, the crosstalk amount of each channel becomes uniform and the optical SNR Will also be uniform. Furthermore, since the total dispersion amount of each channel is made uniform, SP
The difference in waveform deterioration due to the M-GVD effect is also small. this is,
When dispersion compensation is required, it has the effect of facilitating its design.

[0058]

As described above, according to the optical wavelength division multiplexing transmission system of the present invention, the wavelength selection filter for demultiplexing the input wavelength-division-multiplexed optical signal for each wavelength and the wavelength division demultiplexing by the filter. The wavelength converter that converts the wavelength signals of the respective wavelengths according to the control signal so that the wavelengths are switched so that the crosstalk light level is equal to or lower than a predetermined value, and the signals of the respective wavelengths output from the wavelength converters are combined. Since the multiplexer and the wave output device are inserted in the transmission line, the FWM
It is possible to reduce the influence of crosstalk and secure the interval between regenerators.

[Brief description of drawings]

FIG. 1 is a diagram showing a principle of wavelength switching in an optical wavelength division multiplexing transmission system according to the present invention.

FIG. 2 is a graph showing the FWM crosstalk amount after each relay section in the optical wavelength division multiplexing transmission system according to the present invention.

FIG. 3 is a block diagram showing a configuration example (1) of a wavelength conversion device in an optical wavelength division multiplexing transmission system according to the present invention.

FIG. 4 is a block diagram showing a configuration example (2) of the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 5 is a block diagram showing a configuration example (3) of the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 6 is a block diagram showing a configuration example (4) of the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 7 is a block diagram showing a configuration example (5) of the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 8 is a block diagram showing an embodiment (1) of the wavelength converter used in the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 9 is a block diagram showing an embodiment (2) of the wavelength converter used in the wavelength conversion device in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 10 is a diagram showing a wavelength arrangement when the number of wavelengths is 10 in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 11 is a graph showing the FWM crosstalk amount after each relay section in the case where the number of wavelengths is 10 and wavelength switching is not performed in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 12 shows F after each relay section when the number of wavelengths is 10 and wavelength switching is performed in the optical wavelength division multiplexing transmission system according to the present invention.
It is a graph showing the amount of WM crosstalk.

FIG. 13 is a diagram showing an example in which wavelength rotation is performed in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 14 is a graph diagram showing the FWM crosstalk amount after each relay section when wavelength rotation is performed in the optical wavelength multiplexing transmission system according to the present invention.

FIG. 15 is a general system diagram of an optical wavelength division multiplexing transmission system in the case of four relays.

FIG. 16 is a diagram showing an example of wavelength arrangement of an optical wavelength division multiplexing transmission system in the case of four relays.

FIG. 17 is a graph showing the FWM crosstalk amount after each repeater section in the optical wavelength division multiplexing transmission method when the number of wavelengths is 4 and wavelength exchange is not performed.

[Explanation of symbols]

 1, 1-2 Wavelength selection filter 1-1 Demultiplexer 2 Wavelength selector 3 Multiplexer 4,5 Optical amplifier In the figure, the same reference numerals indicate the same or corresponding parts.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04J 14/00 14/02

Claims (6)

[Claims] 1. A wavelength selection filter (1) for demultiplexing an input wavelength-multiplexed optical signal for each wavelength, and wavelength conversion of the signal of each wavelength demultiplexed by the filter (1) according to a control signal. As a result, the wavelength converters (2) that switch the wavelengths to bring the crosstalk light level to a predetermined value or less and the signals of the respective wavelengths output from the wavelength converter (2) are combined and output. A wavelength division multiplexing transmission system comprising a wavelength converter including the wave device (3). 2. The optical wavelength division multiplex transmission system according to claim 1, wherein the control signal designates at least one wavelength conversion device for wavelength conversion. 3. The optical wavelength division multiplex transmission system according to claim 1, wherein the control signal designates a plurality of wavelength conversion devices so that wavelength conversion is performed according to a predetermined rotation. method. 4. The wavelength selective filter (1) according to any one of claims 1 to 3,
Is a demultiplexer (1-1) that demultiplexes the input wavelength-multiplexed optical signal by the number of the multiplexed wavelengths, and the demultiplexer (1-
An optical wavelength multiplex transmission system characterized by comprising a wavelength selection filter (1-2) for selecting different wavelengths from each output optical signal of 1). 5. The optical wavelength division multiplex transmission system according to any one of claims 1 to 4, wherein an optical amplifier (4) for compensating for the loss due to the wavelength converter (2) is provided after the multiplexer (3). )
A wavelength division multiplexing optical transmission system characterized in that 6. The optical wavelength division multiplex transmission system according to claim 1, wherein each wavelength converter (2) is provided between each wavelength converter (2) and the multiplexer (3). An optical wavelength division multiplex transmission system characterized in that an optical amplifier (5) for optically amplifying each other to a uniform level is provided corresponding to conversion efficiency.