JP2002330106A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that when plural stages of Raman amplifiers having the same gain-wavelength characteristic are used, a deviation of a Raman gain-wavelength characteristic becomes larger, and the flatness of the channel waveform of a wavelength multiplex transmission becomes worse, therefore, this needs to be compensated by an equalizer or the like. SOLUTION: The Raman amplifiers are used in combination so that a wavelength band of an upward projected curve including a gain maximum value of a first Raman amplifier and a wavelength band of a downward projected curve including a gain minimum value of a second Raman amplifier overlap each other, and a wavelength band of a downward projected curve including a gain minimum value of the first Raman amplifier and a wavelength band of an upward projected curve including a gain maximum value of the second Raman amplifier overlap each other. The same method is applied for three or more Raman amplifiers repeatedly.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the improvement of gain wavelength characteristic deviation of a Raman amplifier using a wavelength division multiplexing pump light source in a wavelength division multiplexing transmission system.

[0002]

2. Description of the Related Art Optical fibers containing silica as a main component and germanium in a core are widely used. It is known that a Raman amplifier using this optical fiber can obtain the maximum value of the Raman gain at a wavelength that is about 100 nm longer than the pump light source wavelength. Utilizing this phenomenon, a wavelength division multiplexed signal (hereinafter, WDM) is used by using a plurality of light sources having different wavelengths (hereinafter, referred to as wavelength multiplexing pump light source).
Signal). In order that the pump light on the longest wavelength side of the WDM pump light source does not overlap with the signal light on the shortest wavelength side of the WDM signal, the maximum wavelength band of the WDM pump light source needs to be about 100 nm. .

Japanese Patent Application Laid-Open No. 2000-98433 discloses that flattening of Raman gain wavelength characteristics needs to be improved over a wide band in order to perform wavelength multiplex transmission of an optical signal. For that purpose, it is disclosed that it is necessary to consider the wavelength arrangement of the wavelength division multiplexing excitation light source.

[0004] Specifically, regarding the arrangement of a plurality of pumping light sources having different wavelengths, the wavelength interval between adjacent pumping lights is 6 nm.
If less than, when multiplexing pump light of different wavelengths,
A margin cannot be obtained by adding the bandwidth of the multiplexer and the bandwidth of the pumping light source so that the insertion loss of the multiplexer does not increase due to the crosstalk of the multiplexer. On the other hand, if the wavelength interval exceeds 35 nm, a large gain drop near the center of the combined Raman gain bandwidth of the Raman gains generated from adjacent wavelengths occurs, which is not suitable for wavelength division multiplexing transmission. It is said that the following range is required.

In order to reduce the wavelength interval between adjacent pumping light sources as much as possible so that the deviation of the combined Raman gain wavelength characteristic of the Raman amplifier does not increase, the pumping light is divided into forward pumping and backward pumping. Are set to about 6 nm, and the pump light wavelengths λ ₂ , λ ₄ ,... Respectively belonging to the backward pump are arranged between the pump light wavelengths λ ₁ , λ ₃ ,. By arranging dense pumping light wavelengths by setting the pumping light wavelength interval of the amplifier to less than 6 nm, the difference between the maximum and the minimum of the Raman gain wavelength characteristic of the Raman amplifier, or the difference between the maximum and the minimum, that is, the Raman gain wavelength characteristic deviation is high. It is disclosed that the Raman amplifier can be configured to be small enough to enable density WDM transmission.

The Raman amplifier disclosed in Japanese Patent Application Laid-Open No. 2000-98433 handles forward pumping and backward pumping independently, and designates a multiplexing point of forward pumping light as A and a multiplexing point of backward pumping light as B. Then, the section A → B extending in the longitudinal direction of the optical fiber in which stimulated Raman is generated by the forward pumping light and the section B → A extending in the longitudinal direction of the optical fiber in which stimulated Raman is generated by the backward pumping light are common except for the pumping direction. I have.

That is, the invention disclosed in Japanese Patent Application Laid-Open No. 2000-98433 uses a single pump so as to reduce the Raman gain wavelength characteristic deviation using a forward pumping light source and a backward pumping light source so that high-density WDM transmission is possible. An amplifier is configured, and Raman amplification is performed in the same section on the optical transmission system.

[0008]

However, Japanese Patent Laid-Open Publication
A Raman amplifier having a small Raman gain wavelength characteristic deviation as disclosed in Japanese Patent Application No. -98433 also has a larger or smaller Raman gain wavelength characteristic deviation.

That is, with respect to a Raman amplifier combining forward pumping and backward pumping, even if the Raman gain wavelength characteristic deviation is small, if a plurality of Raman amplifiers having the same gain wavelength characteristic are used, the Raman gain wavelength characteristic Since the wavelength determining the maximum value and the wavelength determining the minimum value are the same in each Raman amplifier, the maximum value and the minimum value of the Raman gain wavelength characteristic are accumulated every time the light passes through the amplification stage. The deviation of the gain wavelength characteristic increases,
A large difference in power for each channel results in poor flatness. For example, when a WDM signal is input to an optical amplifier using an erbium-doped optical fiber (EDF) or after output from the optical amplifier, it is necessary to compensate the power of each channel by means such as an equalizer. .

[0010]

According to a first aspect of the present invention, there is provided an optical transmission system including a plurality of Raman amplifiers each including a plurality of pumping light sources having pumping wavelengths different from each other. The section in the longitudinal direction of the optical fiber in which the stimulated Raman is generated by the Raman amplifier is different, and the plurality of Raman amplifiers are characterized by compensating the respective Raman gain wavelength characteristics.

That is, according to the first aspect of the present invention, at the output point of a wavelength division multiplexed signal of an optical transmission system using a plurality of stages of Raman amplifiers, the composite Raman gain wavelength characteristic deviation is the Raman gain wavelength characteristic of one amplifier. The purpose is to obtain an optical transmission system smaller than a value that is assumed to have accumulated the deviation by the number of stages.

According to a second aspect of the present invention, in the optical transmission system according to the first aspect, the plurality of Raman amplifiers are designed with at least two types of pumping wavelength configurations, and are configured with the same pumping wavelength configuration and the same type. When there are a plurality of Raman amplifiers using the amplifying fibers, the Raman amplifiers include those having different set gains.

According to a third aspect of the present invention, in the optical transmission system according to the first or second aspect, at least a first Raman amplifier and a second Raman amplifier are used as the plurality of Raman amplifiers, wavelength of forming the wavelength band of the convex curve above containing a maximum value of Raman gain G _Amax at the wavelength lambda _Amax to form a Raman gain wavelength characteristics of the first Raman amplifier, a Raman gain wavelength characteristic of the second Raman amplifier At λ _Bmin , the wavelength band of the downwardly convex curve including the minimum value of the Raman gain G _Bmin overlaps, and the minimum value of the Raman gain G _Amin at the wavelength λ _Amin forming the Raman gain wavelength characteristic of the first Raman amplifier is calculated. and the wavelength band of convex curve below which includes the wavelength band of the convex curve above containing a maximum value of Raman gain G _Bmax at the wavelength lambda _Bmax forming a Raman gain wavelength characteristic of the second Raman amplifier is it heavy The deviation of the combined Raman gain wavelength characteristic between the first Raman amplifier and the second Raman amplifier is the deviation of the Raman gain wavelength characteristic of the first Raman amplifier and the Raman gain wavelength of the second Raman amplifier. It is characterized by being smaller than the characteristic deviation.

According to a third aspect of the present invention, a gain wavelength characteristic deviation of an optical transmission system using a first Raman amplifier and a second Raman amplifier is determined for each of the first Raman amplifier and the second Raman amplifier. The maximum value and the minimum value of the Raman gain wavelength characteristic do not accumulate and the deviation of the combined Raman gain wavelength characteristic does not increase, and the combined Raman gain wavelength characteristic does not increase in each of the first Raman amplifier and the second Raman amplifier. This is an optical transmission system that can be set smaller than the gain wavelength characteristic deviation.

[0015] The invention of claim 4 is the invention of claim 3, almost equal to the wavelength lambda _Amax and the wavelength lambda _{Bm in,}
The wavelength λ _Amin is substantially equal to the wavelength λ _Bmax .

That is, a fourth aspect of the present invention is an optical transmission system in which the combined Raman gain wavelength characteristic deviation of the optical transmission system using the first Raman amplifier and the second Raman amplifier is minimized.

According to a fifth aspect of the present invention, in any one of the first to fourth aspects of the present invention, the Raman amplifier uses at least one of a lumped constant type Raman amplifier and a distributed constant type Raman amplifier. It is what is being done.

That is, the invention according to claim 5 is a lumped-constant Raman amplifier or distributed Raman amplifier such that the combined Raman gain wavelength characteristic deviation is smaller than the Raman gain wavelength characteristic deviation of each of a plurality of Raman amplifiers constituting the optical transmission system. This is an optical transmission system using a constant-type Raman amplifier.

For example, when a new line is laid as a transmission line, or when a new Raman amplifier is installed on an existing line in which a Raman amplifier is not used, multiple stages of Raman amplifiers are required. When multi-stage Raman amplifiers having characteristics are used, the deviation of the gain wavelength characteristics becomes large, so that the gain wavelength characteristics of the Raman amplifiers compensate each other and each Raman amplifier is concentrated so that the combined gain deviation becomes small. It is necessary to select either a constant Raman amplifier or a distributed constant Raman amplifier. Also, if it is desired to add a Raman amplifier to an existing optical transmission system in which a Raman amplifier is already installed, the existing Raman amplifier gain wavelength characteristic and the gain wavelength characteristic of the additional Raman amplifier are compensated for, It is necessary to select either a lumped-constant Raman amplifier or a distributed-constant Raman amplifier for each additional Raman amplifier so that the combined Raman gain wavelength deviation is reduced.

Here, the theory relating to the present invention will be described.

The Raman amplifier includes a distributed constant type Raman amplifier and a lumped constant type Raman amplifier. Since the distributed constant type Raman amplifier mainly uses the optical fiber transmission line itself as a medium for Raman amplification, considering the distortion generated when the WDM signal is amplified, an effective optical fiber transmission line at the signal band wavelength is used. A core having a core area of about 50 μm ² to 100 μm ² is used. The actual gain of the distributed constant type Raman amplifier, that is, the Raman gain need not exceed the transmission loss of the optical transmission line itself in one repeater section.

Since the lumped-constant type Raman amplifier is used as a relay amplifier, the effective core area Aeff at the signal band wavelength is reduced from about 10 μm ² to about 30 μm ² to enhance the nonlinearity and increase the effective core area. As far as the Raman gain caused by Aeff is concerned, the Raman gain of the lumped constant type Raman amplifier is 10 dB higher than that of the distributed constant type Raman amplifier.
It is designed to be large. Further, since the lumped-constant Raman amplifier is used as a relay amplifier, it is necessary to have a Raman gain exceeding the transmission loss of the highly nonlinear optical fiber constituting the lumped-constant Raman amplifier. That is,
The gain of the lumped Raman amplifier requires a net gain, which is an apparent gain.

The signal-wave, in the case of excitation Koichi wavelength, the Raman gain is approximately given by _{G = exp (g R P 0} L eff / Aeff). Here, g _R is a Raman gain coefficient, P ₀ is a pumping light power at a pumping light input end of an optical fiber constituting the Raman amplifier, L _eff is a length (effective length) at which an effective stimulated Raman effect occurs, α _{When p} is the attenuation per unit length of the excitation light, L _eff = (1 / α _p ) [1−exp (−α _p L)]
It is represented by Here, L is the length of the section in the longitudinal direction of the optical fiber where the induced Raman is generated by the Raman amplifier.

Therefore, in order to experimentally obtain the Raman gain coefficient belonging to the desired wavelength band, the pumping light input end of the optical fiber constituting the Raman amplifier is connected to the pumping light wavelength λ _p at the shortest wavelength side of the desired wavelength band. Is input, and the signal light wavelength λ _s is swept from the short wavelength side to the long wavelength side of the desired wavelength band, and the pump light power at the pump light input end of the optical fiber constituting the Raman amplifier is input. P ₀ , the input terminal signal light input P _{1 of the} Raman amplifier, and the output terminal signal light output P ₂ are measured.

From the input terminal signal light input P ₁ and the output terminal signal light output P ₂ , an apparent gain (net gain) is made corresponding to the fixed pump light wavelength λ _p and the signal light wavelength λ _s to be swept. ) G _N = 10 log (P ₂ / P ₁ ) is obtained, and the attenuation amount α _s per unit length at the signal light wavelength of the optical fiber constituting the Raman amplifier and the length L of the optical fiber constituting the Raman amplifier are determined. seeking transmission loss alpha _s L of the signal light which is the product of the, the actual gain (Raman _{gain) G G = G N + α} s L
Ask by

Excitation of optical fiber constituting Raman amplifier
Excitation light power P at the optical input end₀Is measurable and Raman
Because the length L of the optical fiber constituting the amplifier is known
L_{ef f}Can be obtained by calculation. Gain G is calculated
G = exp (g _RP₀L_eff/ Aeff) relationship
From the equation, g_R/ Aeff is obtained, and Aeff is
Because it is more known, g_RIs obtained. Note that g_RClear
In order to be able to perform measurements in
Pump power P at the pump light input end₀Is +10 dBm
And about -20 dBm for signal light.

Ν _p is the frequency corresponding to the excitation light wavelength λ _p ,
_Assuming that _s is a frequency corresponding to the signal light wavelength λ _s , the above-mentioned Raman gain coefficient g _{R for} the fixed excitation light frequency ν _p and the parameters (ν _p , ν _s ) of the signal frequency ν _s to be swept.
(Ν _p , ν _s ) is obtained. It is theoretically known that the generalized Raman gain coefficient g _R depends on the frequency shift Δν = ν _p −ν _s . Therefore, in order to obtain a generalized Raman gain coefficient g _R (Δν), the parameters (ν _p , ν _s )
_{G R (ν p, ν s} ) corresponding to the relative size of, made to correspond to g _R (Δν) of the same size, the parameter ([nu _p,
ν _s ) may be replaced by a frequency shift Δν = ν _p −ν _s .

As described above, the wavelength (frequency) dependency of the generalized Raman gain coefficient (or g _R / Aeff) is determined, and the wavelength (frequency) versus the Raman gain coefficient (or g
_R / Aeff) table is prepared.

In Raman amplification, the basic concept of how pump light, signal light, and optical fiber attenuation are related to each other is described in “Pump Interactions in a 10
0nmBandwidth Raman Amplifier "IEEE PHOTONICS TECHNO
LOGY LETTERS, VOL.11, No.5, MAY 1999, p.530.

Based on this concept, the pump light, the signal light, and the fundamental equation showing how the attenuation of the optical fiber are related to each other for the simulation of the Raman amplifier. It was assumed that Rayleigh scattering was small and the effect of spontaneous emission was negligible.

The frequency attenuation amount of the optical fiber at [nu alpha _[nu, effective core area Aeff _[nu, as Raman gain coefficient _{g R} (ζν) = g ζν between frequency ζ and the frequency [nu, the frequency [nu
The change in power when the wave of the forward wave power P _ν of the forward wave advances by the distance Z is given by the following equation.

[0032]

(Equation 1)

[0033] Here, the frequency ν _1, ν _2, ..., the power of the WDM signal consisting of _{ν n P sν1, P s ν2} , ..., and P _Esunyuenu, frequency zeta _1, zeta _2, ..., consisting of zeta _m The power of the multiple pump light is P p _{と し 1} , P _pζ2 ,..., P _pζm (where ν ₁ <
ν ₂ <... <ν _n <ζ ₁ <ζ ₂ <... <ζ _m ), and there is no Raman transition between WDM signals.

As described above, since both the WDM signal and the pumping light have discrete spectra, the operation of integration of the equation (1) is replaced by the operation of sum.

When the WDM signal having the frequency ν _k and the power P _sνk is applied to P _ν in the equation (1), the following equation is obtained.

[0036]

(Equation 2)

When the pumping light having the frequency ζ _j and the power P p _ζj is applied to P _ν in the equation (1), the following equation is obtained.

[0038]

(Equation 3)

The power of the WDM signal (P _sν1 , P _sν2 ,
.., P _sνn ) are given at the signal input point and the multiple pump light (P
_{Since pζ1} , _Ppζ2 ,..., _Ppζm ) are given at the pumping light input point, the following _applies to forward pumping, backward pumping, and a combination of both.

First, the case of forward excitation will be described.
(P_sν1, P_sν2, ..., P_sνn), (P_pζ1, P_pζ2,
…, P_pζm) Is the initial value set at Z = 0,
Substituting into the right-hand side of each of the equations and
ν_k, Any ζ_jAgainst P_sνk, P_pζjSeparated by Δz
ΔP_sνk, ΔP_pζjIs obtained. P_sνk+ ΔP_sνk,
P _pζj+ ΔP_pζjAnd P_sνk, P_pζjAs the number
By substituting into the right side of the equation 2 and the equation 3 respectively, z =
P away from point 0 by 2Δz_sνk, P_pζjGot
By repeating this, the output point (P_sν1, P
_sν2, ..., P_sνn), (P_pζ1, P_pζ2, ..., P_pζm)
Is obtained, and the gain is known.

Next, the case of backward excitation will be described.
Multiple excitation light (P_pζ1, P_pζ2, ..., P_pζm) Is Z = L
Transmission loss α_ζkTaking into account
The predicted value of the multiple pump light power at Z = 0 is P_pζk−α
_ζkLet L (k is an integer from 1 to m). W at Z = 0
DM signal light (P_sν1, P_sν2, ..., P_sνn) Initial value and
And multiple pump light power predictions as in the case of forward pumping.
In this case, the equation of Equation 2 is used as it is, and the equation of Equation 3 is used.
Is obtained by using an expression in which the + sign and the-sign are exchanged,
Multiple excitation light power (P_pζ1, P_pζ2, ..., P
_pζm). Set the calculated multiple excitation light power and
Multiple pump light powers within the set error range
For example, the calculated WDM signal light (P_sν1, P
_sν2, ..., P _sνn) Is the solution and number satisfying the equation
Adopt the equation that replaces the sign of equation 3 as a satisfying solution
You. Calculated multiple excitation light power and set multiple excitation light
If the power is out of the set error range, the Z
Double excitation light power (P_pζ1, P_{p ζ2}, ..., P_pζm) Size
Of the WDM signal light (P_s
ν1, P_sν2, ..., P_sνn) Initial value and multiple excitation light
In the same way as in the case of forward excitation,
And Z = L
Pump light power (P_pζ1, P_pζ2, ..., P_pζm)
Ask for. The calculated multiple pump light power and setting at Z = L
Multiplexed pump power matches within set error range
The above calculation is repeated until the calculation is completed.

Next, a case where forward excitation and backward excitation are combined will be described. Here, it is assumed that there is no interaction between forward excitation and backward excitation. Therefore, the gain is obtained by superimposing forward pumping and backward pumping. That is, the sum of the equation of Equation 3 representing pumping light at the time of forward pumping and the equation of Equation 3 representing pumping light at the time of backward pumping, in which + is replaced with-and + is replaced by + P representing the excitation light of the formula
_Subsequent to this, the algorithm of forward excitation and backward excitation is repeated.

As in the above three cases, a simulation is performed using the equations (2) and (3), and the gain wavelength characteristics of the two Raman amplifiers compensate each other, so that the deviation of the combined Raman gain wavelength characteristic is small. The optical transmission system of the present invention can be realized by searching for a numerical example and applying the numerical value to an actual system.

[0044]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic explanatory diagram showing an example of the configuration of the optical transmission system of the present invention.

FIG. 1A is a schematic explanatory view showing an example of the configuration of an optical transmission system when two Raman amplifiers are used. In FIG. 1A, 1 is an optical transmission line, and 2 is a Raman amplifier. In the Raman amplifier 2, the two Raman amplifiers 2 compensate for their Raman gain wavelength characteristics, and the combined Raman gain wavelength characteristic deviation of the two Raman amplifiers becomes the Raman gain wavelength characteristic deviation of each Raman amplifier 2. It is arranged between AB and CD, which are sections where Raman amplification of a WDM signal is to be performed, so as to be smaller. In FIG. 1A, the point B and the point C are located at different positions, but may be located at the same position. In this case, the transmission loss deviation between the BCs of the optical transmission line 1 must be considered. It is possible to obtain the combined Raman gain wavelength characteristic and its deviation without any problem.

FIG. 1B is a schematic explanatory diagram showing an example of the configuration of an optical transmission system when three Raman amplifiers are used. Also in this case, similarly to the case where the two Raman amplifiers 2 are used, the sections where the three Raman amplifiers 2 attempt to Raman amplify the WDM signal are between AB, CD, EF.
Placed between. The configuration of the optical transmission system of the present invention is not limited to these, and the number of connected Raman amplifiers 2 may be any number.

The Raman amplifier 2 may be a forward pumping type, a backward pumping type, a combination of the forward pumping type and the backward pumping type, or a distributed constant type. It may be a lumped constant type. What is important in the embodiment of the present invention is that a plurality of Raman amplifiers compensate for each Raman gain wavelength characteristic, and the combined Raman gain wavelength characteristic deviation of the plurality of Raman amplifiers is the Raman gain wavelength characteristic deviation of one amplifier. The optical transmission system is configured so as to be smaller than a value assumed to have been accumulated by the number of stages.

FIG. 2 is a schematic explanatory view showing an example of a high-power semiconductor laser unit (hereinafter referred to as HPU) used as an excitation light source. In FIG. 2, 3 is an HPU, 4
Is a semiconductor laser, 5 is a polarization maintaining optical fiber, 6 is a polarization combiner that adjusts the polarization state of the two inputted lights so as to be orthogonal, for example, and combines the polarization, 7 is an optical multiplexer, 8 Is an isolator.

In FIG. 2, the semiconductor laser 4 is
a, 4b,..., a polarization maintaining optical fiber 5 (5a, 5b,...), a polarization combiner 6 (6a, 6b,.
..) And a plurality of optical multiplexers 7 (7a, 7b).

In FIG. 2, the same polarization synthesizer 6 is used.
A plurality of semiconductor lasers 4, for example, 4a and 4
b is output at substantially the same wavelength. Although FIG. 2 shows an example in which the excitation wavelength is three, the HPU 3 as shown in FIG. 2 can be actually configured when the excitation wavelength is two or more. Polarization maintaining optical fiber 5 (5
a, 5b,...) have a semiconductor laser 4 inside.
(4a, 4b,...), A light reflecting layer is formed by a fiber grating substantially matching the oscillation light, and the oscillation wavelength of the semiconductor laser 4 (4a, 4b,...) Is stabilized and the oscillation wavelength is narrowed. Can also.

In FIG. 2, the optical multiplexer 7 is a wavelength-selective means such as a WDM coupler or a dichroic mirror (hereinafter referred to as a WDM filter).
It is preferable to use

In the HPU 3 shown in FIG. 2, two semiconductor lasers 4 (4a, 4b; 4c, 4d; 4e, 4f) are respectively connected to polarization maintaining optical fibers 5 (5a, 5b; 5c, 5c).
d; 5e, 5f) through the polarization combiner 6 (6a; 6b;
6c) is connected to the input side. The polarization synthesizer 6
b, 6c are connected to the input side of the optical multiplexer 7b,
The output side of the optical multiplexer 7b and the output side of the polarization combiner 6a are connected to the input side of the optical multiplexer 7a. The output side of the optical multiplexer 7 a is connected to the input side of the isolator 8, and the output side of the isolator 8 is connected to the output end of the HPU 3. In addition, a tap for monitoring the output of the HPU 3 is provided near the output terminal of the HPU 3 as necessary.

Hereinafter, an example of the distributed constant type Raman amplifier used in the present invention will be described. FIG. 3 is a schematic explanatory view showing an example of a distributed constant type Raman amplifier.
Is a WDM signal input point, and 11 is a WDM signal output point.

Here, the WDM signal input point 10 and WD
The M signal output point 11 does not mean something like, for example, an input terminal or an output terminal.
Represents a point A or the like which is an input point of the Raman amplifier in the optical transmission system illustrated in FIG.
Represents a point B or the like which is an output point of the Raman amplifier in the optical transmission system illustrated in FIG. One of the WDM signal input point 10 and the WDM signal output point 11 is
It may be common to things like input terminals and output terminals.

In FIG. 3, the pump light travels from the optical multiplexer 9 to the WDM signal output point 11 in the case of forward pumping, and the direction from the optical multiplexer 9 to the WDM signal input point 10 in the case of backward pumping. The excitation light proceeds. It is preferable to use a WDM filter as the optical multiplexer 9 as in the optical multiplexer 7 of FIG.

The optical fiber used for the distributed constant type Raman amplifier is also used as the optical transmission line 1. In general, since optical transmission lines are often long distances, the length of an optical fiber used for a distributed constant amplifier is generally several tens km to several tens km.
It is several hundred km. The type of optical fiber is non-zero dispersion shift fiber (non-zero dispersion shift fiber).
zero DSF: NZ-DSF).

Hereinafter, an example of a lumped-constant Raman amplifier used in the present invention will be described. FIG. 4 is a schematic explanatory view showing an example of a lumped constant type Raman amplifier.
Denotes a WDM signal input point, 11 denotes a WDM signal output point, and 12 denotes an optical amplification fiber.

Here, a WDM signal input point 10 represents a point A or the like which is an input point of the Raman amplifier in the optical transmission system illustrated in FIG. 1, and a WDM signal output point 11 corresponds to the optical transmission system illustrated in FIG. And WDM signal input point 10 and WDM signal output point 11 in common with the distributed constant type Raman amplifier in that point B, which is the output point of the Raman amplifier in FIG.
Is different from a distributed constant type Raman amplifier in that, for example, the input terminal and the output terminal are often common.

In FIG. 4, the pump light travels from the optical multiplexer 9 to the WDM signal output point 11 in the case of forward pumping, and the direction from the optical multiplexer 9 to the WDM signal input point 10 in the case of backward pumping. The excitation light proceeds. That is, the lumped-constant Raman amplifier in FIG. 4 shows an example of backward pumping. It is preferable to use a WDM filter as the optical multiplexer 9 as in FIG.

Since the lumped-constant type Raman amplifier is generally used as a module including the optical amplification fiber 12, the optical amplification fiber 12 having a large nonlinear refractive index and a large Raman gain coefficient is used. . For example, when the nonlinear refractive index and the Raman gain coefficient
Mode optical fiber (SMF) used as
An optical fiber having a nonlinear refractive index and a Raman gain coefficient several times that of a dispersion-shifted optical fiber (DSF) is used for the optical amplification fiber 12 of the lumped-constant Raman amplifier.
An example is a dispersion compensating optical fiber (DCF).

Example 1 As Example 1, an optical transmission system was constructed by combining a distributed constant type Raman amplifier and a lumped constant type Raman amplifier, and the combined Raman gain wavelength characteristics were simulated.

FIG. 5 is a schematic explanatory view showing an example of the optical transmission system according to the first embodiment. In FIG. 5, two types of Raman amplifiers are used: a backward-pumped distributed constant Raman amplifier 2ad and a backward-pumped lumped constant Raman amplifier 2bc.

Further, the maximum value of the Raman gain is changed to λ11, λ13 by the backward pumping distributed constant type Raman amplifier 2ad.
And the Raman gain wavelength characteristics having the minimum values at λ12 and λ14 are obtained. The minimum values of the Raman gains are set at λ11 ′ and λ13 by the backward pumped lumped-constant Raman amplifier 2bc.
′ And Raman gain wavelength characteristics having local maximum values at λ12 ′ and λ14 ′.

Λ11 and λ11 ', λ12 and λ1
2 ′,... Are substantially equal to each other. Specifically, the wavelength indicating the maximum value of the Raman gain of the distributed constant type Raman amplifier 2ad and the wavelength indicating the minimum value of the Raman gain of the lumped constant type Raman amplifier 2bc are almost equal to each other. The Raman gain of the distributed constant Raman amplifier 2ad and the wavelength of the lumped constant Raman amplifier 2bc showing the local maximum value of the lumped constant type Raman amplifier 2bc are made substantially coincident with each other, so that the Raman The gain wavelength characteristic and the Raman gain wavelength characteristic of the lumped constant type Raman amplifier 2bc compensate each other, and the deviation of the combined Raman gain wavelength characteristic can be made smaller than the deviation of the Raman gain wavelength characteristic of the respective Raman amplifiers 2ad and 2bc. is there.

Here, specific examples of the distributed constant type Raman amplifier 2ad and the lumped constant type Raman amplifier 2bc will be described.

First, a specific example of the distributed constant Raman amplifier 2ad will be described. FIG. 6 shows an example of the wavelength characteristic of the attenuation of the NZ-DSF, FIG. 7 shows the frequency shift characteristic of the Raman gain coefficient, and FIG. 8 shows the wavelength characteristic of the effective core area (Aeff). In this example, the length of the NZ-DSF was set to 100 km.

The HPU 3a will be described with reference to the HPU 3 of FIG. 2. If the oscillation wavelengths of the semiconductor lasers 4a and 4b are 1
383 nm, the oscillation wavelength of the semiconductor lasers 4 c and 4 d is 13
95 nm, the oscillation wavelength of the semiconductor lasers 4e and 4f is 142
It was 2 nm. The output of each wavelength is 250 mW at a wavelength of 1383 nm and 13
95 mW at 95 nm, 230 mW at 1422 nm
It was adjusted to be.

Next, a specific example of the lumped constant type Raman amplifier 2bc will be described. FIG. 9 shows the wavelength-attenuation characteristic of the DCF used as the optical amplification fiber 12, FIG. 10 shows the frequency shift characteristic of the Raman gain coefficient, and FIG. 11 shows the wavelength characteristic of the effective core area (Aeff). In this embodiment, this DCF is used as the optical amplification fiber 12 for 6 km.

The HPU 3b will be described with reference to the HPU 3 of FIG. 2. If the oscillation wavelength of the semiconductor lasers 4a and 4b is 1
388 nm, the oscillation wavelength of the semiconductor lasers 4 c and 4 d is set to 14
02 nm, the oscillation wavelength of the semiconductor lasers 4e and 4f is 142
9 nm.
The waves were polarized so that the polarization states were almost orthogonal. The output of each wavelength is 260 mW at a wavelength of 1383 nm and 217 nm at a wavelength of 1395 nm at the output end of the HPU3.
mW and the wavelength of 1422 nm were adjusted to 166 mW.

Here, the power, frequency, and attenuation of the WDM signal light input to the distributed constant Raman amplifier 2ad, the power, the frequency, the attenuation, and the Raman The simulation was performed by substituting the gain factor. The resulting gain wavelength characteristic is shown by the broken line in FIG.

The gain wavelength characteristic shown by the broken line in FIG.
7d over the entire wavelength band for easy reading of the graph
B has been raised. In the graph of the gain wavelength characteristic indicated by the broken line in FIG. 12, when the difference between the maximum value and the minimum value of the gain is a deviation, the minimum gain value is 14.4 dB at a wavelength of 1492.3 nm.
And a maximum gain of 15.0 d at a wavelength of 1499.0 nm.
B, the deviation of the gain wavelength characteristic is 0.6 dB.

The power, frequency, and attenuation of the WDM signal light input to the lumped constant type Raman amplifier 2bc, the power, the frequency, the attenuation, and the Raman When a simulation is performed by substituting a gain coefficient, a gain wavelength characteristic indicated by a two-dot chain line in FIG. 12 is obtained.

The gain wavelength characteristic indicated by the two-dot chain line in FIG. 12 is raised by 13 dB over the entire wavelength band so that the graph can be easily read. In the graph of the gain wavelength characteristic indicated by the two-dot chain line in FIG. 12, when the difference between the maximum value and the minimum value of the gain is a deviation, the minimum gain value is 8.5 dB at a wavelength of 1499.0 nm, and the maximum gain is at a wavelength of 1491.5 nm. Since the value shows 9.2 dB, the deviation of the gain wavelength characteristic is 0.7 dB.
B.

Next, a simulation result of the combined Raman gain wavelength characteristic will be described. The solid line in FIG. 12 shows a simulation result of the combined Raman gain wavelength characteristic. In the following description, for convenience, the above-described distributed constant Raman amplifier 2ad is a first Raman amplifier, and the above lumped constant Raman amplifier 2bc is a second Raman amplifier.

As shown in FIG. 12, the gain pole of the first Raman amplifier
Small value G_AminIs the wavelength λ_Amin1492.3 nm
And the maximum gain G at 1505.0 nm._AmaxIs the wavelength
λ_{Am ax}1499.0 nm and 1517.2
nm and the gain maximum G of the second Raman amplifier_BmaxIs
Wavelength λ_Bmax1491.5 nm and 150
7.3 nm, minimum gain G_BminIs the wavelength λ_BminTona
1499.0 nm and 1513.3 nm
You. That is, the gain maximum G G of the first Raman amplifier _Amax
Wavelength λ_AmaxIs the minimum gain G of the second Raman amplifier.
_BminWavelength λ_{Bm in}Almost corresponds to the first Raman amplification
Minimum value G_AminWavelength λ_AminThe second Raman
Maximum gain G of amplifier_BmaxWavelength λ_BmaxIs almost compatible
are doing.

FIG. 12 shows that the wavelength band of the upwardly convex curve including the gain maximum value G _Amax of the first Raman amplifier and the second
The wavelength band of the downward convex curve including the minimum gain G _Bmin of the Raman amplifier of the first Raman amplifier overlaps with the wavelength band of the downward convex curve including the minimum gain G _Amin of the first Raman amplifier and the second Raman amplifier. The wavelength bands of the upwardly convex curves including the gain maximum value G _Bmax of FIG.

Since the first Raman amplifier and the second Raman amplifier are connected in series at substantially adjacent points, the gain wavelength characteristic of the first Raman amplifier and the gain wavelength characteristic of the second Raman amplifier Are superimposed to produce a combined Raman gain wavelength characteristic. The gain deviation of the combined Raman gain wavelength characteristic shown by the solid line in FIG.
And the gain deviation of the second Raman amplifier are smaller.

That is, the maximum of the Raman gain wavelength characteristics of the first Raman amplifier and the maximum of the Raman gain wavelength characteristics of the second Raman amplifier do not accumulate, and the deviation of the combined Raman gain wavelength characteristics does not increase. The wavelength band of the upward convex curve including the maximum value of the gain of the Raman amplifier and the wavelength band of the downward convex curve including the minimum value of the gain of the second Raman amplifier overlap with each other,
Since the wavelength band of the downward convex curve including the minimum value of the gain of the first Raman amplifier and the wavelength band of the upward convex curve including the maximum value of the gain of the second Raman amplifier overlap, the first Raman The Raman gain wavelength characteristics of the amplifier and the second Raman amplifier compensate for each other, and the gain deviation of the combined Raman gain wavelength characteristic is smaller than the respective gain deviations of the first Raman amplifier and the second Raman amplifier.

Example 2 An optical transmission system was constructed by combining two distributed constant type Raman amplifiers, and the combined Raman gain wavelength characteristics were simulated.

FIG. 13 is a schematic explanatory view showing an example of the optical transmission system according to the second embodiment. In FIG. 13, the distributed constant type Raman amplifiers 2cd and 2dd both use forward pumping.

Further, the forward pumping distributed constant type Raman amplifier 2cd (having the HPU 3a) has the maximum values of the Raman gain at λ21 and λ23 and the minimum values at λ22 and λ2.
4 is obtained, and a forward-pumped distributed constant Raman amplifier 2dd (having the HPU 3c) is obtained.
As a result, Raman gain wavelength characteristics having the minimum values of Raman gain at λ21 ′ and λ23 ′ and the maximum values at λ22 ′ and λ24 ′ are obtained.

In this case, as in the first embodiment, the deviation of the combined Raman gain wavelength characteristic can be made smaller than the deviation of the Raman gain wavelength characteristic in each Raman amplifier.

Generally, in one Raman amplifier, the deviation of the Raman gain wavelength characteristic is proportional to the magnitude of the Raman gain. Therefore, in order to maximize the offset effect of the gain deviation in the method as in the first embodiment, there is an optimum ratio between the Raman gains of the distributed amplifier and the lumped amplifier. Further, in the second embodiment, there is an optimum ratio between the magnitudes of the Raman gains in the two types of Raman amplifiers regardless of the magnitude of the transmission path loss in each section. Thus, if there is an optimum value for the Raman gain of each Raman amplifier independently of the system design parameters, the optimization of the entire system becomes complicated. Therefore, when it is necessary to consider the above-mentioned optimal ratio of Raman gain,
Optimization can be facilitated by designing the entire system to achieve the optimal ratio. For example, to explain using Embodiment 2, two types of HPUs (3a and 3c)
May be designed so as to achieve the above-described optimum ratio when comparing the accumulated Raman gain of the Raman amplifier using the HPU 3a with the accumulated Raman gain of the Raman amplifier using the HPU 3c using a large number of Raman amplifiers using . In this case, it is not necessary to set the Raman gains of all sections to the same value, and it is possible to assign a value according to the transmission path loss of each section. Since the same can be said for the centralized type, in a system using both the centralized type and the distributed type, the above-mentioned optimization can be performed for each system.

Comparative Example FIG. 14 is a schematic explanatory view showing an example of an optical transmission system of a comparative example. A forward-pumped distributed constant Raman amplifier and a backward-pumped distributed constant Raman amplifier are combined so that a section extending in the longitudinal direction of an optical fiber in which stimulated Raman is generated by a pump light source of each Raman amplifier is common. The amplifiers 2e and 2e 'were configured. And
An optical transmission system was configured by combining the Raman amplifiers 2e and 2e ', and the combined Raman gain wavelength characteristics were simulated.

In this case, each Raman amplifier 2e,
The Raman gain wavelength characteristics in 2e 'are almost the same,
The deviation of the Raman gain wavelength characteristic was about 0.6 dB. The HPU 3x of the Raman amplifier 2e and the Raman amplifier 2e
'HPU3x' and Raman amplifier 2e HPU3x
y and the HPU 3y 'of the Raman amplifier 2e' use pump light sources of substantially the same wavelength.

In FIG. 14, when the combined Raman gain wavelength characteristic deviation (at point D) was measured in the same manner as in the embodiment, it was about 1.1 dB, which was almost doubled. This is because the wavelength indicating the maximum value of the Raman gain of the Raman amplifier 2e and the wavelength indicating the maximum value of the Raman gain of the Raman amplifier 2e 'almost coincide with each other, and the wavelength indicating the minimum value of the Raman gain of the Raman amplifier 2e and the Raman amplifier It is considered that the cause is that the wavelength showing the minimum value of the Raman gain of 2e 'substantially coincides.

That is, when the deviation of the combined Raman gain wavelength characteristic is to be reduced in the system of the comparative example shown in FIG. 14, the wavelength indicating the maximum value of the Raman gain of the Raman amplifier 2e is determined by using the method of the present invention. And the wavelength band of the downward convex curve including the wavelength indicating the minimum value of the Raman gain of the Raman amplifier 2e 'overlaps, and the wavelength indicating the minimum value of the Raman gain of the Raman amplifier 2e. And the Raman amplifier 2e '
It is necessary to consider the arrangement of the pumping wavelengths in the respective Raman amplifiers so that the wavelength band of the upward convex curve including the wavelength indicating the maximum value of the Raman gain overlaps.

In this embodiment, the Raman amplifier may be a forward pump type, a backward pump type, or a combination thereof. The Raman amplifier may be a distributed constant type, a lumped constant type, or a combination thereof. For example, JP 2000
The amplifier described in JP-A-98433 may be regarded as one amplifier and a plurality of amplifiers may be used.

[0090]

As described above, according to the present invention, a plurality of Raman amplifiers compensate each Raman gain wavelength characteristic, and the combined Raman gain wavelength characteristic deviation uses the Raman gain wavelength characteristic deviation of one amplifier. It is possible to configure an optical transmission system smaller than the value assumed to be accumulated by the number of stages. Further, it is possible to reduce the Raman gain wavelength characteristic deviation without using means such as an equalizer.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing an example of an optical transmission system according to an embodiment of the present invention, in which (a) shows an example using two Raman amplifiers, and (b) shows an example using three Raman amplifiers. An example is shown.

FIG. 2 is a schematic explanatory view showing an example of a high-power semiconductor laser unit (HPU) used as an excitation light source.

FIG. 3 is a schematic explanatory diagram illustrating an example of a distributed constant type Raman amplifier.

FIG. 4 is a schematic explanatory diagram showing an example of a lumped constant type Raman amplifier.

FIG. 5 is a schematic explanatory view showing an optical transmission system according to an embodiment of the present invention.

FIG. 6 is a waveform chart showing an example of a wavelength characteristic of an attenuation amount of the NZ-DSF.

FIG. 7 is a waveform chart showing an example of a frequency characteristic of a Raman gain coefficient of the NZ-DSF.

FIG. 8 is a waveform chart showing an example of a wavelength characteristic of an effective core area (Aeff) of the NZ-DSF.

FIG. 9 is a waveform diagram showing an example of a wavelength-attenuation characteristic of a DCF used as an optical amplification fiber.

FIG. 10 is a waveform chart showing an example of a frequency characteristic of a Raman gain coefficient of a DCF used as an optical amplification fiber.

FIG. 11 is a waveform chart showing an example of a wavelength characteristic of an effective core area (Aeff) of a DCF used as an optical amplification fiber.

FIG. 12 is a waveform diagram showing an example of the gain wavelength characteristic of the Raman amplifier used in the optical transmission system of the present invention, wherein the broken line indicates the gain wavelength characteristic of the distributed constant type Raman amplifier;
The dotted line indicates the gain wavelength characteristic of the lumped-constant Raman amplifier, and the solid line indicates the combined Raman gain-wavelength characteristic of the lumped-constant Raman amplifier and the distributed-constant Raman amplifier.

FIG. 13 is a schematic explanatory view showing an optical transmission system according to another embodiment of the present invention.

FIG. 14 is a schematic explanatory diagram illustrating an example of an optical transmission system of a comparative example.

[Explanation of symbols]

1 Optical transmission line 2, 2ad, 2bc, 2cd, 2dd Raman amplifier 3, 3a-3c High power semiconductor laser unit (HP
U) 4, 4a-4f Semiconductor laser 5, 5a-5f Polarization maintaining optical fiber 6, 6a-6c Polarization combiner 7, 7a, 7b Optical multiplexer 8 Isolator 9 Optical multiplexer 10 WDM signal input point 11 WDM signal Output point 12 Optical amplification fiber

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/17 H04B 9/00 E H04J 14/00 14/02 (72) Inventor Sokoko Kado Chiyoda, Tokyo 2-6-1, Marunouchi-ku Furukawa Electric Co., Ltd. F-term (reference) 2K002 AA02 AB30 BA01 CA15 DA10 EA10 GA10 HA24 5F072 AB07 AK06 PP07 QQ07 YY17 5K002 AA06 CA01 CA13 DA02

Claims (5)

[Claims] A plurality of Raman amplifiers each including a plurality of pump light sources having pump wavelengths different from each other;
The plurality of Raman amplifiers have different sections in the longitudinal direction of the optical fiber in which the stimulated Raman occurs, and the plurality of Raman amplifiers compensate each Raman gain wavelength characteristic, and the combined Raman gain of the plurality of Raman amplifiers An optical transmission system characterized in that the wavelength characteristic deviation is smaller than a value assuming that the Raman gain wavelength characteristic deviation of one amplifier is accumulated by the number of stages used. 2. A plurality of Raman amplifiers are designed with at least two types of pumping wavelength configurations, and when there are a plurality of Raman amplifiers using the same type of amplifying fiber with the same pumping wavelength configuration, the Raman amplifiers are set in them. 2. The optical transmission system according to claim 1, wherein the optical transmission system includes ones having different gains. 3. The Raman amplifier according to claim 1, wherein at least a first Raman amplifier and a second Raman amplifier are used as said plurality of Raman amplifiers, and the Raman gain G _{Amax has} a local maximum at a wavelength λ _Amax forming a Raman gain wavelength characteristic of the first Raman amplifier. The wavelength band of the upwardly convex curve including the value and the wavelength band of the downwardly convex curve including the minimum value of the Raman gain G _Bmin at the wavelength λ _Bmin forming the Raman gain wavelength characteristic of the second Raman amplifier overlap with each other. The wavelength band of a downwardly convex curve including the minimum value of the Raman gain G _Amin at the wavelength λ _Amin forming the Raman gain wavelength characteristic of the first Raman amplifier, and the Raman gain wavelength characteristic of the second Raman amplifier. At the wavelength λ _Bmax to be formed, the wavelength band of the upwardly convex curve including the maximum value of the Raman gain G _Bmax overlaps,
The deviation of the combined Raman gain wavelength characteristic between the first Raman amplifier and the second Raman amplifier is the deviation of the Raman gain wavelength characteristic of the first Raman amplifier and the deviation of the Raman gain wavelength characteristic of the second Raman amplifier. The optical transmission system according to claim 1, wherein the optical transmission system is smaller than the deviation. 4. The wavelength λ _Amax is substantially equal to the wavelength λ _Bmin ,
_4. The optical transmission system according to claim 3, wherein the wavelength λ _Amin is substantially equal to the wavelength λ _Bmax . 5. The Raman amplifier according to claim 1, wherein at least one of a lumped-constant Raman amplifier and a distributed-constant Raman amplifier is used. An optical transmission system according to item 1.