JP2001222036A - Raman amplification and optical signal transmission method utilizing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem of being incapable of controlling intensity distribution of exciting light in the longitudinal direction of an optical fiber resulting in no room to improve noise characteristics. SOLUTION: A first exciting light to Raman amplify signal light is made incident from the output terminal of the signal light and the second exciting light, with shorter wavelength than that of the first exciting light, to Raman amplify the first exciting light is made incident from the input terminal of the signal light. A second exciting light is also made incident from the output terminal of the signal light. The first exciting light is also made incident from the input terminal of the signal light. The Raman amplification band of the second exciting light is made not to overlap with the wavelength band of the signal light. The wavelength of the second exciting light is made shorter than the wavelength of the first exciting light by the Raman shift portion of the amplification optical fiber. A multiple light source is used for one or both of the first and the second exciting light. A semiconductor laser is used for the first exciting light. The third exciting light to Raman amplify the second exciting light is guided to the optical transmission line. The signal light is transmitted with the optical transmission line using the Raman amplification.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Raman amplification system used for optical communication and an optical signal transmission method using the Raman amplification system.

[0002]

2. Description of the Related Art The frequency component (wavelength component) of an intensity-modulated optical signal used in current optical communication has a certain width. On the other hand, the optical fiber has a dispersion characteristic that the propagation speed varies depending on the wavelength. Due to these two properties, when an intensity-modulated optical signal propagates through an optical fiber, a signal waveform is distorted due to a difference in propagation speed for each wavelength component. When a pulse is incident as an optical signal, the pulse width after propagation increases, so this phenomenon is called pulse spreading due to dispersion. (Eg, Katsuharu Okamoto, Basics of Optical Waveguides, Corona)

[0003] Digital communication is more resistant to waveform changes than analog communication. However, if the pulse width is widened so as to overlap the preceding and succeeding bits, errors in detection naturally increase remarkably. Therefore, conventionally, the spread of a pulse is suppressed to a small value by using a wavelength having a small dispersion (close to 0), or a wavelength component that has advanced through a medium having dispersion opposite to that of a transmission line is delayed, and a delayed wavelength component is reduced. The pulse that spreads and spreads is restored.

However, in recent optical communication systems,
Non-linear phenomena in an optical fiber become remarkable due to high output and wavelength multiplexing of an optical signal, and it is no longer possible to cope with waveform distortion only from the viewpoint of dispersion. Non-linear phenomena that are mainly considered include self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). In SPM and XPM, the phase of light changes when the refractive index of the optical fiber slightly changes according to the light intensity. Since this phase change causes an instantaneous change in frequency and the amount of change is not constant, irreversible waveform distortion occurs due to the dispersion characteristics of the optical fiber. F
WM is a phenomenon in which, when a polarization field is excited by a plurality of incident lights having different frequencies, a component having a frequency different from that of the incident light is generated, so that light of a new frequency is generated. FWM is particularly pronounced at wavelengths where the dispersion is close to zero. If the light generated by the FWM matches the wavelength used as a signal, errors in detection will increase.

[0005]

There are two approaches as means for preventing the deterioration of transmission characteristics due to the non-linearity of the optical fiber as described above. One is a method of reducing the nonlinear effect by lowering the light intensity in the optical fiber. The other is a method using a transmission method using a non-linear effect. The former method can be realized by simply lowering the input level to the optical fiber or using an optical fiber having a large mode field diameter. The latter method can be realized by using optical soliton communication. However, even if these methods are used, the following problem still remains.

[0006] If the input level to the optical fiber is reduced, the signal-to-noise ratio (S / N ratio) on the receiving side is reduced, so that errors in detection increase. This can also be interpreted as shortening the transmittable distance. Since an optical fiber having a large mode field diameter has a large dispersion slope (wavelength dependence of dispersion), it is difficult to set an optimum dispersion for all wavelength-multiplexed channels. In optical soliton communication, due to perturbations (such as propagation loss and dispersion fluctuation) existing in an actual optical transmission line, a component of a dispersed wave out of the soliton condition is generated, and this causes deterioration of transmission characteristics. As described above, the current optical communication system must be designed with careful consideration of some limiting factors, but if there is no loss in the optical fiber that is the optical transmission line, these limitations will be reduced. Significantly relaxed.
For example, in the lossless transmission line, since the deterioration of the S / N ratio due to the propagation loss is eliminated, the limiting factor related to lowering the input level to the optical fiber is reduced. Further, when a lossless transmission line is applied to optical soliton communication, the generation of dispersed waves is significantly reduced. As a conventional optical transmission line closest to such a lossless transmission line, there is an optical transmission line whose loss has been compensated by Raman amplification.

The Raman amplification system using Raman scattering of an optical fiber has an advantage that an optical transmission line itself becomes an amplification optical fiber and an advantage that an arbitrary wavelength band can be amplified. In the case of a silica-based optical fiber, a gain peak appears in a frequency band that is longer than the wavelength of the pump light and lower in frequency by about 13 THz. For example, wavelengths 1450 nm and 154
The difference between the frequencies of 7 nm is 13 THz. The wavelength difference or frequency difference between the pump light and the gain peak is called Raman shift, and is a value according to the composition of the optical fiber.

In general, a Raman amplification system for communication employs a backward pump configuration in which pump light and signal light propagate in opposite directions as shown in FIG. Since the Raman gain generation mechanism is very fast, in a forward pump configuration in which the pump light and the signal light propagate in the same direction, the fluctuations in the intensity of the pump light are superimposed on the signal waveform as it is, and the transmission characteristics deteriorate significantly. Because. This is also disclosed in paragraphs 0027 to 0030 of JP-A-9-318981.

The general characteristics of the intensity distribution in the longitudinal direction of the pumping light and the signal light in the amplification optical fiber in the Raman amplification system employing the conventional backward pumping structure are shown in FIGS.
FIG. (These calculations are based on GPAgrawal's “Nonl
inear Fiber Optics ”, Chap. 8, Academic Press, RGSm
ith, Applied Optics, Vol. 11, pp. 2489-2494, 1972, J.
Auyeung and A. Yariv, J. Quantum Electron., Vol.QE-14,
pp. 347-352, 1978).

Raman amplifiers to which the Raman amplification method is applied include a distributed type using an optical transmission line as an amplification optical fiber and a centralized type using an amplification optical fiber separately from the optical transmission line. The following description shows an example that assumes a distribution type.
However, even in the case of the centralized type, since the behavior of the signal light and the pumping light in the amplification optical fiber is described by the same equation, the same effect can be obtained only by changing the values of various parameters.

FIG. 22 is a graph showing a change in pump light power and a change in signal light power. The curve a in this graph indicates the signal light power when the incident power of the pump light is 100 mW (curve A), and the curve b indicates that the incident power of the pump light is 20 mW.
The signal light power at 0 mW (curve B) is shown, and the curve c
Indicates the signal light power when the incident power of the pump light is 300 mW (curve C). The straight line d in the graph indicates the signal light power when the pump light is not incident. Line d in the graph
As is clear from FIG. 7, the signal light power attenuates in proportion to the propagation distance unless pumping light is incident. Damping constant is 0.25d
When the pumping light of B / km is incident, Raman amplification occurs, and the signal light power increases. This increase in power is the Raman gain. Curves A to C and curve a in the graph shown in FIG.
As is clear from the relationship between .gamma.-c, the magnitude of the Raman gain is almost proportional to the incident power of the pump light. Since the pumping light also attenuates in proportion to the propagation distance, the pumping light intensity decreases near the signal input end and the Raman gain also decreases. Therefore, the signal light is attenuated at a position near the input terminal, and receives a large gain near the output terminal (the end where the pump light is incident: 50 km from the input terminal in this example). When the signal light intensity is sufficiently small, the attenuation of the pump light is due to the propagation loss, and therefore the longitudinal intensity distribution of the pump light is uniquely determined by the attenuation constant of the amplification optical fiber. The longitudinal distribution of the Raman gain (the longitudinal intensity distribution of the signal light) is determined according to the longitudinal intensity distribution of the pump light.

FIG. 23 is a graph showing the signal light power when the attenuation constant αs of the signal light is changed when the pumping light incident power is constant and its attenuation constant αp is also constant at 0.25 dB / km. . Curve A in this graph is the attenuation constant α
When s = 0.3 dB / km, curve B has a damping constant αs = 0.2
In the case of 5 dB / km, the curve C has an attenuation constant αs = 0.2 dB.
/ Km. As is clear from the graph shown in FIG. 23, the intensity distribution of the signal light in the longitudinal direction of the optical transmission line changes according to the attenuation constant αs of the signal light.

FIG. 24 is a graph showing a change in signal light power when the pump light attenuation constant αp is changed when the pump light incident power is constant and the signal light attenuation constant αs is constant. The curve a in this graph shows the signal light power when the pump light attenuation constant αp = 0.3 dB / km (curve A), and the curve b shows the signal light power when the attenuation constant αp = 0.25 dB / km (curve B). The signal light power is shown, and the curve c shows the signal light power when the attenuation constant αp = 0.2 dB / km (curve C). As is clear from the graph shown in FIG. 24, when the attenuation constant αp of the pumping light is changed, the intensity distribution of the pumping light in the longitudinal direction of the optical transmission line is different, so that the intensity distribution of the signal light in the same direction is changed.

FIG. 25 is a graph showing the relationship between the pump light power and the signal light power when the length of the amplification optical fiber is changed. In this case, the incident power of the pump light is constant, and the attenuation constant of the signal light and the pump light is the same value. From this graph, it can be seen that, when the length of the amplification optical fiber is changed, the intensity distribution of the pump light in the longitudinal direction of the fiber is different, so that the intensity distribution of the signal light in the same direction is also changed. Curves A to E in the graph shown in FIG. 25 indicate that the lengths of the amplification optical fibers are 10 km, 20 km, 30 km, 40 km, and 50 km.
km shows the pump light power, and curves a to e show the signal light power when the amplification optical fibers have the respective lengths.

FIG. 26 is a graph showing a change in signal light power when the pump light incident power is constant, the signal light and the pump light have the same attenuation constant, and the signal light has a different Raman gain coefficient g _R. The curve a in this graph is a gain coefficient g _R = 1/3.
Signal light power at × 10 ⁻¹³ m / W, curve b is signal light power at gain coefficient g _R = ^{と き} × 10 ⁻¹³ m / W, and curve c is gain coefficient g _R = 1 × 10 ^It shows the signal light power at ^-13 m / W. As is clear from this graph, when the magnitude of the Raman gain coefficient g _R is changed, the magnitude of the generated Raman gain changes, and the intensity distribution of the signal light in the longitudinal direction of the optical transmission line changes.

FIG. 27 is a graph showing a change in signal light power when forward pumping and backward pumping are performed with pump light having a constant incident power. A curve a in this graph shows the signal light power when pumping forward, and a curve b shows the signal light power when pumping backward. The value of g _R shown here is a value when the excitation wavelength is 1 μm. As is clear from this graph, when the pumping configuration is different, the intensity distribution of the pumping light in the longitudinal direction of the optical transmission line is different, so that the distribution of the generated Raman gain changes and the distribution of the signal light changes.
The curve A in the graph indicates the power of the forward pumping light, and the curve B indicates the power of the backward pumping light.

As a general behavior of noise characteristics of a transmission system using an optical amplifier, it is known that a signal loss before optical amplification significantly degrades noise characteristics. Therefore, when the amplification effect has a distribution in the longitudinal direction of the optical fiber as in an optical fiber amplifier, a loss at a position near the input end of the amplifier deteriorates noise characteristics. On the other hand, in the case of the backward pumping configuration, the pumping light intensity at the signal light input end is reduced due to the propagation loss in the amplification optical fiber, so that the amplifying action at the signal light input end is also reduced. Therefore, in the backward pumping configuration, the loss at the signal light input terminal becomes relatively large, and there is a problem that the noise characteristic of the amplifier is deteriorated. Therefore, in order to construct a Raman amplifier having good noise characteristics, it is customary to use an optical fiber with the smallest possible loss (for both signal light and pump light) and use an optical fiber as short as possible. there were.

On the other hand, in the case of a distributed Raman amplifier using an optical transmission line as an amplification optical fiber, it is necessary to maintain a high S / N ratio while suppressing a nonlinear effect in the optical transmission line. Therefore, it is ideal that the optical transmission line has a state of a lossless transmission line where the level of the signal light in the longitudinal direction of the transmission line is constant. In this case, the intensity of the pump light is also constant in the longitudinal direction of the optical transmission line. Is desirable. However, the distance that can be realized by the conventional technology is relatively short, and the distance is uniquely determined by the parameters (fiber length, gain coefficient, attenuation constant of pump light and signal light) of the optical fiber constituting the optical transmission line. . This is because the distribution of the excitation light in the longitudinal direction of the optical transmission line could not be controlled.

[0019]

SUMMARY OF THE INVENTION An object of the present invention is to provide a Raman amplification system capable of improving the noise characteristics of an amplifier by controlling the intensity distribution of pump light in the longitudinal direction of an optical transmission line. Compared to the Raman amplification method
An object of the present invention is to provide a Raman amplification method capable of realizing a state closer to a lossless transmission path. Another object of the present invention is to provide an optical signal transmission method using the Raman amplification method.

The first Raman amplification system of the present application is a Raman amplification system utilizing Raman scattering in an optical fiber, and a first pumping light for Raman-amplifying a signal light is supplied to a signal of an amplification optical fiber. A second pump light having a wavelength shorter than the wavelength of the first pump light and a second pump light for Raman-amplifying the first pump light is incident from a signal light input terminal of the optical fiber. .

In the second Raman amplification method of the present application, the second pump light is also incident from the signal light output end of the amplification optical fiber.

In the third Raman amplification method of the present application, the first pump light is also incident from the signal light input end of the amplification optical fiber.

In the fourth Raman amplification method of the present application, the wavelength of the second pumping light is shorter than the wavelength of the first pumping light by the Raman shift of the amplification optical fiber.

In the fifth Raman amplification system of the present application, the Raman amplification band of the second pump light and the wavelength band of the signal light do not overlap.

According to the sixth Raman amplification method of the present application, the wavelength of the second pump light is set to a wavelength slightly shifted from the wavelength shorter than the wavelength of the first pump light by the Raman shift of the amplification optical fiber. Things.

In the seventh Raman amplification system of the present application, one or both of the first and second pump lights are used as a wavelength division multiplex pump light source having a plurality of wavelengths.

The eighth Raman amplification method of the present application uses a semiconductor laser as a light source of the first excitation light.

The ninth Raman amplification method of the present application is to guide third pump light to an optical transmission line and Raman amplify the second pump light.

The first optical signal transmission method of the present application is to propagate an optical signal at a substantially constant level in the longitudinal direction of an optical transmission line by using any one of the first to ninth Raman amplification methods. It is.

The second optical signal transmission method of the present application uses a wavelength multiplexed soliton signal as signal light.

[0031]

(Embodiment 1) A first embodiment of the Raman amplification system of the present invention will be described in detail with reference to FIG. The optical fiber 1 shown in FIG. 1 is an optical fiber of an arbitrary type constituting an optical transmission line or a centralized optical amplifier, for example, SMF, DSF, NZ-D used as an optical transmission line.
SF, LEAF, RDF, dispersion compensating fiber, non-linear device fiber, and the like. In the optical communication system using the optical fiber 1, when the signal light (wavelength λ _S ) propagates through the optical fiber 1, the first pump light (wavelength λ _P1 ) enters from the output end 2 of the signal light, The second pumping light (wavelength λ _P2 ) enters from the input end 3 of the signal light. This allows
Both the first pumping light and the second pumping light are present in the optical fiber 1, the first pumping light is subjected to Raman amplification by the second pumping light and is amplified, and the first pumping light near the signal input terminal 3 is amplified. Is higher than when no second excitation light is present.
Therefore, S / N due to propagation loss on the signal incident end 3 side
The deterioration of the ratio is reduced, and the noise characteristics of the transmission system are improved.
Further, the second pump light enters from the input end 3 of the signal light, so that the second pump light enters from the output end 2 of the signal light.
The intensity of the second pumping light near the signal light input terminal 3 increases, and the gain of the first pumping light from the second pumping light near the signal light input terminal 3 can be increased efficiently. The noise characteristics of the device are also efficiently improved.

In the present embodiment, the wavelength λ _P2 of the second pump light is shorter than the wavelength λ _P1 of the first pump light by the Raman shift of the optical fiber 1 (amplifying optical fiber 1). When both wavelengths have this relationship, the first pump light is most efficiently Raman-amplified by the second pump light. However, the second wavelength wavelength lambda _P2 of the excitation light is the first excitation light lambda _P1
Since amplification is possible with a shorter wavelength, the wavelength λ _P2 of the second pump light is limited to a wavelength shorter than the wavelength λ _P1 of the first pump light by the Raman shift of the amplification optical fiber 1. No need. For example, the wavelength may be slightly shifted from the wavelength shorter than the wavelength λ _P1 of the first pumping light by the Raman shift of the amplification optical fiber 1. With such a slight shift, the gain coefficient of the Raman gain that the first pumping light receives from the second pumping light can be arbitrarily reduced. For example, from a wavelength shorter by Raman shift to 20 to 30
When shifted by about nm, the gain coefficient becomes about half. This serves as means for controlling the intensity distribution of the first pumping light in the longitudinal direction of the amplification optical fiber 1, and can be used to optimize noise characteristics when the input / output level of the amplifier is specified. it can.

As described in the Background Art section, in the forward pumping configuration in which the pumping light and the signal light propagate in the same direction, the intensity fluctuation of the pumping light is superimposed on the signal waveform as it is, and the transmission characteristics are remarkably reduced. Since the light deteriorates, it is desirable that the pump light that generates the Raman gain propagates in the opposite direction to the signal light. Therefore, in the present embodiment, the first pumping light is made incident from the output end 2 of the signal light (in the opposite direction to the signal light) to suppress the deterioration of the transmission characteristics.

When the second pumping light propagates in the same direction as the signal light as in the present embodiment, if the gain band of the second pumping light and the signal light band overlap, the intensity fluctuation of the pumping light causes signal fluctuation. It is superimposed on the waveform as it is, and the transmission characteristics are significantly deteriorated. Therefore, in the present embodiment, the deterioration of the transmission characteristics is suppressed by preventing the gain band of the second pump light propagating in the same direction from overlapping with the band of the signal light. For example, the wavelength of the second pump light and the wavelength of the signal light may be separated by about 20 THz.

It is known that in a transmission system using an optical amplifier, if an optical signal undergoes a loss equal to or greater than the amplification value after being amplified, the noise characteristics are significantly degraded, as in the case where the loss occurs before amplification. Have been. (Published by Ohmsha, Ishio et al., Optical Amplifiers and Their Applications, p. 26). Therefore, when the optical fiber for amplification is relatively long like a Raman amplifier, it is desirable from the viewpoint of noise characteristics to keep the signal light level in the fiber as high as possible. When the input and output levels of the signal light are similar, such as in a multistage relay using a distributed amplifier, the signal light level over the entire optical transmission line must be maintained at the same level or higher than the input and output levels. Is desirable. For example,
GPAgrawal's “NonlinearFiberOptics”, Chap. 8, Acad
As shown in emicPress, the differential coefficient of the signal light intensity in the propagation direction is represented by the following equation, so that the gain coefficient (g _R ) and the loss coefficient (α _S : attenuation coefficient of the signal light) of the amplification fiber.
Is given, the differential coefficient of the signal light intensity in the propagation direction is set to 0.
Is obtained as I _P = α _S / g _R. Therefore, if the excitation light intensity is smaller than α _S / g _R ,
The signal light attenuates as it propagates and increases if it is greater than α _s / g _R. Therefore, it is ideal that the pumping light intensity is α _S / g _R at all positions in the optical transmission line. However, in practice, it is practical to keep the excitation light intensity within a certain range around α _S / g _R. It is a target. By doing so, the level of the signal light in the longitudinal direction of the optical transmission line can be kept close to a constant state, and good noise characteristics can be obtained.

[0036]

(Equation 1)

FIG. 2 shows the intensity distribution of the first excitation light in the longitudinal direction of the optical fiber 1 when the second excitation light is incident and when it is not incident in the Raman amplification method of the present invention shown in the first embodiment. 9 is a graph showing the result of comparing.
When the second pump light is not incident from this graph, the intensity distribution of the first pump light is attenuated exponentially due to the propagation loss of the optical fiber, but when the second pump light is incident, It can be seen that since the first pump light is amplified, the intensity of the first pump light at the signal light incident end is increasing.

FIG. 3 shows a comparison of the signal light intensity distribution in the longitudinal direction of the optical fiber 1 between the case where the second pumping light is incident and the case where the second pumping light is not incident in the Raman amplification system of the present invention shown in the first embodiment. It is a graph which shows a result. According to this graph, the presence of the second pump light has a higher signal light intensity near the signal light input end than the case where the second pump light does not exist, and the noise characteristics are improved. I can hear that.

FIG. 4 shows the case where the second pumping light enters from the input end 3 (FIG. 1) of the signal light and from the output end (FIG. 1) in the Raman amplification system of the present invention shown in the first embodiment. 6 is a graph showing a result of comparing the intensity distribution of the first excitation light in the longitudinal direction of the optical fiber 1 (FIG. 1) with the case. According to this graph, the intensity of the second pumping light near the input end 3 can be larger when the second pumping light enters from the input end 3 of the signal light than when it enters from the output end 2. It can be seen that the gain that the first pumping light receives from the second pumping light is large and the intensity of the first pumping light is large in the vicinity.

FIG. 5 shows the case where the second pumping light is incident from the input end 3 (FIG. 1) of the signal light and from the output end 2 (FIG. 1) in the Raman amplification system of the present invention shown in the first embodiment. 7 is a graph showing the results of comparing the intensity distribution of signal light in the longitudinal direction of the optical fiber 1 (FIG. 1) with the case where the optical fiber 1 (FIG. 1) is used.
According to this graph, the intensity of the signal light near the input terminal 3 is higher when the second pump light is incident from the input end 3 of the signal light than when the second pump light is incident from the output end 2, and the noise characteristics are better. Can be heard.

FIG. 6 shows that the combination of the incident powers of the first pump light and the second pump light is changed under the condition that the gain of the signal light is constant in the Raman amplification method of the present invention shown in the first embodiment. 6 is a graph showing a change in signal light power in the case of the above. Curve A in the graph shown in FIG. 6 indicates that the incident power of the first pumping light is 100 mW and the incident power of the second pumping light is 1100 mW. Curve B indicates that the incident power of the first pumping light is 150 mW. The incident power of the second pump light is 370 m
In the case of W, the curve C shows that the incident power of the first pump light is 200
mW and the case where the incident power of the second pump light is 0 mW. From this graph, it is possible to control the minimum level of the signal light in the middle of the optical fiber by adjusting the power of both the first pumping light and the second pumping light, and by setting the minimum level higher, It can be seen that noise characteristics can be improved.

(Embodiment 2) A Raman amplification system according to a second embodiment of the present invention will be described in detail with reference to FIG. The basic configuration of this embodiment is the same as that of the first embodiment.
The difference is that the second pump light (wavelength λ _p2 ) is incident not only from the input end 3 of the signal light but also from the output end 2 as shown in FIG. In this case, similarly to the first embodiment,
The second wavelength lambda _P2 of the excitation light or the first wavelength shorter than lambda _P1 of the excitation light, the first Raman shift of the optical fiber 1 than the wavelength lambda _P1 of the excitation light (the amplification optical fiber 1) The wavelength can be shorter than the wavelength of the first pumping light, or can be a wavelength slightly shifted from the wavelength shorter than the wavelength λ _P1 of the first pumping light by the Raman shift of the amplification optical fiber 1.

When the second pumping light (wavelength λ _p2 ) is also incident from the output end 2 of the signal light, the intensity distribution of the first pumping light (wavelength λ _p1 ) in the longitudinal direction of the optical fiber 1 is controlled. The degree of freedom is increased, and the intensity distribution of the signal light in the longitudinal direction of the optical fiber 1 can be easily controlled. This method is effective when optimizing noise characteristics under conditions where the input and output levels of signal light are defined, such as in a multistage relay using a distributed amplifier.

FIG. 8 is a graph showing the intensity distribution of the signal light in the longitudinal direction of the optical fiber 1 shown in FIG. 7, and the curve a in the graph represents the signal light in the optical fiber 1 amplified by the conventional Raman amplification method. The curve b shows the intensity distribution of the same signal light when the signal light is amplified by the Raman amplification method of the present invention shown in the first embodiment, and the curve c shows the intensity distribution of the same signal light in the second embodiment. 5 shows an intensity distribution of the signal light when the signal light is amplified by the Raman amplification method of the present invention. From this graph, it can be seen that the use of the Raman amplification method shown in the first embodiment has a higher minimum signal light level in the middle of the optical fiber 1 than the case of using the conventional amplification method, so that the noise is higher. It can be seen that the characteristics are good. In addition, when the Raman amplification method described in the second embodiment is used, the minimum level of the signal light is maintained at a higher level than in the case where the Raman amplification method described in the first embodiment is used. It can be seen that the characteristics are further improved. In the Raman amplification system shown in the first embodiment, the minimum level of the signal light can be made the same as that in the Raman amplification system shown in the second embodiment by adjusting the intensities of the first and second pump lights. it can. However, in the example shown in FIG. 9, it can be said that the case where the Raman amplification method shown in the second embodiment is used is more efficient because the pump power required in total is smaller. FIG. 9 is a graph showing a comparison result of the signal light power between the case where the Raman amplification method according to the first embodiment is used and the case where the Raman amplification method according to the second embodiment is used. A curve a in this graph shows the signal light power when the first pump light is 100 mW and the second pump light is 1100 mW in the Raman amplification method shown in the first embodiment, and a curve b shows the second embodiment. In the Raman amplification method, the first excitation light is 25 mW,
The signal light power when the second pump light is 300 mW × 2 is shown.

FIG. 10 shows a Raman amplification method according to the second embodiment.
The input and output levels and the minimum level of the signal light are the same.
Under one condition, the wavelengths of the first excitation light and the second excitation light
In the longitudinal direction of the optical fiber when the spacing is different
9 is a graph showing the result of comparing the intensity distribution of signal light.
The curve a in this graph is the wavelength λ of the first excitation light._p1Is 14
50 nm, wavelength λ of second excitation light_p2Is 1350nm
Curve (when both wavelength intervals are Raman shifts), the curve b is the first
Excitation light wavelength λ_p1Is 1450 nm, the second excitation light wave
Long λ_p2Is 1325 nm (both wavelength intervals are Raman shift
(If it deviates from the right). First excitation from this graph
Shift the wavelength interval between light and second pump light from Raman shift
This can reduce the maximum level of signal light.
Minimizes degradation of transmission characteristics due to nonlinearity in optical fiber.
You can see that it can be done. The reason for this is that the first excitation light
The longitudinal direction of the optical fiber because the gain coefficient received is small
The intensity distribution of the first excitation light at
The intensity distribution of signal light in the longitudinal direction of the fiber is also gentle.
Because it becomes. FIG. 11 is a diagram showing a second example under the conditions of FIG.
5 is a graph showing an intensity distribution of one excitation light, and a curve a is
Wavelength of one excitation light λ _p1Is 1450 nm and the second excitation light
Wavelength λ_p2Is 1350 nm, the first excitation light
The curve b shows the wavelength λ of the first excitation light._p1Is 1
450 nm wavelength λ of the second excitation light_p2Is 1325 nm
5 shows the intensity distribution of the first excitation light in the case of FIG. This group
From the rough, the wavelength interval between the first pump light and the second
The optical fiber length
The intensity distribution of the first excitation light in the direction
I understand.

In the Raman amplification method according to the second embodiment, the second pump light having the same wavelength (λ _p2 ) is input from the input terminal 3 (FIG. 7) and the output terminal 2 (FIG. 7) of the signal light with the same power. However, the two second pump lights incident from the input end 3 and the output end 2 of the signal light need not necessarily have the same wavelength and the same power. These can be appropriately adjusted according to the intensity distribution of the signal light to be realized in the longitudinal direction of the optical fiber.

(Embodiment 3) A third embodiment of the Raman amplification system of the present invention will be described in detail with reference to FIG. The basic configuration of this embodiment is the same as that of the second embodiment. The difference is that the first pump light (wavelength λ
_p1 ) is incident not only from the output end 2 of the signal light but also from the input end 3. Also in this case, Embodiments 1 and 2
Similarly, the wavelength λ _p2 of the second pump light may be shorter than the wavelength λ _p1 of the first pump light, or the wavelength λ _{p1 of} the first pump light.
_The wavelength is shorter than _p 1 by the Raman shift of the optical fiber 1 (amplifying optical fiber 1), or slightly shifted from the wavelength shorter than the wavelength λ _p1 of the first pump light by the Raman shift of the amplifying optical fiber 1. Wavelength.

When the first pumping light (wavelength λ _p1 ) also enters from the input end 3 of the signal light, the first pumping light (wavelength λ _p1 ) and the second pumping light (wavelength λ _p1 ) in the longitudinal direction of the optical fiber 1. λ
_Since the intensity distribution of _p2 ) can be made more uniform, it is easy to realize a state close to a lossless transmission path. However, since the first pump light propagates in the same direction as the signal light, the input end 3
It is necessary to use a semiconductor laser or the like whose intensity noise is sufficiently small as a light source of the first excitation light incident from the light source.

FIG. 13 shows the configuration of the prior art closest to the Raman amplification method shown in the third embodiment. In the Raman amplification method shown in FIG. 13, since the first pump light propagates in the same direction as the signal light in the third embodiment as in the Raman amplification method of the present invention,
It is necessary to use a semiconductor laser or the like having sufficiently low intensity noise as a light source of the first excitation light incident from the input end 3.

FIG. 14 shows the light when the conventional Raman amplification method is used, when the Raman amplification method according to the present invention shown in the second embodiment is used, and when the Raman amplification method according to the present invention shown in the third embodiment is used. 3 is a graph showing an intensity distribution of signal light in a longitudinal direction of the fiber 1; In any case, the power of the pump light is optimized so that the level fluctuation width of the signal light is minimized. As is clear from this graph, the level variation width of the signal light in the longitudinal direction of the optical fiber 1 can be significantly reduced by using the Raman amplification method of the present invention shown in the third embodiment.

FIG. 15 is a graph showing the intensity distribution of the first pump light in the longitudinal direction of the optical fiber under the conditions of FIG. According to the graph, the level fluctuation of the signal light in the Raman amplification method according to the third embodiment is small because
It can be seen that this is because the level fluctuation of the first excitation light is small.

FIG. 16 is a graph comparing the difference in the relay distance when the level fluctuation width of the signal light is set to be substantially the same. From this graph, in order to realize the same fluctuation width as the level fluctuation width of the signal light (FIG. 14) realized in the Raman amplification method of the present invention shown in the third embodiment, the relay distance is set to 20 km in the conventional Raman amplification method. It can be seen that the Raman amplification method of the present invention shown in Embodiment 2 also needs to be reduced to 25 km.

(Embodiment 4) In order to optimize the noise characteristics, it is desirable that the pump light of the Raman amplifier has a high output, and it is more convenient if the first and second pump lights have a large power range. In this case, one or both of the pump lights can be obtained from a wavelength division multiplex pump light source composed of a pumping semiconductor laser having a plurality of oscillation wavelengths. (Reference: Y. Emori et al., Electronics Letters, vol. 34, pp. 214)
5-2146, 1998). When such a pump light source is used, it is necessary to select the wavelength of the pump light source such that the peak wavelength of the gain and the wavelength of the signal light to be amplified match. For example, 14
When an excitation light source that multiplexes 35 nm, 1450 nm, 1465 nm, and 1480 nm is used, the gain peak wavelength is around 1570 nm. Therefore, the wavelength band is set to include the wavelength of the signal light.

When a wavelength division multiplexing pump light source is used, a phenomenon occurs in which the effective gain coefficient changes in the longitudinal direction of the optical fiber. This is because the gain coefficient at the signal light wavelength is the sum of the coefficients due to the respective pumping lights, but since the respective pumping lights have different attenuation rates, the behavior is different from the case where no multiplexing is performed. The reasons for the different attenuation factors are the wavelength dependence of the loss coefficient and the Raman amplification effect between the pump lights. Therefore, by appropriately selecting the wavelength to be multiplexed, the distribution of the gain coefficient in the longitudinal direction of the optical fiber can be controlled.

(Embodiment 5) When wavelength-division multiplexed pump light is used as first pump light for Raman amplification of signal light,
Since the Raman effect occurs between the pump lights, the shorter wavelength pump light has a higher attenuation rate than the longer wavelength pump light (H. Kidorf
et al., Photonics Technology Letters, pp.530-532,
Vol.11, see Fig.5). For this reason, the noise characteristic of the wavelength band receiving the gain from the short-wavelength pump light is worse than that of the wavelength band receiving the gain from the long-wavelength pump light. For example, when Raman-amplifying the C band (1530 nm-1565 nm) and the L band (1570 nm-1610 nm) simultaneously, excitation light having a wavelength of around 1450 nm is used.
Although the pumping light of about 1490 nm is used, the attenuation rate of the pumping light of 1450 nm is higher than that of the pumping light due to the Raman effect between the pumping lights, and the noise characteristic is worse in the C band. At this time, by setting the wavelength of the second pumping light so that the gain for the pumping light of 1450 nm becomes larger than the gain for the pumping light of 1490 nm, the noise characteristic improvement effect of the C band is improved. It can be larger than the effect. as a result,
It is possible to prevent a difference in noise characteristics from occurring between the C band and the L band.

As shown in FIG. 17, the wavelength of the second pumping light is set to be slightly shorter than the wavelength shorter than the wavelength of 1450 nm by the Raman shift of the amplification optical fiber.
The gain received by the pump light of 1490 nm is smaller than the gain received by the pump light of nm (see FIG. 18). Further, a Raman amplifier using a wavelength division multiplexing pump light source can realize an arbitrary gain shape by appropriately setting the wavelength interval and the wavelength arrangement. Therefore, by using the wavelength multiplexed pump light source as the second pump light, the gain shape for the wavelength band of the first pump light can be freely set, and as a result, the signal amplified by the first pump light It is possible to control the wavelength dependence of the band noise characteristics. FIG. 19 shows an example of the gain shape with respect to the wavelength band of the first pumping light when a wavelength multiplexing pumping light source is used as the second pumping light. In FIG. 19, the gain difference between the 1450 nm pump light and the 1490 nm pump light is smaller than in FIG. In particular, when the first pumping light is multiplexed at a relatively narrow wavelength interval (for example, about 15 nm), if the gain shape can be adjusted by the second pumping light, the wavelength dependence of the noise characteristics of the signal band is reduced. More fine control is possible. This is because the gain profile of the signal band is formed by adding gain profiles obtained from a plurality of pump lights.

(Embodiment 6) In order to suppress the level fluctuation of the signal light, the level fluctuation of the first pumping light may be reduced as much as possible. In the conventional Raman amplification method, the level fluctuation of the first pump light is caused by the propagation loss, and cannot be controlled. In the embodiment of the Raman amplification method of the present invention described so far, the effective loss of the first pump light is reduced by using the Raman gain given to the first pump light by the second pump light. Thus, the level fluctuation of the first excitation light is made smaller than that of the conventional method. However, the level fluctuation of the second pump light is still due to the propagation loss, which is not controlled. Therefore, if a third pumping light for Raman amplification of the second pumping light is introduced, the level fluctuation of the second pumping light can be further reduced. The effect of reducing the fluctuation is expected.

(Embodiment 7) It is well known that soliton in a lossless optical transmission line maintains its waveform not only in the case of a single signal light wavelength but also in the case of a wavelength multiplex system. It has been known. (References: For example, LFMollenauer et al., Soliton propagatio
n in long fibers with periodically compensated los
s, Journal of Quantum Electronics, Vol.QE-22, No. 1,
1986) In addition, it has been proved theoretically and experimentally that a soliton of a single wavelength is transmitted as a soliton even if a lossy transmission line is subjected to loss compensation by direct optical amplification. (Reference: A. Hasegawa, Numerical stud
y of optical solution transmission amplified perio
dically by the stimulated Raman process, Applied O
ptics, Vol.23, No.19, pp.3302-3309,1984, LFMolle
nauer et al., Experimental demonstration of solutionprop
agation in long fibers: loss compensated by Raman
gain, Optics Letters, Vol.10, No.5, pp229-231, 198
5, U.S. Pat. No. 4,558,921) It was once believed that this was also possible with wavelength multiplex systems (references: Patent No. 2688350, LFMollenauer et al., Wavelength division).
multiplexing with solutions inultra-long distance
transmission using lumped amplifiers, Journal of
Lightwave Technology, Vol. 9, No. 3, 1991), and later it became clear that soliton communication would not be realized without proper settings in a wavelength division multiplexing system. (Reference: PVMamyshev et al., Pseudo-phase-matched four-wa
ve mixing in solution wavelength-division multiple
xing transmission, Optics Letters, Vol.21, No.6, 1
996) Further, there is a technique called dispersion compensation soliton as a kind of soliton, and this technique is widely used because practical wavelength multiplex transmission is possible. However, unlike a pure soliton, a pulse spreads in the middle of propagation, so that a non-linear effect caused by the overlap of preceding and following bits causes irreversible waveform distortion.

Known documents indicate that pure wavelength-division multiplexed soliton communication is possible by using an optical transmission line whose dispersion is reduced according to the loss of the optical transmission line. However, such a technique is not practical because existing optical transmission lines cannot be used, and dispersion control of the optical transmission lines is complicated and difficult.

On the other hand, it is expected that pure wavelength-division multiplexed solitons can be propagated by realizing an optical transmission line very close to a lossless transmission line by using distributed amplification such as Raman amplification. . However, in order to realize such an optical transmission line at a conventional technical level, it is not practical because pumping light sources must be arranged at relatively short distances.
As described above, by using the Raman amplification method of the present invention, an optical transmission line closer to a lossless transmission line can be realized over a longer distance than in the related art.
Therefore, it is expected that the wavelength multiplexing soliton communication using this optical transmission line will have significantly improved transmission characteristics as compared with the conventional one. FIG. 20 is an example of a wavelength division multiplexing soliton communication system using an optical transmission line to which the Raman amplification method of the present invention is applied according to the third embodiment. In this system, how close the optical transmission line must be to the lossless transmission line,
That is, since the maximum distance of the transmission path section is determined by how much the allowable value of the level fluctuation of the signal light is given, one section is configured with a shorter distance, and the sections are connected to construct the entire system.

[0061]

The first Raman amplification method according to the present invention has the following effects. (1) Since the first pumping light and the second pumping light are present simultaneously in the amplification optical fiber, the first pumping light is subjected to Raman amplification by the second pumping light, and the second pumping light is The intensity of the first pumping light near the signal input end is larger than in the case where no signal is present, the deterioration of S / N on the signal input side is improved, and the noise characteristics of the transmission system and the amplifier are improved. (2) The power distribution of the first pump light and the second pump light, the wavelength interval, and the pump configuration are appropriately selected according to the length of the amplification optical fiber and the attenuation constant of the signal light and each pump light, and By arbitrarily controlling the longitudinal intensity distribution of the first excitation light for Raman-amplifying the light, the noise characteristics can be optimized. (3) Since the second pumping light enters from the input end of the signal light, the second pumping light intensity near the signal input end is reduced as compared with the case where the second pumping light enters from the output end of the signal light. Can be larger. Therefore, the gain of the first pumping light received from the second pumping light near the signal input terminal can be efficiently increased, and the noise characteristic of the Raman amplifier can be efficiently improved.

The second Raman amplification method according to the present invention has the following effects in addition to the above effects. That is, since the second pumping light is also incident from the output side of the signal light, the degree of freedom in controlling the intensity distribution of the first pumping light in the longitudinal direction of the optical fiber is increased, and the signal in the longitudinal direction of the optical fiber is increased. It becomes easier to control the light intensity distribution. This is effective for optimizing noise characteristics under conditions where the input and output levels of signal light are defined, such as in a multistage relay using a distributed amplifier.

The third Raman amplification method according to the present invention has the following effects in addition to the above effects. (1) Since the first pumping light for Raman-amplifying the signal light also enters from the signal-light input end of the amplifying optical fiber, the vicinity of the signal-light input end is lower than in the first and second Raman amplification systems. Since the intensity of the first pumping light can be easily increased, the noise characteristics of the Raman amplifier can be more efficiently improved. (2) Since the first and second pumping lights are incident from both the input end and the output end of the signal light, the first pumping light in the longitudinal direction of the optical fiber is different from the first and second Raman amplification systems. Since the light intensity distribution can be made more uniform, it is easy to realize a state close to a lossless transmission path.

The fourth Raman amplification method according to the present invention has the following effects in addition to the above effects. That is, since the wavelength of the second pumping light is shorter than the wavelength of the first pumping light by the Raman shift of the amplification optical fiber, the Raman gain that the first pumping light receives from the second pumping light is reduced. It becomes maximum, and the noise characteristic is improved efficiently.

The fifth Raman amplification method of the present invention has the following effects in addition to the above effects. That is, since the Raman amplification band of the second pump light and the wavelength band of the signal light do not overlap, the intensity fluctuation of the second pump light propagating in the same direction as the signal light is superimposed on the signal waveform. , Noise is less likely to occur, and transmission characteristics are improved.

The sixth Raman amplification method of the present invention has the following effects in addition to the above effects. That is, the wavelength of the second pumping light is slightly shifted from the wavelength shorter than the wavelength of the first pumping light by the Raman shift of the amplification optical fiber, so that the first pumping light is The gain coefficient of Raman gain received from light can be arbitrarily reduced. By using this means, the degree of freedom in controlling the longitudinal intensity distribution of the first pump light increases, so that it is easy to optimize the noise characteristics when the input / output level of the amplifier is specified. become.

The seventh Raman amplification method according to the present invention has the following effects in addition to the above effects. That is, since multiple light sources are used for the first, second, or both pump lights, high-output pump light can be obtained, which is convenient for optimizing noise characteristics. In particular, when a wavelength-division multiplexed pump light source having a plurality of wavelengths is used for the first pump light, the wavelength dependence of signal light noise characteristics can be controlled by appropriately selecting the wavelength of the second pump light. It is possible to do. Further, when the second pumping light is a wavelength division multiplexing pumping light source, the wavelength dependence of the noise characteristic of the signal light can be controlled more freely.

The eighth Raman amplification method according to the present invention has the following effects in addition to the above effects. That is, when the first pump light and the signal light propagate in the same direction, the intensity fluctuation of the pump light becomes a fluctuation of the gain and is superimposed on the signal light, and this becomes intensity noise of the signal light and deteriorates transmission characteristics. However, since a semiconductor laser having a small intensity fluctuation is generally used for the first excitation light, this kind of intensity noise can be reduced.

The ninth Raman amplification method according to the present invention has the following effects in addition to the above effects. That is, by guiding the third pump light to the transmission line and Raman-amplifying the second pump light,
Since the effective propagation loss of the second pump light can be reduced, compared with the case where the third pump light is not incident,
The level fluctuation in the longitudinal direction of the second pumping light is reduced, and the level fluctuation in the longitudinal direction of the first pumping light and the signal light can also be reduced. Therefore, a state closer to a lossless transmission path in which the intensity of the signal light is uniform in the propagation direction can be realized.

According to the first optical signal transmission method of the present invention, an optical signal is propagated at a substantially constant level in the longitudinal direction of an optical transmission line by using the first to ninth Raman amplification methods.
Since both the degradation of the S / N ratio and the waveform degradation due to the non-linear effect can be mitigated at the same time, an optical communication system having better transmission characteristics than before can be realized.

The second optical signal transmission method of the present invention has the following effects in addition to the effects of the first optical signal transmission method. (1) In order to enable wavelength multiplex transmission using pure solitons, it is necessary to use a transmission line whose dispersion is reduced according to the loss of the transmission line. This is because the dispersion of the transmission line needs to be a value corresponding to the peak power of the soliton pulse in order to maintain the soliton waveform. on the other hand,
According to the optical signal transmission method of the present invention, since the optical signal is propagated at a substantially constant level in the longitudinal direction of the optical transmission line by using the first to ninth Raman amplification methods, it is not necessary to change the dispersion of the transmission line. . This means that pure wavelength-division multiplexing soliton communication using an existing transmission line, which has been impossible in the past, becomes possible. (2) As one kind of soliton, there is a technique called dispersion-compensated soliton capable of practical wavelength multiplex transmission. Although this technique is more resistant to changes in soliton pulse peak power than pure soliton, it is desirable for this technique that the signal light be at a substantially constant level in the longitudinal direction of the optical transmission line. is there. Therefore, the optical signal transmission method of the present invention works effectively for this technology.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the Raman amplification system of the present invention.

FIG. 2 is a diagram showing a result of comparing the intensity distribution of the first pump light in the longitudinal direction of the optical fiber when the second pump light is incident and when it is not incident in the Raman amplification method shown in FIG.

FIG. 3 is a diagram showing a result of comparing the intensity distribution of signal light in the longitudinal direction of the optical fiber when the second pump light is incident and when it is not incident in the Raman amplification method shown in FIG.

FIG. 4 is a diagram illustrating the Raman amplification method shown in FIG. 1, in which the second pump light is incident from the input end of the signal light and the incident light from the output end of the first pump light in the longitudinal direction of the optical fiber; The figure which shows the result of having compared intensity distribution.

FIG. 5 shows the intensity distribution of the signal light in the longitudinal direction of the optical fiber when the second pumping light enters from the input end of the signal light and when the second pumping light enters from the output end in the Raman amplification method shown in FIG. The figure which shows the result of comparison.

FIG. 6 is a diagram showing a change in signal light when the combination of the first pumping light and the second pumping light is changed under the condition that the signal gain is constant in the Raman amplification method shown in FIG. 1;

FIG. 7 is an explanatory diagram showing a Raman amplification system according to a second embodiment of the present invention.

8 is a diagram showing the intensity distribution of signal light in each of the Raman amplification system shown in FIG. 1, the Raman amplification system shown in FIG. 7, and the conventional Raman amplification system.

9 is a diagram showing pump power required for equalizing the minimum level of the signal light in the Raman amplification system shown in FIG. 1 and the Raman amplification system shown in FIG. 7;

FIG. 10 shows the Raman amplification method shown in FIG. 1 under the condition that the input / output level and the minimum level of the signal light are the same.
The figure which shows the intensity distribution of the signal light at the time of making the wavelength interval of a 1st pumping light and a 2nd pumping light different.

FIG. 11 is a comparison diagram of the intensity distribution of the first pumping light in the longitudinal direction of the optical fiber under the same conditions as those in FIG.

FIG. 12 is an explanatory diagram showing a third embodiment of the Raman amplification system of the present invention.

FIG. 13 is a diagram showing a conventional Raman amplification method closest to the Raman amplification method shown in FIG.

FIG. 14 is a diagram showing an intensity distribution of signal light in a longitudinal direction of the optical fiber.

FIG. 15 is a diagram showing an intensity distribution of the first pump light in the longitudinal direction of the optical fiber under the conditions shown in FIG.

FIG. 16 is a diagram illustrating a difference in a relay distance when the level fluctuation width of the signal light is set substantially the same.

FIG. 17 is an explanatory diagram showing a fifth embodiment of the Raman amplification method of the present invention.

FIG. 18 shows that the gain of one of the wavelength-multiplexed first pump lights is larger than the gain of the other pump light by appropriately selecting the wavelength of the second pump light. The figure which shows that it is possible.

FIG. 19 is a diagram showing an example of a gain shape with respect to a wavelength band of the first pumping light when a wavelength multiplexing pumping light source is used as the second pumping light.

20 is a schematic diagram of a wavelength division multiplexed soliton communication system using an optical transmission line to which the Raman amplification method shown in FIG. 12 is applied.

FIG. 21 is an explanatory diagram showing a conventional Raman amplification method.

FIG. 22 is a diagram showing a change in signal light power when a pump light is not incident, when it is incident, and when the incident power of the pump light is changed in the conventional Raman amplification method.

FIG. 23 is a diagram showing a change in signal light power accompanying a change in the attenuation constant of signal light in a conventional Raman amplification method.

FIG. 24 is a diagram showing changes in pump light and signal light accompanying a change in the attenuation constant of pump light in a conventional Raman amplification method.

FIG. 25 is a diagram showing an intensity distribution of pump light and signal light when the length of an amplification optical fiber is changed in a conventional Raman amplification method.

FIG. 26 is a diagram showing a change in signal light when a Raman gain coefficient is changed in a conventional Raman amplification method.

FIG. 27 is an explanatory diagram showing the intensity distribution of pump light and signal light in the case of forward pumping and backward pumping in a conventional Raman amplification method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Output end of signal light 3 Input end of signal light

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04J 14/02 H04B 10/00 F term (Reference) 2K002 AA02 AB30 BA01 DA10 GA10 HA23 5F072 AK06 JJ20 MM20 QQ07 YY17 5K002 BA05 BA13 CA02 CA13 DA02 DA05 FA01

Claims (11)

[Claims] 1. A Raman amplification system using Raman scattering in an optical fiber, wherein a first pump light for Raman-amplifying a signal light is incident from a signal light output end of an amplification optical fiber, and And a second pumping light having a wavelength shorter than the wavelength of the pumping light and a second pumping light for Raman-amplifying the first pumping light is input from a signal light input end of the optical fiber. 2. The Raman amplification method according to claim 1, wherein the second pumping light is also incident from a signal light output end of the amplification optical fiber. 3. The Raman amplification method according to claim 2, wherein the first pumping light is also incident from a signal light input end of the amplification optical fiber. 4. The apparatus according to claim 1, wherein the wavelength of the second pumping light is shorter than the wavelength of the first pumping light by the Raman shift of the amplification optical fiber. Raman amplification method. 5. The Raman amplification method according to claim 1, wherein the Raman amplification band of the second pump light does not overlap with the wavelength band of the signal light. 6. The method according to claim 1, wherein the wavelength of the second pumping light is a wavelength slightly shifted from a wavelength shorter than the wavelength of the first pumping light by the Raman shift of the amplification optical fiber.
A Raman amplification method according to claim 3. 7. A light source according to claim 1, wherein one or both of the first and second pumping light is a wavelength-division multiplexing pumping light source having a plurality of wavelengths. The Raman amplification method described. 8. The Raman amplification method according to claim 1, wherein a semiconductor laser is used as a light source of the first excitation light. 9. The Raman amplification method according to claim 1, wherein the third pump light is guided to the optical transmission line, and the second pump light is Raman-amplified. 10. An optical signal transmission using the Raman amplification method according to claim 1 to propagate an optical signal at a substantially constant level in the longitudinal direction of an optical transmission line. Method. 11. The optical signal transmission method according to claim 10, wherein wavelength-multiplexed soliton signals are used as signal light.