JP2000330145A - Optical repeater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decrease the noises generated in a part of optical wavelength multiplexing signals by four light wave mixing and to exceedingly increase the transmission distance by providing an optical repeater with first and second means for injecting the exciting light to generate a gain by a distributed Raman effect to the optical wavelength multiplexing signals heading toward an optical amplifier and the optical wavelength multiplexing signal transmitted from the optical amplifier. SOLUTION: This optical repeater 10 is provided with the optical amplifier 1, the first means 2 and 3 for injecting the exciting light to generate the gain by the distributed Raman effect to the optical wavelength multiplexing signal heading toward the optical amplifier 1 in a direction reverse to the signal within an optical fiber 11 connected to the input side of the optical amplifier 1 and the second means 4 and 5 for injecting the exciting light to generate the gain by the distributed Raman effect to the optical wavelength multiplexing signal transmitted from the optical amplifier 1 in the same direction as the direction of the signal within an optical fiber 12 connected to the output side of the optical amplifier 1. As a result, the output light power level of the optical amplifier 1 can be lower by lowering the amplification factor of the optical amplifier 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for optical communication.
The present invention is used for an optical wavelength division multiplexing communication system. The present invention relates to an improvement of an optical repeater that uses an optical amplifier that amplifies an optical signal and an amplifying unit that generates a gain by a distributed Raman amplification effect on an optical signal transmitted through an optical fiber connected to the optical amplifier. .

[0002]

2. Description of the Related Art In an optical wavelength division multiplexing communication system, n optical signals having different wavelengths are transmitted to one optical fiber. That is, in the transmission device, n optical signals having different wavelengths, each of which has been modulated at a high speed (for example, pulse modulated) by an information signal, are multiplexed into one by an optical multiplexer to form one optical fiber of a transmission line. Send to one end. In the receiving device, n optical signals of different wavelengths appearing at the other end of the optical fiber are separated for each wavelength by an optical wavelength demultiplexer to obtain n optical signals, and the n optical signals are demodulated respectively. To extract the information signal.

Since an optical signal is attenuated in accordance with the distance inside an optical fiber serving as a transmission line, the transmitting apparatus amplifies the optical signal by an optical amplifier, transmits the amplified optical signal as a high-power optical signal to the optical fiber, and receives the signal. Even in the apparatus, an optical signal arriving before being input to the optical wavelength demultiplexer is amplified by an optical amplifier.
When the distance of the transmission line is long, an optical amplifier can be inserted in the middle of the transmission line for relay amplification. The input signal minimum optical power level of the optical amplifier is determined from the noise level at the input end of the optical amplifier, and the output signal maximum optical power level of the optical amplifier is determined from the output distortion of the optical amplifier. The maximum length (relay interval) of a transmission line in one section in which a repeater optical amplifier must be installed is determined from the maximum optical power level of the output signal and the attenuation characteristics of the optical fiber. In the optical wavelength division multiplexing communication system, the maximum optical power level of the output signal of the optical amplifier is a function of the multiplex number n.

On the other hand, inside an optical fiber, there is a light dispersion phenomenon in which refracted light is decomposed into a spectrum. This is due to the fact that the refractive index inside the optical fiber varies slightly depending on the optical wavelength, and the light propagation speed in the optical fiber varies according to the wavelength (or frequency) of the transmitted optical signal. When pulse modulation is applied to an optical signal, the wavelength bandwidth of the optical signal is substantially widened. This is because the modulated waveform transmitted at the transmitting end does not directly reach the receiving end due to the dispersion of light. Appears as distortion. This is affected more as the modulation frequency, that is, the speed of the information signal is higher.

In order to avoid this effect, a dispersion shift optical fiber (DSF, Dispersion Shift Optical Fib) transmitting an extremely high-speed information signal (for example, 10 Gb / S) is used.
er) is known. This is devised so that the wavelength of the optical signal can be set to a wavelength at which the dispersion of light becomes small due to the properties of the optical fiber. With the current technology, for example, 1.55 μm DSF can be practically used. The wavelength-division multiplexing communication system also uses such a dispersion-shifted optical fiber to narrow the interval between the wavelengths of n optical signals as much as possible so that all of the n optical signals are intensively reduced in this optical dispersion. Being able to set to a certain wavelength is considered to be advantageous for high-speed signal transmission.

When optical wavelength division multiplexing communication is performed using this dispersion-shifted optical fiber, as described above, in addition to the input signal light power level, output signal light power level, and optical fiber transmission loss of the optical amplifier, An element occurs that limits the maximum length (relay interval) of a transmission line in one section in which a relay optical amplifier must be inserted. This is one of the nonlinear effects of optical fiber, four-wave mixing (FWM, Four W
aves Mixing, a phenomenon in which new light with a frequency of νa + νb−νc is generated from three waves of frequencies νa, νb and νc)
It is due to. The wavelength of the wave generated by this nonlinear effect is generated by n signal light wavelengths, and the generated light may have the same wavelength as another signal light. In this case, this becomes noise. This four-wave mixing occurs in the medium of the optical fiber, and its magnitude is three times the input power.
Since it is proportional to the power, it is extremely effective to reduce the input power to the optical fiber serving as the transmission line to suppress this effect.

In order to solve this problem, the output of the optical amplifier is generated by generating a gain by the distributed Raman effect inside the optical fiber connected to the optical amplifier together with the optical amplifier for relay as described above. It was proposed to reduce the power level (PB Hansenet al., OFC'99 PD8-1).
However, in this proposal, the pumping light is injected only in the direction opposite to the propagation signal.

[0008]

According to the above-mentioned document, the configuration in which the pumping light is injected only in the opposite direction to the signal to generate the gain by the distributed Raman effect as described in the above-mentioned proposal is sufficient for the output of the optical amplifier. The optical power level cannot be reduced, and the generation of four-wave mixing light cannot be reduced sufficiently. Also,
Generally, the wavelength band in which the gain due to the distributed Raman effect can be used is a very narrow wavelength band,
The farther away from the center of the wavelength band where the gain due to the distributed Raman effect occurs, the smaller the available effect is. Therefore, each time the relay is repeated, the wavelength band in which the gain due to the distributed Raman effect occurs becomes substantially narrower. According to the above document, the wavelength interval of n multiplexed optical signals is 5
Even at 0 GHz, at most 4 relays, ie, 3
Only a 35 km transmission is possible. When the wavelength multiplexing interval is set to 100 GHz, this can be performed up to eight relays.
At GHz, the number of channels that can be multiplexed into one optical signal is only 25.

[0009] An object of the present invention is to provide an optical repeater and an optical communication system which further improve this. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical repeater and an optical communication system that can reduce noise generated in a part of an optical wavelength multiplexed signal by four-wave mixing and can greatly increase a transmission distance.

[0010]

SUMMARY OF THE INVENTION An optical repeater (1) of the present invention is provided.
0) is an optical amplifier (1) for amplifying an optical wavelength division multiplexed signal;
Optical fiber (11) connected to the input side of this optical amplifier
First means (2, 3) for injecting pumping light for generating a gain by the distributed Raman effect on an optical wavelength multiplex signal directed to the optical amplifier in a direction opposite to the signal; A second means (4, 5) for injecting, in the same direction as the signal, pumping light for generating a gain by the distributed Raman effect on the optical wavelength multiplex signal transmitted from the optical amplifier in the optical fiber connected to the output side; ). Here, the numbers in parentheses are reference numerals in the drawings of the embodiment (FIG. 1). This is added for easy understanding of the configuration, and is not intended to limit the present invention to the embodiments.

That is, the present invention relates to the above proposal (PB Hansen
et al), the gain of the optical wavelength-division multiplexed signal transmitted through the optical fiber connected to the output side of the relay optical amplifier is generated (by forward pumping) by the distributed Raman effect, For WDM signals transmitted in the optical fiber connected to the input side (by backward pumping)
This is to generate a gain due to the distributed Raman effect. When pumping light is injected into the optical fiber connected to the input side of the optical amplifier, gain due to the distributed Raman effect occurs in the rear part of the optical fiber. When pumping light is injected into the optical fiber connected to the optical fiber, the gain due to the distributed Raman effect occurs in the front part of the optical fiber.
I will say that.

Thus, the output power of the optical amplifier can be reduced by lowering the amplification factor of the optical amplifier.
Since the minimum input optical power level of the optical amplifier is determined by the noise characteristics of the optical amplifier, if the repeater interval is fixed, the amplification gain of the optical amplifier must be reduced to reduce the output optical power level. If it is not necessary to lower the output light power level, the effect of the present invention can be used to increase the relay gain.

The respective effects of forward pumping and backward pumping will be described with reference to simple schematic drawings. FIG. 2 is a diagram showing a distribution of signal light power in an optical fiber. The horizontal axis is the distance from the input end of the optical fiber, and the right end is the output end of the optical fiber. The values displayed in this figure have been normalized.
It is good to understand that one division is one unit length (as a more specific example, this one unit length is 10 units of 1.55 μm DSF).
km). The vertical axis is the signal light power in the optical fiber. This is also normalized and displayed. The scale of the vertical axis indicates that when only an optical amplifier is used without forward pumping or backward pumping, the optical power at the output point of the optical amplifier is “1”.
It is normalized so that The vertical axis is a logarithmic scale.

If there is no forward or backward excitation,
The distribution of the signal light power in the optical fiber attenuates logarithmically in proportion to the distance, and thus becomes as shown by a straight line A in FIG. Then, backward pumping is performed to generate a gain by the distributed Raman effect in the optical fiber connected to the input of the optical amplifier. Now, the gain due to this distributed Raman effect is 10 dB.
If (power ratio 10) is set, the output light power level of the optical amplifier can be reduced by 10 dB. Accordingly, the distribution of the signal light power in this case is such that the straight line A is reduced by 10 dB in parallel, and a gain due to the distributed Raman effect occurs behind the optical fiber, the signal is amplified, and as a whole, as shown by the curve B. become. Here, it is not a practically unnatural value to set the gain by the distributed Raman effect to 10 dB.

If only forward pumping (pumping only on the output side of the optical amplifier) is performed without performing backward pumping, and if the gain due to the distributed Raman effect in this case is also 10 dB, the output light power level of the optical amplifier is reduced by 10 dB. be able to.
In this case, since the gain due to the distributed Raman effect occurs in front of the optical fiber, the distribution of the signal light power is as shown by a curve C. When both backward pumping and forward pumping are performed, the output optical power level of the optical amplifier is reduced to 20.
It can be reduced to dB (one hundredth in power ratio), and the distribution of signal light power in the optical fiber is as shown by a curve D. The shapes of these curves B, C and D are based on theoretical calculations. As described above, by performing forward pumping together with backward pumping according to the present invention, the effect of lowering the signal level of the optical amplifier is synergistic.

As described above, the apparatus of the present invention is extremely effective against the above-mentioned four-wave mixing noise (which is proportional to the cube of the output light power), and an optical amplifier should be installed for the four-wave mixing noise. Factors that limit the distance (relay interval) can be greatly improved. Further, by lowering the output light power level according to the present invention, the mutual interference of the wavelength multiplexed signals is reduced, so that the wavelength multiplexing density can be increased. This is particularly the case for the dispersion-shifted optical fiber (DS
It is advantageous when using F).

In the present invention, the wavelength of the pumping light injected into the optical fiber by the first means (2,3, backward pumping) and the second means (4,5, forward pumping) is determined by an optical amplifier ( It is desirable to set 1) to overlap with the wavelength having the amplification gain in order to reduce the output light power level of the optical amplifier. FIG. 3 is a conceptual diagram of this. In this case,
The gain by the distributed Raman effect is set so that the backward pumping and the forward pumping do not share each other, but the wavelengths of both the backward pumping and the forward pumping overlap.

When the wavelength range in which the gain by the distributed Raman effect is effective is widened, the wavelength of the pumping light of the first means (2, 3, backward pumping) and the second means (4, 5, It can be set so as not to overlap with the wavelength of the excitation light of the forward excitation. FIG. 4 is a conceptual diagram of this.
In this case, the backward pumping and forward pumping share the gain due to the distributed Raman effect.

Another category of the present invention is an optical repeater transmission line, wherein a plurality of the above-described optical repeaters are connected via an optical fiber. Furthermore, when this optical fiber is a dispersion-shifted optical fiber (DSF), the effect is large because the density of wavelength division multiplexing can be increased as described above.

Still another category of the present invention is an optical wavelength division multiplexing communication system.
Injecting pump light for generating a gain by the distributed Raman effect on the optical wavelength multiplex signal transmitted from the optical amplifier in the optical fiber connected to the output side of the optical amplifier for amplifying the optical wavelength multiplex signal. A transmission device including means, an optical amplifier for amplifying an optical wavelength multiplexed signal, and an optical repeater transmission line including first and second means for injecting pump light for generating a gain by the distributed Raman effect; and A receiving device including first means for injecting pumping light for generating a gain by the distributed Raman effect on an optical wavelength multiplexed signal toward the optical amplifier in an optical fiber connected to an input side. Features.

In this optical wavelength division multiplexing communication system, since the output optical signal power level of the optical amplifier can be set extremely small, the possibility that four-wave mixing generated in the optical fiber causes interference can be extremely reduced. As the power level of the optical amplifier decreases, the degree of freedom in design is significantly improved.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Since it is not easy to carry out the test of the present invention using a long-distance optical fiber, the present invention uses a circulating system constituting a pseudo long-distance optical fiber transmission line. A test was conducted to confirm the effect of The test form and test results are described below.

FIG. 5 is a block diagram of a test system for confirming the effect of the present invention. The number of optical multiplexing n = 3 by the transmitting device 20
An optical wavelength-division multiplexed signal modulated by a pseudo information signal (10 Gb / S) with two channels and a channel interval of 50 GHz is created, and the switch SW1 is shortly closed to pass through a 2 × 2 multiplexer 41 as a burst signal. To enter the test optical fiber 11. The optical fiber 11 is a DSF (Dispersion Shift Optical Fiber) having a distance of 80 km.
r), the optical amplifier 1 is provided at the exit of the optical fiber 11.

The switch SW2 at the output end of the optical amplifier 1
Is closed, the burst signal goes around the optical fiber 11. At the input end of the optical amplifier 1, backward pumping is performed on the optical fiber 11 from the pumping light source 2 via the directional coupler 3. A directional coupler 5 is provided at a position corresponding to the output terminal of the optical amplifier 2, and forward excitation is performed from the excitation light source 4. Due to the backward pumping and the forward pumping, gains due to the distributed Raman effect are generated for the optical signals passing through the optical fiber 11, respectively. That is, the burst signal circulating in the optical fiber 11 is amplified by the optical amplifier 1 by this device, and a gain by the distributed Raman effect is generated by backward pumping on the input side, and a distributed Raman effect is generated by forward pumping on the output side. Gain occurs.
The wavelength of the rear pump light source 2 is 1440 nm, and the wavelength of the front pump light source 4 is 1460 nm.

When the burst signal circulates the circulation system of the optical fiber 11 an appropriate number of times, the switch SW2 is opened to stop the circulation, and the burst signal is transmitted to the receiving device 30 via the 2 × 2 multiplexer 41. take in. This receiving device 3
The signal taken into 0 is branched by the 2 × 2 multiplexer 42 and connected to the spectrum analyzer 43 to perform waveform observation. The gain of the optical signal can be measured by observing the waveform.

In the receiving device 30, the optical wavelength division multiplexed signal is separated into 32 optical signals by the optical demultiplexer 32, and each optical signal is restored by the light receiving circuit 35. , The signal error rate can be measured by the error rate measuring device 36.

The dispersion compensating optical fiber inserted into the transmitting device 20 is used to provide chromatic dispersion for eliminating the bit pattern correlation between 16 channels modulated by one modulator. The receiving device 30
The single mode optical fiber inserted into the transmission device is used to cancel the chromatic dispersion value of the dispersion compensating optical fiber inserted into the transmission device. These are operations for increasing the accuracy of the test system, and are not directly related to the configuration of the present invention.

FIG. 6 shows the test results of the wavelength gain characteristics. The horizontal axis indicates the light wavelength, and the vertical axis indicates the gain (in dB). The white circles are the gain characteristics of the optical amplifier 1, the black circles are the gain characteristics due to the distributed Raman effect, and the triangles are the total gain characteristics, each of which is a result measured by the test system shown in FIG. The gain was adjusted so that the total gain (triangular mark) became a span loss (31 dB) of approximately one round section in the use wavelength band indicated by the arrow.

FIG. 7 shows an optical power spectrum after transmission corresponding to 640 km (8 rounds). (A) is the spectrum after transmission of all the channels, and (b) is the channel 1 in the trial.
This is the spectrum after transmission when only 6 is removed. As a result, it can be confirmed that wavelength multiplexing is correctly performed even by long-distance transmission.

FIG. 8 shows the information signal eye patterns before and after the transmission corresponding to 640 km. According to the method of the present invention, 64
An eye pattern appears sufficiently clearly even after transmission of 0 km, and it can be seen that binary identification is possible.

FIG. 9 shows a power penalty at an error rate of 10 (-9 power) of each channel after 640 km transmission. It can be seen that an error rate of 10 (−9 power) is achieved in all channels even after transmission of 640 km.

As a comparative example, in the test system shown in FIG. 5, when the forward excitation was stopped and only the backward excitation was used, the error rate of each channel was 10 (−9) under the same condition of 32 channels at 50 GHz intervals. The maximum transmittable distance achievable by the second power) was 320 km in four relay sections.

[0033]

As described above, the input optical power level of the optical fiber can be reduced by utilizing the optical amplifier and the amplification by the bidirectional distributed Raman effect.
Thereby, distortion due to the nonlinear effect of the optical fiber can be reduced. As a result, high-density wavelength-division multiplex transmission can be realized in the zero-dispersion region where transmission was impossible in the conventional example.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical multiplex communication system according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a distribution of signal light power in an optical fiber for explaining an effect of the present invention.

FIG. 3 is a diagram for explaining the gain of an optical amplifier and the wavelength band of the gain due to the distributed Raman effect.

FIG. 4 is a diagram for explaining the gain of the optical amplifier and the wavelength band of the gain due to the distributed Raman effect (when the wavelength bands of the forward pumping and the backward pumping are different).

FIG. 5 is a block diagram of a test system according to the present invention.

FIG. 6 is a wavelength gain characteristic diagram showing test results of the present invention.

FIG. 7 is a signal light spectrum diagram after transmission corresponding to 640 km, showing test results of the present invention.

FIG. 8 is an information signal eye pattern after transmission corresponding to 640 km, showing a test result of the present invention.

FIG. 9 is a power penalty for each channel at an error rate of 10 (−9 power) after transmission corresponding to 640 km, showing test results of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical amplifier 2 pump light source (for backward pumping) 3 directional coupler 4 pump light source (for forward pump) 5 directional coupler 10 optical repeater 11 optical fiber 12 optical fiber 20 transmitting device 30 receiving device

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04J 14/02 H04B 9/00 J H04B 10/17 10/16 (72) Inventor Masafumi Koga Shinjuku-ku, Tokyo N-Shinjuku 3-chome 19-2 Nippon Telegraph and Telephone Corporation F-term (reference) 2K002 AA02 AB30 BA02 DA10 EA30 GA07 GA10 HA23 5F072 AK06 JJ20 PP07 QQ07 RR01 SS06 YY17 5K002 AA02 AA04 AA06 BA02 BA05 CA01 CA09 CA13 DA02 DA42 FA01 FA02

Claims (8)

[Claims] An optical amplifier for amplifying an optical wavelength division multiplexed signal;
First means for injecting pump light for generating a gain by the distributed Raman effect on an optical wavelength multiplex signal directed to the optical amplifier in an optical fiber connected to an input side of the optical amplifier, and an output of the optical amplifier; Second means for injecting pump light for generating a gain by the distributed Raman effect on the optical wavelength multiplexed signal transmitted from the optical amplifier in the optical fiber connected to the optical fiber. Repeater. 2. The apparatus according to claim 1, wherein a wavelength of the pump light injected into the optical fiber by the first means and the second means is set to overlap a wavelength having an amplification gain of the optical amplifier. Optical repeater. 3. The optical repeater according to claim 2, wherein the wavelength of the excitation light of the first means and the wavelength of the excitation light of the second means are set to overlap. 4. The optical repeater according to claim 2, wherein the wavelength of the excitation light of the first means and the wavelength of the excitation light of the second means are set so as not to overlap. 5. An optical repeater transmission line, wherein a plurality of optical repeaters according to claim 1 are connected via an optical fiber. 6. The optical repeater transmission line according to claim 2, wherein said optical fiber is a dispersion shifted optical fiber (DSF). 7. An optical amplifier for amplifying an optical wavelength division multiplexed signal,
An optical repeater transmission line including first means for injecting pump light for generating a gain by the distributed Raman effect on an optical wavelength multiplexed signal directed to the optical amplifier in an optical fiber connected to an input side of the optical amplifier; Receiving device. 8. An optical repeater transmission line according to claim 4, a receiving device according to claim 7, and transmission from said optical amplifier in an optical fiber connected to an output side of an optical amplifier for amplifying an optical wavelength division multiplexed signal. An optical wavelength division multiplexing communication system comprising: a transmission device including second means for injecting pump light for generating a gain by the distributed Raman effect on the obtained optical wavelength division multiplexed signal.