JP2002341390A - Optical repeating system and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep a total gain of the whole transmission line flat by compensating inter-wavelength gain deviation due to variation in input level to a rare-earth element added fiber amplification part without performing any complicated feedback control. SOLUTION: The optical repeating system is equipped with a Raman amplification part 18 which amplifies signal light by using Raman amplification effect and the rare-earth element added fiber amplification part 14 which amplifies the signal light by using a rare-earth element fiber as an amplifying medium and amplifies and outputs input light L1 having specific light intensity. The Raman amplification part 18 adjusts the light output intensity values of two exciting light sources 16-1 and 16-2 differing in oscillation wavelength and generates an amplification gain spectrum which has a larger gain on the short- wavelength side than on the long-wavelength side and the rare-earth element added fiber amplification part 14 generates an amplification gain spectrum which has a larger gain on the long-wavelength side than on the short- wavelength side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Raman amplification unit for amplifying an optical signal on an optical fiber by a Raman amplification effect, and an erbium-doped fiber amplifier (EDFA).
TECHNICAL FIELD The present invention relates to an optical repeater and an optical transmission system that perform optical amplification by combining with a rare earth-doped fiber amplifier such as an oped fiber amplifier.

[0002]

2. Description of the Related Art In a wavelength division multiplexing optical transmission system, one of the important problems that must be solved in order to achieve high capacity and high speed is how to reduce nonlinear effects in an optical fiber. . Here, an optical wavelength suitable for the conventional optical transmission system is a 1.5 μm band, and an erbium-doped fiber amplifier (EDFA) has been mainly used as an optical amplifier in this band. EDF
In the optical repeater using A, since the EDFA is a lumped-constant-type amplifier, the amplification must be performed periodically and locally. Therefore, a nonlinear effect occurs in the optical fiber where the light intensity is concentrated. This has caused the performance degradation of the optical transmission system.

On the other hand, in a distributed constant type optical amplifier using the Raman optical amplification technique, the amplification operation can be performed using the entire transmission line, and the concentration of signal light intensity can be prevented.
Therefore, by incorporating the Raman amplification technique into the optical transmission system, it is possible to reduce the nonlinear effect. Also,
Since optical amplification is performed using the transmission fiber itself,
This is a desirable optical amplification in that the signal relay interval can be extended. Thus, Raman amplification technology is expected as a key component that increases the flexibility of an optical transmission system.

FIG. 5 is a block diagram showing the configuration of a conventional optical repeater (see Japanese Patent Application Laid-Open No. 11-84440). This optical repeater includes a Raman amplifier 45 and a rare earth-doped fiber amplifier 43 provided at a subsequent stage. The Raman pump light source 44 makes pump light incident on the transmission fiber 41 from the rear through the coupler 42, and generates a Raman amplification gain spectrum G11 shown in FIG. The input light L11 input to the transmission fiber 41 is
The signal is Raman-amplified on 1 and output to the rare-earth-doped fiber amplifier 43 via the coupler 42. The rare earth-doped fiber amplifier 43 further converts the optical signal amplified by the Raman amplifier 45 into a rare earth-doped fiber gain gain spectrum G12.
And output as output light L12.

In this optical repeater, the total gain spectrum is adjusted by adjusting the pump wavelength of the Raman gain gain spectrum G11 and its peak gain so as to offset the decrease in the long wavelength region of the rare earth doped fiber gain gain spectrum G12. G10 is made flat on the desired band.

On the other hand, the document "OAA '99, Nara, ThA3-1 (MT
akeda et al.) also shows an optical repeater that performs optical amplification by combining Raman amplification and EDFA. FIG.
FIG. 1 is a block diagram showing a configuration of this conventional optical repeater. In FIG. 7, the optical repeater includes a Raman amplifier 6
8. A rare earth-doped fiber amplifier 64 and a splitter 67 are provided after the Raman amplifier 68, and an optical spectrum monitor / controller 69 is provided between the splitter 67 and the Raman amplifier 68. Raman amplifier 68
Has a transmission fiber 61, a coupler 62, and a Raman excitation light source 63. The Raman excitation light source 63 has a first excitation light source 66-1, a second excitation light source 66-2, and a wavelength multiplexing unit 65.

[0007] The first pumping light source 66-1 and the second pumping light source 66-2 each output pumping light having a different wavelength and input the same to the wavelength multiplexing unit 65. The wavelength multiplexing unit 65 wavelength-multiplexes each of the inputted pump lights, and pumps them as pump light LP2 onto the transmission fiber 61 via the coupler 62 from behind.
Here, the input light L21 input to the transmission fiber 61
Are Raman amplified on the transmission fiber 61 and the coupler 62
Is output to the rare earth-doped fiber amplifier 64 via
The rare-earth-doped fiber amplifier 64 amplifies the signal light amplified by the Raman amplifier 68 according to the rare-earth-doped fiber amplification gain, and outputs the output light L22 via the splitter 67.

Here, the rare earth-doped fiber amplifying section 64
Is realized by an EDFA, and the gain spectrum of the EDFA has a gain deviation between wavelengths due to wavelength-dependent loss due to the influence of Rayleigh scattering or the like. On the other hand, the splitter 67 extracts a part of the optical signal amplified by the rare-earth-doped fiber amplifier 64 and outputs it to the optical spectrum monitor / controller 69. The optical spectrum monitor / control unit 69 measures the optical spectrum of the input optical signal, and performs feedback control on the first Raman excitation light source 66-1 and the second Raman excitation light source 66-2 based on the measurement result. Is performed to adjust the gain spectrum in the Raman amplifier 68 to compensate for the gain deviation between wavelengths.

[0009]

By the way, in the above-mentioned conventional optical repeater, when the input level fluctuates, as shown in FIG. There is a problem in that a gain deviation occurs between wavelengths in the gain, the flattening of the total gain is disturbed, and the output light is deteriorated.

In particular, in the Raman amplifier 45, which is a distributed optical fiber amplifier, the input level of the signal light and the pump light LP1
Since the light intensity, that is, the Raman gain fluctuates simultaneously,
The influence of the input level fluctuation on the rare earth-doped fiber amplifier 43 is increased.

Further, in the conventional optical repeater shown in FIG. 7, a part of the signal light is extracted through the splitter 67 and feedback control is performed. Since the influence of the signal level fluctuation inputted to the rare earth doped fiber amplifier 64 is large and complicated,
The control itself becomes complicated, the optical repeater itself becomes large, and there is a problem that the cost becomes large.

[0012] The present invention has been made in view of the above,
In an optical repeater that combines Raman amplification and rare-earth-doped fiber amplification, the gain deviation between wavelengths due to input level fluctuations to the rare-earth-doped fiber amplifier is compensated for without complicating feedback control. An object of the present invention is to provide an optical repeater and an optical transmission system capable of maintaining gain flatness.

[0013]

In order to achieve the above object, an optical repeater according to the present invention comprises a Raman amplifier for Raman-amplifying a signal light by using a Raman amplification effect, and a signal using a rare-earth-doped fiber as an amplification medium. In an optical repeater comprising a rare earth-doped fiber amplifying means for amplifying light and amplifying and outputting a signal light having a predetermined light intensity, the Raman amplifying means has a larger gain on a short wavelength side than on a long wavelength side. An amplification gain spectrum is formed, and the rare-earth-doped fiber amplification means forms an amplification gain spectrum having a larger gain on a longer wavelength side than on a shorter wavelength side.

According to the present invention, there is provided a Raman amplifying means for Raman amplifying signal light by using a Raman amplification effect, and a rare earth-doped fiber amplifying means for amplifying signal light using a rare earth-doped fiber as an amplification medium. In an optical repeater that amplifies and outputs a signal light having an intensity, the Raman amplifying means amplifies the gain on the short wavelength side larger than that on the long wavelength side in consideration of the wavelength / gain characteristics of the rare earth doped fiber amplifying means. A gain spectrum is formed, and the rare-earth-doped fiber amplifying means forms an amplification gain spectrum having a larger gain on the long wavelength side than on the short wavelength side.

In the optical repeater according to the next invention, in the above invention, the Raman amplifying means is characterized in that the amplification gain on the short wavelength side is larger than the amplification gain on the long wavelength side by 2 dB or more. .

According to this invention, the Raman amplification means forms a Raman amplification gain spectrum in which the amplification gain on the short wavelength side is specifically 2 dB or more larger than the amplification gain on the long wavelength side.

In the optical repeater according to the next invention, in the above invention, the Raman amplifying means includes a plurality of pumping light sources for outputting lights having different pumping wavelengths, and forms a pumping light for forming an amplification gain spectrum on a long wavelength side. The output of the pump light source that forms an amplification gain spectrum on the short wavelength side is increased compared to the output of the light source.

According to the present invention, the Raman amplifying means includes a plurality of pumping light sources for outputting light having different pumping wavelengths, and has a shorter wavelength than the output of the pumping light source forming an amplification gain spectrum on the longer wavelength side. The output of the pump light source that forms the amplification gain spectrum on the side is increased to form the Raman amplification gain spectrum described above.

The optical repeater according to the next invention is characterized in that, in the above invention, a filter having wavelength transmission dependency is provided between the Raman amplification means and the rare earth doped fiber amplification means. .

According to the present invention, a filter having wavelength transmission dependence is provided between the Raman amplification means and the rare earth-doped fiber amplification means, so that the Raman amplification gain spectrum by the Raman amplification means is further shaped. ing.

The optical repeater according to the next invention is characterized in that, in the above invention, the amplification gain of the Raman amplification means and the amplification gain of the rare-earth-doped fiber amplification means are substantially the same.

According to the present invention, the amplification gain of the Raman amplification means and the amplification gain of the rare-earth-doped fiber amplification means are made substantially the same, so that the flatness of the total amplification gain can be maintained in practice. ing.

An optical transmission system according to the next invention comprises the optical repeater according to any one of the above-mentioned inventions.

According to the present invention, the optical repeater according to any one of the above-mentioned inventions is provided, and the operation and effect of the optical repeater described above are exhibited.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an optical repeater and an optical transmission system according to the present invention will be described below in detail with reference to the accompanying drawings.

Embodiment 1 FIG. 1 is a block diagram showing a configuration of the optical repeater according to the first embodiment of the present invention. In FIG. 1, the optical repeater includes a transmission fiber 1
1 and the Raman amplification unit 18
And a rare earth-doped fiber amplifier 14 connected to the subsequent stage. The Raman amplification unit 18 has a Raman excitation light source 17, and the Raman excitation light L output from the Raman excitation light source 17.
P pumps backward through the coupler 12 to the transmission fiber 11. The input light L1 is input from the input side to the backward-pumped transmission fiber 11, and the input light L1 is
Optical amplification is performed according to the excitation state. Thereafter, the Raman-amplified input light is input to the rare-earth-doped fiber amplifier 14, and is output as light-amplified output light L2.

Here, the Raman excitation light source 17 has a first excitation light source 16-1, a second excitation light source 16-2, and a wavelength multiplexing unit 15, and the wavelength multiplexing unit 15 includes a first excitation light source 16-1.
The pump light output from 1 and the pump light output from the second pump light source 16-2 are wavelength-multiplexed and output to the coupler 12 as Raman pump light LP. Note that an isolator 13 is inserted after the coupler 12 to suppress multiple reflection of the input light L1 and the Raman pumping light LP. Further, the rare earth-doped fiber amplifier 14 is realized by an EDFA.

Incidentally, the rare earth-doped fiber amplifying section 14
, Ie, the EDFA gain has a wavelength / gain characteristic as shown in FIG. 2A due to its physical characteristic.
That is, FIG. 2A shows a case where the input level is lower than the normal output P1 due to a loss in the transmission path or the like, but the input level changes from the output P1 → P2 → P3.
When (P1>P2> P3), the amount of increase in amplification gain on the short wavelength side is greater than the amount of increase in amplification gain on the long wavelength side.

On the other hand, the amplification gain of the Raman amplifier 18 has a wavelength / gain characteristic as shown in FIG. That is, FIG. 2B shows a case where the Raman pumping light is reduced as compared with the normal output intensity R1 due to a loss in the transmission path or the like, but the Raman pumping light has the output intensity R1 → R2 → R
3 (R1>R2> R3), the decrease in the amplification gain on the short wavelength side is greater than the decrease in the gain on the long wavelength side.

In order to generate the wavelength / gain characteristics of the Raman amplifier as shown in FIG. 2B, for example, as shown in FIG. 1, the first pumping light source 16-1 and the second pumping light source 16 having different wavelengths are used. -2. The Raman amplification action has an amplification band of 50 nm centering on a wavelength on the long wavelength side of about 90 nm from the wavelength of the pump light source. So, for example,
The oscillation wavelength of the first excitation light source 16-1 is 1450 nm,
When the oscillation wavelength of the second excitation light source 16-2 is 1500 nm, an amplification band of 1525 nm to 1625 nm can be realized. That is, by using a plurality of excitation wavelengths of the Raman excitation light source 17, an arbitrary amplification band of about 100 nm or more can be realized. Further, for example, the first
By arbitrarily changing the light intensity of the excitation light source 16-1 and the light intensity of the second excitation light source 16-2, the wavelength / gain characteristics shown in FIG. 2B can be obtained.

In the first embodiment, the Raman amplifier 18
By appropriately combining the gain / wavelength characteristics of the optical fiber and the rare-earth-doped fiber amplifier 14, the gain deviation between wavelengths of the rare-earth-doped fiber amplifier 14 due to input level fluctuation of the input light L1 can be compensated without performing complicated feedback control. However, the flatness of the gain over the entire transmission path can be maintained.

The gain between the wavelengths due to the Raman amplification gain and the rare earth-doped fiber amplification gain is reduced by increasing the gain on the short wavelength side of the Raman amplification unit 18 by 2 dB or more as compared with the gain on the long wavelength side. Can be counteracted. Also, for example, the gain deviation between wavelengths can be canceled by making the Raman amplification gain and the rare earth-doped fiber amplification gain approximately the same.

Embodiment 2 FIG. Next, a second embodiment of the present invention will be described. In the first embodiment described above,
In order to generate the Raman amplification gain shown in FIG.
Although a plurality of pumping light sources 16-1 and 16-2 are provided, in this embodiment, a filter 19 that gives a different loss depending on the wavelength is provided between the Raman amplifier 18 and the rare-earth doped fiber amplifier 14. The Raman amplification gain shown in FIG. 2B is provided.

FIG. 3 is a block diagram showing a configuration of the optical repeater according to the second embodiment of the present invention. In this optical repeater, a filter 19 is newly provided in the configuration of the first embodiment, and the filter 19 is disposed between the Raman amplifier 18 and the rare-earth-doped fiber amplifier 14. Other configurations are the same as those of the first embodiment, and the same components are denoted by the same reference numerals.

FIG. 4 is a diagram showing the wavelength transmission characteristics of the filter 19 shown in FIG. As shown in FIG. 4, the wavelength transmission characteristics of the filter 19 have a high light transmittance on the short wavelength side, and the light transmittance is set lower as the wavelength becomes longer. Filter 1 having such wavelength transmission characteristics
9 can be realized by, for example, a fiber grating or a dielectric filter. The filter 1
The wavelength transmission characteristic of No. 9 is not limited to the wavelength transmission characteristic of monotonically decreasing shown in FIG. 4, but actually has an arbitrary characteristic curve for further flattening the total gain of the Raman amplification gain and the rare earth-doped fiber amplification gain. You may make it do.

In the second embodiment, the inter-wavelength gain deviation caused by the Raman amplification gain and the rare earth-doped fiber amplification gain can be more effectively canceled by the filter 19, and the overall gain can be further flattened.

[0037]

As described above, according to the present invention, Raman amplification means for Raman-amplifying signal light using the Raman amplification effect, and rare-earth-doped fiber amplification for amplifying signal light using a rare-earth-doped fiber as an amplification medium. Means for amplifying and outputting a signal light having a predetermined light intensity, wherein the Raman amplifying means takes into account the wavelength / gain characteristics of the rare earth-doped fiber amplifying means, as compared with the longer wavelength side. Since the amplification gain spectrum having a large gain on the short wavelength side is formed, and the rare earth-doped fiber amplifying means is configured to form an amplification gain spectrum having a large gain on the long wavelength side as compared to the short wavelength side, the rare earth addition is performed. Inter-wavelength gain deviation due to input level fluctuation to the fiber amplification means can be compensated without performing complicated feedback control, and the flatness of the overall amplification gain over the entire transmission line is maintained. It is advantageously possible to realize an optical repeater capable.

According to the next invention, a Raman amplification gain spectrum in which the amplification gain on the short wavelength side of the Raman amplification means is specifically larger than the amplification gain on the long wavelength side by 2 dB or more is formed. Therefore, an optical repeater capable of compensating for the gain deviation between wavelengths due to input level fluctuations to the rare-earth-doped fiber amplification means without performing complicated feedback control and maintaining the flatness of the overall amplification gain over the entire transmission line. This has the effect that it can be realized.

According to the next invention, the Raman amplifying means is provided with a plurality of pumping light sources for outputting light having different pumping wavelengths, and is shorter than the output of the pumping light source forming an amplification gain spectrum on the longer wavelength side. Since the output of the excitation light source that forms the amplification gain spectrum on the wavelength side is increased and the Raman amplification gain spectrum described above is formed,
It is possible to actually form an amplified gain spectrum with a large gain on the short wavelength side compared to the long wavelength side, and to perform complex feedback control on the gain deviation between wavelengths due to input level fluctuations to the rare earth doped fiber amplifier. This makes it possible to realize an optical repeater capable of compensating for the loss without compensating and maintaining the flatness of the total amplification gain over the entire transmission path.

According to the next invention, a filter having wavelength transmission dependency is provided between the Raman amplification means and the rare earth-doped fiber amplification means, and the Raman amplification gain spectrum by the Raman amplification means is further shaped. Therefore, an amplified gain spectrum having a large gain on the short wavelength side as compared with the long wavelength side can be formed with high accuracy, and a gain deviation between wavelengths due to input level fluctuation to the rare-earth-doped fiber amplification means can be complicated. This provides an effect that an optical repeater that can compensate without performing feedback control and can maintain the flatness of the overall amplification gain over the entire transmission line even higher can be realized.

According to the next invention, the amplification gain of the Raman amplification means and the amplification gain of the rare earth-doped fiber amplification means are made substantially the same, so that the flatness of the total amplification gain can be maintained in practice. Therefore, the gain deviation between wavelengths due to the input level fluctuation to the rare-earth-doped fiber amplification means can be compensated without performing complicated feedback control, and the flatness of the total amplification gain over the entire transmission line can be reliably maintained. There is an effect that an optical repeater that can be realized can be realized.

According to the next invention, the optical repeater according to any one of the above inventions is provided, and the operation and effect of the optical repeater described above are exhibited. An optical transmission system including an optical repeater capable of compensating for a gain deviation between wavelengths due to input level fluctuation without performing complicated feedback control and maintaining flatness of an overall amplification gain over the entire transmission line. This has the effect that it can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical repeater according to a first embodiment of the present invention.

FIG. 2 is a diagram showing changes in a Raman amplification gain spectrum and a rare earth-doped fiber amplification gain spectrum.

FIG. 3 is a block diagram illustrating a configuration of an optical repeater according to a second embodiment of the present invention.

FIG. 4 is a diagram illustrating an example of a wavelength transmission dependency of the filter illustrated in FIG. 3;

FIG. 5 is a block diagram illustrating a configuration of a conventional optical repeater.

FIG. 6 is a diagram illustrating an overall gain spectrum obtained by the optical repeater illustrated in FIG. 5;

FIG. 7 is a block diagram illustrating a configuration of another example of a conventional optical repeater.

[Explanation of symbols]

REFERENCE SIGNS LIST 11 transmission fiber, 12 coupler, 13 isolator, 14 rare earth-doped fiber amplifier, 15 wavelength multiplexing unit, 16-1 first pump light source, 16-2 second pump light source, 17 Raman pump light source, 18 Raman amplifier, 19
Filter, L1 input light, L2 output light, LP Raman pump light.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04J 14/00 14/02 (72) Inventor Takeo Takeo 2-3-2 Marunouchi 2-chome, Chiyoda-ku, Tokyo 3 Rishi Electric Co., Ltd. CA03 CA13 DA02 FA01

Claims (6)

[Claims] 1. A signal having a predetermined light intensity, comprising: Raman amplification means for Raman-amplifying signal light using a Raman amplification effect; and rare-earth-doped fiber amplification means for amplifying signal light using a rare-earth-doped fiber as an amplification medium. In the optical repeater for amplifying and outputting light, the Raman amplifying means forms an amplification gain spectrum having a large gain on a short wavelength side as compared with a long wavelength side, and the rare earth-doped fiber amplifying means has a small An optical repeater that forms an amplified gain spectrum having a large gain on the long wavelength side. 2. The optical repeater according to claim 1, wherein the Raman amplifying means has an amplification gain on the short wavelength side that is larger than the amplification gain on the long wavelength side by 2 dB or more. 3. The Raman amplifying means includes a plurality of pumping light sources for outputting lights having different pumping wavelengths, and amplifying gain on a short wavelength side as compared with an output of a pumping light source forming an amplification gain spectrum on a long wavelength side. 3. The optical repeater according to claim 1, wherein an output of an excitation light source for forming a spectrum is increased. 4. The filter according to claim 1, wherein a filter having wavelength transmission dependency is provided between the Raman amplification unit and the rare-earth-doped fiber amplification unit. Optical repeater. 5. The optical repeater according to claim 1, wherein an amplification gain of said Raman amplification means and an amplification gain of said rare earth-doped fiber amplification means are made substantially the same. 6. An optical transmission system comprising the optical repeater according to claim 1.