JP2001255564A - Fiber for raman amplification and optical fiber line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a drawback that the Raman gain is limited as a problem of conventional techniques and to improve the wavelength region with the maximum amplification and the wavelength region with a flat gain. SOLUTION: The concentration of germanium added to the core 11 exceeds 30 mol% and germanium or phosphorus is added to the clad 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Raman amplifier and an optical fiber line used in an optical communication system.

[0002]

2. Description of the Related Art FIG. 1 shows a basic configuration of a Raman amplifier (also referred to as a fiber Raman amplifier) according to the prior art and the present invention. The Raman amplification fiber 1 is pumped by the pump light from the pump light source 2 to obtain a Raman gain for the signal light. The pumping light is multiplexed with the signal light using the multiplexer 3 and is incident on the Raman amplification fiber 1. The signal light and the pump light enter the Raman amplification fiber 1 from the silica fiber 4 attached to the optical component such as the multiplexer 3. However, for the sake of simplicity, trivial optical components such as isolators installed before and after the Raman amplification fiber 1 are omitted in FIG. The silica fiber and the Raman amplification fiber 1 are connected by a connection section 5 using fusion splicing or connector connection. The Raman amplification fiber 1 is a fiber housed by being wound around a bobbin (called a Raman fiber) when the Raman amplifier is a centralized amplification type, or laid in the city when the Raman amplifier is a distributed amplification type. Transmission fiber.

FIG. 8 shows an example of the structure of a conventional Raman amplification fiber. This is a case where the Raman amplification fiber is a Raman fiber. The composition of the core 11 is 70 mol% of silicon dioxide (SiO ₂ ), 30 mol% of germanium dioxide (GeO ₂ ), and SiO ₂ (70%) −
GeO ₂ (30%). The composition of the clad 12 is 100 mol% of silicon dioxide (SiO ₂ ) (SiO ₂ (100%)). The magnitude of the Raman gain is mainly determined by the composition of the core and the effective area of the core. In the Raman amplification fiber of this figure, germanium (Ge) is used in the core of the fiber as compared with a normal silica fiber.
Is added at a high concentration to increase the refractive index difference between the cladding and the core.As a result, the effective area of the core is reduced, and the intensity of the excitation light in the core is increased, so that the Raman gain is increased. (Reference: EM Dianov et a
l. , Electron. Lett. , Vol. 34,
pp. 669-670, 1998). In the Raman amplification fiber of this figure, the Raman gain increases as the concentration of GeO ₂ increases (reference: FL Galeen).
er et al. , Appl. Phys. Let
t. , Vol. 32, PP. 34-36, 1978), the concentration of GeO ₂ is higher than that of a normal silica fiber.

[0004]

In the Raman amplification fiber of the prior art, the higher the concentration of GeO ₂ added to the core, the higher the Raman gain. Therefore, the concentration of GeO ₂ is reduced to 30 mol%.
It is high up close. However, when the GeO ₂ concentration is increased, the fiber loss coefficient becomes larger than that of a normal silica fiber having a low GeO ₂ concentration, and breakage such as fiber breakage occurs due to the difference in the thermal expansion coefficient between the core and the clad (see References). : Katsuharu Okamoto, "Basics of Optical Waveguides", Corona Mori, 1992), bending loss at a signal light wavelength (typically 1.5 μm).
ss) is increased. Therefore, in the prior art, the GeO ₂ concentration was limited to about 30 mol%. Incidentally, the thermal expansion coefficient of SiO ₂ is about 4 × 10 ⁻⁷ ° C. ⁻¹ , and the thermal expansion coefficient of GeO ₂ is about one digit or more.

For a numerical example of the Raman gain, GeO ₂
When the concentration is 30 mol%, the Raman gain coefficient is about 3 km ^-1.
W ⁻¹ , the Raman gain is about 20 dB when the Raman fiber length is 5 km and the pump light power is 400 mW.
However, the pump light wavelength and the signal light are 1450, respectively.
nm and 1550 nm.

FIG. 9 shows a typical Raman gain spectrum in the prior art. However, for simplicity, a schematic shape is shown, and a detailed shape is omitted (references:
F. L. Galeener et al. , Appl.
Phys. Leet. , Vol. 32, pp. 34-3
6, 1978). The shape of the Raman gain spectrum is mainly determined by the composition of the core, but in the prior art, the composition of the core is limited to SiO ₂ —GeO ₂ and the concentration of GeO ₂ is 30 mol% or less. Therefore, the maximum amplification wavelength range (frequency band) of the Raman gain is limited. Further, there is a disadvantage that the gain flat wavelength region is limited. However, a large gain flat wavelength region is particularly important in a wavelength division multiplex system. In the example of FIG. 9, the maximum amplification wavelength range is about 100 nm from 1450 to 1550 nm.
And the gain flat wavelength region is about 10 n around 1550 nm.
m. By the way, to expand the gain flat wavelength range,
There is a method of increasing the wavelength of the excitation light over a wide wavelength range, but there is a problem that the excitation light source becomes large and expensive due to the necessity of many excitation lasers (Reference: Y. Emorie).
t al. , OFC, PD19, 1999).

[0007]

To solve the above problems, the present invention uses the following means.

The first means uses a Raman amplification fiber having a core having a germanium added concentration of more than 30 mol% and a clad doped with germanium or phosphorus.

The second means is a Raman amplification fiber used in a system using Raman amplification, comprising: a first fiber having a core and a cladding; and a core and a cladding coupled to the first fiber. A second fiber, wherein the germanium doping concentration of the core of the first fiber is greater than the germanium doping concentration of the core of the second fiber, or the phosphorus doping concentration is greater than the phosphorus doping concentration of the core of the second fiber. The Raman amplification fiber is characterized in that the second fiber is set on the signal light input side.

The third means uses a Raman amplification fiber characterized in that the germanium or phosphorus doping concentration of the fiber core is distributed so as to decrease from the input side to the output side of the signal light. I have.

The fourth means is a Raman amplification fiber used in a system using Raman amplification, comprising: a first fiber having a core and a clad; and a fiber coupled to the first fiber and having a core and a clad. A Raman amplification fiber having a second fiber, wherein the first fiber has a germanium-doped core, and the second fiber has a phosphorus-doped core.

The fifth means uses a Raman amplification fiber having a core doped with both germanium and phosphorus.

The sixth means uses a Raman amplification fiber having a core doped with both germanium and phosphorus, wherein the sum of the germanium concentration and the phosphorus concentration exceeds 30%.

Further, in the seventh means, the first to sixth means are provided.
A Raman amplification system characterized by using a Raman amplification fiber as a transmission fiber is constructed.

[0015]

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

[0016]

FIG. 2 shows the structure of a Raman amplification fiber according to a first embodiment of the present invention. The compositions of the core 11 and the clad 12 are respectively SiO ₂ (40%)-GeO ₂ (6
0%) and SiO ₂ (70%) - it is the GeO ₂ (30%). That is, first, the GeO ₂ concentration of the core is set to a higher value than at most 30% of the prior art to improve the Raman gain. Secondly, the cladding is also GeO ₂
Is added to suppress the difference in GeO ₂ concentration between the core and the clad, thereby suppressing the difference in the coefficient of thermal expansion between the core and the clad.
Third, since the refractive index difference between the core and the clad is also kept at the same level as in the conventional technique, no excessive bending loss occurs at the signal light wavelength. Fourth, fiber loss increases with an increase in GeO ₂ concentration, but the effect of increasing the Raman gain is greater. The composition of the clad may be SiO ₂ -P ₂ O ₅ instead of SiO ₂ -GeO ₂ , so that the refractive index difference between the core and the clad may be maintained at the same level as in the prior art. Here, P ₂ O ₅ is phosphorus oxide.

Regarding the numerical example of the Raman gain, the Raman gain coefficient is about 6 km ^-1 W ^-1 and the Raman fiber length is 5 km.
km, and the Raman gain when the pump light power is 400 mW is about 35 dB. However, the excitation light wavelength and the signal light are 1450 nm and 1550 nm, respectively.
On the other hand, in the prior art, the Raman gain when the pump light power is 400 mW is about 20 dB. Thus, there is a significant gain improvement.

As described above, the Raman gain is improved by increasing the GeO ₂ concentration of the core as compared with the prior art while eliminating excessive deterioration of characteristics of the Raman amplifier. Although the present embodiment shows the case of backward excitation, the same can be said for forward excitation. In addition, the following second to fifth
In the embodiment, the backward excitation is also possible. Further, in the present embodiment and the following second to fifth embodiments, a fiber having a single core and a single clad is described, but a fiber having a double or more core or a fiber having two or more clads is described. The same can be said.

[0019]

Embodiment 2 FIG. 3 shows the structure of a Raman amplification fiber according to a second embodiment of the present invention. Using two Raman amplifying fibers (Raman amplifying fiber-1 and Raman amplifying fiber-1 '), they are connected to each other at a connection portion 5 such as fusion splicing.
To form one Raman amplification fiber (referred to as a composite Raman amplification fiber). The composition of the core 11 and the clad 12 of the Raman amplification fiber-1 is as follows:
GeO ₂ (100%) and SiO ₂ (30%) respectively
-GeO is ₂ (70%). Further, the composition of and the clad 12 '' core 11 'Raman amplifying fiber -1, respectively _{SiO 2 (70%) - GeO} 2 (30%) and a SiO ₂ (100%). The lengths of the Raman amplification fiber-1 and the Raman amplification fiber-1 'are both 2 km. In the present embodiment, the Raman amplification fiber-1 ′ has the same composition as the conventional Raman amplification fiber, but the composition of the Raman amplification fiber-1 is such that the GeO ₂ concentration of the core 11 is the same as in the first embodiment. high. As a result, the Raman gain of the composite Raman amplification fiber is improved. Needless to say, the Raman amplification fiber-1 ′ may be a fiber having a higher GeO ₂ concentration in the core than in the prior art as in the first embodiment. That is, Raman amplification fiber
At least one of the fiber No. 1 and the Raman amplification fiber-1 'has the same composition as the Raman amplification fiber of the first embodiment, and the Raman gain is improved.

In this embodiment, the Raman amplification fiber-1
The GeO ₂ concentration of the core 11 is set higher than the GeO ₂ concentration of the core 11 ′ of the Raman amplification fiber-1 ′. The reason is as follows. Since the signal light is amplified in the composite Raman amplification fiber, the signal light power in the Raman amplification fiber-1 'is generally larger than the signal light power in the Raman amplification fiber-1.
Raman amplification fiber-1 and Raman amplification fiber-
When 1 'is the same fiber, degradation due to non-linear effects (four-wave mixing, cross-phase modulation, etc.) is likely to occur in Raman amplification fiber-1'. Deterioration due to nonlinear effects is
Generally, the higher the GeO ₂ concentration, the greater. Therefore,
Fiber with high GeO ₂ concentration is converted to Raman amplification fiber
1 to suppress degradation due to the non-linear effect. That is, there is an advantage that deterioration due to the non-linear effect in the present embodiment is smaller than that in the first embodiment. The effect of improving the Raman gain is the same.

Regarding a numerical example of the Raman gain, the Raman gain when the pump light power is 400 mW is about 35 dB. However, the pump light wavelength and the signal light are 14
50 nm and 1550 nm. On the other hand, in the prior art, the Raman gain when the pump light power is 400 mW is about 20 dB. Thus, there is a significant gain improvement.

Similar effects were obtained when GeO ₂ was replaced with P ₂ O ₅ . That is, Raman amplification fiber
The compositions of the core 11 and the cladding 12 of the first embodiment are respectively represented by P
₂ O ₅ (100%) and SiO ₂ (30%)-P ₂ O
₅ (70%) and Raman amplification fiber-1 '
The composition of the core 11 ′ and the cladding 12 ′ is SiO ₂ (70%) — P ₂ O ₅ (30%) and SiO ₂ (30%), respectively.
₂ (100%), and Raman amplification fiber-1
And the length of the Raman amplification fiber-1 ′ are both 2
The same effect as in the case of GeO ₂ was obtained when the distance was set to km.

Although the present embodiment has a configuration using two fibers, the same holds when three or more fibers are used. Further, as shown in FIG. 4, it is possible to obtain the same effect as described above by changing the composition of the core and the cladding in the longitudinal direction of the fiber in one fiber. That is, the core 11 has a side end on which the excitation light enters
iO ₂ (70%)-GeO ₂ (30%) (or P ₂ O ₅
(30%)), the concentration of GeO ₂ or P ₂ O ₅ is changed in the longitudinal direction, and the side end where the signal light is incident is GeO 2.
₂ (100%) or P ₂ O ₅ (100%). On the other hand, the cladding 12 also has SiO ₂ (100%) at the side end on which the excitation light is incident, and uses GeO ₂ or P
_By changing the concentration of ₂ O _{5 in} the longitudinal direction, the side end where the signal light is incident becomes SiO ₂ (30%)-GeO ₂ (70%) (or P ₂ O ₅ (70%)). is there.

[0024]

Embodiment 3 FIG. 5 shows a third embodiment of the present invention for Raman amplification.
2 shows a configuration of a fiber. Two Raman amplifiers
Iva (Raman amplification fiber 1 and Raman amplification fiber
1 '), using a connection portion 5 such as fusion splicing them.
And connect them to one Raman amplification fiber (composite Raman
Amplifying fiber). Raman amplification
The composition of core 11 and cladding 12 of Iva-1
Each SiO_Two(70%)-GeO_Two(30%) and Si
O_Two(100%). Also, Raman amplification fiber
-1 'of the core 11' and the cladding 12 '
Each SiO_Two(70%)-P_TwoO_Five(30%) and S
iO_Two(100%). Where P_TwoO_FiveIs phosphorus oxide
It is. Raman amplification fiber-1 and Raman amplification
The length of fiber-1 'is 1 km and 3 km, respectively.
It is. FIG. 8 shows the Raman gain spectrum in this embodiment.
Shown in Raman amplification fiber-1 and Raman amplification
Fiber-1 'spectrum, and their sum.
The overall spectrum for the composite Raman amplification fiber
Is shown. In the Raman amplification fiber-1 ', P _TwoO_Five
, The gain peak wavelength is approximately 150 nm of the pump light wavelength.
on the long wavelength side. On the other hand, Raman amplification fiber-1
Is the same as in the prior art of FIG.
It is about 100 nm longer than the excitation light wavelength.

As shown in FIG. 8, the maximum amplification wavelength range (frequency band) of the Raman gain is expanded from about 100 nm in the prior art to about 150 nm in the present embodiment. Further, in the gain flat wavelength region, although there is some ripple on the spectrum, about 50 nm from about 1550 nm to about 1600 nm is obtained.
nm. On the other hand, the gain flat wavelength region of the prior art is 15
It is about 10 nm near 50 nm. Therefore, according to this embodiment, the maximum amplification wavelength range and the gain flat wavelength range can be improved. Incidentally, the ripple can be easily reduced by means such as increasing the wavelength of the excitation light.

As described above, the gain flat wavelength region is about 10 nm in the related art and about 50 nm in the present invention, and the expression “about” is used. This is because the gain flat wavelength region differs in the application form of the optical amplifier. As an application form, there are differences in the number of relay stages of an optical amplifier, allowable non-flatness, an analog system, a digital system, and the like. A typical definition of the gain average wavelength range is
The wavelength range where the gain drops 3 dB from its peak value (3 dB
Band (wavelength band)), but 1 dB band (wavelength band)
Is also possible. In the present invention, 3 dB in FIG.
Or about 5 dB.

[0027]

Embodiment 4 FIG. 6 shows the configuration of a Raman amplification fiber according to a fourth embodiment of the present invention. This achieves the same effect as the third embodiment. The difference from the third embodiment is that only one Raman amplification fiber is used. The composition of the core 11 and the cladding 12 of the Raman amplification fiber 1 is SiO ₂ (40%)-GeO ₂ (20%)-P ₂ O ₅ (4
0%) and SiO ₂ (70%) - a _{P 2 O 5 (30%)} . The length of the Raman amplification fiber is 4 km. FIG. 7 shows a Raman gain spectrum in this embodiment. A Raman gain spectrum similar to that of the third embodiment is obtained due to the contribution of GeO ₂ and P ₂ O ₅ . That is, according to the present embodiment, the maximum amplification wavelength range and the gain flat wavelength range can be improved.

[0028]

Embodiment 5 FIG. 7 shows the structure of a Raman amplification fiber according to a fifth embodiment of the present invention. Although the configuration is similar to that of the first embodiment, the first embodiment is a case of a centralized amplification type Raman fiber, while the present embodiment is a case of a distribution amplification type transmission fiber laid in a city. . The composition of the core 11 and the cladding 12 was SiO ₂ (40%)-GeO ₂
(60%) and SiO ₂ (70%)-GeO ₂ (30
%), Which is the same as in the first embodiment. The length of the Raman amplification fiber (transmission fiber) is 40 km. The fiber loss coefficient and the loss at a signal light wavelength of 1550 nm are 0.6 dB / km and 24 dB, respectively. Incidentally, comparing the system using the lumped amplification type Raman amplifier with the system using the distributed amplification type Raman amplifier has an advantage that the latter generally has lower noise.

As for the numerical example of the Raman gain, the internal gain of the Raman gain when the pump light power is 400 mW is about 2
5 dB. Thus, the Raman gain makes up for the fiber loss, and the external or net gain of the Raman gain is about 1 dB. All the transmission fibers connecting the nodes may be constituted by the Raman amplification fiber of the present invention, or may be a part thereof.

[0030]

As described above, according to the first to fifth embodiments, according to the present invention, it is possible to solve the problem of the prior art that the Raman gain is limited. Further, the maximum amplification wavelength range and the gain flat wavelength range can be improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a Raman amplifier according to the related art and the present invention.

FIG. 2 is a configuration diagram of a first embodiment of the Raman amplification fiber of the present invention.

FIG. 3 is a configuration diagram of a second embodiment of the Raman amplification fiber of the present invention. .

FIG. 4 is another configuration diagram of the second embodiment of the Raman amplification fiber of the present invention.

FIG. 5 is a configuration diagram of a Raman amplification fiber according to a third embodiment of the present invention.

FIG. 6 is a configuration diagram of a Raman amplification fiber according to a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram of a fourth embodiment of the Raman amplification fiber of the present invention.

FIG. 8 is a diagram showing Raman spectra of the third and fourth embodiments of the present invention.

FIG. 9 is a diagram showing a Raman spectrum of a conventional Raman amplification fiber.

FIG. 10 is a configuration diagram of a conventional Raman amplification fiber.

[Explanation of symbols]

 Reference Signs List 1 fiber for Raman amplification 11 core 12 clad 2 pumping light source 3 multiplexer 4 silica fiber

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Katsumi Iwatsuki, Inventor F-term (reference) 23-1 Otemachi 2-chome, Chiyoda-ku, Tokyo 2H050 AB05X AB05Y AB08Y AC03 AC81 AD00 2K002 AA02 AB30 CA15 DA10 GA10 HA23 4G062 AA06 BB01 BB02 BB09 BB10 CC10 DA01 DA02 DA03 DA04 DA05 DA06 DA08 DA10 DB01 DC01 DD01 DD02 DD03 DD04 DD05 DD06 DD07 DD08 DE01 DF01 EA01 EA10 EB01 EC01 ED01 EE01 EF01 FD01 FC01 FD01 FC01 FD01 FC01 FD01 FC01 FD01 FE01 FF01 FG01 FH01 FJ01 FK01 FL01 GA01 GA10 GB01 GC01 GD01 GE01 HH01 HH03 HH05 HH07 HH09 HH11 HH13 HH15 HH17 HH20 JJ01 JJ03 JJ05 JJ07 JJ10 KK01 KK03 KK05 KK30 KK10 KK01 KK10 NN015

Claims (7)

[Claims] 1. A Raman amplification fiber used in a system using Raman amplification and having a core and a clad, wherein the core contains germanium at a concentration of 30 mol%.
And the cladding is doped with germanium or phosphorus. 2. A Raman amplification fiber used in a system using Raman amplification, comprising: a first fiber having a core and a cladding; and a second fiber having a core and a cladding coupled to the first fiber. Having a fiber,
Whether the germanium doping concentration of the core of the first fiber is greater than the germanium doping concentration of the core of the second fiber,
Alternatively, the Raman amplification fiber has a phosphorus doping concentration higher than the phosphorus doping concentration of the core of the second fiber, and the second fiber is set on the signal light input side. 3. A Raman amplification fiber used in a system using Raman amplification and having a core and a clad, wherein a germanium-doped concentration or a phosphorus-doped concentration of a core of the fiber is changed from an input side of a signal light to an output side. A fiber for Raman amplification, characterized in that the fiber is distributed so as to decrease toward. 4. A fiber for Raman amplification used in a system using Raman amplification, comprising: a first fiber having a core and a clad; and a second fiber coupled to the first fiber and having a core and a clad. A fiber for Raman amplification, comprising a fiber, wherein the first fiber has a core doped with germanium, and the second fiber has a core doped with phosphorus. 5. A Raman amplification fiber used in a system using Raman amplification and having a core and a clad, wherein the core is doped with both germanium and phosphorus. 6. The Raman amplification fiber according to claim 5, wherein the sum of the concentration of germanium and the concentration of phosphorus exceeds 30 mol%. 7. An optical fiber line comprising the Raman amplification fiber according to claim 1 as a transmission fiber.