JP3361409B2 - Optical fiber amplifier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier, and more particularly to an optical fiber amplifier having low noise characteristics.

[0002]

2. Description of the Related Art Generally, when pumping light having a wavelength of 1.48 μm or 0.98 μm is incident on an erbium-doped optical fiber (EDF), erbium ions in the fiber are pumped and a wavelength of 1.55 μm band is emitted. It is known that when light having a wavelength of λ is incident on the optical fiber in the excited state, the incident light is amplified by stimulated emission from erbium ions. An erbium-doped optical fiber amplifier (EDFA) uses this principle. In addition, an optical fiber amplifier using an optical fiber doped with praseodymium is known, and this amplifier can amplify light having a wavelength of 1.3 μm band.

FIG. 2 is a diagram showing the basic construction of an erbium-doped optical fiber amplifier (EDFA) according to the prior art.
Hereinafter, a conventional EDFA will be described with reference to FIG. In FIG. 2, 11 is an input optical connector, and 12
Is an optical multiplexer, 14 is an excitation light source, 18 is an output side optical isolator, 19 is an output optical connector, 33 is an input side optical isolator, 35 is an amplifying optical fiber (EDF), 41 is a front stage fusion of the amplifying optical fiber. It is a place.

As shown in FIG. 2, an EDFA according to the prior art comprises an input side optical isolator 33, a pumping light source 14, an optical multiplexer 12, an amplifying optical fiber 35, and an output side optical isolator 18. .

In the prior art having such a configuration,
The signal light having a wavelength of 1.55 μm to be amplified is input from the input optical connector 11 and input to the input side optical isolator 33.
Is input to the optical multiplexer 12 via. The pumping light and the signal light emitted from the pumping light source 14 are multiplexed by the optical multiplexer 12 and are incident on the amplification optical fiber 35, and the signal light having a wavelength of 1.55 μm is amplified. The amplified signal light having a wavelength of 1.55 μm is output from the output connector 19 through the output side optical isolator 18 provided on the emission side.

Such a configuration can be applied as a general optical fiber amplifier, although the wavelength of the pumping light, the wavelength of the signal light to be amplified, and the material of the amplification optical fiber may change.

The output side optical isolator 18 prevents the reflected light from the output optical connector 19 from returning to the amplification optical fiber 35. In general, an optical connector causes reflection of about 25 dB at worst depending on the connection state. Further, the input side optical isolator 33 prevents slight return light leaking from the output side optical isolator 18 and light returning due to reflection from optical components from returning to the input optical connector 11. These optical isolators prevent oscillation when the amplification optical fiber 35 has a high gain by suppressing return light, and prevent intensity fluctuation of output light due to multiple reflection between optical connectors and between optical components. You can

The above-mentioned optical fiber amplifier according to the prior art is constructed such that at least one optical isolator is installed on the incident side of the amplification optical fiber, and a high gain of 25 dB or more can be obtained with a low noise figure characteristic. Is. Further, such an optical fiber amplifier can be used as a repeater because it can amplify light as it is with low noise. By using optical fiber amplifiers connected in multiple stages as repeaters, it is possible to transmit a signal to be transmitted for several hundred km without converting it into an electric signal.

When the above-mentioned optical fiber amplifier is used by connecting it in multiple stages as a repeater, noise of the optical fiber amplifier is accumulated in the transmission system constituted by this. Assuming that each optical fiber amplifier degrades the noise characteristic by 0.5 dB, in the case of a system in which the optical fiber amplifiers are connected in 10 stages, the noise characteristic is degraded by 5 dB. The deterioration of noise characteristics shortens the transmission distance of the transmission system and increases the cost of the transmission system. Therefore, it is necessary to suppress the deterioration of the noise characteristics of the optical fiber amplifier.

As an optical fiber amplifier capable of improving noise characteristics and suppressing noise, for example, one using light having a wavelength of 0.98 μm as pumping light is known. When the light having the wavelength of 0.98 μm is used as the excitation light, the excitation becomes a transition of three-level system, so that the noise figure can be reduced by about 1 dB to 2 dB as compared with the excitation light having the wavelength of 1.48 μm. . As a conventional technique related to this type of optical fiber amplifier, for example, a technique described in "Optical amplifier and its application 112p" by Ohm Co. is known.

Next, the noise characteristics of the optical fiber amplifier having the above-mentioned structure will be described. In FIG. 2, the loss of the input optical connector 11 is a1, the insertion loss of the optical isolator 33 is a3, the insertion loss of the multiplexer 12 is a2, the fiber of the multiplexer 12 on the input side of the amplification optical fiber (EDF) 35. The noise figure NF1 of the optical fiber amplifier is assumed to be a loss of a fusion-bonded portion 41 with a, a gain of the amplification optical fiber 35 is g, and a spontaneous emission coefficient of the amplification optical fiber is nsp.
Is given as shown in equation (1).

NF1 = {1 + 2 × (g−1) × nsp} / {a1 × a2 × a3 × a4 × g} (1) In the equation (1), the first term of the numerator is the signal light ( The shot noise component of the amplified light), and the second term is the beat noise component of the spontaneous emission light and the signal light generated in the amplification fiber. Further, in the formula (1), shot noise of spontaneous emission light and spontaneous emission light-spontaneous emission light beat noise are small and omitted.

The light emitted from the amplification optical fiber 35 is
Since the signal and noise undergo the same attenuation in the optical components on the output side, it is not necessary to consider the loss on the output side in the noise figure.
When the excitation light having the wavelength of 0.98 μm is used as the excitation light, the spontaneous emission light coefficient nsp is suppressed to a small value and becomes 1 which is an almost ideal value. Gain g of the amplification optical fiber 35
Is usually about 20 dB (100 times), so (g-
1) ≈g, and in this case the noise figure NF
Is given as NF1 = 2 / {a1 * a2 * a4 * a3} (2).

That is, the noise figure NF1 is deteriorated by the loss of the optical component on the incident side. Optical connector 11
Loss a1 of 0 dB to 0.5 dB, loss a of the multiplexer 12
2 is 0 dB to 0.4 dB, and the loss a4 at the fusion-bonded portion 41 is 0.
Although as low as dB to 0.2 dB, these are unavoidable because they are insertion losses of essential components. Further, the largest loss of the components on the input side of the denominator of the equation (2) is the loss a3 due to the optical isolator, which is 0.5 dB.
It is from (0.9) to 1 dB (0.8). Therefore, for example, a1 = 0.1 dB and a2 = 0.3 as typical values.
If dB, a3 = 0.7 dB, and a4 = 0.2 dB,
The noise figure of the optical fiber amplifier configured as described above is NF1.
= 4.3 dB.

As described above, in the conventional optical fiber amplifier, the noise figure of the optical fiber for amplification is 3d.
NF stably lower than 4 dB despite being B
Was hard to get.

[0016]

In the above-described optical fiber amplifier according to the prior art, the optical isolator is placed on the incident side of the amplifying optical fiber, so that it is possible to avoid the deterioration of the noise figure corresponding to the insertion loss of the optical isolator. However, there is a problem that it is difficult to keep the noise figure low.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an optical fiber amplifier having a configuration capable of suppressing the noise figure to a low level.

[0018]

According to the present invention, the above object is to provide an amplification optical fiber, a pumping light source for emitting pumping light for exciting the fiber, and a light to be amplified together with the pumping light. A wavelength-multiplexing wave device for incident on
An optical fiber amplifier including an optical isolator, wherein the optical isolator is the optical fiber for amplification.
It is intended to reduce the noise due to the spontaneous emission light of the optical fiber , and through a multiplexer / demultiplexer arranged on the output side of the amplification optical fiber.
By arranging in the separated signal light path , or by connecting the two amplification optical fibers in series , an optical isolator is arranged in the middle of the two amplification optical fibers as described above. It is achieved by that.

[0019]

By arranging the optical isolator on the output side rather than the incident side of the amplification optical fiber, the deterioration of the noise figure due to the loss of the optical isolator can be reduced, and the deterioration of the noise figure of the optical fiber amplifier can be suppressed to a low level. You can

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical fiber amplifier according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an erbium-doped optical fiber amplifier (EDFA) according to an embodiment of the present invention. In FIG. 1, 13 is an input side optical isolator, and 15
Is an optical fiber for pre-stage amplification, 16 is a demultiplexer / demultiplexer of pumping light and signal light, 17 is an optical fiber for post-stage amplification, 21 is an input side fusion point of the optical fiber for pre-stage amplification, and 22 is an optical fiber for pre-stage amplification 2 is a fusion point of the output side, 23 is a fusion point of the post-stage amplification fiber, and other symbols are the same as those in FIG.

As shown in FIG. 1, an optical fiber amplifier according to an embodiment of the present invention includes two amplifying optical fibers 15 and 17 at the front and rear stages, and an optical isolator 13 between these two amplifying optical fibers. And is provided with an optical fiber loop that passes only the signal light.

In the embodiment of the present invention shown in FIG. 1, the pumping light having a wavelength of 0.98 μm emitted from the pumping light source 14 is output from the input optical connector 11 by the optical multiplexer 12 of the pumping light and the input signal light. The input signal light having a wavelength of 1.55 μm is multiplexed and incident on the amplification optical fiber 15 at the preceding stage. The light output from the amplification optical fiber 15 is pumped with a wavelength of 0.98 μm and a wavelength of 1.5 with a demultiplexer / multiplexer 16.
The signal light of 5 μm is demultiplexed, and the pumping light of wavelength 0.98 μm passes through the demultiplexer / multiplexer 16 and the amplification optical fiber 1 in the subsequent stage
It is incident on 7. The signal light having a wavelength of 1.55 μm separated by the coupler-type demultiplexer / multiplexer 16 goes around the loop of the optical fiber having the optical isolator 13 to make the demultiplexer / demultiplexer 1
After being multiplexed with light having a wavelength of 0.98 μm at 6, the light enters the amplification optical fiber 17 at the subsequent stage. The light emitted from the amplification optical fiber 17 in the latter stage passes through the output side optical isolator 18 and is emitted from the output optical connector 19.

The optical isolator 13 for suppressing the return of the signal light to the incident side is provided in the loop of the optical fiber because the pumping light having a wavelength of 0.98 μm cannot pass through the optical isolator. This is because the signal light and the signal light are separated, the pumping light is made incident on the amplification optical fiber at the subsequent stage, and the signal light is made to pass through the optical isolator and then made incident on the amplification optical fiber at the subsequent stage. Therefore, the loop length of the optical fiber including the optical isolator 13 is not particularly limited, but is about 40 cm.

Next, the noise characteristic of the optical fiber amplifier according to the embodiment of the present invention will be described.

Referring now to FIG. 1, the input optical connector 11
Is a1, the insertion loss of the optical isolator 13 is a3,
The insertion loss of the multiplexer 12 is a2, the loss a4 of the fusion portion 21 on the input side of the amplification optical fiber of the preceding stage is a4, the gain of the amplification optical fiber 15 of the preceding stage is g1, and the naturalness of the amplification optical fiber 15 of the preceding stage is g1. Assuming that the emission light coefficient is nsp1, the gain of the subsequent amplification optical fiber EDF17 is g2, and the spontaneous emission light coefficient of the latter amplification optical fiber 17 is nsp2, the optical fiber amplifier according to the embodiment of the present invention shown in FIG. Noise figure N
F2 is given as shown in Expression (3).

NF2 = {1 + 2 × (g2-1) × nsp2 + 2 × g2 × (g1-1) × nsp1 × a3} / {a1 × a2 × a4 × a3 × g1 × g2} ...... (3 ) In the formula (3), the first term of the molecule is the signal light (amplified light).
Shot noise component, the second term is the latter amplification optical fiber 1
Beat noise term of spontaneous emission and signal light generated in 7.
The term is the noise generated by the beat with the signal light after the spontaneous emission optical noise generated in the amplification optical fiber 15 in the front stage is lost by the optical isolator 13 and then amplified in the amplification optical fiber 17 in the rear stage. In equation (3), the wavelength of 0.
Since the loss of the multiplexer / demultiplexer 16 for the light of 98 μm and the light of 1.55 μm is smaller than that of the optical isolator 13, it is omitted for easy comparison with the prior art.

In the formula (3), the shot noise of spontaneous emission light and the spontaneous emission light-spontaneous emission beat noise are small and omitted, and nsp1 and nsp2 are 1 and g1 and g
If 2 has a value considerably larger than 1, then equation (4) can be obtained from equation (3).

NF2 = {2 × (g2-1)} / {a1 × a2 × a4 × a3 × g1 × g2} + {2 × (g1-1)} / {a1 × a2 × a4 × g1} ...... (4) Formula (4) is the same as Formula (1) when g1 is 1,
The NF is most deteriorated due to the loss a3. As g1 increases, the first term of the equation (4) affected by the loss a3 of the optical isolator 13 decreases, and deterioration due to the loss of the optical isolator decreases. Further, when g2 becomes small, the amplification optical fiber 15 at the front stage before the optical isolator 13 is provided.
When the second term, which is the term of the NF deterioration due to, becomes dominant, and finally g2 becomes 1, the optical isolator 13 is located at the subsequent stage of the amplification optical fiber and does not affect the deterioration of the noise figure. Then, the gain g1 of the amplification optical fiber 15
Is 10 (10 dB) or more, the first term influenced by the loss a3 becomes 1/10 or less, and the influence of the noise characteristic due to the loss of the optical isolator can be reduced to 1/10. That is, in order to improve the noise characteristic of the optical fiber amplifier, the gain g1 of the amplification optical fiber 15 is increased.
Is preferably 10 dB or more.

The output side optical isolator 18 prevents the reflected light from the output optical connector 19 from returning to the amplification optical fiber 17, as in the case of the prior art. Generally, an optical connector has a worst case of 25 dB depending on the connection condition.
It produces some degree of reflection. In addition, the input side optical isolator 13
Is a slight return light leak from the optical isolator 18 on the output side, and light reflected by the optical component is input optical connector 1
It prevents it from returning to 1. These optical isolators suppress the return light to allow the amplification optical fiber 1
It is possible to prevent oscillation when 5 and 17 have a high gain, and to prevent fluctuation in signal light intensity due to multiple reflection between optical connectors and between optical components.

Normally, reflection from optical components such as an optical isolator is suppressed to 1/10000 (-50 dB) or less. Therefore, the optical isolator 13 can sufficiently perform the function as the optical isolator even by the configuration according to the embodiment of the present invention in which the optical isolator 13 is inserted after the amplification optical fiber 15 in the preceding stage. The configuration of this embodiment of the present invention is the same as that of the amplification optical fiber 15 in the preceding stage.
In comparison with the case of the prior art in which the optical isolator 13 is inserted on the incident side before the input connector 11 and the optical isolator 1.
3 is different in that the gain of the amplification optical fiber 15 in the preceding stage is included between the values 3 and 3.

In this case, the loop gain Gr for the return light between the input optical connector 11 and the optical isolator 13 is the gain of the amplification optical fiber 15 as g1 (dB).
Then, it is given as Gr = -25-50 + 2 × g1 (5).

If the loop gain Gr is suppressed to about -30 dB or less, the return light to the input optical connector 11 can be sufficiently suppressed, and the optical fiber amplifier can be made problem-free. That is, in the equation (5), if Gr = −30 dB, g1 = 22.5 dB can be obtained, and if the gain g1 of the amplification optical fiber 15 is up to 22.5 dB, the present invention as described above is performed. With the configuration of the embodiment of FIG.

Although the optical fiber amplifier according to the above-described embodiment of the present invention has been described as using the pumping light having the wavelength of 0.98 μm, the present invention also applies when the pumping light having the wavelength of 1.48 μm is used. Similarly, the influence on the noise characteristic deterioration of the optical isolator can be reduced corresponding to the gain g1 of the amplification optical fiber in the preceding stage. Further, the above-described embodiments of the present invention can be applied to various optical fiber amplifiers.

Next, a specific example of the embodiment of the present invention described above will be described with reference to FIG.

The excitation light emitted from the excitation light source 14 is light having a wavelength of 0.98 μm.
The signal light from the input optical connector 11 of μm is multiplexed by the optical multiplexer 12, and is incident on the amplification optical fiber 15. The power of the pumping light input to the amplification optical fiber 15 is 50 mW, the loss of the input optical connector 11 with respect to the signal light of wavelength 1.55 μm is 0.1 dB, and the optical multiplexer 1
It is assumed that the passage loss of 2 is 0.3 dB. Further, the loss at the fusion splicing point 21 between the optical multiplexer 12 and the amplification optical fiber 15 is 0.2 dB, and the length of the amplification optical fiber 15 is 8 dB.
m.

Of the light emitted from the amplification optical fiber 15, the excitation light of wavelength 0.98 μm passes through the multiplexer / demultiplexer 16 with the signal light of wavelength 1.55 μm, and is transmitted to the next amplification optical fiber 1.
It is incident on 7. On the other hand, the signal light having a wavelength of 1.55 μm separated by the demultiplexer / multiplexer 16 passes through the optical isolator 13, is multiplexed by the demultiplexer / multiplexer 16, and then enters the amplification optical fiber 17. The pass loss of the demultiplexer / multiplexer with respect to the signal light having a wavelength of 1.55 μm is 0.3 dB, and the signal light is subject to a loss of 0.6 dB due to demultiplexing and multiplexing. The loss at the fusion point 22 between the optical fiber of the demultiplexer / multiplexer 16 and the amplification optical fiber 15 and the loss at the fusion point 23 between the optical fiber of the demultiplexer / multiplexer 16 and the amplification optical fiber 17 are 0 respectively. .2
dB, the loss due to the two fusion points is 0.4 dB
become.

The light emitted from the amplification optical fiber 17 passes through the optical isolator 18 on the output side, and the emitted light connector 1
It is output from 9.

The noise characteristic of the optical fiber amplifier according to the above-mentioned example can be obtained by the equation (4) already explained. Then, as described above, the loss a1 of the input optical connector 11 is 0.1 dB, and the insertion loss a of the optical isolator 13 is a.
3 = 0.7 dB, insertion loss a2 of the multiplexer 12 = 0.3 d
B. In addition, the gain g of the amplification optical fiber 15 in the previous stage
1 is g1 = 10 dB. The spontaneous emission light coefficient of the amplification optical fiber 15 is nsp1 = 1 because the power of the excitation light is sufficiently large. The amplification optical fiber 17 in the latter stage
The gain g2 of g2 = 15 dB. The spontaneous emission light coefficient of the amplification optical fiber 17 is set to nsp2 = 1.02 because the power of pumping light is slightly reduced. Further, the insertion loss for inserting the optical isolator 13 in the middle of the two amplification optical fibers is added with the loss associated with the demultiplexer-multiplexer 16, so that a3 in the equation (4) is 1 as a whole. ．
It becomes 7 dB.

By inserting these values into the equation (4), the noise figure in the above-mentioned example can be obtained, and the noise figure NF2 is, as shown in the equation (6), NF2 = {2 × (32- 1) × 1.02} / {0.98 × 0.93 × 0.95 × 0.68 × 316} ＋ {2 × (10-1) × 1.0} / {0.98 × 0.93 × 0.95 × 10} = 0.34 + 2.08 = 2.42 = 3.84dB… … (6)

With the configuration according to the prior art described with reference to FIG. 2, when the loss of the optical component is set as in the above case, the noise figure NF1 becomes NF1 = 2.89 (4.6 dB). Further, in the case of the configuration according to the related art, not only the loss of the optical component on the input side but also the amplification optical fiber is only one. Therefore, the gain of the amplification optical fiber must be set to 25 dB. In this case, the spontaneous emission light in the opposite direction is amplified and the spontaneous emission light coefficient of the input section of the amplification optical fiber deteriorates, resulting in nsp = 1.07.

In the case of the specific example to which the above-described embodiment of the present invention is applied, the amplification of the spontaneous emission light in the opposite direction is suppressed by the optical isolator 13, so that the deterioration of the spontaneous emission light coefficient can be eliminated. That is, in the above-described example, by inserting the optical isolator on the input side at a point where a gain of 10 dB is obtained, the noise figure can be improved by 0.76 dB as compared with the case of the conventional technique.

Next, another concrete example of the embodiment of the present invention will be described with reference to FIG. In this example, an excitation light source having a wavelength of 1.48 μm is used as the excitation light.

In this example, as the multiplexer 12 in FIG. 1, a filter type multiplexer for multiplexing the light having a wavelength of 1.48 μm and the signal light is used. The loss of this multiplexer is 0.5 dB
Is. Excitation light with a wavelength of 1.48 μm has a wavelength of 1.55 μm
Since the signal light of m passes through the optical isolator 13, the demultiplexer / multiplexer 16 as in the case of using the wavelength of 0.98 μm as the pumping light is unnecessary. In this case, the optical fiber amplifier has the optical isolator 13 in the same direction as the optical isolator 18 on the output side to the place of the multiplexer / demultiplexer 16 shown in FIG.
Can be configured by inserting. And the wavelength is 1.48 μm
The nsp with the excitation light of is about 1.58.

The gain g of the two amplifying optical fibers
1, g2 is set to g1 = 10 dB, g2 = 10 dB, and the optical isolator 13 is inserted at a gain of 10 dB, that is, when the isolator 13 is inserted between two amplification optical fibers. The noise figure NF2 of the fiber amplifier is NF2 = 3.83 (5.8 dB).
Becomes

In the case of the conventional technique described with reference to FIG. 2, it is possible to combine the optical isolator 33 with the multiplexer 12 to reduce the total loss. Module loss is 0.9
It becomes dB. As a result, the noise figure NF1 when the multiplexer 12 and the optical isolator 33 of the prior art are combined is NF.
1 = 6.2 dB.

Therefore, when the optical isolator is inserted between the two amplifying optical fibers as in the specific example according to the embodiment of the present invention, the noise figure can be improved by 0.3 dB.

Although the above-described embodiment of the present invention has been described as using the erbium-doped optical fiber as the amplification optical fiber, the present invention is also applicable to the case where the amplification optical fiber having other additives is used. Can be applied. Further, the present invention may be configured such that the amplification optical fibers in the latter stage of the two amplification optical fibers in the above-described embodiment of the present invention are omitted.

In the above-described embodiment of the present invention, the coupler type optical multiplexer / demultiplexer 16 is used. However, the coupler type optical demultiplexer / multiplexer is the optical isolator 1.
In the case of separating the signal light into the loop of the optical fiber having 3, the separation ratio is about 20 dB, and the signal light leaks directly to the amplification optical fiber 17 on the subsequent stage, and as a result, the output signal light changes in level. May occur.

In the present invention, in order to prevent such a situation, as the optical demultiplexer-multiplexer 16, it is possible to use a filter type optical demultiplexer-multiplexer capable of having a separation ratio of 30 dB or more. In this case, the leak amount of the signal light from the optical demultiplexer-multiplexer 16 directly to the amplification optical fiber 17 in the subsequent stage can be extremely reduced, and the level fluctuation of the output signal light can be eliminated. .

[0051]

As described above, according to the present invention, the noise characteristics of the optical fiber amplifier can be improved, and the transmission distance of the optical communication using the optical fiber amplifier can be lengthened. The overall cost can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an erbium-doped optical fiber amplifier (EDFA) according to an embodiment of the present invention.

FIG. 2 is a diagram showing the basic configuration of a conventional erbium-doped optical fiber amplifier (EDFA).

[Explanation of symbols]

11 Input optical connector 12 Optical multiplexer 13, 18 Optical isolator 14 Excitation light source 15, 17, 35 Amplifying optical fiber 16 Optical demultiplexer-multiplexer 19 Output optical connector 21, 22, 23, 41 Fusion point of optical fiber 33 Input side optical isolator

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-3-206426 (JP, A) JP-A-4-96287 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 3/00-3/30

Claims (3)

(57) [Claims] 1. An amplification optical fiber, a pumping light source that emits pumping light that pumps the fiber, a wavelength-multiplexing wave device that causes the amplified light to enter the amplification optical fiber together with the pumping light, and an optical fiber. In an optical fiber amplifier including an isolator, a gain of the amplification optical fiber is 10 dB or more and 22.5 dB or less, and the optical isolator is noise caused by spontaneous emission light of the amplification optical fiber.
In the signal light path separated via the demultiplexer / multiplexer arranged on the output side of the amplification optical fiber.
An optical fiber amplifier characterized by being arranged . 2. A plurality of amplification optical fibers, a pumping light source that emits pumping light that pumps the fibers, and a wavelength-multiplexed wave for entering the amplified light together with the pumping light into the preceding amplification optical fiber. and vessels, in and an optical isolator configured optical fiber amplifier, the gain of the amplifying optical fiber of the preceding pre-SL two amplification optical fiber is more than 10dB 22.5dB below the optical isolator, The above
It also reduces noise due to spontaneous emission of amplification optical fiber.
And is arranged between the two amplification optical fibers.
An optical fiber amplifier arranged in a signal light path separated via a demultiplexer / multiplexer. 3. The amplification optical fiber is an erbium-doped optical fiber, and the excitation light for exciting erbium ions in the fiber is light having a wavelength of 1.48 μm or 0.98 μm, and the amplified light is Wavelength 1.55 μm
3. The optical fiber amplifier according to claim 1, wherein the optical fiber amplifier is the light of