JPH09326519A - Wide range optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To flatten the wavelength characteristics of gains ranging over a wide range. SOLUTION: A signal light S1 and the simulation light 9 sent from a light source 8 are conveyed to Er doped optical fiber 1, they are amplified, the output light only of the Er doped optical fiber 1 is conveyed to an Er doped optical fiber 2, and the light is attenuated so that the prescribed characteristics are obtained. The signal light S3 outputted from the Er doped optical fiber 2 and the simulation light sent from the light source 12 are conveyed to an Er doped optical fiber 3, and they are optically amplified for the second time by the Er doped optical fiber 3. By the superposition of three different wavelength characteristics obtained by the two amplification mediums formed by the Er doped optical fibers 1 and 3 and the loss medium formed by the Er doped optical fiber 2, the wavelength characteristics of the gains which are flat ranging over a wide range can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide band optical fiber amplifier having a flat gain wavelength characteristic over a wide band.

[0002]

2. Description of the Related Art In recent years, optical fiber amplifiers in which a rare earth element such as Er (erbium), Pr (praseodymium), or Nd (neodymium) is added to the core of an optical fiber have reached a practical level. In particular, an Er-doped optical fiber amplifier has a high gain and a high saturation output in the 1.55 μm band, and is therefore expected to be applied to various systems. Among them, 1.53μm to 1.56μ
High-speed, large-capacity, long-distance transmission and optical CATV (Cable Televis) by wavelength multiplex transmission using several or more wavelengths of signal light in m wavelength band
Ion) application to the system is attracting attention. When the Er-doped optical fiber amplifier is applied to such a system, the gain of the Er-doped optical fiber amplifier in the above-mentioned wavelength band is flat in order to suppress the deterioration of the optical S / N characteristic and the crosstalk characteristic. This is very important.

In order to achieve such a high gain and flattening, the present inventors show the Er-doped multicore optical fiber as shown in FIG. 7 and FIG. 8 using this Er-doped multicore optical fiber. Such an optical amplifier is proposed. First, the Er-doped multi-core optical fiber 1 used
00 indicates the primary cladding layer 1 as shown in FIG.
No. 02 is coated, and the glass rods 103 having a plurality of cores 101a to 101g to which a rare earth element (for example, Er) and Al are co-added are assembled, and the glass rods 10 are further assembled.
3 has a structure in which the periphery of 3 is thickly covered with the secondary cladding 104. By using such an Er-doped multi-core optical fiber 100, it is possible to achieve a high gain and a flat gain wavelength characteristic.

There are two reasons for this achievement. First, the first reason is that the Er-doped multi-core optical fiber has a conventional Al core concentration of 1
It is possible to sufficiently increase the amount for one Er-doped optical fiber. The second reason is that when the power of the pumping light in the core is lowered with the conventional optical fiber, the wavelength of 1.535μ
The gain peak near m decreases and the gain-wavelength characteristic gradually becomes flat. As the power is further lowered, the wavelength 1.
Since the gain in the short wavelength region on the 53 μm side is decreased and the gain in the long wavelength region on the 1.56 μm side is increased, that is, a so-called gain-wavelength characteristic that increases to the right from the short wavelength to the long wavelength, the pumping light is lowered. It was known that the gain became extremely low and could not be used as an optical amplifier, but this Er-doped multi-core optical fiber, on the contrary, made active use of this principle. That is, as shown in the drawing, if the core diameter D and the core interval d are optimized so that the excitation light and the signal light propagate almost uniformly in the Er-added cores 101a to 101g, respectively, the core 101a Although the amplification gain of the signal light propagating inside each of the cores 101 to 101g becomes low, its wavelength characteristic becomes almost flat, and after propagating a desired length, each of the cores 101a to 101g. The fact that the internally amplified signal is superposed and the wavelength characteristic of the gain becomes almost flat is used.

Next, the configuration of the optical amplifier of FIG. 8 based on the above-mentioned principle and the result of evaluation of the wavelength characteristic of the gain with respect to it will be described. Er-doped multi-core optical fiber 1
In No. 00, the core spacing d is 1.3 μm, each core diameter is about 2 μm, the cladding diameter is 125 μm, the relative refractive index difference Δ between the core and the cladding is 1.45%, and the mode field diameter is about 8.8 μm ( (Value at wavelength 1.55 μm), the addition amount of Er and Al in each core is 400 ppm and 8500 p
pm, fiber length of about 45 m, relative refractive index difference Δ of 2.19%, mode field diameter of about 5.2 μm (value at wavelength 1.55 μm), Er and Al in each core
Two kinds of fibers having a doping amount of 400 ppm and 17000 ppm and a fiber length of about 20 m were used.

Isolators 105a and 105b for preventing light from propagating in opposite directions are connected to both ends of the Er-doped multi-core optical fiber 100, and WDM (Wavelength Division Mult) is provided inside each of them.
iplexing) couplers 106a and 106b are provided. The signal light S ₁ is input to the isolator 105a,
The amplified signal light S ₂ is output from the isolator 105b.

Optical fibers 108a and 108b connected to pump semiconductor lasers 107a and 107b are coupled to the WDM couplers 106a and 106b, respectively, and pump lights 109a and 109b generated by the pump semiconductor lasers 107a and 107b are coupled. Is an Er-doped multi-core optical fiber 1
It is configured to be able to propagate to 00. When the signal light S ₁ is incident through the isolator 105a,
A laser beam 10 having a short wavelength is emitted from the exciting semiconductor laser 107a.
9a is generated and the laser beam 109a is generated by the WDM coupler 1
When it is incident on the Er-doped multicore optical fiber 100 via 06a, a certain energy level of ions is excited, and an amplification effect by stimulated emission occurs. The amplified optical signal S ₂ is output from the Er-doped multicore optical fiber 100 to the outside through the isolator 105b. Similarly,
Laser light 10 generated by the pumping semiconductor laser 107b
9b is incident on the Er-doped multi-core optical fiber 100 from the rear direction and performs optical amplification on the principle described above. Here, the excitation light is applied from the front and back, but it may be applied either before or after.

Here, the wavelengths of the pumping lights 109a and 109b are set to 980 nm, and the pumping light power of the pumping light 109a is 7
The excitation light power of 0 mW and the excitation light 109b was set to 80 mW. These values are values determined from the fact that the gain wavelength characteristic is suitable for flattening. The wavelength characteristics of gain (Al concentration dependency) in the optical amplifier having the configuration of FIG. 8 are measured in FIGS. 9 and 10. FIG. 9 shows the measurement when the added amount of Al was 8,500 ppm.
Fig. 1 shows the measurement when the addition amount of 17,000ppm.
0. Here, the wavelength characteristics of the respective gains were measured with the signal light power Sp as a parameter in the above two examples. As is clear from FIGS. 9 and 10, 1,54
It can be seen that the gain is flattened from 0 nm to 1,560 nm.

However, as shown in FIGS. 9 and 10, there is a large gain rise near the wavelength of 1,530 nm, and further flattening is desired. JP-A-6-77561 discloses a method for improving this. FIG. 11 shows the configuration of the optical fiber amplifier shown in this publication. As shown in FIG. 11, the optical isolator 201 to which the signal light S ₁ is applied includes an Er ⁺³ ion-doped optical fiber 202, a WDM coupler 203, an optical isolator 204, a bandpass filter (BPF) 205, and a signal light S. _An unexcited Er ⁺³ ion-doped optical fiber 206 that outputs ₂ is cascaded. A pumping light source 207 is coupled to the WDM coupler 203 via a fiber 208.

This optical fiber amplifier has a construction in which a non-pumped Er ⁺³ ion-doped optical fiber 206 is connected to the output end of the optical amplification section by the Er ⁺³ ion-doped optical fiber 202. This unexcited Er ⁺³ ion-doped optical fiber 2
If the supersaturated absorption characteristic of 06 is used, the waveform distortion generated in the optical amplification section is flattened.

[0011]

However, the optical fiber amplifier having the configuration shown in FIG. 11 has the following problems. A non-excited rare earth ion-doped fiber is provided at the output end of the optical amplifying section to flatten the waveform distortion generated in the optical amplifying section, and the gain wavelength characteristic is 1530 nm.
Flattening over the ~ 1560 nm range is difficult.

The reason is that the desired high gain (for example, 35
Gain characteristic curve (Figs. 9 and 10) of an Er-doped optical fiber amplifier having a characteristic of up to 40 dB and unexcited Er
It is mathematically difficult to flatten the gain in the above wavelength range by cascade-connecting the loss characteristics of the doped optical fiber (FIG. 12), and in order to realize this, the gain of the Er-doped optical fiber amplifier is significantly increased. Sacrificed or unexcited Er
It is necessary to add some kind of additive to the doped fiber and manipulate the wavelength characteristic of the loss so that the wavelength characteristic of the gain of the Er-doped optical fiber amplifier becomes flat. However, it is difficult with the current technology.

The present invention has been made in view of the above-mentioned circumstances of the prior art, and an object thereof is to provide a wideband optical fiber amplifier capable of flattening the wavelength characteristic of gain over a wide range.

[0014]

In order to achieve the above object, the present invention provides a first Er-doped optical fiber that optically amplifies incident signal light, and the first Er-doped optical fiber. A first optical fiber connected to the latter stage of the fiber to make the first pumping light incident on the first Er-doped optical fiber from the rearward direction;
WDM coupler, a loss medium that is connected to the first WDM coupler, and has a predetermined attenuation characteristic or wavelength selection characteristic, and a loss medium disposed downstream of the loss medium for signal light from the loss medium. A second Er-doped optical fiber that performs optical amplification, and is connected to a front stage, a rear stage, or both sides of the second Er-doped optical fiber, and the second pumping light is forwardly directed to the second Er-doped optical fiber. A second WDM coupler that propagates from the backward direction or from both directions is provided.

According to this structure, the signal light is optically amplified by the first Er-doped optical fiber based on the first pumping light supplied from the first WDM coupler, and is first amplified by the loss medium in the next stage. The signal light is attenuated so as to correct the characteristics of the Er-doped optical fiber after being amplified. Furthermore, the second Er
Optical amplification is performed by the combination of the doped optical fiber and the second WDM coupler so that a flat wavelength characteristic of gain is obtained over a desired band. As a result, it is possible to obtain one wavelength characteristic by superimposing three different wavelength characteristics by two gain media and one loss medium, and flatten the gain wavelength characteristics over a wide band while maintaining high gain. It will be possible.

A third Er-doped optical fiber can be used as the loss medium. With this configuration, it is possible to propagate the signal light without propagating the pumping light, to perform optical attenuation according to the length of the optical fiber, and to form the loss medium with a simple and low-cost configuration. . An optical fiber grating or an optical filter can be used as the loss medium.

According to this structure, a specific wavelength range can be selectively attenuated, and a flatter wavelength characteristic of gain can be obtained. As the optical fiber grating or optical filter, one having an optical wavelength selection characteristic in the 1530 nm band can be used. According to this configuration, of the gain wavelength characteristics, it is possible to effectively attenuate the peak wavelength band of 1530 nm, and to flatten the gain wavelength characteristics over a wide band.

The first and second Er-doped optical fibers are
As the excitation light, 980 nm band, 1480 nm band, or 1060 nm band can be used. According to this configuration, the noise figure can be reduced by using the 980 nm band,
Cost reduction can be achieved by using the 1480 nm band.
High gain can be achieved by using the 0 nm band.

The first, second or third Er-doped optical fiber has a core made of SiO ₂ glass fiber to which at least 100 ppm Er and at least 1000 ppm Al are added, and at least Ge, P, F.
It is possible to use one containing one kind of the refractive index controlling additive such as. With this configuration, it is possible to easily reduce the loss, increase the gain, and widen the band of the Er-doped optical fiber.

The first Er-doped optical fiber and the second Er-doped optical fiber are each adjusted so that the gain and wavelength characteristics of the signal light output from the second Er-doped optical fiber have desired values. Has set the length of. With this configuration, the gain and the wavelength characteristic can be adjusted according to the length, and the characteristic of the loss medium can be corrected to easily obtain the waveform of the wavelength characteristic of the desired gain.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a connection diagram showing a first embodiment of a broadband optical fiber amplifier according to the present invention.
2 shows the wavelength-loss / gain characteristics in the optical fiber amplifier of FIG.

The length l ₁ of the Er doped optical fiber 1 (first Er-doped optical fiber), the length l ₂ Er-doped optical fiber 2 (third Er-doped optical fiber), and a length l ₃ of the Er
The doped optical fiber 3 (second Er doped optical fiber) is cascade-connected. An optical isolator 4 for advancing signal light in only one direction is connected to the input side of the Er-doped optical fiber 1, and an optical isolator 5 is connected to the output side of the Er-doped optical fiber 3.

The Er-doped optical fiber 1 having the structure shown in FIG. 7 can be used, and its length is 10 m, and Er is
Addition amount is 400ppm, Al addition amount is 17,000pp
m, each core interval is 1.3 μm, and each core diameter is 1.
9 μm, the clad diameter was 125 μm, and the relative refractive index difference between the core and the clad was about 2.1%. The Er-doped optical fiber 2 has a length l ₂ = 2 m. This E
Since the pumping light does not propagate in the r-doped optical fiber 2, the r-doped optical fiber 2 acts as a loss medium, and the loss medium characteristic shown in FIG. 2 is obtained. Further, the Er-doped optical fiber 3 has the same structure and characteristics as the Er-doped optical fiber 1, but its length l ₃ is 8 m.
It is.

A WDM coupler 6 is provided between the Er-doped optical fiber 1 and the Er-doped optical fiber 2. A pumping light source 8 is connected to the WDM coupler 6 via an optical fiber 7.
Are combined. Excitation light 9 generated by the excitation light source 8 (here, the wavelength is 980 nm, the optical power is 100 m
W) is sent to the WDM coupler 6, is sent to the Er-doped optical fiber 1 from the opposite direction (backward direction) to the signal light S _i and is excited.

Further, a WDM coupler 10 is provided on the output side of the Er-doped optical fiber 2, and a pumping light source 12 is coupled to the WDM coupler 10 through an optical fiber 11. The pumping light 13 from the pumping light source 12 enters the Er-doped optical fiber 3 through the WDM coupler 10. Next, the operation of the configuration of FIG. 1 will be described. Signal light S
₁ is usually a wavelength-multiplexed signal light, which is 1530n
Signal light having a wavelength of several waves to several tens of waves is wavelength-multiplexed from a wavelength band of m to 1560 nm. The signal light S _i passes through the optical isolator 4 and enters the Er-doped optical fiber 1. The pumping light 9 generated by the pumping light source 8 is supplied to the Er-doped optical fiber 1 through the WDM coupler 6 and pumped. As a result, the amplified signal light S ₂ is output from the Er-doped optical fiber 1, and this signal light S ₂ is incident on the Er-doped optical fiber 2. The wavelength characteristic of the gain of the signal light S ₂ is shown by the “amplification medium I” in FIG. 2, which shows a downward sloping characteristic.

The Er-doped optical fiber 2 is a pumping light source 8, 1
Since the pumping light is not given from any of the two, the amplification is not performed, and the Er-doped optical fiber 2 functions as a loss medium. As a result, the characteristic shown by the "loss medium" in FIG. 2 is obtained.
The signal light S _{3 that} has passed through the Er-doped optical fiber 2 has the characteristics shown by “S ₃ ” in FIG. The signal light S ₃ passes through the WDM coupler 10 and enters the Er-doped optical fiber 3 together with the pumping light 13 given from the pumping light source 12. The characteristic of this Er-doped optical fiber 3 is "amplification medium II".
Is shown (characteristics shown by dotted lines). This characteristic is downward sloping and the inclination is opposite to the characteristic of the signal light S ₃ . A characteristic obtained by combining the three characteristics of the Er-doped optical fibers 1, 2 and 3 becomes the signal light S ₄ , and has a flat characteristic as shown in the figure.

FIG. 3 is a connection diagram showing a second embodiment of the broadband optical fiber amplifier according to the present invention. While the Er-doped optical fiber 2 is provided between the Er-doped optical fiber 1 and the Er-doped optical fiber 3 in the configuration of FIG. 1, the attenuation optical filter 14 is used in place of the Er-doped optical fiber 2 in FIG. It is provided. This attenuating optical filter 14 has 15
It has the property of attenuating the signal light in the 30 nm band,
For example, a structure in which an interference film filter is inserted in the optical fiber can be used. The attenuating optical filter 14 can realize a loss wavelength characteristic having a narrower band than the loss wavelength of the Er-doped optical fiber 2 of FIG.
Attenuation of signal light in a very narrow wavelength band of 530 nm band can be given.

An optical isolator 4 for advancing signal light in only one direction is connected to the input side of the Er-doped optical fiber 1, and an optical isolator 5 is connected to the output side of the Er-doped optical fiber 3. A WDM coupler 6 is provided between the Er-doped optical fiber 1 and the attenuation optical filter 14. A pumping light source 8 is coupled to the WDM coupler 6 via an optical fiber 7. Further, a WDM coupler 10 is provided on the output side of the attenuation optical filter 14, and an excitation light source 12 is coupled to the WDM coupler 10 through an optical fiber 11.

Since the entire operation in FIG. 3 is the same as that of the configuration in FIG. 1, its explanation is omitted. FIG. 4 is a connection diagram showing a third embodiment of the broadband optical fiber amplifier according to the present invention. The configuration of FIG. 3 has Er-doped optical fibers 1 and E.
While the attenuation optical filter 14 is provided between the r-doped optical fibers 3, in FIG.
In place of 4, an attenuation optical fiber grating 15 is provided. The attenuation optical fiber grating 15 is
It has a characteristic of attenuating the signal light in the 1530 nm band, and has an advantage that it can be easily selected from a very narrow band characteristic to a wide band characteristic. The optical fiber grating 15 is also excellent in fiber matching with the Er-doped optical fiber and the WDM coupler.

FIG. 5 is a connection diagram showing a fourth embodiment of the wideband optical fiber amplifier according to the present invention. Er-doped optical fiber 1 of length l _1, Er-doped optical fiber 3 long Er-doped optical fiber 2 l _2, and a length l ₃ are cascaded. On the input side of the Er-doped optical fiber 1,
An optical isolator 4 for advancing signal light in only one direction is connected, and an optical isolator 5 is connected to the output side of the Er-doped optical fiber 3.

The characteristics, materials, lengths, etc. of the Er-doped optical fiber 1, the Er-doped optical fiber 2 and the Er-doped optical fiber 3 are the same as those in FIG. A WDM coupler 6 is provided between the Er-doped optical fiber 1 and the Er-doped optical fiber 2. This WDM coupler 6 has an optical fiber 7
The excitation light source 8 is coupled via the. Excitation light 9 generated by the excitation light source 8 (here, the wavelength is 980n
m, the optical power is 100 mW) is sent to the WDM coupler 6, and is sent to the Er-doped optical fiber 1 from the direction opposite to the signal light S _i to be excited.

Further, a WDM coupler 10 is provided on the output side of the Er-doped optical fiber 2, and a light receiving element 16 is coupled to the WDM coupler 10 through an optical fiber 11. Further, a WDM coupler 17 is arranged between the Er-doped optical fiber 3 and the optical isolator 5, and a pumping light source 19 is coupled via an optical fiber 18. Excitation light source 1
Reference numeral 9 denotes a semiconductor laser that generates laser light having a wavelength of 980 nm, for example, and this pumping light 20 is propagated through the WDM coupler 17 to the Er-doped optical fiber 3 from the opposite direction (backward direction) to the signal light. At this time, the WDM coupler 10 blocks the pumping light 20 from propagating to the Er-doped optical fiber 2. This is because if the pumping light 20 is not blocked, the Er-doped optical fiber 2 does not become a loss medium but an amplification medium, and the wavelength characteristic of gain is not flat. The pumping light 21 propagating from the opposite direction in the Er-doped optical fiber 3 is received by the light receiving element 16 via the WDM coupler 10 and the optical fiber 11 and monitored.

The overall operation in FIG. 5 and Er
The characteristics of each of the doped optical fibers 1, 2 and 3 are also the same as the configuration of FIG. FIG. 6 is a connection diagram showing a fifth embodiment of the broadband optical fiber amplifier according to the present invention. This configuration is different from the configuration of FIG.
The feature is that a means for injecting the excitation light from the rear direction of the doped optical fiber 3 is provided. That is, Er of length l ₁
Doped optical fiber 1, the length l ₂ Er-doped optical fiber 2,
And the Er-doped optical fiber 3 having a length of l ₃ are cascade-connected. An optical isolator 4 for advancing signal light in only one direction is connected to the input side of the Er-doped optical fiber 1, and an optical isolator 5 is connected to the output side of the Er-doped optical fiber 3.

The characteristics, materials, lengths, etc. of the Er-doped optical fiber 1, Er-doped optical fiber 2 and Er-doped optical fiber 3 are the same as those in the configuration shown in FIG. Er-doped optical fiber 1 and Er-doped optical fiber 2
A WDM coupler 6 is provided between them. This WD
A pumping light source 8 is coupled to the M coupler 6 via an optical fiber 7. Excitation light 9 generated by this excitation light source 8
(Here, the wavelength is 980 nm and the optical power is 100 m.
W) is sent to the WDM coupler 6, is sent to the Er-doped optical fiber 1 from the opposite direction (backward direction) to the signal light S _i and is excited.

Further, a WDM coupler 10 is provided on the output side of the Er-doped optical fiber 2, and a pumping light source 12 is coupled to the WDM coupler 10 through an optical fiber 11. The pumping light 13 from the pumping light source 12 is incident on the Er-doped optical fiber 3 from the front direction through the WDM coupler 10. In addition, the Er-doped optical fiber 3 and the optical isolator 5
The WDM coupler 17 is disposed between the optical fiber 18 and
The excitation light source 19 is coupled via the. The pumping light 20 from the pumping light source 19 passes through the WDM coupler 17 from the direction opposite to the signal light (backward direction) to the Er-doped optical fiber 3
Propagated to. At this time, the WDM coupler 10 causes the pumping light 2
It functions so as to prevent 0 from being propagated to the Er-doped optical fiber 2, so that the Er-doped optical fiber 2 acts as a loss medium.

In the configuration of FIG. 6, the excitation light (13 and 20) is applied to the Er-doped optical fiber 3 from both directions. As a result, the total power can be increased by the three pumping lights, so that a high gain of 40 dB or more can be obtained, and the wavelength characteristic of the gain can be made flat as shown in FIG. Also, the noise figure can be reduced.

Although the wavelength of the excitation light is set to 980 nm in each of the above configurations, the present invention is not limited to this value, and may be 1480 nm, for example.
Further, in the case of using two excitation light sources, one may be 980 nm and the other may be 1480 nm. Specifically, it is possible to achieve a low noise figure by using the 980 nm band,
Cost reduction can be achieved by using the 1480 nm band.
High gain can be achieved by using the 0 nm band.

The Er-doped optical fibers 1, 2 and 3 may be Er-doped optical fibers having a single core, which has been conventionally used. And the Er-doped optical fiber 1
~ 3 core material is SiO ₂ glass (at least Ge
(Containing one kind of refractive index control additive such as germanium), P (phosphorus), F (fluorine) and the like) added with at least 100 ppm of Er and at least 1000 ppm of Al can be used. The cladding material may be SiO ₂ , or at least P, F in SiO _2.
It is possible to use one containing one kind of the refractive index controlling additive such as.

[0039]

As described above, according to the present invention, one wavelength characteristic is obtained by superimposing three different wavelength characteristics by two gain media and one loss medium. Therefore, while maintaining high gain, It becomes possible to flatten the wavelength characteristic of gain over a wide band.

[Brief description of drawings]

FIG. 1 is a connection diagram showing a first embodiment of a broadband optical fiber amplifier of the present invention.

FIG. 2 shows the wavelength in the broadband optical fiber amplifier of FIG.
It is a loss / gain characteristic.

FIG. 3 is a connection diagram showing a second embodiment of the broadband optical fiber amplifier of the present invention.

FIG. 4 is a connection diagram showing a third embodiment of a wideband optical fiber amplifier of the present invention.

FIG. 5 is a connection diagram showing a fourth embodiment of a broadband optical fiber amplifier of the present invention.

FIG. 6 is a connection diagram showing a fifth embodiment of a broadband optical fiber amplifier of the present invention.

FIG. 7 is a cross-sectional view showing an example of a conventional Er-doped multicore optical fiber.

8 is a connection diagram showing a configuration of an optical amplifier using the Er-doped multicore optical fiber of FIG.

FIG. 9 is a wavelength characteristic diagram of gain in the case where the amount of Al added is 8,500 ppm in the optical amplifier having the configuration of FIG.

FIG. 10 is a wavelength characteristic diagram of gain in the case where the amount of Al added is 17,000 ppm in the optical amplifier having the configuration of FIG.

FIG. 11 is a connection diagram showing a configuration of another conventional optical amplifier for which flattening has been achieved.

FIG. 12 is a characteristic diagram showing a loss wavelength characteristic of an Er ⁺³ ion-doped optical fiber.

[Explanation of symbols]

1,2,3 Er-doped optical fiber 4,5 Optical isolator 6,10,17 WDM coupler 8,12,19 Pumping light source Pumping light source 9,13,20 Pumping light 14 Attenuating optical filter 15 Attenuating optical fiber grating S ₁ , S ₄ signal light

Claims (8)

[Claims] 1. A first Er-doped optical fiber that optically amplifies incident signal light, and a first Er-doped optical fiber connected to a subsequent stage of the first Er-doped optical fiber.
A first WDM coupler that causes the pumping light of the above to be incident on the first Er-doped optical fiber from the rear direction; and a loss medium that is connected to the first WDM coupler and has a predetermined attenuation characteristic or wavelength selection characteristic. A second Er-doped optical fiber that is disposed in the latter stage of the loss medium and that optically amplifies the signal light from the loss medium, and is connected to the front stage, the rear stage, or both sides of the second Er-doped optical fiber. And a second WDM coupler that propagates the second pumping light to the second Er-doped optical fiber from the forward direction, the backward direction, or both directions. 2. The broadband optical fiber amplifier according to claim 1, wherein the loss medium is a third Er-doped optical fiber. 3. The broadband optical fiber amplifier according to claim 1, wherein the loss medium is an optical fiber grating. 4. The broadband optical fiber amplifier according to claim 1, wherein the loss medium is an optical filter. 5. The broadband optical fiber amplifier according to claim 3, wherein the optical fiber grating or the optical filter has an optical wavelength selection characteristic in the 1530 nm band. 6. The first and second Er-doped optical fibers have excitation light of 980 nm band, 1480 nm band,
The broadband optical fiber amplifier according to claim 1, wherein the band of 1060 nm is used. 7. The first, second or third Er-doped light
The fiber should be at least 100 ppm Er
SiO with 1000 ppm Al added _TwoSystem glass
A fiber core and at least Ge, P,
Characterized by containing one kind of a refractive index controlling additive such as F
The broadband optical fiber amplifier according to claim 1 or 2. 8. The first Er-doped optical fiber and the second Er-doped optical fiber are configured such that the signal light output from the second Er-doped optical fiber has desired gain and wavelength characteristics. 2. The broadband optical fiber amplifier according to claim 1, wherein each of the lengths is set to.