JPH09138432A - Optical amplifier - Google Patents

Abstract

(57) Abstract: An optical fiber amplifier having low noise and wide band amplification band characteristics is provided. In an optical fiber amplifier configured by connecting in series a plurality of amplifying units using Er-doped fibers having different glass compositions as an amplifying medium, at least a first-stage amplifying unit and a second-stage amplifying unit are provided with an optical isolator. Connected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low noise wide band optical amplifier required in the fields of optical communication and optical measurement.

[0002]

2. Description of the Related Art A fiber-type optical amplifier using an Er-doped optical fiber is capable of optical amplification in the 1.5 μm band where the transmission loss of a quartz fiber for communication is minimum, has no polarization dependence, and has no noise. It is an important component for optical communication because it has excellent characteristics.

FIG. 6 shows a basic configuration (prior art 1) of an amplifier using an Er-doped optical fiber. In FIG. 6, 1
Is an Er-doped optical fiber that is an amplification medium, 2-1 and 2-2
Is a pumping light source section for generating pumping light to the Er-doped optical fiber 1, 3-1 and 3-2 are signal light and pumping light source sections 2-1 and 2-
2. A multiplexing unit that couples the pumping light generated in 2, 4-1 and 4-
Reference numeral 2 is an optical isolator for suppressing oscillation of the optical amplifier. Usually, semiconductor lasers of 980 nm or 1480 nm band oscillation are used for the pumping light source sections 2-1 and 2-2, and the amplifier of FIG. 6 shows a bidirectional pumping configuration for pumping the Er-doped optical fiber 1 from both sides.

In such an optical fiber amplifier, it is an indispensable subject to improve the characteristics of the optical fiber amplifier that the expansion of the amplification band widens the application area of the optical fiber amplifier and the flattening of the amplification gain. In particular, W has been actively researched and developed in the field of optical communication systems in recent years.
In a DM (wavelength division multiplex) optical transmission system, the number of wavelength multiplexes is limited, or in a WDM optical communication system configured by connecting optical amplifiers in multiple stages, the optical signal strength at an optical signal receiving end at each signal wavelength is There was a big problem when configuring the system, such as different systems. That is, there has been a demand for the development of an optical fiber amplifier capable of optically amplifying a WDM signal over a wide wavelength range without gain deviation.

The four methods proposed as methods for improving the gain band characteristic of an optical fiber amplifier are shown below.

1) Al or Al and P are co-doped into an Er-doped optical fiber which is an amplification medium (prior art 2).

2) A wavelength equalizer for compensating the wavelength dependence is added to the optical fiber amplifier (prior art 3).

3) Change the glass host of the Er-doped optical fiber to fluoride glass (prior art 4).

4) Use of an amplifying medium in which two or more kinds of Er-doped optical fibers having different fiber glass compositions are connected (prior art 5).

In method 1 (prior art 2), an Er-doped silica optical fiber not co-doped with Al has a signal wavelength of 1536n.
m and 1552n2 μm had sharp amplification peaks, the addition of Al resulted in a signal wavelength of 155
The amplification peak of 2 μm spreads, and the signal wavelength range 1540 to 1
The amplification band characteristic is improved at 560 nm (see FIG. 7). However, in this configuration, the signal wavelength range 1540 to 1560n
It was difficult to amplify the WDM signal gain at m without gain deviation.

Method 2 (prior art 3) was devised to solve the problems of prior art 2. FIG. 8 shows the configuration of Prior Art 3.
Shown in This technique has a configuration in which a wavelength equalizer 13 for compensating for wavelength dependence is added to the optical amplification unit 7 of the conventional technique 2. The wavelength equalizer 13 can be installed at both the front stage and the rear stage of the optical amplification unit 7. However, in order to avoid the noise characteristic deterioration of the optical amplifier when the wavelength equalizer 13 is installed at the front stage of the optical amplification unit 7, the wavelength equalizer 13 is usually installed at the rear stage. To be done. As the wavelength equalizer 13, a silica optical circuit including a Mach-Zehnder interference circuit is used. By using this configuration, the signal wavelength range 1540 to 1560n
An optical fiber amplifier capable of amplifying the WDM signal at m with a gain deviation of 0.1 dB can be constructed. However, the method of reducing the wavelength dependence using this configuration has a drawback that the cost of the optical fiber amplifier is improved because the wavelength equalizer is added to the optical fiber amplifier and the signal band cannot be set to 1540 to 1560 nm or more.

Method 3 (prior art 4) was devised to solve the problem that the amplification band of prior arts 2 and 3 is limited to the 1540 to 1560 nm band. The amplifier system uses the amplification system shown in FIG. FIG. 9 shows a WDM signal amplification characteristic of an optical fiber amplifier using an Er-doped fluoride fiber as an amplification medium (the signal wavelength is 1532 to 1560 nm, 4
(Amplification characteristics when 8 waves are arranged at an interval of nm). Further, FIG. 9 also shows the characteristics of an optical fiber amplifier (prior art 2) using an Al co-doped Er-doped silica fiber. As shown in the figure, the Er-doped fluoride fiber amplifier can amplify the WDM signal light in the signal wavelength range 1532 to 1560 nm within a gain deviation of 1.5 dB. On the other hand, in the optical fiber amplifier according to the related art 2, the gain deviation is 1.5 dB.
The allowable wavelength range is 1536 to 1560 nm,
The amplification band characteristic can be improved in the short wavelength region by the conventional technique 4. However, there is a problem that the gain deviation is 1.5 dB.

Method 4 (Prior Art 5, JP-A-7-2344)
No. 23), the amplification band of the prior art 4 is 1532 to 1532.
It was developed to improve the gain deviation of 1.5 dB in the 60 nm band. This configuration uses an amplification medium in which two or more kinds of Er-doped optical fibers having different fiber glass compositions are connected. As shown in FIGS. 7 and 9, the amplification band characteristic of the Er-doped optical fiber is changed by changing the glass host of the Er-doped optical fiber. Therefore, by connecting multiple Er-doped fibers with different amplification band characteristics,
Mutual amplification characteristics can be improved to configure an optical fiber amplifier with a wide band and a small gain deviation. FIG. 10 shows an example of the configuration of Prior Art 5. The configuration is a combination of an Al-doped Er-doped silica fiber as the Er-doped optical fiber 1-1, which is an amplification medium, and an Er-doped fluoride fiber, as the Er-doped optical fiber 1-2. Excitation light source unit 2
For 1 and 2-2, 1480 nm band LDs were used. FIG.
Shows the WDM signal amplification characteristics of this optical fiber amplifier (amplification characteristics when the signal wavelength is 1532 to 1560 nm and 8 waves are arranged at 4 nm intervals). With this technique, the gain deviation can be improved from 1.5 dB of the prior art 4 to 1 dB. However, as shown in the characteristic diagram, this configuration has a drawback that the noise characteristic of the optical amplifier is as high as 6 dB (signal wavelength 1532 nm).

[0014]

As described above, in the prior art, optical amplification with a small gain deviation over a wide wavelength band and low noise was impossible.

The present invention has been made in view of such circumstances, and an object thereof is to provide an optical fiber amplifier having low noise and wide band amplification band characteristics.

[0016]

In order to achieve the above object, the optical amplifier according to the present invention is an optical fiber in which an active species made of a rare earth element or a transition metal element having a laser transition level is added to a core or a clad. Type amplifying medium, a pumping light source for optically exciting the active species, and a plurality of optical amplifying sections including optical means for coupling pumping light and amplified light emitted from the pumping light source to the optical fiber type amplifying medium. At least one of the plurality of optical fiber type amplification media uses a glass host different from other optical fiber type amplification media, and at least the first stage optical amplification unit of the plurality of optical amplification units is used. The optical amplification unit in the next stage is connected via an optical isolator.

Here, the optical fiber type amplification medium is Er
The fiber may be selected from among doped silica optical fiber, Er-doped oxide multi-component glass fiber, Er-doped fluoride glass fiber, Er-doped chalcogenide glass fiber, Er-doped telluride glass fiber, The added oxide-based multi-component glass fiber is an aluminosilicate glass fiber, a fluorophosphate glass fiber or a phosphate glass fiber, and the Er-doped fluoride glass fiber is a Zr-based fluoride glass fiber or an In-based fluoride glass fiber. It may be.

At least the pumping light source of the first-stage amplifier is E
A light source that excites the ⁴ I _11/2 level of r is preferable.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The most important feature of the present invention is an optical fiber amplifier constructed by connecting in series a plurality of amplifying sections using Er-doped fibers having different glass compositions as an amplifying medium. A light source that excites Er ⁴ I _11/2 level excitation is used for the first-stage amplifier,
2) At least the first-stage amplification unit and the second-stage amplification unit are connected via an optical isolator.

FIG. 1 shows the basic configuration of the present invention. However,
This configuration is an example using two types of Er-doped fibers having different glass compositions. Reference numerals 1-1 and 1-2 denote Er-doped optical fibers which are amplification media, and the optical fibers 1-1 and 1-
Each of No. 2 has a different glass composition of the fiber. 2-1 and 2-2 are Er-doped optical fibers 1-1 and 1 respectively.
-2, 3-1, 3-2 excitation light source unit for generating excitation light to -2
Is a multiplexer for multiplexing the signal light and the pumping light generated by the pumping light source units 2-1 and 2-2, and 4-1 and 4-2 are optical isolators for suppressing oscillation of the optical amplifier. The excitation light source section 2-1 which is the first feature of the present invention generally uses a 980 nm band laser light source as a light source for exciting ⁴ I _11/2 level excitation of Er. Reference numeral 5 is an optical isolator which is the second feature of the present invention.

The noise figure of the optical fiber amplifier is determined by the noise figure of the amplifying medium and the insertion loss of the incident part (optical coupler 3-1 and optical isolator 4-1 in the figure). As for the noise figure of the optical fiber amplifier, the gain of the preceding amplification medium is G1, the noise figure is NF1, and the gain of the latter amplification medium is G1.
2. If the noise figure is NF2,

[0022]

## EQU1 ## The noise figure of the optical fiber amplifier can be expressed as the insertion loss of the incident part + NF1 + NF2 / G1 (1) (Equation above is E for two different fiberglass compositions.
It represents a case where an amplification medium to which an r-doped optical fiber is connected is used). That is, it is found that the noise figure of the pre-stage amplification medium needs to be reduced in order to reduce the noise of the optical fiber amplifier. Normally, the noise figure of Er-doped fiber, which is an amplification medium, differs depending on the excitation level of Er, and the noise figure in the case of ⁴ I _11/2 level excitation (excitation wavelength is ˜980 nm) is ˜3 dB (quantum limit noise). (Index: 3 dB), the noise figure in the case of ⁴ I _13/2 level excitation (excitation wavelength: ˜1480 nm) is ˜4.1 dB (M. Yamada et al., “Noise ch.
aracteristics fo Er-doped fiber amplifier pumped b
y 0.98 and 1.48 μm laserdiodes ”, IEEE PTL, vol.
2, pp.51-53, 1990). Therefore, in the present invention, in order to reduce the noise of the optical fiber amplifier, the pumping light source unit 2 of the pre-stage amplifier unit is used.
A 980 nm band LD was used for -1.

Further, the optical isolator 5, which is a feature of the present invention, acts to eliminate the following two noise characteristic deterioration factors that occur when a plurality of amplifiers are connected in series.

1) When a plurality of amplifying sections are connected in series to form an amplifier, the _subsequent amplifying section generally has ⁴ I _13/2 level pumping (excitation) to increase the output of the optical fiber amplifier. The wavelength used is ˜1480 nm). However, in the case where the optical isolator 5 is not provided, the 1480n used in the post-stage amplifier is used.
The m-band pump light passes through the Er-doped fluoride fiber in the latter stage and also excites the Er-doped fluoride fiber in the former stage, resulting in an increase in the noise figure in the former stage (980 nm and 1480).
Because it becomes a composite excitation of nm excitation).

2) When the optical isolator 5 is not used,
There is a problem that the spontaneous emission light that is generated in the Er-doped fiber in the latter stage and propagates in the forward direction deteriorates the noise characteristic in the former stage. This deterioration of the noise figure is caused by the reduction of the Er ion number n1 of the ⁴ I _13/2 level which is the amplification initial level in the pre-stage amplification medium due to the amplification of the spontaneous emission light (the amplification end level ⁴ I _15/2 (Increase in the number of ions n2)). That is, if the noise figure uses these n1 and n2,

[0026]

## EQU00002 ## The noise figure can be expressed as 2n1 / (n1-n2) (2), and the noise figure increases due to the amplification of the spontaneous emission light.

Therefore, in the present invention, first, in the first-stage amplifying section, by using a light source that excites the ⁴ I _11/2 level excitation of Er, the noise of the amplification medium is reduced to the quantum limit (noise figure 3 dB). To do. Further, by the optical isolator 5, spontaneous emission light that is generated in the latter-stage amplifier section and is incident on the former-stage amplifier section, which occurs when a plurality of amplifying sections are connected in series, and incident of the latter-stage amplifier section and pump light to the former-stage amplifier section. Both of them can be removed, and the deterioration of the noise characteristics of the front-stage amplification section due to spontaneous emission light and the rear-stage amplification section / pumping light can be eliminated. That is, since the noise reduction of the pre-stage amplifier can be achieved by this method, the noise characteristic of the entire amplifier can be significantly improved as shown in Expression 1 (the amplifier noise is expressed by the first term and the first term in Expression (1)). It is almost determined by the second term, and the third term is usually negligible because G1 is 100 times or more).

In the above description, an optical fiber amplifier formed of two amplifying units has been described, but the same applies to an optical fiber amplifier including a plurality of amplifying units. FIG. 2A shows the configuration of the present invention including three or more amplifying units. 7-1 is the first-stage amplifier, 7-2, 7-
3 is a post-stage amplification section (7-2 is the next-stage amplification section), 5 is an optical isolator inserted between the first-stage amplification section 7-1 and the next-stage amplification section 7-2, and 6-1 and 6-2 are each amplification section. It is an optical isolator inserted between them. However, a configuration without the optical isolators 6-1 and 6-2 is also possible. Further, the noise reduction of the present invention is effective regardless of whether the pumping direction of each amplification unit is the forward pumping system shown in FIG. 2B or the backward pumping system shown in FIG. 2C.

[0029]

The present invention will be described in more detail below with reference to the drawings, but the embodiments disclosed below are merely examples of the present invention and do not limit the scope of the present invention.

Example 1 Example 1 will be described with reference to FIG. 1-1 is an Er-doped silica optical fiber (fiber length 4 m, relative refractive index difference 1.8%, cutoff wavelength 1.1 μm).
m, Al addition concentration 4 wt%, Er addition concentration 1310 pp
m), 1-2 is Er-doped fluoride fiber (glass composition _{ZrF 4 -BaF 2 -LaF 3 -YF 3} -AlF 3 -L
iF-NaF), fiber length 5 m, relative refractive index difference 2.5
%, Cutoff wavelength 1.0 μm, Er addition concentration 1000
ppm), 2-1 is a pumping section (pumping light amount is 80 mW) using a 980 nm band strain quantum well semiconductor laser that generates pumping light to the Er-doped optical fiber 1-1, 2-2 is Er-doped optical fiber 1- A pumping section (pumping light amount is 100 mW) using a semiconductor laser for pumping 1480 nm band oscillation that generates pumping light to 2; 3-1 and 3-2 are signal lights (1510 nm
˜1580 nm) and a pump for synthesizing the pumping light generated by the pumping units 2-1 and 2-2. 3-1 is a WDM fiber coupler and 3-2 is a bulk-type multiplexer. . 4-
1, 4-2 and 5 are polarization independent optical isolators for the 1550 nm band.

FIG. 3 shows the gain band characteristic and the noise characteristic of the fiber type optical amplifier prepared. However, the characteristics were evaluated by the WDM signal amplification characteristic evaluation system shown in FIG. In FIG. 4, 8-1, 8-2, 8-3, 8-4, 8-5, 8-
6, 8-7 and 8-8 are variable wavelength signal light sources, and 9 is a multiplexer for multiplexing the variable wavelength light sources 8-1 to 8-8.
Reference numeral 0 is an optical attenuator for adjusting the amount of incident signal light, 11 is an optical fiber amplifier to be measured, and 12 is an optical spectrum analyzer. Wavelength variable light source 8-1, 8-2, 8-3, 8-
The signal wavelength of 4,8-5,8-6,8-7,8-8 is 15
32, 1536, 1540, 1544, 1548, 15
52, 1556 and 1560 nm. As shown in FIG. 3, the optical fiber amplifiers of this embodiment are 1532 to 1560.
Optical amplification was possible with a gain deviation of 1 dB for a WDM signal of nm, and the noise figure was 4.8 dB or less. Also,
This figure also shows, for comparison, the noise characteristics when the prior art 5 and the optical isolator 5 are removed. It was found that the noise characteristics can be greatly improved by inserting an optical isolator. From the above, it is possible to reduce the noise of the broadband optical fiber amplifier.

It should be noted that the characteristics shown in FIG.
The backward pumping system shown in FIG.
Similar amplification band characteristics and noise characteristics were obtained with the forward pumping system shown in (b).

(Second Embodiment) A second embodiment will be described with reference to FIG. 1-1 is an Er-doped silica optical fiber (fiber length 30 m, relative refractive index difference 0.7%, cutoff wavelength 1.1 μm).
m, Al addition concentration 10000 ppm, Er addition concentration 10
000 ppm), 1-2 is oxide other component glass fiber (glass composition: aluminosilicate glass (SiO ₂ (6
_{5.3wt%) - Al 2 O 3} (4.9wt%) - Li 2
_{O (2.8wt%) - Na 2} O (18wt%) - MgO
(2.2wt%) - CaO (6.5wt %) - Sb 2 O
₃ (0.3 wt%)), fiber length 60 cm, relative refractive index difference 0.7%, cutoff wavelength 1.1 μm, Er addition concentration 10000 ppm), 2-1 is Er addition optical fiber 1-
A pumping section (a pumping light amount is 80 mW) using a 980 nm band strained quantum well semiconductor laser that generates pumping light for 1 to 2-2
Generates excitation light to the Er-doped optical fiber 1-2 14
Excitation unit (excitation light amount is 100 mW) using an excitation semiconductor laser emitting 80 nm band, 3-1 and 3-2 are signal light (1510 nm to 1580 nm) and excitation units 2-1 and 2-
A multiplexer that combines the pumping light generated in 2 with 3-1 is W
A DM fab coupler and a bulk multiplexer 3-2 were used. 4-1, 4-2 and 5 are polarization independent 155
An optical isolator for 0 nm band was used.

FIG. 5 shows the gain band characteristic and the noise characteristic of the fiber type optical amplifier prepared. However, W shown in FIG.
Variable wavelength light sources 8-1, 8-of DM signal amplification characteristic evaluation system
2,8-3,8-4,8-5,8-6,8-7,8-8
Signal wavelengths of 1538, 1542, 1546, 155
0, 1554, 1558, 1560, 1562 nm. As shown in FIG. 5, the optical fiber amplifier of the present embodiment can optically amplify a WDM signal of 1542 to 1558 nm with a gain deviation of 0.5 dB and a noise figure of 4.5 d.
It was B or less. The figure also shows the noise characteristics when the optical isolator 5 is removed for comparison. It was found that the noise characteristics can be greatly improved by inserting an optical isolator. From the above, the noise reduction of the broadband optical fiber amplifier was achieved.

It should be noted that the characteristics shown in FIG.
The backward pumping system shown in FIG.
Similar amplification band characteristics and noise characteristics were obtained with the forward pumping system shown in (b).

(Embodiment 3) In the above Embodiments 1 and 2,
An optical fiber amplifier having two amplifying sections, that is, an Er-doped silica optical fiber and an Er-doped fluoride fiber, and an Er-doped silica optical fiber and an Er-doped oxide other component glass fiber has been described. The structure used was that shown in FIG.
Tables 1 to 12 collectively show the amplification characteristics (amplification band characteristics, noise characteristics) of the optical fiber amplifiers composed of two amplifiers (the characteristics of Examples 1 and 2 are also shown). In all combinations, the noise figure is reduced by inserting the optical isolator 5 as compared with the case without the optical isolator. From the above, the noise reduction of the broadband optical fiber amplifier was achieved.

The characteristics shown in Tables 1 to 12 have the same amplification band even if the front stage amplification section is set to the backward pumping system shown in FIG. 2B or the rear stage amplification section is set to the forward pumping system shown in FIG. 2B. The characteristics and noise characteristics are obtained.

Further, although a pumping light source part of 1480 nm band was used for the backward pumping system, when a pumping light source part of 980 nm band was used (measured by halving the fiber length of each Er-doped fiber used in the subsequent stage). Also, the noise figure can be reduced by inserting the optical isolator 5, and the noise reduction of the wide band optical fiber amplifier can be achieved.

Tables 1 to 12 show an example of an optical fiber amplifier constructed by connecting in series a plurality of amplifying sections using two Er-doped fibers having different compositions as an amplifying medium. , Al co-added E
In addition to r-doped silica fiber, P, F, Ge, Eu, Yb
Co-doped silica fiber (same for multiple co-doped fibers)
Similar noise reduction results were obtained for the combinations using.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

[0044]

[Table 5]

[0045]

[Table 6]

[0046]

[Table 7]

[0047]

[Table 8]

[0048]

[Table 9]

[0049]

[Table 10]

[0050]

[Table 11]

[0051]

[Table 12]

(Example 4) 3 performed in Tables 13 to 14
The amplification characteristics (amplification band characteristic, noise characteristic) of the optical fiber amplifier composed of two amplifiers are shown together. The configuration is shown in Figure 2.
The one shown in (a) was used. The optical isolator 6-
There was no difference in the characteristics when 1 was inserted and when it was not inserted. In all combinations, the noise figure is reduced by inserting the optical isolator 5 as compared with the case without the optical isolator. From the above, the noise reduction of the broadband optical fiber amplifier was achieved.

The characteristics shown in Tables 13 to 14 are the same when the front-stage amplifier section is the backward pumping system shown in FIG. 2C or the rear-stage amplifier section is the forward pumping system shown in FIG. 2B. The amplification band characteristic and noise characteristic were obtained.

A pumping light source unit in the 1480 nm band was used for pumping the next stage amplifying unit and the third stage amplifying unit.
When using the excitation light source section in the nm band (each E used in the subsequent stage
(Measured by halving the fiber length of r-doped fiber)
Also, the noise figure can be reduced by inserting the optical isolator 5, and the noise reduction of the wide band optical fiber amplifier can be achieved.

[0055]

[Table 13]

[0056]

[Table 14]

Although Tables 13 to 14 show examples of optical fiber amplifiers configured by connecting in series a plurality of amplifying sections using three different Er-doped fibers as amplifying media, other examples are shown. Combination, and A
l-doped Er-doped silica fiber, P, F, G
Similar results of noise reduction are obtained also in the case of using e-, Eu-, and Yb-codoped silica fibers (the same applies to a plurality of co-doped fibers).

Therefore, from the results of Examples 1, 2, 3 and 4, the optical fiber according to the present invention is constructed by connecting in series a plurality of amplifying sections using Er-doped fibers having different glass compositions as an amplifying medium. It was confirmed that it is very effective in reducing the noise of the amplifier.

In the above embodiments, the optical fiber amplifier consisting of two and three amplifying sections has been described, but the present invention is not limited to this embodiment, and the optical fiber amplifier consisting of four, five and more amplifying sections is described. Is also possible enough.

[0060]

As described above, according to the present invention, the optical fiber amplifier constituted by connecting in series a plurality of amplifying sections using Er-doped fibers having different glass compositions as an amplifying medium is used as a pre-stage. E for the excitation light source of the amplifier
By using a light source that excites the ⁴ I _11/2 level of r and connecting at least the first-stage amplification section and the second-stage amplification section through an optical isolator, there is an advantage that noise reduction can be easily achieved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of the present invention.

FIG. 2 is a diagram illustrating a configuration of the present invention.

FIG. 3 is a diagram showing amplification characteristics of the optical fiber amplifier of the first embodiment.

FIG. 4 is a diagram illustrating a configuration of a WDM signal amplification characteristic evaluation system.

FIG. 5 is a diagram showing amplification characteristics of an optical fiber amplifier according to a second embodiment.

FIG. 6 is a diagram illustrating a configuration of Conventional Technique 1.

FIG. 7 is an amplification characteristic diagram of an optical fiber amplifier according to prior art 2.

FIG. 8 is a diagram illustrating a configuration of Conventional Technique 3.

FIG. 9 is an amplification characteristic diagram of an optical fiber amplifier of the related art 4.

FIG. 10 is a diagram illustrating a configuration of a conventional technique 5.

FIG. 11 is an amplification characteristic diagram of an optical fiber amplifier according to prior art 5.

[Explanation of symbols]

1-1, 1-2 Er-doped optical fiber that is an amplification medium 2-1, 2-2 Pumping light source unit that generates pumping light for Er-doped optical fiber 3-1 and 3-2 In signal light and pumping light source unit Multiplexing unit for multiplexing generated pump light 4-1 and 4-2 Optical isolator for suppressing oscillation of optical amplifier 5 Optical isolator inserted between first-stage amplification unit 7-1 and second-stage amplification unit 7-2 6-1 and 6-2 Optical isolator 7 inserted between each amplification unit 7 Amplification unit 7-1 First stage amplification unit 7-2 Post stage amplification unit 7-3 Post stage amplification unit 8 Wavelength variable signal light source 9 Wavelength variable light source 8 signal Multiplexer for multiplexing light sources 10 Optical attenuator for adjusting the amount of incident signal light 11 Optical fiber amplifier under test 12 Optical spectrum analyzer 13 Optical equalizer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasunori Oishi 3-19-3 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Shoichi Sudo 3-19-3 Nishishinjuku, Shinjuku-ku, Tokyo No. within Nippon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical fiber type amplification medium in which an active species made of a rare earth element or a transition metal element having a laser transition level is added to a core or a clad, and a pumping light source for optically exciting the active species, It has a plurality of optical amplification units each of which is composed of optical means for coupling pumping light emitted from a pumping light source and light to be amplified to lead to the optical fiber type amplification medium, and at least one of the plurality of optical fiber type amplification media is And a glass host different from the optical fiber type amplification medium, wherein at least the first-stage optical amplification unit and the second-stage optical amplification unit of the plurality of optical amplification units are connected via an optical isolator. Optical amplifier. 2. The optical fiber type amplifying medium is one of an Er-doped silica optical fiber, an Er-doped oxide multi-component glass fiber, an Er-doped fluoride glass fiber, an Er-doped chalcogenide glass fiber and an Er-doped telluride glass fiber. The optical amplifier according to claim 1, which is a fiber selected from 3. The Er-doped oxide multi-component glass fiber is aluminosilicate glass fiber, fluorophosphate glass fiber or phosphate glass fiber, and E
The optical amplifier according to claim 2, wherein the r-doped fluoride glass fiber is a Zr fluoride glass fiber or an In fluoride glass fiber. 4. The optical amplifier according to claim 1, wherein at least the pumping light source of the first-stage amplifying unit is a light source that pumps the ⁴ I _11/2 level of Er.