JP2001189510A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber amplifier which efficiently amplifies the light of long wavelength, especially that of 1.62 μm or longer. SOLUTION: A first excitation light source LD12a which outputs a first excitation light of first wavelength, a second excitation light source LD12b which outputs a second excitation light of a second wavelength different from the first wavelength, multiplexers 14a and 14b which multiplexes the first and second excitation light and the signal light of a third wavelength longer than the first and second wavelength, and an optical amplification fiber 1 in which the first and second excitation light that is multiplexed by the multiplexer as well as the signal light are guided, are provided. The optical amplification fiber 10 absorbs the first excitation light and excited to a first excitation state from a base condition, and absorbs the second excitation light and excited into a second excitation state from the first excitation state, with the signal light amplified by induced emission with the signal light.

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 suitably used in an optical communication network system.

[0002]

2. Description of the Related Art In recent years, with the advent of the advanced information society,
Large-capacity high-speed transmission networks using optical fibers are expanding from long-distance backbone transmission lines to subscriber systems. In such an optical fiber network system, an optical fiber amplifier capable of directly amplifying light is an essential component.

[0003] Optical fiber amplifiers utilizing various nonlinear effects are known. For example, there are optical fiber amplifiers using rare earth element-doped optical fibers, fiber Raman, and fiber Brillouin (for example, Masataka Nakazawa,
Applied Physics, 56, p. 1256 (1987)). Among these optical fiber amplifiers, those using a rare earth element-doped optical fiber have high gain and efficiency, and also have excellent noise characteristics. Generally, a rare earth element doped optical fiber has at least a core and a clad, and the core is doped with a rare earth element. Erbium (Er) -doped optical fiber (hereinafter, referred to as a rare-earth element-doped optical fiber for amplifying light in the 1.55 μm band)
Called "EDF". ) Is excellent.

FIG. 5 schematically shows a conventional optical fiber amplifier (hereinafter referred to as "EDFA") 300 using an EDF.

The EDFA 300 includes an EDF 30 and a pumping laser diode (hereinafter referred to as “LD”) 22.
And a wavelength multiplexer (W) for multiplexing the pump light and the signal light.
average Division Multiple
(hereinafter, referred to as “WDM”) 34 and isolators 36 a and 36 b. EDFA3
00 is inserted and coupled to the optical fiber network system via connectors 38a and 38b.

[0006] The operation of the EDFA 200 will be briefly described below.

Optical fiber 3 connected to connector 38a
2a (for example, a wavelength of 1.55 μm
The band is multiplexed by the WDM 34 with pumping light (for example, a wavelength of 1.48 μm or a wavelength of 0.98 μm) emitted from the pumping LD 22, and propagates to the EDF 30.
The signal light amplified by the EDF 30 is supplied to the connector 38.
The signal is output to the optical fiber 32b connected to the optical fiber 32b. In addition,
The isolators 36a and 36b are connected to the input / output connector 3
8a and 38b, reflection from the pumping LD 22, etc., generation of parasitic oscillation due to scattering in the EDF 30 and the optical fibers 32a and 32b, multiple reflection of signal light, ASE
(Amplified Spontaneous Em
issue) is suppressed, gain and output intensity are improved, and noise characteristics are improved.

[0008]

However, the above-mentioned conventional EDFA is used exclusively for amplifying light in the 1.55 μm band, and even if the length of the EDF is increased, it is at most 1.62 μm. Only light of the following wavelengths could be amplified. Further, if the length of the EDF is increased, it is easily affected by the chromatic dispersion and the polarization mode dispersion of the fiber, and practical characteristics cannot be obtained. further,
An EDF length about five times as large as that of a normal device is required, which leads to an increase in the size of the device.

Some optical fiber amplifiers that can amplify long-wavelength light use the Raman phenomenon (for example, SAE Lewis, et a).
l. , Electronics Letters, Vo
l. 35 (20) 1761, 1999), and there are some technical problems for practical use, such as low light amplification efficiency.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an optical fiber amplifier capable of efficiently amplifying long-wavelength light, particularly light having a wavelength longer than 1.62 μm. And

[0011]

An optical fiber amplifier according to the present invention outputs a first pumping light source for outputting a first pumping light having a first wavelength, and outputs a second pumping light having a second wavelength different from the first wavelength. A second excitation light source, and the first and second excitation lights,
A multiplexer for multiplexing a signal light of a third wavelength longer than any of the first and second wavelengths, and the first and second pump lights and the signal light multiplexed by the multiplexer. And an optical amplification fiber through which the first excitation light is absorbed to be excited from a ground state to a first excitation state, and the second excitation light is absorbed to absorb the first excitation light. A configuration is provided that is excited from the state to the second excited state and amplifies the signal light by stimulated emission by the signal light, thereby achieving the above object.

The first wavelength is 1.48 μm or 0.9.
8 μm, wherein the second wavelength is longer than 1.62 μm;
It may be configured.

[0013] The multiplexer may be a fiber loop mirror type multiplexer.

Preferably, the optical amplification fiber has a core and a clad, and is an optical fiber in which the core is doped with a rare earth element. In particular, erbium (E
r) Doped optical fibers are preferred.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an optical amplification mechanism of an optical fiber amplifier according to the present invention will be described. The present inventor studied the optical amplification mechanism of the optical amplification fiber in detail, and as a result, found that the excited state absorption of the optical fiber (Excited State Abso
1.62 μm
New insight that light with a long wavelength exceeding the limit can be amplified. The present invention has been made based on this finding.

Referring to the energy band diagram shown in FIG. 1, an optical amplification mechanism in a conventional optical fiber amplifier,
The principle of the optical amplification mechanism in the optical fiber amplifier of the present invention will be described by taking a rare earth element doped fiber amplifier as an example.

In a conventional optical fiber amplifier, as shown in FIG. 1 (a), a pumping light (for example, a wavelength band of 1.48 μm or a wavelength of 0.4 mm) is applied to a rare earth element-doped optical fiber for optical amplification.
(98 μm band), electrons of the rare earth element in the optical fiber change from the ground state (Grand State) to the first excited state (First Excited State).
e) is excited. That is, the optical fiber has a ground state absorption (Grand State Absorption).
n) is excited to the first excited state. When signal light (having a longer wavelength than the pump light: for example, 1.55 μm band) is incident on the optical fiber in the first pump state, the electrons in the first pump state emit light having the same wavelength as the signal light. Stimulated emission by so-called signal light occurs.
As a result, the signal light is amplified and emitted from the optical fiber.

On the other hand, rare earth elements have various electronic states, and some exhibit excited state absorption (for example, Er element). By utilizing this excited state absorption, long wavelength light can be amplified.

As shown in FIG. 1B, the rare earth element is excited from the ground state to the first excited state by making the first excitation light incident on the optical fiber. The first pumping light is light having the same wavelength as the conventional pumping light (for example, a wavelength band of 1.48 μm or a wavelength of 0.98 μm). The process of ground state absorption is the same as the conventional pumping light shown in FIG. It is the same as the excitation mechanism in the optical fiber. Further, when the second pumping light (having a longer wavelength than the first pumping light, for example, a wavelength band of 1.63 μm) is incident on the optical fiber together with the first pumping light, the first pumping light is emitted.
The electrons excited to the excited state are excited by the excited state absorption.
It is excited to a second excited state with a higher energy level. The signal light (second light) is applied to the optical fiber in the second excited state.
Longer wavelength than excitation light: for example, wavelength 1.65 μm
When the band is incident, the electrons in the second excited state are relaxed while emitting light having the same wavelength as the signal light (stimulated emission via the second excited state). As a result, the long wavelength signal light is amplified and emitted from the optical fiber.

As an optical fiber exhibiting excited state absorption, E
An r-doped optical fiber (EDF) can be mentioned.
Stimulated emission cross section σe and excited state absorption cross section of EDF
σa _ESAHas the wavelength dependence shown in FIG.

As schematically shown in FIG. 2, the stimulated emission cross section σe (dashed line in the figure) of a typical silica glass EDF doped with Er decreases as the wavelength increases,
The excited state absorption cross section σa _ESA (solid line in the figure) is about 1.
Appearing around 62 μm, increasing with increasing wavelength, they intersect at about 1.63 μm. This EDF has a wavelength region where stimulated emission can occur in a long wavelength region exceeding a wavelength of approximately 1.62 μm and on a longer wavelength side than the wavelength showing the excited state absorption.
By using pump light having a longer wavelength (second pump light), light with longer wavelength (signal light) can be amplified. Note that some glasses other than silica emit excited state absorption (ESA) from a shorter wavelength (for example, about 1.61 μm). When such a glass is used, light having a wavelength shorter than 1.62 μm is used. Can be used as the second excitation light. The wavelength of the second excitation light can be appropriately set to a wavelength range in which excitation state absorption occurs, depending on the material used.

In the present specification, the first excited state and the second excited state are defined as any two of the excited states.
The lower energy level is referred to as a first excited state, and the higher energy level is referred to as a second excited state. The first, second,.
May be different from the first excited state or the second excited state in a strict sense named “・”. In addition, “the electrons in the second excited state are relaxed while emitting light having the same wavelength as the signal light (stimulated emission through the second excited state)” means that the electrons that have been excited to the second excited state at least once. This means stimulated emission in the relaxation process, for example, relaxation from the second excited state to the third excited state in the non-radiation process, and also includes stimulated emission accompanying the relaxation of electrons in the third excited state.

(Embodiment 1) E of the embodiment according to the present invention
FIGS. 3A and 3B schematically show an optical fiber amplifier (hereinafter, referred to as "EDFA") using a DF. Here, an example using a silica-based EDF will be described.

The EDFA 100 shown in FIG.
DF10, first excitation LD 12a, and second excitation LD
12b, a wavelength multiplexer WDM14a for multiplexing the first pump light and the signal light emitted from the first LD 12a,
It has a WDM 14b for multiplexing the second pump light and the signal light emitted from the second LD 12b, and has isolators 16a and 16b. The EDFA 100 is inserted and coupled to the optical fiber network system via connectors 18a and 18b, similarly to a conventional EDFA. The wavelength of the first excitation light emitted from the first excitation LD 12a and the wavelength of the second excitation light emitted from the second excitation LD 12b are different from each other, and at least one of them is longer than 1.62 μm.

The EDFA 100 operates as follows.
The signal light having a wavelength longer than 1.62 μm, which cannot be amplified by the conventional EDFA, is amplified.

Optical fiber 3 connected to connector 18a
2a (for example, a wavelength of 1.66 μm
Band) is the first excitation LD 12a by the WDM 14a.
Excitation light (e.g., wavelength 1.48 μm)
Band or a wavelength of 0.98 μm).
Propagate to In addition, signal light (for example, wavelength 1.66 μm
Band) is the second excitation LD 12b by the WDM 14b.
Excitation light (e.g., a wavelength of 1.63 μm
Band). This signal light is amplified in the EDF 10 by the above-described mechanism involving the excitation state absorption.

The signal light amplified by the EDF 10 is
The signal is output to the optical fiber 32b connected to the connector 18b. The isolators 16a and 16b are provided with reflections from the input / output connectors 18a and 18b and the excitation LDs 12a and 12b, and
It suppresses occurrence of parasitic oscillation, multiple reflection of signal light, and occurrence of ASE due to scattering in F10 and optical fibers 32a and 32b, and improves gain and output intensity and noise characteristics.

The optical fiber amplifier using the EDF of the present embodiment is not limited to the above example, and may be, for example, an optical fiber amplifier 100 'shown in FIG. 3B. In FIG. 3B, components having substantially the same functions as those of the optical fiber amplifier 100 in FIG. 3A are denoted by the same reference numerals, and description thereof will be omitted.

The optical fiber amplifier 10 shown in FIG.
0 'indicates that the first pumping light emitted from the first pumping LD 12a and the second pumping light emitted from the second pumping LD 12b are both introduced into the EDF 10 from the front (on the same side as the signal light). This is different from the optical fiber amplifier 100 shown in FIG. The first pumping light emitted from the first pumping LD 12a and the second pumping light emitted from the second pumping LD 12b are multiplexed by the WDM 14a. The signal light is
The light is multiplexed with the first and second pump lights by the WDM 14b and propagates to the EDF 10. This signal light is transmitted to the EDF 10
After being amplified inside, it is output to the optical fiber 32b connected to the connector 18b via the optical isolator 16b.

The EDFAs 100 and 10 of the present embodiment
0 ′ amplifies signal light having a wavelength longer than 1.62 μm by utilizing the excited state absorption of EDF.
Further, since EDF is used for optical amplification, the efficiency is higher than that of optical amplification using Raman phenomenon. Further, since it is not necessary to lengthen the fiber, there is an advantage that it is hardly affected by Raman scattering, chromatic dispersion and polarization dispersion, and the apparatus can be downsized.

(Embodiment 2) FIG. 4 schematically shows an optical fiber amplifier 200 according to an embodiment of the present invention.

The optical fiber amplifier 200 has a loop-shaped optical fiber 20, a first pumping LD 12a, a second pumping LD 21b, a WDM 14, a coupler 24, and an optical circulator 16. Optical fiber amplifier 200
Is inserted and coupled to the optical fiber network system via connectors 28a and 28b. The loop-shaped optical fiber 20 typically has an EDF as a part. The first light emitted from the first excitation LD 12a
The wavelength of the excitation light and the wavelength of the second excitation light emitted from the second excitation LD 12b are different from each other, and at least one of them is longer than 1.62 μm. The first pumping light (for example, a wavelength of 1.48 μm or 0.98 μm) and the second pumping light (for example, 1.63 μm) are WDM14.
Are multiplexed with each other.

In the optical fiber amplifier 200 of the present embodiment, the signal light and the pump light (the first pump light and the second pump light are multiplexed) are combined by the coupler (multiplexer) 24. Done. Here, as the coupler 24, a wavelength-independent 3
A dB coupler (a coupler having a branching degree of 50%) is used. 3dB
The coupler 24 includes a first port 24a and a second port 24a.
b and one end having a third port 24c and a fourth port 24d. 3dB coupler 2
4 is a single unit, the input from the first port 24a is the third
The input from the second port 24b is also halved to the third and fourth ports 24c and 24d.
The output from the third port 24c is halved to the first and second ports 24a and 24b, and the input from the fourth port 24d is also halved to the first and second ports 24a and 24b. It has characteristics to output. Three
First port 24 provided at one end of dB coupler 24
a and the second port 24b are connected to both ends of the loop-shaped optical fiber 20, respectively.
The optical fiber loop mirror (FLM) is composed of the optical fiber 4 and the looped optical fiber 20.

The pump light emitted from the LDs 12a and 12b and multiplexed by the WDM 14 is supplied to the 3 dB coupler 2
4 is input to the third port 24c. On the other hand, signal light (for example, wavelength 1.66 μm) input from the optical fiber 32a to the optical fiber amplifier 200 via the connector 28a.
The band is input to the first terminal 26a of the optical circulator 26 and output from the second terminal 26b to the fourth port 24d of the 3 dB coupler 24.

The pump light input to the third port 24c of the 3dB coupler 24 and the signal light input to the fourth port 24d are multiplexed in the 3dB coupler 24,
0% each of the first port 24a and the second port 24b
Is output to the loop-shaped optical fiber 20. The loop-shaped optical fiber 20 is typically a rare-earth element-doped optical fiber, and has a mechanism involving excitation state absorption, as in the first embodiment, and a signal emitted by stimulated emission by signal light propagating through the optical fiber 20. Amplify light. The loop-shaped optical fiber 20 may have a region in which the core is doped with a rare earth element in at least a part in the longitudinal direction.

The amplified signal light is again input to the 3 dB coupler 24 from the first port 24a and the second port 24b, and almost all of the signal is output from the fourth port 24d. The amplified signal light is supplied to the circulator 26
Is input to the second terminal 26b, and output from the third terminal 26c. The signal light output from the third terminal 26c of the circulator 26 is output to the optical fiber 32b connected via the connector 28b.

The optical fiber amplifier 200 of this embodiment is
It has the same effects as the optical fiber amplifier of the first embodiment, and further has the features described below.

As described above, the optical fiber amplifier 200 of the present embodiment includes the 3 dB coupler 24 and the 3 dB coupler 24.
The signal light and the pump light are multiplexed by the FLM constituted by the loop-shaped optical fiber 20 connected to one end of the optical fiber amplifier 100.
Instead of one of the WDMs 14a and 14b, an inexpensive 3 dB coupler can be used as illustrated. It is particularly preferable to use a wavelength-independent coupler as the coupler. Further, the coupler is not limited to the 3 dB coupler, and couplers having various branching ratios can be used, and may be a melt drawing fiber type or a bulk mirror type. As the loop-shaped optical fiber 20 constituting the FLM, one having a function of optical amplification can be widely used, but from the viewpoint of performance, it is preferable to use the rare-earth element-doped optical fiber exemplified above, particularly EDF.

The optical circulator 14 of the optical fiber amplifier 100 is a conventional optical fiber amplifier 300 shown in FIG.
As with the two isolators 36a and 36b, the connectors 28a and 28b and the LDs 12a and 1
2b, the occurrence of parasitic oscillation due to the scattering in the EDF 20 and the optical fibers 32a and 32b, the multiple reflection of signal light, and the occurrence of ASE are suppressed, and the gain and output intensity are improved and noise characteristics are improved. Furthermore, since the number of parts can be reduced as compared with the conventional configuration, the number of optical fiber joints can be reduced, and there is an advantage that the performance degradation at the joints can be suppressed. Of course, an optical circulator having four or more terminals may be used as the optical circulator 16.

When an optical fiber deformed into a loop shape is used, the polarization direction of light propagating therethrough may be shifted (changed). If necessary, a polarization control element may be provided to compensate for the deviation in the polarization direction. As the polarization control element, a phase difference plate (1/4 wavelength plate or 1/2
A known polarization control element such as a wave plate, a combination thereof, or a paddle in which an optical fiber is wound around a mandrel can be used. This polarization control element is preferably provided in a loop.

[0041]

According to the present invention, there is provided an optical fiber amplifier capable of efficiently amplifying long-wavelength light by utilizing excited state absorption. For example, by using an optical fiber doped with a rare earth element (particularly, an Er-doped optical fiber) as the optical amplification fiber, light having a wavelength longer than 1.62 μm can be efficiently amplified.

[Brief description of the drawings]

FIG. 1 is an energy band diagram for explaining the principle of an optical amplification mechanism (a) in a conventional optical fiber amplifier and an optical amplification mechanism (b) in an optical fiber amplifier of the present invention.

FIG. 2 is a graph schematically showing the wavelength dependence of a stimulated emission cross section σe and an excited state absorption cross section σa _{ES A of} an Er-doped optical fiber.

FIGS. 3A and 3B are schematic diagrams showing optical fiber amplifiers 100 and 100 ′ according to the first embodiment of the present invention, respectively.

FIG. 4 is an optical fiber amplifier 2 according to a second embodiment of the present invention.
It is a schematic diagram which shows 00.

FIG. 5 is a schematic diagram showing a conventional optical fiber amplifier 300.

[Explanation of symbols]

10, 30 EDF 12a, 12b, 22 LD 20 Loop-shaped optical fiber 16a, 16b, 36a, 36b Isolator 14, 14, a, 14b, 34 Wavelength multiplexer (WDM) 18a, 18b, 28a, 28b, 38a, 38b Connector 24 Coupler 24a, 24b, 24c, 24d Port 26 Optical circulator 26a, 26b, 26c Terminal 32a, 32b Optical fiber 100, 100 ', 200, 300 Optical fiber amplifier

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Kazuo Imamura 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Cable Industry Co., Ltd. Itami Works F-term (reference) 5F072 AB09 AK06 JJ02 JJ04 JJ09 KK30 PP07 RR01 YY17

Claims (4)

[Claims] A first pumping light source that outputs a first pumping light having a first wavelength; a second pumping light source that outputs a second pumping light having a second wavelength different from the first wavelength; (2) a multiplexer for multiplexing the pump light and a signal light of a third wavelength longer than any of the first and second wavelengths; and the first and second pump lights multiplexed by the multiplexer. And an optical amplification fiber through which the signal light is guided, wherein the optical amplification fiber absorbs the first excitation light and is excited from a ground state to a first excitation state, and emits the second excitation light. An optical fiber amplifier that absorbs and is excited from the first excited state to the second excited state, and amplifies the signal light by stimulated emission by the signal light. 2. The method according to claim 1, wherein the first wavelength is 1.48 μm or 0.4 mm.
The optical fiber amplifier according to claim 1, wherein the second wavelength is 98 µm, and the second wavelength is longer than 1.62 µm. 3. The optical fiber amplifier according to claim 1, wherein the multiplexer is a fiber loop mirror type multiplexer. 4. The optical fiber amplifier according to claim 1, wherein the optical amplification fiber has an optical fiber having a core and a clad, and the core is doped with a rare earth element.