JPH05347444A - Optical amplifier - Google Patents

Abstract

(57) [Summary] [Construction] An optical amplifier in which two bidirectional pumping light amplifying units A and B are connected in series via an optical filter 10 and an optical isolator 11, and the above bidirectional pumping light amplifying unit is used. A and B have the first pumping light source 1A or 1 at the first input end.
B is connected, and the first optical coupler 2A or 2B to which the signal light is input from the second input end, and the first optical coupler 2
A rare-earth-doped amplification optical fiber 3A or 3B, one end of which is connected to the output end of A or 2B, and the amplification optical fiber 3A
Or 3B, the pumping light from the second pumping light source 4A or 4B is incident from the other end of the amplifying optical fiber 3A or 3B, and the amplified signal light is propagated from the amplifying optical fiber 3A or 3B. And a second optical coupler 5A or 5B for outputting [Effect] Gain saturation due to up-conversion and ASE can be suppressed at the same time, and the gain efficiency of the optical amplifier is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier used in an optical communication system or the like.

[0002]

2. Description of the ^Related Art In recent years, research on an optical fiber amplifier using a stimulated emission of a transition in a 4f shell by doping a core of an optical fiber with rare earth ions and Er ³⁺ ions has been actively conducted.
Application to optical communication systems in the 1.5 μm band is underway.

By the way, the 1.3 μm band in which the chromatic dispersion of a silica-based optical fiber is zero is a practical wavelength band of an optical communication system along with the 1.5 μm band, and Pr is used as an optical fiber amplifier operating at 1.3 μm. ^{3 +} -doped fluoride fiber amplifiers have been proposed. (Y.Ohishi, T.Kanamori, S.Takaha
shi, E.Snitzer and GHSigel “Technical Digest ofOp
tical Fiber Communication Conference '91 San Dieg
o ”(1991 PD2)) This optical fiber amplifier uses a ZrF ₄ type fluoride glass as a host material Pr ³⁺ as an active ion, and a Pr ^{3+ 1} G ₄ → ³ as shown in the energy diagram of FIG. It utilizes the stimulated emission of the H ₅ transition.

[0004]

However, in this optical fiber amplifier, the energy difference between the ¹ G ₄ level of Pr ³⁺ and the ³ F ₄ level below it is about 3000 c.
Since ^m-1 and smaller and the phonon energy of ZrF ₄ glass which is a host material as large as 500 cm ^{-1, 1}
Phonon emission relaxation from the G ₄ level to the ³ F ₄ level is likely to occur, and the quantum efficiency of the ¹ G ₄ → ³ H ₅ transition is as low as 3%, so the gain coefficient is suppressed to about 0.2 dB / mW. (Y.Ohishi, T.Kanamori, J.Temmyo, M.Wada, M.Yamad
a, M.Shimizu, K.Yoshino, H.Hanafusa, M.Horiguti and S.
Takahashi, Electron.Lett., Vol.27, No.22, p.1995,199
1).

As a mechanism for lowering the gain coefficient, in addition to the above relaxation of phonon emission, as shown in FIG.
¹ G ₄ → ¹ D ₂ up-conversion and ASE (ampl
There is a gain saturation phenomenon due to ified spontaneous emission (superfluorescence). In the case of a Pr ³⁺ -doped fiber amplifier, since the gain coefficient is essentially small, it is a very important technique to suppress the deterioration of the gain characteristic due to these two factors and improve the gain characteristic.

Of the above two mechanisms, it is known that the decrease in gain due to up-conversion can be suppressed by bidirectional pumping. As an optical amplifier that suppresses the gain reduction using this principle, for example, as shown in FIG. 4, a first pumping light source 1 is connected to a first input end, and a second pumping light source 1
A first coupler 2 to which the signal light is input from the input end of
The rare-earth-doped amplification optical fiber 3 having one end connected to the output end of the first coupler 2, and the excitation light from the second excitation light source 4 connected to the amplification optical fiber 3 There is a bidirectional pumping optical amplifier that includes a second optical coupler 5 that outputs amplified signal light that is incident from the other end and that is propagated from the optical fiber 3 (see Japanese Patent Application No. 3-224405). In FIG. 4, signal light is incident on this bidirectional pumping optical amplifier from the signal light source 6 through the optical isolator 7,
When the amplified signal is monitored by the optical spectrum analyzer 9 through the optical isolator 8, a decrease in upconversion is observed. However, the bidirectional pumping optical amplifier having such a configuration cannot suppress ASE,
There is no known optical amplifier capable of suppressing up-conversion and ASE at the same time.

The present invention has been made in view of the above circumstances, and provides an optical fiber amplifier having improved gain characteristics by suppressing a decrease in gain due to up-conversion and ASE.

[0008]

In the present invention, an optical amplifier in which two or more bidirectional pumping optical amplification units are connected in series via an optical filter and an optical isolator is provided. The means.

[0009]

In the basic structure of the conventional optical fiber amplifier, one amplifying optical fiber having a rare earth ion added to the core is used as the amplifying medium. However, in the present invention, at least two or more such optical fibers are used. The most main feature is to pump the amplification optical fibers from both directions in both directions, and couple them in series through an optical filter and an optical isolator that transmit light of a wavelength near the signal light to form an amplification medium. ..

In coupling the amplifying optical fibers with each other, the ASE from one amplifying optical fiber coupled to the optical filter and the optical isolator propagates through the optical filter and the optical isolator. This is to suppress the gain saturation caused by the gain saturation.

[0011]

The present invention will be described in detail below. FIG. 1 shows an embodiment of the optical amplifier of the present invention. In this optical amplifier, two bidirectional pumping light amplification units A and B are connected in series via an optical filter 10 and an optical isolator 11. In this embodiment, each of the bidirectional pumping light amplification units A and B has the same configuration as the conventional bidirectional pumping light amplifier shown in FIG. 4, and the common parts are denoted by the same reference numerals. To simplify the explanation.

That is, the bidirectional pumping light amplification unit A is
As shown in FIG. 1, a first coupler 2A, to which a first pumping light source 1A is connected to a first input end 2Aa, and a signal light is input from a second input end 2Ab, and the first coupler 2A.
Of the rare-earth-doped amplification optical fiber 3A, one end 3Aa of which is connected to the output end 2Ac thereof, and pumping light from the second pumping light source 4A, which is connected to the amplification optical fiber 3A, from the other end 3Ab of the optical fiber 3A. The second optical coupler 5A is provided which outputs the amplified signal light that is incident and propagated from the optical fiber 3A from the output end 5Aa.

Similarly, in the bidirectional pumping light amplifying unit B, the first pumping light source 1B is connected to the first input end 2Ba, and the signal light is input from the second input end 2Bb.
Coupler 2B and the output end 2B of the first coupler 2B
The rare-earth doped amplification optical fiber 3B having one end 3Ba connected to c and the pumping light from the second pumping light source 4B connected to the amplification optical fiber 3B and the other end 3 of the optical fiber 3B.
A second optical coupler 5B that outputs amplified signal light that is incident from Bb and propagated from the optical fiber 3B from an output end 5Ba is provided.

The excitation light sources 1A, 1B, 4A and 4B are
Each is composed of two semiconductor lasers 12, 12 and a polarization combining module 13. As the semiconductor laser 12, an InGaAs strained quantum well semiconductor laser or the like is used.

The optical couplers 2A, 5A and the amplification optical fiber 3A are connected by butt joints 14A, 14A, and the optical couplers 2B, 5B and the amplification optical fiber 3B are connected by butt joints 14B, 14B. It is connected. The optical coupler 2A is for coupling the signal light and the excitation light and making them enter the amplification optical fiber 3A. And the optical couplers 5A, 2B, 5B are
While bidirectionally pumping the amplification optical fiber 3A or 3B, the optical couplers 5A and 2B connect the amplification optical fiber 3
This is for introducing the signal light emitted from A into the amplification optical fiber 3B. The optical coupler 5B is for outputting the amplified signal light from the amplification optical fiber 3B.

As the amplification optical fibers 3A and 3B, P
Not only the r ³⁺ -doped fiber, but also the Er ³⁺ -doped fiber or the fiber in which the core is solely doped with Pr ³⁺ or Er ³⁺ , for example, Pr ³⁺ -Yb
³⁺ (1.3 μm band amplification), Pr ³⁺ -Nd ³⁺ (1.3 μm
Band amplification), Pr ³⁺ -Er ³⁺ (1.3 μm band amplification), Er
³⁺ −Yb ³⁺ (1.5 μm band amplification), Tm ³⁺ −Ho
³⁺ (1.47 μm band amplification), Tm ³⁺ -Ho ³⁺ -Eu
³⁺ (1.47 μm band amplification), Tm ³⁺ (1.65 μm band amplification), Tm ³⁺ -Er ³⁺ (1.65 μm band amplification), or other co-doped fiber may be used, which is appropriately selected according to the application. . Among them, the amplifier using the Pr ³⁺ -doped fiber has an inherently low efficiency as described above, so that the configuration of the present invention has a particularly large effect of improving the amplification characteristic.

Further, the fiber material is not limited to fluoride glass, but oxide glass, chalcogenide glass,
It may be a chloride glass, a bromide glass, an iodide glass, a chalcohalide glass, a mixed halide glass containing a plurality of halide ions, or a crystalline material.

The output end 5Aa of the optical coupler 5A
And the second input end 2Bb of the optical coupler 2B are connected via the optical filter 10 and the optical isolator 11, whereby the bidirectional pumping optical amplification units A and B are connected in series. The optical filter 10 is for removing the ASE power from the fiber 3A propagating with the signal light, and a bandpass filter or the like that transmits light having a wavelength near the signal light is preferably used. Further, the optical isolator 11 is generated from the fiber 3B, and the fiber 3A
This is for blocking the ASE propagating toward.

Further, a signal light source 6 is connected to the second input end 2Ab of the optical coupler 2Aa. A DFB laser or the like is used as the signal light source 6. An optical spectrum analyzer 9 is connected to the output end 5Ba of the optical coupler 5B via an optical isolator 8.

In the optical amplifier having the above structure, the amplification optical fibers 3A and 3B, which are amplification media, are pumping light sources 1A, 4A or 1B, 4B, which are semiconductor lasers 12, 12 and a polarization synthesizing module 13, respectively. Is excited in both directions by. The signal light amplified by the fiber 3A enters the optical fiber 3B via the optical coupler 5A, the optical filter 10, the optical isolator 11, and the optical coupler 2A, and is amplified again. At this time, most of the power of ASE from the fiber 3A propagating with the signal light is removed by the optical filter 10. The ASE generated from the fiber 3B and propagating toward the fiber 3A is blocked by the optical isolator 11 and does not enter the fiber 3A. Further, both the fiber 3A and the fiber 3B are bidirectionally pumped, so that the gain reduction due to the up-conversion is suppressed.

Although two Pr ³⁺ -doped fibers are shown in the configuration of the amplifier here, three or more fibers may be used, and in this case, if the total amount of pumping light is constant, one fiber is used. Since the amount of pumping light incident on a hit can be reduced, it is more effective in suppressing a decrease in gain by up-conversion (see Japanese Patent Application No. 3-224405).
Therefore, the number of semiconductor lasers for amplification increases, but it can be said to be a more preferable configuration.

[0022] (Example) light the amplification optical fiber having the configuration shown in FIG. 1 and Pr ³⁺ doped fiber, the specifications, the core glass composition _{(56) ZrF 4 - (14} ) BdF 2
− (15) PbF ₂ − (3.5) LaF ₃ − (2) YF <sub>3
- (2.5) AlF 3 - (</sub> 7 mol%) LiF, the cladding glass composition _{(47.5) ZrF 4 - (23.5} ) BaF 2
-(2.5) LaF _3- (2) YF _3- (4.5) AlF
_3- (20 mol%) NaF, core diameter 2 μm, relative refractive index difference 3.7%, Pr ³⁺ concentration in core 500 pp
m and the length was 10 m.

The signal light source 6 has an emission wavelength of 1.31 μm.
m semiconductor laser, and the semiconductor lasers 12 ... Of the excitation light source used had an emission wavelength of 1.017 μm. The optical filter 10 has a center wavelength of 1.31 μm,
A bandpass filter with a half width of 3 nm was used.

With respect to the bidirectional pumping optical amplifier of the above embodiment, the dependence of the gain on the pumping strength was examined. The result is shown by a solid line in FIG.

(Comparative Example) With respect to the bidirectional pumping optical amplifier having the configuration shown in FIG. However, the length of the amplification optical fiber 3 is 2
It was set to 0 m. The result is also shown by a broken line in FIG.

From the results shown in FIG. 5, it can be seen that the gain of the embodiment is higher than that of the comparative example for the same excitation intensity. In particular, this tendency becomes remarkable as the excitation intensity increases. This is because the gain saturation due to ASE is suppressed by the above configuration in the embodiment. Further, even in the low excitation intensity range, the embodiment is
A larger gain than that of the comparative example is obtained by dividing the Pr ³⁺ -doped fiber into two, and even if the total pumping intensity is constant, the pumping intensity injected from one fiber end is lowered and This is because the decrease in gain due to conversion is suppressed.

It was confirmed that the gain saturation due to the up-conversion and the ASE could be suppressed by using the Er ³⁺ -doped fiber instead of the Pr ³⁺ -doped fiber in the above example (the result is omitted). Similarly, Pr ³⁺ -Yb ³⁺ (1.3 μm band amplification), Pr ³⁺ -
Nd ³⁺ (1.3 μm band amplification), Pr ³⁺ -Er ³⁺ (1.3
μm band amplification), Er ³⁺ -Yb ³⁺ (1.5 μm band amplification),
Tm ³⁺ -Ho ³⁺ (1.47 μm band amplification), Tm ³⁺ -Ho
³⁺ −Eu ³⁺ (1.47 μm band amplification), Tm ³⁺ (1.65
μm band amplification), Tm ³⁺ -Er ³⁺ (1.65 μm band amplification)
Also in the co-doped fiber such as, the suppression of gain characteristic deterioration due to up-conversion and ASE was confirmed (the result is omitted).

[0028]

As described above, the optical amplifier of the present invention comprises two or more bidirectional pumping optical amplification units connected in series via an optical filter and an optical isolator. Therefore, upconversion and AS
Gain saturation due to E can be suppressed at the same time, and the gain efficiency of the optical amplifier is improved. By using the optical amplifier having the above configuration in an optical communication system, high performance of the system,
Economicization is achieved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of an optical amplifier of the present invention.

FIG. 2 is an energy diagram of Pr ³⁺ .

FIG. 3 shows up-conversion between Pr ³⁺ ions.

FIG. 4 is a schematic diagram showing an example of a conventional optical amplifier.

FIG. 5 is a graph comparing gain characteristics of an optical amplifier according to an embodiment of the present invention and a conventional bidirectional pumping optical amplifier.

[Explanation of symbols]

 1, 1A, 1B Pumping light source 2, 2A, 2B First optical coupler 2Aa, 2Ba First input end 2Ab, 2Bb Second input end 2Ac, 2Bc Output end 3, 3A, 3B Amplifying optical fiber 3Aa, 3Ba One end 3Ab, 3Bb The other end 4, 4A, 4B Pumping light source 5, 5A, 5B Optical coupler 10 Optical filter 11 Optical isolator A, B Bidirectional pumping light amplification unit

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H01S 3/131 8934-4M 3/17 8934-4M (72) Inventor Shoichi Sudo 1-chome, Uchiyuki-cho, Chiyoda-ku, Tokyo No. 6 Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. Two or more bidirectional pumping light amplification units,
An optical amplifier connected in series via an optical filter and an optical isolator. 2. The bidirectional pumping light amplification unit comprises:
To a first pumping light source connected to the input end of the first optical coupler, and a first optical coupler to which signal light is input from the second input end, and a rare earth-doped amplifier whose one end is connected to the output end of the first optical coupler Optical fiber for amplification, and an amplification signal light which is connected to the amplification optical fiber and which receives pumping light from a second pumping light source from the other end of the amplification optical fiber and propagates from the amplification optical fiber. The optical amplifier according to claim 1, further comprising a second optical coupler for outputting.