JPH10144983A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier with an optical fiber which amplifies can amplify signal light while preserving a polarization surface properly, when a polarized surface preservation optical fiber is used as a light-transmitting path and enables various kinds of optical parameters for obtaining required amplification characteristic to be adjusted. SOLUTION: An optical amplifier has an optical fiber DF for amplification, that directly amplifiers signal light by an induction emission effect and various kinds of optical elements such as an excitation light source P and a photocoupler WDM, and an optical fiber DM for amplification and optical elements P, WDM... are connected to one another via optical fibers (a) and (n) for coupling. While the optical fiber DF for amplification is made of a polarized surface preservation optical fiber in which a section has an elliptical core and a rare earth element is doped into the core, the optical fiber (a) located on the passage path of signal light in the optical fibers (a) and (n) for coupling is made of a polarized surface preservation optical fiber in which a stress application part for applying anisotropic distortion to the core is buried in a clad.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier having an amplifying optical fiber for directly amplifying signal light by a stimulated emission effect.

[0002]

2. Description of the Related Art Conventionally, as an optical amplifier used in an optical communication system or the like, an amplification optical fiber doped with a rare earth element such as Er or Nd is provided as an amplification element in a core, and the amplification optical fiber is guided. There is provided a device in which signal light is directly amplified without photoelectric conversion by an emission effect.

On the other hand, when performing coherent optical communication or heterodyne optical detection, a certain polarization state is stored,
It is necessary to be able to transmit phase information stably,
Therefore, a polarization-maintaining optical fiber is used as an optical transmission line.

Conventionally, as such a polarization-maintaining optical fiber, for example, there is a PANDA type optical fiber as shown in FIG. This PANDA polarization maintaining optical fiber a
In the clad d provided on the core c, a pair of stress applying portions e is formed so as to sandwich the core c, and the clad d
A stress field is generated inside the optical fiber due to the difference in material between the core and the stress applying part e, and an anisotropic strain is applied to the core c in the stress field so that the core c has birefringence. is there.

[0005] Such a PANDA polarization maintaining optical fiber has a relatively small transmission loss due to bending and is suitable for use as an optical transmission line.

By the way, when a PANDA type polarization maintaining optical fiber is used for an optical transmission line, in order to amplify the light attenuated during the transmission using the above optical amplifier,
It is necessary to simultaneously preserve the polarization plane of the amplification optical fiber serving as the amplification element.

[0007]

As described above, when a PANDA-type polarization maintaining optical fiber such as that shown in FIG. 3 is used in an optical transmission line, an optical amplifier is constructed in accordance with this. Same PANDA for amplification optical fiber
The use of the PANDA-type optical fiber makes it easy to connect to each other, but when a PANDA-type optical fiber is applied, it is not easy to adjust various optical parameters.

That is, in order to obtain required amplification characteristics as an amplification optical fiber, it is necessary to adjust optical parameters such as a relative refractive index difference Δ and a cutoff wavelength λ _0. Must be performed at the same time as the drawing process, and the stress applying part and preform must be integrated at the same time. Once the drawing is started, even if there is an error in the parameters of the optical fiber, the adjustment is impossible. Yes, all steps must be performed again. When special parameters are required as in an optical fiber doped with a rare-earth element, accumulation of data by repeated PADNA conversion cannot be expected as in the case of ordinary polarization of an optical fiber.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and in the case where a polarization-maintaining optical fiber is used as an optical transmission line, the signal light is kept in a state where the polarization plane is well preserved. It is an object of the present invention to provide an optical amplifier having an amplifying optical fiber that can amplify optical signals and easily adjust various optical parameters for obtaining required amplification characteristics.

[0010]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides an amplifying optical fiber for directly amplifying signal light by a stimulated emission effect and various optical elements such as an excitation light source and an optical coupler. And the following configuration is adopted in an optical amplifier in which the amplification optical fiber and the optical element and the optical element are connected to each other via the coupling optical fiber.

That is, in the optical amplifier of the present invention, the amplifying optical fiber has a core having an elliptical cross section and a polarization-maintaining optical fiber obtained by doping a rare earth element into the core. The coupling optical fiber, which is located on the signal light passage path, is composed of a polarization-maintaining optical fiber in which a stress imparting portion that imparts anisotropic strain to the core is embedded in the cladding. It is characterized by having.

In the above configuration, the amplification optical fiber is
The cross section of the core is elliptical, and the core is doped with a rare earth element so that the polarization plane can be preserved.
There is no need to provide a stress applying portion as in the DA type, and therefore, it is possible to perform preliminary drawing for parameter confirmation,
A polarization-maintaining amplification optical fiber doped with a rare earth element requiring a large number of special parameters can be manufactured relatively easily.

[0013]

FIG. 1 is a configuration diagram of an optical amplifier according to an embodiment of the present invention.

In FIG. 1, DF is an amplification optical fiber for amplifying signal light by a stimulated emission effect. In this embodiment, as shown in FIG. 2, a core h has an elliptical cross section. A clad i having a perfect circular shape is formed on h, and a single-mode polarization-maintaining optical fiber is formed by doping Er (erbium) around the core h or the core h.

The amplification optical fiber DF having this configuration has a core
Since the cross section of h is elliptical so that the polarization plane can be preserved, there is no need to provide a stress imparting part as in the PANDA type. It is possible to relatively easily manufacture a polarization-maintaining amplification optical fiber doped with a rare-earth element which requires various parameters.

The specific specifications of the amplification optical fiber DF are as follows: the short axis length Ls of the core h is 2 μm and the long axis length Ll is 10 μm.
μm, mode-filled diameter Dm, which is a substantial core diameter, is 8 μm
m, the outer diameter Dc of the clad i is 125 μm, the relative refractive index difference Δ between the core and the clad is 1.6%, and the cutoff wavelength λ ₀ is 1.1 μm.
And Er is, for example, 100
About 0 ppm is doped. Note that instead of Er, Nd
It is also possible to dope a rare earth element such as.

P is an excitation light source having a predetermined wavelength (for example, 1.
(48 μm).

Cin and Cout are connectors, and the connectors Cin and Cout are respectively connected to optical transmission lines using polarization maintaining optical fibers (not shown).

I ₁ and I ₂ are isolators provided for preventing unnecessary end face reflection, and WDM is an optical coupler for multiplexing signal light and pump light.

A coupling optical fiber is provided between each of the connectors Cin and Cout, the amplification optical fiber DF, the pump light source P, the isolators I ₁ and I ₂ , and the optical coupler WDM.
They are optically connected to each other via a and n.

Of these coupling optical fibers a and n, except for the coupling optical fiber n connecting between the pumping light source P and the optical coupler WDM, the coupling optical fiber a located on the signal light passage path. Is a so-called PANDA as shown in FIG.
It consists of a polarization-maintaining optical fiber of the type. That is, the PANDA type polarization maintaining optical fiber a has a pair of stress applying portions e for applying anisotropic strain to the core c embedded in the cladding d.

As specific specifications of the coupling optical fiber a, the outer diameter of the core h is 8 μm and the outer diameter of the clad i is 12 μm.
5 μm, the outer diameter of the stress applying portion e is 30 μm, the relative refractive index difference Δ between the core and the clad is 0.3%, and the cutoff wavelength λ ₀ is 1.3.
Those having a size of about μm are used.

The coupling optical fiber n connecting between the excitation light source P and the optical coupler WDM is a general silica-based communication optical fiber, for example, having an outer diameter of a core of 8 μm and a substantial core diameter. Is 10 μm, the outer diameter of the cladding is 125 μm, and the relative refractive index difference Δ between the core and the cladding is Δ
Of 0.3% is used.

The amplifying optical fiber DF and the coupling optical fiber a are connected to each other to reduce connection loss.
They are fused to each other.

In the optical amplifier having the above structure, the pump light source P
From the optical fiber DF via the optical coupler WDM and the isolator I ₁ , and thereby the amplifying optical fiber D
F is pumped.

In this pumping state, when a signal light of a predetermined wavelength (here, a wavelength of 1.55 μm) enters the connector Cin on the incident side through an optical transmission line using a polarization-maintaining optical fiber (not shown), This signal light is transmitted to the optical coupler WD
The light enters the amplifying optical fiber DF via the M, the isolator I ₁ , and the coupling optical fiber a while maintaining the polarization plane.

In the amplification optical fiber DF, the incident signal light is directly amplified by the stimulated emission effect while passing therethrough with the polarization plane kept.

The signal light after being amplified by the amplification optical fiber DF passes through the optical isolator I ₂ and the coupling optical fiber a, and is again output from the output connector Cout in a state where the polarization plane is preserved. The light is emitted to the optical transmission line.

The optical amplifier according to the above-described embodiment is of a so-called forward pump type in which signal light and pump light enter the amplifying optical fiber DF from the same direction. The present invention is also applicable to a so-called backward pumping type in which the light enters the amplifying optical fiber DF from opposite directions, and further to a so-called bidirectional pumping type in which pumping light enters from before and after the amplification optical fiber DF. Applicable.

Further, in this embodiment, a P-type optical fiber is provided between the amplifying optical fiber DF and the isolators I ₁ and I ₂ before and after it.
An ANDA type coupling optical fiber a is provided, but the coupling optical fiber a is omitted and both ends of the amplification optical fiber DF are directly connected to the isolators I ₁ and I ₂ , respectively. You can also.

[0031]

According to the present invention, when a polarization-maintaining optical fiber is used as an optical transmission line, the signal light can be amplified with its polarization plane well preserved. In addition, since the amplification optical fiber serving as an amplification element can easily adjust various optical parameters for obtaining required amplification characteristics, it is possible to provide a relatively inexpensive optical amplifier.

In particular, when the amplification optical fiber and the coupling optical fiber are connected, it is effective to fuse the two, since the connection loss is reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical amplifier according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of an amplification optical fiber included in the optical amplifier of FIG.

FIG. 3 is a sectional view of a coupling optical fiber constituting the optical amplifier of FIG. 1;

[Explanation of symbols]

DF ... amplification optical fiber, P ... pumping light source, WDM ... optical coupler, Cin, Cout ... connector, I _1, I ₂ ... isolator, a, n ... coupling optical fiber.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masataka Nakazawa 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Inside Japan Telegraph and Telephone Corporation (72) Eiichi Yamada 3-19, Nishi-Shinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation

Claims (2)

[Claims] An amplifying optical fiber for directly amplifying a signal light by a stimulated emission effect, and various optical elements such as a pump light source and an optical coupler, between the amplifying optical fiber and the optical element, and between the amplifying optical fiber and the optical element. Are connected to each other via a coupling optical fiber, wherein the amplification optical fiber has a core having an elliptical cross section, and a polarization plane formed by doping a rare earth element in the core. While the storage optical fiber is constituted, the coupling optical fiber, which is located on the signal light passage path, is provided with a stress imparting portion that imparts anisotropic strain to the core and is embedded in the cladding. An optical amplifier comprising a polarization-maintaining optical fiber. 2. The optical amplifier according to claim 1, wherein the coupling optical fiber and the amplification optical fiber are fusion-spliced to each other.