JPH07318744A - Optical composite parts for optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide optical composite parts for an optical fiber amplifier featuring a small size, low loss and high reliability by using optical fibers including rare earth or transition metals. CONSTITUTION:Respective passive parts to be used in the optical fiber amplifier formed by using the optical fibers including rare earth or transition metals; for example, an optical coupler (a) and an optical multiplexer (b) are composed by using optical fiber fusion type parts and an optical isolator (c) of a type non-dependent on polarized light is composed by using parts which are formed by fusion-splicing the core expanded fibers to each other. All parts are composed of parts using the optical fibers at the end faces. Then these parts are integrated and assembled only by fusion splicing, by which the need for precise assembly is eliminated. The end face reflection and loss increase are lessened by the lensless structure. The cause for instability by the space connecting is thus eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier mounting structure using an optical fiber doped with a rare earth element or a transition metal used in the field of optical communication.

[0002]

2. Description of the Related Art A fiber-type optical amplifier includes an optical coupler that couples pumping lights having polarization directions orthogonal to each other, an optical multiplexer that multiplexes the coupled pumping light and signal light, and removes return light. A polarization-independent optical isolator and a rare earth element (eg Er, Pr, Nd, etc.) that amplifies the signal light by the intensity of the excitation light
Alternatively, an optical fiber doped with a transition metal (such as Ti) can be connected in this order.

Elemental components for coupling, multiplexing, and separating light are independently configured, and an optical amplifier is configured by connecting and combining these components by a connector or a fusion splicing. Further, recently, there is one in which each functional element component is partially integrated (for example, a PBS, a multiplexing filter element, an isolator element) and coupled by a lens system. However, the optical fiber amplifier composed of such components has the following drawbacks.

[0004]

In the case of constructing by combining a number of component parts, a large system is formed because a large number of optical parts are connected and the number of connecting parts is large, which causes an increase in loss and reflection. It is difficult to miniaturize.

In the case of using a multi-functional component connected by a lens system, it is necessary to assemble a number of precision adjusting assemblies for each component, which is complicated and time-consuming for assembling.

In the case of (1), it is difficult to reduce the cost because each component is expensive and is used in combination.

Due to the complicated structure and spatial connection, the characteristics change due to the optical axis shift due to environmental changes, and the reliability is poor.

In view of the above-mentioned drawbacks, the present invention has an object to provide a compact and low-loss optical composite component for an optical fiber amplifier which solves these drawbacks.

[0009]

Therefore, in the optical fiber amplifier optical composite part according to the present invention, each component part is constructed by using an optical fiber fusion type part and a core expanding fiber,
By connecting optical fibers to each other, they are optically connected without using a lens system, and by mounting each component integrally, a small size and low loss are realized.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a component structure of an embodiment according to the present invention, FIG. 2 is a detailed view of a portion A of its internal structure, that is, a configuration diagram of a pumping light coupling portion and a multiplexing portion, and FIGS. ) (C) is a cross-sectional view of a polarization-maintaining fiber that constitutes an optical coupler, and FIG.
3 is a detailed configuration diagram of a polarization-independent optical isolator and a connecting portion thereof. FIG. Next, embodiments of the present invention will be described in detail with reference to the drawings.

In FIG. 1, signal light enters from an optical fiber 1 on the left side of the upper part of the figure. The pumping light enters from the two polarization-maintaining fibers 2, 3 on the lower left side of the figure. The polarization-maintaining fibers 2 and 3 to which the pumping light source is propagation-coupled are arranged in two,
It functions as a polarizer that is fusion-bonded and stretched by heating, and in this case, it functions as an optical coupler a for excitation light.

The polarization direction is divided into a horizontal direction and a vertical direction in the stress-applying portion in the cross section of the fiber for propagation coupling (FIG. 3).
(See (a) and (b)). Therefore, when the pumping incident light is incident with the polarization direction of the incident side fiber in a direction perpendicular to the horizontal direction of the stress applying portion, it is coupled and emitted at the tapered coupling portion (see FIG. 3C).

The extinction ratio of this type of polarization beam splitter is about 15 to 20 dB, which is practically sufficient. The pumping light combined there is used as an optical multiplexer b.
At the point of combining with the signal light.

The optical multiplexer b has ordinary optical fibers 1 and 4
And the polarization-maintaining fiber 2 in which the input port end face of the optical fiber 4 is used in the optical coupler a.
Fusion splice with the output port end face. This allows low-loss connection. The pumping light that has entered the optical fiber 4 on the side of the optical multiplexer b is combined with the signal light at the tapered combiner of the multiplexer, and enters the B-section polarization-independent optical isolator c.
As shown in the figure, the optical multiplexer b configured by fusion splicing of optical fibers can combine or demultiplex light of any wavelength by adjusting the coupling length, and the wavelength separation degree is 15
A value of about 20 dB can be realized.

In such fixing of the optical fiber fusion type component (see FIG. 2), expansion and contraction of the fixing member 6 changes the characteristics of the taper joint portion due to environmental changes. It is better to fix it to the package 7 with a material that is hardly affected by changes. As a result, stable operation can be achieved against environmental changes. The pumping light combined with the signal light enters the polarization independent optical isolator c shown in FIG.

The polarization-independent optical isolator c is used between fibers whose polarization directions vary, and is composed of an optical system and a Faraday rotator for separating and coupling propagating light in orthogonal polarization directions. A birefringent crystal typified by TiO ₂ (rutile crystal) is usually used for separating and synthesizing polarized light, but the degree of separation is small at about 5 ° and it is necessary to increase the crystal thickness to separate the two optical axes. was there. In order to reduce the thickness of the birefringent plate used there, a structural birefringent element is used here.

The structural birefringent element is made of, for example, Si (refractive index: 3.5), SiO ₂ (refractive index: 1.4) as a laminated material.
When 5) is used, it can be obtained by configuring the thickness of each layer to be 1/2 or less of the wavelength of the propagating light used. When light is incident on the stacking direction at an angle, it can be used as a birefringent element having a separation angle of about 20 °. Using this, the polarization-independent optical isolator c is formed by arranging a structural birefringent element, a Faraday rotator, a half-wave plate, and a structural birefringent element in this order in a cylindrical magnet. The thickness of the Faraday rotator (Bi-substituted garnet) can be about 500 μm including the thickness of the Faraday rotator (Bi substitution garnet). By connecting the core expansion fibers with each other as in the present invention, a low-loss polarization-independent in-line optical isolator is realized. You can

The expanded core fiber is obtained, for example, by heating an optical fiber from which the protective coating has been removed to about 1400 ° C. to diffuse the additive such as Ge contained in the core for increasing the refractive index. To be As a result, the mode field diameter can be increased to about 2 to 4 times the original size. If the thickness of the isolator element is 500 μm and the mode field diameter of the input / output end face of the core expansion fiber is about 30 μm, a polarization independent isolator with an insertion loss of 0.5 dB or less can be realized.

In this way, the signal light and the pumping light that have passed through the in-line type optical isolator enter an optical fiber doped with, for example, Er of rare earth, the rare earth ions in the fiber are excited by the pumping light, and the excited state is generated. The signal light is amplified by the energy emitted when returning to the steady state from.

Although the case where the optical fiber amplification is used as the forward pumping type has been described above, it may be used as the front and rear pumping type as shown in FIG. 5, and in this case, the backward pumping is performed together with the forward pumping type optical composite component. Molded optical composite parts are required.

The difference between the backward pumping type and the forward pumping type is that the direction of the signal light and the propagation direction of the pumping light are opposite, and the polarization independent optical isolator is located on the side opposite to the rare earth-doped fiber, that is, on the emitting side. That is, a bandpass filter that removes spontaneously amplified optical noise generated during amplification is added to the output side of the optical isolator. With such a structure, an optical fiber amplifier having a large amplification degree can be formed. However, depending on the application, there is no problem even if the optical fiber amplifier is constructed only by the forward pumping optical composite component or the backward pumping composite component of the present invention. Further, a monitor coupler for branching a part of light may be additionally provided before and after the optical multiplexer / demultiplexer used in the present invention. Further, in the embodiment of the present invention, each component is integrally fixed and mounted, but the present invention is not limited to this, and each component is, for example, a coupler, a multiplexer, and an optical isolator as individual components. This also includes a case where each component is mounted, each component is fusion-spliced, and the components are integrally mounted in that state.

[0022]

As described above, the optical composite component for an optical fiber amplifier according to the present invention has the following excellent characteristics.

Since the connecting portion is formed by connecting the optical fibers to each other or the core-expanding fibers to each other, it is not converted into the spatial light using the lens, so that the loss can be configured.

Since the lens system is not used and the optical fiber can be constructed only by fusion splicing, precise assembly adjustment is not required and assembly is easy.

The use of optical fiber fusion-bonded parts, no lenses, and no need for precision assembly processes make mass production excellent and the cost of parts can be reduced.

Since the light is converted into spatial light and there is no narrowing down of the beam in the middle and the fiber is spliced, it is strong and stable against environmental changes.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the internal structure of an optical composite component for an optical fiber amplifier according to the present invention.

FIG. 2 is a diagram showing details of a portion A shown in FIG.

3A, 3B, and 3C are cross-sectional views of polarization-maintaining fibers that form an optical coupler.

FIG. 4 is a diagram showing details of a portion B shown in FIG.

FIG. 5 is a diagram showing an embodiment of a front-back pumping type optical fiber amplifier using an optical composite component according to the present invention.

[Explanation of symbols]

a: optical coupler, b: optical multiplexer, c: polarization independent isolator, 1, 4: optical fiber, 2, 3: polarization maintaining fiber, 5: rare earth-doped fiber 6: fixing member, 7: package

Claims (1)

[Claims] 1. An optical fiber amplifier using an optical fiber doped with a rare earth element or a transition metal, wherein polarization-maintaining fibers are aligned and fused and stretched, and pumping lights input to both polarization-maintaining fibers are orthogonal to each other. The optical coupler to be coupled and the optical fibers are aligned and fused and extended, one optical fiber end face is connected to the output end face of the coupler, and the signal light input to the other optical fiber and the coupler. The output pumping light is multiplexed, and an optical fiber fusion-bonded component such as an optical multiplexer in which the output side fiber core end is enlarged,
A polarization independent optical isolator in which a plurality of elements having a birefringent structure and at least one Faraday rotator are arranged in a magnet, and an input side end face is connected to an output side end face of the optical fiber fusion-type component. An optical composite component for an optical fiber amplifier, wherein an optical fiber having an input end face having an enlarged fiber core end connected to an output end face of the optical isolator is integrally mounted in a package.