JPH0627343A - Optical fiber juncture for optical fiber amplifier - Google Patents

Abstract

PURPOSE:To lower the reflectivity occurring in the Fresnel reflection of a Pr- added fluoride optical fiber and quartz optical fiber indispensable for the higher gain characteristic of an optical amplifier in the juncture of these two fibers necessary in the case of constitution of the optical amplifier of a specific wavelength band. CONSTITUTION:This juncture is formed by holding the ends of the non-quartz optical fiber 1-1 added with a rare earth element having a laser transition level in the core part or clad part and the quartz optical fiber 1-2 connected thereto in housings 7-1, 7-2 and connecting the housings to each other in such a manner that the optical axes thereof are aligned to each other. Dielectric substance films or multilayered dielectric substance films 6 exist over the entire surface at the connection boundary of at least one housing or near the end faces of the fibers and the housings are adhered to each other by interposing an adhesive therebetween.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber connection portion having a low loss and a low reflection characteristic used in an optical fiber amplifier which is required in the fields of optical communication and optical measurement.

[0002]

2. Description of the Related Art An optical fiber in which a rare earth element having a laser transition level is added to a core part or a clad part is attracting attention as an optical fiber for an optical fiber amplifier.
In the signal wavelength 1.5 μm band, an optical fiber amplifier excellent in high efficiency, high output power, polarization independent characteristics, low noise, etc. has been realized by adding an Er element to a silica optical fiber. On the other hand, in the signal wavelength band of 1.3 μm, a Pr-doped fluoride optical fiber has been proposed and is currently being actively researched for realizing the optical fiber amplifier.

FIG. 7 shows the basic structure of a 1.3 μm band optical fiber amplifier (prior art 1). Reference numeral 1-1 represents a Pr-doped fluoride optical fiber, and a high Δ optical fiber having a relative refractive index difference Δ of 2% or more is used to increase the gain coefficient. 1-2 is a high Δ silica optical fiber which will be described in detail later. Reference numeral 9 is an excitation light source for exciting the Pr-doped fluoride optical fiber 1-1 (excitation wavelength is 1.02 μm), 10 is an optical fiber coupler for combining the excitation light generated by the excitation light source 9 and the signal light, 11
Shows a fiber type optical isolator for suppressing the oscillation of the optical amplifier. The optical fibers used in the fiber coupler 10 and the fiber-type optical isolator 11 are silica optical fibers, and the relative refractive index difference Δ is usually 0.3%.

The high Δ quartz optical fiber 1-2 has T at one end A.
The core is expanded by EC (Thermally-Diffused Expanded Core; core expansion technology by thermal diffusion), and the Δ is almost equal to that of the Pr-doped fluoride optical fiber 1-1.
And a cutoff wavelength, and an optical fiber coupler 10 and a Pr-doped fluoride optical fiber 1-1, and a fiber type optical isolator 11 and a Pr-doped fluoride optical fiber 1-1.
The optical fiber coupler 10, the Pr-doped fluoride optical fiber 1-1, and the fiber type optical isolator 1.
1 and the Pr-doped fluoride optical fiber 1-1 are used to improve the coupling efficiency.

When the above-mentioned optical amplifier is actually manufactured, each component is usually manufactured by fusion bonding or an optical connector connection technique which allows low loss and low reflection connection and is highly reliable. But,
The connection between the Pr-doped fluoride optical fiber 1-1 and the high Δ silica optical fiber 1-2 is as follows: (1) Difference in softening temperature of both optical fibers (silica optical fiber ~
1400 ° C, fluoride optical fiber to 300 ° C), the fusion technology cannot be applied.

(2) Since the high Δ optical fibers are connected to each other, the insertion loss due to the misalignment in manufacturing the optical connector becomes large, and the optical connector connecting technology cannot be applied.

Due to the above reasons, there is no effective connecting means, which has been a problem in manufacturing an optical fiber amplifier.

As a method for solving this problem, conventionally, as shown in FIGS. 8 and 9, each optical fiber 1-1 or 1-2 is held by an optical fiber holding housing 7-1 or 7-2. , The optical fiber holding housing 7 so that the optical axes thereof coincide with each other.
After adjusting -1, 7-2, as shown in FIG.
It is considered that the prior art 2 in which the connection is made by using is effective (Japanese Patent Application No. 3-195336). The optical fiber 1-1 or 1-2 is positioned by the V-groove substrates 3-1 and 3-2, and the adhesives 5-1 and 5-2 and the optical fiber fixing plates 4-1 and 4- are used.
It is fixed to the optical fiber housings 2-1 and 2-2 by 2. This technology is applied to Pr-doped fluoride optical fiber 1-1 and high Δ
By using the connection between the quartz optical fibers 1-2,
Optical connection with low loss and excellent environmental resistance is possible.

[0009]

However, in the present technology, P
There is a major problem in reducing the Fresnel reflection that occurs when connecting the r-doped fluoride optical fiber 1-1 and the high Δ silica optical fiber 1-2. For example, the relative refractive index 3.7
%, A cutoff wavelength of 1.0 μm, a core refractive index of 1.56 and a relative refractive index of 2.3%, a cutoff wavelength of 0.7 μm, and a core refractive index of 1.49 when connecting a silica optical fiber, Fresnel reflectance Is -32.8 dB (0.053%).
Therefore, the upper limit of the gain of the optical amplifier using the Pr-doped fluoride optical fiber is about 30 dB due to the Fresnel reflection.
Limited to.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical fiber at a connecting portion between a Pr-doped fluoride optical fiber and a silica-based optical fiber, which is necessary for constructing a 1.3 μm band optical amplifier. High gain characteristics of amplifier (30
It is necessary to reduce the reflectance due to Fresnel reflection of both, which is indispensable for (dB or more).

[0011]

FIG. 1 shows the basic configuration of the present invention. The greatest feature compared to the conventional configuration is that a dielectric film (including a dielectric multilayer film) is present at the interface of at least one of the housings holding the optical fiber.

The present invention will be described below with reference to FIGS. Reference numerals 7-1 and 7-2 denote optical fiber holding casings, which are non-silica optical fibers 1-1 such as Pr-doped fluoride optical fibers or silica optical fibers 1-2, optical fiber casings 2-1. 2-2, V-groove substrates 3-1, 3-2, optical fiber fixing plates 4-1, 4-2, and adhesives 5-1 and 5
-2. The dielectric film 6 is added to the connection end face of one (7-2) of one optical fiber holding housing. However, in the figure, the dielectric film 6 is added to the connection end surface of the optical fiber holding casing 7-2, but the optical fiber holding casing 7
It may be -1 or the connecting end surface of both. further,
Although the dielectric film is added to the entire surface of the connecting portion, it may be provided only near the end face of the optical fiber. Optical fiber holding housings 7-1 and 7
-2, after adjusting the housings so that the optical axes match each other,
As shown in FIG. 2, they are connected to each other using an ultraviolet curing or thermosetting adhesive 8.

In order to reduce the reflectance in the above connection, the refractive index of the adhesive 8 and the characteristics of the dielectric film 6 are precisely adjusted. When the equivalent refractive index of the optical fiber 1-1 is n1 and the equivalent refractive index of the optical fiber 1-2 is n2, the refractive index of the adhesive 8 is adjusted to n1. Further, the dielectric film 6 is manufactured so as to have antireflection characteristics at the boundary surface between the refractive indices n1 and n2. For example, when a single-layer film is used as the dielectric film 6 to realize antireflection characteristics, the refractive index n and the film thickness t of the film must satisfy the following conditions.

N ² = n1 · n2 (1) n · t = λ / 4 (2) However, λ is a used wavelength, and λ = in a 1.3 μm optical amplifier using a Pr-doped fluoride optical fiber. 1.3μ
m.

[0015]

In the present invention, as compared with the conventional structure, by adding an adhesive and a dielectric film whose refractive index is appropriately selected to the connection interface, a silica optical fiber and a non-silica optical fiber such as Pr-doped fluoride optical fiber are added. It is possible to realize an optical amplifier having a high gain characteristic in which the Fresnel reflectance of the connecting portion of the optical fiber is reduced.

[0016]

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.

Embodiment 1 of the present invention with reference to FIGS. 1 and 2.
Will be explained. 1-1 is Zr added with 500 ppm of Pr
It is an F4 type fluoride optical fiber and has a relative refractive index of 3.7.
%, The cutoff wavelength is 1.0 μm, and the core refractive index is 1.56. 1-2 is a high Δ silica optical fiber having a relative refractive index of 2.3%, a cutoff wavelength of 0.7 μm, and a core refractive index of 1.
49. 2-1 and 2-2 are optical fiber housings, 3-
Reference numerals 1 and 3-2 are V-groove substrates, reference numerals 4-1 and 4-2 are optical fiber fixing plates, and the material thereof is glass. 5-1 and 5-2 are ultraviolet curing adhesives. 7-1 and 7-2 are 1-1 and 2
-1,3-1,4-1,5-1 or 1-2,2-2
3 shows an optical fiber holding housing composed of parts 3-2, 4-2 and 5-2. On the connection end face of the optical fiber holding housing 7-2, a mixture of SiO and SiO2 (hereinafter referred to as SiOx film) was prepared as the dielectric film 6 by using a vapor deposition method. Evaporated film 6
From equations (1) and (2), the film thickness is 2138 A and the refractive index is 1.
52. The film thickness was adjusted by a film thickness monitor. on the other hand,
As shown in FIG. 3, the refractive index of the SiOx film can be adjusted by the partial pressure of oxygen during vapor deposition, so the partial pressure of oxygen is 9 × 10 ⁻⁵ Torr.
Setting to r, a SiOx film having a refractive index of 1.52 was formed.

The optical fiber holding casings 7-1 and 7-2 were adjusted so that their optical axes coincided with each other, and then connected using an ultraviolet curing adhesive 8 as shown in FIG. The refractive index of the ultraviolet curable adhesive 8 is 1.56 (wavelength 1.3 μm).

When the reflectance of this connection portion was measured using a return loss measuring instrument, it was -42 dB. Reflection rate without adding dielectric film (SiOx film in this case) -32.8d
The characteristic was improved by about 10 dB compared to B.

Further, as shown in FIG. 4, a 1.3 μm band optical fiber amplifier having the connection portion of the present invention was constructed.
9 is an excitation light source that excites the Pr-doped fluoride optical fiber 1-1 (excitation wavelength is 1.02 μm, Ti sapphire laser is used), and 10 is light for combining the excitation light generated by the excitation light source 9 and the signal light. A fiber coupler 11 is a fiber type optical isolator for suppressing the oscillation of the optical amplifier.
Optical fiber coupler 10, fiber type optical isolator 1
The optical fibers used in No. 1 are silica optical fibers, and the relative refractive index difference Δ is usually 0.3%.

One end of the high Δ silica optical fiber 1-2 which is connected to the optical fiber coupler 10 and one end which is connected to the fiber type optical isolator 11 have their cores enlarged by the TEC treatment, and the optical fiber coupler 10 and Pr-doped fluoride are added. It was used to improve the coupling efficiency between the optical fiber 1-1 and the fiber type optical isolator 10 and the Pr-doped fluoride optical fiber 1-1.

FIG. 5 shows the amplification characteristic. Signal gain 35 dB
An optical amplifier having the above characteristics was constructed. Therefore, it was found that a high gain optical amplifier can be constructed by using the connection portion of the present invention.

In Example 1, a dielectric film (SiOx film in this case) 6 was vapor-deposited on the entire connection end face of the optical fiber holding casing 7-2. However, when the connection part of Example 1 was subjected to the heat cycle test (−30 to 80 ° C.), there were cases where peeling occurred at the connection part. Therefore, in Example 2, considering the connection strength between the optical fiber holding casings, as shown in FIG. 6, the dielectric film (SiOx film) 6 was formed only in the vicinity of the end face of the high Δ silica optical fiber 1-2. ′ Was deposited (film thickness,
The refractive index is the same as in Example 1). The optical fiber holding housing 7-2 'having a dielectric film (SiOx film) 6'added only near the fiber end face is used to perform the same connection as that of the first embodiment, and the same return loss (-42 dB) as that of the first embodiment. Got Also in the amplification experiment, an optical amplifier having a signal gain of 30 dB or more was constructed as in the first embodiment.

Furthermore, a heat cycle test (-
No peeling of the connection part was observed even after 30 to 80 ° C.), and a highly reliable connection part could be constructed.

In the above embodiments, the ZrF4 type fluoride optical fiber was used as the non-quartz type optical fiber, but other fluoride optical fibers such as InF3 type, ZnF2 type, AlF3 type glass, etc. (supervised by Tetsuro Izumitani, "New glass and its physical properties" Chapter 16, Management System Research Institute, 1984, or
Tomozawa and Doremus Edition Treatise on materials scien
ce and technology volum 26, Chapter 4, Academic Press,
(Refer to Inc. 1985, etc.) Fluoride optical fiber using glass, or chloride glass such as ThC14-PbC12-NaCl system other than fluoride glass, AgBr-PbBr2-CsBr-CdBr2 system bromide glass, CdF2-BaCl2 -NaCl-based fluoro-chloride glass,
ZnBr2-TlBr-Tl1 system bromide-iodide glass (see "New Glass Handbook", edited by New Glass Handbook, Maruzen Co., Ltd., 1991), Ge-S system,
Using non-quartz optical fiber made of As-S, Ge-PS, As-Ge-S chalcogenide glass, etc., and further phosphate glass, fluorophosphate glass, Al-silicic acid multi-component glass optical fiber Good. Further, although the ultraviolet curable adhesive is used for connecting the optical fiber holding housing, a thermosetting adhesive may be used, and the material of the fiber housing may be not only glass but also metal or the like. Further, although a single layer film of SiOx is used as the dielectric multilayer film, other dielectric materials such as MgF2 and MgO2 may be used and a multilayer structure of those materials may be used.

[0026]

As described above, according to the present invention, each of the silica-based optical fiber and the non-silica-based optical fiber is held by the optical fiber holding housing, and the housings are arranged so that their optical axes coincide with each other. After adjusting, the dielectric film or (dielectric multilayer film) is added to the connection interface of at least one case at the connection part where the adhesive is used for connection, and the refractive index of the adhesive and the characteristics of the dielectric film are adjusted. Since it is precisely adjusted to have the antireflection property, the reflection property at the connection portion is improved as compared with the case without the dielectric film. Therefore, by applying the connection portion of the present invention to a 1.3 μm band optical fiber amplifier, an optical amplifier having high gain characteristics can be constructed.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a first embodiment of the present invention.

FIG. 2 is a diagram for explaining the first embodiment of the present invention.

FIG. 3 is a diagram showing changes in the refractive index of a SiOx film.

FIG. 4 is an explanatory diagram of an optical fiber amplifier using the present invention.

5 is a diagram for explaining amplification characteristics of the optical fiber amplifier of FIG.

FIG. 6 is a diagram illustrating a second embodiment of the present invention.

FIG. 7 is a diagram illustrating Prior Art 1;

FIG. 8 is a diagram illustrating Prior Art 2.

FIG. 9 is a diagram for explaining Prior Art 2.

[Explanation of symbols]

1-1 ... Non-silica optical fiber such as Pr-doped fluoride optical fiber 1-2 ... Silica optical fiber 2-1 and 2-2
... Optical fiber housing, 3-1 and 3-2 ... V-groove substrate, 4-
1, 4-2 ... Optical fiber fixing plate, 5-1, 5-2 ... Adhesive agent, 6 ... Dielectric film, 7-1, 7-2 ... Optical fiber holding housing, 8 ... Optical fiber holding housing An adhesive that connects each other,
9 ... Excitation light source, 10 ... Optical fiber coupler, 11 ... Fiber type isolator.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasunori Oishi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Shoichi Sudo 1-16-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A non-quartz optical fiber having a core portion or a cladding portion to which a rare earth element having a laser transition level is added and an end portion of the quartz optical fiber connected to the non-quartz optical fiber are held in a housing so that their optical axes coincide with each other. As described above, in the optical fiber connecting portion for optical fiber amplifiers that connect the housings to each other, at least one of the housings has a dielectric film or a dielectric multilayer film on the entire connection interface or near the end face of the fiber, and the housings of the other housings. An optical fiber connecting portion for an optical fiber amplifier, characterized in that the body is adhered by interposing an adhesive. 2. The optical fiber for an optical fiber amplifier according to claim 1, wherein a dielectric film or a dielectric multilayer film which acts as an antireflection film after connection is used as the dielectric film or the dielectric multilayer film. Connection. 3. A relative refractive index difference between the rare earth element-doped non-silica optical fiber and the silica optical fiber is 2% each.
It is above, The optical fiber connection part for optical fiber amplifiers of Claim 1 or 2 characterized by the above-mentioned.