JP3099858B2 - Optical fiber connection - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

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

FIG. 6 shows a basic configuration of a 1.3 μm band optical fiber amplifier according to the prior art. In FIG. 6, a Pr-doped fluoride fiber 01 is a high Δ fiber having a relative refractive index difference Δ of 2% or more to increase the gain coefficient, and high Δ quartz-based fibers 02 are connected to both ends thereof. One end of the high Δ quartz-based fiber 02 opposite to the connection end with the Pr-doped fluoride fiber 01 is a TEC processing portion 02a whose core is expanded by TEC (expansion of thermal diffusion core), and the Pr-doped fluorine is used. Δ which is almost the same as
And a cutoff wavelength. On the other hand, a fiber coupler 03 for inputting excitation light and signal light for exciting the Pr-doped fluoride optical fiber 01 is connected to the high Δ quartz-based fiber 02. Fiber coupler 03
One of the two input ends opposite to the connection end to the high Δ quartz-based fiber 02 is a signal input end 03a, and the other is connected to an excitation light source (excitation wavelength: 1.02 μm) 04. A fiber type optical isolator 05 for suppressing oscillation of the optical amplifier is connected to the other side of the other side of the high Δ quartz-based fiber 02 connected to the Pr-doped fluoride optical fiber 01. Each of the fibers used in the fiber coupler 03 and the fiber type optical isolator 05 is a silica-based optical fiber, and their relative refractive index difference Δ is usually 0.3.
%.

In such an optical amplifier, pump light and signal light from a pump light source 04 are multiplexed by a fiber coupler 03 and input to a Pr-doped fluoride optical fiber 01, and signal light is amplified and output. Note that the high Δ quartz-based fiber 0
2 is between the optical fiber coupler 03 and the Pr-doped fluoride fiber 01 and the fiber type optical isolator 05
It is used to improve the coupling efficiency between the Si and the Pr-doped fluoride fiber 01.

When actually manufacturing the above-described optical amplifier, each component is usually formed by a fusion or optical connector connection technique which can be connected with low loss and low reflection and is excellent in reliability. However, the connection between the Pr-doped fluoride fiber 01 and the high Δ quartz-based optical fiber 02 is based on (1) the difference between the softening temperatures of the two fibers (quartz-based fiber ~ 1400 ° C, fluoride fiber ~
(300 ° C), the fusion technology cannot be applied, (2) high Δ
For connection between fibers, insertion loss due to misalignment in the production of optical connectors increases, and optical connector connection technology cannot be applied. For this reason, a method that replaces fusion or optical connector connection is required.

FIGS. 7 and 8 show a connection method (Japanese Patent Application Laid-Open No. 5-34543 and Japanese Patent Application No. 4-178650) as an alternative to fusion bonding and optical connector connection. As shown in both figures, the optical fiber holding housings 011A and 011B for coupling the respective fibers 01 and 02 include V-groove substrates 012A and 012B having V-grooves for positioning and supporting the fibers 01 and 02, respectively. Optical fiber fixing plates 013A and 013B which are respectively superimposed on V-groove substrates 012A and 012B to hold and fix fibers 01 and 02 on the V-groove;
A and 012B and optical fiber casings 014 respectively holding optical fiber fixing plates 013A and 013B.
A and 014B, and adhesives 015A and 015B for fixing the fibers 01 and 02, respectively. These optical fiber holding housings 011A and 011
To optically couple fibers 01 and 02 using B,
First, the fibers 01 and 02 are positioned and supported by the V-groove substrates 012A and 012B.
A and 015B and the optical fiber fixing plates 013A and 013B, the optical fiber housings 014A and 01A.
4B (FIG. 7). Next, after polishing the connection surfaces of the optical fiber holding housings 011A and 011B, adjusting the positions of the fiber holding housings 011A and 011B so that the optical axes coincide with each other, and then connecting them using an adhesive 016 ( (FIG. 8). Usually, the V-groove substrates 012A and 012B, the optical fiber fixing plates 013A and 01
3B, and optical fiber housings 014A and 014B
Is made of quartz glass or Pyrex glass because of its thermal stability and ease of workability.
As 5A, 015B and 016, an ultraviolet-curing adhesive having a small axis deviation at the time of curing is mainly used.

[0007]

However, the above-described V-groove connection using the optical fiber holding housings 011A and 011B occurs at the time of connection between the Pr-doped fluoride fiber 01 and the high Δ quartz-based optical fiber 02. There is a major problem with reducing Fresnel reflection. For example, a fluoride fiber having a relative refractive index of 3.7%, a cutoff wavelength of 1.0 μm, and a core refractive index of 1.56, 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-based fiber, the Fresnel reflectivity is −32.8.
dB (0.053%). Since the laser oscillation occurs due to the Fresnel reflection, the upper limit of the gain of the optical amplifier using the Pr-doped fluoride fiber is limited to about 30 dB.

As a method for solving the problem of reflection at the end faces of the optical fiber holding casings 011A and 011B,
The technology described in FIGS. 9 and 10 has been previously proposed (Japanese Patent Application No. 4-178650). 9 and 10, the same members as those in FIGS. 7 and 8 are denoted by the same reference numerals.
In this method, a dielectric film (including a dielectric multilayer film) 017 is provided on the connection end face of one optical fiber holding case holding the fiber (the optical fiber holding case 011B holding the fiber 02 in FIG. 9). (FIG. 9). Then, after adjusting the positions of the optical fiber holding casings 011A and 011B so that the optical axes are aligned with each other, the optical fiber holding casings are connected using an ultraviolet curing adhesive 016 as shown in FIG.

However, in order to reduce the reflectance in this connection, it is necessary to precisely adjust the refractive index of the adhesive 016 and the refractive index and the thickness of the dielectric film 017. That is, the equivalent refractive index of the optical fiber 01 is n ₁ ,
When the equivalent refractive index of _{No. 2} is n ₂ , the refractive index of the adhesive 016 is adjusted to n _1, and the refractive index of the dielectric film 017 is n ₁
The need to made to have a non-reflective properties at the interface between the n _2. Furthermore, in the case of realizing non-reflection characteristics using a one-layer film as the dielectric film 017, the refractive index n and the film thickness t of the film must satisfy the following expressions (1) and (2). No.

[0010]

[Number 1] _{n 2 = n 1 · n 2} (1)

[0011]

## EQU2 ## where λ is the wavelength used, and λ = 1.3 μm in a 1.3 μm optical amplifier using a Pr-doped fluoride fiber.
It is.

As described above, in order to form a low-reflection connection using a dielectric film, the refractive index of the adhesive 016 and the refractive index and the film thickness of the dielectric film 017 are precisely adjusted. Therefore, there is a large problem in realizing a connection portion having excellent characteristics with good reproducibility (yield).

On the other hand, as shown in FIG. 11, an optical fiber holding housing 011 holding the respective fibers 01 and 02.
The connection end surfaces 011a of A and 011B are polished so as to be inclined by θ with respect to a vertical axis with respect to the housing side surface 011b (parallel to the optical axis of the held fiber), and the optical fiber holding housings 011A and 011B are polished. After adjusting the positions of the housings so that the respective optical axes coincide with each other, a diagonal connection method in which reflection is prevented by connecting with the use of an ultraviolet curing or thermosetting adhesive 016 is conceivable.

However, in such an oblique connection method, when the optical fiber holding casings 011A and 011B are fixed to the alignment device and the optical axes coincide with each other, as shown in FIG. The alignment must be repeated (in the order of (a) → (b) → (c) → (d) in the figure), it takes a long time to connect, and both optical fiber holding housings 011A are required.
And 011B may not be aligned in a state where they are in close contact with each other and low-loss connection may not be realized. In the figure,
Reference numerals 018A and 018B denote alignment devices, and arrows indicate the alignment directions.

The present invention has been made in view of the above circumstances, and has as its object to provide a non-quartz optical fiber such as a Pr-doped fluoride fiber and a silica optical fiber necessary for forming a 1.3 μm band optical amplifier. Is indispensable for high gain characteristics (30 dB or more) of the optical amplifier.
An object of the present invention is to provide an optical fiber connecting portion which can be manufactured in a short time with low reflection, low loss and high reproducibility.

[0016]

According to the present invention, there is provided an optical fiber connecting portion for connecting a first housing holding an end of a non-quartz optical fiber and the non-quartz optical fiber. Each of the second housing holding the end of the quartz fiber has a connection end face polished so as to be perpendicular to the side wall thereof, and the first and second housings are
In a state where the optical axes of the non-quartz optical fiber and the silica fiber are aligned so that the optical axes thereof coincide with each other, the non-quartz optical fiber and the non-quartz optical fiber are connected via an optical adhesive. The quartz fiber is characterized by being inclined with respect to a vertical axis of the connection end face.

Further, in the above structure, the non-quartz optical fiber is an optical fiber for a fiber amplifier in which a rare earth element or a transition metal having a laser transition level is added to a core portion or a cladding portion. .

[0018]

According to the present invention, since the connection end surface of the housing is perpendicular to the side surface and alignment can be performed in a state in which the housings are in close contact with each other, it is easy to align the housings with each other by the alignment device. On the other hand, since the fiber end faces are obliquely connected to each other, a connection portion having low loss and low reflection characteristics can be realized.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the drawings. However, the embodiments disclosed below are merely examples of the present invention and do not limit the scope of the present invention. . An embodiment of the present invention will be described with reference to FIGS. The non-quartz optical fiber 1 shown in FIG.
Is a ZrF ₄ -based fluoride fiber doped with 500 ppm, having a relative refractive index of 3.7% and a cutoff wavelength of 1.0 μm.
m, the core refractive index is 1.56. Silica-based optical fiber 2
Is a high Δ quartz optical fiber having a relative refractive index of 3.5%,
The cut-off wavelength is 0.9 μm and the core refractive index is 1.49. As shown in FIG. 3, optical fiber holding housings 11A and 11B holding the fibers 1 and 2 respectively include V-groove substrates 12A and 12B having V-grooves 12a for positioning and supporting the fibers 1 and 2, respectively. Optical fiber fixing plates 13A and 13B which are respectively superimposed on V-groove substrates 12A and 12B to hold and fix fibers 1 and 2 on V-groove 12a, V-groove substrates 12A and 12B and optical fiber fixing plates 13A and 13B And housings 14A and 14B for optical fibers, respectively, and adhesives 15A and 15B for fixing the fibers 1 and 2, respectively. here,
The V-grooves 12a of the V-groove substrates 12A and 12B are formed such that the ends of the optical fiber holding housings 11A and 11B on the connection end face 11a side are located at the center in the width direction, but the opposite ends are shifted from the center in the width direction. The axis is inclined at an angle θ (8 ° in this embodiment) with respect to a center line parallel to the side wall 11b of the optical fiber holding housings 11A and 11B. The V-groove substrates 012A and 012B, the optical fiber fixing plates 013A and 013B, and the optical fiber housing 014.
Quartz glass was used as the material of A and 014B.

To optically couple the fibers 1 and 2 using these optical fiber holding housings 11A and 11B,
First, fibers 1 and 2 were connected to V-groove substrates 12A and 12A.
B to position and support the adhesives 15A and 1A.
5B and the optical fiber fixing plates 13A and 13B are fixed to the optical fiber casings 14A and 14B. Next, a connection end face 11a is formed by optically polishing the connection end of the optical fiber holding housings 11A and 11B perpendicularly to the side wall 11b. Thereafter, the optical fiber holding housing 11A is set so that the optical axes of the fibers 1 and 2 coincide with each other.
After the positions of 11B and 11B are adjusted, both are connected using the ultraviolet curing adhesive 16.

FIG. 2 shows the alignment work at this time. As shown in FIG. 2, first, the optical fiber holding casings 11A and 11A
An ultraviolet curable adhesive is applied to the connection end face 11a of 1B, and both are attached to the fiber alignment devices 18A and 18B, respectively. After that, first, coarse adjustment is performed by setting the distance between the connection end faces 11a to about 15 μm or less ((a) in the figure).
That is, they are brought into close contact with each other and finely adjusted so that their optical axes coincide with each other ((b) in the figure). In this state, the connection end face 1
The portion 1a is irradiated with ultraviolet rays to cure the ultraviolet curing adhesive 16 and connect the optical fiber holding housings 11A and 11B2. The UV curable adhesive 16 has a refractive index of 1.56.
(Wavelength 1.3 μm). The alignment time of this connection is 1 minute or less, which is much shorter than the alignment time of about 3 minutes of the oblique polishing connection shown in FIGS.

Further, when 103 connection portions using this connection method were formed and the return loss and the connection loss of all the connection portions were evaluated, the return loss was −41 dB or more, and the connection loss was 0.2 dB or less. It was confirmed that a high-performance connecting portion can be configured with a good yield.

Further, as shown in FIG. 4, a 1.3 μm band optical fiber amplifier having the connecting portion of the present invention was constructed.
A high Δ quartz-based fiber 2 is connected to both ends of the Pr-doped fluoride fiber 1. The high Δ silica-based fiber 2 has a P
One end opposite to the connection end with the r-doped fluoride fiber 1 is a TEC processing unit 2 whose core is enlarged by TEC, respectively.
a. On the other hand, the high Δ quartz-based fiber 2 has P
A fiber coupler 3 for inputting excitation light and signal light for exciting the r-doped fluoride optical fiber 1 is connected. One of two input ends of the fiber coupler 3 opposite to the connection end with the high Δ silica-based fiber 2 is a signal input end 3a.
And the other is an excitation light source (the excitation wavelength is 1.02 μm,
Ti sapphire laser 4) is connected. A fiber-type optical isolator 5 for suppressing oscillation of the optical amplifier is connected to the other side of the other side of the high Δ silica-based fiber 2 connected to the Pr-doped fluoride optical fiber 1. The fibers used in the fiber coupler 3 and the fiber-type optical isolator 5 are silica-based optical fibers, respectively.
The relative refractive index difference Δ is usually 0.3%. In the drawings, 11A and 11B indicate the optical fiber holding housing described above.

In such an optical amplifier, the pump light and the signal light from the pump light source 4 are multiplexed by the fiber coupler 3 and
The signal light is input to the r-doped fluoride optical fiber 1, and the signal light is amplified and output. The high Δ silica-based fiber 2 is used to improve the coupling efficiency between the optical fiber coupler 3 and the Pr-doped fluoride fiber 1 and between the fiber type optical isolator 5 and the Pr-doped fluoride fiber 1. Have been.

FIG. 5 shows the amplification characteristics of the amplifier shown in FIG. This confirmed that an optical amplifier having a signal gain of 30 dB or more was configured. Therefore, it has been found that a high-gain optical amplifier can be configured by using the connection section of the present invention.

The above-mentioned embodiment is used as a non-quartz fiber. 1) InF ₃ -based fluoride fiber (Nishida et al., The 40th Joint Lecture on Applied Physics, Proceedings No. 3, 30p.
-ZL-3, pp 1066, 1993), 2) Ge-S system, As-S system, Ge-PS system, As-
Optical fiber made of glass such as Ge-S chalcogenide glass; 3) chloride glass such as ThCl ₄ —PbCl ₂ —NaCl; bromide glass based on AgBr—PbBr ₂ —CsBr—CdBr ₂ ; CdF ₂ —BaCl ₂ — NaCl-based fluoride-chloride glass, ZnBr ₂ -TlBr-Tl ₁
An optical fiber composed of a bromide-iodide glass ("New Glass Handbook", edited by New Glass Handbook Editing Committee, Maruzen Co., Ltd., 1991). 4) Phosphate glass, fluorophosphate glass, Al-silicate system The same connection as in the example was made using a multi-component glass fiber, and characteristics with a return loss of -40 dB or more were confirmed. However, all the oblique angles θ were 12 °.

The optical fiber connecting portion of the present invention is suitable for use in an optical fiber having a high Δ (high NA) having, for example, a relative refractive index difference of 2% or more. Even when such a fiber is used, it has low loss and low reflection characteristics. Connection can be realized.

The angle θ formed between the center line parallel to the side wall of the optical fiber holding housing and the optical axis of the held optical fiber.
In order to obtain a low reflection characteristic, it is preferable that the thickness be 8 mm or more. In the above embodiment, an ultraviolet curing adhesive is used to connect the fiber holding housing, but a thermosetting adhesive is used. You may. The material of the fiber holding case may be not only glass but also metal.

[0029]

As described above, according to the present invention, the ends of the respective fibers to be connected are held obliquely (at an angle θ) with respect to the side surface of the housing, and the connection end surfaces are formed on the side walls of both fiber holding housings. In order to connect them using the adhesive 3 after adjusting them so that the optical axes of both the fiber holding housings coincide with each other,
Diagonal connection with low loss and low reflection characteristics was realized in a short time. In addition, an optical amplifier having high gain characteristics can be configured by applying the connection section of the present invention to a 1.3 μm band optical fiber amplifier.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating an embodiment of an optical fiber connecting portion according to the present invention.

FIG. 2 is a schematic diagram illustrating alignment when forming an optical fiber connection portion of the present invention.

FIG. 3 is a plan view and a side view illustrating an embodiment of an optical fiber connecting portion according to the present invention.

FIG. 4 is a schematic diagram illustrating an example of an optical amplifier having an optical fiber connection according to the present invention.

FIG. 5 is a graph showing amplification characteristics of the optical amplifier of FIG.

FIG. 6 is a schematic diagram showing an optical amplifier according to the related art.

FIG. 7 is a perspective view illustrating a connection using a V-groove according to the related art.

FIG. 8 is a side view illustrating a connection using a V-groove according to the related art.

FIG. 9 is a perspective view illustrating an improvement of the V-groove connection of FIGS. 7 and 8;

FIG. 10 is a side view illustrating the improvement of the V-groove connection of FIGS. 7 and 8;

FIG. 11 is a plan view illustrating the improvement of the V-groove connection of FIGS. 7 and 8;

FIG. 12 is a schematic diagram illustrating alignment of the V-groove connection in FIG. 11;

[Explanation of symbols]

 Reference Signs List 1 Non-quartz fiber such as Pr-doped fluoride fiber 2 Silica fiber 3 Optical fiber coupler 4 Excitation light source 5 Fiber type isolator 11A, 11B Optical fiber holding housing 12A, 12B V-groove substrate 12a V-groove 13A, 13B Optical fiber fixing Plate 14A, 14B Optical fiber housing 15A, 15B Optical fiber fixing adhesive 16 Adhesive 18A, 18B Optical fiber alignment device

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Shoichi Sudo 1-6-1, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-2-53006 (JP, A) JP-A-Hei 4-360108 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 6/24-6/40

Claims (2)

(57) [Claims] 1. A first housing holding an end of a non-silica-based optical fiber and a second housing holding an end of a silica-based fiber connected to the non-silica-based optical fiber. A connection end face polished so as to be perpendicular to the side wall, and the first and second housings are aligned so that the optical axes of the non-quartz optical fiber and the silica fiber coincide. In this state, the non-quartz optical fiber and the silica fiber are inclined with respect to a vertical axis of the connection end face, respectively, in the optical fiber connecting portion connected via the optical adhesive. Optical fiber connection section. 2. The fiber amplifier according to claim 1, wherein the non-quartz optical fiber is a fiber amplifier optical fiber in which a rare earth element or a transition metal having a laser transition level is added to a core portion or a cladding portion thereof. Optical fiber connection.