JPS59198419A - Production of fiber-shaped directional coupler - Google Patents

Abstract

PURPOSE:To facilitate producing a fiber-shaped directional coupler and make it possible to monitor the coupling state to prevent the loss accompanied with parallel melt-fixing of fibers by heating and stretching only a required part of an optical fiber incorporating two cores preliminarily to perform optical coupling. CONSTITUTION:An optical fiber which has 250mum outside diameter and has cores 21a and 21b which are 140mum apart from each other and have 8mum diameter is used as an optical fiber 21. A single mode optical fiber 23a of a passage having 125mum outside diameter and 8mum core diameter is connected to one end of the fiber 21 having about 10cm length, and a 1.15mum HeNe laser light is led to the core 21a through the fiber 23. The fiber 21 is set to a stretching device, and a part of the fiber 21 is heated locally with oxygen propane flame while observing a near field pattern of the exit end of the fiber 21 with an infrared-ray television to obtain a desired coupling ratio. The smallest diameter in the stretched part is about 10mum, and the length of the stretched part is about 5-10mm. including a tapered part.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a fiber type directional coupler used in the fields of optical communications and optical fiber sensors.

Conventionally, as a directional coupler using an optical fiber, one having a structure as shown in FIG. 1 is known. Figure 1 (
A) is a structure in which parts of two optical fibers 1 and 2 are heated and stretched, and a core 1a is formed in a stretched portion 8.

2a are optically coupled to each other to constitute a fiber type directional coupler. Although this directional coupler has a simple structure, it requires a high degree of skill to fuse and integrate the two optical fibers 1 and 2, and the reproducibility of the coupling ratio etc. is poor. There was a drawback.

FIG. 1(b) shows another conventional method, in which an optical fiber 4 having two adjacent cores 4a, 4b is prepared in advance, and light 77 i/< 5a, 5b, 6a,
6b is connected to form a directional coupler.

The distance between the cores 4a and 4b is approximately 5 μm, and a desired coupling ratio can be achieved by changing the length of the optical fiber 4 (usually several cm). However, in this directional coupler, the spacing between the cores 4a and 4b is as small as about 5 μm, so the optical fiber connected at both ends must be a special fiber with a significantly eccentric core as shown in Figure 1(b). , q optical fibers 5a, 5b, aa.

6b was required, which had a major drawback that it was difficult to connect with ordinary optical fiber (see Figure 1): Miyanaga,
Matsumoto, Proceedings of the 1985 National Conference of the Optical and Radio Division of the Institute of Electronics and Communication Engineers, Volume 2, s12 (Refer to ``Basic Study of Single Mode Optical Fiber Directional Coupler'').

In order to eliminate the above-mentioned drawbacks of the conventional method, the present invention prepares in advance an optical fiber in which two cores are separated by a distance greater than the outer diameter of the mh -y fiber to be connected, and heat-draws a part of this optical fiber. The aim is to obtain optical coupling and to enable the connection of ordinary fibers at both ends. The present invention will be explained in detail below with reference to the drawings.

FIG. 2(a) shows an embodiment of the present invention, in which an optical fiber 22 has two cores 21a and 21b, and a part 2
2 is stretched. Both ends of the optical fiber 22 are ordinary circular fibers with one core in the center/<28a,
28b, 24a, and 24b are connected to form the tail. The distance between the cores 21a and 21b is phi/<28a,
The outer diameter of the fiber 28b+2 is greater than or equal to the outer diameter of the fibers 28b and 24a+24b, and the core portions of the fibers 28a and 24a are aligned with the core 21a at the end face of the optical fiber 21.
The core portion of 4b is connected in alignment with the core 21b. For example, the light incident from the fiber 23a is transmitted to the core 21
a, is partially coupled to the core 21b at the extending portion 22, and is finally emitted to the fibers 24a, 24b.

A specific example will be described below. The optical fiber 21 has an outer diameter of 25
0μm, distance between cores 21a, 21b (distance between core centers): 140μm, diameter of 7A 21a, 21b sμm.

A silica-based optical fiber 21 with a core-cladding relative refractive index difference of 0.25% was used. A normal single mode optical fiber 28a with an outer diameter of 125 μm, a core diameter of 8 μm, and a relative refractive index difference between the core and cladding of 0.25% is connected to one end of the optical fiber 21 having a length of approximately IQ cm, and a 1.15/jm HeNe laser beam is connected to the optical fiber 21. was introduced into the core 21a via the optical fiber 28a. The fiber 21 is set in a fiber drawing device, and while observing the near-field pattern at the output end of the fiber 21 with an infrared television, a part of the fiber 21 is locally heated with a mini-torch using an oxygen-propane flame, and gradually drawn. Once the desired bond ratio was obtained by stretching, the stretching was stopped. The diameter of the stretched part was about 10 μm at the narrowest part, and the length of the stretched part, including the tapered part, was about 5 to 10 lengths.

In the above embodiment, as shown in FIG. 8(a), the fiber 21 is made of a core matrix (core glass body) 82a that includes a portion to become a core in a clad matrix 81 made by drilling a hole in cylindrical quartz glass. , 32b, heated and integrated, and produced using a wire drawing method. Further, the connection between the fiber 21 and the fiber 28a can be achieved by operating a fine movement stage under a microscope, and lens bond can be used as a connecting agent, but it is more reliable to connect the core 121a of the fiber 21 and the fiber 28a. After alignment with 28a, a method of fusing using a C02 laser, mini-torch, or arc discharge is excellent, and connection can be made with a connection loss of 0°2 dB or less. In the method of the present invention, this is shown in Figure 1 ('
This is because a normal optical fiber can be used as the optical fiber 28a, which is different from the conventional method in b).

After the drawing process is completed, the fiber 21 may be cut into a length of several centimeters, leaving the stretched portion 22, and then the four original fibers 28a, z8b, 24a, 24b may be connected again to form a cage. Further, if necessary, it can be used in combination with a laser light source or a photodetector via a lens system.

In the directional coupler manufactured by the method of the present invention, there is almost no excess loss at the joint portion, that is, the extension portion 22. This is the first step in which two separate optical fibers are fused and drawn together.
This is an advantage compared to the conventional method shown in Figure (a), which involves an excess loss of about 0.5 dB. The reason for this is that in the present invention, the two cores 21a and 21b are built into one optical fiber 21 in advance with good parallelism, and no loss occurs due to incomplete parallel fusion of the fibers.

FIG. 2 (bl is another embodiment of the present invention and shows an example of the structure of a polarization-maintaining directional coupler. The optical fiber 25 has two cores 25a and 25b as well as stress birefringence in the core region. Stress applying portions 26a and 26b for imparting stress.

It has 260. Stress applying parts 26a, 26b,
260 is arranged in consideration of symmetry with respect to the cores 25a, 251), and as a result, the cores 25a, 251)
25b has linear polarization maintaining property.

Polarization maintaining 7 fins 628a, 28b, 29a, 29 also have stress applying parts at both ends of the fiber 25.
b are connected, and the distance between the cores 25a, 25b is greater than the outer diameter of the fibers 28'a, 28b, 29a, 29b.

The heating and stretching process is the same as that shown in Fig. 2(a), but
C7 Aiha 28 al 28b p 2 for polarization maintenance
When connecting to 9a + 29b, it is necessary to align the main axis of the fiber, and this operation can be performed under a microscope.

As shown in FIG. 8('b), the fiber 25 is made of a glass body 34 that includes a core portion in a cladding body 83 with holes.
a, 34b % stressed glass body 85a, 35b,
It can be made by inserting 850 and drawing the wire integrally.

The method for manufacturing a fiber-type directional coupler of the present invention can be easily extended to a double-connected structure as shown in FIG. 4th I
Optical fiber 41 with two cores in I (a)
41 is stretched at 45.46 at a distance of 2 m, and the intermediate portion of the stretched portion 45.416 is etched on one side to the core portion and coated with a light absorber 48. The equivalent circuit of this double structure is shown in FIG. 4(k), in which a semiconductor laser is connected to the core part 41, a photodetector is connected to the core part 42, and the core part 4. It is possible to configure an optical gyroscope by connecting an optical fiber coil to the ends of la and 41b.
This type of directional coupler can be expected to find wide application in the field of optical fiber sensors.

Although the embodiment of the present invention has been described with reference to the case where there are two cores, it is possible to easily expand the case where the number of cores is three or more. Q-designed so that the distance between the cores is greater than the outer diameter of the fiber connected to both ends of the fiber having the plurality of cores,
If manufactured, a directional coupler can be realized without deviating from the spirit of the present invention.

As explained above, according to the method for manufacturing a directional coupler of the present invention, only the necessary portions of an optical fiber that has two cores built in are heated and stretched to perform optical coupling, so manufacturing is easy and optical coupling is possible. It can be manufactured while maintaining its condition, and losses associated with parallel fusion of optical fibers can be prevented.

In addition, the distance between the cores of the optical fiber end faces is sufficient to allow connection of ordinary optical fibers, so there is no need to use special eccentric fibers, and there is no loss associated with connection.
．． It can be suppressed to 2 dB or less, and can be easily connected to a laser or a photodetector. In addition, since there is no optical coupling in the non-stretched portion, a coupler with a double structure can be realized in a contiguous form, which is extremely effective when applied to the fields of optical communication and original fiber sensors.

[Brief explanation of the drawing]

1 [1U (a), (b) are structural example diagrams of a directional coupler manufactured by the conventional method, and Figures 2 (al, (t)) are structural example diagrams of a directional coupler manufactured by the method of the present invention. , Figure 3 t
a) and (b) are the second m (a) and (
Fig. 4(a) is an example of the application of the present invention to double purchasing; Fig. 4(b) is an equivalent circuit diagram of Fig. 4(a). 1.2... Optical fiber, la, 2a... Core, 3
...Extension part, 4...Light 7 with adjacent cores 4a, 4b, 5a, 5b, 6a, 6b-...Eccentricity: 177 fibers, 21...knee) (7) Near 7
Optical fiber having 21a, 21b, 22... extension part, 28a, 28b, 24a, 24b-"IQ
77 Iha, 25 ””: Jl tsu (D core 25a
, 25b (!: Stress'("f given fld 26a
, 26b. polarization-maintaining original fiber having 260, 27...extended portion, 28a, 28b, 29a, 29b..., polarization-maintaining 7 fiber, 31-...5)' matrix, 82
a, 32b... Glass body including core portion, 83...
Clad matrix, 84a, 84+b...Glass body including core portion, 85a, a5b,, a5c -...Stress imparting jf 57. Body, 41°41a, 42.42b・
... Core part, 48... Light absorber, 44... Optical fiber having two cores, 415.46... Extension part. Patent applicant: Nippon Telegraph Tunchu Corporation (a) Figure

Claims (1)

[Claims] L A method for manufacturing a fiber/shape-edge directional coupler having optical coupling portions in which a plurality of cores are adjacent to each other and parallel to each other less than the distance at which optical coupling occurs, wherein the distance between core centers is a single core. A multi-core fiber with an outer diameter greater than or equal to that of an optical fiber is prepared in advance,
A method for manufacturing a fiber-type directional coupler, characterized in that a desired portion of the multi-core fiber is heated and stretched to produce an optical coupling portion. The method for manufacturing a fiber-type directional coupler according to claim 1, characterized in that a multi-core fiber having a stress-applying portion that imparts stress birefringence to the core portion G is used as the multi-core fiber.