JPS60154215A - Fiber type directional coupler - Google Patents

Abstract

PURPOSE:To obtain a low-loss coupler by covering a single-mode optical fiber with a core glass part, the 1st clad glass part, and further the 2nd clad glass part with a lower refractive index. CONSTITUTION:Two single-mode optical fibers 1-1a and 2-2a are composed of the 1st clad glass part 21b and the 2nd clad glass part 21c surrounding a center core glass part 21a. Light incident on the core glass part 21a at a fiber end 1 spreads gradually to the whole 1st clad glass part 21b because the core diameter 2A decreases in a taper shape, and then propagates in the single-mode waveguide consists of the 1st clad glass part 21b as the core part and the 2nd clad glass part 21c as the clad part; the ratio of the core diameter decreases much more at a heat-sealed and drawn part than at the fiber end, so the low-loss fiber type directional coupler is constituted without any etching process.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Summary) The present invention relates to a low-loss fiber type directional coupler. Two (prior art) After heating and fusing parts of two single mode optical fibers,
A fiber-type directional coupler constructed by stretching a fused portion is an important optical component necessary for constructing optical circuits for optical communications and optical fiber sensors.゛An example of its structure is shown in Fig. 1 (at).Two single mode optical fibers 1-1a and 2
A part of -2a is fused and stretched. Figure 1 (bl
1A is a sectional view taken along the line AA' in FIG.

For example, light 1 incident on the fiber end IK shown in Figure 1 (al)
゛ is the fusion/stretching part 8, and the optical fiber 2-2allJK
The fibers are combined, split into fiber ends 1a and 2a, and emitted.

Conventionally, in the construction of this type of directional coupler, a single mode optical fiber for long-distance transmission has been used as is.

FIG. 1(c) is a refractive index distribution diagram of the single mode optical fiber, and 5 indicates the refractive index distribution shape. It consists of a core glass part 4a with a high refractive index of two diameters, and a clad glass part 4b around the core glass part 4b having an outer diameter.

By the way, in the fusing/stretching section 8 of the directional coupler, light is coupled and split, but at the same time, some of the light is lost as scattered light, causing so-called excessive loss. The magnitude of this excess loss determines the performance of the fiber type directional coupler. The excess loss depends on the ratio of the outer diameter to the core diameter of the single mode optical fiber used, that is, the size of 5D//2A,
It is generally known that the smaller //2A, the smaller the excess loss.

Since the core diameter 2A cannot be increased unnecessarily because it is necessary to satisfy the single mode condition, efforts are being made to reduce D. 10 Usually, a method is used to reduce the outer diameter of the fiber by etching prior to fusion splicing, and a directional coupler with an excess loss of about 0.5 dB is fabricated.

However, there was a problem in that careful attention was required when handling fibers made thinner by etching.

In addition, in order to achieve an excess loss of about 0.5 dB or less with good reproducibility, a high degree of skill is required in the fusing and stretching operations, which has been a problem in terms of production efficiency.

(Purpose of the invention) The present invention is a method for manufacturing a conventional fiber type directional coupler.

/R) In order to solve the above-mentioned problem, a directional coupler is constructed from a single mode optical fiber with a structure different from that for normal optical transmission, and its purpose is to achieve extremely low loss fiber-type directional coupling. The object of the present invention is to provide a polarization-maintaining fiber type directional coupler with good reproducibility. The present invention will be explained in detail below with reference to the drawings.

(Structure and operation of the invention) Fig. 2 (al is a block diagram of an embodiment of the present invention;
The single mode optical fibers 1-1'a and B-2a have a first clamp 1-1'a surrounding the central core glass part 21a.
It consists of a first cladding glass section 21b and a second cladding glass section 21c. FIG. 2(b) is a cross-sectional view taken along line BB' in FIG. 2fa), FIG. This is a refractive index distribution diagram of a single-mode optical fiber, where the refractive index value is highest at the central core glass part (diameter 2 mm), 1"5y h
"lj5XW5(Me4""ゝ・@2 p 5 y
.

The refractive index is lower than that of the glass portion (diameter D).

The refractive index difference between the core and the first cladding is Δn1, and the refractive index difference between the first cladding and the second cladding is ΔnB. 1°2 people and Δn are set so as to satisfy the single mode condition at the wavelength used. For example, the core glass part 2 of the fiber end 1
1. The core diameter 8A decreases in a 5-tapered shape as the light incident on a passes through the melting, adhesion, and stretching section 8, so that it gradually changes to the first. The cladding glass spreads over the entire glass portion 521b, and eventually propagates through a single mode conductor with the first cladding glass portion as the core portion and the second cladding glass portion 21c as the cladding portion. At fiber end 1, the ratio of the outer diameter to the core diameter is 2A, but due to fusion and
, the ratio of the outer diameter to the core diameter is substantially D/
2B, and a low-loss fiber-type directional coupler can be constructed without the need for an etching process. The light split at the fusion/stretching section 8 is transmitted to the fiber end la,
2a, the tapered structure returns, and the collimated light again proceeds with the core glass portion 81a having the highest refraction value as the core portion, and is emitted from the fibers la and Bh.

The structure of the single mode optical fiber used in the present invention is l
The first clad glass part 21b is used as the core part for optical transmission over a long distance of -2° (%).

Although this is undesirable as part of the multimode light is excited, this is not a problem with a length of several meters required for the construction of a fiber-type directional coupler. Further, if necessary, the conventional single mode optical fiber shown in FIG. 1 can be connected to the fiber ends 1.la, 1.g, 2a.

Next, a specific example of production will be described. A single mode optical fiber with the following structure was used. .

Approximately 2. 11 of two Joki single-mode optical fibers of length tx
After removing the plastic covering near the center, the removed parts were brought into contact in parallel and heated locally with a mini-torch made of oxygen-propane flame to fuse approximately 2u length.

I let it happen. Subsequently, the fused portion was stretched at the same time as heating to form a fused/stretched portion, and a fiber-type directional coupler 2 was formed. During the above process, the wavelength is 1.81 μm from the 7-eyeper end l.
The monitor light is input to the core glass part 21a, the intensity of the light emitted from the core glass part of the fiber end 1a lBa is monitored, and the optical coupling ratio is set to a desired value (usually 50%, that is, 1:
The degree of stretching 5 was adjusted so that the film was divided into equal parts of 1). At the same time, excessive loss was determined from changes in the total emitted light intensity. As a result, a directional coupler with a low excess loss of 0.5 dB or less was easily obtained, and it was not difficult to realize an extremely low loss value of 0.2 dB or less. Some contain 0.0 cl
B and excess loss of 10 were obtained.

For comparative reference, when a conventional directional coupler with the structure shown in Figure 1 was fabricated using an optical fiber with the following specifications, the excess loss was as high as l -2 dB, and was 0.5 In order to reduce the reduction to about dB, the outer diameter of the fiber must be HF
It was necessary to reduce the diameter to about 60 μm by using an aqueous solution. In order to accurately arrange and fuse optical fibers with such narrow outer diameter portions, great care was required, and the work vehicle was slowed down.

The above configuration can be extended to a 95 polarization maintaining fiber type directional coupler using a birefringent single mode optical fiber that stably maintains linearly polarized waves along the principal axis.

FIG. 8(a) shows an example of the structure of a birefringent optical fiber that can be used for constructing a low-loss polarization-maintaining liner type directional coupler, and FIG. 8(b) shows the refraction 1 [・This shows the rate distribution form. FIG. 8(a) is a structural example corresponding to a so-called elliptical clad birefringent optical fiber, in which 81a is a core glass portion, 81b is an elliptical first clad glass portion, and 81C is a second clad glass portion. . Bl
Glass b has a glass composition with a large thermal expansion coefficient lj number, and causes stress-induced refraction in the vicinity of the core glass portion 81a. 8th
As shown in refraction 1 wealth distribution shape 5 in figure (b), the first
The cladding part has a larger refractive index value than the second cladding part, and serves as a core part relatively to the second cladding part! ...(7) Work. A specific structural example is as follows.

This optical fiber was manufactured by the MOVD method. 10 Next, FIG. 8(c) shows that the core glass part 81a has stress applying parts 82a on both sides, and the core glass part 81a has a first cladding glass part 82b and a second cladding glass part 82.
It is surrounded by c. A specific structural example is shown below.

(8) Relative refractive index difference between stress applying part and second cladding part = -0,1
%The refractive index distribution shape of the optical fiber in FIG. 8(c) is
It is shown in Figure (a).

This optical fiber is produced by synthesizing a glass base material consisting of a core glass part and a first cladding glass part by the VAD method.
A second clad glass part is further deposited around it using the OVPO method, and a hole is drilled at the stress applying part position by ultrasonic machining.
Stress-applying glass synthesized by VAD method in the hole part
It was created by inserting a line and drawing the whole thing.

In these polarization-maintaining birefringent optical fibers, prior to the fusion and stretching process necessary for configuring a directional coupler, the direction 8B of the birefringent principal axes of the two fibers is adjusted to 1゜. a)
# (By arranging them in parallel like bl, it is possible to construct a polarization-maintaining fiber-type directional coupler in which the linear polarization state along the main axis is preserved even in the fused/stretched part. Note that 41 and 42 are birefringent optical fibers. !...Figures 4(c) and 4(d) show the polarization-maintaining fiber shape orientation of Figures 4(a) and (b), respectively. A cross section of the fused/stretched part of the coupler is shown.

From the birefringent optical fiber shown in the above structural example, a directional coupler with an excess loss of about 0.8 dB or less can be obtained with good reproducibility, and the extinction ratio, which indicates the quality of linear polarization preservation, is -15 to -15.
It was good at -25 dB. It goes without saying that conventional birefringent optical fibers for optical transmission can be connected to the input and output fiber ends of these directional couplers.

(Effects of the Invention) As explained above, the fiber type directional coupler of the present invention uses a single mode optical fiber having a second clad glass portion with a low refractive index around a first clad glass portion. With 15 components of the optical coupler, extremely low-loss fiber-type directional couplers and polarization-maintaining fiber-type directional couplers can be manufactured with good reproducibility. It is highly effective when applied to the field of fiber sensors. 10 In the above embodiments, a so-called (2XB) type directional coupler using two single mode optical fibers was explained, but the present invention uses an [8X8] type directional coupler using eight fibers.
It is also effective in configuring a shaped directional coupler.

[Brief explanation of the drawing]

Figure 1 (al is a configuration diagram of a conventional fiber-type directional coupler, Figure 1 (bl is a cross-sectional view taken along A-m' in Figure 1 (a), Figure 1 (c) is a diagram for long-distance transmission. Fig. 2(a) is a refractive index distribution diagram of a single mode optical fiber of the present invention.
Side view of the configuration of a bar-shaped directional coupler, Figure 2 (bl is Figure 2 (
2 (cl is a refractive index distribution diagram of the single mode optical fiber of FIG. 2 (a), and FIGS. 8 (a) and (c) are the fiber shapes of the present invention. Structure side views of the birefringent optical fiber used in the configuration of the directional coupler, FIG. 8(b) and (a) are respectively shown in FIG. 8(al,
(Figure showing the refractive index distribution map of cl, Figure 4(a) *(
b) is a side view of the birefringent principal axis arrangement required for the configuration of a polarization-maintaining directional coupler (Fig. 4 (C)). It is a top view of the fusion/stretching part of the directional coupler. 2.
(11) 1-16.2-Fa...single mode optical fiber, 1
B... Fusion/stretching part, 4a... Core glass part. 4b...Clad glass part, 4...Fusion/stretching part cross section, G...Single mode optical fiber refractive index distribution type, 21a
...Core glass part, jllllb...First clad glass 5 lath part, 21c...Second clad glass part, 81a
... Core glass part, 81b... First clad glass part, 81c... Second clad glass part, 82a...
Stress applying portion, 82b...first clad glass portion, 82
c... Second clad glass portion, 8B... Main 1G axis direction, 41.42... Birefringent optical fiber. (1z) E K de Otsu 74--

Claims (1)

[Scope of Claims] 1. In a fiber-type directional coupler 5 made up of a plurality of single-mode optical fibers fused and stretched in two parts, the single-mode optical fiber has a core glass part and a second part surrounding the core glass part. I is characterized by comprising a first clad glass part and a second clad glass part surrounding the first clad glass part and having a refractive index lower than that of the first clad glass part. A fiber type directional coupler. 2. Claim No. 2, characterized in that the single mode optical fiber is a birefringent optical fiber, which is fused and stretched so that its principal axis is parallel to each other at the fused and stretched portion. The fiber type directional coupler described in 11iii.1'.