JPS60260906A - Method for connecting polarization plane preserving optical fiber - Google Patents

Abstract

PURPOSE:To easily connect polarization plane preserving optical fibers with each other in the open air with high accuracy, by making a circularly polarized wave incident from one end of one of the optical fiber and determining the direction, in which the maximum or minimum refractive index is obtained, from the scattered light radiated from the side of the optical fiber, and then, connecting the two optical fibers by making the direction of each fiber coincident with each other. CONSTITUTION:A clockwise circularly polarized wave 2 is made incident on a polarization plane preserving optical fiber 1 and intensity of scattered light radiated from the side of the optical fiber 1 in the directions normal to the optical axis is measured. When, for example, the intensity of the scattered light radiated from the direction C at the position at a distance L from the incident point is zero, the polarized wave in the fiber 1 is a linearly polarized wave and the direction of the wave surface is C. The direction of the maximum refractive index of the fiber 1 becomes the one which is turned counterclockwise by pi/4 radian from the direction C around the optical axis, when the incident point is viewed from the advancing direction of the light. Two fibers 1 and 1' are put together, with the direction of each fiber being made coincident with each other, and fixed to a table 3 with a keeping jig 4, and then, the end faces of both fibers 1 and 1' are heated and connected with each other by a discharge electrode 9.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for connecting polarization-maintaining optical fibers used in optical communications, optical sensors, and the like.

(Prior art) In recent years, in the field of optical communications, polarization-maintaining optical fibers, which transmit light and sound while preserving the polarization plane, have attracted attention as a promising optical transmission path for use in fiber gyros, optical sensors, etc. ing.

In such a polarization-maintaining optical fiber, the fundamental mode of the propagating light is degenerated because the propagation constants of the two propagating lights having polarization planes in two mutually orthogonal directions in the cross section of the core perpendicular to the optical axis are different. is solved, and the polarization plane becomes conserved. To make the propagation constants of light in two orthogonal directions different within the cross section of the core.

If the core has an entirely elliptical cross section, internal stress may be applied to the core so that the refractive index in two orthogonal directions within the cross section of the core differs.

Polarization-maintaining optical fibers were initially used mainly for fiber gyros and optical sensors as mentioned above, but recently polarization-maintaining optical fibers with good polarization preservation properties and low loss have been developed. However, applications to long-distance optical heterodyne communications are now being considered. Long polarization-maintaining optical fibers used for such long-distance communications have an anisotropic refractive index in two orthogonal directions in the cross section of the core perpendicular to the optical axis, and the direction of the maximum refractive index or the minimum This is obtained by connecting opposing polarization-maintaining optical fibers so that the directions of their refractive indices match. The splicing accuracy is expressed by the angular deviation in the direction of the maximum refractive index (hereinafter referred to as the principal axis) of the two polarization-maintaining optical fibers, but the splicing accuracy of the polarization-maintaining optical fibers is
It is said that it is necessary to keep the above-mentioned angular deviation within 2 degrees.

Conventionally, the method of connecting polarization-maintaining optical fibers has been to use an elliptical core at the end face of the polarization-maintaining optical fiber to be connected, or a lens on a stress-applying layer that applies anisotropic strain to the core. There is a method of enlarging the ellipse, aligning the major axis of the ellipse, and then fusion splicing (hereinafter referred to as the enlarging method), or inputting a linearly polarized wave in the optical axis direction of the first polarization-maintaining optical fiber.

While rotating in the same direction as the optical axis of the optical fiber, the output light is also adjusted to be linearly polarized, and the output light of the first polarization-maintaining optical fiber is aligned with the optical axis of the second polarization-maintaining optical fiber. The second polarization-maintaining optical fiber is rotated around the optical axis, and the output light of the second polarization-maintaining optical fiber is adjusted so that it also becomes linearly polarized. A method of attaching and connecting (hereinafter referred to as the spindle adjustment method) has been adopted. 1:1' However, in the conventional enlarging method, the operation is simple because the only thing that is required is to enlarge the optical fiber end surface using an ftL lens or the like to determine the major axis direction of the ellipse.

The connection accuracy is low, and as mentioned above, it is difficult to keep the misalignment of the principal axes of the two polarization-maintaining optical fibers within 2 degrees.

On the other hand, in the main axis adjustment method, as described above, the output light of the second polarization-maintaining optical fiber is adjusted so that it becomes @unpolarized light, so the connection accuracy I was high, but the operation became complicated,
The device may become complicated. Especially when connecting outdoors, such as through a manhole, the input end and output end of the optical fiber are usually far apart, so it is necessary to rotate the input end of the optical fiber as in the main axis adjustment method. However, it was extremely difficult to adjust the emitted light so that it was linearly polarized.

(Object of the Invention) An object of the present invention is to provide a method for connecting polarization-maintaining optical fibers that eliminates the above-mentioned drawbacks, has high connection accuracy, and can be easily operated outdoors.

(Structure of the invention) A method for connecting polarization-maintaining optical fibers according to the present invention is as follows.

A financial connection of two polarization-maintaining optical fibers having a circular outer diameter with the same center as the core glass layer, with an anisotropic strain added to the core glass layer and having the same center as the core. In the method for connecting polarization-maintaining optical fibers, the steps include: inputting circularly polarized waves from end faces of the two polarization-maintaining optical fibers; and observing scattered light intensity from a side surface of the polarization-maintaining optical fibers; observing the direction in which the intensity of the scattered light becomes maximum or zero; and the step of observing the direction in which the intensity of the scattered light becomes maximum or the direction in which the intensity of the scattered light becomes zero within the core cross section of the polarization maintaining optical fiber. A step of determining all the directions in which the refractive index for @-line polarized waves in the direction perpendicular to the optical axis of the optical fiber is maximum or minimum, and the direction of the maximum or minimum refractive index of the two polarization-maintaining optical fibers is matched. and then heating and fusion splicing the two polarization-maintaining optical fibers while reducing the distance between their end faces.

(Principle of the Invention 1 Effect) In the method for connecting polarization maintaining optical fibers according to the present invention,
In particular, circularly polarized light enters from one end of a polarization-maintaining optical fiber, and the scattered light emitted from the side surface of the polarization-maintaining optical fiber determines the direction of the maximum refractive index or the minimum refractive index of the polarization-maintaining optical fiber. We will explain how to make all decisions. Figure 1(a).

(b) is a principle diagram illustrating the entire method.

In FIG. 1(a), 1 is a polarization-maintaining optical fiber,
It is assumed that 2 is a circularly polarized wave incident on a polarization-maintaining optical fiber, and that this circularly polarized wave 1 is a right-handed circularly polarized wave.

The traveling direction of light is the fz axis, and the direction of the maximum refractive index within the core cross section of the polarization maintaining optical fiber 1 is the fx axis.

It is assumed that the direction perpendicular to the direction of the maximum refractive index (the direction of the minimum refractive index) is the kY axis, and the XyZ system forms a right-handed system as shown in FIG. 1(a). At this time, the incident right-handed circularly polarized wave 2 is
It propagates in a polarization-maintaining optical fiber while periodically changing its polarization state. Generally, the distance WI and the total beat distance LB until the polarization state is the same as that of the incident point is the beat length. Also, at a distance of J -LB from the incident point, the left-handed circularly polarized wave has -L
At distance B.

From the positive direction of the X-axis, (as seen from the positive direction of the z-axis) the clock has a linearly polarized wave, and at a distance of -L, from the positive direction of the It becomes a linearly polarized wave with a polarization plane in the direction of radians (45 degrees). At a position where the distance from the point of incidence of the circularly polarized wave is other than the above-mentioned value, the wave becomes elliptically polarized.

Generally, when light passes through a non-vacuum optical medium, scattered light is emitted in a direction perpendicular to the traveling direction of the light, which is well known as the Tyndall effect. It is maximum in the direction perpendicular to the vector, and zero in the direction of the electric field vector. Applying this phenomenon to Figure 1(a), at a position -L from the point of incidence of one circularly polarized wave, A
The scattered light intensity in the direction of (1π radian (45) counterclockwise from the positive direction of the X axis as seen from the direction of light B traveling, direction perpendicular to the plane of polarization) is assumed to be maximum, and the direction of B is (1π radian (45 degrees) clockwise from the positive direction of the X-axis when viewed from the direction of light travel)
In the direction of , the intensity of scattered light in the polarization plane direction becomes zero. Next, at position i of -L from the point of incidence of the 1 circularly polarized wave, the scattered light intensity in the incident direction is zero, and the scattered light intensity in the B direction becomes maximum. Furthermore, at the position -LL from the point of incidence of the circularly polarized wave, the polarization state in the optical fibers 2B and B becomes circularly polarized, so the intensity of the scattered light can be seen from any direction as long as it is perpendicular to the z-axis. Even though.

It becomes - of the maximum strength mentioned above. Furthermore, at a position where the polarization state is elliptically polarized, the scattered light intensity takes a value that is neither zero nor the maximum value.

Up until now, the incident light has been assumed to be a right-handed circularly polarized wave, but if the incident light beam is a left-handed circularly polarized wave, at a position 1L from the incident point of 1 circularly polarized wave, it will move in the direction shown by A in Figure 1 Cal. B It becomes a linearly polarized wave with a polarization plane, and the scattered light in the incoming direction is zero,
-I from the incident point of the 91 circularly polarized wave where the scattered light in the B direction is maximum
At position L, the light is right-handed circularly polarized, and the intensity of the scattered light B is the same even when viewed from a direction perpendicular to the optical axis.

-L from the incident point of the -91 circularly polarized wave of the maximum strong reaction
At the position, the wave becomes a linearly polarized wave with a polarized B wavefront in the direction indicated by B in Figure 1 ta+, and the scattered light in the direction of B becomes zero.
Scattered light in the direction of A is maximum.

At the position beyond LB from the incident point of the circularly polarized wave, as mentioned above,
The phenomenon that occurs between the point of incidence of the circularly polarized wave and the point of incidence repeats throughout the period.

Next, it will be explained using FIG. 1 (bl) that the direction of the maximum refractive index of the polarization-maintaining optical fiber is determined by utilizing the above-mentioned phenomenon. As shown in FIG. 1 (b).

Assume that a right-handed circularly polarized wave 2 is incident on the polarization-maintaining optical fiber 1. The intensity of scattered light emitted from the polarization-maintaining optical fiber 1 in a direction perpendicular to its optical axis is measured, and as shown in FIG. Assume that the intensity of scattered light emitted from the direction C is zero. At this time, the polarization state inside the polarization-maintaining optical fiber 1 is linear polarization, and the direction of the polarization plane is the C direction. From the above-mentioned phenomenon, the direction of the maximum refractive index of the polarization-maintaining optical fiber 1 is the X-axis.

When looking at the direction of the incident point from the direction in which the light travels, it is a direction rotated by 7 degrees (45 degrees) counterclockwise from the direction C around the optical axis.

In this way, the direction of the maximum refractive index of the polarization maintaining optical fiber can be determined.

(Example) An embodiment of the present invention will be described below with reference to the drawings.

FIGS. 2(a) to 2(f) are diagrams illustrating an embodiment of the present invention, in which 1.1' are 11th and 2nd polarization-maintaining optical fibers, respectively. The central axes of both fibers are on the same straight line, and the outer diameters of both fibers are 125 μm. 3 is a stand on which the polarization-maintaining optical fiber 1.1"i is placed, and by holding the polarization-maintaining optical fiber 1,1'i with the holding jig 4,
It is possible to fix the polarization maintaining optical fiber 1.1' to one unit 3.

Further, the polarization-maintaining optical fibers 1 and 1' can be rotated about the optical axis, and can be moved in the direction of the central axis of the polarization-maintaining optical fiber while being fixed to the table 3. 5 is a light source that emits a He-N linearly polarized wave, a circularly polarized wave obtained by combining a laser and a quarter-wave plate, and the 1 circularly polarized light is emitted from the end face of the polarization-maintaining optical fibers 1 and 1'. The light is incident in the optical axis direction. Reference numeral 6 denotes a photodetector that receives scattered light emitted from the side surface of the polarization-maintaining optical fiber II 1' in a direction perpendicular to the optical axis. It is rotatable and movable in the optical axis direction. 7 is an arrow indicating the direction of the polarization plane of linearly polarized waves, 8 is a broken line indicating the direction of the maximum refractive index in the core cross section perpendicular to the optical axis of polarization maintaining optical fiber 1 or 1', and 9 is for fusion splicing. This is a discharge electrode.

First, as shown in FIG. 2(a), polarization preserving light 7 eyeglass 1
2 ft of right-handed circularly polarized waves are incident on the polarization-maintaining optical fiber 1, and the photodetector 6 is rotated in the circular direction of the optical axis of the polarization-maintaining optical fiber 1. The position (based on the light incident point) and direction (based on the horizontal direction) where the scattered light from the side surface first becomes 0 was determined.

According to the above explanation of the principle, at this position of the photodetector 6, the polarization state of one circular polarization-maintaining optical fiber is linearly polarized, and the direction of the polarization plane is 211 (a). ) (D“Ty
s< -jtM K・31"6(D7jUh'l-z"
C+, .

There is. Again, according to the above explanation of the principle, the direction of the maximum refractive index of the polarization-maintaining optical fiber 1 is the direction 7 of the polarization plane when viewed from the direction of light propagation, as shown by 8 in FIG. 2(a). This is the direction rotated counterclockwise by -radians (45 degrees) around the optical axis.

Next, this polarization maintaining optical fiber and photodetector 6 are
While confirming that the intensity of scattered light from the side of the polarization-maintaining optical fiber is zero, rotate around the optical axis (Fig. 2 18).
Rotate it in the direction of the arrow 10 as shown in FIG.
Fixed to stand 3.

Next, the light source 5 is turned in the opposite direction, and the circularly polarized wave is incident on the polarization-maintaining optical fiber 1' as shown in Fig. 2 fc). The position where the scattered light from the side first becomes zero (based on the point of incidence of the light). direction (based on the horizontal direction). At this time, at the position of the photodetector 6 shown in Fig. 2 (fc), the polarization state of the polarization-maintaining optical fiber l' is not linearly polarized, and the direction of the polarization plane is at the position shown in Fig. 2 (cl). The direction of the photodetector 6 is as shown by 7. Also, the direction of the maximum refractive index of the polarization-maintaining optical fiber 1' is determined from the direction of propagation of the light, as shown by 8 in FIG. 2(C). Therefore, it is a direction rotated counterclockwise from the direction 7 of the polarization plane by nirad (451i) around the rotation of the optical axis.

Next, the polarization-maintaining optical fiber 1' and the photodetector 6 are connected to the second polarization-maintaining optical fiber 1' around the optical axis while making sure that the intensity of scattered light from the side surface of the polarization-maintaining optical fiber 1' is zero. Figure fc)
Rotate in the direction of the arrow 10 as shown in FIG. It was fixed to the stand 3 using the

Next, a polarization-maintaining optical fiber 1.1”t-, Fig. 2 (e
), move parallel to the optical axis in the direction shown by the arrow 11, reduce the distance between the end faces of the polarization preserving optical fiber 1', and heat the end faces of the polarization preserving optical fiber 1' with the discharge electrode 9, as shown in Fig. 2. ff). 121C in Fig. 2(e) is determined by the arc during discharge. In addition, after connection.

The directions 8 of the maximum refractive index of the polarization-maintaining optical fiber 1.1' coincided as shown in Fig. 2 (fl).
When a linearly polarized wave with a plane of polarization in the maximum refractive index direction was input to a polarization-maintaining optical fiber, and the amount of extinction ratio deterioration due to the connection was measured by the output light of the polarization-maintaining optical fiber 1', it was found to be about 3 dB. Therefore, the angular deviation in the direction of the maximum refractive index of the two polarization-maintaining optical fibers 1°1' could be kept within 2 degrees.

In this embodiment, the incident circularly polarized wave is right-handedly polarized wave.

Left-handed polarization is also acceptable. Also, in one embodiment.

Although the position and direction where the intensity of scattered light from the side surface of the polarization-maintaining optical fiber becomes zero is determined, the position and direction where it is maximum may be determined. In addition, in this embodiment, He is used as a light source.
-Ne laser was used.

The type of light source is not limited; for example, an infrared light source may be used as long as all appropriate photodetectors are used. Further, in this embodiment, the outer diameter of the polarization-maintaining optical fiber is 125 μm, but a polarization-maintaining optical fiber having an outer diameter other than this may be used. In addition, in this example, the end face of the polarization maintaining optical fiber was fused by arc discharge, but other methods 1, for example,
A heating method using a C02 laser may also be used. Also 1
In this embodiment, the direction of the maximum refractive index of the polarization maintaining optical fiber is determined, but a method of determining the direction of the minimum refractive index orthogonal thereto may also be used. Further, the shape of the glass layer that causes anisotropic strain t 7JO in the core of the polarization maintaining optical fiber is also arbitrary.

(Effect of the invention) According to the present invention, as described above in detail.

With the above configuration, connection accuracy is high, and the direction of the maximum refractive index or minimum refractive index of the polarization-maintaining optical fiber can be set at the connection end, making it possible to connect outdoors. A method for connecting polarization-maintaining optical fibers can be obtained, which has effects such as easy connection even if the output end is far away.

[Brief explanation of drawings]

Figures 1 (a) and (b) are principle diagrams explaining the method for determining the direction of the maximum refractive index of the polarization-maintaining optical fiber used in the present invention, and Figures 2 (al to ge) are diagrams showing one aspect of the present invention. It is a figure explaining an example. 1.1'...Polarization maintaining optical fiber, 2...
...Fuse circularly polarized wave, 3...stand, 4...pressing jig, 5...light source that emits circularly polarized wave, 6...
・Photodetector, 7...Arrow indicating polarization plane direction, 8.
・・・・・・Direction of maximum refractive index of polarization maintaining optical fiber,
9...Discharge electrode, 10...Arrow indicating the rotation direction of the polarized optical fiber, direction 11. (0,) θ Figure 7 Figure 2 Procedural amendment (voluntary) 2. Title of invention Connection method for polarization maintaining optical fiber 3.
Relationship with the case of the person making the amendment Applicant: 33-1 yi, Shiba 5-chome 1, Minato-ku, Tokyo (423) NEC Corporation Representative: Tadahiro Sekimoto 4, Agent: 37 Shiba 5-chome, Minato-ku, Tokyo 108 No. 8 Sumitomo 311
1 Building NEC Co., Ltd. 5, Subject of amendment (1) "Claims" column of the specification (2) "Detailed description of the invention" column (3) "Drawings" column of the specification "Brief Description" column (4) Drawing 6, contents of amendment (1) Claims column should be amended as shown in the attached sheet. (2) On page 3, line 19 of the specification, "the angular deviation in .)" is corrected to ``the angular deviation in the direction of the minimum refractive index.'' (3) On page 6 of the specification, lines 17 to 18, [Fusion splicing by heating while reducing the distance between the end faces] is replaced by ‘bringing the end faces into contact and applying pressure by pushing the other fiber with the end faces. Correct the fusion splice by heating while adding ``K''. (4) On page 8, line 9 of the specification, "in a non-vacuum light medium" is corrected to "in an optical fiber." (5) “Tyndall effect” on page 8, line 11 of the specification [
Tyndall scattering effect. (6) Page 10 of the specification, line 9 “Measure scattered light intensity”
is corrected to "measure the scattered light intensity while rotating the photodetector around the optical axis of the polarization-maintaining optical fiber 1 or moving it in the optical axis direction." (7) Page 10 of the specification, lines 11 to 12 [C
Suppose that the intensity of scattered light emitted from the direction is zero. ”
1. Assume that the above-mentioned scattered light intensity becomes zero for the first time, and that the direction in which the scattered light intensity becomes zero at that time is direction C. ”. (8) On page 11 of the specification, line 8, change “desu. 3 wa” to [
It is. Also, the end faces of both fibers are mirror-finished. 3
is corrected to ``. (9) On page 13 of the specification, from line 16 to line 17, "What is the polarization state of fiber 1'?" is corrected to "What is the polarization state of fiber 1'?" 00) Page 14 of the specification, lines 13 to 14 "Discharging electrode 9 while reducing the distance between the end faces" [The end faces are brought into contact with each other, and the end faces are pressed against each other to discharge while applying pressure. Correct to "electrode 9". 01) On page 14 of the specification, from the 14th line to the 15th line, "Heating of the end surface" is corrected to "Heating of the end surface and the vicinity of the end surface." (12) Page 16 of the specification, line 15: “Figure 1 is (a
). (b)” should be corrected to “Figure 1 (a) and (b) are”. (J3) On page 17 of the specification, line 5, "polarization plane optical fiber" is corrected to "polarization plane maintaining optical fiber" K. 04) Correct Figure 2(e) as shown in the attached drawing. 2 Attachment of attached documents (corrected scope of patent claims) 1 Overexcited paper drawing (second
Figure (e)) One copy corrected His patent claims: ``A glass layer that is anisotropically strained to a core glass layer, and includes a glass layer that has a gold co-centered with the core, and has a circular shape that is co-centered with the core glass layer. In a method for splicing polarization-maintaining optical fibers by fusion splicing end faces of two polarization-maintaining optical fibers having an outer diameter, the step of injecting circularly polarized waves from the end faces of the two polarization-maintaining optical fibers; a step of observing the scattered light intensity from a side surface of the polarization-maintaining optical fiber; a step of observing the direction in which the scattered light intensity becomes maximum or zero; and a step of observing the direction in which the scattered light intensity becomes maximum or zero. determining a direction in which the refractive index for linearly polarized waves in a direction perpendicular to the optical axis of the polarization-maintaining optical fiber is maximum or minimum within the core cross section of the polarization-maintaining optical fiber;
A step of aligning the directions of the maximum or minimum refractive index of the two polarization-maintaining optical fibers, and then applying pressure by pressing the end faces of the two polarization-maintaining optical fibers against each other. While adding
A method for splicing polarization maintaining optical fibers, the method comprising the step of fusion splicing. (C) No. 2 ml Procedural amendment (voluntary) 60.6.27 Showa year, month, day 1, case description 1982 patent application No. 117723 2, title of invention Connection method of polarization-maintaining optical fiber 3 ,
Person making the amendment Relationship to the case Applicant: 5-33-1-4 Shiba, Minato-ku, Tokyo, Agent: 5-37-8 Shiba, Minato-ku, Tokyo, 108 Resident, Mita Building 5, Subject of amendment (1) “Detailed description of the invention” column (2) of the specification F!
Column ``Brief explanation of drawings'' in A specification (3) Drawing 6, contents of amendment (1) 'J specification, page 7, line 6, ``Company (b)'' is r
b) and (C) shall be amended to ``. (2) Page 10, line 6 of the specification [Explain using (b). 1(b)" will be explained using FIG. 1(b) and (C). First, in FIG. 1(b), it is corrected to ``. (3) Insert the following sentence after page 10, line 18 of the specification. On the other hand, when the incident light is left-handed circularly polarized light, it is output from the polarization-preserving optical fiber 1 in a direction perpendicular to its optical axis using the same method as when right-handed circularly polarized light is input. Measure the scattered light intensity and calculate IA I @ (c) K 7J''
1″・Fl(#[17), &M,!5-bL ,,
Assume that the scattered light intensity decreases L to zero at positions , and, and that the direction in which the scattered light intensity becomes 0 at that time is the direction C. In this case, as is clear from the above description, the direction xs of the maximum refractive index of the polarization maintaining optical fiber 1 is
ti, when looking from the direction of light propagation to the direction of incidence, π/4 radians (4
5 degrees) rotated direction. Note that whether the incident light is right-handed circularly polarized or left-handed circularly polarized can be easily determined by using linearly polarized light and a quarter-wave plate. ” (4) Page 16, line 15 of the specification “Figure 1 is (a),
(b) Used for...'' in Figure 1 (a),
(b), (C)II'i amended to "Used in the present invention". (5) Add attached drawings (Figure 1 (C)). 7. Attached documents attached drawings (Figure 1 (C)) 1 copy (C) Iwa 1 Figure

Claims (1)

[Claims] The end faces of two polarization-maintaining optical fibers are made by adding anisotropically strained metal to the glass layer serving as the core, and including a glass layer having the same center as the core, and having a circular outer diameter having the same center as the glass layer serving as the core. A method for connecting polarization-maintaining optical fibers, which is becoming more and more evolving, includes a step of injecting circularly polarized waves from the end faces of the two polarization-maintaining optical fibers, and a step of inputting the scattered light intensity from the side surfaces of the polarization-maintaining optical fibers. a step of observing the direction in which the scattered light intensity increases or becomes zero; and a step of observing the core cross section of the polarization-maintaining optical fiber from the direction in which the scattered light intensity becomes maximum or zero. determining a direction in which the refractive index for linearly polarized waves in a direction perpendicular to the optical axis of the polarization-maintaining optical fiber is maximized or minimized; and the refractive index of the two polarization-maintaining optical fibers. A polarization-preserving optical fiber characterized by comprising the steps of aligning the maximum or minimum directions of the two polarization-preserving optical fibers, and then heating and fusion splicing the two polarization-preserving optical fibers while reducing the distance between the end faces of the two polarization-preserving optical fibers. How to connect fibers.