JPS6083906A - Fiber type optical coupling element and its production - Google Patents

Abstract

PURPOSE:To maintain satisfactorily the quenching ratio of linear polarization and to decrease excessive loss by matching the refractive index of the stress- applying part of an optical fiber to the refractive index of the clad part thereof. CONSTITUTION:Optical fibers 1-1a, 2-2a for maintaining linear polarization of which the main axis directions are adjusted to a desired arrangement are fixed to supporting bases 41, 42 and are partly heated and welded thereto into one body. The welded part 43 is heated and at the same time the base 42 is smoothly moved toward an arrow 44 to stretch the welded part to a tapered shape, thereby forming a welded and stretched part 3. The two fibers are held in place between glass plates 51 and 52 and are dipped in a matching liquid 53 having the refractive index approximate to the refractive index of the clad part if the fiber 2-2a is already adjusted in the main axis direction and 1-1a is before adjustment. The light of an illuminating light source 54 is polarized by a polarizing plate 55a so as to cross the fiber and is thereafter passed through a polarizing plate 55b. The light is observed by a microscope 56 to detect the position of the stress-applying part and the fiber is rotated and is matched to the required position.

Description

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

With the advancement of optical 7-IPA manufacturing technology, single-mode optical fibers that stably preserve linearly polarized waves over long distances along their main axis have been developed, and are called linear polarization-maintaining optical fibers, which are used in optical communications and optical fibers. It is expected that this will lead to new advances in the field of fiber sensors. When using a linear polarization-maintaining optical fiber, optical circuit components connected to the fiber are also required to have linear polarization-maintaining property. Among these, the fiber type optical coupler is an important optical circuit component, and the structure shown in FIG. 1 has heretofore been proposed. In FIG. 7, two linear polarization-maintaining optical fibers 1-1a, 2-28 are partially fused and stretched. Linear polarization maintaining optical fiber is
The cladding part 4bK surrounding the core part 4a has a stress applying part 5, and the fibers are arranged in the cross section 7 of the fusion/stretching part 3 so that the fiber main axes 6a and 6b determined by the stress applying part 5 are aligned parallel to each other. ing. The linearly polarized light 8 incident on the fiber 1 is transmitted along the fiber main axis, is split into the other optical fiber at the fusion/stretching section, and is emitted as linearly polarized light 9.10 from the seven eyepers 1a and 2a, respectively.

Even in the fusing/stretching section 3, the arrangement structure of a single fiber in which the linear polarization state is not destroyed includes the example shown in Fig. 1, and the fusing operation of a single fiber as shown in Fig. 2
It is known that there are three structures obtained by 1 (References M, Iya Kawachi: Electron, L
ett, /g(15'&,2)'i'Otsu2).

Such fiber arrangement operations are performed by observing the stress applying portion positions under a microscope.

The above-mentioned fiber-type optical coupler does maintain some linearly polarized waves well along the main axes of the input and output fibers with an extinction ratio of about 5 dB, but excessive loss occurs in the fusion/stretching section 3. The disadvantage was that the angle was as large as 3 dB and 41 degrees. This means that the excess loss of a non-linear polarization-maintaining fiber-type optical coupler made of an ordinary single-mode optical fiber having no stress-applying portion is about /dB or less. In contrast,
This was a major problem in the use of the conventional linear polarization maintaining fiber type optical coupler shown in FIG.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a fiber type optical coupler with little excess loss and a method for manufacturing the same. A fiber-type optical coupler, which is the first invention of the present invention, has a part of a plurality of linear polarization-maintaining optical fibers each having a stress applying part in the crand part, which are fused and stretched so that the main axis directions of the fibers are aligned. The fiber type optical coupler is characterized in that the refractive index of the stress applying portion of the optical fiber is matched to the refractive index of the cladding portion. Further, the second invention, a method for manufacturing a fiber-type optical coupler, includes manufacturing a fiber-type optical coupler in which a plurality of linear polarization-maintaining optical fibers having stress-applying parts in the cladding parts are bonded and stretched. In this method, prior to fusing and drawing, the position of the stress applying part of the optical fiber is detected using polarized light or ultraviolet light from the side of the optical fiber, and if necessary, each optical fiber is rotated about its central axis. , the principal axes of the plurality of optical fibers are aligned in a desired arrangement.

As a result of intensive study on excessive loss factors, the present inventor found that the refractive index value of the stress-applying part has a large effect on excessive loss. The refractive index value of the stress applying part of the optical fiber constituting the optical fiber is compensated using two or more types of dopants so that it is comparable to the refractive index of the cladding part. M] Stress-applying part center with compensated refractive index / In conventional microscope observation, it is impossible to observe the position from the side, and it is difficult to align the main axis of the fiber, but this can be solved by polarizing the fiber from the side. Alternatively, the problem was solved by observing using ultraviolet light. Hereinafter, the present invention will be described in detail with reference to the drawings.

Figure 3(a) shows the excess loss and stress-applying part relative refractive index difference (refraction of the cladding part) of a fiber-type optical coupler formed by fN from linear polarization-maintaining optical fibers having various stress-applying part refractive index values. It shows the relationship (experimental value) with A side view of the fiber used is shown in Figure 3(b),
The outer diameter of the eyeglass is /, 2jμm.

The core diameter is 1x, 511 m, the difference in refractive index relative to the core portion +θq%, the radius of the stress applying portion 5 is 3θ μm, and the distance between the center of the stressed ear portion and the center of the core is 30 μm. The stress applying portion contains B203 (to lower the refractive index) and Geo2 (to increase the refractive index) as dopants, and the relative refractive index difference is controlled by the balance thereof. Here, two fibers are fused and stretched in the arrangement structure shown in Figure λ(ta), /, 3 μm.
The stretching part is adjusted to have a splitting ratio of approximately 59%; the excess loss shown in Figure 3(a) is the average of the five good connectors out of the 70 connectors fabricated. It shows the value.

From Figure 3, the relative refractive index difference of the stress applying part is -θj~-θ7
It has been confirmed that the excess loss of a conventional fiber-type optical coupler composed of optical fibers of about 3 dB is about 3 dB or more, and the region where the excess loss is about / dB or less is -θ/j% ≦ It can be seen that the difference in refractive index between the stress-applied and stressed folded portions is as narrow as 5%. In Figure 3 (al), as the relative refractive index difference in the stress-applying part moves in the negative direction, the excess loss increases due to the following points. As the diameter becomes smaller, the light spreads widely and is transmitted not only to the core part but also to the cladding part, but if the relative refractive index difference of the stress applying part is negative,
It is estimated that the electric field distribution is disturbed and a conversion from the fundamental mode to a higher order mode occurs, leading to an increase in scattering loss. On the other hand, when the relative refractive index difference is positive, this is considered to be because, in addition to the above factors, undesirable optical coupling to the stress applying portion occurs. The actual M results in Figure 3 (2) correspond to the arrangement shown in Figure 1 ta+, but in the case of the arrangement shown in Figure 1 (bl, (C1), the stress applying part is 2
Since the L6 core is interposed between two cores, the increase in excess loss due to the mismatch in the refractive index values of the L6-applied portion becomes even more significant. Thus, in order to reduce the loss of the fiber-type optical coupler, it is necessary to compensate by using a plurality of dono-kunts so that the refractive index of the stress-applying part is aligned with the 2-rad part.

FIG. 9 is an explanatory diagram of the manufacturing process of the fiber type optical coupler of the present invention. First, two linear polarization maintaining optical fibers 1-1
a, adjust the main axis direction of 2-2a to the desired arrangement shown in Figure 1 and fix it to the support stand 41, 42 (Figure 9 (a))
Next, a part of the fiber is heated with an oxygen/propane flame to fuse it into one piece (Fig. 3(b)). Next, while heating the fused portion 43, the holding table 42 is smoothly moved in the direction of the arrow 44, and the fused portion m43* is stretched in a tapered shape to form the fused/stretched portion 3 (Fig. )).

Fig. 5 is a more detailed illustration of the fiber main axis arrangement process in Fig.
This figure shows a cross-sectional view along . In Figure 5,
Fiber 2-2a has already been adjusted in its main axis direction,
Fiber 1-1a is in a state before adjustment. In the main axis arrangement process, one optical fiber 11a, 2-2a is sandwiched between one glass plate 51, 52, and a matching liquid 53 having a refractive index value close to that of the cladding part of the fiber.
'4 was destroyed. The light from the illumination light source 54 crosses the fiber through a polarizing plate 55a, and then passes through another polarizing plate 55bf. When the polarized light crosses the fiber, the plane of polarization rotates due to the photoelastic effect caused by the presence of the stress-applying portion, and by observing it with the microscope 56, the position of the stress-applying portion can be detected as a difference in brightness. Even if the refractive index value of the stress-applying part matches the refractive index of the cladding part with high precision and the stress-applying part cannot be identified by normal microscopic observation, a photoelastic effect due to stress will occur. The position of the applicator can be known and the fiber can be rotated into any of the configurations shown in Figure 1. After completing the above arrangement operation, the 7 eyepers are fixed to the support base 41, 42, and the glass plate 51
．． 52 is removed to prepare for the primary fusion process. The matching liquid remaining on the side of the fiber is decomposed and vaporized by the acid/propane flame during fusion, so there is no problem.

Ultraviolet light can also be used as a method for arranging the fiber raw glaze. That is, as shown in the embodiment in FIG. 6, the fiber 1-18, 2-2a is sandwiched between glass plates 51, 52 together with a matching liquid 53, and a He-Cd laser is mounted on the side of the fiber. 6,,l(wavelength θ3Ωjμm,
Ultraviolet light from output/θmW) is irradiated. Since the stress-applying portion containing G e 02 as a dopant emits fluorescence in the visible range when irradiated with ultraviolet light, by observing the fluorescence distribution through the microscope 56, the stress-applying portion position,
Therefore, the main axis direction can be detected, which is effective for producing the fiber type optical coupler of the present invention. When microscopic observation is to be performed directly with the eyes without using a television camera or the like, it is desirable to protect the eyes by inserting an ultraviolet cut filter 62 in an appropriate position.

Although the configuration of the present invention has been described above for a (,2x,2) type optical coupler, it is of course equally effective for a (3x3) type using three optical fibers.

In addition to the linear polarization-maintaining original fiber (P, ANDA fiber) taken up in the above embodiments, fiber-type light consisting of similar so-called birefringent fibers (for example, Bow-Tie fiber, elliptical Talarado fiber, etc.) Of course, the present invention can also be applied to connectors.

As explained above, according to the present invention, by matching the refractive index of the stress applying part of the linear polarization maintaining optical fiber with the cladding part, a fiber type optical coupler with an excess loss of less than about /dB can be obtained. can be provided. By using polarized light or ultraviolet light, it is possible to align the main axes of the fibers in the desired direction.
It is possible to stably preserve linear polarization. The fiber-type optical coupler of the present invention is highly effective when used as a component of coherent optical communications that require stable polarization maintenance or optical 7-eyeper interferometer sensors.

[Brief explanation of drawings]

Figure 7 is a structural diagram of a conventional linear polarization maintaining fiber type optical coupler; Relationship diagram between rate difference and excess photocoupler loss, Figure 3 (bl is Figure 3 (a)
Cross-sectional views of the fibers used in the experiment of 1, Figures 7(a) to (
C) is a process diagram for manufacturing a fiber type optical coupler of the present invention, No. 3
The figure is an explanatory diagram showing an embodiment of the fiber main axis alignment method in the present invention, and FIG.
...Linear polarization maintaining optical fiber, 3...
Fusion/stretching part, 4a...core part, 4b...
...Clad part, 5...Stress applying part, 6a. 6b...Fiber main axis, 7...Fusion-stretched section cross section, 8...Incoming polarized wave, 9, 1o...
...Emission polarization, 21... Fusion operation, 41.42
... Support stand, 43 ... Fusion part, 44.
...Stretching direction, 51.52...Glass plate, 53...Refractive index matching liquid, 54...Illumination light source, 55a, 55b...・Polarizing plate, 56...
...Microscope, 61...Ultraviolet light source, (f(e
-cd laser), 62... ultraviolet cut filter. Applicant Nippon Telegraph and Telephone Public Corporation Agent Patent Attorney Tadashi Shiga Takeo 1°) (%) Figure 4) ) Figure 5\55a ↑ Shibushi 54

Claims (1)

[Claims] A fiber-type optical coupler in which parts of a plurality of linear polarization-maintaining optical fibers each having a stress-applying part in the two rad parts are fused and stretched so that the main axes of the fibers are aligned. 2. A fiber type optical coupler according to claim 1, wherein the refractive index of the stress applying portion of the optical 7-eyeper is matched to the refractive index of the cladding portion. 2. In a method for manufacturing a fiber-type optical coupler in which -m of a plurality of linear polarization-maintaining optical fibers having a stress applying part in the cladding part is fused and stretched, the stress of the nine fibers is Detect the position of the applied part on the side of the optical fiber using polarized light or ultraviolet light, and if necessary, rotate each optical fiber about its central axis to align the principal axes of the multiple optical fibers in the desired arrangement. A method for manufacturing a fiber type optical coupler, characterized in that: