JPH02293802A - Polarized wave maintaining optical fiber coupler - Google Patents

Abstract

PURPOSE:To obtain the characteristics of a little loss and a little crosstalk and to allow the application of the optical fiber coupler to all polarized wave maintaining optical fibers by fusion-splicing the polarized wave maintaining optical fibers which are aligned in the main axes of polarized wave to each other to both ends of the optical fiber coupler. CONSTITUTION:This optical fiber coupler consists of two pieces of the polarized wave maintaining optical fibers fusion-spliced with short-sized single mode optical fibers 42a, 42b between the front and rear polarized wave maintaining optical fibers 41a to 41d having the main axes aligned to each other. The main axes of two pieces of the polarized wave maintaining optical fibers are paralleled with each other and the parts of the single mode optical fibers 42a, 42b are fusion-spliced so that the polarized wave is maintained. Namely, the optical fibers of the fusion stretched part 47 of the polarized wave maintaining optical fiber coupler are made of the ordinary single mode optical fibers 42a, 42b and the polarized wave maintaining optical fibers 41a to 41d having the main axes of the polarized wave aligned to each other are fusion-spliced to both ends of the optical fiber coupler. The difference in the coupling degree by the polarized wave is eliminated in this way and the high performance is obtd. The application of the coupler to all the polarized wave maintaining optical fibers is thus possible.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is an optical fiber coupler used for branching optical signals in optical communication systems, sensors, etc., in which polarization is maintained. This invention relates to a structure for realizing an optical fiber coupler.

[Prior art and problems to be solved by the invention] Conventionally, in optical transmission systems that utilize polarized waves, optical signals propagating on one transmission path are branched to a lost propagation path, or the When multiplexing optical signals from two transmission lines onto one transmission line, an optical fiber coupler that maintains polarization is required. In particular, coherent optical transmission requires a polarization diversity optical circuit, and a polarization maintaining optical fiber coupler is used. Furthermore, low-loss polarization-maintaining optical fiber couplers are also used in optical fiber gyros.

By the way, various structures of polarization-maintaining optical fiber couplers have been proposed and put into practical use.

However, polarization-maintaining optical fibers that can achieve low loss and low cross 1-k characteristics in polarization-maintaining optical fiber couplers produced by fusion-drawing are limited to polarization-maintaining optical fibers called PAND fibers. It is being

Figure 6 shows the structure of a polarization-maintaining optical fiber coupler made with PANDA fiber, Figure 7 (a) is a cross-sectional view of the polarization-maintaining optical fiber, and Figure 7 (b) is a cross-sectional view of the polarization-maintaining optical fiber coupler. It is.

In these figures, 11a. 1lb is a polarization maintaining optical fiber, 12a and 12b are cores, 13a and 13b are stress applying parts, 14 is a fusion/stretching part, 2la, 2lb, 24a, 2
4b is a core, 22a and 22b are claddings, 23a. 23
b, 26a, and 26b are applied parts, and 25 is a fused part.

As shown in Figure 6, in a polarization maintaining optical fiber coupler,
The principal axes of polarization are parallel to each other.

However, in the fused/stretched part l4, Fig. 7(a) and (b)
As shown in FIG.
, 26b are deformed. Furthermore, since the stress-applying parts of adjacent polarization-maintaining optical fibers interfere with each other, the cores 24a,
The birefringence of 24b is lower than that before fusion and stretching. Therefore, the crosstalk of the optical fiber coupler deteriorates.

Furthermore, since the stress-applying portion has a low refractive index, loss tends to increase during stretching.

The biggest problem with conventional polarization-maintaining optical fiber couplers is that the degree of coupling between the X and Y axes differs depending on the birefringence present in the fused and stretched parts, as shown in Figure 8. . When the stretching length is long, such as in an optical multiplexing/demultiplexing coupler, this difference becomes even larger, which is extremely inconvenient in practice, and may not be usable depending on the application. In addition, in the production of polarization-maintaining optical fiber couplers whose degree of coupling has broad wavelength characteristics, conventionally, the asymmetry is created by stretching before fusion and stretching. The depth of the loss was deeper than that of a normal optical fiber coupler, and the loss was more than IdB, and there was no solution to reduce the loss.

An object of the present invention is to create a polarization-maintaining optical fiber that has simple branching, optical multiplexing/demultiplexing characteristics, or wide wavelength characteristics, and also has low loss and low crosstalk characteristics, and can be applied to all polarization-maintaining optical fibers. The purpose is to realize couplers.

By the way, conventional single mode optical fiber couplers can easily realize multiplexing/demultiplexing and wide wavelength range functions in addition to simple branching with low loss. Moreover, it has the characteristic that the dependence of the degree of coupling on polarization is extremely small. In addition, in a single mode optical fiber, unless it is left in a straight line and no lateral pressure or bending is applied, there is almost no rotation of the incident linearly polarized wave or any change in the polarization state. The present invention takes advantage of the characteristics of this single mode optical fiber.

There is an idea that it would be possible to use a single-mode optical fiber coupler that has already been completed in this way and to fuse and connect polarization-maintaining optical fibers to both ends of the coupler.

However, in this case, extra length is essential for the single mode optical fiber coupler for fusion and splicing, and it is unavoidable to bend the extra length when storing the optical fiber coupler.
At that time, the state of polarization changes, and as a result, the polarization-maintaining optical fiber coupler no longer functions.

The present invention provides a structure that can solve this problem of polarization fluctuation, and uses a normal single-mode optical fiber as the optical fiber in the fusion/stretching part of the polarization-maintaining optical fiber coupler. Polarization-maintaining optical fibers whose principal axes of polarization coincide with each other are fused and connected to both ends of the fiber.

In this regard, all conventional polarization-maintaining optical fiber couplers are made of polarization-maintaining optical fibers.

[Means for solving the problem] (!) The polarization-maintaining optical fiber coupler according to the first claim includes a short single-mode optical fiber between front and rear polarization-maintaining optical fibers whose principal axes coincide. Consists of two polarization-maintaining optical fibers that are fused and spliced, the main axes of the two polarization-maintaining optical fibers are parallel, and the polarization is maintained by the single mode optical fiber portion being fused and stretched. It is characterized by being an optical coupler.

(2) The polarization-maintaining optical fiber coupler according to the second aspect is characterized in that the single mode optical fibers in the two polarization-maintaining optical fibers have different core parameters or crut parameters. do.

[Function] The polarization-maintaining optical fiber coupler of the present invention uses a normal single-mode optical fiber as the optical fiber at the fusion/stretching part of the polarization-maintaining fiber coupler, and has polarization-maintaining fibers at both ends of the optical fiber coupler. By fusing and splicing polarization-maintaining optical fibers whose principal axes coincide with each other, the difference in coupling degree due to polarization is eliminated, making it possible to apply to all polarization-maintaining optical fibers with high performance.

[Embodiments] Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 5.

(Embodiment 1) FIGS. 1 and 2 are diagrams for explaining the first embodiment of the present invention, and FIG. 1(a) is a structural diagram of a polarization-maintaining optical fiber before fusion, and FIG. Figure (b) is a purchased drawing after fusing and stretching.

In these figures, 41a-4. Id is a polarization maintaining optical fiber, 42a and 42b are single mode optical fibers,
43a to 43d are cores of polarization maintaining optical fibers, 44a to 43d are cores of polarization maintaining optical fibers;
44d is a stress applying part, 45a and 45b are cores of single mode optical fibers, 46a to 46d are fusion splicing points, and 47 is a fusion/stretching part.

In the following, this embodiment will be explained using a wave-maintaining optical fiber called a PANDA fiber as an example.

First, the means for obtaining the polarization-maintaining optical fiber before fusion shown in FIG. 1(a) will be explained.

First, the PANDA fiber 41a with a core parameter relative refractive index difference Δ of -03% and a cutoff wavelength of 1.2071z is fusion-spliced to the single mode optical fiber 422L. The connection loss is less than OIdB at a wavelength of 1.30μ.

Next, the single mode optical fiber 42a is cut to about IO mm, and the PANDA fiber 4lb is fusion spliced again. For this connection, the PA
Rotate the NDA fiber. The result of fusion splicing was 0.15 dB including the splice loss at two locations, and the crosstalk was -45 dB. PANDA fiber 41a,
Even if there is a 1 (1 +

In this way, a single polarization-maintaining optical fiber is manufactured in which the single mode optical fiber 42a is fused between the PAND fibers 4la and 4lb. Similarly, one polarization-maintaining optical fiber is manufactured by fusing the single mode optical fiber 42b between the PAND^7 eyeglasses 41c and 41d.

The two polarization-maintaining optical fibers manufactured in this way are prepared as shown in FIG. Single mode optical fibers 42a, 42b with their main axes arranged in parallel
are fused and stretched as shown in FIG. 2(b). P during Ennaka
Wavelength 1.3 introduced into ANDA fibers 4la and 41c
Measure the 0μ shoulder and 1.55μ wavelength light, respectively. The light of each wavelength is stretched until 100% is coupled with each other, fixed to a substrate, and mounted in a case.

As shown in Figure 2, the wavelength characteristics of the polarization-maintaining multiplexing/demultiplexing coupler fabricated in this way are the same for both X and Y polarizations, and the loss is 1.3 μu at the wavelength.
O at x. IdB. The crosstalk was -37 dB for both the wavelength 1.30μ and the wavelength 1.55μ1, which was an extremely good characteristic.

Furthermore, the deterioration of the cross 1-peak was within 2 dB in the temperature range of -20°C to 80°C.

Note that, in the present invention, a single mode optical fiber 42a . Stress applying parts 442L to 44d of polarization maintaining optical fibers 43a to 43d connected to both sides of 42b
The positions of the two do not necessarily have to coincide with each other, and may be at right angles to each other. Further, the PANDA fiber may be replaced with another polarization-maintaining optical fiber. Also,
PAND^ fiber structure allows only X-polarized waves to propagate,
If a single polarization fiber is applied to the present invention, a polarization-maintaining optical fiber coupler having a function of a polarizer, which has not been reported yet, can be realized.

(Example 2) FIG. 3 is a structural diagram of a polarization-maintaining optical fiber coupler of the present invention having wide wavelength characteristics.

In the figure, 61a to 61d are polarization maintaining optical fibers;
2 is a single mode optical fiber with a cladding outer diameter larger than that of a standard single mode optical fiber, 63 is a standard single mode destination fiber, 64a to 64d are polarization maintaining optical fiber cores,
65a to 65d are stress applying parts, 66 and 67 are cores having the same parameters, and 68 is a fusion/stretching part.

In the following, this embodiment will be explained using a polarization maintaining optical fiber called a PANDA fiber as an example.

62 is a single mode optical fiber with a cladding outer diameter of 135μl, and 63 is a cladding outer diameter of 125μl! The single mode optical fiber 66.67 is a core having a relative refractive index difference Δ-0.3% and a cutoff wavelength of 1.20 μm.

As in the case described in Example 1, first, the core parameter relative refractive index difference Δ-0.3%, the cutoff wavelength 1.25μ
x PANDA fibers 61a, 61c and single mode optical fibers 62, 63 are fusion spliced. The splice loss between the PANDA fibers 61a to 61d and the standard single mode optical fiber 63 is 0.05 dB or less at a wavelength of 1.30μ, while in the case of the single mode optical fiber 62 with a large cladding outer diameter, it is 0. It was lOdB.

Next, the single mode optical fibers 62 and 63 are cut to about 1O■, and the PANDA fiber is cut to 6lb,
61d is fusion spliced. When making this connection, make sure that the polarization main axes of the polarization-maintaining optical fibers match.
Rotate the DA fiber. The result of fusion splicing is 0.20 dB and O. lod
B, and the crosstalk was -43 dB and -45 dB.

Next, the PANDA fibers 61a and 61c are rotated on the setting table of the fusing/stretching device, and each PANDA fiber is
With the principal axes of polarization of the fibers arranged in parallel, single mode optical fibers 62 and 63 are fused and stretched as shown in FIG. 3(b). To monitor the degree of bonding during stretching, PAN
The 1.30μ wavelength light introduced into the DA fiber 6.1a is measured at the output end of the PANDA fiber 61bf31d.

Further, in order to obtain wide wavelength characteristics, the cross-sectional shape of the fused portion is shaped like a figure 8. The fibers were stretched at a wavelength of 1.48 μm to give a maximum coupling of 53%, fixed to a substrate, and mounted in a case. As a result, as shown in FIG. 4, 50% coupling was obtained at wavelengths of 1.30μ and 1.55μ. Moreover, the coupling degree of both the X polarization and the Y polarization of the manufactured PANDA fiber coupler is within 1%. Moreover, the loss is from wavelength 1.30μ shoulder to wavelength 1.55μ
Good characteristics were obtained, with a crosstalk of 0.2 dB to the shoulder and -35 dB at both the shoulder wavelength of 1.30 μl and the wavelength of 155 μl. Further, in the temperature range from -20'C to 80C, the crosstalk degradation was within 2 dB and the coupling degree variation was within 1%.

An optical fiber coupler having such a configuration has a feature that the maximum coupling degree and wide wavelength band can be controlled by changing the difference in the outer diameter of the cladding of the single mode optical fiber.

(Example 3) FIG. 5 shows an example of the present invention having a structure to obtain wide wavelength characteristics.

In the figure, 71a to 71d are PANDA fibers;
2 and 73 are single mode optical fibers, 74a to 74d are cores of polarization maintaining optical fibers, 75a to 75d are stress applying parts, 76 and 77 are cores having different parameters, and 78 is a fusion/stretching part.

In FIG. 5(a), PANDA fibers 71a to 7
1d and the single mode optical fibers 72 and 73 are made in the same manner as in Examples 1 and 2 described above.

The relative refractive index difference of a normal single mode optical fiber used at a wavelength of 1.30 μm or more is about 0.3%, but there is some variation. Focusing on the off-wavelength, cores 76 and 77 have l, 25μIll, and 25μIll, respectively.
1. A single mode optical fiber with a shoulder thickness of 10μ is used.

This optical fiber was lightly fused so that the fusion-cut shape was in the shape of a figure 8, and then it was stretched to a stretching length of 1A], thereby achieving a maximum coupling degree of 50%. Moreover, as described in Example 2, the wavelength characteristics range from wavelength 1.30 μm to wavelength 1.55 μm.
I was able to easily create a flat 3dB coupler up to the shoulder.

Furthermore, this method is practical because wide wavelength characteristics can be freely set using a combination of standardized single-mode optical fibers.

The characteristics of the fabricated polarization-maintaining optical fiber canopler are as follows:
Coupling degree 50% ± 2% at 20 μλ ~ wavelength 1.60 μI,
The degree of coupling of both X and Y polarizations is within 1%, loss is 0.25dl3, crosstalk harm is 39d
B. In the temperature range of -20° C. to 80'° C., the degradation of Talostoke was within 2 dB, and the variation in degree of coupling (t) was within 1%, which is almost the same as the results of Example 2.

In addition, in the optical fiber coupler of the present invention, fusion and
By adjusting the fusion shape, fusion length, and stretching length of the stretching part, the degree of coupling can be varied, and it has optical multiplexing and demultiplexing functions.
In either case, polarization is maintained.

[Effects of the Invention] As described above, according to the present invention, since a single mode optical fiber is used in the fusion/stretching section, a polarization-maintaining optical fiber coupler is provided in which there is no difference in coupling degree due to polarization. be able to.

Additionally, since the length of the single-mode optical fiber is extremely short and it is inside the case, it is not affected by bending or lateral pressure, and the polarization crosstalk is much better than that of conventional polarization-maintaining fiber optic couplers. It is. Moreover, the loss is extremely small, and the degree of coupling and loss are hardly affected by changes in ambient temperature.

Further, according to the present invention, multiplexing/demultiplexing characteristics and wide wavelength characteristics, which have not been achieved with conventional polarization-maintaining optical fiber couplers, can be easily achieved.

Furthermore, according to the present invention, various polarization-maintaining optical fibers that have been reported so far, such as the above-mentioned PANDA fiber, elliptical jacket fiber manufactured by Hitachi, Ltd., and Bokeify fiber manufactured by University of Southampton/< flat clad fiber manufactured by AT&T. This method can be applied to any of the above. Naturally, a combination of PAND fiber and elliptical jacket fiber is also possible.

In addition, during fabrication, in conventional polarization-maintaining optical fiber couplers, the degree of coupling differs between X-polarized waves and y-polarized waves. Although linearly polarized light must be introduced in parallel, the present invention has the advantage of not having to consider polarized light.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining the first embodiment of the present invention, and FIG. 1(a) is a structural diagram before fusion, and FIG.
b) is a structural diagram after fusion splicing and stretching, and FIG. 2 is a wavelength characteristic diagram of the degree of coupling of the produced wave-combining/demultiplexing-maintaining optical fiber coupler. 3 and 4 are explanatory diagrams of a polarization-maintaining optical fiber coupler having wide wavelength characteristics as a second embodiment of the present invention, and FIG. 3(a) is a structural diagram before fusion; FIG. 4(b) is a structural diagram after fusing and stretching, and FIG. 4 is a wavelength characteristic diagram of the degree of coupling of the manufactured polarization-maintaining optical fiber coupler having wide wavelength characteristics. FIG. 5 is a diagram for explaining the third embodiment of the present invention, in which FIG. 5(a) is a structural diagram before fusion, and FIG. 5(b) is a structural diagram before fusion.
It is a jR drawing after stretching. Figure 6 is a structural diagram of a polarization-maintaining optical fiber coupler made with PAND fiber, Figure 7 (a) is a cross-sectional view of the polarization-maintaining optical fiber, and Figure 7 (b) is a diagram of the polarization-maintaining optical fiber coupler. FIG. 8 is a sectional view of , and a wavelength characteristic diagram of the degree of exhaustion. Ifa, Ilb... polarization maintaining optical fibers 12a, 12
b-core, 13a, 13b... stress applying part, l4... fusion/stretching part, 2 1 a, 2 l b, 24 a, 24
1)...-=:+7, 22a, 22b-clad, 2 3 a, 2 3 b, 2 6 a, 2
6 b Stress applying part, 25... Fusion part, 41a to 41d... Polarization maintaining optical fiber, 42a, 4
2b...Single mode optical fiber, 43a to 43d...
- Core of polarization-maintaining optical fiber, 44a to 44d... Stress applying part, 45a, 45b... Core of single mode optical fiber, 4
6a to 46d... Fusion splicing point, 47... Fusion/stretching part, 61a to 61d... Polarization maintaining optical fiber, 62...
Single mode optical fiber 63 with a larger cladding outer diameter than standard single mode optical fiber...Standard single mode optical fiber, 64a to 64
d... Core of polarization maintaining optical fiber, 65a to 65d.
・Stress applying part, 66. 67... Core having the same parameters, 68.
・Fusion/stretching part, 8 1 a ~ 8 1 d = 4AND^ fiber, 8
2.83...Single mode optical fiber, 84a to 84d
...Core of polarization maintaining optical fiber, 5a to 85d...
Stress applying part, 6, 87... Core having different parameters, 88...
...Fusion/stretching part. Kari length (, am) O8 ○ Bo length (, um)

Claims (2)

[Claims] (1) Consisting of two polarization-maintaining optical fibers in which a short single-mode optical fiber is fusion-spliced between front and rear polarization-maintaining optical fibers whose principal axes coincide; 1. A polarization-maintaining optical fiber coupler, characterized in that the main axes of the single-mode optical fibers are parallel, and the portions of these single-mode optical fibers are fused and stretched to serve as an optical coupler in which polarization is maintained. (2) The polarization-maintaining optical fiber coupler according to claim 1, wherein the single-mode optical fibers in the two polarization-maintaining optical fibers have different core parameters or cladding parameters.