JP2002323637A - Manufacturing method of polarization maintaining optical fiber coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a polarization maintaining optical fiber coupler whose excess loss is small. SOLUTION: The manufacturing method of the fiber coupler includes a step of placing two polarization maintaining optical fibers in parallel and a part of the fibers is heated and drawn longitudinally to form a welded and drawn part. After or simultaneous with the manufacturing method includes this forming process, an adjusting step in which a desired coupling characteristic is obtained in the range of wavelengths in use, and in which the cycle of fluctuation in the degree of coupling and that in the excess loss are adjusted so that the excess loss is minimized. Thus, the polarization maintaining optical fiber coupler is manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization maintaining optical fiber coupler useful in the field of optical fiber communication, the field of sensors utilizing optical fibers, and the like, and more particularly to a polarization beam combiner used for coupling and branching different polarizations. (Less than,
PBC) and a polarization beam splitter (hereinafter P
(Abbreviated as BS).

[0002]

2. Description of the Related Art Conventionally, a fusion-stretched optical fiber coupler can be connected to an external optical fiber with low loss, and is excellent in manufacturability, reliability, high power resistance and the like. Are known. Various types of polarization maintaining optical fibers have been proposed, and a typical example is a PANDA type optical fiber (Polarization maintaining fiber).
AND Absorption reducing fiber) is known.

FIGS. 1 (a) and 1 (b) show an example of a PANDA type optical fiber. FIG. 1 (a) is a sectional view cut in a direction perpendicular to the length direction of the optical fiber. (B)
FIG. 3 is a diagram showing a refractive index profile. This PAND
The A-type optical fiber 10 includes a core 11 provided at the center,
Around the core 11, the clad 1 is provided concentrically with the core 11 and has a lower refractive index than the core 11.
2 is provided. The clad 12 is symmetrically arranged around the core 11 and
Two stress applying portions 1 having a circular cross section with a refractive index lower than 2
3 and 13 are provided. In this example, the core 11
Is made of germanium-added quartz glass, the cladding 12 is made of pure quartz glass, and the stress applying section 13 is made of quartz glass to which boron is added in a relatively large amount. Core 11
Outside diameter, outside diameter of stress applying section 13, core 11 and clad 1
2 and the relative refractive index difference between the cladding 12 and the stress applying unit 13 are appropriately set according to desired characteristics and the like.
The outer diameter of the cladding 12 is usually about 125 μm.

The stress applying section 13 has a larger thermal expansion coefficient than the cladding 12. For this reason, in the process of cooling the drawn optical fiber at the time of manufacturing the optical fiber, distortion due to the stress applying portion 13 occurs in the fiber cross section. This distortion causes anisotropic distortion of the core 11. As a result, the polarization degeneracy can be resolved. Here, for the sake of convenience, if two orthogonal polarizations constituting light are X polarization and Y polarization, the propagation constant of the X polarization and the propagation constant of the Y polarization are different values. The distribution of the electromagnetic field is also different. As a result, X polarization and Y
The characteristic of propagating in a state where the polarization is preserved is obtained.

FIG. 2 shows an example of a polarization-maintaining optical fiber coupler.
The two PANDA-type optical fibers 10, 10 are aligned so that their polarization axes are parallel after removing a part of a coating layer made of plastic or the like provided on the surface thereof as necessary. The PANDA type optical fibers 10 and 10 are brought into contact with each other,
The fused and stretched portion (optical coupling portion) 3 is formed by heating and melting and stretching in the longitudinal direction. The polarization axis is a line passing through the center of each of the stress applying sections 13 in each of the PANDA optical fibers 10.

In this polarization maintaining optical fiber coupler, the X polarization propagates while maintaining an electric field vector in the direction of the polarization axis of the PANDA type optical fibers 10 and 10, and the Y polarization expresses an electric field vector orthogonal to this. It is held and propagates through the PANDA type optical fibers 10, 10. In this specification, for convenience, the polarization axis may be referred to as a slow axis (X polarization axis), and an axis orthogonal thereto may be referred to as a fast axis (y polarization axis). If the difference between the degree of coupling of the X-polarized light and the degree of coupling of the Y-polarized light in the fusion stretching part 3 is large, the fusion stretching part 3
The polarization and Y polarization are combined and branched, and PBC and PB
The characteristic as S is obtained.

FIG. 2 shows that X polarization and Y
Arrows indicate the PBS operation in which the polarized light is input, the light is branched in the fusion stretching section 3, and the X-polarized light and the Y-polarized light are output from the two ports B and C, respectively.
In the case of PBC operation, on the contrary, two ports B, C to X
Polarized light and Y-polarized light are input, respectively, are combined at the fusion-spreading section 3, and light combined with these is output from one port A. The degree of coupling refers to one PANDA type optical fiber 1
0 means the ratio of light coupled to the other PANDA type optical fiber 10, and more specifically, the ratio of light output from port B or port C to the sum of light output from ports B and C shown in FIG. It shows the power ratio. In addition, the polarization maintaining optical fiber coupler performs coupling and branching of two polarizations as described above, as well as an optical power combining / branching type that branches optical power at a desired ratio, and branches light for each wavelength. There is also a wavelength combining / branching type.

[0008]

However, the fusion-stretched polarization-maintaining optical fiber coupler has a problem that excess loss tends to increase. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polarization maintaining optical fiber coupler having a small excess loss.

[0009]

According to a first aspect of the present invention, two polarization maintaining optical fibers are arranged in parallel, a part of the length of the polarization maintaining optical fiber is heated, and the fiber is stretched in the length direction to form a fusion-stretched portion. And a period of change of the degree of coupling such that desired coupling characteristics are obtained and excess loss is minimized in the used wavelength range after or simultaneously with the forming step. And a step of adjusting the period of the fluctuation of the excess loss. According to a second invention, in the method for manufacturing a polarization maintaining optical fiber coupler according to the first invention, the adjusting step is performed while monitoring the coupling degree and excess loss. It is a manufacturing method. According to a third aspect, in the method for manufacturing a polarization maintaining optical fiber coupler according to the second aspect, the adjusting step is performed while monitoring the wavelength dependence of the coupling degree and the wavelength dependence of the excess loss. This is a method for manufacturing a polarization maintaining optical fiber coupler. In a fourth aspect of the present invention, in the adjusting step, the cycle of the variation of the coupling degree and the cycle of the variation of the excess loss are adjusted by changing the shape of the fusion-stretched portion. It is a manufacturing method of the polarization maintaining optical fiber coupler according to any of the inventions. A fifth invention is the method for manufacturing a polarization maintaining optical fiber coupler according to the fourth invention, wherein an aspect ratio is used as a parameter for changing the shape. According to a sixth aspect of the present invention, in the method for manufacturing a polarization maintaining optical fiber coupler according to any one of the first to fifth aspects, the polarization maintaining optical fiber coupler is a polarization beam combiner or a polarization beam splitter. This is a method for manufacturing a polarization maintaining optical fiber coupler. A seventh invention is the method of manufacturing a polarization maintaining optical fiber coupler according to any one of the first to sixth inventions, wherein the polarization maintaining optical fiber is a PANDA type optical fiber. An eighth invention is characterized in that another polarization-maintaining optical fiber coupler is manufactured using the manufacturing conditions obtained by the method for manufacturing a polarization-maintaining optical fiber coupler according to any one of the first to seventh inventions. This is a method for manufacturing a polarization maintaining optical fiber coupler. A ninth invention is a polarization maintaining optical fiber coupler obtained by the method for manufacturing a polarization maintaining optical fiber coupler according to any one of the first to eighth inventions.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the present invention, the reason why excess loss increases in a polarization maintaining optical fiber coupler will be described with reference to FIGS. 1 (a), 1 (b) and 2 as well. . That is, as shown in FIG. 3A, in one PANDA type optical fiber 10, the fundamental mode is guided through the core 11. Then, when this is heated and drawn, the diameter of the PANDA type optical fiber 10 is reduced. As a result, the fundamental mode guided through the core 11 oozes into the cladding 12 and couples to the cladding mode as shown in FIG. In this case, no excess loss occurs. However, the basic mode shown in FIG.
When combined with a higher-order mode as shown in FIG. 2B, this mode does not completely return to the fundamental mode propagating through the core 11 again, causing excessive loss. In addition, PANDA
In the optical fiber 10, the stress applying unit 1 is used in the polarization axis.
The presence of 3, 13 causes a phenomenon in which the fundamental mode is coupled to the higher-order mode, resulting in excess loss. However, in the axis perpendicular to the polarization axis, the stress applying portions 13, 13 do not exist, so Therefore, even if the diameter of the PANDA optical fiber 10 is reduced, the fundamental mode is hardly coupled to the higher-order mode, and the problem of excess loss hardly occurs.

In the PANDA type optical fiber 10,
The stress applying portions 13 are present, and the outer diameter thereof is large, for example, about 1/3 to 1/4 of the outer diameter of the clad 12. Therefore, light that has permeated the clad 12 is likely to be coupled to a higher-order mode, and excess loss is likely to be large. Excessive loss is likely to increase due to the presence of the stress applying portions 13 and 13 as described above.
The same applies to polarization maintaining optical fibers such as other Bow-Tie fibers and elliptical jacket fibers, not limited to zero. The loss caused by the stress applying portions 13 is a phenomenon unique to the polarization maintaining optical fiber among various optical fibers. Also in the fusion-stretched polarization maintaining optical fiber coupler shown in FIG.
Excess loss occurs in each of the NDA type optical fibers 10 and 10, and as a result, the excess loss of the polarization maintaining optical fiber coupler increases.

An embodiment of the present invention will be described below to clarify the configuration and effect of the present invention.
The present inventors have disclosed in Japanese Patent Application No. 2000-89814, 2
Polarization maintaining optical fiber (PANDA type optical fiber 1)
By heating, melting and stretching the cores 11 and 11 of (0, 10) as close as possible to form a fusion-stretched portion 3, stretching proceeds as shown in FIG. A method for manufacturing a polarization-maintaining optical fiber coupler that can increase the difference in the degree of coupling between two polarizations from the point in time when the polarization starts to occur. Note that the degree of coupling increases in each of the X-polarization and the Y-polarization as the stretching progresses,
After reaching about 00%, it decreases, and after reaching about 0%, repeats a periodic fluctuation that increases again to draw a wavy curve. In this manufacturing method, the following effects can be obtained. (1) Since the stretching length is short, a polarization maintaining optical fiber coupler having high mechanical strength can be obtained. (2) Since the number of times (the number of transitions) X-polarized or Y-polarized light is coupled from one polarization-maintaining optical fiber to the other polarization-maintaining optical fiber can be reduced, the loss is low. (3) A polarization maintaining optical fiber coupler having a large degree of polarization dependence of the coupling degree in a wide wavelength band can be obtained.

[0013] Therefore, PANDA by this manufacturing method
FIG. 5 shows a state in which the polarization maintaining optical fiber coupler is manufactured without paying attention to the excessive loss due to the diameter reduction of the optical fibers 10 and 10. As can be seen from the graph shown in FIG. 5, the excess loss due to the coupling of the fundamental mode to the higher-order mode as described above is caused by an increase in the extension length of the PANDA-type optical fiber 10, that is, a periodic decrease with a reduction in the diameter. Draws a wavy curve. The peak value of this loss is
Although it depends on the refractive index profile of the polarization maintaining optical fiber,
Empirically, it is about 3 to 10 dB. The cycle of the fluctuation of the excess loss due to the coupling to the higher-order mode is determined by the stress applying unit 1.
Since the distances 3 and 13 are close to each other, the period of the change in the degree of coupling that couples to the core 11 of the adjacent PANDA type optical fiber 10 becomes shorter.

That is, the variation in the degree of coupling and the variation in the excess loss are independently repeated in each cycle with the extension length. In the polarization maintaining optical fiber coupler shown in FIG. 4, when coupling of the Slow axis is almost zero, Fas
The t-axis coupling reaches almost 100%, and at the ideal polarization coupling point where the desired coupling characteristics can be obtained, a PBS operation (synonymous with the PBC operation) for splitting two polarizations is generated. It is clear that can be done.
Then, as shown in FIG. 5, when the ideal polarization coupling point and the point where the excess loss becomes large coincide in the variation of the coupling degree and the variation of the excess loss, the polarization maintaining optical fiber having a large excess loss. It becomes a coupler.

On the other hand, as shown in FIG. 6, when the ideal polarization coupling point coincides with the point where the excess loss is reduced, a polarization maintaining optical fiber coupler having a small excess loss is obtained. In other words, the polarization maintaining optical fiber coupler is designed so that the excess loss due to the stress applying portion of the polarization maintaining optical fiber is almost minimized at the ideal polarization coupling point, so that the polarization having a small excess loss is obtained. A wave holding optical fiber coupler can be obtained. Therefore, for example, after or simultaneously with the formation of the fusion-stretched portion 3, light in the operating wavelength range is input to the polarization-maintaining optical fiber coupler, and the degree of coupling and excess loss are monitored. The fusion-stretched portion 3 is heated and adjusted so that the proper polarization coupling point and the point at which the excess loss becomes the minimum value coincide. The wavelength range used is not particularly limited, but is, for example, 900 to 1650 nm.
It is appropriately selected from the range of the degree.

It is the shape of the fusion-stretched portion 3 that physically changes during the adjustment and affects the degree of bonding and the value of excess loss.
Parameters that define this shape include the minimum outer diameter of the fusion-stretched portion 3, the length of the fusion-stretched portion 3, the aspect ratio, and the like.
In the present invention, the aspect ratio is the ratio (b / a) of the maximum outer diameter b to the minimum outer diameter a of the cross section at the center in the length direction of the fusion-bonded stretched portion 3 as shown in FIG. is there. Usually, the minimum outer diameter a is a length in a direction substantially parallel to the polarization axis, and the maximum outer diameter b is a length in a direction perpendicular to the polarization axis.

Specifically, the cycle of the change of the coupling degree is the core 1
The period of the fluctuation of the excess loss changes depending on the outer diameter of each PANDA type optical fiber 10. For example, if the aspect ratio is increased without changing the outer diameter of the PANDA type optical fiber 10, the period of the fluctuation of the excess loss due to the coupling to the higher-order mode determined only by the outer diameter of one PANDA type optical fiber 10 becomes Although it does not change, the distance between the two cores 11 becomes large.
As a result, the coupling of light generated between the two PANDA optical fibers 10 and 10 due to the stretching becomes slow, and as a result, the period of the change in the coupling degree becomes long. Here, in order to obtain the optimum aspect ratio, for example, it is preferable to apply the following method. That is, after or simultaneously with the formation of the fusion-stretched portion 3, light in the used wavelength range is input to the polarization-maintaining optical fiber coupler to monitor the degree of coupling and excess loss, thereby monitoring the ideal polarization. The heating conditions of the fusion-bonded and stretched section 3 are adjusted so that the wave coupling point and the point at which the excess loss becomes the minimum value substantially match. Generally, a small aspect ratio can be obtained with strong heating (high heating temperature), and a large aspect ratio can be obtained with weak heating (low heating temperature). Therefore, the curves of the degree of bonding as shown in FIGS. 5 and 6 are shifted, for example, to the left in the graph, and if the degree of bonding is to be increased at a shorter stretching length, strong heating is performed. Performs melting by weak heating. In this way, by matching the cycle of the variation of the coupling degree and the cycle of the variation of the excess loss so that a desired degree of coupling is obtained and the excess loss is minimized, in the wavelength region used,
A polarization maintaining optical fiber coupler having a characteristic as shown in FIG. 6 and having a small excess loss can be obtained.

The optimum aspect ratio varies depending on the desired characteristics of the polarization maintaining optical fiber coupler and the polarization maintaining optical fiber (PANDA type optical fiber 10) used. Therefore, usually, it is necessary to individually set each product type or each polarization maintaining optical fiber lot.

In addition, by adjusting the minimum outer diameter defining the above-mentioned shape and the length of the fusion-bonded stretched portion 3 as parameters, the cycle of the variation of the coupling degree and the cycle of the variation of the excess loss can be optimized. As a result, a polarization maintaining optical fiber coupler having a small excess loss can be obtained. In the present embodiment, only the coupling degree of the Y polarization has been described. However, if necessary, the adjustment step is performed by monitoring the coupling degree of the X polarization or both the X polarization and the Y polarization. You can also.

FIG. 7 is a graph showing an example of the relationship between the wavelength and the excess loss of the polarization maintaining optical fiber coupler obtained in this manner. In relation to the wavelength (wavelength characteristic), similarly to the relation to the stretching length, the excess loss repeatedly increases and decreases and draws a wavy curve. In this polarization maintaining optical fiber coupler, when the excess loss is indicated by a broken line A, the center wavelength of the operating wavelength band coincides with the ideal coupling point, and the minimum value of the excess loss is obtained. Wavelength coincides with this center wavelength. Therefore, in the operating wavelength range, an ideal polarization combination is obtained, and the excess loss has a minimum value. On the other hand, the broken line B indicates that the minimum value of the excess loss is shifted to a shorter wavelength side by 20 nm than the center wavelength of the used wavelength range. In this case, the excess loss in the operating wavelength range increases by 1 dB. Therefore, excess loss
In order to suppress it to 1 dB or less, which is preferable for use, it is preferable to adjust the difference between the center wavelength of the used wavelength region and the wavelength at which the minimum value of excess loss is obtained to be 20 nm or less.
If the position where the excess loss of the completed polarization maintaining optical fiber coupler takes the minimum value is shifted from the wavelength region where the optimum polarization combining point is taken, fine adjustment of the conditions in the adjustment step or further heating is performed. , By making adjustments,
By adjusting the period of the variation of the coupling degree and the period of the excess loss, a polarization maintaining optical fiber coupler with a small excess loss can be obtained.

The optical coupling that progresses with the extension of the polarization maintaining optical fiber coupler progresses faster on the long wavelength side and progresses slowly on the short wavelength side. Therefore, the wavelength dependence of the coupling degree,
By monitoring the wavelength dependence of excess loss, it is possible to predict a change in the degree of coupling and excess loss due to stretching. That is, in the manufacture of a polarization maintaining optical fiber coupler, there is usually a wavelength at which the ideal polarization combining point and the position where the excess loss takes a minimum value are matched. In the above-described wavelength dependence of the degree of coupling and excess loss, the phenomenon occurring on the longer wavelength side than the current desired wavelength is the future desired wavelength when the stretching is advanced from the current time. The phenomenon which occurs in appears. Conversely, it can be said that the phenomenon occurring on the short wavelength side is a phenomenon that has occurred at the desired wavelength in the past when the stretching has not progressed from the present time. Therefore, by monitoring the wavelength dependence of the degree of coupling and excess loss, and particularly by observing the degree of coupling and excess loss on the longer wavelength side than the desired wavelength, the degree of coupling and excess The change in loss can be predicted, and adjustments can be made based on this to match the ideal polarization coupling point at the desired wavelength with the location where the excess loss is at a minimum. In addition, since the manufacturing method of the present invention requires that the excess loss be adjusted to a minimum value at the desired wavelength, the optical characteristics of the obtained product necessarily include the desired wavelength. The characteristic as shown in FIG. 7 in which the excess loss becomes the minimum value in the (used wavelength range) is shown.

In this embodiment, PBS (or PB)
Although C) has been described as an example, all of the optical power multiplexing and branching types such as the optical power multiplexing type and the wavelength multiplexing type are manufactured in such a manner that the excess loss is minimized in the extension length or the wavelength characteristic under desired coupling conditions. The present invention can be similarly applied to a fusion-stretched polarization-maintaining optical fiber coupler. In addition, PANDA is used as a commonly used polarization maintaining optical fiber.
Although the type optical fiber has been described as an example, the present invention is not limited to this, and the present invention is also applicable to a case where a polarization maintaining optical fiber such as another Bow-Tie fiber or an elliptical jacket fiber is used.

In this way, if the manufacturing conditions of the polarization maintaining optical fiber coupler which can obtain the desired characteristics are obtained, the same conditions can be obtained by manufacturing another polarization maintaining optical fiber coupler using these conditions. The polarization maintaining optical fiber coupler having the characteristic can be repeatedly produced and mass-produced.

[0024]

As described above, in the present invention,
A polarization maintaining optical fiber coupler having a small excess loss can be obtained by adjusting the cycle of the desired degree of coupling and the cycle of the excess loss.

[Brief description of the drawings]

FIG. 1 shows an example of a PANDA type optical fiber. FIG. 1 (a) is a cross-sectional view cut in a direction perpendicular to the length direction of the optical fiber, and FIG. 1 (b) is a view showing a refractive index profile. It is.

FIG. 2 is a perspective view showing an example of a polarization maintaining optical fiber coupler.

3 (a) to 3 (c) are explanatory diagrams showing states of light guided through a PANDA type optical fiber, and FIG.
FIG. 3A is an explanatory view showing a fundamental mode for guiding one PANDA type optical fiber, FIG. 3B is an explanatory view showing a higher-order mode, and FIG. It is explanatory drawing which showed seepage to the clad of the later fundamental mode.

FIG. 4 is a graph showing the degree of coupling of the polarization maintaining optical fiber coupler used in the example.

FIG. 5 is a graph showing a relationship between a variation in coupling degree and a variation in excess loss with respect to the extension length when a polarization maintaining optical fiber coupler is manufactured without paying attention to excess loss.

FIG. 6 is a graph showing a state in which a change in coupling degree and a change in excess loss with respect to a stretching length are matched.

FIG. 7 is a graph showing variation of excess loss with respect to wavelength.

[Explanation of symbols]

3: fusion-spreading section (optical coupling section); 10: PANDA type optical fiber (polarization maintaining optical fiber).

Claims (9)

[Claims] 1. Two polarization maintaining optical fibers are arranged in parallel,
A forming step of heating a part of the length direction and stretching in the length direction to form a fusion-stretched portion; after the forming step or simultaneously with the forming step, a desired wavelength range is used. A coupling maintaining characteristic is obtained, and an adjusting step of adjusting the period of the variation of the degree of coupling and the period of the variation of the excess loss so that the excess loss becomes a minimum value. Production method. 2. The method for manufacturing a polarization-maintaining optical fiber coupler according to claim 1, wherein the adjusting step is performed while monitoring the degree of coupling and excess loss. Method. 3. The method of manufacturing a polarization-maintaining optical fiber coupler according to claim 2, wherein the adjusting step is performed while monitoring the wavelength dependence of the coupling degree and the wavelength dependence of the excess loss. A method for manufacturing a polarization maintaining optical fiber coupler. 4. The method of claim 1, wherein in the adjusting step, the cycle of the change in the degree of coupling and the cycle of the change in the excess loss are adjusted by changing the shape of the fusion-stretched portion.
4. The method for producing a polarization-maintaining optical fiber coupler according to any one of claims 1 to 3. 5. The method for manufacturing a polarization maintaining optical fiber coupler according to claim 4, wherein in the adjusting step, an aspect ratio is used as a parameter for changing the shape. 6. The method for manufacturing a polarization maintaining optical fiber coupler according to claim 1, wherein the polarization maintaining optical fiber coupler is a polarization beam combiner or a polarization beam splitter. A method for manufacturing a polarization maintaining optical fiber coupler. 7. The method for manufacturing a polarization maintaining optical fiber coupler according to claim 1, wherein the polarization maintaining optical fiber is a PANDA type optical fiber. 8. A method for manufacturing another polarization-maintaining optical fiber coupler using the manufacturing conditions obtained by the method for manufacturing a polarization-maintaining optical fiber coupler according to any one of claims 1 to 7. A method for manufacturing a polarization maintaining optical fiber coupler. 9. A polarization-maintaining optical fiber coupler obtained by the method for manufacturing a polarization-maintaining optical fiber coupler according to claim 1. Description: