JP2004037645A - Polarization holding optical fiber coupler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization holding optical fiber coupler constituted so that the temperature dependence of insertion loss and polarization dependence are reduced. <P>SOLUTION: In the polarization holding optical fiber coupler 30, a single mode optical fiber part is provided in the midst of one part of a polarization holding optical fiber 10, and compound optical fibers 20 and 20 with which the optical axes of the optical fibers 10 and 10 opposed via the single mode optical fiber part are aligned are arranged in parallel. Then a fusion-drawn part is formed by fusing and drawing the single mode optical fiber part. One of the two optical fibers 20 is a single mode optical fiber. The difference between the mode field diameter of the single mode optical fiber part and that of the optical fiber 10 is set to ≤2.0 μm and connection loss per one spot of the connection part of the single mode optical fiber part and the optical fiber 10 is set to ≤0.2 dB. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a polarization-maintaining optical fiber coupler, and more particularly to a polarization-maintaining optical fiber coupler that reduces the temperature dependence of insertion loss.
[0002]
[Prior art]
Various types of polarization-maintaining optical fibers have been proposed. As a typical example, a PANDA type optical fiber (Polarization maintaining AND Absorption reduction fiber) is known.
FIG. 6 is a sectional view showing an example of a PANDA type optical fiber. The PANDA type optical fiber 10 includes a core 11 provided at the center, a clad 12 provided around the core 11 concentrically with the core 11 and having a lower refractive index than the core 11, The stress applying portions 13 are symmetrically arranged around the core 11 and have a lower refractive index than the cladding 12.
[0003]
The stress applying section 13 has a larger thermal expansion coefficient than the cladding 12. Therefore, in the process of cooling the PANDA-type optical fiber 10 obtained by melting and drawing the optical fiber preform, a strain caused by the stress imparting portion 13 occurs in the cross section of the PANDA-type optical fiber 10.
This distortion causes anisotropic distortion of the core 11. As a result, the degeneracy of the polarization is released, and for convenience, if the two orthogonal polarizations constituting the light are X polarization and Y polarization, the propagation constant of the X polarization is different from the propagation constant of the Y polarization. Values, and of course, the distributions of these polarized electromagnetic fields are also different. As a result, a characteristic of propagating in a state where the X polarization and the Y polarization are preserved is obtained.
[0004]
An optical coupler manufactured using a polarization maintaining optical fiber such as the PANDA type optical fiber 10 is a polarization maintaining optical fiber coupler. This polarization maintaining optical fiber coupler is an effective optical component for an optical fiber sensor or coherent optical communication.
FIG. 7 is a perspective view showing an example of a conventional polarization maintaining optical fiber coupler. This polarization maintaining optical fiber coupler 15 removes a part of a coating layer made of plastic or the like provided on the surface of the two PANDA type optical fibers 10 and 10 as necessary, and The PANDA type optical fibers 10, 10 are brought into contact with the claddings 12, 12 in the middle of the PANDA type optical fibers 10, 10 so that the axes are parallel to each other. This is one in which an extending part (optical coupling part) 14 is formed. Note that the polarization axis refers to a straight line passing through the center of each of the stress applying sections 13 in each of the PANDA optical fibers 10.
[0005]
[Problems to be solved by the invention]
The stress generated in the PANDA-type optical fiber 10 utilizes the difference in the thermal expansion coefficient between the clad 12 and the stress-applying section 13, and the stress-applying section 13 increases in the process of cooling the drawn PANDA-type optical fiber 10. This is caused by contraction. Therefore, the stress generated in the PANDA type optical fiber 10 shows temperature dependency. Therefore, in the conventional polarization maintaining optical fiber coupler shown in FIG. 7, since the fusion-spread portion 14 is formed of the PANDA type optical fiber 10, the optical characteristics have a slight temperature dependency due to a change in environmental temperature. there were.
[0006]
As a method for reducing the temperature dependency of a single mode optical fiber coupler, Japanese Patent Application Laid-Open No. H11-52144 discloses a method in which an optical branch coupler (fused and stretched part) of an optical fiber coupler is housed in a protective case, and an adhesive is used. There has been proposed a reinforcing method of bonding and fixing with. However, in the polarization maintaining optical fiber coupler, no method has been proposed for essentially reducing the temperature dependency due to the stress applying portion of the polarization maintaining optical fiber.
In addition, the conventional polarization-maintaining optical fiber coupler has a different X-polarization coupling factor and a Y-polarization coupling factor because the propagation constant of X polarization and the propagation constant of Y polarization are different due to the characteristics of the PANDA type optical fiber. However, only operation with one polarization was realized.
[0007]
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a polarization maintaining optical fiber coupler in which the dependence of insertion loss on temperature and polarization is reduced.
[0008]
[Means for Solving the Problems]
The object is to provide a polarization-maintaining optical fiber coupler in which two optical fibers are arranged in parallel and a fusion-stretched portion formed by fusion-stretching a part of the length in a longitudinal direction is at least one of the two optical fibers. One is a composite optical fiber in which a single-mode optical fiber is interposed in a part of the polarization-maintaining optical fiber, and the optical axes of the polarization-maintaining optical fibers facing each other via the single-mode optical fiber coincide with each other, The fusion-splicing section can be solved by a polarization-maintaining optical fiber coupler formed by fusion-splicing a single-mode optical fiber.
In the polarization-maintaining optical fiber coupler, one of the two optical fibers is preferably a single mode optical fiber.
In the polarization-maintaining optical fiber coupler, the mode field diameter of the single-mode optical fiber interposed in a part of the polarization-maintaining optical fiber and the mode field diameter of the polarization-maintaining optical fiber connected to the single-mode optical fiber Is preferably 2.0 μm or less, and a connection loss per connection portion between the single mode optical fiber and the polarization maintaining optical fiber is 0.2 dB or less.
In the above polarization maintaining optical fiber coupler, it is preferable that the length of the single mode optical fiber interposed in a part of the polarization maintaining optical fiber is 50 mm or less. In the polarization-maintaining optical fiber coupler, it is preferable that the fusion-stretched portion is formed by fusion-stretching two single-mode optical fibers having different propagation constants.
In the polarization-maintaining optical fiber coupler, it is preferable that at least one of the two single-mode optical fibers forming the fusion-spread portion is etched.
In the above-mentioned polarization maintaining optical fiber coupler, it is preferable that a portion other than the fusion-stretched portion is fixed on the reinforcing member with an adhesive.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
FIG. 1 is a schematic configuration diagram showing a first embodiment of the polarization maintaining optical fiber coupler of the present invention. In this drawing, for the sake of convenience, a state is shown in which two optical fibers constituting a polarization maintaining optical fiber coupler are arranged in parallel without being fused and drawn.
FIG. 2 is a schematic configuration diagram illustrating a method for manufacturing the polarization maintaining optical fiber coupler according to the first embodiment.
1 and 2, reference numeral 10 denotes a PANDA-type optical fiber of a polarization maintaining optical fiber, reference numeral 21 denotes a single-mode optical fiber portion composed of a single-mode optical fiber, and reference numeral 20 denotes one of the PANDA-type optical fibers 10. 2 shows a composite optical fiber in which a single mode optical fiber section 21 is interposed.
[0010]
The polarization-maintaining optical fiber coupler 30 according to this embodiment is a fusion splicing in which two composite optical fibers 20, 20 are juxtaposed and a part of the longitudinal direction, that is, the single mode optical fiber portions 21, 21 are fused and drawn. This is one in which the extending portion 23 is formed.
The composite optical fiber 20 has a single mode optical fiber section 21 sandwiched between two PANDA type optical fibers 10 and 10, and both are fusion-spliced. In the composite optical fiber 20, the optical axes of the PANDA type optical fibers 10, 10 facing each other via the single mode optical fiber section 21 are connected so as to be coincident.
[0011]
The PANDA type optical fiber 10 includes a core 11 provided at the center, a clad 12 provided around the core 11 concentrically with the core 11 and having a lower refractive index than the core 11, And two stress applying portions 13, 13 symmetrically arranged about the core 11 and having a lower refractive index than the cladding 12 and having a circular cross section.
The single mode optical fiber section 21 includes a core (not shown) provided at the center, and a clad (not shown) provided concentrically with the core and having a lower refractive index than the core. It is composed of
[0012]
Here, the difference between the mode field diameter of the PANDA type optical fiber 10 and the mode field diameter of the single mode optical fiber section 21 is 2.0 μm or less, and the connection between the PANDA type optical fiber 10 and the single mode optical fiber section 21. Preferably, the connection loss per part of the connection is 0.2 dB or less, more preferably the difference in mode field diameter is 1.0 μm or less, and the connection loss per connection part is 0.1 dB or less.
When the difference between the mode field diameter of the PANDA type optical fiber 10 and the mode field diameter of the single mode optical fiber portion 21 is larger than 2.0 μm, or the PANDA type optical fiber 10 and / or the single mode in which the eccentricity of the core is large. The use of the optical fiber unit 21 is not preferable because the connection loss between them becomes large and causes the insertion loss of the polarization maintaining optical fiber coupler 30.
If the connection loss per connection of the PANDA-type optical fiber 10 and the single-mode optical fiber unit 21 exceeds 0.2 dB, the composite optical fiber 20 has two connection parts. The connection loss of the optical fiber 20 exceeds 0.4 dB. In addition to this, considering the excess loss in the fusion-spreading section 23 of the polarization-maintaining optical fiber 30, the insertion loss becomes larger than that of the conventional polarization-maintaining optical fiber coupler.
[0013]
In the composite optical fiber 20, the length of the single mode optical fiber portion 21 interposed in a part of the polarization maintaining optical fiber 10 is preferably 50 mm or less, more preferably 20 mm or less.
When the length of the single mode optical fiber section 21 exceeds 50 mm, the value of the polarization crosstalk of the composite optical fiber 20 increases, and the polarization maintaining characteristic deteriorates. Further, when the length of the single mode optical fiber section 21 becomes longer, the size of the polarization maintaining optical fiber coupler 30 becomes larger.
Here, the polarization crosstalk is a characteristic index indicating a polarization maintaining characteristic, and when the linearly polarized light polarized in one main axis direction is excited at the input end, how much orthogonal polarization is generated at the output end. It indicates whether power conversion occurs in the wave mode, and is expressed in dB by taking the ratio of both modes.
[0014]
In the polarization maintaining optical fiber coupler 30 of this embodiment, it is preferable that the fusion-stretched portion 23 is formed by fusion-stretching two single-mode optical fiber portions 21 having different propagation constants. If the fusion-stretched portion 23, that is, the optical coupling portion is formed by using the two single-mode optical fiber portions 21 having different propagation constants, the branching ratio becomes substantially flat in a predetermined wavelength band. The polarization maintaining optical fiber coupler 30 can be obtained.
[0015]
In the polarization-maintaining optical fiber coupler 30 of this embodiment, it is preferable that at least one of the two single-mode optical fiber portions 21 forming the fusion-stretched portion 23 is etched. As a method of etching the single mode optical fiber portion 21, an etchant such as hydrofluoric acid that corrodes quartz glass is preferably used. There is also a method of dry-etching a portion to be etched by a burner flame containing a quartz glass corrosive gas such as a fluorine gas.
In this manner, by performing the etching process on the fusion-stretched portion 23, that is, the single-mode optical fiber portion 21 forming the optical coupling portion, a difference can be made between the propagation constants of the two optical fibers after being stretched. As a result, it is possible to obtain the polarization maintaining optical fiber coupler 30 having a small wavelength dependency and a substantially flat branching ratio in a predetermined wavelength band.
[0016]
Further, as a method of reinforcing the polarization maintaining optical fiber coupler 30 of this embodiment, a portion other than the fusion-stretched portion 23 of the polarization maintaining optical fiber coupler 30, that is, the polarization maintaining optical fiber 10 is made of quartz glass. For example, a method of fixing with an adhesive on a reinforcing member (such as a substrate) made of such as is used.
When fixing the polarization-maintaining optical fiber coupler 30 on the reinforcing member with an adhesive, covering the fusion-stretched portion 23 composed of the single-mode optical fiber portion 21 with the adhesive increases the value of polarization crosstalk, The polarization maintaining characteristics deteriorate. Therefore, it is preferable to fix the polarization maintaining optical fiber 10 on the reinforcing member with an adhesive.
Here, as a specific example, in the polarization-maintaining optical fiber coupler 30 in which only the polarization-maintaining optical fiber 10 is covered with the adhesive, the average value of the polarization crosstalk was −27.0 dB, In the polarization maintaining optical fiber coupler 30 in which the single mode optical fiber section 21 was covered with the adhesive, the average value of the polarization crosstalk was −19.1 dB.
[0017]
In the polarization maintaining optical fiber coupler 30 of this embodiment, the fusion extending portion 23 is formed by the two single mode optical fiber portions 21, 21, so that the polarization maintaining light such as the PANDA type optical fiber 10 can be obtained. The temperature dependence and polarization dependence of the insertion loss caused by the fiber can be reduced as much as possible. The polarization-maintaining optical fibers 10 and 10 constituting the composite optical fiber 20 have their optical axes matched in advance, and the fusion-stretched portion 23 has two single-mode optical fiber portions 21 and 21 having no polarization dependence. Therefore, it is not necessary to adjust the polarization axis before the single-mode optical fiber portions 21 and 21 are fused and stretched. Stretching becomes possible. Therefore, the manufacturing cost does not increase.
[0018]
Next, a method of manufacturing the polarization maintaining optical fiber coupler of this embodiment will be described with reference to FIG.
In order to manufacture the polarization maintaining optical fiber coupler of this embodiment, first, a single mode optical fiber portion 21 is sandwiched between PANDA type optical fibers 10 and 10, and these are connected to produce a composite optical fiber 20. At this time, the optical axes of the PANDA type optical fibers 10 and 10 facing each other via the single mode optical fiber portion 21 are precisely adjusted so that they are on the same axis, and the polarization axes of the two are matched. Next, a part of the coating layer made of plastic or the like provided on the surface of the single mode optical fiber portion 21 of the two composite optical fibers 20 is removed as necessary. Next, after the composite optical fibers 20 and 20 are precisely adjusted so that their polarization axes are parallel to each other and are aligned, the single mode optical fiber portions 21 and 21 are brought into contact with each other to heat and melt them. By stretching in the longitudinal direction, a fusion-stretched portion (optical coupling portion) 23 is formed to obtain a polarization-maintaining optical fiber coupler.
[0019]
FIG. 3 is a schematic configuration diagram showing a second embodiment of the polarization maintaining optical fiber coupler of the present invention. In this drawing, for the sake of convenience, a state is shown in which two optical fibers constituting a polarization maintaining optical fiber coupler are arranged in parallel without being fused and drawn. In the figure, the same components as those in the above-described first embodiment are denoted by the same reference numerals, and description thereof will be omitted.
In the polarization-maintaining optical fiber coupler 40 of this embodiment, the composite optical fiber 20 and the single-mode optical fiber section 21 are arranged in parallel, and a portion in the longitudinal direction, that is, the single-mode optical fiber section 21 and the single-mode optical fiber section 21 are fused. The fusion-stretched portion 28 formed by fusion-stretching is formed.
The single-mode optical fiber section 21 includes a core 25 provided at the center, and a clad 26 provided concentrically with the core 25 and having a lower refractive index than the core 25 around the core 25. ing.
[0020]
In the polarization-maintaining optical fiber coupler 40 of this embodiment, the fusion-stretched portion 28 is formed by the single-mode optical fiber portion 21 and the single-mode optical fiber portion 21. Similarly, temperature dependency and polarization dependency can be minimized. Further, since the fusion-spreading section 28 is formed by the single-mode optical fiber section 21 and the single-mode optical fiber section 21 having no polarization dependence, the single-mode optical fiber section 21 and the single-mode optical fiber section 21 are fusion-stretched. It is not necessary to adjust the polarization axis before performing the process, and the fusion stretching can be performed in a short time in the same manner as a normal single mode optical fiber coupler.
In addition, in the polarization maintaining optical fiber coupler 40 of this embodiment, the use amount of the expensive PANDA type optical fiber 10 can be halved, so that the manufacturing cost can be reduced.
In the case of an optical fiber coupler used for monitoring a signal light or a light source, the monitor light emitted from the emission port is not affected by the polarization state, and therefore, like the polarization maintaining optical fiber coupler 40 of this embodiment, The single mode optical fiber section 21 may be used as a branch light output port.
[0021]
Also in the polarization maintaining optical fiber coupler 40 of this embodiment, the fusion-stretched portion 28 is formed by fusion-stretching the single-mode optical fiber portion 21 and the single-mode optical fiber portion 21 having different propagation constants. Is preferred. In this way, it is possible to obtain the polarization maintaining optical fiber coupler 40 in which the branching ratio is substantially flat in a predetermined wavelength band.
Further, also in the polarization maintaining optical fiber coupler 40 of this embodiment, it is preferable that at least one of the single mode optical fiber portion 21 and the single mode optical fiber portion 21 forming the fusion-spread portion 28 is etched. In this way, it is possible to obtain the polarization maintaining optical fiber coupler 40 having a small wavelength dependency and having a substantially flat branching ratio in a predetermined wavelength band.
[0022]
As a method of reinforcing the polarization-maintaining optical fiber coupler 40 of this embodiment, a portion other than the fusion-stretched portion 28 of the polarization-maintaining optical fiber coupler 40, that is, other than the fusion-stretched portion of the single-mode optical fiber portion 21, Is fixed on a reinforcing member (such as a substrate) made of quartz glass or the like with an adhesive. In this way, it is possible to prevent the value of the polarization crosstalk from being reduced and to prevent the polarization maintaining characteristic from deteriorating.
[0023]
The polarization maintaining optical fiber coupler 40 of this embodiment can be manufactured by the same method as the polarization maintaining optical fiber coupler 30 of the first embodiment.
[0024]
Hereinafter, the effects of the present invention will be clarified by showing specific experimental examples with reference to FIGS.
(Experimental example 1)
As an example of the polarization-maintaining optical fiber coupler 30, a 1550 nm band 50% polarization-maintaining optical fiber coupler was used to investigate the temperature dependence and the polarization dependence of the insertion loss. FIG. 4 shows the results.
FIG. 4 shows the insertion loss at the time of slow polarization axis incidence. The white dots indicate the insertion loss of the cross port (the port at which light incident from one incident port is coupled to another optical fiber), and The missing points are insertion losses of through ports (ports in which light incident from one incident port is not coupled to another optical fiber).
From the results in FIG. 4, it was found that the temperature dependence of the insertion loss of the polarization-maintaining optical fiber coupler 30 of this embodiment was sufficiently small at 0.05 dB or less in the range of 0 to 70 ° C. Also, since the fusion-stretched portion is formed by the two single-mode optical fiber portions 21, there is no difference in the optical characteristics between when the slow polarization axis is incident and when the fast polarization axis is incident. It can be said that operation is possible with both X-polarization and Y-polarization.
Also, from the results of FIG. 4, the polarization maintaining optical fiber coupler 30 of this embodiment separates the light incident from one input port into two output ports like a 50% polarization maintaining optical fiber coupler. It has been found that this is suitable for a case where polarization maintaining characteristics are required for both of the lights emitted from the two emission ports.
[0025]
(Experimental example 2)
Next, a plurality of types of polarization maintaining optical fiber couplers 30 were prototyped by changing the length of the single mode optical fiber section 21.
FIG. 5 shows the results of measuring the polarization crosstalk of each polarization-maintaining optical fiber coupler 30.
From FIG. 5, it was found that when the single mode optical fiber section 21 was long, the value of the polarization crosstalk was large, and the polarization maintaining characteristics were deteriorated.
[0026]
【The invention's effect】
As described above, according to the present invention, the polarization and temperature dependence of optical characteristics such as insertion loss are extremely small, and polarization maintaining that can operate with both X polarization and Y polarization is performed. An optical fiber coupler can be obtained. Further, if such a polarization maintaining optical fiber coupler is applied to an optical communication system or the like, the stability of the entire system can be improved. Furthermore, since the basic manufacturing method of the polarization maintaining optical fiber coupler of the present invention is the same as that of the conventional single mode optical fiber coupler, it has high productivity and high reliability. Further, since one of the optical fibers constituting the polarization maintaining optical fiber coupler of the present invention can be an inexpensive single mode optical fiber, the cost can be reduced.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a first embodiment of a polarization maintaining optical fiber coupler of the present invention.
FIG. 2 is a schematic configuration diagram illustrating a method of manufacturing the polarization maintaining optical fiber coupler according to the first embodiment.
FIG. 3 is a schematic configuration diagram illustrating a polarization maintaining optical fiber coupler according to a second embodiment of the present invention.
FIG. 4 is a graph showing the results of investigating the temperature dependence and the polarization dependence of the insertion loss using a 1550 nm band 50% polarization maintaining optical fiber coupler as an example of the polarization maintaining optical fiber coupler of the present invention. is there.
FIG. 5 is a graph showing the results of measuring polarization crosstalk of a polarization maintaining optical fiber coupler manufactured by changing the length of a single mode optical fiber.
FIG. 6 is a sectional view showing an example of a PANDA type optical fiber.
FIG. 7 is a perspective view showing an example of a conventional polarization maintaining optical fiber coupler.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... PANDA type optical fiber, 20 ... Composite optical fiber, 21 ... Single-mode optical fiber part, 23,28 ... Fused extension part, 30,40 ... Polarization maintaining optical fiber coupler

Claims (7)

In a polarization-maintaining optical fiber coupler in which two optical fibers are arranged in parallel and a fusion-stretched portion formed by fusion-stretching a part of the length in the longitudinal direction is provided,
At least one of the two optical fibers has a single-mode optical fiber interposed in a part of the polarization-maintaining optical fiber, and has an optical axis of the polarization-maintaining optical fiber opposed via the single-mode optical fiber. Are matched composite optical fibers,
A polarization maintaining optical fiber coupler, wherein the fusion-splicing portion is formed by fusion-stretching a single-mode optical fiber. The polarization maintaining optical fiber coupler according to claim 1,
A polarization maintaining optical fiber coupler, wherein one of the two optical fibers is a single mode optical fiber. The polarization maintaining optical fiber coupler according to claim 1 or 2,
The difference between the mode field diameter of the single mode optical fiber interposed in a part of the polarization maintaining optical fiber and the mode field diameter of the polarization maintaining optical fiber connected to the single mode optical fiber is 2.0 μm or less. A polarization maintaining optical fiber coupler, wherein a connection loss per connection portion between the single mode optical fiber and the polarization maintaining optical fiber is 0.2 dB or less. The polarization maintaining optical fiber coupler according to any one of claims 1 to 3,
A polarization maintaining optical fiber coupler, wherein a length of a single mode optical fiber interposed in a part of the polarization maintaining optical fiber is 50 mm or less. The polarization maintaining optical fiber coupler according to any one of claims 1 to 4,
A polarization maintaining optical fiber coupler, wherein the fusion extending portion is formed by fusion extending two single mode optical fibers having different propagation constants. The polarization maintaining optical fiber coupler according to any one of claims 1 to 5,
A polarization-maintaining optical fiber coupler, wherein at least one of the two single-mode optical fibers constituting the fusion-spread portion is etched. The polarization maintaining optical fiber coupler according to any one of claims 1 to 6,
A polarization maintaining optical fiber coupler, wherein a portion other than the fusion-stretched portion is fixed on a reinforcing member with an adhesive.

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