JP2001051150A - Manufacture of polarization retaining optical fiber coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization retaining optical fiber coupler having a drawing length shorter than the conventional ones and high polarization dependency of coupling degree. SOLUTION: This manufacturing method comprises juxtaposing two polarization retaining optical fibers, heating a longitudinal part thereof, and drawing them in this longitudinal direction to form a fused drawn part. At this time, the drawing is ended at the time when the periods of coupling degree change of two polarizations accompanied by the drawing are both within two periods to manufacture a polarization retaining optical fiber coupler with the coupling degree of one polarization <=10%, and the coupling degree of the other polarization >=90%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for combining and branching light while maintaining the polarization state of light in an optical fiber, which is useful in the field of optical fiber communication, the field of sensors using an optical fiber, and the like. And a polarization maintaining optical fiber coupler.

[0002]

2. Description of the Related Art An optical mode is composed of X-polarized light and Y-polarized light whose electric field directions are orthogonal to each other. A device that can combine and split these polarized waves is called a polarization beam splitter (hereinafter abbreviated as PBS). PBS is
For example, it is useful for merging or splitting light from an optical fiber gyro that measures angular acceleration using light interference or a light source having linear polarization. In order to realize the characteristics as a PBS, it is necessary to have different coupling characteristics between X polarization and Y polarization.

As such an optical device, a polarization maintaining optical fiber coupler using a polarization maintaining optical fiber has been proposed. Various types of polarization maintaining optical fibers have been proposed, but a typical example is a PANDA type optical fiber (Polalization maintaining AND Absorption reducedfiber).
iber) is known.

FIG. 12 shows an example of a PANDA type optical fiber. This PANDA type optical fiber 10 has a core 11 provided at the center, and a core 11 provided around the core 11 and concentrically with the core 11. A cladding 12 having a lower refractive index than the core 11 and two stress applying portions symmetrically disposed inside the cladding 12 about the core 11 and having a lower refractive index than the cladding 12. 13 and 13. In this example,
The core 11 is made of germanium-doped quartz glass and the clad 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.
Outer diameter of core 11, outer diameter of stress applying section 13, relative refractive index difference between core 11 and cladding 12, cladding 12 and stress applying section 1
The relative refractive index difference of 3 is appropriately set depending on 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 in the core 11, and as a result, the polarization degenerates and X
The polarization propagation constant and the Y polarization propagation constant have different values,
Naturally, the distribution of the electromagnetic fields of these polarizations is also different. As a result, a characteristic of propagating in a state where the X polarization and the Y polarization are preserved is obtained.

FIG. 13 shows an example of a polarization maintaining optical fiber coupler.
Has two PANDA-type optical fibers 10 and 10 arranged in parallel such that their polarization axes are parallel to each other.
Claddings 12, 12 in the middle of the A-type optical fibers 10, 10
Are brought into contact with each other, heated and melted, and stretched in the longitudinal direction to form a fusion-stretched portion (optical coupling portion) 3. Note that the polarization axis is defined by each PANDA
In the optical fiber 10, it refers to a line passing through the centers of the stress applying portions 13, 13. In this polarization maintaining optical fiber coupler, the X polarization propagates while maintaining the electric field vector in the polarization axis direction of the PANDA type optical fibers 10 and 10, and the Y polarization retains the electric field vector orthogonal to this. The light propagates through the PANDA type optical fibers 10 and 10. Then, the X-polarized wave and the Y-polarized wave are merged and branched in the fusion stretching section 3 on the way.

In the conventional polarization maintaining optical fiber coupler, when forming the fusion-spreading section 3, an optical fiber (PAND) is used.
By extending the length of the A-type optical fiber 10), that is, by increasing the extension length, a difference between the coupling degree of the X polarization and the coupling degree of the Y polarization can be realized, and characteristics as a PBS can be imparted. . FIG. 14A is a graph showing the relationship between the stretching length and the degree of coupling of light of the used wavelength. The broken line indicates the coupling characteristic of X polarization, and the solid line indicates the coupling characteristic of Y polarization. In the production of the fusion-spread portion of the conventional polarization maintaining optical fiber coupler, X
Both the polarization and the Y-polarization are held by one polarization-maintaining optical fiber (first
After the optical fiber is coupled to the other polarization maintaining optical fiber (the second optical fiber), the respective polarizations are transferred (coupled) to the first optical fiber again by further extending the drawing, Further, the operation of shifting to the second optical fiber is repeated. In forming the fusion-spreading section 3 using a normal polarization-maintaining optical fiber, the degree of coupling between the Y polarization and the X polarization is small because the coupling of the Y polarization is slightly smaller than the coupling of the X polarization. There is a slight difference in the period of change (transition period). Here, for the sake of convenience, the degree of coupling first increases from 0% to 100%, then changes in the degree of coupling decreases to 0% for one cycle, the degree of coupling increases again to 100%, and further increases to 0%. The change to% is counted as two cycles. Then, when the stretching length becomes long, and this period changes from several periods to several tens, the difference in the coupling degree between the X polarization and the Y polarization becomes large. Then, when the fusion-stretched portion 3 is formed by stretching to the vicinity where the difference in the degree of bonding indicated by the arrow in the graph becomes large, FIG.
As shown in the figure, when the X-polarized light and the Y-polarized light of the used wavelength are input from the input port composed of the same fiber as the output port A, the X-polarized light is output from the output port A. From the port B, characteristics as a PBS that outputs Y-polarized light are obtained.

[0008]

However, in the conventional polarization-maintaining optical fiber coupler, there is a problem that the element length becomes long in order to join and branch the X-polarized light and the Y-polarized light. For example, when a polarization maintaining optical fiber having an outer diameter of 125 μm is used, the stretching length is 60 mm or more, sometimes 1 mm.
It may be about 00 mm. As a result, the fusion-stretched portion becomes extremely thin, the mechanical strength decreases, and reinforcement is required. However, when the reinforcing material is brought into contact with the fusion-bonded stretched portion, the optical characteristics change, so that the reinforcing is difficult. Also,
There is a problem that the wavelength band where X-polarized light and Y-polarized light can be merged and branched is extremely narrow, for example, about 10 nm.

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 shorter extension length than the conventional one and having a large degree of polarization dependence of the degree of coupling. It is another object of the present invention to provide a polarization maintaining optical fiber coupler capable of improving mechanical strength.
Still another object of the present invention is to provide a polarization maintaining optical fiber coupler having a polarization dependency that can be used in a wide wavelength band.

[0010]

In order to solve the above-mentioned problems, a method of manufacturing a polarization maintaining optical fiber coupler according to the present invention comprises:
A method for manufacturing a polarization-maintaining optical fiber coupler in which two polarization-maintaining optical fibers are arranged in parallel, a part of the length of the optical fiber is heated, and is stretched in the length direction to form a fusion-stretched portion.
At the wavelength used, the stretching is completed when the change in the coupling degree of the two polarizations according to the extension length is within two cycles, and when the coupling degree of one polarization is 10% or less and the other polarization is less than 10%. A polarization maintaining optical fiber coupler having a degree of wave coupling of 90% or more is manufactured. In this manufacturing method, by forming the fusion-stretched portion while keeping the cores of the two polarization-maintaining optical fibers as close to each other as possible, two points in the polarization-maintaining optical fiber from the time when light coupling occurs. It is preferable to increase the difference in the degree of polarization coupling. The coupling degree of one polarization is 10% or less and the coupling degree of the other polarization is 90%.
It is preferable to manufacture a polarization maintaining optical fiber coupler having a wavelength band in which the range of 30% or more is maintained is 30 nm or more. Furthermore, the core has a stress applying portion disposed symmetrically with respect to the core in the clad surrounding the core, is a concentric circle of the core, does not cover the stress applying portion, and does not include the stress applying portion therein. It is preferable to manufacture a polarization maintaining optical fiber coupler using a polarization maintaining optical fiber in which the diameter of the largest circle is 20 μm or more. Further, the diameter is more preferably 25 to 30 μm. The diameter is 20 μm.
The birefringence of the polarization-maintaining optical fiber of m or more is preferably 5 × 10 ^{−5 to} 5 × 10 ⁻⁴ . The polarization crosstalk of this polarization maintaining optical fiber is preferably -20 dB.
/ Km or more. Further, the loss of the polarization maintaining optical fiber is preferably 1 dB / km or more. When manufacturing a polarization maintaining optical fiber coupler using this polarization maintaining optical fiber, the length of the lead fiber is set to 10 m.
The following is preferred. In the present invention, it is preferable to use a PANDA type polarization maintaining optical fiber as the polarization maintaining optical fiber.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a graph showing the relationship between the extension length at the time of forming a fusion-stretched portion and the degree of coupling of light having a wavelength of 1550 nm (used wavelength) in the first embodiment.

The polarization maintaining optical fiber used in this embodiment is the following PANDA type optical fiber.

(Characteristics of PANDA type optical fiber) Core diameter (core radius) 6.5 μm (3.25 μm) Cladding diameter 125 μm Relative refractive index difference between core claddings 0.35% Outer diameter of stress applying part 35 μm Stress applying part 55 μm Diameter A 20 μm Working wavelength 1550 nm Mode birefringence at working wavelength 4 × 10 ^-4

The minimum diameter of the fused stretched portion of the obtained polarization maintaining optical fiber coupler is 61 μm, the same shape (aspect ratio) is 1.89, and the stretch length is 17.8 mm. The same shape (aspect ratio) is the ratio between the maximum outer diameter and the minimum outer diameter at the center of the fusion-stretched portion (maximum outer diameter / minimum outer diameter). Table 1 shows the characteristics of this polarization maintaining optical fiber coupler.

[0015]

[Table 1]

As described above, in forming the fusion-bonded stretched portion, as the stretching length becomes longer, the coupling degree of each of the X-polarized wave and the Y-polarized wave increases from 0% to 100%, and becomes 0% again. % Change is repeated. In the present invention, the fusion-stretched portion is formed by heating and melting the two polarization-maintaining optical fibers so that the cores of the two polarization-maintaining optical fibers do not approach each other (so that the distance between the core centers does not become too small). Accordingly, a large coupling degree difference can be generated between the X polarization and the Y polarization from the time when the coupling starts to increase. As a result, as shown in FIG. 1, with respect to the Y polarization, the extension is terminated when the coupling degree reaches 100% for the first time (１ ／ cycle) and the X polarization is stopped. By terminating the drawing when the degree of coupling hardly increases, for example, only the Y-polarized light is mainly coupled from one polarization-maintaining optical fiber to the other polarization-maintaining optical fiber, and X
The fusion-stretched portion can be formed in a state where the polarized waves hardly couple.

Then, as described above, the 2
By adjusting the proximity of the two cores, it is possible to realize a sufficient difference in the degree of coupling between the X polarization and the Y polarization in a wide wavelength range, as shown in FIG. In this example, the range where the degree of coupling of the Y polarization is 90% or more and the degree of coupling of the X polarization is 10% or less is 58 nm, which is very wide. In addition, since the degree of coupling of X polarization is slightly increased on the long wavelength side, the wavelength dependence of X polarization is smaller on the short wavelength side.

In a specific operation for manufacturing the fusion-spreading section, heating conditions are set so that the outer shape of the optical fiber is easily maintained so that the distance between the centers of the cores of the two polarization-maintaining optical fibers does not become too close. It is preferable. For example, a method of setting the temperature of a heating source such as a burner lower than before and increasing the distance from the heating source can be exemplified. In addition, a method in which stretching is performed at high speed and the amount of heat per unit time is reduced can be exemplified. Actually, light of the used wavelength is input from the input side port, and heating and stretching are performed while monitoring the light output from the two output side ports. When a desired degree of coupling is obtained, the operation is completed.

FIG. 3 is a graph showing the relationship between the extension length at the time of forming the fusion-bonded extension portion and the degree of coupling of light having a wavelength of 1550 nm (used wavelength) in the second embodiment. In the second embodiment as well, as in the first embodiment, only two
By heating and melting the core of the polarization maintaining optical fiber so as not to be too close to each other, a fusion-stretched portion is formed.In the fusion-stretched portion, the distance between the centers of the two cores is maintained, and the fusion is performed. From the time when the degree starts to increase, a large coupling degree difference is generated between the X polarization and the Y polarization. In the second embodiment, as shown in FIG. 3, with respect to the Y polarization, the degree of coupling increases once to 100% and then decreases to 0% (1). (Cycle). On the other hand, for the X-polarized wave, the stretching is completed when the degree of coupling first reaches 100% (（cycle).

As a result, also in the second embodiment,
As shown in FIG. 4, it is possible to realize a difference in the coupling degree between the X polarization and the Y polarization in a wide wavelength range. The minimum diameter of the fused extension of this polarization maintaining optical fiber coupler is 41 μm, the same shape (aspect ratio) is 1.98, and the extension length is 24.2 mm. Table 2 shows the characteristics of this polarization maintaining optical fiber coupler.

[0021]

[Table 2]

In this example, the coupling degree of X polarization is 90%
The range in which the degree of coupling of the Y-polarized light is 10% or less is 35 nm, which can be three times or more as compared with about 10 nm in the related art.

Next, a polarization maintaining optical fiber suitable for the present invention will be described. FIG. 5 is a sectional view showing an example of a polarization maintaining optical fiber suitable for the present invention. The polarization maintaining optical fiber of this example is a PANDA type optical fiber. The feature of the PANDA type optical fiber 10 is that
13 is a large distance. This distance is core 1
1 or the concentric circle of the cladding 12, and the stress applying portions 13, 1
3 and is determined based on the diameter A of the largest circle 15 that does not include the stress applying portions 13 and 13 therein. Diameter A is 20 μm or more, preferably 25 to 30 μm
m. When a polarization maintaining optical fiber coupler as shown in FIG. 13 is formed by using the PANDA type optical fiber 10, even if light leaks out of the core 11 in the fusion-spreading section 3, the light is transmitted at a normal operating wavelength. When the light is used, most of the light is located between the stress applying sections 13 and 13 and does not reach the stress applying section 13. Therefore, an optical signal (a mode propagating in the core 10: propagating light) is less likely to be coupled to a higher-order mode, and an increase in excess loss can be suppressed.
When the diameter A is less than 20 μm, excess loss tends to increase. If it exceeds 30 μm, the difference between the propagation constant of X-polarization and the propagation constant of Y-polarization decreases, crosstalk between X-polarization and Y-polarization (polarization crosstalk) deteriorates, and X-polarization and Y-polarization deteriorate.
The state of preserving the polarization may decrease.

The diameter A of the polarization maintaining optical fiber for ordinary communication or the like is about 12 to 17 μm. On the other hand, when the stress applying unit 13 is separated as described above, the stress applied to the core 11 by the stress applying unit 13 decreases, and the birefringence decreases as compared with a normal polarization maintaining optical fiber. Also,
Crosstalk between fast axis (Y polarization axis) and slow axis (X polarization axis) (crosstalk between X polarization and Y polarization)
Tends to deteriorate. Further, the loss may be slightly larger. However, since the length of the fiber used for the polarization maintaining optical fiber coupler is short, even if the conditions of the birefringence, crosstalk and loss of the polarization maintaining optical fiber itself are relaxed as compared with ordinary ones for communication and the like, There is no particular problem in use.

Specifically, the birefringence of the polarization maintaining optical fiber suitable for the present invention is in the range of 5 × 10 ^{-5 to} 5 × 10 ^-4 . The birefringence of a polarization maintaining optical fiber for ordinary communication or the like is about 5 × 10 ⁻⁴ . The crosstalk per unit length is -20 dB / km or more, and is substantially in the range of -20 to -10 dB / km. The crosstalk of a normal polarization maintaining optical fiber is −25 dB / k.
m. The loss per unit length is 1 dB /
km or more. It is substantially 1 to 10 dB / km. Incidentally, the loss of the ordinary polarization maintaining optical fiber is 0.2
It is about 0.3 dB / km.

The lead fiber of the polarization maintaining optical fiber coupler using the polarization maintaining optical fiber is preferably 10 m or less. Substantially 0.5 to 10 m. The lead fiber is, for example, a polarization maintaining optical fiber (PANDA type optical fiber) 10 extending from both ends of the fusion-spreading section 3 two by two and constituting an input / output port as shown in FIG. is there. If the length of the lead fiber is too long, crosstalk and loss of the optical signal when passing through the polarization maintaining optical fiber coupler increase.

The core 11, the clad 12, and the stress applying portion 13 are made of, for example, the same material as that of the related art. The outer diameter of the stress applying section 13, the relative refractive index difference between the core 11 and the cladding 12, and the relative refractive index difference between the cladding 12 and the stress applying section 13 are appropriately set according to desired characteristics. Normally, the mode field diameter of the core 11 varies depending on the diameter of the core 11, the used wavelength, and the like, but is about 4 to 10 μm. The outer diameter of the clad 12 is set to about 125 μm.

FIG. 6 is a graph showing the relationship between the stretching length at the time of forming the fusion-bonded stretching portion and the degree of coupling of light having a wavelength of 980 nm (used wavelength) in the third embodiment. In this embodiment, for Y polarization, 1
Stretching is completed when 00% is reached (１ ／ cycle), and X
For the polarized wave, the stretching is completed when the degree of coupling hardly increases. FIG. 7 shows changes in excess loss of X polarization and Y polarization with increase in extension length, respectively. The excess loss of X polarization hardly changed and the excess loss of Y polarization increased once. It can be seen that it decreases later and approaches zero. Then, the point that the degree of coupling of the Y polarization is sufficiently increased,
The point at which the excess loss of the polarization is close to zero coincides with the point at which the X-polarization and the Y-polarization are stopped.
The characteristic that the difference in the degree of polarization coupling is large and the excess loss is small can be realized. Also, in the third embodiment, as shown in FIG.
The difference in the degree of coupling between the polarized wave and the Y polarized wave can be realized. The minimum diameter of the fusion-stretched part is 58 μm, same shape (aspect ratio)
Is 1.92 and the stretching length is 22 mm. Table 3 shows the characteristics of the polarization maintaining optical fiber coupler.

[0029]

[Table 3]

On the other hand, FIG. 9 shows a drawing length and a wavelength when a polarization maintaining optical fiber coupler is manufactured in the same manner as in the third embodiment using an ordinary PANDA type optical fiber having a small diameter A as described below. It is the graph which showed the relationship of the coupling degree of 980 nm (used wavelength). FIG. 10 shows the stretching length and X at this time.
5 is a graph showing a relationship between polarization and excess loss of Y polarization.

(Characteristics of PANDA type optical fiber) Core diameter (core radius) 6.5 μm (3.25 μm) Cladding diameter 125 μm Relative refractive index difference between core claddings 0.35% Outer diameter of stress applying part 35 μm Stress applying part 51 μm Diameter A 16 μm Working wavelength 980 nm Mode birefringence at working wavelength 5 × 10 ^-4

As is clear from comparison with FIGS. 6 and 7,
In FIG. 10, when the extension length is increased, the excess loss of the X polarization hardly changes, but the excess loss of the Y polarization largely increases and then decreases, but increases again before reaching near zero. Then, as can be seen from FIG. 9, the coupling degree repeatedly increases and decreases under the influence of the fluctuation of the excess loss. Therefore, since the excess loss of the Y-polarized light cannot be reduced to near zero, the characteristics are inferior to the polarization-maintaining optical fiber couplers of the first to third embodiments regardless of the conditions under which the extension is stopped. I can't deny that. Further, if the stretching is stopped at the time when the degree of coupling of the Y polarization is large and the excess loss is small, a practically usable material can be obtained to some extent. However, the range of the stretching length satisfying these conditions is narrow, and the productivity is low. Often low. It should be noted that the effect of the present invention can be obtained to some extent by using a normal polarization maintaining optical fiber as in this example, depending on conditions such as the wavelength used.

As described above, in the present invention, a large difference in the degree of coupling between the X-polarized light and the Y-polarized light from the time when the degree of coupling starts to increase (when light coupling occurs) in the formation of the fusion-bonded stretched portion. Therefore, the following preferable range of the coupling degree in the polarization-maintaining optical fiber coupler can be realized in a range in which the change period of the coupling degree of each polarization is within two cycles. In the polarization-maintaining optical fiber coupler manufactured by the manufacturing method of the present invention, at the wavelength used, the degree of coupling to one polarization is 10% or less, the degree of coupling to the other polarization is at least 90% or more, and Preferably, the wavelength band in which the degree of coupling is maintained is at least 30 nm or more. By realizing such a range of the coupling degree, excellent characteristics as a PBS can be obtained.
Further, by using a polarization maintaining optical fiber having a diameter A of 20 μm or more, a polarization maintaining optical fiber coupler with a small excess loss can be provided.

As described above, the range of the degree of coupling is such that the light of the used wavelength is incident on one of the polarization maintaining optical fibers at the time of producing the fusion-stretched portion, and the degree of coupling of the two polarizations is monitored.
The setting can be made by ending the operation when the desired characteristics are obtained. As shown in the graphs in FIGS. 2, 4, 6, and 9, in the present invention, a large coupling difference occurs between the X polarization and the Y polarization from the time when the coupling starts to increase. , The period of change of the coupling degree of the two polarizations is 2
By realizing the difference in the degree of coupling between the X-polarized light and the Y-polarized light within the period, the extension length does not increase, and the wavelength band in which the above-described range of the degree of coupling can be maintained can be 30 nm or more. In particular, the realization of the characteristics as a PBS in such a wide wavelength band cannot be achieved by the conventional technology.

If the degree of coupling of the two polarized waves is outside the above range, it becomes difficult to join and branch the X polarized waves and the Y polarized waves. Further, if the wavelength band is narrower than 30 nm, the wavelength dependence of the degree of polarization coupling is increased, and the wavelength used is limited. The wavelength used is preferably in the range of 0.6 to 1.7 μm, which is a wavelength band in which a polarization maintaining optical fiber coupler is usually used. Further, it is preferable that the wavelength band is also within this range.

Although the above-described embodiment uses a PANDA type optical fiber, the present invention is not limited to this, and a polarization maintaining optical fiber such as a bow tie fiber or an elliptical jacket fiber can be used. However, FIG.
As shown in the cross-sectional view shown in FIG. 1, it is preferable that portions other than the cladding 12 such as the stress applying portions 13 are not located as much as possible between the cores 11, because the loss due to the absorption of the stress applying portions 13 is small. Most desirably, the two polarization axes are parallel as shown in this sectional view.

As described above, in the present invention, it is possible to obtain a polarization maintaining optical fiber coupler having a short extension length and a large degree of polarization dependence of the coupling degree. Therefore, it is effective to prepare PBS. Further, since the stretching length is short, the mechanical strength can be improved. In addition, the number of times (the number of transitions) that the X polarization or the Y polarization couples from one polarization maintaining optical fiber to the other polarization maintaining optical fiber can be reduced, resulting in low loss. Furthermore, 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. For this reason, for example, it is possible to provide a PBS that is useful for manufacturing an optical circuit that inputs light of multiple wavelengths and simultaneously performs polarization separation or polarization synthesis.

By the way, the powers PA (Z) and PB (Z) of the propagating light at the position Z in the length direction of each of the two optical fibers in the optical fiber coupler are expressed by the following equation (1).

[0039]

(Equation 1)

Here, if the core diameters of the two optical fibers and the relative refractive index difference between the core claddings are equal, β1 = β
2 and δ = 0 and F = 1, so equation (1) is simplified as equation (2) below.

[0041]

(Equation 2)

In the polarization maintaining optical fiber, equation (2) holds for each of the X polarization and the Y polarization. At this time, if the coupling coefficient κ does not depend on the polarization depending on the polarization direction, the polarization-dependent coupling characteristic cannot be obtained at a predetermined wavelength. FIG. 11 (a) is a graph showing that κ depends on the structure of the optical fiber (Reference: Corona Publishing Co., Ltd., Photonics Series, “Basics of Optical Waveguides” by Osamu Okamoto)
p151). As shown in FIG. 11B, D of the horizontal axis D / a is the minimum distance between the centers of the two cores A and B in the fusion-stretched portion, and a is the common radius of the cores A and B. It is. The vertical axis is the normalized light coupling coefficient. V shown in the graph is the normalized frequency of the core of the optical fiber, and is represented by the following equation (3).

[0043]

(Equation 3)

Δ in the equation (3) is represented by the following equation (4).

[0045]

(Equation 4)

This graph shows a case where the normalized frequencies V of the two optical fibers are equal for simplicity. FIG. 11A shows that the coupling coefficient κ greatly varies depending on the normalized frequency V. Further, the normalized frequency V must be a value that guarantees single-mode propagation in the optical fiber constituting the optical fiber coupler. In an optical fiber having a step-type refractive index distribution, a single mode condition is guaranteed when V <= 2.405 is satisfied. In polarization-maintaining optical fibers, single mode conditions are considered for each polarization.

In a polarization maintaining optical fiber coupler, the coupling between X polarizations and the coupling between Y polarizations between two cores is considered. As shown in FIG. 13, when the polarization axes of the two polarization-maintaining optical fibers (PANDA-type optical fibers) 10, 10 are parallel, the coupling between the X-polarized light and the Y-polarized light is theoretically possible. (Polarization crosstalk) need not be considered.

According to the graph shown in FIG. 11A, when the distance between the centers of the cores is large to some extent, the normalized frequency of the X polarization and the normalized frequency of the Y polarization are different in each core. Then, it can be seen that the difference of the coupling coefficient κ between X polarization and Y polarization becomes large. In a normal polarization-maintaining optical fiber, the optical characteristics of X-polarized light and Y-polarized light are slightly different to the extent that they can be distinguished.

For example, when D / a is 12, the normalized frequency VX of X polarization is 1.6, and the normalized frequency VY of Y polarization is 1.4.
At this time, the coupling coefficient of the X polarization takes a value that is about 10 times the coupling coefficient of the Y polarization. At this time, with respect to the Y polarization, the equation (2)
Length L (the length of the fusion-stretched portion) at which the κz becomes π / 2
On the other hand, the product of the Y polarization coupling coefficient κY and L is as follows.

[0050]

(Equation 5)

The product of the X polarization coupling coefficient κX and L is as follows.

[0052]

(Equation 6)

Then, as shown in FIG. 14B, when the X-polarized light and the Y-polarized light are input to the input-side port composed of the same optical fiber as the output-side port A, the Y-polarized light is applied to the port B. 100% binding. On the other hand, the power of the input X polarization is 1
In this case, the ratio of the power of the X-polarized light output from the port B is as follows.

[0054]

(Equation 7)

Therefore, 98% of the X polarization is output from port A, and 100% of the Y polarization is output from port B. That is, the characteristics as a PBS can be obtained.

[0056]

As described above, according to the present invention, the following effects can be obtained. A polarization maintaining optical fiber coupler having a large degree of polarization dependence of the coupling degree with a short extension length can be obtained. Therefore, it is effective to prepare PBS. Further, since the stretching length is short, a polarization maintaining optical fiber coupler having high mechanical strength can be obtained. In addition, the number of times (the number of transitions) that the X polarization or the Y polarization couples from one polarization maintaining optical fiber to the other polarization maintaining optical fiber can be reduced, resulting in low loss. Furthermore, 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. For this reason, it is possible to provide a PBS useful for manufacturing an optical circuit for, for example, simultaneously separating and polarization-multiplexing light of multiple wavelengths. Also, by using a polarization maintaining optical fiber having a large diameter A,
A polarization maintaining optical fiber coupler with a small excess loss can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the extension length and the coupling degree of a polarization maintaining optical fiber coupler according to a first embodiment.

FIG. 2 is a graph showing the relationship between the wavelength and the coupling degree of the polarization maintaining optical fiber coupler of the first embodiment.

FIG. 3 is a graph showing the relationship between the extension length and the coupling degree of the polarization maintaining optical fiber coupler of the second embodiment.

FIG. 4 is a graph showing the relationship between the wavelength and the coupling degree of the polarization maintaining optical fiber coupler of the second embodiment.

FIG. 5 is a cross-sectional view showing an example of a polarization maintaining optical fiber suitable for the present invention.

FIG. 6 is a graph showing the relationship between the extension length and the coupling degree of the polarization maintaining optical fiber coupler of the third embodiment.

FIG. 7 is a graph showing the relationship between the extension length and the excess loss of the polarization maintaining optical fiber coupler of the third embodiment.

FIG. 8 is a graph showing the relationship between the wavelength and the degree of coupling of the polarization maintaining optical fiber coupler of the third embodiment.

FIG. 9 is a graph showing the relationship between the extension length of a polarization maintaining optical fiber coupler and excess loss when a normal PANDA type optical fiber is used.

FIG. 10 is a graph showing the relationship between the extension length of a polarization maintaining optical fiber coupler and excess loss when a normal PANDA type optical fiber is used.

FIG. 11A is a graph showing the relationship between the center-to-center distance between two cores normalized by the core radius, the normalized coupling coefficient, and the normalized frequency;
(B) is an explanatory diagram of values on the horizontal axis of the graph.

FIG. 12 is a sectional view showing an example of a PANDA type optical fiber.

FIG. 13 is an explanatory diagram showing an example of a polarization maintaining optical fiber coupler.

FIG. 14A is a graph showing the relationship between the extension length and the degree of coupling, and FIG. 14B is an explanatory diagram showing the operation of the polarization maintaining optical fiber coupler.

[Explanation of symbols]

Reference numeral 3 denotes a fusion-stretched portion; 10 a PANDA type optical fiber (polarization maintaining optical fiber);

Continued on the front page (72) Inventor Kei Hidaka ▲ Sight ▼ 1440 Murosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Office (72) Inventor Kenji Nishiide 1440 Murosaki, Sakura City, Chiba Prefecture Fujikura Sakura Business In-house (72) Inventor Shigefumi Yamazaki 1440 Mutsuzaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant (72) Inventor Ryoyoshi Matsumoto 1440 Mutsuzaki Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant (72) Inventor Suzuki Yoji 1440, Rokuzaki, Sakura-shi, Chiba F-term in Fujikura Sakura Works (reference) 2H050 AA02 AB03Z AB05X AC44 AC83

Claims (10)

[Claims] 1. Two polarization maintaining optical fibers are arranged in parallel,
In a method for manufacturing a polarization-maintaining optical fiber coupler, which heats a part of its length direction and stretches in this length direction to form a fusion-stretched portion, at the wavelength used, two polarizations associated with the stretching length are produced. Polarization-maintaining light having a coupling degree of less than or equal to 10% and a coupling degree of the other polarization of 90% or more when the extension of the coupling is completed within a period of less than two in each case. A method for manufacturing a polarization maintaining optical fiber coupler, comprising manufacturing a fiber coupler. 2. The method for manufacturing a polarization-maintaining optical fiber coupler according to claim 1, wherein the optical fiber is formed by forming a fusion-spread portion so that the cores of the two polarization-maintaining optical fibers are as close as possible. Wherein the difference between the degrees of coupling of the two polarizations is increased from the point in time when the coupling occurs. 3. The method for manufacturing a polarization-maintaining optical fiber coupler according to claim 1, wherein the coupling degree of one polarization is 10% or less and the coupling degree of the other polarization is 90% or more. A method for manufacturing a polarization-maintaining optical fiber coupler, comprising: manufacturing a polarization-maintaining optical fiber coupler having a wavelength band of 30 nm or more in which is maintained. 4. The method for manufacturing a polarization-maintaining optical fiber coupler according to claim 1, wherein the polarization-maintaining optical fiber is disposed symmetrically with respect to the core in a cladding surrounding the core. The diameter of the largest circle among the circles, which are concentric circles of the core and do not cover the stress applying section and do not include the stress applying section, have a diameter of 20 μm.
m or more, and a method for producing a polarization maintaining optical fiber coupler. 5. The method of claim 4, wherein the diameter is 25 to 30 μm. 6. The method for producing a polarization maintaining optical fiber coupler according to claim 4, wherein the polarization maintaining optical fiber has a birefringence of 5 × 10 ^{-5 to} 5 × 10 ^-4. Of manufacturing a polarization maintaining optical fiber coupler. 7. The method for manufacturing a polarization-maintaining optical fiber coupler according to claim 4, wherein a crosstalk of the polarization-maintaining optical fiber is -20 dB / km or more. A method for manufacturing a polarization maintaining optical fiber coupler. 8. The method for manufacturing a polarization maintaining optical fiber coupler according to claim 4, wherein a loss of the polarization maintaining optical fiber is 1 dB / km or more. A method for manufacturing a holding optical fiber coupler. 9. The method for manufacturing a polarization maintaining optical fiber coupler according to claim 4, wherein the length of the lead fiber of the polarization maintaining optical fiber coupler is 10 m or less. Of manufacturing a polarization maintaining optical fiber coupler. 10. The method of manufacturing a polarization maintaining optical fiber coupler according to claim 1, wherein the polarization maintaining optical fiber is a PANDA type polarization maintaining optical fiber. A method for manufacturing a wave holding optical fiber coupler.