JPH0830774B2 - Elliptical jacket type polarization-maintaining optical fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to an elliptical jacket type polarization-maintaining optical fiber suitable for use as an optical fiber polarizer.

[Prior Art] In order to construct a polarizer with a polarization-maintaining optical fiber, a fiber (absolute single-polarization operation) that exhibits an absolute single-polarization operation in which one of two eigen-polarization modes orthogonal to each other is cut off It is desirable to use one polarization maintaining fiber.
In this regard, the present applicant has filed Japanese Patent Application No. 61-124663,
In Japanese Patent Application No. 61-124664 and Japanese Patent Application No. 61-203751, FIGS. 5 (a) to 5 (c), 6 (a) to 6 (c) and 7 (a), respectively. (C) has proposed an absolute single polarization maintaining fiber.

The optical fiber 61 shown in FIGS. 5A to 5C has a MC (matched cladding) type refractive index distribution on the minor axis side of the elliptical jacket 62 and a W type refractive index distribution on the major axis side. These are formed respectively, and the fact that the cutoff wavelength of the fundamental mode does not exist in the MC type but exists in the W type enables absolute single polarization operation.

On the other hand, the optical fiber 71 shown in FIGS. 6A to 6C has a W-shaped refractive index distribution on both the short axis side and the long axis side of the elliptical jacket 72, but the groove depths are different from each other. It has a. Since the single-mode operating range is determined by the groove depth in W-type fiber, the single-mode operating range of the modes polarized on the minor axis side and the major axis side is determined by making the groove depth different. However, there is an operating region in which one is a low-loss guided mode and the other is a high-loss leaky mode, and this is utilized to enable absolute single polarization operation.

However, these FIG. 5 (a) to (c) and FIG.
In the optical fibers shown in FIGS. (A) to (c), it is necessary to divide the refractive index distributions of the elliptical jackets 62 and 72 in the circumferential direction so that the respective refractive indexes in the minor axis direction and the major axis direction are different. Therefore, there is a problem that the manufacturing is difficult and the yield of obtaining an optical fiber as designed is low.

The optical fiber 81 shown in FIGS. 7 (a) to 7 (c) is an improvement of the problems of the optical fiber shown in FIG. 5 and FIG. In a polarization-maintaining optical fiber having a four-chamber structure in which a clad 2, an elliptical jacket 3 and a support 4 are sequentially provided, the elliptical jacket 3
Has a lower refractive index than the cladding 2 and has a support 4
Has an index of refraction equal to that of the clad 2, and the elliptical jacket 3 has an ellipticity of 25 to 40% and a clad 2 of −0.05 to
The structure has a relative refractive index of −0.03%. Since the elliptical jacket 3 has the same refractive index in the minor axis direction and the major axis direction, it has an advantage that it can be easily manufactured.

[Problems to be Solved by the Invention] However, the elliptical jacket type polarization-maintaining optical fibers of FIGS. 5, 6, and 7 described above try to obtain an optical fiber polarizer by applying appropriate bending. The structure as a polarizer was not optimized because it was not optimal for the application, i.e. the effect of modal birefringence was not taken into account, and therefore its properties, e.g. extinction ratio, insertion loss Etc. could not be said to be the best.

The present invention, by also considering the mode birefringence,
An object of the present invention is to provide an elliptical jacket type polarization-maintaining single-mode optical fiber that exhibits absolute single polarization operation suitable as a polarizer.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a polarization-maintaining optical fiber having a four-layer structure in which a core, a core, and a clad, an elliptical jacket, and a support are sequentially provided on an outer peripheral portion of the core. , The refractive index of the clad and the support is the same as that of silica, and the relative refractive index difference Δ + of the core to the clad is 0.3.
%, The relative refractive index difference Δ− of the elliptical jacket with respect to the cladding is −0.15 to −0.25%, and the mode birefringence B is 4.52 × 10.
^{-Beyond -4} , an elliptical jacket type polarization-maintaining single-mode fiber is constructed.

Further, what is useful in reducing the size of the polarizer is a four-layer polarization-maintaining optical fiber in which a clad, an elliptical jacket, and a support are sequentially provided around a core as a center, and a cladding is used. The refractive index of the support is the same as that of silica, the relative refractive index difference Δ + of the core to the clad is 0.3 to 0.4%, the relative refractive index difference Δ− of the elliptical jacket to the clad is −0.2 to −0.3%, and the mode birefringence B To make an elliptical jacket type polarization-maintaining single-mode fiber with a value of 6.3 × 10 ^-4 or more.

[Function] The relative refractive index difference Δ + of the core with respect to the cladding is 0.3% or less, and the relative refractive index difference Δ− of the elliptical jacket with respect to the cladding Δ−.
The elliptic jacket type polarization-maintaining optical fiber with the parameters of −0.15 to −0.25% and the mode birefringence B of 4.52 × 10 ⁻⁴ or more is not obtained by theoretical analysis by a computer considering the mode birefringence. However, in any case, the structure as a polarizer is optimized within these numerical ranges, and by winding the optical fiber with a certain radius, a polarizer with excellent extinction ratio and insertion loss can be obtained. You can

Also, the relative refractive index difference Δ + of the core to the clad is 0.3
~ 0.4%, the relative refractive index difference Δ- of the elliptical jacket to the cladding is -0.2 to -0.3%, and the mode birefringence B is 6.3 × 10.
Similarly, an elliptical jacket type polarization-maintaining single-mode optical fiber with a parameter of ^-4 or more is also analyzed by taking into account the mode birefringence and performing theoretical analysis so that the bending loss difference between the x polarization and the y polarization becomes large. The structure as is optimized.
In addition, this optical fiber has a smaller winding radius for obtaining a polarizer than in other cases, and a small-sized polarizer can be obtained.

[Embodiment] An embodiment of the present invention will be described below with reference to the accompanying drawings.

1 (a), (b), and (c) are cross-sectional views of an absolute single polarization-maintaining single-mode fiber according to an embodiment of the present invention, respectively.
FIG. 3 is a refractive index distribution diagram in the axial direction and a refractive index distribution diagram in the Y-axis direction. In the figure, 1 is a core made of SiO ₂ glass to which Ge is added, and a clad 2 made of pure SiO ₂ glass is provided on the outer peripheral portion of the core 1 so as to surround the core 1.
Further, an elliptical jacket 3 is provided on the outer peripheral portion of the clad 2 so as to surround the clad 2. The elliptical jacket 3 is made of SiO ₂ glass to which B ₂ O ₃ is added and has a refractive index lower than that of the cladding 2. Further, a support 4 made of pure SIO ₂ glass and having a refractive index equal to that of the cladding 2 is provided on the outer peripheral portion of the elliptical jacket 3 so as to surround it.

1 (a) (and (c), a is the radius of the core 1, b _c is the major axis radius of the elliptical jacket 3, b _t is the minor axis radius of the elliptical jacket 3, and the radius of the cladding 2 is the cladding. It is represented by (1 + δ) a, where δ is the ratio of the thickness to the core radius, and n (r) is the core 1, the clad 2, and the elliptical jacket 3.
And the refractive index of the support 4 is expressed as a function of the distance r from the center O. Δ + is the relative refractive index difference of the core 1 with respect to the cladding 2, and the relative refractive index difference of the elliptical jacket 3 with respect to the Δ− cladding 2.

The groove depth of the refractive index distribution n (r), that is, Δ− is the same on the minor axis side and the major axis side of the elliptical jacket 3, and the relative refractive index difference Δ− is −0.15 to −0.25% for the same silica. Determined within the range of. Further, the width of the groove having the refractive index distribution n (r) is determined so that the ellipticity ε of the elliptical jacket 3 is in the range of 25 to 40%.

Based on such a configuration, the optimum structure of the polarizer is theoretically analyzed by a computer by the finite element method while considering the effect of the mode birefringence B.

In this logic theory analysis, for convenience, the part up to the elliptical jacket 3 is treated as a core, and the normalized phase constant U in this effective core portion and the support 4 around it are treated as a normalized lateral constant. Examine the directional damping constant W and.

To elaborate, first, the major axis of the ellipse (x
Axis) and a minor axis (y-axis) refractive index distribution, regarding x-polarized waves having an electric field in the x-axis direction and y-polarized waves having an electric field in the y-axis direction,
Propagation constants β _x and β _y in the traveling direction are obtained. The difference Δβ = β _x −β _y between the two is the propagation constant difference between the x polarization and the y polarization caused by the non-axial symmetry of the refractive index distribution and the anisotropic distortion, and this propagation constant difference Δβ is as large as possible. Is desired. This is because the normalized mode birefringence B, the propagation constant difference Δβ, and the coupling length CL have the following relationship, and the coupling between both modes can be reduced.

Where k _o (≡2π / λ) is the wave number of light in free space, λ
Is the wavelength of light in free space.

Next, using these values β _x , β _y , and Δβ, the normalized phase constant U _{x in} the effective core portion of x polarization and the normalized phase constant U _y in the effective core portion of x polarization are calculated as follows. Calculate by formula.

However, n _o is the refractive index at the center of the effective core section (r = 0).

On the other hand, the normalized lateral attenuation constants W _x and W _y in the effective clad portion (support portion) for the x polarization and the y polarization are (1),
Using the normalized phase constants U _x , U _y and the normalized frequencies V _x , V _{y of the} equation (2), they are respectively calculated by the following equations.

However, the normalized frequencies V _x and V _y are the refractive indices n in the effective core portion.
_o, the wave number k _o in free space, the major axis radius b _c, the minor axis radius b _t, is expressed as a function of the relative refractive index difference of the effective core portion Δ to the effective cladding portion, Is defined in.

Therefore, the bending loss characteristics that are a problem in optimizing the polarizer are the parameters U _i , W _i , V _{i in the} above equations (1) to (6).
It can be calculated by knowing the electromagnetic field distribution by the finite element method based on (i = x, y), and the refractive index distribution at that time is measured by RNF (Refracted Near Field), The configuration parameters can be determined.

The inventors have empirically sought that the bending loss characteristic of the elliptical jacket type polarization-maintaining optical fiber having an arbitrary structure obtained by the calculation by the theoretical analysis method is extremely close to the measured bending loss characteristic. Therefore, by using the above-mentioned theoretical analysis method, it is possible to obtain the constituent parameters of the elliptical jacket type polarization-maintaining optical fiber having a desired bending loss characteristic and the refractive index distribution which is optimum as a polarizer.

FIG. 2 shows theoretical bending loss characteristics designed for the respective wavelengths of λ of 1.55, 1.30 and 0.85 μm by using the above analysis method, and Table 1 shows the waveguide structure at that time.

However, the normalized mode birefringence B is set to 6.34 × 10 ^−4, and the wavelength is 1.55 μm, and the wavelength is 1.30 μm.
, For any wavelength 0.85μm optical fiber,
The normalized frequency is in the range of 9 <V _x <11,3.5 <V _y <5.55.

From the bending loss characteristics of FIG. 2, in any optical fiber, the bending loss α _y of y-polarized light becomes α _y > 1 dB / m when the bending radius is 40 mm or less. Therefore, if this optical fiber is wound with a radius of 40 mm or less, only the x-polarized wave is effectively formed, and a desired optimum optical fiber polarizer can be obtained.

Actually, an optical fiber for 0.85 μm is bent at a bending radius of 40 mm for 15
As a result of winding only once (3.8 m in length), an optical fiber polarizer having an extinction ratio of -47 dB (circularly polarized wave incident) and an insertion loss of 0.3 dB (x polarized wave incident) was obtained.

The optical radii of the above design example are
Although it becomes a polarizer at 40 mm or less, theoretical analysis revealed that even in other cases, the structure as a polarizer can be optimized under the following conditions. That is, when the refractive index of the clad and the support is the same as that of silica, the clad 2
Relative refractive index difference Δ + of the core 1 to 0.3% or less, and relative refractive index difference Δ− of the elliptical jacket 3 to the cladding 2 is −0.
It is an elliptic jacket type polarization-maintaining optical fiber having a mode birefringence B of 4.52 × 10 ⁻⁴ or more with 15 to −0.25%. An optical fiber in this numerical range also becomes a polarizer when the bending radius is a certain value or less. Therefore, the bending radius of the polarizer is 40
There are cases where it is larger than mm.

By the way, from the bending loss characteristics of Fig. 2, the bending radius is 40mm.
In the following, the bending loss of the y-polarized wave is 1 dB / m or more, so a polarizer can be obtained by winding a few meters, but the insertion loss is 0.5d.
In order to make it B or less, the bending radius is limited to about 15 mm or more due to the bending loss of the x-polarized wave. In order to miniaturize the polarizer and make the entire system compact, it is desirable to realize absolute single polarization operation in the region where the bending radius is small (10 mm or less).

Therefore, by using the design method based on the finite element method considering the mode birefringence, the structural parameters of the elliptical jacket type polarization-maintaining optical fiber are set so that absolute single polarization operation is possible with a bending radius of 10 mm or less. Choose.

First, the refractive index distribution of the elliptical jacket type polarization-maintaining single-mode fiber (hereinafter referred to as “SP optical fiber”) shown in FIG. 1 is as follows. The core 1 is silica glass doped with germanium, the clad 2 is pure silica glass,
The elliptical jacket 3 is made of boron-phosphorus-doped silica glass, and a support 4 made of pure silica glass and having the same refractive index as that of the clad is provided on the outer peripheral portion so as to surround them. The relative refractive index difference Δ + of the core 1 with respect to the cladding 2 is 0.3 to 0.4%, the relative refractive index difference Δ− of the elliptical jacket 3 with respect to the cladding 2 is −0.2 to −0.3%, and the ellipticity ε of the elliptic jacket is 30 to 45%. Mode birefringence B is 6.3
× 10 ^-4 or more.

Based on such a configuration, in consideration of the effect of the mode birefringence B, the optimum structure that operates as a polarizer in a region where the bending radius is small is analyzed by the finite element method described above.

It is confirmed that the bending loss characteristics and the cutoff wavelength obtained by this design method agree with the actual measurement.

Table 2 shows the structural parameters of the SP optical fiber designed for a wavelength of 0.85 μm using the above analysis method.

However, in the calculation, B = 6.3 × 10 ⁻⁴ .

The bending loss characteristics of this SP optical fiber are shown in FIG. 3 and FIG. FIG. 3 shows SP optical fibers (a) and (b) having a relative refractive index difference Δ + = 0.3% with respect to the clad 2 of the core 1,
FIG. 4 shows SP optical fibers (a) and (c) having a relative refractive index difference Δ− = 0.2% of the elliptical jacket 3 with respect to the cladding 2. Any of (a), (b) and (c) shown in Table 2
The SP optical fiber also has a normalized frequency in the range of 9 <V _x <15,4 <V _y <6.

Now, in FIGS. 3 and 4, the bending loss of the y polarization is
Bending radius R1 of 10 dB / m, x polarization field bending loss of 0.1 dB / m
Assuming that the bending radius is R2, the bending loss characteristics of y polarization are 10 dB / m or more and the bending loss of x polarization is 0.1 dB / m or less from the bending loss characteristics of Figs. 3 and 4. Bending radius R is 10mm
It turns out that it exists below. Therefore, the range (R1 <R
In <R2), a small optical fiber polarizer can be obtained by winding this SP optical fiber for several meters.

Actually bend the SP optical fiber (a) for 0.85 μm with a bending radius of 6 mm
As a result of winding 2.5 m, a fiber optic polarizer with characteristics of 0.2 dB insertion loss and -42 dB extinction ratio (when circularly polarized wave was incident) was obtained.

[Advantages of the Invention] As described above, the elliptical jacket type polarization-maintaining single-mode optical fiber of the present invention has a refractive index distribution in which mode birefringence is taken into consideration, and thus a polarizer having excellent characteristics can be obtained. Therefore, an optical measurement system that uses a polarizer, an optical fiber gyro,
It leads to the improvement of functions such as coherent transmission system.

In particular, in the case of the elliptical jacket type polarization-maintaining optical fiber according to claim 2, since an optical fiber polarizer having a small size and excellent characteristics can be obtained, an optical measurement system, an optical fiber gyro, a coherent transmission system or the like using the polarizer can be obtained. Miniaturization,
The function can be improved.

[Brief description of drawings]

1 (a), (b), and (c) are a cross-sectional view, an x-axis direction refractive index distribution diagram, and a y-axis direction refractive index distribution diagram, respectively, of an elliptical jacket type polarization-maintaining optical fiber of the present invention. The figure shows the bending loss characteristics of three types of optical fibers obtained by calculation, FIGS. 3 and 4 are the figures showing the bending loss characteristics of other optical fibers, and FIGS. 5 (a) to 5 (c). , Sixth
FIGS. (A) to (c) and FIGS. 7 (a) to (c) are explanatory views showing a transverse section and a refractive index distribution of an optical fiber already proposed, respectively. In the figure, 1 is a core, 2 is a clad, 3 is an elliptical jacket,
4 is support.

Claims (2)

[Claims] 1. A polarization-maintaining optical fiber having a four-layer structure in which a clad, an elliptical jacket, and a support are sequentially provided around the core around the core, and the refractive index of the clad and the support is the same as that of silica. Elliptic jacket type characterized in that the relative refractive index difference of the core with respect to the cladding is 0.3% or less, the relative refractive index of the elliptical jacket with respect to the cladding is −0.15 to −0.25%, and the mode birefringence is 4.52 × 10 ⁻⁴ or more. Polarization-maintaining optical fiber. 2. In a four-layer polarization-maintaining optical fiber in which a clad, an elliptical jacket, and a support are sequentially provided around the core around the core, the refractive index of the cladding and the support is the same as that of silica, Ellipse characterized by having a relative refractive index difference of 0.3 to 0.4% for the core with respect to the clad, an elliptic jacket with a relative refractive index difference of -0.2 to -0.3% for the clad, and a mode birefringence of 6.3 × 10 ^-4 or more. Jacket-type polarization-maintaining optical fiber.