JPH0774845B2 - 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, an optical fiber (absolute single polarization operation that exhibits an absolute single polarization operation in which one of two orthogonal polarization modes is cut off. It is desirable to use a single polarization maintaining fiber. With respect to this point, the applicant of the present invention discloses in Japanese Patent Application No. 61-124663, Japanese Patent Application No. 61-124664 and Japanese Patent Application No. 61-203751 each of FIGS. 6 (a) to (c). , (A) to (c) of FIG. 7 and (a) to (c) of FIG. 8 have proposed an absolute single polarization maintaining fiber (SPF).

The optical fiber 61 shown in FIGS. 6A to 6C has an 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. By utilizing the fact that the cutoff wavelength of the fundamental mode does not exist in the MC type but does exist in the W type, absolute single polarization operation (ASP operation) is possible.

On the other hand, the optical fiber 71 shown in FIGS. 7A to 7C 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.

Furthermore, the optical fiber 81 shown in FIGS.
6 (a) to (c) and 7 (a) to (c).
In the optical fiber shown in Fig. 2, it is necessary to divide the refractive index distribution 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, so that it is difficult to manufacture. This is an improvement in that the yield for obtaining an optical fiber of the desired value was poor. That is, with the core 1 as the center,
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 on the outer periphery thereof, the elliptical jacket 3 has a lower refractive index than the clad 2, and the support 4 is equal to the clad 2. The elliptical jacket 3 has a refractive index of 25 to 40% and the relative refractive index of the cladding 2 is -0.05 to -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.

However, since the elliptic jacket type polarization-maintaining single-mode optical fibers of FIGS. 6, 7, and 8 described above do not consider the effect of the mode birefringence, the structure as a polarizer is optimized. Therefore, the characteristics, such as extinction ratio and insertion loss, cannot be said to be the best.

In view of this point, the present applicant has not yet publicly known, but by considering the mode birefringence, an elliptical jacket type polarization-maintaining optical fiber showing an absolute single polarization operation suitable as a polarizer, I have already proposed (Japanese Patent Application No. Sho 62-318967)
issue). This has a clad, an elliptical jacket, and a support, which are sequentially provided around the core around the core.
In a polarization-maintaining optical fiber with a layered structure, the refractive index difference between the clad and the support is the same as that of silica, the relative refractive index difference Δ + of the core to the clad is 0.3% or less, and the relative refractive index difference Δ− of the elliptical jacket to the clad is −. −0.15 to −0.25
%, And the mode birefringence B is 4.52 × 10 ⁻⁴ or more, and an elliptical jacket type polarization-maintaining optical fiber is constructed.

The above is the bending loss characteristics of two orthogonal polarizations of a polarization-maintaining optical fiber at a specific wavelength, bending radius, and winding length by the finite element method considering the mode birefringence of the polarization-maintaining optical fiber. The structural parameters are determined so that there is a large difference. In general, a semiconductor laser light source has characteristics of extinction ratio (when circularly polarized wave is incident) of -40 dB or less and insertion loss (x-polarized wave, a mode polarized on the long axis of an ellipse) of 0.5 dB or less.

[Problems to be Solved by the Invention] However, since the absolute single polarization operation range, that is, the wavelength range functioning as a polarizer is narrow, the operation is performed at a wavelength of 0.85 μm, but the center wavelength is deviated from 0.81 μm There is a problem that the characteristics are deteriorated in the light source.

The most important problem is that in the case of the SLD (super luminescent diode), which is the most suitable light source for the rotation angular velocity sensor, its central wavelength is deviated from the central wavelength of the SLD because the spectrum width is wide. And its spectral width is outside the absolute single polarization operating range,
Insertion loss increases and extinction ratio deteriorates. in this case,
It is necessary to match the center wavelength of the absolute single-polarization operating region with the center wavelength of the SLD, but the center wavelength of the SLD differs depending on the manufacturer, and the spectrum width changes depending on the injection current of the SLD. You have to make a child each time.

The present invention is to provide an elliptical jacket type polarization-maintaining optical fiber having a wide wavelength range that functions as a polarizer when the polarizer is configured, and thus the center wavelength of absolute single polarization operation can be arbitrarily selected. is there.

[Means for Solving the Problems] An elliptical jacket type polarization-maintaining single-mode optical fiber of the present invention has a four-layer structure in which a clad, an elliptical jacket, and a support are sequentially provided around a core as a center and are bent. In a polarization-maintaining single-mode fiber that can be effectively applied with absolute single-polarization operation, the outside diameter of the optical fiber should be adjusted so that the outer diameter changes within ± 20 to 30 μm within a strip length of 15 m from the center. The diameter is continuously and uniformly changed.

[Operation] Since the outer diameter of the optical fiber changes continuously and uniformly so that the outer diameter changes within ± 20 to 30 μm in the range of the strip length of 15 m from the center, it was applied to the optical polarizer. In this case, even if the wavelengths are deviated, absolute single polarization operation is possible for all those wavelengths. Therefore, even if the center wavelength of the absolute single polarization operation and the center wavelength of the SLD are deviated by using the SLD with a wide spectrum width as the light source, the center wavelength of the absolute single polarization operation can be arbitrarily changed. The spectrum width does not extend beyond the absolute single polarization operating range, so that increase in insertion loss and deterioration of extinction ratio can be eliminated.

EXAMPLES The present invention will be described below with reference to the drawings.

3 (a), (b), and (c) are transverse sectional views of an absolute single polarization-maintaining single-mode optical fiber which is the premise of the embodiment of the present invention, respectively.
It is a refractive index distribution map of an X-axis direction, and a refractive index distribution map of a Y-axis direction. In the figure, 1 is a core made of SiO ₂ glass to which Ge is added, 2 is a clad made of pure SiO ₂ glass surrounding the outer periphery of the core 1, and 3 is an elliptical jacket 3 surrounding the outer periphery of the clad 2. . This oval jacket 3 is made of B ₂ O ₃
It is made of SiO ₂ glass to which is added and has a refractive index lower than that of the cladding 2. 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.

3 (a) (and (c), a is the radius of the core 1, bc is the major axis radius of the elliptical jacket 3, bt is the minor axis radius of the elliptical jacket 3, and the radius of the cladding 2 is the cladding thickness. The ratio of the core radii is represented by δ, and is represented by (1 + δ) a, where 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% with respect to the 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 theoretical 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 normalized lateral damping constant W when the support 4 around it is treated as an effective cladding portion. Scrutinize.

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,
The propagation constants βx and βy in the traveling direction are obtained. The difference Δβ = β _x −βy between the two is a 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 should be 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.

However, k ₀ (≡2π / λ) is the wave number of light in free space, and λ is the wavelength of light in free space.

Next, using these values βx, βy, and Δβ, the normalized phase constant Ux in the effective core part of the x polarization and the normalized phase constant Uy in the effective core part of the y polarization are calculated by the following equations. .

However, n ₀ is the refractive index at the center (r = 0) in the effective core portion.

On the other hand, x-polarization and y-polarization effective cladding (support)
The normalized lateral damping constants Wx and Wy in the above are (1),
Using the normalized phase constants Ux, Uy and the normalized frequencies Vx, Vy of the equation (2), they are respectively calculated by the following equations.

However, the normalized frequencies Vx and Vy are the refractive indices in the effective core.
n ₀ , wave number in free space k ₀ , major axis radius bc, minor axis radius bt, expressed as a function of relative refractive index difference Δ of the effective core portion with respect to the effective cladding portion, Is defined in.

Therefore, there is a problem in the optimization of the polarizer.
The loss characteristic is the amount U of the above equations (1) to (6). , W , V
Based on (= x, y), the electromagnetic field component is calculated by the finite element method.
By knowing the cloth, it can be calculated, and
The refractive index distribution of mushrooms by RNF (Refracted Near Field)
You can measure and use this to determine the configuration parameters
It

The solid line in FIG. 4 shows the bending loss characteristics obtained by the calculation by the theoretical analysis method. The black circle plots show the measured bending loss values of x polarization, and the white circle plots show the measured bending loss values of y polarization. Show. From this, it can be seen that the bending loss characteristics obtained by the above theoretical analysis method are very similar to the measured bending loss characteristics. Therefore, the elliptic jacket type polarization-maintaining optical fiber having a desired bending loss characteristic can be calculated by the above theoretical analysis method, and the refractive index distribution that is optimum as a polarizer can also be obtained.

FIG. 5 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 <Vx <11, 3.5 <Vy <5.55.

From the bending loss characteristics shown in Fig. 5, the bending loss αy of the y-polarized light is α when the bending radius is 40 mm or less.
y> 1 dB / m. 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.

For example, actually bend an optical fiber for 0.85 μm with a bending radius of 40
Extinction ratio of -47 dB if wound only 15 times (3.8 m in length) in mm
An optical fiber polarizer having characteristics of (circularly polarized wave incident) and insertion loss of 0.3 dB (x polarized wave incident) can be obtained.

Each of the optical fibers in the above design example has a bending radius of 40
Although it becomes a polarizer at mm or less, 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 relative refractive index difference Δ of the core 1 with respect to the clad 2 is Δ.
+ Is 0.3% or less, elliptical jacket 3 for clad 2
Is an elliptic jacket type polarization-maintaining optical fiber having a relative refractive index difference Δ− of −0.15 to −0.25% and a mode birefringence B of 4.52 × 10 ⁻⁴ or more. 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 may be larger than 40 mm.

FIG. 1 is a loss wavelength characteristic (measurement example) of an optical fiber polarizer (bending radius 25 mm, winding length 2.4 m) obtained by using the above technique, and Δλ represents a wavelength width that functions as a polarizer. . However, the outer diameter of the optical fiber is 125 μm.

In the example of the above optical fiber outer diameter of 125 μm, the central wavelength of absolute single polarization operation is 1.29 μm and the range of 1.27 to 1.31 μm
It has the function of a polarizer. Extinction ratio of -41d at wavelength 1.29μm
B, insertion loss is 0.5 dB. At a wavelength of 1.31 μm or more, for example, at a wavelength of 1.33 μm, absolute single polarization operation is possible, but the insertion loss becomes large. At wavelengths below 1.27 μm, Y
Absolute single polarization operation becomes impossible because the polarization is not sufficiently attenuated and the extinction ratio characteristic deteriorates.

FIG. 2 shows the loss wavelength characteristic when the outer diameter of the optical fiber is changed by drawing the same preform. The one-dot chain line shows the loss wavelength characteristic when the outer diameter of the optical fiber is 100 μm, and the solid line shows the loss wavelength characteristic when the outer diameter of the optical fiber is 125 μm.
The dotted line shows the loss wavelength characteristic when the outer diameter of the optical fiber is 150 μm.

As can be seen from FIG. 2, the wavelength range of absolute single polarization operation is
When the outer diameter of the optical fiber is thin, it is shifted to the short axis side, and when it is thick, it is shifted to the long axis side. Therefore, if the outer diameter of the optical fiber is continuously and evenly changed in the range of a strip length of 15 m, the winding strip length may be at most 2 to 3 m. An optical fiber polarizer that matches the center wavelength of a light source using the center wavelength of can be realized.

Actually, the optical fiber outer diameter is 125 μm at the center in the longitudinal direction within the range of 15 m, and ± 25 μ in the front-back direction.
We manufactured a polarization-maintaining optical fiber whose outer diameter was gradually and continuously changed between m and applied it to an optical fiber polarizer. As a result, we could select the operation center wavelength as a polarizer. .

[Effects of the Invention] As described above, when the elliptical jacket type polarization-maintaining optical fiber of the present invention is applied to a polarizer, it becomes a polarizer whose operation center wavelength can be arbitrarily selected, so that the center wavelength of the light source varies. However, it can operate corresponding to the light source. In that sense, the characteristic yield is improved, and the cost of the polarizer can be reduced.

Since it is possible to match the center wavelength of the SLD, which is the light source of the rotation angle sensor, with the operation center wavelength of the polarizer,
Since there is no characteristic deterioration (increased insertion loss, deterioration of extinction ratio) which has been a problem in the past, it leads to improvement of characteristics of various devices using a polarizer, for example, a rotation angle sensor such as an optical fiber gyro.

[Brief description of drawings]

Fig. 1 is a diagram showing an example of loss wavelength characteristics of an optical fiber polarizer when the optical fiber diameter is 125 µm, and Fig. 2 is a case where the outer diameters of optical fibers obtained from the same base material as in Fig. 1 are 100 µm and 150 µm. Showing an example of loss wavelength characteristics of the optical fiber polarizer of FIG. 3, FIGS. 3 (a), 3 (b) and 3 (c) are cross-sectional views of an elliptical jacket type polarization-maintaining optical fiber, a refractive index distribution diagram in the x-axis direction, and FIG. Fig. 4 is a refractive index distribution chart in the y-axis direction.
FIG. 5 is a diagram showing bending loss characteristics by polarization calculation and measurement, FIG. 5 is a diagram showing bending loss characteristics of three kinds of optical fibers obtained by calculation, and FIGS. 6, 7, and 8 are already proposed respectively. It is an explanatory view showing the refractive index distribution of the optical fiber. In the figure, 1 is a core, 2 is a clad, 3 is an elliptical jacket,
4 is support.

Claims (1)

[Claims] 1. A polarization structure which has a four-layer structure in which a clad, an elliptical jacket, and a support are sequentially provided around the core around the core, and bending is applied to enable effective absolute single polarization operation. In a wavefront-maintaining optical fiber, an ellipse characterized by continuously changing the outer diameter of the optical fiber so that the outer diameter changes within ± 20 to 30 μm within a range of a strip length of 15 m from the center. Jacket-type polarization-maintaining optical fiber.