JPH0644088B2 - Polarization-maintaining optical fiber - Google Patents

Description

The present invention relates to a polarization-preserving optical fiber, more specifically, a circular optical fiber for propagating a light wave by using polarized light in only one direction, that is, an optical fiber having a strong property of preserving the polarization plane. Regarding

With the progress of development of optical fibers, realization of optical circuits such as isolators and switch circuit mixers with optical integrated circuits is being developed. The waveguide structure used in an optical integrated circuit is basically a flat plate (slab) type, and the waveguide is a slab structure because it is necessary to polarize light in order to realize a switch circuit in an integrated circuit. It It is desirable to effectively couple such an optical integrated circuit and other optical devices with a circular optical fiber. In that case, it is necessary that the optical fiber can hold the plane of polarization in a specific direction.

Furthermore, it has been proposed to perform various measurements using the polarization plane of light, but in order to put these into practical use, preserving and transmitting the polarization plane of light is a problem that must be solved. . In particular, it is desirable from the viewpoint of transmission efficiency of optical energy and manufacturing that it is possible to transmit while preserving the polarization plane of a circular waveguide (optical fiber) that has come to the stage of practical use.

Conventionally, as a circular optical fiber that preserves the plane of polarization, it is composed of a core, a clad, and a jacket, and by making the thickness of the jacket non-uniform, stress is induced in the core,
The polarization plane is preserved by keeping the refractive index difference (hereinafter referred to as strain birefringence) at a certain level or more in the core part due to the difference in mechanical stress along the transverse directions orthogonal to each other in the core and clad waveguide regions. Optical fibers have been proposed.

(Birefringence in elliptically clad dorosillcate
singlemode fibers. APPLIED OPTICS / Vol. 18, No. 24
15, DEC1979).

That is, in order to preserve the plane of polarization, when the difference between the propagation constants of the polarized waves in the transverse direction orthogonal to each other in the optical fiber is Δβ, The smaller the value of (hereinafter referred to as the coupling length), the stronger the polarization spread. Therefore, when the core is circular, the constant difference Δβ is determined by the difference Δn in the refractive index difference between the two directions of the polarized light. The difference Δn in refractive index is proportional to the difference in strain in these two directions, and this utilizes the difference in coefficient of thermal expansion between the jacket and the cladding.

However, in the above-mentioned proposal, as a means for making the thickness of the jacket non-uniform, it is necessary to deform the jacket in the initial step, which complicates the manufacturing process, and actually, the coupling length L is set. Is only 10 mm or more.

The inventors of the present invention have manufactured the conventional optical fiber by adding a certain amount or more of B ₂ O ₃ to the clad mainly composed of silicon oxide in the optical fiber including the jacket, the clad, and the core. A polarization-maintaining optical fiber with a coupling length of 2 mm or less was realized by slightly changing the method (Japanese Patent Application No. 55-1330, "Polarization-Maintaining Single Mode Optical Fiber").

However, since the glass having B ₂ O ₃ added does not have a relative refractive index difference of 0.7% or more with the SiO ₂ glass, there is a problem that a micro-bending loss or the like becomes large and a low-loss optical fiber cannot be obtained. is there. Further, when the amount of boron oxide (B ₂ O ₃ ) contained in the clad is large, light with a long wavelength (1.2 μm or more) increases lattice vibration absorption, which is essentially low loss as an optical fiber of silicate glass. There is a problem that the loss does not become 1 dB / km or less in the wavelength (1.55 μm band) which is regarded as a wide range.

Therefore, an object of the present invention is to realize an optical fiber which has a large difference Δβ in the propagation constants in the directions of the principal axes orthogonal to each other, which determines the polarization plane preservation, and which is easy to manufacture. A further object of the present invention is to realize an optical fiber with a low transmission loss and a strong polarization plane preservation property.

In order to achieve the above object, the present invention provides a light propagation part corresponding to a conventionally known circular optical fiber, a jacket having an elliptical outer circumference concentrically formed in the light propagation part, and the jacket outer circumference. And an optical fiber composed of the supported portion. According to the above configuration, if the light refractive index of the jacket is set to a constant condition, all the light energy is concentrated only in the light propagating portion, and due to the elliptical jacket,
An optical fiber having a strong polarization plane preservation property can be realized by causing the birefringence to occur due to the difference in the refractive index in the optical axis direction of the light propagating section orthogonal to each other due to the strain stress and increasing the phase propagation constant difference Δβ. In particular, since the jacket, which occupies the majority of the material, has a very small distribution of light energy, it is not necessary to consider the optical transmission loss, and the manufacturing becomes easy.

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

FIG. 1 shows a sectional structure of an embodiment of an optical fiber according to the present invention. As shown in the figure, a circular core 1, a circular clad 2 concentrically formed on the core, a jacket 3 having an elliptical outer circumference formed on the outer circumference of the clad, and a support 4 part formed on the outer circumference of the jacket. . In FIG. 2, the optical refractive index of each layer is highest only in the core 1,
A reference example in which the refractive indexes of the other layers are set to be substantially equal is shown.

In order to obtain such a refractive index distribution, the material of the core 1 is preferably SiO ₂ glass (silicate glass) containing one or both of germanium oxide and phosphorus oxide. The reason is that both dopants have low optical transmission loss. However, the dopants for the core are not limited to both germanium oxide and phosphorus oxide. For example, if the refractive index is higher than that of SiO ₂ glass such as antimony oxide or aluminum oxide, it is not particularly limited. Also, a refractive index lower dopant than SiO ₂ glass like boron oxide, other SiO ₂ refractive index than the glass can be used of course it is used simultaneously with the high dopant.

Next, the preferred material for the cladding 2 is SiO ₂ glass. The absorption loss of SiO ₂ glass is very small. Therefore, if SiO ₂ glass is used as the cladding, the transmission loss of the optical fiber can be reduced. Further, as a constituent material of the elliptical jacket 3, germanium oxide or phosphorus oxide for increasing the refractive index is added to SiO ₂ glass containing boron oxide, and the refractive index of the elliptic jacket 3 is different from that in the reference example shown in FIG. It is the same as the clad 2 and the support 4 of. The material of the elliptical jacket may be based on SiO ₂ glass to which a fluoride compound is added, and to which germanium oxide, phosphorus oxide or the like is added. In FIG. 2, there is no particular limitation as long as it has a large difference in thermal expansion from the support 4 and has the same refractive index as the cladding 2 and the support 3. Support 4
As a constituent material of (1), SiO ₂ glass is preferable because it has a large difference in thermal expansion coefficient from that of the elliptical jacket 3, and also in view of manufacturing problems.

Now, in the case of the optical fiber according to the configuration of the above reference example, the core 1 and the clad 2 form a light propagating portion, and the light energy is as shown by the dotted line 5 in FIG. 2, and is mostly concentrated in the light propagating portion. Therefore, it is not necessary to consider the material of the jacket 3 and the support portion 4 from the viewpoint of transmission loss. Since the jacket 3 may be selected from the viewpoint of causing a large birefringence in the core, manufacturing is extremely advantageous. That is, since the light propagation portion and the portion that generates birefringence are independent of each other, it is possible to change the refractive index distribution in the vicinity of the core without reducing the birefringence distortion amount. Furthermore, there is a degree of freedom in setting the refractive index of the light propagating portion, which will be described later.

In the above-mentioned reference example, although the outer circumference of the support is shown to be circular, the polarization-maintaining optical fiber is preserved only when the linearly polarized light polarized in the optical axis direction is excited. It Therefore, the outer circumference of the main support 4 is elliptic like the elliptical jacket so that the optical axis of the optical fiber (in the case of the optical fiber, it corresponds to the long axis and the short axis of the elliptical jacket) can be known without measurement. It may be configured so that the major axis and minor axis of the elliptical jacket can be known from the knowledge. However, even if the support is not elliptical, it is not necessary to purposely make the support elliptic if the optical axis can be identified.

3 and 4 show the refractive index distribution in the cross-sectional direction of the embodiment of the optical fiber according to the present invention. As described with reference to FIG. 1, since the light propagating portion and the jacket or the support portion are optically independent, the light propagating portion is not limited to that shown in FIG. In the case of FIG. 3, the core has the highest refractive index and a part of the clad portion is lower than the refractive index of the jacket. Therefore, when the optical fiber is bent and used, loss or disturbance of polarization plane occurs. Few. In the case of FIG. 4, the light propagating portion gradually decreases from the center to the outside. It is so-called focused type distribution, and is configured such that the refractive index of the jacket is equal to the minimum refractive index of the light propagating portion. Similar to the above-described principle, the outer circumference of the jacket portion is elliptical, and it is natural that the refractive index of the propagation portion is given birefringence distortion.

Specific examples will be shown below.

Example 1 quartz tube (outer diameter 18Mmfai, inner diameter _{15mmφ) SiO 2 -GeO 2 -B 2} O 3 glass in order on the inner wall surface of, SiO ₂ glass,
The SiO ₂ -GeO ₂ glass is deposited by CVD, and then the tube portion is welded to the -8mmH ₂ O reduced pressure compared to atmospheric pressure, the core consisting of a circular SiO ₂ -GeO ₂ glass, SiO ₂ glass becomes the cladding, SiO ₂ -GeO
Elliptic jacket made of _2- B ₂ O ₃ glass and Si
An optical fiber having a support made of O ₂ glass was prepared. The refractive index of the core of this optical fiber is 1.004n when the hypothesis of the support is n, and the cladding is 1.00.
n, elliptical jacket is 0.999n, core diameter is 8
μm, clad 20 μm, long axis of elliptical jacket is 1
00 μm, short axis 35 μm, support diameter 150 μm
Met. Extinction ratio at a length of 500 m at a wavelength of 0.633 μm of this optical fiber (when linearly polarized light is incident on one optical axis, light of the other optical axis at the exit end and the optical axis in the direction of the incident light) The power ratio was 35 dB, and the transmission loss at the wavelength of 1.55 μm was 0.5 dB / km. Therefore, an optical fiber with low transmission loss and excellent polarization plane preservation characteristics was realized.

Example 2 An optical fiber having a refractive index distribution as shown in FIG. 3 was produced by the same method as in Example 1. The core diameter of this optical fiber is 2
a is 12 μm, 2b is 16 μm, and clad diameter is 30 μm
Then, the refractive index of the core was 1.003n, and n ₂ was 0.997n, where n ₁ is the refractive index of the support. Other parameters are the same as those in the first embodiment. As for the constituent materials, the I portion of the core is SiO ₂ —GeO ₂ glass, and the portion of the core II is SiO ₂ —B ₂ O ₃ glass. The extinction ratio of this optical fiber at a length of 500 m is 30 d at a wavelength of 0.633 μm.
B, the transmission loss was 0.9 dB / km at a wavelength of 1.55 μm.

Example 3 An optical fiber having a refractive index distribution as shown in FIG. 4 was produced by the same method as in Example 1. The core diameter, cladding diameter, etc. of this optical fiber and the constituent materials were the same as in Example 1, and the refractive index of the core was 1.006n in the central portion. The extinction ratio of this optical fiber at a wavelength of 0.633 μm was 36 dB, and the transmission loss was 0.4 dB / km at a wavelength of 1.55 μm.

Example 4 An optical fiber was manufactured by the same method as in Example 1, and the transmission loss and polarization plane preservation performance were compared by changing the constituent material of the core.
Other parameters and materials are the same as those in the example. The table below shows the experimental results.

Example 5 Using the same method and constituent materials as in Example 1, the transmission loss was compared by changing the ratio of the core diameter to the cladding diameter. In this case, if the ratio of the core diameter to the cladding diameter is reduced, the influence of the elliptical jacket on the transmission loss can be reduced. FIG. 5 shows the experimental results, where the horizontal axis shows the ratio of the core diameter to the cladding diameter, and the vertical axis shows the transmission loss at a wavelength of 1.55 μm. From the figure, it can be seen that in order to reduce the transmission loss to 1 dB / km or less, the ratio of the core diameter to the clad diameter needs to be 1/2 or less.

Example 6 An optical fiber was produced in which the outer circumference of the support was an ellipse having an ellipticity of 10% and the axis thereof was in the same direction as the elliptical jacket.
When this optical fiber was used to couple optical fibers together, the outer circumference of the support was elliptical, so it was easy to determine the axes, and the axes of the two optical fibers could be coupled within 0.5 °. It was

[Brief description of drawings]

FIG. 1 is a diagram showing a cross-sectional view of an embodiment of an optical fiber according to the present invention, FIG. 2 is a diagram for explaining a refractive index distribution of an optical fiber in a reference example, and FIGS. Also, FIG. 5 is a diagram showing the refractive index distribution of the embodiment of the optical fiber according to the present invention, and FIG. 5 is a diagram showing the relationship between the ratio of the core diameter to the cladding diameter and the transmission loss. 1 ... Core, 2 ... Clad, 3 ... Jacket, 4 ...
… Support, 5 …… Light energy distribution.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshio Suganuma 1-280, Higashi Koigokubo, Kokubunji City, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (56) Reference JP-A-54-130044 (JP, A) JP-A-56 -27104 (JP, A)

Claims (3)

[Claims] 1. A light propagating portion having a circular cross-sectional shape, a birefringence imparting portion having a non-circular outer periphery formed concentrically with the light propagating portion outside the light propagating portion, and the birefringence imparting portion. The optical propagation part has a support part formed outside, and the light propagation part has a circular core part and a clad part formed concentrically around the core part, and the refractive index of the clad part is the core. And a polarization-preserving optical fiber having a refractive index smaller than that of the birefringence imparting portion. 2. The polarization-maintaining optical fiber according to claim 1, wherein the diameter of the core portion is 1/2 or less of the diameter of the clad portion. 3. A light propagating portion having a circular cross-sectional shape, a birefringence imparting portion having a non-circular outer periphery formed concentrically with the light propagating portion outside the light propagating portion, and the birefringence imparting portion. And a support portion formed on the outside, wherein the refractive index of the light propagating portion gradually decreases from the center toward the outside, and the minimum refractive index thereof is substantially equal to that of the birefringence imparting portion. Polarization-maintaining optical fiber.