JPS6233562B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a polarization-preserving optical fiber, more specifically, a circular optical fiber for propagating light waves using polarization in only one direction, that is, preserving the polarization plane. Regarding optical fiber.

[Background of the invention]

As the development of optical fibers progresses, efforts are being made to realize optical circuits such as isolators and switch circuit mixers using optical integrated circuits. The waveguide structure used in optical integrated circuits is basically a flat plate (slab) type, and in order to realize a switch circuit with an integrated circuit, it is necessary to polarize the light, so the waveguide has a slab structure. . It is desired to effectively couple such optical integrated circuits and other optical devices with a circular optical fiber, but in this case it is necessary that the optical fiber can maintain the plane of polarization in a specific direction.

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

Conventionally, a circular optical fiber that preserves the plane of polarization is composed of a core, a cladding, and a jacket, and by making the thickness of the jacket uneven, stress is induced in the core and the waveguide between the core and the cladding. An optical fiber that preserves the plane of polarization by creating a refractive index difference (hereinafter referred to as strain birefringence) in the core part above a certain level due to the difference in mechanical stress along mutually orthogonal axes within the region. is proposed.

(Birefringence in elliptically clad borosilicate single mode fiber applied optics)
borosillcate singlemode fibers. APPLIED OPTICS” Vol.18, No.2415,
DEC1979).

In other words, in order to preserve the polarization plane, the smaller the value of 2π/Δβ=L (hereinafter referred to as the coupling length), the smaller the value of 2π/Δβ=L (hereinafter referred to as the coupling length), where Δβ is the difference in the transmission constant of the mutually orthogonal lateral polarized waves of the optical fiber. Since the conservation of polarization is strengthened, when the core is circular, this constant difference Δβ is determined by the magnitude Δn of the difference in refractive index in the two directions of polarized light, and this difference in refractive index Δ
n is proportional to the difference in strain in these two directions, and this takes advantage of the difference in coefficient of thermal expansion between the jacket and the cladding.

However, in the above proposal, as a means to make the thickness of the jacket non-uniform, it is necessary to deform the jacket in the initial process, which complicates the manufacturing process. Only those with a diameter of 10 mm or more have been obtained.

The present inventors have developed an optical fiber consisting of the jacket, cladding, and core by adding a certain amount of B ₂ O ₃ to the cladding mainly composed of silicon oxide. By slightly changing the manufacturing method, the bond length can be reduced to 2.
We have realized a polarization-maintaining optical fiber with a diameter of less than mm (Patent Application No. 1330-1988 ``Polarization-maintaining single mode optical fiber'').

However, since glass doped with B ₂ O ₃ does not have a relative refractive index difference of 0.7% or more with SiO ₂ glass, microbending loss increases, making it impossible to obtain a low-loss optical fiber. be. In addition, if the amount of boron oxide (B ₂ O ₃ ) contained in the cladding is large, the long wavelength (1.2 μm)
There is a problem in that the lattice vibration absorption increases with the above light, and the loss does not fall below 1 dB/Km at the wavelength (1.55 μm band), which is originally a low-loss region for silicate glass optical fibers.

[Purpose of the invention]

Therefore, an object of the present invention is to realize an optical fiber that preserves the plane of polarization and has low transmission loss. More specifically, the objective is to realize a polarization-maintaining optical fiber with a loss of 1 dB/Km or less and a large difference in propagation constant Δβ for light with a wavelength of around 1.5 μm.

[Summary of the invention]

In order to achieve the above object, the present invention separates a light propagation member having a circular cross-sectional shape for propagating light and a member for causing birefringence in the light propagation section, and further separates a light propagation member having a circular cross-sectional shape for propagating light and a member for causing birefringence to occur in the light propagation member. A support portion for holding the fastening member is constructed by wrapping the light propagation member and the member that causes birefringence. The light propagation member is made to have a circular cross-sectional shape like a normal optical fiber, and the member that causes birefringence passes through the center of the circle and has different refractive indexes in the directions of optical axes that are orthogonal to each other. In order to achieve this, it is made into a non-circular shape, such as an ellipse, which has a line of symmetry with the orthogonal optical axes, and has a different thickness in the direction of the optical axis (that is, the orthogonal line of symmetry). Further, in order to prevent a large amount of light energy from being distributed in the member that causes birefringence, its refractive index is made to be substantially the same as the refractive index of the outer periphery of the light propagating member. According to the above configuration, all the light energy is concentrated only in the light propagation member, and the non-circular member applies different strain stresses in the orthogonal optical axis direction of the light propagation part, so that the optical energy is concentrated in the orthogonal axial direction of the core. The difference in refractive index causes birefringence and increases the phase propagation constant difference Δβ, making it possible to realize an optical fiber with strong polarization preservation property. In particular, since the distribution of light energy in the non-circular member and the support part, which account for the majority of the material in terms of quantity, is extremely small, there is no need to consider optical transmission loss, and manufacturing is facilitated. According to the present invention, as shown in the example below, the coupling length is about 1 mm, and the wavelength
A polarization-maintaining optical fiber with a low loss of 0.3 dB/Km can be realized even for 1.5 μm light.

Note that polarization preservation expresses the degree of coupling length L=2π/Δβ of an optical fiber, but in the following explanation, it refers to a coupling length of at least 10 mm or less.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows a cross-sectional configuration of an embodiment of an optical fiber according to the present invention. As shown in the figure, it consists of a circular core 1, a circular cladding 2 formed concentrically around the core, a jacket 3 formed on the outer periphery of the cladding and having an oval outer periphery, and 4 parts of supports formed on the outer periphery of the jacket. . As shown in FIG. 2, the optical refractive index of each of the above-mentioned groups is the highest in only the core 1, and the refractive indexes of the other layers are set to be approximately equal.

In other words, the core 1 and the cladding 2 constitute a light propagation section, and the jacket 3 is symmetrical with the axis passing through the center of the core and orthogonal to each other, like an ellipse, and having two lines of symmetry. Consists of members with different thicknesses in different directions.

In order to obtain such a refractive index distribution, the core 1 is made of SiO ₂ glass (silicate glass) containing one or both of germanium oxide and phosphorus oxide. The reason is that both dopants are dopants with low optical transmission loss.

Next, a desirable material for the cladding 2 is SiO ₂ glass. The absorption loss of SiO ₂ glass is very small, so if SiO ₂ glass is used as the cladding, the transmission loss of the optical fiber can be reduced. Furthermore, as the constituent material of the elliptical jacket 3, germanium oxide or phosphorus oxide to increase the refractive index is added to SiO ₂ glass containing boric oxide, and the refractive index of the elliptical jacket 3 is changed from that of other claddings as shown in FIG. 2. It is desirable that the support be the same as 4. In short, it is not particularly limited as long as it has a large thermal expansion difference with the support 4 and has the same refractive index as the clad 2 and jacket 3. As the constituent material of the support 4, SiO ₂ glass is preferable because it increases the difference in coefficient of thermal expansion from the elliptical jacket 3 and from the viewpoint of manufacturing problems. Describing a specific example, the refractive index of each layer is
If the refractive index of SiO ₂ is n, then the core is 100.2n, the portion 2 of the clad SiO ₂ is n, and the clad SiO ₂ +B ₂ O ₃ +
Part 3 of GeO ₂ is 99.99n and the jacket is n. Core diameter 8 μm, outer diameter of SiO ₂ part 4 20 μm
m, the length of the short axis of Clad 2 is 35 μm, the length of the long axis
100 μm, and the outer diameter of the jacket 3 is 150 μm. The coupling length of this optical fiber is 0.8 mm for light with a wavelength of 0.633 μm, and the transmission loss is 1.55 μm.
The transmission loss is extremely small at 0.3dB/Km.

〔Effect of the invention〕

The reason why birefringence occurs in the optical fiber having the structure of the present invention will be explained.

Birefringence is generally caused by anisotropic stress due to geometric deviation from a concentric structure.

In the case of a structure consisting of four layers as shown in Figure 3,
The refractive index n _r1 in the radial direction of the core is n _r1 = n ₁ + (γ ₁ + 3γ ₂ )E/2 (1-ν) [(a
² / ^d2-1 ) _K12 +( ^b2 / ^d2-1 ) _K23 +( ^c2 / ^d2-1 )K
₃₄ ] − (1) K _ij = α _i ΔT _i −α _j ΔT _j n ₁ : Refractive index before stress application γ ₁ , γ ₂ : Photoelastic constant E: Young's modulus, ν: Poisson's ratio a, b, c , d: Core, cladding, jacket, support diameter α _i : Coefficient of thermal expansion of i-layer ΔT _i : Can be expressed as the difference between the softening temperature of the glass of i-layer and room temperature.

Therefore, the refractive index of the core in the x and y directions is n _x
and n _y , the x and y diameters of core 1, cladding, jacket and support are a _x , a _y , b
Assuming that _x , b _y , c _x , c _y and d _x , d _y , substituting these into equation (1) gives n _x = n ₁ + (γ ₁ + 3γ ₂ )E/2 (1-ν) [( a
_x2 / ^{d2-1 )} _K12 + ⁽ _bx2 / ^d2-1 ) _K23 ⁺ ( _cx2 / ^d2-1
)K ₃₄ ]-(2) _ny = n ₁ + (γ ₁ +3γ ₂ )E/2(1-ν) [(a
_{y2 /} ^{d2-1 ) K12} +( _by2 / ^d2-1 ) _K23 +( _cy2 ^{/ d2-1}
)K ₃₄ ]−(3), and the refractive index difference between the core in the x direction and the y direction, δ _o = n _x − _ny , is δ _o = n _x − _ny = (γ ₁ + 3γ ₂ )E/ 2( _{1-ν) [(ax2-ay2)K12 +} ⁽ _bx2 ^- _by2 ⁾ _K23 ⁺ _{( cx2} ^- _cy2 ⁾ _K34 / ^d2 ] -(4) becomes.

Specifically expressing the above formula (4) for the above-mentioned example, δ _o =0.36A ₁ M ₁ ( _ax ² - _{a y} ² ) + A・M _B203 {-(b _x ² - b _y ² )+( c _x ² - c _y ² )}/d ²
−(5). (However, A ₁・M ₁ = K ₁₂ , A・M _B203 = −K ₂₃ = K ₃₄ , and A ₁ and A are proportional constants,
M ₁ and M _B203 are the dopant concentrations in the core and jacket, respectively.) This formula shows that the birefringence in the three-layer structure is 0.36A・M _B203 - (b _x ² - b _y ² ) + (c _x ² _−cy ² )/d
² , and one of the central layers is added to this, resulting in 4 layers, so simply A ₁・M ₁ (a _x ² − a _y ² )
It's just an addition.

Based on the above approximation, the refractive index difference δ _o of the core in the x-axis and y-axis directions is expressed as ax = a _x =
a _y , b _x =b _y , and δ _o =0.36A・M _B203 )(c _x ² - c _y ² )/d ² -(6
) becomes. The larger this value δ _o is, the better the polarization preservation characteristic is. Since there is no parameter b in equation (6),
The size of cladding 2 does not affect birefringence.

As mentioned above, in the optical fiber of the present invention, only the core of the optical fiber causes birefringence, and the optical energy is trapped in the cladding 2 and the core 1 (light transmission part), and birefringence including B ₂ C ₃ etc. Since the distribution of light energy to the members for producing this can be significantly reduced, the jacket 3 can be selected from the viewpoint of producing a large birefringence in the core, which is extremely advantageous in manufacturing. That is, since the light propagation part and the part that generates birefringence are independent, it is possible to change the refractive index distribution near the core without reducing the amount of birefringence distortion. There is a degree of freedom in setting the refractive index of the light propagation section.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a cross-sectional structure of an embodiment of an optical fiber according to the present invention, FIG. 2 is a diagram showing a refractive index distribution of an embodiment of an optical fiber according to the present invention, and FIG. 3 is a diagram showing the effect of the present invention. FIG. 2 is a cross-sectional configuration diagram of an optical fiber having a four-layer structure for explanation. 1: core, 2: cladding, 3: jacket,
4: Support department.

Claims (1)

[Scope of Claims] 1. A core having a circular cross-sectional shape, a cladding provided on the outer periphery of the core, having a refractive index lower than the refractive index of the core, and having a circular outer periphery in cross-section, and a cladding adjacent to the cladding. a birefringence-imparting member that is arranged to produce birefringence in the core due to thermal expansion; and a support that surrounds the core, a cladding, and the birefringence-imparting member; A polarization-maintaining optical fiber characterized by being made of a material with low optical loss. 2. The polarization preserving optical fiber according to item 1, wherein the material of the cladding is silicate glass. 3. The polarization-maintaining optical fiber according to item 2, wherein the birefringence imparting member is made of a material having a refractive index substantially equal to that of silicate glass. 4. The polarization-maintaining optical fiber according to item 2 or 3, wherein the birefringence imparting member surrounds the cladding layer and has an elliptical outer periphery.