JPS6187109A - Optical fiber maintaining plate of polarization - Google Patents

Abstract

PURPOSE:To maintain the plane of polarization and to reduce transmission losses by separating an optical transmission member from a member making the optical transmission part generate birefringence and constituting a supporting part for holding both the members so as to lap both the members. CONSTITUTION:An optical fiber consists of a circular core 1, a circular clad 2 concentrically formed around the core 1, an elliptical jacket 3 formed on the periphery of the clad, and a support part 4 formed on the periphery of the jacket 3. As to the optical refractive indexes of respective parts, only the core 1 is set up to the highest value and other parts are set up to almost equal values. Birefringence is generally generated by anisotropic stress based upon the shift of a shape from its concentrical structure. In the optical fiber, birefringence is generated only in the core of the optical fiber, light is shut in the clad 1 and the core 1 and the light transmitting part is independent of the generation part of birefringence, so that the distribution of refractive indexes around the core can be changed without reducing the distortion of birefringence.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a polarization-maintaining 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]

With the progress of the development of optical fibers, the realization of optical circuits such as isolators, switch circuit mixers, etc. using optical integrated circuits is being developed. The waveguide structure used in optical integrated circuits is basically a flat plate (slab) type, and in order to realize a switch circuit in an integrated circuit, it is necessary to polarize the light, so the waveguide has a slab structure. . It is desirable to effectively couple such optical integrated circuits with other optical devices using circular optical fibers, but in this case it is necessary that the optical fibers be able to 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, circular waveguides (
It is desirable to be able to transmit light while preserving its plane of polarization through optical fibers, both from the standpoint of optical energy transmission efficiency and from the standpoint of manufacturing.

Conventionally, a circular optical fiber that preserves the plane of polarization is composed of a core, cladding, and jacket, and by making the thickness of the jacket uneven, stress is induced in the core.
The plane of polarization can be changed by creating a refractive index difference (hereinafter referred to as strain birefringence) in the core part (hereinafter referred to as strain birefringence) due to the difference in mechanical stress along the mutually orthogonal lateral directions in the waveguide regions of the core and cladding. Optical fiber storage has been proposed.

(Pyrefringence in elliptical clad porosilicate single mode fiber, Abride Optex® B irefringence in ellip
tically cladborosillcate
sinHlemode fibers.

APPLIED 0PTIC5J Vol, 18°Nα
2415. DEC1979).

In other words, in order to preserve the plane of polarization, the smaller the value (hereinafter referred to as the coupling length), where the difference in the transmission constant of the mutually orthogonal horizontal polarized waves of the optical fiber is Δβ, the better the polarization will be preserved. is strengthened, so 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 Δn is determined by the difference in strain in these two directions. This takes advantage of the difference in thermal expansion coefficient between the jacket and 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. is 10III1
I've only gotten more than that.

The present inventors have developed an optical fiber consisting of the jacket, cladding, and core, by adding a certain amount of 8203 to the cladding mainly composed of silicon oxide, and by slightly improving the manufacturing method of conventional optical fibers. By making this change, a polarization-maintaining optical fiber with a coupling length of 211I11 or less was realized (Japanese Patent Application No. 1330-1988 ``Polarization-maintaining single mode optical fiber'').

However, since the glass doped with B2O3 does not have a relative refractive index difference of 0.7% or more with the 5in2 glass, there is a problem that microbending loss and the like increase, making it impossible to obtain a low-loss optical fiber. In addition, when the amount of boron oxide (B203) contained in the cladding is large, lattice vibration absorption increases with light of long wavelengths (1.2 μm or more), which is a low-loss region for a silicate glass optical fiber. There is a problem that the loss does not become less than ldB/km at the wavelength (1.55 μm band).

[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 Δβ in propagation constants for light with a wavelength of around 1.5 μm.

[Summary of the invention]

In order to achieve the above object, the present invention separately mixes a light propagation member with a circular cross-sectional shape for propagating light and a member for causing birefringence in the light propagation section, and furthermore, the light propagation member and the birefringence are mixed together. The holding member that causes the birefringence is made to have a circular cross-sectional shape like a normal optical fiber, and the refractive index of the member that causes the birefringence passes through the center of the circle and the refractive index in the direction of the optical axes that are perpendicular to each other. In order to make the shapes different, a non-circular shape is used, such as an ellipse, with the orthogonal optical axes as a line of symmetry, and the thickness in the direction of the optical axis (that is, the orthogonal line of symmetry) is different. The refractive index should be the same as that of the outer periphery of the member that does not cause birefringence. According to the above configuration, all of the light energy is concentrated only in the light propagation member, and the non-circular member increases the light propagation constant difference Δβ, thereby realizing 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, the bond length is i as shown in the following example.
At around n+m, it is also 0.3 for light with a wavelength of 1.5 μm.
A polarization-maintaining optical fiber with a low loss of dB/km can be realized.

Note that polarization maintaining means that the optical fiber has a coupling length of at least 10 mm or less.

[Embodiments of the invention]

The present invention will be described in detail below 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 clad 2 formed concentrically around the core, a jacket 3 formed around the outer periphery of the clad and having an oval outer periphery, and 4 parts of supports formed around the outer periphery of the jacket. . As shown in FIG. 2, the optical refractive index of each of the layers is set to be the highest only in core 1, and the refractive indexes of the other layers are set approximately equal.

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

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

Next, a desirable material for the cladding 2 is SiO2 glass. The absorption loss of 5i02 glass is very small.
Therefore, if 5i02 glass is used as the cladding,
Optical fiber transmission loss can be reduced. In addition, as a constituent material of the elliptical jacket 3, SiO containing boron oxide is used.
□It is preferable to add germanium oxide or phosphorus oxide to the glass to increase the refractive index so that the refractive index of the elliptical jacket 3 is the same as that of the pond cladding 2 and support 4 as shown in FIG. The material for this elliptical jacket may be one based on SiO2 glass to which a fluoride compound is added, and to which germanium oxide, phosphorus oxide, etc. are added. 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 cladding 2 and jacket 3. As the constituent material of the support 4, 5in2 glass is preferable because it increases the difference in coefficient of thermal expansion from the elliptical jacket 3 and from the viewpoint of manufacturing issues. To describe a specific example, the refractive index of each layer is, where n is the refractive index of SiO2, the core is 100.2n, the portion 4 of the cladding 5i02 is n, and the portion of the cladding 5iQ2+B2O3+Ge○2 is 99.9.
9 n, jacket is n. Core diameter 8μm, 5i0
The outer diameter of the part 4 of 2 is 20 μm, and the short axis length of the cladding 2 is 3
5 μm, the length of the major axis is 100 μm, and the outer diameter of the jacket 3 is 150 μm. The coupling length of this optical fiber is wavelength 0
．． The transmission loss is extremely small at 0.3 dB/km at 633 μm.

〔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 FIG. 3, the refractive index nr□ in the radial direction of the core is Kjj=αiΔTj−αjΔTJ ni: refractive index γ1 before stress application. γ2: Photoelastic constant E: Young's modulus: Bore ratio a Hb HQ g d: Co-cladding, jacket, support diameter αj: Coefficient of thermal expansion of i-layer ΔT Uni Difference between softening temperature of glass in i-layer and room temperature It can be expressed as

Similarly, the refractive index in the y direction can be approximated to the refractive index of an optical fiber having the structure shown in FIG.

Under the above approximation, the refractive index difference δ between the X-axis and y-axis directions of the core. is δn”n r t (C
:x) n r 1 (Cy). Here, C
= (CX + Cy) / 2.

δja=(Cx cy)/(cx+cy). X
The axis is the refractive index difference δ in the Cyy axis direction. is equal to the material birefringence M, and the larger this value is, the better the polarization preservation property is. For example, even when the ellipticity is large and b=cy, that is, the jacket 3 is divided into left and right cladding, the birefringence is expressed by the above equation (2). Since there is no parameter b in equation (2), the size of the cladding 2 does not affect the birefringence.

Therefore, 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).
Since the distribution of light energy to the members for producing birefringence can be significantly reduced, the jacket 3 can be selected from the viewpoint of producing 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 strain. 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 sectional view of a fiber of an embodiment of the optical fiber according to the present invention. 1: core, 2: cladding, 3: jacket, 4: support section. Agent: Patent Attorney Masaru Ogawa No. 4

Claims (1)

[Scope of Claims] 1. A light propagation member having a circular cross-sectional shape, which is provided near the light propagation member, has two lines passing through the center of the circle and orthogonal to each other as object lines, and has a thickness in the direction of the two lines. 1. A polarization-maintaining optical fiber comprising: a member having a non-circular cross-sectional shape having different shapes; and a support member surrounding the light-transmitting member and the non-circular member. 2. In the optical fiber according to item 1, the light propagation member includes a core having a circular cross-sectional shape, and a cross section of the outer periphery formed concentrically around the outer periphery of the core and made of a material having a refractive index lower than that of the core. A polarization-maintaining optical fiber comprising a crut having a circular shape, the non-circular member having a refractive index equal to that of the crut. 3. The optical fiber according to item 1, wherein the non-circular member has an elliptical outer circumference. 4. In the optical fiber according to item 2 or 3, the core is made of silicate glass containing germanium oxide, the crat is made of silicate glass, and the non-circular member is made of silicate glass containing boron oxide. A polarization-maintaining optical fiber constructed of