JPH0572433A - Optical fiber polarizer - Google Patents

Abstract

PURPOSE:To provide the optical fiber polarizer of a high extinction ratio and low excess loss. CONSTITUTION:Stress imparting parts 3 are provided in a clad 2 around a core 1. The optical fiber is constituted by using GeO2, SiO2 for the core 1, SiO2 for a clad 2 and B2O3, SiO2 for the stress imparting parts 3. The specific refractive index difference of the core 1 is specified to DELTA 0.17%, the diameter of the core 1 after fiber formation to 5.5mum, the diameter of the stress imparting parts 3 to 35mum and the spacing between the stress imparting parts 3 to 77m. The specific refractive index difference of the core 1 and clad 2 of such double refractive optical fiber is >=0.13% and <=0.24% and the double refractive index thereof is >=8X10<-4> and, therefore, the optical fiber polarizer having the high extinction ratio and low excess loss is obtd. when this fiber is bent to a coil shape and formed as the optical fiber polarizer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber polarizer used for optical fiber sensors or parts for optical communication and for extracting linearly polarized light from unpolarized light.

[0002]

2. Description of the Related Art Conventionally, bulk type polarizers have been mainly used as polarizers, but they are being replaced by optical fiber polarizers in terms of assembling and handling of optical axis alignment. This is because, in order to use the conventional bulk polarizer in a fiber sensor, a lens must be used to couple the light with the optical fiber, but the optical axis easily shifts due to vibration or heat, and the light quantity fluctuates. Therefore, there is a problem that the reliability of the sensor system is poor.

As this optical fiber polarizer, there is one using a birefringent optical fiber as disclosed in, for example, Japanese Patent Laid-Open No. 62-87906. In this optical fiber, a pair of stress applying portions are embedded in the clad so as to sandwich the core in the center from both sides. An optical fiber polarizer is formed by winding the optical fiber (PANDA fiber) in a coil shape.

[0004]

However, the conventional polarization-maintaining optical fiber is not sufficient to operate as an optical fiber polarizer. Specifically, when an extinction ratio is increased, excess loss also occurs. There was a problem of getting bigger. In addition, since the central wavelength of a light source such as an LD or SLD causes a wavelength change with respect to a temperature change, there is a problem that characteristics related to the extinction ratio and excess loss deteriorate.

It is therefore an object of the present invention to provide an optical fiber polarizer which has a high extinction ratio and a low excess loss, and whose characteristics are not deteriorated even when the wavelength of incident light varies.

[0006]

The optical fiber polarizer of the present invention is formed by bending a birefringent optical fiber having different propagation constants for two orthogonal linearly polarized lights into a coil shape. In the refractive optical fiber, the relative refractive index difference Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ^{2 in} which the refractive indexes of the core and the clad are defined as n ₁ and n ₂ , respectively, is 0.13% or more and 0.24%. Satisfies the following.

[0007]

Operation: An optical fiber polarizer is a birefringent fiber wound to have a loss difference between two polarizations whose polarization planes are orthogonal to each other, and only one polarization is taken out. The loss difference can be adjusted by the relative refractive index difference Δ between the core and the clad and the birefringence B generated in the core. On the other hand, the wavelength (λ _CX , where the bending loss of each of the two orthogonal polarizations is 1 dB,
If the difference Δλ of λ _CY ) can be widened, the excess loss does not increase even if the extinction ratio is increased. Here, the above Δ, B, Δ
There is a relationship shown in FIG. 3 between λ, and the present invention pays attention to this, and widens the wavelength difference Δλ by setting the relative refractive index difference Δ to 0 or 24% or less, thereby providing a high extinction ratio and a low excess. It allows for losses. The optical confinement effect as a fiber is maintained by setting the relative refractive index difference Δ to 0.13% or more.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of the present invention and its embodiments will be described more specifically with reference to the drawings.

FIG. 1 is a block diagram of an optical fiber polarizer according to the present invention, and FIG. 2 is a sectional view of a birefringent fiber used therein. As shown in FIG. 1, the optical fiber polarizer is formed by winding the birefringent fiber 10 in a coil shape. On the other hand, the birefringence optical fiber has a structure in which the stress imparting portion causes a birefringence in the core 1, as shown in FIGS. 2 (a) and 2 (b).
It is configured like. FIG. 2A is a cross-sectional view of an example of a birefringent fiber, and the core 1 is sandwiched from both sides,
A pair of circular stress applying portions 3 are provided in the clad 2. The stress applying section 3 may be deformed into a fan shape as shown in, for example, FIG. 2B, and if it has a structure that gives a desired birefringence in the core 1, when it is used as an optical fiber polarizer. There is no problem, and therefore, the shape is not limited to the shapes shown in FIGS.

As described above as the problem to be solved by the present invention, in the optical fiber polarizer having the above structure, when the extinction ratio of the birefringent fiber is increased, the excessive loss becomes large. By the way, in an optical fiber polarizer, a birefringent fiber is bent in a coil shape to give a difference in loss between two orthogonal polarizations (x polarization and y polarization), and only one polarization (x polarization) is provided. It can be understood that the above-mentioned problem that the extinction ratio and the excess loss increase at the same time because the device is a device that takes out the light source because the difference in loss between the two polarizations whose polarization planes are orthogonal to each other is small. And the loss difference between the two orthogonal polarizations is
A desired value is obtained by the relative refractive index difference Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ² when the refractive indices of the core and the clad are n ₁ and n ₂ , respectively, and the birefringence B generated in the core. Can be adjusted to.

On the other hand, assuming that the wavelengths at which the bending loss of the two orthogonal polarized waves is 1 dB are λ _CX and λ _CY , respectively, the difference in bending loss is large, which means that the wavelength difference Δλ = λ _CX −λ _CY is wide. Is equivalent to Therefore, it can be seen that if the wavelength difference Δλ can be widened, the excess loss does not increase even if the extinction ratio is increased (the bending number is increased), so that the polarizer can operate as a polarizer.

Therefore, the present inventor examined the relationship among the relative refractive index difference Δ, the birefringence index B, and the wavelength difference Δλ, and as a result, FIG.
It was found that there is a relationship shown in. From FIG. 3, the smaller the relative refractive index difference Δ, the wavelength difference Δλ even if the birefringence B is the same.
Since it is spread, the excess loss does not increase even if the extinction ratio is increased, and therefore it is found that the polarizer characteristic is advantageous. However, if the relative refractive index difference Δ between the core and the clad is excessively reduced, the optical confinement effect in the core is lost, stable light transmission as an optical fiber cannot be performed, and the loss increases.

Therefore, according to the optical fiber polarizer of the present invention, from the same figure, the upper limit of the relative refractive index difference Δ is Δλ> 15.
0.24% or less to obtain 0 nm, and the relative refractive index difference Δ
It has been found that the lower limit of 0.13% is a limit at which the loss does not increase in practical handling as a polarizer formed by winding a birefringence optical fiber (the range shown by the shaded area in the figure).
At this time, the minimum required birefringence B is 8 × 10.
^-4 .

From the above results, the optimum relative refractive index difference Δ of the fiber operating as a polarizer is 0.13% or more and 0.1.
It was found to be below 24%. In this way, by determining the structural parameters of the birefringent fiber, by bending the birefringent index optical fiber to form an optical fiber polarizer, a polarizer satisfying both a large extinction ratio and a small excess loss can be manufactured. .. Further, by setting the value of the wavelength difference Δλ at a certain level or more (specifically, 150 nm or more), the characteristics of high extinction ratio and low excessive extinction can be obtained even when the central wavelength of the light source fluctuates (for example, changes due to temperature). An optical fiber polarizer that does not deteriorate can be manufactured.

Hereinafter, further details will be described based on specific examples.

In FIG. 2A, the core 1 and the clad 2 are
The relative refractive index difference Δ was 0.17%, the diameter of the core 1 after fiber formation was 5.5 μm, the diameter of the stress applying portion 3 was 35 μm, and the interval between the stress applying portions 3 was 7 μm. Also, the core 1 is Ge
In O ₂ · SiO _2, the cladding 2 in SiO _2, the stress applying section 3 is constituted respectively by B ₂ O ₃ · SiO _2. The composition of the core 1 and the clad 2 is, for example, the core 1
Of P ₂ O ₅ .SiO ₂ and clad 2 of SiO ₂ , or core 1 of SiO ₂ and clad 2 of F · S.
A combination of iO ₂ is also conceivable, as long as the relative refractive index difference Δ between the core 1 and the clad 2 has a predetermined value. Also, core 1
In addition to B ₂ O ₃ , the material that gives the birefringence effect to
l ₂ O _{3 is} also conceivable. Furthermore, the addition of GeO ₂ (or P ₂ O ₅₎ to B ₂ O ₃ · SiO _2, when matching the refractive index of the cladding 2 and the stress applying section 3, there is no refractive index difference adversely affect the bending loss effect Target.

A base material having the above structural parameters was manufactured, and the cladding 2 was drawn with an outer diameter of 125 μm.
The birefringence index B of the obtained fiber was 1.1 × 10 ⁻³ . Then, this fiber was wound 10 times on a 30 mmφ circular reel, and the bending loss wavelength characteristics of orthogonal two polarizations were examined.
FIG. 4 is a diagram showing the result. As shown,
Practically enough extinction ratio of 30 dB or more, which is a sufficient polarizer characteristic,
The wavelength range where the excess loss is 1 dB or less is 805 to 875 nm
That is, a polarizer having a wide operating wavelength is obtained. At this time, Δλ is 200 nm.

The characteristics were evaluated using an SLD having a central wavelength of 840 nm (half-value width of 10 nm). The extinction ratio was 3
It was 3 dB and the excess loss was 0.1 dB. This SLD is
Center wavelength ± 25 with respect to temperature change from -30 ℃ to + 80 ℃
Although it fluctuates by nm, since the above-mentioned optical fiber polarizer has an operating wavelength in a wide band, no deterioration in characteristics was observed even with a temperature change.

[0019]

As described above, according to the optical fiber polarizer of the present invention, a high extinction ratio and a low excess loss can be realized at the same time by appropriately setting the relative refractive index difference Δ of the structural parameters. Further, since the operating wavelength range of the polarizer is wide, it is stable in characteristics even if the central wavelength of the light source shifts due to temperature changes, and is effective as an optical fiber sensor or optical fiber communication component.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure of an optical fiber polarizer according to the present invention.

FIG. 2 is a cross-sectional view of a birefringent fiber used for the optical fiber polarizer of the present invention.

FIG. 3 is a diagram showing a relationship between a bending loss difference and a birefringence index when a relative refractive index difference is used as a parameter.

FIG. 4 is a diagram showing characteristics of an optical fiber polarizer according to a specific example.

[Simple explanation of symbols]

 1 ... Core 2 ... Clad 3 ... Stress applying part

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Suganuma 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Shigeru Tanaka 1-tani, Sakae-ku, Yokohama-shi, Kanagawa Sumitomo Electric Ki Industry Co., Ltd. Yokohama Works

Claims (2)

[Claims] 1. An optical fiber polarizer formed by bending a birefringent optical fiber having different propagation constants between two polarizations whose polarization planes are orthogonal to each other into a coil shape, wherein a relative refractive index difference between a core and a clad of the birefringent optical fiber. Is 0.13% or more and 0.24% or less, an optical fiber polarizer. 2. The optical fiber polarizer according to claim 1, wherein the birefringent optical fiber has a birefringence of 8 × 10 ⁻⁴ or more.