JPS6287906A - Fiber type polarizer - Google Patents

Abstract

PURPOSE:To prevent an optical axis from the generation of shear by forming specific conditions among the wavelength of laser light from a light source, the longest interrupting wavelength, the refractive indexes of a core and a clad, and a specific refractive index difference in a double refractive optical fiber having different transmission constants for two rectangularly intersected straightly polarized beams. CONSTITUTION:In the double refractive optical fiber 2 having different transmission constants for two rectangularly intersected straightly polarized beams, the wavelength lambda of laser light from the light source, the longest interrupting wavelength lambdaC out of interrupting wavelengths in a higher mode group having electric field distribution other than a Gaussian waveform in the optical fiber 2 and the specific refractive index difference DELTA defined by DELTA=(n<2>1-n<2>2)/2n<2> using the refractive indexes n1, n2 of the core 3 and the clad 4 satisfy the condition expressed by (4.3X10<2>DELTA+0.18<=lambda/lambdaC<=4.3X10<2>DELTA+0.8). Consequently, the size and weight of the optical polarizer can be reduced and optical coupling with another fiber can be easily attained by a connector or fusion splicing.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a fiber polarizer used in fiber sensors and optical communications.

(Prior art) A device that allows only polarized light to pass in a certain direction can be used to generate and detect polarized light, and can be used as a polarizer or an analyzer depending on the application.
lyser). The former is used to create plane polarized light from natural light, and the latter is used to detect the plane of polarization.

FIG. 8 is an explanatory diagram of the conventionally used I) A photon, and 1 is a Glanton-Phnom prism in which one piece of calcite as shown in the figure is glued with Canada balsam in opposite directions.

Of the natural light incident on the polarizer, only the plane-polarized light of the P component is transmitted.

However, in order to use such a conventional bulk type polarizer in a fiber sensor, a lens must be used to couple light with the original fiber.

When a lens or bulk (#i1 element photon is present), the optical axis easily shifts due to vibrations, heat, etc., and the amount of light fluctuates, so the system has a drawback of poor reliability.

(Problems to be Solved by the Invention) In order to eliminate the above-mentioned drawbacks, it is an object of the present invention to provide a fiber-shaped polarizer that utilizes polarization-dependent bending loss in the fiber.

(Means for solving the problem) In a birefringent optical fiber with different propagation constants for two orthogonal linearly polarized lights, the wavelength of the laser light from the light source is λ, and the electric field distribution other than the Gaussian waveform in the original fiber is The longest cutoff wavelength λC of the higher-order mode group
And, using the refractive indices n0 and n2 of the core S and cladding, Δ=(n yo - n2), 4n.

The cutoff wavelength λ with respect to the relative refractive index difference Δ defined by the formula
f is designed so that C satisfies the formula (2) described below.The present invention is entirely composed of original fibers, and is different from conventional fibers in that it can be optically coupled to other original fibers without using lenses or the like. different from the polarizer.

FIG. 1 is a diagram showing an embodiment of the present invention. The fiber polarizer shown in FIG. 1 consists of a stressed PANDA fiber 2 wound around a diameter drum.

FIG. 2 is a cross-sectional view of the PANDA fiber, with core 3
Stress applying portions 5 having a thermal expansion coefficient different from that of the cladding layer are arranged on both sides of the cladding layer. Specifically, glass doped with 15 ml of quartz B2O2 is used in the stress applying part.

FIG. 8 is an explanatory diagram of a method for manufacturing the PANDA 7 AIPA.

In FIG. 8, 6 is a core, into which a stress-applying base material 8 is inserted into the preform 7f, which has been drilled with an ultrasonic drill.
The heater 9 heats 210,004 mm for wire drawing.

FIG. 4 shows the refractive index distribution in consideration of the photoelastic effect due to stress in the PANDA fiber. n and n20 are the refractive indices of the core 8 and the cladding, respectively, when stress is not considered.

FIG. 4 shows the refractive index distribution nx on the y-axis in FIG.
* indicates ny. From FIG. 4, it can be seen that in the core and in the vicinity of the core, defeat > ny(]), but in the cladding, nx #ny (2). In the figure, βx/k or βy/ is the normalized propagation constant of x and y polarized waves. When such a PANDA fiber is bent with radius R (= D/2) as shown in Fig. 5, the phase velocity of the plane wave in the fiber is vR (rl = v-・(] + p) ( 8
It is given by 1. Here, voo is when there is no bending (R
=oo) is the phase velocity. In addition, Fig. 5) shows E (
r> is the electric field distribution of light.

The phase velocity of the plane wave in the cladding increases with the increase of r9, and the phase velocity V in the medium at the position r=r0. z ;
It is radiated into the cladding beyond O/nal〇Here, n
cl is the refractive index of the cladding, and C is the speed of light in vacuum. The point at which radiation begins tr0 is given by VR(ro) = 0/nc7. As shown in Figure 4, β7 is equal to - βy/k = B (5) (
B: mode birefringence] Therefore, the position where the X polarized wave is radiated is troCX) and the position r where the y polarized wave is radiated. It can be seen that there is the following difference between (y).

In this example, B=4. FIXlo.

D=2R=4. Since it is Fl an, rO(x)
-r CY'#7 Am (7). Therefore, it can be seen that the y-polarized wave is radiated from a position closer to the core than the x-polarized wave.

Since the electric field in the cladding decreases exponentially, the X-polarized wave suffers a larger bending loss, whereas the X-polarized wave suffers almost no bending loss, resulting in polarization dependence of bending loss. I understand.

Figure 6 shows the fiber (!1 element (Δ=0.
24i%) and the wavelength dependence of the loss of X polarization. 2R=4. F) cm, number of turns N
= 4, PANDA fiber length L = 1. F) m
In the case of , the extinction ratio is 20 dB or more and the loss of X polarization is 1
The wavelength range below dB is 1.48 μm x 1.61 μm (8) (shaded area in FIG. 6).

The cutoff wavelength of the PANDA fiber used is λC;
Since it is 1 μm, it becomes λ.

FIG. 7 shows the results of experiments conducted with various changes in the relative refractive index difference Δ, the cutoff wavelength λC, the mode birefringence B1, the bending radius R, and the number of turns K. In the area above straight line a in the figure, the extinction ratio is 2.
This is a region where the loss is 0 dB or more, and the region below the straight line is a region where the loss of X polarization satisfies the condition of 1 (iB or less).

That is, the area surrounded by diagonal lines in FIG. 7 is the area that satisfies the characteristics as a polarizer.

Straight lines a and b can be approximated by the following empirical formula: 0 Line a: λ/λ = 4. aXlo Δ10.18 (10
) Straight line b: λ/λ -4,8X 10 Δ+0.8 (
11) Therefore, the range surrounded by diagonal lines is expressed as ◇ After all, when the wavelength of the light source is λ and a fiber with a relative refractive index difference of Δ is used, the cutoff wavelength λC is expressed by the formula (]2)
It can be seen that the design should be designed to satisfy the following.

(Effects of the Invention) As is clear from the above explanation, the fiber polarizer of the present invention is small and lightweight, and optical coupling with other fibers can be easily achieved by using a connector or fusion splice 1. There are advantages.

[Brief explanation of drawings]

Figure 1 is a diagram of an embodiment of the invention, Figure 2 is a cross-sectional view of a PANDA fiber, and Figure 3 is a diagram of a PANDA fiber.
An explanatory diagram of the method for manufacturing a DA fiber, Figure 4 is a PANDA
Figure 5 is a diagram showing the refractive index distribution of each polarized wave of the 7-eye fiber. Figure 5 is a diagram explaining the radiation loss due to the bent fiber. Figure 6 is the fiber fAr when 2R = 4Jα and N = 4.
A diagram showing the wavelength dependence of the +photon extinction ratio and the loss of FIG. 2 is an explanatory diagram of a Grand-Tong-Pnon polarizer. ]... Glan-Thompson prism 2... PANDA fiber 3... Core 4...
Clad 5... Stress applying part 6... Core 7... Preform with holes 8... Stress applying base material 9... Heater 10...
Core patent applicant: Nippon Telegraph and Telephone Corporation (Figure 2, Figure 3, Figure 4)

Claims (1)

[Claims] 1. In a birefringent optical fiber in which the propagation constants for two orthogonal linearly polarized lights are different, the wavelength of the laser beam from the light source is λ, and the optical fiber has an electric field distribution other than a Gaussian waveform. The longest cutoff wavelength λ_C among the cutoff wavelengths of the next mode group and the refractive index n_1 and n of the core and cladding
The cutoff wavelength λ_C is 4.3×10^2Δ+0.18≦λ with respect to the relative refractive index difference Δ Δ=(n_1^2−n_2^2)/2n_1^2 defined as follows using _2. /λ_C≦4.3×
A fiber polarizer characterized by satisfying the condition of 10^2Δ+0.8.