KR100810867B1 - Apparatus and method for residual stress measuring of optical fiber - Google Patents

Abstract

An apparatus and a method for measuring residual stress of an optical fiber are provided to reduce time for measuring the residual stress which determines the variation in refractive index and optical loss in the optical fiber. An apparatus for measuring residual stress of an optical fiber includes light emitting parts(10), first light adjustment parts(20), polarization adjustment parts(30), an interference part, second light adjustment parts(50), and light detection parts(60). The light emitting parts are implemented using HeNe laser. The first light adjustment parts convert light emitted from the light emitting parts into parallel light. The polarization adjustment parts adjust the parallel light to light having polarization perpendicular to an optical fiber shaft. The second light adjustment parts convert the light passed through the interference part into parallel light. The light detection parts show the interference pattern of the light passed through the interference part.

Description

Apparatus and method for residual stress measuring of optical fiber

1 is a conceptual diagram showing a case where the interferometer of the residual stress measuring device of the optical fiber of the present invention is used as a Mach-Zehnder interferometer.

2 is a conceptual diagram showing a case where the interferometer of the residual stress measuring device of the optical fiber of the present invention is used as a Michelson interferometer.

3A is a conceptual diagram illustrating a phase change according to a light propagation path.

Figure 3b is a conceptual diagram for a conventional residual stress measuring method using a polarizer principle.

Figure 3c is a conceptual diagram showing the specific shape and configuration of the polarization control unit of the residual stress measuring device of the optical fiber of the present invention.

4 is a conceptual diagram of a living cell phase measurement apparatus using a conventional interferometer.

The present invention relates to an apparatus and a method for measuring residual stress generated in the process of fabricating an optical fiber.

Normally, residual stresses generated during the fabrication of optical fibers increase the light loss due to light scattering of the optical fibers and cause the refractive index to change due to the photoelastic effect.

In addition, the refractive index change of the optical fiber is an important factor in determining the waveguide characteristics of the optical signal in the optical fiber, and thus it is possible to accurately grasp the optical characteristics of the optical fiber. Therefore, the measurement of residual stress is very important for the production of high quality optical fibers, the development of special optical fibers, and the study of their characteristics.

Here, the basic formula for the measurement of residual stress of a general optical fiber is as follows.

-Relationship between residual stress and phase shift of optical fiber.

Where Δδ is the phase shift to be measured (difference between the phase value in the optical fiber axial direction and the cross-sectional direction), λ is the wavelength of the light source used, σ _z is the residual stress in the optical fiber, and C is the photoelastic coefficient. to be. This equation is a known equation, and Equation 1 can be obtained by Inverse Abel Transformation.

Equation 1

In addition, in the prior art for measuring the phase of a living cell using an interferometer, the phase modulation of 0, π / 2, π, 3π / 2, respectively, using a piezo electric translater (PZT) The intensity distribution of the living cells is measured by the following Equation 2 for each intensity image obtained at this time.

Equation 2

Here, in Equation 2, I ₀ , I _{π / 2} , I _π , and I _{3π / 2} are the intensity of light obtained from the CCD when the phase modulator is given 0, π / 2, π, and 3π / 2, respectively. . That is, each image may be obtained and a phase value may be obtained by Equation 2.

At this time, if the polarized light comes in, different phase values are obtained according to the polarization direction. (This is the case in which the measurement target is a material having polarization characteristics. Therefore, light having polarized light in the axial direction and the cross-sectional direction of each optical fiber is input by the polarization adjusting unit so that each phase value is measured, and a phase shift value is obtained from the difference in the values.

However, the conventional technique for measuring the phase of a living cell using an interferometer has a disadvantage in that it is limited to the case where the measurement target has no special polarization characteristic.

Of course, even if the measurement object has a polarization characteristic, it is possible to measure, but in the prior art has been used mainly limited to the general measurement object without a polarization characteristic. This is due to the lack of interest in objects having polarization characteristics or the lack of optical knowledge such as polarization characteristics.

As a conventional technique for measuring the residual stress of the optical fiber for producing a high quality optical fiber as described above is well shown in the "approximate stress residual device of the optical fiber" of Korea Patent Application No. 10-2002-0014509.

The technique shown in the Republic of Korea Patent Application No. 10-2002-0014509,

A fixing part for fixing the optical fiber to measure the residual stress;

A light generating device for generating light for measuring the residual stress of the optical fiber fixed to the fixed portion, a detector for detecting the light transmitted from the light generating device and transmitted through the optical fiber, and while turning along the circumferential direction of the optical fiber It is provided with a measuring unit for measuring the residual stress of the optical fiber from the phase change of the light transmitted through the optical fiber.

Since the conventional technique is to measure the residual stress while turning in the circumferential direction of the optical fiber, it is possible to accurately measure the residual stress distribution of the optical fiber, thereby having the advantage of measuring the asymmetry of the residual stress of the optical fiber, I have the same problem.

That is, the prior art, as shown in Figure 3a, by using the polarizer principle to measure the phase shift difference (△ δ = δ _z -δ _의 ) in the optical fiber axis direction and cross-sectional direction, and using this to measure the residual stress of the optical fiber Will be measured.

In more detail, as can be seen from the equation of FIG. 3B, the rotation angle and the relative phase of the polarizer exist in the sinus square function.

Therefore, the point where the intensity obtained from the CCD camera becomes zero is found by turning the polarizer.

Then the item in the sine square function, i.e., the value of θ + δ / 2 is also zero. Then, (-2) times of the polarizer rotation angle [theta] at that time (when the intensity of the CCD camera is zero) becomes the relative phase value ([delta] = -2 [theta]). Once the relative phase values are obtained, they can be calculated as residual stresses by the following Abel transformation.

However, the conventional method as described above has to obtain the rotation angle of the polarizer at the position where the intensity of the CCD camera is zero while rotating the polarizer, so that the measurement time is long and the relative phase value obtained according to the amount of the polarizer is rotated. The disadvantage is that resolution is determined.

The present invention is proposed to solve the above conventional problems,

A polarization control unit is added to a conventional technique for measuring the phase distribution of living cells using an interferometer, and the phase distribution in the optical fiber axis direction along the polarization direction of the light source (Axial Phase, δ _z ) and the phase distribution in the optical fiber cross-section direction (Transverse Phase). and δ _⊥ ), and the residual stress measuring device of the optical fiber which makes it possible to easily measure the residual stress existing in the optical fiber from the phase distribution in the axial direction and the sectional direction in the optical fiber measured in this way in a short time and It aims to provide the method.

Residual stress measuring apparatus of the optical fiber of the present invention for achieving the above object

A light generating unit generating light; A first light control unit converting light generated by the light generation unit into parallel light; A polarization control unit for converting the polarized light of the parallel light converted by the first light control unit into polarization parallel to the optical fiber axis direction and polarization perpendicular to the optical fiber axis direction and incident the interferometer part; An interferometer portion for delaying parallel light polarized by the polarization adjusting unit to obtain an arbitrary interference pattern; A second light control unit for maintaining the state of the light passing through the interferometer portion as parallel light; And a light detecting portion for measuring the residual stress of the optical fiber by detecting and measuring the interference fringe image generated by the light passing through the second light control unit.

And the residual stress measuring method of the present invention

a) generating light using the light generating unit;

b) converting the light generated by the light generation unit into parallel light using a first light control unit;

c) converting the polarized light of the parallel light converted by the first light control part into polarized light parallel to the optical fiber axis direction and polarized light perpendicular to the optical fiber axis direction by using the polarization control part to be incident on the interferometer part;

d) an interferometer portion for delaying parallel light polarized by the polarization adjusting unit to obtain an arbitrary interference pattern;

e) maintaining the state of the light passing through the interferometer portion as parallel light using a second light control unit; And

f) measuring the residual stress of the optical fiber by measuring the light passing through the second light adjusting unit by detecting the interference fringe image using the light detecting portion.

1 and 2 are conceptual diagrams of an apparatus for measuring residual stress of an optical fiber according to the present invention.

As shown in Figure 1 and 2, the residual stress measuring apparatus of the optical fiber according to the present invention is basically a light generating unit (10, 80), the first light control unit (20, 90), polarization control unit (30, 100), the interferometer portion, the second light control portion (50, 120) and the light detection portion (60, 130) and the like.

The light generators 10 and 80 are preferably implemented using a helium neon (HeNe) laser because the helium neon (HeNe) laser has a very narrow spectrum and can be considered as a light source having a nearly short wavelength. When light 1 and 71 having short wavelength properties are transmitted through the optical fibers 45 and 114, they may exhibit unique phase change values, respectively.

The first light control units 20 and 90 convert light generated from the light generation unit into parallel light, and are preferably formed of plano-convex lenses.

The plano-convex lens is usually light emitted from the laser used from the light generators 10 and 80 is not parallel light, although small but divergent, that is, light of a spreading shape, the light generator 10, 80) It plays the role of making the light emitted from the laser used in parallel light into parallel light. For this purpose, the light control part is required, and one side is flat and the other side is convex. .

The polarization control units 30 and 90 allow the parallel light passing through the first light control units 20 and 90 to be adjusted to light having polarization parallel to the optical fiber axis and polarization perpendicular to the optical fiber axis.

The polarization control unit (30, 90) is usually implemented using a polarizer, which is formed in the form of a flat plate, the polarizer T.A. There is something called Transmission Axis.

That is, since light is electromagnetic waves (waves that propagate with electric and magnetic fields), when the vibration direction of the electric field is called a polarization direction, light generally has a form in which electromagnetic waves having various polarization directions are mixed.

Accordingly, the polarizer passes only the light having the polarization direction corresponding to T.A. among the light having the various polarization directions, and prevents the light that does not correspond to the T.A. remaining polarization from passing through.

Based on the above principle, in the present invention, as shown in FIG. 3C, the phase of the optical fiber by light having polarization coinciding with the optical fiber axial direction is rotated by using a motor to rotate the polarizer in the direction corresponding to the axial direction of the optical fiber. Firstly, the polarizer is once again rotated in a direction perpendicular to the optical fiber axis direction (cross-sectional direction) to finally measure the phase of the optical fiber due to light having polarization coinciding with the optical fiber cross-sectional direction.

The interferometer portion usually consists of a Mach-Zehnder interferometer portion 41-45, as shown in FIG. 1, or a Michelson interferometer portion 111-114, as shown in FIG. By shifting the phase through the phase shifting method, the light acquires an interference pattern due to the delay of the light propagating phase (the phase delay will lengthen the path the light travels by). .

Here, the interferometer portions 111 to 114 are optical splitters 111, which divides one light into two different lights to make an interference pattern, and creates an interference pattern by causing a certain amount of phase delay. Phase modulator 112 and a mirror 113 that serves to collect the two light is divided into a path so that the interference pattern is generated by reflecting the light that is advanced.

Accordingly, the interference pattern (interference pattern) will come out differently according to the amount of delay of the light path, and each delay is arbitrarily given in four forms of 0, π / 2, π, and 3π / 2.

There is no reason to use the interferometer as a Mach-Zehnometer interferometer, but the Mach-Zehnometer interferometer is the same as the Michelson interferometer in principle.

In the case of the Michelson interferometer, however, since the optical component is less than the Mach-Zehnder interferometer (for example, the optical splitter or the reflection mirror is smaller), the Mach-Zehnder interferometer is easier to construct the interferometer than the Michelson interferometer. (Alignment is easy)

The second light control units 50 and 120 convert the light passing through the interferometer into parallel light and enter the light detecting units 60 and 130.

In the photodetecting portions 60 and 130, the interference fringes of the light passing through the interferometer appear as images, and in the photodetecting portions 60 and 130, four interference fringe images (modulations) modulated by the phase modulators 46 and 112. Images when the phases are 0, π / 2, π, and 3π / 2, respectively, are obtained and the phase distributions (δ _z , δ _⊥ ) are obtained through the obtained four interference fringe images and the above equation.

In this case, the phase distribution measured from the interference fringe images shown in the photodetecting portions 60 and 130 by the light having polarization parallel to the optical fiber axis by the polarization adjusting units 30 and 100 is the phase in the optical fiber axis direction. Phase distribution as measured by the Axial Phase, δ _z , and also from the interference fringe images shown in the photodetecting portions 60, 130 by light having polarization perpendicular to the optical fiber axis by the polarization control units 30, 100. Is defined as the phase distribution (Transverse Phase, δ _⊥ ) in the fiber cross-section direction.

The phase shift value (Δδ = δ _z -δ _⊥ ) can be obtained from the difference between the phase distribution in the optical fiber axial direction and the phase distribution in the cross-sectional direction (δ _z -δ _⊥ ), and the phase shift thus obtained (Δ δ) ), The residual stress of the optical fiber can be obtained by using Inverse Abel Transformation. The related formula is:

Referring to the method of measuring the residual stress of the optical fiber using the residual stress measuring apparatus of the optical fiber according to the present invention configured as described above are as follows.

First, the light (1, 71) from the light source (10, 80) of the light generating unit is converted into parallel light while passing through the plano-convex lens (20, 90) portion of the first light control unit. The light converted into parallel light passes through the polarization control units 30 and 100 of the polarization control unit, thereby being adjusted to light having polarization parallel to the optical fiber axis and polarization perpendicular to the optical fiber axis.

The polarized light passes through the Mach-Zehnder interferometer portions 41 to 45 or the Michelson interferometer portions 111 to 114, and then passes through the part of the plano-convex lens 50 and 120 of the second light control unit to finally detect light. Incidents are entered into portions 60 and 130.

In this case, the interference fringes of the light passing through the interferometer appear as images in the photodetecting portions 60 and 130, and at this time, the four interference fringe images (modulation) by the phase modulators 46 and 112 in the photodetection portions 60 and 130. Images when the phases are 0, π / 2, π, and 3π / 2, respectively, are obtained, and the phase distributions δ _z and δ _⊥ are obtained through the obtained four interference fringe images and the above equation.

In this case, the phase distribution measured from the interference fringe images shown in the photodetecting portions 60 and 130 by the light having polarization parallel to the optical fiber axis by the polarization adjusting units 30 and 100 is the phase in the optical fiber axis direction. Phase distribution as measured by the Axial Phase, δ _z , and also from the interference fringe images shown in the photodetecting portions 60, 130 by light having polarization perpendicular to the optical fiber axis by the polarization control units 30, 100. Is defined as the phase distribution (Transverse Phase, δ _⊥ ) in the fiber cross-section direction.

The phase shift value (Δδ = δ _z -δ _⊥ ) can be obtained from the difference (δ _z -δ _⊥ ) between the phase distribution in the axial direction and the phase distribution in the cross-sectional direction.

Where Δδ is the phase shift to be measured (difference between the phase value in the optical fiber axial direction and the cross-sectional direction), λ is the wavelength of the light source used, σ _z is the residual stress in the optical fiber, and C is the photoelastic coefficient. to be. This equation is a known equation, and the phase shift (△ δ ) thus obtained can be used to find the residual stress of the optical fiber using Inverse Abel Transformation.

As described above, the residual stress measurement apparatus of the optical fiber according to the present invention measures the phase distribution in the axial direction (Axial Phase, δ _z ) and the phase distribution in the cross-sectional direction of the optical fiber (Transverse Phase, δ _⊥ ) together and a car by using a _{(△ δ = δ z -δ ⊥} ) effect, which enables measuring the residual stress of the optical fiber than in a short time by measuring the residual stress of determining the change in refractive index and the optical loss in the optical fiber.

Claims (8)

In the device for measuring the residual stress of the optical fiber, A light generating unit generating light; A first light control unit converting light generated by the light generation unit into parallel light; A polarization control unit for converting the polarized light of the parallel light converted by the first light control unit into polarization parallel to the optical fiber axis direction and polarization perpendicular to the optical fiber axis direction and incident the interferometer part; An interferometer portion for delaying parallel light polarized by the polarization adjusting unit to obtain an arbitrary interference pattern; A second light control unit for maintaining the state of the light passing through the interferometer portion as parallel light; And And a light detecting portion for measuring residual stress of the optical fiber by detecting and measuring an interference fringe image generated by the light passing through the second light adjusting unit. The optical fiber residual stress measuring device, characterized in that for rotating the polarization control unit to create a polarization parallel to the optical fiber axis direction and a polarization perpendicular to the optical fiber axis direction. The method of claim 1, After the light having polarization parallel to the optical fiber axis direction is passed through the interferometer by the polarization control unit, the phase distribution in the optical fiber axis direction (Axial Phase, δ _z ) is measured from the interference fringe image generated by the phase modulator. Optical fiber residual stress measuring device. The method of claim 1, Then by the polarization control unit has passed through the light interferometer with a vertical polarization in the fiber axis direction, and that measures the interference phase distribution (Transverse Phase, δ _⊥) of the optical fiber cross-sectional direction from the pattern image created by the phase modulator Optical fiber residual stress measuring device. The method according to claim 2 or 3, Using the measured difference of the phase distribution (Axial Phase, δ _z ) in the optical fiber axis direction and the phase distribution (Transverse Phase, δ _⊥ ) in the optical fiber cross-section direction (Δδ = δ _z -δ _⊥ ) Equation 1

Residual stress measuring device, characterized in that for measuring the residual stress present in the optical fiber through. In the method for measuring the residual stress of the optical fiber, a) generating light using the light generating unit; b) converting the light generated by the light generation unit into parallel light using a first light control unit; c) converting the polarized light of the parallel light converted by the first light control part into polarized light parallel to the optical fiber axis direction and polarized light perpendicular to the optical fiber axis direction by using the polarization control part to be incident on the interferometer part; d) delaying parallel light polarized by the polarization control unit to obtain an arbitrary interference pattern by using an interferometer portion; e) maintaining the state of the light passing through the interferometer portion as parallel light using a second light control unit; And f) measuring the residual stress of the optical fiber by measuring the light passing through the second light control unit by detecting an interference fringe image using a light detecting portion, In c) And rotating the polarization control part to convert the polarization control part into polarization parallel to the optical fiber axis direction and polarization perpendicular to the optical fiber axis direction using the polarization control part. The method of claim 5, After the light having the polarization parallel to the optical fiber axis direction is passed through the interferometer by the polarization control unit, the phase distribution in the optical fiber axis direction (Axial Phase, δ _z ) is measured from the interference fringe image produced by the phase modulator. Residual stress measurement method characterized in that the optical fiber. The method of claim 5, Then by the polarization control unit has passed through the light interferometer with a vertical polarization in the fiber axis direction, and that measures the interference phase distribution (Transverse Phase, δ _⊥) of the optical fiber cross-sectional direction from the pattern image created by the phase modulator Residual stress measurement method characterized in that the optical fiber. The method according to claim 6 or 7, Using the measured difference of the phase distribution (Axial Phase, δ _z ) in the optical fiber axis direction and the phase distribution (Transverse Phase, δ _⊥ ) in the optical fiber cross-section direction (Δδ = δ _z -δ _⊥ ) Equation 1

Residual stress measurement method characterized in that the residual stress present in the optical fiber through.

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