JP2003042894A - Method and apparatus for measuring out of roundness of core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and apparatus for easily and quickly measuring the out of roundness of a core of an optical fiber blank. SOLUTION: The out of roundness measuring apparatus 1 comprises a container 2 for dipping an optical fiber blank 10 in a liquid, a light source 3 for irradiating the blank 10 with parallel rays of light, a photodetector 4 for receiving the light passed through the blank, hold rotators 5a, 5b for holding and rotating the blank 10, and a core out of roundness measuring unit 6. When the refractive indices of a clad part 10b and a liquid are approximately equal, the light passing through the clad 10b travels straightly without refraction, while the light passing through the core 10a is refracted to arrive at the photodetector due to the refractive index difference between the core 10a and the clad 10b. Hence, an image obtained by the photodetector 4 has specified bright and dark portions. The measuring unit 6 calculates the out of roundness of the core, based on the light intensity distribution of the image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the non-circularity of a core of an optical fiber preform, and an apparatus therefor.

[0002]

2. Description of the Related Art When an optical fiber is produced by drawing an optical fiber preform, the optical characteristics of the produced optical fiber largely depend on the non-circularity of the core of the optical fiber preform.
For example, if an optical fiber is manufactured from an optical fiber preform having a large non-circularity in the core portion, the core non-circularity of the optical fiber itself also deteriorates. Therefore, the polarization mode dispersion characteristic of the optical fiber is deteriorated. When the polarization mode dispersion characteristic deteriorates, there is a problem that optical transmission at a high bit rate becomes difficult. In order to reduce the core noncircularity of the optical fiber preform, it is first necessary to measure the core noncircularity.

[0003]

As a method for determining the core noncircularity of the optical fiber preform, the optical fiber preform is irradiated with light and the light transmitted through the optical fiber preform is observed. There is a method of calculating the refractive index distribution of the material and obtaining the core non-circularity based on this refractive index distribution. As a result of earnest research on such a method, the present inventors have found the following problems. That is, when the optical fiber base material is irradiated with light, the irradiated light is refracted at the side surface of the optical fiber base material and at the boundary between the clad portion and the core portion, so that the image formed by the transmitted light is complicated. Will have different patterns. In order to obtain the core non-circularity, it is necessary to correct a pattern complicated by factors other than the core non-circularity by predetermined data processing. For that purpose, a complicated calculation is required, and a long time is required for the measurement.

Therefore, the present invention has been made in order to solve the above problems, and provides a method and an apparatus for easily measuring the core noncircularity of an optical fiber preform in a short time. With the goal.

[0005]

The core noncircularity measuring method according to the present invention is a core noncircularity measuring method for measuring the core noncircularity of an optical fiber preform having a core and a clad. Then, (a) the optical fiber preform is immersed in a liquid whose refractive index is substantially the same as that of the clad portion, and (b) the optical fiber preform immersed in the liquid is rotated with its central axis as a rotation axis, c)
Parallel light is applied to the side surface of the rotating optical fiber base material, and (d) the light that is applied to the side surface of the optical fiber base material and passes through the optical fiber base material is received and imaged, and (e) the imaging The core non-circularity is obtained based on the light intensity distribution obtained as a result.

According to the above method, the optical fiber preform whose core non-circularity is to be measured is immersed in a liquid whose refractive index is almost the same as that of the cladding. Then, the optical fiber preform immersed in the liquid is rotated about the central axis of the optical fiber preform. Then, the side surface of this optical fiber preform is irradiated with parallel light. Since the refractive index of the clad part and that of the liquid are almost the same, parallel light traveling so as to pass through the clad part is
It goes straight without being refracted at the interface between the liquid and the clad. On the other hand, the parallel light that travels through the core is
Since there is a difference in refractive index between the core part and the clad part, they are refracted at the boundary between the core part and the clad part and proceed to be condensed. As a result, the light received by the light receiving unit exhibits a light intensity distribution including a bright portion and a dark portion. Since such a light intensity distribution is formed by the light refracted at the boundary between the core portion and the clad portion, the core non-circularity can be obtained based on this light intensity distribution.

Further, the core non-circularity measuring method according to the present invention measures the width of the bright portion produced by the light transmitted through the core in the above light intensity distribution, sets the width of the bright portion to Y, and rotates the light. The rotation angle of the fiber base material is obtained, and this rotation angle is φ. The width Y of the bright portion is expressed by the formula Y = A + B × s with respect to the rotation angle φ.
Fitting is performed using inφ, and the non-circularity of the core portion is calculated from the fitting result by the equation D (%) = (2 × B / A) × 1.
Can be estimated based on 00. The core non-circularity can be obtained by performing fitting using the above equation on the graph obtained as a result of measurement and obtaining the values A and B.

Further, when the light transmitted through the optical fiber base material is received and imaged, the light is irradiated onto the pixel surface having a plurality of pixels to form an image of the optical fiber base material, and the image is formed on the pixel surface. It is preferable that at least 1000 or more pixels are included in the portion formed by the core portion of the image. Thereby, the relative value of the diameter of the core portion can be accurately obtained.

Furthermore, the refractive index d _l of the above liquid is
It is preferable that the relationship expressed by 0 ≦ | (d ₁ −d _k ) / d _k | ≦ 0.05 be satisfied with respect to the refractive index d _k of the clad portion. If the refractive index of the liquid in which the optical fiber base material is dipped satisfies such a relationship, the parallel light with which the optical fiber base material is irradiated is hardly refracted at the boundary between the liquid and the outer peripheral surface of the clad portion.

The core non-circularity measuring device for an optical fiber preform according to the present invention is a core non-circularity measuring device for measuring the core non-circularity of an optical fiber preform having a core and a clad. And (a) a container for immersing the optical fiber preform in a liquid having a refractive index substantially equal to that of the core portion, and (b) holding the optical fiber preform immersed in the liquid, and holding the optical fiber preform Holding and rotating means for rotating the optical axis around its central axis as a rotation axis; (c) a light source section arranged to irradiate parallel light onto the side surface of the optical fiber preform immersed in the liquid; An imaging unit provided to receive and image light that has been irradiated to the side surface of the fiber preform and transmitted through the optical fiber preform,
(e) A core part non-circularity measuring part for obtaining the core non-circularity based on the light intensity distribution obtained as a result of imaging by the imaging part. If a liquid having a refractive index almost equal to that of the clad portion of the optical fiber preform is contained in the container and the optical fiber preform is immersed in the liquid, the above measuring method can be preferably carried out, As described above, the core noncircularity of the optical fiber preform can be measured.

Further, the image pickup section has a pixel surface including a plurality of pixels, and light transmitted through the optical fiber preform is irradiated onto the pixel surface to form an image of the optical fiber preform. It is sometimes useful to include at least 1000 pixels in the portion formed by the core portion of the image on the pixel surface.

Further, the rotation holding means further comprises an angle signal output section for outputting an angle signal including the rotation angle φ of the optical fiber preform to be rotated, and the core non-circularity measuring section inputs the angle signal. The rotation angle φ obtained from the angle signal and the width Y of the bright portion generated by the light transmitted through the core portion in the above light intensity distribution were fitted using the formula Y = A + B × sin φ, and the result of this fitting Therefore, it is preferable to estimate the core non-circularity based on the equation D (%) = (2 × B / A) × 100.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of a core non-circularity measuring device and a measuring method using the measuring device according to the present invention will be described below with reference to the drawings. In addition,
In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

FIG. 1 is a schematic view showing the arrangement of a core non-circularity measuring device according to this embodiment. In the figure, a core part non-circularity measuring device 1 generates parallel light to a container 2 in which a part of an optical fiber base material 10 is immersed in a liquid and a side surface of the optical fiber base material 10 immersed in the liquid. Light source unit 3 for irradiation
An image pickup section 4 for picking up an image of light that has been irradiated to the side surface of the optical fiber preform and transmitted through the optical fiber preform, and the optical fiber preform 10.
The optical fiber preform 1 which holds the
The holding / rotating units 5a and 5b that rotate 0, and the core noncircularity measuring unit 6 that obtains the light intensity distribution of the light imaged by the imaging unit 4 and calculates the core noncircularity based on this light intensity distribution.
Have and. The optical fiber preform 10 is, for example, VAD (V
apor phase Axial Deposition) method, MCVD (Modified
Chemical Vapor Deposition) method, OVD (Outside Vapor Deposition)
It is manufactured by an optical fiber preform manufacturing method such as a Deposition) method or a rod in collapse method, and has a core portion 10a and a clad portion 10b. Here, GeO ₂ is uniformly added to the core portion 10a. Also, the clad part 1
The refractive index of 0b is about 1.46 for light with a wavelength of 630 nm, and the relative refractive index difference Δ of the core portion 10a with respect to the cladding portion 10b is about 1%.

The container 2 has a substantially cubic shape with one surface open. Further, an opening through which the optical fiber preform 10 is inserted is provided on a pair of opposing surfaces of the container 2.
A packing material made of a material such as Viton (registered trademark) is provided on the edge of the opening. This packing material has appropriate elasticity and is in close contact with the side surface of the optical fiber preform. Thereby, the opening and the optical fiber are hermetically sealed, and the liquid in the container 2 is contained without leaking. Further, the packing material is adhered to such a degree that the optical fiber preform 10 can rotate about its central axis as a rotation axis. The container 2 needs to be made of a material that transmits parallel light emitted from the light source unit 3 and light that passes through the optical fiber preform 10.
It is preferably composed of quartz glass.

The light source unit 3 has a lamp such as a halogen or xenon lamp. Parallel light R is emitted by this lamp. The light source unit 3 may be composed of a lamp such as a halogen or xenon lamp and an optical system having a predetermined lens. Alternatively, the light source unit 3 may be configured by optically coupling these lamps and the bundle optical fiber and attaching a collimating lens to the end of the optical fiber. With these configurations, the light emitted from this lamp is collimated. The light source unit 3 is provided so as to face the side surface of the optical fiber preform 10.
In particular, it is more preferable that the parallel light from the light source unit 3 is applied to the side surface of the optical fiber preform 10 so as to be orthogonal to the longitudinal direction of the optical fiber preform 10.

The image pickup section 4 is arranged so that the optical fiber preform 10 is located between the image pickup section 4 and the light source section 3.
As a result, the optical fiber preform 1 is emitted from the light source unit 3.
The light transmitted through 0 is incident on the imaging unit 4. The light incident on the image pickup unit 4 is applied to the image pickup surface of the image pickup unit 4, and an image formed by the light transmitted through the optical fiber preform 10 is formed on the image pickup surface. The image pickup surface of the image pickup unit 4 has a plurality of pixels, and photoelectric conversion is performed by each of these pixels. The electric signal obtained as a result of the photoelectric conversion is output to the core non-circularity measuring unit 6 described later. In addition, as the imaging unit 4,
Specifically, it is preferable to use a CCD camera having a predetermined lens or the like.

Further, when the image of the optical fiber preform is projected onto the pixel surface, it is preferable that 1000 or more pixels are included in the portion of the image formed by the core portion. This makes it possible to accurately obtain the shape of the image formed by the light that has passed through the core portion 10a. Specifically, a measurement accuracy of about 0.1% is realized. It should be noted that one pixel is included in the image portion formed by the core portion.
In order to include 000 or more, the image pickup unit 4 may be appropriately configured so as to have a desired number of pixels. Further, an optical system having a predetermined lens is provided between the image pickup unit 4 and the optical fiber preform 10 so that the image portion formed by the core unit includes 1000 or more pixels by adjusting the optical system. Good.

The holding / rotating section 5a has a holder 51 provided with a rotating mechanism for rotating the optical fiber preform 10 with its central axis as the axis of rotation. Further, the holding and rotating unit 5a has an angle signal output unit 52. Angle signal output unit 52
Outputs a rotation angle signal including information about the rotation angle of the optical fiber preform 10. This rotation angle signal is input to the core non-circularity measuring unit 6 described later. The holding / rotating unit 5b holds the optical fiber preform 10. The holding / rotating part 5b is rotatably provided so that the optical fiber preform 10 is rotated about its central axis.

The core non-circularity measuring section 6 inputs an electric signal including image information from the image pickup section 4 and a rotation angle signal from the angle signal output section 52. In addition, the core portion non-circularity measuring unit 6
Calculates the core non-circularity by performing predetermined data processing based on these signals. As the core non-circularity measuring unit 6, for example, an ordinary personal computer can be used.

Then, using the core non-circularity measuring device 1,
Explain the method of measuring the non-circularity of the core along with the principle of measurement
To do. First, insert the optical fiber preform 10 into the opening of the container 2.
Then, the container 2 is penetrated, and the container 2 is attached to the optical fiber preform 10.
Attach. Next, the optical fiber with the container 2 attached
Attach the base material 10 to the holders of the holding and rotating parts 5a and 5b.
It Then, the side surface of the optical fiber preform 10 is completely immersed.
As described above, the core portion 1 of the optical fiber preform 10 is placed in the container 2.
A liquid whose refractive index is substantially the same as that of 0a is added. Where the core part
For a liquid having a refractive index substantially equal to that of 10a, the core portion 10
The refractive index of a is d _kAnd the refractive index of the liquid is d_lAnd when
Formula 0 ≦ | (d_l-D_k) / D_k│ ≦ 0.05 relation
Is satisfied. Further, preferably 0 ≦ | (d_l-D_k)
/ D_kIt is preferable that | ≦ 0.03. Specifically,
Recon-based oil is preferred and may be water.

After putting the above liquid in the container 2,
The optical fiber preform 10 is rotated at a predetermined rotation speed by the holding / rotating unit 5a. Then, the rotating optical fiber preform 1
Collimated light is emitted from the light source unit 3 to the side surface of the portion of No. 0 which is immersed in the liquid. Then, the optical fiber preform 10
The light that has passed through is irradiated onto the imaging surface of the imaging unit 4. Then, the electric signal including the image information generated by the photoelectric conversion by the pixels on the imaging surface is output to the core non-circularity measuring unit 6.
The core non-circularity measuring section 6 calculates the core non-circularity based on the electric signal from the image pickup section 4 and the rotation angle signal from the angle signal output section 52.

Here, the core non-circularity measuring unit 6 is implemented.
Refer to the drawings for the calculation of the core non-circularity
explain. FIG. 2A shows that the optical fiber preform 10 is irradiated.
Model showing how parallel light is transmitted through the optical fiber preform 10.
FIG. 2B is formed on the imaging surface of the imaging unit 4.
It is a figure which shows an example of the image. Parallel as shown in Fig. 2 (a)
Light R is emitted from the light source unit 3 to the optical fiber preform 10.
Be done. Of the parallel light R, a predetermined amount is given to the side surface of the core portion 10a.
Light R traveling at an angle_cbIs the core part 10a
It is refracted due to the difference in refractive index from the rad portion 10b. Refraction
As a result, transmitted light R _caBends to focus. Meanwhile,
Light R traveling so as to pass through the head portion 10b_kbIs a crack
Refractive index of the liquid F around the cladding portion 10b and the cladding portion 10b
Since they are almost equal, they go straight without being refracted. Sanawa
Then, the light R transmitted through the clad portion 10b_kaKeeps going straight,
Light R transmitted through the core portion 10a_caSo that the
And reaches the imaging surface S of the imaging unit 4. As a result, the imaging surface
In the image on S, as shown in FIG.
B₁, And dark area B₂Are formed in stripes.

As described above, since the bright portion M is formed by the light transmitted through the core portion 10a, the relative value of the diameter of the core portion 10a can be obtained from the width Y of the bright portion M. In other words, the width Y of the bright portion M does not necessarily match the diameter of the core portion 10a, but if the positional relationship among the light source unit 3, the optical fiber preform 10, and the imaging unit 4 is kept the same, the width Y The change in the diameter of the core portion 10a can be known by determining the change in Therefore, the core non-circularity can be obtained from the change. The method will be specifically described below.

While rotating the optical fiber preform 10, the width Y of the optical fiber preform 10 at an arbitrary rotation angle φ is measured. Based on the result of this measurement, the width Y is plotted against the rotation angle φ, and the graph G shown in FIG. 3 is obtained. In this measurement, the reference of the rotation angle φ may be appropriately determined, and the angle measured from this reference may be the rotation angle.
Further, when obtaining the width Y, the distance between two positions exhibiting a predetermined light intensity at the boundary between the bright portion M and the dark portion B ₁ and the boundary between the bright portion M and the dark portion B ₂ is obtained, The distance may be the width Y. Further, in order to measure the width Y of the bright portion M with high accuracy, it is necessary to improve the measurement accuracy at the boundary portion between the bright portion M and the dark portion B ₁ and at the boundary portion between the bright portion M and the dark portion B _2. . Therefore, it is preferable to improve the number or density of pixels at these two boundary portions. Regarding the number of pixels, the present inventors consider that it is preferable that there are at least 1000 or more in the range of the bright portion M, the dark portion B ₁ and the dark portion B ₂ on the pixel surface.

Each point (black circle) in the graph G is a point in which the actually measured width Y is plotted. For this point, fitting was performed using the formula represented by Y = A + B × sin φ. The line L in the graph G represents the approximate line actually obtained by this fitting. The fitting variables A and B obtained as a result of this fitting were A = 1.72 and B = 0.02. From these values, the core non-circularity is (2 × B / A) × 100 = (2 × 0.02 / 1.72) ×
It was calculated as 100 = 1.40%.

As described above, according to the measuring method using the core part non-circularity measuring device 1, the optical fiber preform 10 is dipped in the liquid F having a refractive index almost equal to that of the clad part 10b. The refraction of the light passing through the portion 10b is prevented. Therefore, of the light transmitted through the optical fiber preform 10, only the light transmitted through the core portion 10a is refracted. As a result, a bright portion M, a dark portion B ₁ , and a dark portion B ₂ are formed in stripes on the image formed on the image pickup surface of the image pickup unit 4 by the light transmitted through the core portion 10a. From the width Y of the bright portion M, the relative value of the diameter of the core portion 10a can be obtained. Therefore, the width Y is measured while rotating the optical fiber preform 10, a graph in which the width Y is plotted with respect to the rotation angle φ is obtained, and the graph is fitted by a predetermined formula to obtain the core noncircularity. Can be asked. According to the above method, only the light that passes through the core portion 10a is refracted, so that information about the diameter of the core portion 10a can be easily obtained. Therefore, the core noncircularity can be easily and surely obtained without performing a complicated analysis. Moreover, since it does not depend on complicated analysis, the time required for fitting and the like can be shortened.

The present inventors adopted the above-mentioned core noncircularity measuring device and its measuring method, and measured the core noncircularity of various optical fiber preforms. An optical fiber was prepared by drawing an optical fiber preform whose core noncircularity was measured, and the polarization mode dispersion of the optical fiber was measured. It should be noted that, for manufacturing an optical fiber, for example, Japanese Patent Laid-Open No. 6-1
The swing wire drawing method proposed in Japanese Patent No. 71970 and Japanese Patent Laid-Open No. 9-243833 is adopted. This drawing method is a method of imparting a predetermined twist to the optical fiber by guiding the optical fiber at the time of drawing by a guide roller whose rotating shaft oscillates periodically. By imparting a predetermined twist to the optical fiber, the glass softening part is forced to twist resulting in mode coupling between the two polarization modes in the optical fiber that are orthogonal to each other.
Therefore, the spread of the input pulse due to polarization dispersion is 1/100 of that of an optical fiber manufactured without oscillation.
(4Lh) ^1/2 . Where L is the optical fiber length
(m), h is the number of rotations per 1m of optical fiber length (times / m)
Is.

FIG. 4 is a graph showing the relationship between the core non-circularity of the optical fiber preform and the polarization mode dispersion of the optical fiber. As can be seen from the figure, an optical fiber having a polarization mode dispersion value of 0.15 ps / km ^1/2 or less suitable for large capacity transmission when the core noncircularity of the optical fiber preform is 1.0% or less. It turns out that Further, it can be seen from the figure that there is a clear correlation between the core noncircularity measured by the core noncircularity measuring device of the optical fiber preform and the polarization mode dispersion value. Therefore, if the core noncircularity of the optical fiber preform is measured using the above-mentioned core noncircularity measuring device and the optical fiber preform is appropriately selected according to the measurement result, the desired polarization mode dispersion can be obtained. Optical fibers having values can be made. That is, compared with a case where an optical fiber is manufactured from a plurality of optical fiber preforms, polarization mode dispersion is measured, and then an optical fiber having a predetermined specification value is selected, the manufacturing time and manufacturing cost of the optical fiber can be reduced. . Moreover, in the core non-circularity measuring method using the core non-circularity measuring device, as described above, the measurement can be performed easily and in a short time without requiring complicated analysis. From such a point, the effects of the above-described apparatus and method for measuring the core noncircularity of the optical fiber preform can be understood.

Although the core non-circularity measuring device for an optical fiber preform and the measuring method therefor according to the present invention have been described with reference to the embodiments, the present invention is not limited to the above-mentioned embodiments and various modifications are possible. Is possible. For example, in the core non-circularity measuring device 1, the optical fiber preform 10 is formed by using the container 2.
Although only the measured part of was immersed in the liquid,
Instead of the container 2, a container in which the entire optical fiber preform 10 is immersed may be used. Further, the light source unit 3 and the image pickup unit 4 can be provided so as to be movable in the longitudinal direction of the optical fiber preform 10. Thus, the core non-circularity is measured at a plurality of positions along the longitudinal direction.

Further, in the core non-circularity measuring device and the measuring method for the core optical fiber preform according to the present invention,
It is also suitably applied to an optical fiber preform whose refractive index changes in the core part. Even when the refractive index changes in the core part, light is refracted by the difference in the refractive index at the boundary between the outer peripheral surface of the core part and the clad part, so a bright and dark part based on this refraction is formed in the captured image. It Therefore, the relative value of the diameter of the core portion can be obtained based on this brightness.

In the above embodiment, the relative value of the diameter of the core portion 10a is calculated using the width Y of the bright portion M, but the total width of the dark portion B ₁ , the bright portion M and the dark portion B ₂ is used. be able to. Furthermore, in the above embodiment, the formula Y =
The fitting was performed using A + B × sin φ, but the formula used for the fitting may be appropriately changed. Furthermore, the core non-circularity may be obtained based on the maximum value and the minimum value of the width Y of the bright portion M measured while rotating the optical fiber preform 10.

[0033]

As described above, according to the core non-circularity measuring method and the measuring apparatus according to the present invention, the method and the apparatus for easily measuring the core non-circularity in a short time are provided. .

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a core non-circularity measuring device according to the present embodiment.

FIG. 2 (a) is a schematic diagram showing how parallel light emitted to an optical fiber preform passes through the optical fiber preform. FIG. 2B is a diagram showing an example of an image formed on the light receiving surface of the light receiving unit.

FIG. 3 is a graph in which the width Y of the bright portion is plotted against the rotation angle φ.

FIG. 4 is a graph showing the relationship between the core noncircularity of the optical fiber preform and the polarization mode dispersion of the optical fiber.

[Explanation of symbols]

1 ... Core part non-circularity measuring device, 2 ... Container, 3 ... Light source part, 4
... light receiving portion, 5a, 5b ... holding rotating part 6 ... core noncircularity measuring unit, 10 ... optical fiber preform, B _1, B ₂ ... dark space, M ...
Bright portion, R ... parallel light, S ... light receiving surface.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masaaki Hirano             Sumitomoden 1 Taya-cho, Sakae-ku, Yokohama-shi, Kanagawa             Ki Industry Co., Ltd. Yokohama Works F term (reference) 2F065 AA21 AA45 AA60 BB06 BB12                       BB17 BB22 CC23 DD06 FF04                       FF42 FF65 GG02 GG03 HH03                       HH15 JJ03 JJ26 LL01 MM04                       PP13 PP18 QQ17 QQ21 QQ26                       QQ28 QQ31                 2G086 AA02 AA04

Claims (7)

[Claims] 1. A core non-circularity measuring method for measuring a core non-circularity of an optical fiber preform having a core and a clad, wherein the light is contained in a liquid having a refractive index substantially equal to that of the clad. Immersing a fiber base material, rotating the optical fiber base material immersed in the liquid about its central axis as a rotation axis, and radiating parallel light to the side surface of the rotating optical fiber base material, A core characterized by receiving and imaging light that has been transmitted to the side surface of the base material and transmitted through the optical fiber base material, and determines the core noncircularity based on the light intensity distribution obtained as a result of the imaging. Partial non-circularity measurement method. 2. In the light intensity distribution, the width of the bright portion generated by the light transmitted through the core portion is measured, the width of the bright portion is defined as Y, and the rotation angle of the rotating optical fiber preform is determined to obtain the rotation angle. The rotation angle is φ, and the width Y of the bright portion is expressed by the formula Y = A + B with respect to the rotation angle φ.
Xsinφ is used for fitting, and the non-circularity of the core portion is calculated from the fitting result by the equation D (%) =
The core portion non-circularity measuring method according to claim 1, wherein the estimation is performed based on (2 × B / A) × 100. 3. When the light transmitted through the optical fiber base material is received and imaged, the light is irradiated onto a pixel surface having a plurality of pixels to form an image of the optical fiber base material, The core portion non-circularity measuring method according to claim 1, wherein at least 1000 or more of the pixels are included in a portion formed by the core portion of the image on the surface. 4. The refractive index d _{l of the} liquid satisfies the relationship expressed by 0 ≦ | (d ₁ −d _k ) / d _k | ≦ 0.05 with respect to the refractive index d _{k of the} clad portion. The method for measuring a core noncircularity according to claim 1, wherein 5. A core non-circularity measuring device for measuring a core non-circularity of an optical fiber preform having a core part and a clad part, wherein the light is contained in a liquid having a refractive index substantially equal to that of the core part. A container for immersing the fiber preform, a holding and rotating means for holding the optical fiber preform immersed in the liquid, and rotating the optical fiber preform with its central axis as a rotation axis, and immersed in the liquid And a light source unit arranged to irradiate the side surface of the optical fiber preform with parallel light, and to receive and image the light irradiated to the side surface of the optical fiber preform and transmitted through the optical fiber preform. A core part non-circularity comprising: an imaging part provided; and a core part non-circularity measuring part for obtaining a core non-circularity based on a light intensity distribution obtained as a result of imaging by the imaging part. Rate measuring device. 6. The image pickup unit has a pixel surface including a plurality of pixels, and the light transmitted through the optical fiber base material is irradiated onto the pixel surface to form an image of the optical fiber base material. The core portion non-circularity measuring device according to claim 5, wherein at least 1000 or more pixels are included in a portion of the image on the pixel surface formed by the core portion. 7. The rotation holding means further comprises an angle signal output section for outputting an angle signal including a rotation angle φ of the optical fiber preform to be rotated, and the core section non-circularity measuring section includes the angle signal output section. And the rotation angle φ obtained from the angle signal and the width Y of the bright portion generated by the light transmitted through the core portion in the light intensity distribution.
For, and, fitting using the formula Y = A + B × sin φ, and estimating the core non-circularity from the result of this fitting based on the formula D (%) = (2 × B / A) × 100 The core noncircularity measuring device according to claim 5, wherein