JP2519775B2 - Refraction angle measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention scans one cross section along the longitudinal direction from the side surface of a cylindrical transparent body having substantially uniform properties in the longitudinal direction by laser convergent light to The present invention relates to a refraction angle measuring device for determining an internal refractive index distribution of a cylindrical transparent body by measuring a refraction angle, the refraction angle measuring device having a positioning portion for a laser focusing position.

<Prior art and its problems> In order to obtain the internal refractive index distribution of the optical fiber preform, when a light ray travels through a medium whose refractive index is changing, nsin θ = constant (n is the medium Since the light beam is refracted so that the refractive index of θ, θ is an angle indicating the traveling direction of the light beam), this refraction angle is obtained for each incident point (initial condition) of the optical fiber preform that is the subject. , By using these refraction angles to perform integral calculation.

As a concrete device for obtaining the refraction angle based on such a principle, the configuration shown in FIG. 2 is known. In FIG. 2, the light from the He-Ne laser 1 is converged by the condenser lens 2 on a cross section along the longitudinal direction including the axial center of the subject 5 which is the optical fiber preform, and the cladding shown in FIG. It is designed to converge on the Pl surface of the base material 5 composed of 15 and the core 14. The optical fiber preform 5 is held by the holding mechanism 13 in the matching cell 3 filled with the matching oil 4 having substantially the same refractive index as that of the cladding 15 in order to improve the measurement accuracy of the refraction angle. The refracted light that has passed through the optical fiber preform 5 enters the two-dimensional image sensor 8 via the lenses 6 and 7. In this case, the lenses 6 and 7 are selected and arranged so that the refraction angle is converted into position information (displacement information from the optical axis center) on the two-dimensional image sensor 8. The scanning with the beam is performed by the controller 10 reciprocating the stage 9 having the matching cell 3 mounted thereon in the front-back direction of the drawing.

In this way, the position information of the laser beam is captured by the CPU 11 via the camera controller 12.

However, such a measuring device for a base material has the following problems. That is, the light of the laser light source 1 is focused on the Pl plane passing through the axis center shown in FIG. 3 via the lens 2, but when the base material 5 becomes large, When the base material 5 is not straight because the holding point of the base material 5 and the measurement point that is the scanning position of the laser beam are separated, and the Pl surface is determined with the holding point, that is, the position of the stage 9 as a reference. Includes the axis center of FIG.
The light from the laser light source 1 does not converge on the Pl surface.

In this case, since the scanning beam light spreads at the core / clad boundary point P shown in FIG. 3, for example, in the preform for a single mode fiber, the direction of refracted light has a characteristic of abruptly changing due to an abrupt refractive index change. in spite of,
The direction of refracted light is also expanded, and the refraction angle is measured at the maximum intensity position in the intensity distribution of refracted light.
The measurement resolution of this refraction angle must be lowered due to the deviation of the convergent surface.

Therefore, the present invention provides a refraction angle measuring device having a positioning part for aligning a converging surface of a scanning beam light with a Pl plane including an axis center in a device for obtaining an internal refractive index of a cylindrical transparent body. .

<Means and Actions for Solving the Problems> The present invention that achieves the above-described object is to scan a columnar transparent body having uniform characteristics in the longitudinal direction in the width direction with a light beam that converges in one cross section along the longitudinal direction. Then, in the refraction angle measuring device for obtaining the internal refractive index distribution of the cylindrical transparent body by measuring the refraction angle of the transmitted light by this scanning, in the direction orthogonal to both the longitudinal direction and the scanning ray incident direction. An illumination system that irradiates a parallel beam across the cylindrical transparent body, an image receiving optical system that detects the brightness distribution of refracted light of the cylindrical transparent body illuminated by this illumination system, and an image receiving element of this image receiving optical system. Using the symmetry of the luminance distribution obtained by the child, the cylindrical transparent body is arranged such that the cross section passing through the axial center of the cylindrical transparent body and orthogonal to the scanning light incident direction is the converging position of the scanning light beam. Move And a positioning unit having a moving system.

According to such a refraction angle measuring device, it becomes possible to monitor the position of the base material before the measurement and move the base material so that the Pl surface including the center of the base material coincides with the converging point of the scanning beam. The base material can be scanned with a small spot that is laser convergent light, the spatial resolution in refraction angle measurement can be improved, and the refraction angle can be measured with high accuracy.

<Example> An example of the present invention will now be described with reference to FIG. In FIG. 1, part of FIG. 2 is omitted. In FIG. 1, 16 is a constant current source and 17 is this constant current source.
A light source having a comparatively high light source property driven by 16 such as a laser diode, an LED, etc., 18 is a lens for collimating the light from the light source 17, and 19 is an optical fiber preform for the collimated light from the lens 18 in the matching cell 3. 2D image sensor 20 after passing through 5 and matching oil 4 (may be a 1D line sensor)
An image forming lens for forming an image on the monitor 21, a monitor TV for monitoring the light intensity distribution obtained by the image sensor 20, 22 an image processing device for processing the image signal of the monitor TV 21, and 23 an image processing result. It is a controller that moves the holding stage 13 of the optical fiber preform 5 based on the above.

In such a device, the parallel light from the light source 17 illuminates the base material 5 orthogonally to the scanning light beam and is refracted by the refractive index distribution of the base material 5. The intensity distribution of this refracted light in Pl 'in FIG. 3 is imaged on the image sensor 20 via the imaging lens 19.
In this formed image, dark lines (b) and (c) are conspicuously observed on the monitor TV21 in correspondence with the refraction at the core-clad boundary.
At the same time, the refractive index of the matching oil 4 is the cladding part.
If the refractive index does not match that of 15, dark lines (a) and (d) at the outer edge of the cladding are also observed.

In this way, in the single-mode fiber preform in which the dark lines (b) and (c) are clearly observed, the optical fiber preform is separated from the preform 5 so that the axis center of the single-mode fiber preform coincides with the predetermined converging surface of the scanning light. Move to 5 '. Further, in the graded base material in which the boundary line between the dark lines (b) and (c) is not clear, the refractive index of the matching oil 4, that is, the liquid temperature is controlled so that (a) and (d) can be observed,
After that, similar alignment may be performed.

<Specific Example> As the light source 17, an LED having a wavelength of 0.73 μm and an output of 10 mW was used. A plano-convex lens 18 having a diameter of 100 mm collimated the luminous flux and illuminated the base material. As the base material, a base material for a single mode fiber with a diameter of 50 mm was used. The brightness distribution is obtained by the one-dimensional line sensor 20 of 2048 elements via the imaging lens 19, and (b),
(C) Stage so that the midpoint coincides with the pixel of the one-dimensional line sensor that was previously determined as the focus position of the laser light
13 moved. The stage 13 is driven by a pulse motor with a step of 1 μm, and position reproducibility is obtained with a linear scale of 1 μm reading. The maximum variation at N = 20 is 20 μm.
Became.

As a result, when the divergence angle of the scanning beam from the condensing point is 1 °, the divergence due to defocus when the above control is performed has the following values.

20 μm × π × 1/180 = 0.35 μm However, in the case of a base material with a length of 0.5 m and a diameter of 50 mm, there is usually a bending of about 2 mm including the bending due to chucking. The spread becomes the following value.

2,000μm x π x (1/180) = 35μm Therefore, the spatial resolution is significantly improved.

<Effects of the Invention> As described above, when the refraction angle measuring apparatus according to the present invention is used, the Pl plane passing through the center of the base material is located at the condensing point of the laser scanning light. Can be improved.

[Brief description of drawings]

FIG. 1 is a refraction angle measuring apparatus according to an embodiment of the present invention, FIG. 2 is a conventional refraction angle measuring apparatus, and FIG. 3 is a view showing a cross section of a base material. In the figure, 5 and 5'are optical fiber preforms, 16 is a constant current source, 17 is a light source, 18,
Reference numeral 19 is a lens, 20 is an image sensor, 21 is a monitor TV, 22 is an image processing device, and 23 is a controller.

Claims (1)

(57) [Claims] 1. A cylindrical transparent body having uniform characteristics in the longitudinal direction is scanned in the width direction with a light beam that converges in one cross section along the longitudinal direction, and the refraction angle of the transmitted light by this scanning is measured. An illumination system for irradiating a parallel beam across the cylindrical transparent body in a direction orthogonal to both the longitudinal direction and the scanning ray incident direction in a refraction angle measuring apparatus for obtaining the internal refractive index distribution of the cylindrical transparent body. And an image receiving optical system that detects the luminance distribution of refracted light of a cylindrical transparent body illuminated by this illumination system, and the above-mentioned cylindrical shape using the symmetry of the luminance distribution obtained by the image receiving element of this image receiving optical system. A refraction angle provided with a positioning part having a moving system that moves the cylindrical transparent body so that a cross section of the transparent body passing through the axis center thereof and orthogonal to the scanning light incident direction becomes the converging position of the scanning light ray. measuring device.