JPH02275334A - Method and apparatus for measuring distribution of refractive index - Google Patents

Abstract

PURPOSE:To make it possible to perform highly accurate measurement by inputting light rays in the required direction into the preform of optical fiber and cylindrical glass for rod lenses, picking up the image-output projecting light, linearly approximating the spot shape, determining an output angle, and measuring the distribution of refractions. CONSTITUTION:In a preform 1, the distribution of refractive indexes is changed in the radial direction on a stage. Incident light 41 is projected in the direction orthogonal to a central axis 2 of the preform 1. Images 30 - 32 of scattering projected light rays are formed on a projection plane Q. The images are picked up with a TV camera 10. The data of the image picked-up positions are binary-coded in an operating means 13. The data are linearly approximated by a least square method and the like, and a straight line 20 is determined. Coordinates Xc of an intersection (c) between an X axis on a plane P and the straight line 20 are determined with a position computing means 14. The plane P is orthogonal to the axis 2 including the incident light 41. An output angle phi is determined highly accurately based on the data of many positions. A stage is moved, and the output angle phi at each radial position of the preform 1 is obtained by the same way. The distribution of the refractive indexes can be measured highly accurately based on the highly accurate output angles by a refractive-index-distribution operating means 16.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method and apparatus for measuring the refractive index distribution of cylindrical glass used, for example, in optical fiber preforms or rod lenses.

[Conventional technology]

Cylindrical glass used for optical fiber preforms (base materials) and rod lenses has a refractive index in the radial direction that has a substantially square distribution, and a refractive index in the axial direction that is uniform. An optical fiber is formed by drawing this. Accurately measuring the refractive index distribution of the preform before drawing is necessary to obtain a good product. There are various conventional methods for measuring this refractive index distribution. For example, in JP-A-63-95336 or JP-A-63-95337, a light ray is incident from a direction perpendicular to the central axis of an optical fiber preform, and the output angle of the emitted light is determined. A method of measuring refractive weight distribution is disclosed. The method for measuring the refractive index distribution disclosed in Japanese Patent Application Laid-Open No. 63-95336 uses the zero-order diffracted light of the emitted light emitted from the preform and projected onto the observation surface 43 of the TV camera, as shown in FIG. Spot 30. First-order diffracted light spot 31. Second-order diffracted light spot 32...
The 0th order diffracted light spot 30 is two-dimensionally analyzed and taken out, and the X coordinate x of this 0th order diffracted light spot 30 is
The output angle φ is determined from r and the focal ratio 11f of the output optical system as φ=jan-' (xr/fl. Then, from this output angle φ, the preform l
The refractive index distribution n(rl is calculated using the following formula % formula %). The method for measuring the refractive index distribution disclosed in Japanese Patent Application Laid-Open No. 63-95336 is to provide a slit in the output optical system to measure the output light. By extracting only the 0th order diffracted light spot 30,
The refractive index distribution is measured without being affected by high-order diffracted light spots.

[Problem to be solved by the invention]

As shown in FIG. 9, an incident light ray 41 that passes through a plane P perpendicular to the central axis 2 of the optical fiber preform 1 and enters from a direction perpendicular to the central axis 2 is refracted by the preform l, and is refracted by the preform l. The emitted light ray 42 reaches a projection plane Q perpendicular to the plane P. A point a between the incident ray 41 and the outgoing ray 42,
b and c exist on the plane P when there is no change in the refractive index of the preform 1 in the axial direction. Note that d indicates the intersection of the extension line of the incident light ray 41 and the projection plane Q. When glass is grown in the axial direction of the preform I by, for example, the VAD method, striae may occur in the glass, causing fluctuations in the refractive index distribution in the axial direction. When an incident light beam 41 is incident on a preform with striae, an output light beam 4
2 is scattered and dispersed on a straight line 43 passing through point C on the projection plane Q, as shown in FIG. In some cases, the position of the zero-order diffracted light spot having the maximum intensity moves to the position shown by point e in FIG. 8, and the light intensity at point C may become almost zero. For this reason, if the conventional method is to measure the position of the zero-order diffracted light spot to determine the exit angle Φ, as shown in FIG.
In this case, the output angle Φ is not measured accurately and an error occurs. When the refractive index distribution is determined using the exit angle φ with this error, the obtained refractive index distribution is thick (varies) in areas with large striae as shown in Figure 11, making it difficult to obtain an accurate refractive index distribution. The present invention was made to solve this problem, and provides a measuring method and a measuring device that can accurately obtain the refractive index distribution even for preforms with striae. The purpose is to obtain.

[Means for Solving the Problems] A method for measuring refractive index distribution to which the present invention is applied to solve the above problems is to use a cylindrical glass whose refractive index is uniform in the axial direction and whose refractive index distribution changes in the radial direction. In the refractive index distribution measurement method, in which a light ray enters from a direction perpendicular to the central axis of the It is characterized by determining the exit angle by measuring the position of the intersection between the central axis of the glass and a plane that is perpendicular to it and through which the incident light passes. The same (the refractive index distribution measurement device to which the present invention is applied has an input optical system in which a light beam enters from a direction perpendicular to the central axis of a cylindrical glass, and an output light that enters from the input optical system and passes through a coaxial cylindrical glass. an imaging means for detecting a projected image; a calculation means for linearly approximating the spot shape of the emitted light detected by the imaging means; and a plane through which the incident light passes, which is perpendicular to the central axis of the cylindrical glass coaxial with the straight line calculated by the calculation means. and an output angle calculation unit that calculates an output angle from the coordinates of the intersection calculated by the position calculation unit. [Operation] Using the apparatus of the present invention described above In the measurement method of the present invention, all the position data of the scattered image of the emitted light of the light incident perpendicularly to the central axis of the cylindrical glass is binarized by a calculation means and approximated by a straight line.5 This approximated straight line and the incident light are Since the exit angle is determined based on the position of the intersection with the plane through which the incident light passes, more data is required to determine the exit angle, which improves the measurement accuracy of the exit angle. The angle can be found. [Example] Examples of the present invention will be described in detail below. Figures 1 and 2 show a schematic configuration of an example of the present invention,
FIG. 1 is a plan view as seen from the y-axis direction, which is the same direction as the central axis 2 of the preform l for optical fiber, and FIG. 2 is a side view as seen from the x-axis direction, which is perpendicular to the y-axis. In the figure, reference numeral 3 denotes a cell in which the preform 1 is mounted, and the cell 3 is filled with matching oil 4 to eliminate sudden changes in refractive index on the surface of the preform 1. 5 is cell 3
The preform 1 is moved by the moving table 5 in the X-axis and y-axis directions. 6 is a light source consisting of, for example, a He-N e laser oscillator;
Reference numeral 7 denotes an incident optical system, and the incident optical system 7 converges the incident light from the light source 6 so as to be minimized at the center of the preform l. 8 is a screen that forms a projected image of the emitted light 9 emitted from the preform 1 as shown by the projection plane Q in FIG. This is a control unit that calculates the refractive index distribution by determining the exit angle from the position of the projected image obtained by the camera IO. FIG. 3 is a block diagram showing the configuration of the control section 11. In the figure, 12 is a frame memory that stores data of the projected image on the screen 8 observed by the TV camera IO, 13 is a calculation means for binarizing the data stored in the frame memory 12 and performs linear approximation, and 14 is a calculation means 13 A position calculation means calculates the intersection between the straight line obtained in 1 and a plane perpendicular to the central axis 2 of the preform 1 and through which the incident light passes, that is, a plane P in FIG. This is an exit angle calculation means for calculating the angle φ. 16 is the preform l based on the output angle φ calculated by the output angle calculation means.
17 is an output means consisting of a display section and a recording section. When measuring the refractive index distribution of the preform l, first, each projected image 30 to 32 on the screen 8 of the emitted light 9 sent from the light source 6 and scattered through the preform 1 as shown in FIG. Observation is made with a TV camera IO, and the output signal is taken into the frame memory 12. Next, the data of each projection image stored in the frame memory 12 is processed by the calculation means 13.
Binarize with and perform linear approximation using the least squares method,
An approximate straight line 20 shown in the figure is obtained. Thereafter, the position calculating means 14 determines the intersection xc between the approximate straight line 20 and the plane perpendicular to the central axis 2 of the preform 1 and through which the incident light passes, that is, the X axis shown in FIG. The output angle calculation means 15 calculates the output angle φ from the coordinate values of the intersection Xc with the projected image of the incident light as the reference point O, and the calculated output angle φ is stored in the memory 18. This operation is performed at each position in the radial direction of the preform 1 while moving the moving table 5, and the output angle φ at each position is determined and stored in the memory 18. This memory 18
Each output angle φ stored in is sent to the refractive index distribution calculation means 16 to calculate the refractive index distribution n (r) and sent to the output means 17. As described above, for example, as shown in FIG.
FIG. 6 shows the results of measuring the relationship between the incident position r and the output angle φ. As shown in the figure, even if there were regions 51 and 52 with strong striae in the preform 1, no large fluctuation was observed in the change in the exit angle φ. Refractive index distribution n(r) depending on the exit angle φ
By determining this, we were able to obtain the characteristics shown in Figure 7. This measurement was repeated 30 times and the relative refractive index difference ΔΔ=(
l11+-nx) . Furthermore, even when the projected image of the emitted light 9 was not formed on the plane P due to striae, the refractive index distribution could be measured with measurement accuracy comparable to the above accuracy. In the above embodiment, the screen 8 and the TV camera I
Although a case has been described in which the projected image of the emitted light is observed using an image sensor, the same effect as in the above embodiment can be achieved even when the projected image is observed using an image sensor.

【Effect of the invention】

As explained above, according to the present invention, all the positional data of the scattered projected image of the emitted light of the light incident in the direction perpendicular to the central axis of the cylindrical glass is binarized and approximated by a straight line, and this approximated straight line and Since the exit angle is determined by the position of the intersection between the central axis of the coaxial cylindrical glass and the plane that is perpendicular to the plane through which the incident light passes, the amount of data required to determine the exit angle increases, making it possible to improve the detection accuracy of the exit angle and increase the refractive index. Distribution measurement accuracy can be improved. In addition, since the output angle is determined using the approximate straight line obtained by linear approximation of the projection, the output angle can be determined with high accuracy even when the output light is scattered and its image is not on the plane through which the incident light passes. .

[Brief explanation of drawings]

FIG. 1 is a schematic plan view of an embodiment of an apparatus to which the present invention is applied;
Fig. 2 is a side view thereof, Fig. 3 is a block diagram showing the control section of the above embodiment, Fig. 4 is an explanatory diagram showing the operation of the above embodiment, and Fig. 5 is a characteristic showing the refractive index distribution of the preform. 6 is an output angle characteristic diagram measured by the above embodiment, FIG. 7 is a refractive index distribution characteristic diagram obtained from the output angle characteristic, FIG. 8 is an explanatory diagram showing the operation of the conventional example, and FIG. 9 1 is an explanatory diagram showing the principle of refractive index distribution measurement, FIG. 1O is an exit angle characteristic diagram according to a conventional example, and FIG. 11 is a refractive index distribution characteristic diagram obtained according to a conventional example. 1... Preform 8... Screen lO... TV left camera 3... Calculating means 15... Outgoing angle calculating means 2... Cell 6... Light source 9... Outgoing light 12... ...Frame memory 14...Position calculating means Fig. 4 y Fig. 8 Fig. 6 Fig. 7 Fig. 10 Fig. 11 Cow q " Fig. 9

Claims (1)

[Claims] 1. A ray of light is incident from a direction perpendicular to the central axis of a cylindrical glass whose refractive index is uniform in the axial direction and the refractive index distribution changes in the radial direction, and its exit angle is measured to determine the refractive index. In the refractive index distribution measurement method that calculates the distribution, the projected spot shape of the emitted light is approximated by a straight line, and the position of the intersection of this straight line and a plane perpendicular to the central axis of the cylindrical glass and through which the incident light passes is measured. A method for measuring refractive index distribution characterized by determining an exit angle. 2. An entrance optical system that enters a light beam from a direction perpendicular to the central axis of the cylindrical glass, an imaging means that detects a projected image of the outgoing light that enters from the entrance optical system and passes through the coaxial cylindrical glass, and the imaging means a calculation means for linearly approximating the spot shape of the detected emitted light; a position calculation means for determining the position of the intersection of the straight line calculated by the calculation means and a plane perpendicular to the central axis of the coaxial cylindrical glass and through which the incident light passes; 1. An apparatus for measuring a refractive index distribution, comprising an exit angle calculation means for calculating an exit angle from the coordinates of the intersection point calculated by the position calculation means.