JPH08297071A - Method and apparatus for measurement of distribution of refractive index of core base material for optical fiber - Google Patents

Abstract

PURPOSE: To measure the distribution of a refractive index with good accuracy and efficiently by a method wherein a core base material is placed so as to cross one optical path out of two optical paths in the Mach-Zehnder interferometer and an interference image is photographed while the core base material is being moved in the axial direction. CONSTITUTION: A radiated beam from a laser 11 is made incident on the Mach-Zehnder interferometer 10. The interferometer 10 divides the radiated beam into a sample-side optical path and a reference-side optical path. In the sample-side optical path, the radiated beam is passed through a sample cell 9 and a core base material 1, for an optical fiber, which has been immersed in the cell. In the reference-side optical path, the radiated beam is passed through a reference cell. Both beams are put together, and an interference image is focused. The reference image is picked up by a video camera, a video signal is found, and a refractive index in the passage position of the beam in the base material 1 is computed. Then, the refractive index is recorded and displayed. On the other hand, the base material 1 is lowered gradually inside the cell 9 by a driving gear 20, and the position of the beam passing the base material 1 is shifted sequentially. Consequently, the distribution in the axial direction of the refractive index of the base material 1 can be measured with good accuracy and efficiently.

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 axial refractive index distribution of an optical fiber core preform and an apparatus for measuring the refractive index distribution.

[0002]

2. Description of the Related Art An optical fiber is drawn while heating and melting a preform for an optical fiber in which a core base material made of round long glass is covered with a clad glass having a lower refractive index. In order to manufacture an optical fiber with excellent performance, it is preferable that the core base material has a uniform refractive index. However, when the core base material is manufactured by the vapor phase growth method, the refractive index in the axial direction may fluctuate within a range of several mm due to slight temperature fluctuations and fluctuations in the growth rate.
If the core base material changes in the refractive index, the optical fiber using this will have a large optical loss. Therefore, it is necessary to measure the distribution of the refractive index of the core base material in advance and remove the core base material with fluctuation.

[0003]

In such a situation, in the process after the core preform for optical fiber is manufactured,
It is desired to measure the refractive index distribution with high accuracy. In order to measure the refractive index of a transparent material, there is a method of using a geometrical optical refractometer and an interferometric refractometer that utilizes the property of light as a wave. The former refractometer is suitable for measuring the refractive index of the entire transparent material, but it is not necessarily suitable for measuring the refractive index of a minute portion of the transparent material. In that respect, the latter interferometric refractometer can measure the refractive index of the part through which light passes if a transparent material is present in the minute optical path, so the minute part of the material is slightly shifted to measure the refractive index distribution. I am trying to do it. Therefore, the present inventor has adopted the principle of measuring the refractive index by optical interference when measuring the refractive index distribution in the axial direction of the optical fiber core preform.

Therefore, the present invention provides a measuring method and a measuring apparatus by optical interference, which can measure the refractive index distribution of the optical fiber core base material with high accuracy and efficiency.

[0005]

A method for measuring the refractive index distribution of an optical fiber core preform of the present invention made to solve the above-mentioned problems is to divide a laser beam emitted from a single light source into two optical paths, In the interferometer for focusing the interference image with the light passing through the two optical paths, the optical fiber core base material is placed across one optical path of the two optical paths, and the interference image is moved while moving in the axial direction of the base material. The refractive index is calculated from the image signal of the captured image.

The interferometer used in the method of the invention is:
A Mach-Zehnder interferometer is preferred.

The refractive index distribution measuring apparatus for the optical fiber core preform of the present invention is as shown in FIG.
An interferometer 10 having a single laser 11, an optical system for separating a laser beam emitted from the laser 11 into two optical paths, and an optical system for focusing an interference image through the two optical paths,
A drive device 20 for axially moving the optical fiber core preform 1 across one of the two optical paths is installed.

As shown in FIG. 3, the measuring apparatus of the present invention can add a video camera 27 for taking an interference image by an interferometer and a circuit 33 for calculating a refractive index from the video signal.

Interferometer 10 used in the measuring apparatus of the present invention
Is preferably a Mach-Zehnder interferometer (see FIG. 4).

It is preferable that a sample cell 9 containing a refractive index matching liquid 8 is arranged at a position where the optical fiber core base material 1 being measured is dipped across one optical path of the Mach-Zehnder interferometer.

It is preferable that the reference cell 21 containing the refractive index matching liquid 8 is disposed in another optical path.

[0012]

When the optical fiber core preform 1 to be measured is mounted on the measuring apparatus of the present invention and the laser 11 is oscillated, the interferometer 10 focuses the interference image. Since the optical fiber core base material 1 moves in the axial direction of the base material, if the refractive index of the optical fiber core base material 1 varies, the interference image changes according to the variation. Therefore, the distribution of the refractive index can be known.

The refractive index distribution can be automatically obtained by photographing the in-focus interference image with the video camera 27 and calculating the refractive index from the video signal by the circuit 33.

If the interferometer 10 is a Mach-Zehnder interferometer, the laser light passes through the optical fiber core preform 1 to be measured only once (in other types of interferometers, the object to be inspected is laser light). Since the change in the optical path length due to the outer surface (cylindrical surface) of the optical fiber core preform 1 is small, the change in the refractive index appears accurately.

The optical fiber core base material 1 being measured by the Mach-Zehnder interferometer is dipped in a sample cell 9 containing a refractive index matching liquid 8 and the light passing therethrough and the refractive index matching of another optical path are matched. The configuration in which the interference image is focused by the light passing through the reference cell 21 containing the liquid 8 is efficient because it is not necessary to adjust the fine position of the optical system each time measurement is performed. Also, the above-mentioned optical fiber core base material 1
The influence of the change in the optical path length due to the outer surface (cylindrical surface) of can be almost completely removed.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a partial sectional front view of an embodiment of the refractive index distribution measuring apparatus of the present invention.

As shown in the figure, in a measuring apparatus to which the present invention is applied, an interferometer 10 is placed on a pedestal 12, and one of the optical paths of the interferometer 10 penetrates the pedestal 12 to pass through the sample cell 9 And a drive device 20 for moving the optical fiber core preform 1 of the measurement sample up and down in the sample cell 9 is installed.

The interferometer 10 is the Mach-Zehnder interferometer shown in FIG. The Mach-Zehnder interferometer 10 has a beam diffusing lens 13 in the emitting direction of the laser 11, and an incident collimator including an incident plane mirror 14 and an incident lens 15. After the incident collimator, the sample side optical path from the incident side beam splitter plate 16 through the total reflection plane mirror 17 to the emission side beam splitter plate 22, and similarly from the incident side beam splitter plate 16 through the total reflection plane mirror 18 to the emission side beam splitter plate A reference side optical path reaching 22 is configured. Following the beam splitter plate 22 on the output side,
An output collimator including the output lens 23 and the output plane mirror 24 is configured. Further, a video camera 27 having a taking lens 25 is arranged.

In the sample-side optical path of the Mach-Zehnder interferometer 10, a sample cell 9 containing a matching oil 8 which is a refractive index matching liquid and has the same refractive index as that of the optical fiber core base material 1 of the measuring sample. Are placed. Similarly, the same kind of matching oil 8 is put in the reference side optical path and has the same optical path length as the sample cell 9, but the reference cell 21
Is arranged.

As shown in FIG. 2, the optical fiber core preform 1 connected to the drive unit 20 is immersed in the sample cell 9. The optical fiber core base material 1 is held at its upper and lower ends by a tightening mechanism 2 including a thumbscrew and a compression spring.
It is hung on. The up-and-down moving plate 4 is adapted to be screwed into a rotary screw rod 5 which constitutes the drive device 20, penetrate the fixed guide rod 6, and slidably move. Screw rod 5 is motor 7
And bevel gear 3 are connected.

The video camera 27 is connected to the circuit shown in FIG. In the figure, 31 is a video signal processing circuit, 32 is an image interval calculation circuit, 33 is a refractive index calculation circuit, 34 is a video image reproduction circuit, 35 is an image frame memory, 36 is a display device, and 37 is a printer.

Besides, 28 shown in FIG. 1 is a height adjusting table on which the sample cell 9 is mounted, and 29 is a drain for replacing the matching oil 8 in the sample cell 9.

After holding the optical fiber core base material 1 of the sample to be measured with the tightening mechanism 2 in the apparatus of the above embodiment,
The laser 11 is oscillated, the motor 7 is rotated at the same time, and the circuit of FIG. 3 is operated. The outgoing beam from the laser 11 enters the Mach-Zehnder interferometer 10,
It is expanded by the beam diffusion lens 13 and becomes a parallel beam by the incident collimator. This parallel beam is split into a sample side optical path and a reference side optical path by an incident side beam splitter plate 16. In the optical path on the sample side, it passes through the sample cell 9 and the optical fiber core preform 1 immersed therein. In the optical path on the reference side, the light passes through the reference cell 21. When these two parallel beams are combined by the beam splitter plate 22 on the emission side, the optical path lengths are the same, so that interference occurs, and the interference image is focused by the emission collimator.

This interference image is formed on the photoelectric conversion surface of the video camera 27 by the taking lens 25, and a video signal is output. The video signal is subjected to waveform shaping by the video signal processing circuit 31 and converted into a digital signal, and the interval of the interference fringes is calculated from the signal by the image interval calculation circuit 32. The refractive index calculation circuit 33 calculates the refractive index of the optical fiber core preform 1 at the beam passing position based on the distance between the interference fringes. The calculated refractive index is printed by the printer 37. In addition, the video signal of the interference image is reproduced by the video image reproducing circuit 34 for the image, and the frame memory 35 is reproduced.
And then displayed on the display device 36.

On the other hand, since the motor 7 is rotating, the optical fiber core preform 1 is gradually lowered in the sample cell 9 by the driving device 20. Therefore, since the position of the beam passing through the optical fiber core preform 1 is sequentially shifted, if the refractive index of the optical fiber core preform 1 changes, the interference image changes according to the change. Therefore, the printer 37
Or (and) the value of the refractive index output from the display device 36 or the shape of the interference image changes. Therefore, it is possible to know the axial distribution of the refractive index of the optical fiber core preform 1.

In the above embodiment, the Mach-Zehnder interferometer is used as a preferred example, but the present invention is not limited to this, and examples thereof include Michelson interferometer, Jaman interferometer, Rayleigh interferometer, Fabry-Perot interferometer. And any other interferometer can be used.

[0028]

As described above in detail, according to the refractive index distribution measuring method and measuring apparatus of the present invention, the refractive index distribution of the optical fiber core preform can be measured accurately and automatically. become. Therefore, it becomes possible to easily remove the optical fiber core base material whose refractive index is fluctuating, which greatly contributes to the performance improvement of the optical fiber.

[Brief description of drawings]

FIG. 1 is a partial sectional front view of an embodiment of a refractive index distribution measuring device to which the present invention is applied.

FIG. 2 is a diagram showing a main part of the above embodiment.

FIG. 3 is a block diagram of a circuit added to the device of the present invention.

FIG. 4 is an explanatory diagram showing the principle of refractive index distribution measurement in the example of the present invention.

[Explanation of symbols]

1 is an optical fiber core base material, 2 is a tightening mechanism, 3 is a bevel gear, 4 is a vertical moving plate, 5 is a screw rod, 6 is a guide rod, 7 is a motor, 8 is matching oil, 9 is a sample cell, 10 is a Interferometer, 11 is a laser, 12 is a pedestal, 13 is a beam diffusion lens, 14 and 24 are plane mirrors, 15 and 23 are collimator lenses, 16 and 22 are beam splitter plates, 17 and 18 are total reflection plane mirrors, and 20 is a drive. A device, 21 is a reference cell, 25 is a photographing lens, 27 is a video camera, 28 is an adjustment stand, 29 is a drain, 31 is a video signal processing circuit, 32 is an image interval calculation circuit, 33 is a refractive index calculation circuit, and 34 is a video An image reproduction circuit, 35 is an image frame memory, 36 is a display device,
37 is a printer.

Claims (6)

[Claims] 1. An optical fiber which crosses one of two optical paths of an interferometer which separates a laser beam emitted from a single light source into two optical paths and focuses an interference image by the light passing through the two optical paths. A refractive index distribution measuring method for an optical fiber core preform, wherein a core preform for optical fiber is placed and the refractive index is calculated from an image signal of the interference image while moving in the axial direction of the preform. 2. The method for measuring the refractive index distribution of a core preform for an optical fiber according to claim 1, wherein the interferometer is a Mach-Zehnder interferometer. 3. An optical interferometer having two optical paths, which has a single laser, an optical system for separating laser light emitted from the laser into two optical paths, and an optical system for focusing an interference image through the two optical paths. A refractive index distribution measuring device for an optical fiber core preform, wherein a driving device for axially moving the optical fiber core preform across one of the optical paths is installed. 4. The optical fiber core according to claim 3, further comprising a video camera for capturing an interference image focused by the interferometer and a circuit for calculating a refractive index from a video signal of the video camera. A base material refractive index distribution measuring device. 5. The refractive index distribution measuring device for an optical fiber core preform according to claim 3, wherein the interferometer is a Mach-Zehnder interferometer. 6. The optical fiber according to claim 5, further comprising a sample cell containing a refractive index matching liquid for immersing an optical fiber core base material that traverses one optical path of the Mach-Zehnder interferometer. For measuring the refractive index distribution of core materials for automobiles.