JPH07218430A - Method for measuring change in refractive index and shear-interference microscope - Google Patents

Abstract

PURPOSE: To suppress restrictions in the case of measuring an average refractive index by cutting a thin piece taken from a preform in a radial direction, burying in the average refractive index medium of a body, and computing distance difference along a cutting line. CONSTITUTION: A cutting edge along a refractive index change line is formed on a thin piece 1 taken from a preform, and the same is dipped in dipping solution having a refractive index substantially agree with an average refractive index, and is scanned along the cutting edge by a shearing interference microscope. The image of a sample 1 arranged on a scan table 5 is picked up crossing a shearing interferometer on a camera 9 by an objective lens 6. The phase state of the interferometer 7 is changed by a phase shifter 8 controlled via a PC 10 interface. Image information is transferred to a PC 10 by the camera 9 and is displayed by the monitor 11 of the camera. The distance difference on the cutting line of the sample 1 is computed by the PC 10 according to known algorithm, and the refractive index of a cross section area measured at that time is computed using a cross section thickness and a peripheral refractive index.

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 refractive index change, preferably in a macroscopic object, in particular in an optical fiber preform, and a shear interferometer for carrying out this method.

[0002]

2. Description of the Related Art In optimizing a manufacturing process of an optical fiber, it is necessary to measure a refractive index distribution in a large number of preforms (base materials). This measurement must be performed with high accuracy and effect both in the laboratory and in the factory.

Known technical solutions are high adjustment costs and protection costs for considerable environmental conditions when performing interferometric investigations with undisturbed reference waves (mirror interferometer or Michelson interferometer). Requires. While these interferometers allow a complete examination of the entire cross section of a blank specimen, producing the required blank specimen is extremely expensive. However, it is possible to measure samples without the influence of diffusion and temperature. In addition, this interferometer is very sensitive to vibration and costly precautions must be taken before it can be used near its production. "Handbuch de
r Mikroskopie (Microscope Handbook) "Verlag Technik
Berlin, 1973, p. 28
3 describes the measurement of the refractive index of the core of a coated glass fiber by the shear method. This measurement method is only available for fibers with thicknesses up to about 1/10 mm and is not suitable for preforms and other larger objects.

A shear interference microscope is described in US Pat. No. 4,391.
No. 516 is further described.

If the refractive index is to be measured from the reflection value, a high precision intensity measurement is necessary in addition to high preparation costs. Furthermore, the plane to be investigated must be optically polished.
When the average reflectance is about 4.2%, it must be possible to distinguish a difference in reflectance of about 0.01%. Or, when trying to define the refractive index distribution from the function of the preform,
The end faces of the preform must be carefully treated. For that purpose, a well-focused light is introduced into the preform, and the distribution of the light behind the emission surface is measured. The refractive index distribution is estimated from the behavior of emitted light.

[0006]

The object of the present invention is to enable the refractive index change to be selectably and accurately determined while keeping the constraints on preparation cost and environmental conditions as low as possible when measuring the refractive index change. Is the starting point.

[0007]

This object is achieved according to the invention by the features of patent claim 1. Other preferred embodiments are described in the dependent claims. A shear interferometer according to the invention is the subject of claim 4.

The method of the present invention, which is carried out using a shear interferometer, is a method for preforming a thin piece (thickness 20 to 30 μm) taken from an end surface of an object, particularly a preform, on a line where a change in refractive index is to be measured. A radial cut is made by and embedded in the medium of average refractive index of the object and the stroke difference along the cut line is measured. Location-dependent stroke difference, cross-section (uniform)
From the thickness and the refractive index of the embedding medium, the location-dependent refractive index of the object is calculated in a known manner.

With this specimen and commercially available technical equipment, application software can be used to automatically measure changes in the refractive index of the preform.

To that end, a shear interferometer (eg, Jenam) equipped to implement the time-heterodyne method.
ap, Zeiss) is supplemented with a scanning table on which the thin slices of preform are aligned with their cutting edges approximately perpendicular to the shear direction and approximately parallel to the scanning direction.

The scanning direction itself is, for example, "Handbuch der".
Mikroskopie ", Verlag Technik Berlin, 1973, p.
p. 372-374, p. 605.

Alternate measurement and scanning steps are performed via a PC which controls the heterodyne method and processes the measured values. The results of the individual measurements are programmatically joined together without gaps and the desired index change can be determined.

Shear interferometers produce two images laterally offset from the object and operate in so-called self-referencing fashion,
It has the advantage that the two double images are interfered. This eliminates the need for costly adjustments of the blank object, environmental influences acting both in the reference image and in the object image, so that the interferometer is insensitive to temperature fluctuations and vibrations. These properties allow effective operation in relatively harsh environments.

[0014]

Example In the microscope, only a small area 19 is visible on the double image of the cutting edge. By moving the scanning table, the entire area of the cutting line appears on the camera step by step.

The objective lens 6 images the object 1 placed on the scanning table across the shear interferometer 7 on the camera 9. The basic principle of a shear interferometer is, for example, "Handbuc
h der Mikrosopie ", Verlag Technik Berlin, 197
3, p. 273 et seq.

The phase state of the interferometer is changed by the phase shifter 8 which is also controlled via the interface (not shown) of the PC 10 to implement the time heterodyne method. The camera 9 transmits the image information to the PC 10. The image is displayed on the monitor 11 of the camera.

The PC calculates the stroke difference at the cutting edge of the object according to a known algorithm (phase shift method), and then uses the cross-sectional thickness and the ambient refractive index to calculate the refractive index of the cross-sectional area measured at that time. To do.

The results of the cross-sectional areas measured one after the other are arranged in order, and as shown in FIG.
Play at 5. In the region 18, the refractive index of the immersion medium, region 1
6 shows the refractive index of the jacket, and region 17 shows the changing refractive index of the core.

Reference numerals 13 and 14 represent a keyboard and a mouse for operating necessary units.

[0020]

According to the present invention, the adjustment cost is unnecessary for measuring the change in the refractive index, the influence of environmental conditions is small, and highly accurate measurement is possible even under relatively severe environmental conditions.

[Brief description of drawings]

FIG. 1 Thin section 1 of a preform prepared according to this method
FIG. 3 is a schematic view showing the thickness of the same in an exaggerated manner.

FIG. 2 is a view showing a test piece 1 (thin piece) embedded in a dipping medium 2 and covered with a cover glass 3.

FIG. 3 is a schematic diagram showing an interference image seen with a microscope. The area 20 to be evaluated in the field of view of the camera includes a measurement point 21 for the periphery and a measurement point 22 for the sample 1.

FIG. 4 shows the yield wavefront difference c visible in a microscope image, including the wavefronts of two object images a and b displaced in the shearing direction, which are visible by the shear method, and the regions c1 and c2 in which the object stroke difference can be measured. FIG.

FIG. 5 shows a schematic arrangement of the device for the method. 4 represents the illumination system of the microscope. A scanning table 5 is fixed to the microscope and is operated by the PC 10 via an interface (not shown).

FIG. 6 is a schematic diagram showing a measurement result.

[Explanation of symbols]

 1 Test piece 2 Immersion medium 3 Cover glass 4 Lighting system 5 Scanning table 6 Objective lens 7 Shear interferometer 8 Phase shifter 9 Camera 10 PC 11 Camera monitor 12 PC monitor 13 Keyboard 14 Mouse 15 Printer

Claims (4)

[Claims] 1. A method for measuring the change in refractive index, preferably in a macroscopic object, in particular in an optical fiber preform, comprising the steps of: -creating a slice of definite thickness of the object; Creating a cutting edge along the line of refractive index change, -putting the slice in an immersion liquid of a refractive index that substantially matches the average refractive index of the slice, and-using shear interference microscopy along the cutting edge. Determining the location-dependent index of refraction in a known manner from the scanning step, the location-dependent travel difference of the obtained double image of the cutting edge, the thickness of the flakes and the index of refraction of the immersion medium. A method comprising the steps of: 2. A thin piece is obtained from the end face of the preform,
Method according to claim 1, characterized in that the cutting edges are produced by radial cutting of the preform. 3. Method according to claim 1 or 2, characterized in that the thickness of the flakes is 20-30 mm. 4. A scanning table for imaging a slice in immersion liquid having a scan direction substantially perpendicular to the shear direction, the slice having a cutting edge arranged substantially parallel to the scan direction, in particular Shear interference microscope for measuring refractive index changes in preforms.