JP3064329B2 - Refractive index distribution measuring method and refractive index distribution measuring device - Google Patents

Description

The present invention relates to a method and apparatus for measuring a refractive index distribution of an optical element having a refractive index distribution by using total reflection of light. About.

[Conventional technology]

2. Description of the Related Art In recent years, a gradient index optical element having a refractive index distribution has been widely used as an array lens for a pickup of a video disk device or a copying device. Further, in the field of optoelectronics, relatively small refractive index distribution type optical elements such as optical waveguides and flat microlenses having a refractive index distribution are used. In the field of imaging, relatively large apertures are used as lens systems for silver halide cameras, video cameras and microscopes. Various refractive index distribution type optical elements are being put to practical use.

Since the characteristics of these refractive index distribution type optical elements greatly depend on the refractive index distribution state, a method capable of measuring the refractive index distribution shape of each element with high accuracy is required for practical use.

Conventionally, as a method for measuring such a refractive index distribution, a thin sample cut and polished in a direction perpendicular to the central axis of the refractive index distribution is observed with an interference microscope, and an optical path length difference per unit thickness of the thin sample is measured. The vertical interferometry, which measures the distribution of the refractive index by determining the refractive index distribution, or the horizontal direction, in which the light is transmitted in the direction perpendicular to the central axis of the refractive index distribution of the cylindrical measurement sample and the ray tracing is performed, to determine the refractive index distribution Directional interferometry is known.

More recently, a measuring method as described in JP-A-63-275936 has been proposed. This method
It is based on the principle of the well-known Pulfrich refractometer, and as shown in FIG. 5, the measurement surface of a measurement sample 101 for measuring the distribution of the refractive index is changed to a sample setting surface 102A of a hemispherical measurement table 102. Hemisphere other than the installation surface 102A
Measurement table installation surface 102A by condensing lens 103 via 102B
The measurement is performed by making the laser beam 105 converged on the upper measurement point 104 incident thereon. Of the convergent light irradiated to the measurement point 104, the light of the light beam region that is incident at a large incident angle range than the total reflection critical angle phi _c at the point 104 is substantially the same brightness and since the incident light to be totally reflected reflected light is obtained in the optical area 106, since a part of light of the light beam region that is incident at a small incident angle range than the total reflection critical angle phi _c is resulting in transmission emitted from the measurement point 104, darker light than the incident light The reflected light of the region 107 is obtained. Therefore, when observing the light beam reflected from the measurement point 104, a light beam cross section 109 divided into a relatively bright light region 106 and a relatively dark light region 107 with a light-dark boundary 108 interposed therebetween can be measured. Thus, since the light incident with a total reflection critical angle phi _c is reflected as a light-dark boundary 108 is a boundary between the light and dark portions, by measuring the angle at which the ray of light and dark boundary 108 forms the measurement surface normal if, the value is the total reflection critical angle phi _c, a refractive index n of the sample 101 at the measurement point 104 from the known refractive index n ₀ of the measuring table 102 is that obtained from the following equation (1).

n = n ₀ sin φ _c ‥‥ (1) [Problems to be Solved by the Invention] However, these conventional measurement methods have different problems. First, both the vertical interferometry and the horizontal interferometry have high detection accuracy, but in the former case, the greater the difference in refractive index distributed to the measurement sample, the higher the accuracy of the measurement unless the sample is cut and polished. However, depending on the measured object, it is necessary to process it into a very thin sample of several tens of μm.It is very difficult to prepare the sample, and since it is cut and polished, it cannot be measured unless the sample is constantly destroyed. Did not.

In the latter case, there is an effect that the measurement sample can be measured nondestructively, but there are drawbacks in that the shape of the measurement sample is limited to a cylindrical shape, and that it takes time to measure the refractive index distribution.

Next, the method described in Japanese Patent Application Laid-Open No. 63-275936 is an effective method that has a simple structure and almost solves the drawbacks of the above two measurement methods, but is the prototype of the method. was not a problem for refractometer Pulfrich, theoretically appear Goose Hanchen shift during total reflection (Goos-Hanchen shift) is greatly influenced by a reading error of the total reflection critical angle phi _c in this way Nevertheless, since this has not been taken into account, it has been difficult in practice to measure the refractive index distribution with high precision required for practical use of the gradient index optical element.

The present invention has been made in view of the above problems, and provides a refractive index distribution measuring method and a refractive index distribution measuring apparatus that can easily and accurately measure a refractive index distribution shape of a gradient index optical element to be measured. The purpose is to do.

[Means and Actions for Solving the Problems]

In the refractive index distribution measuring method according to the present invention, one surface of a measurement sample having a refractive index distribution is brought into contact with a sample installation surface of a medium member having a larger refractive index than the measurement sample, and electromagnetic waves of a predetermined wavelength are converged through the medium member as convergent light. In the refractive index distribution measuring method in which the critical angle of total reflection is measured by detecting a light-dark boundary of reflected light by total reflection of convergent light, the refractive index is known and Based on the angle of the light-dark boundary measured for a plurality of different calibration members and a correction amount preset by the calculated ideal total reflection critical angle, the alignment error and Goose-Henschen measured for the above-described measurement sample are determined. The critical angle of total reflection including the error due to the shift is corrected so that the alignment error and the error due to the Goos-Henschen shift are eliminated. The refractive index of the measurement sample at the measurement position is determined based on the critical reflection angle, and the above-described processing is repeated by scanning the measurement sample relative to the convergent light, thereby obtaining the refractive index distribution of the measurement sample. It is characterized by having made it.

Further, the refractive index distribution measuring method according to the present invention, for the plurality of calibration members, an approximate expression from the value of the total reflection critical angle measured for each calibration member and the ideal total reflection critical angle of the calibration member. Then, the critical angle of total reflection corrected by the approximate expression is calculated.

The refractive index distribution measuring apparatus according to the present invention includes a light source that generates an electromagnetic wave of a predetermined wavelength, a light beam converting unit that converts the electromagnetic wave emitted from the light source into convergent light, and a sample setting surface that contacts a measurement sample having a refractive index distribution. A medium member having a refractive index larger than that of the sample to be measured, observation means for measuring a total reflection critical angle by reflected light in which convergent light is totally reflected by the sample setting surface, and a measurement sample relative to the convergent light. A scanning means for scanning, wherein a correction preset by an angle of a measured light-dark boundary and an ideal total reflection critical angle for a plurality of calibration members having known and different refractive indexes. The critical angle of total reflection including the alignment error and the error due to Goose-Henschen shift measured for the measurement sample based on the A correction angle correcting means so that an error is eliminated by shift, is characterized in that it comprises a calculating means for calculating the refractive index of the measurement sample by the total reflection critical angle this corrected.

Further, in the refractive index distribution measuring device according to the present invention, the angle correction means may determine, for the plurality of calibration members, a value of a total reflection critical angle measured for each calibration member, and an ideal total reflection critical angle of the calibration member. The method is characterized in that an approximate expression is obtained from the angles and the critical angle for total reflection corrected by the approximate expression is calculated.

Since the refractive index n is a function of the total reflection critical angle φ as is apparent from the equation (1), the correction amount is equal to the angle of the light-dark boundary measured as described above and the ideal total. It is not given only by the relationship with the reflection critical angle,
It is apparent that the refractive index may be obtained from the relationship between the refractive index and the ideal refractive index which can be calculated using the equation (1) and the relationship between the measured angle of the boundary between light and dark and the ideal refractive index.

The critical angle of total reflection measured for the measurement sample includes an alignment error and an error due to Goose-Henschen shift, and the critical angle of total reflection measured based on the correction amount preset by the calibration member is calculated. By correcting, the above two types of errors can be eliminated, and the refractive index of the measurement sample at the measurement position is obtained from the corrected critical angle of total reflection, so that the refractive index distribution shape of the measurement sample can be simplified. And it can measure with high precision.

〔Example〕

Hereinafter, a preferred embodiment of the present invention will be described with reference to FIG.

FIG. 1 is a schematic configuration diagram of a refractive index distribution measuring device,
Is a light source that emits an electromagnetic wave of a predetermined wavelength such as a laser beam, 2 is a light beam converting member that emits the electromagnetic wave of the predetermined wavelength as an incident light beam 3 that is convergent light, and 4 is a hemispherical shape, for example. Incident hemisphere 4a and almost flat sample setting surface
4b is an optical element, that is, a measurement sample, whose refractive index distribution shape can be measured in contact with the sample setting surface 4b, and the refractive index n ₀ of the medium member 4 is 5
Is set to be larger than the refractive index n. 6 is the sample setting surface
A spherical center 7 of the medium member 4 corresponding to an ideal measurement point on the light beam 4b where the incident light beam 3 should converge is used to move the measurement sample 5 relative to the incident light beam 3 along the sample setting surface 4b. A scanning means 9; a reflected light flux in which the incident light flux 3 is reflected at a measurement point on the sample setting surface 4b; 11 an observation means for receiving the reflected light flux 9 and measuring a total reflection critical angle φ '; The angle with respect to the measurement surface normal (line perpendicular to the sample setting surface 4b) at the position where the intensity of the received electromagnetic wave changes most rapidly (the boundary between light and dark) can be measured as the critical angle for total reflection φ ′.
12 is an angle correcting means for correcting an alignment error and an error due to Goose-Henschen shift to be described later with respect to the measured critical angle for total reflection φ ′, and 13 is an angle correcting means based on the corrected critical angle for total reflection φ ″. This is a calculation means for calculating the refractive index n of the measurement sample 5 at the measurement point from the equation (1).

Incidentally, as described above, the total reflection critical angle φ ′ measured by the observation means 11 includes an error due to the Goos-Henschen shift which is essential and cannot be avoided, as in the prior art, together with the alignment error, The means for correcting this will be described below.

FIG. 2 shows only the light rays corresponding to the light-dark boundary between the convergent light fluxes 3 and 9 with respect to the medium member 4 for ease of explanation. In the figure, the incident light beam 3a is ideally the medium member 4
At the measurement point which is the spherical center 6 of the medium member 4 on the sample setting surface 4b, and the total reflection critical angle φ forms an ideal reflected light beam 9a having the same angle as the incident angle. Observed at an ideal angle.

However, actually, the sample setting surface 4b where the incident light beam 3 converges
Since the upper measurement point cannot be precisely adjusted to the spherical center 6 (alignment error), the incident light beam 3b is incident on a point 6 'on the sample setting surface 4b shifted from the spherical center 6 by a distance ΔAL. Furthermore this ray
After sinking into the measurement sample 5 by the Goose-Henschen effect, 3b is totally reflected from a point 6 ″ shifted from the point 6 ′ on the sample setting surface 4b by a distance ΔGH corresponding to the Goose-Henschen shift. beam 9b will be refracted through the medium member 4, the total reflection critical angle, the alignment error of the angle theta _AL and Goose Heng the reflection light 9a constituting the ideal total reflection critical angle φ It will be observed as an angle φ ′ shifted by the angle _{θGH corresponding to the} Shen shift.

In this case, the alignment error can be reduced to some extent by adjustment, but the error due to the Goos-Henschen shift is an essential error that always occurs in the total reflection area and cannot be eliminated. Also, magazine J.Opt.Soc.Am.A / V
ol. 3, No. 4 (April 1986), pp. 550 to 557, a paper by Goll-Hanchen effect ar by HMLai et al.
When the above-described distance error ΔGH was simulated using the method described in “Round and off the critical angle”, it was found that the distance error ΔGH was a large amount several times to several tens times the measurement wavelength. Since this greatly affects the measurement accuracy of the critical angle of total reflection, accurate measurement of the refractive index distribution shape cannot be performed unless correction is made by some method.

Therefore, in the present invention, focusing on the fact that the alignment error is a function of only the wavelength, and the error due to Goose-Henschen shift is a function of the wavelength and the refractive index, The critical angle for total reflection φ ′ measured in step 11 is corrected to a value φ ″ approximating the ideal critical angle for total reflection φ.

That is, a plurality of calibration members 15 having a predetermined refractive index according to the measurement sample 1 and having different refractive indices and being different from each other are selected. An electromagnetic wave of a predetermined wavelength is emitted from the light source 1 in the same procedure as the measurement sample 5 using a refractive index distribution measuring device, and the total reflection critical angle φ ′ of each calibration member 15 is measured (for each wavelength).

Further, based on the known refractive index, the ideal total reflection critical angle φ of each calibration member 15 is back calculated using the above equation (1).

Then, the relationship between each measured critical angle φ ′ and the ideal total reflection critical angle φ is approximated using the following equation so that the square error is minimized using the least squares method. The calculated approximate value φ ″ is set as a corrected total reflection critical angle.

φ ″ = f _λ (φ ′) ‥‥ (2) Accordingly, the equation (2) corrects the measured total reflection critical angle φ ′ of the measurement sample 5 so as to approximate the ideal critical angle φ. The critical angle for total reflection φ ″ can be calculated, and errors due to Goose-Henschen shift and alignment errors can be corrected well.

Note that, in principle, the formula (2) can be obtained by using two types of calibration members 15 having different refractive indices, or by using only one type of electromagnetic wave for measuring the total reflection critical angle φ ′.

Furthermore, the method of approximation expression is not limited to the above least square method, and an approximation method such as spline processing may be used.

Next, the refractive index distribution measuring method according to the present embodiment will be described.

First, each total reflection critical angle φ of the plurality of calibration members 15 having a predetermined refractive index is measured according to each refractive index of the measurement sample 5 to be measured, and the approximate expression (2) is obtained by the above-described method. deep.

Then, the first sample in which the measurement sample 5 is brought into contact with the sample setting surface 4b
In the state shown in the figure, the electromagnetic wave of a predetermined wavelength emitted from the light source 1 is converted into an incident light beam 3 which is a convergent light by the light beam conversion member 2, and the sample setting surface 4b with which the measurement sample 5 comes in contact via the medium member 4. Incident on At this time, the optical axis (3
b) is incident on a point 6 'shifted from the spherical center 6 on the sample setting surface 4b by ΔAL due to an alignment error, and the optical axis (9b) of the reflected light beam 9 is further shifted from the point 6' by an error due to Goose-Henschen shift. The light is totally reflected from the point 6 ″ shifted by ΔGH, and passes through the medium member 4 at an angle (θ _AL + θ _GH ).
The refracted reflected light beam 9 is received by the observation means 11,
The total reflection critical angle φ ′ is measured.

The measured critical angle φ ′ is input to the angle correction means 12, and the square error with the ideal total reflection critical angle φ of the measurement sample 5 is minimized based on a preset approximate expression (2). Is corrected to the critical angle for total reflection φ ″.

Then, the corrected total reflection critical angle φ ″ is calculated by the calculating means 13 according to the above equation (1), where n = n ₀ sin φ ″. What was obtained in this way was
This is the refractive index of the measurement portion of the measurement sample 5 where the refractive index error due to the Goos-Henschen shift or alignment error has occurred, and has a sufficiently high measurement accuracy suitable for practical use.

The scanning unit 7 scans the incident light beam 3 relative to the incident light beam 3 while maintaining the state of contact with the sample setting surface 4b, and repeats the measurement by the above-described method. The refractive index distribution shape of the sample 5 can be measured with high accuracy.

As described above, according to the present embodiment, the refractive index distribution shape of the measurement sample 5 can be measured easily and with high accuracy.

 Hereinafter, specific examples including numerical examples will be described.

Specific Example Next, a specific example of the present invention will be described with reference to FIG.

First, a small-diameter flat lens 5 having a distribution shape in which the refractive index gradually decreases in the radial direction is used as the measurement sample 5. The medium member 4 has a refractive index n ₀ of 1.79896 with a He—Ne laser beam having a wavelength of 632.8 nm, and a radius r of the hemisphere 4a.
= 8mm, using a hemispherical one, and the sample setting surface 4b
Then, the substantially flat surface of the small-diameter flat lens 5a is brought into contact with a matching refractive liquid (not shown) having a refractive index of 1.700 with He-Ne laser light having a wavelength of 632.8 nm.

He-Ne laser having a wavelength of 632.8 nm as a light source 1 and a wavelength of 4
88nm Ar laser, 568.2nm Kr laser and 4 wavelength
A 41.6 nm He-Cd laser is used, and a laser beam of an appropriate wavelength is incident on the light beam conversion member 2 by a switching mirror (not shown) (selected according to the boundary angle between light and dark, a measurement object, and the like). In the light beam converting member 2, after the incident laser light beam is spread about 10 times by an objective lens with a magnification of 50 and an objective lens with a magnification of 5 (both not shown), the working distance (from the lens tip to the sample mounting surface 4b) Distance) long magnification 20
Is converted into an incident light beam 3 converged by an objective lens (not shown), and converged on the sample setting surface 4b.

As the observation means 11, a photodetector is arranged at a distance of about 50 mm from the spherical center 6 of the medium member 4 and rotatably around the spherical center 6, and receives the reflected light flux 9 totally reflected on the sample setting surface 4b. However, the angle at the position where the intensity change of the received light amount is the steepest is detected as the angle of the boundary between light and dark (critical angle of total reflection).

Prior to the measurement of the small-diameter flat lens 5, Expression (2) for determining the correction amount is set.

First, as the calibration member 15, the refractive index of He-Ne laser light having a wavelength of 632.8 nm LaK14 of 1.69429, SF8 of 1.68436, respectively.
1.66854 SF5,1.62108 BaF8,1.60107 SK14,1.52944
Using LLF6 and BK7 of 1.51466, the angle R of the light-dark boundary is measured with the above-mentioned four wavelengths of light from the light source 1 for these seven types of glass materials having known refractive indexes. The relationship between the measured angle R of the light-dark boundary and the ideal critical angle for total reflection φ obtained from the equation (1) based on the known refractive index is determined by the least square method so that the square error is minimized. , (2).

As an example, Table 1 below shows the refractive index n, the ideal critical angle of total reflection φ, and the measured angle R of the light-dark boundary of each calibration member 15 with He-Ne laser light having a wavelength of 632.8 nm.

Based on the above table, the approximate expression obtained by the least square method can be expressed as a linear expression, and the corrected total reflection critical angle φ ″ is expressed as φ ″ = 228.151−1.01459R ‥‥ (3) Can be. This is shown in FIG. 3 in which the black circles indicate the ideal total reflection critical angle φ and the measured angle R of the light-dark boundary. It can be seen that an approximation is obtained.

The above equation (2) assumes the relationship between the measured critical angle for total reflection φ ′ and the corrected critical angle for total reflection φ ″. The former φ ′ is the angle in the normal direction of the sample installation surface and the measured brightness. Since the angle R is uniquely determined from the angle of the boundary (the angle R is not strictly the critical angle for total reflection), the equation (3) can be considered as the same correction equation as the equation (2).

The above equation (3) is input to the angle correction means 12, and
The processing by these means, including the calculation means 13, is performed by a computer.

As the scanning means 7, the small-diameter flat lens 5
An xy stage for two-dimensional scanning is mounted.

As an example, Table 2 shows the measurement results obtained by measuring the small-diameter flat lens 5 with a He-Ne laser beam having a wavelength of 632.8 nm at intervals of 0.2 mm in the diameter direction using the refractive index distribution measuring method and apparatus.

Further, FIG. 4 shows the result as a diagram.
It can be understood that the distribution in which the refractive index n gradually decreases in the radial direction of the measurement sample is measured with high accuracy.

In this specific example, the light source 1 is a coherent light source using a laser. However, an incoherent light source such as a Xe lamp may be used by spectrally separating it with a monochromator or the like, or a bright line light source may be used.

Also, as for the observation means 11, a CCD array, a CCD camera, or the like may be used instead of the photodetector, or visual observation may be used.

The expression (2) for obtaining the correction amount is not limited to the linear expression of the expression (3) in the specific example, but may be set to any other appropriate configuration.

[Effects of the Invention] As described above, since the refractive index distribution measuring method and the refractive index distribution measuring device according to the present invention are provided with the angle correcting means for correcting the measured total reflection critical angle, the refractive index distribution shape of the measurement sample is provided. Can be measured easily and with high accuracy.

[Brief description of the drawings]

FIG. 1 is a principle diagram showing an embodiment of a refractive index distribution measuring apparatus according to the present invention, FIG. 2 is a diagram for explaining an error of a measured total reflection critical angle, and FIG. 3 is a diagram for explaining angle correction in a specific example. Figure,
FIG. 4 is a diagram showing a refractive index distribution shape measured for a small-diameter flat lens in a specific example, and FIG. 5 is a schematic sectional view of a conventional refractive index distribution measuring device. 1 light source, 2 light flux conversion member, 4 medium member, 4b
…… Sample setting surface, 5… Measurement sample, 11… Observation means, 12
... Angle correction means and 13 calculation means.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01M 11/00-11/02 G01N 21/43 CAPLUS (STN) JICST file (JOIS)

Claims (4)

(57) [Claims] 1. A sample setting surface of a medium member having a refractive index higher than that of the measurement material is brought into contact with one surface of the measurement material having a refractive index distribution, and the sample is set via the medium member as a convergent light of an electromagnetic wave of a predetermined wavelength. In a refractive index distribution measuring method in which a light is incident on a measurement position on a surface and a boundary between reflected light and reflected light due to total reflection is detected to measure a critical angle of total reflection, a plurality of calibration members having a known refractive index and different from each other are provided. The alignment error measured for the measurement sample and the goose / correction angle are corrected by a correction amount set in advance based on the angle of the light-dark boundary measured for and the calculated ideal total reflection critical angle.
The critical angle for total reflection including an error due to the Henschen shift is corrected so that the alignment error and the error due to Goose-Henschen shift are eliminated, and the measurement position is determined based on the corrected critical angle for total reflection. The refractive index of the measurement sample is obtained by calculating the refractive index of the measurement sample, and repeating the above processing while scanning the measurement sample relatively to the convergent light. Distribution measurement method. 2. An approximate expression is determined for the plurality of calibration members from the value of the critical angle for total reflection measured for each calibration member and the ideal critical angle for total reflection of the calibration member, and correction is performed using the approximate expression. A method for measuring a refractive index distribution, wherein the calculated critical angle of total reflection is calculated. 3. A light source for generating an electromagnetic wave of a predetermined wavelength, a light beam converting member for converting the electromagnetic wave emitted from the light source into convergent light, and a sample mounting surface for contacting a measurement sample having a refractive index distribution. A medium member having a refractive index higher than that of the measurement sample, observation means for receiving the reflected light in which the convergent light is reflected on the sample setting surface, and measuring the total reflection critical angle; A refractive index distribution measuring apparatus provided with a scanning means for performing a scanning operation, wherein the refractive index is set in advance based on an angle of a light-dark boundary measured for a plurality of calibration members having different known refractive indexes and an ideal total reflection critical angle. The critical angle of total reflection including the alignment error and the error due to Goose-Henschen shift measured for the measurement sample by the corrected amount is converted to the alignment error and Goose-Henschen shift. A refractive index distribution measuring device, comprising: an angle correcting means for correcting so as to eliminate an error caused by the above; and a calculating means for calculating the refractive index of the measurement sample based on the corrected critical angle of total reflection. 4. The angle correction means obtains an approximate expression for the plurality of calibration members from the value of the critical angle for total reflection measured for each calibration member and the ideal critical angle for total reflection of the calibration member. The refractive index distribution measuring apparatus according to claim 3, wherein the apparatus is configured to calculate a critical angle of total reflection corrected by the approximation formula.