JPH07270313A - Measuring method for refractive index and that for nature characteristic - Google Patents

Abstract

PURPOSE:To measure refractive index of air easily using a bicolor interferrometer. CONSTITUTION:A second harmonic having the light from a laser light source as the base wave is generated by a first wavelength conversion element 3a to be made go through the air a distance 2L and is introduced to a second wavelength conversion element 3b through a second beam splitter 4b. The second harmonic generated from the base wave via an optical path 2 is made to interfere with the harmonic via the optical path 2L to obtain an interference fringe signal. A shutter 9 is opened and the second harmonic reaching the second beam splitter 4b directly from the first beam splitter 4a is made to interfere with the second harmonic via the optical path 2L by means of a second beam splitter 4b to obtain an interference fringe signal. Under such a condition, a distance L is determined from changes in the phase of the interference fringe in scanning with the frequency of a laser light. Then, the refractive index of the air is determined with a data analyzer 8 from the distance L and the orders of the interference fringes.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to positioning and dimensional accuracy improvement by a laser interferometer in precision production fields such as electronic industry and mechanical industry, measurement of refractive index of optical materials in optical industry, and liquid / gas in chemical industry. Refractive index measurement method for improving accuracy and efficiency of refractive index measurement, and for measuring and evaluating properties of modified surfaces and coating properties in the mechanical and electronic related production industries. Regarding measurement method.

[0002]

2. Description of the Related Art Conventionally, in an optical interferometer for measuring length, it is necessary to correct the refractive index of air, and a method called a two-color method is known. This two-color method is based on an optical length measurement value due to light wave interference by measuring with a two-color laser light of a second harmonic wave by a wavelength conversion element and a fundamental wave as a light source of a light wave interferometer. The refractive index of air is corrected in real time.

Here, in the above-mentioned two-color interferometer, etc., the interferometric length measurement value by the wavelength λ ₁ of the fundamental wave is L ₁ ,
The interferometric measurement value by the wavelength λ ₂ of the second harmonic is L ₂ , and the wavelength λ
When the refractive index of air at ₂ and n _2, the refractive index n ₂ of the air is expressed by the following equation.

[0004]

[Equation 1]

The coefficient A in the equation (1) is {(n ₂
It is known that it is a constant given by −1) / (n ₂ −n ₁ )}, and its value is about 10 to several hundreds, but the phase of the interference fringe must be measured accurately. Thus, it is possible to obtain a precise refractive index of air even in a relatively short length region. However, an approximate value is sufficient for this, but L
It is necessary to know the value of, and for example, it is necessary to arrange a reflecting mirror of the interferometer on the moving table and perform scanning for a predetermined length.

Further, in the above two-color interferometer, the interference fringe signal is temporarily recorded in a computer or the like, and the phase of the interference fringe is statically calculated after the data collection is completed.

The refractometer as described above has begun to be used for correction of air fluctuations and its refractive index in many scientific and industrial fields. That is, in addition to being used to improve the precision of parts in the advanced electronics and machinery industry, recently, due to automation and industrialization of measurement, there is an increasing demand for use in places where the measurement environment is not stable or in long optical paths. Therefore, the fluctuation of air and the correction of its refractive index have a great influence on the measurement accuracy.

[0008]

However, in order to obtain the refractive index of the medium using the two-color interferometer as described above,
It is necessary to obtain an approximate value of the length L of the medium, and a means such as a movable table for scanning the optical path length of the interferometer that can move in a fixed direction at a constant speed is additionally required, which complicates the measurement system. In addition, it was impossible to measure the refractive index of solids and liquids. Moreover, it is important to know the refractive index of not only gas but also liquid and solid materials in the manufacturing industry such as optical industry and machinery, electronics, chemistry, etc. It is not possible to determine the refractive index of.

Further, it is extremely important to know the measurement and evaluation of the property characteristics of the precision processed surface in the technology for highly functionalized materials and the ultra-precision technology, which have made remarkable progress in recent years, and the characteristics of the coating required for mounting, and devote them to the evaluation. However, in the above method, the phase of the interference fringes was statically measured, so the laser beam of the interferometer was scanned in the direction perpendicular to the surface of the measured object, and the processed surface of the object was scanned. Such as when measuring and evaluating the property characteristics such as
It could not be applied to the measurement of distribution characteristics in a dimensional area.

Therefore, the present invention provides a method of measuring a refractive index and a method of measuring a property, which can easily measure a refractive index of a gas, a liquid or a solid.

[0011]

In order to solve the above problems, in the refractive index measuring method according to the present invention, light obtained from a light source is used as a fundamental wave, and the fundamental wave before passing through the medium to be measured is used. The generated harmonics interfere with the harmonics generated from the fundamental wave that has passed through the non-measuring medium, the order of the interference fringes is obtained based on the interference fringe signal generated by the interference, and the frequency of the light source is changed. The length of the medium to be measured is obtained based on the change in the interference fringes based on the above, and the refractive index of the medium to be measured is measured using the order of the interference fringes and the length of the medium to be measured.

In the property measuring method according to the present invention, the fundamental wave from the light source is applied to the surface of the medium to be measured, and the harmonics generated from the reflected light reflected by the surface of the medium to be measured are The frequency of the reflected light reflected on the surface of the measurement medium is subjected to heterodyne interference with the fundamental wave, the phase of the interference fringe is determined based on the interference fringe signal generated by the interference, and the phase of the interference fringe is determined based on the phase of the interference fringe. The property characteristic of the surface of the measurement medium was measured.

[0013]

Embodiments of the method for measuring the refractive index according to the present invention will now be described in detail with reference to the accompanying drawings.

First, the basic principle of the refractive index measuring method will be outlined. Now, the wave when the wavelength lambda ₁ and lambda respectively n ₁ the refractive index of the medium in the _2, n _2, which is converted and waves propagating converted in advance lambda _2, the lambda ₂ after propagating remains lambda ₁ Assuming that the optical path difference due to the interference with the optical path is 2ΔL, the optical path difference ΔL for the outward path or the return path can be obtained by the following equation (2). Note that L is the length of the medium.

[0015]

[Equation 2]

2ΔL is given by the following equation (3) which is an equation of interference. Note that M is the order of the interference fringes.

[0017]

[Equation 3]

In the above equation (3), the order M of the interference fringes
The By precisely measured and obtain an optical path difference 2.DELTA.L, (2) the refractive index of the medium at the wavelength lambda ₂ from the formula n ₂ is obtained. That is, since it is sufficient to measure only the interference fringes having a single wavelength λ2, it is possible to measure the fraction (phase) of the interference fringes with high resolution. However, since the length L of the medium and the integer part of the order M of the interference fringes are unknowns,
It is not possible to directly obtain the correct refractive index n ₂ .

Therefore, first, the principle of determining the length L of the medium will be described. Now, in an interferometer with an optical path difference of 2 L, when the frequency of the light source is shifted by Δν, the following expression (4) is established. Note that c is the speed of light and m is the order of the interference fringes formed when the frequency of the light source shifts.

[0020]

[Equation 4]

As is clear from the above equation (4), Δ
Knowing ν and measuring m, L is obtained.
For example, if Δν is 10 GHz and the measurement accuracy of the fraction (phase) of m is 1/300, the accuracy of determining L is 50 μm.
Therefore, necessary and sufficient accuracy can be obtained.

Further, in order to obtain the refractive index of the medium, the order M of the interference fringes is necessary, but what is obtained by measuring the interference fringes is only the fractional part (phase) of M, and an integer of M. It is necessary to request a separate section. To do so, the thickness L of the medium
Prepare several kinds of materials that differ significantly, first obtain a rough value of the refractive index by measuring the thin one, and then determine the integer part of M when measuring the thicker one uniquely using the previous value do it.

As described above, by obtaining the length L of the medium and the order M of the interference fringes, the refractive index n ₂ of the medium at the wavelength λ ₂ can be easily obtained from the above equations (2) to (4). You can do it.

A first embodiment of a refractometer which embodies a method of measuring a refraction index based on the above principle will be described with reference to FIG.

In this interferometer, the laser light output from the laser light source 1 enters the first wavelength conversion element 3a after passing through the first lens 2a, and the second light having the incident light as a fundamental wave.
A harmonic is generated and collimated by the second lens 2b, and a part of this second harmonic is generated by the first beam splitter 4
Most of the light (about 96%) is reflected by a toward the second beam splitter 4b and passes through the first beam splitter 4a. The first beam splitter 4a
The fundamental wave and the second harmonic wave that have passed through are transmitted through the air and are reflected by the reflecting mirror 5, and reach the second beam splitter 4b through a propagation path that is substantially parallel to the forward path. The reflecting mirror 5 shown in the drawing
Is composed of two mirror surfaces, and the reflection angle of the laser light which strikes these two surfaces is adjusted appropriately (for example, two mirrors are
By arranging the laser beam at 0 ° and setting the incident angle of the laser beam at 45 °, the forward path and the return path are made substantially parallel. The length L of the medium in this embodiment is the distance from the first beam splitter 4a to the reflecting mirror 5.

Most of the fundamental wave and the second harmonic wave (about 96%) that have reached the second beam splitter 4b are transmitted through the second beam splitter 4b, passed through the second condenser lens 2b, and converted into the second wavelength. The light is focused on the center of the element 3b, and a part of the fundamental wave is converted into the second harmonic. Thus, the second harmonic wave incident on the second wavelength conversion element 3b and the second harmonic wave converted in the second wavelength conversion element 3b interfere with each other, and the interference based on the interference of the second harmonic wave. Stripe signal only first
The first photoelectric detector 7 is selectively transmitted by the filter 6a.
The interference fringe signal is photoelectrically detected by a. Then, the interference fringe detection signal photoelectrically detected by the first photoelectric detector 7a is supplied to the data analyzer 8, and the phase (fractional fraction) of the interference fringe associated with the length L of the optical path is measured.

When performing the above measurement, the shutter 9 provided between the first and second beam splitters 4a and 4b is closed, and the fundamental wave reflected by the first beam splitter 4a and the first The second harmonic is the second beam splitter 4b
I try not to reach.

Next, when the shutter 9 is opened, the fundamental wave and the second harmonic reflected by the first beam splitter 4a reach the second beam splitter 4b, and the fundamental wave and the second harmonic reflected by the reflecting mirror 5 are reached. Only the interference fringe signal of the second harmonic that interferes with the harmonic and selectively passes through the second filter 6b is photoelectrically detected by the second photoelectric detector 7b, and the photoelectric detection signal is supplied to the data analyzer 8. Then, the optical frequency of the laser light source 1 is scanned by the driver 10 by a predetermined Δν, and the phase change of the interference fringes generated at this time is measured to determine the length L of the optical path. Also,
The refractive index of air as a medium can be obtained by calculating the refractive index of air from the measured value L and the measured value of the fraction (phase) of the interference fringes obtained as described above. In addition, in order to obtain the integer part of the interference fringe, the reflection mirror 5
Since it suffices to change the arrangement position of No. 3, there is no need to perform the measurement while changing the length of the medium at a constant speed unlike the conventional two-color interferometer.

The second embodiment shown in FIG. 2 is a refractometer for measuring the refractive index of a solid. The same components as those in the first embodiment are designated by the same reference numerals and their description is omitted. To do.

A part of the light (fundamental wave and second harmonic) reaching the first beam splitter 4a is reflected by the first beam splitter 4a, and the first shutter 9a in the open state.
And, most of the second harmonic wave that reaches the first dichroic mirror 11a that reflects the fundamental wave through the third lens 2c and is transmitted through the first dichroic mirror 11a is the second dichroic mirror 11a.
The light is condensed at the center of the wavelength conversion element 3b and reaches the beam splitter 4. On the other hand, most of the light (fundamental wave and second harmonic) that has passed through the first beam splitter 4a propagates in the transmissive medium 12 to be measured, is reflected by the reflecting mirror 5, and then is again measured medium 12. It propagates through and reaches the second dichroic mirror 11b that transmits the fundamental wave through the fourth lens 2d.

The fundamental wave reaching the second dichroic mirror 11b passes through the second dichroic mirror 11b due to its frequency characteristic, reaches the first dichroic mirror 11a via the second shutter 9b in the open state, and The light is reflected by the 1-dichroic mirror 11a, condensed on the central portion of the second wavelength conversion element 3b, and reaches the beam splitter 4. On the other hand, the second harmonic wave that has reached the second dichroic mirror 11b is reflected by the second dichroic mirror 11b due to its wavelength characteristic, is changed in traveling direction by the reflection mirror 13, and reaches the acousto-optic modulator 14, The optical frequency is shifted by the acousto-optic modulator 14 and reaches the beam splitter 4. The beam splitter 4 in this embodiment uses a half mirror whose transmittance and reflectance are approximately half and half.

A beat signal is generated by interference of the three kinds of second harmonics obtained from the three different routes as described above in the beam splitter 4, and the beat signal is passed through the fifth lens 2e to the filter 6. Then, only the beat signal of the second harmonic wave that has passed through the filter 6 reaches the photoelectric detector 7 and is photoelectrically detected, and the photoelectric detection signal is supplied to the phase measuring device 15 such as a two-phase lock-in amplifier. And
The phase measuring device 15 measures the phase of the interference fringe using the drive signal from the drive oscillator 16 as a reference signal, and supplies the measured value to the data analysis device 8.

In the measurement, first, the first shutter 9a
Is closed and the second shutter 9b is opened, the fraction (phase) of the interference fringes generated by the heterodyne interference is measured, and the order of the interference fringes is obtained. Then, the driver 10 scans the optical frequency of the laser light source 1 with the first shutter 9a opened and the second shutter 9b closed, and the phase change of the interference fringes generated at this time is measured to measure the medium 1 to be measured.
Find the length L of 2. In this way, the refractive index of the measured medium 12 can be obtained from the order of the interference fringes and the length L of the measured medium 12. In the second embodiment, since the laser light passes through a medium (for example, air) different from the medium to be measured 12, some error occurs, but the medium to be measured 12 is not placed. It can be preset with the measured value of. Further, the error caused by the container used to store the gas or the liquid as the medium to be measured can be preset as well.

The third embodiment shown in FIG. 3 is a property measuring apparatus for measuring the property of the machined surface. The same components as those in the first and second embodiments are designated by the same reference numerals. And the description is omitted.

The light (fundamental wave and second harmonic) collimated by the second lens 2b is applied to the surface of the medium to be measured 12 'fixed on the stage 17, and the medium to be measured 1 is measured.
The reflected light reflected by the surface of 2'is passed through the first reflecting mirror 13a and the third lens 2c, and then the dichroic mirror 11 is formed.
To The second harmonic wave that has passed through the dichroic mirror 11 enters the acousto-optic modulator 14, undergoes an optical frequency shift, and then reaches the beam splitter 4 through the second reflecting mirror 13b. On the other hand, the fundamental wave reflected on the surface of the dichroic mirror 11 is condensed on the central portion of the second wavelength conversion element 3b and reaches the beam splitter 4 via the third reflecting mirror 13c.

A beat signal is generated by the interference of the two kinds of second harmonics that have reached the beam splitter 4 as described above, and the beat signal passes through the fourth lens 2d and passes through the filter 7 for removing the fundamental wave. The light is transmitted and photoelectrically detected by the detector 8. The detected signal is phase-measured by the phase measuring device 15 and supplied to the data analysis device 8. Here, when the surface of the measured medium 12 'is scanned with the laser beam while moving the stage 17, the phase of the beat signal changes according to the dispersion characteristic of the surface, and the property characteristic of the modified surface by the ion beam or the like is changed. Can be measured.

In each of the above embodiments, the nonlinear optical crystal such as KTiOPO ₄ is used as the wavelength conversion element to generate the second harmonic from the fundamental wave for the measurement. You may make it measure a refractive index and a property characteristic using higher harmonics, such as a 3rd harmonic and a 4th harmonic. By doing so, the accuracy of measurement of the fraction (phase) of the interference fringes is further enhanced, which is more suitable for highly accurate measurement of the refractive index and property characteristics.

[0038]

As described above, in the refractive index measuring method according to the present invention, the harmonic generated from the fundamental wave before passing through the medium to be measured and the fundamental wave after passing through the medium to be measured. Based on the interference fringe signal obtained by interfering with the generated higher harmonics, it is possible to obtain a highly accurate order of the interference fringes and also to obtain the interference fringes when the optical frequency of the light source is changed by a predetermined width. The length of the medium to be measured can be easily obtained based on the phase change, and the refractive index of the medium to be measured is obtained by using the order of the interference fringes and the length of the medium to be measured, which are obtained with high accuracy and ease. Be done. Moreover, since a large-scale measurement system is not required to obtain the length of the medium to be measured, it becomes possible to supply an inexpensive refractive index measuring device,
Regardless of the property of the medium to be measured, any of gas, liquid, and solid can be measured.

Further, in the property measuring method according to the present invention, the phase of the interference fringe is determined based on the interference fringe signal generated by the heterodyne interference, and the property of the surface of the medium to be measured is determined based on the phase of the interference fringe. Since the characteristic is measured, the characteristic characteristic in the two-dimensional region can be easily obtained by scanning while keeping the distance from the light source to the surface of the medium to be measured constant.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a schematic configuration of a refractive index measuring device according to a first embodiment.

FIG. 2 is a functional block diagram showing a schematic configuration of a refractive index measuring device according to a second embodiment.

FIG. 3 is a functional block diagram showing a schematic configuration of a machined surface property characteristic measuring apparatus according to a third embodiment.

[Explanation of symbols]

 1 Laser Light Source 3a First Wavelength Conversion Element 3b Second Wavelength Conversion Element 4a First Beam Splitter 4b Second Beam Splitter 5 Reflector 7a First Photoelectric Detector 7b Second Photoelectric Detector 8 Data Analysis Device 10 Driver

Claims (4)

[Claims] 1. A light obtained from a light source is used as a fundamental wave, and a harmonic generated from a fundamental wave before passing through a medium to be measured and a harmonic generated from a fundamental wave passing through a non-measuring medium interfere with each other. Then, the order of the interference fringes is obtained based on the interference fringe signal generated by the interference, and the length of the medium to be measured is obtained based on the change of the interference fringes caused by changing the frequency of the light source. A refractive index measuring method, characterized in that the refractive index of the medium to be measured is measured using the order and the length of the medium to be measured. 2. The frequency of a fundamental wave or a harmonic wave is shifted to generate heterodyne interference, and the interference fringes due to the heterodyne interference are used to improve the accuracy of measuring the phase of the interference fringes. The method for measuring the refractive index according to claim 1. 3. The method for measuring the refractive index according to claim 1, wherein the higher harmonic wave is a second higher harmonic wave generated by a nonlinear optical crystal. 4. A fundamental wave from a light source is applied to the surface of the medium to be measured, and the harmonics generated from the reflected light reflected on the surface of the medium to be measured and the frequency of the reflected light reflected on the surface of the medium to be measured are displayed. Heterodyne interference with the shifted fundamental wave, the phase of the interference fringe is determined based on the interference fringe signal generated by the interference, and the property characteristic of the surface of the medium to be measured is measured based on the phase of the interference fringe. A method for measuring property characteristics characterized by the above.