JP2705752B2 - Method of measuring refractive index and method of measuring property characteristics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the improvement of positioning and dimensional accuracy by a laser interferometer in the precision production field of the electronic and mechanical industries, the measurement of the refractive index of an optical material in the optical-related industry, and the liquid and gas in the chemical industry Method for improving the accuracy and efficiency of the refractive index measurement of the surface of the object, and the property characteristics of the processed surface of the object effective for measuring and evaluating the properties of the modified surface and coating in the mechanical and electronic related production industries Related to the measurement method.

[0002]

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

Here, in the above-described two-color interferometer and the like, the interferometric length value at the fundamental wavelength λ _{1 is represented} by L ₁ ,
Let L _{2 be} the interference measurement value at the wavelength λ ₂ of the second harmonic,
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)

Note that the coefficient A in the above equation (1) is ｛(n ₂
-1) / (n ₂ −n ₁ )}, which is known to be a value of about 10 to several hundreds. However, it is necessary to precisely measure the phase of interference fringes. Accordingly, a precise refractive index of air can be obtained even in a relatively short length region. However, for this purpose, approximate values are sufficient, but L
Needs to be known, for example, it is necessary to arrange a reflecting mirror of an interferometer on a movable table and scan by a predetermined length.

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

The refractometer as described above has begun to be used in many scientific and industrial fields for correcting air fluctuations and the refractive index thereof. In other words, in addition to being used for high-precision parts in the advanced electronics and machinery industries, recently, due to the automation and industrialization of measurement, the demand for use in places where the measurement environment is unstable or in long optical paths has increased. This is because the correction of the air fluctuation and the refractive index thereof has a great influence on the measurement accuracy.

[0008]

However, in order to determine 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 it is necessary to separately provide a moving table or the like capable of moving in a constant direction at a constant speed for scanning the optical path length of the interferometer, which complicates the measurement system. In addition, it was impossible to measure the refractive index of solids and liquids. In addition, in the optical-related industries and production industries such as machinery, electronics, and chemistry, it is important to know the refractive index of liquids and solid materials as well as gases. Cannot be determined.

In addition, it is extremely important to know and evaluate the characteristics of the surface properties of precision-machined materials in the technology for the development of highly functional materials and ultra-precision technology, and the characteristics of the coating required for mounting, and to use them for evaluation. However, in the method as described above, since the phase measurement of the interference fringe is performed statically, the laser beam of the interferometer is scanned in a direction perpendicular to the surface of the object to be measured, and the processing surface of the object is processed. As in the case of measuring and evaluating the property characteristics such as
It could not be applied to the measurement of distribution characteristics in a dimensional area.

Accordingly, the present invention provides a method for measuring the refractive index and a method for measuring the properties of a gas, a liquid, and a solid, which can easily measure the refractive index.

[0011]

In order to solve the above-mentioned problems, in the method of measuring a refractive index according to the present invention, light obtained from a light source is used as a fundamental wave, and the fundamental wave before passing through a medium to be measured is used. The generated first harmonic and a second harmonic generated from the fundamental wave passing through the medium to be measured are caused to interfere with each other, and an order of an interference fringe is obtained based on an interference wave signal generated by the interference. determined the length of the measuring medium on the basis of the above interference fringe changes based on continuously varying the frequency of the light source, the measuring with the length of the order and the measuring medium of the interference fringes The refractive index of the medium was measured.

Further, in the method for measuring property characteristics according to the present invention, a fundamental wave from a light source and a wave generated from the fundamental wave are used.
First and harmonic irradiated to the measured medium surface, the fundamental wave and the first high reflected light reflected by the object to be measured medium surface
The above-mentioned group separated into harmonics and reflected on the surface of the medium to be measured
Second harmonic generated from main wave and frequency shift
A first harmonic is to heterodyne interference, the Haitai
The phase of the interference fringe is determined based on the interference wave signal generated by the Rodyne interference, and the property of the surface of the medium to be measured is measured based on the phase of the interference fringe.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of a method for measuring a refractive index according to the present invention will be described in detail with reference to the accompanying drawings.

First, the basic principle of the method for measuring the refractive index will be outlined. Now, assuming that the refractive indices of the medium at wavelengths λ ₁ and λ ₂ are n ₁ and n ₂ , respectively, a wave that is converted to λ ₂ in advance and a wave that is converted to λ ₂ after propagating as λ ₁ Assuming that the optical path difference due to the interference with the optical path is 2ΔL, the optical path difference ΔL for the forward path or the return path is obtained by the following equation (2). Here, L is the length of the medium.

[0015]

(Equation 2)

Further, 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 fringe with a single wavelength λ2, it is possible to measure the fraction (phase) of the interference fringe with high resolution. However, since the length L of the medium and the integer part of the order M of the interference fringes are unknown,
The correct refractive index n ₂ cannot be obtained directly.

Therefore, the principle of obtaining the length L of the medium will be described first. Now, in the interferometer having a 2L optical path difference, when the frequency of the light source is shifted by Δν, the following equation (4) is established. Here, c is the light speed, and m is the order of the interference fringes formed when the frequency of the light source shifts.

[0020]

(Equation 4)

As is apparent from the above equation (4), Δ
When m is measured while knowing ν, L is obtained.
For example, assuming that Δν is 10 GHz and the measurement accuracy of a fraction (phase) of m is 1/300, the determination accuracy of L is 50 μm
Thus, necessary and sufficient accuracy can be obtained.

In order to determine the refractive index of the medium, the order M of the interference fringes is required. However, only the fractional part (phase) of M can be obtained by measuring the interference fringes, It is necessary to ask for a part separately. To do so, the thickness of the medium L
Prepare several kinds of things that are significantly different from each other, first obtain the approximate value of the refractive index by measuring the thin one, and then determine the integer part of M when measuring the thick one uniquely using the previous value do it.

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

A first embodiment of a refractometer embodying a method of measuring a refractive 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 a 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
The light is reflected by the first beam splitter 4b and travels to the second beam splitter 4b. Most of the light (about 96%) passes through the first beam splitter 4a. The first beam splitter 4a
The fundamental wave and the second harmonic transmitted through are propagated in the air, reflected by the reflector 5, and reach the second beam splitter 4b via a propagation path substantially parallel to the outward path. The reflecting mirror 5 shown in the drawing
Is composed of two mirror surfaces, and the reflection angles of the laser light hitting the two surfaces are appropriately adjusted (for example, two mirrors
By setting the angle to 0 ° and setting the incident angle of the laser beam to 45 °), the forward path and the return path are made substantially parallel. Further, the length L of the medium in the present embodiment is a distance from the first beam splitter 4a to the reflection mirror 5.

Most of the fundamental wave and the second harmonic (about 96%) reaching the second beam splitter 4b pass through the second beam splitter 4b and pass through the second condenser lens 2b to the second wavelength converter. 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 incident on the second wavelength conversion element 3b and the second harmonic converted in the second wavelength conversion element 3b interfere with each other, and the interference based on the interference of the second harmonic occurs. First fringe signal only
The light is selectively transmitted through the filter 6a, and the first photoelectric detector 7
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 (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 second The second harmonic is the second beam splitter 4b
So that you don't get to

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, where the fundamental wave and the second harmonic reflected by the reflector 5 are reflected. Only the interference fringe signal of the second harmonic, which interferes with the harmonic and selectively passes through the second filter 6b, is photoelectrically detected by the second photoelectric detector 7b, and supplies the photoelectric detection signal to the data analyzer 8. Then, the optical frequency of the laser light source 1 is scanned by the driver 10 for a predetermined Δν, and the phase change of the interference fringe generated at this time is measured to determine the length L of the optical path. Also,
By calculating the refractive index of the air from the measured value L and the measured value of the fraction (phase) of the interference fringes obtained as described above, the refractive index of the air as the medium can be obtained. In order to determine the integer part of the interference fringes, the reflection mirror 5 is used.
Since it is only necessary to change the disposition position, 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 denoted by the same reference numerals and description thereof is omitted. I do.

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

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

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

At the time of measurement, first, the first shutter 9a
Is closed and the second shutter 9b is opened to measure the fraction (phase) of the interference fringes generated by the heterodyne interference, thereby obtaining the order of the interference fringes. Next, the optical frequency of the laser light source 1 is scanned by the driver 10 while the first shutter 9a is opened and the second shutter 9b is closed, and the phase change of the interference fringe generated at this time is measured to thereby measure the medium 1 to be measured.
2 to determine the length L. 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, a slight error occurs. Can be preset with the measured values of. In addition, an error caused by a container used to store a gas or a liquid as a medium to be measured can be similarly preset.

The third embodiment shown in FIG. 3 is a property characteristic measuring device for measuring the property of the processed surface, and the same components as those in the first and second embodiments are denoted 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
The light reflected by the surface of the dichroic mirror 11a is reflected by the first reflecting mirror 13a and the third lens 2c.
To The second harmonic transmitted through the dichroic mirror 11 enters the acousto-optic modulator 14, undergoes an optical frequency shift, and reaches the beam splitter 4 via the second reflecting mirror 13 b. On the other hand, the fundamental wave reflected on the surface of the dichroic mirror 11 is condensed at the center 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 reaching the beam splitter 4 as described above, and the beat signal passes through a fourth lens 2d to a filter 6 for removing a fundamental wave. The transmitted light is photoelectrically detected by the detector 7 , the detected signal is subjected to phase measurement by the phase measuring device 15, and supplied to the data analyzer 8. Here, when the surface of the medium to be measured 12 ′ is scanned by the laser beam while moving the stage 17, the phase of the beat signal changes according to the dispersion characteristics of the surface, and the property characteristics of the modified surface due to the ion beam or the like are changed. Can be measured.

In each of the above-described embodiments, the second harmonic is generated from the fundamental wave for use in measurement by using a nonlinear optical crystal such as KTiOPO ₄ as the wavelength conversion element. The measurement of the refractive index and the measurement of the property may be performed using higher harmonics such as the third harmonic and the fourth harmonic. In this case, the measurement accuracy of the fraction (phase) of the interference fringes is further improved, and therefore, it becomes more suitable for highly accurate measurement of refractive index and property characteristics.

[0038]

As described above, in the method for measuring the refractive index according to the present invention, the harmonics 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 high-precision interference fringe order, and to obtain the interference fringe when the light 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 using the order of the interference fringes and the length of the medium to be measured which are obtained with high precision and ease. It is done. In addition, since a large-scale measurement system is not required to determine the length of the medium to be measured, it is possible to supply an inexpensive refractive index measuring device,
Regardless of the nature of the medium to be measured, any of gas, liquid, and solid can be measured.

In the method for measuring property characteristics 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 characteristics are measured, the property characteristics in a two-dimensional area 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 the drawings]

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

FIG. 2 is a functional block diagram illustrating 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 an apparatus for measuring properties of a processed surface according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser light source 3a 1st wavelength conversion element 3b 2nd wavelength conversion element 4a 1st beam splitter 4b 2nd beam splitter 5 Reflector 7a 1st photoelectric detector 7b 2nd photoelectric detector 8 Data analyzer 10 Driver

Claims (5)

(57) [Claims] [Claim 1] a fundamental wave light obtained from the light source, the first and harmonics caused from a fundamental wave before passing through the measured medium, a second that caused from the fundamental wave having passed through the measurement medium And the order of the interference fringes is determined based on the interference wave signal generated by the interference, and the frequency of the light source is linked.
It determined the length of the measuring medium on the basis of a change in the interference fringe based on making continue to change, the refractive index of the measuring medium with the length of the order and the measuring medium of the interference fringes A method for measuring a refractive index, characterized in that the refractive index is measured. 2. The method according to claim 1, wherein the light obtained from the light source is a fundamental wave,
First harmonic generated from fundamental before passing through quality
And a second wave generated from the fundamental wave passing through the medium to be measured.
2nd harmonic and shift the frequency of the first harmonic
Is allowed by the heterodyne interference the second harmonic, interference based on the raw Ji interference wave signal by the heterodyne interference
Obtain the order of the interference fringes and change the frequency of the light source continuously.
Measuring the interference fringes based on the
Determine the length of the constant medium, and determine the order of the interference fringes and the medium to be measured.
A method for measuring the refractive index, wherein the refractive index of the medium to be measured is measured using the length of the material . 3. A medium to be measured, wherein light obtained from a light source is used as a fundamental wave.
Through the first nonlinear optical crystal from the fundamental wave before passing through the quality
Passing through the first harmonic and the medium to be measured
From the fundamental wave through the second nonlinear optical crystal
Interference with the second harmonic, and the interference caused by the interference.
The order of the interference fringes is determined based on the interference signal, and the
The change of the interference fringes based on the continuous change of the wave number
The length of the medium to be measured is determined based on the
Of the medium to be measured using the order of
A method for measuring a refractive index, comprising measuring a refractive index of a material. 4. A medium to be measured, wherein light obtained from a light source is used as a fundamental wave.
From the fundamental wave before passing through the first nonlinear optical crystal
Passing through the first harmonic and the medium to be measured
From the fundamental wave through the second nonlinear optical crystal
The second harmonic is obtained and the frequency of the first harmonic is
To cause heterodyne interference with the second harmonic,
Based on the interference signal generated by the heterodyne interference
The order of the interference fringes, and continuously change the frequency of the light source.
Based on the change of the interference fringes based on
The length of the medium to be measured is determined, and the order of the interference fringes and the measured
Measures the refractive index of the medium to be measured using the length of the constant medium
A method of measuring a refractive index. 5. A fundamental wave from a light source and a beam generated from the fundamental wave.
A first harmonic which made irradiated to the measured medium surface, the fundamental wave and the first reflected light reflected by the object to be measured medium surface
After being reflected at the surface of the medium to be measured.
The second harmonic generated from the fundamental and the frequency
Heterodyne interference with the shifted first harmonic
Determining a phase of an interference fringe based on an interference wave signal generated by the heterodyne interference, and measuring a property of the surface of the medium to be measured based on the phase of the interference fringe. .