JP2015105850A - Refractive index measurement method, refractive index measurement device, and method for manufacturing optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive index measurement method which measures a refractive index of an object to be tested with high accuracy.SOLUTION: In a refractive index measurement method, light from a light source 10 is divided into a test light and a reference light, and a refractive index of a specimen 80 is measured by casing interference the test light having passed through the specimen 80 with the reference light then measuring a phase difference between the test light and the reference light. The specimen 80 is placed into a first medium, and a first phase difference is measured, then the specimen 80 is placed into a second medium having a refractive index different from the refractive index of the first medium, and a second phase difference is measured. The refractive index of the specimen 80 is calculated by using the first and second phase differences and the refractive indexes of the first and second media.

Description

  The present invention relates to a refractive index measurement method and a refractive index measurement device, and is particularly useful for measuring the refractive index of an optical element manufactured by molding.

  The refractive index of the mold lens varies depending on the molding conditions. The refractive index of the lens after molding is generally measured by the minimum deflection angle method or the V block method after processing into a prism shape. This processing work takes time and cost. Furthermore, the refractive index of the lens after molding changes due to stress release during processing. Therefore, a technique for measuring the refractive index of the molded lens in a nondestructive manner is necessary.

  Non-Patent Document 1 proposes a method for calculating a refractive index dispersion curve by fitting an interference signal in a spectral region using a function of wavelength. In Patent Document 1, the sum of the optical path lengths of the test object and the medium and the optical path length of the medium are measured for each of the two types of media, and the refractive index is calculated by removing the thickness of the test object.

JP 2012-083331 A

H. Delbarre, C.I. Przygodzki, M .; Tassou, D.M. Boucher. "High-precise index measurement in anisotropy crystals using white-light spectral interferometry." Applied Physics B, 2000, vol. 70, p. 45-51.

  In the method disclosed in Non-Patent Document 1, the thickness of the test object needs to be known. Furthermore, since it is difficult to directly fit an interference signal that is a complex function, the measurement accuracy of the refractive index tends to be low. Moreover, since the measurement method using the low coherence interferometer disclosed in Patent Document 1 is difficult to measure the optical path length with high accuracy, the measurement accuracy of the refractive index tends to be low.

  An object of the present invention is to provide a refractive index measuring method and a refractive index measuring apparatus capable of measuring the refractive index (refractive index dispersion curve) of a test object with high accuracy.

  The refractive index measurement method of the present invention divides light from a light source into test light and reference light, makes the test light incident on the test object, and interferes the test light transmitted through the test object with the reference light. A refractive index measurement method for measuring a refractive index of the test object by measuring a phase difference between the test light and the reference light, wherein the test object is disposed in a first medium. A first measurement step of measuring a first phase difference that is a phase difference between the test light transmitted through the reference light and the reference light, and the second measurement medium disposed in a second medium having a refractive index different from the refractive index of the first medium. A second measurement step of measuring a second phase difference, which is a phase difference between the test light transmitted through the test object and the reference light, the first phase difference, the second phase difference, and the first phase difference A calculating step of calculating the refractive index of the test object using the refractive index of the first medium and the refractive index of the second medium. It is.

  The refractive index measuring device of the present invention includes a light source, test light that divides light from the light source into test light and reference light, makes the test light incident on the test object, and transmits the test light. A phase difference between the test light and the reference light is calculated using an interference optical system that causes the reference light to interfere, a detection unit that detects the interference light between the test light and the reference light, and an interference signal output from the detection unit. A refractive index measuring device having a calculating means for calculating a refractive index of the test object based on the phase difference, wherein the calculating means transmits the test object disposed in the first medium. Transmitted through the test object disposed in the second medium having a first phase difference that is a phase difference between the measured light and the reference light and a refractive index different from the refractive index of the first medium. Using the second phase difference that is the phase difference between the test light and the reference light, the refractive index of the first medium, and the refractive index of the second medium, Serial is characterized by calculating the refractive index of the test object.

  ADVANTAGE OF THE INVENTION According to this invention, the refractive index measuring method and refractive index measuring apparatus which can measure the refractive index of a test object with high precision can be provided.

It is a block diagram of the refractive index measuring device of Example 1 of the present invention. It is a flowchart which shows the procedure which calculates the refractive index of a test object by the refractive index measuring apparatus of Example 1 of this invention. It is a figure which shows the interference signal obtained with the detector of the refractive index measuring device of Example 1 of this invention. It is a block diagram of the refractive index measuring device of Example 2 of the present invention. It is a block diagram of the refractive index measuring device of Example 3 of the present invention. It is a figure which shows the manufacturing process of the manufacturing method of the optical element of Example 4 of this invention.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a block diagram of the refractive index measuring apparatus according to the first embodiment. The refractive index measuring apparatus of the present embodiment is composed of a Mach-Zehnder interferometer, and measures the refractive index of the test object by performing interference measurement. The test object is a refractive optical element such as a lens or a flat plate, and its refractive index and thickness are unknown.

  The refractive index measurement device includes a light source 10, an interference optical system, a container 60 that can accommodate a medium and a test object, a detector 90, and a computer 100, and measures the refractive index of the test object 80.

  The light source 10 is a light source having a wide wavelength band (for example, a supercontinuum light source). The interference optical system divides the light from the light source 10 into light that passes through the test object (test light) and light that does not pass through the test object (reference light), and superimposes the test light and the reference light. Then, the interference light is guided to the detector 90. The interference optical system according to the first embodiment includes a plurality of beam splitters 20 and 21 and a plurality of mirrors 30, 31, 40, 41, 50 and 51.

  The beam splitters 20 and 21 are constituted by, for example, cube beam splitters. The beam splitter 20 transmits part of the light from the light source 10 and reflects the rest at the joint surface 20a of the two prisms. The light transmitted through the joint surface 20a is reference light, and the light reflected by the joint surface 20a is test light. The beam splitter 21 reflects part of the reference light and transmits part of the test light at the joint surface 21a. As a result, the reference light and the test light interfere to form interference light, and the interference light enters the detector 90.

  The container 60 contains a medium (for example, air, water, or oil) and the test object 80. The optical path length of the reference light in the container and the optical path length of the test light preferably coincide with each other in a state where the test object 80 is not arranged in the container. Therefore, it is desirable that the side surface (for example, glass) of the container 60 has a uniform thickness and refractive index, and both side surfaces of the container 60 are parallel. The medium in the container 60 is changed to a medium having a different refractive index by exchange with another medium or addition of another medium.

  The refractive index of the medium is measured by a medium refractive index measuring unit (not shown). The medium refractive index measuring means is composed of, for example, a temperature measuring means for measuring the temperature of the medium and a computer for converting the measured temperature into the refractive index of the medium. The container 60 is provided with a temperature adjustment mechanism (temperature control means) (not shown), and can increase and decrease the temperature of the medium, control the temperature distribution of the medium, and the like. The temperature of the medium is also controlled by the computer 100.

  The mirrors 40 and 41 are, for example, prism type mirrors. The mirrors 50 and 51 are, for example, corner cube reflectors. The mirror 51 has a drive mechanism in the direction of the arrow in FIG. The drive mechanism of the mirror 51 is composed of, for example, a stage with a wide drive range and a piezo element with high drive resolution. The driving amount of the mirror 51 is measured by a length measuring device (not shown) (for example, a laser length measuring device or an encoder). The drive of the mirror 51 is controlled by the computer 100. The optical path length difference between the reference light and the test light can be adjusted by the drive mechanism of the mirror 51.

  The detector 90 includes a spectroscope that separates interference light from the beam splitter 21 and detects the interference light intensity as a function of wavelength (frequency).

  The computer 100 functions as a calculation unit that calculates the refractive index of the test object based on the interference signal output from the detector 90, and also functions as a control unit that controls the drive amount of the mirror 51. The computer 100 includes a CPU and the like. Has been. However, the calculation means for calculating the refractive index of the test object from the interference signal output from the detector 90 and the control means for controlling the drive amount of the mirror 51 and the temperature of the medium can be configured by different computers.

  The interference optical system is adjusted so that the optical path lengths of the reference light and the test light are equal in a state where the test object 80 is not disposed in the container. The adjustment method is as follows.

In the refractive index measurement apparatus of FIG. 1, an interference signal between the reference light and the test light is acquired in a state where the test object 80 is not disposed on the test light path. At this time, the phase difference φ ₀ (λ) and the interference intensity I ₀ (λ) between the reference light and the test light are expressed by Equation 1.

Where λ is the wavelength in the air, Δ ₀ is the difference in optical path length between the reference light and the test light, I ₀ is the sum of the reference light intensity and the test light intensity, and γ is the visibility (visibility). From Equation 1, when Δ ₀ is not zero, the interference intensity I ₀ (λ) is a vibration function. Therefore, in order to make the optical path lengths of the reference light and the test light equal, the mirror 51 may be driven to a position where the interference signal does not become a vibration function. At this time, delta ₀ becomes zero.

Here, the case where the optical path lengths of the test light and the reference light are adjusted to be equal (Δ ₀ = 0) has been described, but how much the current position of the mirror 51 is shifted from Δ0 = 0. If known, it is not necessary to equalize the optical path lengths of the test light and the reference light. The driving amount of the mirror 51 from the position where the optical path lengths of the test light and the reference light are equal (Δ ₀ = 0) can be measured by a length measuring device (not shown) (for example, a laser length measuring device or an encoder).

  FIG. 2 is a flowchart showing a procedure for calculating the refractive index (refractive index dispersion curve) of the test object 80, and “S” is an abbreviation for Step.

First, the test object 80 is placed in a first medium (for example, air) (S10). Next, the first phase difference φ ₁ (λ) between the reference light and the test light in the first medium is measured (S20). Steps S10 and S20 are the first measurement steps.

When the test object 80 is arranged on the test optical path, the interference signal in the spectral region measured by the detector 90 in FIG. 1 is as shown in FIG. 3A and 3B are interference signals measured under conditions where the refractive index of the medium is different. 3A shows an interference signal in the first medium, and FIG. 3B shows an interference signal in the second medium (for example, water). The first phase difference φ ₁ (λ) between the reference light and the test light in the first medium is expressed by Equation 2.

Here, n ^sample (λ) is the phase refractive index of the test object, n ₁ ^medium (λ) is the phase refractive index of the first medium, and L is the geometric thickness of the test object. Λ _{0 in} FIG. 3 indicates a wavelength at which the phase difference φ (λ) takes an extreme value. Since the period of the interference signal becomes longer at wavelengths near λ ₀ , the interference signal is easy to measure. On the other hand, since the period of the interference signal is shortened at a wavelength away from λ ₀ , the interference signal may be too dense to be decomposed. If λ ₀ is out of the measurement range, the mirror 51 may be driven to adjust Δ ₀ .

The first phase difference φ ₁ (λ) can be measured using, for example, the following phase shift method. An interference signal is acquired while driving the mirror 51 minutely. The interference intensity I _k (λ) when the phase shift amount (= drive amount × 2π / λ) of the mirror 51 is δ _k (k = 0, 1,..., M−1) is expressed by Equation 3. .

The first phase difference φ ₁ (λ) is calculated by Equation 4 using the phase shift amount δ _k and the interference intensity I _k (λ). Guidelines to enhance the accuracy of calculation of the phase difference, and minimize the phase shift amount [delta] _k, is to maximize the number of drive steps M. The calculated phase difference is convolved with 2π. Therefore, an operation (unwrapping) for connecting 2π phase jumps is necessary. Note that the phase difference obtained by the phase shift method includes arbitraryness (unknown offset term) that is an integer multiple of 2π. When the first phase difference is measured, the temperature of the first medium is also measured, and the phase refractive index n ₁ ^medium (λ) of the _first ^medium is calculated.

Next, the test object 80 is placed in a second medium (for example, water) having a refractive index different from that of the first medium (S30). Then, the second phase difference φ ₂ (λ) between the reference light and the test light in the second medium is measured (S40). Similar to the first phase difference φ ₁ (λ), the second phase difference φ ₂ (λ) is measured using the phase shift method. When the second phase difference is measured, the temperature of the second medium is also measured, and the phase refractive index n ₂ ^medium (λ) of the _second ^medium is calculated. Steps S30 and S40 are second measurement steps.

Finally, using the first phase difference φ ₁ (λ), the second phase difference φ ₂ (λ), the refractive index of the first medium, and the refractive index of the second medium, the refractive index of the test object ( (Refractive index dispersion curve) is calculated (calculation step S50). Details of the method of calculating the refractive index of the test object are as follows.

When the first phase difference φ ₁ (λ) is fitted by Equation 5, an integer m ₁ and a coefficient A _k (k = 1, 2,..., 6) of the dispersion equation are obtained. When the second phase difference φ ₂ (λ) is fitted by Equation 6, an integer m ₂ and a coefficient B _k (k = 1, 2,..., 6) of the dispersion equation are obtained. That is, the phase refraction of the test object 80 obtained from the phase refractive index n ₁ ^sample (λ) and the second phase difference φ ₂ (λ) of the test object 80 obtained from the first phase difference φ ₁ (λ). The rate n ₂ ^sample (λ) is calculated. Here, the Cauchy dispersion formula is used as a function of the phase refractive index, but another refractive index dispersion formula (for example, a Selmeier dispersion formula) may be used.

The unknown offset terms of the first phase difference and the second phase difference are expressed by 2πm ₁ and 2πm ₂ , respectively. Since the thickness L of the test object 80 is an unknown quantity, the assumed value L + ΔL of the thickness is used in Equations 5 and 6. A difference ΔL between the assumed thickness value L + ΔL and the true value L is a thickness error. As the assumed thickness value, for example, the design thickness of the test object may be used. Equation 5 In Equation 6, the optical path length difference delta ₀ in the optical path length difference delta ₀ second medium in the first medium, it is assumed to be equal or different.

Consider the case where the assumed thickness value is equal to the true value (ΔL = 0). At this time, the phase refractive index of the test object obtained from the phase refractive index n ₁ ^sample (λ) and the second phase difference φ ₂ (λ) of the test object obtained from the first phase difference φ ₁ (λ). n ₂ ^sample (λ) is equal to the true value n ^sample (λ) of the phase refractive index of the test object.

Next, consider a case where the assumed thickness value has an error from the true value L (ΔL ≠ 0). In this case, Equation 5, phase index obtained by fitting Equation _{^{6 n 1 sample (λ),}} n 2 sample (λ) is the refractive index due to the thickness error ΔL error _{Δn 1 (λ), Δn 2} (λ) ^Therefore, it is different from the true value n ^sample (λ) of the phase refractive index of the test object. Refractive index errors Δn ₁ (λ) and Δn ₂ (λ) are expressed by Equation 7.

When the assumed thickness value has a thickness error ΔL, the difference between the phase refractive indexes n ₁ ^sample (λ) and n ₂ ^sample (λ) is expressed by Equation 8.

If the assumed thickness value does not have a thickness error ΔL, the difference between the phase refractive indices n ₁ ^sample (λ) and n ₂ ^sample (λ) is zero. Therefore, the assumed thickness value may be selected so that the phase refractive indexes n ₁ ^sample (λ) and n ₂ ^sample (λ) are equal. The assumed thickness value selected at that time is the thickness of the test object 80, and the calculated phase refractive indexes n ₁ ^sample (λ) and n ₂ ^sample (λ) are the phase refractive index dispersion curves of the test object 80. n ^sample (λ). The thickness of the test object 80 may be any thickness as long as the difference between the phase refractive indexes n ₁ ^sample (λ) and n ₂ ^sample (λ) is substantially zero, and the phase refractive indexes n ₁ ^sample (λ) and n ₂ ^sample (λ) may not be completely equal.

  As described above, the refractive index (refractive index dispersion curve) of the test object 80 is obtained using the first phase difference, the second phase difference, the refractive index of the first medium, and the refractive index of the second medium. ) Is calculated (calculation step S50).

  The calculation method of the refractive index (refractive index dispersion curve) of the test object 80 in the calculation step S50 may be the following method.

First, the unknown offset terms 2πm ₁ and 2πm ₂ are separated from the first phase difference φ ₁ (λ) and the second phase difference φ ₂ (λ) by the fitting of Equations 5 and 6, respectively. When a term Φ ₁ (λ) that separates 2πm ₁ from φ ₁ (λ) and a term Φ ₂ (λ) that separates 2πm ₂ from φ ₂ (λ) are used, a phase index n ₁ ^sample (λ) and n ₂ ^sample (λ) are calculated.

When the phase refractive index n ₁ ^sample (λ) of the ^specimen obtained from the first phase difference is equal to the phase refractive index n ₂ ^sample (λ) of the ^specimen obtained from the second phase difference, Equation 9 The right side of the upper expression and the right side of the lower expression are also equal. When n ₁ ^sample (λ) −n ₂ ^sample (λ) → 0, ΔL → 0, and the upper right side of Expression 9 and the right side of the lower expression are equal. When the upper right side of the formula 9 and the right side of the lower formula are connected by an equal sign, the assumed thickness value L + ΔL is calculated by the formula 10.

The assumed thickness value L + ΔL obtained by Expression 10 is equal to the true value L (that is, ΔL = 0). Substituting the assumed thickness value L + ΔL obtained by Equation 10 into Equation 9, the phase refractive index dispersion curve n ^sample (λ) of the test object 80 is obtained as shown in Equation 11.

  As described above, Formula 9, Formula 10, Formula 11 so that the phase refractive index of the test object obtained from the first phase difference is equal to the phase refractive index of the test object obtained from the second phase difference. Is used to calculate the phase refractive index dispersion curve of the test object.

Φ ₁ (λ) and Φ ₂ (λ) can even separate the unknown offset terms 2πm ₁ and 2πm ₂ from the first phase difference φ ₁ (λ) and the second phase difference φ ₂ (λ). Therefore, Equations 5 and 6 may not be used. Since Φ ₁ (λ) and Φ ₂ (λ) can be expressed by a function that does not include a constant term, for example, by using function fitting like Formula 12 instead of Formula 5 and Formula 6, Φ ₁ (λ ), Φ ₂ (λ) can be extracted.

2πm ₁ and 2πm ₂ can be removed by differentiating the first phase difference φ ₁ (λ) and the second phase difference φ ₂ (λ) with respect to the wavelength, instead of separation by fitting. A differential dφ ₁ (λ) / dλ of the first phase difference and a differential φ ₂ (λ) / dλ of the second phase difference are expressed by Expression 13.

Where n _g1 ^sample (λ) is the group refractive index of the test object obtained from the first phase difference, and n _g2 ^sample (λ) is the group refractive index of the test object obtained from the second phase difference. is there. n _g1 ^medium (λ) is the group refractive index of the first medium, and n _g2 ^medium (λ) is the group refractive index of the second medium. By transforming Equation 13, the group index _n ^{g1 sample} of the object ^{_{(λ), n g2 sample (}} λ) is expressed by Equation 14.

The assumed thickness value L + ΔL that makes n _g1 ^sample (λ) and n _g2 ^sample (λ) calculated by Expression 14 equal is the thickness of the test object. At this time, n _g1 ^sample (λ) and n _g2 ^sample (λ) are equal to the group refractive index dispersion curve _ng ^sample (λ) of the test object. Alternatively, similar to the operations of Equations 10 and 11, the assumed thickness value L + ΔL obtained from the equation of the upper right side and the lower right side of Equation 14 is calculated and substituted into Equation 14 to obtain the group refraction of the test object. The rate dispersion curve _ng ^sample (λ) is calculated (Formula 15).

  As described above, the group refractive index dispersion of the test object is set so that the group refractive index of the test object obtained from the first phase difference is equal to the group refractive index of the test object obtained from the second phase difference. A curve is calculated. The method for calculating the phase refractive index dispersion curve from the group refractive index dispersion curve is as follows.

The phase refractive index N _p (λ) and the group refractive index N _g (λ) have a relationship as shown in Expression 16. However, C is an integral constant.

As can be seen from Equation 16, the calculation from the phase refractive index N _p (λ) to the group refractive index N _g (λ) is one way, but from the group refractive index N _g (λ) to the phase refractive index N _p (λ). The calculation has an arbitrary integration constant C. The phase refractive index N _p (λ) cannot be calculated from information on the group refractive index N _g (λ) alone.

Therefore, calculation of the group refractive index _ng ^sample (λ) of the test object to the phase refractive index n ^sample (λ) requires the assumption of an integration constant C. For example, it is assumed that the integration constant C ^sample of the test object is equal to the integration constant C ^{glass of the} base material from which the test object is based. The integral constant C ^glass of the base material can be calculated using the phase refractive index value of the base material provided by the glass material manufacturer. Using this integration constant C ^glass and Equation 16, the phase refractive index n ^sample (λ) can be calculated from the group refractive index _ng ^sample (λ) of the test object.

Instead of calculating the integration constant C, a method using a difference or ratio between the phase refractive index and the group refractive index can be applied. The phase refractive index calculation method using the difference and the phase refractive index calculation method using the ratio are expressed by Expression 17, respectively. Here, the phase refractive index of the base material is represented by N _p (λ), and the group refractive index of the base material is represented by N _g (λ).

  A guideline for improving the calculation accuracy of the refractive index dispersion curve is to increase the refractive index difference between the first medium and the second medium. In this embodiment, the thickness of the specimen is selected (calculated) so that the refractive index of the specimen obtained from the first phase difference is equal to the refractive index of the specimen obtained from the second phase difference. The refractive index (refractive index dispersion curve) of the test object is calculated. Therefore, the calculation accuracy of the refractive index dispersion curve depends on the calculation accuracy of the thickness of the test object. The denominators of Equations 10 and 15, which are equations for calculating the thickness of the test object, are the difference amounts of the refractive indexes of the first medium and the second medium. Therefore, as the difference in refractive index between the first medium and the second medium is larger, the accuracy of calculating the thickness of the test object is improved, and the accuracy of calculating the refractive index dispersion curve is also improved.

  Since the refractive index distribution of the medium is generated by the temperature distribution of the medium, an error occurs in the calculated refractive index of the test object. Therefore, it is desirable to control the temperature distribution of the medium with a temperature adjusting mechanism (temperature adjusting means) so that the temperature distribution of the medium does not occur. Further, since the error due to the refractive index distribution of the medium can be corrected if the amount of the refractive index distribution is known, it is desirable to have a wavefront measuring device (wavefront measuring means) for measuring the refractive index distribution of the medium.

  In this embodiment, the refractive index of the first medium and the second medium is changed by exchanging the medium in the container 60 or adding another medium. Instead, a container for storing the first medium and a container for storing the second medium may be prepared and replaced together.

  In this embodiment, the phase difference is measured by a combination of mechanical phase shift by the mirror 51 and spectroscopy by the detector 90. Instead, heterodyne interferometry may be used. When heterodyne interferometry is used, the interferometer, for example, arranges a spectroscope immediately after the light source and emits pseudo-monochromatic light, generates a frequency difference between the reference light and the test light by the acousto-optic device, and causes interference. The signal is measured with a detector such as a photodiode. Then, the phase difference is calculated at each wavelength while scanning the wavelength with a spectroscope.

  In this embodiment, a supercontinuum light source is used as the light source 10 having a wide wavelength band. Instead, a super luminescent diode (SLD), a halogen lamp, a short pulse laser, or the like may be used. When scanning the wavelength, a wavelength swept light source may be used instead of the combination of the broadband light source and the spectroscope.

  In this embodiment, a Mach-Zehnder interferometer is used, but a Michelson interferometer may be used instead. In this embodiment, the refractive index and the phase difference are calculated as a function of wavelength, but may be calculated as a function of frequency instead.

  In this embodiment, since the phase difference is calculated from the interference signal and fitting is performed for the phase difference which is a relatively simple function, the fitting accuracy is high. In the present embodiment, the fitting work can be omitted by using Equations 13-15. Further, in this embodiment, the thickness of the test object can be calculated. As described above, according to the refractive index measuring apparatus of the present embodiment, the refractive index (refractive index dispersion curve) of the test object can be measured with high accuracy even if the thickness of the test object is unknown.

  FIG. 4 is a block diagram of the refractive index measuring apparatus according to the second embodiment. An interferometer that measures the refractive index of the medium is added to the refractive index measurement apparatus of the first embodiment. The same configurations as those in the first embodiment will be described with the same reference numerals.

  The light emitted from the light source 10 is split into transmitted light and reflected light by the beam splitter 22. The transmitted light proceeds to the interference optical system for measuring the refractive index of the test object 80, and the reflected light is guided to the interference optical system for measuring the refractive index of the medium. The reflected light is further divided into transmitted light (medium reference light) and reflected light (medium test light) by the beam splitter 23.

  The medium test light reflected by the beam splitter 23 is reflected by the mirrors 42 and 52, passes through the side surface of the container 60 and the medium, is reflected by the mirror 33, and reaches the beam splitter 24. The medium reference light transmitted through the beam splitter 23 is reflected by the mirrors 32, 43, and 53, then passes through the compensation plate 61 and reaches the beam splitter 24. The medium reference light and the medium test light that have reached the beam splitter 24 interfere with each other to form interference light, which is detected by the detection unit 91 configured by a spectroscope or the like. A signal detected by the detector 91 is sent to the computer 100.

  The compensation plate 61 plays a role of correcting the influence of refractive index dispersion due to the side surface of the container 60 and is made of the same material and the same thickness as the side surface of the container 60 (= thickness of the side surface of the container 60 × 2). The compensation plate 61 has an effect of equalizing the optical path length difference between the wavelengths of the medium test light and the medium reference light when the container 60 is empty.

  The mirror 53 has a drive mechanism similar to that of the mirror 51, and is driven in the direction of the arrow in FIG. The drive of the mirror 53 is controlled by the computer 100.

  The procedure for calculating the phase refractive index of the test object 80 of the present embodiment is as follows.

First, the test object 80 is placed in a first medium (for example, oil having a phase refractive index of about 1.5) (S10). The first phase difference is measured in the first medium (S20). Steps S10 and S20 are the first measurement steps. When the first phase difference is measured, the phase difference η ₁ (λ) between the medium reference light and the medium test light in the first medium is also measured using an interferometer for measuring the refractive index of the medium. Is done. The phase difference η ₁ (λ) between the medium reference light and the medium test light in the first medium and its differential dη ₁ (λ) / dλ are expressed by Equation 18.

Where L ^tank is the distance between the side surfaces of the container 60 (the optical path length in the medium of the medium test light), and Δ is the optical path length difference between the medium reference light and the medium test light, which is a known amount. The phase refractive index n ₁ ^medium (λ) of the _first ^medium is obtained by fitting the relational expression of η ₁ (λ) in Expression 18 in the same manner as the method of calculating the phase refractive index n ₁ ^sample (λ) of the test object. It is obtained by doing. The group refractive index n _g1 ^medium (λ) of the first medium can be obtained by modifying the relational expression dη ₁ (λ) / dλ in Expression 18.

  Next, the test object 80 is placed in a second medium (for example, oil having a phase refractive index of about 1.7) (S30). The second phase difference is measured in the second medium (S40). Steps S30 and S40 are second measurement steps. When the second phase difference is measured, the phase difference between the medium reference light and the medium test light in the second medium is also measured using an interferometer for measuring the refractive index of the medium. The refractive index of the second medium is calculated from the phase difference between the medium reference light and the medium test light in the second medium. Finally, the refractive index (refractive index dispersion curve) of the test object 80 is calculated using the first phase difference, the second phase difference, the refractive index of the first medium, and the refractive index of the second medium. (Calculation step S50). In calculation step S50, the refractive index of the test object (refractive index dispersion curve) is set so that the refractive index of the test object obtained from the first phase difference is equal to the refractive index of the test object obtained from the second phase difference. ) Is calculated.

  FIG. 5 is a block diagram of the refractive index measuring apparatus according to the third embodiment. The wavefront is measured using a two-dimensional sensor (wavefront measuring means). In order to measure the refractive index of the medium, a glass prism (reference test object) having a known refractive index and shape is arranged on the optical path of the test light. The same configurations as those in the first and second embodiments will be described with the same reference numerals.

  The light emitted from the light source 10 is split by the spectroscope 95 and enters the pinhole 110 as pseudo-monochromatic light. The wavelength of the pseudo-monochromatic light incident on the pinhole 110 is controlled by the computer 100. The light that has passed through the pinhole 110 and becomes divergent light is collimated into parallel light by the collimator lens 120. The collimated light is split by the beam splitter 25 into transmitted light (reference light) and reflected light (test light).

  The reference light that has passed through the beam splitter 25 passes through the medium in the container 60, is reflected by the mirror 31, and reaches the beam splitter 26. The mirror 31 has a drive mechanism in the direction of the arrow in FIG. 5 and is controlled by the computer 100.

  The test light reflected by the beam splitter 25 is reflected by the mirror 30 and enters the container 60 that houses the medium, the test object 80, and the glass prism 130. Part of the test light passes through the medium and the test object 80. Part of the test light passes through the medium and the glass prism 130. The remaining light of the test light is transmitted only through the medium. Each light transmitted through the container 60 interferes with the reference light in the beam splitter 26 to form interference light, and is detected by the detector 92 (for example, CCD or CMOS sensor) through the imaging lens 121. The interference signal detected by the detector 92 is sent to the computer 100.

  The detector 92 is arranged at a conjugate position with the position of the test object 80 and the glass prism 130. If the phase index of refraction between the test object 80 and the medium is different, the light transmitted through the test object 80 becomes divergent light or convergent light. When the divergent light (converged light) intersects with light transmitted through other than the test object 80, an aperture or the like may be arranged behind the test object 80 (detector 92 side) to cut stray light. The glass prism preferably has a phase refractive index substantially equal to the phase refractive index of the medium so that the interference fringes between the light transmitted through the glass prism 130 and the reference light do not become too dense. The optical path lengths of the test light and the reference light are adjusted to be equal in a state where the test object 80 and the glass prism 130 are not arranged on the test light path.

  The procedure for calculating the phase refractive index of the test object 80 of the present embodiment is as follows.

  First, the test object 80 is placed in the first medium (S10). The first phase difference and the refractive index of the first medium are measured in the first medium by the wavelength scanning by the spectroscope 95 and the phase shift method using the drive mechanism of the mirror 31 (S20). Steps S10 and S20 are the first measurement steps. Next, the test object 80 is placed in the second medium (S30). The second phase difference and the refractive index of the second medium are measured in the second medium (S40). Steps S30 and S40 are second measurement steps. Finally, the refractive index (refractive index dispersion curve) of the test object 80 is calculated using the first phase difference, the second phase difference, the refractive index of the first medium, and the refractive index of the second medium. (Calculation step S50). In calculation step S50, the refractive index of the test object (refractive index dispersion curve) is set so that the refractive index of the test object obtained from the first phase difference is equal to the refractive index of the test object obtained from the second phase difference. ) Is calculated.

  It is also possible to feed back the measurement result of the refractive index using the refractive index measuring device and the refractive index measuring method described in the first to third embodiments to a method for manufacturing an optical element such as a lens.

  FIG. 6 shows an example of a manufacturing process of an optical element using molding.

The optical element is manufactured through an optical element design process, a mold design process, and an optical element molding process using the mold. The molded optical element is evaluated for its shape accuracy, and when the accuracy is insufficient, the mold is corrected and molded again. If the shape accuracy is good, the optical performance of the optical element is evaluated. By incorporating the refractive index measurement method of the present invention into this optical performance evaluation step, it is possible to accurately mass-produce molded optical elements.
If the optical performance is low, the optical element whose optical surface is corrected is redesigned.

  Each embodiment described above is only a representative example, and various modifications and changes can be made to each embodiment in carrying out the present invention.

DESCRIPTION OF SYMBOLS 10 Light source 60 Container 80 Test object 90 Detector 100 Computer

Claims (15)

The light from the light source is divided into the test light and the reference light, the test light is incident on the test object, the test light transmitted through the test object and the reference light are interfered, and the test light and the reference light A refractive index measurement method for measuring a refractive index of the test object by measuring a phase difference of reference light,
A first measurement step of measuring a first phase difference that is a phase difference between the test light transmitted through the test object arranged in the first medium and the reference light;
A second phase difference that is a phase difference between the test light transmitted through the test object arranged in the second medium having a refractive index different from the refractive index of the first medium and the reference light is measured. A second measurement step;
Using the first phase difference, the second phase difference, the refractive index of the first medium, and the refractive index of the second medium to calculate a refractive index of the test object; A method for measuring a refractive index, comprising:   In the calculating step, the thickness of the test object is such that the refractive index of the test object obtained from the first phase difference is equal to the refractive index of the test object obtained from the second phase difference. The refractive index measurement method according to claim 1, wherein the refractive index of the test object is calculated using the thickness of the test object.   In the first measurement step and the second measurement step, the temperature of the medium is measured, and the measured temperature of the medium is converted into the refractive index of the medium, whereby the refractive index of the first medium and the second The refractive index measurement method according to claim 1, wherein the refractive index of the medium is measured.   In the first measurement step and the second measurement step, a reference test object having a known refractive index and shape is arranged in a medium, and light is incident on the reference test object to transmit a transmitted wavefront of the reference test object. And calculating the refractive index of the medium based on the refractive index and shape of the reference specimen and the transmitted wavefront of the reference specimen, thereby calculating the refractive index of the first medium and the second refractive index. The refractive index measurement method according to claim 1, wherein the refractive index of the medium is measured.   In the first measurement step and the second measurement step, the light from the light source is divided into medium test light and medium reference light, the medium test light is incident on the medium, and the medium test light transmitted through the medium By measuring the interference light obtained by causing the medium reference light to interfere, and calculating the refractive index of the medium based on the phase difference between the medium reference light and the medium test light, the refractive index of the first medium and the second refractive index are calculated. The refractive index measurement method according to claim 1, wherein the refractive index of the medium is measured.   The refractive index measurement method according to claim 1, wherein the refractive index distribution of the first medium and the refractive index distribution of the second medium are measured.   The refractive index measurement method according to any one of claims 1 to 6, wherein the temperature distribution of the first medium and the temperature distribution of the second medium are controlled. A light source, an interference optical system that divides light from the light source into test light and reference light, causes the test light to enter the test object, and causes the test light transmitted through the test object to interfere with the reference light; A detection means for detecting interference light between the test light and the reference light; a phase difference between the test light and the reference light is calculated using an interference signal output from the detection means; and the detection target is calculated based on the phase difference. A refractive index measuring device having a calculating means for calculating a refractive index of a specimen,
The calculation means is different from a first phase difference which is a phase difference between the test light transmitted through the test object arranged in the first medium and the reference light, and a refractive index of the first medium. A second phase difference which is a phase difference between the test light transmitted through the test object disposed in the second medium having a refractive index and the reference light, the refractive index of the first medium, the first A refractive index measuring apparatus that calculates the refractive index of the test object using the refractive index of the second medium.   The calculation means calculates the thickness of the test object such that the refractive index of the test object obtained from the first phase difference is equal to the refractive index of the test object obtained from the second phase difference. The refractive index measuring apparatus according to claim 8, wherein the refractive index of the test object is calculated using the thickness of the test object. Having a temperature measuring means for measuring the temperature of the medium;
The calculating means calculates the refractive index of the first medium and the refractive index of the second medium by converting the temperature of the medium measured by the temperature measuring means into the refractive index of the medium. The refractive index measuring device according to claim 8 or 9, characterized by the above. A reference specimen with a known refractive index and shape; and
Having wavefront measuring means for measuring a transmitted wavefront of light incident on the reference specimen placed in a medium;
The calculation means calculates the refractive index of the first medium and the refractive index of the second medium based on the refractive index and shape of the reference specimen and the transmitted wavefront of the reference specimen, respectively. The refractive index measuring device according to claim 8 or 9, wherein An interference optical system that divides light from the light source into medium test light and medium reference light, makes the medium test light incident on the medium, and causes the medium test light transmitted through the medium to interfere with the medium reference light;
Detecting means for detecting interference light between the medium test light and the medium reference light;
9. The calculation unit according to claim 8, wherein the calculation unit calculates a refractive index of the first medium and a refractive index of the second medium based on a phase difference between the medium reference light and the medium test light. The refractive index measuring apparatus according to 9.   The refractive index measuring device according to any one of claims 8 to 12, further comprising: a wavefront measuring unit that measures the refractive index distribution of the first medium and the refractive index distribution of the second medium.   14. The refractive index measurement apparatus according to claim 8, further comprising a temperature control unit configured to control a temperature distribution of the first medium and a temperature distribution of the second medium. Molding the optical element;
And a step of evaluating the molded optical element by measuring the refractive index of the optical element using the refractive index measurement method according to claim 1. A method for manufacturing an optical element.

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