JP7419603B2 - Refractive index measuring device and refractive index measuring method - Google Patents

Description

The present invention relates to a refractive index measuring device and a refractive index measuring method.

Patent Document 1 describes a technique of combining a Michelson interference optical system and a low coherence light source and analyzing the interference signal in order to measure the phase refractive index of a test object having a curved surface or the like with high precision.

Japanese Patent Application Publication No. 2013-186117

In the refractive index measuring device disclosed in Patent Document 1, it is necessary to place the test object in air and in a medium having a phase refractive index different from that of air, such as matching oil, and perform measurements multiple times. As described above, there has been a problem in the past that the measurement procedure is complicated.

The present invention has been made in view of such problems, and an object of the present invention is to provide a refractive index measuring device and a refractive index measuring method that can easily and accurately measure the phase refractive index of a test lens. .

In order to solve the above problems and achieve the objects, a refractive index measuring device according to at least some embodiments of the present invention includes a light source switching unit that selectively switches and emits coherent light and low coherence light. a light source section having a light source section; a light beam splitting section that splits the light beam from the light source section into a measurement light beam and a reference light beam; a reference optical system having a first condensing optical system that focuses the reference light beam on a reference mirror; A measurement optical system having a second condensing optical system that condenses a light beam onto a measurement mirror, and a reference optical system that is provided in the optical path of the reference optical system, and the reference mirror and the first condensing optical system are connected to the optical axis of the reference optical system. a first scanning stage that drives in a direction along the optical axis of the measurement optical system, and a second scanning stage that is provided in the optical path of the measurement optical system and drives the measurement mirror and the lens to be measured in the direction along the optical axis of the measurement optical system. , a detection unit that detects an interference signal between the light flux returning from the reference optical system and the light flux returning from the measurement optical system, and a processor.

The processor switches the light flux from the light source unit into coherent light by the light source switching unit, and connects the reference optical system and the measurement optical system based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system. an optical path length adjustment unit that adjusts the optical path length of the optical path length, a light flux from the reference optical system, and a light flux from the measurement optical system that passes through the test lens while the test lens is held on the second scanning stage; A central thickness calculation unit calculates the central thickness of the lens to be tested based on the interference signal of In this state, the Fourier transform unit converts the interference signal between the reference light beam and the measurement light beam into a complex spectral signal including the spectral phase by Fourier transform analysis, and the calculation result of second-order differentiation of the spectral phase is used to calculate the refractive index dispersion. and a refractive index calculation unit that determines coefficients of the dispersion formula by applying the nonlinear least squares method to the formula.

Further, the refractive index measurement method according to at least some embodiments of the present invention includes a light source light supply step in which coherent light and low coherence light are selectively switched and emitted, and a light flux produced by the light source light supply step is converted into a measurement light flux. a beam splitting step in which the reference beam is divided into a reference beam and a reference beam; a reference beam generation step in which the reference beam is focused on a reference mirror by a first focusing optical system; a first scanning step in which the reference mirror and the first focusing optical system are driven in a direction along the optical axis of the reference optical system; and a first scanning step in which the measuring mirror and the test lens are a second scanning step of driving the measurement optical system in a direction along the optical axis; a detection step of detecting an interference signal between the light flux obtained by the reference light generation step and the light flux obtained by the measurement light generation step; Optical path length that switches the light flux from the supply process to coherent light and adjusts the optical path length between the reference optical system and the measurement optical system based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system. The center thickness of the test lens is calculated based on the interference signal of the adjustment process, the light flux from the reference optical system, and the light flux from the measurement optical system that has passed through the test lens in the second scanning process. The light flux from the thickness calculation process and the light source light supply process is switched to low-coherence light, and while the test lens is held in the second scanning process, the interference signal between the reference light flux and the measurement light flux is analyzed by Fourier transform analysis. A Fourier transform process that converts the spectral phase into a complex spectral signal, and the second-order differential calculation result of the spectral phase is applied to the refractive index dispersion equation using the nonlinear least squares method, and the refractive index coefficients of the dispersion equation are determined. and a calculation step.

According to the present invention, it is possible to provide a refractive index measuring device and a refractive index measuring method that can easily and accurately measure the phase refractive index of a test lens.

FIG. 1 is a schematic system configuration diagram showing a system configuration of a refractive index measuring device according to a first embodiment, a second embodiment, and a third embodiment of the present invention. FIG. 2 is a functional block diagram showing the functional configuration of a processor of a refractive index measuring device according to a first embodiment, a second embodiment, and a third embodiment of the present invention. It is a flow chart which shows an example of refractive index measurement in a 1st embodiment of the present invention. It is a flowchart which shows an example of the process of calculating the center thickness of a test lens. (a), (b), (c), and (d) are diagrams showing a configuration for measuring the center thickness of a test lens. (a), (b), (c), and (d) are diagrams showing measurement results and calculation results regarding interference fringes. It is a flow chart which shows an example of refractive index measurement in a 2nd embodiment of the present invention. (a), (b), (c), (d), (e), and (f) are diagrams showing measurement results and calculation results regarding interference fringes in the second embodiment. FIG. 3 is a diagram showing the relationship between wavelength and phase refractive index. It is a flow chart which shows an example of refractive index measurement in a 3rd embodiment of the present invention.

Prior to describing Examples, the effects of embodiments according to certain aspects of the present invention will be described. In addition, when explaining specifically the effect of this embodiment, a specific example will be shown and demonstrated. However, as in the case of the embodiments described later, the illustrated aspects are only some of the aspects included in the present invention, and there are many variations in the aspects. Therefore, the present invention is not limited to the illustrated embodiments.

The refractive index measuring apparatus of the first embodiment, the second embodiment, and the third embodiment (hereinafter referred to as "the refractive index measuring apparatus of this embodiment") has a basic configuration. The basic configuration of the refractive index measuring device of this embodiment will be explained.

(Basic configuration of refractive index measuring device)
As shown in FIG. 1, a refractive index measuring apparatus 100 of this embodiment is an apparatus that measures the refractive index of a lens 14 to be tested. The type of lens 14 to be tested is not particularly limited, and any suitable convex lens or concave lens can be used. Below, as an example, a case of a biconvex positive lens will be explained.

The refractive index measurement device according to this embodiment is
a light source section having a light source switching section that selectively switches and emits coherent light and low coherence light;
a beam splitter that divides the light beam from the light source into a measurement beam and a reference beam;
a reference optical system having a first condensing optical system that condenses the reference light beam onto a reference mirror;
a measurement optical system having a second condensing optical system that condenses the measurement light flux onto a measurement mirror;
a first scanning stage that is provided in the optical path of the reference optical system and drives the reference mirror and the first focusing optical system in a direction along the optical axis of the reference optical system;
a second scanning stage that is provided in the optical path of the measurement optical system and drives the measurement mirror and the test lens in a direction along the optical axis of the measurement optical system;
a detection unit that detects an interference signal between the light flux returning from the reference optical system and the light flux returning from the measurement optical system;
A processor.

The processor is
The light flux from the light source section is switched to coherent light by the light source switching section, and the optical path length between the reference optical system and the measurement optical system is determined based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system. an optical path length adjustment section that adjusts the
The center of the test lens is determined based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system that has passed through the test lens while the test lens is held on the second scanning stage. a center thickness calculation section that calculates the thickness;
The light flux from the light source is switched to low-coherence light by the light source switching unit, and while the test lens is held on the second scanning stage, the spectral phase of the interference signal between the reference light flux and the measurement light flux is determined by Fourier transform analysis. a Fourier transform unit for converting into a complex spectral signal including;
It has a refractive index calculation unit that applies a calculation result obtained by second-order differentiation of the spectral phase to the dispersion formula of the refractive index by a nonlinear least squares method to determine the coefficients of the dispersion formula. Hereinafter, the refractive index measuring device 100 will be specifically explained.

The refractive index measurement device 100 includes a light source section 1, a Michelson interferometer 19, a photodiode 23 (detection section), a CMOS camera 27 (detection section), a spectrometer 29 (detection section), and a processor 30.

The light source unit 1 includes a low coherence light source 1A that supplies low coherence light, a single frequency laser light source 1B that supplies coherent light, a condensing lens 2A, a condensing lens 2B, a single mode fiber 3, and a low coherence light source 1A that supplies low coherence light. It has a light source switching unit 4 that selectively switches and emits coherence light.

The single frequency laser light source 1B is not particularly limited as long as it has a configuration that generates coherent light, and for example, a laser light source with an appropriate wavelength can be used. The low coherence light source 1A is not particularly limited as long as it has a configuration that generates low coherence light.

The light source switching unit 4 is electrically connected to a processor 30, which will be described later, and the switching operation is controlled by the processor 30. The specific configuration of the light source switching unit 4 depends on the configurations of the low coherence light source 1A and the single frequency laser light source 1B, but for example, a movably supported prism, a configuration in which the optical path is blocked by a light shielding plate, etc. can be used. .

Furthermore, the refractive index measuring device according to this embodiment includes:
The measurement optical system and the reference optical system are formed by a Michelson interferometer,
The Michelson interferometer is
a first laser length measuring device that measures the position of the reference mirror;
and a second laser length measuring device that measures the position of the measurement mirror. This will be explained in detail below. Note that the interferometer is not limited to the Michelson interferometer, and other interferometers (such as a Twyman-Green interferometer) can also be used.

An optical signal from the light source section 1 enters a Michelson interferometer 19. The Michelson interferometer 19 includes a polarizing plate 6, a beam splitter (light beam splitting unit) 7, a reference optical system 12, and a measurement optical system 18. The beam splitting section divides the beam from the light source section 1 into a measurement beam and a reference beam.

The reference optical system 12 includes a first condensing lens 8 (first condensing optical system), a reference mirror 9, a first laser length measuring device 10, and a first scanning stage 11. The first condensing lens 8 is a first condensing optical system that condenses the reference light beam onto the reference mirror 9 . The measurement optical system 18 includes a second condensing lens 13 (second condensing optical system), a test lens 14, a measurement mirror 15, a second laser length measuring device 16, and a second scanning stage 17. . The second condensing lens 13 is a second condensing optical system that condenses the measurement light beam onto the measurement mirror 15.

Optical signals from the reference optical system 12 and the measurement optical system 18 pass through a condenser lens 20 and an aperture 21 and enter a beam splitter 22 . The light beam reflected from the optical path splitting surface of the beam splitter 22 is incident on the photodiode 23.

The light beam that has passed through the optical path splitting surface of the beam splitter 22 passes through the aperture stop 24 and the collimator lens 25, and then enters the beam splitter 26. The light beam that has passed through the optical path splitting surface of the beam splitter 26 enters the CMOS camera 27 . Processor 30 analyzes the optical interference signal from CMOS camera 27.

Further, the light beam (frequency interference signal) reflected by the optical path splitting surface of the beam splitter 26 passes through the condensing lens 28 and enters the spectroscope 29 . Processor 30 analyzes the frequency interference signal.

The processor 30 includes a light source switching section 4, a stage controller 32, a first laser length measuring device 10, a second laser length measuring device 16, a photodiode 23, a CMOS camera 27, a spectrometer 29, a display section 31, and an operation section 33. connected to and controlled.

The low coherence light source 1A is a laser light source with a broadband wavelength. As the low coherence light source 1A, for example, an SLD (super luminescent diode) or an optical frequency comb light source is used. The coherence length of the low coherence light source 1A, that is, the resolution in the optical axis direction is expressed by the following equation (1).

For example, when the center wavelength λ _c of the low coherence light source 1A is 0.675 μm and the band Δλ is 0.008 μm, the coherence length ΔZ _coh is 26 μm.

The single frequency laser light source 1B is a laser light source with a narrow wavelength linewidth. As the single frequency laser light source 1B, for example, a He--Ne laser light source or a semiconductor laser light source is used.

The polarizing plate 6 transmits only a polarized component in one direction of the parallel light emitted from the collimator lens 5.

The processor 30 determines the temperature of the measurement environment with respect to the optical path length L _c between the reference mirror 9 and the first laser length measuring device 10 and the optical path length L _c between the measuring mirror 15 and the second laser length measuring device 16. Equipped with a mechanism to feedback control T, humidity H, and atmospheric pressure P. For example, the refractive index n _air of air can be measured by using Edlen's equation (2) below and the temperature T, humidity H, and atmospheric pressure P of the measurement environment.

Then, the optical path length L _c is measured with high precision using the following equation (3).

L ₀ =L _c ×n _air formula (3)

here,
L _c is the optical path length (mm),
L ₀ is the length measurement value (mm),
n _air is the refractive index of air.

The Michelson interferometer 19 splits the incident light from the light source section 1 into two, a reference light and a measurement light, by the beam splitter 7.

The light beam reflected by the beam splitter 7 enters the reference optical system 12. The light transmitted through the condenser lens 8 is condensed onto the surface of the reference mirror 9 and reflected. In this way, reference light is obtained.

Further, the light beam transmitted through the beam splitter 7 enters the measurement optical system 18. The light that has passed through the condenser lens 13 and the test lens 14 is focused on the surface of the measurement mirror 15 and reflected. In this way, measurement light is obtained.

The reference light and the measurement light return along the optical path they have traveled, are superimposed again by the beam splitter 7, and enter the condenser lens 20.

The first scanning stage 11 and the second scanning stage 17 are connected to a stage controller 32. Processor 30 controls stage controller 32.

The first scanning stage 11 drives a condenser lens 8 and a reference mirror 9. The second scanning stage 17 drives the measuring mirror 15 . Further, the first laser length measuring device 10 measures the amount of movement of the first scanning stage 11. The second laser length measuring device 16 measures the amount of movement of the second scanning stage 17.

The processor 30 measures the stage movement amounts of the first scanning stage 11 and the second scanning stage 17 with high precision at a sampling period of about 1 MHz to 100 kHz via the stage controller 32.

A fringe scanning method is performed in the refractive index measurement step to be described later. Therefore, at least in the direction along the optical axis of the measurement optical system, within a movement range of approximately one wavelength of the wavelength of the light beam of the single-frequency laser light source 1B, this wavelength can be equally divided into a plurality of parts and moved. Precision is required. In this embodiment, the first scanning stage 11 also includes a minute movement mechanism driven by, for example, a piezo element.

The processor 30 is connected to a display unit 31 that displays images of interference fringe images, image data subjected to image processing, calculated numerical data, and the like. The display unit 31 may always display the image captured by the detection unit (photodiode 23, CMOS camera 27, spectrometer 29), but displays the image only when the occurrence of discernible interference fringes is detected. You can do it like this.

The photodiode 23 is a detector that converts an optical signal into an electrical signal. For example, a balanced detector is used as the photodiode 23. Thereby, the signal strength of low coherence interference can be measured with high precision.

FIG. 2 shows the functional configuration of the measurement control section 150 included in the processor 30. The measurement control unit 150 controls the operation of the refractive index measuring device 100 and performs calculations for measuring the refractive index. The measurement control section 150 includes an optical path length adjustment section 151, a center thickness calculation section 152, a Fourier transformation section 153, and a refractive index calculation section 154.

The measurement control section 150 performs overall control of the refractive index measurement apparatus 100. The measurement control unit 150 controls the light source unit 1 to be controlled, the first scanning stage 11, the second scanning stage 17, the stage controller 32, the first laser length measuring device 10, and the second laser It is communicably connected to the length measuring device 16.

Further, the measurement control section 150 is communicably connected to an optical path length adjustment section 151, a center thickness calculation section 152, a Fourier transformation section 153, and a refractive index calculation section 154 in the measurement control section 150. The measurement control unit 150 can send control signals and data to each, and can obtain calculation results from each.

For example, the optical path length information sent from the first laser length measuring device 10 and the second laser length measuring device 16 is sent to the optical path length adjusting section 151 via the measurement control section 150.

Further, the wavefront information sent from the detection section (photodiode 23, CMOS camera 27, spectrometer 29) is sent to the center thickness calculation section 152 and Fourier transform section 153 via the measurement control section 150.

The measurement control unit 150 also includes an operation unit 33 through which an operator inputs operations to control the operation of the refractive index measuring device 100, and information regarding control performed by the measurement control unit 150 and calculation results in text and images. A display section 31 for displaying images is connected thereto.

The operation unit 33 includes operation input means such as a keyboard, a mouse, and operation buttons. Examples of operation inputs performed by the operator include starting and stopping the refractive index measuring device 100, selecting the type of measurement, and operating inputs for driving the first scanning stage 11 and the second scanning stage 17. I can do it. Each operation input is converted into a control signal or numerical information as necessary and sent to the measurement control section 150.

The optical path length adjustment unit 151 switches the light flux from the light source unit 1 to coherent light by the light source switching unit 4, and based on the interference signal between the light flux from the reference optical system 12 and the light flux from the measurement optical system 18. , adjust the optical path lengths of the reference optical system 12 and the measurement optical system 18.

The center thickness calculation unit 152 calculates the light flux from the reference optical system 12 and the light flux from the measurement optical system 18 that has passed through the test lens 14 while the test lens 14 is held on the second scanning stage 17. The center thickness of the lens 14 to be tested is calculated based on the interference signal.

The Fourier transform unit 153 switches the light flux from the light source unit 1 to low coherence light using the light source switching unit 4, and converts the light flux from the light source unit 1 to the detection unit (photodiode 23, The interference signal between the reference light beam and the measurement light beam obtained by the CMOS camera 27 and spectrometer 29 is converted into a complex spectral signal including a spectral phase by Fourier transform analysis.

The refractive index calculation unit 154 applies the calculation result obtained by second-order differentiation of the spectral phase to the refractive index dispersion equation using the nonlinear least squares method, and determines the coefficients of the dispersion equation.

The device configuration of the measurement control unit 150 is a computer including a CPU, memory, input/output interface, external storage device, and the like. This computer executes control programs and arithmetic programs corresponding to each of the functions described above.

(First embodiment)
The flowcharts in FIGS. 3 and 4 (S101 to S110, S201 to S208) show a process of measuring and analyzing the phase refractive index of the test lens 14 using Fourier transform analysis of low coherence interference fringes.

In step S101 in FIG. 3, the center thickness of the lens 14 to be tested is calculated.
FIG. 4, which will be described later, is a flowchart showing the process of calculating the center thickness of the lens 14 to be tested. The step of calculating the center thickness of the test lens 14 is a step commonly used in the first embodiment, the second embodiment, and the third embodiment.

The process of calculating the center thickness of the test lens 14 will be explained using FIG. 4. In step S201, the light source switching unit 4 switches the light beam from the light source unit 1 to coherent light from the single frequency laser light source 1B and emits the coherent light.

In step S201 and step S202, the beam splitter 7 splits the coherent light from the light source unit 1 into a measurement light beam that is transmitted through the splitting surface and a reference light beam that is reflected on the splitting surface.

In step S203, the first condensing lens 8 (first condensing optical system) condenses the reference light beam onto the reference mirror 9.

In step S204, the second condenser lens 13 (second condenser optical system) condenses the measurement light beam onto the measurement mirror 15.

In step S205, the first scanning stage 11 moves the reference mirror 9 and the first condenser lens 8 in a direction along the optical axis of the first condenser lens 8, that is, along the optical axis of the reference optical system 12. drive in the opposite direction.

In step S206, the second scanning stage 17 moves the measurement mirror 15 and the test lens 14 in a direction along the optical axis of the second condensing lens 13, that is, in a direction along the optical axis of the measurement optical system 18. Drive to.

In step S207, the photodiode 23 and CMOS camera 27 detect the interference signal.

In step S208, the center thickness calculation unit 152 calculates the center thickness of the lens 14 to be tested.

Next, the procedure for measuring the center thickness L of the test lens 14 will be described in more detail using FIGS. 5(a), (b), (c), and (d). 5(a), (b), (c), and (d) show the configuration from the second condensing mirror 12 to the second length measuring device 16 in the measurement optical system 18.

As shown in FIG. 5A, the first scanning stage 11 in the reference optical system 12, the CMOS camera 27, and the single frequency laser light source 1B are used in a state in which the test lens 14 is not inserted. By minutely driving the first scanning stage 11, the phase of the reference light beam is changed, and measurement is performed using a fringe scanning method. Let the length measurement value in this state be _L1 . Then, the coefficient of the _Z4 term of the Zernike polynomial (Fringe Zernike polynomial) shown by the following equation (4) is measured.

A Zernike polynomial is an orthogonal polynomial defined on the unit circle. The coefficient of the _Z4 term is related to the amount of defocus.

The defocus amount ΔZ between the focusing point 13a of the second focusing lens 13 and the measuring mirror 15 shown in FIG. 5(a) is calculated by the following equation (5) using the Fringe Zernike coefficient _Z4 term. do.

As shown in FIG. 5(b), the second scanning stage 17 is driven based on the calculated defocus amount ΔZ. The condensing point 13a of the second condensing lens 13 is adjusted with high precision on the surface of the measurement mirror 15. At this time, the length measurement value of the second laser length measuring device 16 that measures the distance to the measurement mirror 15 is assumed to be L ₂ .

Next, as shown in FIG. 5(c), with the lens 14 to be tested inserted, a fringe scanning method (fringe scan method). Then, the coefficient of the _Z4 term of the Zernike polynomial (Fringe Zernike polynomial) shown by equation (3) is calculated.

The second scanning stage 17 has a holding section (not shown) that fixes and holds the lens 14 to be inspected.

As shown in FIG. 5(c), using the defocus amount ΔZ and the second scanning stage 17, the condensing point 13a of the second condensing lens 13 is placed on the surface of the test lens 14 with high precision. adjust. The length measurement value of the second laser length measurement device 16 at that time is assumed to be _L3 .

By using the length measurement values L ₂ and L ₃ of the second laser length measurement device 16, the center thickness L of the test lens 14 can be measured with high accuracy according to the following equation (6).

L=L ₂ - _{L 3} formula (6)

Here, L is the center thickness (mm) of the test lens 14,
L ₂ is the length measurement value (mm) of the second laser length measurement device 16;
L ₃ is the length measurement value (mm) of the second laser length measurement device 16.

Then, as shown in FIG. 5(d), when the lens to be tested 14 is inserted, the second scanning stage 17 is driven based on the center thickness L. Using the CMOS camera 27 and the single frequency laser light source 1B, the condensing point 13a of the second condensing lens 13 is aligned with the interface between the test lens 14 and the measurement mirror 15 with high precision. The length measurement value of the second laser length measurement device 16 at that time is assumed to be L ₄ .

Through the above procedure, the center thickness L of the test lens 14 can be measured, and the condensing point 13a of the second condenser lens 13 can be aligned with the interface between the test lens 14 and the measurement mirror 15 with high precision. be able to.

(Refractive index measurement)
Next, returning to FIG. 3, the procedure for measuring the refractive index will be explained using a flowchart.

In this procedure,
When the second scanning stage holds the lens to be tested,
The processor is
The light source switching unit switches the luminous flux from the light source unit to low coherence light,
Changing the optical path length of the reference beam by driving the first scanning stage in the optical axis direction of the reference optical system,
The detection unit measures interference fringe signals between the reference light beam and the measurement light beam that is transmitted through the test lens and reflected by the measurement mirror. This will be explained in detail below.

In step S101, the center thickness of the lens 14 to be tested is calculated. In step S101, the center thickness is calculated according to the steps from step S201 to step S208 described above.

In step S102, the light source switching unit 4 switches the light emitted from the collimator lens 5 from the single frequency laser light source 1B to the low coherence light source 1A. The coherence length is expressed by the above equation (1).

In step S103, first, the stage controller 32 drives the first scanning stage 11 at high speed to a position where the low coherence interference signal from the low coherence light source 1A is detected by the CMOS camera 27, and then stops it. At this time, the optical path length of the reference optical system 12 and the optical path length of the measurement optical system 18 are equal.

In step S103, the stage controller 32 then drives the first scanning stage 11 at a low speed. Then, a low coherence interference signal from the low coherence light source 1A is acquired by the CMOS camera 27 or the photodiode 23. At this time, the measurement control unit 150 uses the first laser length measuring device 10 to synchronize the low coherence interference signal and the length measurement value with the first laser length measuring device 10, and performs calibration. .

The method for measuring low coherence interference fringes in step S103 is time domain optical coherence tomography (TD-OCT).

In step S104, the Fourier transform unit 153 performs Fourier transform on the low coherence interference signal shown in FIG. 6(a) and analyzes it. As a result, complex spectrum signals shown in FIGS. 6(b) and 6(c) are obtained.

FIG. 6(b) shows the spectral amplitude intensity. FIG. 6(c) shows the spectral phase φ(ω). In the spectrum phase φ(ω) of FIG. 6(c), the signal is folded from −π to +π. Therefore, by performing phase splicing processing (phase unwrapping), the spectral phase φ(ω) shown in FIG. 6(d) is converted into a phase spliced waveform.

The above-mentioned relational expression between the spectral phase φ(ω) and the phase refractive index n _p (ω) is expressed by the following equation (7) using the center thickness L of the test lens 14 described above.

In steps S105, S106, S107, and S108, the nonlinear least squares method is applied to the result of second-order differentiation of the spectral phase φ(ω), for example, to the Sellmeyer dispersion equation of Equation (8) below.

In steps S109 and S110, the refractive index calculation unit 154 can calculate the phase refractive index n _p (ω) from the visible to near-infrared region by calculating the coefficients of the Sellmeyer dispersion equation.

The nonlinear least squares method shown in steps S105 to S109 is the Gauss-Newton method. For example, the Levenberg-Marquardt method may be used.

The Levenberg-Marquardt method generally requires a greater number of operations to converge than the Gauss-Newton method. However, the Levenberg-Marquardt method has the advantage of being able to stably converge even when the initial value is far from the optimal value.

In step S105, initial values A ₀ , B ₀ , C ₀ , D ₀ , E ₀ , and F ₀ of the Sellmeyer variance formula in equation (8) above are set in order to apply the nonlinear least squares method described above. . Note that if the type of glass material is known, the catalog value (value provided by the glass material manufacturer) or theoretical value of the glass material is input as the initial value.

In step S106, the initial values A ₀ , B ₀ , C ₀ , D ₀ , E ₀ , and F ₀ of the Sellmeyer variance equation are applied to the nonlinear least squares algorithm expressed by the following equation (9). Let A _n , B _n , C _n , D _n , E _n , and F _n be the input values when the nonlinear least squares algorithm is repeated n times.

In step S107, the calculation results ΔA _n , ΔB _n , ΔC _n , ΔD _n , ΔE _n , ΔF _n of the nonlinear least squares algorithm are compared with the set threshold values, and it is determined whether the following conditions are satisfied. .

Examples of threshold conditions are shown below.
ΔA _n <10 ^-5
ΔB _n <10 ^-5
ΔC _n <10 ^-5
ΔD _n <10 ⁻⁵
ΔE _n <10 ^-5
ΔF _n <10 ^-5

If the determination result in step S107 is false (No), that is, if the calculation result of the nonlinear least squares algorithm exceeds the threshold, the process advances to step S108. In step S108, the calculation shown in the following equation (10) is performed, and the input values of the (n+1)th nonlinear least squares algorithm A _n+1 , B _n+1 , C _n+1 , D _n+1 , E _n+1 and F _n+1 are calculated.

By repeating steps S106 to S108, the calculated value converges to the set threshold value or less. Then, in step S109, coefficients A, B, C, D, E, and F of the Sellmeyer variance equation are calculated.

In step S110, the phase refractive index n _p (ω) in the visible to near-infrared region can be calculated from the coefficients A, B, C, D, E, and F of the Sellmeyer dispersion equation described above, as shown in FIG. .

(Second embodiment)
Next, a measurement procedure of the refractive index measuring device according to the second embodiment will be explained. FIG. 7 is a flowchart showing a process of measuring and analyzing the phase refractive index of the lens 14 to be tested using Fourier transform phase analysis of frequency interference fringes.

In this procedure, when the second scanning stage holds the lens to be examined,
The processor is
The light source switching unit switches the luminous flux from the light source unit to low coherence light,
By fixing the first scanning stage, the optical path length of the reference beam is made constant;
The detection part is a spectrometer,
A spectrometer measures the frequency interference signal between the reference light beam and the measurement light beam that has passed through the test lens and been reflected by the measurement mirror. This will be explained in detail below.

First, in step S301, the center thickness L of the lens 14 to be tested is measured. The procedure for measuring the center thickness L is the same as the procedure already explained using FIGS. 3 and 4. Therefore, duplicate explanations will be omitted.

In step S302, after measuring the center thickness L of the test lens 14, the light source switching unit 4 switches the light emitted from the collimator lens 5 from the single frequency laser light source 1B to the low coherence light source 1A.

In step S303, the stage controller 32 drives the first scanning stage 11 at high speed to a position where the low coherence interference signal from the low coherence light source 1A is measured by the CMOS camera 27, and then stops the stage. At this time, the optical path length of the reference optical system 12 and the optical path length of the measurement optical system 18 become equal. A frequency interference signal from the low coherence light source 1A is acquired by the spectrometer 29.

The measurement method in step S303 is called Fourier domain optical coherence tomography (FD-OCT). In this embodiment, unlike the time domain optical coherence tomography (TD-OCT) in step S103 (FIG. 3) in the first embodiment, it is not necessary to mechanically drive the first scanning stage 11 during measurement. do not have. However, the optical path length of the reference optical system 12 and the optical path length of the measurement optical system 18 need to be equal.

In step S304, the above-described frequency interference signal shown in FIG. 8(a) is subjected to Fourier transform phase analysis. When the frequency interference signal is subjected to inverse Fourier transform, it can be converted into a time-axis signal having a DC component and two AC components (AC component 1 and AC component 2) shown in FIG. 8(b).

FIG. 8(c) shows a signal obtained by extracting only the AC component 1 using a band-pass filter and shifting the AC component 1 to the origin of the time axis.

FIGS. 8(d) and 8(e) respectively show complex spectral signals obtained when the shifted AC signal 1 is Fourier transformed. FIG. 8(d) shows the spectral amplitude intensity. FIG. 8(e) shows the spectral phase φ(ω).

Next, since the signal is folded from -π to +π, the spectrum phase φ(ω) shown in FIG. 8(e) is obtained by performing phase splicing processing (phase unwrapping). ) is converted into a phase-spliced waveform.

The procedure for calculating the coefficients of the Sellmeyer variance equation from step S305 to step S309 is the same as from step S105 to step S109 (FIG. 3) of the first embodiment.

Then, in step S310, the refractive index of the test lens 14 is calculated.

(Third embodiment)
Next, a measurement procedure of the refractive index measuring device according to the third embodiment will be explained. FIG. 10 is a flowchart showing a process of measuring and analyzing the phase refractive index of the lens 14 to be tested using Fourier transform phase analysis of frequency interference fringes.

In this procedure,
The processor is
If the center thickness of the test lens held on the second scanning stage is greater than or equal to a predetermined thickness,
The light source switching unit switches the luminous flux from the light source unit to low coherence light,
Changing the optical path length of the reference beam by driving the first scanning stage in the optical axis direction of the reference optical system,
The detection unit measures the interference fringe signals of the reference light flux and the measurement light flux transmitted through the test lens and reflected by the measurement mirror,
The processor is
When the center thickness of the test lens placed on the second scanning stage is less than or equal to a predetermined thickness,
The light source switching unit switches the luminous flux from the light source unit to low coherence light,
By fixing the first scanning stage, the optical path length of the reference beam is made constant;
A spectroscope serving as a detection unit measures frequency interference signals between the reference light beam and the measurement light beam that has passed through the test lens and been reflected by the measurement mirror. This will be explained in detail below.

First, in step S401, the center thickness L of the lens 14 to be tested is measured. The procedure for measuring the center thickness L is the same as the procedure already explained using FIGS. 3 and 4. Therefore, duplicate explanations will be omitted.

The flowchart in FIG. 10 shows that the measured center thickness L of the lens 14 to be tested is determined by Fourier transform analysis of low coherence interference fringes (S404, S405) or Fourier transform phase analysis of frequency interference fringes (S404, S405). S406, S407) is used to measure and analyze the phase refractive index.

The Fourier transform analysis of the low coherence interference fringes (S404, S405) and steps S408 to S413 perform the processing described in the first embodiment.

Fourier transform phase analysis of frequency interference fringes (S406, S407) and steps S408 to S413 perform the processing described in the second embodiment.

In step S403, if the measured value of the center thickness L of the lens 14 to be tested is 5 mm≦L, it is determined to be true (Yes), and if 5 mm>L, it is determined to be false (No).

If the determination result in step S403 is true, the process advances to step S404. If the determination result in step S403 is false, the process advances to step S406.

If the value of the center thickness L of the test lens 14 is 5 mm≦L, there is a possibility that the interference fringe interval of the frequency interference fringes measured by the spectrometer 29 may exceed the wavelength resolution of the spectrometer 29. If the interference fringe interval exceeds the wavelength resolution of the spectrometer 29, frequency interference fringes cannot be measured with high precision. Therefore, the process advances to step S404. On the other hand, if 5 mm>L, the process moves to step S406 in which mechanical driving of the first scanning stage 11 is not required during measurement. By proceeding to step S406, the measurement time can be shortened compared to the case of proceeding to step S404.

In this way, according to the present embodiment, the refractive index of the test lens can be measured with high precision and in a short time, corresponding to the center thickness of the test lens.

The refractive index measuring apparatus 100 according to the above-described basic configuration, first embodiment, second embodiment, and third embodiment can perform the following refractive index measuring method.

The refractive index measurement method is
a light source light supply step of selectively switching and emitting coherent light and low coherence light;
a beam splitting step of dividing the light beam from the light source light supply step into a measurement beam and a reference beam;
a reference light generation step of condensing the reference light beam onto a reference mirror by a first condensing optical system ;
a measurement light generation step of condensing the measurement light flux onto a measurement mirror by a second condensing optical system ;
a first scanning step of driving the reference mirror and the first focusing optical system in a direction along the optical axis of the reference optical system;
a second scanning step of driving the measurement mirror and the test lens in a direction along the optical axis of the measurement optical system;
a detection step of detecting an interference signal between the light flux obtained in the reference light generation step and the light flux obtained in the measurement light generation step;
Switch the light flux produced by the light source light supply process to coherent light, and adjust the optical path length between the reference optical system and the measurement optical system based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system. Optical path length adjustment process,
Center thickness calculation that calculates the center thickness of the test lens based on the interference signal between the light flux from the reference optical system and the light flux from the measurement optical system that has passed through the test lens in the second scanning step. process and
The light flux from the light source light supply process is switched to low-coherence light, and while the test lens is held in the second scanning process , the interference signal between the reference light flux and the measurement light flux is analyzed by Fourier transform to obtain a complex signal including the spectral phase. a Fourier transform step for converting into a spectral signal;
The method includes a refractive index calculation step of applying a calculation result obtained by second-order differentiation of the spectral phase to a dispersion formula of the refractive index by a nonlinear least squares method to determine coefficients of the dispersion formula.

The above-mentioned refractive index measurement method is a step of calculating the refractive index described using the flowchart in the first embodiment, the second embodiment, and the third embodiment.

All the components described in the above embodiments can be implemented by appropriately changing the combination or deleting them within the scope of the technical idea of the present invention.

As described above, the present invention is useful for a refractive index measuring device and a refractive index measuring method that can easily and accurately measure the phase refractive index of a test lens.
suitable for

1 Light source section 1A Low coherence light source 1B Single frequency laser light source 2A, 2B Condensing lens 3 Single mode fiber 4 Light source switching section 5 Collimator lens 6 Polarizing plate 7 Beam splitter 8 First condensing lens 9 Reference mirror 10 First Laser length measuring device 11 First scanning stage 12 Reference optical system 13 Second condensing lens 14 Test lens 15 Measuring mirror 16 Second laser length measuring device 17 Second scanning stage 18 Measuring optical system 19 Michael Son interferometer 20 Condensing lens 21 Aperture 22 Beam splitter 23 Photodiode 24 Aperture diaphragm 25 Collimator lens 26 Beam splitter 27 CMOS camera 28 Condensing lens 29 Spectrometer 30 Processor 31 Display section 32 Stage controller 33 Operation section 100 Refractive index measurement Apparatus 150 Measurement control section 151 Optical path length adjustment section 152 Center thickness calculation section 153 Fourier transformation section 154 Refractive index calculation section

Claims (6)

a light source section having a light source switching section that selectively switches and emits coherent light and low coherence light;
a beam splitting section that divides the beam from the light source into a measurement beam and a reference beam;
a reference optical system having a first condensing optical system that condenses the reference light beam onto a reference mirror;
a measurement optical system having a second condensing optical system that condenses the measurement light flux onto a measurement mirror;
a first scanning stage that is provided in the optical path of the reference optical system and drives the reference mirror and the first condensing optical system in a direction along the optical axis of the reference optical system;
a second scanning stage that is provided in the optical path of the measurement optical system and drives the measurement mirror and the test lens in a direction along the optical axis of the measurement optical system;
a detection unit that detects an interference signal between a light flux returning from the reference optical system and a light flux returning from the measurement optical system;
a processor;
The processor includes:
The light beam from the light source section is switched to the coherent light beam by the light source switching section, and based on the interference signal between the light beam from the reference optical system and the light beam from the measurement optical system, the reference optical system and the an optical path length adjustment section that adjusts the optical path length with the measurement optical system;
Based on an interference signal between a light beam from the reference optical system and a light beam from the measurement optical system that has passed through the test lens while the test lens is held on the second scanning stage, a center thickness calculation unit that calculates the center thickness of the test lens;
The light flux from the light source unit is switched to the low coherence light by the light source switching unit, and an interference signal between the reference light flux and the measurement light flux is detected while the test lens is held on the second scanning stage. a Fourier transform unit that converts into a complex spectral signal including a spectral phase by Fourier transform analysis;
A refractive index calculation unit that applies a calculation result obtained by second-order differentiation of the spectral phase to a dispersion formula of the refractive index by a nonlinear least squares method to determine a coefficient of the dispersion formula. measuring device. The measurement optical system and the reference optical system are formed by a Michelson interferometer,
The Michelson interferometer is
a first laser length measuring device that measures the position of the reference mirror;
a second laser length measuring device that measures the position of the measurement mirror;
The refractive index measuring device according to claim 1, characterized in that it has: When the second scanning stage holds the test lens,
The processor includes:
switching the light flux from the light source unit to the low coherence light by the light source switching unit;
changing the optical path length of the reference beam by driving the first scanning stage in the optical axis direction of the reference optical system;
The refractive index measurement according to claim 1, wherein the detection unit measures an interference fringe signal between the reference light beam and the measurement light beam that has passed through the test lens and been reflected by the measurement mirror. Device. When the second scanning stage holds the test lens,
The processor includes:
switching the light flux from the light source unit to the low coherence light by the light source switching unit;
fixing the first scanning stage to make the optical path length of the reference beam constant;
The detection unit is a spectrometer,
2. The refractive index measurement according to claim 1, wherein the spectrometer measures a frequency interference signal between the reference light beam and the measurement light beam that has passed through the test lens and been reflected by the measurement mirror. Device. The processor includes:
When the center thickness of the test lens held on the second scanning stage is greater than or equal to a predetermined thickness,
switching the light flux from the light source unit to the low coherence light by the light source switching unit;
changing the optical path length of the reference beam by driving the first scanning stage in the optical axis direction of the reference optical system;
The detection unit measures interference fringe signals of the reference light flux and the measurement light flux transmitted through the test lens and reflected by the measurement mirror,
The processor includes:
When the center thickness of the test lens placed on the second scanning stage is less than or equal to a predetermined thickness,
switching the light flux from the light source unit to the low coherence light by the light source switching unit;
fixing the first scanning stage to make the optical path length of the reference beam constant;
2. A spectroscope as the detection unit measures a frequency interference signal between the reference light beam and the measurement light beam that has passed through the test lens and been reflected by the measurement mirror. The refractive index measuring device described. a light source light supply step of selectively switching and emitting coherent light and low coherence light;
a luminous flux dividing step of dividing the luminous flux produced by the light source light supply step into a measurement luminous flux and a reference luminous flux;
a reference light generation step of condensing the reference light beam onto a reference mirror by a first condensing optical system ;
a measurement light generation step of condensing the measurement light flux onto a measurement mirror by a second condensing optical system ;
a first scanning step of driving the reference mirror and the first condensing optical system in a direction along the optical axis of the reference optical system;
a second scanning step of driving the measurement mirror and the test lens in a direction along the optical axis of the measurement optical system;
a detection step of detecting an interference signal between the light flux obtained in the reference light generation step and the light flux obtained in the measurement light generation step;
Switching the light flux produced by the light source light supply step to the coherent light, and connecting the reference optical system and the measurement optical system based on an interference signal between the light flux from the reference optical system and the light flux from the measurement optical system. an optical path length adjustment step of adjusting the optical path length of the
A center thickness of the test lens is calculated based on an interference signal between a light flux from the reference optical system and a light flux from the measurement optical system that has passed through the test lens in the second scanning step. Center thickness calculation process,
The light flux produced by the light source light supply step is switched to the low coherence light, and while the test lens is held in the second scanning step, the interference signal between the reference light flux and the measurement light flux is analyzed by Fourier transform analysis. a Fourier transform step of converting into a complex spectral signal including spectral phase;
A refractive index calculation step of applying a calculation result obtained by second-order differentiation of the spectral phase to a dispersion formula of the refractive index by a nonlinear least squares method to determine a coefficient of the dispersion formula. Measuring method.

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