JP4247372B2 - Refractive index measurement method and apparatus - Google Patents

Description

  The present invention relates to a refractive index measuring method and apparatus for measuring an absolute refractive index of a liquid sample.

  For example, Non-Patent Document 1 discloses a refractive index measuring device for measuring the absolute refractive index of a gas sample using a double optical path Michelson interferometer. This double optical path Michelson interferometer makes measurement light of a single wavelength incident on a gas sample and a vacuum, and obtains interference light for each of the gas sample and the vacuum. This refractive index measuring apparatus obtains the absolute refractive index of the sample by measuring the interference light for the gas sample and the interference light for the vacuum.

When performing the measurement, the moving mirror that reflects the measurement light passing through the gas sample and the vacuum is moved. In order to improve the measurement accuracy of the absolute refractive index of the sample, the interference mirror is measured repeatedly by rotating this moving mirror, the absolute refractive index is obtained from each measurement, and the average value of these is calculated as the refractive index of the sample. It may be adopted as.
Kenichi Fujii and three others, “A New Refractometer by Combining a Variable Length Vacuum Cell and a Double-Pass Michelson Interferometer”, Eye Triple E • Transactions on Instrumentation and Measurement, Vol. 46, No. 2, April 1997, p. 191-195

  However, when a liquid sample is repeatedly measured multiple times in the measurement of interference light, the time required for measuring the absolute refractive index becomes longer, so that there is a greater possibility that factors such as temperature fluctuation will adversely affect the measurement.

  When applying the refractive index measuring device as described above to the refractive index measurement of a liquid sample, there are difficulties in obtaining high measurement accuracy and good reproducibility. It must be eliminated as much as possible.

  The present invention has been made in view of such problems in the conventional technology, and an object thereof is to provide a refractive index measurement method and apparatus capable of measuring the absolute refractive index of a liquid sample with high accuracy. .

  In order to achieve the above object, the present invention adopts the following configuration. The refractive index measurement method of the present invention is a method for measuring the absolute refractive index of a sample using a Michelson interferometer having a light source that emits measurement light incident on each of the liquid sample and the vacuum and a moving mirror. .

  In this method, when the movable mirror moves by a set length in which a plurality of measurement intervals are shifted, the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum are acquired for each measurement interval. When the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum are acquired for a plurality of measurement sections while the movable mirror is moved once, based on the phase change amount of the interference wave. Calculate the absolute refractive index of the sample for each measurement interval. For example, the absolute refractive index of the sample can be determined by averaging the absolute refractive index for each measurement interval.

  The procedure for acquiring the phase change amount of the interference wave is to acquire the count value of the number of periods of the interference wave for the interference waveform for the sample and the interference waveform for the vacuum, and to acquire the amplitude value for at least one of the interference waveform of the sample and the vacuum. You may make it include the procedure to do. In this case, the procedure for calculating the absolute refractive index of the sample for each measurement section includes a procedure for obtaining an integer part and a fraction part of the phase change amount expressed in units of the interference wave period based on the acquired count value and amplitude value. . Since the fractional part of the phase change amount expressed by the period of the interference wave can also be used for the calculation of the absolute refractive index, the accuracy is improved.

  Furthermore, the amplitude value of the interference waveform of the sample may be acquired when the phase change of the interference wave with respect to the vacuum is an integral multiple of the period in the procedure of acquiring the count value and the amplitude value. In this case, the absolute refractive index can be calculated by using the fractional part of the phase change amount expressed by the period of the interference wave with respect to the sample, and only the integer phase change amount of the interference wave is obtained for the vacuum. Just do it.

  The procedure for obtaining the fractional part of the phase change amount expressed in units of the period of the interference wave is to obtain the amplitude value of the interference waveform with respect to the sample when counting up the number of periods of the interference wave with respect to the vacuum, and The procedure for determining the phase of the interference waveform based on the amplitude value acquired until the time of one-quarter or less has elapsed, and the amplitude of the interference waveform for the sample acquired when counting up the number of cycles of the interference wave with respect to the vacuum And a procedure for obtaining a fractional part based on the value. At this time, the amplitude value may be acquired only when it corresponds to the start point and end point of each measurement section.

  Alternatively, control may be performed to increase the moving speed of the section in which the amplitude value is not acquired, compared to the moving speed of the moving mirror in the section in which the amplitude value is acquired. Such control can reduce the time required for measurement.

  In another aspect, the present invention provides a refractive index measuring apparatus including the above-described Michelson polarization interferometer. In this refractive index measuring apparatus, the acquisition means calculates the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum when the movable mirror moves by a set length that is arranged by shifting a plurality of measurement sections. Obtain for each measurement interval. The calculation means calculates the absolute refractive index of the sample for each measurement section based on the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum. The absolute refractive index of the sample can be determined based on the absolute refractive index for each measurement interval.

  In this refractive index measuring apparatus, a light source that emits light of a plurality of wavelengths can be used for the Michelson polarization interferometer. In this case, if the absolute refractive index of the sample is determined for the measurement light of each wavelength, the absolute refractive index of the sample with respect to the light of the specified wavelength can be determined based on the determined refractive index.

  In the present invention as described above, since the absolute refractive index of the liquid sample can be obtained for a plurality of measurement sections with one movement of the movable mirror, the absolute refractive index of the sample is determined based on those values. Thus, the absolute refractive index of the sample can be measured with high accuracy.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  The refractive index measuring apparatus in this embodiment is an apparatus that measures the absolute refractive index of a liquid sample using a Michelson polarization interferometer. A liquid used as a density standard solution can be used as a sample. FIG. 1 is a schematic perspective view showing a configuration of an optical system, and FIG. 2 is a schematic diagram for explaining an optical path in the optical system.

  As shown in FIGS. 1 and 2, the Michelson polarization interferometer includes a multi-wavelength laser light source 1, a non-polarization beam splitter 3, an interference optical circuit 4, and a measurement unit 6.

  The multi-wavelength laser light source 1 is a light source that can emit a plurality of laser beams having different wavelengths. Here, a multi-wavelength laser light source 1 having an excitation tube sealed with He gas and Ne gas, an output side mirror disposed at both ends of the excitation tube, and a total reflection mirror is used. By changing the optical distance between the total reflection mirror and the excitation tube, laser beams having wavelengths of 543 nm, 594 nm, 604 nm, 612 nm, and 633 nm can be selectively emitted from the same emission port.

  In this embodiment, laser light emitted in the horizontal direction from the light source 1 is incident on the half-wave plate 2. The half-wave plate 2 rotates the polarization angle of incident light by 45 degrees. The laser light that has passed through the half-wave plate 2 is incident on the cube-type non-polarizing beam splitter 9 and travels in the direction of 90 degrees horizontally with the laser light that travels straight in the beam splitter 9. The laser beam is divided into two light beams.

  The laser beam that has traveled straight through the beam splitter 9 is incident on the cube-type non-polarizing beam splitter 3 and is split into two light beams. One travels straight in the beam splitter 3 and the other travels in the opposite direction to the laser beam that travels in the direction of 90 degrees horizontally by the beam splitter 9.

  At a position away from the non-polarizing beam splitter 3 by a predetermined distance in the traveling direction of the straight laser beam, the traveling direction of the straight laser beam is set in the same direction as the laser beam reflected by the non-polarizing beam splitter 3 in the direction of 90 degrees. A mirror 5 to be changed is provided.

  The laser beam emitted from the multi-wavelength laser light source 1 is split into parallel laser beams traveling in the same direction by the non-polarizing beam splitter 3 and the mirror 5, and both laser beams enter the interference optical circuit 4.

  Hereinafter, the laser light whose traveling direction has been changed by 90 degrees by the mirror 5 is described as vacuum-side laser light L1, and the laser light whose traveling direction has been changed by 90 degrees by the non-polarizing beam splitter 3 is described by sample-side laser light L2.

  The interference optical circuit 4 introduces each laser beam into the vacuum region 63 and the sample region 64 of the measurement unit 6 and generates interference light between the laser beam that has passed through both regions and the reference laser beam.

  The interference optical circuit 4 is disposed above the polarizing beam splitter 41, a cube-shaped polarizing beam splitter 41, a mirror 42 that changes the traveling direction of the laser light that has traveled straight through the polarizing beam splitter 41 vertically downward, and the vertically downward direction. And a corner cube 46 that folds the laser beam incident from a point-symmetrical position vertically downward. Further, quarter wave plates 43, 44 and 45 for rotating the polarization angle of incident light by 90 degrees in two passes are provided above and below the polarization beam splitter 41 and below the mirror 42, respectively.

  As shown in FIG. 2, the vacuum side laser light L1 that has passed through the half-wave plate 2 and entered the interference optical circuit 4 is, for example, in the polarization beam splitter 41, in the incident direction with the s-polarized (lateral) component that travels straight. On the other hand, it is divided into p-polarized light (vertical) components reflected in the direction of 90 degrees.

  The vacuum-side reference laser light L1A, which is a component of the vacuum-side laser light L1 that travels straight through the polarization beam splitter 41, is reflected vertically downward by the mirror 42 and reaches the fixed stage 61 that constitutes the upper surface of the measurement unit 6. The opposite side of the fixed stage 61 is in contact with the vacuum region 63 and the sample region 64. The fixed stage 61 is made of glass, and mirror processing is performed at a position where the vacuum side reference laser beam L1A of the fixed stage 61 is incident.

  The vacuum side reference laser beam L1A is reflected vertically upward by the mirror surface portion. The vacuum-side reference laser beam L1A reflected vertically upward is incident on the polarization beam splitter 41 again through the mirror 42. At this time, the vacuum-side reference laser light L1A is reflected vertically upward by the polarization beam splitter 41 because the polarization angle has changed by 90 degrees because it has passed through the quarter-wave plate 43 twice.

  Then, the vacuum side reference laser light L1A that has reached the corner cube 46 and whose traveling direction has changed vertically downward in the corner cube 46 is incident on the polarization beam splitter 41 again. Since the polarization angle changes 90 degrees after passing through the quarter-wave plate 45 twice, the light travels straight through the polarization beam splitter 41 and reaches the fixed stage 61. The position of the fixed stage 61 is also mirrored in the same manner as described above, and the vacuum side reference laser light L1A is reflected vertically upward by the mirror surface portion. Further, the vacuum-side reference laser light L1A that has passed through the quarter-wave plate 44 twice and whose polarization angle has changed by 90 degrees is reflected by the polarization beam splitter 41 in the direction of incidence on the interference optical circuit 4. At this time, on the incident / exit surface 41a of the polarization beam splitter 41 where the vacuum side laser light L1 enters and exits, the exit position of the vacuum side laser light L1 is located on a horizontal plane different from the incident position due to reflection by the corner cube 46, and the polarized beam The non-polarizing beam splitter 3 and the mirror 5 are not positioned on the optical path of the vacuum side laser light L1 emitted from the splitter 41.

  On the other hand, the vacuum-side measurement laser beam L1B, which is the p-polarized component of the vacuum-side laser beam L1, is reflected vertically downward by the polarization beam splitter 41, and then passes the vacuum-side reference laser beam L2A, but has the same optical order. The light passes through the element and is emitted from the same position as the emission position of the vacuum-side reference laser light L1A.

  In the optical path of the vacuum-side length measuring laser beam L1B, a transparent light introducing window 61a is provided at a position where the vacuum-side measuring laser beam L1B of the fixed stage 61 reaches, and the vacuum-side measuring laser beam L1B is introduced into the light path. It is introduced into the vacuum region 63 through the window 61a and is reflected vertically upward by a mirror surface formed on the upper surface of the moving stage 62 provided at the lower end of the measuring unit 6. The optical path length of the vacuum-side reference laser beam L1A and the optical path length of the vacuum-side measurement laser beam L1B differ by four times (two reciprocations) the optical distance from the upper surface of the fixed stage 61 to the upper surface of the moving stage 62.

  By passing through the polarizing plate 47 arranged so that the laser beam having a polarization angle of 45 degrees can pass, the passing components of the vacuum side reference laser beam L1A and the vacuum side length measuring laser beam L1B interfere with each other, and light corresponding to this optical path difference Interference light having intensity can be obtained.

  Similarly to the vacuum side laser beam L1, the sample side laser beam L2 is divided into the sample side reference laser beam L2A and the sample side measurement laser beam L2B, and passes through the interference optical circuit 4, and the sample side reference laser beam. L2A is reflected on the upper surface of the fixed stage 61, and the sample side measurement laser L2B is reflected on the upper surface of the moving stage 62. The moving stage 62 is immersed in the liquid sample, and the sample-side measurement laser L2B is introduced into the sample region 64 through the light introduction window 61b of the fixed stage 61 and then proceeds through the sample region 64.

  The sample side reference laser light L2A and the sample side measurement laser light L2B are emitted from the same position different from the emission position of the vacuum side laser light L1 of the polarization beam splitter 41. As described above, the sample-side laser light L2 emitted from the interference optical circuit 4 also has a light intensity corresponding to an optical path difference that is four times the optical distance from the upper surface of the fixed stage 61 to the upper surface of the moving stage 62 (two reciprocations). Interference light having

  The interference light is input to the optical sensors 8a and 8b via the mirror 7 and the like. The sensor 8a or 8b detects the light intensity of the interference light. The optical sensor 8 c detects the light intensity of the laser light divided by the non-polarizing beam splitter 2 in order to confirm the intensity of the laser light output from the multi-wavelength laser light source 1.

  The detection signals of the optical sensors 8a and 8b indicate the interference intensity of the vacuum and the sample region 64 with respect to the liquid sample. In this refractive index measuring apparatus, the absolute refractive index of the sample can be obtained from these detection signals.

  The absolute refractive index of a medium is defined by the ratio between the speed of light in vacuum and the speed of light in the medium. Even if the speed of light changes depending on the medium, the vibration period or frequency does not change. Therefore, the absolute refractive index of the medium can be expressed by the ratio between the wavelength in vacuum and the wavelength in the medium.

  When the Michelson interferometer as described above is used, the number of waves included in the optical path increase distance due to the movement of the moving mirror can be observed as the number of periods of the interference wave. For example, the moving distance of the moving stage 62 is given by a quarter of the value obtained by multiplying the wavelength by the number of periods of the interference wave.

  For this reason, the wavelength in the medium can be expressed by a value obtained by multiplying the ratio of the phase change amount of the interference next wave to the vacuum and the phase change amount of the interference wave to the medium by the wavelength in the vacuum. At this time, the absolute refractive index is given by the ratio between the phase change amount of the interference wave with respect to the medium and the phase change amount of the interference wave with respect to the vacuum.

  As for the refractive index of the liquid sample, since there is no laser light source that emits stable light having a wavelength corresponding to the orange sodium D line (589.3 nm) that is normally used, in order to obtain the absolute refractive index at that wavelength, It is necessary to determine the chromatic dispersion of the refractive index of the liquid sample. By calculating the absolute refractive index for each of a plurality of wavelengths near the wavelength corresponding to the sodium D line and interpolating the value of the absolute refractive index at each wavelength with an appropriate chromatic dispersion correction formula, Get the absolute refractive index of. The multi-wavelength laser light source 1 is prepared for this purpose.

FIG. 3 shows the influence of dispersion on pure water at 20 ° C. in the wavelength range of light that can be emitted from the multi-wavelength laser light source 1. In this figure, black dots correspond to the wavelengths of light that can be emitted from the multi-wavelength laser light source 1. White dots correspond to the wavelength of the sodium D line. There is a difference of 150 × 10 ⁻⁶ in the refractive index between the refractive index at a wavelength corresponding to the sodium D line and the refractive index at a wavelength of 594 nm. An apparatus for determining the absolute refractive index of a liquid sample is required to have an accuracy of about 5 × 10 ⁻⁶ and a reproducibility of about 1 × 10 ^−6, so that the absolute refractive index value at each wavelength is interpolated with high accuracy. There is a need.

  In order to accurately obtain the ratio of the interference orders for each wavelength of the light emitted from the multi-wavelength laser light source 1, the refractive index measuring apparatus in this embodiment includes a signal processing means 100 and a control means 200 as shown in FIG. Is provided.

  The signal processing unit 100 includes an acquisition unit 101, a calculation unit 102, and a determination unit 103, and processes a signal from the interferometer according to a command from the control unit 200.

  The acquisition unit 101 acquires the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum for each of the plurality of measurement sections when the moving mirror moves by the set length under the control of the control unit 200. FIG. 5 shows an example of the relationship between the set length and the measurement interval. In this example, a plurality of measurement sections having the same length L are arranged so as to be shifted from the set length by an amount Δ. This shift amount Δ corresponds here to the wavelength of the measurement light in vacuum. In this example, since there are three measurement sections, the set length is equal to a value obtained by adding twice the deviation amount Δ to the length of each measurement section. For each measurement section, the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum are counted. With a single movement of the movable mirror, a plurality of interference wave phase change amounts with respect to the sample and a plurality of interference wave phase change amounts with respect to the vacuum are acquired.

  In order to realize the acquisition unit 101, a signal processing circuit as shown in FIG. 6 can be used. In this signal processing circuit, detection signals from the sensors 8a and 8b are input to the I / V converters 301a and 301b, respectively. The I / V converter 301a or 101b converts the current signal from the sensor 8a or 8b into a voltage signal and outputs the voltage signal to the amplifier 302a or 302b. The signal amplified by the amplifier 302a or 302b is input to the gain controller 303a or 303b. Signals appropriately adjusted for gain are input to zero cross discriminating circuits 304a and 304b, respectively. A comparator can be used for the zero-cross determination circuits 304a and 304b. FIG. 7 shows an input signal to the zero-cross determination circuit 304a or 304b and a signal output from the circuit 304a or 304b to the counter 305a or 305b. In FIG. 7, the upper two signals correspond to the input signal and the output signal of the zero cross determination circuit 304b, and the lower two signals correspond to the input signal and the output signal of the zero cross determination circuit 304a. The counters 305a and 305b respectively count up at the rising edge of the signal input from the zero cross determination circuit 304a. When corresponding to the start point and end point of each measurement section, the count values of the counters 305a and 305b are stored in the memories 306a and 306b, respectively. While the moving stage 62 moves, the counters 305a and 305b are counted up.

  The calculation means 102 calculates the absolute refractive index of the sample for each measurement section based on the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum. An arithmetic circuit 307 including a CPU (Central Processing Unit) and a memory can be used for this calculation function. The arithmetic circuit 307 reads the count value corresponding to each measurement section from the memories 306a and 306b, and divides the value read from the memory 306b by the value read from the memory 306a, thereby obtaining the interference order for the sample and the interference order for the vacuum. A ratio is calculated for each measurement interval. This calculation gives the absolute refractive index of the sample for each measurement interval.

  The determination unit 103 determines the absolute refractive index of the sample based on the absolute refractive index for each measurement section. The arithmetic circuit 307 can also be used for this determination function. The arithmetic circuit 307 determines the absolute refractive index of the sample by averaging the absolute refractive index calculated for each measurement period.

  When it is necessary to determine the absolute refractive index for a plurality of wavelengths as described above, the determining unit 103 determines the absolute refractive index of the sample for the measurement light of each wavelength and designates it based on the determined refractive index. You may make it determine the absolute refractive index of the sample with respect to the light of a wavelength. In this case, when the absolute refractive index of the sample is determined for each wavelength, the arithmetic circuit 307 determines the coefficient of the chromatic dispersion correction equation from those values. Then, from the obtained chromatic dispersion correction formula, the absolute refractive index of the sample with respect to the light of the specified wavelength, such as the wavelength corresponding to the sodium D line, is determined.

  In order to cause the arithmetic circuit 307 to perform the calculation and determination functions as described above, a signal processing program corresponding to the function is read into the memory of the arithmetic circuit 307. The CPU of the arithmetic circuit 307 executes procedures for calculation and determination in accordance with instructions of the signal processing program.

  By the way, in order to ensure sufficient accuracy due to the limitation of the moving range of the moving mirror, not only the integer value obtained from the counter value but also the fractional part of the phase change amount expressed by the period of the interference wave is required. May be. As shown in FIG. 8, the fractional part of the phase change amount expressed in units of the interference wave period corresponds to the phase near the start point of the measurement section, the phase near the end point, or both. By adding the fractional part to the integer part to determine the phase change amount of the interference wave, the accuracy of the absolute refractive index can be improved. It is not always necessary to use a fractional part for both the sample and the vacuum. If the measurement interval is set to a multiple of the integer part of the phase change amount expressed by the period of the interference wave for one of the sample and the vacuum, the interference to the other Only the fractional part of the phase change amount expressed in units of wave cycles may be used. For example, if the measurement interval is set to a multiple of an integer part of the phase change amount expressed in units of the interference wave relative to the vacuum, only the fractional part of the phase change amount expressed in units of the interference wave period relative to the sample is the absolute refractive index. Used to calculate

  What is necessary is just to make the calculation means 102 obtain | require the fractional part of the phase change amount represented by the period unit of the interference wave. For this purpose, the acquisition unit 101 acquires the count value of the number of waves for the interference waveform for the sample and the interference waveform for the vacuum, and acquires the amplitude value for at least one of the interference waveform of the sample and the vacuum. And the calculation means 102 calculates | requires the fraction part of the phase change amount represented by the period unit of the interference wave based on the acquired count value and amplitude value.

  If the arithmetic circuit 307 is also used for this function, as shown in FIG. 6, a hold circuit 308 and an A / D converter 309 are further provided in the signal processing circuit. Here, the hold circuit 308 holds the analog amplitude value output from the gain controller 303a when counting up the number of periods of interference waves with respect to the vacuum. The hold circuit 308 holds the analog amplitude value output from the gain controller 303a even after a predetermined time since the number of waves of the interference waveform with respect to the vacuum is counted up. The predetermined time is a time equal to or less than a quarter of the count-up cycle. The A / D converter 309 converts the held analog value into a digital value and outputs the digital value to the arithmetic circuit 307. Such an amplitude value may be acquired only when corresponding to the start point and end point of each measurement section.

  The arithmetic circuit 307 determines the phase of the interference waveform based on the amplitude value data held when a predetermined time has elapsed since the number of cycles of the interference wave with respect to the vacuum was counted up. For example, if the amplitude value when the number of periods of the interference wave with respect to the vacuum is counted up is positive and the value after a predetermined time is larger than that value, the phase is a value from 0 to a quarter of the wavelength. Determine that there is. If the value after the elapse of a predetermined time is smaller than the amplitude value when the number of cycles of the interference wave with respect to the vacuum is counted up, it is determined that the phase is a value from a quarter to a half of the wavelength.

  Then, the arithmetic circuit 307 obtains a fractional part of the interference order based on the amplitude value of the interference waveform with respect to the sample and the determined phase acquired when counting up the number of interference waves with respect to the vacuum. The value of the fractional part is obtained by substituting the amplitude value when the number of periods of the interference wave with respect to the vacuum is counted up into the inverse sine function, and the value is selected based on the determined phase.

  In addition to the control of the signal processing unit 100, the control unit 200 controls the movement of the moving mirror and the switching of the wavelength. The driving unit 201 drives the moving mirror. Here, the driving means 201 is a motor that drives the moving stage 62.

  As the control means 200, an arithmetic circuit 310 having a CPU and a memory can be used. In order to cause the arithmetic circuit 310 to perform a control function, a control program corresponding to the function is read into the memory of the arithmetic circuit 310. The CPU of the arithmetic circuit 310 executes a procedure corresponding to the refractive index measurement method of the present invention in accordance with an instruction of the control program. FIG. 9 is a flowchart for explaining an example of an operation procedure of the refractive index measuring apparatus.

  When measuring the absolute refractive index of the sample for each wavelength of light emitted from the multi-wavelength light source 1, the CPU of the arithmetic circuit 310 first operates the motor at a low speed (S1). As shown in FIG. 10, when the moving speed of the moving stage 62 rises to the speed V1 and stabilizes (S2), measurement starts (S3), and the counters 305a and 305b start counting (S4). When the counter 305b counts up, the hold circuit 308 holds the analog amplitude value and stores the count values of the counters 305a and 305b in the memories 306a and 306b, respectively. (S5). Further, also when a predetermined time elapses after the counter 305b counts up, the hold circuit 308 holds the analog amplitude value (S6). Here, steps S5 and S6 are repeated until the count value of the counter 305b changes from 0 to 9 (S7). FIG. 7 shows a signal near the start of measurement. White circles in FIG. 7 indicate sampled analog amplitude values. Until the count value of the counter 305b changes from 0 to 9, the analog amplitude value is sampled every time the counter 305b counts up. The analog amplitude value is also sampled every time a predetermined time elapses after the counter 305b counts up.

  When the sampling of a total of 20 analog amplitude values is completed in this way, the CPU of the arithmetic circuit 310 increases the motor speed while the counting is continued by the counters 305a and 305b (S8). When the moving speed of the moving stage 62 increases to a speed V2 larger than the speed V1, and then stabilizes, the state is maintained for a predetermined time (S9). When the designated time elapses, the CPU of the arithmetic circuit 310 decreases the motor speed (S10). When the moving speed of the moving stage 62 decreases to the speed V1 and stabilizes at that speed (S11), the hold circuit 308 holds the analog amplitude value when the counter 305b counts up, just like the start of measurement, and the counters 305a and 305b. Are stored in the memories 306a and 306b, respectively. (S12). Further, also when a predetermined time elapses after the counter 305b counts up, the hold circuit 308 holds the analog amplitude value (S13). Here, after the moving speed of the moving stage 62 is stabilized at the speed V1, the steps S12 and S13 are repeated while the count value of the counter 305b increases by 9 (S14). When the count value increases by 9, the counting by the counters 305a and 305b is stopped (S15), and the measurement at the target wavelength is terminated (S16). FIG. 11 shows a signal near the end of measurement. As shown in FIG. 11, 20 analog amplitude values are sampled near the end of measurement similarly to the vicinity of the start of measurement.

  Of the 20 points near the start of measurement and the 20 points near the end of measurement, 10 points near the start of measurement when the counter 305b counts up and 10 points near the end of measurement are in the measurement period corresponding to 10 measurement intervals. Corresponds to the start and end points. The value counted in each measurement period corresponds to the integer part of the amount of phase change expressed in units of the interference wave period. When the measurement is finished, the CPU of the arithmetic circuit 310 stops the motor (S17) and causes the signal processing circuit to calculate the absolute refractive index.

The CPU of the arithmetic processing circuit 307 calculates the fractional part of the phase change amount expressed in units of the interference wave period based on the amplitude value data corresponding to the start point and end point of each measurement period (S18). The amplitude values at 10 points SA0, SA1,..., SA9 near the start of the measurement period when the counter 305b counts up and at 10 points SA10, SA11,..., SA19 near the end are values X (SA0), X (SA1). ),..., X (SA9) and values X (SA10), X (SA11),..., X (SA19), for example, the fractional part F (SA0) at the point SA0 is
F (SA0) = Sin ⁻¹ (X (SA0) / A)
Given in. However, A is a constant. For selection of the value F (SA0) obtained by this equation, the amplitude value X (SB0) held at the point SB0 when a predetermined time has elapsed from the point SA0 is used. The fractional part is similarly calculated for the other points.

When calculating the fractional part of the phase change amount expressed by the period of the interference wave, the CPU of the arithmetic circuit 307 reads out necessary data from the memory and calculates the absolute refractive index for each measurement period. For the measurement period T1 starting from the point SA0 and ending at the point SA10, the absolute refractive index N (T1) is
N (T1) = (CA (SA10) −CA (SA0) + (1-F (SA0)) + F (SA10)) / (CB (SA10) −CB (SA0))
Can be expressed as However, CA is the count value of the counter 305a at the start point or end point, and CB is the count value of the counter 305b at the start point or end point. The absolute refractive index is similarly calculated for the other measurement periods T2, ..., T10.

  When the absolute refractive index is calculated for each measurement period, the CPU of the arithmetic circuit 307 calculates the absolute refractive index of the sample at the measurement wavelength by averaging the absolute refractive indexes (S19).

  By performing such measurement for each wavelength and calculating the absolute refractive index at each wavelength, the coefficient of the chromatic dispersion correction formula can be obtained, and based on this, the absolute refraction of the sample at the wavelength corresponding to the sodium D-line Rate can be obtained.

  This embodiment does not limit the technical scope of the present invention, and various modifications and applications can be made within the scope other than those already described. For example, instead of setting the measurement interval to a multiple of the integer part of the phase change amount expressed in units of the interference wave to the vacuum, the measurement interval is set to a multiple of the integer part of the phase change amount expressed in units of the interference wave to the liquid sample. May be set. In this case, the absolute refractive index is calculated by using the fractional part of the phase change amount expressed in units of the period of the interference wave with respect to the vacuum. Further, the measurement section may not be a multiple of the integer part of the phase change amount expressed in units of the period of the interference wave of either the sample or the vacuum. In this case, the fractional part of the phase change amount expressed in units of the period of the interference wave for both the sample and the vacuum is used for the calculation of the absolute refractive index.

  Further, the present invention can be applied to other than the above interferometer. For example, in the refractive index measurement apparatus according to the embodiment, the stage for reflecting the measurement light incident on each of the liquid sample and the vacuum is moved, but this stage is fixed, and the interferometer in which the stage for reflecting the reference light moves. You may make it apply this invention to. In addition, the present invention can be applied to an interferometer or other interferometer in which measurement light for obtaining an interference wave for a sample passes not only the sample region but also the vacuum region.

  Since the refractive index measuring device according to the present invention uses the absolute refractive index for a plurality of measurement sections obtained by one movement of the movable mirror, the absolute refractive index of the liquid sample can be measured with high accuracy. It is useful for obtaining the absolute refractive index of a density standard solution at a wavelength corresponding to the sodium D line.

The schematic perspective view which shows the structure of the optical system of the refractive index measuring apparatus in embodiment of this invention. Schematic diagram explaining the optical path in the optical system Diagram for explaining the effect of chromatic dispersion The block diagram which shows the function structure of the refractive index measuring apparatus in embodiment Diagram showing an example of the relationship between the set length and measurement interval The figure which shows the structure of an example of a signal processing circuit Diagram showing the relationship between the interference light detection signal near the start of the measurement period and the zero-cross discrimination signal Illustration for explaining the fractional part of the interference order Flowchart for explaining an example of the operation procedure of the refractive index measuring apparatus The figure which shows the relationship of the speed of the motor under measurement, the detection signal of interference light, and the zero cross discrimination signal Diagram showing the relationship between the interference light detection signal near the end of the measurement period and the zero-cross discrimination signal

Explanation of symbols

8a, 8b, 8c Light receiving sensor 62 Moving stage 100 Signal processing means 101 Acquisition means 102 Calculation means 103 Determination means 200 Control means 201 Driving means 301a, 301b I / V converters 302a, 302b Amplifiers 303a, 303b Gain controllers 304a, 304b Zero cross Discriminating circuit 305a, 305b Counter 306a, 306b Memory 307 Arithmetic circuit 308 Hold circuit 309 A / D converter 310 Arithmetic circuit

Claims (9)

A refractive index measurement method for measuring an absolute refractive index of a sample using a Michelson interferometer including a light source that emits measurement light incident on each of a liquid sample and a vacuum, and a moving mirror,
A procedure for acquiring the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum for each measurement interval when the movable mirror moves by a set length that is arranged by shifting a plurality of measurement intervals,
A refractive index measurement method comprising: calculating an absolute refractive index of a sample for each measurement section based on a phase change amount of an interference wave with respect to a sample and a phase change amount of an interference wave with respect to a vacuum. The procedure for acquiring the phase change amount of the interference wave is to acquire the count value of the number of periods of the interference wave for the interference waveform for the sample and the interference waveform for the vacuum, and to acquire the amplitude value for at least one of the interference waveform of the sample and the vacuum. Including steps to
The procedure for calculating the absolute refractive index of the sample for each measurement section includes a procedure for obtaining an integer part and a fraction part of the phase change amount expressed in units of the interference wave period based on the acquired count value and amplitude value. The refractive index measuring method as described.   The refractive index measurement method according to claim 2, wherein the amplitude value of the interference waveform of the sample is acquired when the phase change of the interference wave with respect to the vacuum is an integral multiple of the period in the procedure of acquiring the count value and the amplitude value.   The procedure for obtaining the fractional part of the phase change amount expressed in units of the period of the interference wave is to obtain the amplitude value of the interference waveform with respect to the sample when counting up the number of periods of the interference wave with respect to the vacuum, and then to the four minutes of the count-up period The procedure for discriminating the phase of the interference waveform based on the amplitude value acquired until the time of 1 or less elapses, and the amplitude value of the interference waveform for the sample acquired when counting up the number of cycles of the interference wave with respect to the vacuum The refractive index measurement method according to claim 3, further comprising a procedure for obtaining a fractional part based on the method.   The refractive index measurement method according to claim 2, wherein the amplitude value is acquired only when corresponding to a start point and an end point of each measurement section.   6. The refractive index measurement method according to claim 5, wherein control is performed to increase the moving speed of a section in which no amplitude value is acquired, compared to the moving speed of a movable mirror in the section in which the amplitude value is acquired.   The refractive index measurement method according to claim 1, wherein the absolute refractive index of the sample is determined by averaging the absolute refractive index for each measurement section. Michelson interferometer equipped with a light source that emits measurement light incident on each of the liquid sample and the vacuum and a moving mirror;
Means for acquiring, for each measurement section, the phase change amount of the interference wave with respect to the sample and the phase change amount of the interference wave with respect to the vacuum when the movable mirror moves by a set length that is arranged by shifting a plurality of measurement sections;
A refractive index measurement device comprising: means for calculating an absolute refractive index of a sample for each measurement section based on a phase change amount of an interference wave with respect to a sample and a phase change amount of an interference wave with respect to a vacuum. The light source is a light source that emits light of a plurality of wavelengths,
9. The refractive index measuring apparatus according to claim 8, wherein the absolute refractive index of the sample is determined for the measurement light of each wavelength, and the absolute refractive index of the sample with respect to the light of the specified wavelength is determined based on the determined refractive index.

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