Interference type refractometer and refractive index measuring method

Abstract

This interference type refractometer comprises a light source (2), a light detector (4) which detects the intensity of received light, an optical system (6; 8; 10; 12) which causes the optical path of light emitted from the light source (2) to branch into a first optical path and a second optical path, then causes the first optical path and the second optical path to merge, and guides interference light of the light from the first optical path and the light from the second optical path to the light detector, a phase modulating unit (18; 20) which is disposed on the first optical path and / or the second optical path and which electrically modulates a phase difference between the light on the first optical path and the light on the second optical path, a control unit (22) which is configured to cause a drive voltage of the phase modulating unit (18; 20) to change over time with a prescribed amplitude and periodicity, to cause the intensity of the interference light detected by the light detector (4, 4') to change over time, a storage unit (24) which stores the intensity of the interference light detected by the light detector (4, 4'), and a calculating unit (26) which performs calculation processing using the intensity of the interference light stored in the storage unit (24), wherein: the control unit (22) is configured to cause the storage unit (24) to store the change over time in the intensity of the interference light in a reference state in which no sample is present on the first optical path and the change over time in the intensity of the interference light in a measurement state in which a sample is present on the first optical path, in association with the change over time in the drive voltage of the phase modulating unit (18; 20); and the calculating unit (26) is configured to obtain an amount of change in the phase of the interference light due to the sample, using a relationship between the change over time in the intensity of the interference light and the change over time in the drive voltage, stored by the storage unit (24), in the reference state, and a relationship between the change over time in the intensity of the interference light and the change over time in the drive voltage in the measurement state, and to obtain the refractive index of the sample on the basis of the amount of change in the phase.

Description

Interference refractometer and refractive index measuring method

[0001] The present invention relates to an interference-type refractometer that can be used as a detector for an analytical device such as a liquid chromatograph, and a method for measuring a refractive index.

[0002] A differential refractometer is known as a detector for liquid chromatography (see, for example, Patent Document 1). A differential refractometer is a device that has a sample cell, through which a sample solution flows, and a reference cell, filled with a reference solution, separated by a transparent partition, and that forms a slit image of light transmitted through the sample cell and the reference cell on the light-receiving surface of a photodetector, and that detects the amount of change in the refractive index of the sample solution flowing through the sample cell by detecting the amount of displacement of the slit image on the light-receiving surface.

[0003] Furthermore, an interferometric refractometer is also known as a detector that utilizes changes in the refractive index of light. Patent Document 2 proposes an interferometric refractometer that uses a Mach-Zehnder interferometer, in which light from a monochromatic light source is split into two optical paths, a sample cell is placed on one optical path, and a compensation cell is placed on the other optical path, and the light that has passed through both optical paths is merged to obtain interference light, which is then focused on the light-receiving surface of a photodetector to detect the intensity of the interference light that changes in accordance with changes in the optical path length of the light that has passed through the sample cell, thereby detecting the refractive index of a sample solution flowing in the sample cell.

[0004] Japanese Patent Application Publication No. 2017-223507

[0005] To improve the refractive index detection sensitivity of a differential refractometer that detects the displacement of a slit image of light, a large displacement of the slit image is required in response to a small change in the refractive index of the sample solution in the sample cell. This requires increasing the optical path length from the sample cell and reference cell to the photodetector and enlarging the light-receiving surface of the photodetector. This requires increasing the overall size of the detector. However, the light source's emission intensity is temperature-dependent. Fluctuations in the temperature inside the instrument during measurement can alter the light source's emission intensity, which in turn alters the intensity of the light focused on the photodetector, potentially leading to false detection of a change in the slit image and thus a change in the refractive index of the sample solution. Therefore, it is necessary to control the temperature of the entire optical system, including the light source and photodetector, to eliminate factors that could lead to false detection of a change in the refractive index of the sample. However, improving the detection sensitivity of a differential refractometer requires increasing the overall size of the instrument, which makes it difficult to control the temperature of the entire optical system. As a result, improving detection sensitivity can result in a decrease in measurement accuracy.

[0006] In addition, in the interference refractometer proposed in Patent Document 2, a phase modulator is placed on one of the branched optical paths, and the phase modulator is operated so that the ratio of the intensity of the interference light to the intensity of the reference light is constant. In order to achieve the high measurement accuracy required of a chromatography detector with this type of interference refractometer, it is necessary to detect slight changes in the ratio of the intensity of the interference light to the intensity of the reference light. However, in this type of method, the intensity of the interference light and the intensity of the reference light are measured while alternately introducing the interference light and the reference light into the detector using an optical modulator. Therefore, fluctuations in the emission intensity of the light source between the time of measuring the interference light intensity and the time of measuring the reference light intensity, and fluctuations in the sensitivity of the detector, result in measurement errors, making it difficult to detect the refractive index of a sample solution with high accuracy and high sensitivity.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an interference refractometer and a refractive index measuring method that are capable of detecting the refractive index of a sample solution with high accuracy and high sensitivity.

[0008] The interferometric refractometer according to the present invention includes a light source, a photodetector that detects the intensity of received light, an optical system that branches the optical path of light emitted from the light source into a first optical path and a second optical path, then merges the first optical path and the second optical path, and guides interference light of the light of the first optical path and the light of the second optical path to the photodetector, a phase modulation unit that is disposed on the first optical path and / or the second optical path and that electrically modulates the phase difference between the light of the first optical path and the light of the second optical path, a control unit that is configured to change the drive voltage of the phase modulation unit with a predetermined amplitude and periodicity over time, thereby changing the intensity of the interference light detected by the photodetector over time, a memory unit that stores the intensity of the interference light detected by the photodetector, and a signal stored in the memory unit. the control unit is configured to store in the memory unit a time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path and a time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path, in association with a time change in the drive voltage of the phase modulation unit; the calculation unit is configured to determine an amount of phase change of the interference light caused by the sample using the relationship between the time change in the intensity of the interference light and the time change in the drive voltage in the reference state and the relationship between the time change in the intensity of the interference light and the time change in the drive voltage in the measurement state, which are stored in the memory unit, and to determine the refractive index of the sample based on the amount of phase change.

[0009] A refractive index measurement method according to the present invention is a method for measuring the refractive index of a sample using an interference refractometer comprising a light source, a photodetector that detects the intensity of received light, and an optical system that branches the optical path of light emitted from the light source into a first optical path and a second optical path, then merges the first optical path and the second optical path, and guides interference light of the light of the first optical path and the light of the second optical path to the photodetector, comprising the steps of: changing, with time, a drive voltage of a phase modulation unit that electrically modulates the phase difference between the light of the first optical path and the light of the second optical path, with a predetermined amplitude and periodicity, thereby changing the intensity of the interference light detected by the photodetector with time; recording the time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path in correspondence with the time change in the drive voltage; and recording the time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path in correspondence with the time change in the drive voltage. The method includes a step of determining the amount of phase change of the interference light caused by the sample using the relationship between the time change in intensity of the interference light and the time change in the drive voltage in the reference state, and the relationship between the time change in intensity of the interference light and the time change in the drive voltage in the measurement state, and a step of determining the refractive index of the sample based on the amount of phase change.

[0010] According to the interferometric refractometer of the present invention, the phase difference between the light in the first optical path and the light in the second optical path is time-modulated by changing the drive voltage of a phase modulation unit arranged on the first optical path and / or the second optical path with a predetermined amplitude and periodicity, and the time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path and the time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path are recorded in association with the time change in the drive voltage of the phase modulation unit, and the amount of phase change of the interference light caused by the sample is determined using the relationship between the time change in the intensity of the interference light in the reference state and the time change in the drive voltage, and the relationship between the time change in the intensity of the interference light in the measurement state and the time change in the drive voltage, and the refractive index of the sample is determined based on the amount of phase change.Therefore, the interferometric refractometer is less susceptible to fluctuations in the emission intensity of the light source than when the difference in intensity of the interference light between the reference state and the measurement state is determined, and the refractive index of the sample can be detected with high sensitivity and high accuracy.

[0011] According to the refractive index measurement method of the present invention, the drive voltage of the phase modulation unit is changed over time with a predetermined amplitude and periodicity to change the intensity of the interference light detected by the photodetector over time, and the time change in the intensity of the interference light in the reference state and the time change in the intensity of the interference light in the measurement state are recorded in correspondence with the time change in the drive voltage, and the amount of phase change of the interference light caused by the sample is determined using the relationship between the time change in the intensity of the interference light in the reference state and the time change in the drive voltage, and the relationship between the time change in the intensity of the interference light in the measurement state and the time change in the drive voltage, and the refractive index of the sample is determined based on the amount of phase change.Therefore, compared to when the difference in intensity of the interference light between the reference state and the measurement state is determined, the refractive index of the sample can be detected with high sensitivity and high accuracy, without being affected by fluctuations in the emission intensity of the light source.

[0012] Fig. 1 is a schematic diagram showing the configuration of an embodiment of an interference refractometer. Fig. 2 is a graph for explaining the difference in the relationship between the phase of the drive voltage and the intensity of interference light between a reference state and a measurement state. Fig. 3 is a diagram showing an example of the change over time of the signal intensity obtained when the drive function of the phase modulation unit in the interference refractometer of the embodiment is set to ΔV=ct. Fig. 4 is a diagram showing an example of the change over time of the signal intensity obtained when the drive function of the phase modulation unit in the interference refractometer of the embodiment is set to ΔV=sinωt. Fig. 5 is a flowchart showing an example of the procedure for measuring the change in the refractive index of a sample using the interference refractometer of the embodiment. Fig. 6 is a schematic diagram showing another embodiment of the interference refractometer.

[0013] Hereinafter, an embodiment of an interference refractometer and a refractive index measuring method of the present invention will be described with reference to the drawings.

[0014] As shown in FIG. 1, the interference refractometer of one embodiment includes a light source 2, a photodetector 4, half mirrors 6 and 8, mirrors 10 and 12, a sample cell 14, a reference cell 16, an electro-optic crystal 18, a driving circuit 20, a control unit 22, a memory unit 24, and a calculation unit 26.

[0015] The light source 2 is a monochromatic light source such as a laser light source that emits light of a predetermined wavelength (e.g., 850 nm). The photodetector 4 has a single light-receiving element and outputs a signal corresponding to the intensity of light incident on the light-receiving surface of the light-receiving element. The light-receiving element of the photodetector 4 is, for example, a photodiode.

[0016] The half mirror 6 is provided at an angle with respect to the optical axis direction of the light emitted from the light source 2, and reflects a portion of the light from the light source 2 and transmits the remaining light, thereby branching the optical path of the light from the light source 2 into two optical paths. The mirror 10 is provided to reflect the light reflected by the half mirror 6 toward the photodetector 4. The half mirror 8 is provided on the optical path of the light reflected by the mirror 10 so as to transmit the light reflected by the mirror 10 and guide it to the photodetector 4. The mirror 12 is provided to reflect the light transmitted through the half mirror 6 toward the half mirror 8. The half mirror 8 is provided to reflect the light reflected by the mirror 12 toward the photodetector 4.

[0017] Here, the optical path that is reflected by the half mirror 6, passes through the mirror 10, and reaches the half mirror 8 is referred to as the first optical path, and the optical path that is transmitted through the half mirror 6, passes through the mirror 12, and reaches the half mirror 8 is referred to as the second optical path. The positional relationship between the half mirror 8, the mirrors 10, and 12 is adjusted so that the optical axis of the light that passes through the first optical path and passes through the half mirror 8 coincides with the optical axis of the light that passes through the second optical path and is reflected by the half mirror 8. As a result, the light that has passed through the first optical path and the light that has passed through the second optical path are multiplexed by the half mirror 8 and enter the photodetector 4 as interference light. In other words, the half mirrors 6, 8, and the mirrors 10, 12 constitute an optical system that splits the optical path of light from the light source 2 into two optical paths, the first optical path and the second optical path, and then merges the first optical path and the second optical path to create interference light that is guided to the photodetector 4.

[0018] The sample cell 14 is a transparent container for allowing a sample solution to flow therein. The sample solution is, for example, an eluate from a separation column of a liquid chromatograph. The sample cell 14 is provided at a position on the first optical path between the mirror 10 and the half mirror 8. Note that the position of the sample cell 14 is not limited to a position between the mirror 10 and the half mirror 8, as long as it is on the first optical path.

[0019] The reference cell 16 is a transparent container for allowing a reference liquid (e.g., a mobile phase in liquid chromatography) to flow therein or for being filled therewith. The reference cell 16 is provided at a position on the second optical path between the mirror 12 and the half mirror 8. Note that the position of the reference cell 16 is not limited to a position between the mirror 12 and the half mirror 8, as long as it is on the second optical path.

[0020] The electro-optic crystal 18 is disposed between the half mirror 6 and the mirror 10 on the first optical path. The electro-optic crystal 18 changes the refractive index of light depending on the strength of the electric field generated inside. For example, the electro-optic crystal 18 is made of lithium niobate (LiNbO ３ ) crystals, etc. can be used.

[0021] Drive circuit 20 is a circuit for applying a variable voltage to electro-optic crystal 18 to form electric fields of various intensities within electro-optic crystal 18. When the intensity of the electric field formed within electro-optic crystal 18 changes, the phase of light passing through electro-optic crystal 18 is modulated. As a result, the phase difference between the phase of light that has passed through the first optical path and the phase of light that has passed through the second optical path changes, and the intensity of the interference light that enters photodetector 4 changes. In other words, electro-optic crystal 18 and drive circuit 20 constitute a phase modulation unit for modulating the phase difference between the phase of light that has passed through the first optical path and the phase of light that has passed through the second optical path.

[0022] The drive voltage of drive circuit 20 is controlled by control unit 22. Control unit 22 is configured to change the drive voltage of drive circuit 20 over time with a predetermined amplitude and periodicity, thereby changing the strength of the electric field formed within electro-optic crystal 18 over time. The drive voltage of drive circuit 20 can be controlled using a periodic function so that the waveform showing the time change depicts various waveforms such as a sine wave, ramp wave, or square wave. The function of control unit 22 is realized by running dedicated software on a computer circuit equipped with a CPU (central processing unit) or the like.

[0023] When the strength of the electric field formed within electro-optic crystal 18 varies over time with a specific amplitude and periodicity, the phase of the light passing through electro-optic crystal 18 is also modulated over time with a specific amplitude and periodicity. As a result, the phase difference between the phase of the light that has passed through the first optical path and the phase of the light that has passed through the second optical path varies over time with a specific amplitude and periodicity, and the intensity of the interference light incident on photodetector 4 varies over time with a specific amplitude and periodicity.

[0024] The memory unit 24 is a storage area for storing the intensity of the interference light detected by the photodetector 4 at very short time intervals, for example, 1 kHz to 10 kHz, thereby correlating and recording the time change in the intensity of the interference light with the time change in the drive voltage of the drive circuit 20. The memory unit 24 is realized by a partial storage area of ​​an information storage device such as a hard disk drive or flash memory.

[0025] The control unit 22 associates the time change in the intensity of interference light detected by the photodetector 4 in a state where no sample is flowing in the sample cell 14, i.e., where only the mobile phase is flowing through the sample cell 14 (hereinafter referred to as the reference state), and the time change in the intensity of interference light detected by the photodetector 4 in a measurement state where the sample is flowing through the sample cell 14, with the time change in the drive voltage of the drive circuit 20, and stores these in the memory unit 24. In both the reference state and the measurement state, the reference cell 16 is filled with or filled with a reference liquid (mobile phase).

[0026] In the measurement state where a sample is flowing through the sample cell 14, the phase of the light that has passed through the first optical path changes compared to the reference state where only the mobile phase is flowing through the sample cell 14 due to a change in the refractive index of the sample flowing through the flow cell 14. As a result, between the reference state and the measurement state, the relationship between the intensity of the interference light detected by the photodetector 4 and the drive voltage of the drive circuit 20 changes depending on the amount of phase change caused by the sample.

[0027] As shown in Fig. 2, when the drive voltage of the drive circuit 20 is changed over time with a constant amplitude and periodicity, the intensity of the interference light detected by the photodetector 4 changes over time like a sine wave. When the sample flows through the sample cell 14 and the measurement state is reached, the relationship between the phase of the drive voltage and the phase of the interference light intensity changes by Δφ from the reference state, as shown by the dashed line in Fig. 2. This phase change Δφ is caused by a change in the refractive index of the sample, and the change in the refractive index of the sample can be determined by calculating the phase change Δφ.

[0028] The calculation unit 26 is configured to determine the change in refractive index of the sample using the relationship between the time change in interference light intensity in the reference state and the measurement state stored in the memory unit 24 and the drive voltage of the drive circuit 20.

[0029] The phase change amount Δφ of the light passing through the first optical path caused by the refractive index change Δn of the sample Ｓ is expressed as follows, where L is the optical path length of the light passing through the sample cell 14: Ｓ =(2π / λ)ΔnL (1)

[0030] The amplitude of the light in the first optical path is A１ , the amplitude of the light in the second optical path is A ２ , the phase of the light in the first optical path is φ １ , the phase of the light in the second optical path is φ ２ Then, the intensity I of the interference light between the light that has passed through the first optical path and the light that has passed through the second optical path can be expressed by the following theoretical formula: I=|E| ２ = A １ ２ +A ２ ２ +2A １ A ２ cos(φ １ -φ ２ ) (2)

[0031] That is, the interference light intensity I Ｒ Is, I Ｒ = A １Ｒ ２ +A ２Ｒ ２ +2A １Ｒ A ２Ｒ cos(φ １Ｒ -φ ２Ｒ ) (3) The interference light intensity I Ｓａｍ Is, I Ｓ = A １Ｓ ２ +A ２Ｓ ２ +2A １Ｓ A ２Ｓ cos(φ １Ｓ -φ ２Ｓ ) (4) Since the phase of the light passing through the second optical path does not change between the reference state and the measurement state, φ ２Ｒ =φ ２Ｓ is.

[0032] The calculation unit 26 calculates each coefficient A by fitting the data of the time change of the interference light intensity in the reference state to the above-mentioned theoretical formula (3). １Ｒ , A ２Ｒ and the phase φ when the interference light intensity is in a specific state (for example, a maximum value). １Ｒ The reference phase φ is １ＲSimilarly, the calculation unit 26 fits the data of the time change in the intensity of interference light in the measurement state to the above-mentioned theoretical formula (4) to obtain each coefficient A １Ｓ , A ２Ｓ and the phase φ when the interference light intensity is in a specific state. １Ｓ The reference phase φ is １Ｓ ' is calculated. The reference phase φ in the reference state １Ｒ ' and the reference phase φ in the measurement state １Ｓ The difference between Δφ′ and Δφ′ is due to the refractive index change Δn of the sample flowing through the sample cell 14. Ｓ =φ １Ｒ '-φ １Ｓ ' (5) The calculation unit 26 calculates Δφ obtained by the above formula (5). Ｓ By applying this to equation (1), the refractive index change Δn of the sample is calculated.

[0033] 3 shows the waveforms of the signal intensity (interference light intensity) of the photodetector 4 in the reference state and the measurement state when the drive voltage V of the drive circuit 20 is controlled using the periodic function ΔV=ct, and FIG. 4 shows the waveforms of the signal intensity (interference light intensity) of the photodetector 4 in the reference state and the measurement state when the drive voltage V of the drive circuit 20 is controlled using the periodic function ΔV=sinωt. As can be seen from these figures, whether the drive voltage of the drive circuit 20 is controlled using the periodic function ΔV=ct or ΔV=sinωt, the waveform of the signal intensity of the photodetector 4 can be made periodic. Furthermore, as can be seen from these figures, by adjusting the amplitude and period of the drive voltage, it is possible to make the relationship between the time change of the drive voltage and the time change of the signal intensity between the reference state and the measurement state sufficiently different to be detectable, and the phase difference Δφ arising from the refractive index of the sample Ｓ It is possible to detect with high sensitivity.

[0034] FIG. 5 is a flow chart showing an example of a method for measuring the refractive index of a sample using the interference refractometer of this embodiment.

[0035] First, the drive voltage of the drive circuit 20 is varied over time with a predetermined amplitude and periodicity (step 101). Next, the time change in the signal intensity (interference light intensity) of the photodetector 4 in a reference state where no sample is flowing in the sample cell 14 and the time change in the signal intensity (interference light intensity) of the photodetector 4 in a measurement state where a sample is flowing in the sample cell 14 are measured over at least one cycle of the drive voltage, and the results are stored in the memory unit 24 (steps 102 and 103).

[0036] Next, the calculation unit 26 fits the measurement data of the time change in the interference light intensity in the reference state and the measurement state stored in the measurement data storage unit 24 to the above-mentioned theoretical formulas (3) and (4), respectively, to obtain the reference phase φ １Ｒ ' and φ １Ｓ ' (step 104). Furthermore, the calculation unit 26 calculates the reference phase φ １Ｒ ' and φ １Ｓ The phase change amount Δφ of the light in the first optical path caused by the sample is calculated by calculating the difference between Ｓ (step 105), and the phase change amount Δφ Ｓ is applied to the above-mentioned formula (1) to calculate the refractive index change Δn of the sample (step 106). Note that this is merely an example, and it is also possible to reverse the order of the steps. For example, the order of steps 103 and 104 may be reversed. Also, some steps may be performed simultaneously in parallel.

[0037] Next, another embodiment of the interference refractometer will be described with reference to FIG.

[0038] The interference refractometer of this embodiment differs from the interference refractometer of FIG. 1 in that the photodetector 4, which is equipped with a single light-receiving element in the interference refractometer of FIG. 1, is changed to a photodetector 4' such as a photodiode array in which the light-receiving surfaces of a plurality of light-receiving elements are arranged in one direction, and the angle of the half mirror 8 is adjusted so that interference fringes of the interference light are formed in the direction of arrangement of the light-receiving surfaces of the light-receiving elements of the photodetector 4'.

[0039] Even with the configuration of this embodiment, the relationship between the time change in the intensity of interference light detected by all of the multiple light-receiving elements of the photodetector 4′ and the time change in the drive voltage of the drive circuit 20 changes between the reference state and the measurement state in the same way as the interference refractometer of FIG. 1, so the calculation unit 26 can determine the refractive index change Δn of the sample using the above-mentioned equations (3) to (5).

[0040] Furthermore, by recording the time-dependent changes in the detection signals of the multiple light-receiving elements of the photodetector 4', it is possible to observe the direction of movement of the interference fringes formed on the light-receiving surface of the photodetector 4', which makes it possible to detect how the refractive index of the liquid flowing through the sample cell 14 is changing.

[0041] In the above-described embodiments, the refractive index change Δn of the sample relative to the reference sample is determined. Determining this refractive index change Δn may also be referred to as determining the refractive index of the sample.

[0042] The above-described examples are merely examples of embodiments of the interference refractometer according to the present invention. The embodiments of the interference refractometer according to the present invention are as follows.

[0043] One embodiment of the interference refractometer according to the present invention comprises: a light source; a photodetector that detects the intensity of received light; an optical system that branches the optical path of light emitted from the light source into a first optical path and a second optical path, then merges the first optical path and the second optical path, and guides interference light of the light of the first optical path and the light of the second optical path to the photodetector; a phase modulation unit that is disposed on the first optical path and / or the second optical path and that electrically modulates the phase difference between the light of the first optical path and the light of the second optical path; a control unit that is configured to change a drive voltage of the phase modulation unit with a predetermined amplitude and periodicity over time, and to change the intensity of the interference light detected by the photodetector over time; a memory unit that stores the intensity of the interference light detected by the photodetector; and a calculation unit that performs calculation processing using the intensity of the interference light stored in the memory unit. The control unit is configured to store in the memory unit the change in intensity of the interference light over time in a reference state where no sample is present on the first optical path and the change in intensity of the interference light over time in a measurement state where a sample is present on the first optical path, in correspondence with the change in the drive voltage of the phase modulation unit over time; and the calculation unit is configured to determine the amount of phase change of the interference light caused by the sample using the relationship between the change in intensity of the interference light over time in the reference state and the change in the drive voltage over time, and the relationship between the change in intensity of the interference light over time in the measurement state and the change in the drive voltage over time, which are stored in the memory unit, and to determine the refractive index of the sample based on the amount of phase change.

[0044] In aspect [1] of the above-described embodiment of the interference refractometer, the calculation unit is configured to obtain reference phases for the reference state and the measurement state by fitting the intensities of the interference light in the reference state and the measurement state at a specific drive voltage to a theoretical formula for the intensity of the interference light, the formula having a phase difference between the light in the first optical path and the light in the second optical path as a variable, and to obtain the amount of phase change by taking the difference between these reference phases.

[0045] A specific example of the above aspect [1] is that the theoretical formula is: I=A １ ２ +A ２ ２+2A １ A ２ cos(φ １ -φ ２ ) where I is the intensity of the interference light, A1 is the amplitude of the light in the first optical path, A2 is the amplitude of the light in the second optical path, and φ １ is the phase of the light in the first optical path, φ ２ is the phase of the light in the second optical path.

[0046] In the above specific example, the calculation unit calculates φ when the time change in the intensity of the interference light in the reference state is in a specific state. １ is the reference phase of the reference state, and φ when the time change in the intensity of the interference light in the measurement state is in the specific state. １ may be determined as the reference phase of the measurement state, and the difference between the reference phase of the reference state and the reference phase of the measurement state may be determined as the phase change amount.

[0047] In addition, in aspect [2] of the above embodiment of the interference refractometer, the phase modulation section includes an electro-optic crystal. This aspect [2] can be combined with the above aspect [1].

[0048] In addition, in aspect [3] of the above embodiment of the interference refractometer, the phase modulation unit is configured to time-modulate only the phase of the light in the first optical path. This aspect [3] can be combined with the above aspects [1] and / or [2].

[0049] In addition, in aspect [4] of the above embodiment of the interference refractometer, the photodetector has a plurality of light-receiving elements whose light-receiving surfaces are arranged in one direction, and the optical system is configured to guide the interference light to the photodetector so that interference fringes are formed in the arrangement direction of the light-receiving surfaces of the light-receiving elements of the photodetector. This aspect [4] can be combined with the above aspects [1], [2], and / or [3].

[0050] One embodiment of the refractive index measurement method according to the present invention is a method for measuring the refractive index of a sample using an interference refractometer including a light source, a photodetector that detects the intensity of received light, and an optical system that branches the optical path of light emitted from the light source into a first optical path and a second optical path, then merges the first optical path and the second optical path, and guides interference light of the light of the first optical path and the light of the second optical path to the photodetector, the method comprising the steps of: changing, with time, a drive voltage of a phase modulation unit that electrically modulates the phase difference between the light of the first optical path and the light of the second optical path, with a predetermined amplitude and periodicity, thereby changing the intensity of the interference light detected by the photodetector with time; recording the time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path in correspondence with the time change in the drive voltage; and recording the time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path in correspondence with the time change in the drive voltage. The method includes a step of determining the amount of phase change of the interference light caused by the sample using the relationship between the time change in intensity of the interference light and the time change in the drive voltage in the reference state, and the relationship between the time change in intensity of the interference light and the time change in the drive voltage in the measurement state, and a step of determining the refractive index of the sample based on the amount of phase change.

[0051] In aspect [1] of the above embodiment of the refractive index measurement method, in the step of calculating the amount of phase change, the intensities of the interference light in the reference state and the measurement state at a specific driving voltage are fitted to a theoretical formula for the intensity of the interference light, which formula has as a variable the phase difference between the light in the first optical path and the light in the second optical path, to calculate reference phases for the reference state and the measurement state, and the amount of phase change is calculated by taking the difference between these reference phases.

[0052] A specific example of the above aspect [1] is that the theoretical formula is: I=A １ ２ +A ２ ２ +2A １ A ２ cos(φ １ -φ ２) where I is the intensity of the interference light, A1 is the amplitude of the light in the first optical path, A2 is the amplitude of the light in the second optical path, and φ １ is the phase of the light in the first optical path, φ ２ is the phase of the light in the second optical path.

[0053] In the above specific example, in the step of calculating the amount of phase change, φ when the time change in the intensity of the interference light in the reference state is in a specific state １ is the reference phase of the reference state, and φ when the time change in the intensity of the interference light in the measurement state is in the specific state. １ may be calculated as the reference phase of the measurement state, and the difference between the reference phase of the reference state and the reference phase of the measurement state may be used as the phase change amount.

[0054] 2 Light source 4, 4' Photodetector 6, 8 Half mirror 10, 12 Mirror 14 Sample cell 16 Reference cell 18 Electro-optic crystal 20 Drive circuit 22 Control unit 24 Memory unit 26 Calculation unit

Claims

1. A light source; a photodetector that detects the intensity of received light; an optical system that branches the optical path of light emitted from the light source into a first optical path and a second optical path, then merges the first optical path and the second optical path, and guides interference light of the light of the first optical path and the light of the second optical path to the photodetector; a phase modulation unit that is disposed on the first optical path and / or the second optical path, and electrically modulates the phase difference between the light of the first optical path and the light of the second optical path; a control unit that is configured to change the drive voltage of the phase modulation unit with a predetermined amplitude and periodicity over time, and to change the intensity of the interference light detected by the photodetector over time; a memory unit that stores the intensity of the interference light detected by the photodetector; and a calculation unit that performs calculation processing using the intensity of the interference light stored in the memory unit, an interference refractometer, wherein the control unit is configured to store in the memory unit a time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path and a time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path in correspondence with a time change in the drive voltage of the phase modulation unit; and the calculation unit is configured to determine an amount of phase change of the interference light caused by the sample using the relationship between the time change in the intensity of the interference light and the time change in the drive voltage in the reference state and the relationship between the time change in the intensity of the interference light and the time change in the drive voltage in the measurement state, both stored by the memory unit, and to determine the refractive index of the sample based on the amount of phase change.

2. The interference refractometer according to claim 1, wherein the calculation unit is configured to obtain reference phases for the reference state and the measurement state by fitting the intensities of the interference light in the reference state and the measurement state at a specific drive voltage to a theoretical equation for the intensity of the interference light, the equation having a variable being the phase difference between the light in the first optical path and the light in the second optical path, and to obtain the amount of phase change by taking the difference between these reference phases.

3. The theoretical formula is I = A １ ２ +A ２ ２ +2A １ A ２ cos(φ １ -φ ２ ), where I is the intensity of the interference light, A1 is the amplitude of the light in the first optical path, A2 is the amplitude of the light in the second optical path, and φ １ is the phase of the light in the first optical path, φ ２ 3. The interferometric refractometer of claim 2, wherein: is the phase of light in the second optical path.

4. The calculation unit calculates φ when the time change in the intensity of the interference light in the reference state is in a specific state. １ is the reference phase of the reference state, and φ when the time change in the intensity of the interference light in the measurement state is in the specific state. １ and determining a difference between the reference phase of the reference state and the reference phase of the measurement state as the amount of phase change.

5. The interference refractometer according to claim 1, wherein the phase modulation portion includes an electro-optic crystal.

6. An interference refractometer according to claim 1, wherein said phase modulation section is provided so as to time-modulate only the phase of the light in said first optical path.

7. An interference refractometer as claimed in claim 1, wherein the photodetector has a plurality of light receiving elements whose light receiving surfaces are arranged in one direction, and the optical system is arranged to guide the interference light to the photodetector so that interference fringes are formed in the arrangement direction of the light receiving surfaces of the light receiving elements of the photodetector.

8. A method for measuring the refractive index of a sample using an interference refractometer comprising a light source, a photodetector for detecting the intensity of received light, and an optical system for branching the optical path of light emitted from the light source into a first optical path and a second optical path, then merging the first optical path and the second optical path, and directing interference light of the light of the first optical path and the light of the second optical path to the photodetector, comprising the steps of: varying over time, with a predetermined amplitude and periodicity, the drive voltage of a phase modulation section that electrically modulates the phase difference between the light of the first optical path and the light of the second optical path, thereby varying over time the intensity of the interference light detected by the photodetector; recording the time change in the intensity of the interference light in a reference state in which no sample is present on the first optical path in correspondence with the time change in the drive voltage; and recording the time change in the intensity of the interference light in a measurement state in which a sample is present on the first optical path in correspondence with the time change in the drive voltage. a step of determining an amount of phase change of the interference light caused by the sample using a relationship between a time change in intensity of the interference light and a time change in the driving voltage in the reference state, and a relationship between a time change in intensity of the interference light and a time change in the driving voltage in the measurement state; and a step of determining a refractive index of the sample based on the amount of phase change.

9. A refractive index measurement method as described in claim 8, wherein in the step of determining the amount of phase change, reference phases for the reference state and the measurement state are determined by fitting the intensities of the interference light in the reference state and the measurement state at a specific driving voltage to a theoretical equation for the intensity of the interference light, the equation having a phase difference between the light in the first optical path and the light in the second optical path as a variable, and the amount of phase change is determined by taking the difference between these reference phases.

10. The theoretical formula is: I = A １ ２ +A ２ ２ +2A １ A ２ cos(φ １ -φ ２ ), where I is the intensity of the interference light, A1 is the amplitude of the light in the first optical path, A2 is the amplitude of the light in the second optical path, and φ １ is the phase of the light in the first optical path, φ ２ 10. The method of claim 9, wherein: is the phase of light in the second optical path.

11. In the step of calculating the amount of phase change, φ when the time change in the intensity of the interference light in the reference state is in a specific state １ is the reference phase of the reference state, and φ when the time change in the intensity of the interference light in the measurement state is in the specific state. １ and determining a difference between the reference phase of the reference state and the reference phase of the measurement state as the amount of phase change.

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