Birefringence measurement method and birefringence measurement system

Abstract

To enable accurate birefringence measurement with a simple configuration.SOLUTION: A birefringence measurement system 100 comprises: a polarization modulation part 104 that modulates a polarized state of light from a light source 102 and irradiates a sample 120 with the light; a polarization analyzing part 106 for analyzing the polarized state of the light that transmits through the sample 120; a detector 108 that detects a light intensity of the emitted light of the polarization analyzing part 106 in a two-dimensional way; and an arithmetic unit 110 that calculates a first Stokes parameter representing a polarized state where there is no sample from the light intensity detected when no sample is present, calculates a second Stokes parameter representing a polarized state where there is a sample from the light intensity detected when the sample is present, projects the first and second Stokes parameters as first and second coordinates on a Poincare spherical surface, respectively, calculates a birefringence phase difference from a distance between the first and second coordinates on the Poincare spherical surface, and calculates a main axis azimuth from the direction of a vector connecting the first and second coordinates.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a birefringence measurement method and a birefringence measurement system. [Background technology]

[0002] In general, to determine the anisotropy of a material, such as birefringence, it is necessary to go through the following steps: (i) irradiating a sample with light of an arbitrary polarization state, (ii) measuring the light transmitted or reflected from the sample, and (iii) analyzing the polarization state of the light that has passed through the sample. These steps are generally called polarization measurement. A system for performing polarization measurement is composed of a light source, a polarization modulation unit that modulates the polarization state of the light from the light source and irradiates the sample with the modulated light, a polarization analysis unit that analyzes the polarization state of the light transmitted or reflected from the sample, and a detector that detects the intensity of the light emitted from the polarization analysis unit (see, for example, Patent Document 1 and Patent Document 2).

[0003] From the viewpoint of analyzing the polarization state of the light irradiated to the sample and its change, the polarization modulation section requires various optical components as shown in Patent Documents 1 and 2. The more optical components are added, the more diverse the modulation of the polarization becomes, and the more types of optical characteristics can be analyzed. However, such a method leads to an increase in the financial cost of the optical system and an increase in the number of analysis procedures, so a method that focuses on a certain anisotropy and simplifies the optical system and analysis procedures is used. One such method is the rotating analyzer method.

[0004] In the rotating analyzer method, it is assumed that the light irradiated to the sample is circularly polarized light, and the polarization analysis unit can easily analyze the birefringence phase difference and the principal axis direction by simply rotating one analyzer to four transmission axis orientations of 0 degrees, 45 degrees, 90 degrees, and 135 degrees. On the other hand, in the rotating analyzer method, a large analysis error occurs when the light incident on the sample is elliptically polarized rather than circularly polarized. Cases where the light becomes elliptically polarized can occur globally or locally due to performance errors of optical elements or oblique incidence on optical elements by a two-dimensional measurement system. To solve this problem, Patent Document 3 enables accurate measurement by subtracting the analysis error as a bias component of the light intensity. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-232550 A [Patent Document 2] JP 2018-194455 A [Patent Document 3] Japanese Patent Application Publication No. 05-281137 Summary of the Invention [Problem to be solved by the invention]

[0006] However, since the method of Patent Document 3 focuses on the bias component of the light intensity, it is considered to be practically difficult to correct the error when the light intensity change is weak, that is, when the birefringence phase difference is small and the phase difference is low. Also, in order to make the difference in light intensity large during the polarization measurement, it is necessary to add one phase plate to the polarization analysis section. Also, since the measurement is performed by changing the rotation condition of this phase plate, it is necessary to perform the procedure of the rotating analyzer method multiple times (twice in Patent Document 3), which is a constraint on the optical system and the measurement procedure.

[0007] The present invention has been made in view of the above problems, and has an object to provide a birefringence measurement method and a birefringence measurement system that are capable of performing accurate birefringence measurement with a simple configuration. [Means for solving the problem]

[0008] A birefringence measurement method according to one aspect of the present invention includes an irradiation step of irradiating a sample with light of a predetermined polarization state by an irradiation unit; an analysis step of analyzing the polarization state of the light transmitted through the sample by an ellipsometry unit; a detection step of receiving the light emitted from the ellipsometry unit by a detector and detecting the light intensity two-dimensionally; and a calculation step of calculating, by a calculation device, a first Stokes parameter as a Stokes parameter representing the polarization state when a sample is not present from a first light intensity which is the light intensity detected by the detector when a sample is not present, and calculating a second Stokes parameter as a Stokes parameter representing the polarization state when a sample is present from a second light intensity which is the light intensity detected by the detector when a sample is present, projecting the first Stokes parameter and the second Stokes parameter onto a Poincaré sphere as first coordinates and second coordinates, respectively, calculating a birefringence phase difference from the distance between the first coordinate and the second coordinate on the Poincaré sphere, and calculating a principal axis orientation from the direction of a vector connecting the first coordinate and the second coordinate.

[0009] A birefringence measurement system according to one embodiment of the present invention includes an irradiation unit that irradiates a sample with light of a predetermined polarization state, an ellipsometry unit that analyzes the polarization state of the light transmitted through the sample, a detector that receives the light emitted from the ellipsometry unit and detects the light intensity two-dimensionally, and a calculation device that calculates a first Stokes parameter as a Stokes parameter representing the polarization state when a sample is not present from a first light intensity that is the light intensity detected by the detector when a sample is not present, calculates a second Stokes parameter as a Stokes parameter representing the polarization state when a sample is present from a second light intensity that is the light intensity detected by the detector when a sample is present, projects the first Stokes parameter and the second Stokes parameter onto a Poincaré sphere as first coordinates and second coordinates, respectively, calculates a birefringence phase difference from the distance between the first coordinate and the second coordinate on the Poincaré sphere, and calculates a principal axis orientation from the direction of a vector connecting the first coordinate and the second coordinate. Effect of the Invention

[0010] According to the present invention, the first Stokes parameter obtained when there is no sample and the second Stokes parameter obtained when there is a sample are projected onto the Poincaré sphere as the first coordinates and the second coordinates, respectively, and the birefringence phase difference and the principal axis direction are calculated based on the first coordinates and the second coordinates on the Poincaré sphere. This makes it possible to accurately measure birefringence with a simple configuration. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing a configuration of a birefringence measurement system according to an embodiment of the present invention. [Diagram 2] 1 is a schematic diagram showing the coordinates of circularly polarized light, which is ideal incident light on a sample, the coordinates of elliptically polarized light, which is actual incident light, and the coordinates of outgoing light on the Poincaré sphere. [Diagram 3] 1 is a graph showing analysis results of birefringence phase difference calculated by a conventional method and birefringence phase difference calculated by a birefringence measurement method according to the present embodiment. [Figure 4] 1 is an example of an analysis result of birefringence retardation in a low retardation region obtained by a conventional method and a birefringence measurement method according to the present embodiment. [Diagram 5] 1 is an example of a birefringence phase difference analysis result obtained using a commercially available birefringence measuring device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. First, the configuration of a birefringence measurement system according to the present embodiment will be described.

[0013] 1, the birefringence measurement system 100 includes a light source 102, a polarization modulation unit 104 that modulates the polarization state of light from the light source 102 and irradiates the light onto a sample 120 (e.g., glass, a Si wafer) placed on a stage, a polarization analysis unit 106 that analyzes the polarization state of the light transmitted through the sample 120, a detector 108 that two-dimensionally detects the light intensity of the light emitted from the polarization analysis unit 106, and a calculation device 110 that calculates the birefringence phase difference and the principal axis direction based on the detection result by the detector 108. The polarization modulation unit 104, the sample 120, the polarization analysis unit 106, and the detector 108 are arranged on the optical path of the light emitted from the light source 102.

[0014] The light source 102 emits light of a predetermined wavelength with a constant light intensity. The polarization modulation unit 104 includes a polarizer 104P and a phase plate 104R. The polarizer 104P transmits linearly polarized light of a specific polarization direction out of the light emitted from the light source 102. The phase plate 104R imparts a phase difference to the linearly polarized light transmitted through the polarizer 104P. An example of the phase plate 104R is a λ / 4 wave plate that delays the phase of the incident linearly polarized light by π / 2 (¼ wavelength).

[0015] The combination of the light source 102 and the polarization modulation unit 104 functions as an irradiation unit that irradiates the sample 120 with light in a predetermined polarization state. The irradiation unit can generate any polarized light (circularly polarized light, elliptically polarized light, or linearly polarized light), but it is preferable to generate circularly polarized light or polarized light equivalent to circularly polarized light (elliptically polarized light very close to circularly polarized light). This is because, when circularly polarized light or elliptically polarized light very close to circularly polarized light is irradiated onto the sample 120, birefringence measurement becomes possible regardless of the principal axis orientation of the sample 120. When the light source 102 that emits light with random polarization is used, circularly polarized light can be generated by tilting the optical axis of the λ / 4 wave plate 45 degrees with respect to the transmission axis (polarization direction) of the polarizer 104P.

[0016] As another configuration of the irradiation unit, a combination of a laser light source that emits linearly polarized light and a λ / 4 wavelength plate may be adopted, or the irradiation unit may be configured by a combination of a light emitting diode (LED) light source that emits circularly polarized light and an expanding means for expanding the beam diameter of the light from the LED light source.

[0017] The polarization analysis unit 106 includes an analyzer 106A that transmits light from the sample 120 in a plurality of different transmission axis orientations. The analyzer 106A can rotate the transmission axis orientation to different angles (for example, 0 degrees, 45 degrees, 90 degrees, and 135 degrees).

[0018] The detector 108 receives the light emitted from the polarization analysis unit 106, detects the light intensity of the emitted light two-dimensionally, and outputs an image signal corresponding to the light intensity at each position. An example of the detector 108 is a two-dimensional array detector such as a charge-coupled device (CCD). Alternatively, other devices such as a device that scans each point on a two-dimensional surface or a polarization camera may be used as long as the polarization state of the light emitted from the polarization analysis unit 106 can be recorded at each point. For example, a polarization camera has a structure in which an analyzer with a different transmission axis direction is incorporated in each pixel, so that one-shot photography is possible without rotating the analyzer. The image signal corresponding to the position information and the light intensity is output to the calculation device 110 via a connection device such as a digital interface or a frame grabber.

[0019] The arithmetic device 110 is a computer including a processor that calculates the birefringence phase difference and the principal axis orientation based on the image signal output from the detector 108, a storage unit 112, a display device that displays the processing results of the processor, etc. The processor stores the image signal output from the detector 108, data required for the calculation processing, and data of the processing results in the storage unit 112. The method of calculating the birefringence phase difference and the principal axis orientation will be described later in detail.

[0020] Next, the birefringence measuring method according to this embodiment will be described in detail.

[0021] First, in a state where the sample 120 is not placed, the detector 108 detects light intensity (hereinafter referred to as first light intensity) in each transmission axis orientation (0 degree, 45 degree, 90 degree, 135 degree) of the analyzer 106A, and the arithmetic device 110 stores the first light intensity in each transmission axis orientation in the storage unit 112. In addition, the arithmetic device 110 calculates a Stokes parameter (hereinafter referred to as first Stokes parameter) representing a polarization state when the sample 120 is not placed, from the first light intensity in each transmission axis orientation, and stores the first Stokes parameter in the storage unit 112.

[0022] Next, with the sample 120 placed, the detector 108 detects the light intensity (hereinafter referred to as the second light intensity) in each transmission axis orientation (0 degree, 45 degrees, 90 degrees, 135 degrees) of the analyzer 106A, and the arithmetic device 110 stores the second light intensity in each transmission axis orientation in the storage unit 112. In addition, the arithmetic device 110 calculates Stokes parameters (hereinafter referred to as the second Stokes parameters) representing the polarization state when the sample 120 is present from the second light intensity in each transmission axis orientation, and stores the second Stokes parameters in the storage unit 112.

[0023] The light intensities at 0 degrees, 45 degrees, 90 degrees, and 135 degrees of the analyzer 106A are I0, I 45 , I 90 , I 135 Then, the Stokes parameters S0, S1, S2, and S3 representing the polarization state are expressed as shown in equation (1).

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[0024] Here, S1 represents a linearly polarized component at 0 degrees and 90 degrees, S2 represents a linearly polarized component at 45 degrees and 135 degrees, and S3 represents a right-handed or left-handed circularly polarized component. In this embodiment, the polarization analysis unit 106 does not have a phase plate and performs analysis including the square root of S3, so it is not possible to distinguish whether S3 is positive or negative. Therefore, in this embodiment, only a hemisphere of the Poincaré sphere described below is targeted, and the birefringence phase difference is calculated in the range of ±90 degrees.

[0025] The Stokes parameters can be expressed as spherical coordinates of radius S0, with S1, S2, and S3 in the x, y, and z directions of the Cartesian coordinate system, respectively. In other words, the Stokes parameters can be projected onto the Poincaré sphere and expressed as coordinates on the Poincaré sphere.

[0026] Here, the light intensity detected by the detector 108 changes according to the light absorption in the sample 120 and the Fresnel reflection on the surface of the sample 120, and therefore the radius S0 of the Poincaré sphere changes. Therefore, the arithmetic device 110 corrects the influence of light absorption and Fresnel reflection by normalizing the radius S0 of each of the first Stokes parameter and the second Stokes parameter to a constant value. Specifically, the Stokes parameters (S0, S1, S2, S3) are converted to (S0 / S0, ​​S1 / S0, S2 / S0, S3 / S0) and normalized so that the radius S0 of the Poincaré sphere becomes 1.

[0027] If S0=1, the coordinates of the circularly polarized light on the Poincaré sphere are point C (S1, S2, S3)=(0, 0, 1) or (0, 0, -1) as shown in FIG. 2. As described above, even if the light incident on the sample 120 from the irradiation unit (light source 102 and polarization modulation unit 104) is set to be circularly polarized, the light incident on the sample 120 may actually be elliptically polarized rather than circularly polarized due to performance errors of optical elements, oblique incidence, etc. As described above, even if the optical axis of the phase plate 104R (λ / 4 wave plate) is set to be inclined 45 degrees with respect to the transmission axis of the polarizer 104P to generate circularly polarized light, the characteristics of the polarizer 104P and the λ / 4 wave plate result in, for example, (S1, S2, S3)=(-0.026, -0.068, 0.997), and the light is not completely circularly polarized. Furthermore, when the angle between the transmission axis of the polarizer 104P and the optical axis of the λ / 4 wave plate is 40 degrees, for example, (S1, S2, S3)=(0.03, 0.171, 0.985), resulting in a higher ellipticity.

[0028] In order to grasp the polarization state of the actual incident light, the arithmetic device 110 calculates a first Stokes parameter from the first light intensity detected when the sample 120 is not placed. Point I in Fig. 2 is an example of a coordinate (hereinafter referred to as the first coordinate) corresponding to the first Stokes parameter obtained when the sample 120 is not placed. Fig. 2 shows an example in which the actual incident light is elliptically polarized. Point O in Fig. 2 is an example of a coordinate (hereinafter referred to as the second coordinate) corresponding to the second Stokes parameter obtained when the sample 120 is placed.

[0029] Assuming that the incident light is perfectly circularly polarized (point C in Fig. 2) as in the conventional rotating analyzer method, the birefringence phase difference and the principal axis direction are calculated based on points C and O (second coordinate) on the Poincare sphere. Under this assumption, the measurement error becomes large especially in the case of low phase difference where the travel distance on the Poincare sphere is small (when the birefringence phase difference is small), and the travel distance and its direction cannot be analyzed accurately.

[0030] On the other hand, in this embodiment, point I (first coordinate) which indicates the polarization state of the actual incident light is obtained, and the birefringence phase difference and the principal axis direction are calculated based on point I (first coordinate) and point O (second coordinate) on the Poincaré sphere as described below, thereby enabling accurate measurement.

[0031] The Stokes parameters (S0, S1, S2, S3) converted into latitude θ and longitude ψ on the Poincaré sphere are given by equations (2) and (3), respectively.

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[0032] The latitude and longitude of the first coordinate on the Poincaré sphere are respectively θ i and ψ i The latitude and longitude of the second coordinate on the Poincaré sphere are denoted as θ o and ψ oThe birefringence phase difference corresponds to the distance between the first coordinate and the second coordinate on the Poincaré sphere. The birefringence phase difference Δ can be calculated using spherical trigonometry, and is given by, for example, Equation (4).

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[0033] The principal axis direction corresponds to the direction of a vector connecting the first coordinate and the second coordinate on the S1-S2 plane. If the S1 component of this vector is dS1 and the S2 component is dS2, then dS1 and dS2 are expressed as in Equation (5). In Equation (5), S 1,i , S 2,i indicate the S1 and S2 components of the first coordinate, respectively, and S 1,o , S 2,o indicate the S1 and S2 components of the second coordinate, respectively.

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[0034] Therefore, the principal axis direction φ is given by equation (6).

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[0035] The birefringence phase difference Δ can provide information on the magnitude of stress in the sample 120, and the principal axis direction φ can provide information on the direction of stress in the sample 120.

[0036] FIG. 3 is a graph showing an example of the analysis result of the birefringence phase difference calculated by the conventional method (rotating analyzer method) and the birefringence phase difference (equation (4)) calculated by the birefringence measurement method according to the present embodiment. The sample phase difference on the horizontal axis of the graph represents the birefringence phase difference of a reference sample, which is measured by a commercially available birefringence measurement device. FIG. 3 shows the measurement results when a sample 120 with a principal axis orientation of 45 degrees is used and the incident light to the sample 120 is approximately circularly polarized light and elliptically polarized light. Here, the first Stokes parameters of the approximately circularly polarized light are (S1, S2, S3)=(-0.115, -0.019, 0.993), and the first Stokes parameters of the elliptically polarized light are (S1, S2, S3)=(-0.008, -0.270, 0.963). These incident lights can be generated by the polarization modulation unit 104 when a wide area is photographed using an optical device such as a lens.

[0037] 3, in the conventional method, an offset component exists even when the incident light is circularly polarized with respect to zero phase difference (horizontal axis), and the offset component becomes larger when the incident light is elliptically polarized. On the other hand, in the birefringence measurement method according to the present embodiment, a zero phase difference (vertical axis) is output with respect to zero phase difference (horizontal axis), and even outside the low phase difference region, an almost accurate value is output, and linearity is ensured.

[0038] When polarized light is incident on the sample 120 and the polarized light transmitted through it is analyzed, the analysis results in the low phase difference region where the error rate due to the offset component of the birefringence phase difference is particularly important. Fig. 4 shows, as an example of low phase difference measurement, the results of simultaneous measurement of air and glass in the case where the incident light to the sample 120 is approximately circularly polarized light and elliptically polarized light, respectively, in the conventional method (rotating analyzer method) and the birefringence measurement method according to this embodiment. Here, the first Stokes parameters of approximately circularly polarized light are (S1, S2, S3) = (-0.026, -0.068, 0.997), and the first Stokes parameters of elliptically polarized light are (S1, S2, S3) = (0.016, 0.143, 0.990).

[0039] As shown in Fig. 4, in the conventional method, even if the incident light is almost circularly polarized, an average birefringence phase difference of 4.4° is output in the air layer, and when the incident light is elliptically polarized, the birefringence phase difference increases further to an average of 8.3°. On the other hand, in the birefringence measurement method according to the present embodiment, when the incident light is almost circularly polarized and when the incident light is elliptically polarized, the birefringence phase difference is 0.44° and 0.38° on average, respectively, and low phase difference including the air layer can be almost correctly measured.

[0040] Fig. 5 shows the measurement results using a commercially available birefringence measuring device (Photonic Lattice, Inc., WPA-100) when the incident light is approximately circularly polarized as in Fig. 4. The birefringence phase difference of the air layer shown in Fig. 5 is an average of 0.43°, so it can be confirmed that the measurement results shown in Fig. 4 using the birefringence measuring method according to this embodiment are correct.

[0041] In this way, even if the light incident on sample 120 such as glass deviates from perfect circular polarization due to errors in individual optical elements, errors in the rotation angle between optical elements, oblique incidence components related to two-dimensional detection, etc., the polarization state when sample 120 is not present can be calculated as the first Stokes parameter and stored in memory unit 112, and when measuring sample 120, the polarization state of the incident light can be subtracted to accurately analyze the change in the polarization state.

[0042] Moreover, even if there is a noise component as disclosed in Patent Document 3, the noise is offset, especially for S1 and S2, because different light intensities are subtracted and calculated as shown in formula (1). Moreover, for S0, the average value of the light intensities obtained in each transmission axis direction of the analyzer 106A is taken as shown in formula (1), so the influence of noise is averaged. From the above, detection focusing on the light intensity change at a low phase difference is possible, compared to the technology of Patent Document 3, which corrects the measured value in the form of a light intensity bias. Furthermore, since there is no need to provide a phase plate in the polarization analysis unit 106 for the purpose of emphasizing the light intensity change, multiple measurements due to the rotation of the phase plate are also unnecessary, and the same result can be obtained with a simpler structure.

[0043] In conventional polarization analysis, a method using a matrix, such as that described in Patent Document 2, was mainstream. By using a matrix, various polarization states can be handled, such as the ability to measure the light absorption rate of a sample. On the other hand, in this embodiment, a matrix is ​​not used, and the birefringence phase difference and the principal axis direction are calculated using coordinates (latitude, longitude) on the Poincaré sphere. If the radius of the Poincaré sphere changes due to changes in light intensity caused by light absorption and Fresnel reflection, the coordinates will shift, making polarization analysis very difficult as is. However, in this embodiment, the Stokes parameters are standardized so that S0 is a constant value, thereby correcting the effects of light absorption and Fresnel reflection, making it easy to perform polarization analysis.

[0044] Furthermore, even if the light incident on the sample 120 deviates from perfect circular polarization, fine correction is not necessary (high degree of freedom), and large-area polarization measurement becomes possible regardless of the influence of oblique incidence, etc.

[0045] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit and scope of the present invention. [Explanation of symbols]

[0046] 100 Birefringence Measurement System 102 Light source 104 Polarization Modulation Section 104P polarizer 104R phase plate 106 Polarization analysis section 106A Analyzer 108 Detector 110 Arithmetic equipment 112 Storage section 120 samples

Claims

1. an irradiation step of irradiating the sample with light in a predetermined polarization state by an irradiation unit; an analysis step of analyzing the polarization state of the light transmitted through the sample by a polarization analysis unit; a detection step of receiving the light emitted from the polarization analysis unit and two-dimensionally detecting the light intensity by a detector; a calculation step of calculating, by a calculation device, a first Stokes parameter as a Stokes parameter representing the polarization state when the sample is not present from a first light intensity which is the light intensity detected by the detector when the sample is not present, calculating a second Stokes parameter as a Stokes parameter representing the polarization state when the sample is present from a second light intensity which is the light intensity detected by the detector when the sample is present, projecting the first Stokes parameter and the second Stokes parameter onto a Poincaré sphere as first coordinates and second coordinates, respectively, calculating a birefringence phase difference from a distance between the first coordinates and the second coordinates on the Poincaré sphere, and calculating a principal axis orientation from a direction of a vector connecting the first coordinates and the second coordinates; A method for measuring birefringence comprising:

2. 2. The birefringence measuring method according to claim 1, wherein in the irradiating step, a polarization modulation section modulates the polarization state of the light emitted from the light source and emits the modulated light.

3. 3. The birefringence measuring method according to claim 2, wherein in the irradiation step, a polarizer transmits linearly polarized light in a specific direction out of the light emitted from the light source, and a phase plate imparts a phase difference to the linearly polarized light transmitted through the polarizer.

4. The birefringence measuring method according to claim 1 , wherein the irradiating step emits circularly polarized light or elliptically polarized light.

5. In the analyzing step, an analyzer transmits light from the sample in a plurality of different transmission axis directions; In the detection step, the first light intensity is detected for each transmission axis direction of the analyzer when the sample is not present, and the second light intensity is detected for each transmission axis direction of the analyzer when the sample is present, 5. The birefringence measuring method according to claim 1, wherein in the calculation step, the first light intensity and the second light intensity are stored in a storage unit for each transmission axis direction of the analyzer.

6. In the calculation step, calculating the first Stokes parameter from the first light intensity stored in the storage unit; calculating the second Stokes parameter from the second light intensity stored in the storage unit; storing the first Stokes parameters and the second Stokes parameters in the storage unit; The birefringence measurement method according to claim 5 , further comprising the steps of: calculating the birefringence phase difference and the principal axis direction based on the first Stokes parameter and the second Stokes parameter stored in the storage unit.

7. 2. The birefringence measurement method according to claim 1, wherein in the calculation step, the first Stokes parameter is standardized by setting a radius of the Poincaré sphere to a constant value based on the first light intensity, and the second Stokes parameter is standardized by setting a radius of the Poincaré sphere to the constant value based on the second light intensity, to calculate the birefringence phase difference and the principal axis orientation.

8. an irradiation unit that irradiates a sample with light in a predetermined polarization state; a polarization analyzer for analyzing the polarization state of light transmitted through the sample; a detector that receives the light emitted from the polarization analysis unit and detects the light intensity two-dimensionally; a calculation device that calculates a first Stokes parameter as a Stokes parameter representing the polarization state when the sample is not present from a first light intensity that is the light intensity detected by the detector when the sample is not present, calculates a second Stokes parameter as a Stokes parameter representing the polarization state when the sample is present from a second light intensity that is the light intensity detected by the detector when the sample is present, projects the first Stokes parameter and the second Stokes parameter onto a Poincaré sphere as first coordinates and second coordinates, respectively, calculates a birefringence phase difference from a distance between the first coordinates and the second coordinates on the Poincaré sphere, and calculates a principal axis orientation from a direction of a vector connecting the first coordinates and the second coordinates; A birefringence measurement system comprising:

9. 9. The birefringence measurement system according to claim 8, wherein the irradiation unit includes a light source and a polarization modulation unit that modulates the polarization state of the light emitted from the light source and emits the modulated light.

10. The birefringence measurement system according to claim 9, wherein the polarization modulation unit includes a polarizer that transmits linearly polarized light in a specific direction from the light emitted from the light source, and a phase plate that imparts a phase difference to the linearly polarized light that has transmitted through the polarizer.

11. The birefringence measurement system according to claim 8 , wherein the irradiating unit emits circularly polarized light or elliptically polarized light.

12. the polarization analysis unit includes an analyzer that transmits light from the sample in a plurality of different transmission axis orientations, The detector detects the first light intensity for each transmission axis direction of the analyzer when the sample is not present, and detects the second light intensity for each transmission axis direction of the analyzer when the sample is present; The birefringence measurement system according to any one of claims 8 to 11, wherein the arithmetic device includes a memory unit, and stores the first light intensity and the second light intensity in the memory unit for each transmission axis orientation of the analyzer.

13. The computing device includes: calculating the first Stokes parameter from the first light intensity stored in the storage unit; calculating the second Stokes parameter from the second light intensity stored in the storage unit; storing the first Stokes parameters and the second Stokes parameters in the storage unit; The birefringence measurement system according to claim 12 , wherein the birefringence phase difference and the principal axis direction are calculated based on the first Stokes parameter and the second Stokes parameter stored in the storage unit.

14. 9. The birefringence measurement system according to claim 8, wherein the arithmetic device standardizes the first Stokes parameter by setting a radius of the Poincaré sphere to a constant value based on the first light intensity, and standardizes the second Stokes parameter by setting a radius of the Poincaré sphere to the constant value based on the second light intensity, thereby calculating the birefringence phase difference and the principal axis orientation.

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