JP5747317B2 - Polarization measuring apparatus and polarization measuring method - Google Patents

Description

  The present invention relates to an apparatus and method for measuring the polarization state of light to be analyzed.

  Conventionally, the rotation analyzer method and the rotation phaser method have been used as methods for measuring the polarization state such as ellipticity of the analysis target light (Patent Document 1). However, in the rotational analyzer method, since the ellipticity is given by a cosine function, the measurement accuracy is deteriorated when the birefringence phase difference of the sample is around 0 ° and 180 °. In addition, since birefringence and optical rotation have wavelength dependence, evaluation for each wavelength is required. However, in the rotational phaser method, the polarization state must be measured by changing the phaser for each wavelength. Cannot be measured efficiently.

In the double rotation method in which the phaser and the analyzer are rotated, the wavelength dispersion in the polarization state can be measured without changing the phaser for each wavelength (Patent Document 2). However, each measurement method requires a driving unit such as a motor as a phaser or analyzer rotation mechanism, which increases the size of the apparatus, and mechanically rotates the phaser or analyzer to increase the measurement time. Become. In addition, when polarization modulation is performed using an optical material such as a Faraday cell or a photoelastic modulator (PEM), high voltage and power consumption are required. Furthermore, the polarization modulation of the light to be analyzed by the rotating phaser and analyzer is a complicated motion including rotation around _three axes (S ₁ axis, S ₂ axis, and S ₃ axis) in the Poincare sphere. The theoretical formula for obtaining the polarization state of the target light is also complicated.

  An apparatus and a method for performing polarization modulation using a liquid crystal variable phase shifter instead of a rotating phase shifter or an analyzer are known (Patent Document 3 and Patent Document 4). However, the apparatus and method of Patent Document 3 can measure the optical characteristics of a sample, but cannot measure the polarization state of arbitrary analysis target light. Moreover, in the apparatus and method of Patent Document 4, since it is necessary to measure the polarization state for each wavelength, it is impossible to efficiently measure the chromatic dispersion of the polarization state.

JP 2005-292028 A JP 2009-085853 A JP 2010-145332 A US Pat. No. 6,744,509

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a polarization measuring device and a polarization measuring method capable of efficiently measuring the polarization state and wavelength dispersion of analysis target light. That is.

In order to achieve the above object, a polarization measuring device according to the present invention is
A polarization measuring device for measuring the polarization state of light to be analyzed,
A polarization modulator for polarization-modulating the light to be analyzed;
An analyzer that transmits a predetermined polarization component of polarization-modulated light; and
Detecting means for detecting the light intensity of the light transmitted through the analyzer;
A calculation unit that calculates the polarization characteristic element of the light to be analyzed based on the light intensity detected by the detection means,
The polarization modulator includes a first variable phase shifter, and a second variable phase shifter disposed between the first variable phase shifter and the analyzer,
The main axis of the second variable phase shifter is inclined by an odd multiple of 45 ° with respect to the main axis of the first variable phase shifter.

Moreover, the polarization measuring method according to the present invention includes:
A polarization measurement method for measuring the polarization state of light to be analyzed,
Polarization-modulating light to be analyzed by a first variable phase shifter;
Polarization-modulating light polarization-modulated by the first variable phase retarder using the second variable phase retarder;
Detecting the light intensity of the linearly polarized light component transmitted through the analyzer from the light modulated by the second variable phase shifter;
Calculating the polarization characteristic element of the light to be analyzed based on the detected light intensity,
The main axis of the second variable phase shifter is inclined by an odd multiple of 45 ° with respect to the main axis of the first variable phase shifter.

  ADVANTAGE OF THE INVENTION According to this invention, the polarization measuring apparatus and polarization measuring method which can measure efficiently the polarization state and its wavelength dispersion of analysis object light can be provided.

It is a schematic block diagram of embodiment of the optical measuring apparatus containing the polarization measuring apparatus which concerns on this invention. It is a Poincare sphere showing the action of a liquid crystal variable phase shifter with a principal axis orientation of 0 °. It is a Poincare sphere showing the action of a liquid crystal variable phase shifter with a main axis direction of 45 °. It is a Poincare sphere showing polarization modulation by two liquid crystal variable phase shifters. It is a Poincare sphere showing the action of an analyzer with a main axis of 90 °. The relationship between the birefringence phase difference of a liquid crystal variable phase retarder in wavelength 486nm, 590nm, and 656nm and an applied voltage is shown. It is a Poincare sphere which shows the polarization state of the transmitted light of the polarizer rotating around the optical axis. The relationship between the rotation angle around the optical axis of the polarizer and the azimuth obtained from the Stokes parameter measurement value of the transmitted light of the polarizer is shown. It is a Poincare sphere showing a polarization state of transmitted light of a liquid crystal variable phase shifter in which a birefringence phase difference is changed. The relationship between the setting value of the birefringence phase difference of the liquid crystal variable phase retarder and the measured value of the birefringence phase difference of the liquid crystal variable phase retarder obtained from the Stokes parameter measurement values is shown. The relationship between the set value of the birefringence phase difference of a liquid crystal variable phase retarder and the ellipticity angle calculated | required from the Stokes parameter measured value is shown. The birefringence phase difference of the quarter wavelength plate measured at wavelength 486nm, 590nm, 656nm is shown.

  FIG. 1 shows an embodiment of an optical measuring device including a polarization measuring device 10 according to the present invention.

  The optical measurement device includes a light source 12 that irradiates the sample S with light so that light that has passed through the sample S as analysis target light enters the polarization measurement device 10. As the light source 12, for example, a white light source such as a halogen lamp can be used.

  The optical measurement device further includes an optical fiber 14 disposed between the light source 12 and the sample S along the optical path L, a collimating lens 16, and an interference filter 18. The optical fiber 14 uses light from the light source 12 as a point light source. For example, an optical fiber having a diameter of 200 μm can be used. The collimating lens 16 converts the light made into a point light source by the optical fiber 14 into parallel light. The interference filter 18 converts the light that has been collimated by the collimating lens into monochromatic light.

  The polarization measuring device 10 includes a polarization modulator 20 that polarizes and modulates analysis target light (light that has passed through the sample S in this embodiment), an analyzer 30 that transmits a predetermined polarization component of the polarization-modulated light, and A detection unit 40 that detects the light intensity of the light transmitted through the analyzer 30 and a calculation unit 60 that calculates a polarization characteristic element of the light to be analyzed based on the light intensity detected by the detection unit 40 are provided.

  The polarization modulator 20 includes two variable phase shifters 21 and 22 that can change the birefringence phase difference. The variable phase shifters 21 and 22 are, for example, liquid crystal variable phase shifters including nematic liquid crystal cells. As the nematic liquid crystal cell, for example, a liquid crystal having a refractive index anisotropy Δn of 0.2 and a cell gap of 5.5 μm can be used.

The first liquid crystal variable phase shifter 21 arranged on the near side of the traveling direction of the light to be analyzed has a main axis (fast axis or slow axis) in an arbitrary direction. When the principal axis direction of the first liquid crystal variable phase retarder 21 is set to 0 °, the first liquid crystal variable phase retarder 21 changes the Stokes parameters S ₂ and S ₃ of arbitrary polarization according to the birefringence phase difference δ _1. Let That is, as shown in FIG. 2, the first liquid crystal variable phase shifter 21 rotates a point P indicating an arbitrary polarization state in the Poincare sphere about the S ₁ axis.

The second liquid crystal variable phase shifter 22 arranged on the back side in the traveling direction of the light to be analyzed has a main axis (phase advance) in a direction inclined by an odd multiple of 45 ° with respect to the main axis direction of the first liquid crystal variable phase shifter 21. Axis or slow axis). Since the principal axis orientation of the first liquid crystal variable phase retarder 21 is 0 °, the principal axis orientation of the second liquid crystal variable phase retarder 22 is an odd multiple of 45 °, and the second liquid crystal variable phase retarder 22 has its birefringence. Stokes parameters S ₁ and S ₃ of arbitrary polarization are changed according to the phase difference δ ₂ . That is, the second liquid crystal variable retarder 22, as shown in FIG. 3, rotating the point P indicating the arbitrary polarization state in the Poincare sphere S ₂ axis.

Since the polarization modulator 20 includes the first liquid crystal variable phase retarder 21 whose principal axis direction is 0 ° and the second liquid crystal variable phase retarder 22 whose principal axis direction is an odd multiple of 45 °, The Stokes parameters S ₂ and S ₃ are changed according to the birefringence phase difference δ ₁ of the first liquid crystal variable phase retarder 21, and then the Stokes parameters S ₁ and S ₃ are changed to the birefringence position of the second liquid crystal variable phase retarder 22. is changed in accordance with the phase difference [delta] _2. That is, as shown in FIG. 4, the polarization modulator 20 moves the point P ₀ indicating the polarization state of the light to be analyzed in the Poincare sphere to the point P ₁ as indicated by the arrow a by rotation about the S ₁ axis. Further, the point P ₁ is moved to the point P ₂ as indicated by the arrow b by rotation around the S ₂ axis.

By setting the principal axis directions of the two liquid crystal variable phase shifters in this way, the polarization modulation of the light to be analyzed by the polarization modulator 20 is only rotated about two axes (S ₁ axis and S ₂ axis) in the Poincare sphere. hints, S ₃ to become a simple movement without the rotation axis, it is possible to simplify the theoretical formula for determining the polarization state of analyzed light.

  The polarization modulator 20 further includes a constant temperature block 23 that surrounds the liquid crystal variable phase shifters 21 and 22, and a temperature control unit 24 that controls the temperature of the liquid crystal variable phase shifters 21 and 22. The temperature control unit 24 includes a temperature sensor 25 that senses the temperature of the liquid crystal variable phase shifters 21 and 22, a Peltier element 26, and a Peltier controller 27 that controls the Peltier element 26 based on the temperature sensed by the temperature sensor 25. .

The analyzer 30 transmits the linearly polarized light component in the specific direction of the light that is polarization-modulated by the polarization modulator 20. The analyzer 30 is a linear polarizer having a principal axis (transmission axis) in a direction inclined by an even multiple of 45 ° with respect to the principal axis direction of the first liquid crystal variable phase retarder 21, and has a Stokes parameter S ₁ of arbitrary polarization. The corresponding vertical linearly polarized light component is transmitted. That is, as shown in FIG. 5, the analyzer 30 moves a point P ₂ indicating an arbitrary polarization state in the Poincare sphere to a point P ₃ projected on the S ₁ axis.

  The detection means 40 detects the light intensity of linearly polarized light that has passed through the analyzer 30. The detection means 40 includes a photo sensor as a light receiving sensor.

  The polarization measuring device 10 includes a calculation unit 60, a control signal generation unit 70, and a storage unit 80 as the control device 50.

  The calculation unit 60 calculates a polarization characteristic element that indicates the polarization state of the light to be analyzed, for example, a Stokes parameter. The polarization characteristic element calculated by the calculation unit 60 may include a Stokes parameter, an ellipticity angle, and an azimuth angle. When the polarization measuring device 10 is configured as a part of the optical measuring device, the calculation unit 60 calculates the optical rotation, birefringence, Mueller matrix, and the like of the sample S based on the Stokes parameter of the analysis target light.

  The control signal generator 70 applies a voltage to each of the first and second liquid crystal variable phase shifters 21 and 22 in order to change the birefringence phase difference between the first and second liquid crystal variable phase shifters 21 and 22. Generate a signal to control.

  The storage unit 80 temporarily stores the light intensity detected by the detection unit 40, the polarization characteristic element of the analysis target light calculated by the calculation unit 60, and the like.

  According to the above configuration, the light intensity can be detected by changing the birefringence phase difference between the two liquid crystal variable phase shifters by changing the voltage applied to the two liquid crystal variable phase shifters. It is possible to easily measure the polarization state such as the Stokes parameter and its wavelength dispersion. In addition, since a polarization modulator including two liquid crystal variable phase shifters is used and measurement is performed without rotating the phase shifter or analyzer, a driving unit for rotating the phase shifter or analyzer is not necessary. Therefore, the apparatus can be downsized, and the driving voltage and power consumption can be reduced. Furthermore, since there is no mechanical drive, the measurement speed can be improved and mechanical failure can be reduced.

  The polarization measuring device 10 according to the embodiment of the present invention has been described above, but the polarization measuring device according to the present invention is not limited to this. The configuration of each part of the polarization measuring device 10 can be replaced with any configuration having the same function.

  The interference filter 18 may be disposed between the analyzer 30 and the detection means 40. Alternatively, instead of using the interference filter 18, the detection means 40 may be a spectral light receiver having a diffraction grating as a spectral means and a light detection array as a light receiving means. As a result, the light incident on the spectroscopic light receiver can be dispersed for each wavelength by the diffraction grating, and the light intensity for each wavelength can be detected simultaneously by the respective light receiving elements constituting the light detection array. Therefore, the chromatic dispersion in the polarization state can be measured more efficiently.

  Subsequently, in order to simplify the mathematical formula of the principle of measurement by the polarization measuring apparatus and the polarization measuring method according to the present invention, the principal axis direction of the first liquid crystal variable phase shifter 21 is set to 0 °, and the second liquid crystal variable phase is set. Description will be made assuming that the principal axis direction of the probe 22 is 45 ° and the principal axis direction of the analyzer 30 is 90 °.

と表される。 When the Stokes parameters at each wavelength λ of the analysis target light are S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ), the Stokes vector S _IN at each wavelength of the analysis target light is

It is expressed.

と表される。ここで、δ_１（λ）は波長λにおける第１の液晶可変位相子２１の複屈折位相差である。このミュラー行列ＬＣ_１によって表される第１の液晶可変位相子２１による偏光変調は、図４のポアンカレ球において矢印ａで表される。したがって、第１の液晶可変位相子２１を通過した光の偏光状態は、ストークスベクトルで

と表され、図４のポアンカレ球において点Ｐ_１で示される。 The Mueller matrix LC ₁ of the first liquid crystal variable phase shifter 21 whose principal axis orientation is 0 ° is

It is expressed. Here, δ ₁ (λ) is a birefringence phase difference of the first liquid crystal variable phase retarder 21 at the wavelength λ. Polarization modulation of the first liquid crystal variable retarder 21 represented by this Mueller matrix LC ₁ is represented by the arrow a in the Poincare sphere of FIG. Therefore, the polarization state of the light that has passed through the first liquid crystal variable phase retarder 21 is a Stokes vector.

It is expressed as, indicated by a point P ₁ in the Poincare sphere of FIG.

と表される。ここで、δ_２（λ）は波長λにおける第２の液晶可変位相子２２の複屈折位相差である。このミュラー行列ＬＣ_２によって表される第２の液晶可変位相子２２による偏光変調は、図４のポアンカレ球において矢印ｂで表される。したがって、第２の液晶可変位相子２２を通過した光の偏光状態は、ストークスベクトルで

と表され、図４のポアンカレ球において点Ｐ_２で示される。 The Mueller matrix LC ₂ of the second liquid crystal variable phase shifter 22 whose principal axis orientation is 45 ° is

It is expressed. Here, δ ₂ (λ) is the birefringence phase difference of the second liquid crystal variable phase retarder 22 at the wavelength λ. Polarization modulation by the second liquid crystal variable retarder 22 represented by this Mueller matrix LC ₂ is represented by the arrow b in the Poincare sphere of FIG. Therefore, the polarization state of the light that has passed through the second liquid crystal variable phase retarder 22 is a Stokes vector.

It is expressed as, indicated by a point P ₂ in the Poincare sphere of FIG.

と表される。このミュラー行列Ａによって表される検光子３０による垂直直線偏光成分の取り出しは、図５のポアンカレ球において矢印ｃで表される。したがって、検光子３０を通過した光のストークスベクトルＳ_ＯＵＴは、

と表される。この偏光状態は、図５のポアンカレ球においてＰ_３で示される。 The Mueller matrix A of the analyzer 30 whose principal axis orientation is 90 ° is

It is expressed. The extraction of the vertical linearly polarized light component by the analyzer 30 represented by the Mueller matrix A is represented by an arrow c in the Poincare sphere in FIG. Therefore, the Stokes vector S _OUT of the light passing through the analyzer 30 is

It is expressed. This polarization state is represented by P ₃ in the Poincare sphere of FIG.

From the equation (7), the light intensity I (δ ₁ (λ), δ ₂ (λ)) of each wavelength detected by the detection means 40 is the S ₀ component of the Stokes vector S _OUT. expressed.

When the birefringence phase differences δ ₁ (λ) and δ ₂ (λ) of the first and second liquid crystal variable phase shifters 21 and 22 are δ ₁ (λ) = δ ₂ (λ) = δ (λ), light The intensity I (δ (λ)) is expressed by the following equation.

The coefficients of the Fourier series when the equation (9) is Fourier-analyzed with respect to δ are expressed by the following equations according to the Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ). It is represented by

Therefore, the Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ) of the analysis target light are respectively expressed by the following equations.

From the above, the apparatus and method according to the present invention change the phase modulation amount δ (λ) and Fourier-analyze the detected light intensity I (δ (λ)) at each wavelength, thereby analyzing each wavelength of the light to be analyzed. Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ) can be calculated.

  As shown in FIG. 6, the relationship between the voltage applied to the liquid crystal variable phase retarder and the birefringence phase difference of the liquid crystal variable phase retarder varies depending on the wavelength. By making it possible to convert the applied voltage into a birefringence phase difference for each wavelength based on the relationship, the Stokes parameters for each wavelength can be measured.

  Further, by obtaining the Stokes parameter, other polarization characteristic elements such as an azimuth angle and an ellipticity angle can be easily obtained. When the polarization measuring device further includes a display unit (not shown), the polarization state of the light to be analyzed can be easily visualized by displaying the Stokes parameter of the light to be analyzed as a Poincare sphere. Further, when the polarization measuring device according to the present invention is incorporated in an optical property measuring device, the optical rotation, birefringence, Mueller matrix, etc. of the sample can be easily measured.

  In order to simplify the mathematical expression of the principle of measurement by the apparatus and method according to the present invention, a first liquid crystal variable phase retarder 21 having a principal axis orientation of 0 ° and a second liquid crystal variable phase retarder 22 having a principal axis orientation of 45 ° The case where the analyzer 30 having the main axis direction of 90 ° is used has been described. However, the principal axis direction of the first liquid crystal variable phase retarder 21 can be set to an arbitrary direction perpendicular to the traveling direction of the light to be analyzed. At this time, the main axis direction of the second liquid crystal variable phase retarder 22 is inclined by an odd multiple of 45 ° with respect to the main axis direction of the first liquid crystal variable phase retarder 21, and the main axis direction of the analyzer 30 is 45 °. The Stokes parameter of the light to be analyzed can be calculated in the same manner by setting the direction to be inclined even times.

  The formula (9) of the light intensity differs depending on the combination of the main axis direction of the second variable phaser 22 and the main axis direction of the analyzer 30 with respect to the main axis direction of the first variable phaser 21. Examples of other three combinations having different light intensity formulas are shown below as first to third modifications.

As a first modification, when the main axis direction of the first variable phase retarder 21 is 0 °, the main axis direction of the second liquid crystal variable phase retarder 22 is 45 °, and the main axis direction of the analyzer 30 is 0 °, The Stokes vector S _{OUT of the} light passing through the analyzer 30 and the light intensity I (δ ₁ (λ), δ ₂ (λ)) of each wavelength detected by the detection means 40 are expressed by the following equations.

When the birefringence phase differences δ ₁ (λ) and δ ₂ (λ) of the first and second liquid crystal variable phase shifters 21 and 22 are δ ₁ (λ) = δ ₂ (λ) = δ (λ), light The intensity I (δ (λ)) is expressed by the following equation.

As a second modification, when the main axis direction of the first variable phase shifter 21 is 0 °, the main axis direction of the second liquid crystal variable phase shifter 22 is −45 °, and the main axis direction of the analyzer 30 is 90 °. The Stokes vector S _{OUT of the} light that has passed through the analyzer 30 and the light intensity I (δ ₁ (λ), δ ₂ (λ)) of each wavelength detected by the detection means 40 are expressed by the following equations.

When the birefringence phase differences δ ₁ (λ) and δ ₂ (λ) of the first and second liquid crystal variable phase shifters 21 and 22 are δ ₁ (λ) = δ ₂ (λ) = δ (λ), light The intensity I (δ (λ)) is expressed by the following equation.

As a third modification, when the main axis direction of the first variable phase shifter 21 is 0 °, the main axis direction of the second liquid crystal variable phase retarder 22 is −45 °, and the main axis direction of the analyzer 30 is 0 °. The Stokes vector S _{OUT of the} light that has passed through the analyzer 30 and the light intensity I (δ ₁ (λ), δ ₂ (λ)) of each wavelength detected by the detection means 40 are expressed by the following equations.

Assuming that the birefringence phase difference δ ₁ (λ) = δ ₂ (λ) = δ (λ) of the first and second liquid crystal variable phase shifters 21 and 22, the light intensity I (δ (λ)) is given by the following equation: It is represented by

Therefore, also in the first to third modified examples, the Stokes parameter S ₀ (λ) of the light to be analyzed is similarly obtained from the Fourier coefficients obtained when the expressions (13), (15), and (17) are Fourier-analyzed with respect to δ. ), S ₁ (λ), S ₂ (λ), and S ₃ (λ) can be calculated. Further, even when the principal axis orientation of the first liquid crystal variable phase retarder 21 is set to an arbitrary direction other than 0 °, the principal axis orientation of the second liquid crystal variable phase retarder 22 with respect to the principal axis orientation of the first liquid crystal variable phase retarder 21 and According to the principal axis orientation of the analyzer 30, the light intensity I (δ (λ)) is analyzed by Fourier analysis based on any one of the formulas (9), (13), (15), and (17). The Stokes parameter of the target light can be calculated similarly.

The main axis direction of the analyzer 30 does not necessarily have to be a direction inclined by an even multiple of 45 ° with respect to the main axis direction of the first liquid crystal variable phase retarder 21. However, for example, when the principal axis orientation of the first liquid crystal variable phase retarder 21 is 0 ° and the principal axis orientation of the second liquid crystal variable phase retarder 22 is −45 °, the principal axis orientation of the analyzer 30 is 45 °. The expression of the intensity of the linearly polarized light that has passed through the analyzer 30 includes all of the Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ) of the light to be analyzed. Thus, the Stokes parameter of the light to be analyzed cannot be obtained. Therefore, the principal axis direction of the analyzer 30 is expressed by the Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), S _{3 of the} light to be analyzed in the formula of the light intensity of the linearly polarized light that has passed through the analyzer 30. It needs to be selected so that all of (λ) are included.

と表される。式（１８ａ）〜（１８ｄ）から、それぞれのストークスパラメータは、以下のように表される。

In order to reduce the number of samplings, the light intensity I (0), I (45) when the phase modulation amount δ of the liquid crystal variable phase shifter is 0 °, 45 °, 90 °, 135 ° instead of the method by Fourier analysis. ), I (90), I (135) can also be used to obtain Stokes parameters. Each light intensity is

It is expressed. From equations (18a) to (18d), the respective Stokes parameters are expressed as follows.

  Therefore, the Stokes parameter of the analysis target light can also be calculated by this method. Note that the voltage applied to the first and second liquid crystal variable phase shifters 21 and 22 in order to set the phase modulation amount δ to 0 °, 45 °, 90 °, and 135 ° differs depending on the wavelength. It is necessary to detect the light intensity at the phase modulation amount δ for each wavelength.

Measurement example 1
The polarization measuring apparatus shown in FIG. 1, which is an embodiment of the present invention, uses a polarizer that can rotate around the optical axis as a sample, and uses the Stokes parameters S ₀ (λ), S of the light to be analyzed that has passed through the sample. ₁ (λ), S ₂ (λ), and S ₃ (λ) were measured. The measurement was performed by rotating the polarizer as a sample by 10 °. Further, the azimuth angle ψ (λ) was obtained from the following equation (20) using the measured Stokes parameters S ₁ and S ₂ .

FIG. 7 shows a Poincare sphere in which measured Stokes parameters S ₁ (λ), S ₂ (λ), and S ₃ (λ) are plotted. FIG. 7 shows that the measured Stokes parameters S ₁ (λ), S ₂ (λ), and S ₃ (λ) are plotted along the equator of the Poincare sphere. This indicates that it is analyzed light of the Stokes parameter S ₃ is generally zero, i.e. the polarization state of light passing through the polarizer as a sample is linearly polarized light. It can also be seen that the azimuth angle of the linearly polarized light changes every measurement (each rotation of the polarizer).

FIG. 8 shows the relationship between the rotation angle of the polarizer around the optical axis and the azimuth angle ψ (λ) obtained from the measured Stokes parameters S ₁ (λ) and S ₂ (λ). FIG. 8 shows that the azimuth angle of linearly polarized light is measured with good linearity with respect to the outgoing polarization angle by the polarizer. Therefore, it can be seen that the Stokes parameters S ₁ (λ) and S ₂ (λ) of the light to be analyzed were accurately measured by the polarization measuring device of the present invention.

Measurement example 2
The polarization measuring apparatus shown in FIG. 1, which is an embodiment of the present invention, uses a liquid crystal variable phase shifter as a sample, and Stokes parameters S ₀ (λ), S ₁ (λ) of the analysis target light transmitted through the sample, S ₂ (λ) and S ₃ (λ) were measured. As the liquid crystal variable phase shifter, a nematic liquid crystal cell having an orientation direction of 45 ° was used. The measurement was performed by changing the birefringence phase difference of the liquid crystal variable phase retarder as a sample by controlling the applied voltage.

The Stokes parameters S ₀ (λ), S ₁ (λ), S ₂ (λ), and S ₃ (λ) of the light to be analyzed that has passed through the liquid crystal variable phase shifter are expressed by the following equation (21a). Using the measured Stokes parameters S ₁ (λ) and S ₃ (λ), the birefringence phase difference δ (λ) was obtained from the equation (21b). Further, the ellipticity angle χ (λ) was obtained from the equation (21c) using the measured Stokes parameters S ₀ (λ) and S ₃ (λ).

FIG. 9 shows a Poincare sphere in which the measured Stokes parameters S ₁ (λ), S ₂ (λ), and S ₃ (λ) are plotted. FIG. 9 shows that the measured Stokes parameters S ₁ (λ), S ₂ (λ), and S ₃ (λ) are plotted along the meridian of the Poincare sphere around the S ₂ axis. This indicates that the polarization state of the light to be analyzed changes between circularly polarized light and linearly polarized light.

FIG. 10 is obtained from the equation (21b) using the set value of the birefringence phase difference of the liquid crystal variable phase retarder set by controlling the applied voltage and the measured Stokes parameters S ₁ (λ) and S ₃ (λ). The relationship with the obtained birefringence phase difference is shown. FIG. 10 shows that the birefringence phase difference of the liquid crystal variable phase retarder obtained from the measurement result corresponds well with the set value. That is, it can be seen from the polarization state of the analysis target light that the optical characteristics of the sample through which the analysis target light is transmitted are accurately measured.

FIG. 11 shows the relationship between the set value of the birefringence phase difference of the liquid crystal variable phase retarder and the ellipticity angle χ (λ) derived from the measured Stokes parameters S ₀ (λ) and S ₃ (λ). From FIG. 11, it can be seen that the ellipticity angle χ (λ) is measured with good linearity. Therefore, it can be seen that the Stokes parameter S ₃ (λ) of the light to be analyzed was also accurately measured by the polarization measuring device of the present invention.

Measurement example 3
The polarization measuring apparatus shown in FIG. 1 which is an embodiment of the present invention was used, and a Stokes parameter of light to be analyzed that was transmitted through the sample was measured using a ¼ wavelength plate made of quartz with a center wavelength of 590 nm. Stokes parameter chromatic dispersion was also measured using interference filters with wavelengths of 486 nm, 590 nm, and 656 nm. Further, from the Stokes parameters measured at the respective wavelengths, the birefringence phase difference of the sample was calculated using Equation (21b) in the same manner as in Measurement Example 2.

FIG. 12 shows the birefringence phase difference of the sample calculated from the Stokes parameter measurement values at wavelengths of 486 nm, 590 nm, and 656 nm. The solid line represents the following birefringence phase difference equation, thickness d = 1.65 μm, and values of ordinary refractive index n _o (λ) and extraordinary refractive index n _e (λ) of quartz at each wavelength. It is the wavelength dispersion curve of the birefringence phase difference of the quarter wavelength plate calculated | required by the used simulation.

  The birefringence phase difference of the sample at each wavelength obtained from the measured value is on the wavelength dispersion curve of the birefringence phase difference obtained by simulation. It can be seen that the chromatic dispersion of the Stokes parameter of the light to be analyzed is accurately measured by the polarization measuring device of the present invention, whereby the wavelength dispersion of the sample through which the light to be analyzed is also accurately measured.

  The polarization measuring apparatus and the polarization measuring method according to the embodiment of the present invention have been described above, but the present invention is not limited to this.

DESCRIPTION OF SYMBOLS 10 Polarization measuring apparatus 12 Light source 14 Optical fiber 16 Collimating lens 18 Interference filter 20 Polarization modulator 21 First liquid crystal variable phase shifter 22 Second liquid crystal variable phase shifter 23 Constant temperature block 24 Temperature control unit 25 Temperature sensor 26 Peltier element 27 Peltier controller 30 Analyzer 40 Detection means 50 Control device 60 Calculation unit 70 Control signal generation unit 80 Storage unit S Sample L Optical path

Claims (6)

A polarization measuring device for measuring the polarization state of light to be analyzed,
A polarization modulator for polarization-modulating the light to be analyzed;
An analyzer for transmitting a predetermined polarization component of the polarization modulated light by the polarization modulator,
Detection means for detecting the light intensity of a predetermined polarization component transmitted through the analyzer;
A calculation unit that calculates a polarization characteristic element of the analysis target light based on the light intensity detected by the detection unit;
The polarization modulator includes a first variable phase shifter, and a second variable phase shifter disposed between the first variable phase shifter and the analyzer,
The main axis of the second variable phaser is inclined by an odd multiple of 45 ° with respect to the main axis of the first variable phaser,
The detecting means has a spectroscopic means and a light receiving means,
The said calculating part is a polarization measuring device which calculates the polarization characteristic element of each wavelength based on the relationship between the applied voltage to each variable phase shifter in each wavelength, and the birefringence phase difference of each variable phase shifter . A controller for controlling a voltage applied to each of the first and second variable phase shifters to change a birefringence phase difference between the first and second variable phase shifters;
The controller controls the applied voltage so that the birefringence phase difference of the first variable phase shifter is always equal to the birefringence phase difference of the second variable phase shifter;
The arithmetic unit Fourier-analyzes the detected light intensity by changing the birefringence phase difference of the first and second variable phase shifters, and based on the obtained Fourier coefficient, the polarization characteristic element of the analysis target light The polarization measuring device according to claim 1, wherein   The polarization measuring apparatus according to claim 1, wherein the spectroscopic means is a diffraction grating or an interference filter. A polarization measurement method for measuring the polarization state of light to be analyzed,
Polarization-modulating the light to be analyzed by a first variable phase shifter;
Polarization-modulating light polarization-modulated by the first variable phase retarder with a second variable phase retarder;
Detecting the light intensity of the linearly polarized light component transmitted through the analyzer out of the light polarized and modulated by the second variable phase shifter;
Calculating a polarization characteristic element of the light to be analyzed based on the light intensity detected by the detection means ,
The main axis of the second variable phaser is inclined by an odd multiple of 45 ° with respect to the main axis of the first variable phaser,
The detection means possess a spectral unit light receiving means,
In the calculation step, a polarization measurement method in which the calculation unit calculates a polarization characteristic element of each wavelength based on a relationship between a voltage applied to each variable phase shifter at each wavelength and a birefringence phase difference of each variable phase shifter . Controlling the voltage applied to each of the first and second variable phase shifters to change the birefringence phase difference of the first and second variable phase shifters;
In the step of controlling the applied voltage, the applied voltage is controlled so that the birefringence phase difference of the first variable phase shifter is always equal to the birefringence phase difference of the second variable phase shifter.
In the calculation step , the light intensity detected by changing the birefringence phase difference of the first and second variable phase shifters is Fourier-analyzed, and based on the obtained Fourier coefficient, the polarization characteristic of the light to be analyzed The polarization measuring method according to claim 4 , wherein the element is calculated. The polarization measuring method according to claim 4 or 5 , wherein the spectroscopic means is a diffraction grating or an interference filter.

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