Calibration method and system of polarization measuring instrument

Abstract

The invention relates to the technical field of optical equipment, and discloses a method and a system for calibrating a polarization measuring instrument, so as to improve the calibration precision. The method comprises the following steps: determining an iteration step length for adjusting an included angle between a polarizing film and a polarization analyzing film; in each iteration process, acquiring a light intensity change curve which is acquired by a detector and corresponds to a circle of rotation of the wave plate, performing fast Fourier transform on the light intensity change curve to obtain a group of coefficients, and calculating an R value corresponding to an included angle between a polarization polarizing film and a polarization analyzing polarizing film in each iteration process according to the group of coefficients; drawing a curve of the included angles and the R values of the polarizing film and the polarization analyzing film, searching two included angles which are separated by 90 degrees and have the difference of the R values closest to 0 in the curve, and determining that the included angle, corresponding to the two included angles, of the polarizing film and the polarization analyzing film is + / -45 degrees; with the included angle of + / -45 degrees between the polarizing film and the polarization analyzing film as the reference, the polarization analyzing film is rotated to be parallel to the polarizing film as the reference.

Description

Technical Field

[0001] The present invention relates to the technical field of optical equipment, and in particular to a calibration method and system for a polarimeter. Background Art

[0002] The polarization angle of the polarization meter is based on the angle of the built-in polarizer of the instrument.

[0003] After the polarimeter is assembled, it needs to be calibrated based on the calibration tool before leaving the factory. During this process, the polarization angle of the built-in analyzer of the polarimeter needs to be measured with an external polarizer to make the polarization angle parallel to the analyzer angle, and the error does not exceed ±0.05°.

[0004] However, in a polarimeter, if a wave plate is placed before the built-in polarizer, combined with the noise interference from the tooling platform and the built-in detector of the polarimeter, how to accurately collect the calibration parameters and achieve calibration becomes a thorny technical problem.

[0005] According to the principle of polarimeter, if the incident light is linearly polarized light, the PD detector will obtain a sine curve after the wave plate rotates one circle. The trough position is theoretically 0 when and only when the polarization source is perpendicular to the polarization analyzer, and the fast axis of the wave plate rotates to be parallel or perpendicular to the polarization source angle. Therefore, when the polarization source angle is rotated slowly to make the curve trough reach the lowest, the polarization source angle is perpendicular to the polarization analyzer angle, and this is the extinction method to find the polarization angle.

[0006] The extinction method measures the extreme point of the trough of the Malus curve, where the slope is 0. Since the stability of the incident laser and PD is not high, high-precision angle recognition cannot be achieved in actual measurement. The trough where the polarization angle of the polarizer is close to 0° (such as -0.1°0.15°, etc.) is closer to the extreme point of the trough than 0°. Therefore, when using the extinction method to find the polarization angle, under normal circumstances, the accuracy is difficult to reach ±0.1°. In addition, in order to ensure that the incident light intensity is not affected by the polarization angle as much as possible, it is also necessary to ensure that the incident light is perfectly circularly polarized light or completely non-polarized light, which brings extremely stringent challenges to the construction of the calibration environment. Summary of the invention

[0007] The present invention aims to disclose a calibration method and system for a polarimeter to improve the calibration accuracy.

[0008] To achieve the above object, in the method disclosed in the present invention, the polarization measuring instrument includes a rotatable wave plate located in front of the incident light beam of the polarizer and a detector located after the polarizer. The method of the present invention includes:

[0009] Step S1, determining an iterative step length for adjusting the angle between the polarizing polarizer and the analyzing polarizer;

[0010] Step S2: In each iteration process, the light intensity variation curve corresponding to one rotation of the wave plate collected by the detector is obtained, and the following five coefficients are obtained after fast Fourier transformation of the light intensity variation curve:

[0011] A0=0.5*S0-0.25*(cos(delta)+1)*S1;

[0012] A2=0.5*S3*sin(delta)*sin(2*fai0);

[0013] B2=0.5*S3*sin(delta)*cos(2*fai0);

[0014] A4=0.25*(cos(delta)-1)*S2*sin(4*fai0)+0.25*(cos(delta)-1)*S1*cos(4*fai0);

[0015] B4=0.25*(cos(delta)-1)*S2*cos(4*fai0)-0.25*(cos(delta)-1)*S1*sin(4*fai0);

[0016] Among them, [S0 S1 S2 S3] is the Stokes vector of the incident light beam emitted by the polarizing plate, delta and fai0 are the phase delay and initial fast axis angle of the rotating wave plate, respectively, and fai is the actual angle of the wave plate; and the light intensity I0 is a function of fai, and the function form according to the Fourier series expansion is:

[0017] I0=A0+A2*cos(2*fai)+B2*sin(2*fai)+A4*cos(4*fai)+B4*sin(4*fai);

[0018] Step S3, calculating the R value corresponding to the angle between the polarizing polarizer and the analyzing polarizer in each iteration process, the calculation formula is:

[0019]

[0020] Step S4, drawing a curve of the angle between the polarizing polarizer and the analyzing polarizer and the R value, in the curve, finding two angles separated by 90° and with R values ​​closest to 0, and determining that the angle between the polarizing polarizer and the analyzing polarizer corresponding to the two angles is in a relationship of ±45°;

[0021] Step S5, taking the angle between the polarizing polarizer and the analyzing polarizer as ±45° as a reference, rotating the analyzing polarizer to be parallel to the polarizing polarizer serving as the reference.

[0022] Preferably, the accuracy of the motors for rotating the wave plate and the analyzing polarizer is 0.02°.

[0023] Preferably, a circular polarizer is disposed before the polarizing polarizer.

[0024] To achieve the above object, the present invention further discloses a calibration system for a polarimeter, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the above method when executing the computer program.

[0025] The present invention has the following beneficial effects:

[0026] Based on the ±45° relationship determined by the present invention, the angle between the polarizing polarizer and the analyzing polarizer is significantly different from other close points, which is easy to distinguish. Thus, on the platform of the calibration tooling, the present invention can adjust the angle between the polarizing polarizer and the analyzing polarizer within ±0.1° based on the reference ±45° relationship, thereby building an extremely strict calibration environment, and thus greatly improving the calibration accuracy.

[0027] The present invention will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0029] Figure 1 It is a diagram of an optical path structure of a calibration system of a polarization measuring instrument disclosed in an embodiment of the present invention.

[0030] Figure 2 It is a flow chart of a calibration method of a polarization measuring instrument disclosed in an embodiment of the present invention.

[0031] Figure 3 It is a schematic diagram of the relationship between the R value and the angle between the polarization source and the polarization analyzer disclosed in the embodiment of the present invention.

[0032] Figure 4 is based on Figure 3 Find a schematic diagram of two angles that are 90° apart and whose R values ​​are closest to 0, and the angle between the two points connected by the same dotted line in the diagram differs by 90°. DETAILED DESCRIPTION

[0033] The embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0034] Example 1

[0035] This embodiment discloses a calibration method for a polarization measuring instrument, which is applied to Figure 1 The polarimeter calibration system shown is used to improve the calibration accuracy.

[0036] exist Figure 1 In the present invention, the polarimeter includes a rotatable wave plate located in front of the incident light beam of the polarizer and a detector (PD) located after the polarizer; the tooling platform for calibrating the polarimeter also includes a polarizing polarizer and a circular polarizer outside the polarimeter.

[0037] like Figure 2 As shown, the method of this embodiment includes the following steps:

[0038] Step S1, determining an iterative step length for adjusting the angle between the polarizing polarizer and the analyzing polarizer.

[0039] Step S2: in each iteration process, obtain the light intensity variation curve corresponding to one rotation of the wave plate collected by the detector, and perform fast Fourier transform on the light intensity variation curve to obtain a set of coefficients.

[0040] In this step, let fai be the actual angle of the wave plate, then the light intensity I0 is a function of fai, and the function form according to the Fourier series expansion is:

[0041] I0=A0+A2*cos(2*fai)+B2*sin(2*fai)+A4*cos(4*fai)+B4*sin(4*fai).

[0042] in:

[0043] A0=0.5*S0-0.25*(cos(delta)+1)*S1;

[0044] A2=0.5*S3*sin(delta)*sin(2*fai0);

[0045] B2=0.5*S3*sin(delta)*cos(2*fai0);

[0046] A4=0.25*(cos(delta)-1)*S2*sin(4*fai0)+0.25*(cos(delta)-1)*S1*cos(4*fai0);

[0047] B4=0.25*(cos(delta)-1)*S2*cos(4*fai0)-0.25*(cos(delta)-1)*S1*sin(4*fai0);

[0048] In the above calculation formula, [S0, S1, S2, S3] is the Stokes vector of the light beam incident on the wave plate emitted by the polarizing plate (well-known alternative expressions include but are not limited to [I, Q, U, V], etc.), delta and fai0 are the phase delay and initial fast axis angle of the rotating wave plate, respectively.

[0049] In this step, each group of coefficients is obtained by FFT of data sampled by the instrument for one cycle, which has excellent stability and accuracy.

[0050] Step S3, calculating the R value corresponding to the angle between the polarizing polarizer and the analyzing polarizer in each iteration process.

[0051] In this step, the calculation formula is:

[0052]

[0053] In this step, R is the ratio of two numbers, which is linearly related to the light intensity and can eliminate the influence of light intensity changes on the measurement results.

[0054] Step S4, draw a curve of the angle between the polarizing polarizer and the analyzing polarizer and the R value, in the curve, find two angles that are 90° apart and whose R value difference is closest to 0, and determine that the angle between the polarizing polarizer and the analyzing polarizer corresponding to the two angles is ±45°.

[0055] In this step, the curve of the angle between the polarizing polarizer and the analyzing polarizer and the R value is as follows: Figure 3 As shown, it is obvious that the R value and the polarization angle show regular changes.

[0056] exist Figure 3 In the extreme point neighborhood, it is still difficult to accurately locate the extreme point due to the influence of laser power fluctuation and PD noise. The curve is symmetrical about the extreme point; therefore, in order to find the extreme point more accurately, two points adjacent to each other at 90° can be found on the curve. If and only if the two points have the same ordinate, the center point of the abscissa of the two points is the corresponding extreme point.

[0057] exist Figure 3 In the figure, the curve is symmetrical according to the peak position. This means that the values ​​of the two points at ±45° from the peak are equal. For this reason, the curve can be further processed as follows:

[0058] The value of each point is subtracted from the value of the next point separated by 90°, and then the absolute value is taken to obtain a new curve. In this new curve, the point closest to 0 is the point of ±45°, that is, Figure 4 The intersection of the dotted line parallel to the horizontal axis and the R value curve is shown, and the other adjacent points have obvious differences from the aforementioned value closest to 0.

[0059] Step S5, taking the angle between the polarizing polarizer and the analyzing polarizer as ±45° as a reference, rotating the analyzing polarizer to be parallel to the polarizing polarizer serving as the reference.

[0060] Preferably, the precision of each motor for rotating the wave plate and the polarizer is 0.02°. Therefore, the iterative compensation determined in step S1 is also 0.02°, and the polarization angle between the polarizer and the analyzer can be well controlled within the range of ±0.1° based on this embodiment.

[0061] Example 2

[0062] This embodiment discloses a calibration system for a polarimeter, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the above method is implemented.

[0063] In summary, the methods and systems disclosed in the above embodiments of the present invention respectively determine that the angle between the polarizing polarizer and the analyzing polarizer is ±45°, which is significantly different from other close points and is easy to distinguish. Thus, on the platform of the calibration tooling, the present invention can adjust the angle between the polarizing polarizer and the analyzing polarizer within ±0.1° based on the reference ±45° relationship, thereby building an extremely strict calibration environment, thereby greatly improving the accuracy of the calibration.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calibrating a polarimeter, wherein the polarimeter comprises a rotatable wave plate located in front of an incident light beam of an analyzer polarizer and a detector located behind the analyzer polarizer, characterized in that: include: Step S1, determining an iterative step length for adjusting the angle between the polarizing polarizer and the analyzing polarizer; Step S2: In each iteration process, the light intensity variation curve corresponding to one rotation of the wave plate collected by the detector is obtained, and the following five coefficients are obtained after fast Fourier transformation of the light intensity variation curve: A0=0.5*S0-0.25*(cos(delta)+1)*S1; A2=0.5*S3*sin(delta)*sin(2*fai0); B2=0.5*S3*sin(delta)*cos(2*fai0); A4=0.25*(cos(delta)-1)*S2*sin(4*fai0)+0.25*(cos(delta)-1)*S1*cos(4*fai0); B4=0.25*(cos(delta)-1)*S2*cos(4*fai0)-0.25*(cos(delta)-1)*S1*sin(4*fai0); Among them, [S0 S1 S2 S3] is the Stokes vector of the incident light beam emitted by the polarizing plate, delta and fai0 are the phase delay and initial fast axis angle of the rotating wave plate, respectively, and fai is the actual angle of the wave plate; and the light intensity I0 is a function of fai, and the function form according to the Fourier series expansion is: I0=A0+A2*cos(2*fai)+B2*sin(2*fai)+A4*cos(4*fai)+B4*sin(4*fai); Step S3, calculating the R value corresponding to the angle between the polarizing polarizer and the analyzing polarizer in each iteration process, the calculation formula is: Step S4, drawing a curve of the angle between the polarizing polarizer and the analyzing polarizer and the R value, in the curve, finding two angles separated by 90° and with R values ​​closest to 0, and determining that the angle between the polarizing polarizer and the analyzing polarizer corresponding to the two angles is in a relationship of ±45°; Step S5, taking the angle between the polarizing polarizer and the analyzing polarizer as ±45° as a reference, rotating the analyzing polarizer to be parallel to the polarizing polarizer serving as the reference.

2. The method according to claim 1, characterized in that The motor that rotates the wave plate has an accuracy of 0.02°.

3. The method according to claim 1, characterized in that The motor that rotates the polarizing plate has an accuracy of 0.02°.

4. The method according to any one of claims 1 to 3, characterized in that: A circular polarizer is disposed in front of the polarizing polarizer.

5. A calibration system for a polarimeter, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the method according to any one of claims 1 to 4 is implemented.

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