A reference direction calibration method for the intensity modulation module of a channel-type polarization spectrometer

Abstract

A method for calibrating the reference direction of an intensity modulation module of a channel-type polarization spectrometer relates to the technical field of optical device calibration and solves the problem of poor accuracy of existing mechanical calibration. The method comprises: placing a light source, a collimator, a calibration polarizer, an intensity modulation module to be calibrated, and a first spectrometer on the same optical axis, with the transmission axis direction of the calibration polarizer corresponding to the 0° linear polarization direction of the linear polarizer; calculating the maximum value of the theoretical light intensity response data of the first spectrometer at different wavelengths, and selecting the wavelength at which the light intensity response reaches a local maximum; rotating the calibration polarizer and simultaneously recording the light intensity response values of the first spectrometer at different wavelengths under the transmission axis directions of the calibration polarizer; processing the light intensity response values of the first spectrometer to obtain the transmission axis direction of the calibration polarizer corresponding to the maximum light intensity response value of the first spectrometer; and using the transmission axis direction of the calibration polarizer as the reference direction of the intensity modulation module to be calibrated. The method has high calibration accuracy, a simple process, and is easy to implement in engineering.

Description

Technical Field

[0001] The present invention relates to the technical field of optical device calibration, and in particular to a reference direction calibration method for an intensity modulation module of a channel-type polarization spectrometer. Background Art

[0002] Polarization detection has outstanding advantages such as penetrating clouds and fog, identifying authenticity, weakening strong light, and intensifying weak light. New polarization spectroscopy measurement instruments have become a research hotspot. In the field of polarization spectroscopy detection, intensity-modulated polarization spectroscopy measurement technology, as an advanced polarization spectroscopy measurement technology, can perform full-static, snapshot, and full-Stokes parameter measurements on targets within a wide band. The core component of the intensity-modulated polarization spectrometer is the Polarimetric Spectral Intensity Modulation (PSIM) module, which consists of two multi-stage wave plates and a polarizer. The fast and slow axis directions and phase factors of the multi-stage wave plates will significantly affect the instrument's measurement results, so precise calibration is required. In addition to the PSIM module, the polarization effects of other optical systems in the intensity-modulated polarization spectrometer will also introduce measurement errors, which also require precise calibration.

[0003] The existing technology requires the use of linearly polarized light with specific azimuth angles (such as 0°, 22.5°, and 45°) in the calibration process of an intensity-modulated polarization spectrometer. These precise angular conditions are relative to the reference direction of the intensity modulation module. Therefore, accurately calibrating the reference direction of the intensity modulation module is a prerequisite for subsequent laboratory calibration of the instrument. In order to obtain the reference direction of the intensity modulation module, it is usually possible to leave a reference direction mark on the light-transmitting surface of the polarizer of the intensity modulation module in advance, and then transfer it multiple times during the optical system assembly process, and finally leave a corresponding direction indication on the polarization spectrometer. However, this method has the problem that the accuracy of the polarizer mark is limited. During the assembly and multi-stage transfer process, the final direction indication accuracy will become worse, making it difficult to meet the requirements of high-precision laboratory calibration. At the same time, since the intensity modulation module is installed after the assembly is completed (or installed in the optical system of the intensity-modulated polarization spectrometer), the polarization extinction method commonly used cannot meet the requirements of high-precision laboratory calibration.

[0004] Therefore, how to calibrate the reference direction of the intensity modulation module, a key component in the intensity modulation polarization spectrometer, to facilitate subsequent instrument system calibration is a technical problem that needs to be urgently solved by technical personnel in this field during the instrument development, installation and calibration process. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a reference direction calibration method for an intensity modulation module of a channel-type polarization spectrometer.

[0006] The technical solutions adopted by the present invention to solve the technical problems are as follows:

[0007] A reference direction calibration method for an intensity modulation module of a channel-type polarization spectrometer, comprising:

[0008] Step 1: Place a light source, a collimator, a calibration polarizer, an intensity modulation module to be calibrated, and a first spectrometer on the same optical axis, with the transmission axis of the calibration polarizer corresponding to the 0° linear polarization direction of the linear polarizer in the intensity modulation module; the calibration polarizer is placed on an adjustment mount, and the transmission axis direction of the calibration polarizer can be adjusted by rotating the adjustment mount;

[0009] Step 2: Calculate the theoretical light intensity response data of the first spectrometer at different wavelengths, and select the wavelength λ' at which the light intensity response reaches a local maximum value according to the theoretical light intensity response maximum value;

[0010] Step 3: Rotate the calibration polarizer and simultaneously record the light intensity response value of the first spectrometer at different transmission axis directions of the calibration polarizer for the light beam with wavelength λ';

[0011] Step 4: Process the light intensity response value of the first spectrometer obtained in step 3 to obtain the transmission axis direction of the calibration polarizer corresponding to when the light intensity response value of the first spectrometer reaches the maximum value. The transmission axis direction is the reference direction of the intensity modulation module to be calibrated.

[0012] The beneficial effects of the present invention are:

[0013] The present invention provides a method for calibrating the reference direction of an intensity modulation module of a channel-type polarization spectrometer, achieving high calibration accuracy, a simple calibration process, and ease of engineering implementation. Based on the present invention, the reference direction of the intensity modulation module can be determined, facilitating both polarization error calibration of the intensity modulation module and overall polarization radiation calibration of the intensity modulation polarization spectrometer. It also serves as a reference for the polarization direction of target light entering the instrument, providing a foundation for subsequent instrument data applications. This method has significant theoretical significance and engineering value for the high-precision calibration and quantitative application of intensity modulation polarization spectroscopy remote sensing instruments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a flow chart of a reference direction calibration method for an intensity modulation module of a channel-type polarization spectrometer of the present invention.

[0015] Figure 2 This is the structural diagram corresponding to step one.

[0016] Figure 3 This is the curve fitting diagram obtained in step 4. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0018] The present invention uses an optical method to calibrate the reference direction of the intensity modulation module. This method achieves superior calibration accuracy compared to conventional mechanical marking methods, while also simplifying the calibration process and facilitating engineering implementation. Prior art methods typically place directional markings on the light-transmitting surface of the intensity modulation module's polarizer beforehand, then transfer the markings multiple times during optical system assembly, ultimately leaving corresponding directional indications on the instrument. However, this method suffers from limited polarizer marking accuracy, further deteriorating the final directional indication accuracy during assembly and multiple transfers. Conventional polarization extinction methods also cannot be implemented even after all optical components of the intensity modulation module are installed.

[0019] A reference direction calibration method for the intensity modulation module of a channel-type polarization spectrometer, such as Figure 1 , specifically a reference direction calibration method for an intensity modulation module in an intensity modulation polarization spectrometer, comprising the following steps:

[0020] Step 1: Place the light source, collimator, calibration polarizer, intensity modulation module to be calibrated and the first spectrometer on the same optical axis, as shown in the following example: Figure 2 The transmission axis direction of the calibration polarizer corresponds to the reference direction of the intensity modulation module obtained by adjustment, that is, the transmission axis direction of the calibration polarizer is aligned (absolute alignment is not required, rough alignment is sufficient) with the reference direction of the intensity modulation module obtained by adjustment, and the reference direction is the 0° linear polarization direction of the linear polarizer in the intensity modulation module. The calibration polarizer is located on an adjustment frame; the transmission axis direction of the calibration polarizer can be adjusted by rotating the adjustment frame.

[0021] Align the optical axes of the stable light source, collimator, calibration polarizer (placed in a precision adjustment frame), intensity modulation module to be calibrated (or intensity modulation polarization spectrometer to be calibrated, i.e., polarization spectrometer with an intensity modulation module), and the first spectrometer. Place the calibration polarizer in the precision adjustment frame. Rotate the precision adjustment frame to roughly align the transmission axis direction of the calibration polarizer with the reference direction left by the intensity modulation module during the installation and adjustment process. The reference direction is the 0° linear polarization direction of the linear polarizer in the intensity modulation module. The alignment here is only for position reference and no accuracy requirement is required.

[0022] The “first” in the above-mentioned first spectrometer does not indicate order or importance, but is only used to distinguish the spectrometer used for calibration from the polarization spectrometer to be calibrated.

[0023] Step 2: Calculate the theoretical light intensity response data of the first spectrometer under light beams of different wavelengths of the light source, and select the wavelength λ' at which the light intensity response reaches a local maximum value according to the theoretical light intensity response maximum value.

[0024] As a preferred embodiment, the method comprises calculating theoretical light intensity response data of the first spectrometer at different wavelengths, observing and / or recording actual light intensity response data of the first spectrometer, and selecting a wavelength λ' at which the light intensity response reaches a local maximum based on the maximum value of the theoretical light intensity response and the maximum value of the actual light intensity response data.

[0025] By theoretically calculating and observing the light intensity response data of the spectrometer, the wavelength near the theoretically calculated value λ0 where the light intensity response reaches a local maximum is selected as the wavelength of interest, λ1. Prioritize wavelengths within the operating bands of the light source and the first spectrometer where the light intensity response is high. λ1 is a wavelength near λ0, for example, λ0-20nm < λ1 < λ0+20nm.

[0026] Step 3: Rotate the calibration polarizer and simultaneously record the light intensity response of the first spectrometer at wavelength λ' (the light beam with wavelength λ0 or λ1) at different transmission axis orientations of the calibration polarizer Pc. Use a precision adjustment mount to rotate the calibration polarizer to change the transmission axis orientation. Simultaneously record the light intensity response of the first spectrometer at wavelength λ' at different transmission axis orientations of the calibration polarizer Pc.

[0027] Step 4: Process the light intensity response value of the first spectrometer obtained in step 3 to obtain the transmission axis direction of the calibration polarizer Pc corresponding to when the light intensity response value of the first spectrometer reaches the maximum value. The transmission axis direction is the reference direction of the intensity modulation module to be calibrated.

[0028] The intensity data of wavelength λ' in the transmission axis directions of different calibration polarizers Pc are processed. By curve interpolation fitting, the transmission axis direction of the calibration polarizer Pc corresponding to the maximum intensity of wavelength λ' is found. This transmission axis direction is the reference direction of the intensity modulation module to be calibrated.

[0029] The following is a specific embodiment. A halogen tungsten lamp is used as the light source, along with a voltage-regulated power supply to ensure light source intensity stability. A transmissive collimator is used as the collimator. A Glan-Thompson prism is used as the calibration polarizer Pc. The intensity modulation module comprises a first quartz multi-stage wave plate R1, a second quartz multi-stage wave plate R2 (thickness ratio d1:d2 = 1:2), and a third polarizer P. A visible-near-infrared spectrometer is used as the first spectrometer. The first quartz multi-stage wave plate R1 has a thickness of d1, and the second quartz multi-stage wave plate R2 has a thickness of d2, with d1:d2 = 1:2.

[0030] The reference direction of the intensity modulation module described in this embodiment is the transmission axis of the third polarizer P in the intensity modulation module, which also serves as the reference direction for all angles in this embodiment. For a single polarizer, its transmission axis direction can be calibrated using polarization extinction. However, the presence of the first and second quartz multi-stage wave plates R1 and R2 makes this approach unsuitable. Therefore, this invention proposes a new optical calibration method to calibrate the reference direction.

[0031] This calibration method uses the principles of polarization optics to derive the spectral response model of an intensity modulation module with unknown azimuth error when linearly polarized light of arbitrary polarization phase angle is incident on it. The intensity response patterns of each wavelength at different polarization phase angles are analyzed, and the correlation between the transmission axis direction of the calibration polarizer and the reference direction of the intensity modulation module to be calibrated is established. The reference direction of the intensity modulation module is calibrated through data analysis. The specific process is as follows:

[0032] like Figure 1 As shown, the optical axes of the stabilized halogen tungsten lamp, collimator, intensity modulation module (or intensity modulation polarization spectrometer) to be calibrated, and visible-near-infrared spectrometer are sequentially aligned. At this point, the visible-near-infrared spectrometer records data of approximately 1 / 2·I(λ), where I(λ) is the radiance of the light source at different wavelengths. A calibration polarizer Pc, housed in a precision adjustment mount, is then placed into the optical path. The mount is rotated to roughly align the transmission axis of the calibration polarizer with the orientation mark left during the intensity modulation module assembly process.

[0033] By theoretically calculating the theoretical light intensity response data of the first spectrometer at different wavelengths

[0034] For a calibration polarizer Pc with a transmission axis direction of θ, the Stokes vector of the transmitted light is:

[0035]

[0036] Among them, I λ That is I(λ).

[0037] For the first quartz multi-order wave plate R1 with an azimuth angle of 0°+ε1 and a phase delay of δ1(λ), its Mueller matrix expression is as follows:

[0038]

[0039] For the second quartz multi-order wave plate R2 with an azimuth angle of 45°+ε2 and a phase delay of δ2(λ), its Mueller matrix expression is as follows:

[0040]

[0041] Where ε1 and ε2 are the azimuth errors of the first quartz multi-order wave plate R1 and the second quartz multi-order wave plate R2, respectively. For the third polarizer P with a transmission axis direction of 0°, its Mueller matrix expression is as follows:

[0042]

[0043] Therefore, for any incident linearly polarized light with a polarization direction of θ, the intensity data (light intensity response data) obtained by the first spectrometer is as follows:

[0044]

[0045] The 4×1 matrix in the above equation is the Stokes vector of the light entering the first spectrometer detector. The first term, 1+A, represents intensity information, which can be obtained by the first spectrometer detector. The specific formula form is detailed in the analysis below. The second, third, and fourth terms, NA, represent the polarization information of the light, which cannot be directly obtained by the detector. Therefore, the specific formula is not listed and NA is used instead.

[0046] Since ε1 and ε2 are azimuth errors, their magnitude is very small, and the second-order small sin is ignored. 2 2ε1, sin 2 2ε2, and sin2ε1·sin2ε2, while cos2ε1≈1, cos2ε2≈1. The expression of A is as follows:

[0047]

[0048] Then, the appropriate specific wavelength λ0 is calculated, and the light intensity response data curve of the visible-near-infrared spectrometer is observed. At or near the theoretical calculated value λ0, the wavelength point that can make the intensity modulation spectrum reach the local extreme value is selected as the final wavelength.

[0049] The calculation formula of λ0 is as follows: Let cosδ2(λ0)≈1, cosδ1(λ0)≈1, that is:

[0050]

[0051] Where m, n, and k are all positive integers, and Δn represents the birefringence coefficient of the multi-stage wave plate in the intensity modulation module. In this embodiment, d1 = 1.0 mm, d2 = 2.0 mm, and the spectral response range of the visible near-infrared spectrometer is 400 nm-1000 nm. According to the material parameters of quartz, Δn = 0.00906 within this wavelength range. Substituting it into formula (7) shows that within the range of 400 nm-1000 nm, the value range of k is 5-11. Taking k = 6 as an example, λ0 ≈ 755 nm.

[0052] The calibration polarizer Pc is rotated using a precision adjustment mount so that its transmission axis direction varies by approximately ±10° around the direction mark left during the intensity modulation module assembly process. The intensity response values for wavelength λ0 at different transmission axis directions of the calibration polarizer Pc are recorded. Finally, the azimuth angle θ of the calibration polarizer Pc that enables wavelength λ0 to reach its extreme point (maximum value) is calculated using curve fitting (see Figure 3). Pc , adjust the transmission axis of the calibration polarizer Pc to the azimuth angle θ through the precision adjustment frame Pc At this time, the transmission axis direction of the calibrated polarizer Pc is the reference direction of the intensity modulation module, a key component in the intensity modulation polarization spectrometer.

[0053] The calibration accuracy of the reference direction calibration method of the intensity modulation module of a channel-type polarization spectrometer of the present invention is better than that of the general mechanical marking method. At the same time, the calibration process is simple and easy to implement in engineering. The present invention utilizes the research results of this method to determine the reference direction of the intensity modulation module, which is convenient for the polarization error calibration of the intensity modulation module and the polarization radiation calibration of the intensity modulation type polarization spectrometer. It can also serve as a reference for the polarization direction of the target light entering the instrument, providing a basis for subsequent instrument data applications (such as polarization navigation, polarization extinction observation, etc.). It has important theoretical significance and engineering value for the high-precision calibration and quantitative application of intensity modulation type polarization spectrum remote sensing instruments.

[0054] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A reference direction calibration method for an intensity modulation module of a channel-type polarization spectrometer, characterized in that: include: Step 1: Place a light source, a collimator, a calibration polarizer, an intensity modulation module to be calibrated, and a first spectrometer on the same optical axis, with the transmission axis of the calibration polarizer corresponding to the 0° linear polarization direction of the linear polarizer in the intensity modulation module; the calibration polarizer is placed on an adjustment mount, and the transmission axis direction of the calibration polarizer can be adjusted by rotating the adjustment mount; Step 2: Calculate the theoretical light intensity response data of the first spectrometer at different wavelengths, and select the wavelength λ' at which the light intensity response reaches a local maximum value according to the theoretical light intensity response maximum value; Step 3: Rotate the calibration polarizer and simultaneously record the light intensity response value of the first spectrometer at different transmission axis directions of the calibration polarizer Pc for the light beam with wavelength λ'; Step 4: Process the light intensity response value of the first spectrometer obtained in step 3 to obtain the transmission axis direction of the calibration polarizer corresponding to when the light intensity response value of the first spectrometer reaches the maximum value. The transmission axis direction is the reference direction of the intensity modulation module to be calibrated.

2. The reference direction calibration method of the intensity modulation module of a channel-type polarization spectrometer according to claim 1, characterized in that: The second step is specifically as follows: calculating the theoretical light intensity response data of the first spectrometer at different wavelengths, observing and / or recording the actual light intensity response data of the first spectrometer, and selecting the wavelength λ' at which the light intensity response reaches a local maximum based on the theoretical light intensity response maximum and the actual light intensity response data.

3. The reference direction calibration method of the intensity modulation module of a channel-type polarization spectrometer according to claim 1, characterized in that: The step three specifically includes: using an adjustment stand to rotate and calibrate the polarizer, with the rotation angle range being ±10°.

4. The reference direction calibration method of the intensity modulation module of a channel-type polarization spectrometer according to claim 1, characterized in that: The step four is specifically to obtain a relationship curve between the light intensity response value of the first spectrometer and the azimuth angle of the calibration polarizer by curve interpolation fitting, and obtain the transmission axis direction of the calibration polarizer corresponding to when the light intensity response value of the first spectrometer reaches the maximum value according to the relationship curve.

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