Self-Calibration Measurement System and Method for Detecting Polarization Spectral Characteristics of Lunar Surface

Abstract

The present invention discloses a self-calibration measurement system and method applicable to the detection of lunar surface polarization spectral characteristics, belonging to the field of optical detection, including: along the propagation direction of incident light, a first achromatic quarter-wave plate, a multi-stage wave plate, a second achromatic quarter-wave plate, a beam splitting prism and two Wollaston prisms are sequentially arranged. The present invention uses the second achromatic quarter-wave plate, the beam splitting prism and two Wollaston prisms to form a polarization phase shift unit. After the incident light passes through the first achromatic quarter-wave plate and the multi-stage wave plate, the polarization information is modulated in the spectral dimension. The polarization phase shift unit shifts the modulated spectral phase by 0°, 180°, 90° and 270° to form two pairs of complementary modulated spectra. After the multi-stage wave plate is rotated by 90°, a secondary measurement is performed. The present invention can use the modulated spectra of the two measurements to extract the phase delay amount of the multi-stage wave plate in real time, and can solve the problem of the phase delay amount offset of the multi-stage wave plate caused by the temperature control fluctuation of the measurement system due to the large temperature difference on the lunar surface.

Description

Technical Field

[0001] The present invention belongs to the field of optical detection, and particularly relates to a self-calibration measurement system and method applicable to the detection of the polarization spectral characteristics of the lunar surface. Background Art

[0002] Since different minerals in lunar soil absorb the energy of specific bands of incident sunlight, its reflected hyperspectrum can reflect the mineral absorption characteristics, and the mineral composition of lunar soil can be identified by using this characteristic spectrum. Secondly, the polarization characteristics of the reflected light contain rich physical characteristics of lunar soil particles, such as particle shape, particle transparency, particle size distribution, etc. Polarization spectral detection of the reflected light on the lunar surface is an important means for inverting the mineral composition and physical properties of lunar soil.

[0003] Traditional time-modulated polarization spectral detection methods can obtain complete spectral-related Stokes parameters through time-sharing intensity measurement. However, this method is easily affected by time mismatch, thus introducing pseudo-polarization characteristics in the spectral output; and this method is mostly used in multi-spectral polarization detection, and the number of measured spectra is limited. In recent years, the channel spectral polarization technology has been proposed, which modulates the polarization information of the incident light in the spectral dimension by using two multi-stage wave plates, and can synchronously obtain the continuous polarization spectral information of the measured target, and can overcome the limitations of the spectral polarization technology based on time modulation. However, this technology is easily affected by environmental disturbances, and the most affected is the multi-stage wave plate in the modulation device. On the one hand, it is reflected in the different birefringences of the wave plate material at different temperatures. Secondly, the thickness of the wave plate is also affected by temperature, resulting in differences in the phase delay amount, and further leading to deviations in the demodulated polarization information. Facing the harsh lunar environment, especially the drastic temperature fluctuations on the lunar surface, this technology requires a highly stable temperature control system to ensure its measurement accuracy, which makes the whole system not only huge and complex, but also has high requirements for temperature control accuracy. Although the use of an achromatic wave plate to overcome the temperature effect of the device has been proposed at present, this method can only achieve complete achromatism at a certain central wavelength, and there are still residual errors introduced by temperature changes at the edge wavelengths. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a self-calibration measurement system and method applicable to lunar surface polarization spectroscopy detection. It is a linearly polarized spectroscopy detection system capable of self-calibration. Along the propagation direction of the incident light, a first achromatic quarter-wave plate, a multi-stage wave plate, a second achromatic quarter-wave plate, a beam splitting prism, and two Wollaston prisms are sequentially arranged. The present invention uses the second achromatic quarter-wave plate, the beam splitting prism, and two Wollaston prisms to form a polarization phase shift unit. After the incident light passes through the first achromatic quarter-wave plate and the multi-stage wave plate, the polarization information is modulated in the spectral dimension. The polarization phase shift unit shifts the modulation spectrum phase by 0°, 180°, 90°, and 270° to form two pairs of complementary modulation spectra. After rotating the multi-stage wave plate by 90° and performing a second measurement, the phase delay of the multi-stage wave plate can be extracted in real time using the modulation spectra of the two measurements, which can solve the problem of the offset of the multi-stage wave plate delay caused by the temperature control fluctuation of the measurement system due to the large temperature difference on the lunar surface. Through the self-calibration of the wave plate delay and the demodulation using the complementary modulation spectra, the polarization spectral information of the reflected light on the lunar surface can be obtained with high precision, which is of great significance for the inversion of the lunar soil mineral composition and physical properties.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A self-calibration measurement system applicable to lunar surface polarization spectroscopy detection includes a first achromatic quarter-wave plate, a multi-stage wave plate, a polarization phase shift unit, and a grating spectrometer sequentially arranged along the propagation direction of the incident light; the fast axis of the first achromatic quarter-wave plate is parallel to the x-axis; the fast axis of the multi-stage wave plate forms an angle of 45° with the x-axis, and the wave plate can rotate around the optical axis; the polarization phase shift unit includes a second achromatic quarter-wave plate, a beam splitting prism, a first Wollaston prism, and a second Wollaston prism sequentially arranged along the propagation direction of the incident light; the fast axis of the second achromatic quarter-wave plate is parallel to the x-axis; the two orthogonal polarization analysis angles of the first Wollaston prism are parallel and perpendicular to the x-axis respectively; the two orthogonal polarization analysis angles of the second Wollaston prism form angles of 45° and 135° with the x-axis respectively.

[0007] The present invention also provides a self-calibration measurement method applicable to lunar surface polarization spectroscopy detection, including the following steps:

[0008] Step 1: After the incident light passes through the measurement module composed of the first achromatic quarter-wave plate, the multi-stage wave plate, and the polarization phase shift unit, four light intensity components with different modulation states are received by the spectrometer; according to the polarization optical principle, the Stokes vector of the light emerging from the Wollaston prism is obtained ; at the same time, the four modulation spectra received by the grating spectrometer in the first measurement are obtained ;

[0009] Step 2: Rotate the fast axis of the multi-stage waveplate to an angle of -45° with the x-axis for the second measurement, and obtain the four modulated spectra received by the grating spectrometer in the second measurement. ;

[0010] Step 3: Calibrate the phase retardation parameter of the multi-stage waveplate in the detection system by using the modulated spectra obtained from the two measurements in Step 1 and Step 2.

[0011] Step 4: When the phase retardation of the multi-stage waveplate is known, use any two pairs of complementary modulated beams emitted by the first Wollaston prism and the second Wollaston prism to restore the intensity spectrum and polarization information of the incident light; add the two complementary modulated spectra to obtain the intensity component of the incident light, and its spectral resolution is the same as the original resolution of the grating spectrometer in the system; subtract the two complementary modulated spectra to obtain a pure modulated spectrum, and restore the polarization component information of the incident light through Fourier transform and inverse transform.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] Through two measurements in the ±45° states of the fast axis of the multi-stage waveplate and combined with the polarization phase shift unit, the present invention can extract the phase retardation of the multi-stage waveplate in real time by using the complementary modulated spectra of the two measurements. This method can well overcome the drawback that the measurement data accuracy deteriorates due to the temperature control fluctuation of the measurement system caused by the large temperature difference on the lunar surface, which leads to the deviation of the retardation of the multi-stage waveplate. At the same time, the detection system can solve the crosstalk problem of the high-frequency components of the target intensity spectrum to the modulation channel by using the complementary modulated spectrum detection method, and can obtain the hyperspectral polarization information with the original spectral resolution of the spectrometer. Combining the above two technical advantages enables the system to obtain the polarization spectral information of the reflected light on the lunar surface with high precision, which is of great significance for the inversion of the mineral composition and physical properties of lunar soil. Description of the Drawings

[0014] Figure 1 It is a schematic optical structure diagram of the self-calibration measurement system for detecting the polarization spectral characteristics of the lunar surface according to the present invention.

[0015] Among them, the reference numerals are: 1 is the first achromatic quarter-wave plate, 2 is the multi-stage waveplate, 3 is the polarization phase shift unit, 3-1 is the second achromatic quarter-wave plate, 3-2 is the beam splitting prism, 3-3 is the first Wollaston prism, 3-4 is the second Wollaston prism, and 4 is the grating spectrometer. Detailed Embodiment

[0016] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0017] As Figure 1 shown, the self-calibration measurement system applicable to the detection of lunar surface polarization spectral characteristics in the embodiment of the present invention is a device for integrated acquisition of spectrum and polarization state, and is sequentially provided with a first achromatic quarter-wave plate 1, a multi-stage wave plate 2, a polarization phase shift unit 3, and a grating spectrometer 4 from left to right according to the light direction; the polarization phase shift unit 3 includes a second achromatic quarter-wave plate 3-1, a beam splitting prism 3-2, a first Wollaston prism 3-3, and a second Wollaston prism 3-4.

[0018] The incident linearly polarized light passes through the first achromatic quarter-wave plate 1, the multi-stage wave plate 2, the second achromatic quarter-wave plate 3-1, and the beam splitting prism 3-2 in sequence, and is divided into two light beams, and then is polarized by the first Wollaston prism 3-3 and the second Wollaston prism 3-4 to form two pairs of complementary modulation light beams, which are received by the grating spectrometer 4.

[0019] Two measurements are made in two states where the fast axis of the multi-stage wave plate 2 is rotated by 45° and -45°. The phase delay amount of the multi-stage wave plate 2 can be calibrated in real time by using the complementary modulation light beams of the two measurements, and the measurement error caused by the delay amount offset of the multi-stage wave plate 2 under different temperature conditions can be avoided. At the same time, by adding a pair of complementary modulation spectra polarized by the first Wollaston prism 3-3 and the second Wollaston prism 3-4, the incident light intensity spectrum information can be directly obtained; by subtracting a pair of complementary modulation spectra, the intensity spectrum can be removed, and only the pure modulation spectrum is left, and the incident light polarization spectrum information can be obtained through Fourier transform and inverse transform. The complementary modulation spectrum detection method can overcome the crosstalk problem of the high-frequency components of the target intensity spectrum to the modulation channel, and can obtain high-spectral polarization information with the original spectral resolution of the grating spectrometer. At the same time, combined with the advantage of real-time calibration of the phase delay amount of the multi-stage wave plate 2, the robustness of the detection system and the data measurement accuracy can be guaranteed under the working environment disturbance.

[0020] Taking the optical axis as the z-axis, an xyz Cartesian coordinate system is constructed. The fast axis of the first achromatic quarter-wave plate 1 is parallel to the x-axis, the fast axis of the multi-stage wave plate 2 forms an angle of 45° with the x-axis, and the multi-stage wave plate 2 can rotate around the optical axis. The fast axis of the second achromatic quarter-wave plate 3-1 is parallel to the x-axis. The two orthogonal polarization analysis angles of the first Wollaston prism 3-3 are parallel and perpendicular to the x-axis respectively; the two orthogonal polarization analysis angles of the second Wollaston prism 3-4 form angles of 45° and 135° with the x-axis respectively. Four grating spectrometers 4 receive two pairs of complementary modulated light beams emitted from the two Wollaston prisms.

[0021] The self-calibration measurement method applicable to the detection of lunar surface polarization spectral characteristics in the embodiments of the present invention includes the following steps:

[0022] Step 1, obtaining the linear polarization Stokes vector of the detected target light beam:

[0023] The linear polarization Stokes vector of the detected target light beam is expressed as:

[0024] (1)

[0025] where is the incident light intensity component, , are the incident light linear polarization components.

[0026] After the incident light passes through the measurement module composed of the first achromatic quarter-wave plate, the multi-stage wave plate and the polarization phase shift unit, the Stokes vector of the modulated light beam detected and emitted by the Wollaston prism is:

[0027] (2)

[0028] where represents the Mueller matrix of the first achromatic quarter-wave plate, represents the Mueller matrix of the multi-stage wave plate, represents the Mueller matrix of the second achromatic quarter-wave plate, represents the Mueller matrix of the Wollaston prism. Substituting the Mueller matrices of each polarization optical device into Equation (2) can obtain the Stokes vector of the modulated light beam.

[0029] Step 2, since the grating spectrometer can only receive the intensity spectrum, extracting the first row of the Stokes vector of the modulated light beam, the four modulated spectra received by the grating spectrometer are:

[0030] (3)

[0031] where is the phase delay introduced by the multi-stage wave plate, is the birefringence of the multi-stage wave plate, is the thickness of the multi-stage wave plate, are the polarization analyzing angles of two Wollaston prisms, which are 0°, 90°, 45°, and 135° respectively; is the wavelength.

[0032] Step 3: Obtain the four modulated spectra received by the grating spectrometer in the second measurement are:

[0033] (4)

[0034] Step 4: Calibrate the delay of the multi-stage wave plate in real time using the complementary modulated spectra of the two measurements:

[0035] (5)

[0036] Or, (6)

[0037] where, 、 respectively represent the modulated spectra corresponding to the 0° and 90° polarization analyzing angles of the first Wollaston prism in the first measurement; 、 respectively represent the modulated spectra corresponding to the 45° and 135° polarization analyzing angles of the second Wollaston prism in the first measurement; 、 respectively represent the modulated spectra corresponding to the 0° and 90° polarization analyzing angles of the first Wollaston prism in the second measurement; 、 respectively represent the modulated spectra corresponding to the 45° and 135° polarization analyzing angles of the second Wollaston prism in the second measurement.

[0038] After obtaining the multi-stage wave plate delay parameter through Equation (5) or Equation (6), the target polarization spectral information can be restored using a pair of complementary modulated beams analyzed by any Wollaston prism. Add the modulated spectra corresponding to the complementary modulated beams to obtain the incident light intensity component:

[0039] (7)

[0040] Subtract the two complementary modulated beams to obtain the pure modulation information :

[0041] (8)

[0042] where is an imaginary number.

[0043] The Fourier transform of Equation (8) gives:

[0044] (9)

[0045] where, represents the function obtained after performing the Fourier transform on Equation (8), represents the independent variable in the optical path difference domain; , represents the modulation optical path difference; is the waveplate birefringence; is the waveplate thickness; the intermediate parameter , represents the Fourier transform operation; is 's complex conjugate.

[0046] It can be seen that the polarization components of the incident light are modulated on two channels, and the carrier frequencies are ± . By performing frequency-domain filtering processing at the corresponding channels, the intensity of the polarization components of the incident light can be restored:

[0047] (10)

[0048] (11)

[0049] where, represents the inverse Fourier transform operation, and Re and Im respectively represent the operations of taking the real part and the imaginary part.

Claims

1. A self-calibration measurement system applicable to the detection of the polarization spectral characteristics on the lunar surface, characterized in that It includes a first achromatic quarter-wave plate, a multi-stage wave plate, a polarization phase shift unit, and a grating spectrometer arranged in sequence along the propagation direction of the incident light; the fast axis of the first achromatic quarter-wave plate is parallel to the x-axis; the fast axis of the multi-stage wave plate forms an angle of 45° with the x-axis, and the multi-stage wave plate can rotate around the optical axis. The polarization phase shift unit includes a second achromatic quarter-wave plate, a beam splitting prism, a first Wollaston prism, and a second Wollaston prism arranged in sequence along the propagation direction of the incident light; the fast axis of the second achromatic quarter-wave plate is parallel to the x-axis; the two orthogonal polarization analysis angles of the first Wollaston prism are parallel and perpendicular to the x-axis respectively; the two orthogonal polarization analysis angles of the second Wollaston prism form angles of 45° and 135° with the x-axis respectively.

2. The self-calibration measurement system applicable to lunar surface polarization spectral characteristic detection according to claim 1, wherein The incident linearly polarized light is successively passed through the first achromatic quarter-wave plate, the multi-stage wave plate, the second achromatic quarter-wave plate, and the beam splitting prism, and then is split into two light beams.

3. The self-calibration measurement system applicable to lunar surface polarization spectral characteristic detection according to claim 2, characterized in that The two light beams are polarization-analyzed by the first Wollaston prism and the second Wollaston prism to form two pairs of complementary modulation light beams, which are received by the grating spectrometer.

4. A self-calibration measurement system applicable to lunar surface polarization spectroscopy detection according to claim 2, characterized in that Add the two pairs of complementary modulation spectra polarization-analyzed by the first Wollaston prism or the second Wollaston prism respectively to directly obtain the incident light intensity component information; subtract the two pairs of complementary modulation spectra to remove the intensity spectrum, leaving only the modulation spectrum, and obtain the incident light polarization component information through Fourier transform and inverse Fourier transform.

5. The self-calibration measurement system applicable to lunar surface polarization spectroscopy detection according to claim 1, wherein There are four grating spectrometers, which respectively receive the two pairs of complementary modulation light beams emitted by the first Wollaston prism and the second Wollaston prism.

6. A self-calibration measurement method applicable to the detection of the polarization spectral characteristics on the lunar surface, characterized in that, It includes the following steps: Step 1: After the incident light passes through the measurement module composed of the first achromatic quarter-wave plate, the multi-stage wave plate, and the polarization phase-shift unit, four light intensity components with different modulation states are obtained and received by the spectrometer; according to the polarization optical principle, the Stokes vector of the light emerging from the Wollaston prism is obtained. ; At the same time, the four modulated spectra received by the grating spectrometer in the first measurement are obtained. ; Step 2: Rotate the fast axis of the multi-stage wave plate to an angle of -45° with respect to the x-axis for the second measurement, and obtain the four modulated spectra received by the grating spectrometer in the second measurement ; Step 3: Calibrate the phase delay parameter of the multi-stage wave plate in the detection system by using the modulation spectra obtained from the two measurements in Step 1 and Step 2. Step 4: When the phase delay of the multi-stage wave plate is known, use any two pairs of complementary modulation light beams emitted by the first Wollaston prism and the second Wollaston prism to restore the intensity spectrum and polarization information of the incident light; add the two complementary modulation spectra to obtain the incident light intensity component, whose spectral resolution is the same as the original resolution of the grating spectrometer; subtract the two complementary modulation spectra to obtain a pure modulation spectrum, and restore the incident light polarization component information through Fourier transform and inverse Fourier transform.

7. A self-calibration measurement method applicable to lunar surface polarization spectral characteristic detection according to claim 6, characterized in that The Stokes vector of the light emerging from the Wollaston prism in step 1 is as follows: （1） Among them, represents the Mueller matrix of the first achromatic quarter-wave plate, represents the Mueller matrix of the multi-stage wave plate, represents the Mueller matrix of the second achromatic quarter-wave plate, represents the Mueller matrix of the Wollaston prism, represents the polarization Stokes vector of the incident light, is the intensity component of the incident light, 、 are the polarization components of the incident light; Substitute the above Mueller matrix into Equation (1), and extract the Stokes vector of the light emerging from the Wollaston prism from the first row to obtain the four modulated spectra received by the grating spectrometer in the first measurement : （2） wherein, is the phase delay introduced by the multi-stage wave plate, is the birefringence of the multi-stage wave plate, is the thickness of the wave plate, is the polarization analyzing angles of two Wollaston prisms, which are 0°, 90°, 45°, 135° respectively; is the wavelength.

8. The self - calibration measurement method applicable to lunar surface polarization spectral characteristic detection according to claim 6, characterized in that, The four modulated spectra received in the second measurement in Step 2 are as follows: （3） wherein, is the phase delay introduced by the multi-stage wave plate, is the birefringence of the multi-stage wave plate, is the thickness of the wave plate, are the polarization analyzing angles of two Wollaston prisms, which are 0°, 90°, 45°, and 135° respectively; is the wavelength.

9. A self-calibration measurement method applicable to lunar surface polarization spectral characteristic detection according to claim 8, characterized in that In the above Step 3, the calibration equation used for calibration is as follows: （4） Or (5) Among them, , respectively represent the modulation spectra corresponding to the 0° polarization angle and 90° polarization angle of the first Wollaston prism during the first measurement; , respectively represent the modulation spectra corresponding to the 45° polarization angle and 135° polarization angle of the second Wollaston prism during the first measurement; , respectively represent the modulation spectra corresponding to the 0° polarization angle and 90° polarization angle of the first Wollaston prism during the second measurement; , respectively represent the modulation spectra corresponding to the 45° polarization angle and 135° polarization angle of the second Wollaston prism during the second measurement.

10. A self-calibration measurement method applicable to lunar surface polarization spectroscopy detection according to claim 9, characterized in that In Step 4, adding the two complementary modulation spectra to obtain the incident light intensity component includes: （6） Subtracting the two complementary modulation spectra: （7） Performing Fourier transform on Equation (7) to obtain: （8） Among them, represents the function obtained after performing the Fourier transform on Equation (8), represents the independent variable in the optical path difference domain; , represents the modulation optical path difference; is the birefringence of the wave plate; is the thickness of the wave plate; intermediate parameter , represents the Fourier transform operation; is the complex conjugate of; i represents the imaginary number; Restoring the incident light Stokes vector spectrum by using the two-channel information in the optical path difference domain, performing optical path difference domain filtering at the corresponding channels by using a filtering function, and performing inverse Fourier transform to restore the incident light polarization component: （9） （10） wherein, represents the inverse Fourier transform operation, Re and Im respectively represent the operations of taking the real part and the imaginary part, , are the polarization components of the incident light.

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