Self-calibration measurement system and method suitable for lunar surface polarization spectrum characteristic detection

Abstract

The invention discloses a self-calibration measurement system and method suitable for lunar surface polarization spectrum characteristic detection, and belongs to the field of optical detection, and the system comprises a first achromatic 1 / 4 wave plate, a multi-stage wave plate, a second achromatic 1 / 4 wave plate, a beam splitter prism and two Wollaston prisms which are sequentially arranged along the propagation direction of incident light. A second achromatic 1 / 4 wave plate, a beam splitter prism and two Wollaston prisms are utilized to form a polarization phase shift unit. After incident light passes through the first achromatic 1 / 4 wave plate and the multi-stage wave plate, polarization information is modulated in a spectrum dimension, the polarization phase shifting unit shifts the phase of a modulation spectrum by 0 degree, 180 degrees, 90 degrees and 270 degrees, and two pairs of complementary modulation spectrums are formed. And rotating the multi-stage wave plate for 90 degrees and then carrying out secondary measurement. According to the method, the phase retardation of the multistage wave plate can be extracted in real time by using the modulation spectrum measured twice, and the problem of multistage wave plate retardation offset caused by temperature control fluctuation of a measurement system due to large temperature difference of the lunar surface can be solved.

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 suitable for detecting the polarization spectral characteristics of the lunar surface. Background Technique

[0002] Since different minerals in lunar soil absorb the energy of specific bands of incident sunlight, their 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 to invert 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. The channel spectral polarization technology proposed in recent years uses two multi-stage wave plates to modulate the polarization information of the incident light in the spectral dimension, and can synchronously obtain the continuous polarization spectral information of the measured target, which 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, which in turn leads to deviations in the demodulated polarization information. Facing the harsh lunar environment, especially the severe temperature fluctuations on the lunar surface, this technology requires a highly stable temperature control system to ensure its measurement accuracy, which makes the entire system not only large 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-calibrating measurement system and method suitable for 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 modulated spectral phase by 0°, 180°, 90°, and 270° to form two pairs of complementary modulated 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 modulated spectra of the two measurements, which can solve the problem of the phase delay 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. Through the self-calibration of the wave plate delay and the demodulation using complementary modulated spectra, high-precision acquisition of the polarization spectral information of the lunar surface reflected light can be achieved, which is of great significance for the inversion of lunar soil mineral composition and physical properties.

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

[0006] A self-calibrating measurement system suitable for lunar surface polarization spectroscopy detection, including 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-calibrating measurement method suitable for 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 modulated spectra received by the grating spectrometer in the first measurement are obtained ;

[0009] Step 2: Rotate the fast axis of the multi-stage wave plate 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 delay parameter of the multi-stage wave plate 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 delay of the multi-stage wave plate 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 simple 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 state where the fast axis of the multi-stage wave plate is ±45°, combined with the polarization phase shift unit, the present invention can extract the phase delay of the multi-stage wave plate in real time by using the complementary modulated spectra of the two measurements. This method can well overcome the drawback that the delay 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, which in turn leads to the deterioration of the measurement data accuracy. 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 spectrum information of the lunar surface reflected light 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 applicable to the detection of lunar surface polarization spectral characteristics of the present invention.

[0015] Among them, the reference numerals are: 1 is the first achromatic quarter-wave plate, 2 is the multi-stage wave plate, 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 clear and understandable, 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 lunar surface polarization spectral characteristic detection in the embodiment of the present invention is a device for integrated acquisition of spectrum and polarization state, and is successively 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 analyzed 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 taken when the fast axis of the multi-stage wave plate 2 is rotated to two states of 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, adding a pair of complementary modulation spectra analyzed by the first Wollaston prism 3-3 and the second Wollaston prism 3-4 can directly obtain the incident light intensity spectrum information; subtracting a pair of complementary modulation spectra can remove the intensity spectrum and leave only the pure modulation spectrum, 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 condition of 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 makes 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 are at 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 lunar surface polarization spectral characteristic detection according to the embodiment 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] Among them, 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 by the Wollaston prism and emitted is is:

[0027] (2)

[0028] 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. 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 can be obtained are:

[0030] (3)

[0031] Among them, 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: Use the complementary modulated spectra of the two measurements to perform real-time calibration on the delay of the multi-stage wave plate:

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

[0038] After obtaining the delay parameter of the multi-stage wave plate through Equation (5) or Equation (6), the target polarization spectral information can be restored by using a pair of complementary modulated beams polarized 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] Performing a Fourier transform on Equation (8) gives:

[0044] (9)

[0045] where represents the function obtained after performing a Fourier transform on Equation (8), represents the independent variable in the optical path difference domain; , represents the modulated optical path difference; is the birefringence of the waveplate; is the thickness of the waveplate; 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, with carrier frequencies of ± . By performing frequency-domain filtering 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 suitable for detecting polarization spectrum characteristics of the lunar surface, characterized in that: It includes a first achromatic 1 / 4 wave plate, a multi-order wave plate, a polarization phase shift unit, and a grating spectrometer which are sequentially arranged along the propagation direction of the incident light; the fast axis of the first achromatic 1 / 4 wave plate is parallel to the x-axis; the fast axis of the multi-order wave plate forms an angle of 45° with the x-axis, and the multi-order wave plate can rotate around the optical axis; The polarization phase shift unit comprises a second achromatic 1 / 4 wave plate, a beam splitter prism, a first Wollaston prism and a second Wollaston prism which are sequentially arranged along the propagation direction of the incident light; the fast axis of the second achromatic 1 / 4 wave plate is parallel to the x-axis; the two orthogonal analysis angles of the first Wollaston prism are parallel and perpendicular to the x-axis respectively; the two orthogonal analysis angles of the second Wollaston prism are respectively at an angle of 45° and 135° to the x-axis.

2. A self-calibration measurement system suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 1, characterized in that: The incident linear polarized light passes through the first achromatic 1 / 4 wave plate, the multi-order wave plate, the second achromatic 1 / 4 wave plate, and the beam splitting prism in sequence, and is then split into two beams.

3. A self-calibration measurement system suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 2, characterized in that: The two light beams are polarized by the first Wollaston prism and the second Wollaston prism to form two pairs of complementary modulated light beams, which are received by the grating spectrometer.

4. A self-calibration measurement system suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 2, characterized in that: A pair of complementary modulation spectra respectively analyzed by the first Wollaston prism or the second Wollaston prism are added together to directly obtain the intensity component information of the incident light; a pair of complementary modulation spectra are subtracted to remove the intensity spectrum, leaving a pure modulation spectrum, and the polarization component information of the incident light is obtained through Fourier transform and inverse transform.

5. The self-calibration measurement system for detecting polarization spectrum characteristics of the lunar surface according to claim 1, characterized in that: There are four grating spectrometers, which respectively receive two pairs of complementary modulated light beams emitted by the first Wollaston prism and the second Wollaston prism.

6. A self-calibration measurement method suitable for detecting polarization spectrum characteristics of the lunar surface, characterized in that: The steps include: 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 principle of polarization optics, the Stokes vector of the Wollaston prism output light is obtained. ; At the same time, the four-beam modulation spectrum received by the grating spectrometer in the first measurement is obtained ; Step 2: Rotate the fast axis of the multi-stage wave plate to an angle of -45° with the x-axis for a second measurement to obtain the four-beam modulated spectrum received by the grating spectrometer for the second measurement. ; Step 3, using the modulation spectra obtained from the two measurements in step 1 and step 2 to calibrate the phase delay parameters of the multi-stage wave plate in the detection system; Step 4: When the phase delay of the multi-stage wave plate is known, use any two pairs of complementary modulated 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 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; subtract the two complementary modulated spectra to obtain a simple modulation spectrum, and restore the polarization component information of the incident light through Fourier transform and inverse transform.

7. A self-calibration measurement method suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 6, characterized in that: The Stokes vector of the light emitted from the Wollaston prism in step 1 for: （1） in, represents the first achromatic quarter-wave plate Mueller matrix, represents the multi-level wave plate Mueller matrix, represents the second achromatic quarter-wave plate Mueller matrix, represents the Wollaston prism-Mueller matrix, represents the Stokes vector of the incident light polarization, is the incident light intensity component, , is the polarization component of the incident light; Substitute the above Mueller matrix into equation (1) and extract the Stokes vector of the Wollaston prism output light: The first line of the grating spectrometer obtains the four-beam modulation spectrum received in the first measurement. : （2） in, is the phase delay introduced by the multi-stage wave plate, is the birefringence of the multi-stage wave plate, is the wave plate thickness, The analysis angles of the two Wollaston prisms are 0°, 90°, 45°, and 135° respectively; is the wavelength.

8. The self-calibration measurement method for detecting polarization spectrum characteristics of the lunar surface according to claim 6, characterized in that: The four-beam modulation spectrum received in the second measurement in step 2 for: （3） in, is the phase delay introduced by the multi-stage wave plate, is the birefringence of the multi-stage wave plate, is the wave plate thickness, The analysis angles of the two Wollaston prisms are 0°, 90°, 45°, and 135° respectively; is the wavelength.

9. A self-calibration measurement method suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 8, characterized in that: In step 3, the calibration equation used for calibration is as follows: （4） or (5) in, , They represent the modulation spectra corresponding to the first Wollaston prism's 0° analyzer angle and 90° analyzer angle in the first measurement respectively; , They represent the modulation spectra corresponding to the second Wollaston prism's 45° analyzing angle and 135° analyzing angle in the first measurement respectively; , They represent the modulation spectra corresponding to the first Wollaston prism's 0° analysis angle and 90° analysis angle in the second measurement respectively; , They respectively represent the modulation spectra corresponding to the second Wollaston prism's 45° analysis angle and 135° analysis angle during the second measurement.

10. A self-calibration measurement method suitable for detecting polarization spectrum characteristics of the lunar surface according to claim 9, characterized in that: In step 4, the incident light intensity component is obtained by adding the two complementary modulation spectra, which comprises: （6） Subtract two complementary modulation spectra: （7） Performing Fourier transform on equation (7) yields: （8） in, represents the function obtained by Fourier transforming equation (8), represents the independent variable in the optical path difference domain; , represents the modulated optical path difference; is the wave plate birefringence; is the wave plate thickness; intermediate parameter , represents Fourier transform operation; for The complex conjugate of ; i represents an imaginary number; The two channel information of the optical path difference domain is used to restore the Stokes vector spectrum of the incident light. The optical path difference domain is filtered using the filter function at the corresponding channel, and the inverse Fourier transform is performed to restore the polarization component of the incident light: （9） （10） in, Represents the inverse Fourier transform operation, Re and Im represent the real part and imaginary part operations respectively. , is the polarization component of the incident light.

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