A wide-spectrum polarization feature detection system and method based on a polarization grating

Abstract

This invention discloses a broadband polarization feature detection system and method based on a polarization grating. The system includes: a polarization state generator for generating broadband polarized light to be measured; a polarization grating for splitting the broadband polarized light into left-handed and right-handed circularly polarized light, and causing dispersion in the horizontal direction; a lens for imaging the intensity pattern formed after beam splitting and dispersion; a CMOS camera for acquiring the imaged intensity pattern; and a terminal device for analyzing and processing the acquired intensity pattern to obtain the spectral intensity ratio of left-handed and right-handed circularly polarized light in the beam to be measured, and calculating the spectral ellipticity and spectral polarization direction of the beam to be measured. This invention features a simple and compact structure, good stability, and fast measurement speed.

Description

Technical Field

[0001] This invention relates to the field of polarization detection technology, and more specifically, to a broadband polarization feature detection system and method based on a polarization grating. Background Technology

[0002] Polarization direction and polarization ellipticity are two main polarization characteristics of polarized light. Rapid and accurate measurement of their relationship with wavelength, i.e., spectral polarization direction and spectral polarization ellipticity, is of great significance in fields such as navigation, chiral substance measurement, protein conformation research, medicine, and chemistry.

[0003] Traditional methods for measuring polarization direction and polarization ellipticity are mostly based on time-modulated polarization detection schemes. These schemes utilize rotating polarizers, electro-optic, acousto-optic, and magneto-optic modulators to achieve time-series modulation of the polarization state. The measured light intensity information is then analyzed to obtain the polarization direction and / or polarization ellipticity. These methods suffer from drawbacks such as complex system structures, slow measurement speeds, and poor stability. In recent years, with advancements in micro / nano fabrication technology, polarization direction measurement methods based on polarization cameras and angular (radial) polarizers have emerged. Furthermore, some specially designed metamaterials can separate left- and right-hand circularly polarized light in the measured light, thereby enabling the measurement of polarization ellipticity. Polarization feature detection methods based on micro / nano devices can effectively overcome the shortcomings of traditional methods, achieving polarization feature detection with a simple and compact structure. However, like traditional methods, for polarization feature detection over a wide spectral range, they still rely on wavelength scanning, resulting in slow measurement speeds. The use of monochromators, spectrometers, and other devices also makes the measurement system complex and bulky.

[0004] In summary, it is necessary to study a broadband polarization feature detection system and method based on a polarization grating that has a simple and compact structure, good stability, and fast measurement speed. Summary of the Invention

[0005] The purpose of this invention is to provide a broadband polarization feature detection system and method based on a polarization grating to overcome the defects of the prior art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A broadband polarization feature detection system based on a polarization grating includes:

[0008] A polarization state generator is used to generate broadband polarized light to be measured.

[0009] A polarization grating is used to split the broadband polarized light to be measured into two beams of left-handed and right-handed circularly polarized light, and to cause dispersion in the horizontal direction.

[0010] Lenses are used to image the intensity patterns formed after beam splitting and dispersion.

[0011] A CMOS camera is used to capture the light intensity pattern of an image.

[0012] And a terminal device, used to analyze and process the acquired light intensity pattern to obtain the ratio of the left and right circularly polarized spectral intensity in the beam to be tested, and to calculate the spectral ellipticity and spectral polarization direction of the polarized beam to be tested.

[0013] Furthermore, the polarization state generating device PSG includes a slit S, a polarizer P, and a quarter-wave plate Q arranged in sequence. The slit S is used to convert broadband light from the source into a narrow vertical beam, and the polarizer P and the quarter-wave plate Q are used to generate broadband polarized light to be measured.

[0014] Furthermore, it also includes an achromatic quarter-wave plate located at the front end of the polarization grating with its fast axis along the horizontal direction. This achromatic quarter-wave plate encodes the polarization direction information of the beam under test into the polarization ellipticity, and calculates the spectral polarization direction information of the beam under test by combining the spectral ellipticity ε'(λ) measured in the current state with the spectral ellipticity ε(λ) measured when the achromatic wave plate is not placed.

[0015] Furthermore, the x-direction of the polarization grating is parallel to the horizontal direction.

[0016] Furthermore, the formula for calculating the spectral ellipticity ε(λ) of the polarized beam under test is as follows:

[0017]

[0018] In the formula, λ is the wavelength, and I L (λ) and I R ε(λ) represents the intensity of the left-hand circularly polarized component and the right-hand circularly polarized component of the spectral ellipticity ε(λ) at wavelength λ. When ε(λ)>0, it represents right-hand circularly polarized light; when ε(λ)<0, it represents left-hand circularly polarized light; when ε(λ)=±1, it represents both right-hand and left-hand circularly polarized light; and when ε(λ)=0, it represents linearly polarized light.

[0019] Furthermore, the formula for calculating the spectral polarization direction α() of the polarized beam to be measured in step S3 is as follows:

[0020]

[0021] The present invention also provides a method for detecting broadband polarization features based on the above-described polarization grating, comprising the following steps:

[0022] S1. Construct the broadband polarization feature detection system based on the polarization grating;

[0023] S2. Monochromatic light of different wavelengths is incident on a polarizing grating, and the wavelength λ and its corresponding pixel position are fitted using the following formula to complete the calibration of each parameter and obtain the relationship between wavelength λ and pixel:

[0024]

[0025] In the formula, λ is the wavelength of the image captured by the CMOS camera, and μ = d λ / x λ d represents the spatial position of the camera during imaging. λ to pixel position x λ The conversion factor, where P is the period of the polarization grating;

[0026] S3. Acquire light intensity images by using a CMOS camera to capture light intensity images corresponding to multiple wavelengths;

[0027] S4. Process the light intensity image obtained in step S3 to obtain the pixel position corresponding to each wavelength.

[0028] S5. Linearly polarized light in the horizontal direction is incident on the polarization grating to obtain a light intensity image. The light intensity image of the linearly polarized light in the horizontal direction is then analyzed and processed to complete the calibration of the diffraction efficiency of the polarization grating.

[0029] Compared with the prior art, the advantages of the present invention are: the present invention has a simple and compact structure, good stability and fast measurement speed. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the structure of a polarization grating.

[0032] Figure 2 This is a conceptual diagram of left- and right-hand circular polarization beam splitting and dispersion of a polarization grating.

[0033] Figure 3 This is a schematic diagram of the structure of the broadband polarization feature detection system based on a polarization grating according to the present invention.

[0034] Figure 4 This is a schematic diagram illustrating the geometric relationship between dispersion and imaging.

[0035] Figure 5 It is a spectral intensity map of left- and right-hand circular polarization obtained from the light intensity image.

[0036] Figure 6 These are light intensity images after placing bandpass filters of different wavelengths.

[0037] Figure 7 It is a pixel location map corresponding to each wavelength.

[0038] Figure 8 This is a graph showing the correspondence between wavelength and pixels.

[0039] Figure 9 This is a polarization grating diffraction efficiency calibration diagram, where (a) is the light intensity image of broadband linearly polarized light in the 0° and 90° directions; (b) is the light intensity ratio of the left and right circularly polarized components of the linearly polarized light in the 90° direction before and after calibration.

[0040] Figure 10 These are light intensity images of six different broadband polarized lights.

[0041] Figure 11 These are intensity distribution curves for six different broadband polarized lights.

[0042] Figure 12 These are the measured and theoretical values ​​of the spectral ellipticity for six different broadband polarized lights.

[0043] Figure 13 (a) shows the light intensity images of left- and right-handed quartz crystals; (b) shows the optical rotation dispersion curves of left- and right-handed quartz crystals based on theory and fitting.

[0044] Figure 14 (a) shows the optical rotation and dispersion curves of five different concentrations of glucose solutions; (b) shows the linear fitting results of the areas enclosed by the optical rotation and dispersion curves of the five different concentrations of glucose solutions and the horizontal axis. Detailed Implementation

[0045] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0046] The structure of a polarization grating is as follows Figure 1 As shown, in the liquid crystal polymer layer, the fast axis of the liquid crystal molecules exhibits a periodic, continuous rotation along the x-direction, but it possesses the same phase retardation δ across the entire device plane. The fast axis direction θ of the liquid crystal molecules varies with the x-position as follows:

[0047]

[0048] In the formula, P is the period of the polarization grating, and represents the spatial (x-direction) length required for the liquid crystal molecules to rotate π along the fast axis.

[0049] The working principle of a polarization grating can be explained by Jones' polarization theory. When left-handed or right-handed circularly polarized light E in =[1 ±i] T At incidence, the Jones vector E of the outgoing light out It can be represented as:

[0050]

[0051] In equation (2), the phase delay δ = 2πΔnd / λ, where Δn is the birefringence of the liquid crystal and d is the thickness of the liquid crystal layer. It can be seen that the outgoing light consists of two parts, the intensity ratio of which is determined by the phase delay of the polarization grating. The polarization state of the first part is the same as that of the incident circularly polarized light; this part is called the normally refracted light (or the 0th order diffracted light). The polarization state of the other part is orthogonal to that of the incident circularly polarized light, and this orthogonal part contains an additional PB phase shift e. ±i2πx / P This introduces a corresponding component into the wave vector of the outgoing light in the x-direction, causing it to deflect in the x-direction. This portion of the light is called anomalously refracted light (or ±1st order diffracted light). It is easy to see that for left- and right-hand circularly polarized incident light, the geometric phase factor is the same in magnitude but opposite in sign. Therefore, the polarization grating exhibits symmetrical characteristics in controlling left- and right-hand circularly polarized light.

[0052] For anomalously refracted light, the angle of refraction β t With the incident angle β i Satisfies the generalized Snell's law:

[0053]

[0054] In equation (3), n i =n t =1 represents the refractive index of the beam before and after incident on the polarization grating, respectively, and dΦ / dx = 2dθ / dx is the phase gradient of the polarization grating PB. Specifically, when the beam is incident orthogonally on the polarization grating, the angle of refraction of the anomalously refracted light is:

[0055]

[0056] As shown in equation (4), when broadband light is incident on a polarizing grating, dispersion will occur, and the larger the wavelength of the light, the larger the dispersion angle. By reasonably selecting the thickness of the liquid crystal layer, the phase delay of the polarizing grating at the designed wavelength is made δ = π. At this time, the outgoing light at this wavelength will only contain ±1st order diffraction light. However, when the incident wavelength deviates from the designed wavelength, the diffraction efficiency of the ±1st order diffraction light will gradually decrease. In order to make the polarizing grating have high diffraction efficiency in a broadband range, liquid crystal layers with different twists can be used to achieve the purpose of achromatic correction.

[0057] See Figure 3As shown, this embodiment discloses a broadband polarization feature detection system based on a polarization grating, comprising: a polarization state generator (PSG) for generating broadband polarized light to be measured; a polarization grating (PG) for splitting the broadband polarized light to be measured into two beams of left-handed and right-handed circularly polarized light, and causing dispersion in the horizontal direction; a lens (L) for imaging the light intensity pattern formed after beam splitting and dispersion; a CMOS camera (sCMOS) for acquiring the imaged light intensity pattern; and a terminal device for analyzing and processing the acquired light intensity pattern to obtain the spectral intensity ratio of left-handed and right-handed circularly polarized light in the beam to be measured, and calculating the spectral ellipticity and spectral polarization direction of the beam to be measured. The polarization state generator (PSG) includes a slit S, a polarizer P, and a quarter-wave plate Q arranged sequentially. The slit S is used to convert the broadband light source into a narrow vertical beam, and the polarizer P and the quarter-wave plate Q are used to generate the broadband polarized light to be measured.

[0058] In this embodiment, the broadband light source, after passing through the slit S, becomes a narrow, vertically oriented beam, which, together with the polarizer P and the quarter-wave plate Q, forms a polarization state generating device PSG to generate the broadband polarized light to be measured. The x-direction of the polarization grating PG is parallel to the horizontal direction. After passing through the polarization grating, the narrow, broadband beam is split into two beams: left-handed and right-handed circularly polarized light, which disperse in the horizontal direction. The left-handed and right-handed circularly polarized spectral distribution patterns formed after beam splitting and dispersion are symmetrically distributed about a vertical line passing through the system's optical axis, and the ratio of the spectral intensities of the left-handed and right-handed circularly polarized components is the same as the ratio of the spectral intensities of the right-handed and left-handed circularly polarized components in the beam under test. The lens L and the CMOS camera (sCMOS) image and acquire the light intensity patterns formed after beam splitting and dispersion. By analyzing and processing the acquired light intensity patterns, the ratio of the left-handed and right-handed circularly polarized spectral intensities in the beam under test can be obtained, and the spectral ellipticity of the polarized beam under test can then be calculated.

[0059] In this embodiment, to further obtain the spectral polarization direction of the beam under test, an achromatic quarter-wave plate (AQ) with its fast axis along the horizontal direction is additionally placed in front of the polarization grating PG to encode the polarization direction information of the beam under test into the polarization ellipticity. By combining the spectral ellipticity measured at this time with the spectral ellipticity measured without placing the achromatic wave plate, the spectral polarization direction information of the beam under test can be calculated.

[0060] Let the spectral ellipticity of the polarized light to be measured be ε(λ), and the intensities of its left-hand circularly polarized component and right-hand circularly polarized component at wavelength λ be I, respectively. L (λ) and I R If ε(λ) is the spectral ellipticity of the polarized light to be measured, then ε(λ) can be calculated from equation (5):

[0061]

[0062] In the formula, τ(λ)=I L (λ) / I R ε(λ) represents the intensity ratio of the left-hand circularly polarized component to the right-hand circularly polarized component of the beam under test at wavelength λ. The rotational polarization of polarized light can be determined by the sign of ε(λ): when ε(λ)>0, it represents right-hand circularly polarized light; when ε(λ)<0, it represents left-hand circularly polarized light; when ε(λ)=±1, it represents both right-hand and left-hand circularly polarized light; and when ε(λ)=0, it represents linearly polarized light.

[0063] To measure the spectral polarization direction α(λ) of the polarized beam under test, an achromatic quarter-wave plate with its fast axis along the horizontal direction needs to be placed in front of the polarization grating. The working principle of this process can be analyzed using Jones polarization theory. Let the Jones vector of the light under test be J. in (λ) (taking horizontally and vertically polarized light as orthogonal bases), then J in (λ) can be expressed by the spectral polarization direction α(λ) and the spectral polarization ellipticity ε(λ) of the light to be measured as follows:

[0064]

[0065] When the polarized light to be measured J in (λ) After passing through an achromatic quarter-wave plate, the Jones vector J(λ) is expressed as:

[0066]

[0067] In the formula, T AQ Let be the Jones matrix of the achromatic quarter-wave plate. The spectral polarization ellipticity ε'(λ) of the light emitted from the achromatic quarter-wave plate can be measured using the aforementioned method. By combining the spectral polarization ellipticity ε(λ) and ε'(λ) measured before and after, the polarization direction of the polarized light to be measured is calculated as follows:

[0068]

[0069] In the system of this embodiment, the intensity information of the left and right circularly polarized light components at different wavelengths is recorded by pixels at different positions in the x-direction of the light intensity image. Therefore, analyzing and establishing the correspondence between wavelength and pixel position is crucial. Figure 4 As shown, the left-hand and right-hand circularly polarized components of the incident light are symmetrically split and dispersed to both sides of the optical axis. For ease of calculation, this embodiment sets the intersection of the x-axis and the system optical axis as the origin of the x-axis. For incident light with wavelength λ, its dispersion angle β λ As given by equation (4):

[0070]

[0071] If a dispersed light beam is collimated by a plano-convex achromatic lens with focal length f, then the spatial position d of the beam with wavelength λ in this collimated beam is... λ It can be given by the formula:

[0072] d λ = f·tan(β) λ (10)

[0073] The correspondence between the wavelength λ after the camera image is formed and the pixel position can be calculated using equations (9) and (10):

[0074] In equation (11), μ = d λ / x λ (Unit: mm / pixel) represents the spatial position d during camera imaging. λ to pixel position x λ The conversion factor is a constant.

[0075] like Figure 5 As shown, the polarization grating only modulates the polarization and wavelength of the light beam in the x-direction, so the acquired intensity modulation image is the same in the y-direction. During image processing, simply performing a Raydon transform on the intensity image along the y-direction yields the intensity of the left and right circularly polarized components for each wavelength (pixel).

[0076] The present invention also provides a method for detecting broadband polarization features based on the above-described polarization grating, comprising the following steps:

[0077] Step S1: Build the broadband polarization feature detection system based on polarization grating.

[0078] Specifically, the model parameters of the optical components used in the system and the experimental conditions are explained. Broadband light source: Hamamatsu EQ-99X-FC plasma source; Slit S: Lubang Optoelectronics (China) S1L-200W slit, 200μm width, 10mm length; Polarizer P: Thorlabs LPVIS100-A polarizer; Quarter-wave plate Q: Thorlabs WPQ10E-633 quarter-wave plate; Achromatic quarter-wave plate AQ: Thorlabs AQWP10M-580 wave plate; Polarizing grating PG: Lubang Optoelectronics (China) PZGS25-5-532 polarizing grating, designed wavelength 532nm, phase delay π; Achromatic plano-convex lens L: focal length f = 60mm; Scientific-grade sCMOS camera: Xintu Optoelectronics (China) Dhyana95 sCMOS camera. Experimental temperature: 20℃, humidity: 55%.

[0079] Before conducting experimental measurements, the system must first be calibrated to ensure measurement accuracy. The calibration process consists of two parts: wavelength radius alignment and polarization grating diffraction efficiency calibration.

[0080] Step S2: In order to establish the actual correspondence between the wavelength and the pixel position of the polarization feature detection system, it is necessary to calibrate each parameter in equation (11) through experiments. To achieve this, this embodiment uses monochromatic light of different wavelengths incident on the polarization grating, and fits each wavelength and its corresponding pixel position through equation (11) to complete the calibration of each parameter, thereby obtaining the relationship between wavelength and pixel.

[0081] Step S3: Acquire light intensity images corresponding to multiple wavelengths using a CMOS camera to obtain light intensity images.

[0082] Specifically, during the calibration process, five bandpass filters (Thorlabs: FKB-V.IS-10, full width at half maximum (FWHM) = 10nm) with center wavelengths of 450, 500, 550, 600, and 650nm were placed sequentially on the broadband light source. The light intensity images corresponding to each wavelength were then acquired using a camera. The acquired light intensity images are shown below. Figure 6 The intensity image of the incident light at 650nm is shown below. Figure 7 As can be seen from the figure, the left and right circularly polarized light beams are symmetrically distributed, and the longer the wavelength of the incident light, the farther the imaging position is from the center of symmetry (i.e., the larger the dispersion angle).

[0083] Step S4: Process the light intensity image obtained in step S3 to obtain the pixel position corresponding to each wavelength.

[0084] Specifically, the five single-wavelength light intensity images are then processed to obtain the pixel positions corresponding to each wavelength. For example... Figure 7 As shown in (a), the size of the light intensity image is 1024×2048 pixels. Performing a Raydon transform on the light intensity image along the y-axis yields the light intensity distribution curves of the "bar" pattern in the x-axis. The pixel positions corresponding to the left and right circularly polarized light of the five wavelengths (the center pixel positions of the left and right "bar" patterns in the five images) are the pixel positions where the maximum values ​​of the left and right portions of the five light intensity distribution curves are located. Next, based on the symmetry between the left and right "bar" patterns, the origin of the x-axis can be determined. Combining the light intensity distribution curves for the five wavelengths, the origin of the image's x-axis is found to be 1013 pixels, which is then used as the "zero point" position of the pixels. Further, the pixel positions corresponding to the five wavelengths (with 1013 pixels as the origin) are obtained as follows: Figure 7 (b) As shown in Table 1.

[0085] Table 1

[0086]

[0087] By combining equation (11) with the results shown in Table 1, the actual correspondence between wavelength and pixel can be obtained:

[0088]

[0089] The fitting results are as follows Figure 8 As shown in the figure, the R-value of the fitting result is... 2 The value is 0.9994, which shows a good fit, and the period P = 5094nm of the fitted polarization grating matches its design value of 5μm well.

[0090] In this embodiment, the system's wavelength resolution is given by the wavelength change between two adjacent pixels in the x-direction of the light intensity image. The wavelength resolution of the system can be obtained by differentiating equation (12) with respect to x. Calculations show that the system's wavelength resolution is less than 1 nm in the range of 450 to 700 nm.

[0091] Step S5: Linearly polarized light in the horizontal direction is incident on the polarization grating to obtain a light intensity image. The light intensity image of the linearly polarized light in the horizontal direction is then analyzed and processed to complete the calibration of the diffraction efficiency of the polarization grating.

[0092] As shown in equation (2), a polarization grating theoretically has the same diffraction efficiency for left- and right-handed circularly polarized light. However, due to the processing precision of the polarization grating and various error factors, the diffraction efficiency of left- and right-handed circularly polarized light will differ at different wavelengths. The polarization feature detection in this system relies on the calculation of the intensity ratio of left- and right-handed circularly polarized light. Therefore, this error in the diffraction efficiency of left- and right-handed circularly polarized light will have a significant impact on the measurement results and must be calibrated.

[0093] Linearly polarized light can be decomposed into two beams of left-handed and right-handed circularly polarized light with equal intensity. When linearly polarized light is incident on an ideal polarization grating, the left-handed and right-handed circularly polarized light of each wavelength will produce the same intensity. Therefore, linearly polarized light can be used to calibrate the diffraction efficiency of a polarization grating. Without loss of generality, if horizontally polarized light is incident on a polarization grating, the resulting intensity image is as follows: Figure 9 As shown in (a), the diffraction efficiency of the polarization grating can be calibrated by analyzing the intensity image of linearly polarized light in the horizontal direction. To verify the calibration effect, linearly polarized light in a 90° direction was incident on the polarization grating, and the resulting intensity image is shown in (a). Figure 9 As shown in (b). Figure 9 (c) shows the results of linearly polarized light before and after calibration in the 90° direction. As can be seen from the figure, calibration can effectively overcome the diffraction efficiency error caused by factors such as the manufacturing process of the polarization grating.

[0094] The following examples further illustrate the spectral polarization ellipticity detection experiment of the present invention.

[0095] To verify the feasibility and accuracy of this system in detecting spectral polarization ellipticity, the spectral ellipticity of different broadband polarized lights was measured. In the experiment, the fast axis of the quarter-wave plate Q was fixed at 90°, and the polarizer P was rotated to 20°, 45°, 60°, 90°, 120°, and 170° to generate a series of polarized lights with different spectral ellipticities. Light intensity images were acquired under these six conditions, and the results are as follows: Figure 10 As shown. Next, a Raydon transform was performed on the six images to obtain the light intensity versus distribution curves, as shown below. Figure 11 As shown. From Figure 10 and Figure 11 It can be seen that as the polarizer rotates, the spectral ellipticity of the generated broadband polarized light also changes, leading to changes in the relative intensity of the left-hand and right-hand spectral components in the light intensity image. Combining equation (5), the spectral ellipticity of the six test beams in the range of 450–700 nm can be calculated. The measured values ​​and theoretical values ​​are as follows: Figure 12 As shown. Further, the RMS error of the spectral ellipticity of the six test beams was calculated, and the results were 1.5%, 3.6%, 1.2%, 1.4%, 1.1%, and 1.2%, respectively. Then, the total RMS error of the six measurements (a total of 1500 measurement points) was calculated, and the result was <1.7%. The measurement results prove the feasibility and accuracy of spectral ellipticity detection.

[0096] The following examples further illustrate the spectral polarization direction detection experiment of the present invention.

[0097] To verify the feasibility and accuracy of the system in detecting polarization direction, the optical rotation and dispersion curves of left-handed (LHQ) and right-handed (RHQ) quartz crystals were measured. The thicknesses of the left-handed and right-handed quartz crystals used in the experiment were 0.240 mm and 0.218 mm, respectively. Figure 3 A quarter-wave plate with a fast axis of 90° is placed in front of the polarization grating of the detection system shown. Removing the quarter-wave plate Q from the polarization state generator yields the optical rotation dispersion curve measurement system based on the polarization grating. First, the light intensity image without a sample is acquired. Then, light intensity images with left-handed and right-handed quartz crystals are acquired respectively. The acquired light intensity images of the left-handed and right-handed quartz crystals are shown below. Figure 13 As shown in (a). Next, the acquired light intensity image is processed, and combined with equation (8), the optical rotation angles of left-handed and right-handed quartz crystals in the range of 450 to 750 nm can be calculated. Finally, the optical rotation dispersion curve formula is used: ρ(λ)=A / λ 2 +B / λ 4By fitting the measured values, the optical rotation and dispersion curves of left- and right-handed quartz crystals were obtained, as shown below. Figure 13 As shown in (b). From Figure 13 As can be seen from (b): the measured values ​​and fitting curves of the optical rotation dispersion of left- and right-handed quartz crystals agree very well with the theoretical curves; particularly low, the R value of the fitting curves of the optical rotation dispersion of left- and right-handed crystals is... 2 The values ​​were 0.9994 and 0.9991, respectively, demonstrating a high goodness of fit. Next, the relative errors between the fitted curves and the theoretical curves of the optical rotation dispersion of left- and right-handed quartz crystals were calculated, with results of 0.98% and 0.81%, respectively, proving that the system has high accuracy in measuring spectral polarization direction.

[0098] To further verify the accuracy of the system in detecting polarization direction, the optical rotation and dispersion curves of five different concentrations of glucose solutions (100, 150, 200, 250, and 300 mg / ml) were measured. The measured results of the optical rotation and dispersion angle are shown in the figure below. Figure 14 As shown in (a), the R-values ​​of the optical rotation and dispersion fitting curves for five glucose solutions of different concentrations are shown. 2 The minimum value of 0.9987 indicates a high goodness of fit. It is well known that the optical rotation angle of a glucose solution is directly proportional to its concentration. This provides a method for quantitatively evaluating the accuracy of experimental measurements. Therefore, this embodiment calculated the areas enclosed by the five fitted curves and the horizontal axis in the 450-700 nm range, and performed linear fitting on these five area values. The results are as follows: Figure 14 As shown in (b). Specifically, the R-squared value of the linear fitting results... 2 The value of 1.000 indicates extremely high linearity between the optical rotation angle and solution concentration in the five measurements. Next, the relative error between the five data points and the fitted line was calculated, yielding a result of 0.5%, further verifying the excellent measurement accuracy of the polarization detection system.

[0099] The feasibility and accuracy of spectral polarization feature detection based on polarization gratings were demonstrated by measuring the spectral ellipticity of broadband polarized light and the optical rotation and dispersion curves of quartz crystal and glucose solution. Compared with traditional methods, the spectral polarization feature detection system based on polarization gratings also exhibits the following two advantages:

[0100] (1) Rapid broadband measurement: The acquisition time for a single image during the measurement process is only 20μs, and the light intensity image processing and analysis takes only 8.74ms (CPU: i7-10700F, 2.90GHz, memory: 16G, software: MatLabR2021a). Therefore, this method can achieve a tracking rate of 50kHz and an output rate of 114Hz. Compared with traditional polarization feature detection methods that rely on wavelength scanning and rotation or active optical elements, this method has a significantly improved measurement speed.

[0101] (2) The system has a compact and simple structure: it can complete the detection of broadband polarization features by using only a polarization grating (the polarization direction measurement requires an additional achromatic quarter-wave plate). Compared with the traditional method that relies on spectrometers and rotating optical elements, the system has a very simple and compact structure.

[0102] Although embodiments of the present invention have been described in conjunction with the accompanying drawings, the patent owner may make various modifications or alterations within the scope of the appended claims, as long as they do not exceed the protection scope described in the claims of the present invention, they shall be within the protection scope of the present invention.

Claims

1. A broadband polarization feature detection system based on a polarization grating, characterized in that, include: A polarization state generator is used to generate broadband polarized light to be measured. A polarization grating is used to split the broadband polarized light to be measured into two beams of left-handed and right-handed circularly polarized light, and to cause dispersion in the horizontal direction. Lenses are used to image the intensity patterns formed after beam splitting and dispersion. A CMOS camera is used to capture the light intensity pattern of an image. And a terminal device, used to analyze and process the acquired light intensity pattern to obtain the ratio of the left and right circularly polarized spectral intensity in the beam to be measured, and to calculate the spectral ellipticity and spectral polarization direction of the polarized beam to be measured. The polarization state generating device PSG includes a slit S, a polarizer P, and a quarter-wave plate Q arranged in sequence. The slit S is used to convert broadband light from the source into a narrow vertical beam, and the polarizer P and the quarter-wave plate Q are used to generate broadband polarized light to be measured. The formula for calculating the spectral ellipticity ε(λ) of the polarized beam to be measured is: In the formula, λ is the wavelength, and I L (λ) and I R ε(λ) represents the intensity of the left-hand circularly polarized component and the right-hand circularly polarized component of the spectral ellipticity ε(λ) of the polarized beam under test at wavelength λ. When ε(λ)>0, it represents right-hand circularly polarized light; when ε(λ)<0, it represents left-hand circularly polarized light; when ε(λ)=±1, it represents both right-hand and left-hand circularly polarized light; and when ε(λ)=0, it represents linearly polarized light. The method for broadband polarization feature detection system based on polarization grating includes the following steps: S1. Construct the broadband polarization feature detection system based on the polarization grating; S2. Monochromatic light of different wavelengths is incident on a polarizing grating, and the wavelength λ and its corresponding pixel position are fitted using the following formula to complete the calibration of each parameter and obtain the relationship between wavelength λ and pixel: In the formula, λ is the wavelength of the image captured by the CMOS camera, and μ = d λ / x λ d represents the spatial position of the camera during imaging. λ to pixel position x λ The conversion factor, where P is the period of the polarization grating; S3. Acquire light intensity images by using a CMOS camera to capture light intensity images corresponding to multiple wavelengths; S4. Process the light intensity image obtained in step S3 to obtain the pixel position corresponding to each wavelength. S5. Linearly polarized light in the horizontal direction is incident on the polarization grating, and the intensity image is obtained. Then, the intensity image of the linearly polarized light in the horizontal direction is analyzed and processed to complete the calibration of the diffraction efficiency of the polarization grating. The formula for calculating the spectral polarization direction α(λ) of the polarized beam to be tested in step S3 is as follows:

2. The broadband polarization feature detection system based on a polarization grating according to claim 1, characterized in that, It also includes an achromatic quarter-wave plate located at the front end of the polarization grating with its fast axis along the horizontal direction. The achromatic quarter-wave plate encodes the polarization direction information of the beam under test into the polarization ellipticity. The spectral polarization direction information of the beam under test is calculated by combining the spectral ellipticity ε′(λ) measured in the current state with the spectral ellipticity ε(λ) measured when the achromatic wave plate is not placed.

3. The broadband polarization feature detection system based on a polarization grating according to claim 1, characterized in that, The x-direction of the polarization grating is parallel to the horizontal direction.

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