Full polarization parameter detection method based on cascade liquid crystal polarization grating

Abstract

The invention discloses a full polarization parameter detection method based on cascaded liquid crystal polarization gratings, and belongs to the technical field of optical polarization detection. A traditional time-sharing polarization detection method cannot detect a dynamic target in real time and lacks a circular polarization detection means. Therefore, the invention provides a snapshot type full-polarization detection method based on two cascaded liquid crystal polarization gratings, and a first liquid crystal polarization grating, a 1 / 4 wave plate, a second liquid crystal polarization grating, a polarizing film, a photoelectric detector, a computer and a polarization state resolving algorithm are sequentially arranged along a light propagation direction. Incident light is sequentially subjected to beam splitting through the first liquid crystal polarization grating, phase modulation through the 1 / 4 wave plate and beam splitting through the second liquid crystal polarization grating, one-time exposure is carried out on the photoelectric detector to form nine separated diffraction orders, the nine separated diffraction orders are synchronously collected, and the full polarization parameter of the incident light is calculated through a polarization state resolving algorithm. The device is compact in structure, free of mechanical moving parts, high in detection speed, high in precision and suitable for full-polarization parameter real-time detection of static and dynamic targets.

Description

Technical Field

[0001] This invention belongs to the field of polarization state detection technology in optics, and more specifically, relates to a method for detecting all polarization parameters based on a cascaded liquid crystal polarization grating. Background Technology

[0002] Polarization is one of the fundamental properties of light, describing the direction of light vibration during propagation and reflecting the microscopic structure of the propagation medium. Polarization measurement is a non-contact, non-destructive measurement method with broad application prospects in deep space exploration, biomedical detection, 3D reconstruction, target enhancement, and target detection and recognition. Currently, traditional polarization measurements employ multi-step time-division polarization measurement methods, which struggle to accurately detect the polarization information of dynamic targets. Furthermore, traditional polarization measurement methods often neglect circular polarization information, which plays a crucial role in chiral material characterization, biological tissue and virus detection, and celestial surface environment analysis. Therefore, rapid full polarization information detection has significant practical implications.

[0003] A liquid crystal polarization grating (LCD) is a polarization-sensitive beam splitter. When light is incident on the LCD, its left-handed and right-handed circularly polarized components are separated and emitted by the +1 and -1 orders, respectively. The emitted intensity is related to the intensity of the two polarization components, thus enabling the conversion of polarization state information into intensity information. The LCD has only three emission orders: ±1 and 0. The +1 and -1 orders emit a pair of orthogonally circularly polarized light, while the 0th order emits the same polarization state as the incident light. The diffraction efficiency can be precisely controlled over a wide range through the fabrication process, providing a crucial device foundation for optimizing energy distribution and improving detection performance in specific application systems.

[0004] Chinese Patent Publication No. CN114018830A discloses a method for detecting linear polarization direction based on a liquid crystal polarization grating. The method includes an embodiment with a quarter-wave plate and a liquid crystal polarization grating, capable of calculating the polarization direction of the incident light by rotating the quarter-wave plate and utilizing the ±1st order diffraction light intensities at two different wave plate angles. However, this method can only detect the linear polarization direction of the incident light, and requires two intensity measurements to complete one linear polarization direction detection. Therefore, a method for detecting all polarization parameters based on a cascaded liquid crystal polarization grating is designed. This method can complete the detection of all polarization states, including circular and linear polarization, with a single intensity measurement, improving the polarization state detection speed and the amount of polarization information acquired. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, the present invention aims to provide a method for detecting full polarization parameters based on a cascaded liquid crystal polarization grating. This method is characterized by its simple structure, convenient operation, and high sensitivity, and is a snapshot-type full polarization state detection method.

[0006] The technical solution of this invention is: a method for detecting the full polarization parameter based on a cascaded liquid crystal polarization grating. Its polarization detection system includes: a first liquid crystal polarization grating (1), a quarter-wave plate (2), a second liquid crystal polarization grating (3), a polarizer (4), a photodetector (5), a computer (6), and a polarization state calculation algorithm (7). The first liquid crystal polarization grating (1), the quarter-wave plate (2), the second liquid crystal polarization grating (3), the polarizer (4), and the photodetector (5) are placed sequentially along the optical path. The diffraction efficiencies of the first liquid crystal polarization grating (1) and the second liquid crystal polarization grating (3) should both be designed to be within the range of 40% to 60%. The photodetector (5) is connected to the computer. (6) The polarization state calculation algorithm (7) runs on the computer (6). The method takes the light with unknown polarization state as the beam to be measured, and obtains the proportion of diffracted light energy by detection by photodetector (5) and calculation by computer (6) as the input of polarization state calculation algorithm (7). The polarization state calculation algorithm (7) inputs the proportion of diffracted light energy into the polarization state detection solution equation set, solves the Stokes vector of the light to be measured by the three equations in the equation set, and then calculates the polarization direction, polarization type and polarization rotation of the light to be measured by the Stokes vector, thereby realizing polarization state detection.

[0007] The specific implementation method of this invention is as follows: 1) According to Figure 1 The system is constructed using the structure diagram shown. Figure 1 Using the spatial rectangular coordinate system as a reference, the fast axis of the quarter-wave plate (2) forms an angle of +45° with the X-axis, and the transmission axis of the polarizer (4) forms an angle of +45° with the X-axis. The first liquid crystal polarization grating (1) and the second liquid crystal polarization grating (3) are cascaded in a manner with orthogonal optical axes. 2) The polarized light with an unknown polarization state is used as the beam to be measured. 3) After the light to be measured passes through the system, it generates 9 diffraction orders on the photodetector (5), and the intensity of each diffraction order is tested. 4) Based on the measured values, the polarization state of the light to be measured is inverted through the polarization state solution algorithm (7).

[0008] The polarization process can be theoretically calculated using the Stokes vector and the Mueller matrix:

[0009] Define the normalized Stokes vector of the incident light as:

[0010]

[0011] in, and The linear polarization parameter representing the incident light. The circular polarization parameter represents the incident light.

[0012] The diffraction efficiencies of the liquid crystal polarization grating (1) and the liquid crystal polarization grating (3) are defined as follows: The intensities of the 0th-order transmitted light and the ±1st-order diffracted light emitted from the liquid crystal polarization grating (1) are respectively

[0013]

[0014]

[0015] The Mueller matrix of a quarter-wave plate (2) whose fast axis forms a 45° angle with the horizontal direction is:

[0016]

[0017] Then, the normalized Stokes vector of the transmitted light and ±1st order diffracted light from the liquid crystal polarization grating (1) after passing through the quarter-wave plate (2) can be obtained as follows:

[0018]

[0019]

[0020] The three transmitted and diffracted beams, after being incident on the liquid crystal polarization grating (3), generate nine outgoing beams with intensities of:

[0021]

[0022] in, The first polarization grating (1) represents the liquid crystal polarization grating (1) The first-order diffracted light is diffracted by the liquid crystal polarization grating (3) to the second-order diffracted light. The intensity of each diffraction order, observed along the beam propagation direction, and the positions of each diffraction order are as follows: Figure 1 As shown.

[0023] For the liquid crystal polarization grating (3), its diffracted light is all circularly polarized light, while the polarization state of the transmitted light is consistent with that of the incident light of the liquid crystal polarization grating (3):

[0024]

[0025]

[0026] in, express The Stokes vector corresponding to the diffracted light. The polarizer behind the liquid crystal polarization grating (3) makes an angle of 45° with the horizontal direction, and its Mueller matrix is...

[0027]

[0028] The intensities of each diffracted light emitted from the polarizer can then be obtained:

[0029]

[0030] definition for The element in the i-th row and j-th column, based on the diffraction characteristics of the liquid crystal polarization grating and the phase delay characteristics of the quarter-wave plate, represents the linear polarization component. After passing through the first liquid crystal polarization grating (1), the transmitted light is retained, converted into circular polarization information by the quarter-wave plate (2), and then diffracted by the second liquid crystal polarization grating (3), finally being converted into circular polarization information. and In the intensity information corresponding to the diffraction order; for the linear polarization component After passing through the liquid crystal polarization grating (1), the quarter-wave plate (2), and the liquid crystal polarization grating (3), this information only exists in the central transmission diffraction order; for the circular polarization component... The first liquid crystal polarization grating (1) polarizes the circularly polarized component of the incident light. Information separation to The diffracted light, since all the diffracted light from the liquid crystal polarization grating is perfectly circularly polarized, is converted into linearly polarized light by the quarter-wave plate (2), and then diffracted by the second liquid crystal polarization grating (3) and the polarizer (4). The information exists in the diffraction orders corresponding to the second diffraction by the second liquid crystal polarization grating (3), that is, the diffraction orders corresponding to the light intensities of the first and third columns in the light intensity matrix. For ease of calculation, based on the calculated intensity of each diffraction order, the diffraction order intensity ratio can be defined to eliminate the diffraction efficiency correlation coefficient acting on the polarization parameter. The intensity ratio of each diffraction order is defined based on the diffraction characteristics of the liquid crystal polarization grating and the phase delay characteristics of the 1 / 4 wave plate, the linear polarization component - Information exists and In the diffraction order corresponding to the energy, the linear polarization component Information exists In the diffraction order corresponding to the energy, the circular polarization component The information is contained in the diffraction orders corresponding to the light intensities in the first and third columns of the light intensity matrix, as shown below:

[0031]

[0032]

[0033]

[0034]

[0035] in, Used to eliminate the effect on The grating diffraction efficiency parameters, , Used to eliminate the effect on The grating diffraction efficiency parameters, Used to eliminate the effect on The diffraction efficiency parameters of the gratings are determined. To improve the signal-to-noise ratio of the system detection, the diffraction efficiency of the two liquid crystal polarization gratings should be optimized to make the intensities of each diffraction image similar. Based on the optimization model derived from the system energy distribution, and ignoring the approximate polarization degree of the incident light itself, the intensities of the nine diffraction orders are respectively... , , and Proportional to each other, an optimization objective function can be designed.

[0036]

[0037] Solving the objective function yields appropriate results. At this point, the intensities of all diffraction orders are completely equal. Under this condition, the energy of the nine diffraction orders is evenly distributed on the detector, maximizing the signal-to-noise ratio of the system. The selection should be close to 50%.

[0038] Based on the definition of intensity ratio, a simplified expression for each polarization parameter can be obtained:

[0039]

[0040]

[0041]

[0042] By calculating the ratio of the intensities at the corresponding image points, the normalized full Stokes vector of the incident light can be obtained:

[0043]

[0044] Based on the normalized full Stokes vector It can further calculate the degree of polarization, linear polarization, circular polarization, and polarization angle parameters to comprehensively describe the polarization characteristics of the incident light.

[0045] polarization degree

[0046]

[0047] linear polarization degree

[0048]

[0049] Circular polarization degree

[0050]

[0051] polarization angle

[0052] Attached Figure Description

[0053] Figure 1 A schematic diagram of the diffraction order distribution emitted from the polarizer, observed along the direction of beam propagation.

[0054] Figure 2 This is a schematic diagram of the structure of a device for detecting the polarization state of a cascaded liquid crystal polarization grating. Detailed Implementation

[0055] The present invention will now be described in detail with reference to the embodiments. Obviously, the described embodiments are only a part of the embodiments of the fully polarization parameter detection method based on cascaded liquid crystal polarization gratings proposed by the present invention, and not all of the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0056] This embodiment follows Figure 2 The system structure shown is used to construct a detection optical path to verify the level-based detection method described in this invention.

[0057] The effectiveness of the full polarization parameter detection method using a liquid crystal polarization grating. The system consists of a first liquid crystal polarization grating (1), a quarter-wave plate (2), a second liquid crystal polarization grating (3), a polarizer (4), and a photodetector (5) arranged sequentially along the optical path. The grating periods of the first liquid crystal polarization grating (1) and the second liquid crystal polarization grating (3) are approximately 28 μm, with diffraction efficiencies of 46% and 48%, respectively, and they are cascaded in an orthogonal manner along their optical axes. The fast axis of the quarter-wave plate (2) forms a 45° angle with the X-axis; the transmission axis of the polarizer (4) also forms a 45° angle with the X-axis. The diffraction efficiencies, the fast axis of the quarter-wave plate (2), and the transmission axis of the polarizer (4) selected in this embodiment are merely examples and should not be construed as limiting the scope of the invention.

[0058] A beam emitted from a laser, modulated into circularly polarized light by a polarizer and another quarter-wave plate, is incident on the system as the test light. By adjusting the angle of the modulation wave plate, linearly polarized light and circularly polarized light are generated as the test targets. After the test light passes through the cascaded grating system, the photodetector (5) collects the intensity distribution of the nine diffraction orders and inputs it into the polarization state calculation algorithm (7) running in the computer (6). The algorithm constructs the following set of equations by calculating the intensity ratio of each diffraction order:

[0059]

[0060] The Stokes vector can then be solved. The experimental results are shown in the table below:

[0061]

[0062] Beneficial effects of the present invention

[0063] (1) The system has a compact structure, requires no mechanical moving parts, and can realize real-time full polarization imaging;

[0064] (2) By using a cascaded grating design, multiple polarization images can be acquired in a single exposure, thus improving detection efficiency;

[0065] (3) It supports broadband imaging in the visible light band and has strong applicability;

[0066] (4) The algorithm is based on rigorous Mueller matrix derivation and has high solution accuracy.

Claims

1. A method for detecting all polarization parameters based on a cascaded liquid crystal polarization grating, characterized in that, This method uses two cascaded liquid crystal polarization gratings and a quarter-wave plate to perform phase modulation and polarization beam splitting on the incident light, collect the intensity of the diffracted light, and calculate the total polarization parameters of the incident light. Its polarization detection optical path includes a first liquid crystal polarization grating (1), a quarter-wave plate (2), a second liquid crystal polarization grating (3), a polarizer (4), a photodetector (5), a computer (6), and a polarization state calculation algorithm (7) running on the computer (6). After the beam under test passes through the first liquid crystal polarization grating (1), some of its left-hand and right-hand circular polarization components are separated. The beam is diffracted to ±1 diffraction order and then phase-modulated by a quarter-wave plate (2), followed by secondary diffraction by a second liquid crystal polarization grating (3), and finally polarized by a polarizer (4). Nine separate diffraction orders are formed on the photodetector (5). The intensity information of each diffraction order collected by the photodetector (5) in one exposure is input into the computer (6). The normalized full Stokes vector of the beam to be tested is calculated by the polarization state calculation algorithm (7) based on the specific ratio of the intensity of each diffraction order, thereby realizing the detection of full polarization parameters.

2. The method for detecting all polarization parameters based on a cascaded liquid crystal polarization grating as described in claim 1, characterized in that, The first liquid crystal polarization grating (1) and the second liquid crystal polarization grating (3) are cascaded in an orthogonal manner, and the diffraction efficiency of both is designed to be in the range of 40% to 60%. After the incident light passes through the grating, diffracted light is generated. By measuring the intensity of the diffracted light, the full polarization parameter information of the measured light is obtained after data analysis.

3. The method for detecting all polarization parameters based on a cascaded liquid crystal polarization grating as described in claim 1, characterized in that, The specific implementation includes the following steps: (1) Use light with an unknown polarization state as the beam to be measured; (2) The first liquid crystal polarizing grating (1), the quarter wave plate (2), the second liquid crystal polarizing grating (3), the polarizer (4) and the photodetector (5) are arranged in sequence according to the optical path. All components are arranged coaxially. The fast axis direction of the quarter wave plate (2) is at a 45° angle with the optical axis direction of the first liquid crystal polarizing grating (1). The transmission axis direction of the polarizer (4) is parallel or perpendicular to the fast axis direction of the quarter wave plate (2). (3) After the beam under test passes through the optical path, it generates 9 diffraction orders on the photodetector (5). The energy of each order is input into the computer (6), and the polarization state calculation algorithm (7) is run to obtain the normalized Stokes vector. The degree of polarization, linear polarization, circular polarization and polarization angle are further calculated based on the Stokes vector to complete the detection of all polarization parameters.

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