CN114018830A

CN114018830A - Linear polarization direction detection method based on liquid crystal polarization grating

Info

Publication number: CN114018830A
Application number: CN202111252506.8A
Authority: CN
Inventors: 郭琦; 宣艳; 张�浩; 赵慧洁
Original assignee: Beihang University
Current assignee: Beihang University
Priority date: 2021-10-27
Filing date: 2021-10-27
Publication date: 2022-02-08
Anticipated expiration: 2041-10-27
Also published as: CN114018830B

Abstract

The invention discloses a linear polarization direction detection method based on a liquid crystal polarization grating. The key technology of the method lies in a polarization detection system and a polarization state calculation algorithm. The core of the polarization detection system is composed of 1/4 wave plate and liquid crystal polarization grating. In actual detection, the angle between the fast axis direction of the quarter-wave plate and the horizontal direction is adjusted to 0° and 45°, and the intensities of the ±1st-order diffracted beams of the liquid crystal polarization grating at the two angles are measured by photodetectors. The polarization state calculation algorithm uses the Stokes calculus to calculate the polarization direction of the incoming and outgoing polarized beams from the intensity of the diffracted beam, so as to realize the polarization state characterization of the linearly polarized light. The advantages of this method lie in that the number of devices used is small, the optical path is simple, the components are easy to obtain, and the cost is low. It is a linear polarization detection method with practical value.

Description

Linear polarization direction detection method based on liquid crystal polarization grating

Technical Field

The invention belongs to the technical field of polarization state detection in the optical field, and particularly relates to a linearly polarized light polarization detection method based on a liquid crystal polarization grating.

Background

Polarization is a fundamental property of light that is not directly observable by the human eye, and thus polarization studies are far less than traditional studies of light. The polarization information of the measurement light is particularly important because polarization is very sensitive to microstructure (sub-micron) changes, which can provide more medium structure information. The polarization measurement method also has the advantages of the traditional optical methods such as non-contact and non-damage methods. Polarization measurements have been used in a breakthrough in sky navigation, ocean exploration, target identification, and biomedical detection for decades.

The liquid crystal polarization grating has polarization separation characteristics. When a beam of natural light enters, the polarization grating separates the beam into two sub-beams with orthogonal circular polarization states according to left-handed and right-handed components, and the two sub-beams are respectively emitted from +1 or-1 level. In general, the emergent light order distribution of the liquid crystal polarization grating with good texture is relatively simple and has three orders of +/-1 and 0, and no high order exists. Meanwhile, as an anisotropic material, the material is sensitive to the polarization state of incident light. When the polarization state of incident light is different, the polarization state and order distribution of diffracted light are also changed accordingly, and this characteristic is referred to as polarization-dependent characteristic. For an ideal polarization grating, the ± 1 order exit light intensities are each about 50% when the incident light is natural light or linearly polarized light. However, when the incident light is circularly polarized, the emergent light is completely emitted from +1 or-1 level, and the emitted level is correspondingly changed according to the difference of the chirality of the incident circularly polarized light.

An embodiment with three Polarization Gratings and two achromatic 1/4 wave plates is proposed in the paper "simple polarized spectroscopy Using Reactive Polarization Gratings", so that a complete stokes vector can be detected under the condition that four intensity measurements are simultaneously performed at each wavelength, and Polarization state detection is realized. However, the system design is divided into three stages, the structure is complicated, and the actual diffraction efficiency of the wave plate and the polarization grating is not 100%, which affects the measurement precision.

Disclosure of Invention

In view of the above problems in the prior art, an object of the present invention is to provide a linear polarization direction detection method based on a liquid crystal polarization grating, and an object of the present invention is to provide a linear polarization detection method which has a simple structure, is convenient to operate, has high sensitivity, and can be miniaturized.

The technical solution of the invention is as follows: a linear polarization direction detection method based on liquid crystal polarization grating. The polarization detection system comprises 1/4 wave plates (1), a liquid crystal polarization grating (2), a photoelectric detector (3) and a computer (4), wherein the photoelectric detector (3) is connected with the computer (4). The 1/4 wave plate (1), the liquid crystal polarization grating (2) and the photoelectric detector (3) are sequentially arranged in the same straight line direction. The method uses elements such as a liquid crystal polarization grating and the like to form a polarization detection light path, measured light passes through an 1/4 wave plate with an adjustable fast axis direction and the liquid crystal polarization grating, diffraction characteristics of the liquid crystal polarization grating are utilized to generate +/-1-order diffraction light beams, intensity of the diffraction light beams is measured, and polarization state information of the measured light is obtained after data analysis.

The specific implementation method of the invention is as follows: 1) and (5) building a system according to a structure diagram shown in the abstract drawing. In a plane perpendicular to the direction of light propagation, the horizontal direction is the x-axis and the vertical direction is the y-axis. 2) Linearly polarized light of unknown polarization direction is used as a measured light beam. 3) The 1/4 wave plate is rotated twice so that its fast axis is at 0 deg. and 45 deg. to the x axis, respectively. 4) And respectively testing the +/-1-order diffraction light intensity of the light after the light is diffracted by the liquid crystal polarization grating under two different wave plate angles. 5) And according to the measured value, inverting the polarization state of the measured light by an algorithm.

The above process can be theoretically calculated by using a stokes vector and a mueller matrix: let the components of the light in the mutually perpendicular x and y directions in a plane perpendicular to the propagation direction be

E_x＝E₀e^i(kz-wt)

E_y＝cE₀e^i(kz-wt+δ)

In which light propagates in the z direction, E₀Let c be the amplitude, the ratio of the amplitude in the y-direction to the amplitude in the x-direction, δ be the phase difference, k be the wave vector, and w be the angular frequency. The resulting normalized stokes vector for the measured light S is

1/4 wave plate is a birefringent single crystal wave plate with a certain thickness. When light is transmitted through the wave plate from normal incidence, the phase difference between the ordinary light (o light) and the extraordinary light (e light) is equal to pi/2 or an odd multiple thereof. If the included angle between the fast axis direction and the horizontal direction (x axis) is θ, the Mueller matrix M is

The Stokes vector of the emergent light S' passing through the 1/4 wave plate is

For a liquid crystal polarization grating, the distribution of each order of emergent light is closely related to factors such as the thickness of the grating, the wavelength and the polarization state of incident light. The diffraction characteristics of the ideal liquid crystal polarization grating are shown in the following table

TABLE 1 diffraction characteristics of ideal liquid crystal polarization gratings

When the half-wave condition Δ nd ═ λ/2 is satisfied,

then

The sum of the light intensity of plus or minus 1 order on the light intensity ratio of +1 order of diffraction is recorded as the energy ratio of +1 order of diffraction; in the same way

Is-1 order light intensity ratio of ±. + -.)The sum of the 1 st order intensities is reported as the-1 st order diffraction energy ratio. The light intensity of +/-1 order of the polarization grating can be respectively measured by the photoelectric detector.

In the process of detecting the direction of linearly polarized light, 1/4 wave plates are respectively rotated to the positions of 0 degree and 45 degrees of theta, the corresponding +/-1-order light intensity of the polarization grating is measured by a photoelectric detector, and the following equation set can be obtained

Solving the system of equations can obtain c and delta, so that the four stokes parameters of the measured light S can be solved.

As shown in FIG. 2, for polarized light, the component E of the polarized light E_pAnd E_sThe phase difference delta is a polarization parameter in a space coordinate system (x-y), and the major axis a and the minor axis b, and the included angle phi of the two coordinate systems is a polarization parameter in a major axis system (x '-y'). The polarization state of fully polarized light can be characterized by the polarization direction, the polarization type, and the polarization rotation direction.

1) Polarization direction: angle psi between the principal axis and the x-axis of the conventional coordinate system

2) Polarization type: describing the shape of the polarized part of the beam, the elliptical shape is represented by the ratio of the minor axis to the major axis, and χ is the ellipticity angle of the polarized light (i.e., the arctangent value of the ratio of the minor axis to the major axis of the ellipse)

When χ is 0: indicating that the light to be measured is in a linear polarization state;

② when

The method comprises the following steps: indicating that the light to be measured is in a circular polarization state;

③ when χ takes other values: indicating that the light to be measured is in an elliptical polarization state.

3) Polarization rotation direction: represents the elliptical rotation direction, χ > 0 is right-handed rotation, χ < 0 is left-handed rotation, but χ is 0 for linear polarization.

Thus, ψ and χ can be represented by the following formulas

And substituting c and delta obtained by solving the equation set into the equation set, so that the polarization direction, the polarization type and the polarization rotation direction of the light to be detected can be known, and the polarization state detection is realized.

The invention has the beneficial effects that:

1) the polarization detection system has the advantages of small number of used devices, simple light path, easily obtained elements and low cost. And the steps for carrying out polarization detection are simple, the operation is convenient, and the linear polarization detection system has practical value.

2) The liquid crystal polarization grating itself exhibits nearly ideal properties, including diffraction efficiencies greater than 99%, strong polarization sensitivity in the first diffraction order, very low incoherent scattering, and suitability for visible and infrared light. The use of liquid crystal polarization gratings for polarization state detection is expected to overcome limitations in speed physical size, complexity and alignment sensitivity.

3) When combined with the 1/4 waveplates, the polarization grating may have +1 order diffraction that is sensitive to stokes parameters, which enhances wavelength separation and polarization selectivity of the polarization grating, and enables polarization detection without the need for active tuning elements.

Drawings

FIG. 1 is a schematic structural diagram of a linear polarization direction detection device based on a liquid crystal polarization grating;

FIG. 2 is a graph of the relationship between polarization state parameters;

FIG. 3 is a schematic view of the apparatus of the embodiment;

FIG. 4 is a liquid crystal polarization grating diffraction pattern;

FIG. 5 is a diagram of the energy ratio of + -1 order diffraction;

FIG. 6 is an optical image of a target;

1/4 wave plate for 1, liquid crystal polarization grating for 2, photoelectric detector for 3, computer for 4, light source for 5, polarizer for 6, light generating module for 7 and detecting module for 8.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in detail below with reference to the accompanying drawings.

Example (b):

the embodiment apparatus is shown in fig. 3, and includes a detected light generation module 7 and a detection module 8; wherein the detected light generation module 7 comprises a light source 5 and a polarizer 6; the detection module 8 is composed of 1/4 wave plate 1, liquid crystal polarization grating 2, photoelectric detector 3 and computer 4, and the photoelectric detector 3 is connected with the computer 4. The light source 5, the polarizer 6, the 1/4 wave plate 1, the liquid crystal polarization grating 2 and the photoelectric detector 3 are sequentially arranged in the same straight line direction.

Linearly polarized light with different polarization directions can be obtained by changing the included angle between the transmission axis of the polarizer and the x-axis direction. In a test experiment, 1/4 wave plates are rotated to enable the fast axis direction of the wave plates to form 0 degrees and 45 degrees with the x axis, a diffraction phenomenon (shown in figure 4) is generated after a to-be-detected light passes through the 1/4 wave plate and the polarization grating, the light intensity of +/-1 order is detected respectively, therefore, the proportion of +/-1 order diffraction energy is obtained, and the representation of the polarization state of the incident linear polarization light is realized through resolving. Using the optical path shown in fig. 3, the corresponding ± 1 st order diffraction energy ratio was measured when the linearly polarized light direction was-80 °, -70 °, -60 ° - …, and the experimental result is shown in fig. 5.

Fig. 5(a) is a diagram of the energy ratio of +1 order diffraction, the abscissa is the polarization direction of incident linearly polarized light, and the ordinate is the corresponding energy ratio of +1 order diffraction. The solid line and the dotted line represent the ratio of +1 order diffraction energy of the 1/4 wave plate when θ is equal to 0 ° and 45 °, respectively. The results of the experimental measurements are indicated by dots in the figure. FIG. 5(b) is a diagram showing the energy ratio of-1 order diffraction. The RMS error of the ratio of the actually measured ± 1 st order diffraction energy of the polarization grating to the theoretical result was calculated to be about 0.4%. For example: when the incident light is linearly polarized light of 45 ° (assumed to be unknown), the ratios of + 1-order diffraction energies of the polarization gratings corresponding to 0 ° and 45 ° of the 1/4 wave plate are 0.008 and 0.499, | χ | is 0.0896, and ψ is 44.9418 °, respectively. Fig. 5 is a photograph taken with the object "beilieve IN YOURSELF" placed before the detection device with the incident light being linearly polarized at 45 °, and with the wave plate rotated 1/4 to 0 ° (fig. 6(a)) and 45 ° (fig. 6(b)), respectively. The target is divided into a + 1-level image and a-1-level image after being diffracted by the liquid crystal polarization grating, and diffracted light beams are all concentrated on the-1 level when the fast axis direction of the 1/4 wave plate is 0 degrees according to the theory; when the fast axis direction of the 1/4 wave plate is 45 degrees, the diffraction of +/-1 order is half. The optical image of the target obtained by observation is in accordance with the theory.

Claims

1. a linear polarization direction detection method based on liquid crystal polarization grating is characterized in that the polarization detection system comprises a quarter wave plate (1), a liquid crystal polarization grating (2), a photodetector (3) and a computer (4) to form , the photodetector (3) is connected to the computer (4); the 1/4 wave plate (1), the liquid crystal polarization grating (2), and the photodetector (3) are sequentially placed in the same linear direction.

2. a kind of linear polarization direction detection method based on liquid crystal polarization grating as claimed in claim 1 is characterized in that, this method uses liquid crystal polarization grating element to form polarization detection optical path, utilizes the diffraction characteristic of liquid crystal polarization grating to make incident linear polarization After the light passes through the grating, a ±1st-order diffracted beam is generated. By measuring the intensity of the diffracted beam, the polarization state information of the measured light is obtained after data analysis.

3. a kind of linear polarization direction detection method based on liquid crystal polarization grating as claimed in claim 1 is characterized in that, comprises the following steps and is specifically implemented:

(1) The linearly polarized light of the unknown polarization direction is used as the measured beam;

(2) be arranged in the plane perpendicular to the light propagation direction, the horizontal direction is the x-axis, and the vertical direction is the y-axis, and the 1/4 wave plate is rotated twice to make its fast axis direction and the x-axis become 0° and 45° respectively;

(3) under the two wave plate angles shown in step (2), respectively, the ±1st-order diffracted light intensity after the test light is diffracted by the liquid crystal polarization grating;

(4) Using the value measured in step (3), invert the polarization state of the measured light through an algorithm.

4. a kind of linear polarization direction detection method based on liquid crystal polarization grating as claimed in claim 1, is characterized in that, described polarization state solution algorithm in the realization process, obtains ± 1 order diffraction energy ratio by detecting Bring into the equation system, solve the two parameters from the two equations, and then obtain the Stokes vector of the light to be measured, and then calculate the polarization direction, polarization type, and polarization rotation of the light to be measured. Polarization state detection.