CN109001904B - Correction method of liquid crystal wavefront corrector based on dynamic LUT - Google Patents
Correction method of liquid crystal wavefront corrector based on dynamic LUT Download PDFInfo
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Abstract
The invention discloses a correction method of a liquid crystal wavefront corrector based on a dynamic LUT (look up table), belonging to the field of liquid crystal wavefront correction. The method provided by the invention has the advantages that the phase modulation characteristics under various temperatures are measured by building the light path and building the LUT data model with high precision, the LUT data model is built by using least square fitting, and reasonable LUT data is obtained by substituting the working temperature of the current spatial light modulator into the LUT data model after the ambient temperature changes. The effect of wavefront correction on incident light is achieved, and the influence of reduction of phase modulation precision of the liquid crystal wavefront corrector caused by fluctuation of ambient temperature is overcome.
Description
Technical Field
The invention relates to a correction method of a liquid crystal wavefront corrector based on a dynamic LUT (look up table), belonging to the field of liquid crystal wavefront correction.
Background
Commercial Liquid crystal spatial light modulators are based on reflective electrically addressed Liquid Crystal On Silicon (LCOS), also small amounts of optically addressed LCOS, and transmissive Liquid crystal devices of about 100 pixels; reflective electrically addressed liquid crystal spatial light modulators (liquid crystal spatial light modulators) can be classified into two types according to the orientation of liquid crystals inside the modulators: 1) liquid crystal spatial light modulators of the amplitude type, which are used basically for modulating light intensity, for example, for display purposes. 2) A liquid crystal spatial Light modulator of a phase type in which alignment layers of upper and lower layers in contact with a liquid crystal layer are aligned in parallel with each other and alignment directions of liquid crystals therein are parallel from the upper layer to the next, is used in an optical system, for example, in an adaptive optical system as a wavefront corrector, and is therefore also called a liquid crystal wavefront corrector herein, and its operation principle, characteristics and application are described in documents [ Zhang Z, youz, Chu d.
The liquid crystal wavefront corrector can be used for high-resolution imaging observation of an astronomical target by a large-aperture optical telescope. The observed target is generally a fixed star and is generally darker and weaker. When light of a star reaches the earth atmosphere, the light can be regarded as plane waves; however, the earth surface has an atmosphere layer with a thickness of 10 to 20 kilometers, on one hand, the refractive index of the atmosphere is not uniform due to temperature fluctuation caused by sunlight irradiation, and on the other hand, the atmosphere absorbs the temperature fluctuation, so that the brightness of the target by the atmosphere is further weakened, and meanwhile, the imaging resolution of the target is reduced by the dynamic disturbance phenomenon of the atmosphere to the optical wavefront, and the imaging quality is seriously influenced. Therefore, people use an adaptive optical system to overcome the disturbance of the atmospheric turbulence, and the adaptive optical wavefront correction system is necessary equipment for an optical telescope with a caliber above meter level. An optical wavefront Adaptive correction system in the technical field of atmospheric Adaptive optics has the function of performing real-time compensation correction on a target optical distortion wavefront continuously entering a telescope to obtain ideal real-time optical imaging, and the working principle of the optical wavefront Adaptive correction system is detailed in Francois rodtier, Adaptive optics in astronomy, Cambridge university Press,1999, Part two. In adaptive optics systems, a wavefront corrector is generally used to compensate for a distorted wavefront, wherein the wavefront corrector includes a deformable mirror, a liquid crystal wavefront corrector, and the like. And compared with a deformable mirror, the liquid crystal wavefront corrector has the advantage of high pixel density.
The liquid crystal wavefront corrector can also be used for shaping the incident laser beam to obtain the required beam. At present, with the increasing application of laser technology, for example, in the fields of laser weapons, laser radars, laser communication, laser automobile lamps, laser processing, laser nuclear fusion, structured light in microscopic imaging, laser display and the like, high-quality laser beams are required for the application, the energy of the laser beams needs to be distributed as required, but the light intensity of the laser beams emitted by a common laser is in gaussian distribution, which limits the application range of the laser beams. Therefore, a great deal of research is being conducted, and one of the technical means adopted is to shape the incident laser beam by using a liquid crystal spatial light modulator to obtain an output beam meeting the requirements. Taking structured light in microscopic imaging as an example, a liquid crystal spatial light modulator and a polarizing plate are introduced into an illumination light path, a proper pattern is applied on the liquid crystal spatial light modulator to generate light and shade (or black and white) spaced stripes, the stripes are projected onto a sample through an objective lens, and the structured light subjected to the stripe pattern is illuminated on a focusing plane of the sample.
In the above application, the liquid crystal wavefront corrector is driven as follows: firstly, a gray picture G1 of M × N pixels is generated by a computer, where M and N are the number of pixels of the liquid crystal spatial light modulator in the x and y directions, respectively; secondly, the gray scale picture is sent to the liquid crystal spatial light modulator through the driving software, at the moment, the gray scale G1 of each pixel on the gray scale picture G1i,j(i is more than or equal to 0 and less than or equal to M, and j is more than or equal to 0 and less than or equal to N) finding out the corresponding gray G2 through LUT (look-up table)i,j(i is more than or equal to 0 and less than or equal to M, and j is more than or equal to 0 and less than or equal to N), thereby obtaining a gray scale map G2; finally, the gray scale G2 is converted into corresponding voltages by analog-to-digital conversion inside the driving circuit board, and the voltages are applied to each pixel of the liquid crystal wavefront corrector, so that incident light is regulated. As can be seen from the above process, the LUT is a key factor affecting the phase modulation characteristics of the liquid crystal wavefront corrector. While the LUT is obtained by measuring the phase modulation characteristics of the liquid crystal wavefront corrector at a certain temperature, the obtained LUT is also suitable for operation only at that temperature, and the LUT can cause phase modulation errors when the temperature changes. Methods for measuring phase modulation characteristics of liquid crystal wavefront correctors are disclosed in the literature [ Zhang Yao super, Hu Li Fang, Peng Zen Zeng, etc. ], research on methods for measuring overdrive matrices of liquid crystal wavefront correctors [ J]Liquid crystal and display, 2014,29(5),709-715.
The LUT is affected by temperature because liquid crystals are relatively temperature sensitive: the liquid crystal phase can only exist in a certain temperature range; and when in the liquid crystal phase, the electro-optical characteristics are closely related to the ambient temperature, which is described in detail in the literature [ Hu Li, Pen Zeng, Wang Qidong, et al. In the application of adaptive optics, when experiments are carried out at different time periods and even different seasons in a day, fluctuation of the ambient temperature is inevitable; the temperature fluctuations are more severe if the experiment is performed in an external field. Therefore, one places the adaptive optics system with the liquid crystal wavefront corrector inside a Coude (Coude) room, which can properly reduce the temperature fluctuation, but there is still a possibility of fluctuation in a small range around room temperature. However, the liquid crystal material is sensitive to the influence of temperature, and the fluctuation of temperature may cause the response speed, the phase modulation depth, and particularly, the LUT (Look-up table) to change, thereby affecting the electro-optical characteristics of the liquid crystal wavefront corrector, such as the phase modulation accuracy, which are very critical for the application of the liquid crystal wavefront corrector. On the other hand, a commercial liquid crystal wavefront corrector generally has no temperature control device, but a liquid crystal wavefront corrector of Meadowlark company also has only a heating device, so that the temperature is prevented from being too low; meanwhile, the LUT of the commercial liquid crystal wavefront corrector is well set at a certain room temperature and wavelength when being shipped from a factory, and is generally not changed during experiments. Therefore, a general liquid crystal wavefront corrector does not have the capability of overcoming temperature fluctuation, and the phase modulation precision of the liquid crystal wavefront corrector is reduced due to temperature change in the application process, so that the actual application effect of the liquid crystal wavefront corrector is influenced. Therefore, overcoming the phase modulation accuracy reduction caused by temperature fluctuation is also an important problem to be solved in the application of the liquid crystal wavefront corrector.
Disclosure of Invention
In order to solve the problems and overcome the influence of temperature on the phase modulation precision of the liquid crystal spatial light modulator, the invention provides a correction method of a liquid crystal wavefront corrector based on a dynamic LUT. The method comprises the steps of constructing a light path, constructing an LUT data model, providing a high-precision LUT calculation method capable of overcoming temperature fluctuation influence, measuring phase modulation characteristics at various temperatures, utilizing least square fitting to establish the LUT data model, and substituting the working temperature of the current spatial light modulator into the LUT data model after the ambient temperature changes to obtain reasonable LUT data.
The technical scheme of the invention is as follows:
through constructing an LUT data model, after ambient temperature changes, through setting up the light path, constructing an LUT data model, after ambient temperature changes, through the temperature of current liquid crystal wavefront correction ware work, substitute LUT data model, obtain reasonable LUT data: (1) operation of liquid crystal wavefront correctorsAmbient temperature of T1Measuring the light intensity response of the liquid crystal wavefront corrector under different gray scales at the wavelength lambda; (2) converting the change of light intensity with gray level into the change of phase with gray level; (3) transforming the obtained variation curve of the phase along with the gray level into a monotonous curve; (4) starting from one end of gray level corresponding to high voltage, taking a phase corresponding to a wavelength, dividing it into N gray levels according to quantization level N, wherein N must be less than or equal to 256, obtaining gray-gray curve, i.e. temperature T1LUT table for time and wavelength λ; (5) when the working environment temperature of the liquid crystal wavefront corrector becomes TNThen, repeating the steps to obtain an LUT table under the temperature and the wavelength lambda; (6) performing least square fitting on the obtained LUT data at different temperatures to obtain mathematical models of the LUT at different temperatures;
in one embodiment, the constructed optical path is: the polarization directions of the polarizer and the analyzer are mutually vertical, the long axis orientation included angle of the polarizer and the liquid crystal molecules of the liquid crystal wavefront corrector is 45 degrees, and according to the principle of polarization optics, the relationship between the light intensity I and the phase difference delta can be expressed as follows:
wherein Imax represents a maximum light intensity value, and δ represents a phase difference; by
The variation of the phase with the light intensity can be expressed as
After light emitted by the laser passes through the polarizer, the transmitted light becomes linearly polarized light, and the polarization direction of the linearly polarized light is consistent with that of the polarizer; after passing through the light splitting prism, a part of the light is transmitted, a part of the light is reflected, and the reflected part reaches the liquid crystal wavefront corrector; after being reflected by the liquid crystal wavefront corrector, the liquid crystal wavefront corrector reaches the beam splitting prism again, and one part of the liquid crystal wavefront corrector is transmitted and the other part of the liquid crystal wavefront corrector is reflected; the transmission part penetrates through the analyzer and finally reaches the photodiode, the electric signal of the photodiode is received by the oscilloscope to obtain light intensity data, and the computer performs correction on the liquid crystal wavefront correctorAnd controlling to receive oscilloscope data.
In one embodiment, the working environment temperature of the liquid crystal wavefront corrector is T1When the wavelength lambda is measured, the light intensity response of the liquid crystal wavefront corrector under different gray levels is as follows: taking a linear LUT given in factory production as an LUT for testing the working of the liquid crystal wavefront corrector; generating 256 gray-scale pictures of 512 x 512 pixels with the gray-scale levels of 0,1 and 2 … 255 respectively; then, the computer sends the gray level images of 512 x 512 data to the liquid crystal wavefront corrector from low gray level to high gray level in sequence, the time for keeping each image is 100ms, and the gray level of the image for driving the liquid crystal wavefront corrector in the test changes along with the time; and the sampling interval of the oscilloscope is 20ms, so that each gray level corresponds to 5 data points, and the condition that the light intensity changes along with time is acquired.
In one embodiment, the transformation of the light intensity variation with gray level into the phase variation with gray level is: averaging the obtained 5 light intensity data at the same gray level, and improving the light intensity response measurement precision at the gray level; after the light intensity data in each gray scale is averaged, 256 gray scale values and corresponding average light intensity relations are obtained.
In one embodiment, the transformation of the obtained phase variation curve with gray level into a monotonic curve is: the intensity data obtained from claim 4 is processed to subtract the minimum intensity and then processed according to the formula
A phase distribution is obtained.
In one embodiment, the least square fitting is performed on the obtained LUT data at different temperatures, and the mathematical model of the LUT at different temperatures is obtained by: fitting by adopting a formula, wherein the formula is as follows:
Z＝Z0+A1*x+A2*x2+A3*x3+A4*x4+A5*x5+B1*y+B2*y2+B3*y3+B4*y4+B5*y5x is the position of the gray level of the picture in the x-axis direction, x is more than or equal to 0 and less than or equal to 255, y is the temperature, and the temperature is more than or equal to 16 DEG Cy is less than or equal to 26 ℃; z is the position of the gray level of the picture in the z-axis direction, and z is more than or equal to 0 and less than or equal to 255; the constant term Z0 and the coefficients a1, a2, A3, a4, A5, B1, B2, B3, B4, B5 of each term are Z0 ═ 8028.22085, a1 ═ 15.4063, a2 ═ 0.73756, A3 ═ 0.02041, a4 ═ 2.83892E-4, A5 ═ 1.55766E-6, B1 ═ 1839.40636, B2 ═ 172.84044, B3 ═ 8.05492, B4 ═ 0.18623, B5 ═ 0.00171; according to a formula, calculating to obtain an LUT (look-up table) at any temperature within the range of y being more than or equal to 16 ℃ and less than or equal to 26 ℃, obtaining a real number by using the formula (I), and rounding to obtain an integer; further, 255 is directly assigned to the data with the driving gray level exceeding 255; if the driving gray level is less than zero, the value is directly assigned to 0; finally, the gray scale data of the LUT is obtained.
In one embodiment, the least square fitting is performed on the obtained LUT data at different temperatures, and the mathematical model of the LUT at different temperatures is obtained by: fitting by adopting a formula, wherein the formula is as follows:
x is the position of the gray level of the picture in the x-axis direction, x is more than or equal to 0 and less than or equal to 255, y is the temperature, and y is more than or equal to 16 ℃ and less than or equal to 26 ℃; z is the position of the gray level of the picture in the z-axis direction, and z is more than or equal to 0 and less than or equal to 255; the constant term z20 and the coefficients a01, a21, a22, a23, B01, B02, B03, B21, and B22 of each term are z20 ═ 1255.55138, a01 ═ 11.1413, B01 ═ 1.74234, B02 ═ 0.17339, B03 ═ 0.000549159, a21 ═ 0.5787, a22 ═ 0.03319, a23 ═ 0.0005792, B21 ═ 0.27354, and B22 ═ 0.00357, respectively; according to the formula (4), the LUT at any temperature within the temperature range of 16 ℃ to y to 26 ℃ can be calculated; rounding the real number obtained by the formula (II) to obtain an integer; further, if there is data exceeding 255, 255 is directly assigned; if the data is less than zero, the value is directly assigned to 0; finally, the gray scale data of the LUT is obtained.
According to the invention, by constructing the light path, constructing the LUT data model, substituting the working temperature of the current spatial light modulator into the LUT data model after the environmental temperature changes, the reasonable LUT data is obtained, and the beneficial effects are as follows: the influence of temperature on the phase modulation precision of the liquid crystal spatial light modulator is overcome, and the correction precision of the liquid crystal wavefront corrector is higher.
Drawings
Fig. 1 is an optical path schematic diagram of phase response characteristic detection of a liquid crystal wavefront corrector of the present invention. After light emitted by the laser 1 passes through the polarizer 2, the transmitted light becomes linearly polarized light, and the polarization direction of the linearly polarized light is consistent with that of the polarizer; after passing through the beam splitter prism 3, a part of the light is transmitted, a part of the light is reflected, and the reflected part reaches the liquid crystal wavefront corrector 4; after being reflected by the liquid crystal wavefront corrector 4, the liquid crystal wavefront reaches the beam splitter prism 3 again, and part of the reflected light is transmitted and part of the reflected light is reflected; the transmission part penetrates through the analyzer 5 and finally reaches the photodiode 6, the electric signal of the photodiode is received by the oscilloscope 7 to obtain the data of light intensity, and the control of the liquid crystal wavefront corrector and the receiving of the oscilloscope data are finished by the computer 8.
Fig. 2 is a graph of the gray scale level of a picture of a liquid crystal wavefront corrector driven under test over time.
Fig. 3 shows the change of the light intensity detected by the photodiode 6 with time.
Fig. 4 shows the relationship between 256 gray values and the corresponding average light intensity, wherein the light intensity is the average value of 5 light intensity data measured in the same gray level.
Fig. 5 is a variation of phase distribution with gray scale, in which the phase is a value in the range of 0 to pi.
Fig. 6 is a curve in which the phase changes monotonically and continuously with the gray scale.
FIG. 7 shows LUT data at 16 ℃.
Correspondence between gray levels of the picture of fig. 8 and the gray levels of the driving
Fig. 9 is data of LUT measured in a temperature range of 16 c to 26 c.
FIG. 10 comparison of first order diffraction efficiencies of different LUTs at room temperature 20 ℃.
Detailed Description
The following detailed description of embodiments of the invention is provided in conjunction with the accompanying drawings: the present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a process are given, but the scope of the present invention is not limited to the following embodiments.
Example 1:
a correction method based on a dynamic LUT liquid crystal wavefront corrector comprises the steps of building a light path, building an LUT data model, verifying the LUT data model, and calculating LUT data according to the LUT data model and the working temperature of the liquid crystal wavefront corrector.
Firstly, building a light path:
as shown in FIG. 1, a GCI-0601 type direct current voltage-regulating optical fiber light source of Daheng company is adopted as a laser (1); two 5004 type polarizing plates with the diameter of 50mm, which are manufactured by the new era science and technology limited company, are adopted as a polarizer (2) and an analyzer (5); a GCC-401102 type beam splitter prism of great permanent new epoch science and technology GmbH is adopted as the beam splitter prism (3); adopting a pure phase modulation type liquid crystal spatial light modulator with 512 multiplied by 512 pixels of American Meadowrark company as a liquid crystal wavefront corrector (4); a photodiode is adopted as (6); an oscilloscope of SDS1102CNL type from SIGLENT corporation is adopted as (7); an industrial personal computer is adopted as a computer (8);
the adopted GCI-0601 type direct current voltage-regulating optical fiber light source can be used as a white light source, and the sampling frequency of the adopted SDS1102CNL type oscilloscope is 100 Mhz. The polarization directions of the polarizer (2) and the analyzer (5) are mutually vertical and form an angle of 45 degrees with the long axis orientation of liquid crystal molecules of the liquid crystal wavefront corrector, under the condition, according to the principle of polarization optics, the relationship between the light intensity I and the phase difference delta can be expressed as follows:
where Imax represents the maximum light intensity value. Thus, is composed of
The variation of the phase with the light intensity can be expressed as
After light emitted by the laser (1) passes through the polarizer (2), the transmitted light becomes linearly polarized light, and the polarization direction of the linearly polarized lightThe polarization directions of the devices are consistent; after passing through the beam splitter prism (3), part of the beam is transmitted, part of the beam is reflected, and the reflected part reaches the liquid crystal wavefront corrector (4); after being reflected by the liquid crystal wavefront corrector (4), the liquid crystal wavefront reaches the beam splitter prism (3) again, and part of the reflected light is transmitted and part of the reflected light is reflected; the transmission part penetrates through the analyzer (5) and finally reaches the photodiode (6), the electric signal of the photodiode is received by the oscilloscope (7) to obtain the data of light intensity, and the control of the liquid crystal wavefront corrector and the receiving of the data of the oscilloscope are finished by the computer (8).
Secondly, constructing an LUT data model:
①, according to the method for constructing the optical path, the optical path as shown in figure 1 is constructed, the working environment of the liquid crystal wave-front corrector is set to be 16 ℃, the linear LUT given by factory is used as the LUT for testing the liquid crystal wave-front corrector to work, 256 gray level pictures of 512 x 512 pixels with the gray levels of 0,1 and 2 … 255 are generated, then the computer (8) sends the gray level pictures of the 512 x 512 data to the liquid crystal wave-front corrector from low to high in sequence, the time for holding each picture is 100ms, the change of the gray level of the picture for driving the liquid crystal wave-front corrector in the test along with the time is shown in figure 2, and the sampling interval of the oscilloscope is 20ms, therefore, each gray level corresponds to 5 data points, the acquired signal is shown in figure 3, and the situation that the light intensity changes along with the time is shown in figure 3;
② averaging the 5 light intensity data at the same gray level to improve the measurement accuracy of light intensity response at the gray level, and averaging the light intensity data in each gray level to obtain 256 gray levels and corresponding average light intensity relationship, as shown in FIG. 4;
③ the light intensity data obtained in step ② is processed to subtract the minimum light intensity and then processed according to the formula
A phase distribution is obtained, which is a value in the range of 0 to pi as shown in fig. 5;
④ processing the data obtained in step ③, when the gray levels are 40, 68, 83, 102, 126 and 206, the data are used as the first, second, third, fourth, fifth and sixth inflection points, and the original data is reserved for the data before the first inflection point, the data between the first inflection point and the second inflection point is turned over downwards by taking the inflection point as the axis, the data of the second inflection point and the third inflection point are translated, and by analogy, all the curves of which the phases change along with the gray level are continuously and monotonously processed to obtain the curves of which the phases change along with the gray level in FIG. 6;
⑤ fifthly, corresponding 32 quantization levels to one wavelength, converting the data obtained in the previous step into new relationship that the gray level changes with the original gray level, i.e. LUT data, as shown in FIG. 7. since there are multiple gray levels of the picture corresponding to one driven gray level in FIG. 7, the gray levels of the picture are one-to-one corresponding by level to obtain the LUT data of FIG. 8, FIG. 8 shows the corresponding relationship between the gray level of the picture and the driven gray level;
⑥ processing the data at other temperatures in the same way as the first to fifth steps to obtain LUT data at the temperature, and obtaining LUT at 16 deg.C, 17 deg.C, … 26 deg.C, as shown in FIG. 9;
⑦ least squares fit the LUT data obtained in step ⑥ at different temperatures, and different methods can be used for the fit;
one way of fitting is to fit using the following equation:
Z＝Z0+A1*x+A2*x2+A3*x3+A4*x4+A5*x5+B1*y+B2*y2+B3*y3+B4*y4+B5*y5(i) wherein x is 0 ≦ 60, y is 16 ≦ 26 ≦ Z is 0 ≦ 255, and Z0 ≦ 8028.22085, a1 ≦ 15.4063, a2 ≦ 0.73756, A3 ≦ 0.02041, a4 ≦ 2.83892E-4, a5 ≦ 1.55766E-6, B1 ≦ 1839.40636, B2 ≦ 172.84044, B3 ≦ 8.05492, B4 ≦ 0.18623, B5 ≦ 0.00171. According to the formula (I), an LUT (look-up table) at any temperature within the range of y being more than or equal to 16 ℃ and less than or equal to 26 ℃ can be obtained by calculation, real numbers are obtained by using the formula (I), and integers are obtained by rounding; further, if the driving gray level exceeds 255, 255 is directly assigned; if the driving gray level is less than zero, the value is directly assigned to 0; finally, the gray scale data of the LUT is obtained.
Another way of fitting is to fit using the following equation:
wherein z20 ═ 1255.55138, a01 ═ 11.1413, B01 ═ 1.74234, B02 ═ 0.17339, B03 ═ 0.000549159, a21 ═ 0.5787, a22 ═ 0.03319, a23 ═ 0.0005792, B21 ═ 0.27354, and B2 ═ 0.00357. According to the formula (II), the LUT at any temperature within the temperature range of 16 ℃ to y to 26 ℃ can be calculated. Rounding the real number obtained by the formula (II) to obtain an integer; further, the driven gray scale has data exceeding 255, and is directly assigned with 255; and if the driven gray scale is less than zero, then directly assigning 0; finally, the gray scale data of the LUT is obtained.
Three, LUT data model validation
Comparing the difference of diffraction efficiencies detected by an imaging camera when the liquid crystal wavefront corrector works under different LUTs when the same blazed grating patterns are loaded at a certain temperature, wherein the more reasonable LUT is, the higher the corresponding first-order diffraction efficiency is; LUT data can be obtained at temperatures including 20 degrees Celsius and others according to formula (I) or formula (II). The experimental light path differs from that of fig. 1 in that a CCD camera is used instead of the photodiode 6, and only one polarizer 2 is used, and its polarization direction is adjusted to be the same as the long axis direction of the liquid crystal molecules in the LCOS. Applying a gray scale pattern with 30 wavelengths on a liquid crystal wavefront corrector, obtaining zero-order and first-order diffraction light spots on a CCD camera, respectively selecting reasonable areas on the camera, removing background noise, summing to respectively obtain first-order and zero-order light intensities, and dividing the first-order light intensity by the sum of the zero-order and first-order light intensities to determine diffraction efficiency. The experiment was performed at room temperature 20 ℃, and the diffraction efficiency was calculated by loading LUT at 16 ℃, 18 ℃,20 ℃ and 24 ℃ with the diffraction images obtained by the camera, and fitting the diffraction efficiency data as shown in fig. 10. As can be seen from fig. 10, the diffraction efficiencies obtained by different LUTs are not the same, and only the LUT corresponding to 20 degrees at room temperature has the highest diffraction efficiency of 61.1%; while operating at 16 c corresponding to the LUT, the diffraction efficiency decreased to 54.8%. The fluctuation of the ambient temperature can have certain influence on the phase modulation of the liquid crystal wavefront corrector, and the diffraction efficiency is reduced by reflecting the influence in the experiment, so that the phase modulation precision of the liquid crystal wavefront corrector can be improved by loading a corresponding LUT according to the actual temperature instead of adopting a traditional fixed LUT for experiment.
Fourthly, calculating LUT data according to the LUT data model
In the operation of the liquid crystal wavefront corrector, LUT data at the temperature is generated by the above formula (i) or (ii) in accordance with the temperature of the operating environment, and the liquid crystal wavefront corrector can be operated in a state of the highest phase modulation accuracy by using the LUT.
Although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (7)
1. A correction method based on a dynamic liquid crystal wavefront corrector is characterized in that the method comprises the steps of building a light path, building an LUT data model, verifying the LUT data model, and substituting the working temperature of the current liquid crystal wavefront corrector into the LUT data model to obtain reasonable LUT data after the environmental temperature changes; the construction steps of the LUT data model are as follows:
(I) the working environment temperature of the liquid crystal wavefront corrector is T1Measuring the light intensity response of the liquid crystal wavefront corrector under different gray scales at the wavelength lambda;
(II) converting the change of light intensity with gray scale into the change of phase with gray scale;
(III) converting the obtained variation curve of the phase along with the gray level into a monotonous curve;
(IV) starting from one end of the gray level corresponding to the high voltage, taking a phase corresponding to one wavelength, dividing the phase into N gray levels according to the quantization level N, wherein N must be less than or equal to 256, and obtaining a gray-gray curve, namely the temperatureIs T1LUT table for time and wavelength λ;
(V) when the temperature of the working environment of the liquid crystal wavefront corrector becomes TNThen, the above steps are repeated to obtain the temperature TNLUT tables at lower and wavelength λ;
and (VI) performing least square fitting on the obtained LUT data at different temperatures to obtain mathematical models of the LUT at different temperatures.
2. The correction method based on the dynamic LUT liquid crystal wavefront corrector according to claim 1, characterized in that the built optical path is: the polarization directions of the polarizer (2) and the analyzer (5) are mutually vertical, the long axis orientation included angle of the polarizer (2) and the liquid crystal molecules of the liquid crystal wavefront corrector is 45 degrees, and according to the principle of polarization optics, the relationship between the light intensity I and the phase difference delta can be expressed as follows:
wherein, ImaxRepresents a maximum light intensity value; by
The variation of the phase with the light intensity can be expressed as
After light emitted by the laser (1) passes through the polarizer (2), the transmitted light becomes linearly polarized light, and the polarization direction of the linearly polarized light is consistent with that of the polarizer; after passing through the beam splitter prism (3), part of the beam is transmitted, part of the beam is reflected, and the reflected part reaches the liquid crystal wavefront corrector (4); after being reflected by the liquid crystal wavefront corrector (4), the liquid crystal wavefront reaches the beam splitter prism (3) again, and part of the reflected light is transmitted and part of the reflected light is reflected; the transmission part penetrates through the analyzer (5) and finally reaches the photodiode (6), the electric signal of the photodiode is received by the oscilloscope (7) to obtain light intensity data, and the computer (8) controls the liquid crystal wavefront corrector to receive the oscilloscope data.
3. A method as claimed in claim 1The correction method of the liquid crystal wavefront corrector based on the dynamic LUT is characterized in that the working environment temperature of the liquid crystal wavefront corrector is T1When the wavelength lambda is measured, the light intensity response of the liquid crystal wavefront corrector under different gray levels is as follows: taking a linear LUT given in factory production as an LUT for testing the working of the liquid crystal wavefront corrector; generating 256 gray-scale pictures of 512 x 512 pixels with the gray-scale levels of 0,1 and 2 … 255 respectively; then, the computer (8) sends the gray level images of 512 multiplied by 512 data to the liquid crystal wavefront corrector from low gray level to high gray level in sequence, the time for keeping each picture is 100ms, and the gray level of the picture for driving the liquid crystal wavefront corrector in the test changes along with the time; and the sampling interval of the oscilloscope is 20ms, so that each gray level corresponds to 5 data points, and the condition that the light intensity changes along with time is acquired.
4. The method of claim 1, wherein the transformation of the variation of light intensity with gray scale to the variation of phase with gray scale is: averaging the obtained 5 light intensity data at the same gray level, and improving the light intensity response measurement precision at the gray level; after the light intensity data in each gray scale is averaged, 256 gray scale values and corresponding average light intensity relations are obtained.
5. The method according to claim 4, wherein the transformation of the obtained phase variation curve with gray level into a monotonic curve is: processing the average intensity data obtained according to claim 4 to subtract the minimum intensity and then to formulate
A phase distribution is obtained in which δ represents the phase difference, I represents the light intensity, ImaxRepresenting the maximum light intensity value.
6. The correction method of claim 1, wherein the obtained LUT data at different temperatures is subjected to least square fitting, and the mathematical model of the LUT at different temperatures is obtained by: fitting by adopting a formula, wherein the formula is as follows:
Z＝Z0+A1*x+A2*x2+A3*x3+A4*x4+A5*x5+B1*y+B2*y2+B3*y3+B4*y4+B5*y5(Ⅰ)
wherein x is the position of the gray level of the picture in the x-axis direction, x is more than or equal to 0 and less than or equal to 255, y is the temperature, and y is more than or equal to 16 ℃ and less than or equal to 26 ℃; z is the position of the gray level of the picture in the Z-axis direction, and 0 ≦ Z ≦ 255, and Z0 ≦ 8028.22085, a1 ≦ 15.4063, a2 ≦ 0.73756, A3 ≦ 0.02041, a4 ≦ 2.83892E-4, a5 ≦ 1.55766E-6, B1 ≦ 1839.40636, B2 ≦ 172.84044, B3 ≦ 8.05492, B4 ≦ 0.18623, B5 ≦ 0.00171; according to a formula, calculating to obtain an LUT (look-up table) at any temperature within the range of y being more than or equal to 16 ℃ and less than or equal to 26 ℃, obtaining real numbers by utilizing the formula (I), rounding to obtain integers, and further directly assigning 255 to the data with the driven gray level exceeding 255; if the driving gray level is less than zero, the value is directly assigned to 0; finally, the gray scale data of the LUT is obtained.
7. The correction method of claim 1, wherein the obtained LUT data at different temperatures is subjected to least square fitting, and the mathematical model of the LUT at different temperatures is obtained by: fitting by adopting a formula, wherein the formula is as follows:
wherein x is the position of the gray level of the picture in the x-axis direction, x is more than or equal to 0 and less than or equal to 255, y is the temperature, and y is more than or equal to 16 ℃ and less than or equal to 26 ℃; z is the position of the gray level of the picture in the Z-axis direction, and Z is equal to or greater than 0 and equal to or less than 255, Z20 is equal to 1255.55138, a01 is equal to-11.1413, B01 is equal to 1.74234, B02 is equal to-0.17339, B03 is equal to-0.000549159, a21 is equal to 0.5787, a22 is equal to-0.03319, a23 is equal to 0.0005792, B21 is equal to 0.27354, and B2 is equal to-0.00357; according to the formula (II), the LUT at any temperature within the temperature range of 16 ℃ to y 26 ℃ can be calculated; rounding the real number obtained by the formula (II) to obtain an integer, and further, directly assigning 255 to the data with the driving gray level exceeding 255; and directly assigning 0 to the data with the driving gray level less than zero to finally obtain the gray level data of the LUT.
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