Phase switching method of a liquid crystal phase modulator for suppressing optical bounce phenomenon

Abstract

The embodiment of the present invention discloses a phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon. It mainly adopts the method of increasing the application time of the overdrive voltage by a specific amount to obtain the fast and accurate phase switching response performance of the liquid crystal phase modulator, so as to realize the suppression of the optical bounce phenomenon caused by the overdrive technology. The calculation method of the application time of the present invention can be applied to any overdrive response switching process of the liquid crystal phase modulator, and through experimental verification, this method can completely eliminate the optical bounce phenomenon, enabling the modulator to have the fastest response speed and accurate phase switching performance.

Description

Technical Field

[0001] The present invention relates to the technical field of liquid crystal phase modulation, and particularly to a fast and accurate response phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon in an overdrive mode Background Art

[0002] The main principle of the overdrive technology is to use a larger voltage difference to achieve a faster response speed and obtain a shorter response time. At present, this technology has been relatively mature in the display industry; however, its application in liquid crystal phase modulators still has technical bottlenecks: the use of the overdrive technology will cause an optical bounce phenomenon during the phase modulation process, and the phase depression generated by this phenomenon will seriously affect the switching response speed and phase response accuracy between two phases. Using the pulse width modulation method can optimize this depression, but it will increase the power loss to a certain extent and requires additional circuit elements, which limits its low-cost nature and hinders its application pace. Only by significantly improving the above deficiencies can this technology be truly applied

[0003] In the application of liquid crystal phase modulators, the main restrictive factor affecting their phase modulation effect is their response time, especially the response time during the switching process of different phase values. The so-called phase switching time refers to the response time required for the liquid crystal to change from the initial phase to the target phase, and the phase change of the modulator is theoretically determined by the rotation of liquid crystal molecules under the electric field. If the voltage difference required for the liquid crystal device corresponding to two switching states is larger, the rotation process of liquid crystal molecules will be faster; if the response time for the change and switching between different phase amounts is shorter, the time resolution of the liquid crystal phase modulation system will be higher, and the covered application scenarios will be richer

[0004] To shorten the liquid crystal response time, the overdrive technology has greatly shortened the response time of liquid crystal devices through a larger voltage difference and has been applied to the display field. In addition, the response speed of liquid crystal can also be increased by doping liquid crystal polymers or gel factors to form polymer networks or gel networks in the liquid crystal system, and at the same time, the threshold voltage of the liquid crystal device is increased and the effective birefringence of the entire liquid crystal system is reduced Summary of the Invention

[0005] The technical problem to be solved by the embodiments of the present invention is to provide a phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon. For the phase switching response process of a liquid crystal phase modulator under the use of the overdrive technology, the present invention optimizes the application time of the transition voltage, thereby eliminating the occurrence of phase depression, and finally can achieve fast and accurate response of liquid crystal phase switching

[0006] To solve the above technical problems, an embodiment of the present invention provides a phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon, including the following steps:

[0007] S1: Build a liquid crystal response characteristic measurement system, as Figure 1 shown, which includes an optical path laser, a first polarizer, a liquid crystal cell, a second polarizer, a photodetector, an oscilloscope, and a signal generator for applying voltage to the liquid crystal cell in sequence. The laser generated by the laser becomes linearly polarized light after passing through the polarizer. Under the action of the electric field, the liquid crystal molecules relax, resulting in a change in the long axis direction of the molecules, which further causes a change in the value of the effective birefringence of the liquid crystal. As time goes by, the polarization direction of the light passing through the liquid crystal cell gradually changes, and the light intensity passing through the analyzer also gradually changes. Finally, the entire process is captured by the photodetector and recorded by the oscilloscope.

[0008] S2: Implementation of the ordinary unoptimized over-driving technology and the measurement method of the phase switching response curve. Usually, over-voltage driving is achieved by applying a transition voltage with a larger voltage difference between the initial voltage and the target voltage to realize a faster response process of the liquid crystal device. The transition voltage is applied at the initial moment. When the liquid crystal device relaxes to the target phase value in the non-stable equilibrium state, the target voltage is immediately applied. This process is the process realized by the ordinary over-driving technology; corresponding Figure 2 to the curve. Among them, point O is the initial phase point, and point A is the target phase value point. The transition voltage is applied within the time period corresponding to the OA phase curve. After the phase value of the liquid crystal device to which the voltage is applied reaches the phase value at point A, the target voltage corresponding to the target phase is immediately applied. Corresponding to the above voltage application process, the change process of the phase curve of the device in this process can be deduced by theoretical calculation or the intensity measurement system of S1, so as to obtain the calculated simulation values and actual measurement values of the response time and phase depression state during the switching between different phase values.

[0009] S3: Calculate the response time when the initial phase reaches the stable phase value under the action of the transition voltage, denoted as t1; calculate the response time when the target phase reaches the stable phase value under the transition voltage, denoted as t2; calculate the response time t3 when the initial phase value first reaches the target phase value under the transition voltage; obtain a specific extension amount of the transition voltage application time as t = t1 - t2 - t3, and the optimized total transition voltage application time is t0 = t + t3; solve the specific response time mentioned during the response switching process between any two phase values;

[0010] S4: Adjust the application time of the transition voltage in the output waveform of the signal generator by a computer and detect whether there is a phase depression in the corresponding phase response curve during this process; if there is a phase depression, obtain two groups of phase response curves of overdrive and optimized overdrive respectively in the experiment, calculate the time for the two groups of curves to respond to the target phase, and if not, use this time as the application time of the optimized overdrive voltage.

[0011] Implementing the embodiments of the present invention has the following beneficial effects: The present invention obtains the fast and accurate phase switching response performance of the liquid crystal phase modulator by increasing the application time of the overdrive voltage by a specific amount, thereby achieving the suppression of the optical bounce phenomenon caused by the overdrive technology. The determination of the optimal values of the application time of the overdrive voltage under any two different phase switches is mainly based on the theoretical simulation calculation of the liquid crystal phase response characteristics during the overdrive process; the calculation principle for determining the optimal application time is as follows: On the premise of a unified accurate arrival determination criterion for the phase, first determine the time t1 for the target phase to reach the stable phase under the transition voltage, secondly determine the time t2 for the initial phase to reach the stable phase under the transition voltage, then determine the response time t3 from the initial phase value to the target phase value under the transition voltage, and finally obtain the optimal extension amount t = t2 - t3 - t1 of the application time of the transition voltage corresponding to the fast and accurate phase response under any two-phase switches. Therefore, the total application time t0 of the final overdrive voltage can be calculated as t0 = t + t3. This calculation method of the application time can be applied to any overdrive response switching process of the liquid crystal phase modulator, and through experimental verification, this method can completely eliminate the optical bounce phenomenon, making the modulator have the fastest response speed and accurate phase switching performance. Description of the Drawings

[0012] Figure 1 is the liquid crystal response characteristic measurement system;

[0013] Figure 2 is the voltage application method of the overdrive technology and the optimized overdrive technology;

[0014] Figure 3 is the phase response curve of the overdrive technology and the optimized overdrive;

[0015] Figure 4 is the method for obtaining the extended application time of the transition voltage;

[0016] Figure 5 is the experimental flow chart of the technology of the present invention. Detailed Embodiments

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0018] A phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon according to an embodiment of the present invention is implemented by the following method.

[0019] Step 1) Establish a liquid crystal response characteristic measurement system, including a laser, a first polarizer, a liquid crystal cell, a second polarizer, a photodetector, an oscilloscope, and a signal generator for applying a voltage to the liquid crystal cell, which are sequentially arranged to form an optical path.

[0020] The implementation of the ordinary unoptimized overdrive technology and the measurement method of the phase switching response curve. Usually, overvoltage driving is achieved by applying a transition voltage with a larger voltage difference between the initial voltage and the target voltage to realize a faster response process of the liquid crystal device. The transition voltage is applied at the initial moment. When the liquid crystal device relaxes to the target phase value in the non-stable equilibrium state, the target voltage is immediately applied. This process is the process realized by the ordinary overdrive technology; corresponding Figure 2 to the curve. Among them, point O is the initial phase point, and point A is the target phase value point. The transition voltage is applied within the time period corresponding to the OA phase curve. After the phase value of the liquid crystal device to which the voltage is applied reaches the phase value of point A, the target voltage corresponding to the target phase is immediately applied. Corresponding to the above voltage application process, the change process of the phase curve of the device in this process can be deduced by theoretical calculation or the intensity measurement system of S1, so that the calculated simulation values and actual measurement values of the response time and phase depression state under the switching between different phase values can be obtained.

[0021] First, construct a simulation program for simulating the director distribution of liquid crystal molecules under different voltages. Starting from the classical Erickson-Leslie equation, simulate and calculate the voltage application process of the liquid crystal cell, and theoretically establish the relationship between the phase delay of the phase modulator and the voltage change. Since directly solving the Erickson-Leslie equation is a very complex task, the finite difference method is used to approximately numerically solve this equation. The basic concept of this method is to obtain the approximate solutions of the first derivative and second derivative of the function by taking the difference after Taylor expansion of the function in two different ways, and the approximate solutions do not contain derivatives, which can greatly simplify the operation.

[0022] This patent conducts simulation calculations on the liquid crystal molecule director based on the Erickson-Leslie equation, the law of rotation, and the finite difference method, and numerically solves the inclination angle distribution of the director. Here, the successive over-relaxation method is used, and the successive over-relaxation factor ω is set to 0.8. This method is based on the iterative formula θ i obtained by solving the Erickson-Leslie equation. Then, with the set successive over-relaxation factor ω, multiply (1 - ω) by the θ i of the previous iteration and multiply ω by the θ i,newAdding the two together, the resulting value is θ i,new Update the result after iteration based on the successive over-relaxation method. The entire iterative process is a process of continuously approaching the actual solution. Generally speaking, the more times the above iterative process is repeated, the more accurate and practical the obtained numerical solution will be.

[0023] Obtain a new θ based on the over-relaxation factor i Iterative formula for θ i,new = θ i + ω(θ i,new - θ i ), which can significantly reduce the number of iterations and thus accelerate the convergence speed on the basis of the original iterative formula, making the program run faster. The specific formula used is as follows:

[0024] ,

[0025] Using this formula, the inclination angle distribution of the initial liquid crystal molecules in each layer of the liquid crystal cell at any voltage can be obtained. Then, using the following formula

[0026]

[0027] θ(z, t) represents the inclination angle of the liquid crystal molecules at a certain molecular layer position at a certain moment; θ m is the inclination angle of the liquid crystal molecules in the middle layer; the direction of the z-axis is perpendicular to the liquid crystal cell, and z = 0 is at the middle layer position of the liquid crystal;

[0028] K is the liquid crystal elastic coefficient, where K 33 = K 11 = K, K = (K 33 + K 11 ) / 2;

[0029] ϒ1 is the viscosity coefficient, and d is the thickness of the liquid crystal device;

[0030] Here, the small-angle approximation method is adopted to calculate the change of the inclination angle of the liquid crystal molecules in each layer with time.

[0031]

[0032] This formula is the calculation formula for the phase delay amount of the liquid crystal device at different times; θ(z, t) still represents the inclination angle of the liquid crystal molecules at a certain molecular layer at a certain moment; n e , n o correspond to the extraordinary light refractive index and the ordinary light refractive index of the liquid crystal respectively; the integral on the right side of the equation is to sum the phase delay amounts of each layer in this formula.

[0033] Among them, the first formula described above shows the variation of the tilt angle of the liquid crystal molecule director with time during free relaxation after the voltage is removed. The second formula calculates the total response phase value of the liquid crystal device based on the tilt angle distribution of each layer of liquid crystal molecules. In this way, the corresponding response time and phase value of the device can be calculated respectively.

[0034] Step 2) Use 0V as the transition voltage. Taking the overvoltage drive from 5V to 3V as an example, based on the program in Step 1, calculate the initial tilt angle distribution corresponding to the liquid crystal cell at 5V and 3V respectively, and the phase value corresponding to the liquid crystal cell at this time.

[0035] Step 3) Based on Steps 1 and 2, calculate the response time t1 of the process from the initial tilt angle distribution in the 5V state to the complete relaxation of the liquid crystal molecules to the stable state of the tilt angle distribution after the voltage is removed.

[0036] Step 4) Based on Steps 1 and 2, calculate the response time t2 for the relaxation from the phase value corresponding to 5V to the phase value corresponding to 3V after the voltage is removed.

[0037] Step 5) Based on Steps 1 and 2, calculate the response time t3 of the process from the initial tilt angle distribution in the 3V state to the complete relaxation of the liquid crystal molecules to the stable tilt angle distribution after the voltage is removed.

[0038] Step 6) Based on the results of Steps 3, 4, and 5, calculate the extension of the transition voltage application time, denoted as t = t1 - t2 - t3.

[0039] Step 7) Based on the simulation results of Steps 5 and 6, calculate the total application time of the optimized transition voltage of the present invention, denoted as t0 = t + t2.

[0040] Step 8) Conduct experimental verification on the theoretical simulation results, and set up the Figure 1 experimental measurement optical path as shown. From left to right, they are: laser light source, first polarizer, liquid crystal cell and the signal generator for applying voltage to it, second polarizer, photodetector, oscilloscope. In the experimental optical path, a helium-neon laser with a wavelength of 632.8nm is used as the light source. The laser emitted by the laser passes through the first polarizer, the liquid crystal cell (the liquid crystal cell selected is the ECB mode LVE-ECB-4), the second polarizer in sequence, and is finally received by the photodetector and finally displayed on the oscilloscope. The voltage applied to the liquid crystal cell is controlled by the signal generator. The default rubbing direction of the liquid crystal cell is 0 degrees, and the first polarizer P1 and the second polarizer P2 form +45 degrees and -45 degrees with the liquid crystal cell respectively.

[0041] Step 9) Use a computer to write the output signal into a signal generator, which is the AFG3011C model produced by Tektronix. Input a self-made overvoltage drive square wave signal through the computer program Arb Express Application. The frequency of this signal is set to 1 HZ, the peak-to-peak voltage is set to 5 V, the transition voltage is set to 0 V, and the target phase voltage can be adjusted accordingly according to the experimental requirements. The oscilloscope is the DSO-X 2012A model produced by Aligent Technologies. One end of it is connected to a photodetector to detect the change in the brightness of the outgoing light during the experiment.

[0042] Step 10) Let the signal generator control the time of each applied voltage on the liquid crystal cell to explore the optimization process of over-driving. To obtain the transmittance, the signal generator is set to first output a square wave signal from 5 V to 0 V, so that the maximum and minimum outgoing light intensities can be received in the photodetector, which is used to normalize the obtained data. Then input the waveform signal required for over-driving, save the pattern observed on the oscilloscope, and output the saved data in tabular form. At this time, the obtained data is the data of the light intensity signal changing with time, and it is also necessary to further process the obtained data through the equation I / Imax = sin 2 δ / 2 to normalize the detected light intensity. The graph corresponding to the processed data is the curve of the phase delay amount changing with the response time.

[0043] Step 11) As Figure 3 shown, point O is the point corresponding to the initial moment. The initial voltage is applied on the liquid crystal cell before point O, and point A is the point corresponding to the target phase. The time from point O to point A is the usual transition voltage application time. During this period, when the phase value of the phase modulator reaches the phase value of point A, the target voltage is immediately applied to maintain the phase modulation device in the target phase state; its entire phase response process corresponds to the OABC curve, as Figure 3 shown. During the process from the starting phase to the target phase, due to the application of the transition voltage, a phase depression is generated in the phase response, corresponding to the response process shown by the black dotted line ABC in Figure 3 . In this embodiment, at room temperature (under the premise that 99.5% of the target phase value is taken as the criterion for reaching the steady state for the 4.2-micron device filled with E7 liquid crystal), the phase response time between the two gray levels of 5 V and 2.7 V is measured to be 22.0 ms under the usual over-driving technology.

[0044] Step 12) As Figure 4 shown, where P is the initial point, M is the point reaching the target phase, which is equivalent to point A in Figure 3 , and M1 represents the point corresponding to the steady state of the target phase value, which is equivalent to Figure 3Point C in [it]; where, A = M, C = M1. Based on the computational simulation in Step 1, the time for the initial phase point P to reach the stable phase value under the normal transition voltage application method is t PN = t N - t P , corresponding to Figure 3 the response process from O through points A and B to the stable point C in [it]. The time t for the stable target phase (corresponding to point M1) to reach stability under the transition voltage MN = t N - t M ; The response time t from the initial phase to the target phase under the transition voltage PM = t M - t P , and finally, the extended transition voltage application time t pr = t PN - t PM - t MN is obtained. Then, the optimized transition voltage application time is t OOD = t pr + t PM . Using t OOD for optimized overdrive, the voltage application method of the optimized overdrive is as shown by the black line segment below Figure 2 , and the finally obtained curve is as shown by the black solid line in Figure 3 . At room temperature (for a 4.2 - micron device filled with E7 liquid crystal, based on the criterion of reaching 99.5% of the target phase value), the phase - switching response time corresponding to 5 volts to 2.7 volts is measured to be 8.8 ms under the optimized overdrive application scenario. It can be seen from Figure 3 that after using t OOD , the time to respond to the target phase is shortened, and the phase depression phenomenon basically disappears.

[0045] Step 13) Tested the phase depression values for the switching responses between specific 7 gray levels. The seven gray levels and the phase response depression values for the mutual switching between the seven gray levels are shown in Table 1. Among them, the phase depression ratio is defined as: the ratio of the difference between the phase value at the lowest point of the phase depression and the target phase value to the difference between the initial phase value and the target phase value.

[0046]

[0047] Table 1

[0048] The phase depression situation after optimizing with the technology of the present invention is shown in Table 2, where the first row and the first column respectively represent the initial phase and the target phase.

[0049]

[0050] Table 2

[0051] Step 14) As Figure 4 shown, using the tOOD obtained in Step 12, by changing the input waveform of the signal generator and controlling the overdrive time, the purpose of optimizing the phase depression phenomenon and shortening the response time can be achieved. In the experiment, two groups of phase response curves of overdrive and optimized overdrive are obtained respectively, and the response time of the two groups of curves to the target phase is calculated. We tested the response time of switching between specific 7 gray levels, and the phase response time of seven gray levels and the switching between seven gray levels is shown in Table 3.

[0052]

[0053] Table 3

[0054] The optimized response time based on the switching method of the present invention is shown in Table 4.

[0055]

[0056] Table 4

[0057] The final shortening ratio of the response time is shown in Table 5. It can be seen that the optimized response time has been significantly improved, and the highest improvement ratio can reach 77.2%.

[0058]

[0059] Table 5

[0060] The above-disclosed is only a preferred embodiment of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Therefore, equivalent changes made according to the claims of the present invention still fall within the scope covered by the present invention.

Claims

1. A phase switching method for a liquid crystal phase modulator for suppressing optical bounce phenomenon, characterized in that, Including the following steps: S1: Establish a liquid crystal response characteristic measurement system, which includes a laser, a first polarizer, a liquid crystal cell, a second polarizer, a photodetector, an oscilloscope, and a signal generator for applying a voltage to the liquid crystal cell, which are sequentially arranged in an optical path; S2: Calculate the response time when the initial phase reaches the stable phase value under the action of the transition voltage, denoted as t1; calculate the response time when the target phase reaches the stable phase value under the transition voltage, denoted as t2; calculate the response time t3 when the initial phase value first reaches the target phase value under the transition voltage; obtain the specific extension amount of the transition voltage application time as t = t1 - t2 - t3, and the optimized total transition voltage application time as t0 = t + t3; solve the specific response time mentioned during the response switching process between any two phase values; S3: Adjust the application time of the transition voltage in the output waveform of the signal generator through a computer and detect whether there is a phase depression in the corresponding phase response curve during this process; if there is a phase depression, respectively obtain two groups of phase response curves of overdrive and optimized overdrive in the experiment, and calculate the time for the two groups of curves to respond to the target phase. If not, use this time as the application time of the optimized overdrive voltage.

2. The phase switching method of the liquid crystal phase modulator for suppressing optical bounce phenomenon according to claim 1, characterized in that, The method for solving the specific response time mentioned during the response switching process between any two phase values includes calculating the initial phase state of the liquid crystal under different voltages, solving the Erickson-Leslie equation by using the finite difference method, and then obtaining the approximate solution of the equation through the finite difference method and the successive over-relaxation method, and calculating the corresponding director distribution to obtain different phase states of the liquid crystal under different voltages.

3. The phase switching method of the liquid crystal phase modulator for suppressing optical bounce phenomenon according to claim 2, characterized in that The S3 further includes the steps of: Steps for obtaining the transmittance of the liquid crystal cell: Set the signal generator to output a square wave signal from overvoltage drive to transition voltage, and receive the maximum and minimum outgoing light intensities from the photodetector to normalize the obtained data; Input the waveform signal required for over-driving, save the pattern observed on the oscilloscope, and output the saved data in tabular form. At this time, the data obtained is the light intensity signal varying with time. It is also necessary to further process the obtained data through the equation I / Imax = sin 2 δ / 2 to normalize the detected light intensity. The graph corresponding to the processed data is a curve showing the phase delay varying with the response time.

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