CN109359436B - A Phase Control Method for Small Beam Deflection Based on Liquid Crystal Spatial Light Modulator - Google Patents

Abstract

The invention discloses a thin beam deflection phase control method based on a liquid crystal spatial light modulator, and belongs to the technical field of non-mechanical beam deflection in an active photoelectric system. The method mainly aims at the problem that the stability of an actual scanning point is easily influenced by alignment errors between a light source and a modulator when the thin-beam high-precision scanning is realized by the original sub-aperture dry method. The phase generation method mainly divides an effective modulation area of a modulator into two groups of symmetrical fan-shaped structures, respectively loads adjacent interval angles which can be scanned by a traditional variable-period grating method, and finally realizes other fine angles between corresponding angles of the two symmetrical fan-shaped areas by changing the area ratio of the two symmetrical fan-shaped areas. Meanwhile, when the center of an input light spot deviates from the center of the modulator panel, the method can ensure that the energy proportion of the light beam falling into the two areas is relatively stable due to the energy complementing effect brought by the double-fan-shaped structure, thereby realizing the high stability of the whole scanning point array. The method can be suitable for centrosymmetric light beams with any millimeter-scale aperture, can greatly relax the adjustment precision limit of the system under the condition of keeping the original radial sub-aperture dry method scanning precision unchanged, and simultaneously improves the resistance of a deflection angle to external mechanical vibration. The improvement on the stability can effectively improve the value of the phased array technology for high-precision light beam deflection control.

Description

A Phase Control Method for Small Beam Deflection Based on Liquid Crystal Spatial Light Modulator

technical field

The invention relates to a phase control method for realizing high stability and high precision beam deflection by utilizing a liquid crystal spatial light modulator, and belongs to the technical field of non-mechanical beam deflection in an active photoelectric control system.

Background technique

Beam deflection technology is widely used in lidar, laser communication, laser imaging and remote sensing, and far-field beam shape control. The traditional beam deflection technology generally relies on mechanical devices to achieve beam deflection control by changing the direction of the optical axis, which has the disadvantages of complex structure, large volume, high cost, and high energy consumption. The new beam deflection technology represented by the optical phased array technology realizes the purely electronic deflection of the beam, overcomes many shortcomings of the traditional mechanical beam deflection technology, and shows its huge application potential in the field of beam deflection.

In recent years, with the continuous improvement of the manufacturing process of the liquid crystal spatial light modulator, the beam deflection and pointing accuracy error based on the conventional variable period grating method has reached the micro-radian level. Through the sub-regional phase loading technology of the scanning panel, the scanning accuracy of the modulator can also reach the sub-microradian level. When the input is a large-aperture plane wave, beam deflection with high scanning accuracy and high stability can be achieved by using the lateral sub-aperture coherent (SAC) phase generation method. For small beams with small apertures, although the radial sub-aperture coherence (RSAC) method can ideally achieve the same precision as the SAC method for large-aperture beam deflection, the actual scanning stability is severely limited by the center of the light source and the modulator. The lateral position deviation of the center is specifically divided into a fixed lateral adjustment error and a small amount of spot position jitter caused by environmental factors. In order to improve the stability of the small-aperture beam deflection system without affecting the accuracy, the present invention improves the original radial sub-aperture coherent method into a symmetrical radial sub-aperture coherent method (SRSAC). When the deviation tolerance is exceeded, the drift of the beam deflection angle is greatly reduced by the method of area area compensation, so as to more effectively ensure the equidistant of the scanning point row.

SUMMARY OF THE INVENTION

The purpose of the present invention is to design a new phase generation method for small-aperture beams, and realize beam scanning with a scanning interval angle of sub-microradian order and numerical stability for center-symmetrical beamlets whose aperture is smaller than the width of the modulator panel. And for different beam apertures and different spot position offsets, the modulator can output a stable scanning angle sequence.

The following specific content of the present invention:

如(1)式所示：At present, the most basic phase generation method to achieve beam deflection is the variable period grating method (VPG). For the desired beam deflection angle θ _ideal , theoretically the overall phase distribution on the modulator panel

As shown in formula (1):

Where d is the pixel width, λ is the wavelength of the incident light, x is the position coordinate with the center of the panel as the coordinate origin, round() and mod() are the rounding function and the remainder function, respectively. For the radial sub-aperture coherent method, the modulator panel is divided into two sector-shaped areas and respectively loaded with phase distributions corresponding to different deflection angles, and the fine adjustment of the angle between the two discrete deflection angles is achieved by adjusting the area ratio of the sector-shaped areas , and its phase distribution is shown in Figure 1. In order to reduce the output angle error caused by the spot shift, the present invention discloses a novel radial region division method—the symmetric radial sub-aperture coherence method. In this method, the modulator panel is divided into two groups of symmetrical sector structures, and the phase distribution diagrams are shown in Figure 2. When there is a slight deviation between the center of the incident light spot and the center of the modulator panel, the symmetrical sector area segmentation method in Figure 2 can make the light energy in the two symmetrical sectors in a certain area complement each other, so that the overall light energy in this area is basically maintained. constant. Figure 3 is a schematic diagram of the specific principle: taking the offset in the x-direction as an example, when the offset vector modulo |δ _in |<<R, the available area change is shown in equation (2):

S ⁺ =2Rsin(β _II /2)·δ _in,x +O(δ _in,x ² ) (2a)

S ^- =2Rsin(β _II /2)·δ _in,x -O(δ _in,x ² ) (2b)

where O(δ _in,x ² ) represents the second order fraction of the x-direction alignment error. The total energy change of the two regions is shown in formula (3):

ΔE _II =(S ⁺ -S ^- )·I(R)=O(δ _in,x ² ) (3a)

ΔE _I =-ΔE _II =O(δ _in,x ² ) (3b)

That is, the partial differential relationship at zero offset is obtained, as shown in equation (4a). Similarly, when there is only a y-direction offset, the partial differential relationship is shown in equation (4b).

So for higher-order continuous binary functions E _I (δ _in,x ,δ _in,y ),E _II (δ _in,x ,δ _in,y ), there is a total differential relationship at zero:

Equation (5) shows that for the symmetric radial subaperture phase generation method disclosed in the present invention, a small alignment error in any direction will not change the energy distribution ratio of the incident beam in the two regions. And the simulation results show that the corresponding angle of the intensity centroid of the final coherent superposition of the two beams mainly depends on the energy distribution of the two regions, and is not sensitive to the complex frequency domain phase shift response caused by the spot translation, that is, the incident spot has a stable energy integration in the two regions. The stability of the deflection angle of the outgoing light can be ensured.

The following is the specific design process of the invention:

Step 1: The computer simulation of the diffraction process is carried out using MATLAB software. Select a scanning interval [θ _I , θ _II ) within the scanning range arbitrarily, and define the normalized angle according to formula (6):

When the area occupancy rate is equal to several discrete typical values, the relationship surface between the output angle error θ _error and the input spot offset vector is simulated respectively. Comparing the existing radial sub-aperture coherence method with the symmetric radial sub-aperture coherence method disclosed in the present invention, the simulation results of the two error surfaces are shown in Figures 4, 5, 6, and 7.

Step 2: Referring to the simulation results in Step 1, the key simulation area occupancy rate and the scanning relationship curve of the normalized deflection angle in the x-direction are shown in Figure 8, respectively. 9 shown.

Step 3: Perform linearization sequence reconstruction on the approximate formula of the symmetric radial sub-aperture scanning curve shown in FIG. 9 , and the specific reconstruction method is similar to the existing radial sub-aperture coherent method. That is to say, the approximate curve is divided into equal parts according to the ordinate, and the corresponding abscissa of a series of equidistant scanning points on the ordinate is recorded, and the obtained abscissa sequence is made into a LUT table for search in actual scanning.

Step 4: Build a specific measurement optical path for experimental verification. The schematic diagram and the actual optical path diagram are shown in Figure 10 and Figure 11 respectively. And analyze the measured data to verify the correctness of the computer simulation results.

Compared with the prior art, the present invention has the following advantages and beneficial effects: the present invention designs a new symmetrical radial sub-aperture coherence method on the basis of the existing radial sub-aperture coherence method. Under the premise of ultra-high scanning precision deflection of the beam, the stability of the system when there is an alignment error is greatly improved. The specific performance is that the adjustment accuracy requirements of the system are significantly reduced, and the tolerance of spatial alignment errors is relaxed to the sub-millimeter level. At the same time, the relative position fluctuations of the modulator and the light source caused by environmental factors will no longer affect the deflection of the beam. angle, so that after linearization correction, it can achieve stable deflection angle and equidistant beam scanning with ultra-high scanning accuracy. This will make the beam deflection technology based on the liquid crystal spatial light modulator more practical in the fields of lidar, laser communication, and laser countermeasures.

Description of drawings

Figure 1: The phase diagram of the existing radial sub-aperture coherence method, the horizontal and vertical coordinates are the number of pixels on the modulator panel, and the total width of the panel is L.

(1) is a phase modulation region, the deflection angle θ _I .

(2) is the second area of phase modulation, the deflection angle θ _II .

(3) is the opening angle of a zone, denoted as α _I .

(4) is the opening angle of the second zone, denoted as α _II .

The central angles of the two fan-shaped regions represent their respective region occupancies: η _I =α _I /2π, η _II =α _II /2π. Restrictions on the value of the deflection angle: the scanning angles of the two regions are both integer multiples of θ _step and θ _step = θ _II - θ _I , and the phase distributions of the respective regions are generated according to formula (1).

Figure 2: Phase schematic diagram of the symmetric radial subaperture coherence method disclosed in the present invention. The "x"-shaped dividing line divides the modulator panel into two distinct phase modulation areas.

(1) and (2) are all one area of phase modulation, the deflection angle θ _I .

(3) and (4) are both phase modulation two-zone, deflection angle θ _II .

(5) is the central angle of the one-zone double sector structure, denoted as β _I .

(6) is the central angle of the two-zone double sector structure, denoted as β _II .

The corresponding relationship between the occupancy rates of the two regions and their respective central angles: η _I =β _I /π, η _II =β _II /π. The constraints on the value of the deflection angle are the same as those of the radial sub-aperture coherent method illustrated in FIG. 1 .

Figure 3: Schematic illustration of how the symmetric radial subaperture method reduces the effects of alignment errors. The modulator phase distribution is the same as in Figure 2. The solid line and the dotted circle represent the incident light spots with and without alignment error, respectively, and the radius is R.

(1) is the area increment S ⁺ of the light spot projected on the second area.

(2) The area reduction S ^- of the light spot projected on the second area.

Figure 4: Contour simulation plot of the x-direction output error surface of the radial subaperture coherence method. The simulated spot radius R=3mm; the abscissa and ordinate are the input errors in the x and y directions respectively, the simulation range is (-0.3mm, 0.3mm) ² , and the surface value is the normalized deflection angle error. (a)(b)(c)(d) are the cases where the division ratios of the two regions are equal to 1/6, 2/6, 3/6, and 4/6, respectively.

Figure 5: Contour simulation plot of the y-direction output error surface of the radial subaperture coherence method. The meanings of the simulated spot radius and surface coordinates are the same as in Figure 4. (a)(b)(c) are the cases where the division ratio of the two regions is equal to 1/6, 2/6, and 4/6, respectively.

FIG. 6 and FIG. 7 are respectively the contour simulation diagrams of the output error curved surfaces in the x-direction and the y-direction of the symmetric radial sub-aperture coherent method. The meaning of the spot radius and surface coordinates is the same as that in Figure 4. (a)(b)(c)(d) are the cases where the division ratios of the two regions are equal to 1/6, 2/6, 3/6, and 4/6, respectively.

Figure 8: Schematic diagram of the radial subaperture coherent scanning curve. The scanning relationship curve between the normalized angle θ _norm and the occupancy rate η _II of the second region in a scanning interval with a span of θ _step was recorded. And recorded the changes of the scanning curve under different alignment errors: "○", "△", "□", "*" represent the x-direction alignment errors are 0, +0.2mm, +0.4mm, +0.6mm respectively scatter point of the scan. When the alignment error in the x-direction is negative, the hash point moves down as a whole, which is symmetrical with when the alignment error is positive. The solid black line is the approximate analytical curve for different sweep scatter points.

Figure 9: Schematic diagram of the coherent scanning curve of the symmetric radial subaperture.

"□" represents the scan scatter when there is no alignment error.

"O" represents the scan scatter with an x-direction alignment error of +0.4 mm.

"*" represents the scan scatter with +0.4mm misalignment in the y-direction.

The solid black line is the uniform approximate analytical curve for all swept scatter points.

Figure 10: Schematic diagram of the angle measurement optical path and main components marked. The dark beam in the figure is the measured beam. The space laser emits 730nm laser and is coupled into the autocollimator through a large-diameter fiber. The large-diameter parallel beam is emitted by the auto-collimator and then reflected back to the auto-collimator by the liquid crystal spatial light modulator. The built-in CCD receives the light spot and outputs the beam deflection angle data according to the position of the light spot. The light-colored beam is the illumination beam and is responsible for aligning the fine adjustment diaphragm of the equivalent beamlet with the modulator panel. After the diaphragm alignment is completed, remove the alignment system to measure the angle.

(1) A 730nm laser that emits the light to be measured.

(2) The rotating frosted glass plate driven by the motor is used for the speckle effect caused by the laser coherence.

(3) Large-diameter optical fiber, which is used to guide the light emitted by the laser into the autocollimator.

(4) All-digital micrometer autocollimator, which converts the light beam input by the fiber into a large-aperture plane wave to emit and receives the reflected light, and then outputs the deflection angle value of the reflected light, and the angle measurement accuracy limit is 0.1 μ _rad . Including substructures (5), (6) (7).

(5) Semi-transparent and semi-return mirror.

(6) Collimating optical system.

(7) The collimator has a built-in CCD camera to receive the spot position. (5) (6) (7) are the structural diagrams of (4), which do not represent the specific internal structure of the instrument.

(8) A goniometric computer for analyzing the signal of (7) and outputting the measured angle data.

(9) Inclined polarizer. It is used to filter out the polarized light components that cannot be modulated by the modulator while avoiding the influence of the reflected light itself.

(10) The diaphragm and its high-precision displacement platform are used to block the large-aperture beam emitted by the autocollimator to be equivalent to a small-aperture plane wave. The diaphragm is placed in close contact with the modulator panel and the center of the two is precisely aligned by means of an alignment system. Afterwards, the displacement stage is fine-tuned to achieve accurate equivalence of different alignment errors.

(11) Silicon-based liquid crystal spatial light modulator, which realizes high-precision beam deflection through wavefront phase modulation.

(12) Computer control system of the modulator.

(13) White light source for lateral illumination of diaphragms and modulator panels.

(14) Mirrors, reflective diaphragms and diffuse reflected light from the modulator panel.

(15) Imaging lens for imaging the diaphragm and modulator panel on the alignment CCD.

(16) The CCD camera used for alignment is used to receive the images of the diaphragm and the modulator panel, and the diaphragm position is finely adjusted by observing the images of the two, so as to realize the alignment of the centers of the two.

Figure 11: The actual measurement optical path and main components are marked. (1) to (7) constitute the measurement system, and the measured 730nm beam is shown by the gray arrow in the figure. (8) to (12) constitute an auxiliary alignment system, and the illuminating white light used for alignment is shown by the white arrow in the figure.

(1) 730nm laser. (2) Rotate the frosted glass plate. (3) Large diameter optical fibers. (4) Silicon-based liquid crystal spatial light modulator. (5) Diaphragm and its high-precision displacement platform. (6) All-digital micrometer autocollimator. (7) Polarizer. (8) White light source. (9), (11) Mirrors. (10) Imaging lens. (12) Align the CCD. Before the actual measurement, the beam alignment is performed first, and after the alignment is completed, the (9) reflector is removed and then the actual deflection angle measurement is performed.

Figure 12: Schematic diagram of the measured scan points with radial sub-aperture coherence method with alignment errors. Figure (12a) shows the scan angle distribution before linearization correction. "○" and "□" respectively represent the measured scatter points when the x-direction alignment error is set to ±0.2mm, and the black solid line is the custom fitting curve. Figure (12b) shows the scan angle distribution after linearization correction. "*", "○", "□" represent the measured scatter points when the x-direction alignment error is set to 0 and ±0.2mm, respectively, and the black straight line is the ideal scanning straight line.

Figure 13: Schematic diagram of the measured scanning points with the symmetric radial sub-aperture coherent method with alignment errors. Figure (13a) shows the scan angle distribution before linearization correction. "○" and "□" represent the measured scatter points when the alignment error in the x-direction is set to 0.2 mm and the alignment error in the y-direction is set to 0.2 mm, and the black solid line is the custom fitting curve. Figure (13b) shows the scan angle distribution after linearization correction. "*", "○", "□" represent the measured scatter points when there is no alignment error, the x-direction alignment error is set to 0.2mm, and the y-direction alignment error is set to 0.2mm, and the black line is the ideal scanning line.

Detailed ways

1. Computer simulation simulates the stability of the symmetric radial sub-aperture coherent method, and compares it with the existing radial sub-aperture coherent method.

1) Set the simulation parameters according to the actual device parameters:

Modulator pixel width: d=15μm; Modulator pixel number: 512×512;

Incident light wavelength: 730nm; incident light form: fundamental mode Gaussian beam waist / finite circular aperture plane wave;

Incident light aperture: 3mm; area scan angle difference: 10μrad;

Scanning minimum angle interval: 0.5μrad.

2) The dependence of the output angle error and the input alignment error of the radial sub-aperture coherent method and the symmetric radial sub-aperture coherent method are simulated respectively.

As shown in Figures 4-7. Comparing the simulation results of the two methods yields several conclusions:

a. Comparing Fig. 4 and Fig. 5, we can see that δ out _,x >> δ _out,y of the radial sub-aperture coherence method under normal circumstances, and it can be seen from Fig. 4 that δ _out,x is approximately independent of δ _in,y , so this The subsequent analysis of the method mainly focuses on the correspondence between δ _out,x and δ _in,x .

b. From Figure 6, the δ _out,x of the symmetric radial sub-aperture method is much smaller than that of the radial sub-aperture method. For the fixed alignment errors in different directions of |δ _in |, δ _out,x are at δ _in along x and y respectively. Take the maximum and minimum values for the direction. The δ _out,y of the two methods are all in the order of one-tenth of a micro-radian, and the influence perpendicular to the scanning direction can be ignored during one-dimensional scanning.

3) Based on the conclusion in 2), simulate the change trend of the scanning point sequence of the two methods when there is an alignment error. For the radial sub-aperture method (Fig. 8), a single-parameter approximate analytical formula is established, as shown in formula (7):

The parameter _δc represents the normalized angular offset at an occupancy rate of 0.5. For the symmetric radial subaperture method (Fig. 9), the trend of the scanning point sequence remains basically stable under different alignment errors, so a unified approximate analytical formula can be established. Considering the connection between the above two phase generation methods, the approximate formula of the symmetric radial subaperture coherence method can be solved by equation (7), such as equation (8):

4) The analytical curve in Fig. 9 is linearized and corrected. The specific method is to divide the simulated curve equally according to the ordinate, and record the corresponding abscissa of a series of equidistant scanning points on the ordinate. The obtained abscissa sequence is made into a LUT table for search in actual scanning.

2. The stability of the symmetric radial sub-aperture coherent method is tested experimentally and compared with the existing radial sub-aperture coherent method.

1) Load the radial sub-aperture method, adjust δ _in,x =0, ±0.2mm. Figure 12a shows two sets of scan point sequences with alignment errors and their self-defined fitting results using equation (7) as the target fitting function. Figure 12b shows the linearized scan point sequence. For the radial sub-aperture method, the alignment error will cause the middle of the final output point sequence to move up or down as a whole, which obviously affects the linear characteristics of the point sequence and the relationship between the scan points. Isometric.

2) Load the symmetrical radial sub-aperture method and adjust δ _in,x =0.4mm or δ _in,y =0.4mm. Figure 13a shows two sets of scan point sequences with alignment errors and their unified approximation formula (8). Figure 13b shows the linearized scan point sequence. Different from the radial sub-aperture method, the three groups of final scan point sequences of the symmetric radial sub-aperture method are all consistent with the ideal scan line. It can be seen from the above computer simulation results that the method can output stable scanning point arrays with linear arrangement and equidistant intervals in the range of |δ _in |<0.4mm.

The deviation of the measured points in Figure 12 and Figure 13 from the theoretical value is a random measurement error caused by the limited accuracy of the measurement system, and does not affect the stability of the actual scanning interval angle.

3. The phase calculation process of the practical application of the symmetric radial subaperture method

1) The control system receives the initial ideal angle value θ, and θ needs to be an integer multiple of the minimum scanning interval angle of 0.5 μrad. Judging the scanning area to which it belongs, the scanning angles corresponding to the first area and the second area are respectively shown in equations (9a, 9b):

θ _I = θ _step ×floor(θ/θ _step ) (9a)

θ _II = θ _I + θ _step (9b)

where floor(x) represents the largest integer not greater than x. The normalized angle θ _norm is calculated from the values of θ _I and θ _II , as shown in formula (6).

2) Calculate the subscript of the linear reconstruction sequence from θ _norm : n=θ _norm ×N. In the present invention, N= _θstep /0.5μrad=20, and the value of corresponding N can be calculated according to the minimum scanning interval angle required by the parameters in the actual use process.

3) look up the LUT table of the second district occupancy rate sequence obtained in step 3, record sequence n _II, the η _II value corresponding to i=n in i, be the actual second district occupancy rate, and the first district occupancy rate is n _I = 1-η _II .

4) According to the area occupancy rate, the area is divided according to Fig. 2. Substitute θ _I and θ _II into θ _ideal in formula (1) to obtain the phase distribution of the two regions. This phase distribution is finally loaded on the modulator panel to achieve a beam deflection of angle θ.

It has been verified that the symmetric radial sub-aperture method (SRSAC) disclosed in the present invention and the existing radial sub-aperture coherent method (RSAC) have the same scanning accuracy, and ideally the two have the same deflection effect on the small aperture beam. . However, when there is an alignment error in the system, the existing radial sub-aperture scanning point sequence will be disturbed, and the scanning interval angle floating error will increase accordingly; while the scanning point sequence of the symmetric radial sub-aperture coherent method will basically not deviate from the ideal straight line, Thereby, the limitation of the system's adjustment accuracy is effectively reduced, and the resistance to external disturbance factors is improved at the same time. This significant improvement in stability will greatly enhance the practical value of liquid crystal phased array beam deflection technology.

Claims (1)

1. A beamlet deflection phase control method based on a liquid crystal spatial light modulator, characterized in that: the effective modulation area of the liquid crystal spatial light modulator is divided into four fan-shaped areas with "×"-shaped dividing lines, and the two pairs are symmetrical to each other. Each of the symmetric sectors constitutes a sub-area; then the phase distributions corresponding to different deflection angles are loaded into the two sub-areas according to the variable period grating method, and the fine-tuning between the corresponding deflection angles of the two sub-areas is realized by changing the central angle of the symmetrical sector area. Deflection angle; the phase modulation structure can realize the complementary energy fluctuation in the area when the incident light spot and the center of the modulator are offset, improve the deflection angle stability when the beam has a lateral alignment error, and realize high-precision beam deflection scanning; the The specific implementation steps of the phase control method are as follows: Step 1: Determine the scanning section [θ _I , θ _II ) according to the input target deflection angle, where θ _I =θ _step ×floor(θ/θ _step ), θ _II =θ _I +θ _step , which are two sub-areas respectively The deflection angle of the first zone and the second zone, floor() is the rounding down function; Step 2: Determine the normalized deflection angle θ _norm =(θ-θ _I )/(θ _II -θ _I ) according to the target deflection angle θ and the scanning section [θ _I ,θ _II ); Step 3: According to the area occupancy sequence table, find the area occupancy rate in the occupancy rate sequence whose serial number is equal to θ _norm ×N, which is the actual second area occupancy rate η _II , and the first area occupancy rate η _I =1-η _II , Among them, N is the number of sampling points in one scanning section; the central sector angles of the first and second regions are respectively β _I = η _I · π, β _II = η _II · π, so that the modulator panel is divided into one area , two sub-regions in the second region, each sub-region has a symmetrical double sector structure; Step 4: Load the phase corresponding to the variable period grating method in the first zone and the second zone respectively,

where d is the pixel width, λ is the wavelength of the incident light, x is the position coordinate with the center of the panel as the coordinate origin, round() and mod() are the rounding function and the remainder function respectively, and then the overall phase can be used to achieve the desired High-precision beam deflection control with deflection angle θ.

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