A method for controlling the light intensity of a multifocal array in a liquid crystal spatial light modulator

Abstract

This invention discloses a method for controlling the intensity of a multifocal array in a liquid crystal spatial light modulator, relating to the field of light field manipulation technology. The method includes: dividing the entrance pupil region into several sector regions with the center of the objective lens as the vertex, and assigning specific phase values ​​to each sector region to obtain a multifocal phase map; loading the multifocal phase map onto the spatial light modulator for phase modulation, and after entrance pupil phase encoding of the objective lens, obtaining a multifocal array on the focal plane; using image processing methods to calibrate the relationship between the central angle of the phase segmentation region and the light intensity distribution of the multifocal array; adjusting the central angle parameter of the phase segmentation region according to the relationship between the central angle and the light intensity distribution to control the light intensity distribution of the multifocal array; setting the corresponding phase distribution to obtain a multifocal array with controllable intensity distribution, focal position, and morphology. This invention provides efficient calculation for controlling the light intensity distribution of the multifocal array, enabling the acquisition of a multifocal array with controllable intensity, position, and morphology.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of light field modulation, and more particularly to a multi-focus array light intensity modulation method for a liquid crystal spatial light modulator. BACKGROUND

[0002] As a common light field modulation device, the spatial light modulator can conveniently and quickly generate a multi-focus light array. The multi-focus light array has important applications in laser micro-nano machining, optical imaging, laser communication, and optical operation and capture. The simultaneous operation of multiple focal points greatly improves efficiency and accuracy. However, the generated multi-focus array has an uneven light field intensity distribution, or in some cases, such as using a special structured light field for machining, different light field intensities are required for parallel machining of light fields with different morphologies.

[0003] Obata et al. used an iterative two-dimensional Fourier transform algorithm to generate a series of computer holograms, each of which was assigned a different laser power and used the dynamic periodic loading of the SLM to generate a multi-focus laser spot that could individually control the position and laser intensity. Shenglong Rao et al. controlled the light intensity between real-time multi-focal points by filling part of the modulation area in the computer hologram with black, which directly reflected the incident light without adding any modulation. However, most traditional methods for controlling the light field intensity distribution between multi-focal points rely on iterative optimization or indirect modulation, which cannot meet the high-precision and high-efficiency energy distribution requirements.

[0004] Therefore, a multi-focus array light intensity modulation method for a liquid crystal spatial light modulator is proposed to solve the problems existing in the prior art, which is a problem that needs to be solved by those skilled in the art. SUMMARY

[0005] Therefore, the present application provides a multi-focus array light intensity modulation method for a liquid crystal spatial light modulator, which aims to solve the problems of uneven light intensity distribution, high computational complexity, and inaccurate energy modulation in the prior art. The present application generates a multi-focus array with controllable light intensity distribution, focal point position, and morphology through an innovative non-iterative sectorial phase segmentation (SPS) method.

[0006] To achieve the above purpose, the present application provides the following technical solutions:

[0007] A multi-focus array light intensity modulation method for a liquid crystal spatial light modulator, comprising the following steps:

[0008] S1, divide the entrance pupil region into several fan-shaped regions with the center of the objective lens entrance pupil as the vertex, and assign specific phase values to the fan-shaped regions to obtain a multi-focus phase map according to the vector diffraction integral theory;

[0009] S2, load the multi-focus phase map on a spatial light modulator for phase modulation, and after the objective lens is phase encoded by a 4f optical system, obtain a multi-focus array on the focal plane;

[0010] S3, calibrate the relationship between the phase segmentation region central angle and the light intensity distribution of the multi-focus array using an image processing method;

[0011] S4, adjust the phase segmentation region central angle parameter according to the relationship between the phase segmentation region central angle and the light intensity distribution to control the light intensity distribution of the multi-focus array;

[0012] S5, according to S1-S4, set the corresponding phase distribution to obtain a multi-focus array with light intensity distribution, focal point position and topography control.

[0013] The above method, optionally, the specific content of dividing the entrance pupil region into several fan-shaped regions in S1 is:

[0014] Divide the entrance pupil region into N fan-shaped regions with the center of the objective lens entrance pupil as the vertex, that is, generate N main regions;

[0015] Divide each main region into M fan-shaped sub-regions, wherein M and N are positive integers, and M corresponds to the number of focal points expected to be generated.

[0016] The above method, optionally, the mth sub-region in the Nth main region is N-m, and the value of m ranges from 1 to M, then the phase region corresponding to the mth focal point in the light array is 1-m, 2-m, 3-m,..., N-m, that is, the mth sub-region of each main region jointly constitutes the phase region of the mth focal point.

[0017] The above method, optionally, the specific content of assigning specific phase values to the fan-shaped regions to obtain a multi-focus phase map in S1 according to the vector diffraction integral theory is:

[0018] Derive a phase displacement expression according to the vector diffraction integral theory;

[0019] Fill the phase displacement expression into the phase segmentation region of the mth focal point to obtain the phase map of the mth focal point, and similarly, obtain the multi-focus phase map.

[0020] The above method, optionally, the phase displacement expression is derived according to the vector diffraction integral theory, and the formula is as follows:

[0021]

[0022] Wherein, (x1, y1, z1) is the rectangular coordinates of the rear light barrier, λ is the wavelength of the laser, NA is the numerical aperture of the objective, N t is the refractive index of the lens immersion medium, (Δx, Δy, Δz) is the position coordinates of the single focal point in three-dimensional space, and R is the aperture stop radius.

[0023] The method described above, optionally, the specific content of S3 is that the multi-focal point array is calibrated by using an image processing method:

[0024] The multi-focal point array is obtained by using an image processing method to obtain a normalized light intensity distribution, and the relationship between the light intensity distribution and the phase segmentation region central angle of the multi-focal point Gaussian light array and the multi-focal point vortex light array structured light array is calibrated, so as to obtain the corresponding relationship between the light intensity distribution and the phase segmentation region area of the multi-focal point light array.

[0025] The method described above, optionally, the specific content of the relationship between the phase segmentation region central angle and the light intensity distribution in S3 is:

[0026] I m =A·Nθ m

[0027] Wherein, A is a constant, θ m is the central angle of the sub-region N-m, which is used for a multi-focal point light array with the same focal point topography, and N is the number of main regions.

[0028] The method described above, optionally, the specific content of S4 is that the phase segmentation region central angle parameter is adjusted according to the relationship between the phase segmentation region central angle and the light intensity distribution, and the light intensity distribution of the multi-focal point array is controlled:

[0029] According to the relationship between the phase segmentation region central angle and the light intensity distribution, the parameters of the phase segmentation region central angle are adjusted, the area of the phase segmentation region at the entrance pupil is controlled, that is, by unevenly segmenting the phase region at the entrance pupil, the parameters of the phase segmentation region central angle are adjusted, and then the light intensity distribution of the multi-focal point array is controlled.

[0030] The method described above, optionally, the specific content of S4 is that the phase segmentation region central angle parameter is adjusted according to the relationship between the phase segmentation region central angle and the light intensity distribution, and the light intensity distribution of the multi-focal point array is controlled:

[0031] A phase region not containing phase information is reserved, the phase region is only activated when the light intensity of a certain focal point needs to be independently adjusted, and the area of the phase region is locally called or released to realize independent control of the light intensity of a single focal point.

[0032] The method described above, optionally, the specific content of S5 is that the corresponding phase distribution is set to obtain a multi-focal point array with light intensity distribution, focal point position and topography control:

[0033] By changing the position parameters (Δx, Δy, Δz) of the phase shift expression, the free movement of the focal point in three-dimensional space is controlled;

[0034] By bringing the phase factor of the special light field into the phase shift expression, the topographic information of the focal point is controlled;

[0035] According to the relationship, the phase segmentation region center angle parameter is adjusted to control the light intensity distribution of the multi-focus array.

[0036] According to the above technical solutions, compared with the prior art, the present application provides a multi-focus array light intensity control method for a liquid crystal spatial light modulator, which has the following advantages:

[0037] Compared with the traditional multi-focus light field intensity distribution control method, the present application is efficient in calculation and accurate in light intensity control, and the generated multi-focus light array has the characteristics of controllable light intensity distribution, focal point position and topography, and has great application prospects in the fields of laser micro-nano machining, light field imaging, laser communication and optical operation and capture. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiment or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and those skilled in the art can obtain other drawings according to the provided drawings without creative labor.

[0039] Figure 1 A flowchart of a multi-focus array light intensity control method for a liquid crystal spatial light modulator provided by the present application;

[0040] Figure 2 A schematic diagram of the entrance pupil region division method provided by the present application;

[0041] Figure 3 A light path setting diagram provided by the present application;

[0042] Figure 4 A schematic diagram of the entrance pupil region phase division provided by the present application;

[0043] Figure 5 A schematic diagram of the phase region division without phase information provided by the present application;

[0044] Figure 6 A 9-focus light array diagram with sequentially decreasing light intensity distribution realized by the present application;

[0045] Figure 7The application provides a different topological charge 4 vortex focal point light array pattern with uniform light intensity distribution. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the application.

[0047] Referring to Figure 1 As shown in the figure, the application discloses a multi-focal array light intensity control method for a liquid crystal spatial light modulator, referred to as a non-iterative sector partitioning (SPS) method, comprising the following steps:

[0048] S1, taking the center of the objective lens entrance pupil as the vertex, dividing the entrance pupil region into a plurality of sector regions, and according to the vector diffraction integral theory, assigning specific phase values to the sector regions to obtain a multi-focal phase pattern;

[0049] S2, loading the multi-focal phase pattern on the spatial light modulator for phase modulation, and after the 4f optical system performs entrance pupil phase encoding on the objective lens, a multi-focal array is obtained on the focal plane;

[0050] Specifically, in this embodiment, the multi-focal array light intensity control method for the liquid crystal spatial light modulator is verified by experiments, and the optical path setting adopted by the application is as shown in the figure. Figure 3 As shown in the figure, the laser used in the experiment is an 800nm femtosecond laser with a repetition frequency of 80MHz. The incident light beam is incident into the spatial light modulator SLM at a certain angle, the calculated multi-focal phase pattern is loaded onto the spatial light modulator SLM in the form of a gray scale pattern, the gray scale value varies from 0 to 255, and the corresponding phase modulation varies from 0 to 2π. The incident light beam is modulated through the electrically controlled birefringence effect and the twisted nematic effect of the liquid crystal molecules in the liquid crystal panel of the spatial light modulator. After the light beam modulated by the spatial light modulator is subjected to entrance pupil phase encoding on the high numerical aperture objective lens through the 4f optical system, a multi-focal array is obtained on the focal plane.

[0051] S3, calibrating the relationship between the phase partition region central angle and the light intensity distribution of the multi-focal array by using an image processing method;

[0052] S4, adjusting the phase partition region central angle parameter according to the relationship between the phase partition region central angle and the light intensity distribution, and controlling the light intensity distribution of the multi-focal array;

[0053] S5, setting the corresponding phase distribution according to S1-S4, and obtaining the multi-focal array with light intensity distribution, focal point position and topography control.

[0054] Further, the entrance pupil area division method is as shown in the description of S1. Figure 2 As shown in S1, the specific content of dividing the entrance pupil area into several fan-shaped regions is:

[0055] The entrance pupil area is divided into N fan-shaped regions with the center of the objective lens entrance pupil as the vertex, that is, N main regions are generated;

[0056] Each main region is divided into M fan-shaped sub-regions, where M and N are positive integers, and M corresponds to the number of focal points expected to be generated.

[0057] Further, the mth sub-region in the Nth main region is denoted as N-m, and the value of m ranges from 1 to M. The phase region corresponding to the mth focal point in the light array is 1-m, 2-m, 3-m,..., N-m, that is, the mth sub-region of each main region collectively constitutes the phase region of the mth focal point.

[0058] Further, the specific content of S1 is to assign specific phase values to the fan-shaped regions according to the vector diffraction integral theory to obtain a multi-focus phase map.

[0059] The phase displacement expression is derived according to the vector diffraction integral theory.

[0060] The phase displacement expression is sequentially filled into the phase segmentation region of focal point m to obtain the phase map of focal point m. By analogy, a multi-focus phase map is obtained.

[0061] Specifically, this method avoids the complex calculation process of traditional iterative algorithms, significantly improves the calculation efficiency, and can accurately control the light intensity distribution of the multi-focus array.

[0062] Further, the phase displacement expression is derived according to the vector diffraction integral theory, and the formula is as follows:

[0063]

[0064] where (x1, y1, z1) is the rectangular coordinate of the rear stop, λ is the wavelength of the laser, NA is the numerical aperture of the objective lens, N t is the refractive index of the lens immersion medium, (Δx, Δy, Δz) is the position coordinate of a single focal point in three-dimensional space, and R is the aperture stop radius.

[0065] Specifically, by vector diffraction integral theory, the light field intensity when the light beam is focused through a high numerical aperture objective lens can be expressed in the form of a vector diffraction integral formula. Combined with the Fourier shift theorem and Fourier transform, the phase displacement expression is derived.

[0066] Further, the specific content of S3 is to calibrate the multi-focus array using image processing methods.

[0067] The normalized light intensity distribution of the multifocal array is obtained using image processing methods. The relationship between the light intensity distribution of the multifocal Gaussian light array and the multifocal vortex light array structured light array and the central angle of the phase segmentation region is calibrated, and the correspondence between the light intensity distribution of the multifocal light array and the area of ​​the phase segmentation region is obtained.

[0068] Furthermore, the specific details regarding the relationship between the central angle of the phase-segmented region and the light intensity distribution in S3 are as follows:

[0069] I m =A·Nθ m

[0070] Where A is a constant, θ m Let Nm be the central angle of the sub-region, used for multifocal optical arrays with identical focal morphology, where N is the number of main regions.

[0071] Further, see Figure 4 As shown, Figure 4 The left image shows the phase division region of focus 1, and the right image shows the phase division region of focus 2. In S4, the central angle parameter of the phase division region is adjusted according to the relationship between the central angle of the phase division region and the light intensity distribution. The specific details of controlling the light intensity distribution of the multi-focus array are as follows:

[0072] By adjusting the parameters of the central angle of the phase segmentation region according to the relationship between the central angle of the phase segmentation region and the light intensity distribution, the area of ​​the phase segmentation region at the entrance pupil can be controlled. That is, by unevenly dividing the phase region at the entrance pupil and adjusting the parameters of the central angle of the phase segmentation region, the light intensity distribution of the multifocal array can be controlled.

[0073] Specifically, in practical applications, for deviations caused by external factors such as optical path collimation, the real-time light intensity distribution can be obtained by light intensity acquisition, and the parameters of the central angle of the phase segmentation region can be further adjusted until the expected light intensity distribution is obtained.

[0074] Furthermore, in S4, adjusting the central angle parameter of the phase-segmented region based on the relationship between the central angle of the phase-segmented region and the light intensity distribution to control the light intensity distribution of the multi-focus array also includes:

[0075] A phase region that does not contain phase information is reserved. The phase region is activated only when the light intensity of a certain focal point needs to be adjusted independently. The independent control of the light intensity of a single focal point is achieved by locally retrieving or releasing the area of ​​the phase region.

[0076] For a specific diagram showing the phase region division without phase information, please refer to [link / reference]. Figure 5 As shown, the P region in the figure is locally retrieved to achieve independent control of the light intensity of a single focal point.

[0077] Furthermore, the specific details of the multifocal array in S5, which sets the corresponding phase distribution to obtain light intensity distribution, focal position, and topography control, are as follows:

[0078] By changing the position parameters (Δx, Δy, Δz) in the phase displacement expression, the free movement of the focus in three-dimensional space can be controlled;

[0079] By incorporating the phase factor of a special light field into the phase shift expression, the morphological information of the focal point can be controlled;

[0080] Adjust the central angle parameter of the phase segmentation region according to the relationship to control the light intensity distribution of the multifocal array.

[0081] Specifically, for example, by substituting the phase factor exp(ilφ) of the vortex light field into the phase displacement expression, we can obtain the phase displacement expression for the vortex light:

[0082]

[0083] See Figure 6 As shown, this is a 9-focal light array with sequentially decreasing light intensity distribution based on the present invention. The average error between the 9 focal points and the desired light intensity distribution is E = 0.0593. See [link / reference]. Figure 7 The image shows a 4-vortex focal array with different topological charge numbers (L = 0, 2, 3, 4) that produces a consistent light intensity distribution based on the present invention. The average error between the array and the desired light intensity distribution is E = 0.0375.

[0084] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0085] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for multi-focal array light intensity control for liquid crystal spatial light modulators, characterized by, The method comprises the following steps: S1, taking the center of the objective lens entrance pupil as the vertex, dividing the entrance pupil region into several fan-shaped regions, and assigning phase values to the fan-shaped regions according to the vector diffraction integral theory to obtain a multi-focal phase map; S2, loading the multi-focal phase map on a spatial light modulator for phase modulation, and obtaining a multi-focal array on the focal plane after phase encoding of the objective lens entrance pupil by a 4f optical system; S3, calibrating the relationship between the phase segmentation region central angle and the light intensity distribution of the multi-focal array by using an image processing method; S4, adjusting the phase segmentation region central angle parameter according to the relationship between the phase segmentation region central angle and the light intensity distribution to control the light intensity distribution of the multi-focal array; S5, setting the corresponding phase distribution according to S1-S4 to obtain a multi-focal array with light intensity distribution, focal point position and topography control; The specific content of S1 is as follows: According to the vector diffraction integral theory, the phase displacement expression is derived; The phase displacement expression is sequentially filled into the phase segmentation region of the focal point m to obtain the phase map of the focal point m, and the same is done for other focal points to obtain the multi-focal phase map; According to the vector diffraction integral theory, the phase displacement expression is derived, and the formula is as follows: wherein, is the straight coordinate of the back aperture, λ is the laser wavelength, is the numerical aperture of the objective, is the refractive index of the lens immersion medium, is the position coordinate of the single focal point in three-dimensional space, is the aperture radius; The specific content of S3 is as follows: The normalized light intensity distribution of the multi-focal array is obtained by using the image processing method, and the relationship between the light intensity distribution and the phase segmentation region central angle of the multi-focal Gaussian light array and the multi-focal vortex light array structure light array is calibrated to obtain the corresponding relationship between the light intensity distribution of the multi-focal light array and the phase segmentation region area; The specific content of the relationship between the phase segmentation region central angle and the light intensity distribution in S3 is as follows: wherein is a constant, is the central angle of the sub-area N-m for a multi-focal light array with identical focal point topography, is the number of main areas; The specific content of S5 is as follows: By changing the position parameter of the phase shift expression to control the free movement of the focus in three-dimensional space; The phase factor of the special light field is brought into the phase displacement expression to control the topography information of the focal point; According to the relationship, the phase segmentation region central angle parameter is adjusted to control the light intensity distribution of the multi-focal array.

2. The method of claim 1, wherein the method is used for a multi-focal array light intensity control for a liquid crystal spatial light modulator. The specific content of S1 is as follows: Taking the center of the objective lens entrance pupil as the vertex, the entrance pupil region is divided into N fan-shaped regions, i.e. N main regions are generated; Each main region is divided into M fan-shaped sub-regions, where M and N are positive integers, and M corresponds to the number of focal points to be generated.

3. A method for multi-focal array light intensity control for a liquid crystal spatial light modulator according to claim 2, wherein, Let the mth sub-region in the Nth main region be -m, m is in the range of 1-M, the phase region corresponding to the mth focus point in the light array is 1-m, 2-m, 3-m... -m, that is, the mth sub-region of each main region jointly constitutes the phase region of the mth focus point.

4. The method for controlling the light intensity of a multifocal array in a liquid crystal spatial light modulator according to claim 1, characterized in that, The specific content of S4 is as follows: According to the relationship between the phase segmentation region central angle and the light intensity distribution, the parameters of the phase segmentation region central angle are adjusted to control the area of the phase segmentation region at the entrance pupil, i.e. by unevenly dividing the phase region at the entrance pupil, the parameters of the phase segmentation region central angle are adjusted to control the light intensity distribution of the multi-focal array.

5. The method for controlling the light intensity of a multifocal array in a liquid crystal spatial light modulator according to claim 1, characterized in that, The specific content of S4 also includes: Reserve the phase area which does not contain phase information, the phase area is only activated when the light intensity of a certain focus needs to be adjusted independently, and independent control of the light intensity of a single focus is achieved by locally calling or releasing the area of the phase area.

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