Method for measuring scattering medium complex transmission matrix and imaging based on binary grating phase shift method

Abstract

The invention relates to the field of imaging after laser penetrates through a scattering medium. A method for measuring a scattering medium complex transmission matrix and imaging based on a binary grating phase shift method comprises the steps that light wavefront is modulated on a digital micromirror device (DMD), modulated light passes through a scattering medium (8), then corresponding intensity speckles are collected by a receiving end (11), and the complex transmission matrix T of the scattering medium is solved based on the binary grating phase shift method; and then reconstructing a target object pattern loaded by the digital micromirror device (3) by using a TVAL3 algorithm based on the T and the intensity speckles. The invention also relates to a device for realizing the method.

Description

Technical Field

[0001] The present invention relates to the field of imaging after laser passes through a scattering medium. Background Art

[0002] Scattering media widely exist in nature, such as turbid water bodies, biological tissues, smoke, etc. Due to the inhomogeneity of the refractive index of the scattering medium, when light passes through the scattering medium for transmission, strong scattering will occur, seriously disturbing the incident wavefront information. At this time, traditional optical imaging techniques are no longer applicable. In recent years, many scholars at home and abroad have studied various wavefront shaping techniques to overcome the influence brought by the scattering effect. Among them, the transmission matrix measurement method is one of the effective methods to realize imaging through the scattering medium and regulate the light transmission path. The method of measuring the complex transmission matrix based on a digital micromirror device (DMD) has a significantly higher measurement speed than a liquid crystal spatial light modulator due to the high frame rate modulation of the DMD. However, since the DMD is a binary amplitude modulation device, to realize the regulation of the light field wavefront, it is necessary to combine phase modulation algorithms, such as the Lee method, the superpixel method, etc. to achieve complex amplitude modulation. When measuring the complex transmission matrix of the scattering medium, using the Lee method, the switching of the modulation basis pattern requires re-optimizing the spatial carrier frequency. At the same time, adjust the experimental optical path so that the diffracted first-order light can be effectively filtered out to obtain the best imaging effect. This operation is rather cumbersome. Therefore, there is an urgent need to develop a simple and efficient method for measuring the complex transmission matrix of the scattering medium based on the DMD and imaging. Summary of the Invention

[0003] The technical problem to be solved by the present invention is: how to avoid the operations of re-optimizing the optimal spatial carrier frequency and adjusting the experimental optical path when switching the modulation basis pattern on the DMD.

[0004] The technical solution adopted by the present invention is: a method for measuring the complex transmission matrix of a scattering medium based on a binary grating phase shift method and imaging. Modulate the light wavefront on a digital micromirror device (3), i.e., DMD. The modulated light passes through the scattering medium (8) and is finally collected by the receiving end (11) to obtain the corresponding intensity speckles. First, solve the complex transmission matrix T of the scattering medium based on the binary grating phase shift method, and then reconstruct the target object pattern loaded on the digital micromirror device (3) using the TVAL3 algorithm based on T and the intensity speckles. The specific steps are as follows

[0005] Step 1: Select a Hadamard basis matrix to modulate the light wavefront. The dimension of the Hadamard basis matrix is N×N;

[0006] Step 2: Replace all elements with a value of -1 in the Hadamard basis matrix with 0, and keep the elements with a value of 1 unchanged. Apply i-step phase shift to the Hadamard basis matrix with only 0 and 1 elements using the binary grating phase shift method, and then superimpose it with the reference light using the "checkerboard" reference pattern to finally generate i×N×N modulated patterns E in ;

[0007] Step 3: Sequentially load i×N×N modulated patterns E onto the digital micromirror device (3) in , and collect the intensity speckles corresponding to each modulated pattern E in at the receiving end (11);

[0008] Step 4: Use the intensity speckles in Step 3 to calculate the output optical field E out through the i-step phase shift formula, and use the formula E out = T·E in to solve for the complex transmission matrix T of the scattering medium;

[0009] Step 5: If an unknown target object pattern is loaded onto the digital micromirror device (3), by receiving the corresponding intensity speckles at the receiving end (11) and combining with the complex transmission matrix T of the scattering medium already obtained in Step 4, the target object pattern can be reconstructed using the TVAL3 algorithm.

[0010] The binary grating phase shift method is a method of realizing phase shift using a set of binary grating matrices with elements of 0 and 1. The number of a set of binary grating matrices is equal to the number of phase shift steps i. Each element in each binary grating matrix has the characteristic of periodic cyclic arrangement, and the number of cyclic periods is equal to the number of phase shift steps i. The periodic cycle means that each row of elements in the binary grating matrix is repeated and arranged with the initial i element values of that row. Each row of the binary grating matrix is determined by repeating the initial four rows of the matrix periodically. In addition, among the i binary grating matrices in the same group, the second binary grating matrix is obtained by shifting the first binary grating matrix as a whole one column to the left and placing the extra first column at the end. Similarly, the remaining binary grating matrices can be obtained.

[0011] An apparatus for implementing the method of the present invention includes a solid-state continuous laser (1), a beam expander (2), a digital micromirror device (3), a first lens (4), an aperture stop (5), a second lens (6), a first microscopic objective lens (7), a scattering medium (8), a second microscopic objective lens (9), a third lens (10), a receiving end (11), and a programmable processor (12) that are arranged in sequence and on the same optical path. The laser emitted by the solid-state continuous laser (1) is expanded by the beam expander (2) and then irradiated onto the digital micromirror device (3) for modulation. The laser modulated by the digital micromirror device (3) sequentially passes through the first lens (4), the aperture stop (5), and the second lens (6) and then is conjugated to the rear aperture of the first microscopic objective lens (7). The first microscopic objective lens (7) converges the light and irradiates it onto the scattering medium (8). The second microscopic objective lens (9) and the third lens (10) cooperate to image the speckle light field passing through the scattering medium (8) onto the receiving end (11). The receiving end (11) receives intensity speckles, and the programmable processor (12) controls the digital micromirror device (3) to load a modulation pattern to modulate the wavefront phase of the incident light, and at the same time records the intensity speckle information received by the receiving end (11).

[0012] The Hadamard basis matrix is an orthogonal basis matrix that is commonly used in various engineering applications such as mobile communication, video coding, and quantum computing. Each element value in the Hadamard basis matrix is 1 or -1. The optical wavefront refers to the surface formed by the mass points that have just started to displace at a certain moment when the light wave propagates. Modulation is to use a baseband signal to control the change of one or several parameters of a carrier signal, and load information on it to form a modulated signal for transmission. A digital micromirror device (DMD) is a binary amplitude modulation device that uses a rotating mirror to realize the opening and closing of an optical switch, and only a binary pattern can be loaded on it.

[0013] The beneficial effects of the present invention are as follows: The present invention applies a phase shift to the Hadamard basis matrix by using the binary grating phase shift method, and superimposes it with the reference light through the "checkerboard" reference pattern, realizing the measurement of the complex transmission matrix of the scattering medium and high-quality imaging. The present invention does not require cumbersome operations to optimize the optimal spatial carrier frequency, and there is no need to adjust the device when switching the modulation basis. Description of the Drawings

[0014] Figure 1 is a schematic diagram of the apparatus of the present invention;

[0015] Figure 2 is a schematic diagram of a binary grating matrix with different phase effects, taking the four-step phase shift of 0, π / 2, π, and 3π / 2 as an example;

[0016] Figure 3 are different reference patterns, from left to right are the "checkerboard" reference pattern and the peripheral reference pattern;

[0017] Figure 4 It is a schematic diagram of the result of realizing high-quality imaging by using a complex transfer matrix. From left to right are the target image and the reconstructed image respectively; among them, 1 - solid continuous laser, 2 - beam expander, 3 - digital micromirror device, 4 - first lens, 5 - aperture stop, 6 - second lens, 7 - first microscopic objective lens, 8 - scattering medium, 9 - second microscopic objective lens, 10 - third lens, 11 - receiving end, 12 - programmable processor. Specific implementation manner

[0018] As Figures 1-4 shown, a method for measuring the complex transfer matrix of a scattering medium and imaging based on the binary grating phase-shift method modulates the light wavefront on the digital micromirror device 3, and the modulated light is finally collected by the receiving end 11 to obtain the corresponding intensity speckles after passing through the scattering medium 8. It is characterized in that: firstly, the complex transfer matrix T of the scattering medium is solved based on the binary grating phase-shift method, and then the target object pattern loaded on the digital micromirror device 3 is reconstructed by using the TVAL3 algorithm based on the complex transfer matrix T and the intensity speckles. The specific steps are as follows

[0019] Step 1: Select a Hadamard basis matrix to modulate the light wavefront, and the dimension of the Hadamard basis matrix is N×N;

[0020] Step 2: Replace all elements with a value of -1 in the Hadamard basis matrix with 0, and keep the elements with a value of 1 unchanged. Apply i-step phase shifts to the Hadamard basis matrix with only 0 and 1 elements by using the binary grating phase-shift method, and then superimpose it with the reference light by using the "checkerboard" reference pattern to finally generate i×N×N modulated patterns E in ;

[0021] Step 3: Sequentially load i×N×N modulated patterns E on the digital micromirror device 3 in , and collect the intensity speckles corresponding to each modulated pattern E at the receiving end 11 in ;

[0022] Step 4: Use the intensity speckles in Step 3 to calculate the output light field E through the i-step phase shift formula out , and use the formula E out = T·E in to solve the complex transfer matrix T of the scattering medium;

[0023] Step 5: If an unknown target object pattern is loaded on the digital micromirror device 3, by receiving the corresponding intensity speckles at the receiving end 11 and combining the complex transfer matrix T of the scattering medium obtained in Step 4, the target object pattern can be reconstructed by using the TVAL3 algorithm.

[0024] The binary grating phase shift method is a method of realizing phase shift by using a binary grating matrix with elements only 0 and 1. The number of a group of binary grating matrices is equal to the number of phase shift steps i. Each element in the binary grating matrix has the characteristic of periodic cyclic arrangement, and the number of cyclic periods is equal to the number of phase shift steps i. The periodic cycle means that each row of elements in the binary grating matrix is determined by repeating the initial i element values of that row, and each row of the binary grating matrix is determined by repeating the initial four rows of the matrix periodically. In addition, among the i binary grating matrices in the same group, the second binary grating matrix is obtained by shifting the first binary grating matrix as a whole one column to the left and placing the extra first column at the end. Similarly, the remaining binary grating matrices can be obtained.

[0025] As Figures 1-4 shown, a device for realizing a method of measuring the complex transmission matrix of a scattering medium and imaging based on the binary grating phase shift method includes a solid continuous laser 1, a beam expander 2, a digital micromirror device 3, a first lens 4, an aperture stop 5, a second lens 6, a first microscopic objective 7, a scattering medium 8, a second microscopic objective 9, a third lens 10, a receiving end 11, and a programmable processor 12 that are sequentially arranged and on the same optical path.

[0026] The laser emitted by the solid continuous laser 1 is expanded by the beam expander 2 and then irradiated onto the digital micromirror device 3 for modulation. The laser modulated by the digital micromirror device 3 sequentially passes through the first lens 4, the aperture stop 5, and the second lens 6 and is conjugated to the rear aperture of the first microscopic objective 7. The first microscopic objective 7 converges the light and irradiates it onto the scattering medium 8. The second microscopic objective 9 and the third lens 10 cooperate to image the speckle light field passing through the scattering medium 8 onto the receiving end 11. The receiving end 11 receives the intensity speckles, and the programmable processor 12 controls the digital micromirror device 3 to load modulation patterns to modulate the wavefront phase of the incident light, and at the same time records the intensity speckle information received by the receiving end 11.

[0027] The Hadamard basis matrix is selected to modulate the light wavefront, and the dimension of the Hadamard basis matrix is N×N; first, all elements with a value of -1 in the Hadamard basis matrix are replaced with 0, and the elements with a value of 1 remain unchanged. Secondly, the binary grating phase shift method is used to apply i-step phase shift to the Hadamard basis matrix with elements only 0 and 1, and then it is superimposed with the reference light by using the "checkerboard" reference pattern, and finally i×N×N modulation patterns E in are generated; the programmable processor 12 controls the digital micromirror device 3 to sequentially load i×N×N modulation patterns E in , and collect the intensity speckles corresponding to each modulation pattern at the receiving end 11; the intensity speckles can calculate the output light field E out through the i-step phase shift formula, and then according to the modulation pattern E in , using the formula E out= T·E in The complex transmission matrix T of the scattering medium can be solved; based on the complex transmission matrix T and the intensity speckle of the unknown target object received by the receiving end 11, the TVAL3 algorithm can reconstruct a high-quality target object pattern, as Figure 4 shown.

[0028] In one embodiment: the scattering medium 8 is frosted glass, multimode optical fiber or artificial scattering medium.

[0029] In one embodiment: the receiving end 11 is a CCD camera or an sCOMS camera.

[0030] In one embodiment: as Figure 2 shown, taking four-step phase shift, i.e., i = 4, as an example for specific illustration, there are a total of four binary grating matrices in a group, which respectively play the role of applying phases 0, π / 2, π, and 3π / 2. If the initial four element values of the first row of the first binary grating matrix are "0011", then the initial four element values of each row of this matrix are determined by periodic cycling according to "0011 - 0110 - 1100 - 1001", and each row is determined by periodic arrangement of the initial four element values of that row; the second binary grating matrix is obtained by shifting the first binary grating matrix one column to the left as a whole and placing the first column at the end; similarly, the remaining two binary grating matrices can be obtained. The three-step phase shift is similar to the above process.

[0031] In one embodiment: the reference pattern is used to superimpose the Hadamard basis matrix after the binary grating phase shift processing and the reference light. As Figure 3 shown, the left figure is a "checkerboard" reference pattern, and the right figure is a peripheral reference pattern. For both patterns, the white areas correspond to the Hadamard basis matrix after the binary grating phase shift processing, and the black areas correspond to the reference light pattern.

[0032] The above are only the preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should also be regarded as within the protection scope of the present invention.

Claims

1. A method for measuring and imaging the complex transmission matrix of a scattering medium based on a binary grating phase shift method, wherein a light wavefront is modulated on a digital micromirror device (3), i.e., a DMD, and the modulated light passes through a scattering medium (8) and is finally collected by a receiving end (11) to obtain a corresponding intensity speckle, characterized in that: First, the complex transmission matrix T of the scattering medium is solved based on the binary grating phase shift method. Then, based on T and the intensity speckle, the target object pattern loaded by the digital micromirror device (3) is reconstructed using the TVAL3 algorithm. The specific steps include the following: Step 1: Select a Hadamard basis matrix to modulate the optical wavefront, where the dimension of the Hadamard basis matrix is ​​N×N; Step 2: Replace all elements with -1 in the Hadamard basis matrix with 0, and keep the elements with 1 unchanged. Use the binary grating phase shift method to apply i-step phase shift to the Hadamard basis matrix with only 0 and 1 elements, and then use the "chessboard" reference pattern to superimpose it with the reference light, and finally generate i×N×N modulation patterns E in ; Step 3: Load i×N×N modulation patterns E on the digital micromirror device (3) in sequence in , at the receiving end (11) collects the data corresponding to each modulation pattern E in The corresponding intensity speckle; Step 4: Using the intensity speckle in step 3, calculate the output light field E through the i-step phase shift formula out , using formula E out =T·E in Solve the complex transmission matrix T of the scattering medium; Step 5: If an unknown target object pattern is loaded on the digital micromirror device (3), the target object pattern can be reconstructed using the TVAL3 algorithm by receiving the corresponding intensity speckle at the receiving end (11) and combining it with the scattering medium complex transmission matrix T obtained in step 4.

2. The method for measuring and imaging the complex transmission matrix of a scattering medium based on a binary grating phase shift method according to claim 1, characterized in that: The binary grating phase shift method is a method of realizing phase shift using a set of binary grating matrices whose elements are 0 and 1. The number of a set of binary grating matrices is equal to the phase shift step number i. The elements in each binary grating matrix have the characteristic of periodic cyclic arrangement. The number of cycles is equal to the phase shift step number i. The periodic cycle means that each row of elements in the binary grating matrix is ​​determined by repeated arrangement of the initial i element values ​​of the row. Each row of the binary grating matrix is ​​determined by repeated arrangement of the initial four rows of the matrix. In addition, between the same group of i binary grating matrices, the second binary grating matrix is ​​obtained by moving the first binary grating matrix to the left as a whole by one column, and placing the extra first column at the end. The remaining binary grating matrices can be obtained in the same way.

3. A device for implementing the method of claim 1, characterized in that: The invention comprises a solid continuous laser (1), a beam expander (2), a digital micromirror device (3), a first lens (4), an aperture stop (5), a second lens (6), a first microscope objective lens (7), a scattering medium (8), a second microscope objective lens (9), a third lens (10), a receiving end (11) and a programmable processor (12) arranged in sequence and on the same optical path. The laser light emitted by the solid continuous laser (1) is expanded by the beam expander (2) and then irradiated onto the digital micromirror device (3) for modulation. The laser light modulated by the digital micromirror device (3) is sequentially transmitted through the laser beam expander (2) and the digital micromirror device (3) is modulated. After passing through a first lens (4), an aperture stop (5) and a second lens (6), the light is conjugated to a rear aperture of a first microscope objective lens (7). The first microscope objective lens (7) converges the light and irradiates the light onto a scattering medium (8). The second microscope objective lens (9) and a third lens (10) work together to image the speckle light field passing through the scattering medium (8) onto a receiving end (11). The receiving end (11) receives the intensity speckle. A programmable processor (12) controls a digital micromirror device (3) to load a modulation pattern to modulate the wavefront phase of the incident light, and simultaneously records the intensity speckle information received by the receiving end (11).

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