A terahertz full-field phase contrast imaging method using two-dimensional galvanometer scanning illumination

Abstract

This invention discloses a terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination. The method involves rapidly changing the position of the illumination beam using a two-dimensional galvanometer and averaging the imaging frames using detector delayed exposure to obtain a partially coherent illumination field with uniform intensity distribution. An object is imaged using a 4f system, acquiring one image plane and two intensity images at different defocus planes. The axial derivative is obtained using a numerical difference method. This axial derivative is then incorporated into an iterative algorithm for the light intensity transport equation. By continuously reducing the difference between the experimentally calculated axial derivative and the calculated axial derivative, the phase value is continuously accumulated. When the iteration meets the convergence condition, the iteration ends, and the final phase distribution of the image plane is obtained.

Description

Technical Field

[0001] This invention is a terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination. As a novel partially coherent illumination method, two-dimensional galvanometer scanning has the characteristics of uniform illumination and good symmetry, providing excellent illumination conditions for non-interference phase-contrast imaging. Background Technology

[0002] Terahertz waves (THz) lie between the infrared and microwave bands, encompassing electromagnetic waves with frequencies ranging from 0.1 to 10 THz. Due to their broad spectrum, low energy, high penetration, and hydrophobicity, terahertz imaging has wide applications in non-destructive testing, biomedical imaging, and materials characterization. The light intensity transmission equation is itself a second-order elliptic partial differential equation, describing the quantitative coupling relationship between the intensity change along the optical axis and the phase of the light wave in the plane perpendicular to the optical axis during propagation. Solving the light intensity transmission equation requires recording multiple intensity images of the sample under test. The axial derivative of the light intensity of the sample is obtained through numerical difference, and then substituted into the solution equation to obtain the phase distribution of the plane under test. For traditional fully coherent illumination methods, due to the high coherence of the light source and the difference in the optical path through the sample and lens, strong interference artifacts appear in the sensor plane, irreversibly degrading image quality. This is addressed by using a partially coherent illumination light field. It can suppress coherent noise caused by highly coherent terahertz sources, improve the overall signal-to-noise ratio of the system, and give full play to the advantages of the light intensity transmission equation for fast full-field imaging. Summary of the Invention

[0003] The purpose of this invention is to image an object using a 4f system, acquiring one image plane and two light intensity images at different defocus planes, and then calculating the phase distribution of the image plane using an iterative algorithm based on the light intensity transfer equation. Because the object and image are strictly conjugate in the 4f system, light intensity information on the defocus plane of the object can be obtained by moving either the object plane or the image plane. Compared to other terahertz full-field phase-contrast imaging methods, this invention offers faster phase solution for the light intensity transfer equation and lower requirements for the coherence of the light source.

[0004] To achieve the above objectives, the technical solution adopted in this invention is a terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination. The optical path of the imaging system implementing this method includes a carbon dioxide-pumped continuous terahertz laser, a large beam diameter dual-axis scanning galvanometer, an F-Theta scanning lens, a circular aperture stop, the sample under test, an electric translation stage, a TPX terahertz lens, and a miniature radiometric calorimeter array with a lens. A carbon dioxide-pumped continuous-wave terahertz laser was used as the radiation source. A large-beam-diameter dual-axis scanning galvanometer was used to rapidly scan the continuous-wave terahertz wave emitted by the laser. An F-Theta scanning lens was used to correct the scanning light field. The output light field was intercepted using a circular aperture stop, strictly aligned with the optical axis. The sample under test and the aperture stop were assembled together, and a motorized translation stage was used to control its movement along the optical axis. The rear surface of the sample was placed at the front focal plane of the terahertz lens. A miniature radiometric calorimeter array with a lens was placed, ensuring that its front focal plane was located at the rear focal plane of the terahertz lens, and its rear focal plane was located at the detector plane. A clear conjugate image I2(r2) of the sample under test was recorded. A defocused image I of the sample under test was acquired using the motorized translation stage. i (r i ), where i = 1, 3.

[0005] This method involves approximating the intensity axial derivative of the intensity image acquired by the detector using a finite difference approach. Due to the quantitative relationship between the object's phase and axial intensity variations, and the strict conjugate relationship between the object and image in a 4f system, the complex amplitude distribution of the image plane can be obtained using an iterative algorithm based on the light intensity transport equation.

[0006] A terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination comprises four steps:

[0007] S1 adjusts the position of the terahertz lens and the camera lens, and the detector records the intensity map of the clear image of the sample, denoted as I2(r2).

[0008] S2 adjusts the defocus distance Δd from the object surface. i The defocus distance of the image plane is denoted as Δz. i Intensity map I of the defocused image of the sample. i (r i ), where i = 1, 3. The object plane defocus distance Δd i Defocus distance Δz from the image plane i The relationship is:

[0009]

[0010] In the formula, f1 represents the focal length of the terahertz lens, and f2 represents the focal length of the lens of the miniature calorimeter array.

[0011] S3 calculates the axial derivative by taking the finite difference between the intensity images I1(r1) and I3(r3) at different defocus positions along the axis:

[0012]

[0013] In the formula, Δz1 represents the distance between I1(r1) and I2(r2), and Δz3 represents the distance between I3(r3) and I2(r2).

[0014] S4 uses an iterative algorithm based on the light intensity transmission equation to obtain the phase distribution of the image plane r2. The solution process consists of the following three steps:

[0015] S4.1 Order Calculate the initial phase of the iteration

[0016]

[0017] In the formula, I max This represents the maximum value of the measured intensity I²(r²). This represents the inverse Laplace transform, where k is the wavenumber.

[0018] S4.2 Calculate the difference in the axial derivative of light intensity:

[0019]

[0020] In the formula Represents the Laplace operation. Represents the Hamiltonian operator. This represents the divergence operation.

[0021] S4.3 Determine if the iteration has ended. When the iteration calculation satisfies the condition that the iteration count n reaches its maximum value, compensate for the phase difference. threshold ε φ or intensity derivative |VJ n |threshold ε J If the iteration ends, the iteration ends; otherwise, return to the second step and enter the next loop.

[0022] Experimental results from typical examples of this invention demonstrate that, compared to other phase-contrast imaging methods such as digital holography and layered imaging, the two-dimensional galvanometer scanning illumination and light intensity transmission equation iterative algorithm achieve better imaging quality. Compared to coaxial digital holography, this method reduces the requirements for light source coherence and sample size, and due to the influence of the coaxial digital holographic twin, it offers better imaging quality. Compared to off-axis digital holography, it reduces the light source coherence requirement. Compared to layered imaging, it significantly reduces data acquisition and processing time, thereby improving imaging efficiency.

[0023] Compared with existing technologies, this invention is a terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination. The axial derivative is obtained by numerically differencing the intensity distributions under three different defocus planes acquired by the detector. The phase distribution of the central image plane is calculated using an iterative algorithm based on the light intensity transport equation. By rapidly changing the position of the illumination beam using a two-dimensional galvanometer and averaging the imaging frames using time-delayed exposure by the detector, a partially coherent illumination field with uniform intensity distribution can be obtained. The spatial coherence of the illumination field can be flexibly adjusted by controlling the spot size and scanning frequency. Furthermore, the fast convergence speed of its iterative algorithm greatly reduces computation time and improves imaging efficiency. Attached Figure Description

[0024] Figure 1 This is a terahertz full-field phase-contrast imaging method utilizing two-dimensional galvanometer scanning illumination. In the figure: 1. FIRL295 carbon dioxide-pumped continuous terahertz laser; 2. GVS112 large-beam-diameter biaxial scanning galvanometer; 3. Terahertz F-Theta scanning lens; 4. Circular aperture combined with the sample under test; 5. Motorized translation stage; 6. Terahertz lens; 7. INOIRXCAM miniature radiometric calorimeter array combined with a silicon lens. Detailed Implementation

[0025] like Figure 1 As shown, a terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination is characterized by the following optical path of the imaging system: 1. A FIRL 295 carbon dioxide-pumped continuous terahertz laser; 2. A GVS112 large-beam-diameter dual-axis scanning galvanometer; 3. A terahertz F-Theta scanning lens with a front focal length of 50 mm, a center thickness of 23 mm, and a rear focal length of 80 mm; 4. A metal circular aperture combined with the sample under test; 5. An electric translation stage; 6. A terahertz TPX lens with a focal length of 65 mm; 7. An INO IRXCAM miniature radiometric calorimeter array (288×384 pixels, pixel size 35×35 μm) combined with an aspherical F / 0.7 silicon lens with a focal length of 44 mm. A FIRL 295 carbon dioxide-pumped continuous terahertz laser 1 serves as the radiation source, with methanol as the working gas. The output frequency is 2.52 THz, corresponding to a center wavelength of 118.83 μm, and a maximum output power of 500 mW. A GVS112 large-beam-diameter dual-axis scanning galvanometer 2 rapidly scans the continuous terahertz wave emitted by laser 1 into a spatially incoherent illumination field. The mechanical scanning angle is 5°, and the frequency F... x =390Hz, F y=400Hz, phase difference of 87.75°; the front focal point of the terahertz F-Theta scanning lens 3 is located at the Y-reflection mirror of the large beam diameter dual-axis scanning galvanometer 2; the metal circular aperture combined with the sample 4 to be tested is located at the rear focal plane of the terahertz F-Theta scanning lens 3 and the front focal plane of the terahertz TPX lens 6, ensuring strict alignment along the optical axis, and is fixed on the one-dimensional motorized translation stage 6; the silicon lens is combined with the INOIRXCAM miniature radiometric calorimeter array 7, with the front focal plane of the silicon lens located at the rear focal plane of the terahertz TPX lens 6, forming a 4f imaging system, using delayed exposure to average the imaging frames and record the image; the position of the sample along the optical axis is adjusted by the one-dimensional motorized translation stage 6, the defocus distance of the object plane is recorded and the defocus distance of the image plane is calculated, the defocus distance set in the experiment is 0.8mm; the number of pixels of the INO IRXCAM miniature radiometric calorimeter array 7 is 288×384, and the pixel size is 35×35μm.

[0026] The method involves approximating the intensity axial derivative of the defocused image acquired by the detector using finite difference. Due to the quantitative relationship between the phase and axial intensity changes of the object and the strict conjugate relationship between the object and the image in the 4f system, the complex amplitude distribution of the image plane can be obtained using an iterative algorithm based on the light intensity transmission equation.

[0027] A terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination comprises four steps:

[0028] S1 adjusts the position of the terahertz lens and the camera lens, and the detector records the intensity map of the clear image of the sample, denoted as I2(r2).

[0029] S2 adjusts the defocus distance Δd from the object surface. i The defocus distance of the image plane is denoted as Δz. i Intensity map I of the defocused image of the sample. i (r i ), where i = 1, 3. The object plane defocus distance Δd i Defocus distance Δz from the image plane i The relationship is:

[0030]

[0031] In the formula, f1 represents the focal length of the terahertz lens, and f2 represents the focal length of the lens of the miniature calorimeter array.

[0032] S3 calculates the axial derivative by taking the finite difference between the intensity images I1(r1) and I3(r3) at different defocus positions along the axis:

[0033]

[0034] In the formula, Δz1 represents the distance between I1(r1) and I2(r2), and Δz3 represents the distance between I3(r3) and I2(r2).

[0035] S4 uses an iterative algorithm based on the light intensity transmission equation to obtain the phase distribution of the image plane r2. The solution process consists of the following three steps:

[0036] S4.1 Order Calculate the initial phase of the iteration

[0037]

[0038] In the formula, I max This represents the maximum value of the measured intensity I²(r²). This represents the inverse Laplace transform, where k is the wavenumber.

[0039] S4.2 Calculate the difference in the axial derivative of light intensity:

[0040]

[0041] In the formula Represents the Laplace operation. Represents the Hamiltonian operator. This represents the divergence operation.

[0042] S4.3 Determine if the iteration has ended. When the iteration calculation satisfies the condition that the iteration count n reaches its maximum value, compensate for the phase difference. threshold ε φ or intensity derivative |VJ n |threshold ε J If the iteration ends, the iteration ends; otherwise, return to the second step and enter the next loop.

[0043] Experimental results from typical examples of this invention demonstrate that, compared to other phase-contrast imaging methods such as digital holography and layered imaging, the two-dimensional galvanometer scanning illumination and light intensity transmission equation iterative algorithm achieve better imaging quality. Compared to coaxial digital holography, this method reduces the requirements for light source coherence and sample size, and due to the influence of the coaxial digital holographic twin, it offers better imaging quality. Compared to off-axis digital holography, it reduces the light source coherence requirement. Compared to layered imaging, it significantly reduces data acquisition and processing time, thereby improving imaging efficiency.

Claims

1. A terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination, characterized in that: The imaging system for implementing this method includes an optical path comprising a carbon dioxide-pumped continuous terahertz laser, a large-beam-diameter biaxial scanning galvanometer, an F-Theta scanning lens, a circular aperture stop, a sample under test, an electric translation stage, a TPX terahertz lens, and a miniature radiometric calorimeter array with lenses; the carbon dioxide-pumped continuous terahertz laser serves as the radiation source; the large-beam-diameter biaxial scanning galvanometer rapidly scans the continuous terahertz wave emitted by the laser, and the F-Theta scanning is used. The lens is used to correct the scanning light field, and the emitted light field is intercepted using a circular aperture stop and aligned with the optical axis. The sample under test and the aperture stop are assembled together, and a motorized translation stage is used to control their movement along the optical axis. The rear surface of the sample is placed at the front focal plane of the terahertz lens, and a miniature radiometric calorimeter array with a lens is placed to ensure that its front focal plane is located at the rear focal plane of the terahertz lens, and its rear focal plane is located at the detector plane. A clear conjugate image I2(r2) of the sample under test is recorded. The defocused image I of the sample under test is acquired using the motorized translation stage. i (r i ), where i = 1, 3; This method involves approximating the intensity axial derivative of the intensity image acquired by the detector using finite difference, taking into account the quantitative relationship between the phase and axial intensity changes of the object and the strict conjugate relationship between the object and the image in the 4f system; and obtaining the complex amplitude distribution of the image plane using an iterative algorithm based on the light intensity transmission equation. The specific implementation steps are as follows: S1 adjusts the position of the terahertz lens and the camera lens, and uses the detector to record the intensity map of the clear image of the sample, denoted as I2(r2); S2 adjusts the defocus distance Δd from the object surface. i The defocus distance of the image plane is denoted as Δz. i Intensity map I of the defocused image of the sample. i (r i ), where i=1,3; the object plane defocus distance Δd i Defocus distance Δz from the image plane i The relationship is: (1)； In the formula, f1 represents the focal length of the terahertz lens, and f2 represents the focal length of the lens of the miniature calorimeter array. S3 calculates the axial derivative by using the finite difference between the intensity images I1(r1) and I3(r3) at different defocus positions along the axis: (2)； In the formula, Δz1 represents the distance between I1(r1) and I2(r2), and Δz3 represents the distance between I3(r3) and I2(r2); S4 uses an iterative algorithm based on the light intensity transmission equation to obtain the phase distribution of the image plane r2. .

2. The terahertz full-field phase-contrast imaging method using two-dimensional galvanometer scanning illumination according to claim 1, characterized in that, The solution process for S4 is as follows: S4.1 Order Calculate the initial phase of the iteration : (3)； In the formula Indicates the measured intensity The maximum value, This represents the inverse Laplace transform, where k is the wavenumber; S4.2 Calculate the difference in the axial derivative of light intensity: ； In the formula Represents the Laplace operation. Represents the Hamiltonian operator. Represents divergence operations; S4.3 Determine if the iteration has ended; when the iteration calculation meets the required number of iterations. The maximum value is reached, which is the compensation phase difference. threshold or intensity derivative threshold If the iteration ends, the iteration ends; otherwise, return to the second step and enter the next loop.

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