CN110989314B - A self-reference digital holographic imaging method based on Fresnel double-sided mirror continuous terahertz wave - Google Patents

Abstract

The invention discloses a continuous terahertz wave self-reference digital holographic imaging method based on a Fresnel double-sided mirror, which can realize a common optical path off-axis interference holography with high time stability only by using fewer optical elements. The advantages of this method are that fewer optical components are required, the optical path structure is relatively compact, and the period of the interference fringes can be flexibly adjusted, which is more helpful to meet the sampling conditions of the detector. Since the intensities of the object light and the reference light on the recording plane are almost equal, interference fringes with more consistent contrast can be obtained, which is more conducive to reproducing the amplitude image and the phase image and improving the accuracy. Under the condition that the recording time is the same and the experimental conditions are unchanged, the invented recording device has been verified by experiments that the present invention has higher time stability than the traditional optical path system based on the Mach-Zehnder interferometer structure and structure.

Description

A self-reference digital holographic imaging method based on Fresnel double-sided mirror continuous terahertz wave

technical field

The invention relates to a self-reference digital holography method for improving system time stability, in particular to a self-reference digital holography imaging method based on Fresnel double-sided mirror continuous terahertz waves, which is a continuous terahertz wave based on a common optical path structure. Hertzian Wave Self-Reference Digital Holographic Imaging Method.

Background technique

Terahertz digital holography can be used to reconstruct object wavefronts in real time. The effectiveness of this full-field lensless terahertz phase-contrast imaging approach has been widely used in off-axis interferometer configurations. The commonly used experimental recording optical path in the terahertz digital holography method is a terahertz off-axis interference structure based on the Mach-Zehnder interferometer, where the object and reference beams have an included angle. However, due to the different propagation paths of the object beam and the reference beam, the two beams Environmental disturbances and vibrations are usually sensitive, and there is an inevitable difference in the split intensity ratio of the two beams. For better temporal stability, a common optical path interferometer structure can be used in off-axis holographic methods, i.e. both the object and reference beams travel along almost the same path.

The present invention aims to propose a continuous terahertz wave self-reference digital holography method based on a Fresnel double-sided mirror, which can realize a common optical path off-axis interference holography with high temporal stability only by using fewer optical elements. The advantage of this method is that the optical path structure is relatively compact, and the period of the interference fringe can be flexibly adjusted, which is more helpful to meet the sampling conditions of the detector. Since the intensities of the object light and the reference light on the recording plane are almost equal, interference fringes with more consistent contrast can be obtained.

SUMMARY OF THE INVENTION

A self-referential continuous terahertz wave illumination digital holographic imaging system, the optical path device of the imaging system comprises a _CO2 pumped terahertz laser 1, a first gold-coated off-axis parabolic mirror 2, a second gold-coated off-axis parabolic mirror 3, The sample to be tested 4 , the first gold-coated mirror 5 , the second gold-coated mirror 6 , and the pyroelectric detector 7 . The _CO2 -pumped terahertz laser 1 is used to output continuous terahertz waves, and the _CO2 -pumped terahertz laser 1 corresponds to the first gold-coated off-axis parabolic mirror 2, the first gold-coated off-axis parabolic mirror 2 and the second gold-coated off-axis parabolic mirror 2 The off-axis parabolic mirrors 3 are arranged correspondingly to form a beam expanding unit, which can triple the diameter of the terahertz wave spot output by the CO ₂ pumped terahertz laser 1, and its propagation direction is parallel; The off-axis parabolic mirror 3 reflects the light path. The object light wavefront 4a scattered by the sample 4 is irradiated to the first gold-coated mirror 5 and the second gold-coated mirror 6. The function of the first gold-coated mirror 5 is to reflect part of the light beam carrying the object information in the object light wave 4a The object light wave 5a is generated and propagated to the pyroelectric detector 7. The function of the second gold-coated mirror 6 is to reflect another part of the object light wave 4a (the part that does not carry object information) from another angle to generate the reference light wave 6a and also propagate to the heat. The pyroelectric detector 7 finally interferes with the object light wave 5a, and the off-axis digital hologram is recorded by the pyroelectric detector 7 .

A self-reference digital holographic imaging method based on the Fresnel double-sided mirror continuous terahertz wave carried out by the above system includes the shooting of a digital hologram, and the spectrum of the hologram is obtained by Fourier transform, and the spectrum filtering will "+1 level" The spectrum is filtered out and moved to the center of the spectral domain, and then the inverse Fourier transform is performed to obtain the hologram, and then the angular spectrum algorithm is used to reconstruct the object light field to obtain the complex amplitude distribution of the object light wave. Amplitude distribution, perform the arctangent operation on the complex amplitude to extract the wrapped phase, and then perform the unwrapping operation to obtain the continuous phase distribution of the reconstructed image.

The process of a continuous terahertz wave self-reference digital holographic imaging method based on Fresnel double-sided mirror is divided into three steps:

1) The terahertz wave 3a is scattered by the tested sample 4 to become a terahertz object wavefront 4a. The part of the tested sample 4 containing the object information only occupies half of the spot area of the terahertz beam 3a. If the object information in the tested sample 4 is sparsely distributed, the part containing the object information propagates to the first gold-coated mirror 5, and the object light wave 5a is generated after reflection; the other part of the terahertz object wavefront 4a, that is, the part that does not carry the object information, is generated by the second gold-coated mirror 6. Reference lightwave 6a. The change of the angle between the first gold-coated mirror 5 and the second gold-coated mirror 6 has a linear relationship with the change of the angle between the object light wave 5a and the reference light wave 6a, so the first gold-coated mirror 5 and the second gold-coated mirror The larger the angle between 6, the larger the angle between the object light wave 5a and the reference light wave 6a. The object light wave 5a and the reference light wave 6a interfere and generate an off-axis digital hologram H(x, y) whose interference fringe period can be flexibly adjusted and the intensity is relatively consistent, and the digital hologram H( x, y).

2) Adjust the period of the interference fringes on the hologram H(x, y), and make the spectrum between "+1 level", "-1 level" and "0 level" in the center of the frequency domain in the spectrum after Fourier transform. The distance increases accordingly, which is more conducive to spectral filtering. Then, the hologram is obtained by inverse Fourier transform, and then the complex amplitude distribution of the object light wave is obtained by using the angular spectrum algorithm. The square of the complex amplitude is used to obtain the amplitude distribution of the hologram in the phenomenon, and the arc tangent operation is performed on the complex amplitude to extract the wrapped phase. , and then perform the unwrapping operation to obtain a continuous phase distribution.

3) The change of the phase leads to the change of the optical path length, revealing the temporal stability of the system. Continuously record multiple interferograms without samples, select the central (100×100 pixel) area, process them into corresponding phase distribution maps, and calculate each pixel in these phase distribution maps through the relationship between phase and optical path. The standard deviation of the optical path length variation of the point, this value can represent the time stability of the system. By comparing the standard deviation of the optical path length of the method of the present invention under the same recording and reproducing conditions with the traditional terahertz off-axis digital holography method based on the Mach-Zehnder interferometer structure after recording the same duration and the same interferogram calculated. Compared with the standard deviation of the optical path length, the standard deviation of the optical path length fluctuation of this system is lower, which means that the method has higher time stability.

The experimental results of the typical embodiments of the present invention show that the self-referential terahertz digital holographic imaging method based on Fresnel double mirrors can effectively improve the terahertz digital holographic imaging device by using only two mirrors, a compact optical path structure time stability.

beneficial effect

A method for self-reference digital holographic phase contrast imaging of terahertz waves based on Fresnel double-sided mirrors. The gold-coated mirror 5 and the gold-coated mirror 6 reflect different parts of the object light wavefront 4a, so that the object light wave 5a and the reference light wave 5a are reflected. The light wave 6a forms a suitable off-axis angle, and finally generates a digital holographic interference pattern with the object and the parametric light wave intensity being relatively consistent on the recording surface where the pyroelectric detector 7 is located. Under the same recording duration and experimental conditions, the proposed recording device only needs a small number of optical elements and can have higher temporal stability than the traditional optical path system based on Mach-Zehnder interferometer structure.

Description of drawings

Figure 1 is a system optical path based on Fresnel double mirror terahertz wave self-reference digital holographic imaging.

Figure 2 shows the principle of a self-reference digital holographic imaging system based on Fresnel double-sided mirror terahertz waves.

In the picture: 1. CO ₂ pumped terahertz laser, 2. The first gold-coated off-axis parabolic mirror, 3. The second gold-coated off-axis parabolic mirror, 4. The sample to be tested, 5. The first gold-coated mirror, 6 , The second gold-plated mirror, 7, the pyroelectric detector.

Detailed ways

Exemplary embodiments of the present invention and their features will be described in detail below with reference to the accompanying drawings.

A continuous terahertz wave self-reference digital holographic imaging system based on Fresnel double-sided mirror, the optical path of the system includes a _CO2 pumped terahertz laser 1, a first gold-coated off-axis parabolic mirror 2 (focal length is 25.4mm), The second gold-coated off-axis parabolic mirror 3 (focal length is 76.2 mm), the sample to be tested 4, the first gold-coated mirror 5, the second gold-coated mirror 6 and the pyroelectric detector 7, as shown in FIG. 1 . The terahertz laser in the experiment is a _CO2 -pumped terahertz laser 1 with a frequency of 2.52THz (corresponding to a central wavelength of 118.83μm), which generates a continuous terahertz wave with a maximum power of 500mW. The pixels of the pyroelectric detector 7 are The number is 320 × 320 pixels, the pixel size is 80 μm × 80 μm, and the sampling frequency is 50 Hz. The sample 4 to be tested in the imaging test is the forewing of a cicada, the size of the hologram measured by the pyroelectric detector 7 is 320×320 pixels, and a hologram is collected.

First, referring to the content of the invention, complete the shooting of the hologram:

A self-reference digital holographic imaging method based on Fresnel double-sided mirror continuous terahertz wave, the process of improving the resolution of the method is divided into three steps:

1) The terahertz wave 3a generates a scattered object light wavefront 4a through the sample under test 4, a part of the object light wavefront 4a becomes the object light wave 5a carrying the object information through the first gold-coated mirror 5, and the other part of the object light wavefront 4a does not. The object light wavefront carrying the object information is reflected by the second gold-coated mirror 6 from different angles to become the reference light wave 6a. As shown in FIG. 2 , the included angle between the first gold-coated mirror 5 and the second gold-coated mirror 6 is α, and the included angle between the object light wave 5a and the reference light wave 6a is 2α. Adjusting the angle between the object light wave 5a and the reference light wave 6a is the key to the experiment of the system, which directly determines the period of the interference fringes, so that it satisfies the sampling condition of the detector 0≤2α≤sin ^-1 (λ/2Δx), Δx in the formula is the pixel size of the pyroelectric detector 7, and the maximum 2α in this system is 47.9°.

2) The object light wave 5a and the reference light wave 6a finally interfere on the recording surface where the pyroelectric detector 7 is located and generate an off-axis hologram H(x,y) expressed as:

H(x,y)＝|O(x,y)| ² +|R(x,y)| ² +R ^* (x,y)O(x,y)+R(x,y)O ^* ( x,y)

Among them, * represents the complex conjugate, and O(x, y) and R(x, y) are the object light wave and the reference light wave, respectively. A spectrogram is obtained after Fourier transform of the above digital hologram H(x, y). The relationship between the terms of each level and the angle 2α in the frequency domain is: sin2α=λp/MΔx, where M is the number of pixels of the pyroelectric detector 7, and p is the “+1-level” spectrum in the frequency domain and the frequency domain The spacing between the central "level 0" spectra, in pixels. The formula shows that the larger the angle 2α is, the larger the interval between the "+1 level" and the central "0 level" spectrum in the frequency domain is, which is more conducive to filtering the "+1 level" spectrum. The included angle in this system is 2α=27.7°, which corresponds to a distance of about 98 pixels between the “+1-level” spectrum and the “0-level” spectrum in the corresponding Fourier spectrogram. After zero-filling the filtered "+1" spectrum, a new spectrum with the same pixel size and the same spectrum center position is obtained, and then inverse Fourier transform is performed on it, and then the angular spectrum algorithm is used to obtain the complex amplitude U ₀ of the reproduced image. (x ₀ ,y ₀ ) distribution:

表示傅里叶变换,

表示傅里叶逆变换，h(xo,yo,x,y)代表系统的传递函数，其具体表达式为：in,

represents the Fourier transform,

Represents the inverse Fourier transform, h(xo,yo,x,y) represents the transfer function of the system, and its specific expression is:

Among them, λ is the wavelength of the terahertz wave, f _x , f _y represent the frequency domain coordinates corresponding to the hologram in the x and y directions, d is the distance between the sample 4 to be tested and the pyroelectric detector 7, in this system d=50.5mm.

The phase information of the object is directly obtained by calculating the arctangent of the complex amplitude of the reproduced image:

代表物体相位，物体相位

的值分布于[-π,π]之间，称为包裹相位。若物体相位起伏大于2π时，还需要对相位分布计算的结果进行解包裹运算，本系统采用最小二乘解包裹算法。in,

Represents the phase of the object, the phase of the object

The value of is distributed between [-π,π], which is called the wrapping phase. If the phase fluctuation of the object is greater than 2π, the result of the phase distribution calculation needs to be unwrapped. This system uses the least squares unwrapping algorithm.

其中L为光程长度，△n为被测样品4的折射率与周围介质折射率之差。计算出两组相位分布图中选取区域的每个像素点的光程长度变化的标准差，这个数据可以代表系统的时间稳定性。经过计算得出本方法的光程标准差值是0.36μm，传统使用基于马赫曾德干涉仪结构在相同计算情况下得出的光程标准差值1.0μm，通过比较得出本系统的光程长度波动的标准差更低，表示本方法具有较高的时间稳定性。3) Continuously record 49 interferograms of the system without the measured object 4, under the same conditions (terahertz laser 1, pyroelectric detector 7, first gold-plated off-axis parabolic mirror 2, second gold-plated 49 interferograms collected by the terahertz off-axis digital holography method based on the Mach-Zehnder interferometer structure were continuously recorded. Perform the above calculations on the two sets of interferograms to obtain phase maps, and intercept the central area (100×100 pixels). According to the relationship between phase and optical path:

Among them, L is the optical path length, and Δn is the difference between the refractive index of the tested sample 4 and the refractive index of the surrounding medium. The standard deviation of the optical path length variation of each pixel in the selected area in the two sets of phase distribution maps is calculated, and this data can represent the time stability of the system. After calculation, the standard deviation of the optical path of this method is 0.36μm, and the standard deviation of the optical path based on the Mach-Zehnder interferometer structure is 1.0 μm under the same calculation conditions. The optical path of this system is obtained by comparison. The standard deviation of the length fluctuation is lower, indicating that the method has higher time stability.

The experimental results of the typical embodiment of the present invention show that the digital holographic interference with the same intensity splitting ratio of the object and the parametric wave and the fringe period can be flexibly adjusted can be generated by the continuous terahertz wave self-reference digital holography system based on the Fresnel double mirror. picture. Under the same recording duration and experimental conditions, the invented recording device requires only a few optical elements and can have higher temporal stability than conventionally used Mach-Zehnder interferometer-based devices.

Although the present invention has been described in detail with reference to specific embodiments, the embodiments of the invention described herein are not intended to be exhaustive or limited to the specific form disclosed. Rather, the embodiment chosen for illustration was chosen to enable those skilled in the art to practice the invention. Variations and modifications exist without departing from the essential scope of the invention as described and defined in the claims below.

Claims (6)

1. A self-reference continuous terahertz wave illumination digital holographic imaging method is provided, and an optical path device of an imaging system for realizing the method comprises CO₂The device comprises a pumping terahertz laser (1), a first gold-plated off-axis parabolic mirror (2), a second gold-plated off-axis parabolic mirror (3), a detected sample (4), a first gold-plated reflecting mirror (5), a second gold-plated reflecting mirror (6) and a pyroelectric detector (7); CO 2₂The pump terahertz laser (1) is used for outputting continuous terahertz waves, CO₂Terahertz pumpThe laser (1) corresponds to the first gold-plated off-axis paraboloidal mirror (2), and the first gold-plated off-axis paraboloidal mirror (2) and the second gold-plated off-axis paraboloidal mirror (3) are correspondingly arranged to form a beam expanding unit which can expand CO₂The diameter of a terahertz wave spot output by the pumping terahertz laser (1) is enlarged by three times, and the propagation directions of the terahertz wave spot are parallel; the sample (4) to be detected is arranged on the reflection light path of the second gold-plated off-axis parabolic mirror (3); the terahertz object light wave (4a) which generates scattering through a detected sample (4) irradiates a first gold-plated reflecting mirror (5) and a second gold-plated reflecting mirror (6), the first gold-plated reflecting mirror (5) is used for reflecting part of light beams carrying object information in the terahertz object light wave (4a) to generate the object light wave (5a) and transmitting the object light wave (5a) to a pyroelectric detector (7), the second gold-plated reflecting mirror (6) is used for reflecting the other part of light beams in the object light wave (4a) from another angle to generate reference light wave (6a) and transmitting the reference light wave (6a) to the pyroelectric detector (7), and finally interferes with the object light wave (5a), and an off-axis digital hologram is recorded through the pyroelectric detector (7), and the terahertz object light wave detection device is characterized in that: the imaging method is divided into three steps,

1) the terahertz wave 3a is scattered into a terahertz object light wave (4a) through a tested sample (4), the part containing object information in the tested sample (4) only occupies half of the area of a light spot of the terahertz light beam (3a), if the object information in the tested sample (4) is sparsely distributed, the part containing the object information is transmitted to a first gold-plated reflecting mirror (5), and the object light wave (5a) is generated through reflection; another part of the terahertz object light wave (4a), namely the part which does not carry object information, generates a reference light wave (6a) through a second gold-plated reflector (6); the change of the angle between the first gold-plated reflector (5) and the second gold-plated reflector (6) is in linear relation with the change of the included angle between the object light wave (5a) and the reference light wave (6a), so that the larger the angle between the first gold-plated reflector (5) and the second gold-plated reflector (6), the larger the included angle between the object light wave (5a) and the reference light wave (6a) is; two beams of light of the object light wave (5a) and the reference light wave (6a) interfere with each other to generate an off-axis digital hologram H (x, y) with flexibly adjustable interference fringe period and consistent intensity, and the digital hologram H (x, y) is recorded by using a pyroelectric detector (7);

2) adjusting interference fringe period on the hologram H (x, y), increasing distances between +1 level and-1 level and 0 level at the center of a frequency domain in the spectrogram after Fourier transform, and performing frequency spectrum filtering; obtaining a hologram through Fourier inverse transformation, obtaining complex amplitude distribution of object light waves by using an angular spectrum algorithm, squaring the complex amplitude to obtain the amplitude distribution of the hologram in the phenomenon, performing arc tangent operation on the complex amplitude to extract a wrapping phase, and performing unwrapping operation to obtain continuous phase distribution;

3) continuously recording a plurality of interferograms without samples, selecting a central area and processing the central area into a corresponding phase distribution graph, and calculating the standard deviation of the optical path length change of each pixel point in the phase distribution graphs through a relational expression of phase and optical path.

2. The self-referenced continuous terahertz wave illumination digital holographic imaging method according to claim 1, wherein: the included angle between the first gold-plated reflector (5) and the second gold-plated reflector (6) is alpha, and the included angle between the object light wave (5a) and the reference light wave (6a) is 2 alpha; sin is equal to or greater than 2 alpha and equal to or less than 0 alpha under the sampling condition of the detector^-1(λ/2 Δ x), where Δ x is the pixel size of the pyroelectric detector (7), and 2 α is 47.9 ° at the maximum.

3. The self-referenced continuous terahertz wave illumination digital holographic imaging method according to claim 1, wherein: the object light wave (5a) and the reference light wave (6a) finally interfere on the recording surface where the pyroelectric detector (7) is located and generate an off-axis hologram H (x, y) which is expressed as follows:

H(x,y)＝|O(x,y)|²+|R(x,y)|²+R^*(x,y)O(x,y)+R(x,y)O^*(x,y)

wherein, represents complex conjugation, and O (x, y) and R (x, y) are object light wave and reference light wave respectively; fourier transform is carried out on the digital hologram H (x, y) to obtain a spectrogram; the relationship between each level term and the included angle 2 α in the frequency domain is: sin2 α ═ λ p/M Δ x, where M is the number of pixels of the pyroelectric detector (7), and p is the interval between the "+ 1 level" spectrum in the frequency domain and the central "0 level" spectrum in the frequency domain, in pixels.

4.The self-referenced continuous terahertz wave illumination digital holographic imaging method according to claim 1, wherein: the larger the included angle 2 alpha is, the larger the interval between the "+ 1 level" and the central "0 level" frequency spectrum in the frequency domain is; the included angle 2 α is 27.7 °, and the distance of 98 pixels is between the "+ 1 level" spectrum and the "0 level" spectrum in the corresponding fourier spectrogram; zero filling is carried out on the filtered + 1-level frequency spectrum to obtain a new frequency spectrum with the same pixel size and consistent frequency spectrum center position, then inverse Fourier transform is carried out on the new frequency spectrum, and the complex amplitude U of the reproduced image is obtained by utilizing an angular spectrum algorithm₀(x₀,y₀) Distribution:

wherein,

which represents the fourier transform of the signal,

denotes the inverse Fourier transform, h (x)_o,y_oAnd x, y) represents a transfer function of the system, and the specific expression is as follows:

wherein λ is the wavelength of the terahertz wave, f_x,f_yThe frequency domain coordinates of the hologram in the x and y directions are shown, and d is the distance between the tested sample (4) and the pyroelectric detector (7).

5. The self-referenced continuous terahertz wave illumination digital holographic imaging method according to claim 1, wherein: the phase information of the object is directly obtained by performing the inverse tangent calculation on the complex amplitude of the reproduced image:

wherein,

representing object phase, object phase

Is distributed in [ -pi, pi ] values]In between, referred to as wrapped phase; if the fluctuation of the object phase is more than 2 pi, unwrapping operation needs to be carried out on the result of phase distribution calculation, and the system adopts a least square unwrapping algorithm.

6. The self-referenced continuous terahertz wave illumination digital holographic imaging method according to claim 1, wherein: after the digital hologram is shot, a frequency spectrum of the hologram is obtained through Fourier transform, the frequency spectrum filter filters out a frequency spectrum of +1 level and moves the frequency spectrum to the center of a spectrum domain, then inverse Fourier transform is carried out to obtain the hologram, then an angular spectrum algorithm is used for object light field reconstruction to obtain complex amplitude distribution of object light waves, the complex amplitude is squared to obtain the amplitude distribution of an existing image, arctangent operation is carried out on the complex amplitude to extract a wrapping phase, and then unwrapping operation is carried out to obtain continuous phase distribution of a reconstructed image.

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