CN110989314B - Continuous terahertz wave self-reference digital holographic imaging method based on Fresnel double-sided mirror - Google Patents
Continuous terahertz wave self-reference digital holographic imaging method based on Fresnel double-sided mirror Download PDFInfo
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Abstract
The invention discloses a continuous terahertz wave self-reference digital holographic imaging method based on a Fresnel double-sided mirror, which can realize common-path off-axis interference holography with high time stability by using less optical elements. The method has the advantages of less required optical elements, compact light path structure, flexible adjustment of the period of the interference fringes and capability of meeting the sampling condition of the detector. Because the intensity of the object light and the intensity of the reference light on the recording plane are almost equal, interference fringes with relatively consistent contrast can be obtained, and the amplitude image and the phase image can be reproduced more favorably to improve the precision. Under the conditions of same recording duration and unchanged experimental conditions, the recording device provided by the invention is proved to have higher time stability than the traditional optical path system based on the Mach-Zehnder interferometer structure through experiments.
Description
Technical Field
The invention relates to a method for improving the time stability of a system by self-reference digital holography, in particular to a continuous terahertz wave self-reference digital holographic imaging method based on a Fresnel double-sided mirror, which is a continuous terahertz wave self-reference digital holographic imaging method based on a common light path structure.
Background
Terahertz digital holography can be used to reconstruct the wavefront of an object in real time. The effectiveness of the full-field lens-free terahertz phase contrast imaging method is widely applied to off-axis interferometer structures. A common experimental recording optical path in a terahertz digital holographic method is a terahertz off-axis interference structure which enables object and reference beams to have an included angle based on a Mach-Zehnder interferometer, but because the propagation paths of the object beam and the reference beam are different, the two beams are generally sensitive to environmental interference and vibration, and the spectral intensity ratio of the two beams is inevitably different. For better temporal stability, a common-path interferometer configuration can be used in the off-axis holographic approach, i.e. both the object beam and the reference beam travel along almost the same path.
The invention aims to provide a continuous terahertz wave self-reference digital holography method based on a Fresnel double-sided mirror, which can realize common-path off-axis interference holography with high time stability by using fewer optical elements. The method has the advantages that the light path structure is compact, the period of the interference fringes can be flexibly adjusted, and the sampling condition of the detector can be met. Since the intensities of the object light and the reference light on the recording plane are almost equal, interference fringes with more uniform contrast can be obtained.
Disclosure of Invention
A self-reference continuous terahertz wave illumination digital holographic imaging system comprises a CO optical path device2The 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, CO2The pumping terahertz laser 1 corresponds to the first gold-plated off-axis parabolic mirror 2, and the first gold-plated off-axis parabolic mirror 2 and the second gold-plated off-axis parabolic mirror 3 are correspondingly arranged to form a beam expanding unit which can enable CO to be absorbed by the beam expanding unit2The 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 measured is arranged on the reflection light path of the second gold-plated off-axis parabolic mirror 3. The object light wave front 4a scattered by the tested sample 4 is irradiated to 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 the light beam carrying object information in the object light wave 4a to generate the object light wave 5a to be transmitted to a pyroelectric detector 7, and the second gold-plated reflecting mirror 6 is used for reflecting the other part (the part not carrying the object information) of the light beam in the object light wave 4a from another angle to generate a reference light waveThe light wave 6a also propagates to the pyroelectric detector 7 and finally interferes with the object light wave 5a, and the off-axis digital hologram is recorded by the pyroelectric detector 7.
The self-reference digital holographic imaging method based on the continuous terahertz waves of the Fresnel double-sided mirror comprises the steps of shooting a digital hologram, obtaining the frequency spectrum of the hologram by utilizing Fourier transform, filtering the frequency spectrum to filter out the +1 level frequency spectrum and moving the frequency spectrum to the center of a spectral domain, then carrying out inverse Fourier transform to obtain the hologram, then reproducing an object optical field by utilizing an angular spectrum algorithm to obtain the complex amplitude distribution of the object optical wave, squaring the complex amplitude to obtain the amplitude distribution of an existing image, carrying out arc tangent operation on the complex amplitude to extract a wrapping phase, and then carrying out unwrapping operation to obtain the continuous phase distribution of the reproduced image.
The continuous terahertz wave self-reference digital holographic imaging method based on the Fresnel double mirror comprises the following three steps:
1) the terahertz wave 3a is scattered into a terahertz object wave front 4a through a tested sample 4, the part of the tested sample 4 containing the object information 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 wavefront 4a, i.e. the part not carrying object information, generates the reference light wave 6a through the second gold-plated mirror 6. The change of the angle between the first gold-plated mirror 5 and the second gold-plated mirror 6 is linear with the change of the angle between the object light wave 5a and the reference light wave 6a, so the larger the angle between the first gold-plated mirror 5 and the second gold-plated mirror 6 is, the larger the 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) The interference fringe period on the hologram H (x, y) is adjusted, and the distances between +1 level and-1 level and 0 level in the center of the frequency domain are correspondingly increased in the spectrogram after Fourier transform, which is more beneficial to performing frequency spectrum filtering. Then, a hologram is obtained through Fourier inverse transformation, the complex amplitude distribution of the object light wave is obtained through an angular spectrum algorithm, the complex amplitude is squared to obtain the amplitude distribution of the hologram in the phenomenon, the complex amplitude is subjected to arc tangent operation to extract a wrapping phase, and then unwrapping operation is performed to obtain continuous phase distribution.
3) The change in phase resulting in a change in optical path length reveals the temporal stability of the system. A plurality of interference patterns without samples are recorded continuously, a central (100 multiplied by 100 pixels) area is selected and processed into corresponding phase distribution patterns, and the standard deviation of the optical path length change of each pixel point in the phase distribution patterns is calculated through a relation formula of phase and optical path, and the value can represent the time stability of the system. By comparing the optical path length standard difference value of the method under the same recording and reproducing condition with the optical path length standard difference value calculated by the traditional terahertz off-axis digital holography method based on the Mach-Zehnder interferometer structure after recording the same time length and the same amplitude of interferograms, the standard difference of the optical path length fluctuation of the system is lower, and the method has higher time stability.
The test result of the typical embodiment of the invention shows that the self-reference terahertz digital holographic imaging method based on the Fresnel double-sided mirror can effectively improve the time stability of the terahertz digital holographic imaging device by using a compact light path structure of only two reflectors.
Advantageous effects
A terahertz wave self-reference digital holographic phase contrast imaging method based on a Fresnel double-sided mirror reflects different parts of object light wave front 4a through a gold-plated reflecting mirror 5 and a gold-plated reflecting mirror 6, so that the object light wave 5a and the reference light wave 6a form a proper off-axis angle, and finally a digital holographic interference pattern with consistent object and reference light wave intensity light splitting is generated on a recording surface where a pyroelectric detector 7 is located. The proposed recording device can have higher time stability than the conventional optical path system based on the Mach-Zehnder interferometer structure with only a small number of optical elements under the same recording time and experiment conditions.
Drawings
FIG. 1 is a system optical path based on Fresnel double-mirror terahertz wave self-reference digital holographic imaging.
Fig. 2 is a principle of a self-reference digital holographic imaging system based on a fresnel double-sided mirror terahertz wave.
In the figure: 1. CO 22The device comprises a pumping terahertz laser, 2, a first gold-plated off-axis parabolic mirror, 3, a second gold-plated off-axis parabolic mirror, 4, a sample to be detected, 5, a first gold-plated reflecting mirror, 6, a second gold-plated reflecting mirror, 7 and a pyroelectric detector.
Detailed Description
Exemplary embodiments of the present invention and features thereof are described in detail below with reference to the accompanying drawings.
A continuous terahertz wave self-reference digital holographic imaging system based on a Fresnel double-sided mirror is disclosed, wherein the light path of the system comprises CO2The device comprises a pumping terahertz laser 1, a first gold-plated off-axis parabolic mirror 2 (with a focal length of 25.4mm), a second gold-plated off-axis parabolic mirror 3 (with a focal length of 76.2mm), a sample to be detected 4, a first gold-plated reflecting mirror 5, a second gold-plated reflecting mirror 6 and a pyroelectric detector 7, as shown in FIG. 1. The terahertz laser in the experiment is CO2The terahertz laser 1 is pumped at a frequency of 2.52THz (corresponding to a center wavelength of 118.83 μm), which generates continuous terahertz waves with a maximum power of 500mW, the number of pixels of the pyroelectric detector 7 is 320 × 320 pixels, the pixel size is 80 μm × 80 μm, and the sampling frequency is 50 Hz. The tested sample 4 of the imaging test is the front wing of the cicada, the size of the hologram measured by the pyroelectric detector 7 is 320 multiplied by 320 pixels, and one hologram is collected.
Referring first to the disclosure of the invention, the taking of a hologram is accomplished:
a continuous terahertz wave self-reference digital holographic imaging method based on a Fresnel double-sided mirror is characterized in that the process of improving the resolution ratio of the method comprises the following three steps:
1) the terahertz wave 3a penetrates through a tested sample 4 to generate a scattered object light wave front 4a, one part of the object light wave front 4a becomes an object light wave 5a carrying object information through a first gold-plated reflecting mirror 5, and the other part of the object light wave front 4a not carrying the object information is subjected to second gold-plated reflecting mirror 6Reflected from different angles as the reference light wave 6 a. As shown in fig. 2, the angle between the first gold-plated mirror 5 and the second gold-plated mirror 6 is α, and the angle between the object light wave 5a and the reference light wave 6a is 2 α. The included angle between the object light wave 5a and the reference light wave 6a is the key of the system experiment, which directly determines the period of the interference fringe to meet the sampling condition of 0-2 alpha-sin-1(lambda/2 delta x), where delta x is the pixel size of the pyroelectric detector 7, and 2 alpha is 47.9 deg. at the maximum in the present system.
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) represented as:
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. And carrying out Fourier transform 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 is 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. The equation shows that the larger the angle 2 α, the larger the separation between the "+ 1 order" and the central "0 order" spectrum in the frequency domain, which is more advantageous for filtering the "+ 1 order" spectrum. The included angle 2 α in the present system is 27.7 °, corresponding to a distance of about 98 pixels between the "+ 1 level" spectrum and the "0 level" spectrum in the 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 the content of the first and second substances,
which represents the fourier transform of the signal,
expressing the inverse Fourier transform, h (xo, yo, x, y) represents the transfer function of the system, and the specific expression is as follows:
wherein λ is the wavelength of the terahertz wave, fx,fyThe frequency domain coordinates of the hologram in the x and y directions are shown, d is the distance between the tested sample 4 and the pyroelectric detector 7, and d is 50.5mm in the system.
The phase information of the object is directly obtained by taking the inverse tangent calculation of the complex amplitude of the reproduced image:
wherein the content of the first and second substances,
representing object phase, object phase
Is distributed in [ -pi, pi ] values]And is called 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.
3) 49 interference patterns of the system under the condition without a measured object 4 are continuously recorded, and 49 interference patterns acquired by a terahertz off-axis digital holography method based on a Mach-Zehnder interferometer structure are continuously recorded under the same condition (under the condition that a terahertz laser 1, a pyroelectric detector 7, a first gold-plated off-axis parabolic mirror 2 and a second gold-plated off-axis parabolic mirror 3 are not changed). The two sets of interferograms are subjected to the above calculation to obtain phase maps, and the central area (100 × 100 pixels) is intercepted. According to the relation between the phase and the optical path:
wherein L is the optical path length, and Δ n is the difference between the refractive index of the sample 4 and the refractive index of the surrounding medium. And calculating the standard deviation of the optical path length change of each pixel point in the selected area in the two groups of phase distribution maps, wherein the data can represent the time stability of the system. The standard difference of the optical path of the method is 0.36 mu m through calculation, the standard difference of the optical path of the system is lower through comparison, which shows that the method has higher time stability, the optical path standard difference is 1.0 mu m through the traditional method based on the Mach-Zehnder interferometer structure under the same calculation condition.
The test result of the typical embodiment of the invention shows that the continuous terahertz wave self-reference digital holographic system based on the Fresnel double mirror can generate the digital holographic interference pattern with the object and reference wave intensities and the splitting ratio consistent and the fringe period can be flexibly adjusted. The inventive recording device requires only a small number of optical elements and has a higher time stability than the conventionally used mach-zehnder interferometer based devices for the same recording duration and experimental conditions.
Although the present invention has been described in detail with reference to particular embodiments, the embodiments of the invention described herein are not intended to be exhaustive or limited to the precise forms disclosed. Rather, the embodiment chosen to illustrate the problem was chosen to enable one skilled in the art to practice the invention. Variations and modifications exist without departing from the true scope of the invention as described and defined in the following claims.
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 the content of the first and second substances,
which represents the fourier transform of the signal,
denotes the inverse Fourier transform, h (x)o,yoAnd 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, fx,fyThe 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 the content of the first and second substances,
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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