KR101077595B1 - Terahertz time domain spectral appatatus and image processing method for reducing sampling number - Google Patents

Abstract

A terahertz time domain spectroscopy apparatus and image processing method are disclosed. In a terahertz time domain spectrometer with terahertz beams as the light source, a much smaller sampling can be achieved by placing a mask designed to pass only waveforms containing projected image or sonogram information in the spatial frequency domain. You can restore the image.

Terahertz time domain spectrum, fourier domain, 4f-system, radon transform, pulse imaging, broad band spectrum , Coherent optical computer

Description

TERAHERTZ TIME DOMAIN SPECTRAL APPATATUS AND IMAGE PROCESSING METHOD FOR REDUCING SAMPLING NUMBER}

The present invention relates to a terahertz time domain spectroscopy apparatus and an image processing method for reducing a sampling number, and more particularly, by placing a mask designed to pass only a waveform including image information on which the entire image is projected in the spatial frequency domain. A terahertz time domain spectroscopy apparatus and image processing method capable of reconstructing an image with a much smaller number of samplings.

In general, a terahertz time domain spectrometer focuses on parabolic mirrors when a two-dimensional image is to be acquired, and places a desired object at this focal point to obtain data for each pixel. That is, the terahertz time domain spectrometer can obtain data pixel by pixel by moving an object directly at the focal point between parabolic mirrors. The method can be imaged in two dimensions. However, the conventional method has a problem that it takes a lot of time in the process of obtaining a pulse at each position.

To alleviate this problem, the meter sections can be configured in arrays or CCDs, but implementing these configurations is quite complex and requires expensive setup.

The present invention provides a terahertz time domain spectroscopy apparatus and an image processing method for obtaining a high resolution two-dimensional image with a small number of samples using a specific mask in the Fourier domain.

The present invention also provides a terahertz time domain spectroscopy apparatus and an image processing method for acquiring a two-dimensional image with a small number of data.

The terahertz time domain spectrometer according to an embodiment of the present invention uses a terahertz beam as a light source, an optical system for forming a two-dimensional image of an object using a time domain, and a mask for modifying a Fourier domain on the optical system. Forming regulators.

In one aspect of the invention, the optical system is a two-dimensional original data obtained by positioning an object consisting of holes in the terahertz spatial domain, positioning a mask in the Fourier domain, and rotating the mask. You can restore the object.

In addition, in one aspect of the present invention, the optical system includes one convex mirror or lens based on the traveling direction of the terahertz beam, one mask on an accurate focal plane in the Fourier domain, and another convex mirror or lens at a focal length from the mask. It is arrange | positioned in order of 1 group, and a measurement part measures in a point form. That is, the wavefronts deformed by the mask in the Fourier domain are all measured by Fourier transform to one point, and at this time, it is defined as measuring at the origin.

In addition, in one aspect of the present invention, when the space occupied by the visible region of the object is small, the optical system can restore the number of samples using a compression sensing theory.

In another aspect of the present invention, the mask samples a portion of the Fourier domain and sums it in the spatial domain to show data. The pulsed data of the time domain summating the electric field passing through the mask is projected in a radial form based on the origin to reduce the size of the terahertz electric field passing through an object (negative aperture) located in two-dimensional space. The obtained one-dimensional sonogram data is obtained for each rotation angle and transmitted in the frequency domain.

In addition, in one aspect of the present invention, a radon transform may be performed using snowogram data to reconstruct a desired 2D image. In order to use it, a basic terahertz frequency without an object must be obtained, and after measurement, a preprocessing process is performed to divide the 1D time data obtained by placing a mask into the fundamental light source frequency in the frequency domain.

In addition, in one aspect of the present invention, the mask is generally made of a metal that can block terahertz light, alternatively may be made of a transmissive SLM or other material that can retard the phase.

In addition, the image processing method according to an embodiment of the present invention comprises the steps of irradiating a light source having a wide range of frequencies to a two-dimensional object, measuring by rotating the pulse data of the one-dimensional time domain with a mask, Dividing the measured pulse data of the time domain into reference pulse data measured without an object and restoring the two-dimensional object by performing inverse radon transformation of the data measured at various angles with respect to the origin.

According to the present invention, two-dimensional image information to be reconstructed using a mask can be obtained from a spectrum of time domain pulses, and a perfect two-dimensional image can be reconstructed using a radon transform.

In addition, according to the present invention, since a much smaller number of fountains can be restored as compared to a method of sampling each general point, data acquisition time for an image can be drastically reduced.

In addition, according to the present invention, by applying compression sensing theory instead of the Radon transform in the reconstruction process, a general resolution problem can be obtained with a smaller number of samples.

Hereinafter, a terahertz time domain spectrometer and an image processing method will be described in detail with reference to the accompanying drawings.

In general, coherent light can be applied to various imaging apparatuses. The present invention limits the terahertz time domain spectroscopy apparatus and image processing method for reconstructing a two-dimensional image using pulses in the terahertz region maintaining coherence. Terahertz pulses are basically generated in terahertz time domain spectroscopy. The spectroscopy device consists of a plane representing the Fourier domain and the spatial domain. The sample to be imaged can be located in the spatial domain, and the frequency information of the sample in the spatial domain can be obtained as temporal information by placing a mask having a hole in the Fourier domain. have.

1 is a view showing the configuration of a terahertz time domain spectroscopy apparatus according to an embodiment of the present invention.

Referring to FIG. 1, the terahertz time domain spectrometer 100 according to an embodiment of the present invention includes a light source 1, an object 2, a convex mirror 3, a mask 4, and a measurement unit 5. ). The light source 1 uses a terahertz beam in the form of a pulse. The terahertz spectroscopy apparatus 100 places an object 2 to obtain an image in parallel light and performs Fourier transform on the focal plane by means of a block plane mirror 3. Place the mask 4 with a hole d away from the axis. The equation for this is shown in Equation 1.

[Equation 1]

로 나타내며, S(ω)는 광원의 스펙트럼 정보가 된다. M(ξ,η)는 푸리에 도메인에 위치하는 마스크(4)를 수식으로 표현한 함수이고, U(ξ,η)는 복원하고자 하는 물체의 함수이다. 특히 U(ξ,η)는 빛을 통과시키는 형태의 물체이므로 평행 광이 입사한다고 보면, 물체가 볼록면 거울(3)에서 떨어진 거리가 어디에 있건 이들의 푸리에 변환은 초점 평면에 위치하게 된다. Here, V (ω) is a Fourier transform type of data measured by the measuring unit 5, and C (ω) is

Denotes spectral information of the light source. M (ξ, η) is a function expressing the mask 4 located in the Fourier domain by a formula, and U (ξ, η) is a function of the object to be restored. In particular, U (ξ, η) is a type of object that allows light to pass, so that when parallel light is incident, the Fourier transform is located in the focal plane wherever the object is separated from the convex mirror 3.

2 is a front view of a perforated mask.

Referring to FIG. 2, the mask 4 is in a state in which a hole is drilled at a position apart from the optical axis by the d in the focal plane, and is represented by Equation 2 below.

[Equation 2]

Here, the diameter (a) of the hole was not taken into account, which was omitted for the sake of simplicity. This delta function (Dirac delta function) has a value corresponding to the variables in it can be expressed simply by applying to the equation (1).

&Quot; (3) "

Equation 3 shows that the radially spread Fourier data of the object to be restored is consistent with the Fourier transform of the pulse data obtained in the time domain. Since k = ω / c in Equation 1, as the frequency is changed, the radial Fourier information of the object coincides with the Fourier transform of the time data. This uses the broad spectral characteristics of terahertz pulses and cannot be used in conventional single wavelength light sources. The resolution in space is determined by the factors d and f in equation (4). d denotes the distance of the hole away from the origin on the mask 4, and f denotes the focal length of the convex mirror 3. Accordingly, the terahertz time domain spectrometer 100 has a higher resolution as d and f become smaller.

Equation (3) is the same as the existing Fourier slice theorem (fourier slice theorem), it is expressed as Equation (4) by converting it into a spatial-temporal equation through an inverse Fourier transform.

&Quot; (4) "

는 2차원 공간 도메인에서 투영한 결과와 같다. 이때 픽셀 한 개의 길이는 델타 함수 안에 있는 인자로 인해 결정된다. In Equation 4, C (ω) is divided and inverse Fourier transform is performed. C (ω) divides and removes the time domain pulse data in free space obtained in the existing system without objects in the Fourier domain. Thus obtained

Is the result of the projection in the two-dimensional space domain. The length of one pixel is determined by the arguments in the delta function.

In order to restore the two-dimensional object in such a process, projection information from various directions must be present, which is obtained by varying the variable θ while rotating the mask 4. Unlike conventional measurement by every pixel, the object can be expressed only in a time domain corresponding to a few θ.

3 is a diagram illustrating an operation flow of an image processing method according to an embodiment of the present invention.

1 and 3, the terahertz time domain spectrometer 100 irradiates a two-dimensional object to a light source having a wide frequency in 310.

The terahertz time domain spectrometer 100 measures the pulse data of the one-dimensional time domain by rotating the mask 4 (320).

The terahertz time domain spectrometer 100 divides the measured time pulse data into reference pulse data without an object (330).

The terahertz time domain spectrometer 100 performs inverse radon conversion of data measured at various angles based on an origin (340).

The terahertz time domain spectroscopy apparatus 100 completes the restoration of the 2D object according to the inverse radon transform performed (350).

The terahertz time domain spectrometer 100 according to the present invention may further reduce the data required for reconstructing the 2D image when the compressed sensing theory is applied. In the case of compression sensing theory, if the object to be restored occupies only a few parts in the space, this information can be used to restore the same resolution as if the number is large with a small sampling. Restored in such a way. Compression sensing theory simplifies the problem with the L1 optimization process and can solve them in several ways, such as FOCUSS, OMP, and Basis Pursuit.

As an example of the mask 4, a case where a hole is drilled has been described, but it is not necessary to use only a hole or a mask or an optical system. The key point of the present invention is that when a pulsed light source having a broadband frequency is used, the light source passing through the object 2 has a sinogram-shaped waveform in each direction when passing through the lens or the convex mirror 3. In this case, the number of sampling required through frequency analysis is reduced, and various modifications and improvements are possible using this.

FIG. 4 shows an example of detector arrangements arranged at an angle with a certain distance that can be improved instead of a perforated mask.

Referring to FIG. 4, instead of the perforated mask shown in FIG. 2, the detector may be arranged at an angle from a center at a predetermined distance. In this case, the 2-f optical system or principle using only one lens or a convex mirror rather than the 4-f optical system is the same. At this time, the image can be easily obtained by rotating the angle and processing the signal measured by each detector without sampling. In this case, two-dimensional images can be directly obtained by using the one-dimensional array detector.

FIG. 5 shows an example of a mask that gives a time delay differently in each direction that can be improved instead of a perforated mask.

Referring to FIG. 5, instead of the perforated mask shown in FIG. 2, an object for slowing a pulse used as a light source at a predetermined distance is arranged by thickness. In this case, rather than blocking the sinograms in each direction, it is delayed and simultaneously received in one waveform. The same waveform can be processed by cutting the received waveform in time. The object that slows down the pulse is silicon or the like that passes through the terahertz light source well. In this case, anti-reflection coating may be applied to prevent the etalon effect.

As described above, the present invention has been described by way of limited embodiments and drawings, but the present invention is not limited to the above embodiments, and those skilled in the art to which the present invention pertains various modifications and variations from such descriptions. This is possible.

Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the claims below but also by the equivalents of the claims.

1 is a view showing the configuration of a terahertz spectroscopy apparatus according to an embodiment of the present invention.

2 is a front view of a perforated mask.

3 is a diagram illustrating an operation flow of an image processing method according to an embodiment of the present invention.

FIG. 4 shows an example of detector arrangements arranged at an angle with a certain distance that can be improved instead of a perforated mask.

FIG. 5 shows an example of a mask that gives a time delay differently in each direction that can be improved instead of a perforated mask.

<Explanation of symbols for the main parts of the drawings>

1: terahertz light source in pulse form

2: object to be restored

3: block face mirror, Fourier transducer

4: mask with holes

5: Measuring unit (ZnTe)

Claims (11)

An optical system that uses a terahertz beam as a light source and forms a two-dimensional image of an object using a time domain; And Control system for forming a mask to modify the Fourier domain on the optical system Including, The optical system, Reconstructing the 2D image using 1D temporal data obtained by rotating the mask with respect to the object located in the terahertz spatial domain, The mask is, The terahertz time domain spectroscopy device is implemented in a form that makes the time delay of the pulse in each direction different. delete Claim 3 was abandoned when the setup registration fee was paid. The method of claim 1, The mask is, A terahertz time domain spectroscopy device that acts as a filter in the spatial frequency domain. The method of claim 1, The optical system, A terahertz time domain spectroscopy device for reconstructing the two-dimensional image using an inverse radon transform. Claim 5 was abandoned upon payment of a set-up fee. The method of claim 1, The optical system, A terahertz time domain spectroscopy apparatus for restoring the 2D image by removing noise or repeatedly applying an optimization process during a restoration process. delete The method of claim 1, The mask is, A terahertz time domain spectroscopy device composed of detectors, each angled at a constant distance from a center, to obtain a signal without an angle scan. The method of claim 1, The optical system, The number of angles to be rotated is arbitrarily determined, and the greater the angle, the closer the image of the original object becomes. delete The method of claim 1, The optical system, A terahertz time domain spectroscopy device for restoring to a smaller number of samples using compression sensing theory when the space occupied by the object to be restored is small. In the image processing method of a terahertz time domain spectroscopy apparatus, An optical system in which the terahertz time domain spectrometer uses a terahertz beam as a light source and forms a two-dimensional image of an object using the time domain; And an adjusting system for forming a mask for modifying a Fourier domain on the optical system. The image processing method, Irradiating the terahertz beam to the object located in the terahertz spatial domain; Rotating the mask to measure pulse data in a one-dimensional time domain; Dividing the measured pulse data of the time domain by reference pulse data measured without an object; And Restoring the 2D image by performing inverse radon transformation of data measured at various angles with respect to the origin; Including, The mask is, The image processing method is implemented in a form that makes the time delay of the pulse in each direction different.

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