Tomography device using terahertz waves

Abstract

The present invention relates to a tomography device using terahertz waves, the device comprising: an interference optical system for irradiating a measurement object with terahertz waves output from a multi-terahertz wave generation unit, which generates a plurality of terahertz waves that have different center frequencies, and generating and providing interference signals by combining signals propagating back from the measurement object and reference signals generated from the terahertz waves; and a tomography unit for generating tomography information about the measurement object by using intensity and phase information about received interference-generating signals while controlling the multi-terahertz wave generation unit to vary the frequency of the terahertz waves and wave number information corresponding to frequency changes of the terahertz waves emitted from the multi-terahertz wave generation unit.

Description

Terahertz wave tomography device

[0001] The present invention relates to a tomography device using terahertz waves, and more specifically, to a tomography device using terahertz waves that is capable of acquiring tomographic information of a measurement target using an interference generation signal generated by changing the frequency of the terahertz waves.

[0002] Terahertz waves possess both the penetrability of electromagnetic waves and the linearity of light waves, so they reflect off metal objects, penetrate non-ionizing materials such as plastic or wood, and absorb moisture. The frequency of terahertz waves is in the 300 GHz to 3 THz band, and this frequency band is called the "terahertz gap (THz Gap)." Various applications are currently underway using these terahertz waves, such as defect inspection by identifying the internal structure of packaging by utilizing the characteristic that they penetrate epoxy, a dielectric material in semiconductors, while reflecting metals; food foreign substance inspection to investigate internal foreign substances and moisture content in food; security inspection for hazardous materials monitoring by utilizing the characteristic that they penetrate dielectric clothing while reflecting metals; and cultural property damage inspection to check for internal wood damage.

[0003] Various devices for generating terahertz waves have been proposed, including domestic registered patent No. 10-1530545.

[0004] Meanwhile, the method of measuring the shape of an object using terahertz waves involves mechanically moving a reference mirror to a mutually different position to generate an optical path difference and measuring the shape from the interference signal obtained therefrom. In this case, a method is required to generate image information more precisely from the interference signal.

[0005] The present invention was devised to solve the above requirements, and aims to provide a tomographic device using terahertz waves that can generate tomographic information of a measurement target using an interference generation signal generated while changing the frequency of the terahertz waves.

[0006] To achieve the above objective, a tomography device using terahertz waves according to the present invention comprises: a multi-terahertz wave generator that generates a plurality of terahertz waves having mutually different center frequencies from light emitted from each of a first laser light source that emits a first laser light of a first center frequency and a wavelength-tunable laser light source that emits a second laser light that is different from the first center frequency and has a variable wavelength; and an interference optical system capable of irradiating a measurement target with the terahertz waves output from the multi-terahertz wave generator and generating and providing an interference generation signal in which a signal propagating backward from the measurement target and a reference signal generated from the terahertz waves output from the multi-terahertz wave generator are combined. A tomography measurement unit is provided that receives an interference generation signal generated and output by the interference optical system while controlling the wavelength of the second laser light emitted from the wavelength-tunable laser light source to be variable, and generates tomography information of the measurement target using intensity and phase information of the received interference generation signal and wavenumber information corresponding to the frequency change of the terahertz wave emitted from the multiple terahertz wave generation unit.

[0007] In addition, the above-described tomography unit comprises: a signal detection unit that detects an interference generation signal output from the interference optical system; a Fourier transform unit that performs a Fourier transform on the interference generation signal output from the signal detection unit; a signal purification unit that removes ghost signals and non-interference signals, excluding the interference eigensignal, from the signal output from the Fourier transform unit; an inverse Fourier transform unit that performs an inverse Fourier transform on the interference eigensignal output through the signal purification unit; a phase spreader unit that matches and outputs a phase value according to the passage of time for the signal output from the inverse Fourier transform unit; a correction function application unit that converts the phase value according to the passage of time generated by the phase spreader unit into wavenumber information according to the passage of time and extracts a correction function through polynomial curve fitting on the relationship between wavenumber and time; and a scale adjustment unit that calculates a corrected interference generation signal by correcting the interference generation signal acquired from the signal detection unit using the correction function extracted from the correction function application unit. The image processing unit generates cross-sectional information of a measurement target using phase information and wavenumber information based on a corrected interference generation signal corrected by the scale adjustment unit above.

[0008] According to the tomography device using terahertz waves according to the present invention, it provides the advantage of supporting the generation of tomographic information of a measurement target by changing the frequency of the terahertz waves.

[0009] FIG. 1 is a drawing showing a tomography device using terahertz waves according to the present invention, and

[0010] FIG. 2 is a drawing showing an example of the photomixer of FIG. 1, and

[0011] FIG. 3 is a drawing showing the detailed structure of the cross-sectional measurement section of FIG. 1, and

[0012] Figure 4 is a graph to explain the process of correcting the interference generation signal by the scale adjustment unit of Figure 3.

[0013] Hereinafter, a tomography device using terahertz waves according to a preferred embodiment of the present invention will be described in more detail with reference to the attached drawings.

[0014] FIG. 1 is a drawing showing a tomography device using terahertz waves according to the present invention.

[0015] Referring to FIG. 1, a tomography device (100) using terahertz waves according to the present invention comprises a multiple terahertz wave generation unit (110), a balanced light conversion unit (150), an interferometric optical system (160), and a tomography measurement unit (200).

[0016] The multiple terahertz wave generator (110) generates multiple terahertz waves with different mutual center frequencies.

[0017] The multi-terahertz wave generating unit (110) is equipped with a first laser light source (111), a wavelength-tunable laser light source (120), an optical coupler (131), an amplifier (EDFA) (133), a polarization-maintaining optical fiber (PMF) (135), and a photomixer (140).

[0018] The first laser light source (111) emits a first laser light of a first center frequency (f1).

[0019] The wavelength-tunable laser light source (120) is controlled by the tomographic measurement unit (200) to emit a second laser light of a second center frequency (f2) different from the first center frequency (f1).

[0020] The wavelength-tunable laser light source (120) emits a second laser light with a variable wavelength. That is, the second center frequency (f2) of the second laser light emitted from the wavelength-tunable laser light source (120) can be varied by being controlled by the tomographic measurement unit (200).

[0021] The optical coupler (131) couples and outputs the first laser light emitted from the first laser light source (111) and the second laser light emitted from the wavelength-tunable laser light source (120).

[0022] The amplifier (EDFA) (133) amplifies the optical signal output from the optical coupler (131). The amplifier (EDFA) (133) may be omitted.

[0023] The polarization-maintaining optical fiber (PMF) (135) supports transmission while maintaining the polarization state of the light output from the amplifier (EDFA) (133). The polarization-maintaining optical fiber (PMF) (135) may be omitted.

[0024] The photomixer (140) mixes light output from the optical coupler (131) through the amplifier (133) and the polarization-maintaining optical fiber (135) to produce terahertz waves (f THZ ) is generated and provided to the interference optical system (160).

[0025] The photomixer (140) mixes multiple terahertz waves (f) with mutually different frequencies in response to the change in wavelength of the second laser light mixed with the first laser light. THZ ) is generated and provided to the interference optical system (160).

[0026] The photomixer (140) can be constructed to include a photodiode (142) and an antenna (144) as exemplified in FIG. 2.

[0027] In this case, the first laser light (f1) and the second laser light (f) coupled in the optical coupler (131) 21 to f 2n When this occurs in the active layer of the photodiode (142), the photodiode (142) modulates the current to correspond to the beating frequency of the incident light, and the terahertz wave (f) through the antenna (144) by the modulated current THZ1 to f THZn ) is radiated.

[0028] The antenna (144) can have a horn structure.

[0029] In particular, when the first laser light (f1) and the second laser light (f2) coupled in the optical coupler (131) are incident on the active layer of the photodiode (142), the photomixer (140) produces a terahertz wave (f) with a frequency changed to correspond to the change in wavelength of the second laser light (f2).THZ1 to f THZn ) radiates.

[0030] Accordingly, the frequency-tuned second laser light (f) of the wavelength-tuned laser (120) through the photomixer (140) 21 to f 2n Multiple terahertz waves (f) with different wavelengths caused by ) THZ1 to f THZn ) is generated and radiated.

[0031] The balanced light conversion unit (150) is installed between the interference optical system (160) and the photomixer (140) so that the terahertz waves emitted from the photomixer (140) can be converted into a balanced beam and transmitted to the beam splitter (162) described later.

[0032] The balanced light conversion unit (150) comprises a first path-converting reflecting member (151) that reflects terahertz waves emitted from a photomixer (140) to a first reflection path different from the incident path, and a second path-converting reflecting member (152) that converts and reflects a beam incident by being reflected from the first path-converting reflecting member (151) into a balanced beam.

[0033] The first path-changing reflective member (151) may be applied such that the reflective surface has a first inclined surface, and the second path-changing reflective member (152) may be applied such that the reflective surface has a concave reflective surface having a parabolic shape.

[0034] Of course, the balanced light conversion unit (150) can be constructed to generate a balanced beam with a structure different from the one shown.

[0035] The interferometric optical system (160) is configured to irradiate a terahertz wave generated by a multi-terahertz wave generator (110) and output through a balanced light converter (150) onto a measurement target (10), and to generate and provide an interference generation signal by combining a signal traveling inversely from the measurement target (10) and a reference signal generated from the terahertz wave output through the balanced light converter (150).

[0036] The interference optical system (160) is equipped with a beam splitter (162), a reference mirror (164), a focusing lens (168), and a base stage driving unit (170).

[0037] The beam splitter (162) transmits a portion of the terahertz waves incident through the second path-changing reflector (152) in the first direction and reflects the remaining portion in the second direction orthogonal to the first direction to the measurement target (10).

[0038] A reference mirror (164) is installed opposite the beam splitter (162) to reflect the terahertz waves traveling in the first direction through the beam splitter (162) back toward the beam splitter (162) and provide them as a reference signal.

[0039] The reference mirror (164) is installed in a fixed position.

[0040] The focusing lens (168) focuses the terahertz waves reflected from the beam splitter (162) onto the measurement target (10).

[0041] The base stage drive unit (170) is controlled by the single-layer measuring unit (200) to move the base stage (175), on which the object to be measured (10) is placed, in a plane parallel to the first direction.

[0042] The cross-sectional measurement unit (200) receives an interference generation signal generated and output by an interference optical system (160) while controlling the wavelength of the second laser light emitted from the wavelength-tunable laser light source (120) to be variable, and generates cross-sectional information of the object to be measured (10) using wavenumber information corresponding to the frequency change of the terahertz wave emitted from the multiple terahertz wave generation unit (110) from the amplitude and phase information of the received interference generation signal.

[0043] That is, the cross-sectional measurement unit (200) receives, through a receiving antenna (192), an interference generation signal generated by the superposition of a signal reflected from the object to be measured (10) in the interferometric optical system (160) and transmitted through the beam splitter (162), and a reference signal reflected from the reference mirror (164) and then reflected again from the beam splitter (162), thereby generating cross-sectional information of the object to be measured (10), and is explained together with reference to FIG. 3.

[0044] The tomographic measurement unit (200) is equipped with a signal detection unit (210), a Fourier transform unit (220), a signal refinement unit (230), an inverse Fourier transform unit (240), a phase unfolding unit (250), a correction function application unit (260), a scale adjustment unit (270), and an image processing unit (280).

[0045] The signal detection unit (210) detects the interference generation signal output from the interference optical system (160).

[0046] The signal detection unit detects the strength and phase information of the signal from the interference generation signal output from the interference optical system (160).

[0047] The Fourier transform unit (220) performs a Fourier transform on the interference generation signal output from the signal detection unit (210).

[0048] The signal purification unit (230) removes ghost signals and non-interference signals, excluding interference-related signals, from the signal output from the Fourier transform unit (210). That is, the signal purification unit (130) purifies only the interference-related signals from the signal output from the Fourier transform unit (210) and outputs them to the inverse Fourier transform unit (240).

[0049] The inverse Fourier transform unit (240) performs an inverse Fourier transform on the interference eigensignal output through the signal refinement unit (230).

[0050] The phase matching unit (250) matches the phase value over time to the signal output from the inverse Fourier transform unit (240) and outputs it.

[0051] The correction function application unit (260) converts the phase value generated over time in the phase unfolding unit (250) into wave number information over time, and calculates a correction parameter value by applying a correction function through polynomial curve fitting to the relationship between wave number and time.

[0052] The scale adjustment unit (270) calculates a corrected interference generation signal by correcting the interference generation signal acquired from the signal detection unit (210) using the correction parameter value calculated from the correction function application unit.

[0053] The image processing unit (270) generates cross-sectional information of the measurement target (10) using phase information and wavenumber information based on the corrected interference generation signal corrected by the scale adjustment unit (270).

[0054] The signal processing process of the tomographic measurement unit (200) is explained in more detail below.

[0055] First, since the wavelength-tunable laser light source (120) has a wavelength (λ2) that changes linearly, the terahertz wave (f) according to the linear variation of the wavelength THz )'s wavenumber(K THz ) is non-linear, as can be confirmed through Equation 1 below. This means that the scale of the data is distorted when processing signals in the Fourier domain (frequency domain).

[0056]

[0057] Here, λ1 is the wavelength of the first laser light emitted from the first laser light source (111).

[0058] Accordingly, the tomography measurement unit (200) processes the Fourier transform data information so that tomography information can be measured precisely by correcting the non-linearity of the wavenumber.

[0059] First, the interference generating signal (I(t)) detected by the signal detection unit (210) can be expressed by the following mathematical formula 2.

[0060]

[0061] Here, k(t) is a wavenumber, t is time, a is a fixed constant value set as the quantum efficiency of the signal detection unit (210) having a value between 0 and 1, Γ is a light source coherence function, Zo is the optical path difference between the reference mirror (164) and the object to be measured (10), Ip(t) is an interference-free signal reflected from the object to be measured (10), and Ir(t) is an interference-free signal reflected from the reference mirror (164).

[0062] Also, in mathematical formula 2 is an interference eigensignal generated by mutual interference after being reflected from the measurement target (10) and the reference mirror (164), respectively, and can be expressed by the following mathematical formula 3 through the Euler formula.

[0063]

[0064] Meanwhile, the interference generation signal (I(t)) detected by the signal detection unit (210) can be expressed by the following mathematical formula 4 when Fourier transformed in the Fourier transform unit (220) described later.

[0065]

[0066] Here, FT[ ] is the Fourier transform operator, and * is the convolution operator.

[0067] In addition, G is a term related to interference and is expressed by the following mathematical formula 5.

[0068]

[0069] Here, μ is a Fourier transform pair of times.

[0070] Also, k1 is the wavenumber component of the delta function (δ()), and k Ω (t) is an exponential function (e () It is the wavenumber component of ) and has a relationship with the wavenumber (K(t)) expressed by the following mathematical formula 6.

[0071]

[0072] For the signal that has been Fourier transformed in the Fourier transform unit (220), the signal refiner (220) extracts only the signal component of the following Equation 7, which is the interference eigensignal term in Equation 5, through a flat-top window or Hamming frequency filter, and removes the ghost signal and non-interference signal, which are the remaining terms in Equation 5.

[0073]

[0074] Next, in the inverse Fourier transform unit (240), an interference eigensignal corresponding to a pure interference signal in the time domain as shown in Equation 8 below can be obtained by performing an inverse Fourier transform on the signal that was Fourier transformed by Equation 7.

[0075]

[0076] Here, the phase of the interference natural signal is expressed by the following mathematical equation 9.

[0077]

[0078] Matching information between the phase (φ(t)) and wavenumber (k(t)) expressed by mathematical formula 9 is generated by the phase unfolding unit (250).

[0079] Therefore, by using the phase (φ(t)) and wavenumber (k(t)) information detected when acquiring a signal in the signal detection unit (210), depth information (zo) from the reference surface of the object to be measured (10) can be obtained. However, as previously explained, terahertz waves (f THz Considering that the wavenumber (k(t)) is nonlinear with respect to the frequency change of ), correction is required from the wavenumber perspective for the signal detected by the signal detection unit (210), and this process is processed in the correction function application unit (260) and the scale adjustment unit (270).

[0080] The phase measurement value of the interference generation signal obtained from the signal detection unit (210) has nonlinearity, and the phase of the interference signal can be expressed in terms of wave number as shown in the following mathematical formula 10.

[0081]

[0082] In addition, the relationship between wavenumber (k(t)) and time (t) in Equation 10 can be corrected using a polynomial curve fitting function.

[0083] The fitting form of the correction function is a polynomial correction function (t) expressed by Equation 11 below. k ) can be applied.

[0084]

[0085] Here, a0, a1, a2, ... are correction parameters.

[0086] Afterwards, using the calculated correction function, the scale of the interference generation signal initially acquired by the signal detection unit (210) is corrected to the time after correction by linear interpolation to generate a corrected interference signal expressed by the following mathematical formula 12.

[0087]

[0088] Here, t n is the time applied to the initially acquired interference-generating signal before calibration, and t n ' is the correction function (t) obtained earlier in the correction function application unit (260). k ) and pre-correction time (t n The difference value from ) is expressed by the following mathematical formula 13.

[0089]

[0090] Through this correction process, a corrected interference signal can be obtained for the initially acquired interference generation signal, as shown in FIG. 4. That is, t calculated through the correction function k The interference generation signals acquired before correction are corrected by linear interpolation to match each other over time.

[0091] In Figure 4, the initially acquired interference generation signal is indicated by a circular mark, and the corrected signal is indicated by a square mark. Additionally, some initial values ​​of the initially acquired interference generation signal may be used as is, while the remaining signals may undergo a correction process.

[0092] From the interference generation signal corrected in this way, the depth (zo) from the surface of the measurement target (10) can be calculated using the previously known wavenumber information and the detected phase information using Equation 9.

[0093] In the depth measurement process, at least three interference generation signals are obtained corresponding to terahertz waves of mutually different frequencies according to the change in wavelength of the wavelength-tunable laser light source (120) at the same measurement position of the object to be measured (10).

[0094] Additionally, the cross-sectional measurement unit (200) controls the base stage driving unit (170) to move the measurement position of the object to be measured (10) on a plane, thereby measuring the depth information of each object to be measured (10) and obtaining overall cross-sectional information on the surface of the object to be measured (10).

[0095] According to the tomography device using terahertz waves described above, it provides the advantage of being able to generate tomographic information of a measurement target by changing the frequency of the terahertz waves.

Claims

1. A multi-terahertz wave generating unit that generates a plurality of terahertz waves having mutually different center frequencies from light emitted from each of a first laser light source that emits a first laser light of a first center frequency and a wavelength-tunable laser light source that emits a second laser light of a wavelength different from the first center frequency and variable wavelength; An interference optical system capable of irradiating a measurement target with terahertz waves output from the multi-terahertz wave generator, and generating and providing an interference generation signal by combining a signal propagating backward from the measurement target and a reference signal generated from the terahertz waves output from the multi-terahertz wave generator; A tomography device using terahertz waves, characterized by comprising: a tomography measurement unit that receives an interference generation signal generated and output from the interference optical system while controlling the wavelength of the second laser light emitted from the wavelength-tunable laser light source to be variable, and generates tomography information of the measurement target using intensity and phase information of the received interference generation signal and wavenumber information corresponding to the frequency change of the terahertz waves emitted from the multiple terahertz wave generation unit.

2. In paragraph 1, the above-mentioned tomography unit A signal detection unit for detecting an interference generation signal output from the above-mentioned interference optical system; A Fourier transform unit that performs a Fourier transform on the interference generation signal output from the above signal detection unit; A signal purification unit that removes ghost signals and non-interference signals, excluding interference eigensignals, from the signal output from the Fourier transform unit; An inverse Fourier transform unit that performs an inverse Fourier transform on the interference eigensignal output through the above signal purification unit; A phase unfolding unit that matches and outputs a phase value over time to the signal output from the above inverse Fourier transform unit; A correction function application unit that converts the phase value generated over time in the above-mentioned phase unfolding unit into wavenumber information over time, and extracts a correction function through polynomial curve fitting on the relationship between wavenumber and time; A scale adjustment unit that calculates a corrected interference generation signal by correcting the interference generation signal acquired from the signal detection unit using a correction function extracted from the correction function application unit; A tomography device using terahertz waves, characterized by comprising: an image processing unit that generates tomographic information of a measurement target using phase information and wavenumber information based on a corrected interference generation signal corrected in the scale adjustment unit above.

3. In paragraph 2, the terahertz wave generating part An optical coupler that couples and outputs the first laser light and the second laser light; A tomography device using terahertz waves, characterized by comprising a photomixer that mixes light output from the optical coupler to generate terahertz waves and provides them to the interferometric optical system.

4. In paragraph 3, the above interference optical system is A beam splitter that transmits a portion of the incident terahertz waves in a first direction and reflects the remaining portion in a second direction orthogonal to the first direction to propagate toward a measurement target; A reference mirror with a fixed position that reflects the terahertz wave passing through the beam splitter and proceeding in the first direction back toward the beam splitter to provide it as the reference signal; and A tomography device using terahertz waves, characterized by having a focusing lens that focuses terahertz waves reflected from the beam splitter and propagating toward a measurement target.

5. In paragraph 4, further comprising a base stage driving unit configured to move the base stage on which the measurement target is placed in a plane parallel to the first direction; and A tomography device using terahertz waves, characterized in that the above-described tomography measurement unit controls the base stage driving unit to move the object to be measured and calculates tomography information for the object to be measured.

6. A tomography device using terahertz waves, further comprising, in claim 5, a balanced light conversion unit that converts the optical path of the multiple terahertz waves emitted from the photomixer into a balanced beam and transmits it to the beam splitter.

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