Spot imaging device and spot imaging method applied to terahertz signals

Abstract

The present disclosure provides a spot imaging device and a spot imaging method for terahertz signals. The device includes: a terahertz signal transmitting module for generating and collimating and expanding a terahertz signal; a beam splitting module for receiving and splitting the terahertz signal to obtain and output a detection signal; a stepped etalon reflection module including a stepped etalon. The stepped etalon includes n levels of steps. The stepped etalon is used to receive the detection signal, rotate around a rotation axis at a preset angular step, so as to reflect the detection signal at different angles to obtain n reflected detection signals. The n reflected detection signals return to the beam splitting module to obtain and output n reflected signals; the reflected signals represent the intensities of n time-domain peaks corresponding to the n levels of steps; a terahertz signal receiving module for converging and processing the n reflected signals to obtain a spot energy intensity distribution image of the terahertz signal.

Description

Technical Field

[0001] The present disclosure relates to the technical field of laser spot measurement instruments, and in particular, to a spot imaging device and a spot imaging method for terahertz signals. Background Art

[0002] The terahertz band is the transition band between millimeter waves and infrared waves, covering a relatively large bandwidth range. Different from other electromagnetic radiation beams, terahertz beams have both the characteristics of electronics and optics, which makes terahertz beams have broad application prospects in the fields of medicine, military, communication, spectroscopy, security inspection and quality inspection, etc. In related applications, the time-domain terahertz signal corresponding to the terahertz pulse can be measured by a terahertz time-domain spectroscopy (THz-Time Domain Spectroscopy, THz-TDS) system. Based on the analysis of THz–TDS, the beam quality and transmission law of the terahertz beam can be measured.

[0003] In the process of implementing the inventive concept of the present disclosure, the inventors found that the following problems generally exist in the related art: the absolute power emitted from the TDS is very limited, the sensitivity of the focal plane array in the terahertz range based on photon or thermal detection is poor, and it is also difficult to directly measure a large-sized collimated terahertz beam after the optical system by the related art and the two-dimensional optoelectronic sampling method; the imaging resolution of the terahertz beam obtained by the existing imaging device for THz–TDS is low, or the process of adjusting the imaging device is complex and prone to errors. Summary of the Invention

[0004] In view of this, the present disclosure provides a spot imaging device and a spot imaging method for terahertz signals, in order to at least partially solve the above-mentioned technical problems.

[0005] An aspect of the present disclosure provides a spot imaging device for terahertz signals. The spot imaging device for terahertz signals includes: a terahertz signal transmitting module for generating and collimating and expanding the above terahertz signal; a beam splitting module for receiving and splitting the above terahertz signal to obtain and output a detection signal; a stepped etalon reflection module, the above stepped etalon reflection module includes a stepped etalon, the above stepped etalon includes n levels of steps, the above stepped etalon is used to receive the above detection signal, rotate around the rotation axis at a preset angular step, so as to reflect the above detection signal at different angles to obtain n reflected detection signals, the direction of the above rotation axis is parallel to the propagation direction of the above detection signal, the above rotation axis passes through the symmetry center of the above stepped etalon, the above n reflected detection signals return to the above beam splitting module, and the above beam splitting module obtains and outputs n reflected signals according to the returned above n reflected detection signals; where n is a positive integer, the above preset angle is obtained by dividing 180° by n, and each of the above reflected signals represents the intensity of n time-domain peaks corresponding to n levels of steps; a terahertz signal receiving module for converging and processing the n above reflected signals to obtain the spot energy intensity distribution image of the above terahertz signal, the above spot energy intensity distribution image is a pixel lattice of size n×n, the above pixel lattice is obtained according to the above stepped etalon, and the pixel lattice energy intensity is obtained according to the n above reflected signals.

[0006] According to an embodiment of the present disclosure, the above step includes a reflecting surface, the plane where the above reflecting surface is located is perpendicular to the plane where the above rotation axis is located, and the above reflecting surface is used to reflect the above detection signal; in the case where the above stepped etalon is in the initial position, the center lines corresponding to the length directions of the above n reflecting surfaces are obtained to obtain n horizontal lines; in the case where the above stepped etalon rotates to a position perpendicular to the above initial position, the center lines corresponding to the length directions of the above n reflecting surfaces are obtained to obtain n vertical lines; n×n intersections are obtained according to the above n horizontal lines and the above n vertical lines; using the above n×n intersections as the pixel point centers and the width of the above reflecting surface as the side length of the pixel points, the above pixel lattice of size n×n is constructed.

[0007] According to an embodiment of the present disclosure, the above terahertz signal transmitting module includes: a transmitting end photoconductive antenna and a transmitting end off-axis parabolic mirror; the port of the above transmitting end photoconductive antenna is arranged at the focal position of the above transmitting end off-axis parabolic mirror for generating the above terahertz signal; the above transmitting end off-axis parabolic mirror is used to perform collimation and expansion operations on the above terahertz signal and output the above terahertz signal.

[0008] According to an embodiment of the present disclosure, the above beam splitting module includes: a metal wire grid beam splitter and a terahertz quarter-wave plate; the metal wire grid beam splitter is disposed on the optical path where the terahertz signal is located, and is configured to pass the terahertz signal whose polarization direction is consistent with that of the metal wire grid beam splitter; the terahertz quarter-wave plate is disposed on the optical path where the terahertz signal is located behind the metal wire grid beam splitter, and is configured to generate a phase delay, and obtain the detection signal according to the terahertz signal whose polarization direction is consistent with that of the metal wire grid beam splitter, the detection signal is circularly polarized light, the terahertz quarter-wave plate is further configured to process the reflected detection signal to cause the reflected detection signal to generate a phase delay, so as to obtain the reflected signal, the polarization direction of the reflected signal is perpendicular to the polarization direction of the metal wire grid beam splitter, and the reflected signal is reflected by the metal wire grid beam splitter to the terahertz signal receiving module.

[0009] According to an embodiment of the present disclosure, the above beam splitting module includes: a high-resistivity silicon lens; the high-resistivity silicon lens is disposed on the optical path where the terahertz signal is located, and the symmetry axis of the cross section of the high-resistivity silicon lens forms a 45° angle with the propagation direction of the terahertz signal, and is configured to split the terahertz signal, the transmission reflection ratio of the splitting is 1:1, output the detection signal propagating along the propagation direction of the terahertz signal that passes through the high-resistivity silicon lens, and is further configured to split the reflected detection signal, and output the reflected signal propagating perpendicular to the propagation direction of the reflected detection signal that is reflected from the high-resistivity silicon lens.

[0010] According to an embodiment of the present disclosure, the above terahertz signal receiving module includes: a receiving end off-axis parabolic mirror, a receiving end photoconductive antenna, and a spot imaging unit; the receiving end off-axis parabolic mirror is configured to sequentially converge n of the above reflected signals; the receiving end photoconductive antenna port is disposed at the focal position of the receiving end off-axis parabolic mirror, and is configured to receive and send n converged reflected signals to the spot imaging unit; the spot imaging unit is configured to respectively obtain n time-domain signals according to the n converged reflected signals, each of the time-domain signals includes the intensities of n time-domain peaks corresponding to n-level steps, and is further configured to solve and obtain the energy intensities corresponding to n×n pixel points in the pixel matrix according to the n time-domain signals and the pixel matrix, and obtain the spot energy intensity distribution image according to the energy intensities.

[0011] According to an embodiment of the present disclosure, the above stepped etalon reflection module further includes: an electronically controlled rotating mirror mount; the electronically controlled rotating mirror mount is configured to drive the stepped etalon to rotate along the rotation axis, so that the stepped etalon reflection module continuously obtains the n reflected detection signals during the process of obtaining the spot energy intensity distribution image of the terahertz signal.

[0012] Another aspect of the present disclosure provides a spot imaging method for terahertz signals. The spot imaging method includes: using a terahertz signal emission module to generate and collimate and expand the above-mentioned terahertz signal; using a beam splitting module to receive and split the above-mentioned terahertz signal to obtain and output a detection signal; using a stepped etalon reflection module, the stepped etalon reflection module includes a stepped etalon, the stepped etalon includes n levels of steps, the stepped etalon receives the above-mentioned detection signal, rotates around a rotation axis at a preset angular step, so as to reflect the above-mentioned detection signal at different angles to obtain n reflected detection signals, the direction of the rotation axis is parallel to the propagation direction of the above-mentioned detection signal, the rotation axis passes through the symmetry center of the stepped etalon, and the above-mentioned n reflected detection signals return to the above-mentioned beam splitting module, so as to use the above-mentioned beam splitting module to obtain and output n reflected signals according to the returned above-mentioned n reflected detection signals; where n is a positive integer, the above-mentioned preset angle is obtained by dividing 180° by n, and each of the above-mentioned reflected signals represents the intensity of n time-domain peaks corresponding to n levels of steps; using a terahertz signal receiving module to converge and process the n above-mentioned reflected signals to obtain a spot energy intensity distribution image of the above-mentioned terahertz signal, the spot energy intensity distribution image is a pixel matrix of size n×n, the pixel matrix is obtained according to the above-mentioned stepped etalon, and the pixel matrix energy intensity is obtained according to the above-mentioned n reflected signals.

[0013] According to an embodiment of the present disclosure, the above-mentioned terahertz signal receiving module includes: a receiving end off-axis parabolic mirror, a receiving end photoconductive antenna, and a spot imaging unit; the above-mentioned using the terahertz signal receiving module to converge and process the n above-mentioned reflected signals to obtain the spot energy intensity distribution image of the above-mentioned terahertz signal includes: using the above-mentioned receiving end off-axis parabolic mirror to sequentially converge the n above-mentioned reflected signals; using the above-mentioned receiving end photoconductive antenna with its port set at the focal position of the above-mentioned receiving end off-axis parabolic mirror to receive and send the n converged reflected signals to the above-mentioned spot imaging unit; using the above-mentioned spot imaging unit, using the rotation scanning spatio-temporal mapping algorithm, according to the above-mentioned n converged reflected signals, respectively obtain n time-domain signals, the time-domain signals include the intensity of n time-domain peaks corresponding to n levels of steps, and also according to the above-mentioned n time-domain signals and the above-mentioned pixel matrix, solve to obtain the energy intensity corresponding to the n×n pixel points in the above-mentioned pixel matrix, and obtain the spot energy intensity distribution image according to the above-mentioned energy intensity.

[0014] According to an embodiment of the present disclosure, using the above spot imaging unit and the rotational scanning spatio-temporal mapping algorithm, n time-domain signals are respectively obtained based on the above n converged reflected signals. The time-domain signals include the intensities of n time-domain peaks corresponding to n-level steps. Also, based on the n time-domain signals and the pixel lattice, the energy intensities corresponding to n×n pixel points in the pixel lattice are solved. The spot energy intensity distribution image is obtained based on the energy intensities and is represented by the following formula:

[0015] a 11 x 11 +a 12 x 12 +a 13 x 13 +...+a nn x nn =e 11

[0016] a 11 x 11 +a 12 x 12 +a 13 x 13 +...+a nn x nn =e 12

[0017] …

[0018] a 11 x 11 +a 12 x 12 +a 13 x 13 +...+a nn x nn =e nn

[0019]

[0020] Where a 11 …a nn are the coefficients corresponding to the pixel points x 11 ..x nn When the pixel points participate in the calculation of the intensity of the time-domain peak, the coefficients corresponding to the pixel points are 1, otherwise 0; e 11 …e nn are the intensities of the time-domain peaks of the 1st to the nth reflecting surfaces obtained from the 1st to the nth reflected detection signals, where the first subscript represents which reflected detection signal, and the second subscript represents which reflecting surface; is the coefficient matrix; is the vector of pixel points to be solved; is a vector of intensity values.

[0021] Based on the above technical solutions, it can be seen that the embodiments of the present disclosure have the following beneficial effects compared with the prior art:

[0022] By applying to the spot imaging device for terahertz signals, the terahertz signals are processed by a terahertz signal collimation and beam expansion module, a polarization beam splitting module, and a stepped etalon reflection module, and the n reflection signals obtained by processing the reflection detection signals at different angles by the signal processing and spot imaging module are processed to obtain the spot energy intensity distribution image of the terahertz signal. The structure of the spot imaging device is simple and does not require frequent manual operation after the positions of each module are fixed. The degree of automation for obtaining the spot energy intensity distribution image of the terahertz signal is high, reducing the measurement complexity and improving the measurement and data processing speed. Moreover, the two-dimensional spatial distribution image of the terahertz signal spot energy intensity is obtained by inverting the n reflection signals, with a small error, and can more realistically restore the actual spot intensity distribution. By adjusting the structural parameters of the spot imaging device, the pixel resolution value and size value of the spot intensity distribution image can be adjusted, further solving the problem that it is difficult to measure the spot of the weak-signal terahertz band in the case of applying to TDS measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematically shows a schematic diagram of the framework of a spot imaging device for terahertz signals according to an embodiment of the present disclosure.

[0024] Figure 2 Schematically shows a schematic diagram of the framework of a spot imaging device for terahertz signals according to another embodiment of the present disclosure.

[0025] Figure 3 Schematically shows a schematic diagram of the structure of a stepped etalon according to an embodiment of the present disclosure.

[0026] Figure 4 Schematically shows a schematic diagram of the flow of a spot imaging method for terahertz signals according to an embodiment of the present disclosure.

[0027] Figure 5 Schematically shows a schematic diagram of the angle of a stepped etalon and the corresponding obtained reflection signals and energy distribution in the spot imaging method according to an embodiment of the present disclosure.

[0028] Figure 6 Schematically shows a schematic diagram of the time-domain signal corresponding to a stepped etalon in the spot imaging method according to an embodiment of the present disclosure.

[0029] Figure 7a Schematically shows a schematic diagram of the original spot energy intensity distribution image of a terahertz signal according to an embodiment of the present disclosure.

[0030] Figure 7b Schematically shown is a schematic diagram of the restored spot energy intensity distribution image of the terahertz signal obtained by using the spot imaging method according to an embodiment of the present disclosure.

[0031] 1 - Transmitting end photoconductive antenna;

[0032] 2 - Transmitting end off-axis parabolic mirror;

[0033] 3 - Metal wire grid beam splitter;

[0034] 4 - Terahertz quarter-wave plate;

[0035] 5 - Step-shaped etalon;

[0036] 6 - Electrically controlled rotating mirror mount;

[0037] 7 - Receiving end off-axis parabolic mirror;

[0038] 8 - Receiving end photoconductive antenna;

[0039] 9 - Spot imaging unit. Detailed implementation manners

[0040] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, for the sake of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, obviously, one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present disclosure.

[0041] The terms used herein are merely for describing specific embodiments and are not intended to limit the present disclosure. The terms "including", "comprising", etc. used herein indicate the presence of features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0042] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0043] In the case of using an expression such as "at least one of A, B, and C, etc.", generally, it should be interpreted according to the meaning that a person skilled in the art usually understands this expression (for example, "a system having at least one of A, B, and C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.).

[0044] In the process of implementing the present disclosure concept, the inventors found the following problems in the related art.

[0045] In the related art of terahertz spot measurement, the methods of laser spot measurement can be referred to. The terahertz beam is processed by methods such as two-dimensional array imaging method, knife-edge method, aperture method, and fiber optic scanning method, etc., to obtain the spot energy intensity distribution image of the terahertz wave.

[0046] Since the output power of the terahertz source is weaker than that of the laser, and the array imaging technology and devices based on the terahertz frequency band are yet to be developed, it is difficult to directly implement imaging measurement by the two-dimensional array imaging method. Even if the spot energy intensity distribution image of the terahertz wave is obtained by the two-dimensional array imaging method, its imaging resolution is low and the imaging spot area is small, which cannot meet the actual requirements.

[0047] Due to the limitations of the measurement instruments and method steps of the knife-edge method and the aperture method, the resolution of the obtained spot energy intensity distribution image is low, and manual operation is required to adjust the instrument during the imaging process, which may lead to measurement errors and the adjustment is also relatively cumbersome.

[0048] Moreover, the fiber optic scanning method is not applicable to the terahertz band. Therefore, a spot imaging device and method for terahertz signals are needed to obtain the spot energy intensity distribution image of a high-resolution terahertz beam without complex steps of manual operation.

[0049] The time-domain terahertz signal corresponding to the terahertz pulse can be generated and detected by a terahertz time-domain spectroscopy (THz-Time Domain Spectroscopy, THz-TDS) system.

[0050] For the measurement of the time-domain terahertz spot, since the terahertz signal is relatively weak, the measurement device of the electro-optic sampling measurement method based on the electro-optic crystal in the related art is complex, and the requirements for the optical path structure and data processing are strict. The operation steps are complex in actual use, and it is difficult to ensure the quality of the spot energy intensity distribution image of the terahertz signal.

[0051] TDS can simultaneously obtain the amplitude information and phase information of terahertz pulses. Picosecond-level terahertz waveforms can be sampled in the time domain by femtosecond laser pulses to obtain information including complete amplitude and phase, and then a broadband spectrum can be obtained by performing a Fourier transform on the time-domain terahertz signal. Optical parameters such as the absorption coefficient and refractive index corresponding to the time-domain terahertz signal can be obtained. For shorter terahertz pulse durations, high-range resolution can be obtained based on time-of-flight (TOF) detection. By measuring the time-domain terahertz signal after emission or transmission, the time difference of the characteristic peaks can be inversely obtained as the spatial distance difference, that is, the time-domain image corresponding to the time-domain terahertz signal can be converted and analyzed to obtain the spatial distance image corresponding to the terahertz signal, so as to obtain the spatial structure information inside the object in the reflection geometry, thereby realizing tomography. The absolute power emitted by TDS is very limited, and the sensitivity of focal plane arrays in the terahertz range based on photon or thermal detection is poor; in addition to traditional methods, two-dimensional electro-optic sampling (2D-EO) is an effective method that can achieve real-time terahertz imaging. A single-shot 2D-EO imaging method using a double-step optical device has also been proposed in related technologies, which obtains spatial imaging in the time domain by splitting the probe laser pulse into sequentially delayed sub-beams. However, in related technologies, the imaging method requires relatively strong terahertz and probe pulses and a high-performance CCD (Charge Coupled Device) camera to record the weak laser pulses modulated by the terahertz field of the Pockels effect. The related technologies and the two-dimensional optoelectronic sampling method are also difficult to directly measure a large-sized collimated terahertz beam after the optical system; for the existing imaging devices of THz–TDS, the obtained terahertz beam imaging resolution is low, or the process of adjusting the imaging device is complex and prone to errors.

[0052] In order to at least partially solve the technical problems existing in the related technologies, one aspect of the present disclosure provides a spot imaging device applied to terahertz signals.

[0053] Figure 1 Schematically shows a schematic diagram of the framework of a spot imaging device applied to terahertz signals according to an embodiment of the present disclosure.

[0054] As Figure 1 shown, the spot imaging device 100 applied to terahertz signals may include: a terahertz signal transmitting module 110, a beam splitting module 120, a stepped etalon reflection module 130, and a terahertz signal receiving module 140.

[0055] The terahertz signal transmitting module 110 can be used to generate and collimate and expand the terahertz signal.

[0056] The beam splitting module 120 can be used to receive and split a terahertz signal, and obtain and output a detection signal.

[0057] The stepped etalon reflection module 130 can include a stepped etalon 5. The stepped etalon 5 includes n levels of steps. The stepped etalon 5 can receive a detection signal and rotate around a rotation axis with a preset angular step, so as to reflect the detection signal at different angles and obtain n reflected detection signals. The direction of the rotation axis is parallel to the propagation direction of the detection signal, and the rotation axis passes through the symmetry center of the stepped etalon. The n reflected detection signals return to the beam splitting module 120, and the beam splitting module 120 obtains and outputs n reflected signals according to the returned n reflected detection signals; where n is a positive integer, the preset angle is obtained by dividing 180° by n, and each reflected signal represents the intensity of n time-domain peaks corresponding to n levels of steps.

[0058] The terahertz signal receiving module 140 can be used to converge and process n reflected signals to obtain an image of the spot energy intensity distribution of the terahertz signal. The image of the spot energy intensity distribution is a pixel matrix of size n×n. The pixel matrix is obtained according to the stepped etalon, and the energy intensity of the pixel matrix is obtained according to n reflected signals.

[0059] According to an embodiment of the present disclosure, the terahertz signal transmitting module 110 can collimate and expand a terahertz signal with a small diameter to obtain a terahertz signal with an adjustable diameter, so that when the terahertz signal is subsequently transmitted to the stepped etalon reflection module 130, the diameter of the terahertz signal can match the size of the stepped etalon. The terahertz signal transmitting module 110 can be a module constructed based on optical elements, and the size of the terahertz signal is adjusted by adjusting the element parameters in the terahertz signal transmitting module 110.

[0060] According to an embodiment of the present disclosure, the stepped etalon reflection module 130 can reflect a detection signal through the stepped etalon 5, and does not change the optical properties of the detection signal while reversing the optical path direction of the detection signal.

[0061] According to an embodiment of the present disclosure, the beam splitting module 120 processes once when the terahertz signal passes through the beam splitting module 120 to the stepped etalon reflection module 130, and also processes the n reflected detection signals one by one again when the stepped etalon reflection module 130 returns the n reflected detection signals, so that the n reflected detection signals no longer propagate along the optical path of the detection signal, but turn the n reflected detection signals to the terahertz signal receiving module 140.

[0062] According to an embodiment of the present disclosure, when the stepped etalon reflection module 130 reflects a detection signal, there is a height difference between the n - level steps of the stepped etalon reflection module 130, which can cause a detection signal to have different intensity spatial distributions at different positions of the stepped etalon 5. Furthermore, the terahertz signal receiving module 140 can receive the intensities of n time - domain peaks corresponding to the n - level steps reflected by each step.

[0063] According to an embodiment of the present disclosure, the terahertz signal receiving module 140 can receive n reflected signals. Among them, each reflected signal can include the intensities of n time - domain peaks caused by the reflections of the n - level steps of the stepped etalon reflection module 130. Based on the intensities of the detected time - domain peaks and an n×n - sized pixel lattice, a system of equations is established. By setting the intensities of each of the n×n pixel blocks as unknowns, the intensities of the n×n time - domain peaks are required. Through corresponding solutions, the intensities of each of the n×n pixel blocks can be obtained, and then the spot energy intensity distribution image of the terahertz signal can be obtained.

[0064] According to an embodiment of the present disclosure, since the terahertz wave signal is weak and invisible, in the spot imaging device 100 for terahertz signals, complex manual adjustment and matching are not required between each module, which can reduce the influence of human error on the terahertz wave signal. In addition, by selecting the optical element material, the transmission losses of the terahertz signal transmitting module 110, the stepped etalon reflection module 130, and 0 during the transmission, modulation, and reception of terahertz - related signals can be reduced. The beam splitting module 120 only changes the polarization state of the terahertz signal, and the spot imaging device 100 does not cause a large change to information such as the shape, size, and intensity distribution of the terahertz spot, so that the obtained spot energy intensity distribution image of the terahertz wave can more truly reflect the information of the terahertz wave.

[0065] According to an embodiment of the present disclosure, by means of a spot imaging device applied to terahertz signals, a terahertz beam is processed by a terahertz signal transmitting module 110, a beam splitting module 120, and a stepped etalon reflection module, and n reflected signals obtained by processing the reflected detection signals at different angles by a terahertz signal receiving module 140 are processed to obtain a spot energy intensity distribution image of the terahertz wave. The spot imaging device has a simple structure and does not require frequent manual operation after the positions of each module are fixed. The degree of automation for obtaining the spot energy intensity distribution image of the terahertz wave is high, reducing the measurement complexity and improving the measurement and data processing speed. Moreover, the two-dimensional spatial distribution image of the spot energy intensity of the terahertz beam is obtained by inverting n reflected signals, with a small error, and can relatively truly restore the actual spot intensity distribution. By adjusting the structural parameters of the spot imaging device, the pixel resolution value and the size value of the spot intensity distribution image can be adjusted, further solving the problem that it is difficult to measure the spot of the weak-signal terahertz band in the case of application to TDS measurement.

[0066] Based on the device described below, through Figure 1 the following will Figures 2 to 3 describe in detail the spot imaging device applied to terahertz signals in the embodiment.

[0067] Figure 2 Schematically shows a schematic diagram of the framework of a spot imaging device applied to terahertz signals according to another embodiment of the present disclosure.

[0068] As Figure 2 shown, the spot imaging device 100 applied to terahertz signals may include: a transmitting end photoconductive antenna 1, a transmitting end off-axis parabolic mirror 2, a metal wire grid beam splitter 3, a terahertz quarter-wave plate 4, a stepped etalon 5, an electronically controlled rotary mirror mount 6, a receiving end off-axis parabolic mirror 7, a receiving end photoconductive antenna 8, and a spot imaging unit 9.

[0069] According to an embodiment of the present disclosure, the terahertz signal transmitting module 110 may include: a transmitting end photoconductive antenna 1 and a transmitting end off-axis parabolic mirror 2.

[0070] The port of the transmitting end photoconductive antenna 1 is disposed at the focal position of the transmitting end off-axis parabolic mirror 2 for transmitting terahertz waves.

[0071] The transmitting end off-axis parabolic mirror 2 is configured to receive terahertz signals and perform collimation and beam expansion operations on the terahertz signals, and obtain and output terahertz signals.

[0072] According to an embodiment of the present disclosure, the initial diameter of the terahertz signal can be controlled by setting the diameter of the transmitting photoconductive antenna 1. The transmitting photoconductive antenna 1 can be a photoconductive antenna. The intensity of the optical wave cross-section of the terahertz signal emitted by the transmitting photoconductive antenna 1 can exhibit a Gaussian distribution with respect to the radial distance. The terahertz signal emitted by the transmitting photoconductive antenna 1 has a certain divergence angle. Therefore, the terahertz signal can be collimated in combination with the off-axis parabolic mirror 2 at the transmitting end for subsequent calculations. However, this is not limited thereto. Those skilled in the art can select a transmission line based on actual needs to send the terahertz signal to the off-axis parabolic mirror 2 at the transmitting end while keeping the transmission and interface losses small. The embodiments of the present disclosure are not limited herein.

[0073] According to an embodiment of the present disclosure, the terahertz signal can be collimated and beam-expanded by the off-axis parabolic mirror 2 at the transmitting end to obtain and output the terahertz signal to the beam splitting module 120. The terahertz signal can be parallel light, and the terahertz signal is linearly polarized light. The diameter of the terahertz signal can be adjusted by setting the parameters of the off-axis parabolic mirror 2 at the transmitting end. However, this is not limited thereto. Those skilled in the art can select a beam expansion and collimation device based on actual needs to output a terahertz signal with a parallelism that meets the actual needs to the beam splitting module 120 without generating reflection losses. The embodiments of the present disclosure are not limited herein.

[0074] According to an embodiment of the present disclosure, the beam splitting module 120 can include: a metal wire grid beam splitter 3 and a terahertz quarter-wave plate 4.

[0075] The metal wire grid beam splitter is disposed on the optical path where the terahertz signal is located and is used to pass the terahertz signal with the same polarization direction as the metal wire grid beam splitter;

[0076] The terahertz quarter-wave plate 4 is disposed on the optical path where the terahertz signal is located after the metal wire grid beam splitter 3. The terahertz quarter-wave plate 4 can be used to generate a phase delay. According to the terahertz signal with the same polarization direction as the metal wire grid beam splitter 3, a detection signal is obtained. The detection signal is circularly polarized light. The terahertz quarter-wave plate is also used to process the reflected detection signal to cause a phase delay in the reflected detection signal to obtain a reflected signal. The polarization direction of the reflected signal is perpendicular to the polarization direction of the metal wire grid beam splitter. The reflected signal is reflected by the metal wire grid beam splitter to the terahertz signal receiving module.

[0077] According to an embodiment of the present disclosure, when setting the metal wire grid beam splitter 3, the terahertz signal can pass through the geometric center of the metal wire grid beam splitter 3 according to the optical path direction of the terahertz signal. When placing the metal wire grid beam splitter 3, it is ensured that the polarization direction of the metal wire grid beam splitter 3 is consistent with the polarization direction of the terahertz signal, that is, the plane where the metal wire grid beam splitter 3 is located forms a 45° angle with the incident plane of the terahertz signal, so that the polarization direction of the terahertz signal coincides with the direction of the metal wire grid stripes of the metal wire grid beam splitter 3, so that the terahertz signal can be transmitted through the metal wire grid beam splitter 3 and reach the terahertz quarter-wave plate 4.

[0078] According to an embodiment of the present disclosure, the terahertz quarter-wave plate 4 can be set on the optical path where the terahertz signal is located, and the terahertz signal passes through the geometric center of the terahertz quarter-wave plate 4.

[0079] According to an embodiment of the present disclosure, the terahertz quarter-wave plate 4 can be composed of a wave plate based on quartz material, or can be composed of a Fresnel prism based on high-density polytetrafluoroethylene material. The terahertz quarter-wave plate 4 can introduce a 90° phase difference between two orthogonal components of the linearly polarized state of the incident terahertz signal, thereby converting the linearly polarized light into circularly polarized light. Similarly, when the circularly polarized light passes through the terahertz quarter-wave plate 4 again, it can be converted from circularly polarized light to linearly polarized light, but the polarization direction of the secondarily modulated linearly polarized light is orthogonal to the polarization direction of the unmodulated linearly polarized light.

[0080] According to an embodiment of the present disclosure, the terahertz quarter-wave plate 4 can cooperate with the metal wire grid beam splitter 3 that splits light according to polarization, and the beam splitting module 120 can achieve almost lossless single-station reflection of the terahertz signal energy.

[0081] According to an embodiment of the present disclosure, the beam splitting module 120 may include: a high-resistivity silicon lens.

[0082] The high-resistivity silicon lens is arranged on the optical path where the terahertz signal is located, and is used for splitting the terahertz signal and outputting a detection signal propagating along the propagation direction of the terahertz signal, and is also used for splitting the reflected detection signal and outputting a reflection signal propagating perpendicular to the propagation direction of the reflected detection signal.

[0083] According to an embodiment of the present disclosure, the high-resistivity silicon lens can be set on the optical path where the terahertz signal is located. The high-resistivity silicon lens is a lens with both sides being flat, and the symmetry axis of the cross-section of the high-resistivity silicon lens can form a 45° angle with the propagation direction of the terahertz signal, so as to split the terahertz signal by the high-resistivity silicon lens to obtain a detection signal.

[0084] According to an embodiment of the present disclosure, the high-resistivity silicon lens functions as a semi-transmissive and semi-reflective mirror in the spot imaging device 100 to split the signal. When the terahertz signal passes through the high-resistivity silicon lens, the split light signal of the part of the terahertz signal that transmits through the high-resistivity silicon lens is used as a detection signal for subsequent processing. The stepped etalon reflection module 130 is arranged on the optical path where the split light signal of the part of the terahertz signal that transmits through the high-resistivity silicon lens is located, so that the stepped etalon reflection module 130 reflects the detection signal. And when the reflected detection signal returns to the high-resistivity silicon lens, the split light signal of the part of the reflected detection signal that reflects out of the high-resistivity silicon lens is used as a reflection signal, so that the terahertz signal receiving module 140 can obtain the spot energy intensity distribution image according to the reflection signal.

[0085] According to an embodiment of the present disclosure, the ratio of the transmittance to the reflectivity of the high-resistivity silicon lens can be adjusted by coating operations. However, considering that the optical signal required for the first beam splitting is transmitted light, and the optical signal required for the second beam splitting is reflected light, in order to maximize the finally detected energy, the ratio of the transmittance to the reflectivity can be selected to be 1:1.

[0086] According to an embodiment of the present disclosure, when the metal wire grid beam splitter 3 and the terahertz quarter-wave plate 4 are selected to form the beam splitting module 120, the energy loss of the terahertz signal in the optical path is small. According to an embodiment of the present disclosure, when the high-resistivity silicon lens is selected to form the beam splitting module 120, there is no need to perform adjustment operations such as adjusting the positional relationship between the metal wire grid beam splitter 3 and the terahertz quarter-wave plate 4 to match the terahertz signal. The device adjustment steps are reduced, the optical path structure is relatively simple, the high-resistivity silicon lens has semi-transmissive and semi-reflective optical properties, and there will be a certain energy loss during the transmission of the optical signal in the optical path, and the optical path volume can be reduced. Those skilled in the art can select optical elements to form the beam splitting module 120 based on actual needs, and the embodiments of the present disclosure do not limit this here.

[0087] According to an embodiment of the present disclosure, the stepped etalon reflection module 130 may include: a stepped etalon 5. Figure 3 Schematically shows a structural diagram of a stepped etalon according to an embodiment of the present disclosure.

[0088] As Figure 3As shown in the figure, the stepped etalon 5 includes n levels of steps. Each step may include a reflective surface, and the plane where the reflective surface is located is perpendicular to the plane where the rotation axis is located. The reflective surface is used to reflect the detection signal. When the stepped etalon is in the initial position, the center lines corresponding to the lengths of the n reflective surfaces are obtained, resulting in n horizontal lines. When the stepped etalon rotates to a position perpendicular to the initial position, the center lines corresponding to the lengths of the n reflective surfaces are obtained, resulting in n vertical lines. n×n intersection points are obtained based on the n horizontal lines and the n vertical lines. Taking the n×n intersection points as the centers of pixel points and the width of the reflective surface as the side length of the pixel points, a pixel lattice of size n×n is constructed.

[0089] According to an embodiment of the present disclosure, the centers corresponding to n×n regions can be set as pixel points, and a pixel lattice of size n×n can be obtained based on the pixel points, so that the terahertz signal receiving module 140 can calculate the energy intensity corresponding to the pixel points within the pixel lattice according to the pixel lattice and the n reflected signals, and obtain the spot energy intensity distribution image according to the energy intensity.

[0090] According to an embodiment of the present disclosure, the stepped etalon can be obtained based on a reflective Michelson stepped grating. The reflective Michelson stepped grating is a blazed grating, whose shape is similar to a multi-step staircase and has regular variations in two dimensions in space. When it is placed in the terahertz signal coordinate system, along the wave propagation direction, the grating changes step by step with the width of each step, and can have time or distance resolution ability in the time-domain signal measured by the TDS system. At the same time, each step changes step by step with the height of each step in the direction perpendicular to the terahertz signal propagation direction, which can reflect the lateral spatial distribution of the intensities at different positions of the terahertz spot. The smaller the height and width of the reflective Michelson step, the higher the resolution. Therefore, based on the time-of-flight ranging principle, a stepped etalon with mutually strictly parallel and optically polished reflective surfaces can be fabricated, which can be used for spot measurement, and the measurement process is simple, with a high degree of automation, a large measurable spot range, and a flexible adjustable resolution, and can effectively meet the current requirements for terahertz signal spot measurement in the time domain.

[0091] According to an embodiment of the present disclosure, the size adjustment of the stepped etalon 5 is relatively flexible, which can adapt to terahertz signals of different sizes, avoiding the limitations of laser spot measurement methods in the related art for large spot and micro-spot measurements, and solving the problem that the energy of terahertz signals in the time domain is weak and their spots are difficult to measure.

[0092] According to an embodiment of the present disclosure, the stepped etalon can be made of aluminum alloy material, with the same length, width, and height for each level, and the reflective surfaces are mutually strictly parallel and optically polished, so as to reduce the reflection loss of the detection signal and maintain the parallelism of the obtained reflected detection signals.

[0093] According to an embodiment of the present disclosure, the stepped etalon reflection module 130 may further include: an electrically controlled rotating mirror holder 6. The electrically controlled rotating mirror holder 6 can drive the stepped etalon 5 to rotate along the rotation axis, so that the stepped etalon reflection module continuously obtains n reflected detection signals during the process of obtaining the spot energy intensity distribution image of the terahertz signal.

[0094] According to an embodiment of the present disclosure, during the process of obtaining the spot energy intensity distribution image of the terahertz signal, the electrically controlled rotating mirror holder 6 can be controlled by a program to automatically drive the stepped etalon 5 to rotate 180° at a preset angle without manual adjustment.

[0095] According to an embodiment of the present disclosure, when the stepped etalon reflection module 130 reflects the detection signal at different angles to obtain n reflected detection signals, the n reflected detection signals return to the beam splitting module 120. When the metal wire grid beam splitter 3 and the terahertz quarter-wave plate 4 are selected to form the beam splitting module 120, it can also be used to reflect the reflected detection signal reflected by the stepped etalon reflection module, so as to obtain and output the reflected signal. When the high-resistivity silicon lens is selected to form the beam splitting module 120, it can also be used to reflect the reflected detection signal reflected by the stepped etalon reflection module, so as to obtain and output the reflected signal.

[0096] According to an embodiment of the present disclosure, the terahertz signal receiving module 140 includes: a receiving end off-axis parabolic mirror 7, a receiving end photoconductive antenna 8, and a spot imaging unit 9.

[0097] The receiving end off-axis parabolic mirror 7 is used to sequentially receive and converge n reflected signals.

[0098] The port of the receiving end photoconductive antenna 8 is set at the focal position of the receiving end off-axis parabolic mirror 7, and is used to receive and send n converged reflected signals to the spot imaging unit.

[0099] According to an embodiment of the present disclosure, in the stepped etalon reflection module 130, the component design of the receiving end off-axis parabolic mirror 7 and the receiving end photoconductive antenna 8 can refer to the description of the transmitting end photoconductive antenna 1 and the transmitting end off-axis parabolic mirror 2 in the above-mentioned terahertz signal transmitting module 110, and will not be elaborated here.

[0100] According to an embodiment of the present disclosure, the spot imaging unit 9 is used for the receiving end photoconductive antenna 8. According to the n converged reflected signals received by the receiving end photoconductive antenna 8, n time-domain signals are obtained, and it is also used to calculate the energy intensity corresponding to the pixel points in the pixel lattice according to the n time-domain signals and the pixel lattice by using the rotation scanning spatio-temporal mapping algorithm, and obtain the spot energy intensity distribution image according to the energy intensity.

[0101] According to an embodiment of the present disclosure, the spot imaging unit 9 may include a photoelectric conversion sub-unit and a computing sub-unit loaded with a computing program. The photoelectric conversion sub-unit pre-processes and performs photoelectric conversion on the signal according to n converged reflection signals to obtain n time-domain signals. The computing sub-unit obtains the spot energy intensity distribution image according to the n time-domain signals and the pixel lattice.

[0102] According to an embodiment of the present disclosure, the spot imaging unit 9 may be a computer loaded with programs for algorithm inversion and automatic control; the spot imaging unit 9 may also be a hardware component such as a chip loaded with programs for algorithm inversion and automatic control; the spot imaging unit 9 may also be other devices including software loaded with programs for algorithm inversion and automatic control. Those skilled in the art can select the type of the spot imaging unit 9 based on actual needs to process the acquired reflection signals and obtain the spot energy intensity distribution image, and the embodiments of the present disclosure do not limit this here.

[0103] According to an embodiment of the present disclosure, the spot imaging unit 9 may further include a control sub-unit loaded with a control program. The control sub-unit may be connected to the electrically controlled rotary mirror frame 6 to control the electrically controlled rotary mirror frame 6 to drive the stepped etalon 5 to rotate along the rotation axis. The control sub-unit may also be connected to the transmitting end photoconductive antenna 1 to obtain the original data of the terahertz beam.

[0104] Figure 4 Schematically shows a schematic flow chart of a spot imaging method applied to a terahertz beam according to an embodiment of the present disclosure.

[0105] As Figure 4 shown, the spot imaging method 400 applied to a terahertz beam includes operations S410 to S440.

[0106] In operation S410, a terahertz signal transmitting module 110 is used to generate and collimate and expand a terahertz signal.

[0107] In operation S420, a beam splitting module 120 is used to receive and split the terahertz signal to obtain and output a detection signal.

[0108] In operation S430, a stepped etalon reflection module 130 is used. The stepped etalon reflection module 130 includes a stepped etalon 5. The stepped etalon 5 includes n levels of steps. The stepped etalon 5 receives the detection signal and rotates around the rotation axis at a preset angular step to reflect the detection signal at different angles to obtain n reflected detection signals. The direction of the rotation axis is parallel to the propagation direction of the detection signal, and the rotation axis passes through the symmetry center of the stepped etalon. The n reflected detection signals return to the beam splitting module, and the beam splitting module 120 obtains and outputs n reflection signals according to the returned n reflected detection signals.

[0109] In operation S440, a terahertz signal receiving module 140 is used to converge and process n reflected signals to obtain a spot energy intensity distribution image of the terahertz signal. The spot energy intensity distribution image is a pixel lattice of size n×n. The pixel lattice is obtained according to a stepped etalon, and the pixel lattice energy intensity is obtained according to the n reflected signals.

[0110] According to an embodiment of the present disclosure, a spot imaging method 400 for terahertz signals converts the time distribution of a time-domain terahertz signal into a spatial intensity distribution through a stepped etalon 5, solves the problem of two-dimensional imaging of a time-domain terahertz signal with weak power, and can effectively meet the current requirements for time-domain measurement of the terahertz signal spot.

[0111] According to an embodiment of the present disclosure, the terahertz signal receiving module 140 may include: a receiving off-axis parabolic mirror, a receiving photoconductive antenna, and a spot imaging unit. Operation S440 may include: using the receiving off-axis parabolic mirror to sequentially converge n reflected signals; using the receiving photoconductive antenna port disposed at the focal position of the receiving off-axis parabolic mirror to receive and send the n converged reflected signals to the spot imaging unit; using the spot imaging unit to respectively obtain n time-domain signals according to the n converged reflected signals by using a rotational scanning spatio-temporal mapping algorithm. The time-domain signals include the intensities of n time-domain peaks corresponding to n levels of steps. Also, according to the n time-domain signals and the pixel lattice, the energy intensities corresponding to the n×n pixel points in the pixel lattice are solved, and a spot energy intensity distribution image is obtained according to the energy intensities.

[0112] According to an embodiment of the present disclosure, according to Figure 2 For the spot imaging device 100 for terahertz signals shown, in operation S440, the spot imaging unit is used to respectively obtain n time-domain signals according to the n converged reflected signals by using a rotational scanning spatio-temporal mapping algorithm. The time-domain signals include the intensities of n time-domain peaks corresponding to n levels of steps. Also, according to the n time-domain signals and the pixel lattice, the energy intensities corresponding to the n×n pixel points in the pixel lattice are solved, and obtaining a spot energy intensity distribution image according to the energy intensities may include: using the spot imaging unit 9 to obtain n time-domain signals according to the n converged reflected signals, so that the spot imaging unit 9 calculates the energy intensity corresponding to the pixel point according to the pixel point and the n time-domain signals by using a rotational scanning spatio-temporal mapping algorithm, and a spot energy intensity distribution image is obtained according to the energy intensities; the pixel point is a point within the area divided by the steps of the stepped etalon 5.

[0113] The following will be based on Figure 4 the method described, and will describe in detail the spot imaging method for terahertz signals in the embodiment through Figures 5 to 7b examples.

[0114] Figure 5Schematically shows a schematic diagram of the angle of a stepped etalon and the corresponding acquired reflection signal and energy distribution in the spot imaging method according to an embodiment of the present disclosure.

[0115] As Figure 5 shown, the left column in the figure is the reflection surfaces of the stepped etalon 5 at 0°, θ°, and 180°. The middle column in the figure is a schematic diagram of the corresponding obtained reflection signals at 0°, θ°, and 180°. In the middle figure, the abscissa is the time axis and the ordinate is the reflection signal energy value. The right column in the figure is the intensity of the time-domain peaks corresponding to the n reflection surfaces of the stepped etalon 5 in one reflection signal at 0°, θ°, and 180°. In the right figure, the abscissa is the 1st to nth steps, the 1st step is the step with the largest height in the stepped etalon 5, and the ordinate is the intensity of the corresponding time-domain peak.

[0116] Set 0° as the initial position of the stepped etalon 5. The following settings are also made: Set that in the current embodiment, the stepped etalon 5 rotates clockwise along the rotation axis, and the rotation axis passes through the center of symmetry of the stepped etalon, that is, at the center of the overall area corresponding to the n reflection surfaces of the stepped etalon 5 as Figure 5 shown. At 0°, the center lines corresponding to the length directions of each step of the stepped etalon 5 are obtained, that is, n horizontal lines are obtained; and after rotating clockwise to 90°, the center lines corresponding to the length directions of each step of the stepped etalon 5 are obtained, that is, n vertical lines are obtained. The n×n foci formed by the n horizontal lines and n vertical lines are as Figure 5 shown as x 11 ..x nn . Define x 11 ..x nn as pixel points, and the pixel points are the midpoints of the pixel blocks of the pixel lattice. The pixel lattice includes n×n pixel blocks of equal size. The pixel blocks obtained by the above method are square, and the side length value thereof is the step width value of the stepped etalon 5. The pixel points are represented by x ij , where i and j ∈ [1, n]; it can be set that i is the i-th row and j is the j-th column. The coordinates of the pixel points can be set as (x, y), where x and y ∈ [1, n]. Set the coordinates of the center point of the etalon rotation as (x0, y0), and x0 = y0 = n / 2.

[0117] Set up a family of lines established with n + 1 edges divided in the length direction of the n-step ladder. The n + 1 lines at each angle are a family, with equal slopes. The distance between adjacent two lines is the step width of the etalon. At 0°, the slope of the family of lines is k0 = 0, and the intercept is b0 = b, where b ∈ (0, n). Then the slope k θ of the stepped etalon 5 at the θ angle and b θThe intercepts can be expressed by the following formulas (1.1) and (1.2) respectively.

[0118]

[0119]

[0120] By rotating the stepped etalon 5, reflection signals at different angles are obtained, and then the intensity values of the corresponding pixel points are obtained. A system of linear equations with multiple unknowns is established for the intensity values of the pixel points. The system of linear equations with multiple unknowns can be expressed by the following formulas (2.1) to formula (2.n 2 ) and, formulas (2.1) to (2.n 2 ) can be converted into the matrix form expressed by formula (2.n 2 + 1).

[0121] a 11 x 11 + a 12 x 12 + a 13 x 13 +... + a nn x nn = e 11 (2.1)

[0122] a 11 x 11 + a 12 x 12 + a 13 x 13 +... + a nn x nn = e 12 (2.2)

[0123] …

[0124] a 11 x 11 + a 12 x 12 + a 13 x 13 +... + a nn x nn = e nn (2.n 2 )

[0125]

[0126] Among them, a 11 …a nn are the coefficients corresponding to the pixel points x 11 ..x nn ; e 11 …enn is the intensity of the time-domain peak of the 1st to the nth reflecting surfaces obtained from the detected signals of the 1st to the nth reflections, where the first subscript represents the detected signal of the nth reflection, and the second subscript represents the nth reflecting surface; is the coefficient matrix; is the vector of pixel points to be solved; is the vector of intensity values.

[0127] According to the embodiments of the present disclosure, for the pixel points x 11 ..x nn the corresponding coefficients can be set as follows. For the pixel points x 11 ..x nn when participating in the calculation of the intensity of the time-domain peak, the coefficient corresponding to the pixel point is 1, otherwise it is 0. For example, in the case of formula (2.1), on the right side of the formula at 0°, the intensity of the time-domain peak of the reflecting surface of the first-order step of the Fabry-Perot etalon 5 is obtained. Correspondingly, the coefficients of x 11 ..x 1n are 1, and the coefficients corresponding to the remaining pixel points not participating are 0. Another example is that during the rotation of the Fabry-Perot etalon, while keeping the position of the pixel lattice unchanged, the relative position of the reflecting surface of the Fabry-Perot etalon to the pixel points changes. It is set that during the rotation of the Fabry-Perot etalon, when the pixel point has not relatively moved out of the step it belongs to, the coefficient corresponding to the pixel point is 1; when the pixel point has relatively moved out of the step it belongs to, the coefficient corresponding to the pixel point is 0. Taking the reflecting surface of the Fabry-Perot etalon 5 at θ° shown in Figure 5 as an example, when calculating the intensity of the time-domain peak of the reflecting surface of the first-order step of the Fabry-Perot etalon 5 at θ°, none of the pixel points of the first-order step have relatively moved out of the step they belong to, so the coefficients corresponding to all pixel points are 1. At this time, the rightmost pixel point of the first-order step is already at the edge of the first-order step; since it is set that the Fabry-Perot etalon 5 rotates clockwise around the rotation axis, after another preset angle, at this time, the rightmost pixel point of the first-order step has relatively moved out of the step it belongs to. Then, when calculating the intensity of the time-domain peak of the reflecting surface of the first-order step after rotating θ° shown in Figure 5 clockwise plus a preset angle, the coefficient corresponding to the rightmost pixel point is 0. The rest of the cases can be deduced by analogy and will not be elaborated here.

[0128] According to the embodiments of the present disclosure, according to the reflected signals corresponding to the detected signals reflected at different angles, in order to obtain the time-domain signals. n equations can be established based on the time-domain signals, and data at greater than or equal to n angles are obtained to establish a system of linear equations of multiple variables so that the system of equations is solvable. Thus, the intensity of each pixel point can be solved, and further the spatial intensity distribution of the terahertz light spot can be obtained.

[0129] According to embodiments of the present disclosure, by changing the manufacturing precision, size of the stepped etalon 5, and the angular step of the rotating mirror mount 6, it is possible to meet a variety of measurement backgrounds and requirements. In addition, the spot imaging device 100 for terahertz signals can also be extended to the spot imaging of systems capable of identifying time-domain signals of electromagnetic waves in other bands by changing the component parameters, such as single-photon counting systems, etc., to achieve laser spot measurement.

[0130] According to embodiments of the present disclosure, when constructing the spot imaging device 100 for terahertz signals, the following specific data can be set.

[0131] Terahertz photoconductive antennas (TAS1230 / TAS1110) of AVANTEST Corporation can be used as the transmitting photoconductive antenna 1 and the receiving photoconductive antenna 8 respectively, and they are fixed at the focal point positions of off-axis parabolic mirrors with a focal length of 2 inches and an aperture of 1 inch at the transmitting end and the detecting end, and the system optical structure is a compact field incidence.

[0132] A metal wire grid beam splitter 3 with a diameter of 5 cm can be placed at 45° on the optical path in the direction parallel to the off-axis parabolic mirror at the transmitting end. A broadband terahertz quarter-wave plate 4 with a diameter of 2 inches and a wavelength coverage range of 100 μm to 717 μm is placed on the metal wire grid beam splitter 3.

[0133] Then, a Sigma electric control rotating mirror mount with a stepped etalon fixed thereon can be placed perpendicular to the optical axis of the beam. The terahertz signal receiving module including the photoconductive antenna and the off-axis parabolic mirror is located in the direction of the vertical reflection of the metal wire grid. The stepped etalon 5 can be set as an etalon made of aluminum alloy. The height corresponding to the highest order of the stepped etalon 5 is 5 cm, the overall length of the stepped etalon 5 is 5 cm, the overall width is 2.5 cm, the stepped etalon 5 includes 50 steps, the interval between adjacent two steps is 0.5 mm, the length of the stepped reflection surface is 50 mm, and the width is 1 mm. During manufacturing, ensure the parallelism and smoothness of the stepped reflection surface. With this specific digital setting, the peak spacing between different stepped reflection surfaces in the time-domain signal reflected by the stepped etalon 5 can be 3.3 ps.

[0134] In the case of actual imaging, the rotation step of the stepped etalon 5 is set to 1°, and the angle of the rotating mirror frame is automatically stepped with the optical axis as the rotation axis to obtain the time-domain signal at each angle. Moreover, in the spot imaging unit 9, signal preprocessing may include signal denoising, smoothing, and taking the absolute value of the peak and valley to obtain the spot energy intensity distribution image subsequently. It can be understood that based on theoretical derivation, when obtaining the reflection signals equal to the number of steps of the stepped etalon 5 (i.e., n reflection signals), and obtaining the intensity of the time-domain peaks of n×n reflection surfaces, the spot energy intensity distribution image of the terahertz signal can be obtained through calculation. However, in actual calculation, the intensity of the time-domain peaks of more than n×n reflection surfaces is obtained for calculation, so as to better eliminate the measurement error in the actual situation, the element matching error of the spot imaging device caused by process problems, and the element errors such as the surface reflectivity of the stepped etalon 5 and the parallelism between the steps of the stepped etalon 5, so as to calculate the spot intensity distribution more accurately and realistically. Preferably, after determining the number of steps n of the stepped etalon 5, the value obtained by 180°÷(n×4) is used as the preset angle value in actual operation, thereby obtaining an image of the spot intensity distribution with better quality. For example, in the case where the above-mentioned stepped etalon 5 includes 50 steps, the specific preset angle value can also be 0.9°.

[0135] To reduce the absorption effect of water vapor in the air on terahertz waves, an acrylic cover can be set to cover the spot imaging device 100 applied to terahertz signals to obtain a sealed space, and the sealed space is filled with dry air until the humidity is 5%RH. During the actual process of obtaining the spot energy intensity distribution image of terahertz signals, the device for filling dry air is kept working to make the humidity of the sealed space 5%RH or less, so as to improve the accuracy of the data obtained in subsequent measurements.

[0136] It can be set to obtain the time-domain signal through the TAS7500TS terahertz time-domain spectrometer system of AVANTEST Corporation.

[0137] Figure 6 A schematic diagram showing the time-domain signal corresponding to the stepped etalon in the spot imaging method according to an embodiment of the present disclosure is schematically shown.

[0138] As Figure 6 shown, the abscissa in the figure is the time axis, and the ordinate is the energy intensity of the time-domain signal. By obtaining the time-domain signal and preprocessing the time-domain signal, the energy intensity of the time-domain signal is obtained, imported into the rotational scanning spatio-temporal mapping algorithm, and through establishing a system of linear equations of multiple variables of the first degree, the spot energy intensity distribution image of the terahertz signal is inversely solved.

[0139] Figure 7aA schematic diagram schematically shows the original spot energy intensity distribution image of a terahertz signal according to an embodiment of the present disclosure.

[0140] Figure 7b A schematic diagram schematically shows the restored spot energy intensity distribution image of a terahertz signal obtained by using a spot imaging method according to an embodiment of the present disclosure.

[0141] As Figure 7a and Figure 7b shown, its x-axis and y-axis represent the spot energy intensity distribution points obtained based on a pixel lattice of size n×n, and its gray value represents the magnitude of the spot energy intensity. Comparing Figure 7a and Figure 7b , the spot imaging method applied to the terahertz signal has a small error and can relatively realistically restore the actual spot intensity distribution.

[0142] The flowcharts and block diagrams in the accompanying drawings schematically show the possible architectures, functions, and operations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in the flowchart or block diagram may represent a module, a program segment, or a part of code, and the above-mentioned module, program segment, or part of code includes one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the blocks may occur in a different order than marked in the accompanying drawings. For example, two consecutive blocks shown may actually be executed substantially in parallel, and they may sometimes be executed in the reverse order, depending on the functions involved. It should also be noted that each block in the block diagram or flowchart, and combinations of blocks in the block diagram or flowchart, can be implemented by a dedicated hardware-based system for performing the specified functions or operations, or can be implemented by a combination of dedicated hardware and computer instructions.

[0143] Those skilled in the art can understand that the features recited in the various embodiments and / or claims of the present disclosure can be combined or combined in various ways, even if such combinations or combinations are not explicitly recited in the present disclosure. In particular, without departing from the spirit and teachings of the present disclosure, the features recited in the various embodiments and / or claims of the present disclosure can be combined and combined in various ways. All such combinations and / or combinations fall within the scope of the present disclosure.

[0144] The embodiments of the present disclosure have been described above. However, these embodiments are merely for the purpose of illustrating the objectives, technical solutions, and beneficial effects of the present disclosure, rather than limiting the scope of the present disclosure. Although the embodiments have been described separately above, this does not mean that the measures in each embodiment cannot be used advantageously in combination. The scope of the present disclosure is defined by the appended claims and their equivalents. Without departing from the scope of the present disclosure and within the spirit and principles of the present disclosure, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should fall within the protection scope of the present disclosure.

Claims

1. A spot imaging device for terahertz signals, comprising: A terahertz signal transmitting module for generating and collimating and expanding the terahertz signal; A beam splitting module for receiving and splitting the terahertz signal to obtain and output a detection signal; A stepped etalon reflection module, the stepped etalon reflection module comprising a stepped etalon, the stepped etalon comprising n levels of steps, the stepped etalon being configured to receive the detection signal, rotate around a rotation axis at a preset angular step, so as to reflect the detection signal at different angles to obtain n reflected detection signals, the direction of the rotation axis being parallel to the propagation direction of the detection signal, the rotation axis passing through the symmetry center of the stepped etalon, the n reflected detection signals returning to the beam splitting module, and the beam splitting module obtaining and outputting n reflected signals according to the returned n reflected detection signals; wherein, n is a positive integer, the preset angle is obtained by dividing 180° by n, and each of the reflected signals represents the intensity of n time-domain peaks corresponding to n levels of steps; Wherein, the step comprises a reflecting surface, the plane where the reflecting surface is located is perpendicular to the plane where the rotation axis is located, and the reflecting surface is configured to reflect the detection signal; In the case where the stepped etalon is in the initial position, obtain the center lines corresponding to the lengths of the n reflecting surfaces to obtain n horizontal lines; In the case where the stepped etalon rotates to be perpendicular to the initial position, obtain the center lines corresponding to the lengths of the n reflecting surfaces to obtain n vertical lines; n×n intersections obtained according to the n horizontal lines and the n vertical lines; and Construct an n×n pixel lattice with the n×n intersections as the centers of pixel points and the width of the reflecting surface as the side length of the pixel points; and A terahertz signal receiving module for converging and processing the n reflected signals to obtain an image of the spot energy intensity distribution of the terahertz signal, the image of the spot energy intensity distribution being an n×n pixel lattice, the pixel lattice being obtained according to the stepped etalon, and the energy intensity of the pixel lattice being obtained according to the n reflected signals.

2. The spot imaging device according to claim 1, wherein, The terahertz signal transmitting module comprises: a transmitting end photoconductive antenna and a transmitting end off-axis parabolic mirror; The port of the transmitting end photoconductive antenna is arranged at the focal position of the transmitting end off-axis parabolic mirror for generating the terahertz signal; and The transmitting end off-axis parabolic mirror is configured to perform collimation and expansion operations on the terahertz signal and output the terahertz signal.

3. The spot imaging device according to claim 2, wherein, The beam splitting module comprises: a metal wire grid beam splitter and a terahertz quarter-wave plate; The metal wire grid beam splitter is arranged on the optical path where the terahertz signal is located for passing the terahertz signal with the same polarization direction as the metal wire grid beam splitter; The terahertz quarter-wave plate is disposed on the optical path of the terahertz signal after the metal wire grid beam splitter, and is used to generate a phase delay. According to the terahertz signal having the same polarization direction as that of the metal wire grid beam splitter, the detection signal is obtained. The detection signal is circularly polarized light. The terahertz quarter-wave plate is further used to process the reflected detection signal to cause a phase delay in the reflected detection signal, thereby obtaining the reflected signal. The polarization direction of the reflected signal is perpendicular to the polarization direction of the metal wire grid beam splitter. The reflected signal is reflected by the metal wire grid beam splitter to the terahertz signal receiving module.

4. The spot imaging device according to claim 2, wherein, The beam splitting module includes: a high-resistivity silicon lens; The high-resistivity silicon lens is disposed on the optical path of the terahertz signal, such that the symmetry axis of the cross-section of the high-resistivity silicon lens forms an angle of 45° with the propagation direction of the terahertz signal. It is used to split the terahertz signal, and the transmission-reflection ratio of the splitting is 1:

1. It outputs the detection signal that is transmitted through the high-resistivity silicon lens and propagates along the propagation direction of the terahertz signal, and is also used to split the reflected detection signal, and outputs the reflected signal that is reflected from the high-resistivity silicon lens and propagates perpendicular to the propagation direction of the reflected detection signal.

5. The spot imaging device according to claim 1, wherein, The terahertz signal receiving module includes: a receiving end off-axis parabolic mirror, a receiving end photoconductive antenna, and a spot imaging unit; The receiving end off-axis parabolic mirror is used to sequentially converge n reflected signals; The receiving end photoconductive antenna port is disposed at the focal position of the receiving end off-axis parabolic mirror, and is used to receive and send n converged reflected signals to the spot imaging unit; and The spot imaging unit is used to respectively obtain n time-domain signals according to the n converged reflected signals. Each time-domain signal includes the intensities of n time-domain peaks corresponding to n levels of steps. It is also used to solve for the energy intensities corresponding to n×n pixel points in the pixel lattice according to the n time-domain signals and the pixel lattice, and obtain the spot energy intensity distribution image according to the energy intensities.

6. The spot imaging device according to claim 1, wherein the stepped etalon reflection module further comprises: An electrically controlled rotating mirror mount; The electrically controlled rotating mirror mount is used to drive the stepped etalon to rotate along the rotation axis, so that the stepped etalon reflection module continuously obtains the n reflected detection signals during the process of obtaining the spot energy intensity distribution image of the terahertz signal.

7. A spot imaging method applied to terahertz signals, comprising: Using a terahertz signal transmitting module to generate and collimate and expand the terahertz signal; Using a beam splitting module to receive and split the terahertz signal, and obtain and output a detection signal; Using a stepped etalon reflection module, the stepped etalon reflection module includes a stepped etalon, the stepped etalon includes n levels of steps, the stepped etalon receives the detection signal, rotates around the rotation axis at a preset angular step, so as to reflect the detection signal at different angles, obtaining n reflected detection signals, the direction of the rotation axis is parallel to the propagation direction of the detection signal, the rotation axis passes through the symmetry center of the stepped etalon, the n reflected detection signals return to the beam splitting module, so as to use the beam splitting module to obtain and output n reflected signals according to the returned n reflected detection signals; wherein, n is a positive integer, the preset angle is obtained by dividing 180° by n, and each reflected signal represents the intensity of n time-domain peaks corresponding to n levels of steps; Wherein, the step includes a reflecting surface, the plane where the reflecting surface is located is perpendicular to the plane where the rotation axis is located, and the reflecting surface is used for reflecting the detection signal; When the stepped etalon is in the initial position, obtain the center lines corresponding to the lengths of the n reflecting surfaces, obtaining n horizontal lines; When the stepped etalon rotates to be perpendicular to the initial position, obtain the center lines corresponding to the lengths of the n reflecting surfaces, obtaining n vertical lines; n×n intersections obtained according to the n horizontal lines and the n vertical lines; and Taking the n×n intersections as the center of pixel points and the width of the reflecting surface as the side length of pixel points, constructing a pixel lattice of size n×n; and Using a terahertz signal receiving module, converging and processing the n reflected signals to obtain the spot energy intensity distribution image of the terahertz signal, the spot energy intensity distribution image is a pixel lattice of size n×n, the pixel lattice is obtained according to the stepped etalon, and the pixel lattice energy intensity is obtained according to the n reflected signals.

8. The spot imaging method according to claim 7, wherein, The terahertz signal receiving module includes: a receiving end off-axis parabolic mirror, a receiving end photoconductive antenna, and a spot imaging unit; The using the terahertz signal receiving module to converge and process the n reflected signals to obtain the spot energy intensity distribution image of the terahertz signal includes: Using the receiving end off-axis parabolic mirror to sequentially converge the n reflected signals; Using the receiving end photoconductive antenna port to be set at the focal position of the receiving end off-axis parabolic mirror, receiving and sending the n converged reflected signals to the spot imaging unit; and Using the spot imaging unit, using the rotational scanning spatio-temporal mapping algorithm, according to the n converged reflected signals, respectively obtaining n time-domain signals, each time-domain signal includes the intensity of n time-domain peaks corresponding to n levels of steps, and also solving according to the n time-domain signals and the pixel lattice to obtain the energy intensity corresponding to the n×n pixel points in the pixel lattice, and obtaining the spot energy intensity distribution image according to the energy intensity.

9. The spot imaging method according to claim 8, wherein, Using the spot imaging unit, by means of the rotational scanning spatio-temporal mapping algorithm, n time-domain signals are respectively obtained based on the n converged reflected signals. Each of the time-domain signals includes the intensities of n time-domain peaks corresponding to n levels of steps. Also, based on the n time-domain signals and the pixel lattice, the energy intensities corresponding to n×n pixel points in the pixel lattice are solved. The spot energy intensity distribution image is obtained based on the energy intensities and is expressed by the following formula: … wherein, is the pixel point corresponding coefficient. When the pixel point participates in the intensity calculation of the time-domain peak, the coefficient corresponding to the pixel point is 1; otherwise, it is 0. is the intensity of the time-domain peak of the first to the nth reflecting surfaces obtained from the first to the nth reflected detection signals, where the first subscript represents the number of the reflected detection signal, and the second subscript represents the number of the reflecting surface. is the coefficient matrix; is the vector of pixel points to be solved; is the intensity value vector.

Images