CN113218909A - Terahertz near field real-time imaging system considering spectral information - Google Patents
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- 238000003384 imaging method Methods 0.000 title claims abstract description 46
- 230000003595 spectral effect Effects 0.000 title claims abstract description 27
- 238000001514 detection method Methods 0.000 claims abstract description 74
- 239000013078 crystal Substances 0.000 claims abstract description 66
- 238000005086 pumping Methods 0.000 claims abstract description 12
- 230000003287 optical effect Effects 0.000 claims abstract description 9
- 238000005259 measurement Methods 0.000 claims abstract description 8
- 230000010287 polarization Effects 0.000 claims description 21
- 229910005540 GaP Inorganic materials 0.000 claims description 12
- HZXMRANICFIONG-UHFFFAOYSA-N gallium phosphide Chemical compound [Ga]#P HZXMRANICFIONG-UHFFFAOYSA-N 0.000 claims description 12
- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 claims description 10
- 238000010586 diagram Methods 0.000 claims description 4
- 238000012360 testing method Methods 0.000 abstract description 3
- 239000000523 sample Substances 0.000 description 14
- 230000005540 biological transmission Effects 0.000 description 5
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- 238000005516 engineering process Methods 0.000 description 3
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 230000003321 amplification Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
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- 230000000630 rising effect Effects 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 229910052594 sapphire Inorganic materials 0.000 description 1
- 239000010980 sapphire Substances 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 239000013589 supplement Substances 0.000 description 1
- 238000001328 terahertz time-domain spectroscopy Methods 0.000 description 1
- 239000010936 titanium Substances 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
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- G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
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- G01J3/12—Generating the spectrum; Monochromators
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
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Abstract
The invention discloses a terahertz near field real-time imaging system giving consideration to spectral information. The femtosecond laser is divided into a pumping path and a detection path, the pumping path generates terahertz pulses, the detection path scans terahertz waveforms, a sample to be detected is placed close to a detection crystal to realize near-field detection, and the image acquisition module realizes optical path multiplexing of terahertz time-domain spectral measurement and terahertz imaging. The terahertz image measuring device is highly modularized in light path design and simple and convenient to operate, terahertz images of various samples can be obtained through testing, the imaging resolution can reach below 50 micrometers, and the real-time frame rate can reach 10 FPS.
Description
Technical Field
The invention belongs to the field of terahertz wave imaging, particularly relates to a terahertz near field real-time imaging system giving consideration to spectral information, and more particularly relates to a near field real-time imaging system based on a transmission type terahertz time-domain spectroscopy system and highly modularized optical path design.
Background
In recent years, the field of terahertz scientific technology has been rapidly developed, and one of the most important applications is terahertz imaging, which can be used as a signal source for object imaging as an electromagnetic wave, as well as visible light, X-ray, ultrasonic wave, and the like, and now the terahertz imaging technology has become a powerful supplement to imaging technologies such as X-ray imaging, millimeter wave imaging, ultrasonic imaging, and the like. The terahertz imaging can be used for obtaining more abundant information than other light sources, such as refractive index and spatial density distribution of materials, water content and distribution of organisms and the like, and has important significance in the fields of biological tissue distinguishing, food and drug quality testing, conductive film nondestructive testing, cultural relic geological exploration, security inspection, hidden object distinguishing, target radar imaging and the like.
However, the resolution of terahertz imaging is limited by the diffraction limit of terahertz wavelength and is far larger than the dimensions of micro-nano structures or biological tissues and cells, so that the terahertz imaging cannot meet the requirement of high-precision observation within a long period of time. Most terahertz imaging systems at present can not be applied to the fields with higher requirements on real-time performance, such as biomedical imaging, based on point-by-point scanning of a two-dimensional displacement table, and the problem of poor consistency of sample measurement results can also exist in scanning imaging.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a terahertz near-field real-time imaging system giving consideration to spectral information.
Technical scheme
The invention provides a terahertz near field real-time imaging system considering spectral information, which comprises a femtosecond laser source, a pumping path, a detection path, a near field detection module and an image acquisition module, wherein the femtosecond laser source generates femtosecond laser which is divided into a first light beam and a detection light beam by a first beam splitter; the first light beam obtains most of energy to be used as a pumping path; the detection light beam obtains a small part of energy as a detection path; the terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing system is characterized in that a terahertz pulse is generated by the incidence of a first light beam to a nonlinear crystal, a detection circuit is used for placing a delay line for scanning the time-domain waveform of the terahertz pulse, a sample to be detected is tightly attached to the detection crystal to be placed to realize near-field detection, the terahertz pulse and the detection light beam irradiate together, the sample and the detection crystal enter the image acquisition module, and the image acquisition module realizes terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing.
Furthermore, the pumping path comprises a grating, a first cylindrical lens, a first quarter wave plate, a second cylindrical lens and a nonlinear crystal, the nonlinear crystal is a lithium niobate crystal, the first light beam is modulated by the chopper and then irradiates the grating, first-order diffraction light emitted by the grating is changed into vertical polarization through the first quarter wave plate, and the vertical polarization light is condensed by the first cylindrical lens and the second cylindrical lens in sequence and then enters the lithium niobate crystal.
Furthermore, the grating scale is 1800/mm, the incident angle of the first light beam is 36.1 degrees, and the first-order diffraction angle of the grating is 58.3 degrees.
Furthermore, the cutting angle of the lithium niobate crystal is 63.7 degrees, and the terahertz pulse is emitted perpendicular to the cutting surface of the lithium niobate crystal and then directly irradiates the detection crystal.
Furthermore, the detection light beam enters a detection path, is sequentially changed into vertical polarization through the delay line and the second half-wave plate, and then is reflected by the second beam splitter to irradiate a detection crystal, wherein the detection crystal is a gallium phosphide crystal.
Furthermore, the terahertz pulse and the detection beam irradiate the sample and the gallium phosphide crystal together, the gallium phosphide crystal is tightly attached to the sample to realize near-field detection, the surface of the gallium phosphide crystal is sequentially plated with a femtosecond laser waveband high-reflection film and a terahertz waveband high-transmission film, the detection beam carrying terahertz information reflected by the gallium phosphide crystal is used as a second beam, the second beam is transmitted by a second beam splitter, and then is condensed by the first lens and the second lens in sequence to enter the image acquisition module.
And further, the second light beam is divided into S polarized light and P polarized light after passing through the quarter-wave plate and the polarization beam splitter in sequence, the S polarized light and the P polarized light are divided into two pairs of light beams through the third beam splitter, one pair of light beams are used as a third light beam and a fourth light beam to enter the electric balance detector, and the other pair of light beams are used as a fifth light beam and a sixth light beam to enter the CMOS camera.
Furthermore, signals detected by the photoelectric balance detector are output to a phase-locked amplifier, and a waveform diagram of the terahertz pulse can be obtained by matching with delay line scanning.
Further, the CMOS camera continuously reads image data with a computer, the CMOS camera continuously collects two frames of images, one frame carries terahertz information, the other frame does not carry terahertz information, the two frames are subtracted to obtain a time domain average image, the time domain average image comprises an S-polarized light spot and a P-polarized light spot, and the two light spots are subtracted to obtain the terahertz image of the sample.
Advantageous effects
The terahertz image acquisition system is highly modularized in light path design, simple and convenient to operate and high in system signal-to-noise ratio, can be used for testing terahertz images carrying spectral information of a plurality of samples, also can be used for independently extracting a certain part of the images to analyze the spectral information of the images, can acquire 600 images per minute under the condition of acquiring pixels of 1024 multiplied by 512, can reach a real-time frame rate of 10FPS, and can reach an imaging resolution of below 50 microns.
Drawings
FIG. 1 is a schematic diagram of a terahertz near-field real-time imaging system considering spectral information;
the system comprises the following components: a femtosecond laser source 1; a first beam splitter 2; a first reflecting mirror 3; a chopper 4; a grating 5; a first cylindrical lens 6; a first quarter wave plate 7; a second cylindrical lens 8; a nonlinear crystal 9; a second reflector 10; a delay line 11; a third reflector 12; a second half wave plate 13; a sample to be tested 14; probing the crystal 15; a second beam splitter 16; a first lens 17; a quarter-wave plate 18; a second lens 19; a fourth mirror 20; a fifth mirror 21; a polarization beam splitter 22; a sixth reflecting mirror 23; a seventh mirror 24; a third beam splitter 25; a photoelectric balance detector 26; a lock-in amplifier 27; a CMOS camera 28; and a computer 29.
Parameter description of key devices in the system: the femtosecond laser source 1 is a titanium sapphire femtosecond pulse laser, the polarization direction is horizontal, the repetition frequency is 1000Hz, the pulse width is 100fs, the central wavelength is 800nm, and the output power is 6W; the grating 5 scale is 1800/mm; the nonlinear crystal 9 is a magnesium-doped lithium niobate crystal with a near stoichiometric ratio, and the cutting angle is 63.7 degrees; the detection crystal 15 is a [110] crystalline phase gallium phosphide crystal with the thickness of 300 mu m, and the surface is sequentially plated with a femtosecond laser waveband high-reflection film and a terahertz waveband high-transmission film; the first beam splitter 2 has a transmission reflectance of 9: 1; second beam splitter 16 transmission reflectance 5: 5; the third beam splitter 25 has a transmission reflectance of 5: 5; the focal length of the first cylindrical lens 6 is 25 cm; the focal length of the second cylindrical lens is 15 cm; the focal length of the first lens 17 is 50 cm; the focal length of the second lens 19 is 30 cm; the stroke of a stepping motor of the delay line 12 is 100mm, and the precision is 0.1 mu m; the CMOS camera 28 exposure delay time is 900 mus and the exposure time is 8 ms.
Fig. 2 shows an imaging result of a terahertz near-field real-time imaging system with spectral information taken into account for testing a metal letter structure with a line width of 200 μm.
Detailed Description
The present invention is further illustrated by the following figures and specific examples, which are to be understood as illustrative only and not as limiting the scope of the invention for use, and modifications of various equivalent forms of the invention which are obvious to those skilled in the art, after reading the present disclosure, are intended to be included within the scope of the appended claims.
The device comprises a femtosecond laser source, a pumping circuit, a detection circuit, a near field detection module and an image acquisition module, wherein the femtosecond laser source generates femtosecond laser which is divided into a first light beam and a detection light beam by a first beam splitter; the first light beam obtains most of the energy, and the probe light beam obtains less of the energy; the first light beam and the detection light beam are respectively used as laser sources of a pumping path and a detection path; the terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing system is characterized in that a terahertz pulse is generated by pumping a nonlinear crystal with a first light beam, a detection circuit is used for placing a delay line for scanning the time-domain waveform of the terahertz pulse, a sample to be detected is tightly attached to the detection crystal and placed to realize near-field detection, the terahertz pulse and the detection light beam jointly expand and irradiate and enter the image acquisition module after the sample and the detection crystal, and the image acquisition module realizes terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing.
The invention will be further explained with reference to the drawings.
As shown in fig. 1, the femtosecond laser generated by the femtosecond laser source 1 is divided into a first beam and a probe beam by the first beam splitter 2. The laser comprises a first beam splitter, a femtosecond laser which obtains 90% of energy after transmission of the femtosecond laser as a first light beam and enters a pumping path after being reflected by a first reflector 3, the first beam is modulated by a chopper 4 and irradiates a grating 5 at an incident angle of 36.1 degrees, the first-order diffraction angle of the grating 5 is 58.3 degrees, the wavefront inclination angle of the first light beam passing through the grating 5 is 63.7 degrees, the wavefront inclined first light beam changes the wavefront inclination mode and is condensed by a first cylindrical lens 6 and a second cylindrical lens 8, the horizontal polarization is changed into the vertical polarization by a first half wave plate 7, a nonlinear crystal 9 with an incident cutting angle of 63.7 degrees is selected as the nonlinear crystal 9, a magnesium-doped near stoichiometric ratio lithium niobate crystal with a cutting angle of 63.7 degrees is selected as the nonlinear crystal 9, the phase matching condition of the first light beam and the generated terahertz is achieved in the crystal, and the terahertz pulse is emitted from the inclined cutting surface of the crystal.
The femtosecond laser with 10% of energy obtained after reflection by the first beam splitter 2 is used as a detection beam, the detection beam is reflected by the second reflector 10 to enter the delay line 11, the delay line 11 is driven by a stepping motor and is used for scanning to obtain a complete terahertz time-domain waveform, the detection beam is reflected by the third reflector 12, and in order to keep the detection beam consistent with the terahertz polarization direction when the detection beam irradiates on the detection crystal 15, the detection beam is changed from horizontal polarization to vertical polarization through the second half-wave plate 13.
The sample 14 to be detected is placed and tightly attached to the detection crystal 15 to realize near-field detection, the detection crystal 15 is gallium phosphide crystal with [110] crystal phase and is 300 mu m thick, a femtosecond laser band high-reflection film and a terahertz band high-transmission film are plated on the detection crystal, terahertz pulses emitted by the nonlinear crystal 9 directly irradiate the sample 14 to be detected, detection light beams irradiate the detection crystal 15 after being reflected by the second beam splitter 16, and the detection light beams carrying terahertz information and reflected by the detection crystal 15 are transmitted through the second beam splitter 16 as second light beams.
When no terahertz electric field acts on the detection crystal 15, the vertically polarized detection light beam reflects a second light beam through the detection crystal 15, the second light beam is condensed by the first lens 17 and the second lens 19, and becomes a circular polarization state after passing through the quarter-wave plate 18, the second light beam in the circular polarization state enters the image acquisition module after passing through the fourth reflector 20 and the fifth reflector 21, the polarization beam splitter 22 splits the second light beam into S-polarized light and P-polarized light, the intensity of the S-polarized light and the P-polarized light is the same at this time, the S-polarized light and the P-polarized light respectively enter the third beam splitter 25 through the sixth reflector 23 and the seventh reflector 24 and are split into two pairs of light beams by the third beam splitter 25, one of the light beams enters the photoelectric balance detector 26 as a third light beam and a fourth light beam, and the other light beam enters the CMOS camera 28 as a fifth light beam and a sixth light beam, and the output difference of the photoelectric balance detector 26 is 0.
When a terahertz electric field acts on the detection crystal 15, the index ellipsoid of the detection crystal 15 is changed due to the linear electro-optic effect, the vertically polarized second light beam is reflected by the detection crystal 15 to form a second light beam, the second light beam is condensed by the first lens 17 and the second lens 19, and is changed into an elliptical polarization state after passing through the quarter-wave plate 18, the elliptical polarization state second light beam enters the image acquisition module after passing through the fourth reflector 20 and the fifth reflector 21, the second light beam is divided into S-polarized light and P-polarized light by the polarization beam splitter 22, at this time, the intensities of the S-polarized light and the P-polarized light are no longer the same, the S-polarized light and the P-polarized light respectively enter the third beam splitter 25 through the sixth reflector 23 and the seventh reflector 24, and are divided into two pairs of light beams by the third beam splitter 25, wherein one pair of light beams as the third light beam and the fourth light beam enters the electrical balance detector 26, the other pair of light beams enters the CMOS camera 28 as a fifth light beam and a sixth light beam, and the difference between the outputs of the photo balance detectors 26 reflects the difference between the two orthogonal polarization components of the second light beam, which is modulated by the detection crystal 15 through the linear electro-optic effect, so that the difference between the outputs of the photo balance detectors 26 is proportional to the intensity of the terahertz electric field impinging on the detection crystal 15.
The third light beam and the fourth light beam are incident on the photoelectric balance detector 26, signals detected by the photoelectric balance detector 26 are received by the lock-in amplifier 27, in order to improve the signal-to-noise ratio, the lock-in amplifier 27 and the chopper 4 form a signal amplification system, the chopper 4 is placed on a first light beam path of a pumping circuit to modulate a generated terahertz signal, the lock-in amplifier 27 is connected with the chopper 4 to obtain a chopping frequency as a reference frequency, the lock-in amplifier 27 performs sampling integration on the received signals under the reference frequency, and the time domain spectrum of the terahertz pulse can be obtained by matching with the scanning of the delay line 12.
The fifth light beam and the sixth light beam are incident into the CMOS camera 28, the CMOS camera 28 is connected with the computer 29, the triggering mode of the CMOS camera 28 is set to be external triggering through programming, the triggering signals are the rising edge and the falling edge of a chopper 4 signal, the exposure mode is delayed exposure, the exposure delay time is set to be 900 microseconds when the chopping frequency is 60Hz, the exposure time is 8 milliseconds, the CMOS camera 28 continuously collects two images under the setting, one frame carries terahertz information, the other frame does not carry terahertz information, the two frames are subtracted to obtain a time domain average image with background noise removed, the time domain average image collected by the CMOS camera 28 comprises an S-polarized light spot and a P-polarized light spot, and the two light spots are subtracted to obtain a terahertz image of the sample to be detected with high signal-to-noise ratio.
The accurate collection of terahertz images in space and time domains can be completed by setting parameters such as trigger modes, exposure time and the like of the CMOS camera 28, 600 terahertz images can be collected by the system every minute under the condition of collecting pixels of 1024 multiplied by 512, the real-time frame rate can reach 10FPS, compared with the traditional two-dimensional point-by-point scanning type terahertz imaging system, the system needs hours for collecting the whole two-dimensional terahertz image, and the system has practical real-time performance.
According to the terahertz near field real-time imaging system which is well established and gives consideration to spectral information as shown in fig. 1, N, J, U three metal letter structures with the line width of 200 microns can be imaged. Schematic diagrams of the three samples and terahertz imaging results are shown in fig. 2.
Claims (9)
1. The terahertz near-field real-time imaging system giving consideration to spectral information is characterized by comprising a femtosecond laser source, a pumping circuit, a detection circuit, a near-field detection module and an image acquisition module, wherein the femtosecond laser source generates femtosecond laser which is divided into a first light beam and a detection light beam by a first beam splitter; the first light beam obtains most of energy to be used as a pumping path; the detection light beam obtains a small part of energy as a detection path; the terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing system is characterized in that a terahertz pulse is generated by the incidence of a first light beam to a nonlinear crystal, a detection circuit is used for placing a delay line for scanning the time-domain waveform of the terahertz pulse, a sample to be detected is tightly attached to the detection crystal to be placed to realize near-field detection, the terahertz pulse and the detection light beam irradiate together, the sample and the detection crystal enter the image acquisition module, and the image acquisition module realizes terahertz time-domain spectral measurement and terahertz imaging optical path multiplexing.
2. The terahertz near-field real-time imaging system considering spectral information as claimed in claim 1, wherein the pump path includes a grating, a first cylindrical lens, a first quarter wave plate, a second cylindrical lens and a nonlinear crystal, the nonlinear crystal is a lithium niobate crystal, the first light beam is modulated by a chopper and then irradiates the grating, the first order diffracted light emitted from the grating is changed into vertical polarization by the first quarter wave plate, and then is condensed by the first cylindrical lens and the second cylindrical lens and then enters the lithium niobate crystal.
3. The terahertz near-field real-time imaging system giving consideration to spectral information as claimed in claim 2, wherein the grating scale is 1800/mm, the incident angle of the first light beam is 36.1 °, and the first-order diffraction angle of the grating is 58.3 °.
4. The terahertz near-field real-time imaging system considering both spectral information and according to claim 2, characterized in that the cutting angle of the lithium niobate crystal is 63.7 °, and the terahertz pulses are emitted perpendicular to the cutting surface of the lithium niobate crystal and then directly irradiate the detection crystal.
5. The terahertz near-field real-time imaging system considering spectral information as claimed in claim 1, wherein the detection light beam enters the detection path, is sequentially changed into vertical polarization through the delay line and the second half wave plate, and then is reflected by the second beam splitter to irradiate the detection crystal, wherein the detection crystal is gallium phosphide crystal.
6. The terahertz near-field real-time imaging system considering both spectral information and claim 5 is characterized in that the terahertz pulses and the detection beams irradiate a sample and a gallium phosphide crystal together, the gallium phosphide crystal is tightly attached to the sample to realize near-field detection, a femtosecond laser band high-reflection film and a terahertz band high-transmission film are sequentially plated on the surface of the gallium phosphide crystal, the detection beams carrying terahertz information reflected from the gallium phosphide crystal are used as second beams, the second beams are transmitted by a second beam splitter, and sequentially condensed by a first lens and a second lens to enter an image acquisition module.
7. The terahertz near-field real-time imaging system giving consideration to spectral information as claimed in claim 6, wherein the second light beam is divided into S-polarized light and P-polarized light after passing through the quarter-wave plate and the polarization beam splitter in sequence, the S-polarized light and the P-polarized light are divided into two pairs of light beams through the third beam splitter, one pair of light beams is used as a third light beam and a fourth light beam to enter the optical balance detector, and the other pair of light beams is used as a fifth light beam and a sixth light beam to enter the CMOS camera.
8. The terahertz near-field real-time imaging system considering both spectral information and according to claim 7, wherein the signal detected by the photoelectric balance detector is output to a lock-in amplifier, and a waveform diagram of the terahertz pulses can be obtained by matching with delay line scanning.
9. The terahertz near-field real-time imaging system considering both spectral information and according to claim 7, characterized in that the CMOS camera continuously reads image data with a computer, the CMOS camera continuously collects two frames of images, one frame carries terahertz information, the other frame does not carry terahertz information, the two frames are subtracted to obtain a time-domain average image, the time-domain average image comprises an S-polarized light spot and a P-polarized light spot, and the two light spots are subtracted to obtain the terahertz image of the sample.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN114235708A (en) * | 2021-12-20 | 2022-03-25 | 天津大学 | Terahertz photoacoustic detection device and method |
| CN114486804A (en) * | 2021-12-29 | 2022-05-13 | 江南大学 | Broadband terahertz wave image reconstruction system |
| CN117706216A (en) * | 2024-02-06 | 2024-03-15 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
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| CN114486804B (en) * | 2021-12-29 | 2024-04-05 | 江南大学 | Broadband terahertz wave image reconstruction system |
| CN117706216A (en) * | 2024-02-06 | 2024-03-15 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
| CN117706216B (en) * | 2024-02-06 | 2024-04-12 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
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