CN112326588A - Terahertz time-domain spectrograph - Google Patents
Terahertz time-domain spectrograph Download PDFInfo
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- CN112326588A CN112326588A CN202011167328.4A CN202011167328A CN112326588A CN 112326588 A CN112326588 A CN 112326588A CN 202011167328 A CN202011167328 A CN 202011167328A CN 112326588 A CN112326588 A CN 112326588A
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
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- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
- 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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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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Abstract
The invention relates to the technical field of spectroscopy, in particular to a terahertz time-domain spectrometer which utilizes terahertz pulses to detect and identify samples. The terahertz time-domain spectrograph comprises a femtosecond laser, a beam splitter, a rapid optical scanning delay device, a photoconductive antenna terahertz transmitter, a photoconductive antenna terahertz detector, a lock-in amplifier, a data processing module and a reflector component; the terahertz transmitter of the photoconductive antenna is positioned between the beam splitter and the reflector component, the reflector component comprises a parabolic reflector set and an ellipsoidal reflector set, and the terahertz detector of the photoconductive antenna is positioned between the reflector component and the lock-in amplifier. The invention can be used for measuring the reflection spectrum or the transmission spectrum of a sample to be measured, and has the advantages of small incident angle, high reflectivity, small attenuation, no aberration, simple and compact structure and the like; and the measurement modes of measuring the transmission spectrum and measuring the reflection spectrum can be flexibly switched according to the attribute of the sample to be measured.
Description
The technical field is as follows:
the invention relates to the technical field of spectroscopy, in particular to a terahertz time-domain spectrometer which utilizes terahertz pulses to detect and identify samples.
Background art:
the terahertz time-domain spectroscopy technology is that terahertz pulses are used for acting with substances, a change curve of terahertz (THz) electric field intensity generated by the action of the terahertz pulses along with time is detected, and data processing is carried out to obtain information such as complex refractive index, dielectric constant and conductivity of a sample. According to different sample attributes, the obtained terahertz electric field intensity distribution is different, the result obtained through data processing is also changed, and different articles can be identified through analyzing the electrochemical information. The detection modes of the traditional terahertz time-domain spectrograph are mainly divided into two types: one is a transmission measurement mode and the other is a reflection measurement mode; the two modes are designed separately and exist simultaneously.
In a terahertz time-domain spectroscopy (thz spectroscopy), which is published by zhangxingning et al, zhejiang university, a mode of collimation and focusing of an off-axis parabolic reflector is adopted to collect a reflection spectrum of a sample; in this paper, the emitted terahertz waves are collimated by a first parabolic reflector and then focused by a second parabolic reflector, and a plane reflector is added behind the second parabolic reflector to collect the sample reflection spectrum, so that the terahertz waves are converged to the sample; however, the volume of the optical path is increased, the number of times of terahertz wave reflection is increased, and the energy attenuation of the terahertz wave is increased.
In the reflection measurement mode, the incident angle theta of the terahertz pulse influences the tested refractive index through 1/cos2 theta, the measurement error is linear to the frequency, and when the refractive index is higher, the error is larger. The calculated transmittance and reflectance are more reliable at normal incidence and reflection, so that the smaller the incidence angle θ is, the better in the reflectometry mode. However, due to the interference of the size of the lens and the size of the support, the incident angle θ cannot be made very small, for example, patent CN108254336A discloses a terahertz spectrometer, which includes a dispersion compensation system, an optical beam splitter, a fiber stretching system, a detection system and a data processing system, wherein the detection system includes a photoconductive transmitting antenna for generating terahertz waves, a photoconductive receiving antenna for receiving terahertz waves and an optical component for focusing terahertz waves, and the optical component includes four lenses, wherein the first lens group includes two lenses, and the second lens group includes two other lenses, which affects the measurement calculation of relevant parameters such as refractive index, and thus causes the measurement error to become large.
In patent CN108254336A, four lenses of the optical assembly are polyolefin lenses, which may be aspheric mirrors or spherical mirrors, for collimating and focusing terahertz waves; because the intensity of the terahertz wave is relatively weak, higher requirements are put on materials for modulating the terahertz wave, for example, high-density polyethylene (HDPE), Polytetrafluoroethylene (PTFE) and the like have high transmittance to the terahertz wave band, and meanwhile, the absorption rate is as high as about 4%/mm, so in the actual optical design, the thickness of the lens is one of the factors which must be considered, and meanwhile, the problems of the structure and the mechanical strength of the lens are also considered in principle, and if the focal power of the lens is positive, the central thickness of the lens is generally not less than 3 mm; when the focal power is negative, the central thickness of the lens is not less than 1/10-1/15 of the aperture of the lens, so that the phenomenon that the imaging effect is influenced due to deformation caused by over-thin thickness during installation and reinforcement is prevented; meanwhile, the thickness of the lens is also an optical structure parameter, and is a variable for optimizing the aberration of the lens focal length correction optical system.
In the prior art, a transmission measurement mode is adopted, but if a lens is adopted to build a transmission measurement light path, terahertz energy loss can be increased, and a transmission measurement mode and a reflection measurement mode can also be adopted simultaneously, for example, patent CN109406441A discloses a terahertz time-domain spectrometer, wherein a first terahertz antenna and a second terahertz antenna are used for receiving terahertz waves transmitted by a sample to be measured and terahertz waves reflected by the sample to be measured, so that a reflection spectrum and a transmission spectrum of the sample to be measured can be conveniently measured, and the characteristics of the sample to be measured can be conveniently and comprehensively analyzed; for another example, patent CN105548083A discloses a dual-optical-path terahertz time-domain spectrometer, which adopts a light splitting mode to divide a laser beam into two paths, each path of laser beam is divided into a pump beam and a probe beam, and the pump beam and the probe beam are respectively reflected and transmitted by the laser beam; the mode of double-optical-path measurement needs two pairs of photoconductive antennas for transmitting and receiving terahertz signals, so that the cost is greatly increased, and the mode is not suitable for being output as an industrial product; in addition, in the light path for measuring the reflection spectrum, a parabolic reflector is adopted as a reflecting element, and the parabolic reflector cannot achieve perfect focusing on an object point through theoretical analysis of optical aberration, so that certain aberration exists, and certain adverse effects are generated on the focusing of terahertz waves and the resolution of a system; more importantly, the axial depth of the optical path can be increased, so that the space volume is enlarged, and the device integration is not facilitated.
The invention content is as follows:
aiming at the defects that the terahertz time-domain spectrograph in the prior art has large incident angle, inaccurate measurement parameters, large attenuation of terahertz waves through a lens, poor focusing effect of a parabolic reflector, high cost for simultaneously measuring a transmission spectrum and a reflection spectrum and the like, the terahertz time-domain spectrograph with low angle measurement, small attenuation and low cost for realizing the switchable two spectrum measurement modes is provided.
The technical scheme adopted by the invention for solving the technical problems is as follows: a terahertz time-domain spectrograph is used for measuring the reflection spectrum or the transmission spectrum of a sample to be measured, and is characterized in that: the device comprises a femtosecond laser, a beam splitter, a rapid optical scanning delay device, a photoconductive antenna terahertz transmitter, a photoconductive antenna terahertz detector, a phase-locked amplifier, a data processing module and a reflector component; the femtosecond laser is used for generating femtosecond laser, the beam splitter is used for dividing the femtosecond laser into pumping light and probe light, and the rapid optical scanning delay device is used for adjusting the time delay of the probe light relative to the pumping light; the photoconductive antenna terahertz transmitter is arranged on a light path of the pump light and positioned between the beam splitter and the reflector assembly, and is used for receiving the pump light and transmitting terahertz waves; the reflecting mirror assembly is arranged on a light path of the pump light, and comprises a parabolic reflecting mirror group and an ellipsoidal reflecting mirror group, the parabolic reflecting mirror group enables the terahertz waves to be transmitted from the sample to be detected, the parabolic reflecting mirror group comprises four parabolic reflecting mirrors, the ellipsoidal reflecting mirror group enables the terahertz waves to be reflected from the sample to be detected, and the ellipsoidal reflecting mirror group comprises two ellipsoidal reflecting mirrors; the photoconductive antenna terahertz detector is arranged on a light path of the pump light and positioned between the reflector component and the lock-in amplifier, and is used for detecting the detection light after time delay and the terahertz wave transmitted or reflected from the sample to be detected and outputting a voltage signal; the phase-locked amplifier is connected with the terahertz detector of the photoconductive antenna and used for receiving the voltage signal output by the terahertz detector of the photoconductive antenna and outputting the detected and restored voltage signal to the data processing module.
Preferably, the femtosecond laser is an erbium-doped fiber femtosecond laser with pulse energy of 100mW, pulse duration less than 60fs and peak dynamic range greater than 1000:1, and the wavelength of the femtosecond laser is 1560 nm.
Preferably, the pump light accounts for 30% of the output power of the femtosecond laser, and the probe light accounts for 70% of the output power of the femtosecond laser.
Preferably, the fast optical scanning delay device adopts a voice coil motor and an optical fiber delay line, and the voice coil motor can control the optical fiber delay line to provide a maximum delay range of 100 ps.
Preferably, the reflectivity of the parabolic mirror to the terahertz wave is greater than or equal to 97%.
Preferably, the photoconductive antenna terahertz transmitter and the photoconductive antenna terahertz detector adopt an optical fiber coupling antenna module for 1560nm laser, and the optical fiber coupling antenna module has an InGaAs/InAlAs based multilayer MESA structure.
Preferably, four parabolic mirrors of the parabolic mirror group are a first parabolic mirror, a second parabolic mirror, a third parabolic mirror and a fourth parabolic mirror, the first parabolic mirror and the second parabolic mirror are sequentially arranged between the photoconductive antenna terahertz transmitter and the sample to be detected, and the third parabolic mirror and the fourth parabolic mirror are sequentially arranged between the sample to be detected and the photoconductive antenna terahertz detector; the two ellipsoidal reflectors of the ellipsoidal reflector group are a first ellipsoidal reflector and a second ellipsoidal reflector which are symmetrically arranged in an opposite mode, the reflecting surface of the first ellipsoidal reflector faces the terahertz transmitter of the photoconductive antenna, and the reflecting surface of the second ellipsoidal reflector faces the terahertz detector of the photoconductive antenna.
More preferably, a distance between the photoconductive antenna terahertz transmitter and the center of the first parabolic mirror is equal to a distance between the photoconductive antenna terahertz detector and the center of the fourth parabolic mirror.
More preferably, a minimum distance between the photoconductive antenna terahertz transmitter and the first ellipsoidal mirror is equal to a minimum distance between the photoconductive antenna terahertz detector and the second ellipsoidal mirror.
More preferably, a distance between the photoconductive antenna terahertz transmitter and the center of the first parabolic mirror is equal to a minimum distance between the photoconductive antenna terahertz transmitter and the first ellipsoidal mirror.
The terahertz time-domain spectrograph can be used for measuring the reflection spectrum or the transmission spectrum of a sample to be measured, and has the advantages of small incident angle, high reflectivity, small attenuation, no aberration, simple and compact structure and the like; and the measurement modes of measuring the transmission spectrum and measuring the reflection spectrum can be flexibly switched according to the attribute of the sample to be measured.
Description of the drawings:
FIG. 1 is a schematic diagram of an optical path structure of a terahertz time-domain spectrometer adopting a parabolic mirror group according to the invention;
FIG. 2 is a schematic diagram of the transmission light path of the parabolic mirror assembly according to the present invention;
FIG. 3 is a schematic diagram of an optical path structure of the terahertz time-domain spectrometer adopting an ellipsoidal mirror group according to the present invention;
FIG. 4 is a schematic diagram of the reflection path of the ellipsoidal mirror group according to the present invention.
The specific implementation mode is as follows:
the invention will be further explained with reference to the accompanying drawings.
As shown in fig. 1 and fig. 3, the terahertz time-domain spectrometer of the present invention is used for measuring a reflection spectrum or a transmission spectrum of a sample 14 to be measured, and specifically includes a femtosecond laser 1, a beam splitter 2, a fast optical scanning delay device 3, a photoconductive antenna terahertz transmitter 4, a photoconductive antenna terahertz detector 5, a lock-in amplifier 6, a data processing module 7, a mirror assembly, and a computer 8; the reflector assembly comprises a parabolic reflector set and an ellipsoidal reflector set which are independent from each other, the ellipsoidal reflector set is adopted when the reflectance spectrum of the sample to be measured 14 is measured, the parabolic reflector set is adopted when the transmittance spectrum of the sample to be measured 14 is measured, and the parabolic reflector set and the ellipsoidal reflector set can be freely replaced, so that the problem of increase of photoconductive antennas is avoided, the cost can be reduced, and the performances such as dynamic range can be ensured.
The femtosecond laser device 1 is used for generating femtosecond laser, the femtosecond laser device 1 serves as an excitation source, and the femtosecond laser serves as a light source of the terahertz time-domain spectrometer; preferably, the femtosecond laser has a wavelength of 1560 nm. The femtosecond laser 1 is connected with the beam splitter 2 through a tail fiber. Specifically, the femtosecond laser 1 is an erbium-doped fiber femtosecond laser of toptica company, the pulse energy is about 100mW, the pulse duration is less than 60fs, and the peak dynamic range is greater than 1000: 1.
The beam splitter 2 is arranged on a light path of the femtosecond laser and is used for dividing the femtosecond laser into two paths: one path of the laser is pump light, the pump light accounts for 30% of the output power of the femtosecond laser, and the pump light is used for generating terahertz waves; the other path is detection light, the detection light accounts for 70% of the output power of the femtosecond laser, and the detection light is used for collinear coupling with the terahertz waves passing through the sample to be detected 14.
The fast optical scanning delay device 3 is arranged on the optical path of the probe light and is used for adjusting the time delay of the probe light relative to the pump light; the fast optical scanning delay device 3 preferably adopts a voice coil motor and an optical fiber delay line, and the voice coil motor can control the optical fiber delay line to provide a maximum delay range of 100 ps.
The terahertz transmitter 4 of the photoconductive antenna is arranged on a light path of the pumping light and is positioned between the beam splitter 2 and the reflector assembly, the beam splitter 2 is connected with the terahertz transmitter 4 of the photoconductive antenna through a polarization maintaining fiber, the pumping light is focused on the terahertz transmitter 4 of the photoconductive antenna through the polarization maintaining fiber, and photoelectron motion generated by pumping light excitation is accelerated under the action of bias voltage to form an instantaneous photocurrent, so that terahertz waves are transmitted.
The reflecting mirror assembly is arranged on the light path of the pump light, and comprises a parabolic reflecting mirror assembly and an ellipsoidal reflecting mirror assembly which are independent of each other, and is used for focusing the terahertz waves on the sample to be measured 14, so that the terahertz waves are transmitted or reflected from the sample to be measured 14, and the terahertz waves transmitted or reflected from the sample to be measured 14 carry measurement information of the sample to be measured 14; the measurement information includes pulse amplitude and phase information of the transmitted or reflected terahertz wave of the sample 14 to be measured, the absorption spectrum and dispersion spectrum of the sample 14 to be measured can be obtained through fast fourier transform, information of different molecular conformations and intermolecular chemical bond interaction can be obtained through chemical analysis and energy calculation, information such as the refraction rate/transmittance/extinction coefficient of the sample 14 to be measured on the terahertz wave of a certain frequency band can be obtained after the pulse amplitude and phase information is processed, and then different samples can be distinguished. When the transmission spectrum of a sample 14 to be measured needs to be measured, a parabolic mirror group is adopted to enable the terahertz waves to transmit the sample 14 to be measured, and the parabolic mirror group comprises four parabolic mirrors; when the reflectance spectrum of the sample 14 to be measured needs to be measured, the terahertz waves are reflected from the sample 14 to be measured by adopting an ellipsoidal mirror group, wherein the ellipsoidal mirror group comprises two ellipsoidal mirrors.
The photoconductive antenna terahertz detector 5 is arranged on the optical path of the pump light, is positioned between the reflector assembly and the lock-in amplifier 6, and is used for detecting the time-delayed detection light and the terahertz wave carrying the measurement information of the sample to be measured 14, and the time-delayed detection light and the terahertz wave carrying the measurement information of the sample to be measured 14 are simultaneously focused on the photoconductive antenna terahertz detector 5; the terahertz transmitter 4 and the terahertz detector 5 of the photoconductive antenna are preferably optical fiber coupling antenna modules which are produced by Menlo Systems and used for 1560nm laser, and the optical fiber coupling antenna modules have multilayer MESA structures based on InGaAs/InAlAs; the terahertz detector 5 of the photoconductive antenna receives the detection light and the terahertz wave to generate a current signal, and because the current signal is weak, usually in the picoampere (pA) level, the current signal is easily submerged in background noise, a preamplifier needs to be used for pre-amplification and main amplification, and a microvolt (muV) level voltage signal is generated after the amplification.
The phase-locked amplifier 6 is connected with the photoconductive antenna terahertz detector 5, is used for receiving a voltage signal generated by the photoconductive antenna terahertz detector 5, and can enable the signal-to-noise ratio to be 10-6The weak signal is successfully detected and restored, and the phase-locked amplification is an effective means for detecting the weak signal. The data processing module 7 is connected with the fast optical scanning delay device 3, the terahertz transmitter 4 of the photoconductive antenna and the lock-in amplifier 6, and the data processing module 7 can process the voltage signal output by the lock-in amplifier 6 and detected and restored, such as acquisition and display. The computer 8 is connected with the data processing module 7, and is used as an upper computer for displaying information such as a time domain, a frequency domain, a calculation result and the like of the sample 14 to be measured.
As shown in fig. 1 and 2, the parabolic mirror group includes four parabolic mirrors, namely a first parabolic mirror 9, a second parabolic mirror 10, a third parabolic mirror 11, and a fourth parabolic mirror 12; the first parabolic reflector 9 and the second parabolic reflector 10 are sequentially arranged between the photoconductive antenna terahertz transmitter 4 and the sample to be measured 14, the first parabolic reflector 9 is used for collimating terahertz waves emitted by the photoconductive antenna terahertz transmitter 4, the second parabolic reflector 10 is used for focusing the collimated terahertz waves on the sample to be measured 14, and the terahertz waves carry measurement information of the sample to be measured 14 after transmitting the sample to be measured 14; the third parabolic reflector 11 and the fourth parabolic reflector 12 are sequentially arranged between the sample to be measured 14 and the photoconductive antenna terahertz detector 5, the third parabolic reflector 11 is used for collimating the terahertz waves carrying the measurement information of the sample to be measured 14, and the fourth parabolic reflector 12 is used for focusing the collimated terahertz waves carrying the measurement information of the sample to be measured 14 onto the photoconductive antenna terahertz detector 5; the reflectivity of the parabolic reflector to the terahertz wave is greater than or equal to 97%, the loss of the terahertz wave is greatly reduced, and the measurement stability and reliability are improved. Preferably, the distance between the photoconductive antenna terahertz transmitter 4 and the center of the first parabolic mirror 9 is equal to the distance between the photoconductive antenna terahertz detector 5 and the center of the fourth parabolic mirror 12.
As shown in fig. 3 and 4, the set of ellipsoidal mirrors 13 comprises two ellipsoidal mirrors, namely a first ellipsoidal mirror 131 and a second ellipsoidal mirror 132, said first ellipsoidal mirror 131 and said second ellipsoidal mirror 132 being symmetrically arranged opposite to each other, the first ellipsoidal mirror 131 and the second ellipsoidal mirror 132 are located between the photoconductive antenna terahertz transmitter 4 and the photoconductive antenna terahertz detector 5, the reflecting surface of the first ellipsoidal mirror 131 faces the photoconductive antenna terahertz transmitter 4, the first ellipsoidal reflector 131 is configured to focus a terahertz wave emitted by the photoconductive antenna terahertz transmitter 4 onto the sample to be measured 14, and the terahertz wave focused onto the sample to be measured 14 carries measurement information of the sample to be measured 14 after interaction with the sample to be measured 14; the reflecting surface of the second ellipsoidal mirror 132 faces the photoconductive antenna terahertz detector 5, the terahertz wave carrying the measurement information of the sample 14 to be measured is reflected from the sample 14 to be measured to the second ellipsoidal mirror 132, and the second ellipsoidal mirror 132 is used for receiving the terahertz wave carrying the measurement information of the sample 14 to be measured and reflected from the sample 14 to be measured and focusing the terahertz wave on the photoconductive antenna terahertz detector 5; the first ellipsoidal reflector 131 and the second ellipsoidal reflector 132 reflect the terahertz waves only 2 times, so that the loss of the terahertz waves is greatly reduced, and the measurement stability and reliability are improved. Preferably, the minimum distance between the photoconductive antenna terahertz transmitter 4 and the first ellipsoidal mirror 131 is equal to the minimum distance between the photoconductive antenna terahertz detector 5 and the second ellipsoidal mirror 132.
In the present invention, the distance between the terahertz transmitter 4 of the photoconductive antenna and the terahertz detector 5 of the photoconductive antenna is fixed and constant, for example 189.8mm, and in order to improve the measurement stability and to improve the degree of freedom of design, it is preferable that the distance between the terahertz transmitter 4 of the photoconductive antenna and the center of the first parabolic mirror 9 is equal to the minimum distance between the terahertz transmitter 4 of the photoconductive antenna and the first ellipsoidal mirror 131, for example 50 mm.
The above contents are further detailed descriptions of the terahertz time-domain spectrometer of the present invention, and do not limit the scope of the present invention, and various modifications and improvements made by those skilled in the art according to the technical solutions of the present invention without departing from the concept of the present invention should be considered as belonging to the protection scope of the present invention.
Claims (10)
1. A terahertz time-domain spectrograph is used for measuring the reflection spectrum or the transmission spectrum of a sample to be measured, and is characterized in that: the device comprises a femtosecond laser, a beam splitter, a rapid optical scanning delay device, a photoconductive antenna terahertz transmitter, a photoconductive antenna terahertz detector, a phase-locked amplifier, a data processing module and a reflector component; the femtosecond laser is used for generating femtosecond laser, the beam splitter is used for dividing the femtosecond laser into pumping light and probe light, and the rapid optical scanning delay device is used for adjusting the time delay of the probe light relative to the pumping light; the photoconductive antenna terahertz transmitter is arranged on a light path of the pump light and positioned between the beam splitter and the reflector assembly, and is used for receiving the pump light and transmitting terahertz waves; the reflecting mirror assembly is arranged on a light path of the pump light, and comprises a parabolic reflecting mirror group and an ellipsoidal reflecting mirror group, the parabolic reflecting mirror group enables the terahertz waves to be transmitted from the sample to be detected, the parabolic reflecting mirror group comprises four parabolic reflecting mirrors, the ellipsoidal reflecting mirror group enables the terahertz waves to be reflected from the sample to be detected, and the ellipsoidal reflecting mirror group comprises two ellipsoidal reflecting mirrors; the photoconductive antenna terahertz detector is arranged on a light path of the pump light and positioned between the reflector component and the lock-in amplifier, and is used for detecting the detection light after time delay and the terahertz wave transmitted or reflected from the sample to be detected and outputting a voltage signal; the phase-locked amplifier is connected with the terahertz detector of the photoconductive antenna and used for receiving the voltage signal output by the terahertz detector of the photoconductive antenna and outputting the detected and restored voltage signal to the data processing module.
2. The terahertz time-domain spectrometer of claim 1, wherein: the femtosecond laser is an erbium-doped fiber femtosecond laser with pulse energy of 100mW, pulse duration less than 60fs and peak dynamic range greater than 1000:1, and the wavelength of the femtosecond laser is 1560 nm.
3. The terahertz time-domain spectrometer of claim 1, wherein: the pump light accounts for 30% of the output power of the femtosecond laser, and the probe light accounts for 70% of the output power of the femtosecond laser.
4. The terahertz time-domain spectrometer of claim 1, wherein: the fast optical scanning delay device adopts a voice coil motor and an optical fiber delay line, and the voice coil motor can control the optical fiber delay line to provide a maximum delay range of 100 ps.
5. The terahertz time-domain spectrometer of claim 1, wherein: the reflectivity of the parabolic reflector to the terahertz waves is greater than or equal to 97%.
6. The terahertz time-domain spectrometer of claim 1, wherein: the terahertz transmitter and the terahertz detector of the photoconductive antenna adopt an optical fiber coupling antenna module for 1560nm laser, and the optical fiber coupling antenna module has an InGaAs/InAlAs based multilayer MESA structure.
7. The terahertz time-domain spectrometer of claim 1, wherein: the four parabolic reflectors of the parabolic reflector group are a first parabolic reflector, a second parabolic reflector, a third parabolic reflector and a fourth parabolic reflector, the first parabolic reflector and the second parabolic reflector are sequentially arranged between the photoconductive antenna terahertz transmitter and the sample to be detected, and the third parabolic reflector and the fourth parabolic reflector are sequentially arranged between the sample to be detected and the photoconductive antenna terahertz detector; the two ellipsoidal reflectors of the ellipsoidal reflector group are a first ellipsoidal reflector and a second ellipsoidal reflector which are symmetrically arranged in an opposite mode, the reflecting surface of the first ellipsoidal reflector faces the terahertz transmitter of the photoconductive antenna, and the reflecting surface of the second ellipsoidal reflector faces the terahertz detector of the photoconductive antenna.
8. The terahertz time-domain spectrometer of claim 7, wherein: the distance between the terahertz transmitter of the photoconductive antenna and the center of the first parabolic mirror is equal to the distance between the terahertz detector of the photoconductive antenna and the center of the fourth parabolic mirror.
9. The terahertz time-domain spectrometer of claim 7, wherein: the minimum distance between the terahertz transmitter of the photoconductive antenna and the first ellipsoidal mirror is equal to the minimum distance between the terahertz detector of the photoconductive antenna and the second ellipsoidal mirror.
10. The terahertz time-domain spectrometer of claim 7, wherein: the distance between the center of the first parabolic mirror and the center of the terahertz transmitter of the photoconductive antenna is equal to the minimum distance between the center of the first parabolic mirror and the center of the terahertz transmitter of the photoconductive antenna.
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