CN116879217A - Spectrum and imaging system based on optical fiber terahertz asynchronous sampling - Google Patents
Spectrum and imaging system based on optical fiber terahertz asynchronous sampling Download PDFInfo
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- 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
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- 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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- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
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- G01J3/2823—Imaging spectrometer
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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
- G01N21/01—Arrangements or apparatus for facilitating the optical investigation
Abstract
The invention discloses a spectrum and imaging system based on optical fiber terahertz asynchronous sampling, which relates to the technical field of terahertz spectrum imaging and comprises a scanning imaging module, a terahertz pulse signal generating module, a terahertz pulse signal receiving module, a signal acquisition module and an asynchronous triggering module; the invention improves the method on the prior asynchronous sampling terahertz spectrum technology, and a plurality of asynchronous trigger signals generated by an asynchronous trigger module respectively asynchronously trigger a terahertz pulse signal generating module, a terahertz pulse signal receiving module and a signal acquisition module, wherein the asynchronous trigger signals have different repetition frequencies, realize the femtosecond laser trigger technology of coherent synthesis trigger, and solve the problems of high adjustment difficulty, unstable trigger signals and the like in the prior two-photon or balance detection trigger acquisition terahertz signal mode.
Description
Technical Field
The invention relates to the technical field of terahertz imaging, in particular to a spectrum and imaging system based on optical fiber terahertz asynchronous sampling.
Background
Terahertz waves (THz) are a collective term for electromagnetic waves having a frequency in the range of 0.1 to 10THz (a wavelength in the range of 3mm to 0.03 mm), and are between microwaves and infrared rays on the electromagnetic spectrum. The terahertz spectrum imaging technology has the characteristics of fingerprint spectrum, high safety, high penetration of dielectric nonpolar materials, transmission/reflection measurement and the like, so that the terahertz spectrum imaging technology has unique advantages in the aspect of nondestructive detection of materials in specific fields, can become an important supplement to traditional detection means such as X-ray imaging, ultrasonic detection, optical detection and the like, and solves some problems which are difficult to solve by the traditional detection means.
Terahertz spectroscopy and imaging measurement systems employ the most widely used terahertz spectroscopy imaging technique based on a mechanical delay line (oscillating delay line or rotating delay line), but the scanning speed of the mechanical delay line is limited due to mechanical inertia. The terahertz spectrum technology based on asynchronous sampling can realize higher terahertz spectrum scanning speed due to discarding the structure of a mechanical delay line. At present, a terahertz asynchronous sampling system mostly adopts a free space system of a titanium sapphire femtosecond laser, and has large volume and high cost; the two-photon or balance detection mode is adopted to trigger and collect terahertz signals, and the problems of high adjustment difficulty, unstable trigger signals and the like exist.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: the existing terahertz spectrum imaging technology has the problems of high adjustment difficulty, unstable trigger signals and the like; the invention aims to provide a spectrum and imaging system based on optical fiber terahertz asynchronous sampling, which improves the method on the existing terahertz spectrum technology of asynchronous sampling, realizes a femtosecond laser triggering technology of coherent synthesis triggering by a plurality of asynchronous triggering signals of an asynchronous triggering module, and solves the problems of high adjustment difficulty, unstable triggering signals and the like in the existing two-photon or balance detection triggering and acquisition terahertz signal mode.
The invention is realized by the following technical scheme:
the scheme provides a spectrum and imaging system based on optical fiber terahertz asynchronous sampling, which comprises:
the scanning imaging module is used for bearing a target sample and carrying out scanning imaging when the terahertz pulse signal irradiates the target sample;
the terahertz pulse signal generation module is used for generating a terahertz pulse signal under the first generation signal and irradiating the terahertz pulse signal onto a target sample of the scanning imaging module;
the terahertz pulse signal receiving module is used for receiving the terahertz pulse signal reflected back on the target sample under the trigger signal;
the signal acquisition module is used for acquiring terahertz pulse signals from the terahertz pulse signal receiving module under the third detection signals;
the asynchronous trigger module is used for generating a first generated signal, a trigger signal and a third detection signal, wherein the first generated signal and the trigger signal have the same pulse repetition frequency, the third detection signal and the trigger signal have the same pulse repetition frequency, and the first generated signal and the third detection signal have different pulse repetition frequencies.
The working principle of the scheme is as follows: the existing terahertz spectrum imaging technology has the problems of high adjustment difficulty, unstable trigger signals and the like; the invention aims to provide a spectrum and imaging system based on optical fiber terahertz asynchronous sampling, which improves the method on the existing terahertz spectrum technology of asynchronous sampling, realizes a femtosecond laser triggering technology of coherent synthesis triggering by a plurality of asynchronous triggering signals of an asynchronous triggering module, and solves the problems of high adjustment difficulty, unstable triggering signals and the like in the existing two-photon or balance detection triggering and acquisition terahertz signal mode. Compared with the existing free space system adopting a titanium sapphire femtosecond laser and an asynchronous sampling system adopting a double laser, the system has the advantages of higher integration level, greatly reduced volume, better stability and environmental adaptation and lower cost;
further preferably, the asynchronous triggering module includes: the device comprises an optical fiber femtosecond laser, a polarization maintaining optical fiber, an attenuator, a delay line and a coupler;
the optical fiber femtosecond laser is used for generating multiple paths of femtosecond lasers:
a part of femtosecond laser attenuates laser power through a polarization maintaining optical fiber and an attenuator to generate a first generation signal and a third detection signal, wherein the first generation signal is input into a terahertz pulse signal generation module, and the third detection signal is input into a terahertz pulse signal receiving module;
and the other part of femtosecond laser is coupled into a trigger signal input signal acquisition module through a coupler after the laser power is attenuated by an attenuator and the delay of a delay line.
In a further optimized scheme, the femto-second laser includes: a first femtosecond laser, a second femtosecond laser, a third femtosecond laser and a fourth femtosecond laser;
the first femtosecond laser generates a first generated signal after attenuating the laser power through the polarization maintaining optical fiber and the attenuator, and the third femtosecond laser generates a third detection signal after attenuating the laser power through the polarization maintaining optical fiber and the attenuator;
the second femtosecond laser is attenuated by the attenuator, delayed by the delay line and enters the coupler, the four femtosecond laser is attenuated by the attenuator, and enters the coupler, and the coupler couples the second femtosecond laser and the four femtosecond laser to obtain a trigger signal.
The further optimization scheme is that the first femtosecond laser and the second femtosecond laser have the same pulse repetition frequency, and the phase difference of the first femtosecond laser and the second femtosecond laser is constant; the third and fourth femtosecond lasers have the same pulse repetition frequency, and the phase difference of the third and fourth femtosecond lasers is constant.
The further optimization scheme is that the first femtosecond laser, the second femtosecond laser, the third femtosecond laser and the fourth femtosecond laser are output by the same optical fiber femtosecond laser, and the optical fiber femtosecond laser is controlled by a standard clock source.
In a further optimization scheme, a fixed difference delta f exists between pulse repetition frequencies of the first femtosecond laser and the third femtosecond laser.
According to a further optimization scheme, the terahertz pulse signal generation module comprises a first photoconductive antenna and a bias source, wherein the bias source is connected to the photoconductive antenna, the terahertz pulse signal is generated after a first generation signal is input into the first photoconductive antenna, and bias voltage is provided by the bias source in the terahertz pulse signal generation process of the photoconductive antenna.
The terahertz pulse signal receiving module comprises a second photoconductive antenna, a bias voltage source and a signal processing unit;
the third detection signal is input into the second photoconductive antenna and then receives the terahertz pulse signal reflected back from the target sample; the second photoconductive antenna transmits the received terahertz pulse signal to the signal processing unit;
the signal acquisition module acquires terahertz pulse signals from the signal processing unit.
The further optimization scheme is that the system further comprises an output module;
and under the third detection signal, the signal acquisition module acquires the terahertz pulse signal, the terahertz spectrum signal and the imaging information of the target sample from the terahertz pulse signal receiving module.
The terahertz pulse signal receiving module is further used for converting the reflected terahertz pulse signal into an electric signal so as to realize transmission spectrum measurement of a target sample.
In the scheme, the scanning imaging module combines the two-dimensional rapid scanning imaging technology based on the terahertz pulse flight time imaging technology, so that the system has the terahertz spectrum measurement function and the terahertz imaging detection function.
Compared with the prior art, the invention has the following advantages and beneficial effects:
the invention provides a spectrum and imaging system based on optical fiber terahertz asynchronous sampling; the method is improved on the prior asynchronous sampling terahertz spectrum technology, and the asynchronous triggering module is used for realizing the femtosecond laser triggering technology of coherent synthesis triggering by a plurality of asynchronous triggering signals, so that the problems of high adjustment difficulty, unstable triggering signals and the like in the existing two-photon or balance detection triggering and acquisition terahertz signal mode are solved. Compared with the existing free space system adopting the femtosecond laser with the titanium sapphire and the asynchronous sampling system adopting the double lasers, the system has the advantages of higher integration level, greatly reduced volume, better stability and environmental adaptation and lower cost.
Drawings
In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the drawings that are needed in the examples will be briefly described below, it being understood that the following drawings only illustrate some examples of the present invention and therefore should not be considered as limiting the scope, and that other related drawings may be obtained from these drawings without inventive effort for a person skilled in the art. In the drawings:
FIG. 1 is a schematic diagram of a spectrum and imaging system based on fiber terahertz asynchronous sampling;
FIG. 2 is a schematic diagram of a dual frequency control principle of an optical fiber femtosecond laser;
FIG. 3 is a schematic diagram of the two-dimensional fast scan imaging technique implementation principle of embodiment 2;
fig. 4 is a schematic diagram of the implementation principle of the spectral imaging technology of the target sample in embodiment 3.
Detailed Description
For the purpose of making apparent the objects, technical solutions and advantages of the present invention, the present invention will be further described in detail with reference to the following examples and the accompanying drawings, wherein the exemplary embodiments of the present invention and the descriptions thereof are for illustrating the present invention only and are not to be construed as limiting the present invention.
Example 1
The present embodiment provides a spectrum and imaging system based on optical fiber terahertz asynchronous sampling, as shown in fig. 1, including:
the scanning imaging module is used for bearing a target sample and carrying out scanning imaging when the terahertz pulse signal irradiates the target sample;
the terahertz pulse signal generation module is used for generating a terahertz pulse signal under the first generation signal and irradiating the terahertz pulse signal onto a target sample of the scanning imaging module;
the terahertz pulse signal receiving module is used for receiving the terahertz pulse signal reflected back on the target sample under the trigger signal;
the signal acquisition module is used for acquiring terahertz pulse signals from the terahertz pulse signal receiving module under the third detection signals;
the asynchronous trigger module is used for generating a first generated signal, a trigger signal and a third detection signal, wherein the first generated signal and the trigger signal have the same pulse repetition frequency, the third detection signal and the trigger signal have the same pulse repetition frequency, and the first generated signal and the third detection signal have different pulse repetition frequencies.
Example 2.
As shown in fig. 1, the present embodiment provides a terahertz asynchronous sampling spectrum and imaging system based on all-fiber coupling, which includes a standard clock source, a fiber femtosecond laser, a terahertz transmitting head, a terahertz detecting head, a bias source, a scanning imaging module, a delay line, an attenuator, a coupler, a PD photoelectric probe, a signal acquisition and amplification module and an upper computer. In the embodiment, the femtosecond laser is transmitted in the optical fiber, so that the stability and the environmental adaptability of the system are greatly improved. And a delay line, an optical fiber coupler and a PD photoelectric probe are adopted to realize high-precision coherent synthesis triggering of the terahertz pulse signal. Terahertz three-dimensional imaging is realized based on the flight time and the two-dimensional scanning imaging technology.
The asynchronous triggering module comprises: the device comprises an optical fiber femtosecond laser, a polarization maintaining optical fiber, an attenuator, a delay line and a coupler;
the fiber femtosecond laser is used for generating multiple paths of femtosecond lasers (1, 2,3 and 4 in the corresponding figures):
a part of femtosecond laser attenuates laser power through a polarization maintaining optical fiber and an attenuator to generate a first generation signal and a third detection signal, wherein the first generation signal is input into a terahertz pulse signal generation module, and the third detection signal is input into a terahertz pulse signal receiving module;
and the other part of femtosecond laser is coupled into a trigger signal input signal acquisition module through a coupler after the laser power is attenuated by an attenuator and the delay of a delay line.
The femtosecond laser includes: a first femtosecond laser (corresponding to fig. 1), a second femtosecond laser (corresponding to fig. 2), a third femtosecond laser (corresponding to fig. 3), and a fourth femtosecond laser (corresponding to fig. 4);
the first femtosecond laser attenuates the laser power through the polarization maintaining optical fiber and the attenuator to generate a first generated signal, and the third femtosecond laser attenuates the laser power through the polarization maintaining optical fiber and the attenuator to generate a third detection signal;
the second femtosecond laser is attenuated by the attenuator, the laser power is delayed by the delay line and then enters the coupler, the four femtosecond laser is attenuated by the attenuator and then enters the coupler, and the coupler couples the second femtosecond laser and the four femtosecond laser to obtain a trigger signal
The first femtosecond laser and the second femtosecond laser have the same pulse repetition frequency, and the phase difference of the first femtosecond laser and the second femtosecond laser is constant; the third and fourth femtosecond lasers have the same pulse repetition frequency, and the phase difference of the third and fourth femtosecond lasers is constant.
The first femtosecond laser, the second femtosecond laser, the third femtosecond laser and the fourth femtosecond laser are output by the same optical fiber femtosecond laser, and the optical fiber femtosecond laser is controlled by a standard clock source.
There is a fixed difference Δf between the pulse repetition frequencies of the first and third femtosecond lasers.
The terahertz pulse signal generation module comprises a first photoconductive antenna and a bias source, wherein the bias source is connected to the photoconductive antenna, the terahertz pulse signal is generated after the first generation signal is input into the first photoconductive antenna, and bias voltage is provided by the bias source in the process of generating the terahertz pulse signal by the photoconductive antenna.
The terahertz pulse signal receiving module comprises a second photoconductive antenna, a bias source and a signal processing unit;
the third detection signal is input into the second photoconductive antenna and then receives the terahertz pulse signal reflected back from the target sample; the second photoconductive antenna transmits the received terahertz pulse signal to the signal processing unit;
the signal acquisition module acquires terahertz pulse signals from the signal processing unit.
The device also comprises an output module;
and under the third detection signal, the signal acquisition module acquires the terahertz pulse signal, the terahertz spectrum signal and the imaging information of the target sample from the terahertz pulse signal receiving module.
In the embodiment, the optical fiber femtosecond laser outputs four paths of femtosecond lasers, wherein the repetition frequency of the first femtosecond laser and the second femtosecond laser is f, and the repetition frequency of the third femtosecond laser and the fourth femtosecond laser is f+Δf; the (first femtosecond laser) laser 1 excites a terahertz transmitting head coupled with an optical fiber after the laser power is attenuated to an allowable range through a polarization maintaining optical fiber and an attenuator, a photoconductive antenna is adopted in the embodiment to generate a terahertz pulse signal, and a bias source provides bias voltage required by exciting the terahertz transmitting head to generate the terahertz pulse; the generated terahertz pulse irradiates on a target sample on a scanning imaging module, the reflected terahertz pulse signal is received by a terahertz transmitting head coupled with an optical fiber, is converted into an electric signal under the excitation of third femtosecond laser (attenuated to an allowable range through an attenuator), is input to a signal acquisition and amplification module for acquisition, amplification and pretreatment of the terahertz signal, finally obtains the terahertz pulse signal, spectrum, image and the like through upper computer software, and displays and stores the terahertz pulse signal, spectrum, image and the like on an upper computer;
after the laser 2 (second femtosecond laser) and the laser 4 (fourth second femtosecond laser) are attenuated to an allowable range through an attenuator, the laser 2 (second femtosecond laser) and the laser 4 are input into a two-in-one optical fiber coupler, an output coherent combined optical signal is converted into an electric signal through a PD photoelectric probe and is input into a signal acquisition and amplification module, and when the amplitude of a trigger electric signal exceeds a set threshold value (as judged by a high-low level comparator) due to coherent combination of the laser 2 and the laser 4, the acquisition of a terahertz pulse signal is started by the trigger signal acquisition and amplification module; and a delay line (manual or electric) is added in the optical path of the laser 2 or the laser 4 and is used for adjusting the time delay length of the trigger signal and the peak value of the acquired terahertz pulse signal so as to adapt to the requirements of different test samples and test scenes. The terahertz pulse signals are rapidly acquired and collected by adopting an optical asynchronous sampling technology, and terahertz spectrum signals are obtained through Fourier transformation; and scanning the target point by a transverse two-dimensional rapid scanning technology to obtain terahertz pulse waveforms of each point, and realizing rapid three-dimensional imaging of the target based on a time-of-flight imaging technology.
In the embodiment, a standard clock source (generally adopting a rubidium clock) provides a time reference for the optical fiber femtosecond laser, so that double-frequency control of the optical fiber femtosecond laser is realized; the repetition frequency of the laser 1 and the laser 2 output by the fiber femtosecond laser is f, the phase difference between the laser 1 and the laser 2 is constant (determined by the optical path delay), the repetition frequency of the laser 3 and the laser 4 is f+Δf, the phase difference between the laser 3 and the laser 4 is constant (determined by the optical path delay), as shown in fig. 2, the repetition frequency of the laser 1 and the laser 3 pulse has a fixed difference Δf, so that the relative time delay between the laser 1 and the laser 3 pulse linearly increases, that is, the pulses of the laser 1 and the laser 3 are supposed to generate the terahertz pulse at a certain moment to coincide in time, and due to the different repetition frequencies, a time difference is formed between the two pulses at the next pulse, and each pulse is sequentially increased by a time difference until the pulse coincides with the next pulse again, thereby realizing the sampling measurement of the terahertz pulse by the laser 3 pulse, and reconstructing the terahertz pulse waveform. The laser 2 and the laser 4 output by the laser trigger the acquisition and amplification module to start acquisition of a terahertz pulse signal through a trigger signal generated by coherent combination.
Example 2
In this embodiment, the scanning imaging module is provided with an imaging detection function at the same time, the imaging detection function is realized based on a terahertz pulse time-of-flight imaging technology and a two-dimensional rapid scanning imaging technology, the basic principle is as shown in fig. 3, and in the longitudinal direction (z direction), high-resolution tomography is realized based on the terahertz pulse time-of-flight imaging technology. The terahertz pulse time-of-flight imaging technology is to realize longitudinal chromatographic resolution of a target by utilizing the time difference of terahertz pulses reflected by different interfaces inside the target, and under the condition of normal incidence, the longitudinal thickness resolution of a target sample is d=cτ/2n g Wherein c is vacuum light velocity, τ is pulse delay time, n g Is the refractive index of the sample. Because the typical pulse width of one terahertz pulse is smaller than 1ps, the longitudinal high-resolution longitudinal tomography with the resolution being better than 30 mu m can be realized by combining the optimization of a signal processing algorithm. In the transverse direction (x y direction), three-dimensional tomography of the target can be finally realized by combining the time-of-flight tomography result of each point in the z direction through point-by-point rapid scanning in the two-dimensional space. When each terahertz pulse is obtained by utilizing an asynchronous sampling technology, accurate position coordinates of the two-dimensional imaging scanning module at the same moment need to be obtained synchronously, which is the key of correctly reconstructing the terahertz three-dimensional image. The system acquires the accurate position coordinates corresponding to each terahertz pulse by acquiring the position signals (such as a grating coding ruler) of the two-dimensional imaging scanning module corresponding to the triggering signals of each terahertz pulse, so that the terahertz three-dimensional image of the detection target is accurately reconstructed.
Example 3
The terahertz pulse signal receiving module of this embodiment may also be performed in a transmission mode, as shown in fig. 4, where the terahertz pulse signal emitted by the terahertz emitting head passes through the target to be measured and then is incident on the terahertz detecting head to be converted into an electrical signal, so as to implement transmission spectrum measurement on the target. And placing the target sample on a two-dimensional scanning module, and synchronously scanning in two dimensions when measuring the spectrum, so that the spectrum imaging of the target can be realized.
The invention controls a laser based on a standard clock source, outputs the working mode of four paths of femtosecond lasers, and compared with the existing free space system adopting the titanium sapphire femtosecond lasers and the asynchronous sampling system of the double lasers, the invention has higher integration level, greatly reduced volume, better stability and environmental adaptation and lower cost; the femtosecond laser triggering technology based on coherent synthesis triggering solves the problems of high adjustment difficulty, unstable triggering signals and the like in the existing two-photon or balance detection triggering and acquisition terahertz signal mode; by combining the terahertz pulse flight time imaging technology with the two-dimensional rapid scanning imaging technology, the system has the terahertz spectrum measurement function and the terahertz imaging detection function.
The foregoing description of the embodiments has been provided for the purpose of illustrating the general principles of the invention, and is not meant to limit the scope of the invention, but to limit the invention to the particular embodiments, and any modifications, equivalents, improvements, etc. that fall within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (10)
1. A fiber-based terahertz asynchronous sampling spectroscopy and imaging system, comprising:
the scanning imaging module is used for bearing a target sample and carrying out scanning imaging when the terahertz pulse signal irradiates the target sample;
the terahertz pulse signal generation module is used for generating a terahertz pulse signal under the first generation signal and irradiating the terahertz pulse signal onto a target sample of the scanning imaging module;
the terahertz pulse signal receiving module is used for receiving the terahertz pulse signal reflected back on the target sample under the trigger signal;
the signal acquisition module is used for acquiring terahertz pulse signals from the terahertz pulse signal receiving module under the third detection signals;
the asynchronous trigger module is used for generating a first generated signal, a trigger signal and a third detection signal, wherein the first generated signal and the trigger signal have the same pulse repetition frequency, the third detection signal and the trigger signal have the same pulse repetition frequency, and the first generated signal and the third detection signal have different pulse repetition frequencies.
2. The optical fiber terahertz asynchronous sampling-based spectroscopy and imaging system according to claim 1, wherein the asynchronous trigger module comprises: the device comprises an optical fiber femtosecond laser, a polarization maintaining optical fiber, an attenuator, a delay line and a coupler;
the optical fiber femtosecond laser is used for generating multiple paths of femtosecond lasers:
a part of femtosecond laser attenuates laser power through a polarization maintaining optical fiber and an attenuator to generate a first generation signal and a third detection signal, wherein the first generation signal is input into a terahertz pulse signal generation module, and the third detection signal is input into a terahertz pulse signal receiving module;
and the other part of femtosecond laser is coupled into a trigger signal input signal acquisition module through a coupler after the laser power is attenuated by an attenuator and the delay of a delay line.
3. The optical fiber terahertz asynchronous sampling-based spectroscopy and imaging system according to claim 2, wherein the femtosecond laser includes: a first femtosecond laser, a second femtosecond laser, a third femtosecond laser and a fourth femtosecond laser;
the first femtosecond laser generates a first generated signal after attenuating the laser power through the polarization maintaining optical fiber and the attenuator, and the third femtosecond laser generates a third detection signal after attenuating the laser power through the polarization maintaining optical fiber and the attenuator;
the second femtosecond laser is attenuated by the attenuator, delayed by the delay line and enters the coupler, the four femtosecond laser is attenuated by the attenuator, and enters the coupler, and the coupler couples the second femtosecond laser and the four femtosecond laser to obtain a trigger signal.
4. A spectroscopic and imaging system based on optical fiber terahertz asynchronous sampling according to claim 3, characterized in that the first and second femtosecond lasers have the same pulse repetition frequency and the phase difference of the first and second femtosecond lasers is constant; the third and fourth femtosecond lasers have the same pulse repetition frequency, and the phase difference of the third and fourth femtosecond lasers is constant.
5. A spectroscopic and imaging system based on fiber terahertz asynchronous sampling according to claim 2, characterized in that the fiber femtosecond laser is controlled by a standard clock source.
6. The optical fiber terahertz sampling-based spectroscopy and imaging system according to claim 4, wherein there is a fixed difference Δf between pulse repetition frequencies of the first and third femtosecond lasers.
7. The optical fiber terahertz asynchronous sampling-based spectrum and imaging system according to claim 1, wherein the terahertz pulse signal generating module comprises a first photoconductive antenna and a bias source, the bias source is connected to the photoconductive antenna, the terahertz pulse signal is generated after the first generated signal is input into the first photoconductive antenna, and the bias voltage is provided by the bias source in the process of generating the terahertz pulse signal by the photoconductive antenna.
8. The optical fiber terahertz asynchronous sampling-based spectrum and imaging system according to claim 1, wherein the terahertz pulse signal receiving module comprises a second photoconductive antenna, a bias source and a signal processing unit;
the third detection signal is input into a second photoconductive antenna and then receives the terahertz pulse signal reflected back on the target sample; the second photoconductive antenna transmits the received terahertz pulse signal to the signal processing unit;
the signal acquisition module acquires terahertz pulse signals from the signal processing unit.
9. The optical fiber terahertz asynchronous sampling-based spectrum and imaging system according to claim 1, further comprising an output module;
and under the third detection signal, the signal acquisition module acquires the terahertz pulse signal, the terahertz spectrum signal and the imaging information of the target sample from the terahertz pulse signal receiving module.
10. The optical fiber terahertz asynchronous sampling-based spectrum and imaging system according to claim 1, wherein the terahertz pulse signal receiving module is further used for converting the reflected terahertz pulse signal into an electric signal so as to realize transmission spectrum measurement of a target sample.
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