Terahertz pulse time-domain spectrum detection system with high signal-to-noise ratio

Abstract

The invention relates to a terahertz pulse time-domain spectrum detection system with a high signal-to-noise ratio, which comprises a terahertz pulse generation light path, a terahertz pulse detection light path and a detection and control circuit based on a lock-in amplifier, the original detection light beam passes through a frequency doubling processing light path to obtain a detection light beam actually entering the nonlinear electro-optical crystal; the frequency doubling processing light path comprises a beam splitter, a delay line device and a beam combiner; the detection light beam is provided with a probe pulse and a mark pulse adjacent to the probe pulse; the phase-locked amplifier detects the alternating information of the probe pulse relative to the mark pulse and periodically appearing at the repetition frequency in the electric signal output by the detection light path, and then the terahertz pulse time-domain spectrum is obtained according to the alternating information. According to the invention, beam splitting-time delay-beam combination processing is carried out on the detection light beam, and system noise introduced in the propagation process of the detection light beam is eliminated by using the measurement principle of a lock-in amplifier, so that the signal-to-noise ratio is improved.

Description

Technical Field

[0001] The present invention relates to the fields of terahertz time-domain spectroscopy technology and weak signal detection technology, and in particular to a terahertz pulse time-domain spectroscopy detection system with a high signal-to-noise ratio. Background Art

[0002] Terahertz waves usually refer to the frequency range from 0.1THz to 10THz (1THz = 10 12 Terahertz pulse light sources are electromagnetic waves with a wavelength range of 30μm to 3mm (Hz). Compared with traditional light sources, terahertz pulse light sources have many unique properties, including transient, broadband, coherent, low energy, etc. Specifically, the typical pulse width of terahertz pulses is below picoseconds and is transient. This property can facilitate time-resolved research on various materials (including liquids, semiconductors, superconductors, biological samples, etc.). By utilizing the coherence of terahertz waves and coherent sampling measurement technology, the interference of background radiation noise can be effectively suppressed. In addition, the frequency band of a single pulse of terahertz waves can cover the range from GHz to tens of THz, which is convenient for analyzing the spectral properties of substances in a large range. Furthermore, the energy of terahertz photons is only a few millielectronvolts, which has lower energy than X-rays and will not destroy the detected substance due to ionization.

[0003] Before the 1980s, due to the lack of efficient terahertz radiation sources and effective terahertz detection methods, people had very limited knowledge of the electromagnetic spectrum in this special band. However, in recent years, with the rapid development of ultrashort pulse laser technology, nonlinear optical technology, and semiconductor technology, the coherent generation and coherent measurement technology of terahertz radiation has become increasingly mature, and terahertz time-domain spectroscopy (THz-TDS) technology has been developed. At present, terahertz waves have been widely studied and applied in various fields. Terahertz time-domain spectroscopy detection technology has great development potential and application value in many fields including biomedical diagnosis, broadband terahertz communication, and non-destructive testing of materials, and has a strong driving force for scientific research, national economic and social development.

[0004] The electro-optic modulation detection scheme is a typical terahertz time-domain spectroscopy detection scheme in the prior art. In this scheme, a pump beam (strong beam) and a probe beam (weak beam) are obtained by splitting the same pulsed laser light source. The pump beam is irradiated on a terahertz emitter (such as a photoconductive antenna), and the short electromagnetic radiation generated lasts for about picoseconds and has a frequency of terahertz. The radiation usually has one or several cycles and has a very wide bandwidth. The terahertz beam (or terahertz electromagnetic radiation, terahertz pulse) is focused on an electro-optical crystal, which changes the ellipsoidal surface of the refractive index of the electro-optical crystal. The linearly polarized probe beam propagates colinearly with the terahertz beam in the crystal, and the phase of the probe beam is modulated by the refractive index of the crystal, while the refractive index at this time has been changed by the electric field of the terahertz pulse. This phase change is converted into an intensity change through a Wollaston prism. A pair of differentially balanced detectors is used to compress background noise. An adjustable delay line device is used to change the time delay of the terahertz pulse relative to the detection pulse. Scanning this delay can obtain the terahertz electric field waveform, and then obtain the terahertz pulse time domain spectrum.

[0005] The TDS signal-to-noise ratio is an important indicator of the terahertz time-domain spectroscopy detection system. The higher the TDS signal-to-noise ratio, the clearer the terahertz time-domain spectroscopy signal of the sample being tested and the easier it is to distinguish the spectral features, thereby improving the accuracy and efficiency of identifying the target substance. Studies have shown that under the same integration time, the TDS signal-to-noise ratio increases with the increase of the modulation and demodulation frequency. However, if modulation is performed directly on the terahertz antenna (for example, by switching the high-frequency switch of the photoconductive antenna to achieve the modulation of the terahertz beam), there is a limitation of 10MHz high-power driving conditions, making it difficult to further increase the modulation and demodulation frequency of the terahertz time-domain spectroscopy detection system.

[0006] On the other hand, modulation technologies other than photoconductive switches also have their shortcomings. For example, in a technology for pulse extraction based on an acousto-optic modulator published by the RIKEN Institute of Physical and Chemical Research in Japan (Rev Sci Instrum 2023,94,043002), since the integer frequency division is at least 2, its modulation and demodulation frequency can only be half of the laser repetition frequency at most (for a typical 80MHz repetition frequency, the highest TDS modulation frequency is 40MHz). If there is a way to further increase the demodulation frequency, it is expected to further improve the signal-to-noise ratio of the terahertz time-domain spectroscopy detection system.

[0007] In summary, for the terahertz time-domain spectroscopy detection system, people expect to further break through the limit and obtain a higher signal-to-noise ratio without increasing the integration time. Summary of the invention

[0008] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a terahertz time-domain spectroscopy detection solution with a higher signal-to-noise ratio without increasing the integration time.

[0009] The invention provides a terahertz pulse time-domain spectrum detection system with a high signal-to-noise ratio, which comprises: a terahertz pulse generation optical path, which generates a terahertz pulse by exciting a photoconductive antenna with a pump light beam, and focuses the terahertz pulse on a sample to be tested placed on a sample stage; a terahertz pulse detection optical path, which collects terahertz pulses carrying information of the sample to be tested and focuses the terahertz pulses on a nonlinear electro-optical crystal for electro-optical sampling based on a detection light beam; wherein the detection light beam irradiates the same position of the nonlinear electro-optical crystal at the same incident angle as the terahertz pulse to achieve the electro-optical sampling; the electro-optically sampled light beam emitted by the nonlinear electro-optical crystal passes through a 1 / 4 glass slide and a Wollaston prism, is received by a balanced detector pair and converted into an electrical signal; and a detection and control circuit, which is used for measuring the electrical signal output by the balanced detector pair and obtaining the terahertz pulse time-domain spectrum carrying information of the sample to be tested; and is used for controlling the terahertz pulse generation optical path and the terahertz pulse detection optical path. It is characterized in that, in the terahertz pulse generating optical path, the pump beam and an original detection beam are obtained by beam splitting the same pulsed laser light source, and the original detection beam is subjected to a frequency doubling processing optical path to obtain the detection beam actually incident on the nonlinear electro-optical crystal; the frequency doubling processing optical path includes a beam splitter, a delay line device and a beam combiner; the beam splitter is used to split the original detection beam into the same first sub-beam and second sub-beam, and the delay line device is used to delay the second sub-beam relative to the first sub-beam by a preset time length, and the preset time length is half a period of the original detection beam; the beam combiner is used to combine the first sub-beam and the second sub-beam, so that the detection beam has a probe pulse and a marker pulse adjacent to it; wherein, the probe pulse and the terahertz pulse act on the same time period The nonlinear electro-optical crystal, and the time period when the marker pulse passes through the nonlinear electro-optical crystal is staggered with the time period when the terahertz pulse acts on the nonlinear electro-optical crystal; the detection and control circuit includes a phase-locked amplifier, and the pulse laser light source is an ultrashort pulse laser, which outputs an electrical signal with the same repetition frequency and synchronization with the emitted laser pulse, and the synchronized electrical signal is input into the phase-locked amplifier as a reference signal; the phase-locked amplifier measures the electrical signal output by the balanced detector pair based on the reference signal input externally, and the phase-locked amplifier detects from the electrical signal output by the balanced detector pair: the alternating information of the probe pulse relative to the marker pulse, which appears periodically at the repetition frequency, and then the terahertz pulse time domain spectrum is obtained according to the alternating information.

[0010] Among them, the frequency doubling processing optical path also includes a first optical element group and a switching device; the high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system has a first working mode and a second working mode. In the first working mode, the frequency doubling processing optical path directly introduces the original detection beam into the nonlinear electro-optical crystal, and in the second working mode, the frequency doubling processing optical path performs frequency doubling processing on the original detection beam to obtain the detection beam having the probe pulse and the marker pulse; the first optical element group is used to directly cause the original detection beam to be incident on the nonlinear electro-optical crystal; the switching device is used to connect the beam splitter, the delay line device and the beam combiner to or remove the optical path from the original detection beam to the nonlinear electro-optical crystal.

[0011] Among them, in the first working mode, in the terahertz pulse generating optical path, the pump light beam is modulated by a chopper or a photoconductive switch with a first frequency to obtain a pump laser pulse with the first frequency, the bias electric field of the photoconductive antenna is driven by a DC signal, and the phase-locked amplifier uses the signal with the first frequency and a set waveform generated internally as a reference signal to measure the electrical signal output by the balanced detector pair, and obtain the terahertz pulse time domain spectrum based on the electro-optical modulation principle.

[0012] Among them, in the second working mode, the pulse laser light source is realized by an ultrashort pulse laser, the repetition frequency of the ultrashort pulse laser is a second frequency, the pump light beam with the second frequency is incident on the photoconductive antenna to excite the terahertz pulse; the second frequency is greater than the first frequency.

[0013] Before using the second working mode to detect the terahertz pulse time domain spectrum, the various optical elements and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated, including the following steps: 1) in the first working mode, the various optical elements and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the actual measurement results of the phase-locked amplifier; 2) in the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode are kept unchanged, and the optical elements of the frequency doubling processing optical path are fine-tuned so that the light intensity difference between the probe pulse and the adjacent marker pulse does not exceed one percent.

[0014] Wherein, the pulse laser light source is a femtosecond laser, at least one aperture is arranged in the frequency doubling processing optical path, and before using the second working mode to detect the terahertz pulse time domain spectrum, the optical components and their installation positions in the terahertz pulse generating optical path and the terahertz pulse detecting optical path are calibrated, including the following steps: 1) in the first working mode, the optical components and their installation positions in the terahertz pulse generating optical path and the terahertz pulse detecting optical path are calibrated according to the actual measurement results of the phase-locked amplifier; 2) in the second working mode, the optical components and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and at an auxiliary calibration frequency, the beam splitter and the optical components in the dual-mode probe optical path for frequency doubling processing are calibrated according to the actual measurement results of the phase-locked amplifier. The delay line device and the beam combiner and their installation positions are calibrated; wherein the auxiliary calibration frequency is a frequency between the first frequency and the second frequency; and 3) in the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode are kept unchanged, and the calibration results of the beam splitter, the delay line device and the beam combiner and their installation positions under the auxiliary calibration frequency are used as the starting position, and then the delay line device is fine-tuned, and at the same time, the relative light intensity of the two sub-beams after splitting by the beam splitter is fine-tuned through the at least one aperture, and the phase-locked amplifier is used to detect the alternating information of the probe pulse relative to the marker pulse that appears periodically at the repetition frequency, until the alternating information is less than a preset threshold, and the fine-tuning is ended.

[0015] Among them, the terahertz pulse generating optical path has a variable displacement stage and a reflector group installed on the variable displacement stage; the detection and control circuit adjusts the time delay of the terahertz pulse relative to the detection light beam by adjusting the variable displacement stage, thereby measuring the electric field signal of the terahertz pulse corresponding to different time delays, thereby obtaining the terahertz pulse time domain spectrum.

[0016] The phase-locked amplifier is a high-frequency digital phase-locked amplifier, which includes: an analog differential circuit, an analog filtering circuit, an analog amplification circuit and a digital phase-locked measurement device, and the digital phase-locked measurement device includes an analog-to-digital conversion unit and an anti-noise phase-locked measurement unit.

[0017] Compared with the prior art, the present invention has at least one of the following technical effects:

[0018] 1. The present application performs beam splitting-delaying-combining processing on the detection beam, thereby obtaining a detection beam with a probe pulse and a marker pulse, and then cleverly utilizes the measurement principle of a phase-locked amplifier to eliminate the system noise introduced during the propagation of the detection beam, thereby improving the signal-to-noise ratio of terahertz pulse time-domain spectroscopy detection. Specifically, the present invention can further shorten the phase-locked integration time while maintaining a sufficient signal-to-noise ratio or further improve the signal-to-noise ratio at the same integration time.

[0019] 2. In some embodiments of the present application, the pulse repetition frequency of pulsed terahertz time-domain spectroscopy detection reaches 80 MHz (or a higher repetition frequency) by direct repetition frequency locking, which significantly improves the theoretical signal-to-noise ratio of terahertz time-domain spectroscopy detection.

[0020] 3. In some embodiments of the present application, the problem of the difficulty of spatial and temporal overlap of the light beams in the repetition frequency direct-locking mode is overcome or alleviated through the mutual calibration of the electro-optical modulation and the repetition frequency direct-locking dual modes, thereby improving the beam quality of the repetition frequency direct-locking detection light and making the actual signal-to-noise ratio closer to its theoretical value.

[0021] 4. In some embodiments of the present application, a dual-mode measurement solution of electro-optical modulation and repetition frequency direct locking is provided. The working mode can be flexibly selected under different working scenarios and the modulation and demodulation frequencies applicable to the optoelectronic elements, thereby taking into account the respective advantages of the two detection modes and obtaining a better signal-to-noise ratio and / or dynamic range.

[0022] 5. Compared with laser pulse extraction, this application eliminates the need for an acousto-optic modulator component, making the optical path simpler and easier to maintain. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram showing the optical path design and electrical connection relationship of a terahertz time-domain spectroscopy detection system in one embodiment of the present application is shown;

[0024] Figure 2 Schematic diagram showing two possible mismatch morphologies that may occur after doubling the laser pulse repetition frequency;

[0025] Figure 3 The oscilloscope measurement image of the combined detection beam obtained after the frequency doubling processing circuit is adjusted in one embodiment of the present application is shown; dV p / V p-mean It represents the ratio of the deviation between the pulses with the largest deviation to the pulse average value, f represents the pulse repetition frequency, and t on the horizontal axis represents time. DETAILED DESCRIPTION

[0026] In order to better understand the present application, a more detailed description will be made of various aspects of the present application with reference to the accompanying drawings. It should be understood that these detailed descriptions are only descriptions of exemplary embodiments of the present application, and are not intended to limit the scope of the present application in any way. Throughout the specification, the same reference numerals refer to the same elements. The expression "and / or" includes any and all combinations of one or more of the associated listed items.

[0027] It should be noted that in this specification, the expressions of first, second, etc. are only used to distinguish one feature from another feature, and do not represent any limitation on the feature. Therefore, without departing from the teaching of this application, the first subject discussed below can also be referred to as the second subject.

[0028] In the drawings, the thickness, size and shape of objects have been slightly exaggerated for ease of explanation. The drawings are only examples and are not drawn strictly to scale.

[0029] It should also be understood that the terms "comprises", "including", "having", "includes" and / or "comprising", when used in this specification, indicate the presence of the stated features, wholes, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, parts and / or combinations thereof. In addition, when expressions such as "at least one of..." appear after a list of listed features, they modify the entire listed features rather than modifying the individual elements in the list. In addition, when describing embodiments of the present application, "may" is used to mean "one or more embodiments of the present application". And, the term "exemplary" is intended to refer to an example or illustration.

[0030] As used herein, the terms "substantially," "approximately," and similar terms are used as terms of approximation, not degree, and are intended to account for the inherent variations in measurements or calculations that would be recognized by one of ordinary skill in the art.

[0031] Unless otherwise defined, all terms (including technical terms) used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. It should also be understood that terms (such as those defined in commonly used dictionaries) should be interpreted as having the same meaning as they have in the context of the relevant technology, and will not be interpreted in an idealized or overly formal sense, unless explicitly defined in this article.

[0032] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application may be combined with each other.

[0033] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0034] According to one embodiment of the present invention, a terahertz pulse time-domain spectroscopy detection system with a high signal-to-noise ratio is provided, which includes: a terahertz pulse generation optical path, a terahertz pulse detection optical path, and a detection and control circuit. The terahertz pulse generation optical path generates a terahertz pulse by exciting a photoconductive antenna with a pump beam, and focuses the terahertz pulse on a sample to be tested placed on a sample stage; the terahertz pulse detection optical path collects the terahertz pulse carrying the information of the sample to be tested and focuses it on a nonlinear electro-optical crystal for electro-optical sampling based on the detection beam; wherein the detection beam irradiates the same position of the nonlinear electro-optical crystal at the same incident angle as the terahertz pulse to achieve the electro-optical sampling; the electro-optically sampled light beam emitted by the nonlinear electro-optical crystal is received by a balanced detector pair through a 1 / 4 glass slide and a Wollaston prism and converted into an electrical signal. The detection and control circuit is used to measure the electrical signal output by the balanced detector pair and obtain the terahertz pulse time-domain spectrum carrying the information of the sample to be tested; and is used to control the terahertz pulse generation optical path and the terahertz pulse detection optical path.

[0035] In this embodiment, in the terahertz pulse generation optical path, the pump beam and an original detection beam are obtained by beam splitting the same pulse laser light source, and the original detection beam is subjected to a frequency doubling processing optical path to obtain the detection beam that is actually incident on the nonlinear electro-optical crystal. The frequency doubling processing optical path includes a beam splitter, a delay line device, and a beam combiner; the beam splitter is used to split the original detection beam into the same first sub-beam and second sub-beam (the same meaning here is at least: the power and polarization direction are consistent), the delay line device is used to delay the second sub-beam relative to the first sub-beam by a preset time length, and the preset time length is half a period of the original detection beam; the beam combiner is used to combine the first sub-beam and the second sub-beam, so that the detection beam has a probe pulse and a marker pulse adjacent to it (a marker pulse used as an internal reference). Among them, the probe pulse and the terahertz pulse act on the nonlinear electro-optical crystal in the same time period (that is, the probe pulse and the terahertz pulse act on the nonlinear electro-optical crystal synchronously), and the time period when the marker pulse passes through the nonlinear electro-optical crystal is staggered with the time period when the terahertz pulse acts on the nonlinear electro-optical crystal. The marker pulse can be used as an internal reference, which will be further explained below.

[0036] In this embodiment, the detection and control circuit includes a phase-locked amplifier, the pulse laser light source is an ultrashort pulse laser, which outputs an electrical signal with the same repetition frequency and synchronization with the emitted laser pulse, and the synchronized electrical signal is input into the phase-locked amplifier as a reference signal; the phase-locked amplifier measures the electrical signal output by the balanced detector pair based on the externally input reference signal, and the phase-locked amplifier detects from the electrical signal output by the balanced detector pair: the alternating information of the probe pulse relative to the marker pulse that appears periodically at the repetition frequency, and then the terahertz pulse time domain spectrum is obtained based on the alternating information. The phase-locked amplifier is very sensitive to the changes between adjacent pulses, especially the alternating information that appears periodically, and is therefore very suitable for capturing such weak signals.

[0037] In fact, the detection pulse will be mixed with some system noise during its propagation in the optical path. For example, the optical path is actually in a non-vacuum environment, and the propagation of the light beam in a non-vacuum environment will inevitably be mixed with some noise. After adding the marker pulse, since the marker pulse and the probe pulse sub-beam pass through the same optical path at the same time period, the noise mixed into the two is highly consistent. Here, the time interval between adjacent marker pulses and probe pulses is extremely short. For various environmental variables in the optical path (such as gas concentration, temperature, humidity, etc.), such a short time interval can be approximated to be the same moment.

[0038] The lock-in amplifier can accurately capture the tiny changes of the probe pulse relative to the marker pulse, and this tiny change contains the terahertz time-domain spectral information. In addition, the terahertz time-domain spectral information has eliminated the system noise introduced by the probe pulse in the optical path, so it has a higher signal-to-noise ratio. Because the marker pulse and the probe pulse actually pass through the same optical path, the marker pulse also carries the same system noise. What the lock-in amplifier captures is actually the tiny changes of the probe pulse relative to the marker pulse. In this process, the system noise introduced by the probe pulse propagating in the optical path has been cleverly eliminated.

[0039] In the existing measurement scheme, the signal measured by the lock-in amplifier actually includes the system noise introduced by the detection light beam propagating in the optical path. Therefore, the scheme of the present invention can theoretically have a higher signal-to-noise ratio.

[0040] Furthermore, in some embodiments, the high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system has a first working mode and a second working mode. The frequency doubling processing optical path also includes a first optical element group and a switching device.

[0041] In the first working mode, the frequency doubling processing optical path directly introduces the original detection beam into the nonlinear electro-optical crystal. In the second working mode, the frequency doubling processing optical path performs frequency doubling processing on the original detection beam to obtain the detection beam having the probe pulse and the marker pulse. The first optical element group is used to directly cause the original detection beam to be incident on the nonlinear electro-optical crystal. The switching device is used to connect or remove the beam splitter, the delay line device and the beam combiner from the optical path from the original detection beam to the nonlinear electro-optical crystal.

[0042] Further, in the first working mode, in the terahertz pulse generating optical path, the pump beam is modulated by a chopper or a photoconductive switch having a first frequency to obtain a pump laser pulse having the first frequency, the bias electric field of the photoconductive antenna is driven by a DC signal, and the phase-locked amplifier uses a signal having the first frequency and a set waveform generated internally as a reference signal to measure the electrical signal output by the balanced detector pair and obtain a terahertz pulse time domain spectrum based on the electro-optical modulation principle. In the second working mode, the pulse laser light source is implemented by an ultrashort pulse laser, the repetition frequency of the ultrashort pulse laser is a second frequency, and the pump beam having the second frequency is incident on the photoconductive antenna to excite the terahertz pulse; the second frequency can be much greater than the first frequency. For example, the second frequency can be 80MHz, while the first frequency is usually not more than 10MHz. For example, in one example, the first frequency can be 1MHz.

[0043] Further, refer to Figure 1 According to one embodiment of the present application, a terahertz pulse time-domain spectroscopy detection system with a high signal-to-noise ratio is provided, which includes a terahertz pulse generation optical path, a terahertz pulse detection optical path, a frequency doubling processing optical path and a detection circuit. The terahertz pulse generation optical path and the terahertz pulse detection optical path constitute a main optical path, which includes an ultrashort pulse laser, a first beam splitting unit, a fixed reflector group, a variable displacement stage, a movable reflector group mounted on the variable displacement stage, a photoconductive antenna PCA, a parabolic mirror group PBM, a sample stage SUT, a nonlinear optical crystal NLC, a 1 / 4 glass slide QWP, a Wollaston prism WP, and a balanced detector pair BT. The frequency doubling processing optical path is used to generate a detection beam, which irradiates the same position of the nonlinear electro-optical crystal at the same incident angle as the terahertz pulse to achieve electro-optical sampling. In this embodiment, the frequency doubling processing optical path has two working modes.

[0044] Among them, the ultrashort pulse laser emits a laser pulse, and two beams of ultrashort pulse lasers of the same source are obtained through the first beam splitting unit, one of which has a larger power, called the pump beam (Pump), which serves as the light source of the main optical path, and the other has a smaller power, called the probe beam (Probe, which can also be called the probe beam). The pump beam passes through the second reflector M2, the movable reflector group MS, and the third reflector M3, and enters the photoconductive antenna PCA. The photoconductive antenna PCA receives the pump beam and radiates terahertz electromagnetic waves under the action of the bias electric field. The terahertz electromagnetic waves are received by the parabolic mirror group PBM and transmitted to the nonlinear optical crystal NLC. The sample stage is set between the multiple parabolic mirrors of the parabolic mirror group PBM to fix the sample to be tested. In this embodiment, the parabolic mirror group PBM has four parabolic mirrors. The terahertz electromagnetic wave radiated by the photoconductive antenna excited by the pump beam is collected and collimated by the first parabolic mirror. The terahertz electromagnetic wave reflected by the first parabolic mirror is transmitted in parallel to the second parabolic mirror. The second parabolic mirror reflects and converges (focuses) the terahertz electromagnetic wave to the sample stage. After the converged terahertz electromagnetic wave penetrates the sample to be measured, it is collected and reflected by the third parabolic mirror in a parallel propagation state, and then reflected and converged (focused) to the nonlinear optical crystal NLC by the fourth parabolic mirror. At this time, the terahertz electromagnetic wave converged to the nonlinear optical crystal NLC has carried the information of the sample to be measured. The detection beam and the terahertz electromagnetic wave carrying the information of the sample to be measured are placed on the same optical axis, and the same area of ​​the nonlinear optical crystal NLC is irradiated. Based on the electro-optical effect of the nonlinear optical crystal NLC, the delay of the detection beam relative to the pump beam is adjusted, and the detection beam can be used to demodulate and sample the terahertz electromagnetic wave carrying the information of the sample to be measured. The light beam emitted by the nonlinear optical crystal NLC passes through the 1 / 4 glass QWP and the Wollaston prism WP in sequence, and is received by the balanced detector pair BT and converted into an electrical signal. The electrical signal carrying the information of the sample under test output by the balanced detector pair BT may be very weak. In this embodiment, the two electrical signals output by the balanced detector pair BT are differentially measured to the phase-locked amplifier, that is, the two electrical signals are differentially measured as the signal to be tested by the phase-locked amplifier, and the two electrical signals are correlated with the reference signal to obtain the accurate values ​​of the phase, amplitude and frequency of the signal to be tested.

[0045] On the other hand, the frequency doubling processing optical path in the present embodiment includes a fixed reflector group, a time delay mirror group, two beam splitters and a beam splitter switching unit. Among them, the beam splitter switching unit can remove a beam splitter from the frequency doubling processing optical path and replace the other beam splitter with a reflector. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system of the present application has two working modes, namely, electro-optical modulation mode and repetition frequency direct locking mode. The beam splitter switching unit can allow the system to switch between the two working modes. In the repetition frequency direct locking mode, the frequency doubling processing optical path performs frequency doubling processing on the original detection beam. In the electro-optical modulation mode, the frequency doubling processing optical path degenerates into an optical path that guides the original detection beam, and no longer performs frequency doubling processing on the original detection beam. In another embodiment, the beam splitter switching unit can be replaced by another switching device. The switching device may, for example, include a movable light shielding plate. In the repetition frequency direct locking mode, the light shielding plate is placed outside the optical path to allow the two sub-beams to be combined. In the electro-optical modulation mode, the light shielding plate is placed in the optical path of one of the sub-beams and blocks the sub-beam. In this way, the output of the beam combiner is actually another sub-beam, which will serve as the detection beam that actually enters the nonlinear electro-optical crystal.

[0046] In this embodiment, the detection circuit is centered on a high-frequency phase-locked amplifier, which has the function of detecting weak electrical signals. The high-frequency phase-locked amplifier can be a high-frequency digital phase-locked amplifier, which can include: an analog differential circuit, an analog filtering circuit, an analog amplification circuit, and a digital phase-locked measurement device, wherein the digital phase-locked measurement device includes an analog-to-digital conversion unit and an anti-noise phase-locked measurement unit. The anti-noise phase-locked measurement unit can be implemented based on the scheme recorded in Chinese patent CN201110380805.X. However, it should be noted that in the electro-optical modulation mode, the anti-noise phase-locked measurement unit uses an internally generated reference signal with a set frequency. The set frequency is consistent with the frequency of the pump beam of the incident photoconductive antenna. In the repetition frequency direct locking mode, the anti-noise phase-locked measurement unit receives the synchronous electrical signal of the ultrashort pulse laser input externally as a reference signal. Based on the reference signal and the input measured signal, the anti-noise phase-locked measurement unit searches for the phase-frequency combination that maximizes the correlation in the phase space and the frequency space, thereby obtaining the precise phase, frequency and amplitude of the measured signal. The concept of relevance can be found in CN201110380805.X and will not be elaborated here.

[0047] In this embodiment, in the electro-optical modulation mode, the bias electric field of the photoconductive antenna PCA is driven by a direct current signal, and a pump laser pulse of the required frequency is realized by a chopper or a photoconductive switch is controlled to perform a high-frequency switching action, and then the pump laser pulse is incident on the photoconductive antenna PCA to excite a terahertz pulse. By adjusting the time delay of the detection beam relative to the pump beam, the terahertz electromagnetic wave carrying the information of the sample to be measured can be sampled and measured at the nonlinear optical crystal NLC. The reference signal of the phase-locked amplifier uses an internally generated reference signal, that is, based on the frequency of the terahertz pulse (actually, the pulse frequency of the pump beam incident on the photoconductive antenna PCA), an internal reference signal of the corresponding frequency and waveform is generated by the phase-locked amplifier, and then correlated with the measured differential signal carrying the information of the sample to be measured (that is, the signal to be measured output by the balanced detector pair BT), thereby obtaining the precise values ​​of the phase, amplitude and frequency of the signal to be measured.

[0048] In the repetition frequency direct locking mode, the bias electric field of the photoconductive antenna PCA can also be driven by a DC signal. At this time, a chopper is no longer set or a high-frequency modulation drive is applied to the photoconductive antenna. Instead, the high-frequency laser pulse output by the ultrashort pulse laser is directly used as the pump pulse for irradiation, and the phase-locked amplifier uses the electrical signal (TTL electrical signal) output by the ultrashort pulse laser that is synchronized with the laser repetition frequency as a reference signal. In other words, the phase-locked amplifier uses an external reference signal.

[0049] In some embodiments of the present invention, when the frequency doubling of the original detection beam is processed, it is necessary to ensure that the probe pulse and the marker pulse in the combined detection beam are as consistent as possible. In particular, the light intensity difference between the probe pulse and the adjacent marker pulse should not exceed the preset threshold. In one embodiment, before using the second working mode to detect the terahertz pulse time-domain spectrum, the optical elements and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated, including the following steps: 1) In the first working mode, the optical elements and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the actual measurement results of the phase-locked amplifier; 2) In the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and the optical elements of the frequency doubling processing optical path are fine-tuned so that the light intensity difference between the probe pulse and the adjacent marker pulse does not exceed one percent. After the fine-tuning is completed, the sample to be tested is placed on the sample stage, and the terahertz pulse time-domain spectrum carrying the information of the sample to be tested is measured by the phase-locked amplifier in the repetition frequency direct lock mode (the second working mode). like Figure 3 As shown in the figure, after the above two-step adjustment, the light intensity difference between the probe pulse and the adjacent marker pulse actually output by the oscilloscope can be within 1%. Figure 3 The tolerance of the amplitude of the displayed signal is 3%. However, this includes the tolerance of different laser pulses themselves. Specifically, although the pulses emitted by the laser are constantly repeated, each pulse may actually be slightly different. Therefore, the 3% tolerance is actually mixed with the difference between different laser pulses themselves. The present invention is actually more concerned with minimizing the difference between the probe pulse and the marker pulse after beam splitting. In the best case, the marker pulse is an exact replica of the probe pulse. Of course, this is not actually possible, and the only way is to minimize the difference between the two. Based on Figure 3 The measured waveform of the combined detection beam and the light intensity difference between the probe pulse and the adjacent marker pulse can be within one percent.

[0050] Furthermore, assuming that more sophisticated fine-tuning means are used, the light intensity difference between the above-mentioned probe pulse and marker pulse may be further reduced to less than one thousandth. However, in the case where the light intensity difference needs to be reduced to less than one thousandth, directly measuring the light intensity of the detection beam, the measurement equipment itself may have a certain impact on the optical path of the terahertz pulse time-domain spectroscopy detection system, thereby affecting the accuracy of the measurement. Therefore, the following embodiment provides a solution for fine-tuning (such as fine-tuning the aperture of one of the sub-beams) using the measured results of the phase-locked amplifier. In this solution, the optical path of the terahertz pulse time-domain spectroscopy detection system and the phase-locked amplifier are directly used to fine-tune the frequency doubling processing optical path, so as to minimize the tolerance of the probe pulse and the marker pulse (especially the tolerance of the light intensity).

[0051] Specifically, in another embodiment, the pulsed laser light source is a femtosecond laser, at least one aperture is set in the frequency doubling processing optical path, and before using the second working mode to detect the terahertz pulse time domain spectrum, the optical elements and their installation positions in the terahertz pulse generating optical path and the terahertz pulse detecting optical path are calibrated, including the following steps: 1) in the first working mode, the optical elements and their installation positions in the terahertz pulse generating optical path and the terahertz pulse detecting optical path are calibrated according to the actual measurement results of the phase-locked amplifier; 2) in the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and at an auxiliary calibration frequency, the optical elements of the dual-mode probe optical path that undergo frequency doubling processing are calibrated according to the actual measurement results of the phase-locked amplifier. The beam splitter, the delay line device, the beam combiner and their installation positions are calibrated; wherein the auxiliary calibration frequency is a frequency between the first frequency and the second frequency; and 3) in the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and the calibration results of the beam splitter, the delay line device, the beam combiner and their installation positions under the auxiliary calibration frequency are used as the starting position, and then the delay line device is fine-tuned, and at the same time, the relative light intensity of the two sub-beams after being split by the beam splitter is fine-tuned through the at least one aperture, and the alternating information of the probe pulse relative to the marker pulse that appears periodically at the repetition frequency is detected by the phase-locked amplifier until the alternating information is less than the preset threshold value, and the fine-tuning is ended. If it is less than the preset threshold value, it can be regarded as zeroing completed. During the zeroing process, no sample is placed on the sample stage. Neither the probe pulse nor the marker pulse carries terahertz information. After the fine adjustment is completed, the sample to be tested is placed on the sample stage, and the terahertz pulse time domain spectrum carrying the information of the sample to be tested is measured by a phase-locked amplifier in the repetition frequency direct locking mode (the second working mode).

[0052] Here, the auxiliary calibration frequency is a relatively low frequency (compared to the working frequency of the second working mode, i.e., the second frequency). When the repetition frequency is low, the relative errors of the light intensity and delay of the two sub-beams after splitting are small. A brief explanation is as follows: when the length of a cycle is 10ns (it corresponds to a lower repetition frequency, note that the values ​​here are only used to illustrate the calibration principle in a schematic way and do not represent the actual values ​​used), then the delay error of 10ps is one thousandth of the length of a cycle. If the length of a cycle is 1ns (it corresponds to a higher repetition frequency), then the delay error of 10ps is also one hundredth of the length of a cycle, which makes it easier to have mismatch problems. Therefore, it can be assumed that at a lower repetition frequency, the adjustment of each component in the self-doubling additional optical path (beam splitter, delay device, beam combiner and related apertures) will be relatively easier and faster, so that the optical path can be adjusted more quickly to a state where there is no mismatch in the detection beam after the combination. Then, the repetition frequency can be increased to a limit state or a state close to the limit, and based on the adjustment result at the lower frequency above (that is, the adjustment result is used as a coarse adjustment step in the solution of the detection beam combination mismatch at a higher repetition frequency), the delay line device and the related aperture can be further fine-tuned. The adjustment step length of this fine-tuning can be much smaller than the adjustment step length of the previous step (coarse adjustment step). According to the measured results of the phase-locked amplifier, a fine-tuning result that makes the measured results optimal (or sufficiently optimized) is obtained. Finally, based on the fine-tuning result, the actual sample to be tested is detected at a preset limit state or a high repetition frequency close to the limit state.

[0053] Furthermore, the terahertz time-domain spectroscopy detection method of the terahertz pulse time-domain spectroscopy detection system with a high signal-to-noise ratio of the present invention is described in detail below with reference to a specific example.

[0054] like Figure 1As shown in the figure, the system uses a laser with a pulse width of 10-200 femtoseconds and a repetition frequency of 80MHz to output laser light, which is divided into two beams, Pump and Probe. The former generates terahertz waves through a variable delay stage and a photoconductive antenna, and then focuses on the detection crystal after passing through a group of four parabolic mirrors; the latter is split, and the repetitive pulse is regenerated at the midpoint of the 12.5ns cycle by a 6.25ns fixed delay line, and the repetition frequency is doubled after combining with another beam, and then after reflection and collimation, it acts on the detection crystal together with the aforementioned terahertz light output port. Because of the terahertz rectification electro-optical effect in the detection crystal, the output visible light of the detection crystal carries terahertz spectral information, and the subsequent 1 / 4 glass slide, Wollaston prism and balanced detector settings allow the presence and strength of terahertz to be detected with a high dynamic range. This optical path involves signal detection above 10MHz, and uses the Saien OE2052 high-frequency lock-in amplifier as a weak signal detection instrument (300MHz bandwidth). Among them, terahertz generation and detection use photoconductive antennas and ZnTe electro-optical crystals, respectively, taking advantage of higher radiation power and faster and richer detection degrees of freedom.

[0055] The switching of the two working modes in the optical path involves the external reference of the laser repetition frequency reference input phase locking, which is only used when the repetition frequency is directly locked, and is changed to an internal reference at the internally set modulation frequency when electro-optical modulation is used; the photoconductive antenna drive is driven by a DC constant potential when the repetition frequency is directly locked, and is changed to an AC drive when the electro-optic modulation is used, and the trigger signal of the AC power supply must be connected from the output signal of the phase-locked amplifier; when the BS beam splitter is switched from the repetition frequency direct lock mode to the electro-optical modulation mode, the delayed optical path is blocked, and only the undelayed detection light is used for electro-optical sampling. In order to facilitate the optimization of terahertz signals, PCA and NLC should be fixed on the lateral 2D of the vertical light and the longitudinal 1D adjustable displacement stage along the propagation of the light, respectively. In order to increase the stability of the optical path and the success rate of adjustment, the laser output light should also be collimated and apertured.

[0056] In one embodiment of the present application, the photoconductive antenna is adjusted by the following scheme: the optical path from M2 to PCA is adjusted so that the laser spot hits the middle of the photoconductive antenna. If necessary, a focusing convex lens should be added in front of the antenna to focus the pump light to the middle of the slit of the photoconductive antenna. The antenna is observed with the ohm range of a multimeter, and the displacement stage is adjusted at the same time. If the resistance of the photoconductive antenna is observed to drop from more than 10 megohms to the minimum (about 60 kilohms), it means that the photoconductive antenna is working normally and is properly positioned.

[0057] In one embodiment of the present application, self-frequency doubling and beam combining are achieved in the detection optical path of the system by the following method: the terahertz generation and detection optical paths are adjusted separately to allow the light to converge at the center of the NLC, wherein the terahertz generation optical path is originally invisible to the naked eye, but the terahertz pulse generation module can be removed and the red laser as the light source can be allowed to pass directly through the parabolic mirror group, thereby checking whether the pump light coincides with the detection light at the NLC.

[0058] In one embodiment of the present application, balanced detection is achieved based on the following scheme.

[0059] Step 1. Connect the detector output to the in+ port of the phase-locked 1st and 2nd channels respectively. Both channels use single-ended detection mode and the range is adjusted to 100mV. Under the external reference triggered by the laser repetition frequency, check the output signal strength of the two detectors at the same time.

[0060] Step 2: Adjust the beam spatial angle and aperture so that the beam hits the center of the detector, the apertures of the two detectors are as small as possible, and the light intensity is roughly the same;

[0061] Step 3, block the terahertz light path and adjust the 1 / 4 glass slide so that the readings of the two light detectors differ by no more than 0.1%;

[0062] Step 4. After the balance adjustment is completed, before scanning the TDS signal, a pair of detector outputs should be connected to the In+ / In- ports of the phase-locked channel 1 respectively, and the differential input mode should be switched back to the reference mode. The internal reference and external reference should be used in the electro-optical modulation and repetition frequency direct lock modes respectively. It should also be confirmed whether the range is adjusted back to 2mV.

[0063] Furthermore, in a preferred embodiment, step 5 may be included, that is, when the terahertz peak is found, the crystal angle of the NLC may be adjusted to make the electro-optical effect most obvious.

[0064] Furthermore, in some embodiments of the present application, while ensuring the normal operation of the electro-optical modulation mode TDS, an attempt is made to challenge the limit signal-to-noise ratio by strictly replicating the optical path (light intensity, polarization, and delay) of the repetition frequency pulse.

[0065] The difficulties include:

[0066] 1. After the 6.25 nanosecond delay, the components in the optical path are combined to see a 160 MHz signal, rather than two 80 MHz misaligned pulses (the common transverse delay and longitudinal intensity misalignments that cannot be synthesized by frequency doubling are as follows: Figure 2 shown);

[0067] 2. The time delay optical path is long, and it is difficult for the two beams of light to coincide with each other in space when they reach the electro-optic crystal;

[0068] 3. On the basis of time and space overlap, the optical polarization of the dual beams should also be consistent, otherwise the signal-to-noise ratio will not be improved.

[0069] In order to solve the problem of simultaneous and spatial overlap of light beams, the specific requirements for laser and optical path adjustment can include the following four items:

[0070] 1. The spot power is between 20 and 200mW;

[0071] 2. Spatial divergence angle <0.6 milliradian;

[0072] 3. Polarization degree>500:1;

[0073] 4. Delay accuracy <1ps.

[0074] In one embodiment of the present application, the variable time-delay stage is adjusted to the middle, and the optical path of the terahertz and probe light is preliminarily measured. From the beam splitter output by the laser to the detection crystal, it should be ensured to be roughly equal. For example, if the probe light is 6.5cm short, the variable stage should be shortened. When it is difficult to approach the limit of the stage, the fixed time-delay mirror pair should be lengthened by 3.25cm, and a pair of apertures should be used to ensure that the optical path is not deviated when the delay mirror is moved.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A terahertz pulse time-domain spectroscopy detection system with high signal-to-noise ratio, comprising: A terahertz pulse generating optical path, which generates a terahertz pulse by exciting a photoconductive antenna with a pump beam, and focuses the terahertz pulse onto a sample to be measured placed on a sample stage; A terahertz pulse detection optical path, which collects terahertz pulses carrying information of the sample to be tested and focuses them onto a nonlinear electro-optical crystal for electro-optical sampling based on a detection beam; wherein the detection beam irradiates the same position of the nonlinear electro-optical crystal at the same incident angle as the terahertz pulse to achieve the electro-optical sampling; the electro-optically sampled light beam emitted by the nonlinear electro-optical crystal passes through a 1 / 4 glass slide and a Wollaston prism, is received by a balanced detector pair and converted into an electrical signal; and A detection and control circuit, which is used to measure the electrical signal output by the balanced detector and obtain the terahertz pulse time domain spectrum carrying the information of the sample being measured; and is used to control the terahertz pulse generation optical path and the terahertz pulse detection optical path; It is characterized in that, in the terahertz pulse generation optical path, the pump beam and an original detection beam are obtained by beam splitting the same pulse laser light source, and the original detection beam is subjected to a frequency doubling processing optical path to obtain the detection beam actually incident on the nonlinear electro-optical crystal; The frequency doubling processing optical path includes a beam splitter, a delay line device and a beam combiner; the beam splitter is used to split the original detection beam into the same first sub-beam and second sub-beam, and the delay line device is used to delay the second sub-beam relative to the first sub-beam by a preset time length, and the preset time length is half a period of the original detection beam; the beam combiner is used to combine the first sub-beam and the second sub-beam, so that the detection beam has a probe pulse and a marker pulse adjacent thereto; wherein the probe pulse and the terahertz pulse act on the nonlinear electro-optical crystal in the same time period, and the time period when the marker pulse passes through the nonlinear electro-optical crystal is staggered with the time period when the terahertz pulse acts on the nonlinear electro-optical crystal; The detection and control circuit includes a phase-locked amplifier. The pulse laser light source is an ultrashort pulse laser, which outputs an electrical signal with the same repetition frequency and synchronization with the emitted laser pulse, and the synchronized electrical signal is input into the phase-locked amplifier as a reference signal. The phase-locked amplifier measures the electrical signal output by the balanced detector pair based on the externally input reference signal, and the phase-locked amplifier detects from the electrical signal output by the balanced detector pair: the alternating information of the probe pulse relative to the marker pulse, which appears periodically at the repetition frequency, and then the terahertz pulse time domain spectrum is obtained based on the alternating information.

2. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 1, characterized in that: The frequency doubling processing optical path also includes a first optical element group and a switching device; The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system has a first working mode and a second working mode. In the first working mode, the frequency doubling processing optical path directly introduces the original detection beam into the nonlinear electro-optical crystal. In the second working mode, the frequency doubling processing optical path performs frequency doubling processing on the original detection beam to obtain the detection beam having the probe pulse and the marker pulse. The first optical element group is used to make the original detection beam directly incident on the nonlinear electro-optical crystal; The switching device is used to connect or remove the beam splitter, the delay line device and the beam combiner into or out of the optical path from the original detection light beam to the nonlinear electro-optical crystal.

3. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 2, characterized in that: In the first working mode, in the terahertz pulse generating optical path, the pump light beam is modulated by a chopper or a photoconductive switch having a first frequency to obtain a pump laser pulse having the first frequency, the bias electric field of the photoconductive antenna is driven by a DC signal, and the phase-locked amplifier uses a signal having the first frequency and a set waveform generated internally as a reference signal to measure the electrical signal output by the balanced detector pair and obtain a terahertz pulse time domain spectrum based on the electro-optical modulation principle.

4. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 3, characterized in that: In the second working mode, the pulse laser light source is implemented by an ultrashort pulse laser, the repetition frequency of the ultrashort pulse laser is a second frequency, the pump light beam with the second frequency is incident on the photoconductive antenna to excite the terahertz pulse; the second frequency is greater than the first frequency.

5. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 4, characterized in that: Before using the second working mode to detect the terahertz pulse time domain spectrum, calibrating each optical element and its installation position in the terahertz pulse generation optical path and the terahertz pulse detection optical path includes the following steps: 1) In the first working mode, calibrating each optical element and its installation position in the terahertz pulse generation optical path and the terahertz pulse detection optical path according to the actual measurement results of the lock-in amplifier; 2) In the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and the optical elements of the frequency doubling processing light path are fine-tuned so that the light intensity difference between the probe pulse and the adjacent marker pulse does not exceed one percent.

6. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 4, characterized in that: The pulse laser light source is a femtosecond laser, at least one aperture is provided in the frequency doubling processing optical path, and before using the second working mode to detect the terahertz pulse time domain spectrum, each optical element in the terahertz pulse generating optical path and the terahertz pulse detecting optical path and their installation positions are calibrated, including the following steps: 1) In the first working mode, calibrating each optical element and its installation position in the terahertz pulse generation optical path and the terahertz pulse detection optical path according to the actual measurement results of the lock-in amplifier; 2) in the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode remain unchanged, and the beam splitter, the delay line device and the beam combiner and their installation positions of the dual-mode probe optical path that perform frequency doubling processing are calibrated according to the actual measurement results of the lock-in amplifier at an auxiliary calibration frequency; wherein the auxiliary calibration frequency is a frequency between the first frequency and the second frequency; and 3) In the second working mode, the optical elements and their installation positions shared by the two modes after calibration in the first working mode are kept unchanged, and the calibration results of the beam splitter, the delay line device and the beam combiner and their installation positions under the auxiliary calibration frequency are used as the starting position, and then the delay line device is fine-tuned. At the same time, the relative light intensity of the two sub-beams after splitting by the beam splitter is fine-tuned through the at least one aperture, and the phase-locked amplifier is used to detect the alternating information of the probe pulse relative to the marker pulse that appears periodically at the repetition frequency until the alternating information is less than a preset threshold, and the fine-tuning is ended.

7. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to any one of claims 1 to 6, characterized in that: The terahertz pulse generating optical path comprises a variable displacement stage and a reflecting mirror group installed on the variable displacement stage; The detection and control circuit adjusts the time delay of the terahertz pulse relative to the detection beam by adjusting the variable displacement stage, thereby measuring the electric field signal of the terahertz pulse corresponding to different time delays, thereby obtaining the terahertz pulse time domain spectrum.

8. The high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system according to claim 7, characterized in that: The lock-in amplifier is a high-frequency digital lock-in amplifier, which includes: an analog differential circuit, an analog filtering circuit, an analog amplifying circuit and a digital lock-in measuring device, wherein the digital lock-in measuring device includes an analog-to-digital conversion unit and an anti-noise lock-in measuring unit.

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