A high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system

Abstract

The present application relates to a kind of high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system, it includes: terahertz pulse generation optical path, terahertz pulse detection optical path and detection and control circuit based on phase-locked amplifier, in terahertz pulse generation optical path, original detection light beam is obtained the detection light beam of actual incidence nonlinear electro-optic crystal by one frequency multiplication processing optical path;The frequency multiplication processing optical path includes beam splitter, delay line device and beam combiner;The detection light beam has probe pulse and adjacent mark pulse;The electrical signal output by the phase-locked amplifier detection optical path detects: probe pulse relative to mark pulse, with the alternating information of the periodic appearance of the repetition frequency, and then according to the alternating information obtains the terahertz pulse time-domain spectroscopy.The present application carries out beam splitting-delay-beam combining processing to detection light beam, utilizes the measurement principle of phase-locked amplifier, excludes the system noise introduced in the propagation process of detection light beam, to improve signal-to-noise ratio.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz time-domain spectroscopy and weak signal detection, in particular, the present application relates to a high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system. BACKGROUND

[0002] Terahertz waves usually refer to electromagnetic waves in the frequency range from 0.1 THz to 10 THz (1 THz = 10 12 Hz) in the electromagnetic spectrum, and the wavelength range is from 30 μm to 3 mm. Compared with traditional light sources, terahertz pulse light sources have many unique properties, including transient, broadband, coherence, low energy, etc. Specifically, the typical pulse width of terahertz pulse is below the picosecond level, which has transient property, and this property can facilitate the time-resolved study of various materials (including liquid, semiconductor, superconductor, biological sample, etc.). By using the coherence of terahertz waves, through coherent sampling measurement technology, the interference of background radiation noise can be effectively suppressed. In addition, the frequency band of a single terahertz pulse can cover from GHz to dozens of THz, which is convenient for analyzing the spectral properties of materials in a large range. Moreover, the energy of terahertz photons is only a few millielectron volts, which has lower energy than X-rays, and will not destroy the detected material due to ionization.

[0003] Before the 1980s, because of the lack of efficient terahertz radiation sources and effective terahertz detection means, people's understanding of the information related to this special waveband of electromagnetic spectrum was very limited. However, in recent years, with the rapid development of ultra-short pulse laser technology, nonlinear optical technology and semiconductor technology, the coherent generation and coherent measurement technology of terahertz radiation has become mature, and then the terahertz time-domain spectroscopy (THz-TDS) technology has been developed. At present, terahertz waves have been widely studied and applied in various fields, and the terahertz time-domain spectroscopy detection technology has great development potential and application value in many fields including biomedical diagnosis, broadband terahertz communication, non-destructive testing of materials, etc., and has a strong driving effect on scientific research, national economy 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 beam splitting from the same pulsed laser source. The pump beam irradiates on the terahertz emitter (such as a photoconductive antenna), and the generated short electromagnetic radiation has a duration of about picoseconds and a frequency of about 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-optic crystal, which changes the ellipsoidal surface of the refractive index of the electro-optic crystal. The linearly polarized probe beam propagates in the crystal in the same direction as the terahertz beam, and the phase of the probe beam is modulated by the refractive index of the crystal, which has been changed by the electric field of the terahertz pulse. This phase change is converted into intensity change by a Wollaston prism. A pair of differential balanced detectors is used to compress the background noise. The time delay of the terahertz pulse relative to the probe pulse is changed by an adjustable delay line device, and the terahertz electric field waveform is obtained by scanning the time delay, and then the terahertz pulse time-domain spectrum is obtained.

[0005] The TDS signal-to-noise ratio is an important indicator of a terahertz time-domain spectroscopy detection system. The higher the TDS signal-to-noise ratio, the clearer the terahertz time-domain spectrum signal of the measured sample and the easier to distinguish the spectral characteristics, thereby improving the identification accuracy and efficiency of 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 the modulation is directly performed on the terahertz antenna (such as by high-frequency switching of the photoconductive antenna to realize the modulation of the terahertz beam), there is a 10MHz high-power driving condition limitation, and it is difficult to further improve the modulation and demodulation frequency of the terahertz time-domain spectroscopy detection system.

[0006] On the other hand, modulation techniques other than photoconductive switches also have their own shortcomings. For example, a technology based on an acousto-optic modulator for pulse extraction published by the Japan Institute of Physical and Chemical Research (Rev Sci Instrum 2023, 94, 043002) has a modulation and demodulation frequency of at most half of the laser repetition frequency (for a typical 80MHz repetition frequency, the highest TDS modulation frequency is 40MHz) due to the integer frequency division of at least 2. If there is a way to further improve 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

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

[0009] The present application provides a terahertz pulse time-domain spectroscopy detection system with high signal-to-noise ratio, which comprises: a terahertz pulse generation light path, which generates terahertz pulses by exciting a light guide antenna with a pump light beam, and focuses the terahertz pulses to a measured sample placed on a sample stage; a terahertz pulse detection light path, which collects the terahertz pulses carrying information of the measured sample and focuses them to a nonlinear electro-optic crystal for electro-optic sampling based on a probe light beam; wherein the probe light beam irradiates the same position of the nonlinear electro-optic crystal at the same incident angle as the terahertz pulses to achieve the electro-optic sampling; the electro-optically sampled light beam emitted by the nonlinear electro-optic crystal passes through a 1 / 4 wave plate and a Wollaston prism, and is received by a balanced detector pair and converted into an electrical signal; and a detection and control circuit for measuring the electrical signal output by the balanced detector pair and obtaining the terahertz pulse time-domain spectrum carrying information of the measured sample; and for controlling the terahertz pulse generation light path and the terahertz pulse detection light path. The feature is that in the terahertz pulse generation light path, the pump light beam and an original probe light beam are obtained by splitting from the same pulsed laser source, the original probe light beam passes through a frequency doubling processing light path to obtain the probe light beam actually incident on the nonlinear electro-optic crystal; the frequency doubling processing light path comprises a beam splitter, a delay line device and a beam combiner; the beam splitter is used to split the original probe light beam into the same first and second sub-beams, 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 probe light beam; the beam combiner is used to combine the first and second sub-beams, so that the probe light beam has a probe pulse and a marker pulse adjacent thereto; wherein the probe pulse and the terahertz pulse act on the nonlinear electro-optic crystal at the same time, and the time period of the marker pulse passing through the nonlinear electro-optic crystal is staggered with the time period of the terahertz pulse acting on the nonlinear electro-optic crystal; the detection and control circuit comprises a phase-locked amplifier, the pulsed laser source is an ultrashort pulse laser, which outputs an electrical signal with the same repetition frequency as the emitted laser pulse and is synchronized, and inputs the synchronized electrical signal 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 the alternating information of the probe pulse relative to the marker pulse, which appears periodically at the repetition frequency, from the electrical signal output by the balanced detector pair, and further obtains the terahertz pulse time-domain spectrum according to the alternating information.

[0010] The frequency doubling optical path further 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 operating mode and a second operating mode. In the first operating mode, the frequency doubling optical path directly guides the original detection beam into the nonlinear electro-optic crystal. In the second operating mode, the frequency doubling optical path performs frequency doubling processing on the original detection beam to obtain the detection beam with the probe pulse and the marker pulse; the first optical element group is used to directly incident the original detection beam onto the nonlinear electro-optic 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-optic crystal.

[0011] In the first operating mode, in the terahertz pulse generation optical path, the pump beam is modulated by a chopper or photoconductive switch with a first frequency to obtain a pump laser pulse with the first frequency. The bias electric field of the optical guide antenna is driven by a DC signal. The lock-in amplifier uses a signal generated internally with the first frequency and a set waveform as a reference signal to measure the electrical signal output by the balanced detector and obtain the time-domain spectrum of the terahertz pulse based on the electro-optic modulation principle.

[0012] In the second operating mode, the pulsed laser source is implemented using an ultrashort pulse laser, the repetition frequency of the ultrashort pulse laser is a second frequency, and the pump beam with the second frequency is incident on the optical guide 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 time-domain spectrum of terahertz pulses, the optical components 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 components and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured results of the lock-in 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 are kept unchanged, and the optical components of the frequency doubling processing optical path are finely adjusted so that the light intensity difference between the probe pulse and the adjacent marker pulse does not exceed one percent.

[0014] Wherein, the pulsed laser source is a femtosecond laser, and at least one aperture is provided in the frequency doubling optical path. Before using the second working mode to detect the time-domain spectrum of the terahertz pulse, the optical components 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 components and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured results of the lock-in 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 are kept unchanged, and at an auxiliary calibration frequency, the beam splitter and the optical components of the dual-mode probe optical path that undergo frequency doubling are calibrated according to the measured results of the lock-in amplifier. The delay line device and the beam combiner and their mounting 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 mounting positions shared by the two modes after calibration in the first working mode are kept unchanged, the calibration results of the beam splitter, the delay line device and the beam combiner and their mounting positions at 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 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 lock-in amplifier until the alternating information is less than a preset threshold, and the fine-tuning ends.

[0015] The terahertz pulse generating optical path includes a variable displacement stage and a mirror assembly mounted 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, and thus obtaining the time-domain spectrum of the terahertz pulse.

[0016] The lock-in amplifier is a high-frequency digital lock-in amplifier, which includes: an analog differential circuit, an analog filter circuit, an analog amplifier circuit, and a digital lock-in measurement device. The digital lock-in measurement device includes an analog-to-digital conversion unit and an anti-noise lock-in measurement unit.

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

[0018] 1. This application performs beam splitting-delay-combining processing on the probe beam to obtain a probe beam with probe pulses and marker pulses. Furthermore, it cleverly utilizes the measurement principle of a lock-in amplifier to eliminate system noise introduced during the probe beam propagation, thereby improving the signal-to-noise ratio (SNR) of terahertz pulse time-domain spectroscopy detection. Specifically, this invention can further shorten the lock-in integration time while maintaining a sufficient SNR, or further improve the SNR within the same integration time.

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

[0020] 3. In some embodiments of this application, the problem of simultaneous spatial and temporal overlap of beams in the repetition frequency direct-locked mode is overcome or alleviated by electro-optic modulation and repetition frequency direct-locked dual-mode mutual calibration, thereby improving the beam quality of the repetition frequency direct-locked probe light and making the actual signal-to-noise ratio closer to its theoretical value.

[0021] 4. In some embodiments of this application, a dual-mode measurement scheme of electro-optic modulation and repetition frequency direct locking is provided. The working mode can be flexibly selected under different working scenarios and the applicable modulation and demodulation frequency determined by the optoelectronic components, so as to take into account the advantages of the two detection modes and obtain better signal-to-noise ratio and / or dynamic range.

[0022] 5. Compared with laser pulse extraction, this application eliminates the acousto-optic modulator element, thus making the optical path simpler and easier to maintain. Attached Figure Description

[0023] Figure 1 A schematic diagram showing the optical path design and electrical connections of a terahertz time-domain spectroscopy detection system according to one embodiment of this application is provided.

[0024] Figure 2 This diagram illustrates two possible mismatch patterns that may occur when the laser pulse repetition frequency is doubled.

[0025] Figure 3 This paper shows an oscilloscope measurement image of the probe beam after beam combining, obtained after adjustment by the frequency doubling processing circuit in one embodiment of this application; the figure shows dV p / V p-mean The value represents the ratio of the deviation between the pulses with the largest deviation to the average value of the pulses, where f represents the pulse repetition frequency and t on the horizontal axis represents time. Detailed Implementation

[0026] To better understand this application, various aspects of this application will be described in more detail with reference to the accompanying drawings. It should be understood that these detailed descriptions are merely illustrative of exemplary embodiments of this application and are not intended to limit the scope of this 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 terms "first," "second," etc., are used only to distinguish one feature from another and do not imply any limitation on the features. Therefore, without departing from the teachings of this application, the first subject discussed below may also be referred to as the second subject.

[0028] In the accompanying drawings, the thickness, size, and shape of the objects have been slightly exaggerated for ease of illustration. The drawings are for illustrative purposes only and are not drawn to scale.

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

[0030] As used herein, the terms “basically,” “approximately,” and similar terms are used as terms of approximation rather than terms of degree, and are intended to describe inherent biases in measured or calculated values ​​that will be recognized by those skilled in the art.

[0031] Unless otherwise specified, all terms used herein (including technical terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms (e.g., those defined in common dictionaries) shall be interpreted as having the meaning consistent with their meaning in the context of the relevant art and shall not be interpreted in an idealized or overly formal sense unless expressly so specified herein.

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other.

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] According to an embodiment of the present invention, a high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system is provided, comprising: a terahertz pulse generation optical path, a terahertz pulse detection optical path, and a detection and control circuit. The terahertz pulse generation optical path uses a pump beam to excite a photoconductive antenna to generate terahertz pulses and focuses the terahertz pulses onto a sample placed on a sample stage. The terahertz pulse detection optical path collects the terahertz pulses carrying information about the sample and focuses them onto a nonlinear electro-optic crystal for electro-optic sampling based on the probe beam. The probe beam illuminates the same position of the nonlinear electro-optic crystal at the same incident angle as the terahertz pulse to achieve the electro-optic sampling. The electro-optically sampled beam emitted from the nonlinear electro-optic crystal passes through a quarter-glass slide and a Wollaston prism, is received by a balanced detector pair, and converted into an electrical signal. The detection and control circuit measures the electrical signal output by the balanced detector pair to obtain the terahertz pulse time-domain spectrum carrying information about the sample; and controls the terahertz pulse generation optical path and the terahertz pulse detection optical path.

[0035] In this embodiment, the terahertz pulse generation optical path is obtained by splitting the pump beam and an original probe beam from the same pulsed laser source. The original probe beam is then processed by a frequency doubling optical path to obtain the probe beam that is actually incident on the nonlinear electro-optic crystal. The frequency doubling optical path includes a beam splitter, a delay line device, and a beam combiner. The beam splitter is used to split the original probe beam into identical first sub-beams and second sub-beams (identical here means at least that the power and polarization direction are the same). The delay line device is used to delay the second sub-beam relative to the first sub-beam for a preset time, which is half a cycle of the original probe beam. The beam combiner is used to combine the first sub-beam and the second sub-beam, so that the probe beam has a probe pulse and an adjacent marker pulse (a marker pulse used as an internal reference). The probe pulse and the terahertz pulse act on the nonlinear electro-optic crystal at the same time (i.e., the probe pulse and the terahertz pulse act synchronously on the nonlinear electro-optic crystal), while the time period during which the marker pulse passes through the nonlinear electro-optic crystal is staggered from the time period during which the terahertz pulse acts on the nonlinear electro-optic 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 lock-in amplifier. The pulsed laser source is an ultrashort pulse laser, which outputs an electrical signal with the same repetition frequency and synchronized with the emitted laser pulse. This synchronized electrical signal is input to the lock-in amplifier as a reference signal. The lock-in amplifier measures the electrical signal output by the balanced detector pair based on the externally input reference signal. Furthermore, the lock-in 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. Based on this alternating information, the time-domain spectrum of the terahertz pulse is derived. The lock-in amplifier is highly sensitive to changes between adjacent pulses, especially periodically occurring alternating information, making it well-suited for capturing such weak signals.

[0037] In reality, the probe pulse is introduced 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 beam in a non-vacuum environment will inevitably introduce some noise. However, with the addition of the marker pulse, since both the marker pulse and the probe pulse sub-beams pass through the same optical path at the same time, the noise introduced by the two is highly consistent. Here, the time interval between adjacent marker pulses and probe pulses is extremely short, and for various environmental variables in the optical path (such as gas concentration, temperature, humidity, etc.), such a short time interval can be approximated as the same moment.

[0038] Lock-in amplifiers can precisely capture minute changes in the probe pulse relative to the marker pulse, and these minute changes contain terahertz time-domain spectral information. Furthermore, this terahertz time-domain spectral information has eliminated the system noise introduced by the probe pulse in the optical path, thus achieving a higher signal-to-noise ratio. Because the marker pulse and the probe pulse actually traverse the same optical path, and the marker pulse carries the same system noise, the lock-in amplifier actually captures the minute changes in 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 existing measurement schemes, the signal measured by the lock-in amplifier actually includes system noise introduced by the propagation of the probe beam in the optical path. Therefore, the solution of this 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 operating mode and a second operating mode. The frequency doubling processing optical path also includes a first optical element group and a switching device.

[0041] In the first operating mode, the frequency doubling optical path directly guides the original probe beam into the nonlinear electro-optic crystal. In the second operating mode, the frequency doubling optical path performs frequency doubling on the original probe beam to obtain the probe beam with the probe pulse and the marker pulse. The first optical element group is used to directly incident the original probe beam onto the nonlinear electro-optic 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 of the original probe beam to the nonlinear electro-optic crystal.

[0042] Further, in the first operating mode, in the terahertz pulse generation optical path, the pump beam is modulated using a chopper or photoconductive switch with a first frequency to obtain a pump laser pulse with the first frequency. The bias electric field of the optical guide antenna is driven by a DC signal. The lock-in amplifier uses a signal generated internally with the first frequency and a set waveform as a reference signal to measure the output electrical signal of the balanced detector and obtain the time-domain spectrum of the terahertz pulse based on the electro-optic modulation principle. In the second operating mode, the pulsed laser source is implemented using an ultrashort pulse laser. The repetition frequency of the ultrashort pulse laser is a second frequency. The pump beam with the second frequency is incident on the optical guide antenna to excite the terahertz pulse. The second frequency can be much greater than the first frequency. For example, the second frequency can be 80 MHz, while the first frequency is usually no more than 10 MHz. For example, in one instance, the first frequency can be 1 MHz.

[0043] Further, refer to Figure 1 According to one embodiment of this application, a high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system is provided, comprising 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 the main optical path, which includes an ultrashort pulse laser, a first beam splitting unit, a fixed mirror group, a variable displacement stage, a movable mirror 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 quarter-glass slide (QWP), a Wollaston prism (WP), and a balanced detector pair (BT). The frequency doubling processing optical path is used to generate a probe beam, which illuminates the same position of the nonlinear electro-optic crystal at the same incident angle as the terahertz pulse, thereby achieving electro-optic sampling. In this embodiment, the frequency doubling processing optical path has two operating modes.

[0044] The system consists of an ultrashort pulse laser emitting laser pulses, which are split into two co-source ultrashort pulse laser beams by the first beam-splitting unit. One beam, with higher power, is called the pump beam and serves as the source of the main optical path. The other beam, with lower power, is called the probe beam. The pump beam passes through the second reflector M2, the movable reflector group MS, and the third reflector M3, and is incident on the optical guide antenna PCA. The optical guide antenna PCA receives the pump beam and radiates terahertz electromagnetic waves under the influence of a bias electric field. These terahertz electromagnetic waves are received by the parabolic mirror group PBM and transmitted to the nonlinear optical crystal NLC. The sample stage is positioned between the multiple parabolic mirrors of the parabolic mirror group PBM to fix the sample under test. In this embodiment, the PBM (Polarized Mirror Assembly) has four parabolic mirrors. The terahertz electromagnetic waves radiated by the pump beam exciting the optical guide antenna are collected and collimated by the first parabolic mirror. The terahertz electromagnetic waves reflected by the first parabolic mirror are transmitted parallel to the second parabolic mirror. The second parabolic mirror reflects and focuses the terahertz electromagnetic waves onto the sample stage. After penetrating the sample under test, the focused terahertz electromagnetic waves are collected and reflected by the third parabolic mirror into a parallel propagation state, and then reflected and focused by the fourth parabolic mirror onto the nonlinear optical crystal (NLC). At this point, the terahertz electromagnetic waves focused onto the NLC carry the information of the sample under test. By aligning the probe beam and the terahertz electromagnetic waves carrying the information of the sample under test on the same optical axis and illuminating the same area of ​​the NLC, and by adjusting the time delay of the probe beam relative to the pump beam based on the electro-optic effect of the NLC, the terahertz electromagnetic waves carrying the information of the sample under test can be demodulated and sampled using the probe beam. The beam emitted from the nonlinear optical crystal (NLC) passes sequentially through a quarter-glass slide (QWP) and a Wollaston prism (WP), and is received by a balanced detector (BT) and converted into an electrical signal. The electrical signal output by the balanced detector to the BT, carrying information about the sample under test, may be very weak. In this embodiment, the balanced detector performs differential measurements on the two electrical signals output from the BT to a lock-in amplifier. That is, the two electrical signals are differentially divided and used as the test signal for the lock-in amplifier. Correlation is then performed between this signal and a reference signal to obtain precise values ​​for the phase, amplitude, and frequency of the test signal.

[0045] On the other hand, the frequency doubling processing optical path in this embodiment includes a fixed reflector group, a time delay mirror group, two beam splitters, and a beam splitter switching unit. The beam splitter switching unit can remove one 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 this application has two operating modes: electro-optic modulation mode and repetition frequency direct-lock mode. The beam splitter switching unit allows switching between the two operating modes. In the repetition frequency direct-lock mode, the frequency doubling processing optical path performs frequency doubling processing on the original probe beam. In the electro-optic modulation mode, the frequency doubling processing optical path degenerates into an optical path guiding the original probe beam, and no longer performs frequency doubling processing on the original probe beam. In another embodiment, another switching device can be used to replace the beam splitter switching unit. The switching device may include, for example, a movable light-shielding plate. In the repetition frequency direct-lock mode, the light-shielding plate is placed outside the optical path, allowing the two sub-beams to be combined. In the electro-optic 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 of the actual incident nonlinear electro-optic crystal.

[0046] In this embodiment, the detection circuit is based on a high-frequency lock-in amplifier, which has the function of detecting weak electrical signals. The high-frequency lock-in amplifier can be a high-frequency digital lock-in amplifier, which may include: an analog differential circuit, an analog filtering circuit, an analog amplification circuit, and a digital lock-in measurement device. The digital lock-in measurement device includes an analog-to-digital conversion unit and a noise-resistant lock-in measurement unit. The noise-resistant lock-in measurement unit can be implemented based on the scheme described in Chinese Patent CN201110380805.X. However, it should be noted that in the electro-optic modulation mode, the noise-resistant lock-in measurement unit uses an internally generated reference signal with a set frequency. This set frequency is consistent with the frequency of the pump beam of the incident optical guide antenna. In the repetition frequency direct lock-in mode, the noise-resistant lock-in measurement unit receives the synchronization electrical signal of an externally input ultrashort pulse laser as a reference signal. Based on the reference signal and the input measured signal, the noise-resistant lock-in measurement unit searches for the phase-frequency combination with the highest correlation in the phase space and frequency space, thereby obtaining the accurate 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, under electro-optic modulation mode, the bias electric field of the photoconductor antenna PCA is driven by a DC signal. A pump laser pulse of the desired frequency is generated through high-frequency switching via a chopper or by controlling a photoconductor switch, and then incident on the photoconductor antenna PCA to excite a terahertz pulse. By adjusting the time delay of the probe beam relative to the pump beam, the terahertz electromagnetic wave carrying the information of the sample under test can be sampled and measured at the nonlinear optical crystal NLC. The reference signal of the lock-in amplifier uses an internally generated reference signal, based on the frequency of the terahertz pulse (actually the pulse frequency of the pump beam incident on the photoconductor antenna PCA). An internal reference signal with the corresponding frequency and waveform is generated internally by the lock-in amplifier and then correlated with the measured differential signal carrying the information of the sample under test (i.e., the signal to be measured output by the balanced detector to the BT), thereby obtaining the precise values ​​of the phase, amplitude, and frequency of the signal under test.

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

[0049] In some embodiments of the present invention, when performing frequency doubling processing on the original probe beam, it is necessary to ensure that the probe pulse and the marker pulse in the combined probe beam are as consistent as possible. In particular, the intensity difference between the probe pulse and its adjacent marker pulse should not exceed a preset threshold. In one embodiment, before using the second operating mode to detect the terahertz pulse time-domain spectrum, the optical elements and their mounting 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 operating mode, the optical elements and their mounting positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured results of the lock-in amplifier; 2) In the second operating mode, the optical elements and their mounting positions shared by the two modes after calibration in the first operating mode remain unchanged, and the optical elements of the frequency doubling processing optical path are fine-tuned so that the intensity difference between the probe pulse and its adjacent marker pulse does not exceed one percent. After fine-tuning, the sample to be tested is placed in the sample stage, and the terahertz pulse time-domain spectrum carrying the information of the sample to be tested is measured by the lock-in amplifier in the repetition frequency direct-lock mode (second operating mode). like Figure 3 As shown, after the above two adjustments, the intensity difference between the measured probe pulse and its adjacent marker pulse output by the oscilloscope can be reduced to within one percent. It should be noted that although...Figure 3 The amplitude tolerance of the displayed signal is within 3%. However, this includes the inherent tolerances of different laser pulses themselves. Specifically, although the laser pulses are continuously repeated, each pulse may actually have slight differences. Therefore, the 3% tolerance actually incorporates the differences between different laser pulses. This invention is actually more concerned with minimizing the difference between the probe pulse and the marker pulse after beam splitting. Ideally, the marker pulse should be a perfect copy of the probe pulse; of course, this is practically impossible, so we can only minimize the difference between them. Based on... Figure 3 The measured waveform of the probe beam after beam combining, and the light intensity difference between the probe pulse and its adjacent marker pulse, can be reduced to less than one percent.

[0050] Furthermore, assuming the use of more refined fine-tuning techniques, it might be possible to further reduce the light intensity difference between the probe pulse and the marker pulse to within one-thousandth. However, for situations requiring a light intensity difference of less than one-thousandth, directly measuring the light intensity of the probe beam might cause the measuring equipment itself to affect the optical path of the terahertz pulse time-domain spectroscopy detection system, thus impacting the measurement accuracy. Therefore, the following embodiment provides a scheme for fine-tuning using the measured results of a lock-in amplifier (e.g., fine-tuning the aperture of one of the sub-beams). This scheme directly uses the optical path of the terahertz pulse time-domain spectroscopy detection system and the lock-in amplifier to fine-tune the frequency doubling processing optical path, minimizing the tolerance (especially the light intensity tolerance) between the probe pulse and the marker pulse.

[0051] Specifically, in another embodiment, the pulsed laser source is a femtosecond laser, and at least one aperture is provided in the frequency doubling optical path. Before using the second working mode to detect the time-domain spectrum of the terahertz pulse, the optical components 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 components and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured results of the lock-in 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 are kept unchanged, and at an auxiliary calibration frequency, the optical components of the dual-mode probe optical path undergoing frequency doubling are calibrated according to the measured results of the lock-in amplifier. The beam splitter, the delay line device, and the beam combiner, along with their installation positions, are calibrated. The auxiliary calibration frequency is a frequency between the first frequency and the second frequency. In the second operating mode, the optical elements shared by both modes after calibration in the first operating mode and their installation positions remain unchanged. The calibration result of the beam splitter, the delay line device, and the beam combiner, along with their installation positions, at the auxiliary calibration frequency is used as the starting position. Then, the delay line device is fine-tuned. Simultaneously, the relative light intensity of the two sub-beams after beam splitting by the beam splitter is fine-tuned using at least one aperture. The alternating information of the probe pulse relative to the marker pulse, appearing periodically at the repetition frequency, is detected using the lock-in amplifier until the alternating information is less than a preset threshold, at which point the fine-tuning ends. If the information is less than the preset threshold, zeroing is considered complete. During zeroing, no sample is placed on the sample stage. Neither the probe pulse nor the marker pulse carries terahertz information. After fine-tuning, the sample to be tested is placed in the sample stage, and the terahertz pulse time-domain spectrum carrying the information of the sample to be tested is measured by a lock-in amplifier in the repetition frequency direct lock-in mode (second working mode).

[0052] Here, the auxiliary calibration frequency is a relatively low frequency (compared to the operating frequency of the second working mode, i.e., the second frequency). At a lower repetition frequency, the relative errors in intensity and time delay between the two sub-beams after beam splitting are smaller. To briefly explain: when a cycle length is 10ns (corresponding to a lower repetition frequency; note that this value is only used as an illustrative example of the calibration principle and does not represent actual usage values), a 10ps time delay error is one-thousandth of a cycle length. However, if a cycle length is 1ns (corresponding to a higher repetition frequency), the same 10ps time delay error is one-hundredth of a cycle length, making mismatch problems more likely. Therefore, it can be assumed that at a lower repetition frequency, adjusting the components in the self-frequency doubling auxiliary optical path (beam splitter, delay device, beam combiner, and related apertures) will be relatively easier and faster, allowing for quicker adjustment of the optical path to a mismatch-free state for the combined probe beam. Then, the repetition frequency can be increased to its limit or near limit. Based on the adjustment results at the lower frequency (i.e., using these results as a coarse adjustment step in the probe beam mismatch solution at higher repetition frequencies), the delay line device and associated aperture can be further fine-tuned. This fine-tuning step size can be much smaller than the adjustment step size in the previous step (coarse adjustment). Based on the measured results of the lock-in amplifier, the fine-tuning result that optimizes (or sufficiently optimizes) the measured results is obtained. Finally, based on the fine-tuning result, the actual sample under test is probed at a preset high repetition frequency, either at or near the limit.

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

[0054] like Figure 1As shown, the system uses a laser with a pulse width of 10-200 femtoseconds and a repetition frequency of 80MHz. The laser output is split into two beams: Pump and Probe. The Pump beam is generated into a terahertz wave by a variable delay stage and an optical guide antenna, and then focused onto the detector crystal after passing through a set of four parabolic mirrors. The Probe beam is split and regenerated at the midpoint of a 12.5ns period by a fixed delay line of 6.25ns. After being combined with another beam, the repetition frequency is doubled. After reflection and collimation, the Probe beam acts on the detector crystal together with the aforementioned terahertz light output port. Due to the terahertz rectification electro-optic effect in the detector crystal, the output visible light of the detector crystal carries terahertz spectral information. Furthermore, the subsequent setup of a quarter-glass slide, a Wollaston prism, and a balanced detector allows for the detection of the presence and intensity of terahertz waves with a high dynamic range. This optical path involves signal detection above 10MHz, using a Sion OE2052 high-frequency lock-in amplifier (300MHz bandwidth) as the weak signal detection instrument. Among them, terahertz generation and detection are achieved using optical guide antennas and ZnTe electro-optic crystals, respectively, taking advantage of higher radiation power and faster and richer detection degrees of freedom.

[0055] The switching between the two operating modes in the optical path involves several key aspects. The external reference for the laser's repetition frequency input phase-locked loop is only used during direct repetition frequency locking, while during electro-optic modulation, it is replaced by an internal reference set at the internal modulation frequency. The optical guide antenna driver uses DC constant potential drive during direct repetition frequency locking, but switches to AC drive during electro-optic modulation, with the AC power supply trigger signal connected from the output signal of the lock-in amplifier. When switching from direct repetition frequency locking mode to electro-optic modulation mode, the BS beam splitter block the delayed optical path, using only the undelayed probe light for electro-optic sampling. To facilitate optimization of the terahertz signal, the PCA and NLC should be fixed on adjustable displacement stages of 2D horizontal direction perpendicular to the light beam and 1D vertical direction along the light beam propagation, respectively. To increase the stability of the optical path and the success rate of adjustment, collimation and an aperture should also be added to the laser output light.

[0056] In one embodiment of this application, the optical guide antenna is adjusted as follows: the optical path from M2 to PCA is adjusted so that the laser spot hits the middle of the optical guide antenna. If necessary, a focusing convex lens should be added in front of the antenna to focus the pump light into the middle of the slit of the optical guide antenna. The antenna is viewed using the ohm setting of a multimeter, and the displacement stage is adjusted at the same time. If the resistance of the optical guide antenna is observed to drop from more than 10 megohms to the lowest level (about 60 kilohms), it indicates that the optical guide antenna is working normally and is properly positioned.

[0057] In one embodiment of this application, the self-frequency doubling and beam combining in the detection optical path of the system is achieved by adjusting the terahertz generation and detection optical paths respectively, so that the light rays meet at the center of the NLC. The terahertz generation optical path is originally invisible to the naked eye, but it can be checked whether the pump light coincides with the detection light at the NLC by removing the terahertz pulse generation module and allowing the red laser, which is the light source, to pass directly through the polished mirror group.

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

[0059] Step 1: Connect the detector outputs to the in+ ports of the first and second channels of the phase-locked loop, respectively. Use single-ended detection mode for both channels and adjust the range to 100mV. Under the external reference triggered by the laser repetition frequency, observe the output signal strength of the two detectors simultaneously.

[0060] Step 2: Adjust the beam space angle and aperture to make the beam hit the center of the detector exactly, and make the apertures of the two detectors as small as possible and the light intensity roughly the same.

[0061] Step 3: Block the terahertz optical path and adjust the 1 / 4 glass slide so that the difference in readings between the two photodetectors does not exceed 0.1%.

[0062] Step 4: After the balance adjustment is completed, before scanning the TDS signal, the outputs of a pair of detectors should be connected to the In+ / In- ports of the first phase-locked channel, respectively. Switch back to differential input mode. In the reference mode, use the internal reference and the external reference in the electro-optic modulation and repetition frequency direct-locked mode, respectively. Also, confirm that the range has been adjusted back to 2mV.

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

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

[0065] The difficulties include:

[0066] 1. After the components in the optical path with a 6.25 nanosecond delay are combined, a 160 MHz signal is observed, instead of two 80 MHz misalignments (common pulse misalignments caused by lateral delay and longitudinal intensity, which cannot be frequency-doubled for synthesis). Figure 2 (as shown);

[0067] 2. Due to the long optical path delay, it is difficult for two beams of light to coincide spatially when they arrive at the electro-optic crystal.

[0068] 3. Based on the spatiotemporal overlap, the optical polarization of the two beams should also be consistent; otherwise, the signal-to-noise ratio will not be high.

[0069] To address the difficulty of simultaneous spatial and temporal overlap of laser beams, the specific requirements for laser and optical path adjustment can include the following four aspects:

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

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

[0072] 3. Polarizability > 500:1;

[0073] 4. Latency accuracy <1ps.

[0074] In one embodiment of this application, the variable time-delay stage is centered, and the optical paths of the terahertz and probe beams are initially measured. The paths from the beam splitter at the laser output to the probe crystal should be approximately equal. For example, if the probe beam is 6.5 cm shorter, the variable displacement stage should be reduced. When it becomes difficult to approach the stage's limit, the fixed time-delay mirror pair should be lengthened by 3.25 cm, and a pair of apertures should be used to ensure the optical path of the moving time-delay mirror remains unbiased.

[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 it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high signal-to-noise ratio terahertz pulse time-domain spectroscopy detection system, comprising: The terahertz pulse generation optical path is generated by a pump beam exciting a photoconductor antenna to generate terahertz pulses, and the terahertz pulses are focused onto the sample to be tested placed on the sample stage. A terahertz pulse detection optical path collects terahertz pulses carrying information about the sample under test and focuses them onto a nonlinear electro-optic crystal for electro-optic sampling based on a probe beam. The probe beam illuminates the same position of the nonlinear electro-optic crystal at the same incident angle as the terahertz pulse to achieve electro-optic sampling. The sampled beam emitted from the nonlinear electro-optic crystal passes through a quarter-glass slide and a Wollaston prism, is received by a balanced detector pair, and converted into an electrical signal. The detection and control circuit is used to measure the electrical signal output by the balanced detector and obtain the time-domain spectrum of the terahertz pulse carrying information about the sample under test; and to control the terahertz pulse generation optical path and the terahertz pulse detection optical path. The feature is that, in the terahertz pulse generation optical path, the pump beam and an original probe beam are obtained by splitting the same pulse laser source, and the original probe beam is processed by a frequency doubling optical path to obtain the probe beam that is actually incident on the nonlinear electro-optic crystal. The frequency doubling optical path includes a beam splitter, a delay line device, and a beam combiner. The beam splitter splits the original probe beam into two identical sub-beams: a first sub-beam and a second sub-beam. The delay line device delays the second sub-beam relative to the first sub-beam for a preset duration, which is half a cycle of the original probe beam. The beam combiner combines the first and second sub-beams, resulting in a probe beam with a probe pulse and an adjacent marker pulse. The probe pulse and the terahertz pulse act on the nonlinear electro-optic crystal at the same time, while the marker pulse passes through the nonlinear electro-optic crystal at a time interval that is offset from the time the terahertz pulse acts on the nonlinear electro-optic crystal. The detection and control circuit includes a lock-in amplifier. The pulsed laser source is an ultrashort pulse laser, which outputs an electrical signal that has the same repetition frequency and is synchronized with the emitted laser pulse. The synchronized electrical signal is input into the lock-in amplifier as a reference signal. The lock-in amplifier measures the electrical signal output by the balanced detector pair based on the externally input reference signal. The lock-in 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. The terahertz pulse time-domain spectrum is then derived based on the alternating information.

2. The high signal-to-noise ratio terahertz pulse time-domain spectral 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 guides the original detection beam into the nonlinear electro-optic 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 with the probe pulse and the marker pulse. The first optical element group is used to directly incident the original probe beam onto the nonlinear electro-optic crystal; The switching device is used to connect or remove the original probe beam from the optical path of the nonlinear electro-optic crystal via the beam splitter, the delay line device, and the beam combiner.

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

4. The high signal-to-noise ratio terahertz pulse time-domain spectral detection system according to claim 3, characterized in that, In the second operating mode, the pulsed laser source is implemented using an ultrashort pulse laser, the repetition frequency of the ultrashort pulse laser is a second frequency, and the pump beam with the second frequency is incident on the optical guide 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 spectral detection system according to claim 4, characterized in that, Before using the second operating mode to detect the time-domain spectrum of terahertz pulses, the optical components and their mounting 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 components and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured 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 optical path are finely adjusted so that the light intensity difference between the probe pulse and the adjacent mark 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 pulsed laser source is a femtosecond laser. At least one aperture is provided in the frequency doubling processing optical path. Before using the second operating mode to detect the terahertz pulse time-domain spectrum, the optical components 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 components and their installation positions in the terahertz pulse generation optical path and the terahertz pulse detection optical path are calibrated according to the measured results of the lock-in amplifier. 2) In the second operating mode, the optical components shared by the two modes after calibration in the first operating mode and their mounting positions remain unchanged. At an auxiliary calibration frequency, the beam splitter, the delay line device, and the beam combiner, which undergo frequency doubling processing in the dual-mode probe optical path, and their mounting positions are calibrated according to the measured results of the lock-in amplifier. The auxiliary calibration frequency is a frequency between the first frequency and the second frequency. 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. The calibration results of the beam splitter, the delay line device, and the beam combiner and their installation positions at the auxiliary calibration frequency are used as the starting position. Then, the delay line device is fine-tuned. 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. The alternating information of the probe pulse relative to the marker pulse, which appears periodically at the repetition frequency, is detected by the lock-in amplifier until the alternating information is less than a preset threshold, and the fine-tuning ends.

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 has a variable displacement stage and a mirror assembly mounted 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, and thus obtaining the time-domain spectrum of the terahertz pulse.

8. The high signal-to-noise ratio terahertz pulse time-domain spectral 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 filter circuit, an analog amplification circuit, and a digital lock-in measurement device. The digital lock-in measurement device includes an analog-to-digital conversion unit and an anti-noise lock-in measurement unit.

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