CN105866061B - The anticoincidence pulse detection device and anticoincidence pulse detection method of THz wave time-domain information - Google Patents

Abstract

The present invention provides a different pulse detection device and a different pulse detection method for terahertz wave time domain information. The different pulse detection device includes a femtosecond laser generating part with a certain repetition frequency, a beam splitting part, a terahertz pulse forming part, The sample holding part to be tested, the terahertz pulse collecting part, the probing laser pulse focusing part, the detecting part, the spatial optical path modulation part, and the terahertz pulse reducing part. Since the spatial optical path length modulation unit can adjust the spatial optical path difference between the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection unit at the same time is an integer multiple of the adjacent pulse interval, the method can be used in When the entire optical path system is determined, the optical path difference is adjusted in real time according to the size of the measured object, so that the design of the entire system is no longer limited by the size of the measured object, and it gets rid of the limitation of the measured object volume by the traditional terahertz time-domain spectroscopy system.

Description

Different pulse detection device and different pulse detection method for terahertz wave time domain information

technical field

The invention belongs to the field of terahertz time-domain scanning, and in particular relates to a different-pulse detection device and a different-pulse detection method for terahertz wave time-domain information.

Background technique

Due to its relatively weak photon energy, terahertz waves are called the optical band for "non-destructive detection". Since the rotational and vibrational energy levels of biomolecules (ie fingerprint spectrum) are mostly in the terahertz band, photons in this band can be used to analyze and manipulate the structure of biomolecules. Therefore, terahertz waves have important application value in biomedical imaging, detection and identification of material components. But overall, research on terahertz waves in these areas is still in the laboratory stage, and large-scale applications still face many challenges. If the power of the existing terahertz radiation source is low, it will limit the penetration of the terahertz wave to the object to be examined; the interference of the surrounding environment on the terahertz radiation, such as the absorption of water vapor; the imaging speed of the terahertz wave spectrum needs to be improved. Among them, an important factor that restricts the miniaturization and practicality of the terahertz system is the terahertz time-domain scanning method.

At present, the acquisition of terahertz wave time domain information is mainly through the linear scanning of the mechanical translation stage. This method requires that the pulsed laser output from the laser source be divided into two beams, one of which is used as pump light to irradiate the semiconductor antenna to generate terahertz radiation; the other beam of pulsed laser is irradiated to the semiconductor antenna as probe light. The spatial optical path of the detection light makes the generated terahertz pulse and the pulse of the detection light reach the detection antenna at the same time, and then use the linear scanning of the translation of the mechanical platform to complete the sampling of the time domain information of the detection light pulse to the terahertz wave, thereby restoring the Time-domain information of terahertz waves. This method can realize accurate time-domain information scanning over a long distance, but requires that the optical path difference between the pump light and the probe light must be equal to complete the time-domain information scanning, so the optical path design and construction process is extremely difficult. Larger is limited by the requirements of the optical path difference; at the same time, the requirements of the entire system for the optical path difference limit the volume of the object to be detected, thus greatly limiting its large-scale application.

Contents of the invention

The present invention is carried out to solve the above problems. It improves on the currently commonly used time-domain scanning method, and provides a device for detecting abnormal pulses of terahertz wave time-domain information and using the device to detect terahertz wave time-domain information. Different pulse detection method of information. The present invention adopts following technical scheme:

The invention provides a different pulse detection device for terahertz wave time domain information, which has: a femtosecond laser generating part with a certain repetition frequency, which outputs a femtosecond laser with a stable repetition frequency; The optical path of the femtosecond laser is used to divide the femtosecond laser into detection laser pulses and detection laser pulses perpendicular to each other; the terahertz pulse forming part is arranged upstream of the detection laser pulse optical path to generate terahertz radiation and form terahertz radiation. Hertz wave; the container for the sample to be inspected is arranged downstream of the detection laser pulse optical path, and is used to accommodate the sample to be inspected; Hertz pulses are collected; the detection laser pulse focusing part is set in the upstream of the detection laser pulse optical path, and is used to focus the detection laser pulse; the detection part detects the combined terahertz pulse and the information of the detection laser pulse at the same time; the space The optical path modulation part adjusts the spatial optical path passed by the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection part at the same time is an integer multiple of the interval between adjacent pulses; and the terahertz pulse reduction part adopts The detection laser pulse samples and scans the time-domain information of the terahertz pulse arriving at the detection part at the same time, and restores the time-domain information of the terahertz pulse.

Further, the femtosecond laser generating part with a certain repetition frequency includes: a femtosecond laser, which outputs a femtosecond laser with a certain repetition frequency, and the end of the resonant cavity is fixed with a piezoelectric ceramic for pumping light into the resonant cavity. A beat frequency signal is formed; the photodetector is set on the optical path of the femtosecond laser, captures the femtosecond laser passing through the mirror, and uses the tunable deflection signal as a reference to perform frequency mixing with the repetition frequency of the femtosecond laser detected by it , to generate an error signal representing the difference between the laser repetition frequency and the standard frequency; the low-pass filter filters out the high-frequency signal in the error signal to obtain a low-frequency error signal; the preamplifier amplifies the low-frequency error signal; the loop The controller outputs the compensation control signal after the amplified error signal enters; and modulates the boost amplifier to drive the piezoelectric ceramic to generate displacement to change the cavity length, so that the repetition frequency dynamically tracks the compensation control signal to realize the locking of the repetition frequency .

Further, the present invention also provides a method for detecting different pulses of terahertz wave time domain information, including the following steps:

Step 1, using a femtosecond laser to output a femtosecond laser with a certain repetition rate, and forming a beat frequency signal after the femtosecond laser enters a resonant cavity provided with piezoelectric ceramics;

Step 2: Use a photodetector to capture the femtosecond laser passing through the mirror, and use the tunable deflection signal as a reference to mix with the repetition frequency of the femtosecond laser detected to generate a representative frequency difference between the laser repetition frequency and the standard frequency error signal;

Step 3, using a low-pass filter to filter out the high-frequency signal in the error signal to obtain a low-frequency error signal;

Step 4, using a preamplifier to amplify the low-frequency error signal;

Step 5, using a loop controller to output a compensation control signal after the amplified error signal enters;

Step 6, using a modulated boost amplifier to drive the piezoelectric ceramic to generate displacement to change the cavity length, so that the repetition frequency can dynamically track and compensate the control signal to realize the locking of the repetition frequency;

Step 7, using a beam splitter to divide the repetition rate-locked femtosecond laser into mutually perpendicular detection laser pulses and detection laser pulses;

Step 8, using a terahertz pulse forming part to generate terahertz radiation to form a terahertz wave;

Step 9, using a terahertz pulse collection unit to converge the terahertz pulses hit on the sample to be tested and reflected by the sample to be tested;

Step 10, using a detection laser pulse focusing unit to focus the detection laser pulse passing through the lens;

Step 11, using the detection part to simultaneously detect the terahertz pulse and the information of the detected laser pulse;

Step 12, using the spatial optical path length modulation unit to adjust the spatial optical path difference between the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection unit at the same time is an integer multiple of the interval between adjacent pulses;

In step 13, the terahertz pulse restoration part is used to sample and scan the time-domain information of the terahertz pulse arriving at the detection part simultaneously with the detection laser pulse, and restore the time-domain information of the terahertz pulse.

In addition, the target pulse of the terahertz wave pulse reaching the detection part is the n-mth one, the target pulse of the detection laser pulse reaching the detection part is the n-wth one, and the two pulses reach the optical path of the detection part at the same time The conditions to be met are:

Δ=(|(n-m)-(n-w)|)c*k/f

Among them, n is an integer, representing the first pulse after passing through the beam splitter; c is the propagation speed of the laser, that is, the speed of light; k is the refractive index of the medium; f is the repetition frequency of the femtosecond laser; n-m represents the terahertz optical path where the pulse reaching the detection part is the mth pulse after passing through the beam splitter; n-w means that the pulse arriving at the detection part in the reference laser pulse optical path is the wth pulse after passing through the beam splitter;

Invention function and effect

According to the different pulse detection device of terahertz wave time domain information and the different pulse detection method of terahertz wave time domain information provided by the present invention, since the spatial optical path modulation unit can adjust the spatial optical path of the terahertz pulse and the detection laser pulse difference, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection part at the same time is an integer multiple of the adjacent pulse interval, so this method can adjust the optical path difference in real time according to the size of the object under the condition that the entire optical path system is determined, so that the entire The design of the system is no longer limited by the size of the measured object, and it gets rid of the limitation of the volume of the measured object by the traditional terahertz time-domain spectroscopy system; at the same time, because different pulses can be used for detection, the design of the entire optical system tends to be more miniaturized, Practical; more importantly, this method breaks the limitation that the time-domain spectroscopy system can only perform close-range material composition analysis, and can realize time-domain information scanning of terahertz waves for both near-field and far-field materials, so that terahertz Time-domain spectral analysis is extended to long-distance component analysis and substance detection.

Description of drawings

Fig. 1 is a structural schematic diagram of a different pulse detection device for terahertz wave time domain information of the present invention;

Fig. 2 is a schematic structural view of a femtosecond laser generator with a certain repetition rate of the present invention;

FIG. 3 is a schematic diagram of the spatial modulation performed by the spatial modulation unit of the present invention;

Fig. 4 is a schematic diagram of the structure of the different pulse detection method for terahertz wave time domain information used in electro-optic crystal sampling and detection of terahertz pulse signals according to the present invention.

Detailed ways

The specific implementation manners of the present invention will be described below in conjunction with the accompanying drawings.

FIG. 1 is a schematic structural diagram of a device for detecting abnormal pulses of terahertz wave time domain information in this embodiment.

As shown in Fig. 1, the abnormal pulse detection device 100 for terahertz wave time domain information includes a femtosecond laser generating part 11 with a certain repetition rate, a beam splitting part 12, a convex lens 13, a semiconductor antenna 14, a parabolic mirror 15, a high-resistance silicon The sheet 16, the sample storage part 17, the terahertz pulse collection part 18, the detection part 19, the detection laser pulse focusing part 20, the spatial optical path length modulation part and the terahertz pulse reduction part not shown in the figure. Among them, the convex lens 13, the semiconductor antenna 14, the parabolic mirror 15, and the high-resistance silicon chip 16 constitute the terahertz pulse forming part.

FIG. 2 is a schematic structural diagram of a femtosecond laser generator with a certain repetition rate in this embodiment.

As shown in Figure 2, the femtosecond laser generator 11 with a certain repetition rate includes a femtosecond laser 111, a photodetector 112, a low-pass filter 113, a preamplifier 114, a loop controller 115, and a modulation boost amplifier 116 .

The femtosecond laser 111 generates a femtosecond laser with a certain repetition frequency, and the end of the resonant cavity is fixed with a piezoelectric ceramic (PZT), which is used to form a beat frequency signal after the pump light enters the resonant cavity, and the frequency is an integer of the beat frequency signal times; the photodetector 112 is arranged on the optical path of the femtosecond laser, which captures a very small amount of femtosecond laser passing through the reflector, and uses the tunable deflection signal as a reference to perform the femtosecond laser repetition rate detected by it. Frequency mixing generates an error signal representing the difference between the laser repetition rate and the standard frequency; the low-pass filter 113 is used to filter out the high-frequency signal in the error signal generated by the photodetector to obtain a low-frequency error signal; the preamplifier 114 It is used to amplify the low-frequency error signal; the loop controller 115 is used to output the compensation control signal after the amplified error signal enters; the modulation boost amplifier 116 is used to drive the piezoelectric ceramic to generate displacement and change the cavity Long, so that the repetition frequency dynamically tracks the compensation control signal, and realizes the locking of the repetition frequency.

As shown in FIG. 1 , the beam splitter 12 is arranged on the optical path of the femtosecond laser whose repetition rate is locked, and is used to divide the femtosecond laser into detection laser pulses and detection laser pulses that are perpendicular to each other. In this embodiment, the beam splitter is a 780nm beam splitter.

The terahertz pulse forming part is composed of a convex lens 13 , a semiconductor antenna 14 , a parabolic mirror 15 , and a high-resistance silicon chip 16 . The convex lens 13 focuses the detection laser pulse, and the semiconductor antenna 14 is irradiated by the focused detection laser pulse to generate terahertz radiation and form a terahertz wave. The parabolic mirror 15 is used to collect and converge the terahertz wave. The high-resistance silicon wafer 16 filters the terahertz wave so that the terahertz wave of the desired frequency passes through. In this embodiment, the parabolic mirror 15 is a gold-plated off-axis parabolic mirror.

The sample holding portion 17 is adjacent to the high-resistance silicon chip and is used to hold the sample to be tested. The terahertz pulse sink collection 18 is used to collect the terahertz pulses hit on the sample to be inspected and reflected back by the sample to be inspected. In this embodiment, the terahertz pulse converging part 18 is a gold-plated off-axis parabolic mirror.

The probing laser pulse focusing unit 20 is arranged upstream of the probing laser pulse optical path for converging the probing laser pulses. In this embodiment, the detection laser pulse focusing part 20 is a convex lens.

The detection part is used to simultaneously detect the terahertz pulse and the detection laser pulse arriving at the same time; the spatial optical path modulation part is used to adjust the spatial optical path of the terahertz pulse and the detection laser pulse, so that the terahertz pulse reaches the detection part at the same time The difference between the detection laser pulses is an integer multiple of the interval between adjacent pulses; the terahertz pulse reduction part uses the detection laser pulse to sample and scan the time-domain information of the terahertz pulse that arrives at the detection part at the same time, and the time-domain information of the terahertz pulse The information is restored to obtain the composition information of the sample to be tested.

FIG. 3 is a schematic diagram of the spatial modulation performed by the spatial modulation unit in this embodiment.

As shown in Figure 1 and Figure 3, it is assumed that the pulse arriving at the detection part in the terahertz optical path is the n-5th pulse, and the pulse arriving at the detecting part in the reference laser pulse optical path is the n-8th pulse, and the two pulses arrive at the detection part at the same time The optical path difference of the part should meet the following conditions:

Δ=(|(n-5)-(n-8)|)c*k/f=3c*k/f

Among them, n is an integer, representing the first pulse after passing through the beam splitter; n-5 represents that the pulse reaching the detection part in the terahertz optical path is the fifth pulse after passing through the beam splitter; n-8 represents the reference laser pulse The pulse that reaches the detection part in the optical path is the eighth pulse after passing through the beam splitter; c is the propagation speed of the laser, that is, the speed of light; k is the refractive index of the medium; f is the repetition rate of the femtosecond laser.

The spatial modulation unit only needs to modulate the optical path difference according to the above formula to realize that two different pulses reach the target object at the same time. At this time, the (n-8)th detection laser pulse can be used to complete the time-domain signal sampling scanning of the (n-5)th terahertz pulse, thereby restoring the terahertz pulse.

In the traditional adjustment method, since the traditional terahertz time-domain detection method does not lock the repetition frequency of the laser source, it is necessary to control the detection optical path and the pulse that reaches the target at the same time is also the n-5th pulse to combine at the target point, and then use the mechanical translation stage The linear scanning of the detection light pulse samples the time-domain information of the terahertz wave, thereby restoring the time-domain information of the terahertz wave.

FIG. 4 is a schematic structural diagram of the method for detecting different pulses of terahertz wave time domain information in this embodiment for sampling and detecting terahertz pulse signals by electro-optic crystals.

The method for detecting abnormal pulses of terahertz wave time domain information will be described in detail below with specific examples.

As shown in Figure 4, the optical path of the electro-optic crystal for sampling and detecting terahertz pulse signals includes a femtosecond laser 11 whose repetition rate is precisely locked, a beam splitter 12, a convex lens 13, a semiconductor antenna 14, a parabolic mirror 15, a high-resistance silicon wafer 16, and a Sample holding part 17, terahertz pulse collecting part 18, probe laser pulse focusing part 20, electro-optic crystal zinc telluride (ZnTe) 21, 1/4 wave plate 22; PBS polarization beam splitter prism 23, photoelectric probe PD24.

As shown in Figure 2 and Figure 4, the method for sampling and detecting terahertz pulse signals by electro-optic crystals using the different pulse detection method of terahertz wave time domain information includes the following steps:

Step 1, the femtosecond laser 111 outputs a femtosecond laser with a certain repetition frequency, and the femtosecond laser enters the resonant cavity as pump light to form a beat frequency signal;

In step 2, the photodetector 112 captures the femtosecond laser light passing through the reflector, and uses the tunable deflection signal as a reference, and performs frequency mixing with the repetition frequency of the femtosecond laser detected by it to generate a signal representing the difference between the repetition frequency of the laser and the standard frequency error signal;

Step 3, the low-pass filter 113 filters out the high-frequency signal in the error signal to obtain a low-frequency error signal;

Step 4, the preamplifier 114 amplifies the low-frequency error signal;

Step 5, the loop controller 115 outputs a compensation control signal after the amplified error signal enters;

Step 6, modulating the step-up amplifier 116 to drive the piezoelectric ceramic to generate displacement to change the cavity length, so that the repetition frequency can dynamically track the compensation control signal to realize the locking of the repetition frequency;

Step 7, the beam splitter 12 divides the repetition rate locked femtosecond laser into mutually perpendicular detection laser pulses and detection laser pulses;

Step 8, the detection laser pulse is focused by the convex lens 13 and then irradiated on the semiconductor antenna 14 to generate terahertz radiation and form a terahertz wave. After the terahertz wave is collected and converged by the parabolic mirror 15 , it is split and filtered by the high-resistance silicon chip 16 , so that the terahertz wave of the required frequency can pass through.

Step 9, the transmitted part of the terahertz wave hits the sample to be tested in the sample storage part 17 and is reflected, and the reflected terahertz pulse is reflected to the terahertz pulse collection part 18 through the high-resistance silicon chip 16 again superior. The terahertz pulse collection part 18 is located on the optical path of the detection laser pulse, which is also a gold-plated parabolic mirror, and the reflected terahertz pulse is collected by the parabolic mirror and converged on the electro-optic crystal zinc telluride 21;

Step 10, after the detection laser pulse focusing part 22 focuses the detection laser pulse, the detection laser pulse passes through the terahertz pulse collection part 18, and is also converged on the electro-optic crystal zinc telluride 21;

Step 11, the spatial optical path modulation unit adjusts the spatial optical path difference between the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the electro-optic crystal zinc telluride 21 at the same time is an integer multiple of the interval between adjacent pulses;

Step 12: Zinc telluride crystal 21 has a birefringence effect under the influence of terahertz pulses. After the two beams are converged, the polarization is adjusted by 1/4 wave plate 22, and the beam is split by PBS beam splitter 23 and then by photoelectric probe PD24. Collect and detect, and use the linear translation stage to scan to complete the sample information detection.

Function and effect of embodiment

According to the device for detecting different pulses of terahertz wave time-domain information and the method for detecting different pulses of terahertz wave time-domain information provided in this embodiment, since the spatial optical path modulation unit can adjust the spatial light passing through the terahertz pulse and the detection laser pulse path difference, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection part at the same time is an integer multiple of the adjacent pulse interval, so this method can adjust the optical path difference in real time according to the size of the measured object under the condition that the entire optical path system is determined, so that The design of the whole system is no longer limited by the size of the measured object, and it gets rid of the limitation of the volume of the measured object by the traditional terahertz time-domain spectroscopy system; at the same time, because different pulses can be used for detection, the design of the entire optical system tends to be more miniaturized , practicability; more importantly, this method breaks the limitation that the time-domain spectroscopy system can only analyze the composition of materials at close range, and can realize the time-domain information scanning of terahertz waves for both near-field and far-field materials, so that the terahertz wave Hertz time-domain spectral analysis is extended to long-distance component analysis and material detection.

Claims (4)

1. A different pulse detection device for terahertz wave time domain information, characterized in that it has: A femtosecond laser generator with a certain repetition rate, which outputs a femtosecond laser with a stable repetition rate; A beam splitter, arranged on the optical path of the femtosecond laser with a locked repetition rate, is used to divide the femtosecond laser into mutually perpendicular detection laser pulses and detection laser pulses; A terahertz pulse forming part is arranged upstream of the optical path of the detection laser pulse, and is used to generate terahertz radiation and form a terahertz wave; The sample-to-be-inspected holding part is arranged downstream of the optical path of the detection laser pulse, and is used to hold the sample to be inspected; a terahertz pulse collection unit, configured to collect the terahertz pulses struck on the sample to be inspected and reflected back by the sample to be inspected; The detection laser pulse focusing part is arranged upstream of the detection laser pulse optical path, and is used to focus the detection laser pulse; a detection unit, simultaneously detecting the information of the terahertz pulse combined here and the detection laser pulse; The spatial optical path modulation unit adjusts the spatial optical path passed by the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection unit at the same time is an integer multiple of the interval between adjacent pulses ;as well as The terahertz pulse restoration part uses the detection laser pulse to sample and scan the time-domain information of the terahertz pulse arriving at the detection part at the same time, and restores the time-domain information of the terahertz pulse. 2. The different pulse detection device of terahertz wave time domain information according to claim 1, characterized in that: Wherein, the femtosecond laser generator with a certain repetition rate includes: The femtosecond laser outputs a femtosecond laser with a certain repetition frequency, and the end of the resonant cavity is fixed with a piezoelectric ceramic, which is used to form a beat frequency signal after the pump light enters the resonant cavity; The photodetector is arranged on the optical path of the femtosecond laser, captures the femtosecond laser passing through the mirror, and uses the tunable deflection signal as a reference to perform frequency mixing with the repetition frequency of the femtosecond laser detected by it, Generate an error signal representing the difference between the repetition rate of the laser and the standard frequency; a low-pass filter for filtering high-frequency signals in the error signal to obtain a low-frequency error signal; a preamplifier, amplifying the low-frequency error signal; a loop controller that outputs a compensation control signal after the amplified error signal enters; and The boost amplifier is modulated to drive the piezoelectric ceramic to generate displacement to change the cavity length, so that the repetition frequency can dynamically track and compensate the control signal to realize the locking of the repetition frequency. 3. A different pulse detection method of terahertz wave time domain information, characterized in that, comprising the following steps: Step 1, using a femtosecond laser to output a femtosecond laser with a certain repetition rate, after the femtosecond laser enters a resonant cavity provided with piezoelectric ceramics, a beat frequency signal is formed; Step 2, using a photodetector to capture the femtosecond laser passing through the mirror, and using the tunable deflection signal as a reference, and mixing it with the repetition frequency of the femtosecond laser detected by it to generate a representative laser repetition frequency and standard an error signal of a frequency difference; Step 3, using a low-pass filter to filter out the high-frequency signal in the error signal to obtain a low-frequency error signal; Step 4, using a preamplifier to amplify the low-frequency error signal; Step 5, using a loop controller to output a compensation control signal after the amplified error signal enters; Step 6, using a modulated boost amplifier to drive the piezoelectric ceramic to generate a displacement to change the cavity length, allowing the repetition frequency to dynamically track the compensation control signal to achieve repetition frequency locking; Step 7, using a beam splitter to split the repetition rate-locked femtosecond laser into mutually perpendicular detection laser pulses and detection laser pulses; Step 8, using a terahertz pulse forming part to generate terahertz radiation to form a terahertz wave; Step 9, using a terahertz pulse converging unit to converge the terahertz pulses hit on the sample to be tested and reflected back by the sample to be tested; Step 10, using a detection laser pulse focusing unit to focus the detection laser pulse passing through the lens; Step 11, using a detection unit to simultaneously detect the information of the terahertz pulse and the detection laser pulse; Step 12, using the spatial optical path length modulation unit to adjust the spatial optical path difference between the terahertz pulse and the detection laser pulse, so that the difference between the terahertz pulse and the detection laser pulse arriving at the detection unit at the same time is adjacent pulses an integer multiple of the interval; Step 13, using a terahertz pulse restoration unit to sample and scan the time domain information of the terahertz pulse arriving at the detection unit at the same time, and restore the time domain information of the terahertz pulse. 4. The different pulse detection method of terahertz wave time domain information according to claim 3, is characterized in that: Wherein, the target pulse of the terahertz wave pulse reaching the detection part is the n-mth one, the target pulse of the detection laser pulse reaching the detection part is the n-wth one, and the two pulses reach the detection part at the same time. The optical path difference should meet the following conditions: Δ=(|(n-m)-(n-w)|)c*k/f Among them, n is an integer, representing the first pulse after passing through the beam splitter; c is the propagation speed of the laser, that is, the speed of light; k is the refractive index of the medium; f is the repetition frequency of the femtosecond laser; n-m represents the terahertz optical path The pulse arriving at the detection part is the mth pulse after passing through the beam splitter; n-w means that the pulse arriving at the detection part in the detection laser pulse optical path is the wth pulse after passing through the beam splitting part.