A thick sample terahertz detection system and method based on phase control

Abstract

This invention discloses a phase-modulated terahertz detection system and method for thick samples, comprising a femtosecond laser, a laser beam splitter, a signal scanning delay line, a reflector A, a photoconductive transmitting antenna, a bias voltage, a phase modulation module, a reflector B, a photoconductive detecting antenna, a terahertz beam splitter, an axonometric lens, a metal plate, a lock-in amplifier, and a host computer. The phase modulation module includes a plane reflector and a high-precision grating encoder. The plane reflector is mounted on a linear displacement stage, and the high-precision grating encoder is used to detect the position of the plane reflector. This invention adds a phase modulation module to a conventional terahertz time-domain spectroscopy system, using phase modulation technology to control the phase of the acquired terahertz signal without changing the sample position. By utilizing the phase continuity of the terahertz signal, multiple terahertz signals under different modulation phases can be matched and restored to obtain the complete terahertz signal of the thick sample.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of terahertz time-domain spectroscopy detection, and particularly relates to a thick sample terahertz detection system and method based on phase control, which is particularly suitable for complete detection of time-domain waveforms of thick samples. BACKGROUND

[0002] Terahertz spectroscopy is between far infrared and sub-millimeter wave, is a frequency band between macro electronics and micro photonics, occupies a unique position in the electromagnetic wave spectrum, has low radiation energy, high signal-to-noise ratio, wide frequency band and other characteristics, can be applied to detection and analysis of most non-metallic materials, and has been widely applied to detection of many types of non-metallic materials such as aerospace composite materials, industrial coating materials and ceramic matrix composites.

[0003] The terahertz time-domain spectroscopy system is based on the principle of coherent detection, and the time and phase information of the terahertz time-domain pulse signal is obtained by adjusting the optical delay line to change the relative phase relationship between the detection femtosecond laser pulse and the terahertz time-domain pulse. The delay range of the delay line determines the size of the detection window of the terahertz time-domain spectroscopy system and the maximum sample thickness that can be detected. In the traditional terahertz time-domain spectroscopy system, the linear delay line has a low scanning frequency, and the sample detection needs a long time. In order to improve the scanning frequency of the terahertz system, a rotating optical delay line is often used instead of a linear delay line.

[0004] For thick detection samples, we are more concerned about whether there are defects in the interior and the type and size of the defects, so the interested thickness position of different detection samples is random, and the sample needs to be detected for full waveform to determine the position of the defects in the sample interior. However, the detection window of the rotating optical delay line is usually short, and only the terahertz focusing lens can be used to realize the collection of the detection waveform of a certain thickness region of the sample, and it is difficult to obtain the complete terahertz reflection signal of the thick sample. SUMMARY

[0005] The present application provides a thick sample terahertz detection system and method based on phase control, which increases a phase control module in the terahertz time-domain spectroscopy detection system, uses phase control technology to control the phase of the terahertz signal, collects the detection waveform of the terahertz signal under multiple different modulation phases, matches and restores the collected signal based on the phase continuity of the terahertz signal, and can obtain the complete time-domain terahertz waveform of the detection sample of different thicknesses while maintaining a high scanning frequency.

[0006] The present application is realized by the following technical solutions:

[0007] The application provides a thick sample terahertz detection system based on phase control, comprising a femtosecond laser, a laser beam splitter, a signal scanning delay line, a mirror A, a photoconductive transmitting antenna, a bias voltage, a phase control module, a mirror B, a photoconductive detecting antenna, a terahertz beam splitter, an axicon lens, a metal plate, a lock-in amplifier and a host computer.

[0008] The femtosecond laser pulses emitted by the femtosecond laser are divided into two beams by the laser beam splitter, one of which is used as pump light and is incident on the photoconductive transmitting antenna through the signal scanning delay line and the mirror A, and the photoconductive transmitting antenna generates a terahertz time-domain pulse under the action of an external bias voltage; the thick sample to be measured is horizontally placed on the metal plate, the terahertz time-domain pulse is divided by the terahertz beam splitter, the transmitted terahertz signal is modulated into a non-diffracted terahertz wave by the axicon lens, and is incident on the thick sample to be measured, and a terahertz echo A1 peak is generated on the upper surface of the thick sample to be measured, and a terahertz reflection echo A2 peak is generated by the interface between the lower surface of the thick sample to be measured and the metal plate, and the terahertz time-domain pulse carrying sample information is reflected by the axicon lens and the terahertz beam splitter again and is incident on the photoconductive detecting antenna.

[0009] The other beam of femtosecond laser pulses is used as probe light and is incident on the photoconductive detecting antenna through the phase control module and the mirror B, and the photoconductive detecting antenna generates an induced current under the simultaneous action of the terahertz time-domain pulse and the probe pulse.

[0010] The phase control module comprises a plane mirror and a high-precision grating encoder, the plane mirror is installed on a linear displacement table, the linear displacement table drives the plane mirror to translate through a stepping motor, and the high-precision grating encoder is used for detecting the position of the plane mirror.

[0011] The host computer is connected to the signal scanning delay line and the phase control module for control, the signal scanning delay line is controlled by the host computer to change the optical path difference between the probe light and the pump light, and the signal is displayed and stored by the host computer through the lock-in amplifier.

[0012] Preferably, the plane mirror is any one of a right-angle mirror, a corner cube prism and a mirror with a curved surface.

[0013] Preferably, the host computer controls the stepping motor to drive the plane mirror to translate, and records the change of the pulse signal through the high-precision grating encoder to obtain the position of the plane mirror.

[0014] Preferably, the delay time range of the phase control module is greater than 1000 ps, and the delay accuracy is less than the required sampling interval of the terahertz time-domain spectrum detection system.

[0015] Furthermore, this invention achieves scanning detection of the entire terahertz time-domain pulse waveform by changing the relative phase relationship between the probe femtosecond laser pulse and the terahertz time-domain pulse.

[0016] This invention also provides a phase-modulated terahertz detection method for thick samples, comprising the following steps:

[0017] Step 1: Construct the phase-modulated terahertz detection system for thick samples;

[0018] Step 2: Adjust the position of the planar mirror in the phase control module to move the peak value of the terahertz reflection signal A1 from the upper surface of the sample to be measured to the middle of the detection window. The position is determined, and the position reading d1 of the high-precision grating encoder of the phase control module is recorded at this time.

[0019] Step 3: Adjust the position of the plane mirror in the phase control module so that the peak value of the terahertz reflection signal A2 from the bottom metal plate of the sample to be measured returns to the middle of the detection window. The position is determined, and the position reading d2 of the high-precision grating encoder of the phase control module is recorded at this time.

[0020] Step 4: By calculating the change in position readings of the high-precision grating encoder between two scans, as well as the delay window size of the signal scan delay line and the overlap of the detection waveform, determine the number of scans required to acquire the complete terahertz detection waveform of the sample to be measured, and the position reading of the high-precision grating encoder of the phase control module during each scan.

[0021] Step 5: Based on the position calculation results of each scan in Step 4, the phase control module is positioned and controlled during multiple scans of the thick sample to be tested, so as to realize the overall terahertz time-domain detection waveform of the thick sample to be tested. For multiple detection data, the phase continuity of the terahertz signal is used to match and restore the acquired signal to obtain the complete terahertz time-domain detection waveform of the thick sample to be tested.

[0022] Preferably, step two specifically includes the following steps:

[0023] The host computer drives a stepper motor to move the planar mirror in the phase modulation module forward, thereby modulating the phase of the probe light until the peak value of the terahertz time-domain detection waveform A1 on the upper surface of the sample is moved to the delay time reading in the detection window. The position is recorded as d1, which indicates the position reading of the high-precision grating encoder in the phase control module at this moment.

[0024] Preferably, step three specifically includes the following steps:

[0025] The host computer drives the stepper motor to adjust the position of the planar reflector in the phase control module system until the peak value of the terahertz time-domain detection waveform A2 on the upper surface of the sample to be measured is reached, and then the position returns to the delay time reading in the detection window. The position is recorded as d2, which indicates the position of the high-precision grating encoder in the phase control module at this moment.

[0026] Furthermore, step four specifically includes the following steps:

[0027] 4.1) Calculate the difference Δd between the two position readings of the high-precision grating encoder in the phase control module:

[0028] Δd=d2-d1

[0029] 4.2) Based on the delay window T of the signal scanning delay line and the overlap ΔT of the detected waveform, the required movement distance of the plane mirror in the phase modulation module for each scan is determined to be Δd′:

[0030]

[0031] Where c is the speed of light in a vacuum, and n0 is the refractive index of air;

[0032] The number of scans m for full-waveform terahertz detection of the thick sample to be tested:

[0033]

[0034] in, The symbol indicates that the calculation result is rounded up, T is the size of the delay window of the signal scanning delay line, and c is the speed of light in a vacuum;

[0035] 4.3) From step 4.2), for a sample of thickness m to be measured, the high-precision grating encoder of the phase control module positions d′ during the i-th scan. i :

[0036]

[0037] Where d1 is the peak value of the terahertz time-domain detection waveform on the upper surface of the sample to be measured, located within the detection window. The position reading of the phase control module is shown when the position is determined, and Δd′ is the distance the phase control module needs to move for each scan.

[0038] Furthermore, step five specifically includes the following steps:

[0039] 5.1) Based on the phase modulation module positioning result d′ in step 4.4) iKeeping the position of the sample to be measured stationary, the stepper motor is controlled by the host computer to move the plane mirror in the phase adjustment module to position d1, thereby completing the scanning of the sample and saving the terahertz time-domain waveform data at that position.

[0040] 5.2) Keeping the position of the sample to be measured unchanged, repeat step 5.1) and move the plane mirror in the phase adjustment module to d′ in sequence. i Position, complete multiple scans of the sample to be measured and the acquisition of terahertz time-domain detection waveforms;

[0041] 5.3) For the collected terahertz time-domain waveform detection data of the thick sample to be tested, extract the data T-ΔT before the detection waveform at the corresponding scanning position of each detection data, and arrange them in sequence according to the scanning order to obtain the complete terahertz time-domain detection waveform of the thick sample to be tested.

[0042] The present invention has the following advantages:

[0043] This invention addresses the problem of signal waveforms exceeding the detection window in thick sample detection. By adding a phase modulation module to a conventional terahertz time-domain spectroscopy system, the phase of the acquired terahertz signal is controlled using phase modulation technology without changing the sample position. By utilizing the phase continuity of the terahertz signal, multiple terahertz signals under different modulation phases are matched and reconstructed to obtain the complete terahertz signal of the thick sample. This method maintains a high scanning frequency for the rotating delay line while meeting the terahertz detection requirements of various non-metallic materials of different thicknesses, including ceramic matrix composites, rubber materials, and resin-based materials. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the optical path of a phase-modulated terahertz detection system for thick samples according to Embodiment 1 of the present invention.

[0045] Figure 2 This is a schematic diagram of the phase control module structure described in Embodiment 1 of the present invention;

[0046] Figure 3 This is a schematic diagram of the terahertz detection principle of the thick sample to be tested in this invention;

[0047] Figure 4 This is a flowchart of a terahertz detection method for thick samples based on phase modulation, as described in Embodiment 2 of the present invention.

[0048] Figure 5 This is a schematic diagram of multiple phase modulations of the terahertz time-domain detection waveform of the thick sample to be tested according to the present invention;

[0049] In the picture:

[0050] 1-Femtosecond laser; 2-Laser beam splitter; 3-Signal scanning delay line; 4-Reflector A; 5-Photoconductive transmitting antenna; 6-Phase modulation module; 7-Reflector B; 8-Photoconductive detection antenna; 9-Terahertz beam splitter; 10-Axis pyramidal lens; 11-Sample to be measured; 12-Metal plate; 13-Bias voltage; 14-Lock-in amplifier; 15-Host computer; 16-Planar reflector; 17-Linear displacement stage; 18-Stepper motor; 19-High-precision grating encoder. Detailed Implementation

[0051] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0052] Example 1

[0053] A phase-modulated terahertz detection system for thick samples, such as Figure 1As shown, the system includes: a femtosecond laser 1, a laser beam splitter 2, a signal scanning delay line 3, a reflector A4, a photoconductive transmitting antenna 5, a bias voltage 13, a phase modulation module 6, a reflector B7, a photoconductive detection antenna 8, a terahertz beam splitter 9, an axonometric lens 10, a metal plate 12, a lock-in amplifier 14, and a host computer 15; wherein, the laser beam splitter 2, the signal scanning delay line 3, the reflector A4, and the photoconductive transmitting antenna 5 are arranged sequentially to form the pump optical path; the laser beam splitter 2, the phase modulation module 9, the signal scanning delay line 3, the reflector A4, and the photoconductive transmitting antenna 5 are arranged sequentially to form the pump optical path; the laser beam splitter 2 ... signal scanning delay line 4, the signal scanning delay line 5, the signal scanning delay line 6, the signal scanning delay line 7, the signal scanning delay line 8, the signal scanning delay line 9, the signal scanning delay line 10, the signal scanning delay line 11, the signal scanning delay line 12, the signal scanning delay line 13, the signal scanning delay line 14, the signal scanning delay line 15, the signal scanning delay line 16, the signal scanning delay line 17, the signal scanning delay line 18, the signal scanning delay line 19, the signal scanning delay line 10, the signal scanning delay line 1 Module 6, reflector B7, and photoconductive detection antenna 8 are arranged sequentially to form the detection optical path; the femtosecond laser pulse emitted by the femtosecond laser 1 is split into two beams by the laser beam splitter 2. One of the femtosecond laser pulses serves as the pump light, which is incident on the photoconductive transmitting antenna 5 via the signal scanning delay line 3 and reflector A4. Simultaneously, under the action of the applied bias voltage 13, the photoconductive transmitting antenna 5 generates a terahertz time-domain pulse; the sample 11 to be measured is placed horizontally on the metal plate 12, and the terahertz time-domain pulse is transmitted through the signal scanning delay line 3 and reflector A4. The terahertz beam splitter splits the terahertz signal, which is then modulated into a diffraction-free terahertz wave by the axial pyramidal lens 10. This wave is incident on the sample 11, where it is reflected from the upper surface to generate a terahertz echo peak A1. This echo passes through the sample 11 and is reflected back to the interface between the lower surface of the sample 11 and the metal plate 12, generating a terahertz reflected echo peak A2. The terahertz time-domain pulse carrying sample information is reflected again by the axial pyramidal lens 10 and the terahertz beam splitter 9, and then incident on the photoconductive detection antenna 8. A femtosecond laser pulse is used as the probe light and is incident on the photoconductive detection antenna 8 through the phase adjustment module 6 and the reflector B7. Under the simultaneous action of the terahertz time-domain pulse and the probe pulse, the photoconductive detection antenna 8 generates an induced current. The host computer 15 is connected to the signal scanning delay line 3 and the phase adjustment module 6 respectively. The host computer 15 controls the operation of the signal scanning delay line 3 to change the optical path difference between the probe light and the pump light. The signal is displayed and stored by the host computer 15 through the lock-in amplifier 14.

[0054] like Figure 2 As shown, the phase control module 6 includes a plane mirror 16 and a high-precision grating encoder 19. The plane mirror 16 is mounted on a linear displacement stage 17, which is driven to translate the plane mirror 16 by a stepper motor 18. The high-precision grating encoder 19 is used for feedback of motion parameters such as position and speed of the plane mirror 16. The plane mirror 16 can be a right-angle mirror, a cornerstone prism, a parabolic mirror, or other reflective devices. The host computer 15 drives the stepper motor 18 to translate the plane mirror 16 on the linear displacement stage 17, while the high-precision grating encoder 19 records the changes in the pulse signal to obtain the position of the plane mirror 16. The movement of the plane mirror 17 changes the optical path of the terahertz detection optical path, altering the optical path difference between the pump optical path and the detection optical path, thereby modulating the phase of the terahertz signal acquired by the signal scanning delay line 3.

[0055] Preferably, the delay time range of the phase modulation module 6 is greater than 1000 ps, ​​and the delay accuracy is less than the required sampling interval of the terahertz time-domain spectroscopy system.

[0056] The working principle of this embodiment is as follows:

[0057] The femtosecond laser pulse emitted from femtosecond laser 1 is split into two beams by laser beam splitter 2. One of the femtosecond laser pulses serves as the pump light, passing through signal scanning delay line 3 and reflector A4 before being incident on photoconductive transmitting antenna 5. Simultaneously, under the action of an applied bias voltage 13, photoconductive transmitting antenna 5 generates terahertz time-domain pulses; as... Figure 3 As shown, the sample to be measured is a terahertz signal-transmissible sample, which can be a single dielectric material or a multilayer composite material. The sample to be measured 11 is placed horizontally on the metal plate 12. The terahertz time-domain pulse is split by a terahertz beam splitter. The transmitted terahertz signal is modulated into a diffraction-free terahertz wave by an axonometric pyramid lens 10 and incident on the sample to be measured 11. A terahertz echo peak A1 is generated by reflection at the upper surface of the sample to be measured 11. This echo passes through the sample to be measured 11, and a terahertz reflected echo peak A2 is generated at the interface between the lower surface of the sample to be measured 11 and the metal plate 12. The terahertz time-domain pulse carrying the sample information is then transmitted again by the axonometric pyramid lens 10. The light is reflected by lens 10 and terahertz beam splitter 9 and incident on photoconductive detection antenna 8. Another femtosecond laser pulse, used as the probe light, is incident on photoconductive detection antenna 8 via phase adjustment module 6 and reflector B7. Under the simultaneous action of the terahertz time-domain pulse and the probe pulse, photoconductive detection antenna 8 generates an induced current (the magnitude of the induced current is proportional to the amplitude of the terahertz time-domain pulse signal and is in phase with the terahertz time-domain pulse signal). Based on the principle of coherent detection, by changing the relative phase relationship between the probe femtosecond laser pulse and the terahertz time-domain pulse, the entire terahertz time-domain pulse waveform can be scanned and detected. The host computer 15 controls the operation of the signal scanning delay line 3, changing the optical path difference between the probe light and the pump light. The signal is then displayed and stored by the host computer 15 via lock-in amplifier 14.

[0058] Example 2

[0059] like Figure 4 As shown, this embodiment is a terahertz detection method for thick samples based on phase modulation, including the following steps:

[0060] Step 1: Construct the terahertz time-domain spectroscopy detection system for thick samples as described in Example 1.

[0061] Step 2: Adjust the position of the planar mirror 16 in the phase control module 6 so that the peak value of the terahertz reflection signal A1 on the upper surface of the sample 11 to be measured moves to the middle of the detection window. The position is recorded, and the position reading d1 of the high-precision grating encoder 19 of the phase control module 6 is recorded at this time.

[0062] Step 3: Adjust the position of the plane mirror 16 in the phase control module 6 so that the peak value of the terahertz reflection signal A2 of the bottom metal plate 12 of the sample 11 to be measured returns to the middle of the detection window. The position is recorded, and the position reading d2 of the high-precision grating encoder 19 of the phase control module 6 is recorded at this time.

[0063] Step 4: By calculating the changes in the position readings of the high-precision grating encoder 19 of the phase control module 6 twice, as well as the size of the delay window of the signal scanning delay line 3 and the overlap of the detection waveform, determine the number of scans required to acquire the complete terahertz detection waveform of the sample 11 to be measured (the waveform between the upper surface detection echo A1 and the lower surface detection echo A2 of the sample 11 to be measured), and the position reading of the high-precision grating encoder 19 of the phase control module 6 during each scan.

[0064] Step 5: Based on the position calculation results of each scan in Step 4, the phase control module 6 is positioned and controlled during multiple scans of the thick sample 11 to be tested, so as to realize the overall terahertz time-domain detection waveform of the thick sample 11 to be tested. For multiple detection data, the phase continuity of the terahertz signal is used to match and restore the acquired signal to obtain the complete terahertz time-domain detection waveform of the thick sample 11 to be tested.

[0065] Further, step two, adjusting the position of the planar mirror 16 in the phase control module 6 system so that the peak value of the terahertz reflection signal A1 on the upper surface of the sample 11 to be measured moves to the middle position in the detection window, and recording the reading d1 of the high-precision grating encoder 19 in the phase control module 6 at this time, specifically includes the following steps:

[0066] The host computer 15 drives the stepper motor 18 to move the planar reflector in the phase adjustment module 6 forward, thereby adjusting the phase of the probe light until the peak value of the terahertz time-domain detection waveform A1 on the upper surface of the sample 11 is moved to the delay time reading in the detection window. The position is recorded, showing the position reading d1 of the high-precision grating encoder 19 in the phase control module 6 at this time.

[0067] Further, step three, adjusting the position of the plane mirror 16 in the phase control module 6 system, so that the peak value of the terahertz reflection signal A2 of the bottom metal plate 12 of the sample 11 to be measured returns to the middle position in the detection window, and recording the position reading d2 of the high-precision grating encoder 19 in the phase control module 6 at this time, specifically includes the following steps:

[0068] The host computer 15 drives the stepper motor 18 to adjust the position of the plane mirror 16 in the phase control module 6 system until the peak value of the terahertz time-domain detection waveform A2 on the upper surface of the sample 11 is reached, and then the position returns to the delay time reading in the detection window. The position is recorded, showing the position reading d2 of the high-precision grating encoder 19 in the phase control module 6 at this time.

[0069] Further, step four, by calculating the change in position readings of the high-precision grating encoder 19 of the phase adjustment module 6 twice, as well as the delay window size of the signal scan delay line 3 and the overlap of the detection waveforms, determines the number of scans required to acquire the complete terahertz detection waveform of the sample 11 to be measured (the waveform between the upper surface detection echo A1 and the lower surface detection echo A2 of the sample 11 to be measured), and the position reading of the high-precision grating encoder 19 of the phase adjustment module 6 during each scan specifically includes the following steps:

[0070] 4.1) Calculate the difference Δd between the two position readings of the high-precision grating encoder 19 in the phase control module 6:

[0071] Δd=d2-d1 (1)

[0072] 4.2) Based on the delay window T of the signal scanning delay line 3 and the overlap ΔT of the detected waveform, the required moving distance Δd′ of the planar reflector 16 in the phase modulation module 6 for each scan can be determined:

[0073]

[0074] Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

[0075] The number of scans m for full-waveform terahertz detection of the thick sample 11 to be tested:

[0076]

[0077] in, The symbol indicates that the calculation result is rounded up, T is the size of the delay window of signal scanning delay line 3, and c is the speed of light in a vacuum.

[0078] 4.3) From step 4.2), it can be obtained that for the sample 11 to be measured with a thickness of m scans, the position d′ of the high-precision grating encoder 19 of the phase control module 6 is determined during the i-th scan. i :

[0079]

[0080] Wherein, d1 is the peak value of the terahertz time-domain detection waveform on the upper surface of the sample 11 to be tested, located within the detection window. The position reading of the phase control module 6 is shown when the position is determined, and Δd′ is the distance that the phase control module 6 needs to move for each scan.

[0081] Furthermore, step five, based on the position calculation results of each scan in step four, realizes the positioning and control of the phase modulation module 6 during multiple scans of the sample 11 to be tested, thereby realizing the overall terahertz time-domain detection waveform of the sample 11 to be tested. For the multiple detection data, the phase continuity of the terahertz signal is used to match and restore the acquired signal to obtain the complete terahertz time-domain detection waveform of the sample 11 to be tested. Specifically, this includes the following steps:

[0082] 5.1) Based on the positioning result d′ of phase modulation module 6 in step 4.4) i Keeping the position of the sample 11 to be measured stationary, the host computer 15 first controls the stepper motor 18 to move the plane mirror 16 in the phase adjustment module 6 to position d1, completes the scanning of the sample 11 to be measured, and saves the terahertz time-domain waveform data at this position.

[0083] 5.2) Keeping the position of the sample 11 to be measured unchanged, repeat step 5.1) and move the plane mirror 16 in the phase adjustment module 6 to d in sequence. i The position is used to complete multiple scans of the thickness sample 11 to be tested and the acquisition of terahertz time-domain detection waveforms.

[0084] 5.3) For the 11 terahertz time-domain waveform detection data of the sample to be tested, extract the data T-ΔT before the detection waveform at the corresponding scan position of each detection data, and arrange them sequentially according to the scan order, such as... Figure 5 As shown, the complete terahertz time-domain detection waveform of the sample 11 to be tested can be obtained. This method can be used in conjunction with a two-dimensional scanning platform. Step five can be repeated to achieve image detection of the sample by scanning two-dimensional points on the sample.

Claims

1. A terahertz detection system for thick samples based on phase modulation, characterized in that, Includes femtosecond laser, laser beam splitter, signal scanning delay line, mirror A, photoconductive transmitting antenna, bias voltage, phase adjustment module, mirror B, photoconductive detection antenna, terahertz beam splitter, axial pyramid lens, metal plate, lock-in amplifier, and host computer; The femtosecond laser pulse emitted by the femtosecond laser is split into two beams by the laser beam splitter. One of the femtosecond laser pulses is used as pump light and is incident on the photoconductive transmitting antenna through the signal scanning delay line and the reflector A. At the same time, under the action of the applied bias voltage, the photoconductive transmitting antenna generates terahertz time-domain pulses. The sample to be measured is placed horizontally on a metal plate. The terahertz time-domain pulse is split by a terahertz beam splitter. The transmitted terahertz signal is modulated into a non-diffraction terahertz wave by an axonometric lens and incident on the sample. The terahertz echo peak A1 is generated by reflection on the upper surface of the sample. The terahertz echo peak A2 is generated by the interface between the lower surface of the sample and the metal plate. The terahertz time-domain pulse carrying the sample information is reflected again by the axonometric lens and the terahertz beam splitter and incident on the photoconductive detection antenna. Another femtosecond laser pulse is used as the probe light. It is incident on the photoconductive detection antenna through the phase modulation module and the reflector B. Under the simultaneous action of the terahertz time-domain pulse and the probe pulse, the photoconductive detection antenna generates an induced current. The phase control module includes a planar reflector and a high-precision grating encoder. The planar reflector is mounted on a linear displacement stage, which is driven to translate the planar reflector by a stepper motor. The high-precision grating encoder is used to detect the position of the planar reflector. The host computer is connected to the signal scanning delay line and the phase adjustment module respectively. The host computer controls the operation of the signal scanning delay line to change the optical path difference between the probe light and the pump light. The signal is then displayed and stored by the host computer after passing through the lock-in amplifier.

2. The terahertz detection system for thick samples based on phase modulation as described in claim 1, characterized in that, The plane mirror can be any one of a right-angle mirror, a cornerstone prism, or a parabolic mirror.

3. The terahertz detection system for thick samples based on phase modulation as described in claim 1, characterized in that, The host computer controls the stepper motor to drive the plane mirror to translate, and at the same time records the changes in the pulse signal through a high-precision grating encoder to obtain the position of the plane mirror.

4. The terahertz detection system for thick samples based on phase modulation as described in claim 1, characterized in that, The phase modulation module has a delay time range greater than 1000 ps and a delay accuracy less than the required sampling interval of the terahertz time-domain spectroscopy detection system.

5. The terahertz detection system for thick samples based on phase modulation as described in claim 1, characterized in that, By changing the relative phase relationship between the probe femtosecond laser pulse and the terahertz time-domain pulse, the scanning and detection of the entire terahertz time-domain pulse waveform can be achieved.

6. A terahertz detection method for thick samples based on phase modulation, characterized in that, Includes the following steps: Step 1: Construct the phase-modulated terahertz detection system for thick samples as described in claim 1; Step 2: Adjust the position of the planar mirror in the phase control module to move the peak value of the terahertz reflection signal A1 from the upper surface of the sample to be measured to the middle of the detection window. The position is determined, and the position reading d1 of the high-precision grating encoder of the phase control module is recorded at this time. Step 3: Adjust the position of the plane mirror in the phase control module so that the peak value of the terahertz reflection signal A2 from the bottom metal plate of the sample to be measured returns to the middle of the detection window. The position is determined, and the position reading d2 of the high-precision grating encoder of the phase control module is recorded at this time. Step 4: By calculating the change in position readings of the high-precision grating encoder between two scans, as well as the delay window size of the signal scan delay line and the overlap of the detection waveform, determine the number of scans required to acquire the complete terahertz detection waveform of the sample to be measured, and the position reading of the high-precision grating encoder of the phase control module during each scan. Step 5: Based on the position calculation results of each scan in Step 4, the phase control module is positioned and controlled during multiple scans of the thick sample to be tested, so as to realize the overall terahertz time-domain detection waveform of the thick sample to be tested. For multiple detection data, the phase continuity of the terahertz signal is used to match and restore the acquired signal to obtain the complete terahertz time-domain detection waveform of the thick sample to be tested.

7. The terahertz detection method for thick samples based on phase modulation as described in claim 6, characterized in that, Step two specifically includes the following steps: The host computer drives a stepper motor to move the planar mirror in the phase modulation module forward, thereby modulating the phase of the probe light until the peak value of the terahertz time-domain detection waveform A1 on the upper surface of the sample is moved to the delay time reading in the detection window. The position is recorded as d1, which indicates the position reading of the high-precision grating encoder in the phase control module at this moment.

8. The terahertz detection method for thick samples based on phase modulation as described in claim 7, characterized in that, Step three specifically includes the following steps: The host computer drives the stepper motor to adjust the position of the planar reflector in the phase control module system until the peak value of the terahertz time-domain detection waveform A2 on the upper surface of the sample to be measured is reached, and then the position returns to the delay time reading in the detection window. The position is recorded as d2, which indicates the position of the high-precision grating encoder in the phase control module at this moment.

9. The terahertz detection method for thick samples based on phase modulation as described in claim 8, characterized in that, Step four specifically includes the following steps: 4.1) Calculate the difference Δd between the two position readings of the high-precision grating encoder in the phase control module: Δd=d2-d1 4.2) Based on the delay window T of the signal scanning delay line and the overlap ΔT of the detected waveform, the required movement distance of the plane mirror in the phase modulation module for each scan is determined to be Δd′: Where c is the speed of light in a vacuum, and n0 is the refractive index of air; The number of scans m for full-waveform terahertz detection of the thick sample to be tested: in, The symbol indicates that the calculation result is rounded up, T is the size of the delay window of the signal scanning delay line, and c is the speed of light in a vacuum; 4.3) From step 4.2), for a sample of thickness m to be measured, the high-precision grating encoder of the phase control module is positioned d during the i-th scan. i ′: Where d1 is the peak value of the terahertz time-domain detection waveform on the upper surface of the sample to be measured, located within the detection window. The position reading of the phase control module is shown when the position is determined, and Δd′ is the distance the phase control module needs to move for each scan.

10. The terahertz detection method for thick samples based on phase modulation as described in claim 9, characterized in that, Step five specifically includes the following steps: 5.1) Based on the phase modulation module positioning result d in step 4.4) i Keeping the position of the sample to be measured stationary, the stepper motor is controlled by the host computer to move the plane mirror in the phase adjustment module to position d1, thereby completing the scanning of the sample and saving the terahertz time-domain waveform data at that position. 5.2) Keeping the position of the sample to be measured unchanged, repeat step 5.1), and move the plane mirror in the phase adjustment module to d in sequence. i The position is used to complete multiple scans of the sample to be measured and the acquisition of terahertz time-domain detection waveforms; 5.3) For the collected terahertz time-domain waveform detection data of the thick sample to be tested, extract the data T-ΔT before the detection waveform at the corresponding scanning position of each detection data, and arrange them in sequence according to the scanning order to obtain the complete terahertz time-domain detection waveform of the thick sample to be tested.

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