Terahertz polarization measurement method and system based on electro-optical sampling

Abstract

The present invention discloses a terahertz polarization measurement method and system based on electro-optical sampling, which belongs to the field of spectral detection. The system includes a femtosecond pulse unit, a terahertz generation unit, a polarization modulation unit and a balanced detection unit. Among them, the polarization modulation unit is used to modulate the detection light into a dual-polarization detection light, and the dual-polarization detection light has two polarization states, and the two polarization states are distributed at intervals in the pulse sequence of the dual-polarization detection light; the balanced detection unit is used to modulate the electric field signals of the S light component and the P light component of the terahertz pulse light on two adjacent pulses of the dual-polarization detection light, respectively, and the full time domain waveform of the terahertz pulse light is detected by a balanced detector. The technical solution provided by the present invention loads two different polarization states at intervals in the adjacent pulses of the detection light through the polarization modulation unit, so that the electric field signals of the S component and the P component can be measured in a single scan, which not only improves the detection efficiency, but also improves the measurement accuracy.

Description

Technical Field

[0001] The present invention relates to the field of spectrum detection, and in particular to a terahertz polarization measurement method and system based on electro-optical sampling. Background Art

[0002] Terahertz waves are electromagnetic waves with frequencies between microwaves and infrared, ranging from 0.1 to 10 THz. Traditional terahertz detection methods only measure a certain polarization component of the terahertz wave, and re-collect and detect it by rotating the electro-optical crystal to obtain the full time domain waveform, including amplitude, phase and polarization information.

[0003] In the prior art, Chinese invention patent CN201610992724.8 discloses a terahertz pulse time domain waveform detection method, which uses circularly polarized light as the detection light for electro-optical sampling of the terahertz time domain waveform. The detection light is modulated in the electro-optic crystal and then divided into two paths by a beam splitter, and each signal light contains the polarization component information of the terahertz wave electric field. The modulation frequency of the pump light is set to f, and this modulated signal is used as the reference frequency of the dual-phase lock-in amplifier. The front end of the dual-phase lock-in amplifier uses a dual-channel multiplexer with a switching frequency of 2f to switch the two optoelectronic signals containing the terahertz component information. The amplitude and polarization direction of the terahertz wave time domain electric field are obtained through the dual-phase lock-in amplifier, and then the terahertz time domain polarization waveform is constructed by changing the delay between the detection light and the terahertz wave.

[0004] However, since this technology has extremely high requirements for the circular deflection of circularly polarized light, which is usually difficult to achieve, it is bound to affect the detection accuracy. In addition, the above scheme requires the use of two sets of balanced detectors, four photodiodes, and a dual-channel multiplexer. The detection method of the prior art is cumbersome and complicated, and the measurement error is large. Moreover, since the detection light is modulated and then split, its light intensity cannot be adjusted separately. According to the measurement principle, the signal-to-noise ratio of one component is reduced by half during the data processing process. Summary of the invention

[0005] Based on this, the technical solution provided by the present invention constructs a detection light pulse sequence containing two polarization states, and loads the two-dimensional polarization information of the terahertz pulse into different time periods of the same pulse time window, thereby realizing the measurement of the full time domain waveform of a single terahertz scan.

[0006] To achieve the above-mentioned purpose, the present invention provides a terahertz polarization measurement system based on electro-optical sampling, comprising: a femtosecond laser unit, used to generate femtosecond pulse light and split it into pump light and detection light output; a terahertz generation unit, used to receive the pump light to generate terahertz pulse light; a polarization modulation unit, used to modulate the detection light into dual-polarization detection light, the dual-polarization detection light has two polarization states, and the two polarization states are distributed at intervals in the pulse sequence of the dual-polarization detection light; and a balanced detection unit, used to combine the dual-polarization detection light with the terahertz pulse light, and modulate the electric field signals of the S light component and the P light component of the terahertz pulse light on two adjacent pulses of the dual-polarization detection light, respectively, and then respectively collect the electric fields of the two adjacent pulses to reconstruct the full time domain waveform of the terahertz pulse light; the full time domain waveform includes amplitude, phase and polarization state information.

[0007] Specifically, the polarization modulation unit includes a beam splitter, a first polarization control module and a second polarization control module; the detection light is divided into at least two beams by the beam splitter, one of which becomes a first polarized light by the first polarization control module, and the other becomes a second polarized light by the second polarization control module; the first polarized light and the second polarized light are both linearly polarized light, and the angle between the polarization directions of the first polarized light and the second polarized light is 45 degrees; the beam splitter is also used to combine the first polarized light and the second polarized light to form a dual-polarized detection light.

[0008] Specifically, the system also includes a first chopper and a second chopper, the first chopper is used to chop the first polarized light, and the second chopper is used to chop the second polarized light; the chopping frequencies of the first chopper and the second chopper are both 2f; the chopping phase difference between the first chopper and the second chopper is π, wherein the pulse repetition frequency of the femtosecond pulse light is 4f.

[0009] Preferably, the first polarization control module and the second polarization control module both include a linear polarizer, a quarter wave plate and a reflector; the polarization direction of the first polarized light is set to a horizontal direction, the polarization direction of the second polarized light is set to an angle of 45 degrees with the horizontal direction, and the intensity of the second polarized light is set to twice the intensity of the first polarized light.

[0010] Preferably, the polarization modulation unit further includes a translation stage for adjusting the time overlap of the first polarized light and the second polarized light.

[0011] Furthermore, the balanced detection unit includes a beam combiner, an electro-optical crystal and a balanced detector; the beam combiner is used to combine the dual-polarization detection light with the terahertz pulse light and focus them on the electro-optical crystal; the balanced detector is connected to a dual-phase lock-in amplifier, and the balanced detector is used to detect the electric field signal of the dual-polarization detection light, and the electric field signals of two adjacent pulses of the dual-polarization detection light are output through two channels of the dual-phase lock-in amplifier respectively, so as to obtain the full time domain waveform of the terahertz pulse light.

[0012] Furthermore, the balanced detector includes a balanced 1 / 4 wave plate, a Wollaston prism and a photodiode detection element; the optical axis direction of the balanced 1 / 4 wave plate is set to be along the polarization direction of the second polarized light, so that the intensities of the two polarization perpendicular components of the dual-polarized detection light are the same when there is no terahertz effect; the Wollaston prism is used to spatially separate pulses of different polarization states of the dual-polarized detection light, and propagate them to the photodiode detection element in different directions to output electrical signals respectively.

[0013] Preferably, the terahertz generating unit includes an excitation element and a terahertz chopper, the pump light interacts with the excitation element to generate a terahertz wave, and the terahertz chopper is arranged on the optical path of the terahertz wave, and is used to chop the terahertz wave into a terahertz pulse light with a repetition frequency of f. The measurement system also includes a delay line, which is used to delay and scan the flight time difference between the pump light and the detection light.

[0014] The present invention also provides a terahertz polarization measurement method based on electro-optical sampling, which uses the terahertz polarization measurement system of the above solution for measurement, and the specific steps are as follows:

[0015] S1: Use a femtosecond laser unit to split the output of pump light and probe light.

[0016] S2: The pump light and the excitation element interact to generate a terahertz wave, and the terahertz wave is chopped into a terahertz pulse light with a repetition frequency of f by a terahertz chopper; the detection light is modulated by a polarization modulation unit so that the detection light becomes a dual-polarization detection light, and the dual-polarization detection light has two polarization states, and the two polarization states are distributed at intervals in the pulse sequence of the dual-polarization detection light.

[0017] S3: Use a balanced detection unit to combine the dual-polarization detection light and the terahertz pulse light and focus them onto an electro-optical crystal, so that the electric field signals of the S light component and the P light component of the terahertz pulse light are respectively modulated on two adjacent pulses of the dual-polarization detection light, and then the electric fields of the two adjacent pulses are respectively collected to reconstruct the full time domain waveform of the terahertz pulse light.

[0018] The beneficial effects achieved by the present invention through the above technical solution are:

[0019] 1) By precisely controlling the polarization state, timing and delay of the optical pulse, the measurement error caused by inaccurate polarization state or time asynchrony is reduced.

[0020] 2) The polarization modulation unit modulates the intensity of the two polarized light beams to 1:2 before balanced detection, avoiding post-processing of the data of the S light component and the P light component of balanced detection, eliminating common mode noise, and improving the signal-to-noise ratio of detection.

[0021] 3) Different polarization states are modulated on two adjacent pulses of the detection light through a polarization modulation unit, and then combined with balanced detection technology, the full time domain waveform information of the terahertz pulse light can be constructed in a single scan, which not only simplifies the detection device and the optical path, avoids manual secondary scanning, improves the detection efficiency, but also improves the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the module structure of a terahertz polarization measurement system according to an embodiment of the present invention.

[0023] Figure 2 Schematic diagram of the optical structure of a terahertz polarization measurement system according to an embodiment of the present invention.

[0024] Figure 3 Schematic diagram of the basic principle of electro-optical sampling according to an embodiment of the present invention.

[0025] Figure 4 Schematic diagram of the control pulse timing of the terahertz polarization measurement system according to an embodiment of the present invention.

[0026] Figure 5 It is a schematic flow chart of a terahertz polarization measurement method according to an embodiment of the present invention.

[0027] Figure 6 1 is a diagram showing test results of a terahertz polarization measurement system according to an embodiment of the present invention.

[0028] Figure 7 This is a full-time-domain terahertz waveform diagram measured by an embodiment of the present invention.

[0029] The symbols in the figure represent:

[0030] 1. Probe light; 2. Terahertz pulse light; 3. Balanced detection unit; 10. Beam splitter; 11. First polarization control module; 12. Second polarization control module; 21. First polarized light; 22. Second polarized light; 31. Dual-polarized probe light; 13. Linear polarizer; 14. 1 / 4 wave plate; 15. Reflector; 16. Translation stage;

[0031] 101. first chopper; 102. second chopper; 201. terahertz chopper; 32. off-axis parabolic reflector; 33. electro-optic crystal; 34. lens; 35. Wollaston prism; 36. balanced 1 / 4 wave plate; 37. photodiode detection element; 38. dual-phase lock-in amplifier. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention more clear, the specific implementation of the present invention is further described in detail below in conjunction with examples. It should be understood that the embodiments described herein are only used to explain the present invention, but are not used to limit the scope of the present invention.

[0033] Embodiment 1

[0034] Please see attached Figure 1 , Figure 1 : It is a schematic diagram of the module structure of the terahertz polarization measurement system of an embodiment of the present invention. As can be seen from the figure, the system includes four modules: a femtosecond laser unit, a terahertz generation unit, a polarization modulation unit, and a balanced detection unit 3. Among them, the femtosecond laser unit is used to generate femtosecond pulse light and split it into pump light and detection light 1 output. The terahertz generation unit is used to receive the pump light to generate terahertz pulse light 2. The polarization modulation unit is used to modulate the detection light 1 into a dual-polarization detection light 31, and the dual-polarization detection light 31 has two polarization states, and the two polarization states are distributed at intervals in the pulse sequence of the dual-polarization detection light 31. The balanced detection unit 3 is used to combine the dual-polarization detection light 31 with the terahertz pulse light 2, and modulate the electric field signals of the S light component and the P light component of the terahertz pulse light 2 on two adjacent pulses of the dual-polarization detection light 31, and then collect the electric fields of the two adjacent pulses through the photodiode detection element 37, and reconstruct the full time domain waveform of the terahertz pulse light 2. The full time domain waveform includes amplitude, phase and polarization state information.

[0035] Exemplarily, the embodiment of the present invention uses a commercial titanium sapphire femtosecond laser with a regenerative amplification mechanism, which can provide femtosecond pulses with a repetition frequency of 1 kHz and a maximum energy of 4 mJ, a central wavelength of 800 nm, and a pulse width of about 100 fs. In one or other embodiments of the present invention, the pulse repetition frequency of the femtosecond pulse is recorded as 4f. The femtosecond pulses from the laser are divided into two beams by a non-polarizing plate beam splitter with a transmission-reflection ratio of 1:9 (T:R), wherein the reflected femtosecond pulses are sent into the terahertz generation unit as pump light, and the transmitted femtosecond pulses are propagated to the polarization modulation unit after passing through the optical delay line as detection light 1.

[0036] Exemplarily, the terahertz generation unit includes an excitation element and a terahertz chopper 201, the pump light interacts with the excitation element to generate a terahertz wave, the terahertz chopper 201 is arranged on the optical path of the terahertz wave, and the chopping frequency of the terahertz chopper 201 is f. In one or other embodiments of the present invention, the terahertz wave is generated by optical terahertz generation technology, such as pumping nonlinear crystal light rectification mechanism, two-color laser field ionization air generation, laser pumping photoconductive antenna generation, etc. In one or other embodiments of the present invention, the terahertz chopper 201 adopts an optical chopper designed with a phase-locked loop motor speed control. In one or other embodiments of the present invention, the excitation element is selected from lithium niobate wafers (LiNbO3).

[0037] Exemplarily, the balanced detection unit 3 includes a beam combiner, an electro-optical crystal 33 and a balanced detector. The beam combiner is used to combine the dual-polarization detection light 31 with the terahertz pulse light 2 and focus them on the electro-optical crystal 33. The balanced detector is connected to a dual-phase lock-in amplifier 38, and the balanced detector is used to detect the electric field signal of the dual-polarization detection light 31, and the electric field signals of two adjacent pulses of the dual-polarization detection light 31 are respectively output through two channels of the dual-phase lock-in amplifier 38 to obtain the full time domain waveform of the terahertz pulse light 2.

[0038] Exemplarily, the beam combiner of the embodiment of the present invention selects an off-axis parabolic reflector 32 with a focal length of 38 mm for reflective focusing of the terahertz pulse light 2. In one or other embodiments of the present invention, the off-axis parabolic reflector 32 is provided with a through hole for transmitting the dual-polarization detection light 31 so as to combine the dual-polarization detection light 31 with the terahertz pulse light 2 on the electro-optical crystal 33.

[0039] Exemplarily, in the balanced detection unit 3, the dual-polarized detection light 31 passes through the through hole of the off-axis parabolic reflector 32, is combined with the terahertz pulse light 2 and focused on the electro-optical crystal 33, and the detection signal is output to the dual-phase lock-in amplifier 38 by the balanced detector, and the signals of the S light component and the P light component of the terahertz pulse light 2 are output through the X channel and the Y channel of the dual-phase lock-in amplifier 38 respectively, so as to obtain the full time domain waveform of the terahertz pulse light 2.

[0040] In one or other embodiments of the present invention, the balanced detector includes a balanced 1 / 4 wave plate 36, a Wollaston prism 35, and a photodiode detection element 37; the optical axis direction of the balanced 1 / 4 wave plate 36 is set to be along the polarization direction of the second polarized light 22, so that when there is no modulation effect of the terahertz pulse light 2, the first polarized light 21 becomes a circularly polarized light after passing through the balanced 1 / 4 wave plate 36, and the polarization direction of the second polarized light 22 remains unchanged after passing through the balanced 1 / 4 wave plate 36. In this way, when the dual-polarized detection light 31 is not acted upon by the terahertz pulse light 2, no matter which polarization state the detection light pulse is in, two mutually perpendicular components of the same intensity can be obtained, so that they can be balanced and incident on the Wollaston prism 35, so as to achieve the purpose of balanced detection. The Wollaston prism 35 is used to spatially separate the pulses of different polarization states of the dual-polarized detection light 31, and propagate to the photodiode detection element 37 in different directions to output electrical signals respectively.

[0041] Please see attached Figure 2 , Figure 2 Schematic diagram of the optical structure of the terahertz polarization measurement system of an embodiment of the present invention. As can be seen from the figure, the polarization modulation unit includes a beam splitter 10, a first polarization control module 11 and a second polarization control module 12. The detection light 1 is divided into at least two beams by the beam splitter 10, one of which becomes a first polarized light 21 by the first polarization control module 11, and the other becomes a second polarized light 22 by the second polarization control module 12. The first polarized light 21 and the second polarized light 22 are both linearly polarized light, and the angle between the polarization directions of the first polarized light 21 and the second polarized light 22 is 45 degrees. In one or other embodiments of the present invention, the polarization direction of the first polarized light 21 is set to the horizontal direction, and the polarization direction of the second polarized light 22 is set to 45 degrees or -45 degrees. The beam splitter 10 is also used to combine the first polarized light 21 and the second polarized light 22 to form a dual-polarization detection light 31. The polarization modulation unit also includes a translation stage 16 for adjusting the time overlap of the first polarized light 21 and the second polarized light 22. In the embodiment of the present invention, the translation stage 16 is a Q545.240 nanometer-level translation stage produced by PI.

[0042] Preferably, the beam splitter 10 in the embodiment of the present invention is made of ultraviolet fused quartz.

[0043] Please see attached Figure 3 , Figure 3: is a basic principle diagram of electro-optical sampling of an embodiment of the present invention. For example, it is assumed that a beam of linearly polarized detection light has a polarization direction along the horizontal direction and passes through a 1 / 2 wave plate, an electro-optic crystal 33 and a balanced 1 / 4 wave plate 36 in sequence. Assuming that the angle between the optical axis of the balanced 1 / 4 wave plate 36 and the optical incident plane of the linearly polarized detection light is 45°, the electro-optic crystal 33 uses a 0.5 mm thick ZnTe (110), and the

[001] direction of the crystal is within the optical incident plane, then after the linearly polarized detection light passes through the balanced 1 / 4 wave plate 36, the expression for the light intensity difference between the P light component and the S light component is:

[0044] .

[0045] Wherein L is the thickness of the electro-optical crystal 33, ω is the angular frequency of the linearly polarized detection light, c is the speed of light in vacuum, n is the refractive index of the electro-optical crystal 33 for the linearly polarized detection light, is the electro-optic coefficient of the electro-optic crystal 33, is the intensity of the linearly polarized probe light, represents the angle between the electric field of the linearly polarized probe light and the optical axis (001) of the ZnTe crystal, represents the angle between the electric field of the terahertz pulse light 2 and the optical axis (001) of the ZnTe crystal. is the electric field intensity of the terahertz pulse light 2, is the X component of the electric field of the terahertz pulse light 2, is the Y component of the electric field of the terahertz pulse light 2. In this configuration, different polarization components of the terahertz pulse light 2 can be measured by changing the electric field of the linearly polarized probe light. When the electric field direction of the linearly polarized probe light is perpendicular to the ZnTe

[001] axis, the X component is measured. When the electric field direction of the linearly polarized probe light is at an angle of 45° to the ZnTe

[001] axis, half of the Y component is measured.

[0046] Therefore, in order to obtain the polarization information of the terahertz pulse light 2, the light intensity of one of the beams is usually doubled during data processing, but this inevitably amplifies the noise of one of the beams by two times, making the electrical signal noise levels of the two beams inconsistent, which ultimately affects the measurement accuracy. In an embodiment of the present invention, the intensity of the first polarized light 21 and the second polarized light 22 are modulated separately by a polarization modulation unit, so that the intensity of the second polarized light 22 is set to twice the intensity of the first polarized light 21. In this way, the intensity of the S light component and the P light component of the combined dual-polarized detection light 31 are equivalent, which not only enables the acquisition of all polarization information of the terahertz pulse light 2 in one scan, avoids repeated scanning, and improves the detection speed, but also avoids the noise amplification caused by data amplification in traditional technologies, improves the measurement signal-to-noise ratio, and thus can more accurately detect the full time domain waveform of the terahertz pulse. Specifically, one or other embodiments of the present invention use a beam splitter 10 with a beam splitting ratio of 5:5 to split the incident detection light 1 into two beams of equal intensity. In particular, the first polarization control module 11 and the second polarization control module 12 both include a linear polarizer 13, a quarter wave plate 14, and a reflector 15, which can adjust the light intensity ratio and polarization state of the first polarized light 21 and the second polarized light 22 within a certain range. In one or other embodiments of the present invention, the polarization direction of the first polarized light 21 is adjusted to a horizontal direction, so that the polarization direction of the second polarized light 22 is adjusted to be positive 45 degrees or negative 45 degrees with the horizontal direction.

[0047] In one or other embodiments of the present invention, the incident angles of the reflectors 15 of the first polarization control module 11 and the reflectors 15 of the second polarization control module 12 are set to zero degrees, even if the split detection light 1 is vertically incident on the reflectors 15 and then reflected back along the original path by the reflectors 15.

[0048] Exemplarily, the dual-polarized detection light 31 formed by combining the first polarized light 21 and the second polarized light 22 is focused by a convex lens with a focal length of 350 mm.

[0049] Exemplarily, the photodiode detection element 37 is a differential circuit composed of two photodiodes and a resistor. The dual-phase lock-in amplifier 38 includes a first phase-sensitive detector and a second phase-sensitive detector, the first phase-sensitive detector is used to multiply the signal with a reference signal, and the second phase-sensitive detector is used to multiply the signal with an orthogonal reference signal. In the demodulation process, the dual-phase lock-in amplifier 38 generates a DC and a high-frequency component, and then the high-frequency component is removed by low-pass filtering to obtain an X channel output and a Y channel output.

[0050] Exemplarily, a lens 34 is provided after the balanced quarter wave plate 36 to reduce the spot of the dual polarization detection light 31 and output it to the Wollaston prism 35. After the dual polarization detection light 31 is split and propagated by the Wollaston prism 35, it is focused on two diodes of the photodiode detection element 37 respectively.

[0051] Exemplarily, the dual-phase lock-in amplifier 38 of the embodiment of the present invention uses the STANFORD SR830 digital dual-phase lock-in amplifier.

[0052] Exemplarily, the first polarization control module 11 includes a first chopper 101, and the second polarization control module 12 includes a second chopper 102. The first chopper 101 is used to chop the first polarized light 21, and the second chopper 102 is used to chop the second polarized light 22. The chopping frequencies of the first chopper 101 and the second chopper 102 are both 2f; and the chopping phase difference between the first chopper 101 and the second chopper 102 is π.

[0053] Please see attached Figure 4 , Figure 4 Schematic diagram of the control pulse timing of the terahertz polarization measurement system according to an embodiment of the present invention. Figure 4 (a) is the pulse timing signal of the femtosecond pulse light, that is, a TTL level signal with the same frequency as the pulse signal of the femtosecond pulse light, and its repetition frequency is 4f. Figure 4 (b) and Figure 4 (c) are the pulse timing signals of the first chopper 101 and the second chopper 102, respectively, and their repetition frequencies are both 2f. Since the chopping phase difference between the first chopper 101 and the second chopper 102 is set to π, the high level of the pulse timing of the first chopper 101 corresponds to the low level of the pulse timing of the second chopper 102, and the low level of the pulse timing of the first chopper 101 corresponds to the high level of the pulse timing of the second chopper 102. In this way, the dual-polarization detection light 31 formed by the combination of the first polarized light 21 and the second polarized light 22 has a pulse repetition frequency of 4f. Figure 4 (d) is the pulse timing signal of the terahertz chopper 201, and its repetition frequency is f. That is, in a pulse cycle of the terahertz chopper 201, there are four femtosecond pulses, and the high-level window corresponds to two femtosecond pulse lights, one of which corresponds to the first polarized light 21, and the other corresponds to the second polarized light 22. Therefore, two consecutive terahertz pulse lights 2 correspond to different polarization states of the detection light, one from the first polarized light 21, and the other from the second polarized light 22.

[0054] The measurement system further comprises a delay line for delaying and scanning the flight time difference between the pump light and the detection light 1 .

[0055] Embodiment 2

[0056] Please see attached Figure 5 , Figure 5 The terahertz polarization measurement method based on electro-optical sampling provided in the embodiment of the present invention is a flow chart of the terahertz polarization measurement method according to the embodiment of the present invention. The terahertz polarization measurement method based on electro-optical sampling provided in the embodiment of the present invention uses the terahertz polarization measurement system described in the first embodiment of the present invention for measurement. The specific steps are as follows:

[0057] S1: Use a femtosecond laser unit to split the output of pump light and probe light 1;

[0058] S2: The pump light and the excitation element interact to generate a terahertz wave, and the terahertz wave is chopped into a terahertz pulse light 2 with a repetition frequency of f by a terahertz chopper 201; the detection light 1 is modulated by a polarization modulation unit so that the detection light 1 becomes a dual-polarization detection light 31, and the dual-polarization detection light 31 has two polarization states, and the two polarization states are distributed at intervals in the pulse sequence of the dual-polarization detection light 31;

[0059] S3: Use a balanced detection unit 3 to combine the dual-polarization detection light 31 and the terahertz pulse light 2 and focus them onto the electro-optical crystal 33, so that the electric field signals of the S light component and the P light component of the terahertz pulse light 2 are respectively modulated on two adjacent pulses of the dual-polarization detection light 31, and the electric fields of the two adjacent pulses are respectively collected by a balanced detector to reconstruct the full time domain waveform of the terahertz pulse light 2.

[0060] Embodiment 3

[0061] The embodiment of the present invention further provides an optical path debugging method of a terahertz polarization measurement system based on electro-optical sampling, which is used to debug the polarization measurement system of the first embodiment of the present invention. The specific steps are as follows:

[0062] S1: Debug polarization modulation unit:

[0063] 1) Adjust the polarization direction of the detection light 1 so that when the angle between the polarization direction of the linear polarizer 13 (including the linear polarizer 13 of the first polarization control module 11 and the second polarization control module 12) and the horizontal direction is zero, the corresponding detection light 1 has the maximum transmittance. Using the detection light 1 as the angle calibration reference of the optical element, set the polarization direction of the linear polarizer 13 of the first polarization control module 11 to zero, and set the polarization direction of the linear polarizer 13 of the second polarization control module 12 to 45 or -45.

[0064] 2) Adjust the time overlap of the first polarized light 21 and the second polarized light 22, remove the first chopper 101 and the second chopper 102, replace the electro-optical crystal 33 of the balanced detection unit 3 with a CMOS or CCD camera that responds better to the detection light 1, use the Daheng Image MER2 series CMOS camera to image the dual-polarized detection light 31, and observe the interference fringes of the first polarized light 21 and the second polarized light 22. In order to better observe the interference, a linear polarizer can be placed between the off-axis parabolic reflector 32 and the beam splitter 10.

[0065] 3) Place the chopper, and adjust the phase of the first chopper 101 and the second chopper 102 so that the chopping phases of the first chopper 101 and the second chopper 102 differ by π. Specifically, connect a pulse train signal with a frequency of 4f to the external trigger interface of the first chopper 101 and the second chopper 102, and set the chopping frequency of the two choppers to 2f. Replace the CMOS or CCD with a photodiode, and use an oscilloscope to measure the signal of the photodiode. Adjust the phase of the first chopper 101 and the second chopper 102 so that when the first polarized light 21 or the second polarized light 22 is blocked alone, the pulse frequency measured by the oscilloscope is 2f, and when there is no obstruction, the pulse frequency measured by the oscilloscope is 4f.

[0066] 4) Adjust the intensity of the first polarized light 21 and the second polarized light 22. Adjust the quarter wave plate 14 of the first polarization control module 11 and the quarter wave plate 14 of the second polarization control module 12 so that the intensity of the second polarized light 22 is twice the intensity of the first polarized light 21. The first polarized light 21 is horizontally polarized, and the angle between the polarization direction of the second polarized light 22 and the polarization direction of the first polarized light 21 is 45 degrees.

[0067] S2: Debugging balance detection unit 3:

[0068] 1) Using a terahertz polarizer, the terahertz pulse light 2 is made into a horizontally polarized linear polarization state.

[0069] 2) Connect the chopped TTL signal of the terahertz chopper 201 to the reference signal interface of the dual-phase lock-in amplifier 38, turn off the second polarized light 22, and only allow the first polarized light 21 to be incident on the electro-optical crystal 33, rotate the electro-optical crystal 33 to maximize the detection signal, and adjust the phase of the reference signal of the dual-phase lock-in amplifier 38 to maximize the output of the X channel and zero the output of the Y channel.

[0070] 3) Rotate the electro-optic crystal 33 to make the X channel output of the dual-phase lock-in amplifier 38 zero.

[0071] 4) Rotate the electro-optic crystal 33 about 35 degrees or 55 degrees to maximize the X channel output of the dual-phase lock-in amplifier 38. At this time, the polarization direction of the terahertz pulse light 2 is parallel to the polarization direction of the first polarized light 21, both are in the horizontal direction and perpendicular to the 001 optical axis direction of the electro-optic crystal 33.

[0072] 5) Turn off the first polarized light 21 and allow only the second polarized light 22 to enter the electro-optical crystal 33 , and adjust the linear polarizer 13 of the second polarization control module 12 so that the X channel output of the dual-phase lock-in amplifier 38 is zero.

[0073] 6) Adjust the terahertz polarizer to make the terahertz pulse light 2 a linear polarization state with vertical polarization, so that the dual-phase lock-in amplifier 38 has only Y channel output and the X channel output is zero. The X channel output of the dual-phase lock-in amplifier 38 only responds to the X component of the terahertz pulse wave, and the Y channel output only responds to the Y component of the terahertz pulse wave.

[0074] 7) Input the first polarized light 21 and the second polarized light 22 to the electro-optical crystal 33 at the same time, adjust the terahertz pulse light 2 to a linear polarization state of 45 degrees, and fine-tune the 1 / 4 wave plate 14 of the first polarization control module 11 and the second polarization control module 12 to make the intensity of the output signal of the X channel and the Y channel of the dual-phase lock-in amplifier 38 the same.

[0075] Please see attached Figure 6 , Figure 6 1 is a test result diagram of the terahertz polarization measurement system of the embodiment of the present invention. The horizontal axis in the figure is the polarization angle of the terahertz polarizer, and the vertical axis is the polarization angle of the terahertz pulse light 2 measured by the terahertz polarization measurement system of the first embodiment. The hollow circles in the figure are the data measured under different polarization angles, and the black solid line is the data fitted by theoretical calculation.

[0076] Please see attached Figure 7 , Figure 7 This is a full-time-domain terahertz waveform diagram measured by an embodiment of the present invention. represents the electric field X component of the terahertz pulse light 2, The Y axis represents the electric field Y component of the terahertz pulse light 2, and the Z axis represents time. The results show that the terahertz polarization measurement system of Example 1 can obtain a terahertz full-time domain waveform diagram with a higher signal-to-noise ratio, including the amplitude, phase, and polarization state of the terahertz pulse light 2 that changes with time.

[0077] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A terahertz polarization measurement system based on electro-optical sampling, characterized in that: include: A femtosecond laser unit, used for generating femtosecond pulse light and splitting it into pump light and detection light output; A terahertz generating unit, used for receiving the pump light to generate terahertz pulse light; A polarization modulation unit, used for modulating the detection light into a dual-polarization detection light, wherein the dual-polarization detection light has two polarization states, and the two polarization states are distributed at intervals in a pulse sequence of the dual-polarization detection light; A balanced detection unit is used to combine the dual-polarization detection light with the terahertz pulse light, and modulate the electric field signals of the S light component and the P light component of the terahertz pulse light on two adjacent pulses of the dual-polarization detection light respectively, and then collect the electric fields of the two adjacent pulses respectively to reconstruct the full time domain waveform of the terahertz pulse light; the full time domain waveform includes amplitude, phase and polarization state information.

2. The measuring system according to claim 1, characterized in that The polarization modulation unit includes a beam splitter, a first polarization control module and a second polarization control module; The detection light is split into at least two beams by the beam splitter, one of which becomes a first polarized light by the first polarization control module, and the other becomes a second polarized light by the second polarization control module; the first polarized light and the second polarized light are both linearly polarized light, and the angle between the polarization directions of the first polarized light and the second polarized light is 45 degrees; The beam splitter is further used to combine the first polarized light and the second polarized light to form dual-polarized detection light.

3. The measuring system according to claim 2, characterized in that Also includes a first chopper and a second chopper, wherein the first chopper is used to chop the first polarized light, and the second chopper is used to chop the second polarized light; The chopping frequencies of the first chopper and the second chopper are both 2f; the chopping phase difference between the first chopper and the second chopper is π, wherein the pulse repetition frequency of the femtosecond pulse light is 4f.

4. The measuring system according to claim 3, characterized in that The first polarization control module and the second polarization control module each include a linear polarizer, a quarter wave plate and a reflector; The polarization direction of the first polarized light is set to be horizontal, the polarization direction of the second polarized light is set to be at an angle of 45 degrees to the horizontal direction, and the light intensity of the second polarized light is set to be twice the light intensity of the first polarized light.

5. The measuring system according to claim 3, characterized in that The polarization modulation unit further includes a translation stage for adjusting the time overlap of the first polarized light and the second polarized light.

6. The measuring system according to claim 1, characterized in that The balanced detection unit includes a beam combiner, an electro-optical crystal and a balanced detector; The beam combiner is used to combine the dual-polarization detection light with the terahertz pulse light and focus them on the electro-optical crystal; The balanced detector is connected to a dual-phase lock-in amplifier, and is used to detect the electric field signal of the dual-polarization detection light. The electric field signals of two adjacent pulses of the dual-polarization detection light are output through two channels of the dual-phase lock-in amplifier respectively to obtain the full time domain waveform of the terahertz pulse light.

7. The measuring system according to claim 6, characterized in that The balanced detector includes a balanced 1 / 4 wave plate, a Wollaston prism and a photodiode detection element; The optical axis direction of the balanced quarter wave plate is set so that the components of the dual-polarization detection light along the two optical axis directions of the Wollaston prism are the same after passing through the balanced quarter wave plate when there is no terahertz pulse light modulation; The Wollaston prism is used to spatially separate pulses of different polarization states of the dual-polarization detection light, and propagate them in different directions to the photodiode detection element to output electrical signals respectively.

8. The measuring system according to claim 1, characterized in that The terahertz generating unit comprises an excitation element and a terahertz chopper. The pump light interacts with the excitation element to generate a terahertz wave. The terahertz chopper is arranged on the optical path of the terahertz wave and is used to chop the terahertz wave into terahertz pulse light with a repetition frequency of f.

9. The measuring system according to claim 1, characterized in that The measurement system further includes a delay line for delaying and scanning the flight time difference between the pump light and the detection light.

10. A terahertz polarization measurement method based on electro-optical sampling, the method using the measurement system of claim 1, characterized in that: S1: Use a femtosecond laser unit to split the output of pump light and probe light; S2: interacting the pump light with the excitation element to generate a terahertz wave, and chopping the terahertz wave into a terahertz pulse light with a repetition frequency of f by a terahertz chopper; Modulating the detection light by a polarization modulation unit so that the detection light becomes a dual-polarization detection light, wherein the dual-polarization detection light has two polarization states, and the two polarization states are alternately distributed in a pulse sequence of the dual-polarization detection light; S3: Use a balanced detection unit to combine the dual-polarization detection light with the terahertz pulse light and focus them onto an electro-optical crystal, so that the electric field signals of the S light component and the P light component of the terahertz pulse light are respectively modulated on two adjacent pulses of the dual-polarization detection light, and then the electric fields of the two adjacent pulses are respectively collected to reconstruct the full time domain waveform of the terahertz pulse light.

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