A THz optical comb spectroscopy detection device and method for frequency conversion and fast scanning

Abstract

The present invention relates to the technical field of terahertz waves, and particularly relates to a THz optical comb spectroscopy detection device and method for frequency conversion and fast scanning. The device includes a signal optical comb module, a sampling optical comb module, a THz radiation generation module, a THz electro-optic sampling detection module, and a signal acquisition and processing module. The signal optical comb module and the sampling optical comb module are equipped with frequency conversion modules for frequency conversion and use the same clock signal as the signal acquisition and processing module. The THz pulse output by two 4F confocal spatial systems in the THz radiation generation module and the sampling pulse output by the second light beam focusing component are jointly input into the THz electro-optic sampling detection module for detecting the change of THz electric field. The signal acquisition and processing module is used to obtain the THz electric field change data from the THz electro-optic sampling detection module and convert it into high-resolution THz spectral information. The present invention can achieve fast frequency conversion and high-resolution THz spectral sampling.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz wave, and particularly to a THz optical comb spectroscopy detection device and method for frequency conversion and fast scanning. Background Art

[0002] Terahertz (THz) lies between the research fields of electromagnetics and optics, and has both characteristics of them, such as safety, penetrability, and material characteristic spectrum resolution characteristics, etc., and has wide applications in the fields of biomedical imaging, security and anti-terrorism, material characterization, etc. The generation and detection of THz are important links in the research of terahertz time-domain spectroscopy (THz-TDS), and the resolution, speed, and sensitivity of spectral measurement are important indicators for THz-TDS to move towards practical application.

[0003] Based on the dual optical comb asynchronous electro-optic sampling technology, by using the beat frequency of two optical combs with slightly different repetition frequencies, the THz time-frequency domain information can be obtained in a non-scanning manner. However, the THz pulse width is usually on the order of several picoseconds, its pulse period is above nanoseconds, and the sampling pulse width is on the order of femtoseconds. Therefore, during the asynchronous sampling process, the interval between two periods is relatively long, which limits the sampling speed. By frequency-converting the repetition frequency of the sampling optical comb, precise sampling of the signal part can be achieved. However, the optical comb light source based on a mode-locked laser is limited by the fixed cavity length, and its repetition frequency tuning range is limited, and large-scale frequency conversion cannot be achieved. The electro-optic frequency comb (electro-optic comb) can be generated by intensity modulation or phase modulation of a CW laser, and its repetition frequency is determined by the driving frequency of the electro-optic modulator, and can be flexibly adjusted in the range of MHz to GHz, and can replace the traditional fixed-frequency light source to achieve frequency conversion. However, the output optical pulse width of the electro-optic comb is limited by the modulated electrical pulse, so only ps pulse output can be achieved; currently, through nonlinear medium broadening and dispersion control, an fs electro-optic comb light source can be obtained, but its repetition frequency is mostly in the GHz range, and the peak power is low, and it cannot generate THz pulses or be used as a sampling pulse for high-resolution sampling. Summary of the Invention

[0004] One of the purposes of the present invention is to provide a THz optical comb spectroscopy detection device for frequency conversion and fast scanning, which can achieve fast frequency conversion and high-resolution THz spectral sampling.

[0005] To achieve the above purpose, a THz optical comb spectroscopy detection device for frequency conversion and fast scanning is provided, including a signal optical comb module, a sampling optical comb module, a THz radiation generation module, a THz electro-optic sampling detection module, and a signal acquisition and processing module; the signal optical comb module, the sampling optical comb module, and the signal acquisition and processing module use the same clock signal;

[0006] The signal optical comb module includes a first signal generator, a first femtosecond electro-optic comb, and a first light beam focusing component connected in sequence;

[0007] The THz radiation generation module includes a first photoconductive antenna, two 4F confocal spatial systems, and a sample cell. The signal optical comb beam collected by the first optical beam focusing component is injected into the first photoconductive antenna, and the generated THz pulse is passed through the two 4F confocal spatial systems. The sample cell is located at the focal points of the two 4F confocal systems; the 4F confocal spatial system includes a pair of off-axis parabolic mirrors;

[0008] The sampling optical comb module includes a second signal generator, a second femtosecond electro-optical comb, and a second optical beam focusing component connected in sequence to output a sampling optical comb; there is an optical comb frequency conversion module between the signal optical comb module and the sampling optical comb module, and the optical comb frequency conversion module is used to perform signal optical comb frequency conversion or sampling optical comb frequency conversion according to the first signal generator and the second signal generator;

[0009] The THz pulses output by the two 4F confocal spatial systems and the sampling pulses output by the second optical beam focusing component are jointly input into a THz electro-optical sampling detection module for detecting changes in the THz electric field;

[0010] The signal acquisition and processing module is used to obtain THz electric field change data from the THz electro-optical sampling detection module and convert it into high-resolution THz spectrum information.

[0011] Furthermore, both the first femtosecond electro-optical comb and the second femtosecond electro-optical comb include a continuous laser, and the optical frequency of the continuous laser is locked to a stable cavity.

[0012] Furthermore, the THz electro-optical sampling detection module includes an electro-optical crystal, a quarter-wave plate, a first mirror, a Wollaston prism, and a balanced detector connected by an optical path; the THz pulses output by the two 4F confocal spatial systems and the focused light beam output by the second optical beam focusing component are jointly input into the electro-optical crystal; the balanced detector is used to detect changes in the THz electric field to obtain THz electric field change data.

[0013] Furthermore, the THz electro-optical sampling detection device includes a second mirror, a third mirror, a second photoconductive antenna, and a current amplifier connected by an optical path. The THz pulse is injected into the second photoconductive antenna, and the sampling pulse is injected into the second photoconductive antenna through the second mirror and the third mirror to form a photocurrent; then the current signal is amplified by the current amplifier to obtain THz electric field change data.

[0014] Furthermore, the signal acquisition and processing module includes a data acquisition card. The data acquisition card is electrically connected to the balanced detector. The data acquisition card includes a Fourier transform sub-module, and the Fourier transform sub-module is used to convert the THz electric field change data into high-resolution THz spectrum information.

[0015] Further, a frequency doubling crystal is provided between the first femtosecond electro-optic comb and the first light beam focusing component; a frequency doubling crystal is also provided between the second femtosecond electro-optic comb and the second light beam focusing component.

[0016] Further, it further includes a third optical comb module, which includes a third signal generator, a third femtosecond electro-optic comb, and a third light beam focusing component connected in sequence, as well as a fourth mirror, a delay line, a fifth mirror, a sixth mirror, and a third photoconductive antenna; a beam splitter is provided between a pair of off-axis parabolic mirrors of the 4F confocal system to jointly input the first THz optical comb output by the first photoconductive antenna and the third THz optical comb output by the third photoconductive antenna into the beam splitter 11 for beam combination.

[0017] Further, when selecting the sampling optical comb for frequency conversion, the optical comb frequency conversion module is used to control the first signal generator to generate a frequency sweep trigger signal and send it to the second signal generator; when selecting the signal optical comb for frequency conversion, the optical comb frequency conversion module is used to control the second signal generator to generate a frequency sweep trigger signal and send it to the first signal generator and the third signal generator.

[0018] Further, it further includes a fourth optical comb module, which includes a fourth signal generator, a fourth femtosecond electro-optic comb, and a seventh mirror connected in sequence. The output light of the fourth femtosecond electro-optic comb is obliquely incident into the sample cell through a mirror.

[0019] The second object of the present invention is to provide a THz optical comb spectroscopy detection method for frequency conversion and fast scanning, which is applied to the above device, and specifically includes the following steps:

[0020] S1. Set the output frequency of the first signal generator to f r1 , and the output frequency of the second signal generator to f r2 ;

[0021] S2. Determine that the frequency conversion object of the optical comb frequency conversion module is the first signal generator or the second signal generator; when the frequency conversion object is the first signal generator, execute step S4; when the frequency conversion object is the second signal generator, execute step S3;

[0022] S3. Make the first signal generator output two signals, one is a sine wave signal, and the first THz optical comb is output through the first photoconductive antenna; the other square wave signal enters the frequency sweep trigger terminal of the second signal generator as a frequency sweep trigger signal, and its period is N times the period of the THz pulse; then execute step S5;

[0023] S4. Make the second signal generator output two signals, one being a sine wave signal as the sampling optical comb, and the other square wave signal entering the frequency-sweeping trigger terminal of the second signal generator as the frequency-sweeping trigger signal, whose period is N times the period of the THz pulse; then perform step S6;

[0024] S5. When the trigger starts, set the sampling pulse frequency to f r2 = f r1 ±Δ f , so that the THz electro-optic sampling detection module obtains THz electric field change data according to the THz pulse, the sampling optical comb and the frequency-sweeping trigger signal;

[0025] S6. When the trigger starts, set the THz pulse frequency to f r1 = f r2 ±Δ f , so that the THz electro-optic sampling detection module obtains THz electric field change data according to the THz pulse, the sampling optical comb and the frequency-sweeping trigger signal;

[0026] S7. Process the THz electric field change data obtained in step S5 or step S6 through the signal acquisition and processing module to convert it into high-resolution THz spectrum information.

[0027] Advantages:

[0028] 1. Compared with traditional terahertz asynchronous sampling, the asynchronous frequency conversion electro-optic sampling technology is incorporated, that is, the sampling pulse is scanned back and forth. For example, when the repetition frequency f r2 of the sampling optical comb is fixed, the scanning time for a single cycle is 1 / Δ f ; and when f r2 jumps between two values of ( f r1 +Δ f ) and ( f r1 -Δ f ), the sampling pulse scans the first THz pulse back and forth, and the scanning time for a single cycle will be shortened, and the shortening factor is the ratio of the entire THz pulse period 1 / f r1 to the THz pulse width (signal part). Frequency conversion fast scanning is realized. Finally, a data acquisition card is used to obtain the THz time-domain spectrum, and high-resolution spectrum information is obtained through Fourier transform, realizing fast high-resolution sampling of the THz time-domain spectrum within the ns time scale.

[0029] 2. The THz optical comb spectrum detection device and method with variable-frequency fast scanning proposed by the present invention do not need to keep the repetition frequencies of the THz optical comb and the sampling optical comb consistent, and can still achieve fast scanning under harmonic conditions. It can also be combined with the three-optical-comb spectrum technology, replace the delay line in the three-optical-comb system with a cascaded double optical comb, and utilize the asynchronous frequency conversion technology to realize the fast measurement of THz two-dimensional spectrum. Thus, the applicable scenarios are more extensive.

[0030] 3. The measurement accuracy of the terahertz optical comb spectrum is further improved. Description of the Drawings

[0031] Figure 1 It is the working framework diagram of asynchronous frequency conversion electro-optic sampling - sampling optical comb frequency conversion in Embodiment 1;

[0032] Figure 2 It is the schematic principle diagram of asynchronous frequency conversion electro-optic sampling - sampling optical comb frequency conversion in Embodiment 1;

[0033] Figure 3 It is the working framework diagram of asynchronous frequency conversion electro-optic sampling - THz optical comb frequency conversion in Embodiment 1;

[0034] Figure 4 It is the schematic principle diagram of asynchronous frequency conversion electro-optic sampling - THz optical comb frequency conversion in Embodiment 1;

[0035] Figure 5 It is the schematic principle diagram of harmonic asynchronous frequency conversion electro-optic sampling - sampling optical comb frequency conversion in Embodiment 2;

[0036] Figure 6 It is the schematic principle diagram of harmonic asynchronous frequency conversion electro-optic sampling - THz optical comb frequency conversion in Embodiment 2;

[0037] Figure 7 It is the structural schematic diagram of the THz optical comb spectrum detection device - sampling optical comb frequency conversion in Embodiment 1;

[0038] Figure 8 It is the structural schematic diagram of the THz optical comb spectrum detection device - THz optical comb frequency conversion in Embodiment 1;

[0039] Figure 9 It is the structural schematic diagram of asynchronous frequency conversion photoconductive sampling - sampling optical comb frequency sweep in Embodiment 3;

[0040] Figure 10 It is the structural schematic diagram of the THz two-dimensional spectrum detection device - sampling optical comb frequency conversion based on asynchronous frequency conversion in Embodiment 4;

[0041] Figure 11 It is the structural schematic diagram of the THz two-dimensional spectrum detection device - THz optical comb frequency conversion based on asynchronous frequency conversion in Embodiment 4;

[0042] Figure 12 Schematic structural diagram of the sampling optical comb frequency conversion for the THz optical comb spectroscopy detection device based on pump-probe in Embodiment 5;

[0043] Figure 13 Diagram of the generating device and spectral pulse width of fs electro-optic comb pulses in Embodiment 5.

[0044] Figure 14 Diagram of electro-optic comb pulses with different repetition frequencies in Embodiment 5. Specific implementation manners

[0045] The following is a further detailed description through specific implementation manners:

[0046] The reference numerals in the accompanying drawings of Embodiment 1 of the specification include: first femtosecond electro-optic comb 1, second femtosecond electro-optic comb 2, first signal generator 3, second signal generator 4, first frequency doubling crystal 5, second frequency doubling crystal 6, trigger terminal 7, hydrogen clock 8, first lens 9, second lens 10, first photoconductive antenna 11, off-axis parabolic mirror 12, sample cell 13, third lens 14, fourth lens 15, electro-optic crystal 16, first mirror 17, quarter-wave plate 18, Wollaston prism 19, balanced detector 20, data acquisition card 21.

[0047] Embodiment 1

[0048] A THz optical comb spectroscopy detection device for frequency conversion and fast scanning, substantially as Figure 7 、 Figure 8 shown, including a signal optical comb module, a sampling optical comb module, a THz radiation generation module, a THz electro-optic sampling detection module, and a signal acquisition and processing module; the signal optical comb module, the sampling optical comb module, and the signal acquisition and processing module use the same clock signal and are all referenced to the hydrogen clock 8.

[0049] The signal optical comb module includes a first signal generator 3, a first femtosecond electro-optic comb 1, and a first light beam collimating assembly connected in sequence; the first femtosecond electro-optic comb 1 includes structures such as a continuous laser, an intensity modulator, an electrical pulse generator, an optical fiber amplifier, a nonlinear optical fiber, and a single-mode optical fiber, and the optical frequency of the continuous laser is locked to a stable cavity.

[0050] The first femtosecond electro-optical comb 1 is a femtosecond optical comb light source generated by intensity modulation with an adjustable repetition frequency. By combining power amplification, spectral broadening, and pulse width compression, two high-power frequency-agile femtosecond electro-optical comb light sources are obtained. The generation method of the femtosecond optical comb light source is as follows: The first signal generator 3 outputs an MHz sine wave, and the electrical pulse generator modulates the sine wave into a ps electrical pulse, which serves as the driving signal for the intensity modulator. After the continuous light is modulated by the intensity modulator, ps optical pulses with a repetition frequency consistent with the sine wave frequency are generated. After the ps optical pulses pass through the fiber amplifier, high-peak-power laser pulses are obtained; spectral broadening is achieved through a nonlinear optical fiber, and pulse width compression is performed using a single-mode optical fiber to obtain a femtosecond optical comb light source with a pulse width less than 300 fs, which serves as the signal optical comb. The frequency is f r1 . After passing through the first frequency doubling crystal 5, it then enters the first light beam focusing component.

[0051] The first light beam focusing component includes a first lens 9 and a second lens 10 that reduce the spot size. The focal lengths are 200 mm and 50 mm respectively, and the spot size is reduced from 2 mm to 500 μm.

[0052] The THz radiation generation module includes a first photoconductive antenna 11, two 4F confocal spatial systems, and a sample cell 13. The signal optical comb focused by the first light beam focusing component is injected into the first photoconductive antenna 11, and the generated THz pulses are passed through the two 4F confocal spatial systems. The sample cell 13 is located at the focal point of the two 4F confocal systems; the 4F confocal spatial system includes a pair of off-axis parabolic mirrors 12;

[0053] The sampling optical comb module includes a second signal generator 4, a second femtosecond electro-optical comb 2, and a second light beam focusing component connected in sequence; the second femtosecond electro-optical comb 2 is the same as the first femtosecond electro-optical comb 1, and both include structures such as a continuous laser, an intensity modulator, an electrical pulse generator, a fiber amplifier, a nonlinear optical fiber, and a single-mode optical fiber. The optical frequency of the continuous laser is locked to a stable cavity. The generated femtosecond optical comb light source serves as the sampling optical comb, and the frequency is f r2 . After passing through the second frequency doubling crystal 6, it then enters the first light beam focusing component.

[0054] The second light beam focusing component includes a third lens 14 and a fourth lens 15 that reduce the spot size. The focal lengths are also 200 mm and 50 mm respectively, and the spot size is reduced from 2 mm to 500 μm.

[0055] An optical comb frequency conversion module is provided between the signal optical comb module and the sampling optical comb module. The optical comb frequency conversion module is used to perform signal optical comb frequency conversion or sampling optical comb frequency conversion according to the first signal generator 3 and the second signal generator 4.

[0056] The optical comb frequency conversion module is used to control the first signal generator 3 or the second signal generator 4 to generate a frequency-sweeping trigger signal and send it to the other party. For example, when the first signal generator 3 is selected to send a frequency-sweeping trigger signal to the second signal generator 4, it belongs to sampled optical comb frequency conversion. As Figure 1 , Figure 2 shown, the first signal generator 3 (corresponding to signal generator a) outputs two signals. One outputs a sine wave signal, and a signal optical comb is generated through the intensity modulation of the first femtosecond electro-optic comb. The other outputs a square wave signal and is connected to the trigger terminal 7 of the second signal generator 4 (corresponding to signal generator b) as the frequency-sweeping trigger signal of the sampled optical comb, and its frequency is f rs , f rs = 1 / N * f r1 , where N represents that the period is N times the period of the THz pulse.

[0057] As Figure 3 , Figure 4 shown, when the second signal generator 4 (corresponding to signal generator b) is selected to send a frequency-sweeping trigger signal to the first signal generator 3 (corresponding to signal generator a), it belongs to signal optical comb frequency conversion (i.e., THz optical comb frequency conversion). The second signal generator 4 will output a square wave signal again and connect it to the trigger terminal 7 of the first signal generator 3 as the frequency-sweeping trigger signal of the signal optical comb, and its frequency is f rs , or f rs = 1 / N * f r2 , where N represents that the period is N times the period of the THz pulse.

[0058] The THz pulse output by the two 4F confocal spatial systems and the sampled pulse output by the second light beam focusing component are jointly input into the THz electro-optic sampling detection module for detecting the change of the THz electric field; the frequency of the THz pulse is f r1 (refer to Figure 2 ), or f r1 = f r2 ±Δ f (refer to Figure 4 ). The frequency of the sampled optical comb is f r2 (refer to Figure 2 ), or f r2 = f r1 ±Δ f (refer to Figure 4 ). In this embodiment, asFigure 7 , Figure 8 As shown, the THz electro-optic sampling detection module includes an electro-optic crystal 16, a quarter-wave plate 18, a first mirror 17, a Wollaston prism 19, and a balanced detector 20 that are optically connected. The THz signal after passing through the sample cell 13 and the output light of the sampling optical comb in the frequency sweep are jointly incident on the electro-optic crystal 16. The quarter-wave plate 18, the Wollaston prism 19, and the balanced detector 20 are used to detect the change of the THz electric field. Sampling points are obtained through the THz electro-optic sampling detection module, and then the sampling waveform is obtained based on the sampling points. For example, when N = 8, positive sampling points are sequentially extracted at different positions in the first N / 2 = 4 cycles before the THz pulse; starting from the end of the 5th cycle, when the square-wave signal is in the falling-edge state, the sampling pulse frequency is changed to f r2 = f r1 + Δ f , and the sampling pulse extracts reverse sampling points in the reverse direction in the last N / 2 = 4 cycles of the THz pulse, and analyzes and processes based on the positive sampling points and the reverse sampling points to achieve asynchronous frequency-converted electro-optic sampling, thereby obtaining THz electric field change data;

[0059] The signal acquisition and processing module is used to obtain the THz electric field change data from the THz electro-optic sampling detection module and convert it into high-resolution THz spectral information. In this embodiment, the signal acquisition and processing module includes a data acquisition card 21. The data acquisition card 21 is electrically connected to the balanced detector 20. The data acquisition card 21 includes a Fourier transform sub-module, and the Fourier transform sub-module is used to convert the THz electric field change data into high-resolution THz spectral information.

[0060] A THz optical comb spectroscopy detection method for frequency-converted fast scanning is applied to the above device, and specifically includes the following steps:

[0061] S1. Set the output frequency of the first signal generator 3 to f r1 , and the output frequency of the second signal generator 4 to f r2 ;

[0062] S2. Determine that the frequency-conversion object of the optical comb frequency-conversion module is the first signal generator 3 or the second signal generator 4; when the frequency-conversion object is the first signal generator 3, execute step S4; when the frequency-conversion object is the second signal generator 4, execute step S3;

[0063] S3. Make the first signal generator 3 output two signals. One is a sine wave signal, which outputs the first THz optical comb through the first photoconductive antenna 11. The other square wave signal enters the frequency-sweeping trigger terminal 7 of the second signal generator 4 as a frequency-sweeping trigger signal, and its period is N times the period of the THz pulse. Then perform step S5;

[0064] S4. Make the second signal generator 4 output two signals. One is a sine wave signal as the sampling optical comb. The other square wave signal enters the frequency-sweeping trigger terminal 7 of the second signal generator 4 as a frequency-sweeping trigger signal, and its period is N times the period of the THz pulse. Then perform step S6;

[0065] S5. When the trigger starts, set the sampling pulse frequency to f r2 = f r1 ±Δ f , and make the THz electro-optic sampling detection module obtain the THz electric field change data according to the THz pulse, the sampling optical comb, and the frequency-sweeping trigger signal. Specifically, set the sampling pulse frequency to f r2 = f r1 -Δ f , and after photoelectric conversion, extract positive sampling points at different positions in the first N / 2 cycles of the THz pulse in sequence. When the square wave signal is in the falling edge state, change the sampling pulse frequency to f r2 = f r1 +Δ f , the sampling pulse extracts reverse sampling points in the reverse direction in the last N / 2 cycles of the THz pulse, and analyzes and processes according to the positive sampling points and the reverse sampling points to achieve asynchronous frequency conversion electro-optic sampling, so as to obtain the THz electric field change data;

[0066] S6. When the trigger starts, set the THz pulse frequency to f r1 = f r2 ±Δ f , and make the THz electro-optic sampling detection module obtain the THz electric field change data according to the THz pulse, the sampling optical comb, and the frequency-sweeping trigger signal. Specifically, set the THz pulse frequency to f r1 = f r2 -Δ f， And after photoelectric conversion, extract positive sampling points at different positions in the first N / 2 cycles of the THz pulse in sequence. When the square wave signal is in the falling edge state, change the THz pulse frequency to f r1 = fr2 +Δ f The sampling pulse extracts reverse sampling points in the latter N / 2 cycles of the THz pulse and analyzes and processes based on the forward sampling points and reverse sampling points to achieve asynchronous frequency conversion electro-optic sampling, thereby obtaining THz electric field change data;

[0067] S7. Process the THz electric field change data obtained in step S5 or step S6 through a signal acquisition and processing module to convert it into high-resolution THz spectrum information.

[0068] Embodiment 2

[0069] The difference between Embodiment 2 and Embodiment 1 is that, as shown in Figure 7 、 Figure 8 the THz optical comb spectrum detection device, harmonic asynchronous frequency conversion electro-optic sampling can be achieved. When the trigger starts, the sampling pulse frequency is set to f r2 = f r1 / n - Δ f , where n is the harmonic number, and forward sampling points are sequentially extracted at different positions in the first N / 2 cycles of the THz pulse; when the square wave signal is in the falling edge state, the sampling pulse frequency is changed to f r2 = f r1 / n +Δ f , the sampling pulse extracts reverse sampling points in the latter N / 2 cycles of the THz pulse and analyzes and processes based on the forward sampling points and reverse sampling points to obtain THz electric field change data. As shown in Figure 5 、 Figure 6 , the harmonic number n = 2, the frequency of the swept-frequency trigger signal is f rs , or f rs = 1 / N * f r2 , N = 16, indicating that the period is 16 times the period of the THz pulse.

[0070] As shown in Figure 5 , when the sampling optical comb is frequency-converted, the repetition frequency f r2 is in ( f r1 / n +Δ f ) and ( f r1 / n -Δ f)A jump between two numerical values is made so that the sampling pulse can still scan the signal part within the THz pulse period while skipping the noise part, achieving frequency conversion fast scanning; as Figure 6 shown, when frequency-converting the THz optical comb, there is the following relationship with the repetition frequency of the sampling optical comb and f r2 : f r1 = f r2 / n ± Δ f , the repetition frequency of the THz optical comb f r1 jumps between two numerical values of ( f r2 / n + Δ f ) and ( f r2 / n - Δ f ) to achieve frequency conversion fast scanning.

[0071] In the THz optical comb spectral detection method for frequency conversion fast scanning, when the device is applied to harmonic asynchronous frequency conversion electro-optic sampling, step S5 further includes the following steps:

[0072] S501. When the trigger starts, set the sampling pulse frequency as f r2 = f r1 / n ±Δ f , so that the THz electro-optic sampling detection module obtains THz electric field change data according to the THz pulse, the sampling optical comb, and the frequency sweep trigger signal;

[0073] Step S6 further includes the following steps:

[0074] S601. When the trigger starts, set the THz pulse frequency as f r1 = f r2 / n ±Δ f , so that the THz electro-optic sampling detection module obtains THz electric field change data according to the THz pulse, the sampling optical comb, and the frequency sweep trigger signal.

[0075] Embodiment III

[0076] The reference numerals in the drawings of Embodiment 3 of the specification include: first femtosecond electro-optic comb 1, second femtosecond electro-optic comb 2, first signal generator 3, second signal generator 4, first frequency doubling crystal 5, second frequency doubling crystal 6, trigger terminal 7, hydrogen clock 8, first lens 9, second lens 10, first photoconductive antenna 11, off-axis parabolic mirror 12, sample cell 13, third lens 14, fourth lens 15, first mirror group 16, second photoconductive antenna 17, current amplifier 18, data acquisition card 19.

[0077] The difference between Embodiment 3 and Embodiment 1 is that, as Figure 9 shown, the THz electro-optic sampling detection device can be replaced with a photoconductive antenna sampling device, and the remaining structures are the same, so no more details will be described. Specifically, it includes a first mirror group 16 (including a second mirror and a third mirror) for optical path connection setting, a second photoconductive antenna 17, and a current amplifier 18. Inject the THz pulse into the second photoconductive antenna 17, and inject the sampling pulse into the second photoconductive antenna 17 through the second mirror and the third mirror to form a photocurrent; then amplify the current signal through the current amplifier 18 to obtain THz electric field change data.

[0078] In this embodiment, the sampling optical comb frequency conversion of the above frequency conversion electro-optic sampling is adopted, and the THz sampling optical comb frequency conversion can also be adopted.

[0079] Embodiment 4

[0080] The reference numerals in the drawings of Embodiment 4 of the specification include: third femtosecond electro-optic comb 1, first femtosecond electro-optic comb 2, third signal generator 3, first signal generator 4, fifth lens 5, sixth lens 6, second mirror group 7, delay line 8, third photoconductive antenna 9, off-axis parabolic mirror 10, beam splitter 11, sample cell 12, first lens 13, second lens 14, first photoconductive antenna 15, second femtosecond electro-optic comb 16, second signal generator 17, trigger terminal 18, hydrogen clock 19, third lens 20, fourth lens 21, electro-optic crystal 22, first mirror 23, 1 / 4 wave plate 24, Wollaston prism 25, balanced detector 26, data acquisition card 27.

[0081] The difference between Embodiment 4 and Embodiment 1 is that, as Figure 10 、 Figure 11 shown, the device is in Figure 7On this basis, it further includes a third optical frequency comb module, and the rest of the structure is the same, so it will not be elaborated here. The third optical frequency comb module includes a third signal generator 3, a third femtosecond electro-optical comb 1, and a third light beam converging component connected in sequence, as well as a fourth mirror, a delay line 8, a fifth mirror, a sixth mirror (the second mirror group 7 includes the fourth mirror, the fifth mirror, and the sixth mirror), and a third photoconductive antenna 9; a beam splitter 11 is provided between a pair of off-axis parabolic mirrors 10 of the 4F confocal system, and the first THz optical frequency comb output by the first photoconductive antenna 15 and the third THz optical frequency comb output by the third photoconductive antenna 9 are jointly input to the beam splitter 11 for beam combination.

[0082] As Figure 10 shown, what is realized is sampling optical frequency comb frequency conversion, and the frequency-sweeping function of the second femtosecond electro-optical comb 16 is still realized by controlling the frequency-sweeping trigger terminal 18 of the second signal generator 17.

[0083] The two third femtosecond electro-optical combs 1 and the first femtosecond electro-optical comb 2 of 780 nm are respectively beam-reduced through two lens groups (the fifth lens 5, the sixth lens 6, and the first lens 13, the second lens 14), and are respectively incident on the third photoconductive antenna 9 and the first photoconductive antenna 15 to generate two THz optical frequency combs. One of the THz optical frequency combs is controlled for time delay through a mirror and a delay line 8 to ensure time synchronization with the other THz pulse. The two optical frequency combs are beam-combined after passing through the off-axis parabolic mirror 10 and the beam splitter 11, and pass through the sample cell 12 at the focal point to excite the nonlinear response of the sample, and generate a nonlinear signal in the THz transmission light field. The frequency-sweeping function of the second femtosecond electro-optical comb 16 is still realized by controlling the frequency-sweeping trigger terminal 18 of the second signal generator 17. After being beam-reduced by the third lens 20 and the fourth lens 21, it is jointly incident on the ZnTe electro-optical crystal 22 with the THz transmission light signal to realize subsequent asynchronous frequency conversion electro-optical sampling. In order to separate the nonlinear signal from the transmission excitation field, the total electric field E 12 acting on the sample by only the third femtosecond electro-optical comb 1 and only the first femtosecond electro-optical comb 2 can be measured. Then, the electric field of the nonlinear signal can be extracted from E NL =E 12 -E1-E2. By performing a double Fourier transform on the optical field of the nonlinear signal, a two-dimensional optical field varying with the excitation frequency and the detection frequency can be obtained. In the two-dimensional spectrum, the absorption peaks of the sample are clustered together along the diagonal direction of the plane, while along the anti-diagonal direction, the intrinsic linewidth of its absorption peaks can be revealed to realize the acquisition of high-resolution two-dimensional spectra.

[0084] As Figure 11As shown, THz optical comb frequency conversion is achieved. The frequency-sweeping function of the first femtosecond electro-optic comb 2 is still realized by controlling the frequency-sweeping trigger terminal 18 of the second signal generator 172. The frequency-sweeping function of the third femtosecond electro-optic comb 1 is still realized by controlling the frequency-sweeping trigger terminal 18 of the third signal generator 3. The principle is the same as Figure 10 the same, and the obtained results are basically the same.

[0085] Combining the THz optical comb spectrum detection of the present invention with the pump-probe technology based on dual optical combs, the process of the THz transmission intensity changing with the time delay between the pump optical comb and the THz optical comb can be observed, replacing the delay line 8 in the traditional pump-probe technology, and realizing the measurement of the THz probe light transient transmission spectrum.

[0086] Embodiment Five

[0087] The reference numerals in the drawings of the fifth embodiment of the specification include: the first femtosecond electro-optic comb 1, the second femtosecond electro-optic comb 2, the first signal generator 3, the second signal generator 4, the trigger terminal 5, the hydrogen clock 6, the first lens 7, the second lens 8, the first photoconductive antenna 9, the off-axis paraboloid mirror 10, the sample cell 11, the third lens 12, the fourth lens 13, the electro-optic crystal 14, the first mirror 15, the quarter-wave plate 16, the Wollaston prism 17, the balanced detector 18, the data acquisition card 19, the fourth femtosecond electro-optic comb 20, the fourth signal generator 21, and the seventh mirror 22.

[0088] The difference between the fifth embodiment and the first embodiment is that, as Figure 12 shown, the device further includes a fourth optical comb module, and the remaining structures are the same, so no further elaboration is made. The fourth optical comb module includes a fourth signal generator 21, a fourth femtosecond electro-optic comb 20, and a seventh mirror 22 connected in sequence. The output light of the fourth femtosecond electro-optic comb 20 is obliquely incident into the sample cell 11 through a mirror.

[0089] The first femtosecond electro-optic comb 1, the second femtosecond electro-optic comb 2, and the fourth femtosecond electro-optic comb 20 (pump optical comb) 20 are all femtosecond electro-optic comb light sources. The diagram of the fs electro-optic comb pulse generation device is as Figure 13 shown in (a) of: First, intensity modulation is performed on a 1550 nm narrow-linewidth continuous laser. The modulation frequency is determined by a signal generator (SG). The picosecond pulse generator (PPG) converts the sine signal of the SG into a 30 ps electrical pulse output and connects it to the intensity modulator as a driving signal to generate a 30 ps optical pulse. Then, after passing through two-stage erbium-doped fiber amplifiers (EDFAs) and a fiber filter (BP), an optical pulse with an average power of 260 mW is injected into a highly nonlinear fiber (HNLF) for spectral broadening, and its coherent spectral range reaches 8 nm (-15 dB), as Figure 13as shown in (b) of. Finally, the negative dispersion provided by the single-mode fiber is used for pulse width compression, and an optical pulse with a pulse width of 260 fs and a power greater than 100 mW can be obtained ( Figure 13 of (c)). The repetition frequency tuning range of the electro-optic comb covers 9 kHz - 1 GHz, Figure 14 showing the electro-optic comb pulse diagrams with repetition frequencies of 10 kHz ( Figure 14 of (a)) and 50 MHz ( Figure 14 of (b)) respectively. The signal generators of the three optical combs (the first signal generator 3, the second signal generator 4, and the fourth signal generator 21) are all referenced to the hydrogen clock 6, and the frequency of the continuous laser is locked to the stable cavity to achieve the locking of the two frequency degrees of freedom of the optical comb. The repetition frequency f r1 of the first femtosecond electro-optic comb 1 is set to 100 MHz, and the repetition frequency f r2 of the second femtosecond electro-optic comb 2 jumps between two values of 100 MHz ± 1 MHz to achieve the fast frequency conversion scanning function. The repetition frequency of the pump optical comb 20 is related to the sample characteristics. For different relaxation times of the sample excited by the pump, the repetition frequencies of each optical comb can be reasonably set to meet the measurement requirements.

[0090] First, the output light of the fourth femtosecond electro-optic comb 20 (pump optical comb) 20 is adjusted in angle by the mirror 22 and then obliquely incident into the sample cell 11 to excite the sample to the excited state, reducing the transmittance of the sample to the THz light; as the excited state decays, the THz transmitted light gradually becomes stronger. After passing through a group of the first lens 7 and the second lens 8 with focal lengths of 200 mm and 50 mm respectively, the spot size of the first femtosecond electro-optic comb 1 is reduced from 2 mm to 500 μm. Subsequently, a femtosecond laser with a power of 100 mW and a spot diameter of 500 μm is injected into the first photoconductive antenna 9 to generate THz waves. The THz waves are focused onto the ZnTe electro-optic crystal 14 after passing through 4 gold-plated off-axis parabolic mirrors 10. The sample cell 11 is located at the focus of the THz transmission optical path. After being reduced in beam size by the third lens 12 and the fourth lens 13 with the same parameters, the sampling optical comb is incident on the ZnTe crystal 14 with a spot diameter of 500 μm. Finally, polarization beam splitting and balanced detection are performed to obtain the current difference between the P light and the S light, thereby obtaining the terahertz time-domain electric field distribution. Using the Fourier transform sub-module of the data acquisition card 19, the THz frequency-domain spectrum can be obtained. Since the THz pulse width is about 10 ps, the THz pulse period is 10 ns, and the repetition frequency difference is 1 MHz, the sampling time for the entire THz period is 1 ns, achieving fast high-resolution sampling of the THz time-domain spectrum.

[0091] This device is applicable to THz two-dimensional spectrum measurement based on asynchronous frequency conversion. The step S1 further includes the following steps:

[0092] S101. The output frequency of the fourth femtosecond electro-optic comb 20 in the fourth optical comb module is f r3 ;

[0093] S102. Set the output frequency of the first femtosecond electro-optic comb 1 in the first optical comb module f r1 and the output frequency of the fourth femtosecond electro-optic comb 20 in the fourth optical comb module f r3 to satisfy the relationship: f r3 - f r1 = Δ f 1;

[0094] The step S2 further includes the following steps:

[0095] S201. When the frequency conversion object is the second signal generator 4, execute step S301; when the frequency conversion objects are the first signal generator 1 and the fourth signal generator 20, execute step S401;

[0096] The step S3 further includes the following steps:

[0097] S301. Make the first signal generator output two signals. One is a sine wave signal, which outputs the first THz optical comb through the first photoconductive antenna; the other square wave signal enters the frequency sweep trigger terminal of the second signal generator as the frequency sweep trigger signal, and its period is N times the period of the THz pulse; then execute step S501;

[0098] The step S4 further includes the following steps:

[0099] S401. Make the second signal generator output two signals. One is a sine wave signal as the sampling optical comb; the other square wave signal simultaneously enters the frequency sweep trigger terminals of the first signal generator 1 and the first signal generator 20 as the frequency sweep trigger signal, and its period is N times the period of the THz pulse; then execute step S601;

[0100] The step S5 further includes the following steps:

[0101] S501. When the trigger starts, set the sampling pulse frequency as f r3 = f r1 ±Δ f 2, so that the THz electro-optic sampling detection module obtains the THz electric field change data according to the double THz pulse, the sampling optical comb and the frequency sweep trigger signal; the repetition frequency of the sampling optical comb is within f r3 = fr2 ±Δ f It scans back and forth within the range of 2, and the repetition frequency of the THz dual optical frequency comb remains unchanged.

[0102] The step S6 further includes the following steps:

[0103] S601. When the trigger starts, set the THz pulse frequency generated by the fourth femtosecond electro-optic comb 20 in the fourth optical frequency comb module as f r3 = f r2 ±Δ f 2, and set the THz pulse frequency generated by the first femtosecond electro-optic comb 1 in the signal optical frequency comb module as f r1 = f r2 -Δ f 1±Δ f 2, so that the THz electro-optic sampling detection module obtains the THz electric field change data according to the dual THz pulses, the sampling optical frequency comb, and the swept-frequency trigger signal; the repetition frequency of the THz optical frequency comb of the fourth optical frequency comb module f r3 Within ( f r2 ±Δ f 2) scans back and forth between two values, and the repetition frequency of the signal optical frequency comb module f r1 Within ( f r2 -Δ f 1±Δ f 2) scans back and forth between two values, and the repetition frequency of the sampling optical frequency comb remains unchanged.

[0104] The step S7 further includes the following steps:

[0105] S701. Combine the THz dual optical frequency combs and focus them on the sample, and excite the sample to be measured through optical perturbation. The THz transmitted optical field carries the nonlinear signal related to the sample; use the sampling optical frequency comb to detect the dynamic process to obtain the dynamic spectral data; form the dynamic spectrum into a matrix and perform mathematical correlation analysis on the matrix to obtain the two-dimensional coherent spectrum.

[0106] (1) The THz optical frequency comb spectroscopy detection device and method with frequency conversion and fast scanning proposed by the present invention realize the generation of an electro-optic comb with a pulse width less than 300 fs through spectral broadening and dispersion control. Its repetition frequency is in the MHz order of magnitude, and the output power is in the order of hundreds of mW, and the peak power can reach the order of MW; an electro-optic comb light source with fast frequency conversion and frequency locking is used as the signal and sampling optical frequency combs, which further improves the measurement accuracy of the terahertz optical frequency comb spectrum;

[0107] (2) Compared with traditional terahertz asynchronous sampling, the asynchronous frequency conversion electro-optic sampling technology is incorporated, that is, the sampling pulse is scanned back and forth, so that only the signal part of the THz pulse is sampled, realizing fast high-resolution sampling of THz time-domain spectroscopy within the ns time scale.

[0108] (3) The THz optical comb spectroscopy detection device and method with frequency conversion fast scanning proposed by the present invention do not need to keep the repetition frequencies of the THz optical comb and the sampling optical comb consistent, and fast scanning can still be achieved under harmonic conditions; it can be combined with the three-optical comb spectroscopy technology, replacing the delay line in the three-optical comb system with a cascaded double optical comb, and using the asynchronous frequency conversion technology to realize fast measurement of THz two-dimensional spectroscopy.

[0109] The above are only embodiments of the present invention. Specific structures and common knowledge such as characteristics well known in the art are described in too much detail here. Those of ordinary skill in the art know all the common general technical knowledge in the technical field to which the invention belongs before the application date or the priority date, are able to know all the existing technologies in this field, and have the ability to apply the conventional experimental means before this date. Those of ordinary skill in the art can, under the inspiration given in this application, combine their own abilities to improve and implement this solution. Some typical well-known structures or well-known methods should not become obstacles for those of ordinary skill in the art to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to explain the content of the claims.

Claims

1. A THz optical comb spectroscopy detection device for frequency conversion and fast scanning, characterized in that: It includes a signal optical comb module, a sampling optical comb module, a THz radiation generation module, a THz electro-optic sampling detection module, and a signal acquisition and processing module; the signal optical comb module, the sampling optical comb module, and the signal acquisition and processing module use the same clock signal; The signal optical comb module includes a first signal generator, a first femtosecond electro-optic comb, and a first light beam collimating assembly connected in sequence; The THz radiation generation module includes a first photoconductive antenna, two 4F confocal spatial systems, and a sample cell. The signal optical comb collimated by the first light beam collimating assembly is injected into the first photoconductive antenna, and the generated THz pulse is passed through the two 4F confocal spatial systems. The sample cell is located at the focal point of the two 4F confocal systems; the 4F confocal spatial system includes a pair of off-axis parabolic mirrors; The sampling optical comb module includes a second signal generator, a second femtosecond electro-optic comb, and a second light beam collimating assembly connected in sequence, and outputs a sampling optical comb; an optical comb frequency conversion module is provided between the signal optical comb module and the sampling optical comb module, and the optical comb frequency conversion module is used for signal optical comb frequency conversion or sampling optical comb frequency conversion according to the first signal generator and the second signal generator; The THz pulse output by the two 4F confocal spatial systems and the sampling pulse output by the second light beam collimating assembly are jointly input into the THz electro-optic sampling detection module for detecting the change of THz electric field; The signal acquisition and processing module is used to obtain the THz electric field change data from the THz electro-optic sampling detection module and convert it into high-resolution THz spectrum information; The THz electro-optic sampling detection module includes an electro-optic crystal, a quarter-wave plate, a first mirror, a Wollaston prism, and a balanced detector connected by optical paths; the THz pulse output by the two 4F confocal spatial systems and the collimated light beam output by the second light beam collimating assembly are jointly input into the electro-optic crystal; the balanced detector is used to detect the change of THz electric field to obtain the THz electric field change data; Or, The THz electro-optic sampling detection module includes a second mirror, a third mirror, a second photoconductive antenna, and a current amplifier connected by optical paths. The THz pulse is injected into the second photoconductive antenna, and the sampling pulse is injected into the second photoconductive antenna through the second mirror and the third mirror to form a photocurrent; Then, the current signal is amplified by the current amplifier to obtain the THz electric field change data.

2. The THz optical comb spectroscopy detection device for frequency conversion fast scanning according to claim 1, characterized in that: Both the first femtosecond electro-optic comb and the second femtosecond electro-optic comb include a continuous laser, and the optical frequency of the continuous laser is locked to a stable cavity.

3. The THz optical comb spectroscopy detection device for frequency conversion fast scanning according to claim 1, characterized in that: The signal acquisition and processing module includes a data acquisition card. The data acquisition card is electrically connected to the balanced detector. The data acquisition card includes a Fourier transform sub-module, and the Fourier transform sub-module is used to convert the THz electric field change data into high-resolution THz spectrum information.

4. The THz optical comb spectroscopy detection device for frequency conversion and fast scanning according to claim 1, characterized in that: A frequency doubling crystal is provided between the first femtosecond electro-optic comb and the first light beam collimating assembly; a frequency doubling crystal is also provided between the second femtosecond electro-optic comb and the second light beam collimating assembly.

5. The THz optical comb spectroscopy detection device for frequency conversion and fast scanning according to claim 1, characterized in that: It further includes a third optical comb module, which includes a third signal generator, a third femtosecond electro-optical comb, and a third light beam focusing component connected in sequence, as well as a fourth reflector, a delay line, a fifth reflector, a sixth reflector, and a third photoconductive antenna; a beam splitter is provided between a pair of off-axis parabolic mirrors of the 4F confocal system to jointly input the first THz optical comb output by the first photoconductive antenna and the third THz optical comb output by the third photoconductive antenna into the beam splitter for beam combination.

6. The THz optical comb spectroscopy detection device for frequency conversion fast scanning according to claim 4, characterized in that: When selecting sampling optical comb frequency conversion, the optical comb frequency conversion module is used to control the first signal generator to generate a sweep trigger signal and send it to the second signal generator; when selecting signal optical comb frequency conversion, the optical comb frequency conversion module is used to control the second signal generator to generate a sweep trigger signal and send it to the first signal generator and the third signal generator.

7. The THz optical comb spectroscopy detection device for frequency conversion fast scanning according to claim 1, characterized in that: It further includes a fourth optical comb module, which includes a fourth signal generator, a fourth femtosecond electro-optical comb, and a seventh reflector connected in sequence, and the output light of the fourth femtosecond electro-optical comb is obliquely incident into the sample cell through the reflector.

8. A THz optical comb spectroscopy detection method for frequency conversion fast scanning, characterized in that: When applied to the device according to any one of claims 1-7, it specifically includes the following steps: S1. Set the output frequency of the first signal generator to f r1 , and the output frequency of the second signal generator to f r2 , | f r1 -f r2 | = Δ f ; S2. Determine that the frequency conversion object of the optical comb frequency conversion module is the first signal generator or the second signal generator; when the frequency conversion object is the first signal generator, execute step S4; when the frequency conversion object is the second signal generator, execute step S3; S3. Make the first signal generator output two signals, one is a sine wave signal, and the first THz optical comb is output through the first photoconductive antenna; the other square wave signal enters the sweep trigger terminal of the second signal generator as a sweep trigger signal, and its period is N times the period of the THz pulse; then execute step S5; S4. Make the second signal generator output two signals, one is a sine wave signal as the sampling optical comb; the other square wave signal enters the sweep trigger terminal of the second signal generator as a sweep trigger signal, and its period is N times the period of the THz pulse; then execute step S6; S5. When the trigger starts, set the sampling pulse frequency to f r2 = f r1 ±Δ f , so that the THz electro-optic sampling detection module obtains THz electric field change data according to the THz pulse, the sampling optical comb, and the swept-frequency trigger signal; S6. When the trigger starts, set the THz pulse frequency to f r1 = f r2 ±Δ f , and enable the THz electro-optic sampling detection module to obtain THz electric field change data according to the THz pulse, sampling optical comb, and swept-frequency trigger signal; S7. Process the THz electric field change data obtained in step S5 or step S6 through the signal acquisition and processing module to convert it into high-resolution THz spectrum information.

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