Continuous scanning half-wave sampling system and method for multi-dimensional coherent spectrum

Abstract

The invention discloses a continuous scanning half-wave sampling system for a multi-dimensional coherent spectrum, which comprises a continuous laser, continuous laser output by the continuous laser and femtosecond laser pulse output by an ultra-short femtosecond laser pulse generation device are combined and then split, and modulation and time delay are respectively carried out; the multi-dimensional coherent femtosecond laser pulse sequence is used for exciting a fluorescence signal generated by a sample and converting the fluorescence signal into a first electric signal, and the obtained continuous laser carrying different modulation frequencies is converted into a second electric signal; and demodulating the second electric signal by taking a total mixing signal obtained by mixing the multiple paths of modulation signals as a reference, and demodulating the first electric signal by using the second electric signal. The invention further discloses a continuous scanning half-wave sampling method for the multi-dimensional coherent spectrum. The half-wave sampling method is utilized to accurately calibrate optical time delay so as to obtain the multi-phase multi-dimensional coherent spectrum. The sampling step length breaks through the limitation of the minimum sampling step length of traditional stepping collection, and the sampling bandwidth is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of multi-dimensional coherent spectroscopy, and particularly relates to a continuous-scanning half-wave sampling system for multi-dimensional coherent spectroscopy, and also relates to a continuous-scanning half-wave sampling method for multi-dimensional coherent spectroscopy. Background Art

[0002] Multi-dimensional coherent spectroscopy is a non-linear ultrafast spectroscopy technique based on the principle of quantum coherence. Since Tanimura and Mukamel first extended the concept of two-dimensional nuclear magnetic resonance spectroscopy in the microwave band to the optical band in 1993, multi-dimensional coherent spectroscopy has become an important tool for studying the microscopic structure and dynamic processes of substances. This technique uses nested optical interferometers to generate a femtosecond laser pulse sequence with precisely controllable optical time delays. By manipulating this pulse sequence to interact with the substance sequentially according to a certain time sequence, its third-order non-linear polarization is excited, and the polarization radiation optical signal under the corresponding optical time delay is collected to construct a time-domain multi-dimensional spectral data matrix. Subsequently, through multi-dimensional Fourier transform, a frequency-domain multi-dimensional coherent spectrum is obtained. Compared with one-dimensional spectroscopy, multi-dimensional coherent spectroscopy can expand the overlapping and crowded one-dimensional spectrum to multiple dimensions, which is beneficial for resolving homogeneous broadening and inhomogeneous broadening, detecting many-body correlations, decoupling quantum state coherent couplings, tracking quantum excitation paths and other complex processes. These technical features have attracted extensive attention in the fields of single atoms and molecules, quantum dots (wells), novel semiconductor two-dimensional materials, chemical and biological macromolecules, etc.

[0003] One of the keys to realizing multi-dimensional coherent spectroscopy lies in maintaining high phase stability for a long time. Compared with one-dimensional spectroscopy, the acquisition time of multi-dimensional spectra increases exponentially, which makes it quite challenging to maintain high phase stability during long-time acquisitions. For this reason, a variety of experimental schemes have emerged. Among them, the active feedback phase-locked non-collinear scheme and the pulse spatio-temporal shaping non-collinear scheme are the two most classical technical schemes. They usually use a grating spectrometer to replace the step-by-step scanning of an optical time axis, thereby effectively shortening the acquisition time of multi-dimensional coherent spectroscopy. However, due to being limited by specific non-linear processes and finite spatial resolution, the above schemes are not applicable to the research of complex quantum systems such as high-order non-linear processes and small-size quantum materials. A newly developed novel phase modulation phase-locked multi-dimensional coherent spectroscopy technique has attracted extensive attention because it can detect a variety of non-linear processes and has diffraction-limited spatial resolution. However, since this scheme needs to step-scan multiple optical time delays in the time domain to obtain a multi-dimensional coherent spectrum, it not only increases the acquisition time of the time-domain multi-dimensional spectral data matrix, but also poses a challenge to maintaining phase stability for a long time, seriously affecting the data quality and experimental efficiency of multi-dimensional coherent spectroscopy. In addition, according to the sampling bandwidth formula (wherein, is the signal bandwidth, (where $\Delta t$ is the optical time-delay sampling step), and the sampling bandwidth is limited by the optical time-delay sampling step. Usually, the multi-dimensional coherent spectroscopy technical solution adopts the optical time-delay stepping acquisition method, and its minimum sampling step is limited by the mechanical structure of the optical delay line, resulting in a limited sampling bandwidth, making it difficult to apply to broadband signal acquisition. At the same time, it will also cause signal undersampling, thereby affecting the correct analysis of the multi-dimensional coherent spectroscopy signal. Summary of the Invention

[0004] The object of the present invention is to provide a continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy and a continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy in view of the above problems existing in the prior art.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy includes a continuous laser. The continuous laser output is combined and then split with the ultra-short femtosecond laser pulse output by an ultra-short femtosecond laser pulse generating device to obtain four collimated lights. Each collimated light passes through a corresponding acousto-optic modulation unit and an optical delay line respectively to generate a corresponding modulated collimated light. Each acousto-optic modulation unit is driven by a corresponding modulation signal, and all modulation signals are output by a direct digital frequency synthesizer signal generator; The four modulated collimated lights are combined and output through a D-shaped mirror as a multi-dimensional coherent femtosecond laser pulse sequence and a continuous laser carrying different modulation frequencies; The multi-dimensional coherent femtosecond laser pulse sequence excites the sample to generate a fluorescence signal, and the fluorescence signal is converted into a first electrical signal by a first photodetector; the continuous laser carrying different modulation frequencies is converted into a second electrical signal by a second photodetector; The direct digital frequency synthesizer signal generator also outputs four modulation signals to a mixing unit to generate a total mixing signal, and the frequency of the total mixing signal is the same as the modulation frequency carried by the fluorescence signal; The first lock-in amplifier demodulates the second electrical signal with the total mixing signal as a reference. One quadrature component of the demodulated second demodulated electrical signal is collected and stored in real time by a synchronous data acquisition unit. The second lock-in amplifier demodulates the first electrical signal with the second electrical signal as a reference to obtain a first demodulated electrical signal. Both quadrature components of the first demodulated electrical signal are synchronously collected and stored by the synchronous data acquisition unit. The sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit based on the zero-crossing point of the second demodulated electrical signal.

[0006] As described above, the ultra-short femtosecond laser pulse generating device includes a laser seed source. The seed laser generated by the laser seed source is parametrically amplified by a non-linear crystal in an optical parametric amplifier to generate an ultra-short femtosecond laser pulse that resonates with the atomic line transition of the sample.

[0007] As described above, there is a spatial offset between the two collimated lights of the ultrashort femtosecond laser pulse and the continuous laser in the direction perpendicular to the horizontal plane.

[0008] As described above, the ultrashort femtosecond laser pulse and the continuous laser are combined by a first beam splitter and then split to obtain a first combined beam and a second combined beam; the first combined beam is split into a first collimated light and a second collimated light by a second beam splitter; after the second combined beam passes through a second optical delay line, it is split by a third beam splitter to obtain a third collimated light and a fourth collimated light; The first collimated light sequentially passes through a first optical delay line and a first acousto-optic modulation unit, and outputs a first modulated collimated light; The second collimated light passes through a second acousto-optic modulation unit, and outputs a second modulated collimated light; The third collimated light passes through a third acousto-optic modulation unit, and outputs a third modulated collimated light; The fourth collimated light sequentially passes through a third optical delay line and a fourth acousto-optic modulation unit, and outputs a fourth modulated collimated light; A fourth beam splitter combines the first modulated collimated light and the second modulated collimated light into a first modulated combined beam; a fifth beam splitter combines the third modulated collimated light and the fourth modulated collimated light into a second modulated combined beam; the first modulated combined beam and the second modulated combined beam are combined by a sixth beam splitter to obtain a total modulated combined beam; the D-shaped mirror separates the multi-dimensional coherent femtosecond laser pulse sequence and the continuous laser carrying different modulation frequencies in the total modulated combined beam; The multi-dimensional coherent femtosecond laser pulse sequence is focused on the sample by a first lens; the fluorescence signal spontaneously emitted by the sample is collected by a second lens and focused on a first photodetector; the first photodetector converts the fluorescence signal into a first electrical signal; the continuous laser carrying different modulation frequencies is converted into a second electrical signal by a second photodetector.

[0009] The modulation frequencies of the modulation signals corresponding to each of the above-mentioned acousto-optic modulation units are different from each other.

[0010] A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy, using the above-mentioned continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy, includes the following steps: Step 1, set up the above-mentioned continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy; Step 2, start the optical delay line to perform continuous scanning of the optical time delay; The first phase-locked amplifier continuously demodulates the second electrical signal, and the second phase-locked amplifier synchronously and continuously demodulates the first electrical signal; the synchronous data acquisition unit synchronously records one quadrature component of the second demodulated electrical signal and two quadrature components of the first demodulated electrical signal; the sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit based on the zero-crossing point of the second demodulated electrical signal, and determines the optical time delay of each sampling point of the first demodulated electrical signal by using the optical period of the second demodulated electrical signal.

[0011] As described above, a continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy further includes the following steps: Step 3: Construct a time-domain spectral data matrix by using the sampling points of the two quadrature components of the first demodulated electrical signal and the corresponding optical time delays; Step 4: Fourier transform the time-domain spectral data matrix to the frequency domain to obtain a complex-phase multi-dimensional coherent spectrum.

[0012] The continuous scanning of the optical time delay in Step 2 as described above specifically includes the acquisition process of two-dimensional coherent spectroscopy: Step 2.1: Fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2: Step the optical time delay between the first collimated light and the second collimated light, and scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.3: Return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed.

[0013] The continuous scanning of the optical time delay in Step 2 as described above includes the acquisition process of three-dimensional coherent spectroscopy: Step 2.1: Fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2: Step the optical time delay between the first collimated light and the second collimated light, and scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.3: Return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed, and then execute Step 2.4; Step 2.4: Step the optical time delay between the second collimated light and the third collimated light, and return to Step 2.1 until all the optical time delay ranges between the second collimated light and the third collimated light are traversed.

[0014] As described above, the sampling time of each sampling point of the first demodulated electrical signal is the time when the amplitude of the second demodulated electrical signal is zero. The optical time delay of the sampling point is the time interval between the sampling time and the time zero point, and the time zero point is the time when the interference intensity of the first demodulated electrical signal is the strongest.

[0015] Compared with the prior art, the present invention has the following beneficial effects: Based on the phase modulation phase-locked multi-dimensional coherent spectroscopy technology, the present invention designs and introduces a continuous scanning half-wave sampling system: using continuous scanning optical time delay to replace the traditional step-by-step acquisition of an optical time axis, effectively shortening the spectral measurement time, reducing the difficulty of maintaining phase stability while shortening the acquisition time; at the same time, using the half-wave sampling method to accurately calibrate the optical time delay, and the sampling step of half wavelength breaks through the limitation of the minimum sampling step of traditional step-by-step acquisition, significantly improving the sampling bandwidth. The present invention realizes the rapid acquisition of multi-dimensional coherent spectroscopy with a bandwidth of 387 THz ( wavelength range, for wavelength) within 3 hours. Compared with the traditional optical time delay step-by-step method (75 THz bandwidth, 12-hour measurement), this method significantly improves the acquisition rate and sampling bandwidth of multi-dimensional coherent spectroscopy. In addition, this continuous scanning half-wave sampling system only adds modules such as continuous scanning optical time delay and half-wave sampling to the original multi-dimensional coherent spectroscopy system, and the experimental device is simple. Moreover, the present invention can be extended to other Fourier transform spectroscopy fields, so the Fourier ultrafast spectroscopy technology also belongs to the content of the present invention. Description of the Drawings

[0016] Figure 1 FIG. is a schematic structural diagram of a continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy proposed by the present invention.

[0017] Figure 2 FIG. is a schematic diagram of half-wave sampling of a continuous laser carrying different modulation frequencies in Embodiment 2 of the present invention. (a) is the change of the amplitude of a quadrature component of the second demodulated electrical signal carrying a modulation frequency of 14 kHz with the optical time delay, and (b) is the 87 change of the amplitude of a quadrature component of the first demodulated electrical signal corresponding to Rb

[0018] Figure 3 FIG. is the complex phase one-dimensional coherent spectroscopy diagram of Rb 87 atoms in Embodiment 2 of the present invention.

[0019] Figure 4 FIG. is the complex phase multi-dimensional coherent spectroscopy diagram of Rb 87 atoms in Embodiment 2 of the present invention.

[0020] Among them, 100 - laser generation module; 101 - continuous laser; 102 - laser seed source; 103 - optical parametric amplifier; 200 - multi-dimensional coherent spectroscopy module; 201 - first beam splitter; 202 - second beam splitter; 203 - third beam splitter; 204 - fourth beam splitter; 205 - fifth beam splitter; 206 - sixth beam splitter; 207 - first optical delay line; 208 - second optical delay line; 209 - third optical delay line; 210 - first acousto-optic modulation unit; 211 - second acousto-optic modulation unit; 212 - third acousto-optic modulation unit; 213 - fourth acousto-optic modulation unit; 214 - D-shaped mirror; 215 - first lens; 216 - sample; 217 - second lens; 218 - first photodetector; 219 - second photodetector; 220 - direct digital frequency synthesis signal generator; 300 - half-wave sampling demodulation module; 301 - first mixer; 302 - second mixer; 303 - third mixer; 304 - first lock-in amplifier; 305 - second lock-in amplifier; 306 - synchronous data acquisition unit; 307 - first acquisition channel; 308 - second acquisition channel; 309 - third acquisition channel; 310 - fourth acquisition channel. Detailed implementation manners

[0021] For the convenience of those of ordinary skill in the art to understand and implement the present invention, the present invention will be further described in detail below in conjunction with embodiments. The embodiments described herein are only used to illustrate and explain the present invention, and are not intended to limit the present invention.

[0022] Embodiment 1

[0023] A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy, as Figure 1 shown. In this embodiment, the Rb 87 atomic complex-phase multi-dimensional coherent spectroscopy of the phase modulation and phase-locked scheme is taken as an example, and it is not limited to the Rb 87 atomic complex-phase multi-dimensional coherent spectroscopy of the phase modulation and phase-locked scheme. Its purpose is to use this continuous scanning half-wave sampling system to achieve fast broadband acquisition of multi-dimensional coherent spectroscopy, improve the spectral data acquisition rate and sampling bandwidth, and reduce the difficulty of maintaining long-term phase stability. Its principle is to use continuous scanning optical time delay to replace the traditional step-by-step acquisition method, and combine the half-wave sampling method to accurately calibrate the optical time delay, break through the minimum sampling step limit, and achieve fast broadband multi-dimensional coherent spectroscopy acquisition, significantly improving the acquisition rate and signal bandwidth.

[0024] The present invention provides a continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy, including: a laser generation module 100, a multi-dimensional coherent spectroscopy module 200, and a half-wave sampling demodulation module 300; The laser generation module 100 is used to generate collimated ultrashort femtosecond laser pulses and continuous laser, and the continuous laser serves as the reference laser; the multi-dimensional coherent spectroscopy module 200 is used to generate a multi-dimensional coherent femtosecond laser pulse sequence with precisely adjustable optical time delay and continuous laser carrying different modulation frequencies; the half-wave sampling demodulation module 300 is used to demodulate the first electrical signal corresponding to the fluorescence signal radiated by the sample 216, and perform half-wave sampling on the second electrical signal corresponding to the continuous laser carrying different modulation frequencies collected synchronously, so as to accurately calibrate the optical time delay of the fluorescence signal radiated by the sample 216 and realize the acquisition of fast broadband multi-dimensional coherent spectroscopy.

[0025] In the embodiment of the present invention, the laser generation module 100 includes: a continuous laser 101, a laser seed source 102, and an optical parametric amplifier 103; The multi-dimensional coherent spectroscopy module 200 includes: a first beam splitter 201, a second beam splitter 202, a third beam splitter 203, a fourth beam splitter 204, a fifth beam splitter 205, a sixth beam splitter 206, a first optical delay line 207, a second optical delay line 208, a third optical delay line 209, a first acousto-optic modulation unit 210, a second acousto-optic modulation unit 211, a third acousto-optic modulation unit 212, a fourth acousto-optic modulation unit 213, a D-shaped mirror 214, a first lens 215, a sample 216, a second lens 217, a first photodetector 218, a second photodetector 219, and a direct digital frequency synthesis signal generator 220; The half-wave sampling demodulation module 300 includes: a first mixer 301, a second mixer 302, a third mixer 303, a first lock-in amplifier 304, a second lock-in amplifier 305, and a synchronous data acquisition unit 306. The connection relationships between the components are as follows: (1) In the laser generation module 100, The continuous laser 101 generates continuous laser. At the same time, an ultrashort femtosecond laser pulse generating device outputs ultrashort femtosecond laser pulses resonant with the atomic line transition of the sample 216. The ultrashort femtosecond laser pulses and the continuous laser are both collimated light, with a spatial offset in the direction perpendicular to the horizontal plane, and are jointly input into the multi-dimensional coherent spectroscopy module 200.

[0026] In this embodiment, the ultrashort femtosecond laser pulse generating device includes a laser seed source 102. The laser seed source 102 generates seed laser, and after the seed laser is parametrically amplified by the nonlinear crystal in the optical parametric amplifier 103, ultrashort femtosecond laser pulses resonant with the atomic line transition of the sample 216 are generated. The specific settings of the continuous laser 101 and the ultrashort femtosecond laser pulse generating device are: the continuous laser with a wavelength of 773 nm generated by the continuous laser 101, and the sample 216 (in this embodiment, the sample 216 is Rb 87The fluorescence signal wavelength of the 794.979 nm of the D1 line transition radiation of the (atom) only differs by 21.979 nm from that of the Rb 87 atomic fluorescence signal phase fluctuation. Meanwhile, the laser seed source 102 outputs a seed laser with a wavelength of 1030 nm. After the seed laser is parametrically amplified by the nonlinear crystal in the optical parametric amplifier 103, an ultrashort femtosecond laser pulse with a central wavelength of 780 nm and a pulse width of 65 fs (femtoseconds) that resonates with the D1 line transition of the sample 216 (i.e., Rb 87 atom) is generated. The ultrashort femtosecond laser pulse and the continuous laser are jointly input into the multi-dimensional coherent spectroscopy module 200. In this embodiment, there is a spatial offset in the direction perpendicular to the horizontal plane between the ultrashort femtosecond laser pulse and the continuous laser, so as to realize the beam splitting of the two lights by using the D-shaped mirror 214 later.

[0027] (2) In the multi-dimensional coherent spectroscopy module 200, A. Four collimated lights are obtained from the ultrashort femtosecond laser pulse and the continuous laser, and the four collimated lights are modulated and optically time-delayed.

[0028] The ultrashort femtosecond laser pulse and the continuous laser are combined and then split to obtain four collimated lights. Each collimated light is modulated by the corresponding acousto-optic modulation unit and obtains an optical time delay through the corresponding optical delay line to obtain the corresponding modulated collimated light. At the same time, each acousto-optic modulation unit is driven by the corresponding modulation signal, and all modulation signals are output by the direct digital frequency synthesizer 220. The modulation frequencies of the modulation signals corresponding to each acousto-optic modulation unit are different from each other; the specific settings of this embodiment are as follows: The optical delay lines of this embodiment include a first optical delay line 207, a second optical delay line 208, and a third optical delay line 209; the acousto-optic modulation units include a first acousto-optic modulation unit 210, a second acousto-optic modulation unit 211, a third acousto-optic modulation unit 212, and a fourth acousto-optic modulation unit 213; The first beam splitter 201 combines the ultrashort femtosecond laser pulse and the continuous laser output by the laser generation module 100 to obtain the total combined light, and splits the obtained total combined light including the ultrashort femtosecond laser pulse and the continuous laser into a first combined light and a second combined light, and the transmission directions of the first combined light and the second combined light are perpendicular to each other; the first combined light is split into a first collimated light and a second collimated light at the second beam splitter 202; after the second combined light passes through the second optical delay line 208, it is split by the third beam splitter 203 to obtain a third collimated light and a fourth collimated light; the second optical delay line 208 is used to precisely control the optical time delay between the second collimated light and the third collimated light; The first collimated light sequentially passes through the first optical delay line 207 and the first acousto-optic modulation unit 210 to output the first modulated collimated light; the first optical delay line 207 is used to precisely control the optical time delay between the first collimated light and the second collimated light; The second collimated light passes through the second acousto-optic modulation unit 211 to output the second modulated collimated light; The third collimated light passes through the third acousto-optic modulation unit 212 to output the third modulated collimated light; The fourth collimated light sequentially passes through the third optical delay line 209 and the fourth acousto-optic modulation unit 213 to output the fourth modulated collimated light; the third optical delay line 209 is used to precisely control the optical time delay between the third collimated light and the fourth collimated light; In this embodiment, the direct digital frequency synthesis signal generator 220 includes four output channels, and each output channel outputs a modulation signal, that is, the direct digital frequency synthesis signal generator 220 outputs a first-channel modulation signal, a second-channel modulation signal, a third-channel modulation signal, and a fourth-channel modulation signal, and each channel modulation signal is divided into two paths. The first branch of the first-channel modulation signal is input to the first acousto-optic modulation unit 210, the first branch of the second-channel modulation signal is input to the second acousto-optic modulation unit 211, the first branch of the third-channel modulation signal is input to the third acousto-optic modulation unit 212, and the first branch of the fourth-channel modulation signal is input to the fourth acousto-optic modulation unit 213; in addition, the second branches of the first-channel modulation signal, the second-channel modulation signal, the third-channel modulation signal, and the fourth-channel modulation signal are simultaneously input to the half-wave sampling demodulation module 300; The first acousto-optic modulation unit 210, the second acousto-optic modulation unit 211, the third acousto-optic modulation unit 212, and the fourth acousto-optic modulation unit 213 are each composed of two convex lenses with a focal length of 200 mm and an acousto-optic modulator. The first acousto-optic modulation unit 210 assigns a first modulation frequency to the first collimated light, the second acousto-optic modulation unit 211 assigns a second modulation frequency to the second collimated light, the third acousto-optic modulation unit 212 assigns a third modulation frequency to the third collimated light, and the fourth acousto-optic modulation unit 213 assigns a fourth modulation frequency to the fourth collimated light: The first collimated light is focused by the first convex lens of the first acousto-optic modulation unit 210 onto the acousto-optic modulator of the first acousto-optic modulation unit 210; the acousto-optic modulator of the first acousto-optic modulation unit 210 is driven by the first branch of the first-channel modulation signal output by the direct digital frequency synthesis signal generator 220 to generate a first-order diffracted light carrying a modulation frequency of 80.000 MHz; then, the second convex lens of the first acousto-optic modulation unit 210 shapes the first-order diffracted light carrying the 80.000 MHz modulation frequency into a collimated first modulated collimated light; The second collimated light is focused by the first convex lens of the second acousto-optic modulation unit 211 onto the acousto-optic modulator of the second acousto-optic modulation unit 211; the acousto-optic modulator of the second acousto-optic modulation unit 211 is driven by the first branch of the second-channel modulation signal output by the direct digital frequency synthesizer 220 to generate the first-order diffracted light carrying a modulation frequency of 80.019 MHz; the second convex lens of the second acousto-optic modulation unit 211 shapes the first-order diffracted light carrying a modulation frequency of 80.019 MHz into collimated second modulated collimated light; The third collimated light is focused by the first convex lens of the third acousto-optic modulation unit 212 onto the acousto-optic modulator of the third acousto-optic modulation unit 212; the acousto-optic modulator of the third acousto-optic modulation unit 212 is driven by the first branch of the third-channel modulation signal output by the direct digital frequency synthesizer 220 to generate the first-order diffracted light carrying a modulation frequency of 80.104 MHz; the second convex lens of the third acousto-optic modulation unit 212 shapes the first-order diffracted light carrying a modulation frequency of 80.104 MHz into collimated third modulated collimated light; The fourth collimated light is focused by the first convex lens of the fourth acousto-optic modulation unit 213 onto the acousto-optic modulator of the fourth acousto-optic modulation unit 213; the acousto-optic modulator of the fourth acousto-optic modulation unit 213 is driven by the first branch of the fourth-channel modulation signal output by the direct digital frequency synthesizer 220 to generate the first-order diffracted light carrying a modulation frequency of 80.109 MHz; the second convex lens of the fourth acousto-optic modulation unit 213 shapes the first-order diffracted light carrying a modulation frequency of 80.109 MHz into collimated fourth modulated collimated light.

[0029] In this embodiment, the modulation of the acousto-optic modulation unit in the 80 MHz range is much faster than the moving speed of the optical delay line. The acousto-optic modulation unit can be placed behind the optical delay line without causing great interference to the experimental results. Therefore, the acousto-optic modulation unit can be placed in front of the optical delay line or behind the optical delay line.

[0030] B. Obtaining a multi-dimensional coherent femtosecond laser pulse sequence and a continuous laser carrying different modulation frequencies from the four-way modulated collimated light.

[0031] The four-way modulated collimated light (the four-way modulated collimated light includes the first modulated collimated light, the second modulated collimated light, the third modulated collimated light, and the fourth modulated collimated light) is combined, and then a multi-dimensional coherent femtosecond laser pulse sequence and a continuous laser carrying different modulation frequencies are output through the D-shaped mirror 214. The specific settings of this embodiment are as follows: The fourth beam splitter 204 combines the first modulated collimated light and the second modulated collimated light into the first modulated combined light; the fifth beam splitter 205 combines the third modulated collimated light and the fourth modulated collimated light into the second modulated combined light; the first modulated combined light and the second modulated combined light are combined by the sixth beam splitter 206 to obtain the total modulated combined light composed of a multi-dimensional coherent femtosecond laser pulse sequence carrying different modulation frequencies and a continuous laser carrying different modulation frequencies. Subsequently, the D-shaped mirror 214 separates the multi-dimensional coherent femtosecond laser pulse sequence with a spatial offset in the total modulated combined light from the continuous laser carrying different modulation frequencies.

[0032] C. The multi-dimensional coherent femtosecond laser pulse sequence excites the sample 216 to generate a fluorescence signal, and a photodetector is used to detect the continuous laser and the fluorescence signal carrying different modulation frequencies respectively.

[0033] The multi-dimensional coherent femtosecond laser pulse sequence excites the sample 216 to generate a fluorescence signal, and the fluorescence signal is converted into a first electrical signal by the first photodetector 218; the continuous laser carrying different modulation frequencies is converted into a second electrical signal by the second photodetector 219; the specific settings of this embodiment are as follows: The multi-dimensional coherent femtosecond laser pulse sequence is reflected by the D-shaped mirror 214 and focused on the sample 216 by the first lens 215. The fluorescence signal spontaneously emitted by the sample 216 is collected by the second lens 217 and then focused on the first photodetector 218; the first photodetector 218 converts the fluorescence signal into a first electrical signal and transmits it to the half-wave sampling demodulation module 300; at the same time, the continuous laser carrying different modulation frequencies is transmitted to the second photodetector 219 through the non-reflective area of the D-shaped mirror 214, and the second photodetector 219 converts the continuous laser carrying different modulation frequencies into a second electrical signal.

[0034] In this embodiment, the first lens 215 is a convex lens with a focal length of 50 mm, which is used to focus the multi-dimensional coherent femtosecond laser pulse sequence on the sample 216; the sample 216 interacts with the multi-dimensional coherent femtosecond laser pulse sequence and emits a fluorescence signal; the second lens 217 is a convex lens with a focal length of 60 mm, which is used to collect the fluorescence signal emitted by the sample 216 and focus the fluorescence signal on the first photodetector 218; the first photodetector 218 is used to detect the fluorescence signal emitted by the sample 216 under the action of the multi-dimensional coherent femtosecond laser pulse sequence; the second photodetector 219 is used to detect the continuous laser carrying different modulation frequencies.

[0035] (3) Half-wave sampling demodulation module 300 The second branch of the modulation signal output from each output channel of the direct digital frequency synthesis signal generator 220 is input to the mixing unit to obtain a total mixing signal. The modulation signal input to the mixing unit from the same output channel of the direct digital frequency synthesis signal generator 220 is the same as the corresponding modulation signal of the acousto-optic modulation unit. The frequencies of the obtained total mixing signal, the modulation frequency carried by the fluorescence signal, the modulation frequency carried by the first electrical signal, and the modulation frequency carried by the second electrical signal are all the same. The total mixing signal and the first branch of the second electrical signal are simultaneously input to the first lock-in amplifier 304. The first lock-in amplifier 304 demodulates the second electrical signal with the total mixing signal as a reference to obtain a second demodulated electrical signal. One quadrature component of the second demodulated electrical signal is divided into two paths and simultaneously input to the synchronous data acquisition unit 306. The first electrical signal and the second branch of the second electrical signal are simultaneously input to the second lock-in amplifier 305. The second lock-in amplifier 305 demodulates the first electrical signal with the second electrical signal as a reference to obtain a first demodulated electrical signal. The two quadrature components of the first demodulated electrical signal are both input to the synchronous data acquisition unit 306. The sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit 306 based on the zero-crossing point of the second demodulated electrical signal. The specific settings of this embodiment are as follows: The mixing unit includes a first mixer 301, a second mixer 302, and a third mixer 303. The second branch of the first-channel modulation signal output from the first channel of the direct digital frequency synthesis signal generator 220 and the second branch of the second-channel modulation signal output from the second channel are mixed by the first mixer 301 to obtain a first beat frequency reference signal of 19 kHz. The second branch of the third-channel modulation signal output from the third channel of the direct digital frequency synthesis signal generator 220 and the second branch of the fourth-channel modulation signal output from the fourth channel are mixed by the second mixer 302 to obtain a second beat frequency reference signal of 5 kHz. The first beat frequency reference signal and the second beat frequency reference signal are mixed by the third mixer 303 to obtain a total mixing signal of 14 kHz. The frequency of the total mixing signal is the same as that of the Rb 87 modulation frequency carried by the complex-phase fluorescence signal of atomic radiation; The first lock-in amplifier 304 demodulates the second electrical signal corresponding to the continuous laser carrying different modulation frequencies with the total mixing signal as a reference; the second lock-in amplifier 305 demodulates the first electrical signal corresponding to the complex-phase fluorescence signal with the second electrical signal as a reference; the synchronous data acquisition unit 306 is provided with four synchronous data acquisition channels (i.e., the first acquisition channel 307, the second acquisition channel 308, the third acquisition channel 309, and the fourth acquisition channel 310), which are used to synchronously record the second demodulated electrical signal corresponding to the continuous laser carrying different modulation frequencies and the first demodulated electrical signal corresponding to the fluorescence signal, wherein both the first acquisition channel 307 and the third acquisition channel 309 are used to record one quadrature component of the second demodulated electrical signal, and the second acquisition channel 308 and the fourth acquisition channel 310 are respectively used to synchronously record the two quadrature components of the first demodulated electrical signal.

[0036] Due to the non-uniform velocity of the movement of the optical delay line, directly scanning and recording the first demodulated electrical signal demodulated by the second lock-in amplifier 305 will result in different degrees of phase errors at each sampling moment, thus hindering the direct determination of the optical time delay of the fluorescence signal of the spontaneous emission of the sample 216 and affecting the correct analysis of the fluorescence signal (the fluorescence signal contains multi-dimensional coherent spectral information). Therefore, the second electrical signal corresponding to the continuous laser carrying different modulation frequencies is demodulated with the total mixing signal generated by the modulation signals output from the four output channels of the direct digital frequency synthesis signal generator 220 to obtain the second demodulated electrical signal (the second demodulated electrical signal is the beat electrical signal corresponding to the continuous laser carrying different modulation frequencies). One of the two quadrature components of the second demodulated electrical signal is used as time calibration, and the sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit 306 based on the zero-crossing point of the second demodulated electrical signal. Since the continuous laser and the ultrashort femtosecond laser pulse are synchronously transmitted in the optical path, the moment when the four pulses in the multi-dimensional coherent femtosecond laser pulse sequence coincide (when the four pulses in the multi-dimensional coherent femtosecond laser pulse sequence coincide, the interference intensity of the first demodulated electrical signal collected is the strongest) is used as the time zero point, and the number of optical cycles of each sampling point in the demodulated second demodulated electrical signal from the time zero point is calculated, that is, half-wave sampling is performed on the second demodulated electrical signal and the first demodulated electrical signal ( ), to determine the optical time delay of each sampling point, so as to accurately obtain the optical time delay obtained by the continuous laser and the ultrashort femtosecond laser pulse, and realize the accurate calibration of the optical time delay of the fluorescence signal radiated by the sample 216. Subsequently, a time-domain spectral data matrix is constructed from the sampling points of the two quadrature components of the first demodulated electrical signal and the corresponding optical time delay, and then a complex-phase multi-dimensional coherent spectrum is obtained. For the specific method, see Embodiment 2.

[0037] Embodiment 2 A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy, using the continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy described in Embodiment 1, specifically includes the following steps: Step 1, set up the continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy described in Embodiment 1; Step 2, start the optical delay line and perform continuous scanning of the optical time delay; For the acquisition of two-dimensional coherent spectroscopy, Step 2.1, fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2, step the optical time delay between the first collimated light and the second collimated light, and scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.3, return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed.

[0038] In this embodiment, for the acquisition of two-dimensional coherent spectroscopy, first fix the first optical delay line 207 and the second optical delay line 208, continuously scan and traverse the optical time delay range of the third optical delay line 209, then step the first optical delay line 207, and then continuously scan and traverse the optical time delay range of the third optical delay line 209, repeat the process of stepping the first optical delay line 207 and scanning and traversing the optical time delay range of the third optical delay line 209 until the optical time delay range of the first optical delay line 207 is traversed to obtain the complex-phase two-dimensional coherent spectroscopy; For the acquisition of three-dimensional coherent spectroscopy, the continuous scanning of the optical time delay includes the following process: Step 2.1, fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2, step the optical time delay between the first collimated light and the second collimated light, and scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.3, return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed, and then execute Step 2.4; Step 2.4, step the optical time delay between the second collimated light and the third collimated light, and return to Step 2.1 until all the optical time delay ranges between the second collimated light and the third collimated light are traversed.

[0039] For the acquisition of the three-dimensional coherent spectrum in this embodiment, the acquisition process of the two-dimensional coherent spectrum is first performed, then the second optical delay line 208 is stepped, and then the above-mentioned acquisition process of the two-dimensional coherent spectrum and the stepping of the second optical delay line 208 are repeated until the second optical delay line 208 is traversed.

[0040] During this process, the first lock-in amplifier 304 synchronously and continuously demodulates the second electrical signals corresponding to the continuous lasers carrying different modulation frequencies, and the second lock-in amplifier 305 synchronously and continuously demodulates the first electrical signals corresponding to the fluorescence signals carrying different modulation frequencies; the synchronous data acquisition unit 306 synchronously acquires and records one quadrature component of the second demodulated electrical signals corresponding to the continuous lasers carrying different modulation frequencies and two quadrature components of the first demodulated electrical signals corresponding to the ultrashort femtosecond laser pulses; the sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signals from the synchronous data acquisition unit 306 based on the zero-crossing point of the second demodulated electrical signal, and determines the optical time delay of each sampling point using the optical period of the second demodulated electrical signal; In this embodiment, the first lock-in amplifier 304 uses the total mixing signal as a reference to demodulate the second electrical signal, obtains two quadrature components of the second demodulated electrical signal carrying a 14 kHz modulation frequency, and takes one of the quadrature components as the time calibration input to the first acquisition channel 307 and the third acquisition channel 309 of the synchronous data acquisition unit 306. In this embodiment, the amplitude of the second demodulated electrical signal corresponding to the continuous laser carrying different modulation frequencies changes with the optical time delay as shown in Figure 2 (a); at the same time, the second lock-in amplifier 305 uses the second electrical signal as a reference to demodulate the first electrical signal, obtains two quadrature components of the first demodulated electrical signal corresponding to the complex-phase fluorescence signal, and the two quadrature components of the first demodulated electrical signal are respectively input to the second acquisition channel 308 and the fourth acquisition channel 310 of the synchronous data acquisition unit 306. In this embodiment, the amplitude of the first demodulated electrical signal corresponding to the complex-phase fluorescence signal changes with the optical time delay as shown in Figure 2 (b); Since the continuous laser and the ultrashort femtosecond laser pulses are synchronously transmitted in the optical path, the optical time delay of the fluorescence signal radiated by the sample 216 can be accurately calibrated by performing half-wave sampling on the second demodulated electrical signal recorded by the synchronous data acquisition unit 306, effectively avoiding the phase error caused by the uneven velocity of the optical delay line.

[0041] The sampling moment of the sampling points of the quadrature components of the first demodulated electrical signal is the moment when the amplitude of the second demodulated electrical signal is zero, and the optical time delay of the sampling point is the time interval between the sampling moment and the time zero point, and the time zero point is defined as the moment when the four pulses in the multi-dimensional coherent femtosecond laser pulse sequence coincide, that is, the moment when the interference intensity of the first demodulated electrical signal is the strongest.

[0042] Step 3: Construct a time-domain spectral data matrix based on the sampling points of the two orthogonal components of the first demodulated electrical signal and the corresponding optical time delays; The sampling interval of the sampling points in the time-domain spectral data matrix is half of the continuous laser optical period, corresponding to a bandwidth of 387 THz, as Figure 3 shown.

[0043] The phase modulation and phase-locking scheme of traditional optical delay step-by-step acquisition is limited by the optical delay line and cannot achieve a half-wavelength sampling step. It takes 12 hours to complete the acquisition of multi-dimensional coherent spectra with a bandwidth of 75 THz and a resolution of 1 THz. However, the half-wave sampling used in the present invention breaks through the limitation of the minimum sampling step of traditional step-by-step acquisition, significantly improves the sampling bandwidth, effectively avoids signal under-sampling, and at the same time significantly improves the spectral acquisition rate. The acquisition time of the time-domain spectral data matrix corresponding to a set of multi-dimensional coherent spectra with a bandwidth of 387 THz and a resolution of 0.1 THz is shortened to 3 hours.

[0044] Step 4: Fourier transform the time-domain spectral data matrix to the frequency domain to finally obtain the complex-phase multi-dimensional coherent spectrum. In this embodiment, the complex-phase multi-dimensional coherent spectrum of Rb 87 atoms is as Figure 4 shown.

[0045] It should be noted that the embodiments described in the present invention are only illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described embodiments or use similar ways to replace them, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy, characterized in that, It includes a continuous laser (101). The continuous laser output by the continuous laser (101) is combined and then split with the ultrashort femtosecond laser pulses output by an ultrashort femtosecond laser pulse generating device to obtain four collimated light beams. Each collimated light beam respectively passes through a corresponding acousto-optic modulation unit and an optical delay line to generate a corresponding modulated collimated light. Each acousto-optic modulation unit is driven by a corresponding path of modulation signal, and all modulation signals are output by a direct digital frequency synthesizer (220). The four modulated collimated light beams are combined and output through a D-shaped mirror (214) to obtain a multi-dimensional coherent femtosecond laser pulse sequence and continuous laser carrying different modulation frequencies. The multi-dimensional coherent femtosecond laser pulse sequence excites a sample (216) to generate a fluorescence signal, and the fluorescence signal is converted into a first electrical signal by a first photodetector (218); the continuous laser carrying different modulation frequencies is converted into a second electrical signal by a second photodetector (219). The direct digital frequency synthesizer (220) also outputs four paths of modulation signals to a mixing unit to generate a total mixing signal, and the frequency of the total mixing signal is the same as the modulation frequency carried by the fluorescence signal. A first lock-in amplifier (304) demodulates the second electrical signal with the total mixing signal as a reference. One quadrature component of the demodulated second demodulated electrical signal is collected and stored in real time by a synchronous data acquisition unit (306). A second lock-in amplifier (305) demodulates the first electrical signal with the second electrical signal as a reference to obtain a first demodulated electrical signal. Both quadrature components of the first demodulated electrical signal are synchronously collected and stored by the synchronous data acquisition unit (306). The sampling unit samples the corresponding data points of both quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit (306) based on the zero-crossing point of the second demodulated electrical signal.

2. The continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that, The ultrashort femtosecond laser pulse generating device includes a laser seed source (102). The seed laser generated by the laser seed source (102) is nonlinearly parametrically amplified by a nonlinear crystal in an optical parametric amplifier (103) to generate ultrashort femtosecond laser pulses resonant with the atomic line transition of the sample (216).

3. The continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, wherein There is a spatial offset in the direction perpendicular to the horizontal plane between the two collimated light beams of the ultrashort femtosecond laser pulses and the continuous laser.

4. A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that, The ultrashort femtosecond laser pulses and the continuous laser are combined and then split by a first beam splitter (201) to obtain a first combined light beam and a second combined light beam; the first combined light beam is split into a first collimated light beam and a second collimated light beam by a second beam splitter (202); after the second combined light beam passes through a second optical delay line (208), it is split by a third beam splitter (203) to obtain a third collimated light beam and a fourth collimated light beam. The first collimated light beam successively passes through a first optical delay line (207) and a first acousto-optic modulation unit (210) and outputs a first modulated collimated light beam. The second collimated light beam passes through a second acousto-optic modulation unit (211) and outputs a second modulated collimated light beam. The third collimated light beam passes through a third acousto-optic modulation unit (212) and outputs a third modulated collimated light beam. The fourth collimated light beam successively passes through a third optical delay line (209) and a fourth acousto-optic modulation unit (213) and outputs a fourth modulated collimated light beam. The fourth beam splitter (204) combines the first modulated collimated light and the second modulated collimated light into the first modulated combined light; the fifth beam splitter (205) combines the third modulated collimated light and the fourth modulated collimated light into the second modulated combined light; the first modulated combined light and the second modulated combined light are combined by the sixth beam splitter (206) to obtain the total modulated combined light; the D-shaped mirror (214) separates the multi-dimensional coherent femtosecond laser pulse sequence in the total modulated combined light from the continuous laser carrying different modulation frequencies; The multi-dimensional coherent femtosecond laser pulse sequence is focused onto the sample (216) by the first lens (215); the fluorescence signal spontaneously emitted by the sample (216) is collected by the second lens (217) and focused onto the first photodetector (218); the first photodetector (218) converts the fluorescence signal into a first electrical signal; the continuous laser carrying different modulation frequencies is converted into a second electrical signal by the second photodetector (219).

5. A continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy according to claim 1, characterized in that, The modulation frequencies of the modulation signals corresponding to each of the acousto-optic modulation units are different from each other.

6. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy, using the continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy described in claim 1, characterized in that, It includes the following steps: Step 1, build the continuous scanning half-wave sampling system for multi-dimensional coherent spectroscopy; Step 2, start the optical delay line to perform continuous scanning of the optical time delay; The first lock-in amplifier (304) continuously demodulates the second electrical signal, and the second lock-in amplifier (305) synchronously and continuously demodulates the first electrical signal; the synchronous data acquisition unit (306) synchronously records one quadrature component of the second demodulated electrical signal and two quadrature components of the first demodulated electrical signal; Based on the zero-crossing point of the second demodulated electrical signal, the sampling unit samples the corresponding data points of the two quadrature components of the first demodulated electrical signal from the synchronous data acquisition unit (306), and determines the optical time delay of each sampling point of the first demodulated electrical signal by using the optical period of the second demodulated electrical signal.

7. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that, It also includes the following steps: Step 3, construct a time-domain spectral data matrix by using the sampling points of the two quadrature components of the first demodulated electrical signal and the corresponding optical time delay; Step 4, perform Fourier transform on the time-domain spectral data matrix to the frequency domain to obtain a complex-phase multi-dimensional coherent spectrum.

8. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that, The continuous scanning of the optical time delay in Step 2 specifically includes the acquisition process of two-dimensional coherent spectroscopy: Step 2.1, fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2, step the optical time delay between the first collimated light and the second collimated light, and scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.3, return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed.

9. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 7, characterized in that The continuous scanning of the optical time delay in Step 2 includes the acquisition process of three-dimensional coherent spectroscopy: Step 2.1, fix the optical time delay between the first collimated light and the second collimated light and the optical time delay between the second collimated light and the third collimated light, and continuously scan and traverse the optical time delay between the third collimated light and the fourth collimated light; Step 2.2: Stepping the optical time delay between the first collimated light and the second collimated light, and scanning and traversing the optical time delay between the third collimated light and the fourth collimated light; Step 2.3: Return to Step 2.2 until all the optical time delay ranges between the first collimated light and the second collimated light are traversed, and then execute Step 2.4; Step 2.4: Stepping the optical time delay between the second collimated light and the third collimated light, and return to Step 2.1 until all the optical time delay ranges between the second collimated light and the third collimated light are traversed and controlled.

10. A continuous scanning half-wave sampling method for multi-dimensional coherent spectroscopy according to claim 6, characterized in that, The sampling moments of the sampling points of the first demodulated electrical signal are the moments when the amplitude of the second demodulated electrical signal is zero. The optical time delay of the sampling point is the time interval between the sampling moment and the time zero point, and the time zero point is the moment when the interference intensity of the first demodulated electrical signal is the strongest.

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