Harmonic beam acquisition structure, FROG trace acquisition device and pulse measurement equipment

Abstract

The invention provides a harmonic light beam acquisition structure, an FROG trace acquisition device and pulse measurement equipment, and the harmonic light beam acquisition structure comprises a beam splitting element, a time delay unit, a nonlinear crystal element, a ghost imaging unit and a spectrometer which are sequentially arranged along a propagation light path of pulse laser. The beam splitting element is used for splitting the pulse laser to obtain a first light beam and a second light beam; the delay unit is used for forming delay between the first light beam and the second light beam; the nonlinear crystal element is used for generating harmonic light beams; the ghost imaging unit is used for carrying out discretization modulation on the harmonic light beams to obtain a modulated harmonic light beam group; and the spectrograph is used for recording spectral information of the modulated harmonic light beam group. The harmonic light beam is discretized and modulated through the ghost imaging element to obtain the speckle light beam at each position of the cross section of the harmonic light beam, so that the spectrograph can record the spectral information at each position of the cross section of the harmonic light beam, and the accuracy of the pulse measurement result is effectively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of pulsed laser, and relates to a pulsed laser measurement technology, in particular to a harmonic beam collection structure, a FROG trace acquisition device and a pulse measurement equipment. BACKGROUND

[0002] The FROG trace (Frequency-Resolved Optical Gating trace, i.e. a two-dimensional time-frequency trajectory generated by a frequency-resolved optical gating method) is a two-dimensional graph for representing the time-domain and frequency-domain distribution of a femtosecond pulse. The time-domain waveform and phase information of the femtosecond pulse can be obtained by inverting the FROG trace based on the femtosecond pulse, so as to realize the measurement of the femtosecond pulse.

[0003] The FROG trace is obtained by a frequency-resolved optical gating method. The traditional frequency-resolved optical gating method mainly includes: splitting a femtosecond pulse laser to obtain two pulse lasers, introducing a time delay to one of the two pulse lasers; and inputting the two pulse lasers into a nonlinear crystal to generate a second harmonic, and then recording the harmonic spectrum under different delay conditions by a spectrometer, so as to construct a corresponding FROG trace. However, due to the spatial averaging effect, the FROG trace obtained by the method is actually the average distribution result of the cross section of the pulse laser. For a non-uniform light beam, such as a Gaussian light beam or a light beam with wavefront distortion, the FROG trace cannot reflect the local time-frequency characteristics of each position on the cross section of the pulse, and the measurement result of the pulse has a large error. SUMMARY

[0004] The application provides a harmonic beam collection structure, a FROG trace acquisition device and a pulse measurement equipment, which are used to solve the problem that the FROG trace obtained by the traditional frequency-resolved optical gating method cannot reflect the local time-frequency characteristics of each position on the cross section of the pulse, and thus the measurement of the non-uniform light beam has a large error.

[0005] In a first aspect, the application provides a harmonic beam collection structure, which comprises: a beam splitting element located on a propagation light path of a pulse laser, used to split the pulse laser to obtain a first light beam and a second light beam; a delay unit located on the propagation light paths of the first light beam and the second light beam, used to form a delay between the first light beam and the second light beam; a nonlinear crystal element located on the propagation light paths of the first light beam and the second light beam; the first light beam and the second light beam are incident to the same position of the nonlinear crystal element after being delayed by the delay unit, so as to generate a harmonic light beam; a ghost imaging unit located on the propagation light path of the harmonic light beam, used to discretely modulate the harmonic light beam to obtain a group of modulated harmonic light beams; and a spectrometer located on the propagation light path of the group of modulated harmonic light beams, used to record the spectrum information of the group of modulated harmonic light beams.

[0006] In an embodiment of the present application, the ghost imaging unit comprises a spatial light modulator located on a propagation path of the harmonic light beam, configured to load a Hadamard matrix to generate a plurality of Hadamard speckle light beams based on the harmonic light beam, each of the Hadamard speckle light beams being used as the modulated harmonic light beam group.

[0007] In an embodiment of the present application, the spatial light modulator is a reflective spatial light modulator.

[0008] In an embodiment of the present application, the spectral instrument and the ghost imaging unit further comprise a first focusing element located on a propagation path of the modulated harmonic light beam group, configured to focus the modulated harmonic light beam group so that the spectral instrument can receive each light beam in the modulated harmonic light beam group.

[0009] In an embodiment of the present application, the delay unit and the nonlinear crystal element further comprise a second focusing element located on a propagation path of the first light beam and the second light beam, configured to focus the first light beam and the second light beam to the same position to be incident on the nonlinear crystal element.

[0010] In an embodiment of the present application, the delay unit comprises a first reflecting element located on a propagation path of the first light beam and a second reflecting element located on a propagation path of the second light beam, the first reflecting element and the second reflecting element being different distances from the beam splitting element to form a delay between the first light beam and the second light beam.

[0011] In an embodiment of the present application, the first reflecting element is located at a fixed position, and the second reflecting element moves in a propagation direction of the second light beam, and the delay unit further comprises a distance measuring element configured to measure a position of the second reflecting element.

[0012] In an embodiment of the present application, the distance measuring element is a laser range finder.

[0013] In a second aspect, the present application provides a FROG trace acquisition device, comprising: a harmonic light beam acquisition structure as described above, configured to record spectral information of a pulsed laser at a plurality of different delay values based on the pulsed laser combined with a preset delay sequence; the delay sequence comprises a plurality of different delay values; a processing unit configured to construct a FROG trace of the pulsed laser based on a modulated harmonic light beam group of the pulsed laser at a plurality of different delays.

[0014] In a third aspect, the present application provides a pulse measurement device, comprising: a FROG trace acquisition device as described above, configured to acquire a FROG trace of a pulsed laser; and a phase retrieval device, configured to perform phase retrieval based on the FROG trace to reconstruct a time-domain waveform and phase information of the pulsed laser.

[0015] As described above, the harmonic beam acquisition structure, the FROG trace acquisition device and the pulse measurement device according to the present application can disperse and modulate the harmonic beam by the ghost imaging unit to obtain the speckle beam at each position of the cross section of the harmonic beam, so that the spectrometer can record the spectral information at each position of the cross section of the harmonic beam, and the FROG trace constructed can reflect the local time-frequency characteristics at each position of the cross section of the pulse, avoiding the error of the non-uniform beam due to the spatial averaging effect, effectively improving the accuracy of the pulse measurement result, and having high industrial application value. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 A schematic diagram of a harmonic beam acquisition structure according to an embodiment of the present application is shown.

[0017] Figure 2 A schematic diagram of a Hadamard matrix according to an embodiment of the present application is shown.

[0018] Figure 3 A schematic diagram of another harmonic beam acquisition structure according to an embodiment of the present application is shown.

[0019] Figure 4 A schematic diagram of a FROG trace acquired according to an embodiment of the present application is shown.

[0020] BRIEF DESCRIPTION OF DRAWINGS

[0021] 11: beam splitting element; 12: time delay unit; 121: first reflecting element; 122: second reflecting element; 123: ranging element; 13: nonlinear crystal element; 14: ghost imaging unit; 15: spectrometer; 16: first focusing element; 17: second focusing element; 18: third reflecting element; 21: pulsed laser; 22: first light beam; 23: second light beam; 24: harmonic light beam; 25: modulated harmonic light beam group. DETAILED DESCRIPTION

[0022] The above description is only used to illustrate the specific and preferred embodiments of the present application. Those skilled in the art can easily understand other advantages and effects of the present application from the above description. The present application can also be implemented or applied by different specific embodiments, and the details in the specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0023] Need to explain, the following examples provided in the diagram only to illustrate the basic idea of the application, the drawing shows only the components related to the application in the present application, not the actual implementation of the number of components, shape and size of the drawing, the actual implementation of each component type, quantity and proportion can be a voluntary change, and its component layout type can be more complex.

[0024] In the prior art, the FROG trace of the pulse laser is usually obtained by the frequency-resolved optical gating method, so as to reconstruct the time-domain waveform and phase information of the pulse laser based on the FROG trace. However, due to the spatial averaging effect, the FROG trace obtained by the conventional frequency-resolved optical gating method is actually the average distribution result of the cross section of the pulse laser, which leads to a large error when the non-uniform beam is measured by the method, and thus the measurement error of the pulse laser is large.

[0025] Based on this, the application provides a harmonic beam acquisition structure, a FROG trace acquisition device and a pulse measurement device. By the ghost imaging principle, the speckle beams at each position of the cross section of the harmonic beam are obtained by discretely modulating the harmonic beam, so that the spectrometer can record the spectral information of each position of the cross section of the harmonic beam. The FROG trace constructed based on the recorded harmonic beam contains the time-frequency characteristics of each position of the cross section of the pulse laser 21, and thus the accuracy of the pulse measurement result is improved.

[0026] The following embodiments of the application provide a harmonic beam acquisition structure, a FROG trace acquisition device and a pulse measurement device, including but not limited to the measurement of ultrafast laser pulses. In order to facilitate understanding of the technical solutions of the application, the following will take the measurement of femtosecond pulses as an example for description.

[0027] The principles and implementation modes of the harmonic beam acquisition structure, the FROG trace acquisition device and the pulse measurement device of the present embodiment will be described in detail below with reference to the accompanying drawings, so that those skilled in the art can understand the harmonic beam acquisition structure, the FROG trace acquisition device and the pulse measurement device of the present embodiment without creative labor.

[0028] As Figure 1As shown, the harmonic beam collection structure provided in this embodiment is used to collect the spectral information of the harmonic beam 24 corresponding to the pulsed laser 21, to construct the FROG trace of the pulsed laser 21 based on the spectral information of the harmonic beam 24, and then to realize the measurement of the pulsed laser 21. Further, in order to obtain the time-frequency characteristics of each position of the pulsed beam cross section and improve the accuracy of the pulse measurement result, the harmonic beam collection structure provided in this embodiment obtains the speckle beams at each position on the cross section of the harmonic beam 24 through discrete modulation, reconstructs the cross-sectional spectral information of the harmonic beam 24 based on the ghost imaging principle, so that the FROG trace obtained based on the harmonic beam 24 can reflect the time-frequency characteristics of each position of the pulsed laser 21 cross section, so as to achieve better pulse measurement effect.

[0029] Specifically, the harmonic beam collection structure sequentially includes a beam splitting element 11, a delay unit 12, a nonlinear crystal element 13, a ghost imaging unit 14 and a spectrometer 15 along the propagation path of the pulsed laser 21. Among them, the beam splitting element 11 is located on the propagation path of the pulsed laser 21, used to split the pulsed laser 21 to obtain the first beam 22 and the second beam 23. Exemplarily, the beam splitting element 11 includes but is not limited to a polarization beam splitter or a half-transmission half-reflection mirror with an angle of 45° between the propagation direction of the pulsed laser 21; the delay unit 12 is located on the propagation path of the first beam 22 and the second beam 23, used to form a delay between the first beam 22 and the second beam 23; the nonlinear crystal element 13 is located on the propagation path of the first beam 22 and the second beam 23, used to generate the harmonic beam 24 based on the first beam 22 and the second beam 23; the spectrometer 15 is located at the end of the light path, used to record the spectral information of the light beam. Based on this, the spectral information of the harmonic beam 24 recorded by the spectrometer 15 can construct the FROG trace of the pulsed laser 21.

[0030] Further, the present embodiment also includes a ghost imaging unit 14, which is located on the propagation path between the nonlinear crystal element 13 and the spectrometer 15. After the harmonic beam 24 is emitted from the nonlinear crystal element 13, it is incident on the ghost imaging unit 14. The ghost imaging unit 14 performs discrete modulation on the harmonic beam 24 to form a modulated harmonic beam group 25, and projects the modulated harmonic beam group 25 to the spectrometer 15, so that the spectrometer 15 can record the spectral information of each position on the cross section of the harmonic beam 24.

[0031] The principle of the ghost imaging unit 14 modulating the harmonic beam 24 to enable the spectrometer 15 to record the spectral information of each position on the cross section of the harmonic beam 24 will be explained below.

[0032] It should be noted that the modulated harmonic beam group 25 includes multiple beams, and each beam is a beam modulated by the ghost imaging unit 14.

[0033] Specifically, the ghost imaging unit 14 includes multiple subunits arranged in a matrix and forming a preset digital pattern. These subunits divide the incident harmonic beam 24 into multiple sub-beams, and each subunit modulates the corresponding sub-beam to form a corresponding speckle beam. Exemplarily, the preset digital pattern includes, but is not limited to, digital patterns such as the Hadamard matrix, sinusoidal speckle, and Fourier-based speckle. It should be noted that these speckle beams constitute a modulated harmonic beam group 25, with a single speckle beam actually corresponding to a pixel in the cross-sectional imaging of the harmonic beam 24.

[0034] The spectrometer 15 records the spectral information of each speckle beam, namely, the distribution of the light intensity of each speckle beam over frequency, and transmits it to an external processing unit. Based on the light intensity distribution of each speckle beam, the external processing unit uses compressed sensing or correlation algorithms to demodulate and restore the spectral information of the cross section of the harmonic beam 24. Based on this, the spectrometer 15 records the spectral information of each speckle beam, that is, the spectral information at each position in the cross section of the harmonic beam 24.

[0035] In some optional embodiments, the ghost imaging unit 14 includes a spatial light modulator, and the preset digital pattern is a Hadamard matrix. That is, the Hadamard matrix is ​​loaded through the spatial light modulator so that the incident harmonic light beam 24 generates a pair of Hadamard speckle beams, and each Hadamard speckle beam is projected as a modulated harmonic light beam group 25 to the spectrometer 15 for recording.

[0036] Among them, such as Figure 2 As shown, the Hadamard matrix is ​​an orthogonal square matrix containing only "+1" and "-1" elements, wherein "+1" and "-1" correspond to the opening and closing of the optical channel, respectively, that is, when the sub-unit is "+1", the light intensity of the sub-beam remains unchanged, and when the sub-unit is "-1", the light intensity of the sub-beam is 0. Based on this, it is possible to achieve separate modulation of each sub-beam, and use correlation calculation or compressed sensing algorithm to demodulate and reconstruct the cross section of the harmonic light beam 24. It should be noted that those skilled in the art should be aware of the specific execution process of demodulating and reconstructing the harmonic light beam 24 using correlation calculation or compressed sensing algorithm, and this embodiment will not be described in detail here. It is worth noting that the Hadamard matrix is ​​easy to implement in hardware due to its fast computing power and good orthogonality, and has a good reconstruction effect, which is conducive to obtaining a more accurate FROG trace.

[0037] Preferably, the spatial light modulator is a reflective spatial light modulator to avoid distortion of the wavefront of each acquired speckle light beam due to transmission delay after the light beam enters the spatial light modulator. For example, the spatial light modulator is a digital micromirror spatial light modulator.

[0038] In some optional embodiments, such asFigure 3 As shown, a first focusing element 16 is further included between the spectrometer 15 and the ghost imaging unit 14, and is located on the propagation path of the modulated harmonic beam set 25, for focusing each beam of the modulated harmonic beam set 25 to avoid the deflection of each beam being too large to be incident on the spectrometer 15, so that the spectrometer 15 can receive each beam of the modulated harmonic beam set 25, and further improve the accuracy of the cross section of the harmonic beam 24 reconstructed based on each beam.

[0039] Exemplarily, the first focusing element 16 is a focusing lens.

[0040] In some optional embodiments, the nonlinear crystal element 13 is a second-order nonlinear crystal or a third-order nonlinear crystal, for generating a second harmonic or a third harmonic, i.e., the harmonic beam 24 is a second harmonic beam or a third harmonic beam. Exemplarily, the nonlinear crystal element 13 is a 23° cut BBO (Beta Barium Borate).

[0041] It should be noted that, since the first beam 22 and the second beam 23 need to be incident on the same position of the nonlinear crystal element 13 to generate the harmonic beam 24 through the harmonic effect in the nonlinear crystal element 13, the second focusing element 17 is further included before the nonlinear crystal element 13. Figure 3 As shown, a second focusing element 17 is further included before the nonlinear crystal element 13. The second focusing element 17 is located on the propagation path of the first beam 22 and the second beam 23, for focusing the first beam 22 and the second beam 23, and the nonlinear crystal element 13 is located at the focal point of the second focusing element 17, so that the first beam 22 and the second beam 23 are incident on the same position of the nonlinear crystal. Exemplarily, the second focusing element 17 is a curved mirror.

[0042] In some optional embodiments, as shown in FIG. 2, the second focusing element 17 is further included before the nonlinear crystal element 13. Figure 3As shown, the delay unit 12 includes a first reflecting element 121 and a second reflecting element 122, the first reflecting element 121 is located on the propagation path of the first light beam 22 for reflecting the first light beam 22, and the second reflecting element 122 is located on the propagation path of the second light beam 23 for reflecting the second light beam 23. Further, the first light beam 22 split by the beam splitting element 11 is reflected to the second focusing element 17 after being incident on the first reflecting element 121 and is reflected again and finally incident on the nonlinear crystal element 13; the second light beam 23 split by the beam splitting element 11 is reflected to the second focusing element 17 after being incident on the second reflecting element 122 and is reflected again and finally incident on the nonlinear crystal element 13; wherein the optical path of the first light beam 22 and the second light beam 23 from the beam splitting element 11 to the nonlinear crystal element 13 is different, thereby forming a delay between the first light beam 22 and the second light beam 23. Specifically, the distances from the first reflecting element 121 and the second reflecting element 122 to the beam splitting element 11 are different to form a delay between the first light beam 22 and the second light beam 23.

[0043] Exemplarily, the first reflecting element 121 and the second reflecting element 122 are both corner reflectors, or both are plane reflectors.

[0044] Notably, the FROG trace is actually a two-dimensional graph jointly constructed based on the harmonic light beams under each delay, thereby correlating the time domain and the frequency domain. Based on this, to obtain the FROG trace, the harmonic light beams 24 corresponding to a plurality of different delay values need to be obtained. Specifically, based on a preset delay sequence, the delay unit 12 is adjusted to form the first light beam 22 and the second light beam 23 with different delays from each other, and the corresponding harmonic light beams 24 are generated through the nonlinear crystal element 13, and then the spectral information thereof is recorded through the spectrometer 15, the wavefront conditions of the harmonic light beams 24 corresponding to each delay are obtained, and the FROG trace is constructed. Wherein, the delay sequence includes each delay value arranged in sequence.

[0045] Based on this, in order to make the first light beam 22 and the second light beam 23 have different delays, the distances from the first reflecting element 121 and the second reflecting element 122 to the beam splitting element 11 are adjusted to make the first light beam 22 and the second light beam 23 have different optical path differences in the propagation process from the beam splitting element 11 to the nonlinear crystal element 13, thereby changing the delay between the first light beam 22 and the second light beam 23 to correspond to each delay value.

[0046] In some optional embodiments, the first reflecting element 121 is located at a fixed position, and the position of the first reflecting element 121 is adjusted to change the time delay between the first light beam 22 and the second light beam 23. Specifically, the second reflecting element 122 is moved in the propagation direction of the second light beam 23, so as to change the optical path of the second light beam 23, and further change the time delay between the first light beam 22 and the second light beam 23.

[0047] Further, as shown in Figure 3 the time delay unit 12 further comprises a distance measuring element 123 for measuring the position of the second reflecting element 122. Specifically, the distance measuring element 123 is a fixedly arranged laser range finder, the laser emitting structure of which is located on the light path of the second light beam 23 incident on the second reflecting element 122, and the laser receiving structure is located on the light path of the second light beam 23 after being reflected by the second reflecting element 122. The distance between the second reflecting element 122 and the laser range finder is calculated based on the time difference between the laser emission and the reception, and the specific position of the second reflecting element 122 is obtained based on the distance between the second reflecting element 122 and the laser range finder, since the position of the laser range finder is fixed. Based on this, the position of the second reflecting element 122 can be accurately obtained and adjusted with high precision, so as to accurately change the time delay between the first light beam 22 and the second light beam 23, and further improve the accuracy of the obtained FROG trace.

[0048] Specifically, as shown in Figure 4 the FROG trace is constructed based on the harmonic light beam spectrum information corresponding to each time delay obtained by the harmonic light beam acquisition structure of the present application, which can display the local time-frequency characteristics of each position on the pulse cross section, and is beneficial to the subsequent measurement of the pulse laser 21.

[0049] Further, as shown in Figure 3 the harmonic light beam acquisition structure further comprises a third reflecting element 18, which is located between the second reflecting element 122 and the second focusing element 17 on the propagation light path of the second light beam 23, and is used for reflecting the second light beam 23 so as to make it incident on the fourth focusing element. Specifically, the third reflecting element 18 is a plane mirror.

[0050] Based on this, the harmonic light beam acquisition structure provided by the present embodiment can obtain the cross-sectional position of the modulated harmonic light beam group 25 by discretely modulating the harmonic light beam 24, and record the spectrum information of each position on the cross section of the harmonic light beam 24 by making the modulated harmonic light beam group 25 incident on the spectrometer 15, so as to improve the accuracy of the pulse measurement result.

[0051] In another aspect, the present application also provides a FROG trace acquisition device, comprising: a harmonic beam acquisition structure and a processing unit in communication with each other. Wherein the harmonic beam acquisition structure records the spectral information of the modulation harmonic beam group 25 of a plurality of different delay values based on a preset delay sequence, and sends it to the processing unit. The processing unit demodulates and reconstructs the corresponding harmonic beam 24 using correlation calculation or compressed sensing algorithm based on the spectral information of the modulation harmonic beam group 25 corresponding to each delay value, and constructs the FROG trace of the pulsed laser based on the harmonic beam 24 corresponding to each delay value. It should be noted that those skilled in the art should know the specific steps and principles of constructing the FROG trace based on the harmonic beam 24 corresponding to each delay value, which will not be specifically explained in this embodiment. It is worth noting that the specific setting and principle of the harmonic beam acquisition structure please refer to the foregoing content, this embodiment will not be repeated here.

[0052] Wherein, each time the spectral information of the modulation harmonic beam group 25 is recorded, the ghost imaging unit 14 loads different Hadamard matrixes to avoid the information redundancy caused by the high correlation between the information recorded each time and the need for more information to perform compressed sensing imaging, thereby reducing the sampling times and reducing the storage pressure. At the same time, the ghost imaging unit 14 dynamically loads the Hadamard matrix to adapt to the change of the wavefront of the light beam with time, avoiding the loss of dynamic information of the modulation harmonic beam group 25 caused by the fixed matrix.

[0053] In yet another aspect, the present application also provides a pulse measurement device, comprising: a FROG trace acquisition device and a phase inversion device in communication with each other. Wherein the FROG trace acquisition device is used to acquire the FROG trace of the pulsed laser 21 and send it to the phase inversion device; the phase inversion device performs phase inversion based on the FROG trace to reconstruct the time-domain waveform and phase information of the pulsed laser 21, thereby realizing the measurement of the pulsed laser 21.

[0054] Exemplarily, the specific execution method of phase inversion based on the FROG trace is: performing phase inversion on the FROG trace through a pre-trained physical information neural network. It should be noted that those skilled in the art should know the specific architecture, construction method and principle of the physical information neural network, which will not be specifically explained in this embodiment.

[0055] It is worth noting that the specific structure and principle of the FROG trace acquisition device please refer to the foregoing content, this embodiment will not be repeated here.

[0056] In summary, the harmonic beam collection structure, the FROG trace acquisition device and the pulse measurement equipment provided by the present application can obtain the spectral information of each position of the cross section of the harmonic beam 24 through the ghost imaging unit 14, and further enable the constructed FROG trace to reflect the local time-frequency characteristics of each position on the cross section of the pulse, especially for the measurement of non-uniform beams, such as Gaussian beams or beams with wavefront distortion, avoiding the spatial averaging effect, and effectively improving the accuracy of the pulse measurement result.

[0057] The description of the corresponding flow or structure of each of the above-mentioned drawings has its own emphasis, and the part not described in detail in a certain flow or structure can refer to the related description of other flows or structures.

[0058] The above embodiments only exemplarily illustrate the principles and effects of the present application, and are not used to limit the present application. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes completed by those skilled in the art without departing from the spirit and technical thought of the present application should be covered by the claims of the present application.

Claims

1. A harmonic beam collection structure, characterized in that: include: A beam splitting element is located on a propagation optical path of the pulsed laser and is used to split the pulsed laser to obtain a first light beam and a second light beam; a delay unit, located on a propagation optical path of the first light beam and the second light beam, and configured to form a delay between the first light beam and the second light beam; a nonlinear crystal element located on a propagation optical path of the first light beam and the second light beam; after being delayed by the delay unit, the first light beam and the second light beam are incident on the same position of the nonlinear crystal element to generate a harmonic light beam; a ghost imaging unit, located on the propagation optical path of the harmonic light beam, for performing discrete modulation on the harmonic light beam to obtain a modulated harmonic light beam group; A spectrometer is located on the propagation optical path of the modulated harmonic light beam group and is used to record spectral information of the modulated harmonic light beam group.

2. The structure according to claim 1, characterized in that The ghost imaging unit includes a spatial light modulator, which is located on the propagation path of the harmonic beam and is used to load a Hadamard matrix to generate multiple Hadamard speckle beams based on the harmonic beam, and each of the Hadamard speckle beams is used as the modulated harmonic beam group.

3. The structure according to claim 2, characterized in that The spatial light modulator is a reflective spatial light modulator.

4. The structure according to claim 2, characterized in that A first focusing element is also included between the spectrometer and the ghost imaging unit. The first focusing element is located on the propagation light path of the modulated harmonic beam group and is used to focus the modulated harmonic beam group so that the spectrometer can receive each light beam in the modulated harmonic beam group.

5. The structure according to claim 1, characterized in that A second focusing unit is also included between the delay unit and the nonlinear crystal element. The second focusing unit is located on the propagation path of the first light beam and the second light beam, and is used to focus the first light beam and the second light beam to the same position to be incident on the nonlinear crystal element.

6. The structure according to claim 1, characterized in that The delay unit includes a first reflecting element and a second reflecting element. The first reflecting element is located on the propagation path of the first light beam, and the second reflecting element is located on the propagation path of the second light beam. The first reflecting element and the second reflecting element have different distances from the beam splitting element to form a delay between the first light beam and the second light beam.

7. The structure according to claim 6, characterized in that The first reflecting element is located at a fixed position, the second reflecting element moves in the propagation direction of the second light beam, and the delay unit further includes a distance measuring element, which is used to measure the position of the second reflecting element.

8. The structure according to claim 7, characterized in that The distance measuring element is a laser rangefinder.

9. A FROG trace acquisition device, characterized in that: include: The harmonic beam collection structure according to any one of claims 1 to 8, which is used to record spectral information of the pulsed laser at multiple different delay values ​​based on the pulsed laser in combination with a preset delay sequence; The delay sequence includes a plurality of different delay values; A processing unit is configured to construct a FROG trace of the pulsed laser based on a group of modulated harmonic beams of the pulsed laser at a plurality of different delays.

10. A pulse measurement device comprising: The FROG trace acquisition device according to claim 9, used to acquire a FROG trace of a pulsed laser; A phase inversion device is used to perform phase inversion based on the FROG trace to reconstruct the time domain waveform and phase information of the pulsed laser.

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