Dispersion Measurement Methods for High Power Laser Systems

Abstract

The present invention discloses a dispersion measurement method for a high-power laser system, which relates to the fields of optical device measurement technology and laser technology. The method is based on the spectrum shaping of ultrashort pulse lasers, and obtains laser pulses with fingerprint spectrum structure and conversion limit pulse width by designing a special spectrum response curve. According to the frequency-time mapping principle, after the laser pulse with fingerprint spectrum structure is transmitted through a dispersive medium (object to be measured), the frequency domain distribution (i.e., spectrum) of the laser pulse will be mapped to the time domain, presenting a state in which the frequency domain distribution corresponds to the time domain waveform. By measuring the information such as the spectrum and time domain waveform of the laser pulse after the dispersion component is connected, and fitting and data analysis and processing with the dispersion model of Taylor expansion, the dispersion measurement of each component in the high-power laser system, such as a stretcher, a compressor, etc., is realized. The method constructs a fingerprint spectrum with a special structure for measurement, which can not only realize the rapid and precise diagnosis of the dispersion of the high-power laser system, obtain the deconstruction of high-order dispersion, but also reduce the statistical fluctuation of the data and the uncertainty of the measurement by increasing the sampling data points, and improve the accuracy of the dispersion measurement.

Description

Technical Field

[0001] The present invention relates to the fields of optical device measurement technology and laser technology, and in particular to a dispersion measurement method. Background Art

[0002] High-power laser systems such as NIF-ARC, OMEGA-EP, LMJ-PETAL, SG-II-9th PW, etc. can output kilojoule-level short-pulse lasers for experimental research on high-energy-density physics such as backlight imaging and fast ignition. Picosecond petawatt laser devices can achieve accurate characterization of laser time-domain characteristics through precise frequency-domain control technology, meet the needs of precision physics experiments and support the safe operation of picosecond petawatt devices. The precise control of the frequency domain of high-power laser systems is directly related to the spectral phase of the entire device (dispersion can be obtained by Taylor expansion of the spectral phase function). The dispersion diagnosis of the entire device is an important basis for outputting precise pulse width in the time domain, and is also a difficult point in the precise control of the time-domain output pulse width in the picosecond petawatt laser device. Through dispersion diagnosis, the dispersion of the device can be better compensated, thereby outputting compressed pulses close to the Fourier transform limit and obtaining higher pulse peak power at the same energy level. Therefore, accurate measurement of dispersion-related parameters is very important.

[0003] There are many methods for measuring dispersion. Taking optical fiber as an example, the three most commonly used methods are: phase shift method, interference method and time delay method. Among them, the most commonly used measurement methods for high-power laser systems are interference method and phase shift method. The time delay method is the simplest method for measuring optical fiber dispersion. It is to inject broadband pulses of different wavelengths into the device to be tested. Due to the existence of dispersion, the time it takes for light pulses of different wavelengths to arrive at the other end is different. The dispersion value can be calculated by measuring the arrival time difference of light pulses of different wavelengths. For example, the document "Comparison of single-mode fiber dispersion measurement techniques" (Lightwave Technol, vol.3, no.5, pp.958-966, 1985) mentioned that it is difficult to accurately obtain the delay value, resulting in low measurement accuracy. The traditional time delay method will lead to statistical uncertainty due to the limited number of test data points, resulting in reduced dispersion measurement accuracy; and dispersion measurement is easily affected by group delay ripples caused by multiple reflections of optical components and fiber polarization mode dispersion. Summary of the invention

[0004] The purpose of this patent is to provide a dispersion measurement method for high-power laser systems to solve the problems of complex device structure, difficult measurement, limited sampling data, inaccurate dispersion measurement accuracy, and susceptibility to other factors in current dispersion measurement.

[0005] The technical solution of the present invention is as follows:

[0006] A method for measuring dispersion of a high-power laser system comprises the following steps:

[0007] S1, the ultrashort pulse laser close to the Fourier transform limit is incident on the angular dispersion element after transmission;

[0008] S2, the angular dispersion element separates the wavelength components of the ultrashort pulse laser in space, and after being transmitted through a lens, enters a spectrum control unit, which is placed at the focal plane of the lens, i.e., on the Fourier transform plane of the lens, and realizes spectrum shaping of the laser pulse by utilizing the Fourier transform property of the lens;

[0009] S3, the laser pulse after spectrum shaping is collimated back to the laser pulse of the limit pulse width through the lens and the angular dispersion element, and the time domain and frequency domain distribution of the laser pulse are in Fourier transform relationship, that is, the output laser pulse after spectrum shaping is the Fourier transform of each frequency component, which is expressed by the following formula:

[0010] E out (t) = FT{E in (ω)·H(ω)}

[0011] Among them, E out (t) is the time domain distribution of the laser pulse, E in (ω) is the initial spectrum of the laser pulse, H(ω) is the spectral transmission function of the spectral control unit, and FT represents Fourier transform;

[0012] S4, inversely deriving the spectral transmission function H(ω) of the spectral control unit according to the required spectral structure, and obtaining a laser pulse with a fingerprint spectral structure and a transformation limit pulse width by designing a special spectral response curve;

[0013] S5. A compressor or a stretcher and other dispersion components are connected to the spectrum control unit. When the laser pulse with a fingerprint spectrum structure and a conversion limit pulse width is transmitted through these dispersion components (the object to be measured), a delay difference is generated due to the different effective optical paths of different wavelength components in the pulse, that is, the pulse is stretched in the time domain. According to the frequency-time mapping principle, the frequency domain distribution (that is, the spectrum) of the laser pulse will be mapped to the time domain, showing a one-to-one correspondence between the frequency domain distribution and the time domain waveform. The output laser pulse passing through the object to be measured is measured and the spectrum information is recorded by a spectrum analyzer, and the time domain waveform is measured and recorded by a high-speed photodiode and a broadband oscilloscope;

[0014] S6, after preprocessing the spectrum information and time domain waveform of the laser pulse described in step S5 using a noise reduction algorithm, select the wavelength interval Δλ of adjacent characteristic peaks in the laser pulse spectrumi , and the time interval Δτ corresponding to the spectrum in the time domain waveform i , save the two as a data set (Δλ i , Δτ i ), and fitted with the Taylor expansion dispersion model, where the Taylor expansion dispersion model is determined by the following formula:

[0015]

[0016] Where ω is the angular frequency (usually expressed in radians per second), and ω0 = 2πc / λ0 (λ0 is the central wavelength), τ(ω) is the group delay time, c is the speed of light in vacuum, and They are group delay, group delay dispersion, third-order dispersion, and fourth-order dispersion;

[0017] S7, according to the frequency-time mapping principle, establish a data model algorithm, and use the collected data group (Δλ i , Δτ i ), search for the dispersion model under this data and The optimal parameter set is the dispersion of each order of the system to be measured.

[0018] Further, the dispersion of each order calculated in step S7 is compared with the data set (Δλ i , Δτ i ) is fitted in the dispersion model, the time domain waveform of the laser pulse is inverted, and compared with the actual measured laser pulse time domain waveform, to further verify the accuracy of the measured dispersion orders under this method.

[0019] The dispersion measurement method of a high-power laser system is applicable to the dispersion measurement of various components in the high-power laser system, such as a stretcher, a compressor, etc.

[0020] In step S2, the angular dispersion element may be a grating, a prism or other optical devices capable of performing angular spectrum separation.

[0021] In step S3, the spectrum control unit may be a liquid crystal modulator, a spectrum shaper, a tunable filter or other optical devices capable of shaping the spectrum of laser pulses.

[0022] In step S5, the time domain waveform of the laser pulse may be measured by a high-speed photodetector such as a photodiode in combination with a broadband oscilloscope, a streak camera or other device capable of high-speed measurement of the pulse time domain waveform.

[0023] The beneficial effects of the present invention are:

[0024] 1) By constructing a fingerprint spectrum for measurement, rapid and precise diagnosis of the dispersion of high-power laser systems can be achieved, and the deconstruction of high-order dispersion can be obtained.

[0025] 2) Different fingerprint spectral structures can not only meet different experimental requirements, but also increase the sampling data points, reduce data statistical fluctuations and measurement uncertainty, and improve the accuracy of dispersion measurement.

[0026] 3) No additional light source is required, and the seed source in the high-power laser system can be directly used to perform dispersion measurement on the object to be measured, thereby achieving low-cost dispersion measurement. This method has the advantages of simple operation, compact structure, and high measurement rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the dispersion measurement process of the high-power laser system of the present invention;

[0028] Figure 2 is a diagram of a dispersion measurement device for a high-power laser system in an embodiment of the present invention;

[0029] Figure 3 is a spectrum diagram of laser pulses after spectrum shaping in an embodiment of the present invention;

[0030] Figure 4 This is a diagram of the measured results of the time domain waveform of the laser pulse after being transmitted through the stretcher in the implementation example of the present invention;

[0031] Figure 5 is a comparison diagram of the actual measurement result of the laser pulse time domain waveform and the inversion result in the implementation example of the present invention; DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0033] Embodiment 1:

[0034] Figure 1 The schematic diagram of the dispersion measurement process of the high-power laser system provided for the implementation example of the present invention is as follows. The ultrashort pulse laser close to the Fourier transform limit is incident on the angular dispersion element after transmission, wherein a diffraction grating is used as the angular dispersion element of this example, and a spatial light modulator is used as an example of the spectrum control unit. Figure 2The dispersion measurement device diagram of the high-power laser system provided as an implementation example of the present invention includes: an ultrashort pulse laser 1, a circulator 2, a diffraction grating 3, a lens 4, a spatial light modulator 5, a computer 6, a reflector 7, a spectrum analyzer 9, a high-speed photodiode 10, a broadband oscilloscope 11 and an object to be measured 12, wherein the spectrum control unit 8 is composed of the spatial light modulator 5, the computer 6 and the reflector 7.

[0035] The diffraction grating 3 separates the wavelength components of the ultrashort pulse laser 1 in space, and enters the spectrum control unit 8 after being transmitted through the lens 4. The spatial light modulator 5 is placed at the focal plane of the lens 4, that is, the Fourier transform plane of the lens, and the Fourier transform property of the lens is used to achieve laser pulse spectrum shaping. After the spectrum shaping, the laser pulse is reflected by the reflector 7, and then collimated by the lens 4 and the diffraction grating 3 to transform the laser pulse of the limit pulse width.

[0036] According to the Fourier transform relationship between the time domain and frequency domain distribution of the laser pulse, that is, the output laser pulse after spectrum shaping is the Fourier transform of each frequency component, which can be expressed by the following formula:

[0037] E out (t) = FT{E in (ω)·H(ω)}

[0038] Among them, E out (t) is the time domain distribution of the laser pulse, E in (ω) is the initial spectrum of the laser pulse, H(ω) is the spectral transmission function of the spectral control unit, and FT represents Fourier transform. The spectral transmission function H(ω) of the spatial light modulator can be inferred according to the required spectral structure, and a laser pulse with a fingerprint spectral structure and a conversion limit pulse width can be obtained by designing a special spectral response curve.

[0039] The spatial light modulator 5 is connected to the computer 6, and the output port of the laser pulse is connected to the spectrum analyzer. The spatial light modulator 5 is calibrated and grayscaled by monitoring the spectrum information of the laser pulse. The wavelength of the laser pulse is calibrated by designing the pixel mask of the spatial light modulator, that is, the wavelength corresponding to each pixel of the spatial light modulator 5 is controlled, so as to obtain the function of each pixel and the wavelength.

[0040] The corresponding relationship between the voltage driving value applied by the spatial light modulator 5 and the phase delay amount is further determined to obtain the spectral transmission function H(ω) of the spatial light modulator. According to the spectral transmission function H(ω) and the function of each pixel and wavelength, the amplitude intensity of each wavelength component is controlled by applying a different voltage driving value to each pixel. In this embodiment, in order to obtain Figure 3The fingerprint spectrum with a multi-peak structure is designed according to the amplitude intensity of each wavelength in the fingerprint spectrum. That is, when realizing the fingerprint spectrum structure, different voltage driving values ​​should be applied to each pixel point. The computer 6 terminal continuously iterates and adjusts to obtain a laser pulse with a fingerprint spectrum structure and a conversion limit pulse width. By designing different spectral response curves, arbitrary waveform output can be achieved to meet different experimental requirements.

[0041] The object to be measured 12 is connected after the spatial light modulator 5. In this embodiment, a stretcher is used as the object to be measured. When the laser pulse after spectrum shaping is transmitted in the stretcher, a delay difference is generated due to the different effective optical paths of different wavelength components in the pulse, that is, the pulse is stretched in the time domain. According to the principle of frequency-time mapping, the frequency domain distribution (i.e., spectrum) of the laser pulse will be mapped to the time domain, showing a state of one-to-one correspondence between the frequency domain distribution and the time domain waveform.

[0042] The spectrum analyzer 9 measures and records the spectrum information of the output laser pulse after spectrum shaping and passing through the stretcher. Figure 3 is the laser pulse spectrum after spectrum shaping. Further, the high-speed photodiode 10 and the broadband oscilloscope 11 measure and record the time domain waveform. Figure 4 This is the actual measurement result of the time domain waveform of the laser pulse after it is transmitted through the stretcher. Among them, the high-speed photodiode has the characteristics of high bandwidth and fast response speed; the broadband oscilloscope has the performance of high bandwidth and high sampling rate.

[0043] After the measured laser pulse spectrum information and time domain waveform are preprocessed by using the noise reduction algorithm, the wavelength interval Δλ of adjacent characteristic peaks in the laser pulse spectrum is selected. i , and the time interval τ corresponding to the spectrum in the time domain waveform i , save the two as a data set (Δλ i , Δτ i ), and fitted with the Taylor expansion dispersion model, where the Taylor expansion dispersion model is determined by the following formula:

[0044]

[0045] Where ω is the angular frequency (usually expressed in radians per second), and ω0 = 2πc / λ0 (λ0 is the central wavelength), τ(ω) is the group delay time, c is the speed of light in vacuum, and They are group delay, group delay dispersion, third-order dispersion and fourth-order dispersion.

[0046] According to the frequency-time mapping principle, a data model algorithm is established, using the collected data set (Δλ i , Δτ i), search for the dispersion model under this data and The optimal parameter set is the dispersion of each order of the system to be measured.

[0047] Furthermore, the dispersion of each order calculated in the above steps is compared with the data set (Δλ i , Δτ i ) is fitted in the dispersion model, the time domain waveform of the laser pulse is inverted, and compared with the actual measured laser pulse time domain waveform. Figure 4 The comparison between the actual measurement results of the laser pulse time domain waveform and the inversion results is shown. The two fit well, further verifying the accuracy of measuring each order of dispersion under this method. The measured data results are as follows: Group delay (The error with the theoretical value is about 0.02%), group velocity dispersion (The error with the theoretical value is about 0.45%), third-order dispersion Fourth-order dispersion

[0048] The above description is only a preferred embodiment of the present invention and the technical principles used. It should be understood by those skilled in the art that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above implementation examples, the present invention is not limited to the above implementation examples, and may also include more other equivalent implementation examples without departing from the concept of the present invention, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A dispersion measurement method for a high-power laser system, characterized in that: The following steps are involved: S1, the ultrashort pulse laser close to the Fourier transform limit is incident on the angular dispersion element after transmission; S2, the angular dispersion element separates the wavelength components of the ultrashort pulse laser in space, and after being transmitted through a lens, enters a spectrum control unit, which is placed at the focal plane of the lens, i.e., on the Fourier transform plane of the lens, and realizes spectrum shaping of the laser pulse by utilizing the Fourier transform property of the lens; S3, the laser pulse after spectrum shaping is collimated back to the laser pulse of the limit pulse width through the lens and the angular dispersion element, and the time domain and frequency domain distribution of the laser pulse are in Fourier transform relationship, that is, the output laser pulse after spectrum shaping is the Fourier transform of each frequency component, which is expressed by the following formula: E out (t)=FT{E in (ω)·H(ω)} Among them, E out (t) is the time domain distribution of the laser pulse, E in (ω) is the initial spectrum of the laser pulse, H(ω) is the spectral transmission function of the spectral control unit, and FT represents Fourier transform; S4, inferring the spectral transmission function H(ω) of the spectral control unit according to the required spectral structure, and obtaining a laser pulse with a fingerprint spectral structure and a conversion limit pulse width through a spectral response curve; S5. A dispersion component is connected behind the spectrum control unit. When the laser pulse having a fingerprint spectrum structure and a conversion limit pulse width is transmitted through the dispersion component, a delay difference is generated due to the different effective optical paths of different wavelength components in the pulse, that is, the pulse is broadened in the time domain. According to the frequency-time mapping principle, the frequency domain distribution of the laser pulse, that is, the spectrum, is mapped to the time domain, presenting a one-to-one correspondence between the frequency domain distribution and the time domain waveform. The output laser pulse passing through the object to be measured is measured and the spectrum information is recorded by a spectrum analyzer, and the time domain waveform is measured and recorded by a high-speed photodiode and a broadband oscilloscope; S6, after preprocessing the spectrum information and time domain waveform of the laser pulse described in step S5 using a noise reduction algorithm, select the wavelength interval Δλ of adjacent characteristic peaks in the laser pulse spectrum i , and the time interval Δτ corresponding to the spectrum in the time domain waveform i , save the two as a data set (Δλ i , Δτ i ), and fitted with the Taylor expansion dispersion model, where the Taylor expansion dispersion model is determined by the following formula: Where ω is the angular frequency, and ω0 = 2πc / λ0, λ0 is the central wavelength, τ(ω) is the group delay time, c is the speed of light in vacuum, and They are group delay, group delay dispersion, third-order dispersion, and fourth-order dispersion; S7, according to the frequency-time mapping principle, establish a data model algorithm, and use the collected data group (Δλ i , Δτ i ), search for the dispersion model under this data and The optimal parameter set is the dispersion of each order of the system to be measured.

2. A dispersion measurement method for a high power laser system as claimed in claim 1, characterized in that: The dispersion component includes a stretcher and a compressor.

3. The method for measuring dispersion of a high-power laser system according to claim 1, characterized in that: The dispersion of each order calculated in step S7 can be compared with the data set (Δλ i ,Δτ i ) is fitted in the dispersion model to invert the time domain waveform of the laser pulse.

4. The method for measuring dispersion of a high-power laser system according to claim 1, characterized in that: The angular dispersion element is a grating, a prism or other optical devices capable of performing angular spectrum separation.

5. The method for measuring dispersion of a high-power laser system according to claim 1, characterized in that: The spectrum control unit is a liquid crystal modulator, a spectrum shaper, a tunable filter or other optical devices capable of shaping the laser pulse spectrum.

6. The method for measuring dispersion of a high-power laser system according to claim 1, characterized in that: The laser pulse time domain waveform measurement device is a high-speed photoelectric detector combined with a broadband oscilloscope, a streak camera or other devices capable of performing high-speed measurement of pulse time domain waveforms.

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