CN115127779A - Dispersion measurement method of high-power laser system - Google Patents
Dispersion measurement method of high-power laser system Download PDFInfo
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Abstract
The invention discloses a dispersion measurement method for a high-power laser system, and relates to the fields of optical device measurement technology, laser technology and the like. The method is based on the spectrum shaping of ultrashort pulse laser, and laser pulses with fingerprint spectrum structures and conversion limit pulse widths are obtained by designing a special spectrum response curve. According to the frequency-time mapping principle, after laser pulses with a fingerprint spectrum structure are transmitted through a dispersion medium (an object to be measured), the frequency domain distribution (namely, spectrum) of the laser pulses can be mapped to a time domain, and a state that the frequency domain distribution corresponds to the time domain waveform one by one is presented. The dispersion measurement of each component in the high-power laser system, such as a stretcher, a compressor and the like, is realized by measuring information such as laser pulse spectrum, time domain waveform and the like after the laser pulse spectrum and the time domain waveform are accessed into the dispersion component, and fitting and data analysis processing are carried out on the information and a dispersion model expanded by Taylor. The method can realize rapid and precise diagnosis of dispersion of a high-power laser system and obtain deconstruction of high-order dispersion by constructing the fingerprint spectrum with a special structure for measurement, and can reduce data statistical fluctuation and measurement uncertainty and improve the precision of dispersion measurement by increasing sampling data points.
Description
Technical Field
The invention relates to the fields of optical device measurement technology, laser technology and the like, in particular to a dispersion measurement method.
Background
High-power laser systems such as NIF-ARC, OMEGA-EP, LMJ-PETAL, SG-II-9th PW and the like can output short pulse laser of kilojoule level, and are used for experimental researches of high-energy density physics such as backlight imaging, fast ignition and the like. The picosecond tile laser device can realize the accurate representation of laser time domain characteristics through the precise regulation and control technology of a frequency domain, and meets the requirement of precise physical experiments to support the safe operation of the picosecond tile laser device. The precise regulation and control of the frequency domain of the high-power laser system is directly related to the spectrum phase (the dispersion can be obtained by Taylor expansion of a spectrum phase function) of the whole device, the dispersion diagnosis of the whole device is an important basis for outputting precise pulse width in a time domain, and is also a difficulty for precisely controlling the time domain output pulse width in a picosecond beat laser device, and the dispersion of the device is better compensated through the dispersion diagnosis, so that a compression pulse close to the Fourier transform limit is output, and higher pulse peak power is obtained under the same energy level. Therefore, accurate measurement of the dispersion related parameter is very important.
There are many methods for measuring chromatic dispersion, such as optical fiber, among which the three most commonly used methods are: phase shift methods, interference methods, and time delay methods. The measurement methods commonly used in high power laser systems are interferometry and phase shift. The time delay method is the simplest and most convenient method for measuring the optical fiber dispersion, broadband pulses with different wavelengths are respectively injected into a device to be measured, due to the existence of dispersion, the time for the optical pulses with different wavelengths to reach the other end is different, and the dispersion value can be calculated by measuring the arrival time difference of the optical pulses with different wavelengths. Such as that described in the document "Comparison of Single-mode fiber dispersion measurement techniques" (Lightwave technique, vol.3, No.5, pp.958-966, 1985), the measurement accuracy is low due to the difficulty in obtaining the time delay value accurately. The traditional time delay method can cause statistical uncertainty due to the limitation of the number of test data points, so that the dispersion measurement precision is reduced; also, the dispersion measurement is susceptible to group delay ripple and fiber polarization mode dispersion caused by multiple reflections from the optical element.
Disclosure of Invention
The invention aims to provide a dispersion measurement method for a high-power laser system, and solves the problems that the existing dispersion measurement device is complex in structure, difficult to measure, limited in sampling data, inaccurate in dispersion measurement precision, easy to be influenced by other factors and the like.
The technical scheme of the invention is as follows:
a dispersion measurement method of a high-power laser system comprises the following steps:
s1, transmitting the ultra-short pulse laser close to the Fourier transform limit and then making the ultra-short pulse laser incident to an angular dispersion element;
s2, the angular dispersion element spatially separates each wavelength component of the ultrashort pulse laser, the ultrashort pulse laser enters the spectrum control unit after being transmitted through the lens, the spectrum control unit is placed at a lens focal plane, namely on a Fourier transform surface of the lens, and the property of Fourier transform of the lens is utilized to realize the spectrum shaping of the laser pulse;
s3, the laser pulse after spectrum shaping is collimated by the lens and the angular dispersion element and back to transform the laser pulse with the limit pulse width, and the laser pulse is in Fourier transform relation according to the time domain and frequency domain distribution of the laser pulse, namely the output laser pulse after spectrum shaping is Fourier transform of each frequency component, and is represented by the following formula:
E out (t)＝FT(E in (ω).H(ω)}
wherein, E out (t) is the temporal distribution of the laser pulses, E in (omega) is the initial spectrum of the laser pulse, and H (omega) is the light of the spectrum control unitSpectral transmission function, FT denotes fourier transform;
s4, reversely deducing a spectrum transmission function H (omega) of the spectrum control unit according to a required spectrum structure, and designing a special spectrum response curve to obtain laser pulses with a fingerprint spectrum structure and a transformation limit pulse width;
s5, connecting a compressor or stretcher with other dispersive components behind the spectrum control unit, when the laser pulse with fingerprint spectrum structure and conversion limit pulse width is transmitted by these dispersive components (objects to be measured), generating delay difference due to different effective optical paths of different wavelength components in the pulse, i.e. the pulse is stretched in time domain, according to the frequency-time mapping principle, the frequency domain distribution (i.e. spectrum) of the laser pulse can be mapped to the time domain, presenting the state of one-to-one correspondence of the frequency domain distribution and the time domain waveform, measuring and recording the spectrum information by the optical spectrum analyzer for the output laser pulse passing through the objects to be measured, and measuring and recording the time domain waveform by the high-speed photodiode and the broadband oscilloscope;
s6, preprocessing the spectral information and the time domain waveform of the laser pulse in the step S5 by a noise reduction algorithm, and selecting the wavelength interval delta lambda of adjacent characteristic wave crests in the laser pulse spectrum i And the time interval Δ τ corresponding to the spectrum in the time domain waveform i Storing both as a data set (Δ λ) i ,Δτ i ) And fitting with a Taylor-expanded dispersion model, wherein the Taylor-expanded dispersion model is determined by the following formula:
where ω is the angular frequency (usually expressed in radians per second), and ω is 0 ＝2πc/λ 0 (λ 0 As the center wavelength), τ (ω) is the group delay time, c is the speed of light in vacuum,
and
group delay, group delay dispersion, third order dispersion and fourth order dispersion, respectively;
s7, establishing a data model algorithm according to the frequency-time mapping principle, and utilizing the collected data set (delta lambda) i ,Δτ i ) Searching for dispersion models under the data
And
and the optimal parameter set is the chromatic dispersion of each order of the system to be tested.
Further, the chromatic dispersion of each order calculated in the step S7 and the data set (Δ λ) are combined i ,Δτ i ) And fitting in a dispersion model, inverting the time domain waveform of the laser pulse, comparing the time domain waveform with the actually measured time domain waveform of the laser pulse, and further verifying the accuracy of the measured chromatic dispersion of each order under the method.
The dispersion measurement method of the high-power laser system is suitable for dispersion measurement of components in the high-power laser system, such as a stretcher, a compressor and the like.
In step S2, the angular dispersive element may be a grating, a prism, or other optical device capable of angular spectral separation.
In step S3, the spectrum control unit may be a liquid crystal modulator, a spectrum shaper, a tunable filter, or other optical device capable of performing laser pulse spectrum shaping.
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 strip camera, or other device capable of high speed measurement of the pulse time domain waveform.
The invention has the beneficial effects that:
1) by constructing the fingerprint spectrum for measurement, the rapid and precise diagnosis of the dispersion of the high-power laser system can be realized, and the deconstruction of the high-order dispersion can be obtained.
2) Different fingerprint spectrum structures not only can satisfy different experimental demands, can also increase the sampling data point, reduce data statistics fluctuation and measure uncertainty, improve dispersion measurement's precision.
3) The method has the advantages of simple operation, compact structure, high measurement speed and the like.
Drawings
FIG. 1 is a schematic diagram of a dispersion measurement process for a high power laser system according to the present invention;
FIG. 2 is a diagram of a dispersion measuring device for a high power laser system in an exemplary embodiment of the invention;
FIG. 3 is a spectrum of a spectrally shaped laser pulse spectrogram in an exemplary embodiment of the present invention;
fig. 4 is a graph of the actual measurement result of the time domain waveform of the laser pulse after being transmitted by the stretcher in the embodiment of the present invention;
FIG. 5 is a graph comparing the actual measurement result and the inversion result of the laser pulse time domain waveform in the embodiment of the present invention;
Detailed Description
The invention is described in further detail below with reference to the figures and specific examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Example 1:
fig. 1 is a schematic diagram of a process for measuring dispersion of a high-power laser system according to an embodiment of the present invention. Ultra-short pulse laser light close to the fourier transform limit is transmitted and then incident on an angular dispersion element, 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 a spectrum control unit. Fig. 2 is a diagram of a dispersion measuring device of a high-power laser system provided for an embodiment of the invention, which includes: the system comprises 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 a spectrum control unit 8 is composed of the spatial light modulator 5, the computer 6 and the reflector (7).
The diffraction grating 3 spatially separates each wavelength component of the ultrashort pulse laser 1, and the separated components are transmitted through the lens 4 and then enter the spectrum control unit 8. The spatial light modulator 5 is placed at the focal plane of the lens 4, namely on the Fourier transform surface of the lens, and the spectral shaping of the laser pulse light is realized by utilizing the Fourier transform property of the lens. The laser pulse after the spectrum shaping is reflected by the reflector 7, and then is collimated by the lens 4 and the diffraction grating (3) to return the laser pulse with the transformation limit pulse width.
The time-domain and frequency-domain distributions of the laser pulse are in fourier transform relationship, that is, the output laser pulse after the spectral shaping is fourier transform of each frequency component, which can be expressed by the following formula:
E out (t)＝FT{E in (ω).H(ω)}
wherein E is out (t) is the temporal distribution of the laser pulses, 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 the Fourier transform. The spectral transmission function H (omega) of the spatial light modulator can be reversely deduced according to the required spectral structure, and laser pulses with fingerprint spectral structures and conversion limit pulse widths are obtained by designing a special spectral response curve.
And the spatial light modulator (5) is connected with the computer (6), the output port of the laser pulse is connected with the spectrum analyzer, and the calibration and the gray scale calibration of the spatial light modulator (5) are carried out by monitoring the spectrum information of the laser pulse. The wavelength calibration of the laser pulse is performed by designing a pixel mask of the spatial light modulator, that is, the wavelength controlled correspondingly for each pixel of the spatial light modulator 5, so as to obtain a function of each pixel and the wavelength.
Further, the correspondence between the voltage drive value applied to the spatial light modulator 5 and the phase delay amount is determined, and the spectral transmission function H (ω) of the spatial light modulator is obtained. The amplitude intensity of each wavelength component is controlled by applying different voltage drive values to each pixel according to the spectral transmission function H (ω) and the function of each pixel and wavelength. In this embodiment, in order to obtain a fingerprint spectrum having a multi-peak structure as shown in fig. 3, a special spectral response curve is designed according to the amplitude intensity of each wavelength in the fingerprint spectrum, that is, different voltage driving values to be applied to each pixel point when the fingerprint spectrum structure is implemented are continuously iterated and adjusted at the computer 6 terminal to obtain laser pulses having a fingerprint spectrum structure and a transform limit pulse width. By designing different spectral response curves, arbitrary waveform output can be realized so as to meet different experimental requirements.
The object 12 to be measured is connected behind the spatial light modulator 5, in this embodiment, a stretcher is used as the object to be measured, and when the laser pulse after the spectral shaping is transmitted in the stretcher, delay difference is generated due to 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 (i.e., spectrum) of the laser pulse is mapped to the time domain, and the state that the frequency domain distribution corresponds to the time domain waveform one by one is presented.
The spectrally shaped output laser pulse passing through the stretcher is measured and the spectral information is recorded by the spectrum analyzer 9, and fig. 3 is a spectrum of the spectrally shaped laser pulse. Further, the time domain waveform is measured and recorded by the high-speed photodiode 10 and the broadband oscilloscope 11, and fig. 4 is a time domain waveform actual measurement result diagram after the laser pulse is transmitted by the stretcher. The high-speed photodiode has the characteristics of high bandwidth, high response speed and the like; the broadband oscilloscope has the performances of high bandwidth, high sampling rate and the like.
After the measured laser pulse spectrum information and time domain waveform are subjected to data preprocessing by adopting a noise reduction algorithm, the wavelength interval delta lambda of adjacent characteristic wave crests in the laser pulse spectrum is selected i And a time interval τ in the time domain waveform corresponding to the spectrum i Storing both as a data set (Δ λ) i ,Δτ i ) And fitting with a Taylor-expanded dispersion model, wherein the Taylor-expanded dispersion model is determined by the following formula:
where ω is the angular frequency (usually expressed in radians per second), and ω is 0 ＝2πc/λ 0 (λ 0 As the center wavelength), τ (ω) is the group delay time, c is the speed of light in vacuum,
and
group delay, group delay dispersion, third order dispersion and fourth order dispersion, respectively.
Establishing a data model algorithm according to a frequency-time mapping principle, and utilizing the acquired data set (delta lambda) i ,Δτ i ) Searching for the dispersion model under the data
And
and the optimal parameter set is the chromatic dispersion of each order of the system to be tested.
Further, the chromatic dispersion of each order and the data set (delta lambda) obtained by the calculation in the step are compared i ,Δτ i ) Fitting in a dispersion model, inverting the time domain waveform of the laser pulse, and comparing the time domain waveform with the actually measured laser pulse time domain waveform, wherein the comparison between the actual measurement result and the inversion result of the laser pulse time domain waveform is shown in FIG. 4, the fitting conditions of the two are good, and the accuracy of measuring the dispersion of each order under the method is further verified. The data results obtained from the measurements are as follows: group delay
(error value from theoretical value is about 0.02%), group velocity dispersion
(error from theoretical value is about 0.45%), third-order dispersion
Fourth order dispersion
The foregoing description is only exemplary of the invention and the application of the principles thereof. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in more detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.
Claims (6)
1. A dispersion measurement method of a high-power laser system is characterized by comprising the following steps:
s1, transmitting the ultra-short pulse laser close to the Fourier transform limit and then making the ultra-short pulse laser incident to an angular dispersion element;
s2, the angular dispersion element spatially separates wavelength components of the ultrashort pulse laser, the wavelength components are transmitted through the lens and enter the spectrum control unit, the spectrum control unit is placed at a lens focal plane, namely a Fourier transform surface of the lens, and the property of Fourier transform of the lens is utilized to realize the spectrum shaping of the laser pulse;
s3, the laser pulse after the spectrum shaping is collimated by the lens and the angular dispersion element and then transformed back to the laser pulse with the limit pulse width, and the laser pulse is in Fourier transform relationship according to the time domain and frequency domain distribution of the laser pulse, namely the output laser pulse after the spectrum shaping is Fourier transform of each frequency component, and is expressed by the following formula:
E out (t)＝FT{E in (ω)·H(ω)}
wherein E is out (t) is a laserTemporal distribution of pulses, E in (ω) is the initial spectrum of the laser pulse, H (ω) is the spectral transmission function of the spectral control unit, FT represents the fourier transform;
s4, reversely deducing a spectrum transmission function H (omega) of the spectrum control unit according to a required spectrum structure, and obtaining laser pulses with a fingerprint spectrum structure and a transformation limit pulse width through a spectrum response curve;
s5, connecting a dispersion component behind the spectrum control unit, when the laser pulse with fingerprint spectrum structure and conversion limit pulse width is transmitted by the dispersion component, generating delay difference due to different effective optical paths of different wavelength components in the pulse, namely the pulse is widened in the time domain, according to the frequency-time mapping principle, the frequency domain distribution (i.e. spectrum) of the laser pulse is mapped to the time domain, presenting a state that the frequency domain distribution corresponds to the time domain waveform one by one, measuring and recording the spectrum information by a spectrum analyzer for the output laser pulse passing through the object to be measured, and measuring and recording the time domain waveform by a high-speed photodiode and a broadband oscilloscope;
s6, preprocessing the spectral information and the time domain waveform of the laser pulse in the step S5 by a noise reduction algorithm, and selecting the wavelength interval delta lambda of adjacent characteristic wave crests in the laser pulse spectrum i And the time interval Δ τ corresponding to the spectrum in the time domain waveform i Storing both as a data set (Δ λ) i ,Δτ i ) And fitting with a Taylor-expanded dispersion model, wherein the Taylor-expanded dispersion model is determined by the following formula:
where ω is the angular frequency and ω is 0 ＝2πc/λ 0 (λ 0 As the center wavelength), τ (ω) is the group delay time, c is the speed of light in vacuum,
and
group delay, group delay dispersion, third order dispersion and fourth order dispersion, respectively;
s7, establishing a data model algorithm according to the frequency-time mapping principle, and utilizing the acquired data set (delta lambda) i ,Δτ i ) Searching for the dispersion model under the data
And
and the optimal parameter set is the chromatic dispersion of each order of the system to be tested.
2. The method of claim 1 wherein the dispersive and dispersive element comprises a stretcher and a compressor.
3. The method of claim 1, wherein the chromatic dispersion of each order calculated in step S7 is compared with a data set (Δ λ) i ,Δτ i ) And fitting in a dispersion model, and inverting the time domain waveform of the laser pulse.
4. The method of claim 1, wherein the angular dispersive element is a grating, prism or other optical device capable of angular spectral separation.
5. The method of claim 1, wherein the optical spectrum control unit is a liquid crystal modulator, an optical spectrum reshaper, a tunable filter or other optical device capable of performing laser pulse spectral shaping.
6. The dispersion measurement method of a high power laser system according to claim 1, wherein the time domain waveform measurement of the laser pulse is performed by a high speed photodetector such as a photodiode in combination with a broadband oscilloscope, a fringe camera or other device capable of high speed measurement of pulse time domain waveform.
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