CN105700157B - Based on the smoothing by spectral dispersion method of compound grating in inertially confinement fusion device - Google Patents
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Abstract
The present invention relates in a kind of inertially confinement fusion device based on compound grating smoothing by spectral dispersion method and its even sliding device.This method is the one smoothing by spectral dispersion device based on compound grating of addition in inertially confinement fusion device optical transport chain preamplifier system or after main amplification system；The smoothing by spectral dispersion device is made of phase-modulator and compound grating；Wherein phase-modulator generates bandwidth needed for the compound grating carries out spectrum angle dispersion to make to laser beam generation time phase-modulation；Compound grating is used to control laser beam dispersion direction.The present invention carries out subregion spectral dispersion to laser beam, to realize the multi-direction sweeping of laser beam speckle, so that laser beam be made to realize comprehensive even cunning in far-field focus within the shorter time of integration, that is, improves the uniformity that laser beam irradiates target surface.For compound grating used in the present invention compared with existing special grating, difficulty of processing is smaller, and structure is relatively easy and is easier to heavy caliber processing.
Description
Technical Field
The invention relates to a light beam smoothing technology in a laser-driven inertial confinement fusion device, in particular to a spectral dispersion smoothing method based on a composite grating in a laser-driven Inertial Confinement Fusion (ICF) device and a smoothing device thereof.
Background
In a laser-driven Inertial Confinement Fusion (ICF) device, because physical experiments require a high-power laser device to precisely control the light field distribution of a target surface, various airspace beam smoothing technologies and time domain beam smoothing technologies are widely adopted to control the uniformity of a far-field focal spot of a laser beam in the ICF device, namely improve the uniformity of irradiation of the target surface. In the existing spatial domain beam smoothing technology, a Continuous Phase Plate (CPP) is often used to control the far-field focal spot profile of a laser beam, but due to coherent superposition among sub-beams inside the laser beam, a speckle structure exists inside the far-field focal spot; the speckle structure can cause various parameter instability effects in the interaction process of the laser beam and the target pill, so that the compression symmetry of the laser beam on the target pill is reduced, the existence of speckles is inhibited by adopting a time domain light beam smoothing technology, and the harm of various parameter instability effects is reduced. The existing time domain light beam smoothing technology mainly comprises polarization smoothing, spectral angular dispersion smoothing and other technologies.
The polarization smoothing technology is characterized in that optical elements such as a birefringent optical wedge and a polarization rotating plate are used for changing the polarization state in the cross section of a light beam, so that the decoherence superposition of sub-light beams in the light beam is realized, and the far-field focal spot contrast is reduced. The method has the advantages that the focal spot can be instantly smoothed, the method is effective for improving the uniformity of the far-field focal spot at the initial stage of target surface irradiation, the birefringent wedge is mainly used for smoothing of a single light beam and can be damaged by stimulated Raman scattering, and the polarization rotation plate has the smoothing effect only when multiple light beams are superposed.
The Spectral angular Dispersion (SSD) Smoothing technique is to perform time phase modulation on a laser beam and perform Spectral angular Dispersion by using a grating, thereby realizing sweeping of far-field speckles of the laser beam, and improving uniformity of far-field focal spots within a thermal Smoothing time of a plasma. At present, a one-dimensional SSD (1D-SSD) has the defect that the far-field speckle of a laser beam can only be swept in the grating dispersion direction, namely the x direction or the y direction, so that the inside of the far-field speckle of the laser beam is subjected to striped light intensity modulation along the grating dispersion direction, a two-dimensional SSD smoothing technology is provided, and the beam smoothing of the laser beam in two orthogonal directions of the x direction and the y direction is realized; however, the unavoidable coherence phenomenon of the laser beam in the far field exists in the two-dimensional SSD, and then a three-direction SSD smoothing technology is provided, and the frequency of the near-field laser beam tends to be random by the three-direction SSD smoothing technology, so that the irradiation uniformity of the target surface is further improved. However, the three-direction SSD smoothing technology has the disadvantages of complicated optical path structure, difficult adjustment, etc.
In order to make up for the deficiency of linear grating in the single dispersion direction, circular gratings, spiral gratings and array gratings are proposed. It is hoped that better beam smoothing effect can be obtained by the dispersion mode of special circular grating, spiral grating and array grating. The circular grating has obvious even and smooth effect on focal spots in the radial direction, namely, has better inhibition effect on diffraction fringes around circumferential scribed lines. Disadvantageously, diffraction fringes are present in large numbers due to the absence of spectral angular dispersion in the circumferential direction. The dispersion direction of the spiral grating is in the circumferential direction, and the closer to the center of the circumference, the denser the grating lines are, thus increasing the processing difficulty. However, far from the center of the circumference, since the amount of dispersion caused by the grating is small, the total line of the grating needs to be increased to obtain a proper dispersion effect, which further increases the processing difficulty. The array grating introduces a plurality of dispersion directions, however, theoretical research shows that the increase of the number of grating units does not further improve the smooth effect of the focal spot, and meanwhile, the increase of the number of units increases the actual processing difficulty.
Disclosure of Invention
The invention aims to overcome the defects and shortcomings in the prior art and provides a novel method for uniformly smoothing spectral dispersion based on a composite grating in an inertial confinement fusion device and a spectral dispersion uniform smoothing device for realizing the method. The method is characterized in that a spectral dispersion smoothing device based on a composite grating is added in an optical transmission chain of an inertial confinement fusion device, so that the smooth of far-field focal spots of laser beams in different directions can be realized, the uniformity of far-field irradiation of the laser beams is improved within a short integration time, and the uniformity of irradiation of the laser beams on a target surface is improved.
The spectral dispersion smoothing method and the smoothing device based on the composite type grating in the inertial confinement fusion device can be used in the inertial confinement fusion device with direct drive and indirect drive so as to improve the uniformity of irradiation on a target surface.
In order to achieve the above object, the present invention is achieved by the following technical means.
The design concept of the invention is as follows: a spectral dispersion smoothing device based on a composite grating is added in a pre-amplification system or after a main amplification system in an optical transmission chain of a laser-driven inertial confinement fusion device, time phase modulation is firstly carried out on a laser beam, and then regional spectral dispersion is carried out on the laser beam according to the characteristics of the laser beam so as to realize multidirectional sweeping of far-field speckles of the laser beam, so that omnibearing smoothing of far-field focal spots of the laser beam is realized in short integration time, namely the uniformity of irradiation of the laser beam on a target surface is improved.
The spectral dispersion smoothing device based on the composite grating consists of a phase modulator and the composite grating; the phase modulator is used for generating time phase modulation on the laser beam so as to generate a bandwidth required by the compound type grating for spectral angular dispersion; the composite grating is used to control the dispersion direction of the laser beam. In order to ensure the uniform and smooth effect of the composite grating and reduce the processing difficulty of the composite grating, the adopted composite grating is a planar transmission grating. Meanwhile, in order to ensure symmetrical transmission of the laser beam after dispersion, a central axis of the composite grating and a central axis of an incident laser beam are collinear, and a dispersion area of the composite grating is divided into two parts: the circular area outside the central axis of the composite grating is used as a dispersion area i, the radius of the circular area is a certain fixed value, the dispersion is a dispersion mode I, the dispersion mode can be one-dimensional dispersion or radial dispersion, the area outside the circular area is used as a dispersion area II, the dispersion is a dispersion mode II, and the dispersion mode is circumferential dispersion or radial dispersion. In the optical transmission chain, a laser beam output by the seed optical output unit sequentially passes through a pre-amplification system, a phase modulator, a composite grating, a main amplification system, a frequency conversion unit and a Continuous Phase Plate (CPP), and is finally focused to a target surface through a focusing lens; when the laser beam modulated by the phase modulator passes through the composite grating, the laser beam in the circular region is dispersed according to a dispersion mode I, and the laser beam outside the circular region is dispersed according to a dispersion mode II. When reasonable dispersion areas and dispersion modes are selected for the dispersion areas of the two parts, speckles in far-field focal spots can be quickly swept in multiple directions, so that the modulation structure of the focal spots in the dispersion direction is smoothed in short integration time, and the irradiation uniformity of a laser beam on a target surface is improved.
The invention provides a spectral dispersion smoothing method based on a composite grating in an inertial confinement fusion device, which comprises the following steps of:
(1) adding a spectral dispersion smoothing device in an optical transmission chain of a laser-driven inertial confinement fusion device, and adjusting the modulation frequency omega of a phase modulator in the devicemA parameter value;
(2) performing time phase modulation on incident laser beams in the optical transmission chain by using the phase modulator in the step (1);
(3) in the step (2), the incident laser beam in the optical transmission chain modulated by the phase modulator passes through a composite grating in the spectral dispersion smoothing device;
(4) in the step (3), the incident laser beam passes through the composite grating, a dispersion region of the composite grating is divided into a dispersion region i and a dispersion region ii, and the laser beam respectively passes through the dispersion region i and the dispersion region ii of the composite grating simultaneously to carry out spectral dispersion in different directions;
(5) after the laser beams in the optical transmission chain in the step (4) are subjected to spectral dispersion, the laser beams sequentially pass through a beam expander, a main amplification system and a continuous phase plate in an inertial confinement fusion device, and are finally focused to a target surface through a focusing lens; when the far-field focal spot of the laser beam is subjected to spectral dispersion in different directions through the composite grating, the speckles in the far-field focal spot are quickly swept in respective dispersion directions, so that the target surface is uniformly smoothed in short integration time, namely the irradiation uniformity of the target surface is improved.
In the above technical solution, the spectral dispersion smoothing device added in step (1) is added in a pre-amplification system in an optical transmission chain of an inertial confinement fusion device, or after a main amplification system.
In the above technical solution, the added spectral dispersion smoothing device in step (1) is composed of a phase modulator and a composite grating.
In the above technical scheme, the outer diameter of the composite grating in the spectral dispersion smoothing device is the same as the aperture of the incident laser beam.
After the laser beam is modulated by the phase modulator in step (2) of the present invention to obtain the spectrum broadening, the electric field strength of the laser beam can be represented as:
in the formula E0For the initial laser beam field, omega0For the angular frequency, delta and omega, of the initial laser beammRespectively modulation depth and modulation frequency.
In the step (3), the laser beam in the optical transmission chain is subjected to spectral dispersion by the composite grating, and the expression is as follows:
wherein,
wherein r is the radius of the circular dispersion region i,the dispersion coefficient of the circular area i of the composite grating,the dispersion direction of a composite grating circular dispersion area i;the dispersion coefficient of the composite grating dispersion region ii,is a dispersion direction of the composite grating dispersion region ii, and
when the circular area i of the composite grating adopts one-dimensional linear dispersion,
when the circular area i of the composite grating is dispersed with radial color,
when the composite grating region ii employs circumferential dispersion,
when radial dispersion is employed in the composite grating region ii,
in equations (5) to (8), radial dispersion is generally achieved using a circular line, and circumferential dispersion is achieved using a star-line grating.
In the above integration time Δ t, the far-field light intensity distribution is:
in the formula, Δ t is an integration time,for the near-field optical field distribution of the laser beam,for additional phase of a Continuous Phase Plate (CPP), "FT" represents the fourier transform and "|" represents the absolute value.
The smoothing device for realizing the spectral dispersion smoothing method based on the composite type grating in the inertial confinement fusion device comprises a seed light output unit, a pre-amplification system, a lens, a beam expander, a main amplification system, a frequency conversion unit, a continuous phase plate, a focusing lens and a target surface; according to the invention, the device also comprises a spectral dispersion smoothing device, wherein the spectral dispersion smoothing device consists of a phase modulator and a composite grating; describing according to the light path: in the optical transmission chain, laser beams output from the seed optical output unit sequentially pass through a pre-amplification system, a phase modulator, a composite grating, a main amplification system, a frequency conversion unit and a continuous phase plate and are finally focused on a target surface through a focusing lens; when the far-field focal spots of the laser beams are subjected to dispersion in different directions through the composite grating, speckles inside the far-field focal spots are quickly swept in respective dispersion directions, so that the target surface is uniformly smoothed within short integration time, namely the irradiation uniformity of the laser beams on the target surface is improved.
In the above technical solution, the spectral dispersion smoothing device is located in a pre-amplification system or behind a main amplification system in an optical transmission chain.
In the above technical solution, the phase modulator in the spectral dispersion smoothing device is an electro-optical modulator to realize sinusoidal phase modulation of the laser beam in the optical transmission chain.
In the above technical scheme, the outer diameter of the composite grating in the spectral dispersion smoothing device is the same as the aperture of the incident laser beam.
In the above technical scheme, the composite grating in the spectral dispersion smoothing device is a planar transmission grating.
In the technical scheme, the composite grating in the spectral dispersion smoothing device is formed by processing a plane transmission grating with a high damage threshold value.
According to the spectral dispersion smoothing method and device based on the composite type grating in the inertial confinement fusion device, on the premise that the continuous phase plate is used in an optical transmission chain of the inertial confinement fusion device, the spectral dispersion smoothing device can be placed in a prevention large system or behind a main amplification system, so that speckles inside far-field focal spots of laser beams in a far-field focal spot of the optical transmission chain are rapidly swept in different dispersion areas along different directions, and the purpose of smoothing the far-field speckles of the laser beams in the optical transmission chain in the dispersion direction of the composite type grating is achieved at the same time.
Compared with the prior art, the invention has the advantages and beneficial technical effects as follows:
1. the invention provides a spectral dispersion smoothing method based on a composite grating in a laser driving system of an inertial confinement fusion device for the first time, and provides a spectral dispersion smoothing device for realizing the method, so that the beam smoothing of laser beam far-field speckles in an optical transmission chain in the dispersion direction of the composite grating is realized. The spectral dispersion smoothing device utilizes the composite grating to carry out regional dispersion which is different from the dispersion of the traditional array type grating, the traditional array type grating has more units, and the division mode of the dispersion region is also different; if the dispersion direction of the circular grating is radial, and the dispersion direction of the spiral grating is circumferential, the dispersion directions are all single.
2. Compared with the existing 1D-SSD, the spectral dispersion smoothing method and the smoothing device based on the composite grating in the inertial confinement fusion device can effectively reduce the generation of fringe-shaped intensity modulation of laser beams in a far field due to the increase of the dispersion mode; compared with 2D-SSD, the laser beam subjected to composite grating dispersion can effectively avoid the coherence phenomenon in a far field.
3. According to the spectral dispersion smoothing method and device based on the composite type grating in the inertial confinement fusion device, the composite type grating is used for carrying out regional dispersion on a laser beam, and because the dispersion in the circumferential direction is used in the dispersion region ii, when the composite type grating in the spectral dispersion smoothing device is placed in a pre-amplification system, the phenomenon that the via hole efficiency of a spatial filter is reduced due to the fact that the size of a focal spot introduced by the grating dispersion is increased can be effectively avoided; when the composite type grating in the spectral dispersion smoothing device is placed behind the main amplification system, the problem of hole blockage at the injection hole of the fusion black cavity caused by the fact that the size of a focal spot introduced by grating dispersion is enlarged can be effectively avoided.
4. The invention relates to a spectral dispersion smoothing method and a smoothing device based on a composite grating in an inertial confinement fusion device, wherein the composite grating in the spectral dispersion smoothing device and the existing special grating are as follows: compared with a circular grating, a spiral grating, an array grating and the like, the scribing density of the circular grating, the spiral grating, the array grating and the like in the dispersion area i is small, so that the processing difficulty is small, the structure is relatively simple, and the large-caliber processing is easier.
Drawings
FIG. 1 is a schematic structural diagram of a smoothing device arranged in a pre-amplification system for implementing a composite grating-based spectral dispersion smoothing method in an inertial confinement fusion device in embodiment 1 of the invention;
FIG. 2 is a schematic structural diagram of a smoothing device for implementing a composite grating-based spectral dispersion smoothing method in an inertial confinement fusion device according to embodiment 2 of the present invention, which is placed behind a main amplification system;
FIG. 3 is a schematic structural diagram of a composite grating in a smoothing device of the inertial confinement fusion device based on the spectral dispersion smoothing method of the composite grating;
FIG. 4 is a focal spot of a composite grating-based spectral dispersion smoothing method in an inertial confinement fusion device according to the present invention with a continuous phase plate, wherein (a) is the focal spot of a composite grating beam; (b) a focal spot for an existing 1D-SSD beam; (c) the focal spot of the existing spiral grating beam;
FIG. 5 is a schematic diagram showing the dispersion direction of the method for homogenizing spectral dispersion based on composite grating in the inertial confinement fusion device of the present invention, wherein (a) is the direction of homogenizing dispersion of the composite grating beam; (b) the direction of the existing 1D-SSD light beam is uniform and smooth; (c) the direction of the existing spiral grating light beam is uniform and smooth;
FIG. 6 is the FOPAI curve of the focal spot of the composite grating, 1D-SSD, spiral grating in the case of the composite grating-based spectral dispersion smoothing method in the inertial confinement fusion device of the present invention with a continuous phase plate;
FIG. 7 is a curve showing the variation of the luminous flux Contrast ratio along with the integral time Δ t in the case of the three kinds of beam smoothing schemes of the composite type grating, the 1D-SSD, and the spiral grating when the continuous phase plate is provided in the dispersion smoothing method based on the composite type grating spectrum in the inertial confinement fusion device of the present invention.
In fig. 1, 1 a seed light output unit, 2 a pre-amplification system, 3 lenses, 4 a phase modulator, 5 a composite grating, 6 a beam expander, 7 a main amplification system, 8 a frequency conversion unit, 9 a continuous phase plate, 10 a focusing lens, 11 a target surface and 12 a spectral dispersion smoothing device.
Detailed Description
The present invention will be described in further detail with reference to the following detailed description of specific embodiments thereof, which should be pointed out in the attached drawings, wherein the embodiments are only used for further description of the present invention and are not intended to limit the scope of the present invention.
The structure of the spectral dispersion smoothing method based on the composite grating in the inertial confinement fusion device and the spectral dispersion smoothing device for realizing the method, which is arranged in a pre-amplification system, is shown in figure 1. The device comprises a seed light output unit 1, a pre-amplification system 2, a lens 3, a beam expander 6, a main amplification system 7, a frequency conversion unit 8, a continuous phase plate 9, a focusing lens 10 and a target surface 11; the device is characterized by further comprising a spectral dispersion smoothing device 12, wherein the spectral dispersion smoothing device 12 is composed of a phase modulator 4 and a composite type grating 5. Describing according to an optical path: the laser beam output by the seed light output unit 1 sequentially passes through a pre-amplification system 2, a lens 3, a phase modulator 4 and a composite grating 5 in a spectral dispersion smoothing device 12, a beam expander 6, a main amplification system 7, a frequency conversion unit 8, a continuous phase plate 9 and a focusing lens 10, and finally reaches a target surface 11; as can be further seen from fig. 1, after the laser beam in the optical transmission chain is subjected to time phase modulation by the phase modulator 4, the laser beam passes through the composite grating 5, and at this time, the laser beam simultaneously passes through the dispersion region i and the dispersion region ii of the composite grating 5 to be subjected to dispersion in different directions; after being subjected to spectral dispersion, the laser beam sequentially passes through a main amplification system 7, a frequency conversion unit 8 and a continuous phase plate 9, and is finally focused on a target surface 11 through a focusing lens 10; when the far-field focal spot of the laser beam is subjected to dispersion in different directions through the composite grating 5, the speckles in the far-field focal spot are quickly swept in respective dispersion directions, so that the target surface 11 is uniformly smoothed in a short integration time, namely, the irradiation uniformity of the target surface is improved.
In order to ensure the symmetric transmission of the laser beam after dispersion, the central axis of the composite grating 5 and the central axis of the incident laser beam are collinear, and the incident laser beam divides the dispersion area of the composite grating 5 into two parts: namely, a circular dispersion area outward from the central axis of the composite grating 5 is divided into a dispersion area i, the dispersion of the dispersion area i is a dispersion mode i, the dispersion mode i can be one-dimensional dispersion or radial dispersion, and the radius of the circular dispersion area is an area with a certain fixed value; the dispersion region outside the circular dispersion region is divided into dispersion regions II, and the dispersion is dispersion mode II, which may be circumferential dispersion or radial dispersion.
In the fig. 2 of the present invention, the structure of the spectral dispersion smoothing device for realizing the spectral dispersion smoothing method based on the composite type grating in the inertial confinement fusion device, which is placed behind the main amplification system, is shown in fig. 2. Describing according to an optical path: the laser beam output by the seed light output unit 1 sequentially passes through a pre-amplification system 2, a lens 3, a beam expander 6, a main amplification system 7, a frequency conversion unit 8, a phase modulator 4 and a composite grating 5 in a spectrum dispersion smoothing device 12, a continuous phase plate 9 and a focusing lens 10, and finally reaches a target surface 11; as can be further seen from fig. 2, after the laser beam in the optical transmission chain is modulated by the time phase, the laser beam passes through the composite grating 5, and at this time, the laser beam simultaneously passes through the dispersion region i and the dispersion region ii of the composite grating 5 to perform dispersion in different directions; after being subjected to spectral dispersion, the laser beam passes through a continuous phase plate 9 and is finally focused to a target surface 11 through a focusing lens 10; when the far-field focal spot of the laser beam is subjected to dispersion in different directions through the composite grating 5, the speckles in the far-field focal spot are quickly swept in respective dispersion directions, and the target surface 11 is uniformly smoothed in a short integration time, namely the irradiation uniformity of the target surface is improved.
In fig. 3, a central dispersion region i is a circular dispersion region outward from a central axis of the composite grating 5; the outer dispersion region ii is a dispersion region other than the circular dispersion region.
In fig. 4, the focal spot of the composite grating-based spectral dispersion smoothing method with the continuous phase plate is seen from the figure, wherein (a) the focal spot of the composite grating beam has no obvious diffraction fringes; (b) diffraction fringes exist in the reticle direction of the existing focal spot of the 1D-SSD light beam; (c) the existing focal spot of the spiral grating beam has diffraction fringes in the radial direction.
In fig. 5, the dispersion of the composite grating is divided into two directions, a dispersion region i is one-dimensional linear dispersion, and a dispersion region ii is circumferential dispersion; the existing 1D-SSD is one-dimensional linear dispersion; spiral gratings have been known to be radially dispersive.
Example 1
In the embodiment, a structural schematic diagram that a smoothing device based on a spectral dispersion smoothing method of a composite grating in an inertial confinement fusion device shown in fig. 1 is arranged in a pre-amplification system 2 is adopted.
The phase modulator 4 in the spectral dispersion smoothing device used in the embodiment is an electro-optical modulator, and the position where the composite grating is placed is in a pre-amplification system; the dispersion direction of the dispersion region i of the composite grating 5 is one-dimensional x-direction dispersion, and the dispersion coefficient isThe dispersion region ii of the composite grating is circumferentially dispersed, the total grating line M is 20000, the dispersion areas of the composite grating dispersion region i and the dispersion region ii are the same, and the integration time Δ t is 10 ps.
The aperture of the incident laser beam is 30mm, and the outer diameter of the used composite grating 5 is the same as the aperture of the laser beam.
The spectral dispersion smoothing method based on the composite grating comprises the following specific operation steps:
(1) adding a spectral dispersion smoothing device into a laser-driven inertial confinement fusion device optical transmission chain pre-amplification system 2, and adjusting the parameter value of a time phase modulator 4 in the device, wherein the modulation frequency omega ism＝17GHZAnd modulation depth δ 2.38;
(2) performing sinusoidal time phase modulation on laser beams in an optical transmission chain by using the phase modulator 4 in the step (1);
(3) in the step (2), after an incident laser beam in the optical transmission chain is modulated by the phase modulator 4, the incident laser beam passes through the composite grating 5, and the outer diameter of the composite grating is the same as the aperture of the incident laser beam by 30 mm;
(4) in the step (3), the incident laser beam passes through the composite light 5, and the dispersion region of the composite grating 5 is divided into a dispersion region i and a dispersion region ii, which respectively pass through the dispersion region i and the dispersion region ii of the composite grating 5 at the same time to carry out dispersion in different directions; the dispersion region i is one-dimensional x-direction dispersion with a dispersion coefficient ofThe dispersion area ii is circumferential dispersion, and the total grating ruling M of the dispersion area is 20000;
(5) after the incident laser beam in the optical transmission chain is subjected to spectral dispersion in the step (4), the incident laser beam sequentially passes through a beam expander 6, a main amplification system 7, a frequency conversion unit 8 and a continuous phase plate 9 and is finally focused on a target surface 11 through a focusing lens 10; when the far-field focal spot of the laser beam is subjected to dispersion in different directions through the composite grating 5, the speckles in the far-field focal spot are quickly swept in respective dispersion directions, so that the target surface 11 is uniformly smoothed in a short integration time, namely, the irradiation uniformity of the target surface 11 is improved.
For ease of comparison, the spectral dispersion smoothing method of example 1 was compared to a typical one-dimensional 1D-SSD whose parameters were chosen according to the literature (S.Skupsky, R.W.Short, T.Kessler, et al., improved scrubber beam uniformity using the angular dispersion of frequency modulated. J.appl.Phys.66, 3456 (1989)), as was the modulation frequency ω of the phase-phase modulation, and a helical gratingm2.5GHz, modulation depth delta 12, and grating dispersion coefficientWherein the parameters of the spiral grating are according to the literature (Rui Zhang, Xiaomin Zhang, Zhan Sui, and Hai Ming. research on target irregular using linear modulated light and specific grating dispersion&Laser Technology,43: 1073-.
Fig. 4 to 7 show the improvement effect of the far-field focal spot uniformity by the composite grating-based spectral dispersion smoothing method in embodiment 1, and the results obtained from the respective diagrams fully illustrate the effectiveness and feasibility of the composite grating-based spectral dispersion smoothing method and the smoothing device of the present invention.
Fig. 4 shows a comparison of the focal spot distribution in the three schemes of radial smoothing, 1D-SSD, radial smoothing and 1D-SSD combined use with a continuous phase plate 9. In fig. 4, (a) is a composite grating focal spot, there is no obvious stripe-shaped light intensity modulation inside the focal spot, and the size of the focal spot is not changed obviously; (b) the focal spot is the existing 1D-SSD focal spot, the focal spot is uniform and smooth in the y direction, and obvious stripe-shaped light intensity modulation exists; (c) for the existing spiral grating focal spot, the focal spot has obvious stripe-shaped light intensity modulation in the radial direction.
FIG. 5 is a schematic diagram showing dispersion directions under three schemes of radial smoothing method, 1D-SSD, and combined use of radial smoothing and 1D-SSD. In fig. 5, (a) is the direction of smoothing of the composite grating, the dispersion thereof is divided into two directions, the dispersion region i is one-dimensional linear dispersion, and the dispersion region ii is circumferential dispersion; (b) the direction of the existing 1D-SSD light beam is uniform, the 1D-SSD is dispersion in the x direction, and the direction of the existing spiral grating light beam is uniform, and the uniform direction is radial dispersion. In order to quantitatively analyze the improvement degree of the focal spot uniformity, the improvement degree is evaluated by using a focal spot light flux Contrast (Contrast), and the smaller the focal spot light flux Contrast is, the better the focal spot uniformity is; the formula is as follows:
in the formula Ii,j(xf,yf) Is (x)f,yf) The light intensity at the location; i ismeanIs the average light intensity.
In order to quantitatively analyze the improvement degree of the beam smoothing technology on the hot spot inside the focal spot, a Fractional Power above Intensity (FOPAI) curve is adopted for evaluation, and the FOPAI curve is shifted to the left to show that the improvement effect on the hot spot inside the focal spot is better; the formula is as follows:
wherein A is the focal spot area, ImeanIs the average light intensity.
In quantitative analysis of the degree of improvement in focal spot uniformity and internal hot spots, calculations were performed for a region of 90% energy around circle ratio.
The light FOPAI and the flux contrast curve of the light intensity distribution of the focal spot shown in fig. 4 are calculated by using the equations (10) and (11), respectively, to obtain the curves shown in fig. 6 and 7.
Fig. 6 shows that, when the continuous phase plate 9 is used, FOPAI curves of the focal spot under the respective actions of the composite type grating smoothing method, the 1D-SSD and the spiral grating are substantially the same, and the hot spot inside the focal spot is not increased significantly.
As shown in fig. 7, when the continuous phase plate 9 is used, the contrast of the light flux of the focal spot is close to that of the beam under the action of the composite grating smoothing method, the 1D-SSD and the spiral grating, and the beam smoothing effect is equivalent.
The demonstration of the embodiment 1 and the diagrams 4 to 7 shows that the spectral dispersion smoothing method and the smoothing device based on the composite type grating in the inertial confinement fusion device can realize the effective smoothing of the target surface within a short integration time of 10 ps.
Example 2
In the embodiment, a structural schematic diagram that a smoothing device based on a spectral dispersion smoothing method of a composite grating in an inertial confinement fusion device shown in fig. 2 is arranged behind a main amplification system 7 is adopted.
The dispersion region i of the composite grating 5 used in this example has a one-dimensional x-direction dispersion with a dispersion coefficient ofThe dispersion region ii of the composite grating is radial dispersion with a dispersion coefficient ofThe dispersion areas of the composite grating dispersion region i and the dispersion region ii are the same, and the integration time Δ t is 10 ps.
The aperture of the incident laser beam is 360mm, and the outer diameter of the used composite grating is the same as the aperture of the incident laser beam.
Other operation processes and steps are the same as those in embodiment 1, and similarly, after an incident laser beam in the optical transmission chain is modulated by the phase modulator 4, the laser beam passes through the composite grating 5, and at this time, the laser beam respectively and simultaneously passes through the dispersion region i and the dispersion region ii of the composite grating 5 to carry out dispersion in different directions; after being subjected to spectral dispersion, the laser beam passes through a frequency conversion unit 8 and a continuous phase plate 9 and is finally focused on a target surface 11 through a focusing lens 10; when the far-field focal spot of the laser beam is subjected to dispersion in different directions through the composite grating 5, the speckles in the far-field focal spot are quickly swept in respective dispersion directions, so that the target surface 11 is uniformly smoothed in a short integration time, namely, the irradiation uniformity of the target surface is improved.
When the composite grating dispersion area i adopts one-dimensional linear dispersion and the dispersion area ii adopts radial dispersion, the uniformity of target surface irradiation can be effectively improved.
The embodiments of the present invention only give specific application examples, but it is still considered to be covered by the present invention that a variety of composite grating-based spectrally dispersed beam smoothing structures for inertial confinement fusion devices can be designed according to the above teachings for researchers in laser driving systems for inertial confinement fusion.
Claims (2)
1. A spectral dispersion smoothing method based on composite type grating in an inertial confinement fusion device is characterized by comprising the following steps:
(1) in the optical transmission chain of the laser-driven inertial confinement fusion device, a spectral dispersion smoothing device (12) is added, and the modulation frequency omega of a time phase modulator (4) in the device is adjustedmA parameter value;
(2) performing time phase modulation on incident laser beams in the optical transmission chain by using the time phase modulator (4) in the step (1);
(3) the incident laser beam modulated by the time phase modulator (4) in the step (2) passes through a composite grating (5) in a spectral dispersion smoothing device (12);
(4) in the step (3), the dispersion area of the incident laser beam is divided into a dispersion area i and a dispersion area ii by the composite grating (5), and the laser beam respectively passes through the dispersion area i and the dispersion area ii to carry out dispersion in different directions;
(5) after the laser beam is subjected to spectral dispersion in the step (4), the laser beam sequentially passes through a beam expander (6), a main amplification system (7), a frequency conversion unit (8) and a continuous phase plate (9) in an inertial confinement fusion device, and is finally focused to a target surface (11) through a focusing lens (10); when the far-field focal spot of the laser beam is subjected to spectral dispersion in different directions through the composite grating (5), the speckles inside the far-field focal spot are quickly swept in respective dispersion directions, so that the target surface is uniformly smoothed in short integration time, namely, the irradiation uniformity of the target surface is improved.
2. A method for smoothing spectral dispersion based on a composite grating according to claim 1, characterized in that the added spectral dispersion smoothing device (12) in step (1) is composed of a time phase modulator (4) and a composite grating (5).
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