阅读说明：本技术 激光惯性约束聚变直接驱动的连续动态聚焦装置和方法 (Continuous dynamic focusing device and method directly driven by laser inertial confinement fusion ) 是由 刘德安 韩璐 崔子健 李展 杨帅帅 张盼 朱健强 于 2019-11-08 设计创作，主要内容包括：一种激光惯性约束聚变直接驱动高功率激光器的连续动态聚焦装置和方法。该装置由透镜状电光晶体、电压源和两片平行金属电极构成。本发明利用能承受高功率密度的电光晶体来实现在高功率激光系统终端的连续动态聚焦,避免了现有连续动态聚焦方案对激光在高功率激光系统中的传输、放大以及频率转换过程造成的影响,为实现高功率激光装置的连续动态聚焦提供了一种新的方案。(A continuous dynamic focusing device and method for a laser inertial confinement fusion direct-driven high-power laser. The device consists of a lens-shaped electro-optic crystal, a voltage source and two parallel metal electrodes. The invention realizes the continuous dynamic focusing at the high-power laser system terminal by using the electro-optic crystal which can bear high power density, avoids the influence of the prior continuous dynamic focusing scheme on the transmission, amplification and frequency conversion processes of laser in the high-power laser system, and provides a new scheme for realizing the continuous dynamic focusing of the high-power laser device.)

1. A continuous dynamic focusing device of a laser inertial confinement fusion direct-driven high-power laser is characterized by comprising: the device comprises a lenticular electro-optic crystal (1), two parallel metal electrodes (2) and a voltage source (3); the planes of the two parallel metal electrodes (2) on the two sides of the lenticular electro-optic crystal (1) are vertical to the optical axis of the lenticular electro-optic crystal (1); the two parallel metal electrodes (2) are respectively connected with the positive electrode and the negative electrode of the voltage source (3), and the voltage output by the voltage source (3) is continuously adjustable.

2. A method of continuous dynamic focusing using the continuous dynamic focusing apparatus of a high power laser of claim 1, comprising the steps of:

1) a focusing lens (4) is arranged between the output terminal of the laser system and the lenticular electro-optical crystal (1), the focusing lens (4) and the lenticular electro-optical crystal (1) form a lens group, the distance between the focusing lens (4) and the lenticular electro-optical crystal (1) is a lens distance D, and the position f of a light beam focus is determined according to the condition that when the voltage applied between parallel metal electrodes at two sides of the lenticular electro-optical crystal (1) is U-0₀And the position of the focal point of the light beam when the maximum voltage is applied is f₀+ Δ f, the position adjustment range of the beam focus is Δ f, and a proper focal length f is selected₁And the lens distance D, and calculating the required initial focal length f of the lens-shaped electro-optical crystal (1) by the following formula₂：

f₀＝D+f_b，

Wherein the content of the first and second substances,

2) the connecting line of the front and back surface poles of the lenticular electro-optical crystal (1) and the y_E(or x)_E) The directions are parallel, and the polarization direction of the incident beam is along x_E(or y)_E) Direction, said x_EOr y_EThe main axis of a refractive index ellipsoid of the lenticular electro-optic crystal (1) in an electric field, the planes of the two parallel metal electrodes (2) positioned at the two sides of the lenticular electro-optic crystal (1) are vertical to the optical axis (z axis) of the lenticular electro-optic crystal (1); the two parallel metal electrodes (2) are respectively connected with the positive electrode and the negative electrode of the voltage source (3);

3) the voltage output by the voltage source (3) is continuously adjusted according to the requirement, so that the intensity of a uniform electric field where the lens-shaped electro-optical crystal (1) is positioned is continuously changed, the refractive index of the electro-optical crystal is changed, the focal length of the lens-shaped electro-optical crystal (1) is continuously changed, and the continuous dynamic focusing of the laser beam output by the high-power laser is realized.

Technical Field

The invention relates to laser inertial confinement fusion, in particular to a continuous dynamic focusing device and a method directly driven by the laser inertial confinement fusion.

Background

The research of laser inertial confinement fusion has important significance on the research of new energy development, laboratory celestial body physics and high-energy physics, and the driving mode of the laser inertial confinement fusion can be roughly divided into direct driving and indirect driving. In the direct drive mode, the size of the target pellet containing the fusion fuel is gradually reduced during the laser irradiation process, which results in the increase of the surface unevenness of the target pellet and the great loss of laser energy, and greatly reduces the energy coupling efficiency between the laser and the target pellet. The position and the size of the laser focusing spot are dynamically adjusted, so that the size of the laser focusing spot is always consistent with that of the target pellet, the energy coupling efficiency between the laser and the target pellet can be greatly improved, and fusion ignition is favorably realized. At present, many laser fusion devices have adopted a dynamic focusing method to improve the energy coupling efficiency between the laser and the target pellet, and the implementation of the dynamic focusing method can be roughly divided into a discrete type and a continuous type.

Discrete dynamic focusing is the generation of focal spots of decreasing size at specific locations and times by manipulating different beams. The dynamic focusing schemes in Nike, OMEGA, NIF in the united states and HiPER devices in europe fall into this category. Although this kind of dynamic focusing scheme improves the energy coupling efficiency between the laser and the target pellet, it puts high demand on the connection between pulses, and in addition, because only a part of the light beam is used for irradiating the target pellet at each moment, the peak value of the energy output of the laser system is limited, and the total energy of the laser system cannot be simultaneously used for driving the laser inertial confinement fusion, which results in the improvement of the demand on the energy of a single light beam. If the position and the size of a laser focusing spot can be continuously controlled, the output capacity of a laser system can be fully utilized, the requirement on single beam energy is reduced while higher energy coupling efficiency is obtained, and the method has important significance for avoiding the damage of optical devices in the laser system. Due to the limitation of response speed and damage threshold, dynamic light field modulation devices such as liquid crystal spatial light modulators cannot be applied to large laser fusion driving devices, and at the present stage, continuous dynamic focusing schemes applicable to large laser fusion driving devices mainly include two schemes: an electro-optic crystal with an electrode is placed at the front end of the system, a non-uniform electric field is generated in the crystal through the electrode, so that the lens-shaped refractive index gradient change is generated in the crystal, the wave front of a light beam is controlled by controlling the electric field intensity, and the position and the size of a focusing light spot are changed; the laser pulse of the broadband is focused by the focusing grating at the output end of the system, the broadband pulse is generated at the front end of the system by the scheme, the lights with different wavelengths in the broadband pulse have different time delays by the dispersion technology and the like, and the continuous change of the position of a focusing light spot along with the time can be realized by the focusing grating at different moments and the characteristic that the focal length of the focusing grating changes along with the wavelength. In the former, due to the introduction of modulation at the front end of the system, the transmission of a light beam in a subsequent optical path is affected, for example, a hole blocking effect and phase mismatch during frequency conversion are easily caused; the latter use of a broadband beam also results in reduced efficiency of laser amplification and frequency conversion. In order to ensure the beam quality and the output power while realizing dynamic focusing, the amplification and transmission of the laser main body system should be avoided as much as possible, so that it is of great significance to explore a new efficient and concise scheme without interfering with the laser system.

Disclosure of Invention

The invention aims to provide a continuous dynamic focusing device and a continuous dynamic focusing method suitable for a high-power laser aiming at the defects of the technical scheme, and the continuous dynamic control of the focal position of a light beam is realized under the condition of not influencing the amplification and transmission of a laser main body system. The invention focuses light beams by utilizing the lenticular electro-optical crystal in the uniform intensity electric field with adjustable field intensity, and utilizes the electro-optical effect of the electro-optical crystal to enable the field intensity of the uniform intensity electric field in which the lenticular electro-optical crystal is positioned to be continuously changed by adjusting the output voltage of the voltage source, thereby controlling the change of the refractive index of the crystal, changing the focal length of the lenticular electro-optical crystal and realizing the continuous dynamic regulation and control of the position of a laser focusing light spot.

The technical solution of the invention is as follows:

a continuous dynamic focusing device of laser inertial confinement fusion direct drive high-power laser is characterized by comprising: the device comprises a lenticular electro-optic crystal, two parallel metal electrodes and a voltage source; the planes of the two parallel metal electrodes on the two sides of the lenticular electro-optic crystal are vertical to the optical axis of the lenticular electro-optic crystal; the two parallel metal electrodes are respectively connected with the anode and the cathode of the voltage source, and the voltage output by the voltage source is continuously adjustable.

The continuous dynamic focusing method of the continuous dynamic focusing device utilizing the high-power laser is characterized by comprising the following steps of:

1) a focusing lens is arranged between the output terminal of the laser system and the lens-shaped electro-optical crystal to form a lens group, the distance between the focusing lens and the lens-shaped electro-optical crystal is a lens interval D, and the position f of the light beam focus is determined according to the condition that the voltage applied between the parallel metal electrodes at the two sides of the lens-shaped electro-optical crystal is 0₀And the position of the focal point of the light beam when the maximum voltage is applied is f₀+ Δ f, the position adjustment range of the beam focus is Δ f, and a proper focal length f is selected₁And the lens distance D, and calculating the initial focal length f of the needed lens-shaped electro-optical crystal on the basis of the lens distance D and the formula₂：

f₀＝D+f_b，

Wherein the content of the first and second substances,

2) the connection line of the front and back surface poles of the lens-shaped electro-optical crystal is connected with y_E(or x)_E) Parallel, the polarization direction of the incident beam should be along x_E(or y)_E) Direction, said y_EOr x_EThe metal electrodes are arranged on the two sides of the lenticular electro-optic crystal, and the planes of the two parallel metal electrodes are vertical to the optical axis of the lenticular electro-optic crystal; the two parallel metal electrodes are respectively connected with the positive electrode and the negative electrode of the voltage source;

3) the voltage output by the voltage source is continuously adjusted according to the requirement, so that the intensity of a uniform electric field where the lens-shaped electro-optical crystal is positioned is continuously changed, the refractive index of the electro-optical crystal is changed, the focal length of the lens-shaped electro-optical crystal is continuously changed, and the continuous dynamic focusing of the output laser beam of the high-power laser is realized.

The invention has the technical effects that:

1. the method comprises the steps of utilizing a lenticular electro-optical crystal to attach spherical wave front to a laser beam to generate a focusing effect, utilizing the electro-optical effect, and continuously controlling the field intensity of a uniform electric field where the lenticular electro-optical crystal is located by a voltage source to change the refractive index of the electro-optical crystal so as to change the focal length of the lenticular electro-optical crystal, thereby realizing the continuous adjustment of the position of a focusing light spot;

2. by continuously adjusting the output voltage of the voltage source, the continuous dynamic adjustment of the position of a focusing light spot in a single light beam can be realized, and the problems of laser pulse connection and limitation of the output capacity of a laser, which are faced by multi-light beam discrete dynamic focusing in the conventional direct drive, are solved;

3. the application of the electro-optical crystal with high damage threshold can ensure that the system is applied to the output end of a high-power laser system, and avoids the influence on laser shaping, frequency conversion and the like when the conventional continuous dynamic focusing scheme is regulated and controlled at the front end;

4. in practical application, according to the focusing distance actually needed, the modulation range of dynamic focusing can be controlled by adjusting parameters such as the relative positions of the focusing lens, the focusing lens and the lenticular electro-optical crystal.

Drawings

FIG. 1 is a schematic view of a laser inertial confinement fusion directly driven continuous dynamic focusing apparatus of the present invention;

FIG. 2 shows the electric field distribution between two parallel metal electrodes;

FIG. 3 is a schematic diagram showing the change of the principal axis coordinate of refractive index when an electric field is applied to a DKDP crystal along the optical axis direction;

FIG. 4 is a schematic diagram of a continuous dynamic focusing method suitable for laser inertial confinement fusion direct drive;

FIG. 5 is a graph showing the modulation range of dynamic focusing as a function of the lens spacing when the focusing distance and the voltage source output voltage are determined for dynamic focusing using the method of FIG. 4;

fig. 6 is a relationship between a focus position change amount and a voltage value after determining a focus distance, a focus lens focal length, and a lens-shaped electro-optical crystal position when dynamic focusing is performed by the method of fig. 4.

Detailed Description

The invention is described in detail below with reference to the accompanying drawings.

Please refer to fig. 1. FIG. 1 is a schematic view of a continuous dynamic focusing device suitable for laser inertial confinement fusion direct drive of the present invention, and it can be seen from the figure that the continuous dynamic focusing device of laser inertial confinement fusion direct drive high power laser of the present invention comprises: the device comprises a lenticular electro-optic crystal 1, two parallel metal electrodes 2 and a voltage source 3; the planes of the two parallel metal electrodes 2 on the two sides of the lenticular electro-optical crystal 1 are perpendicular to the optical axis of the lenticular electro-optical crystal 1; the two parallel metal electrodes 2 are respectively connected with the positive electrode and the negative electrode of the voltage source 3, and the voltage output by the voltage source 3 is continuously adjustable.

When light beams emitted by a laser pass through the lenticular electro-optical crystal 1, the light beams are focused, a uniform electric field with adjustable field intensity is generated between two parallel metal electrodes by utilizing the characteristic that the refractive index of the electro-optical crystal changes along with the electric field through the continuous adjustment of the voltage source 3, and the refractive index of the lenticular electro-optical crystal 1 continuously changes under the action of the electric field, so that the continuous adjustment of the focal position of the light beams is realized.

The following theoretical analysis is conducted on the continuous dynamic focusing device and method applicable to laser inertial confinement fusion direct drive according to the present invention by an embodiment, the lenticular electro-optic crystal 1 of the embodiment is a DKDP crystal, since the DKDP crystal has a higher damage threshold and electro-optic coefficient, and the electro-optic coefficient increases with the increase of deuterium content, the lenticular DKDP crystal with deuterium content of 98% is taken as an example in the theoretical analysis:

in FIG. 1, the optical axis of the lens-shaped electro-optic crystal 1 is along the z-axisTo the direction of the crystal optical axis (z axis) of the lenticular DKDP crystal 1, the plane of the two parallel metal electrodes 2 is perpendicular, the laser is incident along the x axis, the polarization direction is perpendicular to the z axis, and when passing through the lenticular DKDP crystal 1, the laser is subjected to exp [ -jk (x axis)²+z²)/(2f)]Where k is 2 pi/λ and f is the focal length, produces a focusing effect. When no electric field is applied, the focal length f of the lens-shaped DKDP crystal 1 is determined by the curvature radius of the front and back surfaces and the refractive index n of o light in the crystal_oJointly determining:

wherein R is₁、R₂Is the radius of curvature of the front and back surfaces of the lenticular DKDP crystal 1.

If the distance between the two metal electrodes 2 is d and the voltage is U, the electric field strength between the two metal electrodes 2 is: e ═ U/d, and the electric field direction is parallel to the crystal optical axis (z axis) direction, so the electric field component E_x＝E_y＝0，E_zE, the electric field profile is shown in fig. 2. Because the DKDP crystal is a negative uniaxial crystal and belongs to a tetragonal crystal system,group of dots, the electro-optical tensor gamma of this type of crystal_ijIn, only gamma₄₁And gamma₆₃The refractive index of the principal axis of the crystal is independent electro-optic coefficient, and when no electric field is applied, the refractive index of the principal axis of the crystal is as follows: n is_x＝n_y＝n_o，n_z＝n_eAnd no > n_eRefractive index n corresponding to light of different wavelengths_o、n_eCan be calculated by Sellmeier Equation. When the lenticular DKDP crystal 1 is oriented along the z-axis, the strength is E_zWhen the crystal is in an external electric field, the main axis of the crystal refractive index ellipsoid is deflected, and the refractive index of the main axis of the new refractive index ellipsoid is as follows:

in this case, the DKDP crystal is changed from a uniaxial crystal to a biaxial crystal, and the principal axis of the index ellipsoid thereof is changed from x₀、y₀Rotated 45 ° about the z-axis to become x_E、y_EAs shown in fig. 3. Therefore, when the lenticular DKDP crystal 1 is manufactured, it is necessary to make the connecting line of the front and rear surface poles along x_EOr y_EDirections, i.e. their principal planes and z-y_EFace or z-x_EPlane parallel, and correspondingly, the direction of polarization of the incident beam is along y_EOr x_EAnd (4) direction. In this case, n in the formula (1)_oN in formula (2)_{x_E}Or n_{y_E}Instead, the focal length becomes a function of the electric field strength, and therefore, by adjusting the voltage to continuously vary the electric field strength, continuous adjustment of the focal length of the lenticular DKDP crystal can be achieved.

FIG. 4 is a schematic diagram of the continuous dynamic focusing method suitable for laser inertial confinement fusion direct drive of the present invention: a focusing lens 4 is placed at the front end of the continuous dynamic focusing system, so that the focusing lens 4 and the lenticular DKDP crystal 1 form a lens group, and the distance between the focal point and the lenticular DKDP crystal 1 in the figure, i.e. the back focal length of the lens group, is as follows:

wherein the lens pitch D is the distance between the focusing lens 4 and the lenticular DKDP crystal 1, f₁、f₂The focal lengths of the focusing lens 4 and the lenticular DKDP crystal 1, respectively. Defining a focusing distance f of a lens group₀The distance between the focal point and the focusing lens 4 is known from the geometrical relationship

f₀＝D+f_b (4)

The focusing distance f is shown by the formulas (3) and (4)₀Is the focal length f of the focusing lens 4₁Focal length f of lens-shaped DKDP crystal 1₂As a function of the distance D between the lenses, by adjusting f₁、f₂D controls the modulation range of the dynamic focus, since f₂As a function of the output voltage of the voltage source 3, and thus at f₁D and initial focal length f of lenticular DKDP crystal 1₂In certain cases, the focusing distance f can be made by continuously varying the output voltage of the voltage source 3₀A continuous change occurs. In practical application, the focusing lens 4 and the lens spacing D with proper focal lengths can be selected according to the focusing distance and the modulation range, and the required initial focal length of the lenticular electro-optical crystal 1 can be calculated through the equations (3) and (4) on the basis of the selected focusing lens 4 and the lens spacing D.

An example of our simulation of the above-described continuous dynamic focusing method is given below:

suppose a focus distance f₀3.0m, the focal length of the focusing lens 4 is f₁A lens-shaped DKDP crystal 1 having a suitable focal length is placed behind the focusing lens 4 at a distance D such that the focal position of the light beam is spaced from the focusing lens 4 by a distance f₀The initial focal length of the desired lenticular DKDP crystal 1 can be found by the formulae (3) and (4) when it is 3.0 m. A voltage is applied to the two parallel metal electrodes 2 by the voltage source 3, the distance D between the two parallel metal electrodes 2 is 40.0mm, and when the voltage U is 10kV, a relationship curve between a position change amount Δ f of the beam focus relative to the time when U is 0 and the lens distance D is shown in fig. 5. The wavelength of the incident light used in the simulation was 632.8nm, along y_EIncident, polarization direction along x_E. The set of curves gives the relationship between the modulation range of the focal position and the lens separation D over a certain voltage range, according to which the focal length f of the focusing lens 4 in the system can be adjusted₁And optimizing the lens distance D, and calculating the focal length of the needed lens-shaped DKDP crystal 1.

From the curves in fig. 5 we choose to focus the focal length f of the lens 4₁When the lens pitch D is equal to 0.75m and 1.0m, the beam focal point can be calculated by equations (3) and (4) such that f is the focal point of the beam₀1 focal length f of lenticular DKDP crystal required at 3.0m₂(ii) -0.28m, when the lenticular DKDP crystal 1 is in the shape of a concave lens, due to DKDP crystal γ₆₃< 0, in order to focus the distance f during the voltage increase₀As the voltage increases, i.e., the focus position change Δ f is positive, the connection line of the front and rear surface poles of the lenticular DKDP crystal 1 should be the same as y in FIG. 3, as shown in formula (2)_EParallel, incident beam of lightThe polarization direction should be along x_EAnd (4) direction. Under the above conditions, the focus position variation Δ f is plotted against the voltage U as shown in fig. 6.

It can be seen that the invention utilizes the characteristic that the refractive index of the electro-optic crystal changes along with the change of the electric field intensity, and the refractive index of the lenticular electro-optic crystal 1 is continuously changed by adjusting the output voltage of the voltage source 3, thereby realizing the continuous dynamic adjustment of the focal position in the order of hundreds of microns, and the optimization of the dynamic focusing modulation range can be realized by adjusting the lens distance D in the lens group. Because the DKDP crystal has a higher damage threshold, the device can be applied to the output end of a high-power laser driver, and continuous dynamic adjustment of the position of a focusing light spot of output laser is realized. Particularly, in laser inertial confinement fusion, the position and the size of a focusing light spot are continuously adjusted in the order of hundreds of microns, so that the energy coupling efficiency between laser and a target pellet can be effectively improved. The device is simple, can realize continuous dynamic regulation and control of hundreds of microns of laser beam focusing spot positions under the condition of a single light beam, avoids the problems of laser pulse connection and limitation of the output capacity of a laser in a discrete dynamic focusing scheme, and meanwhile, compared with the conventional continuous dynamic focusing scheme, the system used in the invention is positioned at the tail end of a laser system, avoids the influence on transmission, amplification and frequency conversion of the laser in the system, and has important application value for a large laser driving system.