CN102590845B - Soft X ray double-frequency grating shearing interference system - Google Patents

Abstract

The invention provides a soft X-ray dual-frequency grating shearing interference system, which uses a dual-frequency grating as a soft X-ray shearing interference element, and utilizes the two groups of -1 order diffracted beams of the grating to automatically Shear interferometry, wavefront detection of an X-ray laser passing through a plasma. The shearing interferometer adopts the imaging system of two multilayer spherical mirrors, which can obtain a clear plasma boundary. The present invention successfully applies the shearing interference system to the soft X-ray band, utilizes the characteristics of the common optical path of the shearing interference system, solves the problem of difficult adjustment of the medium optical path of the Mach-Zehnder interferometer test system, and uses the shearing method Diagnosing plasma electron density provides key techniques.

Description

A Soft X-ray Dual-frequency Grating Shear Interferometry System

technical field

The present invention relates to the technical field of shearing interference systems, in particular to a soft X-ray dual-frequency grating shearing interference system, which is a shearing interferometer system working in the short-wave band (soft X-ray band). The density diagnosis of the plasma is carried out by using soft X-rays, which uses the shear interference pattern of the X-ray wavefront after passing through the plasma to obtain information such as the density of the plasma.

Background technique

Prior art 1 (see literature: Jorge Filevich, Jorge J. Rocca, Mario C. Marconi, et al. "picosecond-resolution soft-x-ray laser plasma interferometry". Applied Optics, Vol.43, 3938-3946, 2004 ) for plasma diagnostics using a Mach-Zehnder (M-Z) interferometry system. The working principle of this method is simple, and the result processing is also relatively convenient. In interferometric diagnosis, after the probe laser beam is split by the grating, one beam as the object light passes through the plasma to be measured, and interferes with the other laser beam as the reference light to form interference fringes. The disturbance of the volume causes the change of the optical path, which causes the movement of the interference fringes. According to the number of moving fringes, the change of the optical path is calculated, and then the distribution of the plasma electron density can be obtained according to the formula. The disadvantages of this method are: 1. Using gratings for beam splitting and combining elements, the object light and reference light pass through different paths, and it is difficult to adjust the optical path. Since the coherence length of X-rays is only 200 μm, it is difficult to adjust the equal optical path; 2. The middle element of the optical path There are many devices and are relatively complex, and the surface shape of the components will interfere with the interference fringes.

Prior art 2, 3 (see literature: J.C.Wyant. "double Frequency Grating Lateral shearinterferometer" Applied Optics, Vol12, 2057-2060, 1973 and refer to literature: Ming Hai et al., "Double Frequency Grating Schlieren Shear Interferometer on Temperature Field Diagnosis", Acta Optics Sinica, 14(2), 214-218, 1994) uses dual-frequency gratings as shearing optical elements, designs shearing interference systems, and tests the surface shape changes of transmitted objects in the visible light band. The characteristics of this method are: firstly, the working band is the visible light band, and the shearing interference fringes are formed directly by using two sets of -1 order interference orders of the dual-frequency grating, and the characteristics of the optical element under test are calculated by using the formula; secondly: the system adopts Transmission diffraction for shearing interferometry; finally, there is a large deformation in the system. Due to the shearing interference of the diffracted light of the grating, the spot deformation is large, and the measured component cannot be clearly imaged. Therefore, the boundary of the measured optical component Can not be accurately positioned.

In prior art 4 (referring to literature: J.C.Wyant. "White Light Extended Source Shearing Interferometer". Applied Optics, Vol.13, 200-202, 1974.) in the shearing interferometer that gives, mentions that in the interference system Regarding the problem of diffraction aberration, it is first mentioned in the literature that when the requirements for aberration are high, the second grating is used to eliminate it, but when the requirements for the boundary positioning of the measured object are high, the aberration of the system is still too large. Secondly, it is mentioned in the system that the grating method is used again, and the efficiency in the X-ray band is low. Adding the grating as an optical element again will cause the light intensity to be too weak, and the imaging system cannot recognize the cropped image.

At present, the M-Z interference system is used to diagnose the plasma density, and its data processing is simple, but due to the short coherence length of soft X-rays, it is difficult to adjust the equal optical path of the system. In order to simplify the plasma test system, the present invention proposes to use a shearing interference system to diagnose and test the plasma. The system uses an X-ray dual-frequency grating and can form a clear boundary for the target to be tested. There is no system using this method yet.

Contents of the invention

The purpose of the present invention is to solve the problem that the current shearing interference system cannot be used in the short wave band (especially the soft X-ray band) for plasma diagnosis. The system uses a soft X-ray dual-frequency grating as the shearing interference element, and uses the two beams of the dual-frequency grating with a small angle-first-order diffracted light to perform shearing interference and test the measured plasma density distribution. At the same time, the system structure design of two imaging mirrors eliminates the aberration problem caused by the diffraction element and solves the boundary blur problem of the measured plasma.

The technical scheme that the present invention adopts is as follows:

A soft X-ray dual-frequency grating shearing interference system is characterized in that it includes a first spherical imaging mirror, a soft X-ray dual-frequency grating, a second spherical imaging mirror, an X-ray CCD camera and a computer processing system, and its position The relationship is as follows:

The distance between the first spherical imaging mirror and the measured target surface is equal to the focal length of the first spherical imaging mirror, the angle between the incident light and the first spherical imaging mirror is 1.5-3.5°, and the incident light passes through the measured plasma; The distance between the soft X-ray dual-frequency grating and the first spherical imaging mirror is not limited, and the incident angle of the probe X-ray on the surface of the grating is between 82° and 84°; the incident light is diffracted by the soft X-ray dual-frequency grating , the -1 order light has two beams, and the angle between them is determined by the frequency of the soft X-ray dual-frequency grating and the angle of the incident light; after the two sets of -1 order diffracted light are reflected by the second spherical imaging mirror, Incidence on the X-ray CCD camera 6, the distance between the second spherical imaging mirror and the X-ray CCD camera equals the focal length of the second spherical imaging mirror; the output end of the X-ray CCD camera and the input end of the computer processing system connected.

Wherein, the first spherical imaging mirror and the second spherical imaging mirror are multi-layer dielectric film mirrors.

Wherein, the measured plasma 1 is located on the focal plane of the first spherical imaging mirror 3 , and the X-ray CCD camera 6 is located on the focal plane of the second spherical imaging mirror 5 .

Wherein, the shearing interference element is a soft X-ray dual-frequency grating 4, and the grating has a diffraction efficiency higher than 10% in the soft X-ray band.

Wherein, the shearing interference element is a soft X-ray dual-frequency grating 4, which can generate a shearing angle between two beams within the range of 0.0004° to 0.0025°.

Compared with the prior art, the present invention has the following advantages:

1. The use of spherical mirror 1 and spherical mirror 2 in this system can image the target to be measured, and the boundary is clear, which is of great significance for the boundary definition of the target in the test laser plasma.

2. The cutting amount is determined by the frequency of the dual-frequency grating and the distance between the dual-frequency grating and the CCD.

3. The optical path difference between the two interference beams of the shearing system is very small, and it can work in the X-ray band with a very short coherence length.

4. In the above system, the key link is the imaging of the shearing system by the imaging system composed of the shearing interference element (dual-frequency grating 4), the spherical mirror 3 and the spherical mirror 5:

(1) Since the space of the measured plasma in the system is mainly between 0-300 μm on the target surface, it is required that the image of the target surface on the CCD be clear. The magnification is known, and the distribution characteristics of the plasma on the target surface can be precisely located from the image on the CCD. And this cannot be realized in prior art 2,3.

(2) The soft X-ray dual-frequency grating can generate two shearing interference beams with a small angle and perform shearing interference without optical path difference, but this type of shearing interference element is difficult to realize in the X-ray band.

Description of drawings

Fig. 1 is a schematic structural diagram of an embodiment of the soft X-ray dual-frequency grating shearing interference system of the present invention.

Fig. 2 is the schematic diagram of dual-frequency grating (being shear element) in the soft X-ray dual-frequency grating shearing interference system of the present invention: Fig. 2 (a) is the overall schematic diagram of dual-frequency grating surface; Fig. 2 (b) is dual-frequency Schematic cross-section of a grating.

Figure 3 is a schematic diagram of shear fringes obtained by plasma; Figure 3(a) is a static shear diagram without plasma; Figure 3(b) is a shear interference diagram of plasma.

In the figure, 1 is the measured plasma, 2 is the target surface to be measured, 3 is the first spherical imaging mirror, 4 is the soft X-ray dual-frequency grating, 5 is the second spherical imaging mirror, 6 is the X-ray CCD camera , 7 is a computer processing system.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Referring first to FIG. 1 , FIG. 1 is a schematic structural diagram of an embodiment of a transverse shearing interferometer according to the present invention. It can be seen from the figure that the transverse shearing interferometer of the present invention is composed of a first spherical imaging mirror 3 , a soft X-ray dual-frequency grating 4 , a second spherical imaging mirror 5 , an X-ray CCD camera 6 and a computer processing system 7 .

After the probe X-ray laser beam passes through the measured plasma 1 (generated after the target surface 2 is bombarded by the laser), the probe X-ray laser wavefront contains information such as the density of the measured plasma, and the incident angle is 2.5 ° angle, incident on the first spherical imaging mirror 3 coated with a multi-layer dielectric film, the distance between the spherical mirror and the measured plasma is the focal length of the first spherical imaging mirror 3. After being reflected by the first spherical imaging mirror 3, it is incident on the soft X-ray dual-frequency grating 4. The surface of the soft X-ray dual-frequency grating is coated with a 30nm gold film, and the frequencies of the soft X-ray dual-frequency grating are respectively 1000 lines/mm and 1003 lines/mm, the grating lines are perpendicular to the incident surface (the plane formed by the incident beam and the normal of the grating surface), and after the incident beam is diffracted by the dual-frequency grating, there are two beams at the -1 level, and the two beams respectively contain plasma body information. After the two beams are reflected by the second spherical imaging mirror 5 , the two beams are coherently imaged on the X-ray CCD camera 6 . The distance between the second spherical imaging mirror 5 and the X-ray CCD camera 6 is equal to the focal length of the second spherical imaging mirror 5 . The images are collected by the X-ray CCD camera 6 and input to the computer processing system 7 for image processing.

The width of the incident beam in the Z-axis direction is L _ZI , the width in the Y-axis direction is L _YI , the width of the diffracted beam in the Z-axis direction is L _ZO , the width in the Y-axis direction is L _YO , the incident angle is θ ₀ , and the diffraction angle is θ _-1 , there is no diffraction in the Z-axis direction, and there is diffraction in the Y-axis direction, then the relationship between the size of the diffracted beam after passing through the grating and the size of the incident beam is:

L _ZO = L _ZI , $L_{\mathrm{YO}}=L_{\mathrm{YI}}*\frac{\cos {\mathrm{\θ}}_0}{\cos {\mathrm{\θ}}_{-1}}---\left(1\right)$

Therefore, the magnification ratio relationship between the plasma-generating target and its image on the CCD in the optical path system in Figure 1:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span>$ $\mathrm{<span\; class="notranslate">\; Ky</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; Kz</span>}<span\; class="notranslate">\; *</span>\frac{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Since the plasma generating target surface is on the focal plane of the first spherical imaging mirror 3 , the light beam diffracted by the grating is imaged on the X-ray CCD camera 6 through the second spherical imaging mirror 5 again. The target surface is imaged on the X-ray CCD camera 6 through grating diffraction, which will cause inconsistencies in the magnification of the two dimensions of the target surface, but there will be no aberrations due to the diffraction of the grating, and the target surface that generates plasma can be clearly imaged on the CCD. Body boundaries can be clearly defined.

Example:

Fig. 1 is a structural diagram of an embodiment of the present invention, and its concrete structure and parameters are described as follows:

20mm×6mm，其材料为石英玻璃，其反射膜为多层介质膜，反射波段为13～15nm。X射线激光在第一块球面成像镜3的入射角度为1.5°。第一块球面成像镜3的球面反射镜的曲率半径为700mm，待测靶与球面反射镜3的距离为350mm。第一块球面成像镜3和双频光栅4的夹角在94.5°，软X射线激光在双频光栅4的入射角为84°。软X射线双频光栅尺寸为60mm×60mm×10mm，软X射线双频光栅的线密度分别为1000线/毫米和1003线/毫米，表面镀30nm金膜。经双频光栅衍射后，-1级衍射光与光栅法线夹角为78.7°，第二块球面成像镜5与软X射线双频光栅的夹角为77.2°，-1级衍射光以1.5°入射角入射到球面成像镜5。第二块球面成像镜5的尺寸为

20mm×6mm，为介质膜反射镜，反射波段为13～15nm。第二块球面成像镜5球面基底曲率半径为4200mm。第二块球面成像镜5与X射线CCD相机6的距离为2100mm。X射线CCD相机6与第二块球面成像镜5的夹角在1.5°，经成像镜5反射的X射线激光垂直入射到X射线CCD相机6的探测面上。The size of the first spherical imaging mirror 3 is

20mm×6mm, the material is quartz glass, the reflection film is a multi-layer dielectric film, and the reflection band is 13-15nm. The incident angle of the X-ray laser to the first spherical imaging mirror 3 is 1.5°. The radius of curvature of the spherical mirror of the first spherical imaging mirror 3 is 700 mm, and the distance between the target to be measured and the spherical mirror 3 is 350 mm. The angle between the first spherical imaging mirror 3 and the dual-frequency grating 4 is 94.5°, and the incident angle of the soft X-ray laser on the dual-frequency grating 4 is 84°. The size of the soft X-ray dual-frequency grating is 60mm×60mm×10mm, the linear density of the soft X-ray dual-frequency grating is 1000 lines/mm and 1003 lines/mm, and the surface is coated with 30nm gold film. After being diffracted by the dual-frequency grating, the angle between the -1-order diffracted light and the grating normal is 78.7°, the angle between the second spherical imaging mirror 5 and the soft X-ray dual-frequency grating is 77.2°, and the -1-order diffracted light has an angle of 1.5° ° incident angle to the spherical imaging mirror 5. The size of the second spherical imaging mirror 5 is

20mm×6mm, it is a dielectric film mirror, and the reflection band is 13-15nm. The radius of curvature of the spherical base of the second spherical imaging mirror 5 is 4200mm. The distance between the second spherical imaging mirror 5 and the X-ray CCD camera 6 is 2100mm. The angle between the X-ray CCD camera 6 and the second spherical imaging mirror 5 is 1.5°, and the X-ray laser reflected by the imaging mirror 5 is vertically incident on the detection surface of the X-ray CCD camera 6 .

The probe X-ray laser, after passing through the measured target surface 2 (no laser bombards the target, so no plasma is produced), is incident on the first spherical imaging mirror 3, and after being sheared by the system, the result shown in (a) in Figure 3 is obtained. The static interference fringes shown. Use the laser to bombard the target surface 2 to generate plasma, and use the probe X-ray laser again to pass through the measured plasma 1 and the measured target surface 2, and after being sheared and imaged by the system, it is obtained on the X-ray CCD camera 6 as shown in Figure 3 (b) Shear interference fringes shown. From Figure 3(a) and (b), we can clearly see the curvature of the interference fringes when there is plasma. Therefore, density detection of plasma can be realized.

Fig. 2 is a schematic diagram of a dual-frequency grating (that is, a shearing element) in the soft X-ray dual-frequency grating shearing interference system of the present invention.

Its specific structure and parameters are described as follows:

The line densities of the dual-frequency gratings are 1000 lines/mm and 1003 lines/mm respectively, and the beat frequency of two sets of gratings is 3 lines/mm. The etching depths of the two groups of gratings with a line density of 1000 lines/mm and 1003 lines/mm are both 12-14 nm. The material is quartz, and the surface shape is better than 1/10 wavelength (wavelength is 632.8nm). The surface of the grating is coated with 30-40nm gold film.

The experimental results are shown in Figure 3 as an example. When the incident beam does not pass through the plasma, the shear interference fringes are straight and have equal periods, and there are only two groups of images of the target on the X-ray CCD, as shown in Figure 3(a). When passing through the plasma, the plasma brings disturbance and bending of shear interference fringes, as shown in Fig. 3(b).

The parts not described in detail in the present invention belong to the well-known technology in the art.

Claims (1)

1. A soft X-ray dual-frequency grating shearing interference system, characterized in that it includes a first spherical imaging mirror (3), a soft X-ray dual-frequency grating (4), and a second spherical imaging mirror (5), X-ray CCD camera (6) and computer processing system (7), its position relation is as follows: The distance between the first spherical imaging mirror (3) and the measured target surface (2) is equal to the focal length of the first spherical imaging mirror (3), and the angle between the incident light and the first spherical imaging mirror (3) is 2.5°, The incident light passes through the measured plasma (1); the distance between the soft X-ray dual-frequency grating (4) and the first spherical imaging mirror (3) is not limited, and the incident angle of the probe X-ray on the grating surface between 82° and 84°; after the incident light is diffracted by the soft X-ray dual-frequency grating (4), there are two beams of -1-order light, and the included angle is determined by the frequency of the soft X-ray dual-frequency grating (4) and the incident light determined by the angle; the two sets of -1 order diffracted light are reflected by the second spherical imaging mirror (5), and then incident on the X-ray CCD camera (6), the second spherical imaging mirror (5) and the X-ray CCD camera ( 6) The distance between them is equal to the focal length of the second spherical imaging mirror (5); the output end of the X-ray CCD camera (6) is connected to the input end of the computer processing system (7); The first spherical imaging mirror (3) and the second spherical imaging mirror (5) are multilayer dielectric film mirrors; The measured plasma (1) is located on the focal plane of the first spherical imaging mirror (3), and the X-ray CCD camera (6) is located on the focal plane of the second spherical imaging mirror (5); The shearing interference element is a soft X-ray dual-frequency grating (4), which has a diffraction efficiency higher than 10% in the soft X-ray band; The shearing interference element is a soft X-ray dual-frequency grating (4), which can generate two shearing interference beams with an included angle in the range of 0.0004-0.0025°; specifically: The measured plasma (1) is generated after the target surface (2) is bombarded by laser, after the probe X-ray laser beam passes through the measured plasma (1), the probe X-ray laser wavefront contains the measured The density information of the plasma is incident on the first spherical imaging mirror (3) coated with a multi-layer dielectric film at an angle of incidence of 2.5°. The distance between the spherical mirror and the measured plasma is the first spherical surface The focal length of the imaging mirror (3) is incident on the soft X-ray dual-frequency grating (4) after being reflected by the first spherical imaging mirror (3). The surface of the soft X-ray dual-frequency grating is coated with a 30nm gold film, and the soft X-ray The frequencies of the dual-frequency grating are 1000 lines/mm and 1003 lines/mm respectively. The grating lines are perpendicular to the incident surface, that is, the plane formed by the incident beam and the normal of the grating surface. After the incident beam is diffracted by the dual-frequency grating, its -1 order has Two beams, the two beams respectively carry plasma information, after the two beams are reflected by the second spherical imaging mirror (5), the two beams are coherently imaged on the X-ray CCD camera (6), the second spherical The distance between the imaging mirror (5) and the X-ray CCD camera (6) is equal to the focal length of the second spherical imaging mirror (5), and the image is collected by the X-ray CCD camera (6) and input to the computer processing system 7 for image processing; The width of the incident beam in the Z-axis direction is L _ZI , the width in the Y-axis direction is L _YI , the width of the diffracted beam in the Z-axis direction is L _ZO , the width in the Y-axis direction is L _YO , the incident angle is θ ₀ , and the diffraction angle is θ _-1 , there is no diffraction in the Z-axis direction, and there is diffraction in the Y-axis direction, then the relationship between the size of the diffracted beam after passing through the grating and the size of the incident beam is: ${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; ZO</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; ZI</span>}}<span\; class="notranslate">\; ,</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; YO</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; YI</span>}}<span\; class="notranslate">\; *</span>\frac{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$ Therefore, the magnification ratio relationship between the plasma target and its image on the CCD in the optical system: ${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; Z</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; Ky</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; Kz</span>}<span\; class="notranslate">\; *</span>\frac{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; cos</span>}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ Since the plasma-generating target surface is on the focal plane of the first spherical imaging mirror (3), and the light beam diffracted by the grating is imaged on the X-ray CCD camera (6) through the second spherical imaging mirror (5) again, The target surface is imaged on the X-ray CCD camera (6) through grating diffraction, which will cause inconsistency in the magnification of the two dimensions of the target surface, but there will be no aberration due to grating diffraction, and the target surface that generates plasma can be clearly imaged on the CCD , the boundary of the plasma can be clearly defined.

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