A vortex optical diagnostic device for a free electron laser device

Abstract

The present invention provides a vortex light diagnostic device for a free electron laser device, comprising a light inlet window, a light inlet aperture, a porous array plate, a light exit aperture and a camera arranged in sequence, and a vacuum chamber for accommodating the light inlet aperture, the porous array plate and the light exit aperture, wherein the light inlet window is installed on one side of the vacuum chamber, and the camera is installed on the opposite side of the vacuum chamber through a light transmission vacuum tube; the camera obtains a far-field interference pattern of the vortex light, thereby determining the topological charge and spiral direction of the vortex light. The vortex light diagnostic device for a free electron laser device of the present invention realizes vacuum conditions, and designs and processes the required porous array plate based on the wavelength of the vortex light to be diagnosed, and based on the intensity distribution of the far-field interference pattern of the porous array, the topological charge and spiral direction of the vortex light in the extreme ultraviolet to X-ray band can be efficiently and accurately diagnosed.

Description

Technical Field

[0001] The invention relates to a diagnostic device for topological charge and spiral direction of vortex light in the extreme ultraviolet to X-ray band generated by a free electron laser device. Background Art

[0002] Vortex light is a special light beam whose electromagnetic wavefront has a spiral phase structure, which gives the light beam angular momentum. This type of light beam has applications in many fields, including optical communications, microscopic imaging, optical manipulation, and quantum information. The diagnostic method of vortex light is mainly used to determine its characteristic parameters, such as the topological charge (also called the quantum number or helicity of the optical vortex) and the spiral direction.

[0003] With the rapid development of free electron laser devices, vortex light in the extreme ultraviolet to X-ray band with extremely high brightness and ultra-short pulse width based on free electron laser devices has become one of the most cutting-edge research directions. The research on diagnostic methods for vortex light is also very urgent. At present, there are mainly the following technologies for diagnosing the characteristic parameters of vortex light in the extreme ultraviolet to X-ray band:

[0004] Interference method: By interfering the vortex light to be measured with the reference beam, the topological charge of the vortex light is extracted from the interference image. For example, Bahrdt and colleagues in Germany conducted experimental research on the third-generation storage ring light source BESSY II device and observed for the first time the generation of X-ray vortex light based on spiral undulators. In the experiment, the method of interfering X-ray vortex light with X-ray plane waves was used to diagnose the topological charge and spiral direction of X-ray vortex light, see the literature [J. Bahrdt et al., First observation of photons carrying orbit angular momentum inundulators radiation. Physical Review Letters, 111:034801, 2013]. The disadvantage of this method is that optical devices of corresponding wavelengths are required to realize the processes of light transmission, splitting, and beam combining. Optical lenses of specific wavelengths need to be customized, and the implementation process is complicated and costly.

[0005] Phase recovery technology: For the diagnosis of ultraviolet vortex light, a wavefront detector can be used to collect the intensity distribution and wavefront characteristics of the light, and then a specific optical reconstruction algorithm can be used to recover the phase information of the light beam from the measured light intensity distribution, thereby characterizing the phase structure of the vortex light. See the literature [F. Sanson et al., Hartmannwavefront sensor characterization of a high charge vortex beam in the extremeultraviolet spectral range. Optics letters,Vol.43,No.12, 15 June 2018]. This method requires the development of a corresponding Hartmann wavefront detector to complete the diagnosis of vortex light. See the literature [M. Pascal et al., Hartmann wave-front measurement at 13.4nm with λEUV / 120 accuracy]. The wavefront detector is processed by machining 65 × 65 square holes in a rectangular array on a 0.08mm thick nickel plate with a side length of 15mm × 15mm. The side length of the square hole is 0.08mm, and the square hole is further rotated 25° to prevent the diffracted light fields of the square holes from interfering with each other. Therefore, the measurement process is relatively complex to realize the diagnosis of vortex light based on the light field data collected by the detector and combined with the light field image reconstruction algorithm.

[0006] Forked Gratings: A specially designed forked diffraction grating can decompose vortex light into multiple beams with different topological charges. The characteristics of the original vortex light can be diagnosed by analyzing these beams, see [Koh Saitoh et al., Measuring the orbit angular momentum of electron vortex beams using a forked grating. Physical Review Letters, 111, 074801, 2013.]. The disadvantage of this method is that it requires the design and processing of the required forked diffraction grating, which is difficult and costly to process.

[0007] Invention patent document CN104792412B discusses the detection system composed of a metaholographic surface, a coupled grating, and a silicon-based detector, which matches the topological charge of the vortex light with the plasma wave generated for the metal surface, and then collects the plasma wave through four detectors to determine the characteristics of the input vortex light. During the measurement process, the center of the vortex light needs to coincide with the center of the metaholographic surface, and in addition, a surface plasma wave optical field database corresponding to the vortex light characteristics needs to be established, and the implementation process is complicated.

[0008] However, in the field of free electron lasers, research has found that there are no literature or patents discussing the use of multi-aperture far-field interference methods to diagnose the topological charge and spiral direction of vortex light. Summary of the invention

[0009] The object of the present invention is to provide a vortex light diagnostic device for a free electron laser device, so as to achieve efficient measurement of the topological charge and spiral direction of vortex light in the extreme ultraviolet to X-ray band.

[0010] In order to achieve the above-mentioned purpose, the present invention provides a vortex light diagnostic device for a free electron laser device, wherein the vortex light diagnostic device comprises a light inlet window, a light inlet aperture, a multi-hole array plate, a light exit aperture and a camera arranged in sequence along the propagation direction of the vortex light, and a vacuum chamber for accommodating the light inlet aperture, the multi-hole array plate and the light exit aperture, wherein the light inlet window is installed on a first side of the vacuum chamber, and the camera is installed on a second side of the vacuum chamber opposite to the first side through a light transmission vacuum tube; the camera is configured to obtain a far-field interference pattern of the vortex light, so as to determine the topological charge number and spiral direction of the vortex light.

[0011] The vortex light is vortex light in the extreme ultraviolet to X-ray band, the diameter of the micropores of the porous array plate ranges from several micrometers to tens of micrometers; the number of micropores of the porous array plate is greater than 2 times the topological charge of the vortex light to be measured.

[0012] The shape of the micro-wells of the multi-well array plate is set to be circular or polygonal.

[0013] The micropores of the porous array plate are distributed along concentric circles in a rotationally symmetrical manner, and the diameter of the concentric circles ranges from tens of micrometers to hundreds of micrometers.

[0014] The material of the porous array plate is a stainless steel thin plate.

[0015] The multi-hole array plate is mounted on a three-axis translation platform.

[0016] The light entrance aperture, the three-axis translation stage and the light exit aperture are fixed on the same optical flat plate in the vacuum cavity.

[0017] A clamp is provided between the multi-hole array plate and the three-axis translation stage, and the clamp is used to fix the multi-hole array plate to the three-axis translation stage.

[0018] The number of the multi-hole array plates is multiple and the parameters are different from each other, and the multiple multi-hole array plates are fixed on the three-axis translation platform by the same clamp.

[0019] The length of the light transmission vacuum tube is on the order of several meters; the distance between the light entrance aperture and the light exit aperture is about 1 meter.

[0020] The vortex light diagnostic device for a free electron laser device of the present invention realizes vacuum conditions to reduce the absorption of air on the light intensity in the ultraviolet to X-ray band, which is conducive to the lossless transmission of vortex light; and the required porous array plate is designed and processed based on the wavelength of the vortex light to be diagnosed, and based on the intensity distribution of the far-field interference image of the porous array, the topological charge and spiral direction of the vortex light in the extreme ultraviolet to X-ray band can be efficiently and accurately diagnosed, and the single-shot diagnosis of the vortex light pulse characteristics can be realized. In addition, the device does not require optical devices such as spectroscopes, reflectors, and lenses, and has a simple structure, providing an accurate diagnostic tool for the study of the method of generating vortex light by a free electron laser device and the application of vortex light. In addition, the vortex light diagnostic device for a free electron laser device of the present invention can efficiently diagnose the topological charge and vortex direction of vortex light in the extreme ultraviolet to X-ray band through improvements in the size and number of micropores, the diameter of the micropores distributed along the circumference, and the transmission distance of the light field, so that vortex light with a larger topological charge can also be confirmed and distinguished. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the principle diagram of the multi-hole interferometry method for diagnosing the topological charge of vortex light.

[0022] Figure 2 Schematic diagram of the structure of a vortex light diagnostic device for a free electron laser device according to an embodiment of the present invention.

[0023] Figure 3 Schematic diagram of the installation method of the multi-hole array plate.

[0024] Figures 4A-7D This is a simulation diagram of the diagnostic results of the vortex light topological charge based on the far-field interference pattern of the porous array plate.

[0025] Figures 8A-10D This is a simulation diagram of the diagnostic results of the spiral direction of the vortex light based on the far-field interference pattern of the porous array plate. DETAILED DESCRIPTION

[0026] like Figure 2The figure shows a vortex light diagnostic device for a free electron laser device according to an embodiment of the present invention, which is installed in a beamline station of a free electron laser device and is used to diagnose the topological charge of extreme ultraviolet and X-ray vortex light. Figure 2 As shown, the vortex light diagnostic device includes a light inlet window 1, a light inlet aperture 4, a porous array plate 8, a light exit aperture 5 and a camera 7 arranged in sequence along the propagation direction of the vortex light, and a vacuum chamber 2 for accommodating the light inlet aperture 4, the porous array plate 8 and the light exit aperture 5, wherein the light inlet window 1 is installed on the first side of the vacuum chamber 2, and the camera 7 is installed on the second side of the vacuum chamber 2 opposite to the first side through a light transmission vacuum tube 6. Thus, all devices are located in the vacuum environment defined by the vacuum chamber 2 to reduce the absorption of air to the light intensity in the ultraviolet to X-ray band, which is conducive to the lossless transmission of the vortex light. The length of the light transmission vacuum tube 6 is on the order of several meters to increase the transmission distance of the porous interference light field, thereby obtaining the far-field interference pattern of the vortex light.

[0027] Among them, the porous array plate 8 is located at Figure 1 The camera 7 is located on the plane of the microwell array plate in Figure 1 In the imaging plane, the porous array plate 8 and the camera 7 are separated by a distance Z, which is on the order of several meters, thereby realizing the porous interferometry method to diagnose the topological charge of the vortex light.

[0028] In this embodiment, the diameter of the light inlet window 1 is 35 mm to allow the vortex light to pass through. The distance between the light inlet aperture 4 and the light exit aperture 5 is about 1 m. Since the light inlet aperture 4 and the light exit aperture 5 are located on both sides of the porous array plate 8, the light inlet aperture 4 and the light exit aperture 5 play the role of collimating the light path to ensure that the light passing through the center of the aperture can directly reach the camera 7.

[0029] In the present embodiment, the material of the porous array plate 8 is a light-proof material such as a stainless steel sheet, so that light can pass through the micropores, and light at other positions is blocked and cannot pass through. The diameter of the micropores of the porous array plate 8 is in the range of a few microns to tens of microns, and the diameter of the micropores is generally 100 to 1000 times the wavelength of the vortex light. The number of micropores of the porous array plate 8 is N5. The shape of the micropores of the porous array plate 8 can be set to polygons such as circles, triangles, and quadrilaterals. In other embodiments, for the detection of high-order vortex light, the number of micropores of the porous array plate 8 needs to meet the following requirements: the number of micropores N of the porous array plate 8 is greater than 2 times the topological charge of the vortex light to be measured.

[0030] The micropores of the porous array plate 8 are distributed along concentric circles in rotational symmetry, and the diameter of the concentric circles ranges from tens of micrometers to hundreds of micrometers. That is, even if the shape of the micropores is a triangle or a quadrilateral, the micropores of the porous array plate 8 still satisfy the characteristics of rotational symmetry.

[0031] The porous array plate 8 is fixed on the three-axis translation stage 3, and the light entrance aperture 4, the three-axis translation stage 3, and the light exit aperture 5 are fixed on the same optical flat plate in the vacuum chamber 2. A clamp 9 is provided between the porous array plate 8 and the three-axis translation stage 3, and the function of the clamp 9 is to fix the porous array plate 8 to the three-axis translation stage 3. The three-axis translation stage can move in the horizontal and vertical directions to adjust the position of the porous array plate 8, so that the hollow position of the vortex light is within the circumference of the porous array plate 8, ensuring that the micropores of the porous array plate 8 can collect the vortex light intensity and phase information.

[0032] like Figure 3 As shown, the number of porous array plates 8 can be multiple and have different parameters from each other, and they are fixed on the three-axis translation stage 3 by the same fixture 9. In this embodiment, the number of installed porous array plates 8 is 7, and the diameters of the micropores of the porous array plates 8 increase in order from left to right. Therefore, in the process of diagnosing vortex light, firstly, 7 porous array plates 8 are fixed to the fixture 9 at the same time, and according to the wavelength of the vortex light, the three-axis translation stage 3 is controlled to insert the porous array plate 8 with specific parameters into the transmission light path to collect the light field information of the vortex light and obtain the corresponding far-field interference diagram, so as to diagnose the characteristics of vortex light of different wavelengths through the porous array plates 8 with different parameters.

[0033] In addition, the vortex light diagnostic device for the free electron laser device may further include a vacuum control system connected to the vacuum chamber 2 and an image acquisition system connected to the camera 7 .

[0034] The specific principle of vortex light diagnosis is as follows: For the vortex light of the Laguerre-Gaussian beam type, in the cylindrical coordinate system In the vortex light field It can be expressed as:

[0035] (1)

[0036] in, is the distance from any position of the light spot to the center point in the horizontal direction. It's the size of the waist. for Laguerre polynomial of order, It is a direction marker. It is a radial mark. is the azimuth, is the distance of light transmission, and i is the sign of an imaginary number.

[0037] Assuming the number of micropores on the porous array plate 8 is N, the light field after the vortex light passes through the porous array plate is for:

[0038] (2)

[0039] in, is the transmission function of the circular aperture, , is the center coordinate of the nth microhole, x, y is the lateral position of the light field, is the pore radius, It is a vortex light field.

[0040] Interference intensity distribution of vortex light after passing through a porous array plate It can be expressed by the light field Fourier transform, as shown in formula (3):

[0041] (3)

[0042] in , is the wavelength of light, is the radius of the microwell circular array, is the azimuth coordinate of the nth hole, is the distance from the porous array plate to the image plane, It is a direction marker.

[0043] The diagnostic method implemented by the vortex light diagnostic device for a free electron laser device comprises the following steps:

[0044] Step S1: Install the vacuum chamber 2 to the beamline station;

[0045] Step S2: Install the light inlet window 1 and the camera 7 on the vacuum chamber 2, and install the light inlet aperture 4, the porous array plate 8 and the light exit aperture 5 inside the vacuum chamber 2, and make the light inlet window 1, the light inlet aperture 4, the porous array plate 8, the light exit aperture 5 and the camera 7 on the same optical axis to achieve collimation of the light path and ensure that the vortex light can reach the camera 7.

[0046] In the process of realizing the collimation of the optical path, a visible light band laser is used to collimate the optical path, so that the laser passes through the light inlet window 1, the center of the light inlet aperture 4, the center of the porous array plate 8 and the center of the light exit aperture 5 in sequence, and finally reaches the camera 7.

[0047] Step S3: The ultraviolet or X-ray vortex light generated by the free electron laser device passes through a vortex light diagnostic device for the free electron laser device to obtain a far-field interference pattern, and the far-field interference pattern is compared with the simulation result to determine the topological charge and spiral direction of the vortex light to be measured.

[0048] The vortex light generated by the free electron laser device first enters the light window 1 and reaches the porous array plate 8. When the light passes through the micropores, diffraction occurs, and then the diffracted light continues to be transmitted forward. During the transmission process, the diffracted light fields further interfere with each other to produce porous interference fringes. Finally, the intensity distribution of the interference fringes is collected by a camera to obtain the far-field interference diagram of the vortex light. By comparing the far-field interference diagram of the vortex light collected by the camera with the simulation results, the topological charge and spiral direction of the vortex light to be measured can be directly determined.

[0049] Specific diagnostic process:

[0050] Taking the diagnosis of 13.5nm extreme ultraviolet vortex light as an example, assuming that the topological charges of the vortex light are 1, 2, and 3 respectively, the micropores of the porous array plate are designed to be distributed in a 6-hole circle, the diameter of the micropores is 30 microns, and the diameter of the concentric circles where the micropores are located is 300 microns. In addition, it is necessary to further determine the diameter of the concentric circles on the porous array plate 8 according to the beam waist size of the vortex light to ensure that the micropores can collect a stronger vortex light field. A camera is installed 6 meters away from the porous array plate to collect the interference distribution diagram of the vortex light field. Before diagnosing the vortex light, a laser collimation measurement system in the visible light band is used to ensure that the light passes through aperture 1, the porous array plate, and aperture 2 once until it reaches the camera. After completing the alignment of the optical path, the diagnostic system is sealed, and then further vacuum treatment is performed to ensure that the vacuum degree of the box is less than 10 -3 mBar. Finally, the vortex light to be diagnosed is transmitted to the diagnostic system, and the position of the porous array plate is fine-tuned by a high-precision translation stage to obtain a high-quality far-field interference pattern of the vortex light. By comparing the far-field interference pattern with the simulation results, the topological charge of the vortex light to be measured can be directly determined. In addition, multiple porous array plates can be installed in the box at one time, and the distance between the camera and the porous array plate can be adjusted to diagnose vortex light of other wavelengths, such as vortex light in the X-ray band.

[0051] Figures 4A-7D This is a simulation diagram of the diagnosis results of the vortex optical topological charge based on the far-field interference pattern of the porous array plate. Figure 4A-4D The intensity distribution diagram of ultraviolet 13.5nm vortex light is shown; Figure 5A-5D The phase distribution diagram of the vortex light is shown. Since each phase distribution diagram corresponds to a micro-hole interference result, the topological charge of the vortex light field can be determined based on the interference diagram of the micro-hole light field. Figure 6A-6D It shows the far-field interference pattern of the vortex light obtained when the number N of microholes in the porous array plate 8 is 6; Figure 7A-7D The far-field interference diagram of the vortex light is shown when the number of micropores N of the porous array plate 8 is 7. Figures 4A-7A , Figure 4B-Figure 7B , Figure 4C-Figure 7C , Figure 4D-Figure 7DThe corresponding topological charges are l = 0, 1, 2, and 3. Figure 6A-6D It can be seen that the interference patterns corresponding to the topological charge number l=0, 1, 2, 3 of the vortex light are circular bright spot array, hexagonal array, 6-tooth gear array and 6-leaf clover array respectively.

[0052] Figures 8A-10D This is a simulation diagram of the diagnostic results of the spiral direction of the vortex light based on the far-field interference pattern of the porous array plate. Figure 8A-8D shows the intensity distribution of the vortex light, Figure 9A-9D shows the phase distribution diagram of the vortex light, Figures 10A-10D FIG. 8 shows the far-field interference pattern of the vortex light obtained when the number of micropores N of the porous array plate 8 is 7. Figures 10A-10D As shown in Figure 1, the spiral direction of the vortex light can be determined based on the far-field interference pattern when N=7. Figures 8A-10A , Figure 8B-Figure 10B , Figure 8C-Figure 10C , Figure 8D-Figure 10D The phase distribution and far-field interference patterns of the vortex light with topological charge of l=2 and -2, and l=3 and -3 are shown respectively. By comparison, it can be determined that the far-field interference patterns of the two opposite spiral directions are mirror-imaged about the horizontal axis.

[0053] The above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. The above embodiments of the present invention can also be modified in various ways. All simple, equivalent changes and modifications made according to the claims and the description of the present invention fall within the scope of protection of the claims of the present invention. The contents not described in detail in the present invention are all conventional technical contents.

Claims

1. A vortex optical diagnostic device for a free electron laser device, characterized in that: The invention comprises a light inlet window, a light inlet aperture, a porous array plate, a light exit aperture and a camera which are sequentially arranged along the propagation direction of the vortex light, and a vacuum chamber for accommodating the light inlet aperture, the porous array plate and the light exit aperture, wherein the light inlet window is installed on a first side of the vacuum chamber, and the camera is installed on a second side of the vacuum chamber opposite to the first side through a light transmission vacuum tube; the camera is configured to obtain a far-field interference pattern of the vortex light so as to determine the topological charge number and spiral direction of the vortex light; the micropores of the porous array plate are distributed symmetrically along concentric circles with the center of the circle being rotated; the vortex light first reaches the porous array plate, diffracts when passing through the micropores, and then the diffracted light continues to be transmitted forward, and during the transmission process, the diffracted light fields further interfere with each other to generate porous interference fringes, so that the camera collects the far-field interference pattern of the vortex light.

2. The vortex light diagnostic device for a free electron laser device according to claim 1, characterized in that: The vortex light is vortex light in the extreme ultraviolet to X-ray band, the diameter of the micropores of the porous array plate ranges from several micrometers to tens of micrometers; the number of micropores of the porous array plate is greater than 2 times the topological charge of the vortex light to be measured.

3. The vortex light diagnostic device for a free electron laser device according to claim 1, characterized in that: The shape of the micro-wells of the multi-well array plate is set to be circular or polygonal.

4. The vortex light diagnostic device for a free electron laser device according to claim 1, characterized in that: The diameter of the concentric circles ranges from tens of microns to hundreds of microns.

5. The vortex light diagnostic device for a free electron laser device according to claim 1, characterized in that: The material of the porous array plate is a stainless steel thin plate.

6. The vortex light diagnostic device for a free electron laser device according to claim 1, characterized in that: The multi-hole array plate is mounted on a three-axis translation platform.

7. The vortex light diagnostic device for a free electron laser device according to claim 6, characterized in that: The light entrance aperture, the three-axis translation stage and the light exit aperture are fixed on the same optical flat plate in the vacuum cavity.

8. The vortex light diagnostic device for a free electron laser device according to claim 6, characterized in that: A clamp is provided between the multi-hole array plate and the three-axis translation stage, and the clamp is used to fix the multi-hole array plate to the three-axis translation stage.

9. The vortex light diagnostic device for a free electron laser device according to claim 8, characterized in that: The number of the multi-hole array plates is multiple and the parameters are different from each other, and the multiple multi-hole array plates are fixed on the three-axis translation platform by the same clamp.

10. The vortex light diagnostic device for a free electron laser device according to claim 8, characterized in that: The length of the light transmission vacuum tube is on the order of several meters; the distance between the light entrance aperture and the light exit aperture is 1 meter.

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