Off-axis light beam quality detection method based on metasurface

Abstract

The invention relates to the technical field of laser beam quality detection, and particularly discloses an off-axis beam quality detection method based on a metasurface, and the method is realized based on the following system: the system comprises a light source, and a to-be-detected beam emitted by the light source is red light with unknown wavefront phase distribution; the light beam processing system is composed of a polarizer and a 1 / 4 wave plate; the light beam separation system is used for separating light beams in a working area of the light beam separation system at a certain angle; the off-axis light beam quality analysis system consists of a CCD (Charge Coupled Device) camera and an analysis computer; and the off-axis detection light path is used for receiving the off-axis light beam deflected by the metasurface and guiding the off-axis light beam to a light beam quality analyzer (such as a CCD camera, a wavefront sensor and the like). According to the off-axis light beam quality detection method based on the metasurface, original optical elements do not need to be dismantled or moved, the troubleshooting process is greatly simplified, a large amount of debugging time and labor cost are saved, and the off-axis light beam quality detection method based on the metasurface is particularly suitable for an integrated or sealed light path system or a light path system with the high stability requirement.

Description

Technical Field

[0001] This invention relates to the field of laser beam quality testing technology, specifically to an off-axis beam quality testing method based on metasurfaces. Background Technology

[0002] With the rapid development of optical technology, the number of optical components integrated into complex laser optical path systems is increasing daily. Manufacturing errors, assembly deviations, or performance degradation due to long-term use in each optical component (such as lenses, mirrors, modulators, etc.) can all lead to a decrease in beam quality, such as wavefront distortion, uneven intensity distribution, and mode degradation. These localized or cumulative defects can severely affect the final output performance of the optical path and the reliability of experimental results.

[0003] Currently, beam quality testing primarily relies on coaxial testing methods, which require directly incident the beam under test onto a beam quality analyzer (such as a wavefront sensor or beam profilometer) for measurement. However, these instruments are typically bulky and must be inserted into the optical path to operate. When diagnosing faults in complex optical systems, it is often necessary to repeatedly disassemble and reassemble optical components to insert into the testing equipment, locating the problematic device through a step-by-step elimination process. This process is cumbersome, time-consuming, and labor-intensive, and easily introduces assembly and adjustment errors, affecting system stability. Furthermore, coaxial testing methods struggle to achieve real-time, online quality monitoring, failing to provide immediate warnings when problems occur, and only allowing for post-incident troubleshooting, thus reducing system reliability and maintenance efficiency.

[0004] To reduce interference with the main optical path, off-axis detection techniques (such as using beam splitters for sampling) have emerged. However, these techniques typically split the entire beam, significantly reducing the power of the main optical path. Furthermore, the sampled beam may still be affected by aberrations inherent in the beam splitter itself, making it difficult to achieve high spatial resolution local diagnostics. Therefore, there is an urgent need for a non-invasive, high spatial resolution, real-time online beam quality detection method to meet the debugging, monitoring, and maintenance needs of complex optical systems. Summary of the Invention

[0005] To solve the above technical problems, the present invention is implemented through the following technical solution: an off-axis beam quality detection method based on metasurfaces, implemented based on the following system, the system comprising: The light source emits a beam of red light with an unknown wavefront phase distribution. The beam processing system consists of a polarizer and a quarter-wave plate; A beam separation system is used to separate beams within its working area at a certain angle; The off-axis beam quality analysis system consists of a CCD camera and an analysis computer. Off-axis detection optical path: Receives the off-axis beam deflected by the metasurface and guides it to a beam quality analyzer (such as a CCD camera, wavefront sensor, etc.). Through image stitching and algorithm reconstruction, it obtains a spectrum of quality information such as the intensity distribution and wavefront phase of the entire beam cross section. The core of this system lies in utilizing the metasurface's ability to flexibly control the wavefront phase of a light beam. Combined with a scanning motion mechanism and a detection and analysis unit, it forms an online diagnostic device that can be integrated into the optical path. The scanning step size is equal to the working area size parameter of the metasurface. When performing beam quality detection for visible light, the step size is less than 100μm. For a light spot with a diameter of 2mm, each 100μm scanning area is equivalent to dividing the beam into 400 regions for sampling and reconstruction. The detection accuracy can meet the requirements for reconstructing the wavefront. During the scanning process, each scanning area does not overlap, but the scanning area must be continuous without any jumps.

[0006] The detection method includes the following steps: The metasurface is moved under control to a certain region of the beam, and off-axis phase modulation is applied to the micro-region beam to deflect it to the detection optical path; The detection unit acquires the quality parameters of the beam in the micro-area; The metasurface is then moved to the next region, and the above process is repeated. Finally, a complete two-dimensional distribution map of beam quality is synthesized through data processing.

[0007] Preferably, the light source is composed of a helium-neon laser with a wavelength of 632.8 nm.

[0008] Preferably, the polarizer in the beam processing system is used only when the beam polarization state is unknown; if the beam polarization state is known, this component can be omitted.

[0009] Preferably, the beam separation system is composed of a metasurface element equipped with a scanning drive device, and the metasurface element is composed of a subwavelength-scale nanostructure array.

[0010] It has the following beneficial effects: Non-invasive online detection without disassembling the optical path: The system only needs to insert an extremely thin metasurface element (usually on the order of micrometers) and a compact scanning mechanism into the optical path. There is no need to remove or move the original optical components, which greatly simplifies the troubleshooting process and saves a lot of debugging time and labor costs. It is especially suitable for integrated, sealed or high-stability optical path systems.

[0011] Micro-area sampling has minimal impact on the main optical path: the metasurface only guides a tiny local area of ​​the beam off-axis each time, and most of the light energy continues to propagate along the original path. The system's impact on the power loss, intensity distribution, and wavefront distortion of the main beam is negligible, achieving truly near-zero interference monitoring.

[0012] High spatial resolution and global diagnostic capability: Through the micro-nano structure design and precise scanning control of metasurfaces, local beam analysis with spatial resolution of tens of micrometers or even higher can be achieved. Furthermore, by scanning to cover the entire beam cross section, global, pixel-level quality mapping can be realized, accurately locating anomalies caused by specific devices or regions in the optical path.

[0013] Supports real-time and long-term monitoring: Since it has almost no impact on the main optical path, the system can be embedded in the optical path for a long time to achieve real-time and continuous monitoring of beam quality. Once beam quality degradation is detected, an alarm can be triggered immediately, which facilitates timely maintenance and improves the reliability and automation level of system operation.

[0014] The system is compact and easy to integrate: the metasurface components are thin and light, the scanning mechanism is small, the whole system structure is simple, and it is easy to embed into existing optical paths. It is especially suitable for space-constrained precision optical platforms, laser processing systems, optical communication modules, or scientific research-grade complex optical path experimental devices.

[0015] With strong compatibility and wide range of applications, it can adapt to the beam detection needs of different wavelengths and modes by designing different metasurface phase patterns. It is suitable for various laser systems from ultraviolet to infrared bands and has broad application prospects in industrial processing, scientific experiments, medical instruments and communications. Attached Figure Description

[0016] Figure 1 This is the optical path diagram of the off-axis beam quality detection system of the present invention; Figure 2 This is a schematic diagram of the grazing incidence of the beam in this invention; Figure 3 The diagram shows the relationship between the deflection angle of the separated beam and the deflection angle of the original beam in this invention. Figure 4 This invention compares the beam wavefront detected by the off-axis beam quality detection system with the theoretical value. Figure 5 This is a graph showing the relationship between the phase of the emitted beam and the rotation angle of the nanostructure in this invention.

[0017] In the diagram: 1. Light source; 2. Beam to be tested; 3. Polarizer; 4. Quarter wave plate; 5. Metasurface element; 6. Off-axis detection optical path; 7. CCD camera; 8. Analysis computer. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] For the first embodiment, please refer to... Figures 1-5 This invention provides a technical solution: an off-axis beam quality detection method based on metasurfaces, comprising: Light source 1 consists of a helium-neon laser with a wavelength of 632.8nm, and the emitted beam 2 is red light with an unknown wavefront phase distribution. The beam processing system consists of a polarizer 3 and a quarter-wave plate 4; The beam separation system comprises a metasurface element 5 equipped with a scanning drive device. The metasurface element 5 is composed of a subwavelength-scale nanostructure array, used to separate beams within its working region at a certain angle. The relationship between the propagation angle of the separated beam and the grazing incidence angle of the beam under test is as follows: Figure 3 As shown, note that the acceptable range for the incident grazing angle of this beam quality testing device is -20° to 20°. If the change in the incident grazing angle of the beam within its spot area exceeds this range, we consider it to exceed the concept of beam quality testing because its wavefront change is too large. Therefore, this standard should not be used for beam quality testing. The working area of ​​metasurface 5 is a circular region with a diameter of 100 μm. Metasurface 5 uses silica glass as a substrate, and an array of elliptical cylindrical microstructures made of amorphous silicon is grown on its surface. The major axis dimension of the microstructure is fixed at 290 nm, the minor axis dimension is 150 nm, and the height is 120 nm. These microstructure parameters are optimized to ensure that at a wavelength of 632.8 nm, when the beam propagation direction is perpendicular to metasurface 5 (… Figure 2 (b) Efficient phase modulation and precise 45° separation of the beam; The off-axis beam quality analysis system consists of a CCD camera 7 and an analysis computer 8; Off-axis detection optical path 6: Receives the off-axis beam deflected by the metasurface and guides it to the beam quality analyzer. Through image stitching and algorithm reconstruction, it obtains quality information spectrum such as light intensity distribution and wavefront phase of the entire beam cross section. The detection target in this embodiment is the spatial distribution of the spherical equiphase surface of the test beam 2 emitted by the helium-neon laser. Ideally, the laser beam should be a flat-top beam with a perfectly flat wavefront equiphase surface, but in reality, the beams output by lasers are Gaussian beams, and their wavefront equiphase surface is only a portion of a sphere. During the movement of the metasurface 5 driven by the scanning drive device, the beam at positions other than the center position is incident on the metasurface 5 at a certain angle, such as... Figure 2 As shown, the exit angle of the separated beam 6 is not constant at 45°, and its correspondence with the incident beam angle is as follows: Figure 3 As shown.

[0020] Based on the above correspondence, the spatial distribution of the wavefront equiphase surface of the beam under test 2 can be obtained through the following steps: The scanning drive device drives the metasurface 5 to perform point-by-point scanning. The micro-area portion of the beam to be measured 2 is incident on the elliptical cylindrical microstructure array of the metasurface 5. After phase modulation by the microstructure, it is separated at a specific angle to form a separated beam 6, which is then incident on the CCD camera 7. CCD camera 7 collects information such as the spot position and light intensity distribution of the separated beams 6 at various locations, and transmits it to the analysis computer 8 in real time; Based on the acquired image information and combined with system installation parameters (such as the distance between the metasurface and the CCD camera, the microstructure array period, etc.), the separation angle of the beam at each scanning point is derived using a geometric optics calculation model. The derivation process is as follows: Figure 1 As shown, in the ideal perpendicular incidence state, the exit separation angle is 45° from the original beam. When we scan and sample on the moving metasurface, the corresponding spot position of the separated beam will also change. However, during the scanning process, the change in the beam's incident angle is from... Figure 2 (a) to Figure 2 (b) then to Figure 2 (c) In this process, it can be clearly seen that if the beam to be measured is as follows: Figure 2 The imperfect flat-top light shown indicates that the movement distance of its separated spot is not equal to the scanning step size. The principle is simple: the movement distance of the separated spot equals the scanning step size plus the displacement caused by the wavefront change. Therefore, when sampling each scanning point using the CCD camera 7, by comparing the lateral displacement of the spot and subtracting the step size, we can obtain the displacement of the spot caused by the wavefront phase change. After obtaining this displacement, we can then use the two-angle tangent difference formula...

[0021] here The difference in the exit angle between two adjacent sampling points is denoted as... The tangent of each angle is equal to the distance between its spot position and the principal optical axis divided by the longitudinal distance d between the CCD camera 7 and the metasurface. In this embodiment, the longitudinal distance d is 24 cm. Also, because the beam divergence angle in this embodiment is less than 1°, therefore... We approximate the angle to 45°, which is 1, and the final formula simplifies to:

[0022] in This refers to the displacement measured due to the change in the wavefront, as mentioned above. Using this formula, the distribution and magnitude of the emission angle at the corresponding position can be determined. Then, through... Figure 3 Based on the correspondence, the wavefront isophase surface distribution information of the beam under test 2 at the corresponding position is obtained. Finally, by combining the principles of calculus with the scanning step size of the scanning drive device, the spatial distribution of the wavefront equiphase surface of the beam under test 2 is reconstructed.

[0023] Test results as follows Figure 4 As shown, the spatial distribution of the equiphase surface of the beam wavefront measured by this system is in high agreement with the theoretical value, verifying the phase modulation accuracy of the metasurface elliptical cylindrical microstructure (major axis 290nm, minor axis 150nm, height 120nm) and the overall detection reliability of the system.

[0024] Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art and related fields based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described and explained in the present invention, unless otherwise specified or limited, shall be implemented according to conventional means in the art.

Claims

1. A method for detecting the quality of an off-axis beam based on a metasurface, characterized in that, include: The light source (1) emits a beam (2) of red light with an unknown wavefront phase distribution. The beam processing system consists of a polarizer (3) and a quarter-wave plate (4); A beam separation system is used to separate beams within its working area at a certain angle; The off-axis beam quality analysis system consists of a CCD camera (7) and an analysis computer (8); Off-axis detection optical path (6): Receives the off-axis beam deflected by the metasurface and guides it to the beam quality analyzer. Through image stitching and algorithm reconstruction, it obtains the quality information spectrum such as the light intensity distribution and wavefront phase of the entire beam cross section. The detection method includes the following steps: The metasurface is moved under control to a certain region of the beam, and off-axis phase modulation is applied to the micro-region beam to deflect it to the detection optical path; The detection unit acquires the quality parameters of the beam in the micro-area; The metasurface is then moved to the next region, and the above process is repeated. Finally, a complete two-dimensional distribution map of beam quality is synthesized through data processing.

2. The method for detecting off-axis beam quality based on metasurfaces according to claim 1, characterized in that: The light source (1) is composed of a helium-neon laser with a wavelength of 632.8 nm.

3. The method for detecting off-axis beam quality based on metasurfaces according to claim 1, characterized in that: The polarizer (3) in the beam processing system is only used when the beam polarization state is unknown. If the beam polarization state is known, this component can be omitted.

4. The method for detecting off-axis beam quality based on metasurfaces according to claim 1, characterized in that: The beam separation system consists of a metasurface element (5) equipped with a scanning drive device, which is composed of a subwavelength scale nanostructure array.

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