Method for processing computer-generated holography based on femtosecond laser

Abstract

The invention belongs to the technical field of laser processing, and particularly relates to a method for processing computer-generated holography based on femtosecond laser, which comprises the following steps of: 1, designing a CGH pattern which is adaptive to femtosecond laser and is processed on a quartz glass substrate according to an optical detection requirement; 2, processing parameters of a femtosecond laser are set, and lattice damage and thermal distortion of the quartz glass substrate are reduced through the non-thermal processing characteristic of the femtosecond laser; according to the method, the non-thermal processing characteristic of femtosecond laser is adopted, a mask photoetching step necessary for a traditional process is omitted, processing characteristic constraint and substrate error compensation during CGH pattern design are combined, and real-time deviation compensation of a high-precision online confocal sensor is matched, so that the problems of environmental pollution caused by traditional reactive ion etching and structure falling caused by physical vapor deposition are solved, and the quality of the product is improved. And the problems of long processing period and insufficient precision caused by thermal damage are solved, and high-efficiency and high-precision processing of the CGH on the quartz glass substrate is realized.

Description

Technical Field

[0001] This invention belongs to the field of laser processing technology, specifically a method for calculating holography based on femtosecond laser processing. Background Technology

[0002] In the field of modern optical manufacturing and inspection, especially for the inspection of high-precision aspherical and freeform surfaces, computational holograms have become an indispensable core component due to their excellent wavefront compensation capabilities. A computational hologram is a diffractive optical element that is designed by computer coding and fabricated on a substrate to precisely control the phase of light waves.

[0003] Currently, the mainstream methods for preparing computational holograms mainly include two technical paths: reactive ion etching and physical vapor deposition.

[0004] Reactive ion etching typically requires the fabrication of a physical photomask, followed by the transfer of the pattern onto a photoresist through photolithography and development processes. Finally, plasma is used to selectively etch the quartz glass substrate. While this method is mature, it has significant drawbacks: the process is cumbersome, involving multiple steps such as exposure, development, etching, and photoresist removal, resulting in a long preparation cycle and high costs. Furthermore, the use of reactive gases such as fluorine during etching may cause environmental pollution and damage the crystal structure of the substrate material, affecting the final optical performance of the device.

[0005] Physical vapor deposition (PVD) typically requires spin-coating photoresist as a sacrificial layer onto the substrate surface. After photolithography and development to form a mask, multiple thin films are deposited in a vacuum chamber to form a relief structure. Finally, a lift-off process is needed to remove the photoresist and excess film. The drawbacks of this method are: stress matching issues between the multilayer film and the substrate, as well as between the film layers, can easily lead to structural warping or detachment; the lift-off process is difficult and can easily produce residual photoresist or damage the fine structure, resulting in a low yield and difficulty in controlling the roughness of the structural edges, making it difficult to meet the stringent requirements of ultra-high precision optical inspection for wavefront aberrations.

[0006] Regardless of the technology mentioned above, they are all essentially mask-dependent processes, meaning that patterns must be defined using pre-made masks. This fundamentally limits the flexibility and efficiency of the process, making it difficult to adapt to the needs of modern optical component R&D and manufacturing, which require small batches, multiple varieties, and rapid iteration.

[0007] In addition, when processing hard and brittle materials such as quartz glass, traditional laser processing (such as nanosecond and picosecond lasers) is used, the large heat input can easily generate a heat-affected zone, which can cause thermal distortion, microcracks or even melting and re-condensation of the material, seriously affecting the phase accuracy and diffraction efficiency of the computational hologram.

[0008] Therefore, the present invention provides a method for computing holography based on femtosecond laser processing. Summary of the Invention

[0009] In order to overcome the shortcomings of existing technologies and solve the problems of cumbersome process flow, environmental pollution, easy detachment of structure and thermal damage in traditional mask-dependent processing methods.

[0010] The technical solution adopted by this invention to solve its technical problem is as follows: A method for calculating holography based on femtosecond laser processing, comprising the following steps:

[0011] Step 1: Based on the optical inspection requirements, design a CGH pattern adapted for femtosecond laser processing on a quartz glass substrate;

[0012] Step 2: Set the processing parameters of the femtosecond laser to reduce lattice damage and thermal distortion of the quartz glass substrate by utilizing its non-thermal processing characteristics;

[0013] Step 3: Pre-treat the quartz glass substrate;

[0014] Step 4: Place the pre-treated quartz glass substrate on a three-dimensional precision moving stage. Control the movement of the stage with a computer so that the femtosecond laser beam is focused and directly scanned and processed according to the designed CGH pattern.

[0015] Step 5: Perform optical performance testing on the processed CGH.

[0016] Furthermore, if the test results do not meet the requirements, the processing parameters or pattern design details can be adjusted and the process can be reprocessed to form a closed-loop optimization.

[0017] Preferably, in step one, when designing the CGH pattern, the heat-affected zone, processing threshold, and refractive index change of quartz glass during femtosecond laser processing are used as constraints to iteratively optimize the phase distribution of the CGH.

[0018] Furthermore, the Gerchberg-Saxton (GS) algorithm is used for iterative optimization, while compensating for the surface shape error of the quartz glass substrate.

[0019] Preferably, in step four, a high-precision online confocal sensor is used to monitor the relative position of the laser focus and the workpiece surface in real time. When a deviation is detected, the subsequent laser scanning path or focus position is dynamically adjusted for real-time compensation.

[0020] Furthermore, the deviation threshold is set to ±1μm to ensure stable machining accuracy.

[0021] Preferably, the CGH pattern includes:

[0022] The main holographic region employs phase-type coding to generate +1st order diffraction light;

[0023] Align with the holographic region and use amplitude-type encoding to generate +5th order diffraction light;

[0024] The minimum fringe period of the main holographic region is smaller than the minimum fringe period of the aligned holographic region.

[0025] Furthermore, the minimum fringe period of the main holographic region is no greater than 2.3 μm, which is suitable for the processing limit of femtosecond lasers.

[0026] Preferably, in step two, the pulse width of the femtosecond laser is 200-400 femtoseconds, and the repetition frequency is 10-50kHz.

[0027] Furthermore, the laser wavelength is preferably 800nm, 532nm, or 1064nm to match the absorption characteristics of quartz glass.

[0028] Preferably, the pretreatment in step three includes: ultrasonic cleaning for 10-15 minutes each in acetone, ethanol and deionized water, followed by chemical mechanical polishing.

[0029] Furthermore, the surface roughness Ra of the substrate after chemical mechanical polishing is ≤1nm, which meets the requirements of high-precision processing.

[0030] Preferred, including:

[0031] Femtosecond laser systems are used to generate femtosecond laser beams;

[0032] An optical focusing system is used to focus a femtosecond laser beam;

[0033] A three-dimensional precision moving stage is used to support and precisely move a quartz glass substrate;

[0034] The monitoring and compensation system is used to monitor the processing status in real time and dynamically compensate for deviations.

[0035] The device is configured to perform the direct scanning process.

[0036] Furthermore, the device also includes a computer and control module to enable coordinated control of all components, eliminating the need for additional photolithography mask equipment.

[0037] Preferably, the femtosecond laser system further includes a spatial light modulator and a cylindrical lens, the cylindrical lens being used to fine-tune the beam to match the spatial light modulator.

[0038] Furthermore, the femtosecond laser system is equipped with a collimating lens, a beam expander, and a filter in sequence to optimize beam quality.

[0039] Preferably, the monitoring and compensation system includes a CCD monitoring module and a high-precision online confocal sensor, used to monitor the laser focal position in real time and dynamically adjust the scanning path or focal position.

[0040] Furthermore, the CCD monitoring module and the confocal sensor linkage control module form a closed-loop control.

[0041] Preferably, the CGH pattern data format generated in step one is the GDSII or OASIS standard integrated circuit mask data format.

[0042] Furthermore, the data file contains key coordinate information of the pattern, which can be directly read and executed by the femtosecond laser direct writing system.

[0043] The beneficial effects of this invention are as follows:

[0044] 1. The method for computational holography based on femtosecond laser processing described in this invention relies on the non-thermal processing characteristics of femtosecond lasers to avoid harmful gas pollution generated by traditional etching processes, while eliminating the risk of structure detachment in physical vapor deposition processes. The processing is green and safe, and can protect the quartz glass substrate to the greatest extent, reduce lattice damage and thermal distortion, and ensure the stability of the component structure.

[0045] 2. The method for computational holography based on femtosecond laser processing described in this invention eliminates the photolithography masking step required in traditional processing, simplifies the process through direct scanning, and, combined with the integrated logic of intelligent design and real-time compensation, significantly shortens the processing cycle, improves processing flexibility, and precisely adapts to the characteristics of quartz glass to achieve high-precision processing results, meeting the stringent requirements of optical inspection for component performance. Attached Figure Description

[0046] The invention will now be further described with reference to the accompanying drawings.

[0047] Figure 1 This is a flowchart of a method for computational holography based on femtosecond laser processing;

[0048] Figure 2 This is a schematic diagram of the femtosecond laser processing device in this invention;

[0049] Figure 3 This is a schematic diagram illustrating the transformation of the CGH phase function into a line function;

[0050] Figure 4 This is a schematic diagram of the overall effect of CGH;

[0051] Figure 5 This is a schematic diagram of the microstructure of CGH.

[0052] In the diagram: 1. Femtosecond laser; 2. Collimating lens 1; 3. Beam expander 1; 4. Filter; 5. Collimating lens 2; 6. Spatial light modulator; 7. Objective lens; 8. Stage; 9. Imaging lens; 10. CCD camera; 11. Computer; 12. Control module. Detailed Implementation

[0053] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the protection scope of this application.

[0054] Example 1: High-precision CGH machining for aspherical mirror inspection (core application scenario)

[0055] This embodiment addresses the testing requirements of a parabolic aspherical mirror with a 100mm aperture and a vertex curvature radius of 200mm. It describes the fabrication of a high-precision phase-type CGH with a diffraction efficiency ≥85% and a wavefront error ≤λ / 50 (λ=632.8nm). The specific steps are as follows:

[0056] 1. CGH Pattern Design

[0057] Target wavefront definition: Using Zemax optical design software, import the parameters of the aspherical mirror to be tested (aperture 100mm, vertex curvature radius 200mm, quadratic surface coefficient -1), calculate its aberration distribution with the reference spherical wave, and obtain the target wavefront phase function φ_target(x,y) to be compensated; the complex amplitude of the target wavefront is expressed as U_target(x,y)=A_target(x,y)*exp [i•φ_target(x,y)], where A_target(x,y) is set to a constant 1 (pure phase CGH design).

[0058] Encoding type selection: According to claim 4, the main holographic region uses pure phase encoding (high diffraction efficiency), and the alignment holographic region uses amplitude encoding (high positioning accuracy).

[0059] Iterative optimization and constraint setting: The Gerchberg-Saxton (GS) algorithm was used to perform 1500 iterations of Fourier transform. According to claim 2, the heat-affected zone threshold (≤1μm), processing threshold (10μJ / cm²), and quartz glass refractive index change (≤0.001) of femtosecond laser processing were used as constraints to optimize the CGH phase distribution φ_CGH(x,y). During the iteration process, the "amplitude = 1" constraint was applied to the CGH plane, and the "amplitude = A_target" constraint was applied to the target plane.

[0060] Substrate error compensation: The surface shape error φ_substrate(x,y) of the quartz glass substrate is measured by Zygo interferometer, and the error is compensated according to the formula φ_CGH_corrected(x,y)=φ_CGH_designed(x,y)-φ_substrate(x,y) to avoid the impact of substrate unevenness on the detection accuracy.

[0061] Discretization and sampling: The CGH plane is divided into a 2048×2048 pixel grid with a pixel spacing of Δx=Δy=1μm. The sampling frequency satisfies 1 / Δx>2f_xmax and 1 / Δy>2f_ymax (f_xmax and f_ymax are the highest spatial frequencies of the target wavefront) to avoid aliasing distortion.

[0062] Pattern partitioning design:

[0063] Main holographic region: located inside the white ring (inner diameter 20mm, outer diameter 80mm), producing +1st order diffraction light, with a minimum fringe period of 2.3μm;

[0064] Alignment with the holographic region: located outside the white ring (inner diameter 85mm, outer diameter 95mm), producing +5th order diffraction light with a minimum fringe period of 6.4μm.

[0065] Data format generation: As per claim 10, the final phase distribution is converted into the GDSII standard integrated circuit mask data format, which includes polygon vertex coordinates for controlling the femtosecond laser scanning path.

[0066] 2. Femtosecond laser parameter settings

[0067] A Ti:sapphire femtosecond laser with a center wavelength of 800nm ​​is selected, and the parameters are set according to claim 5:

[0068] Pulse width: 300 femtoseconds (within the optimal range of 200-400 femtoseconds);

[0069] Repetition frequency: 20kHz (within the 10-50kHz range, balancing machining accuracy and efficiency);

[0070] Single pulse energy: 10μJ, processing power adjusted to 50mW;

[0071] Focused spot size: Focused to submicron level (0.8μm) through a high numerical aperture objective lens (NA=0.85).

[0072] 3. Pretreatment of quartz glass substrate

[0073] A quartz glass substrate with a diameter of 150 mm and a thickness of 5 mm (purity ≥ 99.99%) was selected.

[0074] Pretreatment is performed according to claim 6: ultrasonic cleaning is performed sequentially in acetone, ethanol and deionized water for 12 minutes each (power 100W, frequency 40kHz) to remove surface oil and impurities.

[0075] Chemical mechanical polishing (CMP) was used with SiO2 polishing slurry (50nm particle size), polishing pressure of 3psi, rotation speed of 30rpm, and polishing time of 30 minutes to achieve a substrate surface roughness Ra≤1nm and flatness≤λ / 20 (λ=632.8nm).

[0076] 4. Femtosecond laser processing

[0077] Device configuration: Assemble the processing device according to claims 7-9: femtosecond laser system (including femtosecond laser, collimating lens, beam expander, filter, spatial light modulator (SLM, model: Holoeye PLUTO-NIR), cylindrical lens) → optical focusing system (objective lens NA=0.85) → three-dimensional precision moving stage (positioning accuracy ±100nm) → monitoring and compensation system (CCD camera + high-precision online confocal sensor (model: Keyence LK-G80));

[0078] Clamping and focusing: Fix the pretreated substrate on the vacuum chuck of the worktable, start the confocal sensor, and adjust the height of the worktable to make the focus accurately aligned with the substrate surface (deviation ≤ ±0.2μm).

[0079] Scanning process: Import GDSII format data, control the stage to move along a "snake-like scanning" path (scanning speed 50μm / s), and simultaneously emit laser beam to etch CGH microstructures on the substrate surface;

[0080] Real-time compensation: According to claim 3, the confocal sensor collects 1000 sets of focal position data per second. When platform vibration, material surface undulation or environmental vibration is detected, causing a deviation > ±1μm, the control module adjusts the subsequent scanning path (X / Y axis) or focal position (Z axis) in real time to compensate for the deviation.

[0081] 5. Post-processing and optical performance testing

[0082] Cleaning and drying: After processing, CGH is ultrasonically cleaned in deionized water for 5 minutes (power 50W) to remove processing debris, and then dried with nitrogen (purity 99.99%) (pressure 0.3MPa).

[0083] Performance testing:

[0084] The phase distribution was detected using a Zygo interferometer, and the wavefront error (RMS) was λ / 55 (≤λ / 50).

[0085] The diffraction efficiency was measured using a diffraction efficiency meter. The +1st order diffraction efficiency was 82% (below the 85% threshold).

[0086] Surface roughness was measured using atomic force microscopy (AFM), with Ra = 0.5 nm.

[0087] Closed-loop optimization: After adjusting the femtosecond laser repetition frequency to 25kHz and reducing the scanning speed to 45μm / s, and reprocessing the local area, the diffraction efficiency was improved to 86%, meeting the high-precision detection requirements of aspherical mirrors.

[0088] II. Example 2: CGH fabrication for beam shaping (expanded application scenarios)

[0089] The specific steps for processing the CGH used to shape a Gaussian beam into a flat-top beam in this embodiment are as follows:

[0090] 1. CGH Pattern Design

[0091] The target wavefront is defined as the complex amplitude distribution of the flat-top beam (amplitude uniformity ≥ 95%), and the reverse propagation of the target wavefront towards the CGH plane is calculated using angular spectrum theory.

[0092] The GS algorithm was iterated 1200 times, taking into account the constraints of femtosecond laser processing, to optimize the phase distribution;

[0093] According to claim 10, generate OASIS standard mask data format.

[0094] 2. Femtosecond laser parameter settings

[0095] A femtosecond laser with a center wavelength of 1064nm was selected, with a pulse width of 200 femtoseconds, a repetition frequency of 50kHz, and a processing power of 30mW.

[0096] 3. Substrate pretreatment

[0097] A quartz glass substrate with a diameter of 50 mm and a thickness of 3 mm was selected. After ultrasonic cleaning for 10 minutes each, the surface roughness Ra was ≤0.5 nm after chemical mechanical polishing.

[0098] 4. Processing and Testing

[0099] The scanning speed was set to 80 μm / s, and the deviation was compensated for in real time by a confocal sensor.

[0100] After post-processing, the beam quality was tested using a beam quality analyzer. The beam uniformity after shaping was 96%, which meets the beam shaping requirements.

[0101] III. Example 3: CGH fabrication for curved microfluidic chips (complex application scenarios)

[0102] In this embodiment, CGH is fabricated on a curved microfluidic chip substrate with a curvature radius of 50mm. The specific steps are as follows:

[0103] 1. CGH Pattern Design

[0104] During the design process, the surface shape error φ_substrate (x,y) of the curved substrate is compensated by the formula φ_CGH_corrected (x,y)=φ_CGH_designed (x,y)-φ_substrate (x,y) to ensure optical performance; and GDSII format data is generated.

[0105] 2. Femtosecond laser parameter settings

[0106] A femtosecond laser with a center wavelength of 532nm was selected, with a pulse width of 400 femtoseconds, a repetition frequency of 10kHz, and a processing power of 20mW.

[0107] 3. Processing and Testing

[0108] After clamping, the confocal sensor tracks the surface profile in real time (sampling frequency 500Hz), and the Z-axis of the worktable is dynamically adjusted to ensure that the focal point is always located in the direction of the surface normal.

[0109] The scanning speed was 50 μm / s. After post-processing, the CGH diffraction efficiency was 82% and the structural integrity was good.

[0110] IV. Example 4: Apparatus for implementing the above method

[0111] 1. Device Composition

[0112] Femtosecond laser system: includes femtosecond laser (1), collimating lens 1 (2), beam expander (3), filter (4), collimating lens 2 (5), spatial light modulator (SLM), (6), cylindrical lens; after collimation, beam expansion and filtering, the laser beam is finely adjusted in aspect ratio by cylindrical lens, matched with SLM rectangular pixel array, and SLM loads CGH phase-encoded data to achieve wavefront modulation;

[0113] Optical focusing system: A high numerical aperture objective lens (7) with NA=0.85 is used to focus the laser beam to a submicron-level spot;

[0114] Three-dimensional precision moving stage (8): adopts high-precision motor and guide rail, with positioning accuracy of ±100nm, to realize precise three-dimensional movement of substrate;

[0115] Monitoring and compensation system: including CCD monitoring module (10), imaging lens (9) and high-precision online confocal sensor; CCD collects sample reflected light through imaging lens and monitors processing status in real time; confocal sensor collects focal position data in real time and sends signals to control module (12) to dynamically adjust scanning path or focal position;

[0116] Computer (11) and control module (12): The computer stores design data, controls SLM loading and workbench movement, and receives and processes monitoring data; the control module drives the various components to work together.

[0117] 2. Working principle

[0118] The computer sends the CGH pattern data to the SLM, which performs phase modulation on the incident femtosecond laser. The modulated laser is focused onto the substrate surface by the objective lens, and the stage moves along a preset path to achieve scanning processing. The monitoring and compensation system monitors the processing status and focal position in real time, dynamically compensates for deviations, and ensures processing accuracy.

[0119] The method for computational holography based on femtosecond laser processing disclosed in this application combines the non-thermal processing characteristics of femtosecond lasers with intelligent design, monitoring, and compensation systems to achieve direct, rapid, and high-precision computational holography processing on quartz glass substrates. This method effectively solves the problems of cumbersome process flow, environmental pollution, easy structural detachment, and thermal damage associated with traditional mask-dependent processing methods, providing a new and efficient solution for the field of high-precision optical inspection.

[0120] In light of current practical needs, the above-described embodiments of this invention are not limited to these specific implementations. Any changes made within the scope of knowledge possessed by those skilled in the art, without departing from the concept of this invention, still fall within the protection scope of this invention.

Claims

1. A method of femtosecond laser processing based on computer holography, characterized by, The method comprises the following steps: Step 1: design a CGH pattern suitable for femtosecond laser machining on a quartz glass substrate according to optical detection requirements; Step 2: set the machining parameters of the femtosecond laser, and use its non-thermal machining characteristics to reduce the lattice damage and thermal distortion of the quartz glass substrate; Step 3: pretreat the quartz glass substrate; Step 4: place the pretreated quartz glass substrate on a three-dimensional precision moving workbench, and make the femtosecond laser beam scan and machine according to the designed CGH pattern after focusing by computer control of the workbench movement; Step 5: detect the optical performance of the machined CGH.

2. The method of claim 1, wherein the femtosecond laser processing and the computer holography are based on, In step 1, the thermal influence zone, machining threshold and refractive index change of the quartz glass in the femtosecond laser machining process are taken as constraint conditions when designing the CGH pattern, and the phase distribution of the CGH is iteratively optimized.

3. The method of claim 1, wherein the femtosecond laser processing and the computer holography are based on, In step 4, a high-precision online confocal sensor is used to monitor the relative position of the laser focal point and the workpiece surface in real time, and when a deviation is detected, the subsequent laser scanning path or focal point position is dynamically adjusted for real-time compensation.

4. The method of claim 1, wherein the femtosecond laser machining computer hologram is based on, The CGH pattern comprises: a main holographic area using phase-type coding for generating +1 order diffracted light; an alignment holographic area using amplitude-type coding for generating +5 order diffracted light; wherein the minimum fringe period of the main holographic area is smaller than the minimum fringe period of the alignment holographic area.

5. The method of claim 1, wherein the femtosecond laser machining computer hologram is based on, In step 2, the pulse width of the femtosecond laser is 200-400 femtoseconds, and the repetition frequency is 10-50 kHz.

6. The method of claim 1, wherein the femtosecond laser machining computer hologram is based on, The pretreatment in step 3 comprises: sequentially ultrasonic cleaning in acetone, ethanol and deionized water for 10-15 minutes, and then chemical mechanical polishing.

7. An apparatus for implementing the method of femtosecond laser processing based on computer holography according to claim 1, characterized in that, The method comprises: a femtosecond laser system for generating a femtosecond laser beam; an optical focusing system for focusing the femtosecond laser beam; a three-dimensional precision moving workbench for carrying and accurately moving the quartz glass substrate; a monitoring and compensation system for real-time monitoring of the machining state and dynamic compensation of deviations; wherein the device is configured to perform the direct scanning machining.

8. The apparatus for a method of femtosecond laser processing computed holography according to claim 7, wherein, The femtosecond laser system further comprises a spatial light modulator and a cylindrical lens for fine-tuning the light beam to match the spatial light modulator.

9. The apparatus for a method of femtosecond laser processing computed holography according to claim 7, wherein, The monitoring and compensation system comprises a CCD monitoring module and a high-precision online confocal sensor for real-time monitoring of the laser focal point position and dynamic adjustment of the scanning path or focal point position.

10. The method of claim 1, wherein the femtosecond laser machining computer hologram is based on, The CGH pattern data generated in step 1 is in GDSII or OASIS standard integrated circuit mask data format.

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