A pump-probe online characterization system and method for the laser processing zone of lithium niobate.

Abstract

This invention discloses a pump-probe online characterization system and method for lithium niobate laser processing zones. The system includes a pump source, a probe source, a beam splitting / combining module, a time delay module, a polarization control module, a focusing imaging module, a sample support module, a photoelectric detection module, a data processing module, and a processing feedback control module. By establishing a correlation model between time-resolved response characteristic parameters and processing defects, refractive index modification degree, heat accumulation level, microcrack risk, and the scale of the modified region, non-contact, in-situ, online diagnosis and processing status assessment of the femtosecond laser processing zone can be achieved. This system can be integrated with a femtosecond laser processing platform to form a closed-loop control chain of processing-detection-feedback-reprocessing. It features fast response speed, high temporal resolution, precise spatial positioning, high detection sensitivity, and strong online adaptability, making it suitable for online quality control of lithium niobate surface microstructures, internal waveguides, locally modified regions, and photonic devices.

Description

Technical Field

[0001] This invention relates to pump-probe online characterization technology, and more particularly to a pump-probe online characterization system and method for lithium niobate laser processing zones. Background Technology

[0002] Lithium niobate is a functional crystalline material with excellent electro-optic, nonlinear optical, piezoelectric, and photorefractive properties, and it has significant application value in integrated photonics, nonlinear frequency conversion, and optical waveguide devices. Currently, using femtosecond lasers to write functional units such as micro / nano structures, waveguides, and locally modified refractive index regions onto or within the surface of lithium niobate has become an important method for fabricating high-performance lithium niobate photonic devices. Existing technologies include studies on using pump-probe techniques for characterizing the transient response of materials, and attempts to integrate online detection methods such as Raman spectroscopy with femtosecond laser processing, achieving the acquisition of molecular structure information and preliminary closed-loop control of processing parameters during the processing.

[0003] However, existing technologies still have the following problems: First, online detection methods such as Raman spectroscopy have limited time resolution and cannot capture ultrafast processes such as carrier dynamics, thermal relaxation, defect state capture, and stress wave propagation at the femtosecond to picosecond scales. These processes precisely determine the modification quality, microcrack risk, and refractive index perturbation degree of the lithium niobate processing zone. Second, existing pump-probe systems are mostly used for offline basic research and lack spatial coordinate mapping, time synchronization, and real-time feedback linkage design with femtosecond laser processing platforms, making it difficult to directly apply them to online diagnosis at the processing site. Third, there is a lack of dedicated characteristic parameter models for the properties of lithium niobate materials, making it impossible to effectively convert transient optical response signals into processing quality evaluation indicators. Fourth, most existing solutions stop at detection or alarm, failing to form a complete closed-loop control link of processing-detection-feedback-reprocessing. This results in the inability to correct processing parameters in a timely manner when they deviate from the optimal range, easily leading to increased sample damage and higher device scrap rates. Summary of the Invention

[0004] Purpose of the Invention: The purpose of this invention is to provide an online pump-probe characterization system and method for the laser processing zone of lithium niobate, in order to solve the following problems existing in the prior art: lack of in-situ, time-resolved detection means in the processing process; difficulty in establishing a direct correlation between transient response information and processing quality; inability to detect abnormal states such as overprocessing, heat accumulation, microcrack initiation, and insufficient modification in a timely manner; separation of detection and processing, making it difficult to form online closed-loop control; and lack of dedicated system integration for the laser processing zone of lithium niobate in existing pump-probe schemes.

[0005] Technical solution: The pump-probe online characterization system for the laser processing zone of lithium niobate according to the present invention includes: A pump source is used to provide excitation light pulses to the lithium niobate laser processing area to excite transient carrier response, thermal response, defect state response, stress response and / or refractive index perturbation response in the processing area; A detection light source is used to provide detection light to the processing area and read the transient optical response signal of the processing area under a preset delay condition; The beam splitter / combiner module is used to split, combine, guide, and couple the pump light and probe light. The time delay module is used to adjust the time delay of the probe light relative to the pump light; A focusing imaging module is used to focus the pump light and probe light onto the target area of ​​the lithium niobate sample and to image and locate the target area. The sample carrier module is used to fix the lithium niobate sample and realize the two-dimensional or three-dimensional displacement of the sample relative to the light spot; The photoelectric detection module is used to receive reflected signals, transmitted signals, scattered signals, polarization-related signals, or phase-related signals output from the target area and convert them into electrical signals. The data processing module is used to amplify, demodulate, fit, extract features, and determine the state of the electrical signal to obtain the characterization results of the processing area. The characterization results are used to evaluate the degree of modification, defect risk, thermal damage level, refractive index disturbance and / or processing quality status of the lithium niobate laser processing zone.

[0006] Preferably, the system further includes a polarization control module, which is used to control the polarization state of the pump light and / or probe light to obtain polarization-resolved transient response information of the lithium niobate processing area.

[0007] Preferably, the data processing module outputs processing quality evaluation results by establishing a correlation model between time-resolved response feature parameters and processing status. The correlation model is an empirical model, a fitting model, a lookup table model, a machine learning model, or a combination thereof.

[0008] Preferably, the system further includes a processing feedback control module, which adjusts the processing laser energy, repetition frequency, scanning speed, focusing position, scanning trajectory, number of scans, or dwell time based on the characterization results.

[0009] Preferably, the output wavelength of the pump light source is 343nm to 1064nm, preferably 515nm or 1030nm, and the pulse width is less than 10ps, preferably less than 1ps; the output wavelength of the detection light source is 400nm to 1550nm, preferably 780nm to 850nm, and the detection light source is a continuous light source, a pulsed light source or a supercontinuum light source.

[0010] Preferably, the system further includes a scanning imaging module for performing point scanning, line scanning, surface scanning, or multi-point parallel scanning on the processing area to form a two-dimensional or three-dimensional spatiotemporal response distribution map.

[0011] Preferably, the sample carrier module shares a sample stage with the femtosecond laser processing platform, shares a precision displacement platform, or is mechanically or controllably linked with the processing platform.

[0012] Preferably, the time delay module is a mechanical delay line, an electro-optic delayer, an optical fiber delayer, an acousto-optic delayer, or a combination thereof; the beam splitter / combiner module includes a beam splitter, a dichroic mirror, a polarizing beam splitter, a half-wave plate, a quarter-wave plate, a neutral density attenuator, a filter, or a combination thereof.

[0013] Preferably, the focusing imaging module includes a microscope objective, a tube mirror, a coaxial illumination module, and a CCD or CMOS imaging device, wherein the numerical aperture of the microscope objective is 0.1 to 0.95.

[0014] Preferably, the photoelectric detection module includes a photodiode, a balance detector, an avalanche photodetector, a photomultiplier, a high-speed oscilloscope acquisition unit, a lock-in amplifier, or a combination thereof.

[0015] Preferably, the data processing module includes a signal preprocessing unit, a time-resolved curve fitting unit, a feature extraction unit, a state recognition unit, and a parameter feedback unit.

[0016] The method for online characterization using the system includes the following steps: S1, locate the area to be processed in the lithium niobate sample and establish the mapping relationship between the detection coordinates and the processing coordinates; S2, coupling the pump light and the probe light to the area to be processed, the area being processed, or the area after processing; S3, by adjusting the delay time of the probe light relative to the pump light through the time delay module, at least one transient optical response signal is acquired at multiple delay points; S4, Demodulate, fit and extract features from the transient optical response signal to obtain response feature parameters; S5, evaluate the processing status, degree of modification, degree of thermal damage, defect risk and / or refractive index disturbance level of the lithium niobate processing zone based on the response characteristic parameters; S6 outputs online characterization results based on the evaluation results and selectively adjusts the laser processing parameters based on feedback.

[0017] Preferably, the response characteristic parameters include one or more of the following: peak change, rise time, full width at half maximum (FWHM), recovery time constant, double exponential decay constant, long-lifetime wake component, steady-state offset, oscillation component frequency, signal area, and polarization difference component.

[0018] Preferably, the transient optical response signal includes one or more of the following: transient reflectivity change signal, transient transmittance change signal, transient scattering intensity change signal, transient polarization rotation signal, transient birefringence change signal, and transient phase change signal; the processing status assessment includes one or more of the following: over-processing warning, under-processing discrimination, thermal accumulation assessment, microcrack risk assessment, stress accumulation assessment, refractive index modification effect assessment, and waveguide quality prediction.

[0019] Preferably, the processing laser energy, scanning speed, focusing depth, number of repeated scans, polarization direction, trajectory spacing, or processing pause strategy are further adjusted based on the evaluation results.

[0020] Preferably, the time delay scan range is -20ps to 10ns, and more preferably -5ps to 2ns.

[0021] The method is applicable to surface wire grid processing, surface phase structure processing, internal waveguide writing, local refractive index modification, coupled microstructure processing, periodic modification region preparation, or combinations thereof, of lithium niobate.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. This invention can directly acquire time-resolved optical response information of the lithium niobate processing area without damaging the sample, significantly improving detection timeliness and enabling in-situ reading of the transient response of the processing area. It is suitable for monitoring the entire processing process, and can be used for pre-processing area assessment, online inspection during processing, and rapid quality verification after processing. It is applicable to different types of processed objects such as surface gratings, phase structures, internal waveguides, locally modified areas, coupling areas, and periodic modulation areas.

[0023] 2. This invention can not only reflect carrier dynamics, but also characterize various processing-related effects such as thermal accumulation, defect state changes, stress evolution and refractive index perturbation.

[0024] 3. This invention can be deeply integrated with a femtosecond laser platform. The sample stage, coordinate system, control module and data link can be shared with the processing platform, which can easily form an online closed-loop control system. Through abnormal state early warning and real-time parameter correction, problems such as over-processing, crack damage and insufficient modification can be reduced, which is conducive to improving processing consistency and device yield. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the optical path structure of the pump-probe online characterization system of the present invention.

[0026] Figure 2 This is a schematic diagram showing the spatial relationship between the pump spot and the probe spot in the sample area.

[0027] Figure 3 This is a schematic diagram illustrating the closed-loop linkage between online characterization and the femtosecond laser processing platform. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the present invention is not limited to the following embodiments.

[0029] Figure 1 This invention describes the overall optical path structure of the pump-probe online characterization system for lithium niobate laser processing areas, which includes a pump source, a probe source, a beam splitting / combining module, a time delay module, a focusing imaging module, a sample carrying module, a photoelectric detection module, and a data processing module. These modules work together to achieve online detection of the transient optical response of the processing area.

[0030] Figure 2 The spatial relationship between the pump spot and the probe spot in the target area of ​​the lithium niobate sample was demonstrated. The probe spot can be located in the central area or the adjacent area of ​​the pump spot to adapt to different spatial resolution detection requirements.

[0031] Figure 3 This indicates the closed-loop linkage process between the online characterization system and the femtosecond laser processing platform of the present invention. That is, through processing, detection, signal analysis, state discrimination and parameter feedback adjustment, the online monitoring and closed-loop optimization control of the lithium niobate laser processing process is realized.

[0032] Example 1: In-line reflective pump-detection system integrated into a surface wire mesh processing platform The pump-probe online characterization system is integrated to the side of the femtosecond laser surface processing platform, and the detection coordinates and processing coordinates are unified through a shared sample stage. The pump light is a 515nm pulsed light, with the single pulse energy controlled in the range of 5nJ to 50nJ; the probe light is an 808nm continuous light, which is attenuated by neutral density and then combined with the pump light at a dichroic mirror, and both are focused onto the target area on the lithium niobate surface through an objective lens with NA=0.4.

[0033] In this embodiment, the pump spot diameter is approximately 18 μm, and the probe spot diameter is approximately 8 μm. The probe spot is located in the central region of the pump spot to improve the detection sensitivity of the localized modification core region. The time delay range is set to -2 ps to 500 ps, ​​with a step size of 2 ps. During the processing, an online detection scan is initiated after every 20 wire grids are completed, and the transient reflectivity change signal is extracted through a lock-in amplifier.

[0034] Experimental results show that when the processing single-pulse energy is within a suitable range, the response curve exhibits a significant transient peak and a recoverable wake, with a moderate recovery time. When the processing energy exceeds the threshold, the long-life wake component in the curve is significantly enhanced, indicating increased local heat accumulation, increased defect state density, or increased material damage, which can serve as an overprocessing warning indicator. Based on these results, the system automatically reduces subsequent processing energy by 5% to 15%, thereby suppressing overheating and boundary degradation.

[0035] Example 2: Transmission-pump-probe characterization of the internal waveguide writing region For the waveguide writing region inside lithium niobate, the pump light maintains a 515nm pulse mode, while the probe light operates in an 800nm ​​pulse mode to achieve higher time resolution. A transmission-type probe structure is employed, extending the delay range to 2ns, to characterize slower thermal relaxation processes and defect-related slow recovery processes.

[0036] After waveguide writing was completed, different writing trajectory regions were scanned and detected. The results showed that regions with larger peak responses and moderate recovery times were highly consistent with regions showing good continuity of modified bands in subsequent microscopic images and lower insertion loss in guided mode testing. Conversely, regions with excessively high peak values ​​and obvious long tails typically corresponded to larger thermal accumulation, uneven boundaries, or enhanced local damage. Therefore, a three-dimensional discriminant relationship between peak response, recovery time, and waveguide quality can be established for online prediction of internal waveguide quality.

[0037] Example 3: Polarization-resolved pump-probe for stress birefringence and crack risk assessment In this embodiment, a polarizer, analyzer, and waveplate are introduced into the probe optical path to control and analyze the polarization state of the probe light, thereby obtaining the polarization-resolved transient response of the processing area. This scheme is suitable for scenarios where stress redistribution is significant on or within the lithium niobate surface, and is particularly suitable for early warning of crack initiation and stress birefringence changes.

[0038] By comparing transient reflection differential signals or transmission differential signals under different polarization directions, response information related to local stress anisotropy can be obtained. When a region exhibits an abnormally enhanced slow recovery component or abrupt change in differential component in a specific polarization direction, it can be determined that there is a high risk of stress concentration, which may trigger processing pause, energy rollback, or scan path replanning.

Claims

1. A pump-probe online characterization system for the laser processing zone of lithium niobate, characterized in that, include: A pump source is used to provide excitation light pulses to the lithium niobate laser processing area to excite transient carrier response, thermal response, defect state response, stress response and / or refractive index perturbation response in the processing area; A detection light source is used to provide detection light to the processing area and read the transient optical response signal of the processing area under a preset delay condition; The beam splitter / combiner module is used to split, combine, guide, and couple the pump light and probe light. The time delay module is used to adjust the time delay of the probe light relative to the pump light; A focusing imaging module is used to focus the pump light and probe light onto the target area of ​​the lithium niobate sample and to image and locate the target area. The sample carrier module is used to fix the lithium niobate sample and realize the two-dimensional or three-dimensional displacement of the sample relative to the light spot; The photoelectric detection module is used to receive reflected signals, transmitted signals, scattered signals, polarization-related signals, or phase-related signals output from the target area and convert them into electrical signals. The data processing module is used to amplify, demodulate, fit, extract features, and determine the state of the electrical signal to obtain the characterization results of the processing area. The characterization results are used to evaluate the degree of modification, defect risk, thermal damage level, refractive index disturbance and / or processing quality status of the lithium niobate laser processing zone.

2. The system according to claim 1, characterized in that, The system also includes a polarization control module, which is used to control the polarization state of the pump light and / or probe light to obtain polarization-resolved transient response information of the lithium niobate processing area.

3. The system according to claim 1, characterized in that, The data processing module outputs processing quality evaluation results by establishing a correlation model between time-resolved response feature parameters and processing status. The correlation model can be an empirical model, a fitting model, a lookup table model, a machine learning model, or a combination thereof.

4. The system according to claim 1, characterized in that, The system also includes a processing feedback control module, which adjusts the processing laser energy, repetition frequency, scanning speed, focusing position, scanning trajectory, number of scans, or dwell time based on the characterization results.

5. The system according to claim 1, characterized in that, The pump light source has an output wavelength of 343nm to 1064nm and a pulse width of less than 10ps; the detector light source has an output wavelength of 400nm to 1550nm and is a continuous light source, a pulsed light source, or a supercontinuum light source.

6. The system according to claim 1, characterized in that, The system also includes a scanning imaging module, which is used to perform point scanning, line scanning, surface scanning or multi-point parallel scanning on the processing area to form a two-dimensional or three-dimensional spatiotemporal response distribution map.

7. The system according to claim 1, characterized in that, The sample carrying module shares a sample stage and a precision displacement platform with the femtosecond laser processing platform, or is mechanically or controllably linked with the processing platform; the time delay module is a mechanical delay line, an electro-optic delayer, an optical fiber delayer, an acousto-optic delayer, or a combination thereof; the beam splitting / combining module includes a beam splitter, a dichroic mirror, a polarizing beam splitter, a half-wave plate, a quarter-wave plate, a neutral density attenuator, a filter, or a combination thereof; the focusing imaging module includes a microscope objective, a tube mirror, a coaxial illumination module, a CCD or CMOS imaging device, wherein the numerical aperture of the microscope objective is 0.1 to 0.95; the photoelectric detection module includes a photodiode, a balanced detector, an avalanche photodetector, a photomultiplier, a high-speed oscilloscope acquisition unit, a lock-in amplifier, or a combination thereof; the data processing module includes a signal preprocessing unit, a time-resolved curve fitting unit, a feature extraction unit, a state recognition unit, and a parameter feedback unit.

8. A method for online characterization using the system of claim 1, characterized in that, Includes the following steps: S1, locate the area to be processed in the lithium niobate sample and establish the mapping relationship between the detection coordinates and the processing coordinates; S2, coupling the pump light and the probe light to the area to be processed, the area being processed, or the area after processing; S3, by adjusting the delay time of the probe light relative to the pump light through the time delay module, at least one transient optical response signal is acquired at multiple delay points; S4, Demodulate, fit and extract features from the transient optical response signal to obtain response feature parameters; S5, evaluate the processing status, degree of modification, degree of thermal damage, defect risk and / or refractive index disturbance level of the lithium niobate processing zone based on the response characteristic parameters; S6 outputs online characterization results based on the evaluation results and selectively adjusts the laser processing parameters based on feedback.

9. The method according to claim 8, characterized in that, The response characteristic parameters include one or more of the following: peak change, rise time, full width at half maximum (FWHM), recovery time constant, double exponential decay constant, long-life wake component, steady-state offset, oscillation component frequency, signal area, and polarization difference component.

10. The method according to claim 8, characterized in that, The transient optical response signal includes one or more of the following: transient reflectivity change signal, transient transmittance change signal, transient scattering intensity change signal, transient polarization rotation signal, transient birefringence change signal, and transient phase change signal; the processing status assessment includes one or more of the following: over-processing warning, under-processing discrimination, thermal accumulation assessment, microcrack risk assessment, stress accumulation assessment, refractive index modification effect assessment, and waveguide quality prediction.

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