A method and device for detecting surface defects of fused quartz based on whispering gallery mode resonance

Abstract

The application discloses a fused quartz surface defect detection method and device based on echo wall mode resonance and belongs to the technical field of optical fibers, which comprises a 355nm pumping laser module, a homogenization and focusing module, a polarization maintaining fiber coupler unit, a near-infrared harmonic detection light source, a three-dimensional scanning platform, a spectrum analysis and signal acquisition module for collecting WGM micro-ring resonator transmission spectrum, and a control and data processing unit for realizing synchronous control, data acquisition and signal processing of the device and establishing a corresponding database between WGM spectrum characteristics and EPR signal characteristics. The application is non-contact, non-destructive, high in sensitivity and spatial resolution, and is suitable for defect screening and quality control of large-aperture fused quartz optical elements in high-power laser systems.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, and more specifically, this invention relates to a method and apparatus for detecting defects on the surface of fused silica based on whispering wall mode resonance. Background Technology

[0002] Fused silica is widely used in high-power laser systems due to its wide bandgap, low absorption, and high laser damage threshold. However, micro- and nano-scale defects generated during manufacturing and service can reduce system reliability, and traditional destructive testing methods are difficult to apply to such large-aperture, high-cost components.

[0003] Most existing devices use expensive equipment to detect surface defects in fused silica. For example, application number CN202411441493.2, "A Method for Detecting Subsurface Damage of Fused Silica Optical Elements Based on Intrinsic Point Defects," relies on a high-precision ultraviolet laser confocal microscope. This equipment is expensive to manufacture and maintain, and compared to traditional fluorescent labeling methods, it has higher requirements for laboratory hardware, making it difficult to popularize in small and medium-sized processing enterprises. Moreover, the fluorescence signal acquisition wavelength (440-480nm) requires precise calibration, and the parameter settings of the pseudo-colorization function (such as color mapping and brightness threshold) depend on the operator's experience. Parameter deviations can easily lead to distortion of tomographic images, affecting the accuracy of 3D reconstruction.

[0004] Meanwhile, existing technologies still lack systematic solutions for pump-probe spectrum isolation, Q-stability, thermal-scattering effect decoupling, and two-dimensional mapping. Summary of the Invention

[0005] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0006] To achieve these objectives and other advantages of the present invention, the present invention provides a method and apparatus for detecting defects on the surface of fused silica based on echo-gallery mode resonance, comprising:

[0007] 355nm pump laser module;

[0008] The homogenization and focusing module, used for beam shaping and energy homogenization of the pump light, is located at the output end of the 355nm pump laser module.

[0009] The WGM microring resonator is located at the output of the homogenization and focusing module.

[0010] A tapered fiber coupling unit is used to enhance the evanescent field component outside the optical fiber, and it is adjacent to the WGM microring resonator at a coupling interface.

[0011] A near-infrared harmonic detection source is used to input probe light into a tapered fiber, and its output window is aligned with the tapered fiber coupling unit.

[0012] A three-dimensional scanning platform is used to accurately locate the spatial position of the WGM microring resonator relative to the surface of the sample under test. The optical element under test is placed on the platform and is located in the coupling interface area.

[0013] A spectral analysis and signal acquisition module for acquiring the transmission spectrum of WGM microring resonators is connected to the tapered fiber coupling unit for signal processing.

[0014] The control and data processing unit is used to realize the synchronous control, data acquisition and signal processing of the device, and to establish a corresponding database between WGM spectral features and EPR signal features. It is connected to the three-dimensional scanning platform for signal transmission.

[0015] In this process, incident light emitted from the near-infrared resonant detector light source generates an evanescent field via a tapered fiber coupling unit, which is then coupled and embedded into the WGM microring resonator. The evanescent field propagates on the ring surface of the WGM microring resonator and excites a whispering gallblade mode. Part of the light energy is coupled out of the WGM microring resonator and transmitted to the spectral analysis and signal acquisition module through the tapered fiber coupling unit.

[0016] Preferably, the structure of the tapered fiber coupling unit includes:

[0017] An optical fiber, one end of which is aligned with the output window of the near-infrared resonant detection light source, and the other end of which is connected to the input signal of the spectral analysis and signal acquisition module, wherein the WGM microring resonator is located in the tapered region of the optical fiber;

[0018] The optical fiber has an isolator and a polarization and power control module sequentially arranged in the output window of the near-infrared harmonic detection light source.

[0019] Preferably, the WGM microring resonator uses fused silica or silicon dioxide as the waveguide material to form a low surface roughness microring-waveguide coupling structure; the surface of the WGM microring resonator is in an open environment or thinly covered, so that the evanescent field can interact with the near surface of the sample under test.

[0020] The WGM microring resonator exhibits detectable resonant drift and linewidth variation in response to refractive index and loss perturbations induced by 355nm pumping.

[0021] Preferably, the control and data processing unit uses a computer program to perform correlation modeling on the EPR signal characteristic parameters and WGM spectral characteristic parameters to establish a database of spectral response-defect types for the determination of optical component defect types and lifetime prediction.

[0022] Preferably, one of the methods for detecting defects on the surface of fused silica based on echo-gallery mode resonance includes:

[0023] S1. Make the WGM microring resonator and the tapered fiber coupling unit critical or undercoupled, place the optical element under test on the three-dimensional scanning platform, and then focus the 355nm pump laser module on the target area on the surface of the optical element under test to establish the pump-probe optical path isolation and filtering.

[0024] S2. Use a 355nm laser to generate local excitation on the surface of the optical element under test, so that the defect-induced refractive index perturbation, scattering loss and absorption loss changes appear in the near-surface volume of the pump spot position, while ensuring that the pump energy density is lower than the material damage threshold.

[0025] S3. The light emitted by the near-infrared resonant detector source is coupled into the excitation tapered fiber coupling unit. When the light passes through the tapered region, it is coupled into the near field of the WGM microring resonator through the evanescent field. The light is coupled into the cavity of the WGM microring resonator and forms a whispering-gallery mode resonance through total internal reflection in the cavity of the WGM microring resonator.

[0026] At this point, the transmission or reflection resonance spectrum is obtained through the tapered fiber coupling unit; the resonance wavelength is extracted. λ res Calculate the resonant wavelength drift Δλ and the quality factor Q = λ res / Δλ;

[0027] S4. Based on the recorded state of the 355nm pump laser module being turned on, the state of the 355nm pump laser module being turned off, or the state of the 355nm pump laser module being modulated, determine whether there is a defect at the corresponding position of the pump spot. The defect indication quantity includes at least: resonance wavelength drift Δλ and quality factor change ΔQ or linewidth change Δ(1 / Q).

[0028] S5. By moving the pump spot, the optical element under test, or the WGM microring resonator, a grid scan is performed to form a two-dimensional distribution map of Δλ and ΔQ, and the defect distribution and its intensity index are output.

[0029] Preferably, in step S3, a 355nm laser is used to generate localized excitation on the surface of the optical element under test. Microscopic changes such as chemical bond breaking, material densification, or color center formation may occur in the local area, causing a change in the refractive index n Δn. This is due to the WGM resonance wavelength. λ res Satisfy the following formula:

[0030]

[0031] Where R is the geometric radius of the micro-ring. n effThis represents the effective refractive index of the WGM mode, where m is the angular quantum number of the WGM. λ res Let be the resonant wavelength of the WGM with wavenumber m.

[0032] When the refractive index of the environment changes, the resonant wavelength will shift accordingly by Δλ; at the same time, the scattering loss introduced by the defect causes the WGM quality factor Q to decrease.

[0033] Preferably, the radius of the WGM microring resonator is 30–200µm, the effective waveguide width is 300–1000nm, the gap between the WGM microring resonator and the tapered fiber coupling unit is 80–400nm, and the static quality factor Q ≥ 1×10 5 .

[0034] Preferably, the near-infrared harmonic detection light source is located in the 1260–1625 nm band.

[0035] Preferably, the 355nm pump laser module has a spot diameter of 20–300µm and a modulation frequency of 10Hz–10kHz, and the signal-to-noise ratio is improved by phase-locked loop or differential algorithm.

[0036] Preferably, the pump-probe optical path is equipped with a short-pass filter, a long-pass filter, and a dichroic mirror to achieve spectral isolation between the 355nm and probe bands, thus preventing pump crosstalk from entering the spectrometer.

[0037] The present invention has at least the following beneficial effects:

[0038] In summary, this invention provides a method and apparatus for detecting surface defects in fused silica based on whispering-gallery mode resonance. A 355nm pump laser module excites local defects on the surface of the component under test. Through an evanescent field coupling structure formed by a WGM microring resonator and a tapered fiber coupling unit, the resonance spectrum changes are acquired by a spectral analysis and signal acquisition module using time-resolved sampling, enabling dynamic response measurement to refractive index perturbations. Simultaneously, the control and data processing unit incorporates EPR technology to identify the chemical structure of the defect center, thereby achieving a correspondence between the macroscopic optical damage response and the microscopic defect type.

[0039] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0040] Figure 1 This is a general structural diagram of the device in this invention;

[0041] Figure 2 This shows the spectral line broadening of different component surface qualities in this invention;

[0042] Figure 3 This is a two-dimensional distribution diagram of Δλ and ΔQ in this invention;

[0043] The corresponding labels for each structure are as follows:

[0044] 1. 355nm pump laser module; 2. Beam shaping; 3. Mirror I; 4. Focusing mirror; 5. Mirror II; 6. WGM microring resonator; 7. Tapered fiber coupling unit; 8. Optical element under test; 9. 3D scanning platform; 10. Absorption trap; 11. Near-infrared resonant detector source; 12. Isolator; 13. Polarization and power control module; 14. Control and data processing unit; 15. Spectral analysis and signal acquisition module; 16. Optical fiber. Detailed Implementation

[0045] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0046] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0047] like Figure 1 - Figure 3 As shown, a device for detecting defects on the surface of fused silica based on whispering-gallery mode resonance includes: a 355nm pump laser module 1;

[0048] The homogenization and focusing module for beam shaping 2 and energy homogenization of pump light is located at the output end of the 355nm pump laser module 1.

[0049] WGM microring resonator 6, which is located at the output end of the homogenization and focusing module;

[0050] A tapered fiber coupling unit 7 is used to enhance the evanescent field component outside the fiber 16, and it is adjacent to the WGM microring resonator 6 at a coupling interface.

[0051] The near-infrared harmonic detection source 11, used to input probe light into the tapered fiber 16, has its output window aligned with the tapered fiber coupling unit 7.

[0052] A three-dimensional scanning platform 9 is used to accurately locate the spatial position of the WGM microring resonator 6 relative to the surface of the sample under test. The optical element under test 8 is placed on the platform and is located in the coupling interface area.

[0053] The spectral analysis and signal acquisition module 15 is used to acquire the transmission spectrum of the WGM microring resonator 6 and is signal-connected to the tapered fiber coupling unit 7.

[0054] The control and data processing unit 14 is used to realize the synchronous control, data acquisition and signal processing of the device, and to establish a corresponding database between WGM spectral features and EPR signal features. It is connected to the three-dimensional scanning platform 9 via signal.

[0055] In this process, the incident light emitted from the near-infrared resonant detector 11 generates an evanescent field via the tapered fiber coupling unit 7, and then couples and embeds it into the WGM microring resonator 6. The evanescent field propagates on the ring surface of the WGM microring resonator 6 and excites a whispering gallblade mode. Part of the light energy is coupled out of the WGM microring resonator 6 and transmitted to the spectral analysis and signal acquisition module 15 through the tapered fiber coupling unit 7.

[0056] Working principle:

[0057] ①The 355nm pump laser module 1 is a continuous or quasi-continuous laser with an ultraviolet band of 355 nm and an output power of 1 W. The output light of the 355nm pump laser module 1 is homogenized and focused by the homogenization and focusing module and then irradiates the surface of the optical element under test 8 (the fused silica optical element under test) to induce defects and local refractive index changes.

[0058] ② The homogenization and focusing module consists of beam shaping 2, mirror I 3, focusing mirror 4 and mirror II 5. It is used to perform beam shaping and energy homogenization on the light emitted by the 355nm pump laser module 1 to ensure that the light spot forms a uniform energy distribution on the surface of the optical element 8 under test.

[0059] In actual use, the laser is reflected by the reflector II5 onto the optical element 8 under test, and then the other side of the reflection by the optical element 8 under test is provided with an absorption trap 10.

[0060] When the laser light reaches the optical element under test 8, it is reflected and scattered, which may cause some of the pump light to be ineffectively absorbed. By setting up the absorption trap 10, the pump light can be more evenly distributed on the optical element under test 8.

[0061] ③ The WGM microring resonator, also known as a Whispering Gallery Mode (WGM) microring resonator, operates based on the total internal reflection effect. When incident light propagates within the ring cavity, it generates a high quality factor (static quality factor Q ≥ 1 × 10⁻⁶ under air coupling conditions). 6 The standing wave mode achieves extremely high sensitivity to changes in the external refractive index;

[0062] In practical applications, the preferred range for the microring cavity radius of the WGM microring resonator 6 is 30–200 μm; the preferred range for the microring cavity thickness of the WGM microring resonator 6 is 2–5 μm.

[0063] ④ The tapered fiber coupling unit 7 is used to enhance the evanescent field component outside the fiber 16; the evanescent field refers to the energy field formed near the interface when a light wave propagates from an optically denser medium to an optically less dense medium and undergoes total internal reflection;

[0064] ⑤ The three-dimensional scanning platform 9 can adjust the spacing between the WGM microring resonator 6 and the tapered fiber coupling unit 7, thereby achieving efficient evanescent field coupling and mode matching;

[0065] Coupling is the transmission and exchange process from the tapered fiber coupling unit 7 to the WGM microring resonator 6. This is a non-contact optical energy transfer method.

[0066] In practical applications, the preferred spacing range is 80–400 nm.

[0067] ⑥ The optical element 8 under test is a high-purity fused silica optical element, whose surface may contain microscopic defects such as non-bridging oxygen hole centers, E' centers, and peroxy free radicals.

[0068] Non-bridged oxygen hole centers, paramagnetic centers formed by oxygen vacancies capturing a hole, are the primary reason why fused silica emits red light under irradiation.

[0069] The E' center, associated with oxygen vacancies and containing a family of silicon dangling defects with an unpaired electron, is the key defect causing the "tailing" of the UV cutoff edge in fused silica.

[0070] Peroxy radicals, the peroxy bond (-OO-) structure formed in the network and the active radicals derived therefrom, are one of the main factors contributing to the degradation of the optical properties of fused silica in the deep ultraviolet band (<200 nm).

[0071] ⑦ Near-infrared resonant detection light source 11 is used to input detection light into tapered fiber coupling unit 7. The light will enter the cavity of WGM microring resonator 6 and be confined within the cavity of WGM microring resonator 6 through continuous total internal reflection to form a whispering gallery mode.

[0072] In practical applications, the preferred wavelength range is 1260–1625 nm.

[0073] ⑧ The spectral analysis and signal acquisition module 15 is used to acquire the transmission spectrum of WGM and measure its resonance wavelength, linewidth and intensity changes in real time;

[0074] ⑨ The control and data processing unit 14 is used to realize the synchronous control, data acquisition and signal processing of the device, and to establish a corresponding database between WGM spectral characteristics and EPR (electron paramagnetic resonance, EPR) signal characteristics;

[0075] ⑩ This device has low manufacturing cost and simple manufacturing process. All materials involved in the device are common optical and engineering materials, and the structure is simple. The optical detection function of this invention is rich and diverse, applicable to the detection of various defect types, and the detection sensitivity and spatial resolution can be independently optimized by adjusting the pump energy, detection wavelength and polarization state. This invention has a wide range of applications, not only suitable for small-aperture, low-energy general scientific experiments, but also has broad application prospects in large-aperture, high-power laser devices due to its high reliability and the high damage threshold characteristics of the materials.

[0076] In summary, this invention provides a method and apparatus for detecting surface defects in fused silica based on whispering-gallery mode resonance. A 355nm pump laser module 1 excites local defects on the surface of the component under test. Through an evanescent field coupling structure formed by a WGM microring resonator 6 and a tapered fiber coupling unit 7, the resonance spectrum changes are acquired by a spectral analysis and signal acquisition module 15 using time-resolved sampling, enabling dynamic response measurement to refractive index perturbations. Simultaneously, the control and data processing unit 14 incorporates EPR technology to identify the chemical structure of the defect center, thereby achieving a correspondence between the macroscopic optical damage response and the microscopic defect type.

[0077] As described above, the structure of the tapered fiber coupling unit 7 includes:

[0078] One end of the optical fiber 16 is aligned with the output window of the near-infrared resonant detector 11, and the other end is connected to the input signal of the spectral analysis and signal acquisition module 15. The WGM microring resonator 6 is located in the tapered region of the optical fiber 16.

[0079] An isolator 12 and a polarization and power control module 13 are sequentially arranged in the output window of the near-infrared harmonic detection light source 11 on the optical fiber 16.

[0080] Working principle:

[0081] When the probe light from the near-infrared resonant probe light source 11 enters the tapered region of the optical fiber 16, the optical field cannot be completely confined within the core of the optical fiber 16, and its evanescent field extends to the outside of the optical fiber 16. Since the WGM microring resonator 6 is located in the tapered region of the optical fiber 16, the evanescent field outside the optical fiber 16 will overlap and interact with the WGM microring resonator 6, so that the light energy in the optical fiber 16 will be efficiently coupled into the cavity of the WGM microring resonator 6.

[0082] The optical fiber 16 is a tapered optical fiber prepared by a fused taper process. In actual use, the waist diameter of the tapered region is less than 1 μm.

[0083] Isolator 12 ensures unidirectional signal flow during transmission, preventing interference and damage from reverse signals;

[0084] The polarization and power control module 13 is used to adjust the polarization state and power of the probe light to obtain stable coupling efficiency and spectral quality.

[0085] In the above scheme, the WGM microring resonator 6 uses fused silica or silicon dioxide as the waveguide material to form a low surface roughness microring-waveguide coupling structure; the surface of the WGM microring resonator 6 is in an open environment or thin-layer cladding, so that the evanescent field can interact with the near surface of the sample under test.

[0086] The WGM microring resonator 6 exhibits detectable resonant drift and linewidth variation in response to refractive index and loss perturbations induced by 355nm pumping.

[0087] Among them, ① in actual use, the coupling region formed between the WGM micro-ring resonator 6 and the tapered fiber coupling unit 7 is thermo-induced or piezoelectrically tuned to stabilize the coupling state, so that the short-time drift of Q is less than 0.5%.

[0088] ② In actual use, the upper limit of the pump energy density of the 355nm pump laser module 1 is set to ≤0.2×the material single-pulse damage threshold to ensure that there is no damage during the entire detection process.

[0089] In the above scheme, the control and data processing unit 14 uses a computer program to model the correlation between the EPR signal characteristic parameters and the WGM spectral characteristic parameters in order to establish a database of spectral response-defect types for the determination of optical component defect types and lifetime prediction.

[0090] Working principle:

[0091] Electron paramagnetic resonance (EPR) detection was performed in the same region irradiated by the 355nm pump laser module 1 to identify paramagnetic defect centers generated by pump excitation; the defect centers included non-bridged oxygen hole centers (NBOHC), E′ centers, peroxy radicals or other photo-induced color centers;

[0092] The control and data processing unit 14 uses a computer program to model the correlation between the EPR signal characteristic parameters (g factor, peak width, integral intensity) and the WGM spectral characteristic parameters (Δλ, ΔQ) to establish a "spectral response-defect type" database for the determination of optical component defect types and lifetime prediction.

[0093] Example 1

[0094] A method for detecting surface defects in fused silica based on echo-gallery mode resonance, applied to the aforementioned device for detecting surface defects in fused silica based on echo-gallery mode resonance, comprising:

[0095] The operator makes the WGM microring resonator 6 and the tapered fiber coupling unit 7 form critical or undercoupled, then places the optical element under test 8 on the three-dimensional scanning platform 9, and then focuses the 355nm pump laser module 1 on the target area on the surface of the optical element under test 8 to establish the pump-probe optical path isolation and filtering.

[0096] The operator uses a 355nm laser to generate local excitation on the surface of the optical element under test 8, so that the defect-induced refractive index perturbation, scattering loss and absorption loss changes appear in the near-surface volume at the pump spot position; when the ultraviolet pump laser of the 355nm pump laser module 1 irradiates the surface of the optical element under test 8, microscopic changes such as chemical bond breaking, material densification or color center formation may occur in the local area, thereby causing a change in refractive index n Δn; at the same time, it is ensured that the pump energy density is lower than the material damage threshold;

[0097] The light emitted by the near-infrared resonant probe light source 11 is coupled into the excitation tapered fiber coupling unit 7. When the light passes through the tapered region, it is coupled into the near field of the WGM microring resonator 6 through the evanescent field. The light is coupled into the cavity of the WGM microring resonator 6 and will propagate along the cavity wall due to total internal reflection on the smooth circular inner surface of the WGM microring resonator 6, forming an optical resonance mode, namely the whispering gallery mode.

[0098] A portion of the resonant light within cavity 6 of the WGM microring resonator will also be coupled back into the tapered fiber coupling unit 7 via the same evanescent field coupling mechanism, and ultimately received by the spectral analysis and signal acquisition module 15.

[0099] At this time, the spectral analysis and signal acquisition module 15 acquires the transmission or reflection resonance spectrum through the tapered fiber coupling unit 7; and extracts the resonance wavelength. λ res Calculate the resonant wavelength drift Δλ and the quality factor Q = λ res / Δλ; In this embodiment, when the near-infrared resonant detector light source operates in the approximately 1550 nm band, the resonant wavelength λres in the pump-off state is approximately 1550.000 nm, the corresponding resonant wavelength drift Δλ is approximately 10 pm, and the calculated quality factor Q is approximately 1.5 × 10⁻⁶. 5 When the 355 nm pump laser is activated, due to changes in the refractive index and absorption loss of the measured region, the resonant wavelength may undergo a redshift or blueshift of approximately 10–30 pm, while the spectral lines broaden to 15–30 pm, corresponding to a decrease in the quality factor to approximately (0.5–1.0) × 10⁻⁶. 5 When the change in the resonance parameter exceeds the system noise baseline, it can be determined that there is an absorption defect at the pump spot location.

[0100] The system records the WGM resonance spectra obtained when the 355nm pump laser module 1 is in the off, on, and modulated states, respectively (e.g., Figure 2 As shown in the figure, the difference results of the resonance spectrum under different operating conditions are used as the basis for defect judgment. In this embodiment, in the pump-off state, the resonance wavelength λres remains stable, with a short-term drift of less than 2 pm and a relative fluctuation of the quality factor Q of less than 3%. When the 355 nm pump laser is turned on, due to the change in the refractive index and absorption loss of the measured area, the resonance wavelength can drift by about 10–30 pm relative to the off state, and the quality factor Q can decrease by about 10%–40%. Furthermore, when the pump laser is modulated at a frequency of 100 Hz–1 kHz, by performing differential processing on the resonance spectrum obtained before and after modulation, the resonance wavelength difference component Δλ_diff synchronized with the pump modulation can be extracted. Its amplitude can be in the range of 5–20 pm, which is significantly higher than the system noise baseline, thereby improving the sensitivity of identifying absorption defects. To further determine the microstructure type of defects, in-situ or near-in-situ EPR analysis is performed on the same region. The preferred EPR system is an X-band (approximately 9.4 GHz) instrument, which can detect the g-factor and hyperfine structure signals of typical paramagnetic defects such as non-bridged oxygen hole centers (NBOHC), E' centers, and peroxy radicals.

[0101] Determine whether there is a defect at the corresponding location of the pump spot, wherein the defect indication includes at least: resonant wavelength drift Δλ and quality factor change ΔQ or linewidth change Δ(1 / Q).

[0102] When the WGM spectrum shows a significant decrease in Q value and Lorentz broadening, if the characteristic spectral line of the E' center also appears in the EPR signal, it can be confirmed that the defect is the main type causing optical loss. By comparing different WGM spectral line change patterns with EPR signal types, a database of "WGM spectral characteristics - EPR defect types" can be established to achieve a qualitative or semi-quantitative correlation between defect type and optical response.

[0103] By using grid scanning, the resonant wavelength shift Δλ and the quality factor change ΔQ can be obtained point by point within the measured area. The control and data processing unit 14 then performs two-dimensional reconstruction of these parameters, resulting in a two-dimensional distribution diagram of Δλ and ΔQ as shown below. Figure 3 As shown, it is used to characterize the spatial distribution of defects and their relative intensity.

[0104] In this embodiment, in the two-dimensional distribution map of Δλ, the defect region can appear as a locally enhanced resonant wavelength drift "hot spot," the spatial range of which is related to the pump spot size and scanning step size; while in the two-dimensional distribution map of ΔQ, the region corresponding to the same spatial location can show a significant decrease in the quality factor, reflecting the additional scattering or absorption loss introduced by the defect. By comparing and analyzing the Δλ and ΔQ distribution maps, the consistency of the defect location can be confirmed.

[0105] Among them, ① in this example Figure 2 In the diagram, ΔλA (and ΔλFWHM1) is the full width at half maximum (FWHM) of the resonance under loss perturbation in defect region 1, and ΔλB (and ΔλFWHM0) is the full width at half maximum (FWHM) of the resonance in the intrinsically defect-free region.

[0106] Full width at half maximum (FWHM) Δλ is used to characterize the spectral linewidth characteristics of a whispering-gallery mode resonator. Its size is closely related to the total loss level within the cavity. The cavity loss includes, but is not limited to: intrinsic absorption loss of the resonator material, absorption loss induced by surface or bulk defects, and additional dissipation loss introduced by the pump laser.

[0107] By detecting the changes in the full width at half maximum (FWHM) of the resonance spectral lines at different spatial locations, the spatial distribution characteristics of the intracavity loss level can be reflected. Furthermore, by comparing and analyzing the full width at half maximum (FWHM) of the defective and non-defective regions, the additional intracavity loss component introduced by the defects can be extracted, thereby characterizing the absorption-type defects.

[0108] ② Δλ mainly reflects the resonant frequency shift effect caused by refractive index perturbation, while ΔQ is used to characterize the optical loss intensity related to defects, thereby achieving a comprehensive characterization of the spatial distribution and intensity characteristics of absorption defects;

[0109] Simultaneously, characteristic parameters such as g-factor, peak width, and integral intensity of the EPR test are acquired. The control and data processing unit uses a preset algorithm to model the correlation between the EPR characteristic parameters and the WGM optical response parameters (including Δλ, ΔQ, and intensity attenuation rate), establishing a database of correspondences between fused silica defect types and optical response characteristics. In the implementation, the algorithm may include, but is not limited to: a rule matching algorithm based on threshold determination, a multi-parameter statistical analysis algorithm, or a data-driven model based on supervised or semi-supervised learning, the output of which can be used as input for defect identification, damage prediction, and optical component life assessment.

[0110] In summary, this invention enables in-situ identification and type determination of microscopic defects on the surface of fused silica optical elements under non-destructive conditions. Combining the high sensitivity of WGM optical resonance with the chemical identification capability of EPR spectroscopy, it allows for multimodal correlation analysis of macroscopic damage processes and microscopic defect structures. Furthermore, it features high detection sensitivity, achieving sub-picometer wavelength resolution, and high spatial resolution, enabling micrometer- to nanometer-scale local detection. It also allows for the coordinated measurement and database establishment of WGM and EPR signals, providing quantitative evidence for laser tolerance analysis and lifetime prediction of optical elements.

[0111] In Example 1, a 355nm laser is used to generate localized excitation on the surface of the optical element 8 under test. Microscopic changes such as chemical bond breaking, material densification, or color center formation may occur in the local area, causing a change in refractive index n Δn. Due to the WGM resonance wavelength... λ res Satisfy the following formula:

[0112]

[0113] Where R is the geometric radius of the micro-ring. n eff This represents the effective refractive index of the WGM mode, where m is the angular quantum number of the WGM. λ res Let be the resonant wavelength of the WGM with wavenumber m.

[0114] When the refractive index n of the environment changes, the resonant wavelength λ res A corresponding frequency shift Δλ will occur; simultaneously, the scattering loss introduced by the defect leads to a decrease in the WGM quality factor Q, as detailed in the reference. Figure 2 The spectral line broadening of different component surface qualities is shown. By coupling the near-infrared resonant probe light source 11 with the evanescent field of the WGM microring resonator 6 (the portion of the light wave entering the cladding during total internal reflection in the WGM microring resonator 6 (i.e., the evanescent wave) can couple the signal in the WGM microring resonator 6 to the tapered fiber coupling unit 7), the drift of the resonant wavelength, linewidth variation, and intensity attenuation in the transmission spectrum can be monitored in real time. By analyzing these parameters, highly sensitive in-situ detection of the formation, evolution, and optical loss characteristics of ultraviolet laser-induced defects can be achieved.

[0115] Meanwhile, the transient curves of Δλ(t) and ΔQ(t) were obtained by time-resolved acquisition, and the thermal-carrier relaxation time constant related to the defect was extracted from them.

[0116] In Example 1, the radius of the WGM microring resonator 6 is 30–200µm, the effective waveguide width is 300–1000nm, the gap between the WGM microring resonator 6 and the tapered fiber coupling unit 7 is 80–400nm, and the static quality factor Q ≥ 1×10 5 .

[0117] In Example 1, the near-infrared harmonic detection light source is located in the 1260–1625 nm band, preferably 1550 nm; the spectrometer or spectrometer has a resolution better than 0.02 nm.

[0118] In Example 1, the 355nm pump laser module 1 has a spot diameter of 20–300µm and a modulation frequency of 10Hz–10kHz, and the signal-to-noise ratio is improved by phase-locked loop or differential algorithm;

[0119] Meanwhile, the polarization and power control module 13 is used to select the TE / TM eigenmodes (TE mode, electric vector perpendicular to the propagation direction; TM mode, magnetic vector perpendicular to the propagation direction) of the whispering-gallery mode microring resonator, so that the thermal-refractive index effect and the scattering loss effect are separated under different polarizations.

[0120] In Example 1, the pump-probe optical path is equipped with a short-pass filter, a long-pass filter, and a dichroic mirror to achieve spectral isolation between the 355nm and probe bands, thus preventing pump crosstalk from entering the spectrometer.

[0121] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0122] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A device for detecting defects on the surface of fused silica based on echo-gallery mode resonance, characterized in that, include: 355nm pump laser module; The homogenization and focusing module, used for beam shaping and energy homogenization of the pump light, is located at the output end of the 355nm pump laser module. The WGM microring resonator is located at the output of the homogenization and focusing module. A tapered fiber coupling unit is used to enhance the evanescent field component outside the optical fiber, and it is adjacent to the WGM microring resonator at a coupling interface. A near-infrared harmonic detection source is used to input probe light into a tapered fiber, and its output window is aligned with the tapered fiber coupling unit. A three-dimensional scanning platform is used to accurately locate the spatial position of the WGM microring resonator relative to the surface of the sample under test. The optical element under test is placed on the platform and is located in the coupling interface area. A spectral analysis and signal acquisition module for acquiring the transmission spectrum of WGM microring resonators is connected to the tapered fiber coupling unit for signal processing. The control and data processing unit is used to realize the synchronous control, data acquisition and signal processing of the device, and to establish a corresponding database between WGM spectral features and EPR signal features. It is connected to the three-dimensional scanning platform for signal transmission. Among them, the incident light emitted from the near-infrared resonant detection light source generates an evanescent field through the tapered fiber coupling unit, and then couples and embeds it into the WGM microring resonator. The evanescent field propagates on the ring surface of the WGM microring resonator and excites the whispering gallblade mode. Part of the light energy is coupled out of the WGM microring resonator and transmitted to the spectral analysis and signal acquisition module through the tapered fiber coupling unit. The WGM microring resonator uses fused silica or silicon dioxide as the waveguide material to form a low surface roughness microring-waveguide coupling structure; the surface of the WGM microring resonator is in an open environment or thinly covered, so that the evanescent field can interact with the near surface of the sample under test. The WGM microring resonator exhibits detectable resonant drift and linewidth variation in response to refractive index and loss perturbations induced by 355nm pumping.

2. The fused silica surface defect detection device based on echo-gallery mode resonance as described in claim 1, characterized in that, The structure of the tapered fiber coupling unit includes: An optical fiber, one end of which is aligned with the output window of the near-infrared resonant detection light source, and the other end of which is connected to the input signal of the spectral analysis and signal acquisition module, wherein the WGM microring resonator is located in the tapered region of the optical fiber; The optical fiber has an isolator and a polarization and power control module sequentially arranged in the output window of the near-infrared harmonic detection light source.

3. The fused silica surface defect detection device based on echo-gallery mode resonance as described in claim 1, characterized in that, The control and data processing unit uses a computer program to model the correlation between EPR signal characteristic parameters and WGM spectral characteristic parameters, in order to establish a database of spectral response-defect types for the determination of optical component defect types and lifetime prediction.

4. A method for detecting surface defects in fused silica based on echo-gallery mode resonance, comprising using the fused silica surface defect detection device based on echo-gallery mode resonance as described in any one of claims 2-3, characterized in that, include: S1. Make the WGM microring resonator and the tapered fiber coupling unit critical or undercoupled, place the optical element under test on the three-dimensional scanning platform, and then focus the 355nm pump laser module on the target area on the surface of the optical element under test to establish the pump-probe optical path isolation and filtering. S2. Use a 355nm laser to generate local excitation on the surface of the optical component under test, so that the defect-induced refractive index perturbation and scattering / absorption loss change appear in the near-surface volume of the pump spot position, while ensuring that the pump energy density is lower than the material damage threshold. S3. The light emitted by the near-infrared resonant detector source is coupled into the excitation tapered fiber coupling unit. When the light passes through the tapered region, it is coupled with the WGM microring resonator in the near field through the evanescent field. The light is coupled into the cavity of the WGM microring resonator and forms a whispering-gallery mode resonance through total internal reflection in the cavity of the WGM microring resonator. At this point, the transmission or reflection resonance spectrum is obtained through the tapered fiber coupling unit; Extracting resonant wavelengths λ res 1. Calculate the half-height and full width Δλ, and then calculate the quality factor Q = λ res / Δλ; S4. Based on the recorded state of the 355nm pump laser module being turned on, the state of the 355nm pump laser module being turned off, or the state of the 355nm pump laser module being modulated, determine whether there is a defect at the corresponding position of the pump spot. The defect indication quantity includes at least: resonance wavelength drift Δλ and quality factor change ΔQ or linewidth change Δ(1 / Q). S5. By moving the pump spot, the optical element under test, or the WGM microring resonator, a grid scan is performed to form a two-dimensional distribution map of Δλ and ΔQ, and the defect distribution and its intensity index are output.

5. The method for detecting defects on the surface of fused silica based on echo-gallery mode resonance as described in claim 4, characterized in that, In S3, a 355nm laser is used to generate localized excitation on the surface of the optical element under test. Microscopic changes such as chemical bond breaking, material densification, or color center formation may occur in the local area, causing a change in the refractive index n Δn. This is due to the WGM resonance wavelength. λ res Satisfy the following formula: Where R is the geometric radius of the micro-ring. n eff This represents the effective refractive index of the WGM mode, where m is the angular quantum number of the WGM. λ res The resonant wavelength of the WGM with wavenumber m; When the refractive index of the environment changes, the resonant wavelength will shift accordingly by Δλ; at the same time, the scattering loss introduced by the defect causes the WGM quality factor Q to decrease.

6. The method for detecting defects on the surface of fused silica based on echo-gallery mode resonance as described in claim 4, characterized in that, The WGM microring resonator has a radius of 30–200µm, an effective waveguide width of 300–1000nm, a gap of 80–400nm between the WGM microring resonator and the tapered fiber coupling unit, and a static quality factor Q ≥ 1 × 10⁻⁶. 5 .

7. The method for detecting defects on the surface of fused silica based on echo-gallery mode resonance as described in claim 4, characterized in that, The near-infrared tunable detection light source is located in the 1260–1625 nm band.

8. The method for detecting defects on the surface of fused silica based on echo-gallery mode resonance as described in claim 4, characterized in that, The 355nm pump laser module has a spot diameter of 20–300µm and a modulation frequency of 10Hz–10kHz. The signal-to-noise ratio is improved through phase-locked loop or differential algorithms.

9. The method for detecting defects on the surface of fused silica based on echo-gallery mode resonance as described in claim 4, characterized in that, The pump-probe optical path is equipped with short-pass / long-pass filters and a dichroic mirror to achieve spectral isolation between the 355nm and probe bands, thus preventing pump crosstalk from entering the spectrometer.

Images