A method and system for detecting absorption defects on the surface of an optical element

Abstract

The present invention discloses a method and system for detecting absorptive defects on the surface of an optical element, which relates to the field of optical element detection, including: S1, based on the principle of frustrated total reflection imaging in a micro-area, using EMCCD to scan the probe light evanescent wave image I on a standard glass slide under frustrated total reflection conditions; S2, after or before the acquisition of the probe light evanescent wave image I, scanning the probe light evanescent wave image II on the surface of the element under unpumped conditions; S3, based on the set coordinates and path, completing the scanning within the full aperture range of the optical element to be measured; S4, splicing and comparing each probe light evanescent wave image I and each probe light evanescent wave image II, respectively, to obtain the relative light absorption intensity of the absorptive defects within the full aperture range of the optical element. The present invention provides a method and system for detecting absorptive defects on the surface of an optical element, which can ensure a high-sensitivity detection effect under high efficiency.

Description

Technical Field

[0001] The present invention relates to the field of optical element detection, and more specifically, to a method and system for detecting absorption defects on the surface of an optical element. Background Art

[0002] There are a large number of functional optical devices with high transmission, high reflection and polarization splitting in high-energy laser devices. When there are absorption defects in the optical matrix material or surface film, the transmitted laser energy will be strongly absorbed and converted into heat through the photoelectric or photoacoustic effect. Once the temperature rise or stress generated by this thermal field effect exceeds the tolerance limit of the optical material, it can cause laser-induced damage to the optical device. Therefore, the detection of absorption defects in the used optical elements is of great significance for evaluating the manufacturing process level of the optical elements and ensuring the stable operation of the high-energy laser system.

[0003] The sources of absorptive defects mainly come from absorptive impurities, scratches, microcracks and nodule defects of the film layer introduced during the polishing or coating process of the component surface. They have a micro-mesoscopic cross-scale distribution characteristic from submicron to millimeter, and the absorption coefficient is usually in the weak absorption range of 0.1ppm~100ppm. Although traditional optical material absorption characteristic detection methods such as spectrophotometry and calorimetry are efficient, the detection accuracy is only one thousandth to one ten-thousandth, which cannot meet the requirements for absorptive defect detection of high-precision processed optical components. Although the photothermal weak absorption detection technology based on the principle of photothermal deflection or thermal lens has high sensitivity (theoretical value ~1ppm), its single-point detection area is only tens of microns, and it takes more than 1 month to detect optical components above several centimeters. Summary of the invention

[0004] An object of the present invention is to solve at least the above problems and / or disadvantages and to provide at least the advantages which will be described hereinafter.

[0005] In order to achieve these purposes and other advantages of the present invention, a method for detecting absorptive defects on the surface of an optical element is provided, comprising:

[0006] S1. Based on the principle of micro-area frustrated total reflection imaging, the probe light evanescent wave image on the standard glass slide under the condition of frustrated total reflection is scanned by EMCCD under the condition of laser pumping I.

[0007] S2, after or before the acquisition of the probe light evanescent wave image I, scanning the probe light evanescent wave image II of the component surface under the unpumped condition;

[0008] S3, based on the set coordinates and path, the optical element to be measured is horizontally moved step by step in the X and Y directions through the three-dimensional translation mechanism, and returns to S1-S2 after each step until the scanning of the full aperture range of the optical element to be measured is completed;

[0009] S4, respectively splicing and comparing the evanescent wave images I and II of each probe light to obtain the relative light absorption intensity of the absorptive defects within the full aperture range of the optical element;

[0010] By comparing the probe light evanescent wave image I and the probe light evanescent wave image II of the same detection area, the relative light absorption intensity of the absorption defect is reflected;

[0011] The light absorption intensity of the absorptive defects in the laser pumping area can be characterized by the change in the intensity of the evanescent wave light field of the probe light before and after laser pumping.

[0012] Preferably, in S1, the principle of micro-area frustrated total reflection imaging is:

[0013] Based on the evanescent wave formation theory, the maximum spatial distance of the evanescent wave from the component surface is d for:

[0014]

[0015] In the above formula, l is the wavelength of the incident probe light, i is the total internal reflection angle, n 1 is the refractive index of the optical element to be measured, n 0 is the refractive index of air.

[0016] When the distance between the standard glass slide and the surface of the optical element to be measured is less than d When the evanescent wave enters the standard glass slide, it forms frustrated total reflection imaging. Under this condition, the evanescent wave can be detected by the EMCCD.

[0017] Preferably, before S1, it also includes:

[0018] S10, the continuous laser emitted by the continuous pump laser is incident vertically from the lower surface of the optical element, and converges to the upper surface of the optical element to be measured after beam expansion;

[0019] S11, adjusting the distance between the optical element to be tested and the pump laser focusing lens to form a pump light focusing spot with a diameter distribution range of 10µm~1000µm on the upper surface of the element;

[0020] S12, incident the continuous probe laser beam emitted by the continuous probe laser from the lower surface of the optical element to be measured to the upper surface of the optical element to be measured at an incident angle greater than or equal to the total internal reflection, and overlap with the focused spot of the pump light of S2 on the upper surface;

[0021] S13, place a standard glass slide above the upper surface of the optical element to be tested, and when the distance between the standard glass slide and the surface of the element to be tested is less than d When , frustrated total reflection is formed, where d is the maximum spatial distance of the evanescent wave from the surface of the element.

[0022] A system for detecting absorption defects on the surface of an optical element, which is applied to a method for detecting absorption defects on the surface of an optical element, comprises:

[0023] A continuous pump laser is arranged below the optical element to be measured and has an output wavelength that is consistent with or close to the laser loading wavelength actually applied to the optical element to be measured;

[0024] A beam expanding telescope and a focusing lens are used to spatially shape and converge the continuous laser light emitted by the continuous pump laser onto the upper surface of the optical element to be measured;

[0025] A continuous probe laser is arranged below the optical element to be measured and has an incident angle greater than or equal to the total internal reflection angle;

[0026] Optical absorption trap I spatially corresponding to the continuous pump laser;

[0027] Light absorption trap II coordinated with the reflection path of the continuous probe laser;

[0028] A standard glass slide parallel to the upper surface of the optical element to be measured and having an air gap between the standard glass slide and the optical element to be measured;

[0029] An EMCCD is placed obliquely above the standard glass slide to detect the evanescent wave image of the probe light;

[0030] A three-dimensional translation mechanism that cooperates with the optical element to be measured.

[0031] Preferably, in S10, the power density of the continuous pump laser is controlled to be close to the laser damage threshold of the optical element to be measured.

[0032] Preferably, in S13, the standard glass slide is a high-precision surface glass slide with a centimeter-level aperture, and its reflective surface is less than or equal to 1 / 50 l, l is the wavelength of the incident probe light;

[0033] The standard glass slide is parallel to the upper surface of the optical element to be measured, and there is an air gap with a width of ≤1µm between the two.

[0034] Preferably, in S1, the EMCCD is configured to adopt an active cooling type, and the dynamic range is greater than or equal to 16 bits.

[0035] The present invention includes at least the following beneficial effects: This patent proposes a photothermal weak absorption defect detection method based on the principle of evanescent wave imaging. Since the light intensity of the evanescent wave under the condition of suppressed total reflection has an exponentially decaying dependence on the thickness change of the air gap, it theoretically has the advantage of high sensitivity detection of surface absorptive defects of optical components. At the same time, based on the surface imaging detection technology, the single defect detection range can be increased to the millimeter level. Compared with the traditional photothermal weak absorption detection technology, the detection efficiency can be improved by nearly a thousand times, which is of great industry application value.

[0036] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A schematic diagram of the system layout for detecting absorption defects on the surface of an optical element according to the present invention;

[0038] Among them, continuous pump laser-1, beam expansion telescope-2, focusing lens-3, light absorption trap I-4, continuous probe laser-5, light absorption trap II-6, standard glass slide-7, EMCCD -8, three-dimensional translation mechanism-9, control unit-10, optical element to be tested-11. DETAILED DESCRIPTION

[0039] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement the invention with reference to the description.

[0040] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0041] It should be noted that in the description of the present invention, the orientation or positional relationship indicated by the term is based on the orientation or positional relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "Ⅰ" and "Ⅱ" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0042] In the description of the present invention, it should be noted that, unless otherwise clearly stipulated and limited, the terms "installed", "provided with", "mounted / connected", "connected", etc. should be understood in a broad sense. For example, "connected" can be a fixed connection, a detachable connection, or an integral connection. It can be a mechanical connection or an electrical connection. It can be a direct connection or an indirect connection through an intermediate medium. It can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0043] like Figure 1 As shown, a system for detecting absorption defects on the surface of optical components mainly includes:

[0044] 1. Continuous pump laser subsystem: including a continuous pump laser 1 whose output wavelength is consistent with or close to the laser loading wavelength actually used in the optical element 11 to be measured, a beam expansion telescope 2 and a focusing lens 3 for beam shaping and energy convergence, so as to realize strong laser pumping on the upper surface of the optical element to be measured. According to the different requirements of the defect type of the optical element to be measured and the size of the focused light spot, the output power of the pump laser is usually required to be ≥10W (the specific power depends on the size of the single pump detection area). At the relative position of the continuous pump laser 1, a light absorption trap I 4 matching its output light is arranged;

[0045] 2. Continuous probe laser subsystem: It is composed of a continuous probe laser 5 with a working band of visible light and a power of mW level. When in use, it is incident from the lower surface of the optical element and reaches the upper surface with an incident angle greater than or equal to the total internal reflection angle, and accurately coincides with the pump light on the upper surface. At the same time, a light absorption trap II 6 is set at a position that matches the reflection path of the continuous probe laser 5.

[0046] 3. Evanescent wave detection subsystem: It consists of a standard glass slide 7 and a high dynamic range EMCCD (also known as electron multiplying CCD, which is a high-end photoelectric detection product with extremely high sensitivity in the detection field) 8. The standard glass slide 7 is a high-precision surface glass slide with a centimeter-level aperture. The reflective surface is generally required to be ≤1 / 50λ. The glass slide and the upper surface of the optical element to be measured are strictly parallel through a high-precision clamping device, and an air gap with a width of ≤1µm is formed. A high dynamic range EMCCD 8 is placed obliquely above the standard glass slide 7 along the incident direction of the probe light to capture the evanescent wave. In order to obtain high-sensitivity detection results, the EMCCD 8 is generally required to be actively cooled and have a dynamic range of ≥16bit.

[0047] 4. Automatic control and data acquisition system: including an electric three-dimensional translation mechanism 9 for clamping the optical element to be measured and realizing three-dimensional program-controlled movement, a control unit 10 for controlling or switching the output of the continuous pump laser 1, the output of the continuous probe laser 5, and the action of the three-dimensional translation mechanism, and an image processing unit for splicing the images collected by the EMCCD 8.

[0048] The workflow of the system for detecting absorption defects on the surface of optical components is as follows:

[0049] S1. Before testing, firstly precisely adjust the Z axis of the stage in the vertical up and down direction to ensure that the width of the air gap formed between the standard glass slide 7 and the upper surface of the optical element to be tested is ≤ d, and a stable evanescent wave image of the probe light can be formed on the EMCCD 8;

[0050] S2. After the detection starts, the three-dimensional translation stage carries the optical element to be tested to perform stepping movements in the X and Y directions in the horizontal direction according to the coordinates and paths set by the program. The laser control unit 10 turns off the laser output every time it steps, collects the evanescent wave image of the probe light on the surface of the element under the condition of no pumping, and then opens the laser output light gate to collect the evanescent wave image of the probe light on the surface of the element under the condition of laser output again.

[0051] EMCCD 8 collects and outputs the evanescent wave image of the probe light in the same detection area before and after heat pumping in real time. By referring to the change in the evanescent wave light intensity ∆I, the degree of thermal pack deformation in the laser irradiation area on the component surface can be calculated, which ultimately reflects the relative light absorption intensity of the absorptive defects.

[0052] By combining step scanning with image stitching, the two-dimensional distribution and relative intensity information of absorption defects within the full aperture range of the optical element can be realized, which can be used to evaluate the control level of absorption defects on the surface of the optical element and predict the anti-laser damage performance of the element under strong laser loading.

[0053] Example 1

[0054] The main implementation principle of the present invention is "continuous heat pumping + micro-area frustrated total reflection imaging". Compared with traditional absorption defect detection methods, it can achieve high-efficiency and high-sensitivity detection of absorption defects on the surface of optical components. Specifically, the light source of heat pumping is a high-power solid continuous laser that is consistent with the actual application wavelength of the optical component. The continuous laser (or quasi-continuous) emitted by the laser is incident from the lower surface of the optical component in a vertical incident form, and after beam expansion, it is converged through a short-focus lens to reach the upper surface of the optical component to be tested (the absorption defect test surface of the component). By adjusting the distance to the optical component to be tested, a pump light focus spot with a diameter distribution range of about 100µm~1000µm can be formed on the upper surface of the component. Another continuous probe laser beam in the visible light band is also emitted from the lower surface of the optical component to be tested at an incident angle greater than or equal to total internal reflection (i c , whose value is acrsin(n1 / n0)) is incident on the upper surface of the optical element to be tested and coincides with the pump spot on the upper surface. Based on the evanescent wave formation theory, when the incident light is incident on the interface (upper surface of the optical element to be tested) between the optically dense medium (optical element to be tested) and the optically sparse medium (air) at an incident angle greater than or equal to total internal reflection, an evanescent wave will be formed in the air at a distance of about the wavelength scale from the surface of the optical element. The maximum spatial distance of the evanescent wave from the surface of the element is d for:

[0055]

[0056] In the formula, l is the wavelength of the incident probe light, i is the total internal reflection angle, n 1 is the refractive index of the optical element to be measured, n 0 is the refractive index of air, and the intensity of the evanescent wave is d At this time, a standard glass slide is placed above the upper surface of the optical element to be tested. When the distance between the standard glass slide and the surface of the optical element to be tested is less than d When the evanescent wave enters the standard glass slide, it forms frustrated total reflection. Under this condition, the evanescent wave can be detected by the high dynamic range imaging EMCCD above the standard glass slide.

[0057] Since the intensity of the evanescent wave decays exponentially with the width of the air gap, and the refractive index of the optical element to be measured will also affect d Therefore, when there are absorptive defects in the area irradiated by the pump spot, the defective part absorbs the laser energy and generates a temperature gradient with the non-defective part, thereby generating thermal expansion and stress fields. This will cause the optical film or substrate material at the defective part to have a raised thermal pack deformation. The lateral and longitudinal dimensions of the deformation are related to the light absorption intensity of the defect, the defect scale, the thermal diffusion length of the matrix material and the thermal expansion coefficient. The lateral scale is usually in the range of 1µm~100µm, and the longitudinal height is a few nm~hundreds of nm. Since the width of the air gap that produces frustrated total reflection is only a few hundred nm, when thermal pack deformation occurs, it will directly lead to a decrease in the width of the air gap, thereby enhancing the frustrated total reflection effect. At the same time, the temperature field generated after the defect absorption will also cause the refractive index of the optical material near the defect to change (n1 changes to n1′). According to the maximum spatial distance d The calculation formula will also affect the strength of the frustrated total reflection effect.

[0058] Therefore, the heat pack deformation and material refractive index change caused by defect absorption will affect the suppressed total reflection effect with high sensitivity, thereby affecting the evanescent wave light field intensity detected by the high dynamic range EMCCD. The intensity change of the evanescent wave in the laser pumping area is detected by the EMCCD, and then the light absorption intensity of the absorptive defects in the laser pumping area is reflected with high sensitivity.

[0059] In actual detection, the power density of the pump laser can be controlled to a level close to the laser damage threshold of the optical element to be tested, thereby maximizing the thermal pack effect generated by the defect to improve the detection sensitivity (it should be noted that the thermal pack effect here refers to the phenomenon that when high-energy laser hits the surface of the material, the surface of the material will bulge due to heat). When the laser pumping within the optical pumping range does not cause obvious thermal pack deformation and material refractive index change, the high dynamic range EMCCD observes that the evanescent wave has no intensity change, proving that there is no obvious absorption defect in the part to be tested; surface thermal pack deformation and refractive index change of nearby materials occur around the defect, resulting in the high dynamic range EMCCD being able to observe obvious evanescent wave intensity changes, proving that there is a high absorption defect in the part to be tested.

[0060] The above solution is only an illustration of a preferred embodiment, but is not limited thereto. When implementing the present invention, appropriate replacement and / or modification can be performed according to user needs.

[0061] The number of devices and processing scales described here are used to simplify the description of the present invention. Applications, modifications and variations of the present invention will be obvious to those skilled in the art.

[0062] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and the embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily realized. Therefore, without departing from the general concept defined by the claims and equivalent scope, the present invention is not limited to the specific details and the illustrations shown and described here.

Claims

1. A method for detecting absorptive defects on the surface of an optical element, characterized in that: include: S1. Based on the principle of micro-area frustrated total reflection imaging, the probe light evanescent wave image on the standard glass slide under the condition of frustrated total reflection is scanned by EMCCD under the condition of laser pumping I. S2, after or before the acquisition of the probe light evanescent wave image I, scanning the probe light evanescent wave image II of the component surface under the unpumped condition; S3, based on the set coordinates and path, the optical element to be measured is horizontally moved step by step in the X and Y directions through the three-dimensional translation mechanism, and returns to S1-S2 after each step until the scanning of the full aperture range of the optical element to be measured is completed; S4, respectively splicing and comparing the evanescent wave images I and II of each probe light to obtain the relative light absorption intensity of the absorptive defects within the full aperture range of the optical element; By comparing the probe light evanescent wave image I and the probe light evanescent wave image II of the same detection area, the relative light absorption intensity of the absorption defect is reflected; The light absorption intensity of the absorptive defects in the laser pumping area can be characterized by the change in the light field intensity of the evanescent wave of the probe light before and after laser pumping. It is characterized in that, in S1, the principle of micro-area frustrated total reflection imaging is: Based on the evanescent wave formation theory, the maximum spatial distance of the evanescent wave from the component surface is d for: In the above formula, λ is the wavelength of the incident probe light, θ is the total internal reflection angle, n 1 is the refractive index of the optical element to be measured, n 0 is the refractive index of air; When the distance between the standard glass slide and the surface of the optical element to be measured is less than d When , the evanescent wave will enter the standard glass slide, forming frustrated total reflection imaging. Under this condition, the evanescent wave can be detected by the EMCCD. Prior to S1, it also included: S10, the continuous laser emitted by the continuous pump laser is incident vertically from the lower surface of the optical element, and converges to the upper surface of the optical element to be measured after beam expansion; S11, adjusting the distance between the optical element to be tested and the pump laser focusing lens to form a pump light focusing spot with a diameter distribution range of 100µm~1000µm on the upper surface of the element; S12, incident the continuous probe laser beam emitted by the continuous probe laser from the lower surface of the optical element to be measured to the upper surface of the optical element to be measured at an incident angle greater than or equal to the total internal reflection, and overlap with the focused spot of the pump light of S2 on the upper surface; S13, place a standard glass slide above the upper surface of the optical element to be tested, and when the distance between the standard glass slide and the surface of the element to be tested is less than d When , frustrated total reflection is formed.

2. A system for detecting absorption defects on the surface of an optical element, which is applied to the detection method of absorption defects on the surface of an optical element as claimed in claim 1, characterized in that: include: A continuous pump laser is arranged below the optical element to be measured and has an output wavelength that is consistent with or close to the laser loading wavelength actually applied to the optical element to be measured; A beam expanding telescope and a focusing lens are used to spatially shape and converge the continuous laser light emitted by the continuous pump laser onto the upper surface of the optical element to be measured; A continuous probe laser is arranged below the optical element to be measured and has an incident angle greater than or equal to the total internal reflection angle; Optical absorption trap I spatially corresponding to the continuous pump laser; Light absorption trap II coordinated with the reflection path of the continuous probe laser; A standard glass slide parallel to the upper surface of the optical element to be measured and having an air gap between the standard glass slide and the optical element to be measured; An EMCCD is placed obliquely above the standard glass slide to detect the evanescent wave image of the probe light; A three-dimensional translation mechanism matched with the optical element to be measured; In S13, the standard glass slide is a high-precision surface glass slide with a centimeter-level aperture, and its reflective surface is less than or equal to 1 / 50 λ ; The standard glass slide is parallel to the upper surface of the optical element to be measured, and there is an air gap with a width of ≤1µm between the two.

3. The system for detecting absorptive defects on the surface of an optical element according to claim 2, characterized in that: In S10, the output power of the continuous pump laser is ≥10 W, and the power density is controlled to be close to the laser damage threshold of the optical element to be measured.

4. The system for detecting absorptive defects on the surface of an optical element according to claim 2, wherein: In S1, the EMCCD is configured to use active cooling and a dynamic range greater than or equal to 16 bits.

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