Three-dimensional measurement device and measurement method for laser damage of optical elements

Abstract

The present invention discloses a three-dimensional measurement device and method for laser damage of optical elements, belonging to the technical field of three-dimensional measurement. The device uses a microscopic objective lens in a reflection holographic optical path to achieve large-curvature illumination and magnified imaging, realizes field-of-view expansion and continuous adjustment of wavefront curvature through sample space translation, uses the holographic method to obtain the reflected light information and illumination light distribution when the sample is translated, and then numerically reconstructs the three-dimensional transmittance distribution of the optical element to be measured by using these reflected light information and illumination light distribution, so as to realize the three-dimensional measurement of laser damage of the optical element. The present invention can measure the three-dimensional distributions of transmittance and refractive index near the laser damage point of the optical element, and can provide parameter basis for the laser damage repair of the optical element.

Description

Technical Field

[0001] The present invention relates to a three-dimensional measurement device and method for laser damage of optical elements, belonging to the technical field of three-dimensional measurement. Background Art

[0002] Laser is the light generated by stimulated emission of atoms. As is well known, high-intensity laser can cause injuries to the human eyes, skin, etc. Similarly, high-intensity laser can also cause damage to various optical elements, which is called laser damage. As the main components of various laser systems, the laser damage of optical elements such as lenses and crystals can seriously affect the load capacity of high-power laser devices. These laser damages modulate the intensity and phase of the output beam of the device, causing some small-size bright spots with high intensity to appear during subsequent propagation, resulting in damage to the optical elements due to the local light intensity exceeding the damage threshold. In addition, the occurrence of laser damage to each optical element in the laser system has an obvious cascading effect. After a laser damage occurs to an optical element, the diffraction effect at the damage point will cause more downstream elements to be damaged, resulting in a rapid decline in the load capacity of the laser device. Therefore, timely detection of laser damage to optical elements can repair them when the damage size is small to avoid irreversible damage and cascading damage. However, due to the complexity of high-power laser damage, there is no ideal measurement method to perform high-precision three-dimensional measurement of the laser damage of optical elements quantitatively so far.

[0003] Currently, the most commonly used damage detection method in the field of high-power lasers is the dark-field imaging method. The optical path of the dark-field imaging method is simple and easy to use. The damage of optical elements can be detected online by using a hand-held light source and visual observation, so it is widely used in the field of high-power lasers. However, the dark-field imaging method can only determine the number, approximate position and size of laser damage points in the optical element, but cannot accurately locate the damage, nor can it quantitatively measure the changes in the transmission intensity and transmission wavefront of the element caused by the damage. Therefore, the dark-field imaging method can only be used as a visual observation tool to check the state of the element, but cannot be used to accurately evaluate the damage situation. Traditional measuring instruments such as Hartmann wavefront sensors and interferometers can measure the phase change of transmitted light. In theory, they can measure the changes in transmittance and transmitted wavefront near the laser damage point of the optical element in combination with the intensity imaging method. However, the resolution of the Hartmann sensor is too low to be applicable to the detection of small-size laser damage; while the interferometer has high precision and resolution, but it is inconvenient to use due to its large volume and high environmental requirements. Moreover, both of these two measurement techniques are based on two-dimensional measurement and cannot provide the three-dimensional distribution information of laser damage of optical elements.

[0004] CN102156133A discloses a three-dimensional measurement method for high-power laser bulk damage of KDP crystals. This method rotates the KDP sample at multiple angles to obtain two-dimensional phase distributions at different viewing angles, and then uses the inverse Fourier transform method to reconstruct the three-dimensional distribution. However, in essence, it is composed of a clear image at the focal position and blurred images at other positions, so it is essentially impossible to obtain an accurate clear image, and thus the reconstructed three-dimensional distribution is also inaccurate. CN111879708A provides a device and method for measuring the three-dimensional structure of laser damage. By collecting four interference phase-shift images at each depth position and using the four-step phase-shift method to solve the phase to obtain the phase distribution at the corresponding depth position, a set of phase distributions at different depth positions is obtained, thereby reconstructing the three-dimensional phase distribution. This method has a high precision, but because it uses the four-step phase-shift method, a piezoelectric ceramic needs to be added to change the phase shift amount of the reference optical path, and the device is relatively complex. Summary of the Invention

[0005] In order to reduce the complexity of the measurement device as much as possible while ensuring the accuracy of three-dimensional laser damage measurement, the present invention provides a laser damage three-dimensional measurement device and method that are convenient and simple to use and have high measurement accuracy, which can measure the three-dimensional distributions of the transmittance and refractive index near the laser damage point of an optical element, providing a parameter basis for the laser damage repair of the optical element.

[0006] A three-dimensional measurement device for laser damage of an optical element, the device includes a laser 1, a beam splitter 2 is placed along the direction of the laser beam emitted by the laser 1, and the beam splitter 2 divides the laser beam into a reflected beam and a transmitted beam;

[0007] A first optical attenuator 3 and a first mirror 4 are sequentially arranged along the direction of the reflected beam; the first mirror 4 is used to adjust the direction of the reflected beam to be parallel to the direction of the transmitted beam, and a first spatial filter 5, a first collimating lens 6 and a second mirror 7 are sequentially arranged along the adjusted direction of the reflected beam, and an imaging device 8 is arranged according to the direction of the beam reflected by the second mirror 7;

[0008] A second optical attenuator 9, a second spatial filter 10, a second collimating lens 11, a focusing lens 12, a small hole aperture 13, a first long working distance objective lens 14, and a beam splitting prism 15 are sequentially arranged along the direction of the transmitted beam. Translation stages 16 and second long working distance objective lenses 17 are arranged in two directions after the beam splitting prism 15 splits the light, and the position of the imaging device 8 is adjusted so that the imaging device 8 can receive the beam passing through the second long working distance objective lens 17; the translation stage 16 is used to place the optical element to be measured;

[0009] The first spatial filter 5, the first collimating lens 6, the second spatial filter 10, the second collimating lens 11, the focusing lens 12, the small aperture stop 13, the first long working distance objective lens 14, the beam splitting prism 15, and the second long working distance objective lens 17 are all perpendicular to the laser beam and their centers are kept on the optical axis.

[0010] Optionally, during the measurement process, the optical element to be measured is fixed on the translation stage 16, and the translation stage 16 moves at equal intervals along a one-dimensional direction in a plane perpendicular to the axial direction. The imaging device 8 records a set of corresponding holographic interference patterns when the translation stage 16 moves.

[0011] Optionally, the device further includes a plane mirror; during the measurement process, the optical element to be measured is replaced with the plane mirror and fixed on the translation stage 16, and the imaging device 8 records a holographic interference pattern of the illumination light.

[0012] Optionally, the focal lengths of the first long working distance objective lens 14 and the second long working distance objective lens 17 are not less than 10 mm, and the working distances are not less than 30.5 mm.

[0013] Optionally, the diameter of the small aperture stop 13 is not greater than 5 mm.

[0014] Optionally, the minimum pixel unit of the imaging device 8 is less than or equal to 7.4 μm, and the resolution is at least 2048×2048.

[0015] Optionally, the translation stage 16 is an electrically controlled translation stage.

[0016] This application also provides a three-dimensional measurement method for laser damage of an optical element. The method regards the optical element to be measured as being composed of several layers of "optical slices", where each layer of "optical slice" has the same depth in the optical axis direction and has uniform refractive index and intensity transmittance at the same time. Based on the above three-dimensional measurement device for laser damage of an optical element, by fixing the optical element to be measured on the translation stage 16 and moving it at equal intervals, the complex amplitude information of each layer of "optical slice" is obtained, and the three-dimensional distribution of laser damage of the optical element to be measured is obtained according to the complex amplitude information of each layer of "optical slice".

[0017] Optionally, the method includes:

[0018] Step 1: Turn on the laser 1. The optical element to be measured is fixed on the translation stage 16, and the translation stage 16 moves at equal intervals along a one-dimensional direction in a plane N times, where N is an odd number. The imaging device 8 records a corresponding set of holographic interference patterns, and the intensity is marked as I(x, y; n), where n = 1, 2, … N; where x and y represent the spatial coordinate distribution of the interference pattern collected by the imaging device 8 after the nth movement.

[0019] The distance of each equal-spacing movement is fixed as the minimum pixel unit of the imaging device 8;

[0020] Step 2: Perform Fourier transform on the holographic interferogram, Denote the Fourier transform of I(x, y; n) as f In is the spectrum after Fourier transform, including the +1 order spectrum f In1 and the -1 order spectrum f In2 ;

[0021] Step 3: Set a zero matrix f Inc , with the same matrix size as f In . Take out the +1 order spectrum f In1 from f In , and replace the area in the central region of f Inc with the same size as f In1 with f In1 to obtain the updated spectrum matrix f' Inc ;

[0022] Step 4: Diffract the updated spectrum matrix f' Inc over the transmission distance d to the surface of the optical element to be measured, and the reflected complex amplitude distribution on the surface of the optical element to be measured is Denote the diffraction over the transmission distance d for f' Inc ;

[0023] Step 5: Replace the optical element to be measured with a plane mirror and fix it on the translation stage 16. The imaging device 8 records a holographic interferogram, with the intensity marked as I illu (x, y);

[0024] Step 6: Use the same method as in Steps 2 to 4 to obtain the complex amplitude distribution of the illumination light, and mark it as the illumination complex amplitude distribution on the surface of the optical element to be measured as E illu_1 (x, y);

[0025] Step 7: Diffract the illumination complex amplitude E illu_1 (x, y) over the transmission distance k*Δz to obtain the illumination complex amplitude distribution of the k-th layer of the optical element as Δz represents the distance between adjacent "optical slices";

[0026] Step 8: Remove the illumination light of the complex amplitude distribution E(x, y; n) on the surface of the optical element to be measured at the k-th layer of the optical element, and obtain R(x, y; n) = E(x, y; n) / E illu_k (x, y) (n = 1, 2,... N);

[0027] Step 9: Taking the complex amplitude of R(x, y; (N + 1) / 2) as the reference region, translate R(x, y; n) (n = 1, 2, … N) along one-dimensional direction by ((N + 1) / 2 - n) pixels to obtain R’(x, y; n), where n = 1, 2, … N, so as to ensure that R’(x, y; n) corresponds to the same region of the optical element to be measured;

[0028] Step 10: Accumulate and sum R’(x, y; n), and the complex amplitude information of the k-th layer of the optical element to be measured is obtained as

[0029]

[0030] Step 11: Repeat Steps 7 to 10 to obtain the complex amplitude information of each layer of the optical element to be measured, and further obtain the imaging results of the optical element to be measured at different depths, so as to obtain the three-dimensional distribution of laser damage of the optical element to be measured.

[0031] The beneficial effects of the present invention are as follows:

[0032] (1) The three-dimensional measurement device and method for laser damage of optical elements disclosed by the present invention can effectively separate the complex amplitude information of the laser-damaged optical element by depth, so as to realize three-dimensional measurement, and has the advantages of simple device structure, convenient measurement and high measurement accuracy.

[0033] (2) The three-dimensional measurement device and method for laser damage of optical elements disclosed by the present invention are applicable to optical elements such as lenses and crystals, and can accurately measure the transmittance change, refractive index change and three-dimensional morphology of the laser damage point and its surrounding area of such optical elements, which is helpful for analyzing the cause of optical element damage and can provide parameter basis for laser damage repair of optical elements. Description of the Drawings

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0035] Figure 1 It is a diagram of the three-dimensional measurement device for laser damage of optical elements of the present invention;

[0036] Among them, 1 - laser, 2 - beam splitter, 3 - first optical attenuator, 4 - first mirror, 5 - first spatial filter, 6 - first collimating lens, 7 - second mirror, 8 - imaging device, 9 - second optical attenuator, 10 - second spatial filter, 11 - second collimating lens, 12 - focusing lens, 13 - aperture stop, 14 - first long working distance objective lens, 15 - beam splitting prism, 16 - translation stage, 17 - second long working distance objective lens. Detailed implementation mode

[0037] To make the objectives, technical solutions and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

[0038] Embodiment 1:

[0039] This embodiment provides a three-dimensional measurement device for laser damage of optical elements. Refer to Figure 1 , the device includes: a laser 1, a beam splitter 2 is placed along the laser beam direction of the laser, and the beam splitter 2 divides the beam into a reflected beam and a transmitted beam;

[0040] A first optical attenuator 3, a first mirror 4, a first spatial filter 5, a first collimating lens 6 and a second mirror 7 are sequentially arranged along the reflected beam direction; an imaging device 8 is arranged according to the direction of the beam after being reflected by the second mirror 7;

[0041] A second optical attenuator 9, a second spatial filter 10, a second collimating lens 11, a focusing lens 12, an aperture stop 13, a first long working distance objective lens 14, a beam splitting prism 15, a translation stage 16 and a second long working distance objective lens 17 are sequentially arranged along the transmitted beam direction. The position of the imaging device 8 is adjusted so that the imaging device 8 can receive the beam passing through the second long working distance objective lens 17; the translation stage 16 is used to place the optical element to be measured;

[0042] The imaging device 8 and the translation stage 16 are both connected to a computer, so as to accurately control the translation stage 16 to move at equal intervals through the computer subsequently, and the imaging device 8 correspondingly records holograms.

[0043] All the above optical elements are perpendicular to the laser beam and their centers are kept on the optical axis. The focal lengths of the first and second long working distance objective lenses are at least 10 mm and the working distances are at least 30.5 mm. The diameter of the aperture stop is not greater than 5 mm, and the corresponding aperture stop can be selected according to the actual situation, such as an aperture stop with a diameter of 2 mm. The minimum pixel unit of the imaging device 8 is less than or equal to 7.4 microns and the resolution is at least 2048×2048.

[0044] The working process of the device is as follows: Turn on the laser 1, place the optical element to be measured on the translation stage 16, adjust the first optical attenuator 3 and the second optical attenuator 9 to appropriate positions, and control the translation stage 16 to translate at equal intervals along a one-dimensional direction in a plane perpendicular to the axial direction ( Figure 1 in the z direction in Figure 1 is the axial direction) by a computer. The imaging device 8 records a set of holograms related to the optical element; then, replace the optical element to be measured with a plane mirror fixed on the translation stage 16, and the imaging device 8 records a hologram related to the illumination light. The collected holographic data can be used for measuring the three-dimensional distribution of laser damage to the optical element.

[0045] The complex amplitude information can reflect the structural information of the object. Therefore, the three-dimensional distribution of laser damage to the optical element can be reflected by calculating the complex amplitude of each 'layer' slice.

[0046] Embodiment 2

[0047] This embodiment provides a method for three-dimensional measurement of laser damage to an optical element. The method is based on the measurement method of the three-dimensional measurement device for laser damage to an optical element provided in Embodiment 1. The method includes the following steps:

[0048] 1) Turn on the laser 1 as described above. The optical element with damage to be measured is fixed on the translation stage 16. The translation stage 16 moves N times at equal intervals along a one-dimensional direction in a plane, and the imaging device 8 records a corresponding set of holographic interference patterns with the intensity marked as I(x, y; n) (n = 1, 2,... N); as Figure 1 shown, the translation stage 16 moves at equal intervals along the x or y direction in a plane, and the distance of each equal interval movement is fixed as the minimum pixel unit of the imaging device 8.

[0049] 2) Perform a Fourier transform on the holographic interference pattern, indicating performing a Fourier transform on I(x, y; n), and f In is the spectrum after the Fourier transform, including the +1 order spectrum f In1 and the -1 order spectrum f In2 ;

[0050] 3) Set a zero matrix f Inc , with the matrix size being the same as that of f In . Take out the +1 order spectrum f In1 from f In , and replace the area in the central region of f Inc with the same size as that of f In1 with f In1 to obtain an updated spectrum matrix f' Inc ;

[0051] 4) The spectrum matrix f' IncThe transmission distance d is to the surface of the optical element to be measured for damage, and the complex amplitude distribution of the reflected light on the surface of the optical element is obtained as E(x, y; n) = F d {f’ Inc}(n = 1, 2, … N), indicating the diffraction transmission distance d for f’ Inc ;

[0052] 5) Replace the optical element with a plane mirror and fix it on the translation stage 16. The imaging device 8 records a holographic interferogram, and the intensity is marked as I illu (x, y);

[0053] 6) Use the same method as in 2)-4) to obtain the complex amplitude distribution of the illumination light, which is marked as E illu_1 (x, y) as the complex amplitude distribution of the illumination light on the upper surface of the optical element;

[0054] 7) Diffract the illumination light complex amplitude E illu_1 (x, y) obtained in 6) by a transmission distance of k * Δd millimeters to obtain the illumination light complex amplitude distribution of the k-th layer of the optical element as

[0055] 8) Remove the illumination light of the reflected light complex amplitude E(x, y; n) of the optical element on the k-th layer of the optical element to obtain R(x, y; n) = E(x, y; n) / E illu_k (x, y) (n = 1, 2, … N);

[0056] 9) Taking the complex amplitude of R(x, y; (N + 1) / 2) as the reference area, translate R(x, y; n) (n = 1, 2, … N) along the one-dimensional (x) direction by ((N + 1) / 2 - n) pixels to obtain R’(x, y; n) (n = 1, 2, … N), so as to ensure that R’(x, y; n) corresponds to the same area of the object;

[0057] 10) Accumulate and sum R’(x, y; n) to obtain the complex amplitude information of the k-th layer of the glass optical element as

[0058]

[0059] 11) Repeat steps 7)-10) to obtain the complex amplitude information of each layer of the optical element to be measured, and then obtain the imaging results of the optical element to be measured at different depths, and obtain the three-dimensional distribution of the laser damage of the optical element to be measured.

[0060] Some steps in the embodiments of the present invention can be implemented by software, and the corresponding software program can be stored in a readable storage medium, such as an optical disc or a hard disk, etc.

[0061] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A three-dimensional measuring device for laser damage of an optical element, characterized in that, The device includes a laser (1), and a beam splitter (2) is placed along the direction of the laser beam emitted by the laser (1). The beam splitter (2) divides the laser beam into a reflected beam and a transmitted beam; A first optical attenuator (3) and a first mirror (4) are sequentially arranged along the direction of the reflected beam; the first mirror (4) is used to adjust the direction of the reflected beam to be parallel to the direction of the transmitted beam. A first spatial filter (5), a first collimating lens (6), and a second mirror (7) are sequentially arranged along the adjusted direction of the reflected beam. An imaging device (8) is arranged according to the direction of the beam after reflection by the second mirror (7); A second optical attenuator (9), a second spatial filter (10), a second collimating lens (11), a focusing lens (12), a pinhole aperture (13), a first long working distance objective lens (14), and a beam splitting prism (15) are sequentially arranged along the direction of the transmitted beam. A translation stage (16) and a second long working distance objective lens (17) are arranged in two directions after the beam splitting prism (15) splits the beam, and the position of the imaging device (8) is adjusted so that the imaging device (8) can receive the beam passing through the second long working distance objective lens (17); the translation stage (16) is used to place the optical element to be measured; The first spatial filter (5), the first collimating lens (6), the second spatial filter (10), the second collimating lens (11), the focusing lens (12), the pinhole aperture (13), the first long working distance objective lens (14), the beam splitting prism (15), and the second long working distance objective lens (17) are all perpendicular to the laser beam and their centers are kept on the optical axis.

2. The three-dimensional measurement device for laser damage of an optical element according to claim 1, characterized in that, During the measurement process of the device, the optical element to be measured is fixed on the translation stage (16). The translation stage (16) moves equidistantly along a one-dimensional direction in a plane perpendicular to the axial direction. The imaging device (8) records a set of corresponding holographic interference patterns when the translation stage (16) moves.

3. The three-dimensional laser damage measurement device for an optical element according to claim 1, characterized in that, The device further includes a plane mirror; during the measurement process, the optical element to be measured is replaced with the plane mirror and fixed on the translation stage (16), and the imaging device (8) records a holographic interference pattern of the illumination light.

4. The three-dimensional measurement device for laser damage of an optical element according to claim 1, wherein, The focal lengths of the first long working distance objective lens (14) and the second long working distance objective lens (17) are not less than 10 mm, and the working distances are not less than 30.5 mm.

5. The three-dimensional measurement device for laser damage of an optical element according to claim 1, wherein The diameter of the pinhole aperture (13) is not greater than 5 mm.

6. The three-dimensional measurement device for laser damage of an optical element according to claim 1, characterized in that, The minimum pixel unit of the imaging device (8) is less than or equal to 7.4 microns, and the resolution is at least 2048×2048.

7. The three-dimensional measurement device for laser damage of an optical element according to claim 1, wherein The translation stage (16) is an electrically controlled translation stage.

8. A three-dimensional measurement method for laser damage of an optical element, characterized in that The method regards the optical element to be measured as being composed of several "optical slices", where each "optical slice" has the same depth in the optical axis direction and simultaneously has a uniform refractive index and intensity transmittance. The method is based on the three-dimensional laser damage measurement device for an optical element according to any one of claims 1-7. By fixing the optical element to be measured on the translation stage (16) and moving it at equal intervals, the complex amplitude information of each "optical slice" is obtained, and the three-dimensional laser damage distribution of the optical element to be measured is obtained according to the complex amplitude information of each "optical slice".

9. The method according to claim 8, wherein The method includes: Step 1: Turn on the laser (1). The optical element to be measured is fixed on the translation stage (16). The translation stage (16) moves N times at equal intervals along a one-dimensional direction in a plane. N is an odd number. The imaging device (8) records a corresponding set of holographic interference patterns, with the intensity marked as I(x, y; n), where n = 1, 2,..., N; among them, x and y represent the spatial coordinate distribution of the interference pattern collected by the imaging device (8) after the nth movement; the distance of each equal-interval movement is fixed as the minimum pixel unit of the imaging device (8). Step 2: Perform a Fourier transform on the holographic interferogram, which means performing a Fourier transform on I(x, y; n), and f In is the spectrum after the Fourier transform, including the +1 order spectrum f In1 and the -1 order spectrum f In2 ; Step 3: Set a zero matrix f Inc , with the same matrix size as f In . Take out the +1 level spectrum f In1 from f In , and replace the area in the central region of f Inc with the same size as f In1 using f In1 to obtain the updated spectrum matrix f' Inc ; Step 4: The updated spectral matrix f' Inc is transmitted to the surface of the optical element to be measured at a transmission distance d, and the reflected complex optical amplitude distribution on the surface of the optical element to be measured is obtained as denotes the diffraction transmission distance d for f' Inc at a diffraction transmission distance d; Step 5: Replace the optical element to be measured with a plane mirror and fix it on the translation stage (16). The imaging device (8) records a holographic interferogram with the intensity marked as I illu (x, y); Step 6: Obtain the complex amplitude distribution of the illumination light using the same method as in Steps 2 to 4, and mark it as the complex amplitude distribution of the illumination light on the surface of the optical element to be measured, denoted as E illu_1 (x, y); Step 7: Convert the illumination light amplitude E obtained in step 6 to illu_1 The (x,y) diffraction transmission distance k*Δz, the complex amplitude distribution of the illumination light of the kth layer of the optical element is obtained as follows: Δz represents the distance between adjacent "optical sections"; Step 8: Remove the illumination light of the complex amplitude distribution E(x, y; n) on the k-th layer of the optical element from the surface of the optical element to be measured, and obtain R(x, y; n) = E(x, y; n) / E illu_k (x, y); Step 9: Taking the complex amplitude of R(x, y; (N + 1) / 2) as the reference region, translate R(x, y; n) along the one-dimensional direction by ((N + 1) / 2 - n) pixels to obtain R'(x, y; n), so as to ensure that R'(x, y; n) corresponds to the same region of the optical element to be measured. Step 10: Accumulate and sum up R’(x, y; n) to obtain the complex amplitude information of the k-th layer of the optical element to be measured as Step 11: Repeat steps 7 to 10 to obtain the complex amplitude information of each layer of the optical element to be measured, and then obtain the imaging results of the optical element to be measured at different depths, and obtain the three-dimensional distribution of the laser damage of the optical element to be measured.

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