A laser-damage resistant optical reflective film system and its design method

Abstract

This invention discloses a laser-damage-resistant optical reflective film system and its design method. The invention proposes adding an electric field modulation layer to the outside of an initial quarter-wavelength-thick reflective film system. By constructing a relationship between the physical thickness and refractive index of the newly added electric field modulation layer, the internal electric field of the thin film is optimized. For the reflective film system with the added electric field modulation layer, a model is established to determine the peak electric field when the nodule defect is present. Then, a particle swarm optimization algorithm is used to obtain the refractive index of the newly added electric field modulation layer when the peak electric field is minimized in the defective model. Finally, the physical thickness of the newly added electric field modulation layer is calculated based on the obtained refractive index. This invention solves the limitations of current laser-damage-resistant optical reflective film systems that only consider the electric field distribution or only consider the nodule defect during the design process.

Description

Technical Field

[0001] This invention belongs to the field of optical thin film design, specifically relating to an optical reflective film system and design method that resists laser damage. Background Technology

[0002] In laser systems, optical thin-film elements are crucial components for laser generation, conversion, and transmission, but they are also highly susceptible to damage. Damage to the optical thin film in a laser system affects the system's lifespan and stability, and the laser damage threshold of the optical thin-film element limits the output power of the laser system. Therefore, the design and fabrication of optical thin films for laser systems must consider not only spectral characteristics but, more importantly, the film's resistance to laser damage. Extensive research has shown that the internal electric field distribution of the thin film affects its resistance to laser damage; electric field peaks located in layers with low extinction coefficients and low electric fields at the film interface often exhibit higher resistance to laser damage. Simultaneously, the presence of nodular defects within the thin film enhances the internal electric field, reducing its resistance to laser damage.

[0003] A design method for the electric field distribution of a tilted-incident high-reflectivity thin-film laser, authorized by announcement number CN104330895B, achieves optimal distribution of the laser electric field by pre-setting the extinction coefficient of the thin-film material and numerically optimizing the absorptivity of the high-reflectivity film structure to minimize the absorptivity, thus avoiding strong electric field distribution at the thin-film interface. Another method, authorized by announcement number CN104032266B, improves the laser damage threshold of near-infrared high-reflectivity films by determining the maximum incident angle and designing a reflective film system with a certain angle width to reduce the electric field in nodules and improve the reflective film's resistance to laser damage. Both methods are simple and easy to operate, and the designed reflective film systems can achieve good electric field distribution or insensitivity to defects, thereby improving the thin film's resistance to laser damage. However, certain limitations remain. If only the electric field distribution is considered, the presence of nodule defects within the reflective film during the deposition process can enhance the electric field, as the increased peak electric field leads to a decrease in the laser damage threshold of the thin film. Conversely, if only the influence of nodule defects is considered, the peak electric field in a reflective film without nodule defects may occur at the weak film interface, resulting in reduced laser damage resistance. Current technology cannot simultaneously achieve a designed reflective film that possesses both a good electric field distribution and insensitivity to defects. Summary of the Invention

[0004] The purpose of this invention is to address the limitations of current anti-laser damage optical reflective film systems that only consider electric field distribution or nodule defects during the design process, and to propose a design method for anti-laser damage optical reflective film systems.

[0005] This invention proposes adding an electric field modulation layer to the outside of an initial quarter-wavelength-thickness reflective film system. By constructing a relationship between the physical thickness and refractive index of the newly added electric field modulation layer, the internal electric field of the thin film is optimized. For the reflective film system with the added electric field modulation layer, a model with nodule defects is established in electromagnetic simulation software to obtain the peak electric field when nodule defects are present. Then, a particle swarm optimization algorithm is used to obtain the refractive index of the newly added electric field modulation layer when the peak electric field is minimized in the defective model, thereby reducing the impact of nodule defects on the peak field strength. The physical thickness of the newly added electric field modulation layer is then calculated based on the obtained refractive index. This invention uses the refractive index of the electric field modulation layer as a bridge to connect the electric field distribution problem and the electric field enhancement problem caused by nodule defects in the design process of reflective film systems, solving the limitations caused by current laser-damage-resistant optical reflective film systems that only consider electric field distribution or nodule defects in the design process.

[0006] The reflective film system designed in this invention has both a good electric field distribution, with the peak electric field avoiding the film interface and residing in the low-refractive-index film layer, while the electric field decreases in the high-refractive-index film layer; at the same time, it is insensitive to nodule defects, reducing the electric field enhancement caused by nodule defects. It is mainly used for the design of reflective films with 0-degree incident light in laser systems.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A method for designing an optical reflective film system resistant to laser damage, characterized by the following steps:

[0009] 1) Design the initial membrane structure using TFCale membrane system design software: Sub|(HL)^ n H|Air;

[0010] H is a high refractive index film layer with a thickness of one-quarter wavelength, L is a low refractive index film layer with a thickness of one-quarter wavelength, n is the number of times the initial reflective film stack is stacked, Sub refers to the substrate side, and Air refers to the air side.

[0011] To ensure that the final designed film system's reflectivity meets the requirements, the initial film system's reflectivity must first be guaranteed to meet the requirements. Therefore, the specific value of the number of stacking times, n, is determined by the reflectivity requirements.

[0012] 2) Set m pairs of electric field modulation layers on the outer side of the initial membrane system designed in step 1);

[0013] The electric field modulation layer is composed of alternating low-refractive-index and high-refractive-index film layers.

[0014] The pair of electric field control layers consists of a low-refractive-index film and a high-refractive-index film. m represents the number of pairs of electric field control layers, where 1 ≤ m ≤ 4.

[0015] The low-refractive-index film layer in the electric field control layer is preferably a SiO2 film layer.

[0016] The refractive indices of the high-refractive-index films in the electric field modulation layer are n1, n2, ..., n. m .

[0017] The physical thickness of the low-refractive-index film in the newly added i-th pair of electric field modulation layers is determined by the following formula.

[0018]

[0019] The physical thickness of the high-refractive-index film in the newly added i-th pair of electric field modulation layers is determined by the following formula.

[0020]

[0021] In the above formula, and Let represent the physical thicknesses of the low-refractive-index film and the high-refractive-index film in the newly added i-th pair of electric field control layers, respectively, 1≤i≤m≤4; To determine the physical thickness of the high-refractive-index film in the (i-1)th pair of electric field modulation layers, when i = 1, n is the physical thickness of the outermost high-refractive-index layer in the initial film; n' is the refractive index of the low-refractive-index SiO2 layer in the electric field-controlled layer; n i The refractive index of the high-refractive-index film in the newly added i-th pair of electric field modulation layers; n i-1 Let n be the refractive index of the high-refractive-index film in the (i-1)th pair of electric field modulation layers. When i = 1, n i-1 λ is the refractive index of the outermost high-refractive-index layer in the initial film; λ is the reference wavelength of the reflectivity film system.

[0022] The physical thickness of the electric field control layer is constrained by the formula, so that the electric field inside the film is initially optimized and an initially optimized electric field control layer is obtained.

[0023] The physical thickness of the newly added electric field control layer in the formula is related to the refractive index of the newly added electric field control layer.

[0024] 3) Based on the initial film system designed in step 1) and the preliminary optimized electric field control layer obtained in step 2), a new reflective film system is obtained. The electric field distribution of the reflective film system without nodule defects is obtained by simulating the reflective film system in electromagnetic simulation software. A model of the film system with nodule defects is established in electromagnetic simulation software to obtain the peak electric field in the film layer with nodule defects.

[0025] The formula for the nodule defect model is D = sqrt(cdt), where D is the surface diameter of the nodule defect, d is the seed diameter, t is the film thickness, and c is a parameter related to the actual coating. In this paper, c is taken as 8 to represent the ideal nodule defect model.

[0026] The electromagnetic simulation software is LumericalFDTD.

[0027] 4) Using the refractive index of the film layer in the electric field control layer as a bridge, the electric field distribution of the reflective film system without nodule defects is linked to the peak electric field enhancement in the film layer with nodule defects. The refractive index range of the high refractive index film material is set, and the refractive index of the high refractive index film layer in the electric field control layer when the peak electric field is minimized is determined by the particle swarm optimization algorithm.

[0028] The high refractive index film material is obtained by doping with one or more materials with high damage threshold or high thermal conductivity, and is further selected as Al2O3, Ta2O5, HfO2, or Nb2O5.

[0029] The refractive index range of the high refractive index film material is from the minimum refractive index to the maximum refractive index among the selected high refractive index materials.

[0030] 5) Based on the refractive index of the high-refractive-index film in the electric field control layer determined in step 4), substitute it into the physical thickness formula in step 2) to calculate the thickness of the electric field control layer, and obtain the optimized electric field control layer. The initial film system designed in step 1) and the optimized electric field control layer form an optical reflection film system that resists laser damage.

[0031] The resulting reflective film system has both a good electric field distribution and is insensitive to defects, thus improving the film's resistance to laser damage.

[0032] The high-reflectivity film system designed in this invention not only meets the requirements in terms of spectral characteristics, but also has a good electric field distribution inside the film when there are no nodule defects, with the peak electric field avoiding the film interface and the electric field of the high refractive index film being relatively small; when nodule defects are present, it is not sensitive to defects, thereby improving the film's resistance to laser damage.

[0033] A laser-damage-resistant optical reflective film system comprises an initial film system and an electric field modulation layer. Let λ = 550 nm, the reflective film system structure is obtained through steps 1) to 5). The initial film system structure is Sub|(HL)^ 6H|Air, the high refractive index film material H is selected as Ta2O5, and the low refractive index film material L is selected as SiO2. The thickness of each layer from the substrate side to the air side is as follows: 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm, 94.18nm, 61.59nm. Among them, the odd-numbered layers are Ta2O5 films, and the even-numbered layers are SiO2 films. The electric field control layer consists of 3 pairs of 6 layers, with odd-numbered layers being low-refractive-index SiO2 films and even-numbered layers being high-refractive-index films. The refractive indices of the high-refractive-index films are 2.18, 2.36, and 2.36, respectively. The thicknesses of each layer from the initial film system side to the air side are 145.63 nm, 37.08 nm, 154.58 nm, 28.05 nm, 162.08 nm, and 24.05 nm, respectively.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] Existing technologies can optimize the electric field within a film system, allowing the electric field peak to avoid the film interface and reside in the low-refractive-index film, while reducing the electric field in the high-refractive-index film. Alternatively, the influence of nodule defects on electric field enhancement can be reduced through film system design. However, these methods only consider one aspect and have limitations. Existing technologies rarely simultaneously consider both the distribution of the electric field in the reflective film system and the enhancement of the peak electric field within the film caused by nodule defects. This invention optimizes the internal electric field by adding an electric field modulation layer outside the initial quarter-wavelength thickness film system and establishing a relationship between the physical thickness and refractive index of the film layer within the electric field modulation layer. An electromagnetic simulation software model is then used to establish a model of the film system with nodule defects after the addition of the electric field modulation layer. The physical thickness of the electric field modulation layer in the model is constrained by a derived thickness formula. In the electromagnetic simulation software, a particle swarm optimization algorithm is used to determine the refractive index and thickness of the high-refractive-index film layer in the electric field modulation layer when the peak electric field is minimized. This invention uses the refractive index of the film layer in the newly added electric field modulation layer as a bridge to connect the electric field distribution problem in the design process of reflective film systems with the electric field enhancement problem caused by nodule defects. The reflective film system designed by this invention has both good electric field distribution and insensitivity to nodule defects. It is more comprehensive, simple, easy to operate, and has significant effects. Attached Figure Description

[0036] Figure 1 This is a graph showing the internal electric field distribution when the initial membrane system has no nodule defects.

[0037] Figure 2 This is a schematic diagram of the structure of an anti-laser damage optical reflective film system according to the present invention.

[0038] Figure 3This is a curve showing the internal electric field distribution of an anti-laser damage optical reflective film system of the present invention when there are nodules or defects.

[0039] Figure 4 This is a graph showing the internal electric field distribution of the initial film system and the laser-damage-resistant optical reflective film system of the present invention when there are nodular defects. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, but the present invention is not limited thereto.

[0041] We chose to design a 550nm high-reflectivity film. First, we set the initial film structure: Sub|(HL)^ 6 H|Air; The high-refractive-index film material H is selected as Ta2O5 (refractive index 2.2325@550nm), and the low-refractive-index film material L is selected as SiO2 (refractive index 1.4599@550nm). The following parameters are entered into the film system design software TFCale: incident angle 0°, reference wavelength 550nm, and initial film system structure Sub|(HL)^ 6 The H|Air initial film system consists of 13 layers, with the thicknesses of each layer from the substrate side to the air side as follows: 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm. At this point, the reflectivity of the film system at 550 nm is 99.25%. The internal electric field distribution of the initial film system without defects is as follows... Figure 1 As shown, the peak electric field is at the film interface, and the maximum electric field in the high refractive index material is 0.79959.

[0042] Three pairs of electric field modulation layers, totaling six layers, were set on the outer side of the initial membrane system. The structure of the membrane system after setting the electric field modulation layers is as follows: Figure 2 As shown. The low-refractive-index material in the electric field modulation layer is SiO2 (refractive index 1.4599@550nm), and the high-refractive-index materials have refractive indices n1, n2, and n3. Substituting these values ​​into the formula, the film thickness of the electric field modulation layer is:

[0043]

[0044]

[0045]

[0046]

[0047]

[0048]

[0049] In FDTD, a film system model with nodule defects is established. The thickness of the electric field control layer is constrained by the aforementioned formula, with the nodule seed diameter set to 200 nm. An electric field monitor is used to monitor the electric field in the film layer to obtain the peak electric field when defects are present. Then, the refractive indices n1, n2, and n3 of the high-refractive-index film in the electric field control layer are considered as variables. Since the high-refractive-index film is obtained by doping two or more materials from Al2O3 (refractive index 1.77@550nm), Ta2O5 (refractive index 2.2325@550nm), HfO2 (refractive index 1.99@550nm), and Nb2O5 (refractive index 2.36@550nm), the range of these indices is 1.77–2.36. Then, a particle swarm optimization algorithm is used to optimize the calculation to find that the refractive indices of the high-refractive-index film in the electric field control layer are 2.18, 2.36, and 2.36 when the peak electric field is minimized.

[0050] Substituting n1 = 2.18, n2 = 2.36, and n1 = 2.36 into the above formula for the physical thickness of the film, the physical thicknesses of the films in the electric field control layer from the initial film system side to the air side are 145.63 nm, 37.08 nm, 154.58 nm, 28.05 nm, 162.08 nm, and 24.05 nm, respectively. Among them, the odd-numbered layers are low-refractive-index SiO2 films, and the even-numbered layers are high-refractive-index films.

[0051] Therefore, the final physical thickness of the film system, from the substrate side to the air side, is as follows: 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 94.18 nm, 61.59 nm, 145.63 nm, 37.08 nm, 154.58 nm, 28.05 nm, 162.08 nm, and 24.05 nm. The odd-numbered layers are low-refractive-index SiO2 layers; the 2nd, 4th, 6th, 8th, 10th, and 12th layers are Ta2O5 layers; the 14th layer has a refractive index of 2.18; the 16th layer has a refractive index of 2.36; and the 18th layer has a refractive index of 2.36. At this point, the reflectivity of the film system at 550 nm is 99.74%.

[0052] Figure 3This is a graph showing the internal electric field distribution of a laser-damage-resistant optical reflective film system of the present invention without nodule defects. It can be seen that, compared to the initial film system's electric field distribution, the peak electric field shifts from the film interface to the SiO2 film layer, which has a higher laser damage threshold. Furthermore, the maximum electric field in the high-refractive-index layer decreases from 0.79959 in the initial film system to 0.27832, which to some extent improves the film's resistance to laser damage.

[0053] Figure 4 The graphs show the internal electric field distribution curves of the initial film system and the laser-damage-resistant optical reflective film system of the present invention when there are nodule defects. The dashed line represents the electric field distribution curve when there is a nodule defect with a diameter of 200 nm in the initial film system, at which point the peak electric field is 2.28725. The solid line represents the internal electric field distribution curve of the laser-damage-resistant optical reflective film system of the present invention when there is a 200 nm nodule defect, at which point the peak electric field is 1.79455. It can be seen that the film system designed in this invention can reduce the electric field enhancement caused by nodule defects and achieve sensitivity to defects.

[0054] In summary, the film system of the present invention can not only meet the reflectivity requirements, but also have a good electric field distribution when there are nodules or defects. The peak electric field is in the low refractive index film layer, and it is relatively insensitive to defects when there are nodules or defects.

Claims

1. A method for designing an optical reflective film system resistant to laser damage, characterized in that, Includes the following steps: 1) Design the initial membrane structure using TFCale membrane system design software: Sub|(HL)^ n H|Air; H is a high refractive index film layer with a thickness of one-quarter wavelength, L is a low refractive index film layer with a thickness of one-quarter wavelength, n is the number of times the initial reflective film stack is stacked, Sub refers to the substrate side, and Air refers to the air side. 2) Set m pairs of electric field modulation layers on the outer side of the initial membrane system designed in step 1); The electric field modulation layer is composed of alternating low-refractive-index and high-refractive-index film layers; The refractive indices of the high-refractive-index films in the electric field modulation layer are n1, n2, ..., n. m ; The physical thickness of the low-refractive-index film in the newly added i-th pair of electric field modulation layers is determined by the following formula; The physical thickness of the high-refractive-index film in the newly added i-th pair of electric field modulation layers is determined by the following formula; In the above formula, and Represent the physical thicknesses of the low-refractive-index film and the high-refractive-index film in the newly added i-th pair of electric field control layers, 1≤i≤m≤4; n' is the refractive index of the low-refractive-index SiO2 film in the electric field control layer; n i Let n be the refractive index of the high-refractive-index film in the i-th pair of electric field modulation layers; i-1 Let n be the refractive index of the high-refractive-index film in the (i-1)th pair of electric field modulation layers. When i = 1, n i-1 λ is the refractive index of the outermost high-refractive-index layer in the initial film; λ is the reference wavelength of the reflectivity film system. The physical thickness of the electric field control layer is constrained by the formula, so that the electric field inside the film layer is initially optimized, resulting in an initially optimized electric field control layer. 3) Based on the initial film system designed in step 1) and the preliminary optimized electric field control layer obtained in step 2), a new reflective film system is obtained. The electric field distribution of the reflective film system without nodule defects is obtained by simulating the reflective film system in electromagnetic simulation software. A model of the reflective film system with nodule defects is established in electromagnetic simulation software to obtain the peak electric field in the film layer with nodule defects. 4) Using the refractive index of the film layer in the electric field control layer as a bridge, the electric field distribution of the reflective film system without nodule defects is linked to the peak electric field enhancement in the film layer with nodule defects. The refractive index range of the high refractive index film material is set, and the refractive index of the high refractive index film layer in the electric field control layer when the peak electric field is minimized is determined by the particle swarm optimization algorithm. 5) Based on the refractive index of the high-refractive-index film in the electric field control layer determined in step 4), substitute it into the physical thickness formula in step 2) to calculate the thickness of the electric field control layer, and obtain the optimized electric field control layer. The initial film system designed in step 1) and the optimized electric field control layer form an optical reflection film system that resists laser damage.

2. The design method for laser-damage-resistant optical reflective film system according to claim 1, characterized in that, In step 2), the pair of electric field control layers consists of a low-refractive-index film and a high-refractive-index film.

3. The design method for an anti-laser damage optical reflective film system according to claim 1, characterized in that, In step 2), m is the logarithm of the electric field control layer, 1≤m≤4.

4. The design method for laser-damage-resistant optical reflective film system according to claim 1, characterized in that, In step 2), the low refractive index film in the electric field control layer is a SiO2 film.

5. The design method for an anti-laser damage optical reflective film system according to claim 1, characterized in that, In step 3), the formula for the nodule defect model is D = sqrt(cdt), where D is the surface diameter of the nodule defect, d is the seed diameter, t is the film thickness, and c is a parameter related to the actual coating.

6. The design method for an anti-laser damage optical reflective film system according to claim 1, characterized in that, In step 3), the electromagnetic simulation software is LumericalFDTD.

7. The design method for an anti-laser damage optical reflective film system according to claim 1, characterized in that, In step 4), the high refractive index film material is obtained by doping with one or more of Al2O3, Ta2O5, HfO2, and Nb2O5. The refractive index range of the high refractive index film material is from the minimum refractive index to the maximum refractive index among the selected high refractive index materials.

8. An optical reflective film system resistant to laser damage designed according to the design method of any one of claims 1 to 7.

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