Ultra-wideband low-absorption low-stress high-reflection film based on gradient chirp structure

Abstract

The invention provides an ultra-wideband low-absorption low-stress high-reflection thin film based on a gradual change chirp structure, and belongs to the technical field of optical thin films. Different from a mode of mutually combining and superposing high and low refractive index materials of a traditional structured film system to control the reflection bandwidth, the invention provides a logarithmic function gradient chirp structure, the reflectivity index is adjusted by adjusting the total number of film layers, and the reflection bandwidth is adjusted by adjusting head and tail optical thickness coefficients. The problem that the reflectivity is suddenly reduced in a multi-reflection-band superposition reflection gap area when the broadband high-reflection dielectric film is designed is effectively avoided, and the problem of surface distortion caused by thin film stress is reduced by adjusting the film thickness ratio of the stress regulation and control layer to the gradual change main reflection layer. From the perspective of membrane system design, the thickness of the membrane layer is reduced, the preparation period and difficulty are reduced, problems and risks caused by stress are reduced, membrane layer absorption is effectively reduced, and the absorption and stress can be controlled within a proper range in combination with a post-treatment means to effectively support safe and stable operation of the system.

Description

Technical Field

[0001] The invention belongs to the field of laser films, and in particular relates to a broadband high-reflection low-absorption low-stress laser film, and a stress compensation method for reducing and regulating the reflectivity reduction of an ultra-broadband reflective film at a reflection gap. Background Art

[0002] The rapid development of laser technology is inseparable from the promotion of optical thin film technology. Laser reflective film is one of the most important components in laser systems. Highly reflective film used to reduce the energy loss of reflected light in the resonant cavity. With the development of tunable laser technology applications, higher and higher requirements are put forward for the output of tunable lasers, such as wider bandwidth, stronger power, and higher and higher requirements for reflectivity and threshold. Reflective films are mainly divided into two categories: metal reflective films and dielectric reflective films: reflectors made of metal films have the advantages of low plating cost, thin film layer and low stress, but metal reflective films have problems such as large absorption loss and low reflectivity. Obviously, it cannot meet the requirements of high-performance optical systems. Traditional metal films or the combination of metal films and dielectric films cannot achieve high reflectivity and high resistance to laser damage. All-dielectric films are required to achieve a higher laser damage threshold. The reflection band is widened by superimposing reflective film stacks with different central wavelengths, but the film stacking method causes resonant absorption amplification at the junction wavelength of the film stack, resulting in a local depression in the reflection curve and a significant decrease in local reflectivity.

[0003] As the bandwidth of broadband high-reflection films becomes wider and wider, the film thickness will inevitably increase, which will further increase the film stress and lead to stress-induced surface distortion. In precision optical systems, especially large-aperture systems, it is usually required to achieve stable beam quality control over a long period of time, obtain excellent focal spot main lobe energy distribution, controllable side lobe energy distribution, and low light scattering, so there are strong restrictions on the amount of wavefront distortion. When the surface control accuracy of thin film elements is low, the output beam quality will be significantly reduced, and in severe cases, beam control will not be able to be achieved. The traditional broadband reflective film surface deviation control technology uses double-sided coating, and the film stack of the corresponding band λ / 4 wavelength is plated on the back to compensate for the stress problem on the front, but it can only coat the reflection bands of different bands on the back for stress compensation, and cannot achieve precise stress regulation. Or the same film system is plated on the back to achieve the effect of compensating stress (application number 202210072105.2). The present invention is an adjustable stress broadband high-reflection thin film structure, which is different from the original reflective film stack design and adopts a full-dielectric reflective film with a gradual thickness change of a logarithmic function. It is not subject to the hardness requirements of the reflective film stack and can directly select the number of film layers that need to be plated on the front and back surfaces according to the stress conditions, thereby accurately adjusting the film thickness on the front and back surfaces of the substrate to perform stress compensation. Summary of the invention

[0004] The present invention proposes a logarithmic function gradient thickness variation film system structure for the design of broadband high-reflection dielectric films, which effectively avoids the problem of local depression of reflectivity reduction in the reflection gap area caused by superimposing different central wavelength reflection film stacks. In terms of stress regulation, it is different from the traditional front and back surfaces plated with different central wavelength reflections to bring balanced stress. The overall thickness of the film system of the present invention presents a logarithmic function gradient thickness variation film system structure, which can achieve the effect of balanced stress by adjusting the film thickness ratio of the front and back surfaces of the substrate according to the required film thickness. Its characteristic is that the film system is separated at any position in the middle of the film system and the front and back surfaces of the substrate are plated for stress regulation, which effectively reduces the stress-induced surface distortion problem of broadband high-reflection films.

[0005] The solution of the present invention is as follows:

[0006] The ultra-wideband low-absorption, low-stress, high-reflection film based on the gradient chirp structure is characterized by comprising a main reflection gradient film layer, a substrate, and a stress-regulating gradient film layer. The film system structure is as follows:

[0007] AIR / y1Hy2L…y n-1 Hy n L) / SUB / y n+1 Hy n+2 L…y k-1 Hy k L) / AIR,

[0008] Where SUB represents the substrate, AIR represents air, H and L represent high and low refractive index materials with an optical thickness of λ / 4, n is the number of main reflective coating layers, kn is the number of layers that need stress regulation, k is the total number of layers before and after the substrate, y1 to y k is the optical thickness control coefficient. The optical thickness changes of the main reflective film layer and the stress control film layer generally change gradually according to a logarithmic function relationship. The function expression is: Where x represents the xth layer of film, y represents the optical thickness control coefficient corresponding to the film layer, and a and b are the functional relationship constants solved by determining the optical thickness coefficients of the first and last layers according to the index requirements.

[0009] Specifically, it is based on the coating index bandwidth and reflectivity requirements combined with the central wavelength, the refractive index of the high refractive index material and the low refractive index material. According to the index requirements for the target bandwidth and wavelength range, the optical thickness control coefficients corresponding to the first and last layers are adjusted; according to the index requirements for reflectivity, the total number of film layers k is selected (the more the total number of film layers, the higher the reflectivity). Under the premise of meeting the index requirements, considering factors such as bonding strength, adhesion, and production cost, the film structure with the thinnest physical thickness is given priority to, so as to clarify the optical thickness control coefficients corresponding to the first and last layers of the film layer, that is, y1 and a1 for the first layer, y for the last layer k and a k . Bring in The two unknowns a and b in the function expression can be solved in the expression to determine the functional relationship.

[0010] The optical thickness of each layer of coating material is determined through this functional relationship, and the physical thickness of each layer is calculated using the optical thickness and the material refractive index. After clarifying the logarithmic function gradient film structure and the broadband reflective film structure, stress compensation is performed through the film thickness ratio of the front and back surfaces of the substrate, that is, the film layer on the front and back surfaces of the substrate as a whole satisfies the logarithmic function gradient film structure relationship. The high-reflective film layer of the main wavelength band of the broadband reflector is plated on the main reflective layer, and the film layer on the other side of the substrate for stress regulation according to the main reflective layer is the stress regulation film layer, and the film thickness ratio on both sides of the substrate is judged, that is, the ratio of the main reflective film layer thickness to the stress regulation film layer thickness is in the range of 0.9-1.1. If the above requirements are met, the logarithmic function gradient film structure design is completed.

[0011] Based on the gradient chirp structure, an ultra-wideband low-absorption, low-stress, high-reflection film is realized. The optical thickness variation trend of the dielectric film layer of the combination of high and low refractive index dielectric materials conforms to Functional relationship, its structural characteristics are that the changes between film layers are continuous and uninterrupted, and the front and back coefficients of the optical thickness are adjusted to control the reflection bandwidth, and the reflectivity is improved by changing the total number of film layers.

[0012] The high refractive index dielectric material is TiO2, Ta2O5, ZrO2, HfO2, Nb2O5, sulfide or Si.

[0013] The low refractive index dielectric material is SiO2, MgF2, Al2O3.

[0014] The substrate layer material is quartz glass, K9 (BK7) or CaF2. The substrate can be a plane mirror or a curved mirror (excluding notched and irregularly shaped mirrors).

[0015] The optical thickness adjustment coefficients y1 to y k The range is between 0.2-2, and the total number k of film layers before and after plating on the substrate is between 40-200.

[0016] Compared with the prior art, the technical effects of the present invention are:

[0017] The present invention can effectively reduce the problem of broadband reflective mirrors being affected by the reflection gap, resulting in a drop in reflectivity, thereby improving the use efficiency. The relationship between the bandwidth, reflectivity and film thickness of the film system is comprehensively considered using a logarithmic function gradient thickness change relationship. Compared with the traditional method of using different central wavelength reflection films to stack and add to plate broadband reflection films, the present invention can effectively reduce the film thickness drop, greatly reducing the difficulty and cost of preparation. In terms of stress compensation, the film system with a logarithmic function gradient thickness change is different from the traditional method of using a film stack with a corresponding band of λ / 4 wavelength coated on the front and back surfaces to compensate for the stress problem on the front side. The number of film layers on the front and back surfaces can be freely selected to achieve appropriate stress compensation, effectively reducing the problem of stress-induced surface distortion. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the design process of the present invention;

[0019] Figure 2 It is a schematic diagram of the film structure of the present invention;

[0020] Figure 3 This is the overall spectrum diagram with a total of 175 film layers;

[0021] Figure 4 The overall spectrum diagram for increasing the number of layers to 195;

[0022] Figure 5 This is the overall optical thickness diagram of the 175-layer film system;

[0023] Figure 6 This is the overall physical thickness diagram of the 175-layer film system;

[0024] Figure 7 It is the physical thickness diagram of 1-108 layers of stress-controlled film;

[0025] Figure 8 It is the spectrum of 1-108 layers of stress-controlled film;

[0026] Fig. 9 It is the physical thickness diagram of the main reflective film layers 109-175;

[0027] Fig.10 Spectrum of the main reflective film layer 109-175 DETAILED DESCRIPTION

[0028] The present invention is further described with reference to the embodiments.

[0029] Example 1

[0030] The design process of this embodiment is as follows Figure 1 As shown, the structural diagram is as follows Figure 2As shown, the design reflectivity is required to be greater than 99.9%, the use wavelength is 500-1100nm, the bandwidth is 600nm, and the surface accuracy is as high as possible.

[0031] The design steps are as follows:

[0032] 1. According to the band, the central wavelength is selected as 800nm. According to the actual coating needs, the high refractive index material is Ta2O5 and the low refractive index material is SiO2. The refractive index parameters of the high and low refractive index materials are given by the Cauchy formula Determine, as shown in Table 1:

[0033] <![CDATA[A0]]> <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[SiO2]]> 1.44293 1.1622618e-2 -3.705533e-4 <![CDATA[Ta2O5]]> 2.01486 3.01116301e-2 -7.635062e-4

[0034] Table 1: Refractive index parameters of high and low refractive index materials

[0035] 2. According to the index requirements, the wavelength is 500-1100nm and the bandwidth is 600nm. The reflection bandwidth and position can be adjusted by changing the optical thickness control coefficient or adjusting the central wavelength position. After adjustment, Figure 1 As shown, the optical thickness coefficient of the first layer is determined to be y1≤0.55, and the optical thickness coefficient of the last layer is y k ≥1.5 can meet the reflection bandwidth requirements and optical performance requirements. The increase in film thickness will bring many problems such as preparation cost difficulty and adhesion. The thinnest film system is based on meeting the index requirements;

[0036] 3. According to the index requirement, the reflectivity is greater than 99.9%. The minimum number of film layers that meet the optical performance requirements is k = 175 layers; the more film layers there are, the higher the reflectivity. The reflectivity spectrum of 175 layers is as follows Figure 3 As shown, the reflectivity spectrum of the total number of film layers increased to 195 layers using the same structure with the head and tail coefficients unchanged is as follows Figure 4 As shown;

[0037] The relationship between the optical thickness function and the The optical thickness of the first layer is 0.55, and the optical thickness of the 175th layer is 1.5. The function curve can be determined by fitting the functional relationship (1, 0.55) and (175, 1.5). b=0.55, the functional relationship expression is The 1 / 4 wavelength optical thickness of each layer can be determined as Figure 5 As shown, the physical thickness of each layer is determined Pt x is the physical thickness of the xth layer of the film, λ is the central wavelength, and N is the refractive index of the material. Figure 6As shown. Combining the index requirements and the plating process, the thickness of the 175-layer broadband reflective film is determined to be 19627nm, ensuring that the film thickness ratio of the front and rear surfaces of the substrate meets the ratio requirement (i.e., the film thickness ratio meets 1±0.1). The 175 layers are divided into two parts: (1) Stress control film layer: The 1st to 108th layers are plated on the rear surface of the substrate as stress control film layers with a film thickness of 9744.4nm. The physical film thickness of the 1st to 108th stress control layers is as follows: Figure 7 As shown, the physical film thickness spectrum curve of the 1st to 108th stress regulation layer is as follows Figure 8 As shown in (2) main reflective film layer: the 109th to 175th layers are plated on the front surface of the substrate as the main reflective film layer with a film thickness of 9882.3nm. The physical film thickness of the 109th to 175th main reflective film layer is as follows: Fig. 9 As shown, the spectrum of the 109th to 175th main reflection film layers is as follows Fig.10 The stress regulating film layer and the main reflective film layer are respectively plated on both sides of the substrate, and the film thickness ratio is: Meet the film thickness ratio requirements;

[0038] The coating process is as follows: the substrate is ultrasonically cleaned for 15 minutes and then rinsed and wiped clean with deionized water. After the vacuum chamber in the coating machine is cleaned, the substrate is placed in the coating machine, the vacuum chamber door is closed, and vacuum is evacuated. The background vacuum degree of the vacuum chamber in the coating machine reaches 1×10 -6 Pa, heat the substrate to keep its temperature at 80°C; before coating, perform ion beam etching on the substrate, bombard the substrate with an Ar ion source, use an Ar ion source voltage of 650V, and etch to a depth of 150nm; use a dual ion beam sputtering device to coat the above-treated substrate, the main ion source voltage is 1250V, argon is used as the main ion source gas, the auxiliary ion source voltage is 650V, the argon and oxygen flow ratio is 1:5, the oxygen filling amount is 15sccm, and the deposition rate of the film material molecules on the substrate is controlled to be approximately 0.2nm / s.

[0039] The interferometer measurement is 632.8nm, 0° reflection surface shape, substrate geometric size: Ф50mm×5mm measurement effective aperture: Ф45mm, the test results show that the film surface shape is significantly improved as shown in Table 2:

[0040]

[0041]

[0042] Table 2: Comparison of surface data of blank substrate and film structure with logarithmic function gradient thickness change on both sides of the substrate

[0043] Absorption analysis: The test mode is 0° incidence, the test wavelength is 1064nm, and 12 points are randomly selected on the sample surface for measurement. The test data are shown in Table 3:

[0044] Test Points Measurement value (PPM) 1 51.7 2 52.1 3 53.1 4 52.8 5 52.9 6 53.1 7 52.5 8 52.2 9 52.6 10 52.2 11 52.6 12 52.1

[0045] Table 3: Absorption test data of film structure with gradual thickness variation in logarithmic function on both sides of substrate

[0046] Comparative Example 1

[0047] The reflectivity is greater than 99.9% by superimposing reflection bands, using a broadband anti-reflection film with a wavelength of 500-1100nm and a film thickness of 25117nm. The interferometer is measured to be 632.8nm, 0° reflection surface shape, substrate geometric dimensions: Ф50mm×5mm, effective measurement aperture: Ф45mm, and the surface test data are as follows in Table 4:

[0048]

[0049]

[0050] Table 4: Surface data of blank substrate and reflective tape superimposed film structure

[0051] Absorption test: 0° incidence, test wavelength: 1064nm; 12 points are randomly selected on the sample surface for measurement. The test data are shown in Table 5 below:

[0052] Test Points Measurement value (PPM) 1 98.3 2 98.5 3 99.2 4 98.5 5 97.4 6 96.8 7 95.0 8 96.7 9 95.7 10 96.4 11 95.0 12 94.2

[0053] Table 5: Reflection band superimposed film structure absorption data

[0054] By comparing the logarithmic function gradient thickness variation film structure plated on both sides of the substrate with the traditional reflective film stacking structure in comparative example 1, the stress-induced surface distortion problem and the thin film absorption data are significantly reduced.

[0055] Comparative Example 2

[0056] The reflectivity of the film system with logarithmic function gradient thickness variation is greater than 99.9% on one side, and a broadband anti-reflection film with a wavelength of 500-1100nm is used, with a film thickness of 19627nm. The interferometer is measured to be 632.8nm, 0° reflection surface shape, substrate geometric dimensions: Ф50mm×5mm, effective measurement aperture: Ф45mm, and surface test data are shown in Table 6 below:

[0057]

[0058]

[0059] Table 6: Blank substrate and single-side logarithmic function gradient thickness variation film structure surface data

[0060] Absorption test: 0° incidence, test wavelength: 1064nm; 12 points are randomly selected on the sample surface for measurement. The test data are shown in Table 7 below:

[0061] Test Points Measurement value (PPM) 1 77.0 2 77.2 3 72.1 4 75.7 5 78.0 6 77.1 7 77.8 8 78.0 9 78.0 10 74.8 11 78.0 12 77.8

[0062] Table 7: Absorption data of film structure with gradual thickness variation of single-side plating logarithmic function

[0063] By comparing the data of the film system structure with a logarithmic function gradient thickness change coated on both sides of the substrate and on one side of the substrate with that of Example 2, although the stress-induced surface distortion problem and the thin film absorption data are lower than those of the traditional reflective film stack plus structure, they are still higher than those of the film system structure with a logarithmic function gradient thickness change coated on both sides of the substrate.

Claims

1. An ultra-wideband low-absorption, low-stress, high-reflection film based on a gradient chirp structure, characterized in that: It includes a main reflective film layer, a substrate, and a stress regulating film layer, and the film structure expression is: AIR / y1Hy2L…y n-1 Hy n L) / SUB / y n+1 Hy n+ 2L…y k-1 Hy k L) / AIR, where SUB represents the substrate, AIR represents air, H and L represent high and low refractive index materials with an optical thickness of λ / 4, respectively, n is the number of main reflective coating layers, k is the total number of film layers before and after the substrate, kn is the number of film layers that need stress regulation, y1 to y k is the optical thickness control coefficient. The optical thickness of the main reflective film layer and the stress control film layer gradually changes according to a logarithmic function. The function expression is: Wherein, x represents the xth layer of the film (1≦x≦k), y represents the optical thickness control coefficient corresponding to the film layer, a and b are the functional relationship constants solved after determining the optical thickness coefficients of the first and last layers according to the design index requirements, the total number of film layers is determined according to the reflectivity required by the index, and the function growth trend is determined according to the reflection bandwidth required by the index. The film thickness ratio of the main reflection film layer and the stress control film layer of the film structure is changed to perform stress adjustment.

2. The ultra-wideband low-absorption, low-stress, high-reflection film based on a gradient chirp structure according to claim 1, characterized in that: The film thickness ratio between the front and rear surfaces of the substrate, that is, the ratio of the film thickness of the main reflective film layer to the film thickness of the stress regulating film layer, is in the range of 0.9-1.

1.

3. The ultra-wideband low-absorption, low-stress, high-reflection film based on a gradient chirp structure according to claim 1, characterized in that: The film optical thickness control coefficients y1 to y k The values ​​show a continuous logarithmic function gradient relationship and y1 to y k The range is between 0.2-2, and the total number k of the front and rear film layers of the substrate is between 40-200.

4. The ultra-wideband low-absorption, low-stress, high-reflection film based on a gradient chirp structure according to any one of claims 1 to 3, characterized in that: The high refractive index material is TiO2, Ta2O5, ZrO2, HfO2, Nb2O5, Si or sulfide.

5. The ultra-wideband low-absorption, low-stress, high-reflection film based on a gradient chirp structure according to any one of claims 1 to 3, characterized in that: The low refractive index material is SiO2, MgF2 or Al2O3.

6. The method according to any one of claims 1 to 3, wherein the ultra-wideband low-absorption, low-stress, high-reflection film is realized based on a gradient chirp structure, characterized in that: The substrate material is quartz glass, K9 (BK7) or CaF2, the substrate is a plane mirror or a curved mirror, and does not include a notched special-shaped mirror.

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