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

Abstract

The application provides a kind of ultra-wideband low-absorption low-stress high-reflection film based on gradual chirp structure, and belongs to the technical field of optical film.The application is different from the way that high and low refractive index materials are combined and superimposed to control the reflection bandwidth of traditional regular film system, and a logarithmic function gradual chirp structure is provided, the reflectivity index is adjusted by adjusting the total number of film layers, the reflection bandwidth is adjusted by adjusting the optical thickness coefficient at the beginning and end, the problem of sudden drop of reflectivity in the reflection gap area caused by the superimposition of multiple reflection bands when designing wideband high-reflection dielectric film is effectively avoided, and the film thickness ratio of stress control layer and gradual main reflection layer is adjusted to reduce the problem of face shape distortion caused by film stress.The application reduces the film layer thickness from the film system design point of view, reduces the preparation cycle and difficulty, reduces the problems and risks caused by stress, effectively reduces the absorption of film layer, and can combine post-processing means to control the absorption and stress within a suitable range to effectively support the safe and stable operation of the system.

Description

Technical Field

[0001] The present 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 at the reflection gap of an ultra-broadband reflective film. Background Art

[0002] The rapid development of laser technology is inseparable from the driving force behind optical thin film technology. Laser reflective films are one of the most critical components in laser systems. Highly reflective films are used to minimize energy loss from reflected light in the resonant cavity. With the advancement of tunable laser technology, the output requirements of tunable lasers are increasingly demanding, such as wider bandwidth, higher power, and increasingly stringent reflectivity and threshold requirements. Reflective films are primarily categorized into two types: metallic and dielectric. Mirrors made from metallic films offer advantages such as low coating costs, thin films, and low stress. However, metallic reflective films suffer from high absorption losses and low reflectivity, clearly failing to meet the requirements of high-performance optical systems. Traditional metallic films or combinations of metallic and dielectric films cannot achieve high reflectivity and resistance to laser damage. All-dielectric films are required to achieve a high laser damage threshold. Broadening the reflection band is achieved by stacking reflective films with different center wavelengths. However, this stacking approach results in resonant absorption amplification at the wavelength boundary between the stacks, leading to localized dips in the reflection curve and a significant decrease in 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 stress of the film 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, it will lead to a significant decrease in the output beam quality. In severe cases, beam control will not be possible. The traditional broadband reflective film surface deviation control technology uses double-sided coating, and the film stack of the corresponding wavelength band λ / 4 is coated on the back to compensate for the stress problem on the front. However, it can only coat the reflection bands of different wavelengths on the back for stress compensation, and cannot achieve precise stress control. 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 a stress-adjustable 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, effectively avoiding the problem of localized depression of reflectivity in the reflection gap region caused by superimposing reflective films with different central wavelengths. In terms of stress regulation, unlike the traditional method of plating different central wavelengths on the front and back surfaces to bring about balanced stress, the overall thickness of the film system of the present invention presents a logarithmic function gradient thickness variation film system structure. The film thickness ratio of the front and back surfaces of the substrate can be adjusted according to the required film thickness to achieve the effect of balanced stress. 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, effectively reducing 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 including 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 requiring stress control, 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 high-refractive index materials and the refractive index of low-refractive index materials. 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 determine 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 layers on the front and back surfaces of the substrate as a whole meet 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] Ultra-broadband low-absorption, low-stress, high-reflection films are realized based on a gradient chirp structure. The optical thickness variation trend of the dielectric film layer of the combination of high and low refractive index dielectric materials is consistent with Function relationship, its structural characteristics are that the changes between the 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, or Al2O3.

[0014] The substrate layer material is quartz glass, K9 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 present invention has the following technical effects:

[0017] The present invention can effectively reduce the reflectivity drop caused by the reflection gap in broadband reflectors, thereby improving their efficiency. By using a logarithmic function gradient thickness variation relationship to comprehensively consider the relationship between film bandwidth, reflectivity, and film thickness, compared to the traditional method of coating broadband reflective films using stacks of reflective films with different center wavelengths, the present invention can effectively reduce film thickness drop, greatly reducing preparation difficulty and cost. Regarding stress compensation, the use of a film system with a logarithmic function gradient thickness variation differs from the traditional method of compensating for front-side stress by coating the front and back surfaces with corresponding λ / 4 wavelength film stacks. The number of film layers on the front and back surfaces can be freely selected to achieve appropriate stress compensation, effectively reducing 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 Schematic diagram of the film structure of the present invention;

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

[0021] Figure 4 The overall spectrum graph 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 The physical thickness diagram of the stress-controlled film layer from 1 to 108 layers;

[0025] Figure 8 Spectrum diagram of 1-108 layers of stress-controlled film;

[0026] Figure 9 This is the physical thickness map of the main reflective film layers 109-175;

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

[0028] The present invention will be 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 operating 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 high and low refractive index materials are determined 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 center 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. Increasing the film thickness will bring many problems such as preparation cost difficulty and adhesion. The thinnest film system should be based on the requirements of the indicators.

[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 film layer is increased to 195 layers with the same structure and the head and tail coefficients unchanged. 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 back surfaces of the substrate meets the proportional requirements (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 back surface of the substrate as stress control film layers with a 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 control 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 thickness of 9882.3nm. The physical film thickness of the 109th to 175th main reflective film layer is as follows: Figure 9 As shown, the spectrum of the 109th to 175th main reflection film layers is as follows Figure 10 As shown. 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 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 showed a 632.8 nm, 0° reflection surface shape, substrate geometry: Ф50mm×5mm, effective aperture: Ф45mm. The test results showed that the film surface shape was significantly improved as shown in Table 2:

[0040]

[0041]

[0042] Table 2: Comparison of surface profile data of a blank substrate and a film structure with logarithmic gradient thickness variation 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 Point 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: Comparison example 1 of the absorption test data of the film structure with gradual thickness variation using logarithmic function on both sides of the substrate

[0046] A broadband anti-reflection coating with a wavelength of 500-1100 nm and a thickness of 25117 nm was used to create a reflectivity greater than 99.9% using a superimposed reflective strip. Interferometer measurements revealed a wavelength of 632.8 nm, a 0° reflective surface, and substrate dimensions of Ø50 mm × 5 mm. The effective aperture was Ø45 mm. Surface profile test data is shown in Table 4 below:

[0047]

[0048]

[0049] Table 4: Surface data of blank substrate and reflective tape superimposed film structure

[0050] Absorption test: 0° incidence, test wavelength: 1064nm; 12 points on the sample surface were randomly selected for measurement. The test data is shown in Table 5 below:

[0051] Test Point 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

[0052] Table 5: Absorption data of reflection band superimposed film structure

[0053] 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.

[0054] Comparative Example 2

[0055] A logarithmic gradient thickness coating structure was used to achieve a single-sided reflectivity greater than 99.9%. A broadband anti-reflection coating with a wavelength of 500-1100 nm and a thickness of 19627 nm was used. Interferometer measurements revealed a 0° reflective surface shape of 632.8 nm. The substrate geometry was Ø50 mm × 5 mm, with an effective aperture of Ø45 mm. The surface profile data is shown in Table 6 below:

[0056]

[0057]

[0058] Table 6: Surface data of blank substrate and film structure with logarithmic function gradient thickness change on one side of substrate

[0059] Absorption test: 0° incidence, test wavelength: 1064nm; 12 points on the sample surface were randomly selected for measurement. The test data is shown in Table 7 below:

[0060] Test Point 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

[0061] Table 7: Absorption data of film structure with logarithmic function gradient thickness change on one side

[0062] By comparing the data of the film system structure with logarithmic function gradient thickness change coated on both sides of the substrate and coated on one side of the substrate with that of comparative 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 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 chirped structure, characterized in that: It includes a main reflective film layer, a substrate, and a stress regulating film layer. 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 require stress control, 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 optical thickness coefficients of the first and last layers determined according to the design index requirements and then substituted into the above function expression to solve the functional relationship constant terms, 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, and the film thickness ratio of the main reflective 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 chirped structure according to claim 1, characterized in that: The film thickness ratio of the front and back 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 chirped 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 back film layers of the substrate is between 40-200.

4. The method according to any one of claims 1 to 3, wherein the method comprises: The high refractive index material is TiO2, Ta2O5, ZrO2, HfO2, Nb2O5, Si or sulfide.

5. The method according to any one of claims 1 to 3, wherein the method comprises: The low refractive index material is SiO2, MgF2 or Al2O3.

6. The method according to any one of claims 1 to 3, wherein the method comprises: The substrate material is quartz glass, K9 or CaF2, and the substrate is a plane mirror or a curved mirror, and does not include a notched or special-shaped mirror.

Images