Bandwidth and splitting ratio adjustable light splitting film and regulation method

Abstract

The application discloses a bandwidth and light splitting ratio adjustable light splitting film and a regulation method, and belongs to the technical field of optical films. The structure of the light splitting film comprises, from bottom to top, a substrate, an interface anti-reflection film, a virtual interface layer and air. The interface anti-reflection film is a multilayer interference type anti-reflection film composed of high and low refractive index films or a wideband anti-reflection film based on a gradually changing refractive index, and the reflection from the virtual interface layer to the substrate is substantially zero. The virtual interface layer is a film layer with a thickness of zero, and the interface between the virtual interface layer and the air realizes wideband uniform light splitting. The regulation method adjusts the bandwidth by the subdivision degree of the sublayer of the gradually changing refractive index anti-reflection film, and adjusts the light splitting ratio by the refractive index of the virtual interface layer. The light splitting film and the regulation method provided by the application can realize uniform light splitting with an arbitrary bandwidth, the light splitting ratio is adjustable, the film system structure is simple, and the design and preparation are easy.

Description

Technical Field

[0001] This invention belongs to the field of optical thin film technology, specifically relating to a beam splitter with adjustable bandwidth and splitting ratio and a control method thereof. Background Technology

[0002] Beam splitters are components in optical systems that separate light beams, allowing the energy of a beam to be used in separate parts of the optical path according to a specific ratio. Beam splitters are widely used in various fields such as spectral analysis, optical systems, optoelectronic instruments, and display technology, and their performance directly affects the degree to which the instrument or system achieves its final function.

[0003] Based on the type of thin-film material used, beam splitters are generally divided into metallic beam splitters and dielectric beam splitters. Metallic beam splitters typically utilize the semi-transparent properties of ultrathin metal layers and the strong dispersion characteristics of the metal's optical constants as a function of wavelength to achieve beam splitting over a wide wavelength range. However, the untunable dispersion curve of the metal itself and the optical losses caused by strong absorption result in poor beam splitting efficiency, making it impossible to maintain a consistent splitting ratio across a wide wavelength range. Furthermore, the unstable chemical properties of metallic films, such as easy oxidation and corrosion, as well as their fragile mechanical properties, such as poor mechanical strength and weak bonding, can easily lead to a series of reliability issues. Dielectric beam splitters, on the other hand, use dielectric materials throughout the film layers, avoiding the significant absorption caused by metallic film layers. This can improve the laser damage threshold in some high-power laser applications. At the same time, dielectric beam splitters offer good environmental reliability and durability, making them a more practical and reliable choice for beam splitters. Dielectric beam splitters generally utilize the interference effect between film layers to enhance light reflection or transmission, thereby controlling the splitting ratio. However, the interference characteristics of light fluctuate with wavelength, which limits the applicability of dielectric beam-splitting films to a narrow wavelength range, making it difficult to achieve uniform beam splitting over an ultra-wide wavelength range. Furthermore, although beam-splitting films are a common type of optical thin film, the design principles of beam-splitting film systems have long been insufficiently understood, resulting in suboptimal designs for film structure and splitting efficiency. Addressing the current problems of inconsistent spectral splitting efficiency, high absorption loss, and narrow splitting bandwidth in beam-splitting films, this paper proposes an analytical design method to design the structure of beam-splitting films, thereby achieving quantitative adjustment of the splitting band and splitting ratio. This approach has significant scientific and practical value and represents an important direction for the development of optical thin films. Summary of the Invention

[0004] To address the shortcomings of the existing technology, the purpose of this invention is to provide a beam splitter with adjustable bandwidth and splitting ratio, and a control method thereof, which can achieve a uniform splitting ratio in an ultra-wide band, and can also achieve arbitrary splitting ratio control within a certain band range.

[0005] To achieve the above objectives, the technical solution proposed by this invention is as follows:

[0006] A beam splitter with adjustable bandwidth and splitting ratio is constructed. The structure of the beam splitter, from bottom to top, includes a substrate, an interface antireflection film, a virtual interface layer, and air. The substrate is a material that is completely transparent in the working wavelength band. The interface antireflection film is a multilayer interference antireflection film composed of high and low refractive index films or a broadband antireflection film based on a gradient refractive index film. The virtual interface layer is a virtual film layer with zero thickness, and its effective refractive index is calculated by the splitting ratio of the beam splitter. The gradient refractive index refers to the change in refractive index of the film layer from the refractive index of the substrate to the refractive index of the virtual interface layer along the thickness direction of the film layer. An equivalent layer with the structure xL(2-2x)H xL is used to form a gradient refractive index distribution in the thickness direction, where H and L represent high and low refractive index film layers with a thickness of 1 / 4 of the optical thickness of the reference wavelength, respectively, and x represents the film layer thickness coefficient, which is taken in the range of 0 to 1 according to the thickness distribution of the gradient refractive index film.

[0007] The method for controlling a beam splitter with adjustable bandwidth and splitting ratio includes the following steps:

[0008] (1) Obtain the spectrophotometric ratio R / T, and assume the refractive index of the virtual interface layer is n. eff or 1 / n eff To achieve an interface reflectance of R between the air and the virtual interface layer, n can be obtained. eff = (1+R) 0.5 ) / (1-R 0.5 );

[0009] (2) Obtain the spectral bandwidth, with the shortest wavelength being λ. min The longest wavelength is λ max Calculate λ max / λ min value;

[0010] (3) For λ max / λ min For a beam splitter with a value <2, a conventional multilayer interferometric design can be used, with (2λ) max λ min ) / (λ max +λ min Using λ as the reference wavelength, a single-wavelength antireflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interfacial refractive index of the film system is close to n. eff or 1 / n eff ;

[0011] (4) For λ max / λ min ≥2 spectral film, with 3λ max / 8 is used as the reference for the total optical thickness of the initial film system, with 2λ minUsing / 3 as the reference wavelength for the initial film system, and based on the equivalent layer theory, the graded refractive index film is constructed. The stacking of the equivalent layers satisfies that the total thickness just exceeds 3λ. max / 8, thus forming a broadband interface antireflective film;

[0012] (5) Optimize the initial membrane structure obtained in step (3) or (4) with the spectral ratio R / T and band requirements as the optimization targets. The optimized membrane structure that meets the index requirements can be obtained quickly.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] 1. The analytical design method ensures the adjustable characteristics of the spectral bandwidth and spectral ratio in principle. The design concept based on the gradient refractive index reduces the requirements for the precision of film thickness control and ensures the manufacturing tolerance.

[0015] 2. The all-dielectric spectrophotometer system avoids the absorption loss of metal spectrophotometers. The introduction of the virtual interface layer ensures the consistency of the splitting ratio in different bands in principle, making it easy to achieve wide-band uniform splitting.

[0016] 3. As low as 3λ max The total optical thickness of / 8 ensures controllable film stress, thereby providing more reliable optical film quality and longer lifespan. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the spectrophotometer structure of the present invention.

[0018] Figure 2 This is a flowchart illustrating the control of the bandwidth and splitting ratio of the spectral splitting film according to the present invention.

[0019] Figure 3 This is an optical admittance distribution diagram of the initial film system of the 400-700nm beam splitter in Example 1 of the present invention.

[0020] Figure 4 The reflection spectrum of the 400-700nm spectrophotometer with a spectrophotometer ratio R / T = 1:1 in Example 1 of this invention is shown.

[0021] Figure 5 The reflection spectrum of the 2.5-15μm spectrophotometer with a spectrophotometer ratio R / T = 2:3 in Example 2 of this invention is shown.

[0022] The markings in the diagram are as follows: 1-substrate, 2-interface antireflective film, 3-virtual interface layer, 4-air. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific examples.

[0024] A beam-splitting film with adjustable bandwidth and splitting ratio, its structure is as follows: Figure 1 As shown, the structure of the beam splitter, from bottom to top, includes a substrate 1, an interface antireflection film 2, a virtual interface layer 3, and air 4. The substrate 1 is a material that is completely transparent in the working wavelength band. The interface antireflection film 2 is a multilayer interference antireflection film composed of high and low refractive index films or a broadband antireflection film based on a gradient refractive index film. The virtual interface layer 3 is a virtual film layer with zero thickness, and its effective refractive index is calculated by the beam splitter ratio of the beam splitter. The gradient refractive index refers to the change in refractive index of the film layer from the refractive index of the substrate to the refractive index of the virtual interface layer 3 along the thickness direction of the film layer. An equivalent layer with the structure xL(2-2x)H xL is used to form a gradient refractive index distribution in the thickness direction, where H and L represent high and low refractive index film layers with a thickness of 1 / 4 of the optical thickness of the reference wavelength, respectively, and x represents the film layer thickness coefficient, which is taken in the range of 0 to 1 according to the thickness distribution of the gradient refractive index film.

[0025] The method for controlling the bandwidth and splitting ratio of the adjustable beam splitter film is as follows: Figure 2 As shown, it includes the following steps:

[0026] (1) Obtain the spectrophotometric ratio R / T, and assume that the refractive index of the virtual interface layer 3 is n. eff or 1 / n eff To achieve an interface reflectivity of R between air layer 4 and virtual interface layer 3, n can be obtained. eff = (1+R) 0.5 ) / (1-R 0.5 );

[0027] (2) Obtain the spectral bandwidth, with the shortest wavelength being λ. min The longest wavelength is λ max Calculate λ max / λ min value;

[0028] (3) For λ max / λ min For a beam splitter with a value <2, a conventional multilayer interferometric design can be used, with (2λ) max λ min ) / (λ max +λ min Using λ as the reference wavelength, a single-wavelength antireflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interfacial refractive index of the film system is close to n. eff or 1 / n eff ;

[0029] (4) For λ max / λ min ≥2 spectral film, with 3λ max / 8 is used as the reference for the total optical thickness of the initial film system, with 2λ minUsing / 3 as the reference wavelength for the initial film system, and based on the equivalent layer theory, the graded refractive index film is constructed. The stacking of the equivalent layers satisfies that the total thickness just exceeds 3λ. max / 8, thus forming a broadband interface antireflective film;

[0030] (5) Optimize the initial membrane structure obtained in step (3) or (4) with the spectral ratio R / T and band requirements as the optimization targets. The optimized membrane structure that meets the index requirements can be obtained quickly.

[0031] Working Principle: For beam-splitting films, there are few explicitly documented analytical design methods in textbooks and literature, thus their design technology requires further research and development. However, the design principles of another type of thin film controlling transmittance (reflectance) across different wavelength bands—anti-reflection coatings—are provided in many documents and optical thin film textbooks. Therefore, we add a virtual interface layer 3 to the film system design, ensuring that the reflectance and transmittance at the interface between air 4 and this virtual interface layer 3 meet the required beam splitting ratio. This transforms the beam-splitting film system design into an anti-reflection coating design from the virtual interface layer 3 to the substrate 1. Thus, we can utilize mature anti-reflection coating design theory to analyze and obtain the initial design film system of the beam-splitting film, greatly increasing the theoretical basis and feasibility of the beam-splitting film design.

[0032] There are two main design methods for antireflective coatings: one is to achieve the antireflection effect based on the principle of destructive interference of multilayer films, and the other is to achieve the interface reflectivity by designing a gradient refractive index film so that the cumulative reflectivity of all interfaces is still very small, thereby achieving the antireflection effect.

[0033] For the first scenario, we utilize the equivalent interface optical admittance theory of optical thin films to design the equivalent optical admittance of the reference wavelength of the antireflective coating 2 to be the same as that of the virtual interface layer 3. Based on Fresnel's formula, we can obtain that the reflectance of this reference wavelength incident from the virtual interface layer 3 to the substrate 1 is zero, thus achieving the antireflection effect. Simultaneously, our design based on a virtual layer with a half-wavelength thickness ensures that the reflectance of the reference wavelength remains unchanged, and the equivalent optical admittance in the band near the reference wavelength approximates the reflectance equivalent admittance curve, thereby enabling the designed beam-splitting film system to have similar reflectance values ​​within this band.

[0034] In the second scenario, a graded refractive index film design method is employed. Based on the equivalent layer theory, and utilizing the property that a symmetrical multilayer film can be equivalent to a single-layer film with a specific refractive index, a composite film with a refractive index that gradually changes along the film thickness can be constructed. Since the refractive indices of adjacent equivalent layers are very close, the interfacial reflectivity between equivalent layers is approximately zero. Therefore, all layers of the final antireflective film 2 exhibit a broadband antireflection effect, allowing light to pass from the virtual interface layer 3 onto the substrate 1. The overall reflection of the beam-splitting system is determined by the interfacial reflectivity between air 4 and the virtual interface layer 3, thus achieving a broadband and uniform beam-splitting effect.

[0035] Specific examples: In order to verify the advantages of the technical solution of this invention, namely, that the splitting bandwidth and splitting ratio can be designed by analytical method to achieve a wide-band uniform splitting effect, the invention will be further explained below with specific examples.

[0036] Example 1: 400-700nm spectral density splitter with a ratio R / T of 1:1

[0037] For the design of a beam splitter in the visible light band, we select a beam splitter with a 1:1 splitting ratio in the 400-700nm band as an example. The substrate 1 is quartz (refractive index 1.46), the high-refractive-index material H is TiO2 (refractive index 2.4), and the low-refractive-index material L is SiO2 (refractive index 1.46). Firstly, regarding the 1:1 splitting ratio requirement, considering the absence of an absorbing dielectric film, the reflectivity needs to be 50%, corresponding to the effective refractive index n of the virtual interface layer 3. eff = 5.828. For the splitting bandwidth λ max / λ min When 1.75 < 2, we choose a multilayer interference film approach to design the interface antireflection film 2, with a reference wavelength of 509 nm. While striving for a well-ordered film structure, we can select the 0.247H 0.362L aH 1L 1H film system as the initial film system for interface antireflection film 2. This yields an optical admittance value close to 5.828 at the reference wavelength of 509 nm, indicating good antireflection effect from the virtual interface layer 3 to the substrate 1. Here, the third layer, aH, is a dummy layer for the reference wavelength, where 'a' is zero or a positive even number. By comparing the optical admittance curves of different film systems (different 'a' values) at 400-700 nm with the equivalent optical admittance curve at 50% reflectivity, as shown... Figure 3 As shown, the film system 0.247H 0.362L 4H 1L 1H has the closest admittance curve, so it is used as the initial film system. Further optimization of the initial film system to meet the requirement of more uniform spectral distribution in the 400-700nm wavelength range yields the optimized film system 0.27H 0.364L 4.096H 1.091L 0.918H. The reflectance spectra of the initial and optimized film systems are calculated as follows... Figure 4 As shown, both have a refraction ratio of approximately 1:1 in the 400-700nm band, while the optimized film system has a more uniform refraction ratio distribution in this required band.

[0038] Example 2: 2.5-15μm spectrophotometer with a spectrophotometer R / T ratio of 2:3

[0039] The 2.5-15 μm wavelength range is a common band in Fourier transform infrared spectrometers. The spectrometer is one of its core components, and achieving uniform spectral dispersion within this range is crucial for improving the accuracy of the Fourier transform. Considering the limitations imposed on the selection of the refractive index of the thin film material, we chose a spectrometer with an R / T ratio of 2:3 for demonstration. In this example, without sacrificing effectiveness, we selected a material with a refractive index of 1.52 as the substrate, a material with a refractive index of 4 as the high-refractive-index layer H, and a material with a refractive index of 1.52 as the low-refractive-index layer L. Following the working principle and control method described above, a reference wavelength of 1.667 μm was selected, based on the spectral bandwidth λ. max / λ min In the case where 6 > 2, we chose a gradient refractive index film system design, selecting 45 degrees as the incident angle. The film system structure used is as follows:

[0040] Initial membrane system: 0.9L 0.2H 0.9L 0.7L 0.6H 0.7L 0.5L 1H 0.5L 0.4L 1.2H 0.4L 0.3L 1.4H 0.3L 0.2L 1.6H 0.2L 0.1L 1.8H 0.1L 2H;

[0041] Optimized membrane system: 0.013H 2.688L 0.056H 2.541L 0.148H 2.269L 0.303H 1.885L 0.528H 1.432L 0.832H 0.945L 1.22H 0.43L 2.236H

[0042] By removing impurities from the ultrathin films of the optimized membrane system, a simplified membrane system with more practical feasibility for development can be obtained, as shown in the following structure:

[0043] Simplified membrane system: 0.141H 2.495L 0.315H 1.344L 0.597H 1.51L 2.689H 1.222L 0.249H

[0044] Simulation analysis of the above three film systems yields theoretical reflectance spectra such as... Figure 5As shown, the reflectivity is close to 40% in all wavelength ranges, but the initial film system has the largest spectral fluctuation, while the optimized film system has the smallest spectral fluctuation and the best spectral uniformity. The simplified film system has the simplest film structure and still has good spectral uniformity, making it a suitable choice for practical research.

Claims

1. A method for controlling a beam splitter with adjustable bandwidth and splitting ratio, characterized in that, The structure of the spectral splitter, from bottom to top, includes a substrate, an interface antireflection film, a virtual interface layer, and air. The substrate is a material that is completely transparent to light in the working wavelength band; The interface antireflection film is a multilayer interference antireflection film composed of high and low refractive index films or a broadband antireflection film based on a graded refractive index film. The virtual interface layer is a virtual film layer with zero thickness, and its effective refractive index is calculated by the spectral ratio of the spectrophotometer. The gradient refractive index refers to the change in refractive index of the film layer from the refractive index of the substrate to the refractive index of the virtual interface layer along the thickness direction of the film layer. An equivalent layer with the structure xL (2-2x)H xL is used to form a gradient refractive index distribution in the thickness direction. Here, H and L represent high and low refractive index films with a thickness of 1 / 4 of the optical thickness of the reference wavelength, respectively, and x represents the film thickness coefficient, which is taken in the range of 0 to 1 according to the thickness distribution of the gradient refractive index film. The method includes the following steps: (1) Obtain the spectrophotometric ratio R / T, and assume that the refractive index of the virtual interface layer is n. eff or 1 / n eff To achieve an interface reflectance of R between the air and the virtual interface layer, n can be obtained. eff =(1+R 0.5 ) / (1-R 0.5 ); (2) Obtain the splitting bandwidth, with the shortest wavelength being λ. min The longest wavelength is λ max Calculate λ max / λ min value; (3) For λ max / λ min The beam splitter with a value <2 uses a conventional multilayer interferometric design with (2λ) as the basis. max λ min ) / (λ max +λ min Using λ as the reference wavelength, a single-wavelength antireflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interfacial refractive index of the film system is close to n. eff or 1 / n eff ; (4) For λ max / λ min ≥2 spectral film, with 3λ max / 8 is used as the reference for the total optical thickness of the initial film system, with 2λ min Using / 3 as the reference wavelength for the initial film system, and based on the equivalent layer theory, the graded refractive index film is constructed. The stacking of the equivalent layers satisfies that the total thickness just exceeds 3λ. max / 8, thus forming a broadband interface antireflective film; (5) Optimize the initial membrane structure obtained in step (3) or (4) with the spectral ratio R / T and band requirements as the optimization targets. The optimized membrane structure that meets the index requirements can be obtained quickly.

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