Bandwidth and splitting ratio adjustable beam splitting film and regulation and control method

Abstract

The invention discloses a light splitting film with adjustable bandwidth and splitting ratio and a regulation and control method, and belongs to the technical field of optical thin films. The structure of the light splitting film sequentially comprises a substrate, an interface anti-reflection film, a virtual interface layer and air from bottom to top. The interface anti-reflection film is a multi-layer interference type anti-reflection film formed by a high-refractive-index film and a low-refractive-index film or a broadband anti-reflection film achieved based on a gradient refractive index, and reflection from a virtual interface layer to a substrate is basically zero. The virtual interface layer is a film layer with the thickness being zero, and broadband uniform light splitting is achieved on the interface of the virtual interface layer and the air. According to the regulation and control method, the bandwidth is regulated and controlled through the subdivision degree of the sublayers of the antireflection film with the gradient refractive index, and the splitting ratio is regulated and controlled through the refractive index of the virtual interface layer. According to the light splitting film and the regulation and control method provided by the invention, uniform light splitting of any bandwidth can be realized, the splitting ratio is adjustable, and the film system is simple in structure and easy to design and prepare.

Description

Technical Field

[0001] The invention belongs to the technical field of optical films, and in particular relates to a spectroscopic film with adjustable bandwidth and splitting ratio and a control method. Background Art

[0002] Beam splitting film is a component that realizes beam separation in the optical system. It can separate the beam energy according to a specific ratio for separate use. Beam splitting film is widely used in many fields such as spectral analysis, optical systems, optoelectronic instruments, display technology, etc. The quality of its performance indicators is directly related to the degree of realization of the final function of the instrument or system.

[0003] According to the type of thin film material used, spectroscopic films are generally divided into metal spectroscopic films and dielectric spectroscopic films. Metal spectroscopic films generally use the semi-transparent properties of ultra-thin metal layers and the strong dispersion properties of metal optical constants that vary with wavelength to achieve wide-band spectroscopic performance. However, the uncontrollable dispersion curve of the metal itself and the optical loss caused by strong absorption lead to poor spectroscopic efficiency, and the spectroscopic ratio cannot maintain a consistent ratio within a wide band. At the same time, the unstable chemical properties of metal films such as easy oxidation and corrosion, as well as the fragile mechanical properties such as poor mechanical strength and weak bonding force, are also prone to cause a series of reliability problems. Dielectric spectroscopic films use dielectric materials for all film layers, avoiding the obvious absorption caused by the metal film layer, which can increase the laser damage threshold in some strong laser applications. At the same time, dielectric spectroscopic films have good environmental reliability and durability, and are a more practical and reliable choice of spectroscopic films. Dielectric spectroscopic films generally use the interference effect of light between film layers to achieve light reflection or transmission enhancement, and then adjust the spectroscopic ratio. However, the interference characteristics of light fluctuate with the change of wavelength, which means that dielectric spectrometer films are generally only applicable to a narrow band range, and it is difficult to achieve uniform spectrometry in an ultra-wide band range. In addition, although spectrometer films are a common type of optical film, the design principle of spectrometer film system has not been fully analyzed for a long time, resulting in the film system structure and spectrometer efficiency of the spectrometer film not being able to achieve the optimal design. In view of the problems of inconsistent spectral spectrometer efficiency, large absorption loss, and narrow spectrometer bandwidth of current spectrometer films, an analytical design method is used to design the structure of the spectrometer film, thereby achieving quantitative adjustment of the spectrometer band and the spectrometer ratio. This has important scientific significance and application value, and is an important development direction in the current field of optical films. Summary of the invention

[0004] In view of the above-mentioned deficiencies of the prior art, the purpose of the present invention is to provide a splitter film with adjustable bandwidth and splitting ratio and a control method, which can achieve a uniform splitting ratio in an ultra-wide band and can also achieve the control of any splitting ratio within a certain band.

[0005] In order to achieve the above-mentioned purpose, the technical solution proposed by the present invention is:

[0006] A spectroscopic film with adjustable bandwidth and splitting ratio is constructed. The structure of the spectroscopic film includes a substrate, an interface anti-reflection film, a virtual interface layer and air from bottom to top. The substrate is a material that is completely transparent in the working band. The interface anti-reflection film is a multilayer interference anti-reflection film composed of high and low refractive index films or a broadband anti-reflection film based on a gradient refractive index film. The virtual interface layer is a virtual film layer with a thickness of zero, and its effective refractive index is calculated through the splitting ratio of the spectroscopic film. The gradient refractive index refers to the refractive index of the film layer changing from the refractive index of the substrate to the refractive index of the virtual interface layer along the thickening direction of the film layer, and an equivalent layer with a structure of xL(2-2x)H xL is used to form a refractive index distribution that changes gradually in the thickness direction, wherein H and L represent high and low refractive index film layers with an optical thickness of 1 / 4 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 regulating a spectroscopic film with adjustable bandwidth and splitting ratio comprises the following steps:

[0008] (1) Obtain the splitting ratio R / T, assuming that the refractive index of the virtual interface layer is n eff or 1 / n eff , so that the interface reflectivity between air and virtual interface layer is R, we can get n eff =(1+R 0.5 ) / (1-R 0.5 );

[0009] (2) Obtain the spectral bandwidth, the shortest wavelength is λ min , the longest wavelength is λ max , calculate λ max / λ min value;

[0010] (3) For λ max / λ min <2 beam splitter film can be designed with conventional multilayer interference film to (2λ max λ min ) / (λ max +λ min ) is the reference wavelength, and a single-wavelength anti-reflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interface refractive index of the film system is close to n eff or 1 / n eff ;

[0011] (4) For λ max / λ min ≥2 beam splitter film, 3λ max / 8 as the total optical thickness reference of the initial film system, with 2λ min / 3 is used as the reference wavelength of the initial film system. Based on the equivalent layer theory, the gradient refractive index film is constructed. The stacking of the equivalent layers satisfies the total thickness just exceeding 3λ. max / 8, thus forming a broadband interface anti-reflection film;

[0012] (5) Taking the splitting ratio R / T and the wavelength requirements as the optimization targets, the initial film system structure obtained in step (3) or (4) is optimized, and the optimized film system that meets the index requirements can be quickly obtained.

[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 splitting bandwidth and splitting ratio in principle. The design concept based on the gradient refractive index reduces the control accuracy requirements for the film thickness and ensures the manufacturing tolerance;

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

[0016] 3. As low as 3λ max The total optical thickness of / 8 ensures that the stress of the film layer is controllable, thus providing more reliable optical film quality and long life. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the structure of the beam splitter film of the present invention.

[0018] Figure 2 The present invention is a flow chart for regulating the bandwidth and splitting ratio of the beam splitting film.

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

[0020] Figure 4 This is the reflection spectrum of the 400-700nm splitting ratio R / T=1:1 splitting film of Example 1 of the present invention.

[0021] Figure 5 This is the reflection spectrum of the 2.5-15 μm splitting film with a splitting ratio of R / T=2:3 in Example 2 of the present invention.

[0022] Markings in the figure: 1-substrate, 2-interface anti-reflection film, 3-virtual interface layer, 4-air. DETAILED DESCRIPTION

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

[0024] A spectroscopic film with adjustable bandwidth and splitting ratio, the structure of which is as follows Figure 1 As shown, the structure of the spectroscopic film includes a substrate 1, an interface anti-reflection film 2, a virtual interface layer 3 and air 4 from bottom to top; the substrate 1 is a material that is completely transparent in the working band; the interface anti-reflection film 2 is a multilayer interference anti-reflection film composed of high and low refractive index films or a broadband anti-reflection film based on a gradient refractive index film; the virtual interface layer 3 is a virtual film layer with a thickness of zero, and its effective refractive index is calculated by the splitting ratio of the spectroscopic film; the gradient refractive index refers to the refractive index of the film layer changing from the refractive index of the substrate to the refractive index of the virtual interface layer 3 along the thickening direction of the film layer, and an equivalent layer with a structure of xL(2-2x)H xL is used to form a refractive index distribution that changes gradually in the thickness direction, wherein H and L represent high and low refractive index film layers of 1 / 4 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.

[0025] The method for regulating the spectral film with adjustable bandwidth and splitting ratio, the regulation process is as follows Figure 2 As shown, the following steps are included:

[0026] (1) Obtain the splitting ratio R / T, assuming that the refractive index of the virtual interface layer 3 is n eff or 1 / n eff , so that the interface reflectivity between air 4 and virtual interface layer 3 is R, we can get n eff =(1+R 0.5 ) / (1-R 0.5 );

[0027] (2) Obtain the spectral bandwidth, the shortest wavelength is λ min , the longest wavelength is λ max , calculate λ max / λ min value;

[0028] (3) For λ max / λ min <2 beam splitter film can be designed with conventional multilayer interference film to (2λ max λ min ) / (λ max +λ min ) is the reference wavelength, and a single-wavelength anti-reflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interface refractive index of the film system is close to n eff or 1 / n eff ;

[0029] (4) For λ max / λ min ≥2 beam splitter film, 3λ max / 8 as the total optical thickness reference of the initial film system, with 2λ min / 3 is used as the reference wavelength of the initial film system. Based on the equivalent layer theory, the gradient refractive index film is constructed. The stacking of the equivalent layers satisfies the total thickness just exceeding 3λ. max / 8, thus forming a broadband interface anti-reflection film;

[0030] (5) Taking the splitting ratio R / T and the wavelength requirements as the optimization targets, the initial film system structure obtained in step (3) or (4) is optimized, and the optimized film system that meets the index requirements can be quickly obtained.

[0031] Working principle: For the spectroscopic film, there are not many clear analytical design methods recorded in textbooks and literature, so its design technology needs further research and development. The design principle of another type of film that controls the transmittance (reflectivity) of the film band - anti-reflection film (anti-reflection film) is provided in many documents and optical film textbooks. Therefore, we add a virtual interface layer 3 to the film system design so that the reflectivity and transmittance at the interface between the air 4 and the virtual interface layer 3 meet the required splitting ratio, so that the design of the spectroscopic film system can be converted into the anti-reflection film design from the virtual interface layer 3 to the substrate 1. At this point, we can use the mature anti-reflection film design theory to analyze and obtain the initial design film system of the spectroscopic film, which greatly increases the theoretical basis and feasibility of the spectroscopic film design.

[0032] There are two main design methods for anti-reflection films: one is to achieve the anti-reflection effect based on the principle of interference destructiveness of multi-layer film interference, and the other is to achieve extremely low reflectivity of the interface based on the design of gradient refractive index film, so that the accumulated reflectivity of all interfaces is still very small, thereby achieving the anti-reflection effect.

[0033] For the first case, we use the equivalent interface optical admittance theory of optical thin films to design the equivalent optical admittance of the reference wavelength of the interface anti-reflection film 2 to be the same as that of the virtual interface layer 3. Based on the Fresnel formula, we can obtain that the reflectivity of the reference wavelength incident from the virtual interface layer 3 to the substrate 1 is zero, that is, the anti-reflection effect is achieved. At the same time, our design of the virtual layer based on the half-wavelength thickness can ensure that the reflectivity of the reference wavelength is not changed, and the equivalent optical admittance of the band near the reference wavelength is used to approach the reflectivity equivalent admittance curve, so that the designed spectroscopic film system has similar reflectivity values ​​within the band.

[0034] For the second case, a gradient refractive index film design method is adopted. Based on the equivalent layer theory, the characteristic that a symmetrical multilayer film can be equivalent to a single-layer film with a specific refractive index can be used to construct a composite film layer whose refractive index gradually changes along the thickness direction of the film layer. Since the refractive indices of the adjacent equivalent layers are very close, the interface reflectivity between the equivalent layers is approximately equal to zero. Therefore, all the film layers of the final interface anti-reflection film 2 show a broadband anti-reflection effect of light incident from the virtual interface layer 3 to the substrate 1. The reflection of the overall splitting film system is determined by the interface reflectivity of the air 4 and the virtual interface layer 3, thereby achieving a broadband uniform splitting effect.

[0035] Specific example: In order to verify the advantages of the patented technical solution of the present invention, that is, the splitting bandwidth and splitting ratio can be designed by analytical methods to achieve a wide-band uniform splitting effect, the present invention is further explained below in conjunction with specific examples.

[0036] Example 1: 400-700nm splitting ratio R / T = 1:1 splitting film

[0037] For the design of the spectroscopic film in the visible light band, we selected the spectroscopic film with a splitting ratio of 1:1 in the 400-700nm band as an example. The substrate 1 is quartz (refractive index is 1.46), and the high refractive index material H is TiO 2 (refractive index is 2.4), the low refractive index material L is SiO 2 (refractive index is 1.46). First, for the requirement of 1:1 splitting ratio, in the absence of absorbing medium film, the reflectivity needs to be 50%, and the corresponding effective refractive index of the virtual interface layer 3 is n eff =5.828. For the splitting bandwidth λ max / λ min =1.75<2, we choose a multilayer interference film method to design the interface anti-reflection film 2, and the reference wavelength is 509nm. While trying to choose a regular film structure, we can choose a 0.247H 0.362L aH 1L 1H film system as the initial film system of the interface anti-reflection film 2, and obtain an optical admittance value close to 5.828 at a reference wavelength of 509nm, that is, it has a good anti-reflection effect from the virtual interface layer 3 to the substrate 1, where the third layer aH is a virtual layer of the reference wavelength, and a is zero or a positive even number. By comparing the optical admittance curves of 400-700nm of different film systems (different a values) with the optical admittance curves of 50% reflectivity, as shown in Figure 3As shown in the figure, it can be seen that the film system 0.247H 0.362L 4H 1L 1H is the film system structure that is closest to the admittance curve, so it is used as the initial film system. Further optimizing the initial film system to meet the more uniform light splitting requirements in the 400-700nm band, we can obtain the optimized film system of 0.27H 0.364L 4.096H 1.091L 0.918H. By calculating the reflectivity spectrum curves of the initial film system and the optimized film system, as shown in Figure 4 As shown, it can be seen that both have a splitting ratio of approximately 1:1 in the 400-700nm band, and the optimized film system has a more uniform splitting ratio distribution in this required band.

[0038] Example 2: 2.5-15μm splitting ratio R / T = 2:3 splitting film

[0039] The band 2.5-15μm is a common band of Fourier transform infrared spectrometer, among which the beam splitter is one of the core components of the spectrometer. Achieving uniform beam splitting of 2.5-15μm plays an important role in improving the accuracy of Fourier transform. Taking into account the limitations on the selection of refractive index of thin film materials, we chose a beam splitter film with R / T=2:3 as a demonstration. In this example, without losing effectiveness, we chose a refractive index material of 1.52 as the substrate, a refractive index material of 4 as the high refractive index layer H, and a refractive index material of 1.52 as the low refractive index layer L. According to the above working principle and steps of the control method, the reference wavelength of 1.667μm is selected, and according to the beam splitting bandwidth λ max / λ min =6>2, we chose a gradient refractive index film design, selected 45 degrees as the incident angle, and the film structure used is as follows:

[0040] Initial film system: 0.9L 0.2H 0.9L 0.7L 0.6H 0.7L 0.5L 1H 0.5L 0.4L 1.2H 0.4L0.3L1.4H 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 0.303H1.885L0.528H 1.432L 0.832H 0.945L 1.22H 0.43L 2.236H

[0042] By removing the ultra-thin film of the optimized film system, a simplified film system with more practical research feasibility can be obtained, the structure of which is as follows:

[0043] Simplified membrane system: 0.141H 2.495L 0.315H 1.344L 0.597H 1.51L 2.689H 1.222L0.249H

[0044] By simulating and analyzing the above three film systems, we can obtain theoretical reflection spectra such as Figure 5 As shown. It can be seen that the reflectivity is close to 40% in the spectral band, but the spectrum fluctuation of the initial film system is the largest, the spectrum fluctuation of the optimized film system is the smallest, and it has the best spectral uniformity. The film layer structure of the simplified film system is the simplest, and it still has good spectral uniformity, which can be used as the actual film system selection for research and development.

Claims

1. A spectroscopic film with adjustable bandwidth and splitting ratio, characterized in that: The structure of the beam splitting film includes, from bottom to top, a substrate, an interface anti-reflection film, a virtual interface layer and air; The substrate is a material that is completely light-transmissive in the working wavelength range; The interface anti-reflection film is a multilayer interference anti-reflection film composed of high and low refractive index films or a broadband anti-reflection film based on a gradient refractive index film; The virtual interface layer is a virtual film layer with a thickness of zero, and its effective refractive index is calculated by the splitting ratio of the splitting film; The gradient refractive index refers to the refractive index of the film layer changing from the refractive index of the substrate to the refractive index of the virtual interface layer along the direction of film thickening, and an equivalent layer with a structure of xL(2-2x)H xL is used to form a refractive index distribution gradient in the thickness direction, wherein H and L represent high and low refractive index film layers with an optical thickness of 1 / 4 of the reference wavelength, respectively, and x represents the film thickness coefficient, which takes a value in the range of 0 to 1 according to the thickness distribution of the gradient refractive index film.

2. A method for controlling the spectral film with adjustable bandwidth and splitting ratio as claimed in claim 1, characterized in that: The method comprises the following steps: (1) Obtain the splitting ratio R / T, assuming that the refractive index of the virtual interface layer is n eff or 1 / n eff , so that the interface reflectivity between air and virtual interface layer is R, we can get n eff =(1+R 0.5 ) / (1-R 0.5 ); (2) Obtain the spectral bandwidth, the shortest wavelength is λ min , the longest wavelength is λ max , calculate λ max / λ min value; (3) For λ max / λ min <2 beam splitter film can be designed with conventional multilayer interference film to (2λ max λ min ) / (λ max +λ min ) is the reference wavelength, and a single-wavelength anti-reflection film is constructed based on a stack of high and low refractive index films of regular thickness, so that the equivalent interface refractive index of the film system is close to n eff or 1 / n eff ; (4) For λ max / λ min ≥2 beam splitter film, 3λ max / 8 as the total optical thickness reference of the initial film system, with 2λ min / 3 is used as the reference wavelength of the initial film system. Based on the equivalent layer theory, the gradient refractive index film is constructed. The stacking of the equivalent layers satisfies the total thickness just exceeding 3λ. max / 8, thus forming a broadband interface anti-reflection film; (5) Taking the splitting ratio R / T and the wavelength requirements as the optimization targets, the initial film system structure obtained in step (3) or (4) is optimized, and the optimized film system that meets the index requirements can be quickly obtained.

Images