A mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal, its fabrication method and application

Abstract

This invention discloses a mid-to-far-infrared ultra-wideband neutral beamsplitter based on KBr crystal, its fabrication method, and its applications, relating to the field of optical thin film technology. The KBr crystal-based mid-to-far-infrared ultra-wideband neutral beamsplitter includes a KBr crystal substrate; a front surface adhesive layer disposed on the front surface of the KBr crystal substrate; an ultra-wideband neutral beam splitter disposed on the surface of the front surface adhesive layer; a rear surface adhesive layer disposed on the rear surface of the KBr crystal substrate; and an ultra-wideband protective film disposed on the surface of the rear surface adhesive layer. The front and rear surface adhesive layers are both magnesium fluoride thin films; the ultra-wideband neutral beam splitter has a multilayer dielectric film structure; and the ultra-wideband protective film is a barium fluoride thin film. This invention innovatively introduces a double-sided composite thin film structure of "front surface adhesive layer + beam splitter system" and "rear surface adhesive layer + protective layer".

Description

Technical Field

[0001] This invention relates to the field of optical thin film technology, specifically to a mid-to-far-infrared ultrawideband neutral beam splitter based on KBr crystal, its fabrication method, and its application. Background Technology

[0002] In infrared hyperspectral remote sensing and atmospheric sounding systems, interferometric spectroscopy techniques (such as Fourier transform infrared spectrometers, FTIR) have become an important technical approach for space observation and Earth-atmosphere system research due to their high spectral resolution and high throughput. The beam splitter, as the core optical element of FTIR, works by splitting an incident broadband beam into transmitted and reflected light. The two beams are then reflected by a moving mirror and a fixed mirror, and re-merge to generate interference fringes. Compared to traditional dispersive grating beam splitting, interferometric beam splitting using high-performance beam splitters improves the optical transmission efficiency of the infrared channel and achieves a higher signal-to-noise ratio because it eliminates the need for slits and spatial frequency separation. In fields such as meteorological satellites, deep space exploration, and military security, these beam splitters help systems efficiently acquire Earth observation data with ultra-wide spectral coverage, playing a crucial role in atmospheric temperature and humidity profile inversion, trace gas monitoring, and all-weather target identification under complex climatic conditions.

[0003] With the further development of ultrawide spectral imaging technology and advanced materials analysis technology in space remote sensing, the operating band coverage of various detection systems is constantly expanding. Traditional infrared interferometric spectrometers typically require multiple beamsplitters of different frequency bands (e.g., using different materials for mid-wave and long-wave) and switching them via mechanical mechanisms to cover the entire infrared band. This not only increases the complexity of the rear optical path structure design and reduces the utilization rate of spectral channel resources, but also reduces the spatial reliability of the system due to the introduction of mechanical moving parts. Therefore, achieving ultrawide spectral splitting using a single element has gradually become an important development direction. Beamsplitters based on KBr crystals, with their wide infrared transmission and low absorption characteristics, can achieve continuous, efficient, uniform, and neutral beam splitting in the 2–25 μm band. This ultrawide spectral beamsplitter has good spectral compatibility, can meet the transmission requirements of mid- and long-wave infrared signals under a shared optical path, and replaces the traditional complex switching system with a compact, non-moving-part broadband beam splitting design, thereby reducing optical transmission loss in the transmission optical path and reducing the weight of the optical system.

[0004] However, existing KBr crystal-based beam splitters still face significant technical challenges in practical fabrication and application. KBr crystals themselves have low hardness, a high coefficient of thermal expansion, and strong hygroscopic properties. This leads to difficulties in matching the internal stress of the thin film when commonly used mid- and far-infrared high and low refractive index materials (such as germanium and zinc sulfide) are directly deposited onto the KBr crystal substrate surface under conventional processing conditions. This results in problems such as film delamination, cracking, or surface distortion. Furthermore, without effective isolation and protection, KBr crystal substrates are susceptible to moisture damage due to environmental humidity, exhibiting poor environmental stability. These material characteristics and processing defects severely restrict the engineering application of KBr ultrawideband beam splitters in high-end commercial instruments and complex space environments.

[0005] Therefore, there is an urgent need to provide a KBr crystal-based mid- and far-infrared ultrawideband neutral beam splitter and its fabrication method that can improve film adhesion and environmental stability. Summary of the Invention

[0006] To address the technical bottlenecks in the preparation and application of existing KBr crystal-based beam splitters, such as difficulty in matching internal stress in the film layer, easy moisture absorption and deliquescence of the substrate, and low spectral efficiency, the present invention aims to provide a mid-far-infrared ultrawideband neutral beam splitter based on KBr (potassium bromide) crystal and its preparation method.

[0007] A further technical problem to be solved by the present invention is to provide an application of a mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A mid-to-far-infrared ultrawideband neutral beam splitter based on KBr crystal, comprising:

[0010] KBr crystal substrate;

[0011] A front surface adhesive layer is set on the front surface of the KBr crystal substrate;

[0012] An ultra-broad spectrum neutral spectral film is disposed on the surface of the front surface adhesive layer;

[0013] A back surface bonding layer disposed on the back surface of a KBr crystal substrate; and,

[0014] An ultra-broad spectrum protective film is applied to the surface of the adhesive layer on the rear surface;

[0015] The front and rear surface adhesive layers are both magnesium fluoride films; the ultra-wideband neutral spectral film is a multilayer dielectric film structure; and the ultra-wideband protective film is a barium fluoride film.

[0016] The ultra-wideband neutral spectral film is a multilayer dielectric film composed of three materials. The ultra-wideband neutral spectral film includes alternating deposited high refractive index film, medium refractive index film and low refractive index film. The high refractive index film is made of germanium, the medium refractive index film is made of zinc sulfide and the low refractive index film is made of barium fluoride.

[0017] The ultrawideband neutral spectral film comprises, from the front surface adhesive layer outward, a bandwidth-broadening equivalent matching stack composed of barium fluoride and zinc sulfide, and a spectral film composed of a zinc sulfide film layer and a germanium film layer.

[0018] The bandwidth-strengthened equivalent matching layer membrane structure is (k1L k2M). n The film structure of the spectral splitting film is (a1H a2M a3H); where L represents a barium fluoride film with an optical thickness of one-quarter of the center wavelength, M represents a zinc sulfide film with an optical thickness of one-quarter of the center wavelength, and H represents a germanium film with an optical thickness of one-quarter of the center wavelength; n is the number of stacking periods, 1≤n≤8; k1, k2, a1, a2, and a3 are film thickness optimization coefficients.

[0019] The front and rear adhesive layers have the same thickness.

[0020] The thickness of both the front and rear surface adhesive layers is 20 nm; the thickness of the ultra-wide spectrum protective film is 150 nm to 250 nm.

[0021] The above-mentioned method for fabricating a mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal includes the following steps:

[0022] Vacuum degree better than 2×10 -3 Under high vacuum conditions of Pa, the KBr crystal substrate is heated to 170℃~200℃; a front surface bonding layer and an ultra-wide spectrum neutral spectral film are sequentially deposited on the front surface of the KBr crystal substrate using physical vapor deposition technology, and a rear surface bonding layer and an ultra-wide spectrum protective film are sequentially deposited on the rear surface of the KBr crystal substrate.

[0023] Before depositing the pre- and post-deposition surface bonding layers, the surface of the KBr crystal substrate is pre-cleaned by bombarding it with an ion beam. The ion beam bombardment uses a Hall ion source, and the operating parameters of the ion beam bombardment are: anode voltage of 120 V to 150 V and anode current of 3 A to 5 A.

[0024] The deposition rate of barium fluoride film in ultra-wideband neutral spectral film is 0.6~0.8 nm / s, the deposition rate of zinc sulfide film in ultra-wideband neutral spectral film is 2.1~2.8 nm / s, and the deposition rate of germanium film in ultra-wideband neutral spectral film is 0.15~0.25 nm / s.

[0025] The KBr crystal-based mid-to-far-infrared ultrawideband neutral beam splitter is used to uniformly split incident light in the 2–25 μm band.

[0026] The beneficial effects of this invention are as follows:

[0027] (1) This invention innovatively introduces a double-sided composite thin film structure consisting of a "front surface adhesive layer + beam splitting film system" and a "rear surface adhesive layer + protective layer". Magnesium fluoride (MgF2) is used as the bottom adhesive layer on both the front and rear surfaces, which improves the adhesion between the film layer and the substrate, as well as the bonding force between the film layers. At the same time, the BaF2 protective layer deposited on the back of the substrate above the adhesive layer forms a physical isolation from external moisture, improving the long-term reliability of the beam splitter in complex spaces and high humidity environments.

[0028] (2) Based on the wide infrared transmission characteristics of the KBr crystal substrate and the optimized broadband neutral beam splitting film system design, this invention can achieve efficient and uniform beam splitting in the continuous spectral range of 2 to 25 μm. This design allows the optical system to replace the traditional multi-beam splitter mechanical switching structure with a single broadband beam splitting element, which helps to reduce optical path transmission loss and improve system integration and reliability.

[0029] (3) The preparation process of the present invention is reasonable and feasible, and the physical and optical properties of the product are stable. It not only meets the spectroscopic requirements of infrared hyperspectral detection systems such as deep space exploration, atmospheric temperature and humidity profile inversion, and trace gas monitoring, but is also an indispensable key optical component of high-end commercial infrared Fourier spectrometers (FTIR). Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal of the present invention.

[0031] Figure 2 The design curves for the KBr crystal-based mid-far-infrared ultrawideband neutral beam splitter of this invention are shown for a 2~25μm ultrawideband beam splitter at a 30° incident angle.

[0032] Figure 3 The measured spectral curves of the KBr crystal-based mid-far-infrared ultrawideband neutral beam splitter of the present invention are shown in the 3~20 μm band.

[0033] In the attached figures, the labels are as follows: 1-ultra-wideband neutral spectral film; 2-front surface adhesive layer; 3-KBr crystal substrate; 4-back surface adhesive layer; 5-ultra-wideband protective film; 6-incident beam; 7-reflected beam; 8-transmitted beam. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] This embodiment developed a mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal with an incident angle of 45° and a wavelength range of 3~20 μm.

[0036] like Figure 1 As shown, the mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal in this embodiment is a composite stacked structure consisting of a KBr crystal substrate 3, a front surface adhesive layer 2 and an ultrawideband neutral beam splitter 1 sequentially placed on the front surface of the KBr crystal substrate 3, a rear surface adhesive layer 4 and an ultrawideband protective film sequentially placed on the rear surface of the KBr crystal substrate 3.

[0037] Because KBr crystals have low hardness, a high coefficient of thermal expansion, and are extremely hygroscopic, conventional infrared film materials (such as Ge and ZnS) are prone to peeling off when directly deposited on a KBr crystal substrate due to low adhesion and difficulty in matching film stress. This embodiment innovatively uses magnesium fluoride (MgF2) films as both the front and rear surface adhesive layers 2 and 4, respectively, improving film adhesion. The physical thickness of both layers is controlled at 20 nm. This thickness design not only improves film adhesion but also ensures the optical performance of the beam splitter in the ultra-wide wavelength range of 3–20 μm.

[0038] The ultra-wideband neutral spectrophotometer 1 is a multilayer dielectric film composed of three materials. It includes alternating high-refractive-index, medium-refractive-index, and low-refractive-index layers. The high-refractive-index layer is made of germanium, the medium-refractive-index layer is made of zinc sulfide, and the low-refractive-index layer is made of barium fluoride. Preferably, the multilayer dielectric film of the ultra-wideband neutral spectrophotometer, from the front surface adhesive layer outwards, includes a bandwidth-broadening equivalent matching stack composed of barium fluoride and zinc sulfide, and a spectrophotometer layer composed of a zinc sulfide layer and a germanium layer. The specific structure of the bandwidth-broadening equivalent matching stack is (k1L k2M). nThe film structure of the spectral splitting film is (a1H a2M a3H); where L represents a barium fluoride film with an optical thickness of one-quarter of the center wavelength, M represents a zinc sulfide film with an optical thickness of one-quarter of the center wavelength, and H represents a germanium film with an optical thickness of one-quarter of the center wavelength; n is the number of stacking periods, which is determined by the bandwidth of the working band, 1≤n≤8; k1, k2, a1, a2, and a3 are film thickness optimization coefficients.

[0039] In this embodiment, the film structure of the ultrawideband neutral spectrophotometer 1 together with the front surface adhesive layer 2 is: substrate / 20nm MgF2 / (k1L k2M). n (a1H a2M a3H) / air.

[0040] Figure 2 This is the design curve of the KBr crystal-based mid-far-infrared ultrawideband neutral beamsplitter of the present invention, for a 2-25 μm ultrawideband spectral beamsplitter at a 30° incident angle. It is used to demonstrate the wideband adaptability and neutral beamsplitting potential of the design method of the present invention. For the 3-20 μm working band that this embodiment actually needs to achieve, [the curve is shown in the original text]. Figure 2 The corresponding design method is based on theory. In the initial structure, n is taken as 6. After optimization by substituting the actual process parameters of the film layer, the film system structure of the ultrawide spectrum neutral spectrophotometer 1 and the front surface adhesive layer 2 in this embodiment is as follows:

[0041] KBr crystal substrate / 20nm MgF2 / 0.767L 0.524M 2.08L 0.723M 2.329L 0.833M 2.29L 1.097M 1.817L 1.906M 1.034L 0.823M 0.538H 2.268M 4.059H / air.

[0042] Furthermore, a 20 nm back surface adhesive layer 4 is formed on the back surface of the KBr crystal substrate 3, and a barium fluoride (BaF2) thin film is further deposited on the surface of the back surface adhesive layer 4 as an ultra-broadband protective film 5 to improve the moisture-proof and barrier properties of the substrate. In this embodiment, the thickness of the ultra-broadband protective film 5 is controlled within the range of 150 nm to 250 nm. This thickness range improves the environmental protection effect while reducing the impact of absorption on the transmission spectral dispersive performance.

[0043] like Figure 1 As shown, the incident beam 6 is incident from the air side onto the front surface of the beam splitter. Part of the light is reflected to form the reflected beam 7; the other part of the light passes through the beam splitter to form the transmitted beam 8.

[0044] The specific steps of the fabrication process for the neutral beam splitter described in this embodiment are as follows:

[0045] (1) Place the cleaned KBr crystal substrate 3 in a vacuum chamber equipped with a diffusion pump system and seal the vacuum chamber. Wait until the background vacuum level reaches 1×10⁻⁶. -1 Pa ~ 1×10 -32 When Pa, start baking, set the baking temperature of KBr crystal substrate 3 to 180℃ and maintain it. Keep the KBr crystal substrate at the coating temperature for 2 hours to ensure uniform heating.

[0046] (2) When the vacuum degree reaches 2×10 -3 At Pa, the Hall ion source was turned on to pre-clean the KBr crystal substrate. The ion source anode voltage was set to 120 V, the anode current to 4 A, and the argon gas flow rate to 25 sccm; the pre-cleaning time was 10 min, and the Hall ion source was turned off after completion.

[0047] (3) A MgF2 bonding layer is deposited on the front surface of the KBr crystal substrate 3 to form the front surface bonding layer 2: electron beam evaporation deposition is used, the electron gun beam current is set to 45 mA, and the evaporation rate is set to 0.4 nm / s.

[0048] (4) Deposit an ultra-wide spectrum neutral spectral film 1 on the surface of the front surface adhesive layer 2 according to the above design; specifically, according to the above design, first deposit BaF2 film and ZnS film of corresponding thickness on the front surface adhesive layer 2 alternately to form a bandwidth broadening equivalent matching stack, and then deposit ZnS film and germanium film of corresponding thickness on the bandwidth broadening equivalent matching stack to form a spectral film layer.

[0049] The BaF2 films were deposited using electron beam evaporation, with the electron gun current set to 80 mA and the evaporation rate set to 0.8 nm / s. The ZnS films were deposited using electron beam evaporation, with the electron gun current set to 45 mA and the evaporation rate set to 2.8 nm / s. The Ge films were deposited using electron beam evaporation, with the electron gun current set to 230 mA and the evaporation rate set to 0.25 nm / s.

[0050] (5) A MgF2 bonding layer was deposited on the back surface of the KBr crystal substrate 3 to form a back surface bonding layer 4: electron beam evaporation deposition was used, with the electron gun beam current set to 45 mA and the evaporation rate set to 0.4 nm / s.

[0051] (6) A 150 nm thick ultra-wide spectrum protective film 5 is deposited on the surface of the back surface adhesive layer 4.

[0052] (7) After deposition, strictly control the heating system and use a cooling gradient of 0.5 ℃ / min to slowly cool the substrate to room temperature with the furnace, and then open the furnace to take out the part.

[0053] Figure 3The measured spectral curves of the KBr crystal-based mid-far-infrared ultrawideband neutral beam splitter of the present invention are shown in the 3~20 μm band. Figure 3 In this context, T and R represent the transmittance and reflectance of the beam splitter at an incident angle of 45°, respectively, and T / R is the transmittance / reflectance ratio. For spectral efficiency, from Figure 3 As can be seen from the data, the KBr crystal-based mid-to-far-infrared ultrawideband neutral beam splitter prepared in this embodiment exhibits a T:R ratio of approximately 0.9-1.2 within the ultrawide spectral range of 3–20 μm. This technology achieves efficient and uniform ultrawideband neutral beam splitting. The optical and physical performance of this KBr crystal-based mid-to-far-infrared ultrawideband neutral beam splitter is stable, meeting the stringent requirements of space optical systems.

[0054] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention. The above embodiments are provided only for the purpose of describing the present invention and are not intended to limit the present invention. Parts not described in detail in this specification are well-known in the art and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. All equivalent substitutions and modifications made without departing from the spirit and principle of the present invention should be covered within the scope of the present invention.

Claims

1. A mid-to-far-infrared ultra-wideband neutral beam splitter based on KBr crystal, characterized in that, The KBr crystal-based mid- and far-infrared ultrawideband neutral beam splitter includes: KBr crystal substrate; A front surface adhesive layer is set on the front surface of the KBr crystal substrate; An ultra-broad spectrum neutral spectral film is disposed on the surface of the front surface adhesive layer; A back surface bonding layer disposed on the back surface of a KBr crystal substrate; and, An ultra-broad spectrum protective film is applied to the surface of the adhesive layer on the rear surface; The front and rear surface adhesive layers are both magnesium fluoride films; the ultra-wideband neutral spectral film is a multilayer dielectric film structure; and the ultra-wideband protective film is a barium fluoride film.

2. The mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 1, characterized in that, The ultrawideband neutral spectral film is a multilayer dielectric film composed of three materials. The ultrawideband neutral spectral film includes alternating deposited high refractive index film, medium refractive index film and low refractive index film; wherein, the material of the high refractive index film is germanium, the material of the medium refractive index film is zinc sulfide and the material of the low refractive index film is barium fluoride.

3. The mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 2, characterized in that, The multilayer dielectric film of the ultrawideband neutral spectral film includes, from the front surface adhesive layer outward, a bandwidth-broadening equivalent matching stack composed of barium fluoride and zinc sulfide, and a spectral film composed of zinc sulfide film and germanium film.

4. The mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 3, characterized in that, The equivalent matching layer membrane structure for bandwidth broadening is (k1L k2M). n The film structure of the spectral splitting film is (a1H a2M a3H); where L represents a barium fluoride film with an optical thickness of one-quarter of the center wavelength, M represents a zinc sulfide film with an optical thickness of one-quarter of the center wavelength, and H represents a germanium film with an optical thickness of one-quarter of the center wavelength; n is the number of stacking periods, 1≤n≤8; k1, k2, a1, a2, and a3 are film thickness optimization coefficients.

5. The mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 1, characterized in that, The thickness of the front and rear surface adhesive layers is equal.

6. The mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 5, characterized in that, The thickness of both the front and rear surface adhesive layers is 20 nm; the thickness of the ultra-wide spectrum protective film is 150 nm to 250 nm.

7. The method for fabricating a mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to any one of claims 1 to 6, characterized in that, Includes the following steps: Vacuum degree better than 2×10 -3 Under high vacuum conditions of Pa, the KBr crystal substrate is heated to 170℃~200℃; a front surface bonding layer and an ultra-wide spectrum neutral spectral film are sequentially deposited on the front surface of the KBr crystal substrate using physical vapor deposition technology, and a rear surface bonding layer and an ultra-wide spectrum protective film are sequentially deposited on the rear surface of the KBr crystal substrate.

8. The method for fabricating a mid-far-infrared ultra-wideband neutral beam splitter based on KBr crystal according to claim 7, characterized in that, Before depositing the pre- and post-contamination surface adhesive layers, the surface of the KBr crystal substrate is pre-cleaned by bombarding it with an ion beam. The ion beam bombardment uses a Hall ion source, and the operating parameters of the ion beam bombardment are: anode voltage of 120V to 150V and anode current of 3A to 5A.

9. The method for fabricating a mid-far-infrared ultrawideband neutral beam splitter based on KBr crystal according to claim 7, characterized in that, The deposition rate of barium fluoride film in ultra-wideband neutral spectral film is 0.6~0.8 nm / s, the deposition rate of zinc sulfide film in ultra-wideband neutral spectral film is 2.1~2.8 nm / s, and the deposition rate of germanium film in ultra-wideband neutral spectral film is 0.15~0.25 nm / s.

10. The application of the KBr crystal-based mid-to-far-infrared ultra-wideband neutral beam splitter according to any one of claims 1 to 6, characterized in that, The KBr crystal-based mid-to-far-infrared ultrawideband neutral beam splitter is used to split incident light in the 2–25 μm band.

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