High-optical-density infrared narrow-band passband and wide-cutoff-band filter based on CaF2 substrate

Abstract

This invention discloses an infrared narrow-band passband and wide-cutoff-band high-optical-density filter based on a CaF2 substrate. It comprises a front film system and a back film system designed on both sides of the CaF2 substrate, with each film system consisting of alternating deposits of Ge and ZnS. The front film system is: 0.762(0.5HL0.5H)^M1 + 1.246(0.5LH.5L)^M2; the back film system is: 0.56(0.5HL0.5H)^M3 + 0.39(0.5HL0.5H)^M4 + 0.28(0.5HL0.5H)^M5 + 0.2(0.5HL0.5H)^M6L. The resulting filter, when tested with a spectrometer, achieves a cutoff bandwidth of 1000nm-7100nm and 8500nm-20000nm, with an average cutoff optical density reaching OD4; the transmission band of 7669±120nm has an average transmittance of 98.85%, and a half-wavelength of 470nm±30nm.

Description

Technical Field

[0001] This invention relates to the field of filter technology, and in particular to a high optical density infrared narrow-band passband cutoff-band filter based on a CaF2 substrate. Background Technology

[0002] Methane gas (mainly CH4) detectors are widely used in security protection, detection and monitoring, and other fields. With the rapid development of technology, the accuracy requirements for filters in uncooled infrared thermal imagers are becoming increasingly stringent. As an important optical component of the filter, the filter must not only have a narrow transmission band at a specific wavelength, but also a wide cutoff band. The cutoff band should also have a high optical density to improve the signal-to-noise ratio.

[0003] Therefore, filters are needed to achieve high transmittance at the center wavelength of 7669nm, an average transmittance of 97% in the 7549nm-7789nm range, ultra-wide cutoff bands in the 1000nm-7000nm and 8600nm-20000nm ranges, and an average optical density of OD4. Currently, my country solves this technical problem by using two or three filters stacked together; no products achieving this effect with a single filter have been reported. Stacking multiple filters leads to complex and bulky structures, and unstable optical sensing performance, directly resulting in inconsistent product quality. This invention can solve the above problems with a single filter, and all indicators are improved compared to imported filters. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the above-mentioned technologies by providing a high-density infrared narrow-band passband and wide cutoff band filter based on a CaF2 substrate. This filter achieves a narrow-band high transmittance of 7669±120nm with an average transmittance of 98.83%, a half-wavelength of 460nm (7460nm-7920nm), and wide cutoff bands of 1000nm-7100nm and 8600nm-20000nm, with an average optical density of OD4 in the cutoff region.

[0005] To achieve the above objectives, this invention designs an infrared narrow-bandpass wide-cutoff-band high-optical-density filter based on a CaF2 substrate. It comprises a front-side film system and a back-side film system designed on both sides of the CaF2 substrate, with each film system composed of alternately deposited Ge and ZnS optical materials. The front-side film system on one side of the substrate is based on a long-pass base film system (0.5HL0.5H)^M and a short-pass base film system (0.5LH0.5L)^M, with the coefficients set as follows: 0.762(0.5HL0.5H)^M1 + 1.246(0.5LH.5L)^M2. The back-side film system on the other side of the substrate is based on a long-pass base film system (0.5HL0.5H)^M, with the coefficients set as follows: 0.56(0.5HL0.5H)^M3 + 0.39(0.5HL0.5H)^M4. 0.28(0.5HL0.5H)^M50.2(0.5HL0.5H)^M6L; where: H represents the high refractive index material Ge, L represents the low refractive index material ZnS, and M, M1, M2, M3, M4, M5, and M6 represent the iteration number, wherein the iteration number of M1 and M2 is 6, 7, or 8; and the iteration number of M3, M4, M5, and M6 is 10.

[0006] This invention also provides a method for fabricating a high-density infrared narrow-band passband and cutoff-band filter based on a CaF2 substrate. Using the aforementioned CaF2-based infrared narrow-band passband and cutoff-band high-density filter substrate and its designed front and back film systems, a Guangchi OTFC-1300 vacuum coating equipment is used to deposit the front and back film systems on a calcium fluoride substrate. This vacuum coating equipment is equipped with a plasma ion source, two e-shaped electron guns, a 12-point acupoint crucible, and a ring crucible. The vacuum system is a combination of a diffusion pump, a Roots pump, and a mechanical pump, and crystal control is used to monitor the physical thickness. Before coating the film, the calcium fluoride substrate needs to be ultrasonically cleaned and degreased in an acetone solution, and then wiped with a lint-free cloth soaked in a 2:1 ether-ethanol mixture. To ensure film uniformity, the wiped calcium fluoride substrate is placed on a fixture, and then the fixture with the substrate is fixed to the coating substrate. On the umbrella frame, the vacuum chamber door is closed, and the coating machine is evacuated. The workpiece tray speed is adjusted to 25 RPM. When the vacuum level reaches 2.0*10-3 Pa (or higher), the front-side coating system is deposited. During the deposition of the calcium fluoride substrate, when the temperature rises to 200℃, the substrate is cleaned using a plasma ion source for 300-600 seconds. Coating is then performed according to the physical thickness of each coating obtained from the front-side coating system. After the coating begins, ion beam irradiation is used throughout the deposition process. After the coating is completed, the temperature begins to drop. Once it reaches room temperature, the filter is removed, its surface is cleaned, and it is flipped and placed on the clamping ring. Then, the clamping ring with the front-side coating substrate is placed on the workpiece tray. The equipment is evacuated, and the workpiece tray speed is adjusted to 25 RPM to prepare for the deposition of the back-side coating system. Coating is then performed according to the physical thickness of the second coating system. After the coating begins, ion beam irradiation is used throughout the deposition process. After the coating is completed, the temperature begins to drop. Once it reaches room temperature, the filter is removed.

[0007] This invention also provides the application of a CaF2-based infrared narrow-band passband and wide-cutoff-band high-optical-density filter in methane gas detection.

[0008] This invention uses calcium fluoride as a substrate, which, compared to conventional silicon and germanium substrates, exhibits very high transmittance near 7669nm (center wavelength, transmission band of narrowband filters) (reaching approximately 94% even without coating) and good cutoff band in the 16000nm-20000nm range (e.g., ...). Figure 8 As shown in the figure, this makes it easier to reduce the design difficulty of narrowband filters.

[0009] This invention is based on the Fabry-Perot optical thin film design theory and long-pass optical thin film design theory to design and fabricate film systems using electron beam evaporation deposition and ion source-assisted deposition processes. The front side of the filter is designed based on the Fabry-Perot optical thin film design theory to obtain the front film system of the substrate material; the back side of the filter is designed based on the long-pass optical thin film design theory to obtain the front film system of the substrate material. This results in a CaF2-based infrared narrow-bandpass wide-cutoff-band high-optical-density filter with a predetermined material and physical thickness applied to both the front and back sides of the calcium fluoride substrate.

[0010] The technical advantages of the CaF2-based infrared narrow-band, wide-cutoff-band, high-optical-density filter obtained by this invention are as follows: 1. The cutoff bandwidth ranges from 1000nm to 7100nm and 8500nm to 20000nm, with an average cutoff optical density reaching OD4. 2. The average transmittance of the transmission band at 7669±120nm reaches 98.86%, and the half-wavelength is 470nm±30nm. This solves the problem of balancing the narrow bandwidth of a single-piece methane detection infrared filter with a wide cutoff band of 1000nm-7100nm and 8500nm-20000nm. Furthermore, all indicators are improved compared to imported filters, and this invention fills a gap in the domestic market. Attached Figure Description

[0011] Figure 1 This is the spectral curve of the front-side film system design in Example 1;

[0012] Figure 2 This is a spectral curve of the back-side film system design in Example 1;

[0013] Figure 3 This is a spectral curve of the filter obtained in Example 1;

[0014] Figure 4 This is the spectral curve of Example 2;

[0015] Figure 5 This is the spectral curve of Example 3;

[0016] Figure 6 This is the spectral curve of Example 4;

[0017] Figure 7 This is the spectral curve of Example 5;

[0018] Figure 8 This is a spectrum curve of a calcium fluoride substrate;

[0019] Figure 9 This is a schematic diagram of the filter system structure.

[0020] In the schematic diagram of the filtering system structure: 1. Narrowband filter; 2. Filter wheel; 3. Servo motor; 4. Electric driver; 5. Motor encoder. Detailed Implementation

[0021] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0022] Example 1:

[0023] This embodiment provides an infrared narrow-bandpass wide-cutoff-band high-optical-density filter based on a CaF2 substrate. It comprises a front-side film system and a back-side film system designed on both sides of the CaF2 substrate. Each film system is composed of alternately deposited Ge and ZnS optical materials. The front-side film system on one side of the substrate is based on a long-pass base film system (0.5HL0.5H)^M and a short-pass base film system (0.5LH0.5L)^M. After applying a coefficient, the front-side film system is: 0.762(0.5HL0.5H)^M1

[0024] 1.246(0.5LH.5L)^M2; The back film system on the other side of the substrate is based on the long-wavelength pass basic film system (0.5HL0.5H)^M, and after coefficients, the back film system is set as: 0.56(0.5HL0.5H)^M3 0.39(0.5HL0.5H)^M4 0.28(0.5HL0.5H)^M5 0.2(0.5HL0.5H)^M6 L; where: H represents the high refractive index material Ge, L represents the low refractive index material ZnS, M, M1, M2, M3, M4, M5, and M6 represent the iteration number, wherein the iteration number of M1 and M2 is 6, 7, or 8; the iteration number of M3, M4, M5, and M6 is 10; wherein the layer thickness of the substrate is 1-5mm. In this embodiment, the physical thickness of the CaF2 material substrate is selected as 3mm.

[0025] For iteration numbers M1 and M2, if less than 5 is selected, the obtained spectral effect cannot meet the design requirements, and the cutoff region has secondary peaks that affect the cutoff effect; if more than 8 is selected, the cutoff region also has secondary peaks that affect the cutoff effect, and the obtained effect is not ideal.

[0026] In this embodiment, M1 and M2 in the front film system are selected as 6, and M3, M4, M5, and M6 in the back film system are selected as 10. (M3 to M5 are selected as 10 because after the back surface is optimized, the software will automatically optimize the optimal number of layers. 10 is just a reference value. It is better to design it to be larger so that the design effect is better. The same applies to the following embodiments.) to obtain the best overall spectral effect.

[0027] The front and back film systems were optimized using the Variable Metric method in TFCalc software, and the physical thicknesses of the front and back film layers were calculated as shown in Tables 1 and 2, respectively. (It is well known to those skilled in the art that the physical thickness of each coating layer is obtained by optimizing the determined film system using the Variable Metric method in TFCalc software based on the designed film system, and will not be described in detail here.)

[0028] Table 1: Physical thickness of the front side in Example 1

[0029]

[0030]

[0031] The spectral curve obtained after separately depositing the front-side film system on the substrate in this embodiment is shown in the figure below. Figure 1 As shown.

[0032] Table 2: Physical thickness of the back side in Example 1

[0033]

[0034]

[0035]

[0036] The spectral curve obtained in this embodiment after separately depositing the back-side film system on the substrate is shown in the figure below. Figure 2 As shown.

[0037] This embodiment also provides a method for fabricating a high-density infrared narrow-band passband cutoff-band filter based on a CaF2 substrate. Using the aforementioned substrate and the designed front and back film systems, the first and second film systems are deposited using a Guangchi OTFC-1300 vacuum coating machine. This equipment is equipped with a plasma ion source, two e-shaped electron guns, a 12-point acupoint crucible, and a ring crucible. The vacuum system is a combination of a diffusion pump, a Roots pump, and a mechanical pump. Crystal control is used to monitor the physical thickness.

[0038] Before depositing the thin film, the calcium fluoride substrate needs to be ultrasonically cleaned and degreased in an acetone solution, and then wiped with a lint-free cloth soaked in a 2:1 mixture of ether and ethanol. To ensure film uniformity, the wiped calcium fluoride substrate is placed on a fixture, and then the fixture with the substrate is fixed to the coating machine frame. The vacuum chamber door is then closed, the coating machine is evacuated, and the workpiece tray speed is adjusted to 25 RPM.

[0039] The first film system is deposited when the vacuum level reaches 2.0*10-3 Pa (or higher). During the deposition of the calcium fluoride substrate, when the temperature rises to 200℃, the substrate is cleaned using a plasma ion source for 300-600 seconds. Deposition begins according to the physical thickness of the first film system. After deposition begins, ion beam irradiation assists the film deposition process throughout. After deposition is completed, the temperature begins to drop. Once it reaches room temperature, the filter is removed, its surface is cleaned, and it is flipped and placed on the clamping ring. The clamping ring with the substrate is then placed on the workpiece tray. The equipment is evacuated, and the workpiece tray rotation speed is adjusted to 25 R / min, ready for the deposition of the second film system.

[0040] The first film system is deposited on the front side when the vacuum level reaches 2.0*10-3 Pa (or higher). During the deposition of the calcium fluoride substrate, when the temperature rises to 200℃, the substrate is cleaned using a plasma ion source for 300-600 seconds. The second film system is then deposited according to its physical thickness. After deposition begins, ion beam irradiation is used throughout the entire process to assist thin film deposition. The process steps for the front and back films are basically the same, differing only in the number of layers and their physical thickness. After deposition, the temperature is lowered until it reaches room temperature, at which point the filter is removed.

[0041] The optical filter designed and finally manufactured in this embodiment, after depositing the front and back film layers on the substrate material, has the following spectral curve: Figure 3 As shown in the figure, the spectral curve indicates that, according to spectrometer testing, the cutoff bandwidth of this filter ranges from 1000nm to 7100nm and from 8500nm to 20000nm, with an average cutoff optical density reaching OD4. The average transmittance at the transmission band of 7669±120nm reaches 98.85%, and the half-wavelength is 496nm.

[0042] Example 2:

[0043] The infrared narrow-band passband and wide-cutoff-band high-optical-density filter based on CaF2 substrate provided in this embodiment uses the same film system and production method as in Embodiment 1. The difference is that in this embodiment, the physical thickness of the CaF2 material substrate is selected as 1 mm. In the front film system, M1 and M2 are both 7, and in the back film system, M3, M4, M5, and M6 are all 10. The front and back film systems are optimized using the Variable Metric method of TFCalc software, and the physical thicknesses of the front and back film layers are calculated as shown in Tables 3 and 4.

[0044] Table 3: Physical thickness of the front side in Example 2

[0045]

[0046]

[0047] Table 4: Physical thickness of the back side in Example 2

[0048]

[0049]

[0050]

[0051] The filter produced according to the above physical data, after spectral detection, yields the following results: Figure 4 The spectral curves shown indicate that the filter has a cutoff bandwidth of 1000nm–7100nm and 8500nm–20000nm, with an average cutoff optical density reaching OD4.2. The transmission band of 7669±120nm has an average transmittance of 98.85% and a half-wavelength of 472nm.

[0052] Example 3:

[0053] The infrared narrow-band passband and wide-cutoff-band high-optical-density filter based on a CaF2 substrate provided in this embodiment uses the same film system and production method as in Embodiment 1. The difference is that the physical thickness of the CaF2 material substrate is selected as 5mm in this embodiment. The front film system M1 and M2 are both 8, and the back film systems M3, M4, M5, and M6 are all 10. The front and back film systems are optimized using the Variable Metric method in TFCalc software, and the physical thicknesses of the front and back film layers are calculated as shown in Tables 5 and 6, respectively.

[0054] Table 5: Physical thickness of the front side in Example 3

[0055]

[0056]

[0057] Table 6: Physical thickness of the back side in Example 3

[0058]

[0059]

[0060]

[0061]

[0062] The filter produced according to the above physical data, after spectral detection, yields the following results: Figure 5The spectral curve shown indicates that, according to spectrometer testing, the cutoff bandwidth of this filter ranges from 1000nm to 7100nm and from 8500nm to 20000nm, with an average cutoff optical density reaching OD4.2. The average transmittance of the transmission band at 7669±120nm reaches 98.86%, and the half-wavelength is 486nm.

[0063] Example 4:

[0064] The infrared narrow-band passband and wide-cutoff-band high-optical-density filter based on a CaF2 substrate provided in this embodiment uses the same film system and production method as in Embodiment 1. The difference is that the physical thickness of the CaF2 material substrate is selected as 4 mm in this embodiment. In the front film system, M1 is 6 and M2 is 8, while in the back film system, M3, M4, M5, and M6 are 10. The front and back film systems are optimized using the Variable Metric method in TFCalc software, and the physical thicknesses of the front and back film layers are calculated as shown in Tables 7 and 8, respectively.

[0065] Table 7. Front view of the third embodiment

[0066]

[0067]

[0068] Table 8. Back side of the third embodiment

[0069]

[0070]

[0071]

[0072] The filter produced according to the above physical data, after spectral detection, yields the following results: Figure 6 The spectral curves shown indicate that the filter has a cutoff bandwidth of 1000nm–7100nm and 8500nm–20000nm, with an average cutoff optical density reaching OD4.2. The transmission band at 7669±120nm has an average transmittance of 98.81% and a half-wavelength of 493nm.

[0073] Example 5:

[0074] The infrared narrow-band passband and wide-cutoff-band high-optical-density filter based on a CaF2 substrate provided in this embodiment uses the same film system and production method as in Embodiment 1. The difference is that the physical thickness of the CaF2 material substrate is selected as 2 mm in this embodiment. In the front film system, M1 is 8 and M2 is 6, while in the back film system, M3, M4, M5, and M6 are 10. The front and back film systems are optimized using the Variable Metric method in TFCalc software, and the physical thicknesses of the front and back film layers are calculated as shown in Tables 9 and 10, respectively.

[0075] Table 9: Front view of Example 5

[0076] number of floors Material Physical thickness (nm) 1 Ge 201.58 2 ZnS 669.26 3 Ge 385.78 4 ZnS 615.37 5 Ge 328.29 6 ZnS 634 7 Ge 333.57 8 ZnS 695.7 9 Ge 347.35 10 ZnS 742.14 11 Ge 366.16 12 ZnS 690.2 13 Ge 366.44 14 ZnS 676.07 15 Ge 326.29 16 ZnS 583.51 17 Ge 216.21 18 ZnS 378.87 19 Ge 681.22 20 ZnS 1061.1 21 Ge 541.67 22 ZnS 1058.26 23 Ge 558.96 24 ZnS 1048.38 25 Ge 549.54 26 ZnS 1161.33 27 Ge 614.14 28 ZnS 1089.33 29 Ge 529.68 30 ZnS 499.86

[0077] Table 10: Back side of Example 5

[0078]

[0079]

[0080] The filter produced according to the above physical data, after spectral detection, yields the following results: Figure 7 The spectral curve shown indicates that, according to spectrometer testing, the cutoff bandwidth of this filter ranges from 1000nm to 7100nm and from 8500nm to 20000nm, with an average cutoff optical density reaching OD4.2. The average transmittance of the transmission band at 7669±120nm reaches 98.59%, and the half-wavelength is 451nm.

[0081] Example 6:

[0082] This embodiment describes the application of the high-density infrared narrow-bandpass cutoff-band filter based on a CAF2 substrate obtained by the present invention in methane gas detection, such as... Figure 9 As shown, the device comprises a narrowband filter 1, a filter wheel 2, a servo motor 3, an electric driver 4, and a motor encoder 5. The device uses a mechanical structure to connect the servo motor 3 to the filter wheel 2. Four circular holes with a diameter of 25mm are pre-set on the filter wheel 2. One of these holes is left empty, the second hole is fitted with a full absorption filter as a blackbody window, and the other two holes are fitted with corresponding narrowband filters. The center wavelengths of the two narrowband filters are 3310nm and 7669nm, respectively. Since methane gas (mainly CH4) has two characteristic infrared absorption peaks, 3310nm and 7669nm, the filter with a center wavelength of 7669nm is the high-density infrared narrowband passband cutoff band filter based on a CAF2 substrate obtained in the above embodiment of the present invention.

[0083] Methane gas exhibits its strongest absorption peak at 3310 nm. However, besides methane, most alkanes, alkenes, and alkynes also have absorption peaks in the 3200 nm–3400 nm range. Narrowband filters cannot achieve sufficiently narrow passbands, allowing other gases to be detected without accurate identification; in other words, it's impossible to determine whether the detected gas is methane. Methane, on the other hand, has a strong absorption peak at 7669 nm. Other organic gases do not show significant characteristic absorption peaks in this wavelength range. Therefore, by using a narrowband filter with a center wavelength of 3310 nm and adding a narrowband filter with a center wavelength of 7669 nm, the presence of methane gas can be accurately detected. The specific methods for analyzing the data obtained through these filters are well-known to those skilled in the art and will not be described in detail here.

Claims

1. A high-density infrared narrow-bandpass cutoff-band filter based on a CaF2 substrate, comprising a front-side film system and a back-side film system designed on both sides of the CaF2 substrate, each film system being composed of alternately deposited Ge and ZnS optical materials, characterized in that... The front membrane system on one side of the substrate is based on the long-pass basic membrane system (0.5HL0.5H)^M and the short-pass basic membrane system (0.5LH0.5L)^M, and after coefficients, the front membrane system is defined as: 0.762(0.5HL0.5H)^M1, 1.246(0.5LH0.5L)^M2; the back membrane system on the other side of the substrate is based on the long-pass basic membrane system (0.5HL0.5H)^M, and after coefficients, the back membrane system is defined as: 0.56(0.5HL0.5H)^M3, 0.39(0.5HL0.5H)^M4, 0.28(0.5HL0.5H)^M5, 0.2(0.5HL0.5H)^M6. L; where: H represents the high refractive index material Ge, L represents the low refractive index material ZnS, M, M1, M2, M3, M4, M5, and M6 represent the iteration number, wherein the iteration number of M1 and M2 is 6, 7, or 8; and the iteration number of M3, M4, M5, and M6 is greater than 10.

2. A method for fabricating an infrared narrow-bandpass wide-cutoff-band high-optical-density filter based on a CaF2 substrate according to claim 1, characterized in that: The OTFC-1300 vacuum coating equipment from Guangchi was used to deposit front and back films on calcium fluoride substrates. This equipment is equipped with a plasma ion source, two e-shaped electron guns, a 12-point acupoint crucible, and a ring crucible. The vacuum system is a combination of diffusion pumps, Roots pumps, and mechanical pumps, and physical thickness is monitored using crystal control. Before coating, the calcium fluoride substrate is ultrasonically cleaned in acetone solution to remove oil, and then wiped with a lint-free cloth soaked in a 2:1 ether-ethanol mixture. To ensure film uniformity, the wiped calcium fluoride substrate is placed on a fixture, and then the fixture with the substrate is fixed to the coating machine frame. The vacuum chamber door is then closed, and the coating machine is evacuated. The workpiece tray speed is adjusted to 25 RPM. When the vacuum level reaches 2.0 × 10⁻³ Pa or higher... The front-side coating process begins. During the deposition of the calcium fluoride substrate, when the temperature rises to 200℃, the substrate is cleaned using a plasma ion source for 300-600 seconds. Coating begins according to the physical thicknesses of each coating obtained from the front-side coating. Ion beam irradiation assists in film deposition throughout the process. After coating, the temperature begins to drop. Once it reaches room temperature, the filter is removed, its surface is cleaned, and it is flipped and placed on a clamping ring. The clamping ring with the front-side coating substrate is then placed on the workpiece tray. The equipment is evacuated, and the workpiece tray speed is adjusted to 25 R / min to prepare for the back-side coating. Coating begins according to the physical thicknesses of the second coating system. Ion beam irradiation assists in film deposition throughout the process. After coating, the temperature begins to drop. Once it reaches room temperature, the filter is removed.

3. The application of the high optical density infrared narrow-band passband cutoff band filter based on CaF2 substrate as described in claim 1 in methane gas detection.

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