A method for preparing a linear gradient narrowband filter

Abstract

The present invention discloses a method for preparing a linear gradient narrowband filter. The substrate is a glass strip. The linear gradient characteristic of the film thickness distribution is achieved by adjusting the geometric configuration of the vacuum evaporation system, that is, the relative position between the glass strip substrate and the film material evaporation source. The geometric configuration mainly includes: the distance h between the plane of the glass strip substrate and the evaporation plane of the evaporation source, the distance L between the center of the evaporation source and the rotation center axis of the substrate, and the distance ρ between any film thickness observation point selected on the glass strip substrate of the fixture disk and the rotation center axis. The film thickness distribution is to calculate the relationship between the film thickness ratio t / t0 and ρ / L under different geometric configurations h / L. When h / L>1.5, t / t0 shows a very good linear relationship with the change of ρ / L when ρ / L>0.8, thereby obtaining various filters with continuous linear gradient of film thickness. h / L is usually selected from 1.5 to 2.5, and the present invention preferably selects h / L=1.8. The filter adopts a three-cavity Fabry-Perot structure.

Description

Technical Field

[0001] The invention relates to a method for preparing a linear gradient narrowband filter, and belongs to the technical field of design and manufacture of optical thin film filters. Background Art

[0002] Narrowband filters with gradient film thickness or center wavelength can usually be divided into two types: linear strip gradient narrowband filters and circular gradient narrowband filters. The spectral characteristics of linear strip gradient narrowband filters are linear gradients in a certain direction as the position of the strip filters changes, while the spectral characteristics of circular gradient narrowband filters are gradients in the circumferential direction as the angle of the circular filter changes.

[0003] Gradient narrowband filters can replace bulky prism or grating monochromators to form portable and fast spectrometers, or they can be integrated with CCD or CMOS array detectors to form miniature spectrometers. Using gradient narrowband filters allows for compact instrumentation and greatly simplifies the optical and mechanical systems, resulting in advantages such as light weight, small size, high stability, and excellent reliability. Furthermore, because the passband position and shape, as well as the cutoff degree and bandwidth, of gradient narrowband filters can be freely designed, they offer advantages such as high resolution and high optical efficiency. Due to these characteristics, gradient narrowband filters have been successfully applied in a wide range of fields, including optical communications, aerospace, field exploration, atmospheric monitoring, television imaging, and infrared alarm systems, enabling spectral separation, imaging, analysis, correction, and secondary spectral separation / cutoff.

[0004] The preparation of gradient narrowband filters is relatively difficult. Generally speaking, there are two main methods: the baffle method and the ion etching method. The baffle method adds one or even two baffles of a specific shape above or below the glass substrate. During film preparation, the baffle and the glass substrate are moved or rotated relative to each other, and the rate and frequency of movement or rotation are controlled to achieve the desired continuous gradient film thickness. The ion etching method exposes and develops the photoresist under a specific mask, and then uses ion etching to transfer the pattern to the spacer layer of the narrowband filter. The gradient characteristics of this method can be a continuous gradient film thickness or a step-by-step film thickness change. The narrowband filters produced by this method generally have only a single-cavity structure.

[0005] The present invention aims to prepare a strip-shaped linear narrowband filter with continuously gradient thickness of each thin film layer on a highly transparent substrate. The continuous gradient of film thickness is achieved by adjusting the geometric configuration of the thin film vacuum evaporation system. The so-called geometric configuration adjustment is to adjust the relative position between the strip-shaped glass substrate and the thin film material evaporation source, so that the film thickness distribution on the strip-shaped glass substrate exhibits a continuous linear gradient as its position changes. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a linear gradient narrowband filter, in which a linear narrowband filter with a continuously gradient film thickness is prepared on a strip glass substrate. The gradient of the film thickness is achieved by adjusting the geometric configuration of the thin film vacuum evaporation system, so it is necessary to first briefly introduce the vacuum evaporation system and its film thickness monitoring system.

[0007] To achieve the above object, the specific technical solution adopted by the present invention is:

[0008] A linear gradient narrowband filter is prepared. It is a strip-shaped linear narrowband filter with gradually varying film thicknesses on a highly transparent substrate. The highly transparent substrate is a strip of optical glass. The film thickness gradient is achieved by adjusting the geometric configuration of the thin film vacuum evaporation system, specifically the relative position between the strip of glass substrate and the film material evaporation source. This allows the film thickness distribution on the strip of glass substrate to exhibit a continuous linear gradient as its position changes. The narrowband filter comprises a multi-cavity Fabry-Perot structure.

[0009] Furthermore, the strip glass substrate is placed on the circular metal aluminum fixture disk according to a specific position calculated by design. The strip glass substrate can be directly regarded as a part of the circular workpiece fixture disk, and a film thickness monitoring piece is placed in the center of the fixture disk.

[0010] Furthermore, the geometric configuration of the thin film vacuum evaporation system mainly includes three parameters: one is the distance between the plane of the strip glass substrate and the evaporation plane of multiple evaporation sources, referred to as the height h; the second is the distance L between the center of the evaporation source and the central axis of rotation of the substrate; and the third is the distance ρ between any film thickness observation point selected on the fixture disk strip glass substrate and the central axis of rotation of the substrate.

[0011] Furthermore, the film thickness distribution refers to the relationship between the normalized film thickness ratio t / t0 and ρ / L calculated under different geometric configurations h / L, where t0 and t are the film thickness at the center point of the workpiece fixture disk and the film thickness at any film thickness observation point selected on the strip glass substrate, respectively.

[0012] Furthermore, when h / L>1.5, the film thickness ratio t / t0 shows a very good linear relationship with the change of ρ / L when ρ / L>1, so various filter films with continuous linear gradient of film thickness can be obtained.

[0013] Furthermore, since the gradient of the continuous linear change in film thickness decreases with the increase of h / L, and the resolution increases with the increase of h / L, h / L can be selected according to the film thickness gradient change and resolution requirements.

[0014] Furthermore, to avoid the use of a large vacuum evaporation system and to ensure excellent repeatability of the film thickness distribution, h / L is usually selected to be 1.5 to 2.5. In the present invention, h / L is preferably 1.8.

[0015] Furthermore, the linear gradient narrowband filter adopts a three-cavity Fabry-Perot structure: G[(HL) 4 HH(LH) 4 M] 3 A, where G is the glass substrate, H, L, and M are the high refractive index film, the low refractive index film, and the lower refractive index film, respectively, and A is the incident air.

[0016] Furthermore, the high refractive index film is TiO2, the low refractive index film is SiO2, and the lower refractive index film is MgF2.

[0017] Furthermore, the high refractive index film TiO2 and the low refractive index film SiO2 are evaporated by electron beam, and the lower refractive index film MgF2 is evaporated by molybdenum boat resistor.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] There are two main methods for fabricating linearly graded narrowband filters: the baffle method and ion etching. The baffle method involves adding one or more baffles of a specific shape above (or below) a glass substrate. During film deposition, the baffles and the glass substrate are moved or rotated relative to each other, controlling the rate and frequency of these movements to achieve the desired continuous, linearly graded film thickness distribution. This method is currently the most widely used because, compared to ion etching, it not only eliminates the need for complex and expensive ion etching systems but also offers a simpler fabrication process. However, its disadvantages include: 1. The baffles significantly increase the loss of evaporated film material; 2. They increase film evaporation time, reducing fabrication efficiency; 3. Repeated adjustments to the baffle shape are required to achieve a satisfactory linearly graded film thickness distribution; 4. Due to the baffles, the film deposited on the substrate is intermittent, resulting in changes in the film structure; and 5. Large amounts of film can easily accumulate on the baffles, and when these break, they can contaminate the substrate or the growing film. Ion etching involves exposing and developing the photoresist under a specific mask, then transferring the pattern to the spacer layer of the narrowband filter using ion etching, resulting in a linearly gradient film thickness. This method not only requires an expensive etching system but also a complex process. Furthermore, it can generally only produce gradient narrowband filters with a single-cavity structure, and the gradient characteristics often result in a film thickness distribution consisting of discontinuous micro-steps.

[0020] The present invention achieves a linear narrowband filter with a continuous gradient of thickness across each layer by adjusting the geometric configuration of the thin film vacuum evaporation system. Adjusting the geometric configuration involves adjusting the relative position between the strip glass substrate and the thin film material evaporation source. This adjustment requires no additional equipment and is simple to implement. The resulting continuous linear gradient is inherent to the specific geometric configuration, making it reliable and stable. Thin film researchers have long focused on improving film uniformity and expanding the uniform area, often using geometric configurations with h / L = 1.35 to 1.45 for research. The optimal result is a film thickness nonuniformity of 0.04% in the central region of the workpiece holder with h / L = 1.405 and ρ / L < 0.6. Unfortunately, no specific applications have been discovered for the continuous linear gradient of film thickness distribution produced by larger h / L and ρ / L, which would provide a highly effective new method for directly fabricating linearly gradient narrowband filters. The only drawback of this method is the sacrifice of the area S1 in the center of the workpiece holder. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the vacuum evaporation system and film thickness monitoring system configuration of the vacuum coating machine.

[0022] Figure 2 Schematic diagram showing the main parameters of the geometric configuration of the vacuum evaporation system.

[0023] Figure 3 This is the relationship curve of the normalized film thickness ratio t / t0 calculated for h / L = 1.0, 1.4, 1.5, 2.5, and 3.5 as in ρ / L, where L = 200 mm is taken in the calculation.

[0024] Figure 4 1 is a calculation curve of the normalized film thickness ratio t / t0 varying with ρ / L for h / L=1.5, 2.5 and 3.5 applicable to the present invention, with L=200 mm.

[0025] Figure 5 1 is a calculated curve of the normalized film thickness ratio t / t0 varying with ρ / L when h / L=1.8, which is preferably used in the present invention, and L=100 mm.

[0026] Figure 6 It is a schematic diagram of the workpiece fixture disk of the present invention, its film thickness monitoring piece and the placement position of the strip glass substrate.

[0027] Figure 7 It is a structural schematic diagram of the three-cavity linear gradient narrowband filter of the present invention.

[0028] Figure 8 The three-cavity linear gradient narrowband filter G[(HL) 4 HH(LH) 4 M] 3Transmission spectrum curve of A.

[0029] Figure 9 It is a single cavity linear gradient narrowband filter G|(HL) 4 HH(LH) 4 Transmission spectrum curve of M|A.

[0030] Figure 10 It is the transmission spectrum curve of the three-cavity linear gradient narrowband filter of the present invention at a peak center wavelength of 600 nm.

[0031] Figure 11 It is a transmission spectrum curve of the three-cavity linear gradient narrowband filter of the present invention at a peak center wavelength of 700 nm.

[0032] Figure 12 The transmission spectrum curve of the three-cavity linear gradient narrowband filter of the present invention at different ρ with a peak center wavelength of 600-700 nm. DETAILED DESCRIPTION

[0033] Figure 1 This is a schematic diagram of the vacuum evaporation system and film thickness monitoring system configuration of the vacuum coating machine. Figure 1 In the vacuum coating machine, the vacuum chamber 1 is evacuated to achieve a high vacuum. A substrate heating system 2 is installed within the vacuum chamber 1 to heat the substrate to a set temperature before coating. Immediately below the heating system 2 is the filter workpiece fixture 3, which holds the coated glass substrate 4. The workpiece fixture 3 is a circular aluminum disk that rotates evenly and rapidly during coating. A film thickness monitoring plate 5 is placed at the center of the fixture, surrounded by multiple substrates 4 to be coated, to form the designed linear gradient filter. The thin film material is composed of a resistance heating evaporation source 6 and an electron beam evaporation source 8. Each evaporation source has its own characteristics and can be selected based on the properties of the evaporated material. The evaporation plane 9 of the resistance heating evaporation source 6 and the electron beam evaporation source 8 are coplanar. A baffle 7, 7', controls the evaporation material vapor above each evaporation source, allowing for precise control of the thickness of each layer of the filter film. The high-refractive-index TiO2 and low-refractive-index SiO2 thin films are evaporated by electron beam evaporation source 8, while the even lower-refractive-index MgF2 is evaporated by resistance heating evaporation source 6. The film thickness monitoring system, consisting of a light source 10, a control filter 11, a film thickness monitoring plate 5, a focusing lens 12, and a photoelectric receiver 13, precisely controls the thickness of each layer of the filter.

[0034] The concept of the present invention is described in detail below.

[0035] 1. Adjust the geometry to get linear gradient characteristics

[0036] By adjusting the geometric configuration of the vacuum evaporation system, that is, adjusting the relative position between the strip glass substrate and the thin film material evaporation source, the film thickness distribution on the strip glass can be made to show a continuous linear gradient as its position changes. Figure 2 As shown in Figure 1, the geometric configuration of the vacuum evaporation system mainly includes three parameters: the distance h between the plane of the strip glass substrate and the evaporation planes of the multiple evaporation sources; the distance L between the center of the evaporation source and the rotation axis of the substrate; and the distance ρ between any film thickness observation point selected on the clamping disk strip substrate and the rotation axis of the substrate. Based on these three parameters, the film thickness distribution of the planar evaporation source under different geometric configurations h / L can be derived, that is, the relationship between the normalized film thickness ratio t / t0 and ρ / L:

[0037]

[0038] Where t0 and t are the film thickness on the monitoring piece at the center of the workpiece fixture disk and the film thickness at the selected film thickness observation point on the strip substrate, respectively. t / t0 is also called the normalized relative thickness. Figure 3 The relationship curve of t / t0 and ρ / L calculated by the above calculation formula of the present invention for h / L=1.0, 1.4, 1.5, 2.5, 3.5 is shown, and L=200mm is taken in the calculation. Figure 3 It can be seen that 1) in order to pursue the largest uniform film thickness area, the geometric configuration should be h / L = 1.4, which is why thin film engineers choose the geometric configuration h / L = 1.4; 2) if the uniform area is to be further increased, h can only be increased, which means that a larger coating machine is needed, because when h / L = 1.4 remains unchanged, increasing h will increase L, and increasing L will increase ρ. Except for h / L = 1.4, thin film engineers rarely pay attention to the case of h / L>1.5, because when h / L>1.5, the uniform area will be greatly reduced, which is obviously not conducive to the preparation of uniform thin films. However, as Figure 4 The three film thickness distribution curves shown for h / L = 1.5, 2.5, and 3.5 show that when ρ / L > 0.8, the film thickness distribution t / t0 exhibits a very good linear relationship with ρ / L. This linear relationship is suitable for preparing linear gradient filters for strip substrates. Moreover, when h / L > 1.5, the linear variation curve of t / t0 with ρ / L is more stable than when h / L < 1.5, which makes the preparation of linear gradient characteristics more repeatable. Moreover, as h / L increases, the thickness gradient becomes smaller and the linearity becomes better. This means that increasing h or decreasing L is very useful for changing the gradient and improving linearity. The specific h, L, and h / L should be determined based on factors such as the length of the glass substrate, the linear wavelength range, the gradient, and the linearity requirements.

[0039] 2. Determine the position of the strip glass substrate

[0040] For smaller vacuum chambers, the distance between the center of the planar evaporation source and the substrate rotation axis can be selected as L = 100 mm. Once the position of the evaporation source is determined, the film thickness distribution curve of t / t0 versus ρ / L when the geometric configuration h / L = 1.8 can be calculated using formula (1), as shown in the following example: Figure 5 As shown. Figure 5 From the curve, we can see that the film thickness distribution curve of t / t0 with ρ / L can be considered to have three parts: the first part is the uniform film thickness area with ρ / L = 0 to 0.4; the second part is the film thickness gradient nonlinear area with ρ / L = 0.4 to 0.8; and the third part is the film thickness gradient linear area with ρ / L = 0.8 to 2. Obviously, this third part is the coating glass substrate placement area required by the present invention. Figure 6 It is clearer to see it in conjunction with the workpiece fixture tray. The first and second parts are the unusable S1 area. Except for a film thickness monitoring piece 5 placed in the center, no strip glass substrates to be coated are placed. All strip glass substrates are arranged regularly in the S2 area. O is the center point of the fixture plate, so the position of the strip glass substrate can be placed from ρ1 to ρ2. In the present invention, ρ1 corresponds to Figure 5 The curve's ρ / L = 0.8 represents the thick end of the film thickness (long-wavelength side), while ρ2 corresponds to ρ / L = 2, representing the thin end of the film thickness (short-wavelength side). Without changing the evaporation source position L and the geometric configuration h / L, ρ2 can be changed by changing the size of the coating machine's vacuum chamber.

[0041] Table 1 below lists the normalized relative thickness t / t0 corresponding to the linear region of film thickness gradient with a ρ / L gradient from 0.8 to 2.0 with a step size of 0.2. Since L is assumed to be 100 mm, the corresponding ρ (mm) can be calculated. Once the wavelength of the thin end short-wave side is set to ρ / L = 2, the corresponding ρ / L or ρ long-wave side wavelength (nm) can be calculated using a simple proportional relationship due to the linear relationship of ρ / L = 0.8 to 2.0. For example, if the wavelength of the thin end short-wave side is set to 400 nm, the wavelength of the thick end long-wave side can be easily calculated to be 865 nm, and the film thickness monitoring wavelength can be calculated to be 930 nm. This means that when the film thickness monitoring film uses a wavelength of λ0 = 930nm, the wavelength of the long-wavelength side of the ρ1 end of the glass strip can be λ1 = 865nm, and the wavelength of the short-wavelength side of the ρ2 end can be λ2 = 400nm. This means that on a glass strip with a length of ρ2 - ρ1 = 120mm, the linear gradient wavelength width reaches 465nm. This linear gradient wavelength width far exceeds the spectral width of the visible light region. Conversely, if the wavelength of the long-wavelength side of the thick end is set, the position of the wavelength of the short-wavelength side of the thin end can be calculated using a proportional relationship. For example, if the linear gradient wavelength width is required to be exactly within the visible light region of 700-400nm, and the wavelength of the long-wavelength side of the thick end is set at ρ / L = 1, then the wavelength of the short-wavelength side of the thin end, 400nm, can be calculated to be at ρ / L = 1.85 (i.e., t / t0 = 0.497), resulting in a film thickness monitoring wavelength of 804nm. This means that when the film thickness monitoring film uses a wavelength of λ0 = 804 nm, the wavelength of the long-wavelength side of the glass strip can be λ1 = 700 nm at ρ1 = 100 mm, and the wavelength of the short-wavelength side can be λ2 = 400 nm at ρ2 = 185 mm. This means that the wavelength width of the linear gradient achieved on a glass strip with a length of ρ2 - ρ1 = 85 mm is 300 nm. Analysis of ρ / L and t / t0 in Table 1 shows that for the geometric configuration h / L = 1.8 and L = 100 mm, the linearity of ρ / L from 1.0 to 1.8 is very good. The linearity at the two extremes, 0.8 to 1.0 and 1.8 to 2.0, is slightly worse. Therefore, to improve linearity, the glass strip should be placed as close as possible to ρ / L = 1.0 to 1.8.

[0042] Table 1

[0043] ρ / L 0 0.8 1.0 1.2 1.4 1.6 1.8 2.0 <![CDATA[t / t0]]> 1.0 0.93 0.87 0.79 0.70 0.61 0.52 0.43 ρ(mm) 0 80 100 120 140 160 180 200 λ(nm) <![CDATA[λ 0(监控波长) ]]> <![CDATA[0.93λ0]]> <![CDATA[0.87λ0]]> <![CDATA[0.79λ0]]> <![CDATA[0.70λ0]]> <![CDATA[0.61λ0]]> <![CDATA[0.52λ0]]> <![CDATA[0.43λ0]]>

[0044] If the evaporation source position L is further reduced, the area of ​​S1 can be further reduced, thereby increasing the length and wavelength width of the linear gradient of the strip glass substrate.

[0045] If the evaporation source position L is kept unchanged and the geometric configuration h / L is increased, it is expected that the linear gradient length of the strip glass substrate will be increased, the gradient will be reduced, and the linearity will be improved.

[0046] 3. Narrowband filter structure

[0047] In order to improve the resolution and light efficiency of spectral spectrometry, the linear gradient narrowband filter adopts a three-cavity Fabry-Perot structure: G[(HL) 4 HH(LH) 4 M] 3 A, where G is a glass strip, H, L, and M are high-refractive-index film, low-refractive-index film, and even lower-refractive-index film, respectively, and A is incident air. The high-refractive-index film is TiO2, the low-refractive-index film is SiO2, and the even lower-refractive-index film is MgF2. Figure 7 This is a schematic diagram of the structure of a three-cavity linear gradient narrowband filter, where (HL) 4 It is the first reflection film of the Fabry-Perot cavity, HH is the lowest order spacer layer, (LH) 4 It is the second reflective film of the Fabry-Perot cavity, and M is the cavity coupling layer or anti-reflection layer. The superiority of this three-cavity linear gradient narrowband filter is clear when comparing the transmittance curve with the single-cavity structure. Figure 8 and Figure 9 Although they all use the same structure, the same number of layers and the same materials, Figure 8 It is a three-cavity narrowband filter, and Figure 9 Is a single cavity narrowband filter: G(HL) 4 HH(LH) 4 While the transmission curves of these two filters are similar in terms of central wavelength, peak transmittance, and half-width (FWHM), the three-cavity structure exhibits a narrow rectangular transmission curve with good cutoff performance, while the single-cavity structure exhibits a bell-shaped transmission curve with low cutoff. This results in a difference of several orders of magnitude in the signal-to-noise ratio of the filters. This is because the signal in the signal-to-noise ratio is the rectangular area formed by the half-width, while the half-width of the narrowband filter mentioned above is only 3nm. The noise is the integrated area of ​​all light in the entire receiver response spectrum, excluding the signal light. Due to the very broad background spectrum, the signal is likely to be submerged in the background noise. Generally speaking, single-cavity filters are unusable in many situations.

[0048] 4. Elimination of secondary transmission peaks on both sides of the main transmission peak

[0049] Linear gradient narrowband filters will have transmission secondary peaks on both the short-wave and long-wave sides of the central wavelength of the main transmission peak, often called sideband transmission secondary peaks. Of course, if these transmission secondary peaks are within the response spectrum of the receiver, they must be eliminated in actual use. Figure 8As shown, if the central wavelength of the main transmission peak is 650nm, secondary transmission peaks will appear for shortwave wavelengths less than 553nm and for longwave wavelengths greater than 780nm. Generally, these secondary transmission peaks need to be eliminated. There are two common methods for eliminating secondary transmission peaks. The first is to use a linear gradient cutoff filter, similar to the method described in the present invention, to subsequently cut off the secondary transmission peaks generated by the linear gradient narrowband filter. Linear gradient cutoff filters include longwavepass cutoff filters that cut off shortwave wavelengths but transmit longwave wavelengths, and shortwavepass cutoff filters that cut off longwave wavelengths but transmit shortwave wavelengths. These filters can precisely eliminate the secondary transmission peaks on the shortwave and longwave sides of the linear gradient narrowband filter, respectively. The second method is to use a complex common cutoff filter. This common cutoff filter is generally composed of a longwavepass cutoff filter and a shortwavepass cutoff filter composed of multiple uniform film layers. A high-transmittance window is formed in the linear gradient region of the narrowband filter, with wide wavelength cutoff bands formed on both sides of the window. The specific width and range of the cutoff band are determined by the receiver's spectral response band. Since the common cutoff filter has many layers and a large thickness, it is often placed separately between the receiver and the linear gradient narrowband filter. The above-mentioned transmission sub-peak is beyond the scope of this invention and will not be described in detail.

[0050] As an embodiment 1, the requirements for the linear gradient narrowband filter are as follows: 1) the wavelength range of the linear gradient modulation is 600 to 700 nm; 2) the average half-width of the narrowband filter is 3 nm; 3) the average transmittance of the main transmission peak of the filter is >90%; 4) the length and width of the strip glass substrate are 30 mm and 5 mm respectively; 5) the average gradient is <5 nm / mm; 6) the nonlinearity is <3%.

[0051] Figure 1 This is a schematic diagram of the vacuum evaporation system and film thickness monitoring system of a vacuum coating machine. The coated glass substrate 4 of the linear gradient narrowband filter is positioned on a metal aluminum disk on a workpiece fixture 3 according to specific requirements and rotated evenly and rapidly during coating. A film thickness monitoring plate 5 is placed at the center of the fixture. Using the photoelectrode value method, the thickness of each filter layer can be precisely controlled using this plate.

[0052] 1. Determine the geometric configuration and position of the strip glass substrate

[0053] use Figure 5The film thickness distribution curve of normalized thickness t / t0 versus ρ / L for the geometric configuration h / L = 1.8 and L = 100 mm can be used to determine the position of the strip glass substrate on the workpiece fixture plate. Because the linear gradient wavelength range is 600-700 nm, if the wavelength of the thick end, long-wave side, 700 nm, is set at ρ / L = 1, its corresponding normalized thickness t / t0 = 0.87 and ρ1 = 100 mm (also see Table 1). Then, the wavelength of the thin end, short-wave side, 600 nm, should be set at: t / t0 = 600 * 0.87 / 700 = 0.746, corresponding to ρ / L = 1.3, or ρ2 = 130 mm. The monitoring wavelength λ0 = 700 / 0.87 = 804 nm. That is to say, when the monitoring wavelength λ0 = 804nm, the wavelength λ1 = 700nm on the long-wave side of the strip glass substrate at ρ1 = 100mm and the wavelength λ2 = 600nm on the short-wave side at ρ2 = 130mm can be achieved. That is, the wavelength width of the linear gradient on the strip glass substrate with a length of ρ2-ρ1 = 30mm is 100nm, and the average gradient is 3.3nm / mm. The nonlinearity obtained by this configuration is 1.2%. The calculation method of nonlinearity δ is: δ = △Ymax / Y*100%, where △Ymax is the linear error, that is, the maximum deviation between the actual normalized thickness t / t0 distribution curve and the fitted straight line, and Y is the total value of t / t0, which is obtained by Figure 5 We can get △Ymax=0.01 (this number is small, it is best to Figure 5 ), Y takes the average value (0.87+0.746) / 2=0.81, so the nonlinearity δ=1.2%.

[0054] To further improve the nonlinearity, the thick end of the glass strip can be moved from ρ1 = 100 mm to ρ1 = 130 mm. This will bring the nonlinearity δ close to zero. This results in a ρ / L = 1.3 for the long-wavelength side of the thick end at a wavelength of 700 nm, corresponding to a normalized thickness t / t0 = 0.746 and ρ1 = 130 mm. Meanwhile, a ρ / L = 1.55 for the short-wavelength side of the thin end at a wavelength of 600 nm, corresponding to t / t0 = 0.639 and ρ2 = 155 mm. Clearly, the average gradient now becomes 4 nm / mm, similar to the linear dispersion of a prism or grating monochromator, which varies with wavelength.

[0055] 2. Determine the narrowband filter structure

[0056] In order to improve the resolution and light efficiency of spectral spectrometry, the linear gradient narrowband filter adopts a three-cavity Fabry-Perot structure: G[(HL) 4 HH(LH) 4 M] 3A, where G is a strip glass substrate, H represents a high refractive index film with an optical thickness of λ0 / 4, L represents a low refractive index film with an optical thickness of λ0 / 4, M represents an even lower refractive index film with an optical thickness of λ0 / 4, and A is the incident medium, air. The high refractive index film is TiO2, the low refractive index film is SiO2, and the even lower refractive index film is MgF2. In the linear gradient narrowband filter, (HL) 4 It is the first reflection film of the Fabry-Perot cavity, HH is the lowest order spacer layer, (LH) 4 It is the second reflection film of the Fabry-Perot cavity, and M is the cavity coupling layer or the last anti-reflection layer. The advantage of this three-cavity linear gradient narrowband filter is that the transmission curve presents a narrow rectangle and the cutoff is very high. Figure 9 Compared with the single-cavity Fabry-Perot structure gradient narrowband filter shown in the figure, the signal-to-noise ratio shows a difference of several orders of magnitude. Generally speaking, a single-cavity filter cannot meet practical requirements in many cases, so this embodiment uses a three-cavity structure.

[0057] During preparation, the high-refractive-index TiO2 film and the low-refractive-index SiO2 film are evaporated by heating using an electron beam evaporation source, while the lower-refractive-index MgF2 film is evaporated by heating using a resistance heating evaporation source.

[0058] Three-cavity structure G[(HL) 4 HH(LH) 4 M] 3 Although A has a large number of film layers, all of them are uniform films with an optical thickness of λ0 / 4. Therefore, the thickness of each filter layer can be determined using the photodiode value method using a single monitoring wavelength λ0 and film thickness monitoring film 5. The photodiode value method converts changes in transmitted light into changes in the electrical signal of a photodetector. According to the principle of optical interference, each extreme value of transmittance corresponds to an optical thickness of exactly λ0 / 4. Therefore, by simply controlling the maximum or minimum value of the electrical signal, the monitoring film 5 can monitor all the film layers of the three-cavity structure, which is very convenient.

[0059] 3. Elimination of secondary transmission peaks on both sides of the main transmission peak

[0060] Since the linear gradient modulation wavelength range of this embodiment is 600-700nm and the wavelength width is only 100nm, it is not necessary to eliminate the transmission secondary peaks on both sides of the main transmission peak. This is because the linear gradient narrowband filter itself does not produce sideband transmission secondary peaks within the wavelength range of 600-700nm. Figure 10 and Figure 11 It can be clearly seen that Figure 10 The main transmission peak center wavelength is 600nm, and its sideband transmission secondary peak on the long-wave side appears in the region with a wavelength greater than 750nm; Figure 11The main transmission peak of the linear gradient modulation is centered at 700nm, while its short-wavelength sideband transmission peaks are all located in the wavelength range below 590nm. This demonstrates that the main transmission peak of the linear gradient modulation has no sideband transmission peaks other than the main transmission peak within the wavelength range of 600-700nm. If the receiver's response spectrum is also 600-700nm, the linear gradient narrowband filter of this embodiment can be used directly. Otherwise, a common cutoff filter is required to eliminate the transmission peaks on both sides of the wavelength range of 600-700nm.

[0061] Figure 12 The transmittance curve of a linearly graded narrowband filter with a modulation wavelength of 600-700 nm according to this embodiment varies with the central wavelength or substrate position. Only a portion of the curve is shown. In reality, because all film layer thicknesses are continuously graded without steps, the transmittance curve also exhibits an uninterrupted, continuous gradient. The figure shows that the average half-width of the narrowband filter is 3 nm. The term "average half-width" is used here because the filter half-width increases with the central wavelength of the main transmission peak, an inherent characteristic of narrowband filters. The average transmittance of the filter's main transmission peak is approximately 98%. The length and width of the linearly graded narrowband filter are 30 mm and 5 mm, respectively, with an average gradient of 3.3 nm / mm and a nonlinearity of 1.2%.

[0062] It should be pointed out that the gradient and nonlinearity vary with the position of the strip glass substrate. This may seem like a disadvantage, but it is actually an advantage because it is equivalent to increasing the design adjustment parameters, making it easier to meet various technical requirements.

Claims

1. A method for preparing a linear gradient narrowband filter, characterized in that: Adopt vacuum coating machine, including: Vacuum chamber; a workpiece fixture disk disposed within the vacuum chamber; a strip-shaped glass substrate mounted on the workpiece fixture disk; a plurality of evaporation sources disposed at the bottom of the vacuum chamber; The preparation method comprises the following steps: 1) The distance between the plane where the strip-shaped glass substrate is in close contact with the workpiece fixture plate and the evaporation planes of the multiple evaporation sources, referred to as height h; The distance between the evaporation point of the evaporation source and the rotation center axis of the workpiece fixture disk is L; The distance ρ between any film thickness inspection point selected on the strip-shaped glass substrate and the rotation center axis of the workpiece fixture disk; 2) Calculate the relationship between the normalized film thickness ratio t / t0 and ρ / L for different geometric configurations h / L, and obtain film thickness distribution curves for multiple geometric configurations, where t0 and t are the film thickness at the center of the workpiece fixture disk and the film thickness at any selected film thickness observation point on the strip glass substrate, respectively; The relationship between normalized film thickness ratio t / t0 and ρ / L: (1) 3) Selecting a curve with a large linear area and good linearity from multiple geometric configuration film thickness distribution curves as the optimal geometric configuration film thickness distribution curve, arranging strip glass substrates according to the optimal geometric configuration film thickness distribution curve, and manufacturing to obtain a linear gradient narrowband filter; The optimal geometric configuration film thickness distribution curve has h / L of 1.8, L = 100 mm, ρ / L = 1.0 ~ 1.8, and the strip glass substrate is placed between ρ / L = 1.0 ~ 1.8; The linear gradient narrowband filter adopts a three-cavity Fabry-Perot structure: G[(HL) 4 HH(LH) 4 M] 3 A, where G is the glass substrate, H, L, and M are high refractive index film, low refractive index film, and lower refractive index film, respectively, and A is the incident air; The high refractive index film is TiO2, the low refractive index film is SiO2, and the even lower refractive index film is MgF2.

2. The method for preparing a linear gradient narrowband filter according to claim 1, wherein: In step 1), the evaporation planes of the multiple evaporation sources are in the same plane.

3. The method for preparing a linear gradient narrowband filter according to claim 1, wherein: In step 1), the distances L between the plurality of evaporation sources and the central axis of rotation of the workpiece fixture disk are all equal.

4. The method for preparing a linear gradient narrowband filter according to claim 1, wherein: In step 3), the high refractive index film TiO2 and the low refractive index film SiO2 are evaporated by electron beam, and the lower refractive index film MgF2 is evaporated by molybdenum boat resistor.

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