Ultra-wide tuning range narrow-band optical filter using FP cavity high-order harmonics and design method of ultra-wide tuning range narrow-band optical filter

Abstract

The invention provides an ultra-wide tuning range narrow-band optical filter utilizing FP cavity high-order harmonics and a design method of the ultra-wide tuning range narrow-band optical filter. The optical filter comprises an input reflecting layer, a composite cavity layer and an output reflecting layer which are stacked in sequence, the composite cavity layer comprises a metal layer and two single-side cavity layers which are arranged on the two sides of the metal layer and are equal in thickness; by adjusting the thickness of the single-side cavity layer, the filtering center wavelength is adjusted; the larger the thickness of the single-side cavity layer is, the larger the filtering center wavelength is. The metal layer is introduced into the middle of the FP cavity of the optical filter, so that a first-order harmonic peak is effectively inhibited, second-order harmonic waves are reserved, and a sideband at a short wave position is reduced, so that the full width of half peak of filtering is reduced, and meanwhile, the working range of the FP optical filter is widened to the range of 550nm-850nm by utilizing high-order resonance. The optical filter is simple in structure process, easy to implement and simple in structure, and the central wavelength of the optical filter can be adjusted only by adjusting the thickness of the silicon dioxide cavity layers adjacent to the two sides of the metal layer.

Description

Technical Field

[0001] The invention belongs to the technical field of optical devices, and relates to the structural design of a filter, and in particular to an ultra-wide tuning range narrow-band filter using high-order harmonics of an FP cavity and a design method thereof. Background Art

[0002] In the field of optical technology, filters are indispensable and important optical components, and their performance plays a decisive role in the accuracy and efficiency of optical signal processing and screening. With the continuous development of science and technology, the requirements for filters in various fields are becoming more and more stringent, especially in terms of tuning range, transmittance, spectral selectivity, stability and process flow.

[0003] There are many types of filters on the market, but all of them have problems to varying degrees. Traditional filters have obvious shortcomings in tuning range, and usually cannot cover a wider wavelength range. This makes it impossible to accurately screen out the required wavelength when facing multi-band optical signals, greatly limiting their application in complex optical systems. Moreover, their manufacturing process is complex, which further increases production costs and production cycles. In terms of transmittance, the peak transmittance of many traditional filters is low, resulting in a large amount of light energy being lost during the filtering process, seriously weakening the overall efficiency of the optical system. In addition, the large bandwidth leads to poor spectral selectivity, making it difficult to clearly distinguish light signals of adjacent wavelengths, thereby reducing imaging quality and detection accuracy.

[0004] Traditional thin-film Fabry-Perot (FP) cavity filters have certain unique advantages, such as being relatively insensitive to polarization and angle changes, thanks to the fact that their resonance mechanism is independent of polarization and angle changes. Its structure consists of a low-refractive-index cavity layer sandwiched between two distributed Bragg reflectors or metal reflectors, and filtering of incident light is achieved based on the principle of interference-induced cavity resonance. However, filters based on the FP cavity structure are not perfect. Its free spectral range (FSR) is limited, and in order to maintain high color purity, the operating wavelength range has to be limited to a narrow range, which largely restricts its application in scenarios that require wide-band working, and cannot fully meet the needs of modern optical technology for high-performance filters.

[0005] In the research of filters based on the FP cavity principle, although the progress of nano-processing technology has brought certain improvements, it still faces many challenges. On the one hand, the research on transmissive FP cavity filters in the visible light range is relatively insufficient, and the potential of FP cavity in this band has not been fully explored. On the other hand, the existing FP cavity filters have the problem of high mirror alignment accuracy in practical applications, which not only increases the difficulty and cost of the preparation process, but also a slight deviation will lead to increased light loss, which will seriously affect its performance. In addition, its working range is restricted by factors such as reflector materials and coatings, cavity length, temperature and power, which limits its application in a wider range of scenarios.

[0006] Most micro filters use different resonance mechanisms to filter incident light, and common resonance mechanisms include plasma resonance, Mie resonance, guided mode resonance, and cavity resonance. Specifically, filters based on metal metasurfaces and using the principle of plasma resonance have the characteristics of a wide operating wavelength range and a relatively simple manufacturing process. However, the absorption of light by metals will limit their transmission performance.

[0007] In view of the many shortcomings of the existing technology, this study is committed to proposing an innovative high-order FP cavity filter, aiming to provide a better performance and more adaptable solution for the development of multispectral imaging technology to meet the growing practical application needs in various fields. Summary of the invention

[0008] In view of the shortcomings of the prior art, the present invention proposes an ultra-wide tuning range narrow-band filter using high-order harmonics of the FP cavity. In the first aspect, the present invention provides an ultra-wide tuning range narrowband filter using high-order harmonics of an FP cavity, comprising an input reflection layer, a composite cavity layer and an output reflection layer stacked in sequence; a metal layer is disposed in the middle of the composite cavity layer; the material of the metal layer is platinum or aluminum; the thickness of the metal layer is 5 nm to 15 nm.

[0009] Preferably, the composite cavity layer further comprises two single-sided cavity layers of equal thickness arranged on both sides of the metal layer; the thickness of the single-sided cavity layer is 170 nm to 310 nm; by adjusting the thickness of the single-sided cavity layer, the filtering center wavelength is adjusted within the range of 550 nm to 850 nm; the greater the thickness of the single-sided cavity layer, the greater the filtering center wavelength.

[0010] Preferably, the single-sided cavity layer is made of silicon dioxide.

[0011] Preferably, the material of the metal layer is platinum.

[0012] Preferably, the thickness of the metal layer is 10 nm.

[0013] Preferably, the input reflective layer and the output reflective layer both include a silicon dioxide layer and two silicon layers respectively arranged on both sides of the silicon dioxide layer.

[0014] Preferably, in the input reflection layer, along the direction close to the composite cavity layer, the thicknesses of the first silicon layer, the silicon dioxide layer, and the second silicon layer are 45nm-65nm, 110nm-130nm, and 30nm-50nm, respectively; in the output reflection layer, along the direction close to the composite cavity layer, the thicknesses of the first silicon layer, the silicon dioxide layer, and the second silicon layer are 45nm-65nm, 110nm-130nm, and 30nm-50nm, respectively.

[0015] Preferably, the output reflective layer is arranged on a quartz substrate.

[0016] In a second aspect, the present invention provides a method for designing a filter with a specified filtering wavelength, which comprises the following steps: according to a target wavelength in the range of 550 nm to 850 nm, setting the thickness of two single-sided cavity layers in a composite cavity layer so that the filtering center wavelength of the filter matches the target wavelength; an input reflection layer, a first single-sided cavity layer, a metal layer, a second single-sided cavity layer and an output reflection layer are stacked in sequence; the material of the metal layer is platinum or aluminum; the thickness of the metal layer is 5 nm to 15 nm.

[0017] In a third aspect, the present invention provides a filtering method, which uses the filter designed by the aforementioned filter design method to obtain light of a target wavelength.

[0018] The present invention has the following beneficial effects: 1. The present invention introduces a metal layer in the middle of the filter FP cavity to effectively suppress the first-order resonance peak and retain the second-order harmonic, so that the sideband at the short-wavelength is reduced, thereby reducing the half-maximum full width of the filter, and at the same time, the working range of the FP filter is widened to the range of 550nm-850nm by utilizing high-order resonance.

[0019] 2. The structure and process of the present invention are simple and easy to implement. The structure is simple, and the central wavelength of the filter can be adjusted by only adjusting the thickness of the silicon dioxide cavity layer on both sides adjacent to the metal layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the structure of the filter in Example 1 of the present invention.

[0021] Figure 2 Graph showing the variation of the transmission spectrum with the thickness of the composite cavity layer in Example 1 of the present invention.

[0022] Figure 3 This is a comparison diagram of resonance peaks between filters made of different metal layer materials provided in Examples 1 to 4 of the present invention.

[0023] Figure 4 This is a scanning diagram of the effect of different thicknesses of the metal layer on the central wavelength in Example 5 of the present invention.

[0024] Figure 5 Spectral scanning diagrams of the metal layer at different positions in the cavity in Example 1 of the present invention and Comparative Example 1.

[0025] Figure 6 This is a schematic diagram of the structure of a filter lacking a metal layer provided in Comparative Example 2 of the present invention.

[0026] Figure 7 It is a comparison diagram of the resonance peaks of Example 1 of the present invention and Comparative Example 2.

[0027] Figure 8 This is a scanning diagram showing the transmission peak changing with the thickness of the composite cavity layer in Comparative Example 2 of the present invention.

[0028] Fig. 9 This is a scanning diagram showing the transmission peak changing with the thickness of the composite cavity layer in Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further explained below in conjunction with the accompanying drawings.

[0030] Example 1 like Figure 1 As shown, an ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity includes an input reflection layer, a composite cavity layer, an output reflection layer and a quartz substrate 1 stacked in sequence from top to bottom.

[0031] The input reflective layer includes a first silicon layer 10 with a thickness of 55 nm, a first silicon dioxide layer 9 with a thickness of 120 nm, and a second silicon layer 8 with a thickness of 40 nm, which are stacked in sequence from top to bottom.

[0032] The composite cavity layer adopts an FP cavity structure in which a metal platinum layer is introduced, which includes a first single-sided cavity layer 7 with a thickness of 170nm-310nm, a metal layer 6 with a thickness of 10nm, and a second single-sided cavity layer 5 with a thickness of 170nm-310nm stacked from top to bottom. The materials of the first single-sided cavity layer 7 and the second single-sided cavity layer 5 are both silicon dioxide. The material of the metal layer 6 is platinum (Pt). In some other embodiments, the material of the first single-sided cavity layer 7 and the second single-sided cavity layer 5 can also be any one of MgO (magnesium oxide), TiO2 (titanium dioxide), and MgF2 (magnesium difluoride).

[0033] The output reflective layer includes a third silicon layer 4 with a thickness of 40 nm, a second silicon dioxide layer 3 with a thickness of 120 nm, and a fourth silicon layer 2 with a thickness of 55 nm.

[0034] The input reflection layer and the output reflection layer arranged opposite to each other in this embodiment can not only reduce the full width at half maximum, but also reduce the energy loss to the maximum extent and retain the high transmittance. The silicon layer in the reflector can significantly suppress the short-wave stray band.

[0035] The thickness of the above materials are the best thicknesses currently obtained after a large number of simulations based on basic theory, which can ensure the optimal effect of the structure. The thickness of the first silicon layer 10, the first silicon dioxide layer 9, the second silicon layer 8, the third silicon layer 4, the second silicon dioxide layer 3, and the fourth silicon layer 2 can all be However, it should be noted that when the thickness of the proposed filter is not the optimal thickness proposed in this embodiment, the overall effect of the proposed filter will be affected.

[0036] When the incident light 11 is vertically irradiated on the upper surface of the filter (i.e., the outer side of the input reflection layer), the light passes through the input reflection layer and is reflected multiple times between the parallel input reflection layer and the output reflection layer to form interference. Due to the introduction of the metal layer 6, the interference order of the FP cavity is changed; in the composite cavity layer provided in this embodiment, the metal platinum layer with a thickness of 10nm can suppress the first-order harmonic, so that the interference order m=2, only the high-order resonant wave remains, and the low-order resonant wave is almost completely lost due to absorption or scattering by the metal layer; therefore, this embodiment can utilize the second harmonic with a narrower half-peak full width, thereby improving the resolution of the filter.

[0037] In the FP cavity, the resonance has a specific electric field distribution characteristic. The electric field intensity of the odd-order resonance reaches the maximum in the middle area of ​​the cavity, while the electric field of the even-order resonance is the minimum in this area. When the metal layer 6 is placed in the middle of the composite cavity layer, it will have a strong impact on the electric field due to its special physical properties. Specifically, it can disrupt the strong electric field distribution of the odd-order resonance in the middle of the cavity, making it difficult for the odd-order resonance to form, thereby effectively suppressing the odd-order resonance. In this way, the second-order resonance peak is highlighted, and its effective FSR becomes a separation from the fourth-order resonance peak, rather than a separation from the third-order resonance peak, thereby expanding the effective FSR of the second-order resonance peak. The goal of using high-order resonance (mainly second order) to expand the operating wavelength range and improve the spectral resolution has been successfully achieved, while maintaining the narrow bandwidth characteristics, effectively improving the overall performance of the filter.

[0038] Multiple filters with different total thickness d of the composite cavity layer (the values ​​of cavity thickness d are 350nm, 396.7nm, 443.4nm, 490.1nm, 536.8nm, 583.5nm, and 630nm) were tested respectively, and the transmission spectra of each filter were obtained as shown in Figure 2 shown; from Figure 2It can be seen that by adjusting the thickness of the two single-sided cavity layers in the composite cavity layer, the transmission center wavelength of the filter can be moved within the range of 550nm-850nm. According to the resonance formula of the FP cavity: = (where m is the resonance order, is the m-order resonance wavelength, n is the refractive index of the cavity material, is the refraction angle, is the cumulative reflection phase of the two reflection layers) The change of cavity thickness d will directly affect the optical path difference, and then change the wavelength that meets the resonance condition. When the cavity thickness increases, the optical path difference increases. According to the formula, the resonant wavelength will move toward the long wavelength direction (i.e., red shift occurs); conversely, when the cavity thickness decreases, the resonant wavelength will move toward the short wavelength direction (i.e., blue shift occurs). Figure 5 As shown, by adjusting the thickness of the first single-sided cavity layer 7 and the second single-sided cavity layer 5 adjacent to the metal layer, the central wavelength can be moved within the range of 550nm-850nm to achieve the selection of different wavelengths. Therefore, the output light wavelength of the ultra-wide tuning range narrow-band filter provided in this embodiment can be selected within the wide wavelength range of 550nm-850nm.

[0039] It can be seen that compared with other narrow-band filters based on FP cavities, this embodiment has outstanding advantages such as working range (550nm-850nm), high transmission peak (average transmittance exceeds 70%), narrow half-maximum full width (10nm-15nm), low sideband, few layers and simple structure, simple process and easy mass production, high optical quality and light energy utilization.

[0040] Example 2 A filter, the difference between this comparative example and embodiment 1 is that the material of the metal layer 6 is different; in this embodiment, the material of the metal layer 6 is gold (Au), and the other parameters remain the same.

[0041] Example 3 A filter, the difference between this comparative example and embodiment 1 is that the material of the metal layer 6 is different; in this embodiment, the material of the metal layer 6 is silver (Ag), and the other parameters remain the same.

[0042] Example 4 A filter. The difference between this comparative example and Example 1 is that the material of the metal layer 6 is different. In this example, the material of the metal layer 6 is aluminum (Al), and the other parameters remain the same.

[0043] The transmission spectra provided in Examples 1-4 are compared, and the results are as follows: Figure 3As shown; it can be seen that when the material used for the metal layer 6 is platinum, the first-order harmonic is completely suppressed, leaving only the second-order harmonic, and the sideband at the short wave is also reduced; when the material used for the metal layer 6 is gold or silver, although the second-order harmonic is still retained, there is an additional peak with higher transmittance at the long wave, which is not conducive to the monochromaticity of the structure; when the material used for the metal layer 6 is aluminum, the transmittance of the second-order harmonic drops to 50%. Although the sideband reduction effect at the short wave is the best, considering the importance of transmittance, we finally choose metal platinum as the best metal material for the structure.

[0044] Example 5 A filter, the difference between this comparative example and embodiment 1 is that the thickness of the metal layer 6 is different; in this embodiment, the thickness of the metal layer 6 takes other values ​​other than 10nm within the range of 5nm to 30nm, such as 5nm, 15nm, 20nm, 25nm, and 30nm.

[0045] When the filter cavity thickness is 350nm and other parameters are constant, the scanning diagram of the thickness of the metal layer 6 from 5nm to 30nm is as follows: Figure 4 As shown; Figure 4 In the figure, the wavelength corresponding to the first white dotted box from top to bottom is the central wavelength of the filter when the cavity thickness is 350nm, and the wavelength corresponding to the second white dotted box is the sideband of the filter when the cavity thickness is 350nm.

[0046] Depend on Figure 4 It can be seen that as the thickness of the metal layer 6 increases, the transmittance decreases significantly. This is because when the thickness of the metal layer changes, the phase difference of the light in the cavity changes, which may cause the original constructive interference condition to be destroyed and become destructive interference, so that the intensity of the transmitted light is weakened, that is, the transmittance is reduced. When the thickness of the metal layer 6 is 5nm, although its transmittance is higher than that of the metal layer 6 with a thickness of 10nm, its side band is also correspondingly higher than that of the metal layer 6 with a thickness of 10nm. This phenomenon is obviously not conducive to the monochromatic performance of the structure. Based on the comprehensive consideration of the peak-to-peak value of the central wavelength transmission and the full width at half maximum, and after rigorous simulation analysis, it is finally determined to select a metal layer 6 with a thickness of 10nm as the most ideal thickness. In this way, it can ensure the optimal transmittance in the entire working range, and ensure that the minimum full width at half maximum is obtained in the entire working range, thereby meeting the performance requirements of high transmittance and narrow bandwidth, providing a solid foundation and reliable guarantee for the performance optimization of the filter.

[0047] Comparative Example 1 A filter, the difference between this comparative example and embodiment 1 is that: the position of the metal layer 6 in the composite cavity layer is different; in this embodiment, the metal layer 6 is arranged at a position other than the center position of the composite cavity layer; for example, it is arranged at a third position of the composite cavity layer close to the input reflection layer, or it is arranged at a third position of the composite cavity layer close to the output reflection layer.

[0048] When the thickness of the filter cavity is 350 nm and other parameters are constant, the scanning image obtained for the position of the variable metal layer 6 in the cavity is as follows: Figure 5 As shown, for a clearer display, three white dotted frames are used to circle the part where the central wavelength changes with the position of the metal layer 6. As can be seen from the figure, the effect of the metal layer 6 at different positions in the cavity presents a symmetrical characteristic. Specifically, when the metal layer 6 is located at the upper or lower position of the cavity, it has a suppressive effect on the even-order harmonics (mainly the second-order harmonics); and when the metal layer 6 is in the middle position of the cavity, it mainly suppresses the odd-order harmonics (mainly the first-order harmonics). When we put the metal layer 6 at the upper or lower position of the cavity, the metal layer 6 will have a suppressive effect on the second-order resonance. Although this suppression is not complete, the energy at the second-order resonance peak has been weakened to the point where it will not significantly affect the monochromaticity of the first-order resonance. When the metal layer 6 is placed at one-third of the thickness of the cavity, it can be observed that the metal layer 6 has a suppressive effect on the transmission peaks at both the first-order resonance and the second-order resonance.

[0049] Therefore, it is necessary to place the metal layer 6 in the middle of the composite cavity layer (eg Figure 1 As shown in the figure), the first-order resonance is almost completely suppressed, thereby obtaining a single transmission peak (the white dotted box in the middle of the figure), which effectively ensures the monochromaticity of the structure. At the same time, given that the high-order resonance itself has the characteristics of narrow bandwidth, we can obtain a transmission peak with good monochromaticity and narrow half-maximum full width. From the basic principle of the FP cavity, this change in the position of the metal layer 6 changes the electric field distribution and the interference pattern of light in the cavity. When the metal layer 6 is located above and below the cavity, it changes the energy distribution of the electric field enhancement area corresponding to the second-order resonance, thereby weakening the second-order resonance; when it is located in the middle of the cavity, it can interfere with the strongest electric field area of ​​the first-order resonance, thereby suppressing the first-order resonance.

[0050] Comparative Example 2 like Figure 6 As shown, a filter is shown. The difference between this comparative example and Example 1 is that no metal layer 6 is provided, and the materials and thicknesses of the other layers are the same.

[0051] The transmission spectra of the filter provided by Example 1 (the total thickness of the first single-sided cavity layer 7 plus the metal layer 6 plus the second single-sided cavity layer 5 is 350 nm) and Comparative Example 1 (the thickness of the cavity layer is 350 nm) are compared. The results are as follows: Figure 7shown. Figure 7 In the figure, the solid line and the dotted line correspond to the transmission spectra of Example 1 and Comparative Example 1, respectively.

[0052] It can be seen that the comparative example 1 lacking the metal layer 6 has a double peak in the transmission spectrum. However, in the embodiment 1 with the metal layer 6, the first-order harmonic in the transmission spectrum is suppressed, leaving only the second-order harmonic, thereby providing a transmission spectrum with high transmittance, low sideband and narrow bandwidth.

[0053] In order to better illustrate the important role played by the metal layer 6 in the filter, a comparative simulation of the central wavelength change is performed for Example 1 and Comparative Example 1 with different total thicknesses of the composite cavity layer. The results are as follows: Figure 8 and Fig. 9 shown. Figure 8 The figure shows the transmission peak of the comparative example 1 without the metal layer 6 as the total thickness of the cavity increases. Figure 8 It can be seen that the maximum cutoff band of the structure is 610nm~840nm (such as Figure 8 The range framed by the white dotted line in the middle), under the condition that the known cut-off band must be larger than the working band, the maximum working range of the filter structure at this time is only 610nm~840nm, and it can only screen the transmission wavelength in the fixed band of 610nm~840nm. If it exceeds this band, relevant interference will be formed due to the existence of multiple peaks. As mentioned in the resonance formula of the FP cavity in Example 1, the reason for the appearance of multiple peaks in the FP cavity is that for a given cavity thickness, different integer multiples of wavelengths may meet the resonance conditions, resulting in multiple transmission peaks at different wavelengths. For example, in the resonance formula of the FP cavity mentioned in Example 1, in order to obtain a larger FSR, lower-order resonances, such as the first-order resonance wave, are usually preferred. Because as the resonance order m increases, the FSR of higher-order resonances will decrease rapidly (when the reflection phase is ignored, the FSR of different-order resonances at λ0 is approximately ). However, multispectral imaging requires not only a large FSR, but also a narrow bandwidth to obtain more spectral channels and higher resolution. However, although the low-order resonance has a large FSR, its bandwidth is relatively wide. , resulting in lower spectral resolution. On the contrary, higher-order resonances such as the second-order resonance peak can reduce the FWHM to about , but the FSR also becomes smaller, thus limiting the operating wavelength range. This makes it difficult to achieve both large FSR and narrow bandwidth in the traditional FP cavity filter design, and a trade-off needs to be made between the two.

[0054] Therefore, in Example 1, a structure in which a metal layer 6 is introduced in the middle of the cavity, the scanning diagram of the central wavelength of the obtained filter changing with the cavity thickness is shown as follows: Fig. 9 As shown. Fig. 9As shown, the introduction of metal layer 6 suppresses odd-order harmonics (mainly first-order and third-order harmonics), and the second-order resonance peak is highlighted, and its effective FSR becomes separated from the fourth-order resonance peak instead of the third-order resonance peak, thereby expanding the effective FSR of the second-order resonance peak. The goal of expanding the working wavelength range and improving the spectral resolution by using high-order resonance (mainly second-order) is successfully achieved, while maintaining the narrow bandwidth characteristics.

[0055] See also Fig. 9 In the range framed by the white dotted line, as the thickness of the composite cavity gradually increases, the central wavelength of the filter gradually increases within the range of 550nm~850nm; at the same time, it can be seen that the filter provided in Example 1 has an energy of almost 0 in the band range of 400nm~1000nm except for the central wavelength, and all are black areas. Although there are some white bright parts, these energies are not as strong as the energy of the central wavelength and do not affect the effect; therefore, compared with Comparative Example 1, Example 1 expands the cutoff band to 400nm~1000nm. Therefore, the ultra-wide tuning range narrowband filter provided by the present invention can filter out light of any transmission wavelength within the working range of 550nm~850nm in the band of 400nm~1000nm, which undoubtedly provides strong technical support for multi-spectral imaging.

[0056] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively detailed in order to clearly explain the invention verification process of the inventor, but it is not to be understood as limiting the scope of protection of the patent of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and optimizations can be made without departing from the principles of the present invention, which all belong to the scope of protection of the present invention.

Claims

1. An ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity, comprising an input reflection layer, a composite cavity layer and an output reflection layer stacked in sequence; characterized in that: A metal layer (6) is arranged in the middle of the composite cavity layer; the material of the metal layer (6) is platinum or aluminum; and the thickness of the metal layer (6) is 5 nm to 15 nm.

2. According to claim 1, a narrowband filter with ultra-wide tuning range using high-order harmonics of the FP cavity, characterized in that: The composite cavity layer also includes two single-sided cavity layers of equal thickness arranged on both sides of the metal layer (6); the thickness of the single-sided cavity layer is 170 nm to 310 nm; by adjusting the thickness of the single-sided cavity layer, the filtering center wavelength is adjusted within the range of 550 nm to 850 nm; the greater the thickness of the single-sided cavity layer, the greater the filtering center wavelength.

3. The ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity according to claim 2, characterized in that: The material of the single-sided cavity layer is silicon dioxide.

4. The ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity according to claim 1, characterized in that: The material of the metal layer is platinum.

5. The ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity according to claim 4, characterized in that: The thickness of the metal layer is 10 nm.

6. The ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity according to claim 1, characterized in that: The input reflection layer and the output reflection layer both include a silicon dioxide layer and two silicon layers respectively arranged on both sides of the silicon dioxide layer.

7. The ultra-wide tuning range narrowband filter using high-order harmonics of the FP cavity according to claim 1, characterized in that: In the input reflection layer, along the direction close to the composite cavity layer, the thicknesses of the first silicon layer, the silicon dioxide layer, and the second silicon layer are 45nm-65nm, 110nm-130nm, and 30nm-50nm, respectively; in the output reflection layer, along the direction close to the composite cavity layer, the thicknesses of the first silicon layer, the silicon dioxide layer, and the second silicon layer are 45nm-65nm, 110nm-130nm, and 30nm-50nm, respectively.

8. The ultra-wide tuning range narrowband filter using FP cavity high-order harmonics according to claim 1, characterized in that: The output reflection layer is arranged on the quartz substrate (1).

9. A method for designing a filter with a specified filtering wavelength, characterized in that: The following steps are involved: According to a target wavelength in the range of 550 nm to 850 nm, the thicknesses of the two single-sided cavity layers in the composite cavity layer are set so that the filter center wavelength of the filter is matched with the target wavelength; the input reflection layer, the first single-sided cavity layer, the metal layer (6), the second single-sided cavity layer and the output reflection layer are stacked in sequence; the material of the metal layer (6) is platinum or aluminum; the thickness of the metal layer (6) is 5 nm to 15 nm.

10. A filtering method, characterized in that: The filter designed by the filter design method as claimed in claim 9 is used to obtain light of a target wavelength.

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