A cavity length control method and device for a high-flatness tunable filter

Abstract

The present invention discloses a cavity length control method and device for a high-flatness tunable filter, comprising: obtaining an ideal capacitance value and obtaining a driving voltage based on the ideal capacitance value; applying the driving voltage to an annular piezoelectric ceramic block disposed between upper and lower circular substrates outside an FP cavity, causing the annular piezoelectric ceramic block to deform, thereby changing the spacing between the upper and lower circular substrates; obtaining the support capacitance value of at least one annular support capacitor group disposed on the upper and lower circular substrates outside the FP cavity, respectively, and a reference capacitance value connected to the annular support capacitor group, and obtaining a measured capacitance value based on inversion of the two; and adjusting the driving voltage based on the ideal capacitance value and the measured capacitance value, thereby adjusting the spacing between the upper and lower circular substrates. This method lays the foundation for subsequent performance improvement by specially positioning the annular piezoelectric ceramic block and the annular support capacitor group, and designing a cavity length adjustment step. Testing has shown that during the cavity length adjustment process, high flatness is maintained while achieving higher precision.

Description

Technical Field

[0001] The present invention relates to the technical field of tunable filters, and in particular to a cavity length control method and device for a high-flatness tunable filter. Background Art

[0002] As a miniaturized spectroscopic structure, the FP cavity has shown extremely broad application prospects in many fields such as infrared spectral imaging, military reconnaissance and warning, remote sensing and telemetry of earth resources and environment, and deep space exploration.

[0003] Compared to gratings, prisms, and Fourier transform spectrometers, FP cavity tunable filters offer significant advantages when used in spectrometers, including high spectral resolution, narrow passband, tunability, high light throughput, compactness, and high precision. The FP cavity primarily adjusts the wavelength of its transmitted light passband by controlling the distance between the upper and lower reflectors. Therefore, the precision of the position between the reflectors is a key factor affecting the accuracy of the FP cavity's transmitted wavelength.

[0004] Currently, a common solution is to design a capacitive sensor on the FP cavity. The size of this capacitance is closely related to the spacing between the upper and lower reflectors. By monitoring the capacitance value, the spacing between the upper and lower reflectors can be monitored, ultimately achieving displacement control of the spacing. However, the current cavity length adjustment method is affected by the error in the capacitance value, which limits the accuracy of the closed-loop feedback of the spacing between the upper and lower reflectors. Therefore, ensuring a small error in the capacitance value and maintaining high flatness of the capacitor metal layer during the cavity length adjustment process has become the key to determining the spectral resolution and spectral measurement accuracy of the spectrometer. Summary of the Invention

[0005] In response to the problems in the related art, the present invention proposes a cavity length control method and device for a high-flatness adjustable filter, which can achieve precise adjustment of the cavity length while ensuring that the capacitance flatness is sufficiently high.

[0006] To this end, the specific technical solutions adopted in the present invention are as follows:

[0007] A cavity length control method for a high-flatness tunable filter, comprising:

[0008] Step 1: Obtain a wavelength to be used, convert the wavelength to be used into an ideal capacitance value, and obtain a driving voltage based on the ideal capacitance value;

[0009] Step 2: Applying a driving voltage to an annular piezoelectric ceramic block disposed between the upper and lower circular substrates outside the FP cavity causes the annular piezoelectric ceramic block to deform, causing the distance between the upper and lower circular substrates to change;

[0010] Step 3: Obtain the support capacitance value of at least one group of annular support capacitors respectively provided on the upper and lower circular substrates outside the FP cavity, and the reference capacitance value connected to the annular support capacitor group, and obtain the measured capacitance value based on the inversion of the two;

[0011] Step 4: Adjust the driving voltage based on the ideal capacitance value and the measured capacitance value, and then adjust the distance between the upper and lower circular substrates, thereby adjusting the distance between the circular reflectors connected to the circular substrates in the FP cavity to achieve cavity length control of the FP cavity.

[0012] This cavity length control method for a high-flatness tunable filter lays the foundation for subsequent performance improvements by specially positioning the annular piezoelectric ceramic block and the annular support capacitor group, as well as designing the steps for adjusting the cavity length. Testing has shown that during the cavity length adjustment process, higher accuracy is achieved while ensuring high flatness.

[0013] Furthermore, the number of annular support capacitor groups affects the accuracy of cavity length adjustment. The number of annular support capacitor groups is 3, and the support capacitance value is the sum of the capacitance values ​​of the three annular support capacitors. The three annular support capacitors are connected in parallel and in series with the reference capacitor. The annular support capacitor group and the reference capacitor are connected in series and connected to an AC signal with a phase difference of 180°, which immediately generates an error voltage. The error voltage is used as the input value of the error control algorithm for inversion to obtain the measured capacitance value. The error control algorithm includes PID or LQR, wherein the AC signal includes any one of a sine wave, a square wave or a triangle wave.

[0014] The present application also provides a high-flatness tunable filter for realizing the cavity length control method of the above-mentioned high-flatness tunable filter, comprising: a circular substrate 1 and a circular substrate 2, wherein the shapes and sizes of the circular substrate 1 and the circular substrate 2 are consistent, the circular substrate 1 and the circular substrate 2 are respectively connected to a circular reflector, an annular piezoelectric ceramic block is connected between the circular substrate 1 and the circular substrate 2 on the outer circumference of the circular reflector, the annular piezoelectric ceramic block seals the circular reflector inside, and an annular support capacitor is respectively connected to the outer circumference of the annular piezoelectric ceramic block on the circular substrate 1 and the circular substrate 2 to form an annular support capacitor group, the annular support capacitor group is at least one group, the annular support capacitor group is connected in series with a reference capacitor, and the non-series ends of the two are respectively input with an AC signal with a phase difference of 180°. The core components of the high-flatness tunable filter are all circular or annular designs, and the positional relationship of the components is coordinated so that the capacitors and the reflector are subjected to uniform force during the cavity length adjustment process, thereby achieving high flatness.

[0015] Furthermore, the annular support capacitor on the circular substrate 1 includes several arc-shaped support capacitors 1, and the annular support capacitor on the circular substrate 2 includes several arc-shaped support capacitors 2. Several arc-shaped support capacitors 1 form a quasi-circular ring shape, and several arc-shaped support capacitors 2 form a quasi-circular ring shape. Every two arc-shaped support capacitors 2 correspond to one arc-shaped support capacitor 1, and the leads of the capacitors are connected to the two arc-shaped support capacitors 2. The two arc-shaped support capacitors 2 serve as the upper and lower electrodes of the capacitor group, and the arc-shaped support capacitor 1 serves as the middle electrode of the capacitor group to form a three-electrode capacitor group.

[0016] Furthermore, the annular support capacitor includes an annular support structure and an annular metal layer. The annular support structure is fixedly connected to the circular substrate one or the circular substrate two. The annular metal layer is plated on the surface of the annular support structure as an electrode plate of the capacitor.

[0017] Furthermore, the circular reflector is connected to the circular substrate one or the circular substrate two through a circular support structure. The height of the circular support structure is adjustable. The circular support structure uses an adhesive material to bond the circular reflector. A protective film is provided on the surface of the circular reflector.

[0018] Furthermore, there are three arc-shaped support capacitors and six arc-shaped support capacitors. Arc-shaped support capacitors 1 and arc-shaped support capacitors 2 are evenly arranged on corresponding concentric rings. This design allows a range of cavity lengths to be taken at intervals of 120° on the circumference of the annular piezoelectric ceramic. The three groups of cavity lengths are added together as the overall cavity length, avoiding errors introduced by the maximum, minimum or mutation points of the cavity length, thereby ensuring the cavity length accuracy of the adjustable filter.

[0019] The beneficial effects of the present invention are:

[0020] 1. This cavity length control method for a high-flatness tunable filter solves the problems of excessive and uneven deformation of the capacitor during cavity length adjustment in the existing FP cavity, which leads to the inability to ensure flatness, and the poor cavity length adjustment accuracy, which affects the spectral resolution and spectral measurement accuracy.

[0021] 2. This high-flatness tunable filter uses annular piezoelectric ceramics instead of the commonly used three-point or multi-point piezoelectric drive method. Combined with the annular support capacitor, the upper and lower reflectors and the capacitor metal layer are subjected to the same stress, which can make the upper and lower reflectors and the capacitor metal layer produce smaller deformation when pushed, and the deformation is evenly distributed in all directions, thereby improving the flatness of the FP cavity tunable filter at different gaps, and improving the transmission wavelength accuracy and spectral resolution.

[0022] 3. This high-flatness tunable filter uses capacitors to surround a circular reflective layer, and designs an arc-shaped capacitor feedback structure to surround the upper and lower reflective mirrors to form three groups of capacitor feedback channels. The capacitor design with a three-layer metal layer structure can effectively control the FP cavity capacitance parameters and realize a closed-loop position feedback function. Compared with the three-point and multi-point piezoelectric block drive methods, under the same piezoelectric drive voltage conditions, the annular piezoelectric ceramic structure makes the deformation of the capacitor metal layer and the upper and lower reflective layers more uniform, and the flatness of the capacitor metal layer can be improved by 1-2 orders of magnitude. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 Schematic diagram of the connection of the cavity length control circuit of the first embodiment of the present invention.

[0025] Figure 2 A schematic structural diagram of an adjustable filter according to a second embodiment of the present invention.

[0026] Figure 3 The internal structure of the tunable filter of the second embodiment of the present invention is shown in FIG. Figure 1 .

[0027] Figure 4 The internal structure of the tunable filter of the second embodiment of the present invention is shown in FIG. Figure 2 .

[0028] Figure 5 This is a transmission simulation diagram of the tunable filter according to the second embodiment of the present invention.

[0029] Figure 6 FIG. 4 is a capacitance-distance relationship curve diagram of the tunable filter according to the second embodiment of the present invention.

[0030] Figure 7 This is a graph showing the capacitance-spacing relationship of a tunable filter using three-point piezoelectric ceramics.

[0031] Figure 8 This is a graph showing the capacitance-spacing relationship of a tunable filter using four-point piezoelectric ceramics.

[0032] In the picture:

[0033] 1. Circular substrate 1; 2. Circular substrate 2; 3. Circular reflector; 4. Ring-shaped piezoelectric ceramic block; 5. Arc-shaped support capacitor 2; 6. Arc-shaped support capacitor 1. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention. Example 1

[0035] A cavity length control method for a high flatness tunable filter, such as Figure 1 Shown, including:

[0036] Step 1: In the terminal controller, input the wavelength to be used, and the terminal controller converts the wavelength to be used into an ideal capacitance value. The terminal controller is a human-computer interaction platform for realizing system task scheduling. It can be various forms of software and hardware systems, such as a computer host computer, which performs functions such as parameter distribution, status detection, and real-time feedback. The terminal controller inputs the ideal capacitance value into the error processing unit, and the error processing unit controls the annular piezoelectric ceramic block drive unit to output the driving voltage. The error processing unit has the capabilities of error voltage acquisition, analog-to-digital conversion, and logical calculation.

[0037] Step 2: Apply a driving voltage to the annular piezoelectric ceramic block disposed between the upper and lower circular substrates outside the FP cavity. The annular piezoelectric ceramic block deforms to change the distance between the upper and lower circular substrates.

[0038] Step 3: Obtain the support capacitance values ​​of the three groups of annular support capacitors respectively arranged on the upper and lower circular substrates outside the FP cavity, and the reference capacitance value of the annular support capacitor group. The reference capacitor is set in the hardware circuit. The reference capacitor is a high-precision, high-stability capacitor. The capacitance of this capacitor remains unchanged or changes slightly and can be ignored during the entire working process. The support capacitance value is the sum of the capacitance values ​​of the three groups of annular support capacitors. The three groups of annular support capacitors are connected in parallel and then connected in series with the reference capacitor. AC signals with a phase difference of 180° are input to the non-series ends of the three groups of annular support capacitors and the reference capacitor respectively. The AC signal includes any one of a sine wave, a square wave or a triangular wave. After the three groups of annular support capacitors are connected in parallel and then connected in series with the reference capacitor, an error voltage is generated. The error voltage is used as the input value of the error control algorithm to the error processing unit. The error control algorithm is such as the PID and LQR algorithms. The error control algorithm calculates the measured capacitance value through the error voltage, generates a control signal based on the difference between the measured capacitance value and the ideal capacitance value, and outputs it to the annular piezoelectric ceramic drive unit.

[0039] The calculation process of the measured capacitance value is as follows:

[0040] The reference capacitance value is C ref, V and V' are input AC voltages with a phase difference of 180°, C ref It is known that the measured capacitance value is C fp , the error voltage is V err , V err For actual measurement,

[0041] C fp =(C ref *V err -C ref *V) / (V'-V err ).

[0042] Step 4: The annular piezoelectric ceramic drive unit receives the control signal from the error processing unit, thereby adjusting the drive voltage. The adjusted drive voltage causes the annular piezoelectric ceramic block to change its deformation, thereby adjusting the spacing between the upper and lower circular substrates, thereby adjusting the spacing between the circular reflectors connected to the circular substrates in the FP cavity, and realizing cavity length control of the FP cavity. Example 2

[0043] The current filter is a capacitance sensor designed on the FP cavity. The size of the capacitance is closely related to the spacing between the upper and lower reflectors. By monitoring the capacitance value, the spacing between the upper and lower reflectors can be monitored, and finally the displacement control of the spacing can be achieved. However, since the existing capacitor structure is composed of metal layers plated on the upper and lower substrates of the FP cavity, in the process of the thruster pushing the upper and lower substrates to change the spacing between the reflectors, the substrate material will be deformed due to stress, causing the capacitor metal layer to no longer remain flat, thereby causing an error in the capacitance value. This error limits the accuracy of the closed-loop feedback of the spacing between the upper and lower reflective layers. Therefore, ensuring the high flatness of the capacitor metal layer during the movement of the upper and lower substrates has become the key to determining the spectral resolution and spectral measurement accuracy of the spectrometer. Based on this, the present application proposes a high-flatness adjustable filter for realizing a cavity length control method of a high-flatness adjustable filter in Example 1, such as Figure 2-Figure 4As shown, it includes a circular substrate 1 and a circular substrate 2, and the circular substrate 1 and the circular substrate 2 are respectively connected to a circular reflector 3, and an annular piezoelectric ceramic block 4 is connected between the circular substrate 1 and the circular substrate 2 at the outer circumference of the circular reflector 3. The circular substrate 1 is connected to three arc-shaped support capacitors 16 on the outer circumference of the annular piezoelectric ceramic block 4, and the circular substrate 2 is connected to six arc-shaped support capacitors 25 on the outer circumference of the annular piezoelectric ceramic block 4. The six arc-shaped support capacitors 25 form a quasi-circular ring, and the three arc-shaped support capacitors 16 are connected to the outer circumference of the annular piezoelectric ceramic block 4. Capacitor 1 6 forms a quasi-circular ring, and the arc-shaped support capacitor 1 6 and the arc-shaped support capacitor 2 5 are evenly arranged on the corresponding concentric rings. Every two arc-shaped support capacitors 2 5 correspond to one arc-shaped support capacitor 1 6. The leads of the capacitors are connected to the two arc-shaped support capacitors 2 5. The two arc-shaped support capacitors 2 5 serve as the upper and lower electrodes of the capacitor group, and the arc-shaped support capacitor 1 6 serves as the middle electrode of the capacitor group to form a three-electrode capacitor group. The annular support capacitor group is connected in series with a reference capacitor, and the non-series ends of the two are respectively input with AC signals with a phase difference of 180°. It should be noted that the arc-shaped support capacitor 1 6 and the arc-shaped support capacitor 2 5 both include an annular support structure and an annular metal layer. The annular support structure is fixedly connected to the circular substrate 1 1 or the circular substrate 2 2. The annular metal layer is plated on the surface of the annular support structure as an electrode plate of the capacitor. The circular reflector 3 is connected to the circular substrate 1 1 or the circular substrate 2 2 through the circular support structure. The height of the circular support structure is adjustable. The circular support structure uses an adhesive material to bond the circular reflector 3. The adhesive material can be made of a material with good adhesion properties such as titanium to ensure the reliability of the connection between the reflective layer and the support structure while not affecting the optical properties of the reflective layer. A protective film is provided on the surface of the circular reflector 3. The protective film is used to protect the upper and lower circular reflectors 3 to prevent them from being oxidized and mechanically damaged during use. The protective film material should have good optical transparency, chemical stability and wear resistance to extend the service life of the reflector without affecting the transmission and reflection of light, thereby ensuring the long-term stable operation of the filter.

[0044] Arc-shaped support capacitor 1 6 and arc-shaped support capacitor 2 5 surround the annular piezoelectric ceramic block 4, and an arc-shaped capacitor feedback structure is designed to surround the upper and lower reflecting mirrors to form three groups of capacitor feedback channels. This three-layer metal layer capacitor design can effectively control the FP cavity capacitance parameters and realize closed-loop position feedback function.

[0045] Regarding the closed-loop feedback principle: the capacitance value is determined by the area of ​​the capacitance layer, the spacing between the capacitor plates, and the spacing between the plates. For the capacitance layer of the FP tunable filter, after the structural design is completed, the capacitance layer area and the dielectric between the plates remain unchanged and are fixed constants. If the spacing between the capacitor plates is twice the cavity length of the FP cavity tunable filter, when a voltage is applied to the annular piezoelectric ceramic block 4, the FP cavity length changes, and the capacitance value of the capacitance layer changes immediately. Ideally, one transmission wavelength corresponds to one cavity length, and the piezoelectric ceramic block needs to be pushed or compressed a certain distance to drive the upper substrate to stretch or compress, thereby increasing or decreasing the capacitance value. By monitoring the capacitance value in real time through the peripheral circuit, the FP cavity length can be monitored. By adjusting the deviation between the actual change in cavity length and the ideal change, the driving voltage of the piezoelectric ceramic block is adjusted, and then the FP cavity length is changed, realizing a closed-loop feedback from piezoelectric ceramic driving voltage-FP cavity length-capacitance value-piezoelectric ceramic driving voltage. Compared with the three-point and multi-point piezoelectric block driving methods, under the same piezoelectric driving voltage conditions, the annular piezoelectric ceramic block 4 structure makes the deformation of the capacitor metal layer and the upper and lower reflective layers more uniform, and the flatness of the capacitor metal layer can be improved by 1-2 orders of magnitude.

[0046] It should be noted that circular substrate 1 and circular substrate 2 are spaced and parallel to each other, forming the basic framework of the entire filter and providing a mounting and support platform for other components. Materials with stable optical properties and high mechanical strength, such as quartz and glass, are preferred to ensure structural reliability and stable optical performance under various operating conditions.

[0047] The circular ring structure of the annular piezoelectric ceramic block 4 causes the upper and lower reflectors and the capacitor metal layer to be subjected to the same stress, which can cause the upper and lower reflectors and the capacitor metal layer to produce smaller deformation when pushed, and the deformation is evenly distributed in all directions, thereby improving the flatness of the FP cavity tunable filter at different gaps and improving the transmission wavelength accuracy and spectral resolution.

[0048] The circular reflector 3 is composed of a single or multiple layers of metal or inorganic materials and has high reflectivity. Typically, its reflectivity for light within a specific wavelength range is greater than 90%, effectively reflecting incident broadband light back and forth between the two reflectors. As light propagates between the reflectors, light with wavelengths that meet the cavity length requirements undergoes constructive interference, allowing it to pass through the filter; light that does not meet the cavity length requirements undergoes destructive interference and is filtered out. This high-reflectivity reflector design is key to achieving the FP cavity filtering function, ensuring the filter has excellent optical selectivity and highly flat transmittance characteristics.

[0049] The parameters of the high-flatness tunable filter of this embodiment are as follows: the radius of circular substrate 1 and circular substrate 2 are both 15 mm, and the thickness is 5 mm; the radius of the circular support structure is 7 mm, and the thickness is 1 nm; the bonding radius is 7 mm, and the thickness is 4 nm; the radius of the circular reflector 3 is 7 mm, and the thickness is 40 nm; the radius of the protective film is 7 mm, and the thickness is 20 nm; the initial cavity length is 275 nm; the height of the annular support structure is 1 mm, and the thickness of the annular metal layer is 84 nm.

[0050] The filter of the present application uses three groups of capacitors packaged in parallel to form an adjustable filter capacitor group. The adjustable filter capacitor group is connected in series with a reference capacitor, and an error voltage is generated at the connection end of the adjustable filter capacitor group and the reference capacitor. The purpose of packaging the three groups of capacitors in parallel to form an adjustable filter capacitor group is to take a range of cavity lengths at every 120° interval on the circumference of the annular piezoelectric ceramic block 4, and add the three groups of cavity lengths as the overall cavity length, thereby avoiding errors introduced by the maximum, minimum or mutation point of the cavity length, and ensuring the accuracy of the cavity length of the adjustable filter.

[0051] During use of the filter of the present application, the circular substrate 2 is fixed, and the annular piezoelectric ceramic block 4 has an inverse piezoelectric effect. After voltage is applied, the annular piezoelectric ceramic block 4 generates strain. Depending on the positive or negative applied voltage, the annular piezoelectric ceramic block 4 is stretched or compressed. It is defined that when a positive voltage is applied to the annular piezoelectric ceramic block 4, the annular piezoelectric ceramic block 4 is stretched, and the two ends of the annular piezoelectric ceramic block 4 are fixedly connected to the circular substrate 1 and the circular substrate 2. The circular substrate 1 is subjected to the thrust of the annular piezoelectric ceramic block 4 at the connection with the annular piezoelectric ceramic block 4, pushing the circular substrate 1 to move forward. Since the substrate material is only subjected to stress at the circular connection, the substrate material will produce elastic deformation when the material is subjected to stress. The uneven stress distribution causes different sizes of deformations in different positions of the reflective layer, capacitor layer, etc. that have been plated on the substrate, resulting in changes in the distance between the reflective mirror and the capacitor layer.

[0052] Comparing the filter of the present application with the traditional three-point structure and the traditional multi-point structure, it was found that the traditional three-point structure: the maximum deviation of capacitance flatness: 36.1nm; the maximum capacitance deviation: 0.585pf; the traditional multi-point structure: the maximum deviation of capacitance flatness: 67nm; the maximum capacitance deviation: 1.207pf; the filter of the present application: the maximum deviation of capacitance flatness: 0.4nm; the maximum capacitance deviation: 0.057pf, indicating that the filter of the present application can adjust the cavity length more accurately while ensuring capacitance flatness. The transmission spectrum of the filter of the present application is shown as follows Figure 5 As shown, and as Figure 6-Figure 8 As shown, it can be seen that the capacitance accuracy of the filter of the present application is significantly better than that of the existing three-point and four-point piezoelectric ceramics, so the cavity length adjustment of the filter of the present application is more precise.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A cavity length control method for a high-flatness tunable filter, characterized in that: include: Step 1: Obtain a wavelength to be used, convert the wavelength to be used into an ideal capacitance value, and obtain a driving voltage based on the ideal capacitance value; Step 2: Applying a driving voltage to an annular piezoelectric ceramic block disposed between the upper and lower circular substrates outside the FP cavity causes the annular piezoelectric ceramic block to deform, causing the distance between the upper and lower circular substrates to change; Step 3: Obtain the support capacitance value of at least one group of annular support capacitors respectively provided on the upper and lower circular substrates outside the FP cavity, and the reference capacitance value of the reference capacitor connected to the annular support capacitor group, and obtain the measured capacitance value based on the inversion of the two; Step 4: Adjust the driving voltage based on the ideal capacitance value and the measured capacitance value, and then adjust the distance between the upper and lower circular substrates, thereby adjusting the distance between the circular reflectors connected to the circular substrates in the FP cavity to achieve cavity length control of the FP cavity. The high-flatness tunable filter comprises: a circular substrate 1 (1) and a circular substrate 2 (2), wherein the circular substrate 1 (1) and the circular substrate 2 (2) are respectively connected to a circular reflector (3), an annular piezoelectric ceramic block (4) is connected between the circular substrate 1 (1) and the circular substrate 2 (2) at the outer circumference of the circular reflector (3), and an annular support capacitor is respectively connected to the circular substrate 1 (1) and the circular substrate 2 (2) at the outer circumference of the annular piezoelectric ceramic block (4) to form an annular support capacitor group, wherein the annular support capacitor group is at least one group, and the annular support capacitor group is connected in series with a reference capacitor, and the non-series ends of the two are respectively input with AC signals with a phase difference of 180 degrees.

2. A cavity length control method for a high-flatness tunable filter according to claim 1, characterized in that: The annular support capacitor group is connected in parallel with the reference capacitor in series. The non-series ends of the annular support capacitor group and the reference capacitor are respectively input with AC signals with a phase difference of 180 degrees. The AC signal includes any one of a sine wave, a square wave or a triangle wave.

3. A cavity length control method for a high-flatness tunable filter according to claim 2, characterized in that: The support capacitance value is the sum of the capacitance values ​​of the annular support capacitance group. When the annular support capacitance group and the reference capacitance are connected in series and energized, an error voltage is generated. The error voltage is used as an input value of an error control algorithm for inversion to obtain the measured capacitance value. The error control algorithm includes PID or LQR.

4. The cavity length control method of a high-flatness tunable filter according to claim 1, characterized in that: The annular support capacitor on the circular substrate 1 (1) includes a plurality of arc-shaped support capacitors 1 (6), and the annular support capacitor on the circular substrate 2 (2) includes a plurality of arc-shaped support capacitors 2 (5). The plurality of arc-shaped support capacitors 1 (6) form a quasi-circular ring shape, and the plurality of arc-shaped support capacitors 2 (5) form a quasi-circular ring shape. The arc-shaped support capacitors 1 (6) and the arc-shaped support capacitors 2 (5) are correspondingly arranged.

5. The cavity length control method of a high-flatness tunable filter according to claim 4, characterized in that: Every two arc-shaped support capacitors 2 (5) correspond to one arc-shaped support capacitor 1 (6), and the leads of the capacitors are connected to the two arc-shaped support capacitors 2 (5). The two arc-shaped support capacitors 2 (5) serve as the upper and lower electrodes of the capacitor group, and the arc-shaped support capacitor 1 (6) serves as the middle electrode of the capacitor group, forming a three-electrode capacitor group.

6. The cavity length control method of a high-flatness tunable filter according to claim 1, characterized in that: The annular support capacitor comprises an annular support structure and an annular metal layer, wherein the annular support structure is fixedly connected to the circular substrate one (1) or the circular substrate two (2), and the annular metal layer is plated on the surface of the annular support structure as an electrode plate of the capacitor.

7. The cavity length control method of a high-flatness tunable filter according to claim 1, characterized in that: The circular reflector (3) is connected to the circular substrate 1 (1) or the circular substrate 2 (2) via a circular support structure, and the height of the circular support structure is adjustable.

8. The cavity length control method of a high-flatness tunable filter according to claim 7, characterized in that: The circular support structure is bonded to the circular reflector (3) using an adhesive material, and a protective film is provided on the surface of the circular reflector (3).

9. The cavity length control method of a high-flatness tunable filter according to claim 4, characterized in that: There are three arc-shaped support capacitors 1 (6) and six arc-shaped support capacitors 2 (5). The arc-shaped support capacitors 1 (6) and the arc-shaped support capacitors 2 (5) are evenly arranged on corresponding concentric rings.

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