Thermal compensation birefringent filter and imaging device

Abstract

The present invention provides a thermal compensation birefringent filter and an imaging device. The thermal compensation birefringent filter adopts a four-stage series interference polarization unit design, with the third and fourth stage interference polarization units placed in the middle, and the first and second stage interference polarization units placed on both sides. Each interference polarization unit is based on the principle of interference polarization and is based on two birefringent crystals with different refractive index temperature coefficients. The phase difference change caused by temperature change is compensated by the reasonable placement of the optical axis. The birefringence of crystals of different materials is different, and the thermo-optical coefficient is also different. The problem of spectral line drift caused by temperature change is solved, so that the working wavelength is always stable at 656.3nm (i.e., the Hα line of the sun), and the fluctuation range of the central wavelength of the spectral line does not exceed 0.002nm. The thermal compensation birefringent filter designed by the present invention has no electrical control, temperature control and mechanical devices in the system, which can reduce the physical size of the instrument and improve the stability of the instrument.

Description

Technical Field

[0001] The present invention relates to the technical field of astronomical optics, and in particular to a thermal compensation type birefringent filter and an imaging device. Background Art

[0002] The structure of the sun includes the solar core, radiation layer, troposphere and solar atmosphere. At present, the study of solar physics is through direct observation of its intensity and magnetic field. Solar physical phenomena such as flare eruption, coronal ejection, solar storm, etc. are all closely related to the temporal and spatial evolution and distribution of the solar magnetic field. Therefore, the measurement of the solar magnetic field is a very critical technical means in solar physics research. With the deepening of solar physics research, it is necessary to observe various physical activity phenomena in the solar atmosphere from multiple bands, multiple perspectives, multiple scales, high precision and high resolution, so higher requirements are put forward for the accuracy and stability of solar observation equipment. At present, the observation of solar physics is mainly based on ground-based and space-based telescopes. However, the complex external environment changes have a great influence on the measurement accuracy of the instrument. The main reason is that temperature changes will cause changes in the refractive index and physical size of the optical component material, which will cause changes in the system optical path difference and phase, and finally lead to the drift of the instrument spectral image. It is very important to solve the influence of temperature changes on the instrument.

[0003] The traditional method to solve temperature changes is to achieve high-precision temperature control of 0.02°C through a two-level temperature control system. Its defect is the complex control system. The introduction of the temperature control system makes the overall size of the instrument larger. It can still meet the requirements for ground-based observations, but for air-based solar observation equipment, it will increase the launch cost and reduce stability. Secondly, the temperature is compensated by rotating the wave plate, which will introduce multiple stepper motors. The complex mechanical and control structure increases the size and mass of the instrument. In addition, in order to match the refractive index of the optical element, it is necessary to fill the filter with silicone oil and seal the instrument. Therefore, the existence of the mechanical rotating device will affect the stability of the instrument.

[0004] In addition, the existing technology also attempts to use liquid crystal phase retarders for temperature compensation. The advantage is that the temperature control system is abandoned, and the instrument is relatively more compact. However, liquid crystal is voltage-driven, so there is still a control system, and the phase delay of the liquid crystal itself is also related to temperature changes. When the temperature changes, the calibrated voltage and phase relationship is no longer applicable, and the measurement accuracy will also decrease to a certain extent.

[0005] Currently, the rapid development of solar physics research urgently requires simultaneous observation of imaging and spectroscopy. The instruments that can meet this requirement include: spectrometers or a combination of light arrays and spectrometers. However, the instruments designed in the above-mentioned existing technologies are relatively complex and the stability of the observation system is poor. Summary of the invention

[0006] In view of the problem of spectral line drift caused by temperature environment changes in astronomical optical systems for solar imaging measurements in the prior art, the present invention aims to provide a thermally compensated birefringent filter. Based on the principle of polarization interference and the differences in thermo-optical coefficients of different birefringent crystals, temperature compensation of the birefringent filter is achieved by reasonably configuring the optical axis direction and the thickness of the crystal, thereby reducing the physical size of the instrument, the complexity of the system, and improving the stability of the instrument.

[0007] According to a first aspect of the present invention, a thermal compensation birefringent filter is provided, which is arranged between an entrance pupil and a detector. The thermal compensation birefringent filter is composed of four interference polarization units, which are arranged coaxially from the entrance pupil to the detector as a second-order interference polarization unit, a third-order interference polarization unit, a fourth-order interference polarization unit and a first-order interference polarization unit connected in series in sequence, wherein:

[0008] Each level of interference polarization unit includes, from left to right, a polarizer, an Iceland spar crystal, a half wave plate, an Iceland spar crystal, an ADP crystal, and a polarizer; wherein the optical element arrangement direction and optical axis placement position of the polarizer, the Iceland spar crystal, the half wave plate, the Iceland spar crystal, the ADP crystal, and the polarizer of each level of interference polarization unit are the same, and the thickness of the Iceland spar crystal and the ADP crystal are different;

[0009] In one of the first-stage interference polarization units, from left to right, the transmission directions of the two linear polarizers are parallel, the front-stage linear polarizer is used as a polarizer, and the rear-stage linear polarizer is used as an analyzer. An Iceland spar crystal, a half-wave plate, an Iceland spar crystal, and an ADP crystal are sequentially added between the two linear polarizers;

[0010] - The fast axis of the front stage Iceland spar crystal is 45 degrees to the transmission direction of the polarizer, and the fast axis of the rear stage Iceland spar crystal is -45 degrees to the transmission direction of the polarizer, so that the fast axes of the two Iceland spar crystals of equal thickness are perpendicular to each other;

[0011] -The optical axis direction of the half-wave plate is consistent with the transmission direction of the polarizer;

[0012] -The optical axis direction of the ADP crystal is consistent with the fast axis direction of the subsequent Iceland spar crystal;

[0013] After being polarized by the linear polarizer in the front stage, the polarized light will cause an optical path difference when passing through the birefringent crystal composed of Iceland spar crystal and ADP crystal, which will cause interference in the polarizer in the rear stage. The thermal compensation is achieved by using Iceland spar crystal and ADP crystal at the same time, which reduces the sensitivity of the filter to temperature changes.

[0014] As an example of the present invention, the thermally compensated birefringent filter proposed in the present invention is particularly suitable for the detection imaging optical path of the Hα line of solar radiation, operating in the band of 656.3nm; and further obtains the required interference order according to the required half-width at half maximum, that is, the phase delay amount of each interference polarization unit, determines the crystal thickness, and achieves the purpose of reducing size and improving stability.

[0015] As an example of the present invention, in the order of the first-order interference polarization unit, the second-order interference polarization unit, the third-order interference polarization unit, and the fourth-order interference polarization unit, the thickness of the Iceland spar crystal and the thickness of the APD crystal in the latter-order interference polarization unit are respectively twice the thickness of the Iceland spar crystal and the thickness of the APD crystal corresponding to the previous-order. Correspondingly, the interference orders of the second- to fourth-order interference polarization units are also twice the interference orders of the previous-order interference polarization units. For example, if the interference order of the first-order interference polarization unit is m, the interference orders of the second- to fourth-order interference polarization units are 2m, 4m, and 8m, respectively.

[0016] According to the design of the thermal compensation birefringent filter proposed by the present invention, different birefringent crystal materials are used to compensate the phase difference change caused by temperature change through the reasonable placement of the optical axis. The birefringence of crystals of different materials is different, and the thermo-optic coefficient is also different. Based on this, the problem of spectral line drift caused by temperature change is solved. The thermal compensation birefringent filter proposed by the present invention realizes the temperature compensation of the instrument. The system does not have any electrical control, temperature control and mechanical devices, which reduces the physical size of the instrument and greatly improves the stability of the instrument.

[0017] According to a second aspect of the present invention, an imaging device using the above-mentioned thermal compensation type birefringent filter is also provided, comprising the above-mentioned thermal compensation type birefringent filter and a detector, wherein the above-mentioned thermal compensation type birefringent filter is arranged between the incident pupil and the detector;

[0018] A thermally compensated birefringent filter is used to achieve a high stability filter for air-based and ground-based solar observations. The light beam entering through the entrance pupil is filtered and emitted into the detector for real-time imaging.

[0019] It should be understood that all combinations of the aforementioned concepts and the additional concepts described in more detail below can be considered as part of the inventive subject matter of the present disclosure as long as such concepts are not mutually inconsistent. In addition, all combinations of the claimed subject matter are considered as part of the inventive subject matter of the present disclosure.

[0020] The foregoing and other aspects, embodiments and features of the present invention can be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the present invention, such as the features and / or beneficial effects of the exemplary embodiments, will be apparent from the following description or learned from the practice of the specific embodiments according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For clarity, not every component is labeled in every figure. Embodiments of various aspects of the present invention will now be described by way of example and with reference to the accompanying drawings, in which:

[0022] Figure 1 Schematic diagram of a thermally compensated birefringent filter according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of the center spectrum line drift caused by reducing the operating temperature by 5°C when no temperature compensation design is used.

[0024] Figure 3 It is a schematic diagram of the central spectrum line when the temperature is lowered by 5°C based on the thermal compensation birefringent filter designed by the present invention. DETAILED DESCRIPTION

[0025] The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. The specific embodiments described in the specific embodiments are only used to explain the present invention and are not used to limit the present invention. The present invention is the result of the National Natural Science Foundation of China's general project "Key Technology Research of Snapshot Linear Stokes Parametric Polarization Spectrometer" (12073056).

[0026] Combination Figure 1 As shown, the thermally compensated birefringent filter according to an exemplary embodiment of the present invention is a core device in a solar imaging measurement telescope. The temperature compensation is based on the thermo-optical effect of the birefringent crystal. The temperature compensation of the instrument is achieved by the difference in the thermo-optical coefficients of the two birefringent crystals. The temperature compensation method based on the thermo-optical effect replaces the traditional temperature control or mechanical rotation of the half-wave plate, avoids the use of complex electrical control and mechanical transmission devices, reduces the size of the observation instrument, and greatly improves the stability of the instrument, thereby solving the problem of the overall size of the instrument being too large and the stability being reduced due to the existing temperature compensation scheme being too complicated. Especially for space solar observation, the instrument must be miniaturized and reliable.

[0027] The thermal compensation type birefringent filter proposed by the present invention is based on the principle of interference polarization and two birefringent crystals with different refractive index temperature coefficients, so that their fast axis directions are perpendicular to each other to achieve optical path difference subtraction, that is, the spectrum line drift caused by temperature change is compensated by the thermo-optical effect, so that the working wavelength is always stable at 656.3nm (that is, the Hα line of the sun), and the fluctuation range of the center wavelength of the spectrum line caused by temperature change does not exceed 0.002nm. According to the requirements of temperature compensation, the spectral line position of the peak wavelength needs to be stabilized, based on the central transmittance peak wavelength of 656.3nm, the thickness of the first-level Iceland spar crystal and the corresponding interference order are calculated according to the half-height full width FWMH0.025nm, and then the thickness of the Iceland spar and ADP crystal is calculated according to the principle of thermo-optical effect to achieve temperature compensation.

[0028] Combination Figure 1 In the example shown, the thermally compensated birefringent filter proposed in the present invention adopts a four-stage series interference polarization unit design. Since the third and fourth levels are more sensitive to temperature, the third and fourth level interference polarization units are placed in the middle, and the first and second level interference polarization units are placed on both sides, that is, the temperature-sensitive interference polarization unit is placed in the middle, and the insensitive unit is placed on the outside, and the detector unit is used to receive image information.

[0029] Each level of interference polarization unit includes, from left to right, a polarizer, an Iceland spar crystal, a half wave plate (45° half wave plate), an Iceland spar crystal, an ADP crystal and a polarizer.

[0030] The use of half-wave plates is to expand the field of view of the instrument and meet the requirements of wide field of view. Each stage contains two linear polarizers with high extinction ratio, the first one is a polarizer, and the second one is an analyzer. Between the two linear polarizers, an Iceland spar crystal, a half-wave plate, an Iceland spar crystal, and an ADP crystal are placed in sequence. The Iceland spar crystal provides phase delay. Two Iceland spar crystals of equal thickness are placed with their fast axes perpendicular to each other.

[0031] A 45° half-wave plate is placed between two Iceland spar crystals to form a wide field of view stage.

[0032] The role of the ADP crystal is to compensate for phase changes caused by temperature changes.

[0033] like Figure 1As shown, light enters the thermally compensated birefringent filter through the entrance pupil. Based on the principle of interference polarization, it passes through each level of narrow-band filtering and thermal compensation for spectral drift caused by temperature changes, and is finally imaged on the detector in real time, realizing the filtering effect of thermal compensation, so that the working wavelength of the filter is always stable at 656.3nm, and the fluctuation range of the central wavelength of the spectral line caused by temperature changes is less than or equal to 0.002nm, realizing a high-stability filter for air-based and ground-based solar observation needs, and filtering the light beam incident through the entrance pupil.

[0034] The thermal compensation birefringent filter proposed in the present invention first determines the working band of the birefringent filter, 656.3nm, that is, the Hα line of the sun, and obtains the required interference order according to the required half-height full width, that is, the phase delay amount of each interference polarization unit; then, based on the thermal compensation method, two birefringent crystals with different refractive index temperature coefficients are used in each level of interference polarization unit, and the optical axis orientations thereof are such that their optical paths are subtracted, and the intensity formula after adding the compensation crystal ADP is determined according to the Mueller matrix; then, the required optical path difference of each level of interference polarization unit is obtained according to the interference order, and the thickness of the Iceland spar crystal and the ADP crystal is calculated according to the thermo-optical effect, that is, the thickness of the Iceland spar crystal and the ADP crystal is The relationship between the thickness of the crystal is used to achieve the complementarity of the optical path difference caused by temperature change, and the thickness information of the birefringent crystal material in each interference polarization unit is determined as well as the specific placement method, including: in order to achieve the function of wide field of view, the Iceland spar crystal is divided into two, and the optical axes are placed crosswise, and a half-wave plate is placed in the middle, and the fast axis of the half-wave plate is at a 45° angle to the fast axis of the Iceland spar crystal; the entire thermal compensation birefringent filter adopts a four-level design, the third and fourth level interference polarization units are placed in the middle, and the first and second level interference polarization units are placed on both sides, that is, the temperature-sensitive interference polarization unit is placed in the middle, and the insensitive unit is placed on the outside, and the detector unit is used to receive image information.

[0035] Therefore, the light passes through the entrance pupil and enters the thermally compensated birefringent filter. Based on the principle of interference polarization, it undergoes narrow-band filtering at each level and is finally imaged on the detector unit, thereby achieving the filtering effect.

[0036] Combination Figure 1 As shown, an embodiment of the present invention provides a thermally compensated birefringent filter, whose optical structure includes four interference polarization orders Ⅰ, Ⅱ, Ⅲ, and Ⅳ.

[0037] like Figure 1The exemplary thermally compensated birefringent filter is arranged between the detector 22 and the incident pupil 23, and is composed of four levels of interference polarization units. From the incident pupil 23 to the detector 22, the units are coaxially arranged to form a second-level interference polarization unit, a third-level interference polarization unit, a fourth-level interference polarization unit and a first-level interference polarization unit connected in series in sequence, that is, the temperature-sensitive third and fourth-level interference polarization units are placed in the middle, and the insensitive first and second-level interference polarization units are placed on both sides, and the detector unit is used to receive image information.

[0038] Each level of interference polarization unit includes, from left to right, a polarizer, an Iceland spar crystal, a half wave plate, an Iceland spar crystal, an ADP crystal and a polarizer; the optical element arrangement direction and optical axis placement position of the polarizer, Iceland spar crystal, half wave plate, Iceland spar crystal, ADP crystal and polarizer of each level of interference polarization unit are the same, and the thicknesses of the Iceland spar crystal and the ADP crystal are different.

[0039] like Figure 1 The thermally compensated birefringent filter of the example shown specifically includes: a linear polarizer 1, an Iceland spar crystal 2, a half wave plate 3, an Iceland spar crystal 4, an ADP crystal 5, a linear polarizer 6, an Iceland spar crystal 7, a half wave plate 8, an Iceland spar crystal 9, an ADP crystal 10, a linear polarizer 11, an Iceland spar crystal 12, a half wave plate 13, an Iceland spar crystal 14, an ADP crystal 15, a linear polarizer 16, an Iceland spar crystal 17, a half wave plate 18, an Iceland spar crystal 19, an ADP crystal 20, a linear polarizer 21, a detector 22, and an entrance pupil 23.

[0040] Four interference polarization units are connected in series. In order to reduce the influence of temperature on the system, the temperature-sensitive interference polarization unit is placed in the middle and the insensitive unit is placed on the outside to minimize the influence of temperature.

[0041] In one of the first-stage interference polarization units, from left to right, the transmission directions of the two linear polarizers are parallel, the front-stage linear polarizer is used as a polarizer, and the rear-stage linear polarizer is used as an analyzer. An Iceland spar crystal, a half-wave plate, an Iceland spar crystal, and an ADP crystal are added between the two linear polarizers in sequence:

[0042] - The fast axis of the front stage Iceland spar crystal is 45 degrees to the transmission direction of the polarizer, and the fast axis of the rear stage Iceland spar crystal is -45 degrees to the transmission direction of the polarizer, so that the fast axes of the two Iceland spar crystals are perpendicular to each other;

[0043] -The optical axis direction of the half-wave plate is consistent with the transmission direction of the polarizer;

[0044] -The optical axis direction of the ADP crystal is consistent with the fast axis direction of the subsequent Iceland spar crystal;

[0045] After being polarized by the linear polarizer in the front stage, the polarized light will cause an optical path difference when passing through the birefringent crystal composed of Iceland spar crystal and ADP crystal, which will cause interference in the polarizer in the rear stage. The thermal compensation is achieved by using Iceland spar crystal and ADP crystal at the same time, which reduces the sensitivity of the filter to temperature changes.

[0046] Taking the first-order interference polarization unit as an example, two linear polarizers 16 and 21 have parallel transmission directions, and Iceland spar crystal 17, half-wave plate 18, Iceland spar crystal 19 and ADP crystal 20 are added in between, wherein the optical axis of Iceland spar crystal 17 is 45 degrees to the transmission direction of the polarizer, and the optical axis of Iceland spar crystal 19 is -45 degrees to the transmission direction of the polarizer. The optical axis direction of the half-wave plate 18 is consistent with the transmission direction of the polarizer, and its purpose is to achieve a large observation field of view. The optical axis direction of the ADP crystal 20 is consistent with the direction of the Iceland spar 19. When the polarized light after being polarized by the polarizer passes through the Iceland spar and ADP crystal as birefringent crystals, it will cause an optical path difference, causing interference in the second polarizer. At the same time, thermal compensation is achieved through the Iceland spar and ADP crystal, which greatly reduces its sensitivity to temperature changes.

[0047] The composition and working mechanism of the first interference polarization unit described above, the arrangement of optical elements and the direction of optical axis placement of the other three interference polarization units are the same as those of the first one. The difference is the thickness of the Iceland spar and ADP crystal, which will not be repeated here.

[0048] In the embodiment of the present invention, in the order of the first-order interference polarization unit, the second-order interference polarization unit, the third-order interference polarization unit, and the fourth-order interference polarization unit, the thickness of the Iceland spar crystal and the thickness of the APD crystal in the latter-order interference polarization unit are respectively twice the thickness of the Iceland spar crystal and the thickness of the APD crystal corresponding to the previous-order. Since the thickness of the latter unit is twice that of the former, the interference orders of the second- to fourth-order interference polarization units are respectively twice the interference order of the former-order interference polarization unit.

[0049] In an embodiment of the present invention, the birefringence of the Iceland spar crystal is 0.1695, the full width at half maximum FWMH≤0.025nm, and the interference order m of the first-order interference polarization unit is determined to be 1454 according to the Mueller matrix, and the interference orders of the II-IV-order interference polarization units are determined to be 2m, 4m and 8m respectively.

[0050] It can be further confirmed that in the interference polarization units of levels I, II, III and IV, the thickness combinations of Iceland spar crystal and ADP crystal are: (7.5345, 7.1205), (15.0690, 14.2410), (30.1380, 28.4820) and (60.2760, 56.9640).

[0051] The application principle of the present invention is further described below in conjunction with specific embodiments.

[0052] The embodiment of the present invention takes the spectral imaging of the solar Hα (656.3nm) line as an example, requiring that the fluctuation range of the peak wavelength of the transmittance be within ±0.002nm, and the full width at half maximum FWMH of the spectrum line be ≤0.025nm.

[0053] At present, there is no design that realizes filter temperature compensation based on the mechanism of crystal thermo-optical effect. The existing thermal compensation instruments are relatively large in size and have low stability in the presence of an external control unit.

[0054] By using the method proposed in the present invention, a high-stability filter that can meet the needs of air-based and ground-based solar observations can be designed.

[0055] Under the condition of a traditional optical filter, the transmittance τ of the embodiment of the present invention can be expressed as:

[0056]

[0057] where μ = n e -n0, is the birefringence of the crystal, l0 is the thickness of the iceland spar of the first-level unit, λ=656.3nm, the interference polarization units are arranged in series, and the thickness of each unit crystal after the first level is calculated according to d k =2 k l0 design.

[0058] make Represents the optical path difference produced by the birefringent crystal. Differentiating it can give the formula for the full width at half maximum of the spectrum line:

[0059]

[0060] Among them, the birefringence of Iceland spar is μ = 0.1695. Therefore, according to the scientific band λ = 656.3nm and the full width at half maximum of the spectrum line FWMH ≤ 0.025nm, supplemented by the optical interference theory, from formulas (1) and (2), it can be obtained that the interference order of unit I is m = 1454. Since the thickness of the latter unit is twice that of the former, the interference order of units II-IV is also twice that of the former. This data will be used later when calculating the thickness of Iceland spar and ADP crystal materials.

[0061] Considering that Iceland spar and ADP crystal form a level (such as level I), the fast axes of the two crystals are placed perpendicular to each other to achieve thermo-optical compensation. Based on the theory of matrix optics, the intensity formula of each level unit can be obtained using the Mueller matrix method. For the level I interference polarization unit:

[0062]

[0063] Among them, M L represents the Mueller matrix of a linear polarizer, is the Mueller matrix of Iceland spar, is the Mueller matrix of ADP, S in is the incident Stokes parameter, and the relationship between the output light intensity and the input light intensity can be finally obtained:

[0064]

[0065] Where δ1 and δ2 are the phase differences between Iceland spar and ADP crystal respectively. According to the interference order of the first-order interference polarization unit obtained above, m=1454, the relationship between δ1 and δ2 can be obtained from equation (4):

[0066]

[0067] Therefore, while achieving thermo-optical compensation of temperature, the total phase difference between the two crystals needs to satisfy formula (5).

[0068] According to formula (4), the total optical path difference σ = σ1-σ2, σ1 is the optical path difference produced by the Iceland spar crystal, σ2 is the optical path difference produced by the ADP crystal, and the total optical path difference satisfies the following relationship:

[0069]

[0070] Where l1 is the thickness of the Iceland spar, l2 is the thickness of the ADP crystal

[0071] When the temperature changes, the crystal phase difference will change. Temperature compensation is to make the optical path difference Δσ caused by temperature change = 0. Formula (4) is a function of temperature T and wavelength λ, then:

[0072]

[0073] From dσ=0, we get Simplifying the above formula, we can get:

[0074]

[0075] Further simplifying the above formula yields:

[0076]

[0077] in and are the thermo-optical coefficients of Iceland spar and ADP crystal, that is, the phase change per unit length when the temperature changes by 1 degree (unit: mm -1 ℃ -1 ).

[0078] It can be seen that when the change in optical path difference caused by temperature change is zero, σ1 and σ2 satisfy formula (7). Therefore, by solving equations (5) and (7), the thicknesses of l1 and l2 can be obtained, which means that the scientific goal of the filter is met and temperature compensation is achieved at the same time.

[0079] The known quantities μ1=0.1695、μ1=0.0454、 Substituting in the solution, we can get:

[0080] l1=7.5345

[0081] l2=7.1205

[0082] Therefore, the thickness combinations of Iceland spar and ADP crystals in levels I, II, III, and IV are: (7.5345, 7.1205), (15.0690, 14.2410), (30.1380, 28.4820), and (60.2760, 56.9640).

[0083] If the operating temperature is lowered by 5°C, if the temperature compensation design method is not adopted, the central spectrum line will drift away from the scientific band, such as Figure 2 Based on the thermal compensation design structure proposed by the present invention, the spectrum line is not affected by temperature changes and is still near the spectrum line 656.3nm, as shown in FIG. Figure 3 shown.

[0084] In summary, by selecting Iceland spar and ADP materials as a combination of interference polarization, the optical axes are perpendicular to each other, and temperature compensation of the instrument can be achieved based on the thermo-optical effect. This design structure can abandon the traditional temperature control system, electrical control system, and mechanical transmission system, reduce the physical size, and greatly improve the stability of the system.

[0085] Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. A person with ordinary knowledge in the technical field to which the present invention belongs may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be determined by the definition of the claims.

Claims

1. A thermally compensated birefringent filter, characterized in that: The thermal compensation birefringent filter is arranged between the incident pupil and the detector, and is composed of four-level interference polarization units, which are arranged coaxially from the incident pupil to the detector to form a second-level interference polarization unit, a third-level interference polarization unit, a fourth-level interference polarization unit and a first-level interference polarization unit connected in series in sequence, wherein: Each level of interference polarization unit includes, from left to right, a polarizer, an Iceland spar crystal, a half wave plate, an Iceland spar crystal, an ADP crystal, and a polarizer; wherein the optical element arrangement direction and optical axis placement position of the polarizer, the Iceland spar crystal, the half wave plate, the Iceland spar crystal, the ADP crystal, and the polarizer of each level of interference polarization unit are the same, and the thickness of the Iceland spar crystal and the ADP crystal are different; In one of the first-stage interference polarization units, from left to right, the transmission directions of the two linear polarizers are parallel, the front-stage linear polarizer is used as a polarizer, and the rear-stage linear polarizer is used as an analyzer. An Iceland spar crystal, a half-wave plate, an Iceland spar crystal, and an ADP crystal are sequentially added between the two linear polarizers; - The fast axis of the front stage Iceland spar crystal is 45 degrees to the transmission direction of the polarizer, and the fast axis of the rear stage Iceland spar crystal is -45 degrees to the transmission direction of the polarizer, so that the fast axes of the two Iceland spar crystals are perpendicular to each other; -The optical axis direction of the half-wave plate is consistent with the transmission direction of the polarizer; -The optical axis direction of the ADP crystal is consistent with the fast axis direction of the subsequent Iceland spar crystal; After being polarized by the linear polarizer at the front stage, the polarized light will cause optical path difference when passing through the birefringent crystal composed of Iceland spar crystal and ADP crystal, which will cause interference in the polarizer at the rear stage. The thermal compensation is achieved by using Iceland spar crystal and ADP crystal at the same time, which reduces the sensitivity of the filter to temperature changes. The two Iceland spar crystals in each stage of interference polarization unit adopt the same structure and design and have the same thickness, and their fast axes are 90° to achieve optical path difference subtraction; According to the order of the first-order interference polarization unit, the second-order interference polarization unit, the third-order interference polarization unit, and the fourth-order interference polarization unit, the thickness of the Iceland spar crystal and the thickness of the APD crystal in the interference polarization unit of the latter order are respectively twice the thickness of the Iceland spar crystal and the thickness of the APD crystal corresponding to the former order; The interference orders of the II-IV interference polarization units are respectively twice the interference order of the previous interference polarization unit.

2. The thermally compensated birefringent filter according to claim 1, wherein: The thermal compensation birefringent filter is suitable for the detection imaging optical path of Hα rays of solar radiation and operates in the wavelength band of 656.3nm.

3. The thermally compensated birefringent filter according to claim 1, wherein: The birefringence of the Iceland spar crystal is 0.1695, and the full width at half maximum FWMH is ≤0.025nm. The interference order m of the first-order interference polarization unit is determined to be 1454, and the interference orders of the second-fourth-order interference polarization units are determined to be 2m, 4m and 8m respectively.

4. The thermally compensated birefringent filter according to claim 1, wherein: In the I, II, III and IV level interference polarization units, the thickness combinations of Iceland spar crystal and ADP crystal are: (7.5345, 7.1205), (15.0690, 14.2410), (30.1380, 28.4820) and (60.2760, 56.9640), and the thickness unit is mm.

5. The thermally compensated birefringent filter according to claim 1, wherein: The thermal compensation birefringent filter compensates for the spectral line drift caused by temperature change based on the thermo-optic effect, so that the working wavelength of the filter is always stable at 656.3nm, and the fluctuation range of the central wavelength of the spectral line caused by temperature change is less than or equal to 0.002nm.

6. An imaging device, characterized in that: A device comprising a detector and the thermally compensated birefringent filter according to any one of claims 1 to 5, wherein the thermally compensated birefringent filter is arranged between an entrance pupil and the detector; A thermally compensated birefringent filter is used to achieve a high stability filter for air-based and ground-based solar observations. The light beam entering through the entrance pupil is filtered and emitted into the detector for real-time imaging.

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