Wide-spectrum intermediate infrared polarization separation device and preparation method and application thereof

Abstract

The invention relates to a wide-spectrum mid-infrared polarization separation device and a preparation method and application thereof.The wide-spectrum mid-infrared polarization separation device comprises a first reflector and at least two second reflectors based on silicon carbide crystals, and the reflecting face of the first reflector is arranged relative to an incident beam and the reflecting face of at least one second reflector; the incident angle of the first reflector relative to the second reflectors is adjustable, the two adjacent second reflectors are arranged at intervals according to a preset Brewster angle, and the incident beam is separated into TE polarized light and TM polarized light. The polarization separation device has the advantages of being simple in structure, convenient to install and capable of achieving efficient and low-loss polarization separation in a wide-spectrum middle-infrared band, and the preparation method has the advantages of being low in production cost, high in yield and suitable for mass production. The polarization separation device overcomes the problems of narrow working bandwidth, lower damage threshold, higher cost and the like of the traditional polarization element, and can be widely applied to the fields of thermal imaging, molecular spectrum, free space communication and the like.

Description

Technical Field

[0001] This invention relates to the technical field of optical polarization devices, and in particular to a broadband mid-infrared polarization separation device, its preparation method, and its application. Background Technology

[0002] Mid-infrared polarization devices have demonstrated significant application value in fields such as thermal imaging, molecular spectroscopy, and free-space communication. Traditional polarization elements, such as wire-grid polarizers and birefringent prisms, suffer from narrow operating bandwidths, low damage thresholds, and high costs, particularly in the mid-infrared band (e.g., 12–14 μm), where achieving efficient, wide-band polarization separation remains a technical challenge. While the physical principles of polarization separation using the Brewster angle effect are relatively mature, limitations in material optical properties and interface losses make it difficult to achieve high-efficiency, wide-band operation in practical systems. Existing matrix materials (calcite, YVO4, rutile, and MgF2, etc.) struggle to simultaneously achieve a grazing incidence Brewster angle and low absorption loss in this band, leading to increased reflections during polarization separation and consequently significant energy loss.

[0003] Chinese Patent Publication No. CN119960091A discloses a wide-band infrared polarization device based on mercury halide crystals, comprising a right-angled triangular prism based on a mercury halide single crystal and a hexagonal prism based on a mercury halide single crystal; the mercury halide is Hg2Cl2 or Hg2Br2. Due to the low birefringence of existing crystal devices and their inability to meet the requirement of a wide spectral coverage in the visible-long-wave infrared band, polarization beam combining and splitting technologies face the challenge of a shortage of matrix materials for crystal devices. The aforementioned wide-band infrared polarization device based on mercury halide crystals can achieve both wide-band polarization beam splitting and combining applications, particularly in the mid-to-long-wave infrared band. Its polarization beam splitting function enables the generation of high-power lasers, and the Brewster structure angle design eliminates reflection loss and beam interference, contributing to higher transmittance and extinction ratio across the entire wavelength range.

[0004] Although mercury halide crystals have significant potential in the field of mid- and far-infrared polarizing devices due to their wide transmission range and high birefringence, the existing technologies mentioned above have the following drawbacks: 1) The growth and preparation of mercury halide crystals are greatly affected by process parameters such as temperature, pressure, and chemical composition, resulting in low product yield and relatively high production costs, which directly limits the large-scale application of mercury halide crystals in mid- and far-infrared polarizing devices; 2) Mercury halide crystals are easily affected by environmental factors such as humidity and temperature changes, which can lead to fluctuations in their optical performance. For example, in high humidity environments, chemical reactions may occur on the crystal surface, forming an impurity layer, thereby reducing the transmittance and extinction ratio of the device. Temperature changes can also cause thermal expansion and contraction of the crystal, altering its internal structure and affecting the polarization beam splitting effect; 3) The structures of the aforementioned devices are complex, and the structural angle is crucial to the transmittance performance of infrared polarization combiners or beam splitters. This necessitates extremely high precision during installation, as any slight deviation can lead to a reduction in polarization beam splitting performance. The installation and debugging process requires specialized equipment and technicians, increasing the cost of use and the difficulty of maintenance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the first objective of this invention is to provide a broadband mid-infrared polarization separation device, which has the advantages of simple structure, easy installation, and efficient and low-loss polarization separation in a broadband mid-infrared band.

[0006] The second objective of this invention is to provide a method for preparing a broadband mid-infrared polarization separation device, which has the advantages of low production cost, high yield, and suitability for mass production.

[0007] The third objective of this invention is to provide an application of a broadband mid-infrared polarization separation device that overcomes the problems of narrow operating bandwidth, low damage threshold, and high cost of traditional polarization elements, and can be widely used in fields such as thermal imaging, molecular spectroscopy, and free-space communication.

[0008] To achieve the first objective mentioned above, the present invention provides the following technical solution: A broadband mid-infrared polarization separation device includes a first reflector and at least two second reflectors based on silicon carbide crystals. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surfaces of at least one second reflector, respectively. The incident angle of the first reflector relative to the second reflector is adjustable. Adjacent second reflectors are arranged at intervals according to a predetermined Brewster angle and separate the incident beam into TE polarized light and TM polarized light.

[0009] Furthermore, the first reflector is driven by an optical rotation adjustment frame to complete the incident angle adjustment.

[0010] Furthermore, the first reflector is configured as a gold-film planar reflector, wherein the gold film is made of aluminum alloy or Invar alloy, has a thickness of 100~200nm, and has a reflectivity of over 98% in the 2~20µm wavelength range.

[0011] Furthermore, the silicon carbide crystal is configured as 4H-SiC.

[0012] Furthermore, the silicon carbide crystal surface is arranged according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°.

[0013] Furthermore, the thickness of the second reflector is 1~3mm.

[0014] Furthermore, the surface of the second reflector undergoes ultra-precision polishing, with a roughness of no more than 50nm and a surface shape accuracy exceeding λ / 4.

[0015] Furthermore, adjacent second mirrors are arranged at intervals of more than 70° Brewster angle in the mid-infrared band of 12.5~14.3μm.

[0016] To achieve the second objective mentioned above, the present invention provides the following technical solution: A method for fabricating a broadband mid-infrared polarization separation device includes a process of crystal orientation polishing and ultra-precision polishing of a second reflector, and a process of assembling a first reflector and at least two second reflectors according to a predetermined Brewster angle.

[0017] To achieve the third objective mentioned above, the present invention provides the following technical solution: Applications of a broadband mid-infrared polarization separation device in thermal imaging devices, molecular spectroscopy devices, free-space communication devices, infrared polarization spectrometers, gas sensing devices, and thermal radiation control devices.

[0018] In summary, the beneficial technical effects of the present invention are as follows: 1. Because the broadband mid-infrared polarization separation device of the present invention uses a silicon carbide reflective module with a grazing incidence Brewster angle greater than 70°, since silicon carbide has extreme optical anisotropy in the 12.5–14.3 μm band, it can naturally generate this large Brewster angle, thus achieving the effect of efficient separation of transverse electric (TE) polarized light after a single or very few reflections. In principle, this significantly reduces the optical loss caused by multiple reflections in traditional designs, and realizes high-efficiency, low-loss polarization separation over a wide spectrum.

[0019] 2. In the broadband mid-infrared polarization separation device of the present invention, a silicon carbide reflective element with a preset tilt angle formed by polishing is preferably used. Because this design enables the propagation path of light at the crystal interface to achieve optimal matching with the anisotropy of the material, the purity and efficiency of polarization separation are further improved, and unnecessary interface reflections are further reduced.

[0020] 3. The preparation method of the present invention, by performing crystal orientation polishing and ultra-precision polishing on the silicon carbide reflective element (so that its surface roughness is no greater than 50nm), and using a high-precision (no less than 0.1°) angle adjustment component for assembly and calibration, achieves the effect of accurately realizing and stably maintaining the preset grazing incidence Brewster angle, thus ensuring the consistency and reliability of the device's performance throughout the entire operating band.

[0021] 4. The application of this invention, by integrating the above-mentioned high-efficiency and low-loss polarization separation device into a mid-infrared optical system, achieves the effects of improving the image signal-to-noise ratio for thermal imaging devices, providing high-purity polarized excitation light for molecular spectroscopy devices, and enhancing the signal anti-interference capability for free-space communication devices, thus expanding its practical value in high-end optical instruments and systems. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the broadband mid-infrared polarization separation device of Embodiment 1 of the present invention.

[0023] Figure 2 These are the detection images from Embodiments 1 to 5 of the present invention. Detailed Implementation

[0024] To make the technical means, creative features, objectives and effects of this invention clearer and easier to understand, the invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Example

[0025] Example 1: A broadband mid-infrared polarization separation device disclosed in this invention includes a first reflector and a pair of second reflectors based on silicon carbide crystals. This device is designed for a target wavelength band with a center wavelength of approximately 14.3 μm. Specific structural parameters are as follows. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surface of one of the second reflectors, respectively. The first reflector is driven by an optical rotation adjustment frame to complete the adjustment of the incident angle. In this embodiment, it is arranged with an incident angle of 141°. The first reflector is set as a gold film plane reflector. The gold film is made of aluminum alloy, has a thickness of 150nm, and has a reflectivity of more than 98% in the 2~20µm wavelength range. The silicon carbide crystal is set as 4H-SiC, the surface of 4H-SiC is set according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°; correspondingly, the size of the second reflector is 20mm×20mm, the thickness is 2mm, and the surface of the second reflector is treated with ultra-precision polishing, the roughness does not exceed 50nm, and the surface accuracy exceeds λ / 4. The two second reflectors are arranged 500μm apart from each other according to the Brewster angle of 78° in the mid-infrared band of the target, so as to ensure that the TM polarized light can achieve near-reflection-free transmission in the target band. In the optical path design of this embodiment, the incident TM polarized light is received by the polarized light collection device after two reflections, which has good optical path separation performance.

[0026] Example 2: A broadband mid-infrared polarization separation device disclosed in this invention includes a first reflector and a pair of second reflectors based on silicon carbide crystals. This device is designed for a target wavelength band with a center wavelength of approximately 13.5 μm. Specific structural parameters are as follows. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surface of one of the second reflectors, respectively. The first reflector is driven by an optical rotation adjustment frame to complete the incident angle adjustment. In this embodiment, it is arranged with an incident angle of 140°. The first reflector is set as a gold film plane reflector. The gold film is made of Invar alloy, has a thickness of 100nm, and has a reflectivity of more than 98% in the 2~20µm wavelength range. The silicon carbide crystal is set as 4H-SiC, the surface of 4H-SiC is set according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°; correspondingly, the thickness of the second reflector is 2mm, and the surface of the second reflector is treated with ultra-precision polishing, the roughness does not exceed 50nm, and the surface accuracy exceeds λ / 4. The two second reflectors are arranged 500μm apart from each other according to the Brewster angle of 80° in the mid-infrared band of the target, so as to ensure that the TM polarized light can achieve near-reflection-free transmission in the target band. In the optical path design of this embodiment, a key operating point in the broadband band is demonstrated. The incident TM polarized light is received by the polarized light collection device after two reflections, which has good optical path separation performance.

[0027] Example 3: A broadband mid-infrared polarization separation device disclosed in this invention includes a first reflector and a pair of second reflectors based on silicon carbide crystals. This device is designed for a target wavelength band with a center wavelength of approximately 13.0 μm. Specific structural parameters are as follows. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surface of one of the second reflectors, respectively. The first reflector is driven by an optical rotation adjustment frame to complete the adjustment of the incident angle. In this embodiment, it is arranged with an incident angle of 138.75°. The first reflector is set as a gold film plane reflector. The gold film is made of aluminum alloy, has a thickness of 125nm, and has a reflectivity of more than 98% in the 2~20µm wavelength range. The silicon carbide crystal is set as 4H-SiC, the surface of 4H-SiC is set according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°; correspondingly, the thickness of the second reflector is 2mm, and the surface of the second reflector is treated with ultra-precision polishing, the roughness does not exceed 50nm, and the surface accuracy exceeds λ / 4. The two second reflectors are arranged 500μm apart according to the Brewster angle of 82.5° in the mid-infrared band of the target, so as to ensure that the TM polarized light can achieve near-reflection-free transmission in the target band; In the optical path design of this embodiment, by optimizing the incident angle of the first reflector and the relative spatial position of the second reflector, the incident TM polarized light is received by the polarized light collection device after one reflection. This single-reflection optical path design significantly shortens the optical path, helps to reduce energy loss and optical path deviation caused by multiple reflections, thereby making the device structure more compact and the transmission efficiency higher.

[0028] Example 4: A broadband mid-infrared polarization separation device disclosed in this invention includes a first reflector and a pair of second reflectors based on silicon carbide crystals. This device is designed for a target wavelength band with a center wavelength of approximately 12.65 μm. Specific structural parameters are as follows. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surface of one of the second reflectors, respectively. The first reflector is driven by an optical rotation adjustment frame to complete the adjustment of the incident angle. In this embodiment, it is arranged with an incident angle of 138.5°. The first reflector is set as a gold film plane reflector. The gold film is made of Invar alloy, has a thickness of 175nm, and has a reflectivity of more than 98% in the 2~20µm wavelength range. The silicon carbide crystal is set as 4H-SiC, the surface of 4H-SiC is set according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°; correspondingly, the thickness of the second reflector is 2mm, and the surface of the second reflector is treated with ultra-precision polishing, the roughness does not exceed 50nm, and the surface accuracy exceeds λ / 4. The two second reflectors are arranged 500μm apart according to the Brewster angle of 83° in the mid-infrared band of the target, so as to ensure that the TM polarized light can achieve near-reflection-free transmission in the target band. In the optical path design of this embodiment, by optimizing the incident angle of the first reflector and the spatial arrangement of the second reflector, another core operating point in the broadband band is demonstrated. The incident TM polarized light is received by the polarized light collection device after one reflection, which has good optical path separation performance.

[0029] Example 5: A broadband mid-infrared polarization separation device disclosed in this invention includes a first reflector and a pair of second reflectors based on silicon carbide crystals. This device is designed for a target wavelength band with a center wavelength of approximately 12.65 μm. Specific structural parameters are as follows. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surface of one of the second reflectors, respectively. The first reflector is driven by an optical rotation adjustment frame to complete the adjustment of the incident angle. In this embodiment, it is arranged with an incident angle of 138.25°. The first reflector is set as a gold film plane reflector. The gold film is made of aluminum alloy, has a thickness of 200nm, and has a reflectivity of more than 98% in the 2~20µm wavelength range. The silicon carbide crystal is set as 4H-SiC, the 4H-SiC surface is set according to the

[100] crystal orientation, and the deviation angle of the

[100] crystal orientation relative to the crystal plane normal does not exceed 0.5°; correspondingly, the thickness of the second reflector is 2mm, and the surface of the second reflector is treated with ultra-precision polishing, the roughness does not exceed 50nm, and the surface accuracy exceeds λ / 4. The two second reflectors are arranged 500μm apart according to the Brewster angle of 83° in the mid-infrared band of the target. In addition, a crystal orientation rotation process is introduced to increase the effective Brewster angle at this wavelength to 83.5° to ensure that the TM polarized light can achieve near-reflection-free transmission at the target band. In the optical path design of this embodiment, the incident TM polarized light is received by the polarized light collection device after one reflection, which has good optical path separation performance. This reflects the synergistic effect of optical path optimization (reducing the number of reflections) and material modification (crystal orientation rotation process), thereby achieving more superior polarization separation performance in the high-frequency band.

[0030] Example 6: This invention discloses a method for preparing a broadband mid-infrared polarization separation device. The difference from Example 1 is that it includes a process of crystal orientation polishing and ultra-precision polishing of the second mirror, and a process of assembling the first mirror and at least two second mirrors according to a predetermined Brewster angle.

[0031] Example 7: This is an application of a broadband mid-infrared polarization separation device disclosed in this invention. The difference from Example 1 is that the device is used in thermal imaging devices, molecular spectroscopy devices, free-space communication devices, infrared polarization spectrometers, gas sensing devices, and thermal radiation control devices.

[0032] Comparative Example 1: This is a broadband mid-infrared polarization separation device disclosed in this invention. The difference from Example 1 is that silicon carbide crystal is replaced with fused silica.

[0033] This comparative example aims to illustrate that using common infrared materials with small Brewster angles significantly complicates the polarization separation device structure. This example aims to achieve polarization separation performance comparable to Example 2 (operating wavelength 13.5 μm, Brewster angle 80°), but uses fused silica as the second mirror. Because fused silica has a low refractive index in this wavelength band, its Brewster angle is only about 60°. To achieve similar polarization separation purity, the TM polarized light needs to undergo multiple reflections between multiple second mirrors to accumulate the separation effect. Simulations and experiments have verified that at least four reflections are required to achieve acceptable polarization separation. The results show that using materials with small Brewster angles significantly increases the number of second mirrors and the system size, leading to a more complex optical path structure, increased insertion loss, and a substantial increase in assembly and adjustment difficulty.

[0034] Comparative Example 2: This is a broadband mid-infrared polarization separation device disclosed in this invention. The difference from Example 1 is that the silicon carbide crystal is replaced with calcium fluoride crystal.

[0035] This comparative example further demonstrates the extreme negative impact of a continued decrease in the Brewster angle. The device structure is the same as Comparative Example 1, but the material of the second reflector is replaced with calcium fluoride crystal. Calcium fluoride has a lower refractive index in the mid-infrared band, and its Brewster angle is only about 50°. In this configuration, the polarization selection effect obtained by TM polarized light in each reflection is further weakened. To achieve effective polarization separation, up to 6 reflections are required. Due to the increased number of reflections, the optical path transmission efficiency drops sharply, completely losing the compact structure and high-efficiency transmission characteristics pursued by this invention.

[0036] To quantitatively verify the Brewster angle and corresponding TM-polarized light reflectivity of the 4H-SiC second reflector, a core component of the polarization separation device of this invention, at different wavelengths, the experiment employed a Fourier transform infrared spectrometer (FTIR) with a linear grating polarizer to generate pure TM-polarized light, and used a high-precision goniometer for angle control. During the measurement process, a collimated TM-polarized infrared beam was irradiated onto the surface of 4H-SiC crystal samples from the same batch as those in Examples 1-5 at different incident angles. The reflected light intensity was collected using an FTIR detector, and the reflectivity at a specific wavelength was calculated by combining it with the incident light intensity. By scanning the incident angle, the point of lowest reflectivity was determined; the angle corresponding to this point is the experimentally measured Brewster angle, and the reflectivity at this point reflects the theoretical minimum reflection loss at that wavelength. Each data point was measured three times and the average value was taken.

[0037] Experimental results show that 4H-SiC crystals exhibit excellent non-reflective properties at the Brewster angle across a broad mid-infrared band from 14.3 μm to 12.65 μm, providing a reliable physical basis for achieving efficient polarization separation. Specifically, referring to... Figure 2 At a wavelength of 14.3 μm, the Brewster angle was 65.5°, and the reflectance of Example 1 was 3.9%; at 13.5 μm, the Brewster angle was 66.2°, and the reflectance of Example 2 was 3.6%; at 13.0 μm, the Brewster angle was 66.8°, and the reflectance of Example 3 was 3.4%; at 12.65 μm, the Brewster angle was 67.5° without crystal orientation rotation, and the reflectance of Example 4 was 3.82%; after crystal orientation rotation, the Brewster angle was 67.8°, and the reflectance of Example 5 was 3.96%. Figure 2 The test results shown by the dashed line are in good agreement with the simulation results shown by the solid line, which fully demonstrates that the mirror based on 4H-SiC crystal has stable and low-loss optical performance over a wide spectral range.

[0038] In contrast, traditional infrared materials such as silicon dioxide, calcium fluoride, and germanium generally have low Brewster angles in the mid-infrared band, typically in the range of 50-60°. To achieve sufficient polarization separation purity, four or more reflections are often required, resulting in complex system structures and large volumes. The cumulative losses introduced by multiple reflections also make it difficult to meet the optical efficiency requirements of high-end applications. The silicon carbide-based polarization separation device used in this invention, with its inherent high Brewster angle characteristics, solves the technical problem of the coexistence of losses and structural complexity in traditional solutions from a physical perspective.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A broadband mid-infrared polarization separation device, characterized in that: It includes a first reflector and at least two second reflectors based on silicon carbide crystals. The reflecting surfaces of the first reflector are arranged opposite to the incident beam and the reflecting surfaces of at least one second reflector, respectively. The incident angle of the first reflector relative to the second reflector is adjustable. Adjacent second reflectors are arranged at intervals between each other according to a predetermined Brewster angle and separate the incident beam into TE polarized light and TM polarized light.

2. The broadband mid-infrared polarization separation device according to claim 1, characterized in that: The first reflector is driven by an optical rotation adjustment frame to complete the incident angle adjustment.

3. The broadband mid-infrared polarization separation device according to claim 1, characterized in that: The first reflector is a gold-coated planar reflector. The gold coating is made of aluminum alloy or Invar alloy, has a thickness of 100~200nm, and has a reflectivity of over 98% in the 2~20µm wavelength range.

4. The broadband mid-infrared polarization separation device according to claim 1, characterized in that: The silicon carbide crystal is configured as 4H-SiC.

5. A broadband mid-infrared polarization separation device according to claim 4, characterized in that: The silicon carbide crystal surface is arranged according to the [100] crystal orientation, and the deviation angle of the [100] crystal orientation relative to the crystal plane normal does not exceed 0.5°.

6. A broadband mid-infrared polarization separation device according to claim 4, characterized in that: The thickness of the second reflector is 1~3mm.

7. A broadband mid-infrared polarization separation device according to claim 4, characterized in that: The surface of the second reflector is treated with ultra-precision polishing, with a roughness of no more than 50nm and a surface shape accuracy of more than λ / 4.

8. A broadband mid-infrared polarization separation device according to claim 1, characterized in that: The two adjacent second mirrors are arranged at intervals of more than 70° Brewster angle in the mid-infrared band of 12.5~14.3μm.

9. A method for preparing a broadband mid-infrared polarization separation device according to any one of claims 1 to 8, characterized in that: This includes the process of crystal orientation grinding and ultra-precision polishing of the second mirror, as well as the process of assembling the first mirror and at least two second mirrors according to the predetermined Brewster angle.

10. The application of a broadband mid-infrared polarization separation device according to any one of claims 1 to 8 in thermal imaging devices, molecular spectroscopy devices, free-space communication devices, infrared polarization spectrometers, gas sensing devices, and thermal radiation control devices.

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