Low-loss deep ultraviolet orthogonal polarization prism

Abstract

The invention relates to the technical field of optical polarization devices, and discloses a low-loss deep ultraviolet orthogonal polarization prism which comprises a left prism, a right prism and an air gap or other low-refractive-index materials, the low-refractive-index materials comprise magnesium fluoride, the left prism is made of a birefringent crystal material, and the right prism is made of an isotropic material; the inclined surfaces of the prisms on the two sides are mechanically fixed and packaged by optical cement or sealant; the prism only comprises a right-angle prism made of birefringent crystal materials, so that the optical loss of the corresponding wavelength is small; through special parameter design of a birefringent crystal right-angle prism, a right-angle prism made of isotropic materials, an air gap or a low refractive index, light in two different polarization directions can be vertically output. By adopting a composite prism structure of a single birefringent crystal, an isotropic crystal and a low-refractive-index material, the length of a birefringent material in a light path is remarkably shortened, and an absorption path is compressed from the source.

Description

Technical Field

[0001] The present invention relates to the technical field of optical polarization devices, and particularly to a low-loss deep ultraviolet orthogonal polarization prism. Background Art

[0002] A polarization prism is an important device in an optical system for separating or controlling light of different polarization states. In the ultraviolet band, especially in the deep ultraviolet region (150–360 nm), the demand for high-efficiency polarization separation devices has increased significantly. It is widely used in high-precision scenarios such as lithography, laser measurement, and scientific experiments. However, deep ultraviolet photons have high energy, which poses more stringent requirements on material transmittance and structural design. Especially in terms of loss control and volume integration, there are still significant bottlenecks in existing solutions.

[0003] Current mainstream deep ultraviolet polarization devices are mostly based on birefringent crystals such as Ca 3 (BO 3 ) 2 , α-BBO, etc., and adopt a double-crystal gluing or air-gap combination structure. For example, Patent CN201010601239.6 proposes a cuboid "hamburger" structure with α-BBO and LiF crystals juxtaposed. Although it has certain transmission performance, it has a large volume, a long optical path, and non-negligible internal losses. Another example is Patent CN201120241418.3, which is formed by gluing two right-angled triangular prisms. The structure is simplified, but the output optical path is uncontrollable, and the polarization directions are not orthogonal, which limits its practicality. Patent CN201110377795.4 proposes to fix birefringent crystals with deep optical glue. Although it is convenient for assembly, there is a risk of adhesive layer aging and increased absorption under ultraviolet high-energy irradiation.

[0004] Other similar designs have not solved the core problems either. Patent CN201811546251.4 combines α-BBO and CLBO crystals with optical glue, and has poor stability and is prone to de-bonding. Patent CN202102135U adopts an isomorphic double-crystal structure. Although the angles are symmetric, the polarization separation efficiency is limited. Patent CN202122682099.6 introduces a mixed structure of Li 2 B 4 O 7 and quartz crystals, attempting to balance loss and transmittance, but no obvious advantage is shown in actual measurement. In most solutions, the optical axis direction is not optimized, the P / S light output directions are disordered, the system light adjustment is complex, and the practicality is insufficient.

[0005] In summary, existing deep ultraviolet polarization prisms generally face three core problems: first, the crystal path is long, the material absorption is significant, and the transmittance is low; second, the bonding structure is prone to aging, reducing long-term stability; third, the polarization light output direction is not ideal, and orthogonal and synchronous applications cannot be achieved. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the present invention provides a low-loss deep ultraviolet orthogonal polarization prism, which solves the problems of large absorption loss of the existing birefringent polarization prism due to the large absorption loss of the existing prism birefringent crystal in the deep ultraviolet band and the difficulty of orthogonal output.

[0007] To achieve the above objectives, the present invention is realized through the following technical solutions: A low-loss deep ultraviolet orthogonal polarization prism, comprising:

[0008] A left prism, a right prism, and an air gap or other low-refractive-index material, the low-refractive-index material includes magnesium fluoride, the left prism is a birefringent crystal material, and the right prism is an isotropic material; the inclined surfaces of the two prisms are mechanically fixed or encapsulated with sealant;

[0009] The prism only includes a right-angle prism made of a single piece of birefringent crystal material, so that the light loss of the corresponding wavelength is small;

[0010] Through the special parameter design of the birefringent crystal right-angle prism, the isotropic material right-angle prism, the air gap or the low refractive index, non-linearly polarized deep ultraviolet light can be made to output two lights with different polarization directions perpendicular to each other after passing through the present polarization prism.

[0011] Preferably, the birefringent crystal material is a negative uniaxial crystal, selected from one of α-BBO crystal, β-BBO crystal, Ca 3 (BO 3 ) 2 crystal or CsLiB 6 O 10 crystal, and its applicable wavelength range is 150 nm to 360 nm. The incident surface and the exit surface of the birefringent crystal can be coated with an antireflection film corresponding to the laser wavelength.

[0012] Preferably, the optical axis direction of the birefringent crystal material is parallel to the incident surface of the prism, that is, the plane where the incident light and the exit light are located, and perpendicular to the light propagation direction.

[0013] Preferably, the apex angle α of the left birefringent crystal prism satisfies: when the light is perpendicularly incident from the surface of the left prism, when the e-light exits to the air gap (or low-refractive-index material) and reaches the surface of the right prism, the incident angle for the right prism is the Brewster angle of the right prism material, and can be calculated by the following formula:

[0014]

[0015] θ B =arctg(n 1 );

[0016] where, n 1 is the refractive index of the right isotropic crystal, θ Bis its Brewster angle, n e is the e - light refractive index of the left - hand birefringent crystal material.

[0017] Preferably, the base angle β of the left - hand prism satisfies: when light is incident perpendicularly from the surface of the left - hand prism, the o - light is totally reflected by the B and A surfaces and reaches the C surface, and the direction of the outgoing light is perpendicular to the original light, and is calculated by the following formula:

[0018] sin(2α - β)·n o =cosβ;

[0019] where, n o is the o - light refractive index of the left - hand birefringent crystal refractive index, and α is the apex angle of the birefringent crystal.

[0020] Preferably, the range of the apex angle α is between the total - reflection angles of o - light and e - light corresponding to the laser wavelength of its crystal material, that is, ensuring that the o - light undergoes total reflection on the B surface and the e - light is transmitted on the B surface.

[0021] Preferably, the isotropic material is selected from one of fused quartz SiO 2 , calcium fluoride CaF 2 , barium fluoride BaF 2 or magnesium fluoride MgF 2 . The refractive index range of the isotropic material at the 190nm band is 1.35 to 1.57. The left - hand apex angle of the isotropic material crystal satisfies: being complementary to the apex angle α of the left - hand birefringent crystal prism.

[0022] Preferably, for the isotropic material crystal, the base angle β′ satisfies: when light is incident on the crystal interior at the Brewster angle from the left - hand side and exits from the B′ surface after passing through the crystal refraction, the direction of the light is horizontal, and is calculated by the following formula:

[0023]

[0024] where, n 1 represents the refractive index of the right - hand prism of the isotropic crystal.

[0025] Preferably, the thickness of the air gap or low - refractive - index material is 0.01mm to 1mm, and the parallelism error of the inclined surfaces of the two prisms is less than 30 arc - seconds. The outgoing surface of the right - hand prism can be coated with a deep - ultraviolet antireflection film.

[0026] Preferably, the extinction ratio of the prism is better than 5×10 -6 , and the overall transmittance is greater than 85%.

[0027] The present invention provides a low - loss deep - ultraviolet orthogonally polarized prism, having the following beneficial effects:

[0028] 1. The present invention adopts a composite prism structure of a single birefringent crystal + an isotropic crystal + a low-refractive-index material, significantly shortening the length of the birefringent material in the optical path, compressing the absorption path from the source, achieving the technical effect of effectively reducing the overall material absorption loss and improving the system transmittance in the deep ultraviolet band. Different from the prior art that requires splicing or gluing two birefringent crystals, this solution solves the problems of high penetration loss and heavy device caused by long optical path, making the system more compact and more adaptable.

[0029] 2. By strictly controlling the apex angle of the birefringent crystal, the present invention makes the e-ray incident on the right crystal at the Brewster angle, minimizing the interface reflection. This geometric design method brings a significant increase in transmittance, especially more obvious in the band below 200 nm. Compared with the traditional design that ignores the Brewster condition and causes non-negligible reflection loss at the interface between two materials, the present invention accurately matches the angle parameters, solving the problems of high interface energy reflection and poor utilization rate.

[0030] 3. In the geometric design of the birefringent prism and the isotropic prism, the present invention adopts an asymmetric angle matching method, separating the total reflection path of the o-ray from the transmission path of the e-ray, and finally forming mutually perpendicular P-ray and S-ray at the output end. This structure effectively realizes the spatial orthogonal output of polarized light. Different from the prior art in which the output angles of the two polarization directions overlap or are non-orthogonal, this solution breaks through the bottleneck of uncontrollable polarization direction and low output coupling efficiency, and is applicable to high-end optical application scenarios with spatial separation requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the prism structure of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0033] Please refer to the attached Figure 1 :

[0034] Embodiment 1: A deep ultraviolet polarization separation module applicable to the 193 nm band

[0035] This embodiment focuses on deep ultraviolet applications in the 193nm band and is suitable for scenarios with extremely high requirements for polarization purity in lithography mask detection systems. The left prism is processed from an α-BBO crystal. The crystal is cut along the X-axis orientation to ensure that its optical axis is parallel to the incident plane and orthogonal to the light propagation direction, improving the polarization separation efficiency. The apex angle of the prism is set to 48.5°, and the base angle is controlled to 41.5°. The thickness of the entire crystal does not exceed 2.2mm.

[0036] To reduce the absorption of short-wave ultraviolet light by the birefringent crystal, its path length is deliberately compressed in the design. Only a single short crystal is used, without splicing or multi-segment structure. The right prism is made of MgF 2 material, with a refractive index of about 1.38, which adapts to the Brewster incidence condition. The geometric angles are complementary to those of the left prism, ensuring that the extraordinary ray (e-ray) is incident at a nearly critical condition with almost no reflection loss.

[0037] An air gap of 0.2mm is introduced between the two prisms, and mechanical positioning parts are used to maintain the spacing consistency. The parallelism error is controlled within ±20 arcseconds. The edges are sealed with UV glue to prevent degumming caused by ultraviolet irradiation. Both the incident surface and the exit surface adopt a super-polishing process with an Ra value < 0.5nm. A special deep ultraviolet antireflection coating (AlF 3 +MgF 2 combination) is deposited on the exit surface, with a central wavelength of 193nm.

[0038] Under this scheme, the transmittance of the e-ray reaches 92%, and the total reflection efficiency of the ordinary ray (o-ray) > 98%. Finally, the light rays in the two polarization directions are orthogonally output at a 90° angle. The transmittance of the system is greatly improved. Compared with the traditional thick birefringent prism structure, not only is the volume reduced, but the thermal drift is also significantly reduced, which is especially suitable for the long-term stable operation requirements of high-precision scanning optical devices.

[0039] Example 2: For a 248nm laser polarization beam shaping system

[0040] This example is for a laser polarization shaping device in the 248nm band, which adapts to the beam control system of a KrF laser. A CsLiB 6 O 10 single crystal is used as the left birefringent prism. Its negative uniaxial property is suitable for separating linearly polarized light. The cutting angle of the crystal is precisely controlled to be 44.2° at the apex angle and 45.8° at the base angle, with a thickness set at 2.5mm, and the crystal orientation tolerance does not exceed ±0.3°.

[0041] To enhance the stability of the component structure and reduce the influence of mechanical stress on the crystal, a 0.05mm thick MgF 2 solid thin sheet is introduced between the two prisms to replace the air gap. It not only has low refractive characteristics but also has a certain flexibility, effectively resisting vibration and shock. CaF is used on the right side 2The material has a refractive index of approximately 1.46. The angular parameters are strictly complementary to the left side, ensuring that the incident angle of the extraordinary ray is close to the Brewster angle (about 56°). When the light passes through the interface, the reflection loss is extremely low, and the measured average reflectivity is <0.15%.

[0042] After assembly, a He-Cd laser is used for alignment fine-tuning. The relative attitude of the prism is corrected through a piezoelectric stage until the polarization beam separation angle reaches 90° ± 0.1°. The working stability of the device is tested for 48 hours under the condition of temperature control ± 0.5°C, and no polarization direction deviation phenomenon occurs.

[0043] Compared with the existing cumbersome system that uses a Glan-Thompson polarizer for registration and then beam splitting, this solution significantly simplifies the optical path, reduces the number of components, and at the same time avoids the common ultraviolet degradation problem of the glued structure, greatly improving the service life of the device and the laser coupling efficiency.

[0044] Example 3: Polarizer assembly matching a 266nm nanosecond laser system

[0045] This solution is particularly suitable for the polarized light beam splitting module of a 266nm nanosecond pulsed laser system and has good adaptability in precision detection instruments and laser marking systems. The left prism uses Ca 3 (BO 3 ) 2 crystal. The material thickness is 2.0mm, the apex angle is processed to 49°, and the base angle is controlled at 41°. The crystal is completed by CNC ultra-precision cutting through a five-axis processing equipment, and the edge chamfering treatment prevents chipping and microcracks.

[0046] The right prism uses fused silica (SiO 2 ). The refractive index is approximately 1.49 (in the 266nm band). The structural angle design makes the extraordinary ray coming out of the left prism enter in the form of the Brewster angle, with almost no reflection loss. The system reflectivity is reduced to less than 0.2%. There is no adhesive in the middle. It is assembled using a vacuum cavity clamping structure, and a piezoelectric fine-tuning mechanism is set to ensure that the crystal optical path alignment is maintained within an accuracy of ±1 arcsecond during long-term operation.

[0047] The surface roughness of all optical surfaces is <0.5nm. A broadband antireflection film is deposited using ion beam assisted deposition technology. The central wavelength is 266nm, effectively suppressing the multiple reflections caused by film layer interference. During long-term use, this assembly shows extremely strong anti-thermal drift ability. The temperature rise is within 5°C, and the polarization separation angle change is less than 0.02°.

[0048] Compared with the structure using the traditional PBS + mirror combination method, this embodiment is more compact, has higher energy utilization rate, and at the same time the system integration difficulty is greatly reduced, which is suitable for the embedded integration use scenario of batch optical modules.

[0049] Comparative Example 1 (corresponding to Example 1): Optimization design of double-crystal structure without crystal axis

[0050] In this comparative example, the same α-BBO crystal material as in Example 1 is selected. However, instead of using a single right-angle prism in the structure, the crystal is cut into two sections, and each section is processed into an isosceles right-angle prism with a thickness of 2.2 mm. The two crystal sections are bonded and fixed with UV photosensitive glue, and the bonding surface is not treated with an air gap. Mechanical positioning and assembly are not used either. Instead, preliminary alignment is directly completed through a planar fixture.

[0051] The crystal optical axis direction is not strictly defined. Only the default spindle direction from the factory is adopted, and no three-dimensional crystal axis angle calibration is performed. The bonding surface is polished conventionally, and the surface roughness is about 2 nm. No fine adjustment of the optical axis alignment is carried out, and the total thickness of the entire crystal structure increases to more than 4.4 mm.

[0052] This structural form is a typical segmented and glued crystal design, which has been used in a linear polarization beam splitting device in an existing deep ultraviolet optical path system. However, due to the lengthened crystal path in this scheme, the overall absorption increases. At the same time, the glue interface is prone to aging and yellowing in the short wavelength band, affecting the transmission efficiency. In addition, the optical axis deviation caused by the splicing error of the double crystals will lead to unstable polarization purity and cannot achieve spatial control of the outgoing polarization angle.

[0053] Comparative Example 2 (corresponding to Example 1): Structure without air gap + poor control of fitting error

[0054] In this comparative example, an α-BBO crystal is used on the left side, and MgF 2 crystal is used on the right side. The materials are the same as those in the example. The geometric angle design refers to the same specifications, but there are differences in the structural combination method: no air layer is set between the two prisms, and no low-refractive-index intermediate film is introduced either. The prisms are in direct contact and are clamped and positioned with an elastic bracket, and no precision adjustment mechanism is used.

[0055] During the assembly process, the bevel fitting is completed by manual alignment, and no high-precision angle control mechanism is set. After measurement, the bevel parallelism error is between 90 arcseconds and 110 arcseconds, which is much higher than the requirement of ≤20 arcseconds in the example. Limited by the assembly method, obvious light ray deviation and polarization distortion problems appear in the multi-batch assembly of this structure.

[0056] This structural design is common in low-cost polarizers or teaching systems for roughly separating polarized light. However, due to the unencapsulated interface, the structural stability is poor, and it is easy for air moisture to penetrate, resulting in the deterioration of the MgF 2 surface. At the same time, the e-light direction cannot be stably controlled. This comparative example aims to highlight the improvement value of the present invention in the air gap structure, fitting accuracy, and assembly process.

[0057] Comparative Example 3 (corresponding to Example 2): Structural configuration without matching Brewster angle

[0058] The overall structure of this comparative example is similar to that of Example 2, using CsLiB 6 O 10 Crystal and CaF 2 Crystal is the main material, and its thickness parameters are basically the same. However, in terms of angle design, the Brewster angle matching optimization is not performed based on the refractive index difference between materials. The top angle of the left prism is designed to be 45°, and there is no polarization path design for the laser wavelength.

[0059] Crystal processing does not set a specific optical axis direction. The incident light may form a non-ideal angle with the optical axis, further weakening the birefringence characteristics. At the prism interface, only standard MgF 2 The film (thickness 0.05±0.1mm) is used as a cushion layer. It has not been calibrated for laser parallelism. The output surface has no anti-reflection film treatment. The surface roughness is about 4nm, which is the standard for ordinary fine polishing.

[0060] The structure is assembled without a matching alignment platform and is directly adjusted manually, resulting in insufficient positioning accuracy and serious error accumulation. This solution intends to simulate the simplified assembly process of a traditional polarizing prism module, which is widely seen in general optical components. This comparative example compares the optical path structure, interface angle optimization, coating technology and other aspects, fully reflecting the system performance improvement brought about by the present invention in terms of Brewster condition control, optical axis precision and multi-faceted anti-reflection.

[0061] Comparative Example 4 (corresponding to Example 3): Traditional design with non-orthogonal output of spatial angles

[0062] This comparative example material still uses Ca 3 (BO 3 ) 2 Birefringent crystals and fused silica (SiO 2 ) isotropic crystal, and the structure is a typical symmetrical right-angle prism splicing form. Different from the third embodiment, this structure does not adopt the orthogonal beam splitting strategy, and the top angles of the left and right prisms are designed to be 45°, rather than matching the angles according to the reflection / transmission path of the polarized light.

[0063] The e-light and o-light paths are not geometrically separated, there is overlap and deviation in the output light directions, there is a lack of spatial angle control mechanism, and no piezoelectric fine-tuning device is introduced. The prism is assembled using hot pressing, which is fixed once and cannot be adjusted, and the fitting error is irreversible.

[0064] This design is used for universal polarization modules in commercial optical path platforms. When used, it requires additional configuration of beam splitters or adjustment mirrors for angle compensation. This comparison mainly reflects the planning ability of the present invention in the spatial separation structure in the polarization direction, highlighting the design advantage of orthogonal output.

[0065] Comparative Example 5 (Suitable for Comprehensive Comparison): Using a Commercially Available PBS Polarization Beam Splitter + External Reflector Structure

[0066] This comparative example is not a prism structure, but a polarization processing solution commonly used in current commercially available systems. It uses a cubic PBS (polarizing beam splitter). After the light beam is separated by the PBS, it enters the P-light and S-light channels respectively. To achieve the regulation of the separation angle, two high-reflection mirrors and two motorized turntables need to be configured outside the system to adjust the optical path height and direction respectively.

[0067] The PBS cube uses multilayer dielectric films for polarization control. The film layers are generally not applicable to the deep ultraviolet band. Especially in the band below 250 nm, the transmittance drops sharply. The system structure is complex, the optical path needs additional debugging, and the optical path and output direction need to be calibrated for each channel, which is time-consuming for debugging.

[0068] This structure does not have the characteristic of high integration and is difficult to be used in occasions with limited volume or operating in a vibrating environment. The integrated, dual-functional polarization structure of the present invention is obviously superior to such split optical paths, especially having systematic advantages under the requirements of high stability and low loss.

[0069] Experiment 1: Influence of the path length of the birefringent crystal on loss

[0070] Experimental description

[0071] Purpose

[0072] Verify whether the shortening of the crystal path length can significantly reduce the optical loss in the system in the deep ultraviolet band. Compare the influence of the structural differences between Example 1 of the present invention and Comparative Example 1 on the transmission performance.

[0073] Experimental materials and equipment

[0074] α-BBO birefringent crystal (single piece / double-piece splicing)

[0075] Deep ultraviolet nitrogen laser (wavelength: 193 nm, output power: 5 mW)

[0076] High-precision optical power meter (measurement accuracy: ±0.01 mW)

[0077] High-sensitivity spectrometer (UV band)

[0078] Laser collimator, blackbody background plate, constant temperature control platform (25 °C)

[0079] Data acquisition card, LabVIEW recording program

[0080] Experimental steps

[0081] Install the crystal structure of Example 1 in the optical path to ensure that the laser beam is vertically incident.

[0082] Record the incident light power Pin and the transmitted light power Pout output after passing through the crystal, repeat the measurement three times and take the average value.

[0083] The structure was replaced with that of comparative example 1 (dual crystal splicing structure), the laser parameters were kept unchanged, and Pin and Pout were measured repeatedly.

[0084] Calculate the loss rate of each structure L = (Pin-Pout) / Pin×100%

[0085] The overall transmission curve at 193nm was collected using a spectrometer to compare the strength differences of the transmission spectrum.

[0086] All experimental operations were performed at a constant temperature of 25±0.5°C to avoid the influence of thermal expansion on the internal stress and refractive state of the crystal.

[0087] Table name: Comparison of transmittance and loss rate of different crystal structures at 193nm

[0088]

[0089] Summary (based on mechanism)

[0090] Crystal absorption is not uniform. Once the path becomes longer, the attenuation of UV light increases nonlinearly. Although the splicing interface is physically transparent, it becomes an internal boundary. The reflection is weak but the accumulation is serious. This explains why the loss of the dual crystal is more than twice as high. The single-piece structure avoids interface foldback, which is clearly confirmed by the experiment.

[0091] Looking at the internal structure of the crystal, the group delay of α-BBO for e-light at 193nm is already critical. The increase in the path makes this characteristic easier to amplify and the equivalent absorption area expands. Therefore, even if the two crystal materials are exactly the same, the total loss after splicing will be much greater than 2 times the theoretical value due to a slight angle deviation. This is not a simple linear superposition problem.

[0092] Surprisingly, the peak transmission value also dropped a lot. It was not because the laser was unstable, but because the interface after splicing had subtle scattering in the short-wave band. It was difficult to detect with the naked eye, but the slight fluctuations in the spectrum curve in the range of 192-195nm seemed to reveal the uncertainty of the interface stress. This phenomenon shows that a compact structure alone may not be enough, and shortening the crystal path is the fundamental solution.

[0093] Experiment 2: Effect of Brewster Angle Optimization Design on Reflectivity

[0094] Experimental Description

[0095] Purpose

[0096] Explore whether the optimized design of Brewster angle in the second embodiment of the present invention can indeed reduce the deep ultraviolet interface reflectivity, thereby improving the overall transmission efficiency of the system.

[0097] Experimental equipment and configuration

[0098] Deep ultraviolet KrF laser (wavelength 248 nm, pulse width 18 ns)

[0099] Precision adjustable angle platform (minimum step 0.05°)

[0100] Optical reflectometer, adjustable incident angle range 30°–65°

[0101] Experimental crystal samples: Embodiment 2 (optimized Brewster angle structure), Comparative Example 3 (traditional 45° structure)

[0102] Constant temperature workbench (control temperature maintained at 24.8–25.2 °C)

[0103] Multi-channel acquisition card, filter set, signal stabilizer

[0104] Experimental steps

[0105] Install the crystal of Embodiment 2 on the platform, adjust the laser to be vertically incident with p-polarized light, and adjust the angle to make the incident plane coincide with the optical axis plane.

[0106] In the range from 35° to 60°, record the reflection intensity every 2.5°, and control the accuracy within ±0.005 mW.

[0107] Repeat Step 2, and record the reflectivity at the corresponding angles for the crystal of Comparative Example 3 (standard unmatched angle).

[0108] All tests are scanned 3 times, take the average value and analyze the error range.

[0109] Convert the measured data into reflectivity (R), and record the angle corresponding to the minimum R value as the approximate Brewster angle.

[0110] Compare the reflection suppression capabilities of the two groups of data near the optimal angle.

[0111] Table name: Reflectivity data of comparative test for optimized design of Brewster angle

[0112]

[0113] Summary (combined with the design mechanism of Brewster angle)

[0114] The suppression of interface reflection does not solely rely on film layer design. Under specific wavelength conditions, precise regulation of the geometric structure often plays a decisive role. When the incident angle approaches the critical refraction angle between materials, the reflectivity can be significantly reduced. At this time, light traverses the interface along the path with the minimum reflection loss. In particular, the extraordinary ray (e-ray) exits from the high refractive index surface in a critically inclined manner with almost no residence. This effect is not accidental but the result of a refined design based on the refractive index matching of optical materials.

[0115] In contrast, the structure adopted in Comparative Example 3 fails to achieve the optimal matching of the incident angle. Although the angles on the surface are seemingly similar, a tiny 1° difference shows a significant increase in reflection at the 248 nm wavelength band, resulting in obvious light rebound at the interface. Part of the energy cannot be transmitted and undergoes reverse loss. This result indicates that the setting of the so-called "Brewster angle" cannot be arbitrarily selected. It is restricted by the high coupling among the crystal's optical axis direction, the polarization state of the incident light, and the incident azimuth. Any mismatch in these factors will disrupt the transmission efficiency of the system.

[0116] Analysis from the perspective of experimental data can also corroborate the above mechanism. Among all the tested angles, the reflectivity of Comparative Example 3 is generally higher than that of Example 2, and at some angles, it even exceeds twice that of the latter. This phenomenon indicates that the key to reflection control lies not in surface modification but in the integrated design of the internal geometry and optical path behavior. With a reasonable structure setting, reflection is naturally suppressed; without a proper structure match, no matter how the film layer is optimized, it is difficult to make up for the energy loss. This "hidden logic" in the design is precisely the core value for achieving high-efficiency transmission in deep ultraviolet systems.

[0117] Experiment 3: Influence of Orthogonal Output of Polarization Directions on Polarization Control

[0118] Experiment Description

[0119] Purpose

[0120] Evaluate the effectiveness of the polarization light spatial orthogonal output structure in Example 3 of the present invention in terms of output angle control. By detecting the angle between the P-ray and the S-ray, analyze the advantages of the structure of the present invention in the control precision of the polarization separation direction.

[0121] Experimental Configuration and Materials

[0122] Laser Source: Pulsed Solid-State Laser (266 nm, linearly polarized)

[0123] Samples to be Measured: Example 3 (orthogonal output structure), Comparative Example 4 (without angle optimization design)

[0124] Two Precision Optical Turntables (rotation resolution 0.01°)

[0125] High-Sensitivity CCD Imaging System (equipped with a polarization measurement unit)

[0126] High-precision polarization angle detector (polarization accuracy < 0.1°)

[0127] Spatial beam analyzer, three-axis micro-motion platform, black cavity background noise suppression device

[0128] Measurement software: including vector decomposition and spatial angle calculation modules

[0129] Experimental steps

[0130] Install Example 3 in the main optical path, adjust the laser incident direction to be perpendicular and aligned with the crystal exit surface, and keep the system free of reflection interference.

[0131] Use the CCD system to capture the propagation directions of the e-ray (S polarization) and o-ray (P polarization) in space respectively, and measure their actual exit angles.

[0132] Repeat the above steps and perform the same operations using the structure of Comparative Example 4.

[0133] Perform three groups of measurements for each structure, extract the spatial separation angle α, and record the polarization offset error Δφ.

[0134] Analyze whether the angle between the P-ray and S-ray is stably maintained at 90°, as well as the stability, range, and fluctuation trend of the angle offset.

[0135] All measurements are carried out under a vibration isolation platform to isolate the influence of mechanical vibration on the polarization direction.

[0136] Table name: Data comparison table for orthogonal output angles of polarized light

[0137]

[0138] Summary (combining the mechanism of polarization spatial separation)

[0139] The spatial angle output of polarized light is not only a matter of the light's direction, but also an embodiment of the beam splitting strategy within the structure. Example 3 optimizes the path through the crystal geometry, completing the angle pre-separation internally, so that the e-ray and o-ray are naturally orthogonal after exiting. This is not a surface adjustment, but an active shaping of the light direction by the structure geometry, based on the propagation path constraint under the birefringence principle.

[0140] For Comparative Example 4, although it is similar in form, the difference in the propagation angles of polarized light was not considered in the design. The setting of the crystal apex angle does not form a control relationship with the axial light exit direction, resulting in the deviation of the P-ray and S-ray in space and the formation of a crossover between the output angles. Since stable separation conditions are not established within the structure, the angle between the exiting lights fluctuates significantly, indicating an inherent deficiency in its direction control ability.

[0141] In the third embodiment of the present invention, a spatial coupling correction mechanism is introduced in the design. The polarization path is geometrically bounded, and the orthogonal condition is set internally. In this way, the system can directly output stable mutually orthogonal polarized light without relying on a post-stage compensation structure. The orthogonal accuracy is high and the output direction does not drift with the incident fluctuations. This advantage has significant engineering value for systems that require high polarization direction control accuracy.

[0142] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A low-loss deep ultraviolet orthogonal polarization prism, characterized in that: include: A left prism, a right prism and an air gap or other low refractive index material, wherein the low refractive index material includes magnesium fluoride, the left prism is a birefringent crystal material, and the right prism is an isotropic material; the inclined surfaces of the prisms on both sides are mechanically fixed or sealed with sealant; The prism only comprises a right angle prism of birefringent crystal material, so that the light loss of the corresponding wavelength is small; Through the special parameter design of birefringent crystal right-angle prism, isotropic material right-angle prism, air gap or low refractive index, non-linearly polarized deep ultraviolet light can be output in two different polarization directions perpendicular to each other after passing through the polarizing prism.

2. A low-loss deep ultraviolet orthogonal polarization prism according to claim 1, characterized in that: The birefringent crystal material is a negative uniaxial crystal selected from α-BBO crystal, β-BBO crystal, Ca3(BO3)2 crystal or CsLiB6O 10 A type of crystal, the applicable wavelength range of which is 150nm to 360nm, the incident surface and the output surface of the birefringent crystal can be coated with an anti-reflection film corresponding to the laser wavelength.

3. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The optical axis direction of the birefringent crystal material is parallel to the incident surface of the prism, that is, the plane where the incident light and the outgoing light are located, and is perpendicular to the light propagation direction.

4. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The vertex angle α of the left birefringent crystal prism satisfies: when the light is incident vertically from the left prism surface, when the light of e-light emitted to the air gap (or low refractive index material) reaches the right prism surface, the incident angle for the right prism is the Brewster angle of the right prism material, which can be calculated by the following formula: θ B =arctg(n1); Where n1 is the refractive index of the isotropic crystal on the right side, θ B is the Brewster angle, n e is the e-light refractive index of the birefringent crystal material on the left.

5. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The bottom angle β of the left prism satisfies: when the light is incident vertically from the left prism surface, when the light is totally reflected from the B and A surfaces to the C surface, the direction of the outgoing light is perpendicular to the original light, and is calculated by the following formula: sin(2a-b)·n o =cosβ; Among them, n o is the refractive index of the birefringent crystal on the left, o is the refractive index of light, and α is the apex angle of the birefringent crystal.

6. The low-loss deep ultraviolet crossed polarization prism according to claim 4, characterized in that: The range of the vertex angle α is between the total reflection angle of o light and the total reflection angle of e light of the crystal material corresponding to the laser wavelength, that is, it is ensured that the o light is totally reflected at the B surface, while the e light is transmitted at the B surface.

7. A low-loss deep ultraviolet crossed polarization prism according to claim 6, characterized in that: The isotropic material is selected from one of fused quartz SiO2, calcium fluoride CaF2, barium fluoride BaF2 or magnesium fluoride MgF2, and its refractive index ranges from 1.35 to 1.57 in the 190nm band. The left vertex angle of the isotropic material crystal satisfies: being complementary to the vertex angle α of the left birefringent crystal prism.

8. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The bottom angle β′ of the isotropic material crystal satisfies that when light is incident into the crystal from the left at the Brewster angle and emerges from the B' surface after refraction, the direction of the light is horizontal and is calculated by the following formula: Where n1 represents the refractive index of the isotropic crystal right prism.

9. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The thickness of the air gap or low refractive index material is 0.01 mm to 1 mm, and the parallelism error of the inclined surfaces of the prisms on both sides is less than 30 arc seconds. The exit surface of the right prism can be coated with an anti-reflection film corresponding to the laser wavelength.

10. The low-loss deep ultraviolet crossed polarization prism according to claim 1, characterized in that: The extinction ratio of the prism is better than 5×10 -6 , the overall transmittance is greater than 85%.

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