Non-contact detection sensors that can mitigate the effects of environmental fluctuations integrate differential optical paths

Abstract

This invention belongs to the field of non-contact high-precision component testing technology. In order to meet the requirements of surface measurement accuracy of non-contact high-precision components and further reduce the impact of environmental fluctuations, this invention proposes a "non-contact detection sensor integrated differential optical path that can mitigate the impact of environmental fluctuations". It adopts a two-stage system design: the first stage adopts a dual-frequency heterodyne interference optical path, which can achieve a resolution of 0.3nm under four-fold path subdivision. By symmetrically placing the measuring reflector c and the reference reflector c, the interference optical path presents geometric symmetry. The ΔΦ caused by environmental interference can be reduced by differential subtraction. A differential optical path is designed on the basis of the dual-frequency heterodyne interference optical path and integrated with the differential confocal optical path of the second stage, which can realize high-precision tracking of the surface of the component under test.

Description

Technical Field

[0001] This invention belongs to the field of non-contact high-precision component testing technology. Background Technology

[0002] The use of complex freeform surface optical elements can increase the design freedom of optical systems, greatly improve system performance, and reduce the number of components required. They are widely used in remote sensing optics, semiconductor systems, head-mounted displays, and other technical fields. However, complex freeform surface optical elements require high surface accuracy, which inevitably increases the accuracy requirements and detection efficiency of the measuring equipment. High-precision measuring equipment is highly susceptible to the effects of environmental temperature, humidity, and air pressure. The Edlen formula is used to estimate the impact of environmental fluctuations on surface accuracy measurement. The air refractive index is a function of air temperature, air pressure, humidity, and CO2 content in the air. The differential formula near the normal state is derived as follows:

[0003] δn=(0.00268δp-0.929δt-0.00042δf)×10 -6 ,

[0004] In the formula, δp, δt, and δf represent the changes in air pressure, temperature, and humidity relative to normal conditions, respectively. For example, when the dead distance of the measurement branch is 20 mm, the sensitivity to air pressure, temperature, and humidity is 0.054 nm / Pa, 18.58 nm / ℃, and 0.0084 nm / Pa, respectively, which clearly demonstrates the negative impact of environmental fluctuations on surface shape accuracy measurement. Therefore, designing a non-contact detection sensor with resistance to environmental fluctuations is of great significance for the accuracy of surface shape measurement of complex freeform optical elements.

[0005] The differential optical path integrated into the sensor in this invention uses two principles: differential confocal principle and dual-frequency heterodyne interference principle.

[0006] like Figure 1 As shown, the optical path of the differential confocal principle consists of a differential laser beam that passes sequentially through a polarizing beam splitter a and a quarter-wave plate a, and is focused onto the surface of the device under test by the objective lens a. After reflection, it passes through the quarter-wave plate a and returns to the polarizing beam splitter a. After further reflection by the polarizing beam splitter a, it reaches a semi-transparent mirror a located in the reflection direction of the polarizing beam splitter a. The beams reflected and transmitted by the mirror are converged by a converging mirror a. Two pinholes a are located at Rayleigh distances from the focal plane in front of the focal point of the converging mirror. A photodetector a is placed behind the pinholes. By differentially processing the two signals, the differential confocal measurement curve I is obtained. diff =I A -I B ,

[0007] Among them, I A I BI represents the light intensity signal detected by the two photodetectors. diff By determining the zero point of the curve, the precise focal position can be obtained, ensuring that the sensor can accurately focus.

[0008] like Figure 2 As shown, the optical path of the dual-frequency heterodyne interference principle consists of linearly polarized light with frequencies f0 and f1 that are perpendicular to each other, emitted by a dual-frequency laser, which passes through a beam splitter and the reflected light is used as the reference signal for the heterodyne interferometer. The transmitted light is split into p-beams and s-beams by the polarizing beam splitter b. The p-beam, with a frequency of f1, is transmitted through a quarter-wave plate b, incident on a reference mirror b, reflected by the reference mirror b, and then sequentially passes through another quarter-wave plate b and the polarizing beam splitter b. After being reflected by the polarizing beam splitter b, it passes through a pyramidal prism b, is reflected again by the polarizing beam splitter b, and then passes through the reference mirror b, through another quarter-wave plate b, and finally through the polarizing beam splitter b before being collected by the heterodyne detector. The other beam, s-beam, with a frequency of f0, is reflected by the measuring mirror b, then passes through a quarter-wave plate b, incident on the polarizing beam splitter b, transmitted through a pyramidal prism b, and then through the polarizing beam splitter b again. After being transmitted through the polarizing beam splitter b, it passes through a quarter-wave plate b, is reflected by the reference mirror b, and then through another quarter-wave plate b and the polarizing beam splitter b before being collected by the heterodyne detector. Thus, the two beams are combined by the polarizing beam splitter b to form a measurement signal with a frequency of f1. m =f2-f0±Δf, where

[0009] In the formula, c is the speed of light, and v is the velocity of the measuring mirror b. Using binomial expansion, when the ratio 2v / c is very small, higher-order terms can be ignored, and the Doppler frequency difference can be obtained as:

[0010]

[0011] The classic formula for dual-frequency heterodyne interferometry length measurement is:

[0012]

[0013] Where λ is the laser wavelength of the dual-frequency laser, and N is the number of interference fringe cycles, i.e., the number of cycles between frequencies f0 and f... m The two beams of light are converted from a photoelectric detection signal to a measurement signal I formed by the interference beam. M When the measuring mirror b moves, the length of the measuring arm in the interferometer will change, and at this time the measurement signal I... M There will be a phase change of ΔΦ.

[0014] Although the aforementioned differential confocal and dual-frequency heterodyne interference optical paths can mitigate the impact of environmental fluctuations to some extent, they still cannot meet the accuracy requirements for non-contact high-precision component surface measurement. Summary of the Invention

[0015] To meet the accuracy requirements of non-contact high-precision component surface measurement and further mitigate the impact of environmental fluctuations, this invention proposes a "non-contact detection sensor integrated differential optical path that can mitigate the impact of environmental fluctuations".

[0016] Non-contact detection sensors that can mitigate the effects of environmental fluctuations integrate differential optical paths, such as... Figure 3 As shown, the laser 31 is located on the right side of the optical path and is perpendicularly incident on the first right-angle prism 32, which is on the same horizontal line. The differential confocal beam is deflected by 90° and propagates downwards, passing sequentially through the corner bevel prism c33 located directly below, the polarizing beam splitter c34, the first quarter-wave plate c35 at its bottom, and the objective lens c36, converging on the surface of the device under test. The bottom of the first right-angle prism 32, the corner bevel prism c33, the first quarter-wave plate c35, and the center of the objective lens c36 coincide with the differential confocal optical axis. The beam converged on the surface of the device under test returns along the original path and passes through... After reflection, the beam splitter c34 is perpendicularly incident on the second 1 / 4 wave plate c37 located to its left. A semi-transparent and semi-reflective mirror c38 is set to the left of the beam splitter c34, and its center spatial position coincides with the horizontal center of the beam splitter c34. The reflector 39 is located directly above the semi-transparent and semi-reflective mirror c38. The two converging mirrors c315, the pinhole c, and the photodetector c (not marked in the figure) are coaxial with the semi-transparent and semi-reflective mirror c38 and the reflector 39 from right to left. The pinhole c is located at the Rayleigh distance from the focal plane in front of the focal point of the two converging mirrors c315.

[0017] The dual-frequency heterodyne laser interferometer 316 emits linearly polarized light with frequencies f0 and f1, whose vibration directions are perpendicular to each other. The light first passes through the second quarter-wave plate c37, and is then split by the polarizing beam splitter c34, resulting in a transmitted p-beam and a reflected s-beam. The transmitted p-beam passes through the third quarter-wave plate c310 to the right of the polarizing beam splitter c34, coinciding with the horizontal center of the polarizing beam splitter c34. It then passes through the second right-angle prism 311 to the right of the third quarter-wave plate c310. The transmitted light is refracted 90° and incident perpendicularly into the reference mirror c312, which is at the same horizontal position as the objective lens c36. After reflection, it passes sequentially through the second right-angle prism 311, the third quarter-wave plate c310, and the polarizing beam splitter c34, and is then reflected back by the pyramidal prism c33. After being reflected by the polarizing beam splitter, the light enters the third quarter-wave plate c310 again, and after being reflected by the second right-angle prism 311, it is perpendicularly incident on the reference mirror c312 and returns along the same path, then is transmitted through the polarizing beam splitter c34. The reflected light s-ray passes through the first quarter-wave plate c35 and is perpendicularly incident on the measuring mirror c313, which is at the same horizontal position as the objective lens c36. The measuring mirror c313 and the objective lens c36 are fixed together on the servo device. After being reflected by the measuring mirror c313, the light enters the first quarter-wave plate c35, the polarizing beam splitter c34, and the cornerstone prism c33. After being reflected back, it is projected by the polarizing beam splitter c34, enters the first quarter-wave plate c35 again, and is perpendicularly incident on the measuring mirror c313 again. It returns to the polarizing beam splitter c34 and is emitted out, forming a beat frequency with the transmitted light p-ray.

[0018] Technical effects:

[0019] This invention employs a two-stage system design: the first stage uses a dual-frequency heterodyne interference optical path, achieving a resolution of 0.3nm with four-fold path subdivision. A new optical quality surface is added by cutting the top of the cornerstone prism c. By symmetrically placing the measuring mirror c and the reference mirror c, the interference optical path exhibits geometric symmetry. Due to the symmetry of the optical path, the ΔΦ caused by environmental interference is approximately equal for the reference arm and the measuring arm. Differential subtraction can reduce the phase drift caused by environmental disturbances. A differential optical path is designed based on the dual-frequency heterodyne interference optical path and integrated with the differential confocal optical path of the second stage, enabling high-precision tracking of the surface of the component under test. To achieve the purpose of differential between the reference optical path and the measuring optical path, the second right-angle prism folds the transmitted p-beam by 90°, making the environmental fluctuations encountered by the transmitted p-beam and the reflected s-beam approximately equal in their optical paths, thereby greatly reducing environmental interference. Furthermore, the measuring mirror c is connected to the objective lens c, and a servo device simultaneously drives the objective lens c and the measuring mirror c to measure the precision component.

[0020] This non-contact detection sensor integrates a differential optical path, which ensures that the non-contact detection sensor has a resolution at the nm level and a measurement accuracy at the sub-nm level. It also greatly reduces the impact of environmental fluctuations on measurement accuracy and has strong anti-interference and stability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the optical path structure based on the differential confocal principle.

[0022] Among them: differential laser 11, polarizing beam splitter a12, quarter wave plate a13, objective lens a14, semi-transparent mirror a15, converging mirror a16, photodetector a17, pinhole a18.

[0023] Figure 2 This is a schematic diagram of the optical path structure based on the dual-frequency heterodyne interference principle.

[0024] Among them: dual-frequency laser 21, beam splitter 22, quarter-wave plate b23, polarizing beam splitter b24, corner bevel prism b25, reference mirror b26, measuring mirror b27, and dual-frequency signal acquisition and data processing card 28.

[0025] Figure 3 This is a schematic diagram of the optical path structure of the present invention.

[0026] Among them: laser 31, first right-angle prism 32, corner bevel prism c33, polarizing beam splitter c34, first quarter-wave plate c35, objective lens c36, second quarter-wave plate c37, semi-transparent mirror c38, reflector 39, third quarter-wave plate c310, second right-angle prism 311, reference reflector c312, measuring reflector c313, integrated signal acquisition and data processing card 314, converging mirror c315, dual-frequency heterodyne laser interferometer 316. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art without creative effort using the embodiments of the present invention are within the scope of protection of the present invention.

[0028] Optical component selection and parameter requirements:

[0029] The light source can be a laser 31, and a laser with high power stability should be selected, such as a He-Ne laser with a wavelength of 633nm.

[0030] The first right-angle prism 32 and the second right-angle prism 311 have aluminum-plated bevels and anti-reflective coatings on their right-angle surfaces.

[0031] The c33 bevel of the angle cone prism is coated with aluminum for polarization maintenance, and its bottom surface and the optical quality surface formed after cutting are coated with an anti-reflection coating.

[0032] The polarizing beam splitter C34 has an extinction ratio of at least 1000:1 due to the surface coating of an antireflection film.

[0033] The first quarter-wave plate C35, the second quarter-wave plate C37, and the third quarter-wave plate C310 all have anti-reflective coatings applied to their light-transmitting surfaces.

[0034] The splitting ratio of the semi-transparent mirror C38 is 50:50±2%, and the reflector 39 has a high-reflection coating of 45° on its reflective surface. The semi-transparent mirror C38 and the reflector 39 meet the requirement of 45°±30.

[0035] The reference mirror C312 and the measuring mirror C313 have high-reflectivity coatings (0°) on their reflecting surfaces.

[0036] The objective lens c36 has a magnification of 100×, a focal length of 2mm, and a working distance of 3-6mm. The specific model can be the Mitutoyo Mplan Apo 100x.

[0037] Converging lens 315, focal length 50mm.

[0038] The noise equivalent sensitivity of the two photodetectors is better than The saturation light power is greater than 30μW.

[0039] The dual-frequency heterodyne laser interferometer 316 has a selectable resolution better than 1nm, an accuracy better than 0.4ppm, and a beam diameter of 3mm.

[0040] The light-transmitting surface dimensions of the aforementioned optical components need to be matched with each other. In order to ensure the size of the integrated probe, the components should be selected with standard sizes such as 12.7mm, 20mm, and 25.4mm to facilitate subsequent replacement or matching with mounting brackets. They can be fixed together by means of epoxy resin bonding to achieve the integration and folding of differential confocal and dual-frequency heterodyne optical paths, which is beneficial to improving stability and heat transfer.

[0041] Optical component reference dimensions:

[0042] The first right-angle prism 32 has dimensions of 5mm×5mm×5mm, and the second right-angle prism 311 has dimensions of 25.4mm×25.4mm×25.4mm.

[0043] The corner cube prism c33 has a diameter of 25.4mm. The top corner of the corner cube prism c33 is cut off, resulting in a height of 3mm.

[0044] The polarizing beam splitter c34 has a size of 25.4 mm.

[0045] The first quarter wave plate, C35, has a diameter of 25.4 mm; the second quarter wave plate, C37, has a diameter of 12.7 mm; and the third quarter wave plate, C310, has a diameter of 25.4 mm.

[0046] The semi-transparent and semi-reflective mirror c38 and the reflector 39 have dimensions of 9×12mm and a thickness of 2mm.

[0047] The reference reflector c312 has a size of 25.4 mm, a central light aperture diameter of 4 mm, and a thickness of 3 mm; the measuring reflector c313 has a size of 28 mm, a central light aperture diameter of 4 mm, and a thickness of 5 mm.

[0048] The converging lens 315 has a diameter of 8mm.

[0049] The two pinholes have a diameter of 30μm.

[0050] The servo device uses a voice coil motor drive based on electromagnetic principles as the driving method for the probe motion system. It is a special type of single-phase two-pole direct drive motor. The current flowing through the coil can generate a proportional force, causing the coil to move axially within the air gap. It features simple structure, small size, high acceleration, and fast response. Alternatively, a piezoelectric ceramic drive based on the inverse piezoelectric effect can be used, which can generate precise displacement with positioning accuracy at the nanometer level, but the response speed is slower. In summary, a piezoelectric ceramic drive is chosen for small motion ranges, while a voice coil motor drive is chosen for large motion ranges.

[0051] The He-Ne laser beam is emitted horizontally. First, mechanical components are used to ensure that the positions of each optical element meet the tolerance requirements. The subsequent assembly and adjustment process is carried out in three steps: First, the focal plane position is determined by an interferometer; then, two pinholes are fixed at 0.5 mm in front of and behind the focal plane, respectively, and the beam quality analyzer is used to determine the beam's alignment with the lens axis; finally, a waist hole with two degrees of freedom is set at the pinhole fixing position. The pinhole is adjusted, and the maximum light intensity detected by the photoelectric detection module is used as the evaluation standard. At this point, the pinhole is fixed to complete the final assembly and adjustment process.

[0052] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. Integrated differential optical path for non-contact detection sensor with reduced impact of environmental fluctuations, characterized in that, The laser (31) is located on the right side of the light path, vertically enters the first right-angle prism (32) on the same horizontal line, and the differential confocal light beam is deflected by 90° downward to propagate through the corner cube prism c (33), the polarization beam splitter prism c (34), the first 1 / 4 wave plate c (35) at the bottom thereof, and the objective lens c (36) in turn, and converges on the surface of the element to be measured, wherein the center of the first right-angle prism (32), the corner cube prism c (33), the first 1 / 4 wave plate c (35), and the objective lens c (36) coincide with the differential confocal optical axis; the light beam converging on the surface of the element to be measured returns to the original path and is reflected by the polarization beam splitter prism c (34) to vertically enter the second 1 / 4 wave plate c (37) located on the left side thereof; a half-transmission half-reflection mirror c (38) is arranged on the left side of the polarization beam splitter prism c (34), and the center spatial position thereof coincides with the horizontal center of the polarization beam splitter prism c (34); the reflecting mirror (39) is located directly above the half-transmission half-reflection mirror c (38); the two converging lenses c (315), the pinhole c, and the photodetector c are coaxial with the half-transmission half-reflection mirror c (38) and the reflecting mirror (39) from right to left in turn, and the pinhole c is located at the Rayleigh distance from the focal surface of the converging lens c (315) focal point. The dual-frequency heterodyne laser interferometer (316) emits linearly polarized light with frequencies f0 and f1 in mutually perpendicular vibration directions, first passes through the second 1 / 4 wave plate c (37), is split by the polarization beam splitter prism c (34) to generate one transmitted light p and one reflected light s, the transmitted light p passes through the third 1 / 4 wave plate c (310) on the right side of the polarization beam splitter prism c (34), which coincides with the horizontal center of the polarization beam splitter prism c (34), and then passes through the second right-angle prism (311) on the right side of the third 1 / 4 wave plate c (310), the transmitted light p is deflected by 90° and vertically enters the reference mirror c (312) on the same horizontal line as the objective lens c (36), is reflected by the second right-angle prism (311), the third 1 / 4 wave plate c (310), the polarization beam splitter prism c (34) in turn, and then enters the third 1 / 4 wave plate c (310) again after being reflected by the corner cube prism c (33) and the polarization beam splitter prism c, and is returned to the polarization beam splitter prism c (34) after being reflected by the second right-angle prism (311) and vertically entering the reference mirror c (312) again; the reflected light s passes through the first 1 / 4 wave plate c (35) and vertically enters the measurement mirror c (313) on the same horizontal line as the objective lens c (36), the measurement mirror c (313) and the objective lens c (36) are fixed on the servo device, is reflected by the measurement mirror c (313), enters the first 1 / 4 wave plate c (35), the polarization beam splitter prism c (34), and the corner cube prism c (33) in turn, is projected by the polarization beam splitter prism c (34) after being reflected, enters the first 1 / 4 wave plate c (35) again, and enters the measurement mirror c (313) again to return to the polarization beam splitter prism c (34) to generate beat frequency with the transmitted light p.

2. The non-contact detection sensor integrated differential optical path capable of mitigating the effects of environmental fluctuations according to claim 1, wherein, The first right-angle prism (32) and the second right-angle prism (311) are aluminized on the inclined surface, and the right-angle surface is coated with an anti-reflection film; The corner prism c (33) is aluminized on the inclined surface, and the bottom surface is coated with an anti-reflection film on the optical quality surface formed after cutting; The polarizing beam-splitting prism c (34) is coated with an anti-reflection film on the surface, and the extinction ratio is at least 1000:1; The first 1 / 4 wave plate c (35), the second 1 / 4 wave plate c (37) and the third 1 / 4 wave plate c (310) are all coated with an anti-reflection film on the light-transmitting surface; The half-transmission half-reflection mirror c (38) has a splitting ratio of 50:50±2%, the reflecting surface of the mirror (39) is coated with a high-reflection film at 45°, and the half-transmission half-reflection mirror c (38) and the mirror (39) satisfy 45°±30"; The reflecting surface of the reference mirror c (312) and the measuring mirror c (313) is coated with a high-reflection film at 0°; The objective lens c (36) has a magnification of 100×, a focal length of 2mm and a working distance of 3-6mm, and the specific model is Sanfeng Mplan Apo 100x; The converging lens (315) has a focal length of 50mm.

3. The non-contact detection sensor integrated differential optical path capable of mitigating the effects of environmental fluctuations according to claim 2, wherein, The first right-angle prism (32) has a size of 5mm×5mm×5mm, and the second right-angle prism (311) has a size of 25.4mm×25.4mm×25.4mm; The corner prism c (33) has a size of 25.4mm in diameter, and the top corner of the corner prism c (33) is cut off with a height of 3mm; The polarizing beam-splitting prism c (34) has a size of 25.4mm; The first 1 / 4 wave plate c (35) has a size of 25.4mm in diameter, the second 1 / 4 wave plate c (37) has a size of 12.7mm in diameter, and the third 1 / 4 wave plate c (310) has a size of 25.4mm in diameter; The half-transmission half-reflection mirror c (38) and the mirror (39) have a size of 9×12mm and a thickness of 2mm; The reference mirror c (312) has a size of 25.4mm, a central light-transmitting hole diameter of 4mm and a thickness of 3mm, and the measuring mirror c (313) has a size of 28mm, a central light-transmitting hole diameter of 4mm and a thickness of 5mm; The converging lens (315) has a size of 8mm in diameter; The two pinhole light-transmitting hole diameters are 30μm.

4. The non-contact detection sensor integrated differential optical path capable of mitigating the effects of environmental fluctuations according to claim 1, wherein, The servo device is a voice coil motor driving device.

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