A Highly Stable Zero-Difference Differential Interferometer Based on Intrinsic Optical Symmetry

Abstract

This invention relates to a highly stable zero-difference differential interferometer based on inherent optical symmetry, belonging to the field of optical measurement technology. It addresses the challenge of balancing low temperature drift and ease of use in differential interferometer design. The invention comprises a frequency-stabilized laser, a differential interferometer mirror assembly, a measurement light mirror, a reference light mirror, and an interference signal calculation module. Linearly polarized light emitted by the frequency-stabilized laser is incident perpendicularly onto a polarizing beam splitter. The measurement light mirror is positioned in the output path of the second quarter-wave plate, the reference light mirror is positioned in the output path of the fifth quarter-wave plate, and the interference signal calculation module is positioned in the output path of the polarizing beam splitter next to the frequency-stabilized laser. The inherent symmetry between the reference and measurement arms in the differential interferometer structure of this invention results in theoretically zero thermal drift, good structural thermal stability, simpler optical path arrangement, and easy integration into near-high-end equipment.

Description

Technical Field

[0001] This invention belongs to the field of optical measurement technology, specifically relating to a high-stability zero-difference differential interferometer based on optical intrinsic symmetry. Background Technology

[0002] Zero-difference laser interferometers are a core technology and device for achieving nanometer-level precision displacement measurement. This technology, due to its ability to combine large-scale and high-resolution measurements with non-contact operation, is widely used in key fields such as precision manufacturing, semiconductor lithography, and metrology. However, because laser interferometers are highly sensitive, environmental vibrations and thermal disturbances can significantly impact measurement results. For example, in high-end optical component processing equipment, vibration is unavoidable during cutting operations. To achieve high-precision motion detection in such environments, a highly stable differential interferometric structure is needed to convert external interference into common-mode noise for suppression. Furthermore, under continuous, long-duration, high-speed cutting conditions, multi-source thermal excitation causes significant machine tool thermal drift, with a temperature drift of ±0.1℃ still present even under temperature control (Moore Nanotechnology Systems, Nanotech 250 UPL). MP Specification, [EB / OL] 2023-07-21). Therefore, there is an urgent need for a differential interferometer with low thermal drift to meet the requirements for precision displacement measurement in high-precision machining processes.

[0003] Among commonly used differential interferometers, Agilent (now Keysight) describes the impact of mirror thermal drift error on interferometer displacement measurements in its product documentation. They have also conducted thermal drift tests on multiple interferometer mirror assemblies (Agilent Laser and Optics User's Manual), showing that the thermal drift coefficient of their differential mirror assemblies can reach 150 nm / ℃, and even after compensation, it remains at 50 nm / ℃. The temperature drift coefficient of the Renishaw RLD10-DI differential probe, commonly used in industrial applications, also reaches 50 nm / ℃. Its thermal drift error can range from several nm to tens of nm.

[0004] To eliminate or suppress thermal drift errors in interferometers, in 1995, Shigeru Hosoe performed a full-path precise calculation of the optical path temperature drift of the reference arm and the measuring arm (Hosoe S. Highly precise and stable displacement-measuring laser interferometer with differential optical paths[J]. Precision engineering, 1995, 17(4): 258-265.), deriving the mechanical thermal expansion coefficient of the mirror assembly material, the temperature coefficient of refractive index, and the thermal error model of the optical path structure on the mirror assembly, and designing a low-temperature drift center-difference interferometer structure. Agilent, based on the symmetrical construction of the shared pyramid of the reference arm and the measuring arm, designed two center-difference interferometer structures based on pyramid offset, achieving an extremely low temperature drift of 2 nm / ℃. In 2024, Simon Rerucha et al. designed a central differential interferometer structure based on the principle of equal optical path length between the reference arm and the measuring arm, achieving a temperature drift of 2.5 nm / ℃ (Rerucha S, Hola M, Oulehla J, et al. Thermally compensated common-pathdifferential interferometer with reduced long-term zero-drifts[J]. Measurement Science & Technology, 2024(9):35.). However, central differential interferometers have strict requirements for optical path arrangement, making them inconvenient for integration into high-end equipment.

[0005] In summary, in existing differential interferometer research, the general differential structure is in high demand in high-end processing equipment due to its simple and easy optical path layout. However, its optical structure is asymmetrical and has large thermal drift error. On the other hand, the central differential optical path has the advantage of structural symmetry and can achieve low temperature drift design. However, its optical path structure requires central symmetry, which places high demands on the equipment under test and makes it inconvenient to integrate into industrial equipment. Summary of the Invention

[0006] The problem this invention aims to solve is the difficulty in balancing low temperature drift and ease of use in current differential interferometer designs. It aims to meet the demand in high-end processing equipment for a low-temperature drift and simple, easy-to-integrate differential interferometer, and proposes a high-stability, zero-difference differential interferometer based on optical intrinsic symmetry.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A high-stability zero-difference differential interferometer based on optical intrinsic symmetry includes a frequency-stabilized laser, a differential interferometer mirror group, a measurement light mirror, a reference light mirror, and an interference signal calculation module; The differential interference mirror group includes a polarizing beam splitter, a first quarter-wave plate, a first pyramidal prism, a second quarter-wave plate, a second pyramidal prism, a third quarter-wave plate, a third pyramidal prism, a fourth quarter-wave plate, a fourth pyramidal prism, a fifth quarter-wave plate, a fifth pyramidal prism, a sixth quarter-wave plate, and a sixth pyramidal prism. The polarizing beam splitter has a second quarter-wave plate and a fifth quarter-wave plate attached to its left end face, a first quarter-wave plate, a third quarter-wave plate, and a fifth pyramidal prism attached to its upper end face, and a fourth quarter-wave plate, a sixth quarter-wave plate, and a second pyramidal prism attached to its right end face. The first pyramidal prism is attached above the first quarter-wave plate; the third pyramidal prism is attached above the third quarter-wave plate; the fourth pyramidal prism is attached above the fourth quarter-wave plate; and the sixth pyramidal prism is attached above the sixth quarter-wave plate. The linearly polarized light emitted by the frequency-stabilized laser is incident perpendicularly on the polarization beam splitter. The measuring light reflector is set in the output optical path of the second quarter-wave plate, the reference light reflector is set in the output optical path of the fifth quarter-wave plate, and the interference signal calculation module is set in the output optical path of the polarization beam splitter next to the frequency-stabilized laser.

[0008] Furthermore, the polarization state of the transmitted light from the polarizing beam splitter is horizontally polarized, and the polarization state of the reflected light from the polarizing beam splitter is vertically polarized; the fast axis direction of the first quarter-wave plate, the second quarter-wave plate, the third quarter-wave plate, the fourth quarter-wave plate, the fifth quarter-wave plate, and the sixth quarter-wave plate forms an angle of 45° with the horizontal direction.

[0009] Furthermore, the measuring light reflector and the reference light reflector are plane mirrors coated with a reflective film. Before use, the distances from the measuring light reflector and the reference light reflector to the differential interference mirror group are equal. During use, the measuring light reflector moves relative to the reference light reflector.

[0010] Furthermore, the linearly polarized light emitted by the frequency-stabilized laser is incident perpendicularly onto the polarization beam splitter and is split into transmitted light and reflected light.

[0011] Furthermore, the total optical path L experienced by the beam in the mirror group is measured. m The expression is:

[0012] ;

[0013] Among them, L m1To measure the optical path length of the beam from the point of beam splitting to the first quarter-wave plate, L QWP1 For the first quarter-wave plate optical path, L RR1 L is the optical path length of the first pyramidal prism. m2 To measure the optical path length of the beam as it travels from the first quarter-wave plate to the beam-splitting surface of the polarizing beam splitter, L m3 To measure the optical path length of the beam as it travels from the beam-splitting surface of the polarizing beam splitter to the left end face of the polarizing beam splitter, L QWP2 For the second quarter-wave plate optical path, L PBS For the optical path length of the polarizing beam splitter, L RR2 L is the optical path length of the second pyramidal prism. QWP3 For the third quarter-wave plate optical path, L RR3 The optical path length of the third pyramidal prism.

[0014] Furthermore, the total optical path traveled by the reference beam within the mirror group The expression is:

[0015] ;

[0016] in, As the reference beam travels from the point of beam splitting to the fourth quarter-wave plate, L QWP4 For the fourth quarter-wave plate optical path, L RR4 For the optical path length of the fourth pyramidal prism, L QWP5 For the fifth quarter-wave plate optical path, As a reference, the optical path length of the beam as it travels from the left end face of the polarizing beam splitter to the beam splitting surface of the polarizing beam splitter is... As the reference beam travels from the beam-splitting surface of the polarizing prism to the fifth-angled pyramidal prism, L RR5 L is the optical path length of the fifth pyramidal prism. QWP6 For the optical path length of the sixth quarter wave plate, L RR6 The optical path length of the sixth-angle pyramidal prism.

[0017] Furthermore, L m1 and All originate from the same point inside the polarizing beam splitter. Based on the fact that the coefficient of thermal expansion is equal at all points inside the selected cubic polarizing beam splitter, then L... m1 and For equal lengths, as temperature changes, the following always holds true:

[0018] ;

[0019] Based on the selected cubic polarizing beam splitter and the fact that the coefficient of thermal expansion is equal at all points inside, as the temperature changes, we always have:

[0020] .

[0021] Furthermore, given that the first, third, fourth, and sixth quarter-wave plates have the same size and material, and that the coefficient of thermal expansion is equal at all points within them, then as the temperature changes, the following always holds true:

[0022] ;

[0023] Since the second and fifth quarter-wave plates have the same size and material, and the coefficient of thermal expansion is equal at all points inside, then as the temperature changes, we always have:

[0024] .

[0025] Furthermore, given that the first, third, fourth, and sixth pyramidal prisms have the same dimensions and material, and that the coefficient of thermal expansion is equal at all points within them, then as the temperature changes, the following always holds true:

[0026] ;

[0027] The remaining second and fifth pyramidal prisms have the same dimensions and material, and the coefficient of thermal expansion is equal at all points inside them. Therefore, as the temperature changes, the following always holds true:

[0028] ;

[0029] The optical path lengths experienced by the measurement beam and the reference beam in a high-symmetry zero-difference differential interferometer based on intrinsic optical symmetry are exactly equal.

[0030] Furthermore, the measurement beam and the reference beam are combined at the same position on the beam-splitting surface of the polarizing prism, and emitted from the front face of the polarizing prism in the same direction to form a composite beam;

[0031] The phase difference between the two beams is calculated by the interference signal solving module after the synthesized beam is processed. The displacement value of the measuring optical mirror is then measured using a high-symmetry, zero-difference differential interferometer based on optical intrinsic symmetry. L for:

[0032] ;

[0033] Where n is the refractive index of the medium, This represents the output wavelength of the frequency-stabilized laser in a vacuum.

[0034] The beneficial effects of this invention are:

[0035] The present invention describes a highly stable zero-difference differential interferometer based on inherent optical symmetry. The differential interferometer mirror group effectively suppresses the influence of environmental interference, such as refractive index fluctuations, air turbulence, and mechanical vibrations, which simultaneously and to the same extent affect both optical paths. During differential calculations, these shared interferences are canceled out. The inherent symmetry between the reference arm and the measurement arm in the differential interferometer structure ensures that the theoretical thermal drift of the zero-difference laser interferometer is zero, resulting in excellent structural thermal stability.

[0036] The present invention describes a high-stability zero-difference differential interferometer based on inherent optical symmetry. The differential interference structure is constructed with the reference arm optical path and the measurement arm optical path parallel, resulting in a simpler optical path arrangement and easier integration into near-high-end equipment. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure of a high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to the present invention; Figure 2 This is a schematic diagram of the differential interference mirror assembly described in this invention; Figure 3 This is a front view of the differential interference mirror assembly described in this invention; Figure 4 This is a right view of the differential interference mirror assembly described in this invention; Figure 5 This is a top view of the differential interference mirror assembly described in this invention; Figure 6 The optical path diagrams for the measurement beam and reference beam in the internal interferometer group of a high-stability zero-difference differential interferometer based on optical intrinsic symmetry are shown in the present invention. The yellow light is the total beam before beam splitting and after beam combining, the red light is the measurement beam, and the blue light is the reference beam. Figure 7 This invention provides a two-dimensional optical path diagram of the measurement beam and reference beam within an internal interferometer array of a high-stability zero-difference differential interferometer based on intrinsic optical symmetry. In the figure, 1 is the frequency-stabilized laser, 2 is the differential interferometer mirror group, 3 is the measurement light reflector, 4 is the reference light reflector, 5 is the interference signal calculation module, 6 is the polarization beam splitter, 7 is the first quarter-wave plate, 8 is the first pyramidal prism, 9 is the second quarter-wave plate, 10 is the second pyramidal prism, 11 is the third quarter-wave plate, 12 is the third pyramidal prism, 13 is the fourth quarter-wave plate, 14 is the fourth pyramidal prism, 15 is the fifth quarter-wave plate, 16 is the fifth pyramidal prism, 17 is the sixth quarter-wave plate, and 18 is the sixth pyramidal prism. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described specific embodiments are merely a part of the embodiments of the invention, and not all of them. The components of the specific embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations, and the invention may also have other embodiments.

[0039] Therefore, the following detailed description of specific embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected specific embodiments of the invention. All other specific embodiments obtained by those skilled in the art based on these specific embodiments without inventive effort are within the scope of protection of this invention.

[0040] To further understand the invention's content, features, and effects, the following specific embodiments are provided, along with accompanying drawings. Figure 1 - Appendix Figure 7 Detailed explanation is as follows: A high-stability zero-difference differential interferometer based on optical intrinsic symmetry includes a frequency-stabilized laser 1, a differential interferometer mirror group 2, a measurement light mirror 3, a reference light mirror 4, and an interference signal calculation module 5; The differential interference mirror group 2 includes a polarizing beam splitter 6, a first quarter-wave plate 7, a first cornerstone prism 8, a second quarter-wave plate 9, a second cornerstone prism 10, a third quarter-wave plate 11, a third cornerstone prism 12, a fourth quarter-wave plate 13, a fourth cornerstone prism 14, a fifth quarter-wave plate 15, a fifth cornerstone prism 16, a sixth quarter-wave plate 17, and a sixth cornerstone prism 18;

[0041] The polarizing beam splitter 6 has a second quarter-wave plate 9 and a fifth quarter-wave plate 15 attached to its left end face, a first quarter-wave plate 7, a third quarter-wave plate 11 and a fifth pyramidal prism 16 attached to its upper end face, and a fourth quarter-wave plate 13, a sixth quarter-wave plate 17 and a second pyramidal prism 10 attached to its right end face. The first pyramidal prism 8 is attached above the first quarter-wave plate 7; the third pyramidal prism 12 is attached above the third quarter-wave plate 11; the fourth pyramidal prism 14 is attached above the fourth quarter-wave plate 13; and the sixth pyramidal prism 18 is attached above the sixth quarter-wave plate 17.

[0042] The linearly polarized light emitted by the frequency-stabilized laser 1 is incident perpendicularly on the polarization beam splitter 6. The measuring light reflector 3 is set in the output optical path of the second quarter-wave plate 9, the reference light reflector 4 is set in the output optical path of the fifth quarter-wave plate 15, and the interference signal calculation module 5 is set in the output optical path of the polarization beam splitter 6 next to the frequency-stabilized laser 1.

[0043] Furthermore, the polarization state of the transmitted light from the polarizing beam splitter 6 is horizontally polarized, and the polarization state of the reflected light from the polarizing beam splitter 6 is vertically polarized; the fast axis direction of the first quarter-wave plate 7, the second quarter-wave plate 9, the third quarter-wave plate 11, the fourth quarter-wave plate 13, the fifth quarter-wave plate 15, and the sixth quarter-wave plate 17 forms an angle of 45° with the horizontal direction.

[0044] Furthermore, the measuring light reflector 3 and the reference light reflector 4 are plane mirrors coated with a reflective film. Before use, the distances from the measuring light reflector 3 and the reference light reflector 4 to the differential interference mirror group 2 are equal. During use, the measuring light reflector 3 moves relative to the reference light reflector 4.

[0045] Furthermore, the linearly polarized light emitted by the frequency-stabilized laser 1 is incident perpendicularly on the polarization beam splitter 6 and is split into transmitted light and reflected light.

[0046] Furthermore, the total optical path L experienced by the beam in the mirror group is measured. m The expression is:

[0047] ;

[0048] Among them, L m1 To measure the optical path length of the beam from the point of beam splitting to the first quarter-wave plate 7, L QWP1 For the first quarter-wave plate, 7 optical path lengths, L RR1 For the first cornerstone prism, the optical path is 8, L m2 To measure the optical path length of the beam as it travels from the first quarter-wave plate 7 to the beam-splitting surface of the polarizing beam splitter 6, L m3 To measure the optical path length of the beam as it travels from the beam-splitting surface of polarizing beam splitter 6 to the left end face of polarizing beam splitter 6, L QWP2 For the second quarter-wave plate, 9 optical path lengths, L PBS For a polarizing beam splitter with a 6-optical-path length, L RR2 For the second corner prism, the optical path is 10, L QWP3 For the third quarter-wave plate, the optical path is 11, L RR3 The optical path length of the third corner prism is 12.

[0049] Furthermore, the measurement beam, after passing through the polarizing beam splitter, passes through the first quarter-wave plate, is reflected by the first cornerstone prism, and then passes through the first quarter-wave plate again. At this point, the polarization state of the measurement beam becomes vertically polarized under the two transformations of the first quarter-wave plate.

[0050] The measurement beam, after its polarization state is converted to vertical polarization, propagates to the beam-splitting surface of the polarizing beam splitter, where it is reflected and transmitted through the second quarter-wave plate. It then carries the target displacement information at the measurement light reflector and is reflected again, transmitting through the second quarter-wave plate once more. At this point, the polarization state of the measurement beam becomes horizontally polarized under the two transformations of the second quarter-wave plate.

[0051] The measurement beam, after undergoing a second polarization conversion to a horizontal polarization state, propagates to the beam-splitting surface of the polarizing beam-splitting prism, is transmitted, and is reflected by the second pyramidal prism. After passing through the polarizing beam-splitting prism and the second quarter-wave plate, it is reflected a second time at the measurement light reflecting mirror and is transmitted through the second quarter-wave plate again. At this point, the polarization state of the measurement beam becomes vertically polarized under the two transformations of the second quarter-wave plate.

[0052] The measurement beam, after undergoing a third polarization conversion to a vertical polarization state, propagates to the beam-splitting surface of the polarizing beam-splitting prism, where it is reflected and transmitted through the third quarter-wave plate. It is then reflected again by the third cornerstone prism and transmitted through the third quarter-wave plate once more. At this point, the polarization state of the measurement beam becomes horizontally polarized under the two transformations of the third quarter-wave plate, and finally transmits through the beam-splitting surface of the polarizing beam-splitting prism.

[0053] Furthermore, the total optical path traveled by the reference beam within the mirror group The expression is:

[0054] ;

[0055] in, As the reference beam travels from the point of beam splitting to the fourth quarter-wave plate 13, L QWP4 For the fourth quarter-wave plate, the optical path is 13, L RR4 For the fourth pyramidal prism, the optical path is 14, L QWP5 For the fifth quarter-wave plate, 15 optical path length. As a reference, the optical path length of the beam as it travels from the left end face of the polarizing beam splitter 6 to the beam splitting surface of the polarizing beam splitter 6 is... As a reference beam, L is the optical path length experienced by the beam as it propagates from the beam-splitting surface of polarizing beam splitter 6 to the fifth cornerstone prism 16. RR5 For the fifth pyramidal prism, the optical path is 16, L QWP6 For a sixth quarter-wave plate with a path length of 17, L RR6 It is a sixth-angle pyramidal prism with an optical path of 18.

[0056] Furthermore, the reference beam, after being reflected by the polarizing beam splitter, is transmitted through the fourth quarter-wave plate, reflected by the fourth pyramidal prism, and transmitted through the fourth quarter-wave plate again. At this point, the polarization state of the reference beam becomes horizontally polarized under the two transformations of the fourth quarter-wave plate.

[0057] The reference beam, after its polarization state is converted to horizontal polarization, propagates to the beam-splitting surface of the polarizing beam-splitting prism, is transmitted, and then passes through the fifth quarter-wave plate. It is reflected at the reference light mirror and then transmitted through the fifth quarter-wave plate again. At this point, the polarization state of the reference beam becomes vertically polarized due to the two transformations by the fifth quarter-wave plate.

[0058] The measurement beam, after undergoing a second polarization conversion to a horizontal polarization state, propagates to the beam-splitting surface of the polarizing beam-splitting prism, is transmitted, and is reflected by the second pyramidal prism. After passing through the polarizing beam-splitting prism and the second quarter-wave plate, it is reflected a second time at the measurement light reflecting mirror and is transmitted through the second quarter-wave plate again. At this point, the polarization state of the measurement beam becomes vertically polarized under the two transformations of the second quarter-wave plate.

[0059] The reference beam, after undergoing a third polarization conversion to a horizontal polarization state, propagates to the beam-splitting surface of the polarizing beam-splitting prism, where it is transmitted and then passes through the sixth quarter-wave plate. After being reflected by the sixth cornerstone prism, it is transmitted through the sixth quarter-wave plate again. At this point, the polarization state of the reference beam becomes vertically polarized under the two transformations of the sixth quarter-wave plate, and finally reflects at the beam-splitting surface of the polarizing beam-splitting prism.

[0060] Furthermore, L m1 and All originate from the same point inside the polarizing beam splitter 6. Based on the fact that the coefficient of thermal expansion is equal at all points inside the selected cubic polarizing beam splitter, then L m1 and For equal lengths, as temperature changes, the following always holds true:

[0061] ;

[0062] Based on the selected cubic polarizing beam splitter and the fact that the coefficient of thermal expansion is equal at all points inside, as the temperature changes, we always have:

[0063] .

[0064] Furthermore, given that the first quarter-wave plate 7, the third quarter-wave plate 11, the fourth quarter-wave plate 13, and the sixth quarter-wave plate 17 have the same dimensions and material, and that the coefficient of thermal expansion at all points within them is equal, then as the temperature changes, the following always holds true:

[0065] ;

[0066] Since the second quarter-wave plate 9 and the fifth quarter-wave plate 15 have the same size and material, and the coefficient of thermal expansion is equal at all points inside, then as the temperature changes, the following always holds true:

[0067] .

[0068] Furthermore, given that the first pyramidal prism 8, the third pyramidal prism 12, the fourth pyramidal prism 14, and the sixth pyramidal prism 18 have the same dimensions and material, and that the coefficient of thermal expansion at all points inside them is equal, then as the temperature changes, the following always holds true:

[0069] ;

[0070] The remaining second-angled pyramidal prism 10 and fifth-angled pyramidal prism 16 have the same dimensions and material, and the coefficient of thermal expansion at all points inside them is equal. Therefore, as the temperature changes, the following always holds true:

[0071] ;

[0072] The optical path lengths experienced by the measurement beam and the reference beam in a high-symmetry zero-difference differential interferometer based on intrinsic optical symmetry are exactly equal.

[0073] Furthermore, the measuring beam and the reference beam are combined at the same position on the beam-splitting surface of the polarizing beam splitter 6, and emitted from the front end face of the polarizing beam splitter 6 in the same direction to form a composite beam.

[0074] The phase difference between the two beams is calculated by the interference signal calculation module 5 after the synthesized beam is processed. The displacement value of the measuring optical reflector 3 is then measured using a high-symmetry, zero-difference differential interferometer based on optical intrinsic symmetry. L for:

[0075] ;

[0076] Where n is the refractive index of the medium, λ is the output wavelength of the frequency-stabilized laser 1 in a vacuum.

[0077] Combined with L m and As can be seen from the simplified expression, even with temperature fluctuations, the optical path lengths of the measurement beam and the reference beam in this embodiment are always exactly the same. Because of the structure of the optical path center difference and keeping the measurement arm and the reflection arm on the same side of the polarization beam splitter, the influence of thermal drift on the optical path difference can be effectively suppressed. Therefore, the thermal stability of the interferometer group using the differential optical path design method described in this embodiment is higher than that of the traditional zero-difference interferometer group.

[0078] The differential interference mirror group includes a polarizing beam splitter (PBS), six quarter-wave plates (QWP), and six pyramidal prisms (RR).

[0079] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0080] Although this application has been described above with reference to specific embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of this application. In particular, as long as there is no structural conflict, the features in the specific embodiments disclosed in this application can be combined with each other in any way. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, this application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A highly stable zero-difference differential interferometer based on intrinsic optical symmetry, characterized in that, Includes a frequency-stabilized laser (1), a differential interferometer mirror group (2), a measurement light mirror (3), a reference light mirror (4), and an interference signal calculation module (5); The differential interference mirror group (2) includes a polarizing beam splitter (6), a first quarter-wave plate (7), a first corner prism (8), a second quarter-wave plate (9), a second corner prism (10), a third quarter-wave plate (11), a third corner prism (12), a fourth quarter-wave plate (13), a fourth corner prism (14), a fifth quarter-wave plate (15), a fifth corner prism (16), a sixth quarter-wave plate (17), and a sixth corner prism (18). The polarizing beam splitter (6) has a second quarter-wave plate (9) and a fifth quarter-wave plate (15) attached to its left end face, a first quarter-wave plate (7), a third quarter-wave plate (11) and a fifth pyramidal prism (16) attached to its upper end face, and a fourth quarter-wave plate (13), a sixth quarter-wave plate (17) and a second pyramidal prism (10) attached to its right end face. The first pyramidal prism (8) is attached above the first quarter-wave plate (7); the third pyramidal prism (12) is attached above the third quarter-wave plate (11); the fourth pyramidal prism (14) is attached above the fourth quarter-wave plate (13); and the sixth pyramidal prism (18) is attached above the sixth quarter-wave plate (17). The linearly polarized light emitted by the frequency stabilized laser (1) is incident perpendicularly onto the polarization beam splitter (6). The measuring light reflector (3) is set on the output optical path of the second quarter-wave plate (9), the reference light reflector (4) is set on the output optical path of the fifth quarter-wave plate (15), and the interference signal calculation module (5) is set on the output optical path of the polarization beam splitter (6) next to the frequency stabilized laser (1).

2. The high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 1, characterized in that, The polarization state of the transmitted light of the polarizing beam splitter (6) is horizontally polarized, and the polarization state of the reflected light of the polarizing beam splitter (6) is vertically polarized; the fast axis direction of the first quarter-wave plate (7), the second quarter-wave plate (9), the third quarter-wave plate (11), the fourth quarter-wave plate (13), the fifth quarter-wave plate (15) and the sixth quarter-wave plate (17) forms an angle of 45° with the horizontal direction.

3. The high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 2, characterized in that, The measuring light reflector (3) and the reference light reflector (4) are plane mirrors coated with a reflective film. Before use, the distances from the measuring light reflector (3) and the reference light reflector (4) to the differential interference mirror group (2) are equal. During use, the measuring light reflector (3) moves relative to the reference light reflector (4).

4. A high-stability zero-difference differential interferometer based on inherent optical symmetry according to claim 3, characterized in that, The linearly polarized light emitted by the frequency stabilized laser (1) is incident perpendicularly onto the polarization beam splitter (6) and is split into transmitted light and reflected light.

5. A high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 4, characterized in that, The total optical path L of the measurement beam in the mirror group m The expression is: ； Among them, L m1 To measure the optical path length of the beam from the point of beam splitting to the first quarter-wave plate (7), L QWP1 For the first quarter-wave plate (7), L RR1 For the optical path of the first corner prism (8), L m2 To measure the optical path length of the beam as it travels from the first quarter-wave plate (7) to the beam-splitting surface of the polarizing beam splitter (6), L m3 To measure the optical path length of the beam as it travels from the beam-splitting surface of the polarizing beam splitter (6) to the left end face of the polarizing beam splitter (6), L QWP2 For the second quarter-wave plate (9), L PBS For the optical path length of the polarizing beam splitter (6), L RR2 For the optical path of the second pyramidal prism (10), L QWP3 For the third quarter-wave plate (11), L RR3 The optical path of the third corner prism (12).

6. A high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 5, characterized in that, The total optical path traveled by the reference beam in the mirror group The expression is: ； in, As the reference beam travels from the point of beam splitting to the fourth quarter-wave plate (13), L QWP4 For the fourth quarter-wave plate (13), L RR4 For the optical path of the fourth pyramidal prism (14), L QWP5 For the fifth quarter-wave plate (15), As a reference, the optical path length of the beam traveling from the left end face of the polarizing beam splitter (6) to the beam splitting surface of the polarizing beam splitter (6) is as follows: As the reference beam travels from the beam-splitting surface of the polarizing beam splitter (6) to the fifth pyramidal prism (16), L RR5 For the optical path of the fifth pyramidal prism (16), L QWP6 For the optical path of the sixth quarter-wave plate (17), L RR6 The optical path of the sixth-angle pyramidal prism (18) is given.

7. A high-stability zero-difference differential interferometer based on inherent optical symmetry according to claim 6, characterized in that, L m1 and All originate from the same point inside the polarizing beam splitter (6). Based on the fact that the thermal expansion coefficients of all points inside the selected cubic polarizing beam splitter are equal, then L m1 and For equal lengths, as temperature changes, the following always holds true: ； Based on the selected cubic polarizing beam splitter and the fact that the coefficient of thermal expansion is equal at all points inside, as the temperature changes, we always have: 。 8. A high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 7, characterized in that, Since the first quarter-wave plate (7), the third quarter-wave plate (11), the fourth quarter-wave plate (13), and the sixth quarter-wave plate (17) have the same size and material, and the coefficient of thermal expansion at all points inside are equal, then as the temperature changes, the following always holds true: ； Since the second quarter-wave plate (9) and the fifth quarter-wave plate (15) have the same size and material, and the coefficient of thermal expansion at all points inside them is equal, then as the temperature changes, we always have: 。 9. A high-stability zero-difference differential interferometer based on optical intrinsic symmetry according to claim 8, characterized in that, Given that the first pyramidal prism (8), the third pyramidal prism (12), the fourth pyramidal prism (14), and the sixth pyramidal prism (18) have the same size and material, and that the coefficient of thermal expansion at all points inside them is equal, then as the temperature changes, the following always holds true: ； The remaining second-angled pyramidal prism (10) and fifth-angled pyramidal prism (16) have the same dimensions and material, and the coefficient of thermal expansion at all points inside them is equal. Therefore, as the temperature changes, the following always holds true: ； The optical path lengths experienced by the measurement beam and the reference beam in a high-symmetry zero-difference differential interferometer based on intrinsic optical symmetry are exactly equal.

10. A high-stability zero-difference differential interferometer based on inherent optical symmetry according to claim 9, characterized in that, The measuring beam and the reference beam are combined at the same position on the beam splitting surface of the polarizing beam splitter (6) and emitted from the front end of the polarizing beam splitter (6) in the same direction to form a composite beam. The phase difference between the two beams is calculated by the interference signal calculation module (5) after the synthesized beam is processed. The displacement value of the measuring optical mirror (3) is then measured by a high-symmetry, zero-difference differential interferometer based on optical intrinsic symmetry. L for: ； Where n is the refractive index of the medium, The output wavelength of the frequency-stabilized laser (1) in vacuum.

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