A heterodyne laser interferometry device and method for linear expansion coefficient measurement

Abstract

The application discloses a heterodyne laser interferometer device and method for linear expansion coefficient measurement, comprising a dual-frequency laser, a beam splitter, a first optical fiber coupler, a front light splitting device, a rear light splitting device, a measurement reference device, a second optical fiber coupler, a collection card and an upper computer; the dual-frequency laser emits orthogonal linearly polarized light; the front light splitting device outputs light which is perpendicularly incident on the front surface of the measurement reference device and is reflected multiple times; the rear output light is perpendicularly incident on the rear surface of the measurement reference device and is reflected multiple times; the reflected light, after passing through the front light splitting device and the rear light splitting device, is incident on the second optical fiber coupler together with a reference signal; the collection card collects the measurement signal of the second optical fiber coupler and the reference signal of the first optical fiber coupler and transmits them to the upper computer; and the upper computer calculates the thermal expansion amount and the linear expansion coefficient. The application can solve the problems of insufficient spatial accuracy of the thermal expansion amount, inaccurate baseline of the reference accuracy, and uncontrollable linear change in the expansion change.

Description

Technical Field

[0001] This application belongs to the field of optical measurement technology, and specifically relates to a heterodyne laser interferometry measuring device and method for measuring the coefficient of linear expansion. Background Technology

[0002] In modern high-end manufacturing and scientific research, the demand for precise measurement of the microscopic properties of materials is increasing daily. Semiconductor manufacturing pursues higher integration and smaller chip sizes, and precision instruments are also developing towards ultra-precision, making sub-nanometer-level measurement technology a key support. In these applications, the thermal expansion characteristics of materials have a significant impact on product performance and stability. For example, in high-end optical equipment, if the coefficient of thermal expansion of lens materials is not well controlled, deformation will occur with temperature fluctuations, leading to a decline in optical performance; semiconductor chips generate heat during operation, and mismatch in material thermal expansion can induce internal stress, affecting chip lifespan and reliability. Therefore, developing high-precision thermal expansion coefficient measurement devices is crucial.

[0003] The technology for measuring the coefficient of thermal expansion has undergone several stages of development. Early methods using fiber optic displacement sensors and dial indicators had relatively low accuracy, with an uncertainty of only 10⁻⁶ / K, which was insufficient to meet modern high-precision requirements. With technological advancements, inductive micrometers and laser interferometers have been widely used, significantly improving measurement accuracy. For example, the laser interferometry structure from PTB in Germany achieves an uncertainty of 2*10⁻¹⁰ / K. Currently, measurement methods based on laser interferometers are mainstream, while emerging measurement techniques such as ultrafast lasers are also beginning to show promise.

[0004] Despite significant advancements in existing technologies, considerable limitations remain. Regarding spatial positioning accuracy, sub-nanometer measurements demand more precise positioning of the measurement reference, a standard that current measuring devices struggle to meet. The accuracy of thermal expansion measurement also needs further improvement. While some technologies have achieved high precision, there is still room for enhancement to meet even higher precision requirements, and higher-magnification signal superposition techniques urgently need development. Furthermore, in terms of real-time measurement error correction, current measurement technologies cannot effectively address errors arising from multi-point and simultaneous left-right measurements of thermal expansion. They struggle to obtain the average change in material thermal expansion in real time, impacting the reliability of measurement results and failing to meet the stringent accuracy and stability requirements of high-end applications. Summary of the Invention

[0005] The purpose of this application is to provide a heterodyne laser interferometry apparatus and method for measuring the coefficient of linear expansion. This addresses the problems in the prior art, such as insufficient spatial accuracy of thermal expansion, inaccurate baseline accuracy, and uncontrollable linear changes in expansion.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] A heterodyne laser interferometry device for measuring the coefficient of linear expansion includes a dual-frequency laser, a beam splitter, a first fiber coupler, a pre-beam splitter, a post-beam splitter, a measurement reference device, a second fiber coupler, a data acquisition card, and a host computer.

[0008] The dual-frequency laser emits two orthogonally linearly polarized beams with a frequency difference of 1-10MHz. After being split by a beam splitter, the transmitted light is incident on the pre-splitter device, and the reflected light is incident on the first fiber coupler.

[0009] The pre-splitting device performs polarization splitting, optical path reflection and polarization modulation on the transmitted light, and the output light is perpendicularly incident on the front surface of the measuring reference device and reflected multiple times.

[0010] The rear beam splitter performs polarization beam splitting, optical path reflection and polarization state modulation on the reflected light, and the output light is incident perpendicularly on the rear surface of the measurement reference device and reflected multiple times.

[0011] After passing through the pre-splitter and post-splitter, the reflected light is incident on the second fiber coupler along with the reference signal of the first fiber coupler.

[0012] The acquisition card acquires the measurement signal from the second fiber coupler and the reference signal from the first fiber coupler, and transmits them to the host computer.

[0013] The host computer is used to calculate thermal expansion and linear expansion coefficient.

[0014] In one possible implementation, the pre-splitting device includes a first polarizing beam splitter, a first right-angle prism, a first quarter-wave plate, a second right-angle prism, and a first inclined plane reflecting prism.

[0015] The first polarizing beam splitter polarizes the transmitted light. The transmitted horizontally polarized light is transmitted through the first temperature control cavity, the second polarizing beam splitter of the rear beam splitter, and reflected by the third right-angle prism. It then returns to the front beam splitter, is transmitted through the first polarizing beam splitter, and is reflected multiple times by the first right-angle prism, the first quarter-wave plate, and the front surface of the measurement reference device before finally entering the second fiber coupler.

[0016] The reflected vertically polarized light passes through the first inclined reflective prism and the rear beam splitter, and is reflected multiple times by the rear surface of the measurement reference device before finally entering the second fiber coupler.

[0017] In one possible implementation, the rear beam splitter includes a second inclined reflective prism, a second polarizing beam splitter, a third right-angle prism, a second quarter-wave plate, a third quarter-wave plate, and a fourth right-angle prism.

[0018] The reflected light is reflected by the second inclined reflective prism, the second polarizing beam splitter, and the third right-angle prism. Its polarization state is modulated by the second quarter-wave plate and the third quarter-wave plate. It is then incident perpendicularly onto the rear surface of the measurement reference device and reflected multiple times. After being reflected by the fourth right-angle prism and the rear beam splitter, it is incident onto the second fiber coupler.

[0019] In one possible implementation, the measuring reference device includes a first measuring reference mirror and a first temperature control cavity;

[0020] The first measuring reference mirror has planar reflective mirrors on both the front and rear sides, and is placed in the first temperature control cavity to reflect laser light and generate thermal expansion with temperature changes.

[0021] The first temperature control cavity is used to control the temperature environment of the first measuring reference mirror and provide thermal expansion measurement conditions.

[0022] In one possible implementation, the first quarter-wave plate is disposed between the first right-angle prism and the first measurement reference mirror to convert horizontally linearly polarized light into circularly polarized light, so that the laser is incident perpendicularly on the front surface of the first measurement reference mirror and achieves four round-trip reflections. After each reflection, the polarization state is switched, and finally, 4 times Doppler frequency shift information is superimposed.

[0023] In one possible implementation, the second and third quarter-wave plates of the rear beam splitter are sequentially disposed between the rear surface of the second polarizing beam splitter and the first measuring reference mirror. They are used to convert vertically linearly polarized light into circularly polarized light that is incident on the rear surface. The light is superimposed with 4 times Doppler frequency shift information through four round trip reflections, and the reflected light is folded back through the fourth right-angle prism.

[0024] In one possible implementation, both the first and second fiber couplers are polarization-maintaining fiber couplers, used to separate the optical signals of the reference optical path and the measurement optical path to ensure the polarization state consistency of the dual-frequency laser.

[0025] In one possible implementation, the measurement reference device further includes a second measurement reference mirror and a second temperature control cavity. The second measurement reference mirror has the same structure as the first measurement reference mirror and is used as a standard cavity mirror for spatial position comparison interferometry. The second measurement reference mirror is reflected four times back and forth through a fourth quarter-wave plate to obtain 4 times Doppler frequency shift information.

[0026] In one possible implementation, the rear beam splitter further includes a fifth right-angle prism, which replaces the inclined reflective prism in part of the reflection path. The light path is redirected through two internal reflections to ensure that the reflected light is perpendicularly incident on the rear surface of the first measurement reference mirror and undergoes multiple reflections.

[0027] In a first aspect, a heterodyne laser interferometry method for measuring the coefficient of linear expansion is provided, comprising the following steps:

[0028] Step 1: Two orthogonally linearly polarized beams with a frequency difference of 1-10MHz are generated by a dual-frequency laser, and then split into transmitted and reflected light by a beam splitter;

[0029] Step 2: Input the transmitted light into the pre-splitter device. After polarization beam splitting, optical path reflection and polarization modulation, it forms four round trips to the front surface of the first measurement reference mirror. Each round trip is superimposed with Doppler frequency shift information, and finally outputs a measurement signal containing 4 times Doppler frequency shift.

[0030] Step 3: Input the reflected light into the post-beam splitter. After polarization beam splitting, optical path reflection and polarization state modulation, it forms four round trips to the rear surface of the first measurement reference mirror. Each round trip is superimposed with Doppler frequency shift information, and finally outputs a measurement signal containing 4 times Doppler frequency shift.

[0031] Step 4: Interfere the measurement signals output by the pre-splitter and post-splitter with the reference signal of the first fiber coupler in the second fiber coupler to obtain the Doppler frequency shift difference signal containing the synchronous anisotropic measurement results of the left and right sides;

[0032] Step 5: Acquire the Doppler frequency shift signal using the acquisition card and transmit it to the host computer;

[0033] Step 6: The host computer uses the formula:

[0034] The Doppler signal of the first fiber coupler can be represented as: f1-f2;

[0035] The Doppler signal of the second fiber coupler can be expressed as: f1-f2±8Δf;

[0036] Let λ represent the phase change under multiple reflections from the first measurement reference mirror, and let λ represent the incident wavelength of the dual-frequency laser. The length change under thermal expansion can be expressed as:

[0037]

[0038] Where ΔL represents the magnitude of the thermal expansion change of the first measuring reference mirror in the first temperature control cavity under temperature change conditions, the coefficient of thermal expansion can be expressed as:

[0039]

[0040] dl(t) / dt represents the amount of material length deformation before and after the temperature increases by 1℃.

[0041] Compared with the prior art, this application has the following beneficial effects:

[0042] This application provides a heterodyne laser interferometry device for measuring the coefficient of linear expansion. By utilizing the Doppler superposition characteristics of dual-frequency lasers, Doppler frequency shift information is superimposed on each laser reflection. Combined with synchronous and out-of-direction measurement of the left and right end faces, the equivalent average value of thermal expansion is obtained in real time, which significantly improves the measurement accuracy and solves the problems of insufficient spatial positioning accuracy and low accuracy of thermal expansion measurement in the prior art.

[0043] In one possible implementation, the horizontally polarized light is reflected four times back and forth on the front surface of the first measurement reference mirror by polarization splitting of the first polarization beam splitter and light path reflection of the right-angle prism. This superimposed 4 times Doppler frequency shift information, combined with the multi-incident base point design, allows for precise spatial positioning of the material with the coefficient of thermal expansion, thereby improving the accuracy of the measurement.

[0044] In one possible implementation, the polarization modulation of the second and third quarter-wave plates enables four round-trip reflections of vertically polarized light on the rear surface, and the 4-fold Doppler frequency shift information is simultaneously superimposed. The synchronous measurement of the left and right end faces can obtain anisotropic length measurement references, and the influence of temperature non-uniformity is eliminated through mathematical calculations, thereby improving measurement reliability.

[0045] In one possible implementation, the first temperature-controlled cavity provides a stable temperature gradient environment, and the dual-plane reflection design of the first measurement reference mirror ensures synchronous measurement of the left and right end faces. The average thermal expansion parameter is obtained through real-time phase decoupling, reducing the interference of environmental factors such as temperature, air pressure, and vibration on the measurement results.

[0046] In one possible implementation, the polarization state switching of a quarter-wave plate enables multiple round-trip reflections of a single polarized light on the same surface, avoiding optical path overlap interference. At the same time, the linear measurement range is extended by superimposing a 4-fold frequency shift, improving the measurement accuracy to the sub-nanometer level, which is superior to the dual-optical-path Michelson structure in the prior art.

[0047] In one possible implementation, the four round-trip reflections of the rear surface form a symmetrical optical path with the front surface, synchronously acquiring the expansion information of the left and right end faces. By decoupling the phase of the dual-optical-path frequency shift signals, the equivalent average thermal expansion is calculated in real time, eliminating the error caused by the temperature gradient at a single measurement point and improving the reliability of the results.

[0048] In one possible implementation, a polarization-maintaining fiber coupler ensures that the polarization states of the dual-frequency lasers remain orthogonal during transmission, avoiding signal attenuation caused by polarization crosstalk, improving the stability of the interference signal, thereby enhancing the accuracy of the baseline for calculating thermal expansion and solving the problem of inaccurate baseline in the prior art.

[0049] In one possible implementation, a dual-reference mirror design provides spatial positional comparison. By synchronously measuring with the standard cavity mirror and the measuring cavity mirror, the reference offset caused by uneven temperature field or mechanical vibration in the first temperature-controlled cavity is eliminated, further improving the positioning accuracy of the measurement reference.

[0050] In one possible implementation, the internal reflection design of the right-angle prism reduces energy loss in the optical path and improves the stability of the optical path through a reflection path with a fixed angle. This avoids the incident angle deviation caused by installation errors of the inclined reflecting prism, and further improves the positioning accuracy of the measurement reference.

[0051] A heterodyne laser interferometry method for measuring the coefficient of linear expansion achieves high-precision acquisition of thermal expansion by synchronously measuring the left and right end faces in opposite directions and superimposing the frequency shift of four round-trip reflections. Combined with a real-time phase decoupling algorithm, random errors in temperature changes are eliminated, and the average thermal expansion parameter is obtained. The reliability and accuracy of the measurement results are significantly better than those of existing single-optical-path or dual-optical-path interferometric structures. Attached Figure Description

[0052] Figure 1 A schematic diagram of the overall structure of a heterodyne laser interferometry device for measuring the coefficient of linear expansion provided in this application;

[0053] Figure 2 A schematic diagram of the overall structure of another heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion provided in this application;

[0054] Figure 3 This is a schematic diagram of the overall structure of another heterodyne laser interferometry device for measuring the coefficient of linear expansion provided in this application.

[0055] The attached figures are labeled as follows: 1. Dual-frequency laser; 2. Beam splitter; 3. First fiber coupler; 4. Pre-splitting device; 401. First polarizing beam splitter prism; 402. First right-angle prism; 403. First quarter-wave plate; 404. Second right-angle prism; 405. First inclined plane reflecting prism; 406. Fourth quarter-wave plate; 5. Rear beam splitting device; 501. Second inclined plane reflecting prism; 502. Second polarizing beam splitter prism; 503. Third right-angle prism; 504. Second quarter-wave plate; 505. Third quarter-wave plate; 506. Fourth right-angle prism; 507. Fifth right-angle prism; 6. Measurement reference device; 601. First measurement reference mirror; 602. First temperature control cavity; 603. Second measurement reference mirror; 604. Second temperature control cavity; 7. Second fiber coupler; 8. Data acquisition card; 9. Host computer. Detailed Implementation

[0056] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0057] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0058] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "a plurality of" means two or more, unless otherwise explicitly defined. The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings.

[0059] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a communication connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0060] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0061] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0062] like Figure 1 As shown, this application discloses a heterodyne laser interferometry device for measuring the coefficient of linear expansion. The heterodyne laser interferometry device for measuring the coefficient of linear expansion may include a dual-frequency laser 1, a beam splitter 2, a first fiber coupler 3, a pre-beam splitter 4, a post-beam splitter 5, a measurement reference device 6, a second fiber coupler 7, a data acquisition card 8, and a host computer 9.

[0063] The dual-frequency laser 1 can emit two orthogonally linearly polarized beams with a frequency difference of 1-10MHz. After being split by the beam splitter 2, the transmitted light is incident on the pre-splitter 4, and the reflected light is incident on the first fiber coupler 3.

[0064] Optionally, the dual-frequency laser 1 is a helium-neon laser with a frequency difference of 5 MHz, emitting orthogonally linearly polarized light (horizontally polarized f1 and vertically polarized f2) with a wavelength of 632.8 nm. The beam splitter 2 splits the light into transmitted light (70% energy) and reflected light (30% energy). The transmitted light enters the pre-splitter 4, and the reflected light is used as a reference signal to directly enter the first fiber coupler 3.

[0065] The pre-splitter 4 performs polarization splitting, optical path reflection and polarization modulation on the transmitted light, and the output light is incident perpendicularly on the front surface of the first measurement reference mirror 601 and reflected multiple times.

[0066] The rear beam splitter 5 performs polarization beam splitting, optical path reflection and polarization state modulation on the reflected light, and the output light is incident perpendicularly on the rear surface of the first measurement reference mirror 601 and reflected multiple times.

[0067] After passing through the pre-splitter 4 and the post-splitter 5, the reflected light is incident on the second fiber optic coupler 7 along with the reference signal of the first fiber optic coupler 3.

[0068] Optionally, the pre-splitter 4 separates the transmitted light into f1 (horizontally polarized) and f2 (vertically polarized). f1 is reflected by the post-splitter 5 and returns to the pre-splitter 4, modulated into circularly polarized light, and incident vertically onto the front surface of the first measurement reference mirror 601. After completing four round trip reflections, it enters the second fiber coupler 7.

[0069] After being modulated by the rear beam splitter 5, f2 is incident on the rear surface of the first measurement reference mirror 601, and after completing four round-trip reflections, it enters the second fiber coupler 7.

[0070] The acquisition card 8 acquires the measurement signal from the second fiber optic coupler 7 and the reference signal from the first fiber optic coupler 3, and transmits them to the host computer 9. The acquisition card 8 acquires the signals synchronously, and the host computer 9 calculates the thermal expansion and linear expansion coefficient using formulas.

[0071] In this embodiment, by utilizing the Doppler superposition characteristics of dual-frequency lasers, Doppler frequency shift information is superimposed on each laser reflection. Combined with synchronous and out-of-direction measurement of the left and right end faces, the equivalent average value of thermal expansion is obtained in real time, which significantly improves the measurement accuracy and solves the problems of insufficient spatial positioning accuracy and low thermal expansion measurement accuracy in the prior art.

[0072] In one possible embodiment, the pre-splitting device 4 may include a first polarizing beam splitter 401, a first right-angle prism 402, a first quarter-wave plate 403, a second right-angle prism 404, and a first inclined plane reflecting prism 405.

[0073] The first polarizing beam splitter 401 polarizes and splits the transmitted light. The transmitted horizontally polarized light is transmitted through the first temperature control cavity 602, the second polarizing beam splitter 502 of the rear beam splitter 5, and reflected by the third right-angle prism 503. It then returns to the front beam splitter 4, and after being reflected multiple times by the first right-angle prism 402, the first quarter-wave plate 403, and the front surface of the measurement reference device 6, it finally enters the second fiber optic coupler 7.

[0074] The reflected vertically polarized light passes through the first inclined reflective prism 405 and the rear beam splitter 5, and is reflected multiple times by the rear surface of the measurement reference device 6 before finally entering the second fiber coupler 7.

[0075] In this embodiment, the horizontally polarized light is reflected four times back and forth on the front surface of the first measuring reference mirror 601 by the polarization beam splitting of the first polarization beam splitter 401 and the light path reflection of the right-angle prism, and the information of 4 times Doppler frequency shift is superimposed. Combined with the multi-incident base point design, the material with thermal expansion coefficient is accurately located in space, thereby improving the accuracy of the measurement.

[0076] In one possible embodiment, the rear beam splitter 5 includes a second inclined reflective prism 501, a second polarizing beam splitter 502, a third right-angle prism 503, a second quarter-wave plate 504, a third quarter-wave plate 505, and a fourth right-angle prism 506.

[0077] The reflected light is reflected by the second inclined reflective prism 501, the second polarizing beam splitter prism 502 and the third right-angle prism 503, and then its polarization state is modulated by the second quarter-wave plate 504 and the third quarter-wave plate 505. It is then incident perpendicularly onto the rear surface of the measuring reference device 6 and reflected multiple times. After being reflected by the fourth right-angle prism 506 and the rear beam splitter 5, it is incident onto the second fiber optic coupler 7.

[0078] Specifically, the second inclined reflective prism 501 of the rear beam splitter 5 receives the vertically polarized light f2 from the front beam splitter 4, reflects it through the second polarizing beam splitter prism 502 to the third right-angle prism 503, and after two internal reflections returns to the second polarizing beam splitter prism 502. It then passes through the second quarter-wave plate 504 and is converted into circularly polarized light, which is then incident vertically on the rear surface of the first measurement reference mirror 601. After four round trip reflections, the reflected light is refracted by the fourth right-angle prism 506, its polarization state is switched by the third quarter-wave plate 505, and finally enters the second fiber coupler 7.

[0079] In this embodiment, the polarization modulation of the second quarter-wave plate 504 and the third quarter-wave plate 505 enables four round-trip reflections of vertically polarized light on the rear surface, and simultaneously superimposes 4 times Doppler frequency shift information. The synchronous measurement of the left and right end faces can obtain anisotropic length measurement references. The influence of temperature non-uniformity is eliminated through mathematical calculations, thereby improving measurement reliability.

[0080] In one possible embodiment, the measurement reference device 6 may include a first measurement reference mirror 601 and a first temperature control cavity 602.

[0081] The first measuring reference mirror 601 has planar reflective mirrors on both the front and rear sides and is placed inside the first temperature control cavity 602 to reflect laser light and generate thermal expansion with temperature changes.

[0082] The first temperature control cavity 602 is used to control the temperature environment of the first measuring reference mirror 601 and provide thermal expansion measurement conditions.

[0083] Specifically, the first measuring reference mirror 601 is made of fused silica material with a low coefficient of thermal expansion, and the flatness of its front and rear surfaces is better than λ / 20. It is placed inside the first temperature control cavity 602 with a temperature control accuracy of ±0.01℃. When the temperature of the first temperature control cavity 602 changes, the first measuring reference mirror 601 undergoes linear expansion or contraction, and the optical path difference of the laser reflected from its front and rear surfaces changes accordingly. The amount of thermal expansion is reflected by the Doppler frequency shift signal.

[0084] In this embodiment, the first temperature control cavity 602 provides a stable temperature gradient environment, and the dual-plane reflection design of the first measurement reference mirror 601 ensures synchronous measurement of the left and right end faces. The average thermal expansion parameter is obtained through real-time phase decoupling, reducing the interference of environmental factors such as temperature, air pressure, and vibration on the measurement results.

[0085] In one possible embodiment, the first quarter-wave plate 403 is disposed between the first right-angle prism 402 and the first measurement reference mirror 601 to convert horizontally linearly polarized light into circularly polarized light, so that the laser is incident perpendicularly on the front surface of the first measurement reference mirror 601 and achieves four round-trip reflections. After each reflection, the polarization state is switched, and finally, 4 times Doppler frequency shift information is superimposed.

[0086] Specifically, the first quarter-wave plate 403 is located between the first right-angle prism 402 and the first measurement reference mirror 601. Horizontally polarized light f1 is converted into right-hand circularly polarized light after passing through the wave plate and is incident perpendicularly onto the front surface. After the first reflection, the circularly polarized light is converted into left-hand circularly polarized light, which is reflected by the first polarizing beam splitter prism 401 as vertically linearly polarized light f2. After the second reflection, it is converted into right-hand circularly polarized light again, which is transmitted through the prism as horizontally linearly polarized light f1. This process is repeated four times, resulting in a final superposition of four times the Doppler frequency shift (±4Δf).

[0087] In this embodiment, the polarization state switching of a quarter-wave plate enables multiple round-trip reflections of a single polarized light on the same surface, avoiding optical path overlap interference. At the same time, the linear measurement range is extended by superimposing a 4-fold frequency shift, improving the measurement accuracy to the sub-nanometer level, which is superior to the dual-optical-path Michelson structure in the prior art.

[0088] In one possible embodiment, the second quarter-wave plate 504 and the third quarter-wave plate 505 of the rear beam splitter 5 are sequentially disposed between the second polarizing beam splitter prism 502 and the rear surface of the first measuring reference mirror 601, for converting vertically linearly polarized light into circularly polarized light that is incident on the rear surface, and superimposing 4 times Doppler frequency shift information through four round-trip reflections, and the reflected light is reflected back through the fourth right-angle prism 506.

[0089] Optionally, the vertically polarized light f2 is converted into circularly polarized light by the second quarter-wave plate 504, incident on the rear surface, and after the first reflection, it is switched to horizontally linearly polarized light f1, which is transmitted through the second polarizing beam splitter 502. After the second reflection, it is converted back to circularly polarized light, and after the third quarter-wave plate 505, it is switched to vertically linearly polarized light f2, and so on, with a 4-fold frequency shift after four round trip reflections. The reflected light is refracted 90° by the fourth right-angle prism 506 to ensure that the optical path is closed.

[0090] In this embodiment, the four round-trip reflections of the rear surface form a symmetrical optical path with the front surface, synchronously acquiring the expansion information of the left and right end faces. By decoupling the phase of the dual-optical-path frequency shift signals, the equivalent average thermal expansion is calculated in real time, eliminating the error caused by the temperature gradient at a single measurement point and improving the reliability of the results.

[0091] In one possible embodiment, the first fiber coupler 3 and the second fiber coupler 7 are both polarization-maintaining fiber couplers, used to separate the optical signals of the reference optical path and the measurement optical path to ensure the polarization state consistency of the dual-frequency laser.

[0092] Optionally, both the first fiber coupler 3 and the second fiber coupler 7 are polarization-maintaining fiber couplers with a polarization extinction ratio > 20dB, ensuring the consistency of polarization states between the reference optical path (f1-f2) and the measurement optical path (f1-f2±8Δf). The reference signal is directly provided by the reflected light from the beam splitter 2, and the measurement signal, after being modulated by the pre- and post-beam splitting devices 5, is combined with the reference signal in the second fiber coupler 7 to form a stable interference signal.

[0093] In this embodiment, the polarization-maintaining fiber coupler ensures that the polarization states of the dual-frequency lasers remain orthogonal during transmission, avoids signal attenuation caused by polarization crosstalk, improves the stability of the interference signal, thereby improving the baseline accuracy of thermal expansion calculation and solving the problem of inaccurate baseline in the prior art.

[0094] In one possible embodiment, such as Figure 2 As shown, the measurement reference device 6 also includes a second measurement reference mirror 603 and a second temperature control cavity 604. The second measurement reference mirror 603 has the same structure as the first measurement reference mirror 601 and is used as a standard cavity mirror for spatial position comparison interferometry. The fourth quarter-wave plate 406 realizes four round-trip reflections of the second measurement reference mirror 603 to obtain 4 times Doppler frequency shift information.

[0095] Optionally, a second temperature control cavity 604 is added next to the first temperature control cavity 602, housing a second measurement reference mirror 603 with the same structure as the first measurement reference mirror 601. Vertically polarized light f2, after being modulated by the fourth quarter-wave plate 406, is incident on the front surface of the second measurement reference mirror 603, undergoing four round-trip reflections and resulting in a superimposed Doppler frequency shift of 4 times (±4Δf). By comparing the frequency shift signals of the two reference mirrors, spatial positioning errors are decoupled.

[0096] In this embodiment, the dual reference mirror design provides spatial position comparison. By synchronously measuring with the standard cavity mirror and the measuring cavity mirror, the reference offset caused by uneven temperature field or mechanical vibration in the first temperature control cavity 602 is eliminated, further improving the positioning accuracy of the measurement reference.

[0097] In one possible embodiment, such as Figure 3 As shown, the rear beam splitting device 5 also includes a fifth right-angle prism 507, which is used to replace the inclined reflective prism in part of the reflection path. It achieves optical path reversal through two internal reflections, ensuring that the reflected light is perpendicularly incident on the rear surface of the first measurement reference mirror 601 and undergoes multiple reflections.

[0098] Optionally, in the rear beam splitter 5, the fifth right-angle prism 507 replaces part of the inclined surface reflecting prism. The vertically polarized light f2 is incident on the fifth right-angle prism 507 through the first inclined surface reflecting prism 405. The light path direction is changed through two internal reflections to ensure that it is incident perpendicularly to the rear surface of the first measuring reference mirror 601.

[0099] The vertically polarized light f2 is internally reflected twice by the fifth right-angle prism 507, then reflected sequentially by the second polarizing beam splitter prism 502 and the third right-angle prism 503, and then modulated into circularly polarized light by the second quarter-wave plate 504 and the third quarter-wave plate 505, and then incident vertically on the rear surface of the first measuring reference mirror 601, completing four round-trip reflections.

[0100] The reflected light is reflected by the fourth right-angle prism 506 and the second polarizing beam splitter prism 502 and the second inclined surface reflecting prism 501 of the rear beam splitter 5, and finally enters the second fiber optic coupler 7.

[0101] In this embodiment, the internal reflection design of the right-angle prism reduces energy loss in the optical path, and the fixed-angle reflection path improves the stability of the optical path, avoids the incident angle deviation caused by the installation error of the inclined reflection prism, and further improves the positioning accuracy of the measurement reference.

[0102] A heterodyne laser interferometry method for measuring the coefficient of linear expansion includes the following steps:

[0103] Step 1: Two orthogonally linearly polarized beams with a frequency difference of 1-10MHz are generated by dual-frequency laser 1, and then split into transmitted and reflected beams by beam splitter 2.

[0104] Specifically, the dual-frequency laser 1 outputs orthogonally linearly polarized light with f1 = 474MHz and f2 = 474MHz + 5MHz. The beam splitter 2 has a transmission ratio of 7:3. The transmitted light enters the pre-splitter 4, and the reflected light enters the first fiber coupler 3 as a reference signal (f1-f2 = 5MHz).

[0105] Step 2: Input the transmitted light into the pre-splitter 4. After polarization beam splitting, optical path reflection and polarization modulation, it forms a beam that is incident on the front surface of the first measurement reference mirror 601 four times. Each round trip is superimposed with Doppler frequency shift information, and finally outputs a measurement signal containing 4 times Doppler frequency shift.

[0106] Optionally, after f1 undergoes four round-trip reflections by the pre-splitter 4, the measured signal is f1-f2+4Δf.

[0107] Step 3: The reflected light is input into the rear beam splitter 5. After polarization beam splitting, optical path reflection and polarization state modulation, it is formed to be incident on the rear surface of the first measurement reference mirror 601 four times. Doppler frequency shift information is superimposed once for each round trip, and finally the measurement signal containing 4 times Doppler frequency shift is output.

[0108] Optionally, after f2 undergoes four round-trip reflections by the rear beam splitter 5, the measured signal is f1-f2-4Δf.

[0109] Step 4: Interfere the measurement signals output by the pre-splitter 4 and the post-splitter 5 with the reference signal of the first fiber coupler 3 in the second fiber coupler 7 to obtain the Doppler frequency shift difference signal containing the synchronous and out-of-direction measurement results of the left and right sides.

[0110] Optionally, the two measurement signals and the reference signal interfere in the second fiber coupler 7 to form a beat frequency signal ±8Δf, and the acquisition card 8 acquires the signal at a sampling rate of 1000Hz.

[0111] Step 5: Acquire the Doppler frequency shift signal through acquisition card 8 and transmit it to host computer 9.

[0112] Step 6: The host computer 9 is based on the formula:

[0113] The Doppler signal of the first fiber coupler 3 can be represented as: f1-f2;

[0114] The Doppler signal of the second fiber coupler 7 can be expressed as: f1-f2±8Δf;

[0115] Let λ represent the phase change under multiple reflections of the first measurement reference mirror 601, and let λ represent the incident wavelength of the dual-frequency laser. The length change under thermal expansion can be expressed as:

[0116]

[0117] Where ΔL represents the magnitude of the change in thermal expansion of the first measuring reference mirror 601 under temperature change conditions in the first temperature control cavity 602, the coefficient of thermal expansion can be expressed as:

[0118]

[0119] dl(t) / dt represents the amount of material length deformation before and after the temperature increases by 1℃.

[0120] The host computer 9, based on the phase difference Calculate thermal expansion The coefficient of linear expansion α = ΔL / (L) o ΔT), where L0=100mm, ΔT=10℃.

[0121] In this embodiment, high-precision acquisition of thermal expansion is achieved by synchronously measuring the left and right end faces in opposite directions and superimposing the four-fold frequency shift of four round-trip reflections. Combined with a real-time phase decoupling algorithm, random errors in temperature changes are eliminated, and the average thermal expansion parameter is obtained. The reliability and accuracy of the measurement results are significantly better than those of existing single-optical-path or dual-optical-path interference structures.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications to the technical solutions described in the foregoing embodiments, or equivalent substitutions for some or all of the technical features, do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion, characterized in that, It includes a dual-frequency laser (1), a beam splitter (2), a first fiber optic coupler (3), a pre-splitting device (4), a post-splitting device (5), a measurement reference device (6), a second fiber optic coupler (7), a data acquisition card (8), and a host computer (9); The dual-frequency laser (1) emits two beams of orthogonally linearly polarized light with a frequency difference of 1-10MHz. After being split by a beam splitter (2), the transmitted light is incident on the pre-splitter (4), and the reflected light is incident on the first fiber coupler (3). The pre-splitter (4) performs polarization splitting, optical path reflection and polarization modulation on the transmitted light, and the output light is perpendicularly incident on the front surface of the measuring reference device (6) and reflected multiple times. The rear beam splitter (5) performs polarization beam splitting, optical path reflection and polarization state modulation on the reflected light, and the output light is incident perpendicularly on the rear surface of the measurement reference device (6) and reflected multiple times. After passing through the pre-splitter (4) and the post-splitter (5), the reflected light is incident on the second fiber coupler (7) together with the reference signal of the first fiber coupler (3); The acquisition card (8) acquires the measurement signal of the second fiber coupler (7) and the reference signal of the first fiber coupler (3), and transmits them to the host computer (9); The host computer (9) is used to calculate the thermal expansion and linear expansion coefficient; The pre-splitting device (4) includes a first polarizing beam splitter (401), a first right-angle prism (402), a first quarter-wave plate (403), a second right-angle prism (404), and a first inclined plane reflecting prism (405); The first polarizing beam splitter (401) polarizes and splits the transmitted light. The transmitted horizontally polarized light is transmitted through the first temperature control cavity (602), the second polarizing beam splitter (502) of the rear beam splitter (5), and reflected by the third right-angle prism (503). It then returns to the front beam splitter (4), is transmitted through the first polarizing beam splitter (401), and is reflected multiple times by the first right-angle prism (402), the first quarter-wave plate (403), and the front surface of the measurement reference device (6), and finally enters the second fiber coupler (7). The reflected vertically polarized light passes through the first inclined reflective prism (405) and the rear beam splitter (5), and is reflected multiple times by the rear surface of the measurement reference device (6) before finally entering the second fiber coupler (7). The measurement reference device (6) further includes a second measurement reference mirror (603) and a second temperature control cavity (604). The second measurement reference mirror (603) has the same structure as the first measurement reference mirror (601) and is used as a standard cavity mirror for spatial position comparison interferometry measurement. The second measurement reference mirror (603) is reflected four times back and forth through a fourth quarter-wave plate (406) to obtain 4 times Doppler frequency shift information.

2. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 1, characterized in that, The rear beam splitting device (5) includes a second inclined reflective prism (501), a second polarizing beam splitting prism (502), a third right-angle prism (503), a second quarter-wave plate (504), a third quarter-wave plate (505), and a fourth right-angle prism (506); The reflected light is reflected by the second inclined reflector prism (501), the second polarizing beam splitter prism (502) and the third right-angle prism (503), and then its polarization state is modulated by the second quarter-wave plate (504) and the third quarter-wave plate (505). It is then incident perpendicularly onto the rear surface of the measuring reference device (6) and reflected multiple times. After being reflected by the fourth right-angle prism (506) and the rear beam splitter (5), it is incident onto the second fiber coupler (7).

3. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 1, characterized in that, The measuring reference device (6) includes a first measuring reference mirror (601) and a first temperature control cavity (602); The first measuring reference mirror (601) has planar reflective mirrors on both the front and rear sides, and is placed in the first temperature control cavity (602) to reflect laser light and generate thermal expansion with temperature changes; The first temperature control cavity (602) is used to control the temperature environment of the first measuring reference mirror (601) and provide thermal expansion measurement conditions.

4. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 1, characterized in that, The first quarter-wave plate (403) is disposed between the first right-angle prism (402) and the first measurement reference mirror (601) to convert horizontal linearly polarized light into circularly polarized light, so that the laser is incident perpendicularly on the front surface of the first measurement reference mirror (601) and achieves four round-trip reflections. After each reflection, the polarization state is switched, and finally, 4 times Doppler frequency shift information is superimposed.

5. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 2, characterized in that, The second quarter-wave plate (504) and the third quarter-wave plate (505) of the rear beam splitter (5) are sequentially disposed between the rear surface of the second polarizing beam splitter prism (502) and the first measuring reference mirror (601). They are used to convert vertically linearly polarized light into circularly polarized light and incident on the rear surface. The light is superimposed with 4 times Doppler frequency shift information through four round-trip reflections. The reflected light is folded back through the fourth right-angle prism (506).

6. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 1, characterized in that, The first fiber coupler (3) and the second fiber coupler (7) are both polarization-maintaining fiber couplers, used to separate the optical signals of the reference optical path and the measurement optical path to ensure the polarization state consistency of the dual-frequency laser.

7. The heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion according to claim 1, characterized in that, The rear beam splitter (5) also includes a fifth right-angle prism (507), which replaces the inclined reflective prism in part of the reflection path. It achieves optical path reversal through two internal reflections, ensuring that the reflected light is perpendicularly incident on the rear surface of the first measurement reference mirror (601) and undergoes multiple reflections.

8. A method of using the heterodyne laser interferometry apparatus for measuring the coefficient of linear expansion as described in any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Two orthogonally linearly polarized beams with a frequency difference of 1-10MHz are generated by a dual-frequency laser (1), and then split into transmitted and reflected beams by a beam splitter (2); Step 2: Input the transmitted light into the pre-splitter (4), and after polarization beam splitting, optical path reflection and polarization modulation, it forms four round trips to be incident on the front surface of the first measurement reference mirror (601). Each round trip is superimposed with Doppler frequency shift information, and finally outputs a measurement signal containing 4 times Doppler frequency shift. Step 3: Input the reflected light into the rear beam splitter (5). After polarization beam splitting, optical path reflection and polarization state modulation, it forms four round trips to the rear surface of the first measurement reference mirror (601). Each round trip is superimposed with Doppler frequency shift information, and finally outputs a measurement signal containing 4 times Doppler frequency shift. Step 4: Interfere the measurement signals output by the pre-splitter (4) and the post-splitter (5) with the reference signal of the first fiber coupler (3) in the second fiber coupler (7) to obtain the Doppler frequency shift difference signal containing the synchronous anisotropic measurement results of the left and right sides; Step 5: Acquire the Doppler frequency shift signal through the acquisition card (8) and transmit it to the host computer (9); Step 6: The host computer (9) is based on the formula; The Doppler signal of the first fiber coupler (3) is represented as: f1-f2; The Doppler signal of the second fiber coupler (7) is expressed as: f1-f2±8 f; φ represents the phase change under multiple reflections from the first measurement reference mirror (601), λ represents the incident wavelength of the dual-frequency laser, and the length change under thermal expansion is expressed as: in, L represents the magnitude of the change in thermal expansion of the first measuring reference mirror (601) in the first temperature control cavity (602) under temperature change conditions. The coefficient of thermal expansion is expressed as: dl(t) / dt represents the amount of material length deformation before and after the temperature increases by 1℃.

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