Heterodyne laser interference measuring device and method for measuring linear expansion coefficient

Abstract

The invention discloses a heterodyne laser interference measurement device and method for linear expansion coefficient measurement. The heterodyne laser interference measurement device comprises a dual-frequency laser, a spectroscope, a first optical fiber coupler, a front light splitting device, a rear light splitting device, a measurement reference device, a second optical fiber coupler, an acquisition card and an upper computer, the double-frequency laser emits orthogonal linearly polarized light; the output light of the front light splitting device vertically enters the front surface of the measurement reference device and is reflected for multiple times; the rear output light vertically enters the rear surface of the measurement reference device and is reflected for multiple times; the reflected light passes through the front light splitting device and the rear light splitting device, and then enters a second optical fiber coupler together with a reference signal; the acquisition card acquires a measurement signal of the second optical fiber coupler and a reference signal of the first optical fiber coupler and transmits the measurement signal and the reference signal to the upper computer; and the upper computer calculates the thermal expansion amount and the linear expansion coefficient. The problems that the thermal expansion amount is insufficient in space precision, the reference accuracy base line is inaccurate, and the linear change in the expansion change is uncontrollable can be solved.

Description

Technical Field

[0001] The present application belongs to the field of optical measurement technology, and in particular relates to a heterodyne laser interferometry measurement device and method for measuring linear expansion coefficient. Background Art

[0002] In modern high-end manufacturing and scientific research, the demand for precise measurement of material microscopic properties is increasing. Semiconductor manufacturing pursues higher integration and smaller chip sizes, and precision instruments are also developing towards ultra-precision, making sub-nanometer measurement technology a key support. In these application scenarios, the thermal expansion characteristics of materials have a significant impact on product performance and stability. For example, if the thermal expansion coefficient of lens materials in high-end optical equipment is not well controlled, they will deform when the temperature fluctuates, resulting in a decrease in optical performance. Semiconductor chips generate heat during operation, and the mismatch in thermal expansion of materials will induce internal stress, affecting the chip's lifespan and reliability. Therefore, the development of high-precision thermal expansion coefficient measurement equipment is crucial.

[0003] Thermal expansion coefficient measurement technology has undergone several stages of development. Early fiber optic displacement sensors and push-rod micrometers offered relatively low accuracy, with thermal expansion measurement uncertainties as low as 10⁻⁶ / K, which was insufficient to meet modern high-precision requirements. Technological advancements have led to the widespread use of inductive micrometers and laser interferometers, significantly improving measurement accuracy. For example, the laser interferometer structure developed by Germany's PTB achieves an uncertainty of up to 2 x 10⁻⁶ / K. Currently, laser interferometer-based measurement methods have become mainstream, while emerging measurement techniques such as ultrafast lasers are also emerging.

[0004] Although existing technologies have achieved many results, they still have obvious limitations. In terms of spatial positioning accuracy, sub-nanometer measurements require more precise positioning of the measurement benchmark, and existing measurement devices are difficult to meet this standard. The measurement accuracy of thermal expansion also needs to be further improved. Although some technologies have achieved high accuracy, there is still room for improvement in the face of higher precision requirements, and higher-power signal superposition technology is urgently needed. In addition, in terms of real-time measurement error correction, existing measurement technologies cannot effectively deal with the errors generated when measuring thermal expansion at multiple points and when measuring left and right synchronously. It is difficult to obtain the mean change of the thermal expansion of the material in real time, which affects the reliability of the measurement results and cannot meet the strict requirements of high-end application scenarios for measurement accuracy and stability. Summary of the Invention

[0005] The purpose of the embodiments of the present application is to provide a heterodyne laser interferometry device and method for measuring the linear expansion coefficient, so as to solve the problems existing in the above-mentioned background technology, such as insufficient spatial accuracy of thermal expansion, inaccurate reference accuracy baseline, and uncontrollable linear changes in expansion changes.

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

[0007] A heterodyne laser interferometer measuring device for linear expansion coefficient measurement, comprising a dual-frequency laser, a spectroscope, a first optical fiber coupler, a pre-splitting device, a post-splitting device, a measurement reference device, a second optical fiber coupler, an acquisition card, and a host computer;

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

[0009] After the pre-beam splitting device performs polarization splitting, optical path reflection and polarization modulation on the transmitted light, the output light is vertically incident on the front surface of the measurement reference device and is reflected multiple times;

[0010] The rear-mounted light splitting device performs polarization splitting, optical path reflection, and polarization state modulation on the reflected light, and the output light is vertically incident on the rear surface of the measurement reference device and is reflected multiple times;

[0011] After passing through the pre-splitting device and the post-splitting device, the reflected light and the reference signal of the first optical fiber coupler are incident on the second optical fiber coupler;

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

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

[0014] In a possible implementation, the pre-beam splitting device includes a first polarization beam splitting prism, a first right-angle prism, a first quarter-wave plate, a second right-angle prism, and a first oblique reflecting prism;

[0015] The first polarization beam splitter prism performs polarization beam splitting on the transmitted light. The transmitted horizontally polarized light passes through the first temperature-controlled cavity, the second polarization beam splitter prism of the post-beam splitting device, and is reflected by the third right-angle prism. The light then returns to the pre-beam splitting device, passes through the first polarization beam splitter prism, 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 oblique reflecting prism and the rear light splitting device, and is reflected multiple times by the rear surface of the measurement reference device, and finally enters the second optical fiber coupler.

[0017] In a possible implementation, the post-beam splitting device includes a second oblique reflecting prism, a second polarization beam splitting prism, a third right-angle prism, a second quarter-wave plate, a third quarter-wave plate, and a fourth right-angle prism;

[0018] After being reflected by the second oblique reflecting prism, the second polarization splitting prism and the third right-angle prism, the reflected light is polarized by the second quarter-wave plate and the third quarter-wave plate, vertically incident on the rear surface of the measurement reference device and reflected multiple times, and after being reflected by the fourth right-angle prism and the rear splitting device, is incident on the second optical fiber coupler.

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

[0020] The front and rear sides of the first measurement reference mirror are both plane reflective mirrors, which are placed in the first temperature-controlled cavity and are used to reflect laser light and generate thermal expansion with temperature changes;

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

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

[0023] In one possible implementation, the second quarter-wave plate and the third quarter-wave plate of the post-beam splitting device are sequentially arranged between the second polarization splitting prism and the rear surface of the first measurement reference mirror, and are used to convert vertical linear polarized light into circularly polarized light that is incident on the rear surface, and superimpose 4 times the Doppler frequency shift information through four round-trip reflections, and the reflected light is returned through the fourth right-angle prism.

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

[0025] In one possible implementation, the measurement reference device also includes a second measurement reference mirror and a second temperature-controlled cavity. The second measurement reference mirror has the same structure as the first measurement reference mirror and is used as a standard cavity mirror to perform spatial position comparison interference measurement. Four round-trip reflections of the second measurement reference mirror are achieved through a fourth quarter-wave plate to obtain 4 times Doppler frequency shift information.

[0026] In one possible implementation, the rear spectrometer further includes a fifth right-angle prism, which is used to replace the oblique reflection prism in the partial reflection path, realize light path steering through two internal reflections, and ensure vertical incidence and multiple reflections of the reflected light on the rear surface of the first measurement reference mirror.

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

[0028] Step 1: Generate two orthogonal linearly polarized beams with a frequency difference of 1-10 MHz through a dual-frequency laser, and split them into transmitted light and reflected light through a beam splitter;

[0029] Step 2: The transmitted light is input into the pre-splitting device. After polarization splitting, optical path reflection and polarization modulation, it is incident on the front surface of the first measurement reference mirror four times. The Doppler shift information is superimposed once each round trip, and the final output is a measurement signal containing a 4-fold Doppler shift;

[0030] Step 3: The reflected light is input into the post-splitting device. After polarization splitting, optical path reflection, and polarization state modulation, it is incident on the rear surface of the first measurement reference mirror four times. The Doppler shift information is superimposed once each round trip, and the final output is a measurement signal containing a 4-fold Doppler shift.

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

[0032] Step 5: Collect the Doppler frequency shift difference signal through an acquisition card and transmit it to a host computer;

[0033] Step 6: The host computer is based on the formula:

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

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

[0036] represents the phase change under multiple first-measurement reference mirror reflection states, λ represents the incident wavelength of the dual-frequency laser, and the length change under thermal expansion can be expressed as:

[0037]

[0038] Wherein, ΔL represents the change in thermal expansion of the first measurement reference mirror when the temperature of the first temperature-controlled chamber changes. The thermal expansion coefficient can be expressed as:

[0039]

[0040] dl(t) / dt represents the length deformation of the material before and after the temperature rises by 1°C.

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

[0042] The present application provides a heterodyne laser interferometer measurement device for linear expansion coefficient measurement. By utilizing the Doppler superposition characteristics of a dual-frequency laser, Doppler frequency shift information is superimposed on each laser reflection. Combined with synchronous anisotropic measurement of the left and right end faces, the equivalent average value of the thermal expansion amount is obtained in real time, significantly improving measurement accuracy and solving the problems of insufficient spatial positioning accuracy and low thermal expansion measurement accuracy in the prior art.

[0043] In one possible implementation, the horizontally polarized light is reflected four times on the front surface of the first measurement reference mirror through polarization splitting by the first polarization splitter prism and optical path reflection by the right-angle prism. The 4-fold Doppler frequency shift information is superimposed. Combined with the multi-incident base point design, the thermal expansion coefficient material is precisely positioned in space, thereby improving the measurement accuracy.

[0044] In one possible implementation, polarization modulation of the second and third quarter-wave plates is used to achieve four round-trip reflections of vertically polarized light on the rear surface, and 4x Doppler frequency shift information is synchronously superimposed. Synchronous measurement of the left and right end faces can obtain an anisotropic length measurement benchmark. Mathematical operations can eliminate the influence of temperature unevenness and improve 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 parameters are obtained through real-time phase decoupling, reducing the interference of environmental factors such as temperature and air pressure vibration on the measurement results.

[0046] In one possible implementation, the polarization state of a quarter-wave plate is switched to achieve multiple round-trip reflections of single-polarized light on the same surface, avoiding interference from overlapping optical paths. At the same time, the linear measurement range is expanded by 4x frequency shift superposition, and the measurement accuracy is improved to the sub-nanometer level, which is superior to the dual-path Michelson structure in existing technologies.

[0047] In one possible implementation, four round-trip reflections from the rear surface form a symmetrical optical path with the front surface, synchronously acquiring expansion information for the left and right end faces. Through phase decoupling of the dual-path frequency-shifted signals, the equivalent average thermal expansion is calculated in real time, eliminating errors caused by temperature gradients 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 and improving the stability of the interference signal, thereby enhancing the baseline accuracy of thermal expansion calculations and solving the problem of inaccurate baselines in existing technologies.

[0049] In one possible implementation, a dual reference mirror design provides spatial position comparison. Through synchronous measurement of the standard cavity mirror and the measurement cavity mirror, the reference offset caused by the 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 through a fixed-angle reflection path, improves the stability of the optical path, avoids the incident angle deviation caused by installation errors of the bevel reflection prism, and further improves the positioning accuracy of the measurement benchmark.

[0051] A heterodyne laser interferometry method for measuring the linear expansion coefficient achieves high-precision acquisition of thermal expansion through simultaneous anisotropic measurement of the left and right end faces and the superposition of a 4x frequency shift from four round-trip reflections. Combined with a real-time phase decoupling algorithm, it eliminates random errors in temperature changes and obtains the average thermal expansion parameter. The reliability and accuracy of the measurement results are significantly better than those of existing single- or dual-path interference structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a schematic diagram of the overall structure of a heterodyne laser interferometer measurement device for linear expansion coefficient measurement provided by the present application;

[0053] Figure 2 This is a schematic diagram of the overall structure of another heterodyne laser interferometer measurement device for linear expansion coefficient measurement provided by the present application;

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

[0055] 1. Dual-frequency laser; 2. Spectrum; 3. First fiber coupler; 4. Pre-beam splitting device; 401. First polarization beam splitting prism; 402. First right-angle prism; 403. First quarter-wave plate; 404. Second right-angle prism; 405. First bevel reflecting prism; 406. Fourth quarter-wave plate; 5. Post-beam splitting device; 501. Second bevel reflecting prism; 502. Second polarization beam splitting 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 chamber; 603. Second measurement reference mirror; 604. Second temperature control chamber; 7. Second fiber coupler; 8. Acquisition card; 9. Host computer. DETAILED DESCRIPTION

[0056] Hereinafter, only certain exemplary embodiments are briefly described. As will be appreciated by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit or scope. Therefore, the drawings and description are to be regarded as illustrative in nature and not restrictive.

[0057] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0058] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly indicate the number of the technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of such features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise specifically defined. The specific embodiments of this application are further described in detail below with reference to the accompanying drawings.

[0059] In this application, unless otherwise expressly specified or limited, terms such as "installed," "connected," "connect," and "fixed" should be understood in a broad sense. For example, they may refer to fixed connection, detachable connection, or integration; mechanical connection, electrical connection, or communication; direct connection or indirect connection through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0060] In this application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Moreover, a first feature being "above," "above," and "above" a second feature includes the first feature being directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature includes the first feature being directly above and obliquely above the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0061] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0062] like Figure 1 As shown, the present application provides a heterodyne laser interferometer measurement device for measuring linear expansion coefficient, which may include a dual-frequency laser 1, a spectroscope 2, a first optical fiber coupler 3, a pre-splitting device 4, a post-splitting device 5, a measurement reference device 6, a second optical fiber coupler 7, an acquisition card 8 and a host computer 9.

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

[0064] Optionally, the dual-frequency laser 1 uses a helium-neon laser with a frequency difference of 5 MHz, emitting orthogonal linearly polarized light (horizontally polarized light f1, vertically polarized light 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-beam splitting device 4, and the reflected light directly enters the first fiber coupler 3 as a reference signal.

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

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

[0067] After passing through the pre-splitting device 4 and the post-splitting device 5 , the reflected light and the reference signal of the first optical fiber coupler 3 are incident on the second optical fiber coupler 7 .

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

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

[0070] The acquisition card 8 collects the measurement signal of the second optical fiber coupler 7 and the reference signal of the first optical fiber coupler 3, and transmits them to the host computer 9. The acquisition card 8 synchronously collects the signals, and the host computer 9 calculates the thermal expansion and linear expansion coefficient through the formula.

[0071] In the embodiment of the present application, through the Doppler superposition characteristics of the dual-frequency laser, the Doppler frequency shift information is superimposed on each laser reflection. Combined with the synchronous anisotropic measurement of the left and right end faces, the equivalent average value of the thermal expansion amount 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 a possible embodiment, the pre-beam splitting device 4 may include a first polarization beam splitting prism 401 , a first right-angle prism 402 , a first quarter-wave plate 403 , a second right-angle prism 404 and a first oblique reflecting prism 405 .

[0073] The first polarization beam splitter prism 401 performs polarization beam splitting on the transmitted light. The transmitted horizontally polarized light passes through the first temperature control cavity 602, the second polarization beam splitter prism 502 of the post-beam splitting device 5, and is reflected by the third right-angle prism 503. The light then returns to the pre-beam splitting device 4, 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 is finally incident on the second fiber coupler 7.

[0074] The reflected vertically polarized light passes through the first oblique reflecting prism 405 and the post-beam splitting device 5 , and is reflected multiple times by the rear surface of the measurement reference device 6 , and finally enters the second optical fiber coupler 7 .

[0075] In the embodiment of the present application, through the polarization splitting of the first polarization splitter prism 401 and the light path reflection of the right-angle prism, four round-trip reflections of the horizontally polarized light are achieved on the front surface of the first measurement reference mirror 601, and the 4-fold Doppler frequency shift information is superimposed. Combined with the multi-incident base point design, the thermal expansion coefficient material is accurately positioned in space, thereby improving the accuracy of the measurement.

[0076] In a possible embodiment, the post-beam splitting device 5 includes a second oblique reflecting prism 501 , a second polarization 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 .

[0077] After being reflected by the second bevel reflecting prism 501, the second polarization splitting prism 502 and the third right-angle prism 503, the reflected light is polarized by the second quarter-wave plate 504 and the third quarter-wave plate 505, vertically incident on the rear surface of the measurement reference device 6 and reflected multiple times, and after being reflected by the fourth right-angle prism 506 and the post-split device 5, it is incident on the second fiber coupler 7.

[0078] Specifically, the second oblique reflecting prism 501 of the post-beam splitting device 5 receives vertically polarized light f2 from the pre-beam splitting device 4. The light is then reflected by the second polarization beam splitting prism 502 to the third right-angle prism 503. After two internal reflections, the light returns to the second polarization beam splitting prism 502. The light is then converted into circularly polarized light by the second quarter-wave plate 504 and perpendicularly incident on the rear surface of the first measurement reference mirror 601. After four round-trip reflections, the reflected light is then returned by the fourth right-angle prism 506, has its polarization state switched by the third quarter-wave plate 505, and finally enters the second fiber coupler 7.

[0079] In an embodiment of the present application, polarization modulation of the second quarter-wave plate 504 and the third quarter-wave plate 505 is used to achieve four round-trip reflections of vertically polarized light on the rear surface, and synchronous superposition of 4-fold Doppler frequency shift information. The synchronous measurement of the left and right end faces can obtain anisotropic length measurement benchmarks, and mathematical operations are used to eliminate the influence of temperature unevenness, thereby improving measurement reliability.

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

[0081] The first measurement reference mirror 601 has plane reflective mirror surfaces on both the front and rear sides and is placed in the first temperature control cavity 602 for reflecting laser light and generating thermal expansion with temperature changes.

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

[0083] Specifically, the first measurement reference mirror 601 is made of fused quartz with a low thermal expansion coefficient, with front and rear surface flatness better than λ / 20. It is placed within the first temperature-controlled chamber 602 and has a temperature control accuracy of ±0.01°C. When the temperature of the first temperature-controlled chamber 602 changes, the first measurement reference mirror 601 undergoes linear expansion or contraction, and the optical path difference between the front and rear surfaces of the mirror changes accordingly. This change is reflected in the Doppler frequency shift signal.

[0084] In an embodiment of the present application, the first temperature-controlled 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 parameters are obtained through real-time phase decoupling, reducing the interference of environmental factors such as temperature and air pressure vibration on the measurement results.

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

[0086] Specifically, the first quarter-wave plate 403 is positioned between the first right-angle prism 402 and the first measurement reference mirror 601. Horizontally polarized light f1 is converted to right-handed circularly polarized light after passing through the wave plate and is incident perpendicularly on the front surface. After the first reflection, the circularly polarized light is converted to left-handed circularly polarized light, which is then reflected by the first polarization beam splitter 401 as vertically linearly polarized light f2. After the second reflection, it is again converted to right-handed circularly polarized light, which is then transmitted through the prism as horizontally linearly polarized light f1. This process repeats four times, completing a round-trip reflection cycle. Ultimately, a four-fold Doppler shift (±4Δf) is generated by the forward and reverse superposition of the two reflections.

[0087] In an embodiment of the present application, by switching the polarization state of a quarter-wave plate, multiple round-trip reflections of single polarized light on the same surface are achieved, avoiding interference from overlapping optical paths. At the same time, the linear measurement range is expanded by superposition of a 4-fold frequency shift, and the measurement accuracy is improved to the sub-nanometer level, which is superior to the dual-path Michelson structure in the prior art.

[0088] In a possible embodiment, the second quarter-wave plate 504 and the third quarter-wave plate 505 of the rear spectroscopic device 5 are sequentially arranged between the second polarization splitting prism 502 and the rear surface of the first measurement reference mirror 601, and are used to convert vertical linear polarized light into circularly polarized light that is incident on the rear surface, and superimpose 4 times the Doppler frequency shift information through four round-trip reflections, and the reflected light is returned by the fourth right-angle prism 506.

[0089] Optionally, the vertically polarized light f2 is converted to circularly polarized light by the second quarter-wave plate 504, incident on the rear surface, and switched to horizontally polarized light f1 after the first reflection. This light is then transmitted through the second polarization beam splitter 502. After the second reflection, it is converted to circularly polarized light, which is then switched to vertically polarized light f2 by the third quarter-wave plate 505. This process continues in this manner, with four round-trip reflections resulting in a four-fold frequency shift. The reflected light is then folded back 90° by the fourth right-angle prism 506 to ensure a closed optical path.

[0090] In an embodiment of the present application, the four round-trip reflections of the rear surface form a symmetrical optical path with the front surface, and the expansion information of the left and right end faces is obtained synchronously. Through the phase decoupling of the dual-optical path frequency shift signal, 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 a possible embodiment, the first fiber coupler 3 and the second fiber coupler 7 are both polarization-maintaining fiber couplers, which are used to separate the optical signals of the reference optical path and the measurement optical path to ensure the polarization consistency of the dual-frequency laser.

[0092] Optionally, both the first fiber coupler 3 and the second fiber coupler 7 utilize polarization-maintaining fiber couplers with a polarization extinction ratio greater than 20 dB, ensuring polarization consistency 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. The measurement signal is modulated by the pre- and post-beam splitting device 5 and then combined with the reference signal in the second fiber coupler 7 to form a stable interference signal.

[0093] In the embodiment of the present application, the 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 improving the benchmark accuracy of the thermal expansion calculation and solving the problem of inaccurate baseline in the prior art.

[0094] In one possible embodiment, 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 to perform spatial position comparison interference measurement. The fourth quarter-wave plate 406 is used to realize four round-trip reflections of the second measurement reference mirror 603 to obtain 4 times Doppler frequency shift information.

[0095] Optionally, a second temperature-controlled chamber 604 is added adjacent to the first temperature-controlled chamber 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 superimposing a 4x Doppler frequency shift (±4Δf). By comparing the frequency shift signals of the two reference mirrors, spatial positioning errors are decoupled.

[0096] In the embodiment of the present application, the dual reference mirror design provides spatial position comparison. Through the synchronous measurement of the standard cavity mirror and the measurement cavity mirror, the reference offset caused by the uneven temperature field or mechanical vibration in the first temperature control cavity 602 is eliminated, and the positioning accuracy of the measurement reference is further improved.

[0097] In one possible embodiment, Figure 3 As shown, the rear spectrometer 5 further includes a fifth right-angle prism 507, which is used to replace the oblique reflection prism in the partial reflection path, realizes light path steering through two internal reflections, and ensures vertical incidence and multiple reflections of the reflected light on the rear surface of the first measurement reference mirror 601.

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

[0099] After two internal reflections by the fifth right-angle prism 507, the vertically polarized light f2 is reflected in sequence by the second polarization 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. The light is vertically incident on the rear surface of the first measurement reference mirror 601, completing four round-trip reflections.

[0100] The reflected light is reflected by the fourth right-angle prism 506 and the second polarization beam splitting prism 502 and the second oblique reflecting prism 501 of the post-beam splitting device 5 , and finally enters the second optical fiber coupler 7 .

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

[0102] A heterodyne laser interferometry method for linear expansion coefficient measurement comprises the following steps:

[0103] Step 1: Generate two orthogonal linearly polarized light beams with a frequency difference of 1-10 MHz through a dual-frequency laser 1, and split them into transmitted light and reflected light through a beam splitter 2.

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

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

[0106] Optionally, after f1 is reflected back and forth four times by the pre-beam splitting device 4, the measured signal is f1-f2+4Δf.

[0107] Step 3: The reflected light is input into the post-splitting device 5. After polarization splitting, optical path reflection and polarization state modulation, it forms four round trips incident on the rear surface of the first measurement reference mirror 601. Each round trip is superimposed with Doppler frequency shift information, and finally a measurement signal containing 4 times the Doppler frequency shift is output.

[0108] Optionally, after f2 is reflected back and forth four times by the post-beam splitting device 5, the measured signal is f1-f2-4Δf.

[0109] Step 4: The measurement signals output by the pre-splitting device 4 and the post-splitting device 5 are interfered with the reference signal of the first optical fiber coupler 3 in the second optical fiber coupler 7 to obtain a Doppler frequency shift difference signal containing synchronous anisotropic measurement results on the left and right sides.

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

[0111] Step 5: The Doppler frequency shift difference signal is collected by the acquisition card 8 and transmitted to the host computer 9.

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

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

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

[0115] represents the phase change of the first measurement reference mirror 601 under the reflection state for multiple times, λ represents the incident wavelength of the dual-frequency laser, and the length change under the thermal expansion state can be expressed as:

[0116]

[0117] Wherein, ΔL represents the change in thermal expansion of the first measurement reference mirror 601 when the temperature of the first temperature control chamber 602 changes. The thermal expansion coefficient can be expressed as:

[0118]

[0119] dl(t) / dt represents the length deformation of the material before and after the temperature rises by 1°C.

[0120] The host computer 9 is based on the phase difference Calculating thermal expansion Linear expansion coefficient α=ΔL / (L o ΔT), where L0 = 100 mm, ΔT = 10°C.

[0121] In this embodiment, high-precision acquisition of thermal expansion is achieved through simultaneous anisotropic measurement of the left and right end faces and the superposition of 4 times 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 parameters are obtained. The reliability and accuracy of the measurement results are significantly better than those of existing single- or dual-path interference structures.

[0122] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that modifications to the technical solutions described in the aforementioned embodiments, or equivalent replacement of some or all of the technical features therein, do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A heterodyne laser interferometer measuring device for linear expansion coefficient measurement, characterized in that: It comprises a dual-frequency laser (1), a spectroscope (2), a first optical fiber coupler (3), a pre-splitting device (4), a post-splitting device (5), a measurement reference device (6), a second optical fiber coupler (7), an acquisition card (8) and a host computer (9); The dual-frequency laser (1) emits two beams of orthogonal linear polarized light with a frequency difference of 1-10 MHz. After being split by the beam splitter (2), the transmitted light is incident on the pre-beam splitting device (4), and the reflected light is incident on the first optical fiber coupler (3). After the pre-beam splitting device (4) performs polarization splitting, light path reflection, and polarization modulation on the transmitted light, the output light is vertically incident on the front surface of the measurement reference device (6) and is reflected multiple times; The rear-mounted light splitting device (5) performs polarization splitting, light path reflection, and polarization state modulation on the reflected light, and the output light is vertically incident on the rear surface of the measurement reference device (6) and is reflected multiple times; After passing through the pre-splitting device (4) and the post-splitting device (5), the reflected light and the reference signal of the first optical fiber coupler (3) are incident on the second optical fiber coupler (7); The acquisition card (8) acquires the measurement signal of the second optical fiber coupler (7) and the reference signal of the first optical fiber coupler (3), and transmits them to the host computer (9); The host computer (9) is used to calculate the thermal expansion amount and the linear expansion coefficient.

2. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 1, characterized in that: The front-end light splitting device (4) comprises a first polarization light splitting prism (401), a first right-angle prism (402), a first quarter-wave plate (403), a second right-angle prism (404), and a first oblique reflection prism (405); The first polarization beam splitter prism (401) performs polarization beam splitting on the transmitted light. The transmitted horizontally polarized light is transmitted through the first temperature control cavity (602), the second polarization beam splitter prism (502) of the rear beam splitter device (5), and is reflected by the third right-angle prism (503). The light then returns to the front beam splitter device (4), is transmitted through the first polarization beam splitter prism (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), before finally being incident on the second optical fiber coupler (7). The reflected vertically polarized light passes through the first oblique reflecting prism (405) and the rear light splitting device (5), and is reflected multiple times by the rear surface of the measurement reference device (6), and finally enters the second optical fiber coupler (7).

3. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 1, characterized in that: The post-beam splitting device (5) comprises a second oblique reflection prism (501), a second polarization 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 oblique reflecting prism (501), the second polarization splitting prism (502) and the third right-angle prism (503), and then modulated in polarization state by the second quarter-wave plate (504) and the third quarter-wave plate (505). The reflected light is vertically incident on the rear surface of the measurement reference device (6) and is reflected multiple times. After being reflected by the fourth right-angle prism (506) and the rear-placed splitting device (5), the reflected light is incident on the second optical fiber coupler (7).

4. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 1, characterized in that: The measurement reference device (6) comprises a first measurement reference mirror (601) and a first temperature control chamber (602); The first measurement reference mirror (601) has plane reflective mirror surfaces on both its front and rear sides, and is placed in the first temperature control cavity (602) for reflecting laser light and generating thermal expansion as the temperature changes; The first temperature control chamber (602) is used to control the temperature environment of the first measurement reference mirror (601) to provide thermal expansion measurement conditions.

5. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 2, characterized in that: The first quarter-wave plate (403) is arranged between the first right-angle prism (402) and the first measurement reference mirror (601), and is used to convert horizontal linear polarized light into circularly polarized light, so that the laser is vertically incident on the front surface of the first measurement reference mirror (601) and realizes four round-trip reflections. After each reflection, the polarization state is switched, and finally 4 times Doppler frequency shift information is superimposed.

6. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 3, characterized in that: The second quarter-wave plate (504) and the third quarter-wave plate (505) of the rear-mounted light splitting device (5) are sequentially arranged between the second polarization beam splitting prism (502) and the rear surface of the first measurement reference mirror (601), and are used to convert vertical linear polarized light into circularly polarized light incident on the rear surface, and to superimpose 4 times Doppler frequency shift information through four round-trip reflections, and the reflected light is returned through the fourth right-angle prism (506).

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

8. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 1, characterized in that: The measurement reference device (6) further comprises a second measurement reference mirror (603) and a second temperature-controlled 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 to perform spatial position comparison interference measurement. The fourth quarter-wave plate (406) is used to realize four round-trip reflections on the second measurement reference mirror (603) to obtain 4-fold Doppler frequency shift information.

9. The heterodyne laser interferometer measuring device for linear expansion coefficient measurement according to claim 1, characterized in that: The rear-mounted light splitting device (5) further includes a fifth right-angle prism (507) for replacing the oblique reflection prism in the partial reflection path, achieving light path steering through two internal reflections, and ensuring vertical incidence and multiple reflections of the reflected light on the rear surface of the first measurement reference mirror (601).

10. A heterodyne laser interferometry method for linear expansion coefficient measurement according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: Generate two orthogonal linearly polarized light beams with a frequency difference of 1-10 MHz by a dual-frequency laser (1), and split them into transmitted light and reflected light by a beam splitter (2); Step 2: The transmitted light is input into the pre-splitting device (4), and after polarization splitting, light path reflection and polarization modulation, it is incident on the front surface of the first measurement reference mirror (601) four times, and Doppler frequency shift information is superimposed once each time, and finally a measurement signal containing 4 times the Doppler frequency shift is output; Step 3: inputting the reflected light into the post-beam splitting device (5), and after polarization splitting, light path reflection and polarization state modulation, the reflected light is incident on the rear surface of the first measurement reference mirror (601) four times, and each round trip is superimposed with Doppler frequency shift information, and finally outputting a measurement signal containing 4 times the Doppler frequency shift; Step 4: interfering the measurement signals output by the front optical splitter (4) and the rear optical splitter (5) with the reference signal of the first optical fiber coupler (3) in the second optical fiber coupler (7) to obtain a Doppler frequency shift difference signal containing synchronous anisotropic measurement results on the left and right sides; Step 5: collecting the Doppler frequency shift difference signal through an acquisition card (8) and transmitting it to a host computer (9); Step 6: The host computer (9) is based on the formula; The Doppler signal of the first optical fiber coupler (3) can be expressed as: f1-f2; The Doppler signal of the second optical fiber coupler (7) can be expressed as: f1-f2±8Δf; represents the phase change of the first measurement reference mirror (601) in the reflection state for multiple times, λ represents the incident wavelength of the dual-frequency laser, and the length change in the thermal expansion state can be expressed as: Wherein, ΔL represents the change in thermal expansion of the first measurement reference mirror (601) in the first temperature control chamber (602) under a temperature change state, and the thermal expansion coefficient can be expressed as: dl(t) / dt represents the length deformation of the material before and after the temperature rises by 1°C.

Images