Common-mode denoising ultra-high-precision small-angle measurement reference device capable of realizing continuous measurement

Abstract

The invention provides a common-mode de-noising ultra-high-precision small-angle measurement reference device capable of realizing continuous measurement, which comprises a first light path mechanism, a second light path mechanism, a partial reflection mirror (5) and a beat frequency measurement module, and is characterized in that a second medium pool (14) filled with a second medium is arranged on the emergent light end surface of a laser generation device; the narrow-band interference sheet and the sheet-shaped cavity are both located in the second medium. According to the common-mode denoising ultra-high-precision small-angle measurement reference device, two sets of angle measurement devices are adopted for angle measurement, and angle change is converted into change of frequency difference of laser output by the two sets of devices. A part of common-mode noise is effectively eliminated while the measurement range of the detector is not exceeded, so that the angle measurement limit reaches 0.00001 ''magnitude, and the device can be applied to MEMS, precise instruments and optical systems to meet the application requirements of such high-precision equipment.

Description

Technical Field

[0001] This invention relates to a small-angle measurement reference device, and particularly to a common-mode noise reduction ultra-high precision small-angle measurement reference device that enables continuous measurement. Background Technology

[0002] Improving the accuracy of small-angle measurements is crucial in many technological fields, including scientific research, engineering, manufacturing, and navigation. In scientific experiments and research, enhancing the accuracy of small-angle measurements helps scientists more accurately observe and measure minute angular changes, thereby improving the reliability and accuracy of experiments.

[0003] In engineering fields such as micro-mechanical systems (MEMS), precision instruments, and optical systems, improving the accuracy of small-angle measurements can enhance the performance and stability of these systems. Currently, the most commonly used angle measurements are based on the principle of laser interferometry (e.g., the technical solutions disclosed in Chinese invention patents CN 104330054A, CN 1963384A, CN 102384730 B, and CN 102506768 B), which converts changes in angle into changes in laser interference fringes. However, this method achieves relatively low accuracy in small-angle measurements and cannot meet the demands for high-precision metrology. According to existing technology, the highest accuracy achievable with current laser small-angle reference devices is 0.001″. With the development of technology, applications such as lithography machines, lidar, precision robots, and virtual reality are increasingly demanding high-precision measurements of small angles. Further improving the accuracy of small-angle measurements has become a requirement for these high-precision technological applications. Summary of the Invention

[0004] The purpose of this invention is to improve the accuracy of small-angle measurements while meeting measurement requirements. Based on converting angle measurement into laser frequency measurement, a common-mode noise reduction ultra-high precision small-angle measurement reference device for continuous measurement is proposed.

[0005] In their preliminary research, the inventors designed an ultra-high precision small-angle measurement reference device for continuous measurement. This device transforms angle measurement into laser frequency measurement using a narrowband interferometer laser, and achieves continuous measurement by incorporating a sheet-like cavity in the optical path. Specifically, as shown... Figure 1As shown, the small-angle measurement reference device includes a laser generator, a reflector 108, and a beat frequency measurement module arranged sequentially on the optical path. The laser generator's output light end face is provided with a second medium pool 102 filled with a second medium. The front and rear walls of the second medium pool 102 are transparent and flat, perpendicular to the optical path. The refractive index of the second medium is greater than that of vacuum. A narrow-band interferometer 103 and a sheet-like cavity 104 are fixed parallel to each other on a high-precision horizontal rotating stage 105. The high-precision horizontal rotating stage 105 is inverted above the second medium pool 102 (i.e., the stage surface faces the second medium pool), and in the horizontal projection, the narrow-band interferometer 103 and the sheet-like cavity 104 are each equidistant from the axis of the rotating stage. A partial reflector is provided on the outer side of the rear wall of the second medium pool 102. 107. Narrowband interferometer 103, sheet cavity 104, and partial reflector 107 are all located in the optical path. The light output from the laser generator enters the second dielectric cell 102 from the front wall, and after passing through the narrowband interferometer 103, the out-of-band light is filtered out to obtain narrowband light. The narrowband light leaves the second dielectric cell 102 from the rear wall after passing through the sheet cavity 104, and returns to the laser generator after being reflected by the partial reflector 107. When the oscillation in the cavity reaches the threshold, the laser to be measured is output by the reflector 108. The laser to be measured is input to the beat frequency measurement module, which includes a first half-wave plate 109 disposed in the output reflected light direction of the reflector 108, a polarizing beam splitter 112, a broadband detector 113 disposed in the output light direction of the polarizing beam splitter 112, and a frequency counter 114. The laser under test is completely reflected after passing through the first half-wave plate 109 and entering the polarizing beam splitter 112. The comb teeth emitted by the optical comb system 110 pass through the second half-wave plate 111 and enter the polarizing beam splitter 112. The comb teeth and the laser under test are combined and reach the broadband detector 113. The broadband detector 113 is connected to the frequency counter 114, and the beat frequency is obtained from the frequency counter 114. When the high-precision horizontal rotary stage 105 rotates, causing a change in the beat frequency, the change in beat frequency Δf is obtained by Δf = f0 - f1. Where f0 is the frequency of the laser under test when the high-precision horizontal rotary stage is in its initial position, and f1 is the frequency of the laser under test after the high-precision horizontal rotary stage has rotated.

[0006] According to the formula f=cλ -1 Then df = -cλ -2 dλ, where f is the frequency, c is the speed of light, and λ is the wavelength, and the rotation angle of the interferometer has a linear relationship with the transmission wavelength K nm / degree, it is known that the rotation angle of the interferometer satisfies the formula:

[0007]

[0008] Where K is the value of the thickness D of the plate cavity when it satisfies the rotation angle of the interference plate. When the rotation angle θ of the interferometer plate is equal to that of the plate cavity and the wavelength changes caused by both are approximately equal, the rotation angle of the interferometer plate... With respect to the slope of the θ′ curve:

[0009]

[0010] However, the frequency fluctuation of the laser under free-running conditions reaches tens of kHz, which affects the resolution of angle measurements. Furthermore, comparing the beat frequency of a single laser wavelength standard exceeds the frequency measurement range of the detector, failing to meet measurement requirements. Therefore, further improvements are needed to the structure of the small-angle measuring device and its measurement method.

[0011] The present invention builds upon previous research by adding an angle measurement device to form a common-mode noise reduction device. Utilizing two angle measurement devices with sheet-like cavities of different thicknesses, the aim is to synchronize the cavity mode changes and interference frequency selection caused by rotating the sheet-like cavities in both devices. By comparing the beat frequencies of the two angle measurement devices, the frequency difference between the two devices is obtained while eliminating common-mode noise, and the output frequency difference is controlled within 100 GHz. This ensures that angle measurement is achieved without exceeding the detector's frequency measurement range. This addresses the issues of large frequency fluctuations and output frequency ranges exceeding the detector's measurement range encountered in previous research under free-running conditions, achieving ultra-high precision continuous measurement of small angles.

[0012] Therefore, this invention provides a common-mode noise reduction ultra-high precision small-angle measurement reference device for continuous measurement. The device includes a first optical path mechanism and a second optical path mechanism, a partial reflector 5, and a beat frequency measurement module. The first optical path mechanism includes a first laser generator, and a high-precision horizontal rotating stage 7 is arranged on the output optical path of the first laser generator.

[0013] The interference plate 3 and the first plate-shaped cavity 4 are fixed in parallel under the rotating table;

[0014] The second optical path mechanism includes a second laser generating device and a second narrowband interferometer 33 and a second sheet-like cavity 44 located on the output optical path of the second laser generating device. The second sheet-like cavity 44 is fixed below the first sheet-like cavity 4 and their front surfaces are on the same plane. The second narrowband interferometer 33 is fixed below the first narrowband interferometer 3 and is on the same plane.

[0015] The laser generator has a second medium pool 14 filled with a second medium at the output light end face. The front and rear walls of the second medium pool 14 are transparent and flat, perpendicular to the light path. The refractive index of the second medium is greater than that of vacuum. The first narrow-band interference plate 3, the first sheet-like cavity 4, the second narrow-band interference plate 33, and the second sheet-like cavity 44 are all located in the second medium. In horizontal projection, the distances of the first narrow-band interference plate 3, the first sheet-like cavity 4, the second narrow-band interference plate 33, and the second sheet-like cavity 44 from the axis of the rotating stage are equal. Partial reflectors 5 are provided on the outer side of the rear wall of the second medium pool 14. The partial reflectors 5 are all located on the optical paths of the first and second optical paths.

[0016] The initial wavelengths of the first laser generator and the second laser generator are equal, but the thicknesses of the first sheet cavity 4 and the second sheet cavity 44 are not equal.

[0017] In the first and second optical path mechanisms, the light output from each laser generator enters the second dielectric cell, passes through a narrowband interferometer, and is filtered to remove out-of-band light, resulting in narrowband light. The narrowband light then returns to the laser generator after passing through a sheet-like cavity and a partial reflector 5. When the oscillation within the cavity reaches a threshold, the partial reflector 5 outputs the first and second laser beams under test, respectively. The two laser beams under test are input to the beat frequency measurement module, which performs beat frequency measurement on the two laser beams. When the frequencies of the first and second laser beams under test change by Δf1 and Δf2, respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

[0018] The following example uses the first optical path mechanism, combined with... Figure 2-3 Its working principle is explained in detail below:

[0019] like Figure 2 As shown in (a), when the plate-shaped cavity in the first optical path mechanism rotates by 0° (the incident light is perpendicular to the plate-shaped cavity, and the incident angle is 0°): Cavity length When the sheet-like cavity rotates by θ, the cavity length Therefore, when the angle of the sheet-like cavity rotates from 0° to θ°, the resulting change in cavity length is: Where D1 is the thickness of the sheet-like cavity in optical path 1, n eff L is the refractive index of the second medium, L is the length from the laser output surface to the front surface of the mirror, L1 is the length from the laser output surface to the front surface of the sheet cavity, and L2 is the length from the rear surface of the sheet cavity to the front surface of part of the mirror.

[0020] The relationship between the frequency change c / 2L and the change of half wavelength based on the cavity length is as follows:

[0021]

[0022] The relationship between the resonant cavity jitter ΔL and the cavity mode frequency change Δf can be obtained as follows:

[0023]

[0024] Where f is the frequency, c is the speed of light, and λ0 is the initial wavelength of the laser output.

[0025] Known The relationship between the change in cavity mode wavelength Δλ caused by the rotation of the plate-like cavity and the thickness of the plate-like cavity and its rotation angle θ can be obtained:

[0026]

[0027] On the other hand, for the first optical path mechanism, assuming the angle between the normal of the sheet-like cavity and the incident light is 0°, the cavity length is... When the sheet-like cavity rotates by θ, the cavity length Therefore, when the plate-like cavity is rotated by an angle θ, the cavity mode wavelength can be obtained at this time. Where n eff Let n be the refractive index of the second medium, n be a positive number, D1 be the thickness of the sheet cavity in ray 1, L be the length from the laser output surface to the front surface of the partial reflector, L1 be the length from the laser output surface to the front surface of the sheet cavity, and L2 be the length from the sheet cavity to the front surface of the partial reflector.

[0028] In this invention, the narrowband interferometer rotation angle The relationship between the wavelength θ′ and the transmission wavelength is expressed as follows:

[0029]

[0030] λ0 is the transmission wavelength when the narrowband interferometer is perpendicular to the beam, and n eff2 Let be the refractive index of the narrowband interferometer. Then, in the first optical path mechanism, the change in transmission wavelength caused by the rotation of the interferometer is: Δλ′=λ0-λ′. Since the fixed positions of the narrowband interferometer and the plate-like cavity are collinear with the axis of the rotating stage and the optical path, the rotation angle of the narrowband interferometer is... The rotation angle θ is equal to that of the sheet-like cavity.

[0031] Therefore, since the fixed positions of the sheet-like cavity and the interferometer of this invention are collinear with the axis of the rotating stage and the optical path, when the rotating stage rotates by an angle θ, the narrowband interferometer and the sheet-like cavity rotate simultaneously by an angle θ. By selecting the thickness of the sheet-like cavity, the change in cavity mode wavelength caused by the rotation of the sheet-like cavity by an angle θ is proportional to the rotation of the narrowband interferometer. The changes in transmission wavelength caused by the angle are equal (or approximately the same. In this invention, "approximately the same" is understood as having a very small error and can be considered as phase), which allows the cavity mode frequency to change synchronously with the transmission peak frequency of the interferometer, such as... Figure 2 As shown in (b), at this time, the cavity mode frequency 2b in the first optical path mechanism changes from f1 to f1′, and the change in the center frequency 1b of the transmission peak of the interferometer is Δf1. Their synchronous change can ensure that f1′-f1=Δf1, so that the output laser frequency can change continuously, thereby converting the continuous change of angle into a continuous change of frequency, and realizing continuous small angle precise measurement.

[0032] Based on the same principle, for the second optical path mechanism, another sheet-like cavity with a thickness of D2 (D2≠D1) is selected and placed in the second optical path mechanism, so that it also satisfies the same condition as the change in cavity mode wavelength caused by the rotation angle θ of the sheet-like cavity and the rotation of the narrowband interference plate. The changes in transmitted wavelength caused by the angle are equal. For example... Figure 2 As shown in (b), in the second optical path mechanism, the cavity mode frequency 22b changes from f1 to f1', at which point the change in the center frequency 11b of the transmission peak of the interference plate is Δf2. Their synchronous changes ensure that f1' - f1 = Δf2. Because the thicknesses of the sheet-like cavities in the two optical paths are different, the laser frequency changes caused by rotating the sheet-like cavities by the same angle θ in each path are different. Figure 2 As can be seen from (b), the frequency difference between the two optical paths is Δf = Δf2 - Δf1.

[0033] Specifically, the changes in laser frequency generated by the output laser of the two optical path mechanisms as the rotation angle θ of the rotary table changes are as follows:

[0034]

[0035] The frequency difference between the two laser beams is:

[0036]

[0037] The frequency difference between the two laser beams is obtained by comparing their beat frequencies, and then converted into an angle.

[0038] In this invention, the thickness of the two sheet-like cavities should be selected to ensure that the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interferometer at the same angle does not exceed 20 pm, so as to ensure that the laser frequency of each channel is continuously output.

[0039] In this invention, the laser generating device is a conventional setup in the art, typically including a laser diode 1 and a first collimating lens 2 sequentially arranged on optical path 1, and a laser diode 11 and a first collimating lens 22 sequentially arranged on optical path 2. The operating wavelengths of the two laser diodes can be aligned using a cold atom optical frequency reference.

[0040] In this invention, the laser generating devices in the first and second optical path mechanisms are conventional configurations in the art, typically including laser diodes and collimating lenses sequentially arranged in their respective optical paths. The operating wavelength of the laser diodes can be any wavelength; preferably, for ease of acquisition or economic considerations, a 780nm wavelength laser diode can be used. Those skilled in the art can also choose laser diodes with other operating wavelengths, such as 420nm, 850nm, or 1250nm laser diodes. The operating wavelengths of the laser diodes in the first and second optical path mechanisms can be aligned using a cold atom optical frequency reference.

[0041] Beat frequency measurement is a prior art in this field. In a preferred embodiment, the beat frequency measurement module includes a reflector 6, a first half-wave plate 9, a second half-wave plate 10, a polarizing beam splitter 11, a broadband detector 12, and a frequency counter 13 disposed in the direction of reflected light output from the polarizing beam splitter 11, all positioned in the direction of reflected light output from the polarizing beam splitter 11. The laser beam under test output from the first optical path mechanism is reflected by the reflector 6 to the first half-wave plate 9, then passes through the first half-wave plate 9 and enters the polarizing beam splitter 11 where it is completely reflected. The laser beam under test output from the second optical path mechanism passes through the second half-wave plate 10 and enters the polarizing beam splitter prism, where it is combined with the laser beam under test output from the first optical path mechanism and reaches the broadband detector 12. The broadband detector 12 is connected to the frequency counter 13, and the beat frequency of the two laser beams is obtained from the frequency counter 13. When the rotation of the rotating stage causes a change in the beat frequency, the change in beat frequency Δf is obtained by Δf = Δf2 - Δf1, and then converted into a rotation angle.

[0042] As an alternative implementation, the reflector 6 is placed in the second optical path. Based on the same principle, the two measured laser beams are combined and then reach the broadband detector 12.

[0043] In this invention, the broadband detector, frequency counter, and optical comb system are all existing devices in the art, such as a 50 GHz broadband detector.

[0044] As an alternative technical solution, a prism-type partial reflective film 51 can be used instead of the partial reflector 5. When this technical solution is adopted, the transparent solid medium 4 can be bonded to the prism-type partial reflective film 51 to improve the mechanical robustness of the system. Generally, a prism-type partial reflective film 51 with a high retroreflection coefficient can achieve the technical solution of this invention. For example, the microprism-type reflective film disclosed in Chinese invention patent application CN 201410823342.3 is used. This prism-type partial reflective film is set in a direction perpendicular to the incident light, and the incident light returns along the original path within a rotation angle range of ±20°, which can realize the intracavity oscillation output of the measured laser.

[0045] The high-precision horizontal rotary stage of the present invention is a commercially available product, such as the small rotary displacement stage (PDR1C) sold by Thorlabs.

[0046] According to a preferred embodiment, the present invention further includes an ultra-low expansion coefficient glass base 52, on which the laser generating device, the first narrowband interferometer 3 and the first sheet cavity 4 on the first optical path mechanism, the laser generating device, the second narrowband interferometer 33 and the second sheet cavity 44 on the second optical path mechanism, as well as the high-precision horizontal rotary stage 7 and the rotary stage fixing base 8 are fixed together on the base made of ultra-low expansion coefficient glass. This can be used to reduce the influence of cavity length jitter on the frequency of the laser under test during free operation, and stabilize the laser frequency fluctuation to the Hz level. This setting can further improve the angle measurement resolution.

[0047] In this invention, the ultra-low expansion glass can be ultra-low expansion glass (ULE) sold by Corning Incorporated, USA.

[0048] Since the frequency fluctuation of the freely operating narrowband interferometer laser in a single angle measurement device is on the order of tens of kHz, the angle measurement limit can be converted into a laser frequency measurement of tens of kHz, corresponding to an angle measurement limit on the order of approximately 0.0001″. This invention uses two angle measurement devices combined into one unit, eliminating some common-mode noise and reducing the frequency fluctuation of the freely operating narrowband interferometer laser in the entire device to the order of several kHz. This converts the angle measurement limit into a laser frequency measurement of several kHz, corresponding to an angle measurement limit on the order of approximately 0.00001″. When the frequency fluctuation of the freely operating narrowband interferometer laser is further stabilized at the order of Hz using an ultra-low expansion coefficient glass substrate, the corresponding angle measurement resolution can be achieved on the order of approximately 0.00000001″, resulting in a significant improvement in the ultra-high precision angle measurement resolution for small angles.

[0049] The common-mode noise reduction ultra-high precision small-angle measurement reference device of the present invention uses two sets of angle measurement devices to measure angles, converting changes in angle into changes in the frequency difference of the laser outputs of the two sets of devices. While remaining within the detector's measurement range, it effectively eliminates a portion of common-mode noise, bringing the angle measurement limit to the order of 0.00001". This device can be applied to MEMS, precision instruments, and optical systems, meeting the application requirements of such high-precision equipment. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the small-angle measurement reference device previously studied by the inventor.

[0051] Among them, 101, laser diode; 102, second dielectric cell; 103, narrowband interference plate; 104, sheet cavity; 105, high-precision horizontal rotating stage; 106, rotating stage fixture; 107, partial reflector; 108, reflector; 109, first half-wave plate; 110, optical comb system; 111, second half-wave plate; 112, polarizing beam splitter; 113, broadband detector; 114, frequency counter.

[0052] Figure 2 A schematic diagram of the principle of a small-angle measurement reference device;

[0053] Figure 3 This is a schematic diagram of the small angle measuring device in Example 1;

[0054] Figure 4 This is a schematic diagram of the small angle measuring device in Example 2;

[0055] Figure 5 This illustrates the relationship between the rotation angle of the sheet-like cavity and the cavity mode frequency under different thicknesses D in Example 1.

[0056] Figure 6 This illustrates the relationship between the rotation angle of the sheet-like cavity and the change in cavity mode wavelength under different thicknesses D in Example 1.

[0057] Figure 7 This is a comparison diagram of the relationship between the rotation angle of the sheet cavity and the cavity mode wavelength under different thicknesses D in Example 1, and the relationship between the rotation angle of the interferometer and the transmission wavelength.

[0058] Among them, 1. First laser diode, 11. Second laser diode, 2. First collimating lens, 22. Second collimating lens, 14. Second dielectric cell, 3. First narrowband interferometer, 33. Second narrowband interferometer, 4. First sheet cavity, 44. Second sheet cavity, 5. Partial reflector, 51. Prism-type partial reflective film, 52. Ultra-low expansion coefficient glass base, 6. Reflector, 7. High-precision horizontal rotating stage, 8. Rotating stage fixing base, 9. First half-wave plate, 10. Second half-wave plate, 11. Polarizing beam splitter prism, 12. Broadband detector, 13. Frequency counter. Detailed Implementation

[0059] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0060] Example 1 examines the effect of different sheet cavity thicknesses on measurement results.

[0061] like Figure 3 The small-angle measurement reference device shown has two lasers whose operating wavelengths are calibrated to 780nm by cold atom optical frequency reference.

[0062] The device mainly includes a laser diode 1, a first collimating lens 2, a first narrowband interferometer 3, and a first sheet-like cavity 4 (made of glass and filled with air) arranged sequentially on a first optical path mechanism; and a laser diode 11, a first collimating lens 22, a second narrowband interferometer 33, and a first sheet-like cavity 44 (made of glass and filled with air) arranged sequentially on a second optical path mechanism, with a beat frequency measurement module located after the two optical paths. A high-precision horizontal rotary stage 7 is fixed on a rotary stage mounting base 8. In this system, the first narrowband interferometer 3 and the first sheet-like cavity 4 are fixed parallel to each other on the precision rotating platform 7. The second narrowband interferometer 33 is bonded parallel to the bottom of the first narrowband interferometer 3, and similarly, the second sheet-like cavity 44 is placed below the first sheet-like cavity 4, aligned and bonded along the front surface. The two sheet-like cavities have different thicknesses. The high-precision horizontal rotating stage 7 is inverted above the second dielectric cell 14, and the first narrowband interferometer 3, the first sheet-like cavity 4, the second narrowband interferometer 33, and the first sheet-like cavity 44 are all inserted into the second dielectric cell 14 and placed within the second dielectric. In the horizontal projection position relationship, the rotation axes of the narrowband interferometer, the sheet-like cavity, and the high-precision horizontal rotating stage 7 in the first and second optical path mechanisms are all located on the optical path. The distances between the center of the narrowband interferometer and the center of the sheet-like cavity and the axis of the high-precision horizontal rotating stage 7 are equal. Optical components such as mirrors and partial mirrors are also located on the optical path. In the height position relationship, the narrowband interferometer and the sheet-like cavity in the first and second optical path mechanisms can be passed through by laser light. At this point, assume that the normal of the narrowband interferometer has an initial angle α with respect to the optical path direction, and α < 90°, and the initial angle between the plate-like cavity and the optical path is also α. Therefore, when the precision rotating platform rotates counterclockwise by θ around the center, the angle of the interferometer and the angle between the plate-like cavity and the optical path increase synchronously (i.e., α′ = α + θ).

[0063] The beat frequency measurement module mainly includes a first half-wave plate 9, a second half-wave plate 10, a polarizing beam splitter 11, a broadband detector 12, and a frequency counter 13.

[0064] During operation, the light output from the 780nm laser diode 1 in the first optical path mechanism is collimated and enters the second dielectric cell 14. After passing through the first narrowband interferometer 3 to filter out out-of-band light, narrowband light is obtained. This narrowband light passes through the first sheet-like cavity 4 and reaches the front surface of the partial reflector 5. After passing through the partial reflector 5, reflected light is obtained that is collinear with and in the opposite direction to the narrowband light. This reflected light is fed back to the first laser diode 1. When the intracavity oscillation reaches the threshold, the first measured laser 1 is output through the partial reflector 5. Similarly, the second optical path mechanism outputs the second measured laser.

[0065] The first and second laser beams under test arrive at the beat frequency measurement module. The beat frequency measurement module compares the beat frequencies of the two laser beams to obtain their beat frequencies. When the frequencies of the first and second laser beams under test change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

[0066] To convert the change in beat frequency Δf into the change in rotation angle θ of the plate cavity, firstly, the curves of the cavity mode frequency change Δf and the cavity mode wavelength change Δλ caused by the rotation angle θ of the plate cavity are determined by measurement:

[0067] In this embodiment, the cavity length is set to L = 10 cm, λ0 = 780 nm, and the refractive index is n. eff =1.5, select sheet-like cavities of different thicknesses, D=40mm, D=45mm, D=50mm, D=55mm, and D=60mm respectively. Plot the curves of cavity mode frequency change Δf and cavity mode wavelength change Δλ caused by the rotation angle θ of each sheet-like cavity, as shown below. Figure 5 , 6 As shown.

[0068] Depend on Figure 5-6 It can be seen that when the rotation angle θ of the sheet cavity changes continuously within the range of 0 to 14°, the cavity mode frequency and wavelength change accordingly.

[0069] The results show that when the interferometer and the plate-shaped cavity are rotated clockwise / counterclockwise in one direction, the angle α between the interferometer and the incident light decreases / increases, the center frequency of the transmission peak of the corresponding interferometer increases / decreases, the transmission wavelength of the interferometer decreases / increases, the angle change of the plate-shaped cavity is the same as that of the interferometer (decreases / increases), the cavity length of the resonant cavity decreases / increases, the cavity mode frequency also increases / decreases, and the cavity mode wavelength decreases / increases.

[0070] Taking θ = 10° as an example, from Figure 5 and Figure 6 It can be seen that when D = 45 mm, the change in cavity mode frequency caused by rotating the plate-shaped cavity is Δf = 1.335 × 10⁻⁶. 12 The cavity mode wavelength change is Δλ = -2.707 nm. When D = 50 mm, Δf = 1.483 × 10⁻⁶ Hz. 12 Hz, Δλ = -3.008 nm. When D = 55 mm, Δf = 1.632 × 10⁻⁶. 12 HzHz, Δλ=-3.309nm.

[0071] To synchronize the cavity mode frequency with the center frequency of the transmission peak of the interferometer, thus addressing the issue of limited laser frequency tunability caused by the rotation of the plate cavity, and to ensure that the cavity mode wavelength caused by the rotation of the plate cavity changes synchronously with the transmission wavelength caused by the rotation of the interferometer, the rotation angle of the narrowband interferometer is determined. The expression relating the wavelength to the transmission wavelength λ′ is as follows:

[0072]

[0073] Figure 6 The calculated IF curve is given. When θ = 10°, the transmission wavelength λ′ = 777.1 nm of the interferometer is obtained. The calculated change in transmission wavelength Δλ′ = -2.9 nm is between Δλ = -2.707 nm and Δλ = -3.008 nm. Therefore, the optimal range for the thickness D of the sheet cavity is 45 mm to 50 mm.

[0074] That is, when the thickness D of the plate cavity is 45mm to 50mm, the change in cavity mode wavelength Δλ caused by the rotation of the plate cavity is approximately the same as the change in transmission wavelength Δλ′ caused by the rotation of the interferometer and is therefore considered equal. Thus, the cavity mode frequency and the transmission frequency of the interferometer change synchronously, so that the output laser frequency can change continuously, thereby realizing the transformation of continuous angle change measurement into continuous frequency change measurement.

[0075] Therefore, the thicknesses D1 and D2 of the first sheet-like cavity 4 and the second sheet-like cavity 44 range from 45mm to 50mm, with D1 ≠ D2, and the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interferometer at the same angle does not exceed 20pm. Finally, the measured beat frequency Δf is converted into an angular change:

[0076]

[0077] This invention employs two sets of angle measurement devices to form a single device, thereby eliminating a portion of common-mode noise and reducing the frequency fluctuation of the freely operating narrowband interferometer laser to the order of 3 kHz. This is based on the angle between the narrowband interferometer and the incident light. The relationship with the transmission wavelength shows that when the interferometer is rotated from 0° to 20°, the slope of the transmission wavelength change with the angle is in the range of 0-1.1 nm / °.

[0078] For example, when the laser and the interferometer rotate slightly around 20°, K is approximately 1. Therefore, the relationship between the change in the interferometer angle and the change in the transmitted wavelength is 1 nm / °. For a 780 nm laser wavelength, a 1 nm change in wavelength corresponds to a 490 GHz change in frequency. Thus, the relationship between the change in the interferometer angle and the change in the transmitted frequency is 490 GHz / °. Therefore, the angle change corresponding to a 3 kHz frequency fluctuation is approximately 6.12 × 10⁻⁶. -9 ° (0.000022041″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.00001″.

[0079] Example 2

[0080] The difference from Example 1 is that the partial reflector 5 is replaced by a prism-type partial reflective film 51, which adopts the microprism-type reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0081] The laser diode 1, first collimating lens 2, first narrowband interferometer 3, and first sheet-like cavity 4 on the first optical path mechanism, and the laser diode 11, first collimating lens 22, second narrowband interferometer 33, and second sheet-like cavity 44 on optical path 2, including the prism-type partial reflective film 51, high-precision horizontal rotating stage 7, and rotating stage fixing base 8, are fixed together on a base 52 made of ultra-low expansion coefficient glass. The ultra-low expansion coefficient characteristics of the base further overcome the interference caused by mechanical deformation. The rest of the structure remains unchanged.

[0082] Similarly, the thicknesses D1 and D2 of the first sheet-like cavity 4 and the second sheet-like cavity 44 are set within the range of 45mm to 50mm. Using a high-precision horizontal rotating stage 7, the angles between the incident light and the first narrow-band interferometer 3 and the second narrow-band interferometer 33, as well as the angles between the incident light and the first sheet-like cavity 4 and the second sheet-like cavity 44, are adjusted. This causes the center position of the transmission peak of the interferometer in both optical paths to move with the angle, highly synchronized with the frequency change of their corresponding specific cavity modes with the angle, achieving a wide range of continuously tunable laser frequencies. Then, the beat frequency Δf obtained by comparing the beat frequencies of the output lasers from the two optical paths is converted into an angular change.

[0083] Because ultra-low expansion coefficient glass can stabilize the frequency fluctuations of a free-running narrowband interferometer laser within the Hz range, based on the angle between the narrowband interferometer and the incident light... The relationship between the interferometer and the transmitted wavelength shows that when the interferometer rotates from 0° to 20°, the slope of the transmitted wavelength change with angle is in the range of 0-1.1 nm / °. When the laser and the interferometer rotate at a small angle of around 20°, k is approximately 1, meaning the relationship between the change in the interferometer angle and the change in transmitted wavelength is 1 nm / °. For a 780 nm laser wavelength, a 1 nm change in wavelength corresponds to a 490 GHz change in frequency. Therefore, the relationship between the change in the interferometer angle and the change in transmitted frequency is 490 GHz / °. Thus, the angle change per 1 Hz is approximately 2.04 × 10⁻⁶. -12 ° (0.000000073469″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.00000001″.

[0084] Compared to Example 1, where the frequency fluctuation of the laser during free operation was about 3kHz after eliminating some common-mode noise, this example uses ultra-low expansion coefficient glass to improve the stability of the frequency fluctuation of the laser during free operation to the Hz level, so that the corresponding angle measurement resolution can be achieved to the order of about 0.00000001″, thus achieving a significant improvement in angle measurement resolution.

Claims

1. A common-mode noise-reducing, ultra-high precision small-angle measurement reference device for continuous measurement, characterized in that... The device includes a first optical path mechanism and a second optical path mechanism, a partial reflector (5) and a beat frequency measurement module. The first optical path mechanism includes a first laser generator. A high-precision horizontal rotating stage (7) is set on the output optical path of the first laser generator. A first narrow-band interference plate (3) and a first sheet-like cavity (4) are fixed in parallel under the rotating stage. The second optical path mechanism includes a second laser generating device and a second narrowband interference plate (33) and a second sheet cavity (44) located on the output optical path of the second laser generating device. The second sheet cavity (44) is fixed below the first sheet cavity (4) and their front surfaces are in the same plane. The second narrowband interference plate (33) is fixed below the first narrowband interference plate (3) and is in the same plane. The laser generator has a second medium pool (14) filled with a second medium on the output light end face. The front and rear walls of the second medium pool (14) are transparent and flat and perpendicular to the light path. The refractive index of the second medium is greater than that of vacuum. The first narrow band interference plate (3), the first sheet cavity (4), the second narrow band interference plate (33), and the second sheet cavity (44) are all in the second medium. In the horizontal projection, the distance between the first narrow band interference plate (3), the first sheet cavity (4), the second narrow band interference plate (33), and the second sheet cavity (44) is equal to the axis of the rotating stage. Partial reflectors (5) are provided on the outer side of the rear wall of the second medium pool (2). The partial reflectors (5) are all located on the optical paths of the first optical path mechanism and the second optical path mechanism. The initial wavelengths of the first laser generating device and the second laser generating device are equal, but the thicknesses of the first sheet cavity (4) and the second sheet cavity (44) are not equal. In the first optical path mechanism and the second optical path mechanism, the light output by each laser generator enters the second dielectric cell, passes through the narrowband interferometer and is filtered out of the out-of-band light to obtain narrowband light; the narrowband light returns to the laser generator after passing through the sheet cavity and the partial reflector (5). When the oscillation in the cavity reaches the threshold, the first laser and the second laser under test are output by the partial reflector (5) respectively; the two lasers under test are respectively input to the beat frequency measurement module, and the beat frequency measurement module beats the two lasers under test. When the frequency of the first laser under test and the frequency of the second laser under test change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

2. The common-mode noise reduction ultra-high precision small-angle measurement reference device according to claim 1, characterized in that... The first laser generating device includes a first laser diode (1) and a first collimating lens (2) arranged sequentially on the optical path, and the center wavelength of the transmission peak of the first narrowband interferometer (3) is matched with the wavelength of the first laser diode (1); the second laser generating device includes a second laser diode (11) and a second collimating lens (22) arranged sequentially on the optical path, and the center wavelength of the transmission peak of the second narrowband interferometer (33) is matched with the wavelength of the second laser diode (11).

3. The common-mode noise reduction ultra-high precision small-angle measurement reference device according to claim 1, characterized in that... The beat frequency measurement module includes a reflector (6), a first half-wave plate (9), a second half-wave plate (10), a polarizing beam splitter (11), a broadband detector (12) and a frequency counter (13) set in the output light direction of the polarizing beam splitter (11). The reflector is located in the first optical path, the first half-wave plate is located in the output light direction of the reflector (6), and the second half-wave plate (10) is located in the output light direction of the second sheet-like cavity (44). The first laser beam under test is reflected by the mirror (6) and then enters the polarization beam splitter (11) through the first half-wave plate (9) and is completely reflected. The second laser beam under test enters the polarization beam splitter (11) through the second half-wave plate (10). The two laser beams under test are combined and reach the broadband detector (12). The broadband detector (12) is connected to the frequency counter (13). The frequency counter (13) measures the beat frequency of the two laser beams under test and obtains the change in beat frequency Δf.

4. The common-mode noise reduction ultra-high precision small-angle measurement reference device according to claim 1, characterized in that... A prism-type partial reflective film (51) is used to replace a partial reflector (5).

5. The common-mode noise reduction ultra-high precision small-angle measurement reference device according to claim 1, characterized in that... The device also includes an ultra-low expansion coefficient glass base (52), on which the laser diode, collimating lens, narrow band interference plate, sheet cavity and prism-type partial reflective film (51), high-precision horizontal rotating stage (7) and rotating stage fixing base (8) of the first optical path mechanism and the second optical path mechanism are fixed.

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