Ultra-high precision small angle measurement method and instrument based on common mode noise cancellation double laser frequency measurement

Abstract

The application provides a common mode noise elimination double laser frequency measurement ultra-high precision small angle measurement reference device, comprising a first light path mechanism and a second light path mechanism arranged in parallel and a beat frequency measurement module, the light path mechanism comprises a laser generating device, a narrow band interference sheet, a micro prism array and a first ring piezoelectric module, which are fixed on a high precision horizontal rotating table (7). The common mode noise elimination double laser frequency measurement ultra-high precision small angle measurement reference device changes the cavity length by rotating the cavity mirror to cause the frequency change, converts the angle change into the change of the frequency difference of the output laser of the two sets of devices, selects the mode by using the narrow band interference sheet for auxiliary adjustment, makes the cavity mode change highly synchronized with the interference frequency selection, realizes the non-mode jumping of the laser output frequency, guarantees the continuous tunability of the output frequency with the rotation angle, solves the continuous measurement problem, and further realizes the ultra-high precision measurement of the small angle.

Description

Technical Field

[0001] This invention relates to a small-angle measurement reference device, and particularly to an ultra-high precision small-angle measurement reference device with common-mode noise reduction dual-laser frequency 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 further improve the accuracy of small angle measurement by converting angle measurement into laser frequency measurement, and to propose an ultra-high precision small angle measurement reference device with common-mode noise reduction dual laser frequency measurement.

[0005] In their previous research, the inventors designed a small-angle measurement reference device based on interferometric frequency selection and microprism array feedback. This device transforms angle measurement into laser frequency measurement using a narrowband interferometer laser. Utilizing the ultra-high sensitivity of laser frequency to cavity length in an external cavity semiconductor laser, a microprism array is integrated with a piezoelectric module of thickness D. The center of the rear surface of the piezoelectric module is placed at the axis of the rotating stage. Even if the perpendicular distance between the rear surface of the microprism array and the axis is D, rotating the microprism array and the piezoelectric module changes the optical path length of the laser passing through the prism array, thereby changing the cavity length and ultimately the cavity mode frequency. Specifically, as shown... Figure 1As shown, the small-angle measurement reference device includes a laser generator and a beat frequency measurement module arranged sequentially on the optical path. A high-precision horizontal rotating stage 107 is arranged on the output optical path of the laser generator. A narrow-band interferometer 104, a microprism array 105, and a piezoelectric module 106 are fixed on the high-precision horizontal rotating stage 107. The rear surface of the microprism array 105 is bonded to the front surface of the piezoelectric module 106, and the center of the rear surface of the piezoelectric module 106 is located at the axis of the rotating stage. A polarization beam splitter is arranged between the laser generator and the narrow-band interferometer 104. The laser generator output light is filtered out of the band by a narrowband interferometer 104, resulting in narrowband light. This narrowband light is reflected by a microprism array 105, then returns to the laser diode via a polarizing beam splitter 103. When the intracavity oscillation reaches a threshold, the laser under test is reflected by the polarizing beam splitter 103. This laser under test is input to a beat frequency measurement module, which beats the laser under test with the comb teeth output from the optical comb system 110 to obtain the beat frequency. When the frequency of the laser under test changes by Δf, the change in beat frequency is equal to Δf. The rotation angle of the narrowband interferometer is obtained by measuring the change in beat frequency Δf.

[0006] Where f0 is the measured laser frequency when the high-precision horizontal rotary table is in its initial position, and f1 is the measured laser frequency after the high-precision horizontal rotary table has rotated.

[0007] 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 an approximately linear relationship with the transmission wavelength within ±1°, K nm / degree, indicating the rotation angle of the interferometer. Satisfying the formula:

[0008]

[0009] Where K is the value of the piezoelectric module thickness D that satisfies the rotation angle of the interferometer. When the rotation angle θ of the microprism array is equal to that of the microprism array and the wavelength changes caused by both are approximately equal, the rotation angle of the interferometer is... With respect to the slope of the λ′ curve:

[0010]

[0011] However, the frequency fluctuation of the measured laser under free-running conditions reaches tens of kHz, which affects the resolution of angle measurements. Simultaneously, 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 to the structure and measurement methods of small-angle measuring devices are needed to simplify measurements and increase accuracy. Meeting the high-precision metrology requirements for small angles in applications such as lithography machines, lidar, precision robotics, and virtual reality remains a key technical challenge in this field.

[0012] The present invention builds upon previous research by adding an angle measurement device to form a common-mode device. Specifically, two angle measurement devices utilize ring-shaped piezoelectric modules of different thicknesses to support the vertical distance of the microprism array to the axis. This aims to synchronize the cavity mode changes and interference frequency selection caused by the rotation of the microprism array in both devices. By comparing the output frequencies of the two angle measurement devices through beat frequency comparison, the frequency difference between the two devices is obtained while eliminating common-mode noise. Furthermore, the output frequency difference is controlled within 100 GHz, ensuring that angle measurement does not exceed 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.

[0013] To address this, the present invention provides an ultra-high precision small-angle measurement reference device for common-mode noise reduction dual-laser frequency measurement. The device includes a first optical path mechanism and a second optical path mechanism arranged in parallel and a beat frequency measurement module. The first optical path mechanism includes a first laser generator. A high-precision horizontal rotating stage 7 is arranged on the output optical path of the first laser generator. A first narrow-band interferometer 3, a first microprism array 4, and a first annular piezoelectric module 5 are fixed in parallel on the high-precision horizontal rotating stage 7. The rear surface of the first microprism array 4 is bonded to the front surface of the first annular piezoelectric module 5, and the center of the rear surface of the first annular piezoelectric module 5 is located at the axial center of the rotating stage.

[0014] The second optical path mechanism includes a second laser generator and a second narrowband interference plate 33, a second microprism array 44, and a second annular piezoelectric module 55 arranged in parallel. The rear surface of the second microprism array 44 is bonded to the front surface of the second annular piezoelectric module 55, and the rear surface of the second annular piezoelectric module 55 is in the same plane as the rear surface of the first annular piezoelectric module 5, and the center of the rear surface of the second annular piezoelectric module 55 is located at the axis of the rotating stage.

[0015] The initial wavelengths of the first laser diode 1 and the second laser diode 11 are equal;

[0016] The thickness D1 of the first annular piezoelectric module 5 is not equal to the thickness D2 of the second annular piezoelectric module 55;

[0017] In each optical path mechanism, the light output by each laser generator is filtered out of the band after passing through its respective narrowband interferometer to obtain narrowband light; the narrowband light is reflected back to the laser diode after passing through its respective microprism array; when the intracavity oscillation reaches the threshold, it is output by its respective microprism array to obtain the first test laser and the second test laser; the two test lasers are respectively input to the beat frequency measurement module, which beats the two test lasers to obtain the beat frequency; when the frequency of the first test laser and the frequency of the second test laser change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

[0018] In this invention, the first and second microprism arrays can be realized by a prism-type ultra-high intensity reflective film with a high retroreflection coefficient, such as the microprism-type reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0019] In each optical path of this invention, the rear surface of the microprism array is bonded to the front surface of the annular piezoelectric module, and the center of the rear surface of the annular piezoelectric module is located at the axis of the rotating stage. Therefore, the vertical distance D between the rear surface of the microprism array and the axis of the rotating stage can be adjusted by adjusting the voltage of the annular piezoelectric module to compensate for the thickness error of the annular piezoelectric module and the deviation error between the axis position and the center position of the rear surface of the annular piezoelectric module, thereby optimizing the device.

[0020] In this invention, the first and second optical path mechanisms are substantially identical; for example, their laser diodes and narrowband interferometers have the same parameters, differing only in the thickness of the annular piezoelectric module, thus resulting in different resonant cavity lengths. In each optical path, each optical element should be located within the optical path to allow laser light to pass through.

[0021] In this invention, for ease of explanation rather than limitation, the position of the laser output device in the optical path is referred to as "front" and the position of the annular piezoelectric module in the optical path is referred to as "rear". Therefore, taking the microprism array as an example, the side facing the laser output device is called the "front surface" and the other side is called the "rear surface".

[0022] Taking the first optical path mechanism as an example, combined with Figure 2-3 Its working principle is explained in detail below:

[0023] like Figure 2 As shown, when the microprism array (bonded together with the narrowband interferometer and the annular piezoelectric module) is perpendicular to the optical path, the incident angle is 0°. Let the cavity length L = L1, where L1 is the length from the laser output surface to the front surface of the microprism array when the incident angle is 0°. When the rotating stage rotates by an angle θ, the microprism array also rotates by θ, and the incident angle is θ, with the cavity length... Therefore, when the rotating stage rotates and the microprism array rotates from an incident angle of 0° to θ (°), the resulting change in cavity length is: Where D is the thickness of the annular piezoelectric module, which is the vertical distance between the microprism array and the axis.

[0024] The relationship between the frequency change c / 2L and the change of half wavelength based on the cavity length is as follows: The relationship between the resonant cavity jitter ΔL and the cavity mode frequency change Δf in the first optical path mechanism can be obtained as follows:

[0025] Where f is the frequency and λ0 is the initial wavelength emitted by the laser generator (the initial wavelengths of the first and second laser generators are the same, both denoted as λ0).

[0026] Known The relationship between the change in wavelength Δλ1 of the first cavity mode caused by the rotation angle θ of the microprism array and the perpendicular distance D1 between the microprism array and the axis and its rotation angle θ can be obtained:

[0027] Assuming the incident angle of the laser incident on the microprism array is 0°, the cavity length is... When the microprism array rotates by θ, the cavity length Therefore, when the microprism array is rotated by an angle θ, the cavity mode wavelength can be obtained at this time. Where n is a positive number, D1 is the thickness of the first annular piezoelectric module, i.e., the vertical distance between the microprism array and the axis, and L1 is the length from the laser output surface to the front surface of the microprism array when the incident angle is 0°.

[0028] On the other hand, in the first optical path mechanism, the rotation angle of the first narrowband interferometer The relationship between the wavelength and the transmission wavelength λ′ is as follows:

[0029]

[0030] λ0 is the initial transmission wavelength when the narrowband interferometer is perpendicular to the light beam, and n eff Let be the refractive index of the narrowband interferometer. Then, the change in transmission wavelength Δλ′ caused by the rotation of the interferometer is: Δλ′=λ0-λ′.

[0031] Since the narrowband interferometer of this invention is placed parallel to the microprism array, when the rotating stage rotates by an angle θ, the rotation angle of the narrowband interferometer... The rotation angle θ of the microprism array is equal to that of the microprism array. Therefore, by adjusting the perpendicular distance D1 between the first microprism array and the axis, the change in cavity mode wavelength caused by rotating the microprism array by an angle θ is equal to the change in the narrowband interferometer plate. angle The resulting changes in transmission wavelength are equal or approximately the same (in this invention, "approximately the same" is understood as having an extremely small error and can be considered equal), which allows the cavity mode frequency to change synchronously with the center frequency of the transmission peak of the interferometer, such as... Figure 2 As shown in (b), the cavity mode frequency 2b in the first optical path changes from f1 to f1'. At this time, 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. This transforms the continuous change of angle into a continuous change of frequency, solves the laser frequency mode skipping phenomenon caused by the rotation of a single interferometer, and realizes continuous small angle measurement.

[0032] Similarly, for the second optical path mechanism, the thickness of the second annular piezoelectric module is set to D2 (D2≠D1), that is, the vertical distance between the microprism array and the axis of the rotating stage is D2, so that it also satisfies the condition that the change in cavity mode wavelength caused by the rotation angle θ of the microprism array is equal to the change in transmission wavelength caused by the rotation angle θ of the narrowband interferometer. Figure 2 As shown in (b), in the second optical path mechanism, the cavity mode frequency 22b changes from f1 to f″1. At this time, the change in the center frequency 11b of the interference plate transmission peak is Δf2. Their synchronous changes ensure that f″1 - f1 = Δf2. Because the thicknesses of the annular piezoelectric modules in the two optical paths are different, i.e., the distances from the microprism arrays to the axis are different, although the rotation angle θ is the same, the changes in cavity mode frequency caused by the rotation angle θ are different for the two microprism arrays. Figure 2 As can be seen from (b), the frequency difference between the two optical paths input to the polarizing beam splitter is Δf = Δf2 - Δf1.

[0033] The changes in laser frequency produced by the two optical paths output lasers as the rotation angle θ of the rotary table change are as follows: The frequency difference between the two laser beams is: Where c is the speed of light. The frequency difference between the two laser beams is obtained by comparing their beat frequencies, and this frequency difference is then converted into the rotation angle of the rotating platform.

[0034] Based on the linear relationship K′ (Hz / degree) between the rotation angle of the narrowband interferometer and the change in beat frequency Δf, the relationship between the change in beat frequency Δf and the rotation angle of the narrowband interferometer is:

[0035]

[0036] Wherein, K′ is the slope of the curve of rotation angle θ versus frequency change. The value of K′ can be obtained by measuring the rotation angle of the narrowband interferometer and the frequency change of the two measured lasers, or by the following formula:

[0037]

[0038] In this invention, the first laser generating device includes a first laser diode (1) and a first collimating lens (2) sequentially arranged in the first optical path, and the center wavelength of the transmission peak of the first narrowband interferometer (3) matches 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) sequentially arranged in the second optical path, and the center wavelength of the transmission peak of the second narrowband interferometer (33) matches the wavelength of the second laser diode (11).

[0039] In this invention, the laser generating device is a conventional setup in the art. The first and second optical path mechanisms each include a laser diode and a collimating lens sequentially arranged along their respective optical paths. The laser diode can operate in any wavelength band; preferably, for ease of acquisition or economic reasons, 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 1550nm laser diodes. However, the first and second laser diodes operate in the same wavelength band.

[0040] Thus, in the first and second optical path mechanisms, the diverging light output from the laser diode is collimated into parallel light by a collimating lens. This parallel light is then filtered out of the band by a narrow-band interferometer, resulting in narrow-band light. This narrow-band light reaches the front surface of the microprism array, where it is reflected in the opposite direction and along the same line. This reflected light is fed back to the laser diode, and when the intracavity oscillation reaches a threshold, it is output from the microprism array as the laser under test. The two laser beams under test reach the beat frequency measurement module for beat frequency comparison. By measuring the change in beat frequency Δf, the formula is used... The frequency change is converted into a change in the rotation angle θ.

[0041] Beat frequency measurement is a prior art in this field. As a preferred embodiment, it includes a reflector 6 positioned in the direction of light emission from the second annular piezoelectric module 55, a first half-wave plate 9 positioned in the direction of light reflection from the reflector 6, a second half-wave plate 10 positioned in the direction of light emission from the first annular piezoelectric module 5, a polarizing beam splitter 15 with the output light from the first half-wave plate 9 and the second half-wave plate 10 as input, and a broadband detector 12 and a frequency counter 13 positioned in the direction of light output from the polarizing beam splitter 15.

[0042] The second laser beam under test is reflected completely after passing through the first half-wave plate 9 and entering the polarization beam splitter 15. The first laser beam under test is then passed through the second half-wave plate 10 and entering the polarization beam splitter 15. 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.

[0043] In this invention, since the first and second optical path mechanisms have the same structure except for the thickness of the annular piezoelectric module, as another implementation, the reflector 6 can also be disposed in the first optical path structure, that is, the reflector 6 is disposed in the direction of the emitted light from the first annular piezoelectric module 5. Through the reflection of the reflector 6, the first and second measured lasers are also input into the polarizing beam splitter to achieve beat frequency measurement.

[0044] 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.

[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] In one embodiment of the present invention, the first narrowband interferometer 3 can be bonded to the front surface of the first microprism array 4, and the first narrowband interferometer 3, the first microprism array 4, and the first annular piezoelectric module 5 can be fixed together as a whole on the rotating stage; the second narrowband interferometer 33 can be bonded to the front surface of the second microprism array 44, and the second narrowband interferometer 33, the second microprism array 44, and the second annular piezoelectric module 55 can be fixed together as a whole on the first annular piezoelectric module 5. That is, the narrowband interferometer, the microprism array, and the annular piezoelectric module are integrated into one unit and fixed together on the rotating stage, thereby improving the mechanical robustness of the system.

[0047] According to a preferred embodiment, the present invention further includes an ultra-low expansion coefficient glass base 14, on which the laser diode, collimating lens, integrated narrowband interferometer-microprism array-annular piezoelectric module, high-precision horizontal rotating stage 7, and rotating stage fixing base 8 in the first and second optical paths are fixed. 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.

[0048] In this invention, the ultra-low expansion glass can be ultra-low expansion glass (ULE) sold by Corning Incorporated, USA.

[0049] On the other hand, the present invention also provides a method for continuously measuring small angles, the method comprising the following steps:

[0050] (1) The laser emitted by the first laser diode passes through the first narrowband interference plate to obtain narrowband light; the narrowband light is reflected and returned to the first laser diode after passing through the first microprism array; when the cavity oscillation reaches the threshold, the first measured laser is output by the first microprism array.

[0051] The laser emitted by the second laser diode passes through the second narrowband interferometer to obtain narrowband light; the narrowband light is reflected back to the second laser diode after passing through the second microprism array; when the intracavity oscillation reaches the threshold, the second measured laser is output by the second microprism array.

[0052] The narrowband interferometer and the microprism array are fixed in parallel on a high-precision horizontal rotating stage. A ring-shaped piezoelectric module is bonded to the rear surface of the microprism array. The center of the rear surface of the ring-shaped piezoelectric module is located at the axis of the rotating stage. Therefore, when the rotating stage rotates, the narrowband interferometer and the microprism array change simultaneously with the incident angle and the change in angle is equal.

[0053] (2) When the high-precision horizontal rotary table rotates, the narrowband interference plates of the first optical path mechanism and the second optical path mechanism rotate synchronously with the microprism array. Based on the different values ​​of the thickness D of the annular piezoelectric module in each optical path mechanism, the curve of the change in cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array is plotted. Δλ and the rotation angle θ of the microprism array satisfy the following relationship:

[0054]

[0055] Where L is the length from the output light end face of the laser diode to the front surface of the microprism array in each optical path mechanism, λ0 is the initial wavelength of the laser, D is the thickness of the annular piezoelectric module, and Δλ is the change in cavity mode wavelength caused by the rotation angle of the microprism array being θ.

[0056] (3) The rotation angle of the narrowband interferometer in each optical path mechanism is obtained by the following formula. The relationship between the wavelength and the transmission wavelength λ′ is expressed as follows:

[0057]

[0058] λ0 is the initial transmission wavelength when the narrowband interferometer is perpendicular to the light beam, and n eff Let be the refractive index of the first and second narrowband interferometers. Then, the change in transmission wavelength Δλ′ caused by the rotation of the interferometers is: Δλ′=λ0-λ′.

[0059] (4) For the first optical path mechanism and the second optical path mechanism, the two laser beams under test in step (1) are input to the beat frequency measurement module. The beat frequency measurement module performs beat frequency measurement on the two laser beams under test of the first optical path mechanism and the second optical path mechanism to obtain the beat frequency difference Δf = Δf2 - Δf1.

[0060] (5) When the value of the thickness D1 of the first annular piezoelectric module satisfies Furthermore, the condition Δλ1=Δλ1′ is met, and the value of the thickness D2 of the second annular piezoelectric module satisfies the following conditions. And the condition Δλ2=Δλ2′ (i.e., the change in cavity mode wavelength Δλ1 caused by the rotation angle θ of the first microprism array and the rotation of the first narrowband interferometer) The change in transmission wavelength Δλ1′ caused by the angle is approximately equal to that caused by the rotation of the second microprism array by an angle θ. The change in cavity mode wavelength Δλ2 caused by the rotation of the second narrowband interferometer is equal to that caused by the rotation of the second narrowband interferometer. The changes in transmission wavelength caused by the angle are approximately equal (Δλ2′), and D1≠D2. When 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, the rotation angle θ of the microprism array is calculated. The relationship between the change in beat frequency Δf and the rotation angle θ of the microprism array is:

[0061] Where c is the speed of light.

[0062] Based on the linear relationship K′ (Hz / degree) between the rotation angle of the narrowband interferometer and the change in beat frequency Δf, the relationship between the change in beat frequency Δf and the rotation angle of the narrowband interferometer is:

[0063]

[0064] Where K′ is the slope of the curve of rotation angle versus frequency change. The value of K′ can be obtained by measuring the rotation angle of the narrowband interferometer and the frequency change of the two measured lasers, or by the following formula:

[0065]

[0066] The small-angle measurement reference device of this invention uses a narrow-band interferometer for narrow-band filtering, filtering the broadband light emitted by the laser diode into a narrow-band beam. With optical feedback from the microprism array, it achieves the output of the measured laser whose frequency is extremely sensitive to the rotation angle, thus reflecting small changes in the rotation angle in the frequency difference Δf between the two measured laser beams. Specifically, by adjusting the voltage of the annular piezoelectric module to optimize its own thickness D (i.e., the perpendicular distance between the rear surface of the microprism array and the axis), the change in cavity mode frequency can be synchronized with the change in the center frequency of the transmission peak of the interferometer. This solves the problems of laser frequency mode jumping caused by a single interferometer rotation and the small frequency tunability range caused by a single change in cavity length (rotating the microprism array).

[0067] Since the frequency fluctuation of the freely operating narrowband interferometer laser in a single angle measuring 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 employs parallel first and second optical path mechanisms to combine two angle measuring devices 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 0.0001″. 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 0.000001″, significantly improving the resolution of ultra-high precision angle measurement for small angles.

[0068] The ultra-high precision common-mode noise-canceling dual-laser frequency measurement of this invention employs two sets of angle measurement devices to measure angles, converting changes in angle into changes in the frequency difference of the laser outputs from the two devices. While remaining within the detector's measurement range, it effectively eliminates a portion of common-mode noise, achieving an angle measurement limit on 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

[0069] Figure 1 A schematic diagram of the small-angle measurement reference device researched by the inventor in the early stage;

[0070] Among them, 101, laser diode; 102, collimating lens; 103, first polarizing beam splitter; 104, narrowband interferometer; 105, microprism array; 106, piezoelectric module; 107, high-precision horizontal rotating stage; 108, rotating stage fixture; 109, first half-wave plate; 110, optical comb system; 111, second half-wave plate; 112, second polarizing beam splitter; 113, broadband detector; 114, frequency counter.

[0071] Figure 2 A schematic diagram of the principle of a small-angle measurement reference device;

[0072] Figure 3 This is a schematic diagram of the small angle measuring device in Example 1;

[0073] Figure 4 This is a schematic diagram of the small angle measuring device in Example 2;

[0074] Figure 5 This illustrates the relationship between the rotation angle of the microprism array and the change in cavity mode frequency in Example 1.

[0075] Figure 6 This illustrates the relationship between the rotation angle of the microprism array and the change in its cavity mode wavelength in Example 1.

[0076] Figure 7 This is a comparison diagram of the relationship between the rotation angle of the microprism array and the cavity mode wavelength and the relationship between the rotation angle of the interferometer and the transmission wavelength in Example 1.

[0077] Among them, 1. First laser diode, 2. First collimating lens, 3. First narrowband interference plate, 4. First microprism array, 5. First annular piezoelectric module;

[0078] 11. Second laser diode; 22. Second collimating lens; 33. Second narrowband interferometer; 44. Second microprism array; 55. Second annular piezoelectric module.

[0079] 6. Reflector; 7. High-precision horizontal rotary stage; 8. Rotary stage mounting base; 9. First half-wave plate; 10. Second half-wave plate; 12. Broadband detector; 13. Frequency counter; 14. Ultra-low expansion rate glass base; 15. Polarizing beam splitter. Detailed Implementation

[0080] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0081] Example 1 examines the influence of the thickness D of the annular piezoelectric module, i.e., the different perpendicular distances between the rear surface of the microprism array and the axis, on the measurement results.

[0082] like Figure 3 The small-angle measurement reference device shown has a laser operating at a wavelength of 780nm. The device mainly comprises a first laser diode 1, a first collimating lens 2, a first narrow-band interferometer 3, a first microprism array 4, and a first annular piezoelectric module 5, sequentially arranged on the first optical path. The first narrow-band interferometer 3, the first microprism array 4, and the first annular piezoelectric module 5 are sequentially bonded and fixed together, with the center of the rear surface of the first annular piezoelectric module 5 located at the axis of the rotating stage. It is fixed on a high-precision horizontal rotating stage 7, which is in turn fixed on a rotating stage mounting base 8. Similarly, the second optical path mechanism includes a second laser generator and a second narrowband interference plate 33, a second microprism array 44, and a second annular piezoelectric module 55, which are also bonded and fixed together. The rear surface of the second annular piezoelectric module 55 is in the same plane as the rear surface of the first annular piezoelectric module 5, and the center of the rear surface of the second annular piezoelectric module 55 is also located at the axial center of the rotating stage. For example, the second annular piezoelectric module 55 is fixed directly above the first annular piezoelectric module 5.

[0083] A reflector 6 is positioned behind the second annular piezoelectric module 5. A beat frequency measurement module is positioned along the direction of the reflected light from the reflector. When the high-precision horizontal rotating stage 7 rotates counterclockwise by θ around its center, the first and second narrow-band interferometers and the second and second prism-type ultra-high-intensity reflective films simultaneously rotate by θ.

[0084] The initial wavelengths of the first laser diode 1 and the second laser diode 11 are equal, both being 780nm.

[0085] The beat frequency measurement module mainly includes a reflector 6, a first half-wave plate 9, a second half-wave plate 10, a polarizing beam splitter 15, a broadband detector 12, and a frequency counter 13.

[0086] During operation, the light output from the 780nm laser diode in the first optical path mechanism is collimated and then filtered out of the band by a narrow-band interferometer to obtain narrow-band light. This narrow-band light reaches the front surface of the microprism array and, after passing through the microprism array, is reflected in the same direction as the narrow-band light. This reflected light is fed back to the laser diode, and when the intracavity oscillation reaches a threshold, it is output through the microprism array as the first laser under test. Similarly, the second optical path mechanism obtains the second laser under test.

[0087] Two laser beams are brought to 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 change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

[0088] To convert the beat frequency change Δf into the microprism array rotation angle change θ, we first determine the cavity mode wavelength change Δλ curve caused by the microprism array rotation angle θ:

[0089] In this embodiment, the cavity length is set to L = 10 cm, λ0 = 780 nm, and annular piezoelectric modules with different thicknesses D are selected, i.e., the perpendicular distance between the microprism array and the axis, are D = 21 mm, D = 23 mm, D = 25 mm, and D = 27 mm, respectively. The cavity mode frequency change Δf curve caused by the rotation angle θ of each microprism array and the cavity mode wavelength change Δλ curve caused by the rotation angle θ of the microprism array are plotted, as shown below. Figure 5 , 6 As shown.

[0090] When θ changes continuously within the range of 0 to 14°, the cavity mode frequency and cavity mode wavelength of the microprism array change accordingly.

[0091] Taking θ = 10° as an example, from Figure 4 and Figure 5It can be seen that when D = 21 mm, the cavity mode frequency change Δf caused by rotating the microprism array is 1.25 × 10⁻⁶. 12 The cavity mode wavelength change is Δλ = -2.527 nm. When D = 23 mm, Δf = 1.37 × 10⁻⁶ Hz. 12 Hz, Δλ=-2.768nm. When D=25mm, Δf=1.48×10 12 Hz, Δλ=-3nm.

[0092] Then, based on the rotation angle of the narrowband interferometer The expression relating the wavelength to the transmission wavelength λ′ is as follows:

[0093]

[0094] Obtained through settlement Figure 7 The IF curve shows that when θ = 10°, the transmission wavelength λ′ = 777.1 nm of the interferometer. The calculated change in transmission wavelength Δλ′ = -2.9 nm is between Δλ = -2.768 nm and Δλ = -3 nm. Therefore, the optimal range for the vertical distance D between the microprism array and the axis is 23 mm to 25 mm.

[0095] Therefore, the thicknesses D1 and D2 of the first annular piezoelectric module 5 and the second annular piezoelectric module 55 range from 23mm to 25mm, but it is essential 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 20pm. Finally, the measured beat frequency Δf is converted into an angular change.

[0096]

[0097] Two sets of angle measurement devices were combined to form a single device, eliminating some common-mode noise and reducing the frequency fluctuation of the laser in the freely operating narrowband interferometer to the order of 200 Hz. Based on the relationship between the beat frequency change Δf and the rotation angle of the narrowband interferometer, when the angle between the laser and the interferometer is 2°, the relationship between the beat frequency and the change in angle rotation is 1.8 GHz / °. Therefore, the angle change corresponding to a 200 Hz frequency fluctuation is approximately 1.11 × 10⁻⁶. -7 ° (0.0004″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.0001″.

[0098] Example 2

[0099] The difference from Embodiment 1 is that the laser generator, narrowband interferometer, microprism array, and annular piezoelectric module on the first and second optical path mechanisms, along with the high-precision horizontal rotating stage and rotating stage mounting base, are fixed together on a base 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.

[0100] Similarly, the thicknesses D1 and D2 of the first annular piezoelectric module 5 and the second annular piezoelectric module 55 are set within the range of 23mm to 25mm. 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 microprism array 4 and the second microprism array 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.

[0101] Because ultra-low expansion coefficient glass can stabilize the frequency fluctuations of a freely operating narrowband interferometer laser within the Hz range, the change in beat frequency Δf and the rotation angle of the narrowband interferometer are related. From the relationship, we can see that when the angle between the laser and the interferometer is 2°, the relationship between the beat frequency and the change in angle rotation is 1.8 GHz / °. Therefore, the angle change corresponding to each 1 Hz is approximately 5.56 × 10⁻⁶. -10 ° (0.000002″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.000001″.

[0102] Compared to Example 1, where the frequency fluctuation of the laser during free operation was about 200Hz 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 0.000001″, thus achieving a significant improvement in angle measurement resolution.

[0103] This invention relates to a common-mode noise-reducing dual-laser frequency measurement ultra-high precision small-angle measurement reference device, which is based on the ultra-high sensitivity of laser frequency to cavity length in an external cavity semiconductor laser to achieve continuous measurement of small angles. This invention utilizes the ultra-high sensitivity of laser frequency to cavity length, changing the cavity length by rotating the cavity mirror, causing a frequency change, and converting the change in angle into a change in the frequency difference of the laser output from the two devices. A narrow-band interferometer is used for mode selection, ensuring that the cavity mode change is highly synchronized with the interferometric frequency selection, achieving mode-free laser output frequency, guaranteeing the continuous tunability of the output frequency with the rotation angle, solving the problem of continuous measurement, and thus achieving ultra-high precision measurement of small angles. This common-mode noise-reducing dual-laser frequency measurement ultra-high precision small-angle measurement reference device can meet the application needs of aerospace laser gyroscopes, precision optical devices, construction, and automotive fields.

Claims

1. A high-precision small-angle measurement reference device for common-mode noise reduction dual-laser frequency measurement, the device comprising a first optical path mechanism and a second optical path mechanism arranged in parallel and a beat frequency measurement module, wherein, The first optical path mechanism includes a first laser generating device. A high-precision horizontal rotating stage (7) is set on the output optical path of the first laser generating device. The first narrow-band interference plate (3), the first micro-prism array (4) and the first annular piezoelectric module (5) are fixed in parallel on the high-precision horizontal rotating stage (7). The rear surface of the first micro-prism array (4) is bonded to the front surface of the first annular piezoelectric module (5) and the center of the rear surface of the first annular piezoelectric module (5) is located at the axis of the rotating stage. The second optical path mechanism includes a second laser generator and a second narrowband interference plate (33), a second microprism array (44), and a second annular piezoelectric module (55) arranged in parallel. The rear surface of the second microprism array (44) is bonded to the front surface of the second annular piezoelectric module (55), and the rear surface of the second annular piezoelectric module (55) is in the same plane as the rear surface of the first annular piezoelectric module (5), and the center of the rear surface of the second annular piezoelectric module (55) is located at the axis of the rotating stage. The initial wavelengths of the first laser diode (1) and the second laser diode (11) are equal; The thickness D1 of the first annular piezoelectric module (5) is not equal to the thickness D2 of the second annular piezoelectric module (55); In each optical path mechanism, the light output from each laser generator is filtered out of the band by its respective narrowband interferometer, resulting in narrowband light. This narrowband light is then reflected back to the laser diode after passing through its respective microprism array. When the intracavity oscillation reaches a threshold, it is output from its respective microprism array, yielding the first and second laser beams under test. The two laser beams are respectively input to the beat frequency measurement module, which performs beat frequency measurement on the two beams. When the frequencies of the first and second laser beams change... and When, the change in beat frequency is equal to .

2. The ultra-high precision small angle measurement reference device according to claim 1, characterized in that... The first microprism array (4) and the second microprism array (44) are prism-type ultra-high intensity reflective films (71).

3. The 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 first optical path, and the center wavelength of the transmission peak of the first narrowband interferometer (3) matches 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 second optical path, and the center wavelength of the transmission peak of the second narrowband interferometer (33) matches the wavelength of the second laser diode (11).

4. The ultra-high precision small angle measurement reference device according to claim 1, characterized in that... The beat frequency measurement module includes a reflector (6) set in the direction of light emitted from the second annular piezoelectric module (55), a first half-wave plate (9) set in the direction of light reflected from the reflector (6), a second half-wave plate (10) set in the direction of light emitted from the first annular piezoelectric module (5), a polarizing beam splitter (15) with the output light of the first half-wave plate (9) and the second half-wave plate (10) as input, a broadband detector (12) set in the direction of light output from the polarizing beam splitter (15), and a frequency counter (13). The second laser beam under test is completely reflected after passing through the first half-wave plate (9) and entering the polarization beam splitter (15). The first laser beam under test is then passed through the second half-wave plate (10) and entering the polarization beam splitter (15). 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) obtains the beat frequency of the two laser beams under test and obtains the change in beat frequency. .

5. The ultra-high precision small angle measurement reference device according to claim 1, characterized in that... The first narrowband interference plate (3) is bonded to the front surface of the first microprism array (4), and the first narrowband interference plate (3), the first microprism array (4) and the first annular piezoelectric module (5) are fixed together as a whole and fixed on the rotating platform. The second narrowband interference plate (33) is bonded to the front surface of the second microprism array (44), and the second narrowband interference plate (33), the second microprism array (44), and the second annular piezoelectric module (55) are fixed together as a whole and fixed on the first annular piezoelectric module (5).

6. The 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 (14), on which the laser diode, collimating lens, narrow band interferometer, microprism array, annular piezoelectric module, high-precision horizontal rotary stage (7) and rotary stage fixing base (8) are directly or indirectly fixed.

7. The ultra-high precision small angle measurement reference device according to claim 1, characterized in that... The annular piezoelectric module is a piezoelectric ceramic.

8. A method for ultra-high precision small-angle measurement using common-mode noise-reducing dual-laser frequency measurement, the method comprising the following steps: (1) The laser emitted by the first laser diode passes through the first narrowband interference plate to obtain narrowband light; the narrowband light is reflected and returned to the first laser diode after passing through the first microprism array; when the cavity oscillation reaches the threshold, the first measured laser is output by the first microprism array. The laser emitted by the second laser diode passes through the second narrowband interferometer to obtain narrowband light; the narrowband light is reflected back to the second laser diode after passing through the second microprism array; when the intracavity oscillation reaches the threshold, the second measured laser is output by the second microprism array. The narrowband interferometer and the microprism array are fixed in parallel on a high-precision horizontal rotating stage. A ring-shaped piezoelectric module is bonded to the rear surface of the microprism array. The center of the rear surface of the ring-shaped piezoelectric module is located at the axis of the rotating stage. Therefore, when the rotating stage rotates, the narrowband interferometer and the microprism array change simultaneously with the incident angle and the change in angle is equal. (2) When the high-precision horizontal rotary table rotates, the narrowband interferometers of the first and second optical path mechanisms rotate synchronously with the microprism array. Based on the different values ​​of the thickness D of the annular piezoelectric module in each optical path mechanism, the rotation angle of the microprism array in each optical path mechanism is plotted as follows: The change in cavity mode wavelength caused by time curve, Rotation angle with the microprism array The following relationship must be satisfied: Where L is the length from the output light end face of the laser diode to the front surface of the microprism array in each optical path mechanism. Where λ is the initial wavelength of the laser, and D is the thickness of the annular piezoelectric module. The rotation angle of the microprism array is The change in cavity mode wavelength caused by time; (3) The rotation angle of the narrowband interferometer in each optical path mechanism is obtained by the following formulas. With transmission wavelength The relational expression is as follows: The initial transmission wavelength when the narrowband interferometer is perpendicular to the light beam. Let be the refractive index of the first and second narrowband interferometers, then the change in transmission wavelength caused by the rotation of the interferometers is... for: , (4) For the first optical path mechanism and the second optical path mechanism, the two laser beams to be measured in step (1) are input to the beat frequency measurement module, and the beat frequency measurement module performs beat frequency measurement on the two laser beams to be measured in the first optical path mechanism and the second optical path mechanism to obtain the beat frequency difference. , and These represent the changes in the first and second measured laser frequencies, respectively. (5) When the value of the thickness D1 of the first annular piezoelectric module satisfies and The condition is met, and the value of the thickness D2 of the second annular piezoelectric module satisfies... and Calculate the rotation angle of the microprism array under the following conditions: 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 20 pm. The change in the beat frequency Rotation angle with microprism array The relationship is: Where c is the speed of light.

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