Ultrahigh-precision small-angle measurement method and instrument for common-mode denoising dual-laser frequency measurement

Abstract

The invention provides a common-mode denoising dual-laser frequency measurement ultrahigh-precision small-angle measurement reference device, which comprises a first light path mechanism, a second light path mechanism and a beat frequency measurement module, the first light path mechanism and the second light path mechanism are arranged in parallel, and the light path mechanism comprises a laser generation device, a narrow-band interference sheet, a microprism array and a first annular piezoelectric module. And the components are fixed on a high-precision horizontal rotating table (7). According to the ultrahigh-precision small-angle measurement benchmark device for common-mode noise elimination double-laser frequency measurement, the cavity length is changed by rotating the cavity mirror to cause frequency change, the variable quantity of the angle is converted into the change of the frequency difference of laser output by the two devices, the narrow-band interference sheet is used for assisting adjustment to perform mode selection, the cavity mode change and interference frequency selection are highly synchronous, and the measurement accuracy is improved. According to the invention, mode hopping of the laser output frequency is avoided, continuous tunability of the output frequency along with the rotation angle is ensured, the problem of continuous measurement is solved, and small-angle ultra-high-precision measurement is realized.

Description

Technical Field

[0001] The invention relates to a small angle measurement reference device, in particular to an ultra-high precision small angle measurement reference device with common mode noise elimination and dual laser frequency measurement. Background Art

[0002] Improving the accuracy of small-angle measurement is crucial to many technical fields, involving multiple technical applications such as scientific research, engineering, manufacturing, navigation, etc. In scientific experiments and research, improving the accuracy of small-angle measurement can help scientists observe and measure tiny angle changes more accurately, thereby improving the reliability and accuracy of experiments.

[0003] In engineering fields, such as micromechanical systems (MEMS), precision instruments, and optical systems, improving the accuracy of small-angle measurement can improve the performance and stability of these systems. At present, the most commonly used angle measurements are based on the principle of laser interferometry (for example, the technical solutions disclosed in Chinese invention patents CN 104330054A, CN 1963384A, CN 102384730 B, and CN 102506768 B), that is, the change in angle is converted into the change in laser interference fringes. The small-angle measurement accuracy achieved by this type of method is relatively low and can no longer meet the demand for high-precision measurement. According to the records of the prior art, the highest accuracy that can be achieved by the currently implemented laser small-angle reference device is 0.001". With the development of science and technology, the requirements for high-precision measurement of small angles in application technologies such as lithography machines, laser radars, precision robots, and virtual reality are increasing. Further improving the accuracy of small-angle measurement has become a requirement for the application of these high-precision technologies. Summary of the invention

[0004] The purpose of the present invention is to further improve the accuracy of small angle measurement, convert the angle measurement into laser frequency measurement, and propose an ultra-high precision small angle measurement reference device with base common mode noise elimination and dual laser frequency measurement.

[0005] In the previous research work, the inventor designed a small angle measurement reference device based on interference frequency selection and micro-prism array feedback, and a small angle measurement reference device based on a narrow-band interference plate laser that converts angle measurement into laser frequency measurement. The ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser is utilized to integrate the micro-prism array with a piezoelectric module with a thickness of D, and the center position of the rear surface of the piezoelectric module is placed at the axis position of the rotating table. Even if the vertical distance between the rear surface of the micro-prism array and the axis is D, the optical path length of the laser through the prism array is changed by rotating the micro-prism array and the piezoelectric module, so as to change the cavity length of the resonant cavity and thus change the cavity mode frequency. Specifically, as Figure 1As shown, the small angle measurement reference device comprises a laser generating device and a beat frequency measurement module which are sequentially arranged on an optical path, wherein a high-precision horizontal rotating table 107 is arranged on the output light optical path of the laser generating device, a narrow-band interference plate 104, a micro-prism array 105 and a piezoelectric module 106 are fixed on the high-precision horizontal rotating table 107, the rear surface of the micro-prism array 105 is bonded to the front surface of the piezoelectric module 106 and the rear surface center of the piezoelectric module 106 is at the axis center position of the rotating table, and a polarization splitting prism is arranged between the laser generating device and the narrow-band interference plate 104. Mirror 103; the light output by the laser generating device is filtered out of the band after passing through the narrowband interference plate 104 to obtain narrowband light; the narrowband light is reflected after passing through the microprism array 105, and returns to the laser diode after passing through the polarization beam splitter prism 103. When the oscillation in the cavity reaches the threshold, the polarization beam splitter prism 103 reflects and outputs the measured laser; the measured laser is input to the beat frequency measurement module, and the beat frequency measurement module beats the measured laser with the comb teeth output by the optical comb system 110 to obtain the beat frequency. When the measured laser frequency changes by Δf, the change in the beat frequency is equal to Δf. The rotation angle of the narrowband interference plate is obtained by measuring the change in the beat frequency Δf.

[0006] Among them, f 0 is the measured laser frequency when the high-precision horizontal rotating stage is at the initial position, f 1 It is the frequency of the laser being measured after the high-precision horizontal rotating stage rotates.

[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. The rotation angle of the interference plate and the transmission wavelength have an approximate linear relationship within the range of ±1°. K nm / degree, so the rotation angle of the interference plate is Satisfy the formula:

[0008]

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

[0010]

[0011] However, since the frequency fluctuation of the measured laser under free-running conditions reaches tens of kHz, it will affect the resolution of angle measurement; at the same time, the standard beat frequency comparison of a single laser wavelength will exceed the frequency measurement range of the detector and cannot meet the measurement requirements. Therefore, it is necessary to further improve the structure of the small angle measurement device and its measurement method to make the measurement simpler and more accurate to meet the needs of high-precision measurement of small angles in application technologies such as lithography machines, laser radars, precision robots, and virtual reality. This is still a technical problem that this technical field wants to solve.

[0012] The idea of ​​the present invention is to add a set of angle measurement devices on the basis of previous research work to form a common mode device. Specifically, the two sets of angle measurement devices use annular piezoelectric modules of different thicknesses to support the vertical distance of the microprism array to the axis, so as to achieve the synchronization of the cavity mode changes caused by the rotating microprism array in the two sets of devices and the interference frequency selection, and compare the output frequencies of the two sets of angle measurement devices. While eliminating the common mode noise, the frequency difference of the two sets of devices is obtained, and the output frequency difference range is controlled within 100GHz, so that it can achieve angle measurement without exceeding the frequency measurement range of the detector. In order to solve the problem that the frequency fluctuation range of the measured laser is large under free running conditions in the previous research work and the output frequency range exceeds the measurement range of the detector, ultra-high precision continuous measurement of small angles can be achieved.

[0013] To this end, the present invention provides an ultra-high-precision small-angle measurement reference device for common-mode noise elimination 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 comprises a first laser generating device, a high-precision horizontal rotating table 7 is arranged on the output light path of the first laser generating device, a first narrow-band interference plate 3, a first microprism array 4 and a first annular piezoelectric module 5 are fixed in parallel on the high-precision horizontal rotating table 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 rear surface center of the first annular piezoelectric module 5 is at the axial center position of the rotating table;

[0014] The second optical path mechanism includes a second laser generating device and a second narrow-band 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 at the axial center position of the rotating table;

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

[0016] The thickness D of the first annular piezoelectric module 5 1is not equal to the thickness D of the second annular piezoelectric module 55 2 ;

[0017] In each optical path mechanism, the light output by each laser generating device passes through its own narrow-band interference plate, and then out-of-band light is filtered out to obtain narrow-band light; the narrow-band light is reflected by its own micro-prism array and returns to the laser diode, and when the intracavity oscillation reaches a threshold value, it is output by its own micro-prism array to obtain a first measured laser and a second measured laser; the two measured laser beams are respectively input to the beat frequency measurement module, and the beat frequency measurement module beats the two measured laser beams to obtain a beat frequency. When the frequency of the first measured laser and the frequency of the second measured laser change Δf respectively, 1 and Δf 2 When , the change in the beat frequency is equal to Δf = Δf 2 -Δf 1 .

[0018] In the present invention, the first and second micro-prism arrays can be realized by a prismatic super-strong reflective film with a high retroreflection coefficient, for example, the micro-prismatic reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0019] In each optical path of the present 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 position of the turntable. Therefore, the vertical distance D between the rear surface of the microprism array and the axis center of the turntable 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 the present invention, the first and second optical path structures are substantially the same, for example, their laser diodes and narrowband interference plates have the same parameters, but the thickness of the annular piezoelectric module is different, so they have different resonant cavity lengths. In each optical path, each optical element should be located in the optical path so that the laser can pass through.

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

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

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

[0024] When the half wavelength is changed according to the cavity length, the frequency change c / 2L is related to: The relationship between the resonant cavity jitter ΔL and the cavity mode frequency change Δf in the first optical path mechanism is obtained as follows:

[0025] Where f is the frequency, λ 0 is the initial wavelength emitted by the laser generator (the initial wavelengths of the first laser generator and the second laser generator are the same, both denoted as λ 0 ).

[0026] Known The wavelength change of the first cavity mode Δλ caused by the rotation of the microprism array by an angle θ can be obtained as 1 The vertical distance D between the microprism array and the axis 1 And the relationship between the rotation angle θ:

[0027] Assuming that the incident angle of the laser incident on the microprism array is 0°, the cavity length When the microprism array rotates by θ, the cavity length So when the microprism array rotates at an angle of θ, the cavity mode wavelength can be obtained: Where n is a positive number, D 1 is the thickness of the first annular piezoelectric module, that is, the vertical distance between the microprism array and the axis, and L1 is the length from the laser output light 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 first narrow-band interference plate is rotated by an angle The relationship with the transmission wavelength λ′ is as follows:

[0029]

[0030] λ 0 is the initial transmission wavelength when the narrowband interference plate is perpendicular to the light, n effis the refractive index of the narrowband interference plate, then the change in transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ 0 -λ′.

[0031] Since the narrow-band interference plate of the present invention is placed parallel to the microprism array, when the rotating platform rotates by an angle θ, the rotation angle of the narrow-band interference plate is is equal to the rotation angle θ of the microprism array. Therefore, by adjusting the vertical distance D between the first microprism array and the axis 1 The change in the cavity mode wavelength caused by rotating the microprism array by an angle θ is similar to the change in the narrow-band interference plate by rotating angle The transmission wavelength change caused by the above method is equal or approximately the same (in the present invention, "approximately the same" means that the error is extremely small and can be regarded as equal), so that the cavity mode frequency and the center frequency of the transmission peak of the interference plate can change synchronously, such as Figure 2 As shown in (b), the cavity mode frequency 2b in the first optical path is determined by f 1 Change to f 1 ', the change of the center frequency 1b of the interference plate transmission peak is Δf 1 , they change synchronously to ensure f 1 '-f 1 =Δf 1 , so that the output laser frequency can change continuously, thereby converting the continuous change of angle into continuous change of frequency, solving the laser frequency mode hopping phenomenon caused by the rotation of a single interference plate, and realizing continuous small angle measurement.

[0032] Similarly, for the second optical path mechanism, the thickness of the second annular piezoelectric module is set to D 2 (D 2 ≠D 1 ), that is, the vertical distance between the microprism array and the axis of the rotating stage is D 2 , so that it also satisfies the same requirement that the change in the cavity mode wavelength caused by the microprism array rotating at an angle θ is equal to the change in the transmission wavelength caused by the narrow-band interference plate rotating at an angle θ. Figure 2 As shown in (b), the cavity mode frequency 22b in the second optical path mechanism is f 1 Change to f″ 1 At this time, the change in the center frequency 11b of the interference plate transmission peak is Δf 2 , they change synchronously to ensure f″ 1 -f 1 =Δf 2 Since the thickness of the annular piezoelectric modules in the two optical paths is different, that is, the distances from the microprism arrays in the two paths to the axis are different, although the rotation angle θ is the same, the changes in the cavity mode frequency caused by the rotation angle θ of the microprism arrays in the two paths are different. Figure 2As can be seen from (b), the frequency difference between the two light paths input to the polarization beam splitter is Δf = Δf 2 -Δf 1 .

[0033] The changes in laser frequency generated by the output lasers of the two optical paths as the rotation angle θ of the rotating stage changes are: The frequency difference between the two laser beams is: Where c is the speed of light. That is, the beat frequencies of the two laser beams are compared to obtain the frequency difference between the two laser beams, and then the frequency difference is converted into the rotation angle of the rotating stage.

[0034] According to the linear relationship K′ (HZ / degree) between the rotation angle of the narrowband interference plate and the change in the beat frequency Δf, it is known that the relationship between the change in the beat frequency Δf and the rotation angle of the narrowband interference plate is:

[0035]

[0036] Wherein, K′ is the slope of the curve of rotation angle θ and 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 the present invention, the first laser generating device comprises a first laser diode (1) and a first collimating lens (2) arranged in sequence on a first optical path, and the transmission peak center wavelength of the first narrow-band interference plate (3) matches the wavelength of the first laser diode (1); the second laser generating device comprises a second laser diode (11) and a second collimating lens (22) arranged in sequence on a second optical path, and the transmission peak center wavelength of the second narrow-band interference plate (33) matches the wavelength of the second laser diode (11).

[0039] In the present invention, the laser generating device is a conventional setting in the art, and in the first and second optical path mechanisms, respectively includes a laser diode and a collimating lens arranged in sequence on the optical path. Among them, the working band of the laser diode can be any band. Preferably, from the perspective of easy acquisition or economical saving, a 780nm band laser diode can be used. Those skilled in the art can also choose laser diodes with other working wavelengths, such as 420nm, 850nm, 1550nm laser diodes, etc. However, the first laser diode and the second laser diode have the same band.

[0040] In this way, in the first and second optical path mechanisms, the divergent light output by the laser diode is collimated into parallel light by the collimating lens, and the parallel light is filtered out of the band after passing through the narrow-band interference plate to obtain narrow-band light; the narrow-band light reaches the front surface of the microprism array, and after passing through the microprism array, the reflected light in the opposite direction of the narrow-band light is obtained; the reflected light is fed back to the laser diode, and when the oscillation in the cavity reaches the threshold, it is output from the microprism array as the measured laser. The two measured laser beams arrive at the beat frequency measurement module for beat frequency comparison, and by measuring the change in the beat frequency Δf, the formula is used The frequency change is converted into a change in the rotation angle θ.

[0041] Beat frequency measurement belongs to the prior art in this field. As a preferred embodiment, it includes a reflector 6 arranged in the direction of the outgoing light of the second annular piezoelectric module 55, a first half-wave plate 9 arranged in the direction of the reflected light of the reflector 6, a second half-wave plate 10 arranged in the direction of the outgoing light of the first annular piezoelectric module 5, a polarization beam splitter prism 15 using the output light of the first half-wave plate 9 and the second half-wave plate 10 as input, a broadband detector 12 and a frequency counter 13 arranged in the direction of the output light of the polarization beam splitter prism 15;

[0042] The second measured laser is incident on the polarization beam splitter prism 15 through the first half-wave plate 9 and is completely reflected. The first measured laser is incident on the polarization beam splitter prism 15 through the second half-wave plate 10. The two measured laser beams are combined and arrive at 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 measured laser beams and obtains the change Δf of the beat frequency.

[0043] In the present invention, since the first optical path mechanism and the second optical path mechanism have the same structure and only the thickness of the annular piezoelectric module is different, as another implementation, the reflector 6 can also be arranged in the first optical path structure, that is, the reflector 6 is arranged in the outgoing light direction of the first annular piezoelectric module 5. Through the reflection of the reflector 6, the first and second measured lasers are also input into the polarization beam splitter prism to achieve the beat frequency measurement.

[0044] In the present invention, the broadband detector, frequency counter and optical comb system are all existing devices in the art, for example, a 50 GHz broadband detector is used.

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

[0046] As an embodiment of the present invention, the present invention can bond the first narrowband interference sheet 3 to the front surface of the first microprism array 4, and the first narrowband interference sheet 3, the first microprism array 4 and the first annular piezoelectric module 5 are fixed as a whole and fixed together on the rotating table; the second narrowband interference sheet 33 is bonded to the front surface of the second microprism array 44, and the second narrowband interference sheet 33, the second microprism array 44 and the second annular piezoelectric module 55 are fixed as a whole and fixed together on the first annular piezoelectric module 5. That is, the narrowband interference sheet, the microprism array and the annular piezoelectric module are integrated into one and fixed together on the rotating table, thereby improving the mechanical robustness of the system.

[0047] According to a preferred embodiment, the present invention also includes an ultra-low expansion glass base 14, on which the laser diode, collimating lens, integrated narrow-band interference plate-microprism array-annular piezoelectric module and high-precision horizontal rotating table 7 and rotating table fixing seat 8 on the first and second optical paths are fixed, which can be used to reduce the influence of cavity length jitter on the frequency of the measured laser during free operation, and stabilize the laser frequency fluctuation to the Hz level. This arrangement can further improve the angle measurement resolution.

[0048] In the present invention, the ultra-low expansion glass may be ultra-low expansion glass (ULE) sold by Corning Incorporated of the United States.

[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 light emitted by the first laser diode is passed through the first narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by the first micro-prism array and returns to the first laser diode; when the intracavity oscillation reaches a threshold value, the first micro-prism array outputs a first measured laser light;

[0051] The laser light emitted by the second laser diode is passed through the second narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by the second micro-prism array and returns to the second laser diode, and when the oscillation in the cavity reaches a threshold value, the second micro-prism array outputs a second measured laser light;

[0052] The narrowband interference plate and the microprism array are fixed in parallel on a high-precision horizontal rotating platform, and 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 position of the rotating platform. Therefore, when the rotating platform rotates, the narrowband interference plate and the microprism array change at the same time as the incident angle, and the angle change amount is equal;

[0053] (2) When the high-precision horizontal rotating stage rotates, the narrow-band interference plates of the first optical path mechanism and the second optical path mechanism rotate synchronously with the microprism array. According to the different values ​​of the thickness D of the annular piezoelectric module in each optical path mechanism, a curve of the change in the cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array is drawn. Δλ and the rotation angle θ of the microprism array satisfy the following relationship:

[0054]

[0055] Wherein, 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, λ 0 is the initial wavelength of the laser, D is the thickness of the annular piezoelectric module, and Δλ is the change in the cavity mode wavelength caused by the rotation angle of the microprism array to θ;

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

[0057]

[0058] λ 0 is the initial transmission wavelength when the narrowband interference plate is perpendicular to the light, n eff is the refractive index of the first and second narrowband interference plates, then the change in transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ 0 -λ′,

[0059] (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 beats the two laser beams to be measured in the first optical path mechanism and the second optical path mechanism to obtain a beat frequency difference Δf=Δf 2 -Δf 1 ;

[0060] (5) When the thickness D of the first annular piezoelectric module 1 The value of satisfies And Δλ 1 =Δλ 1 ' condition, and the thickness of the second annular piezoelectric module D 2 The value of satisfies And Δλ 2 =Δλ 2 ' condition (i.e. the change in the cavity mode wavelength Δλ caused by the rotation of the first microprism array by an angle θ 1 Rotate with the first narrowband interference plate Transmission wavelength change due to angle Δλ 1 ′ is approximately equal, and the change in the cavity mode wavelength Δλ caused by the rotation of the second microprism array by an angle θ 2Rotate with the second narrowband interference plate Transmission wavelength change due to angle Δλ 2 ′ are approximately equal), and D 1 ≠D 2 , when the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interference plate at the same angle does not exceed 20 pm, the rotation angle θ of the microprism array is calculated. The relationship between the change Δf of the beat frequency and the rotation angle θ of the microprism array is:

[0061] Where c is the speed of light.

[0062] According to the linear relationship K′ (HZ / degree) between the rotation angle of the narrowband interference plate and the change in the beat frequency Δf, it is known that the relationship between the change in the beat frequency Δf and the rotation angle of the narrowband interference plate is:

[0063]

[0064] Wherein, K′ is the slope of the curve of rotation angle and 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 the present invention uses a narrow-band interference plate to perform narrow-band filtering, filters the broadband light emitted by the laser diode into a narrow-band beam, and realizes the output of the measured laser whose laser frequency is extremely sensitive to the rotation angle under the light feedback of the microprism array, so that the small angle change of the rotation angle is reflected in the frequency difference Δf of the two measured laser beams. Among them, by adjusting the voltage of the annular piezoelectric module to optimize its own thickness D (i.e., the vertical distance between the rear surface of the microprism array and the axis), the change of the cavity mode frequency can be synchronized with the change of the central frequency of the transmission peak of the interference plate, so as to solve the problem of laser frequency mode hopping caused by the rotation of a single interference plate and the small frequency tunable range caused by the single change of the cavity length (rotation of the microprism array).

[0067] Since the frequency fluctuation of the free-running narrowband interference plate laser of a single set of angle measurement device is in the order of tens of kHz, the angle measurement limit can be converted into a laser frequency measurement of tens of kHz, and the corresponding angle measurement limit is about 0.0001″. The present invention adopts the first and second optical path mechanisms arranged in parallel to form the two sets of angle measurement devices into one set of devices, thereby eliminating a part of the common mode noise, reducing the frequency fluctuation of the free-running narrowband interference plate laser of the whole set of devices to the order of several kHz, and converting the angle measurement limit into a laser frequency measurement of several kHz, and the corresponding angle measurement limit is about 0.0001″. When the frequency fluctuation of the free-running narrowband interference plate laser is further stabilized at the order of Hz through an ultra-low expansion rate glass base, the corresponding angle measurement resolution can be achieved at the order of about 0.000001″, achieving a substantial improvement in the angle measurement resolution of ultra-high precision small angle metrology.

[0068] The ultra-high precision common-mode noise-eliminating dual laser frequency measurement of the present invention uses two sets of angle measurement devices to measure angles, and converts the change in angle into the change in the frequency difference of the laser output by the two sets of devices. While not exceeding the measurement range of the detector, a part of the common-mode noise is effectively eliminated, so that the angle measurement limit reaches the order of 0.00001". The device can be applied to MEMS, precision instruments and optical systems to meet the application requirements of such high-precision equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 This is a schematic diagram of the structure of the small angle measurement reference device previously studied by the inventor;

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

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

[0072] Figure 3 This is a schematic structural diagram of a small angle measurement device according to Embodiment 1;

[0073] Figure 4 This is a schematic structural diagram of a small angle measurement device according to Embodiment 2;

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

[0075] Figure 6 The relationship between the rotation angle of the microprism array and the change of the cavity mode wavelength in Example 1;

[0076] Figure 7 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 interference plate and the transmission wavelength in Example 1;

[0077] Among them, 1. a first laser diode, 2. a first collimating lens, 3. a first narrow-band interference plate, 4. a first microprism array, 5. a first annular piezoelectric module;

[0078] 11. a second laser diode, 22. a second collimating lens, 33. a second narrow-band interference plate, 44. a second microprism array, 55. a second annular piezoelectric module;

[0079] 6. Reflector, 7. High-precision horizontal rotating table, 8. Rotating table fixing seat, 9. First half-wave plate, 10. Second half-wave plate, 12. Broadband detector, 13. Frequency counter, 14. Ultra-low expansion glass base, 15. Polarization beam splitter prism. DETAILED DESCRIPTION

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

[0081] Example 1 investigates the influence of the thickness D of the annular piezoelectric module, that is, the different vertical 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 working band of 780nm. The device mainly includes a first laser diode 1, a first collimating lens 2, a first narrowband interference plate 3, a first microprism array 4 and a first annular piezoelectric module 5 which are sequentially arranged on a first optical path. The first narrowband interference plate 3, the first microprism array 4 and the first annular piezoelectric module 5 are sequentially bonded and fixed as a whole, so that the rear surface center of the first annular piezoelectric module 5 is at the axis center position of the rotating table, and is fixed on a high-precision horizontal rotating table 7, and the high-precision horizontal rotating table 7 is fixed on a rotating table fixing seat 8. Similarly, the second optical path mechanism includes a second laser generating device and a second narrow-band interference plate 33, a second microprism array 44 and a second annular piezoelectric module 55 which are also bonded and fixed as one body, 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 also at the axial position of the rotating table, for example, the second annular piezoelectric module 55 is fixed directly above the first annular piezoelectric module 5.

[0083] A reflector 6 is arranged behind the second annular piezoelectric module 5. A beat frequency measurement module is arranged in the direction of reflected light from the reflector. When the high-precision horizontal rotating stage 7 rotates counterclockwise about the center by θ, the first and second narrow-band interference plates and the second and second prism-type super-strong reflective films rotate by θ at the same time.

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

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

[0086] When working, the light output by the 780nm laser diode of the first optical path mechanism is collimated, and then the out-of-band light is filtered out by the narrow-band interference plate to obtain narrow-band light. The narrow-band light reaches the front surface of the micro-prism array, and after passing through the micro-prism array, the reflected light in the opposite direction of the narrow-band light is obtained; the reflected light is fed back to the laser diode, and when the oscillation in the cavity reaches the threshold, it is output through the micro-prism array as the first measured laser. Similarly, the second optical path mechanism obtains the second measured laser.

[0087] The two measured laser beams arrive at the beat frequency measurement module. The beat frequency measurement module compares the beat frequencies of the two measured laser beams to obtain the beat frequencies of the two laser beams. When the first measured laser frequency and the second measured laser frequency change Δf respectively 1 and Δf 2 When , the change in the beat frequency is equal to Δf = Δf 2 -Δf 1 .

[0088] In order to convert the change of the beat frequency Δf into the change of the microprism array rotation angle θ, firstly, the change of the cavity mode wavelength Δλ caused by the rotation angle of the microprism array is determined by measurement:

[0089] In this embodiment, the cavity length is set to L = 10 cm, λ 0 =780nm, select annular piezoelectric modules with different thicknesses D, that is, the vertical distances between the microprism array and the axis are D = 21mm, D = 23mm, D = 25mm, and D = 27mm respectively. By measuring and plotting the cavity mode frequency change Δf curve caused by the rotation angle of each microprism array being θ and the cavity mode wavelength change Δλ curve caused by the rotation angle of the microprism array being θ, as shown in Figure 5 , 6 shown.

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

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

[0092] Then, according to the narrowband interference plate rotation angle The relationship expression with the transmission wavelength λ′:

[0093]

[0094] Obtained through settlement Figure 7 From the IF curve, we know that when θ=10°, the transmission wavelength λ′ of the interference plate is 777.1nm. By calculation, the transmission wavelength change of the interference plate is Δλ′=-2.9nm, which is between Δλ=-2.768nm and Δλ=-3nm. Therefore, the optimal range of the vertical distance D between the microprism array and the axis is 23mm~25mm.

[0095] Therefore, the thickness D of the first annular piezoelectric module 5 and the second annular piezoelectric module 55 is 1 , D 2 The value range is 23mm~25mm, but it must be ensured that the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interference plate at the same angle does not exceed 20pm. Finally, the measured beat frequency Δf is converted into angle change:

[0096]

[0097] Two sets of angle measurement devices are used to form a set of devices, which eliminates part of the common mode noise and reduces the frequency fluctuation of the narrow-band interference plate laser to the order of 200 Hz when the whole device is free to run. According to the relationship between the change of the beat frequency Δf and the rotation angle of the narrow-band interference plate, when the angle between the laser and the interference plate is 2°, the relationship between the beat frequency and the change of the angle rotation is 1.8 GHz / °. Therefore, the angle change corresponding to the 200 Hz frequency fluctuation is about 1.11×10 -7 °(0.0004″), so the corresponding angle measurement resolution can be achieved at the order of 0.0001″.

[0098] Example 2

[0099] The difference from Example 1 is that the laser generating device, narrow-band interference plate, microprism array and annular piezoelectric module on the first and second optical path mechanisms are fixed together with the high-precision horizontal rotating table and the rotating table fixing seat on a base made of ultra-low expansion coefficient glass, and the ultra-low expansion coefficient characteristics of the base are used to further overcome the interference caused by mechanical deformation. The rest of the structure remains unchanged.

[0100] Similarly, the thickness D of the first annular piezoelectric module 5 and the second annular piezoelectric module 55 are 1 , D 2 The range is set to 23mm to 25mm. Through the high-precision horizontal rotating stage 7, the angles between the incident light and the first narrowband interference plate 3 and the second narrowband interference plate 33, as well as the angles between the incident light and the first microprism array 4 and the second microprism array 44 are adjusted, so that the center position of the transmission peak of the interference plate in the two optical paths moves with the angle, and the corresponding specific cavity mode frequency changes with the angle, respectively, and the laser frequency can be continuously tuned over a large range. Then, the beat frequency Δf obtained by beating the output lasers under test of the two optical paths is converted into an angle change.

[0101] Since the ultra-low expansion glass can stabilize the frequency fluctuation of the free-running narrow-band interference plate laser at the Hz level, according to the change of the beat frequency Δf and the rotation angle of the narrow-band interference plate, From the relationship, it can be seen that when the angle between the laser and the interference plate is 2°, the relationship between the beat frequency and the angle rotation change is 1.8GHz / °, so the angle change corresponding to each 1Hz is about 5.56×10 -10 °(0.000002″), so the corresponding angle measurement resolution can be achieved at the order of 0.000001″.

[0102] Compared with the frequency fluctuation of about 200 Hz during free operation of the laser after eliminating part of the common mode noise in Example 1, the present embodiment uses ultra-low expansion glass to improve the stability of the frequency fluctuation during free operation of the laser to the Hz level, so that the corresponding angle measurement resolution can be achieved at the order of about 0.000001", achieving a significant improvement in the angle measurement resolution.

[0103] The ultra-high precision small-angle measurement reference device with common-mode noise elimination and dual laser frequency measurement of the present invention is a small-angle measurement reference device that realizes continuous measurement based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. With the ultra-high sensitivity of the laser frequency to the cavity length, the present invention changes the cavity length by rotating the cavity mirror, causing a frequency change, and converts the angle change into a change in the frequency difference of the laser output by the two sets of devices. The mode selection is performed by auxiliary adjustment using a narrow-band interference plate, so that the cavity mode change is highly synchronized with the interference frequency selection, and the laser output frequency does not jump mode, ensuring the continuous tunability of the output frequency with the rotation angle, solving the problem of continuous measurement, and then realizing ultra-high precision measurement of small angles. The ultra-high precision small-angle measurement reference device with common-mode noise elimination and dual laser frequency measurement of the present invention can meet the application needs of laser gyroscopes, precision optical devices, construction, and automobiles in the fields of aerospace.

Claims

1. An ultra-high precision small angle measurement reference device for common mode noise elimination 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 comprises a first laser generating device, a high-precision horizontal rotating platform (7) is arranged on the output light path of the first laser generating device, a first narrow-band interference plate (3), a first microprism array (4) and a first annular piezoelectric module (5) are fixed in parallel on the high-precision horizontal rotating platform (7), a rear surface of the first microprism array (4) is bonded to a 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 center position of the rotating platform; The second optical path mechanism comprises a second laser generating device and a second narrow-band interference plate (33) arranged in parallel, a second microprism array (44) and a second annular piezoelectric module (55), the rear surface of the second microprism array (44) is bonded to the front surface of the second annular piezoelectric module (55), the rear surface of the second annular piezoelectric module (55) and the rear surface of the first annular piezoelectric module (5) are in the same plane, and the center of the rear surface of the second annular piezoelectric module (55) is located at the axial center position of the rotating table; 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 by each laser generating device passes through its own narrow-band interference plate, and out-of-band light is filtered out to obtain narrow-band light; the narrow-band light is reflected after passing through its own microprism array and returns to the laser diode. When the intracavity oscillation reaches a threshold value, it is output by its own microprism array to obtain a first measured laser and a second measured laser; the two measured laser beams are respectively input to the beat frequency measurement module, and the beat frequency measurement module beats the two measured laser beams. When the frequency of the first measured laser and the frequency of the second measured laser change by Δf1 and Δf2 respectively, the change in the beat frequency is equal to Δf=Δf2-Δf1.

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

3. The ultra-high precision small angle measurement reference device according to claim 1 is characterized in that The first laser generating device comprises a first laser diode (1) and a first collimating lens (2) which are sequentially arranged on a first optical path, and the transmission peak center wavelength of the first narrow-band interference plate (3) matches the wavelength of the first laser diode (1); the second laser generating device comprises a second laser diode (11) and a second collimating lens (22) which are sequentially arranged on a second optical path, and the transmission peak center wavelength of the second narrow-band interference plate (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 comprises a reflector (6) arranged in the direction of the light emitted by the second annular piezoelectric module (55), a first half-wave plate (9) arranged in the direction of the light reflected by the reflector (6), a second half-wave plate (10) arranged in the direction of the light emitted by the first annular piezoelectric module (5), a polarization beam splitter prism (15) using the output light of 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) arranged in the direction of the light output by the polarization beam splitter prism (15); The second measured laser beam is incident on the polarization beam splitter prism (15) through the first half-wave plate (9) and is completely reflected. The first measured laser beam is incident on the polarization beam splitter prism (15) through the second half-wave plate (10). The two measured laser beams are combined and arrive at 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 measured laser beams and obtains the change amount Δf of the 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); the first narrowband interference plate (3), the first microprism array (4) and the first annular piezoelectric module (5) are fixed as a whole and fixed together on the rotating platform; The second narrowband interference plate (33) is bonded to the front surface of the second microprism array (44); the second narrowband interference plate (33), the second microprism array (44) and the second annular piezoelectric module (55) are fixed as a whole and fixed together 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 glass base (14), and the laser diode, collimating lens, narrow-band interference plate, microprism array, annular piezoelectric module, high-precision rotating table (7) and rotating table fixing seat (8) are directly or indirectly fixed on the ultra-low expansion glass base (14).

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

8. An ultra-high precision small angle measurement method for common mode noise elimination dual laser frequency measurement, the method comprising the following steps: (1) The laser light emitted by the first laser diode is passed through the first narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by the first micro-prism array and returns to the first laser diode; when the intracavity oscillation reaches a threshold value, the first micro-prism array outputs a first measured laser light; The laser light emitted by the second laser diode is passed through the second narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by the second micro-prism array and returns to the second laser diode, and when the oscillation in the cavity reaches a threshold value, the second micro-prism array outputs a second measured laser light; The narrowband interference plate and the microprism array are fixed in parallel on a high-precision horizontal rotating platform, and 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 position of the rotating platform. Therefore, when the rotating platform rotates, the narrowband interference plate and the microprism array change at the same time as the incident angle, and the angle change amount is equal; (2) When the high-precision horizontal rotating stage rotates, the narrow-band interference plates of the first optical path mechanism and the second optical path mechanism rotate synchronously with the microprism array. According to the different values ​​of the thickness D of the annular piezoelectric module in each optical path mechanism, a cavity mode wavelength change Δλ curve caused by the rotation angle θ of the microprism array in each optical path mechanism is drawn respectively. Δλ and the rotation angle θ of the microprism array satisfy the following relationship: Wherein, 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 the cavity mode wavelength caused by the rotation angle of the microprism array to θ; (3) The rotation angle of the narrow-band interference plate in each optical path mechanism is obtained by the following formula: The relationship between the transmission wavelength λ′ is as follows: λ0 is the initial transmission wavelength when the narrowband interference plate is perpendicular to the light, n eff is the refractive index of the first and second narrowband interference plates, then the change in transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ0-λ′, (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 a beat frequency measurement module, and the beat frequency measurement module performs beat frequency measurement on the two laser beams to be measured from the first optical path mechanism and the second optical path mechanism to obtain a beat frequency difference Δf=Δf2-Δf1, where Δf1 and Δf2 are changes in the frequency of the first laser beam to be measured and the frequency of the second laser beam to be measured, respectively; (5) When the value of the thickness D1 of the first annular piezoelectric module satisfies And Δλ1=Δλ1′, and the value of the thickness D2 of the second annular piezoelectric module satisfies When the conditions of Δλ2=Δλ2′ and D1≠D2 are met, and the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interference plate at the same angle does not exceed 20 pm, the rotation angle θ of the microprism array is calculated. The relationship between the change Δf of the beat frequency and the rotation angle θ of the microprism array is: Where c is the speed of light.

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