Common-mode denoising ultrahigh-precision continuous-measurement small-angle measurement method and instrument

Abstract

The invention provides an ultrahigh-precision small-angle measurement reference device for common-mode denoising dual-laser frequency measurement, which comprises a first light path mechanism, a second light path mechanism, a partial reflection mirror (5) and a beat frequency measurement module, wherein the first light path mechanism and the second light path mechanism are arranged in parallel; the fixing positions of the narrow-band interference sheet and the transparent solid medium are in axial symmetry with the diameter, perpendicular to the light path, of the rotating table as the axis and are not parallel. According to the invention, two sets of angle measuring devices are adopted to realize small-angle measurement, a part of common-mode noise is eliminated, the frequency fluctuation of the narrow-band interference sheet laser during free operation of the whole set of device is reduced to hundreds of Hz magnitude, and the corresponding angle measurement limit is about 0.0001 ''magnitude. When the frequency fluctuation of the free operation narrow-band interference sheet laser is further stabilized at the Hz magnitude through the ultra-low expansion rate glass base, the corresponding angle measurement resolution can achieve about 0.000001 ''magnitude, and the ultra-high precision angle measurement resolution of small angle measurement is greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a small angle measurement reference device, in particular to a common mode noise elimination ultra-high precision continuous measurement small angle measurement reference device based on interference frequency selection principle. BACKGROUND

[0002] The improvement of small angle measurement precision is crucial to many technical fields, involving scientific research, engineering, manufacturing, navigation and other technical applications. In scientific experiments and research, improving the precision of small angle measurement can help scientists more accurately observe and measure small angle changes, thereby improving the reliability and accuracy of experiments.

[0003] In the engineering field, such as micro-mechanical system (MEMS), precision instruments and optical systems, improving the precision of small angle measurement can improve the performance and stability of these systems. At present, the most commonly used angle measurement is based on the principle of laser interference (for example, the technical solutions disclosed in Chinese invention patents CN 104330054A, CN 1963384A, CN 102384730B and CN 102506768B), that is, the change of angle is converted into the change of laser interference fringe. This kind of method realizes low precision of small angle measurement, which has been unable to meet the demand of high precision measurement. According to the records of the prior art, the highest precision that the current laser small angle reference device can achieve is 0.001". With the development of technology, such as photolithography, laser radar, precision robot, virtual reality and other application technologies, the demand for high precision measurement of small angle is increasing, and further improving the precision of small angle measurement has become the requirement of these high-tech applications. SUMMARY

[0004] The purpose of the present application is to further improve the precision of small angle measurement, based on the conversion of angle measurement to laser frequency measurement, a small angle measurement reference device with high resolution and continuous measurement is proposed.

[0005] The inventors designed a new ultra-high precision small angle measurement reference device in the previous research work, which is a small angle measurement reference device based on narrow-band interference sheet laser that converts angle measurement to laser frequency measurement. Specifically, as shown in Figure 1As shown, the small-angle measurement reference device includes a laser generator, a reflector group, and a beat frequency measurement module arranged sequentially on the optical path. A high-precision horizontal rotating stage 105 is positioned between the laser generator and the reflector group. A narrow-band interferometer 103 and a transparent solid medium 104 are fixed on the high-precision horizontal rotating stage 105. The fixed positions of the narrow-band interferometer 103 and the transparent solid medium 104 are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel. The light output from the laser generator is filtered out of the band by the narrow-band interferometer 103, resulting in narrow-band light. The narrow-band light returns to the laser diode after passing through the transparent solid medium 104 and the reflector group. When the intracavity oscillation reaches a threshold, the reflector group outputs the laser under test. The laser under test is input to the beat frequency measurement module, which performs a beat frequency measurement by comparing the laser under test with the comb teeth output from the optical comb system. When the frequency of the laser under test changes by Δf, the change in the beat frequency is equal to Δf. The rotation angle of the narrowband interferometer can be calculated by measuring the change in beat frequency Δf.

[0006] Where Δf = f0 - f1, f0 is the measured laser frequency when the high-precision horizontal rotary table is in the 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 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 narrowband interferometer has a linear relationship K (nm / degree) with the change in transmission wavelength Δλ within ±1°, it is known that the change in beat frequency Δf is related to the rotation angle of the narrowband interferometer. The relationship is:

[0008]

[0009] Where K is the value of the thickness D of the transparent solid medium (104) when it satisfies the rotation angle of the interference plate. When the rotation angle θ of the interferometer and the transparent solid medium are equal and the wavelength changes caused by both are approximately equal, the rotation angle of the interferometer... The slope of the curve with respect to λ′, and the value of K, can be obtained by measuring the rotation angle of the narrow-band interferometer and the change in transmission wavelength, or by using the following formula:

[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. Furthermore, comparing the beat frequency of a single laser wavelength standard exceeds the frequency measurement range of the detector, failing to meet measurement requirements. Therefore, further improvements are needed to the structure of the small-angle measuring device and its measurement method.

[0012] The present invention builds upon previous research by adding a small-angle measurement device, forming a common-mode device using two identical structures. By employing transparent solid media of different thicknesses in the two angle measurement devices, the aim is to synchronize the cavity mode changes and interference frequency selection caused by rotating the transparent solid media in each device. The output frequencies of the two angle measurement devices are compared by beat frequency comparison, eliminating common-mode noise and obtaining the frequency difference between the two devices. This frequency difference is controlled within 100 GHz, ensuring angle measurement without exceeding the detector's frequency measurement range. This addresses the issues of large frequency fluctuations and output frequency ranges exceeding the detector's measurement range encountered in previous research under free-running conditions, achieving ultra-high precision continuous measurement of small angles.

[0013] To this end, the present invention provides a common-mode noise reduction, ultra-high precision, and continuously measurable small-angle measurement reference device, comprising a first optical path mechanism and a second optical path mechanism arranged in parallel, a partial reflector 5, and a beat frequency measurement module. The first optical path mechanism includes a first laser generator, and a high-precision horizontal rotating stage 7 is arranged on the output optical path of the first laser generator. The fixed positions of the first narrow-band interferometer 3 and the first transparent solid medium 4 are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel.

[0014] The second optical path mechanism includes a second laser generating device and a second narrowband interferometer 33 and a second transparent solid medium 44 located on the output optical path of the second laser generating device. The second transparent solid medium 44 is fixed above the first transparent solid medium 4 and their front surfaces are located on the same plane. The second narrowband interferometer 33 is fixed above the first narrowband interferometer 3.

[0015] Part of the reflector 5 is located in the output light direction of the first transparent solid medium 4 and the second transparent solid medium 44;

[0016] The initial wavelengths of the first laser generating device and the second laser generating device are equal, but the thicknesses of the first transparent solid medium 4 and the second transparent solid medium 44 are not equal.

[0017] In the first and second optical path mechanisms, the light output from each laser generator is filtered out of the band by a narrow-band interferometer, resulting in narrow-band light. This narrow-band light then returns to the laser generator after passing through a transparent solid medium and a set of mirrors. When the intracavity oscillation reaches a threshold, the first and second laser beams under test are output by a portion of the mirrors 5. The two laser beams are input to the beat frequency measurement module, which performs beat frequency measurement on them. 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. By measuring the change in beat frequency, the rotation angle of the rotary table is converted, thus enabling small-angle measurement.

[0018] 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 transparent solid medium, 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.

[0019] 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 the "front" or "upstream" direction, and the position of some of the reflectors in the optical path is referred to as the "rear" or "downstream" direction.

[0020] Taking the first optical path mechanism as an example, combined with Figures 2-3 Its working principle is explained in detail below:

[0021] like Figure 2 As shown in (a), when the first transparent solid medium of the first optical path mechanism is perpendicular to the optical path (i.e., the incident angle is 0°): cavity length Therefore, when the first transparent solid medium rotates from 0° to θ, the cavity length... The resulting change in cavity length is: Where D1 is the thickness of the first transparent solid medium, n eff Let n be the refractive index of the first transparent solid medium, n be a positive number, L be the length from the output surface of the first laser to the partial reflector, L1 be the length from the output surface of the first laser to the front surface of the first transparent solid medium, and L2 be the length from the rear surface of the first transparent solid medium to the front surface of the partial reflector.

[0022] The relationship between the frequency change c / 2L and the change of half wavelength based on the cavity length is as follows:

[0023]

[0024] The relationship between the resonant cavity jitter ΔL and the cavity mode frequency change Δf can be obtained as follows:

[0025]

[0026] Where f is the frequency, c is the speed of light, and λ0 is the initial wavelength of the first laser.

[0027] Regarding wavelength variation, it is known that wavelength and frequency satisfy the following relationship. The relationship between the change in cavity mode wavelength Δλ1 caused by the rotation of the first transparent solid medium and the thickness of the transparent solid medium and its rotation angle θ can be obtained:

[0028]

[0029] Assuming the incident angle of the first transparent solid medium is 0°, the cavity length is...

[0030] When the transparent solid medium rotates by θ, the cavity length Therefore, when the transparent solid medium is rotated by an angle of θ, the wavelength at that time can be obtained. Where n eff Let n be the refractive index of the transparent solid medium, n be a positive number, D1 be the thickness of the transparent solid medium in ray 1, L be the length from the laser output surface to the front surface of the partial reflector, L1 be the length from the laser output surface to the front surface of the transparent solid medium, and L2 be the length from the rear surface of the transparent solid medium to the front surface of the partial reflector.

[0031] In this invention, the narrowband interferometer rotation angle The relationship between the wavelength and the transmission wavelength λ′ is expressed by the following formula:

[0032]

[0033] λ′0 is the transmission wavelength when the narrowband interferometer is perpendicular to the beam, and n eff2 Let be the refractive index of the first narrowband interferometer. Then, the change in transmission wavelength Δλ′ caused by the rotation of the interferometer is: Δλ′=λ0-λ′. Since the positions of the narrowband interferometer and the transparent solid medium are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel, the rotation angle of the narrowband interferometer is... The rotation angle θ is equal to that of the transparent solid medium.

[0034] Therefore, since the positions of the narrowband interferometer and the transparent solid medium in this invention are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel, when the rotation angle of the narrowband interferometer is... At the same time, the transparent solid medium undergoes an angle rotation of θ, and at this time... Equal to θ. By setting the thickness D1 of the transparent solid medium, the change in cavity mode wavelength caused by rotating the transparent solid medium by an angle θ is equal to the change in the narrowband interferometer plate. The changes in transmission wavelength caused by the angle are equal (or approximately the same. In this invention, "approximately the same" is understood as having an error that is extremely small and can be considered equal), which allows the cavity mode frequency of the first optical path to change synchronously with the transmission peak frequency 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, realizing continuous small angle measurement.

[0035] Similarly, for the second optical path mechanism, a transparent solid medium with a thickness of D2 (D2≠D1) is placed in the second optical path, so that it also satisfies the same condition as the change in cavity mode wavelength caused by the rotation angle θ of the transparent solid medium and the rotation of the narrowband interferometer. The changes in transmission wavelength caused by the angle are equal, and like Figure 2 As shown in (b), in the second optical path, the cavity mode frequency 22b changes from f1 to f1″. At this time, the change in the center frequency 11b of the transmission peak of the interferometer is Δf2. Their synchronous changes ensure that f1″ - f1 = Δf2. Because the thicknesses of the transparent solid media in the two optical paths are different, the changes in cavity mode frequency caused by rotating the transparent solid media by the same angle θ in each path are different. Figure 2 As can be seen from (b), the frequency difference between the two optical paths is Δf = Δf2 - Δf1.

[0036] Specifically, the changes in laser frequency generated by the output laser from the two optical paths as the rotation angle θ of the rotary table changes are as follows:

[0037]

[0038] The frequency difference between the two measured laser beams is:

[0039]

[0040] The beat frequencies of the two laser beams are compared, and the frequency difference Δf between the two laser beams is obtained by measurement, and then converted into an angle.

[0041] 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:

[0042]

[0043] 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 Δf of the two measured lasers, or by the following formula:

[0044]

[0045] In this invention, the thickness of the first and second transparent solid media must be selected to ensure that the difference between the cavity mode wavelength and the transmission wavelength of the interferometer at the same angle in their respective optical paths does not exceed 20 pm, so as to ensure continuous output of laser frequency for each path.

[0046] In this invention, the laser generating devices in the first and second optical path mechanisms are conventional configurations in the art, typically including laser diodes and collimating lenses sequentially arranged in their respective optical paths. The operating wavelength of the laser diodes can be any wavelength; preferably, for ease of acquisition or economic considerations, a 780nm wavelength laser diode can be used. Those skilled in the art can also choose laser diodes with other operating wavelengths, such as 420nm, 850nm, or 1250nm laser diodes. The operating wavelengths of the laser diodes in the first and second optical path mechanisms can be aligned using a cold atom optical frequency reference.

[0047] In this invention, within their respective optical paths, a transparent solid medium of appropriate thickness and a narrowband interferometer are fixed together on a high-precision horizontal rotating stage. The placement of the transparent solid medium and the narrowband interferometer is symmetrical about the diameter of the rotating stage perpendicular to the optical path and is not parallel. Figure 2 As shown in (a), at time t0, there is an initial angle α between the narrowband interferometer and the optical path, and an initial angle β between the transparent solid medium and the optical path, where β = α. At time t1, when the high-precision horizontal rotary stage rotates counterclockwise by θ around the center, the angle of the interferometer increases (i.e., α′ = α + θ), and the center frequency of the transmission peak of the interferometer increases accordingly; the angle of the transparent solid medium decreases (i.e., β′ = β - θ), the cavity length of the resonant cavity decreases, and the cavity mode frequency also increases. When a transparent solid medium of appropriate thickness is selected so that the change in cavity mode frequency is synchronized with the change in the center frequency of the transmission peak of the interferometer, the problems of laser frequency mode jumping caused by the rotation of a single interferometer and the small frequency tunable range caused by the rotation of a single transparent solid medium can be avoided, thereby realizing continuous change of output laser frequency and thus continuous measurement.

[0048] In a preferred embodiment of this invention, the reflector assembly includes a portion of the reflectors disposed along the optical path. Preferably, a focusing lens and a collimating lens may also be disposed before and after the portion of the reflectors, respectively. Those skilled in the art can also selectively add / remove optical elements along the optical path based on laser design principles.

[0049] Beat frequency measurement is a prior art in this field. In a preferred embodiment, the beat frequency measurement module includes a reflector 6 disposed in the second optical path and a first half-wave plate 9 disposed in the direction of the reflected light output from the reflector 6, placing the reflector 6 in the second optical path. A second half-wave plate 10, a polarizing beam splitter 11, a broadband detector 13 disposed in the output light direction of the polarizing beam splitter 11, and a frequency counter 13 are disposed in the first optical path. The second laser beam to be measured is reflected by the reflector 6, passes through the first half-wave plate 9, and enters the polarizing beam splitter 11, where it is completely reflected. The first laser beam to be measured passes through the second half-wave plate 10 and then enters the polarizing beam splitter 11. The two laser beams are combined and reach the broadband detector 12, which is connected to the frequency counter 13. The beat frequency is obtained from the frequency counter 13. When the rotating stage rotates, causing a change in the beat frequency, the change in beat frequency Δf is obtained through Δf = Δf2 - Δf1, and then converted into a rotation angle.

[0050] As an alternative implementation, the reflector 6 is placed in the first optical path. The first laser to be measured is reflected by the reflector 6 and then enters the polarization beam splitter 11 and is completely reflected. The second laser to be measured enters the polarization beam splitter 11, and the two laser beams are combined to reach the broadband detector 12.

[0051] 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.

[0052] As an alternative technical solution, a prism-type partial reflective film 51 can be used instead of the partial reflector 5. When this technical solution is adopted, the transparent solid medium 4 can be bonded to the prism-type partial reflective film 51, improving the mechanical robustness of the system. Generally, prism-type partial reflective films with high retroreflection coefficients can achieve the technical solution of this invention. For example, the microprism-type reflective film disclosed in Chinese invention patent application CN 201410823342.3 is used. This prism-type partial reflective film is set in a direction perpendicular to the incident light, and the incident light returns along the original path within a rotation angle range of ±20°, which can achieve intracavity oscillation output of the measured laser.

[0053] 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.

[0054] According to a preferred embodiment, the present invention further includes an ultra-low expansion coefficient glass base 52, on which the laser diode, narrowband interferometer, transparent solid medium, prism-type partial reflective film 51, high-precision horizontal rotary stage 7 and rotary stage fixing base 8 on the first and second optical path mechanisms 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.

[0055] In this invention, the ultra-low expansion glass can be ultra-low expansion glass (ULE) sold by Corning Incorporated, USA.

[0056] On the other hand, the present invention also provides a common-mode ultra-high precision continuous measurement method for small angles, the method comprising the following steps:

[0057] (1) The laser emitted by the first laser diode passes through the first narrowband interference plate to obtain narrowband light; the narrowband light passes through the first transparent solid medium and is reflected by the first partial reflector, returning to the first laser diode; when the cavity oscillation reaches the threshold, the first measured laser is output by the first partial reflector.

[0058] The first narrowband interferometer and the first transparent solid medium are fixed on a high-precision horizontal rotating stage. The fixed positions of the first narrowband interferometer and the first transparent solid medium are symmetrical about the diameter of the rotating stage perpendicular to the laser optical path and are not parallel.

[0059] The laser emitted by the second laser diode passes through the second narrowband interferometer to obtain narrowband light; the narrowband light passes through the second transparent solid medium and is reflected by the second partial reflector, returning to the second laser diode; when the oscillation in the cavity reaches the threshold, the second measured laser is output by the second partial reflector.

[0060] The second narrowband interferometer is fixed above the first narrowband interferometer, and they are in the same plane. The second transparent solid medium is fixed above the first transparent solid medium, and their front surfaces are in the same plane.

[0061] The first laser diode and the second laser diode have the same initial frequency, and the first transparent solid medium and the second transparent solid medium have the same refractive index but different thicknesses.

[0062] (2) When the high-precision horizontal rotary stage rotates, the narrow-band interferometer rotates synchronously with the transparent solid medium. Based on the different thickness values ​​of the transparent solid medium in each optical path mechanism, the curve of the change in cavity mode wavelength caused by the change of the transparent solid medium with the rotation angle θ is plotted. The relationship between the change in cavity mode wavelength Δλ caused by the rotation of the transparent solid medium in the corresponding optical path mechanism and the thickness of the transparent solid medium and its rotation angle θ is as follows:

[0063]

[0064] Where L is the length from the output light end face of the laser diode to the front surface of the partial reflector, c is the speed of light, λ0 is the initial wavelength of the laser output, D is the thickness of the transparent solid medium, and n is the wavelength of light. eff Δλ1 and Δλ2 are the wavelength changes caused by the rotation angle θ of the first and second transparent solid media, respectively.

[0065] (3) The rotation angle of the narrowband interferometer is obtained by the following formula: The curve of the change in transmission wavelength Δλ′ caused by time:

[0066]

[0067] λ′0 is the transmission wavelength when the narrowband interferometer is perpendicular to the beam, and n eff2 Let Δλ′ be the refractive index of the narrowband interferometer, and Δλ′ be the refractive index of the interferometer.

[0068] (4) The two laser beams under test 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 obtain the change in beat frequency Δf.

[0069] (5) Calculate the rotation angle θ of the transparent solid medium based on the change in beat frequency Δf. The relationship between the change in beat frequency Δf and the rotation angle θ (°) of the narrowband interferometer is:

[0070]

[0071] 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 Δf of the two measured lasers, or by the following formula:

[0072]

[0073] Since the frequency fluctuation of the freely operating narrowband interferometer laser in a single angle measurement device is on the order of tens of kHz, the angle measurement limit can be converted into a laser frequency measurement of tens of kHz, corresponding to an angle measurement limit on the order of approximately 0.0001″. The common-mode ultra-high precision continuously measurable small angle measurement reference device of this invention uses two identical structures to form one device, 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 hundreds of Hz. This converts the angle measurement limit into a laser frequency measurement of hundreds of Hz, 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 to 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″, realizing a significant improvement in the resolution of ultra-high precision small angle measurement.

[0074] The common-mode noise-canceling dual-laser frequency measurement ultra-high precision small-angle measurement reference device of the present invention uses two sets of angle measurement devices to measure the angle, converting the change in angle into a change in the frequency difference of the laser output from the two sets of devices. While not exceeding the detector's measurement range, it effectively eliminates a portion of common-mode noise, bringing the angle measurement limit to the order of 0.0001″. This device can be applied to MEMS, precision instruments, and optical systems, meeting the application requirements of such high-precision equipment. Attached Figure Description

[0075] Figure 1 This is a schematic diagram of the small-angle measurement reference device previously studied by the inventor.

[0076] Among them, 101, laser diode; 102, first collimating lens; 103, narrowband interferometer; 104, transparent solid medium; 105, high-precision horizontal rotary stage; 106, rotary stage mounting base; 107, focusing lens; 108, partial reflector; 109, second collimating lens; 110, reflector; 111, first half-wave plate; 112, optical comb system; 113, second half-wave plate; 114, polarizing beam splitter; 115, broadband detector; 116, frequency counter.

[0077] Figure 2 This is a schematic diagram of the principle of the small angle measuring reference device of the present invention;

[0078] Figure 3 This is a schematic diagram of the small angle measuring device in Example 1;

[0079] Figure 4 This is a schematic diagram of the small angle measuring device in Example 2;

[0080] Figure 5This illustrates the relationship between the rotation angle and the cavity mode frequency for transparent solid media of different thicknesses D in Example 1.

[0081] Figure 6 This illustrates the relationship between the rotation angle of transparent solid media of different thicknesses D and the change in their cavity mode wavelength in Example 1.

[0082] Figure 7 This is a comparison diagram of the relationship between the rotation angle of the transparent solid medium with different thicknesses D and the cavity mode wavelength, and the relationship between the rotation angle of the interferometer and the transmission wavelength in Example 1.

[0083] Among them, 1. First laser diode, 11. Second laser diode, 2. First collimating lens, 22. Second collimating lens, 3. First narrowband interferometer, 33. Second narrowband interferometer, 4. First transparent solid medium, 44. Second transparent solid medium, 5. Partial reflector, 51. Prism-type partial reflective film, 52. Ultra-low expansion coefficient glass base, 6. Reflector, 7. High-precision horizontal rotating stage, 8. Rotating stage fixing base, 9. First half-wave plate, 10. Second half-wave plate, 11. Polarizing beam splitter prism, 12. Broadband detector, 13. Frequency counter. Detailed Implementation

[0084] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0085] Example 1 examines the effect of different transparent solid medium thicknesses D on measurement results.

[0086] 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 optical path mechanism and a second optical path mechanism arranged in parallel. The first optical path mechanism includes a first laser diode 1, a first collimating lens 2, a first narrow-band interferometer 3, and a first transparent solid medium 4 (made of white glass with a refractive index of n). eff =1.5), wherein the fixed positions of the first narrowband interferometer 3 and the first transparent solid medium 4 are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel; the second optical path mechanism is located directly above the first optical path mechanism, including the second laser diode 11, the second collimating lens 22, the second narrowband interferometer 33, and the second transparent solid medium 44 (made of white glass with a refractive index of n). eff =1.5), the second transparent solid medium 44 is fixed above the first transparent solid medium 4 and their front surfaces are in the same plane, the second narrow band interferometer 33 is fixed above the first narrow band interferometer 3 and they are in the same plane. Each optical device in the first and second optical paths should be in its own optical path.

[0087] The first laser diode 1 and the second laser diode 11 are both 780nm laser diodes. The center frequencies of the first narrowband interferometer 3 and the second narrowband interferometer 33 are both 780nm. The thickness D1 of the first transparent solid medium 4 is not equal to the thickness D2 of the second transparent solid medium 44.

[0088] The first collimating lens 2, the first narrowband interference plate 3, and the first transparent solid medium 4 are fixed on a high-precision horizontal rotating stage 5.

[0089] Part of the reflector 5 is located in the output light direction of the first transparent solid medium 4 and the second transparent solid medium 44; a second half-wave plate 10 is set in the first optical path, a reflector 6 is set in the second optical path, and a first half-wave plate 9, a polarizing beam splitter 11, a broadband detector 13 and a frequency counter 13 are set in the output light direction of the reflector 6 to form a beat frequency measurement module.

[0090] Since the initial fixed positions of the first narrowband interferometer 3 and the first transparent solid medium 4 are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel, let's assume that the narrowband interferometer and the optical path have an initial angle α and α < 90°, and the transparent solid medium and the optical path have an initial angle β, then β = α (e.g., ...). Figure 2 (As shown in a). Therefore, when the precision rotating platform rotates counterclockwise by θ around the center, the angle of the interferometer increases (i.e., α′=α+θ), the angle of the transparent solid medium decreases (i.e., β′=β-θ), and the angles between the narrowband interferometer 3 and the transparent solid medium 4 and the optical path change respectively.

[0091] During operation, the 780nm laser in the first optical path is collimated and then filtered by a first narrowband interferometer to remove out-of-band light, resulting in narrowband light. This narrowband light passes through a first transparent solid medium and reaches the front surface of a first partial reflector. After passing through the first partial reflector, it is reflected as collinear and in the opposite direction to the narrowband light. This reflected light is fed back to the laser diode. When the intracavity oscillation reaches a threshold, the first measured laser is output through the first partial reflector. Similarly, the second optical path outputs the second measured laser.

[0092] 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 respective beat frequencies. When the rotary table rotates, causing the frequencies of the two laser beams to change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

[0093] To convert the beat frequency change Δf into the rotation angle change θ of the transparent solid medium, firstly, the cavity mode wavelength change Δλ curve caused by the rotation angle θ of the transparent solid medium is determined by measurement:

[0094] In this embodiment, the cavity length is set to L = 10 cm, λ0 = 780 nm, and a first transparent solid medium of different thicknesses is selected with a refractive index of n. eff =1.5, with thicknesses of D1 = 40mm, D1 = 45mm, D1 = 50mm, D1 = 55mm, and D1 = 60mm. The change in cavity mode wavelength Δλ caused by rotation angle λ of each transparent solid medium was measured and plotted, as shown below. Figure 5 , 6 As shown.

[0095] When θ changes continuously within the range of 0 to 14°, the frequency and wavelength of the first transparent solid medium cavity mode change accordingly.

[0096] The results show that when the first interferometer and the first transparent solid medium are rotated clockwise / counterclockwise in one direction, the angle α between the interferometer and the incident light decreases / increases, the center frequency of the transmission peak of the interferometer increases / decreases, and the transmission wavelength of the interferometer decreases / increases; the angle β between the transparent solid medium and the incident light increases / decreases, the cavity length of the resonant cavity decreases / increases, and the cavity mode frequency also increases / decreases.

[0097] Taking θ = 10° as an example, from Figure 5 and Figure 6 It can be seen that when D1 = 40mm, the change in cavity mode frequency caused by rotating the transparent solid medium is Δf1 = -1.187 × 10⁻⁶. 12 The cavity mode wavelength variation is Δλ1 = 2.407 nm. When D1 = 45 mm, Δf1 = -1.335 × 10⁻⁶ Hz. 12 Hz, Δλ1=2.707nm. When D1=50mm, Δf1=-1.483×10 12 Hz, Δλ1=3.008nm.

[0098] To synchronize the cavity mode frequency with the center frequency of the transmission peak of the interferometer, thus addressing the issue of limited tunable laser frequency caused by the rotation of the first transparent solid medium, and to ensure that the cavity mode wavelength caused by the rotation of the first transparent solid medium changes synchronously with the transmission wavelength caused by the rotation of the first interferometer, the rotation angle of the narrowband interferometer is determined. The expression relating the wavelength to the transmission wavelength λ′ is as follows:

[0099]

[0100] Calculated Figure 6From the IF curve, we can see that when θ = 10°, the transmission wavelength λ1′ = 777.1 nm of the interferometer is obtained. The change in transmission wavelength Δλ1′ = -2.9 nm is calculated. This value is between Δλ1 = 2.707 nm and Δλ1 = 3.008 nm. Therefore, the optimal range for the thickness D1 of the first transparent solid medium is 45 mm to 50 mm.

[0101] At this time, the change in cavity mode wavelength Δλ1 caused by the rotation of the first transparent solid medium is approximately the same as the change in transmission wavelength Δλ1′ caused by the rotation of the first interferometer, and is therefore considered equal. Thus, the cavity mode frequency changes synchronously with the transmission frequency of the interferometer, allowing the output laser frequency to change continuously, thereby realizing the conversion of continuous angle change measurement into continuous frequency change measurement.

[0102] Similarly, the value of D2 also ranges from 45mm to 50mm, but D1 ≠ D2, and the difference between the wavelengths of the two cavity modes and the transmission wavelength of the interferometer at the same angle does not exceed 20pm. Finally, the beat frequency difference Δf between the two measured laser beams is converted into an angle change:

[0103]

[0104] Two sets of angle measurement devices were combined into one system, eliminating some common-mode noise and reducing the frequency fluctuation of the freely operating narrowband interferometer laser to the order of 300 Hz. Based on the relationship between the beat frequency change Δf and the rotation angle of the narrowband interferometer, for example, when the laser and interferometer rotate at a small angle of around 5°, the relationship between the beat frequency and the change in angle rotation is 1.5 GHz / °. Therefore, the angle change corresponding to a 300 Hz frequency fluctuation is approximately 2.04 × 10⁻⁶. -7 ° (0.000733″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.0001″.

[0105] Example 2

[0106] The difference from Example 1 is that the partial reflector 5 can be replaced by a prism-type partial reflective film 51, specifically the microprism-type reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0107] The first laser diode 1, first collimating lens 2, first narrowband interferometer 3, and first transparent solid medium 4 on the first optical path mechanism, and the second laser diode 11, second collimating lens 22, second narrowband interferometer 33, and second transparent solid medium 44 on the second optical path mechanism, along with the prism-type partial reflective film 51, high-precision horizontal rotary stage 7, and rotary stage mounting base 8, are fixed together on a base 52 made of ultra-low expansion coefficient glass. The ultra-low expansion coefficient characteristics of the base further overcome the interference caused by mechanical deformation. The rest of the structure remains unchanged.

[0108] Similarly, the thicknesses D1 and D2 of the first transparent solid medium 4 and the second transparent solid medium 44 are set within the range of 45mm to 50mm. Using a high-precision horizontal rotating stage 7, the angles between the incident light and the first narrow-band interferometer 3 and the second narrow-band interferometer 33, as well as the angles between the incident light and the first transparent solid medium 4 and the second transparent solid medium 44, are adjusted. This causes the center position of the transmission peak of the interferometers in the two optical paths to move with the angle, highly synchronized with the change in frequency of their corresponding specific cavity modes with the angle, achieving a wide range of continuously tunable laser frequencies. Then, the beat frequency of the laser output from each of the two optical paths is compared, and the resulting beat frequency change Δf is converted into an angle change using the same calculation method as in Example 1.

[0109] Since ultra-low expansion coefficient glass can stabilize the frequency fluctuations of a freely operating narrowband interferometer laser within the Hz range, based on the relationship between the beat frequency change Δf and the rotation angle of the narrowband interferometer, when the laser rotates at an angle of approximately 5° with the interferometer, the relationship between the beat frequency and the change in angle rotation is 1.5 GHz / °. Therefore, a 1 Hz frequency fluctuation corresponds to an angle change of approximately 6.78 × 10⁻⁶. -10 ° (0.000002443″), which enables the corresponding angle measurement resolution to be on the order of approximately 0.000001″.

[0110] Compared with Example 1, this example uses ultra-low expansion coefficient glass to improve the stability of frequency fluctuations during laser free operation to the Hz level, so that the corresponding angle measurement resolution can be achieved to the order of approximately 0.000001″, thus achieving a significant improvement in angle measurement resolution.

[0111] The small-angle measurement reference device of this invention employs two sets of angle measurement devices to achieve small-angle measurements, eliminating some common-mode noise and reducing the frequency fluctuation of the freely operating narrowband interferometer laser to the order of several hundred Hz. This transforms the angle measurement limit into a laser frequency measurement of several hundred Hz, corresponding to an angle measurement limit of approximately 0.0001″. When the frequency fluctuation of the freely operating narrowband interferometer laser is further stabilized at the Hz level using an ultra-low expansion coefficient glass substrate, the corresponding angle measurement resolution can reach approximately 0.000001″, achieving a significant improvement in the ultra-high precision angle measurement resolution for small-angle metrology.

Claims

1. A common-mode noise-reducing, ultra-high precision, continuously measurable small-angle measurement reference device, comprising a first optical path mechanism and a second optical path mechanism arranged in parallel, a partial reflector (5), and a beat frequency measurement module, characterized in that... 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 fixed positions of the first narrow band interference plate (3) and the first transparent solid medium (4) are symmetrical about the diameter of the rotating stage perpendicular to the optical path and are not parallel. The second optical path mechanism includes a second laser generating device and a second narrowband interferometer (33) and a second transparent solid medium (44) located on the output optical path of the second laser generating device. The second transparent solid medium (44) is fixed above the first transparent solid medium (4) and their front surfaces are located on the same plane. The second narrowband interferometer (33) is fixed above the first narrowband interferometer (3). Part of the reflector (5) is located in the output light direction of the first transparent solid medium (4) and the second transparent solid medium (44); The initial wavelengths of the first laser generating device and the second laser generating device are equal, and the thicknesses of the first transparent solid medium (4) and the second transparent solid medium (44) are not equal; In the first optical path mechanism and the second optical path mechanism, the light output by each laser generator is filtered out of the band after passing through a narrowband interferometer to obtain narrowband light; the narrowband light returns to the laser generator after passing through a transparent solid medium and a mirror group. When the oscillation in the cavity reaches the threshold, the first and second lasers under test are output by a portion of the mirrors (5); the two lasers under test are respectively input to the beat frequency measurement module, and the beat frequency measurement module beats the two lasers under test. When the frequency of the first laser under test and the frequency of the second laser under test change by Δf1 and Δf2 respectively, the change in beat frequency is equal to Δf = Δf2 - Δf1.

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

3. The small angle measuring reference device according to claim 1, characterized in that... The beat frequency measurement module includes a reflector (6), a first half-wave plate (9), a second half-wave plate (10), a polarizing beam splitter (11), a broadband detector (12) and a frequency counter (13) set in the output light direction of the polarizing beam splitter (11). The reflector (6) is located in the second optical path, the first half-wave plate (9) is located in the output light direction of the reflector (6), and the second half-wave plate (10) is located in the output light direction of the first transparent solid medium (4). The second laser beam under test is reflected by the mirror (6) and then enters the polarization beam splitter (11) through the first half-wave plate (9) and is completely reflected. The first laser beam under test enters the polarization beam splitter (11) through the second half-wave plate (10). The two laser beams under test are combined and reach the broadband detector (12). The broadband detector (12) is connected to the frequency counter (13). The frequency counter (13) measures the beat frequency of the two laser beams under test and obtains the change in beat frequency Δf.

4. The small angle measuring reference device according to claim 1, characterized in that... A prism-type partial reflective film (51) is used to replace a partial reflector (5).

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

6. A method for ultra-high precision small-angle measurement using common-mode noise reduction 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 passes through the first transparent solid medium and is reflected by the first partial reflector, returning to the first laser diode; when the cavity oscillation reaches the threshold, the first measured laser is output by the first partial reflector. The first narrowband interferometer and the first transparent solid medium are fixed on a high-precision horizontal rotating stage. The fixed positions of the first narrowband interferometer and the first transparent solid medium are symmetrical about the diameter of the rotating stage perpendicular to the laser optical path and are not parallel. The laser emitted by the second laser diode passes through the second narrowband interferometer to obtain narrowband light; the narrowband light passes through the second transparent solid medium and is reflected by the second partial reflector, returning to the second laser diode; when the oscillation in the cavity reaches the threshold, the second measured laser is output by the second partial reflector. The second narrowband interferometer is fixed above the first narrowband interferometer, and they are in the same plane. The second transparent solid medium is fixed above the first transparent solid medium, and their front surfaces are in the same plane. The first laser diode and the second laser diode have the same initial frequency, and the first transparent solid medium and the second transparent solid medium have the same refractive index but different thicknesses. (2) When the high-precision horizontal rotary table rotates, the narrow-band interferometer rotates synchronously with the transparent solid medium. Based on the different thickness values ​​of the transparent solid medium in each optical path mechanism, the curve of the change in cavity mode wavelength caused by the change of the transparent solid medium with the rotation angle θ is plotted. The relationship between the change in cavity mode wavelength Δλ caused by the rotation of the transparent solid medium in each optical path mechanism and the thickness of the transparent solid medium and its rotation angle θ is as follows: Where λ0 is the initial wavelength of the laser output, L is the length from the output light end face of the laser diode to the front surface of the partial reflector, D is the thickness of the transparent solid medium, and n is the wavelength of the laser output. eff θ is the refractive index of the transparent solid medium, and Δλ is the change in cavity mode wavelength caused by the rotation angle θ of the transparent solid medium in the corresponding optical path mechanism. (3) The rotation angle of the narrowband interferometer is obtained by the following formula: The curve of the change in transmission wavelength Δλ′ caused by time: λ′0 is the transmission wavelength when the narrowband interferometer is perpendicular to the beam, and n eff2 Let Δλ′ be the refractive index of the narrowband interferometer, and Δλ′ be the refractive index of the interferometer. (4) The two laser beams under test 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 obtain the change in beat frequency Δf. (5) Calculate the rotation angle θ of the transparent solid medium based on the change in beat frequency Δf. The relationship between the change in beat frequency Δf and the rotation angle θ (°) of the narrowband interferometer is: 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 Δf of the two measured lasers, or by the following formula: Where c is the speed of light.

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