High-resolution small-angle measurement reference device based on laser frequency measurement

Abstract

The invention provides a high-resolution small-angle measuring device based on laser frequency measurement, which comprises a laser generating device, a reflecting mirror group and a beat frequency measuring module, and is characterized in that a narrow-band interference sheet (3) and a transparent solid medium (4) are fixed on a high-precision horizontal rotating table (5); the fixing positions of the narrow-band interference sheet (3) and the transparent solid medium (4) are axially symmetric and are not parallel to each other by taking the diameter, perpendicular to the light path, of the rotating table as the axis; and when the high-precision horizontal rotating table (5) rotates to cause the frequency change delta f of the measured laser, the variable quantity of the beat frequency is equal to delta f. According to the small-angle measuring device, angle measurement is converted into frequency measurement by means of the high sensitivity of laser frequency to the rotation angle of the transparent solid medium, and the cavity mode frequency and the transmission frequency of the interference piece are synchronously changed by adding the interference piece; the problem that the tunable range of laser frequency is small due to rotation of a transparent solid medium is solved, and continuous measurement of micro angles is achieved.

Description

Technical Field

[0001] The invention relates to a small angle measuring device, in particular to a high-resolution small angle measuring reference device based on laser frequency measurement and based on the interference frequency selection principle. 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. Based on converting the angle measurement into the laser frequency measurement, a small angle measurement device with high resolution and continuous measurement is proposed.

[0005] In the previous research work, the inventor designed a new ultra-high precision small angle measurement device, which is a small angle measurement device based on narrow-band interference plate laser and converts angle measurement into laser frequency measurement. Specifically, Figure 1As shown, the small angle measurement device includes a laser diode 101, a first collimating lens 102, a narrowband interference plate 103, a focusing lens 106, a partial reflector 107, a second collimating lens 108 and a reflector 109 arranged in sequence on the optical path, an optical comb system 111 and a beat frequency measurement module are arranged in the reflected light output direction of the reflector 109, and a high-precision horizontal rotating stage 104 is fixed on a rotating stage fixing seat 105. Among them, the narrowband interference plate 103 is fixed on the precision rotating platform, so the narrowband interference plate 103 can rotate in the horizontal direction to change the angle between the incident light and the narrowband interference plate. The beat frequency measurement module mainly includes a first half-wave plate 110, a second half-wave plate 112, a polarization beam splitter prism 113, a broadband detector 114 and a frequency counter 115. The light output by the laser generating device passes through the narrowband interference plate 103, and the out-of-band light is filtered out to obtain narrowband light; the narrowband light returns to the laser diode after passing through the reflector group, and when the intracavity oscillation reaches the threshold, the reflector group 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 to obtain the beat frequency. When the high-precision horizontal rotating stage 104 rotates to cause the measured laser frequency to change Δf, the change in the beat frequency is equal to Δf. By measuring the change in the beat frequency Δf, the rotation angle of the narrowband interference plate can be calculated.

[0006] Wherein, Δf=f0-f1, f0 is the measured laser frequency when the high-precision horizontal rotating stage is located at the initial position, and f1 is the measured laser frequency after the high-precision horizontal rotating stage rotates.

[0007] According to the formula f = cλ -1 , df=-cλ -2 dλ, where f is the frequency, c is the speed of light, λ is the wavelength, and there is a linear relationship K (nm / degree) between the rotation angle of the narrowband interference plate and the transmission wavelength λ′. It is known that the relationship between the change in the beat frequency Δf and the rotation angle of the narrowband interference plate is:

[0008]

[0009] Wherein, K is the slope of the curve of the rotation angle θ and the transmission wavelength λ′ when the narrowband interference plate rotates. The value of K can be obtained by measuring the rotation angle of the narrowband interference plate and the wavelength change of the measured laser, or by the following formula:

[0010]

[0011] However, since the length L of the resonant cavity is an integer multiple of half a wavelength, when the transmission center frequency changes due to the rotation of the narrowband interference plate, the change in the beat frequency Δf is an integer multiple of the longitudinal mode interval. Therefore, when the narrowband interference plate rotates continuously, the change in the beat frequency Δf does not change continuously, but is discontinuous and has a jump phenomenon, so continuous small angle measurement cannot be achieved.

[0012] The idea of ​​the present invention is to attach a transparent solid medium of a specific thickness to the object to be measured by utilizing the ultra-high sensitivity of the laser frequency to the cavity length of the resonant cavity on the basis of the previous research work, and to change the angle between the transparent solid medium and the optical path, thereby changing the optical path length of the laser through the solid medium, that is, the cavity length of the resonant cavity. The thickness of the transparent solid medium is selected to control the change in the cavity length of the resonant cavity, so that the change in the cavity mode frequency caused by the change in the cavity length of the resonant cavity is synchronized with the change in the central frequency of the transmission peak of the interference plate caused by the rotation of the interference plate, ensuring that the laser frequency changes continuously to solve the mode hopping phenomenon that may occur in continuous rotation in the previous research work, and then the change in the laser output frequency is converted into the rotation angle by detecting the change in the laser output frequency, so as to achieve ultra-high precision continuous measurement of the rotation angle.

[0013] To this end, the present invention provides a small angle measuring device with high resolution and continuous measurement, the device comprises a laser generating device, a reflector group and a beat frequency measuring module which are sequentially arranged on an optical path, wherein a high-precision horizontal rotating table 5 is arranged between the laser generating device and the reflector group, a narrow-band interference plate 3 and a transparent solid medium 4 are fixed on the high-precision horizontal rotating table 5, the fixed positions of the narrow-band interference plate 3 and the transparent solid medium 4 are symmetrical and non-parallel with the diameter of the rotating table perpendicular to the optical path as an axis, the light output by the laser generating device is filtered out of the band after passing through the narrow-band interference plate 3, and narrow-band light is obtained; the narrow-band light returns to the laser diode after passing through the transparent solid medium 4 and the reflector group, and when the oscillation in the cavity reaches a threshold value, the reflector group outputs the measured laser; the measured laser is input to the beat frequency measuring module, and the beat frequency measuring module beats the measured laser with the comb teeth output by the optical comb system to obtain the beat frequency, and when the measured laser frequency changes by Δf, the change in the beat frequency is equal to Δf.

[0014] Taking white glass as a transparent solid medium, combined with Figure 2-3 The working principle is explained as follows:

[0015] like Figure 2 As shown in (a), when the white glass is perpendicular to the light path (i.e. the incident angle is 0°): the cavity length Therefore, when the white glass rotates from 0° to θ°, the change in cavity length is:

[0016] 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 is obtained as follows:

[0017]

[0018] where n eff is the refractive index of white glass, D is the thickness of white glass, L1 is the length from the laser output light surface to the front surface of the transparent solid medium, L2 is the length from the back surface of the transparent solid medium to the front surface of the partial reflector, and θ is the incident angle (i.e. the angle between the normal of the white glass and the laser).

[0019] Known The relationship between the change in cavity mode wavelength Δλ caused by the rotation of white glass and the thickness of the white glass and its rotation angle θ can be obtained: Where λ0 is the initial wavelength of the laser output.

[0020] Therefore, since the positions of the narrowband interference plate and the transparent solid medium of the present invention are symmetrical and non-parallel with the diameter of the rotating platform perpendicular to the optical path, when the rotating platform rotates at an angle of θ, the narrowband interference plate and the transparent solid medium rotate at an angle of θ at the same time. By selecting the thickness of the white glass so that the change in the cavity mode wavelength caused by the rotation of the white glass at an angle of θ is equal to the change in the transmission wavelength caused by the rotation of the narrowband interference plate at an angle of θ (or approximately the same. In the present invention, "approximately the same" is understood as the error is extremely small and can be regarded as equal), the cavity mode frequency and the interference plate transmission peak frequency can be changed synchronously, thereby solving 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 rotation of the white glass, and realizing continuous measurement of the rotation angle.

[0021] In the present invention, the narrowband interference plate is rotated by an angle The relationship between the transmission wavelength λ′ is expressed as follows:

[0022]

[0023] λ′0 is the initial transmission wavelength of the narrowband interference plate when it is perpendicular to the light, n eff2 is the refractive index of the narrowband interference plate, then the change in the transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ0-λ′. Since the positions of the narrowband interference plate and the transparent solid medium are symmetrical and not parallel to the diameter of the rotating stage perpendicular to the optical path, the rotation angle of the narrowband interference plate is It is equal to the rotation angle θ of the transparent solid medium.

[0024] As another embodiment of the present invention, the relationship between the rotation angle of the transparent solid medium and the cavity mode wavelength is:

[0025] (1) Assuming that the incident angle between the transparent solid medium and the incident light is 0°, the cavity length When the transparent solid medium rotates by θ, the cavity length Therefore, when the transparent solid medium rotates at an angle of θ, the cavity mode wavelength can be obtained: where n eff is the refractive index of the transparent solid medium, n is a positive number, D is the thickness of the transparent solid medium, L is the length from the laser output light surface to the front surface of the partial reflector, L1 is the length from the laser output light surface to the front surface of the transparent solid medium, and L2 is the length from the rear surface of the transparent solid medium to the front surface of the partial reflector.

[0026] (2) Therefore, by selecting the thickness of the transparent solid medium so that the change in the cavity mode wavelength caused by the rotation of the transparent solid medium is equal to (or approximately the same as) the change in the transmission wavelength caused by the rotation of the interference plate, the cavity mode frequency and the transmission peak frequency of the interference plate can be changed synchronously, thereby solving the problems of laser frequency mode hopping caused by a single rotation of the interference plate and the small frequency tunable range caused by a single rotation of the white glass.

[0027] 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°. Knm / degree, so the rotation angle of the interference plate is Satisfy the formula:

[0028]

[0029] Among them, K is when the value of D satisfies And Δλ=Δλ′, the rotation angle The slope of the λ′ curve is:

[0030]

[0031] According to a particularly preferred embodiment, the transparent solid medium 4 is white glass with a thickness of 45 to 50 mm.

[0032] In the present invention, the laser generating device is a conventional setting in the art, and generally includes a laser diode 1 and a first collimating lens 2 which are sequentially arranged on the optical path. The working wavelength of the laser diode 1 can be any wavelength. Preferably, from the perspective of easy acquisition or economical saving, a 780nm wavelength laser diode can be used. Those skilled in the art can also select laser diodes with other working wavelengths, such as 420nm, 850nm, 1550nm laser diodes, etc.

[0033] In the present invention, a transparent solid medium 4 with a suitable thickness is fixed together with a narrow-band interference plate 3 on a high-precision horizontal rotating table 5, and the transparent solid medium 4 and the narrow-band interference plate 3 are placed in a symmetrical and non-parallel position with respect to the rotating table diameter perpendicular to the optical path. Figure 2 As shown in (d), at time t0, it is assumed that there is an initial angle α between the narrowband interference plate and the direction of the optical path, and an initial angle β between the transparent solid medium and the optical path, and β=α. At time t1, when the high-precision horizontal rotating stage 5 rotates counterclockwise about the center by θ, the angle of the interference plate increases (i.e., α′=α+θ), and the corresponding center frequency of the interference plate transmission peak increases; 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 to synchronize the change in cavity mode frequency with the change in center frequency of the interference plate transmission peak, the problem of laser frequency mode hopping caused by a single rotation of the interference plate and the small frequency tunable range caused by a single rotation of the white glass can be avoided, thereby achieving continuous change of the output laser frequency and further achieving continuous measurement.

[0034] In the present invention, a preferred embodiment is to arrange a second collimating lens 9 between the partial reflector 8 and the reflector 10. Those skilled in the art can also selectively increase / decrease optical elements according to the laser design principle.

[0035] In this way, the divergent light output by the laser diode is collimated into parallel light by the first 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 partial reflector after passing through the transparent solid medium and the focusing lens, and the reflected light in the opposite direction of the narrow-band light is obtained after passing through the partial reflector; the reflected light is returned to the laser diode, and when the oscillation in the cavity reaches the threshold, the partial reflector outputs the laser, and the output laser to be measured is obtained after passing through the second collimating lens. The measured laser reaches the beat frequency measurement module after being reflected by the reflector, and the beat frequency is compared with the optical comb system, and the change in the beat frequency Δf is converted into the change in the rotation angle θ by measuring the change in the beat frequency.

[0036] Beat frequency measurement belongs to the prior art in this field. As a preferred embodiment, the beat frequency measurement module includes a first half-wave plate 11 arranged in the direction of the reflected light of the reflector 10, a polarization beam splitter prism 14, a broadband detector 16 and a frequency counter 16 arranged in the direction of the output light of the polarization beam splitter prism 14; the laser to be measured is completely reflected after entering the polarization beam splitter prism 14 through the first half-wave plate 11, and the comb teeth emitted by the optical comb system 12 are incident on the polarization beam splitter prism after passing through the second half-wave plate 13. The optical comb and the laser to be measured are combined and arrive at the broadband detector 15. The broadband detector 15 is connected to the frequency counter 16, and the beat frequency is obtained by the frequency counter 16. When the beat frequency changes due to the rotation of the rotating table, the beat frequency change Δf is obtained by Δf=f0-f1.

[0037] 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.

[0038] As an alternative technical solution, a prismatic super-strong reflective film 81 can be used to replace part of the reflector 8. When this technical solution is adopted, the transparent solid medium 4 can be bonded to the prismatic super-strong reflective film 81, and there is no need to install the focusing lens 7 and the second collimating lens 9, thereby improving the mechanical robustness of the system. Generally, a prismatic super-strong reflective film 81 with a high retroreflection coefficient can implement the technical solution of the present invention, for example, the micro-prismatic reflective film disclosed in Chinese invention patent application CN 201410823342.3 is used. The prismatic super-strong reflective film is set in a direction perpendicular to the incident light, and the incident light is returned along the original path within a rotation angle range of ±20°, and the intracavity oscillation output of the measured laser can be achieved.

[0039] 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.

[0040] According to a preferred embodiment, the present invention further comprises an ultra-low expansion glass base 82, and the laser diode 1, the first collimating lens 2, the narrow-band interference plate 3, the transparent solid medium 4, the prism-type partially reflective film 81, the high-precision horizontal rotating table 5 and the rotating table fixing seat 6 are fixed on the base made of ultra-low expansion glass, 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 setting can further improve the angle measurement resolution. Since the second collimating lens does not interfere with the frequency stability, the second collimating lens can be placed on the ultra-low expansion glass base.

[0041] In the present invention, the ultra-low expansion glass may be ultra-low expansion glass (ULE) sold by Corning Incorporated of the United States.

[0042] On the other hand, the present invention also provides a small angle measurement method with high resolution and continuous measurement, the method comprising the following steps:

[0043] (1) The laser light emitted by the laser diode is passed through a narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by a partial reflector after passing through a transparent solid medium and returns to the laser diode; when the oscillation in the cavity reaches a threshold value, the measured laser light is output by the partial reflector;

[0044] The narrowband interference plate and the transparent solid medium are fixed on a high-precision horizontal rotating table. The fixed positions of the narrowband interference plate and the transparent solid medium are symmetrical and non-parallel with the diameter of the rotating table perpendicular to the laser light path. Therefore, when the rotating table rotates, the angles between the narrowband interference plate and the transparent solid medium and the laser light path change simultaneously and the angle changes are equal.

[0045] (2) When the high-precision horizontal rotating stage rotates, the narrowband interference plate rotates synchronously with the transparent solid medium. According to different values ​​of D, a curve of the change in cavity mode wavelength Δλ caused by the rotation angle θ of the transparent solid medium is drawn. Δλ and the rotation angle θ of the transparent solid medium satisfy the following relationship:

[0046]

[0047] Where L is the length from the output light end 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, n eff is the refractive index of the transparent solid medium, Δλ is the change in the cavity mode wavelength caused by the rotation angle θ of the transparent solid medium;

[0048] (3) The rotation angle of the narrowband interferometer is obtained by the following formula The relationship between the transmission wavelength λ′ is as follows:

[0049]

[0050] λ′0 is the initial transmission wavelength of the narrowband interference plate when it is perpendicular to the light, n eff2 is the refractive index of the narrowband interference plate, then the change in transmission wavelength caused by the rotation of the interference plate is: Δλ′=λ0-λ′;

[0051] (4) The laser under test in step (1) is input to a beat frequency measurement module, and the beat frequency measurement module beats the laser under test with the comb teeth output by the optical comb system to obtain a beat frequency;

[0052] (5) Calculate the rotation angle θ of the transparent solid medium. The relationship between the change Δf of the beat frequency and the rotation angle θ of the transparent solid medium is:

[0053]

[0054] When the value of D satisfies When Δλ=Δλ′ (i.e. the change in the cavity mode wavelength Δλ caused by the rotation of the transparent solid medium by an angle θ is equal to the change in the wavelength of the narrow-band interference plate by the rotation of the narrow-band interference plate by an angle θ), The transmission wavelength change Δλ′ caused by the angle is equal), K is the rotation angle in step (2) The slope of the λ′ curve is:

[0055]

[0056] As a preferred embodiment, when L=10 cm, the transparent solid medium 4 is white glass with a thickness of 45-50 mm.

[0057] The small angle measuring device of the present invention adopts narrow band interference plate to filter 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 partial reflector, so that the small angle change of the rotation angle is reflected in the change amount Δf of the measured laser and the beat frequency of the light comb. The transparent solid medium is used to change the cavity length, so that 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 transparent solid medium, such as white glass).

[0058] Since the stability of the optical comb frequency can reach the E-14 level and the frequency fluctuation is in the Hz level, while the frequency fluctuation of the free-running narrowband interference plate laser 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″. When the frequency fluctuation of the free-running narrowband interference plate laser is further stabilized at the Hz level through an ultra-low expansion rate glass base, the corresponding angle measurement resolution can be achieved at the order of about 0.00000001″, achieving a significant improvement in the angle measurement resolution of ultra-high precision small angle metrology.

[0059] The present invention utilizes the structure and principle of a small angle measuring device based on the interference frequency selection principle to realize an angle measuring instrument with an angle measurement limit of 0.0001″, which 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

[0060] Figure 1 This is a schematic diagram of the structure of a small angle measurement reference device previously studied by the inventor;

[0061] Figure 2 This is a schematic diagram of the principle of a small angle measurement device previously studied by the inventor and in accordance with Example 1;

[0062] Figure 3 This is a schematic structural diagram of a small angle measurement device according to Embodiment 1;

[0063] Figure 4 This is a schematic structural diagram of a small angle measurement device according to Embodiment 2;

[0064] Figure 5is the relationship between the rotation angle of the transparent solid medium and the change of the cavity mode frequency in Example 1;

[0065] Figure 6 is the relationship between the rotation angle of the transparent solid medium and the change of the wavelength of its cavity mode in Example 1;

[0066] Figure 7 is a comparison diagram of the relationship between the rotation angle of the transparent solid medium and the cavity mode wavelength and the relationship between the rotation angle of the interference plate and the transmission wavelength in Example 1;

[0067] Among them, 1. laser diode, 2. first collimating lens, 3. narrow-band interference plate, 4. transparent solid medium, 5. high-precision horizontal rotating table, 6. rotating table fixing seat, 7. focusing lens, 8. partial reflector, 81. prism-type super-strong reflective film, 82. ultra-low expansion rate glass base, 9. second collimating lens, 10. reflector, 11. first half-wave plate, 12. optical comb system, 13. second half-wave plate, 14. polarization beam splitter prism, 15. broadband detector, 16. frequency counter.

[0068] 101. Laser diode; 102. First collimating lens; 103. Narrowband interference plate; 104. High-precision horizontal rotating stage; 105. Rotating stage fixing seat; 106. Focusing lens; 107. Partial reflecting mirror; 171. Prism-type super-strong reflective film; 172. Ultra-low expansion glass base; 108. Second collimating lens; 109. Reflecting mirror; 110. First half-wave plate; 111. Optical comb system; 112. Second half-wave plate; 113. Polarization beam splitter prism; 114. Broadband detector; 115. Frequency counter. DETAILED DESCRIPTION

[0069] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0070] Example 1 Investigating the influence of different transparent solid medium thicknesses on the measurement results

[0071] like Figure 3 The small angle measuring device shown in the figure has a laser operating band of 780nm. The device mainly includes a laser diode 1, a first collimating lens 2, a narrow-band interference plate 3, a transparent solid medium 4 (made of white glass with a refractive index of n eff =1.5) focusing lens 7, partial reflector 8, second collimating lens 9 and reflector 10, an optical comb system 12 and a beat frequency measurement module are arranged in the reflected light output direction of the reflector 10, and a high-precision horizontal rotating stage 5 is fixed on the rotating stage fixing seat 6. Among them, the narrow-band interference plate 3 and the transparent solid medium 4 are fixed on the precision rotating platform 5, and their placement positions are symmetrical and non-parallel with the rotating stage diameter perpendicular to the optical path (such as Figure 2(d)), at this time, assuming that there is an initial angle α between the narrowband interference plate and the direction of the optical path and α<90°, and an initial angle β between the transparent solid medium and the optical path, then β=α. Therefore, when the precision rotating platform rotates counterclockwise around the center by θ, the angle of the interference plate 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 interference plate 3 and the transparent solid medium 4 and the optical path change respectively.

[0072] The beat frequency measurement module mainly includes a first half-wave plate 11 , a second half-wave plate 13 , a polarization beam splitter prism 14 , a broadband detector 15 and a frequency counter 16 .

[0073] The optical comb system adopts Sanglok Lee et.al, Laser frequency stabilization in the10 -14 range via optimized modulation transfer spectroscopy on the 87 The 780nm optical comb system disclosed in the Rb D2 line (Opt. Lett. 48(4), 1020-1023(2023)). The system uses modulation transfer spectroscopy (MTS) to achieve high-performance laser frequency stabilization on the Rb 87D2 transition line. The frequency stability of the two frequency-stabilized external cavity semiconductor lasers (ECDL) reaches Short-term stability, up to 10 5 s is only more than 2×10 -12 The structure consists of an MTS mounted on an optical table: a 87 Wedge-shaped vapor cell for Rb (Thorlabs, GC19075-RB87). AMP, amplifier (Mini-Circuits, ZFL-500LN+); BE1 and BE2 are beam expanders; EOM, electro-optic modulator (Qubig, PM7-NIR_5); FC, fiber collimator; FG, signal generator; GT-P, Glan-Thompson polarizer; half-wave plate; LPF, low-pass filter (Mini-Circuit, SLP-1.9+, SLP-5+); mixer, phase detector (Mini-Circuits, ZRPD-1+); OS, oscilloscope; PBS, polarization beam splitter; PD, photodiode (Thorlabs, PDA1 0A2).

[0074] When working, the light output by the 780nm laser diode is collimated, filtered out of the band by a narrow-band interference plate, and then passes through a transparent solid medium, and then reaches a partial reflector to be reflected after passing through a focusing lens. When the oscillation in the resonant cavity reaches the threshold, the laser is output by the partial reflector and the measured laser is obtained after passing through a second collimating lens.

[0075] The measured laser reaches the beat frequency measurement module after reflection. The beat frequency measurement module compares the measured laser with the comb teeth output by the optical comb system to obtain the beat frequency. When the rotating table rotates and causes the measured laser frequency to change by Δf, the change in the beat frequency is equal to Δf.

[0076] In order to convert the change in the beat frequency Δf into the change in the rotation angle θ of the transparent solid medium, firstly, the change in the cavity mode wavelength Δλ caused by the rotation angle θ of the transparent solid medium is determined by measurement:

[0077] In this embodiment, the cavity length is set to L = 10 cm, λ0 = 780 nm, and transparent solid media of different thicknesses are selected. The refractive index of the transparent solid medium is n eff =1.5, and the thicknesses are D=40mm, D=45mm, D=50mm, D=55mm, and D=60mm respectively. Plot the change Δλ of the cavity mode wavelength caused by the rotation angle θ of each transparent solid medium, as shown in Figure 5 , 6 shown.

[0078] When θ changes continuously within the range of 0 to 14°, the cavity mode frequency and the cavity mode wavelength of the transparent solid medium change accordingly.

[0079] The results show that when the interference plate and the transparent solid medium are rotated clockwise / counterclockwise in a single direction, the angle α between the interference plate and the incident light decreases / increases, the corresponding center frequency of the interference plate transmission peak increases / decreases, and the transmission wavelength of the interference plate decreases / increases; when the angle β between the transparent solid medium and the incident light increases / decreases, the length of the resonant cavity decreases / increases, the cavity mode frequency also increases / decreases, and the cavity mode wavelength decreases / increases.

[0080] Taking θ=14° as an example, Figure 5 and Figure 6 It can be seen that when θ=14°D=40mm, the cavity mode frequency change Δf caused by rotating the transparent solid medium is -2.355×10 12 Hz, the cavity mode wavelength changes Δλ=4.776nm. When D=45mm, Δf=-2.649×10 12 Hz, Δλ=5.373nm. When D=50mm, Δf=-2.944×10 12 Hz, Δλ=5.97nm.

[0081] Then, the rotation angle of the narrowband interference plate is obtained by the formula: Curve of the change in transmission wavelength Δλ′ caused by:

[0082] In order to make the cavity mode frequency change synchronously with the center frequency of the interference plate transmission peak, in order to solve the problem of small tunable range of laser frequency caused by the rotation of transparent solid medium, the cavity mode wavelength of transparent solid medium and the transmission wavelength of interference plate are changed synchronously. The relationship expression with the transmission wavelength λ′:

[0083]

[0084] Depend on Figure 7 From the IF curve, it can be seen that when θ=14°, the transmission wavelength λ′ of the interference plate is 774.3nm. By calculation, the transmission wavelength change of the interference plate is Δλ′=-5.7nm, which is between Δλ=5.373nm and Δλ=5.97nm. Therefore, the optimal value range of the thickness D of the transparent solid medium is 45mm~50mm.

[0085] Under this condition, the change in the cavity mode wavelength Δλ caused by the rotation of the transparent solid medium and the change in the transmission wavelength Δλ′ caused by the rotation of the interference plate are approximately the same and are considered equal. Therefore, the cavity mode frequency changes synchronously with the transmission frequency of the interference plate, that is, Figure 2 As shown in (e), the cavity mode frequency changes from f1 to f1', and the change in the center frequency of the interference plate transmission peak is Δf. They change synchronously, so that the output laser frequency can change continuously, thereby realizing continuous small angle measurement.

[0086] Finally, the measured change in beat frequency Δf is used to calculate the rotation angle θ of the transparent solid medium:

[0087]

[0088] When D = 45 mm to 50 mm, and θ changes slightly around 14° (i.e., 14°±1°), Δλ = Δλ′. K is The slope of the λ′ curve is:

[0089]

[0090] Since the frequency stability of the optical comb is at the E-14 level, its frequency fluctuation range is about Hz. Based on the mode selection principle of the narrowband interferometer, the frequency fluctuation of the measured laser output by the free-running interferometer laser is about 30kHz. From the relationship between the interference plate and the transmission wavelength, it can be seen that when the interference plate rotates from 0 to 20°, the slope of the change of the transmission wavelength with the angle is in the range of 0-1.1nm / °. For example, when the incident angle is rotated at a small angle near 13°, K is about 0.725, and the relationship between the change of the interference plate angle and the change of the transmission wavelength is 0.725nm / °. For a 780nm laser wavelength, a wavelength change of 1nm corresponds to a frequency change of 490GHz. Therefore, the relationship between the change of the interference plate angle and the change of the transmission frequency is 355.25GHz / °. Therefore, the angle change corresponding to a 30kHz frequency fluctuation is about 8.43×10 -8° (0.00030348″), so that the corresponding angle measurement resolution can reach about 0.0001″.

[0091] Example 2

[0092] The difference from Example 1 is that the partial reflector 8 can be replaced by a prismatic partial reflective film 81, using the micro-prismatic reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0093] The narrowband interference plate and the prism-type partial reflective film are bonded together to reduce the laser frequency change caused by mechanical vibration, so that the angle measurement can be performed more accurately. In addition, the laser diode 1, the first collimating lens 2, the narrowband interference plate 3, the transparent solid medium 4, the prism-type partial reflective film 81, the high-precision horizontal rotating table 5 and the rotating table fixing seat 6 are fixed on the base 82 made of ultra-low expansion rate glass, and the ultra-low expansion rate characteristics of the base are used to further overcome the interference caused by mechanical deformation.

[0094] During operation, the divergent light output by the laser diode 1 is collimated into parallel light by the first collimating lens 2, and the parallel light is filtered out of the band after passing through the narrow-band interference plate 3 to obtain narrow-band light; the narrow-band light passes through the transparent solid medium 4 to reach the prism-type partial reflective film 81, and after passing through the prism-type partial reflective film 81, the reflected light that is collinear and opposite to the narrow-band light is obtained; the reflected light returns to the laser diode 1 after passing through the transparent solid medium 4 and the interference plate 3, and when the oscillation in the cavity reaches the threshold, the prism-type partial reflective film 81 outputs the measured laser, and the measured laser is reflected by the reflector 10 and reaches the beat frequency measurement module, and the beat frequency is compared with the optical comb system, and the change in the beat frequency Δf is converted into the change in the rotation angle θ of the transparent solid medium by measuring the change in the beat frequency.

[0095] Since 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 angle between the narrow-band interference plate and the incident light, From the relationship between the interference plate and the transmission wavelength, it can be seen that when the interference plate rotates from 0 to 20°, the slope of the change of the transmission wavelength with the angle is in the range of 0-1.1nm / °. When the incident angle is around 13° and a small angle rotation occurs, K is about 0.725, that is, the relationship between the change of the interference plate angle and the change of the transmission wavelength is 0.725nm / °. For a 780nm laser wavelength, a wavelength change of 1nm corresponds to a frequency change of 490GHz. Therefore, the relationship between the change of the interference plate angle and the change of the transmission frequency is 355.25GHz / °. Therefore, the angle change corresponding to each 1Hz is about 2.81×10 -12 °(0.000000010116″), so the corresponding angle measurement resolution can reach the order of 0.00000001″.

[0096] Compared with the frequency fluctuation of about 30kHz when the measured laser is freely running in Example 1, in this embodiment, the laser diode 1, the first collimating lens 2, the narrow-band interference plate 3, the transparent solid medium 4, the prism-type partially reflective film 81, the high-precision horizontal rotating table 5 and the rotating table fixing seat 6 are fixed on a base 82 made of ultra-low expansion coefficient glass. The ultra-low expansion coefficient glass is used to improve the stability of the frequency fluctuation of the measured laser during free running to the Hz level, so that the corresponding angle measurement resolution can achieve about 0.00000001″, achieving a significant improvement in the ultra-high precision angle measurement resolution of small angle measurement.

[0097] The small angle measuring device of the present invention can realize a small angle measuring device based on the principle of interference frequency selection. Compared with the prior art, the present invention overcomes the limitation of the angle index measured by the laser interferometer measurement method, and converts the angle measurement into the frequency measurement by virtue of the high sensitivity of the laser frequency to the rotation angle of the transparent solid medium (such as white glass). By adding an interference plate, the cavity mode frequency and the transmission frequency of the interference plate are changed synchronously to solve the problem of the small tunable range of the laser frequency caused by the rotation of the transparent solid medium, thereby realizing a small angle measuring device with the ability to continuously measure tiny angles. It can meet the application needs of laser gyroscopes for aerospace, high-end equipment manufacturing, and precision optical devices.

Claims

1. A high-resolution small-angle measurement device based on laser frequency measurement, the device comprising a laser generating device, a reflector group and a beat frequency measurement module arranged in sequence on an optical path, characterized in that A high-precision horizontal rotating table (5) is arranged between the laser generating device and the reflector group. The narrowband interference plate (3) and the transparent solid medium (4) are fixed on the high-precision horizontal rotating table (5). The fixed positions of the narrowband interference plate (3) and the transparent solid medium (4) are symmetrical and non-parallel with the diameter of the rotating table perpendicular to the optical path as an axis. The light output by the laser generating device passes through the narrowband interference plate (3) and the out-of-band light is filtered out to obtain narrowband light. The narrowband light returns to the laser diode after passing through the transparent solid medium (4) and the reflector group. When the oscillation in the cavity reaches a threshold value, the reflector group outputs the measured laser. The measured laser is input to the beat frequency measurement module. The beat frequency measurement module beats the measured laser with the comb teeth output by the optical comb system to obtain the beat frequency. When the high-precision horizontal rotating table (5) rotates to cause the measured laser frequency to change Δf, the change in the beat frequency is equal to Δf.

2. The small angle measuring device according to claim 1, characterized in that The laser generating device comprises a laser diode (1) and a first collimating lens (2) which are sequentially arranged on an optical path, and the transmission peak center wavelength of the narrow-band interference plate (3) matches the wavelength of the laser diode (1).

3. The small angle measuring device according to claim 1, characterized in that The reflector group comprises a focusing lens (7) and a partial reflector (8) which are sequentially arranged on the optical path.

4. The small angle measuring device according to claim 1, characterized in that The beat frequency measurement module comprises a reflector (10), a first half-wave plate (11) arranged in the emission direction of reflected light of the reflector (10), a polarization beam splitter prism (14), a broadband detector (15) arranged in the output light direction of the polarization beam splitter prism (14), and a frequency counter (16); the laser to be measured is incident on the polarization beam splitter prism (14) through the first half-wave plate (11) and then completely reflected; the comb teeth emitted by the optical comb system are incident on the polarization beam splitter prism after passing through the second half-wave plate (13); the optical comb and the laser to be measured are combined and then arrive at the broadband detector (15); the broadband detector (15) is connected to the frequency counter (16); the frequency counter (16) measures the beat frequency of the optical comb and the laser to be measured to obtain a variation Δf of the beat frequency.

5. The small angle measuring device according to claim 1, characterized in that A prism-type super-strong reflective film (81) is used to replace part of the reflector (8).

6. The small angle measuring device according to claim 1, characterized in that The device also includes an ultra-low expansion glass base (82), on which the laser diode (1), the first collimating lens (2), the narrow-band interference plate (3), the transparent solid medium (4), the prism-type partially reflective film (81), the high-precision horizontal rotating stage (5) and the rotating stage fixing seat (6) are fixed.

7. The small angle measuring device according to claim 1, characterized in that A second collimating lens (9) is arranged between the partial reflecting mirror (8) and the reflecting mirror (10).

8. A method for measuring small angles with high resolution and continuous measurement, the method comprising the following steps: (1) The laser light emitted by the laser diode is passed through a narrow-band interference plate to obtain narrow-band light; the narrow-band light is reflected by a partial reflector after passing through a transparent solid medium and returns to the laser diode; when the oscillation in the cavity reaches a threshold value, the measured laser light is output by the partial reflector; The narrowband interference plate and the transparent solid medium are fixed on a high-precision horizontal rotating table, and the fixed positions of the narrowband interference plate and the transparent solid medium are symmetrical and non-parallel with the diameter of the rotating table perpendicular to the laser light path as the axis; (2) When the high-precision horizontal rotating stage rotates, the narrowband interference plate rotates synchronously with the transparent solid medium. According to different D values, a curve of the change in cavity mode wavelength Δλ caused by the change in the rotation angle θ of the transparent solid medium is drawn. Δλ and the rotation angle θ of the transparent solid medium satisfy the following relationship: Where L is the length from the output light end 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, n eff is the refractive index of the transparent solid medium, Δλ is the change in the cavity mode wavelength caused by the rotation angle θ of the transparent solid medium; (3) The rotation angle of the narrowband interference plate is obtained by the following formula: Curve of the change in transmission wavelength Δλ′ caused by: λ′0 is the transmission wavelength when the narrowband interference plate is perpendicular to the light beam, n eff2 is the refractive index of the narrowband interference plate, Δλ′=λ0-λ′, λ′ is the rotation angle of the narrowband interference plate. The transmission wavelength at (4) The laser under test in step (1) is input to a beat frequency measurement module, and the beat frequency measurement module beats the laser under test with the comb teeth output by the optical comb system to obtain a beat frequency; (5) Calculate the rotation angle θ of the transparent solid medium. The relationship between the change Δf of the beat frequency and the rotation angle θ of the transparent solid medium is: Among them, K is when the value of D satisfies When Δλ=Δλ′, the slope of the curve of rotation angle θ and λ′ in step (3) is:

9. The small angle measurement method according to claim 8, characterized in that When L=10 cm, the transparent solid medium (4) is white glass with a thickness of 45 to 50 mm.

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