Small-angle measurement reference device based on interference frequency selection and microprism array feedback

Abstract

The invention provides a small-angle measuring reference device based on interference frequency selection and microprism array feedback, which comprises a laser generating device and a beat frequency measuring module, and is characterized in that a narrow-band interference sheet (4), a microprism array (5) and a piezoelectric module (6) are fixed on a high-precision horizontal rotating table (7) in parallel; the rear surface of the microprism array (5) is bonded with the front surface of the piezoelectric module (6), and the center of the rear surface of the piezoelectric module (6) is located at the axis position of the rotating table. And when the frequency of the measured laser is changed by delta f, the variable quantity of the beat frequency is equal to delta f. By means of ultrahigh sensitivity of laser frequency to cavity length, the cavity length is changed by rotating the cavity mirror to cause frequency change, then angle measurement is converted into frequency measurement, mode selection is carried out through auxiliary adjustment of the narrow-band interference sheet, cavity mode change and interference frequency selection are highly synchronous, and mode hopping of laser output frequency is avoided. The continuous tunability of the output frequency along with the rotation angle is ensured, and small-angle ultrahigh-precision measurement is realized.

Description

Technical Field

[0001] The invention relates to a small angle measurement reference device, in particular to a small angle measurement reference device based on interference frequency selection and microprism feedback. Background Art

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

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

[0004] The purpose of the present invention is to further improve the accuracy of small angle measurement, convert the angle measurement into the laser frequency measurement, and propose a small angle measurement reference device based on interference frequency selection and microprism array feedback.

[0005] In the previous research work, the inventor designed a high-resolution small-angle measurement device based on laser frequency measurement, which converted the angle measurement into a small-angle measurement reference device based on a narrow-band interference plate laser for laser frequency measurement, and realized continuous measurement by setting a transparent solid medium sheet in the optical path. Specifically, Figure 1As shown, the small angle measurement device includes a laser diode 201, a first collimating lens 202, a narrowband interference plate 203, a transparent solid dielectric plate 204, a focusing lens 207, a partial reflector 208, a second collimating lens 209 and a reflector 210 arranged in sequence on the optical path, an optical comb system and a beat frequency measurement module are arranged in the reflected light output direction of the reflector 210, and a high-precision horizontal rotating platform 205 is fixed on a rotating platform fixing seat 206. Among them, the narrowband interference plate 203 and the transparent solid dielectric plate 204 are fixed on the precision rotating platform 205, and their placement positions are symmetrical and non-parallel with the rotating platform diameter perpendicular to the optical path as the symmetry axis, so when the precision rotating platform 205 rotates, the angles between the narrowband interference plate 203 and the transparent solid dielectric plate 204 and the optical path change respectively. The beat frequency measurement module mainly includes a first half-wave plate 211, a second half-wave plate 213, a polarization beam splitter prism 214, a broadband detector 215 and a frequency counter 216. The light output by the laser generating device passes through the narrowband interference plate 203, and the out-of-band light is filtered out to obtain narrowband light; the narrowband light returns to the laser diode 201 after passing through the reflector group, and when the oscillation in the cavity 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 212 to obtain the beat frequency, and the rotation angle of the narrowband interference plate is obtained by measuring the change Δf of the beat frequency.

[0006] However, due to the special placement of the narrowband interference plate 203 and the transparent solid medium plate 204, the algorithm is more complicated when calculating the angle change by measuring the change in the beat frequency Δf. Therefore, further improving the structure of the small angle measurement device and its measurement method to make the measurement simpler and more accurate to meet the needs of high-precision measurement of small angles in application technologies such as lithography machines, laser radars, precision robots, and virtual reality is still a technical problem that the technical field wants to solve.

[0007] The idea of ​​the present invention is to utilize the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser, integrate the microprism array with a piezoelectric module with a thickness of D, and place the center position of the rear surface of the piezoelectric module at the axis position of the rotating table, even if the vertical distance between the rear surface of the microprism array and the axis is D, by rotating the microprism array and the piezoelectric module to change the optical path length of the laser through the prism array, the cavity length of the resonant cavity is changed, and then the cavity mode frequency is changed. At the same time, a narrow-band interference plate is used to assist in the adjustment of the mode selection, so that the center position of the transmission peak of the interference plate changes with the angle, and is highly synchronized with the change of the cavity mode frequency with the angle, so as to achieve the laser output frequency without mode hopping, and ensure the continuous tunability of the output frequency with the rotation angle, thereby achieving continuous ultra-high precision continuous measurement of small angles.

[0008] To this end, the present invention provides a small angle measurement reference device based on interference frequency selection and microprism array feedback, the device comprises a laser generating device and a beat frequency measurement module which are sequentially arranged on an optical path, wherein a high-precision horizontal rotating table 7 is arranged on the output light optical path of the laser generating device, a narrow-band interference plate 4, a microprism array 5 and a piezoelectric module 6 are fixed on the high-precision horizontal rotating table 7, the rear surface of the microprism array 5 is bonded to the front surface of the piezoelectric module 6 and the rear surface center of the piezoelectric module 6 is at the axis position of the rotating table, and the laser generating device and the narrow-band interference plate 4 are connected to each other. A polarization beam splitter prism 3 is arranged between the laser generating device and the laser diode. The light outputted by the laser generating device is filtered out of the band after passing through the narrow-band interference plate 4 to obtain narrow-band light. The narrow-band light is reflected after passing through the microprism array 5 and returns to the laser diode after passing through the polarization beam splitter prism 3. When the oscillation in the cavity reaches the threshold value, the measured laser is reflected and outputted by the polarization beam splitter prism 3. The measured laser is inputted into the beat frequency measurement module. The beat frequency measurement module beats the measured laser with the comb teeth outputted by the optical comb system 10 to obtain the beat frequency. When the frequency of the measured laser changes by Δf, the change in the beat frequency is equal to Δf.

[0009] In the present invention, the micro-prism array 5 can be realized by a prismatic super-strong reflective film 71 with a high retroreflection coefficient, for example, the micro-prismatic reflective film disclosed in Chinese invention patent application CN 201410823342.3.

[0010] In the present invention, the rear surface of the microprism array 5 is bonded to the front surface of the piezoelectric module 6 and the center of the rear surface of the piezoelectric module 6 is located at the axis position of the turntable. Therefore, the vertical distance D between the rear surface of the microprism array 5 and the axis center of the turntable can be adjusted by adjusting the voltage of the piezoelectric module to compensate for the thickness error of the piezoelectric module and the deviation error between the axis position and the center position of the rear surface of the piezoelectric module, thereby optimizing the device.

[0011] In the present invention, the optical elements should all be located in the optical path to allow the laser to pass through.

[0012] In the present invention, for the sake of convenience of explanation rather than limitation, the position of the laser output device in the optical path is referred to as the "front", and the position of the piezoelectric module in the optical path is referred to as the "rear". Therefore, taking the microprism array as an example, the side facing the laser output device is referred to as the "front surface", and the other side is referred to as the "rear surface".

[0013] Combination Figure 2-3 The working principle is explained as follows:

[0014] like Figure 2As shown in the figure, when the microprism array (bonded to the piezoelectric module) is perpendicular to the optical path, the incident angle is 0°. Let the resonant cavity length L = L1, where L1 is the length from the laser output surface to the front surface of the microprism array when the incident angle is 0°. When the rotation angle of the microprism array is θ, the cavity length Therefore, when the rotating stage rotates and the microprism array rotates from an incident angle of 0° to θ°, the change in cavity length is: Where D is the thickness of the piezoelectric module, that is, the vertical distance between the microprism array and the axis.

[0015] 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:

[0016] Where f is the frequency, c is the speed of light, and λ0 is the initial wavelength emitted by the laser generator.

[0017] Known The relationship between the change in cavity mode wavelength Δλ caused by the rotation of the microprism array by an angle of θ and the vertical distance D between the microprism array and the axis and its rotation angle θ can be obtained:

[0018] On the other hand, the narrowband interference plate rotation angle The relationship with the transmission wavelength λ′ is as follows:

[0019]

[0020] λ0 is the transmission wavelength when the narrowband interference plate is perpendicular to the light beam, n eff is the refractive index of the narrowband interference plate, then the change in transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ0-λ′.

[0021] Since the narrow-band interference plate of the present invention is placed parallel to the microprism array, when the rotating platform rotates by an angle θ, the rotation angle of the narrow-band interference plate is The rotation angle θ of the microprism array is equal to that of the microprism array. Therefore, by adjusting the vertical distance D between the microprism array and the axis, the change in the cavity mode wavelength caused by the microprism array rotating at an angle θ is equal to the rotation angle of the narrow-band interference plate. angle The transmission wavelength change caused is equal or approximately the same (in the present invention, "approximately the same" is understood to mean that the error is extremely small and can be regarded as equal), which can make the cavity mode frequency and the interference plate transmission peak frequency change synchronously, thereby solving the problems of laser frequency mode hopping caused by the rotation of a single interference plate and the small frequency tunable range caused by a single microprism array, and realizing a large range of continuous measurement of the rotation angle.

[0022] As another embodiment of the present invention, the relationship between the rotation angle of the microprism array (integrated with the piezoelectric module) and the wavelength is:

[0023] (1) Assuming that the angle between the microprism array and the incident light is 0°, the cavity length When the microprism array rotates by θ, the cavity length So when the microprism array rotates at an angle of θ, the cavity mode wavelength can be obtained: Where n is a positive number, D is the thickness of the piezoelectric module, that is, the vertical distance between the front surface of the microprism array and the axis. L1 is the length from the laser output light surface to the microprism array when the incident angle is 0°.

[0024] (2) Therefore, by setting the vertical distance between the microprism array and the axis, the change in the cavity mode wavelength caused by the rotating microprism array is equal to (or approximately the same as) the change in the transmission wavelength caused by the rotating 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 the rotation of a single interference plate and the small frequency tunable range caused by the single rotating microprism array.

[0025] 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:

[0026]

[0027] Among them, K is when the value of D satisfies When Δλ=Δλ′, the rotation angle The slope of the λ′ curve is:

[0028]

[0029] In the present invention, the laser generating device is a conventional setting in the art, and generally includes a laser diode 1 and a collimating lens 2 arranged in sequence on the optical path. The working band of the laser diode 1 can be any band, and preferably, from the perspective of easy acquisition or economical saving, a 780nm band laser diode can be used. Those skilled in the art can also select laser diodes with other working wavelengths, such as 420nm, 850nm, 1550nm laser diodes, etc.

[0030] 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 front surface of the microprism array, and after passing through the microprism array, the reflected light in the opposite direction of the narrow-band light is obtained; the reflected light is fed back to the laser diode, and when the oscillation in the cavity reaches the threshold, the reflected light output by the polarization beam splitter prism is used as the measured laser. The measured laser reaches the beat frequency measurement module, and the beat frequency is compared with the optical comb system. By measuring the change in the beat frequency Δf, it is converted into a change in the rotation angle θ.

[0031] 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 9 arranged in the output reflected light direction of the first polarization beam splitter prism 3, a second polarization beam splitter prism 12, a broadband detector 13 and a frequency counter 14 arranged in the output light direction of the second polarization beam splitter prism 12; the measured laser is completely reflected after being incident on the second polarization beam splitter prism 12 through the first half-wave plate 9, the comb teeth emitted by the optical comb system 10 are incident on the second polarization beam splitter prism 12 after passing through the second half-wave plate 11, and the comb teeth and the measured laser are combined to reach the broadband detector 13, and the broadband detector 13 is connected to the frequency counter 14, and the beat frequency is obtained by the frequency counter 14. When the beat frequency changes due to the rotation of the rotating stage, the beat frequency change Δf is obtained by Δf=f0-f1. Among them, f0 is the measured laser frequency when the high-precision horizontal rotating stage is at the initial position, and f1 is the measured laser frequency after the high-precision horizontal rotating stage rotates.

[0032] 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.

[0033] 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.

[0034] As another embodiment of the present invention, the present invention can bond the narrowband interference plate 4 to the front surface of the microprism array 5, that is, the narrowband interference plate, the microprism array and the piezoelectric module are integrated into one and fixed together on a rotating table to improve the mechanical robustness of the system.

[0035] According to a preferred embodiment, the present invention also includes an ultra-low expansion glass base 15, on which the laser diode 1, the collimating lens 2, the first polarization beam splitter prism 3, the integrated narrow-band interference plate-microprism array-piezoelectric module, the high-precision horizontal rotating table 7 and the rotating table fixing seat 8 are fixed. This can be used to reduce the influence of cavity length jitter on the frequency of the measured laser during free operation, and stabilize the laser frequency fluctuation to the Hz level. This arrangement can further improve the angle measurement resolution.

[0036] In the present invention, the ultra-low expansion glass may be ultra-low expansion glass (ULE) sold by Corning Incorporated of the United States.

[0037] On the other hand, the present invention also provides a method for continuously measuring a small angle, the method comprising the following steps:

[0038] (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 the microprism array and returns to the laser diode through a polarization beam splitter prism; when the oscillation in the cavity reaches a threshold value, the polarization beam splitter prism reflects and outputs the measured laser light;

[0039] The narrowband interference plate and the microprism array are fixed in parallel on a high-precision horizontal rotating platform, a piezoelectric module is bonded to the rear surface of the microprism array, and the center of the rear surface of the piezoelectric module is located at the axis position of the rotating platform, so when the rotating platform rotates, the narrowband interference plate and the microprism array change at the same time as the incident angle, and the angle change amount is equal;

[0040] (2) When the high-precision horizontal rotating stage rotates, the narrow-band interference plate rotates synchronously with the microprism array. According to different values ​​of the thickness D of the piezoelectric module, a curve of the change in the cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array is drawn. Δλ and the rotation angle θ of the microprism array satisfy the following relationship:

[0041]

[0042] Wherein, L is the length from the output light end face of the laser diode to the front surface of the microprism array, λ0 is the laser output wavelength, D is the thickness of the piezoelectric module, and Δλ is the change in the cavity mode wavelength caused by the rotation angle of the microprism array to θ;

[0043] (3) The rotation angle of the narrowband interferometer is obtained by the following formula The relationship between the transmission wavelength λ′ is as follows:

[0044]

[0045] λ0 is the transmission wavelength when the narrowband interference plate is perpendicular to the light beam, n effis the refractive index of the narrowband interference plate, then the change in transmission wavelength Δλ′ caused by the rotation of the interference plate is: Δλ′=λ0-λ′;

[0046] (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;

[0047] (5) Calculate the rotation angle θ of the microprism array. The relationship between the change Δf of the beat frequency and the rotation angle θ of the microprism array is:

[0048]

[0049] 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 microprism array 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:

[0050]

[0051] The small angle measurement reference device of the present invention adopts a narrow band interference plate to perform narrow band filtering, filters the broadband light emitted by the laser diode into a narrow band beam, and realizes the output of the measured laser whose laser frequency is extremely sensitive to the rotation angle under the light feedback of the micro prism array, so that the small angle change of the rotation angle is reflected in the change amount Δf of the measured laser and the comb tooth beat frequency. Among them, by adjusting the voltage of the piezoelectric module to optimize its own thickness D (i.e., the vertical distance between the rear surface of the micro prism array and the axis), the change of the cavity mode frequency can be synchronized with the change of the central frequency of the transmission peak of the interference plate, so as to solve the problem of laser frequency mode hopping caused by the rotation of a single interference plate and the small frequency tunable range caused by the single change of the cavity length (rotation of the micro prism array).

[0052] 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.

[0053] The present invention utilizes a small angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in an external cavity semiconductor laser 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

[0054] Figure 1 This is a schematic diagram of the structure of the small angle measurement reference device previously studied by the inventor;

[0055] Among them, 201, laser diode, 202, first collimating lens, 203, narrow-band interference plate, 204, transparent solid medium plate; 205, high-precision horizontal rotating stage, 206, rotating stage fixing seat, 207, focusing lens, 208, partial reflection mirror, 209, collimating lens, 210, reflection mirror, 211, first half-wave plate, 212, optical comb system, 213, second half-wave plate, 214, polarization beam splitter prism, 215, broadband detector, 216, frequency counter.

[0056] Figure 2 It is a schematic diagram of the principle of a small angle measurement reference device;

[0057] Figure 3 This is a schematic structural diagram of a small angle measurement device according to Embodiment 1;

[0058] Figure 4 This is a schematic structural diagram of a small angle measurement device according to Embodiment 2;

[0059] Figure 5 is the relationship between the rotation angle of the microprism array and the change of the cavity mode frequency in Example 1;

[0060] Figure 6 The relationship between the rotation angle of the microprism array and the change of the cavity mode wavelength in Example 1;

[0061] Figure 7 is a comparison diagram of the relationship between the rotation angle of the microprism array and the cavity mode wavelength and the relationship between the rotation angle of the interference plate and the transmission wavelength in Example 1;

[0062] Among them, 1. laser diode, 2. collimating lens, 3. first polarization beam splitter prism, 4. narrow-band interference plate, 5. microprism array, 6. piezoelectric module, 7. high-precision horizontal rotating stage, 8. rotating stage fixing seat, 9. first half-wave plate, 10. optical comb system, 11. second half-wave plate, 12. second polarization beam splitter prism, 13. broadband detector, 14. frequency counter. DETAILED DESCRIPTION

[0063] The following examples are used to explain the technical solutions of the present invention in a non-limiting manner.

[0064] Example 1 investigates the influence of the thickness D of the piezoelectric module, that is, the different vertical distances between the rear surface of the microprism array and the axis on the measurement results

[0065] like Figure 3 The small angle measurement reference device shown has a laser operating band of 780nm. The device mainly includes a laser diode 1, a collimating lens 2, a first polarization beam splitter prism 3, a narrowband interference plate 4, a microprism array 5 and a piezoelectric module 6 which are arranged in sequence on the optical path. An optical comb system 10 and a beat frequency measurement module are arranged in the output reflected light direction of the first polarization beam splitter prism 3. A high-precision horizontal rotating table 7 is fixed on a rotating table fixing seat 8. The microprism array 5 and the piezoelectric module 6 are integrated into one and are fixed on the high-precision horizontal rotating table 7 in parallel with the narrowband interference plate 4. The center of the rear surface of the piezoelectric module 6 with a thickness of D is placed at the axis center of the rotating table (therefore, when the rotating table rotates, the distance between the rear surface of the microprism array and the axis center of the rotating table is always D), and the narrowband interference plate 4 is parallel to the microprism array 5 (such as Figure 2 As shown), when the high-precision horizontal rotating stage 7 rotates counterclockwise around the center by θ, the narrow-band interference plate and the prism-type super-strong reflective film rotate by θ at the same time, that is, φ=θ.

[0066] The beat frequency measurement module mainly includes a first half-wave plate 9 , a second half-wave plate 11 , a second polarization beam splitter prism 12 , a broadband detector 13 and a frequency counter 14 .

[0067] 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 87Wedge-shaped vapor cell for Rb (Thorlabs, GC19075-RB87). AMP, amplifier (Mini-Circuits, ZFL-500LN+); BE1 and BE2 are beam expanders; EOM is electro-optic modulator (Qubig, PM7-NIR_5); FC is fiber collimator; FG is signal generator; GT-P is Glan-Thompson polarizer; half-wave plate; LPF is 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, PDA10A2).

[0068] During operation, the light output by the 780nm laser diode is collimated, and then the out-of-band light is filtered out by a narrow-band interference plate to obtain narrow-band light. The narrow-band light reaches the front surface of the microprism array, and after passing through the microprism array, the reflected light in the opposite direction of the narrow-band light is obtained; the reflected light is fed back to the laser diode, and when the oscillation in the cavity reaches the threshold, the reflected light is output by the polarization beam splitter prism as the measured laser.

[0069] The laser to be measured reaches the beat frequency measurement module. The beat frequency measurement module compares the laser to be measured with the comb teeth output by the optical comb system to obtain the beat frequency. When the frequency of the laser to be measured changes by Δf, the change in the beat frequency is equal to Δf.

[0070] In order to convert the change of the beat frequency Δf into the change of the microprism array rotation angle θ, firstly, the change of the cavity mode wavelength Δλ caused by the rotation angle of the microprism array is determined by measurement:

[0071] In this embodiment, the cavity length is set to L = 10 cm, λ0 = 780 nm, and piezoelectric modules with different thicknesses D are selected, that is, the vertical distances between the microprism array and the axis are D = 20 mm, D = 22 mm, D = 24 mm, D = 25 mm, and D = 28 mm. The cavity mode wavelength change Δλ curve caused by the rotation angle θ of each microprism array is plotted, as shown in Figure 5 , 6 shown.

[0072] When θ changes continuously in the range of 0 to 14°, the cavity mode frequency and wavelength caused by the microprism array change accordingly.

[0073] Taking θ=10° as an example, Figure 5 and Figure 6 It can be seen that when D = 22 mm, the cavity mode frequency change Δf caused by rotating the microprism array is 1.3×10 12 Hz, the cavity mode wavelength changes Δλ=-2.647nm. When D=24mm, Δf=1.42×1012 Hz, Δλ=-2.888nm. When D=25mm, Δf=1.48×10 12 Hz, Δλ=-3nm.

[0074] Then, according to the narrowband interference plate rotation angle The relationship expression with the transmission wavelength λ′:

[0075]

[0076] get Figure 7 From the IF curve, it can be obtained that when θ=10°, the transmission wavelength λ′ of the interference plate is 777.1nm, that is, the transmission wavelength change Δλ′ of the interference plate is -2.9nm, which is between Δλ=-2.888nm (corresponding to D=24mm) and Δλ=-3nm (corresponding to D=25mm). Therefore, the optimal range of the vertical distance D between the microprism array and the axis is 24mm~25mm.

[0077] When D is selected to be 24mm~25mm, when the microprism array rotates at a small angle near θ=10°, the change in cavity mode wavelength Δλ caused by the rotation of the microprism array and the change in transmission wavelength Δλ′ caused by the rotation of the interference plate are approximately the same and are considered equal. Therefore, the cavity mode frequency and the transmission frequency of the interference plate change synchronously, so that the output laser frequency can change continuously, thereby realizing a large range of continuous small-angle measurement.

[0078] Finally, the measured change in beat frequency Δf is used to calculate the rotation angle θ of the microprism array:

[0079]

[0080] That is, when D = 24 mm to 25 mm, and θ changes slightly around 10° (i.e., 10° ± 1°), Δλ = Δλ′, so θ can be calculated by this formula. K is The slope of the λ′ curve is:

[0081]

[0082] 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 of the interference plate (i.e. the angle between the laser and the normal of the interference plate) is 10°, k = 0.55, then the relationship between the change of the interference plate angle and the change of the transmission wavelength is 0.55nm / °. 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 269.5GHz / °. Therefore, the angle change corresponding to a frequency fluctuation of 30kHz is approximately 1.11×10 -7 °(0.00040074″), so the corresponding angle measurement resolution can be achieved at the order of 0.0001″. That is, when the frequency change is not greater than 30kHz, it is regarded as the system's own fluctuation and cannot be measured. When the frequency change is greater than 30kHz, the angle change can be calculated by measuring the frequency change.

[0083] Example 2

[0084] The difference from Example 1 is that the narrowband interference plate 4 is bonded to the front surface of the microprism array 5, that is, the narrowband interference plate 4, the microprism array 5 and the piezoelectric module 6 are fixed as a whole, which reduces the laser frequency change caused by mechanical vibration and can measure the angle more accurately. In addition, the laser diode 1, the collimating lens 2, the first polarization beam splitter prism 3, the narrowband interference plate 4, the microprism array 5, the piezoelectric module 6, the high-precision horizontal rotating table 7 and the rotating table fixing seat 8 are fixed on the base 15 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. The rest of the structure remains unchanged.

[0085] Similarly, the vertical distance D between the microprism array 5 and the axis is set in the range of 24 mm to 25 mm. The incident angles of the narrowband interference plate 4 and the microprism array 5 are changed by the high-precision horizontal rotating stage 7. At this time, when the microprism array rotates at a small angle near θ=10°, the center position of the transmission peak of the interference plate moves with the angle and is highly synchronized with the change of the cavity mode frequency with the angle, thereby realizing continuous tunability of the laser frequency over a large range.

[0086] 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 incident angle of the narrow-band interference plate 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 wavelength with angle is in the range of 0-1.1nm / °. For example, when the incident angle of the interference plate is 10°, k = 0.55, that is, the relationship between the change of the interference plate angle and the change of the transmission wavelength is 0.55nm / °. 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 269.5GHz / °. Therefore, the angle change corresponding to each 1Hz is about 3.71×10 -12 °(0.000000013358″), so the corresponding angle measurement resolution can reach about 0.00000001″.

[0087] Compared with the frequency fluctuation of about 30kHz when the measured laser is free running in Example 1, the present embodiment uses ultra-low expansion glass to improve the stability of the frequency fluctuation of the measured laser when it is free running to the Hz level, so that the corresponding angle measurement resolution can be achieved at the order of about 0.00000001", achieving a significant improvement in the angle measurement resolution.

[0088] The present invention is a small-angle measurement reference device based on interference frequency selection and microprism feedback, which can realize continuous measurement based on the ultra-high sensitivity of laser frequency to cavity length in an external cavity semiconductor laser. The present invention uses the ultra-high sensitivity of laser frequency to cavity length, changes the cavity length by rotating the cavity mirror, causes frequency changes, and then converts angle measurement into frequency measurement, uses narrow-band interference plate to assist in adjustment for mode selection, makes cavity mode changes highly synchronized with interference frequency selection, realizes laser output frequency without mode hopping, ensures continuous tunability of output frequency with rotation angle, solves continuous measurement problems, and then realizes ultra-high precision measurement of small angles. It can meet the application needs of laser gyroscopes, precision optical devices, construction, and automobiles in the fields of aerospace.

Claims

1. A small angle measurement reference device based on interference frequency selection and microprism array feedback, the device comprising a laser generating device and a beat frequency measurement module arranged in sequence on an optical path, wherein: A high-precision horizontal rotating table (7) is arranged on the optical path of the output light of the laser generating device; the narrow-band interference plate (4), the micro-prism array (5) and the piezoelectric module (6) are fixed in parallel on the high-precision horizontal rotating table (7); the rear surface of the micro-prism array (5) is bonded to the front surface of the piezoelectric module (6) and the center of the rear surface of the piezoelectric module (6) is located at the axis position of the rotating table; a first polarization beam splitting prism (3) is arranged between the laser generating device and the narrow-band interference plate (4); the light output by the laser generating device is transmitted through the optical path of ... narrow-band interference plate (4) is transmitted through the optical path of the narrow-band interference plate (4); the light output by the narrow-band interference plate (4) is transmitted through the optical path of the narrow-band interference plate (4); the light output by the narrow-band interference plate (4) is transmitted through the optical path of the narrow-band interference plate (4); the light output by the narrow-band interference plate (4) is transmitted through the optical path of the narrow-band interference plate (4); the light output by the narrow-band interference plate (4) is transmitted through the optical path of After passing through a narrow-band interference plate (4), out-of-band light is filtered out to obtain narrow-band light; the narrow-band light is reflected after passing through a micro-prism array (5), and returns to a laser generating device after passing through a polarization beam splitter prism (3); when the oscillation in the cavity reaches a threshold value, the polarization beam splitter prism (3) reflects and outputs a measured laser; the measured laser is input into 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 a beat frequency. When the frequency of the measured laser changes by Δf, the change in the beat frequency is equal to Δf.

2. The small angle measurement reference device according to claim 1, characterized in that The micro-prism array (5) is a prism-type super-strong reflective film (71).

3. The small angle measurement reference device according to claim 1, characterized in that The laser generating device comprises a laser diode (1) and a collimating lens (2) which are sequentially arranged on an optical path, and the transmission peak center wavelength of the narrow-band interference plate (4) matches the wavelength of the laser diode (1).

4. The small angle measurement reference device according to claim 1, characterized in that The beat frequency measurement module comprises a first half-wave plate (9) arranged in the emission direction of light reflected by the first polarization beam splitter prism (3), a second polarization beam splitter prism (12), a broadband detector (13) and a frequency counter (14) arranged in the output direction of light from the second polarization beam splitter prism (12); The measured laser is incident on the second polarization beam splitter prism (12) through the first half-wave plate (9) and is completely reflected. The comb teeth emitted by the optical comb system (10) are incident on the second polarization beam splitter prism (12) through the second half-wave plate (11). The optical comb and the measured laser are combined and arrive at a broadband detector (13). The broadband detector (13) is connected to a frequency counter (14). The frequency counter (14) measures the beat frequency of the optical comb and the measured laser to obtain a change Δf of the beat frequency.

5. The small angle measurement reference device according to claim 1, characterized in that The narrowband interference plate (4) is bonded to the front surface of the microprism array (5); the narrowband interference plate (4), the microprism array (5) and the piezoelectric module (6) are fixed as a whole and fixed together on the rotating platform.

6. The small angle measurement reference device according to claim 1, characterized in that The device also includes an ultra-low expansion glass base (15) and a rotating table fixing base (8), a high-precision horizontal rotating table (7) is fixed on the rotating table fixing base (8), and a laser diode (1), a collimating lens (2), a first polarization beam splitter prism (3), a narrow-band interference plate (4), a microprism array (5), a piezoelectric module (6), a high-precision horizontal rotating table (7) and a rotating table fixing base (8) are fixed on the ultra-low expansion glass base (15).

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

8. A method for continuously measuring small angles, 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 the microprism array and returns to the laser diode through a polarization beam splitter prism; when the oscillation in the cavity reaches a threshold value, the polarization beam splitter prism reflects and outputs the measured laser light; The narrowband interference plate and the microprism array are fixed in parallel on a high-precision horizontal rotating platform, a piezoelectric module is bonded to the rear surface of the microprism array, and the center of the rear surface of the piezoelectric module is located at the axis position of the rotating platform. When the rotating platform rotates, the incident angles of the laser and the narrowband interference plate and the microprism array change simultaneously and the angle changes are equal; (2) When the high-precision horizontal rotating stage rotates, the narrow-band interference plate rotates synchronously with the microprism array. According to different values ​​of the thickness D of the piezoelectric module, a curve of the change in the cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array is drawn. Δλ and the rotation angle θ of the microprism array satisfy the following relationship: Wherein, L is the length from the output light end face of the laser diode to the front surface of the microprism array, λ0 is the initial wavelength emitted by the laser generator, D is the thickness of the piezoelectric module, and Δλ is the change in the cavity mode wavelength caused by the rotation angle of the microprism array to θ; (3) The rotation angle of the narrowband interferometer is obtained by the following formula The relationship between the transmission wavelength λ′ is as follows: λ0 is the transmission wavelength when the narrowband interference plate is perpendicular to the light beam, n eff is the refractive index of the narrowband interference plate, then the wavelength change Δλ′ caused by the rotation of the interference plate is: Δλ′=λ0-λ′; (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 microprism array. The relationship between the change Δf of the beat frequency and the rotation angle θ of the microprism array is: When the value of D satisfies When Δλ=Δλ′, K is the rotation angle in step (3) The slope of the λ′ curve is:

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