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

Abstract

The application provides a small-angle measurement reference device based on interference frequency selection and micro-prism array feedback, comprising a laser generation device and a beat frequency measurement module, a narrow-band interference sheet (4), a micro-prism array (5) and a piezoelectric module (6) are fixed in parallel on a high-precision horizontal rotating table (7), the rear surface of the micro-prism 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; when the measured laser frequency changes by Δf, the change amount of the beat frequency is equal to Δf. The application utilizes the ultra-high sensitivity of the laser frequency to the cavity length, changes the cavity length by rotating the cavity mirror to cause the frequency change, and then converts the angle measurement into the frequency measurement, utilizes the narrow-band interference sheet to assist the adjustment to select the mode, makes the cavity mode change highly synchronized with the interference frequency selection, realizes the non-mode jumping of the laser output frequency, guarantees the continuous tunability of the output frequency with the rotation angle, and realizes the ultra-high precision measurement of the small angle.

Description

TECHNICAL FIELD

[0001] The present application relates to a small angle measurement reference device, in particular to a small angle measurement reference device based on interference frequency selection and micro-prism feedback. BACKGROUND

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

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

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

[0005] The inventor designed a high-resolution small angle measurement device based on laser frequency measurement in the early research work, which converts the measurement of angle into the measurement of laser frequency based on a small angle measurement reference device of narrow-band interference sheet laser, and realizes continuous measurement by setting a transparent solid medium sheet in the optical path. Specifically, as shown in FIG. 1, the device includes a laser source 1, a narrow-band interference sheet 2, a solid medium 3, a micro-prism array 4, a detector 5 and a data processing unit 6. Figure 1As shown, the small-angle measuring device includes, in sequence arranged on the light path, a laser diode 201, a first collimating lens 202, a narrow-band interference sheet 203, a transparent solid medium sheet 204, a focusing lens 207, a partial mirror 208, a second collimating lens 209, and a mirror 210, a light comb system and a beat frequency measuring module are arranged in the reflected light output direction of the mirror 210, and a high-precision horizontal rotating table 205 is fixed on a rotating table fixing seat 206. Among them, the narrow-band interference sheet 203 and the transparent solid medium sheet 204 are fixed on the precision rotating platform 205, and their placement positions are symmetrical and non-parallel with the rotating table diameter perpendicular to the light path as the symmetry axis, so when the precision rotating platform 205 rotates, the included angles of the narrow-band interference sheet 203 and the transparent solid medium sheet 204 with the light path change respectively. The beat frequency measuring module mainly includes a first half-wave plate 211, a second half-wave plate 213, a polarization beam splitter prism 214, a wide-band detector 215 and a frequency counter 216. The light output by the laser generating device is filtered out of the light outside the band after passing through the narrow-band interference sheet 203, and narrow-band light is obtained; the narrow-band light returns to the laser diode 201 after passing through the mirror group, and when the intracavity oscillation reaches the threshold, the measured laser is output by the mirror group; the measured laser is input to the beat frequency measuring module, the beat frequency measuring module performs beat frequency on the measured laser and the comb teeth output by the light comb system 212 to obtain the beat frequency, and the change amount Δf of the beat frequency is measured to obtain the rotation angle of the narrow-band interference sheet.

[0006] However, due to the special placement position of the narrow-band interference sheet 203 and the transparent solid medium sheet 204, the algorithm is relatively complex when calculating the angle change by measuring the change amount Δf of the beat frequency. Therefore, further improving the structure of the small-angle measuring device and its measuring method to make the measurement simpler and more accurate to meet the demand of high-precision measurement of small angles in application technologies such as photolithography machines, laser radars, precision robots, virtual reality and the like is still a technical problem to be solved in the technical field.

[0007] The idea of the present application is to utilize the ultra-high sensitivity of laser frequency to cavity length in an external cavity semiconductor laser, integrate a micro-prism array with a piezoelectric module having a thickness 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 micro-prism array and the axis is D, the optical path length of the laser passing through the prism array is changed by rotating the micro-prism array and the piezoelectric module, the cavity length is changed, and the cavity mode frequency is changed. At the same time, the narrow-band interference sheet is used for auxiliary adjustment to select the mode, so that the center position of the interference sheet transmission peak changes with the angle, and is highly synchronized with the change of the cavity mode frequency with the angle, the laser output frequency does not jump mode, the continuous tunability of the output frequency with the rotation angle is ensured, and the continuous ultra-high precision continuous measurement of small angles is realized.

[0008] To this end, the application provides a small angle measurement reference device based on interference frequency selection and micro-prism 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 output light path of the laser generating device, a narrow-band interference sheet 4, a micro-prism array 5 and a piezoelectric module 6 are fixed on the high-precision horizontal rotating table 7, the rear surface of the micro-prism 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 a polarization beam splitter 3 is arranged between the laser generating device and the narrow-band interference sheet 4; the light output by the laser generating device is filtered to obtain narrow-band light after passing through the narrow-band interference sheet 4; the narrow-band light is reflected after passing through the micro-prism array 5 and returned to the laser diode after passing through the polarization beam splitter 3, and the measured laser is reflected and output by the polarization beam splitter 3 when the cavity oscillation reaches the threshold; the measured laser is input into the beat frequency measurement module, the beat frequency measurement module performs beat frequency on the measured laser and the comb teeth output by the optical comb system 10 to obtain a beat frequency, and when the frequency of the measured laser changes by Δf, the change amount of the beat frequency is equal to Δf.

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

[0010] In the application, the rear surface of the micro-prism 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, so that the vertical distance D between the rear surface of the micro-prism array 5 and the axis of the rotating table 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, and the device is optimized.

[0011] In the application, all optical elements should be located in the optical path so that the laser can pass through.

[0012] In the application, for the convenience of explanation but not limitation, the position of the laser output device in the optical path is referred to as "front", the position of the piezoelectric module in the optical path is referred to as "rear", and therefore, taking the micro-prism array as an example, one side facing the laser output device is referred to as "front surface" and the other side is referred to as "rear surface".

[0013] In combination Figures 2-3 The working principle is explained as follows:

[0014] For example Figure 2As shown, when the micro-prism array (which is integrated with the piezoelectric module) is perpendicular to the light path, the incident angle is 0°, and the resonant cavity length L is L1 at this time, L1 being the length from the output light surface of the laser to the front surface of the micro-prism array when the incident angle is 0°. When the rotation angle of the micro-prism array is θ, the cavity length L is L1+Dθ. Therefore, when the rotation table rotates and the micro-prism array rotates from an incident angle of 0° to an angle of θ, the change in the cavity length is Dθ. Where D is the thickness of the piezoelectric module, i.e. the vertical distance between the micro-prism array and the axis.

[0015] According to the relationship between the change in the cavity length and the half wavelength, the relationship between the frequency change c / 2L and the change in the cavity length is as follows: The relationship between the resonant cavity fluctuation ΔL and the change in the cavity mode frequency Δf is as follows:

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

[0017] It is known that The relationship between the change in the cavity mode wavelength Δλ caused by the rotation of the micro-prism array by an angle of θ and the vertical distance D between the micro-prism array and the axis and the rotation angle θ of the micro-prism array is as follows:

[0018] On the other hand, the rotation angle of the narrow-band interference sheet is The relationship between the transmission wavelength λ' and the rotation angle of the narrow-band interference sheet is as follows:

[0019]

[0020] λ0 is the transmission wavelength when the narrow-band interference sheet is perpendicular to the light beam, n is the refractive index of the narrow-band interference sheet, and Δλ' = λ0 - λ' is the change in the transmission wavelength caused by the rotation of the interference sheet. eff

[0021] Since the narrow-band interference sheet and the micro-prism array are parallel in the present application, the rotation angle of the narrow-band interference sheet is equal to the rotation angle θ of the micro-prism array when the rotation table rotates by an angle of θ. Therefore, the change in the cavity mode wavelength caused by the rotation of the micro-prism array by an angle of θ and the adjustment of the vertical distance D between the micro-prism array and the axis is equal to or approximately the same (in the present application, "approximately the same" means that the error is extremely small and can be considered equal) as the change in the transmission wavelength caused by the rotation of the narrow-band interference sheet by an angle of θ, so that the cavity mode frequency and the transmission peak frequency of the interference sheet change synchronously, thereby solving the problems of the laser frequency mode jumping caused by the rotation of a single interference sheet and the small frequency tunable range caused by a single micro-prism array, and realizing a large-range continuous measurement of the rotation angle.

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

[0023] (1) Assuming that the angle between the microlens array and the incident light is 0°, the cavity length is When the microlens array is rotated by θ, the cavity length is Therefore, when the microlens array is rotated by θ, the cavity mode wavelength at this time can be obtained where n is a positive number, D is the thickness of the piezoelectric module, i.e., the vertical distance between the front surface of the microlens array and the axis. L1 is the length from the laser output surface to the microlens array when the incident angle is 0°.

[0024] (2) Therefore, by setting the vertical distance between the microlens array and the axis, the change in the cavity mode wavelength caused by rotating the microlens array is equal to (or approximately equal to) the change in the transmission wavelength caused by rotating the interference plate, so that the cavity mode frequency and the interference plate transmission peak frequency change synchronously, thereby solving the problems of laser frequency mode jumping caused by single interference plate rotation and small frequency tuning range caused by single rotating microlens array.

[0025] 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 the interference plate rotation angle and the transmission wavelength have an approximate linear relationship Knm / degree within ±1°, so that the interference plate rotation angle satisfies the formula:

[0026]

[0027] where K is the rotation angle when the value of D satisfies and Δλ = Δλ' conditions, the rotation angle and the slope of the λ' curve:

[0028]

[0029] In the present application, the laser generating device is a conventional arrangement in the art, and generally includes a laser diode 1 and a collimating lens 2 arranged in sequence in the optical path. Among them, the working waveband of the laser diode 1 can be any waveband, and preferably, a 780 nm waveband laser diode can be used from the perspective of convenience and economy. Those skilled in the art can also choose other working wavelength laser diodes, such as 420 nm, 850 nm, 1550 nm 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, the parallel light is filtered to remove the out-of-band light after passing through the narrow-band interference sheet, and narrow-band light is obtained; the narrow-band light reaches the front surface of the micro-prism array, and the reflected light which is collinear with the narrow-band light is obtained after the micro-prism array; the reflected light is fed back to the laser diode, and when the intracavity oscillation reaches the threshold, the reflected light output by the polarizing beam splitter is used as the measured laser. The measured laser reaches the beat frequency measurement module and is compared with the optical comb system, and the change amount Δf of the beat frequency is converted into the change of the rotation angle θ.

[0031] The beat frequency measurement belongs to the prior art in the field. As a preferred embodiment, the beat frequency measurement module comprises a first half-wave plate 9 arranged in the direction of the output reflected light of the first polarizing beam splitter 3, a second polarizing beam splitter 12, a wideband detector 13 arranged in the direction of the output light of the second polarizing beam splitter 12, and a frequency counter 14; the measured laser is totally reflected after entering the second polarizing beam splitter 12 through the first half-wave plate 9, and the comb teeth emitted by the optical comb system 10 enter the second polarizing beam splitter 12 after passing through the second half-wave plate 11, and the comb teeth and the measured laser are combined to reach the wideband detector 13, the wideband detector 13 is connected to the frequency counter 14, and the beat frequency is obtained by the frequency counter 14. When the rotation of the rotation table causes the beat frequency to change, the change amount Δf of the beat frequency is obtained by Δf = f0-f1. Wherein, f0 is the frequency of the measured laser when the high-precision horizontal rotation table is in the initial position, and f1 is the frequency of the measured laser after the high-precision horizontal rotation table is rotated.

[0032] In the present application, the wideband detector, the frequency counter and the optical comb system all belong to the prior art devices in the field, for example, a wideband detector with a frequency of 50GHz is used.

[0033] The high-precision horizontal rotation table of the present application is a commercially available product, for example, a small rotation displacement table (PDR1C) sold by Thorlabs company.

[0034] As another embodiment of the present application, the narrow-band interference sheet 4 can be bonded to the front surface of the micro-prism array 5, that is, the narrow-band interference sheet, the micro-prism array and the piezoelectric module are integrated together and fixed on the rotation table, thereby improving the mechanical robustness of the system.

[0035] According to a preferred embodiment, the application further comprises a glass base 15 with ultra-low expansion rate, and the laser diode 1, the collimating lens 2, the first polarization beam splitter prism 3, the integrated narrow-band interference film-micro-prism array-piezoelectric module, the high-precision horizontal rotating table 7 and the rotating table fixing base 8 are fixed on the base made of glass with ultra-low expansion rate, which can be used to reduce the influence of cavity length fluctuation on the frequency of the measured laser when it is running freely, and stabilize the laser frequency fluctuation to the order of Hz, and this arrangement can further improve the angle measurement resolution.

[0036] In the application, the glass with ultra-low expansion rate can be the ultra-low expansion glass (ULE) sold by Corning Incorporated, USA.

[0037] In another aspect, the application also provides a method for continuously measuring a small angle, which comprises the following steps:

[0038] (1) the laser emitted by the laser diode passes through the narrow-band interference film to obtain narrow-band light; the narrow-band light is reflected by the micro-prism array and returned to the laser diode through the polarization beam splitter prism, and when the oscillation in the cavity reaches the threshold, the measured laser is reflected and output by the polarization beam splitter prism;

[0039] The narrow-band interference film and the micro-prism array are fixed in parallel on a high-precision horizontal rotating table, the rear surface of the micro-prism array is bonded with a piezoelectric module, and the center of the rear surface of the piezoelectric module is located at the axis of the rotating table, so that when the rotating table rotates, the narrow-band interference film and the micro-prism 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 table rotates, the narrow-band interference film and the micro-prism array rotate synchronously, and according to the value of the thickness D of the piezoelectric module, a curve of the cavity mode wavelength change amount Δλ caused by the rotation angle θ of the micro-prism array is drawn, and Δλ and the rotation angle θ of the micro-prism 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 micro-prism array, λ0 is the output wavelength of the laser, D is the thickness of the piezoelectric module, and Δλ is the cavity mode wavelength change amount caused by the rotation angle θ of the micro-prism array;

[0043] (3) the rotation angle θ of the narrow-band interference film is obtained by the following formula: and the relationship expression of the transmission wavelength λ' is as follows:

[0044]

[0045] λ0 is the transmission wavelength of the narrow-band interference film when it is perpendicular to the light beam, n effLet be the refractive index of the narrowband interferometer. Then, the change in transmission wavelength Δλ′ caused by the rotation of the interferometer is: Δλ′=λ0-λ′;

[0046] (4) The laser to be tested in step (1) is input to the beat frequency measurement module, and the beat frequency measurement module beats the laser to be tested with the comb teeth output by the optical comb system to obtain the beat frequency.

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

[0048]

[0049] When the value of D satisfies And when Δλ=Δλ′ (i.e., the change in cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array and the rotation of the narrowband interferometer) The change in transmission wavelength Δλ′ caused by the angle is equal), K is the rotation angle in step (2). With respect to the slope of the λ′ curve:

[0050]

[0051] The small-angle measurement reference device of this invention uses a narrow-band interferometer for narrow-band filtering, filtering the broadband light emitted by the laser diode into a narrow-band beam. With optical feedback from the microprism array, it achieves the output of the measured laser whose frequency is extremely sensitive to the rotation angle, thus reflecting small changes in the rotation angle in the change Δf between the measured laser and the comb beat frequency. Specifically, by adjusting the voltage of the piezoelectric module to optimize its own thickness D (i.e., the perpendicular distance between the rear surface of the microprism array and the axis), the change in cavity mode frequency can be synchronized with the change in the center frequency of the transmission peak of the interferometer. This solves the problems of laser frequency mode hopping caused by a single interferometer rotation and the small frequency tunability range caused by a single change in cavity length (rotating the microprism array).

[0052] Since the frequency stability of an optical comb can reach the E-14 level, with frequency fluctuations on the order of Hz, while the frequency fluctuations of a free-running narrowband interferometer laser are on the order of tens of kHz, the angle measurement limit can be converted into laser frequency measurement at tens of kHz, corresponding to an angle measurement limit on the order of approximately 0.0001″. When the frequency fluctuations of the free-running narrowband interferometer laser are further stabilized to the order of Hz using an ultra-low expansion coefficient glass substrate, the corresponding angle measurement resolution can be achieved on the order of approximately 0.00000001″, resulting in a significant improvement in the ultra-high precision angle measurement resolution for small angle measurements.

[0053] The application utilizes a small-angle measurement reference device based on the ultra-high sensitivity of laser frequency to cavity length in an external cavity semiconductor laser to realize an angle measurement instrument with an angle measurement limit of 0.0001" order, which can be applied to MEMS, precision instruments and optical systems to meet the application requirements of such high-precision devices. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 Structure diagram of a small-angle measurement reference device in a previous study of the inventor;

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

[0056] Figure 2 Principle diagram of the small-angle measurement reference device;

[0057] Figure 3 Structure diagram of the small-angle measurement device in Example 1;

[0058] Figure 4 Structure diagram of the small-angle measurement device in Example 2;

[0059] Figure 5 Relationship between the rotating angle of the micro-prism array and the cavity mode frequency in Example 1;

[0060] Figure 6 Relationship between the rotating angle of the micro-prism array and the cavity mode wavelength in Example 1;

[0061] Figure 7 Comparison diagram of the relationship between the rotating angle of the micro-prism array and the cavity mode wavelength and the relationship between the rotating angle of the interference sheet and the transmission wavelength in Example 1;

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

[0063] The following examples are used to non-limitingly explain the technical solutions of the application.

[0064] Example 1 examines the effect of the thickness D of the piezoelectric module, i.e., the different perpendicular 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 at a wavelength of 780nm. The device mainly includes a laser diode 1, a collimating lens 2, a first polarizing beam splitter 3, a narrow-band interferometer 4, a microprism array 5, and a piezoelectric module 6, arranged sequentially along the optical path. An optical comb system 10 and a beat frequency measurement module are positioned in the direction of the output reflected light from the first polarizing beam splitter 3. A high-precision horizontal rotating stage 7 is fixed on a rotating stage mounting base 8. The microprism array 5 and the piezoelectric module 6 are integrated into one unit and fixed parallel to the narrow-band interferometer 4 on the high-precision horizontal rotating stage 7. The center of the rear surface of the piezoelectric module 6, which has a thickness of D, is placed at the axis of the rotating stage (therefore, when the rotating stage rotates, the distance between the rear surface of the microprism array and the axis of the rotating stage is always D). The narrow-band interferometer 4 is parallel to the microprism array 5 (e.g., ...). Figure 2 As shown in the figure, when the high-precision horizontal rotary table 7 rotates counterclockwise by θ around the center, the narrow band interference plate and the prism-type ultra-high intensity reflective film rotate simultaneously by θ, 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 polarizing beam splitter 12, a broadband detector 13, and a frequency counter 14.

[0067] The optical comb system adopts Sanglok Lee et al., Laser frequency stabilization in the 10 -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)) utilizes modulation-transfer spectroscopy (MTS) to achieve high-performance laser frequency stabilization on the rubidium 87D2 transition line, with the frequency stability of the two frequency-stabilized external cavity semiconductor lasers (ECDLs) reaching [value missing]. Short-term stability, until 10 5 s only exceeds 2×10 -12 Its structure includes an MTS mounted on an optical platform: a micrometer-sized metal surrounding a... 87Rb wedge-shaped vapor chamber (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, Gran-Thompson polarizer; half-wave plate; LPF, low-pass filter (Mini-Circuit, SLP-1.9+, SLP-5+); mixer, phase detector (Mini-Circuits, ZRPD-1+); operating system, oscilloscope; PBS, polarization beam splitter; PD, photodiode (Thorlabs, PDA10A2).

[0068] During operation, the light output from the 780nm laser diode is collimated and then filtered out of the band by a narrowband interferometer to obtain narrowband light. This narrowband light reaches the front surface of the microprism array and is reflected by the microprism array, which is collinear with and opposite to the narrowband light. This reflected light is fed back to the laser diode. When the intracavity oscillation reaches the threshold, the reflected light is output by a polarization beam splitter as the laser to be measured.

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

[0070] To convert the beat frequency change Δf into the microprism array rotation angle change θ, we first determine the cavity mode wavelength change Δλ curve caused by the microprism array rotation angle θ:

[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, i.e., the perpendicular distance between the microprism array and the axis, which are D = 20 mm, D = 22 mm, D = 24 mm, D = 25 mm, and D = 28 mm, respectively. The curves of the change in cavity mode wavelength Δλ caused by the rotation angle θ of each microprism array are plotted, as shown below. Figure 5 , 6 As shown.

[0072] When θ changes continuously within 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, from 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 × 10⁻⁶. 12 The cavity mode wavelength change is Δλ = -2.647 nm. When D = 24 mm, Δf = 1.42 × 10⁻⁶ Hz.12 Hz, Δλ = -2.888 nm. When D = 25 mm, Δf = 1.48 x 10 12 Hz, Δλ = -3 nm.

[0074] Then, according to the rotation angle of the narrow-band interference film The relationship expression of the transmission wavelength λ' and the rotation angle θ of the narrow-band interference film is:

[0075]

[0076] The relationship expression of the transmission wavelength λ' and the rotation angle θ of the narrow-band interference film is: Figure 7 The IF curve is obtained, and when θ = 10°, the transmission wavelength λ' of the interference film is 777.1 nm, i.e. the transmission wavelength variation Δλ' of the interference film is -2.9 nm, which is between Δλ = -2.888 nm (corresponding to D = 24 mm) and Δλ = -3 nm (corresponding to D = 25 mm), and thus the optimal value range of the vertical distance D of the micro-prism array and the axis is 24 mm to 25 mm.

[0077] When D is selected as 24 mm to 25 mm, when the micro-prism array is rotated at a small angle near θ = 10°, the wavelength variation Δλ caused by the rotation of the micro-prism array is approximately equal to the transmission wavelength variation Δλ' caused by the rotation of the interference film, and thus the cavity mode frequency and the transmission frequency of the interference film are synchronously changed, so that the output laser frequency can be continuously changed, thereby realizing a large-range continuous small-angle measurement.

[0078] Finally, the rotation angle θ of the micro-prism array is calculated by using the measured variation Δf of the beat frequency:

[0079]

[0080] That is, when D = 24 mm to 25 mm and the small-angle variation of θ is near 10° (i.e. 10° ± 1°), Δλ = Δλ', and thus the formula can be used to calculate θ. K is the slope of the λ' curve under this condition. The slope of the λ' curve:

[0081]

[0082] Since the frequency stability of the optical comb is in the order of E-14, the frequency fluctuation range thereof is about Hz. Based on the mode selection principle of the narrow-band interference film, the frequency fluctuation of the measured laser output by the free-running interference film laser is about 30 kHz. According to the incident angle The relationship with the transmission wavelength can be known that when the interference piece is rotated from 0 to 20°, the slope of the transmission wavelength change with the angle is in the range of 0-1.1 nm / °. For example, when the interference piece incident angle (i.e. the angle between the laser and the normal line of the interference piece) is 10°, k = 0.55, then the relationship between the interference piece angle change and the transmission wavelength change is 0.55 nm / °, and for the 780 nm laser wavelength, the wavelength change of 1 nm corresponds to the frequency change of 490 GHz, so the relationship between the interference piece angle change and the transmission frequency change is 269.5 GHz / °, and therefore the angle change amount corresponding to the 30 kHz frequency fluctuation is about 1.11 x 10 -7 °(0.00040074″), so that the corresponding angle measurement resolution can be realized about 0.0001″ order. That is, when the frequency change amount is not greater than 30 kHz, it is considered as the fluctuation of the system itself and cannot be measured, and when the frequency change amount is greater than 30 kHz, the angle change amount can be calculated by measuring the frequency change amount.

[0083] Example 2

[0084] The difference from example 1 is that the narrow-band interference piece 4 is bonded to the front surface of the micro-prism array 5, that is, the narrow-band interference piece 4, the micro-prism array 5 and the piezoelectric module 6 are fixed as a whole, reducing the laser frequency change caused by mechanical vibration, and the angle measurement can be more accurate. In addition, the laser diode 1, the collimating lens 2, the first polarization beam splitter prism 3, the narrow-band interference piece 4, the micro-prism array 5, the piezoelectric module 6, the high-precision horizontal rotation table 7 and the rotation table fixing seat 8 are fixed on the base 15 made of ultra-low expansion glass, and the interference caused by mechanical deformation is further overcome by the ultra-low expansion characteristic of the base. The rest of the structure remains unchanged.

[0085] Similarly, the vertical distance D between the micro-prism array 5 and the axis is set to be in the range of 24 mm-25 mm, and the incident angle of the narrow-band interference piece 4 and the incident angle of the micro-prism array 5 are changed by the high-precision horizontal rotation table 7. At this time, when the micro-prism array is rotated at a small angle near θ = 10°, the interference piece transmission peak center position moves with the angle and is highly synchronized with the change of the cavity mode frequency with the angle, realizing the large range continuous tunable of the laser frequency.

[0086] Since the ultra-low expansion glass can stabilize the frequency fluctuation of the free-running narrow-band interference piece laser to the order of Hz, according to the narrow-band interference piece incident angle It can be known from the relationship with the transmission wavelength that when the interference piece is rotated from 0 to 20°, the slope of the wavelength change with the angle is in the range of 0-1.1nm / °. For example, when the incident angle of the interference piece is 10°, k=0.55, that is, the relationship between the change of the angle of the interference piece and the change of the transmission wavelength is 0.55nm / °, and for the 780nm laser wavelength, the wavelength change of 1nm corresponds to the frequency change of 490GHz, so the relationship between the change of the angle of the interference piece and the change of the transmission frequency is 269.5GHz / °, and therefore the angle change corresponding to 1Hz is about 3.71x10 -12 °(0.000000013358″), so that the corresponding angle measurement resolution can be realized to about the order of 0.00000001″.

[0087] Compared with the frequency fluctuation of about 30kHz of the measured laser when running freely in the embodiment 1, the embodiment utilizes the ultra-low expansion glass to improve the stability of the frequency fluctuation of the measured laser when running freely to the order of Hz, so that the corresponding angle measurement resolution can be realized to about the order of 0.00000001″, and the angle measurement resolution is greatly improved.

[0088] The small-angle measurement reference device based on the interference frequency selection and the micro-prism feedback can realize the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in the external cavity semiconductor laser. The small-angle measurement reference device is capable of realizing the continuous measurement of the small-angle measurement reference device based on the ultra-high sensitivity of the laser frequency to the cavity length in

Claims

1. A small-angle measurement reference device based on interferometric frequency selection and microprism array feedback, the device comprising a laser generator and a beat frequency measurement module sequentially arranged in the optical path, wherein, A high-precision horizontal rotating stage (7) is set on the output optical path of the laser generator. A narrow-band interferometer (4), a micro-prism array (5), and a piezoelectric module (6) are fixed parallel to each other on the high-precision horizontal rotating stage (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 of the rotating stage. A first polarizing beam splitter (3) is set between the laser generator and the narrow-band interferometer (4). The light output from the laser generator passes through... After passing through the narrowband interferometer (4), the out-of-band light is filtered out to obtain narrowband light; the narrowband light is reflected after passing through the microprism array (5), and returns to the laser generator after passing through the polarization beam splitter (3). When the oscillation in the cavity reaches the threshold, the laser under test is reflected and output by the polarization beam splitter (3); the laser under test is input to the 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 the beat frequency. When the frequency of the laser under test changes by Δf, the change in beat frequency is equal to Δf.

2. The small angle measuring reference device according to claim 1, characterized in that... The microprism array (5) is a prism-type ultra-high intensity reflective film (71).

3. The small angle measuring reference device according to claim 1, characterized in that... The laser generating device includes a laser diode (1) and a collimating lens (2) arranged sequentially in the optical path, and the center wavelength of the transmission peak of the narrowband interferometer (4) matches the wavelength of the laser diode (1).

4. The small angle measuring reference device according to claim 1, characterized in that... The beat frequency measurement module includes a first half-wave plate (9) set in the direction of the reflected light output of the first polarizing beam splitter (3), a second polarizing beam splitter (12), a broadband detector (13) set in the direction of the output light of the second polarizing beam splitter (12), and a frequency counter (14). The laser under test is reflected after passing through the first half-wave plate (9) and entering the second polarization beam splitter (12). The comb teeth emitted by the optical comb system (10) are reflected after passing through the second half-wave plate (11) and entering the second polarization beam splitter (12). The optical comb and the laser under test are combined and reach the broadband detector (13). The broadband detector (13) is connected to the frequency counter (14). The frequency counter (14) measures the beat frequency of the optical comb and the laser under test and obtains the change in beat frequency Δf.

5. The small angle measuring 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), and the narrowband interference plate (4), the microprism array (5) and the piezoelectric module (6) are fixed together as a whole and fixed on the rotating platform.

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

7. The small angle measuring 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 emitted by the laser diode is passed through a narrowband interference plate to obtain narrowband light; the narrowband light is reflected by a microprism array and returned to the laser diode by a polarizing beam splitter. When the oscillation in the cavity reaches the threshold, the laser to be measured is reflected out by the polarizing beam splitter. The narrowband interferometer and the microprism array are fixed in parallel on a high-precision horizontal rotating stage. A piezoelectric module is bonded to the rear surface of the microprism array. The center of the rear surface of the piezoelectric module is located at the axis of the rotating stage. When the rotating stage rotates, the incident angle of the laser with the narrowband interferometer and the microprism array changes simultaneously and the amount of angle change is equal. (2) When the high-precision horizontal rotary table rotates, the narrow-band interference plate and the microprism array rotate synchronously. Based on different values ​​of the piezoelectric module thickness D, the curve of the change in cavity mode wavelength Δλ caused by the rotation angle θ of the microprism array is plotted. Δλ and the rotation angle θ of the microprism array satisfy the following relationship: Where 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 cavity mode wavelength caused by the rotation angle θ of the microprism array. (3) The rotation angle of the narrowband interferometer is obtained by the following formula. The relationship between the wavelength and the transmission wavelength λ′ is expressed as follows: λ0 is the transmission wavelength when the narrowband interferometer is perpendicular to the beam, and n eff Let be the refractive index of the narrowband interferometer. Then, the wavelength change Δλ′ caused by the rotation of the interferometer is: Δλ′=λ0-λ′; (4) The laser to be tested in step (1) is input to the beat frequency measurement module, and the beat frequency measurement module beats the laser to be tested with the comb teeth output by the optical comb system to obtain the beat frequency. (5) Calculate the rotation angle θ of the microprism array. The relationship between the change in beat frequency Δf 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). With respect to the slope of the λ′ curve: 。

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