Large frequency difference heterodyne gas laser interferometer range finder

Abstract

The application discloses a large-frequency-difference heterodyne gas laser interferometer ranging device, which comprises a frequency-stabilized helium-neon laser tube, an acousto-optic frequency shifter, a light splitting and combining assembly and a signal processing module; the frequency-stabilized helium-neon laser tube outputs frequency-stabilized laser, and the acousto-optic frequency shifter performs frequency shift on incident light to obtain frequency-shifted light; the light splitting and combining assembly proportionally distributes double-frequency laser, which is respectively used for frequency stabilization control and heterodyne interference ranging; and the signal processing module locks zero point frequency based on equal-intensity zero-crossing point recognition technology to perform ranging. Compared with the prior art, the application has the advantages that a large-frequency-difference heterodyne gas laser interferometer ranging device meeting the demand of large length and high-speed measurement is provided.

Description

Technical Field

[0001] This invention relates to the field of high-speed laser ranging technology, specifically to a large frequency difference heterodyne gas laser interferometer ranging device. Background Technology

[0002] In existing helium-neon dual-frequency laser interferometers based on the Zeeman effect, the frequency difference of the Zeeman effect dual-frequency laser is determined by both the magnetic field strength and the atomic energy level splitting characteristics. The Zeeman splitting frequency difference of the helium-neon laser is usually only 1-2 MHz (typical value 1.5 MHz). Its theoretical upper limit is limited by the magnetic field strength threshold and the physical limit of energy level splitting; long-term magnetic field action will cause the magnetic core material to accumulate hysteresis loss, with a frequency drift of up to 100kHz per year, requiring regular calibration.

[0003] This frequency difference range is insufficient to meet the measurement requirements of long-length, high-speed dynamic targets. Frequency drift also causes measurement accuracy to decay over time, which restricts the application of laser interferometers in precision metrology, aerospace and industrial automation. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the above-mentioned technical defects and provide a high-frequency difference heterodyne gas laser interferometer ranging device that meets the requirements for long-length and high-speed measurement.

[0005] To solve the above-mentioned technical problems, the technical solution provided by the present invention is: a large frequency difference heterodyne gas laser interferometer ranging device, including a frequency-stabilized helium-neon laser tube, an acousto-optic frequency shifter, a beam splitter and combiner assembly, and a signal processing module; The frequency-stabilized helium-neon laser tube outputs a laser with a stable frequency, and the acousto-optic frequency shifter shifts the frequency of the incident light to obtain frequency-shifted light. The beam splitter and beam combiner includes a first-stage beam splitter, a second-stage beam splitter and a third-stage beam splitter. The first-stage beam splitter splits the laser into two beams, one of which is incident on the acousto-optic frequency shifter. The second-stage beam splitter combines the frequency-shifted light with another beam to form a dual-frequency laser, and the third-stage beam splitter distributes the dual-frequency laser proportionally for frequency stabilization control and heterodyne interferometric ranging, respectively. The signal processing module uses equal intensity zero-crossing identification technology to lock the zero-point frequency for ranging.

[0006] Preferably, the third-stage beam splitter splits the dual-frequency laser into two beams in a 2:8 ratio, with 20% of the light used for frequency stability detection and 80% used for heterodyne interferometry ranging.

[0007] Preferably, the frequency difference of the frequency-shifted light is between 1 MHz and 100 MHz.

[0008] Preferably, a temperature control system is also included; The temperature control system is attached to the surface of the acousto-optic frequency shifter to suppress the frequency drift of the acousto-optic crystal caused by temperature fluctuations. It includes a semiconductor cooling chip, an H-bridge drive circuit, a measurement and control circuit using a PID algorithm, and a digital temperature sensor. The semiconductor cooling chip is attached to the surface of the acousto-optic frequency shifter via a heat-conducting sheet; The H-bridge drive circuit amplifies the drive signal output by the microcontroller to control the heating or cooling mode of the semiconductor cooling chip. The measurement and control circuit monitors the temperature of the acoustic-optical frequency shifter in real time based on a digital temperature sensor and adjusts the power of the semiconductor cooling chip.

[0009] Preferably, it also includes an angle adjustment device, which includes a support frame, a locking screw, and a worm gear transmission assembly; The support frame fixes the spatial position of the acoustic-optical frequency shifter, and the locking screws cooperate with the support frame to lock the acoustic-optical frequency shifter. The worm gear transmission assembly finely adjusts the incident angle of the acoustic-optical frequency shifter so that the incident light strictly satisfies the Bragg diffraction condition.

[0010] Preferably, the laser tube adjustment device includes two clamping structures and a C-shaped sleeve; The clamping structure is provided with multiple threaded holes, and the frequency-stabilized helium-neon laser tube is fixed to the base by bolts. The C-shaped sleeve is a non-closed heat-insulating plastic structure, which is fitted on the outside of the frequency-stabilized helium-neon laser tube to protect the laser tube and reduce temperature interference.

[0011] Preferably, it also includes an optical structure, which is an inverted Kepler telescope structure that collimates the beam and compresses the laser divergence angle.

[0012] Preferably, the reflector group includes three reflectors disposed within the device to change the direction of the light path.

[0013] The advantages of this invention compared to the prior art are: This invention solves the technical difficulty of high-speed measurement in high-precision measurement due to the small frequency difference in continuous wave laser heterodyne interferometry. The device has a compact structure and stable performance. This invention can meet the requirements of high-precision and high-speed measurement of long targets. This solution utilizes the principle of acousto-optic frequency shifting, which enables a single-frequency helium-neon laser to achieve continuous optical frequency modulation. When strictly meeting the Bragg diffraction condition, i.e. the Bragg angle, it can achieve a diffraction efficiency of over 98%. When PID temperature control is used to reduce the temperature change of the acousto-optic crystal, the resulting dual-frequency laser has less frequency difference drift and a simple structure. The solution of this invention greatly expands the traditional fixed-frequency-difference dual-frequency laser, enabling it to be used for long-distance, high-speed, dynamic target motion characteristic measurement. This solution provides a novel laser heterodyne interferometry measurement system that is compact, stable, fast, accurate, long-distance, and environmentally stable, and can be widely applied to the measurement of various moving target characteristics. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of a large frequency difference heterodyne gas laser interferometer ranging device according to the present invention; Figure 2 This is a schematic diagram of a large frequency difference heterodyne gas laser interferometer ranging device provided by this invention patent from another angle; Figure 3 This is a partial view provided by the present invention patent.

[0015] Figure 4 This is a partial view provided by the angle adjustment device of this invention patent.

[0016] Figure 5 This invention patent provides the principle of equal power frequency stabilization for lasers.

[0017] Figure 6 This invention patent provides the principle of the equal light intensity signal phase measurement method. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings.

[0019] Combined with appendix Figure 1-6 As shown, the acousto-optic frequency shifting scheme overcomes the limitations of physical energy levels, allowing for continuous adjustment of the frequency difference range via ultrasonic frequency: the adjustable frequency difference accuracy reaches 0.1Hz. Continuous tuning is achieved through direct digital synthesis (DDS) control of the ultrasonic frequency, enabling adaptation to different measurement needs (such as full-range coverage from micrometer-level displacement to meter-level travel) without the need for component replacement. Temperature-controlled acousto-optic crystals, such as... Figure 1 As shown, the low-phase-noise ultrasonic drive source together ensures frequency stability.

[0020] A large-frequency-difference heterodyne gas laser interferometer ranging device includes a base 3, a frequency-stabilized helium-neon laser tube 1, a laser tube adjustment device 2, a first-stage beam splitter 7, three reflectors 6, 9, and 13, an acousto-optic frequency shifter 15, a driver 5, a temperature control system 11, a heat-conducting sheet 12, a second-stage beam splitter 14, a third-stage beam splitter 10, a signal processing module 16 and 17, an angle adjustment device 8, a photoelectric fixer 18, a photodetector 19, and an optical structure 20. The ranging device of this large frequency difference heterodyne gas laser interferometer uses base 3 as the installation reference. The frequency-stabilized helium-neon laser tube 1 is fixed by the laser tube adjustment device 2 with heat-insulating plastic C-type sleeve and clamping structure. The outer wall of the laser tube is wrapped with heating wire and covered with insulation layer. Photodetectors 18 and 19 work with the frequency stabilization control circuit of helium-neon laser tube in the signal processing module. By detecting the power difference signal between the two lights as the closed-loop feedback signal, the frequency stabilization control algorithm and the fuzzy-prediction PID digital controller are studied to adjust the heating wire to stabilize the 633nm laser cavity length. The helium-neon laser emitted from the laser tube is collimated by the optical structure 20, compressing the laser divergence angle. After the first-stage beam splitter 7 splits the laser beam 1:1, three reflectors 6, 9, and 13 change the direction of the light path. One of the beams is incident on the acousto-optic frequency shifter 15 installed on the angle adjustment device 8 (composed of a support frame, locking screws, and a transmission gear set, used for fine-tuning the Bragg angle). The sinusoidal signal generation circuit of the driver 5 provides a 1M 100MHz frequency shift signal. The temperature control system 11 stabilizes the temperature of the frequency shifter to suppress frequency drift through a semiconductor cooling chip (attached to the frequency shifter via a heat-conducting sheet), an H-bridge drive circuit, and a measurement and control circuit with an integrated PID algorithm. After the frequency-shifted light and the original frequency light are combined by the second-stage beam splitter 14, the third-stage beam splitter 10 splits the light at a 2:8 ratio, with 20% of the light entering signal processing modules 16 and 17. Figure 5 As shown, the photoelectric detection circuit provides raw data for frequency difference detection. The photoelectric detection circuit converts the light beat signal into current, and the signal processing circuit completes current-voltage conversion, multi-stage amplification, and PID algorithm to regulate the frequency difference. 80% of the light is used for heterodyne interferometric ranging. The entire device achieves high-precision ranging with large frequency difference through optical path optimization, temperature closed-loop, and dual frequency stabilization control (light source frequency stabilization + frequency difference stabilization control).

[0021] In addition, the formula for calculating the Bragg angle is: Where λ is the incident light wavelength and λs is the wavelength after frequency shifting.

[0022] Furthermore, the ranging principle of this invention is based on the principle and method of frequency conversion ranging. A frequency synthesizer generates a bandwidth within 15MHz, controlled by a microprocessor, to sequentially modulate a helium-neon laser at a fixed frequency of 161.7Hz until the distance to be measured becomes an integer multiple of half the modulation wavelength. The zero-point frequency at this point is then measured. If the integer number is N, the measured distance is: To address the multivalued problem, the instrument modifies the frequency to detect another zero point and accurately measure its frequency fi. Let N be the integer number at this point, then the distance is: If the frequency increases, the integer number N also increases, therefore they must be related. From the above three formulas: Where Δf is the frequency difference between two adjacent zero points, and its value remains constant within a certain distance; C0 is the speed of light in vacuum, taken as 299,792,458 m / s; n0 is the atmospheric refractive index under reference meteorological conditions (temperature t=15℃, atmospheric pressure P=760 mmHg, CO content 0.03%, and dry atmosphere), calculated by the formula to be 1.000284514844; f1 and fi are the frequency values ​​of the 1st and ith zero points, respectively. Therefore, accurately measuring the frequency of each zero point is crucial for achieving high-precision ranging. Thus, in using an acousto-optic frequency shifter (AOFS) to modulate the frequency of continuous waves...

[0023] To precisely control the frequency shift, equal-intensity zero-crossing identification technology is employed, such as... Figure 6 As shown, a sinusoidal signal with a fixed frequency and a certain frequency shift is used to modulate the frequency, so that the modulation voltage falling on the modulator is a sinusoidal frequency modulation voltage.

[0024] This causes the photodetector to exhibit a different current-time distribution when the center frequency deviates from the zero frequency compared to when it is at zero. Under the control of the internal logic circuit and calculation control unit, the frequency modulation drive will adjust the frequency value locked at zero according to the polarity and magnitude feedback of the correction voltage.

[0025] The frequency shift range is approximately 100 MHz, corresponding to a wavelength shift range of approximately 2.67 × 10⁻⁴ nm (632.99987 nm ~ 633.00013 nm). Figures a, b, and c represent the center frequency f, respectively. 中 Greater than, equal to, or less than zero frequency f 零 The situation is as follows. As shown in Figures a and c, the modulation changes at the comparison frequency are accompanied by unequal light intensity amplitudes at the detector diode entrance. The difference is that in the first case, the maximum and minimum light intensity values ​​occur correspondingly at (f...). 中 +Δf) and (f 中 -Δf); while the third case is the opposite. The most significant is the second case (Figure b), which is equivalent to f. 中 =f 零 In this case, when the comparison frequency is frequency modulated, an equal amplitude light intensity signal will be generated, which is the basic working state to be found—zero position.

[0026] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

[0027] The working principle of this invention is as follows: A third-stage beam splitter allocates light in a 2:8 ratio, with 20% of the light input to the signal processing module. After photoelectric detection, flow-voltage conversion, and PID algorithm stabilization of the frequency difference, 80% of the light is used for heterodyne interferometric ranging. A frequency-stabilized helium-neon laser tube, through a cavity wound with a heating wire and an insulating layer, combined with photoelectric detection and fuzzy PID algorithm to adjust the cavity length to stabilize the light source frequency, achieves stable output over a wide frequency difference. The device has a compact structure, meets the requirements of long-distance, high-speed, and sub-nanometer precision measurement, and is suitable for precision metrology, aerospace, and other fields.

[0028] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0029] The present invention and its embodiments have been described above. This description is not restrictive, and the accompanying drawings are only one embodiment of the present invention; the actual structure is not limited thereto. In conclusion, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A ranging device for a large frequency difference heterodyne gas laser interferometer, characterized in that: Includes a frequency-stabilized helium-neon laser tube, an acousto-optic frequency shifter, a beam splitter and combiner assembly, and a signal processing module; The frequency-stabilized helium-neon laser tube outputs a laser with a stable frequency, and the acousto-optic frequency shifter shifts the frequency of the incident light to obtain frequency-shifted light. The beam splitter and beam combiner includes a first-stage beam splitter, a second-stage beam splitter and a third-stage beam splitter. The first-stage beam splitter splits the laser into two beams, one of which is incident on the acousto-optic frequency shifter. The second-stage beam splitter combines the frequency-shifted light with another beam to form a dual-frequency laser, and the third-stage beam splitter distributes the dual-frequency laser proportionally for frequency stabilization control and heterodyne interferometric ranging, respectively. The signal processing module uses equal intensity zero-crossing identification technology to lock the zero-point frequency for ranging.

2. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: The third-stage beam splitter splits the dual-frequency laser into two beams in a 2:8 ratio, with 20% of the light used for frequency stability detection and 80% used for heterodyne interferometry ranging.

3. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: The frequency difference of the frequency-shifted light is between 1 MHz and 100 MHz.

4. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: It also includes a temperature control system; The temperature control system is attached to the surface of the acousto-optic frequency shifter to suppress the frequency drift of the acousto-optic crystal caused by temperature fluctuations. It includes a semiconductor cooling chip, an H-bridge drive circuit, a measurement and control circuit using a PID algorithm, and a digital temperature sensor. The semiconductor cooling chip is attached to the surface of the acousto-optic frequency shifter via a heat-conducting sheet; The H-bridge drive circuit amplifies the drive signal output by the microcontroller to control the heating or cooling mode of the semiconductor cooling chip. The measurement and control circuit monitors the temperature of the acoustic-optical frequency shifter in real time based on a digital temperature sensor and adjusts the power of the semiconductor cooling chip.

5. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: It also includes an angle adjustment device, which comprises a support frame, a locking screw, and a worm gear transmission assembly; The support frame fixes the spatial position of the acoustic-optical frequency shifter, and the locking screws cooperate with the support frame to lock the acoustic-optical frequency shifter. The worm gear transmission assembly finely adjusts the incident angle of the acoustic-optical frequency shifter so that the incident light strictly satisfies the Bragg diffraction condition.

6. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: The laser tube adjustment device includes two clamping structures and a C-shaped sleeve; The clamping structure is provided with multiple threaded holes, and the frequency-stabilized helium-neon laser tube is fixed to the base by bolts. The C-shaped sleeve is a non-closed heat-insulating plastic structure, which is fitted on the outside of the frequency-stabilized helium-neon laser tube to protect the laser tube and reduce temperature interference.

7. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: It also includes an optical structure, which is an inverted Kepler telescope structure, to collimate the beam and compress the laser divergence angle.

8. The ranging device for a large frequency difference heterodyne gas laser interferometer according to claim 1, characterized in that: The reflector group includes three reflectors housed within the device, used to change the direction of the light path.

Images