Laser heterodyne interferometric absolute distance measurement method and system based on internal phase modulation

Abstract

The application provides a laser heterodyne interference absolute distance measurement method and system based on internal phase modulation, and relates to the technical field of laser precision measurement. The method comprises the following steps: performing internal phase modulation on a distributed feedback semiconductor laser; dividing a laser beam into measurement light and reference light; performing frequency shift processing on the reference light; combining the frequency-shifted reference light with the measurement light returned by a target to generate a heterodyne interference signal; detecting the heterodyne interference signal and demodulating the phase modulation depth introduced by the internal phase modulation; and calculating the absolute distance according to the linear relationship between the phase modulation depth and the optical path difference. The system comprises a distributed feedback semiconductor laser, a light splitting module, a frequency shift module, an interference optical path, a photoelectric detection module and a signal processing module. The application combines internal phase modulation and heterodyne interference, simplifies the system structure while ensuring high measurement accuracy, reduces the cost, and can introduce a reference interferometer to realize real-time self-calibration and eliminate the measurement error caused by laser frequency jitter.

Description

Technical Field

[0001] This invention relates to the field of laser precision measurement technology, and in particular to a laser heterodyne interferometry absolute distance measurement method and system based on internal phase modulation. Background Technology

[0002] Laser absolute distance measurement technology has significant application value in fields such as industrial large-size measurement, precision manufacturing, and aerospace assembly. With the increasing demands for measurement accuracy and range in modern industry, high-precision, large-range absolute distance measurement methods have become a research hotspot.

[0003] Currently, laser absolute distance measurement methods mainly include the pulse time-of-flight method, laser modulation absolute ranging method, synthetic wavelength interferometry, and dual-comb absolute ranging method. The pulse time-of-flight method calculates the distance by measuring the round-trip time of a laser pulse over the distance to be measured. This method has a relatively simple structure and can achieve large-scale measurements. However, its measurement accuracy is usually limited to the millimeter level due to the accuracy of the time interval measurement and the response speed of the photodetector, making it difficult to meet the measurement requirements of sub-micron or even nanometer level precision.

[0004] Laser-modulated absolute ranging methods obtain distance information by modulating the amplitude or polarization state of a laser beam and measuring the phase delay of the modulated signal during propagation. While these methods can achieve high measurement resolution, they require complex modulation and demodulation circuits, resulting in a relatively complex system structure. Furthermore, their measurement accuracy is susceptible to modulation instability and environmental interference.

[0005] Synthetic wavelength interferometry (SWIFT) uses multiple lasers of different wavelengths to form a composite wavelength, and measures the absolute distance by measuring the interference phase of the composite wavelength. This method can balance measurement range and accuracy, but it requires multiple frequency-stabilized laser sources, resulting in high system costs. Furthermore, it has stringent requirements for wavelength stability and environmental conditions, which limits its application and promotion in industrial settings.

[0006] The dual-comb absolute ranging method uses two optical combs with a small difference in repetition frequency for optical sampling, and calculates the distance by measuring the time delay of the interferogram. This method has extremely high measurement accuracy and resolution, but the system is extremely complex, requiring precise phase-locked control and complex data processing, resulting in high equipment costs. Currently, it is mainly limited to laboratory research environments.

[0007] Traditional laser frequency-shifting heterodyne interferometers are often used in precision displacement measurement, providing a displacement-time curve by measuring the displacement of a target point over an observation period. However, traditional heterodyne interferometers can only measure relative displacement and cannot directly obtain absolute distance, which limits their application in situations requiring absolute position information.

[0008] In summary, existing laser absolute distance measurement technologies generally suffer from problems such as complex system structure, high cost, limited measurement accuracy, or the ability to measure only relative displacement, making it difficult to simultaneously meet the comprehensive requirements of high precision, large range, simple structure, and moderate cost. Therefore, developing a laser absolute distance measurement method that is simple in structure, low in cost, and can maintain high measurement accuracy is of significant practical importance. Summary of the Invention

[0009] To address the technical problems of complex system structure, high cost, limited measurement accuracy, and the inability of traditional heterodyne interferometers to measure absolute distance in existing laser absolute distance measurement technologies, this invention provides a laser heterodyne interferometry absolute distance measurement method and system based on internal phase modulation.

[0010] The technical solution provided by this invention is as follows: First aspect: The laser heterodyne interferometry absolute distance measurement method based on internal phase modulation provided by this invention includes: S1: Internal phase modulation of a distributed feedback semiconductor laser to modulate its output frequency into a laser beam. S2: Divide the laser beam into a measurement beam and a reference beam; S3: The reference beam is subjected to frequency shifting processing to produce a fixed frequency shift; S4: Combine the frequency-shifted reference beam with the measurement beam after it has been reflected or returned from the target to generate optical mixing and form a heterodyne interference signal; S5: Detect the heterodyne interference signal and demodulate the phase modulation depth introduced by the inner phase modulation from it; S6: Based on the linear relationship between the phase modulation depth and the optical path difference, the absolute distance of the target under test is calculated.

[0011] The second aspect: The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation provided by this invention includes: A distributed feedback semiconductor laser in which the injection current is modulated to produce an output laser beam whose frequency is modulated by the internal phase. A beam splitting module is used to split the laser beam into a measurement beam and a reference beam; A frequency shifting module is disposed in the optical path of the reference beam and is used to perform frequency shifting processing on the reference beam; An interference optical path is used to combine the measurement beam reflected or returned by the target with a frequency-shifted reference beam to generate optical mixing and form a heterodyne interference signal. The photoelectric detection module is used to detect the heterodyne interference signal and convert it into an electrical signal; The signal processing module is used to demodulate the phase modulation depth introduced by the internal phase modulation from the electrical signal, and calculate the absolute distance of the target under test according to the linear relationship between the phase modulation depth and the optical path difference.

[0012] Furthermore, the beam splitting module and the interference optical path are fiber-optic structures, including fiber couplers, optical circulators, and fiber collimators; or they are free-space structures, including polarization beam splitters, mirrors, quarter-wave plates, and non-polarization beam splitters.

[0013] Furthermore, it also includes a reference interferometer with a fixed optical path difference, used to calibrate measurement errors caused by laser frequency jitter or modulation instability in real time.

[0014] Furthermore, the reference interferometer and the measurement interferometer share the reference optical branch where the frequency shift module is located. By synchronously demodulating the phase modulation depth in the reference interference signal and the measurement interference signal, the calibrated absolute distance is calculated by substituting it into the calibration formula.

[0015] Furthermore, the step of demodulating the phase modulation depth from the heterodyne interference signal or the signal processing module specifically includes: The heterodyne interference signal is mixed and low-pass filtered using orthogonal carrier signals to obtain a pair of orthogonal interference signals; Phase demodulation is performed on the orthogonal interference signals to obtain a phase signal containing internal modulation information; The amplitude of the internal modulation frequency component is extracted from the phase signal, and this amplitude is the phase modulation depth.

[0016] Furthermore, the signal processing module includes: The quadrature carrier mixing unit is used to mix the electrical signal with two quadrature carrier signals respectively; The low-pass filter unit is used to filter out the high-frequency components after mixing and output a pair of orthogonal interference signals. A phase demodulation unit is used to perform operations on the orthogonal interference signals to obtain a phase signal containing internal modulation information; The inner modulation depth extraction unit is used to extract the amplitude of the inner modulation frequency component from the phase signal as the phase modulation depth.

[0017] Furthermore, the phase demodulation uses the CORDIC algorithm to calculate Arctan and combines it with phase unwrapping processing; the internal modulation depth extraction is implemented using a digital lock-in amplifier or a fast Fourier transform algorithm.

[0018] Furthermore, in the step of calculating the absolute distance to the target being measured, the scaling factor in the linear relationship is determined by pre-calibration or real-time calibration.

[0019] Furthermore, the real-time calibration method includes: At the reference point, acquire and record the first internal modulation depth of the measuring interferometer; At the target point, the second internal modulation depth of the measuring interferometer and the third internal modulation depth of the reference interferometer are simultaneously acquired and recorded. The optical path difference is fixed based on a pre-calibrated reference interferometer, and the calibrated absolute distance is calculated using the first, second, and third inner modulation depths. The beneficial effects of the technical solution provided by this invention include at least the following: (1) In this invention, by performing internal phase modulation on a distributed feedback semiconductor laser, a low-frequency modulation signal is directly loaded onto the laser beam. Heterodyne interferometry is used with the frequency shifting frequency of an acousto-optic frequency shifter as the carrier wave. Absolute distance measurement is achieved by demodulating the linear relationship between the internal modulation depth and the optical path difference. Compared with the traditional pulse time-of-flight method and laser modulation absolute ranging method, this invention adopts a heterodyne interferometry structure, and the measurement signal is located in the high-frequency carrier region, which effectively avoids low-frequency noise interference and significantly improves the measurement resolution and accuracy. At the same time, only the internal modulation function needs to be added to the conventional heterodyne interferometer, without the need for complex modulation and demodulation circuits or multi-wavelength light sources. The system structure is greatly simplified and the cost is significantly reduced.

[0020] (2) In this invention, a reference interferometer with a fixed optical path difference is introduced, which shares an acousto-optic frequency shift branch with the measurement interferometer, and synchronously demodulates the internal modulation depth in both the reference and measurement interferometric signals. When laser frequency jitter or modulation instability causes fluctuations in the amplitude of the internal modulation frequency change, the internal modulation depth of the reference interferometer can reflect this change in real time. The measurement results are corrected using a calibration formula, eliminating measurement errors caused by laser frequency instability. Compared to measurement systems without a calibration mechanism, this invention achieves self-calibration, maintaining high-precision measurements even under long-term operation and complex environmental conditions, significantly improving the stability and reliability of the system.

[0021] (3) In this invention, orthogonal carrier mixing and digital signal processing techniques are used to demodulate heterodyne interference signals. Phase demodulation is achieved through the CORDIC algorithm, and the inner modulation depth is extracted by combining digital phase-locked loop or fast Fourier transform. This technical solution makes the signal processing process digital and modular, ensuring both the accuracy and speed of phase demodulation, and facilitating flexible implementation in FPGA or host computer. Compared with traditional analog demodulation methods, this invention can effectively suppress the effects of circuit noise and temperature drift, improve the anti-interference capability of measurement, and provide convenient conditions for subsequent algorithm upgrades and functional expansion, thereby enhancing the adaptability and maintainability of the system. Attached Figure Description

[0022] Figure 1 A schematic diagram of the fiber optic principle of a laser heterodyne interferometric absolute distance measurement system based on internal phase modulation, provided in an embodiment of the present invention; Figure 2 A free-space schematic diagram of the laser heterodyne interferometry absolute distance measurement system based on internal phase modulation provided in an embodiment of the present invention; Figure 3 A schematic diagram of the analog circuit principle for the down-conversion of the measurement signal of the laser heterodyne interferometry absolute distance measurement system based on internal phase modulation provided in an embodiment of the present invention; Figure 4 A schematic diagram illustrating the principles of two algorithms for extracting the inner phase modulation depth in a laser heterodyne interferometric absolute distance measurement system based on inner phase modulation, provided in an embodiment of the present invention. Figure 5 A schematic diagram of the fiber optic system structure of the laser heterodyne interferometry absolute distance measurement system based on internal phase modulation provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of the free-space system structure of the laser heterodyne interferometry absolute distance measurement system based on internal phase modulation, provided in an embodiment of the present invention.

[0023] In the diagram: MUL: multiplier; FFT: fast Fourier transform; MOD: modulus of complex numbers; LPF: low-pass filter. Detailed Implementation

[0024] This invention provides a laser heterodyne interferometry absolute distance measurement method based on internal phase modulation, and the processing flow may include the following steps: S1: Internal phase modulation of a distributed feedback semiconductor laser to modulate its output frequency into a laser beam.

[0025] In this step, a distributed feedback semiconductor laser is selected as the light source. Low-frequency modulation of the injection current of the distributed feedback semiconductor laser is used to achieve phase modulation of its output laser beam. This modulation frequency is denoted as the internal modulation frequency. According to the current modulation principle of semiconductor lasers, changes in the injection current will cause corresponding changes in the laser's output frequency, thereby achieving internal phase modulation.

[0026] S2: Divide the laser beam into a measurement beam and a reference beam.

[0027] This step is achieved through a beam splitting module. The laser beam, after internal phase modulation, is split into two paths: one as the measurement beam and the other as the reference beam. The beam splitting can be achieved using a fiber optic coupler or free-space optical elements such as a polarization beam splitter.

[0028] S3: The reference beam is subjected to frequency shifting processing to produce a fixed frequency shift.

[0029] This step utilizes an acousto-optic frequency shifter. A reference beam is guided into the acousto-optic frequency shifter while a radio frequency (RF) drive signal is applied to it; the drive frequency is denoted as the carrier frequency. When the laser beam passes through the acousto-optic frequency shifter, Bragg diffraction occurs, resulting in a fixed frequency shift relative to the incident light. This frequency shift is equal to the frequency of the RF drive signal. This fixed frequency shift will serve as the carrier signal for subsequent heterodyne interference.

[0030] S4: Combine the frequency-shifted reference beam with the measurement beam after it has been reflected or returned from the target to generate optical mixing and form a heterodyne interference signal.

[0031] This step is achieved through an interference optical path. The frequency-shifted reference beam and the measurement beam, reflected or returned from the target, are combined, and the two beams undergo optical mixing to form a heterodyne interference signal. The frequency of this interference signal includes a carrier frequency component, and its phase contains a modulation term introduced by internal phase modulation. The depth of internal modulation is related to the optical path difference between the measurement beam and the reference beam.

[0032] S5: Detect the heterodyne interference signal and demodulate the phase modulation depth introduced by the inner phase modulation.

[0033] This step first converts the heterodyne interference signal into an electrical signal using a photodetector. Then, the electrical signal is processed to demodulate the inner modulation depth. The specific demodulation process includes: mixing and low-pass filtering the measurement signal using orthogonal carrier signals to obtain a pair of orthogonal interference signals; performing phase demodulation on the orthogonal interference signals to obtain a phase signal containing the inner modulation information; and extracting the amplitude of the inner modulation frequency component from the phase signal, which is the inner modulation depth.

[0034] S6: Based on the linear relationship between the phase modulation depth and the optical path difference, the absolute distance of the target under test is calculated.

[0035] This step is based on the linear relationship between the internal modulation depth and the optical path difference. According to the principle of internal modulation in semiconductor lasers, the internal modulation depth is proportional to the optical path difference between the measurement light and the reference light, and the proportionality coefficient is determined by the amplitude of the laser frequency change caused by internal modulation. After obtaining the internal modulation depth through demodulation in step S5, the optical path difference between the measurement light and the reference light can be calculated by combining it with the pre-calibrated proportionality coefficient, thereby obtaining the absolute distance of the target.

[0036] This invention also provides a laser heterodyne interferometry absolute distance measurement system based on internal phase modulation, applied to the aforementioned laser heterodyne interferometry absolute distance measurement method based on internal phase modulation. In conjunction with the method and system provided by this invention, a possible implementation includes: The internal phase modulated laser heterodyne interferometry absolute distance measurement system provided by this invention can be implemented in two ways, such as... Figure 1 and Figure 2 As shown, these are fiber-optic and free-space implementations, both using DFB lasers (distributed feedback semiconductor lasers, which offer advantages such as high spectral purity, narrow linewidth, high integration, and large modulation bandwidth) as their light source. The principles behind both implementations are identical, and the resulting measurement signals share the same mathematical model. A brief description follows. Figure 1 and Figure 2 The principle of the system shown.

[0037] For fiber optic measurement systems, DFB lasers (common wavelengths are 1550nm or 1310nm) achieve internal phase modulation by changing the injection current. The modulated laser beam is then output via fiber optic coupling. After passing through an optical isolator and a 1×2 fiber coupler, it is split into two paths. One path passes through an optical circulator and collimator to serve as the measurement light, while the other path passes through an acousto-optic frequency shifter (AOM), where its frequency changes. The amount of frequency change is determined by the AOM's radio frequency drive frequency; this other path serves as the reference light for the interferometer. The measurement light returns after passing through a pyramidal reflector. After passing through a collimator and a circulator, the light is coupled with a frequency-shifted reference light in a 2×2 fiber coupler to generate optical mixing, forming two interference signals with a 180° phase difference. The two interference signals are received by a balanced photodetector, which outputs the final measurement signal. This signal is processed by analog circuitry (including quadrature carrier mixing and low-pass filtering) and then acquired by a data acquisition card and uploaded to an FPGA or host computer. The phase demodulation of the measurement signal is performed on the FPGA or host computer software platform to obtain the depth of the internal modulation, and then the measured distance is calculated. For a free-space measurement system, the DFB laser outputs a laser beam into free space. A collimating lens group modulates the diverging output light into a collimated beam. This collimated light passes through a polarizer and is then split into horizontally polarized light (P-beam) and vertically polarized light (S-beam) by a polarizing beam splitter (PBS). The P-beam serves as the measurement light, and the S-beam serves as the reference light. The measurement light then passes through a polarizing beam splitter and a quarter-wave plate before reaching a cornerstone reflector at the distance to the target. After reflection by the cornerstone reflector, it passes parallel to the original light path and passes through the quarter-wave plate again, becoming S-beam. This S-beam is then reflected by the polarizing beam splitter. The reference light passes through a reflector and an acousto-optic frequency shifter (AOM) before being combined with the measurement light at the non-polarizing beam splitter (BS). The two beams have the same polarization state, forming an optical mixer. The optical mixer signal is received by a photodetector and converted into an electrical signal. This signal is then processed by analog circuitry, converted from analog to digital, and sent to an FPGA or host computer for demodulation. The principle of the analog processing circuit is as follows: Figure 3 As shown, the heterodyne interferometric measurement signal is mixed with two orthogonal carrier signals and low-pass filtered to achieve frequency down-conversion, resulting in a pair of orthogonal interference signals.

[0038] Building such Figure 1 or Figure 2 The heterodyne interferometry system is described, and the modulation frequency of the DFB laser is set to be... The modulation signal (i.e., carrier frequency) of the acousto-optic frequency shifter is After the system is turned on, the photodetector can obtain a measurement signal in the following form ( Figure 2 The system signal needs to be filtered by DC blocking. In the formula, A The amplitude of the signal. To measure the sum of the initial optical path difference between the light and the reference light, the phase caused by the displacement of the target object, and the phase noise caused by laser jitter, The depth of the internal modulation is related to the optical path difference between the measurement light and the reference light. Based on the principle of internal modulation in semiconductor lasers, we can obtain... The calculation formula is as follows: In the formula, The optical path difference between the measuring light and the reference light, The amplitude of the laser frequency variation generated by internal modulation. At the speed of light. Utilizing Figure 3 The circuit shown processes the aforementioned measurement signals, where the AOM modulated signal and its quadrature signals are generated by a DDS implemented using an FPGA. After processing, a pair of orthogonal interference signals are obtained:

[0039] After sampling by an ADC, the pair of orthogonal interference signals are fed into an FPGA or host computer. Phase demodulation of the orthogonal signals can then be achieved using digital signal processing methods in the FPGA or host computer software. Generally, the CORDIC algorithm is used to calculate the Arctan, and then combined with phase unwrapping, the phase of the interference signal can be obtained. After obtaining the phase, the inner modulation depth can be extracted using digital phase-locked loops or Fourier transform methods. The digital signal processing flow is as follows: Figure 4 As shown, the digital phase-locked loop (PLL) method involves mixing the original internal modulation signal with the aforementioned phase demodulation signal, and then processing the mixture using a low-pass filter with a cutoff frequency lower than the internal modulation frequency to obtain the depth of the internal modulation. The Fourier transform method uses the Fast Fourier Transform algorithm to obtain the spectrum of the phase demodulated signal and extract the internal modulation frequency. The amplitude of the component is the depth of internal modulation. .

[0040] The system design ensures that the optical path length of the measuring light is greater than that of the reference light, and the calculation starting point for absolute distance measurement is designed according to application requirements, that is... Figure 1 and Figure 2 Reference points in the middle. Figure 1 The reference point needs to be designed outside the collimator, meaning it should be located in the air medium. At the reference point, a measurement signal is acquired, and the internal modulation depth is obtained using the aforementioned hardware and digital signal processing methods, denoted as... .

[0041] Move the corner cube prism to the desired distance measurement position, collect the measurement signal, and obtain the modulated depth at the target position using the same method described above, denoted as . Then the distance from the target to the reference point can be calculated: In the formula, is the refractive index of air.

[0042] When the internally modulated drive signal remains stable, the amplitude of the frequency change caused by the internal modulation of the DFB laser is... The parameter can be calibrated by keeping it constant and measuring a known distance. In actual calibration, the corner cube prism can be continuously moved from a reference point to an arbitrarily set target point along the direction of the distance to be measured. Precision displacement measurement equipment such as a laser heterodyne interferometer or a sinusoidal phase-modulated laser interferometer can be used to collect and accumulate the continuous displacement of the corner cube prism, thus obtaining a known distance. Assuming the known distance is measured as... The demodulation depth obtained at this distance is The formula for calibrating the laser frequency change is: Once this parameter is calibrated, arbitrary absolute distance measurements can be achieved.

[0043] However, maintaining long-term stability of internal modulation is very difficult in practical engineering. Firstly, there is the stability of the DFB injection modulation current (i.e., the stability of the laser driver source). Secondly, the DFB laser itself also exhibits frequency jitter, thus the amplitude of internal modulation frequency variation is significant. Maintaining long-term stability is difficult. Therefore, it is necessary to perform real-time calibration of the distance measurement results to eliminate measurement errors caused by the instability of the internal modulation frequency variation. Introducing a reference interferometer into the aforementioned measurement system enables real-time calibration, giving the measurement system self-calibration capabilities. Figure 5 and Figure 6 Schematic diagram of a laser heterodyne interferometric absolute distance measurement system for self-calibration using a reference interferometer. They are in... Figure 1 and Figure 2 The system shown introduces a reference interferometer. Structurally, the measurement beam and reference beam are split into two separate paths based on the original interferometer, and then combined in pairs to form the reference interferometer and the measurement interferometer. The reference interferometer has a fixed optical path difference. The reference interferometer and the measurement interferometer share the branch containing the acousto-optic frequency shift, therefore, their output signals have the same frequency shift.

[0044] During distance measurement, the optical path difference between the two arms of the reference interferometer remains constant (denoted as ). The change in the demodulated internal modulation depth then reflects the amplitude of the frequency change. The change in is denoted as the internal phase modulation depth obtained by demodulation using the reference interferometer. Let the modulation depth within the reference point measured by the interferometer be denoted as . The internal modulation depth of the target point Then the formula for calculating the absolute distance being measured is: In the formula Even if the coefficients are undetermined, they can still be calibrated using a known distance measurement. Let's assume the known distance is... The corresponding internal modulation depth is ,but The calibration formula is: Once this parameter is calibrated, self-calibrated absolute distance measurement can be achieved. First, at the reference point, the demodulated internal modulation depth of the measurement signal is acquired. Next, move the corner cube prism to the target distance to be measured, and simultaneously acquire the reference signal and the measurement signal, demodulating the internal modulation depth of the reference interferometer. and the internal modulation depth of the measuring interferometer Substituting these three values ​​into formula (6) yields the measured distance. .

[0045] The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation provided by this invention can perform the laser heterodyne interferometry absolute distance measurement method based on internal phase modulation and achieve the same or similar technical effects. To avoid repetition, this invention will not elaborate further.

[0046] The beneficial effects of the technical solutions provided in the embodiments of the present invention include at least the following: (1) In this invention, by performing internal phase modulation on a distributed feedback semiconductor laser, a low-frequency modulation signal is directly loaded onto the laser beam. Heterodyne interferometry is used with the frequency shifting frequency of an acousto-optic frequency shifter as the carrier wave. Absolute distance measurement is achieved by demodulating the linear relationship between the internal modulation depth and the optical path difference. Compared with the traditional pulse time-of-flight method and laser modulation absolute ranging method, this invention adopts a heterodyne interferometry structure, and the measurement signal is located in the high-frequency carrier region, which effectively avoids low-frequency noise interference and significantly improves the measurement resolution and accuracy. At the same time, only the internal modulation function needs to be added to the conventional heterodyne interferometer, without the need for complex modulation and demodulation circuits or multi-wavelength light sources. The system structure is greatly simplified and the cost is significantly reduced.

[0047] (2) In this invention, a reference interferometer with a fixed optical path difference is introduced, which shares an acousto-optic frequency shift branch with the measurement interferometer, and synchronously demodulates the internal modulation depth in both the reference and measurement interferometric signals. When laser frequency jitter or modulation instability causes fluctuations in the amplitude of the internal modulation frequency change, the internal modulation depth of the reference interferometer can reflect this change in real time. The measurement results are corrected using a calibration formula, eliminating measurement errors caused by laser frequency instability. Compared to measurement systems without a calibration mechanism, this invention achieves self-calibration, maintaining high-precision measurements even under long-term operation and complex environmental conditions, significantly improving the stability and reliability of the system.

[0048] (3) In this invention, orthogonal carrier mixing and digital signal processing techniques are used to demodulate heterodyne interference signals. Phase demodulation is achieved through the CORDIC algorithm, and the inner modulation depth is extracted by combining digital phase-locked loop or fast Fourier transform. This technical solution makes the signal processing process digital and modular, ensuring both the accuracy and speed of phase demodulation, and facilitating flexible implementation in FPGA or host computer. Compared with traditional analog demodulation methods, this invention can effectively suppress the effects of circuit noise and temperature drift, improve the anti-interference capability of measurement, and provide convenient conditions for subsequent algorithm upgrades and functional expansion, thereby enhancing the adaptability and maintainability of the system.

[0049] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. The scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A laser heterodyne interferometry absolute distance measurement method based on internal phase modulation, characterized in that, include: S1: Internal phase modulation of a distributed feedback semiconductor laser to modulate its output frequency into a laser beam. S2: Divide the laser beam into a measurement beam and a reference beam; S3: The reference beam is subjected to frequency shifting processing to produce a fixed frequency shift; S4: Combine the frequency-shifted reference beam with the measurement beam after it has been reflected or returned from the target to generate optical mixing and form a heterodyne interference signal; S5: Detect the heterodyne interference signal and demodulate the phase modulation depth introduced by the inner phase modulation from it; S6: Based on the linear relationship between the phase modulation depth and the optical path difference, the absolute distance of the target under test is calculated.

2. A laser heterodyne interferometry absolute distance measurement system based on internal phase modulation, characterized in that, include: A distributed feedback semiconductor laser in which the injection current is modulated to produce an output laser beam whose frequency is modulated by the internal phase. A beam splitting module is used to split the laser beam into a measurement beam and a reference beam; A frequency shifting module is disposed in the optical path of the reference beam and is used to perform frequency shifting processing on the reference beam; An interference optical path is used to combine the measurement beam reflected or returned by the target with a frequency-shifted reference beam to generate optical mixing and form a heterodyne interference signal. The photoelectric detection module is used to detect the heterodyne interference signal and convert it into an electrical signal; The signal processing module is used to demodulate the phase modulation depth introduced by the internal phase modulation from the electrical signal, and calculate the absolute distance of the target under test according to the linear relationship between the phase modulation depth and the optical path difference.

3. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 2, characterized in that, The beam splitting module and interference optical path are fiber-optic structures, including fiber couplers, optical circulators, and fiber collimators; or they are free-space structures, including polarization beam splitters, mirrors, quarter-wave plates, and non-polarization beam splitters.

4. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 2, characterized in that, It also includes a reference interferometer with a fixed optical path difference, used to calibrate measurement errors caused by laser frequency jitter or modulation instability in real time.

5. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 4, characterized in that, The reference interferometer and the measurement interferometer share the reference optical branch where the frequency shift module is located. By synchronously demodulating the phase modulation depth in the reference interference signal and the measurement interference signal, the calibrated absolute distance is calculated by substituting it into the calibration formula.

6. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 2, characterized in that, The step of demodulating the phase modulation depth from the heterodyne interference signal or the signal processing module specifically includes: The heterodyne interference signal is mixed and low-pass filtered using orthogonal carrier signals to obtain a pair of orthogonal interference signals; Phase demodulation is performed on the orthogonal interference signals to obtain a phase signal containing internal modulation information; The amplitude of the internal modulation frequency component is extracted from the phase signal, and this amplitude is the phase modulation depth.

7. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 6, characterized in that, The signal processing module includes: The quadrature carrier mixing unit is used to mix the electrical signal with two quadrature carrier signals respectively; The low-pass filter unit is used to filter out the high-frequency components after mixing and output a pair of orthogonal interference signals. A phase demodulation unit is used to perform operations on the orthogonal interference signals to obtain a phase signal containing internal modulation information; The inner modulation depth extraction unit is used to extract the amplitude of the inner modulation frequency component from the phase signal as the phase modulation depth.

8. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 7, characterized in that, The phase demodulation uses the CORDIC algorithm to calculate Arctan and combines it with phase unwrapping processing; the internal modulation depth extraction is implemented using a digital lock-in amplifier or a fast Fourier transform algorithm.

9. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 2, characterized in that, In the step of calculating the absolute distance to the target being measured, the scaling factor in the linear relationship is determined by pre-calibration or real-time calibration.

10. The laser heterodyne interferometry absolute distance measurement system based on internal phase modulation according to claim 9, characterized in that, The real-time calibration methods include: At the reference point, acquire and record the first internal modulation depth of the measuring interferometer; At the target point, the second internal modulation depth of the measuring interferometer and the third internal modulation depth of the reference interferometer are simultaneously acquired and recorded. The optical path difference is fixed based on a pre-calibrated reference interferometer, and the calibrated absolute distance is calculated using the first, second, and third inner modulation depths.

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