Single-frequency laser interference vibration measurement system and method based on optical fiber sensing

Abstract

The invention discloses a single-frequency laser interference vibration measurement system and method based on optical fiber sensing. The system comprises a laser, a beam splitter, a circulator, a beam combiner, a coupler, a Faraday rotating mirror, a photoelectric detector and a data acquisition and processing module. Single-frequency laser emitted by the first laser is divided into two paths through the beam splitter, one path serves as signal light and irradiates the surface of a measured object after passing through the first circulator and the beam combiner, and reflected light returns through an original path; one path of interference signals is deflected by 90 degrees through a circulator II and a Faraday rotating mirror and then enters a photoelectric detector, the other path of interference signals directly enters the photoelectric detector, orthogonal I / Q signals are output, vibration displacement is extracted through a data acquisition and processing module by adopting a phase calculation method, and the vibration displacement is calculated through a phase calculation method. And the vibration speed is obtained through differential calculation. By optimizing the optical path design and introducing orthogonal signal processing, the system cost is reduced, and the measurement precision and the anti-interference capability are improved.

Description

Technical Field

[0001] The present invention relates to the field of laser interference technology, and in particular to a single-frequency laser interference vibration measurement system and method based on optical fiber sensing. Background Art

[0002] Laser vibration measurement technology has the characteristics of high measurement accuracy, large measurement range and no interference with the vibrating surface to be measured. It has become the technology with the greatest development potential and application prospects in the field of vibration measurement. Laser vibration measurement technology can not only measure the macroscopic and microscopic features of objects at the nanometer or even sub-nanometer level, and realize ultra-precision value transfer and value traceability; it can also accurately describe and record the spatial-temporal behavior process of matter from ultra-low frequency to high frequency at the micro-nanometer level to reveal the interaction mechanism and scientific laws between substances.

[0003] Laser vibration measurement technology mainly includes laser triangulation, light intensity method, speckle interferometry, holographic interferometry, single-frequency laser interferometry, dual-frequency laser interferometry and other methods. Laser triangulation and light intensity method belong to direct detection methods. Laser triangulation has a small measurement range, low measurement accuracy and low resolution, and is not suitable for measuring high-frequency and high-speed vibrations. The light intensity method is easily disturbed by the environment and light source, and has low measurement accuracy. These two methods are not suitable for high-precision measurement. Speckle interferometry is very sensitive to environmental disturbances, and can only measure simple harmonic vibrations, but cannot measure non-harmonic motion and impact. The holographic interferometry processing process is time-consuming and cannot perform real-time measurements. The dual-frequency laser interferometry method has the advantages of small signal influence by DC drift and strong anti-interference ability, but due to its limited measurement speed, the nonlinear measurement error is large, and it is difficult to correct and compensate in real time, which restricts its measurement accuracy. In addition, it has expensive acousto-optic frequency shifting devices and a relatively complex optical path structure.

[0004] There are currently two main implementation schemes for single-frequency laser interferometry vibration measurement. One is to build an interference system through discrete optical components, and the other is to design silicon photonic chips to integrate optical devices and implement the interference system on the chip. Both schemes have their own advantages and disadvantages. Discrete optical components are simple to implement, but the device cost is high and the system volume is large. The silicon photonic chip solution uses the latest silicon photonic integration technology, which can achieve functions in a smaller volume, but the design is difficult, the development and tape-out cycle is long, and there is also a high threshold. Summary of the invention

[0005] Technical purpose: In view of the shortcomings of the existing single-frequency laser interferometer vibration measurement method, the present invention discloses a single-frequency laser interferometer vibration measurement system and method based on optical fiber sensing, which reduces system complexity and manufacturing cost by simplifying optical design and signal processing, while improving the accuracy and stability of vibration measurement.

[0006] Technical solution: To achieve the above technical objectives, the present invention adopts the following technical solution: A single-frequency laser interferometer vibration measurement system based on optical fiber sensing, comprising: Laser 1, used for emitting continuous single-frequency laser; The beam splitter is used to split the single-frequency laser into two paths, wherein the light beam of the first light path passes through the circulator 1 and the beam combiner respectively, and is combined with the single-frequency reference laser emitted by the laser 2 and then irradiated to the surface of the object to be measured, and the light beam reflected by the object to be measured returns through the beam combiner and the circulator 1, and enters the coupler as the signal light; the light beam of the second light path is directly mixed with the light beam reflected from the first light path as the local oscillator light through the coupler; A coupler is used to mix the two light beams and generate two interference light signals, one of which is deflected 90° by a Faraday rotator mirror after passing through circulator 2; The photodetector 1 and the photodetector 2 are used to receive two interference light signals and convert them into I signal and Q signal respectively; The data acquisition and processing module is used to solve the I signal and Q signal to obtain the vibration information of the object being measured.

[0007] The present invention also discloses a single-frequency laser interferometer vibration measurement method based on optical fiber sensing, which is applied to the above-mentioned single-frequency laser interferometer vibration measurement system based on optical fiber sensing, and specifically comprises the following steps: The laser emits a continuous single-frequency laser, and the beam is divided into two paths by a beam splitter; One beam passes through circulator 1 and beam combiner in sequence, and then is combined with the single-frequency reference laser emitted by laser 2 and irradiated to the surface of the object to be measured. The beam reflected by the object to be measured then returns through the beam combiner and circulator 1 and enters the coupler. The other beam directly enters the coupler as the local oscillator light. The two light beams are mixed through a coupler to generate two interference light signals; One of the two interference light signals passes through circulator 2 and is deflected 90° by the Faraday rotator mirror before entering photodetector 2, and the other directly enters photodetector 1; Photodetector 1 and photodetector 2 receive the mixed interference light signal and convert it into I signal and Q signal respectively; The data acquisition and processing module processes the I signal and Q signal to extract the vibration information of the object.

[0008] Beneficial effects: The single-frequency laser interferometer vibration measurement system and method based on optical fiber sensing provided by the present invention has the following beneficial effects: The present invention adopts a dual optical path design, and adjusts the polarization direction of the mixed light beam through a Faraday rotator to ensure that the polarization states of the signal light and the local oscillator light match, which not only improves the stability of the interference signal and avoids signal attenuation and interference caused by polarization mismatch, but also greatly reduces the sensitivity of the system to vibration environment and temperature fluctuations through the stable polarization control of the Faraday rotator. Traditional optical interference systems usually rely on complex mixing devices and multi-stage polarization control elements, while the present invention replaces the traditional mixer with a Faraday rotator. Compared with a complex optical mixer, the optical path complexity is reduced, the number of discrete optical devices used and the influence of the mixer on the system performance are reduced, and at the same time, the adverse influence of the use of the Faraday rotator on the system performance is reduced to a certain extent by optimizing the optical path structure, thereby ensuring the measurement accuracy and performance of the system.

[0009] Orthogonal I and Q signals are generated through photodetectors, and phase information is obtained using an orthogonal solution method to avoid the phase ambiguity problem of traditional methods. Orthogonal signal processing ensures that the phase information can be continuously solved throughout the entire cycle; and the phase change is directly linked to the displacement through a calculation formula. This optical phase-based solution method can achieve nanometer-level displacement accuracy; real-time sampling and solution of I / Q signals are suitable for dynamic vibration measurement scenarios and support high-frequency vibration analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0011] Figure 1 It is a structural schematic diagram of the single-frequency laser interferometer vibration measurement system of the present invention; Figure 2 The figure is a flow chart of the single-frequency laser interferometer vibration measurement method of the present invention. DETAILED DESCRIPTION

[0012] The present invention will be described more clearly and completely below by way of a preferred embodiment in conjunction with the accompanying drawings, but the present invention is not limited to the scope of the embodiment.

[0013] like Figure 1 As shown, the present invention discloses a single-frequency laser interferometer vibration measurement system based on optical fiber sensing, including a laser 1, a laser 2, a beam splitter, a circulator 1, a circulator 2, a beam combiner, a coupler, a Faraday rotator mirror, a photodetector 1, a photodetector 2 and a data acquisition and processing module, wherein: Laser 1 is used to emit continuous single-frequency laser. Laser 1 uses 1550nm polarization-maintaining narrow-linewidth laser as the signal source; The beam splitter is used to split the single-frequency laser into two paths, wherein the light beam of the first optical path passes through the circulator 1 and the beam combiner respectively, and is combined with the single-frequency reference laser emitted by the laser 2 and irradiated to the surface of the object to be measured. The light beam reflected from the object to be measured returns through the beam combiner and the circulator 1, and enters the coupler as the signal light. The laser 2 is used for debugging the optical path and calibrating the system, and a 650nm single-mode laser is used as the indicator light source. The circulator 1 is used to guide the signal light into the beam combiner and guide the light beam reflected from the object to be measured back to the system. The beam combiner is used to combine the light beam of the laser 1 and the light beam of the laser 2 into a coaxial light beam, which is irradiated to the surface of the object to be measured.

[0014] The light beam of the second optical path is directly mixed with the light beam reflected from the first optical path through the coupler as the local oscillator light; A coupler is used to mix two light beams and generate two interference light signals, one of which is deflected 90° by a Faraday rotator after passing through a second circulator. The second circulator and the Faraday rotator are used to adjust the polarization direction of the mixed light beam to adapt to the input characteristics of the photodetector. For laser interferometers, according to Nyquist's sampling theorem, there is a problem of long acquisition time due to the large amount of collected data. Therefore, the existing laser interferometer technology uses a mixer to achieve spectrum shifting from high-frequency bandpass signals to low-frequency bandpass signals. Mixing has relatively high requirements on the waveform of the input signal. The distortion of the original input signal forms high-frequency noise after Fourier transform, causing frequency interference and nonlinear distortion of the mixed output. Therefore, it is also necessary to connect a low-pass filter after mixing to filter the high-frequency component. The above method realizes spectrum shifting through mixing and low-pass filtering of external devices, greatly reducing the sampling rate required for collecting laser interference signals, thereby shortening the time required to process interference signals. However, due to the introduction of external mixers and low-pass filters, the complexity of the overall optical path increases, the number of discrete optical devices required increases, the cost of system construction increases, and the probability of introducing external interference increases significantly. In particular, the presence of the mixer will lead to the following problems: (1) The mixer may introduce phase noise or frequency drift, especially in the frequency conversion process. Since the mixer relies on the phase relationship between two signals of different frequencies, when the phase of one of the signals is unstable, it may affect the phase lock of the interference measurement system; (2) When the mixer mixes frequency signals with each other, it may generate multiple output signals of different frequencies. These frequency components may be different from the original interference signal frequency, resulting in unexpected signal superposition or frequency interference in the measurement process, affecting the interference effect of the system; (3) The working principle of the mixer is based on the mixing of signals of different frequencies. This process may introduce time delay or phase delay, especially when the mixer is used for low-frequency signal conversion, the time delay effect will be more obvious.

[0015] The present invention creatively uses a Faraday rotator to replace a mixer, and optimizes the optical path structure to reduce the complexity of the optical path structure and the construction cost. A new single-frequency laser interferometer is constructed using a Faraday rotator, and the Faraday rotator is used as a phase regulator. By adjusting the angle of the rotating mirror or the intensity of the magnetic field, the phase change of the light wave is accurately controlled to achieve modulation and demodulation of the interference signal. In addition, the use of a Faraday rotator can also reduce the influence of the external magnetic field on the light wave propagation path and improve the measurement accuracy of the system.

[0016] When building a Faraday rotator into a new single-frequency laser interferometer, it is necessary to improve the optical path structure of other parts of the system. The newly designed optical path structure also realizes the leap from theory to practice in applying the Faraday rotator to the single-frequency laser interferometer. The introduction of the Faraday rotator will inevitably bring about magnetic field dependence, light loss and scattering problems. By changing the external optical path structure of the Faraday rotator, the influence of the magnetic field dependence, light loss and scattering problems on the system measurement is reduced. Ultimately, compared with the existing measurement system, the new single-frequency laser interferometer vibration measurement system designed by the present invention reduces the complexity of the optical path, the number of discrete optical devices used, and the influence of the mixer on the system performance. At the same time, by optimizing the optical path structure, the adverse effects of the use of the Faraday rotator on the system performance are reduced to a certain extent, ensuring the system measurement accuracy and performance.

[0017] The photodetector 1 and the photodetector 2 are used to receive two interference light signals and convert them into I signal and Q signal respectively; The data acquisition and processing module is used to digitally process the I signal and the Q signal, and calculate the vibration displacement and vibration velocity information of the measured object through phase resolution. The data acquisition and processing module includes a signal processing module, an analog-to-digital conversion module, an algorithm processor module, and a data upload module.

[0018] The signal processing module is used to pre-process the analog signal output by the photodetector, including filtering, gain adjustment, and orthogonal signal generation; the analog-to-digital conversion module is used to convert the analog signal output by the signal processing module into a digital signal for subsequent algorithm processing; the algorithm processor module is responsible for extracting the vibration information of the object from the digitized signal, including the calculation of phase, displacement, and velocity; the data upload module is used to upload the calculated vibration information to the host computer to realize the display and storage of vibration information.

[0019] The working principle of the single-frequency laser interferometer vibration measurement system of the present invention is as follows: First, starting from the wave equation of light, the wave equation of the electric field of light changing with time and distance is:

[0020] in, represents the amplitude of the electric field, represents the frequency of light, represents the wavelength of light, represents the initial phase of the light wave; According to the above wave equation, the electric field equations of the signal light and the local oscillator light can be obtained, which are:

[0021] in, and represent the electric field of the signal light and the electric field of the local oscillator light respectively, and represent the electric field amplitude of the signal light and the electric field amplitude of the local oscillator light respectively, and They represent the frequency of the signal light and the frequency of the local oscillator light respectively.

[0022] The calculation formula for the electric field superimposed by the electric fields of the signal light and the local oscillator light on the photosensitive surface of the photodetector is:

[0023] According to the square law characteristics of the photodetector, its output current and electric field The relationship between them is:

[0024] Where R is the current response coefficient of the photodetector, Indicates the integration time, which is much larger than the optical frequency period time; According to the above formula, we can finally get two orthogonal current signals and :

[0025] in, represents a noisy light field; The orthogonal current signal is passed through the transimpedance amplifier to obtain the orthogonal voltage signal. The IQ signal expression under ideal conditions is:

[0026] in, and Represent the I signal and Q signal respectively, and Represent the amplitude of I signal and Q signal respectively, Indicates the propagation distance of the signal light, represents the propagation distance of the local oscillator light, t represents the time, Indicates the wavelength of light.

[0027] The above formula gives the relationship between phase and displacement in one-way measurement. In actual measurement, the influence of object displacement on phase should take into account the two-way optical path. The relationship between displacement and phase can be expressed as:

[0028] in, Indicates the vibration displacement of the object being measured, Indicates the phase change of the signal light.

[0029] By taking differential derivatives based on the displacement and sampling rate, the vibration velocity of the object can be obtained as:

[0030] in, Indicates the vibration speed of the object being measured, Indicates the sampling time interval.

[0031] like Figure 2 As shown, the present invention also provides a single-frequency laser interferometer vibration measurement method based on optical fiber sensing, which is used to implement a single-frequency laser interferometer vibration measurement system based on optical fiber sensing described in the present invention, and specifically includes the following steps: Laser 1 emits a 1550nm continuous single-frequency laser, which is split into two beams by a beam splitter. One beam is used as signal light to illuminate the surface of the object being measured, and the other is used as local oscillator light to mix with the signal light and serve as interference reference light. One beam passes through circulator 1 and beam combiner in sequence, and is combined with the single-frequency reference laser emitted by laser 2 and irradiated to the surface of the object to be measured. The beam reflected by the object to be measured then returns through the beam combiner and circulator 1 and enters the coupler as signal light. The other beam directly enters the coupler as local oscillator light. The signal light path is specifically as follows: the light beam emitted by laser one passes through circulator one and then enters the beam combiner, and is combined with the visible light beam emitted by laser two to form a coaxial light beam. The combined light beam is irradiated to the surface of the object to be measured. Under the action of vibration, the surface reflected light of the object to be measured carries the dynamic change information of the path length, and the reflected light returns to the beam combiner, is guided to the coupler through circulator one, and is mixed with the local oscillator light.

[0032] The specific path of the local oscillator light is as follows: another light beam separated by the laser is directly output from the beam splitter and enters the coupler, where it is superimposed with the returned signal light to generate a mixed light signal.

[0033] The two light beams are mixed through a coupler to generate two interference light signals; One of the two interference light signals passes through circulator 2 and is deflected 90° by the Faraday rotator mirror before entering photodetector 2, and the other directly enters photodetector 1; Photodetector 1 and photodetector 2 receive the mixed interference light signal and convert it into an I signal and a Q signal respectively; The data acquisition and processing module processes the I signal and the Q signal to extract the vibration information of the object, wherein the vibration information includes the vibration displacement and the vibration velocity.

[0034] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A single-frequency laser interferometer vibration measurement system based on optical fiber sensing, characterized in that: include: Laser 1, used for emitting continuous single-frequency laser; The beam splitter is used to split the single-frequency laser into two paths, wherein the light beam of the first light path passes through the circulator 1 and the beam combiner respectively, and is combined with the single-frequency reference laser emitted by the laser 2 and then irradiated to the surface of the object to be measured, and the light beam reflected by the object to be measured returns through the beam combiner and the circulator 1, and enters the coupler as the signal light; the light beam of the second light path is directly mixed with the light beam reflected from the first light path as the local oscillator light through the coupler; A coupler is used to mix the two light beams and generate two interference light signals, one of which is deflected 90° by a Faraday rotator mirror after passing through circulator 2; The photodetector 1 and the photodetector 2 are used to receive two interference light signals and convert them into I signal and Q signal respectively; The data acquisition and processing module is used to solve the I signal and Q signal to obtain the vibration information of the object being measured.

2. The single-frequency laser interferometer vibration measurement system based on optical fiber sensing according to claim 1, characterized in that: The I signal and Q signal output by photodetector 1 and photodetector 2 are expressed as: ； in, and Represent the I signal and Q signal respectively, and Represent the amplitude of I signal and Q signal respectively, Indicates the propagation distance of the signal light, represents the propagation distance of the local oscillator light, t represents the time, Indicates the wavelength of light.

3. The single-frequency laser interferometer vibration measurement system based on optical fiber sensing according to claim 1, characterized in that: The data acquisition and processing module is based on a two-way optical path design. The vibration displacement calculation formula of the measured object is as follows: ； in, Indicates the vibration displacement of the object being measured, represents the phase change of the signal light, Indicates the wavelength of light.

4. The single-frequency laser interferometer vibration measurement system based on optical fiber sensing according to claim 3, characterized in that: The vibration velocity calculation formula of the measured object is as follows: ； in, Indicates the vibration speed of the object being measured, Indicates the sampling time interval.

5. The single-frequency laser interferometer vibration measurement system based on optical fiber sensing according to claim 1, characterized in that: The data acquisition and processing module includes a signal processing module, an analog-to-digital conversion module, an algorithm processor module and a data upload module.

6. The single-frequency laser interferometer vibration measurement system based on optical fiber sensing according to claim 1, characterized in that: Laser 1 uses a 1550nm polarization-maintaining narrow-linewidth laser as a signal source, and Laser 2 uses a 650nm single-mode laser as an indicator light source.

7. A single-frequency laser interferometer vibration measurement method based on optical fiber sensing, applied to a single-frequency laser interferometer vibration measurement system based on optical fiber sensing as claimed in any one of claims 1 to 6, characterized in that: The specific steps include: The laser emits a continuous single-frequency laser, and the beam is divided into two paths by a beam splitter; One beam passes through circulator 1 and beam combiner in sequence, and is combined with the single-frequency reference laser emitted by laser 2 and irradiated to the surface of the object to be measured. The beam reflected by the object to be measured then returns through the beam combiner and circulator 1 and enters the coupler as signal light. The other beam directly enters the coupler as local oscillator light. The two light beams are mixed through a coupler to generate two interference light signals; One of the two interference light signals passes through circulator 2 and is deflected 90° by the Faraday rotator mirror before entering photodetector 2, and the other directly enters photodetector 1; Photodetector 1 and photodetector 2 receive the mixed interference light signal and convert it into an I signal and a Q signal respectively; The data acquisition and processing module processes the I signal and Q signal to extract the vibration information of the object.

8. The single-frequency laser interferometer vibration measurement method based on optical fiber sensing according to claim 7, characterized in that: The electric fields of the signal light and the local oscillator light are expressed as: ； in, and represent the electric field of the signal light and the electric field of the local oscillator light respectively, and represent the electric field amplitude of the signal light and the electric field amplitude of the local oscillator light respectively, and represent the frequency of the signal light and the frequency of the local oscillator light respectively, Indicates the propagation distance of the signal light, represents the propagation distance of the local oscillator light, t represents the time, represents the wavelength of light, Represents the initial phase of the light wave.

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