Heterodyne Doppler laser vibration meter based on all optical fibers

Abstract

The utility model discloses a heterodyne Doppler laser vibration meter based on all optical fibers. The vibration meter adopts a double-light-path differential design, and the system divides laser into a measuring light beam and a reference light beam which are respectively guided into a measuring light path and a reference light path which are parallel to each other. The measuring light path emits laser to an object to be measured, and a return signal of the laser comprises composite information of object vibration and system errors after being demodulated. Meanwhile, a reference light path forms a closed loop inside, and a demodulation signal of the reference light path can accurately extract system errors introduced by devices such as a frequency shifter. The signal processing system subtracts the two demodulated signals in real time, so that homologous errors are eliminated, and pure and real vibration data are obtained. According to the utility model, a complex phase-locked loop is replaced by front-end optical path difference, so that the measurement precision, stability and environmental adaptability are remarkably improved.

Description

Technical Field

[0001] This utility model relates to the field of laser measurement technology, and in particular to a heterodyne Doppler laser vibration meter based on an all-fiber optic cable. Background Technology

[0002] Laser Doppler vibration measurement technology is widely used in many fields such as industrial inspection, aerospace, biomedicine and civil engineering due to its advantages of high measurement accuracy, wide dynamic range and non-contact measurement.

[0003] Currently, most mainstream laser Doppler vibrometers employ a coherent heterodyne optical path structure. In this structure, the frequency accuracy of the frequency shift modulator (usually an acousto-optic modulator) within the heterodyne optical path is one of the key factors limiting the overall accuracy of the vibrometer. The crystal oscillator (crystal oscillator) signal source upon which commonly used frequency shifters rely typically only achieves an accuracy of 10 to several tens of ppb. This error directly influences the measurement results, making it difficult to further improve the vibration measurement accuracy. Furthermore, the frequency shift is also affected by ambient temperature, external vibration, and the inherent error of the crystal oscillator itself, exhibiting irregular random fluctuations.

[0004] To address this issue, existing technologies typically employ an analog or digital phase-locked loop (PLL) at the signal processing end, using algorithms to track and compensate for changes in frequency shift. However, the effectiveness of the PLL mechanism is limited by its loop filter design, and its locking and tracking performance deteriorates when the vibration frequency range of the measured object varies significantly, resulting in errors in the final measurement results. Therefore, fundamentally eliminating the errors introduced by the frequency shifter and improving the accuracy and stability of vibration measurement is a pressing technical problem that needs to be solved in this field. Utility Model Content

[0005] The purpose of this invention is to provide a heterodyne Doppler laser vibrometer based on an all-fiber optic cable to solve the problems mentioned in the background art, including:

[0006] A light source used to generate laser light;

[0007] A beam splitting system is used to split the laser into a measurement beam that enters the measurement optical path and a reference beam that enters the reference optical path.

[0008] The measurement optical path includes a measurement beam emitted to the object under test via an optical probe and a measurement signal light reflected back. The measurement signal light beats with the local oscillator to generate a measurement beat frequency signal containing vibration information of the object under test and frequency shifter error information.

[0009] A reference optical path in which the signal light in the reference beam beats the local oscillator light directly without being reflected by the object under test, so as to generate a reference beat frequency signal containing only frequency shifter error information.

[0010] A signal processing system is configured to subtract the reference beat frequency signal from the measured beat frequency signal to eliminate common frequency shifter error information, thereby obtaining accurate vibration information of the object under test.

[0011] Furthermore, the beam splitting system includes a first beam splitter, a second beam splitter, and a third beam splitter. The laser beam is split into a local oscillator beam and a signal beam by the first fiber beam splitter. The local oscillator beam and the signal beam enter the second and third fiber beam splitters, respectively, and are split into two paths again. One path is used as a measurement beam and enters the measurement optical path, while the other path is used as a reference beam and enters the reference optical path.

[0012] Furthermore, the splitting ratio of the first fiber beam splitter is from 1:99 to 20:80.

[0013] Furthermore, the fiber length in the reference optical path is extended to compensate for the optical path difference between the reference optical path and the measurement optical path.

[0014] Furthermore, the signal processing system is configured to correct the reference beat frequency signal by setting different delay compensation amounts when the distance between the optical probe and the object under test changes.

[0015] Furthermore, both the measurement optical path and the reference optical path include a balanced detector for receiving beat frequency signals.

[0016] Furthermore, the optical probe includes an indicator laser, and the beam of the indicator laser and the beam of the measurement signal light are combined by a wavelength division multiplexer and then emitted from the optical probe.

[0017] Furthermore, the optical probe integrates a waveplate for changing the polarization state of light. The waveplate and the polarization beam splitter together form a circulator structure to guide the transmission and reception path of the measurement signal light.

[0018] Furthermore, an adjustable optical attenuator is provided in the optical path used to transmit the local oscillator light in the measurement optical path, which is used to adjust the optical power ratio between the local oscillator light and the measurement signal light.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention adds a reference optical path parallel to the measurement optical path, ensuring that both the measurement and reference signals contain the same error introduced by the frequency shifter. By differentially processing the two signals, the frequency shifter error can be directly and accurately eliminated without relying on a complex phase-locked loop mechanism. This front-end optical path compensation method makes the measurement results unaffected by changes in ambient temperature, external vibrations, or crystal oscillator frequency drift, resulting in stable operation and more accurate measurement results.

[0021] This invention employs a dual-optical-path demodulation and subtraction signal processing method, avoiding the complex phase-locked loop design and associated problems such as loop filtering and phase detection in traditional technologies. The system structure is simplified, and the signal processing flow is direct and clear, thereby improving the reliability and environmental adaptability of the entire system. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this utility model, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the system structure of a specific embodiment of the present invention.

[0024] Figure 2 This is a waveform diagram of the frequency shifter error changing over time, obtained by demodulating the reference optical path signal according to an embodiment of the present invention. The diagram shows the results of 12 repeated measurements.

[0025] Figure 3 According to an embodiment of this utility model, the vibration displacement-time spectrum is finally obtained by subtracting the original measurement signal from the reference error signal. The left figure shows the single-frequency vibration measurement result, and the right figure shows the swept-frequency vibration measurement result. Detailed Implementation

[0026] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present utility model.

[0027] In this utility model, unless otherwise explicitly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances.

[0028] Example 1:

[0029] Reference Figure 1 This invention provides a heterodyne Doppler laser vibrometer based on an all-fiber optic cable. The vibrometer includes a light source, a beam splitting system, a measuring optical path, a reference optical path, and a signal processing system.

[0030] In this embodiment, the light source is a sub-laser. The laser emitted by the sub-laser is split into local oscillator light and signal light after passing through the first fiber optic beam splitter. The splitting ratio can be set within the range of 1:99 to 20:80, preferably 5:95 or 10:90. Subsequently, the local oscillator light and the signal light enter the second and third fiber optic beam splitters, respectively, and are split into two paths again. One path enters the measurement optical path, and the other path enters the reference optical path.

[0031] The construction and operation of the reference optical path are as follows: the reference signal light from the third beam splitter and the reference local oscillator light from the second beam splitter directly enter a 2x2 fiber coupler for beat frequency measurement without passing through an external measurement stage, and are then received by a balanced detector. Specifically, to compensate for the optical path difference between the reference optical path and the measurement optical path caused by the different physical paths, the fiber used for signal light transmission in the reference optical path in this embodiment is intentionally extended to ensure that the two signals are aligned in time.

[0032] The construction and operation of the measurement optical path are as follows: The measurement signal light from the third beam splitter first passes through a fiber polarization beam splitter. Simultaneously, an indicator laser emitting visible light has its beam combined with the measurement signal light beam via a wavelength division multiplexer. The combined light enters an optical probe integrating an optical telescope structure and is then emitted by the probe onto the surface of the object under test. The telescope is designed with an achromatic structure, ensuring that the indicator laser and the measurement signal light have the same focal plane, facilitating aiming and focusing.

[0033] The optical probe also integrates a waveplate for changing the polarization state of the light. This waveplate, together with the fiber polarization beamsplitter in front of the probe, forms a fiber optic circulator structure. The waveplate can be a quarter-waveplate, a half-waveplate, or other conventional waveplates; in this embodiment, a quarter-waveplate is preferred to achieve the above function. This allows the measurement signal light reflected from the surface of the object under test to be guided to the third port of the beamsplitter when passing through it, entering the fiber optic coupler used for beat frequency. Simultaneously, the measurement local oscillator light from the second beamsplitter, after passing through an adjustable optical attenuator, also enters the aforementioned fiber optic coupler. The purpose of the adjustable optical attenuator is to flexibly adjust the optical power ratio of the local oscillator light to the measurement signal light to obtain the optimal beat frequency signal-to-noise ratio. The measurement signal light and the measurement local oscillator light beat in the coupler, and the generated measurement beat frequency signal is then received by another balanced detector.

[0034] The working process of a signal processing system is as follows:

[0035] The signal processing system is responsible for demodulating and analyzing the two beat frequency signals from the measurement optical path and the reference optical path. Its core task is to extract the true vibration information through differential processing. The process is as follows:

[0036] The system performs quadrature phase demodulation on the beat frequency signal returned from the measurement optical path. Since this signal light is reflected by the object under test, the demodulated result is a composite signal, containing both Doppler frequency shift information caused by the object's vibration and a common error introduced by the frequency shifter. We define this composite signal, which includes both vibration and error, as the original measurement signal.

[0037] Simultaneously, the system performs the same demodulation algorithm on the beat frequency signal returned from the reference optical path. Because the reference optical path is an internal, closed loop that does not involve the measurement of external objects, its demodulation result can accurately separate the system's own error sources, that is, an error signal that purely reflects the frequency change of the frequency shifter over time. Figure 2 Typical waveforms of the error signal obtained from multiple measurements are shown, verifying that it can be stably extracted.

[0038] Finally, the signal processing system performs a crucial differential cancellation step: subtracting the pure error signal from the original measurement signal, which contains both vibration and error. Since the frequency shifter errors in the two signals originate from the same source and are synchronized, this subtraction operation can accurately and completely cancel them out, ultimately yielding high-precision data that reflects only the true vibration of the object under test. Figure 3 The vibration displacement-time spectrum obtained using this method is shown, and the results are clear and stable, fully demonstrating the effectiveness of this differential processing method.

[0039] Example 2:

[0040] This embodiment is a further optimization based on Embodiment 1. In actual measurements, the distance between the optical probe and the object under test may change, which will cause a change in the total optical path of the measurement optical path. To address this, the signal processing system of this embodiment is further configured to allow the user to set an adjustable delay compensation in the signal processing software according to the actual measurement distance. This delay compensation is applied to the reference signal to correct the time difference introduced by the change in measurement distance, thereby ensuring accurate elimination of frequency shifter errors at different working distances.

[0041] In summary, by adding a reference optical path and employing differential signal processing, this invention effectively eliminates the error of the frequency shifter at the front end of the optical path, simplifies the complexity of the back-end signal processing, and significantly improves the accuracy and stability of the vibration measurement system.

[0042] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

Claims

1. A heterodyne Doppler laser vibrometer based on an all-fiber optic cable, characterized in that, include: A light source used to generate laser light; A beam splitting system is used to split the laser into a measurement beam that enters the measurement optical path and a reference beam that enters the reference optical path; The measurement optical path includes a measurement beam emitted to the object under test via an optical probe and a measurement signal light reflected back. The measurement signal light beats with the local oscillator to generate a measurement beat frequency signal containing vibration information of the object under test and frequency shifter error information. A reference optical path in which the signal light in the reference beam beats the local oscillator light directly without being reflected by the object under test, so as to generate a reference beat frequency signal containing only frequency shifter error information. A signal processing system is configured to subtract the reference beat frequency signal from the measured beat frequency signal to eliminate common frequency shifter error information, thereby obtaining accurate vibration information of the object under test.

2. The laser vibrometer according to claim 1, characterized in that: The beam splitting system includes a first beam splitter, a second beam splitter, and a third beam splitter. The laser beam is split into a local oscillator beam and a signal beam by the first fiber beam splitter. The local oscillator beam and the signal beam enter the second and third fiber beam splitters, respectively, and are split into two paths again. One path is used as a measurement beam and enters the measurement optical path, while the other path is used as a reference beam and enters the reference optical path.

3. The laser vibrometer according to claim 2, characterized in that: The splitting ratio of the first fiber optic beam splitter is 1:99 to 20:

80.

4. The laser vibrometer according to claim 2, characterized in that: The signal processing system is configured to correct the reference beat frequency signal by setting different delay compensation amounts when the distance between the optical probe and the object under test changes.

5. The laser vibrometer according to claim 1, characterized in that: Both the measurement optical path and the reference optical path include a balanced detector for receiving beat frequency signals.

6. The laser vibrometer according to claim 1, characterized in that: The optical probe includes an indicator laser. The beam of the indicator laser and the beam of the measurement signal light are combined by a wavelength division multiplexer and then emitted from the optical probe.

7. The laser vibrometer according to claim 6, characterized in that: The optical probe integrates a waveplate for changing the polarization state of light. The waveplate and the polarization beam splitter together form a circulator structure to guide the transmission and reception path of the measurement signal light.

8. The laser vibrometer according to claim 1, characterized in that: An adjustable optical attenuator is provided in the optical path used to transmit the local oscillator light in the measurement optical path, which is used to adjust the optical power ratio between the local oscillator light and the measurement signal light.

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