Optical fiber sensing noise suppression system based on differential inverse frequency modulation

Abstract

The present invention discloses a fiber optic sensing noise suppression system based on differential reverse frequency modulation, comprising: a sensing optical path module comprising: a laser, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter, a sensing optical fiber, a reference optical fiber, a sensitive unit, a photoelectric conversion unit, a first modulation frequency output circuit, a second modulation frequency output circuit and a demodulation circuit. The fiber optic sensing noise suppression system based on differential reverse frequency modulation of the present invention simultaneously performs differential reverse frequency shift modulation on the sensing light and the reference light, so that at any time, the frequency band where the secondary Rayleigh scattering noise is located is different from the frequency band where the carrier signal is located, and the secondary Rayleigh scattering noise is filtered out by designing a demodulation circuit bandpass filter. At the same time, the modulation frequency adopts a method of driving with a homologous signal and synchronously changing the modulation frequency in the opposite direction, so that when the two beams of light interfere, the partial common mode of the dynamic change of the modulation frequency is offset, which greatly simplifies the demodulation circuit.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing systems, and in particular relates to an optical fiber sensing noise suppression system based on differential reverse frequency modulation. Background Art

[0002] Fiber optic sensing technology has become an important technical means in the fields of modern manufacturing, medical treatment, military, etc., among which heterodyne interferometry is one of the most important technologies in fiber optic sensing. It realizes the detection and demodulation of optical signals based on the principle of interference, and can achieve high-precision measurement of light parameters such as amplitude, phase, and frequency. It has the advantages of high sensitivity, high resolution, and good reliability. It is currently widely used in fiber optic communication, spectral analysis, biomedicine, materials science, and industrial manufacturing. Therefore, heterodyne interferometry technology has broad application prospects and market demand, and at the same time, it also puts forward higher requirements for its detection performance.

[0003] In practical applications, due to the secondary Rayleigh scattering effect in the optical fiber, this part of the scattered light will interact with the original signal, causing the phase and amplitude of the signal to change, thereby affecting the measurement accuracy and sensitivity of the optical heterodyne interference signal detection. At present, a method for eliminating the influence of secondary Rayleigh scattering by frequency modulation is proposed to address the influence of secondary Rayleigh scattering on heterodyne interference detection in optical fiber transmission, which is called frequency modulation and demodulation technology (FM-AM technology). Since secondary Rayleigh scattering is relatively uniformly distributed in the frequency range of the optical signal, by modulating the frequency of the optical signal, during the frequency modulation process, the secondary Rayleigh scattered signal will be modulated outside the detection bandwidth, while the measured signal falls within the detection bandwidth, so that the two can be effectively separated from the frequency band, and the noise caused by secondary Rayleigh scattering can be suppressed. However, frequency modulation and demodulation technology has certain requirements for parameters such as the frequency and modulation depth of the optical signal. If the frequency and modulation depth are not properly selected, the amplitude and phase information of the demodulated signal will be affected, reducing the sensitivity and accuracy of the system. At the same time, special demodulators and signal processing algorithms are required in the demodulation process, which increases the technical difficulty and computational complexity of the system, thereby limiting the practical application of frequency modulation and demodulation technology in eliminating secondary Rayleigh scattering. Summary of the invention

[0004] The present invention provides a fiber optic sensing noise suppression system based on differential inverse frequency modulation to solve the above-mentioned technical problems, and specifically adopts the following technical solutions:

[0005] An optical fiber sensing noise suppression system based on differential reverse frequency modulation comprises: a laser, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter, a sensing optical fiber, a reference optical fiber, a sensitive unit, a photoelectric conversion unit, a first modulation frequency output circuit, a second modulation frequency output circuit and a demodulation circuit;

[0006] The output of the laser is divided into two paths, one path is input into the first acousto-optic frequency shifter as sensing light, and the other path is input into the second acousto-optic frequency shifter as reference light, the first acousto-optic frequency shifter is respectively connected to the first modulation frequency output circuit and the sensing optical fiber, the sensing optical fiber is connected to the sensitive unit, the sensitive unit is used to convert the physical quantity to be detected into the phase change of the sensing light, the second acousto-optic frequency shifter is respectively connected to the second modulation frequency output circuit and the reference optical fiber, the sensing light output from the sensitive unit and the reference optical fiber interferes with the reference light beam, and then is input into the photoelectric conversion unit to be converted into a microwave electrical signal, and the converted microwave electrical signal is input into the demodulation circuit for demodulation;

[0007] The first modulation frequency output circuit and the second modulation frequency output circuit respectively output two frequency-synchronized, differentially reverse-changing microwave signals to the first acousto-optic frequency shifter and the second acousto-optic frequency shifter, and the first acousto-optic frequency shifter and the second acousto-optic frequency shifter perform frequency shift modulation on the optical signal based on the received microwave signal.

[0008] Furthermore, the demodulation circuit includes a bandpass filter, and the converted microwave electrical signal is input into the bandpass filter.

[0009] Furthermore, the demodulation circuit further includes a lock-in amplifier, an A / D converter and a first digital microcontroller connected in sequence, and the lock-in amplifier is connected to the bandpass filter.

[0010] Furthermore, the two channels of frequency synchronously differentially reversely changing homologous microwave signals output by the modulation frequency output circuit are dynamically changing frequency values, thereby dynamically changing frequency modulation is performed on the first acousto-optic frequency shifter and the second acousto-optic frequency shifter.

[0011] Furthermore, the first modulation frequency output circuit and the second modulation frequency output circuit use the same source signal.

[0012] Further, the first modulation frequency output circuit and the second modulation frequency output circuit are an integrated circuit, and the integrated circuit comprises: a crystal oscillator, a first frequency synthesizer, a second frequency synthesizer and a second digital microcontroller;

[0013] The crystal oscillator is connected to the first frequency synthesizer and the second frequency synthesizer, the second digital microcontroller is connected to the first frequency synthesizer and the second frequency synthesizer, the first frequency synthesizer is connected to the first acousto-optic frequency shifter, and the second frequency synthesizer is connected to the second acousto-optic frequency shifter.

[0014] Furthermore, the first frequency synthesizer and the second frequency synthesizer are direct digital frequency synthesizers.

[0015] Furthermore, the photoelectric conversion unit is a photodiode.

[0016] Furthermore, the laser generates a narrow linewidth optical signal and generates two outputs through a coupler, which are respectively input into the first acousto-optic frequency shifter and the second acousto-optic frequency shifter.

[0017] The invention is beneficial in that the fiber optic sensing noise suppression system based on differential reverse frequency modulation is provided, and the sensing light and the reference light in the fiber heterodyne interference system are simultaneously differentially reversed frequency modulated, so that at any time, the frequency band of the secondary Rayleigh scattering noise generated by the interference between the sensing light and the secondary Rayleigh scattered light, and between the reference light and the secondary Rayleigh scattered light is different from the frequency band of the signal after the direct interference between the sensing light and the reference light. Then, by designing the filter bandwidth of the post-photodiode, the secondary Rayleigh scattering noise is filtered out.

[0018] The benefit of the present invention lies in that the fiber optic sensing noise suppression system based on differential reverse frequency modulation is provided, which adopts the method of homologous signal driving, frequency synchronization generation, and reverse change of modulation frequency, so that when two light beams interfere, the part of the modulation frequency that changes dynamically can be eliminated in common mode, which avoids the introduction of noise due to the dynamic change of modulation frequency on the one hand, and greatly simplifies the design difficulty of the demodulation circuit on the other hand. Ultimately, the detection signal-to-noise ratio of the long-distance fiber optic sensing system can be improved to ensure the detection performance of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0020] Figure 1 is a schematic diagram of an optical fiber sensing noise suppression system based on differential reverse frequency modulation of the present invention;

[0021] Figure 2 is a schematic diagram of a modulation frequency output circuit according to an embodiment of the present invention;

[0022] Figure 3 It is a schematic diagram of the formation of secondary Rayleigh scattering noise in optical fiber sensing;

[0023] Figure 4 is a schematic diagram of a demodulation circuit according to an embodiment of the present invention;

[0024] Laser 1, first acousto-optic frequency shifter 2, second acousto-optic frequency shifter 3, sensing optical fiber 4, reference optical fiber 5, sensitive unit 6, photoelectric conversion unit 7, first modulation frequency output circuit 8, second modulation frequency output circuit 9, demodulation circuit 10. DETAILED DESCRIPTION

[0025] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0026] like Figure 1 The present invention shows an optical fiber sensing noise suppression system based on differential reverse frequency modulation, which includes a laser 1, a first acousto-optic frequency shifter 2, a second acousto-optic frequency shifter 3, a sensing optical fiber 4, a reference optical fiber 5, a sensitive unit 6, a photoelectric conversion unit 7, a first modulation frequency output circuit 8, a second modulation frequency output circuit 9 and a demodulation circuit 10. The laser 1, the first acousto-optic frequency shifter 2, the second acousto-optic frequency shifter 3, the sensing optical fiber 4, the reference optical fiber 5, the sensitive unit 6 and the photoelectric conversion unit 7 constitute a sensing optical path module, and the first modulation frequency output circuit 8, the second modulation frequency output circuit 9 and the demodulation circuit 10 constitute a modulation and demodulation circuit module.

[0027] The output of the laser 1 is divided into two paths, one path is input as sensing light into the first acousto-optic frequency shifter 2, and the other path is input as reference light into the second acousto-optic frequency shifter 3. The first acousto-optic frequency shifter 2 is respectively connected to the first modulation frequency output circuit 8 and the sensing optical fiber 4, the sensing optical fiber 4 is connected to the sensitive unit 6, and the sensitive unit 6 is used to convert the physical quantity to be detected into the phase change of the sensing light, and the second acousto-optic frequency shifter 3 is respectively connected to the second modulation frequency output circuit 9 and the reference optical fiber 5.

[0028] The first modulation frequency output circuit 8 and the second modulation frequency output circuit 9 respectively output two frequency-synchronous, differentially reverse-changing microwave signals to the first acousto-optic frequency shifter 2 and the second acousto-optic frequency shifter 3. The first acousto-optic frequency shifter 2 and the second acousto-optic frequency shifter 3 perform frequency shift modulation on the optical signal based on the received microwave signal.

[0029] After the sensing light output by the sensitive unit 6 and the reference optical fiber 5 interferes with the reference light beam, it is input to the photoelectric conversion unit 7 to be converted into a microwave electrical signal, and the converted microwave electrical signal is input to the demodulation circuit 10 for demodulation, and the phase change sensed by the sensitive unit 6 is extracted, thereby obtaining information of external physical quantities. Specifically, the photoelectric conversion unit 7 is a photodiode.

[0030] In the embodiment of the present application, the laser 1 is a narrow linewidth laser 1, and the narrow linewidth ensures the detection noise floor performance of the entire optical fiber sensing system. The laser 1 generates a narrow linewidth optical signal and generates two outputs through a coupler, which are respectively input into the first acousto-optic frequency shifter 2 and the second acousto-optic frequency shifter 3. The laser 1 outputs two narrow linewidth optical signals, and its frequency value is expressed as .

[0031] The spatial angle of the diffracted light output by the acousto-optic frequency shifter is related to its sideband. After diffraction, the positive and negative first-order sidebands are separated in space. When the frequency of the microwave signal input to the first acousto-optic frequency shifter 2 is , the frequency of the microwave signal input to the second acousto-optic frequency shifter 3 is When , the two acousto-optic frequency shifter outputs select the positive first-order sideband and the negative first-order sideband respectively, that is, the optical signal frequency of the sensor light output is , the optical signal frequency of the reference light output is .

[0032] In the implementation of the present application, the first modulation frequency output circuit 8 and the second modulation frequency output circuit 9 use the same source signal. Specifically, the same source signal is used to drive, the frequency is generated synchronously, and the modulation frequency changes inversely. Figure 2 As shown, the first modulation frequency output circuit 8 and the second modulation frequency output circuit 9 are an integrated circuit, and the integrated circuit includes: a crystal oscillator, a first frequency synthesizer, a second frequency synthesizer, and a second digital microcontroller. The crystal oscillator is connected to the first frequency synthesizer and the second frequency synthesizer, the second digital microcontroller is connected to the first frequency synthesizer and the second frequency synthesizer, the first frequency synthesizer is connected to the first acousto-optic frequency shifter 2, and the second frequency synthesizer is connected to the second acousto-optic frequency shifter 3.

[0033] The two-way frequency synchronous differential reverse-changing homologous microwave signals output by the modulation frequency output circuit are dynamically changing frequency values, thereby dynamically changing frequency modulation is performed on the first acousto-optic frequency shifter 2 and the second acousto-optic frequency shifter 3. Specifically, the crystal oscillator is preferably a low phase noise crystal oscillator. The first frequency synthesizer and the second frequency synthesizer are direct digital frequency synthesizers (DDS). The clock source selects a low phase noise crystal oscillator and divides it into two outputs, and synchronizes the working state of the two DDS. The change in modulation frequency is a periodic change, and a frequency change waveform such as a sine wave or a triangle can be selected. In the present application, the digital microcontroller synchronously configures the two DDSs according to the preset triangular wave frequency change waveform, so as to achieve the following. Figure 2 The two frequency signal outputs shown are expressed as and .

[0034] The first acousto-optic frequency shifter 2 and the second acousto-optic frequency shifter 3 input the modulation frequency output of the first modulation frequency output circuit 8 and the second modulation frequency output circuit 9 respectively. and , and frequency shift modulation is performed on the optical signal. The outputs of the two acousto-optic frequency shifters respectively select the positive first-order sideband and the negative first-order sideband, that is, the frequency of the sensing optical signal output by the first acousto-optic frequency shifter 2 is , the reference optical signal frequency output by the second acousto-optic frequency shifter 3 is .

[0035] The sensing fiber 4 and the reference fiber 5 generate Rayleigh scattering due to the slight inhomogeneity in the optical fiber (such as slight wavelength deviation, material composition difference, etc.). The back-Rayleigh scattered light propagates in the opposite direction along the optical fiber, and the secondary Rayleigh scattered light caused by it will continue to propagate along the optical fiber, thereby forming secondary Rayleigh scattering noise at the place where the two light beams are combined.

[0036] The sensing light is transmitted in the sensing optical fiber 4, as shown in the attached Figure 3 As shown, assuming that the optical signal is in the optical fiber Rayleigh scattering occurs at Secondary Rayleigh scattering occurs at , and the time delay caused by Rayleigh scattering is Since the frequency of the sensing light in the sensing optical fiber 4 is The scattered light that reaches the beam interference point after secondary Rayleigh scattering will change dynamically with the time period. This produces a frequency difference , thereby distinguishing the secondary Rayleigh scattering noise after interference from the target detection signal in terms of frequency band.

[0037] The reference light is transmitted in the reference optical fiber 5, similarly Figure 3 As shown, assuming that the optical signal is in the optical fiber Rayleigh scattering occurs at Secondary Rayleigh scattering occurs at , and the time delay caused by Rayleigh scattering is Since the reference light frequency in the reference optical fiber 5 is The scattered light that reaches the beam interference point after secondary Rayleigh scattering will change dynamically with the time period. This produces a frequency difference , thereby distinguishing the secondary Rayleigh scattering noise after interference from the target detection signal in terms of frequency band.

[0038] The sensitive unit 6 converts the change of the external measured physical quantity into the change of the optical signal phase, frequency and other parameters, which are characterized by frequency here, and the frequency change caused by it is expressed as The frequency of the sensing light signal output after passing through the sensitive unit 6 can be expressed as .

[0039] The photodiode converts the optical signal after the interference between the sensing light output by the sensitive unit 6 and the reference light output by the reference optical fiber 5 into a microwave electrical signal. Figure 4 As shown, the frequency of this microwave signal can be expressed as , the part where the modulation frequency changes dynamically As the common mode part is cancelled when the optical signal interferes, the microwave signal only contains the microwave carrier frequency. And the frequency information reflecting the changes of the external measured physical quantity In addition, the carrier frequency of the secondary Rayleigh scattering noise generated by the interference between the sensing light and the secondary Rayleigh scattered light, and between the reference light and the secondary Rayleigh scattered light can be expressed as .

[0040] like Figure 4 As shown, in the embodiment of the present application, the demodulation circuit 10 includes a bandpass filter, and the converted microwave electrical signal is input into the bandpass filter. The center is ,and The demodulation circuit 10 also includes a lock-in amplifier, an A / D converter, and a first digital microcontroller connected in sequence, and the lock-in amplifier is connected to the bandpass filter. The demodulation circuit 10 synchronously demodulates the microwave signal output by the photodiode, and uses a lock-in amplifier to extract the frequency change information caused by the external measured physical quantity. , thus ultimately achieving high-precision photoelectric sensing of external measured physical quantities.

[0041] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form, and any technical solution obtained by equivalent replacement or equivalent transformation falls within the protection scope of the present invention.

Claims

1. A fiber optic sensing noise suppression system based on differential inverse frequency modulation, characterized in that: It includes: a laser, a first acousto-optic frequency shifter, a second acousto-optic frequency shifter, a sensing optical fiber, a reference optical fiber, a sensitive unit, a photoelectric conversion unit, a first modulation frequency output circuit, a second modulation frequency output circuit and a demodulation circuit; The output of the laser is divided into two paths, one path is input into the first acousto-optic frequency shifter as sensing light, and the other path is input into the second acousto-optic frequency shifter as reference light, the first acousto-optic frequency shifter is respectively connected to the first modulation frequency output circuit and the sensing optical fiber, the sensing optical fiber is connected to the sensitive unit, the sensitive unit is used to convert the physical quantity to be detected into the phase change of the sensing light, the second acousto-optic frequency shifter is respectively connected to the second modulation frequency output circuit and the reference optical fiber, the sensing light output from the sensitive unit and the reference optical fiber interferes with the reference light beam, and then is input into the photoelectric conversion unit to be converted into a microwave electrical signal, and the converted microwave electrical signal is input into the demodulation circuit for demodulation; The first modulation frequency output circuit and the second modulation frequency output circuit respectively output two microwave signals with frequency synchronization, differential and reverse changes to the first acousto-optic frequency shifter and the second acousto-optic frequency shifter, and the first acousto-optic frequency shifter and the second acousto-optic frequency shifter perform frequency shift modulation on the optical signal based on the received microwave signal; The modulation frequency of the first modulation frequency output circuit inputting the first acousto-optic frequency shifter is The modulation frequency of the second acousto-optic frequency shifter input to the second modulation frequency output circuit is , to perform frequency shift modulation on the optical signal, the first acousto-optic frequency shifter and the second acousto-optic frequency shifter output respectively select the positive first-order sideband and the negative first-order sideband, and the frequency of the sensing light signal output by the first acousto-optic frequency shifter is , the reference optical signal frequency output by the second acousto-optic frequency shifter is , where f c is the frequency of the optical signal output by the laser.

2. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 1, characterized in that: The demodulation circuit comprises a bandpass filter, and the converted microwave electrical signal is input into the bandpass filter.

3. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 2, characterized in that: The demodulation circuit further includes a lock-in amplifier, an A / D converter and a first digital microcontroller connected in sequence, and the lock-in amplifier is connected to the bandpass filter.

4. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 1, characterized in that: The two channels of frequency synchronously differentially reversely changing homologous microwave signals output by the modulation frequency output circuit are dynamically changing frequency values, thereby dynamically changing frequency modulation is performed on the first acousto-optic frequency shifter and the second acousto-optic frequency shifter.

5. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 1, characterized in that: The first modulation frequency output circuit and the second modulation frequency output circuit use the same source signal.

6. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 5, characterized in that: The first modulation frequency output circuit and the second modulation frequency output circuit are an integrated circuit, and the integrated circuit comprises: a crystal oscillator, a first frequency synthesizer, a second frequency synthesizer and a second digital microcontroller; The crystal oscillator is connected to the first frequency synthesizer and the second frequency synthesizer, the second digital microcontroller is connected to the first frequency synthesizer and the second frequency synthesizer, the first frequency synthesizer is connected to the first acousto-optic frequency shifter, and the second frequency synthesizer is connected to the second acousto-optic frequency shifter.

7. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 6, characterized in that: The first frequency synthesizer and the second frequency synthesizer are direct digital frequency synthesizers.

8. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 1, characterized in that: The photoelectric conversion unit is a photodiode.

9. The fiber optic sensing noise suppression system based on differential inverse frequency modulation according to claim 1, characterized in that: The laser generates a narrow linewidth optical signal and generates two outputs through a coupler, which are respectively input into the first acousto-optic frequency shifter and the second acousto-optic frequency shifter.

Images