A high-precision FBG sensing device and method based on optical signal frequency shift modulation

Abstract

The application discloses a high-precision FBG sensing device and method based on optical signal frequency shift modulation, relates to the fields of fiber Bragg grating sensing and optical signal frequency shift modulation, and comprises a light source (1), an optical frequency shift modulator (2), a sweep frequency radio frequency source (3), a 90-degree electric bridge (4), a probe light output optical fiber (5), an optical circulator (6), an FBG sensor (7), an FBG sensor array (8), a sensing light receiving optical fiber (9), a high-speed photoelectric detector (10), a data acquisition device (11), a data processing device (12), a light source synchronous control signal line (13) and a radio frequency source synchronous control signal line (14). The high-precision FBG sensing method of the application replaces the vibration applied at the sensor by constructing the frequency shift modulated incident light signal, avoids the problems of complex sensor structure and temperature sensing only, and thus directly improves the strain and temperature sensing precision on the FBG sensor without special design or the FBG sensor already completed.

Description

Technical Field

[0001] This invention relates to the fields of fiber Bragg grating sensing and optical signal frequency shift modulation, and particularly to a high-precision FBG sensing device and method based on optical signal frequency shift modulation. Background Technology

[0002] Fiber Bragg grating (FBG) sensing technology, due to its wavelength demodulation, is less affected by factors such as light source power fluctuations, fiber connection losses, and light polarization states. It exhibits stable performance in measuring external physical quantities such as temperature and strain, and possesses good multiplexing capabilities and resistance to electromagnetic interference, making it widely used in fields such as medicine, power, and structural health monitoring. With increasing demands for measurement accuracy, high-precision FBG sensing methods that overcome the limitations of FBG reflection spectral width have attracted growing attention from researchers both domestically and internationally. Traditional high-precision sensing methods apply periodic strain to the FBG while simultaneously acquiring the time-domain variation signals of light intensity at various wavelengths in the spectrum. The ratio of the second harmonic component to the first harmonic component after Fourier transform of the time-domain signal at each wavelength is then calculated to obtain the intensity ratio curve for each wavelength, which is used to replace the FBG spectrum. Although this method significantly reduces the FBG spectral width and effectively improves temperature sensing accuracy, it requires additional periodic vibrations to be applied to the FBG sensor as a prerequisite for generating the periodic time-domain signals at each point, thus limiting its application to temperature sensing and preventing strain sensing. Because each sensor requires a vibration device, the structure is complex, and the important advantages of FBG, such as electromagnetic insulation, resistance to high humidity and heat, and high safety, are diminished, limiting its application scenarios. Furthermore, when upgrading existing FBG sensors that do not employ high-precision sensing methods, the only option is to remove and reinstall them, further restricting the ability to upgrade from a standard FBG sensing system to a high-precision FBG sensing system. Summary of the Invention

[0003] This invention aims to overcome the shortcomings of the prior art by proposing a high-precision FBG sensing device and method based on optical signal frequency shift modulation. By applying rapid periodic frequency shift modulation to a narrow-linewidth laser with wavelength scanning, a frequency-shift modulated optical signal is constructed. This optical signal is injected into the sensing fiber where the FBG is located, and the corresponding reflected or transmitted optical signals are received and collected. This generates a periodically changing time-domain light intensity signal at each wavelength point of the FBG, which is used to calculate the ratio of the second harmonic component to the first harmonic component of the time-domain signal at each wavelength point, thus breaking through the limitation of the FBG spectral width on sensing accuracy.

[0004] The technical solution adopted in this invention is as follows:

[0005] A high-precision FBG sensing device based on optical signal frequency shift modulation includes a light source 1, an optical frequency shift modulator 2, a swept frequency radio frequency source 3, a 90-degree bridge 4, a probe light output fiber 5, an optical circulator 6, an FBG sensor array 8 composed of multiple FBG sensors 7, a sensing light receiving fiber 9, a high-speed photodetector 10, a data acquisition device 11, a data processing device 12, a light source synchronization control signal line 13, and a radio frequency source synchronization control signal line 14.

[0006] Light source 1 is used to emit a narrow-linewidth laser with a wavelength that steps with time under the control of a synchronization control signal, and outputs it to optical frequency shift modulator 2; wherein, during the Nth sensing, the wavelength of the output narrow-linewidth laser is λ. N ;

[0007] The frequency sweeping radio frequency source 3 is used to generate a frequency sweeping signal under the control of the synchronization control signal and output it to the 90-degree bridge 4;

[0008] The 90-degree bridge 4 is used to divide the sweep frequency signal into two sweep frequency modulation signals, I and Q, and output them to the optical frequency shift modulator 2;

[0009] Optical frequency shift modulator 2 is used to modulate the input wavelength λ under the control of a sweep frequency modulation signal. N A narrow-linewidth laser is periodically frequency-shifted and reciprocated, generating a periodically frequency-shifted modulated optical signal. This signal is then sequentially injected into an FBG sensor array 8 composed of FBG sensors 7 through a probe output fiber 5 and an optical circulator 6. The wavelength of the periodically frequency-shifted modulated optical signal is within λ′. N -Δλ~λ′ N It changes back and forth within the range of +Δλ. f0 is the center frequency of the frequency variation range, Δf is the amplitude of the frequency variation, and c is the speed of light in vacuum; the wavelength cyclic variation period is T, and the corresponding main frequency of the timing signal is F0 = 1 / T;

[0010] The FBG sensor array 8 is used to return the sensing light carrying the spectral information of each FBG sensor 7 in the array. The returned sensing light passes through the optical circulator 6 and the sensing light receiving fiber 9 in sequence and enters the high-speed photodetector 10.

[0011] The high-speed photodetector 10 is used to convert the sensing light into an electrical signal, which is then recorded by the data acquisition device 11 and transmitted to the data processing device 12.

[0012] The data processing device 12 is used to perform high-precision demodulation of the spectral information of each FBG sensor 7, obtain high-precision strain and temperature sensing information based on the demodulated information, and then send out synchronization control signals, which are transmitted to the light source 1 and the sweep frequency radio frequency source 3 respectively through the light source synchronization control signal line 13 and the radio frequency source synchronization control signal line 14.

[0013] A high-precision FBG sensing method based on optical signal frequency shift modulation includes the following steps:

[0014] Step 1: Under the control of the synchronization control signal, the light source emits light with a wavelength of λ. N The narrow-linewidth laser enters the optical frequency shift modulator; the sweep frequency source emits periodic cyclic sweep signals under the control of the synchronization control signal, and is converted into I and Q sweep frequency modulation signals by a 90-degree bridge, and then enters the optical frequency shift modulator.

[0015] Step 2: Under the control of the sweep modulation signal, the optical frequency-shift modulator periodically reciprocates the input narrow-linewidth laser, outputting a periodically frequency-shift modulated optical signal. This signal serves as the probe light, sequentially passing through the probe output fiber and an optical circulator before being injected into an FBG sensor array composed of FBG sensors. The wavelength of the periodically frequency-shift modulated optical signal varies with the frequency-shift modulation signal at λ′. N -Δλ and λ′ N The periodic cyclical change between +Δλ f0 is the center frequency of the frequency variation range, Δf is the amplitude of the frequency variation, and c is the speed of light in vacuum; the wavelength cyclic variation period is T, and the corresponding main frequency of the timing signal is F0 = 1 / T;

[0016] Step 3: In the FBG sensor array, when the center wavelength λ′ of the probe light... N When the light is positioned at different positions in the reflection spectrum of the FBG sensor, the returned sensing light carries the spectral information of each FBG sensor. The returned sensing light passes sequentially through the optical circulator and the sensing light receiving fiber into the high-speed photodetector. The high-speed photodetector converts the sensing light into an electrical signal, which is recorded by the data acquisition device and transmitted to the data processing device. The electrical signal contains the time-domain signal waveform of the sensing light with spectral information. The closer to the spectral peak, the more second harmonic components with frequency 2F0 and the fewer first harmonic components with frequency F0 in the time-domain signal waveform.

[0017] Step 4: The data processing device performs a Fast Fourier Transform on the time-domain spectral information of each FBG sensor to obtain the frequency-domain signal. It then divides the 2F0 frequency component I(2F0) by the F0 frequency component I(F0) to obtain the intensity ratio S(λ′) at the corresponding wavelength position. N ) = I(2F0) / I(F0);

[0018] Step 5: After completing the intensity ratio calculation at a wavelength position, the data processing device sends a synchronization control signal, which is transmitted to the light source and the frequency sweeping radio frequency source respectively through the light source synchronization control signal line and the radio frequency source synchronization control signal line. The light source steps the wavelength of the narrow linewidth laser to the next wavelength position, while the radio frequency source continues to synchronously output the frequency sweep signal, returning to step 1.

[0019] Compared with the prior art, the high-precision FBG sensing device and method based on optical signal frequency shift modulation of the present invention has the following positive effects:

[0020] 1. By periodically frequency-shifting and modulating a narrow-linewidth laser with wavelength scanning, a frequency-shifting and modulated optical signal is constructed as a probe light, replacing the traditional method that requires the vibration of an FBG sensor to generate peak shifts. This generates time-domain signals of various wavelengths, avoiding the problems of complex high-precision FBG sensor structures and the inability to sense only temperature.

[0021] 2. By dividing the second harmonic component by the first harmonic component of the time-domain signal corresponding to each wavelength position, the resulting intensity ratio curve significantly reduces the width of the reflection peak or transmission peak of the FBG sensor, breaking through the limitation of the FBG spectral width on the sensing accuracy and effectively improving the FBG sensing accuracy.

[0022] 3. In the process of calculating the relative curve, the signal intensity at two point frequency positions, the second harmonic and the first harmonic, in the frequency domain is used to remove the influence of white noise introduced during the acquisition of light intensity signal on the demodulation result, effectively avoiding the problem of reduced sensing accuracy when the optical signal-to-noise ratio of FBG signal is low.

[0023] 4. By constructing a frequency-shift modulated optical signal from the incident light, the vibration applied to the sensor is replaced by the traditional method, avoiding the problems of complex high-precision FBG sensor structure and the inability to sense only temperature. This allows for direct improvement of strain and temperature accuracy on FBG sensors that do not require special design. For ordinary FBG sensors that have already been mounted, there is no need to remove or replace them. Simply connect this sensing device to the FBG sensor to achieve improved sensing accuracy. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of a high-precision FBG sensing device based on optical signal frequency shift modulation according to the present invention;

[0025] Figure 2 This is a schematic diagram showing the frequency change of a periodic cyclic sweep signal emitted by a sweep frequency radio frequency source over time.

[0026] Figure 3 This is a schematic diagram of periodic frequency shift modulation optical signal generation;

[0027] Figure 4 This is a schematic diagram illustrating the principle of time-domain signal generation by periodic frequency-shift modulated light at different center wavelength positions;

[0028] Figure 5 It is the sensing optical time-domain signal waveform that is far from the peak position of the FBG reflection peak or transmission peak.

[0029] Figure 6It is the time-domain signal waveform of the sensed light corresponding to the peak position of the FBG reflection peak or transmission peak.

[0030] Figure 7 This is a comparison diagram of the intensity ratio curve obtained by demodulation in this invention and the FBG spectrum;

[0031] Figure 8 This is a magnified view of the peak position of the intensity ratio curve obtained by demodulation in this invention and the comparison diagram of FBG spectrum. Detailed Implementation

[0032] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are implemented based on the technical solution of the present invention and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0033] Example 1: High-precision FBG sensing device based on optical signal frequency shift modulation

[0034] like Figure 1 The diagram shown is a schematic of the device. The device includes a light source 1, an optical frequency shift modulator 2, a swept-frequency radio frequency source 3, a 90-degree bridge 4, a probe light output fiber 5, an optical circulator 6, an FBG sensor array 8 consisting of multiple FBG sensors 7, a sensing light receiving fiber 9, a high-speed photodetector 10, a data acquisition device 11, a data processing device 12, a light source synchronization control signal line 13, and a radio frequency source synchronization control signal line 14.

[0035] in:

[0036] Light source 1: A narrow-linewidth laser (linewidth 10kHz~100MHz) with wavelength-stepped scanning (step size 0.1pm~0.1nm) is used to emit stepped narrow-linewidth laser light under the control of a synchronization control signal, and the output is sent to optical frequency shift modulator 2; wherein, during the Nth sensing, the wavelength of the output narrow-linewidth laser is λ. N .

[0037] Optical frequency shift modulator 2: Employs a lithium niobate crystal electro-optic frequency shift modulator with a maximum operating frequency exceeding 20 GHz, used to modulate the input wavelength λ under the drive of a sweep frequency modulation signal. N A narrow-linewidth laser is periodically frequency-shifted and reciprocated to generate a periodically frequency-shifted modulated optical signal, which is then injected as probe light into an FBG sensor array 8 composed of FBG sensors 7 through a probe light output fiber 5 and an optical circulator 6.

[0038] Frequency sweeping source 3: A frequency sweeping source with a maximum sweeping range of 5GHz to 20GHz is used to generate a sweeping modulation signal to drive the optical frequency shift modulator under the control of the synchronization control signal, and output it to the 90-degree bridge 4.

[0039] 90-degree bridge 4: A 90-degree bridge with a working frequency band covering the set sweep frequency range is used to convert a single sweep frequency modulation signal into two sweep frequency modulation signals, I and Q, required by the optical frequency shift modulator, and output them to the optical frequency shift modulator 2.

[0040] Detection light output fiber 5: adopts standard single-mode fiber, and the transmission light wavelength range covers the detection light wavelength.

[0041] Optical Circulator 6: Uses a standard fiber optic circulator, with the working optical wavelength covering the probe optical wavelength.

[0042] FBG Sensor 7: Made of FBG fiber grating, it includes a temperature sensor and a stress sensor. The reflection or transmission peak is within the detection range of the probe light, which is used to convert the physical quantity to be sensed into a shift in the wavelength of the reflection or transmission peak.

[0043] FBG sensor array 8: Composed of FBG sensors 7 connected in series, with each sensor's reflection or transmission peak maintaining a safe distance; used to return sensing light carrying the spectral information of each FBG sensor 7 in the array, the returned sensing light passes sequentially through optical circulator 6 and sensing light receiving fiber 9 into high-speed photodetector 10.

[0044] Fiber 9 for sensing light receiving: It adopts standard single-mode fiber, and the transmission light wavelength range covers the detection light wavelength.

[0045] High-speed photodetector 10: It adopts a high-speed photodetector with a maximum response frequency of 100MHz to 200MHz, and the range of received light wavelengths covers the detection light wavelength; it is used to convert the sensing light into an electrical signal, which is then recorded by the data acquisition device 11 and transmitted to the data processing device 12.

[0046] Data acquisition device 11: A data acquisition card is used as the acquisition device, with a maximum sampling rate of 20MSps to 1GSps, for acquiring time-domain signals converted by the photodetector.

[0047] Data processing device 12: Using a computer, FPGA, or ARM as the data processing device, it can perform data processing such as fast Fourier transform, demodulate and calculate the intensity ratio curve, calculate the peak change, and convert it into the temperature or strain value to be measured; it is used to perform high-precision demodulation of the spectral information of each FBG sensor 7, obtain high-precision strain and temperature sensing information based on the demodulated information, and then send out synchronization control signals, which are transmitted to the light source 1 and the sweep frequency radio frequency source 3 respectively through the light source synchronization control signal line 13 and the radio frequency source synchronization control signal line 14.

[0048] Light source synchronization control signal line 13: Uses a standard coaxial cable to transmit synchronization signals for controlling the light source.

[0049] RF source synchronization control signal line 14: Uses a standard coaxial cable to transmit synchronization signals for controlling the RF element.

[0050] Example 2: High-precision FBG sensing method based on optical signal frequency shift modulation

[0051] The method includes the following steps:

[0052] Step 1: Under the control of the synchronization control signal, the light source emits light with a wavelength of λ. N A narrow-linewidth laser beam enters the optical frequency-shift modulator; the swept-frequency radio source emits light under the control of a synchronization control signal. Figure 2 The periodic cyclic sweep signal shown is a sinusoidal signal with a frequency varying between f0-Δf and f0+Δf, where f0 is the center frequency of the frequency range and Δf is the amplitude of the frequency variation. The periodic cyclic sweep signal is converted into I and Q sweep modulation signals by a 90-degree bridge and then enters the optical frequency shift modulator.

[0053] Step 2: The optical frequency shift modulator, under the control of the frequency sweep modulation signal, performs periodic reciprocating frequency sweep modulation on the input narrow linewidth laser, and outputs... Figure 3 The periodically frequency-shift modulated optical signal shown is used as the probe light, which is sequentially injected into an FBG sensor array composed of FBG sensors after passing through the probe light output fiber and an optical circulator. The wavelength of the periodically frequency-shift modulated optical signal varies with the frequency-shift modulation signal at λ′. N -Δλ and λ′ N The periodic cyclical changes between +Δλ, where λ' M The modulated center wavelength at the Nth measurement satisfies... Δλ is the amplitude of the modulated wavelength scan, which satisfies c is the speed of light in a vacuum. The wavelength scanning amplitude value is selected between 0.05nm and 0.5nm depending on the grating conditions, the long cycle variation period T is 100ns to 100μs, and the corresponding main frequency F0 is 10kHz to 10MHz; Figure 3 λ′ M The center wavelength of the modulated output during the Mth measurement is given. A complete spectral measurement process requires multiple measurements, and the modulated center wavelength needs to cover the spectral range to be measured.

[0054] Step 3: In the FBG sensor array, such as Figure 4 As shown, when the center wavelength of the probe light is λ′ N At different positions in the FBG sensor's reflection spectrum, the periodic reciprocating change of wavelength results in different waveforms of reflected light intensity. The returned sensing light, carrying spectral information from each FBG sensor, passes sequentially through an optical circulator and a sensing light receiving fiber before entering a high-speed photodetector. The high-speed photodetector converts the sensing light into... Figure 5 and Figure 6 The electrical signal shown is recorded by the data acquisition device and transmitted to the data processing device. The electrical signal is a time-domain signal waveform of the sensing light containing spectral information. The closer to the spectral peak, the more second harmonic components with frequency 2F0 and fewer first harmonic components with frequency F0 in the time-domain signal waveform. Conversely, the farther away from the peak region, the more first harmonic components with frequency F0 and fewer second harmonic components with frequency 2F0 in the time-domain signal waveform.

[0055] Step 4: The data processing device performs a Fast Fourier Transform on the time-domain spectral information of each FBG sensor to obtain the frequency-domain signal. It then divides the 2F0 frequency component I(2F0) by the F0 frequency component I(F0) to obtain the intensity ratio S(λ′) at the corresponding wavelength position. N ) = I(2F0) / I(F0);

[0056] Step 5: After calculating the intensity ratio at a given wavelength, the data processing device sends a synchronization control signal, which is transmitted to the light source and the sweeping RF source respectively via the light source synchronization control signal line and the RF source synchronization control signal line. The light source steps the wavelength of the narrow-linewidth laser to the next wavelength position, while the RF source continues to synchronously output the sweeping signal. Returning to Step 1, the spectral information at the next wavelength position is demodulated, completing the intensity ratio calculation at the corresponding position. This process is repeated to demodulate the spectral information at each wavelength position within the range of the detected spectrum, resulting in... Figure 7 , Figure 8 The intensity ratio curve S(λ) shown by the dashed line has the same peak value as the grating peak value. It also significantly reduces the width of the grating reflection peak or transmission peak, thereby achieving high-precision peak position demodulation. Based on the wavelength correspondence between the FBG sensor and the sensed physical quantities such as strain and temperature, high-precision FBG sensing is achieved.

Claims

1. A high-precision FBG sensing device based on optical signal frequency shift modulation, characterized in that, It includes a light source (1), an optical frequency shift modulator (2), a swept frequency radio frequency source (3), a 90-degree bridge (4), a probe light output fiber (5), an optical circulator (6), an FBG sensor array (8) composed of multiple FBG sensors (7), a sensing light receiving fiber (9), a high-speed photodetector (10), a data acquisition device (11), a data processing device (12), a light source synchronization control signal line (13), and a radio frequency source synchronization control signal line (14); The light source (1) is used to emit a narrow-linewidth laser with a wavelength that steps with time under the control of a synchronization control signal, and output it to the optical frequency shift modulator (2); wherein, during the Nth sensing, the output narrow-linewidth laser wavelength is λ. N ; The sweep frequency source (3) is used to generate a sweep frequency signal under the control of the synchronization control signal and output it to the 90-degree bridge (4); The 90-degree bridge (4) is used to divide the sweep frequency signal into two sweep frequency modulation signals, I and Q, and output them to the optical frequency shift modulator (2); The optical frequency shift modulator (2) is used to modulate the input wavelength λ under the control of the sweep frequency modulation signal. N A narrow-linewidth laser is periodically frequency-shifted and reciprocated to generate a periodically frequency-shifted modulated optical signal. This signal is then injected into an FBG sensor array (8) composed of FBG sensors (7) via a probe output fiber (5) and an optical circulator (6). The wavelength of the periodically frequency-shifted modulated optical signal is within λ′. N -Δλ~λ′ N It changes back and forth within the range of +Δλ. f0 is the center frequency of the frequency variation range, Δf is the amplitude of the frequency variation, and c is the speed of light in vacuum; the wavelength cyclic variation period is T, and the corresponding main frequency of the timing signal is F0 = 1 / T; The FBG sensor array (8) is used to return the sensing light carrying the spectral information of each FBG sensor (7) in the array. The returned sensing light passes through the optical circulator (6) and the sensing light receiving fiber (9) in sequence and enters the high-speed photodetector (10). A high-speed photodetector (10) is used to convert the sensing light into an electrical signal, which is then recorded by the data acquisition device (11) and transmitted to the data processing device (12). The data processing device (12) is used to perform high-precision demodulation of the spectral information of each FBG sensor (7), obtain high-precision strain and temperature sensing information based on the demodulated information, and then send out synchronization control signals, which are transmitted to the light source (1) and the sweep frequency radio frequency source (3) respectively through the light source synchronization control signal line (13) and the radio frequency source synchronization control signal line (14).

2. A high-precision FBG sensing method based on optical signal frequency shift modulation, characterized in that, Includes the following steps: Step 1: Under the control of the synchronization control signal, the light source emits light with a wavelength of λ. N The narrow-linewidth laser enters the optical frequency shift modulator; the sweep frequency source emits periodic cyclic sweep signals under the control of the synchronization control signal, and is converted into I and Q sweep frequency modulation signals by a 90-degree bridge, and then enters the optical frequency shift modulator. Step 2: Under the control of the sweep modulation signal, the optical frequency-shift modulator periodically reciprocates the input narrow-linewidth laser, outputting a periodically frequency-shift modulated optical signal. This signal serves as the probe light, sequentially passing through the probe output fiber and an optical circulator before being injected into an FBG sensor array composed of FBG sensors. The wavelength of the periodically frequency-shift modulated optical signal varies with the frequency-shift modulation signal at λ′. N -Δλ and λ′ N The periodic cyclical change between +Δλ f0 is the center frequency of the frequency variation range, Δf is the amplitude of the frequency variation, and c is the speed of light in vacuum; the wavelength cyclic variation period is T, and the corresponding main frequency of the timing signal is F0 = 1 / T; Step 3: In the FBG sensor array, when the center wavelength λ′ of the probe light... N When the light is positioned at different positions in the reflection spectrum of the FBG sensor, the returned sensing light carries the spectral information of each FBG sensor. The returned sensing light passes sequentially through an optical circulator and a sensing light receiving fiber into a high-speed photodetector. The high-speed photodetector converts the sensing light into an electrical signal, which is recorded by the data acquisition device and transmitted to the data processing device. The electrical signal contains the time-domain signal waveform of the sensing light with spectral information. The closer the waveform is to the spectral peak, the more second harmonic components with frequency 2F0 and the fewer first harmonic components with frequency F0 are present in the time-domain signal waveform. Step 4: The data processing device performs a Fast Fourier Transform on the time-domain spectral information of each FBG sensor to obtain the frequency-domain signal. It then divides the 2F0 frequency component I(2F0) by the F0 frequency component I(F0) to obtain the intensity ratio S(λ′) at the corresponding wavelength position. N ) = I(2F0) / I(F0); Step 5: After completing the intensity ratio calculation at a wavelength position, the data processing device sends a synchronization control signal, which is transmitted to the light source and the frequency sweeping radio frequency source respectively through the light source synchronization control signal line and the radio frequency source synchronization control signal line. The light source steps the wavelength of the narrow linewidth laser to the next wavelength position, while the radio frequency source continues to synchronously output the frequency sweep signal, returning to step 1.

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