A method and device for externally modulated Brillouin light correlation domain reflection based on large modulation center frequency

Abstract

The present invention discloses a method and device for externally modulated Brillouin optical correlation domain reflectometry based on a large modulation center frequency. The method employs a function generator output signal as the input of a voltage-controlled oscillator, which in turn outputs a radio frequency signal with a large modulation center frequency and sinusoidal frequency variation over time as the radio frequency input of an electro-optical modulator. A laser emits continuous detection laser light, which is optically isolated and polarization-controlled before being modulated into modulated light by an electro-optical modulator. One path of the modulated light is amplified, filtered, and selected for a 1-order modulated light signal. This light is then passed through a circulator to obtain forward-propagating pump light, while the other path is passed through a delay fiber to control the order of the correlation peak and obtain reference light. The pump light generates backward spontaneous Brillouin scattered Stokes light in the sensing fiber to be measured. This Stokes light carries temperature information and is then photoelectrically converted and collected for analysis after colliding with the reference light. This method avoids the spectrum aliasing inherent in external modulation systems, effectively improving the spatial resolution and measurement range of the sensing system.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber temperature sensing, and in particular relates to a method and device for externally modulated Brillouin light correlation domain reflection based on a large modulation center frequency. Background Art

[0002] Temperature is a crucial environmental parameter, and its sensing and monitoring are widely needed in a wide range of fields, including aerospace, energy, electricity, transportation, and communications. Due to the advantages of optical fiber, such as its resistance to electromagnetic interference, compact size, and lightweight, temperature sensing based on optical fiber sensing technology has been extensively researched and applied. Many applications require continuous, distributed, high-spatial-resolution temperature monitoring of a specific area, necessitating distributed temperature measurement optical fiber sensing technology.

[0003] Currently, the fiber optic sensing technologies that can achieve continuous distributed temperature monitoring mainly include distributed fiber optic sensing technology based on Raman scattering and distributed fiber optic sensing technology based on Brillouin scattering. Among them, distributed fiber optic sensing technology based on Raman scattering has the advantage of high temperature measurement sensitivity, but it has the problem of low spatial resolution. Brillouin Optical Correlation-Domain Reflectometry (BOCDR) technology, which has the advantage of single-ended measurement, can achieve high spatial resolution temperature measurement, but it has high requirements for hardware equipment. On this basis, external modulation BOCDR technology has been developed to externally modulate the light source. However, the output light of the external modulation light source system contains 0th-order modulated light that is difficult to filter out. If the modulation amplitude exceeds a certain value, its Rayleigh scattering will cause spectral aliasing when it overlaps with the local reference light, which seriously restricts the spatial resolution of the external modulation BOCDR system. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the deficiencies of the above-mentioned prior art and provide a method and device for externally modulated Brillouin optical correlation domain reflection based on a large modulation center frequency. The large modulation center frequency is used to suppress the aliasing effect of beat spectrum noise in the external modulation system on the Brillouin scattering signal, which can avoid the spectrum aliasing problem inherent in the external modulation system. The method can be applied to distributed temperature or strain sensing of optical fibers, effectively improving the spatial resolution and measurement range of the sensing system.

[0005] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0006] A method for correlation domain reflection of externally modulated Brillouin light based on a large modulation center frequency, comprising:

[0007] Step 1: The function generator output signal is used as the input of the voltage controlled oscillator. The voltage controlled oscillator outputs a radio frequency signal with a large modulation center frequency and a frequency that varies sinusoidally with time, which is used as the radio frequency input of the electro-optical modulator.

[0008] Step 2: The laser emits a continuous detection laser which passes through an optical isolator and a polarization controller and is then modulated into a modulated light by an electro-optical modulator;

[0009] Step 3: The modulated light is amplified by an optical amplifier, the optical bandpass filter selects the -1 order modulated light signal, and then passes through a circulator to obtain the forward propagating pump light. The other path passes through a delay fiber to control the order of the correlation peak and obtains the reference light.

[0010] Step 4: The pump light propagates to the sensing fiber to be measured, generating Stokes light through backward spontaneous Brillouin scattering in the sensing fiber to be measured. The Stokes light carries temperature information and is combined with the reference light to complete the photoelectric conversion through the photodetector. The conversion result is collected and analyzed by the spectrum analyzer.

[0011] To optimize the above technical solutions, specific measures taken also include:

[0012] The above method also includes changing the frequency of the output signal of the function generator in a certain step size to achieve the movement of the correlation peak, thereby achieving distributed temperature sensing with high spatial resolution.

[0013] The above large modulation center frequency satisfies:

[0014] f c ≥3Δf+f Bmax

[0015] Among them, f c is the modulation center frequency, Δf is the modulation amplitude, f Bmax is the maximum Brillouin frequency shift of the sensing fiber to be measured.

[0016] The wavelength of the laser is selected in the C band.

[0017] The above-mentioned sensing optical fiber to be tested is G657 optical fiber.

[0018] The spectrum analyzer performs normalization processing on the acquired Brillouin gain spectrum after accumulating and averaging the spectrum and removing the background noise, so as to eliminate the influence of the overall power jitter of the measurement signal.

[0019] An externally modulated Brillouin optical correlation domain reflectometry device based on a large modulation center frequency includes a modulated light generating unit, a function generator, a voltage-controlled oscillator, a delay fiber, a photodetector, and a spectrum analyzer;

[0020] The modulated light generating unit includes a laser, an optical isolator, a polarization controller, an electro-optical modulator, an optical amplifier, and an optical bandpass filter connected in sequence;

[0021] The function generator is connected to the electro-optic modulator via a voltage-controlled oscillator;

[0022] The optical bandpass filter is connected to one end of the sensing optical fiber to be measured and one end of the delay optical fiber respectively through a circulator;

[0023] The other ends of the sensing optical fiber to be measured and the delay optical fiber are connected to a photoelectric detector, and the output end of the photoelectric detector is connected to a spectrum analyzer.

[0024] The present invention has the following beneficial effects:

[0025] The present invention utilizes a voltage-controlled oscillator to generate a large modulation center frequency, suppresses the aliasing effect of beat spectrum noise on the Brillouin scattering signal in the external modulation system, increases the upper limit of the modulation amplitude in the external modulation system, and can obtain higher spatial resolution and a larger temperature measurement range.

[0026] The present invention designs a corresponding new BOCDR device for the externally modulated Brillouin optical correlation domain reflectometry method based on a large modulation center frequency. Using this device, a 17.2m long G657 optical fiber is used to achieve continuous distributed measurement, with a spatial resolution of 11.6cm and a temperature measurement error of 0.17°C, which is one order of magnitude lower than that of similar methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of a method and device for externally modulated Brillouin light correlation domain reflection based on a large modulation center frequency;

[0028] Figure 2 1 is a schematic diagram of the process for determining the parameters of the large modulation center frequency of the present invention;

[0029] Figure 3 This is the Brillouin frequency shift-temperature coefficient calibration result diagram of the tested optical fiber;

[0030] Figure 4 This is a three-dimensional diagram of the Brillouin gain spectrum when the tested optical fiber is heated to 46.8°C;

[0031] Figure 5 The Brillouin frequency shift and spatial resolution results when the tested optical fiber is heated to 46.8°C are shown. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0033] Although the steps in the present invention are arranged with numbers, they are not intended to limit the order of the steps. Unless the order of the steps is clearly stated or the execution of a step requires other steps as a basis, the relative order of the steps can be adjusted. It is understood that the term "and / or" used herein refers to and covers any and all possible combinations of one or more of the associated listed items.

[0034] The present invention provides an externally modulated Brillouin optical correlation domain reflection method based on a large modulation center frequency. The beat spectrum noise inherent in the externally modulated Brillouin optical correlation domain reflection system has a center frequency value equal to the system modulation center frequency, and a bandwidth twice the system modulation amplitude. The center frequency of the Brillouin scattering signal is the Brillouin frequency shift, and the signal bandwidth is four times the system modulation amplitude. Based on this, it is analyzed that the modulation center frequency of the external modulation system should be greater than the sum of three times the system modulation amplitude and the system measurement dynamic range. When this condition is met, the spectrum aliasing problem of the external modulation system can be avoided. By using this method, the modulation amplitude and the upper limit of the measurement range of the system can also be increased, and the spatial resolution of the system can be further improved. Specifically, the following steps are included:

[0035] Step 1: The function generator output signal is used as the input of the voltage controlled oscillator. The voltage controlled oscillator outputs a radio frequency signal with a large modulation center frequency and a frequency that varies sinusoidally with time, which is used as the radio frequency input of the electro-optical modulator.

[0036] Step 2: The laser in the modulated light generating unit emits a continuous detection laser which passes through an optical isolator and a polarization controller and is then modulated into a modulated light by an electro-optical modulator;

[0037] Step 3: The modulated light is amplified by an optical amplifier, the optical bandpass filter selects the -1 order modulated light signal, and then passes through a circulator to obtain the forward propagating pump light. The other path passes through a delay fiber to control the order of the correlation peak and obtains the reference light.

[0038] Step 4: The pump light propagates to the sensing fiber to be measured, generating Stokes light through backward spontaneous Brillouin scattering in the sensing fiber to be measured. The Stokes light carries temperature information and is combined with the reference light to complete the photoelectric conversion through the photodetector. The conversion result is collected and analyzed by the spectrum analyzer.

[0039] In the embodiment, the frequency of the output signal of the function generator is changed in a certain step size to achieve the movement of the correlation peak, thereby achieving distributed temperature sensing with high spatial resolution.

[0040] The large modulation center frequency satisfies:

[0041] f c ≥3Δf+f Bmax

[0042] Among them, f c is the modulation center frequency, Δf is the modulation amplitude, f Bmax is the maximum Brillouin frequency shift of the sensing fiber to be measured.

[0043] The laser wavelength is selected in the C band.

[0044] The spectrum analyzer accumulates and averages the acquired Brillouin gain spectrum, removes the background noise, and then performs normalization processing to eliminate the influence of the overall power jitter of the measurement signal.

[0045] Accordingly, the present invention provides an externally modulated Brillouin optical correlation domain reflectometry device based on a large modulation center frequency, comprising a modulated light generating unit, a function generator, a voltage-controlled oscillator, a delay fiber, a photodetector, and a spectrum analyzer;

[0046] The modulated light generating unit includes a laser, an optical isolator, a polarization controller, an electro-optical modulator, an optical amplifier, and an optical bandpass filter connected in sequence;

[0047] The function generator is connected to the electro-optic modulator via a voltage-controlled oscillator;

[0048] The optical bandpass filter is connected to one end of the sensing optical fiber to be measured and one end of the delay optical fiber respectively through a circulator;

[0049] The other ends of the sensing optical fiber to be measured and the delay optical fiber are connected to a photoelectric detector, and the output end of the photoelectric detector is connected to a spectrum analyzer.

[0050] In the embodiment, the modulated light generating unit is composed of a laser, an optical isolator, a polarization controller, an electro-optical modulator, an optical amplifier, and an optical bandpass filter connected in sequence; the continuous detection laser emitted by the laser passes through the optical isolator and is modulated into modulated light by the electro-optical modulator. At the same time, the function generator generates a frequency f mThe signal is used as the input of the voltage-controlled oscillator (VCO). The VCO then outputs a RF signal whose frequency varies sinusoidally with time, which serves as the RF input of the electro-optical modulator. This signal is then amplified by an optical amplifier, filtered through an optical bandpass filter to select the -1-order modulated optical signal, and then passed through a circulator to obtain the forward-propagating pump light. The pump light generates backward spontaneous Brillouin scattered Stokes light in the fiber under test (FUT). This Stokes light carries temperature information and is then combined with the reference light. The photoelectric conversion is then completed by a photodetector (PD), and the conversion result is collected and analyzed by a spectrometer (ESA). The delay fiber controls the order of the correlation peak. Selecting a higher-order correlation peak can enhance sensitivity to correlation peak shifts. Subsequently, varying the frequency of the function generator output signal by a specific step size can shift the correlation peak, enabling distributed temperature sensing with high spatial resolution.

[0051] To optimize the above technical solution, specific measures include averaging the Brillouin gain spectrum measured by the spectrum analyzer, removing the noise floor, and then normalizing it to eliminate the impact of overall power jitter in the measured signal. The laser wavelength is selected in the C-band, making it suitable for use at lower modulation frequencies, thereby achieving a longer detection range.

[0052] Example 1

[0053] In order to improve the spatial resolution of the measurement, a larger modulation amplitude is generally selected. When the modulation amplitude is large, the center frequency is f c The beat frequency noise will be B To avoid signal aliasing caused by beat frequency noise, a new method and device for externally modulated Brillouin optical correlation domain reflectometry (CDRS) based on a large modulation center frequency is proposed. The present invention will now be further described in detail with reference to the accompanying drawings and embodiments.

[0054] like Figure 1 As shown, the laser output frequency is f l The narrow linewidth continuous light with a power of about 11dBm (corresponding to a central wavelength of 1549.912nm) is modulated by the electro-optical modulator after passing through the isolator (ISO) and polarization controller (PC). Before modulation, the function generator outputs a frequency of f m The signal of this frequency is the modulation frequency of the system. After driving the voltage controlled oscillator, the output frequency is f RF The RF signal of (t) can be expressed as

[0055] f RF(t) = f c +Δf sin(2πf m t) (1)

[0056] Among them, f c is the modulation center frequency, and Δf is the modulation amplitude. The RF signal is injected into the RF input of the electro-optical modulator, and the output frequency is a modulated signal of f(t). It can be expressed as

[0057]

[0058] By continuously changing the output frequency f of the function generator m The correlation peak can be shifted by using a delay fiber (6.0400 to 6.0490 MHz, in 0.25 kHz steps). Selecting a delay fiber approximately 2 km long can increase the order of the system's correlation peak, achieving "sensitization" of the correlation peak shift and enabling distributed measurement.

[0059] The wavelength of the laser can be selected within the C-band range, and the wavelength is selected to achieve spontaneous Brillouin scattering of the optical fiber.

[0060] like Figure 2 As shown in the figure, since a large modulation amplitude Δf is required to obtain high spatial resolution, a bandpass filter with a large bandwidth needs to be selected. Due to the uneven passband of the filter and the uneven amplification of the erbium-doped fiber amplifier, the pump light injected into the optical fiber under test also includes a small amount of frequency f l The power of the 0th order signal is about 3% to 5% of the -1st order modulated light. Figure 2 (a) is shown. Then in the reference optical path, in addition to the central frequency f l -f c -1 order modulation signal, also includes a frequency of f l The -1 order modulated light is the main component of the reference light, and its 3dB linewidth is about twice the modulation amplitude Δf.

[0061] The scattered light in the optical fiber to be tested mainly includes the scattered light with a center frequency of f l -f c -f B , Brillouin scattered light with a linewidth of 2Δf and a center frequency of f l -f c , Rayleigh scattered light with a linewidth of 2Δf, and a central frequency of f l Rayleigh scattered light with a narrow line width equal to the laser line width. B is the Brillouin frequency shift, which is generally in the range of 10 to 12 GHz for quartz optical fibers. It is worth noting that the scattered light entering the photodetector also contains the central frequency f l -f cRayleigh scattered light (after filtering, the center frequency is f l The power of the Rayleigh scattered light signal is small and can be ignored).

[0062] The reference light beats with the filtered scattered signal, and the beat spectrum is as follows: Figure 2 (b) is shown. The center frequency is f B The beat signal with a line width of 4Δf is the center frequency of f l -f c -f B The Brillouin scattering signal with a center frequency of f l -f c The result of the reference light beating, the center frequency is f c , the beat signal with a line width of 2Δf is the center frequency f l -f c The Rayleigh scattered signal with the center frequency f l The result of shooting with the reference light. Figure 2 (b) It can be seen that the center frequency is f B The beat frequency signal corresponds to the Brillouin scattering signal carrying the temperature information to be measured, and the center frequency is f c The beat frequency signal has nothing to do with the Brillouin scattering signal and is called beat spectrum noise. When the modulation amplitude Δf is small, the noise will not be affected by the center frequency f. B However, from the above analysis, it can be seen that in order to improve the spatial resolution of the measurement, a larger modulation amplitude Δf is generally selected. When the modulation amplitude Δf is large, the center frequency is f c The beat frequency noise will be B The beat frequency signal is aliased, such as Figure 2 (c) In order to avoid signal aliasing caused by beat spectrum noise, the modulation amplitude Δf should have an upper limit, which should satisfy

[0063]

[0064] If the maximum Brillouin frequency shift of the sensing fiber to be measured is f Bmax , modulation center frequency f c Should meet

[0065] f c ≥3Δf+f Bmax (4)

[0066] The output voltage of the function generator is controlled, and the modulation amplitude of the modulated electrical signal is 1.25 GHz and the modulation center frequency is 14.7 GHz.

[0067] The sensing fiber under test is a G657 fiber with a total length of 17.2m. Its left end is connected to a circulator. A 0.6m long water bath heating section is located 12.1m from the connection point. With room temperature at approximately 22°C, the heating section temperature is set to 25°C, 30°C, 40°C, 50°C, and 60°C. The measured Brillouin frequency shift is shown in Figure 1. Figure 3 As shown, it can be seen that the temperature and the Brillouin frequency shift value are linearly related. The Brillouin frequency shift temperature coefficient obtained by linear fitting is 1.15MHz / ℃, and the fitting correlation coefficient R 2 The value of is close to 1.

[0068] Figure 4 To change the water bath temperature to 46.8°C, the signal processing results near the heating section clearly show changes in the Brillouin frequency shift. During measurement, the signal at each measurement location was accumulated and averaged 1000 times. The direct measurement results were then subtracted from the noise floor and normalized to eliminate the effects of overall power jitter in the measured signal.

[0069] The result after subtracting the background noise is fitted with Lorentzian and the Brillouin frequency shift is extracted as Figure 5 As shown. According to the commonly used definition method of spatial resolution, the experimental spatial resolution of the BOCDR system can be expressed by the average value of the optical fiber length corresponding to 10% to 90% of the rising edge and the falling edge. In the rising edge, the distance for the Brillouin frequency shift to change from 10% amplitude to 90% amplitude is calculated to be 11.5 cm. At the same time, in the falling edge, the distance for the Brillouin frequency shift to change from 90% amplitude to 10% amplitude is calculated to be 11.7 cm. Therefore, it is believed that the spatial resolution of the system measurement is 11.6 cm. At the same time, the average Brillouin frequency shift of the heating section is 10.71899 GHz, corresponding to a measured temperature of 46.64°C, and the temperature measurement error is 0.17°C. In general, the system spatial resolution of the method proposed in the present invention is high, and the measurement error is one order of magnitude lower than that of similar methods.

[0070] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be included therein. Any reference sign in a claim should not be construed as limiting the claim to which it relates.

[0071] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A method for externally modulated Brillouin light correlation domain reflection based on a large modulation center frequency, characterized in that: include: Step 1: The output signal of the function generator is used as the input of the voltage-controlled oscillator. The voltage-controlled oscillator outputs a radio frequency signal with a large modulation center frequency and a frequency that varies sinusoidally with time, which is used as the radio frequency input of the electro-optical modulator. Step 2: The laser emits a continuous detection laser which passes through an optical isolator and a polarization controller and is then modulated into a modulated light by an electro-optical modulator; Step 3: The modulated light is amplified by an optical amplifier, the optical bandpass filter selects the -1 order modulated light signal, and then passes through a circulator to obtain the forward propagating pump light. The other path passes through a delay fiber to control the order of the correlation peak and obtains the reference light. Step 4: The pump light propagates to the sensing fiber to be tested, generating Stokes light through backward spontaneous Brillouin scattering in the sensing fiber to be tested. The Stokes light carries temperature information and is combined with the reference light to complete the photoelectric conversion through the photodetector. The conversion result is collected and analyzed by the spectrum analyzer. The large modulation center frequency satisfies: f c ≥3Δf+f Bmax Among them, f c is the modulation center frequency, Δf is the modulation amplitude, f Bmax is the maximum Brillouin frequency shift of the sensing fiber to be measured.

2. The method of externally modulated Brillouin light correlation domain reflectometry based on a large modulation center frequency according to claim 1, characterized in that: The method also includes changing the frequency of the output signal of the function generator with a preset step size to achieve the movement of the correlation peak, thereby achieving distributed temperature sensing with high spatial resolution.

3. The method of externally modulated Brillouin light correlation domain reflectometry based on a large modulation center frequency according to claim 1, characterized in that: The laser wavelength is selected in the C band.

4. The method of externally modulated Brillouin light correlation domain reflectometry based on a large modulation center frequency according to claim 1, characterized in that: The sensing optical fiber to be tested is a G657 optical fiber.

5. The method of externally modulated Brillouin light correlation domain reflectometry based on a large modulation center frequency according to claim 1, characterized in that: The spectrum analyzer accumulates and averages the acquired Brillouin gain spectrum, removes the background noise, and then performs normalization processing to eliminate the influence of the overall power jitter of the measurement signal.

6. A Brillouin optical correlation domain reflectometry device based on a large modulation center frequency, used to implement the method according to any one of claims 1 to 5, characterized in that: The device includes a modulated light generating unit, a function generator, a voltage controlled oscillator, a delay optical fiber, a photodetector and a spectrum analyzer; The modulated light generating unit includes a laser, an optical isolator, a polarization controller, an electro-optical modulator, an optical amplifier, and an optical bandpass filter connected in sequence; The function generator is connected to the electro-optic modulator via a voltage-controlled oscillator; The optical bandpass filter is connected to one end of the sensing optical fiber to be measured and one end of the delay optical fiber respectively through a circulator; The other ends of the sensing optical fiber to be measured and the delay optical fiber are connected to a photoelectric detector, and the output end of the photoelectric detector is connected to a spectrum analyzer.

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