Distributed fiber optic sensing system for measuring temperature and strain based on OCDR-SOCF

Abstract

A distributed fiber optic sensing system for measuring temperature and strain based on OCDR-SOCF (Optical Detection and Conversion) technology includes a laser, an isolator, a collimator, a polarization controller, a coupler, an acousto-optic modulator (AOM), a polarization diversity photodetector (PDD), and a signal acquisition and processing module. The laser light generated by the laser passes through the isolator and enters a Michelson interferometer. One path of the laser light is calibrated by the collimator and injected into the optical fiber. The returned Rayleigh backscattered light passes through the polarization controller and is then injected into the coupler. The other path of the laser light passes through the acousto-optic modulator and is then injected into the coupler. The two signals enter the polarization diversity photodetector, where they interfere and generate a current signal, which is then output to the signal acquisition and processing module. The signal acquisition and processing module synthesizes the coherence function to achieve simultaneous fiber temperature and strain sensing. This system facilitates measurement, reduces system complexity, and reduces system cost.

Description

Technical Field

[0001] The present invention relates to a technology in the field of optical fiber, specifically a distributed optical fiber sensing system for measuring temperature and strain based on optical coherence domain reflectometry with synthetic coherence function (OCDR-SOCF). Background Art

[0002] Distributed sensing of strain or temperature plays a vital role in numerous fields, including energy, construction, aerospace, security, and the military. Currently, a variety of distributed fiber-optic sensing systems are commercially available that utilize different methods for measuring temperature and strain, including intensity-modulated optical heterodyne time-domain detection (OTDR), intensity-modulated optical frequency-domain reflectometry (OFDR), and optical frequency-domain scanning OFDR. However, intensity-modulated OTDR and OFDR require high-speed modulation and detection, heterodyne OTDR requires a mechanically movable mirror to adjust the time difference between the reference pulse and the signal pulse, and optical frequency-domain scanning OFDR is limited by the nonlinearity of the frequency sweep. Summary of the Invention

[0003] In response to the shortcomings of existing technology systems, such as complex structure, low spatial resolution, large influence of frequency sweep nonlinearity, and only being able to measure vibration, the present invention proposes a distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF. The system measures the backscattered Rayleigh light in the optical fiber through OCDR-SOCF, and through the correspondence between the change in Rayleigh light frequency and the change in optical fiber strain and temperature, a synthetic coherence function is constructed to realize the sensing system for measuring optical fiber temperature and strain simultaneously, which has high spatial resolution and anti-interference ability.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF. The system comprises: a laser, an isolator, a collimator, a polarization controller, a coupler, an acousto-optic modulator, a polarization diversity photoelectric detector, and a signal acquisition and processing module. The system comprises: a laser generated by the laser enters a Michelson interferometer after passing through the isolator; one path of laser light is calibrated by the collimator and injected into an optical fiber; the returned back Rayleigh scattered light passes through the polarization controller and is injected into the coupler; the other path of laser light passes through the acousto-optic modulator and is injected into the coupler; the two paths of signals enter a polarization diversity photoelectric detector, interfere with each other, and obtain a current signal, which is then output to the signal acquisition and processing module. The signal acquisition and processing module realizes sensing of optical fiber temperature and strain by synthesizing a coherence function.

[0006] The splitting ratio of the coupler is 50 / 50.

[0007] The signal acquisition and processing module includes: a data acquisition card and a data processor which are connected to each other.

[0008] Technical Effects

[0009] Compared with the prior art, the present invention realizes the simultaneous measurement of strain and temperature by using the optical coherence domain reflectometry method of synthetic coherence function, thereby reducing the system complexity and thus the system cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a simplified structural diagram of the present embodiment;

[0011] Figure 1 Middle: main laser 1, RF signal source 2, optical isolator 3, first coupler 4, circulator 5, collimator 6, optical fiber 7, acousto-optic modulator 8, polarization controller 9, second coupler 10, polarization diversity photodetector 11, data acquisition card 12, data processor 13;

[0012] Figure 2 (a) Schematic diagram of the relationship between the laser frequency and the injected current added by the RF signal source;

[0013] Figure 2 (b) is a schematic diagram of the coherence function of the embodiment;

[0014] Figure 3 Schematic diagram of distributed measurement in an embodiment;

[0015] Figure 4 Schematic diagram of the change of coherence function with frequency and distance in the embodiment;

[0016] Figure 5 Schematic diagram of power spectrum of the embodiment. DETAILED DESCRIPTION

[0017] like Figure 1As shown in FIG. 1 , a distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF according to the present embodiment includes: a main laser 1, a radio frequency signal source 2, an optical isolator 3, a first coupler 4 and a circulator 5 constituting a Michelson interferometer, a collimator 6, an optical fiber 7, an acousto-optic modulator 8, a polarization controller 9, a second coupler 10, a polarization diversity photodetector 11, a data acquisition card 12, and a data processor 13. The radio frequency signal source 2 is connected to the main laser 1, which is in turn connected to the optical isolator 3 and the first 50 / 50 coupler 4. The two outlets of the first 50 / 50 coupler 4 are connected to the polarization diversity photodetector 11. Port a of the circulator 5 is connected to an acousto-optic modulator 8, and port b of the circulator 5 is connected to a collimator 6 and an optical fiber 7 in sequence. The returned back Rayleigh scattered light returns from port b of the circulator 5 and is emitted from port c. Port c is connected to a polarization controller 9. The light beam emitted from port c of the optical fiber coupler 4 is modulated by the acousto-optic modulator 8 to remove DC noise. The light beam is then injected into a second 50 / 50 coupler 10 together with the output light from port c of the optical circulator 5 after passing through the polarization controller 9. After the polarization diversity photodetector 11 generates a current signal, the data acquisition card 12 and the data processor 13 collect and process the current signal.

[0018] The master laser 1 generates a laser with a constant wavelength and power. Under the control of the radio frequency signal source 2, the relationship between the frequency of the generated laser and the injection current added by the radio frequency signal source 2 is as follows: Figure 2 As shown in (a), it is a rectangular wave with gradually increasing amplitude.

[0019] The current signal output by the polarization diversity photodetector 11 is Among them: the main laser has an amplitude E0, the reference light E ref (t)=E0 exp[j(2πf0t-2πft+φ(l))], backscattered Rayleigh light f0 is the average oscillation frequency, φ is the phase fluctuation of the light source, R(z) is the reflectivity amplitude at distance z, Δn(z) is the normalized refractive index fluctuation, the related term <Δn(z1)Δn(z2)> is non-zero only when z1=z2, k is the wave number, and the coherence function of the light source is F is a parameter that describes the shape of the power spectrum.

[0020] The power of the backscattered Rayleigh light is the square of its optical electric field intensity, specifically:

[0021] When the RF signal source 2 generates a rectangular wave injection current with gradually increasing amplitude, the spectrum of the light source modulated by it has 2N peaks, and the coherence function of the light source is Synthetic coherence function γ 2 like Figure 2 As shown in (b), in the figure, N=40, γ 2 The peak indicates the position where the reflected light is coherent with the reference light, where: N is the number of modulations, and the frequency interval f between a certain position z of the optical fiber and each order s relationship satisfaction i d Corresponding to the number of cycles of triangular wave used for measurement, by changing the frequency interval f of the waveform s Each coherence peak can be swept.

[0022] When using Figure 3 The sinusoidal current shown directly modulates the master laser 1, and the measurement range corresponds to the spacing between the correlation peaks Where: n is the refractive index of the optical fiber, f2 is the modulation frequency of the laser, and the spatial resolution is described as f1 is the modulation amplitude of the laser.

[0023] The data processing is as follows: the data processor 13 changes the output of the light source at a fixed frequency interval to obtain the function P of the power of the backscattered Rayleigh light with respect to the frequency v and the distance z for the current signal obtained at different times Ta and Tb. a (ν,z) and P b (ν,z) and calculate the cross-correlation term between the two Where: x = a, b, when the temperature and strain do not change between Ta and Tb, then R ab (f,z) takes its maximum value at f = 0, otherwise, it takes its maximum value at f = Δv. ab (f,z) Find the corresponding frequency change ∆v at the maximum value at different positions z.

[0024] The frequency change △v, temperature change △T, strain change Δε satisfy Where: v0 is the nominal laser frequency.

[0025] Through specific actual experiments, by simulating the measurement process of a section of optical fiber, the return light frequency and intensity spectrum of the optical fiber 0.3m to 1m section were measured. The input light frequency was Figure 2 After the signal is modulated, it is swept from 200MHz to 700MHz with a frequency interval of 3MHz. The obtained coherence function changes with frequency and distance as shown in Figure 4 As shown, calculate its power spectrum Pa(v,z); after changing the temperature parameter (0.1℃), re-simulate the measurement with the same data to obtain the power spectrum Pb(v,z). Plot Pa and Pb on the same graph and get Figure 5As can be seen in the figure, the power spectrum undergoes a significant frequency shift, compensating for the change in laser frequency caused by temperature variations. By calculating the cross-correlation terms of Pa and Pb, we obtain a frequency shift of △v = -1.33MHz. Compared with the nominal laser frequency, the temperature change is approximately 0.0993°C, which agrees well with the preset value.

[0026] Compared to OTDR and OFDR temperature and strain measurement methods, the present invention uses optical coherence time-domain reflectometry (OCTDR) based on synthetic coherence functions to measure temperature and strain changes. This method has a simple structure and is free from the nonlinearity of frequency sweeping. It also offers higher spatial resolution and anti-interference capabilities.

[0027] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. A distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF, characterized in that: include: Laser, isolator, collimator, polarization controller, coupler, acousto-optic modulator, polarization diversity photoelectric detector and signal acquisition and processing module, wherein: the laser light generated by the laser enters the Michelson interferometer after passing through the isolator, one path is calibrated by the collimator and injected into the optical fiber, and the returned back Rayleigh scattered light is injected into the coupler after passing through the polarization controller; the other path is injected into the coupler through the acousto-optic modulator, and the two signals enter the polarization diversity photoelectric detector to interfere and obtain the current signal, which is then output to the signal acquisition and processing module. The signal acquisition and processing module realizes the sensing of the temperature and strain of the measured optical fiber through the synthesis of the coherence function, that is, the current signal is collected and processed by the data acquisition card and the data processor, specifically: the data processor obtains the current signals at different times Ta and Tb, that is, changes the output of the light source at a fixed frequency interval to obtain the function of the power of the back Rayleigh scattered light with respect to the frequency v and the distance z. and And calculate the cross-correlation between the two , ,in: , when the temperature and strain do not change between Ta and Tb, then It takes the maximum value at f=0, otherwise, it takes the maximum value at the frequency f=△v. Therefore, in the cross-correlation term Find the corresponding frequency change △v at the maximum value at different positions z; The frequency change △v, temperature change △T, strain change satisfy , where: v0 is the nominal laser frequency.

2. The distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF according to claim 1 is characterized in that include: The invention relates to a main laser, a radio frequency signal source, an optical isolator, a first coupler and a circulator constituting a Michelson interferometer, a collimator, an optical fiber, an acousto-optic modulator, a polarization controller, a second coupler, a polarization diversity photodetector, a data acquisition card, and a data processor. The radio frequency signal source is connected to the main laser, which is sequentially connected to the optical isolator and the first 50 / 50 coupler. The two outlets of the first 50 / 50 coupler are respectively connected to port a of the circulator and the acousto-optic modulator. The port b of the circulator is sequentially connected to the collimator and the optical fiber. Returned back Rayleigh scattered light returns from port b of the circulator and is emitted from port c. Port c is connected to the polarization controller. The light beam emitted from port c of the optical fiber coupler is modulated by the acousto-optic modulator, DC noise is removed, and the light beam is injected into the second 50 / 50 coupler together with the output light from port c of the optical circulator after passing through the polarization controller. After the polarization diversity photodetector generates a current signal, the data acquisition card and the data processor collect and process the current signal.

3. The distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF according to claim 1 or 2, characterized in that: The current signal output by the polarization diversity photoelectric detector , where n is the refractive index of the fiber, the main laser has an amplitude E0, and the reference light , backscattered Rayleigh light , f0 is the average oscillation frequency, is the phase fluctuation of the light source, R(z) is the reflectivity amplitude at distance z, is the normalized refractive index fluctuation, and the related terms It is non-zero only when z1=z2, k is the wave number, and the coherence function of the light source is , F is a parameter describing the shape of the power spectrum; the power of the backscattered Rayleigh light is the square of its optical electric field intensity, specifically: .

4. The distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF according to claim 1 or 2, characterized in that: When the RF signal source generates a rectangular wave injection current with gradually increasing amplitude, the spectrum of the light source modulated by it has 2N peaks, and the coherence function of the light source is , where: N is the number of modulations, the frequency interval f between a certain position z of the optical fiber and each order s relationship satisfaction ,i d Corresponding to the number of cycles of triangular wave used for measurement, by changing the frequency interval f of the waveform s Each coherence peak can be swept.

5. The distributed optical fiber sensing system for measuring temperature and strain based on OCDR-SOCF according to claim 1 or 2, characterized in that: When the main laser is modulated directly by a sinusoidal current, the measurement range corresponding to the spacing between the correlation peaks is , where n is the fiber refractive index, f2 is the modulation frequency of the laser, and the spatial resolution is described as , f1 is the modulation amplitude of the laser.

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