A distributed optical fiber loss, temperature and strain measurement system

Abstract

The present invention discloses a distributed optical fiber loss, temperature and strain measurement system, comprising a first laser, a second laser, a first coupler, a second coupler, a semiconductor optical amplifier, a pulse signal source, an erbium-doped optical fiber amplifier, a circulator, a first detector, a second detector, an acquisition card, a spectrum analyzer and a computer. The present invention measures the distributed loss, temperature and strain of an optical fiber by simultaneously measuring the Rayleigh scattering and Brillouin scattering of the optical fiber. The distributed loss of the optical fiber is measured by using the optical time domain reflectometer technology based on Rayleigh scattering, and the distributed Brillouin gain spectrum information of the optical fiber is measured by using the BOTDR technology. The frequency shift and gain size of the optical fiber loss and Brillouin gain spectrum are combined to achieve the differentiated measurement of the optical fiber temperature and strain.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensors, and in particular to a distributed optical fiber loss, temperature and strain measurement system. Background Art

[0002] Distributed fiber optic sensing can measure information along the optical fiber and is widely used in long-distance line monitoring. By measuring different scattered light, different physical information of the optical fiber can be obtained. Among them, the Brillouin Optical Time Domain Reflectometry (BOTDR) technology based on Brillouin scattering can measure the temperature and strain information of the optical fiber and is widely used in various lines and engineering applications.

[0003] In power transmission application scenarios such as optical fiber composite overhead ground wire (OPGW), due to the limited amount of optical fiber sensing, it is necessary to multiplex temperature sensing and strain sensing on one optical fiber, and temperature demodulation needs to be independent of strain demodulation. However, the Brillouin frequency shift of the optical fiber at any position measured by the traditional BOTDR is related to the temperature and strain of the optical fiber at the same time, making it difficult to decouple the temperature and strain. As a result, it is difficult for BOTDR to perform qualitative analysis of the frequency shift phenomenon in practical applications, limiting its application in some scenarios. Summary of the invention

[0004] The present invention provides a distributed optical fiber loss, temperature and strain measurement system, which measures the distributed loss, temperature and strain of optical fiber by simultaneously measuring the Rayleigh scattering and Brillouin scattering of optical fiber. The distributed loss of optical fiber is measured by using the optical time-domain reflectometer (OTDR) technology based on Rayleigh scattering, and the distributed Brillouin gain spectrum information of optical fiber is measured by using the BOTDR technology. The frequency shift and gain size of the optical fiber loss and Brillouin gain spectrum are combined to achieve the differentiated measurement of optical fiber temperature and strain.

[0005] The above object of the present invention is achieved by the following technical solution: a distributed optical fiber loss, temperature and strain measurement system, characterized in that it includes a first laser, a second laser, a first coupler, a second coupler, a semiconductor optical amplifier, a pulse signal source, an erbium-doped fiber amplifier, a circulator, a first detector, a second detector, an acquisition card, a spectrum analyzer and a computer;

[0006] The first laser and the second laser are connected to the two input ports of the first coupler respectively, an output end of the first coupler is connected to the optical input port of the semiconductor optical amplifier, a pulse signal source is connected to the electrical input port of the semiconductor optical amplifier, an output port of the semiconductor optical amplifier is connected to the optical input port of the erbium-doped fiber amplifier, an optical output port of the erbium-doped fiber amplifier is connected to a port of the circulator, the next port of the circulator along the light transmission direction is connected to the optical fiber to be tested, the third port of the circulator along the light transmission direction is connected to the input port of the optical filter, the output port of the optical filter is connected to the first detector, the acquisition card and the computer in sequence, the other output port of the optical filter and the other output of the first coupler are respectively connected to the two output ports of the second coupler, the output end of the second coupler is connected to the second detector, and the second detector is connected to the spectrum analyzer and the computer in sequence.

[0007] Furthermore, the first laser and the second laser are both narrow linewidth lasers, the linewidth of the first laser is 0.08nm to 0.2nm, and the linewidth of the second laser is 0.01nm to 1nm; the wavelength difference between the first laser and the second laser is not less than 3nm, and the filtering wavelengths of the two output ports of the optical filter correspond to the wavelengths of the first laser and the second laser.

[0008] Furthermore, the first coupler and the second coupler are both 3dB couplers, and the splitting ratio is 50:50.

[0009] Furthermore, the pulse width of the pulse signal source is 10ns to 1000ns.

[0010] Furthermore, the semiconductor optical amplifier has an extinction ratio greater than 40 dB and a rising edge less than 10 ns, and the pulses output by the pulse signal source drive the semiconductor optical amplifier to modulate direct current light into pulse light with a high extinction ratio.

[0011] Furthermore, the optical filter is an optical wavelength division multiplexer.

[0012] Furthermore, the process of using the system to obtain the loss, temperature, and strain information of the optical fiber is as follows:

[0013] S1 obtains the OTDR curve through the first detector;

[0014] S2 uses the OTDR curve to calculate the optical fiber loss curve;

[0015] S3 obtains the Brillouin gain spectrum through a spectrum analyzer;

[0016] S4 calculates the Brillouin frequency shift f and the fiber Brillouin gain g at a single position through the Brillouin gain spectrum;

[0017] S5 can obtain the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single location;

[0018] S6 obtains the optical fiber strain information through the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information.

[0019] Furthermore, S2 uses the OTDR curve to calculate the optical fiber loss curve, which is:

[0020]

[0021] Wherein, R0 is the OTDR measurement result of the starting point of the optical fiber, and R(z) is the intensity of any point z on the optical fiber on the OTDR curve. The OTDR curve is the Rayleigh scattered light intensity of each point on the optical fiber.

[0022] Further, in S3, specifically, the BOTDR obtains the Brillouin gain spectrum of each point of the optical fiber through a spectrum analyzer;

[0023] In S4, specifically, a Lorentz fitting is performed on each gain spectrum, the maximum value of the fitting signal is the Brillouin gain at that point of the optical fiber, and the spectrum frequency corresponding to the Brillouin gain is the Brillouin frequency shift.

[0024] Furthermore, the specific method in which S5 can obtain the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single position is:

[0025]

[0026] Among them, g0 is the Brillouin gain at the starting point of the optical fiber, T0 is the reference temperature at the starting point of the optical fiber, g(z) is the Brillouin gain of point z on the optical fiber, r(z) is the loss at point z on the optical fiber, so the temperature T(z) of point z on the optical fiber can be obtained, and T0 can be obtained by measuring the air temperature at the starting point of the optical fiber with an electronic thermometer.

[0027] Furthermore, the method S6 for obtaining the optical fiber strain information through the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information is specifically as follows:

[0028] f=f0+A(T-T0)+Bε

[0029] Among them, f0 is the Brillouin frequency shift of the optical fiber at temperature T0, A is the temperature coefficient of the optical fiber, B is the strain coefficient of the optical fiber, ε is the strain of the optical fiber, f and T are the frequency shift and temperature of a certain point z on the optical fiber, which can also be recorded as f(z) and T(z).

[0030] In summary, the beneficial effects of the present invention are as follows: the present invention solves the problem that temperature and strain cannot be decoupled in general BOTDR systems by adopting clock-synchronized dual-wavelength pulses and multi-parameter cross-demodulation of BOTDR and OTDR, thereby increasing the applicability of the system in different application scenarios and the practical value of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the structural principle of the present invention. DETAILED DESCRIPTION

[0032] The specific implementation of the present invention will be described in detail below with reference to the accompanying drawings.

[0033] Example: See Figure 1 A distributed optical fiber loss, temperature, and strain measurement system includes: a first laser, a second laser, a first coupler, a second coupler, a semiconductor optical amplifier, a pulse signal source, an erbium-doped fiber amplifier, a circulator, a first detector, a second detector, an acquisition card, a spectrum analyzer, and a computer.

[0034] The first laser and the second laser are connected to the two input ports of the first coupler respectively, an output end of the first coupler is connected to the optical input port of the semiconductor optical amplifier, a pulse signal source is connected to the electrical input port of the semiconductor optical amplifier, an output port of the semiconductor optical amplifier is connected to the optical input port of the erbium-doped fiber amplifier, an optical output port of the erbium-doped fiber amplifier is connected to the a port of the circulator, the b port of the circulator is connected to the optical fiber to be tested, the c port of the circulator is connected to the input port a of the optical filter, the b output port of the optical filter is connected to the first detector, the acquisition card and the computer in sequence, the c port of the optical filter and the other output of the first coupler are connected to the two output ports of the second coupler respectively, the output end of the second coupler is connected to the second detector, and the second detector is connected to the spectrum analyzer and the computer in sequence.

[0035] Wherein, the first laser and the second laser are both narrow linewidth lasers, the wavelength difference between the first laser and the second laser is greater than 3nm, the linewidth of the first laser is 0.08nm to 0.2nm, and the linewidth of the second laser is 0.01nm to 1nm. In one embodiment, the center wavelength of the first laser output light is 1550nm, the linewidth of the first laser is 0.2nm, the center wavelength of the second laser output light is 1553, the linewidth of the second laser is 0.1nm, and the filtering wavelengths of the optical filter b and c ports correspond to the wavelengths of the first laser and the second laser. The first coupler and the second coupler are 3dB couplers, that is, the splitting ratio is 50:50. The pulse width of the pulse signal source is 10ns to 1000ns. The extinction ratio of the semiconductor optical amplifier is greater than 40dB, and the rising edge is less than 10ns. The pulse output by the pulse signal source drives the semiconductor optical amplifier to modulate the DC light into a pulse light with a high extinction ratio. The bandwidth of the first detector is 125MHz, and the bandwidth of the second detector is 125MHz. The acquisition card sampling rate is 250MSa / s.

[0036] With optical fiber as the medium, the first laser and the second laser are divided into two paths after passing through the first coupler. One path of light passes through a semiconductor optical amplifier, is controlled by a pulse signal source, and is modulated into pulse light. After being amplified by an erbium-doped fiber amplifier, the pulse light enters the optical fiber to be tested from the b port through the circulator a port. The Rayleigh scattered light and Brillouin scattered light in the optical fiber to be tested return through the circulator b port and enter the a port of the optical filter from the c port. The optical filter outputs the corresponding scattered light sources generated by the first laser and the second laser from the b port and the c port of the optical filter respectively. The output light of the b port is directly detected by the first detector to obtain an OTDR signal, and the data is collected by the acquisition card and transmitted to the computer. The other path of light from the first coupler and the scattered light from the c port of the optical filter interfere at the second coupler. The interference light is converted into photoelectricity by the second detector, and the electrical signal enters the spectrum analyzer to obtain the BOTDR signal, which is transmitted to the computer. Finally, the collected OTDR and BOTDR are jointly analyzed in the computer to obtain the optical fiber distributed loss, temperature and strain information.

[0037] In one implementation, the optical filter is specifically an optical wavelength division multiplexer.

[0038] The BOTDR measurement obtains the distributed optical fiber Brillouin gain spectrum, and the OTDR measurement obtains the distributed optical fiber loss. For a certain position of the optical fiber, its optical fiber Brillouin frequency shift f, optical fiber Brillouin gain g, and optical fiber loss r can be obtained. Among them, the Brillouin frequency shift f is proportional to the temperature and strain of the optical fiber, and the optical fiber Brillouin gain g is related to the temperature and loss of the optical fiber. Therefore, the loss, temperature, and strain information of the optical fiber can be obtained through the following process:

[0039] S1 obtains the OTDR curve through the first detector;

[0040] S2 uses the OTDR curve to calculate the optical fiber loss curve;

[0041] S3 obtains the Brillouin gain spectrum through a spectrum analyzer;

[0042] S4 calculates the Brillouin frequency shift f and the fiber Brillouin gain g at a single position through the Brillouin gain spectrum;

[0043] S5 can obtain the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single location;

[0044] S6 obtains the optical fiber strain information through the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information.

[0045] Ultimately, simultaneous measurement of distributed optical fiber loss, temperature, and strain is achieved.

[0046] In one implementation, S2 uses the OTDR curve to calculate the optical fiber loss curve:

[0047]

[0048] Where R0 is the OTDR measurement result at the starting point of the optical fiber, and R(z) is the intensity of any point z on the optical fiber on the OTDR curve. The OTDR curve is the Rayleigh scattered light intensity at each point on the optical fiber.

[0049] In one embodiment, S3 obtains the Brillouin gain spectrum through a spectrum analyzer. Specifically, BOTDR obtains the Brillouin gain spectrum of each point of the optical fiber through a spectrum analyzer.

[0050] In one implementation, the Brillouin frequency shift f at a single position can be calculated in S4 through the Brillouin gain spectrum. Specifically, each gain spectrum is subjected to Lorentz fitting, and the maximum value of the fitting signal is the Brillouin gain at that point of the optical fiber, and the spectrum frequency corresponding to the Brillouin gain is the Brillouin frequency shift.

[0051] In one implementation, the specific method in S5 for obtaining the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single position is:

[0052]

[0053] Among them, g0 is the Brillouin gain at the starting point of the optical fiber, T0 is the reference temperature at the starting point of the optical fiber, g(z) is the Brillouin gain of point z on the optical fiber, and r(z) is the loss at point z on the optical fiber. Therefore, the temperature T(z) of point z on the optical fiber can be obtained. T0 can be obtained by measuring the air temperature at the starting point of the optical fiber using an electronic thermometer.

[0054] In one embodiment, the method of obtaining the optical fiber strain information by using the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information in S6 is specifically as follows:

[0055] f=f0+A(T-T0)+Bε

[0056] Among them, f0 is the Brillouin frequency shift of the optical fiber at temperature T0, A is the temperature coefficient of the optical fiber, B is the strain coefficient of the optical fiber, ε is the strain of the optical fiber, and f and T are the frequency shift and temperature of a certain point z on the optical fiber, which can also be recorded as f(z) and T(z). According to the formula, when the temperature and Brillouin frequency shift of the optical fiber are measured, the strain at this time can be calculated. The A of ordinary single-mode optical fiber is about 1MHz / ℃, and B is about 0.05MHz / με.

[0057] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A distributed optical fiber loss, temperature and strain measurement system, characterized in that: It includes a first laser, a second laser, a first coupler, a second coupler, a semiconductor optical amplifier, a pulse signal source, an erbium-doped fiber amplifier, a circulator, a first detector, a second detector, an acquisition card, a spectrum analyzer and a computer; The first laser and the second laser are connected to two input ports of the first coupler respectively, an output end of the first coupler is connected to the optical input port of the semiconductor optical amplifier, a pulse signal source is connected to the electrical input port of the semiconductor optical amplifier, an output port of the semiconductor optical amplifier is connected to the optical input port of the erbium-doped fiber amplifier, an optical output port of the erbium-doped fiber amplifier is connected to a port of the circulator, a next port of the circulator along the light transmission direction is connected to the optical fiber to be tested, a third port of the circulator along the light transmission direction is connected to the input port of the optical filter, an output port of the optical filter is connected to the first detector, an acquisition card and a computer in sequence, another output port of the optical filter and another output of the first coupler are connected to two output ports of the second coupler respectively, an output end of the second coupler is connected to the second detector, and the second detector is connected to the spectrum analyzer and the computer in sequence; The process of using this system to obtain the loss, temperature, and strain information of optical fiber is as follows: S1 obtains the OTDR curve through the first detector; S2 uses the OTDR curve to calculate the optical fiber loss curve; S3 obtains the Brillouin gain spectrum through a spectrum analyzer; S4 calculates the Brillouin frequency shift f and the fiber Brillouin gain g at a single position through the Brillouin gain spectrum; S5 obtains the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single location; S6 obtains the optical fiber strain information through the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information; S2 uses the OTDR curve to calculate the fiber loss curve: Where R0 is the OTDR measurement result of the starting point of the optical fiber, and R(z) is the intensity of any point z on the optical fiber on the OTDR curve. The OTDR curve is the Rayleigh scattered light intensity of each point on the optical fiber. The specific method of S5 to obtain the fiber temperature information through the fiber Brillouin gain g and fiber loss r at a single position is: Among them, g0 is the Brillouin gain at the starting point of the optical fiber, T0 is the reference temperature at the starting point of the optical fiber, g(z) is the Brillouin gain of point z on the optical fiber, r(z) is the loss at point z on the optical fiber, and thus the temperature T(z) of point z on the optical fiber is obtained. T0 is obtained by measuring the air temperature at the starting point of the optical fiber with an electronic thermometer.

2. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: The first laser and the second laser are both narrow linewidth lasers, the linewidth of the first laser is 0.08nm to 0.2nm, and the linewidth of the second laser is 0.01nm to 1nm; the wavelength difference between the first laser and the second laser is not less than 3nm, and the filtering wavelengths of the two output ports of the optical filter correspond to the wavelengths of the first laser and the second laser.

3. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: The first coupler and the second coupler are both 3dB couplers, and the splitting ratio is 50:

50.

4. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: The pulse width of the pulse signal source is 10ns to 1000ns.

5. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: The semiconductor optical amplifier has an extinction ratio greater than 40 dB and a rising edge less than 10 ns. The pulses output by the pulse signal source drive the semiconductor optical amplifier to modulate direct current light into pulse light with a high extinction ratio.

6. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: The optical filter is an optical wavelength division multiplexer.

7. The distributed optical fiber loss, temperature and strain measurement system according to claim 1, characterized in that: In S3, specifically, the BOTDR obtains the Brillouin gain spectrum of each point of the optical fiber through a spectrum analyzer; In S4, specifically, a Lorentz fitting is performed on each gain spectrum, the maximum value of the fitting signal is the Brillouin gain at that point of the optical fiber, and the spectrum frequency corresponding to the Brillouin gain is the Brillouin frequency shift.

8. The distributed optical fiber loss, temperature and strain measurement system according to claim 7, characterized in that: S6 The method for obtaining the optical fiber strain information through the optical fiber Brillouin frequency shift f at a single position and the optical fiber temperature information is specifically as follows: f=f0+A(T-T0)+Bε Among them, f0 is the Brillouin frequency shift of the optical fiber at temperature T0, A is the temperature coefficient of the optical fiber, B is the strain coefficient of the optical fiber, ε is the strain of the optical fiber, and f and T are the frequency shift and temperature of a certain point z on the optical fiber, respectively.

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