CN102102998A - Distributed sensing system based on weak Bragg reflection structure - Google Patents

Abstract

The invention provides a distributed sensing system based on a weak Bragg reflection optical fiber, which includes a SG-DBR tunable laser (1), an electro-optical modulator (4), an optical isolator (5), and an optical circulator (6 ), weak Bragg reflection periodic structure fiber (7), they are connected in sequence; SG-DBR tunable laser (1) is controlled by wavelength tuning control circuit (2), electro-optic modulator (4) is controlled by pulse function generator (3) , an optical detector (8) is connected to the optical circulator (6), the control signal output by the wavelength tuning control circuit (2) and the electro-optic modulator (4) and the electrical signal output by the optical detector (8) enter the data together The acquisition card (9) is sent to the data analysis device (10) after processing. The invention realizes long-distance distributed sensing and alarming of parameters such as temperature, stress and vibration, has high detection sensitivity and positioning accuracy, is easy to realize, has low cost and is reliable in operation.

Description

Distributed sensing system based on weak Bragg reflection structured optical fiber

technical field

The invention relates to the field of sensing technology, in particular to the application of weak Bragg reflection periodic structure optical fiber in distributed sensing. Based on the continuous backscattering effect and wavelength modulation sensing characteristics of distributed optical fibers, it combines the innovative technology of "Optical Wavelength Time Domain Reflectometry (OWTDR)" for parameter detection and positioning, and obtains the spatial field distribution of the parameters to be measured along the optical fiber. Distributed sensing and alarm system based on weak Bragg reflection structured optical fiber.

Background technique

The intelligence of materials and structures is a challenging topic in the 21st century. The use of intelligent structure analysis to monitor engineering structure damage and evaluate the working status of high-risk equipment is of great significance to prevent catastrophic accidents and protect the safety of people's lives and property. Therefore, fiber optic sensing technology has developed rapidly in recent years and has attracted more and more attention. Among them, Distributed Optical Fiber Sensing (DOFS) has become a research hotspot in the world. There are three main types of distributed optical fiber sensing technologies that have been reported so far: ① optical time domain reflectometry (OTDR) technology based on fiber backscattering; ② long-distance interferometric technology (Interferometer); Quasi-distributed sensing technology, etc.

Distributed sensing based on optical backscattering technology is realized by optical time domain reflection detection technology: determine the size of the parameter to be measured according to the change of the characteristics of the backscattered light (light intensity, polarization state, frequency, etc.), and determine the position according to the echo time , thus obtaining the spatial distribution of the measured parameters, and the spatial resolution is determined by the pulse width. This type of sensor receives and transmits light at the same end of the optical fiber. A single light source works and the system is simple, but the backscattered light is very weak, the sensitivity of single detection is low, and the real-time performance of multiple average measurements is poor. very high. At the same time, optical intensity sensing is characterized by changes in the optical power of the output signal, which is easily affected by many factors in long-distance detection, and it is difficult to guarantee measurement accuracy and reliability; optical polarization sensing requires polarization-maintaining optical fiber, which is expensive , and extracting polarization state evolution information is technically difficult.

A mix of long-range Mach-Zehnder, Sagnac, and Michelson interferometers enables distributed measurements of time-varying disturbances. For example, dual interferometer structures such as Sagnac/Mach-Zed, Sagnac/Michelson, Sagnac/Sagnac, Mach-Zed/Mach-Zed, and differential ring/ring , including single light source single detector and single light source double detector types. This type of sensor relies on forward-transmitted optical signal interference sensing and specific structure positioning, and has the advantages of high sensitivity, dynamic signal detection, long sensing distance, and good real-time performance. However, interference is manifested as a change in the optical power of the output signal, which is easily affected by many factors in long-distance detection, and measurement accuracy and reliability are difficult to guarantee; polarization fading in communication optical cables as sensing fibers is difficult to solve; phase modulation The type of interference principle makes it difficult to realize multi-point simultaneous detection; the structure is complex. Therefore, interferometric technology is difficult to be practical in long-distance distributed sensing.

FBG sensing is manifested as center wavelength modulation (or wavelength encoding). The change of external parameters can be measured by monitoring the movement of FBG reflection wavelength. The detection ability is not affected by factors such as light source power fluctuations, fiber bending loss, and detector aging. Suitable for long-term security monitoring. However, a major problem faced by FBG-based distributed sensing is that the multiplexing capacity of the system is limited by the bandwidth of the light source and the filter. Generally, an array can only multiplex about 15 gratings. is not enough. Therefore, the research on fiber grating multiplexing technology has received widespread attention. This method solves the problem that the number of multiplexing in wavelength division multiplexing technology is limited by the bandwidth of the light source and wavelength demodulator, but it still needs to demodulate the reflected light wavelength to determine the maximum temperature. , and the biggest disadvantage is that it is impossible to distinguish the specific location where the overheating occurs.

Contents of the invention

The technical problem to be solved by the present invention is to provide a distributed sensing system based on a weak Bragg reflection optical fiber in view of the problems existing in the existing distributed optical fiber sensing technology, so that it can control the temperature, stress, vibration and other parameters. Widely used in long-distance sensing and alarming, the invention has high detection sensitivity and vibration positioning accuracy, and the system is easy to implement, low in cost and reliable in operation.

The present invention solves its technical problem and adopts the following technical solutions:

Distributed sensing system based on weak Bragg reflection structured fiber, including SG-DBR tunable laser, electro-optic modulator, optical isolator, optical circulator, weak Bragg reflection periodic structured fiber, which are connected in turn; SG-DBR tunable laser Controlled by the wavelength tuning control circuit, the electro-optic modulator is controlled by the pulse function generator, and the optical circulator is also connected with a photodetector, the control signal output by the wavelength tuning control circuit and the electro-optical modulator and the electrical signal output by the photodetector enter together The data acquisition card is sent to the data analysis equipment after processing.

The weak Bragg reflection periodic structure optical fiber includes a reference grating with a protection device and an identical weak Bragg reflection periodic structure connected in sequence, and an optical circulator is connected to the reference grating with a protection device.

The optical fiber with a weak Bragg reflection structure includes an outer layer of an ultraviolet transparent coating layer and an inner layer of an optical fiber core, and weak Bragg reflection structures are sequentially distributed on the optical fiber core.

Compared with the prior art, the present invention mainly has the following advantages:

First, the present invention uses a weak Bragg reflection structure (WBRS) as the sensing unit, the reflectivity is several orders of magnitude higher than that of the backscattering technology, the detection is easier and the measurement accuracy is higher, and the reflectivity can be flexibly designed according to the application requirements .

Second, the present invention detects wavelength changes rather than intensity changes, and is not affected by light source power fluctuations, so its performance is more reliable.

Third, the present invention adopts an identical weak Bragg reflection structure (WBRS), and the retroreflectivity of each position is extremely low, which can satisfy long-distance detection without missing measurement information of any position. Therefore, the sensing distance and detection density will be qualitatively improved.

Fourth, the present invention adopts optical wavelength time domain reflectometry (OWTDR) technology to realize detection and positioning at the same time without wavelength demodulation, high real-time performance and low cost.

The invention can break through the predicament of the current optical fiber sensing technology, greatly improve the capacity and sensing distance of the sensing system, and can be widely used in bridge safety control, power transmission and transformation line monitoring, regional perimeter safety warning, nuclear power plant safety monitoring and seabed monitoring and other monitoring areas.

Description of drawings

Figure 1 is a schematic diagram of the principle of a distributed sensing and alarm system based on a weak Bragg periodic structured optical fiber.

Fig. 2 is a schematic diagram of the structure of the weak Bragg reflection periodic structure optical fiber of the present invention. In the figure: In the figure: 1. Sampling grating distributed Bragg reflection (SG-DBR) tunable laser; 2. Wavelength tuning control circuit; 3. Pulse function generator; 4. Electro-optic modulator; 5. Optical isolator; 6. Optical circulator; 7. Weak Bragg reflection periodic structure fiber (WBF); 8. Optical detector; 9. Data acquisition card; 10. Data analysis equipment; 11. Reference grating with protection device; 12. Identical weak Bragg Periodic reflection structure (WBRS); 13. UV transparent coating; 14. Weak Bragg reflection periodic structure; 15. Optical fiber core.

Detailed ways

The invention provides a distributed sensing alarm method and system based on weak Bragg reflection periodic structure optical fiber.

The invention engraves a series of identical weak Bragg reflection structures (WBRS) on the long-distance optical fiber as the sensing unit of the system. The first WBRS is a reference unit (11 in Figure 1), which is placed in a protection device with a constant environment, and its central wavelength is used as a reference wavelength, and the central wavelengths of other WBRS are used as a standard to judge the amount of wavelength drift. The sampled grating distributed Bragg reflection (SG-DBR) tunable laser is used as the system light source, driven by the wavelength tuning control circuit, and the programmed output wavelength changes according to the set channel interval and conversion rate. SG-DBR is a current injection laser, which can achieve high tuning performance: tuning bandwidth is 30nm, channel spacing is 1pm, wavelength conversion time is 100ns, and line width is 4MHz. The narrow-band DC light output by the SG-DBR laser becomes pulsed light through the electro-optic modulator driven by the pulse function generator, and enters the weak Bragg reflection periodic structured fiber (WBF) through the optical isolator and optical circulator. The backscattered pulsed light of the WBRS is coupled into the photodetector through the optical circulator. The output electrical signals of the wavelength tuning control circuit, the pulse function generator and the detector are collected and analyzed through data, and the detection results of the distributed sensing are measured and calculated, and an abnormal alarm signal is issued.

The invention adopts a high-speed multi-channel synchronous data acquisition card to collect output electrical signals of a wavelength tuning control circuit, a pulse function generator and a detector, and then sends the three-channel signals to a computer for data analysis. The output signal of the wavelength tuning control circuit reflects the change of the lasing wavelength of the laser with time, the output signal of the pulse function generator reflects the distribution of the pulse sequence of the incident WBF over time, and the output signal of the detector reflects the backscattered light signal in the WBF over time. The change. n+1 WBRS are engraved on the optical fiber at intervals, the first of which is the reference unit. It is assumed that the wavelength tuning period is T _d =mT ₀ , and the holding time of each wavelength is T ₀ . The wavelength tuning rate is consistent with the pulse modulation repetition frequency, that is, only one optical pulse is emitted in each T ₀ . Taking the pulse emission time as the reference, if the WBRPS can reflect the emitted light pulse, the photodetector should detect the reflected light pulse at the time position of delay τ ₀ , τ ₁ , τ ₂ , . . . , τ _n respectively. In this case, the central wavelength and spatial position of WBRPS are related to time. Therefore, although the light intensity of the reflected signal detected by the photodetector only changes with time, it is closely related to the central wavelength and position of WBRPS. In each T ₀ , record the light intensity values of the reflected signal at τ ₀ , τ ₁ , τ ₂ ,..., τ _n time positions respectively, then in the entire tuning period T _d of the wavelength, each WBRPS will be measured The reflectivity for wavelengths λ ₁ to λ _m , thus yielding the reflectance spectrum of each WBRPS. Because each WBRPS is affected by different external parameters, the corresponding central wavelength of the reflection spectrum is different. Comparing the central wavelength λ _ri (i=1, 2, 3...n) of the sensing unit WBRPS with the central wavelength λ _r0 of the reference unit WBRPS, the central wavelength shift of each WBRPS can be calculated to obtain the field distribution. Therefore, the concept of "Optical Wavelength Time Domain Reflectometry (OWTDR)" is proposed, that is, according to the data analysis of the optical signal in the two-dimensional space of the wavelength domain and the time domain, the distribution of the parameter to be measured is sensed and located.

与应力

的线性变化关系为WBRS is a fiber optic filter that couples modes with opposite transmission directions and reflects specific wavelengths. When affected by temperature, stress, strain or vibration of the external environment, the central wavelength of its narrow-band weak reflection spectrum moves linearly. Taking stress sensing as an example, the central wavelength of WBRS

with stress

The linear variation relationship is

                          （1）

(1)

为应力

引起的中心波长移动，

为光纤的弹光系数。in,

for stress

The shift of the center wavelength caused by

is the elastic-optic coefficient of the fiber.

Using the same WBRS as the sensing unit, the entire bandwidth of the light source can be provided to any WBRS, and the dynamic range and multiplexing capacity of the measurement can be greatly improved. However, the central wavelength and stress characteristics of the same WBRS are completely consistent, so they cannot be encoded by wavelength. In order to be able to distinguish the spatial position while measuring, refer to the positioning principle of OTDR technology, the DC light output by the light source is pulse-modulated and then input to the WBRS array engraved on the WBF, and the time delay between the backscattered light pulse and the input light pulse can be measured. The spatial position of the reflected WBRS is calculated. A high-precision tunable laser is used as the light source of the sensing system to realize wavelength scanning. The output wavelength of the program-controlled tuning laser changes periodically and continuously, and only outputs light of a specific wavelength at each moment. If the central wavelength of the WBRS is consistent with this wavelength, the input light pulse is reflected, otherwise it is completely transmitted. The wavelength of the laser changes continuously for one cycle within the tuning bandwidth, corresponding to all values in the entire measurement range, so that the field distribution along the WBF can be measured.

Preferred embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

The invention provides a distributed optical fiber sensing method, which is characterized in that it is based on the optical wavelength time domain reflection type sensing of the weak Bragg reflection structure optical fiber: the SG-DBR tuning laser 1 is used as the system light source, driven by the wavelength tuning control circuit 2, The program-controlled output wavelength changes according to the set channel interval and conversion rate. The narrow-band DC light output by the light source after modulation is converted into pulsed light by the electro-optic modulator 4 driven by the pulse function generator 3. The pulsed light is optically isolated Device 5 and optical circulator 6 incident WBF 7. Weakly reflective Bragg reflective fiber (WBF) 7 is an optical fiber engraved with periodic low-reflectivity Bragg gratings. The backscattered pulsed light generated by the WBRS in this optical fiber 7 enters the optical detector 8 through the optical circulator 6 . The output electrical signals of the wavelength tuning control circuit 2, the pulse function generator 3, and the optical detector 8 are processed by the data acquisition card 9, and then sent to the data analysis device 10. After analysis and comparison, the distributed sensing detection results are calculated and an abnormality is issued. Alarm.

The invention provides a distributed optical fiber sensing positioning method, which is characterized in that the optical wavelength time domain reflection OWTDR type positioning is integrated with the wavelength modulation sensing technology and the OTDR positioning technology: the light source outputs a narrow pulse sequence with a certain repetition frequency, and simultaneously outputs a wavelength Periodic continuous tuning changes to realize wavelength scanning from the source. At a certain moment, a laser pulse of a specific wavelength is input into the fiber 7 with a weak Bragg reflection structure. If the central wavelength of a certain structural unit 12 at this time drifts to be consistent with the incident laser wavelength due to the influence of external parameters, the input light pulse is in this periodic structure. The position is backscattered, part of the pulse light intensity is reflected, and then a light pulse is returned to reach the photodetector; otherwise, the incident light pulse will be completely transmitted and transmitted forward until it encounters the structural unit that can reflect this wavelength. On the one hand, through the relationship between the reflected light intensity and the laser wavelength, the central wavelength drift can be calculated, thereby calculating the value of the external parameter effect; on the other hand, through the relationship between the reflected light intensity and time, the The time delay between the reflected light pulse and the input light pulse is measured, so as to calculate the spatial position of the external parameter. The wavelength of the laser changes rapidly and continuously for one cycle within the tuning bandwidth, which corresponds to traversing all values in the entire measurement range, so the field distribution along the distributed optical fiber can be measured to locate the measurement object.

As shown in Figure 1, the present invention provides a distributed sensing and alarm system based on weak Bragg reflection optical fiber, which includes a SG-DBR tunable laser 1, a pulse function generator 3, an electro-optic modulator 4, and an optical isolator 5 , optical circulator 6, WBF 7 and photodetector 8, the above-mentioned devices are connected in sequence to form a sensing optical path. Among them, the SG-DBR tunable laser 1 is controlled by the wavelength tuning control circuit 2, and the electro-optical modulator 4 is controlled by the pulse function generator 3. These two control signals and the electrical signal output by the optical detector 8 enter the data acquisition card 9 together. After processing, it is sent to the data analysis equipment 10; the weak Bragg reflection periodic structure optical fiber 7 is composed of a reference grating 11 with a protection device and a WBRS 12 laid in the warning area.

As shown in FIG. 2 , the present invention performs sensing based on a weak Bragg reflection structured optical fiber (WBF) 7 . The optical fiber 7 with a weak Bragg reflection structure includes an outer ultraviolet transparent coating layer 13 and an inner optical fiber core 15 . Weak Bragg reflection structure fiber (WBF) 7 is a special sensing fiber, its fiber core 15 is sequentially distributed with Weak Bragg reflection structure (WBRS) 14, and these reflection structures are continuously written on the fiber through ultraviolet light irradiation to avoid The splicing loss when a single fiber grating sensor is connected in series can achieve longer sensing distance and multiplexing number of sensors.

The refractive index modulation intensity and grating length of the weak Bragg reflection structure (WBRS) of the present invention are smaller than those of common Bragg fiber gratings, and the central wavelength reflectivity is as low as below 10%.

The weak Bragg reflection periodic structure optical fiber of the present invention is designed based on a carrier optical fiber whose coating layer is transparent to 355nm wavelength ultraviolet light, and a series of weak Bragg reflection periodic structures 14 with identical physical structures and optical properties are continuously distributed on it with a certain delay fiber. . The physical essence of this structure is that the refractive index of the fiber core is periodically distributed, and the depth and length of the refractive index modulation can be flexibly designed. The optical properties of this structure are reflected (P _{s )} for a certain wavelength in the incident broadband light (P _in ), and completely transmitted (P _t ) for other wavelengths, but it is different from the strong reflection of the Bragg grating, it The transmission characteristic of is a narrow bandwidth weak reflection, so we call it "weak Bragg reflection periodic structure". When the signal light enters the optical fiber and reaches the position of the weak Bragg reflection periodic structure, a weaker part of the light is reflected by the periodic structure whose central wavelength is consistent with the wavelength of the signal light, and most of the remaining light continues to travel forward until reaching the next periodic structure Location. These structures are sensitive to external parameters such as temperature and stress changes, and can be used as sensing units. Therefore, the entire optical fiber is capable of distributed sensing and detection of external parameters along the optical fiber, so it is called "distributed sensing optical fiber".

由折射率调制强度和光栅长度决定，即According to coupled wave theory and transmission matrix analysis method, the central wavelength reflectivity of fiber Bragg grating and bandwidth

Determined by the refractive index modulation strength and the grating length, that is

                               （2）

(2)

                        （3）

(3)

为耦合系数，

为栅区长度，

为光纤光栅中纤芯的折射率调制幅度，为光栅的周期，

为光纤中传播模式的有效折射率，

为Bragg（布拉格）中心波长。in,

is the coupling coefficient,

is the gate length,

is the refractive index modulation amplitude of the fiber core in the fiber grating, is the period of the grating,

is the effective refractive index of the propagating mode in the fiber,

is Bragg (Pragg) center wavelength.

According to formulas (2) and (3), weak Bragg reflection periodic structures with different central wavelengths and reflectivities can be designed by changing the refractive index modulation intensity and the length of the modulation region. It is entirely feasible to fabricate a Bragg reflection periodic structure with specific reflectivity and bandwidth by using weak photosensitivity fiber and long phase template. Through theoretical analysis and calculation, the intensity of ultraviolet light and the length of scanning and writing can be obtained, and a weak Bragg reflection periodic structure with reflectivity weaker than 1% can be produced.

The above embodiments are only for further description of the present invention, and do not constitute a limitation to the technical solution of the present invention.

Claims (3)

1. A distributed sensing system based on a weak Bragg reflection optical fiber, characterized in that it includes a SG-DBR tunable laser (1), an electro-optic modulator (4), an optical isolator (5), an optical circulator (6 ), weak Bragg reflection periodic structure fiber (7), they are connected in sequence; SG-DBR tunable laser (1) is controlled by wavelength tuning control circuit (2), electro-optic modulator (4) is controlled by pulse function generator (3) , an optical detector (8) is connected to the optical circulator (6), the control signal output by the wavelength tuning control circuit (2) and the electro-optic modulator (4) and the electrical signal output by the optical detector (8) enter the data together The acquisition card (9) is sent to the data analysis device (10) after processing. 2. The distributed sensing system according to claim 1, characterized in that: the weak Bragg reflection periodic structure optical fiber (7) includes a reference grating (11) connected in sequence with a protection device and an identical weak Bragg reflection periodic structure (12), the optical circulator (6) is connected to the reference grating (11) with a protection device. 3. The distributed sensing system according to claim 1, characterized in that: the weak Bragg reflection structure optical fiber (7) includes an outer ultraviolet transparent coating (13) and an inner optical fiber core (15), Weak Bragg reflection structures (14) are sequentially distributed on the fiber core (15).

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