CN102175268A - Quasi-distributed sensor network based on time division multiplex and matched optical fiber grating technology - Google Patents

Abstract

The invention discloses a quasi-distributed optical fiber grating sensing network system based on time division multiplexing and matching optical fiber grating technology, which can accommodate multiple sets of optical fiber grating pairs with center reflection wavelength matching in one optical grating sensing network. A narrow pulse signal sequence is used to directly modulate the broadband light source at high speed. The optical pulse emitted by the broadband light source enters the sensing fiber grating and matching fiber grating sequence through the isolator and circulator. The optical pulse varies with the position of the fiber grating during propagation. The light pulse sequences reflected back with different time delays are photoelectrically converted by photodetectors to form electrical pulse signal sequences. The arrangement position of the pulse sequence in the time domain is related to the spatial position of the reflected fiber grating in the sensor network, so that the signal is processed based on the high-speed circuit, and different gratings are distinguished according to the time position relationship of the pulse signal, and according to the pulse The peak intensity of the signal obtains the wavelength variation of each sensing grating respectively.

Description

Quasi-distributed sensor network based on time division multiplexing and matched fiber grating technology

technical field

The invention relates to the technical field of a method for measuring environmental parameters such as pressure, strain or temperature by using time division multiplexing and matching grating in the field of fiber grating measurement or sensing.

Background technique

Fiber Bragg Grating (or Fiber Bragg Grating, Fiber Bragg Grating - FBG) is currently a hot spot in the research and application of fiber optic sensing. Fiber Bragg Grating sensors have the advantages of anti-electromagnetic interference, high precision, long effective service life, strong wavelength coding multiplexing ability, etc., and have been widely used in the field of sensing in recent years.

One of the outstanding advantages of fiber grating sensors is that it can realize quasi-distributed sensing, that is, engrave multiple fiber gratings on one or more optical fibers, and use multiplexing technology to realize quasi-distributed sensing of various sensing quantities. Measurement. On this basis, the research focuses on the design and development of fiber Bragg grating sensor network. The multiplexing technology that is currently researched and applied more is the wavelength division multiplexing technology (as shown in Figure 1, the center wavelength of each fiber grating is different). In the FBG sensing system using WDM technology, the FBG demodulation speed and system lifetime are limited by the key F-P mechanical scanning optical filter. Using edge filters to demodulate the intensity of the central reflection wavelength variation of fiber gratings can avoid the limitation of mechanical devices, but usually only one fiber grating can be accommodated in the same fiber, and it is impossible to use one fiber to connect multiple gratings in series for quasi-distribution formula measurement. On the other hand, some technologies are also based on time-division multiplexing technology, but they can only use the modulated pulse sequence of a narrow-band light source to enter the sensing grating sequence, and all sensing fiber gratings can only use the same center due to the limited bandwidth of the light source. Wavelength but different reflectivity (generally, the reflectivity near the light source is lower and increases one by one) for time-domain analysis (as shown in Figure 2, the central wavelength of the grating is the same, but the reflectivity is different). Such a solution mainly has the problems of large crosstalk between different reflection points and insufficient sensing accuracy.

Contents of the invention

In view of the shortcomings of the prior art, the purpose of the present invention is to provide a fiber grating sensor network technology with a faster demodulation rate and a larger capacity, so that it can accommodate multiple groups of center reflection wavelength matching in a grating sensor network fiber grating pair.

The purpose of the present invention is achieved by the following means:

A quasi-distributed sensing network based on time-division multiplexing and matched fiber grating technology. The broadband light source is directly modulated by a high-frequency narrow pulse sequence signal, and then enters the sensing fiber grating sequence and matching fiber with edge filtering function after an optical circulator. After the grating sequence is finally received by the photodetector, the high-speed circuit detects and analyzes the sensing signal of the fiber grating based on the principle of optical time domain reflection.

The invention utilizes a narrow pulse signal sequence to directly modulate the broadband light source at high speed. The optical pulse emitted by the broadband light source enters the sensing fiber grating and matching fiber grating sequence through the isolator and the circulator. The light pulse sequences reflected back with different time delays are converted by photodetectors to form electrical pulse signal sequences. The arrangement position of the pulse sequence in the time domain is related to the spatial position of the reflected fiber grating in the sensor network, so that the signal is processed based on the high-speed circuit, and different gratings are distinguished according to the time position relationship of the pulse signal, and according to the pulse The peak intensity of the signal obtains the wavelength variation of each sensing grating respectively. Multiple sets of center-reflected wavelength-matched FBG pairs can be accommodated in a grating sensing network.

The present invention utilizes the advantages of time-division multiplexing networking and the high-speed matching grating to effectively solve the problem that in the traditional solution, for the same series network, the continuous optical signals reflected by the fiber grating at different positions cannot be effectively distinguished when superimposed on the photodetector. The problem makes the fiber grating sensor network can perform quasi-distributed measurement with a faster demodulation rate, which has important use value.

The accompanying drawings are as follows:

Figure 1 is a schematic diagram of a typical wavelength division multiplexing fiber grating sensor network.

Fig. 2 is a schematic diagram of a typical time-division multiplexing FBG sensor network.

Fig. 3 is a schematic diagram of the structure of the fiber grating sensing network proposed by the present invention.

Figure 4 is a schematic diagram of matching fiber grating sensing.

FIG. 5 is a schematic diagram of another structure of the fiber grating sensing network proposed by the present invention.

Fig. 6 is a schematic diagram of the relationship between the pulse sequence received by the photodetector and the arrangement position of the fiber grating.

Fig. 7 is a structural diagram of a typical implementation system of the present invention.

Fig. 8 is an application example of the present invention.

FIG. 9 is a schematic diagram of a larger sensor network based on the present invention.

Detailed ways:

The present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figure 3, the broadband light source 100 is directly modulated by a narrow pulse sequence signal 101, and then enters the sensor after passing through the optical circulator 102 (an isolator can also be added between the light source 100 and the optical circulator 102 to further improve the protection of the light source). Fiber Bragg grating sequence 103, the central reflection wavelength of each grating in the sensing fiber grating sequence is different, so that the broadband optical pulse will be reflected back to a part by each fiber grating respectively; the reflected light will enter the edge filtering function after passing through the optical circulator 102 again The matched fiber grating sequence 104 performs edge filtering on the reflected light respectively; the light pulse after the edge filtering reaches the photodetector 105 for photoelectric conversion to generate an electric pulse sequence signal, and the position sequence of the electric pulse sequence signal in time is related to the quasi-distributed The arrangement position of the measured fiber grating in the fiber link is related; use the high-speed circuit 106 to process and analyze the obtained electrical pulse sequence signal, judge the reflection signals of the fiber grating at different positions according to the order in time, and According to the peak height change of the pulse signal (according to the matching fiber grating principle described later), the variation of the reflected wavelength of the sensing fiber grating center is calculated.

More carefully, the sensing FBG sequence 103 is composed of a series of FBGs with different central wavelengths (103 ₁ , 103 ₂ , ... 103 _N ), and the matching FBG sequence 104 is also composed of a series of FBGs with different central wavelengths Fiber Bragg Grating (104 ₁ , 104 ₂ , ...104 _N ), however, for the fiber Bragg gratings at the same position in the two sequences, they have exactly the same characteristics (such as the central wavelength) to realize the matching function. The principle of using matched gratings for sensing is shown in Figure 4. In the initial state, the two gratings (FBG1 and FBG2) have the same characteristics and completely overlap in the spectrum. A grating is not affected by the outside world, as a reference grating), the coincidence between the two gratings (the shaded part in Figure 4) will also change, so it is only necessary to detect the power change corresponding to the overlapping part of the two gratings. Realize the sensing of the external conditions, and because the two fiber gratings reflect exactly the same temperature, the influence of temperature on the sensing results can be eliminated. The matching fiber grating sequence and the sensing fiber grating sequence are placed in the same external environment to eliminate temperature effects or undergo initial temperature difference calibration to achieve temperature-insensitive strain or pressure sensing. Therefore, this method has the advantages of simple structure, temperature insensitivity, and high-speed sensing (only power detection is required).

A similar principle can also be carried out in the manner shown in Figure 5. After the broadband light source 200 is driven and modulated at high speed by 201, it enters the fiber grating sequence through the circulator 202. The difference from Figure 3 is that the two fiber grating sequences (transmission Sensing FBG sequence and matching FBG sequence) are distributed on the same link, wherein the sensing FBG sequence is 203 ₁ , 203 ₂ ... 203 _N , while the matching FBG sequence is 203 ₁ ', 203 ₂ ',...203 _N '. The arrangement is in the order of 203 ₁ , 203 ₁ ′, 203 ₂ , 203 ₂ ′…203 _N , 203 _N ′. Similar to Figure 3, gratings with different serial numbers have different central reflection wavelengths, and fiber gratings with the same serial number (FBG _i and FBG _i ￠, such as 203 ₁ and 203 ₁ ') in the two sequences have the same properties as a pair of matched gratings (original central wavelength), where the sensing grating is used to measure changes in external parameters, while the matching grating serves as a reference. The principle has been illustrated in Figure 4. It is worth noting that as a pair of matched gratings (FBG _i and FBG _i ¢), the distance between these two gratings should be smaller than the spatial resolution of the system detection (determined by the high-speed circuit and the nature of the light source). The optical signal reflected by the entire sensor network passes through the circulator 202 and enters the photodetector 204 for reception, and then is analyzed and processed by the high-speed circuit 205 .

In addition, the measurement principle based on time-division multiplexing is somewhat similar to that of optical time-domain reflectometry. The broadband light source emits an optical pulse signal sequence under the direct modulation of a narrow pulse signal sequence. The time domain width of the pulse signal depends on the minimum length of the optical fiber connection line between the fiber gratings in the fiber grating array for quasi-distributed measurement. Generally, it can be determined by the following Calculated by the formula

Where t is the time domain width of the narrow pulse signal, △L is the minimum length of the fiber connecting line between the fiber gratings in the fiber grating array for quasi-distributed measurement, n is the effective refractive index of the fiber, c is the light in vacuum transmission speed. The emission time interval T between adjacent narrow pulses depends on the total length of the fiber connection line in the fiber grating array for quasi-distributed measurement, and can generally be calculated by the following formula

Among them, L is the total length of the optical fiber connection line in the fiber grating array for quasi-distributed measurement, n is the effective refractive index of the optical fiber, and c is the propagation speed of light in vacuum. After the light pulse sequence is detected by the photodetector, the arrival time of each light pulse in the sequence varies with the reflection position in the sensing fiber grating sequence, as shown in FIG. 6 .

In addition, since the original performance of the two fiber grating sequences is the same, if the two sequences are placed in the same external environment (mainly referring to temperature), due to the temperature insensitivity of the matching fiber grating, accurate perception of parameters such as strain or pressure can be achieved .

Figure 7 describes our specific implementation example. The overall experimental setup is based on the principle shown in Figure 3. The broadband light source uses a superluminescent light-emitting diode (SLED), and its modulation bandwidth can reach more than 100MHz. In the experiment, we used four groups of matching gratings with different wavelengths as the sensor network nodes, four of which were used as sensing sequences, and the other four were used as edge filter matching sequences. The photodetector uses an integrated PIN-FET with a bandwidth of 50MHz and a photoelectric conversion efficiency greater than 100mV/mW.

In order to prevent the reflected light caused by end face reflection from damaging the broadband light source, the light pulse first passes through an isolator, then enters a port 1 of the circulator, and then comes out from port 2 to be coupled to a fiber grating sequence for quasi-distributed sensing ( Fiber Bragg Gratings with 4 different center wavelengths), the light pulse encounters the Fiber Bragg Gratings with different center reflection wavelengths at different positions during the propagation process, and part of it is reflected to form a light pulse with less intensity, and its shape in the spectrum is the same as The position depends on the fiber grating forming the reflection. In order not to cause "crosstalk" phenomenon, a certain interval is required between the reflected wavelengths of the fiber grating center, generally 2nm is appropriate (5nm in the experiment). The optical pulses reflected by each fiber grating form an optical pulse sequence due to the time difference, enter the port 2 of the circulator, and then couple out from port 3 to the matching fiber grating sequence for edge filtering. In order to effectively demodulate the change of the FBG center reflection wavelength in the FBG sequence for quasi-distributed sensing, in the matching FBG sequence, it is necessary to set There are no special requirements for the spatial arrangement position and the length of the connecting fiber for the matched fiber gratings (4 in the experiment). Due to the edge filtering effect of the corresponding matched grating in the optical pulse sequence in the matched fiber grating sequence, the intensity of the optical pulse will change approximately linearly with the change of the reflected wavelength difference between the matched fiber grating and the center within a certain range. Finally, the light pulse sequence reaches the PIN-FET detector, and the arrival time of each light pulse in the sequence varies with the reflection position in the sensing FBG sequence (as shown in Figure 6). PIN-FET transforms the light pulse sequence into the electrical pulse sequence with the same time sequence, which is analyzed and processed by the high-speed circuit. In the experiment, the drive circuit and analysis circuit of SLED are integrated into the same module.

It can be seen from this embodiment that since the reflected light of different fiber gratings reaches the photodetector at different times, it is easy to distinguish the reflected light of fiber gratings, thereby using multiple sensors in a series sensor network (optical fiber) Matched grating pairs for quasi-distributed measurements. The change of the central reflection wavelength of a node (fiber grating) in the sensing fiber grating sequence leads to the misalignment of the central wavelength of the corresponding node (fiber grating) of the matching fiber grating sequence, resulting in the peak value of the pulse signal detected by the photodetector within the corresponding time period Changes in intensity.

The photodetector analyzes the change of the central wavelength of each independent fiber grating in the sensing fiber grating sequence caused by the external influence according to the change of the reflected power at different times.

It can be widely used in distributed strain/pressure monitoring occasions, and it is not expected to be related to temperature, such as the strain state of high-speed railway tracks, the state of fan blades, the safety of bridge structures, the crack monitoring of oil (gas) pipelines, etc., as shown in Figure 8 Show. For some different applications, different network structures can be used (Figure 3 or Figure 5). For example, in the monitoring of fan blades, the structure in Figure 3 is more suitable, while in the monitoring of strain status of high-speed railway tracks, the structure in Figure 5 The structure is more convenient (put the matching grating in a state of no external strain).

In addition, the present invention can be further combined with multiplexing methods such as space division or wavelength division to form a larger sensor network. As shown in the example in Figure 9, after splitting the modulated broadband light source, it can be used in different spatial locations (Structure) shared by multiple sensor networks to achieve a wider range of monitoring.

To sum up, the present invention has the advantages of no moving parts, high speed, low crosstalk, etc., and is widely used, therefore, has great application prospects.

Claims (5)

1. based on the quasi-distributed sensing network of time division multiplex and matched fiber grating technology, it is characterized in that, wideband light source is directly modulated by the high-frequency narrow-pulse sequence signal, behind optical circulator, enter sensor fibre grating sequence then and have the matched fiber grating sequence of edge filter function, after fiber grating sensing signal is detected and analyzes based on the optical time domain reflection principle by high speed circuit after the photodetector reception.

2. the quasi-distributed sensing network based on time division multiplex and matched fiber grating technology according to claim 1 is characterized in that, has the fiber grating sequence FBG of sensing function ₁, FBG ₂FBG _iFBG _NBe connected on the same optical fiber link, have the matched fiber grating sequence FBG of edge filter function ₁￠, FBG ₂￠ ... FBG _i￠ ... FBG _N￠ is connected on the same optical fiber link, and sensor fibre grating sequence and matched fiber grating sequence are positioned at the different port of optical circulator, and the light of sensor fibre grating sequence reflected back is once more by entering the matched fiber grating sequence behind the optical circulator.

3. the quasi-distributed sensing network based on time division multiplex and matched fiber grating technology according to claim 1 is characterized in that, sensor fibre grating sequence FBG ₁, FBG ₂FBG _iFBG _NWith matched fiber grating sequence FBG ₁￠, FBG ₂￠ ... FBG _i￠ ... FBG _N￠ is connected on the same optical fiber link and forms sensing network, and its arrangement mode is according to FBG ₁, FBG ₁￠, FBG ₂, FBG ₂￠ ... FBG _i, FBG _i￠ ... FBG _N, FBG _NThe order of ￠ is formed, and the sensor fibre grating is used to measure external parameter to be changed, and matched fiber grating directly enters photodetector and treatment circuit by the light of whole sensing network reflected back after by optical circulator as a reference.

4. the quasi-distributed sensing network based on time division multiplex and matched fiber grating technology according to claim 1, it is characterized in that, sensor fibre grating sequence has different foveal reflex wavelength with each fiber grating in the matched fiber grating sequence, and the fiber grating of same sequence number has identical archicenter reflection wavelength in two sequences.

5. the quasi-distributed sensing network based on time division multiplex and matched fiber grating technology according to claim 3, it is characterized in that, the fiber grating of same sequence number is right as matched fiber grating in sensor fibre grating sequence and the matched fiber grating sequence, and its space length is less than the spatial resolution of detection system.

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