CN114235018A - Temperature-adaptive FBG demodulation method and system - Google Patents
Temperature-adaptive FBG demodulation method and system Download PDFInfo
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- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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Abstract
The invention discloses a temperature self-adaptive FBG demodulation method and a system, comprising the following steps: selecting VCSELs meeting set requirements based on the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control; simultaneously selecting the wavelength of a methane absorption peak and the wavelength of the FBG, so that the scanning wavelength range of the VCSEL always covers the central wavelength of the FBG, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range; and in each preset temperature range, determining the FBG central wavelength value and the sensed variation according to the wavelength position of the methane absorption peak and the position of the FBG central wavelength. The invention has the beneficial effects that: under the condition of not using a temperature control device, the change value of the FBG central wavelength can be effectively measured, so that the whole sensor measuring system meets the requirements of low power consumption, low cost and miniaturization.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing/photoelectric detection, in particular to a temperature-adaptive FBG demodulation method and system.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
In the monitoring of the roof stress and the drilling stress of the underground coal mine roadway, a traditional coal mine mechanical roof separation sensor and a traditional drilling stress sensor are often used, and the sensors are simple in structure, low in cost and widely applied to coal mines. The existing roof dynamic monitoring means mainly comprise a mechanical manual monitoring technology and an electronic monitoring technology. However, because the sensors have the defects of low measurement accuracy and difficulty in real-time online monitoring, in actual use, a professional is required to regularly go into a well for inspection and recording, so that hidden dangers cannot be found in time, and faults cannot be cleared and dangers cannot be eliminated in time. The general electronic top plate stress and drilling hole stress sensor is mainly based on the working principle of a strain gauge, the resistance of the strain gauge is changed by external stress change, and the change of output voltage generated by the change of the resistance of the strain gauge can be measured by using an electric bridge, so that the purpose of detecting the stress change is achieved. Because the underground working environment is high humidity and much dust, the electronic element is easily influenced by underground water and dust, and the problems of short service life of the sensor, easy electromagnetic interference on signal transmission, easy data false alarm and the like are caused.
Compared with the traditional electronic sensor, the FBG (fiber Bragg grating) stress sensor has the advantages of no electricity, intrinsic safety, strong anti-electromagnetic interference capability, easiness in multiplexing, simple structure and the like. The stress changes monitored by the FBG stress sensor are demodulated and calculated by the FBG wavelength demodulator. In the FBG demodulator, the change of the measured physical quantity can be calculated by detecting the drift amount of the FBG central wavelength. Currently, various FBG demodulation techniques are widely applied to FBG demodulators, such as edge filtering, tunable F-P filter, interferometric demodulation, matched FBG, wavelength scanning laser, etc. Most of the methods use a wide-spectrum light source or a DFB light source, and the light source needs to be controlled at constant temperature, so that the whole sensing system has high power consumption, high production cost and high maintenance difficulty.
Aiming at the applications of monitoring the temperature of the underground coal mine roadway, the stress of the top plate and the like, the explosion-proof characteristic of the FBG demodulator is also required to be considered. The prior art discloses an FBG demodulation device based on VCSEL (vertical cavity surface emitting laser) and an operating method thereof, wherein 5 absorption peaks of acetylene gas in a C wave band 1527-1530nm are used as reference wavelengths to measure the central wavelength of the FBG, and compared with the traditional demodulation method, the method effectively reduces partial power consumption of a demodulator. Generally, the temperature/wavelength coefficient of the C-band VCSEL is about 0.11 nm/deg.c, and since 5 absorption peaks of acetylene gas at 1527-. However, when the laser uses a temperature control system (TEC), the power consumption and cost of the whole sensing system increase accordingly, and the system structure becomes complicated, especially the power consumption cannot meet the intrinsic safety requirement.
Disclosure of Invention
In order to solve the problems, the invention provides a temperature self-adaptive FBG demodulation method and a temperature self-adaptive FBG demodulation system, wherein methane gas with a larger distribution range of an absorption peak is selected as a reference wavelength to demodulate the central wavelength of the FBG, and meanwhile, the wavelength scanning range of a VCSEL and the central wavelength of the FBG are selected and matched, so that the change value of the central wavelength of the FBG is effectively measured under the condition of not using a temperature control device, and the whole sensor measurement system meets the requirements of low power consumption, low cost and miniaturization.
In some embodiments, the following technical scheme is adopted:
a temperature adaptive FBG demodulation method comprising:
selecting VCSELs meeting set requirements based on the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control; simultaneously selecting the wavelength of a methane absorption peak and the wavelength of the FBG, so that the scanning wavelength range of the VCSEL always covers the central wavelength of the FBG, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range;
and in each preset temperature range, determining the FBG central wavelength value and the sensed variation according to the wavelength position of the methane absorption peak and the position of the FBG central wavelength.
In other embodiments, the following technical solutions are adopted:
a temperature adaptive FBG demodulation system comprising:
the optical fiber coupling device comprises a VCSEL, an optical fiber isolator, an optical fiber splitter, an optical fiber coupler and an FBG sensor which are connected in sequence; one port of the optical fiber coupler is connected with the photoelectric detection circuit and the analog-to-digital converter, the output of the analog-to-digital converter is connected with the microprocessor, and the output end of the microprocessor is connected with the VCSEL after being sequentially connected with the digital-to-analog converter and the current driving circuit;
the optical fiber branching unit is provided with an input port and eight output ports, and one path of optical fiber of each output port is directly transmitted to the photoelectric detector through the coupler; one path of optical fiber passes through the coupler and a reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; the eight paths of optical fibers are input into the FBG sensor after passing through the coupler, and are output to the photoelectric detector after being reflected by the sensing FBG;
by adopting the temperature self-adaptive FBG demodulation method, the microprocessor receives the environmental temperature and pressure measurement data, the VCSEL is driven by the sawtooth wave current driving circuit, and the wavelength range corresponding to the driving current of the VCSEL is determined according to the current VCSEL temperature and driving current change value measured by the thermistor in the VCSEL to form periodic wavelength scanning;
and adjusting the scanning range of the wavelength of the VCSEL according to the position of the methane absorption peak in the VCSEL working temperature range to cover at least one methane absorption peak wavelength.
Compared with the prior art, the invention has the beneficial effects that:
(1) the invention selects methane gas with larger distribution range of absorption peaks as reference wavelength to demodulate the FBG central wavelength, and simultaneously selects and matches the wavelength scanning range of the VCSEL and the FBG central wavelength to effectively measure the change value of the FBG central wavelength under the condition of not using a temperature control device, so that the whole sensor measuring system meets the requirements of low power consumption, low cost and miniaturization.
(2) Because the power consumption of the whole measuring system is very low, the system can be driven by a battery to operate, and a mobile sensor node of a wireless sensor network can be made by adding the wireless transmission module, so that the practicability and flexibility of the sensor are greatly improved. By adopting the scheme, the power consumption of the system and the cost of the demodulation device are reduced, the size is reduced, the device not only meets the intrinsic safety requirement, but also is convenient to construct and install, and the problem of difficulty in getting electricity in the underground coal mine can be solved.
(3) The method of the invention does not need a laser temperature control device, and can carry out reference and compensation by dynamically selecting 1 to 2 absorption peaks which can be scanned by the VCSEL at the current temperature under different working temperatures, so that the VCSEL can normally work in a specific range. The FBG demodulation method and the FBG demodulation system with low power consumption and self-adaptive temperature are particularly suitable for demodulating signals of physical quantity sensors for measuring and monitoring temperature, roof stress, drilling stress and the like in a coal mine underground roadway by using FBGs.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
Drawings
FIGS. 1(a) - (b) are schematic diagrams of the relationship between the wavelength value of VCSEL and the methane absorption peak and the effective working area of FBG, respectively, determined to be used in the embodiment of the present invention;
FIG. 2 is a schematic diagram of a low-power consumption and temperature-adaptive FBG demodulation device for coal mine stress monitoring according to an embodiment of the invention;
the system comprises a VCSEL (vertical cavity surface emitting laser), a fiber isolator, a fiber splitter, a first fiber coupler, a second fiber coupler, a reference gas chamber, a photoelectric detector, a linear transimpedance amplifier, an analog-to-digital converter, a microprocessor, a digital-to-analog converter, a current driving circuit, a communication interface, a FBG (fiber Bragg Grating) sensor and a FBG (fiber Bragg Grating) sensor, wherein the VCSEL 1, the fiber isolator 2, the fiber splitter 3, the first fiber coupler 4, the second fiber coupler 5, the reference gas chamber 6, the photoelectric detector 7, the linear transimpedance amplifier 8, the analog-to-digital converter 9, the microprocessor 10, the digital-to-analog converter 11, the current driving circuit 12, the communication interface 13 and the FBG sensor are arranged in a distributed mode.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Example one
In one or more embodiments, disclosed is a temperature adaptive FBG demodulation method, including the following processes:
step (1): determining the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control;
specifically, the output wavelength characteristics of the VCSEL under the condition without temperature control are mainly shown in the following two points:
1) in the absence of a laser temperature control device, the size of the central wavelength of the VCSEL, or the position of the wavelength on the spectrum, varies with the ambient temperature of the laser, and the temperature of the VCSEL has an approximately linear relationship with the scanning wavelength: k is a temperature coefficient, and K values of different VCSELs are different. So that the center wavelength can vary from 1642-1654nm to 1646-1658nm when the temperature of the different VCSELs varies from 0 ℃ to 40 ℃.
2) In the wavelength range of 1640-1660nm, the driving current of the VCSEL approximates a quadratic curve relation with the scanning wavelength: λ ═ aI2+ bI + c, when the VCSEL operates under the driving of a sawtooth wave scanning current with amplitude change of 1-14mA, the VCSEL wavelength change range is about 5-6 nm, and the VCSEL wavelength scanning range is formed. Therefore, when selecting the laser according to the wavelength characteristics of the VCSEL, it is required that the shortest wavelength of the different VCSELs at 0 ℃ to the longest wavelength of the VCSELs at 40 ℃ can cover the whole wavelength range of 1640-1660nm in the operating temperature range of 0 ℃ to 40 ℃, and 1 to 2 methane absorption peaks exist in the VCSEL scanning wavelength range at each temperature.
The VCSEL is selected according to the following parameters: wavelength values λ @0 ℃ (nm) and λ @40 ℃ (nm) of the VCSEL at 0 and 40 ℃, respectively, VCSEL wavelength vs. drive current variation Δ λ/Δ I (nm/mA) over a temperature variation range of 0-40 ℃: 0.5nm/mA, VCSEL wavelength and temperature change relation delta lambda/delta T (nm/DEG C) in the scanning current change range: 0.122 nm/deg.C, the variation of VCSEL output power and drive current in the temperature variation range of 0-40 deg.C, and the maximum drive current range of VCSEL. By analyzing these parameters, the VCSEL can be selected for a range of wavelength variations, or a range of wavelength sweeps, at different operating temperatures.
The present example determines through a number of experimental tests that the relationship Δ λ/Δ T (nm/deg.c) between the wavelength variation of the VCSEL used and the temperature variation at the center wavelength current is 0.116 nm/deg.c to 0.128 nm/deg.c, and the average value is 0.122 nm/deg.c; and the relation delta lambda/delta I (nm/mA) between the wavelength change of the VCSEL and the change of the driving current is 0.418 nm/mA-0.550 nm/mA, the average value is 0.5nm/mA, the relation between the output power of the VCSEL and the change of the driving current in the temperature change range of 0-40 ℃, and the maximum driving current range of the VCSEL. Determining the wavelength scanning range of the VCSEL at different working temperatures according to the group of relations, and further determining the maximum driving current and wavelength of the VCSEL at a low temperature, namely realizing the maximum scanning wavelength of the VCSEL without temperature control; determining the minimum driving current and wavelength of the VCSEL at high temperature, namely realizing the minimum scanning wavelength of the VCSEL without temperature control; finally, the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control is determined.
Step (2): and selecting the VCSEL meeting the set requirement, and selecting the wavelength of the methane absorption peak and the wavelength of the FBG at the same time, so that the scanning wavelength range of the VCSEL always covers the central wavelength (FBG peak wavelength) of the FBG, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range.
For the selection of the wavelength of the methane absorption peak, it is known that methane gas molecules have a set of unique infrared absorption spectrum lines in the infrared band, and the spectral position of the lines does not change with the change of the external environmental conditions, so that each absorption peak has an inherent absorption wavelength in the spectral range, and the absorption peaks form a set of 'known wavelength scales' in the spectrum, and the position of the FBG reflection peak in the spectral range and the wavelength variation of the FBG reflection peak caused by the external measurement can be measured and calculated by using the set of known wavelength scales as a reference. In the embodiment, several strong intrinsic absorption peaks of methane gas in 1640-1660nm wave band are selected as known wavelength scales: 1640.374nm,1642.914nm,1645.561nm,1648.234nm,1650.961nm,1653.723nm,1656.546nm and 1659.413nm, and the interval between every two absorption peaks in 8 methane absorption peaks is about 2.72nm on average.
In this example, the wavelength sweep ranges from the threshold current to the inflection point current of the VCSEL to be used at 0 ℃ and 40 ℃ are determined to be 4.366-5.527nm and 2.870-3.676nm, respectively, and the average values are 4.822nm (at 0 ℃) and 3.265nm (at 40 ℃), respectively, through a large number of experimental tests, and further, the overall wavelength sweep range of the VCSEL at 0-40 ℃ is determined to be average: 4.96nm +2.411nm +1.6325 nm-9.0035 nm, wherein 4.96nm is the average value of the central wavelength shift, 2.411nm is half of the wavelength scanning range at 0 ℃, and 1.6325nm is half of the wavelength scanning range at 40 ℃. It is known that the average interval of the methane absorption peaks is 2.72nm, and further that when the VCSEL temperature is changed from 0 ℃ to 40 ℃, the VCSEL can cover at most 4 and at least 3 methane absorption peaks in the whole wavelength scanning range.
For the selection of the FBG wavelength, the selection of the FBG wavelength needs to satisfy the following three conditions:
1) the FBG wavelength plus the measured variation should always be within each scanning wavelength range of the VCSEL;
2) 1 to 2 methane absorption peaks exist in each scanning wavelength range of the VCSEL;
3) the above two conditions can be satisfied within the temperature range of 0-40 ℃. Thus, the scanning wavelength range of the VCSEL can always cover the central wavelength of the FBG, the wavelength variation introduced by sensing and 1 to 2 corresponding methane absorption peaks in the temperature range of 0 to 40 ℃.
FBG wavelength matches VCSEL scanning wavelength range: the maximum scanning wavelength of the VCSEL at low temperature and the minimum scanning wavelength at high temperature determine an effective wavelength scanning range (the wavelength range jointly covered by the low temperature and the high temperature), and the working wavelength of the grating needs to be selected within the effective wavelength scanning range; the range is the wavelength range that VCSELs can scan at any temperature from low temperature to high temperature, and the effective wavelength scanning range is determined according to the temperature difference from low temperature to high temperature.
Taking a top plate delamination sensor and a drilling stress sensor as examples, according to the measuring range (wavelength variation range: about 1-1.2 nm) of the top plate delamination sensor, the measuring range (wavelength variation range: about 0.8-1 nm) of the drilling stress sensor and FBG parameters (central wavelength, 3dB bandwidth and the like), the VCSEL scans the maximum wavelength at low temperature and needs to reach the central wavelength of the FBG +2nm at the current temperature, specifically, the central wavelength of the FBG +1.2 (maximum variation of the sensor wavelength) +0.2nm (prestress during sensor packaging) +0.3nm (allowance, so that the condition that the central wavelength of the grating) +0.3nm (FBG wavelength 3dB bandwidth) cannot be scanned due to error variation is prevented; the minimum scanning wavelength of the VCSEL at high temperature needs to be smaller than the central wavelength of the FBG at the current temperature by-0.4 nm, specifically, the central wavelength of the FBG at the current temperature is-0.3 nm (FBG wavelength bandwidth) +0.2nm (prestress during sensor packaging) and-0.3 nm (allowance is provided to prevent the central wavelength of the grating from being not scanned due to error change). Taking an FBG with a central wavelength of 1648.5 ± 0.2nm (room temperature, usually about 20 ℃) as an example, an FBG sensor corresponding to a demodulation device mainly comprises a temperature sensor, a top plate separation layer, a borehole stress sensor and the like, and the full range of the sensor is within 1.2nm, so that a VCSEL matched with a sensor (temperature sensor, top plate sensor and borehole) with a central wavelength of 1648.5 ± 0.2nm needs to scan a maximum wavelength of 1650.5 ± 0.2nm at a low temperature and a minimum wavelength of 1648.1 ± 0.2nm at a high temperature, that is, the effective wavelength scanning range of the VCSEL needs to satisfy:
1648.1 + -0.2 nm-1650.5 + -0.2 nm (about 2.4 nm).
And (3): determining the FBG central wavelength value and the sensed variation according to the wavelength position of the methane absorption peak and the FBG central wavelength within each preset temperature range;
the preset temperature range is as follows: 0-20 ℃ and 20-40 ℃. In each temperature range, the scanning range of the VCSEL wavelength covers 1-2 methane absorption peaks, and the FBG wavelength change is measured and calculated by using 1-2 corresponding methane absorption peaks as wavelength references. And determining the FBG central wavelength value and the sensed variation according to the wavelength position of the methane absorption peak and the position of the FBG central wavelength. Under different working temperatures, different methane absorption peaks are dynamically selected and locked, so that the position of the central wavelength of the FBG can be demodulated by the demodulation system under the conditions of high temperature and low temperature according to the position of the methane absorption peak.
In this example, the covered methane absorption peaks are in order: 1645.561nm and 1648.239nm (0 to 20 ℃), 1648.239nm and 1650.959nm (20 to 40 ℃), as shown in FIGS. 1(a) - (b); the methane absorption peak covered in the above set temperature range is one example due to the difference in the wavelength scanning range of different VCSELs. Measuring the temperature of a VCSEL chip by using a thermistor inside the VCSEL, thereby determining the working temperature range of the VCSEL, and further determining 1-2 methane absorption peaks covered in the VCSEL wavelength scanning range according to the preset temperature range; then, a driving circuit of the VCSEL is tuned by utilizing the sawtooth wave, so that the wavelength of the VCSEL is changed, and periodic wavelength scanning is formed; different calculation methods are selected according to the number of the appeared methane absorption peaks:
1) when two methane absorption peaks exist in the wavelength scanning range of the VCSEL, the corresponding wavelength change value represented between every two scanning sampling points is calculated by utilizing the known wavelength values of the two methane absorption peaks, and then the wavelength value corresponding to the FBG peak is calculated by measuring the position of the corresponding sampling point when the FBG peak is measured and adding the known wavelength value of one methane absorption peak and the corresponding wavelength change value represented between every two scanning sampling points. Taking the example that two absorption peaks of 1648.239nm and 1650.959nm appear in the wavelength scanning range of the selected VCSEL when the VCSEL is operated at 20-40 ℃, the FBG value is calculated as follows:
2) when only one methane absorption peak exists in the VCSEL wavelength scanning range, for example at 20 ℃, the methane absorption peak is switched, the absorption peak is positioned at the center of the VCSEL wavelength scanning range, and the condition that only one methane absorption peak exists is that the methane absorption peak is 1648.239nm, 1650.959nm or 1653.757nm …; for example, at 40 ℃, the VCSEL wavelength scan becomes smaller, so there are cases where only one methane absorption peak occurs, which may be 1648.239nm, 1650.959nm, 1650.959nm, 1653.757nm …; the wavelength of the absorption peak is determined according to the temperature value measured by the thermistor of the laser, and then the bandwidth value of the absorption peak at the position of 3dB is calculated by utilizing the normalized gas absorption peak. Because the pressure of the reference air chamber is unchanged, the width value of the absorption peak value 3dB bandwidth of the reference air chamber is also unchanged, and the central wavelength of the FBG is calculated and demodulated by measuring the number of two sampling points corresponding to the 3dB bandwidth, the number of sampling points corresponding to the FBG peak value and the wavelength value of the absorption peak. Taking the VCSEL operating at 20 ℃ as an example, the VCSEL wavelength scanning range shows an absorption peak of 1648.239nm, and the FBG value is calculated as follows:
3) when any methane absorption peak exists in the wavelength scanning range of the VCSEL, the current parameter of the VCSEL at 20 ℃ is preset, the driving current value corresponding to the sampling point position is obtained by measuring the sampling point position corresponding to the peak value of the FBG, and then the FBG central wavelength corresponding to the sampling point can be calculated according to the quadratic curve relation of the wavelength driving current of the VCSEL at 20 ℃, the linear relation of the VCSEL wavelength and the temperature value of the VCSEL. The FBG value was calculated as follows:
and (4): the method adopts the methane absorption peak wavelength in the VCSEL wavelength scanning range as a fixed wavelength reference point, and corrects the measurement temperature deviation caused by different positions of the thermistor in the VCSEL and the VCSEL chip by using the fixed methane absorption peak wavelength so as to accurately measure the FBG peak wavelength and the variation value thereof.
Because the positions of the thermistor in each VCSEL and the VCSEL chip are different, the scanning wavelength corresponding to the actual scanning wavelength range of the VCSEL or a certain sampling point has certain deviation from the scanning wavelength range calculated by the microprocessor according to the current temperature or the scanning wavelength corresponding to the certain sampling point, so that the FBG peak wavelength which is actually demodulated has deviation. In the embodiment, the methane absorption peak wavelength in the wavelength scanning range of the VCSEL is used as a fixed wavelength reference point, the fixed methane absorption peak wavelength is used for correcting the measurement temperature deviation generated by different positions of the thermistor in the VCSEL and a VCSEL chip, the demodulation precision is improved, and the purpose of accurately measuring the FBG peak wavelength and the variation value thereof is achieved.
The specific correction method comprises the following steps: actual methane absorption peak λ1The corresponding sampling point is X, and the wavelength corresponding to the sampling point X calculated according to the laser current wavelength parameter is lambda2Since the VCSEL wavelength is approximately linear with temperature, the corrected temperature error is:
example two
In one or more embodiments, disclosed is a temperature adaptive FBG demodulation system, referring to fig. 2, including:
the device comprises a VCSEL (1), a fiber isolator (2), a fiber splitter (3), fiber couplers (4) and (5), a reference gas chamber (6), a photoelectric detector (7), a linear transimpedance amplifier (8), an analog-to-digital converter (9), a microprocessor (10), a digital-to-analog converter (11), a current driving circuit (12), a communication interface (13) and an FBG sensor (14).
The VCSEL (1) is connected with the optical fiber isolator (2) and then is connected with the input end of a 1 x 8 optical fiber branching device (3), the first path to the sixth path of the optical fiber branching device are connected with a 2 x 1 optical fiber coupler (4) and then are connected with an FBG sensor (14), and the other path of two ports of the 2 x 1 optical fiber coupler (4) is connected with a photoelectric detection circuit (7) and an analog-to-digital converter (9); a seventh path and an eighth path of the optical fiber branching unit are respectively connected with 2 x 2 optical couplers (5), wherein one path of output end of the seventh path of 2 x 2 optical fiber coupler is connected with one FBG (14), and the other end of the seventh path of 2 x 2 optical fiber coupler is connected with one photoelectric detector (7), a linear transimpedance amplifier (8) and an analog-to-digital converter (9); one output end of the eighth path of 2 x 2 optical fiber coupler (5) is connected with one FBG (14), and the other end of the 2 x 2 optical fiber coupler is connected with the methane gas chamber (6), the photoelectric detector (7), the linear transimpedance amplifier (8) and the analog-to-digital converter (9). The output of the analog-to-digital converter (9) is connected with a microprocessor (10), the output end of the microprocessor is connected with a digital-to-analog converter (11) and a communication interface (13), and the digital-to-analog converter (11) is connected with the VCSEL (1) through a current driving circuit (12).
The 1 x 8 optical fiber branching unit is provided with an input port and eight output ports and is used for uniformly distributing a light source to eight paths of output, and after the light is split by the eight paths of output, one path of optical fiber is directly output to a photoelectric detector through a coupler; one path of optical fiber passes through the coupler and a reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; the eight paths of optical fibers are input into the FBG sensor after passing through the coupler, and are output to the photoelectric detector after being reflected by the sensing FBG.
The left side of the 2 x 1 optical fiber coupler is provided with two ports, the right side of the 2 x 1 optical fiber coupler is provided with one port, one end of the port on the left side is used for being connected with the output end of the optical fiber branching unit, the other end of the port on the left side is used for being connected with the photoelectric detector, and one end of the port on the right side is used for being connected with the FBG sensor. The light emitted by the VCSEL enters the right port from one end of the left port and then enters the two ends of the left port from the right port to be output.
Two sides of the 2 multiplied by 2 optical fiber coupler are respectively provided with two ports, one end of the port on the left side is used for connecting the output end of the optical fiber branching unit, the other end of the port on the left side is used for connecting the photoelectric detector, one end of the port on the right side is used for connecting the FBG sensor, and the other end of the port on the right side is used for connecting the methane reference air chamber and the photoelectric detector. The light emitted by the VCSEL enters the two ends of the right port from one end of the left port and is output, and then enters the two ends of the left port from one end of the right port and is output.
Because the optical fiber coupler has the characteristic of bidirectional light transmission, the optical fiber isolator (2) is added between the light source (1) and the optical fiber branching unit (3), so that the light reflected by the FBG can be isolated, and the interference of the reflected light on the light source is reduced.
The working steps of the temperature adaptive FBG demodulation system of the embodiment are as follows:
1) the temperature and pressure sensing device measures ambient temperature and pressure and sends the measurements to the microprocessor. The microprocessor drives the VCSEL through the sawtooth wave current driving circuit, and determines a wavelength range corresponding to the driving current of the VCSEL according to the current VCSEL temperature measured by the thermistor in the VCSEL and the change value of the driving current, so as to form periodic wavelength scanning. And according to the position of the methane absorption peak in the VCSEL working temperature range, adjusting the scanning range of the VCSEL wavelength to cover 1 to 2 methane absorption peak wavelengths.
2) Light emitted by the VCSEL is uniformly distributed to eight paths of output through the optical fiber branching unit, the first path output to the eight paths of output of the optical fiber branching unit respectively enter the FBG sensor through 82 x 2 optical fiber couplers, and when the wavelength of incident light is coincident with the central wavelength of the FBG, incident light reflected by the FBG enters the photoelectric detector through the 2 x 2 optical fiber couplers. Meanwhile, in the seventh path of the optical fiber splitter, the other path of the output light after passing through the 2 × 2 coupler is directly connected with a photodetector for detecting the change of the light intensity of the light source. In the eighth optical path of the optical fiber splitter, the output light of the optical fiber splitter is connected with a methane reference gas chamber in another optical path passing through the 2 × 2 coupler, and then is connected with a photoelectric detector. The first path to the eighth path of light entering the photoelectric detector after being reflected by the coupler and the FBG is a detection signal of the central wavelength of the FBG, the seventh path of light entering the photoelectric detector through the coupler is a reference light source signal, and the eighth path of light entering the photoelectric detector through the coupler and the reference gas chamber is a methane reference gas chamber signal.
3) In the photoelectric detection circuit, 1 path of reference light source signal, 1 path of reference air chamber signal and 8 paths of FBG center wavelength signals respectively enter 10 photoelectric detectors, the photoelectric detectors convert optical signals into corresponding current signals, the current signals are amplified into analog voltage signals through 10 linear trans-group amplifiers, and the analog-to-digital converter converts the analog voltage signals into digital voltage signals. The microprocessor carries out normalization operation on the received reference gas chamber signal and each FBG central wavelength detection signal by using a reference light source signal so as to eliminate the influence caused by the power change of a light source;
4) and the microprocessor determines the wavelength of a methane absorption peak of a reference gas chamber signal according to the preset VCSEL current, the temperature parameter and the current temperature of the VCSEL, performs peak detection on the normalized FBG central wavelength detection signal, and records the number of sampling points at the FBG reflection peak. According to the algorithm described in the first embodiment, the wavelength variation of the FBG is measured by using 1 to 2 methane absorption peaks, and the temperature error caused by the position difference of the thermistor is corrected.
5) The microprocessor can calculate corresponding parameters such as temperature variation, roof displacement or drilling stress according to a corresponding relation between temperature and wavelength of a temperature sensor calibrated in advance, a corresponding relation between displacement of a roof separation sensor and wavelength, or a corresponding relation between stress of a drilling stress sensor and wavelength, the measured parameters can be transmitted to a network serial server in a RS485 wired communication interface or wireless modes such as Lora/Nb-lot/WiFi, and a user can inquire related information through a cloud platform or a mobile phone APP.
The system of the embodiment has low power consumption, can be driven by a battery to run, and can be made into a mobile sensor node of a wireless sensor network by the aid of the wireless transmission module, so that the practicability and flexibility of the sensor are greatly improved. The system of the embodiment not only reduces the system power consumption and the cost of the demodulation device, but also reduces the volume, meets the intrinsic safety requirement, is convenient to construct and install, and can solve the problem of difficult underground electricity taking of a coal mine.
The sensing and demodulating structure of the system is simple, no mechanical moving part exists, the power consumption of the system is reduced, and the production and debugging cost is reduced. The device can be powered by a battery (3.7V/2000m A), an intermittent working mode is selected according to the actual requirements on the site, the power consumption of the system is less than 200mW, and the working time of the whole device is as long as 6 months. Under the condition of nearly equal measurement accuracy, compared with the TEC, the power consumption of the system is reduced by about one order of magnitude, and the technical problem of large power consumption of the conventional demodulator is solved.
The FBG demodulation system of the embodiment has the volume (164 multiplied by 108 multiplied by 47 in length, width and height) of only the volume 1/3 of the conventional FBG demodulation device, is low in production cost, and is suitable for the application occasions of monitoring discrete measurement points such as coal mine underground roadway temperature, roof separation stress, drilling stress and the like. And the wireless transceiving interfaces such as an RS485 wired communication interface and Zigbee/Lora/Nb-lot/WiFi and the like are supported. Because the power consumption of the demodulation system is greatly reduced, the wireless communication mode can be selected according to actual requirements on occasions with difficult power supply. The demodulation system has a self-diagnosis function (whether the light source and the FBG spectrum work normally) and a self-calibration function (real-time self-calibration and temperature compensation by using a methane gas absorption peak).
Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive efforts by those skilled in the art based on the technical solution of the present invention.
Claims (10)
1. A temperature adaptive FBG demodulation method, comprising:
selecting VCSELs meeting set requirements based on the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control; simultaneously selecting the wavelength of a methane absorption peak and the wavelength of the FBG, so that the scanning wavelength range of the VCSEL always covers the central wavelength of the FBG, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range;
and in each preset temperature range, determining the FBG central wavelength value and the sensed variation according to the wavelength position of the methane absorption peak and the position of the FBG central wavelength.
2. The method for temperature-adaptive FBG demodulation as claimed in claim 1, further comprising: the method adopts the methane absorption peak wavelength in the VCSEL wavelength scanning range as a fixed wavelength reference point, and corrects the measurement temperature deviation caused by different positions of the thermistor in the VCSEL and the VCSEL chip by using the fixed methane absorption peak wavelength so as to accurately measure the FBG peak wavelength and the variation value thereof.
3. The method for demodulating temperature-adaptive FBG according to claim 1, wherein determining the range of operating temperature variation that different VCSELs can adapt to without temperature control specifically comprises:
determining the relationship between the wavelength change of the VCSEL and the temperature change at the central wavelength current, the relationship between the wavelength change of the VCSEL and the change of the driving current, the relationship between the output power of the VCSEL and the change of the driving current in a set temperature change range and the maximum driving current range of the VCSEL through experiments;
determining the wavelength scanning range of the VCSEL at different working temperatures, and further determining the maximum scanning wavelength and the minimum scanning wavelength for realizing the temperature-free control of the VCSEL; finally, the working temperature variation range which can be adapted by different VCSELs under the condition of no temperature control is determined.
4. The method as claimed in claim 1, wherein selecting the VCSEL meeting the setting requirement includes:
in the set working temperature range of 0-40 ℃, the shortest wavelength of the VCSEL at the lowest temperature to the longest wavelength of the VCSEL at the highest temperature can be covered in the set waveband range; while at least one corresponding methane absorption peak is present in the VCSEL scanning wavelength range at each of the temperature ranges 0-20 c and 20-40 c.
5. The temperature-adaptive FBG demodulation method as claimed in claim 1, wherein the scanning range of the VCSEL wavelength covers 1-2 methane absorption peaks in each of the temperature ranges of 0-20 ℃ and 20-40 ℃, and the FBG center wavelength value and the sensed variation are determined according to the wavelength position of the methane absorption peak and the position of the FBG center wavelength; the method specifically comprises the following steps:
when two methane absorption peaks exist in the wavelength scanning range of one VCSEL, the known wavelength values of the two methane absorption peaks are utilized to calculate the corresponding wavelength change value represented between every two scanning sampling points;
and obtaining a wavelength value corresponding to the FBG peak value based on the corresponding sampling point position when the FBG peak value is measured, the known wavelength value of one methane absorption peak and the corresponding wavelength change value represented between every two scanning sampling points.
6. The method as claimed in claim 1, wherein the scanning range of the VCSEL wavelength covers 1 or 2 methane absorption peaks in each preset temperature range, and the FBG center wavelength value and the sensed variation are determined according to the wavelength position of the methane absorption peak and the FBG center wavelength position; the method specifically comprises the following steps:
when only one methane absorption peak exists in a VCSEL wavelength scanning range, determining the wavelength of the absorption peak according to the temperature value measured by the sensor;
determining two bandwidth values of the absorption peak value at the position of 3dB and the positions of two corresponding scanning sampling points after the methane absorption peak is normalized; calculating the corresponding wavelength variation value represented between the two scanning sampling points;
and obtaining the central wavelength value of the FBG based on the wavelength of the methane absorption peak, the position of the sampling point corresponding to the FBG peak value and the wavelength variation value.
7. The method as claimed in claim 1, wherein the scanning range of the VCSEL wavelength covers 1-2 methane absorption peaks in each preset temperature range, and the FBG center wavelength value and the sensed variation are determined according to the wavelength position of the methane absorption peak and the FBG center wavelength position; the method specifically comprises the following steps:
when any methane absorption peak exists in a VCSEL wavelength scanning range, current parameters of the VCSEL at a set temperature are preset, and the central wavelength of the FBG is calculated and demodulated by measuring the position of a sampling point corresponding to the peak value of the FBG and utilizing the relation between the number of sampling points at the FBG reflection peak value and the VCSEL driving current value, the quadratic curve relation between the VCSEL driving current and the scanning wavelength, the linear relation between the VCSEL temperature and the scanning wavelength and the temperature value of the VCSEL.
8. A temperature adaptive FBG demodulation system, comprising:
the optical fiber coupling device comprises a VCSEL, an optical fiber isolator, an optical fiber splitter, an optical fiber coupler and an FBG sensor which are connected in sequence; one port of the optical fiber coupler is connected with the photoelectric detection circuit and the analog-to-digital converter, the output of the analog-to-digital converter is connected with the microprocessor, and the output end of the microprocessor is connected with the VCSEL after being sequentially connected with the digital-to-analog converter and the current driving circuit;
the optical fiber branching unit is provided with an input port and eight output ports, and one path of optical fiber of each output port is directly transmitted to the photoelectric detector through the coupler; one path of optical fiber passes through the coupler and a reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; the eight paths of optical fibers are input into the FBG sensor after passing through the coupler, and are output to the photoelectric detector after being reflected by the sensing FBG;
the temperature adaptive FBG demodulation method according to any one of claims 1 to 7, wherein the microprocessor receives environmental temperature and pressure measurement data, drives the VCSEL through a sawtooth wave current driving circuit, and determines a wavelength range corresponding to the driving current of the VCSEL according to the current VCSEL temperature and driving current variation values measured by the thermistor inside the VCSEL to form periodic wavelength scanning;
and adjusting the scanning range of the wavelength of the VCSEL according to the position of the methane absorption peak in the VCSEL working temperature range to cover at least one methane absorption peak wavelength.
9. The temperature adaptive FBG demodulation system according to claim 8, wherein the microprocessor determines the wavelength of the methane absorption peak of the reference gas chamber signal according to the preset VCSEL current, the temperature parameter and the current VCSEL temperature, performs peak detection on the normalized FBG center wavelength detection signal, and records the number of sampling points at the FBG reflection peak;
the method of claim 2, wherein the wavelength variation of the FBG is measured by using the methane absorption peak, and the temperature error caused by the position difference of the thermistor is corrected.
10. The temperature-adaptive FBG demodulation system according to claim 8, characterized in that the demodulated sensor temperature and stress information is transmitted to a network serial server through a wired communication interface or a wireless mode so as to query relevant information on a cloud platform or a mobile phone APP.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN117387673A (en) * | 2023-12-08 | 2024-01-12 | 山东星冉信息科技有限公司 | Fiber bragg grating demodulation method and device based on reference air chamber |
| CN117387673B (en) * | 2023-12-08 | 2024-02-23 | 山东星冉信息科技有限公司 | Fiber bragg grating demodulation method and device based on reference air chamber |
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