CN114235018B - Temperature-adaptive FBG demodulation method and system - Google Patents
Temperature-adaptive FBG demodulation method and system Download PDFInfo
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- G01D5/00—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
- G01D5/26—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
- G01D5/32—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
- G01D5/34—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
- 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
- G01D5/35306—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 using an interferometer arrangement
- G01D5/35309—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 using an interferometer arrangement using multiple waves interferometer
- G01D5/35316—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 using an interferometer arrangement using multiple waves interferometer using a Bragg gratings
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Abstract
The invention discloses a temperature self-adaptive FBG demodulation method and system, comprising the following steps: selecting VCSELs meeting set requirements based on the working temperature change range which can be adapted to different VCSELs under the condition of no temperature control; simultaneously selecting the methane absorption peak wavelength and the FBG wavelength, so that the scanning wavelength range of the VCSEL always covers the FBG center wavelength, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range; and determining the FBG center wavelength value and the change after sensing according to the wavelength position of the methane absorption peak and the position of the FBG center wavelength in each preset temperature range. The invention has the beneficial effects that: under the condition of not using a temperature control device, the change value of the FBG center wavelength can be effectively measured, so that the whole sensor measurement 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 self-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 roadway of the coal mine, the traditional mechanical roof separation sensor and the drilling stress sensor of the coal mine are often required to be used, and the sensor has a simple structure and low cost and is widely applied to the coal mine. The existing dynamic monitoring means of the top plate mainly comprise a mechanical manual monitoring technology and an electronic monitoring technology. However, because the sensor has the defects of low measurement precision and difficult real-time on-line monitoring, in actual use, professionals are required to go into the well periodically to carry out inspection and recording, hidden dangers cannot be found in time, and faults cannot be cleared and dangers cannot be removed in time. The general electronic roof stress and drilling 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 output voltage change 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 working environment in the pit is high in humidity and multiple in dust, electronic components are easily affected by water and dust in the pit, so that the service life of the sensor is short, signal transmission is easily affected by electromagnetic interference, and data false alarm is easy to solve.
Compared with the traditional electronic sensor, the FBG (fiber Bragg Grating) stress sensor has the advantages of no electrification, intrinsic safety, strong electromagnetic interference resistance, easy multiplexing, simple structure and the like. The stress variation monitored by the FBG stress sensor is demodulated and calculated by an FBG wavelength demodulator. In the FBG demodulator, the variation of the measured physical quantity can be calculated by detecting the shift amount of the FBG center wavelength. Currently, various FBG demodulation techniques have been widely applied to FBG demodulators, such as an edge filtering method, a tunable F-P filter method, an interference demodulation method, a matching FBG method, a wavelength scanning laser method, and the like. 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 larger power consumption, high production cost and great maintenance difficulty.
Aiming at the applications of coal mine underground roadway temperature, roof stress monitoring and the like, the explosion-proof characteristic of the FBG demodulator also needs to be considered. The prior art discloses a FBG demodulation device based on VCSEL (vertical cavity surface emitting laser) and a working method thereof, wherein 5 absorption peaks of acetylene gas in C wave band 1527-1530nm are used as reference wavelengths to measure the center wavelength of the FBG, and compared with the traditional demodulation method, the method effectively reduces part of power consumption of the demodulator. In general, the temperature/wavelength coefficient of the C-band VCSEL is about 0.11nm/°c, and since 5 absorption peaks of 1527-1530nm are fixedly used in the optical grating demodulation system, a temperature control system must be added to the laser to ensure that the scanning wavelength of the laser can cover the entire wavelength range of 1527-1531nm, so as to avoid that the scanning wavelength of the laser deviates from the wavelength range of 5 absorption peaks of acetylene due to a large environmental temperature change, which makes the demodulation system not work normally. However, when the laser uses a temperature control system (TEC), the power consumption and cost of the whole sensing system are correspondingly increased, the system structure becomes complicated, and especially the power consumption cannot meet the intrinsic safety requirement.
Disclosure of Invention
In order to solve the problems, the invention provides a temperature-adaptive FBG demodulation method and system, which adopts methane gas with a relatively large absorption peak distribution range as a reference wavelength to demodulate the center wavelength of the FBG, and simultaneously, the wavelength scanning range of the VCSEL and the center wavelength of the FBG are selected and matched, so that the change value of the center 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 change range which can be adapted to different VCSELs under the condition of no temperature control; simultaneously selecting the methane absorption peak wavelength and the FBG wavelength, so that the scanning wavelength range of the VCSEL always covers the FBG center wavelength, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in the set temperature range;
and determining the FBG center wavelength value and the change after sensing according to the wavelength position of the methane absorption peak and the position of the FBG center wavelength in each preset temperature range.
In other embodiments, the following technical solutions are adopted:
a temperature adaptive FBG demodulation system comprising:
the VCSEL, the optical fiber isolator, the optical fiber splitter, the optical fiber coupler and the FBG sensor are sequentially connected; 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 digital-to-analog converter and the current driving circuit in sequence and then is connected with the VCSEL;
the optical fiber splitter is provided with an input port and eight output ports, and one path of optical fiber of the output ports is directly transmitted to the photoelectric detector through the coupler; one path of optical fiber passes through the coupler and the reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; eight paths of optical fibers are input into an FBG sensor after passing through a coupler, and are transmitted to a photoelectric detector after being reflected by a sensing FBG;
by adopting the temperature self-adaptive FBG demodulation method, the microprocessor receives the ambient temperature and pressure measurement data, drives the VCSEL through the sawtooth current driving circuit, and determines the wavelength range corresponding to the driving current of the VCSEL according to the current VCSEL temperature and the variation value of the driving current measured by the thermistor in the VCSEL to form periodic wavelength scanning;
and adjusting the scanning range of the VCSEL wavelength according to the methane absorption peak position in the VCSEL working temperature range so as to cover at least one methane absorption peak wavelength.
Compared with the prior art, the invention has the beneficial effects that:
(1) According to the invention, methane gas with a larger absorption peak distribution range is selected as a reference wavelength to demodulate the FBG center wavelength, and meanwhile, the wavelength scanning range of the VCSEL and the FBG center wavelength are selected and matched, so that the change value of the FBG center wavelength is effectively measured under the condition that a temperature control device is not used, and the whole sensor measurement 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 operated by using a battery, and a wireless transmission module is added, so that a mobile sensor node of a wireless sensor network can be manufactured, and the practicability and flexibility of the sensor are greatly improved. By adopting the scheme, the system power consumption and the demodulation device cost are reduced, the volume is reduced, the device not only meets the intrinsic safety requirement, but also is convenient to construct and install, and meanwhile, the problem of difficult underground electricity taking of a coal mine can be solved.
(3) The method does not need a laser temperature control device, and can dynamically select 1 to 2 absorption peaks which can be scanned by the VCSEL at the current temperature to reference and compensate under different working temperatures, so that the VCSEL can normally work in a specific range. The FBG demodulation method and the system with low power consumption and self-adaption temperature are particularly suitable for signal demodulation of physical quantity sensors such as FBG used for measuring, monitoring temperature, roof stress, drilling stress and the like in underground coal mine roadways.
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 correspondence between the wavelength values of VCSELs and the methane absorption peaks and the effective working areas of FBGs, respectively, determined for use in the embodiments of the present invention;
FIG. 2 is a schematic diagram of a low-power-consumption, temperature-adaptive FBG demodulation device for coal mine stress monitoring in an embodiment of the invention;
the system comprises a VCSEL, a fiber isolator, a fiber splitter, a first fiber coupler, a second fiber coupler, a reference air 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 and a FBG sensor, wherein the VCSEL, the fiber isolator, the fiber splitter, the first fiber coupler, the second fiber coupler, the reference air chamber, the photoelectric detector, the linear transimpedance amplifier, the analog-to-digital converter, the microprocessor, the digital-to-analog converter, the current driving circuit, the communication interface and the FBG sensor.
Detailed Description
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. 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 in accordance with the present application. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
Example 1
In one or more embodiments, a temperature-adaptive FBG demodulation method is disclosed, comprising the following processes:
step (1): determining the working temperature change range which can be adapted to different VCSELs under the condition of no temperature control;
specifically, the output wavelength characteristics of the VCSEL without temperature control are mainly exhibited at the following two points:
1) Without a laser temperature control device, the magnitude of the center wavelength of the VCSEL, or the position of the wavelength in the spectrum, varies with the ambient temperature of the laser, with the temperature of the VCSEL being approximately linear with the scanning wavelength: λ=kt, K is the temperature coefficient, and the 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 1640-1660nm band, the drive current of VCSEL approximates a conic relationship with scan wavelength: λ=ai 2 +bI+c, the VCSEL wavelength variation range is about 5-6 nm when the VCSEL is operated under a sawtooth sweep current with a magnitude variation of 1-14mA, forming a sweep range of VCSEL wavelengths. Thus, when selecting lasers based on VCSEL wavelength characteristics, it is required that the shortest wavelength of different VCSELs at 0 ℃ to the longest wavelength of VCSELs at 40 ℃ can be covered over the entire 1640-1660nm band, and that there are 1 to 2 methane absorption peaks in the VCSEL scanning wavelength range at each temperature.
The VCSEL is selected according to several parameters: the wavelength values of VCSEL at 0 and 40 ℃ are λ@0 ℃ (nm) and λ@40 ℃ (nm), respectively, the relationship of VCSEL wavelength to drive current variation Δλ/ΔI (nm/mA) over a temperature variation range of 0-40 ℃): relationship of VCSEL wavelength to temperature change Δλ/ΔT (nm/. Degree.C.) over a scan current variation range of 0.5 nm/mA: 0.122 nm/DEG C, a relationship between VCSEL output power and drive current variation over a temperature range of 0-40 ℃ and a maximum drive current range for the VCSEL. By analysis of these parameters, VCSELs of varying wavelength ranges, or wavelength scan ranges, at different operating temperatures can be selected.
In the embodiment, a plurality of experimental tests prove that the relation delta lambda/delta T (nm/DEG C) between the wavelength change and the temperature change of the VCSEL at the central wavelength current is 0.116 nm/DEG C-0.128 nm/DEG C, and the average value is 0.122 nm/DEG C; and the relationship of VCSEL wavelength variation and drive current variation, deltalambda/DeltaI (nm/mA), is 0.418nm/mA to 0.550nm/mA, the average value is 0.5nm/mA, the relationship of VCSEL output power and drive current variation in the temperature variation range of 0-40 ℃ and the maximum drive current range of the VCSEL. Determining wavelength scanning ranges of the VCSEL at different working temperatures according to the groups of relations, and further determining the maximum driving current and the wavelength of the VCSEL at low temperature, namely the maximum scanning wavelength for realizing the non-temperature control of the VCSEL; determining the minimum driving current and the minimum wavelength of the VCSEL at high temperature, namely the minimum scanning wavelength for realizing the non-temperature control of the VCSEL; and finally, determining the working temperature change range which can be adapted to different VCSELs under the condition of no temperature control.
Step (2): and selecting the VCSEL meeting the set requirements, and simultaneously selecting the methane absorption peak wavelength and the FBG wavelength, so that the scanning wavelength range of the VCSEL always covers the FBG center wavelength (FBG peak wavelength), 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 absorption peak of methane, it is known that the methane gas molecules have a unique set of infrared absorption spectral lines in the infrared band whose spectral position is constant with the change of the external environmental conditions, so that in this spectral range, each absorption peak has an intrinsic absorption wavelength, which forms a set of "known wavelength scales" in the spectrum, by means of which the position of the reflection peak of the FBG in this spectral range, and the variation of the wavelength of the reflection peak of the FBG caused by the external measurement, can be measured and calculated as a reference. In the embodiment, a plurality of strong inherent absorption peaks of methane gas in the wavelength band of 1640-1660nm are selected as known wavelength scales: the average separation of every two absorption peaks in the 1640.374nm, 1642.910 nm,1645.561nm,1648.234nm,1650.961nm,1653.723nm,1656.546nm and 1659.413nm,8 methane absorption peaks was about 2.72nm.
In the embodiment, a plurality of experimental tests determine that the wavelength scanning range values from the threshold current to the inflection point current of the VCSEL at 0 and 40 ℃ are 4.366-5.527nm and 2.870-3.676nm respectively, and the average value is 4.822nm (at 0 ℃) and 3.265nm (at 40 ℃), so that the average value of the whole wavelength scanning range of the VCSEL at 0-40 ℃ is determined to be: 4.96nm+2.4115 nm+1.6325nm= 9.0035nm, where 4.96nm is the center wavelength shift average, 2.411nm is half of the wavelength sweep at 0 ℃,1.6325nm is half of the wavelength sweep at 40 ℃. The average interval of the methane absorption peaks is known to be 2.72nm, and it is further known that the VCSEL can cover up to 4 and at least 3 methane absorption peaks in the entire wavelength scan range of the VCSEL when the VCSEL temperature is changed from 0 ℃ to 40 ℃.
For the choice of FBG wavelength, the choice of FBG wavelength needs to satisfy the following three conditions:
1) The FBG wavelength plus the amount of variation caused by the measurement should always be within each scanned wavelength range of the VCSEL;
2) There will be 1 to 2 methane absorption peaks in each scanned wavelength range of the VCSEL;
3) Both conditions can be satisfied within the temperature range of 0-40 ℃. Thus, the scanning wavelength range of the VCSEL can always cover the FBG center wavelength, the wavelength variation introduced by sensing and 1 to 2 corresponding methane absorption peaks in the temperature range of 0-40 ℃.
The FBG wavelength matches the VCSEL scan 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 (a wavelength range jointly covered by low temperature and high temperature), and the working wavelength of the grating needs to be selected in the effective wavelength scanning range; the range is a wavelength range which can be scanned by any temperature from the low temperature to the high temperature of the VCSEL, and the effective wavelength scanning range is determined according to the temperature difference from the low temperature to the high temperature.
Taking a roof separation layer and a drilling stress sensor as an example, according to the measuring range of the roof separation layer sensor (the wavelength change range is about 1-1.2 nm), the measuring range of the drilling stress sensor (the wavelength change range is about 0.8-1 nm) and FBG parameters (the central wavelength, the 3dB bandwidth and the like), the VCSEL scans the maximum wavelength at a low temperature to reach the FBG central wavelength +2nm at the current temperature, specifically the FBG central wavelength +1.2 (the maximum change amount of the sensor wavelength) +0.2nm (the prestress when the sensor is packaged) +0.3nm (the allowance prevents error change from causing scanning of the FBG central wavelength) +0.3nm (the 3dB bandwidth of the FBG wavelength); the minimum wavelength of the VCSEL for scanning at high temperature needs to be smaller than the FBG center wavelength-0.4 nm at the current temperature, specifically the FBG center wavelength-0.3 nm (FBG wavelength bandwidth) +0.2nm (prestress during sensor packaging) -0.3nm at the current temperature (allowance for preventing error change from causing scanning of the grating center wavelength). Taking an example of an FBG with a central wavelength of 1648.5 ±0.2nm (at room temperature, usually about 20 ℃) as the FBG sensor corresponding to the demodulation device mainly comprises a temperature sensor, a roof separation layer sensor, a drilling stress sensor and the like, and the full range of the sensor is within 1.2nm, so that the maximum wavelength of the VCSEL matched with the sensor with the central wavelength of 1648.5 ±0.2nm (temperature, roof and drilling) needs to reach 1650.5 ±0.2nm at low temperature, and the minimum wavelength of the VCSEL needs to be less than 1648.1 ±0.2nm at high temperature, namely the effective wavelength scanning range of the VCSEL needs to be satisfied:
1648.1 + -0.2 nm to 1650.5 + -0.2 nm (about 2.4 nm).
Step (3): determining the FBG center wavelength value and the change after sensing according to the wavelength position of the methane absorption peak and the position of the FBG center wavelength in each preset temperature range;
the preset temperature range is as follows: 0-20 ℃ and 20-40 ℃. Within each temperature range, the sweep range of VCSEL wavelengths covers 1-2 methane absorption peaks, and FBG wavelength variation is calculated by measuring with 1 to 2 corresponding methane absorption peaks as wavelength references. The value of the center wavelength of the FBG and the change after sensing are determined according to the wavelength position of the methane absorption peak and the position of the center wavelength of the FBG. Under different working temperatures, different methane absorption peaks are dynamically selected and locked, so that the demodulation system can demodulate the position of the FBG center wavelength according to the position of the methane absorption peak under the conditions of high temperature and low temperature.
In this example, the methane absorption peaks covered were 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 different VCSEL wavelength scan ranges. Measuring the temperature of a VCSEL chip by using a VCSEL internal thermistor, 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 utilizing a sawtooth wave to tune a drive circuit of the VCSEL to change the wavelength of the VCSEL so as to form periodic wavelength scanning; different calculation methods are selected according to the number of the occurring methane absorption peaks:
1) When two methane absorption peaks exist in the VCSEL wavelength scanning range, calculating a corresponding wavelength variation value represented between every two scanning sampling points by using the known wavelength values of the two methane absorption peaks, and then calculating the wavelength value corresponding to the FBG peak value by measuring the position of the corresponding sampling point when the FBG peak value and adding the known wavelength value of one methane absorption peak and the corresponding wavelength variation value represented between every two scanning sampling points. Taking the two absorption peaks of 1648.239nm and 1650.959nm in the VCSEL wavelength scanning range as an example when the selected VCSEL works at 20-40 ℃, the FBG value calculation method is 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 situation that only one methane absorption peak exists can be 1648.239nm, 1650.959nm and 1653.757nm …; for example, at 40 ℃, the VCSEL wavelength scan range becomes smaller, so there is a case 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 first determined based on the temperature value measured by the laser thermistor, and then the bandwidth value of the absorption peak at 3dB is calculated after normalization using this gas absorption peak. Since the pressure of the reference air chamber is unchanged, the width value of the 3dB bandwidth of the absorption peak of the reference air chamber is also unchanged, and the center 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 and the absorption peak wavelength value. Taking the VCSEL working at 20 ℃ as an example, the wavelength scanning range of the VCSEL has an absorption peak of 1648.239nm, and the FBG value calculating method is as follows:
3) When any methane absorption peaks exist in the VCSEL wavelength scanning range, presetting current parameters of the VCSEL at 20 ℃, firstly obtaining the magnitude of a driving current value corresponding to a sampling point position by measuring the sampling point position corresponding to the FBG peak value, and then calculating the FBG center wavelength corresponding to the sampling point 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 calculation method is as follows:
step (4): and the methane absorption peak wavelength in the VCSEL wavelength scanning range is used as a fixed wavelength reference point, and the fixed methane absorption peak wavelength is used for correcting the measurement temperature deviation caused by the difference of the positions of the thermistor and the VCSEL chip in the VCSEL so as to accurately measure the FBG peak wavelength and the variation value thereof.
Because the internal thermistor of each VCSEL is different from the VCSEL chip in position, the scanning wavelength corresponding to the actual scanning wavelength range or a sampling point of the VCSEL has certain deviation with the scanning wavelength corresponding to the scanning wavelength range or the sampling point calculated by the microprocessor according to the current temperature, so that the deviation exists in the actually demodulated FBG peak wavelength. In the embodiment, the methane absorption peak wavelength in the VCSEL wavelength scanning range is used as a fixed wavelength reference point, and the fixed methane absorption peak wavelength is used for correcting the measurement temperature deviation generated by different positions of the thermistor and the VCSEL chip in the VCSEL, so that the demodulation precision is improved, and the purpose of accurately measuring the FBG peak wavelength and the change value thereof is achieved.
The specific correction method comprises the following steps: actual methane absorption peak lambda 1 The corresponding sampling point is X, and the wavelength corresponding to the sampling point X calculated according to the laser current wavelength parameter is lambda 2 Because VCSEL wavelength is approximately linear with temperature, the corrected temperature error is:
example two
In one or more embodiments, a temperature adaptive FBG demodulation system is disclosed, referring to fig. 2, comprising:
the optical fiber coupler comprises a VCSEL (1), an optical fiber isolator (2), an optical fiber splitter (3), an optical fiber coupler (4) (5), a reference air 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 the 1X 8 optical fiber splitter (3), the first path to the sixth path of output of the optical fiber splitter are connected with the 2X 1 optical fiber coupler (4) and then are connected with the FBG sensor (14), and the other path of the two ports of the 2X 1 optical fiber coupler (4) is connected with the photoelectric detection circuit (7) and the analog-to-digital converter (9); the seventh path and the eighth path of the optical fiber branching device are respectively connected with 2X 2 optical couplers (5), wherein one path of output end of the seventh path of 2X 2 optical fiber couplers is connected with an FBG (14), and the other end of the seventh path of 2X 2 optical fiber couplers is connected with a photoelectric detector (7), a linear transimpedance amplifier (8) and an analog-to-digital converter (9); one output end of the eighth path of 2 multiplied by 2 optical fiber coupler (5) is connected with an FBG (14), and the other end of the 2 multiplied by 2 optical fiber coupler is connected with a methane chamber (6), a photoelectric detector (7), a linear transimpedance amplifier (8) and an analog-to-digital converter (9). The output of the analog-to-digital converter (9) is connected with the microprocessor (10), the output end of the microprocessor is connected with the digital-to-analog converter (11) and the communication interface (13), and the digital-to-analog converter (11) is connected with the VCSEL (1) through the current driving circuit (12).
The 1X 8 optical fiber splitter is provided with an input port and eight output ports, and is used for uniformly distributing a light source to eight paths of outputs, and after the light source is split, one path of optical fiber is directly transmitted to the photoelectric detector through the coupler; one path of optical fiber passes through the coupler and the reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; eight paths of optical fibers are input into the FBG sensor after passing through the coupler, and are transmitted to the photoelectric detector after being reflected by the sensing FBG.
The 2X 1 optical fiber coupler has two ports on the left side, one port on the right side, one end of the left side port is used for connecting with the output end of the optical fiber splitter, the other end is used for connecting with the photoelectric detector, and one end of the right side port is used for connecting with the FBG sensor. Light emitted by the VCSEL enters the right side port from one end of the left side port, and then enters the two ends of the left side port from the right side port to be output.
Two ports are respectively arranged on two sides of the 2X 2 optical fiber coupler, one end of the left port is used for being connected with the output end of the optical fiber branching device, the other end of the left port is used for being connected with the photoelectric detector, one end of the right port is used for being connected with the FBG sensor, and the other end of the right port is used for being connected with the methane reference air chamber and the photoelectric detector. Light emitted by the VCSEL enters and is output from one end of the left side port to two ends of the right side port, and then enters and is output from one end of the right side port to two ends of the left side port.
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 splitter (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 the ambient temperature and pressure and sends the measured values to the microprocessor. The microprocessor drives the VCSEL through the sawtooth wave current driving circuit, and determines the wavelength range corresponding to the driving current of the VCSEL according to the current VCSEL temperature and the variation value of the driving current measured by the thermistor in the VCSEL, so as to form periodic wavelength scanning. And according to the methane absorption peak position in the VCSEL working temperature range, the scanning range of the VCSEL wavelength is adjusted to cover 1 to 2 methane absorption peak wavelengths.
2) The light emitted by the VCSEL is uniformly distributed to eight paths of outputs through the optical fiber splitters, the first to eight paths of outputs of the optical fiber splitters enter the FBG sensor after passing through 8 2X 2 optical fiber couplers respectively, and when the wavelength of the incident light is overlapped with the central wavelength of the FBG, the incident light reflected by the FBG enters the photoelectric detector through the 2X 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 photoelectric detector for detecting the change of the light intensity of the light source. In the eighth path of the optical fiber splitter, the output light is connected with a methane reference air chamber in the other path of the optical fiber splitter passing through the 2×2 coupler, and then is connected with a photoelectric detector. The first to eighth paths of light entering the photoelectric detector after being reflected by the coupler and the FBG are detection signals of the center 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 air chamber is a methane reference air 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, then the corresponding current signals are amplified into analog voltage signals through 10 linear group-crossing amplifiers, and the analog voltage signals are converted into digital voltage signals through an analog-to-digital converter. The microprocessor performs normalization operation on the received reference air chamber signal and each FBG center wavelength detection signal by using a reference light source signal so as to eliminate the influence caused by the power change of the light source;
4) The microprocessor determines the wavelength of the methane absorption peak of the reference air chamber signal according to the preset VCSEL current, temperature parameters and the current temperature of the VCSEL, carries out peak detection on the normalized FBG center wavelength detection signal, and records the sampling point number at the FBG reflection peak. According to the algorithm described in embodiment one, the corresponding 1 to 2 methane absorption peaks are used to measure the wavelength variation of the FBG and correct the temperature errors caused by the different thermistor positions.
5) The microprocessor can calculate corresponding parameters such as temperature variation, top plate displacement, drilling stress and the like according to the corresponding relation between the temperature of the pre-calibrated temperature sensor and the wavelength, the corresponding relation between the displacement of the top plate separation layer sensor and the wavelength, or the corresponding relation between the drilling stress sensor stress and the wavelength, the measured parameters can be transmitted to the network serial server in a wireless mode such as an RS485 wired communication interface or Lora/Nb-lot/WiFi, and the 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 operated by using a battery to drive, and can be manufactured into a mobile sensor node of a wireless sensor network by adding a wireless transmission module, so that the practicability and the flexibility of the sensor are greatly improved. The system of the embodiment not only reduces the system power consumption and the demodulation device cost, but also reduces the volume, meets the intrinsic safety requirement, is convenient for construction and installation, and can solve the problem of difficult underground electricity taking of a coal mine.
The sensing and demodulation structure of the system of the embodiment is simple, and no moving parts exist, so that 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/2000 m A), an intermittent working mode is selected according to the actual requirements of 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. Compared with TEC, the system power consumption is reduced by about one order of magnitude under the condition of approaching the same measurement precision, and the technical problem of high power consumption of the conventional demodulator is solved.
The volume (164×108×47 length, width and height) of the FBG demodulation system of the embodiment is only 1/3 of that of the existing FBG demodulation device, and the FBG demodulation system is low in production cost and suitable for application occasions of monitoring discrete measuring points such as underground coal mine roadway temperature, roof separation stress and drilling stress. And wireless receiving and transmitting interfaces such as RS485 wired communication interfaces, zigbee/Lora/Nb-lot/WiFi and the like are supported. Because the power consumption of the demodulation system is greatly reduced, a wireless communication mode can be selected according to actual requirements in some 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 utilizing the absorption peak of methane gas).
While the foregoing description of the embodiments of the present invention has been presented in conjunction with the drawings, it should be understood that it is not intended to limit the scope of the invention, but rather, it is intended to cover all modifications or variations within the scope of the invention as defined by the claims of the present invention.
Claims (9)
1. A temperature-adaptive FBG demodulation method, comprising:
selecting VCSELs meeting set requirements based on the working temperature change range which can be adapted to different VCSELs under the condition of no temperature control; simultaneously selecting the methane absorption peak wavelength and the FBG wavelength, so that the scanning wavelength range of the VCSEL always covers the FBG center wavelength, the wavelength variation introduced by sensing and at least one corresponding methane absorption peak in a set temperature range;
determining the FBG center wavelength value and the change after sensing according to the wavelength position of the methane absorption peak and the position of the FBG center wavelength in each preset temperature range;
within each temperature range of 0-20 ℃ and 20-40 ℃, the scanning range of VCSEL wavelength covers 1-2 methane absorption peaks, and the value of FBG center wavelength and the variation after sensing 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 a VCSEL wavelength scanning range, calculating a corresponding wavelength variation value represented between every two scanning sampling points by using known wavelength values of the two methane absorption peaks;
based on the position of the corresponding sampling point when the FBG peak value is measured, the known wavelength value of one methane absorption peak and the corresponding wavelength variation value represented between every two scanning sampling points, the wavelength value corresponding to the FBG peak value is obtained.
2. A temperature-adaptive FBG demodulation method as claimed in claim 1, further comprising: and the methane absorption peak wavelength in the VCSEL wavelength scanning range is used as a fixed wavelength reference point, and the fixed methane absorption peak wavelength is used for correcting the measurement temperature deviation caused by the difference of the positions of the thermistor and the VCSEL chip in the VCSEL so as to accurately measure the FBG peak wavelength and the variation value thereof.
3. A temperature-adaptive FBG demodulation method according to claim 1, characterized in that the determination of the operating temperature variation range that different VCSELs can adapt to without temperature control comprises:
determining the relation between VCSEL wavelength variation and temperature variation at the central wavelength current, the relation between VCSEL wavelength variation and drive current variation, the relation between VCSEL output power and drive current variation in a set temperature variation range and the maximum drive current range of the VCSEL through experiments;
determining wavelength scanning ranges of the VCSEL at different working temperatures, and further determining the maximum scanning wavelength and the minimum scanning wavelength for realizing the non-temperature control of the VCSEL; and finally, determining the working temperature change range which can be adapted to different VCSELs under the condition of no temperature control.
4. A temperature-adaptive FBG demodulation method according to claim 1, characterized in that the selection of VCSELs meeting the set requirements comprises:
within 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 within the set wave band range; at least one corresponding methane absorption peak exists in the VCSEL scanning wavelength range at each of the temperature ranges of 0-20 ℃ and 20-40 ℃.
5. A temperature-adaptive FBG demodulation method as claimed in claim 1, characterized in that 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 position of the FBG center wavelength; 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;
after the methane absorption peak is normalized, determining two bandwidth values of the absorption peak at 3dB and two corresponding scanning sampling point positions of the two bandwidth values; calculating a corresponding wavelength variation value represented between two scanning sampling points;
and obtaining the central wavelength value of the FBG based on the methane absorption peak wavelength, the sampling point position corresponding to the FBG peak value and the wavelength variation value.
6. A temperature-adaptive FBG demodulation method as claimed in claim 1, characterized in that 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 position of the FBG center wavelength; the method specifically comprises the following steps:
when any methane absorption peaks exist in a VCSEL wavelength scanning range, presetting current parameters of the VCSEL at a set temperature, and calculating and demodulating a central wavelength value of the FBG by measuring sampling point positions corresponding to FBG peak values and utilizing the relation between the sampling point numbers at the FBG reflection peak values and VCSEL driving current values, the quadratic curve relation between VCSEL driving current and scanning wavelength, the linear relation between VCSEL temperature and scanning wavelength and the temperature value of the VCSEL.
7. A temperature adaptive FBG demodulation system, comprising:
the VCSEL, the optical fiber isolator, the optical fiber splitter, the optical fiber coupler and the FBG sensor are sequentially connected; 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 digital-to-analog converter and the current driving circuit in sequence and then is connected with the VCSEL;
the optical fiber splitter is provided with an input port and eight output ports, and one path of optical fiber of the output ports is directly transmitted to the photoelectric detector through the coupler; one path of optical fiber passes through the coupler and the reference air chamber for wavelength calibration and then is transmitted to the photoelectric detector; eight paths of optical fibers are input into an FBG sensor after passing through a coupler, and are transmitted to a photoelectric detector after being reflected by a sensing FBG;
adopting the temperature self-adaptive FBG demodulation method as claimed in any one of claims 1 to 6, wherein the microprocessor receives the ambient temperature and pressure measurement data, drives the VCSEL through a sawtooth current driving circuit, and determines the wavelength range corresponding to the driving current of the VCSEL according to the current VCSEL temperature and the variation value of the driving current measured by the thermistor in the VCSEL to form periodic wavelength scanning;
and adjusting the scanning range of the VCSEL wavelength according to the methane absorption peak position in the VCSEL working temperature range so as to cover at least one methane absorption peak wavelength.
8. The temperature adaptive FBG demodulation system of claim 7 wherein the microprocessor determines the wavelength of the methane absorption peak of the reference plenum signal according to a preset VCSEL current, a temperature parameter and a VCSEL current temperature, performs peak detection on the normalized FBG center wavelength detection signal, and records the number of sampling points at the FBG reflection peak.
9. The system of claim 7, wherein the temperature and stress information of the demodulated sensor is transmitted to the network serial server through a wired communication interface or a wireless mode to query related information in the cloud platform or the mobile phone APP.
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