CN100501375C - Microcavity double-grating and optical-fiber gas sensing system and method - Google Patents

Abstract

A micro cavity gas sensing system with double-grid comprises wide band light source, optical fiber, wide band coupler, micro cavity gas transducer with double-grid, spectrum demodulator and computer. Its sensing method includes calibrating out constant of transducer and influence factor of temperature on sensing system then sensing out concentration of measured gas.

Description

Double grid optical fiber microcavity gas sensing system and method for sensing

Technical field

The invention belongs to analytical instrument, aerochemistry, safety technique field, be specifically related to a kind of double grid optical fiber microcavity gas sensing system and method for sensing.

Background technology

Each great especially big coal mining accident is all brought huge property loss, brings the loss that can't retrieve especially for mining industry workman's life.The one of the main reasons of coal mining accident is the blast that gas causes.Detection ﹠ Controling to coal mine gas (major component is methane CH4) gas concentration are effective ways of evading the colliery serious accident.Firedamp sensor and system thereof are the critical equipment of the mine gas comprehensive regulation and hazard prediction, are subjected to people's extensive attention.

Methane sensing is a kind of gas sensing.At present, the mine gas sensor mainly contains semiconductor gas sensor, electrochemical sensor, carrier catalyst element, the interference of light or infrared absorption type sensor etc.The precision of semiconductor gas sensor is low, drift is big, stability and poor selectivity.The precision of electrochemical sensor and good stability, its deficiency be the life-span short, price is high.The carrier catalysis firedamp sensor has convenient and the low advantage of price, its deficiency is that reaction is slow, poor stability, life-span are short, easily fluctuate activation and poisoning, humiture influence are big, to the gas poor selectivity, resist high dense impact poor, have a potential safety hazard etc.Good stability is hanged down in the drift of light interference type firedamp sensor, and its deficiency is poor selectivity, disturbs greatly.According to passing the light form, the infrared absorption type firedamp sensor can be divided into two kinds of free space formula and waveguide fiber formulas, and its advantage is the precision height, drift is low, selectivity good, life-span and calibration cycle are long.The deficiency of free space formula firedamp sensor is that dust tightness is poor, the humiture influence is big, poor stability, price height.Optical fiber gas sensor can have evanescent wave type and the absorption sensor of White air chamber according to the mode of action of light, but its common advantage is remote measurement, electric insulation, no potential safety hazard.The deficiency of fast travelling waves of optical fibre type firedamp sensor is the making difficulty, signal noise is big, precision is low, detection system is complicated.The absorption firedamp sensor of optical fiber White air chamber has that volume is big, collimation assembling difficulty, vibrationproof and deficiency such as temperature stability is poor, environmental suitability weak, light loss and interference are big.

Close with the present invention be based on that optical fiber White absorbs air chamber and short period fiber grating (the short period fiber grating is also referred to as Bragg fiber grating or bragg grating) and the optical fiber gas sensing system that forms, the sensor-based system of two kinds of structures is arranged at present.One class sensor-based system is the reflection source that obtains wavelength-modulated by piezoelectric ceramics and fiber grating, being transferred to White absorption air chamber is absorbed by methane gas, obtain respectively by the second harmonic component light signal after the gas absorption and the optical signal transmissive of fiber grating with 2 photodetectors, get the methane sensing data after being divided by.Another kind of is dual-wavelength difference formula fiber grating gas sensing system: its light source light is after White absorption air chamber is absorbed by methane gas, by the light of two wavelength of 2 short period fiber grating reflected backs; This two wavelength light is separated by 2 fiber couplers and 1 fibre optic isolater or 2 fiber optical circulators, passes to 2 photodetectors more respectively and is converted into electric signal, obtains the gas density data by data Collection ﹠ Processing System at last.These sensor-based systems have advantages such as remote measurement, precision height, electric insulation, no potential safety hazard; Its shortcoming comprises all deficiencies (as described above) of the absorption firedamp sensor of optical fiber White air chamber, comprises also that simultaneously sensor cost height (fibre optic isolater or circulator costliness), system architecture complexity, ambient temperature effect big (can not temperature compensation) wait deficiency.These deficiencies remain new technology and method and are overcome and improve.

Summary of the invention

At the prior art above shortcomings, the object of the present invention is to provide a kind of have structure and debug simple, can detect simultaneously that gas density and temperature, its sensor bulk are little, collimation easily, the double grid optical fiber microcavity gas sensing system of lower, the vibrationproof of cost and characteristics such as temperature stability is good, light loss and interference are little;

Another object of the present invention provides a kind ofly can carry out temperature compensation correction, gas density method for sensing that precision is high.

Technical scheme of the present invention is as follows:

Double grid optical fiber microcavity gas sensing system is characterized in that: be made up of wideband light source A, optical fiber B1～B3, wide-band coupler Cw, double grid optical fiber microcavity firedamp sensor S, spectrum demodulator D and computing machine E; Wideband light source A is connected to the input end of wide-band coupler Cw by optical fiber B1, the output terminal of wide-band coupler Cw is connected to the front end of double grid optical fiber microcavity firedamp sensor by optical fiber B3, another input end of wide-band coupler Cw is connected to the optic fibre input end of spectrum demodulator D by optical fiber B2, and the FPDP of spectrum demodulator D is connected to computing machine E by the data-interface line;

Described double grid optical fiber microcavity firedamp sensor comprises first optical fiber and second optical fiber, first self-focus lens and second self-focus lens, the first short period fiber grating and the second short period fiber grating, supporting mass, wherein first optical fiber end face is positioned at the focus of first self-focus lens and is fixed together with first self-focus lens, this first self-focus lens is fixedly connected on the supporting mass, on supporting mass, be fixedly connected with second self-focus lens at a distance of first self-focus lens, one segment distance place, space between first self-focus lens and second self-focus lens constitutes microcavity, an end face of second optical fiber is positioned at the focus of second self-focus lens and is fixed together with second self-focus lens, on second optical fiber with the first short period fiber grating and the second short period fiber grating; The reflection kernel wavelength of the first short period fiber grating is outside the absorbing band of methane gas, the reflection kernel wavelength of the second short period fiber grating is in the absorbing band of methane gas, and the interval between the reflection kernel wavelength of the first short period fiber grating and the second short period fiber grating is greater than half of the three dB bandwidth sum of its reflection wavelength band; Second optical fiber is the rear end of double grid optical fiber microcavity firedamp sensor, and first optical fiber is the front end of double grid optical fiber microcavity firedamp sensor.

In the double grid optical fiber microcavity firedamp sensor of this sensor-based system, the end face of first optical fiber that is positioned at the first self-focus lens focus is vertical with the optical axis of first self-focus lens; The end face of second optical fiber that is positioned at the second self-focus lens focus is vertical with the optical axis of second self-focus lens; The light shaft coaxle of the optical axis of first self-focus lens and second self-focus lens, the distance between first self-focus lens and second self-focus lens is about 3-500mm; The 3dB wavelength bandwidth at the reflection kernel wavelength place of the first short period fiber grating and the second short period fiber grating all is about 0.02-0.5nm, the reflectivity at the reflection kernel wavelength place of these two short period fiber gratings is all greater than 50%, distance between these two short period fiber gratings is greater than 0.5mm, these two short period fiber gratings to the distance of any end face of second optical fiber greater than 5mm.

In double grid optical fiber microcavity firedamp sensor, be connected with an over cap on the supporting mass; There is bleeder vent the position of over cap between first self-focus lens and second self-focus lens, near two the short period fiber grating place bleeder vent is arranged, be coated with dustproof ventilative thin layer at each bleeder vent place; Be connected with the first buffer fiber sheath and the second buffer fiber sheath at both ends respectively by solidifying glue near over cap and supporting mass.

This sensor-based system obtains the method for sensing of methane gas concentration, its concrete steps are: primary constant, the temperature correction parameter of (i) demarcating double grid optical fiber microcavity firedamp sensor in this sensor-based system earlier, (ii) then by the reflected light perception gas density and the temperature information of double grid optical fiber microcavity firedamp sensor, obtain described catoptrical spectroscopic data and be transferred to computing machine by spectrum demodulator, (iii) calculate sensitivity coefficient by computing machine according to spectroscopic data, primary constant, temperature correction parameter at last, calculate the methane gas concentration value again; It is characterized in that: 1) light of wideband light source A is transferred to first optical fiber of double grid optical fiber microcavity firedamp sensor by optical fiber B1, wide-band coupler Cw and optical fiber B3, behind first self-focus lens collimation, become directional light and enter microcavity, absorbed by the methane gas in the microcavity, be coupled to second optical fiber through second self-focus lens again, returned near described two short period fiber grating reflection kernel wavelength narrow band light by the first short period fiber grating on second optical fiber and the fiber grating reflection of second short period; 2) described two short period fiber gratings reflect the light that returns becomes directional light and enters microcavity once more behind second self-focus lens collimation, absorbed again by the methane gas in the microcavity, be coupled to first optical fiber through first self-focus lens again, and be transferred to spectrum demodulator through optical fiber B3, wide-band coupler Cw and optical fiber B2; 3) computing machine obtains described two catoptrical spectrum of short period fiber grating by spectrum demodulator, with the catoptrical amplitude of the first short period fiber grating as the reference signal, with the catoptrical amplitude of the second short period fiber grating as the gas density signal, with the catoptrical reflection kernel wavelength of the second short period fiber grating as temperature signal, computing machine calculates the measured value of methane gas concentration again according to described reference signal, gas density signal, temperature signal, primary constant, sensitivity coefficient when temperature correction parameter calculates actual sensing temperature; Computing formula when its demarcation and sensing is respectively:

(a) the sensor primary constant a of Biao Dinging ₀For

$a_0=\ln \left[\frac{I_2\left(0\right)}{I_1\left(0\right)}\right]$

Wherein, I ₁(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the first short period fiber grating;

I ₂(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the second short period fiber grating;

(b) temperature correction parameter k ₀

$k_0=\frac{1}{C_0}\{\ln \left[\frac{I_2\left(1\right)}{I_1\left(1\right)}\right]-a_0\}$

Wherein, C ₀It is the reference concentration of timing signal methane gas;

I ₁(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the first short period fiber grating;

I ₂(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the second short period fiber grating;

a ₀It is the sensor primary constant of demarcating;

(c) temperature correction parameter γ

$\mathrm{\γ}=({\mathrm{\λ}}_{2T}-{\mathrm{\λ}}_0)\sqrt{\frac{k_0{C}_0}{k_0{C}_0+a_0-\ln \left[\frac{I_2\left(2\right)}{I_1\left(2\right)}\right]}-1}$

Wherein, λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating;

λ _2TBe to demarcate temperature (T _C) and λ _2T≠ λ ₀The time, the reflection kernel wavelength of the second short period fiber grating;

C ₀It is the reference concentration of timing signal methane gas;

k ₀Be the temperature correction parameter k that timing signal obtains ₀

a ₀It is the sensor primary constant that timing signal obtains;

I ₁(2) be to demarcate temperature (T _C) and described sensor place gas density be C ₀The time, the reflected light amplitude of the first short period fiber grating;

I ₂(2) be to demarcate temperature (T _C) and described sensor place gas density be C ₀The time, the reflected light amplitude of the second short period fiber grating;

(d) the sensitivity coefficient k of described double grid optical fiber microcavity firedamp sensor during sensing

$k=\frac{k_0}{1+{\left(\frac{{\mathrm{\λ}}_2-{\mathrm{\λ}}_0}{\mathrm{\γ}}\right)}^2}$

Wherein, k ₀Be the temperature correction parameter k that timing signal obtains ₀

γ is the temperature correction parameter γ that timing signal obtains;

λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating;

λ ₂The reflection kernel wavelength of the second short period fiber grating that obtains when being sensing;

(e) concentration C of the tested methane gas that sensor-based system obtains during sensing is:

$C=\frac{1}{k}\{\ln \left[\frac{I_2\left(C\right)}{I_1\left(C\right)}\right]-a_0\}$

The sensitivity coefficient of the described double grid optical fiber microcavity firedamp sensor that calculates when wherein, k is sensing;

a ₀It is the sensor primary constant that timing signal obtains;

I ₁The reflected light amplitude of the first short period fiber grating when (C) being sensing;

I ₂The reflected light amplitude of the second short period fiber grating when (C) being sensing.

Compared to existing technology, this sensor-based system and method for sensing have following advantage:

(a) the double grid optical fiber microcavity firedamp sensor Stability Analysis of Structures in this sensor-based system, volume is little, and temperature stability is good, and anti-high dense impact is good, and reaction is fast, and the life-span is long, does not need frequent adjustment, and electric insulation does not burn, no potential safety hazard (intrinsic safety type sensor); (b) self-focus lens is based on the micro lens of graded index, and the matching that is absorbed air chamber and optical fiber by its microcavity that constitutes is good, and collimation and assembling are easy, and vibration resistance is strong, the coupling good stability; (c) this sensor-based system can be realized remote measurement, and non-relay telemeter distance can reach more than 20 kilometers, and system wiring is simple; (d) this sensor-based system and method for sensing thereof have been eliminated the influence that temperature is measured gas density, have also eliminated the influence that light source and light path are disturbed, and also can obtain temperature information simultaneously, and the sensing accuracy height can reach 10ppm.

Description of drawings

Fig. 1 is the structural drawing and the partial enlarged drawing thereof of the double grid optical fiber microcavity gas sensing system that the present invention relates to;

Fig. 2 is the structural drawing of the double grid optical fiber microcavity firedamp sensor that the present invention relates to;

Fig. 3 is the H-H cut-open view of Fig. 2.

Embodiment

Further specify the present invention below in conjunction with accompanying drawing:

As shown in Figure 1, sensor-based system of the present invention is made up of wideband light source A, optical fiber B1～B3, wide-band coupler Cw, double grid optical fiber microcavity firedamp sensor S, spectrum demodulator D and computing machine E.Its constructive method is: centre wavelength is 1330nm, the wideband light source A of bandwidth 40nm is connected to the input end of wide-band coupler Cw by optical fiber B1, the output terminal of wide-band coupler Cw is connected to the front end of double grid optical fiber microcavity firedamp sensor by optical fiber B3, another input end of wide-band coupler Cw is connected to the optic fibre input end of spectrum demodulator D by optical fiber B2, the data output end of spectrum demodulator D is by gpib interface (or USB, RS232,485 standard data interfaces such as grade) bus is connected to computing machine E, and double grid optical fiber microcavity firedamp sensor places tested methane gas.Wherein, the band of wideband light source is 1310～1350nm, comprises one or more absorbing bands of methane gas (major component is methane CH4).

Referring to Fig. 2 and Fig. 3, the structure of the double grid optical fiber microcavity firedamp sensor in the sensor-based system of the present invention comprises: the first buffer fiber sheath 1, first optical fiber 2, supporting mass 3, curing glue 4, first self-focus lens 5, second self-focus lens 6, curing glue 7, the first short period fiber grating 8, the second short period fiber grating 9, second optical fiber 10, the second buffer fiber sheath 11, over cap 12, dustproof ventilative thin layer 13 and curing glue 14.Wherein, be the first short period fiber grating 8 and the second short period fiber grating 9 of 5mm on second optical fiber 10 with length; The fiber end face 10mm that the first short period fiber grating 8 is nearest, its reflection kernel wavelength is 1327nm, and the three dB bandwidth of its reflection wavelength band is 0.2nm, and the reflectivity at its reflection kernel wavelength place is 95%; The fiber end face 17mm that the second short period fiber grating 9 is nearest, its reflection kernel wavelength is 1331.5nm, and the three dB bandwidth of its reflection wavelength band is 0.2nm, and the reflection efficiency at its reflection kernel wavelength place is 95%; Supporting mass is about 80mm, and the external diameter of first self-focus lens 5 and second self-focus lens 6 is 4mm; By solidifying glue 4 and solidifying glue 7, first optical fiber 2 and first self-focus lens 5, second optical fiber 10 and second self-focus lens 6 all with optical axis be fixedly connected on the supporting mass 3, distance between these two self-focus lenses is 12mm, first optical fiber 2 is positioned at the focus of second self-focus lens 6 as the front end of double grid optical fiber microcavity firedamp sensor from the first short period fiber grating, 8 nearest end faces on second optical fiber 10; On supporting mass 3, be connected with over cap 12 by solidifying glue; This over cap 12 is equipped with the bleeder vent of long 12mm arc length 7mm at centre bit, the bleeder vent of long 8mm arc length 6mm is arranged, the inner arc radius 3mm of over cap in the over cap end near the short period fiber grating; Be coated with dustproof breathable films 13 at two bleeder vent places of over cap, this dustproof breathable films is by solidifying the gluing surface that is attached to over cap; Bleeder vent on the over cap and dustproof ventilative thin layer thereof can stop entering of dust, but allow methane gas enter absorbing light in the microcavity, can make temperature influence the short period fiber grating fast, make described sensor sense temperature and gas information better; Be connected with the first buffer fiber sheath 1 and the second buffer fiber sheath 11 at both ends respectively by solidifying glue 14, avoid damaging with protection optical fiber near over cap and supporting mass; In over cap end surface sign " rear end " arranged near the short period fiber grating.

In double grid optical fiber microcavity firedamp sensor of the present invention, the end face of first optical fiber 2 that is positioned at first self-focus lens, 5 focus places is vertical with the optical axis of first self-focus lens 5, and the end face of second optical fiber 10 that is positioned at second self-focus lens, 6 focus places is vertical with the optical axis of second self-focus lens 6; The reflection kernel wavelength of the second short period fiber grating 9 is better in the 3dB of methane gas absorption spectrum band, and is best near the absorption peak wavelength of methane gas; Space between two self-focus lenses constitutes a microcavity, as the miniature absorption air chamber of methane gas, the about 3mm of its diameter.

Sensor-based system of the present invention obtains the method for sensing of methane gas concentration, and its concrete steps and feature thereof are:

Step (i) is demarcated double grid optical fiber microcavity firedamp sensor primary constant a earlier ₀, temperature correction parameter k ₀And γ: (a) to be placed in temperature be that normal temperature and gas density are in 0 the gas box to double grid optical fiber microcavity firedamp sensor, and computing machine obtains that the amplitude of narrow band central wavelength is respectively I near 1327nm and the 1331.5nm ₁(0) and I ₂(0); (b) double grid optical fiber microcavity firedamp sensor places gas density C ₀In=2% the gas box, gas box places the adjustable constant temperature oven of temperature, change the temperature of constant temperature oven, computing machine determines that by near the amplitude of the narrow band central wavelength 1331.5nm this amplitude is hour temperature T r, and at temperature T r point constant temperature, the amplitude that this computer-chronograph obtains near the narrow band central wavelength of 1327nm is I ₁(1), obtains that the centre wavelength and the amplitude of narrow band is respectively λ near the 1331.5nm ₀And I ₂(1), this λ ₀Near 1331.5nm; (c) double grid optical fiber microcavity firedamp sensor places gas density C ₀In=2% the gas box, it is T that gas box places temperature _CIn=10 ℃ the constant temperature oven, the amplitude that computing machine obtains near the narrow band central wavelength of 1327nm is I ₁(2), obtain that the centre wavelength and the amplitude of narrow band is respectively λ near the 1331.5nm _2TAnd I ₂(2); (d) in the described computing formula of above-mentioned data substitution that computing machine will obtain, obtain sensor primary constant a ₀, temperature correction parameter k ₀And γ.

Step is the reflected light perception gas density and the temperature information of double grid optical fiber microcavity firedamp sensor (ii), obtained described catoptrical spectroscopic data and be transferred to computing machine by spectrum demodulator: (a) light of the wideband light source A of 1310～1350nm band is by optical fiber B1, wide-band coupler Cw and optical fiber B3 are transferred to first optical fiber of double grid optical fiber microcavity firedamp sensor, behind first self-focus lens collimation, become directional light and enter microcavity, absorbed by the methane gas in the microcavity, be coupled to second optical fiber through second self-focus lens again, returned near 1327nm and the 1331.5nm wavelength narrow band light by the first short period fiber grating on second optical fiber and the fiber grating reflection of second short period; (b) reflecting 1327nm and near the narrow band light the 1331.5nm wavelength returned becomes directional light and enters microcavity once more behind second self-focus lens collimation, absorbed again by the methane gas in the microcavity, be coupled to first optical fiber through first self-focus lens again, and be transferred to spectrum demodulator through optical fiber B3, wide-band coupler Cw and optical fiber B2; (c) spectrum demodulator is obtained 1327nm and near the narrow-band spectrum data of 1331.5nm wavelength, and is sent to computing machine.

Step (iii) calculates sensitivity coefficient by computing machine according to spectroscopic data, primary constant, temperature correction parameter at last, calculates the methane gas concentration value again: (a) computing machine obtains 1327nm and near the narrow-band spectrum data of 1331.5nm wavelength; (b) computing machine obtains that the amplitude of narrow-band spectrum central wavelength is I near the 1327nm wavelength ₁(C), obtain that the amplitude of narrow-band spectrum central wavelength is I near the 1331.5nm wavelength with as with reference to signal ₂(C), obtain that the centre wavelength of narrow-band spectrum is λ near the 1331.5nm wavelength with as the gas density signal ₂With as temperature signal, according to described computing formula, temperature signal λ ₂, the λ that obtains of step (i) ₀And temperature correction parameter k ₀The sensitivity coefficient k of described double grid optical fiber microcavity firedamp sensor when calculating actual sensing with γ etc.; (c) the sensor primary constant a that obtains according to described computing formula, step (i) of computing machine ₀, the reference signal I that (iii) obtains of step ₁(C) and gas density signal I ₂(C) and sensitivity coefficient k etc. calculate the measured value C of methane gas concentration.Obtain the concentration value of tested methane gas like this with regard to sensing.This measured value has been eliminated the influence of disturbing on temperature, light source and the light path, and its precision can reach 10ppm.

Claims (5)

1, double grid optical fiber microcavity gas sensing system is characterized in that: be made up of wideband light source A, optical fiber B1～B3, wide-band coupler Cw, double grid optical fiber microcavity firedamp sensor S, spectrum demodulator D and computing machine E; Wideband light source A is connected to the input end of wide-band coupler Cw by optical fiber B1, the output terminal of wide-band coupler Cw is connected to the front end of double grid optical fiber microcavity firedamp sensor by optical fiber B3, another input end of wide-band coupler Cw is connected to the optic fibre input end of spectrum demodulator D by optical fiber B2, and the FPDP of spectrum demodulator D is connected to computing machine E by the data-interface line;

Described double grid optical fiber microcavity firedamp sensor S comprises first optical fiber (2) and second optical fiber (10), first self-focus lens (5) and second self-focus lens (6), the first short period fiber grating (8) and the second short period fiber grating (9), supporting mass (3), wherein an end face of first optical fiber (2) is positioned at the focus of first self-focus lens (5) and is fixed together with first self-focus lens (5), the end face of first optical fiber (2) is vertical with the optical axis of first self-focus lens (5), this first self-focus lens (5) is fixedly connected on the supporting mass (3), go up at a distance of first self-focus lens (5) fixedly connected second self-focus lens in 3-500mm place (6) at supporting mass (3), space between first self-focus lens (5) and second self-focus lens (6) constitutes microcavity, an end face of second optical fiber (10) is positioned at the focus of second self-focus lens (6) and is fixed together with second self-focus lens (6), the end face of second optical fiber (10) is vertical with the optical axis of second self-focus lens (6), and the light shaft coaxle of first self-focus lens and second self-focus lens, second optical fiber (10) is gone up with the first short period fiber grating (8) and the second short period fiber grating (9), distance between the two short period fiber gratings is greater than 0.5mm, and two short period fiber gratings to the distance of any end face of second optical fiber (10) greater than 5mm; The reflection kernel wavelength of the first short period fiber grating (8) is outside the absorbing band of methane gas, the reflection kernel wavelength of the second short period fiber grating (9) is in the absorbing band of methane gas, and the interval between the reflection kernel wavelength of the first short period fiber grating (8) and the second short period fiber grating (9) is greater than half of the three dB bandwidth sum of its reflection wavelength band; Second optical fiber (10) is the rear end of double grid optical fiber microcavity firedamp sensor, and first optical fiber (2) is the front end of double grid optical fiber microcavity firedamp sensor.

2, double grid optical fiber microcavity gas sensing system according to claim 1, it is characterized in that: in described double grid optical fiber microcavity firedamp sensor, first optical fiber (2), first self-focus lens (5) and supporting mass (3) are fixed together by solidifying glue (4); Second optical fiber (10), second self-focus lens (6) and supporting mass (3) are fixed together by solidifying glue (7); The 3dB wavelength bandwidth at the reflection kernel wavelength place of the first short period fiber grating (8) and the second short period fiber grating (9) all is about 0.02-0.5nm, and the reflectivity at its reflection kernel wavelength place is all greater than 50%.

3, double grid optical fiber microcavity gas sensing system according to claim 1 and 2 is characterized in that: on described supporting mass (3), be connected with an over cap (12) by solidifying glue (4) and solidifying glue (7); There is bleeder vent the position of over cap (12) between first self-focus lens (5) and second self-focus lens (6), locate bleeder vent near the first short period fiber grating (8) and the second short period fiber grating (9), be coated with dustproof ventilative thin layer (13) at each bleeder vent place; Be connected with the first buffer fiber sheath (1) and the second buffer fiber sheath (11) at both ends respectively by solidifying glue (14) near over cap (12) and supporting mass (3).

4, obtain the method for sensing of methane gas concentration by claim 1 or 2 described sensor-based systems, its concrete steps are: the primary constant of (i) demarcating double grid optical fiber microcavity firedamp sensor in the described sensor-based system earlier, temperature correction parameter, (ii) then by the reflected light perception gas density and the temperature information of double grid optical fiber microcavity firedamp sensor, obtain described catoptrical spectroscopic data and be transferred to computing machine by spectrum demodulator, (iii) at last by computing machine according to spectroscopic data, primary constant, temperature correction parameter calculates sensitivity coefficient, calculates the methane gas concentration value again; It is characterized in that:

1) light transmission of wideband light source A is to first optical fiber (2) of double grid optical fiber microcavity firedamp sensor, behind first self-focus lens (5) collimation, become directional light and enter microcavity, absorbed by the methane gas in the microcavity, be coupled to second optical fiber (10) through second self-focus lens (6) again, returned near the narrow band light of described two short period fiber grating reflection kernel wavelength by first short period fiber grating (8) on second optical fiber (10) and the reflection of the second short period fiber grating (9);

2) described two short period fiber gratings reflect the light that returns becomes directional light and enters microcavity once more behind second self-focus lens (6) collimation, absorbed again by the methane gas in the microcavity, be coupled to first optical fiber (2) through first self-focus lens (5) again, and be transferred to spectrum demodulator;

3) computing machine obtains described two catoptrical spectrum of short period fiber grating by spectrum demodulator, with the catoptrical amplitude of the first short period fiber grating (8) as the reference signal, with the catoptrical amplitude of the second short period fiber grating (9) as the gas density signal, with the catoptrical reflection kernel wavelength of the second short period fiber grating (9) as temperature signal, computing machine is according to described reference signal, the gas density signal, temperature signal, primary constant, sensitivity coefficient when temperature correction parameter calculates actual sensing temperature calculates the measured value of methane gas concentration again;

Computing formula when its demarcation and sensing is respectively:

(a) the sensor primary constant a of Biao Dinging ₀For

$a_0=\ln \left[\frac{I_2\left(0\right)}{I_1\left(0\right)}\right]$

Wherein, I ₁(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the first short period fiber grating (8);

I ₂(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the second short period fiber grating (9);

(b) temperature correction parameter k ₀

$k_0=\frac{1}{C_0}\{\ln \left[\frac{I_2\left(1\right)}{I_1\left(1\right)}\right]-a_0\}$

Wherein, C ₀It is the reference concentration of timing signal methane gas;

I ₁(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating (9) is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the first short period fiber grating (8);

I ₂(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating (9) is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the second short period fiber grating (9);

a ₀It is the sensor primary constant of demarcating;

(c) temperature correction parameter γ

$\mathrm{\γ}=({\mathrm{\λ}}_{2T}-{\mathrm{\λ}}_0)\sqrt{\frac{k_0{C}_0}{k_0{C}_0+a_0-\ln \left[\frac{I_2\left(2\right)}{I_1\left(2\right)}\right]}-1}$

Wherein, λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating (9);

λ _2TBe to demarcate temperature (T _C) and λ _2T≠ λ ₀The time, the reflection kernel wavelength of the second short period fiber grating (9);

C ₀It is the reference concentration of timing signal methane gas;

k ₀Be the temperature correction parameter k that timing signal obtains ₀

a ₀It is the sensor primary constant that timing signal obtains;

I ₁(2) be to demarcate temperature (T _C) and described sensor place gas density be C ₀The time, the reflected light amplitude of the first short period fiber grating (8);

I ₂(2) be to be C demarcating temperature (TC) and described sensor place gas density ₀The time, the reflected light amplitude of the second short period fiber grating (9);

(d) the sensitivity coefficient k of described double grid optical fiber microcavity firedamp sensor during sensing

$k=\frac{k_0}{1+{\left(\frac{{\mathrm{\λ}}_2-{\mathrm{\λ}}_0}{\mathrm{\γ}}\right)}^2}$

Wherein, k ₀Be the temperature correction parameter k that timing signal obtains ₀

γ is the temperature correction parameter γ that timing signal obtains;

λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating (9);

λ ₂The reflection kernel wavelength of the second short period fiber grating (9) that obtains when being sensing;

(e) concentration C of the tested methane gas that sensor-based system obtains during sensing is:

$C=\frac{1}{k}\{\ln \left[\frac{I_2\left(C\right)}{I_1\left(C\right)}\right]-a_0\}$

The sensitivity coefficient of the described double grid optical fiber microcavity firedamp sensor that calculates when wherein, k is sensing;

a ₀It is the sensor primary constant that timing signal obtains;

I ₁The reflected light amplitude of the first short period fiber grating (8) when (C) being sensing;

I ₂The reflected light amplitude of the second short period fiber grating (9) when (C) being sensing.

5, obtain the method for sensing of methane gas concentration by the described sensor-based system of claim 3, its concrete steps are: the primary constant of (i) demarcating double grid optical fiber microcavity firedamp sensor in the described sensor-based system earlier, temperature correction parameter, (ii) then by the reflected light perception gas density and the temperature information of double grid optical fiber microcavity firedamp sensor, obtain described catoptrical spectroscopic data and be transferred to computing machine by spectrum demodulator, (iii) at last by computing machine according to spectroscopic data, primary constant, temperature correction parameter calculates sensitivity coefficient, calculates the methane gas concentration value again; It is characterized in that:

1) light transmission of wideband light source A is to first optical fiber (2) of double grid optical fiber microcavity firedamp sensor, behind first self-focus lens (5) collimation, become directional light and enter microcavity, absorbed by the methane gas in the microcavity, be coupled to second optical fiber (10) through second self-focus lens (6) again, returned near the narrow band light of described two short period fiber grating reflection kernel wavelength by first short period fiber grating (8) on second optical fiber (10) and the reflection of the second short period fiber grating (9);

2) described two short period fiber gratings reflect the light that returns becomes directional light and enters microcavity once more behind second self-focus lens (6) collimation, absorbed again by the methane gas in the microcavity, be coupled to first optical fiber (2) through first self-focus lens (5) again, and be transferred to spectrum demodulator;

3) computing machine obtains described two catoptrical spectrum of short period fiber grating by spectrum demodulator, with the catoptrical amplitude of the first short period fiber grating (8) as the reference signal, with the catoptrical amplitude of the second short period fiber grating (9) as the gas density signal, with the catoptrical reflection kernel wavelength of the second short period fiber grating (9) as temperature signal, computing machine is according to described reference signal, the gas density signal, temperature signal, primary constant, sensitivity coefficient when temperature correction parameter calculates actual sensing temperature calculates the measured value of methane gas concentration again;

Computing formula when its demarcation and sensing is respectively:

(a) the sensor primary constant a of Biao Dinging ₀For

$a_0=\ln \left[\frac{I_2\left(0\right)}{I_1\left(0\right)}\right]$

Wherein, I ₁(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the first short period fiber grating (8);

I ₂(0) be that described sensor place gas density is 0 o'clock, the reflected light amplitude of the second short period fiber grating (9);

(b) temperature correction parameter k ₀

$k_0=\frac{1}{C_0}\{\ln \left[\frac{I_2\left(1\right)}{I_1\left(1\right)}\right]-a_0\}$

Wherein, C ₀It is the reference concentration of timing signal methane gas;

I ₁(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating (9) is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the first short period fiber grating (8);

I ₂(1) be that described sensor place gas density is C ₀And when the reflection kernel wavelength of the second short period fiber grating (9) is positioned at gas absorption spectrum peak wavelength, the reflected light amplitude of the second short period fiber grating (9);

a ₀It is the sensor primary constant of demarcating;

(c) temperature correction parameter γ

$\mathrm{\γ}=({\mathrm{\λ}}_{2T}-{\mathrm{\λ}}_0)\sqrt{\frac{k_0{C}_0}{k_0{C}_0+a_0-\ln \left[\frac{I_2\left(2\right)}{I_1\left(2\right)}\right]}-1}$

Wherein, λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating (9);

λ _2TBe to demarcate temperature (T _C) and λ _2T≠ λ ₀The time, the reflection kernel wavelength of the second short period fiber grating (9);

C ₀It is the reference concentration of timing signal methane gas;

k ₀Be the temperature correction parameter k that timing signal obtains ₀

a ₀It is the sensor primary constant that timing signal obtains;

I ₁(2) be to demarcate temperature (T _C) and described sensor place gas density be C ₀The time, the reflected light amplitude of the first short period fiber grating (8);

I ₂(2) be to demarcate temperature (T _C) and described sensor place gas density be C ₀The time, the reflected light amplitude of the second short period fiber grating (9);

(d) the sensitivity coefficient k of described double grid optical fiber microcavity firedamp sensor during sensing

$k=\frac{k_0}{1+{\left(\frac{{\mathrm{\λ}}_2-{\mathrm{\λ}}_0}{\mathrm{\γ}}\right)}^2}$

Wherein, k ₀Be the temperature correction parameter k that timing signal obtains ₀

γ is the temperature correction parameter γ that timing signal obtains;

λ ₀It is the peak wavelength of gas absorbing band at the reflection kernel wavelength place of the second short period fiber grating (9);

λ ₂The reflection kernel wavelength of the second short period fiber grating (9) that obtains when being sensing;

(e) concentration C of the tested methane gas that sensor-based system obtains during sensing is:

$C=\frac{1}{k}\{\ln \left[\frac{I_2\left(C\right)}{I_1\left(C\right)}\right]-a_0\}$

The sensitivity coefficient of the described double grid optical fiber microcavity firedamp sensor that calculates when wherein, k is sensing;

a ₀It is the sensor primary constant that timing signal obtains;

I ₁The reflected light amplitude of the first short period fiber grating (8) when (C) being sensing;

I ₂The reflected light amplitude of the second short period fiber grating (9) when (C) being sensing.

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