CN104535090A - Wavelength-matched double FBG demodulation systems based on cascaded long period grating - Google Patents

Abstract

The invention discloses wavelength-matched double FBG demodulation systems based on a cascaded long period grating. Light emitted from a broadband light source is smoothed by a cascaded long period grating (CLPG) and then divided into two paths of light by a first Y-shaped optical fiber coupler: after one path of the light goes through a second Y-shaped optical fiber coupler and a first fiber bragg grating (FBG) sensor, reflected light is transformed to be a first path of light intensity signal by a first photoelectric detection module; after the other path of the light goes through a third Y-shaped optical fiber coupler and a second fiber bragg grating (FBG) sensor, the reflected light is transformed to be a second path of light intensity signal by a second photoelectric detection module; the first path of light intensity signal and the second path of light intensity signal go through a data acquisition card, and then are sent into a PC computer to be processed. The wavelength-matched double FBG demodulation systems based on the cascaded long period grating use the wideband characteristics of the cascaded long period grating, and each detect the power changes of signals in the central wavelength of one FBG in a cascaded long period grating transmission spectrum positive and negative slop linearity region, therefore, the bad effects brought by noise are eliminated, and the precision of the detected signals is improved.

Description

Based on Wavelength matched two FBG demodulating systems of cascaded long-period grating

Technical field

The present invention relates to the field such as optical fiber communication, Fibre Optical Sensor, specifically a kind of Wavelength matched two FBG demodulating systems based on cascaded long-period grating.

Background technology

Fiber Bragg Grating FBG (FBG) sensor is as a kind of comparatively ripe optical sensor, can effectively overcome conventional sensing systems long-time stability, permanance, electromagnetism interference and distributed etc. in deficiency, and to the environment parameter such as temperature and strain, there is higher sensitivity, be easy to imbed the health detection that intelligent structure inside realizes structure.Wavelength interrogation technique is one of gordian technique of FBG sensor-based system.Usual employing fiber grating matching matrix, tunable Fabry Perot cell method etc. carry out the demodulation of Wavelength-encoding.Wherein fiber grating matching matrix structure is simple but precision is not high; Tunable Fabry Perot cell method precision is higher but expensive.Cascade-connection long period fiber grating (CLPG) is formed by connecting by the identical even long period fiber grating (LPG) of two parameters and one section of general single mode fiber, can obtain spectrum property more better than single LPG after cascade.Adopt cascade-connection long period fiber grating to build FBG demodulating system, although this system has, price is low, structure is simple, demodulation speed is fast.But often doped with the noise that light source shake and other labile factor of system etc. are brought in monitored signal, bring comparatively big error to system, reduce the precision of system.

Summary of the invention

For prior art Problems existing, the present invention utilizes the wide spectral characteristics of CLPG, propose a kind of Wavelength matched two FBG demodulating systems based on cascaded long-period grating, utilize the wide spectral characteristics of cascade-connection long period fiber grating, the signal power variations of each monitoring FBG central wavelength in the positive negative slope range of linearity of cascaded long-period grating transmission spectrum, thus the adverse effect eliminated except noise brings, improve the precision of monitored signal.

The technical scheme that the present invention takes is:

Based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, the light that wideband light source sends is after cascade-connection long period fiber grating CLPG filtering, be divided into two-way light through the first y-type optical fiber coupling mechanism again: a road light after the second y-type optical fiber coupling mechanism and the first optical fiber bragg grating FBG sensor, reflected light through the first Photoelectric Detection module converter be first via light intensity signal; After another Lu Guangjing the 3rd y-type optical fiber coupling mechanism and the second optical fiber bragg grating FBG sensor, reflected light through the second Photoelectric Detection module converter be the second road light intensity signal, first via light intensity signal, the second road light intensity signal, after data collecting card, are sent into PC and are processed.

Described cascade-connection long period fiber grating CLPG is in series by least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascaded long-period grating after series connection is crossing with the reflectance spectrum of optical fiber bragg grating FBG, and this intersection point is positioned at cascade-connection long period fiber grating CLPG transmitted spectrum linearity range.

The wavelength coverage of described wideband light source is 1200nm ~ 1600nm.

Described second y-type optical fiber coupling mechanism, the 3rd y-type optical fiber coupling mechanism comprise three ports respectively: 1# port, 2# port, 3# port, and wherein 1# port connecting fiber bragg grating FBG, 2# port connect the reflected light port that input light, 3# port are optical fiber bragg grating FBG.

Described first Photoelectric Detection module, the second Photoelectric Detection module are equal: connected to form successively by photodetector, temperature-compensation circuit, zeroing circuit, two-stage amplifying circuit and extraneous output interface.

The present invention is based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, technique effect is as follows:

1), the Wavelength matched two FBG demodulation schemes based on cascaded long-period grating of the present invention's proposition, make use of the wide spectral characteristics of CLPG, the signal power variations of each monitoring FBG central wavelength in the positive negative slope range of linearity of CLPG transmission spectrum, eliminate often doped with the noise effect that light source shake and other labile factor of system etc. are brought in monitored signal, the precision of the therefore larger system that improves.

2) the present invention adopts light intensity demodulation method, is the change of light intensity, changes light signal into electric signal by photodetector by the change transitions of Fiber Bragg Grating FBG centre wavelength, is convenient for measuring the record of result, stores and controls.Photodetector and modulate circuit conveniently make module, change Fiber Bragg Grating FBG and mate long-period gratings and can adopt this modular circuit, saving cost.

Accompanying drawing explanation

Fig. 1 is the Wavelength matched two FBG demodulating system schematic diagram that the present invention is based on cascaded long-period grating.

Fig. 2 is the spectrogram of the present invention two FBG and CLPG.

Fig. 3 is the connection diagram of coupling mechanism of the present invention.

Fig. 4 is reflective light intensity (by photodetector voltage V1, V2 reflection) the temperature variant working curve of a FBG under high-temperature condition of the present invention, the 2nd FBG.

Fig. 5 is that the present invention can obtain V=V1-V2 and high temperature relation curve according to V1 and V2.

Embodiment

Based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, the light that wideband light source 1 sends is after cascade-connection long period fiber grating CLPG filtering, two-way light is divided into again: a road light is after the second y-type optical fiber coupling mechanism 4 and the first optical fiber bragg grating FBG sensor 5, and reflected light is converted into first via light intensity signal through the first Photoelectric Detection module 6 through the first y-type optical fiber coupling mechanism 3; After another Lu Guangjing the 3rd y-type optical fiber coupling mechanism 7 and the second optical fiber bragg grating FBG sensor 8, reflected light is converted into the second road light intensity signal through the second Photoelectric Detection module 9, first via light intensity signal, the second road light intensity signal, after data collecting card 10, are sent into PC (11) machine and are processed.

Described cascade-connection long period fiber grating CLPG is in series by least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascaded long-period grating after series connection is crossing with the reflectance spectrum of optical fiber bragg grating FBG, and this intersection point is positioned at cascade-connection long period fiber grating CLPG transmitted spectrum linearity range.

The wavelength coverage of described wideband light source 1 is 1200nm ~ 1600nm.

Described second y-type optical fiber coupling mechanism 4, the 3rd y-type optical fiber coupling mechanism 7 comprise three ports respectively: 1# port, 2# port, 3# port, and wherein 1# port connecting fiber bragg grating FBG, 2# port connect the reflected light port that input light, 3# port are optical fiber bragg grating FBG.

Described first Photoelectric Detection module 6, second Photoelectric Detection module 9 is equal: connected to form successively by photodetector, temperature-compensation circuit, zeroing circuit, two-stage amplifying circuit and extraneous output interface.

Principle analysis:

Because cascade-connection long period fiber grating CLPG is transmission-type optical fibre device, insertion loss is low, and system make use of the transmitted light of cascade-connection long period, and optical fiber Bragg raster is reflection type optical fiber device, reflectivity is high, and system then make use of the reflected light of optical fiber bragg grating FBG.When after optical fiber Bragg raster center wavelength variation, can there is respective change by the filtered light intensity of cascaded long-period grating CLPG in optical fiber bragg grating FBG reflected light signal.Adopt high-speed photodetector that light signal (variable quantity of output intensity) is converted into electric signal, the light signal that light path part exports is more weak, after opto-electronic conversion, obtain voltage signal also fainter, devise that signal conditioning circuit amplifies signal, filtering etc.The anti-variable quantity releasing Fiber Bragg Grating FBG centre wavelength of the electric signal exported by modulate circuit.

Described cascade-connection long period fiber grating CLPG is in series by least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascaded long-period grating after series connection is crossing with the reflectance spectrum of Bragg grating, and this intersection point is positioned at cascaded long-period grating transmitted spectrum linearity range.The transmitted spectrum of cascade-connection long period is different from the spectrum of common long-period gratings, because this system utilizes the linear filter characteristic of cascaded long-period grating, therefore only analyzes linear section.The slope of the linear section of cascaded long-period grating is much larger than single long-period gratings, and therefore comparatively large through the filtered light intensity variable quantity of cascaded long-period grating, facilitate photodetector to monitor, multiple long period cascades can significantly improve the precision of demodulation.

The reflectance spectrum of Fiber Bragg Grating FBG is crossing with the linearity range of the transmitted spectrum of cascaded long-period grating, to ensure the luminous power linear change after modulating.The wavelength coverage of cascade-connection long period linear section is greater than single long-period gratings, adopts the program can increase the wavelength coverage of demodulation.This invention is all optical fibre structure, tuning without the need to mechanical part, and demodulation speed depends on the processing speed of the bandwidth sum back-end circuit in photodetector, and the bandwidth of high-speed photodetector is generally several GHz, and therefore demodulation speed of the present invention is high.Select the cascaded long-period grating matched according to the centre wavelength of Fiber Bragg Grating FBG, the centre wavelength of Fiber Bragg Grating FBG must be positioned at left side linearity range or the right linear section of cascaded long-period grating spectral centroid wavelength.(left side linearity range is positioned at) when the centre wavelength of Fiber Bragg Grating FBG is less than long-period gratings, along with the increase of Fiber Bragg Grating FBG centre wavelength, luminous power corresponding to resonance wavelength after cascade-connection long period modulation reduces gradually, otherwise luminous power increases; When the centre wavelength of Fiber Bragg Grating FBG is greater than long-period gratings, along with the increase of Fiber Bragg Grating FBG centre wavelength, luminous power corresponding to resonance wavelength increases gradually, otherwise luminous power reduces.

Concrete principle of the present invention is:

T is used respectively in two linear zones of the transmission spectrum T (λ) of cascaded long-period grating ₁(λ) (negative slope linear zone) and T ₂(λ) (positive slope linear zone) represents, then have:

T ₁(λ)＝A ₁λ+B ₁(1-1)

T ₂(λ)＝A ₂λ+B ₂(1-2)

In formula: A ₁and A ₂be the slope of two straight lines, B ₁and B ₂it is the intercept of two straight lines.

The reflectance spectrum R of two FBG ₁(λ) and R ₂(λ) can be expressed as:

R ₁(λ)＝R _B1exp[-α ₁(λ-λ _B1) ²] ${\mathrm{\α}}_1=\frac{4\ln 2}{{b_1}^2}---(1-3)$

R ₂(λ)＝R _B2exp[-α ₂(λ-λ _B2) ²] ${\mathrm{\α}}_2=\frac{4\ln 2}{{b_2}^2}---(1-4)$

In formula: λ _b1and λ _b2be respectively the centre wavelength of FBG1 and FBG2, R _b1and R _b2be respectively the reflectivity of FBG1 and FBG2 in central wavelength, b ₁and b ₂be respectively the halfwidth degree of FBG1 and FBG2.

The output voltage V of two photodetectors ₁and V ₂can be expressed as:

$V_1=\underset{-\∞}{\overset{\∞}{\∫}}{\mathrm{\β}}_1{T}_1\left(\mathrm{\λ}\right)R_1\left(\mathrm{\λ}\right)\mathrm{d\λ}---(1-5)$

$V_2=\underset{-\∞}{\overset{\∞}{\∫}}{\mathrm{\β}}_2{T}_2\left(\mathrm{\λ}\right)R_2\left(\mathrm{\λ}\right)\mathrm{d\λ}---(1-6)$

In formula: β ₁and β ₂for constant, this constant is relevant with the factor such as the photoelectric conversion factor of the splitting ratio of coupling mechanism, optical path loss and photodetector.

By formula (1-1) – (1-4) brings formula (1-5) and (1-6) into and integration can obtain:

$V_1={\mathrm{\β}}_1(A_1{R}_{B1}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_1}}{\mathrm{\λ}}_{B1}+B_1{R}_{B1}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_1}})=K_1{\mathrm{\λ}}_{B1}+C_1---(1-7)$

$V_2={\mathrm{\β}}_2(A_2{R}_{B2}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_2}}{\mathrm{\λ}}_{B2}+B_2{R}_{B2}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_2}})=K_2{\mathrm{\λ}}_{B2}+C_2---(1-8)$

In formula:

$\begin{array}{cc}{K}_1={\mathrm{\β}}_1{A}_1{R}_{B1}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_1}}& C_1={\mathrm{\β}}_1{B}_1{R}_{B1}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_1}}\end{array}$

$\begin{array}{cc}{K}_2={\mathrm{\β}}_2{A}_2{R}_{B2}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_2}}& C_2={\mathrm{\β}}_2{B}_2{R}_{B2}\sqrt{\frac{\mathrm{\π}}{{\mathrm{\α}}_2}}\end{array}$

Assuming that there is identical change, then V in two FBG (temperature or strain) centre wavelength under external influence ₁and V ₂can be rewritten as:

V ₁＝K ₁(λ _B1+Δλ)+C ₁＝K ₁·Δλ+(K ₁λ _B1+C ₁) (1-9)

V ₂＝K ₂(λ _B2+Δλ)+C ₂＝K ₂·Δλ+(K ₂λ _B2+C ₂) (1-10)

During monitor strain, the centre wavelength drift of FBG can be expressed as:

Δλ＝(1-p _e)λ _B·ε (1-11)

In formula: p _efor strain optical coefficient, λ _bfor Bragg wavelength.

According to formula (1-9)-(1-11), all can monitor strain ε by FBG1 sensor or FBG2 sensor.

Consider the impact of the error that system noise is brought, V ₁and V ₂can be rewritten as:

V ₁＝K ₁·Δλ+(K ₁λ _B1+C ₁)+n ₁(t) (1-12)

V ₂=K ₂Δ λ+(K ₂λ _b2+ C ₂)+n ₂in (t) (1-13) formula: n ₁(t) and n ₂t () represents that system noise is to output voltage V respectively ₁and V ₂impact.

The transmission spectrum of cascaded long-period grating has near-sighted symmetry characteristic in positive negative linear slope district, therefore A ₁≈-A ₂.Assuming that the spectrum of FBG1 and FBG2 have symmetry and etc. square characteristic, then have R _b1=R _b2, b ₁=b ₂.Assuming that in FBG1 and FBG2 sensing process, there is the photoelectric conversion factor of identical coupling mechanism splitting ratio, optical path loss and photodetector, then β ₁=β ₂.Therefore K ₁≈-K ₂.V ₁and V ₂record under identical conditions simultaneously, can think that system noise is to V ₁and V ₂impact identical, therefore n ₁(t)=n ₂(t).Make V=V ₁-V ₂, then have:

V＝2K ₁·Δλ+K ₁(λ _B1-λ _B2)+C ₁-C ₂(1-14)

Although V ₁and V ₂the impact of system noise can be subject to, but through V ₁-V ₂process process, V is only relevant with measured signal, thus can effective filtering appts noise.

Concrete steps of the present invention are:

The first step, the light of wideband light source 1 is divided into two-way light by the first y-type optical fiber coupling mechanism 3 after cascade-connection long period fiber grating CLPG filtering;

Second step, this two-way light encourages corresponding optical fiber bragg grating FBG sensor respectively by after each self-corresponding coupling mechanism;

3rd step, the light of more than 90% reflects through optical fiber bragg grating FBG, by entering corresponding photodetector after respective coupling mechanism;

4th step, after the center wavelength variation of optical fiber bragg grating FBG, the Fiber Bragg Grating FBG output intensity generation linear change after cascade-connection long period fiber grating CLPG modulates;

5th step, the optical power change of two-way light is converted into the change of voltage signal by photodetector respectively, after signal condition, exports PC 11 to process by data collecting card 10.

Below in conjunction with accompanying drawing 4 and accompanying drawing 5, the present invention is further detailed explanation.

Insert in DH401CT temperature control box by a FBG (centre wavelength is 1527nm) and the 2nd FBG (centre wavelength is 1533nm), the maximum temperature of this temperature control box is 360 DEG C simultaneously.During experiment, the temperature in temperature control box is raised to 115 DEG C from 20 DEG C, and every minor tick 5 DEG C, is incubated 10min after temperature is raised to setting value.Utilize demodulating equipment of the present invention to measure output voltage, result as shown in Figure 4.Voltage V can be found out ₁raise with temperature and reduce, degree of fitting is 0.9996, according to data fitting result V ₁be 0.49137mv/ DEG C with temperature relation.Voltage V ₂raise with temperature and increase, degree of fitting is 0.9994, according to data fitting result V ₂be 0.49437mv/ DEG C with temperature relation.According to V ₁and V ₂v=V can be obtained ₁-V ₂with high temperature relation curve, Fig. 5 can find out V and high temperature linear, degree of fitting is 0.9998, is 0.98574mv/ DEG C according to data fitting result V and temperature relation.

The high temperature of improved system and low temperature test show that improved system system accuracy compared with original system is significantly improved.The sensitivity of original system is 0.49mv/ DEG C, and wavelength resolution is 0.005nm, and temperature resolution is 0.5 DEG C; The sensitivity of improved system is 0.98mv/ DEG C, and wavelength resolution is 0.0025nm, and temperature resolution is 0.25 DEG C.

Claims (5)

1., based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that,

The light that wideband light source (1) sends after cascade-connection long period fiber grating CLPG filtering, then is divided into two-way light through the first y-type optical fiber coupling mechanism (3):

One road light is after the second y-type optical fiber coupling mechanism (4) and the first optical fiber bragg grating FBG sensor (5), and reflected light is converted into first via light intensity signal through the first Photoelectric Detection module (6);

After another Lu Guangjing the 3rd y-type optical fiber coupling mechanism (7) and the second optical fiber bragg grating FBG sensor (8), reflected light is converted into the second road light intensity signal through the second Photoelectric Detection module (9),

First via light intensity signal, the second road light intensity signal, after data collecting card (10), are sent into PC (11) and are processed.

2. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described cascade-connection long period fiber grating CLPG is in series by least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascaded long-period grating after series connection is crossing with the reflectance spectrum of optical fiber bragg grating FBG, and this intersection point is positioned at cascade-connection long period fiber grating CLPG transmitted spectrum linearity range.

3., according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, the wavelength coverage of described wideband light source (1) is 1200nm ~ 1600nm.

4. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described second y-type optical fiber coupling mechanism (4), the 3rd y-type optical fiber coupling mechanism (7) comprise three ports respectively: 1# port, 2# port, 3# port, and wherein 1# port connecting fiber bragg grating FBG, 2# port connect the reflected light port that input light, 3# port are optical fiber bragg grating FBG.

5. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described first Photoelectric Detection module (6), the second Photoelectric Detection module (9) are equal: connected to form successively by photodetector, temperature-compensation circuit, zeroing circuit, two-stage amplifying circuit and extraneous output interface.