CN101261281A - Optical fibre acceleration sensor based on ultra- short cavity optical fibre laser - Google Patents

Abstract

The invention relates to an optical fiber acceleration sensor based on an ultrashort cavity optical fiber laser, which is characterized by comprising: an equal strength socle beam which is an isoceles triangle and can convert an end inertia force into an even strain on the surface; an inertial quality block which is fixed on one end of the equal strength socle beam to produce inertia forces; and an optional fiber laser which is fixed on the central axis line above the equal strength socle beam to convert the strain into the change of the lasing wavelength of the laser.

Description

Optical fiber acceleration transducer based on ultra-short cavity optical fibre laser

Technical field

The invention belongs to sensory field of optic fibre, be specifically related to a kind ofly, be suitable for the high precision measurement in fields such as the measurement of microvibration of industry, military field and earthquake motion monitoring, building monitoring and demarcation, metering based on the optical fiber acceleration transducer of ultra-short cavity optical fibre laser as core component.

Background technology

Acceleration transducer is an important sensor commonly used in shock resistance, anti-vibration measurement, seismic monitoring, the inertial navigation and guidance systems such as ordnance, vehicle, boats and ships.The principle of work of inertia-type accelerometer is at inertial space inertial force or the displacement that produces when mass is made acceleration movement with the perception measured piece to be set, and measures this inertial force or displacement and can measure relevant acceleration.Traditional acceleration transducer adopts the structure of electromagnetic type more, because the characteristics of electromagnetic measurement itself determine, unavoidably brings shortcomings such as electromagnetic noise interference, complex structure, easy installation.

Along with the development of photoelectron technology, fiber grating sensing technology has become the focus of research in recent years.Fiber grating is a photosensitivity of utilizing fiber optic materials, forms the space phase grating in fiber core.When multi-wavelength signals went into to inject optical fiber, certain wavelength signals (Bragg wavelength) that satisfies the optical grating reflection condition can be coupled into backward wave and return along the former road of optical fiber.The ultimate principle of optical fiber grating sensing is: the variation of physical quantitys such as temperature, strain and stress can cause the pitch of fiber grating and the variation of effective refractive index, thereby the Bragg wavelength of fiber grating reflection is drifted about, just can obtain the information of corresponding temperature, strain and stress by detection fiber grating Bragg wavelength change.Because the transducing signal of fiber grating directly is modulated to optical wavelength, is not subjected to intensity of light source fluctuating, junction loss and bending loss of optical fiber etc. to influence good stability; Be convenient to utilize connect a plurality of fiber gratings of wavelength-division multiplex technique to form the distributed sensing networks simultaneously, therefore obtain huge development.The successful direct measurement that is applied to stress, strain, temperature of fiber grating at present, and by means of mechanical hook-up, imbed technology etc. and also can realize measurement, as optical fibre grating acceleration sensor, fiber-optical grating temperature sensor, optical fiber grating sonic device etc. physical quantitys such as displacement, power, acceleration.

Sensor based on fiber grating is a passive device, promptly detects the variation by fiber grating narrowband reflection center wavelength of light, obtains the information of measured signal.Because the reflected light bandwidth of fiber grating can't further be dwindled (about 0.1nm), supporting with it wavelength detection resolution only is inferior pm magnitude, although taked various enhanced sensitivity measures, sensitivity and dynamic range all can not reach higher level.

Based on the fiber laser of fiber grating, utilize the reflection characteristic and the frequency-selecting effect of fiber grating, form optical fiber laser structure by respectively connect an identical fiber grating of centre wavelength at one section high-gain rare earth doped fiber two ends.Because the output laser signal of above-mentioned laser instrument has extremely narrow live width (can reach the KHz magnitude at present), coherent length can reach hundred kilometers, by designing suitable energy converter and demodulating system, can be used for multiple field sensing measurement.And this scheme is than highly sensitive several orders of magnitude of traditional fiber grating sensor.

Based on the sensor of the fiber laser of fiber grating, not only highly sensitive in traditional fiber-optic grating sensor, and possess big, the easy multiplexing characteristics of networking of, light weight little, dynamic range equally as anti-electromagnetic interference (EMI), volume.

Summary of the invention

The objective of the invention is to, a kind of optical fiber acceleration transducer based on ultra-short cavity optical fibre laser is provided, it can reach and improve several order of magnitude sensitivity of sensing sensitivity.The present invention utilizes ultra-short cavity optical fibre laser as basic sensing element, utilizes equi intensity cantilever as energy converter, realizes acceleration and other vibration informations of object are carried out the high-precision sensing measurement.

A kind of optical fiber acceleration transducer based on ultra-short cavity optical fibre laser of the present invention is characterized in that, comprising:

One equal strength semi-girder, this equal strength semi-girder are an isoceles triangle shape, and end inertial force is converted into surface strain uniformly;

One inertial mass, this inertial mass is fixed on the end of equi intensity cantilever, is used to produce inertial force;

One fiber laser, this fiber laser are fixed on the axis above the equi intensity cantilever, the strain on the semi-girder is converted into the variation of laser instrument excitation wavelength.

Wherein said fiber laser is DBR type fiber laser or DFB type fiber laser.

Wherein said DBR type fiber laser comprises: the rare earth doped fiber of a high-gain, the two ends of the rare earth doped fiber of this high-gain are manufactured with the fiber grating of coupling.

Wherein the fiber grating of DBR type fiber laser is connected with tail optical fiber respectively.

Wherein said DFB type fiber laser comprises: the rare earth doped fiber of a high-gain and making grating in the above.

The two ends that wherein are produced on the fiber grating on the high-gain Active Optical Fiber are connected with tail optical fiber respectively.

Useful result of the present invention is:

Ultra-short cavity optical fibre laser as sensing element provides extremely narrow laser linewidth, is different from the reflected signal of fiber grating, makes further high precision demodulation heat transfer agent become possibility;

Content of the present invention not only can be used as acceleration transducer and uses, and by adjusting the natural frequency of vibration of the beam of uniform strength and mass system, can also be used for the vibration velocity sensor and the displacement transducer of different measuring scope;

With traditional electric sensor, no electronic component and electric signal transmit in the sensor provided by the invention relatively, and sensor is not subjected to the interference of electromagnetic field, and electrical insulating property and security are good;

Laser instrument can be any among DBR or the DFB, and the working method of laser instrument can adopt two kinds of optional modes of reflection or transmission;

Equi intensity cantilever has guaranteed on the semi-girder that strain equates that everywhere fiber grating chirp phenomenon can not occur, and then has guaranteed beating of lasing mode can not occur on the fiber laser;

The demodulation scheme of non-equilibrium M-Z is adopted in the demodulation of wavelength, can realize high-precision Wavelength demodulation;

The scheme of phase demodulating is adopted in the signal demodulation, has improved the dynamic range of system greatly;

The signal of sensor measurement is the fast vibration signal, can ignore gradual temperature effect;

Can choose at random the frequency-selecting fiber grating of different centre wavelengths, the excitation wavelength that changes laser instrument is to satisfy functional need;

Laser instrument possesses natural wavelength characteristic, easily adopts the mode of wavelength-division multiplex, realizes arrayed applications easily than interfere type acceleration transducer and intensity modulation type sensor.So system not only can spot measurement, also can realize arrayed applications.

Description of drawings

In order to further specify technical characterictic of the present invention and effect, the present invention is described further below in conjunction with drawings and Examples, wherein:

Fig. 1 is the acceleration transducer synoptic diagram of fiber laser;

Fig. 2 is a Ultrashort cavity DBR type fiber laser synoptic diagram;

Fig. 3 is a DFB type fiber laser synoptic diagram;

Fig. 4 is a fiber laser acceleration transducer synoptic diagram;

Fig. 5 is the optical fiber laser model selection synoptic diagram;

Fig. 6 is fiber laser single longitudinal mode, narrow linewidth output map (heterodyne testing scheme);

Fig. 7 is a high precision Wavelength demodulation system schematic diagram;

Fig. 8 is a fiber laser acceleration transducer arrayed applications synoptic diagram.

Embodiment

Be example with DBR type optical fiber laser acceleration sensor below, further specify measuring principle of the present invention, concrete structure and embodiment in conjunction with the accompanying drawings.

See also Fig. 1, shown in Figure 2, a kind of optical fiber acceleration transducer based on ultra-short cavity optical fibre laser of the present invention comprises:

One equal strength semi-girder 5, this equal strength semi-girder 5 is an isoceles triangle shape, and end inertial force is converted into surface strain uniformly;

One inertial mass 6, this inertial mass 6 is fixed on the end of equi intensity cantilever 5, is used to produce inertial force;

One fiber laser 8, this fiber laser 8 are fixed on the axis above the equi intensity cantilever 5, the strain on the semi-girder is converted into the variation of laser instrument excitation wavelength.

Wherein said fiber laser is DBR type fiber laser 8 or DFB type fiber laser 8 ' (Fig. 3).

Wherein said DBR type fiber laser 8 (Fig. 2) comprising: the rare earth doped fiber 12 of a high-gain, the two ends of the rare earth doped fiber 12 of this high-gain are manufactured with the fiber grating 11,13 of coupling, should, this fiber grating 11,13 is connected with tail optical fiber 10,14 respectively.

Wherein said DFB type fiber laser 8 ' (Fig. 3, wherein identical parts adopt identical label) comprising: the rare earth doped fiber 12 of a high-gain and making grating 11 ' in the above, the two ends of this fiber grating 11 ' are connected with tail optical fiber 10,14 respectively.

Please in conjunction with consulting Fig. 4, Fig. 4 is a fiber laser acceleration sensor system structural representation of the present invention.This system comprises pump light source 1, and Wavelength demodulation system 2 closes/wave-dividing device 3, and sensor and package casing 4 pass light optical fiber 9, and acceleration transducer 30 passes light optical fiber 9 and close by connecting/and wave-dividing device 3 is connected with Wavelength demodulation system 2 with pump light 1; Show as Fig. 1, acceleration transducer 30 comprises fiber laser 8, equi intensity cantilever 5, inertial mass 6, trip bolt 7 etc., wherein, the beam of uniform strength 5 fastened screws 7 are fixed on the package casing 4, modes such as fiber laser 8 employings are bonding, embedding are fixed on the top central authorities of the beam of uniform strength 5, are used for the strain that inertial force causes is changed into the long variation in laser instrument 8 chambeies; Show as Fig. 2, fiber laser 8 is mixed rare earth (as erbium) optical fiber 12 (also claiming Active Optical Fiber), is connected tail optical fiber 10 and 14 formations by catoptron, high-gain that the fiber grating 11 and 13 of two centre wavelength unanimities constitutes laserresonator, wherein, two couplings of the two ends welding of high-gain rare earth doped fiber grating 11 and 13 adopts fusing mode to realize that low-loss connects between them.

The basic functional principle of fiber laser acceleration transducer: the fiber laser acceleration transducer is in vibration environment, the relative motion of mass 6 and acceleration transducer shell 4, cause equi intensity cantilever 5 to produce crooked as transducer, cause that the fiber laser 8 that is cemented in above the semi-girder 5 produces axial stretching or compressive strain, the length that is the FP resonator cavity of laser instrument is modulated by outer signals, and then caused the change that laser instrument is exported laser center wavelength, by the high-precision Wavelength demodulation system 2 anti-vibration acceleration information of releasing.

The modulation principle of fiber laser:

Fiber laser adopts the structure of DBR (Distributed BraggReflector, distributed Blatt reflective), as shown in Figure 2: the fiber grating 11 of two couplings and 13 and the rare earth doped fiber 12 of high-gain form a F-P resonator cavity.Under the acting in conjunction of FBG bandpass filtering and the frequency-selecting of F-P chamber, the specific part in the broadband fluorescence Spectra that the pumping laser effect produces down comes constantly to be amplified and finally form laser in the back reflective in the chamber.Reasonably select rare earth doped fiber kind and length, FBG coupling wavelength and pumping laser, can obtain the continuous output of single longitudinal mode laser.Change that the pump light incident direction can realize reflecting or the working method of transmission.Its modeling principle schematic is as shown in Figure 5:

The fiber grating 11,13 of two centre wavelength unanimities constitutes the catoptron of F-P resonator cavity in the accompanying drawing 5, as the fiber grating 11 of big reflection bandwidth, high reflectance with play the fiber grating 13 of the narrow reflection bandwidth of frequency-selecting effect, big reflection bandwidth can also increase fiber grating between mate redundance; Pattern 17 in the F-P chamber is:

f＝mc/2nL，m＝1，2，3…(1)

Mode spacing is

Δf＝c/2nL(2)

But because the bandpass characteristics of fiber grating, the part longitudinal mode that only partly is arranged in narrow linewidth fiber grating 13 reflectance spectrums 16 has the possibility (as the grey of accompanying drawing 5 and the longitudinal mode of black) of resonance; Because the bandwidth of whole FBG only has about 0.12nm, the gain level unanimity of the rare earth doped fiber in this scope reaches threshold value sharp ejaculation narrow-linewidth laser at first and consumes the exciting that the population upset suppresses other patterns (accompanying drawing 5 grey) in the free schema competition of laser instrument so be arranged in the resonant mode (accompanying drawing 5 black) of FBG reflectivity peak in several longitudinal modes of competition.

Fiber laser 8 as sensor is being subjected under the external environment effect, can not have the jump sudden change of pattern.Effects of strain on the equi intensity cantilever causes the rare earth doped fiber of fixed fiber laser 8 thereon and fiber grating coupling to produce corresponding axial strain on 11,13, and as for the F-P chamber, the long variation of the ejected wave that strain causes can be represented with two parts:

$\frac{\mathrm{\Δ\λ}}{\mathrm{\λ}}={\left(\frac{2\mathrm{nL}}{K}\right)}^{\′}/2\mathrm{nL}/K=\frac{\mathrm{\ΔL}}{L}+\frac{\mathrm{\Δn}}{n}=\mathrm{\ϵ}+\frac{\mathrm{\Δn}}{n}=(1-P_e)\·\mathrm{\ϵ}---\left(3\right)$

And for FBG, strain causes wherein that the cardiac wave long wavelength is changed to:

$\frac{\mathrm{\Δ\λ}}{\mathrm{\λ}}={(2n\·\mathrm{\Λ})}^{\′}/2\mathrm{n\Λ}=\frac{\mathrm{\Δ\Λ}}{\mathrm{\Λ}}+\frac{\mathrm{\Δn}}{n}=\mathrm{\ϵ}+\frac{\mathrm{\Δn}}{n}=(1-P_e)\·\mathrm{\ϵ}---\left(4\right)$

Be that both have same result.So during the common strain of F-P resonator cavity and fiber grating, two coupling gratings still mate, institute's lectotype is constant according to the relative position in fiber grating reflectance spectrum 15,16, the frequency-selecting result is stable, can not occur because the sharp power swing of penetrating that the pattern that the dislocation of F-P chamber mode of resonance and reflection peak causes is beated and the variation of mode reflection rate causes.

From the modeling process as can be seen, the resonator cavity of fiber laser 8 long 12 is short more, and longitudinal mode 17 is big more at interval; Simultaneously, frequency-selecting FBG live width 16 is narrow more, can exciting and to participate in the longitudinal mode of mode competition few more.So, sharp the penetrating of the shortest active resonant cavity length 12 and the minimum easy more realization single longitudinal mode of FBG reflection bandwidth 13, length as band optical fiber grating 11 in the laser instrument is 5mm, rare earth doped fiber 12 length of high-gain are about 15mm, the length of narrow linewidth fiber grating 13 is about 10mm, and laser instrument length amounts to about 30mm.Help follow-up enhanced sensitivity encapsulation and miniaturization, single longitudinal mode swashs penetrates figure as shown in Figure 6.

Sensing principle is analyzed:

According to Elasticity knowledge, deflection when being subjected to perpendicular to the load of middle plane, the beam of uniform strength 5 takes place, and the curvature of the beam of uniform strength is a constant when deflection is little, and the strain of each point can be expressed as above the beam of uniform strength:

$\mathrm{\&epsiv;}=\frac{6\mathrm{<figure-callout\; id="2"\; label="L\; Ebh"\; filenames="A20071006423800211.png"\; state="\{\{state\}\}">L</figure-callout>}}{{\mathrm{Ebh}}^{}}\&CenterDot;P---\left(5\right)$

Wherein, E is the elastic modulus of material, and L, h, b are respectively the width of length, thickness and the stiff end of beam, for the ultra-short cavity optical fibre laser 8 that is cemented in above the beam of uniform strength 5, does the time spent being subjected to axial stress, and its wavelength variations is:

$\frac{\mathrm{\&Delta;\&lambda;}}{\mathrm{\&lambda;}}=\mathrm{\&eta;}(1-P_e)\&CenterDot;\mathrm{\&epsiv;}---\left(6\right)$

Wherein, η is the paste transfer coefficient, and the glue and the technology that adopt when pasting between fiber-grating laser 8 and the equi intensity cantilever 5 are relevant, and Pe is the elasto-optical coefficient of optical fiber, is generally 0.22.

When carrying out acceleration analysis, be rigidly connected to this fiber laser acceleration transducer 4 on the tested point in some way, if vibration acceleration is a at the component perpendicular to middle plane, the inertial force that causes of mass 6 is F=ma so, direction is with acceleration direction unanimity, promptly applied size on the top of the beam of uniform strength 5 for the load of F and cause deflection of beam, at this moment, the wavelength variations that causes is:

$\frac{\mathrm{\&Delta;\&lambda;}}{\mathrm{\&lambda;}}=\mathrm{\&eta;}(1-P_e)\&CenterDot;\mathrm{\&epsiv;}=\mathrm{\&eta;}(1-P_e)\&CenterDot;\frac{6\mathrm{<figure-callout\; id="2"\; label="L\; Eb\; h"\; filenames="A20071006423800211.png"\; state="\{\{state\}\}">L</figure-callout>}}{\mathrm{Eb}{h}^{}}\&CenterDot;\mathrm{ma}---\left(7\right)$

As can be seen from the above equation, promptly wavelength variable quantity Δ λ is proportional with acceleration a.

The wave length shift that temperature causes is a tempolabile signal for the wave length shift that vibration acceleration causes, can remove by low-pass filtering.

For the optical-fiber laser vibration acceleration, transducing signal is to export in the mode of Wavelength-encoding, the sensitivity that can define sensor is the wavelength variations (using the unit of the gravity acceleration g=9.8m/s2 of a standard as acceleration usually) that unit acceleration that sensor is subjected on sensitive direction is done time spent output, promptly

$s=\frac{\mathrm{\&Delta;\&lambda;}}{a}(\mathrm{pm}/g)---\left(8\right)$

So formula (7) can be expressed as again:

$s=\frac{\mathrm{\&Delta;\&lambda;}}{g}=0.78\&times;9.8\&times;\frac{6\mathrm{<figure-callout\; id="2"\; label="L\; Eb\; h"\; filenames="A20071006423800211.png"\; state="\{\{state\}\}">L</figure-callout>}}{\mathrm{Eb}{h}^{}}\&times;m\&times;\mathrm{\&lambda;}(\mathrm{pm}/g)---\left(9\right)$

Wherein, transfer coefficient η is 1, and elasto-optical coefficient is 0.78.For the beam of uniform strength and mass system, its natural frequency of vibration is expressed as:

$f=\frac{1}{2\mathrm{\&pi;}}\sqrt{\frac{{\mathrm{<figure-callout\; id="3"\; label="Ebh"\; filenames="A20071006423800211.png"\; state="\{\{state\}\}">Ebh</figure-callout>}}^3}{6(m_1+0.2m_2)l^3}}---\left(10\right)$

Wherein, m ₁And m ₂It is respectively the quality of mass and beam.When doing acceleration analysis, must be by the frequency of measuring acceleration less than the natural frequency of vibration of acceleration transducer, in earthquake motion monitoring field, earthquake motion all is a low frequency signal, the natural frequency of vibration of for this reason setting acceleration is about 150Hz.

The high precision solution conditioning technology:

Although fiber laser and the center wavelength shift amount that fiber grating produces under the same strain effect be identical (suc as formula 3 and formula 4 show), but because the laser linewidth very narrow (KHz) that fiber laser sends, laser linewidth is about the 10-6 of fiber grating three dB bandwidth (15GHz).So supporting corresponding Wavelength demodulation technology can realize that transducer sensitivity improves 106 times, thereby realize the purpose of high-acruracy survey acceleration.

Fiber laser hydrophone Wavelength demodulation technology is based on the ultimate principle of interfere type sensor, by non-equilibrium interferometer the fiber laser hydrophone wavelength change is converted into phase change, adopt high-resolution phase place to produce carrier wave (PGC) scheme then and carry out phase-detection, thereby realize the nautical receiving set Wavelength demodulation.The demodulating system principle is shown in 2 in the accompanying drawing 7.18 is isolator among the figure, and 19 is signal generator, and 20 for being wrapped in the arm length difference on the PZT, and 21 is non-equilibrium interferometer, and 22 is photodetector, and 23 is phase demodulation modules.The laser of being modulated by vibration signal in the acceleration transducer enters the variation that non-equilibrium interferometer 21 is converted into phase place after by isolator 18, because the coherent length of flashlight is very long, interferometer 21 can adopt big arm length difference 20 that very little wavelength shift is zoomed into phase change, adopts PGC demodulating system 23 that it is detected reduction.Relation between fiber laser wavelength variations and interferometer phase change is shown below:

$\mathrm{\&Delta;\&phi;}=\frac{\mathrm{\&pi;nd}}{{\mathrm{\&lambda;}}^2}\&CenterDot;\mathrm{\&Delta;\&lambda;}---\left(11\right)$

Interferometer by the design different parameters is realized different Wavelength demodulation precision with PGC phase demodulating system.

Illustrate below by example:

Adopt alloy steel material as the beam of uniform strength, its various parameters are as shown in the table:

b(mm)	l(mm)	h(mm)	E(GPa)	M(g)	S(pm/g)	f(Hz)
							10	60	2.1	144	10	6.63	154

If the wavelength resolution of high sensitivity Wavelength demodulation system is 5 * 10 ^-6Pm, thus the minimum acceleration size that this sensor can be offered an explanation be 0.75 μ g!

And based on the sensor of fiber grating, its light signal that returns demodulated equipment is the reflectance spectrum of fiber grating, its live width is about about 15GHz, because the reflected light bandwidth of fiber grating can't further be dwindled, can not obtain high-precision wavelength resolution (inferior pm magnitude) as Ultrashort cavity light laser instrument acceleration sensor system, so its sensing sensitivity is also well below said system.

Optical fiber laser acceleration sensor 4 of the present invention not only can spot measurement, and can arrayed applications, as shown in Figure 8.Owing to the wavelength characteristic that fiber laser itself has, when series connection group battle array, can at first adopt the method for wavelength-division multiplex.Pump light is absorbed and is inspired the narrow-linewidth laser of respective wavelength successively in laser instrument, by increase corresponding partial wave unit 24 and detector array 25 can realize the distributing arrayed applications of multimetering in demodulating system.

Claims (6)

1. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser is characterized in that, comprising:

One equal strength semi-girder, this equal strength semi-girder are an isoceles triangle shape, and end inertial force is converted into surface strain uniformly;

One inertial mass, this inertial mass is fixed on the end of equi intensity cantilever, is used to produce inertial force;

One fiber laser, this fiber laser are fixed on the axis above the equi intensity cantilever, the strain on the semi-girder is converted into the variation of laser instrument excitation wavelength.

2. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser according to claim 1 is characterized in that, wherein said fiber laser is DBR type fiber laser or DFB type fiber laser.

3. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser according to claim 2, it is characterized in that, wherein said DBR type fiber laser comprises: the rare earth doped fiber of a high-gain, the two ends of the rare earth doped fiber of this high-gain are manufactured with the fiber grating of coupling.

4. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser according to claim 3 is characterized in that wherein the fiber grating of DBR type fiber laser is connected with tail optical fiber respectively.

5. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser according to claim 2 is characterized in that, wherein said DFB type fiber laser comprises: the rare earth doped fiber of a high-gain and making grating in the above.

6. the optical fiber acceleration transducer based on ultra-short cavity optical fibre laser according to claim 5 is characterized in that, the two ends that wherein are produced on the fiber grating on the high-gain Active Optical Fiber are connected with tail optical fiber respectively.

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