CN112067843A - Optical fiber acceleration measuring device based on fiber core mismatch - Google Patents
Optical fiber acceleration measuring device based on fiber core mismatch Download PDFInfo
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- CN112067843A CN112067843A CN202010931459.9A CN202010931459A CN112067843A CN 112067843 A CN112067843 A CN 112067843A CN 202010931459 A CN202010931459 A CN 202010931459A CN 112067843 A CN112067843 A CN 112067843A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
- G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
- G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
- G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
- G01P15/093—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by photoelectric pick-up
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/02—Optical fibres with cladding with or without a coating
- G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
- G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
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Abstract
The invention provides an optical fiber acceleration measuring device based on fiber core mismatch. The device consists of a light source, a single-mode fiber, an optical circulator, an accelerometer based on fiber core mismatch, a photoelectric detector and a signal processing device. The accelerometer is formed by welding a single mode fiber, a large-core-diameter fiber and a double-clad fiber with an inclined grating in sequence, wherein the welded fiber penetrates through a preformed hole of the mounting plate and is arranged in the elastic sleeve. Light emitted by the light source is output from the end a of the circulator through the end b through the single-mode optical fiber, the end b is connected with the accelerometer, light reflected by the inclined grating is output to the photoelectric detector through the port b of the circulator and the port c of the circulator and converted into electric signals, and finally the electric signals are processed by the signal processing device. When the external vibration frequency changes, the resonance peak of the inner cladding of the double-cladding fiber tilt grating changes, and the measurement of the vibration frequency and the acceleration of the object is realized by monitoring the resonance peak. The invention has the advantages of small volume, quick response, no temperature influence and the like.
Description
(I) technical field
The invention relates to an optical fiber acceleration measuring device based on fiber core mismatch, in particular to a reflective acceleration sensor based on a double-cladding optical fiber tilt grating, which can measure vibration frequency and acceleration and belongs to the technical field of optical fiber sensing.
(II) background of the invention
The acceleration is a very important parameter index in the motion process of the object, and represents and measures the dynamic characteristic of the object. The accurate measurement of the acceleration is widely applied to various technical fields of mechanical vibration measurement, traffic condition monitoring, seismic wave signal detection of oil and gas exploration, structure monitoring of buildings, inertial navigation and guidance systems of aerospace and the like.
The acceleration signal measurement usually uses the principle of inertia to measure the corresponding acceleration by sensing the displacement or strain generated by the inertial force. The acceleration measurement method is various, such as mechanical, electromagnetic, electromechanical, and the like. However, the accuracy and sensitivity of the conventional mechanical measurement are not very high. In some special environments, the sensitivity of the acceleration sensor is required to be high, and the sensor is required to have the performances of resisting complex electromagnetic interference and the like. Conventional electromechanical or piezoelectric sensors are not capable of performing in response to such sensing environments and requirements. In this case, the optical fiber sensor can satisfy the requirements under such severe conditions.
The optical fiber sensing technology uses an optical fiber as a physical medium, and uses an optical signal as a sensitive information carrier, when some physical characteristic quantities (such as temperature, refractive index, pressure and the like) of the external environment of the optical fiber slightly change, the optical signal transmitted in the optical fiber also changes, and the changes can be detected by using a special optical signal processing technology. Compared with the traditional transmission medium, the optical fiber has the characteristic of low loss, and simultaneously has the advantages of large dynamic range and wide working frequency band; in the actual use process, the optical fiber is easily influenced by the measured physical quantity of the outside, and meanwhile, the optical fiber can still work well in a complex power plant and magnetic field environment because the optical fiber has light weight, small volume and is easy to bend. Therefore, the optical fiber sensor has more practicability than the traditional piezoelectric or electromechanical sensor under the extreme environments of easy combustion and explosion, small working space, serious electromagnetic field influence and the like.
Due to the advantages of the optical fiber sensor, in recent years, the optical fiber accelerometer draws high attention of researchers and obtains more and more researches and applications. Patent CN102721827A proposes an optical fiber interference accelerometer, which uses a control unit to generate two beams of orthogonal polarized light, a sensing optical fiber is wound on a mass block, and an acceleration to be measured is transmitted to an elastic body, so that the elastic body is deformed correspondingly, thereby generating a phase difference in the two orthogonal polarized lights of the sensing optical fiber, and detecting the phase difference, so as to obtain the acceleration to be measured. However, the method needs more components and parts, is complex to manufacture, and the elastomer and the mass block have larger volumes and are not suitable for extreme environments. Patent CN108680767A proposes a fiber accelerometer based on grating, which utilizes femtosecond laser micromachining technology to cut the cladding of the fiber, make the inertial element in the fiber, and carve the grating at the vibrating arm, when measuring the acceleration, the stress generated by vibration will concentrate on the vibrating arm, thereby changing the length and center wavelength of the grating, and obtaining the change of the acceleration by measuring the wavelength shift. However, the grating type wavelength measurement accelerometer is susceptible to external environment changes, for example, when the temperature changes, the central wavelength is also shifted, cross interference is generated, and the method needs femtosecond finish machining, is difficult to manufacture, has high cost, and is difficult to realize batch production.
Disclosure of the invention
The invention aims to provide an optical fiber acceleration measuring device based on fiber core mismatch, which is used for directly measuring the target acceleration under the condition of eliminating temperature cross interference.
In order to achieve the purpose, the invention adopts the scheme that:
the utility model provides an optic fibre acceleration measuring device based on fibre core mismatch which characterized in that: the device comprises a light source, a single-mode fiber, an optical circulator, a photoelectric detector, a signal processing device and an accelerometer based on fiber core mismatch, wherein the accelerometer is formed by sequentially welding the single-mode fiber, a large-core-diameter fiber and a double-clad fiber with an inclined grating, the welded accelerometer passes through a preformed hole of a mounting plate, the fiber at one end extends out of the mounting plate and is arranged in an elastic sleeve, and the other end of the accelerometer is fixed on the mounting plate.
Light emitted by the broadband light source is input from an a port and output from a b port of the optical circulator through a single-mode optical fiber, the output light is transmitted to the accelerometer based on fiber core mismatch, and a transmission schematic diagram of the accelerometer based on fiber core mismatch is shown in fig. 4. Light transmitted along the fiber core of the single-mode fiber is transmitted to the double-cladding fiber through the large-core-diameter fiber, and is excited by the inclined grating of the fiber core of the double-cladding fiber to resonate in a cladding transmitted backwards, wherein a resonance peak of the inner cladding is transmitted to the fiber core of the large-core-diameter fiber and is finally transmitted into the fiber core of the single-mode fiber, the light is input from a port b of the optical circulator, and a port c of the optical circulator is output to the photoelectric detector and the signal processing device. When the external vibration frequency changes, the resonance peak intensity of the inner cladding of the double-cladding fiber tilt grating changes, and the measurement of the vibration frequency and the acceleration of the object is realized by monitoring the resonance intensity. Temperature changes can cause the wavelength of the core mode resonant peak and the cladding mode resonant peak to change, so that the problem of cross talk caused by temperature is eliminated by monitoring the intensity of the inner cladding resonant peak only. In order to eliminate the influence of the light reflected by the end face of the optical fiber on the sensor, the tail end of the accelerometer is subjected to antireflection treatment.
The working principle of the invention is as follows:
the tilted grating core refractive index modulation is at an angle with respect to the core axis. When light from a light source passes through the tilted grating, in addition to reflecting the Bragg wavelength within the core, a portion of the core energy can be back-coupled into the fiber cladding, with each of these back-coupled cladding modes having its own particular coupling wavelength and mode field distribution.
The relationship between the effective refractive index of each cladding mode and its coupling wavelength can be expressed by the phase matching condition:
λBragg＝(ncore+ncore)Λ/cosθ
λclad,i＝(nclad,i+ncore)Λ/cosθ
wherein the subscript i represents the modulus, ncoreAnd nclad,iThe effective refractive indices of the core and cladding modes (ith order), respectively, a is the pitch of the grating when the grating is not tilted, and theta is the tilt angle of the grating, i.e., the angle between the grating and the axial normal of the fiber.
The transmission spectrum of the tilted grating is in the shape of an optical comb, where each resonance peak corresponds to an order cladding mode, and its spectral position (wavelength) depends on the effective refractive index of the corresponding cladding mode.
The optical fiber used by the invention is a double-clad optical fiber, an inner clad layer and an outer clad layer are arranged outside a fiber core, the refractive index difference exists between the two clad layers, and the fiber core is engraved with an inclined grating. The length of the inclined grating is 10mm-50 mm; the angle of inclination is not more than 20 deg..
The double-clad optical fiber consists of a fiber core, an inner cladding and an outer cladding, wherein the diameter of the fiber core is approximately the same as that of a single-mode optical fiber, and the diameter of the outer cladding is 125 mu m.
In order to make the inner cladding resonant mode enough to facilitate the measurement of acceleration, the diameter of the inner cladding of the double-clad optical fiber is not less than 16 μm.
The double-clad optical fiber core and the inner cladding as well as the inner cladding and the outer cladding have refractive index differences which can be the same or different.
The refractive index change of the double-clad optical fiber of the present invention may be either a step type or a graded type.
The light source is a broadband light source, and the output spectrum range of the light source covers the output spectrum of the inclined grating.
In order to effectively couple the backward-transmitted inner cladding mode into the core of the single-mode fiber, the diameter of the core of the large-core-diameter fiber is not less than that of the inner cladding of the double-clad fiber.
In the invention, the optical fiber extending out of the mounting plate is used as an inertia element, so the length of the optical fiber extending out of the mounting plate directly influences the resonance frequency and the acceleration sensitivity of the acceleration sensor, the specific length of the optical fiber extending out of the mounting plate needs to be designed according to the measurement requirement of actual vibration, and the length of the optical fiber extending out of the mounting plate is 20mm-100 mm.
Light emitted by a light source is input from an a port of the optical circulator through a single-mode fiber and output from a b port of the optical circulator, the output light is transmitted to the accelerometer based on fiber core mismatch, the output light is transmitted to the double-clad fiber through the large-core-diameter fiber, cladding resonance is excited by the inclined grating of the double-clad fiber, a backward transmitted inner cladding resonance peak is transmitted into the single-mode fiber through the large-core-diameter fiber, the output light is input from a b port of the optical circulator, and the output light is output to the photoelectric detector and the signal processing device through a c port. When the external vibration frequency changes, the intensity of the resonance peak of the inner cladding of the double-cladding fiber tilt grating changes, and the measurement of the vibration frequency and the acceleration of the object is realized by monitoring the resonance intensity. Because the power of the inner cladding is monitored, the change of temperature does not affect the coupling strength, and only the wavelength is changed, the cross interference of temperature is eliminated by monitoring the strength of a certain resonant peak of the inner cladding, and the temperature is measured by monitoring the wavelength of the fiber core mode because the Bragg resonant peak exists in the fiber core and is sensitive to the temperature.
The invention has the beneficial effects that:
1. the sensing device designed by the invention can measure the temperature and the acceleration at the same time, eliminates the cross interference caused by the temperature change, and because the inner cladding and the outer cladding exist, the acceleration measurement is only carried out by monitoring the resonance peak of the inner cladding, so the change of the refractive index of the external environment can not influence the resonance of the inner cladding, and the interference caused by the external environment is eliminated;
2. the invention converts the change of the acceleration into the change of the intensity of the output resonant wavelength, avoids the instability caused by adopting wavelength drift detection, and can eliminate the influence caused by the fluctuation of the light source by monitoring the Bragg resonant peak power;
3. the optical fiber sensing device has the advantages of small volume, high sensitivity, high temperature resistance, corrosion resistance, quick response, strong operability and the like.
(IV) description of the drawings
FIG. 1 is a fiber optic acceleration measurement device based on fiber core mismatch;
FIG. 2 is a schematic diagram of an accelerometer structure based on core mismatch;
FIG. 3 is a side view of an accelerometer based on core mismatch;
FIG. 4 is a schematic representation of accelerometer optical signal transmission based on core mismatch;
FIG. 5 is a schematic cross-sectional view of a step-index double-clad fiber;
FIG. 6 is a schematic cross-sectional view of a step-index large core optical fiber;
FIG. 7 is a double-clad fiber tilted grating transmission spectrum.
(V) detailed description of the preferred embodiments
The following describes an embodiment of the fiber core mismatch-based optical fiber acceleration measuring device according to the present invention with reference to the accompanying drawings:
example 1
The device is shown in fig. 1 and comprises a light source 1, a single-mode fiber 2, an optical circulator 3, a photoelectric detector 4, a signal processing device 5 and an accelerometer 6 based on fiber core mismatch.
The accelerometer structure based on fiber core mismatch is shown in fig. 2, and the preparation process is as follows:
1) carrying out hydrogen-loaded pretreatment on the optical fiber: the double-clad fiber 9 used in this embodiment is a step-type double-clad fiber, and its cross section is shown in fig. 5, and includes a core 9-1, an inner cladding 9-2, and an outer cladding 9-3, and the refractive index of the fiber changes in a step-type manner. The double-clad optical fiber is placed in a container filled with hydrogen, the pressure is 8MPa, the temperature is room temperature, after 240 hours, hydrogen molecules can be diffused into the fiber core of the double-clad optical fiber, the photosensitivity of the fiber core is increased, and if the hydrogen carrying time is shortened, the temperature can be properly increased or the pressure can be properly increased.
2) Inclined grating engraving: ultraviolet incident light is focused on a phase mask plate after passing through a beam expander and a focusing lens, the mask plate is parallel to a double-clad optical fiber, focused ultraviolet light irradiates the optical fiber through the mask plate, the mask plate is rotated to enable the mask plate to generate a certain inclination angle relative to the axial direction of the optical fiber, the writing time and the ultraviolet light energy are controlled, the inclined grating 10 with a high extinction ratio is obtained, the transmission spectrum of the inclined grating is shown in fig. 7, and the diameter of the inner cladding of the used optical fiber is 20 micrometers.
Preferably, the ultraviolet incident light is ultraviolet pulse laser with energy of 7mJ and frequency of 100Hz output by 193nm excimer laser.
3) Welding: and taking down the optical fiber with the carved grating, cutting the optical fiber at a position which is about 10mm away from the grating, and welding the cut double-clad optical fiber with the large-core-diameter optical fiber 8. The large-core fiber used is a step-type multimode fiber, and is composed of a core 8-1 and a cladding 8-2, the cross section of the large-core fiber is shown in FIG. 6, and the diameter of the core is equal to that of the inner cladding of the double-cladding fiber. The other end of the large-core optical fiber is connected with the single-mode optical fiber.
4) Packaging: and (3) taking the welded optical fiber off the fusion splicer, penetrating through a preformed hole of the mounting plate 7, extending a part of the optical fiber out of the mounting plate, placing the optical fiber in the elastic sleeve 9, and finally fixing the optical fiber on the mounting plate by using glue. The prepared accelerometer based on the core mismatch is shown in fig. 2, and fig. 3 is a side view of the accelerometer.
Preferably, the length of the double-cladding inclined grating region is 10mm, and the inclination angle is 3 degrees.
Preferably, the diameter of the core of the double-clad optical fiber is 9 μm, the diameter of the inner cladding is 20 μm, and the diameter of the outer cladding is 125 μm.
Preferably, the length of the optical fiber extending out of the mounting plate part of the accelerometer based on the mismatch of the fiber cores is 25 mm.
During measurement, the mounting plate of the accelerometer based on fiber core mismatch is fixed on an object to be measured, light emitted by a light source is input from an a port and output from a b port of the optical circulator through a single-mode fiber, the output light is transmitted to the accelerometer, and a transmission schematic diagram of an optical signal in the accelerometer is shown by an arrow in fig. 4. Light transmitted along the fiber core 2-1 of the single-mode fiber is transmitted to the double-clad fiber 9 through the large-core-diameter fiber 8, and is excited by the inclined grating 10 of the fiber core of the double-clad fiber to carry out cladding resonance after transmission, wherein the resonance peak of the inner cladding is transmitted to the fiber core 8-1 of the large-core-diameter fiber, and is finally transmitted into the fiber core 2-1 of the single-mode fiber, and is input from the port b of the optical circulator, and the port c is output to the photoelectric detector and the signal processing device.
Because one end of the optical fiber is fixed on the object to be tested along with the mounting plate, and the extended part is used as an inertial element, when the object to be tested vibrates, the vibration is transmitted to the accelerometer 6 based on fiber core mismatch, and the extended part drives the inclined grating to vibrate along with swinging, so that the coupling strength of the resonance peak of the inner cladding is changed, the power of the output optical signal is changed, and the vibration information is obtained by monitoring the power.
Since the double clad optical fiber used in the present invention has an inner and an outer cladding, as shown in fig. 5. The inner cladding is not in direct contact with the external environment, so that the resonance peak of the inner cladding is not influenced when the refractive index of the external environment is changed. Because of the low order modes at the inner cladding-core interface, any slight fiber vibration can cause a change in its transverse electric field amplitude profile, and thus a change in the resonant peak power. The invention can realize the measurement of the acceleration by detecting the power of the resonance peak of the inner cladding layer. The fiber core mode is a Bragg resonance peak which is only sensitive to temperature and axial strain, and cross interference generated by temperature, axial strain and light source output power fluctuation can be eliminated by detecting the Bragg peak. When the external temperature is relatively stable, the acceleration can be measured by monitoring the power of a certain wave band, and the demodulation mode is greatly simplified.
Claims (8)
1. The utility model provides an optic fibre acceleration measuring device based on fibre core mismatch which characterized in that: the device consists of a light source, a single-mode fiber, an optical circulator, an accelerometer based on fiber core mismatch, a photoelectric detector and a signal processing device; the accelerometer is formed by welding a single mode fiber, a large core diameter fiber and a double-clad fiber with an inclined grating in sequence, the welded accelerometer passes through a preformed hole of the mounting plate, one end of the fiber extends out of the mounting plate and is arranged in the elastic sleeve, and the other end of the fiber is fixed on the mounting plate; light emitted by the light source is transmitted to the accelerometer from the single-mode optical fiber through the optical circulator, is transmitted to the double-clad optical fiber through the large-core-diameter optical fiber, is excited to be clad to resonate through the inclined grating of the double-clad optical fiber, and is transmitted to the single-mode optical fiber through the large-core-diameter optical fiber through the inner cladding resonant peak of backward transmission, and is output to the photoelectric detector and the signal processing device through the optical circulator.
2. The apparatus of claim 1, wherein the tilted grating has a length of 5mm to 50 mm; the angle of inclination is not more than 20 deg..
3. The apparatus of claim 1, wherein the double-clad fiber comprises a core, an inner cladding and an outer cladding, the outer cladding having a diameter of 125 μm.
4. A fiber core mismatch-based optical fiber acceleration measuring device according to claim 1, wherein the diameter of the inner cladding of the double-clad optical fiber is not less than 16 μm.
5. The apparatus of claim 1, wherein the double-clad fiber has refractive index differences between the core and the inner cladding and between the inner and outer cladding, and the core has a refractive index higher than the cladding.
6. A fiber optic core mismatch-based acceleration measuring device according to claim 1, wherein the change of the refractive index of said double-clad fiber is step-shaped or graded-shaped.
7. A fiber core mismatch-based optical fiber acceleration measuring device according to claim 1, wherein the core diameter of the large-core optical fiber is not smaller than the inner cladding diameter of the double-clad optical fiber.
8. A fiber optic core mismatch-based acceleration measuring device according to claim 1, wherein the length of the optical fiber extending outside the mounting plate is 20mm-100 mm.
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Cited By (1)
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