CN112067843A - An Optical Fiber Acceleration Measurement Device Based on Fiber Core Mismatch - Google Patents

Abstract

The invention provides an optical fiber acceleration measurement device based on fiber core mismatch. The device is composed of a light source, a single-mode fiber, an optical circulator, an accelerometer based on fiber core mismatch, a photodetector, and a signal processing device. The accelerometer is formed by welding a single-mode fiber, a large-core fiber, and a double-clad fiber with an inclined grating in sequence. The welded fiber passes through the reserved hole of the mounting plate and is placed in an elastic sleeve. The light emitted by the light source is output from the input end b of the circulator through the single-mode fiber, and the end b is connected to the accelerometer. The light reflected by the inclined grating is output to the photodetector through the input port c of the b port of the circulator and converted into an electrical signal. Finally processed by the signal processing device. When the external vibration frequency changes, the resonance peak of the inner cladding of the double-clad fiber tilt grating changes. By monitoring the resonance peak, the vibration frequency and acceleration of the object can be measured. The present invention has the advantages of small size, fast response, not affected by temperature and the like.

Description

An Optical Fiber Acceleration Measurement Device Based on Fiber Core Mismatch

(1) Technical field

The invention relates to an optical fiber acceleration measurement device based on fiber core mismatch, in particular to a reflective acceleration sensor based on a double-clad optical fiber tilt grating, which can measure vibration frequency and acceleration, and belongs to the technical field of optical fiber sensing.

(2) Background technology

Acceleration is a very important parameter index in the process of object motion, which characterizes and measures the dynamic characteristics of the object. Accurate measurement of acceleration is currently widely used in various technical fields such as mechanical vibration measurement, traffic situation monitoring, seismic wave signal detection in oil and gas exploration, structural monitoring of buildings, and inertial navigation and guidance systems in aerospace.

The acceleration signal measurement usually uses the principle of inertia, and measures the corresponding acceleration by sensing the displacement or strain generated by the inertial force. There are various measurement methods of acceleration, such as mechanical, electromagnetic, electromechanical and so on. However, the accuracy and sensitivity of traditional mechanical methods are relatively low. In some special environments, the sensitivity of the acceleration sensor is required to be high, and at the same time, the sensor is required to have the performance of anti-complex electromagnetic interference. For such detection environments and requirements, traditional electromechanical or piezoelectric sensors are not up to the task. At this point, fiber optic sensors are able to meet the requirements of such harsh situations.

Optical fiber sensing technology uses optical fiber as the physical medium and optical signal as the sensitive information carrier. When some physical characteristics of the external environment of the optical fiber (such as temperature, refractive index, pressure, etc.) change slightly, the optical signal transmitted in the optical fiber will also Changes occur, which can be detected using special optical signal processing techniques. Compared with the traditional transmission medium, the optical fiber has the characteristics of low loss, and the optical fiber has the advantages of large dynamic range and wide working frequency; The mass is very light, the volume is small, and it is easy to be bent, and it can still work well in the complex power plant and magnetic field environment. Therefore, fiber optic sensors are more practical than traditional piezoelectric or electromechanical sensors in extreme environments that are prone to combustion, explosion, small workspace, and severe electromagnetic fields.

Due to the above advantages of fiber optic sensors, fiber optic accelerometers have attracted the attention of researchers in recent years, and have been more and more studied and applied. Patent CN102721827A proposes an optical fiber interferometric accelerometer, which uses the control unit to generate two beams of orthogonally polarized light, the sensing fiber is wound on the mass block, and the acceleration to be measured is transmitted to the elastic body, so that the elastic body is deformed accordingly, so that the A phase difference is generated in the two orthogonal polarized lights of the sensing fiber, and the acceleration to be measured can be obtained by detecting the phase difference. However, this method requires more components, is more complicated to manufacture, and the elastomer and mass are bulky, which is not suitable for extreme environments. Patent CN108680767A proposes a grating-based optical fiber accelerometer. Using femtosecond laser micromachining technology, the fiber cladding is cut, the inertial element is built into the fiber, and the grating is engraved at the vibrating arm. , the stress generated by the vibration will be concentrated on the vibrating arm, thereby changing the length and center wavelength of the grating, and the change of the acceleration can be obtained by measuring the wavelength shift. However, this grating-type wavelength measurement accelerometer is easily affected by changes in the external environment. For example, when the temperature changes, the center wavelength will also shift, resulting in cross-interference, and this method requires femtosecond finishing, which is difficult to manufacture. The cost is high, and it is difficult to achieve mass production.

(3) Contents of the invention

The purpose of the present invention is to provide an optical fiber acceleration measurement device based on fiber core mismatch, so as to solve the direct measurement of the target acceleration under the condition of eliminating the temperature cross-interference.

For achieving the above object, the scheme that the present invention adopts is:

An optical fiber acceleration measurement device based on fiber core mismatch is characterized in that: the device is composed of a light source, a single-mode fiber, an optical circulator, a photodetector, a signal processing device and an accelerometer based on fiber core mismatch, The accelerometer is composed of a single-mode fiber, a large-core fiber, and a double-clad fiber with an inclined grating, which are welded in sequence. The welded accelerometer passes through the reserved hole of the mounting plate, and one end of the fiber extends out of the mounting plate and is placed in the elastic sleeve. Inside the pipe, the other end is fixed on the mounting plate.

The light emitted by the broadband light source is input from the a port of the optical circulator and output from the b port through the single-mode fiber. The output light is transmitted to the accelerometer based on fiber core mismatch. The transmission diagram of the accelerometer based on fiber core mismatch is shown in Figure 4. Show. The light propagating along the core of the single-mode fiber is transmitted to the double-clad fiber through the large-core fiber, and the backward-propagating cladding resonance is excited by the inclined grating of the double-clad fiber core, wherein the resonance peak of the inner cladding is transmitted to the large The core of the core diameter fiber is finally transmitted into the core of the single-mode fiber, which is input from the b port of the optical circulator, and the c port is output to the photodetector and the signal processing device. When the external vibration frequency changes, the resonant peak intensity of the inner cladding of the double-clad fiber grating will change. By monitoring the resonant intensity, the vibration frequency and acceleration of the object can be measured. The temperature change will cause the wavelength of the resonance peak of the core mode and the resonance peak of the cladding mode to change. Therefore, the crosstalk problem caused by temperature can be eliminated by monitoring only the resonance peak intensity of the inner cladding mode. In order to eliminate the influence of the light reflected from the end face of the optical fiber on the sensor, the tail end of the accelerometer is de-reflected.

The working principle of the present invention:

The tilt grating core refractive index modulation is angled relative to the core axis. When the light emitted by the light source passes through the inclined grating, in addition to reflecting the Bragg wavelength in the core, part of the core energy can also be back-coupled into the fiber cladding. Each of these back-coupled cladding modes has own specific 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:

λ _Bragg =(n _core +n _core )Λ/cosθ

λ _clad,i =(n _clad,i +n _core )Λ/cosθ

where the subscript i represents the modulus, n _core and n _clad,i are the effective refractive indices of the core and cladding modes (i-th order), respectively, Λ is the grating pitch when the grating is not tilted, and θ is the tilt angle of the grating , that is, the angle between the grid and the fiber axial normal.

The transmission spectrum of a tilted grating is comb-shaped, in which each resonance peak corresponds to an order cladding mode whose spectral position (wavelength) depends on the effective refractive index of the corresponding cladding mode.

The optical fiber used in the present invention is a double-clad optical fiber, which is formed with two layers of cladding layers outside the core, and there is a refractive index difference between the two cladding layers, and the core is engraved with an inclined grating. The length of the inclined grating is 10mm-50mm; the angle of inclination is not more than 20°.

The double-clad optical fiber of the present invention is composed of a core, an inner cladding and an outer cladding, the diameter of the core is approximately the same as that of the single-mode optical fiber, and the diameter of the outer cladding is 125 μm.

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-cladding fiber according to the present invention is not less than 16 μm.

The double-clad optical fiber core and the inner cladding and the inner cladding and the outer cladding of the present invention all have refractive index differences, and the two refractive index differences may be the same or different.

The refractive index change of the double-clad optical fiber of the present invention can be a step type or a graded type.

The light source of the present invention is a broadband light source, and the output spectrum range covers the output spectrum of the inclined grating.

In order to effectively couple the inner cladding mode of the backward transmission into the core of the single-mode fiber, the diameter of the core of the large core diameter fiber of the present invention should not be smaller than the diameter of the inner cladding of the double-clad fiber.

In the present invention, the optical fiber extending out of the mounting plate itself is used as an inertial element, so the extended length has a direct impact on the resonant frequency and acceleration sensitivity of the acceleration sensor. According to the actual vibration measurement requirements, the specific extended length needs to be designed. The length of the optical fiber extending out of the mounting plate according to the present invention is 20mm-100mm.

The light emitted by the light source is input from the a port of the optical circulator through the single-mode fiber, and output from the b port. The output light is transmitted to the accelerometer based on the core mismatch, and is transmitted from the large core diameter fiber to the double-clad fiber. The cladding resonance excited by the inclined grating of the fiber, and the inner cladding resonance peak of the backward transmission is transmitted into the single-mode fiber through the large core diameter fiber, which is input from the b port of the optical circulator, and the c port is output to the photodetector and signal processing device. When the external vibration frequency changes, the resonant peak intensity of the inner cladding of the double-clad fiber tilt grating changes. By monitoring the resonant intensity, the vibration frequency and acceleration of the object can be measured. Because the power of the inner cladding is monitored, the change of temperature will not affect the coupling strength, but only the wavelength. Therefore, by monitoring only the strength of a certain resonance peak of the inner cladding, the cross-interference of temperature is eliminated, and Since there is a Bragg resonance peak in the fiber core, which is sensitive to temperature, the temperature can be measured by monitoring the wavelength of the core mode.

The beneficial effects of the present invention are:

1. The sensing device designed by the present invention can measure temperature and acceleration at the same time, eliminating the cross-interference caused by temperature changes, and because there are two cladding layers inside and outside, the acceleration measurement can only be done through the resonance of the inner cladding layer. Therefore, the change of the refractive index of the external environment will not affect the resonance of the inner cladding, eliminating the interference caused by the external environment;

2. The present invention converts the change of acceleration into the change of the intensity of the output resonance wavelength, avoids the instability caused by the wavelength drift detection, and can eliminate the influence caused by the fluctuation of the light source by monitoring the Bragg resonance peak power;

3. The optical fiber sensing device has the advantages of small size, high sensitivity, high temperature resistance, corrosion resistance, fast response, and strong operability.

(4) Description of drawings

Fig. 1 is an optical fiber acceleration measurement device based on fiber core mismatch;

Figure 2 is a schematic diagram of the structure of an accelerometer based on core mismatch;

Figure 3 is a side view of an accelerometer based on core mismatch;

FIG. 4 is a schematic diagram of optical signal transmission of an accelerometer based on fiber core mismatch;

Figure 5 is a schematic cross-sectional view of a step-type double-clad optical fiber;

FIG. 6 is a schematic cross-sectional view of a step-type large core diameter optical fiber;

Figure 7 is the transmission spectrum of the double-clad fiber tilt grating.

(5) Specific implementation methods

A specific embodiment of the optical fiber acceleration measurement device based on fiber core mismatch of the present invention will be described below with reference to the accompanying drawings:

Example 1

The device is shown in FIG. 1 , including a light source 1 , a single-mode fiber 2 , an optical circulator 3 , a photodetector 4 , a signal processing device 5 and an accelerometer 6 based on fiber core mismatch.

The structure of the accelerometer based on core mismatch is shown in Figure 2, and the preparation process is as follows:

1) Optical fiber hydrogen-carrying pretreatment: the double cladding 9 used in this embodiment is a step-type double cladding fiber, and its cross section is shown in Figure 5, including a core 9-1, an inner cladding 9-2, and an outer cladding 9-3, the refractive index of the optical fiber changes in a step-like manner. Put the double-clad optical fiber into a container filled with hydrogen, the pressure is 8MPa, and the temperature is room temperature. After 240 hours, the hydrogen molecules can be diffused into the core of the double-clad optical fiber to increase the photosensitivity of the core. To shorten the hydrogen carrying time, the temperature can be increased or the pressure can be increased appropriately.

2) Tilt grating engraving: the incident ultraviolet light is focused on the phase mask after passing through the beam expander and focusing lens, the mask is kept parallel to the double-clad fiber, the focused ultraviolet light is irradiated on the fiber through the mask, and the mask is rotated The template is used to generate a certain inclination angle of the mask plate relative to the axis of the optical fiber, and the writing time and ultraviolet light energy are controlled to obtain the inclined grating 10 with high extinction ratio. .

Preferably, the ultraviolet incident light is an ultraviolet pulsed laser with an energy of 7 mJ and a frequency of 100 Hz output by a 193 nm excimer laser.

3) Welding: Take off the optical fiber with the engraved grating, cut it at a distance of about 10mm from the grating, and weld the cut double-clad optical fiber with the large-core fiber 8 . The large-core-diameter fiber used is a step-type multimode fiber, which is composed of a core 8-1 and a cladding 8-2, and its cross-section is shown in Figure 6. The core diameter is equal to the inner cladding diameter of the double-clad fiber. The other end of the large core diameter fiber is connected to the single mode fiber.

4) Encapsulation: Take off the welded optical fiber from the fusion splicer, pass through the reserved hole of the mounting plate 7, part of the optical fiber extends out of the mounting plate, and place it in the elastic sleeve 9, and finally use the glue to fix the optical fiber on the mounting plate. . The prepared accelerometer based on fiber core mismatch is shown in Fig. 2, and Fig. 3 is a side view of the accelerometer.

Preferably, the length of the double-clad inclined grating grid region is 10mm, and the inclination angle is 3°.

Preferably, the core diameter 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 from the part of the fiber core mismatch-based accelerometer that extends out of the mounting board is 25 mm.

During measurement, the mounting plate of the accelerometer based on fiber core mismatch in the present invention is fixed on the object to be measured, and the light emitted by the light source is input from the a port of the optical circulator through the single-mode fiber, and output from the b port, and the output light is transmitted to the optical circulator. The accelerometer, the schematic diagram of the transmission of the optical signal in the accelerometer is shown by the arrow in Figure 4. The light propagating along the single-mode fiber core 2-1 is transmitted to the double-clad fiber 9 through the large-core fiber 8, and the backward-propagating clad resonance is excited via the inclined grating 10 of the double-clad fiber core, wherein the inner cladding The resonance peak is transmitted to the core 8-1 of the large-core fiber, and finally transmitted to the core 2-1 of the single-mode fiber, which is input by the b port of the optical circulator, and the c port is output to the photodetector and the signal processing device.

Since one end of the optical fiber is fixed on the object to be measured with the mounting plate, and the extended part is used as an inertial element, when the object to be measured vibrates, the vibration is transmitted to the accelerometer 6 based on fiber core mismatch, and the extended part is With the swing, the inclined grating is driven to vibrate, which causes the coupling strength of the inner cladding resonance peak to change, so that the output optical signal power changes. By monitoring the power, the vibration information can be obtained.

Since the double-clad optical fiber used in the present invention has two layers of cladding layers inside and outside, as shown in FIG. 5 . The inner cladding is not in direct contact with the external environment, so that when the refractive index of the external environment changes, the resonance peak of the inner cladding is not affected. Due to the existence of low-order modes at the interface between the inner cladding and the core, any slight vibration of the fiber may cause a change in its transverse electric field amplitude distribution and thus a change in the resonant peak power. The invention can realize the measurement of acceleration by detecting the resonance peak power of the inner cladding. The core mode of the fiber is the Bragg resonance peak, which is only sensitive to temperature and axial strain. By detecting the Bragg peak, the cross-interference caused by temperature, axial strain and fluctuations in the output power of the light source can be eliminated. When the external temperature is relatively stable, the acceleration measurement can be realized by monitoring only the power of a certain band, which greatly simplifies the demodulation method.

Claims (8)

1. an optical fiber acceleration measuring device based on fiber core mismatch, it is characterized in that: described device is composed of light source, single mode fiber, optical circulator, accelerometer based on fiber core mismatch, photodetector and signal processing device The accelerometer is composed of a single-mode optical fiber, a large core diameter optical fiber and a double-clad optical fiber with an inclined grating. Inside the elastic sleeve, the other end is fixed on the mounting plate; the light emitted by the light source is transmitted from the optical circulator through the single-mode fiber, and the output light is transmitted to the accelerometer, and is transmitted from the large-core fiber to the double-clad fiber, through the double-clad fiber. The slanted grating of the fiber excites the cladding resonance, and the back-transmitted inner cladding resonance peak is transmitted into the single-mode fiber through the large-core fiber, and is output by the optical circulator to the photodetector and signal processing device. 2 . The optical fiber acceleration measurement device based on fiber core mismatch according to claim 1 , wherein the length of the inclined grating is 5 mm-50 mm, and the inclination angle is not more than 20°. 3 . 3 . The optical fiber acceleration measurement device based on core mismatch according to claim 1 , wherein the double-clad optical fiber is composed of a core, an inner cladding and an outer cladding, and the diameter of the outer cladding is 125 μm. 4 . 4 . The optical fiber acceleration measurement device based on core mismatch according to claim 1 , wherein the inner cladding diameter of the double-clad optical fiber is not less than 16 μm. 5 . 5. A kind of optical fiber acceleration measurement device based on core mismatch according to claim 1, wherein there is a refractive index difference between the double-clad optical fiber core and the inner cladding and between the inner cladding and the outer cladding , the core refractive index is higher than the cladding refractive index. 6 . The optical fiber acceleration measurement device based on fiber core mismatch according to claim 1 , wherein the refractive index change of the double-clad optical fiber is a step type or a graded type. 7 . 7 . The optical fiber acceleration measurement device based on core mismatch according to claim 1 , wherein the core diameter of the large-core-diameter optical fiber is not less than the inner cladding diameter of the double-clad optical fiber. 8 . 8 . The optical fiber acceleration measurement device based on fiber core mismatch according to claim 1 , wherein the length of the optical fiber extending out of the mounting plate is 20mm-100mm. 9 .

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