CN110608797A - Cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber laser - Google Patents

Abstract

The invention discloses a cylindrical cantilever beam vibration sensor based on a dual-channel DFB fiber laser, which mainly solves the problems of low sensitivity and poor directivity of the existing cylindrical cantilever beam vibration sensor. It consists of a metal base, two sections of active optical fiber engraved with DFB gratings, a pump source, two wavelength division multiplexers, a 3dB coupler, two metal clamping blocks, two photodetectors, signal acquisition and Processing end components. Two sections of active optical fibers engraved with DFB gratings are fixed together in parallel, clamped by two metal clamping blocks and fixed in the metal base. The pigtails of two sections of active optical fibers engraved with DFB gratings are led out through the small holes on the back of the metal base for signal processing and demodulation. The invention realizes high sensitivity and high-quality detection of very low frequency signals, avoids the use of off-core optical fiber, and has a simpler structure.

Description

Cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber laser

technical field

The invention relates to an optical fiber vibration sensor, in particular to a cylindrical cantilever beam vibration sensor based on a distributed feedback (DFB) optical fiber laser.

Background technique

Optical fiber sensors are widely used in the research field of hydrophones due to their small size, light weight, corrosion resistance, insensitivity to electromagnetic interference, and high sensitivity.

Under water, the only thing that can be transmitted remotely at present is sound waves. Underwater early warning, hydrological environment detection, oil exploration, and underwater communication all use sound waves. Because low-frequency sound waves travel farther and have lower losses, especially very low-frequency (frequency below 100 Hz) sound waves are the focus of underwater acoustic detection research. Traditional optical fiber vibration sensors used for underwater acoustic detection mainly include light intensity modulation type and optical phase modulation type. However, due to light source fluctuations, optical fiber disturbances, temperature disturbances, etc., the background noise is strong. Especially the 1/f noise of the light source greatly affects the low-frequency detection effect and reduces the minimum detectable signal size. As a kind of fiber optic vibration sensor, the cantilever vibration sensor is miniaturized and uses a grating as a strain sensing structure. It mainly uses the change of Bragg wavelength to realize sensing, and it is basically not affected by the disturbance of the light source. It can detect even The low-frequency effect is good, and it has gradually become a research hotspot.

The current cantilever beam vibration sensor has many kinds of structures, such as traditional flat beam, cylindrical beam, equal strength beam, hollow beam, cantilever beam and so on. For example, Nankai University (patent application number: 200420028862.7) directly pastes the grating on the flat beam, and measures the strain by measuring the change of the wavelength of the returned light.

The cantilever vibration sensor based on DFB fiber laser is developed on the basis of the traditional cantilever fiber grating sensor. The detection method of this kind of sensor is to detect the strain of the cantilever beam by measuring the frequency change of the DFB output light. The sensor directly uses the DFB fiber laser as the sensing core, which greatly simplifies the structure of the system; and has the advantages of fiber grating sensing. For example, low-frequency detection has high sensitivity, low background noise, high precision, large measurement bandwidth and strong reliability.

The main principle of the current cantilever beam vibration sensor based on DFB fiber laser is to attach an active optical fiber with a grating on the cantilever beam. According to the document titled "High-resolution distributed-feedback fiber laser dc magnetometer based on the Lorentzianforce" published on the 20th issue of IOP SCIENCE in 2009, for cantilever beams, the sensitivity of strain detection is related to the position of the grating to the neutral plane into a positive correlation. According to the formula, for a beam undergoing pure bending (i.e. no torsion, no shear), the strain at position x is:

△ε(x)＝±d _n K(x)

Among them, d _n represents the distance from the point to the neutral axis, K(x) represents the curvature of the point, and ± represents whether it is above or below the neutral axis, and the up and down are determined according to the vibration direction. Therefore, for the cantilever beam under the same bending condition, the farther it is from the neutral axis, the greater the strain, and thus the higher the detection sensitivity to vibration. For a uniform and symmetrical structure, such as a cylinder, the neutral axis is at its geometric centerline; and for the case where two cylinders are adhered in parallel, the neutral axis is at the midpoint of the line connecting the centers of the two cylinders. According to the article titled "Miniaturization of AcousticVector Sensors Enabled by Viscous Fluids: Towards Fiber Laser Hair Sensors" published by Lou, JW et al. on IEEE SENSORS in 2013. Compared with the cylindrical cantilever vibration sensor that uses eccentric optical fiber to increase sensitivity (increase the distance between the fiber core and the neutral axis), the flat cantilever beam vibration sensor has higher detection sensitivity. And in the directivity test, the directivity of the cylindrical cantilever beam is extremely poor. As a result, cylindrical beams are rarely used in practical applications. However, the low-frequency frequency response characteristic of the cylindrical beam structure is relatively flat, and the low-frequency sensitivity and noise floor are better than traditional fiber optic vibration sensors, so there is still a great application prospect. At present, there are no reports on improving the detection sensitivity and directionality of cylindrical cantilever beam vibration sensors by using dual-channel DFB.

Contents of the invention

The invention proposes a cylindrical cantilever beam vibration sensor based on a dual-channel DFB fiber laser, which mainly solves the problems of low sensitivity and poor directionality of the existing cylindrical cantilever beam vibration sensor, and realizes high sensitivity and high-quality detection of very low frequency signals. Moreover, the use of eccentric optical fibers is avoided, and the structure is simpler.

The technical scheme adopted in the present invention is:

The invention consists of a metal base, two sections of active optical fibers engraved with DFB gratings, a pump source, two wavelength division multiplexers, a 3dB coupler, two metal clamping blocks, two photodetectors, signal acquisition Composed with the processing end.

Each wavelength division multiplexer has three ports a, b, and c. Port a is the pump input port. The input bandwidth of port a covers the wavelength range of the output light of the pump source. Port b is used for pump light output and signal light. Input, port c is the signal light output port, the output bandwidth of port c covers the wavelength range of signal light returned by the active optical fiber engraved with DFB grating;

The metal base is a cube, with a large hole on the front and a small hole on the back. The centers of the large hole and the small hole are coaxial, and there are four fixing holes on the bottom of the metal base;

The metal clamping block is a semi-cylindrical body, the diameter of which matches the large hole, and there are grooves on the two metal clamping blocks, and the diameter of the groove matches the diameter of the active optical fiber engraved with DFB grating;

Two active optical fibers engraved with DFB gratings are fixed together in parallel, the preferred example is parallel bonding, and other methods such as sleeve packaging can also be used for fixing. One end of two sections of active optical fiber engraved with DFB grating is clamped by two metal clamping blocks through the groove, and only the part engraved with grating is exposed to form a cantilever beam as the strain sensing part. Two metal clamping blocks are inserted into the large hole. The pigtails of the two active optical fibers engraved with DFB gratings are led out through the small holes on the back of the metal base, and are respectively connected to the ports of the two wavelength division multiplexers. The c ports of the two wavelength division multiplexers are respectively connected to the photodetectors, the a ports of the two wavelength division multiplexers are connected to the two output ports of the 3dB coupler, and the input port of the 3dB coupler is connected to the output port of the pump source connected. The output terminals of the two photodetectors are connected with the signal acquisition and processing terminals.

The grating pitch, gate length, type and concentration of dopant ions of the two active optical fibers engraved with DFB gratings are the same, and the center frequency is determined according to the detection requirements. In this example, the preferred diameter is 125um, and the center wavelength of the output light is 1550nm ;The central wavelength of the pump output light in the pump source is determined according to the wavelength requirements of the detection signal light. For example, when the signal light wavelength is 1550nm, the corresponding pump light wavelength can be selected as 980nm;

The method for realizing low-frequency detection by the present invention is as follows: the pump light is output from the pump source, and after entering the 3dB coupler, it is divided into two paths with equal light intensity, and the pump light that is divided into two paths passes through the wavelength division multiplexer. Ports a and b enter into the active optical fiber engraved with DFB grating. Under the excitation of the pump light and the frequency selection of the grating, the active optical fiber generates reverse signal light respectively, and then the signal light is transmitted from the two active Return through the optical fiber, output through the c-port of the two-wavelength division multiplexer, enter the photodetector, and judge the external low-frequency vibration by detecting the frequency change trend and magnitude of the signal light at the signal acquisition and processing end. The specific judgment method is as follows :

When the metal base is fixed and there is no vibration, the output light frequencies of the two signal lights are the same.

When the outside world vibrates, the cantilever beam composed of two DFB gratings will vibrate and produce strain when subjected to external vibrations, which will cause the output light frequency of the signal light to change. From the strain to the change of the output light frequency, the basic principle is expressed as the following formula:

Among them, Δυ represents the frequency change, and its positive and negative are the same as the positive and negative of the strain, υ represents the original optical frequency, κ represents the coupling coefficient of the grating, Δε(x,t) represents the strain at time t at x, x ₁ and x ₂ respectively are the start and end positions of the raster.

When the vibration direction is in the direction of the line connecting the centers of the two DFBs, since the two DFBs are on both sides of the neutral plane, the positive and negative strains will be opposite, resulting in a larger pitch of the gratings written on the two DFBs and a larger pitch of the other. The output light frequency of the laser with smaller grating pitch becomes smaller, and the output light frequency of the laser with smaller grating pitch becomes larger. Since the strain is measured by the difference between the two output light frequencies, the detection sensitivity of the cantilever beam vibration sensor is the largest at this time; When the vibration direction is in the vertical direction of the line connecting the centers of the two DFBs, since the two DFBs are on the center plane, the strain of the two DFBs is the same, and the change trend of the grating pitch is also the same, so the change trend of the output light frequency is the same, and the result after the difference is close to 0, the same as when there is no external vibration, the detection sensitivity of the cantilever beam vibration sensor is the lowest at this time; when the vibration direction is at a certain angle (0-90°) with the center line of the two DFBs, the vibration can be decomposed into In the vertical and parallel directions connecting the two DFB centers, the vertical direction does not cause frequency difference, and the parallel direction causes frequency difference, but the strain in this direction is a component of the total strain, which must be smaller than the total strain, so the detection sensitivity at this time It is lower than the case where the vibration direction is on the line connecting the centers of the two DFBs. It can be obtained from this that the sensor has excellent directivity.

The beneficial technical effect of the present invention is:

1. The present invention greatly improves the directivity of the cylindrical cantilever beam vibration velocity hydrophone because of the parallel fixed structure of two identical DFBs.

2. Since the present invention uses two DFBs to be fixed in parallel, its structure is simpler compared with the use of eccentric optical fibers, and the distance between the fiber core and the neutral plane is increased, the sensitivity to strain detection is improved, and the sensitivity enhancement effect is improved. it is good.

3. The present invention uses two DFB output optical frequencies to make a difference. For the frequency change caused by the same strain, the difference result is twice the frequency change value of each DFB. Compared with the sensor sensitivity of a single DFB structure, the sensitivity is improved to double.

4. The symmetry of the structure of the present invention is good, and the directivity of the cantilever beam detection is improved. It only needs to expand and add two identical cantilever beams on the metal base, and three cantilever beams are perpendicular to each other to form a three-dimensional axis, the three-dimensional vibration sensing can be realized.

In a word, the present invention greatly improves the detection directionality of the cylindrical cantilever beam through the bonding and use of two DFB lasers, and provides a basis for subsequent three-dimensional detection. At the same time, due to the push-pull-like structure and the differential demodulation method, the sensitivity is doubled. Furthermore, compared with the off-core fiber, the two-way DFB bonding increases the distance between the fiber core and the neutral plane, and further improves the sensitivity.

Description of drawings

Fig. 1 is an overall structural block diagram of the present invention;

Fig. 2 is three views of the assembly of the sensing part of the present invention;

Fig. 3 is three views of metal base;

Fig. 4 is three views of the metal clamping block;

Among them: 1 is the metal base, 2 is the large hole on the front of the metal base, 3 is the small hole on the back of the metal base, 4, 5 are two sections of active optical fiber engraved with DFB grating, 6 is the pump source, 7, 8 There are two wavelength division multiplexers, each of which has three ports a, b, and c, 9 is a 3dB coupler, 10, 11 are two metal clamping blocks, 12, 13 are metal clamping blocks The grooves opened, 14a, 14b, 14c, 14d are fixing holes at the bottom of the metal base, 15, 16 are photodetectors, and 17 is a signal collection and processing terminal.

Detailed ways

The present invention will be further described in conjunction with the above drawings, and the specific embodiments described here are only used to explain the present invention, and are not intended to limit the present invention.

Fig. 1 is a structural schematic diagram of the present invention. The present invention consists of a metal base 1, two sections of active optical fibers 4 and 5 engraved with DFB gratings, a pump source 6, two wavelength division multiplexers 7 and 8, a 3dB coupler 9, and two metal clamping blocks 10 and 11, two photodetectors 15 and 16, and a signal acquisition and processing terminal 17.

Each wavelength division multiplexer has three ports a, b, and c. Port a is the pump input port. The input bandwidth of port a covers 980nm. Port b is used for pump light output and signal light input, and port c is for signal At the optical output end, the signal output optical bandwidth covers 1550nm, the central wavelength of the output light of the pump source 6 is 980nm, and the central wavelength of the signal light is 1550nm;

The metal base 1 is a combined cube composed of two cubes, with a large hole 2 on the front and a small hole 3 on the back, the centers of the large hole and the small hole are coaxial, and four fixing holes 14a are opened on the bottom of the metal base 1 , 14b, 14c, 14d; the parameters of the metal base are: the base is 70mm×70mm×8mm, the cube on the base is 40mm×70mm×40mm; the diameter of the large hole 2 is 20mm, and the depth is 50mm; the diameter of the small hole 3 is 5mm, The depth is 20mm.

The metal clamping blocks 10 and 11 are semi-cylindrical, and the diameter matches the large hole 2. Both metal clamping blocks 10 and 11 are provided with grooves 12 and 13. The diameters of the grooves 12 and 13 are the same as those engraved with DFB. Active fiber diameters are matched. The structural parameters are taken as follows: the diameter of the clamping blocks 10, 11 is 20mm, the height is 70mm, and the diameter of the grooves 12, 13 is taken as 125um. ;

The two active optical fibers 4 and 5 engraved with DFB gratings have a grating pitch of 51nm, a gate length of 50mm, the type of doped ion is bait ion and the concentration can be 300ppm, the diameter is 125um, and the central wavelength of the output light is 1550nm;

Two active optical fibers 4 and 5 engraved with DFB gratings are glued together in parallel, and one end of two sections of active optical fibers engraved with DFB gratings is clamped by two metal clamping blocks 10, 11 through grooves 12 and 13, so that only The grating part is exposed to form a cantilever beam, which is used as the strain sensing part. Two metal clamping blocks 10 , 11 are inserted into the large hole 2 . The pigtails of the two active optical fibers 4 and 5 engraved with DFB gratings are led out through the small hole 3 on the back of the metal base 1, and are connected to the b ports of the two wavelength division multiplexers 7 and 8 respectively. The c ports of two wavelength division multiplexers 7 and 8 are connected with photodetectors 15 and 16 respectively, and the a ports of two wavelength division multiplexers 7 and 8 are connected with the two output ends of 3dB coupler 9, and the 3dB coupling The input terminal of the device 9 is connected with the output terminal of the pumping source 1. The output ends of the two photodetectors 15 and 16 are both connected to the signal acquisition and processing end 17 .

When the external vibration propagates to the device, the cantilever beam composed of 4 and 5 will be strained. If the vibration direction is in the direction of the line connecting the centers of 4 and 5, the strains of the two DFBs will be the same in theory, but one is compression and the other is tension. , resulting in a larger output optical frequency of the two DFBs and a smaller one. The output light is drawn out through the pigtail to perform demodulation and difference of the optical frequency, so that the strain information of the cantilever beam can be obtained, and then the vibration information can be obtained.

The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. All equivalent structural transformations made by using the description of the present invention and the contents of the accompanying drawings, or directly or indirectly used in other related technical fields, are all The same reasoning is included in the patent protection scope of the present invention. The invention is intended to cover modifications within the spirit and scope of the appended claims.

Claims (7)

1. A cylindrical cantilever beam vibration sensor based on dual-way DFB fiber laser, it is characterized in that the sensor consists of metal base (1), two sections of active optical fibers (4) and (5) engraved with DFB grating, pump Pu source (6), two wavelength division multiplexers (7) and (8), a 3dB coupler (9), two metal clamping blocks (10) and (11), two photodetectors (15 ) and (16), signal acquisition and processing end (17) form; Each wavelength division multiplexer (7) and (8) has three ports a, b, and c, port a is the pump input end, the input bandwidth of port a covers the output light wavelength range of the pump source (6), and port b The port is used for pumping light output and signal light input, the c port is the signal light output port, and the output bandwidth of the c port covers the wavelength range of the signal light returned by the active optical fibers (4) and (5) engraved with DFB gratings; The metal base (1) is a cube, with a large hole (2) on the front and a small hole (3) on the back, the centers of the large hole and the small hole are coaxial, and four fixed holes (14a), (14b), (14c), (14d); The metal clamping blocks (10) and (11) are semi-cylindrical, and the diameter matches the large hole (2), and grooves (12) and (13) are all arranged on the two metal clamping blocks (10) and (11). ), the diameter of the grooves (12) and (13) matches the diameter of the active optical fiber (4) and (5) engraved with DFB grating; The grating pitch, length, dopant ion type and concentration of the two sections of active optical fibers (4) and (5) engraved with DFB gratings are all the same, and their center frequency is determined according to the detection requirements, and the output light center of the pump source (6) The wavelength is determined according to the wavelength requirements of the detection signal light; Two sections of active optical fibers (4) and (5) engraved with DFB gratings are fixed together in parallel, and one end of the two sections of active optical fibers (4) and (5) engraved with DFB gratings is held by two metal clamping blocks (10 ) and (11) are clamped by the grooves (12) and (13), and only the grating engraved part is exposed, and two metal clamping blocks (10) and (11) are inserted into the large hole (2), two sections The pigtails of the active optical fibers (4) and (5) engraved with DFB gratings are led out through the small hole (3) on the back of the metal base (1), and are respectively connected with two wavelength division multiplexers (7) and (8) The b port of two wavelength division multiplexers (7) and (8) is connected with two photodetectors (15) and (16) respectively, two wavelength division multiplexers (7) and ( Port a of 8) is respectively connected to the two output ends of the 3dB coupler (9), and the input end of the 3dB coupler (9) is connected to the output end of the pumping source (6). The output ends of the two photodetectors (15) and (16) are both connected to the signal acquisition and processing end (17). 2. The cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber lasers as claimed in claim 1, characterized in that two sections of active optical fibers (4) and (5) that are engraved with DFB gratings are bonded in parallel. 3. The cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber lasers as claimed in claim 1, characterized in that two sections of active optical fibers (4) and (5) engraved with DFB gratings are packaged in parallel with a sleeve. 4. The cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber lasers as claimed in claim 1, wherein the diameter of the active optical fibers (4) and (5) engraved with DFB gratings is 125um. 5. The cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber lasers as claimed in claim 1, characterized in that the output light center wavelength of the active optical fiber (4) and (5) engraved with DFB grating is 1550nm. 6. The cylindrical cantilever beam vibration sensor based on dual-channel DFB fiber lasers as claimed in claim 5, characterized in that the center wavelength of the pumping source (6) output light is 980nm. 7. the cylindrical cantilever beam vibration sensor based on dual-way DFB fiber laser as claimed in claim 1, it is characterized in that the active optical fiber (4) and (5) doping erbium ion that is engraved with DFB grating, doping concentration is 300ppm.