CN111812350B

CN111812350B - FBG (fiber Bragg Grating) tapered optical fiber flow velocity sensor based on femtosecond laser writing

Info

Publication number: CN111812350B
Application number: CN202010712844.4A
Authority: CN
Inventors: 陈敏; 郭允; 徐贲
Original assignee: China University of Metrology
Current assignee: China University of Metrology
Priority date: 2020-07-22
Filing date: 2020-07-22
Publication date: 2022-08-02
Anticipated expiration: 2040-07-22
Also published as: CN111812350A

Abstract

The invention provides a tapered optical fiber flow velocity sensor based on FBG (fiber Bragg Grating) inscribed by femtosecond laser, wherein a sensing head is formed by inscribing the FBG on a tapered optical fiber by the femtosecond laser point by point method. It is characterized in that: the diameter of the waist is 9-11 μm, the length of the waist is 5-10mm, the inclination angle of the tail end of the waist is 7.5-8.5 degrees, the length of the FBG is 2-5mm, and the Bragg wavelength is 1450-1560 nm. The intensity demodulation method is utilized to convert the optical signals into electric signals, a functional relation between flow velocity and voltage is established, and the flow velocity is inverted by monitoring the change of output voltage, so that the sensing measurement of the flow velocity is realized. The invention has the advantages of simple structure, small volume, electromagnetic interference resistance, corrosion resistance, high temperature resistance, high sensitivity, distributed sensing and the like, and can be applied to flow velocity measurement of micro-flow liquid and air.

Description

FBG (fiber Bragg Grating) tapered optical fiber flow velocity sensor based on femtosecond laser writing

Technical Field

The invention provides a tapered optical fiber flow velocity sensor based on a femtosecond laser writing FBG (fiber Bragg Grating), and belongs to the technical field of optical fiber sensing.

Background

The flow rate describes the displacement of a liquid or gas per unit of time, which is an important parameter in energy and metering, industrial production, medical and other practical applications. The earliest flow rate sensors used mechanical rotors, which were relatively sophisticated in technology but had a low degree of measurement accuracy. Ultrasonic wave current meter, electromagnetic current meter and laser doppler current meter etc. have good measurement accuracy, but equipment is expensive, and measuring range is little and easily receive electromagnetic wave interference.

In recent years, the optical fiber sensor provides a new development direction for flow velocity measurement by virtue of the advantages of high sensitivity, small sensing head volume, light weight, electromagnetic field interference resistance, good waterproof performance, convenience in realizing distributed sensing and the like. More and more flow velocity sensors also combine the classical flow velocity principle with the optical fiber sensing technology, and continuously explore new flow velocity detection methods. If a model building et al provides an optical fiber through hole type flow velocity rotary slurry sensor, the sensor has low starting flow velocity, but has a complex structure and a small measurement range; the Liarmy and the like design a reflection-type intensity modulation optical fiber sensor by using a micro-pressure probe and a diaphragm, so that the flow velocity measurement in the range of 0-0.8m/s is well realized, but the design of the micro-probe of the sensor is not ideal, and the temperature compensation property is poor; the Chenjian military and the like design a Fiber Bragg Grating (FBG) high-sensitivity flow velocity sensor by utilizing a FBG and a Venturi tube, and the sensor has high sensitivity but a narrow dynamic sensing range; the flow velocity sensor based on the constant strength cantilever beam structure design of double FBGs such as the poplar and the shulian well realizes temperature compensation, but the sensor is based on the design of a Venturi tube, has a complex structure and is not easy to realize the measurement of the flow velocity of fluid in an underground fracture. Therefore, the optical fiber flow velocity sensor which has the advantages of simple exploration structure, high accuracy and wide measurement range and can be used for distributed sensing has important practical significance in the aspects of energy, medical treatment, environmental protection and the like.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides the tapered optical fiber flow velocity sensor based on the femtosecond laser writing FBG, which has the advantages of simple structure, small volume, electromagnetic interference resistance, corrosion resistance, high temperature resistance, high sensitivity, distributed sensing and the like.

The technical scheme adopted by the invention for solving the technical problem is as follows: a tapered optical fiber flow velocity sensor based on FBG (fiber Bragg Grating) inscribed by femtosecond laser comprises a single-wavelength laser, a circulator, an optical fiber sensing head, a photoelectric detector, an A/D (analog/digital) data acquisition unit and a microprocessor; the method is characterized in that: the sensing head is made by writing FBG (fiber Bragg Grating) on the taper waist part of the single-mode optical fiber after tapering and thinning by femtosecond laser by a point-by-point method, wherein the diameter of the taper waist of the sensing head is 9-11 mu m, and the length of the taper waist is 5-10 mm. The length of FBG is 2-5mm, the Bragg wavelength is 1450-1560nm, and the inclination angle of the end of the cone waist is 7.5-8.5 deg.

Compared with the prior art, the invention has the beneficial effects that:

1. the sensing head is made of common single-mode optical fiber, and has the advantages of low price, simple structure and small volume.

2. The FBG is engraved on the sensing head by the femtosecond laser point-by-point method, compared with a phase mask method, the engraving process is faster and more flexible, photosensitive treatment on optical fibers is not needed, the preparation is simple, high temperature resistance is realized, and the FBG sensing head is suitable for severe environments.

3. The FBG inscribed in the tapered superfine optical fiber of the sensing head has the advantages of narrow bandwidth, small grating size and high measurement precision, supports a multiplexing system, and has wider application range.

Drawings

FIG. 1 is a schematic diagram of a sensor head according to the present invention;

FIG. 2 is a schematic diagram of an application system of the present invention;

in the figure: 1. the optical fiber sensing device comprises a single-mode optical fiber, 2 a single-mode optical fiber conical waist part, 3 FBG, 4 a single-wavelength laser, 5 a circulator, 6 an optical fiber sensing head, 7 a photoelectric detector, 8A/D data acquisition units and 9 a microprocessor.

Detailed Description

FIG. 1 is a schematic structural diagram of a sensing head of the present invention, in which a tapered single-mode fiber is obtained by discharge drawing of a common single-mode fiber, wherein the diameter of the taper waist is 9-11 μm, the length of the taper waist is 5-10mm, and the inclination angle of the end of the taper waist is 7.5-8.5 °. The FBG is inscribed on the cone waist by femtosecond laser by a point-by-point method, the length is 2-5mm, and the Bragg wavelength is 1450-1560 nm.

Fig. 2 is a schematic diagram of an application system of the present invention, which includes a single-wavelength laser, a circulator, an optical fiber sensing head, a photodetector, an a/D data acquisition unit, and a microprocessor, wherein the laser, the optical fiber sensing head, and the photodetector are respectively connected to three ports of the circulator, the photodetector converts the acquired optical signal into an electrical signal, and then the data is acquired by the a/D data acquisition unit connected thereto, and finally the data is processed by the microprocessor.

The working principle of the invention is specifically described with reference to fig. 1 and 2: the output wavelength of the single-wavelength laser is set to be slightly larger than the wavelength of the output spectrum peak of the FBG, input light is reflected by the FBG through the circulator to enter the photoelectric detector, an optical signal is converted into an electric signal, namely, the corresponding light intensity under the wavelength is converted into the voltage value, the voltage value is output to the A/D data acquisition unit for acquisition, and finally the microprocessor receives the data signal. When fluid flows along the conical sensing head and passes through the conical transition region, due to the action of the fluid, the conical waist part can generate a flow-induced vibration phenomenon, the conical waist swings to cause the FBG to generate bending deformation, the grating interval and the effective refractive index are changed, and accordingly the Bragg wavelength of the FBG is deviated, namely the light intensity corresponding to the selected monitoring wavelength is changed, and the voltage value output to the microprocessor is changed. Different flow rates correspond to different output voltage values, so that a functional relation between the flow rate and the voltage is established, and the calibration of the sensor is completed. When the sensor is applied to the environment with the flow speed to be measured, the voltage value output at a certain flow speed can be measured, namely the magnitude of the corresponding flow speed can be inverted, so that the sensing of the flow speed is realized. The invention adopts intensity modulation and demodulation, is faster than wavelength demodulation, and greatly improves the response speed of the device.

Claims

1. The utility model provides a toper optic fibre velocity of flow sensor based on FBG is write to femto second laser, includes single wavelength laser ware, circulator, optical fiber sensing head, photoelectric detector, AD data acquisition unit and microprocessor, its characterized in that: the optical fiber sensing head is made by writing FBG on the taper waist part of the single-mode optical fiber after tapering and thinning by femtosecond laser;

the diameter of the cone waist of the optical fiber sensing head is 9-11 μm, and the length of the cone waist is 5-10 mm;

the length of the FBG is 2-5mm, the Bragg wavelength is 1450-1560nm, and the inclination angle of the tail end of the cone waist is 7.5 ^° -8.5 ^° ；

The output wavelength of the single-wavelength laser is set to be larger than the wavelength of the output spectrum peak of the FBG, input light is reflected by the FBG through the circulator to enter the photoelectric detector, an optical signal is converted into an electric signal and then output to the A/D data acquisition unit for acquisition, and finally the microprocessor receives the data signal; when fluid flows along the conical sensing head and passes through the conical transition region, due to the action of the fluid, the conical waist part can generate a flow-induced vibration phenomenon, and the conical waist swings to cause the FBG to generate bending deformation and change the grating distance and the effective refractive index, so that the Bragg wavelength of the FBG is shifted, and the voltage value output to the microprocessor is changed; different flow rates correspond to different output voltage values, so that a functional relation between the flow rate and the voltage is established, and the calibration of the sensor is completed.