Fiber fabry-pero micro pressure sensor

A new fiber fabry-perot pressure sensor is designed. It is made by photolithography, silicon wafer etching, anode bonding, microelectromechanical system technology and so on. It is suitable for the pressure measurement of micro-pressure environment under harsh environment and small space. The structure of the transmitter and the way to make it are described in detail. The design smartly uses the Fiber optic flange, the optical fiber end face is parallel to the sensitive film, thus a high-quality Fabry-Pero-cavity is formed. The structure also contributes to the stability of the initial cavity, the sensor error is reduced. The experimental demodulation system is established, the pressure, temperature and other characteristics are tested in detail. The experiment result shows that within the pressure range of 0~0.1MPa, the sensor has a good linearity, high repeatability and a sensitivity of 61.6  m/MPa.


Introduction
Micropressure sensor is the most commonly used pressure sensor in industrial practice,widely used in various industrial automatic control environment, it is involved in oil pipelines, water and electricity, railway transportation, intelligent building, production of automatic control and machine tools and other industries [1.2].But in the industrial security zone, against bad conditions, micro pressure leak in confined Spaces and test requirements for pipeline microflow have not been satisfied, need small size, high anti-interference, high sensitivity, micropressure sensor products with wide range and not lose the precision.In recent years, the development direction of micropressure sensor is starting to shift to new materials, new mechanism and new structure, the process is being developed from silicon micropressure sensor [3,4] to optical micropressure sensor.
Meanwhile, fiber fabry-perot(F-P) sensor is developing rapidly, it has been applied to severe electromagnetic interference and high temperature and other harsh conditions.The optical fiber F-P interferometric pressure sensor structure is simple, low cost, it is the most commonly used interferometric fiber pressure sensor.It not only has the characteristics of general optical fiber sensor good reliability, anti-electromagnetic interference, corrosion resistance, etc ,but also has the characteristics of high measure precision, high dynamic range, linearity and so on.This paper designs and develops a fiber F-P micropressure sensor, make full use of the advantages of optical fiber sensor, the disadvantages of traditional micro pressure sensor in small space and strong electromagnetic environment are overcome, and linearity is good, the sensitivity is high.The sensor is made of microelectromechanical system(MEMS) technology and suitable for mass serialproduction.

sensor design
The structure of the sensor is shown in figure 1   the center deflection of silicon membrane and pressure P is The h is sensitive film thickness, y is the sensitive film center deflection.In general(y<h/2),( 1) can be approximated to A linear equation: .

Analysis of experimental results
The pressure test system of micropressure sensor is shown in figure 4,the broadband source with the optical fiber sensor

Conclusions
The design and manufacture of optical fiber F-P micro pressure sensor, a number of tests have been carried out.The results show that the micro-pressure sensor is stable, good hysteresis, high sensitivity, and it is easy to draw materials, Compact construction, the advantages of low production cost, suitable for mass production.
, among them a is the fiber optic flange, b is the fiber and ceramic inserts, c is the glass ring,d is monocrystalline silicon piece, also called the pressure sensitive membrane of the sensor.The light travels vertically through the fiber, part of it is reflected off the fiber end, the other part passes through the air cavity, the silicon sensitive film is reflected back into the optical fiber, two beams of light interfere in the optical fiber, the F-P interference cavity was formed under the surface of the fiber optic end and the MATEC Web of Conferences 139, 00200 (2017) sensitive film.When external pressure changes, the sensitive membrane changes under the action of pressure, that changes the length of the F-P cavity, the phase of the reflection spectrum is changed.By means of the demodulation of its reflection spectra, you get the length of the F-P cavity.

Figure 1
Figure 1 Schematic diagram of the structure

Figure 2
Figure 2 Sensor production process In this sensor, the mechanical model of silicon sensitive film with glass ring bonding can be abstracted as radius r, a round plate of surrounding solids, the surface is subjected to uniformly distributed pressure P, then the relationship between external pressure, r is the sensitive membrane radius, E is young's modulus, v is poisson's ratio.For monocrystalline materials, E is 160GPa,v is 0.22.The assignment simulate, the results are shown in figure3,different colors represent different degrees of deflection,the specific values are given by the ruler in the figure, the deflection of the circular film center is

Si720 is used as
the light source of the sensor.The light passes through the coupler and divides into two equal paths, all the way as the incident light enter fiber F-P micro pressure sensor, the light reflected by the sensor passes through the same coupler into the sensing analyzer.The spectral signal of the reflected light is collected by the sensor analyzer to find the cavity length under certain pressure.Due to the membrane deflection and pressure are linear relationships, therefore, the corresponding MATEC Web of Conferences 139, 00200 (2017) pressure can be determined according to the change of lumen length.

Figure 3 Figure 4
Figure 3 Simulation results of the sensitive membrance.(a)Front view; (b)side view

Figure 5
Figure 5 Reflectance spectra of micro pressure sensor

Figure 6
Figure 6 Relationship between pressure and cavity length of the micro pressure sensor

Figure 7
Figure 7 Cavity length changes during pressure rising and dropping The experimental group also tested the repeatability of

Figure 8
Figure 8 Temperature characteristic of the sensor