CN1632489A - Optical Fiber MEMS Pressure Sensor and Its Multiplexing Structure - Google Patents

Abstract

It is a light fiber micro-electron mechanical system pressure sensor and its complex structure. The said sensor end polished fiber end is connected with the boron and silicon underlay and their outer end is connected with the light fixed epoxide resin and there is monocrystalline silicon on the underlay. Its complex structure wide band light source is connected with the tunable filter controller and tunable filter. The output end of the filter is connected with the array wave-guide grating. The output end of the array wave-guide grating is separately connected with photoelectricity detector and sensor through fiber adapter, light single direction isolator and light fiber coupler.

Description

Optical Fiber MEMS Pressure Sensor and Its Multiplexing Structure

                      

The invention relates to a sensor multiplexing system for measuring pressure, which is mainly used for the measurement of absolute and relative pressure, and especially uses the dual-wavelength cavity length demodulation principle and multiplexing technology to measure multi-point pressure to realize distributed pressure measurement.

Background technique

With the continuous development of microelectronics technology and silicon-based microsensors, pressure sensors have rapidly developed towards miniaturization, high performance and suitable for mass production. The continuous development of optical fiber sensing technology makes the organic combination of optical multiplexing technology and optical fiber sensing technology, and realizes the distributed measurement of physical quantities.

Fig. 3 is a brief structural explanatory diagram of an optical fiber MEMS pressure sensor commonly used at present. Published in Opt.Eng.40(4)598-604 (April 2001) "Optically interrogatedMEMS pressure sensors for propulsion applications" by JieZhou et al. 14 is a single crystal silicon film, 12 is a borosilicate glass substrate, 15 is a Fabry-Perot optical interference cavity formed by silicon film and borosilicate glass through anodic bonding process, and the optical fiber 13 is closely connected with the glass through epoxy resin 16 glued together.

Among the above structures, the Fabry-Perot optical interference cavity is realized by the following process. First, the borosilicate glass is photolithographically prepared to make a good pattern; then the borosilicate glass not masked by photoresist is etched with BOE etching solution, and the cavity with the required thickness is obtained by etching; finally, the borosilicate glass is bonded with the borosilicate glass by anodic bonding process. The silicon wafers are tightly bonded together to make a Fabry-Perot optical interference cavity.

However, in such devices, due to the difficulty of quantitative etching of borosilicate glass, the alignment of glass and silicon wafers is not easy to achieve, etc., making the production of optical fiber MEMS (micro-electro-mechanical system) pressure sensors more difficult. It is not easy to realize industrialization and mass production.

Figure 4 is a schematic structural diagram of the fiber optic MEMS pressure sensor Fabry-Perot cavity length demodulation system used by Jie Zhou in the above article. 51 is a light-emitting diode (LED), 52 is a 2×2 coupler, 53 is a sensor probe, 54, 56, 57, and 58 are photodetectors, 55-1, 55-2 are band-pass filters, 61 It is a data acquisition card.

In the above-mentioned demodulation system, the demodulation of the cavity length of the Fabry-Perot cavity is realized by the following method. First, the light emitted by the LED passes through a 2×2 coupler, and a part of the light enters the sensor probe to form multi-beam interference; the other part of the light passes through a 2×2 coupler (50:50), enters the photodetector 57 and has a band Through the photodetector 58 of the filter 55-2, the signals V _L1 and V _L2 are collected by the data acquisition card (61) and processed by computer software. This signal is used to monitor the stability of the light source LED. The light reflected by the sensor probe is detected by a photodetector 54 and a 56 with a bandpass filter 55-1, and the detected signals V _S1 and V _S2 are collected by a data acquisition card (61), and the signals V _S1 and V _S2 The ratio of V S1 and V S2 is the reflectivity of the sensor probe; when the pressure-sensitive film is under pressure, the spectrum of the reflected light of the sensor probe shifts, and the ratio of V _S1 and V _S2 changes, and the pressure on the sensor probe is measured by detecting the change of the ratio.

The above example uses LED as the light source, and the LED light source will drift with the temperature change, so the sensitivity of the sensor output will be affected. Moreover, it is difficult to combine this cavity length demodulation method with multiplexing technology to form a distributed measurement network. It can only measure single-point pressure and cannot realize distributed pressure measurement.

Contents of invention

Technical problem: the object of the present invention is to overcome the above-mentioned shortcoming in prior art, provide a kind of optical fiber MEMS pressure sensor and its multiplexing structure, in order to obtain a kind of completely novel optical fiber MEMS pressure sensor and its signal demodulation complex use the system.

Technical solution: The sensor probe of the present invention uses a single crystal silicon film made by MEMS microfabrication technology as a pressure sensitive film, and the etched shallow cylindrical tank silicon wafer and borosilicate glass substrate are bonded through an anodic bonding process. Tightly bond together to form a Fabry-Perot cavity, and measure the pressure on the single crystal silicon film by detecting the change of the Fabry-Perot cavity length L; optical fiber MEMS pressure sensor signal demodulation and multiplexing system , using broadband light source, tunable filter, tunable filter controller and arrayed waveguide grating, and then combined with arrayed waveguide grating to distribute the light of different wavelengths to the corresponding output channel, and then use the dual wavelength method to demodulate The variation of the cavity length of each Fabry-Perot cavity is obtained, and the variation of the cavity length corresponds to the pressure value of the pressure sensitive membrane one by one, so that the measurement of the distributed pressure can be realized.

The sensor includes: a borosilicate glass substrate, an optical fiber with end face polishing, a single crystal silicon film, a Fabry-Perot cavity, and a light-cured epoxy resin; wherein the polished end face of the end face polished optical fiber is in contact with the borosilicate glass substrate The outer end of the connecting surface is connected by light-curing epoxy resin; on the borosilicate glass substrate is a single crystal silicon film, and a cavity is provided between the lower part of the center of the single crystal silicon film and the borosilicate glass substrate Namely: the Fabry-Perot cavity, in which a pit with a trapezoidal cross-section is arranged on the upper part of the center of the single crystal silicon film.

The multiplexing structure of the fiber optic MEMS pressure sensor includes a broadband light source, a control tunable filter, an arrayed waveguide grating, a tunable filter controller, a fiber optic adapter, a fiber optic coupler, an optical one-way isolator, a photodetector, a sensor The broadband light source and the tunable filter controller are connected with the tunable filter, the output end of the tunable filter is connected with the arrayed waveguide grating, and the output end of the arrayed waveguide grating is coupled through a fiber optic adapter, an optical one-way isolator, and a fiber optic The devices are respectively connected to the photodetector and the sensor, and the pressure on the single crystal silicon film is measured by detecting the change of the cavity length L of the Fabry-Perot cavity of the sensor.

The broadband light source is an ASE (Amplified Spontaneous Emission) stabilized light source or an SLED (Superluminescent Light Emitting Diode) light source. The wavelength tuning range of the tunable filter in the multiplexing structure is 1520nm-1620nm, and the AWG (arrayed waveguide grating) has 40 channels.

The sensor of the invention combines the sensor probe made by using the MEMS microfabrication technology and the dual-wavelength demodulation and multiplexing system for the cavity length of the optical fiber Fabry-Perot sensor.

The basis of the present invention is to use the MEMS microfabrication process to obtain the core element, that is, the Fabry-Perot interference cavity; combined with the dual-wavelength demodulation method and multiplexing technology, by measuring the Fabry-Perot interference caused by the deformation of the silicon diaphragm under pressure. —The change of the length of the Perot cavity is used to detect the pressure on multiple sensors to realize distributed pressure measurement.

The core technology of the dual-wavelength demodulation and multiplexing system of fiber Fabry-Perot sensor cavity length in the present invention is to effectively combine the broadband light source with the tunable filter and its controller to replace the expensive tunable laser, and The arrayed waveguide grating AWG is used to distribute light waves of a certain wavelength to each optical fiber sensing channel, and the dual-wavelength cavity length demodulation method and wavelength division/time division multiplexing technology are effectively used to measure multi-point pressure to realize distributed pressure measurement.

The cavity length demodulation and multiplexing system utilizes a dual-wavelength demodulation method, which includes:

Broadband light source; tunable filter that outputs wavelength tunable light according to the specified output mode; tunable filter controller controls the output mode of output wavelength of tunable filter; in order to prevent the reflected light from the sensor from reflecting into the light source and affecting the sensor system The accuracy and stability of the optical path is used in the optical one-way isolator to prevent the light reflected back through the 2×2 (50:50) fiber coupler (6) from entering the AWG;

At moment 1, part of the light of wavelength λ ₁ is coupled into the photodetector (8-1) through the channel (a), and another part of the light is coupled into the optical fiber MEMS pressure sensor probe, and reflected back, using the photodetector (8-2 ) to detect its light intensity signal, and obtain the reflectance R(λ ₁ ) of the light with a wavelength of λ ₁ ; at time 2, obtain the reflectance R(λ ₂ ) of the light with a wavelength of λ ₂ in the same way, and then use The change of the cavity length of the sensor (9-2) can be obtained by the dual-wavelength cavity length detection principle;

Using the same method as above, the chamber length changes of other sensors (such as the sensor (9-1) in the embodiment) can be detected to realize distributed pressure measurement.

The tunable light source is realized by amplifying spontaneous emission ASE light source, tunable filter and tunable filter controller, and then combined with the arrayed waveguide grating to distribute the light of different wavelengths to the corresponding output channels. The tunable filter is a tunable optical filter based on the Fabry-Perot cavity structure; the tunable filter controller uses an asymmetric piezoelectric ceramic drive power supply.

The sensor probe uses a single crystal silicon film as the pressure sensitive film, and is made of silicon-based MEMS microfabrication technology, which can realize mass production. The wavelength tuning range of the tunable filter in the sensor is 1520nm-1620nm, and the AWG has 40 channels.

Beneficial effects: the sensor of the present invention uses light as the sensing medium, and has excellent characteristics of anti-electromagnetic interference and prevention of explosion.

The sensor of the present invention has extremely high sensitivity and linearity. The sensor probe adopts the single crystal silicon thin film obtained by the combination of reactive ion etching and deep etching as the pressure sensitive magic, which avoids the residual stress instability of the silicon film formed by epitaxy. So that the sensor has a stable, high-precision pressure measurement performance.

The Fabry-Perot cavity length demodulation system of the sensor of the present invention adopts a dual-wavelength demodulation method, which can effectively reduce the light intensity fluctuation of the light source in the sensing system, the disturbance of the sensing optical path, and the disturbance of the detector. Errors caused by factors such as drift, fiber transmission loss, and device magnification changes.

The sensor of the invention effectively combines the stabilized broadband light source with the tunable filter and its controller to replace the expensive tunable laser. Moreover, the detection optical path is simple, which greatly reduces the cost of the sensor system and increases the possibility of practical application and industrialization.

In the sensor demodulation and multiplexing system of the present invention, the tunable filter adopts a Fabry-Perot cavity structure, which has the characteristics of high filtering precision, more flexible filtering spectral shape, faster speed and smaller size. Moreover, the piezoelectric ceramic crystal is used to adjust the length of the Fabry-Perot cavity, which can obtain extremely high resolution.

In the sensor demodulation and multiplexing system of the present invention, the piezoelectric ceramic crystal in the tunable filter controller adopts an asymmetric drive power supply, which effectively solves the defect of low-voltage output saturation distortion when a single power supply is used, and also avoids the use of symmetrical The problem of high device requirements and low power supply efficiency in dual power supply has the advantages of simple circuit, good linearity, good repeatability, and low cost.

The sensor system of the present invention can form a distributed pressure measurement network by using multiplexing technology. The tunable filter can use its controller to output light of a certain wavelength according to a certain timing, so that the multiplexing technology can be used to measure the pressure value of multiple points at the same time, and the pressure distribution in the environment can be obtained.

The sensor system of the present invention uses an arrayed waveguide grating instead of an optical fiber array as an optical wave splitter, which greatly reduces the volume and more effectively uses the multiplexing technology to realize the measurement of distributed pressure, and with the continuous development of the arrayed waveguide grating technology, more The arrayed waveguide grating with the number of channels is constantly becoming practical, so that the distribution measurement of more pressure points can be realized by using the arrayed waveguide grating.

The sensor of the present invention effectively eliminates the crosstalk of the signal in a certain channel coupled to the signal quantity of another channel due to the proper use of the one-way optical isolator.

Therefore, the sensor of the present invention can realize the dual-wavelength demodulation and multiplexing system of the sensor probe obtained by using the MEMS microfabrication process combined with the cavity length of the optical fiber Fabry-Perot sensor, which has high precision, high sensitivity and good reliability. And can be used to measure the optical fiber MEMS pressure sensor of distributed pressure.

Description of drawings

The present invention will be described in detail below in conjunction with the accompanying drawings of the embodiments.

Fig. 1 is a structural schematic diagram of an optical fiber MEMS absolute pressure sensor in the present invention. In the figure, there are: borosilicate glass substrate 12, optical fiber with end face polished 13 single crystal silicon film 14, Fabry-Perot cavity 15, light-cured epoxy resin 16, cavity length L, and pressure P.

Fig. 2 is a schematic diagram of the multiplexing structure of the optical fiber MEMS pressure sensor in the present invention. In the figure are: broadband light source 1, control tunable filter 2, arrayed waveguide grating 3, tunable filter controller 4, fiber optic adapter 5, fiber coupler 6, optical one-way isolator 7, photodetector 8- 1, 8-2, sensors 9-1, 9-2.

Figure 3 is a structural schematic diagram of the optical fiber MEMS absolute pressure sensor produced by the University of Cincinnati in the United States.

Figure 4 is a schematic diagram of the multiplexing structure of the optical fiber MEMS absolute pressure sensor produced by the University of Cincinnati in the United States.

Fig. 5 is a schematic diagram of the tunable filter in the present invention.

Fig. 6 is a schematic diagram of a micro-displacement drive power supply for a tunable filter piezoelectric ceramic crystal in the present invention.

Fig. 7 is a schematic diagram of the relationship between the cavity length obtained by using the dual-wavelength demodulation method and the corresponding ratio.

Detailed ways

Since the borosilicate glass 12 and the single crystal silicon film 14 are bonded together by a vacuum anodic bonding process, the internal air pressure of the Fabry-Perot chamber is approximately equal to the ambient air pressure in the vacuum anodic bonding chamber, so it can be used as an absolute Reference pressure for the pressure sensor.

The light of wavelength λ is coupled into the Fabry-Perot cavity through the optical fiber, and the light is reflected back and forth on the upper surface of the borosilicate glass and the lower surface of the single crystal silicon film to form multi-beam interference. When the single crystal silicon film 14 is under pressure, it will Bending occurs so that the cavity length L of the Fabry-Perot cavity becomes L′. According to the multi-beam interference principle of the Fabry-Perot cavity, for light with a wavelength of λ, the interference light intensity will change, resulting in a wavelength The reflectivity of λ light changes, and in the following dual-wavelength demodulation and multiplexing system of the optical fiber Fabry-Perot sensor cavity length, the reflectivity change of different wavelengths combined with the dual-wavelength demodulation method is used to obtain the pressure-induced The change of the length of the Fabry-Perot cavity ΔL (ΔL=L'-L). The sensor includes: a borosilicate glass substrate 12, an optical fiber 13 with end face polishing, a single crystal silicon film 14, a Fabry-Perot cavity, and a light-cured epoxy resin 16; The substrates are connected, and the outer ends of the connection surfaces are connected by light-cured epoxy resin; on the borosilicate glass substrate is a single crystal silicon film, and a One cavity is the Fabry-Perot cavity 15, and a pit with a trapezoidal cross section is provided on the upper part of the center of the single crystal silicon film.

The multiplexing structure of the fiber optic MEMS pressure sensor includes broadband light source 1, control tunable filter 2, arrayed waveguide grating 3, tunable filter controller 4, fiber optic adapter 5, fiber optic coupler 6, optical one-way isolator 7. Photodetectors 8-1, 8-2, sensors 9-1, 9-2; the broadband light source and the tunable filter controller are connected to the tunable filter, and the output end of the tunable filter is connected to the arrayed waveguide grating connected, the output end of the arrayed waveguide grating is respectively connected to the photodetector and the sensor through the fiber adapter, the optical one-way isolator and the fiber coupler, and the change of the cavity length L of the Fabry-Perot cavity of the sensor is used to measure the single crystal The pressure on the silicon membrane. The broadband light source is an ASE (Amplified Spontaneous Emission) stabilized light source or an SLED (Superluminescent Light Emitting Diode) light source. The wavelength tuning range of the tunable filter in the multiplexing structure is 1520nm-1620nm, and the AWG (arrayed waveguide grating) has 40 channels.

In Figure 2, if no optical isolator is used, the light output from channel a is reflected by the sensor probe and recoupled into channels a and b, so that the returned light interferes with the output light of the arrayed waveguide grating, making the optical fiber Crosstalk occurs in the MEMS pressure sensor multiplexing system, which greatly affects the sensitivity of sensor multiplexing and the measurement accuracy of distributed pressure. Therefore, an optical one-way isolator 7 is used in the optical path to prevent coupling via 2×2 (50:50) optical fibers The light reflected by the device 6 enters the arrayed waveguide grating, which effectively avoids crosstalk between optical signals. The broadband light source works in the C+L band (1525nm-1610nm), and provides a stable light source for a long time. The main part of the light source is a gain medium erbium-doped fiber and a high-performance pump laser. By controlling the output of the pump laser, the output power is guaranteed. Stablize.

In order to effectively utilize the broadband light source, the operating wavelength range of the tunable filter is also selected as (1520nm-1620nm), and the controller of the tunable filter uses a piezoelectric ceramic crystal to adjust the length of the built-in Fabry-Perot cavity of the controller. , to control the output wavelength of the tunable filter, a theoretically almost unlimited resolution can be obtained.

According to the existing cost-effectiveness in the market and the actual demand of the pressure distribution measurement points of the pressure sensor, the arrayed waveguide grating with 40 channels can be selected to realize the measurement of 20 distributed pressures.

In the present invention, the change of the Fabry-Perot cavity cavity length is demodulated by a dual-wavelength method, and R(λ ₁ ) and R(λ ₂ ) are reflections of wavelength λ ₁ and λ ₂ light in the sensor probe respectively At this time, R(λ ₁ ) and R(λ ₂ ) include the light intensity fluctuation of the light source, the disturbance of the sensing optical path, the drift of the detector, the optical fiber transmission loss and the change of device magnification and other factors. . The dual-wavelength method uses the ratio of R(λ ₁ ) to R(λ ₁ ), R(λ ₂ ) (as shown in Equation 1) as the detection signal of the Fabry-Perot cavity length, effectively avoiding the above error.

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

The specific multiplexing technology is to use the stable wavelength allocation technology of the arrayed waveguide grating, and two channels can be used to measure the change of the cavity length of a sensor Fabry-Perot cavity in real time, so that the pressure value of each sensor point can be measured with high precision , to achieve distributed pressure measurement.

In Figure 5, the end faces of two sections of optical fiber are coated with high-reflection film, and a Fabry-Perot cavity is formed between the two end faces. The length of the Fabry-Perot cavity is controlled by controlling the displacement of the piezoelectric ceramic crystal. Thereby controlling the output of light with a certain wavelength and realizing the function of tunable filtering.

Figure 6 is an asymmetrical piezoelectric ceramic crystal drive power supply to control a tunable filter. In the figure, Vi is the 0~5V control signal output by the signal source. T1 and T2 form a differential amplifier circuit. In order to ensure the symmetry of the differential amplifier circuit, the parameters of the two tubes should be as consistent as possible. T3, R5, R6, D1, D2 constitute a constant current source, constant current 2mA. Static output is 0V, T5, R9, D1, D2 constitute a constant current source, the constant current is 2.5mA. D3, D4, and D5 realize voltage translation and complete voltage amplification. T6~T9 are power output, the maximum undistorted output voltage range is -7~+220V, R7 and R4 are voltage deep negative feedback network, select the precision resistor with appropriate resistance value, so that when the input signal changes between 0~+5V , the output varies between 0 and +200V. T10, T11, T12, and T13 play the role of overcurrent protection. It is verified by experiments that the linearity error of the driving power supply is less than 0.2%, the frequency response can reach above 1.5kHz, and there is no saturation distortion when the output voltage is low, and the output wavelength of the tunable filter can be well controlled.

In Fig. 7 is the variation curve of I(h, λ) and Fabry-Perot cavity length L obtained by using the dual-wavelength method, which has good linearity and sensitivity.

As an experimental example of the present invention, a single crystal silicon circular film with a diameter of 600 microns and a thickness of 20 microns can be obtained, and 40 channel arrayed waveguide gratings are selected. The broadband light source works within the range of [1525, 1610] nm, and 20 points can be realized. The distributed pressure measurement, the pressure measurement range is [0, 2.5]Mpa.

Therefore, with the help of the present invention, the optical fiber MEMS pressure sensor and its multiplexing system with good precision, good sensitivity, high reliability, anti-electromagnetic interference, and ability to measure distributed pressure can be manufactured by using MEMS processing technology.

Claims (4)

1, a kind of optical fiber microelectronic pressure sensor for mechanical system is characterized in that described sensor comprises: optical fiber (13), monocrystalline silicon membrane (14), Fabry-Perot cavity (15), the photo-curing epoxy resin (16) of Pyrex substrate (12), end face polishing; Wherein the polished end faces and the Pyrex substrate (12) of the optical fiber (13) of end face polishing join, and the outer end of its joint face is connected by photo-curing epoxy resin (16); Going up at Pyrex substrate (12) is monocrystalline silicon membrane (14), be provided with a cavity promptly between the bottom of monocrystalline silicon membrane (14) central authorities and Pyrex substrate (12): Fabry one Perot cavity (15), being provided with a cross sectional shape on the central top of monocrystalline silicon membrane (14) is trapezoidal pit.

2, a kind of multiplexing structure of optical fiber microelectronic pressure sensor for mechanical system as claimed in claim 1 is characterized in that this structure comprises wideband light source (1), control tunable optic filter (2), array waveguide grating (3), tunable optic filter controller (4), fiber adapter (5), fiber coupler (6), light one-way isolator (7), photodetector (8-1,8-2), sensor (9-1,9-2); Wideband light source (1) and tunable optic filter controller (4) join with tunable optic filter (2), the output terminal of tunable optic filter (2) and array waveguide grating (3) join, the output terminal of array waveguide grating (3) connects photodetector (8-1,8-2), sensor (9-1,9-2) respectively by fiber adapter (5), light one-way isolator (7), fiber coupler (6), the pressure size that the measure of the change monocrystalline silicon membrane (14) of the chamber of the Fabry-Perot cavity (15) by detecting sensor long (L) is subjected to.

3, the multiplexing structure of optical fiber microelectronic pressure sensor for mechanical system as claimed in claim 2 is characterized in that: described wideband light source (1) is amplified spontaneous emission light source or super-radiance light emitting diode.

4, the multiplexing structure of optical fiber microelectronic pressure sensor for mechanical system as claimed in claim 2 is characterized in that: tunable optic filter in this multiplexing structure (2) wavelength tuning range is 1520nm-1620nm, and array waveguide grating has 40 passages.

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