CN102590112A - Surface microstructure silicon cantilever enhancement type optical-thermal spectrum trace gas detection method and device - Google Patents

Abstract

A surface microstructure silicon cantilever enhanced photothermal spectrum trace gas detection method and device, including a tunable laser, a reflective object, a surface microstructure silicon cantilever, a concave mirror, an optical fiber, a fiber coupler, a continuous laser, and a laser control devices, photodetectors, and signal processing systems. After the modulated light emitted by the tunable laser passes through the gas to be detected, it is reflected by the reflected object onto the concave mirror, and the concave mirror focuses the received reflected light on the silicon surface of the cantilever beam, and the cantilever beam absorbs the light energy and undergoes photothermal deflection And to generate resonance, the higher the gas concentration, the greater the light intensity absorbed by the gas, the smaller the light energy absorbed by the cantilever beam, and the smaller the resonance amplitude. The end face of the optical fiber and the metal surface of the cantilever beam form a Fab cavity with adjustable cavity length. The amplitude of the cantilever beam can be demodulated to obtain the absorption spectrum of the gas, and then the concentration of the detected trace gas can be obtained. The device has the advantages of low cost, small size, simple structure, convenient use, strong mobility, high detection sensitivity, and ability to work in field environments, and can be widely used for long-distance detection of multiple or multi-component trace gas components and concentrations .

Description

Surface Microstructure Silicon Cantilever Beam Enhanced Photothermal Spectroscopy Trace Gas Detection Method and Device

technical field

The invention relates to gas detection technology, especially a photothermal spectrum trace gas detection method and device for remote detection of trace gas based on a surface microstructure silicon cantilever beam working in an open environment, which can be widely used in remote It is widely used in fields such as explosive detection, environmental monitoring, and detection of toxic and hazardous gases.

Background technique

The development of trace gas long-distance detection technology is of great significance for atmospheric environment monitoring, long-distance detection of explosives and detection of biological and physiological states. Absorption spectroscopy gas detection technology has the advantages of large measurement range, multi-component measurement, continuous monitoring, etc., and has gradually become an ideal trace gas concentration detection tool. Absorption spectroscopy gas detection technologies mainly include differential absorption spectroscopy, tunable laser diode absorption spectroscopy, laser-induced fluorescence, and photoacoustic spectroscopy. Among them, photoacoustic spectroscopy has always been one of the most important development directions of trace gas detection technology due to its high sensitivity, strong anti-interference ability, large dynamic range, and detector response independent of incident wavelength.

In order to improve the detection sensitivity of photoacoustic spectroscopy, people have been working on improving the structure of the photoacoustic cell and using more sensitive microphones to improve and develop this technology. For example, in 2002, the photoacoustic spectroscopy group of Nijmegen University in the Netherlands used the photoacoustic spectroscopy system built by the optical parametric oscillator to increase the detection sensitivity of ethane to the level of 10ppt. The quartz tuning fork-enhanced photoacoustic spectroscopy gas device, and real-time measurement of water vapor concentration under atmospheric pressure, its detection normalized equivalent noise is 5.9×10 ^-10 cm ^-1 W/Hz ^1/2 . Finland V.KOSKINEN et al. proposed a "cantilever-enhanced photoacoustic spectroscopy detection device" to detect the concentration of carbon dioxide gas, and its detection normalized equivalent noise is 1.7×10 ^-10 cm ^-1 W/Hz ^1/2 . Although this type of method has high detection sensitivity, due to the use of photoacoustic cells, it is limited that this method can only be detected in the area where trace gases exist, and cannot be used for long-distance detection. Therefore, it is greatly restricted in the application of detecting toxic, flammable and explosive dangerous gases.

In order to overcome the above shortcomings and realize the long-distance detection of trace gases by photoacoustic spectroscopy, the Oak Ridge National Laboratory under the US Department of Energy proposed a photoacoustic spectroscopy detection system for long-distance detection of explosives in 2008. In this system, the laser light emitted by the quantum laser irradiates the object to be detected at a distance of 20m, and after being reflected by the reflector to the concave mirror, it is focused on the quartz tuning fork, thereby causing the tuning fork to vibrate. Due to the piezoelectric effect of the tuning fork, the vibrating quartz tuning fork will generate a piezoelectric current signal, which is demodulated at the resonance frequency by a lock-in amplifier to obtain the absorption spectrum of the gas sample. However, this detection method uses a quartz tuning fork as a light energy absorbing device, and its light absorption rate is low, which limits the further improvement of detection sensitivity. In addition, if the detection sensitivity of the tuning fork is to be improved, a larger laser energy is required, which greatly increases the risk of detecting flammable and explosive gases. Later, the laboratory proposed a cantilever beam-based explosive detection system, which can detect three typical explosives within a range of 1 meter. Compared with the quartz-enhanced photoacoustic spectroscopy detection system, the system has Higher detection accuracy. However, due to the very low absorption efficiency of the silicon nitride cantilever beam for incident light, its ability to detect weak light signals is greatly reduced, making it impossible to perform long-distance spectral detection; moreover, the cantilever beam has low flexibility and low thermal deflection efficiency , so it is necessary to use an expensive high-precision position-sensitive detection system to pick up the vibration amplitude of the cantilever beam, resulting in a complex system structure and high prices.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies in the prior art, and propose a photothermal spectroscopy detection method and device based on surface microstructured silicon cantilever beams for trace gas detection in an open environment, which is a cheap, The utility model has the advantages of small volume, simple structure, convenient use, high detection sensitivity, and the ability to work in field environments, and can be used for a long-distance detection method and device for multiple or multi-component trace gases.

For solving the technical problem of the present invention, the technical scheme adopted is:

A surface microstructure silicon cantilever enhanced photothermal spectrum trace gas detection method, the method is to let the pulsed laser emitted from the tunable laser pass through the gas to be detected, be absorbed and incident on a reflective object, and after passing through the reflective object Reflected on the concave mirror, the received light energy is focused on the surface microstructured silicon cantilever with the concave mirror, and the light energy is absorbed by the surface microstructured silicon cantilever to resonate, and at the same time pass through the metal surface based on the end face of the optical fiber and the cantilever The tunable fiber-optic F-P demodulation system picks up the vibration signal of the surface microstructure silicon cantilever beam. When the cantilever beam resonates, the cavity length of the F-P cavity changes, resulting in periodic changes in the intensity of reflected interference light. The resonance signal of the cantilever beam is obtained by demodulating the intensity of the optical signal, and finally the concentration of the detected gas is inverted by a signal processing system.

A photothermal spectrum trace gas detection device based on a surface microstructured silicon cantilever for realizing the above method, which includes a tunable laser, a reflective object, a surface microstructured silicon cantilever, a concave mirror, an optical fiber, a fiber coupler, a continuous laser, a single Point photodetectors, signal processing systems, optical fibers, fiber optic and laser controllers.

The surface microstructured silicon cantilever has a double-layer structure, and the upper layer (that is, the light-absorbing layer) is the surface microstructured silicon. It uses femtosecond laser pulses and sulfur hexafluoride etching gas to etch multiple microcone structures on the surface, and uses the method of sulfur hexafluoride concentration etching with a pressure between 60Kpa and 80Kpa to increase the concentration of doped micropyramids on the surface. The concentration of sulfur element in the structural silicon changes the energy band structure of the surface microstructure silicon, thereby improving the light absorption efficiency of the cantilever beam in the ultraviolet to infrared band, and the absorbance in this band is relatively flat, realizing the ultraviolet to infrared The non-selective absorption of the whole band has non-selective absorption characteristics for the laser wavelength, which can meet the requirements of wide-spectrum detection, and can detect the concentration of various trace gases or multiple groups of trace gases.

This structure can greatly improve the absorption efficiency of the detected laser light, and greatly increase the photothermal conversion coefficient of the structure. Under the same excitation light energy, the vibration amplitude of the cantilever beam is significantly increased, thereby greatly improving the detection sensitivity of the system. . The silicon thickness of the microstructure on the upper surface is 2.5-28 microns. The surface structure of the microcone is micron and nanometer in size and is in the shape of a "pyramid". The ratio of the height of the cone to the diameter of the bottom surface is between 2 and 4. The thickness of the cantilever beam is 3-30 microns, and the lower layer of the cantilever beam (ie, the reflective layer) is made of metal material, generally gold, silver or aluminum. When the laser is irradiated on the silicon surface of the cantilever beam, elastic thermal expansion occurs. Due to the different thermal expansion coefficients of the two materials, the surface microstructure silicon cantilever beam undergoes periodic photothermal deflection. Based on the principle of photothermal deflection, a high quality factor (high Q value ) cantilever beam to pick up the gas absorption light intensity signal, which is a signal picking method with high signal-to-noise ratio and has strong immunity to environmental noise.

The structural relationship of the device is as follows: the tunable laser, the reflective object and the concave mirror are set on the same optical path, the surface microstructured silicon cantilever is placed at the focal point of the concave mirror, and the surface microstructured silicon of the surface microstructured silicon cantilever faces the concave mirror , the metal surface and the end face of the optical fiber form a Fab cavity, the continuous laser and the fiber coupler are connected through the optical fiber, the fiber coupler is connected with the optical fiber, the photodetector and the fiber coupler are connected by the optical fiber, and the photodetector receives the light The signal is converted into an electrical signal. The output end of the photodetector is electrically connected to the input end of the signal processing system. The signal processing system performs filtering, denoising, and data processing on the electrical signal. The output end of the laser controller is connected to the tunable laser The input terminal is electrically connected to perform wavelength scanning and frequency modulation on the laser.

The optical fiber generally adopts a single-mode quartz optical fiber or a polarization-maintaining optical fiber, and a F-P cavity is formed between its end face and the metal surface of the silicon cantilever beam with a microstructure on the surface, and its length is generally 1 to 10 times the wavelength emitted by the continuous laser.

The tunable laser generally adopts distributed feedback laser, quantum cascade laser or vertical cavity surface emitting laser, etc., whose central wavelength is consistent with the absorption peak of the gas to be detected, and can be adjusted near the absorption peak, for example, when detecting carbon dioxide gas , a tunable laser with a center wavelength of 1580nm and an adjustment range of ±1nm can be used.

The advantages of the present invention relative to the prior art are as follows:

First, the cantilever beam is made of composite surface microstructure silicon, which has a high absorption rate in the ultra-wide spectral region from ultraviolet to infrared, including visible light, so it can use one device to achieve the requirements of wide-spectrum detection ; In the case of not changing the system structure, lasers with different wavelengths are selected as excitation light sources, and the concentration detection of multiple or multi-component trace gases can be completed without changing the cantilever beam detection device.

Second, the present invention uses the surface microstructured silicon cantilever beam structure as a resonant device, which is a detector with a narrow-band filtering function for vibration signals, and has immunity to ambient light noise, which will enable the system to be used in an open space environment The anti-interference ability is greatly improved, and it has the characteristics of small size, low cost and portability.

Third, the device can work in an open environment without using a photoacoustic cell to reduce the impact of external noise on the system's detection signal-to-noise ratio, so it is very suitable for the detection of trace gases at long distances, especially for toxic Compared with the traditional photoacoustic spectroscopy detection system, it has great advantages in the detection of flammable and explosive trace gases.

Fourth, the traditional electro-demodulation method is replaced by the optical fiber method, which has high sensitivity, high detection accuracy and good stability. It can effectively eliminate external electromagnetic interference and has the function of fire and explosion protection.

Description of drawings

Fig. 1 Schematic diagram of a surface microstructure silicon cantilever enhanced photothermal spectroscopy trace gas detection device;

Figure 2 Schematic diagram of the cantilever beam structure;

Figure 3 Schematic diagram of the optical pickup of the tunable F-P demodulation system;

Figure 4 Schematic diagram of silicon surface microstructure;

Figure 5 is a schematic diagram of the absorption efficiency curve of the surface microstructure silicon cantilever beam.

Detailed ways

The photothermal spectroscopy trace gas detection device based on the surface microstructured silicon cantilever working in an open environment is shown in Figure 1. The tunable laser 1 is on the same optical path as the reflective object 2 and the concave mirror 4. The surface microstructured silicon cantilever Beam 3 is placed at the focal point of concave mirror 4 . The metal surface 311 of the surface microstructure silicon cantilever 3 and the end face 51 of the optical fiber 5 form a Fab cavity, the continuous laser 7 is connected with the fiber coupler 6 through the optical fiber 10, the fiber coupler 6 is connected with the optical fiber 5, and the photodetector 8 It is connected with the optical fiber coupler 6 by an optical fiber 11, and the photodetector receives the optical signal and converts it into an electrical signal. The output end of the photodetector 8 is electrically connected to the input end of the signal processing system 9, and the signal processing system 9 performs filtering, denoising, data processing, etc. on the electrical signal. The output end of the laser controller 12 is electrically connected to the input end of the tunable laser 1, and its functions include performing wavelength scanning and frequency modulation on the laser.

During operation, the laser controller 12 modulates the tunable laser 1, and the modulation frequency is 1/2 of the natural frequency f ₀ of the surface microstructure silicon cantilever beam 3 . The laser controller 12 controls the central wavelength of the tunable laser 1 at the position of the absorption peak of the measured gas by means of current sweeping. The modulated light emitted by the tunable laser 1 reaches the reflective object 2 after passing through the measured gas. The reflective object 2 reflects the light absorbed by the gas to the concave mirror 4, and the concave mirror 4 focuses it on the cantilever beam 3. The cantilever beam 3 resonates after absorbing the energy. The light emitted from the light source 7 passes through the coupler 6, and then irradiates the metal surface 311 of the cantilever beam through the optical fiber 5. The fiber end face 51 and the cantilever beam metal surface 311 form a Fab cavity with a cavity length tuning, wherein the fiber end face 51 is fixed. Part of the laser light in the fiber 5 is reflected back to the fiber 5 by the fiber end face 51; the other part is reflected by the metal surface 311 of the cantilever beam. The interference light enters the photodetector 8 through the fiber coupler 6, and the photodetector outputs an electrical signal corresponding to the intensity of the interference light and enters the signal processing system 9. The signal processing system 9 performs data processing on the electrical signal, and then calculates the measured gas concentration.

The structure of the cantilever beam 3 is shown in FIG. 2 , which is a double-layer structure, and the upper layer is silicon 32 with surface microstructure. The lower layer is a metal material 31 with a thermal expansion coefficient different from that of the microstructured silicon on the surface. The surface microstructure silicon 32 is shown in Figure 4. The surface microstructure silicon is formed on the insulating layer with a three-layer structure by using a femtosecond laser in an environment of sulfur hexafluoride corrosion gas with a pressure between 60Kpa and 80Kpa. A plurality of microcone structures 321 are etched on the upper surface of the insulating layer, the middle layer of silicon on the insulating layer is etched away with hydrofluoric acid, and the uppermost layer is peeled off to obtain an ultra-thin flexible surface microstructure silicon 32 with a thickness of 2.5 -28 microns, and then plate a layer of metal material 31 on the back of the microstructure silicon to obtain the cantilever beam 3 with a total thickness of 3-30 microns.

The principle of the cantilever beam 3 picking up the light intensity absorbed by the gas to be measured is shown in Figure 3. Based on the principle of photothermal deflection, the cantilever beam 3 absorbs the light intensity signal focused by the concave mirror 4 after being absorbed by the gas, and then the following occurs: For the vibration shown in the figure, the vibration amplitude has a linear relationship with the light intensity signal absorbed by the gas. The higher the gas concentration, the greater the light intensity absorbed by the gas, the smaller the light intensity incident on the cantilever beam 3 , and the smaller the amplitude of the cantilever beam 3 . The Fab cavity formed by the metal surface 311 of the cantilever beam and the end face 51 of the optical fiber can detect the intensity of the interference light entering the optical fiber 5 to obtain the intensity of light absorbed by the gas.

The absorption efficiency curve of the silicon cantilever beam with the surface microstructure is shown in FIG. 5 , the cantilever beam has high absorption efficiency from ultraviolet to infrared region, and the absorption efficiency curve is relatively flat. the

Claims (10)

1. A surface microstructure silicon cantilever beam enhanced photothermal spectroscopy trace gas detection method, characterized in that the method is to allow the pulsed laser light sent from the tunable laser to pass through the gas to be detected, and the pulsed laser light is absorbed and incident on the On the reflective object, after passing through the reflective object, it is reflected on the concave mirror, and the received light energy is focused on the surface microstructured silicon cantilever beam by the concave mirror. The tunable optical fiber FAP demodulation system composed of the fiber end face and the metal surface of the cantilever picks up the vibration signal of the surface microstructure silicon cantilever, that is, when the cantilever resonates, the cavity length of the FAP cavity changes, resulting in The intensity of reflected interference light changes periodically, and the resonance signal of the cantilever beam is obtained by demodulating the intensity of the light signal, and finally the concentration of the detected gas is inverted by the signal processing system; The surface microstructure silicon cantilever beam (3) has a double-layer structure, the upper layer is the surface microstructure silicon, and the lower layer is metal. When the pulsed laser irradiates the surface of the cantilever silicon beam, elastic thermal expansion occurs. Since the thermal expansion coefficients of the two layers of materials are different, Periodic photothermal deflection of surface microstructured silicon cantilever beams; The modulation frequency of the tunable laser (1) is 1/2 of the resonant frequency of the surface microstructure silicon cantilever beam, and its central wavelength is consistent with the absorption peak of the detected gas; The cavity length of the Faper cavity is 1-10 times of the emission wavelength of the continuous laser. 2. The surface microstructure silicon cantilever enhanced photothermal spectroscopy trace gas detection method according to claim 1, characterized in that the surface microstructure silicon cantilever (3) has a thickness of 3-30 microns, and the upper layer The surface microstructure of silicon contains multiple micropyramidal structures with a thickness of 2.5-28 microns; The upper layer of the silicon cantilever beam with the surface microstructure is obtained by the following method: in a sulfur hexafluoride gas environment with a gas pressure between 60Kpa and 80Kpa, the upper surface of the silicon on the insulating layer is etched with a femtosecond laser A plurality of micro-cone structures, and then use hydrofluoric acid to etch away the middle layer of silicon on the insulating layer, and finally peel off the uppermost surface to obtain ultra-thin surface micro-structured silicon. 3. A surface microstructure silicon cantilever enhanced photothermal spectroscopy trace gas detection device for realizing the method of claim 1 or 2, characterized in that it includes a tunable laser (1), a reflective object (2), a surface Microstructure silicon cantilever (3), concave mirror (4), optical fiber (5), fiber coupler (6), continuous laser (7), photodetector (8), signal processing system (9), optical fiber (10 ), optical fiber (11) and laser controller (12); The surface microstructured silicon cantilever (3) has a double-layer structure, the upper layer is the surface microstructured silicon, and the lower layer is metal. When the laser irradiates the surface of the cantilever silicon, elastic thermal expansion occurs. Due to the different thermal expansion coefficients of the two layers of materials, the surface Periodic photothermal deflection of microstructured silicon cantilever beams; The tunable laser (1) is set on the same optical path as the reflective object (2) and the concave mirror (4), the surface microstructure silicon cantilever beam (3) is placed at the focus of the concave mirror (4), the surface microstructure silicon The microstructure silicon on the surface of the cantilever beam (3) faces the concave mirror (4), the metal surface (311) and the end face (51) of the optical fiber (5) form a F-P cavity, and the continuous laser (7) and the fiber coupler (6) pass through The optical fiber (10) is connected, the optical fiber coupler (6) is connected with the optical fiber (5), the photodetector (8) and the optical fiber coupler (6) are connected by the optical fiber (11), and the photodetector receives the optical signal And convert it into an electrical signal, the output end of the photodetector (8) is electrically connected to the input end of the signal processing system (9), and the signal processing system (9) performs filtering, denoising, data processing on the electrical signal, and laser control The output end of the laser (12) is electrically connected to the input end of the tunable laser (1), and the laser is subjected to wavelength scanning and pulse frequency modulation; The modulation frequency of the tunable laser (1) is 1/2 of the resonant frequency of the surface microstructure silicon cantilever beam, and its central wavelength is consistent with the absorption peak of the detected gas; The cavity length of the Faper cavity is 1-10 times of the emission wavelength of the continuous laser. 4. The surface microstructure silicon cantilever enhanced photothermal spectrum trace gas detection device according to claim 3, characterized in that, the thickness of the surface microstructure silicon cantilever (3) is 3-30 microns, and the upper surface Microstructured silicon contains multiple micropyramidal structures with a thickness of 2.5-28 microns. 5. The surface microstructure silicon cantilever beam enhanced photothermal spectrum trace gas detection device according to claim 3, characterized in that, the microcone structure is of micron and nanometer size, in a pyramid shape, and the height of the cone is the same as The ratio of the base diameters is between 2 and 4. 6. The surface microstructure silicon cantilever beam-enhanced photothermal spectrum trace gas detection device according to claim 3, characterized in that the upper surface microstructure silicon is obtained by the following method: when the gas pressure is between 60Kpa and 80Kpa In the sulfur hexafluoride gas environment between them, on the upper surface of the silicon on the insulating layer, a plurality of microcone structures are etched with a femtosecond laser, and then the middle layer of silicon on the insulating layer is etched away with hydrofluoric acid, and finally The uppermost surface is peeled off to obtain ultra-thin surface microstructured silicon. 7. The surface microstructure silicon cantilever enhanced photothermal spectrum trace gas detection device according to claim 3, wherein the tunable laser adopts distributed feedback laser, quantum cascade laser or vertical cavity surface emission laser. 8 . The surface microstructured silicon cantilever enhanced photothermal spectrum trace gas detection device according to claim 3 , wherein the photodetector is a single-point photodetector. 9. The surface microstructure silicon cantilever beam enhanced photothermal spectrum trace gas detection device according to claim 3, wherein the optical fiber adopts a single-mode silica optical fiber or a polarization-maintaining optical fiber, and its end face is in contact with the microscopic surface of the surface. The metallic surfaces of the structural silicon cantilever form a Fappel cavity. 10 . The surface microstructure silicon cantilever enhanced photothermal spectrum trace gas detection device according to claim 3 , wherein the metal material of the lower layer of the cantilever is gold, silver or aluminum. 11 .

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