Distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers

Abstract

This invention discloses a distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow-core optical fiber. The device includes a semiconductor laser, a hollow-core photonic crystal fiber, a quartz tuning fork, an impedance amplifier, a control and data acquisition system, and a computer. The laser light generated by the semiconductor laser is transmitted into the hollow-core photonic crystal fiber. The gas to be tested enters the hollow-core photonic crystal fiber through the micropores and absorbs the laser energy, causing localized heating and generating a photoacoustic signal. This photoacoustic signal overflows from the micropores, causing the quartz tuning fork to vibrate periodically, generating a current signal. This current signal is converted into a voltage signal by the impedance amplifier, acquired and demodulated by the control and data acquisition system, and finally processed by the computer to determine the concentration of the gas to be tested. The detection device of this invention has advantages such as high sensitivity, low loss, and low cost.

Description

Technical Field

[0001] This invention relates to a spectral trace gas detection device, specifically a distributed photoacoustic spectral trace gas detection device based on micropores in hollow optical fibers. Background Technology

[0002] The detection of trace gases (gases with a volume fraction much less than 1%) is of great significance in fields such as medical diagnosis, atmospheric environmental monitoring, and manned spaceflight. Quartz-enhanced photoacoustic spectroscopy (QEPAS) is an important spectroscopic method for the detection of trace gases, possessing advantages such as small size, high selectivity, and low cost. In recent years, combining optical fiber with QEPAS has gradually become a hot topic. However, multi-point spatial distributed measurement based on optical fiber remains a technical challenge. Light propagates in optical fiber according to the principle of total internal reflection. When light reaches the interface between the fiber core and cladding, it does not return entirely to the first medium; a trace amount of light wave is injected into the second medium, known as an evanescent wave. The smaller the diameter of the fiber cladding, the greater the intensity of the evanescent wave. Therefore, the optical fiber needs to be drawn to form a fiber taper. The fiber taper is placed between the two strands of a quartz tuning fork, and then both are placed in an environment filled with the gas to be measured. A tunable laser propagates through an optical fiber to the fiber taper, generating an evanescent wave. The gas absorbs some of the energy of the evanescent wave, causing a local temperature increase, which in turn generates sound waves. This ultimately causes the quartz tuning fork to vibrate periodically. The quartz tuning fork has its maximum vibration amplitude when the frequency of the sound wave equals the resonant frequency of the quartz tuning fork. The sound waves in the gap between the forks of the quartz tuning fork cause it to vibrate symmetrically. According to the piezoelectric effect of the quartz tuning fork, this symmetrical vibration generates a current signal. Finally, the concentration of the gas being measured can be determined by demodulating the generated current signal.

[0003] In quartz-enhanced photoacoustic spectroscopy, the photoacoustic signal intensity is directly proportional to the optical power, that is:

[0004] S∝k×P (1)

[0005] Where S is the intensity of the photoacoustic signal, k is the proportionality coefficient, and P is the laser power. In the quartz-enhanced photoacoustic spectroscopy technology based on fiber evanescent waves, increasing the signal intensity of the evanescent wave can increase the intensity of the photoacoustic signal.

[0006] Studies have shown that the smaller the cladding diameter of an optical fiber, the greater the intensity of the evanescent wave. When the cladding diameter is greater than 2 μm, the intensity of the generated evanescent wave changes slowly, and the proportion of the evanescent wave intensity to the total laser power is less than 10%. Therefore, in order to improve the intensity of the photoacoustic signal, the diameter of the drawn fiber taper should be less than 2 μm. In this way, after proportionally reducing the overall structure of the optical fiber, the core diameter is approximately 0.1 μm, while the normal core diameter is 9–125 μm. Drawing the fiber taper will inevitably affect its toughness, making the taper region too taut, thereby reducing the fiber's resistance to external interference. Even slight shaking can easily lead to fiber breakage.

[0007] Furthermore, uneven core diameter variations during fiber drawing cause losses, and dust accumulation on the fiber taper surface leads to evanescent wave leakage. In short, generating evanescent waves through fiber drawing introduces numerous unavoidable losses, thus limiting the sensor's detection limits and sensitivity. Summary of the Invention

[0008] To improve the detection limit and sensitivity of sensors, this invention provides a distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers. This detection device has advantages such as high sensitivity, low loss, and low cost.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] A distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fiber includes a semiconductor laser, a hollow photonic crystal fiber, a quartz tuning fork, an impedance amplifier, a control and data acquisition system, and a computer, wherein:

[0011] The hollow-core photonic crystal fiber has fine holes punched in its side;

[0012] The hollow-core photonic crystal fiber is placed horizontally in the center of the two prongs of the quartz tuning fork, ensuring that the two prongs of the quartz tuning fork and the perforated part of the hollow-core photonic crystal fiber are on the same horizontal line.

[0013] The perforated portion of the hollow photonic crystal fiber and the quartz tuning fork are placed in an environment filled with the gas to be tested.

[0014] The control and data acquisition system modulates the laser wavelength output by the semiconductor laser. The laser light generated by the semiconductor laser is transmitted to a hollow photonic crystal fiber. The gas to be tested enters the hollow photonic crystal fiber through a small hole and absorbs the laser energy, causing local heating and generating a photoacoustic signal. The photoacoustic signal overflows from the small hole, causing a quartz tuning fork to vibrate periodically. According to the piezoelectric effect of the quartz tuning fork, the vibration causes the quartz tuning fork to generate a current signal. This current signal is converted into a voltage signal by an impedance amplifier, which is then acquired and demodulated by the control and data acquisition system. Finally, the signal is processed by a computer to determine the concentration of the gas to be tested.

[0015] A method for distributed photoacoustic spectroscopy trace gas detection based on hollow fiber micropores using the above-mentioned device includes the following steps:

[0016] Step 1: The control and data acquisition system superimposes low-frequency sawtooth wave signals and high-frequency sine wave signals to modulate the output wavelength of the semiconductor laser and scan the resonant frequency of the quartz tuning fork.

[0017] Step 2: The laser generated by the semiconductor laser is transmitted to the hollow photonic crystal fiber. The gas to be tested enters the hollow photonic crystal fiber through a small hole, absorbs the laser energy, and generates a photoacoustic signal. The photoacoustic signal overflows from the small hole.

[0018] Step 3: When the photoacoustic signal overflows, it is located in the center of the two prongs of the quartz tuning fork, which in turn causes the quartz tuning fork to vibrate, generating a piezoelectric effect and ultimately producing a current signal.

[0019] Step 4: The current signal generated by the quartz tuning fork is converted into a voltage signal by an impedance amplifier and then amplified;

[0020] Step 5: The amplified voltage signal is acquired and demodulated by the control and data acquisition system, and then processed by the computer to finally determine the concentration of the gas to be measured.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. This invention eliminates the need to draw optical fibers to generate evanescent waves, avoiding unnecessary losses caused by drawing optical fibers and also avoiding the poor toughness of optical fibers caused by drawing optical fibers, which is of great significance for improving the detection performance of sensors.

[0023] 2. This invention utilizes femtosecond lasers to drill holes at different locations on the same optical fiber. Quartz tuning forks are placed at these holes and placed in different environments to achieve multi-point spatial distributed measurement. Attached Figure Description

[0024] Figure 1This is a schematic diagram of a distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers.

[0025] Figure 2 The positional relationship between the fine aperture and the quartz tuning fork strands;

[0026] Figure 3 The results show the distributed measurement of water vapor concentration at three different spatial locations. Detailed Implementation

[0027] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0028] This invention provides a distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers, such as... Figure 1 As shown, the device includes a semiconductor laser 1, a hollow-core photonic crystal fiber 2, a quartz tuning fork 3, an impedance amplifier 4, a control and data acquisition system 5, and a computer 6, wherein:

[0029] The hollow-core photonic crystal fiber 2 has holes punched in its side;

[0030] The hollow-core photonic crystal fiber 2 is placed horizontally in the center of the two prongs of the quartz tuning fork 3, ensuring that the two prongs of the quartz tuning fork 3 and the perforated part of the hollow-core photonic crystal fiber 2 are on the same horizontal line.

[0031] The control and data acquisition system 5 modulates the laser wavelength output by the semiconductor laser 1. The laser generated by the semiconductor laser 1 is transmitted to the hollow-core photonic crystal fiber 2. The perforated part of the hollow-core photonic crystal fiber 2 and the quartz tuning fork 3 are placed in an environment filled with the gas to be tested. The gas to be tested can enter the hollow-core photonic crystal fiber 2 through the small hole and absorb the laser energy, causing local heating and generating a photoacoustic signal. The photoacoustic signal overflows from the small hole. At this time, the photoacoustic signal is exactly located in the center of the two forks of the quartz tuning fork 3. Therefore, the generated photoacoustic signal will cause the quartz tuning fork 3 to vibrate periodically. According to the piezoelectric effect of the quartz tuning fork 3, the vibration will cause the quartz tuning fork 3 to generate a current signal. This current signal is converted into a voltage signal by the impedance amplifier 4, which is then acquired and demodulated by the control and data acquisition system 5. Finally, the signal is processed by the computer 6 to determine the concentration of the gas to be tested.

[0032] The present invention also provides a method for distributed photoacoustic spectroscopy trace gas detection based on hollow fiber micropores using the above-mentioned device, the specific implementation process of which is as follows:

[0033] Step 1: The control and data acquisition system 5 superimposes the low-frequency sawtooth wave signal and the high-frequency sine wave signal to modulate the output wavelength of the semiconductor laser 1 and the resonant frequency of the scanning quartz tuning fork.

[0034] Step 2: The laser generated by semiconductor laser 1 is transmitted to hollow photonic crystal fiber 2. The gas to be tested enters the hollow photonic crystal fiber 2 through a small hole, absorbs the laser energy, and generates a photoacoustic signal. The photoacoustic signal overflows from the small hole.

[0035] Step 3: When the photoacoustic signal overflows, it is located in the center of the two prongs of the quartz tuning fork 3, which in turn causes the quartz tuning fork 3 to vibrate, generating a piezoelectric effect and ultimately producing a current signal.

[0036] Step 4: The current signal generated by the quartz tuning fork 3 is converted into a voltage signal and amplified by the impedance amplifier 4.

[0037] Step 5: The amplified voltage signal is acquired and demodulated by the control and data acquisition system 5, and then processed by the computer 6 to finally determine the concentration of the gas to be measured.

[0038] In this invention, the semiconductor laser 1 is a continuously tunable distributed feedback semiconductor laser, and its output is a single longitudinal mode output.

[0039] In this invention, in order to improve the intensity of the photoacoustic signal, the laser power output by the semiconductor laser 1 should be greater than 15mW.

[0040] In this invention, the hollow-core photonic crystal fiber 2 has an air hole structure that runs through the entire length of the fiber, which can confine more than 90% of the light within it, thus enabling it to transmit both light and gas. This invention utilizes the characteristics of femtosecond lasers—short duration and high pulse power density—to perform femtosecond laser drilling on the side of the hollow-core photonic crystal fiber.

[0041] In this invention, to facilitate the escape of photoacoustic signals from the hollow photonic crystal fiber 2, the diameter of the hole drilled by the femtosecond laser should not be too small, but it should not exceed the diameter of the fiber either; the hole diameter should be between 30 and 50 μm.

[0042] In this invention, the aperture of the hollow-core photonic crystal fiber 2 should be located precisely at the center of the two prongs of the quartz tuning fork 3, and the vertical distance between the aperture and the top of the prongs of the quartz tuning fork 3 should not be too large or too small. If the vertical distance is too small, some sound waves will overflow from the top of the prongs of the quartz tuning fork 3, resulting in a loss of sound wave energy. If the vertical distance is too large, the equivalent torque of the sound waves acting on the quartz tuning fork 3 will decrease, causing a smaller vibration amplitude of the prongs of the quartz tuning fork 3. The vertical distance between the two should be 0.7 mm (e.g., ...). Figure 2 As shown in the figure, the quartz tuning fork 3 has the greatest elastic deformation at this location.

[0043] In this invention, in order to better realize multi-point spatial distribution measurement, the hollow core photonic crystal fiber 2 has no less than 5 holes, and quartz tuning forks 3 are placed at the holes and placed in different environments.

[0044] In this invention, in order to reduce the laser energy loss in the hollow-core photonic crystal fiber 2, the loss of the hollow-core photonic crystal fiber 2 should be less than 0.2 dB / m.

[0045] In this invention, the hollow-core photonic crystal fiber 2 can be replaced with an optical waveguide.

[0046] In this invention, the positions of the hollow photonic crystal fiber 2 and the quartz tuning fork 3 can be set to off-axis, that is, the quartz tuning fork 3 is placed on one side of the hollow photonic crystal fiber 2, so that the axis of the fine hole passes through the center of the two prongs of the quartz tuning fork 3.

[0047] In this invention, since a lower resonant frequency quartz tuning fork 3 is beneficial to the enhancement of photoacoustic signals, the resonant frequency of quartz tuning fork 3 should be less than 30kHz.

[0048] In this invention, the resistance of the transimpedance amplifier 4 should be greater than 1MΩ.

[0049] In this invention, the control and data acquisition system 5 modulates the laser wavelength at a frequency that is half the resonant frequency of the quartz tuning fork 3, and uses second harmonic demodulation technology to demodulate the signal.

[0050] In this invention, the control and data acquisition system 5 is connected to the computer 6, and real-time control and signal processing are performed on the computer 6 through software.

[0051] Example:

[0052] In this embodiment, a semiconductor laser with an output wavelength of 1.395 μm was used. Three micro-holes with a diameter of 50 μm were fabricated on a 5 m long hollow-core photonic crystal fiber. The water vapor concentration in the air was measured in corridors, indoors, and outdoors. The results are as follows: Figure 3 As shown. By Figure 3 It is evident that the distributed photoacoustic spectroscopy gas sensing device based on micropores in hollow optical fibers can achieve spatial distribution measurement of gas concentration.

Claims

1. A distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers, characterized in that... The detection device includes a semiconductor laser, a hollow-core photonic crystal fiber, a quartz tuning fork, a transimpedance amplifier, a control and data acquisition system, and a computer, wherein: The hollow-core photonic crystal fiber has fine holes punched in its side; The hollow-core photonic crystal fiber is placed horizontally in the center of the two prongs of the quartz tuning fork, ensuring that the two prongs of the quartz tuning fork and the perforated part of the hollow-core photonic crystal fiber are on the same horizontal line. The perforated portion of the hollow photonic crystal fiber and the quartz tuning fork are placed in an environment filled with the gas to be tested. The control and data acquisition system modulates the laser wavelength output by the semiconductor laser. The laser light generated by the semiconductor laser is transmitted to a hollow photonic crystal fiber. The gas to be tested enters the hollow photonic crystal fiber through a small hole and absorbs the laser energy, causing local heating and generating a photoacoustic signal. The photoacoustic signal overflows from the small hole, causing a quartz tuning fork to vibrate periodically. According to the piezoelectric effect of the quartz tuning fork, the vibration causes the quartz tuning fork to generate a current signal. This current signal is converted into a voltage signal by a transimpedance amplifier, which is then acquired and demodulated by the control and data acquisition system. Finally, the signal is processed by a computer to determine the concentration of the gas to be tested.

2. The distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers according to claim 1, characterized in that... The semiconductor laser is a continuously tunable distributed feedback semiconductor laser with a single longitudinal mode output and a laser power greater than 15 mW.

3. The distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers according to claim 1, characterized in that... The hollow-core photonic crystal fiber has a loss of less than 0.2 dB / m, the hole diameter of the hollow-core photonic crystal fiber is 30 to 50 μm, the number of holes is not less than 5, and quartz tuning forks are placed at the holes and placed in different environments.

4. The distributed photoacoustic spectroscopy trace gas detection device based on hollow fiber micropores according to claim 1 or 3, characterized in that... The vertical distance between the aperture of the hollow photonic crystal fiber and the tip of the quartz tuning fork strand is 0.7 mm.

5. The distributed photoacoustic spectroscopy trace gas detection device based on hollow fiber micropores according to claim 1, characterized in that... The hollow photonic crystal fiber and the quartz tuning fork are positioned off-axis, that is, the quartz tuning fork is placed on one side of the hollow photonic crystal fiber, so that the axis of the fine hole passes through the center of the two prongs of the quartz tuning fork.

6. The distributed photoacoustic spectroscopy trace gas detection device based on micropores in hollow optical fibers according to claim 1, characterized in that... The resonant frequency of the quartz tuning fork is less than 30 kHz.

7. The distributed photoacoustic spectroscopy trace gas detection device based on hollow fiber micropores according to claim 1, characterized in that... The resistance of the transimpedance amplifier is greater than 1 MΩ.

8. The distributed photoacoustic spectroscopy trace gas detection device based on hollow fiber micropores according to claim 1, characterized in that... The control and data acquisition system modulates the laser wavelength at a frequency that is half the resonant frequency of the quartz tuning fork, and uses second harmonic demodulation technology to demodulate the signal.

9. A method for detecting trace gases using distributed photoacoustic spectroscopy based on micropores in hollow optical fibers using the detection device described in any one of claims 1-8, characterized in that... The method includes the following steps: Step 1: The control and data acquisition system superimposes low-frequency sawtooth wave signals and high-frequency sine wave signals to modulate the output wavelength of the semiconductor laser and scan the resonant frequency of the quartz tuning fork; Step 2: The laser generated by the semiconductor laser is transmitted to the hollow photonic crystal fiber. The gas to be tested enters the hollow photonic crystal fiber through a small hole, absorbs the laser energy, and generates a photoacoustic signal. The photoacoustic signal overflows from the small hole. Step 3: When the photoacoustic signal overflows, it is located in the center of the two prongs of the quartz tuning fork, which in turn causes the quartz tuning fork to vibrate, generating a piezoelectric effect and ultimately producing a current signal. Step 4: The current signal generated by the quartz tuning fork is converted into a voltage signal by a transimpedance amplifier and then amplified; Step 5: The amplified voltage signal is acquired and demodulated by the control and data acquisition system, and then processed by the computer to finally determine the concentration of the gas to be measured.

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