Spectral trace gas detection device and method based on lithium niobate integrated chip

Abstract

The invention discloses a spectrum trace gas detection device and method based on a lithium niobate integrated chip, and the device comprises a signal generator, a tunable laser, a conical lens fiber, a gas chamber, a lithium niobate integrated chip, a signal amplifier, a data collector, and a computer. The lithium niobate integrated chip comprises a lithium niobate optical waveguide, a lithium niobate tuning fork and a signal export electrode. According to the invention, the tuning fork is innovatively miniaturized and is integrated with the optical waveguide on the lithium niobate sheet. After the laser is incident to the optical waveguide, the laser is irradiated to the side surface of the micro lithium niobate tuning fork by utilizing the interaction of an evanescent field and external gas in the transmission process after being emitted from the optical waveguide, and an electric signal generated after the micro lithium niobate tuning fork vibrates due to the light-induced thermal effect is led out by an electrode on the tuning fork. The tuning fork and optical waveguide integrated structure can significantly reduce the size of the system, does not need to adjust the laser light path, and improves the portability of the sensor.

Description

Technical Field

[0001] This invention relates to a spectral trace gas detection device and method, specifically to a spectral trace gas detection device and method based on a lithium niobate integrated chip. Background Technology

[0002] Photothermal elastic spectroscopy utilizes a quartz tuning fork as the sensing element, combining the photothermal elastic effect with the high quality factor of the quartz tuning fork to significantly improve the system's detection performance. This technology boasts advantages such as fast response, high sensitivity, good selectivity, and online monitoring capabilities, thus gaining widespread application. Despite the advancements in photothermal elastic spectroscopy, traditional systems suffer from large component sizes and low integration. Furthermore, the piezoelectric coefficient of traditional quartz tuning forks is inferior to other piezoelectric materials, limiting the maximum piezoelectric signal they can generate. These factors collectively restrict the system's detection performance and compactness, making the detection of low-concentration gases in confined spaces a continued challenge.

[0003] The traditional photothermoelastic spectroscopy sensing technique works by injecting a modulated laser into a gas chamber. The gas being measured absorbs the laser energy according to the Lambert-Beer law. The emitted laser, after being focused, directly illuminates the base of the interdigitated fingers on the surface of a quartz tuning fork. Due to the photothermoelastic effect, the modulated laser causes the quartz tuning fork to undergo periodic thermal expansion. When the modulation frequency of the laser matches the resonant frequency of the quartz tuning fork, the fork resonates, increasing the vibration amplitude. Due to the piezoelectric effect of quartz, charge accumulates on the vibrating surface of the tuning fork, generating a current signal. By demodulating and analyzing the collected current signal, the gas concentration information can be deduced. Researchers have explored various approaches to improve the performance of photothermoelastic spectroscopy, including designing multi-path cells, optimizing the quartz tuning fork structure, and increasing optical power. In designing multi-path cells, this mainly involves using double-sided mirrors to create a multi-reflection cavity to increase the interaction distance between the light and the gas, thereby increasing the signal amplitude. In terms of quartz tuning fork structure optimization, the main approaches include designing quartz tuning forks of different shapes to increase the amplitude of the electrical signal on the tuning fork surface and reducing the resonant frequency of the quartz tuning fork to increase the energy accumulation time, thereby increasing the signal amplitude. Regarding increasing optical power, increasing laser power allows gas molecules to absorb more energy, thus linearly increasing the intensity of the signal being measured.

[0004] Current photothermal spectroscopy detection technology still has the following core problems:

[0005] (1) The system has low integration and its size is difficult to further reduce. Optical components such as collimating lenses, focusing lenses, quartz tuning forks, and multi-path cells are still independent parts that need to be assembled and fixed by mechanical brackets. The assembly gaps and connection structures between the parts result in a large overall system size, which is difficult to meet the integration requirements of portable devices.

[0006] (2) The optical path adjustment is complex and the stability is poor. The distributed optical path requires adjustment of the relative positions of the laser, tuning fork and gas chamber. The adjustment process is time-consuming and sensitive to environmental vibration. In the field detection scenario, slight vibration can cause the optical path to deviate, requiring repeated calibration, which seriously affects the continuity and accuracy of detection and reduces the detection accuracy of the system.

[0007] (3) The piezoelectric coefficient of the quartz tuning fork used is too low. The commonly used quartz piezoelectric coefficient is only 2.3 pC / N, and the electromechanical coupling coefficient is 0.3, resulting in weak signal strength of the sensing system.

[0008] (4) While increasing laser power can enhance signal strength, it also significantly increases system power consumption. The operation of high-power lasers requires a larger capacity power supply, which not only increases the size and weight of the equipment but also significantly increases energy consumption costs. In addition, increasing laser power will significantly exacerbate the generation of thermal noise, which will interfere with the detection signal, reduce the signal-to-noise ratio of the system, and thus affect the accuracy and reliability of the detection. Summary of the Invention

[0009] To address the aforementioned problems with existing photothermal elastic spectroscopy detection technologies, this invention provides a spectral trace gas detection device and method based on a lithium niobate integrated chip.

[0010] The objective of this invention is achieved through the following technical solution:

[0011] A trace gas detection device based on a lithium niobate integrated chip includes a signal generator, a tunable laser, a conical lens fiber, a gas chamber, a lithium niobate integrated chip, a signal amplifier, a data acquisition unit, and a computer, wherein:

[0012] The lithium niobate integrated chip is placed in the gas chamber. The lithium niobate integrated chip includes a lithium niobate optical waveguide, a lithium niobate tuning fork, and a signal output electrode. The lithium niobate optical waveguide and the lithium niobate tuning fork are distributed at both ends of the lithium niobate integrated chip. The bottom surface of the lithium niobate tuning fork is hollowed out and suspended. It is connected to the chip substrate by the root of the tuning fork. The signal output electrode is etched on the upper surface of the vibrating arm of the lithium niobate tuning fork.

[0013] The signal generator drives a tunable laser to output laser light and modulates the laser. The laser light is incident on a lithium niobate optical waveguide using a conical lens fiber. During transmission in the lithium niobate optical waveguide, the laser interacts with the gas to be measured and then exits to illuminate the side of a lithium niobate tuning fork. The lithium niobate tuning fork is forced to vibrate periodically, generating a piezoelectric signal. The piezoelectric signal is extracted by the signal output electrode, and the piezoelectric signal is collected and amplified by a signal amplifier, outputting a voltage signal. The data acquisition unit collects the amplified voltage signal and transmits the collected data to a computer, which then calculates the gas concentration.

[0014] A method for spectral trace gas detection based on a lithium niobate integrated chip using the above-mentioned device includes the following steps:

[0015] Step 1: Start the signal generator, set the output signal parameters, and drive the tunable laser to output laser with a specific wavelength and power to ensure that the laser power after transmission meets the minimum power requirement for the vibration of the tuning fork.

[0016] Step 2: After the laser is output from the tunable laser, it enters the tapered lens fiber. The position of the fiber end face and the lithium niobate integrated chip is adjusted to ensure that the beam is incident in-plane into the lithium niobate optical waveguide to achieve optimal beam coupling.

[0017] Step 3: The lithium niobate integrated chip is placed in a sealed gas chamber, the internal gas pressure is kept constant, and the gas to be tested is introduced into the chamber.

[0018] Step 4: The gas to be tested interacts with the modulated laser. When the light from the lithium niobate waveguide is emitted to the side of the lithium niobate tuning fork, it begins to vibrate due to the photothermoelastic effect.

[0019] Step 5: The piezoelectric signal generated by the forced vibration of the lithium niobate tuning fork is extracted by the signal extraction electrode and transmitted to the signal amplifier. The amplification factor of the signal amplifier is adjusted according to the signal strength to amplify the piezoelectric signal into a voltage signal with a suitable amplitude.

[0020] Step 6: The computer is responsible for controlling the data acquisition unit. Based on the preset sampling frequency and A / D conversion accuracy, it accurately acquires the amplified voltage signal and efficiently transmits the acquired data to the computer.

[0021] Step 7: The computer analyzes and processes the collected data using the calibrated linear relationship, retrieves the concentration information of the gas to be measured, and displays and stores it.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. This invention utilizes lithium niobate crystals to fabricate optical waveguides and miniature lithium niobate tuning forks. Lithium niobate, as an important ferroelectric crystal, has a piezoelectric coefficient of up to 30 pC / N, which is 13 times that of quartz. The electromechanical coupling coefficient of lithium niobate is 0.68, which is also significantly better than that of quartz. Lithium niobate has become one of the preferred materials in the field of photonic chips.

[0024] 2. Compared with traditional photothermoelastic spectroscopy, this invention innovatively miniaturizes the tuning fork and integrates it with an optical waveguide on a lithium niobate sheet. After the laser is incident on the optical waveguide, it interacts with the external gas using an evanescent field during transmission. The laser beam then exits from the waveguide and illuminates the side of the miniature lithium niobate tuning fork. The electrical signal generated by the vibration of the miniature lithium niobate tuning fork due to the photothermal effect is extracted by the electrodes on the tuning fork. This integrated structure of the tuning fork and optical waveguide significantly reduces the size of the system, eliminates the need for laser path adjustment, and thus improves the portability of the sensor. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a trace gas detection device based on a lithium niobate integrated chip.

[0026] Figure 2 A top view of a lithium niobate integrated chip;

[0027] Figure 3 for Figure 2 The main view. Detailed Implementation

[0028] 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.

[0029] This invention provides a trace gas detection device based on a lithium niobate integrated chip, such as... Figure 1 As shown, the device includes a signal generator 1, a tunable laser 2, a conical lens fiber optic 3, a gas chamber 4, a lithium niobate integrated chip 5, a signal amplifier 6, a data acquisition unit 7, and a computer 8. Figure 2 and Figure 3As shown, the lithium niobate integrated chip 5 includes a lithium niobate optical waveguide 5-1, a lithium niobate tuning fork 5-2, and a signal output electrode 5-3. The lithium niobate optical waveguide 5-1 and the lithium niobate tuning fork 5-2 are distributed at both ends of the lithium niobate integrated chip 5 and are both air-clad. The lithium niobate optical waveguide 5-1 is arranged in a curved structure. The bottom surface of the lithium niobate tuning fork 5-2 is hollowed out and suspended to ensure vibration. It is connected to the chip substrate by the root of the tuning fork. The signal output electrode 5-3 is etched on the upper surface of the lithium niobate tuning fork 5-2. The signal generator 1 drives the tunable laser 2 to output laser light and modulates the laser light. The laser light is incident on the lithium niobate optical waveguide 5-1 placed in the gas chamber 4 using a conical lens fiber 3. After the laser light interacts with the gas during transmission in the lithium niobate optical waveguide 5-1, it is emitted and irradiates the side of the lithium niobate tuning fork 5-2. Because the modulated laser light irradiates the side wall of the lithium niobate tuning fork 5-2, the tuning fork begins to vibrate periodically due to the photothermoelastic effect. The lithium niobate tuning fork 5-2 generates a piezoelectric signal through forced vibration. The signal extraction electrode 5-3 on the surface extracts the piezoelectric signal. A signal amplifier 6 collects and amplifies the piezoelectric signal, outputting a voltage signal. A data acquisition unit 7 collects the amplified voltage signal and transmits the data to a computer 8. The computer 8 then calculates the gas concentration. Detailed implementation process:

[0030] Step 1: Fabrication of lithium niobate integrated chip 5:

[0031] Step 1: Select an X-cut lithium niobate crystal substrate and ultrasonically clean it sequentially with acetone, ethanol, and deionized water for 10-15 minutes to remove surface oil and impurities. Dry it for later use.

[0032] Steps 1 and 2: Fabrication of lithium niobate optical waveguides using electron beam etching process 5-1: After coating the substrate surface with adhesive, exposure, and development, the desired waveguide structure is formed;

[0033] Step 13: Fabrication of miniature lithium niobate tuning forks using deep reactive ion etching process 5-2: After coating, exposing, and developing the corresponding area of ​​the substrate, etching is performed to form the tuning fork arm and root structure. The etching depth is 50-80% of the substrate thickness to ensure a stable connection between the tuning fork and the substrate.

[0034] Step 14: Fabrication of signal output electrodes using vapor deposition process 5-3: Au layers are vapor deposited on both sides of the tuning fork arm, and the electrode pattern is defined by photolithography to complete the electrode fabrication.

[0035] Step 2: Start signal generator 1, set output signal parameters, drive tunable laser 2 to output laser with specific wavelength and power, and ensure that the laser power after transmission meets the minimum power requirement for the vibration of the tuning fork.

[0036] Step 3: After the laser is output from the tunable laser 2, it enters the conical lens fiber 3. The position of the fiber end face and the lithium niobate integrated chip 5 is adjusted to ensure that the laser beam is incident in-plane into the lithium niobate optical waveguide 5-1 to achieve optimal beam coupling.

[0037] Step 4: The lithium niobate integrated chip 5 is placed in the sealed gas chamber 4, the internal gas pressure of the gas chamber is kept constant, and the gas to be tested is introduced into the chamber.

[0038] Step 5: The gas to be tested interacts with the modulated laser. When it exits from the lithium niobate optical waveguide 5-1 and reaches the side of the lithium niobate tuning fork 5-2, it begins to vibrate due to the photothermoelastic effect.

[0039] Step 6: The piezoelectric signal generated by the forced vibration of the lithium niobate tuning fork 5-2 is extracted by the signal extraction electrode 5-3 and transmitted to the signal amplifier 6. The amplification factor of the signal amplifier 6 is adjusted according to the signal strength to amplify the piezoelectric signal into a voltage signal with a suitable amplitude.

[0040] Step 7: The computer 8 is responsible for controlling the data acquisition unit 7. Based on the preset sampling frequency and A / D conversion accuracy, it accurately acquires the amplified voltage signal and efficiently transmits the acquired data to the computer 8.

[0041] Step 8: The computer uses the calibrated linear relationship to analyze and process the collected data, retrieve the concentration information of the gas to be measured, and display and store it.

[0042] In this invention, the lithium niobate integrated chip 5 uses an X-cut lithium niobate crystal with a substrate thickness of 0.5~1.0 mm and a side length of 8~10 mm. The lithium niobate optical waveguide 5-1 is fabricated using electron beam etching technology, with a waveguide width of 0.5~2 μm, a depth of 0.3~1 μm, and a total length of 5~10 cm. It employs a bent waveguide structure with a bending radius of 100~150 μm, a transmission loss of less than 0.5 dB / cm, and a coupling loss of less than 6 dB / cm. The lithium niobate tuning fork 5-2 has interdigitated fingers with a length of 0.3~0.5 cm, a width of 0.5~1.0 mm, and a natural frequency of less than 10 kHz. The signal output electrode 5-3 is fabricated using an evaporation process with Au material, an electrode thickness of 100~200 nm, and covers more than 60% of the tuning fork interdigitated finger surface.

[0043] In this invention, in order to generate a larger acoustic signal and improve detection sensitivity, the incident laser power should be >30 mW.

[0044] In this invention, the end face diameter of the tapered lens fiber 3 should be 2~5 μm to ensure that the light spot can be coupled into the lithium niobate integrated chip 5 at a higher ratio.

[0045] In this invention, the interior of the air chamber 4 needs to be polished to a surface roughness of less than 0.1 μm, so as to reduce the interference of scattered light reflection on the inner wall of the air chamber. At the same time, the sealing performance of the air chamber needs to ensure that the internal air pressure fluctuation is within ±0.1 kPa.

[0046] In this invention, in order for the lithium niobate tuning fork 5-2 to receive light waves and photothermal elasticity more effectively, the natural frequency of its vibrating arm needs to match the absorption frequency generated by the target gas.

[0047] In this invention, the amplification factor of the signal amplifier 6 must be adjustable between 10 and 1000 times, and the noise it introduces should be less than 1 μV, so as to fully meet the amplification requirements of signals of different intensities and ensure high-quality signal transmission.

[0048] In this invention, the sampling frequency of the data acquisition unit 7 must be higher than 2.5 times the highest frequency of the target signal to ensure the integrity and accuracy of the acquired data. Furthermore, its A / D conversion accuracy should be no less than 16 bits.

Claims

1. A trace gas detection device based on a lithium niobate integrated chip, characterized in that... The device includes a signal generator, a tunable laser, a conical lens fiber, a gas chamber, a lithium niobate integrated chip, a signal amplifier, a data acquisition unit, and a computer, wherein: The lithium niobate integrated chip is placed in a gas chamber and includes a lithium niobate optical waveguide, a lithium niobate tuning fork, and a signal output electrode. The lithium niobate optical waveguide and the lithium niobate tuning fork are distributed at both ends of the lithium niobate integrated chip. The bottom surface of the lithium niobate tuning fork is hollowed out and suspended. It is connected to the chip substrate by the root of the tuning fork. The signal output electrode is etched on the upper surface of the vibrating arm of the lithium niobate tuning fork. The signal generator drives a tunable laser to output laser light and modulates the laser. The laser light is incident on a lithium niobate optical waveguide using a conical lens fiber. During transmission in the lithium niobate optical waveguide, the laser interacts with the gas to be measured and then exits to illuminate the side of a lithium niobate tuning fork. The lithium niobate tuning fork is forced to vibrate periodically, generating a piezoelectric signal. The piezoelectric signal is extracted by the signal output electrode, and the piezoelectric signal is collected and amplified by a signal amplifier, outputting a voltage signal. The data acquisition unit collects the amplified voltage signal and transmits the collected data to a computer, which then calculates the gas concentration.

2. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The lithium niobate integrated chip uses an X-cut lithium niobate crystal with a substrate thickness of 0.5–1.0 mm and a side length of 8–10 mm. The lithium niobate optical waveguide has a width of 0.5–2 μm, a depth of 0.3–1 μm, and a total length of 5–10 cm. It adopts a bent waveguide structure with a bending radius of 100–150 μm, a transmission loss of less than 0.5 dB / cm, and a coupling loss of less than 6 dB / cm. The lithium niobate tuning fork has a finger length of 0.3–0.5 cm, a width of 0.5–1.0 mm, and a natural frequency of less than 10 kHz. The signal output electrode is made of Au with a thickness of 100–200 nm, covering more than 60% of the tuning fork finger surface.

3. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1 or 2, characterized in that... The method for fabricating the lithium niobate integrated chip is as follows: Step 1: The chip substrate is ultrasonically cleaned with acetone, ethanol and deionized water for 10-15 minutes in sequence to remove surface oil and impurities, and then dried for later use. Step 2: Fabricate lithium niobate optical waveguides using electron beam etching process; Step 3: Prepare lithium niobate tuning forks using deep reactive ion etching process; Step 4: Prepare the signal extraction electrode using a vapor deposition process.

4. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The laser power is >30 mW.

5. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The end face diameter of the tapered lens fiber is 2~5 μm.

6. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The interior of the air chamber is polished, with a surface roughness of less than 0.1 μm, and the internal air pressure fluctuation is within the range of ±0.1 kPa.

7. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The natural frequency of the vibrating arm of the lithium niobate tuning fork is matched with the absorption frequency generated by the gas being tested.

8. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The signal amplifier has an adjustable amplification factor between 10 and 1000 times, and its noise is less than 1 μV.

9. The spectral trace gas detection device based on a lithium niobate integrated chip according to claim 1, characterized in that... The sampling frequency of the data acquisition device is 2.5 times higher than the highest frequency of the target signal, and its A / D conversion accuracy should be no less than 16 bits.

10. A method for detecting trace gases using the above-mentioned apparatus based on a lithium niobate integrated chip, comprising the following steps: Step 1: Start the signal generator, set the output signal parameters, and drive the tunable laser to output laser light of a specific wavelength and power to ensure that the transmitted laser power meets the minimum power requirement for the vibration of the tuning fork. Step 2: After the laser is output from the tunable laser, it enters the tapered lens fiber. The position of the fiber end face and the lithium niobate integrated chip is adjusted to ensure that the beam is incident in-plane into the lithium niobate optical waveguide to achieve optimal beam coupling. Step 3: The lithium niobate integrated chip is placed in a sealed gas chamber, the internal gas pressure is kept constant, and the gas to be tested is introduced into the chamber. Step 4: The gas to be tested interacts with the modulated laser. When the light from the lithium niobate waveguide is emitted to the side of the lithium niobate tuning fork, it begins to vibrate due to the photothermoelastic effect. Step 5: The piezoelectric signal generated by the forced vibration of the lithium niobate tuning fork is extracted by the signal extraction electrode and transmitted to the signal amplifier. The amplification factor of the signal amplifier is adjusted according to the signal strength to amplify the piezoelectric signal into a voltage signal with a suitable amplitude. Step 6: The computer is responsible for controlling the data acquisition unit, accurately acquiring the amplified voltage signal according to the preset sampling frequency and A / D conversion accuracy, and efficiently transmitting the acquired data to the computer. Step 7: The computer analyzes and processes the collected data using the calibrated linear relationship, retrieves the concentration information of the gas to be measured, and displays and stores it.

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