Ppb detection rate grade gas detection method and system based on near-infrared light source

Abstract

The invention discloses a ppb detection rate grade gas detection method and system based on a near-infrared light source. According to the structure, a near-infrared tunable laser serves as a light source to be matched with an erbium-doped optical fiber amplifier to amplify laser light intensity, a nonlinear resonance photoacoustic cell with a high quality factor serves as a gas chamber and a resonant cavity to amplify photoacoustic signals, and interference of water vapor on an optical mirror surface is avoided while an effective absorption path is improved by being matched with a near-concentric optical multi-pass structure. The carbon monoxide photoacoustic detection system of the ppb detection rate grade gas detection method based on the near-infrared light source is autonomously established and comprises a laser controller (1), a near-infrared laser source (2), a collimator (3), a nonlinear resonance photoacoustic cell (4), an optical multi-pass structure (5), a microphone (6), an erbium-doped optical fiber amplifier (7), a lock-in amplifier (8), an acquisition card (9) and an upper computer (10). The system has the advantages of simultaneous detection of various gases, high detection rate and the like.

Description

Technical Field

[0001] The invention belongs to the field of laser technology and relates to a ppb detection rate level gas detection method based on a near-infrared light source. Background Art

[0002] Photoacoustic spectroscopy (PAS) has been widely studied due to its advantages such as compact structure, fast response speed, low sample gas consumption, high detection rate, and continuous real-time and in situ monitoring.

[0003] The PAS-based carbon monoxide sensor utilizes quantum cascade laser (QCL) to excite the strong fundamental rotational vibration band (4.7 μm) of carbon monoxide in the mid-infrared (MIR), and its minimum detectable limit (MDL) can be as low as ppb level. However, the price of QCL is not easily accepted by most people, and its price is as high as thousands of dollars. In addition, the threshold current of QCL is large, so the power consumption is high, which is about several watts. Therefore, the operation of QCL requires additional active air cooling or water cooling to meet the heat dissipation requirements that match the high power consumption, which makes the system more complicated and expensive, and is not suitable for widespread application.

[0004] Near-infrared diode lasers are attractive alternatives to QCLs in gas sensing applications. The typical emission wavelengths of commercially available near-infrared diode lasers are in the spectral range of 0.5 to 2 μm, covering the gas vibration overtone spectrum. They can be easily operated at room temperature, have long lifetimes (>10 years) and are low cost. Their narrow linewidth (<1 MHz), wide tuning range, and strong modulation capability make them ideal for highly selective and low-detection trace gas detection. Typically, they are provided with pigtailed single-mode fibers for easy beam delivery and efficient beam coupling into different gas sensor configurations. The output power of near-infrared diode lasers is only a few tens of milliwatts, but it can be easily boosted to watt levels by using telecom fiber amplifiers. As mentioned above, near-infrared diode lasers have great potential for trace gas detection. However, the gas absorption line intensities in the near-infrared region are 2 to 4 orders of magnitude lower than those in the mid-infrared region, so there are still limitations. Fortunately, techniques to enhance photoacoustic signals have been widely developed and reported. By adopting these techniques, the weaker gas absorption line intensities in the near-infrared region can be compensated. Summary of the invention

[0005] In order to overcome the shortcomings of the prior art, the present invention uses a near-infrared laser to develop a low detection rate photoacoustic sensor to detect trace carbon monoxide. The near-infrared tunable laser source increases the excitation power through an erbium-doped fiber amplifier. The shape of the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell is designed with a hyperbolic cosine function and has a high quality factor, thereby producing a high photoacoustic signal resonance enhancement. The hyperbolic cosine-shaped nonlinear resonant photoacoustic cell is tightly coupled with a near-concentric optical multi-pass structure to further enhance the photoacoustic signal. The present invention compensates for the weak absorption line intensity of carbon monoxide in the near-infrared spectral region and achieves a background noise normalized detection rate of the ppb order.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: A ppb detection rate-level gas detection method based on a near-infrared light source adopts a near-infrared tunable laser source and cooperates with an erbium-doped fiber amplifier to amplify the laser power; a nonlinear resonant photoacoustic cell and a coordinated optical multi-pass structure are adopted.

[0007] The wavelength range of the near-infrared tunable laser source can cover the entire C-band wavelength, or any wavelength within the C-band.

[0008] The gain range of the erbium-doped fiber amplifier covers or belongs to the wavelength of the C band.

[0009] The nonlinear resonance photoacoustic cell contains a nonlinear resonance tube, and the shape of the nonlinear resonance photoacoustic cell is designed to be closely combined with the shape of the light beam in the optical multi-pass structure.

[0010] The nonlinear resonance tube is an axisymmetric closed tube with a variable cross section, and its shape is described by a nonlinear elementary function or an interpolation spline.

[0011] The nonlinear resonance tube has axial rotational symmetry and is composed of a hyperbolic resonance tube and two buffer cavities; the hyperbolic resonance tube adopts a double horn structure, and the symmetry axis is set along the Z axis. The shape of the hyperbolic resonance tube is given by the upper hyperbolic cosine function, and the δ winding coefficient is used to describe the expansion degree of the resonance tube.

[0012] The optical multi-pass structure enables multiple reflections of the laser light, using a concave mirror and / or a mirror structure.

[0013] In the optical multi-pass structure, the reflector is placed outside the photoacoustic cell to prevent the humidity of the gas to be measured from affecting the reflectivity of the reflector.

[0014] The photoacoustic sensing system comprises a laser controller, a near-infrared laser source, a collimator, an erbium-doped fiber amplifier, a nonlinear resonance photoacoustic cell, an optical multi-pass structure, a microphone, a phase-locked amplifier, an acquisition card and a host computer; the laser controller controls the near-infrared laser source, the laser emitted by the near-infrared laser source is amplified by the erbium-doped fiber amplifier, and then enters the nonlinear resonance photoacoustic cell through the collimator, and is reflected back and forth in the optical multi-pass structure, the microphone is placed in the photoacoustic cell to collect photoacoustic signals, and transmits them to the phase-locked amplifier, the phase-locked amplifier transmits the results to the acquisition card, and then the acquisition card sends them to the computer; the acquisition card is also connected to the laser controller and the phase-locked amplifier, and is responsible for the output of the modulation signal and the reference signal; the photoacoustic sensing system is used to realize laser control, photoacoustic signal excitation and resonance enhancement, effective absorption path enhancement, microphone signal preamplification, phase-locked amplification detection, and display of carbon monoxide concentration inversion results based on LabVIEW program. Beneficial effects of the present invention

[0015] The present invention provides a ppb detection rate-level gas detection method based on a near-infrared light source: 1) achieving enhancement of near-infrared gas detection photoacoustic signals; 2) achieving enhancement of effective absorption paths while avoiding interference of water vapor on optical mirrors; 3) achieving low detection rate detection of absorbing gases in the C band.

[0016] The carbon monoxide gas photoacoustic sensing system constructed by the present invention and using a ppb detection rate-level gas detection method based on a near-infrared light source realizes low detection rate detection of carbon monoxide, and the background noise normalized detection rate can reach the ppb level. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the structure of the hyperbolic cosine nonlinear resonance photoacoustic cell.

[0018] Figure 2 Schematic diagram of a nearly concentric optical multi-pass structure.

[0019] Figure 3 Schematic diagram of the structure of the ppb detection rate level gas photoacoustic sensing system based on near-infrared light source.

[0020] In the figure, there are a laser controller 1, a laser source 2, a collimator 3, a nonlinear resonant photoacoustic cell 4, an optical multi-pass structure 5, a microphone 6, an erbium-doped fiber amplifier 7, a phase-locked amplifier 8, an acquisition card 9 and a host computer 10. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the scope of the present invention. Example 1: Construction of a carbon monoxide gas photoacoustic sensing system based on a near-infrared light source with a ppb detection rate level gas detection method

[0022] In order to achieve low detection rate detection of carbon monoxide gas, a carbon monoxide gas photoacoustic sensing system with ppb detection rate level gas detection method based on near-infrared light source was built.

[0023] Step 1: Design a hyperbolic cosine-shaped nonlinear resonant photoacoustic cell, which consists of a hyperbolic resonant tube and two buffer cavities. Figure 1 The shape of the hyperbolic resonant tube in the photoacoustic cell can be given by the upper hyperbolic cosine function, where the δ winding coefficient is used to describe the expansion degree of the resonant tube. Finite element analysis was performed based on the harmonic fully linearized Navier-Stokes equations to simulate the pressure distribution in the photoacoustic cell. Finally, the resonant tube length, resonant tube radius, winding coefficient and buffer cavity length of the hyperbolic cosine-shaped nonlinear resonant photoacoustic cell were optimized to 40 mm, 3 mm, and 120 mm, respectively. -1 and 12 mm, the resonance frequency is 6871 Hz and the high quality factor is 90.4. The nonlinear resonance photoacoustic cell can be any shape other than a cylinder.

[0024] Step 2: Figure 2 What is shown is a near-concentric optical multi-pass structure. The near-concentric optical multi-pass structure consists of two silver-coated concave mirrors with a focal length of 25 mm and a diameter of 25.4 mm. The two concave mirrors are placed coaxially and 100 mm apart to meet the near-concentric condition. The number of reflections is determined by the incident distance, the incident angle and the tilt angle of the mirror. By adjusting these three parameters to 0.6 mm, 13.48° and 0.19° respectively, up to 20 reflections can be achieved. The near-concentric optical multi-pass structure can use concave mirrors with different focal lengths, and can be composed of other structures or mirrors.

[0025] Step 3: Figure 3 This is a structural diagram of a carbon monoxide gas photoacoustic sensing system based on a near-infrared light source and a ppb detection rate-level gas detection method. The temperature of the detection laser (1566 nm butterfly distributed feedback tunable laser) is stabilized at 25°C by a laser temperature controller. The acquisition card provides modulation and scanning signals. A modulation signal consisting of a 0.1 Hz sawtooth wave and a 3232 Hz sine wave is generated and sent to the laser controller to drive the laser so that the laser scans the carbon monoxide absorption peak and modulates.

[0026] Step 4: The laser is amplified by an erbium-doped fiber amplifier and then emitted. The laser beam is collimated by a collimator and coupled to a nearly concentric optical multi-pass structure to achieve multiple reflections.

[0027] Step 4: The carbon monoxide gas to be measured absorbs the laser after passing into the photoacoustic cell to generate a photoacoustic signal, and the microphone collects the photoacoustic signal.

[0028] Step 5: The signal collected by the microphone is input to the phase-locked amplifier. The phase-locked amplifier demodulates the second harmonic signal according to the reference signal provided by the acquisition card, and collects it to the host computer through the data acquisition card. The LabVIEW program of the host computer realizes the inversion of signal to concentration and the display of results.

[0029] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0030] The above-mentioned implementation case only expresses one embodiment of the present invention, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A ppb detection rate level gas detection method based on near infrared light source, characterized in that A near-infrared tunable laser source is used in conjunction with an erbium-doped fiber amplifier to amplify the laser light intensity, and a nonlinear resonance photoacoustic cell is used in conjunction with an optical multi-pass structure to increase the absorption path; the nonlinear resonance photoacoustic cell contains a nonlinear resonance tube, and the shape of the nonlinear resonance photoacoustic cell is designed to be closely combined with the shape of the light beam in the optical multi-pass structure.

2. The method according to claim 1, characterized in that The wavelength range of the near-infrared tunable laser source can cover the entire C-band wavelength or any wavelength within the C-band.

3. The method according to claim 1, characterized in that The gain range of the erbium-doped fiber amplifier covers or belongs to the entire C-band wavelength.

4. The method according to claim 1, characterized in that: The nonlinear resonance tube is an axisymmetric closed tube with a variable cross section, and its shape is described by a nonlinear elementary function or an interpolation spline.

5. The method according to claim 1, characterized in that The optical multi-pass structure enables multiple reflections of the laser light, using a concave mirror and / or a mirror structure.

6. The method according to claim 1, characterized in that In the optical multi-pass structure, the reflector is placed outside the photoacoustic cell to prevent the humidity of the gas to be measured from affecting the reflectivity of the reflector.

7. A ppb detection rate level gas detection photoacoustic sensing system based on near infrared light source, characterized in that: It comprises a nonlinear resonance photoacoustic cell and an optical multi-pass structure; a near-infrared light source cooperates with an erbium-doped fiber amplifier to perform power amplification; the nonlinear resonance photoacoustic cell contains a nonlinear resonance tube, and the shape of the nonlinear resonance photoacoustic cell cooperates closely with the shape of the light beam in the optical multi-pass structure.

8. The photoacoustic sensing system according to claim 7, characterized in that: The invention comprises a laser controller, a near-infrared laser source, a collimator, an erbium-doped fiber amplifier, a nonlinear resonance photoacoustic cell, an optical multi-pass structure, a microphone, a phase-locked amplifier, an acquisition card and a host computer; the laser controller controls the near-infrared laser source, the laser emitted by the near-infrared laser source is amplified by the erbium-doped fiber amplifier, then enters the nonlinear resonance photoacoustic cell through the collimator, and is reflected back and forth in the optical multi-pass structure; the microphone is placed in the photoacoustic cell to collect photoacoustic signals, and transmits them to the phase-locked amplifier; the phase-locked amplifier transmits the results to the acquisition card, and then the acquisition card sends them to the computer; the acquisition card is also connected to the laser controller and the phase-locked amplifier, and is responsible for the output of the modulation signal and the reference signal; the photoacoustic sensing system is used to realize laser control, photoacoustic signal excitation and resonance enhancement, effective absorption path enhancement, microphone signal preamplification, phase-locked amplification detection, and display of carbon monoxide concentration inversion results based on LabVIEW program.

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