Trace gas photoacoustic detection device based on gain modulation fiber laser

Abstract

The invention discloses a trace gas photoacoustic detection device based on a gain modulation fiber laser, comprising: a mid-infrared fiber laser module for obtaining gain modulation pulse laser output in a mode of modulation pumping; gas to be detected is arranged in the photoacoustic detection module, the generated sound pressure is detected based on the gain modulation pulse laser to obtain a sound signal, and an acoustic resonator in the photoacoustic detection module is matched with the frequency of the gain modulation pulse laser; and the signal acquisition and processing module is used for carrying out data processing and concentration inversion on the sound signal to obtain a concentration detection result. According to the invention, by directly modulating the driving current of the pumping laser diode, the gain modulation operation of the laser is realized, the pulse laser output accurately matched with the resonant frequency of the photoacoustic cell is generated, the internal modulation mode avoids the mechanical or acoustic noise introduced by the traditional external modulator, the system structure is simplified, and the detection sensitivity is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of trace gas detection, and particularly to a trace gas photoacoustic detection device based on a gain-modulated fiber laser. BACKGROUND

[0002] High-sensitivity detection of trace gases has important application value in the fields of environmental monitoring, industrial safety, medical diagnosis, etc. As a high-sensitivity gas detection method, photoacoustic spectroscopy achieves the detection of trace gases by measuring the acoustic signals generated after the absorption of light energy by gas molecules. However, the existing photoacoustic detection systems mainly have the following technical problems: The existing technology generally adopts a technical scheme of "continuous laser + external modulation", that is, a semiconductor laser (DL) or a quantum cascade laser (QCL) is used to output continuous light, and then an external device is used to modulate the laser. The wavelength modulation method or the intensity modulation method based on a chopper / acousto-optic modulator is adopted.

[0003] (1) Limitations of the wavelength modulation method: Only applicable to narrow-line-width gases: The wavelength modulation spectroscopy (WMS) technology relies on high-frequency fine tuning of the laser wavelength, that is, the wavelength modulation method only performs slight modulation near the center wavelength of the laser, so as to scan the sharp absorption peak of the gas. This is effective for gases with very narrow absorption lines (such as many gases in the near-infrared band). However, for gases with wide absorption lines, the depth of wavelength modulation is limited, and the entire absorption line type cannot be effectively covered, resulting in low modulation efficiency, a significant reduction in signal response, and a serious decline in detection sensitivity and accuracy. The modulation range is much smaller than the absorption spectrum range of VOCs and other broadband gases, making it difficult to achieve effective matching and resulting in a significant decline in detection sensitivity.

[0004] System complexity: To achieve high-precision wavelength modulation, the injection current and temperature of the laser need to be precisely controlled, increasing the complexity and cost of the system.

[0005] (2) Disadvantages of the intensity modulation method: Introducing additional noise: When a chopper or an acousto-optic modulator is used for intensity modulation, mechanical chopper method introduces additional mechanical vibration, and acousto-optic modulation method introduces acoustic noise. Both methods introduce additional noise, which seriously degrades the signal-to-noise ratio (SNR).

[0006] At the same time, such modulation methods themselves cause at least 50% of the light power loss, limiting the detection sensitivity and causing a significant decline in the energy utilization efficiency of the system. In addition, these two types of modulation devices are large in size and complex in control, and are easily disturbed by the environment, thereby reducing the integration and long-term stability of the photoacoustic detection system.

[0007] (3) Light source technology limitation: the commonly used photoacoustic detection light source currently mainly adopts a distributed feedback semiconductor laser, but the output power of the laser is low, and the output wavelength is mainly concentrated in the near-infrared band (1-2 mu m), the molecular absorption cross section of the band is small, which limits the detection sensitivity. The mid-infrared band is the molecular fingerprint region, which contains the fundamental frequency vibration absorption peaks of numerous bond chemical bonds, and has a higher absorption cross section, but the existing mid-infrared photoacoustic detection light source has many limitations. The quantum cascade laser cannot realize laser output below 3.5 mu m, and cannot match the C-H stretching vibration wavelength of the core chemical bond of the organic gas, which limits the gas detection category and sensitivity; the band-to-band cascade laser can match many stretching vibrations, but the output power is low, only tens of milliwatts, which limits the improvement of gas detection accuracy; the optical parametric oscillator can realize high-power tunable wavelength output, but its volume is large, the price is high, and the stability is poor, which seriously limits the commercialization and miniaturization development of the photoacoustic detection system.

[0008] Therefore, it is urgent to develop a new type of photoacoustic detection system, which can not only reduce the complexity of the photoacoustic detection system and improve the robustness of the system, but also improve the trace gas detection accuracy of the photoacoustic detection scheme. SUMMARY

[0009] The purpose of the present application is to overcome the shortcomings of the prior art and provide a trace gas photoacoustic detection device based on a gain-modulated fiber laser.

[0010] The purpose of the present application is achieved by the following technical solutions: In a first aspect, the present application provides a trace gas photoacoustic detection device based on a gain-modulated fiber laser, comprising: A mid-infrared fiber laser module obtains gain-modulated pulsed laser output by modulating the pump; A photoacoustic detection module contains a gas to be measured, detects the sound pressure generated based on the gain-modulated pulsed laser to obtain a sound signal, and the acoustic resonator in the photoacoustic detection module is matched with the gain-modulated pulsed laser frequency; A signal acquisition and processing module processes the sound signal and concentration inversion to obtain a concentration detection result.

[0011] Further, the mid-infrared fiber laser module comprises a driving power supply, a pump laser diode, a front cavity mirror, a gain fiber and a rear cavity mirror. The driving power supply provides a modulation current to drive the pump laser diode to realize modulated pulsed laser output, and the modulated pulsed laser output is coupled into the gain fiber, a resonant cavity is constructed by using the front cavity mirror and the rear cavity mirror, and the gain-modulated pulsed laser output is realized.

[0012] Further, the pump laser diode outputs 976 nm pump light, i.e. modulated pulse laser, and the driving power supply generates a square wave modulation current with a frequency of 1 kHz-35 kHz to realize periodic modulation of pump power.

[0013] Further, the photoacoustic detection module comprises a fiber collimator, an acoustic resonator and an electronic amplifier. The collimator is used to collimate laser light to match the light aperture of the acoustic resonator, the acoustic resonator is used to pass the gas to be measured, realize resonance enhancement of acoustic waves, and collect acoustic signals, and the collected acoustic signals are amplified by the electronic amplifier. The periodic modulation of pump power of the driving power supply is matched with the acoustic resonator.

[0014] Further, the acoustic resonator comprises a photoacoustic cell, a quartz tuning fork and a cantilever beam.

[0015] Further, the signal acquisition and processing module comprises a lock-in amplifier, a data acquisition card and a computer. The lock-in amplifier is used for detection of acoustic signals of a specific frequency, the data acquisition card is used for analog-digital signal conversion and transmission, and the computer is used for data processing, storage and inversion of gas concentration information.

[0016] The beneficial effects of the present application are: In an exemplary embodiment of the present application, by directly modulating the driving current of the pump laser diode, gain modulation operation of the laser is realized, and pulse laser output matching the resonance frequency of the photoacoustic cell is generated. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The trace gas photoacoustic detection device based on the gain modulation fiber laser provided in an exemplary embodiment of the present application; Figure 2 The trace gas photoacoustic detection device based on the gain modulation fiber laser provided in an exemplary embodiment of the present application; In the figure, 1 is a driving power supply, 2 is a pump laser diode, 3 is a front cavity mirror, 4 is a gain fiber, 5 is a rear cavity mirror, 6 is a fiber collimator, 7 is an acoustic resonator, 8 is an electronic amplifier, 9 is a lock-in amplifier, 10 is a data acquisition card, and 11 is a computer. DETAILED DESCRIPTION

[0018] The technical solutions of the present application will be described clearly and completely below with reference to the drawings. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0019] In the description of the present application, it should be noted that the directions or positional relationships indicated by "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like are based on the directions or positional relationships described in the drawings, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, "first", "second" are only for the purpose of description and cannot be understood as indicating or implying relative importance.

[0020] In the description of the present application, it should be noted that unless otherwise explicitly specified and limited, "mounting", "connection" and "connection" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0021] In addition, the technical features involved in the different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0022] Reference is made to Figure 1 , Figure 1 The trace gas photoacoustic detection device based on gain-modulated fiber laser provided in an exemplary embodiment of the present application is shown, which comprises: A mid-infrared fiber laser module obtains gain-modulated pulsed laser output by modulating pumping; A photoacoustic detection module has a gas to be measured therein, detects the sound pressure generated based on the gain-modulated pulsed laser to obtain a sound signal, and an acoustic resonator in the photoacoustic detection module is matched with the frequency of the gain-modulated pulsed laser; A signal acquisition and processing module performs data processing and concentration inversion on the sound signal to obtain a concentration detection result.

[0023] Specifically, in the present exemplary embodiment, the mid-infrared fiber laser module obtains gain-modulated pulsed laser output by modulating pumping, and the proposed gain-modulated laser can realize resonator frequency matching directly by modulating pumping, thereby simplifying the system structure and realizing pulsed signal enhancement.

[0024] More specifically, by directly modulating the driving current of the pump laser diode, the gain modulation operation of the laser is realized, and the pulsed laser output that precisely matches the resonance frequency of the photoacoustic cell is generated. This internal modulation method avoids the mechanical or acoustic noise introduced by traditional external modulators, simplifies the system structure, and improves the detection sensitivity. At the same time, by utilizing the pulsed property of the gain-modulated laser, the resonance acoustic response is improved, further reducing the detection limit. In terms of effect: (1) Improved detection sensitivity: Utilizing the high peak power of the pulsed laser, a super-strong acoustic pressure is excited in a short time, achieving a significant enhancement of the fundamental resonance; (2) Promote the miniaturization of the detection structure: without additional modulating devices such as mechanical chopper, acousto-optic modulator, function generator, etc., the system volume is reduced by 30%; (3) Low system cost: the cost of fiber laser is reduced by more than 60% compared with QCL and OPO; (4) Good stability: gain modulation avoids mechanical vibration caused by chopper, and detection limit is reduced by 1.5 times; (5) Wide wavelength coverage: near-infrared gain-modulated fiber laser can achieve 1-2.5 μm wavelength coverage, and mid-infrared gain-modulated fiber laser can achieve 2.7-3.9 μm wavelength coverage, covering the overtone vibration region and the molecular fingerprint region, which can match the strong absorption lines of various gases, achieving ultra-high detection precision.

[0025] The following will describe the specific implementation of each module, please refer to Figure 2 : More preferably, in an exemplary embodiment, the mid-infrared fiber laser module comprises: a driving power supply 1, a pump laser diode 2, a front cavity mirror 3, a gain fiber 4, and a rear cavity mirror 5. Wherein, the driving power supply 1 provides a modulated current to drive the pump laser diode 2 to realize a modulated pulsed laser output, and is coupled into the gain fiber 4, and a resonant cavity is constructed by the front cavity mirror 3 and the rear cavity mirror 5 to realize the gain-modulated pulsed laser output.

[0026] Specifically, in the present exemplary embodiment, the pump laser diode 2 outputs 976 nm pump light and couples into the gain fiber 4. The front cavity mirror 3 and the rear cavity mirror 5 at both ends of the gain fiber 4 constitute a laser resonant cavity. The driving power supply 1 generates a square wave modulated current with a frequency of 1 kHz-35 kHz adjustable to realize periodic modulation of pump power (which can match various acoustic resonators such as photoacoustic cell, cantilever beam, and quartz tuning fork). When the pump power is higher than the threshold value, the laser outputs pulsed laser. By precisely controlling the modulation depth and duty cycle, stable gain-modulated pulsed laser output is obtained.

[0027] More preferably, in an exemplary embodiment, the photoacoustic detection module comprises: a fiber collimator 6, an acoustic resonator 7, and an electronic amplifier 8; The collimating lens 6 is used to collimate the laser to match the light aperture of the acoustic resonator 7, the acoustic resonator 7 is used to pass the gas to be measured, realize the resonance enhancement of the acoustic wave, and collect the acoustic signal, and the collected acoustic signal is amplified by the electronic amplifier 8. The periodic modulation of the pump power of the driving power supply 1 is matched with the acoustic resonator 7.

[0028] Specifically, in the present exemplary embodiment, the gain-modulated pulsed laser output by the mid-infrared fiber laser module is collimated by the fiber collimator 6 and then enters the acoustic resonator 7, while the gas to be measured is passed into the acoustic resonator 7. The acoustic pressure generated is detected by an acoustic transducer (such as a microphone, a quartz tuning fork, or a cantilever beam) in the acoustic resonator 7, and the acoustic signal is amplified by an electronic amplifier (8).

[0029] More preferably, in an exemplary embodiment, the signal acquisition and processing module comprises: a lock-in amplifier 9, a data acquisition card 10, and a computer 11. The lock-in amplifier 9 is used for the detection of acoustic signals of specific frequencies, the data acquisition card 10 is used for the conversion and transmission of analog-digital signals, and the computer 11 is used for data processing, storage, and the inversion of gas concentration information. The acoustic signals of specific frequencies are acoustic signals related to the modulation frequency.

[0030] Specifically, in the present exemplary embodiment, the acoustic signal is amplified by the electronic amplifier 8 and then sent to the lock-in amplifier 9. The reference signal of the lock-in amplifier 9 is provided by an internal crystal oscillator and is set to be synchronized with the laser modulation signal. Through phase-sensitive detection technology, noise is effectively suppressed, and the photoacoustic signal related to the modulation frequency is extracted. The data acquisition card 10 is used for data acquisition. The data is transmitted to the computer 11 for data processing and concentration inversion. The software system realizes functions such as spectral scanning, data storage, and concentration calibration.

[0031] Working principle: When the gain-modulated pulsed laser passes through the gas sample to be measured, the specific vibration mode in the gas molecules will absorb the laser, and the molecules will transition from the ground state to the excited state. This process can be described by a two-level system of gas absorption: where N1 is the number of excited state molecules, N0 is the number of ground state molecules, Φ( t ) is the modulation laser radiation density, τ v-t is the molecular vibration relaxation time, σis the gas absorption cross section. The excited state molecules relax by collision and convert the absorbed light energy into heat, resulting in local temperature rise and pressure change, generating an equivalent heat source: where, h is the Planck constant, v is the photon frequency. Due to the periodic modulation of the laser pulse, the temperature and pressure of the gas also change periodically, generating acoustic waves with the same modulation frequency. The acoustic resonator is equivalent to a bandpass filter, which amplifies acoustic waves with the same resonant frequency. Under the small absorption approximation, the acoustic pressure can be expressed as a function of the heat source: where, c is the sound speed, β is the thermal expansion coefficient of the gas, C p is the heat capacity of the gas. The fundamental resonant component of the acoustic wave is detected by a lock-in amplifier: where, S PA is the detected photoacoustic signal. By measuring the amplitude of the acoustic signal, the acoustic response of different concentrations of gas is obtained, and the detection sensitivity is further calibrated, so that the accurate detection of the concentration of the gas to be detected can be realized.

[0032] Obviously, the above embodiments are only examples for the purpose of clarity, and are not limiting of the embodiments. For those skilled in the art, on the basis of the above description, other different forms of changes or variations can also be made. Here, it is not necessary and impossible to exhaust all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A trace gas photoacoustic detection device based on a gain-modulated fiber laser, characterized in that: The application relates to a mid-infrared fiber laser module, a photoacoustic detection module and a signal acquisition and processing module. The mid-infrared fiber laser module comprises a driving power supply, a pumping laser diode, a front cavity mirror, a gain fiber and a rear cavity mirror. The driving power supply provides a modulated current to drive the pumping laser diode to realize modulated pulse laser output, and the modulated pulse laser is coupled into the gain fiber. The front cavity mirror and the rear cavity mirror are used to build a resonant cavity to realize the gain modulated pulse laser output.

2. The gain-modulated fiber laser based trace gas photoacoustic detection apparatus of claim 1, wherein: The pumping laser diode outputs 976 nm pumping light, i.e. the modulated pulse laser. The driving power supply generates a square wave modulated current with a frequency of 1 kHz-35 kHz to realize periodic modulation of the pumping power.

3. The gain-modulated fiber laser based trace gas photoacoustic detection apparatus of claim 2, wherein: The photoacoustic detection module comprises a fiber collimating lens, an acoustic resonator and an electronic amplifier.

4. The gain-modulated fiber laser based trace gas photoacoustic detection apparatus according to claim 2 or 3, characterized in that: The collimating lens is used to collimate the laser to match the light aperture of the acoustic resonator. The acoustic resonator is used to introduce the measured gas, realize resonance enhancement of the acoustic wave, collect the acoustic signal and amplify the collected acoustic signal through the electronic amplifier. The periodic modulation of the pumping power of the driving power supply is matched with the acoustic resonator.

5. The gain-modulated fiber laser based trace gas photoacoustic detection apparatus of claim 4, wherein: The acoustic resonator comprises a photoacoustic cell, a quartz tuning fork and a cantilever beam.

6. The gain-modulated fiber laser based trace gas photoacoustic detection apparatus of claim 1, wherein: The signal acquisition and processing module comprises a lock-in amplifier, a data acquisition card and a computer. The lock-in amplifier is used to detect the acoustic signal with a specific frequency. The data acquisition card is used to convert and transmit the analog-digital signal. The computer is used to process and store data and invert the gas concentration information. The acoustic signal with the specific frequency is the acoustic signal related to the modulation frequency.

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