1. A single-light-path trace gas online detection method based on cavity enhancement is characterized by comprising the following steps: the method specifically comprises the following steps:

step one, establishing a single-light-path trace gas online detection device based on cavity enhancement, which comprises a signal generator (10), a laser driver (20), a tunable semiconductor laser (30), a light beam coupler (40), an optical spherical cavity (50), a photoelectric detector (60), a phase-locked amplifier (70) and an upper computer (80); the output end of the signal generator (10) is connected with the input end of the laser driver (20), and the output end of the laser driver (20) is connected with the input end of the tunable semiconductor laser (30); the light beam coupler leads laser emitted by the tunable semiconductor laser (30) into the optical spherical cavity and leads the reflected light out of the optical spherical cavity; the optical spherical cavity is provided with a gas inlet and a gas outlet; the input end of the photoelectric detector receives the light beam led out by the light beam coupler, the output end of the photoelectric detector is connected with the input end of the lock-in amplifier, and the output end of the lock-in amplifier is connected with the upper computer;

secondly, generating a low-frequency trapezoidal wave and a high-frequency sine wave by using a signal generator (10), modulating the low-frequency trapezoidal wave by the high-frequency sine wave, inputting the modulated low-frequency trapezoidal wave into a laser driver (20), and controlling the wavelength and the light intensity of the output laser of the tunable semiconductor laser (30); synchronously modulating the light intensity of the output laser by a high-frequency sine wave;

inputting gas to be detected from a gas inlet (51) of the optical spherical cavity (50), introducing a modulated laser signal into the optical spherical cavity (50) by using the optical beam coupler (40), and leading out the laser signal after the laser signal is fully contacted with the gas to be detected through multiple reflections in the optical spherical cavity (50) by using the optical beam coupler (40);

step four, the photoelectric detector (60) converts the received laser signal into an electric signal and transmits the electric signal to the lock-in amplifier (70), and the lock-in amplifier (70) extracts signal information; the specific method comprises the following steps:

step 4.1, acquiring a depression generated after the laser signal is absorbed by gas by a phase-locked amplifier (70):

A＝[P₁,P₂,.....,P_N-1,P_N] (1)

wherein A represents the signal collected by the phase-locked amplifier (70), P₁,P₂,.....,P_N-1,P_NRespectively representing N data points acquired in a rising edge acquisition period; further processing the acquired signals:

step 4.2, introducing an optical power normalization coefficient m:

wherein L is_botAnd L_topRespectively the upper and lower bottom amplitude values of the acquired trapezoidal wave signal, and the normalization coefficient m is used for representing the real-time optical power condition of the laser signal and the gas to be measured after absorption; the gas absorption signal B after processing is therefore:

and fifthly, the lock-in amplifier (70) transmits the processed and demodulated gas absorption signal to an upper computer (80), and the upper computer (80) obtains the concentration of the detected gas through algorithm inversion, so that the online detection of the gas is realized.

2. The on-line detection method for the single-light-path trace gas based on the cavity enhancement as claimed in claim 1, wherein: the tunable semiconductor laser (30) is a quantum cascade laser with a radiation wavelength of 9.55 μm.

3. The on-line detection method for the single-light-path trace gas based on the cavity enhancement as claimed in claim 1, wherein: the light beam coupler (40) is one of a prism light beam coupler, a grating light beam coupler or a micro-nano structure coupler.

4. The on-line detection method for the single-light-path trace gas based on the cavity enhancement as claimed in claim 1, wherein: the optical spherical cavity (50) is a silicon dioxide cavity with the diameter of 150mm, and the inner surface of the cavity is plated with a metal mirror.

5. The on-line detection method for trace gas based on single light path of cavity enhancement as claimed in claim 1 or 4, wherein: the metal mirror surface plated in the optical spherical cavity (50) is one of silver, gold, aluminum or platinum.

6. The on-line detection method for trace gas based on single light path of cavity enhancement as claimed in claim 1 or 4, wherein: the on-line detection device further comprises a heat sink for mounting the tunable semiconductor laser (30).

7. The on-line detection method for the single-light-path trace gas based on the cavity enhancement as claimed in claim 1, wherein: the photodetector (60) is a photomultiplier tube.

8. The on-line detection method for the single-light-path trace gas based on the cavity enhancement as claimed in claim 1, wherein: the frequency of the low-frequency trapezoidal wave is 220Hz, and the frequency of the high-frequency sine wave is 2.55 kHz.

9. The on-line detection method for trace gas based on single light path of cavity enhancement as claimed in claim 1 or 4, wherein: the optical path of the laser signal after multiple reflections in the optical spherical cavity (50) is not less than 6.3 m.

Background

The exhaled endogenous gas has landmark information closely related to the physiological metabolism of the human body, and the diseases can be rapidly diagnosed in a noninvasive mode by detecting the components and the concentration of the corresponding exhaled gas. Tunable semiconductor laser absorption spectroscopy (TDLAS) is used as a spectroscopy measurement method with high sensitivity, high specificity and simple and stable measurement system, and real-time online detection of exhaled gas is easy to realize. In the gas detection system by absorption spectroscopy, a gas chamber is a place where light and gas act, the effective optical path of the gas chamber directly determines the detection precision of the system, and the stability of the gas chamber structure influences the stability of the system.

Reference 1(CN112304899) discloses a gas online detection device based on a cavity enhancement technology, which includes a tunable semiconductor laser, an optical cavity, a photodetector, a data acquisition and processing system, a gas compression cavity, a pressure sensor, a TEC semiconductor refrigerator, a microprocessor, a display screen, and the like. The device uses an enhanced Fabry-Perot (FP) optical cavity, realizes higher detection accuracy by compressing the volume of gas to improve the concentration, but has larger cavity size, low optical path/cavity size ratio, excessively complex cavity design, high-reflectivity cavity mirror, high manufacturing cost, complex stabilizing device and poor system stability.

The comparison document 2(CN102706832) discloses a laser infrared gas analyzer based on TDLAS-WMS, which includes a laser, a laser driving circuit, a temperature control circuit, an optical system with an optical cavity, a main detector, a reference detector, an intensity modulation cancellation circuit, a phase-locked amplification circuit, and a data acquisition and display circuit; the device can fundamentally eliminate the influence of intensity modulation by introducing division operation in the intensity modulation elimination circuit and combining a space double-light-path differential detection method, but in the double-light-path differential detection method, the mismatching problem of two photoelectric detectors is the root cause of the distortion of absorption peak signals.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a single-light-path trace gas online detection method based on cavity enhancement, and a single-light-path detection system is designed based on a TDLAS technology, and not only can the absorption signal of the gas be demodulated, but also the interference of non-absorption power fluctuation to the absorption signal can be eliminated by matching with a power normalization coefficient through an improved absorption peak extraction algorithm; meanwhile, the optical spherical cavity is adopted, the optical path is increased by increasing the reflection times, the optical path/cavity size ratio is greatly improved, and the problems of poor stability, high cost, large size and the like in the conventional cavity enhanced TDLAS technology are solved.

A single-light-path trace gas online detection method based on cavity enhancement specifically comprises the following steps:

the method comprises the following steps of firstly, building an online detection device of the single-light-path trace gas based on cavity enhancement, wherein the online detection device comprises a signal generator, a laser driver, a tunable semiconductor laser, a light beam coupler, an optical spherical cavity, a photoelectric detector, a phase-locked amplifier and an upper computer. The signal generating end of the signal generator is connected with the input end of the laser driver, and the output end of the laser driver is connected with the input control end of the tunable semiconductor laser. The light beam coupler leads laser light emitted by the tunable semiconductor laser into the optical spherical cavity and leads the reflected light out of the optical spherical cavity. The optical spherical cavity is provided with a gas inlet and a gas outlet. The input end of the photoelectric detector receives the light beam led out by the light beam coupler, the output end of the photoelectric detector is connected with the input end of the phase-locked amplifier, and the output end of the phase-locked amplifier is connected with the upper computer.

Preferably, the tunable semiconductor laser is a quantum cascade laser with a radiation wavelength of 9.55 μm.

Preferably, the beam coupler is one of a prism beam coupler, a grating beam coupler or a micro-nano structure coupler.

Preferably, the optical spherical cavity is a silicon dioxide cavity with the diameter of 150mm, and the inner surface of the cavity is plated with a metal mirror.

Preferably, the metal mirror surface plated in the optical spherical cavity is one of silver, gold, aluminum or platinum.

Preferably, the online detection device further comprises a heat radiator for mounting the tunable semiconductor laser.

Preferably, the photodetector is a photomultiplier tube.

And step two, generating a low-frequency trapezoidal wave and a high-frequency sine wave by using a signal generator, modulating the low-frequency trapezoidal wave by the high-frequency sine wave, inputting the modulated low-frequency trapezoidal wave into a laser driver, controlling the wavelength and the light intensity of the laser output by the tunable semiconductor laser, and synchronously modulating the light intensity of the output laser by the high-frequency sine wave.

Preferably, the frequency of the low-frequency trapezoidal wave is 220Hz, and the frequency of the high-frequency sinusoidal wave is 2.55 kHz.

And step three, inputting the gas to be detected from a gas inlet of the optical spherical cavity, introducing the modulated laser signal into the optical spherical cavity by the optical beam coupler, and leading out the laser signal after the laser signal is fully contacted with the gas to be detected through multiple reflections in the optical spherical cavity.

Preferably, the optical path length of the laser signal after multiple reflections in the optical spherical cavity is not less than 6.3 m.

And step four, converting the received laser signal into an electric signal by the photoelectric detector and transmitting the electric signal to the phase-locked amplifier, acquiring the depression generated by the gas absorption in the laser signal by the phase-locked amplifier through an improved absorption peak extraction algorithm, and demodulating the gas absorption signal by matching with a power normalization coefficient. The specific method comprises the following steps:

step 4.1, acquiring a depression generated after the laser signal is absorbed by gas by a phase-locked amplifier:

A＝[P₁,P₂,.....,P_N-1,P_N] (1)

wherein A represents the signal collected by the phase-locked amplifier, P₁,P₂,.....,P_N-1,P_NEach representing N data points acquired during a rising edge acquisition period. Further processing the collected signals to obtain S₁,S₂,.....,S_N-1,S_N：

Step 4.2, introducing an optical power normalization coefficient m:

wherein L is_botAnd L_topThe normalized coefficient m is used for representing the real-time optical power condition of the laser signal and the gas to be measured after absorption. The gas absorption signal B after processing is therefore:

and step five, the lock-in amplifier transmits the processed and demodulated gas absorption signal to an upper computer, and the upper computer obtains the concentration of the detected gas through algorithm inversion, so that the online detection of the gas is realized.

The invention has the following beneficial effects:

a single-light-path absorption peak demodulation method is adopted, a gas to be detected is filled in through a sealed air hole, low-frequency trapezoidal waves are used for scanning in a certain range according to the current and temperature tuning characteristics of a laser, meanwhile, high-frequency sinusoidal signals are superposed to be used as carriers for modulation, high-power laser which is emitted by a quantum cascade laser and is subjected to high-frequency modulation is emitted into an optical spherical cavity from one side of a beam coupler, the high-power laser is reflected for a plurality of times in the optical spherical cavity to be absorbed by the gas to be detected, then the laser is emitted into a photoelectric detector from the other side of the beam coupler, the photoelectric detector converts the laser with the information of the gas to be detected from an optical signal into an electric signal, then the electric signal is used for demodulating the absorption signal of the gas through a special absorption peak extraction algorithm by matching with a power normalization coefficient, absorption signal information is obtained from an upper computer, then data processing is carried out, and the inversion of the concentration of the gas exhaled by the algorithm, and the signal data and the like are displayed and stored, so that the method has the advantages of high sensitivity, high resolution and real-time continuous online detection of multi-component gas at the same time, and can be widely applied to the fields of public environments, medical treatment and the like.

Drawings

FIG. 1 is a flow chart of an on-line gas detection method;

FIG. 2 is a schematic structural diagram of an on-line detection device in an embodiment.

Detailed Description

The invention is further explained below with reference to the drawings;

as shown in fig. 1, a single-light path trace gas online detection method based on cavity enhancement specifically includes the following steps:

step one, an online detection device 100 based on the cavity-enhanced single-optical-path trace gas is built, as shown in fig. 2, and includes a signal generator 10, a laser driver 20, a tunable semiconductor laser 30, a light beam coupler 40, an optical spherical cavity 50, a photoelectric detector 60, a lock-in amplifier 70, and an upper computer 80. The signal generator 10 has a signal generating terminal connected to an input terminal of the laser driver 20, and an output terminal of the laser driver 20 is connected to an input control terminal of the tunable semiconductor laser 30. The beam coupler 40 directs laser light emitted by the tunable semiconductor laser 30 into the optical spherical cavity 50 and directs the reflected light out of the optical spherical cavity 50. The optical spherical cavity 50 is provided with a gas inlet 51 and an outlet 52. The input end of the photodetector 60 receives the light beam from the beam coupler 40, the output end is connected to the input end of the lock-in amplifier 70, and the output end of the lock-in amplifier 70 is connected to the upper computer 80. The tunable semiconductor laser 30 is a quantum cascade laser with a radiation wavelength of 9.55 μm mounted on a heat sink, and can cover absorption of gases such as ammonia, ethylene, and carbon dioxide within a scanning range. The beam coupler 40 is a prism beam coupler. The optical spherical cavity 50 is a silicon dioxide cavity with the diameter of 150mm, and the inner surface of the cavity is plated with a silver mirror. The photodetector 60 is a photomultiplier tube.

And step two, generating a 220Hz trapezoidal wave and a 2.55kHz sine wave by using a signal generator, modulating the low-frequency trapezoidal wave by the sine wave, inputting the modulated low-frequency trapezoidal wave into a laser driver, controlling the wavelength and the light intensity of the laser output by the tunable semiconductor laser to change according to the driving current and the temperature, and synchronously modulating the light intensity of the output laser by the 2.55kHz sine wave.

Inputting the gas to be measured from a gas inlet 51 of the optical spherical cavity, introducing the modulated laser signal into the optical spherical cavity 50 by the optical beam coupler 40, leading out the laser signal after the laser signal is fully contacted with the gas to be measured through multiple reflections in the optical spherical cavity 50 by the optical beam coupler 40, and leading out the optical path of the laser signal after the laser signal is reflected for multiple times in the optical spherical cavity 50 to be not less than 6.3 m.

Step four, the photodetector 60 converts the received laser signal into an electrical signal and transmits the electrical signal to the lock-in amplifier 70, the lock-in amplifier 70 collects the depression generated by the gas absorption in the laser signal through an improved absorption peak extraction algorithm, and demodulates the gas absorption signal by matching with the power normalization coefficient. The specific method comprises the following steps:

step 4.1, the lock-in amplifier 70 collects the depression generated after the laser signal is absorbed by gas:

A＝[P₁,P₂,.....,P_N-1,P_N] (1)

where A represents the signal collected by the lock-in amplifier 70, P_NRepresenting the nth data point acquired during a rising edge acquisition period. Further processing the acquired signals:

step 4.2, introducing an optical power normalization coefficient m:

wherein L is_botAnd L_topThe normalized coefficient m is used for representing the real-time optical power condition of the laser signal and the gas to be measured after absorption. The gas absorption signal B after processing is therefore:

through the above data processing, the trapezoidal wave concave portion is leveled, and at the same time, the influence of the non-absorbed power fluctuation on the measurement result can be eliminated.

And step five, the lock-in amplifier 70 transmits the processed and demodulated gas absorption signal to the upper computer 80, and the upper computer 80 obtains the concentration of the gas to be detected through algorithm inversion, so that the online detection of the gas is realized.