CN108872148A - High-precision particulate in air concentration detection method based on Fibre Optical Sensor - Google Patents

Abstract

The invention discloses a high-precision air particle concentration detection method based on optical fiber sensing. With the gradual increase of the concentration of tiny particles in the alignment air chamber, the loss in the optical fiber ring cavity increases accordingly, and the decay time of the pulse signal Corresponding changes occur, and the ring-down time of the corresponding standard gas sample is obtained through detection based on multiple sets of standard gas samples with known particle concentrations, and the linear relationship curve between the particle concentration and the ring-down time is established, and then according to the measured attenuation time of the gas sample to be tested The concentration of particulate matter in the gas sample to be measured can be obtained by combining the linear relationship curve between particle concentration and ring-down time, and then realize the detection of PM _2.5 in the air. The invention has high detection sensitivity, wide measurement range, fast analysis speed, simple operation, low cost and convenient real-time monitoring, and can be used for vehicle exhaust emission measurement, indoor smog concentration measurement, environmental pollution index measurement, fire alarm and the like.

Description

High-precision airborne particle concentration detection method based on optical fiber sensing

technical field

The invention belongs to the technical field of particle pollutant detection in the air, and in particular relates to a high-precision detection method for particle concentration in the air based on optical fiber sensing.

Background technique

In recent years, haze weather has seriously affected people's life and health, and the research on particle detection technology is urgent. The core technology of detecting particle concentration is to design accurate and reliable sensors. Compared with China, the research on photoelectric sensors abroad started earlier, and the technology is relatively mature. Photoelectric sensors have been widely used in military, aerospace, industrial control and detection technology and other fields. At present, the detection methods of PM _2.5 in the air mainly include gravimetric method, micro-vibration balance method, β-ray absorption method, light scattering method, etc. The sensors used in these methods are relatively complicated, expensive and greatly affected by external factors.

Contents of the invention

The present invention provides a high-precision detection method for particulate matter concentration in the air based on optical fiber sensing to solve the problems of complex structure, high price, and greater influence from external factors existing in relevant particle concentration detection devices at home and abroad. Erbium-doped fiber amplifier (EDFA) is used to compensate the loss and increase the decay time of the measurement system, which effectively solves the problems of rising optical pulse curve and small number of pulse peaks. It has high sensitivity, wide measurement range, fast analysis speed, simple operation and low cost. And convenient real-time detection and other characteristics, can be used for vehicle exhaust emission measurement, indoor smoke concentration measurement, environmental pollution index measurement, fire alarm, etc.

The present invention adopts the following technical scheme to solve the above-mentioned technical problems, a high-precision airborne particle concentration detection method based on optical fiber sensing, characterized in that: the first optical fiber coupler, 3.6km single-mode optical fiber, and erbium-doped The fiber amplifier, the second isolator, the alignment gas chamber and the second fiber coupler form a fiber ring cavity. The first fiber coupler is connected with the first isolator and the laser source through the optical fiber in turn, and the laser source is connected with the semiconductor laser through the line in turn. The modulator is connected to the signal source, and the second fiber coupler is connected to the photodetector through the optical fiber. The photodetector is connected to the oscilloscope through the line. When the signal source generates a series of pulse waves and connects to the semiconductor laser modulator, the semiconductor laser modulator controls The output power and wavelength of the laser source, the modulated optical pulse train is coupled into the fiber loop through the first isolator and the 10% port of the first fiber coupler, and then passes through the 3.6km single-mode fiber, erbium-doped fiber amplifier, The second isolator and the alignment gas chamber, 90% of the output light in the fiber ring cavity is connected to the first fiber coupler through the second fiber coupler, and 10% of the output light in the fiber ring cavity is transmitted by the fiber through the second fiber coupler The loop is connected to the photodetector, and the output periodic sequence of the attenuated pulse is converted into an electrical signal by the photodetector, and finally displayed on the digital oscilloscope. The alignment gas chamber is filled with the marked gas sample or the gas sample to be measured. With the gradual increase of the concentration of tiny particles in the gas chamber, the loss in the optical fiber ring cavity increases accordingly, and the decay time of the pulse signal changes accordingly. Establish a linear relationship curve between particle concentration and ring-down time, and then obtain the concentration of particles in the gas sample to be measured according to the measured ring-down time of the gas sample to be tested and combined with the linear relationship curve between particle concentration and ring-down time. Then realize the detection of PM _2.5 in the air.

Preferably, the erbium-doped fiber amplifier is composed of a section of low-gain erbium-doped fiber, a pump laser and a three-port WDM coupler, and the light pulses output by the low-gain erbium-doped fiber, pump laser and WDM coupler are respectively connected to Three ports on the WDM coupler.

Preferably, the alignment air chamber has four ports, which are light inlet, light outlet, air inlet and gas outlet, wherein the light inlet is connected to the second isolator through an optical fiber, and the light outlet is connected to the second isolator through an optical fiber. The fiber optic coupler is connected, the gas inlet is connected with the standard gas sample or the gas sample to be tested, and the gas outlet is connected with the gas collection device or the outside atmosphere.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention uses an erbium-doped fiber amplifier to compensate the coupling loss when the optical pulse is coupled into the fiber ring cavity and increases the attenuation time of the measurement system, thereby effectively improving the measurement accuracy;

2. The measurement system of the present invention is carried out under constant temperature conditions, and is insensitive to cross-sensing of other factors such as bending, strain, and refractive index;

3. The measurement system of the present invention has a good linear response, and the detection method effectively solves the problems of rising pulse curve and small number of pulse peaks. Far beyond the comprehensive performance of existing PM _2.5 detectors on the market;

4. The invention has high detection sensitivity, wide measurement range, fast analysis speed, simple operation, low cost, and convenient real-time monitoring, and can be used for automobile exhaust emission measurement, indoor smoke concentration measurement, environmental pollution index measurement, fire alarm, etc.

Description of drawings

Fig. 1 is the optical path connection figure of measuring system among the present invention;

Figure 2 is the change curve of the peak point of the ring-down curve with different particle concentrations;

Fig. 3 is the relationship curve between the decay time of the optical fiber cavity ring-down spectroscopy system and the particle concentration.

In the figure: 1-signal source, 2-semiconductor laser modulator, 3-laser source, 4-the first isolator, 5-the first fiber coupler, 6-3.6km single-mode fiber, 7-erbium-doped fiber amplifier, 8-second isolator, 9-alignment air chamber, 10-second fiber coupler, 11-photodetector, 12-oscilloscope, 13-optical fiber.

Detailed ways

The above-mentioned contents of the present invention are described in further detail below through the embodiments, but this should not be interpreted as the scope of the above-mentioned themes of the present invention being limited to the following embodiments, and all technologies realized based on the above-mentioned contents of the present invention all belong to the scope of the present invention.

Example

A high-precision airborne particle concentration detection method based on optical fiber sensing consists of a first optical fiber coupler 5, a 3.6km single-mode optical fiber 6, an erbium-doped optical fiber amplifier 7, a second isolator 8, and an alignment gas connected sequentially through an optical fiber 13. Chamber 9 and the second fiber coupler 10 form a fiber ring cavity, and the erbium-doped fiber amplifier 7 is made up of a low-gain erbium-doped fiber, a pump laser and a three-port WDM coupler, and the low-gain erbium-doped fiber, pump laser The optical pulses output by the WDM coupler are respectively connected to the three ports of the WDM coupler. The first fiber coupler 5 is connected with the first isolator 4 and the laser source 3 through the optical fiber 13 in turn, and the laser source 3 is connected with the semiconductor through the circuit in turn. The laser modulator 2 is connected to the signal source 1, the second fiber coupler 10 is connected to the photodetector 11 through the optical fiber 13, and the photodetector 11 is connected to the oscilloscope 12 through the line, when the signal source 1 generates a series of pulse waves connected to the semiconductor Laser modulator 2, the semiconductor laser modulator 2 controls the output power and wavelength of the laser source 3, and the modulated optical pulse train is coupled into the fiber loop through the first isolator 4 and the 10% port of the first fiber coupler 5 , passing through the 3.6km single-mode fiber 6, the erbium-doped fiber amplifier 7, the second isolator 8 and the alignment air chamber 9 in turn, 90% of the output light in the fiber ring cavity is connected to the first fiber coupling through the second fiber coupler 10 5, 10% of the output light in the fiber ring cavity is connected to the photodetector 11 by the fiber loop through the second fiber coupler 10, and the output periodic sequence of the attenuated pulse is converted into an electrical signal by the photodetector 11, and finally displayed on On the digital oscilloscope 12, the alignment air chamber 9 has four ports, which are light inlet, light outlet, air inlet and air outlet, wherein the light inlet is connected to the second isolator 8 through an optical fiber 13, and the light outlet is connected to the second isolator 8 through an optical fiber. 13 is connected with the second optical fiber coupler 10, the air inlet is connected with the standard gas sample or the gas sample to be tested, the gas outlet is connected with the gas collection device or the outside atmosphere, and the alignment gas chamber 9 is filled with the marked gas sample or the gas to be tested Sample, as the concentration of tiny particles in the alignment gas chamber gradually increases, the loss in the fiber ring cavity increases accordingly, and the decay time of the pulse signal changes accordingly. According to the detection of multiple groups of standard gas samples with known particle concentrations Corresponding to the ring-down time of the standard gas sample, establish a linear relationship curve between the particle concentration and the ring-down time, and then obtain the gas to be tested according to the measured ring-down time of the gas sample to be tested and combined with the linear relationship curve between the particle concentration and the ring-down time The concentration of particulate matter in the sample, and then realize the detection of PM _2.5 in the air.

The principle of the present invention is based on optical fiber ring cavity ring-down spectroscopy technology. A series of pulse waves generated by the signal source are sent to the semiconductor laser modulator through the "analog modulation input" port to be modulated into pulsed light, and enter the fiber ring cavity through the first fiber coupler. Through the 3.6km single-mode optical fiber in the optical fiber ring cavity, the erbium-doped optical fiber amplifier, the second optical fiber isolator, and the alignment gas chamber, it comes out from one end of the second optical fiber coupler and is connected to the photodetector. In order to improve the measurement accuracy of the ring-down time, it is necessary to increase the number of pulses. Therefore, an erbium-doped fiber amplifier is used to compensate the loss of the fiber loop. Through amplifier experiments with different lengths of erbium-doped fibers, it was found that if the erbium-doped fibers are too long, laser light will be generated, but if the erbium-doped fibers are too short, the gain is not enough and the pulse interval is relatively small. Therefore, a 2m-long erbium-doped optical fiber optical amplifier and a 980nm pump laser were built in the laboratory. Under the same conditions, the attenuation spectra of EDFA placed inside and outside the cavity were compared. When the EDFA was placed inside the cavity, the peak intensity and number of pulse signals were greater, which was more conducive to detecting the ring-down time. Compared with conventional CRD signal processing, the ring-down system with EDFA in the cavity has a great improvement in the improvement of the sensitivity of particle size concentration measurement.

The time-domain analysis method usually determines the optical loss in the optical fiber loop by monitoring the attenuation lifetime of the optical pulse introduced into the optical fiber loop. In the FLRDS system, the modulated pulsed light enters the fiber loop through the first fiber coupler, and then goes back and forth in the fiber loop for many times. During each round trip, a small amount of light will be lost due to the loss in the fiber ring-down cavity. As a result, the light in the ring cavity decays exponentially with time, and the ring-down signal of the cavity can be expressed as:

I _t =I ₀ exp(-t/τ) (1)

Among them, I ₀ is the initial light intensity, and τ is the ring-down time of the optical cavity.

The exponentially decaying signal is obtained by detecting and recording a small amount of leaked light each time passing through the second coupler by the photodetector. In the particle size concentration measurement experiment, the attenuation in the optical fiber ring-down cavity changes with the change of the particle size concentration. When the loss in the ring cavity is lower, the attenuation time τ is larger. Thus τ reflects the losses caused by changes in particle size concentration and can be obtained by measuring τ. The decay time τ of the optical cavity is defined as the time required for the light to decay to 1/e of its initial intensity, the expression is

In the formula, d is the length of the fiber ring cavity, c is the speed of light, n is the refractive index of the fiber ring cavity, and A is the total loss during each cavity ring-down process, including the inherent loss of the fiber ring cavity and the lens and high reflection mirror Butt loss. In the fiber ring cavity, the inherent loss of the fiber ring cavity mainly comes from the absorption of the fiber ring cavity, the insertion loss of the fiber coupler, the scattering loss of the fiber and the connection loss of the fiber.

A photodetector and oscilloscope are used to monitor the decay signal and record the analysis data for each round trip. In the laboratory, the experiment was carried out with the smog environment to simulate the haze environment. Fig. 2 is the variation curve of the peak point of the light pulse attenuation curve in the range of 0-1 mg/cm ³ with particle concentration. It can be seen from the figure that as the concentration of particulate matter increases, the attenuation curve also gradually declines. By fitting the relationship between the particle concentration and the ring-down time τ of 10 groups (as shown in Figure 3), it is found that the ring-down time and the particle concentration present a good linear relationship, that is, the ring-down time can reflect the change of the particle concentration. The experimental results show that when the EDFA with low-gain and low-noise Erbium-doped fiber is used for signal amplification in the ring-down cavity, the number of pulses can be increased and the sensitivity of particle concentration measurement can be improved. It can be seen from the results that in the FLRDS system, the correlation coefficient between particle concentration and ring-down time is 0.99424. The optical fiber ring cavity ring-down spectroscopy system is compact in structure, low in cost, and convenient for real-time measurement. It can be used for the detection of PM _2.5 in the air, and provides convenience for industrial monitoring, environmental monitoring and other fields.

The above embodiments have described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What are described in the above embodiments and description are only to illustrate the principles of the present invention. Without departing from the scope of the principle of the present invention, there will be various changes and improvements in the present invention, and these changes and improvements all fall within the protection scope of the present invention.

Claims (3)

1. A high-precision air particle concentration detection method based on optical fiber sensing, characterized in that: the first optical fiber coupler, 3.6km single-mode optical fiber, erbium-doped optical fiber amplifier, second isolator, alignment gas The chamber and the second fiber coupler form a fiber ring cavity, the first fiber coupler is connected with the first isolator and the laser source through the fiber in turn, the laser source is connected with the semiconductor laser modulator and the signal source through the line in turn, and the second fiber coupler The photodetector is connected to the photodetector through an optical fiber, and the photodetector is connected to the oscilloscope through a line. When the signal source generates a series of pulse waves, it is connected to the semiconductor laser modulator, and the semiconductor laser modulator controls the output power and wavelength of the laser source. The optical pulse train is coupled into the fiber loop through the first isolator and the 10% port of the first fiber coupler, and then passes through the 3.6km single-mode fiber, the erbium-doped fiber amplifier, the second isolator and the alignment gas chamber, and the fiber 90% of the output light in the ring cavity is connected to the first fiber coupler through the second fiber coupler, and 10% of the output light in the fiber ring cavity is connected to the photodetector from the fiber loop through the second fiber coupler to attenuate the pulse The output periodic sequence is converted into an electrical signal by the photodetector, and finally displayed on the digital oscilloscope. The alignment gas chamber is filled with marked gas samples or gas samples to be tested. As the concentration of tiny particles in the alignment gas chamber gradually increases, The loss in the optical fiber ring cavity increases accordingly, and the decay time of the pulse signal changes accordingly. According to the standard gas samples with known particle concentration in multiple groups, the ring-down time of the corresponding standard gas sample is obtained through detection, and the relationship between particle concentration and ring-down time is established. The linear relationship curve, and then according to the measured ring-down time of the gas sample to be tested and combined with the linear relationship curve between the particle concentration and the ring-down time, the concentration of particulate matter in the gas sample to be tested is obtained, and then the detection of PM _2.5 in the air is realized. 2. the high-precision airborne particle concentration detection method based on optical fiber sensing according to claim 1, is characterized in that: the erbium-doped fiber amplifier is composed of a section of low-gain erbium-doped optical fiber, a pump laser and a three-port WDM Composed of couplers, the low-gain erbium-doped fiber, the pump laser and the optical pulses output by the WDM coupler are respectively connected to the three ports of the WDM coupler. 3. The method for detecting the concentration of particulate matter in the air with high precision based on optical fiber sensing according to claim 1, characterized in that: the alignment air chamber has four ports, which are respectively light inlet, light outlet, and air inlet and the gas outlet, wherein the light inlet is connected to the second isolator through an optical fiber, the light outlet is connected to the second optical fiber coupler through an optical fiber, the inlet is connected to the standard gas sample or the gas sample to be measured, and the gas outlet is connected to the gas collection device or The outside world communicates with each other.