CN204177736U - The trace gas detection device in chamber is swung based on two-way light decay - Google Patents

Abstract

The utility model discloses a trace gas detection device based on a double-path optical ring-down cavity, which belongs to the technical field of gas detection. The trace gas detection device includes a light source unit, an optical ring-down unit, and a signal receiving and processing unit, the light source unit includes an interconnected light source and an optical beam splitter, and the optical ring-down module includes two identical optical ring-down cavity, the signal receiving and processing unit includes two optical detectors and computing modules respectively connected to the two optical detectors, the optical input ends of the two optical ring-down cavities and the two output ends of the optical beam splitter are respectively connected, and the ring-down optical signal output ends of the two optical ring-down cavities are respectively connected with the two optical detectors. Compared with the prior art, the utility model uses a dual optical ring-down cavity to obtain two ring-down curves at the same time, so that the concentration of the gas to be measured can be obtained conveniently and quickly.

Description

Trace gas detection device based on dual optical ring-down cavity

technical field

The utility model relates to a trace gas detection device based on an optical ring-down cavity, in particular to a trace gas detection device based on a two-way optical ring-down cavity, which belongs to the technical field of gas detection.

Background technique

With the continuous development of petroleum, coal, chemical industry, and automobile industry, a large amount of polluting, toxic, harmful, flammable and explosive waste gas is produced in the process of human production activities. Although the concentration of these gases is very low, they have varying degrees of impact on human health and the atmospheric environment. Therefore, trace gas analysis and detection technology has a wide range of application requirements in many fields.

Cavity Ring Down Spectroscopy (CRDS) is a high-sensitivity absorption spectrum detection technology developed rapidly in the past 20 years. It has extremely high sensitivity and resolution, and is not affected by light intensity fluctuations of the light source. , suitable for the measurement of weak absorption spectra. Since O’Keefe et al. proposed pulsed cavity ring-down spectroscopy in 1998, the research on optical cavity ring-down technology in spectral measurement has become increasingly extensive.

Initially, CRDS was built on an optical resonant cavity formed by two high-reflection mirrors, and the loss information in the cavity was obtained by measuring the light intensity decay rate in the resonant cavity. Due to the need to ensure the high reflection and low loss characteristics of the resonator, that is to say, it is required to coat the two mirrors with high reflectivity and ensure the precise alignment of the cavity mirrors, the application development of CRDS in some specific environmental conditions is limited. After 2001, fiber ring ring-down cavity, fiber-end coated ring-down cavity and fiber Bragg grating pair ring-down cavity appeared successively. These fiber-type ring-down cavity not only improved the limitations of the above-mentioned traditional With unique advantages, it has been used in the fields of absorption detection and sensing of various gases and liquids. In 2001, Stewart et al. first proposed an active optical fiber ring cavity ring-down spectroscopy technology. The system uses a DFB laser as a light source to excite the optical fiber ring formed by connecting the optical fiber with a coupler to obtain a ring-down signal, and introduces gas into the optical fiber ring. The absorption cell is used to detect the tracer gas. Because the insertion loss of the absorption pool is very large (about 1dB), the ring-down time in the cavity is very small (about 100ns), so a 980nm laser-pumped erbium-doped gain fiber is added in the fiber ring to prevent excessive loss in the ring. The optical signal is compensated, and the ring-down time of micron level is obtained. But its disadvantage is that it increases the complexity of the system while improving the detection accuracy, and it is difficult to control and achieve stable gain compensation near the wavelength of 1550nm. In 2002, Brown et al. proposed a passive fiber ring ring down cavity, which bends a section of multimode fiber into a ring and connects it with a fiber connector, and couples the light pulse into the fiber ring by illuminating the bend of the fiber, and uses a photomultiplier tube to Receive the transmitted signal at another bend, and get the double exponential ringdown signal, which respectively correspond to the attenuation of the optical pulse in the fiber cladding and fiber core in the ring. The device directly bends the fiber into a ring, and between the aligned fiber end faces or Organic dyes are introduced into the microcavity for sensing, the total loss of the system is low, the ring-down time is long, and the structure is simple, which can realize the analysis of the loss mechanism of the optical fiber communication system and the high-precision sensing of microfluidics. But its defect is that the ring down time is not long enough, so the detection range is small. In 2004, Tarsa and Lehmann et al. introduced a section of bitapered optical fiber into the optical fiber ring ring cavity as a sensing unit, and measured the concentration of octyne solution by detecting the absorption of the evanescent field leaked by the optical fiber bicone, and obtained results better than Other evanescent absorption detection methods. They also applied it to bending and axial strain measurement, connecting a section of single-mode double-tapered optical fiber to a 2.2Km single-mode optical fiber ring, and applying axial strain through a conversion platform controlled by a stepping motor. Due to the unreasonable design of the fiber optic bicone, the ring-down time has a nonlinear response to the axial displacement, and the sensitivity is not high enough, so further optimization of the design is needed. In 2007, Andachi et al. used a tunable picosecond pulse laser to excite the Bragg grating ring-down cavity and the fiber ring ring-down cavity respectively, and opened a 100 μm micro-gap in the fiber ring for the introduction of methylene blue fuel for absorption detection, and obtained Lower minimum detection limit.

The above-mentioned existing CRDS-based trace gas detection devices have their own advantages and disadvantages. However, these existing detection devices use a single ring-down cavity to measure the ring-down time twice. After the ring-down time of the gas, the introduction of the measured gas is not easy to operate, and it is easy to introduce dopant gas, which reduces the detection accuracy, and it is impossible to measure two ring-down curves at the same time conveniently and quickly.

Utility model content

The technical problem to be solved by the utility model is to overcome the deficiencies in the prior art, and provide a trace gas detection device based on a dual-channel optical ring-down cavity, which can obtain two ring-down curves at the same time by using a dual-channel optical ring-down cavity, so that it can be conveniently Quickly obtain the concentration of the gas to be measured.

The utility model specifically adopts the following technical solutions to solve the above-mentioned technical problems:

A trace gas detection device based on a dual-channel optical ring-down cavity, including a light source unit, an optical ring-down unit, and a signal receiving unit

and a processing unit, the light source unit includes an interconnected light source and an optical beam splitter, the optical ring-down module includes two identical optical ring-down cavities, and the signal receiving and processing unit includes two photodetectors and a A computing module connected to two optical detectors respectively, the optical input ends of the two optical down-down cavities are respectively connected to the two output ends of the optical beam splitter, and the down-down optical signal output ends of the two optical down-down cavities are connected to the The two photodetectors are connected separately.

The optical ring-down cavity in the above-mentioned technical scheme can adopt the optical resonant cavity formed by two high-reflection mirrors, or the existing or future technologies such as the optical fiber ring-down cavity. Considering the various advantages of the optical fiber ring-down cavity, preferably , the optical ring-down cavity is an optical fiber ring-down cavity.

Compared with the prior art, the utility model has the following beneficial effects:

The utility model can obtain two ring-down curves at the same time by using two parallel optical down-down cavities, so that the side gas can be measured conveniently and quickly; the utility model further adopts an optical fiber down-down cavity to solve the traditional Because the optical path of the ring-down cavity is not long enough and the ring-down time is small, the measurement accuracy is higher.

Description of drawings

Fig. 1 is the structural representation of a preferred embodiment of the present utility model; The meaning of each label among the figure is as follows:

1. Light source; 2. Fiber coupler (split ratio 50:50); 3 and 4, isolator; 5, 6, 9 and 10, fiber Bragg grating; 7 and 8, photonic crystal fiber; 11 and 12, focusing lens; 13 and 14, Photoelectric detector; 15 and 16, data acquisition card; 17, computer.

Detailed ways

Below in conjunction with accompanying drawing, technical scheme of the present utility model is described in detail:

The utility model is a trace gas detection device based on a dual-channel optical down-down cavity, which includes a light source unit, an optical down-down unit, and a signal receiving and processing unit. The light source unit includes a light source and an optical beam splitter connected to each other. The ring-down module includes two identical optical ring-down cavities, and the signal receiving and processing unit includes two photodetectors and computing modules respectively connected to the two photodetectors, and the optical input of the two optical ring-down cavities The two output ends of the optical beam splitter are respectively connected to each other, and the ring-down optical signal output ends of the two optical ring-down cavities are respectively connected to the two optical detectors.

The optical ring-down cavity in the utility model can adopt various existing or future technologies, and the utility model preferably adopts an optical fiber ring-down cavity, especially an optical fiber ring ring-down cavity, a fiber-end coated ring-down cavity or an optical fiber grating R&D cavity.

In order to prevent the light in the optical ring-down cavity from having an impact on the light source and the optical beam splitter, the utility model is further provided with a device in the connecting optical path between each optical ring-down cavity and the optical beam splitter, which can make the light The optical isolator transmitted from the beam splitter to the optical drop-down cavity can effectively prevent the light in the optical drop-down cavity from entering the optical beam splitter and the light source.

The light source in the utility model is preferably a fiber pulse laser, and the optical beam splitter is preferably a 50:50 optical beam splitter.

Fig. 1 shows a preferred embodiment of the present invention, the measuring device includes a light source unit, an optical ring down unit and a signal receiving and processing unit. As shown in Figure 1, the light source unit includes a light source 1 and Fiber coupler (or optical beam splitter) 2; the optical ring down unit includes a pair of fiber Bragg gratings (FBG) 5, 9 connected by photonic crystal fiber 7, and a pair of fiber Bragg gratings connected by photonic crystal fiber 8 6, 10, photonic crystal fiber can only support one mode transmission in a wide bandwidth range, has a high nonlinear coefficient, and is beneficial to reduce the dispersion loss when the light source is transmitted in the fiber, and has a photonic crystal with a larger aperture The optical fiber, as an air chamber, can increase the number of reflections and effectively improve the measurement accuracy; as shown in the figure, The two output ends of the fiber coupler 2 are respectively connected to the fiber Bragg grating 5 and the Bragg grating 6 via the optical isolator 3 and the optical isolator 4; The photodetector 13, the photodetector 14, the data acquisition card 15, the data acquisition card 16 that are connected with the photodetector 13, the photodetector 14 respectively, and the computer 17 that is connected with the data acquisition card 15, the data acquisition card 16; In order to enhance optical signal collection, in this embodiment, a focusing lens 11 and a focusing lens 12 are respectively arranged between the fiber Bragg grating 9 and the electrical detector 13 and between the fiber Bragg grating 10 and the detector 14 .

Fiber Bragg grating 5, photonic crystal fiber 7, and fiber Bragg grating 9 form a fiber Bragg grating ring-down cavity, and fiber Bragg grating 6, photonic crystal fiber 8, and fiber Bragg grating 10 form another fiber Bragg grating ring-down cavity. The materials, lengths, diameters and other parameters of the components of the two fiber grating ring-down cavities are exactly the same.

The light emitted by light source 1 is The fiber coupler 2 is equally divided into two paths, which respectively enter into two fiber grating ring-down cavities via the optical isolator 3 and the optical isolator 4 and form oscillations therein. The photodetector 13 and the photodetector 14 respectively receive the The down-down optical signal converged by the focusing lens 12 and converted into an electrical signal, is collected by the data acquisition card 15 and the data acquisition card 16 and then input into the computer 17, and the computer 17 can draw two fiber grating attenuation signals according to the collected signals. The ring-down curve in the oscillating chamber can be used to invert the concentration information of the gas to be measured.

According to the basic principle of optical cavity ring-down spectroscopy and Beer's law of absorption spectroscopy, with the ring-down of the light wave, the light intensity signal that decays with the exponential can be detected outside the cavity for:

In the formula, is the initial light intensity incident on the cavity, is the ring-down time in the cavity. The ring down time is derived by calculation for:

In the formula, is the cavity length, is the speed of light, is the absorption coefficient of the gas, is the concentration of the measured gas, is the reflectivity of the two-cavity mirror. Then by fitting the ring-down time of the two optical fiber ring-down cavities and , inversion to obtain the trace gas concentration.

When using the above device for trace gas detection, the specific process is as follows:

1) After the light source 1 is started, the emitted laser passes through the laser with a split ratio of 50:50 fiber optic coupler, by The optical fiber coupler divides the light source into two equal-intensity lights whose light intensity is half of the original light intensity;

2) Pour a high-concentration calibration gas with a known concentration into the upper (or lower) optical fiber ring-down cavity, let this optical path be channel I, and inject the gas to be measured into the lower (or upper) optical fiber ring-down cavity, Let this optical path be channel II. After the initial light source is divided into two light sources, it passes through the isolators in channel I and channel II at the same time, and enters two optical fiber ring-down cavities at the same time, forming oscillations in them respectively;

3) The photodetectors 13 and 14 respectively receive the ring-down optical signals output by the two optical fiber ring-down cavities focused by the focusing lenses 11 and 12, and convert them into electrical signals, which are then processed by the data acquisition cards 15 and 16. The data is collected and output to the computer 17 to obtain the ring-down curve of the calibration gas and the ring-down curve of the gas to be measured;

4) Set the ring-down curve of the calibration gas obtained , and the ring-down curve of the gas to be measured , calculate and deduce the ring-down time of the calibration gas according to the ring-down curve of the calibration gas , and calculate and deduce the ring-down time of the gas to be measured according to the ring-down curve of the gas to be measured . by the ring down time and The expression shows that the ring-down time is about the gas absorption coefficient function of calibration gas concentration Known, after the measurement system is determined, the optical path and the reflectivity of the fiber Bragg grating If both are fixed values, the concentration of the gas to be measured can be inverted by the fitting method .

It should be noted that the signal acquisition, signal processing, ring-down curve drawing and inversion of the concentration of the gas to be measured according to the ring-down curve involved in the above-mentioned measurement process are all existing mature technologies, and the utility model is not an improvement on the existing contribution of technology. The contribution of the utility model to the prior art lies in the improvement of the hardware structure of the detection device.

Claims (7)

1. the trace gas detection device in chamber is swung based on two-way light decay, comprise light source cell, light decay swings unit and Signal reception and processing unit, it is characterized in that, described light source cell comprises interconnective light source, beam splitter, described light decay swings module and comprises two identical light decays and swing chamber, the computing module that described Signal reception and processing unit comprise two photo-detectors and be connected respectively with two photo-detectors, two output terminals of light input end and beam splitter that described two light decays swing chamber are connected respectively, two light decays swing declining of chamber and swing light signal output end and be connected respectively with two photo-detectors.

2. trace gas detection device as claimed in claim 1, is characterized in that, it is that optical fiber declines and swings chamber that described light decay swings chamber.

3. trace gas detection device as claimed in claim 2, is characterized in that, described optical fiber decline swing chamber be fiber optic loop decline swing chamber, the coating of fine end declines and swings chamber or fiber grating and decline and swing chamber.

4. trace gas detection device as claimed in claim 3, is characterized in that, described optical fiber declines and swings chamber is that fiber grating declines and swings chamber, comprises the Fiber Bragg Grating FBG connected by photonic crystal fiber for a pair.

5. trace gas detection device as claimed in claim 1, it is characterized in that, described beam splitter is 50:50 beam splitter.

6. trace gas detection device as claimed in claim 1, it is characterized in that, described light source is fiber pulse laser.

7. trace gas detection device as claimed in claim 1, is characterized in that, swings in the connection light path of chamber and described beam splitter to be provided with one light can be made to light decay by beam splitter to be swung the optoisolator transmitted in chamber at each light decay.