CN104819957A - CRDS principle-based gas concentration measurement system of continuously adjustable laser light source - Google Patents

Abstract

The invention relates to a gas concentration measurement system based on the CRDS principle of a continuously adjustable laser light source, which includes a part of the optical path device and a part of the circuit device; the part of the optical path device includes a laser, an optical isolator, an optical collimator, a wavelength meter, an optical Passive cavity; the circuit components include a photoelectric detector, a control circuit, a PZT cavity length modulator, a high-speed data acquisition card and a computer. The invention only uses light of one polarization direction for detection, and the connection with the optical collimator is connected by a polarization-maintaining optical fiber, which saves the optical isolator in the system, and can obtain a better polarized laser beam; and solves the problem that in the ring cavity, The problem that the S component and the P component have different resonant frequencies and therefore cannot resonate at the same time. The invention simplifies and omits some devices with polarization beam splitting and adjusting functions, improves the light energy utilization rate of the laser, realizes the switching function of the laser through current tuning, thereby omitting the AOM and its related control system.

Description

A Gas Concentration Measurement System Based on CRDS Principle with Continuously Adjustable Laser Source

technical field

The invention relates to the technical field of CRDS measurement systems, in particular to a gas concentration measurement system based on the CRDS principle of a continuously adjustable laser light source.

Background technique

In the traditional CRDS measurement system, the high-quality optical passive cavity used to contain the gas to be measured usually adopts a two-mirror straight cavity. The intensity of the pulsed beam established in the cavity decays with time. For a cavity, the attenuation speed of the light intensity in the cavity is related to the reflectivity of the cavity mirror, the distance between the two cavity mirrors and the speed of light in the cavity. When the optical passive cavity is filled with the gas to be measured, the attenuation speed of the light intensity in the cavity increases. Under good conditions, the light intensity in the cavity will decay with a single e number. In this way, the intracavity absorption spectrum is obtained by plotting the relationship between the decay rate of the intracavity light intensity and the corresponding laser wavelength.

The traditional pulsed CRDS measurement system has several problems to be solved urgently. The laser usually uses a pulsed semiconductor laser, and the data acquisition rate is limited by the pulse frequency of the laser. In addition, due to the small overlap between the laser frequency of the pulsed laser and the natural spectral frequency of the cavity, the stable light intensity coupled into the optical passive cavity and established in the cavity will be small due to the influence of the pulse frequency of the pulsed laser.

The traditional CRDS measurement system adopts Drever's PDH method. Drever et al. used the phase characteristics of the optical passive cavity to form a frequency stabilization system, and used the frequency modulation spectrum technology to obtain the dispersion curve of the optical optical passive cavity as a frequency discrimination curve. The laser is slightly phase modulated to produce two sidebands distributed on both sides of the carrier frequency, with equal amplitudes but opposite initial phases. Due to the influence of the optical passive cavity or other media, the amplitude or phase of the two sidebands has an uneven change, the double-beat signal cannot be completely canceled, and the receiver will output a frequency signal. The output signal is used to generate a frequency discrimination curve similar to the phase shift curve of an optical passive cavity, so that an effective error signal can be generated in a small range to adjust the cavity length. Therefore, the system is not easy to lose lock, and the anti-interference ability very strong.

Contents of the invention

The invention aims to solve the technical problems in the prior art, and provides a gas concentration measurement system based on the CRDS principle of a continuously adjustable laser light source.

In order to solve the problems of the technologies described above, the technical solution of the present invention is specifically as follows:

A gas concentration measurement system based on the CRDS principle of a continuously adjustable laser light source, including some components of the optical path and some components of the circuit;

The optical path components include a laser, an optical isolator, an optical collimator, a wavelength meter, and an optical passive cavity; the laser contains the absorption light frequency of the gas to be measured; the function of the optical isolator is to protect the laser from feedback The influence of light and the polarization characteristics of light in the control branch; the function of the optical collimator is to adjust the quality of the outgoing beam of the laser, so as to facilitate mode matching with the optical passive cavity;

The parts of the circuit include a photodetector, a control circuit, a PZT cavity length modulator, a high-speed data acquisition card and a computer;

After a stable optical path is formed in the cavity of the optical passive cavity, a strong optical signal is transmitted through the output mirror of the optical passive cavity, and the outgoing light is divided into two beams by the beam splitter, and one beam of light is detected by a photodetector to obtain a laser Information on whether the matching with the optical passive cavity is good or not, through the spot distribution of the light emitted from the cavity, it is determined that the laser only excites the fundamental mode Gaussian beam corresponding to the optical passive cavity; the other beam is detected by a photodetector to obtain Judgment of light intensity threshold accumulation in the cavity and measurement of decay rate; finally, strong ring-down spectrum is obtained by measuring light intensity decay rate.

In the above technical solution, a pumping and filling device for assisting the low-loss optical cavity is also provided.

In the above technical solution, the laser is a DFB laser.

In the above technical solution, the optical passive cavity is composed of two plane mirrors and one spherical mirror, wherein the two plane mirrors are the input mirror and the output mirror respectively, and all three mirrors have ultra-high reflectivity corresponding to the absorption frequency of the gas to be measured.

In the above technical solution, the polarization state of the laser used is S-component polarized light, and when determining the mirror coating in the optical passive cavity and the distance between the mirrors, it is necessary to perform installation modulation for the S-component.

In the above technical solution, the laser beam expander and mode matching mirror are used to match the laser output light mode with the eigenmode of the optical passive cavity, and the transverse spot size of the beam is adjusted through the diaphragm, and the irregular transverse distribution Part of the light is intercepted.

The present invention has following beneficial effect:

1) The solution of triangular annular cavity and related problems. The triangular annular cavity has the following advantages: (1) It can realize a long absorption path in a limited cavity material. For a tuned system, this can reduce the cavity longitudinal mode interval, thereby improving the spectral resolution of the system; (2) In addition to the input and output cavity mirrors, the addition of folding mirrors facilitates the adjustment of the PZT cavity length modulator in the tuned system. Installation; (3) When the laser is selected to be incident into the cavity from the folding mirror, it can prevent the light beam from being directly reflected back to the laser, thereby saving the optical isolator. It is characterized in that it uses a triangular ring cavity and adopts a laser light source with polarization characteristics, and only uses light in one polarization direction for detection, and the connection with the optical collimator is connected by a polarization-maintaining optical fiber, so as to save the optical isolator in the system , and can obtain a better polarized laser beam; and solve the problem that in the ring cavity, the S component and the P component have different resonance frequencies and therefore cannot resonate at the same time.

2) The wavelength locking system of the laser. In the present invention, the DFB laser with tunable function is adopted. Because the laser light source has the function of temperature and current tuning, the wavelength meter is used to determine the temperature and current tuning coefficient of the laser, and the data control method can be adopted in the product. By controlling The temperature and current realize the precise tuning of the frequency of the laser and the frequency locking system between the laser and the optical passive cavity. The temperature and current control of the above laser is combined with the current high-precision wavelength meter, and the frequency of the laser is stabilized by error feedback tuning. Compared with the traditional PDH method, this method not only reduces the number of control devices in the instrument, reduces the difficulty of the control system, but also realizes the frequency stabilization of the laser light source for a long time.

3). The present invention realizes multi-point measurement on the absorption spectrum line. Accurately measuring and controlling temperature and pressure is critical for quantitative analysis. Traditional cavity ring-down spectroscopy (CRDS) instruments measure absorbance at only two points, the wavelength of maximum absorption and the baseline wavelength, between which the laser is emitted alternately, leading to multiple sources of noise and error . First, simple statistics show that single-point peak height measurement is limited because it is affected by shot noise (double-mirror chamber); second, this method assumes that the laser is always at the center of the spectral line, which is No, even if the laser wavelength does not drift away from the center of the spectral line, the baseline and peak lines will drift away, unless the temperature and pressure of the equipment can be accurately and stably controlled. Third, if the absorption line of the non-target gas overlaps with the absorption line of the target gas, even if the wavelength is close to the selected baseline of the target gas, the equipment for measuring the two points cannot identify it. All of these will lead to inaccurate measurement results. In fact, the present invention adjusts the laser wavelengths corresponding to different positions of the absorption line along the absorption line of the spectrum of the target gas, and calculates the concentration of the target gas by finely matching the absorption peaks.

4), increase the mode detection system. A CCD is connected to one end of the detector to detect the distribution of the light spot emitted from the optical passive cavity, which ensures that the laser is coupled with the fundamental mode of the optical passive cavity. In addition, the system is also supplemented with a set of high-precision laser wavelength calibration system and vacuum system (not shown in the figure), which are used for the determination of DFB laser wavelength and tuning coefficient, and the pumping and filling of low-loss optical cavity. In addition to the hardware, the normal operation of the spectroscopy system also needs to be equipped with corresponding software. The functions to be completed by the software part include: coordinating the work of each module of the hardware, collecting and analyzing cavity ring-down signals, and displaying cavity loss values and spectral curves, etc. By using a DFB laser with polarization characteristics, a certain optical isolation function, and current tunable, only one polarization direction is used to detect the gas in the cavity. Through such simplification, some devices with polarization beam splitting and adjusting functions are omitted, and the light energy utilization rate of the laser is improved; the switching function of the laser is realized through current tuning, thereby omitting the AOM and its related control system.

Description of drawings

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Figure 1 is a schematic diagram of the system configuration of the measurement system.

Fig. 2 is a schematic diagram of the measurement system of the present invention.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings.

Figure 1 is the preferred system for the present invention, with optical connections indicated by dotted lines and electronic connections indicated by solid lines. The system includes: a tunable single-frequency light source 1, a ring-down optical passive cavity 20 connected in the optical path, a sample detection electronic system 30, an indicating electronic system 40, a locking electronic system 50 and the optical passive cavity between optical connections. The connection of the locking system between the optical passive cavity and the laser is an electronic connection. The indicator electronics system 50 is electronically connected to the sample detection.

The schematic diagram of the measurement system designed by the present invention is shown in Figure 2. The system uses the DFB (Distributed Feed-Back) laser as the light source, and realizes the rapid shutdown of the continuous laser through the current modulation of the DFB laser; through parameters such as laser beam shaping and cavity structure The mode coupling of the two is realized by the design of the two; the frequency matching between the two is realized by tuning the injection current of the DFB laser and scanning the cavity length of the passive cavity. Therefore, compared with the traditional spectroscopic system, the system can save system cost and reduce system volume. As can be seen from the above figure, the hardware of the system can be divided into two parts, optical and electrical, with a total of 7 modules. Among them, the components of the optical path mainly include lasers, optical isolators, optical collimators, and low-loss optical cavities; the components of the circuit mainly include photodetectors, control circuits, PZT cavity length modulators, high-speed data acquisition cards, and computers.

The laser light source 1 should contain the absorption light frequency of the gas to be measured, and the output beam is a fundamental-mode Gaussian beam with polarization characteristics. The ideal laser bandwidth is 2MHz, and the laser output power is 10mW. Laser 1 is a DFB laser with temperature and current tuning functions . For safety reasons, a Fabry optical isolator 2 is usually connected behind the laser 1. The optical isolator 2 has two functions, one is to protect the laser from the feedback light; the other is to control the polarization characteristics of the light in the branch . The same cavity mirror has different reflectivity for light with different polarization characteristics (S, P), and we choose the S component as the probe light. The light emitted from the optical isolator 2 is polarized light in the S direction. The optical passive cavity is composed of two plane mirrors and one spherical mirror. The two plane mirrors are the input mirror 6 and the output mirror 8 respectively. All three mirrors have ultra-high reflectivity corresponding to the absorption frequency of the gas to be measured. One end of the spherical mirror 7 is connected with a cavity length adjusting device 9 (PZT). By adjusting the distance between the mirrors, the resonance frequency of the fundamental mode mode existing in the stable cavity is the absorption frequency of the gas to be measured.

The function of the optical collimator 3 is to adjust the quality of the beam emitted by the laser, so as to facilitate the mode matching with the optical passive cavity. The optical collimator 3 is connected with an aperture device 5 to control the size of the light spot, which is used to filter out the edge light with poor beam distribution characteristics. Through the function of the matching lens 4, the incident light beam is efficiently coupled into the optical passive cavity, and the light beam coupled into the optical passive cavity will go back and forth in the cavity for many times to establish a pulse light intensity signal with a certain intensity, and finally the outgoing light is transmitted by the Specular exit.

After forming a stable optical path in the cavity, a strong optical signal is transmitted through the output mirror 8 of the optical passive cavity, and the outgoing light is divided into two beams by the beam splitter, and one beam is detected by the detector CCD14 to obtain the matching between the laser and the passive cavity Whether the information is good or not, it is determined that the laser 1 only excites the fundamental mode Gaussian beam corresponding to the optical passive cavity through the spot distribution of the cavity exit light. For the specific theory and experimental process, please refer to the relevant literature. The other beam of light is detected by a photodetector 13 to obtain the judgment of the threshold accumulation of light intensity in the cavity and the measurement of the decay rate. Finally, the strong ring-down spectrum is obtained by measuring the decay rate of light intensity.

Apparently, the above-mentioned embodiments are only examples for clear description, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or changes derived therefrom are still within the scope of protection of the present invention.

Claims (6)

1. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source, is characterized in that, comprises light path part device and circuit part device;

Described light path part device comprises laser instrument, optoisolator, optical collimator, wavemeter, optical passive chamber; Described laser instrument comprises the absorption light frequency of gas to be measured; The effect of described optoisolator is protection laser instrument from polarisation of light characteristic in the impact of feedback light and controlling brancher; The effect of described optical collimator is the quality regulating laser emitting light beam, is convenient to carry out pattern match with optical passive chamber;

Described circuit part device comprises photodetector, control circuit, the long modulator in PZT chamber, high-speed data acquisition card and computing machine;

Formed after stablizing light path in the chamber in described optical passive chamber, at the light signal that optical passive chamber output mirror transmission is stronger, emergent light is divided into two-beam by beam splitter, light beam is obtained laser instrument and the whether good information of optical passive chamber match condition by photodetector detection, by distributing to the hot spot of chamber emergent light, determine the fundamental-mode gaussian beam that laser instrument has only excited optical passive chamber corresponding; Another light beam is detected by photodetector, obtains the judgement of Intensity threshold accumulation and the measurement of rate of decay in chamber; Strong ring-down spectroscopy is obtained finally by the measurement of light intensity attenuation speed.

2. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source according to claim 1, is characterized in that, what be also provided with auxiliary low damage optics cavity takes out aerating device.

3. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source according to claim 1, is characterized in that, described laser instrument is Distributed Feedback Laser.

4. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source according to claim 1, it is characterized in that, described optical passive chamber is made up of two level crossing one spherical mirrors, wherein two level crossings are respectively input minute surface and export minute surface, and three mirror contact lens all crosses the ultra-high reflectivity corresponding with gas absorption frequency to be measured.

5. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source according to claim 4, it is characterized in that, laser instrument polarization state used is S component polarization light, to the mirror plated film in optical passive chamber and the timing really of mirror spacing, all need to carry out installation modulation for S component.

6. the gas concentration measurement system based on CRDS principle of continuously adjustabe LASER Light Source according to claim 1, it is characterized in that, by laser beam expander and pattern match mirror, the eigenmodes of laser emitting optical mode with optical passive chamber is mated, and adjusted by the horizontal spot size of diaphragm to light beam, the irregular part light of cross direction profiles is tackled.