Cavity ring-down spectroscopy-based 14CO2 abundance detection device

Abstract

The invention relates to the technical field of laser spectrum, in particular to a 14CO2 abundance detection device based on cavity ring-down spectroscopy. In order to improve the detection sensitivity of 14CO2, a narrow-linewidth reference light source is obtained by using an optical feedback technology, detection light is locked to the reference light source through an optical phase-locked loop technology, a narrow-linewidth detection light source capable of being precisely tuned is obtained, and then measurement of 14CO2 cavity ring-down spectroscopy is carried out.

Description

Technical Field

[0001] The present invention relates to the field of laser spectroscopy technology, and in particular to a method based on cavity ring-down spectroscopy. 14 CO2 abundance detection device. Background Art

[0002] Carbon-14 is a radioactive isotope of carbon. In nature, carbon-14 atoms are produced when neutrons from cosmic rays collide with nitrogen-14 nuclei. Carbon-14 atoms spontaneously decay into nitrogen-14 atoms, while releasing beta rays (electrons) and neutrinos. The half-life of carbon-14 is about 5730 years. Carbon-14 participates in the cycle within organisms and is constantly renewed. However, when an organism dies, there is no new carbon-14 to replenish it, and the carbon-14 remaining in the body will continue to decay, so the carbon-14 in the organism 14 C / 12 C will continue to decrease. Therefore, fossil energy buried underground for many years 14 C / 12 C is zero, so by monitoring the emissions from a region 14 CO2 can assess local fossil energy emissions. In addition, in response to global warming, monitoring carbon isotopes ( 14 Carbon-14 analysis is also used in a variety of other fields, such as dating, marine science, and geology.

[0003] The natural abundance of carbon-14 is 1.2×10 -12 Therefore, ultra-sensitive carbon-14 abundance measurement tools are needed. Liquid scintillation counter is a traditional carbon-14 detection method. The disadvantage of this method is that the sample amount required for measurement is large (about 1g pure carbon), the measurement time is long, and it cannot be detected in situ. The sensitivity of accelerator mass spectrometer (AMS) can reach 1×10 -15 , is the world's most sensitive carbon-14 detection technology. However, the accelerator mass spectrometer needs to prepare the sample as graphite, and the sample preparation process requires a lot of professional manual operations, the sample preparation cycle is long, and the size of the AMS is also very large. Because of the problems of these two technologies, it is impossible to achieve on-site and online monitoring of carbon-14. Laser spectroscopy technology is the next generation of carbon-14 measurement technology, which is expected to achieve real-time online measurement. The earliest laser absorption spectroscopy technology measurement 14 The CO2 abundance is measured by multi-path cell technology, but the multi-path cell is extended 14 The CO2 absorption path is usually only tens to hundreds of meters, and the sensitivity that can be obtained is limited. In addition, the large size of the multi-path cell requires more 14 CO2 samples, these shortcomings limit its application range.

[0004] Cavity ring-down spectroscopy is the most widely used 14 CO2 spectral measurement method. It uses a high-precision Fabry-Perot cavity to improve the laser and 14 The interaction path of CO2 molecules improves detection sensitivity. In the high-fineness cavity, 14 The CO2 absorption signal will be amplified by 2F / π times, where F is the finesse of the optical cavity.

[0005] Cavity ring-down spectroscopy is currently used to measure 14 The most widely used measurement light source for CO2 is quantum cascade laser (QCL), because this laser has a compact size and a wide wavelength tuning range. However, its line width is large, and the cavity mode line width of the high-precision optical cavity is very small, so the laser line width is much larger than the cavity mode line width, so the coupling efficiency between the laser and the optical cavity is not high. The low coupling efficiency leads to extremely low transmitted light intensity, so the signal-to-noise ratio of the ring-down signal is low, which increases the influence of detector noise and electronic noise and reduces 14 CO2 detection sensitivity.

[0006] In order to solve the above problems, it is necessary to develop a cavity ring-down spectroscopy technique for measuring narrow linewidth light sources. 14 The abundance of CO2. Summary of the invention

[0007] In view of the above problems, the present invention provides a method based on cavity ring-down spectroscopy. 14 The CO2 abundance detection device uses optical feedback technology to obtain a narrow-linewidth reference light source, and uses optical phase-locked loop technology to lock the detection light to the reference light source to obtain a precisely tunable narrow-linewidth detection light source. 14 The present invention can improve the coupling efficiency between the ring-down cavity and the detection light, thereby improving 14 CO2 detection sensitivity.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a cavity ring-down spectroscopy-based 14 A CO2 abundance detection device, comprising a first laser, a second laser and a radio frequency generator;

[0010] The first laser emits a laser as a reference light, the reference light is split into a first reference light through a first polarization beam splitter, and is reflected by the first polarization beam splitter to split into a second reference light, the first reference light is coupled into the ultra-stable cavity through a quarter wave plate, a first reflector, and a first matching lens in sequence; the second reference light enters the first detector through the beam splitter;

[0011] The second laser emits a laser as a detection light. The detection light is reflected by the second polarization beam splitter to separate a first detection light, and the second detection light is separated through the second polarization beam splitter. The first detection light is reflected back to the second laser by the second reflector. The second detection light passes through the isolator, is reflected by the third polarization beam splitter to separate a third detection light, and the fourth detection light is separated through the third polarization beam splitter. The third detection light enters the first detector through the beam splitter, and generates a beat signal with the second reference light. The beat signal and the radio frequency signal generated by the radio frequency generator enter the phase detector to generate an error signal. The error signal enters the loop filter to generate a control signal. The control signal enters the second laser. The fourth detection light passes through the second matching lens, the acousto-optic modulator, and the window of the vacuum chamber in sequence to enter the ring-down cavity in the vacuum chamber, and then enters the second detector. The output end of the second detector is connected to the input end of the pulse / delay generator, and the output end of the pulse / delay generator outputs a shutdown signal to the acousto-optic modulator.

[0012] Furthermore, a first piezoelectric ceramic is disposed on the first reflector, and a second piezoelectric ceramic is disposed on the ring-down cavity.

[0013] Further, the feedback phase is adjusted by adjusting the length of the first piezoelectric ceramic so that the feedback phase is always an integer multiple of 2π, and the length of the ring-down cavity is changed by changing the voltage of the second piezoelectric ceramic.

[0014] Furthermore, the temperature of the ring-down cavity is maintained at 170K, and the pressure is controlled at 1 kPa.

[0015] Furthermore, the first laser and the second laser are both quantum cascade lasers.

[0016] Compared with the prior art, the present invention has the following advantages:

[0017] 1. Compared with other optical phase-locked loops, the present invention first uses strong optical feedback to pre-narrow the line width of the slave laser to reduce the difficulty of phase locking, and then performs phase locking.

[0018] 2. Compared with other measurements 14 The cavity ring-down spectroscopy technology of CO2 abundance locks the detection light to the reference light through an optical phase-locked loop, and achieves precise control of the detection light frequency by tuning the RF source. 14 Precision measurement of CO2 spectrum. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Based on cavity ring-down spectroscopy 14 Schematic diagram of the CO2 abundance detection device.

[0020] Figure 2 Spectrum of the beat frequency signal locked by the optical phase-locked loop. DETAILED DESCRIPTION

[0021] In order to further illustrate the technical solution of the present invention, the present invention is further described below through embodiments.

[0022] like Figure 1 As shown, a method based on cavity ring-down spectroscopy in this embodiment 14 A CO2 abundance detection device, comprising a first laser 1, a second laser 13 and a radio frequency generator 10;

[0023] The first laser 1 emits a laser as a reference light, and the reference light is separated into a first reference light through a first polarization beam splitter 2, and is reflected by the first polarization beam splitter 2 to separate a second reference light. The first reference light is coupled into an ultra-stable cavity 7 through a quarter wave plate 3, a first reflector 4, and a first matching lens 6 in sequence. A first piezoelectric ceramic 5 is arranged on the first reflector 4. The leakage light of the front mirror of the ultra-stable cavity 7 returns to the first laser 1 through the original path to form optical feedback. The feedback rate is adjusted by adjusting the quarter wave plate 3, and the feedback phase is adjusted by adjusting the length of the first piezoelectric ceramic 5 in real time through the servo system, so that the feedback phase is always an integer multiple of 2Π, so as to obtain a reference light with narrow line width and stable frequency; the second reference light enters the first detector 9 through the beam splitter 8;

[0024] The second laser 13 emits a laser as a detection light, which is reflected by the second polarization beam splitter 14 to separate a first detection light, which is then passed through the second polarization beam splitter 14 to separate a second detection light, and the first detection light is reflected back to the second laser 13 by the second reflector 15 to form a strong optical feedback to pre-narrow the line width of the detection light; the second detection light passes through the isolator 16, is reflected by the third polarization beam splitter 17 to separate a third detection light, which is then passed through the third polarization beam splitter 17 to separate a fourth detection light, and the third detection light enters the first detector 9 through the beam splitter 8 to generate a beat signal with the second reference light, and the beat signal is The RF signal generated by the RF generator 10 enters the phase detector 11 to generate an error signal, and the error signal enters the loop filter 12 to generate a control signal, and the control signal enters the second laser 13 to stabilize the frequency of the detection light and narrow the line width; the fourth detection light passes through the second matching lens 18, the acousto-optic modulator 19, and the window of the vacuum chamber 20 in sequence and enters the ring-down cavity 21 in the vacuum chamber 20, and then enters the second detector 23, the output end of the second detector 23 is connected to the input end of the pulse / delay generator 24, and the output end of the pulse / delay generator 24 outputs a shutdown signal to the acousto-optic modulator 19.

[0025] By scanning the RF generator 10, the detection light resonates with the ring-down cavity 21. When the voltage of the second detector 23 reaches the threshold, the pulse / delay generator 24 drives the acousto-optic modulator 19 to send a shutdown signal, and then the acousto-optic modulator 19 cuts off the light entering the ring-down cavity 21 to generate a ring-down signal. Multiple measurements are made to suppress noise. By changing the voltage of the second piezoelectric ceramic 22, the length of the ring-down cavity is changed to improve the spectral resolution, and then the program is processed to obtain 14 CO2 absorption spectrum, and then infer 14 CO2 concentration, simultaneously measured 12 The CO2 concentration can be obtained 14 The relative abundance of C.

[0026] In this embodiment, a Stirling refrigerator and a temperature control system are used to maintain the temperature of the ring-down cavity 21 at 170K, and a pressure control technology is used to control the pressure of the ring-down cavity 21 at 1 kPa.

[0027] The first laser 1 and the second laser 13 of this embodiment are both quantum cascade lasers with a central wavelength of 4.527 um and a maximum power of 70 mW.

[0028] The first detector 9 of this embodiment is a mercury cadmium telluride infrared detector, and the high-frequency cutoff frequency is greater than 1 GHz.

[0029] The AOM 19 of this embodiment has an operating wavelength of 4500 nm, a center frequency of 41 MHz, a diffraction efficiency of 85%, and an extinction ratio of 1000:1.

[0030] In addition, the principle of the present invention is: first, using the first laser 1 as a reference light source, using optical feedback technology to lock the first laser 1 in a high-precision ultra-stable cavity with precise temperature control and ultra-high vacuum, so that the first laser 1 works in an ultra-narrow linewidth state, and the wavelength is long-term stable at 14 Near the characteristic absorption wavelength of CO2. Use the second laser 13 as the detection light source, first pre-narrow the line width of the second laser 13 through strong optical feedback, and then lock the second laser 13 to the first laser 1 through an optical phase-locked loop, so as to obtain a narrow line width detection light source that can be precisely tuned, so that the detection light and the high-fineness ring-down cavity can be efficiently coupled. Use a Stirling refrigerator to control the low temperature of the ring-down cavity, so that the temperature of the ring-down cavity is 170K, and the temperature fluctuation is less than 0.1K. Use pressure control technology to make the pressure in the ring-down cavity 1kPa. Thereby reducing the broadening of the spectral line and reducing the 13 Finally, the interference of CO2 and N2O 14 The CO2 gas sample was subjected to cavity ring-down precision spectroscopy measurement to obtain 14 CO2 spectral signal, obtained 14 CO2 abundance. And through multiple averaging methods, the noise is reduced and the detection sensitivity is improved.

[0031] Figure 2 OPLL represents the spectrum of the beat signal locked by only the optical phase-locked loop, and OF-OPLL represents the spectrum of the beat signal locked by the optical phase-locked loop combined with strong optical feedback. Figure 2 It can be seen that the spectrum of the beat signal after locking with strong optical feedback has a stronger noise suppression effect at high frequencies and a higher signal amplitude at the center frequency, indicating that the locking performance of the system is improved after combining with strong optical feedback.

[0032] The above shows and describes the main features and advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and scope of the equivalent elements of the claims are included in the present invention.

[0033] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method based on cavity ring-down spectroscopy 14 The CO2 abundance detection device is characterized by: It comprises a first laser (1), a second laser (13) and a radio frequency generator (10); The first laser (1) emits laser light as reference light, the reference light passes through a first polarization beam splitter (2) to split into a first reference light path, and is reflected by the first polarization beam splitter (2) to split into a second reference light path, the first reference light path passes through a quarter wave plate (3), a first reflector (4), and a first matching lens (6) in sequence to couple into an ultra-stable cavity (7); the second reference light path passes through a beam splitter (8) to enter a first detector (9); The second laser (13) emits a laser as a detection light. The detection light is reflected by the second polarization beam splitter (14) to generate a first detection light. The second detection light is transmitted through the second polarization beam splitter (14) to generate a second detection light. The first detection light is reflected back to the second laser (13) by the second reflector (15). The second detection light passes through the isolator (16) and is reflected by the third polarization beam splitter (17) to generate a third detection light. The fourth detection light is transmitted through the third polarization beam splitter (17). The third detection light enters the first detector (9) through the beam splitter (8) and generates a beat signal with the second reference light. The beat signal is transmitted to the radio frequency transmitter (11). The radio frequency signal generated by the generator (10) enters the phase detector (11) to generate an error signal, the error signal enters the loop filter (12) to generate a control signal, the control signal enters the second laser (13), the fourth path of detection light passes through the second matching lens (18), the acousto-optic modulator (19), and the window of the vacuum chamber (20) in sequence to enter the ring-down cavity (21) in the vacuum chamber (20), and then enters the second detector (23), the output end of the second detector (23) is connected to the input end of the pulse / delay generator (24), and the output end of the pulse / delay generator (24) outputs a shut-off signal to the acousto-optic modulator (19).

2. A cavity ring-down spectroscopy-based method according to claim 1 14 The CO2 abundance detection device is characterized by: The first path of detection light is reflected back to the second laser (13) by the second reflector (15), and the line width of the laser is pre-narrowed by using strong optical feedback to reduce the difficulty of phase locking, and then phase locking is performed.

3. A cavity ring-down spectroscopy-based method according to claim 1 14 The CO2 abundance detection device is characterized by: The third path of detection light enters the first detector (9) through a beam splitter (8) and generates a beat frequency signal with the second path of reference light. The beat frequency signal and the radio frequency signal generated by the radio frequency generator (10) enter a phase detector (11) to generate an error signal. The error signal enters a loop filter (12) to generate a control signal. The control signal enters a second laser (13). The detection light is locked to the reference light through an optical phase-locked loop, and precise control of the detection light frequency is achieved by tuning the radio frequency source.

4. A cavity ring-down spectroscopy-based method according to claim 1 14 The CO2 abundance detection device is characterized by: A first piezoelectric ceramic (5) is arranged on the first reflector (4), and a second piezoelectric ceramic (22) is arranged on the ring-down cavity (21).

5. The method according to claim 1 based on cavity ring-down spectroscopy 14 The CO2 abundance detection device is characterized by: The feedback phase is adjusted by adjusting the length of the first piezoelectric ceramic (5) so that the feedback phase is always an integer multiple of 2π, and the length of the ring-down cavity (21) is changed by changing the voltage of the second piezoelectric ceramic (22).

6. The method according to claim 1 based on cavity ring-down spectroscopy 14 The CO2 abundance detection device is characterized by: The temperature of the ring-down cavity (21) is maintained at 170K, and the pressure is controlled at 1 kPa.

7. The method according to claim 1 based on cavity ring-down spectroscopy 14 The CO2 abundance detection device is characterized by: The first laser (1) and the second laser (13) are both quantum cascade lasers.

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