High-speed And Strong Absorption Gas Sensing Technology Based On External Modulation

Nowadays,gas sensors have been widely used in fields such as safety production,health monitoring,environmental protection,energy-saving,and basic research.The gas sensing method based on absorption spectroscopy is a non-contact measurement method.Compared with the commonly used gas sensing methods based on semiconductor devices,the gas sensing method based on absorption spectroscopy doesn't require an adsorption process to complete the measurement.It has the advantages of fast measurement speed,anti-electromagnetic interference,and non-contact measurement.However,in terms of combustion detection and high-speed fluid measurement,the measurement speed of the gas sensing method needs to be further improved.In extreme environments,there is also the problem of not being able to accurately measure strong absorption signals.This thesis takes the laser frequency external modulation as the core and takes the measurement of acetylene gas as an example to carry out research work on high-speed and strong-absorption gas sensing.The main research contents are as follows:Aiming at the problem of slow measurement speed in existing fast gas sensing methods,this thesis proposes a fast gas sensing scheme based on an optical linear chirp chain.This scheme first applies the laser frequency external modulation to obtain an optical linear chirp chain with a repetition frequency of 2.5 MHz.Then this optical linear chirp chain is used to detect fast gas processes with a time resolution of400 ns,a spectral resolution of?10 MHz,and a duty cycle of 100%,and the uncertainty is less than 1%.The use of a non-local mean filtering algorithm improves the signal-to-noise ratio of the absorption spectrum without reducing the time resolution and spectral resolution of the measurement results.In the experiment,this method was used to detect the rapid gas process that occurred within 4 ms,and 10000frames of effective absorption spectrum signals were obtained.In order to realize the detection of fast gas processes occurring under normal pressure,this thesis proposes a fast gas sensing scheme based on an extended optical linear chirp chain.This scheme first generates linear chirped light pulses through laser frequency external modulation.Each time the light pulse passes through the optical loop,its frequency will move by 8 GHz.By setting a reasonable optical loop length and pulse width of linear chirped light pulses,11 linear chirped light pulses can be connected end to end in the time domain and frequency domain to form an extended optical linear chirp chain with a spectral width of 88 GHz,The minimum detectable limit of this program is 260 ppm.This method breaks through the restriction of the electronic instrument bandwidth to the detection spectrum width and improves the gas sensing performance based on external modulation of the laser frequency.Aiming at the restriction of the performance of hardware instrum ents and equipment on the measurement speed,this thesis proposes a fast gas sensing scheme combined with compressed sensing.This scheme can increase the measurement speed by 14 times without adding additional equipment and save 93%of storage space.Compressed sensing technology can collect signals at a sampling rate lower than the minimum sampling rate required by the Nyquist sampling law,and obtain a reconstructed absorption spectrum that is highly consistent with the original signal through a reconstruction algorithm.It is verified by simulation experiments that only7%of the total data can be used to reconstruct a reconstructed spectral signal that is highly consistent with the original absorption spectral signal,and the magnitude of the residual is only 10^-6.In the experiment,the measurement speed was increased by14 times using this scheme,and the correlation between the reconstructed absorption spectrum and the original absorption spectrum was 0.9952.In order to achieve accurate detection of strong absorption signals,this thesis proposes a microwave photon measurement technology based on externally modulated lasers.This technology uses laser frequency external modulation to obtain swept-frequency sideband signals and use them as probe light.By detecting the beat signal between the carrier and the sideband,the measurement of strongly a bsorbing gas(absorption rate?99.999%)is realized,and the absorption detection range of about 50 d B is provided.The frequency linearity error of the absorp tion spectrum obtained by this scheme is only 2×10^-4.Using this technology in the reverse direction,a microwave photonic filter with a 20 GHz adjustable frequency and a band rejection ratio greater than 60 d B can be produced.This thesis first proposes a fast gas sensing scheme based on an optical linear chirp chain,which uses the optical linear chirp chain as the detection light to realize fast gas sensing.Secondly,a fast gas sensing scheme based on optical loop spread spectrum is proposed,which expands the frequency sweep range of the optical linear chirp chain through the optical loop,so that the technology can be used for rapid gas process measurement under normal pressure.A fast gas sensing scheme combined with compressed sensing technology is also proposed to further improve the measurement speed of fast gas sensing.Finally,a microwave photon measurement technique is proposed for the measurement of strong absorption signals.The sensing scheme proposed in this thesis is expected to be used for aero-engine testing and strong absorption environment testing.