Characteristics And Application Study Based On Continuous Wave Cavity Ring-Down Spectroscopy For Gas Concentration Measurement

Gas concentration monitoring for industrial safety and human life is of great significance. Toxic, flammable and explosive gases produced, to varying degrees, in industrial production seriously pollute the environment and threaten human health and industrial safety. With the rapid development and infiltration of spectroscopy, chemical analysis, materials science, laser technology and optical fiber technology, there have been developed new methods and theories on gas concentration measurement including cavity ring down, film absorption and cavity enhanced absorption. Continuous wave cavity ring down spectroscopy (CW-CRDS) has become a research hotspot in recent years due to its higher spectral resolution and sensitivity. In this paper, characteristics of CRDS and the effect of relevant parameters are studied in depth, and gas concentration measurements based on CRDS are investigated theoretically and experimentally.Gas molecule near infrared selective absorption theory is discussed in detail, the theoretical basis for measuring gas absorption is presented and absorption spectra of various gases are investigated. Moreover, fundamental principles of CRDS technology used in gas concentration measurement are given and meanwhile, the intrinsic characteristics of passive optical cavities, along with concepts of laser and CRD modes, are introduced.Two methods for the coupling between laser frequency and cavity modes are presented and the relationship between output electric fields and incident light frequencies, scanning speed and dielectric absorption in both methods is deduced detailedly. Reflectivity of different cavity mirrors, together with the relationship between scanning speed and transmission characteristics, are in depth studied.The cause and suppression of two kinds of noises in the ring down cavity are analyzed in detail, and it's pointed out that as for the continuous wave CRD technology, while the TEM00 mode is excited in the cavity, high order transverse modes, TEMmn, are also excited by the scatter coupling on the mirror's surface, and when these modes degenerate approximately, even if coupling intensity of each reflection is very low, the coupled modes can still cause beating noise in ring down signals, which is the first noise. In experiments, a diaphragm placed on a special position in the cavity is used to restrain high order transverse modes, thus this noise can be reduced effectively.The second noise is the beating noise brought by residual strain caused by linear birefringence of cavity mirrors. On the high reflective coatings of the cavity mirrors, birefringence due to linear residual strain or stress also makes both polarization states of TEM00 mode away from degeneracy, providing two new eigen polarization states that are very close to linear polarization. Since polarization relies on the loss of high reflective mirrors, the new eigenstates are no longer orthogonal, thus beating noise is generated in ring down signals. With two polarizers placed in front of and behind the mirror respectively, this noise can be removed .The laser detuning method used for rapidly shutting off the incident laser is proposed, and an additional optical switch is not required to shut the light source, which makes the system simpler. On this basis, a new CW CRDS gas concentration measurement system is designed, and by using this system gas concentration is analyzed experimentally.Fiber loop ring down spectroscopy (FLRDS) is a highly sensitive absorption spectroscopy technology based on the combination of CRDS and optical fiber sensing technology. A dual loop gas concentration measurement system is designed to overcome the defects of FLRDS. Erbium doped fiber amplifier (EDFA) with gain stability is designed and relevant experiments are performed. Effects of amplified spontaneous emission (ASE) noise on the measurement accuracy of the system are investigated, and in order to eliminate the ASE noise the adaptive filtering method is brought forward. Finally, ring down characteristics for gases of different concentrations are analyzed and the relationship between ring down time and gas concentration is also studied.