| Laser absorption spectroscopy?LAS?is a powerful tool for the study of interaction between electromagnetic field and molecule,atom or ion.It therefore has been widely used in detection of atmospheric pollution,investigation of precision spectra,pathologic diagnostic,national defense security and so on.And the development of high sensitive and resolution spectroscopy has been highlighted in scientific field.In the recent decades,based on direct absorption spectroscopy?DAS?,people have proposed various improved techniques,e.g.modulation spectroscopy,multipass cell technique and cavity enhanced spectroscopy.Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy?NICE-OHMS?is one of the most sensitive and resolution laser absorption spectroscopy,which is based on the combination of frequency modulation spectroscopy?FMS?to restrain 1/f noise and cavity enhanced absorption spectroscopy?CEAS?to increase the interaction length between the laser and the targeted gas.This dissertation is focused on NICE-OHMS,aimed to improve its detection sensitivity and applicability on trace gas detection and frequency standard.The contents can be divided into 6 parts:1.We introduce the principle,implementation method and the theoretical model of NICE-OHMS by the knowledge about modulation techniques,optical cavity technique,spatial mode match,Pound-Drever-Hall?PDH?locking and DeVoe-Brewer?DVB?locking;Then the limitation factors to the performance and the corresponding solutions are analyzed.2.A Whispering gallery mode?WGM?laser based NICE-OHMS has been established.The optical cavity is composed of two plane-concave mirrors.The reflectivity of the mirror is 99.994%in 1530 nm,indicating a high finesse of 55000.And the cavity length is 40 cm,corresponding to equivalent absorption length of 14km.By the feedback control of the laser PZT and acoustic-optic modulator?AOM?,PDH locking with a high bandwidth of 100 kHz has been realized.The functions of electro-optic modulator?EOM?,AOM and circuit model for radio frequency signal have been introduced.Then,the simulation of the NICE-OHMS signal under different pressure was implemented and the optimum pressure for maximum signal amplitude was offered.Based on this system,the C2H2 and CO2 transitions have been investigated and the detection sensitivity analysed by Allan-Werle plot has been obtained,resulting in 6.6×10-14 cm-1.3.A novel strategy named as differential NICE-OHMS which is based on the simultaneous measurement of transmission and reflection NICE-OHMS has been proposed.With consideration of cavity mirror loss,spatial mode match and impedance match,the transmission and reflection function of cavity have been offered.The relationship of molecular and background signal in transmission and reflection have been theoretically analyzed.Based on this,we propose differential NICE-OHMS and give the optimal condition for the maximal signal.We have verified this novel strategy by the measurement of C2H2 transition based on a erbium-doped fiber laser based NICE-OHMS system.The detection sensitivity has been evaluated to be4.7×10-14 cm-1.4.A balanced-detected NICE-OHMS system has been present.We introduce its principle and experimental setup.The efficiency of this novel method has been demonstrated by the measurement of background signals and Allan-Werle plot.And a detection sensitivity of 2.2×10-14cm-1 has been obtained.According to the measurement with different laser power,the white noise response has been found to be proportional to square root of power,indicating the system is shot noise limited.5.We have applied NICE-OHMS to the measurement the C2H2 from smokers'breath.The influence from interference between adjacent transitions on Doppler-Broadened and sub-Doppler NICE-OHMS has been studied.A novel method to restrain etalon noise by dithering the cavity position has been introduced.We analyze the efficiency of this method by the comparison of background signals with and without dither.Then,we measure the C2H2 in two smokers'breath.It is,to our knowledge,the first time to apply NICE-OHMS to practical application filed.6.A frequency standard has been obtained by sub-Doppler NICE-OHMS.The laser frequency has been locked to the CO2 transition center in 1.57?m with sub-Doppler signal acted as error signal.The laser linewidth has been evaluated by?-separation line method and relative frequency stabilization has been analyzed by Allan plot based on the frequency spectrum of the error signal.Finally,we have applied this frequency standard to frequency comb based cavity enhanced system which is used for the measurement of optical cavity modes.In conclusion,in this dissertation,three novel NICE-OHMS system,i.e.WGM based NICE-OHMS,differential NICE-OHMS and balanced detected NICE-OHMS,have been introduced which,successively produced three unprecedented detection sensitivity.And the first realization of shot noise limited Doppler-Broadened NICE-OHMS has been achieved.And,the first attempt to the implementation of NICE-OHMS to the practical application filed,which is for the measurement of C2H2in smokers'breath,has been introduced. |
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