Optical Feedback Cavity Enhanced Absorption Spectroscopy And Its Application In CH₄ And C₂H₂ Gas Detection

Abundant substance wealth has been created due to world’s industrialization in the 21th century.Meanwhile,some issues,such as air,water and soil pollutions have been emerging,so our habitat are overburdened.Methane（CH₄）and carbon dioxide（CO₂）are both crucial greenhouse gas,and are involved oxidation reaction of atmosphere.It plays an important role in the climate change.Acetylene（C₂H₂）,propane（C₃H₈）and other industrial gases are easily explosive gases in the air.Therefore,the measurement of trace gases is of significant importance in environment protection and safeties of lives and wealth.Laser absorption spectroscopy has been extensively used in gas detection due to its characteristics of high sensitivity,rapid response and on-line measurement.Optical Feedback Cavity Enhanced Absorption Spectroscopy（OF-CEAS）is a novel technique and it has been proposed and developed in recent years.The frequency of diode laser is locked into the external cavity if the phase and intensity of feedback laser fulfil the resonant requirement.Under this condition,the linewidth of laser is tremendously narrowed.Therefore,the sensitivity of OF-CEAS could be immensely improved due to high coupling efficiency.In this thesis,the theory of OF-CEAS was investigated,including Lambert Beer’s law,broadening mechanism and three common profile functions.The transmitted cavity mode signal is briefly introduced under optical feedback.The possible reasons of etalon effect in the OF-CEAS were investigated and the Ariy function was used to simulate the etalon noise in the transmitted signal.It was discovered that the optical fringes are from the interference of the incident mirror of cavity.It was validated through replacing the three mirrors with different thickness.Using an attenuator to control feedback rate in OF-CEAS system,an experimental setup was constructed for measuring the CH₄ gas.In the system,the cavity length is 60 cm,and its free spectral region（FSR）is approximate 250 MHz.The reflectivity of cavity mirror approaches 99.8%,so the corresponding finesse of the cavity is about 1569.Therefore,the effective pathlength of the cavity approaches 599 m.In experiment,the feedback intensity of laser was adjusted by a neutral density filter,and the feedback phase was regulated by an XYZ configuration stage and a piezo actuator.The stage is served for static adjustment and piezo is used for dynamic locking.A single mode distributed feedback laser（DFB）was employed to observe the absorption line of CH₄ at 1.65μm.Based on polarization-dependent phase elements to control feedback rate in OF-CEAS system,an experimental apparatus was constructed for measuring the C₂H₂ gas.In the setup,polarization optics（half wave plate,quarter wave plate and polarization splitting prism）were used to adjust the intensity of feedback laser.In comparison with the previous one,the polarized optical system can guarantee the fulfilling the feedback requirement but does not loose laser intensity.A method for reducing the optical interference was proposed based on fitting the etalon spectra by Ariy function.The signal without background could be derived via the observed signal subtracting the fitting.An apparatus was developed and verified by measuring the spectral line of C₂H₂.It was proved that the method can efficiently suppress the environmental influence to OF-CEAS.The sensitivity of the system was estimated at 7.143×10^-8（1σ）.