Frequency Metrology Based On Cavity Enhanced Molecular Spectroscopy

Molecular spectroscopy,as a technical means to study molecular structure and intermolecular interactions,has important applications in various fields.In the field of basic scientific research,high-precision molecular spectroscopy can be used to test physical theories,determine fundamental physical constants,and as molecular frequency standards.The main work of this thesis is to measure the saturated absorption spectroscopy of molecules using different cavity-enhanced absorption spectroscopy techniques.The saturated absorption spectra of ro-vibrational transitions of molecules are used to stabilize laser frequencies.The work of the thesis mainly includes the following parts:In chapter 1,four different cavity-enhanced absorption spectroscopy techniques and their characteristics are introduced.Applications of cavity-enhanced absorption spectroscopy in precise measurement of molecular transition frequencies are reviewed.In chapter 2,the configuration of cavity-enhanced absorption spectroscopy instrument is introduced in detail,including the home-made diode lasers,the design of F-P cavity and frequency calibration of lasers and optical combs,mode matching between F-P cavity and laser,and principles of PDH locking and DVB locking.Eventually,we can carry out different cavity-enhanced spectroscopy with one experimental setup.In chapter 3,four kinds of cavity-enhanced absorption spectroscopy techniques are used to measure saturated absorption spectra of acetylene.The sensitivity and frequency precision of different methods are compared,and we verify the consistency among measurements of different methods.The saturated absorption spectrum of HD was measured by this experimental setup,and it was found that the profile of the HD line near 1.4 μm was different from that observed in normal saturated absorption spectra of molecules.In chapter 4,frequency stabilization of laser is carried out by using cavity-enhanced saturated absorption spectroscopy.Experimentally,we used two different methods,namely,calibrating the laser frequency by saturated absorption spectroscopy and locking the laser to the molecular transition line.Comparison between the two methods indicates that the long-term stability of the laser frequency in both methods can reach 40 Hz(2E-13),and a better stability of 10 Hz(5E-14)has been achieved by locking the laser to a molecular transition line.