Ultrasensitive high resolution laser spectroscopy and its application to optical frequency standards

Advanced laser stabilization techniques now enable one to lock laser frequencies onto line centers of natural atomic/molecular resonances with unprecedented precision and accuracy. In this dissertation we discuss our effort in utilizing these techniques to establish visible optical frequency standards. By summarizing our earlier results on frequency measurements of the {dollar}sp{lcub}87{rcub}{dollar}Rb D{dollar}sb2{dollar} line at 780 nm {dollar}sp{lcub}127{rcub}{dollar}I{dollar}sb2{dollar} hyperfine transitions at 532 nm, we show the advantage of using a higher quality reference line, usually characterized by its narrower linewidth, higher attainable signal-to-noise ratio and lower sensitivity toward external perturbations.; We then present a novel approach of cavity-enhanced frequency modulation spectroscopy for ultra-sensitive detections. The powerful utility of this new technique in the field of frequency standards is demonstrated by probing saturated molecular overtone transitions in the visible and near infrared. Weakly-absorbing gases such as {dollar}rm Csb2Hsb2{dollar} and C{dollar}sb2{dollar}HD are placed inside an external high-finesse resonator to enhance their detection sensitivities. A frequency modulation technique is employed to achieve a shot noise limited signal-to-noise ratio. The rf modulation frequency is chosen to match the cavity's free spectral range in order to avoid the cavity-induced conversion of laser frequency noise into amplitude noise. The molecular saturated dispersion signal is directly recovered after demodulation of the cavity transmitted light. A record high integrated absorption sensitivity of {dollar}5times 10sp{lcub}-13{rcub} (1times 10sp{lcub}-14{rcub}{dollar}/cm) (at 1 second averaging time) has been obtained.; Systematic studies on this new technique are presented on topics of detection sensitivity, signal line shape, signal size and slope, and pressure dependent linewidth broadening and linecenter shift. A Nd:YAG laser is stabilized on the P(5) transition in the ({dollar}nusb2+3 nusb3{dollar}) overtone band of C{dollar}sb2{dollar}HD at 1.064 {dollar}mu{dollar}m. Its absolute frequency is established. The excellent signal-to-noise ratio produces a frequency Allan variance of {dollar}3times10sp{lcub}-13{rcub}{dollar} at 1 second averaging, improving to {dollar}1times 10sp{lcub}-14{rcub}{dollar} after 800 sec. Selection of slow molecules with low power and low gas pressure produces a linewidth thirteen times below the room temperature transit time limit. The {dollar}rm Csb2Hsb2{dollar} ({dollar}nusb1+3 nusb3{dollar}) overtone band near 790 nm is also studied by a tunable Ti:Sapphire laser. The possibility of establishing optical reference frequency networks based on molecular ro-vibrational transitions in overtone bands is discussed.; As an important and effective tool in optical frequency metrology, an efficient optical frequency comb (OFC) generator has also been developed. A heterodyne beat signal between a HeNe laser and a tunable diode laser parked as far as 1.5 terahertz away is demonstrated with the comb generator. The future use of the OFC generator in our phase coherent optical frequency chain is discussed.