15min:

JUN YE, Quantum Optics Group, CalTech, Pasadena CA 91125; MA LONG-SHENG, Quantum Physics Lab, East China Normal University, Shanghai, PR China; JOHN L. HALL AND MATTHEW S. TAUBMAN, JILA, University of Colorado and NIST, Boulder CO 80309-0440.

Improved physical understanding of the REAL sensitivity-limiting processes, along with better technical solutions for cavity-enhanced optical heterodyne spectroscopy recently have allowed remarkable sensitivity improvements, presently at the absorption level of 5.2 x10^-13 for a 1 s integration . Here we review the several problems which led to this NICE-OHMS solution, and report recent progress with active control of the Residual Amplitude Modulation produced by the Electro-Optic Modulator.

Also, saturated absorption spectra near 1064 nm for HCCD, HCCH, and CO₂ are presented. The two additional lines are ¹²C₂H₂ (2₁ + ₂ + ₅) R(12) and ¹²C¹⁶O₂ (2₁ + 3₃) R(6) , with their respective transition dipole moments of 50 µDebye and 6 µDebye. They are both weaker than our usual C₂HD(₂ + 3 ₃) P(5) transition, which has a transition dipole moment of ~70 µDebye, but all are recovered with excellent signal-to-noise ratios. The absolute resonance center frequencies of all three transitions have been measured (+/- 25 kHz) using as reference a Nd:YAG laser locked via frequency doubling on the a₁₀ hyperfine-structure component of the R(56) 32-0 I₂ transition. The C₂H₂ resonance is about 4-fold weaker than that of C₂HD, while the pressure broadening rate of 34(1) MHz/Torr (FWHM) is similar. For the CO₂ transition, however, the saturated absorption signal is much weaker, by more than a factor of 350, and shows an elegant and unexpected lineshape which is believed to result from nearly overlapping one- and two-photon transitions.