# OPTICAL ABSORPTION SENSITIVITY BETTER THAN $1 \times 10^{-12}$

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 Title: OPTICAL ABSORPTION SENSITIVITY BETTER THAN $1 \times 10^{-12}$ Creators: Ye, Jun; Long-Sheng, Ma; Hall, J. L.; Taubman, Matthew S. Issue Date: 1998 Publisher: Ohio State University Abstract: 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 $5.2 \times 10^{-13}$ for a 1 s $integration^{a}$. 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 $CO_{2}$ are presented. The two additional lines are $^{12}C_{2}H_{2} (2\nu_{1} + \nu_{2} + \nu_{5}) R(12)^{b}$ and $^{12}C^{16}O_{2} (2\nu_{1} + 3\nu_{3}) R(6)^{c}$, with their respective transition dipole moments of $50 \mu Debye$ and $6 \mu Debye$. They are both weaker than our usual $C_{2}HD(\nu_{2} + 3 \nu_{3}) P(5)$ transition, which has a transition dipole moment of $70\mu$Debye, 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 $a_{10}$ hyperfine-structure component of the $R(56) 32-0 I_{2}$ transition. The $C_{2}H_{2}$ resonance is about 4-fold weaker than that of $C_{2}HD$, while the pressure broadening rate of 34(1) MHz/Torr (FWHM) is similar. For the $CO_{2}$ 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. Description: $^{a}$ Jun Ye. Long$\cdot$Sheng Ma. and John L. Hall, J. Opt. Soc. Am. B15, 6 (Jan 1998). $^{b}$ K. Nakagawa, T. Katsuda, M. de Labachelerie, and M. Ohtsu, Opt. Comm. 107, 369 (1994). $^{c}$ P. Fritschel and R. Weiss, Appl. Opt. 31, 1910 (1994). Author Institution: Quantum Optics Group; Quantum Physics Lab, East China Normal University; Quantum Physics Lab, JILA, University of Colorado and NIST URI: http://hdl.handle.net/1811/18840 Other Identifiers: 1998-MI-02