The line position of the very weak S(2) transition of deuterium in the 2-0 band has been measured with a Comb-Assisted Cavity Ring Down spectrometer.The high sensitivity spectra were recorded at 5 and 10 mbar with a Noise Equivalent Absorption, a min ,of 8Â 10 -11 cm À1 . The line positions at 5 and 10 mbar were measured with sub-MHz accuracy (460 and 260 kHz, respectively). After correction of the line pressure-shift, the frequency at zero pressure of the S(2) transition of the first overtone band was determined to be 187 104 299.51 ± 0.50 MHz. This value agrees within 1.7 MHz with the frequency obtained from the best available ab initio calculations and corresponds to only 15% of the claimed theoretical uncertainty.

In this letter we demonstrate the possibility to determine the temporal and spectral structure (spectrogram) of a complex light pulse exploiting the ultrafast switching character of a nonthermal photoinduced phase transition. As a proof, we use a VO₂ multifilm, undergoing an ultrafast insulator-to-metal phase transition when excited by femtosecond near-infrared laser pulses. The abrupt variation in the multifilm optical properties, over a broad infrared/visible frequency range, is exploited to determine, in situ and in a simple way, the spectrogram of a supercontinuum pulse produced by a photonic crystal fiber. The determination of the structure of the pulse is mandatory to develop pump-probe experiments with frequency resolution over a broad spectral range (700–1100 nm)

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Doppler-free saturated-absorption Lamb dips were measured on weak rovibrational lines of (12)C(16)O2 between 6189 and 6215 cm(-1) at sub-Pa pressures using optical feedback frequency stabilized cavity ring-down spectroscopy. By referencing the laser source to an optical frequency comb, transition frequencies for ten lines of the 30013←00001 band P-branch and two lines of the 31113←01101 hot band R-branch were determined with an accuracy of a few parts in 10(11). Involving rotational quantum numbers up to 42, the data were used for improving the upper level spectroscopic constants. These results provide a highly accurate reference frequency grid over the spectral interval from 1599 to 1616 nm.

Extreme frequency accuracy and high sensitivity are obtained with a novel comb-locked cavity-ring-down spectrometer operating in the near-infrared from 1.5 to 1.63 μm. A key feature of our approach is the tight frequency locking of the probe laser to the comb, ensuring very high reproducibility and accuracy to the frequency axis upon scanning the comb repetition rate, as well as an efficient light injection into a length-swept high-finesse passive cavity containing the gas sample. Spectroscopic tests on the (30012) ← (00001) P14e line of CO2 at ∼1.57 μm demonstrate an accuracy of ∼17 kHz on the line center frequency in a Doppler broadening regime over the time scale of about 5 min, corresponding to four consecutive spectral scans of the absorption line. Over a single scan, which consists of 1500 spectral points over 75 s, the limit of detection is as low as 5.7 × 10(-11) cm(-1).