Abstract
Overcoming the Doppler broadening limit is a cornerstone of precision spectroscopy. Nevertheless, the achievement of a Doppler-free regime is severely hampered by the need of high field intensities to saturate absorption transitions and of a high signal-to-noise ratio to detect tiny Lamb-dip features. Here we present a novel comb-assisted spectrometer ensuring over a broad range from 1.5 to 1.63 μm intra-cavity field enhancement up to 1.5 kW/cm2, which is suitable for saturation of transitions with extremely weak electric dipole moments. Referencing to an optical frequency comb allows the spectrometer to operate with kHz-level frequency accuracy, while an extremely tight locking of the probe laser to the enhancement cavity enables a 10−11 cm−1 absorption sensitivity to be reached over 200 s in a purely dc direct-detection-mode at the cavity output. The particularly simple and robust detection and operating scheme, together with the wide tunability available, makes the system suitable to explore thousands of lines of several molecules never observed so far in a Doppler-free regime. As a demonstration, Lamb-dip spectroscopy is performed on the P(15) line of the 01120-00000 band of acetylene, featuring a line-strength below 10−23 cm/mol and an Einstein coefficient of 5 mHz, among the weakest ever observed.
Highlights
The saturated absorption regime has been recognized since the 70’s as an extremely powerful approach to push the frequency precision of an optical spectrometer far below the limit set by Doppler broadening[1]
This is in practice a challenge since the higher is Leff the steeper is the slope of the frequency-to-amplitude response given by the cavity modes, with the consequence that any residual frequency noise between laser and cavity may severely impact on ΔI/I and on αmin
This is confirmed by the green trace reported in the figure, which refers to a measurement performed before the cavity where ΔI/I is mostly affected by laser intensity noise: this remains a factor greater than 4 below the frequency noise contribution over the typical 2-s-long measurement time taken by each frequency scan
Summary
The saturated absorption regime has been recognized since the 70’s as an extremely powerful approach to push the frequency precision of an optical spectrometer far below the limit set by Doppler broadening[1]. The noise level of 1.7·10−11 cm−1 over a 7-minute-long measurement remained nearly 2 orders of magnitude above the limit of detection of the same spectrometer in a linear absorption regime, equal to 5∙10−13 cm−1 over 1 s33 This derives from two main issues of Doppler-free CRDS spectroscopy: i) the shorter ring-down time that occurs at the centre of the absorption line, where the dip takes place, and ii) the failure of the exponential function for the fit of ring-down decays in a saturated regime. This makes the fit more susceptible to fluctuating conditions such as the optical power coupled into the cavity at the beginning of the decay. With more versatile and tunable laser sources such lock and the sensitivity limit was found to be degraded by one or even two or three orders of magnitude[38,39], which explains why NICE-OHMS failed to be applied to broad line surveys in a saturated regime
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