Daniele ROMANINI
Absorption line shape metrology is one of the hottest topics of high resolution spectroscopy at present.The accurate measurement of molecular absorption features both on the frequency (X) and on the absorption (Y) axis has crucial implications in various fields, including atmospheric remote sensing[1], isotopic ratio measurements[2] and a redetermination of the Boltzmann constant[3]. This has fostered considerable theoretical progress (e.g. [4]) in quest of a line shape model, which accounts correctly for collision-induced velocity changes and dephasings.
Our group is committed to contributing high quality experimental absorption lines for testing models at ever-increasing signal-to-noise ratios and to tackle the aforementioned urgent and fascinating applications. To this end, we have
proposed and developed a novel spectrometer combining a narrow, stable and tunable laser with an ultra-sensitive [5] cavity ring-down absorption measurement: Optical Feedback Frequency Stabilized Cavity Ring-Down Spectroscopy (OFFS-CRDS).
As the heart of our setup, we have developed a subkilohertz linewidth laser[6], which is based on feedback photons coming from inside a home-made highly stable V-shaped reference cavity.
Being reinjected into the laser medium, they counteract the random walk of laser phase due to spontaneous emission. This leads to a linewidth narrowing by almost four orders of magnitude and all-optical locking to a reference cavity resonance, thereby reducing frequency drifts to below 20 Hz/s.
This laser source is continuously tunable over 1 THz around 1590 nm by selecting a reference cavity mode and using an innovative single-sideband suppressed-carrier modulation scheme[6], which allows for frequency shifting over up to 40 GHz with millihertz accuracy and excellent spectral purity. Its physical basis are multiple voltage-controlled interferences in an integrated electro-optic Mach-Zehnder modulator
(MZM)[7].
In a final step, the ring-down spectroscopy cavity is locked onto our single-sideband tuned optical feedback stabilized laser, thereby transferring its frequency stability and optimizing CRDS transmission. The versatility of the MZM makes the use of an acousto-optic modulator for triggering ring-down events obsolete. The same holds for an additional electro-optic modulator normally used for generating the PDH error signal.Because of its stable and narrow probe laser, high repetition rate and shot-noise-limited ring-down acquisition, the performance features of our new OFFS-CRDS spectrometer are unprecedented. It combines subkilohertz frequency stability and resolution with terahertz-wide tunability as well as absorption detection limits at the
10-13 cm-1 level. The experimental setup as well as a characterization of its performance have been already published[8],[9],[10].