Comb-locked Lamb-dip spectrometer.
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/cm(2), 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.
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, P.za L. da Vinci 32, 20133 Milano, Italy.|Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Davide
D
Gatti
Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Riccardo
R
Gotti
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, P.za L. da Vinci 32, 20133 Milano, Italy.|Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Alessio
A
Gambetta
Oclaro Inc. - via F. Fellini, 4, 20097 San Donato Milanese, Italy.
Michele
M
Belmonte
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, P.za L. da Vinci 32, 20133 Milano, Italy.|Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Gianluca
G
Galzerano
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, P.za L. da Vinci 32, 20133 Milano, Italy.|Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Paolo
P
Laporta
Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, P.za L. da Vinci 32, 20133 Milano, Italy.|Dipartimento di Fisica - Politecnico di Milano, Via Gaetano Previati 1/C, 23900 Lecco, Italy.
Marco
M
Marangoni