title: Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit.
abstract: A comprehensive investigation of the frequency-noise spectral density of a free-running midinfrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white-noise plateau, corresponding to an intrinsic linewidth of a few hundred hertz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.
authors.affiliation: Istituto Nazionale di Ottica (INO)-CNR, Largo Fermi 6, 50125 Firenze FI, Italy. saverio.bartalini@ino.it
authors.forename: S
authors.initials: S
authors.lastname: Bartalini
authors.forename: S
authors.initials: S
authors.lastname: Borri
authors.forename: P
authors.initials: P
authors.lastname: Cancio
authors.forename: A
authors.initials: A
authors.lastname: Castrillo
authors.forename: I
authors.initials: I
authors.lastname: Galli
authors.forename: G
authors.initials: G
authors.lastname: Giusfredi
authors.forename: D
authors.initials: D
authors.lastname: Mazzotti
authors.forename: L
authors.initials: L
authors.lastname: Gianfrani
authors.forename: P
authors.initials: P
authors.lastname: De Natale