Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit.
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.
Bartalini
S
S
Istituto Nazionale di Ottica (INO)-CNR, Largo Fermi 6, 50125 Firenze FI, Italy. saverio.bartalini@ino.it
Borri
S
S
Cancio
P
P
Castrillo
A
A
Galli
I
I
Giusfredi
G
G
Mazzotti
D
D
Gianfrani
L
L
De Natale
P
P