Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 mid-infrared 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 Hz. 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.

15
The categories of this paper are: nucl-ex
19
The categories of this paper are: eess
4
The categories of this paper are: cond-mat
10
The categories of this paper are: q-bio
9
The categories of this paper are: nlin
24
The categories of this paper are: dg-ga
14
The categories of this paper are: math-ph
21
The categories of this paper are: adap-org
22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
2
The categories of this paper are: math
16
The categories of this paper are: stat