Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A quantum cascade laser-based mid-IR frequency metrology system with
  ultra-narrow linewidth and $1\times 10^{-13}$-level frequency instability   We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and
frequency metrology with quantum cascade lasers (QCLs). We have implemented
frequency stabilization of a QCL to an ultra-low expansion (ULE) reference
cavity, via upconversion to the near-IR spectral range, at a level of
$1\times10^{-13}$. The absolute frequency of the QCL is measured relative to a
hydrogen maser, with instability $<1\times10^{-13}$ and inaccuracy
$5\times10^{-13}$, using a frequency comb phase-stabilized to an independent
ultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated by
fiber noise. Active suppression of fiber noise could result in sub-10 Hz
linewidth.

31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
4
The categories of this paper are: cond-mat
2
The categories of this paper are: math
6
The categories of this paper are: astro-ph
37
The categories of this paper are: supr-con
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
29
The categories of this paper are: funct-an
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
18
The categories of this paper are: econ