  We propose and characterize experimentally a new source of optical frequency
combs for performing multi-heterodyne spectrometry. This comb modality is based
on a frequency shifting loop seeded with a CW monochromatic laser. The comb
lines are generated by successive passes of the CW laser through an
acousto-optic frequency shifter. We report the generation of frequency combs
with more than 1500 mutually coherent lines, without resorting to non-linear
broadening phenomena or external electronic modulation. The comb line spacing
is easily reconfigurable from tens of MHz down to the kHz region. We first use
a single acousto-optic frequency comb to conduct self-heterodyne interferometry
with a high frequency resolution (500 kHz). By increasing the line spacing to
80 MHz, we demonstrate molecular spectroscopy on the sub-millisecond time
scale. In order to reduce the detection bandwidth, we subsequently implement an
acousto-optic dual-comb spectrometer with the aid of two mutually coherent
frequency shifting loops. In each architecture, the potentiality of
acousto-optic frequency combs for spectroscopy is validated by spectral
measurements of hydrogen cyanide in the near-infrared region.
