Dual-comb spectroscopy of fundamental vibrational transitions

Spectroscopy of fundamental vibrational transitions offers a
label-free alternative for high-chemical contrast measurements.
These transitions can be interrogated either directly by using
mid-infrared light or indirectly through Raman scattering. This
thesis aims to advance dual-comb spectroscopy to improve the
acquisition speed, resolution and spectral coverage of vibrational
spectroscopy. Dual-comb spectroscopy is a time domain technique,
which combines optical frequency combs -coherent light sources with
a spectrum constituted of discrete evenly spaced lines - and
Fourier transform spectroscopy. For linear spectroscopy, a
mid-infrared optical parametric oscillator was developed and
characterized. Its idler-pulse duration can be as short as a few
cycles (~3 to 6 cycles), with a central wavelength tunable from
2180nm to 3732nm (2679cm-1 - 4587cm-1), allowing more than 2500nm
(2861 cm-1) of total coverage while maintaining an average power of
tens of milliwatts. The high peak power of this system was
exploited for spectral broadening; generation of phase-coherent
supercontinua was achieved in waveguides, made from either silicon
or chalcogenide glass, producing octave spanning spectra ~1500nm to
3300nm (3030cm-1 - 6666cm-1) for silicon and from ~1600nm beyond
3860nm (2590 cm-1 - 6250 cm-1) for chalcogenide glass). Two optical
parametric oscillator were constructed, advancing toward a
dual-comb mid-infrared spectrometer. Since the optical parametric
oscillators are not stabilized, an additional correction scheme was
set up and characterized. Coherent Raman scattering was also
investigated, as a means to access optically active and inactive
fundamental vibrational transitions. Several spectroscopy setups
were developed to measure the Raman blue or red shifted light in
forward and backward scattered direction as well as a differential
detection between blue and red shifted light. There is a dead time
between consecutive interferograms existent, up to a factor of 1000
larger than the measurement time. This dead time could be reduced
by an order of magnitude using a laser with ~1GHz and a laser with
100MHz repetition rate instead of two lasers with ~100MHz
repetition rate. All implementations achieved excellent acquisition
times (in the microsecond range), signal-to-noise ratios up to 1000
and spectral coverage of about ~1200 cm-1). These advantages
enabled measuring spectrally resolved images, in a first
rudimentary microscopy-setup.