Yu, Yi
Description
Bright, broadband, mid-infrared (MIR) sources are useful for
microscopy and spectroscopy as well as many other areas of
science and technology. Among the available sources,
supercontinuum (SC) sources stand out because of their high
brightness and continuous spectral coverage. SC generation
involve a range of nonlinear optical effects including self-phase
modulation, four-wave mixing, stimulated Raman scattering, etc.
Anomalous dispersion plays a major role...[Show more] and interacts with
nonlinearity leading to the creation of optical solitons that are
essential to create a broad spectrum.
The goal of this PhD was to generate practical, octave-spanning
MIR SC sources spanning at least 2-10 μm using optical
waveguides. To achieve this, firstly, it was necessary to
identify the best nonlinear materials for SC generation.
Chalcogenide glasses were chosen due to their high third-order
optical nonlinearity, low nonlinear absorption and good
transparency in the MIR. The potential of chalcogenides for SC
generation was first demonstrated using bulk samples leading to a
SC spectrum covering more than one octave.
A challenge with chalcogenides is that they typically have long
zero dispersion wavelengths (ZDWs) (beyond 5 μm) and this makes
it difficult to pump them directly in the anomalous dispersion
region. Two approaches were used to overcome this: 1) the
dispersion was engineered via waveguide design to shift the
anomalous region to shorter wavelengths; and 2) long-wavelength
femtosecond pump sources were developed with appropriate powers
to pump them.
Both optical fibers and planar waveguides were explored and the
measured SC spectra were compared with simulations based on the
split-step Fourier method. Dispersion-engineered, step-index
fibers were drawn by collaborators in China whilst
dispersion-engineered rib waveguides were fabricated in house.
Both allowed the first ZDWs to be shifted to wavelengths around 3
μm, however, ZDW below 3 μm was incompatible with the need for
the waveguide to operate to beyond 10 μm.
Simulations showed that MIR SC generation required a pump pulses
in the 3-5 μm band with duration of a few hundred fs. We
developed laser-seeded optical parametric amplifiers (OPA) pumped
with femtosecond pulses from mode-locked Yb lasers, to create
either 330 fs or 200 fs pulses tunable around 4 μm. In addition,
we demonstrated a method for chirping and compressing the OPA
pulses down to <60 fs which is needed to create a coherent SC
spectrum.
Combining the dispersion design and the femtosecond MIR OPA
system, SC spectra more than two octaves wide with moderate
average output powers (10s mW) were obtained from both fibers and
waveguides. For the step-index chalcogenide fibers, typical
experimental SC spectra covered the ranges of 2-10 μm or 2.2-12
μm depending on the fiber composition, however, due to their
circular symmetry, the output was generally unpolarised. The
fibers were also multimode over some of the SC spectrum. Both
these deficiencies could be overcome by moving to a planar
waveguide design. A tri-layer rib waveguide allowed the
production of a linearly-polarized SC spanning from 2.0 μm to
10.8 μm. This source was used successfully for demonstrations of
MIR spectroscopy.
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