Mid-infrared supercontinuum generation in chalcogenides

Abstract

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 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.