Mid-Infrared Supercontinuum based Spectroscopic OCT

This thesis provides a thorough review of mid-infrared (mid-IR) supercontinuum (SC) laser sources and their application within optical coherence tomography (OCT). A strong emphasis has been put on the development of the SC light sources as the performance of the light source in terms of power, spectral range, and noise directly impacts the quality, and acquisition speed of the OCT images.
The beautiful theory behind the nonlinear dynamics that is responsible for SC generation has been applied in numerical modelling. The numerical simulations are a useful tool to investigate a large set of fiber, and input pulse parameters. This has resulted in the prediction of a pump modulation scheme that can provide a more flat blue edge of commercially relevant SC sources based on input pulses with picosecond duration. It has also been applied to investigate SC generation in the curious case of a highly nonlinear fiber with an oscillating dispersion. Numerical simulations have been further used to describe a novel type of of SC dynamics, which is important in the last stage in cascaded SC generation. In the cascaded SC sources, that we consider, a large number of solitons are created in the first fiber stages. The last stage is a chalcogenide fiber in which the solitons are coupled into normal dispersion. After initial nonlinear broadening the pulses are temporally dispersed until a temporal overlap is created between different spectral components, allowing a red-shift of power through Raman interaction. Also in cascaded mid-IR SC generation a novel noise reduction mechanism has been shown both numerically and experimentally. The initial nonlinear broadening of solitons - also in the last stage of the cascade - results in spectral overlap between each subpulse, providing an effective averaging of subpulses within each pulse, thus reducing pulse-to-pulse fluctuations. We experimentally show a reduction from above 35 % to below 25 % in relative intensity noise across a broad bandwidth. Based on cascaded mid-IR SC sources, OCT is performed with a centre wavelength of 4 µm. A centre wavelength in the mid-IR typically allows increased penetration in samples compared to the more conventional near-IR systems. The OCT system consists of an SC laser source, a Michelson interferometer, and detection based on near-IR detection after nonlinear upconversion of the mid-IR light. A characterisation of the OCT system is provided showing 11.9 µm axial resolution, 60 dB sensitivity, and 3.9 mm 6 dB sensitivity roll of distance. The applicability of mid-IR OCT in non-destructive testing is shown on a set of paper samples. The 4 µm centre wavelength allowed penetrating through paper with a thickness of 90 µm, such that the thickness of the sample could be measured simultaneously with the refractive index. It was further shown that the OCT system can show the roughness of the paper surface, and that it detect defects in the cases of tears, voids and contamination by a droplet of oil. By spectrally subdividing the OCT images in the post processing it is possible to obtain spectral information such as spectrally dependent scattering or absorption of the sample. This data analysis technique was applied to show a proof-of-concept of
spatially and temporally resolved imaging of CO2 gas in channels inside a 3D printed epoxy resin cube.