Online PhD defence by Shreesha Rao Delanthabettu Shivarama

Title:Low-noise supercontinuum sources utilizing normally dispersive fibers

Principal supervisor: Professor Ole Bang, DTU Fotonik Denmark

Evaluation Board
Alexander Heidt, Institute of Applied Physics, University of Bern, Switzerland
Michael Frosz, Max Planck Institute for the Science of Light, Erlangen, Germany

Assoc. Prof. Jesper Lægsgaard, DTU Fotonik, Denmark

Master of the Ceremony
Assoc. Prof. Christos Markos, DTU Fotonik, Denmark

We have investigated low-noise supercontinuum generation by utilizing all-normal dispersion fibers, pumped by a femtosecond source. Spectrally resolved narrow-band relative intensity noise measurements were performed to study and demonstrate the low pulse-to pulse fluctuation in the generated supercontinuum. In addition to ultra-broad bandwidth, spatial coherence, and high average power, supercontinuum sources can now be low-noise. We then used the generated supercontinuum to demonstrate that the low-noise property enable their application.

We have demonstrated that low-noise supercontinuum sources can fill the existing lack of a broadband and coherent light sources in the near-infrared regime that is suitable for nanoscale Fourier transform infrared spectroscopy, in demodulation based apertureless scattering type scanning near-field optical microscopy. We performed ultra-high resolution scanning near-field optical microscopy based broadband nano-spectroscopy to study surface plasmon polaritons at 40 nm spatial resolution on high quality monocrystalline goal-flakes. We were able to identify both the edge-launched and tip-launched surface plasmon polaritons in the entire bandwidth of the low-noise supercontinuum. We then used the low- noise supercontinuum to study shot-noise limited ultra-high resolution spectral-domain optical coherence tomography. We have made an one-to-one comparison of the noise properties of a ultra-high resolution spectral-domain optical coherence tomography system around 1.37 μm using an all-normal dispersion fiber based low-noisesource to the noise properties of the system when two commercially available supercontinuum sources displaying high pulse-to-pulse fluctuation. Using the low-noise source, we have demonstrated that ultra-high resolution spectral-domain optical coherence tomography now can be operated in the shot-noise limited detection regime. With the shot-noise limited detection, the available sensitivity is no longer determined by the light source, but by fundamental physics. In general, the ultra-high resolution spectral-domain optical coherence tomography system using all-normal dispersion fiber based low-noise supercontinuum shows darker background, higher contrast, and improved penetration compared to one using the traditional supercontinuum sources. This improvement in ultra-high resolution spectral-domain optical coherence tomography can help in detection of skin cancer at an early stage. In addtion, femtosecond pumped, directional supercontinuum generation in fibers with two-zero dispersion wavelengths is studied. We have investigated the presence of addition dispersive waves from non-degenerate four-wave mixing. In addition, we have studied interesting effects on soliton, including a novel effect that we have named soliton boomerang effect.

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Wed 03 Jun 20
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