Manipulating Supercontinuum Generation and Its Applications

This dissertation, "Manipulating Supercontinuum Generation and Its Applications" by Yi, Josephine, Qiu, 裘一, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Due to the large penetration depth in tissues, ultra-broadband supercontinuum (SC) in the 1m wavelength range, which is regarded as a diagnostic window in bio-photonics, represents a versatile light source for a wide range of bio-imaging and spectroscopy applications. In particular, dispersive Fourier transform (DFT) has recently emerged as anultrafast optical technique forimaging and spectroscopy. Thus, In order to employ the SC source for DFT, it has to exhibit ultra-broad bandwidth as well as good temporal stability- the two important metrics for practical high-speed bio-imaging and spectroscopy applications. In this thesis, we first demonstrate stabilized and enhanced SC generation (in the anomalous dispersion regime) at 1m by a minute continuous-wave (CW) seeding scheme. By introducing a weak CW which is around 200,000 times weaker than the pump, a significant broadening in the SC bandwidth and an improvement in the temporal stability is obtained. This seeding scheme allows, for the first time,1m DFT at a spectral acquisition rate of 20MHz with good temporal stability-paving the way toward realizing practical real-time, ultrafast biomedical spectroscopy and imaging. For the DFT part, instead of using the regular specialty 1m single mode fiber (SMF) as the dispersive elements, we here explore and demonstrate the feasibility of using the standard telecommunication single-mode fibers (e.g. SMF28 and dispersion compensating fiber (DCF)) as few-mode fibers (FMFs) for optical time-stretch confocal microscopy in the 1m region. By evaluating group velocity dispersion (GVD) of different FMF modes and thus the corresponding time-stretch performances, we show that the fundamental modes (LP01) of SMF28 and DCF, having sufficiently high dispersion-to-loss ratios, are particularly useful for practical time-stretch spectroscopy and microscopy in the 1m region, without the need for the specialty 1m(single mode fiber) SMF. More intriguingly, the ability of selective modal excitation in FMFs also enables us to utilize the higher-order FMF modes (e.g. LP11) for time-stretch imaging. Such additional degrees of freedom create a new avenue for optimizing and designing the time-stretch operations, such as by tailored engineering of the modal-dispersion as well as the GVD of the individual FMF modes. In the search for stable and efficient SC generation for practical bio-imaging and spectroscopy, we also numerically investigate the active enhancement of the seeded-SC generation pumped in the normal dispersion regime. Similarly, we introduced a minute CW seeding scheme, more specifically seeding spectrally coincides with the Raman gain peak of the pump. With this design, the SC bandwidth can be enhanced to more than one octave, even when the pump is far away from zero dispersion wavelengths (ZDW) ( 100nm) in the normal dispersion regime. This new seeding mechanism opens opportunities to expand the scope of active seeding mechanism for enhancing SC generation to the normal dispersion regime. DOI: 10.5353/th_b5317053 Subjects: Laser pulses, Ultrashort