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Eigenwillig, Christoph (2012): New rapidly wavelength-swept light sources for optical coherence tomography and picosecond pulse generation. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis deals with research on novel, semiconductor-based, ultrafast and widely tunable wavelength-swept light sources with respect to different applications. The main focus was on the young technology of Fourier domain mode locked (FDML) lasers, where the insertion of a kilometer-long fiber delay line enables to tune a narrowband spectral filter synchronously to the roundtrip time of light in the resonator. In this way, very high sweep speeds become feasible. A very successful application in the field of biomedical imaging is optical coherence tomography (OCT), where FDML lasers allow for very large image acquisition rates. One important part of the research work was the development and characterization of novel concepts of wavelength-swept light sources improving performance and applicability in OCT. In this context, two novel modes of operation of FDML lasers have been demonstrated. On the one hand, an FDML laser with a highly linear time-frequency sweep characteristic was realized for the first time and allowed for OCT imaging at 1300 nm based on simplified numerical image processing. On the other hand, the first subharmonic FDML laser was implemented and successfully used for OCT imaging at 1300 nm. Here, light passes the same fiber delay line several times during each laser cavity roundtrip. In case of reduced sweep range, subharmonic FDML operation enabled an inherent multiplication of the effective sweep rate by a factor of ten, reaching 570 kHz. Another important achievement was the demonstration of a new type of ultrafast wavelength-swept light sources, where superluminescent light alternately passes a cascade of different gain elements and spectral filters which have to be tuned out of phase in order to compensate for the transit time of light. Different implementations operated at 1300 nm and at 1060 nm enabled effective sweep rates of up to 340 kHz. Ultrafast OCT imaging of the human retina was shown. The second part of the research work focused on the demonstration and investigation of a novel approach of short pulse generation, where light within the wavelength sweeps of an FDML laser is temporally compressed by a subsequent pass through 15 km of highly dispersive fiber. The achievable temporal pulse width was an indicator for the coherence properties and the quality of mode-locking of the FDML laser. This became evident in the very critical dependence on the FDML sweep frequency as well as due to the results of comparable pulse generation experiments based on using an incoherent wavelength-swept light source. With a dispersion compensated FDML laser, operated at 1560 nm, pulse durations of 60-70 ps at a repetition rate of 390 kHz were achieved. Although the generation of bandwidth-limited pulses was not feasible, it was shown that the electric field within the wavelength sweeps of the FDML laser must at least be partially coherent. Due to remaining uncompensated higher order chirp, the optical bandwidth was limited to 6 nm and the pulse energy was restricted. Pulse energies of 5.6 nJ have been achieved using erbium-doped fiber amplification prior to temporal compression.