Cascaded Quadratic Soliton Compression in Waveguide Structures.

Hairun Guo

Research output: Book/ReportPh.D. thesis

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The generation of few-cycle, high-intensity laser pulses is of great interests in a variety of research and application fields such as time-resolved spectroscopy, bio-chemical imaging with two-photon absorptions, medical treatment, material characterization, coherent supercontinuum generation and tera-hertz wave generation. Commercial pulsed lasers including the solid state system and the pulsed fiber laser have promised the generation of energetic femto-second pulses with the temporal duration around much more than tens of femto-seconds. Therefore, pulse compression technologies could be used to further push such multi-cycle pulses into few-cycle and even single-cycle. In this thesis, we investigate the high order soliton compression in quadratic nonlinear waveguide structures, which is a one-step pulse compression scheme making use of the soliton regime -- with the spontaneous cancelation between the Kerr nonlinear effects and the dispersive effects in the medium. A Kerr-like nonlinearity is produced through the cascaded phase mismatched quadratic process, e.g. the second harmonic generation process, which can be flexibly tuned in both the sign and the amplitude, making possible a strong and self-defocusing Kerr effect so that the soliton is created and the soliton self-compression happens in the normal dispersion region. Meanwhile, the chromatic dispersion in the waveguide is also tunable, understood as the dispersion engineering with structural designs. Therefore, compared to commonly used two-step compression scheme with e.g. hollow-core photonic crystal fibers plus a dispersion compensation component, our scheme, called the cascaded quadratic soliton compression (CQSC), provides a simpler setup with larger tunability on the nonlinearity, and could avoid the problem with the self-focusing Kerr effects when under the self-defocusing regime. On the other hand, CQSC in quadratic waveguides seems highly complementary to that in quadratic bulk crystals. With bulk crystals dealing with high-energy, low-repetition-rate and large-beam-size pulses, quadratic waveguides could operate low-energy, high-repetition-rate pulsed lasers with the beam finely confined by the waveguide structure. Therefore, nano-joule, mega-hertz fiber laser pulses with ~100 fs durations can be compressed to few-cycle. We investigate quadratic waveguides with both small and large refractive index (RI) changes. Robust wafer bonding is proposed as a fabrication technology to achieve a waveguide with large RI change, which could substantively extend the guidance band of the waveguide in near- and mid-infrared ranges, and meantime evoke flexible dispersion engineering so that a broadband normal dispersion region can be achieved. Through numerical simulations as well as experiments, we find out that CQSC in small-RI-changed waveguides is mainly targeting the communication band in the near-infrared range, where the waveguide is naturally suitable for producing a self-defocusing Kerr-like cascaded nonlinearity so that quasi-phase-matching technology is not necessarily needed. In large-RI-changed waveguides, CQSC is extended to the mid-infrared range to generate single-cycle pulses with purely nonlinear interactions, since an all-normal dispersion profile could be achieved within the guidance band. We believe that CQSC in quadratic waveguides is an effective pulse compression scheme with compact and simple setups, and could have potentials in many applications.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages160
Publication statusPublished - 2014

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