Femtosecond few-cycle mid-infrared laser pulses

Xing Liu

Research output: Book/ReportPh.D. thesisResearch

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The few-cycle pulses of mid-infrared (mid-IR, wavelength 2-10 microns) have attracted increasing attention owing to their great potentials for high order harmonic generation, time-resolved spectroscopy, precision of cutting and biomedical science.In this thesis, mid-IR frequency conversion by the ultra-fast soliton was proposed, which exploit optical solitons in near-IR for generating the mid-IR pulses. Firstly, we show numerically that ultrashort self-defocusing temporal solitons colliding with a weak pulsed probe in the near-IR can convert the probe to the mid-IR. A near perfect conversion efficiency is possible for a high effective soliton order. The near-IR self-defocusing soliton can form in a quadratic nonlinear crystal (beta-barium borate) in the normal dispersion regime due to cascaded (phase-mismatched) second-harmonic generation, and the mid-IR converted wave is formed in the anomalous dispersion regime between λ = 2.2 − 2.4 μm as a resonant dispersive wave. This process relies on non-degenerate four-wave mixing mediated by an effective negative cross-phase modulation term caused by cascaded soliton-probe sum-frequency generation. Secondly, a self-pumped soliton-driven tunable mid-IR optical parametric amplifier is demonstrated for the first time in a standard periodically poled lithium niobate (PPLN) bulk crystal. The new type of resonant radiation is generated through a three wave mixing (TWM) process. The poling pitch gives a parametrically tunable resonant radiation from λ =4.2 - 5.5 μm with only one fixed pump wavelength, a feature absent in Kerr media. Finally, we experimentally observe supercontinuum generation spanning 1.5 octaves, generated in a 10 mm long silicon-rich nitride waveguide pumped by 100 pJ femtosecond pulses from an erbium fiber laser. The waveguide has a large nonlinear Kerr coefficient and two zero dispersion wavelengths giving broadband anomalous dispersion centered around the pump wavelength. This is achieved by slightly increasing the silicon content over stoichiometric silicon nitride and waveguide geometry engineering. The spectral broadening relies on exciting a soliton and two dispersive waves. In the same waveguide using orthogonal pump polarization, optical wave-breaking occurs as the pump dispersion becomes normal. The numerical simulations indicate that the supercontinua are highly coherent. Through numerical simulations and experiments, we find out soliton based on generating the mid-IR is a compact and simple approach, and could have great potentials.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages98
Publication statusPublished - 2016

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