COPULCO · Cascaded Optical Pulse Compressor

Cascaded Optical Pulse Compressor

The goal of the project is to develop an efficient optical pulse compression to ultra-short (sub-20 fs) visible and near-infrared femtosecond pulses in one quadratic nonlinear crystal. Nowadays most of accessible femtosecond pulses are delivered from mode-locked solid-state oscillators and fiber lasers with the active materials. Besides the popular Ti:sapphire technology, currently femtosecond laser wavelengths are mainly focused at the wavelength of 1.03 microns by using Yb-doped or 1.56 microns by using Er-doped gain materials. Their pulse durations are severely limited to several hundreds of femtoseconds due to the narrow gain bandwidth. Generation of optical few-cycle pulses inside the chirped-pulse-amplification (CPA) systems requires nonlinear phase shift to broaden the spectrum, which may lead to the distorted pulses due to excessive detrimental nonlinear effects. However for many applications such as ultra-short pump-probe spectroscopy, micromachining with nanoscale precision, etc., shorter pulses are desirable. Pulse compression to few-cycle duration from hundreds of femtosecond duration out of the energetic solid-state and fiber amplifiers, would provide a stable and compact optical source. The cascaded optical pulse compressor (COPULCO) investigated in this project is based on cascaded second-order nonlinear processes and implemented by using quasi-phase-matching (QPM) techniques. An extremely simple method is to use phase-mismatched second-harmonic generation (SHG). In this process the input pulse is frequency converted to the SH (having double frequency of the input pulse). Energy exchange occurs periodically between the input and SH pulses under large phase-mismatching condition. The input pulse accumulates a huge nonlinear phase shift due to the difference in the phase velocities of the two pulses upon propagation. The effective nonlinear phase shift is equivalent to the cubic (Kerr) nonlinearity, and self-defocusing cascaded quadratic nonlinearity is always required to counterbalance the material Kerr nonlinearity (self-focusing). Cascading quadratic nonlinearity generates a frequency chirp and eventually the dispersion in the nonlinear crystal compresses the input pulse temporally by using the soliton effect. Employing QPM technology in cascaded quadratic soliton compression is highlighted because of the benefit of engineering the cascaded quadratic nonlinearity through the phase mismatch. This property is not possible in standard Kerr media. The advantage of the cascaded quadratic soliton compression is that it is generic and very simple: it is based on only a single active optical component through engineering one QPM nonlinear crystal. The cascaded quadratic nonlinearity was engineered through varying the phase mismatch in ways of multi-section QPM structure with different domain periods. By adjusting the residual QPM phase mismatch and keeping a low effective soliton number in each section, soliton compression in the stationary regimes, soliton compression of pulses with small soliton numbers can achieve a small pedestal and high pulse quality. This reduces the detrimental pedestal of the compressed pulses and improves the pulse quality, opening up for more sensitive applications such as pump-probe spectroscopy. Temporal few-cycle soliton compression in near-infrared regions is investigated both in bulk and in waveguide devices. Another work done in this project is to present new nonlinear wave equation in frequency domain (NWEF) to accurate modeling of ultrashort pulse interaction with minimal approximations. This model automatically includes all of the possible phase-matching conditions and the different wave-mixing possibilities (three- and four-wave mixing, third harmonic generation and parametric up- and down-conversion). We then use the NWEF as a platform for investigating soliton compression to few-cycle duration in cascaded SHG and mid-IR generation from different frequency generation. The paper was spotlighted by the Optical Society of America, and was on the top 10 most downloaded papers in February and March 2013 in J. Opt. Soc. Am. B. We study the anisotropic nature of the Kerr nonlinear response in BBO nonlinear crystal, which has been extensively used as a nonlinear crystal in the cascading experiments because of its decent quadratic nonlinear coefficient. We show our own measurements on Kerr cubic tensor components and compare with various experiments in the literature. We present for the first time a complete list that we propose as reference of the four major cubic tensor components in BBO. The impact of using the cubic anisotropic response in ultrafast cascading experiments in BBO is also discussed. Currently the majority of femtosecond lasers operate in the near-IR wavelength and cannot directly deliver mid-IR femtosecond pulses due to lack of suitable laser gain media. The standard way to generate mid-IR is to use optical parameter amplification of the near-IR pulses, which needs to overcome bandwidth limitations of frequency conversion to achieve few-cycle pulses. We present that the energetic mid-IR few-cycle pulses can be generated through optical Cherenkov radiation in ultrafast nonlinear optical process, especially through cascaded soliton compression of phase-mismatched SHG. The radiation originates from soliton propagation perturbed by high-order dispersion and is simply considered as a narrow band phase matching with the soliton. We show a new concept of spectral coupling with soliton eigenstate to explain soliton spectral tunneling effect in a coupler-like wave-number profile and linear dispersive wave in a leaking profile. Group-velocity matching is another significant premise for decent soliton spectral switching with high efficiency and broad conversion. This new soliton phase-matching condition is very important consideration to greatly improve the efficiency of frequency conversion through optical Cherenkov radiation. Besides, understanding the ultra-fast dynamics and accurately modeling few-cycle pulse propagation in the cascaded quadratic nonlinear media has fundamental physical interests in the field of femtosecond nonlinear optics. We also investigate the necessary conditions of dispersive wave generation accompanied with the few-cycle pulse compression, which enables high efficient wavelength conversion to mid-IR wavelengths.

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