Spectral broadening enhancement in silicon waveguides through pulse shaping

Abstract

Spectral broadening in silicon waveguides is usually inhibited at telecom wavelengths due to some adverse effects related to semiconductor dynamics, namely, two-photon and free-carrier absorption (FCA). In this Letter, our numerical simulations show that it is possible to achieve a significant enhancement in spectral broadening when we properly preshape the input pulse to reduce the impact of FCA on spectral broadening. Our analysis suggests that the use of input pulses with the correct skewness and power level is crucial for this achievement. © 2012 Optical Society of America OCIS codes: 130.4310, 190.4360, 190.4390. Nowadays, continuous advances in nanophotonics technologies have spurred on interest in the study of silicon-based optical materials [1]. Strong confinement of light in nanoengineered silicon-on-insulator (SOI) waveguides results in a huge effective nonlinearity and the ability for dispersion engineering (see, e.g., [2,3]). These achievements have opened up the possibility of performing previously demonstrated signal processing functionalities (mainly based in nonlinear kilometric fibers) at chip scale with relatively low optical power [4]. These Si-based components offer the benefits of low cost (complementary metal-oxide-semiconductorcompatible large-scale-fabrication processes) and low power consumption. However, at the relevant wavelength region around 1.5 μm, Kerr-based spectral broadening or self-phase modulation (SPM) is accompanied by an orchestra of different nonlinear phenomena arising from the semiconductor carrier dynamics [5][6][7]. Specifically we mention the absorption and dispersion of free carriers produced by two-photon absorption (TPA), which are not present in conventional silica-based devices. The net effect results in a depletion of the achievable spectral broadening for a Gaussian input pulse in comparison with the case when only SPM is acting [8][9][10].Additionally, the phenomenon under study is extremely sensitive to the input pulse characteristics due to the inherent nonlinear nature of the spectral broadening. In fact, up to some extent, pulse shaping techniques have demonstrated to be effective in controlling the nonlinear broadening in photonic crystal fibers [11,12] and other nonlinear materials [13][14][15], using both single-pass [15] and self-learning adaptive configurations [11][12][13][14]. Besides, these techniques offer valuable insight in understanding the pulse dynamics through propagation [11,16]. In this Letter, we show that the proper manipulation of the pulse phase enhances spectral broadening even in the presence of TPA and free-carrier absorption (FCA).Let us remind the reader that the dynamics of an optical pulse propagating in an SOI nanowaveguide can be described in mathematical terms by [9](1)where A is the electric-field envelope, ν 0 represents the carrier frequency, β 2 stands for the group velocity dispersion (GVD) parameter, n 2 is the Kerr coefficient, A eff is the effective area of the waveguide, γ 0 denotes the nonlinear coefficient of t...

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