Linear and Nonlinear Optics of Broad-Band Laser Pulses: Diffraction.

The typical spectrally limited laser pulse in the near-infrared region is narrow-band up to 40-50 fs. Its spectral width Δk is much smaller than the carrying wavenumber k0 (Δk ≪ k0) . For such kinds of pulses, on distances of a few diffraction lengths, the diffraction is of a Fresnel's type and their evolution can be described correctly in the frame of the well-known paraxial evolution equation. The technology established in 1985 of amplification through chirping of laser pulses triggered remarkable progress in laser optics along with the construction of femtosecond (fs) laser facilities producing high intensity fields of the order of 1015-1021 W/cm2. However, the duration of the pulse was quickly shortened from picoseconds down to 5-6 fs, which have a broad-band nature (Δk ∼ k0). The linear and nonlinear propagation dynamics of broad-band pulses is quite different form their narrow-band counterparts. Here, we review the appropriate theoretical approach to study the evolution of the pulse. Moreover, we shed light on the different diffraction regimes inherent to both narrow-band and broad-band laser pulses and compare them to unveil the main differences. Using this very method, in subsequent papers, we will investigate the influence of the dispersion and nonlinearity on the laser pulse propagation in isotropic media.

Latin America and the Caribbean: Strategies to Fight Hidden Hunger.

The Latin American and the Caribbean (LAC) region has shown significant progress in the decline of hunger and malnutrition in the last decade. However, besides the food availability, the region is still affected by undernourishment, and even more worryingly it is witnessing increasing prevalence of the "double burden" in malnutrition by having a coincidence in the prevalence of undernourishment as insufficient calorie intake and overweight/obesity as excess body fat. The governments in the LAC region recognize the problem and have many nutrition-related and nutrition-sensitive policies in place. This paper proposes a model that intends to enrich the wide gamma of nutrition-sensitive policies and add value to the holistic and sustainable approach in eradicating hidden hunger. The community-based nutrition strategy model consists of 4 main strategies: (1) diversification of agricultural production; (2) training and technical assistance; (3) identification and support of local producers' organizations and (4) supporting the establishment of agro-industry. It considers education as a core cross-cutting strategy and contemplates education in nutrition, sanitation, finance, and sustainable agriculture. The model includes education, training, technical assistance, and infrastructure in its interventions aiming at the improvement of the human, physical, social, and natural capital in the community. It also contemplates baseline assessment of wide gamma of indicators and continuous monitoring and evaluation which would guarantee its successful implementation and necessary adaptation to reach the desired objectives. The model regards reaching dietary diversity as its primary objective but considers income generation as another important element. As such it incorporates nutrition strategies as a part of a wider rural development strategy and suggests a sustainable approach to building healthy and resilient communities.