Focusing of diode laser beams: a partially coherent Lorentz model

Diode lasers develop very fast and are widely used in various optical equipment. Diode laser sources produce fields that show fundamental variations with respect to the canonical Gaussian beam. A partially coherent Lorentz model is employed to describe the far field of a single mode diode laser beam. This paper is concerned with laser junctions significantly narrower than the wavelength. Two lenses system are placed in front of the laser diode, so that. the diverging beam is transformed into a converging beam. The across spectral density function in the plane perpendicular to the diode junction is considered in detail, and subsequently employed to predict the light intensity at various beam cross sections near the focus by using the generalized Huygens diffraction integral. The intensity profile at a focused spot produced by a partially coherent Lorentz beam is investigated and compared with that of Gaussian Schell-model beam. It is shown that it has a simple form but fairly describes the optical field in the focal region. The theoretical results are well fit to the practical results in this model and the variations between theory and the experiments are quite less than that of in Gaussian beams. Since Gaussian beams have a minimum uncertainty field, i.e. it possesses the minimum achievable angular spreading once the spatial extension is fixed. Since diode lasers produce highly diverging fields, a Gaussian description for the transverse fields fails. In this case, our results show that partially coherent Lorentz model is a better approximation, and the numerical simulation and discussions are given in detail.