Optimization of tapered semiconductor optical amplifiers for picosecond pulse amplification

Abstract

We present results of a fundamental device simulation study aimed at an optimization of high-power tapered semiconductor optical amplifiers for picosecond pulse amplification. Thereby, the microscopic light-matter coupling is described on the basis of a spatially resolved Maxwell-BlochLangevin description that takes into account many-body-carrier interactions, energy transfer between the carrier and phonon systems and, in particular, the spatiotemporal interplay of stimulated and amplified spontaneous emission and the noise caused by spontaneous emission. Extensive simulation runs reveal the microscopic physical processes which are responsible for the beam quality of the amplified picosecond pulses and allow us to give concrete suggestions for the optimization of the amplifier structure and geometry. We discuss the influence of a curved waveguide or spatially patterned current contact, and identify the length of the unpumped area as the most critical parameter to be carefully optimized. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2149177͔The need for high output power and good spatial and spectral purity, often required by applications in, e.g., ultrafast nonlinear optics or communication systems, has lead in recent years to device designs for high-power highbrightness semicondcutor laser sources 1,2 and to the realization of new laser amplifiers with improved beam quality. 3-7The laser amplifier system consisting of a a laser pulse source and a semiconductor amplifier is a very commonly used configuration since it allows the separate optimization of device properties responsible for pulse properties and output power. The spatial and spectral properties of a pulsed signal that has been amplified in a high-power semiconductor laser thereby strongly depend on both the geometry of the laser ͑in particular the waveguide geometry͒ and the specific realization of the current injection via an electronic contact. More recent investigations 8 on tapered laser amplifiers have revealed the influence of the tapered angle and the length of the waveguide section on the beam quality and power of the amplified signal. It could be shown that a tapered angle that has been adjusted to the diffraction angle of the injected Gaussian beam and a waveguide length of a few 100 m provide good beam quality and high small signal gain with only small contributions of amplified spontaneous emission ͑ASE͒ in the emitted radiation.While previous studies have mainly addressed the amplification of continuous-wave ͑cw͒ optical signals, in this letter, we focus on an investigation of the design of the waveguide section and the electronic contact of tapered laser amplifiers ͑InGaAs, 2 -3 mm length͒ that should be optimized for picosecond pulse amplification. In particular, we will analyze the influence of these design parameters with respect to the spatiotemporal light field dynamics and quality of the picosecond pulses. Table I summarizes the fundamental material and device parameters used in the modeling. More details on the background ...