Nonuniform carrier distribution in semiconductor optical amplifiers

This work discusses the experimental results and theoretical analysis of the longitudinal distribution of the spontaneous emission, amplified spontaneous emission, hole-electron pairs, and gain in semiconductor optical amplifiers. A novel, yet simple, technique was developed enabling, for the first time, the measurement of these distributions. This was accomplished with a high degree of spatial resolution using a single-mode micro-lensed fiber tip positioned perpendicular to the active region and moved longitudinally along the stripe. The cornerstone of this study centers on the theoretical analysis of the small portion of the isotropic spontaneous emission, emanating from the optical cavity, which is captured by the lensed fiber. Spectral as well as integrated power measurements were made along the length of the cavity. These spectral and integrated power measurements provide a direct link to the carrier concentration and gain along the optical cavity. The distribution of this spontaneous emission along the amplifier, and its relationship to other parameters, provide information about the carriers.; Since the common mode of operation for linear optical amplifiers is deep in saturation, the devices in this study were analyzed in regimes significantly below the small-signal gain. While large carrier density non-uniformity occur with output powers equal to or greater than Psat, a significant amount can occur in amplifiers even with small input signals. In these amplifiers, the higher carder concentrations produce much higher internal gain coefficients making them more prone to non-uniform carrier density distributions. Moreover, even in semiconductor lasers, where the carrier concentration and the gain are pinned at the onset of lasing to rather pedestrian levels (approximately 1 x 1018 cm-3 and of 3 dB, respectively), previous theoretical analysis for more than a decade postulated that a significant spatial distribution occur. These measurements and analysis provide evidence, for the first time, of the spatial distribution of light and carriers within these amplifiers and provide, as well, support to theories related to Fabry-Perot laser structures.