Optical properties of Er in Er-doped Zn2Si0.5Ge0.5O4 waveguide amplifiers

There is a growing need for compact, efficient integrated waveguide optical amplifiers for use in optoelectronic communication. Zn₂Si_0.5Ge_0.5O₄ (ZSG) doped with Er (ZSG-Er) is a promising new host material due to the high concentration of Er that can be incorporated and the high optical activity of the incorporated Er. In this paper, the absorption and emission cross sections of Er in ZSG-Er (to the authors´ knowledge, for the first time) are measured both through photoluminescence spectra and direct gain and absorption measurements. Peak absorption and emission cross sections are about 3×10^-24 m² from a Landenburg-Fuchtbauer analysis of the photoluminescence spectra, comparable to measurements on other oxide-based glass amplifiers. The population statistics of the excited Er level, along with the excited-state lifetime, are determined through a novel frequency-domain method in which the spontaneous emission power at 1550 nm is measured as a function of frequency under a modulated 980-nm input. The determined lifetime of 2 ms is comparable to the 2.3 ms measured using a conventional pump-probe technique. The novel analysis technique yields the population statistics of the excited Er atoms and the lifetime of the excited Er state under given pumping conditions independent of the unknown and variable coupling in and out of the waveguide. This method predicts zero net gain at 70 mW, about what is observed. Comparison of calculated gain and absorption based on Er density and measured cross sections with measured gains suggest that only about 20%-30% of the Er in the material is optically active. A 4.7-cm-long sample demonstrated a signal enhancement of ∼13dB. Cavity characteristics were measured using an analysis of coherent reflection under no pumping. The facet reflectivity was determined to be 0.27, and the scattering/absorption loss was 1.05/cm, for a total distributed loss of 1.65/cm in a 4-cm cavity. These losses, compared with an estimated achievable gain of 0.25/cm under full inversion, suggest that optically pumped lasing at this concentration is not possible. Measurements of both the cross sections and population statistics, compared with actual gain and absorption properties, give insight into the - contribution of the Er dopant under different conditions and can be used to model and improve rare-earth-based amplifiers.