Er/sup 3+/-doped Silica-based Planar Lightwave Circuits

Recent progress in optical communication systems has emphasized the importance of integrated optical circuits such as light sources, amplifiers, and multi/demultiplexers. Active integrated optical circuits which use optical amplification have been made mainly with semiconductor waveguides. Recently, optical amplification was achieved in E?'-doped glass planar waveguides [1,2] which initiated the study of active integrated optical circuits [3]. The integration of Ef"doped waveguides with silica-based planar lightwave circuits (PLCs) [4] opens up the possibility of creating a new family of active integrated circuits. In this talk, the technology of E?+-doped silica-based PLCs is presented. First, fabrication techniques and amplification characteristics of E?'-doped waveguides are discussed. Then, their preliminary applications to active PLCs are described. A key element in obtaining a high gain in short waveguides is uniform doping at high E?' concentrations. Quenching in E?' systems caused from upconversion [5] and the upconversion is accelerated in clusters because the upconversion energy transfer rate is in proportion to r (r: distance between neighboring ions). Therefore, we need to use host glass materials with high solubilities of E?' ions in order to obtain efficient amplification in planar waveguides. Since the solubility of rare earth ions in silica is low, codoping using phosphorus or aluminum is necessary to increase the E?' concentrations to a sufficient level of about 0.5 wt% in silica-based waveguides [6]. We have developed a technique for fabricating low-loss E?-doped silica-based waveguides made by flame hydrolysis deposition and reactive ion etching using phosphorus as a codopant [2]. A gain of 0.7 dB/cm is obtained at a wavelength of 1534 nm in waveguides with an E?' concentration of 0.5 wt%. Based on a design calculated using the amplifier theory including the upconversion [7], we demonstrated a 24 dB-gain planar waveguide amplifier with a noise figure of 3.8 dB using the 0.5 wt% Er3+-doped 35 cm-long waveguide and 980 nm laser diode pump sources [8]. Active PLCs, including optical sources, amplifiers and filters, have been demonstrated by integrating E?'-doped waveguides with various waveguide circuit elements. As integrated light sources, waveguide lasers with different cavity configurations have been reported. E?'-doped Y-branched waveguide lasers, which use an interferometric effect in the multiple cavity in order to control oscillation modes [9], were demonstrated. Wavelengthtunable oscillation in the 1.5 pm telecommunication window was obtained by applying electric power to a thermo-optic phase shifter integrated in a branch of the waveguide [lo]. An E3'doped ring laser equipped with a directional coupler generated output light with narrow linewidth of 200 kHz in a 9 cm-long ring cavity [ 1 11. More recently, single-longitudinal-mode oscillation was achieved in E?'-doped waveguide lasers with integrated Bragg reflectors [12]. The Bragg reflectors were photo-imprinted using 193 nm UV light exposure in Ge0,-free P,O,-codoped waveguides sensitized by H,-loading. Further integration of these light sources may lead to use in WDM optical communication systems. A waveguide amplifiers integrated with a 1540/980 nm directional coupler which