Efficient, low parasitics 1.3 μm InGaAsP electroabsorptionwaveguide modulators on semi-insulating substrate

The high-speed operation of waveguide modulators is limited by the RC time constant, where the capacitance is primarily due to the parasitics. As part of an effort to reduce the parasitic capacitance, the authors have demonstrated a 1.3 μm InGaAsP electroabsorption waveguide modulator on semi-insulating InP substrate. A parasitic capacitance smaller than 10 fF was achieved by fabricating the bonding pad on an isolated mesa and by making use of an air bridge. For a 400 μm long waveguide modulator, an operating voltage as small as 2 V at an on-off ratio of 10 dB was obtained by optimizing InGaAsP bandgap for the operating laser wavelength. The device consists of a five-layer structure on semi-insulating substrate grown by liquid-phase epitaxy. These layers are undoped-InP buffer layer, undoped-InGaAsP waveguide layer, undoped-InP spacer layer, p-InP cladding layer, and p-InGaAsP contact layer