GaAs‐ALGaAs double‐heterostructure coupled‐waveguide optical modulators and their high‐frequency modulation characteristics

Abstract

AbstractIn this paper, a GaAs‐AlGaAs double‐heterostructure (DH) coupled‐waveguide optical modulator, which is important as a multifunction optical component and for possible monolithic integration, has been studied in detail. A DH device with pn‐junction structure, which is expected to have the lowest switching voltage, has been fabricated and the optical modulation characteristics have been measured at 1.06 μm wavelength. At a reverse bias of 13 V, an extinction ratio of 11 dB has been obtained. However, the cutoff frequency has been limited to about 250 MHz, which was lower than the calculated value of 1.1 GHz obtained using a simple lumped‐element circuit analysis. Therefore, the optical modulation characteristics have been analyzed assuming the device as a distributed‐element circuit line, and also taking into account the external parasitic impedancees. Thus it has been concluded that a reduction of the top‐electrode resistance and an increase of the conductance across the device cross‐section are necessary to improve the high‐frequency optical modulation characteristics.To verify these results, a DH Schottky contact structure, which essentially possesses no ohmic contact resistance with a thick topelectrode, has been fabricated on a MBE‐grown wafer. At a reverse bias of 16 V, an extinction ratio of 12 dB and a cut‐off frequency of 1.5 GHz have been obtained successfully. It has also been concluded that an even thicker top‐electrode and an improved mounting method which minimizes the external parasitic elements are necessary for further improvement of the optical modulation frequency characteristics.