Millimetre-wave InP antenna for monolithic integration

Introduction: Millimetre-wave fibre wireless applications [1] could profit from a small antenna that can be fully integrated with the microwave and optical components of the system. This can lead to smaller and more cost-effective fibre-fed antenna units. Active integrated antennas [2] are another application that could benefit from a directly integrated antenna. Printed antennas are the obvious choice for monolithic integration, owing to their low profile, planar characteristics. However, the materials used to fabricate the microwave=optical devices (e.g. gallium arsenide or indium phosphide (InP)) are typically of high permittivity (er > 10). The design of conventional printed antenna elements on such substrates is generally avoided, as they suffer from a narrow bandwidth (<6%) and excessive loss due to surface-wave excitation. To ensure that the antenna is not a limiting factor in the system bandwidth, wideband antenna elements are desired. Most of the MMIC=OEIC integratable antennas presented in the literature (e.g. [3]) exhibit a narrow bandwidth and=or indicators of poor efficiency. A coplanar waveguide (CPW)-fed aperture stacked patch (ASP) [4] has exhibited excellent performance on high dielectric constant feed materials by using a combination of high and low permittivity layers, similar to that described in [5]. Material systems based on InP have the potential to realise the active and passive devices required for a fibre-wireless base station. InP optoelectronic integrated circuit (OEIC) modules [6] avoid concerns with interconnections such as wire bonding or butt-coupling, and have the potential to yield reliable and cost-effective modules. OEICs may use a CPW transmission structure, so only single-sided wafer processing is required and the need for vias is eliminated. In this Letter, a broadband millimetre-wave antenna fabricated on an InP wafer is presented, which is suitable for integration with microwave=optical circuits. The geometry investigated is the CPW-fed aperture coupled patch (ACP), which utilises the antenna design principles of [4] and [5].