Efficient Antenna/Electro-Optic Front-Ends for Fiber Radio Applications Rod Waterhouse and Dalma Novak

Pharad, LLC, Glen Burnie, MD

I Introduction The application of RF photonic links for signal

remoting in wireless communication systems continues to evolve with recent developments in RF photonic device technologies, transmission techniques and receiver architectures. The replacement of conventional RF components such as the interconnecting coaxial cable between antenna base-stations, to exploit the various advantages of optical fiber as a transmission medium is now well-established. The next phase in the evolution of RF photonic systems is to improve RF link performance, thereby enabling photonic technologies to be competitive with traditional electronic RF front-end architectures.

Progress has been made in the realization of high gain, low noise figure (NF) RF photonic links using high power low noise fiber lasers, high E/O efficiency broadband lithium niobate (LiNbO3) optical modulators, and high power, high saturation current optical detectors [1]. We are now at a stage where it may be possible to remove the RF amplification stage of the wireless base-station altogether; particularly for picocellular wireless systems which feature shorter radio link distances. Doing so, would create a base-station module that consists of an antenna/electro-optic front-end that draws minimal DC power, thereby potentially reducing the overall cost of the system implementation. The development of such a front-end requires an integrated approach, whereby the photonic devices and antenna are designed in unison, sharing common wafers, materials, electrodes and structural supports. In fact, the required approach is similar in concept to the evolution of hybrid integrated circuits to realize efficient Monolithic Microwave Integrated Circuits (MMICs) and Optical Electrical Integrated Circuits (OEICs). To maximize efficiency and performance, all aspects of the base-station front-end must be designed together.

In this paper, we present strategies in which printed antennas can be directly integrated with photonic components thereby creating efficient antenna/electro-optic front-ends for fiber radio applications. We present a variety of configurations highlighting the advantages and issues associated with each approach. In Section II we discuss how dipoles and slots can be integrated with photonic components. In Section III we present patch antenna configurations that can be easily integrated with photonic devices that overcome many of the shortcomings of using simple dipoles or slots. In Section IV we explore the potential integration of traveling wave antenna structures with photonic components to yield an efficient antenna/electro-optic front-end. The efficient integration of antennas with photonic components is a very challenging problem, however with novel approaches, efficient solutions can be achieved.

II Dipole/Slot Integration Printed slots, monopoles and dipoles are probably the simplest form of antenna structure that can be integrated with a photonic device. These antennas are relatively straightforward to design, can be developed on any material (LiNbO3, InP or polymers), fed with most forms of transmission lines such as co-planar waveguide (CPW) and strip line, and can provide reasonably efficient solutions. Fig. 1 shows a wideband printed monopole developed on a thin layer of material with a dielectric constant similar to InP. The antenna is easily coupled to a CPW transmission line (as shown in Fig. 1) and can operate efficiently over a frequency range greater than an octave.

Fig. 1 Photograph of a printed wideband monopole developed

on high dielectric constant material Although it may appear that dipoles and slot antennas would be a good choice for realizing an efficient antenna/electro-optic front-end, this is not the case. The fundamental issue associated with these forms of simple antenna structures is that they are omni-directional in nature and therefore couple strongly to their surrounding environment. This coupling results in a severe degradation of their performance, including: significant reduction in the bandwidth of operation; reductions in efficiency due to the excitation of leaky wave modes, and also severe scalping in the radiation patterns leading to nulls in the coverage area. Cavity backed versions of these antennas can mitigate some of these issues to a degree, however overall there are simply better antenna configurations that are more suited to this challenging problem. At the same time however, utilizing slots and dipoles in more complicated configurations, such as that examined in later sections, can be useful. III Patch Antenna Integration Patch antennas are probably the best solution when attempting to develop an efficient antenna/electro-optic front-end, however this was not always thought to be the case. Early research on patch antennas found that although the structural

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characteristics were compliant with integration with active devices (MMICs or photonic components), the efficiency and return loss bandwidth performance was compromised simply because of the high dielectric constant of the materials used to make the devices. The high dielectric constant material would cause the excitation of surface waves which can dramatically reduce the overall efficiency of the antenna; in some cases by more than 3 dB. It was not until more recently that multi-layer patch antenna approaches were created that enabled highly efficient integrated solutions that can operate in this manner over large bandwidths. The two fundamental multi-layered patch antenna configurations are: the hi-lo stacked patch [2]; and the aperture coupled stacked patch [3]. Both approaches rely on the use of parasitic patches to draw power out of the lower layer (the material used to develop the photonic device) thereby creating an efficient solution. We have applied these techniques to a variety of materials including LiNbO3 and InP and the overall efficiencies of these antennas are greater than 90 %. These forms of antennas when integrated with photonic components can be thought of as ‘flip-chip’ solutions, where the layers with conductors are bonded to the photonic material layer. Recently we created a variation of the hi-lo stacked patch which not only highlights how easily it can be integrated with photonic components, but it also outperforms the original form of the antenna. Fig. 3 shows a schematic of a CPW fed version of the antenna. Here the driven patch (developed on the photonic component wafer) resides above a cavity created in the support structure used to house the photonic device. The cavity is air-filled. Using such a configuration with the parasitic elements of a conventional hi-lo stacked patch almost doubles the -10 dB return loss bandwidth while maintaining an efficiency greater than 95 %.

Fig. 2 Schematic of a CPW fed cavity backed hi-lo stacked

patch Importantly, these integrated patch antenna configurations do not suffer the scalping in the radiation patterns usually encountered for dipole or slot solutions. Fig. 3 shows an example of the measured radiation patterns in one of the principal planes for the antenna presented in Fig. 2. As can be seen in Fig. 3 there no undulation in the gain in the upper hemisphere.

Fig. 3 H-plane radiation patterns of the cavity backed hi-lo

stacked patch

IV Traveling Wave Antenna Integration Traveling wave based antennas can be directly integrated with photonic components and yield efficient solutions, however there is a fundamental issue associated with this approach. Whether a printed quasi-Yagi antenna (which uses parasitic uni-planar dipoles to ensure an efficient solution) or a tapered slot is used, these antennas need to be electrically large to operate efficiently. This equates to requiring a large amount of wafer space dedicated to the antenna; which is not really a viable option given the expense of a photonic material wafer. The only exception to this would be if a polymer photonic device was to be considered, where the material is relatively cheap. Fig. 4 shows a polymer modulator integrated with a tapered slot antenna [4]. It should be noted to date, there are still many potential issues associated with polymer devices such as stability and insertion loss, although improvements are being made in this area.

Fig. 4 Photograph of an integrated polymer modulator and

tapered slot antenna

V Conclusions Several techniques that enable an antenna to be

efficiently integrated with photonic components have been described. These approaches can lead to highly efficient antenna/electro-optic front-ends for radio applications.

References [1] S. Pappert, “MTO Electronics & Photonics, ‘The RF Story’”, DARPA

MTO presentation, 2007. [2] W. S. T. Rowe et al, IEEE Trans. Ant. Propn., Vol. 51, No. 6, (2003), pp

1413 - 1415. [3] W. S. T. Rowe et al, IEEE Trans. Ant. Propn.,Vol 51, No. 9, (2003), pp

2484 - 2486. [4] MDA BAA on ‘Development and Integration of Polymer Components for

Next Generation Radar’, 2004.

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