Photonic Antennas

 

Photonic Antennas

 C.P. Liu, C.H. Chuang, and Tabassam Ismail

Partners

Queen Mary University
Rutherford Appleton Laboratory  

Description

As wireless communication systems move to higher frequencies, such as the 5 GHz band for the IEEE 802.11a and European HIPERLAN type 2 (H/2) standards, coaxial cable based signal distribution for in-building distributed antenna systems, which has previously been used for cellular radio applications in the 900 MHz and 1.8GHz bands becomes impractical because of high feeder loss. Also, at higher frequencies increased propagation losses have led to strong interest in the use of active array antennas to optimise coverage to a population of users. Wireless over fibre techniques can be used to deliver microwave communication signals between the Radio Network Controller (RNC) and the antenna points for mobile communication systems. For example, the Passive Integrated Picocells (PIPs) concept distributes these signals through optical fibre as microwave modulated optical signals, with a combined detector/modulator at each base station to provide a transceiver radio path. This approach allows several operators to provide different format services, such as cellular radio and wireless data, but share a common optical network infrastructure.

This interdisciplinary research project brings together the Antenna Engineering Group at Queen Mary (QMUL) and the Ultra-fast Photonics (UP) Laboratory at University College London (UCL) with the Millimetre Wave Technology Group, Space Science and Technology Department, Rutherford Appleton Laboratory (MMT) to combine their extensive expertise on microwave/millimetre wave antenna technology, electromagnetic modelling, optical communications, microwave photonics technology and semiconductor device/antenna integration with the objective of creating a new integrated structure, the Photonic Antenna, having widespread application in wireless communication and short range radar. 

One such structure is the Photonic Band Gap (PBG) antenna.  PBGs offer the opportunity to control and manipulate wave propagation as a result of their being formed from small-scale periodic geometric structures. Such materials offer pass and stop bands to electromagnetic waves in the same way semiconductors offer these properties to electrons. At QMUL research has been conducted on the application of PBGs to planar diplexer antenna designs that make it possible to integrate millimetre-wave front-end systems on single substrates.

In this project, the PBG antenna concept would be used and extended to provide low array element mutual coupling, improved antenna radiation patterns and to suppress distortion products generated from nonlinearity within the optoelectronic components and therefore improve the proposed Photonic Antenna system efficiency and maximise the system dynamic range. Currently a novel Embedded Uni-planar Compact-PBG scheme is being investigated to overcome the strong backward radiation caused by the usual approach of placing the PBG patterns on the ground plane, as illustrated in Figure 1.

Figure 1.  (a) Uni-planar PBG antenna structure. (b) Coupled split slot ring BG element. (c) Measured improved radiation pattern of structure.

The optoelectronic component used in a PBG antenna is the Multiple Quantum Well (MQW) Asymmetric Fabry-Perot Modulator/Detector (AFPM).  Such MQW electro-absorption modulators exhibit high modulation sensitivity, and more importantly, they can be used as modulators and detectors simultaneously.  The Ultra-fast Photonics Laboratory at UCL has designed and fabricated high-speed air-bridged modulators with modulation bandwidths exceeding 15GHz, the highest reported to date for InGaAsP/InGaAsP MQW AFPMs.  The semiconductor wafer layer structure, the schematic cross-section and a picture of a fabricated air-bridged AFPM are shown in Figure 2.

 

Figure 2: Layer structure, schematic cross-section and a photo of a fabricated air-bridged AFPM.

An example of how an AFPM can be used as modulator and detector in a point-to-point radio link is shown in Figure 3.

 

Figure 3.  Passive picocell system.

At the moment, the effective radiated power is limited by inefficient impedance matching of the AFPM to the separate antenna and by optical saturation in the AFPM.  We in this project intend to integrate the AFPM optimally with a PBG antenna, which performs diplexing, filtering and beam shaping operations as well as impedance matching. We further intend to increase the effective radiated power to levels useful for outdoor applications by moving to new AFPM semiconductor materials systems and integrating Photonic Antennas into fibre-fed phased array antennas.

The proposed basic antenna element with integrated AFPM is shown in Figure 4.  Here dual rectangular patches tuned to the TX/RX bands are constructed on an Embedded Uni-planar Compact-PBG (Figure 1), with different periodicity under each path, so providing isolation between the Transmit and Receive feed to the AFPM.  This ensures that the output RF signal from the AFPM is efficiently diverted to the Transmit antenna and sees a good match.  Similarly the incoming RF optimally excites the RX patch and thus channels the available power efficiently to the AFPM.

 

Figure 4.  Proposed self-diplexing PBG antenna with integrated AFPM.