FOCUSED ISSUE FEATURE RF Engineering Meets Optoelectronics Stavros Iezekiel, Maurizio Burla, Jonathan Klamkin, David Marpaung, and José Capmany ©ISTOCKPHOTO.COM/CHAINATP ntegrated microwave photonics (IMWP) is field of IMWP and to describe how it may potentially concerned with applying integrated photonics be applied to improve the performance and capabili- technology to microwave photonic systems. It ties of microwave and millimeter-wave (mmWave) is one of the most active and exciting areas of systems. Just as the microwave monolithic integrated current research and development in micro- circuit (MMIC) has revolutionized active microwave Iwave photonics (MWP), building upon the impres- circuits, IMWP is poised to open up new applica- sive foundations of integrated photonics in various tions for microwave engineering that take advantage systems involving material platforms such as indium of the unique functionalities offered by photonics, phosphide (InP) and silicon nitride (Si3N4). The aim especially with regard to its large bandwidth. of this article is to explain to the wider microwave Working on the assumption that most microwave engineering community the significance of the new engineers may be somewhat unfamiliar with the Stavros Iezekiel ([email protected]) is with the Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus. Maurizio Burla ([email protected]) is with the Institut National de la Recherche Scientifique-Énergie, Matériaux et Télécommunications, Varennes, Quebec, Canada. Jonathan Klamkin ([email protected]) is with the Department of Electrical and Computer Engineering, Boston University, Massachusetts, United States. David Marpaung ([email protected]) is with the Centre for Ultrahigh Bandwidth Devices for Optical Systems, Institute of Photonics and Optical Science, School of Physics, University of Sydney, New South Wales, Australia. José Capmany ([email protected]) is with the Instituto de Telecomunicaciones y Aplicaciones Multimedia, Ciudad Politécnica de la Innovación, Valencia, Spain. Digital Object Identifier 10.1109/MMM.2015.2442932 Date of publication: 7 August 2015 28 1527-3342/15©2015IEEE September 2015 RF Engineering Meets Antenna RFRF RFRF In OutOut Downconversion Filter and E/O O/E Optoelectronics LNA OpticalOptical Fiber and Processing Figure 1. An analog optical fiber link as an example of an MWP system for transporting RF signals over optical fiber. In many links, the E/O module consists of a laser and optical intensity modulator, with the RF input being applied as the drive voltage to the modulator. The O/E module consists of a photodiode that recovers the microwave signal as a photocurrent. LNA: low-noise amplifier. prospects of regular MWP technology, we begin by 100 briefly describing the main characteristics, advan- tages, and uses of MWP. We then move on to describe how IMWP will enable performance improvements and new functionalities for microwave systems based Microstrip on photonics. Our overview will discuss various s Delay (dB) 10 n IMWP material platforms and devices, and we will also outline how IMWP can be used in microwave sys- tems, focusing specifically on the important example of antenna beamforming. 1.0 Surface Acoustic WaveSuperconductor Microstrip What Is Microwave Photonics? Single-Mode Optical MWP is an interdisciplinary field that unites the Propagation Loss for 1- Fiber at 1.55 nm domains of RF engineering and optoelectronics [1]–[4]; 0.1 an MWP system will typically contain a mix of micro- 0.1 1.0 10 100 wave and photonic components configured such that Frequency (GHz) microwave signals are transported (and, quite often, Figure 2. A comparison of the propagation loss, based on processed) in the optical part of the spectrum using a a delay of 1 ns versus frequency, between three microwave variety of photonic components, such as lasers, optical transmission media and optical fiber. In the case of optical fiber, and, in the most basic systems, photodiodes. A fiber, the frequency corresponds to that of the RF input good example of a first-generation system is an analog signal (as shown in Figure 1); the effects of E/O and O/E intensity-modulation optical link of the type shown conversion are not accounted for. (Figure adapted from [4].) in Figure 1, which consists of a device for converting microwaves to a microwave-modulated optical sig- The motivation for this approach is the large band- nal [electrical-to-optical (E/O) converter], a fiber over width and relatively low loss per unit length of optical which the microwave signal propagates as the nar- fiber compared with that of conventional microwave rowband modulation of an optical carrier, and then a transmission media (Figure 2) in addition to its elec- photodetector for conversion back to the microwave tromagnetic immunity and mechanical flexibility—all domain [optical-to-electrical (O/E) conversion]. of which make the approach ideally suited to antennas September 2015 29 the remote location of antennas, where con- Antenna ventional microwave cables would incur too RF RF O/O In Out much loss. Filter and E/O Processing of O/E In the 30 years since the earliest work on LNA Signals with analog links, the field of MWP has attracted Photonics immense interest and generated many new Figure 3. Analog optical processing of microwave signals. In this developments from both the research com- advanced application of MWP benefiting from photonics’ broad munity and the commercial sector. Emerg- bandwidth, not only are RF signals transported in the optical domain, ing applications for future 5G networks and but a variety of signal processing functions can also be implemented via subterahertz systems, including radio-over optical input–optical output (O/O) components. fiber (ROF), indicate that MWP is set to be a subject of increasing importance [5]. located remotely over several kilometers. It should be From one perspective, MWP deals with the appli- noted that the contributions of the E/O and O/E con- cation of microwave engineering to high-speed pho- version stages are not accounted for in Figure 2, and tonic devices—e.g., in the design of traveling-wave the square-law nature of photodetection means that (TW) electrodes for optical intensity modulators or an optical loss of approximately 0.2 dB/km will trans- that of matching networks for analog links (such as late into a corresponding electrical loss of 0.4 dB/km. that shown in Figure 1). In fact, the Mach–Zehnder Moreover, the presence of active devices in both the modulator (MZM) is an early example of the integra- E/O and O/E stages means that, in addition to the tion of guided-wave photonics with TW structures overall link gain, it is also necessary to evaluate link that highlights the need for performance optimization performance with respect to noise figure, third-order through such integration. intermodulation products, and spurious free dynamic The real significance of MWP, however, is as a dis- range (SFDR). Nevertheless, over several kilometers, ruptive technology in which the application of pho- an analog optical fiber link can prove attractive for tonic components, systems, and techniques enhances Physical World MWP Applications Signal Acquisition, Transport — Fiber/Wireless Communication — Biomedical Imaging Optical Sensing, — Wireless Sensor Networks User Transport, — Internet of Things Interface Acquisition — Satellite/Distributed Antenna Systems Analog Signal Processing Engine Analog-to-Digital ASIC/FPGA/DSP Analog Processing Software-Based Conversion (ADC) Digital Hardware Digital Processing Analog Optical Photonically Digital Optical Signal Assisted ADC Processing Processing MWP Functionalities — Tunable and Reconfigurable Filters — Frequency Multiplication — Optoelectronic Oscillators — Turnable Tr ue Time Delay/Phase Shift — Arbitrary Waveform Generation — Instantaneous Frequency Measurement — Frequency Up-/Downconverters — Optical Phase Locked Loops and RF — Optically Assisted ADC Transceivers — Beamforming Networks Figure 4. Conceptual rendering of an analog signal processing engine within the broad context of multiple ICT systems. ASIC: application-specific integrated circuit; FPGA: field-programmable gate array. 30 September 2015 microwave and mmWave systems, yielding perfor- steered arrays, which, in turn, represent an ideal example mance improvements and adding new functionalities of IMWP systems, as illustrated in Figure 5. However, that would not be possible using an electronic-based there are also benefits to be had from IMWP for simpler approach. Looked at from this perspective, it is more MWP systems such as analog optical links, where IMWP accurate to say that MWP deals with the generation, pro- devices can be used to improve dynamic range through cessing, and distribution of microwave and mmWave tunable optical filtering [18] or novel link architectures signals by optical means, benefiting from photonics’ requiring balanced detection [19]. inherently unique advantages, such as low loss (inde- Integrated photonics has the potential to change pendent of frequency), high bandwidth, and immunity the power-scaling laws of high-bandwidth systems to electromagnetic interference [1]–[4]. Moreover, MWP through proper architectural choices that combine adds fundamental value by enabling key functionalities photonics with electronics to optimize performance, to be realized in microwave systems using photonics, power, footprint, and cost [20]. In particular, analog as suggested in Figure 3. These