Since the first radio transmission, the size of the radio spectrum has doubled every 30 months, following an exponential increase in data transfer rates in wireless communications. The next generation 5G systems would require increase in wireless data transfer rates to above 100Gb/s that may be achieved by using millimeter-wave (30–300GHz) and terahertz (300–3000GHz) frequency ranges as carriers. Microwave photonics by combining photonics and high-frequency electronics at the chip level, can provide a cost-effective method of achieving the increased bandwidth required for 5G networks.
Microwave photonics (MWP) is an emerging interdisciplinary area that investigates the interaction between microwave and optical waves for the generation, processing, control, distribution and measurement of microwave, millimeter-wave and THz-frequency signals. It brings unique advantages inherent to photonics, such as low loss (independent of frequency), high-bandwidth and immunity to electromagnetic interference.
The aim of microwave photonics is using photonic devices to achieve microwave functions which are difficult or impossible for electronic techniques. MWP is being applied in applications ranging from defense applications, such as radar and electronic warfare systems, to civil applications, such as wireless and satellite communications, imaging and instrumentation. MWP has created new opportunities for information and communication (ICT) systems and networks such as distributed antenna systems and optical signal processing, as well as emerging fields such as converged fiber-wireless and in-home networks, medical imaging systems using terahertz (THz) waves, wireless body and personal area networks, instrumentation and the Internet of Things.
The availability of cost-effective telecom-based components such as lasers, modulators, and photodiodes and the ability to use optical fibers to transmit RF signals over long distances to remote antenna units has been critical in the growth of MWP.
MWP brings the fundamental added value of enabling the realization of key functionalities in microwave systems such as microwave oscillators, signal processing, antenna beam steering, analog transmission, arbitrary waveform generation, analog-to-digital convertor, filtering, frequency up/down conversion and instantaneous measurement that are either complex or even not directly possible in the radiofrequency domain.
However the widespread use and application of this technology is currently limited by the high cost, bulky, complex and power consuming nature of its systems. “Typical space, weight and power (SWAP) figures for commercial MWP systems are around 0.04-0.2 m2 in size, 1.5- 10 kg in weight and 15-20 W in power consumption, making them unsuitable for mass production and widespread use required by the next generation and emerging applications,” write Daniel Pérez, Ivana Gasulla, and José Capmany from Spain. The major challenge that MWP researchers have to overcome is therefore related to reduction of SWAP figures.
Integrated microwave photonics (IMWP) deals with the application of integrated photonics technologies to microwave photonics (MWP) systems. During the last 5 years, IMWP has become probably the most active area of current research and development in the discipline of MWP, capitalizing upon the outstanding progress of integrated photonics in various material platforms such as indium phosphide (InP), Silicon on Insulator (SOI) and silicon nitride (Si3N4).
Microwave photonics for 5G Communications
One key application of MWP is the transport and distribution of radio or wireless signals over optical fiber. In a typical configuration, distributed and remote antennas gather various wireless-based services (mobile units or wireless sensor nodes, for example) and transmit their radio frequency (RF) signals to the central office through optical fiber links.
Currently researchers are working on defining next generation of wireless communication, i.e., 5G. 5G communications require a multi Gb/s data transmission in its small cells. For this purpose millimeter wave (mm-wave) RF signals are the best solutions to be utilized for high speed data transmission. Wireless access network (WAN) at millimeter wave bands (30–300 GHz) has large bandwidth which offers an alternative for high speed indoor/hotspot communication to be utilized for 5G. For generating the high-frequency mm-wave electrical signals, using conventional electronics becomes less financially attractive, therefore, there is a high concern to directly generate mm-wave signals in the optical domain.
The researchers are developing microwave photonics based generation, modulation and distribution of 60 GHz frequency band signals to be applied in 5G. The generation of photonics based millimeter based source is based on principle of mixing two or more coherent longitudinal modes of laser beam with frequency spacing equal to the wanted mm-wave. As the longitudinal modes beat with each other in the photodiode, the required electrical mm-wave could be generated.
DARPA has been investigating the development of microwave photonic technology that has the potential to dramatically reduce all of the SWAP-C parameters of a traditional RF telecommunications link. DARPA has identified three dominant components that could enable a photonic link: low-noise/high-power laser diodes, low-loss/low-drive-voltage electro-optic modulators, and high-power/highly linear photodiodes.
The microwave photonics is one of the most promising directions in modern radar engineering. Application of microwave photonics components provides a significant improvement in certain characteristics of radar, such as dramatically improving informational content and resolution range of radar; increasing range of target detection; high noise immunity; performance stability under changing climate, primarily, temperature conditions; lessened weight and size parameters of antenna systems and lessened cost.
The next generation of radar systems needs to be based on software-defined radio to adapt to variable environments, with higher carrier frequencies for smaller antennas and broadened bandwidth for increased resolution.
However, it is very difficult for the state of the art electronic technologies to operate the microwave components in multiple frequency bands without sacrifice of the performance. In addition, the calibration of the system would be a critical issue because the electrical devices always have different insertion losses, phase shifts and reflection coefficients in different frequency bands.
Today’s digital microwave components (synthesizers and analogue-to-digital converters) suffer from limited bandwidth with high noise at increasing frequencies, so that fully digital radar systems can work up to only a few gigahertz, and noisy analogue up- and down conversions are necessary for higher frequencies.
In contrast, Photonics provide high precision and ultrawide bandwidth allowing both the flexible generation of extremely stable radio-frequency signals with arbitrary waveforms up to millimetre waves, and the detection of such signals and their precise direct digitization without downconversion.
The U.S. Army Research Laboratory (ARL) has developed and patented next-generation phased array radar based upon photonic integrated circuits.
A team of researchers from Italy, including from the National Laboratory of Photonic Networks has reported in Nature about the development and the field trial results of a fully photonics-based coherent radar demonstrator carried out within the project PHODIR (‘Photonics-based fully digital radar’). Europe seventh framework programme (FP7) established a project named GAIA (Photonics front-end for next-generation SAR applications, Oct. 2012—Oct. 2015), to develop the photonic technologies required in future array antenna systems for the implementation of the next generation synthetic-aperture radar (SAR) applications for future Earth observation missions.
MIT Lincoln Laboratory has been developing several MWP subsystems that leverage its technology to realize next-generation wideband EW receivers and simultaneous transmit and receive (STAR) systems. They have recently achieved and reported a compact wideband channelized electronic intelligence (ELINT) receiver architecture that leverages high-Q silicon-photonic filter PIC, low-noise lasers, and high-saturation photodetectors.
The Laboratory demonstrated a prototype system for a two-channel ELINT receiver; the system performed both channelization and photonic frequency down-conversion from X band to baseband (IF ~1.5 GHz) with image rejection >85 dB using advanced photonic filtering. This level of performance is necessary for ELINT receivers operating in dense signal environments.
The Lincoln is also developing Simultaneous transmit and receive (STAR), a critical system need in electronic attack and electronic protection systems, The Laboratory’s RF system architecture for STAR operation utilizes an adaptive RF canceller. The function of the RF canceller is to adaptively replicate the response of the surrounding environment, adjusting the transfer function, to match an inverted version of the mutual coupling between the transmitter and receiver. At Lincoln, a photonic tapped-delay-line RF canceller architecture for STAR is being developed. In this effort, low-loss photonic taps are achieved through a fiber Bragg grating (FBG) written into the core of a low-loss optical fiber.
They are also focusing on Integration of multiple photonic components on a common substrate that is highly desired not only to reduce the cost, size, weight and power (SWaP) of photonic subsystems, but also to improve the optical system performance through reduction of parasitic losses and physical delay lengths. The most basic photonic integrated circuit (PIC) for an optical communication link would include lasers, modulators, filters, amplifiers, and photodetectors.
Photonic integrated circuits for millimeter-wave wireless broadband communications
“We have now succeeded in combining photonics and high-frequency electronics at the chip level, by developing so-called radio frequency light engines. These provide a cost-effective method of achieving the increased bandwidth required for 5G networks. As part of the iPHOS research project funded by the European Commission, in collaboration with III-V Lab and University College, London, we integrated a complete photonic transmitter system in a single chip. This chip received an electrical input and generated a modulated millimeter-wave electronic signal in a coplanar microstrip line ready for transmission to an antenna,” writes Guillermo Carpintero, associate professor in the Department of Electronic Technology, University of Madrid Leganés, Spain.
The development of integrated photonic components has been facilitated by the establishment of new facilities in Europe. In particular, collaboration between academia and industry with the support of the European Commission has resulted in multi-project wafer production runs that are commercially available to circuit designers. These production runs, which use fabrication foundries, have the advantage that circuit designers can share the costs of the design tools, fabrication processes, and maintenance of the facilities. Each foundry uses a specific material substrate, such as indium phosphide or silicon, has a specific set of components that master onto that substrate, and provides designers with a standardized set of photonic components that are optimized to enable high performance.
Unfortunately, the integration of broadband antennas onto the substrate materials is hampered by the high dielectric constant of the substrates, which does not allow RF signals to be radiated efficiently. This indicates a need for new integration techniques, as different components require specific sets of material parameters that cannot be achieved using a single substrate. Instead of monolithic integration of all the circuit elements in a single chip, the focus has shifted to heterogeneous or hybrid integration. Heterogeneous integration comprises combining multiple material substrates on a chip-scale form factor to ‘mix and match’ a variety of devices and materials to provide the best substrate for each function. Hybrid integration, on the other hand, involves combining multiple preprocessed chips, writes Guillermo Carpintero, associate professor in the Department of Electronic Technology, University of Madrid Leganés, Spain.
The current focus of hybrid and heterogeneous integration is predominantly on silicon components. However, polymer-based hybrid components have also attracted attention, owing to their potentially low cost and the possibility of integrating in one chip broadly tunable lasers and phase shifters for phased-array transmitters. Also, the low permittivity of polymer materials allows the integration of broadband antennas. Recently, in collaboration with the Fraunhofer Heinrich Hertz Institute (HHI) and Osaka University, we demonstrated the advantages of dual polymer-based distributed Bragg reflector lasers based on the HHI PolyBoard integration structure for generating high frequencies.
RF photonics requires operation at high optical power levels that semiconductor-based devices such as silicon and InP cannot achieve. It is estimated that the power needed is 10 to 50 times higher than semiconductor-based devices can support. There is need for materials such as glasses and lithium niobate. Need Q filters in glass-based materials, such as CF2.
RF photonics systems are unique in that packaging requires the integration and assembly of multiple materials: lithium niobate, InP, gallium arsenide (GaAs), silica, silicon, rare earths (erbium) and glass (CF2). These disparate materials make assembly, packaging, and integration particularly challenging for \systems that operate up to 100 GHz.
“The nation that develops the consolidation technology to integrate disparate platforms will have a significant strategic (military) advantage,” said Dr. Arthur C Paolella from Harris.
Integrated microwave photonics (IMWP)
Spain Researchers Daniel Pérez, Ivana Gasulla and José Capmany, in their paper, describe the two salient approaches that are available for the implementation of IMWP chips from a functional point of view. On one hand, there is Application-Specific Photonic Integrated Circuits (ASPICs), where a particular circuit and chip configuration is designed to optimally perform a particular MWP functionality. Examples on different functionalities recently reported will be presented, including tunable filtering, optoelectronic oscillation, instantaneous frequency measurement, frequency up and down conversion, etc.
On another hand, in a radically different approach, the universal MWP signal processor architecture that can be integrated on a chip and is capable of performing all the main functionalities by suitable software programming of its control signals. This last approach is inspired by the flexibility of digital signal processors, where a common hardware is shared by multiple functionalities through a software-defined approach (or programmability), leading to significant cost reduction in the hardware fabrication.
In such concept, the central element is a reconfigurable optical core included after external modulation (represented as an E/O device) and prior to detection (represented as an O/E device). The role of the optical core is to provide the required routing/switching functionalities and also the implementation of reconfigurable processing elements such as FIR/IIR filtering, delay lines and phase shifting operations either in standalone or in cascade configuration.
EAGER: SAPPHIRE BASED INTEGRATED MICROWAVE PHOTONICS
The National Science Foundation’s division of Electrical, Communications & Cyber Systems, through the Electronics, Photonics and Magnetic Devices program, has awarded two electrical engineering professors a grant of just over $250,000 to develop a new Integrated Microwave Photonics chip. The awarded project, entitled “EAGER: Sapphire Based Integrated Microwave Photonics,” will be headed by Samir El-Ghazaly, principal investigator and distinguished professor in electrical engineering, and Shui-Qing “Fisher” Yu, co-principal investigator and associate professor in electrical engineering.
This project proposes to develop a new “Integrated Microwave Photonics chip” which could potentially integrate several functions of microwave photonic components on a single chip and also offer reduced size-weight-and-power at a very low cost. If successful, the developed technology would find tremendous applications in defense systems, such as Radar, and many civilian applications, such as cell phones, sensing, and datacom.
This project proposes to utilize R-plane sapphire as a transformative, high-performance and self-consistent IMWP platform which provides a feasible approach for realizing fully-integrated MWP systems. The proposed approach enables the integration of complete sets of microwave and optical components such as light sources, analog and digital signal processing circuits, light detectors, control circuits, and Silicon on Sapphire (SOS) radio-frequency (RF) circuits all-in-one sapphire platform to achieve high-performance and low-cost mixed-signal optical links. Sapphire has a lower refractive index with an index difference of 0.3 with Si3N4.
Therefore, it could leverage the mature Si3N4 low-loss waveguide technology to produce similar low-loss waveguide-based passive components by drop-in replacing quartz wafers with sapphire wafers. For RF applications, the sapphire platform has a potential to obtain much higher dynamic range due to low-loss optical waveguides while the competing Si-photonics platform combined with off-chip 1.55 micron laser suffers from the strong two-photon absorption and therefore has a limited dynamic range. As a transparent substrate, sapphire would enable a versatile 3-D photonics/electronics integration architecture.
This project aims to, first, study the “feasibility” of the proposed approach by identifying and investigating key “fundamental challenges”, then, conduct a proof-of-concept study to provide an effective route for overcoming the identified obstacles, and, eventually, provide a conclusive recommendation as to whether the proposed research is feasible. As a fully integrated solution to fundamentally address the most important technical challenge in IMWP, if successful, the new platform would find tremendous applications in defense systems, such as Radar signal processing, and many civilian applications. The broad wavelength coverage enables on-chip sensing applications. It could potentially replace the current Si-photonics for datacom and be used in harsh environments such as space and nuclear applications.
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