# Integrated Microwave Photonics for Wideband Signal Processing
## 1 Introduction  
The quest for greater speed, bandwidth, and reconfigurability has led to an active pursuit in the field of microwave photonics (MWP) for the generation, transmission, and processing of radio frequency (RF) signals in the optical domain. While initially, defense and radar applications were the main beneficiaries of this technology, the scope has since expanded to encompass many new areas of developments. Bringing together the worlds of radiofrequency (RF) engineering and optoelectronics, the field of MWP has recently attracted great interest from both the research and the commercial communities where its importance has been identified in emerging applications including medical imaging, future 5G networks, and subterahertz systems. It offers the unique abilities to enable key functionalities in microwave systems such as filtering, arbitrary waveform generation, frequency up/down conversion and instantaneous measurement that are either complex or even not directly possible in the radiofrequency domain [1; 2; 3; 4; 5; 6; 7; 8; 9]. As the role of MWP develops from niche to widespread applications, it becomes important to replace systems that rely exclusively on discrete optoelectronic devices and fiber-based components. These systems are bulky, expensive, and power-hungry, making them economically unfeasible to be mass produced for a widespread use. The challenge has thus intensified to implement reproducible, compact, lightweight, low power consumption and low-cost MWP systems that are physically and economically competitive against their electronic counterparts. This calls for an evolution of MWP components in the form of integrated circuits which aims to incorporate many devices and optical functionalities on a single photonic chip to achieve high speed and wideband signal processing functions [4; 5; 6; 7; 8; 9].  
The area of integrated MWP signal processors has been a tremendously active area of current research and development, and is the technology best positioned to realize compact versions of existing MWP systems with similar or even better performance. Work in this field has attracted a great deal of attention in recent years and has been reported by several groups spanning various platforms such as silicon photonics [5; 6; 7], III-V semiconductors [8], and Si\({}_{4}\)N\({}_{3}\) (TripleX) technologies [9].  
In this paper, we present recent new developments in integrated microwave photonic signal processing with a focus on the key signal processing building blocks for RF systems and photonic links, which include an integrated microwave photonic phase shifter, time delay line, and frequency tunable microwave filter for amplitude control. Finally, we will give a broad view on the realization of these full functionalities from a system link perspective of a general-purpose MWP system. Specifically, the future of inline photonic signal processing systems with cascaded functionalities and the potential approaches for realizing full platform integration of MWP systems will be discussed. The attractive use of MWP techniques such as optoelectronic oscillators (OEO) for sensing applications is also explored. We present the prospects of transferring these OEO technologies onto an integrated platform with a demonstration of an on-chip sensing probe for enhanced resolution and interrogation speed. Because of the vast scope of research in this area and the high volume of publications, we would like to point out that this paper, although covering a wide range of integrated MWP, will not provide an exhaustive list of published papers in the field.