# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 1 Introduction  
The low-loss and large available time-bandwidth product capability of fiber optic systems make them attractive not only for signal transmission, but also for the generating, measuring and processing of microwave signals [1; 2; 3; 4]. Combining microwave engineering and optoelectronics provides superior features of microwave photonics including immunity to electromagnetic interference (EMI), wide bandwidth capability, and light weight. Additionally, since the fractional bandwidth of a microwave signal modulated onto an optical carrier is small, these qualities are virtually independent of the microwave frequency, making photonics suitable for broadband applications. Microwave photonics brings new opportunities to revolutionize the microwave field. Its key benefits, including inherent speed and reconfigurability, have led to diverse applications such as electronic warfare, wired and wireless communications, radio astronomy, and terahertz spectroscopy and imaging, for tackling the problems that are difficult to solve or cannot be solved by conventional electronic approaches.  
From a system perspective, the advantage of microwave photonics is its wide bandwidth capability, which makes it attractive for multi-function systems. This is significant in radars, electronic warfare and communication systems. Key subsystems that are required include widely tunable filters with high-resolution, wideband phase shifters, and hybrid couplers with a tunable coupling ratio and phase difference. For example, in radar systems, clutter reduction using a microwave photonic filter allows processing high-frequency signals directly in the optical domain, which is compatible withadvanced phased array radar systems that use fiber for signal transport [5]. The filter is an important element for the RF front end. There is a great deal of interest to make the filter widely tunable so that only one filter is required to achieve multi-band operation, which is very difficult to achieve using electronic approaches. Moreover, the microwave signal phase shifting operation in optical domain is of particular interest because it enables the operation over a wide bandwidth, together with a full phase shift range of 0\({}^{\circ}\) to 360\({}^{\circ}\) while keeping the amplitude of the microwave signal constant and fine tuning resolution, which are the fundamental requirements of phased array radar systems [6]. These requirements again cannot be achieved using electronic approaches. In other applications such as next-generation wireless communication systems, microwave photonic couplers are required for combining and splitting microwave signals with a tunable coupling ratio and phase difference at extremely high frequencies [7]. Since optical fiber is used as the transmission medium, it is attractive to perform high-speed and adaptive signal processing directly in the optical domain.  
A number of techniques such as delay line [8; 9], Sagnac loop interferometer [10], re-modulation [11], stimulated Brillouin scattering (SBS) [12; 13], dual parallel modulation [14], fiber Bragg gratings (FBGs) [15] and liquid crystal on silicon (LCOS) based optical processor [16] have been reported to implement various microwave photonic devices. Amongst them, microwave photonic devices based on the LCOS technology have a number of advantages such as simple structure, robust performance, parallel signal processing and reconfigurability, which are the focus of this paper. We present three types of microwave photonic devices for notch filtering, phase shifting and coupling operations. We also present a new multi-function signal processor, which is based on using an LCOS based optical processor to realize multiple true time delay, frequency-independent phase shift and amplitude control operations for phased array antenna systems. Experimental results are presented, which demonstrate the multi-function signal processor operating over a very wide frequency range.