# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 2 LCOS Based Optical Processor Operating Principle and Functions
Note that the LCOS based optical processor can be placed in any location between the phase modulator and the photodetectors to process the RF phase modulated optical signals. The performance of a microwave photonic device can be described by RF power gain, noise figure, spurious free dynamic range (SFDR) and signal-to-noise ratio (SNR) [23; 24] as a fiber optic link. It is desirable for the structure shown in Figure 2 to have high power optical sources, an optical phase modulator with low insertion loss and a small switching voltage, and photodetectors with high power handling capability and high responsivity to maximize an LCOS based microwave photonic device performance. The structure shown in Figure 2 does not involve any electrical component. Hence its upper operating frequency is only limited by the bandwidth of the optical phase modulator and the photodetectors for electrical-to-optical and optical-to-electrical signal conversion process respectively. Photodetectors with a >100 GHz bandwidth are commercially available [25]. Over 100 GHz bandwidth optical phase modulators have been demonstrated [26]. However, commercial off-the-shelf optical  
Figure 1: The schematic of an liquid crystal on silicon (LCOS) based optical processor. Figure 2: The structure of a microwave photonic device based on LCOS technology. phase modulators have an around 65 GHz bandwidth [27], which limits the upper operating frequency of an LCOS based microwave photonic device. The lower operating frequency of an LCOS based microwave photonic device is limited by the bandwidth of the optical filter inside the LCOS based optical processor, which is 10 GHz for a commercial LCOS based optical processor (WaveShaper 4000A, Finisar, Sunnyvale, CA, USA). Techniques to extend the lower operating frequency of an LCOS based microwave photonic device down to a few gigahertz are presented in the following sections. The use of an optical phase modulator in an LCOS based microwave photonic device has the advantage of requiring no DC bias voltage and, hence, no bias drift problem, which is present in the conventional fiber optic links that use an optical intensity modulator for RF signal modulation. An LCOS based optical processor has a very low polarization dependent loss and its performance is insensitive to changes in environmental condition. Therefore microwave photonic devices based on LCOS technology have a stable performance and hence are suitable for practical use. On the other hand, microwave photonic notch filters implemented using an FBG based delay line technique have a coherent interference problem [8] and microwave photonic phase shifters implemented using a dual-parallel Mach Zehnder modulator (DPMZM) have a bias drift problem. An LCOS based microwave photonic device does not generate additional noise components other than those in a fiber optic link, whereas using an FBG based delay line technique and SBS to implement a notch filter causes phase-induced intensity noise [28] and SBS noise [29], respectively. An LCOS based optical processor has no effect on the output RF signal nonlinearity. On the other hand, microwave photonic phase shifters implemented by controlling the bias voltages of a DPMZM or using SBS alter the output RF signal nonlinearity. This degrades the system dynamic range performance. In terms of cost, using an LCOS based optical processor to realize a single microwave signal processing operation could be expensive compared to techniques that use FBGs or a DPMZM. However, an RF modulated optical signal inside an LCOS based optical processor can be split into multiple signals with different performance characteristics, which is useful in phased array antenna systems. In this case, the overall cost of an LCOS based microwave photonic signal processor is similar or even lower than that implemented using other techniques. For example, an SBS based microwave photonic phase shifter requires a microwave signal generator to generate a pump frequency into an SBS medium to obtain an SBS effect.