# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 2 LCOS Based Optical Processor Operating Principle and Functions  
The schematic showing the operation principle of an LCOS based optical processor is depicted in Figure 1. The input optical signal is dispersed by a diffraction grating before its spectral components hit a 2D LCOS array [17]. The LCOS array consists of a matrix of reflective liquid crystal elements. By applying voltages to these matrix elements, they can add individual phase shifts to the reflected signals. As the wavelengths are separated on the LCOS chip, each wavelength can be controlled independently and can be switched or filtered without interfering with other wavelengths. In short, an LCOS based optical processor is capable of controlling the amplitude and phase of different frequency components. There are various versions of LCOS-based optical processors. They are referred to as opto-VLSI processor [18], WaveShaper [17] and spatial light modulator [19], and have different performance. Amongst them, the WaveShaper manufactured by Finisar is the most widely used LCOS based optical processor. It has the ability to provide accurate and independent control on the amplitude and phase of the optical signal routed out from each output port with a frequency resolution of 1 GHz across the whole C band via programming the power splitting profile (PSP) file. Although the filter bandwidth and the frequency setting resolution of current commercial LCOS based optical processors are limited to 10 GHz and 1 GHz respectively, there are strong driving forces to increase the LCOS resolution. This comes from important consumer electronics applications such as TV and projection systems, 3D imaging, and defense. WaveShaper has a settling time of less than 500 ms. An LCOS based optical processor with a fast response time is demanded in many applications such as phased array antenna systems to achieve fast beam steering. Various approaches have been proposed to improve the LCOS based optical processor response time to less than 10 ms [20]. Other than resolution and response time, crosstalk, insertion loss and polarization dependent loss are also important in an LCOS based optical processor [21]. WaveShaper has a low insertion loss of less than 5 dB and a polarization dependent loss of less than 0.4 dB. Its performance is insensitive to environmental perturbation. Note that controlling the amplitude and phase of different frequency components has recently been demonstrated in a monolithic chip fabricated using 180 nm SOI CMOS technology [22]. This indicates that microwave photonic devices based on LCOS technology can be made to have a small size. They can be fabricated with other components for future communication or military applications. Figure 2 shows the structure of an LCOS based microwave photonic device for realizing various signal processing operations described in the following sections. One or more optical sources, which can be telecommunication-type lasers such as DFB lasers or Fabry Perot (FP) lasers, generate continuous wave (CW) light with single or multiple wavelengths at around 1550 nm. The CW lights from the optical sources are launched into a lithium niobate optical phase modulator operated based on electro-optic effect. An RF signal is applied to the phase modulator RF input port. The output of the phase modulator consists of single- or multiple-wavelength optical carriers and pairs of anti-phase RF modulation sidebands adjacent to the carriers. An LCOS based optical processor is used to provide independent control on the amplitudes and phases of the carriers and sidebands. It can also split the processed optical signals to different output ports, which are connected to optical components, e.g., a dispersive medium to introduce different time delays for different frequency components to implement a microwave photonic filter based on a discrete time signal processing technique, before being detected by photodetectors. RF signals after signal processing are obtained from the photodetector outputs.