# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 6 Multi-Function Microwave Photonic Signal Processors for Phased Array Antenna Systems  
A true time delay beam steering technique implemented using true time delay elements or time shifters can avoid the beam squint problem in wideband phased array antenna systems [62]. Unlike a phase shifter, which provides a tunable frequency-independent phase shift, a time shifter has a linear phase response with a tunable slope. Widely and continuously tunable time delays are needed to obtain a wide beam steering range. For example, considering a phased array antenna system with a half wavelength antenna separation operates at 12 GHz (X-band). The required time delay between adjacent elements in order to obtain a beam direction of 60\({}^{\circ}\) left of the boresight is 36.1 ps. This implies that a 288.8 ps time delay is needed for a 9 \(\times\) 9 phased array to achieve a \(\pm\)60\({}^{\circ}\) beam steering range. It is difficult for an electronic device to realize such a large time delay in a continuously tunable mode. A conventional method to implement true time delay beam steering is by using a series of switches. In this case, the signal time delay is controlled in a discrete fashion. Many switches are needed for realizing a high-resolution phase shift resulting in a bulky and complex structure. Combining true time delay beam steering with phase-based beam steering using phase shifters can simplify the implementation of phased array [63]. In this case, the total array is partitioned into sub-arrays. Time shifting is applied in feeding the sub-arrays with phase shifting in the excitation of the individual element. Squinting in the sub-arrays is negligible due to their reduced size. Many microwave photonic techniques for realizing true time delay [64; 65; 66] or phase shifting operation, have been reported. The phase response profile of an LCOS based optical processor can be designed to have a tunable linear phase slope to obtain the time-shifting operation. Additionally, the power splitting function in the commercial LCOS based optical processor enables a single-wavelength laser source to generate multiple independently tunable true time delays suitable for phased array antenna systems. However, a commercial LCOS based optical processor can only realize a limited amount of time shift ranged from \(-\)32 ps to +32 ps [65] so that a 0\({}^{\circ}\)-360\({}^{\circ}\) phase shift cannot be obtained for microwave signals with frequencies below 30 GHz. This limits the beam steering range. This problem can be solved by a new LCOS based time and phase shifter, which has a structure as shown in Figure 2. CW light from a single-wavelength laser source is phase modulated by an RF signal generating a pair of anti-phase RF modulation sidebands together with an optical carrier. The phase response profile of the LCOS based optical processor is designed to introduce a linear phase shift to the left sideband and a constant phase shift to the right sideband. The left sideband beat with the optical carrier at the photodetector generates an RF signal with a linear slope phase response. The right sideband beat with the optical carrier at the photodetector generates an RF signal with a  
Figure 15: The measured (**a**) S\({}_{21}\) amplitude response, (**b**) S\({}_{21}\) phase response, (**c**) S\({}_{41}\) amplitude response and (**d**) S\({}_{41}\) phase response of the photonics-based microwave hybrid coupler designed to have a 3 dB coupling ratio and a tunable phase. Port 3 of the hybrid coupler was terminated by a 50 \(\Omega\) terminator. constant phase shift independent to the RF signal frequency. Therefore the overall phase response of the output RF signal has a linear slope plus a constant phase shift. Unlike the previously reported LCOS based time shifter [65], there is no removal of an RF modulation sideband in the new LCOS based time and phase shifter. Hence the proposed structure has a high output RF signal power, which leads to a high SNR performance.