# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 3 Flat Passband and Narrow Notch Microwave Photonic Filters
This demonstrates that the novel all-optical microwave photonic notch filter has an excellent long term stability performance, which cannot be achieved by many reported notch filtering structures. For example, the measurements in [38] show there is a more than 20 MHz change in notch frequency over 12 h in a conventional SBS based microwave photonic notch filter. The notch depth is also reduced to only around 10 dB. The change in the notch filter response shape is due to the modulator bias drift and the SBS effect is sensitive to changes in environmental conditions. An active control loop, which involves an LCOS based optical processor, is required in the SBS based microwave photonic notch filter to stabilize the notch frequency and the notch depth for long-term operation. Continuous notch frequency tuning in the primary and secondary tap distribution impulse response microwave photonic notch filter was also demonstrated. A wavelength-tunable laser with a 16 dBm output power and an FP laser followed by an optical filter to generate six wavelengths were used as the primary and secondary optical sources respectively. The solid line in Figure 5 shows the notch filter frequency response with a notch at 23.68 GHz. The response was very stable with a notch depth of \(>\)40 dB. The amplitudes of the ripples in the notch filter passband were around 1 dB over the operating frequency range, which was 84% of the filter FSR. The figure shows that the notch frequency can be tuned to 23.29 GHz and 24.05 GHz by adjusting the phases of the secondary taps. The \(-\)6 dB  
Figure 3: (**a**) The Sinc function distribution impulse response and (**b**) primary and secondary tap distribution impulse response microwave photonic notch filter. notch width, as well as the FSR, remained the same and the notch depth of >40 dB was maintained while tuning the notch frequency. Microwave Photonic Phase Shifters with Continuously Tunable 0\({}^{\circ}\)-360\({}^{\circ}\) Phase Shift  
Microwave phase shifting is an important signal conditioning function that is required in signal processing for defense electronic countermeasure systems [39] and in phased-array beamforming networks for radar and satellite communication systems [40]. The phase shifter needs to provide a tunable 0\({}^{\circ}\)-360\({}^{\circ}\) phase shift and to maintain a constant amplitude in the system bandwidth. The main problem with electrical microwave phase shifters is that the phase change varies significantly depending on the frequency for a given control voltage, i.e., the phase is not flat over broad bandwidths. This introduces phase distortion to the broadband signal. Furthermore, the electrical phase shifters can only realize discrete phase shifts in a limited frequency range [41]. Moreover, it is important for the phase shifter to have a simple structure for use in multi-element phased array antenna applications [42]. Techniques based on microwave photonics can offer a solution to these limitations. Microwave photonic phase shifters implemented using different techniques such as SBS [43], optical carrier and RF modulation sidebands amplitude and phase controls via a DPMZM [14], a polarization modulator [44], nonlinear optical loop mirrors [45] and an optical filter with a nonlinear phase response [46], have been reported. A photonic microwave phase shifter implemented using LCOS technology has the advantages of simple structure and robust performance. The power splitting function in a commercial LCOS based optical processor enables the realizing of multiple phase shifts in a single unit suitable for phased array antenna systems. An LCOS based microwave photonic phase shifter has a structure as shown in Figure 2. Phase shifting is achieved by adjusting the relative optical phase of the carrier and the sideband within the full 0\({}^{\circ}\) to 360\({}^{\circ}\) range by means of an LCOS based optical processor, and this phase shift is directly translated to the RF signal after photodetection.