# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 3 Flat Passband and Narrow Notch Microwave Photonic Filters  
Microwave photonic notch filters have applications, such as distributed fiber-fed antennas in defense systems [23], where specific microwave frequency components need to be eliminated. A frequency response with a flat passband and a narrow notch width is required to avoid attenuation on other frequency components around the desired null. Previous approaches to obtain a flat passband and narrow notch microwave photonic filter include techniques based on discrete time signal processing [9] and SBS [30]. The former approach can be implemented using either an infinite impulse response (IIR) or finite impulse response (FIR) delay line structure. Microwave photonic notch filter based on an IIR delay line structures have a limited free spectral range (FSR), which reduces the filter operating frequency range. Furthermore, the notch frequency cannot be tuned continuously. Unlike discrete time signal processing, the frequency response of an SBS based microwave photonic notch filter is not periodic and the notch frequency can be tuned continuously over a wide frequency range. However, the performance of an SBS based microwave photonic filter is affected by environmental perturbation because the SBS effect is dependent on the input light polarization state. Furthermore, a recent study found that the frequency response of an SBS based microwave photonic filter is dependent on the input RF signal power [31]. In an antenna remoting application, the RF signal into a microwave photonic system is received by an antenna where the RF signal power is unknown. The RF power dependent frequency response characteristic of an SBS based microwave photonic filter prevents the filter from being used in practice. Conventional FIR delay line filters require a complex structure to realize a high-resolution frequency response. Tuning the notch frequency is difficult in a high-resolution FIR delay filter. However, the advancement of multi-wavelength optical sources and the amplitude and phase control functions in an LCOS based optical processor enable a high-resolution and continuously tunable FIR delay line notch filter to be realized using a simple structure. The structure of a microwave photonic notch filter based on LCOS technology and the FIR delay line technique is shown in Figure 2. Here the optical source is a multi-wavelength optical source generating different-wavelength CW light into an optical phase modulator. The LCOS based optical processor connected at the output of the phase modulator has two functions in the microwave photonic notch filter. One is that it filters out one sideband of the phase modulated optical signal to obtain single sideband (SSB) modulation to avoid the dispersion-induced power fading problem [32]. Another is that it independently controls the amplitude and phase of the carriers and the remaining sidebands. The optical signals after signal processing in an LCOS based optical processor output port pass through a dispersive medium, which introduces different time delays to different-wavelength optical signals. The delayed optical signals are detected by a photodetector, which generates a notch filter shape frequency response via discrete time signal processing [8]. Each wavelength from the multi-wavelength optical source can generate one delayed optical signal or tap formed by beating between the carrier and the corresponding sideband at the photodetector. The tap amplitude is proportional to the product of the carrier and the sideband amplitude. The phase shift of the tap equals the phase difference between the carrier and the corresponding sideband. A flat passband notch filter response can be obtained by programming the LCOS based optical processor such that the taps have a sinc function distribution in the filter impulse response as illustrated in Figure 3a [33]. It can be seen from the figure that the tap distribution is symmetrical. The middle two taps have the largest and equal amplitudes, and the same phase.