# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 3 Flat Passband and Narrow Notch Microwave Photonic Filters
The next pair of taps has a 180\({}^{\circ}\) phase difference relative to the middle two taps. The resolution of the notch filter can be increased, which consequently reduces the notch width, by extending the number of taps in pair with this sinc function distribution. This can be obtained by increasing the number of wavelengths generated by the optical source. Techniques such as spectrum slicing a broadband optical source, multi-wavelength erbium-doped fiber laser [34], or a wideband parametric frequency comb generator can be used to generate a large number of wavelengths to realize a high-resolution notch filter response. A commercial laser array, which generates 56 different-wavelength CW light [35], can also be used as an optical source for the notch filter. Since the tap separation is proportional to the wavelength separation and the dispersion parameter of the dispersive medium, which can be made to be small, the structure can realize a large FSR frequency response. It should be noted that the ability to independently control the amplitude and the phase shift of each tap enables a highly reconfigurable notch filtering operation to be realized. A flat passband and narrow notch microwave photonic filter can also be obtained by using a primary and secondary tap distribution impulse response as illustrated in Figure 3b [36]. In this case, the impulse response is asymmetrical and the taps have unequal separations. The amplitude of the primary tap, which is generated by a high power laser source, is equal to the sum of amplitudes of all the secondary taps. The secondary taps are generated by a multi-wavelength optical source. The secondary taps are designed to have the same time delay \(\tau\) except for the first secondary tap, which needs to have 0.5\(\tau\) time delay to the primary tap. The time delay \(\tau\) determines the filter FSR. Thanks to the optical phase control function provided by the LCOS based optical processor, the notch frequency can be continuously tuned over the full FSR by designing the phase shift of each tap. Note that while tuning the notch frequency, the notch width and the FSR of the notch filter remain unchanged. This shows the advantage of the optical phase control notch frequency tuning technique compared to the conventional technique that uses a wavelength tunable laser and a wavelength dependent time delay element to change the fundamental time delay of the structure, which alters not only the notch frequency but also the response shape. An FP laser generating eight different wavelengths was used as an optical source and a 20 km standard single mode fiber was used as a dispersive medium to realize a microwave photonic notch filter with a sinc function distribution impulse response. The measured frequency response and group delay response of the microwave photonic notch filter are shown in Figure 4a. The filter \(-\)6 dB notch width was 5.1% of the \(-\)3 dB operating frequency range. The measured amplitude ripple in the notch filter passband was <1 dB. The measured notch depth was over 35 dB. The group delay response in Figure 4a shows \(<\)\(\pm\)25 ps ripple in the filter passband, which satisfies the requirements of radar applications [37]. Continuous notch frequency tuning was demonstrated by controlling the phases of different-wavelength optical carriers and sidebands via the LCOS based optical processor. Figure 4b shows the notch filter responses with the notch frequency tuned from 22.16 GHz to 21.84 GHz. The notch depth of \(>\)35 dB was maintained while tuning the notch frequency. The stability of the microwave photonic notch filter based on LCOS technology was investigated by measuring the eight-tap notch filter frequency response for 8 h. The eight-tap notch filter frequency response was recorded every 15 min. The results show the notch depth remains \(>\)35 dB throughout the 8-h period and the notch filter passband amplitude has a <1 dB variation.