# Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology
## 3 Flat Passband and Narrow Notch Microwave Photonic Filters
Figure 4: **(a)** The measured frequency response and group delay response within the notch filter \(-3\) dB operating frequency range. **(b)** Measured tunable notch filter response with a notch frequency of 21.84 GHz (blue dashed line) and 22.16 GHz (red solid line) obtained by introducing different phase shifts to the radio frequency (RF) modulation sidebands. Figure 5: The measured tunable frequency responses of the microwave photonic notch filter with a primary and secondary tap distribution impulse response. The notch frequency is 23.29 GHz (blue dashed-dotted line), 23.68 GHz (black solid line) and 24.05 GHz (red dashed line). The optical source generates single-wavelength CW light, which is phase modulated by an RF signal in an optical phase modulator. The RF phase modulated optical signal is processed by an LCOS based optical processor and is detected by a photodetector. The amplitude response profile of the LCOS based optical processor is designed to fix the carrier and the right sideband amplitude, as shown in Figure 6[47], while changing the phase difference between the carrier and the right sideband to realize an RF signal phase shift. The phase difference is introduced by designing the LCOS based optical processor phase response to fix the carrier phase to 0\({}^{\circ}\) and to alter the right sideband phase to the desired value. Note that the left sideband at the frequency of 0 to around 20 GHz away from the optical carrier is included at the phase shifter output. The inclusion of the left sideband compensates for the unwanted effect caused by the limited LCOS resolution. This extends the LCOS based microwave photonic phase shifter operating frequency range to cover all the X, K\({}_{\text{u}}\), K, and K\({}_{\text{a}}\) bands. Figure 7 shows the measured amplitude and phase response of the LCOS based microwave photonic phase shifter. The experimental results demonstrate that the structure is capable to realize a full phase shift range from \(-180^{\circ}\) to \(180^{\circ}\) with <4\({}^{\circ}\) phase deviation over the 7.5 GHz-26.5 GHz frequency range. The amplitude response is also flat with <3 dB variation over almost 20 GHz frequency range. The maximum measurement frequency of 26.5 GHz was limited by the bandwidth of the network analyzer used in the experiment. The stability of the phase shifter was investigated by measuring the phase shifter output amplitude and phase for a 14 GHz input RF signal over a period of time. Less than 0.2 dB amplitude variation and less than 2\({}^{\circ}\) phase deviation were obtained over the measurement period of almost 5 h. This demonstrates that the LCOS based microwave photonic phase shifter has an excellent long term stability performance. In the case of a microwave photonic phase shifter based on controlling the bias voltages of a DPMZM, there is less than 0.5\({}^{\circ}\) drift in the RF signal phase in 2000 s in an open laboratory environment [48]. However, it was pointed out that performance degradation occurs in the long-term operation due to fluctuations in the environmental temperature and temperature control is required to improve long-term stability. Figure 6: The amplitude and phase response profile of an LCOS based optical processor together with the optical carrier and the RF phase modulation sidebands showing the operation principle of the microwave photonic phase shifter based on LCOS technology. Figure 7: The measured (**a**) amplitude and (**b**) phase response of the microwave photonic phase shifter based on LCOS technology. The removal of one of the RF modulation sidebands in the LCOS based microwave photonic phase shifter reduces the output RF signal amplitude. This consequently degrades the system RF power gain and SNR, which are the important performance parameters of a phase shifter [24]. Figure 8 shows another design of the LCOS amplitude and phase response profiles, which is aimed to realize a continuously tunable 0\({}^{\circ}\)-360\({}^{\circ}\) phase shift with a high SNR performance [49].