# Ripple Suppression in Broadband Microwave Photonic Phase Shifter Frequency Response
## 3 Experimental Results  
Experiments were set up to investigate the ripples in the frequency response of a MPPS and to verify that using a low cost optical filter can improve MPPS frequency response performance. The experimental setup is shown in Figure 5. The laser source (Keysight N7714A) was a wavelength tunable laser operated at 1557.8 nm. The laser output power was 16 dBm. A dual-polarization DPMZM was connected after the laser source. It was formed by two DPMZMs, a 90\({}^{\circ}\) polarization rotator, and a polarization beam combiner (PBC). Each DPMZM consisted of two MZMs connected in parallel and an OPS at one of the MZM outputs. No RF signal was applied to the lower DPMZM, and the two MZMs were biased at the maximum and minimum transmission point, respectively. Therefore, the optical carrier passed through only one MZM but was blocked by the other MZM. Thus, by controlling the voltage into the OPS, the optical carrier phase could be tuned. The upper DPMZM was driven by a pair of RF signals with 90\({}^{\circ}\) phase difference via a 4-40 GHz bandwidth 90\({}^{\circ}\) hybrid coupler (Marki Microwave QH-0440 [11]) with the frequency response shown in Figure 2 (red line). A SSB-SC RF modulated optical signal could be generated by designing the bias voltages into the upper DPMZM [8]. Therefore, the dual-polarization DPMZM had the same function as an integrated OFS and OPS structure. A single compact modulator bias controller (PlugTech MBC-DPIQ-01 [12]) was used to control the modulator bias voltages. A polarization controller (PC) and a polarizer at the output of the dual-polarization DPMZM were employed to ensure the carrier and the sideband had the same polarization state. This was followed by an optical filter, which had 1.5 dB insertion loss. Since the laser source had high output optical power and the optical filter had low insertion loss, the average optical power after the optical filter was \(-\)4 dBm. No optical amplifier was used in the experiment. The carrier with a tunable optical phase shift from the lower DPMZM and the single RF modulation sideband from the upper DPMZM beat at a 50 GHz bandwidth photodetector (U2t XPDV2120R), which was connected to a 43.5 GHz bandwidth vector network analyzer (VNA) (Agilent N5224A) to display the phase shifter amplitude and phase response. The optical spectrum at the output of the dual-polarization DPMZM was measured on an optical spectrum analyzer (OSA), as shown in Figure 6, when the upper DPMZM was driven by a 32 GHz RFsignal. This shows that, for a high input RF signal frequency of 32 GHz, the unwanted left sideband was only 16 dB below the wanted right sideband due to the 90\({}^{\circ}\) hybrid coupler had large amplitudes and phase imbalances at high frequencies. Note that even though the carrier at the output of the upper DPMZM could not be fully suppressed, the residual carrier was 39.3 dB below the carrier from the lower DPMZM. Therefore, the residual carrier had a negligible effect on the MPPS performance. The amplitude and phase response of the MPPS were first measured without an optical filter and the results are shown in Figure 7. This was done by programming the bias controller to alter the DC voltage into the OPS inside the lower DPMZM in the dual-polarization DPMZM. The experimental results show there are 4.2 dB amplitude variations and a \(\pm 12^{\circ}\) phase deviation for a \(-180^{\circ}\) to \(180^{\circ}\) phase shift over the 4 to 40 GHz frequency range. Note that the amount of MPPS amplitude variation and phase deviation were different for different phase shifts. It can be seen from the figure that \(\pm 45^{\circ}\) phase shifts have large amplitude variations and \(\pm 90^{\circ}\) phase shifts have large phase deviations, which are consistent with the simulation results given in Figure 4.  
An optical filter with an optical spectrum as shown in Figure 8 was connected after the polarizer to suppress the unwanted sideband.