# Microwave Photonic Signal Processing and Sensing Based on Optical Filtering
## 2 Optical Filtering for Single Sideband Modulation and Spectral Characterization  
RF-over-fiber (RoF) is an effective technique for wireless-optical systems including 5G, because it provides advantages including simpler antenna remote units (as no frequency conversion is required), centralized frequency channel management, and central office (CO) equipment sharing, as well as the capability to readily support multiple wideband signals [5,18]. However, an important disadvantage is dispersion-induced fading within the fiber, which becomes more of a problem as the RF spectrum is pushed to higher frequencies. This limitation can be overcome by using OSSB modulators. The OSSB scheme is also widely used in microwave photonic controlled phased array antennas and phase shifters [19,20], and in the optical vector network analyzer systems because it enables the important feature of direct mapping between the optical and electrical domains while also suppressing the dispersion-induced power penalty [21, 22, 23]. Various approaches have been reported for generating OSSB modulation. The conventional technique [24] and its extensions [25] comprise the use of an electrical hybrid coupler to obtain orthogonal RF signals that are applied to the optical modulator. This approach suffers from a limited modulated bandwidth, principally because of the use of the electrical hybrid coupler. All-optical approaches to realize OSSB have been presented, based on optical filters to eliminate one of the optical sidebands in an intensity modulation or phase modulation scheme [18,26]. This has been demonstrated by different approaches such as phase-shift fiber Bragg gratings [27] or stimulated Brillouin scattering (SBS) [28], or whispering gallery mode resonators, which are limited to either fixed frequency operation or narrowband operation [29]. A particularly attractive approach is to use optical microring resonator structures because they are integrated and hence they are small and exhibit excellent performance [30,31]. OSSB has been achieved via the use of a ring resonator assisted Mach-Zehnder interferometer to suppress the optical sideband for RF signals between 1.2 GHz and 2.2 GHz [32]. In Reference [33], the use of a parallel dual-ring modulator and a quadrature hybrid coupler enabled the operation at 30 GHz. In addition to dispersion penalty elimination in optical fibre microwave transmission, OSSB with large sideband suppression across wide RF frequencies also presents new applications to optical vector network analyzer for measuring the spectral response of both the amplitude and phase of optical components. An OSSB with a sideband suppression of 23 dB has been achieved via an optical bandpass filter based on high order coupled-resonator optical waveguides (CROW) [34], which reduces RF power ripple to around +1 dB for wideband transmission of RF signals in a dispersive medium with a group delay slope of 330 ps/nm. When using the OSSB modulator in an optical vector network analyzer application, it can be noted that as the order of the microring CROW filter increases, the sideband suppression ratio is also increased, thus causing an obvious reduction in both the magnitude and the phase measurement errors. For a 6 th-order CROW filter used for measuring the optical notch filter as a testing example, the simulated maximum magnitude deviation and the phase deviation are less than 0.4 dB and 2\({}^{\circ}\), respectively [35]. In the experiment, 3200 effective points over a span of 20 GHz have been obtained, thus resulting in a measurement resolution of 6 MHz. It can be noted that the resolution can be easily improved to kHz by increasing the measurement points and altering the RF sweep range, without the need for adjusting of the rest of the setup.