# Microwave Photonic Signal Processing and Sensing Based on Optical Filtering
## 3 Optical Filtering for Single Passband Microwave Photonic Filter  
Microwave photonic filters provide advantages of extremely wide tunability, reconfigurability and electromagnetic immunity [37]. Conventional microwave photonic filters are based on finite impulse response (FIR) structures consists of multi-taps optical signals where they are weighted, delayed, and summed before the photodetection [37; 38; 39; 40; 41; 42; 43; 44]. While the FIR structure has inherent flexibility in realizing arbitrary transfer function, it presents undesired spectral periodicity of multiple harmonic passbands with a period given by the free spectral range [1; 2; 3]. Therefore, for wideband and continuously tunable applications, it is particularly important to realize microwave photonic filters that have a single passband to avoid the crosstalk and spectral overlapping from undesired RF passbands within the spectral range of interest. To solve this problem, optical filtering techniques have been implemented to produce single passband microwave photonic filters and can be broadly divided into two principal categories. The first one builds upon the spectrum slicing techniques to provide a cost-effective solution for achieving high-resolution signal processing [45; 46; 47]. In [47], single passband microwave photonic filter structure was achieved by controlling the dispersion-induced RF fading and the carrier suppression effect,  
Figure 1: (**a**) Schematic of the experimental setup of the OSSB modulation system, (**b**) experimental results of the measured OSSB at 20 GHz, (**c**) experimental results of the measured OSSB at 35 GHz (where the blue line represents the upper sideband suppression of the left edge while the red line represents the lower sideband suppression of the right edge of the fabricated dual-ring weak EIT-like notch filter), and (**d**) the sideband suppression ratio as a function of frequency from 5–35 GHz. so that the notch frequencies fall on the periodic spectral resonances. The development in single passband spectrum-sliced microwave photonic filter are either based on the Fourier-transform of optical spectrum into RF domain [11; 48; 49; 50], or the shifted dispersion-induced RF fading using a dual-input Mach-Zehnder interferometer configuration [9; 51; 52; 53]. Optical filtering techniques have also been extensively applied to apodize broadband optical source with a Gaussian windowing function, hence greatly increasing the filter sidelobe suppression ratio [9; 51; 52; 53]. A shape-invariant single passband microwave photonic filter with an improved sidelobe suppression to nearly 30 dB has been demonstrated in Reference [9] by implementing the Gaussian optical spectrum. The second category is based on utilising optical filters to manipulate the RF sidebands of the modulation schemes and to realise single passband microwave photonic signal processors through direct mapping from the optical domain to the electrical domain [54; 55; 56; 57; 58; 59; 60; 61]. This technique features a simple configuration to achieve an extremely wide frequency tuning range. The majority of the techniques are based on the phase modulation scheme due to the absence of bias drift problems. The concept of using stimulated Brillouin scattering (SBS) based filtering of optical signals to realise narrowband RF selectivity is an attractive approach for implementing high-resolution single passband filters, and this operates by breaking the RF phase modulation sideband symmetry using the SBS gain and loss spectrum [54; 55; 56; 57; 58; 59; 60; 61]. An RF filter with a bandwidth of about 20.6 MHz, less than \(\pm\)1.2 dB of amplitude fluctuation and over 30 dB of out-of-band rejection ratio within the whole tuning range of 20 GHz has been obtained by using optical fiber as the gain medium [56]. Optical and microwave photonic filtering techniques based on the SBS effect have subsequently been presented using miniaturization by means of photonic integrated waveguides.