# Microwave Photonic Signal Processing and Sensing Based on Optical Filtering
## 4 Microwave Photonic Sensing Based on Optical Filtering  
A need exists for sensors that have extremely high resolution, or which need to operate with immunity to electromagnetic interference, as well as exhibiting inertness in chemical and biological applications, compactness, light-weight, and ability to operate in harsh environments. Photonic sensors can fulfil these requirements. Techniques based on microwave photonics, in which the wavelength shift of the optical sensor is converted directly into the variation of an RF frequency signal are particularly attractive [70, 71]. The hybrid approach of using microwave photonics as an extension to conventional optical sensors has demonstrated significant merits in enhancing the sensing speed, sensitivity, and resolution, where a minute optical changes determined by the sensing information can be converted into a large variation in the RF domain [70, 71, 72, 73]. The use of FIR microwave photonic filters with discrete multi-taps for optical sensing method translates the targeted measurand information on optical delays between different taps of the filter into RF frequency variation of the microwave photonic filters [74, 75]. For example, a two-tap incoherent notch microwave photonic filter based transverse load sensor has been implemented to determine wavelength separation changes of the fiber Bragg gratings by detecting the notch resonance frequency shifts of the RF filter response [75]. Another technique to implement the interrogation of optical sensors is via an optoelectronic oscillator (OEO) whose microwave oscillation frequency is determined by the resonant wavelength of the optical device that is employed for sensing. In the OEO based sensing topology, the resonance of the optical filtering device based on the measurand information determines  
Figure 3: **(a)** Measured RF responses, (**b**) \(-10\)-dB bandwidths, and (**c**) shape factors at \(-10\)-dB/\(-3\)-dB bandwidth for single passband microwave photonic filter based on single and double notch filters. Figure 2: Schematics and illustration of the (**a**) single passband microwave photonic based on optical double notch filter, and (**b**) the optical double notch filter. the generated microwave oscillation frequency [71]. The OEO offers a high-speed and high-resolution sensing solution by translating the optical domain resonance wavelength change caused by a strain, temperature, refractive index or transverse load to a frequency change of an RF electrical spectrum [72]. Various OEO based sensing architectures have been reported [76; 77; 78; 79; 80; 81; 82], which includes using phase-shifted fiber Bragg grating (PS-FBG) as strain sensors [76], polarization-maintaining PS-FBG for temperature-insensitive transverse load sensing [77; 78], temperature sensors using a single laser source [79], or a Mach-Zehnder interferometer structure built by a broadband optical light source [80], where the frequency drifting problem of the single laser source was eliminated [81]. Looking to the future, the development of a compact and portable OEO sensor in a miniaturized platform has attracted significant interest. An integrated photonic sensor based on an OEO with an on-chip sensing probe that is capable of realizing highly sensitive and high resolution optical sensing has been presented [82]. The integrated SOI single ring resonator exhibits an optical notch filtering response and implements a RF single bandpass filter in an OEO system configuration. The side modes of the OEO were effectively suppressed by more than 30 dB, leaving a dominant peak RF signal at the oscillating frequency, and due to the high thermal coefficient of the SOI based microring resonator and the oscillator configuration.