# Microwave Photonic Signal Processing and Sensing Based on Optical Filtering
## 4 Microwave Photonic Sensing Based on Optical Filtering
Thus, a small temperature change generated on the microring resonator sensor will be reflected by a large RF frequency shift in the OEO output that can be measured with very high temperature resolution of 0.02 \({}^{\circ}\)C.  
Direct optical-domain to RF-domain mapping by manipulating the two sidebands of an optical carrier provides a potential effective realization of a microwave resonant based optical sensing system [83; 84]. Conventional approaches operate by converting a change in the measurand into (i) an optical wavelength shift in the resonance spectrum, which is then interrogated by an optical spectrum analyzer (OSA) [85; 86], or (ii) into an intensity variation [87]. Both these approaches are limited by the interrogation speed and resolution, especially when using microring resonators where degradation in the quality (Q) factor due to fabrication errors arising from processing uncertainties is inevitable. Alternative hybrid approaches using OEO based sensors by means of a SOI based single all-pass microring resonator [82] are also largely dependent on the bandwidth of the optical filter due to the mode selectivity, which means a high-Q microring resonator is still essential and critical in the system. To address these issues, we recently reported a novel microwave photonic sensing interrogation technique to provide high notch rejection with narrow width throughout the entire sensing range in combination with a reflective SOI based microring resonator based optical notch filter as a sensing probe, as displayed in Figure 4[88]. By adjusting the rejection depth of the optical broadband notch enhancement filter, the two sidebands can be made equal in amplitude, while they have an anti-phase relationship, thus an ultra-high rejection is achieved after detecting the photocurrent, thus removing the need for a high-Q microring optical notch filter. Since the microring is essentially a point sensor, it also enables the capability to perform remote measurements at locations with limited accessibility. This system adopts a simple configuration, requiring only a fixed laser diode, a conventional phase modulator (PM), a wideband optical notch equalization filter, a reflective single microring as a sensing window, and a photodiode (PD) to convert and enlarge the temperature changes in the microwave photonic/RF domain. The complexity and cost of the system can be further reduced by implementing the broadband optical equalisation filter by means of a fixed optical notch filter in practical applications. A temperature sensor has been demonstrated experimentally as a proof-of-concept. When ambient temperature was varied between 23.69 \({}^{\circ}\)C to 25.37 \({}^{\circ}\)C, an ultra-high notch rejection of the microwave photonic filter of about 60 dB has been achieved over a large RF range, as depicted in Figure 4b. A small, temperature-dependent shift in the optical resonant frequency leads to a large change in the RF notch frequency location that produces a high sensitivity of 11.57 GHz/\({}^{\circ}\)C, as illustrated in Figure 4c. It demonstrates the ability to convert a 0.03 \({}^{\circ}\)C small temperature change, which is limited by the equipment resolution of the temperature controller, to a significant frequency shift as large as 324 MHz, whereas by comparison if conventional optical spectrum interrogator techniques were used such that a minor temperature change would not be detectable. This demonstrated a sensing probe system that is robust, easy to be operated, and creates more flexibility for various point measurements with significantly improved sensitivity, which is highly attractive for applications that require accurate detection.