# Integrated Microwave Photonics for Wideband Signal Processing
## 5 Integrated MWP for Optical Sensing Applications  
In traditional optical sensing systems, where a change in the sensor measurand is converted into a shift in the optical sensing resonant wavelength, an optical interrogator such as an optical spectrum analyzer is required to convert the wavelength shift into an electrical signal. This, however, raises the issues of sensitivity and speed limitations. In contrast, sensors based on microwave photonic signal processing techniques can conveniently convert the change in the optical domain to a corresponding variation in the microwave domain. The ability to perform measurements in the RF domain, which provides higher frequency resolution, enables a superior method for high resolution monitoring of miniscule changes in the measurand, thus enabling high performance sensing [74]. A much higher interrogation speed can also be obtained since the microwave frequency can readily be measured by a digital signal processor (DSP) with high speed and high resolution. One technique to implement the interrogation of optical sensors is via an optoelectronic oscillator (OEO) system, which is an important system for microwave photonics that enables the generation of high-frequency and stable microwave signals by combining the unique advantages of electronic and photonic components [75; 76]. The conventional optoelectronic oscillator structure requires an electrical bandpass filter in an optoelectronic feedback path to select the desired oscillation frequency. However, the low-frequency operating range and limited tunability of typical electronic filters impose restrictions on the bandwidth,  
Figure 3: (**a**) Schematics and (**b**) illustration of the proposed single passband microwave photonic filter (SPMPF). noise and stability of generated oscillations, making these optoelectronic oscillators unsuitable for high-frequency applications. To overcome this fundamental limitation, MWP filters based on the direct filtering method can be employed to replace electronic filters. Using these optical filtering devices as the sensing probe, the microwave oscillation frequency can be determined by the resonant wavelength of the optical device. However, the practicality of current OEO-based sensor schemes, which are typically comprised of bulk components of large physical size, is limited. The challenge is now to transfer the OEO-based topologies onto an integrated platform in order to develop compact and portable solutions for sensing environment variables. The first steps to developing an integrated OEO-based system was first initiated in [77] where we presented an integrated photonic sensor based on an optoelectronic oscillator with an on-chip sensing probe that is capable of realizing high sensitivity and high resolution. This work engaged the use of a SOI microring resonator to continuously select different RF oscillation modes. As an application example, the targeted measurand was chosen to be temperature. Figure 4a shows the working principle of the proposed OEO based optical sensor that employs an integrated microring resonator as the sensing probe element. The chip was placed under a heat sink with temperature controllability provided by a thermoelectric cooler (Newport) to emulate the effects of disturbance in the ambient temperature. The frequency response of the generated electrical signal at 24.78 \({}^{\circ}\)C is shown in Figure 4b, which depicts an electrical signal at 14.3 GHz. The inset of Figure 4b shows the zoomed in response which illustrates a narrowband signal with a linewidth of 0.1 MHz. The next most dominant mode is located at 4.9 MHz away from the peak oscillation mode and shows a mode suppression of 30.6 dB. To investigate the temperature sensing performance, the temperature of the nanochip sensor was varied from 24.27 \({}^{\circ}\)C to 25.29 \({}^{\circ}\)C. Figure 4c shows the superimposed spectra of the generated microwave frequencies at different temperature points.