## 4 Applications of Integrated Photonics
### Sensing Applications  
A key benefit of using integrated photonics in sensing applications is the substantial decrease in both size and power consumption compared to conventional optical systems [60; 62]. By integrating multiple photonic components, such as WGs, modulators, and detectors, onto a single chip, the need for bulky optical setups is eliminated. This miniaturization allows for the deployment of sensors in environments where space is at a premium or where traditional sensors cannot be easily installed. For instance, in medical diagnostics, integrated photonic sensors can be used for in vivo monitoring of biological parameters, providing real-time data with minimal invasiveness [57, 63]. These sensors can detect minute changes in biological markers, leading to early diagnosis and improved patient outcomes [64]. Integrated photonics also excels in environmental sensing applications [65, 66]. Photonic sensors can perceive a wide range of environmental parameters, including temperature, humidity, pressure, and the presence of various chemicals and pollutants [67, 68, 69, 70]. The extraordinary sensitivity and selectivity of photonic sensors make them ideal for monitoring air and water quality. For example, integrated photonic sensors can detect trace amounts of pollutants in water, providing crucial information for maintaining safe drinking water standards. Additionally, their ability to operate in harsh environments makes them suitable for use in industrial settings, where they can monitor conditions in real time and alert operators to potential hazards. The study conducted by [PERSON] et al. presented an integrated channel WG-based fluorescent immunosensor designed to swiftly and simultaneously identify up to 32 contaminants (Figure 3a) [71]. The utilization of WG tapers by the sensor enhanced the efficiency of both the excitation and collection of fluorescent signals, even in instances of fluorophore bleaching that occur in solid surface bioassays. Figure 3b illustrates a cross-sectional representation of a sensing area, which includes the WG, isolation layer, and surface chemistry. Figure 3c depicts the propagation of light along a WG chip, offering insight into the sensor's operational mechanism. This pioneering use of WGs in immunosensors marked the first demonstration of the ability to detect microcystin-LR (MC-LR) in lake water with an optimized WG geometry. To achieve this, a WG chip was activated using 3-mercaptoethanoltrimethoxysilane/N-(4-maleimidobutyryloxy) succinimide (MTS/GMBS), which enabled a BSA-MC-LR conjugate to be immobilized. The immunosensor successfully detected MC-LR in real lake samples, even at sub-microgram per liter concentrations (e.g., 0.5 ug/L), with recovery rates ranging from 84% to 108%. These results confirmed the sensor's potential application for measuring MC-LR in real water samples [71]. Another notable application of integrated photonics in sensing is in the field of telecommunications [72]. Photonic sensors are employed to monitor the health of optical networks, detecting issues such as signal degradation and fiber breaks with high precision [73]. This capability is essential for maintaining the reliability and performance of high-speed communication networks. Integrated photonic sensors can be embedded within the network infrastructure, providing real-time data that enables proactive maintenance and rapid response to faults [74]. The use of integrated photonics in sensing is also advancing the development of quantum sensors, which exploit quantum phenomena to achieve unprecedented levels of sensitivity and accuracy. Integrated photonics provides a platform for developing compact and scalable quantum sensors, which can be used in various applications, from gravitational wave detection to navigation and timing. The integration of quantum sensing elements on a photonic chip opens up new possibilities for fundamental research and practical applications, potentially revolutionizing fields such as biosensing [75], metrology [76], and geophysics [77].