Integrated optics is a field of study and technology that focuses on the design, fabrication, and application of optical devices and systems using integrated circuit technology. It involves the integration of various optical components, such as waveguides, couplers, modulators, detectors, and lasers, into a single substrate. One of the key advantages of integrated optics is its compatibility with electronic integrated circuits. This compatibility enables seamless integration of optical and electronic functionalities onto the same chip, allowing efficient data transfer between optical and electronic domains. This synergy is crucial for applications such as optical interconnects in high-speed communication systems, optical sensing interfaces, and optoelectronic integrated circuits. This entry presents a brief study on some of the widely used and commercially available optical platforms and fabrication methods that can be used to create photonic integrated circuits.
Integrated optics refers to the field of study and technology that involves the integration of various optical components, such as waveguides (WGs), lasers, modulators, detectors, and filters, onto a single photonic integrated circuit (PIC) 
. It aims to miniaturize and integrate optical functions and devices onto a single platform, similar to how electronic circuits are integrated onto a microchip 
. The major benefit of integrated optics is its ability to manipulate and control light signals on a small scale, leading to compact and highly efficient optical systems. By integrating different optical components onto a chip, it becomes possible to perform complex optical operations, such as signal generation, modulation, amplification, routing, and detection, all of which occur within a compact and stable platform.
Optical signals have a much higher bandwidth and can transmit data at significantly higher speeds than electrical signals 
. These features make integrated optics particularly advantageous in high-speed data communication and interconnected applications. Optical communication systems can transmit large amounts of data over long distances with low signal degradation, enabling faster and more efficient data transfer. Integrated optics can achieve low signal losses, especially when using WGs made of low-loss materials, such as silicon or indium phosphide. This feature enables long-distance transmission without significant degradation of the optical signal. In contrast, electrical signals suffer from losses due to resistance, capacitance, and inductance, especially at higher frequencies 
Optical signals are immune to electromagnetic interference (EMI), which can disrupt or degrade electrical signals in electronic circuits. This benefit makes integrated optics more suitable for applications where EMI is a concern, such as those operating in high-noise environments or near electromagnetic radiation sources. Integrated optics allows the integration of multiple optical components and functionalities onto a single chip or substrate. This integration reduces the size, weight, and complexity of optical systems compared to traditional electronic circuits 
. It also empowers the development of compact, highly integrated photonic circuits, making them suitable for applications where space is limited or portability is important.
Optical signals can carry a vast amount of data simultaneously due to their high bandwidth 
. This property is crucial in applications that require high data capacity, such as data centers, telecommunications networks, and high-performance computers. Integrated optics provides a scalable and efficient solution for handling large data volumes 
. It is important to note that integrated optics and electronic circuits are often complementary technologies, and their selection depends on the specific application requirements. Some applications may benefit from the integration of both optics and electronics, leveraging the strengths of each technology for optimal performance and functionality.
The main building block of integrated optics is the WG, which is a structure that guides and confines light within a material, typically a semiconductor or a dielectric 
. WGs can be fabricated using various techniques, including lithography, etching, and deposition processes. The basic principle behind an optical WG is total internal reflection, which occurs when light travels from a medium with a higher refractive index to a medium with a lower refractive index at an angle larger than the critical angle. This technique permits the light to be confined within the WG structure. The dimensions of the WG, such as the width, height, and thickness, are designed to ensure that the light remains trapped and guided along the desired path. The refractive index contrast between the core and the cladding layers of the WG is an imperative parameter for determining the dimensions of the WG 
. A higher refractive index contrast enables better confinement and tighter light propagation within the WG structure. Thus, the dimensions of the WG will vary depending on the refractive indices of the core and cladding materials.
This entry briefly describes the most widely used optical platforms, such as silicon-on-insulator (SOI), indium phosphide (InP), polymers, silicon nitride (Si3N4), and lithium niobate (LiNbO3). Lithography plays a crucial role in defining the resolution of a waveguide in integrated photonic devices. Thus, it is important to choose an appropriate patterning method for the realization of the best possible features of the photonic device. The most used lithography techniques, which include ultra-violet lithography (UVL), laser lithography (LL), electron beam lithography (EBL), and nanoimprint lithography (NIL) are discussed. In the end, reactive ion etching (RIE) and chemical wet etching methods are presented for the fabrication of the WG structures. The entry then ends with the concluding remarks.