Waveguides serve as conduits to transport light in an optical medium such as an optical fiber or photonic integrated circuit, in the same way metallic wires transport electrons in an integrated circuit. However, unlike traditional wires, waveguides can function as passive elements. Optical waveguides can be few micrometers long, as in the case in photonic circuits, or thousands of kilometers, as in the case of optical fibers. Waveguides confine the light as it propagates within a high-refractive-index medium, known as the core, surrounded by a lower-refractive-index medium, called the cladding. Waveguides confine light within a high-refractive-index region, known as the core, which is surrounded by a lower-refractive-index region called the cladding. This confinement is achieved through total internal reflection (TIR), a fundamental optical phenomenon detailed in Chap. 3 .