# Integrated Photonics for IoT, RoF, and Distributed Fog-Cloud Computing: A Comprehensive Review
## 6 Key Enablers of High-Speed Networks Technology
### Wavelength Division Multiplexing (WDM)  
PICs with WDM functionalities are essential building blocks for enabling high-throughput and energy-efficient optical communication in Fog and Cloud computing environments. Unlike discrete optical systems, WDM PICs integrate components such as modulators, multiplexers, and photodetectors on a single chip, reducing footprint, latency, and power consumption. This subsection highlights several key implementations of WDM transmitters and receivers using silicon photonics. These examples demonstrate how PIC-based WDM architectures are well-suited to the bandwidth, latency, and integration demands of next-generation Fog and Cloud networks. Traditional WDM systems based on discrete components struggle with scalability and integration density. Recent advances have produced ultra-compact, broadband, multi-dimensional multiplexed PICs that significantly increase data throughput while minimizing footprint [73]. These innovations support the real-time processing and high-speed transmission of large data volumes. Their integration also reduces system complexity and power consumption, supporting the energy-efficiency goals of modern data centers [74]. As Fog computing brings processing closer to the edge and Cloud systems scale globally, PIC-based multiplexing becomes essential for seamless, high-capacity optical communication. Figure 7 illustrates two key demonstrations of silicon photonic WDM-integrated circuits. In part (A), a 40-channel WDM receiver PIC is shown, which integrates a silicon nitride arrayed waveguide grating (AWG) with germanium photodetectors. The device achieves flattened passbands and supports single-channel operation at data rates up to 40 Gb/s, as shown in the eye diagram inset. In part (B), a compact 10 \(\times\) 25 Gb/s DWDM transmitter is presented, which integrates single-drive push-pull Mach--Zehnder modulators and a silicon nitride AWG on a 5 \(\times\) 8 mm\({}^{2}\) chip. Each channel exhibits a clear eye-opening at 25 Gb/s after wavelength multiplexing, demonstrating the viability of dense WDM transmission using silicon PICs [75]. Another key device that can be used for long-haul, Fog and Cloud optical networks is the Mach-Zehender modulator (MZM). Figure 8 depicts a schematic view of a single-drive MZM designed in silicon-on-insulator (SOI) technology with a 250 nm active silicon layer. This modulator uses transversal electric (TE) mode grating couplers to couple light between single-mode optical fibers and the integrated waveguides, followed by linear tapers for mode adaptation. Multimode interference (MMI) couplers symmetrically split and recombine optical signals across two interferometric branches. The modulation functionality is realized through carrier-depletion effects in one branch, induced by the doping and reverse biasing of silicon waveguides connected via metal via to an RF coplanar transmission line. The other branch includes a delay line to ensure appropriate phase shifts for effective modulation. This configuration provides a compact, CMOS-compatible platform, enabling efficient optical modulation suitable for high-speed optical communication systems [76]. One device that can act as a filter and as a modulator is the microring. Figure 9 illustrates the detailed structure and operational principles of the two-segment Z-shape junction silicon micro-ring modulators (MRMs) [77]. Panel (a) presents the layout of the five-channel dense-wavelength division multiplexing (DWDM) array, highlighting variations in the ring radius to facilitate evenly spaced resonance wavelengths. Panel (b) details the Z-shape doping profile, showing its specific arrangement within the silicon waveguide,  
Figure 8: Schematic representation of the designed Mach–Zehnder modulator in silicon-on-insulator (SOI) technology, adapted from [76]. The modulator includes transversal electric (TE) mode grating couplers, linear tapers, multimode interference (MMI) couplers, and doped waveguide sections for modulation using the carrier depletion effect.