# Integrated Photonics for IoT, RoF, and Distributed Fog-Cloud Computing: A Comprehensive Review
## 6 Key Enablers of High-Speed Networks Technology
### Coherent Optics  
PIC technology plays a critical role in enabling ultra-high spectral efficiency, long-distance transmission, and scalable optical capacity for next-generation Fog and Cloud computing infrastructures. By integrating advanced modulation formats such as QAM and coherent on-chip detection, PICs allow for compact, low-loss, and high-bit-rate transmission systems that maximize spectral usage [78]. These functions are difficult to achieve with discrete components due to alignment challenges and signal degradation. Additionally, PICs support ultra-dense wavelength division multiplexing (UDWDM) and polarization multiplexing within a single platform, enhancing throughput without increasing fiber infrastructure [79]. Their ability to reduce insertion losses and incorporate on-chip amplifi  
Figure 9: Device design of the silicon micro-ring modulators (MRMs) with two-segment Z-shape junctions, taken from [77]. (a) Schematic diagram of the 5 five-channel dense-wavelength division multiplexing (DWDM) MRMs array. (b) Cross-sectional diagram illustrating the Z-shape doping profile within the waveguide. (c) Simulated transverse electric (TE) optical mode profile. (d) Electric field distribution at a reverse bias voltage of \(-3\) V. (e) Finite-difference time-domain (FDTD) simulated optical field distribution within the coupling region of the microring modulator to a bus waveguide for monitoring. Light blue corresponds to low field intensity, while cyan, green, yellow, and red represent progressively higher field intensities, with red indicating the strongest electric field regions.  
cation extends transmission reach and lowers the need for regeneration, which is essential for high-capacity Cloud backbones and low-latency Fog deployments.  
Moreover, PICs are ideally suited for the high-speed interconnection of geographically distributed data centers--a growing requirement for edge analytics, content delivery, and global-scale computing. Unlike traditional interconnects based on discrete optical elements, PIC-based transceivers integrate coherent optics, modulators, multiplexers, and FEC, supporting long-haul links over hundreds to thousands of kilometers with low power consumption and minimal signal degradation [80; 81]. These integrated transceivers also support software-defined networking (SDN), enabling dynamic bandwidth allocation and real-time traffic management across optical backbones [74]. Together, these features make PICs foundational to building energy-efficient, scalable, and globally interconnected Fog and Cloud computing systems.  
Figure 11 presents a four-channel PIC coherent transmitter module demonstrating advanced hybrid integration techniques at both the chip and package levels [82]. Panel (a) provides an overview of the multi-chip module comprising InP-based HCSEL laser sources, silicon-organic hybrid (SOH) IQ modulators, and single-mode fibers interconnected by photonic wire bonds (PWBs). Panel (b) details the structure of the SOH Mach-Zehnder modulators, emphasizing the precise deposition of electro-optic material within silicon slot waveguides for efficient modulation. Panel (c) outlines the experimental setup used for coherent transmission experiments, involving modulation via an arbitrary-waveform generator, propagation through 75 km of fibre, and subsequent coherent detection. Finally, panel (d) shows representative constellation diagrams along with measured bit error ratios at high symbol rates, underscoring the feasibility of achieving reliable high-speed communication with this hybrid integration approach.  
Figure 10: The diagram of microring resonator (MRR)-based transceiver consisting of drivers, microring modulators (MRMs), receivers, MRR demultiplexer (DEMUX), and thermal tuners. Figure taken from [58].