# Integrated Photonics for IoT, RoF, and Distributed Fog-Cloud Computing: A Comprehensive Review
## 6 Key Enablers of High-Speed Networks Technology
### Energy Performance Trends in Photonic Integration  
To address the need for a more systematic presentation of performance metrics, we include two figures summarizing the technological evolution of photonic integrated circuits (PICs). Figure 4 shows the evolution of integration densities (measured in number of components per PIC) across different platforms, including monolithic InP, monolithic silicon, and heterogeneous InP/Si or GaAs/Si systems. This timeline highlights the rapid growth in integration scale, particularly in silicon-based platforms, which have reached over 40,000 components per chip. Also, projections indicate that the integration density of heterogeneous InP/Si systems--featuring on-chip lasers and optical amplifiers--is expected to rise significantly in the near future.  
Figure 5 [50], compares the energy-efficiency (in pJ/bit) of various PIC platforms over time, including both academic and commercial implementations. It provides a benchmark for evaluating power consumption trends and highlights the progress toward sub-picojoule efficiencies--an essential enabler for sustainable Fog and IoT photonic computing. PIC platforms offer a pathway to significantly more power-efficient and cost-effective systems for the future scaling of optically enabled technologies. Together, these figures illustrate the trajectory of performance improvements and help contextualize the architectures discussed in Section 4 in terms of practical scalability, energy profile, and technological maturity.  
Several definitions used in Figure 5 are presented as follows: QSFP (Quad Small Formfactor Pluggable) is a compact, hot-pluggable transceiver used in data communication applications. It supports four lanes of data in each direction (quad = 4) and is typically used in 40 G and 100 G Ethernet. PSM4 (Parallel Single Mode 4-lane) is an optical link standard  
Figure 4: Historical evolution of the number of components per PIC for InP, Si, and heterogeneous platforms. Figure taken from [49; 50].  
for 100 G Ethernet that uses four parallel single-mode fibers in each direction. Each lane operates at 25 Gbps, totaling 100 Gbps. CWDM4 (Coarse Wavelength Division Multiplexing 4-lane) is a standard for 100 G Ethernet that multiplexes four wavelengths--rather than fibers--onto a single pair of single-mode fibers, with each wavelength carrying 25 Gbps. QSFP-DD (Quad Small Form-factor Pluggable Double Density) is an evolution of the QSFP form factor. The \"DD\" denotes double density, offering eight electrical lanes instead of four, and supports 200 G and 400 G data rates. DR4 (100 G/400G Direct Reach 4-lane) is a standard optical interface used primarily in 400 G Ethernet. It employs four lanes of 100 G PAM4 (4-level Pulse Amplitude Modulation) signaling over parallel single-mode fiber. CPO (Co-Packaged Optics) refers to the integration of optical components (e.g., lasers, modulators) directly onto the same substrate or package as electronic switch or Application-Specific Integrated Circuit (ASIC) chips.  
PIC technology significantly reduces power consumption per transmitted bit compared to traditional discrete optical transceivers by integrating key photonic elements--lasers, modulators, and detectors--onto a single chip. This consolidation minimizes losses from external connections, fiber couplings, and alignment errors [46]. Integrated architectures also reduce parasitic capacitances and shorten interconnect distances, lowering drive voltages and energy usage per bit [51]. Advanced techniques like monolithic laser integration and hybrid assembly further enhance thermal management, cutting the overhead from temperature control systems [52]. These innovations position PICs as a highly energy-efficient solution for data center interconnects, Cloud computing, and other high-performance networking environments.