Present demand is covered by Telecom and Cloud companies, with the former covering Long Haul, metro, and access, while the latter covers inside and between data centers. The infrastructure required to support these ecosystems is not only restricted by its associated real estate, but also by its power consumption. These limitations have been driving the use of more complex signal processing techniques, along with technology developments for their implementation.
Complex modulation schemes (amplitude, phase, and light polarization), and multiplexation (time, wavelength) are fundamental methods to increase data throughput in current optical systems. Innovations in three areas are driving current technology improvements: electro-optical/opto-electrical transducers (i.e., modulators and photo detectors), analog high-speed circuits (e.g., Drivers and TIAs), and processing circuits (e.g., re-timers and DSPs).
Processes for monolithically integrated solutions have been established, however, optimization of each component of the transceiver is possible only when each of them is realized in the technology most suitable for its implementation. For example, optical components were previously implemented mainly in III-V technologies, but now they are being moved to silicon (SiPho), because it offers higher level of integration and higher capability for high volume manufacturability. Analog high-speed circuits were traditionally implemented in III-V technologies due to their speed and high-voltage tolerance, however, developments in optical devices (reducing required voltages) and SiGe technologies (with increased speed and integrated CMOS) have positioned SiGe as one of the preferable technologies for high-speed analog circuits. Circuits for retiming (CDR) or signal processing (DSP) achieve better performance in CMOS technologies because of its higher capability of integration (i.e., small feature size), which also scales down power consumption.
Optimization of the transceiver, by using the optimum technology for each of its components, is a feasible approach thanks to packaging developments. Co-package and in-package solutions use 2D/3D integration, materials better suited for high frequency, and rely on wafer level testing and assembly for its realization.
In this presentation, the evolution of optical communications to the modern coherent system is briefly described. Then, system requirements are used to derive the technology requirements to implement next generation analog high-frequency circuits. The transmitter and receiver of a coherent optical system are described using typical circuit topologies, to describe the limitations imposed by technology metrics. On the transmitter, the maximum voltage swing allowed within the limits imposed by reliability and degradation over lifetime are described. On the receiver, a trans-impedance amplifier (TIA) is described, along with the limitations imposed by the technology and their impact on circuit performance. Further limitations dictated by FEOL devices (other than transistors), the relevance of substrate, passive devices parasitics, and limitations introduced in the BEOL are reviewed.