The advancement of digital coherent technologies has dramatically increased the system capacity per single-core single-mode fiber to the point that we can now approach the Shannon limit by utilizing high-order modulation formats and high-coding gain forward error correction (FEC) codes. Because the required energy per bit increases exponentially the closer we get to the Shannon limit, extending the available optical bandwidth by using ultrawideband wavelength-division multiplexing (WDM) and/or spatial-division multiplexing (SDM) is indispensable for increasing the system capacity with high energy efficiency. However, simple extensions of wavelength resources and spatial parallelization dramatically increase the number of transceivers (TxRxs) in proportion to the wavelength/spatial multiplicity. The key to achieving cost- and energy-efficient systems is to reduce the system complexity by using high-density integration and broadband optelectronics. In this article, we overview and discuss the recent advances of coherent optical transceivers integrated with an optical front end and digital signal processing (DSP)/application-specific integrated circuit (ASIC). We then present the transponder architectures and the challenges involved in applying them for massive parallelized transmission systems.

Silicon, well established as an electronics platform, was first seriously explored as an optical waveguide platform in 1987 by Soref and Bennett. Light at fiber-optic communication wavelengths can pass through crystalline silicon with very high transmission. Silicon waveguides can be made by etching into the thin crystalline silicon layer on a silicon-on-insulator wafer. Because of the high refractive index of silicon compared to the surrounding oxide, these waveguides can bend with radii less than five micrometers and make compact splitters/combiners, filters, and polarization elements. High-speed optical modulators can be made by rapidly changing free electron and hole densities in the silicon in the optical path, and photodetectors can be made by epitaxially growing germanium on the silicon, both using electrically connected p-n junctions. The only element missing to complete an optical integration platform is an integrated light source, rendered very difficult because of the indirect bandgap of silicon and germanium. While to date, no practical electrically pumped light emitter has been deployed based on silicon itself, multiple hybrid and heterogeneous solutions have been reported.

Optical coherent systems, which employ the interference of a received optical signal with a local laser to achieve high signal-to-electrical-noise ratio and capture the magnitude, phase, and polarization of the signal, have grown dramatically in the last decade due to improvements in digital signal processors. The optics of both the transmitter and receiver have grown in complexity, and its deployment has benefitted greatly from the integration offered by silicon photonics. Today’s coherent systems range from 1.2-Tb/s fiber-optic communication transceivers to optical coherence tomography medical instruments. This paper will give an overview of optical coherent systems, present the state-of-the-art in silicon photonic components shipping today, and discuss where silicon photonics is going in this field.