Optical Communications for Short Reach

Systems modulating, transporting, and detecting lightwaves have tremendously evolved in the past four decades. The first systems which were relying on intensity modulation with direct detection and those using the same modulation and detection principles today have little in common. Not only have electro-optic components drastically evolved, digital signal processing is now the work horse of long reach fiber-optic systems and it is making its way to shorter distances. Today we are observing a shift in the needs in fiber-optic communication systems targeting distances around 10 to 100 km, which should be of the highest throughput but at the same time of low cost, power consumption and foot-print. The reasons leading to this new market segment are manifold. New applications such as mobile optical fronthaul, massive machine-to-machine communication, real-time augmented and virtual reality, latency-sensitive/time-critical applications such as autonomous vehicles and vehicle-to-anything communication as well as haptic control over the Internet are a few examples. The massive deployment of such interconnects dictates the requirement to be of low cost, foot-print and power consumption. Solutions relying on direct detection are generally perceived as the way to achieve this goal. As a result, digital direct detect transceivers modulating and recovering one or multiple degrees of freedom of a lightwave are currently widely investigated as potential solutions. As opposed to digital coherent transceivers which allow a linear mapping of the four-dimensional optical field to a four-dimensional photocurrent, digital direct detect transceivers often require nonlinear signal processing to increase the throughput of the system and/or complex receiver front-ends. In this paper we review the evolution of fiber-optic communication systems, from intensity modulation-direct detection to coherent and back to direct detection, assisted by DSP. We address the main impairments preventing increasing the bitrate-reach metric for systems relying on intensity modulation with direct detection. We summarize a few solutions modulating and recovering an increasing number of degrees of freedom of the lightwave while maintaining a self-beating direct detection mechanism. We conclude with remarks on the use of coherent technologies to address this market segment.