Modulation and multiplexing in optical communication systems

Abstract

Digital electronics and optical transport The rapid transition from analog to digital systems over the past ~50 years has enabled universal processing of all kinds of information, fundamentally without loss of quality [1]. Breakthroughs in digital semiconductor technologies and their enormous ability to scale [2] have enabled cost-effective mass-production of richly functional yet highly reliable and power-efficient microchips that are found in virtually any electronic device today, from high-end internet routers to low-end consumer electronics. Closely coupled to the generation, processing, and storage of digital information is the need for data transport, ranging from short on-chip [3] and board-level [4,5] data buses all the way to long-haul transport networks spanning the globe [6,7] and to deep-space probes collecting scientific data [8], cf. Fig. 1 [5,10]. Each of these very different applications brings its own set of technical challenges, which can be addressed using electronic, radio-frequency (RF), or optical communication systems. Among the different communication technologies, optical communications generally has the edge over baseband electronic or RF transmission systems whenever high aggregate bit rates and/or long transmission distances are involved. Both advantages are deeply rooted in physics: First, the high optical carrier frequencies allow for high-capacity systems at small relative bandwidths. For example, a mere 2.5% bandwidth at a carrier frequency of 193 THz (1.55 μm wavelength) opens up a 5-THz chunk of continuous communication bandwidth. Such “narrow-band” systems are much easier to design than systems with a large relative bandwidth. Second, transmission losses at optical frequencies are usually very small compared to baseband electronic or RF technologies. Today’s optical telecommunication fibers exhibit losses of less than 0.2 dB/km; the loss of typical coaxial cables supporting ~1 GHz of bandwidth is 2 to 3 orders of magnitude higher. In free-space systems optical beams have much smaller divergence angles than in the microwave regime, at the expense of significantly exacerbated antenna pointing requirements, though. The narrow beam width favorably translates into the system’s link budget, in particular in space-based systems where atmospheric absorption is less of a problem. Apart from the above two major advantages, other considerations sometimes come into play, such as the unregulated spectrum in the optical regime or the absence of electromagnetic interference.