Abhay M. Joshi, Shubhashish Datta, and Andrew Crawford
Discovery Semiconductors, Inc.
119 Silvia Street
Ewing, NJ 08628, USA
Tel: 609-434-1311, Fax: 609-434-1317
The implementation of multilevel modulation formats, in conjunction with coherent detection, will significantly increase the information capacity of future fiber-optic links through increased spectral efficiency.
Since the late 1980s, fiber-optic networks have steadily become the bedrock for the ever-expanding global telecommunications system. Early fiber-optic links, such as the eighth transatlantic telecommunications cable (TAT-8) installed in 1988, were relatively simple systems by today's standards and used on-off signaling to transmit a few hundred megabits per second (Mbit/s) over a single optical fiber.
The year 1992 saw the deployment of wavelength-division multiplexing (WDM) that allowed multiple optical channels, each allocated a unique optical carrier frequency, to co-propagate over the same optical fiber. Coupled with innovations in optical amplification, WDM triggered an exponential growth in information capacity of a single optical fiber and heralded the "Information Age."
Modern WDM networks, incorporating as many as 80 optical channels with 50 GHz grid spacing, have reached the spectral limit of optical fiber amplifiers used in these systems. As a result, further expansion of information capacity through incremental "horizontal stacking" of optical channels is not feasible. However, the global demand for information, primarily fueled by video traffic, is expected to continue its growth Coherent detection of high-level modulation formats, supplemented by WDM, has lately emerged as the solution to further upgrade the fiber-optic backbone networks.