Modulation Optimization for Noncoherent Optical Systems

There has been a significant improvement in high-speed electronics and optical components over the past few years, and this is one of the key enabling factors for high data rate transmission. Moreover, multilevel modulation formats, which encode information onto the carrier’s amplitude and phase, are necessary to increase the spectral efficiency of communication systems. For short-haul low cost optical links, as the ones used in data centers and access networks for example, noncoherent optical communication with directly modulated lasers and direct detection receivers is more cost-effective than using coherent communication systems. Such types of noncoherent communications use the intensity of the optical carrier to carry information. These systems can be modeled as an additive white Gaussian noise channel with input constrained to being nonnegative. Subcarrier modulation, a concept studied in the wireless infrared communications context, allows the use of in-phase and quadrature phase (I/Q) modulation formats for such types of noncoherent optical systems. We propose a novel quaternary subcarrier modulation format in [Paper A] which is a hybrid between on-off keying (OOK) and ternary phase-shift keying. At asymptotically high signal-to-noise ratios, this hybrid format offers a 1.2 dB average electrical power gain and 0.6 dB average optical power gain compared to OOK, making it the most power-efficient format for noncoherent optical systems with a spectral efficiency of 1 bit/s/Hz. However, for systems that are limited by laser nonlinearities, we show that OOK is the best choice at this spectral efficiency. In [Paper B], we complement the theoretical results obtained in [Paper A] by carrying out an experimental link analysis, thus comparing the different modulation formats with spectral efficiency of 1 bit/s/Hz. In agreement with the theory, the proposed modulation format is more power efficient in terms of average optical power compared to the other modulation formats under study. However, the