Fiber Distribution Networks with Direct and External Modulation of Digital QAM-Signals

Abstract

This We compare in detail the both available methods of intensity modulation in C-band: a direct one controlling injection current of distributed feedback laser or vertical cavity surface emitting laser, also an external one using electroabsorption modulator or lithium-niobate Mach-Zehnder modulator. In all cases, the same 64-position quadrature modulated signal at the radio-frequency carrier propagates over fiber fronthaul link of an analog Radio-over-Fiber system. Two limiting factors were taken into account: parasitic frequency chirp and fiber dispersion. In the result, the fiber distance of the link under study reaches within standard limit of 8% up to 33 km for a vertical cavity surface emitting laser, up to 55 km for a distributed feedback laser, up to 58 km for an electroabsorption modulator, and up to 62 km for a Mach-Zehnder modulator that are comparable with known data for individual devices under test. During the study of link at the carrier frequencies up to 10 GHz an intriguing effect was found for the case of a vertical cavity surface emitting laser, which consists in a drop in the slope of the error values with their increasing that is most likely explained by the presence of inverse frequency dependence of the adiabatic chirp factor’s magnitude and phase.