Influence of Current Pulse Shape on Directly Modulated Systems using Positive and Negative Dispersion Fibers

Abstract

The proliferation of high-bandwidth applications has given rise to a growing interest, between network providers, on upgrading networks to deliver broadband services to homes and small businesses. There has to be a good balance between the total cost of the infrastructures and the services that can be offered to the end users, as they are very sensitive to equipment outlay, requiring the use of low-cost optical components. Coarse Wavelength Division Multiplexing (CWDM) is an ideal solution to the tradeoff between cost and capacity (Thiele, 2007). This technology uses all or part of the 1270 to 1610 nm wavelength fiber range with an optical channel separation of about 20 nm. This channel separation allows the use of low-cost, uncooled, Directly Modulated Lasers (DML). The main advantages of these devices associated with uncooled operation are (Nebeling, 2007); No integration of TEC and cooler required, less complexity for control electronics, reduced power consumption, only laser diode current required, lower device cost. Otherwise, the direct modulation of the laser current leads to a modulation of the carrier density giving rise to a chirp frequency. This results in a broadened linewidth and a laser wavelength drift (Henry, 1982; Linke, 1985). Since wavelength chirp was recognized several years ago, (Koch, 1988; Hinton, 1993) many papers have addressed the causes and implications of chirp on the optical system performance (Cartledge, 1989; Hakki, 1992; Horche, 2008). However, a large part of the research has focused on fiber transmission properties. The idea of these studies is, generally, based on considerations regarding the interaction of the laser frequency chirp with the fiber dispersion. It is known that if the chirp parameter (-factor) is positive, as is always the case for directly modulated lasers, then the frequency components of the leading edge of the pulse will be blue-shifted and the trailing edge red-shifted. If the pulses are transmitted over a negative dispersion fiber where the blue wavelength is slower than the red, to some extent, pulse compression and significant transmission performance improvement is expected (Morgado & Cartaxo, 2001; Tomkos et al., 2001). To counteract the effect of chromatic dispersion, the system can be moved to a non-zero negative dispersion-shifted fiber, but this method has some problems that are difficult to resolve when it comes to a CWDM metropolitan or access network; The traffic must be interrupted. This method may only be used for upgrading long wavelength channels where