Directly Modulated Laser Intrinsic Parameters Optimization for WDM Systems

Review on the chirping characteristics of directly modulated laser diodes under high transmission speeds in optical fiber networks

“…The frequency chirp typically causes variations in pulse width, which combined with the fiber dispersion to degrade the digital system performance (Ahmed et al, 2014;del Río Campos et al, 2011;Hakki, 1992;Tucker, 1985;Yamamoto et al, 1987). Another serious limitation of frequency chirp is that it causes shifting in the output wavelength away from the actual laser wavelength, which is a critical issue in the directly modulated WDM systems in order to avoid channel interference (del Río Campos & Horche, 2008).…”

“…On the other hand, the adiabatic chirp refers to the frequency shift between the laser steady-state output powers at "1" and "0" levels (Ahmed et al, 2012). The adiabatic chirp causes a shift in the emitted wavelength, while the transient chirp causes variations in pulse width (del Río Campos & Horche, 2008). The parameters controlling frequency chirps are the linewidth enhancement factor (α-factor) (Henry, 1982;Osinski & Buus, 1987) and the gain suppression coefficient (ε) (Hangauer & Wysocki, 2015;Koch & Linke, 1986;Wang et al, 1993).…”

“…The parameters controlling frequency chirps are the linewidth enhancement factor (α-factor) (Henry, 1982;Osinski & Buus, 1987) and the gain suppression coefficient (ε) (Hangauer & Wysocki, 2015;Koch & Linke, 1986;Wang et al, 1993). The α-factor describes how the refractive index changes with the injected carrier density and its effect on LDs' dynamic properties (del Río Campos & Horche, 2012;Vahala et al, 1983). The simultaneous changes in the optical gain and refractive index of the active region, due to the changes in the injected carrier density, result in an intensity-phase coupling.…”

“…However, the findings reported in (Mahmoud, 2007) revealed that the gain suppression causes enhancement in the modulated signal quality at 2 and 10-Gbps. On the other hand, the findings given in (del Río Campos & Horche, 2008) revealed that at 10-Gbps modulation, the adiabatic chirp is the main contributor to the degradation of directly modulated wavelength deviation multiplexing (WDM) system quality. In addition, the findings given in (Mutalip, 2010) confirmed that, in directly modulated WDM systems, frequency chirp reduces the transmission bit rate and vice versa at long fiber lengths.…”

“…Although most of these previous studies focused on the impact of laser chirp on the system performance, there is still room for a more in-depth investigation into the influence of laser design and structure parameters (α-factor and ε) on the frequency chirp and its effects on the fiber length, especially at a high modulation bit rates. In addition, it is important to study on the influence of these laser parameters on the shifting of the output wavelength away from the actual laser wavelength, which is a critical issue in WDM system performance avoiding the channel interference (del Río Campos & Horche, 2008del Río Campos et al, 2010). This article presents a review on the effect of laser parameters such as linewidth enhancement factor and nonlinear optical gain suppression on the chirping characteristics of directly modulated LDs under high transmission bit rates.…”