The selection of an optical transmitter is a fundamental decision in designing a photonic link, often boiling down to the choice between a Directly Modulated Laser (DML) and an externally modulated system (like a CW laser coupled with a TFLN or LiNbO3 modulator). While both serve to convert electrical signals to light, their underlying architecture leads to distinct performance trade-offs, making each suitable for different types of communication and RF links.
The DFB DML laser, as discussed, is prized for its simplicity, compact size, low threshold current, and cost-effectiveness. The direct modulation approach allows for a single, integrated component to perform both light generation and modulation. This makes DMLs the preferred solution for many metropolitan and short-to-medium-haul data center applications where cost and power consumption are major factors, and where the performance requirements are met by its inherent bandwidth limits (typically up to 18 GHz). The core limitation of direct modulation, however, is the unavoidable phenomenon of "chirp"—a shift in the laser's output wavelength as the injection current changes. This chirp interacts with fiber chromatic dispersion, limiting the effective transmission distance and data rate, especially for very high-speed digital signals over long spans.
In contrast, externally modulated systems overcome the chirp limitation. By using a CW laser (such as a high-power DFB laser like the NY55D series) as a stable, unmodulated source, and passing its output through a dedicated external modulator, the modulation is applied with minimal or zero chirp. This configuration enables ultra-long-haul transmission at extremely high data rates (e.g., 400G and beyond) and is necessary for applications requiring the highest possible bandwidth (e.g., >100 GHz offered by TFLN modulators). Externally modulated links also typically offer a higher optical output power capability.
Therefore, the choice is dictated by the link's requirements: DMLs are the optimal choice for high-fidelity analog RF links, military applications, and cost-sensitive digital links where bandwidths are up to ∼20 GHz and transmission distance is moderate. External modulation, using a high-power CW source like a DFB laser, is essential for maximizing speed and distance in the most demanding digital and coherent communication networks. This complementary relationship ensures that the high-performance DFB laser family, whether used directly modulated or as a CW source, remains central to all tiers of modern optical transmission.