Chirped Managed Laser for Multilevel Modulation Formats: A Semi-Analytical Approach for Efficient Filter Design

The performance of a directly modulated laser (DML) in a direct detect system is greatly impaired by its inherent frequency chirp and chromatic dispersion of the optical fiber, limiting both transmission speed and reach. Chirped managed lasers (CMLs) were introduced as a solution to extend the transmission reach and improve receiver sensitivity by tailoring the spectrum of the DML using an optical filter. In this work, we present a simple semi-analytical approach for deterministically designing the optical filter for CMLs. This approach can be applied to design the filter for both non-return-to-zero (NRZ) and multi-level modulation formats (e.g., PAM4). We also show that as the modulation speed increases, optical filtering alone may not be sufficient to extend the reach and efficient digital signal processing (DSP) should be utilized as well. With the aid of a proper optical filter and DSP, we experimentally demonstrate 35 Gbaud PAM4 transmission over 20 km of standard single mode fiber, showing the potential of a low-cost C-band DML-based transceiver solution.