Compensation of directly modulated distributed feedback laser frequency chirps in optical orthogonal frequency division multiplexing intensity-modulation and direct-detection passive optical network systems

A simple and effective technique capable of compensating frequency chirps associated with directly modulated distributed feedback (DFB) lasers (DMLs) is proposed, which utilises an electrical analogue circuit (EAC) and an optical phase modulator. The EAC produces a phase signal mimicking the original phase of the DML-modulated optical signal, and the optical phase modulator driven by the phase signal compensates for the DML frequency chirp in the optical domain. In optical orthogonal frequency division multiplexing (OOFDM) passive optical network (PON) systems utilising DMLs and intensity-modulation and direct-detection (IMDD), the proposed technique can almost completely alleviate the DML frequency chirp effect. In particular, compared to uncompensated conventional DML-based OOFDM IMDD PON systems, the proposed technique can not only improve the transmission capacity by approximately 25% over a wide transmission distance range but also considerably enhance the system performance robustness to variations in DML operating conditions. In addition, optimum EAC design parameters are also identified, based on which the present technique offers system operating condition-independent transmission performance.