Experimental and theoretical study of intrinsic and clipping-induced distortions in CATV DFB laser diodes

Calculation of distortion performances of a given distributed feedback (DFB) laser for different frequency plans is a well-known problem when working with CATV AM-VSB optical links. One generally takes safety margins or perform measurements on the lasers with the requested frequency plans when it is possible. After investigating the most important theories proposed in the scientific literature, we have come to the conclusion that laser distortion can be described as the weighted contribution of two different distortion sources: leakage current and intrinsic distortion due to the laser oscillation mechanism itself. On the basis of the two preceding ideas we predict the evolution of the second-order two-tone intermodulation (IMD2) response as a function of frequency and operating point by cascading Volterra models of the laser leakage current and rate equations. The total composite second order (CSO) distortion can then be predicted with a “10.log” adding law and compared to distortion measurements, including clipping effects