Validity of perturbation approximation in bistable external cavity LDs

Studies have been made on the validity of perturbation approximation when analyzing the rate equations of semiconductor lasers respectively subject to small gain and loss modulations.It is found that for a current or gain modulated diode laser,the neglected second order small quantity in the perturbation method is comparable to the square of the retained first order small quality.For a loss modulation where the threshold carrier density may change,the neglected term may become comparable to the retained first order small quantity.Taking an external cavity laser diode(ECLD) as an example,calculations indicate that the perturbation analysis may become invalid under loss modulation.