Observation of second derivatives of the electrical characteristics of double-heterostructure junction lasers

Measurements of the second current derivative of the external voltage d²V/dI²generated by stripe-geometry (AlGa)As junction lasers are reported and correlated with corresponding measurements of the first derivative dV/dI as a function of current. In the vicinity of the lasing threshold, a pronounced negative peak occurs in the second-derivative signal as a result of the voltage saturation induced by the stimulated emission. With further increases in current, the second derivative assumes nonzero positive values for lasers with incomplete saturation of the junction voltage, while complete saturation is characterized by a near-zero value for the derivative. A pronounced fine structure observed in the second derivative at currents above threshold indicates the presence of perturbations in the saturated state of a nominally well-behaved laser.