Studying Dynamic And Static Characteristics Of Grating Tuned Bistable External Cavity Semiconductor Lasers

Tunable external cavity semiconductor lasers(ECLD) are useful for generating frequency tunable laser radiation of narrow spectral width,and providing important system for studying interactions between light and matters The dynamic and static characteristics of the bistable output of an ECLD have been studied in this work under the condition of strong feedback.The equivalent cavity model is used to deduce the threshold condition of the ECLD,and the expression of the threshold carrier density N(v) when ECLD is tuned to oscillate at different frequencies has been obtained. Using the expression of N(v) and the carrier dependent refractive index,the simple basic equation describing the bistable characteristics has been derived after choosing an appropriate reference frequency Nf.Because quasi-Fermi levels of a laser diode(LD) vary with the carrier density,it is predicted that a new type of hysteresis loop should occur for the current passing LD while a hysteresis loop appears on the power-frequency curve of the ECLD. An explicit analytical expression for the frequency width of the hysteresis loop and the condition for the formation of the hysreresis loop has been deduced. The influence of several vital parameters(the linewidth enhancement factor or,reflectivities of the facet adjacent to the external cavity R and the external reflector R\) of the ECLD on the bistable characteristics have been discussed briefly.Comparing the difference between the small-signal approximation analytical and numerical solutions,it is realized that the small-signal approximation is not an approximate choice for the analysis of the dynamic characteristic of the bistable laser. Solving the rate equation numerically,the influence of the factor a and the reflectivity R on the transient characteristics of the ECLD has been analyzed when the shape of the hysteresis loop looks like a parallelogramapproximately. The result shows that the variation of the power-jump magnitude dependents on the factor a and the reflectivity R. As the power-jump magnitude increases,specially,if the system jumps up,the initial output pulse will get stronger,the peak will come earlier. When the ECLD jumps down,at the first stage,the output power reduces quickly,and then experiences an oscillation process which will appear later ,the intensity of the oscillation at the initial stage will increase.