Effect of gain localization in circular-grating distributed feedback lasers

We explore the influence of gain localization on the lasing performance of circular-grating distributed feedback (CDFB) lasers. The effect is studied in an optically pumped CDFB laser resonator based on a waveguide of the conjugated polymer poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-phenylene vinylene]. Variations in lasing threshold and slope efficiency are determined as a function of the radius of the optical excitation. The experimental lasing results are compared with predictions from a theoretical analysis based on an adaptation of the transfer matrix method. We find that a strong localization of the gain near the center of the CDFB laser can lead to both a substantial reduction in threshold and increase in output efficiency. As the excitation radius changes from a 90 to a 15 mu m radius, the threshold energy decreases from 5.3 to 0.29 nJ, and the surface-emitted output efficiency increases by an order of magnitude. A simple model is developed that confirms that the significant reduction in threshold can be explained by an enhanced overlap of the population inversion with the resonant mode. (C) 2005 American Institute of Physics.