Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift

Abstract

An analysis of second-order distributed feedback lasers (DFB) with central grating phaseshift is performed. The devices have an active grating (i.e., DFB) section, passive grating sections (i.e., DBRs); and the active grating is formed at a metal-semiconductor interface. Coupled-mode theory and the transfer matrix method are employed. It is found that a central grating phaseshift, /spl Delta//spl phi/, of 180/spl deg/ causes the laser to radiate in a beam of symmetric near-field amplitude profile, in sharp contrast to conventional second-order DFB lasers which radiate in beams of asymmetric near-field amplitude profile. In turn the far-field profile becomes a single-lobe beam pattern. Thus, a means to fundamentally obtain surface emission in an orthonormal single-lobe beam from a second-order DFB/DBR device has been found. The orthornomal-beam emission is achieved at no penalty in device efficiency. External differential quantum efficiencies, /spl eta//sub d/, in excess of 70% can be obtained, and the guided-field intensity profile is substantially uniform. The effects of the lengths of the DFB section (L/sub DFB/) and of each of the DBR sections (L/sub DBR/) on device performance are analyzed and optimal values are found to occur for L/sub DFB/ in the 500-700 /spl mu/m range and for L/sub DBR/ in the 600-700 /spl mu/m range. One can obtain /spl eta//sub D/ values as high as 76% from devices with 80% of the energy in the central lobe, and moderate threshold gains (i.e., 40 cm/sup -1/). Threshold gains as low as 25 cm/sup -1/ can also be obtained from highly efficient devices (i.e., /spl eta//sub D//spl cong/70%), at some penalty in guided-field uniformity. In either case the intermodal discrimination is quite high (70-75 cm/sup -1/). Gratings with half-wave (i.e., /spl pi/) phaseshifts have been fabricated by using the dual-tone photoresist method, and the concept has been experimentally proven: orthonormal, single-lobe emission in a diffraction-limited beam from 1500 /spl mu/m-long devices. Extension to two-dimensional (2-D) large-aperture: 200 /spl mu/m/spl times/1500 /spl mu/m; surface emitters is quite possible, which should allow for the emission of watts of coherent CW power in a stable, single mode. The 2-D structure represents a defect-free, second-order active photonic lattice.