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

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4 Author(s)
Shuang Li ; Reed Center for Photonics, Univ. of Wisconsin, Madison, WI, USA ; G. Witjaksono ; S. Macomber ; D. Botez

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, Δφ, of 180° 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, ηd, 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 (LDFB) and of each of the DBR sections (LDBR) on device performance are analyzed and optimal values are found to occur for LDFB in the 500-700 μm range and for LDBR in the 600-700 μm range. One can obtain ηD values as high as 76% from devices with 80% of the energy in the central lobe, and moderate threshold gains (i.e., 40 cm-1). Threshold gains as low as 25 cm-1 can also be obtained from highly efficient devices (i.e., ηD≅70%), at some penalty in guided-field uniformity. In either case the intermodal discrimination is quite high (70-75 cm-1). Gratings with half-wave (i.e., π) 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 μm-long devices. Extension to two-dimensional (2-D) large-aperture: 200 μm×1500 μm; 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.

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IEEE Journal of Selected Topics in Quantum Electronics  (Volume:9 ,  Issue: 5 )