Spatial mode control of concentric-circle-gratingsurface-emitting semiconductor lasers

by Stephen Craig Olson

Institution: University of Rochester
Year: 2017
Posted: 02/01/2018
Record ID: 2155238
Full text PDF: http://hdl.handle.net/1802/32427


The concept of controlling the spatial emissionpatterns from concentric-circle-grating distributed-feedbacksemiconductor lasers is an intriguing and important idea. Theconcentric-circle-grating structure provides a uniquetwo-dimensional mechanism for providing feedback within an opticalwaveguide cavity as well as for coupling light out normal to thesurface. Although the azimuthally polarized, circularly symmetricfundamental mode has previously been observed from optically pumpedcircular-grating lasers, single-spatial-mode lasing in radiallypolarized, higher-azimuthal-order modes has also been observed fromthese devices, leading to the question of deterministic spatialmode control. These higher-azimuthal-order spatial modes have beenpredicted theoretically using a vector-scattering approach.
Inherent to the concept of mode control is thefabrication technology of circular gratings. Electron-beamlithography has been shown to be the only method to date capable ofproducing the small-period (~250 nm) circular gratings that arenecessary for circular emission patterns; however, such systems arecomplex and inevitable errors arise during the course offabrication. An investigation of electron-beam system errorscoupled with dry etching requirements has yielded improvedunderstanding into the processes required to produce high-qualitycircular gratings.
Using such fabricationtechniques and understanding their limitations, controlling theoutput of a circular-grating laser is deemed feasible. Varyingparameters such as grating depth and gain distribution foroptically pumped semiconductor lasers shows that coarse control ispossible using these simple methods. Alternative mechanisms thatmay require tighter tolerances on grating fabrication are alsoevaluated theoretically, including chirped and tapered gratings aswell as spiral and elliptical gratings. These inhomogeneousgratings are shown to have varying effects on the mode spectrum andmode intensity profiles inside the laser cavity as well as toimprove mode discrimination of the concentric-circlegratingsurface-emitting semiconductor laser.