Surface-Emitting Active Photonic Lattices for Watt-Range Coherent Power

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

What is proposed is the study and use of periodic dielectric structures with modulated optical gain, so called active photonic lattices (APLs), for the realization of watt-range coherent, surface-emitted powers from two-dimensional (2-D) horizontal-cavity devices with 2nd-order gratings of novel design. In sharp contrast to conventional APLs, the proposed devices have gain in the low-index lattice sites. In turn, long range (coherent) coupling via traveling waves will become possible via resonant leaky-wave coupling between the low-index lattice sites. The proposed 2-D APLs combine antiguided phase-locked arrays with surface emission from 2nd-order DFB/DBR grating structures. Unlike all previously reported 2nd-order grating DFB surface emitters, the proposed structures employ central grating phase shifts of around pi, which insure emission in an orthonormal, single-lobe beam at no penalty in device efficiency. In addition, the grating structure, besides insuring single-longitudinal-mode operation, will act as a highly effective selector of a single lateral mode: the in-phase array mode. As a consequence large-aperture (200mm x 1200mm) coherent sources of nearly uniform 2-D guided-field profiles will be capable to operate in a stable, single diffraction-limited beam to watt-range CW output powers. First, a Bloch-function model will be developed for analyzing the proposed 2-D APL-type device. Then, upon applying the model for device-design optimization, the device structure will be fabricated by metal-organic chemical vapor deposition, followed by grating fabrication via holographic interferometry and wet chemical etching. To create the necessary grating p phase shift, a dual-tone photoresist will be used. To create the phase-locked array (in the lateral direction), narrow periodic trenches will be etched into the top surface, and then high-index material (GaAs) will be regrown as to provide an antiguided array. After depositing the necessary metallization on the device p-side, a stripe window will be created in the device n-side metallization, to allow 1st-order diffracted light to be emitted in a direction normal to the laser-chip surface. Initial work will be done on 20-element arrays to prove the concept of lateral-array-mode selection via the 2nd-order diffraction grating as well as to confirm enhancement in beam brightness due to the grating existence only in the antiguided-array cores. 40-element arrays will then be developed. For effective lateral-mode selection, longitudinally tapered DBR reflectors will be used. For analysis of such intermodal discrimination the beam-propagation method will be employed. Devices will be fabricated (by using standard photolithrography and wet chemical etching in the DBR-grating (reflector) regions).