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).
