Robert Amantea

Two-dimensional coherent laser arrays using grating surface emission

The concepts, fabrication, and operating characteristics of monolithic two-dimensional, coherent AlGaAs laser arrays are presented. The arrays consist of 100 (10*10) active elements fabricated from a single-quantum-well graded-index separate-confinement heterostructure laser geometry. A surface relief grating is used for feedback and outcoupling. The elements of the array are index-guided ridge lasers with evanescent or Y coupling in the lateral direction and injection coupling longitudinally. The far field emanating from a 60- mu m*5-mm aperture, measures 0.01 degrees *1 degrees . These arrays emit more than 1 W peak power into a 2-AA wavelength interval. By adjusting the drive current to the electrically independent gain sections of these arrays, the angular position of the far-field beam can be steered. >

High-sensitivity uncooled microcantilever infrared imaging arrays

The structure and operation of a new uncooled thermal infrared imaging detector is described which is composed of bimaterial, thermally sensitive microcantilever structures that are the moving elements of variable plate capacitors. The heat sensing microcantilever structures are integrated with CMOS control and amplification electronics to produce a low cost imager that is compatible with silicon IC foundry processing and materials. The bimorph sensor structure is fabricated using amorphous hydrogenated silicon carbide (a-SiC:H) as the low thermal expansion coefficient material, and gold as the high thermal expansion coefficient bimaterial (14 x 10-6/K). Amorphous hydrogenated silicon carbide is an ideal material in this application due to its very low thermal conductivity (0.34 W/m-K) and low thermal expansion coefficient (4x10-6/K). High resistivity (200-400 Ω/sq) thin Ti/W films are used as the infrared resonant cavity absorber and low thermal loss electrical interconnect to the substrate electrical contacts. A temperature coefficient of capacitance, ΔC/C, (equivalent to TCR for microbolometers) above 20% has been measured for these structures, and modeling of the performance of these devices indicates sensor performance in the range NETD < 5 mK and thermal time constants in the 5 -10 msec range are feasible with this technique. Our development efforts have focused on the fabrication of 320 x 240 imaging arrays with 50 micron pitch pixels. A number of these arrays have been fabricated with performance characteristics that are predicted by a detailed thermo-electro-optical-mechanical model of the sensor. The sensor design and the results from measurements of the thermo-electromechanical and optical properties of the detector arrays will be discussed.

Silicon-based reconfigurable antennas-concepts, analysis, implementation, and feasibility

We report on an innovative reconfigurable antenna concept with significant practical relevance based on the dynamic definition of metal-like conductive plasma channels in high-resistivity silicon that are activated by the injection of DC current. The plasma channels are precisely formed and addressed using current high-resolution silicon fabrication technology. These dynamically defined plasma-reconfigurable antennas enable frequency hopping, beam shaping, and steering without the complexity of RF feed structures. This concept shows promise for delivering the performance and capabilities of a phased array, but at a reduced cost. However, challenges such as p-i-n biasing circuit complexity and their nonlinearities, as well as antenna efficiency, would still require further investigations.

Coherent, monolithic two-dimensional (10×10) laser arrays using grating surface emission

Two‐dimensional, coherent AlGaAs laser arrays consisting of 100 (10×10) active elements have been fabricated using single quantum well laser structures. A surface relief grating is used both for feedback and outcoupling. The elements of the array are index‐guided ridge lasers. In one array design, the elements are coupled laterally by evanescent field overlap while in the second design, the coupling is by Y branches. Longitudinal coherence is achieved by injection coupling. The far field, emanating from a 60 μm by 5 mm aperture, measures 0.01°×1°. Both types of arrays emit more than 1 W peak power. The mode spectrum of the emitted power is contained in a ∼2–3 A wavelength interval at ∼1 W.

Characteristics of coherent two-dimensional grating surface emitting diode laser arrays during CW operation

Recent progress in the development of monolithic two-dimensional coherent grating surface emitting (GSE) laser arrays is presented. Such GSE arrays have operated continuously to more than 3 W/surface and pulsed to more than 30 W/surface. They have achieved continuous wave (CW) threshold current densities of under 140 A/cm/sup 2/ with CW differential quantum efficiencies of 20-46% per surface. Linewidths in the 40-100 MHz range were obtained with output powers of 100-300 mW per surface. The arrays typically consist of 10-30 mutually injection coupled gain sections with 10 laterally coupled ridge-guided lasers in each gain section. A single GaInAs strained-layer quantum well with a graded-index separate confinement heterostructure (GRINSCH) geometry allows junction down mounting with light emission through the transparent GaAs substrate. A surface relief grating is used for feedback and outcoupling. >

Mode discrimination in distributed feedback grating surface emitting lasers containing a buried second-order grating

The operating characteristics of a novel grating surface emitting laser structure containing a buried second-order grating have been studied both experimentally and theoretically. This device consists of a pumped distributed feedback (DFB) section that is terminated at each end by unpumped distributed Bragg reflector (DBR) sections. A single continuous second-order grating layer and waveguide layer extend throughout the active and passive sections so that there is essentially no fabricated optical discontinuity at the interface between the active and passive end sections. Single mode operation (>30 dB side mode suppression) and single-lobed far fields with negligible sidelobes are observed up to more than 5*I/sub th/ with spectral linewidths as narrow as 1 MHz. >

Electronic beam steering in monolithic grating‐surface‐emitting diode laser arrays

Electronic beam steering has been demonstrated in both one‐ and two‐dimensional injection‐coupled grating‐surface‐emitting diode laser arrays. By appropriately varying the drive current to the electrically independent gain sections of an injection‐coupled grating‐surface‐emitting laser array, the angular position of the far‐field output can be steered. Experimental results for two‐dimensional surface‐emitting arrays are presented, as well as a theoretical model which shows that beam steering is a general property of injection‐coupled surface‐emitting arrays.

Uncooled IR imager with 5-mK NEDT

The bi-material concept for room-temperature infrared imaging has the potential of reaching an NE(Delta) T approaching the theoretical limit because of its high responsivity and low noise. The approach, which is 100% compatible with silicon IC foundry processing, utilizes a novel combination of surface micromachining and conventional integrated circuits to produce a bimaterial thermally sensitive element that controls the position of a capacitive plate coupled to the input of a low noise MOS amplifier. This approach can achieve the high sensitivity, the low weight, and the low cost necessary for equipment such as helmet mounted IR viewers and IR rifle sights. The pixel design has the following benefits: (1) an order of magnitude improvement in NE(Delta) T due to extremely high sensitivity and low noise, (2) low cost due to 100% silicon IC compatibility, (3) high image quality and increased yield due to ability to do offset and sensitivity corrections on the imager, pixel-by-pixel; (4) no cryogenic cooler and no high vacuum processing; and (5) commercial applications such as law enforcement, home security, and transportation safety. Two designs are presented. One is a 50 micrometer pixel using silicon nitride as the thermal isolation element that can achieve 5 mK NE(Delta) T; the other is a 29 micrometer pixel using silicon carbide that provides much higher thermal isolation and can achieve 10 mK NE(Delta) T.

A numerical investigation of wave interactions in dielectric waveguides with periodic surface corrugations

The modal properties of planar multilayered waveguides with a rectangular surface corrugation are investigated. A rigorous full Floquet numerical analysis is performed for the fundamental TE mode of the infinite periodic structure. The algorithm is based on a boundary element solution of the integral wave equation in the grating region. A generalized transverse resonance-type matrix equation is sought that matches all continuity, periodicity, and boundary conditions. The resonant solutions of this characteristic equation represent all the surface and leaky waves supported by the structure. The exact dispersion characteristics, as well as the amplitudes of the space harmonics are computed and discussed in connection with radiation losses and coupling mechanisms near resonant Bragg conditions. In particular, a specific double-heterostructure GaAs/AlGaAs waveguide geometry is examined in detail. >

Coherent, monolithic two-dimensional strained InGaAs/AlGaAs quantum well laser arrays using grating surface emission

Two‐dimensional coherent strained‐layer InGaAs/AlGaAs quantum well laser arrays consisting of 100 (10×10) active elements have been fabricated and characterized. The central lobe of the far field has a full width at half power of 0.04°×1°. Observation of about 2 W peak power from either the substrate or the junction surface, with differential quantum efficiencies from each side of about 40%, is reported. The mode spectrum of the emitted power is contained in a ∼2 A wavelength interval at ∼2 W.