Abstmct-Crosstalk due to material absorption in a twowaveguide, symmetric directional coupler switch is investigated. In a material with absorption, it is not possible to completely eliminate the crosstalk by adjusting the coupling length. The coupling length for minimum crosstalk differs from that of lossless systems. Theoretical limits of the lowest achievable crosstalk and the corresponding coupling lengths are calculated. The results show that the effect of absorption on crosstalk is more severe when the devices are designed for low crosstalk. The increase in crosstalk due to absorption can be as high as 20 dB. The material absorption is thus a critical parameter in designing low crosstalk devices.
Material properties and performance of metamorphic optoelectronic integrated circuits grown by molecular beam epitaxy on GaAs substratesA technique for the heteroepitaxy of GaAs/Si films having reduced threading dislocation density is presented. The important attribute of this technique is the suppression of three-dimensional Volmer-Weber island formation during initial deposition. This suppression is achieved by deposition of a stoichiometric GaAs buffer layer by a migration-enhanced epitaxy technique on silicon at 348 K to a thickness greater than the ''monolithic thickness,'' h m . Subsequent GaAs films deposited by conventional molecular beam epitaxy on buffer layers of thickness greater than h m possess structural and optical characteristics that exceed those for state-of-the-art GaAs/Si layers: an x-ray full width at half maximum ͑FWHM͒ of 110 arcsec with a dislocation density at the film surface of 3ϫ10 6 cm Ϫ2 and a concomitant 4 K photoluminescence FWHM of 2.1 meV. The p-i-n structures suitable for use as light-emitting diodes ͑LEDs͒ that were grown on the reduced threading dislocation density GaAs/Si and by means of forward-and reverse-bias measurements, demonstrated an ideality factor of nϭ1.5, an increased reverse-bias breakdown electric field of 2.1ϫ10 7 V/m, and an intrinsic region resistivity of 4ϫ10 7 ⍀ cm for LEDs of increasingly smaller mesa size.
Abtrmt-Results are presented which demonstrate the optical generation and transmission of a microwave signal by mixing two high-qualih optical signals from diode-laser-pumped NdYAG ring lasers, resulting in a narrow microwave line width (less than 5 kHz line width at 3 dB and less than-115 dBc/Hz at 300 kHz from line center) and broad microwave tunabilih (dc to 52 GHz). A 111-V semiconductor waveguide with a doping wperlattice active region is used to optoelectronicallg provide 20 dB of amplitude control and up to 517 of phase shift. This approach can be straightforwardly implemented using integrated optics and fiber-optic links for control of phased array antenna elements.
The wet oxidation kinetics of AlAs layers of interest in vertical cavity surface emitting laser (VCSEL) fabrication are investigated in detail. The process is modeled as a diffusion-reaction process. For oxidation over a long time interval, variation of the oxidation rate with the variation of the radius of the etched mesa of the VCSEL is observed. A theory has been developed to obtain the rate equation of the oxidation process and the dependence of the oxidation rate on the size of the VCSEL is explained.
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