James Albert Walker

Dynamic spectral power equalization using micro-opto-mechanics

We present a voltage-controlled spectral attenuator for gain shaping and power equalization in wavelength division multiplexed single-mode fiber systems. A micro-opto-mechanical modulator array, where electrostatic deflection of a silicon nitride quarter-wave dielectric layer suspended over a silicon substrate creates a column of variable reflectivity mirrors, is packaged using bulk optics and a diffraction grating to disperse the input spectrum across the device and collect the reflected light into a separate output fiber. The packaged component has 9-dB excess loss, 20-dB dynamic range and 10-/spl mu/s response. We demonstrate equalization of the amplified spontaneous emission spectrum from an erbium-doped fiber amplifier and of individual laser signals with 10-dB initial variation to less than 0.5-dB variation over a 24-nm passband-free spectrum.

Optical modulator with independent control of attenuation and spectral tilt

We describe here a device based on the mechanical anti-reflection switch, or MARS modulator for use in WDM networks. The MARS modulator is a broad spectrum, high contrast reflection modulator based on the vertical movement of an optical membrane above a substrate. The movement is via electric force on the membrane induced by a bias applied between the membrane and the substrate.

The AMOEBA chip: an optoelectronic switch for multiprocessor networking using dense-WDM

We present AMOEBA: a single-chip asynchronous multiprocessor optoelectronic bit-sliced arrayed crossbar switch intended to provide switched interconnection between multiple processors in a distributed computing environment. AMOEBA relies on optoelectronic-VLSI integration, free-space optical interconnects, and wavelength-and-space-division multiplexed networking on single-mode fiber. We report the implementation and testing of a first generation, 16-channel prototype of the switch and a compact opto-mechanical transceiver package that accomplishes the free-space-to-fiber interfacing.

Passband-free dynamic WDM equalization

We demonstrate passband-free power equalization over a 24 nm spectrum using free-space optical demultiplexing and a micromechanical variable-reflectivity mirror strip. The device has 20 dB dynamic range, 10 microsecond response and no static power dissipation.

32 channel WDM graphic equalizer

We have fabricated a packaged 32 channel active intensity equalizer for WDM fiber networks using free-space optics and surface normal MARS (mechanical antireflection switch) modulators. This design could be altered to allow automatic level control by monitoring the output level with a 1% beamsplitter before the grating and a detector array adjacent to the modulators.

Dual-Function Detector–Modulator Smart-Pixel Module

We describe a smart-pixel circuit that permits the use of a GaAs/AlGaAs multiple quantum well diode to be used both as a detector for data input and a modulator for data output. The module provides the ability to double the number of inputs or outputs to the array and is well suited to cascaded optoelectronic system architectures that require bidirectional communition.

Growth of GaAs light modulators on Si by gas source molecular‐beam epitaxy for 850 nm optical interconnects

The growth of GaAs quantum well modulators on Si for photonic switching applications is reported. Comparison of modulator’s quantum confined Stark effect atop different miscut Si surfaces demonstrate the need for a highly ordered array of bilayer steps as an initial Si surface condition for heteroepitaxy. Proton implantation into the SiO2/Si system via an electron cyclotron resonance plasma to lower the Si oxide desorption temperature, while perserving step ordering of the surface, is explored.

Silicon micromachines for lightwave networks: can little machines make it big?

Silicon micromechanics is an emerging field which is beginning to impact upon almost every area of science and technology. In areas as diverse as the chemical, automotive, aeronautical, cellular and optical communications industries, silicon micromachines are becoming the solution of choice for many problems. In this paper, we describe what they are, how they are built and show how they have the potential to revolutionize lightwave systems. Devices such as optical switches, variable attenuators, active equalizers, add/drop multiplexers, optical crossconnects, gain tilt equalizers, data transmitters and many others are beginning to find ubiquitous application in advanced lightwave systems. We show examples of these devices and describe some of the challenges in attacking the billions of dollars in addressable markets for this technology.

Low‐temperature growth of 850 nm quantum well modulators for monolithic integration to very large scale integrated electronics on GaAs

We report a new, low‐temperature process for monolithically integrating 850 nm modulators to pre‐existing very large scale integrated GaAs electronics. We find pristine GaAs surfaces form below 400 °C in the presence of energetic protons generated by an electron cyclotron resonance plasma. While 850 nm superlattices grown at 430 °C exhibit nearly perfect lattice structure when examined by x‐ray diffraction, nonradiative centers are detected with photoluminescence. Transmission spectroscopy reveals the excitons deteriorate too rapidly under bias voltage to fully utilize the quantum confined Stark effect. Efforts to reduce nonradiative trap generation and improve modulator performance under low‐temperature growth appear more feasible when growth is performed under tightly controlled stoichiometric conditions than with arsenic pulsed layer molecular beam epitaxy methods.We report a new, low‐temperature process for monolithically integrating 850 nm modulators to pre‐existing very large scale integrated GaAs electronics. We find pristine GaAs surfaces form below 400 °C in the presence of energetic protons generated by an electron cyclotron resonance plasma. While 850 nm superlattices grown at 430 °C exhibit nearly perfect lattice structure when examined by x‐ray diffraction, nonradiative centers are detected with photoluminescence. Transmission spectroscopy reveals the excitons deteriorate too rapidly under bias voltage to fully utilize the quantum confined Stark effect. Efforts to reduce nonradiative trap generation and improve modulator performance under low‐temperature growth appear more feasible when growth is performed under tightly controlled stoichiometric conditions than with arsenic pulsed layer molecular beam epitaxy methods.

Operation of a 50-element two-dimensional smart-pixel receiver array

Two-dimensional arrays of optical receivers become interesting as we try to pack more information into smaller spaces, as might be encountered in optical interconnect applications. We have realized a two-dimensional array of such receivers, and coupled their outputs off-chip both electrically and optically, by using a previously described hybrid process whereby multiple-quantum-well modulator/detectors are attached to CMOS circuits. We have demonstrated 622 Mb/s operation of receivers in the array, and found that roughly a 2.5 dB penalty is incurred for operation of nearly the whole array as compared to an isolated element at 311 Mb/s.