Photonic page buffer based on GaAs multiple-quantum-well modulators bonded directly over active silicon complementary-metal-oxide-semiconductor (CMOS) circuits.

We present a 2-kbit, 50-Mpage/s, photonic first-in, first-out page buffer based on gallium arsenide/aluminium-gallium arsenide multiple-quantum-well diodes that are flip-chip bonded to submicrometer silicon complementary-metal-oxide-semiconductor circuits. This photonic chip provides nonvolatile storage (buffering), asynchronous-to-synchronous conversion, bandwidth smoothing, tolerance to jitter or skew, spatial format conversion, wavelength conversion, and independent flow control for the input and the output channels. It serves as an interface chip for parallel-accessed optical bit-plane data. It represents the first smart-pixel array that accomplishes the vertical integration of multiple-quantum-well modulators and detectors directly over active silicon VLSI circuits and provides over 340 transistors per optical input-output. Results from high-speed single-channel testing and real-time array operation of the photonic page buffer are reported.

Photonic page buffer based on GaAs multiple-quantum-well modulators bonded directly over active silicon complementary-metal-oxide-semiconductor (CMOS) circuits: errata.

Owing to printing errors, [Appl. Opt. 35, 2439 (1996)] several figures were illegible. The figures are reprinted and briefly reviewed.

Wavelength optimization of quantum-well modulators in smart pixels.

We extend our recent general discussion of electroabsorption and refraction in multiple-quantum-well modulators to determine the optimum modulator design for smart-pixel applications. In addition to the optimum operating wavelength shift, from that of the zero-voltage exciton, we determine the optimum number of quantum wells, and we calculate the reflectivity change and the contrast ratio obtainable. This analysis is undertaken for both simple detectors and modulators, meaning that they are antireflection coated, as well as for devices that include Fabry-Perot resonators. The optimization is performed on a figure of merit that is inversely proportional to the incident optical read energy required on a device to switch another, downstream device. We maximize the figure of merit to minimize the optical read energy. An interesting result is that there should be no significant improvement in our smart-pixel circuit figure of merit with the use of Fabry-Perot resonant modulators and detectors. Our results are, of course, material-system specific, but for the 850-nm AlGalAs/GaAs quantum-well system the optimum wavelength shift from the exciton location is approximately 6 nm. The general trends and approach are applicable to other material systems.

Picosecond switching of GaAs/AlGaAs smart pixels.

We demonstrate that GaAs/AlGaAs smart pixels can be operated with picosecond and subpicosecond laser pulses. Switching times as short as 200 ps are measured. Our results confirm the theoretical prediction that input mode-locked pulses are more advantageous than the more typical square-wave pulses. They also suggest that use of picosecond mode-locked pulses for optical output could produce operating speeds in the gigahertz range.

Field-effect-transistor self-electro-optic-effect-device (FET-SEED) electrically addressed differential modulator array.

We describe a 6 × 6 array of electrically addressed field-effect-transistor self-electro-optic-effect-device differential modulators in which each element has a single-stage amplifier to permit an input voltage of less than 1 V to control the output modulators, which can operate at as high as 10 V. The variations in the switching voltages across the array are less than ±70 mV, and the individual array elements are operated at as high as 2 Gbits/s. We also measure cross talk between adjacent elements within the array, measure the dependence of the switching time on the input voltage swing, and calculate the dependence of the switching time that is due to the photocurrent of the modulators.

Fabry-Perot-enhanced self-electro-optic-effect devices.

Optically bistable symmetric self-electro-optic-effect devices that use an asymmetric Fabry-Perot resonator are demonstrated and assessed. Contrast ratios exceeding 30 for low bias levels (under 6 V) are measured for low intensities. The Fabry-Perot self-electro-optic-effect device is also shown to saturate readily with intensity, with the contrast ratio decreasing by a factor of 3 for an irradiance of 10 microW/microm(2). Wavelength sensitivity measurements and calculations are also presented.

Photonic ring counter using batch-fabricated symmetric self-electro-optic-effect devices.

We describe a photonic ring counter that demonstrates cascadability of batch-fabricated symmetric self-electrooptic-effect devices at a bit rate of 12.5 Mbits/sec for conventional operation and at a bit rate of 50 Mbits/sec when the devices are operated as optical signal sense amplifiers. In each of these experiments the average laser power incident upon each device was less than 1 mW. The system switching speeds agree well with speeds calculated by using the measured switching energies of the devices.

Optical digital processor using arrays of symmetric self-electrooptic effect devices.

Four arrays of thirty-two GaAs symmetric self-electrooptic effect devices were optically interconnected to form a looped-pipeline optical digital processor. Several circuits were demonstrated, including two shift registers and a decoder circuit. Clock frequencies of up to 1 MHz were attained. Possible extensions to and limitations of this system are described.

Cascaded operation of arrays of symmetric self-electro-optic effect devices.

Two identical optical modules were used to demonstrate the cascaded operation of 32-element arrays of symmetric self-electro-optic effect devices. The devices had 5 microm x 10 microm optical windows spaced on a square grid with a 20-microm spacing. They were operated as optically interconnected inverters at 1.1 MHz. The optical power was provided by two current modulated laser diodes per array, each with a maximum output power of 9 mW. The operation of the devices as logic gates is optically implemented but not demonstrated.

Quantum well optical tri-state devices.

We demonstrate quantum well tri-state logic devices for possible use in optical bus architectures. These optical devices are analogous to the tri-state devices often used in electronic buses, where each device can be actively on, actively off, or disabled with at most one device on the bus active at a time. We show two methods of generating these tri-state data, one using tri-state quantum well modulators and one using optical tri-state self-electrooptic effect devices, and we demonstrate a simple optical bus consisting of two such devices. Finally, we comment on the limitations on the number of devices that can be connected to a bus of this type.

Optical logic using electrically connected quantum well PIN diode modulators and detectors.

We present new optoelectronic logic devices or circuits consisting of electrically connected quantum well PIN diodes capable of implementing any boolean logic function. One class of circuits uses single beams to represent the logic levels and compares their intensities to a locally generated reference signal. A second class of circuits routes signals as differential pairs. The connections of diodes in these circuits resemble the transistor connections in NMOS and CMOS logic families. We demonstrate simple optical programmable logic arrays (e.g., E = AB + CD) using both of these classes of circuits.

Module for optical logic circuits using symmetric self-electrooptic effect devices.

An optical module designed to perform cascadable optical logic using arrays of symmetric self-electrooptic effect devices (S-SEEDs) is described. The operation of an array of 7 x 3 devices with optical windows spaced by 20 microm is demonstrated including both array preset and individual device switching. The issues leading to the design of this optical system are detailed. This work illustrates some of the issues which must be considered when designing systems using small reflecting electrooptic devices such as SEEDs and free-space optics in digital systems.