David Chapman

Planning for Conjunctive Goals

Abstract The problem of achieving conjunctive goals has been central to domain-independent planning research; the nonlinear constraint-posting approach has been most successful. Previous planners of this type have been complicated, heuristic, and ill-defined. I have combined and distilled the state of the art into a simple, precise, implemented algorithm (TWEAK) which I have proved correct and complete. I analyze previous work on domain-independent conjunctive planning; in retrospect it becomes clear that all conjunctive planners, linear and nonlinear, work the same way. The efficiency and correctness of these planners depends on the traditional add/delete-list representation for actions, which drastically limits their usefulness. I present theorems that suggest that efficient general purpose planning with more expressive action representations is impossible, and suggest ways to avoid this problem.

What Are Plans For

What plans are like depends on how theyre used. We contrast two views of plan use. On the plan-as-program view, plan use is the execution of an effective procedure. On the plan-as-communication view, plan use is like following natural language instructions. We have begun work on computational models of plans-as-communications, building on our previous work on improvised activity and on ideas from sociology.

Widely tunable single-mode quantum cascade laser source for mid-infrared spectroscopy

We demonstrate a compact, single-mode quantum cascade laser source continuously tunable between 8.7 and 9.4μm. The source consists of an array of single-mode distributed feedback quantum cascade lasers with closely spaced emission wavelengths fabricated monolithically on a single chip and driven by a microelectronic controller. Our source is suitable for a variety of chemical sensing applications. Here, we use it to perform absorption spectroscopy of fluids.

High-Quality 150 mm InP-to-Silicon Epitaxial Transfer for Silicon Photonic Integrated Circuits

The integration of dissimilar materials is of great interest to enable silicon photonics and enable optical interconnects in future microprocessors. The wavelength transparency of Si in the telecom window 1.3–1.6 m is another compelling reason to integrate microphotonics and microelectronics. A major challenge for this integration is the incompatibility of the III–V compound and Si semiconductors used to implement microphotonics and microelectronics, respectively. Si and InP have an 8.1% lattice mismatch, making heteroepitaxial growth of InGaAsP compounds on Si with low misfit dislocation density difficult. 1

A program testing assistant

This paper describes the design and implementation of a program testing assistant which aids a programmer in the definition, execution, and modification of test cases during incremental program development. The testing assistant helps in the interactive definition of test cases and executes them automatically when appropriate. It modifies test cases to preserve their usefulness when the program they test undergoes certain types of design changes. The testing assistant acts as a fully integrated part of the programming environment and cooperates with existing programming tools such as a display editor, compiler, interpreter, and debugger.

Slab-coupled optical waveguide photodiode

We report the first high-current photodiode based on the slab-coupled optical waveguide concept. The device has a large mode (5.8 times 7.6 mum) and ultra-low optical confinement (Gamma ~ 0.05%), allowing a 2-mm absorption length. The maximum photocurrent obtained was 250 mA (R = 0.8-A/W) at 1.55 mum.

Improving the efficiency of high-power semiconductor optical amplifiers

We demonstrate a 71% increase in electrical-to-optical conversion efficiency in a slab-coupled optical waveguide amplifier (SCOWA) by employing multiple contacts. At a saturated output power of 26.4 dBm, the amplifier has an efficiency of 11.5% and a gain of 15 dB.

150 mm InP-to-Silicon Direct Wafer Bonding for Silicon Photonic Integrated Circuits

A low-temperature direct wafer bonding process was developed to realize the high-quality transfer of the largest (150 mm in diameter) available InP-based epitaxial structure onto the prepatterned silicon-on-insulator (SOI) substrate. Over 95% bonding yield and a void free bonding interface was obtained. A InGaAsP multiple quantum-well (MQW) diode laser structure is well preserved after bonding, as indicated by the high-resolution X-ray diffraction (XRD) rocking curve measurement. XRD omega scans of the bonded wafers over a 9×9 matrix revealed a small bowing of only 64.12 µm, showing that the III-V epitaxial surface is relatively flat with low-strain. The first InGaAsP-based MQW hybrid Si evanescent racetrack ring lasers have been realized, showing comparable performance as previously InAlGaAs ring lasers.

Arrays of 128x32 InP-Based Geiger-Mode Avalanche Photodiodes

Arrays of InP-based avalanche photodiodes operating at 1.06-μm wavelength in the Geiger mode have been fabricated in the 128x32 format. The arrays have been hermetically packaged with precision-aligned lenslet arrays, bump-bonded read-out integrated circuits, and thermoelectric coolers. With the array cooled to -20C and voltage biased so that optical cross-talk is small, the median photon detection efficiency is 23-25% and the median dark count rate is 2 kHz. With slightly higher voltage overbias, optical cross-talk increases but the photon detection efficiency increases to almost 30%. These values of photon detection efficiency include the optical coupling losses of the microlens array and package window.

Reliable large format arrays of Geiger-mode avalanche photodiodes

The fabrication of reliable InP-based Geiger-mode avalanche photodiode arrays is described. Arrays of up to 256 times 64 elements have been produced and mated to silicon read-out circuits forming single-photon infrared focal plane imagers for 1.06 and 1.5 mum applications.