Robert J. Feuerstein

Photo-, cathodo-, and electroluminescence from erbium and oxygen co-implanted GaN

Efficient Er-related photo-, cathodo-, and electroluminescence at 1539 nm was detected from Er and O co-implanted n -type GaN on sapphire substrates. Several combinations of Er and O implants and postimplant annealing conditions were studied. The Er doses were in the range (0.01–5)×10 15 ions/cm 2 and O doses (0.1–1)×10 16 ions/cm 2 . GaN films implanted with 2×10 15 Er 2+ /cm 2 at 350 keV and co-implanted with 10 16 O + /cm 2 at 80 keV yielded the strongest photoluminescence intensity at 1539 nm. The annealing condition yielding the strongest Er-related photoluminescence intensity was a single anneal at 800 °C (45 min) or at 900 °C (30 min) in flowing NH 3 . The optimum O:Er ratio was found to be between 5:1 and 10:1. Co-implanting the GaN:Er films with F was also found to optically activate the Er, with slightly (20%) less photoluminescence intensity at 1539 nm compared to equivalent GaN:Er,O films. The Er-related luminescence lifetime at 1539 nm was found to depend on the excitation mechanism. Luminescence lifetimes as long as 2.95±0.15 ms were measured at 77 K under direct excitation with an InGaAs laser diode at 983 nm. At room temperature the luminescence lifetimes were 2.35±0.12, 2.15±0.11, and 1.74±0.08 ms using below-band-gap excitation, above-band-gap excitation, and impact excitation (reverse biased light emitting diode), respectively. The cross sections for Er in GaN were estimated to be 4.8×10 −21 cm 2 for direct optical excitation at 983 nm and 4.8×10 −16 cm 2 for impact excitation. The cross-section values are believed to be within a factor of 2–4.

A STUDY OF NEODYMIUM-CHELATE-DOPED OPTICAL POLYMER WAVEGUIDES

The problem of doping optical quality polymers with chelated Nd3+ has been studied. A number of material systems were evaluated. The best results in terms of optical quality at a high Nd3+ concentration were achieved by doping a fluorinated polyimide with fluorinated neodymium chelate. Slab and channel waveguides were fabricated in this material system. Optical absorption and luminescence studies were carried out.

Electroluminescence from erbium and oxygen coimplanted GaN

Room temperature operation of erbium and oxygen coimplanted GaN m‐i‐n (metal–insulator–n‐type) diodes is demonstrated. Erbium related electroluminescence at λ=1.54 μm was detected under reverse bias after a postimplant anneal at 800°C for 45 min in flowing NH3. The integrated light emission intensity showed a linear dependence on applied reverse drive current.

Cathodoluminescence study of erbium and oxygen coimplanted gallium nitride thin films on sapphire substrates

The cathodoluminescence(CL) of erbium and oxygen coimplanted GaN(GaN:Er:O) and sapphire (sapphire:Er:O) was studied as a function of temperature. Following annealing, the 1.54 μm intra‐4f‐shell emission line was observed in the temperature range of 6–380 K. As the temperature increased from 6 K to room temperature, the integrated intensity of the infrared peak decreased by less than 5% for GaN:Er:O, while it decreased by 18% for sapphire:Er:O. The observation of minimal thermal quenching by CL suggests that Er and O dopedGaN is a promising material for electrically pumped room‐temperature optical devices emitting at 1.54 μm.

First demonstration of multihop all-optical packet switching

Experimenta1 demonstration of all-optical multihop routing through 2/spl times/2 photonic packet switches is described for the first time. As each packet traverses a switch, the functions of routing, contention resolution, header regeneration, and header reinsertion are performed. Three all-optical hops are demonstrated for the payload without optical amplification. The number of hops is currently limited by /spl minus/13 dB optical loss per switch pass. Each packet is coded using six discrete optical wavelengths, making this the widest bit-parallel photonic switch reported to date.<<ETX>>

Modeling photobleached optical polymer waveguides

One of the most efficient ways to prepare nonlinear optical polymer channel waveguides is by photobleaching. To control the index profile precisely and to design and improve the performance of active electro-optical devices, modeling of the photobleaching process is important. We report our phenomenological bleaching model, which uses a stretched exponential time dependency technique that predicts the index profile for polymer channel waveguides and present design rules for active optical switches and modulators. One way to verify the bleaching model is to calculate the effective index and compare this with our measured effective index obtained with prism-coupling techniques. The bleaching model shows good agreement with experiments.

Optoelectronic time-of-flight design and the demonstration of an all-optical, stored program, digital computer

The recent demonstration of an all-optical, stored-program, digital computer by our group focused on high-speed optoelectronic design. It was made possible by a new digital design method known as time-of-flight design. A rudimentary, but general-purpose, proof of principle computer was built, which is all-optical in the sense that all signals connecting logic gates and all memory are optical in nature. LiNbO/sub 3/ directional couplers, electrooptic switches, are used to perform logic operations. In addition to demonstrating stored program operation in an optoelectronic digital computer, the system demonstrated the feasibility of the new design method, which does nor use any flip-flops or other bistable devices for synchronization or memory. This potentially allows system clock rates of the same order as device bandwidth. This paper describes how the time-of-flight design method was motivated by the special properties of optoelectronic digital design. The basic principles of the method we employed will be discussed along with some of its potential advantages. The experimental work with digital optical circuits leading up to and including the stored program computer experiment will then be discussed. Finally, the future potential of time-of-flight design in high-bandwidth optoelectronic systems will be discussed. >

Digital optical counter using directional coupler switches

We describe the design and implementation of a bit-serial, four-bit, binary optical counter. The counter was designed and simulated using a digital optical simulation program developed for this purpose. It consists of five switches, a 4-bit fiber loop memory to store the count, four splitters, and fibers to interconnect the components. The counter is presently limited to a clock rate of 50 MHz because of the propagation delay in the single-bit time feedback loop. As designed, the same hardware may be used to count any even number of bits simply by changing the lengths of two fiber loops. The counter is unique in that it does not employ latches or other synchronizing memory elements, rather relying on a time-of-flight architecture. We describe the system issues involved in construction of the counter as well as the novel requirements on the switch drive electronics. We then outline the issues still to be addressed for the current counter and conclude with suggested design alternatives to improve its operation and increase its clock rate.

Implementation of a fiber-optic delay-line memory

The construction and operation of a 50-MHz 64 x 16 bit fiber-optic bit-serial delay-line memory is described. It consists of LiNbO(3) directional coupler switches, fused-fiber couplers, and a 4.17-km fiber loop. It is a subsystem of a bit-serial optical computer under construction by our group. We discuss delay and clock source stability requirements for the long delay line in the face of a limited phase error tolerance. The reliability testing of the memory subsystem is described. The degradation of data in the memory loop as the phase error tolerance is exceeded by a small amount is studied through the temperature dependence of the memory loop. Data are presented for the memory-loop stability with respect to temperature variations. The memory subsystem design and construction is presented. The results of these experiments support the feasibility of a 100-MHz 128 x 16 bit memory.

Implementation of a general-purpose stored-program digital optical computer

The implementation of what we believe to be the first stored-program digital optical computer is described. The implementation domain consists of lithium niobate directional couplers that are modified to provide optical control and are interconnected with single-mode fiber. The architecture is also the first to employ time-of-flight synchronization. That is, there are no flip-flops used as synchronizing memory elements. Synchronization is achieved by the precise timing of the arrival of information at all points of interaction. The design is a minimal one, employing only 62 directional couplers. Previous papers have discussed the primary architecture and synchronization conditions for the machine. Here we focus on the secondary architecture, construction, debugging, and performance of the machine.