Thomas McEwen

Using Tungsten Oxide Based Thin Films for Optical Memory and the Effects of Using IR Combined with Blue/Blue-Green Wavelengths

Utilizing a form of tungsten oxide as the physical medium for optical memory, we show that infrared irradiation provides heat to the lattice structure while blue or blue-green light provides electronic excitation to initiate a photochromic chemical change. We have constructed and tested a spin stand based demonstration system for this form of optical memory. Tungsten oxide offers potential advantages in storage density, storage capacity, write speed, signal to noise, processing flexibility, data robustness, and overall, cost.

Experimental results for a photonic time reversal processor for the adaptive control of an ultra wideband phased array antenna

This paper describes a new concept for a photonic implementation of a time reversed RF antenna array beamforming system. The process does not require analog to digital conversion to implement and is therefore particularly suited for high bandwidth applications. Significantly, propagation distortion due to atmospheric effects, clutter, etc. is automatically accounted for with the time reversal process. The approach utilizes the reflection of an initial interrogation signal from off an extended target to precisely time match the radiating elements of the array so as to re-radiate signals precisely back to the targets location. The backscattered signal(s) from the desired location is captured by each antenna and used to modulate a pulsed laser. An electrooptic switch acts as a time gate to eliminate any unwanted signals such as those reflected from other targets whose range is different from that of the desired location resulting in a spatial null at that location. A chromatic dispersion processor is used to extract the exact array parameters of the received signal location. Hence, other than an approximate knowledge of the steering direction needed only to approximately establish the time gating, no knowledge of the target position is required, and hence no knowledge of the array element time delay is required. Target motion and/or array element jitter is automatically accounted for. Presented here are experimental results that demonstrate the ability of a photonic processor to perform the time-reversal operation on ultra-short electronic pulses.

Compensated Crystal Assemblies for Type-II Entangled Photon Generation in Quantum Cluster States

Spontaneous downconversion yields photons for Quantum-Optical-Gate development though their generation is probabilistic. Optimized efficiency requires control over the spectral wavefunction, generally achieved via spectral filtering which sacrifices most downconverted photons. Selecting crystal parameters to address the issue has been demonstrated, but no natural media enable this for 800 nm applications with optimal detection. Synthesizing parameters with super-lattices of known crystals was also analyzed but two-crystal experiments were insufficient to exploit it. Prototype twelve-crystal-assemblies have now been fabricated and the first results are reported here. We review implications for further work and discuss how methods described here enhance efficiency in applications of entangled photons requiring multi-crystal sources, such as cluster states, entanglement swapping, and teleportation.

A MIMO-inspired rapidly switchable photonic interconnect architecture

It is well-known that interconnect issues pose a significant bottleneck with regard to improving the performance of high-speed integrated systems such as a cluster of computer processing units. Power, speed (bandwidth), and size all affect the computational performance and capabilities of future systems. High-speed optical processing has been looked to as a means for eliminating this interconnect bottleneck. Presented here are the results of a study for a novel optical (integrated photonic) processor which would allow for a high-speed, secure means for arbitrarily addressing a multiprocessor system. This paper will present analysis, simulation, and optimization results for the architecture as well as considerations for a proof-of-concept level system design. The architecture takes advantage of spatial and wavelength diversity and in this regard may be regarded as a Multiple Input Multiple Output (MIMO) architecture. A given node to be addressed, rather than having a wired metal contact as an output, has as a radiating laser source that has been modulated with the data to be conveyed to another point in the system. Each processor node radiates a different optical wavelength. Each individual wavelength is chosen, for example, to correspond to the wavelengths associated with a WDM ITU grid. All wavelengths are incident on a coherent fiber bundle which acts as an array receiver. Unlike conventional phased arrays, the receive elements are spaced many wavelengths apart giving rise to a large number of grating lobes. It will be shown that by using appropriate photonic/optical signal processing methods any node of the processor cluster can be randomly and rapidly addressed using high-speed phase shifters (electrooptic or others) as control elements. The diversity techniques employed achieve high gain and a narrow beamwidth in the direction of the desired node and high attenuation with regard to the signals from all other nodes. As is often the case of MIMO-bases systems, overall performance exceeds that of diffraction limited array processing. In addition to the interconnect application discussed, the methods described in this paper can also be applied to other applications where rapid electrical (non-mechanical) optical beamsteering is required such as raster scanned laser radar systems and tracking, guidance, and navigation systems.

A photonic recirculating delay line for analog-to-digital conversion and other applications

Experimental results for a photonic recirculating delay line for high-speed, high-resolution Analog-to-Digital Converted (ADC) and other applications is presented. The approach modifies an analog fiber optic link with a recirculating optical loop as a means to store a time-limited microwave signal so that it may be digitized by using a slower, conventional electronic ADC. Detailed analytical analysis of the dynamic range and noise figure shows that under appropriate conditions the microwave signal degradation is sufficiently small so as to allow the digitization of a multi-gigahertz signal with a resolution greater than 10 effective bits. Experimental results provided support the theory.

Ultra-stable harmonically mode-locked erbium-doped waveguide laser

Generation of stable pulses and a frequency stabilized optical comb are two key requirements for Fourier Based Arbitrary Waveform Generation (AWG) techniques. The longitudinal mode spacing of the laser must remain as stable as possible to permit effective isolation and processing of the modes for waveform synthesis. The short and long term temporal stability ultimately limits the systems precision as well as its operability in fielded systems. A packaged erbium-doped waveguide provided a highly compact gain medium for the harmonically mode-locked laser design. Stability was achieved by use of an intracavity etalon for frequency stabilization of the optical comb, a Pound-Drever- Hall (PDH) method, and an active bias feedback loop for low frequency noise suppression. The temperature was controlled to limit cavity length variation, and the contribution to stability of each method is quantitatively assessed. The systems stable operating time was increased from hours to greater than a day, and the timing jitter is demonstrated to be lower than that of commercially available erbium-doped fiber laser (EDFL) systems. Applications to optical signal synthesis and Laser Radar are briefly discussed.

A spinning disk test stand for two-color, tungsten oxide based optical memory system

We have constructed and tested a spin stand based demonstration system for a new form of optical memory. Based on a new two color storage process which utilizes a form of tungsten oxide as the physical medium, this system offers potential advantages in storage density, write speed, signal to noise, processing flexibility, data robustness, and overall cost. In this paper we describe the system and its performance on prototype disks. Issues regarding choices of wavelengths, write power and speed are discussed.

Examination of tungsten oxide based thin films for optical memory

Because of its well known electrochromic and photochromic properties, and its relative ease of large scale manufacture, tungsten oxide has long been a candidate for display applications. This use was first suggested by Deb in 1973, and numerous patents and papers in the primary literature followed. Tungsten oxide based electrochromic processes have been reviewed by Granquist. We are currently exploring the medium’s photochromic and electrochromic properties for memory and switching purposes. Photochromism refers to the fact that exposing the medium to light causes the material to change color. The light is thought to induce the formation of free charge carriers which can be mobile oxygen ions and radicals as well as electrons that initiate the chemistry leading to the color change. The process we have studied is represented by the chemical reaction below. This process is distinct from chemistry originally described by Deb and reviewed by Granquist in that subbandgap exploitation can be employed.