Biomedical subjects

Kelvin H Wagner

Publications and source records attributed to Kelvin H Wagner.

5 recordsLinked to original sources

PubMed2006

Sluggish light for radio-frequency true-time-delay applications with a large time-bandwidth product.

A new radio-frequency (RF) photonic technique for achieving large RF time delays has been experimentally demonstrated using femtosecond pulses modulated by an acousto-optic tunable filter in a frequency-mapped and Doppler-shifted modulation scheme. A short optical delay line with length of 240 mum produces nearly 3 micros RF time delay after optical heterodyne detection, resulting in an effective slow-light velocity of 86 m/s. A delay-to-pulse-width ratio of 20 based on this technique has been observed, with a larger fractional delay foreseeable.

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PubMed2006

Broadband radio-frequency spectrum analysis in spectral-hole-burning media.

We demonstrate a 20 GHz spectrum analyzer with 1 MHz resolution and >40 dB dynamic range using spectral-hole-burning (SHB) crystals, which are cryogenically cooled crystal hosts lightly doped with rare-earth ions. We modulate a rf signal onto an optical carrier using an electro-optic intensity modulator to produce a signal beam modulated with upper and lower rf sidebands. Illuminating SHB crystals with modulated beams excites only those ions resonant with corresponding modulation frequencies, leaving holes in the crystal's absorption profile that mimic the modulation power spectrum and persist for up to 10 ms. We determine the spectral hole locations by probing the crystal with a chirped laser and detecting the transmitted intensity. The transmitted intensity is a blurred-out copy of the power spectrum of the original illumination as mapped into a time-varying signal. Scaling the time series associated with the transmitted intensity by the instantaneous chirp rate yields the modulated beam's rf power spectrum. The homogeneous linewidth of the rare-earth ions, which can be <100 kHz at cryogenic temperatures, limits the fundamental spectral resolution, while the medium's inhomogeneous linewidth, which can be >20 GHz, determines the spectral bandwidth.

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PubMed2006

Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers.

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.

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PubMed2004

Optoelectronic implementation of a 256-channel sonar adaptive-array processor.

We present an optoelectronic implementation of an adaptive-array processor that is capable of performing beam forming and jammer nulling in signals of wide fractional bandwidth that are detected by an array of arbitrary topology. The optical system makes use of a two-dimensional scrolling spatial light modulator to represent an array of input signals in 256 tapped delay lines, two acousto-optic modulators for modulating the feedback error signal, and a photorefractive crystal for representing the adaptive weights as holographic gratings. Gradient-descent learning is used to dynamically adapt the holographic weights to optimally form multiple beams and to null out multiple interference sources, either in the near field or in the far field. Space-integration followed by differential heterodyne detection is used for generating the system's output. The processor is analyzed to show the effects of exponential weight decay on the optimum solution and on the convergence conditions. Several experimental results are presented that validate the system's capacity for broadband beam forming and jammer nulling for linear and circular arrays.

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PubMed2002

Optical finite impulse response neural networks.

The finite impulse response neural network is described in detail. Different algorithms capable of temporal back-propagation are considered, including a novel modification to the conventional algorithm, called the delayed-feedback back-propagation algorithm. We present and analyze different optoelectronic processors making use of adaptive volume holograms and three-dimensional optical processing. Two single-layer architectures are presented: the input delay plane architecture and the output delay plane architecture. By combining them it is possible to implement both forward and backward propagation in two multi-layer architectures: the first making use of the conventional temporal back-propagation and the second making use of delayed-feedback back-propagation.

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