C. Michael Jefferson

Modulation coding for pixel-matched holographic data storage

We describe a digital holographic storage system for the study of noise sources and the evaluation of modulation and error-correction codes. A precision zoom lens and Fourier transform optics provide pixel-to-pixel matching between any input spatial light modulator and output CCD array over magnifications from 0.8 to 3. Holograms are angle multiplexed in LiNbO(3):Fe by use of the 90 degrees geometry, and reconstructions are detected with a 60-frame/s CCD camera. Modulation codes developed on this platform permit image transmission down to signal levels of ~2000 photons per ON camera pixel, at raw bit-error rates (BERs) of better than 10(-5). Using an 8-12-pixel modulation code, we have stored and retrieved 1200 holograms (each with 45,600 user bits) without error, for a raw BER of <2x10(-8).

Volume holographic data storage at an areal density of 250 gigapixels/in. 2

One thousand volume holographic data pages, each containing 1x10(6)pixels , are stored in a common volume of LiNbO(3) :Fe by use of the 90 degrees geometry. An effective transverse aperture of 1.6 mm x 1.6mm , realized by repetition of this experiment at each of the eight surrounding locations, results in a demonstrated areal density of 394pixels/mum (2) (254 Gpixels/in. (2)) . Short-focal-length Fourier optics provide a tightly confined object beam at the crystal; the reference beam is angle multiplexed. Data pages retrieved with a 1024 x 1024 CCD camera are processed to remap bad spatial light modulator pixels and to compensate for global and local pixel misregistration and are then decoded with a strong 8-bits-from-12-pixels modulation code. The worst-case raw bit-error rate (BER) before error correction was 1.1x10(-3) , sufficient to deliver a user BER of 10(-12) at an overall code rate of 0.61 user bits per detector pixel. This result corresponds to 1.08% of the well-known theoretical volumetric density limit of 1/lambda(3) .

Beam shaping with a plano-aspheric lens pair

A pair of plano-aspheric lenses can be used to transform a collimated, radially symmetric, Gaussian beam to a radially symmetric flat-top beam. Diffraction of the output beam due to the choice of irradiance profile, as well as the finite aperture of the optics, must be considered if a propagating beam is required. Choosing both lenses to be positive, one can show that the aspheric surfaces are strictly convex, which facilitates fabrication by magnetorheological figuring. A fused silica lens pair is demonstrated, which can be used at any wavelength from 250 to 1550 nm to transform a Gaussian to a flat-top beam. Measurements of both the irradiance profile and phase of the output beam are presented and compared to the ideal design. These optics transform 78% of the total input beam power into the flat-top region of the output beam, which is uniform to better than 5% rms. For applications requiring uniform illumination, this represents a fourfold improvement in power utilization over the Gaussian input. The output wavefront is flat to a quarter wave at 514 nm, resulting in a beam that propagates approximately 0.5 m without significant change in profile.

Gray-Scale Data Pages for Digital Holographic Data Storage

The prospects for gray-scale (or multilevel) digital holographic data storage are theoretically and experimentally investigated. A simple signal-to-noise ratio (SNR) partitioning argument shows that when SNR scales as 1 over the number of holograms squared, five gray levels (log(2) 5 bits/pixel) would be expected to result in a 15% capacity increase over binary data pages. However, the additional signal-dependent noise sources present in practical systems create a baseline SNR that reduces both the optimal number of gray levels and the resulting gain in capacity. To implement gray-scale recording experimentally, we adapt the predistortion technique previously developed for binary page-oriented memories [Opt. Lett. 23, 289 (1998)]. Several new block-based modulation codes for decoding gray-scale data pages are introduced. User capacity is evaluated by an experimental technique using LiNbO(3) :Fe in the 90 degrees geometry. Experimental results show that a balanced modulation code with three gray levels provides a 30% increase in capacity (as well as a 30% increase in readout rate) over local binary thresholding.

Experimental evaluation of user capacity in holographic data-storage systems

An experimental procedure for determining the relation between the number of stored holograms and the raw bit-error rate (BER) (the BER before error correction) of a holographic storage system is described. Compared with conventional recording schedules that equalize the diffraction efficiency, scheduling of recording exposures to achieve a uniform raw BER is shown to improve capacity. The experimentally obtained capacity versus the raw-BER scaling is used to study the effects of modulation and error-correction coding in holographic storage. The use of coding is shown to increase the number of holograms that can be stored; however, the redundancy associated with coding incurs a capacity cost per hologram. This trade-off is quantified, and an optimal working point for the overall system is identified. This procedure makes it possible to compare, under realistic conditions, system choices whose impact cannot be fully analyzed or simulated. Using LiNbO(3) in the 90 degrees geometry, we implement this capacity-estimation procedure and compare several block-based modulation codes and thresholding techniques on the basis of total user capacity.

Characterization of phase change memory materials using phase change bridge devices

Having become one of the most promising candidates for future nonvolatile memory applications, phase change random access memory is now driving an intensive search for phase change materials with optimized properties. In this paper, phase change bridge devices are utilized as a unique material test vehicle, allowing systematic extraction of crucial material parameters such as resistance contrast, switching speed, threshold voltage, and set and reset power. Bridge devices fabricated from undoped Ge15Sb85 are presented that reproducibly switch between set and reset states with one decade resistance contrast using current pulses as short as 10 ns. Since devices are fabricated in the amorphous-as-deposited phase, an interesting intermediate device state can be produced, which we attribute to a crystalline center region that fails to completely bridge the amorphous active device volume. The subtle differences between the amorphous-as-melt-quenched and amorphous-as-deposited phases are explored using both bridg...

Figure of merit for the optical aperture used in digital volume holographic data storage

A 4-focal length, telocentric holographic storage system architecture usually employs an aperture stop. The aperture plays an important role in determining the areal density of storage as well as the extent of inter-symbol interference. We propose a figure-of-merit for such an aperture. For simplicity, we assume that a zero-forcing equalizer is used to compensate for inter-symbol interference caused by the point spread function.

A solid-state 193-nm laser with high spatial coherence for sub-40-nm interferometric immersion lithography

We have developed a solid-state 193-nm laser source operating at 5-kHz that generates a near-diffraction-limited TEM00 beam with 35 mW average power. The frequency spectrum is Gaussian, with a linewidth ~7-pm (FWHM), corresponding to a coherence length of ~2-mm. The output beam also has a very high degree of spatial coherence. This source was used in an interferometric liquid-immersion lithography test stand to produce 40- and 35-nm half-pitch grating structures over a ~0.6-mm field of view with a commercially available chemically-amplified photoresist.

Frequency-domain measurements of spectral hole patterns burned with phase-coherent pulses

Abstract We provide the first detailed characterization of spectral hole patterns created by coherent pulse pairs of which the optical phases could be varied in a controlled manner. The pulse pairs form hole patterns with a period given by the inverse of the time between the leading edges of the pulses. The envelopes of the hole patterns reflect the Fourier transforms of the pulses and the phase difference of the pulses appears as a phase shift of the population gratings in the frequency domain. The data are analyzed with a theoretical model which includes the pulse spectra and temporal parameters, the pulse phases, non-linear excitation, dephasing times and laser jitter.

Invited Article: High resolution angle resolved photoemission with tabletop 11 eV laser

We developed a table-top vacuum ultraviolet (VUV) laser with 113.778 nm wavelength (10.897 eV) and demonstrated its viability as a photon source for high resolution angle-resolved photoemission spectroscopy (ARPES). This sub-nanosecond pulsed VUV laser operates at a repetition rate of 10 MHz, provides a flux of 2 × 10(12) photons/s, and enables photoemission with energy and momentum resolutions better than 2 meV and 0.012 Å(-1), respectively. Space-charge induced energy shifts and spectral broadenings can be reduced below 2 meV. The setup reaches electron momenta up to 1.2 Å(-1), granting full access to the first Brillouin zone of most materials. Control over the linear polarization, repetition rate, and photon flux of the VUV source facilitates ARPES investigations of a broad range of quantum materials, bridging the application gap between contemporary low energy laser-based ARPES and synchrotron-based ARPES. We describe the principles and operational characteristics of this source and showcase its performance for rare earth metal tritellurides, high temperature cuprate superconductors, and iron-based superconductors.