We have developed a solid-state 193-nm laser source operating at 5-kHz that generates a near-diffraction-limited TEM<sub>00</sub>
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.
Methods will be described for achromatizing a previously described highly efficient refractive laser beam reshaper which allow a propagating, collimated, diffraction limited flat-top laser beam to be generated over a 200 nm spectral interval. Although the beam reshaper utilizes two aspheric lenses, the achromatizers detailed in this paper use only conventional spherical refracting elements. This allows a practical and cost effective path to upgrade such a beam reshaper to broad wavelength operation.
We review recent progress made towards commercializable read-write, fast-access holographic data storage. This includes a recent demonstration of high areal density holographic storage, systems architectures for extending this high density to high capacity using phase-conjugate readout, and recent experimental progress along these lines. Other topics include using signal processing to relieve alignment and distortion constraints, optical elements for improving beam uniformity, and most importantly, requirements and prospects for improved photorefractive materials for two-color, gated nonvolatile holographic storage.
A system of two plano-aspheric lenses which transforms a collimated, radially symmetric Gaussian beam to a flattop is described. To minimize diffraction of the output beam, the lenses were designed to accept essentially all (99.7%) of the input beam, and the output intensity distribution was chosen to have a controlled roll-off, given by the Fermi-Dirac function. Both aspheric surfaces were convex, simplifying fabrication by the technique of magnetorheological figuring. The optics were made of fused silica and, with suitable antireflection coatings, a single prescription can be used at any wavelength from 250 to 1550 nm. Measurements of the output intensity distribution were made by directly illuminating a CCD sensor with the flattop beam, and these results were compared quantitatively to the theoretical design. At the 8 mm diameter output aperture, 78% of the total beam power is contained in a region with 5% rms intensity variation, representing a fourfold improvement in power utilization over the Gaussian input. The beam propagates approximately 0.5 m without significant change in the intensity profile. Both the intensity uniformity and the propagation range are believed to be limited by the accuracy of the aspheric surfaces. It was verified that expanding the linear dimensions of the beam by the factor m increases by m<SUP>2</SUP> the range over which the beam retains its shape as it propagates. The optics have been successfully used in a holographic data storage test stand and for deep-UV interferometric lithography.
Holographic data storage allows the simultaneous search of an entire database by performing multiple optical correlations between stored data pages and a search argument. We have recently developed fuzzy encoding techniques for this fast parallel search and demonstrated a holographic data storage system that searches digital data records with high fidelity. This content-addressable retrieval is based on the ability to take the two-dimensional inner product between the search page and each stored data page. We show that this ability is lost when the correlator is defocussed to avoid material oversaturation, but can be regained by the combination of a random phase mask and beam confinement through total internal reflection. Finally, we propose an architecture in which spatially multiplexed holograms are distributed along the path of the search beam, allowing parallel search of large databases.
Volume holographic storage combines fast, parallel readout (because each hologram stores a large data page) with high density (because many holograms are multiplexed within the same volume). Phase-conjugate readout has been proposed as a way to eliminate the precision optics that recent demonstrations have relied upon to image the pixels of the input spatial light modulator (SLM) onto those of the output detector array. However, hologram multiplexing with the phase-conjugate approach requires multiple pairs of phase- conjugate beams, which are extremely difficult to create and maintain. We have developed a two-step recording process which combines the advantages of phase-conjugate holography with the simplicity of using the same multiplexed reference beam for recording and readout. The data-bearing object beam first passes completely through a long storage crystal, and is then temporarily stored in a second holographic storage material. This buffer hologram is immediately read with a phase-conjugate reference beam, reconstructing a phase- conjugate object beam which travels back into the storage crystal. This new object beam can now be recorded, and then later reconstructed, with a multiplexed reference beam at any of the spatial storage locations. We describe the advantages and limitations of this technique, the materials requirements for the buffer hologram, and describe a test platform designed to implement this technique.
Holographic storage has the potential to become a digital data storage technology with fast readout and high density. Computer users have come to expect, however, that data retrieved from their storage devices will be retrieved error- free (with a probability of error less than 10<SUP>-12</SUP>). In both conventional storage devices and holographic data storage, achieving this degree of reliability involves a good understanding of the data channel and a combination of careful hardware engineering, signal processing, and coding. At the IBM Almaden Research Center, we have leveraged the expertise acquired with 1-dimensional, time-dependent data channels found in magnetic and optical data storage systems, to develop unique and highly effective signal processing and coding algorithms to optimize the performance of the 2-dimensional, space-dependent digital holographic data storage channel. Crucial to our efforts has been the high-performance holographic data storage platform we built in 1996. This tool has allowed us to characterize and perturb a real holographic data channel, and implement and evaluate new data-coding and signal processing algorithms. This rapid feedback loop between ideas, implementation, and results both aids in selecting fruitful approaches and yields deeper understanding of the underlying data channel. In this paper, we discuss the holographic digital data storage channel as divided into five parts: the optical path, pre-processing (how the data gets into the holograms), post-processing (manipulation of raw data just after optical detection), conversion into binary 0's and 1's, and error-correction (using added redundancy). Optimizing the channel involves maximizing the system performance (density, speed) while minimizing complexity (and thus cost) and maintaining the required degree of reliability.
KEYWORDS: Signal to noise ratio, Point spread functions, Holograms, Holography, Digital holography, Data storage, Fourier transforms, Spatial light modulators, Charge-coupled devices, Volume holography
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.
SC565: Introduction to Refractive Laser Beam Shaping Optics
This course covers the design and use of refractive optics to transform the transverse intensity profile of a laser beam. Typically, beam shaping is used to convert the Gaussian profile emitted by a stable resonator or single-mode fiber to a more uniform, flat-top profile, but the formalism used is general enough to accomodate other profile transformations as well. The course describes the advantages of beam shaping and presents an overview of the many possible methods to perform the intensity profile transformation. It procedes to a detailed study of refractive beam shaping, including the choice of a suitable output intensity profile, limitations on beam shaping imposed by diffraction, and the calculation of the necessary aspheric refractive surfaces. Finally, we discuss practical issues pertaining to the use of beam shaping optics, such as input beam preparation, alignment of the beam shaping elements, diagnostics, propagation effects, dispersion, and the resizing and relaying of the output beam.