Integrated holographic filters for flat passband optical multiplexers.

Lithographically rendered, slab-waveguide-based, volume holographic filters are shown, via fabrication and test, capable of providing fully integrated, single-mode compatible, flat-topped and low loss filtering for wide bandwidth multiplexers as, for example, used in coarse wave-division multiplexing (CWDM). Single-mode compatibility is preserved since the filters operate via multi-path interference like thin-film filters rather than the angular dispersion typically utilized by grating type devices. Flexible apodization, entirely consistent with simple binary etch, is employed to provide steep passband falloff. High reflectivity and wide bandwidth is enabled through a tailored dual core waveguide geometry providing for mode concentration on the diffractive elements.

Diffraction-limited performance of flat-substrate reflective imaging gratings patterned by DUV photolithography.

We report on the first demonstration of flat substrate imaging gratings fabricated by deep ultraviolet (DUV) photoreduction lithography, which uniquely offers sub-100-nm resolution and spatial coherence over centimeter scales. Reflective focusing gratings, designed according to holographic principle, were fabricated on 300-mm silicon wafers by immersion DUV lithography. Spatial coherence of the fabrication process is evident in measured diffraction-limited imaging function. Flat-substrate gratings, with lines of arbitrary spacing and curvature, offer both dispersion and general spatial wavefront transformation combining the function of multiple optical elements. Fabrication at the sub-100-nm resolution level allows high-line-count, low-order efficient gratings even into the deep ultraviolet region. Nanoreplication of gratings at the wafer level provides a pathway to devices of ultimate low cost.

Planar-waveguide integrated spectral comparator.

A cost-effective yet robust and versatile dual-channel spectral comparator is presented. The silica-on-silicon planar-waveguide integrated device includes two holographic Bragg-grating reflectors (HBRs) with complementary spectral transfer functions. Output comprises projections of input signal spectra onto the complementary spectral channels. Spectral comparators may be useful in optical code-division multiplexing, optical packet decoding, spectral target recognition, and the identification of molecular spectra. HBRs may be considered to be mode-specific photonic crystals.

Instability and pulse area quantization in accelerated superradiant atom-cavity systems.

We study the interaction of pulsed excitation fields with optically thin atomic ensembles subject to strong cavity-accelerated superradiance. In homogeneously broadened atom-field systems the excitation of superradiant instabilities is seen to lead to a quantization of the total pulse area accrued during excitation and subsequent superradiant decay. In inhomogeneously broadened systems, quasidiscretized area values and pulse area gaps (i.e., forbidden area values) are found. Predicted area discontinuities are demonstrated in cryogenically coherence-stabilized Tm(3+) ions.

Planar holographic optical processing devices.

Time-domain optical processing implemented through linear spectral filtering offers unique potential for future high-bandwidth communications systems. One key to realization of this potential is the development of robust, cost-effective, fully integrated filtering devices. A new spectral filtering device concept, derived from the unique properties of index holograms stamped or otherwise written in thin planar waveguide slabs, is described. The holograms that are described provide for high-resolution spectral filtering while at the same time mapping general input spatial waveforms to desired output waveforms.

Spatially distributed spectral storage.

A new technique for overcoming the effects of excitation-induced frequency shifts in swept-carrier optical memories is proposed and demonstrated. Spectral storage capacity increases of more than 2 orders of magnitude were achieved through implementation of the method.

Laser frequency stabilization by means of optical self-heterodyne beat-frequency control.

We propose and demonstrate a novel frequency-stabilization scheme that provides linewidth narrowing and at the same time is compatible with high-speed frequency agility. The method relies on sensing and control of a heterodyne beat signal derived from a fiber interferometer and functions in the absence of a fixed reference frequency. Our demonstration utilizes a short-external-cavity diode laser equipped with an intracavity electro-optic crystal for frequency correction. The stabilization method is shown to suppress impressed laser frequency modulation by nearly 2 orders of magnitude. Without further modification of our scheme we also demonstrate laser frequency tuning control.

Experimental observation of photon echoes and power-efficiency analysis in a cavity environment.

We present what is, to our best knowledge, the first experimental demonstration of echo signals generated from a sample in an optical cavity. The configuration allows us to use an optically thin sample and still have high effective absorption as well as relatively uniform excitation in the longitudinal direction within the sample. We observed echo signal power of up to twice that of certain input fields. The observed efficiency is much greater than the normally expected few percent. By using an input field encoded with a bit sequence, we also show that the cavity does not introduce significant distortion into the reproduced signal.

Simple high-coherence rapidly tunable external-cavity diode laser.

We describe a simple electro-optically activated external-cavity diode laser designed to provide high-speed, high-coherence tuning over gigahertz frequency ranges. Tuning as fast as 23 GHz/ micros is demonstrated. Coherence measurements indicate transform-limited quiescent laser linewidth in observation windows as wide as ~100 micros and transform-limited chirps. A self-heterodyne, cross-correlation-based coherence diagnostic is developed for characterizing phase coherence during high-speed chirps.

Single-sideband spectral holographic optical memory.

Spectral holography provides for the storage of data-encoded, time-domain optical signals in frequency-selective recording materials. This capability may form the basis for entirely new classes of ultrahigh-speed and -density optical storage devices. We demonstrate the implementation of swept-carrier spectral-holographic data storage in a single-sideband mode. Temporal-waveform information originally encoded in doublesideband format is re-created but stripped of one of its sidebands. The process described can be used to achieve higher spectral storage densities or as an optical single-sideband conversion device.

Experimental demonstration of swept-carrier time-domain optical memory.

Swept-carrier time-domain optical memory [Opt. Lett. 17, 535 (1992)] has been proposed as a means of storing optical data streams in inhomogeneously broadened spectral recording materials. The process provides for full utilization of the material storage bandwidth regardless of the data rate and permits storage of data sequences with durations exceeding the material dephasing time. We report what we believe is the first experimental demonstration of this storage process. Our experiments, implemented with frequency-swept diode lasers and Tm(3+):YAG storage material, demonstrate single-event storage of 98-bit data streams whose duration is an order of magnitude longer than the material dephasing time.

Spatial routing of optical beams through time-domain spatial-spectral filtering.

We propose a novel new method of temporal-waveform-controlled high-speed passive spatial routing of optical beams. The method provides for the redirection of optical signals contained within a single input beam into output directions that are specified entirely by temporal information encoded on the waveform of each incident signal. The routing is effected by means of deflection from spectrally structured spatial gratings that may be optically programmed into materials with or without intrinsic frequency selectivity.

Heterodyne detection of swept-carrier frequency-selective optical memory signals.

Swept-carrier time-domain optical memories [Opt. Lett. 17, 535 (1992)] are unique in that memory output signals occur cotemporally with the excitation field (read field) responsible for their generation. We demonstrate that this fact provides for the convenient detection of memory signals through their heterodyne interference with the cotemporal read field. Memory implementation is dramatically simplified.

Demonstration of 8-Gbit/in.(2) areal storage density based on swept-carrier frequency-selective optical memory.

Using swept-carrier frequency-selective optical data storage techniques to record multikilobit-long data streams at single spatial locations, we have achieved an areal storage density of 8 Gbits/in.(2) and a density-input/output bandwidth product of 1.5 x 10(17) bits/(in.(2) s). The latter quantity is among the highest demonstrated in the context of nonparallelized two-dimensional optical or magnetic recording systems. Extrapolation of the present results to full material utilization suggests an achievable storage density of the order of 100 Gbits/in.(2).

Optical bit-rate conversion and bit-stream time reversal by the use of swept-carrier frequency-selective optical data storage techniques.

We have demonstrated real-time all-optical bit-rate conversion and bit-sequence time reversal, using a variation of the swept-carrier frequency-selective optical storage/retrieval process. Working in the Mbit/s regime, we obtained order-of-magnitude bit-rate enhancements and reductions. Output signals appear in a time-forward or time-reversed format after an adjustable buffering storage time. Analysis of the method of its potential indicates applicability at substantially higher data rates.

Swept-carrier time-domain optical memory.

A new time-domain approach to frequency-selective optical data storage is described. In this approach, a spatial storage cell is exposed to two cotemporal, spatially angled, frequency-chirped excitation fields whose duration can greatly exceed the homogeneous dephasing time of the storage material employed. The two beams have a fixed frequency offset, and one is temporally encoded with data. The new method averts several of the drawbacks of existing approaches to time- and frequency-domain optical memories.

Use of phase-noisy laser fields in the storage of optical pulse shapes in inhomogeneously broadened absorbers.

It has been demonstrated [see, e.g., W.R. Babbitt and T. W. Mossberg, Opt. Commun. 65, 185 (1988)] that coherent optical processes can be employed to store and reproduce temporal sequences of optical data, thereby providing a mechanism for advanced optical memories. We find that excitation pulse phase noise can be used to extend the range of experimental conditions under which the storage process is effective and discuss the use of phase noise to achieve secure data storage.