The Linear Perspective to Soliton Dynamics

Unlike linear waves, solitons create their own channel as they travel in a uniform medium, remaining localized and preserving their shape. Whereas linear waves always pass through one another, solitons can be dramatically altered by collisions. They can annihilate one another, fuse, or create multiple solitons. These phenomena turn out to be important to the emerging technology of light guiding light and light written circuitry. To pursue this technology, we need to understand how such waves interact in an arbitrary nonlinear medium.

Massive Soliton WDM Transmission at N × 10 Gbit/sec, Error-free Over Transoceanic Distances

We have demonstrated massive wavelength division multiplexing (WDM), over transoceanic distances, in multiples of 10 Gbit/sec. The vital ingredients to this success were first, solitons, second, sliding-frequency guiding filters, and third, the use of "dispersion-tapered" fiber spans between amplifiers, i.e., spans for which D(z) tends to follow (here in step-wise approximation), the same exponential decay profile as the signal energy. Although the first two ingredients and their benefits are by now well known, the third, at least in this context, is both novel and vital.

Bright Temporal Soliton-like Pulses in Self-defocusing Media

Bright temporal solitons are generating a great deal of interest because of their possible use in long distance optical fiber communications. They maintain their temporal shape by cancelling the detrimental effects of both the dispersion and the Kerr nonlinearity. Until recently, the only demonstration of bright temporal solitons has been in silica optical fibers, which possess a self-focusing nonlinearity (n2 > 0) and an anomalous dispersion (β2 < 0) for λ > 1312 nm. The nonlinear Schrödinger equation (NLS), which describes the propagation of an optical pulse through a nonlinear optical medium with chromatic dispersion, allows bright temporal solitons as long as the nonlinear refraction (n2) is of opposite sign to the dispersion (β2). Hence, a medium with normal dispersion and self-defocusing nonlinearity should also support bright temporal solitons.

New Semiconductor Materials Offer Promise for Ultra-fast Optical Devices

Future communications and computing systems will require advanced capabilities to handle the increasing requirements for ever-faster and higher-bandwidth operation. To move beyond the system limits imposed by electronic components, researchers are investigating the use of all-optical components for ultrafast operations such as switching and time-division multiplexing and demultiplexing.

Manakov Spatial Solitons

The Manakov soliton is a two-component soliton that was first considered by Manakov in the early 1970s. Based on the work of Zakharov and Shabat, Manakov found that the coupled nonlinear Schrödinger (CNSE) equations with special choice of the coefficients in front of nonlinear terms can be solved exactly. This system is inte¬grable and solitons have therefore a number of special properties which might be useful in practice.

Novel Resonant Structures for Laser Light Modulation

We have recently developed and demonstrated, for the first time, novel resonant grating/waveguide structures, that can modulate laser light at relatively high rates. We believe that these can be incorporated into arrays to form compact spatial light modulators operating at several hundred megahertz.

Optical Modulation with a Resonant Tunneling Diode

Since the discovery that the resonant tunneling diode (RTD) has sufficient negative differential resistance for practical devices most work has concentrated on entirely electronic devices for microwave applications and for sub-millimeter wave generation where oscillation at frequencies up to 720 GHz have been achieved. Recently we reported on an optoelectronic application of a resonant tunneling diode in which we used a simple direct integration scheme to achieve optoelectronic modulation from the electric field associated with the RTD, by embedding the RTD directly in an optical waveguide.

Optoelectronics Technology Consortium 32-Channel Parallel Fiber Optic Transmitter/Receiver Array Testbed

In the data communications industry, there are emerging requirements for a short distance (tens to hundreds of meters), high-speed (200 Mbit/sec-1 Gbit/sec) data bus for large computing environments, clustered parallel computing systems, and datacom switching. In response to these requirements, a parallel optical fiber interconnect has been developed by the Optoelectronics Technology Consortium (OETC), an ARPA-funded industry alliance including IBM, AT&T, Honeywell, and Lockheed Martin. This year, IBM completed testing of a 32-channel OETC fiber optic transmitter/receiver array in a product testbed, and announced future availability of a commercial product called "Jitney" based on the OETC prototype.

Opto-Electronic Microwave Oscillator

Photonic applications are important in RF communication systems to enhance many functions including remote transfer of antenna signals, carrier frequency up or down conversion, antenna beam steering, and signal filtering. Many of these functions require reference frequency oscillators. However, traditional microwave oscillators cannot meet all the requirements of photonic communication systems that need high frequency and low phase noise signal generation. Because photonic systems involve signals in both optical and electrical domains, an ideal signal source should be able to provide electrical and optical signals. In addition, it should be possible to synchronize or control the signal source by both electrical and optical means.

Simultaneous Laser Diode Emission and Detection for Optical Fiber Sensors

Reflective fiber optic intensity sensors often use a coupler to guide part of the reflected light back to a photodetector. We have demonstrated a sensor that requires no coupler, and instead uses the emitting laser diode for photodetection. A laser diode operating at constant current can detect light reflected into the junction region if the terminal voltage is monitored. We used a self-detecting source to sense rotation using a simple magneto-optic transducer and single-fiber sensor system.

Compact and Parallel Free-Space Optoelectronic Interconnection and Logic Operations with Optical Thyristors

For several years there has been a realization that electronics is facing limits in both the speed and parallelism that may be achieved with conventional wiring. This is particularly apparent for chip-to-chip and board-to-board interconnects. Optics has been widely studied as a suitable high-speed interconnection medium to overcome this interconnection bottleneck. It is attractive due to its better immunity to capacitative and inductive crosstalk, signal dispersion, and electromagnetic interference.

Three Dimensional Reconstruction of Random Radiation Sources

The degree of the spatial coherence of an electromagnetic field is a useful function mainly for two reasons. First it provides information on the spatial coherence of light sources. Second, due to the Van Cittert-Zernike theorem, knowledge of the coherence distribution induced by a source, enables one to compute its shape. Explicitly, it is manifested in this theorem that the two-point degree of coherence in the far field of a quasi-monochromatic, spatially incoherent light source is proportional to the Fourier transform of the source's planar intensity distribution. Therefore, by measuring the two-point degree of coherence in the far field, one can image the source distribution. This imaging technique is, among others, the theoretical basis of the very long base-line interferometers used in astronomy. However, this technique has been limited to imaging of planar two-dimensional objects.

Room Temperature, Mid-Infrared Quantum Cascade Lasers

The quantum cascade (QC) laser1 is a new optical source in which one type of carrier, typically electrons, cascading down an electronic staircase...

Achievement of the Saturation Limit and Energy Extraction in a Discharge Pumped Table-Top Soft X-ray Amplifier

Amajor goal in ultrashort wavelength laser research is the development of compact "table-top" amplifiers capable of generating soft X-ray pulses of substantial energy that can impact applications. Such development motivates the demonstration of gain media generated by compact devices, that can be successfully scaled in length to reach gain saturation. At this condition, which occurs when the laser intensity reaches the saturation intensity, a large fraction of the energy stored in the laser's upper level can be extracted. To date, gain saturation had only been achieved in a few soft X-ray laser transitions in plasmas generated by some of the world's largest laser facilities.

Multiple-wavelength Vertical Cavity Laser Arrays with Wide Wavelength Span and High Uniformity

Vertical-cavity surface-emitting lasers (VCSELs) are promising for numerous applications. In particular, due to their inherent single Fabry-Perot mode operation, VCSELs can be very useful for wavelength division multiplexing (WDM) systems allowing high bandwidth and high functionalities. Multiple wavelength VCSEL arrays with wide channel spacings ( 10 nm) provide an inexpensive solution to increasing the capacity of local area networks without using active wavelength controls.

New Techniques in Wideband Terahertz Spectroscopy

In recent years, remarkable progress has been made in the development of spectroscopic capabilities for coherent terahertz (THz) measurements. This spectral region is one of great interest because of the abundance of excitations in molecular systems and condensed media. It also represents a region in which the dielectric properties of materials are of critical importance for high frequency electronics and optoelectronics. A key ingredient to the significant advances in this field is the development of broadband, optically driven sources and detectors of terahertz radiation. The ready availability of laser pulses with durations of ~10 fsec suggests the potential for extending the bandwidth of coherent spectroscopy to significantly higher frequencies. By using materials with an instantaneous nonlinear optical response for both emission and detection, we may be able to capture much of this enormous bandwidth.

Interferometric Optical Tweezers

Optical trapping of micron-size, dielectric microspheres using a single beam gradient force (Fig. la) was first demonstrated by Ashkin in 1986. Since then, extensive research and development of this technique has turned it into a practical device (known as optical tweezers) which has been used in a wide variety of biological and biomedical applications.

Optical Patterning of Three-Dimensional Spatio-Tensorial Micro-Structures in Polymers

One challenging requirement for the design of devices for photonic applications is to achieve complete manipulation of molecular order. The great latitude and flexibility of optical methods offers interesting prospects for material engineering using light-matter interactions. Efficient spatial modulation of polymer macroscopic properties is usually achieved using holographic recording of an interference pattern between intense light-waves. For second-order optical nonlinear processes, a full control of the molecular orientation is mandatory. However, patterning with polarized monochromatic beams results only in molecular alignment. We report on a new, purely optical technique based on a non-classical holographic process with coherent mixing of dual-frequency fields. It enables efficient and complete three-dimensional spatio-tensorial control of polymer micro-structures.

Spontaneous Density Grating Formation in Hot Atomic Vapor

Recently a new gain mechanism has been observed in a nonlinear optical system: The spontaneous formation of a density grating in an atomic vapor through interaction with a strong pump field. A sodium filled cell is pumped by a high intensity (I 104 W/cm2) circularly polarized laser beam detuned from resonance and is probed by a weak field degenerate in frequency with the pump and with the same polarization. The probe beam is introduced into the cell in two different geometrical configurations: Nearly parallel (angle 5°) and nearly antiparallel (same angle, but opposite direction) to the pump. For sufficiently high pump intensity, and for appropriate values of detuning and atomic density, the probe beam displays a gain as large as 30% (pumping only a small fraction of the probe cross section) at the expense of the pump, only in the nearly counterpropagating geometry.

Single-Atom Quantum Logic Gate and Schrödinger Cat State

One of the fundamental tenets of quantum mechanics is the existence of superposition states, or states whose properties simultaneously possess two or more distinct values. Although quantum superpositions and entanglements seldom appear outside of the microscopic quantum world, there is growing interest in the creation of "big" superpositions and massively entangled states for use in applications such as a quantum computer. We report first steps toward this goal by demonstrating a fundamental two-bit quantum logic gate and a "Schrödinger cat"-like state of motion with a single trapped 9Be+ ion. Both experiments allow sensitive measurements of decoherence mechanisms which will play an important role in the feasibility of quantum computation.

Polarization-entangled Photons and Quantum Dense Coding

Entangled states of particles form the cornerstone of the newly emerging field of quantum information: they are central to tests of nonlocality, have been proposed for use in quantum cryptography schemes, and would arise automatically in the operation of quantum computers. Polarization-entangled photons are preferable because they are easier to handle.

Excitation of a Schrödinger Cat State Within an Atom

Experiments in a number of laboratories over the past few years have explored the classical limit of a single atom. In this limit, the electron wave function takes on the form of a spatially localized wave packet moving with the classical orbital period around a classical Keplerian orbit of near macroscopic dimensions. In various experiments the diameter of this orbit ranges from approximately 100 to 100,000 nm. The behavior of the atom, even in this limit, is quite rich, displaying a range of classical as well as distinctly quantum features.

Excess Quantum Noise Fluctuations in Unstable-resonator Lasers

Experiments completed during the past year confirm the existence of a sizable excess quantum noise factor in lasers using unstable optical resonators or, more generally, resonators with nonorthogonal oscillation modes.

Self-Trapping of Partially Spatially Incoherent Light Beams

Here, we report the first observation of self-trapping of a "partially" spatially incoherent optical beam in a nonlinear medium. Self-trapping occurs in both transverse dimensions, when diffraction is exactly balanced by photorefractive self-focusing. We have used the photorefractive nonlinearity associated with photorefractive solitons as the self trapping mechanism and generated a stable, two-dimensional, 30-μm wide, spatially incoherent self-trapped beam.

Supramolecular Enhancement of Second-Order Optical Nonlinearity

Only noncentrosymmetric molecules can possess a second-order nonlinear response, i.e., they have a nonvanishing first molecular hyperpolarizability. Polar molecules with donor and acceptor groups connected by a conjugated π-electron system are traditional organic second-order materials (Fig. 1). For macroscopic noncentrosymmetry, such molecules are poled in a host material using a static electric field. The nonlinear coefficients of poled materials are proportional to μβ where μ is the permanent dipole moment of the molecules and β is the vectorial part of the first hyperpolarizability.

Stopping Light in its Tracks

To control the speed of a light pulse without absorbing its photons, or distorting its shape, is a challenging problem. However, this has been accomplished using fiber gratings, as part of a joint research program of the University of Sydney, the Australian Photonics Research Centre, Lucent Technologies, and the University of Toronto.

Isotropic Liquid Crystal Fiber Structures for Passive Optical Limiting of Short Laser Pulses

Ever since the invention of the laser, there has been a need to protect the eye or sensitive optical sensors from damage by overexposure. The problem has become increasingly difficult with the advent of frequency agile high power pulsed lasers, which negate fixed line filters or optoelectronics/mechanical devices; all-optical or nonlinear optical means have to be used. In this context, various device concepts and nonlinear optical materials are being investigated. To satisfy such stringent requirements, it has become necessary to optimize both the device function and the material responses by specialized optical configurations. One means of achieving this is to use fiber or waveguide geometry in which highly intensity dependent (optical limiting) processes occur more efficiently due to the spatial confinement over distances much longer than the Rayleigh range of tightly focused lasers.

Texture in Binary Images

IImage texture is one of the important parameters in the field of digital image processing. In displayed images, it affects the reproduction of the local average gray level, because usually there is a certain amount of pixel overlap. In image perception, it may result in the appearance of false contours between regions with different textures. There is a demand for a quantitative description of textural characteristics in the various fields of digital image processing, of which digital halftoning is one.

Photonic Signal Processing for Biomedical and Industrial Ultrasonic Probes

Ultrasonics has been widely used in medical, indus¬trial, and scientific applications. In medical applications, ultrasonics is an essential diagnostic method in internal medicine, urology, and vascular surgery. High-Intensity Focussed Ultrasound (HIFU) and lithotripsy applications use relatively low ultrasonic frequencies (< 100 KHz), while a 5-15 MHz band is typically used in diagnostic external cavity imaging ultrasound. Today, with endoscopic applications in mind, a very high ultrasonic frequency, e.g., 100 MHz, probe with high (> 50%) instantaneous bandwidths is highly desirable as higher frequencies give higher imaging resolution and smaller physical dimensions of the front-end intracavity transducer array.

Atomic Lifetimes From Molecular Spectroscopy

Although molecular properties are clearly related to the properties of the constituent atoms, it has seldom been possible to make precision measurements of these atomic properties by examining the molecules. Over the last year or so, however, molecular spectroscopy has been shown to be a powerful technique for determining atomic lifetimes and has provided the most precise alkali lifetimes yet reported, at levels ranging from 0.3% to 0.03%.

Multiphoton Ionization with Precise Intensity Control

In the presence of strong laser fields (> 1012 W/cm2), atoms and molecules can simultaneously absorb many photons to exceed the ionization limit, leading to the ejection of photoelectrons. The analysis of photoelectron kinetic energy spectra provides valuable insight into atomic and molecular structures. The kinetic energy can be determined by measuring the time-of-flight of the electrons over a known distance.

Atomic Streak Camera Sees Rydberg Atoms Falling Apart

Highly excited or Rydberg atoms are an ideal quantum laboratory. In a Rydberg atom, the loosely bound electron moves in a large Kepler orbit around the atomic nucleus and is very sensitive to external perturbations. For instance, by applying a moderate electric field, the behavior of the quantum system is drastically influenced. A static field of a few Kilovolts per centimeter is sufficient to change the bound Rydberg atom into a system in which the electron can escape. Within a few picoseconds (10-12 sec) the atom falls apart. It is an experimental challenge to detect how this decay actually happens. Does the electron come out immediately, or does the atom emit the electron in subsequent bursts of probability, that are signatures of the quantum nature of the system?

Bragg Scattering from an Optical Lattice

For many years the structure of solid crystals has been studied by Bragg scattering of x-rays. X-rays, with wavelengths on the order of a few tenths of a nanometer (a few angstroms) irradiate a crystal where the distance between the atoms is also a few angstroms. Only when the wavelength, atomic spacing, and angles are such that there is constructive interference of the scattering of x-rays from the regularly spaced planes of atoms does one see the strong reflection of x-rays characteristic of Bragg scattering.

Two-dimensional Photonic Bandgap Structures at 850 nm

The long-predicted benefits of photonic bandgap (PBG) technology, such as complete control over the spontaneous emission of an excited atomic system, are starting to become possible. Following initial doubt about the technological feasibility, a full bandgap was demonstrated at microwave frequencies several years ago.

Nanoparticle-Enhanced Photodetection

A properly placed layer of metal nanoparticles can increase the optical absorption within a thin photodetector.1 Acting much like an array of microscopic antennas, the particles collect a fraction of the light that falls within their resonance bandwidth, coupling it into guided modes within the photodetector layer.

Massive Soliton WDM Transmission at N × 10 Gbit/sec, Error-free Over Transoceanic Distances

We have demonstrated massive wavelength division multiplexing (WDM), over transoceanic distances, in multiples of 10 Gbit/sec.1 The vital ingredients to this success were first, solitons, second, sliding-frequency guiding filters, and third, the use of "dispersion-tapered" fiber spans between amplifiers, i.e., spans for which D(z) tends to follow (here in step-wise approximation), the same exponential decay profile as the signal energy.

Intracavity Phase Modulated Transmitter for Hybrid Lidar-Radar

This paper discusses the development of a microwave-modulated transmitter using a bulk phase modulator for a novel hybrid lidar-radar application. Aerial lidar (light detection and ranging) is used for underwater surveillance. A pulse of blue-green optical radiation is transmitted from an airborne platform, and target information is extracted from the detected echo. Attenuation, dispersion, backscatter clutter, and particularly the lack of coherent signal processing, limit the performance of lidar.

An Intuitive User Interface for Remote Adjustment of Optical Elements

As part of an ongoing effort to improve the imaging of the Multiple Mirror Telescope (MMT) south of Tucson, our group is concerned with developing adaptive techniques. When a star's light passes through the earth's turbulent atmosphere, the wavefront is distorted and the imaging of a ground-based telescope suffers. The Center for Astronomical Adaptive Optics (CAAO) builds instruments which measure and correct for this wavefront aberration. The wavefront sensing part of the instrument was rebuilt using 18 Picomotors™ to simplify alignment.

Nonlinear Optics Using Atomic Coherence Effects

Nonlinear optical mixing of existing laser frequencies to access portions of the spectrum where lasing action is not easily obtainable is common practice today. Various techniques, including important new ones like quasi-phasematch¬ing in tailored nonlinear-media are aimed at efficient generation in the region of the spectrum from just under 200 nm in the ultraviolet to about a few microns in the infrared.

1997 Funding for R&D Up: Poised to Plummet toward 2002

The results are in for 1997 federal appropriations for science. According to AAAS, Congress appropriated $74 billion for R&D—an increase of 4.0% from last year. About $14.8 billion of the total goes for basic research, an increase of 2.7%. R&D funding kept ahead of inflation, but the slope downward will have to get steeper to balance the budget by 2002.

New Terahertz Beam Imaging Device

Recently a new electro-optic detection system has been used to characterize the temporal and spatial distribution of free-space broadband, pulsed electromagnetic radiation (THz beams). This detection system, which uses an electro-optic crystal sensor, provides diffraction-limited spatial resolution, femtosecond temporal resolution, DC-THz spectral bandwidth, and sub¬milli-volt per centimeter field detectability. The sensitivity and bandwidth of the electro-optic detectors are comparable or superior to conventional ultrafast photo-conductive dipole antennas and liquid helium cooled bolometers. Advantages intrinsic to electro-optic detection include nonresonant frequency response, large detector area, high scan rate, low optical probe power, and large linear dynamic range.

Capital Eye: 1997 Funding for R&D Up: Poised to Plummet toward 2002

New Terahertz Beam Imaging Device

A New Technique for Remote Sensing

New Focus Student Essay Winners: Intracavity Phase Modulated Transmitter for Hybrid Lidar-Radar

An Intuitive User Interface for Remote Adjustment of Optical Elements

Nonlinear Optics Using Atomic Coherence Effects