The Optical Vortex Soliton

Until recently, optical solitons with two transverse degrees of freedom were not known to exist, and thus, nonlinear optical engineering often required the use of waveguides to constrain diffraction. Catastrophic beam collapse can occur once this constraint is relaxed in a self-focusing system. In a self-defocusing system, on the other hand, a new class of soliton phenomena, including dark soliton stripes and grids,1 and optical vortex solitons,2 have been observed. Because these effects occur in a bulk medium, without the constraints of a waveguide, new techniques may be possible, especially in the areas of planar spatial light modulation, image processing, optical switching, and optical interconnection.

Phase Sensitive Amplifiers for Ultra-long Distance Soliton Propagation

Lumped erbium-doped fiber amplifiers have been demonstrated as effective devices for compensating loss in long-distance soliton communication systems. More recently, various filtering schemes have been proposed as a way of reducing the Gordon-Haus jitter present in such systems, thereby increasing the maximum allowable bit-rate distance product. As a possible alternative to erbium-doped amplifiers, the use of lumped phase-sensitive parametric amplifiers has been proposed. A chain of such amplifiers should lead to a higher bit-rate-distance product because no spontaneous emission noise is present.

A New Optical Model of the Human Eye

The optical apparatus of the eye represents a low-pass spatial filter at the front end of the visual system that is a potential limiting factor for any visual task. Apart from defocus, which the eye automatically corrects by a reflexive change in optical power of its internal lens, the major optical factors limiting visual performance are chromatic aberration, spherical aberration, and diffraction at the pupil.1 To help gauge the potential magnitude and significance of these optical factors, and to provide a useful design element for optical engineers, we have developed an accurate, mathematically tractable model of the human eye that takes account of these three primary factors.

Afterimages and Multiplexing

There is much current interest in the functions of parallel pathways in vision and, in particular, the question of where and how the opponent-color and luminance signals are formed. According to several lines of evidence, chromatic and achromatic signals both share a common pathway from the retina to the brain; namely, the "sustained" or parvocellular pathway. Our recent results imply that the same multiplexing arrangement also holds for chromatic and achromatic afterimages, surely the most "sustained" phenomena in normal vision.

The Telescopes of Galileo

Moiré methods are a versatile set of techniques for in-plane and out-of-plane displacement and deformation measurement, topographic and shape determination, and slope and curvature contouring. The basis of these moire methods are gratings. Deformation of the specimen cause distortions in the grating lines. Visualization of these distortions are enhanced using the moire effect that is achieved by superposing the deformed specimen grating on a reference grating representing the undeformed state of the specimen.

Time-reversal Operator for the State of Polarization

The concept of an Ortho-Conjugated Mirror (OCM), or Faraday Rotator Mirror, was first demonstrated in 1989 and has recently found application in the field of fiber optic circuits1 and fiber optic current sensors. The OCM polarizes the light and conveys the time-reversal properties displayed by the Phase Conjugated Mirror (PCM) for the optical phase. Both devices effectively retrace optical paths, i.e., circuits where light maintains a mirror symmetry, and both devices return the light in the opposite state, which means reverse propagation direction for the PCM and orthogonal state of polarization (SOP) for the OCM.

Magnetorheological Finishing

Magnetorheological (MR) suspensions, used in optical manufacturing, constitute a new and novel optical finishing technology. These suspensions consist of micron-sized magnetic particles in a carrier liquid and are representative of a class of "intelligent" fluids, i.e. media with controllable properties. They can exhibit rapid and reversible changes in their structure. As an example, the rheological property of viscosity varies by over two orders of magnitude with the application of a magnetic field. The ability to induce changes in mechanical properties has been used in a variety of actuators from vibration isolators and shock absorbers to robotic positioning devices by the Byelocorp Scientific staff, and now to the figuring and finishing of optics.

Holographic Laser Radar

Holographic Laser Radar (HLR) is an emerging method for performing fine-resolution, 3-D imaging. It uses a frequency-tunable laser source, electronic holography recording methods and digital Fourier processing to recover 3-D representations of imaged targets. The resulting images are useful primarily for inspection applications.

Free-space WDM Optical Mesh-connected Bus Interconnect

Have you ever wondered how the above-ground public transportation in a crowded urban area such as New York city is handled? The next time you visit "the Big Apple," note that most of the city buses follow routes along either east-west oriented streets or north-south oriented avenues in Manhattan. By taking three consecutive bus rides (turning corners twice), you can reach any destination in this mesh-structured city. The concept behind this transportation pattern can be found in a communication network, commonly known as the Clos 3-stage network. The mesh-connected bus (MCB) topology is one way of laying out the Clos network in a 2D format. It has been shown that any permutation can be arranged, without experiencing internal blocking, in three global routing steps; permuting along the row buses, then along the column buses, and finally along the row buses.

30 dB Contrast GaAlInAs Multiple Quantum Well Asymmetric Reflection Modulator at 1.3 µm

High-speed optical communication interconnects and networks will require the use of high-contrast optical processing elements to fully use the bandwidth available in state-of-the-art optical fibers. Semiconductor quantum well modulators may be used for such applications. We have demonstrated an all-optical GaAlInAs/AlInAs multipe quantum well (MQW) asymmetric reflection modulator for use at 1.3 μm with an on/off contrast ratio exceeding 1000:1 and an insertion loss of 2.2 dB at a pump intensity of 30 kW/cm2, corresponding to a carrier density of 4.5X1017 cm-3. The recovery time of the modulator is measured at ~ 725 psec, indicating that the operating speed of the device approaches 1 GHz.

2-D WDM Optical Interconnections Using Multiple-wavelength VCSELs for Simultaneous and Reconfigurable Communication Among Many Planes

The ability to efficiently connect many high-speed ports or "smart" pixels is of critical importance for large-capacity communications. High bandwidth, 2-D optical planes can be used to achieve such an interconnection and avoid eventual electronic bottlenecks. However, existing solutions do not efficiently resolve a situation in which one plane is required to simultaneously and reconfigurably communicate with many subsequent planes. One previous solution involves etching large holes in each plane's substrate establishing only a predetermined static configuration between any two planes.

Acousto-optic Switch Matrices

The vast information capacity of optical fiber is widely touted, but substantial problems remain—multiplexing many sources of data onto the same optical carrier. The technique of time division multiplexing involves sorting and packing bits into ever smaller time slots as the information rate increases—something that becomes ever more difficult with increasing capacity. A very different two-tiered approach is to subdivide the useful optical spectrum into many independent wavelength channels, each of which carries a more manageable electronic information bandwidth.

High-Repetition Rate Broadly Tunable Femtosecond Sources

High-repetition rate, broadly tunable femtosecond sources are particularly important for the study of ultrafast processes because high repetition rates yield higher signal-tonoise ratios in such experiments, and because broad tunability allows a greater variety of materials and processes to be studied. Since the first demonstration of the broadly tunable femtosecond optical parametric oscillator (fsec OPO) several years ago, dramatic advances in the development of such sources have occurred. Initially, only the Rh6G fsec dye laser was available as a pump source. This made the operation of the fsec OPO very difficult and the output power relatively low. With the development of the relatively high-power modelocked Ti:sapphire lasers, the situation has changed fundamentally. Using the fs Ti:sapphire laser as a pump, high repetition rate fsec OPOs producing hundreds of milliwatts broadly tunable from 900 nm to over 3.5 μm have been demonstrated. This development paved the way for the introduction of commercial fsec OPOs.

High-Order Harmonic Generation

Extremely high-order harmonic radiation generated from the interaction of intense (>1014 W/cm2) subpicosecond laser pulses with dense gases offers research scientists a new source of coherent soft x-ray radiation. Harmonic radiation extending to the 33rd harmonic of 1064 nm radiation was first reported in 1989. However, during the past year the most dramatic developments in this new area of nonlinear optics have occurred. Harmonic radiation extending below 7 nm was reported by several groups, many of the observations were well described by both quantum and classical theories, the coherence of this XUV source was demonstrated, and coherent control of the output XUV polarization using a second field of different frequency was shown.

Uncooled Lasers for Deployment of Fiber in the Loop

Among other factors, the wide-spread deployment of fiber in the loop (FITL) is hindered by the lack of availability of low-cost laser transmitters that emit at 1.3 μm and operate reliably over a temperature range of —40° to 85°C. To date, commercial laser transmitters have relied on thermoelectric (TE) coolers to maintain the laser temperature constant against the variations in the ambient temperature. However, the TE cooler and associated controller add substantial costs to the laser transmitter. A significant new advancement over existing approaches has been made by prototyping a laser that incorporates a strained quantum well structure based on the AlGaInAs/InP material system.

Single-mode Large-aperture Vertical Cavity Surface Emitting Laser

Vertical cavity surface emitting lasers (VCSEL) are promising for optical interconnects, communications, and signal processing. The most promising aspect lies in the prospect of eliminating low yield and high cost laser fabrication steps such as wafer lapping, cleaving, dicing, and facet coatings. Moreover, the topology of a vertical cavity facilitates on-wafer testing, pre-process screening, and the fabrication of large 2D arrays.

Emission Linewidth in Semiconductor Microcavity Lasers

The lower limit of the emission linewidth in lasers is determined by the spontaneous emission into the laser mode. This spontaneous emission coupling can be modified through the design of the laser structure. In semiconductor microcavity lasers, coupling efficiencies up to 30% have been achieved. Generally, the spontaneous emission rate depends on the photon emission probability of the active material and of the density of states (DOS) of the corresponding photon modes (in semiconductors it depends on the product of the electron-hole occupation probabilities and the carrier density of states). Whereas the carrier DOS in semiconductors is strongly influenced by many-body effects in the electron-hole plasma, the photon DOS is determined by the laser structure.

Noise Gratings Recorded in Silver Halide Volume Holograms

With recent advances in holographic technology, many types of holographic optical elements (HOEs) are being used in different optical systems. Typically these HOEs are recorded as volume phase holograms. Bleached photographic emulsions are an important medium for making volume phase HOEs because of the relatively high sensitivity and ease of processing of the material, improved processing chemistries, and the repeatability of results. However, photographic emulsions consist of fine-grained silver halide crystals suspended in a gelatin base, so the effect of noise gratings on holograms recorded in this material is significant.

Resonance- domain Diffractive Optics

Momentum Gaps and Laser Stability

Gaps that appear, through resonant interactions, in the dispersion relation of a particle or guided wave have had a rich history in areas of physics ranging from the study of solids to that of guided wave phenomena. Kogelnik and Shank1 first outlined and applied the concept to laser structures and coined the term distributed feedback (DFB) to describe lasing near a Bragg resonance. DFB lasers have found applications in many areas of optics and optoelectronics, particularly those that require single-mode, narrow linewidth operation.

Optical Detection of Atoms Near Surfaces

Since the advent of lasers in the field of atomic or molecular spectroscopy, the investigation of small concentrations of particles a few Angstroms apart from metallic or dielectric surfaces has been a challenge. We have developed a new laser-based method that combines two-photon high-resolution spectroscopy with organized monolayer film growth on planar (≤ λ/1000) surfaces. Layers of organized fatty acid films are prepared on epitaxially grown metal films on mica, and atoms adsorbed on top of the fatty acid films are examined spectroscopically in an ultrahigh vacuum (UHV), p0 ≤ 10-10 mbar, environment.

Detection of Minority Species in Microdroplets: Enhancement of Stimulated Raman Scattering

Spontaneous Raman scattering has served as a useful spectroscopic technique since its discovery. However, the weak signal prevents its application in dynamic environments. Moreover, the Raman spectrum can be overwhelmed by fluorescence from even trace impurities. Stimulated Raman scattering (SRS) is intense, but a minimum sample length is needed to provide the Raman gain. SRS is useful only in detecting majority species because of depletion of the pump laser by the SRS from the strongest-gain Raman mode.

Optical Detection of Magnetic Resonance of a Single Molecular Spin

Toward Ultrafast Movies of Moving Atoms

By combining ultrashort laser pulses techniques with scanning tunneling microscopy (STM), we have developed an instrument that obtains simultaneous 2 psec time resolution and 50 Å spatial resolution. This is a nine orders of magnitude improvement over the time resolution previously attainable with STM. We have used this instrument to measure the response of the tunneling gap to excitation by a subpicosecond electrical pulse. Our technique is not limited to STM, and can be implemented in a variety of scanning probe microscopies, allowing the observation of ultrafast dynamics on the atomic scale.

Intrinsic Thermal Phase Noise Limit in Optical Fiber Interferometers

Optical fiber interferometers are used for several high sensitivity measurement applications, such as acoustic sensors, magnetometers, accelerometers, and gyroscopes. variety of noise sources limit the phase shift detection sensitivity attainable using fiber interferometric sensors. The most often quoted (and desired) intrinsic minimum detectable phase shift in interferometric optical systems is the shot, or quantum noise limit. In interferometric optical fiber sensors, the shot noise limit often can be realized; minimum detectable phase shifts of a few μrad/Hz at frequencies above a few hundred Hz are readily achieved with modest detectable power levels of a few microwatts. When shot noise dominates, minimum detectable phage shift performance improvements can be obtained simply by increasing the detected power levels.

Time-dependent Emission Spectra from Molecular Wave Packets

Wave packet states of atoms and molecules play an important role in studying the boundary between the classical and quantum domains. Such states are essentially nonstationary. Characterizing them requires determining the dynamics of the probability distribution of the particular degree of freedom in which the wave packet is excited. One way to accomplish this goal is to track the wave packet via the time-dependent spectrum of spontaneous emission. We have performed such measurements for a wave packet in the nuclear degree of freedom of a sodium dimer. This technique permits tracking of the nuclear wave packet in a single excited electronic state over a substantial fraction of its periodic trajectory. It has important applications for studying wave packet excitations in quantum confined electronic microstructures and larger molecules.

Number-phase Uncertainty Relations

The uncertainty principle lies at the heart of quantum mechanics. Any pair of noncommuting variables satisfy a form of uncertainty relationship, and this sets bounds on the measurement precision achievable for these quantities. In quantum optics, one such pair of variables are the photon number and the phase of an optical field. They satisfy an uncertainty relation ΔnΔφ≥(1/2)|‹|Ψ [n, φ] |Ψ›| where |Ψ› represents the state of the field. One important point to note is that the lower bound of this relation depends on the state. Thus, one needs a way of determining the values of both the uncertainty product and the expectation value of the commutator [n, φ].

Photorefractive Polymers Achieve Net Gain, High Diffraction Efficiency and Speed

The protection of eyes and sensors from exposure to intense, frequency-agile lasers, using passive optical limiters, is an active area of research. Material and device requirements include high transmission of low intensity light over a broad range of wavelengths, wide field-of-view, a fast yet persistent temporal response, and the ability to avoid or recover from optical damage. Research efforts to optimize the nonlinear optical properties of materials used in optical limiters have resulted in the development of many new and interesting materials.

Absorption of High Intensity Femtosecond Pulses in Semiconductor Amplifiers

For picosecond or longer pulses the gain saturation model provides an adequate physical basis for understanding pulse propagation in semiconductor amplifiers. In this model only amplification of the output pulse energy with respect to the input can occur because the gain is saturated and the carrier density approaches its value at transparency (at transparency the propagation is lossless). We have observed extreme departures from the gain saturation model predictions in numerical simulations of femtosecond pulse propagation in semiconductor amplifiers. In particular, we predict absorption of high intensity input pulses even though the input pulse spectrum is fully inside the gain region.

Nonlinear Optics and Photonic Applications of Photorefractive Polymeric Composite Materials

Until very recently the photorefractive effect has been observed exclusively in inorganic crystals, such as BaTiO3, doped GaAs, BSO, and others. The organic polymeric photorefractive films define an entirely new and very promising class of media exhibiting photorefractive response. The crucial functions for photorefractivity are photoconductivity and electro-optic activity. Known inorganics show high X(2) nonlinearities and moderate to high carrier mobilities. However, there is an inherent limitation is imposed on the photorefractive figure of merit, n3reff/є (where n is the refractive index, reff the effective electro-optic coefficient, and є the material's dielectric constant), for these systems due to the ionic polarizability origin of their nonlinearities resulting in high values of є.1 Secondly, these materials are very difficult to fabricate and process. Organics differ from their inorganic counterparts in that they are easy to process and, due to the localized nature of their electronic properties, combine low dielectric constants and relatively high electro-optic coefficients.

Advanced Modeling and Simulation of Self-electro-optic Effect Devices

Optical computing and photonic switching are new and emerging technologies for future parallel processing systems. Optically controllable switching elements based on multiple quantum wells (MQW), especially the self-electro-optic effect devices (SEEDs), are well-suited for applications in these areas and are now developed in different variants for mass fabrication of GaAs/AlGaAs integrated circuits.

Optical Matrix Multiplier: Grating Degeneracy Recycled

Grating degeneracy in volume holography has been a source of crosstalk noise during the readout of stored holograms. Special arrangement of input pixels1 (e.g., fractal sampling) is often needed to avoid crosstalk noise in optical storage, interconnections, and neural networks. Recently the authors showed that grating degeneracy can be used to perform summation operations in a matrix-matrix multiplication. This is the first useful application of grating degeneracy ever reported.

Birefringent Bistability in Ferroelectric Waveguides

Some ferroelectric materials are known to possess outstanding transverse electro-optic properties, namely, the electric-field-controlled birefringence. Great efforts have been made to study ferroelectric thin films to determine their use in integrated optoelectronic devices where light may be modulated or switched in ferroelectric waveguides by electric signals. It is well known that the remanent polarization in a ferroelectric material can be switched between two opposite directions with an external electric field; however, much to everyone's dissatisfaction, the polarization reversal causes no consequence for optical waves propagating in normal ferroelectric materials because the birefringence of a ferroelectric material depends quadratically on polarization. Nonvolatile light modulation and binary switching by means of polarization reversal in normal ferroelectric materials are thus prohibited.

How Retroreflectors Really Work

Retroreflectors, which reflect light from automobile headlamps back in the same direction from which the light originated, are invaluable safety devices. They are entirely passive, requiring no power supply or maintenance. They make street signs, lane dividers, bicyclists, joggers, and road crews highly visible. Although less than 100 years old, the devices are now ubiquitous and indispensable. Even so, most people do not know how they work, and those who think they do are probably wrong. Certainly the explanation I was given when I began my study of optics (and which I have heard from others as well) was demonstrably incorrect.

Light Action! Amazing Experiments for Children

Light is still an enigma, although some of the best minds have tried to figure out exactly what it is. Thousands of years ago, the Greek engineer, Hero, thought that light was sent out of our eyes as a "feeler." Since then, we have tried to quantify it, measure it, and theorize about it. Although we still do not know exactly what it is, we do know what we can do with it. When great scientists are stumped they may resort to their playful imaginations. Playing allows their minds to become free and, although playing generates ideas that may appear foolish and absurd, occasionally, a "foolish" idea breaks a discovery wide open. It is not difficult to teach kids how to play with science, and optics is a great place to begin.

Environmental Standards Revisited

Although we have mentioned ISO standards for environmental tests for optical instruments before in this column, it is a good time to bring them up again. Some optical instrument manufacturers have looked at the draft standards and realized what the impact will be on their field if the standard is adopted in its present form. To review the situation briefly, there are two standards being drafted by ISO/TC172/SC1/WG3: Environmental standards, ISO 9022-Environmental test methods, and ISO 10109-Environmental requirements. ISO 9022 describes nearly 20 basic types of tests that can be used to determine how well optical instruments withstand simulated adverse operating conditions. Most of the 20 parts of this standard have been adopted internationally.

Photorefractive Spatial Solitons

Light solitons in space (spatial solitons) exhibit a dual behavior of both waves and particles. They form when light interacts with the medium of propagation in a manner that exactly compensates for diffraction. This results in self-trapping of the light beam. It is obvious, however, that, since the medium is required to modify its properties in the presence of light, a strong light-matter interaction is required, i.e., the material must possess optical nonlinearities. All optical solitons that have thus far been discovered are a consequence of material non-linearities that are proportional to the absolute light intensity (Kerr-like solitons). Therefore, they typically require intensities of the order of kW-MW/ cm2 for their operation threshold. On the other hand, photorefractive (PR) materials, which have been studied over the last two decades, possess strong nonlocal nonlinearities that do not depend upon the absolute light intensity. It was not initially obvious, however, that these materials are also capable of forming optical solitons.