December, 1988

Subpicosecond UV Kinetic Absorption Spectroscopy

J. Misewich, J. H. Glownia, J. E. Rothenberg, and P. P. Sorokin

We explain these spectra in terms of a model based upon the transient behavior of the radiating polarization induced by the probe continuum pulse as it interacts with the population of two-level atoms resulting from the photolysis pulse.

Laser Depletion Spectroscopy

To perform depletion spectroscopy, a radiating core-excited level is used as a reference level from which to access the autoionizing manifold, as illustrated in the figure. The radiating level is impulsively excited, and a detector monitors the resulting fluorescence from this level. A tunable dye laser is passed through the excited vapor to transfer the fluorescing atoms to other core-excited levels nearby. As the laser is scanned in frequency, a level is encountered, and the excited population is transferred to it, resulting in a depletion in the amount of fluorescence observed from the reference level. The location of the depletion signal, as a function of dye laser frequency, determines the energy of the accessed level, relative to the reference level. The shape of the depleted signal, as a function of laser intensity, can be analyzed to determine the oscillator strength and Lorentz width of the transition.

More About Resonance Fluorescence

The resonant interaction between a two-state atomic transition and coherent light is the simplest and most widely studied example of the quantized interaction between light and matter. Surprisingly, this problem is still not exhausted as a source of new insights into the quantum statistics of optical fields and their interaction with atoms. Two areas of current research continue to add to our understanding of this problem—the study of nonclassical light, particularly squeezed light, and the study of intracavity atom-field interactions.

Major Breakthrough in Tunable Solid-State Lasers

In this review, a description of the spectroscopic and laser properties of forsterite laser crystal is presented.

Room-Temperature Operation of the CO:MGF2 Laser

Room-temperature operation of the Co:MgF2 laser is of significance for applications, typically outside of a laboratory environment, where the need for cryogenic cooling can present major problems.

Traveling-Wave Geometry for Short Wavelength Lasers

Stanford researchers have demonstrated a new pumping geometry that promises to be a significant step in the development of practical photoionization-pumped short-wavelength lasers. Using this method, they have observed single-pass, fully saturated gain near 100 nm in both Xe and Cs.

Research in Nonlinear Polymer Materials and Devices

A new mutually pumped phase conjugator (MPPC) was discovered at the Rockwell International Science Center. This conjugator is called Bird-Wing Phase Conjugator (BWPC) because the beam path inside the crystals bears resemblance to a pair of bird wings.

A Violet to Mid-Infrared Barium Borate Optical Parametric Oscillator

The combination of nonlinear optical materials developments and improvements in optical pump sources are leading to rapid advances in many areas of nonlinear frequency conversion and the generation of tunable coherent radiation. The demonstration of barium borate (BaB2O4) Optical Parametric Oscillator (OPO) pumped by a single-axial-mode 355-nm source is an example of one such advance. An average output power of 140 mW with power conversion efficiency of 13% was achieved in the OPO signal wave, and combined conversion efficiency of 24% was achieved in signal and idler waves. This BaB2O4 OPO was continuously tuned from 412 run to 2.55 μm limited by the infrared transmission range of the crystal. Linewidths of 23 GHz were observed with a simple plane parallel OPO cavity, and single mode parametric oscillation was achieved when the OPO was injection seeded with narrow bandwidth 532-nm or 1.064-μm radiation available from the pump source.

High Efficiency Second Harmonic Generation of a CW Frequency Stable Laser

Nonlinear optics is often used to convert a laser output to other frequencies. In Second Harmonic Generation (SHG), for example, the laser is focused into a nonlinear crystal that produces an output beam at twice the laser frequency. The conversion efficiency of this process scales as the incident laser power. High-peak-power pulsed lasers are therefore easily converted with high efficiencies, but lower power continuous wave (cw) lasers have proven to be more difficult to convert efficiently.

Upconversion Lasers Excited By Pairs and Trios

Recent experiments in Er:YLF and other crystals have demonstrated efficient laser action from energy levels populated entirely by cooperative upconversion of pump energy from pairs and trios of excited Er ions.1 This has revealed a general method for pumping solid state laser materials that is quite distinct from processes relying on the absorption of one or more photons by single ions. The new method permits excitation of high energy states by long wavelength radiation. For UV solid state lasers, it therefore offers an alternative to the use of deep ultraviolet pump sources that typically exhibit shallow penetration and can cause deleterious color center formation. Also, pair or multi-atom interactions can be exploited to achieve inversions on new transitions that may be normally self-quenching.

Optical Fibers with Negative Group-Velocity Dispersion in the Visible

Optical pulse compression is one of the more powerful methods to obtain ultrashort optical pulses. For wavelengths longer than 1.3 μm, an optical pulse can be temporally compressed in an optical glass fiber due to the interplay between self-phase modulation (intensity-dependent refractive index) and negative group-velocity dispersion (GVD). For wavelengths shorter than 1.3 μm, the dispersion in the fiber is positive; hence, an external grating pair has to be employed to temporally compress the pulses. However, with the discovery of optical fiber gratings1 it is possible to obtain negative GVD for visible wavelengths.

Determination of the Average Time Interval Between Two Photons with Sub-Optical Period Accuracy

The basic idea is to mix the two photons to be studied with a beam splitter, and then to detect the reflected and transmitted photons with two photodetectors in coincidence. A variable time delay Τ between the photons is introduced by moving the beam splitter slightly toward one or the other detector. If the two incident photons happen to be in the form of two identical wave packets, then destructive interference of the two-photon probability amplitude at the beam splitter leads to zero photon coincidences when the two wave packets overlap in time (Τ = 0).

High Speed GAAS Lasers and Detectors on Silicon for Optical Interconnects

High performance GaAs/AlGaAs lasers and detectors have been fabricated for the first time on Si substrates. This important development establishes the feasibility of bringing together monolithically the electronic switching technology of Si and the optoelectronic technology of GaAs/AlGaAs, thereby opening the way to a new class of integrated optoelectronic circuits and to optical interconnect technology.

Large Area Epitaxial Gaas and ALxGA1-x as Films on Arbitrary Substrates

Thin-film semiconductors have always represented a tradeoff between material quality and ease of preparation. Photonic devices require the highest quality epitaxial films, in which the atoms are in exact registry with an underlying crystal. But they must be grown on, and are accompanied by, cumbersome and expensive bulk single crystal wafer substrates.

Compact, Low-Loss Semiconductor Waveguide Bends

Monolithic opto-electronic integrated circuits, integrating active and passive optical and electronic components on a single chip are the subject of active research at numerous laboratories around the world, particularly for potential applications in optical communications systems.

Bragg Multiplex Holograms Integrated with Planar Waveguides

Significant progress in high-resolution phase holographic materials such as dichromated gelatin (DCG), PVA-based polymers, PMMA-based polymers, and DCG/ polymer grafts, has stimulated several attractive applications of high-efficiency Bragg holography, including holographic notch filters, Lippmann holographic mirrors and holographic optical elements (HOEs), display holograms, holographic gratings, solar-control holowindows, and, recently, VLSI Holoplanar™ interconnects (based on Bragg multiplex holograms, integrated with planar waveguides1), developed in POC's optics lab.

Binary Optics: A New Approach to Optical Design and Fabrication

Binary optics uses computer-aided design (CAD) tools and very-large-scale-integration (VLSI) electronic circuit manufacturing technology to create novel optical devices and to provide design freedom and new materials choices for refractive optical elements. With conventional optics, we polish a surface to the desired profile. With binary optics, we use high-resolution lithography (the design and replication process used in electronic circuit fabrication) to transfer a binary surface relief pattern to the optical element, and we etch this pattern into a surface using ion-beam etchers.

Stratified Volume Holographic Optical Elements

In this method, the distributed optical inhomogeneities that characterize a typical hologram are approximated by a discrete sequence of physically and mathematically simplified elements: infinitesimally thin phase and/or polarization modulation layers, interleaved with optically homogeneous layers of finite thickness. By approximating the distributed grating with a sufficiently large number of these discrete elements, the resulting numerical model of the grating can be brought arbitrarily close to that of the desired distributed bulk grating.

16 Gb/s Lightwave Transmission by Optical Time-Division Multiplexing

Optical data at a bit rate of 16 Gb/s were recently transmitted over optical fiber in an experiment at AT&T Bell Laboratories. This is the highest transmission bit-rate reported for any single-channel lightwave system —an achievement made possible using optical technology to time-division multiplex and demultiplex the data.

Modulation Instability Based Fiber Devices for Ultrafast All-Optical Switching

For the ultrafast applications, fibers themselves are an attractive medium in which to do the switching. For a fused silica fiber, the response time of the nonlinearity is about 6 femtoseconds (10-15sec). A figure of merit for nonlinear optical devices is n2/α, where n2 is the nonlinear index coefficient and a is the absorption coefficient. In fibers, this ratio is favorable because of the small value of α. Also, because of the excellent guiding properties of fibers and the low loss near 1.5 μm, long length of fibers can be used, thereby reducing the power requirements. Although long length of fibers introduces a latency (delay between the inputs and outputs), the fast response times permit use of a pipelined architecture. Furthermore, there exists a mature fiber technology that can be exploited if devices are made out of fibers.

All-Optical Repeater

Lightwave communications systems today rely on electronic devices to perform the necessary signal processing. This is nowhere more evident than in the commercial optical repeater in which the only two optical components are a photodetector and an injection laser. With the advent of photonic integrated circuits, future systems could use "all-optical" repeaters to process signals optically. The advantages of this all-optical repeater include simplicity (no need to convert from optical to electrical signals and back again) and potential high speed

Experimental Observation of the Fundamental Dark Optical Soliton

In recent years, optical fibers have been shown to be almost ideal media for the study of nonlinear optical phenomena and for the exploitation of those phenomena. An important focus for this work has been to explore the ultimate limits on the information capacity of optical fibers.

Soliton Transmission Over Many Thousands of Kilometers in Loss Compensated Fiber

Researchers at AT&T Bell Laboratories have demonstrated the transmission of soliton pulses over many thousands of kilometers of fiber path without the use of electronic regeneration.1 Instead, Raman gain from pump light periodically injected along the fiber cancelled the normal fiber loss, so that the non-dispersive solitons (based on index nonlinearity) could maintain both their strength and width throughout the entire trip. This achievement is thought to foreshadow the development of all-optical long distance transmission systems featuring ultra high bit rates and high reliability.

In Situ Studies of Crystal Growth

The last two decades have been marked by steady progress in our ability to grow artificial semiconductor materials and structures, and in their use to explore and critically examine increasingly sophisticated aspects of semiconductor and low-dimensional physics. We have now reached the point where further advances depend on real-time, in situ measurement and control of the growth process itself. But the high pressure, highly reactive environments of many growth technologies prevent the use of the electron- and ion-beam spectroscopies that have been extensively developed for surface studies.

Holographic Neural Network for Word-Break Recognition

In recent years, there has been interest in optical implementations of neural networks in the hope of developing optical supercomputers to solve computationally massive problems such as pattern recognition by emulating the human brain. Many experimental demonstrations have shown that optics can provide parallel processing, globally distributed information storage, and analogue computing, and hence is a good match with neural networks. For example, work on optical associative memories has beautifully demonstrated auto-associative recall: recalling full information from a partial or partially incorrect cue.

Optical Novelty Filters

Optical computers have been promoted as the remedy for the bottlenecks of electronic computers, but as Top: Protozoa without novelty filter. Algae is also visible. Bottom: Protozoa seen with novelty filter. Algae in background is no longer seen. yet no one has made a digital optical computer that can perform even the simplest of tasks, such as adding two 2-digit numbers. Analog optical computers are much easier to make than digital optical computers, and for some applications analog computers are better. A frog, for example, uses an analog computer to quickly compute the velocity and the predicted position of a careless fly, and the frog obviously does not need a digitally-precise value for the fly's velocity.

Optical Symbolic Computing

The goal of this project is to simulate and implement a demonstration optical inference engine. The approach is to develop an optical data representation for knowledge and then to construct simulation software for all optical components to validate system behavior and reduce implementation risk. The next step is to design, simulate, and implement optical computation modules for the basic operations required for inference; optical storage and data handling modules required for data-dependent computation; and the data-dependent control required for inference.

Focusing Neutral Atoms

In recent work, our group has studied the use of laser ablation to produce focused beams of neutral atoms. Laser ablation has been investigated as a method of producing cold, high density beams of refractory elements (those elements that do not readily vaporize). Typically, a laser incident on a slab of material causes a thin layer to sublimate. Studies of the beams produced by this method are complicated by changes in surface characteristics caused by the removal of particles from the material.

Surface roughness, interferometers dominate standards discourse

Major topics for discussion at the OSA Standards Committee Aug. 15 meeting in San Diego were the specification of spatial bandwidth in the measurement of surface finish, the calibration or standardization of interferometric wavefront measurement instrumentation, and the writing of an optical measurement procedures guide. The discussion of surface finish measurement centered around the need to specify more than just an rms surface roughness number. To see that this is necessary, we can consider that any surface profile can be represented by an infinite sum of spatial Fourier components.

Atom Gratings and Guides

When one thinks of gratings, a number of things come to mind—conventional ruled gratings, phase gratings, and even temporary density gradients in atomic vapors—all of which share the important property that they diffract light.

Lorenz-Like and Shil'nikov Chaos in Lasers

Irregular pulsations in many kinds of lasers have been shown to arise from deterministic nonlinear dynamics instead of from stochastic noise. Recently two special cases of chaotic pulsations have been found that differ from the classic routes to chaos of period doubling and quasi-periodicity. These are instead special classes of what has been called the intermittency route to chaos.


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