In the past five years, there has been a revolution in the field of ultrafast laser technology. Femtosecond lasers are now simple and turn-key, with output powers orders of magnitude higher than were available only a decade ago. Nonlinear frequency conversion techniques can be used to generate femtosecond pulses through the visible and infrared, and high field effects allow this range to be extended to the far-IR and x-ray regions of the spectrum. New measurement techniques have also been devised, which can extract the complete waveform of a femtosecond pulse, allowing the complete determiniation of both the amplitude and phase of pulses as short as only a few optical cycles. Finally, the pulse generation mechanisms close to the fundamental limits of operation of these systems have been understood.
Considerable interest has arisen recently in the prospect of using specially crafted ultrashort laser pulses for coherent control of atomic and molecular systems. The aim in these studies is to utilize control over femtosecond optical waveforms as a tool to manipulate constructive and destructive interferences associated with quantum mechanical wave packets motions, which could ultimately lead to optical manipulation of chemical reactions and in the shorter term should make possible preparation of well-defined quantum mechancial states for precise spectroscopic determination of molecular Hamiltonians. At Purdue we have initiated a project aimed at applying these coherent control concepts in a new setting--namely, in specially designed layered semiconductor materials. A key motivation for using layered semiconductors as a coherent control laboratory is the ability to engineer the Hamiltonian through the epitaxial growth process, so that one may have better knowledge of and control over the Hamiltonian than one has in studies of complex molecules. Here we review the femtosecond pulse shaping technology crucial for our coherent control studies and discuss our plans and progress in applying pulse shaping to manipulate coherent charge oscillations in double coupled quantum wells and superlattices in the GaAs/GaAlAs material system.
This contribution focuses on applying the unique optoelectronic THz beam system we have developed to THz time-domain spectroscopy (TDS). The work developing the system is first reviewed. The emerging technique of THz-TDS is introduced, and some past applications are described. We then present a theoretical and experimental comparison between THz-TDS and the well-established Fourier transform spectroscopy (FTS). The extraordinary dynamic range of THz-TDS allows the study of exceptionally optically dense materials; specific past examples and new opportunities are discussed. The topic of difficult samples is explored for samples, such as flames, posing exceptional problems for FTS, but which can be relatively easily measured with THz-TDS.
Advances in femtosecond laser technology now make it possible to reliably generate laser pulses in the terawatt energy range with 20 fs pulse duration, using laser system which fits on a single optical table. We have demonstrated two such systems: a 10 Hz, 3 TW sysetm, and a 1kHz, 0.05 TW system. These lasers make possible new studies of strong-field laser-matter interactions, and the generation of reliable femtosecond pulsed soft x-rays.
This paper deals with the implementation of multichannel wavelength-division-multiplexing (WDM) to more fully utilize the enormous fiber bandwidth for high-speed data transmission. The main research activities described in this paper relate to the all-optical network. The fundamental systems assumptions include: i) all-optical processes can potentially avoid an optoelectronic speed bottleneck, and ii) wavelength shifting of an incoming optical channel will be required in a reconfigurable WDM network in which there are fewer wavelengths than total users. The two main projects are: i) simultaneous all-optical packet-header replacement and wavelength shifting, and ii) all-optical TDM-to-WDM data format conversion. Project 1: We experimentally demonstrate a method simultaneous all-optical header replacement and wavelength shifting. These functions, important in a dynamically-reconfigurable WDM network, are realized by using cross-gain compression in semiconductor optical amplifier and by using three-level modulation of a probe laser only when the header bits require changing. We simultaneously replace the 8-bit modulation of a probe laser only when the header bits require changing. We simultaneously replace the 8-bit header of a 1-Gbit/s packet and wavelength-shift the entire packet by 19 nm. A receiver sensitivity of -27 dBm at 10-9 BER is measured when using a 416-bit NRZ data packet. Our approach requires no guard bits and allows both the header and data to be at the same bit rate and wavelength. Project 2: We demonstrate both single-stage and two-stage all-optical TDM-to- WDM data format converters based on cross gain compression in an SOA. Our method simultaneously time demultiplexes and wavelength shifts each channel's data within a TDM packet. We convert a 2-Gbit/s 4-channel TDM data stream at 1552 nm into four 500-Mb/s WDM channels at 1535, 1537, 1540, and 1571 nm. Moreover, we extend this single-stage approach by demonstrating a two-stage TDM-to-WDM converter that selectively time demultiplexes and wavelength shifts a 2-Gbit/s TDM input data stream into two 500-Mb/s WDM channels after the second stage. We find that low power penalties (< 2 dB can be achieved by using this scheme for both the single- and two-stage cases. This conversion scheme may be quite useful in WDM switching nodes. This work is performed in the context of a three-investigator multidisciplinary research program sponsored by the NSF initiative on all-optical networks.
In this paper, we introduce a novel self-routed wavelength-addressable switching network (SWANET) that provides wavelength-transparent optical data paths between end-point, configured by wavelength-coded optical signals. The network is based on a new scheme for encoding destination addresses using a sequence of wavelengths. This allows large networks to be constructed using a moderate number of available wavelengths. The multistage switch architecture may be used either as a circuit switch or as a packet switch. We describe the architecture of SWANET and the design of its switch nodes. We also analyze the effect of fiber dispersion on the transmission time of the header and evaluate the tradeoffs involved in minimizing the header transmission time.
We describe a novel approach for implementing WDM networks that does not rely on fixed wavelength channels. Rather than tuning to a preassigned fixed wavelength channel any two nodes that require a link select a currently unused portion of the spectrum. Rather than tying to make trnamitters and receivers tune to fixed wavelength channels, the medium access control unit determines the approximate channel wavelength to be used for a given link. The receiver then dynamically locks onto this channel.
We develop algorithms for the design of optical packet-switched networks. These algorithms are aimed at upgrading an existing fiber-based, network infrastructure, such as the NSFNET backbone, by using wavelength division multiplexing (WDM) technology. The network architecture employs wavelength-routing optical switches which enable the establishment of all-optical, WDM channels, called lightpaths. A set of lightpaths may be used as a virtual topology over which packet-switched traffic may be transported ina store-and-forward manner. The packet forwarding is done by electronic packet routers which are attached to the wavelength-routing optical switches. This paper examines issues related to the upgrading of an existing fiber-based, packet-switched, electronic network to accommodate WDM, and to the optimal choice of the virtual topology based on changing traffic demands. We demonstrate how the total information carrying capacity of the network can be enhanced by using WDM.
We report on a variety of methods for imaging into highly scattering biological tissue, using electronic holography, in combination with a variety of other methods. We present the basic principles of each method and give experimental results. The experiments are carried out with a cooled CCD scientific camera and extensive digital processing.
The resonance Raman spectrum of the simple peptide N-methylacetamide (NMA) is very different in the vapor phase than when dissolved in aqueous solution. The major difference is that the amide I mode, primarily involving C equals O stretching, is very strong in the vapor phase spectrum but this mode is essentially nonexistent in the aqueous solution spectrum. Since resonance Raman scattering for compounds of this type reflects the geometry change associated with electronic excitation, this spectral difference suggests that there is a large effect of solvation on the geometry of the excited electronic state. The present work describes the developemnt of quantatitive ab initio quantum chemical procedures for calculating resonance Raman spectra with application to NMA both isolated and in solution. A simple cluster model involving hydrogen bonding of water molecules to NMA provides the major effect of aqueous solvation on resonance Raman spectra.
The unique structure of a protein is encoded in its characteristic sequence of amino acids; the processes by which this linear sequence collapses into a unique 3D structure remains an unsolved problem that represents one of the most challenging issues in fundamental biomolecular science. This so-called protein folding problem is the second half of the genetic code. Studies of this biological problem are complicated by the need to study dynamic behavior involving small populations of transient species in a solution environment. However, the use of advanced transient laser spectroscopy techniques based on intrinsic chromophores provides a powerful means to study this problem. Specifically, time-resolved phosphorescence of tryptophan (Trp) provides a means to study the dynamics associated with different regions of the protein surrounding the emitting Trp residue. Using these methodologies, we are able to study, in real time, the later stages of unfolding and refolding of the bacterial protein alkaline phosphatase, a nonspecific monoesterase. Results show the presence of several intermediate states, including states with significantly altered core structure that still exhibit complete biological activity. Moreover, the refolding of alkaline phosphatase following denaturation in either chaotropic denaturants or low pH reveals a relatively fast refolding leading to the biologically active state, while laser spectroscopy measurements show a soft core which is annealing to the native-like state on a time-scale long compared to the return of activity. The active refolded protein is also initially characterized by an increase in susceptibility to denaturant. The slow annealing of the core is consistent with the presence of high energy barriers that separate fully active, long-lived, kinetic intermediate states along the folding pathway, a description suggested in the rugged energy landscape model.
A review is presented of the role of local field effects in establishing the nonlinear optical properties of materials. Special emphasis is placed on the role of local field effects for the case of composite nonlinear optical materials. There is great need for optical materials that possess large optical nonlinearities while simultaneously possessing other desirable properties such as fast response, low attenuation, and high damage threshold. One avenue for the development of such materials is to form a composite of two or more components in such a manner that the properties of the composite are more desirable than those of their individual components. In this article, we examine how one might do so, especially from the point of view of how local field effects come into play in establishing the response of composite materials. However, before turning our attention to composite materials, we present a brief summary of local field effects for the case of homogeneous materials.
The status of ZnSe-based blue-green diode lasers evolved extremely rapidly from 'non- existent' in 1990 to 'room temperature continuous-wave operation' in 1993. Such rapid progress can be attributed, in the author's opinion, to four critical materials-related breakthroughs reported over an exciting two-year time frame. The nature of these four breakthroughs will be reviewed in this paper as will their significance with regard to their impact on the performance of blue-green diode lasers.
The integration of thin film optoelectronic devices with host substrates such as circuits, waveguides, and micromachines offers to the systems engineer the freedom to choose the optimal materials to achieve performance and cost objectives. In essence, the bonding of the thin film components becomes an integral part of the system packaging. The fabrication and integration of thin film compound semiconductor optoelectronic devices with a number of host substrates is presented. Thin film devices have been integrated with silicon circuits, and movable micromachines, and have been used to demonstrate three dimensionally interconnected systems. The three dimensionally integrated systems include detector arrays bonded directly on top of circuits for massively parallel processing of images, and vertical optical interconnections have been demonstrated between stacked layers of silicon circuits (which are transparent to the wavelength of light used) for a massively parallel processing architecture based upon low memory and high input/output.
Digital electronics has become so low-cost that practically all electronic systems of any kind have digital chips at their heart--that is, information is shuttled from subsystem to subsystem in digital format as its flow is controlled by digital control signals generated by digital subsystems. In general, analog or optical subsystems are used for signal transmission between digital subsystems. As 'packaging' refers to the elements of a system that are used to interconnect bare dies to bare dies, one can think of analog and optical systems as packaging and refer to insertion of analog and optical components into a digital system as an alternative form of packaging. In this paper we will discuss optoelectronic packaging reserach as research associated with inserting optics in electronic systems. Optoelectronics has proven to be the technology of choice for long-haul communications as well as compact disc player redout. In order for the applications of optoelectronics to become widespread, much will have to be done to solve problems associated with the generation and reception of the optical signals and problems associated with the compatibility of digital and analog electronics with optoelectronic components. These problems include laser-to-waveguide alignment, electrical modulation of optical signals, and receiver complexity. These research issues will be discussed in the following manuscript.
Experiments conducted at Colorado State University, which resulted in the first demonstration of large soft x-ray amplification in a discharge-created plasma, are reviewed. The most recent measurements, conducted in capillary discharges up to 20 cm in length, have yielded an amplification of approximately equals exp(14) in the 46.9 nm line of Ne-like argon. The dependence of the line intensity with discharge parameters and the dynamics of the capillary plasma column under lasing conditions are reported. Prospects for laser operation at shorter wavelengths are also discussed.
The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University is building an interferometric array of telescopes for high resolution imaging at optical and infrared wavelengths. The 'CHARA Array' will initially consist of five 1-m diameter telescopes arranged in a Y-shaped configuration with a maximum baseline of approximately 350 m. The facility will be located on Mt. Wilson, near Pasadena, California, a site noted for its stable atmoshperic conditions that often gives rise to exceptional image quality. The Array will be capable of submilliarcsecond imaging and will be devoted to a broad program of science aimed at fundamental stellar astrophysics in the visible and the astrophysics of young stellar objects in the infrared (2.2 micrometers ) spectral regions. This project has been supported by the National Sceince Foundation through Phase A feasibility and Phase B preliminary design stages, and NSF awarded 5.6 million dollars towards the construction of the facility in October 1994. Georgia State University is committed to providing an additional 5.8 million dollars in construction funds. The CHARA Array is expected to be operational late this decade. This paper will provide a summary overview of the project.
Forward scattering solutions, whereby one predicts the scattered electromagnetic field given the goemetry, find numerous engineering applications in computer aided design. Therefore, a fast way to solve the forward scattering problem will impact a number of areas, like high- speed circuits, integrated optics, antenna analysis, remote sensing, geophysical sensing, and inverse scattering. We will describe several fast methods developed in our group to solve the forward scattering problem rapidly. These methods involve solving the volume integral equation of scattering as well as surface integral equation of scattering. Different strategies are used to accelerate the solutions of these integral equations. Both iterative and direct solution techniques will be considered. The computational complexity and memory requirement of various scattering algorithms will be discussed. In inverse scattering, one reconstructs the physical goemetry of a scatterer from the measured scattered field. Hence, it finds applications in image and profile reconstructions. When using a linear method, multiple scattering within a scatterer is ignored. By using a nonlinear inverse scattering method, such multiple scattering effect is accounted for. Image and profile reconstruction using such nonlinear inverse algorithm can remove artifacts that linear methods would not remove. We will discuss the use of the distorted Born iterative method and local shape function method to reconstruct a scattering object.
Microscopy using vacuum ultraviolet and soft x-ray radiation offers high resolution, high contrast, and chemical sensitivity. Holography and contact printing are unique in their potential to achieve wavelength-limited resolution because they eliminate optical elements and their imperfections. Both methods, however, lack magnification and require high resolution films and methods to recover images. We present our results on source development and film characterization.
Over the last decade, free-space optical interconnects have been investigated for their potential use for inter- and intra-chip communication. Among the unique characteristics of free-space interconnects are their 3D nature, lower power requirements and potential for higher connectivity and compactness. On the other hand, 3D assembly which can are emerging to package many electronic chips in a single 3D assembly which can provide single chip like performance. These 3D packaging techniques are especially useful when applications of interest require simultaneously algorithms with functions that do not fit in a few chips and a systems implementation with a small volume and weight. At UCSD, jointly with Hughes Research, we are investigating the potential impact of using free-space optical interconnects to communicate between several such 3D electronic modules. This approach relies on implementing long module to module communication by free-space optical interconnects conserving the benefits of 3D packaging of chips at the system level. This is derived by the good match that exists in the characteristics and capabilities of these electronic and optical 3D techniques in terms of I/O density, bandwidth, and small volume and power.