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Optics & Photonics News Magazine
December 1991 Issue

Feature Articles

2-D Monolithic Multiple-Wavelength Diode Laser Array

Optical sources capable of THz data rates are essential for applications ranging from optical fiber communications, to fully interconnected large computers, to real-time optical signal and image processing. The more conventional approach to achieve high data rate is to use a single high speed or mode-locked diode laser. However, their bandwidths are limited to a few tens to a hundred GHz. Moreover, the laser as well as its driver and packaging becomes very expensive as its speed increases.

by Connie Chang-Hasnain, Bellcore
Visible Ii-Vi Laser Diodes

One current trend in laser diode research is to push toward shorter wavelengths. Due to limitations in their bandgaps, it is not likely that the III-V compound semiconductors will produce lasers with emission wavelengths significantly shorter than 600 nm. Therefore, the quest for green and blue injection lasers is forcing researchers to look elsewhere. Our recent demonstration of 490-535 nm laser diodes from ZnSe-based materials suggests that a new era of laser development using the wide bandgap II-VI semiconductors has begun.

by J. M. DePuydt, M. A. Haase, J. Qiu, and H. Cheng
Dressed-State Two-Photon Laser

Though the possibility of two-photon laser action was first discussed over 25 years ago, it has been realized only under a very limited set of circumstances and never, in the optical regime, under continuous-wave conditions. This is unfortunate, since the two-photon laser has been predicted to display a number of interesting properties such as bistable operation, unique threshold behavior, and, in some cases, squeezing.

by D. J. Gauthier, Q. Wu, S. E. Morin, and T. W. Mossberg
Intensity-Invariant Joint Transform Correlator

The optical joint transform correlation results from the diffraction of a collimated beam from a prerecorded interference pattern, produced by the Fourier transform of a target and a reference. The joint transform can be seen as the Young interference generated by multiple pairs of point sources separated by the same distance. When the intensities of the target and reference are not identical, the generated Young fringes cannot achieve a unity visibility, and, as a consequence, the final correlation intensity (which is the light diffracted by the recorded fringe pattern) is not a maximum.

by Suganda Jutamulia and Don A. Gregory
Demonstration: A Photorefractive Correlator for Robotic Applications

Recent advances in solid state lasers, liquid crystal spatial light modulators, computer generated holograms, and nonlinear optical materials have opened new opportunities for photonic technologies in the field of parallel processing and computing. To explore some industrial applications of these technologies, the Commission of the European Communities started in 1989 an ESPRIT* project, bringing together, in a precompetitive consortium, industrial partners (Thomson-CSF in France, KRUPP in Germany), a national research center (RISØ in Denmark), and a university (Universitat Erlangen in Germany).

by Henri Rajbenbach
Vertical Cavity Surface Emitting Lasers: Toward Applications

Recent developments in vertical cavity surface emitting (VCSE) laser device research have made possible for the first time, high yield, one or two dimensional arrays of almost arbitrary size and geometry. This has generated intense interest in application of these devices to optical interconnects, optical data storage, optical scanners, and optical signal processing and computing. To realize the use of VCSE laser arrays in such applications, it is necessary to establish ways of optically or electronically controlling them and to investigate technologies for incorporating them as components in larger electronics systems. This article focuses on recent progress in addressing and integration of MBE-grown VCSE laser arrays toward applications to optical interconnect or signal processing systems.

by A. C. Von Lehmen
Photorefractive Particle Image Velocimetry

Optical signal processing techniques are attractive when the input is a two-dimensional transparency. With the help of the photorefractive effect, this processing is achieved quite easily in real-time using holographic schemes. We have recently demonstrated particle image velocimetry (or PIV) over a large area of a transparency in parallel using a photorefractive correlator.1 In such an optical system, the input is a double exposure photograph of a particle flow. The autocorrelation function of this flow is carried out in a two-step procedure. First, the Fourier transform of an area of the transparency and a plane reference wave induce a phase grating in a photorefractive BSO crystal. The reference wave is then switched off and the hologram is read out with the Fourier transformed object beam itself to yield autocorrelation on the output screen. The result consists of two side-peaks and a central spot in the case of a uniform particle velocity. The distance between the peaks and their relative orientation determine the velocity and direction of the flow.

by A. Marrakchi, P. M. Petersen and P. Buchhave
Compact And Robust Pattern Recognizer Using A Micro-Laser Array

Although the classical coherent optical processor has been widely used in many different fields of analog optical computing, pattern recognition, neural networks, and optical interconnects, its bulky volume and critical alignment requirements make its integration difficult. Furthermore, for an optical computing/processing system to be practical, it is important to be integrable with semiconductor technology to facilitate the interfacing with other devices. The recent invention of surface-emitting laser diode arrays (SELDAs) opened many possibilities in the field of optical computing. In particular, the compact two-dimensional nature of SELDA makes possible compact optical signal processing systems.

by Eung Gi Paek, A. Von Lehmen, J. R. Wullert II, and R. Martin
Stressed-Lap Polishing of Large, Fast Aspheres

The present decade is the setting for a renaissance in the design and construction of ground-based optical and infrared telescopes. A number of projects worldwide will use primary mirrors 8 m in diameter or larger.1 Demands of economy and performance drive these telescopes to short focal lengths. The cost of both the telescope and its enclosure increase rapidly with focal length, and the telescope's stiffness and tracking accuracy benefit from shorter length.

by H. M. Martin
Rapid Optical Fabrication With Computer Controlled Optical Surfacing

The design needs for large optical components for advanced strategic defense systems and reconnaissance systems presented the optical industry with fabrication capability requirements that could not be met by conventional fabrication methods rooted in telescope making techniques. Faced with the lack of enabling production technology for this type of optics, Itek undertook to develop a new generation of optical fabrication technology.

by Robert A. Jones and Wiktor J. Rupp
Degradation Mechanisms In High Power Algaas Lasers

As optical designs continually improve, limiting conditions on the image quality ultimately depend on the quality of the components. Surface roughness scatters light, reducing intensity and contrast. Quantitatively, optical surfaces can be evaluated using the bidirectional reflectance distribution function (BRDF). This statistical, Fourier-transform approach used to characterize surfaces gives an rms roughness of the surface. Quantifying contamination— dust particles on a mirror, for instance—is not nearly as well-developed.

by Wade C. Tang, Hal J. Rosen, Peter Vettiger and Dave J. Webb
Universal Dispersion And Band-Gap Scaling Of N2 In Solids

Using the "Z-scan" technique, which can separately and accurately measure nonlinear absorption (NLA) and nonlinear refraction (NLR), two-photon absorption (2PA) and the bound electronic Kerr effect (n2) have been measured in a large number of inorganic solids. As a result of this large database and detailed characterization, a universal predictive capability for both the NLA and NLR of solids has been developed over a range of band-gaps from 0.2 to 10 eV. It is found that n2 displays strong dispersion and sign reversal as ћω approaches the band-gap energy Eg.

by M. Sheik-Bahae, D. J. Hagan, and E. W. Van Stryland
Hyper-Rayleigh Scattering In Solution

A problem with the experimental determination of the first-order optical hyperpolarizability β (second order nonlinear optical polarizability) has been that centrosymmetric structures of individual molecules with a non-zero microscopic hyperpolarizability β do not possess a macroscopic second-order susceptibility p. For this reason, the first measurements of β have been performed on crystals without centrosymmetry, Langmuir-Blodgett films, and poled polymers. Electric field induced second harmonic generation has been the only technique that allows the determination of optical hyperpolarizabilities in solution, if the values for the dipole moment, the local field correction factors, and the third-order hyperpolarizability are known.

by Koen Clays and Andre Persoons
Highly Nonlinear Phenomena In Optical Waveguides

An exciting aspect of nonlinear integrated optics is the prediction that the properties of guided waves can become power-dependent when one or more of the guiding media is nonlinear. It is useful to classify these effects as either weakly or highly nonlinear, depending on the ratio of the nonlinear change in refractive-index to the linear index difference between the guiding media. In the weakly nonlinear regime where the ratio is small, the field profiles can be approximated by the linear guided modes and only the propagation constants are power-dependent. In contrast, in the highly nonlinear regime, the ratio is larger than or equal to one, and both the propagation constants and field profiles become power-dependent.

by Ewan M. Wright, David R. Heatley and George I. Stegeman
Coherent Transient Optical Phenomena In Semiconductors

Coherent transient spectroscopy of resonances in gas phase atoms and molecules has provided considerable information on irreversible decay processes in these systems due, for example, to collisions. However, despite the presence of strong resonances in semiconductors such as excitons, the rapid dephasing times even at liquid helium temperatures made the study of coherent optical phenomena difficult.

by Duncan Steel, Jagdeep Shah
Squeezed Lasers

A simple mechanism whereby lasers can "lase" squeezed light has recently been discovered. Unlike previous schemes, the pump does not have to be regular; rather, the atoms themselves regularize the pumping process. This is significant because optical measurements are ultimately limited in their sensitivity by the light's quantum mechanical noise.

by C. M. Savage and T. C. Ralph
Photonic Band Structure

There now exist three-dimensionally periodic dielectric structures that are to photon waves as semiconductor crystals are to electron waves. That is, these photonic crystals have a photonic bandgap, a band of frequencies in which electromagnetic waves are forbidden, irrespective of propagation direction in space. Photonic bandgaps provide for spontaneous emission inhibition and allow for a new class of electromagnetic micro-cavities.

by E. Yablonovitch
Atom Interferometers

Years of discussions about possible ways to make atom interometers bore fruit this year as several groups demonstrated different experimental routes to the realization of a working atom interferometer. These realizations built on advances in atom slowing and trapping, the manipulation of atoms using light forces, and the construction of nanofabricated structures for diffracting atoms. These advances culminated this year in the use of nanofabrication technology by a group in Konstanz, Germany, to realize Young's experiment for He atoms, in which atoms displayed interference fringes as a result of passing through a double slit.

by David Pritchard
Electromagnetically Induced Transparency

For about 15 years, the atomic physics community has been aware of a phenomena that is termed population trapping. When two lasers with frequencies that couple two relatively metastable lower states to a common upper state are applied to an atom, the population of the atom is frozen or trapped in the lower states. As a result of a destructive quantum mechanical interference, the common upper state is empty, and (in steady state) the contribution of the atom to the susceptibility is zero. It is therefore possible to render an opaque media transparent at what would otherwise be a strongly absorbing atomic transition.

by S. E. Harris
Ultra-Compact Optical Waveguide Couplers For Monolithic Integration

Monolithic integration of waveguides for on-chip optical signal processing is a technology fundamental to enhanced optoelectronic circuit functionality. A major drawback to monolithic optics, however, has been the large size (typically many millimeters) required for optical directional couplers with their associated input/output branching guides, which is incompatible with the relatively high cost of III-V semiconductor materials. This year two quite different, yet complementary, new techniques were demonstrated for making extremely compact couplers suitable for monolithic integration.

by R. J. Deri, E. C. M. Pennings, R. J. Hawkins and L. B. Soldano
Ultra-Low-Power Integrated Acousto-Optic Filter

The acousto-optic filter has gained increasing attention in wavelength-division multiplexed optical communication because it offers the unique and powerful capacity to extract many different wavelength channels (simultaneously and independently) from one optical fiber and divert all or part of each signal to a second fiber. This parallel-processing capability means that each wavelength in a fixed fiber network has an independently configurable matrix of interconnections.

by D. A. Smith, J. J. Johnson and J. E. Baran
Fiber Optic Interconnects For The IBM System/390

In the past year, IBM announced the System/390™ family of mainframe computers. Along with many other performance enhancements, this system was among the first in the computer industry to use fiber optic I/O channels for high speed data transfer. Known as ESCON (Enterprise Systems Connection)™ Channels, these optical links provide significantly higher bandwidth, data rates, and distance than the parallel electrical cables they replace.

by Casimer DeCusatis, Allen Huffman, George DeMario, and Daniel Stigliani
Optical Clock Distribution Using A Mode-Locked Semi-Conductior Laser Diode System

Optical clock distribution has been achieved by using a modelocked semiconductor laser diode system as the master timing device. The optical clock has demonstrated a fanout of 1024, while maintaining less than 12 psec of jitter on the recovered clock signal for periods of over one hour. This type of clock distribution technique demonstrates a method to overcome fundamental problems associated with conventional electronic clock distribution techniques.

by Peter J. Delfyett, Davis H. Hartmann and S. Zuber Ahmad
Programmable Micro-Optics Components

Micro-lenses and micro-gratings are now routinely fabricated on a variety of substrates such as glass and semiconductors using well-established photolithographic techniques. These optical components are so small that they can be arranged in large two-dimensional arrays for use in a variety of applications in image processing and parallel interconnections. Our aim is to make these micro-optic components programmable. For this purpose, we have suggested and demonstrated the use of addressable two-dimensional spatial light modulators (SLMs).

by A. Marrakchi, S. F. Habiby, and J. R. Wullert II
Surface Dynamics on The Femtosecond Time Scale: A Real-Time Study of Desorption

Energy flow between a molecule and a surface is critical in determining the evolution of surface processes such as absorption, desorption, and fragmentation. Energy transfer between an adsorbate and the substrate occurs on a very short time scale, typically in the range of 10-15-10-10 s. Information on these rapid processes has generally been inferred from spectroscopic data. Recently, however, significant progress has been made in probing surface dynamics directly in the time domain. Elegant linear and nonlinear optical techniques have been applied to determine lifetimes and dephasing rates of vibrational excitation in adsorbates.

by T. F. Heinz, M. M. T. Loy, and J. A. Misewich
Self-Modelocking: Ultimate Simplicity In Ultrashort Pulse Generation

For the past 25 years or so, there have been many sustained research programmes devoted to the study of the techniques by which ultrashort (≤ 10-12 s) laser pulses can be generated. Whereas spectral purity can be ensured by constraining the laser to operate on a single axial or longitudinal mode of the resonator, the production of picosecond or shorter pulses demands that many longitudinal resonator modes must be precisely locked in phase by a process that is referred to as modelocking.

by W. Sibbett
Ballistic 2D-Imaging Through Tissue Scattering Walls

By adding an ultrafast time gate, the detectability of small tumors located inside the breast can be significantly improved. Time-resolved techniques such as the Kerr effect and holography, can be used to separate out the ballistic and snake components (least distorted image) from the diffusive component (most image information lost).

by L. Wang, P. P. Ho, R. R. Alfano
Electronic Holography For Imaging Through Tissue Using Femtosecond Gated Pulses

Imaging through and into highly scattering media is a major problem of optics. One potential application is for medical imaging—to see through biological tissue. One imaging method uses short pulses (picosecond or subpicosecond) and the principle of the first arriving light. Here, a short pulse of light enters the medium and emerges as a greatly elongated pulse. The light that is scattered least emerges first, and the more scattered light emerges later. The least scattered light, being distorted the least, will form the best image of any absorbers located behind or embedded within the material. Gating methods are then used to separate this first arriving light from the rest.

by E. Leith, E. Arons, H. Chen, Y. Chen, D. Dilworth, J. Lopez, R. Masri, J. Rudd and J. Valdmanis
Characterizing The Hubble Space Telescope Using Retrieval Alogorithms

Soon after launch, it was discovered that the Optical Telescope Assembly (OTA) of the Hubble Space Telescope (HST) suffers from a large amount of spherical aberration. An accurate characterization of the aberrations and state of alignment of the HST is important for several reasons: (1) for the design of replacement instruments that will contain correction optics, planned for installation in 1993; (2) for the accurate alignment of the secondary mirror of the telescope; and (3) to analytically compute noise-free point-spread-functions (PSFs) to optimally deblur the images presently being collected by the HST. The analytic PSFs are particularly important for the HST's Wide-Field/ Planetary Camera (WF/PC) for which the PSF is highly space-variant.

by J. R. Fienup
Atmospheric Compensation Using Laser Beacons

Researchers at MIT Lincoln Laboratory and the Air Force Phillips Laboratory have demonstrated real-time correction of turbulence induced atmospheric wavefront distortions using laser beacon adaptive optics. These experiments could have a profound impact on the course of astronomy during the next decade.

by Robert Q. Fugate
Kaleidoscopes: More Than Fun

In so few instances has history produced a single item that has brought such brilliance to an era as the kaleidoscope did 180 years ago. Yet this simple optical device, often described as merely a method of viewing reflections of an object, has impacted daily life far more than as an early toy. The kaleidoscope reveals many of the basic principles of optics, and for this reason is also a great educational tool.

by Stephen D. Fantone
Large Two-Dimensional Arrays Of Phase-Locked Vertical Cavity Surface Emitting Diode Lasers

Coherent arrays of vertical cavity surface emitting lasers (VSCELs) are useful for applications involving high power coherent optical beams, e.g., optical computing, multichannel optical interconnects, and free space communications. One of the unique properties of VCSELs is their low roundtrip gain, generated in active regions as short as ~10 nm, which requires high-Q optical cavities to achieve lasing in these structures. However, this feature makes it possible to define a VCSEL element simply by patterning one of the laser mirrors, which allows realization of high density, two dimensional (2D) arrays of VCSELs with high inter-element optical coupling.

by Eli Kapon and Meir Orenstein
Lasing In Sub-Micron Wide Vertical Cavity Microlasers

Coherent arrays of vertical cavity surface emitting lasers (VSCELs) are useful for applications involving high power coherent optical beams, e.g., optical computing, multichannel optical interconnects, and free space communications. One of the unique properties of VCSELs is their low roundtrip gain, generated in active regions as short as ~10 nm, which requires high-Q optical cavities to achieve lasing in these structures. However, this feature makes it possible to define a VCSEL element simply by patterning one of the laser mirrors, which allows realization of high density, two dimensional (2D) arrays of VCSELs with high inter-element optical coupling.

by Eli Kapon and Meir Orenstein