December 1995

Absorption Lineshape and Propagation Effects in Multiple Quantum Well Structures

Jérôme Faist, Federico Capasso, Carlo Sirtori, Deborah L. Sivco, Albert L. Hutchinson and Alfred Y. Cho, AT&T Bell Laboratories, Murray Hill, N.J.

Quantum Cascade Laser with a Vertical Transition and an Electron Bragg Reflector.

Spectrodetector: Novel Monolithic Wavelength Meter and Photodetector

Recently, we have introduced and demonstrated a novel approach using the ratio of two back-to-back detectors with a wavelength dependent reflector (Distributed Bragg Reflector, DBR) in between.5 The DBR is designed to have a weak wavelength-dependent reflectance which provides the necessary spectral dependent ratio for the two detectors. This ratio has a one-to-one correspondence with the signal wavelength, and thus can be used as a wavelength meter. Each photodetector can still be used as a receiver for data in addition to the wavelength reading.

Second Harmonic Generation: Toward an All-Optical Transistor

One of the goals in the development of all-optical alternatives to electronic processing has been development of an optical equivalent to the semiconductor transistor. Different schemes have been suggested for all-optical (AO) modulators and transistors, most of them based on third-order nonlinearities. More recently, after the experiments by DeSalvo et al., attention has been paid to the cascading of second-order nonlinearities. The possibility of using cascaded quadratic effects to exploit the coherent nature of parametric interactions has spurred interest toward analog AO processes in second-order material systems, first among them the transistor.

Depth of Focus Enhancement and Twisted Beams Using Radial Harmonic Pupil Filters

It is well known that when a plane wave crosses a simple spherical lens, it is focused at the lens's front focus. When one measures the intensity profile along the optical axis, one sees a bell-like curve around the focus, i.e., a strong intensity peak in the focal point that falls off rapidly as we move away from it on either side. The goal we address is avoiding this typical behavior of the light.

Optical Technique for Simulating Severe Phase Distortion Effects on Imaging System Performance

Anew technique was recently introduced that allows study of imaging system performance in the presence of controlled phase distortions. Previous methods for introducing phase distortions into an image relied on a technique such as using a cell of heated water having turbulent refractive index fluctuations along the path of light propagation, or heating air to produce strong phase distortions. The latter method was shown to produce turbulence that was reasonably homogeneous, isotropic, and, within a specified range of parameters, to agree with the Kolmogorov model for the phase structure function.

Refraction and Diffraction in the First-Order Born Approximation

Recently it was demonstrated that refraction, reflection, and diffraction of a field distribution at an interface can be treated by the first-order Born approximation. Therefore, the Born approximation can be applied to various design problems like the design of blazed phase gratings or surface relief elements. Even "classical" optical lens systems, consisting of single refracting surfaces, can be treated with this new approach. The calculation of the transmitted and reflected field can be performed by the use of a fast Fourier algorithm only.

High-Efficiency Multilayer Dielectric Diffraction Gratings

Diffraction gratings have been produced by mechanical ruling since 1883 and by interferometric or "holographic" techniques since the early 1970s. Whether produced by mechanical ruling or interferometric exposure, high diffraction efficiency is achieved in conventional reflection gratings by overcoating the grooves with a metal that exhibits high reflectance over the wavelength of interest. Most metallic gratings exhibit a diffraction efficiency determined by the shape and depth of the groove profile and the reflectivity of the metal. Due to the inherent broadband reflectivity of metals, frequency selectivity is accomplished only by dispersion. Finally, the low threshold for optical damage of metallic gratings limits their use with high power lasers.

Low-Threshold Optical Switching in Non-uniform Nonlinear Distributed Feedback Structures

Photonic band structure of a nonlinear distributed feedback (DFB) device can be altered by increasing the intensity of the input light. Local changes in refractive index of the DFB structure lead to a variety of all-optical effects including switching, multistability, low-velocity energy transfer, pulse generation, and pulse shaping. However, a strictly periodic nonlinear DFB (NLDFB) must be operated at prohibitively high intensity levels, requiring both high-power sources and low-absorption nonlinear materials.

Optical Characterization of Photonic Microstructures

Microstructured photonic materials are expected to show full photonic bandgap (PBG) effects if the periodic arrangement of the photonic lattice is appropriately chosen and there is sufficent refractive index contrast. A PBG crystal can be seen as a three-dimensional Bragg reflector for photons. Such materials may eventually lead to a new generation light emitting devices with ultralow threshold, high efficiency, and very low noise. The fabrication of suitable three-dimensional photonic crystals at optical wavelengths is difficult and may not be practical for some time, which is why our work is focused on lower-dimensional patterns embedded in a waveguide structure.

Experimental Demonstration of Photorefractive Resonator for Adaptive Fault-Tolerant Coupling

Recently, we have successfully demonstrated that a PSPCR pumped by an injected laser beam can indeed automatically sense and track the location of the fiber facet and diffract the input beam into the fiber.

Excitation of Morphology Dependent Resonances of Microspherical Cavities Using Optical Fibers

Optical cavities are used extensively to enhance processes that depend on the interaction between light and matter due to their strong frequency selectivity and sharp dispersion near resonance. Most of the cavities that are used in optics have relatively large dimensions and modest Qs (<10,000). A new type of optical resonator, which has been the focus of increased attention this year as a possible photonic device, is the spherical dielectric microparticle (SDM).

Small-Bore Hollow-Glass Waveguides for Broadband, Infrared Transmission

Anew class of hollow glass waveguides has been developed for the delivery of both broadband and infrared laser energy. These hollow waveguides have the advantages of high power thresholds, low insertion loss, no end reflection, ruggedness, and small beam divergence. While these waveguides were initially developed for CO2 laser power delivery, they are today one of the most attractive alternatives for the transmission of infrared radiation for broadband spectroscopic and radiometric fiber sensors as well as for laser power delivery in surgical and industrial applications.

Observation of Two Dimensional Spatial Solitary Waves in a Quadratic Medium

Using intense fields, newly developed second order materials, such as KTP, which show a quadratic response with the optical field, can produce large phase distortions. Such effects are more commonly associated with an ultrafast change in the refractive index following the intensity of light. In a few materials with large second order response such as KTP, the coupling (energy exchange) between the fundamental and second harmonic fields can be the dominant process inducing large phase front distortions.

Transmission of Eight 20 Gb/sec Channels over 232 km of Conventional Single-Mode Fiber

Installed telecommunication lightwave systems are now being upgraded with erbium-doped-fiber-amplifier (EDFA) optical repeaters. Less expensive than full regenerators which receive, reshape, and re-transmit the optical signal, the EDFAs also allow future capacity increases through the use of wavelength-division multiplexing. These amplifiers operate at approximately 1555 nm, where the loss of conventional step-index fiber, used in almost all installed systems, is a low 0.2 dB/km.

Multiple Colliding Pulse Mode-Locked Quantum Well Lasers

We present a multiple colliding pulse mode-locked (MCPM) laser configuration, that can generate one, two, three, or four pulses in the cavity, giving first (fundamental) to fourth harmonics of the repetition rate.

Micromachined Wavelength Tunable Optoelectronic Devices with Record Tuning

Much as the vacuum tube and radio transformed communications, optical devices and fiber optic communications seem destined to change our lives. Wavelength division multiplexing (WDM) will be an important tool in this transformation. The extent that it will be implemented strongly depends on the availability of low-cost, wavelength-engineered optoelectronic devices and loosening of stringent WDM network requirements.

Enhanced Nonlinear Optical Response of Nano-Composite Materials

Is it possible to combine two or more optical materials in such a manner that the effective nonlinear optical susceptibility of the composite material exceeds those of its constituents? Recent theoretical and experimental results indicate it is possible, and suggest that the construction of composite materials may constitute a generally useful method for obtaining materials with desirable nonlinear optical properties.

Polarization Dependence of Ultrafast Nonlinear Refraction in Semiconductors at the Half-Bandgap

All-optical switching using half-bandgap non-linearities in semiconductors has received an increasing amount of attention. This is due to an increased understanding of the nonlinear optical mechanisms (in particular, AlGaAs at communication wavelengths has a relatively large nonlinear refraction coefficient—on the order 10^3 times larger than silica) and the maturing of fabrication technologies which allow development of efficient all-optical devices in the 1.55 μm telecommunications window.

Holographic Grating Formation in dye- or Fullerene-C60-Doped Liquid Crystals

Liquid crystals possess many unique physical, optical, and electro-optical and nonlinear optical properties. In particular, nematic liquid crystals (NLC) are characterized by large birefringence that spans a broad spectral range, and large susceptibility to ac, dc, and optical fields. Researchers at the Pennsylvania State University recently discovered a nonlinear electro-optical effect in dye- or fullerene C60-doped NLC films. They have demonstrated, by theory and experiments, the possibility of using low power lasers of a few milliwatts, in conjunction with a small dc field to induce persistent realignment of the liquid crystal director axis (the equivalence of the c-axis or symmetry axis of a birefringent crystal), and therefore a "stored" refractive index change in these films.

Quasi-Phasematched Optical Parametric Oscillators in Periodically Poled LiNbO3

Quasi-phasematching (QPM) is an alternative to birefringent phasematching that is especially useful for optical parametric oscillators (OPOs). QPM can be implemented in ferroelectrics, such as LiNbO3, by building into the crystal a grating of domain-reversed regions. Because the grating is controlled by the design of a lithographic mask, phasematching can be achieved independent of inherent material properties. Thus, QPM permits non-critical phasematching of any wavelength at any temperature, within the transparency range of the material. In addition, QPM interactions can use the largest element of the nonlinear susceptibility.

Gain Theory of Wide Gap Semiconductors

Semiconductor lasers based on wide-bandgap semiconductor compounds have substantial application potential because they can provide output with wavelengths covering almost the entire optical spectrum, including the range from green/blue to ultraviolet. At present, operation of II-VI semiconductor lasers has been demonstrated at room temperature with injection current pumping. Light emitting diodes (LEDs) based on group III-nitride heterostructures are commercially available, and lasing in bulk GaN was recently achieved using optical pumping.

High Brightness, Eye-safe Lasers

The 1.5 μm spectral region is of significant interest because it is "eye-safe" with low loss atmospheric and fiber transmission. Applications involving these special features have driven a significant amount of research on high power laser sources at this wavelength.

Young's Double-Slit Interferometry within an Atom

Young's double-slit interference is fundamental to our understanding of the coherence properties of any wave phenomenon. It has played a major role in the development of optical coherence theory. The quantum mechanical deBroglie waves described by Schrödinger's equation have coherence properties that can be studied in much the same way as optical waves.

Gain in Strongly Confined Quantum Dots

Quasi-zero-dimensional systems have been proposed as superior gain media for a laser because they should exhibit a delta-function like density of states and therefore a high inversion and a low lasing threshold. We present theoretical and experimental studies of the ultrafast dynamics of optical gain in strongly confined CdSe quantum dots in a glass matrix, which serve as a model system for quasi-zero-dimension semiconductors.

Visual System Modeling: Putting the Pieces Together

Studying the optical performance of the human eye is a central problem in physiological optics. In recent years, the topic has received increasing attention because of the potential applications in ophthalmic optics. The double pass (or ophthalmoscopic) technique has been used over the last 40 years as an objective method to estimate ocular image quality. It records the light reflected back in the retina when the eye forms an image of an object test. This external retinal image (aerial image) is used to estimate aberrations of the eye, point and line spread functions, and the ocular modulation transfer function (MTF).

Watching Proteins Fold with Transient Laser Spectroscopy

The processes by which a protein's amino acid sequence forms a unique three-dimensional structure remains one of the most challenging issues in fundamental and applied biomolecular science. Studies of this biological problem are complicated by the need to study dynamical 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.

Magnetic Resonance Imaging with Laser-Polarized Noble Gases

In its present form, magnetic resonance imaging (MRI) produces images by mapping the hydrogen nuclei in the tissues of the body. A new implementation of MRI using laser-polarized noble gases has recently been demonstrated wherein lasers are used to enhance the MR signal from noble gases such as3He and 129Xe, making them easily observable in a conventional MRI scanner. Initial experiments have yielded spectacular MR images of the lungs of laboratory animals, and recently, images of human lungs. This technology should provide functional information that may be important in evaluating and treating a wide range of respiratory problems such as emphysema and asthma.

Optical Information Processing Using Free Space Interconnects And Smart Pixel Arrays

The emergence of page-oriented optical memories as viable optical storage technologies requires the development of an efficient optoelectronic interface between the memory and the electronic computer. This interface must exhibit some intelligence without substituting for the electronic processor itself. For example, the interface could filter data on-the-fly from the optical memory to the electronic computer, thus alleviating any potential bottlenecks that may arise from the different data rates used by the two technologies.


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