Low threshold electrically-pumped vertical-cavity surface-emitting micro-lasers

Vertical-cavity surface-emitting lasers, show promise for a variety of applications. Use of large coherently-coupled arrays could provide high power, cheap laser sources. Smaller arrays could accomplish high-speed communication communication between electronic chips, overcoming a bottleneck that presently limits the speed of computers. In the longer term, arrays of laser logic gates may be used for photonic switching in communication networks or for general purpose computing. For information processing applications, minimizing the threshold current is essential. The lowest threshold edge-emitting lasers contain a single quantum well and require ~0.55 mA. Minimum thresholds will be attained by minimizing the volume of active material in the laser, which in turn requires high-reflectivity mirrors.

Single and multiple element 4-pass phase conjugate master oscillator power amplifier using diode lasers

Scaling laser diodes to high optical powers demands large emission apertures. Simple broad-area structures or laser diode arrays suffer from degradation of both the spectral and spatial mode as the emitting aperture increases in size. In addition, when multiple elements are combined, proper phasing is a difficult and sometimes impossible task.

Optical switching in polydiacetylene-based directional couplers

Waveguide directional-coupler devices have been fabricated from thin films of solution-processible polydiacetylenes. When operated at a wavelength of 1.06 μm, these devices exhibit a variety of nonlinear transmission and switching phenomena due to both fast electronic and slow thermal nonlinearities in the polymer. Such results, obtained on prototype device structures, give important information on the operating conditions and constraints relevant for future polymer based on nonlinear optical devices.

Excitonic and phonon-mediated optical Stark effect in a conjugated polymer

The last several years have been marked by steady progress in the understanding of optically induced non-linearity of reduced-dimension excitons. The two-dimensional excitons associated with GaAs-AlGaAs quantum wells have been extensively investigated. For nonresonant excitation, the nonlinear response is dominated by anharmonic interactions among virtual excitons and virtual carriers. Virtual particles exist only in the presence of an optical field; their influence on optical properties disappear with the passing of that field. One well known signature of this instantaneous nonlinear response is the optical Stark effect, i.e., a blue shift of the exciton transition for below resonance excitation.

Nonlinear optical birefringence in liquid suspensions of shaped microparticles

The optical properties of microparticle suspensions have been of considerable interest for well over a century. Recent research on the nonlinear optical properties of shaped microparticles suspension demonstrates that such media still command the attention of the optics community.

Phase matching with time varying inhomogeneous fields

Bistability and chaos of counterpropagating laser beams

We have studied the mutual interaction of two counterpropagating laser beams in a sodium atomic vacuum.

Lorenz-like dynamics in coherently pumped lasers

Avery active and profound revolution in science ignited some 25 years ago with the realization, after the pioneering work of Lorenz, that, just as quantum systems and systems with a large number of particles are unpredictable, low-dimensional, deterministic nonlinear systems also are able to display transitions not only from disorder to order (synergetic behavior) but also from order to complexity (temporal and spatio-temporal instabilities and chaos).

Second harmonic generation in periodically-poled LiNbO3

Compact, solid-state sources of coherent blue light have applications in biomedicine, displays, printing, and optical storage. Because blue semiconductor diode lasers are not currently available, techniques for the efficient nonlinear optical frequency conversion of infrared diode lasers are being developed. Materials with high nonlinearity and good optical quality are therefore in demand. Engineering the optical properties of a well-studied material such as lithium niobate (LiNbO3 ) provides an alternative to the search for entirely new materials. Using quasiphase- matching in LiNbO3 , we have accomplished collinear cw second harmonic generation (SHG) with wavelengths and nonlinear coefficients that have been impossible to phase-match using only the material birefringence.

Intersubband far infrared emission pumped by resonant tunneling in superlattices

Far infrared emission has been obtained from a semiconductor superlattice electrically pumped by sequential resonant tunneling. The experiments performed by Helm and co-workers1 realize an idea that has been incubating since the conception of semiconductor super-lattices and artificial engineered bandstructures.2,3 They are the first experiments that use tunneling to create hot carriers capable of emitting radiation and establish a fundamental step toward the realization of an intersubband laser.

Semiconductor quantum wire lasers

The performance of semiconductor lasers has been improved by the use of quantum confinement in one dimension. In these conventional quantum well (QWL) lasers, the charge carriers recombine and emit light in extremely thin (usually less than 100 A) layers of a lower band gap semiconductor such as GaAs cladded by a higher band gap barrier compound, e.g., AlGaAs. The quantum confinement of carriers in the direction normal to the QWL plane gives rise to discrete energy levels, which effectively narrows the spectral profile of the optical gain. This, in turn, leads to lower threshold currents, higher modulation bandwidths, narrower spectral linewidths, and reduced temperature sensitivity.

Observation of optically pumped intersubband emission from quantum wells

There is growing interest in new devices based on confined states in quantum wells. Optical transitions between subbands are of special interests because they can be tuned over most of the far- and mid-infrared spectrum. Spontaneous emission from transitions between subbands in isolated quantum wells has been observed by Helm et al. by electrically heating the electrons. We describe here the observation of radiative intersubband transitions by optically excited carriers in a quantum-well structure.

Strain-induced lateral confinement of excitons in semiconductor quantum wells

There is a worldwide effort to achieve efficient and controlled confinement of carriers to "quantum wires" and "quantum dots". Use of these structures, of dimensionality lower than the by-now-familiar quantum wells, is expected to improve the performance of lasers and nonlinear optical devices. With varying degrees of success, teams of researchers have made quantum wires and dots in various ways—for example, by etching freestanding structures, by defining buried structures by ion implantation, and by growing them directly on patterned or unpatterned substrates.

Quantum-confined Franz-Keldysh effect in CdTe quantum dots in glass

There has been considerable interest in the electric-field dependence of optical properties of semiconductor quantum-well structures. We have extended these electric-field studies to quantum dots, i.e., to structures confined in all three dimensions. The new CdTe quantum-dot glass first reported here is especially suited for electric-field effects because of the large Bohr radius, aB = 73 Å; it has as many as six quantum-confinement peaks in the absorption spectrum with dot radii, ro = 36 Å. The observed changes in the absorption spectrum (see top figure) are interpreted to arise mainly from a red shift of the lowest transition and the redistribution of the oscillator strength as forbidden transitions are opened up by the symmetry-breaking action of the external field. This interpretation is made plausible by a spherical-coordinate-system calculation (upper figure), that neglects the Coulomb interaction; it is similar to that for cuboidal structures by Miller, Chemla, and Schmitt-Rink, and predicts a differential absorption spectrum in good qualitative agreement with the data.

Optical learning machine for multi-category classification

The recent increased activity in the area of neural networks is mainly due to their potential to mimic the computational ability of humans, in particular the ability to learn. While the ordinary computer requires precise rules to perform a given task, the neural computer has the ability to learn its own rules from examples presented to it. This gives the neural computer the potential to solve problems not possible by artificial rule-based systems.

Femtosecond multiple pulse impulsive stimulated Raman scattering

Ongoing advances in ultrafast laser technology have led to the generation and spectroscopic use of pulses whose duration is shorter than the time required for most nuclear motions—e.g., molecular translations or rotations, lattice or molecular vibrations, etc. In principle, these elementary motions can now be observed in real time.

Atomic fountains and clocks

Momentum exchange between light and atoms causes mechanical forces which, in recent years, have been used to slow thermal atoms to speeds on the order of a few photon recoils (~10 cm/sec). At these low speeds, atomic trajectories are significantly altered by gravitational forces. Recently, laser cooled atoms have been launched upwards on vertical trajectories at speeds for which gravity reverses the atom's direction in approximately 30 cm.

Laser cooling of neutral atoms

The past year has seen major advances and surprises in laser cooling of neutral atoms in optical molasses— an arrangement of counterpropagating laser beams that strongly damps the motion of atoms. For atoms at rest in equal intensity, oppositely propagating laser beams, the total force is, of course, zero. But for atoms moving in light tuned to a frequency below the atomic resonance, the traditional view of optical molasses was that the Doppler shift causes atoms to absorb light more strongly from the beams that oppose their motion. This produces a damping force that both cools the atoms and provides a viscous confinement within the intersecting laser beams (although there is no restoring force). In particular, the simple theory for two level atoms predicted that the lowest atomic kinetic energy would be hΥ/4, where Υ is the decay rate of the atomic state excited by the laser. For sodium atoms cooled on the yellow D-line transition, this corresponds to a temperature of 240 μK.

Two mode fibers for self-referencing force sensors and mode filtering

The advent of microprocessor controlled engines in automobiles has prompted an increase in sensor research. Optical sensors are being considered because of the immunity to EMI and the potential for interfacing to optical buses for future auto networks. A formidable problem is the measurement of cylinder pressure to gain information regarding the current state of an engine. With the pressure pulse data, a feedback control can be used to optimize engine performance. The peak combustion pressure location versus crank angle allows optimization for reduced emissions and better fuel economy. Effectively, with this system operational, the automobile is self tuning in real time.

Optical time-shifters for microwave phased array antenna steering

In electronically steered phased array antennae, pointing is accomplished by controlling the relative microwave phase between successive antenna elements in the array. The phase of these radiation elements can be shifted by using lumped-element components typically less than a wavelength long. This approach suffers, however, from the drawback that the constructive interference condition is satisfied only over a narrow frequency band, giving rise to undesirable phenomena such as "beam-squinting." An alternative technique known as true-time-delay steering imposes differential delays between successive antenna elements in real time. This approach is inherently broadband, but is presently implemented by routing the RF signals through bulky coaxial cables or waveguides whose length can be as long as LsinΘm, where L and Θm are, respectively, the aperture size and maximum steering angle of the antenna.

Control of plasmas parameters by ultrashort pulse multiphoton ionization

Multiphoton ionization has been widely studied because of its implications for atomic and molecular physics. Probably the most active area of current research concerns the energy of the electrons produced as a result of MPI. Experiments have found that electrons can absorb considerably more energy than the minimum required for ionization. The absorption process has become known as above-threshold ionization (or ATI). Strangely, it is rarely noted that the products are plasmas and that a detailed knowledge of both MPI and ATI could be extremely important to plasma physics.

Real-time enhancement of submicron defects using photorefractives

We have developed a new approach using photorefractives for real-time inspection of periodic masks or cracks and defects in non-periodic objects. The approach is based on Fourier transform, holographic recording of the object, filtering, and phase-conjugate readout using a photorefractive crystal. These processes are performed simultaneously to allow real-time operation. The object to be inspected (top part of the figure) is placed in the input plane, and the defect-enhanced image in the bottom part appears at the output plane, in a time limited only by the time constant of the photorefractive material. This time constant is material and light intensity dependent and ranges in our experiments from 50 to 250 msecs. This method differs from previous approaches in that all operations are carried out in the Fourier domain, no object dependent mask is needed, and real-time operation is achieved without the need for careful alignment of filters and masks.

The back-lighted Thyratron

A new optically triggered switch that is useful for high power applications was discovered1 and is being developed at USC. The optically triggered switch is a back lighted thyratron (BLT), a name that is indicative of its method of operation and the dense glow characteristic of its discharge.

Multifacet mirrors for the extreme ultraviolet

Until recently, the prospects for efficient normal-incidence reflectors for the extreme-ultraviolet (XUV) spectrum, nominally wavelengths between 10 and 100 nm, have been very limited. In this spectral range, all materials have significant absorption and the index of refraction does not deviate much from unity. The few exceptions with moderately high (≥ 40%) reflectance at 0° include: 1) SIC, either single-crystal or chemically vapor deposited surfaces, with reflectance between 40 and 50% for wavelengths longer than 60 nm, and 2) multilayer interference reflector stacks of Mo and Si films with peak reflectance-performance just over 60% at 13 nm and lower values for designs optimized for adjacent wavelengths between 10 and 20 nm. Indeed, full development of the multilayer option is currently a major goal of a number of research groups around the world.

Production of kinoforms by single point diamond machining

Kinoforms, are a subset of holographic optical elements. They have the advantage of being highly efficient and they are computer-generated, not optically constructed, so desired wavefront deformations can be readily achieved.

Hybrid diffractive-refractive telescope

Many optical design advantages including smaller lens curvatures and larger aperture elements may be obtained from mixing diffractive and refractive elements to form hybrid lenses. These advantages are also found in hybridizing conventional optical systems. Hybrid diffractive-refractive optical systems are particularly interesting due to recent advances in highly efficient, low scatter, broadband fabrication techniques such as bleached silver halide, dichromated gelatin, photopolymer recording materials, and blazed surface-relief techniques.

Wide field diffractive lenses for imaging, scanning, and Fourier transformation

Diffractive optical elements offer the potential to reduce significantly the weight, size, and cost of a variety of optical systems that currently use refractive and/or reflective components. Unfortunately, the utility of conventional holographic lenses has been plagued by uncorrected field aberrations, particularly coma and astigmatism. However, recent advances in surface-relief diffractive lenses provide the optical designer with enough degrees of freedom to eliminate third-order coma, astigmatism, and field curvature.

High-performance, externally modulated analog fiber-optic links

Fiber-optic links are finding an increasing number of uses in analog RF systems. In most applications, information is impressed on the optical carrier by modulating the current of a semiconductor diode laser, a process referred to as direct modulation. Alternatively, the laser is operated continuously, and an external modulator is coupled to the laser output. Some advantages of the latter approach, which has been known for some time, are now being recognized.

Dynamic optical interconnects

Dynamic optical interconnects are reconfigurable routing networks that can interconnect high bandwidth optical data paths. The optical crossbar is an example of an attractive architecture that can be implemented using photorefractives. Once the routing network has been established, optical inputs are reflected, refracted, or diffracted passively to their respective outputs. Upon termination of the task, the network can be reconfigured and adapted to new routing requirements. The major advantage of this approach is the high optical transmission bandwidth (GHz), although the reconfiguration time may be slow (on the order of msecs or (μsecs).

Optical modulation using silicon

Light modulators are important components for optical communications systems and optical interconnection strategies. III-V semiconductors, as well as non-semiconductor crystals such as LiNbO3, are the traditional materials of choice for these modulators, but there is mounting evidence that silicon can also be used. The central role of silicon in the microelectronics industry makes this an interesting possibility.

Acoustically-tuned integrated-optic filter for many-channel wavelength switching

Twenty years ago, Harris introduced the concept of a narrowband, tunable optical filter based on the interaction of light and sound, but the practical use of such devices has been unimpressive until now, largely because the devices were bulky, power hungry, and not very efficient However, by combining the best aspects of integrated optics technology and the methods of surface acoustic wave electronics, we have fabricated sub-nanometer-bandwidth integrated-optic acoutically-tunable optical filters (ATOFs) that are compact, low in power consumption, and electronically-tunable over hundreds of nanometers.

Guided-wave magneto-optic Bragg cells and applications

Although magneto-optics in waveguide structures1 was among the areas actively pursued during the early phase of guided-wave optics research, research activity in this area was greatly reduced between the mid-70s and early 1980s. However, a revival of interest in this area has taken place recently.

Advances in waveguide photodetectors

Many signal processing functions needed for future broadband optical communications and switching networks will be most effectively realized in waveguide-based systems where optoelectronic integrated circuits serve as intelligent control points capable of recognizing and processing the optical signal as required. Fast photo-detectors that can detect light in a waveguide are key to the success of these systems. The year saw significant developments in the integration of photodetectors with both semiconductor and non-semiconductor waveguides.

Acousto-optic photonic switch

In recent years, there has been interest in developing photonic switches. A promising class of space-division architectures is based on dynamic beam steering, where input light is deflected to one of several output ports. Such an architecture uses the three-dimensional processing capability of optics, allowing nonblocking NxN switches to be constructed with only N deflectors. In addition to the low hardware complexity, low losses are possible even for large switches because light must encounter only one deflector, regardless of the number of output ports. Optical crosstalk is due to diffractive spreading of output beams in this type of switch and, typically, only nearest neighbor contributions are significant; therefore, crosstalk can also be limited to low levels when N is large.

16-channel heterodyne broadcast network at 155 Mb/sec

There has been considerable interest this year in exploring heterodyne reception techniques for use in fiber distribution networks, especially broadband-ISDN networks. These will likely be implemented using conventional fiber technology, but could later be upgraded using heterodyne receiver technology. Heterodyne systems provide several key advantages: the ability to transmit many channels simultaneously, to tune the receiver to select a single channel, and to detect smaller signals. The current thinking is that B-ISDN networks will use a double-star topology, employ a SONET digital transmission standard, and also include broadcast video.

Optical oceanography: The role of microparticles in attenuation of marine light

Historically, the treatment of optical oceanography has been limited to studies of relatively large (i.e. >1μm) organic (phytoplankton) and inorganic (sands, silts, and clays) particles. These particles have been considered the dominant variables in the attenuation of light (natural or artificial) in the sea, and their respective spectral absorption and scattering curves have been the focus of numerous applications of inversion techniques. Ultimately, the goal of these studies, irrespective of whether they are primarily theoretical or experimental, is to make use of characteristic optical signatures to define the particulate content of oceanic water masses. In this way, oceanographically significant processes such as primary production, current dynamics, and sediment transport can be provided with a unique tracer, since suspended particles are considered quasi-conservative in most marine environments.

Acousto-optics

During the past half century, the problem of strong two-dimensional acousto-optic interaction has been attacked by a variety of analytic and numerical methods. The model most frequently used is one in which a rectangular, non-spreading sound column is illuminated by a single plane wave of coherent light. The analysis then proceeds along the classical lines of wave propagation in periodic media. When framed in terms of the coupled modes of the unperturbed medium, this leads to an infinite set of coupled equations—the well known Raman-Nath equations. Using the eigenmodes of the perturbed medium results in Mathieu functions.

A new twist to optical polarization theory: Color U(2) computer graphics

The mathematical theory of optical polarization has been used many different ways since it was originated by Stokes and Poincare. In terms of group theoretical acronyms, polarization theory is called an "R(3)-SU(2) homomorphism," that is, a relation between a real three-dimensional (vector) space and a complex two-dimensional (spinor) space. The three-vector description of a two-level quantum system was popularized by Feynman, Vernon, and Hellwarth. Since then, other examples of quasi-spin vectors have been introduced in areas such as molecular physics.

Nonlinear Chinese tea

In a recent report, Hong-Jun Zhang and co-workers from the Institute of Physics, Chinese Academy of Sciences, report on strong nonlinear Kerr effects in Chinese tea, herbal medicine, and solutions of chlorophyll. They describe observation of self-focusing and self-phase modulation at powers around 5 mw, using a He-Ne laser. Their observed time constant is of the order of 100 msec and they calculate a nonlinear Kerr coefficient of about 3 x 10-5 e.s.u. Their solution is contained in a sample cell of 10 mm length. Illumination is by means of a 2 mm laser beam focused by a 30 mm lens into the cell.

New subgroup looks at laser standards

Anew subcommittee on Electro-Optical Systems has been added to the six international optics standards subcommittees now active in ISA/TC 172—Optics and Optical Instruments. Formation of the new group—SC 9—was prompted by the work that the EEC standards organization—CEN— has already begun on laser standards. The German Institute for Standards (DIN) will serve as the secretariat for SC 9.

A passively mode-locked Ti: Al203 laser using a nonlinear coupled cavity

Atitanium sapphire laser is a versatile laser with wide tunability (670 nm to 1100 nm) and high power capability. Researchers at MIT have recently demonstrated tunable femtosecond pulse generation in a titanium sapphire laser using a passive nonlinear external cavity. The coupled-cavity titanium sapphire laser provides a versatile source at a significant reduction in cost and complexity over short pulse dye laser technology.

High power, long life continuous-wave monolithic laser diode arrays

Multistripe monolithic AlGaAs laser diode arrays show considerable promise as sources of high power, high efficiency narrow bandwidth optical energy. Examples of applications include: a) optical pumping of solid state laser media, b) infrared illumination, c) laser soldering, and d) eye surgery. Power levels of 38 W have been demonstrated at room temperature under continuous-wave (cw) conditions for twenty 10-stripe lasers spaced along a 1 cm bar (2 mm total aperture width). We report cw operation up to 76 W and 55 W from 1 cm laser diode arrays with aperture widths of 3 mm and 2 mm, at heat-sink temperatures of 0° C and 23° C, respectively. We also show that a projected lifetime in excess of 5,000 hours is obtained at 10 W at 20° C heatsink temperature.