bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial pattern formation in multiwave mixing
Authors: Grynberg Gilbert
Published: February 2023

A nonlinear medium interacting with two counterpropagating pump beams is potentially unstable. This instability is related to a mirrorless four-wave mixing oscillation.1 Recent experiments done in vapors2 show that the behavior is strongly different according to the sign of the frequency detuning from resonance. When the laser frequency ω is larger than the atomic resonance frequency ω0, the oscillation appears as a sharp conical emission, the oscillation frequency being ω. In some cases the cone is fragmented into a hexagon. When ω < ω0, the oscillation is more difficult to achieve and appears as a very broad cone.
Compression of optical pulses from gain-switched laser diodes using a modified grating-pair compressor
Authors: Wiesenfeld J. M. , Kuznetsov M. , Radzihovsky L.
Published: February 2023

We have produced optical pulses shorter than 6 ps and at repetition rates between 2 and 8 GHz by compressing chirped pulses from gain-switched distributed feedback (DFB) semiconductor diode lasers operating near 1.3 μm. The chirp from a gain- switched DFB laser is approximately linear from blue to red during the portion of peak intensity of the pulse. Hence, by passage through a medium with normal dispersion, the duration of the pulse can be reduced. Previously, pulses from gain-switched single-mode diode lasers were compressed using optical fiber as the medium of normal dispersion.
Hybrid guided modes in biaxial dielectric planar waveguides
Authors: Maldonado Theresa A. , Gaylord Thomas K.
Published: February 2023

The guiding layer of a planar waveguide may be naturally biaxial or may become biaxial (e.g., in an electrooptic modulator). The goals of the present work are (1) to determine the effect of principal axes orientation on the effective index of a guided mode and (2) to present a quantitative classification scheme for hybrid modes in biaxial waveguides.
Resolution and signal-to-noise trade-offs in confocal scanning microscopy
Authors: Conchello José-Angel , Hansen Eric W.
Published: February 2023

The confocal scanning microscope achieves improved depth discrimination by the action of a small aperture in front of the detector. The confocal aperture rejects out- of-focus contributions and, in the case of fluorescence microscopy, results in a 3-D optical transfer function (OTF) that does not suffer from the missing-cone problem found in nonconfocal microscopes. Reducing the size of the aperture results in better depth discrimination, but it also reduces the amount of light reaching the detector and therefore reduces the image signal-to-noise ratio (SNR).
Model of dual rod pathways
Authors: Buck Steven L. , Knight Roger
Published: February 2023

We offer a model of dual rod pathways that incorporate recent psychophysical, physiological, and anatomical work. Flicker sensitivity studies suggest that one rod pathway, π0, has greater absolute sensitivity but poorer high temporal frequency responses than the other pathway, π0 (Conner, 1982; Connor and MacLeod, 1977; Sharpe, Stockman, and MacLeod, in press). The dual pathways may arise from differential electrical coupling of rods and cones via gap junctions as a function of adaptation level (Yang and Wu, 1989), so that π0 signals travel through rod pathways and π0 signals travel through cone pathways (Smith, Freed, and Sterling, 1986) before being recombined at ganglion cells or beyond. So far, the importance of dual rod pathways for processing other than temporal information is unclear. Our model of dual rod pathways generalizes their role to incremental and spatial processing and makes testable predictions to challenge the model. Predictions include more conelike spatial sensitivity for π0, differential dependence of π0 and π0 increment sensitivity on spatial and temporal parameters, and different interactions of cone signals with π0 and π0 in detection and adaptation. Support for the model comes in part from kinks in increment t.v.i. curves observed in various labs at –2 to 0 log scot td (the same as for flicker data).
Accommodation and chromatic aberration: effect of spatial frequency
Authors: Kruger Philip B. , Stevens Debra , Mathews Steven
Published: February 2023

The longitudinal chromatic aberration of the eye produces color fringes at edges that can have a profound influence on accommodation. We used grating targets to study the influence of spatial frequency on sensitivity to these chromatic cues. A high speed infrared optometer was used to monitor accommodation while the subject viewed gratings in a Badal optometer. Dioptic vergence was modulated sinusoidally (1-3D) by moving a sine wave grating (1-10.5 c/d) toward and away from the subject's eye at 0.2 Hz. Three special lenses were used to modify the chromatic aberration of the eye; giving four stimulus conditions: (1) normal (usual chromatic aberration was present); (2) neutralized (no aberration was present); (3) doubled (aberration was twice as large as usual); and (4) reversed (aberration was opposite to normal). Gain and phase plots show that subjects are most sensitive to intermediate spatial frequencies (3-5 c/d), whether the effects of chromatic aberration are present or not. Furthermore, while subjects display broad individual differences in sensitivity to these cues, such sensitivity seems to be independent of spatial frequency.
Dynamic bistability of an optical ring circuit having photorefractive gain, loss, and a photorefractive feedback coupling element: theory
Authors: Crouch David D. , Hermanns Anno , Anderson Dana Z.
Published: February 2023

We discuss the properties of an optical circuit with gain and loss provided by two-beam coupling in a photorefractive medium and with a photorefractive element as a programmable mirror to provide feedback. As with an ordinary ring resonator having photorefractive gain and loss the circuit can exhibit reversible or irreversible bistability and self-pulsing. In the limit that the feedback element has a response that is slow compared to the gain and loss, its behavior is much like the conventional case. However, whereas a conventional ring resonator has a well-defined detuning characteristic dependent on the round trip length, in the present circuit a fast photorefractive element continually compensates for changes in the round trip path length. The bistability and self-pulsing characteristics are strongly affected by the relative time constants of the three photorefractive media and by the relative magnitudes of the small signal gain and loss. In particular, the on state of the ring has a memory time determined by the time constant of the feedback element.
Floquer methods applied to atoms in intense laser fields
Authors: Shakeshift Robin
Published: February 2023

In the Floquet method, the dynamics of an atom in a monochromatic laser field is described within a time-independent framework. The atomic state vector, which satisfies the time-dependent Schrodinger equation, is replaced by exp (–iEt/ħ)|F(t)), where E is the quasienergy of the atom and where |F(t)〉 is periodic in t with period equal to the cycle time of the laser field. If |F(t)〉 is expanded in a Fourier series with terms exp (–inωt)t)|Fn〉, where ω is the laser frequency, the harmonic components |Fn〉 satisfy a set of time-independent coupled equations. The nth harmonic component represents the (real or virtual) absorption of n photons by the atom. In an ionization process (the active electron is initially bound) the quasienergy is complex; it has a real part which includes the AC Stark shift, and an imaginary part which is proportional to the total ionization rate. Large scale calculations of total ionization rates for hydrogen and helium have been performed. Departures from perturbation theory can be substantial. A knowledge of the harmonic components gives the time-dependent atomic dipole moment, which has been used to calculate harmonic generation rates in hydrogen. Further, absorption cross sections for electrons scattering from protons have been calculated.
Model for imaging three-dimensional, incoherently radiating objects with focal imaging systems
Authors: Sitter David N. , Rhodes William T.
Published: February 2023

The 3-D imaging properties of focal imaging systems are space variant and therefore cannot be described by a 3-D convolution with a point spread function. A model is presented that describes the imaging properties of focal imaging systems for incoherently radiating, 3-D object distributions.
Recent advances in phase conjugate laser amplifiers
Authors: Rockwell David A.
Published: February 2023

During the past decade, it has become clear that phase conjugation offers an approach whereby the performance of laser amplifiers can be significantly improved and extended. Early applications of phase conjugation to solid state lasers have demonstrated improved beam quality and the possibility of compensating static or thermally induced birefringence. This presentation reviews recent advances in phase conjugate amplifiers that establish significant additional capabilities. For example, phase conjugation is presently being applied to alternative laser amplifiers, such as semiconductor diode lasers and carbon dioxide lasers. Phase conjugation also offers the possibility of coherently coupling multiple parallel amplifiers, thereby allowing energy scaling well in excess of the level traditionally set by the maximum available size of any particular amplifier medium. In systems requiring precise pointing of a laser beam relative to a fixed reference direction, phase conjugation can be employed to eliminate any deviation in the pointing direction introduced by optical components or mounts.
Design and performance of a high average power zigzag slab laser
Authors: Summers Mark A. , Boyd Robert D. , Hackel Lloyd A. , Swift Charles D. , Seppala Lynn G.
Published: February 2023

The objective of this laser design is to efficiently extract energy from a low gain, glass zigzag slab in a single, near diffraction-limited laser pulse short enough to be efficiently frequency converted. This task is made difficult because the zigzag slab gain and optical quality are limited by thermally induced stresses resulting from repetitive pulse operation. Efficient extraction in these conditions can be achieved if the laser cavity losses are low to achieve a favorable gain/loss and if the propagation effects are controlled to uniformly illuminate the slab aperture.
A 551-nm diode laser pumped upconversion laser
Authors: Tong F. , Macfarlane R. M. , Risk W. P. , Lenth Wilfred
Published: February 2023

There have recently been several reports of visible upconversion lasers that are pumped by cw dye lasers.1,2 We report the first operation to our knowledge of an upconversion laser pumped by a semiconductor diode laser. Using a GaAlAs diode laser array as the pump source, upconversion lasing at 551 nm was observed in LiYF4:Er3+ (5%) at 41 K. The ~791-nm pump light populates the long-lived (~5 ms) intermediate 4I11/2 state of Er3+ and an efficient energy transfer upconversion process involving two ions in 4I11/2 results in excitation of the 4S3/2 upper laser level. When the near infrared pump laser was turned off rapidly the green laser continued to oscillate for several hundred microseconds, demonstrating that upconversion pumping occurs predominantly by energy transfer rather than sequential two-photon absorption. The relatively poor spectral overlap of the multimode output spectrum (~3-nm band-width) of the laser array and the 0.8-nm wide Er3+ absorption lines resulted in only 15% absorption of the pump light in the 3mm long laser crystal which had mirror coatings directly deposited on spherically polished faces. As a consequence, the pump threshold of 225 mW of incident diode laser power was relatively high; with 340 mW of pump power the green output was 100 µW. A much lower pump threshold of 13 mW of incident power was observed with a cw near infrared narrowband dye laser. With 130 mW of dye laser power a 551-nm output of 4.2 mW was achieved. This latter result suggests the use of index guided single-mode GaAlAs diode lasers for efficient pumping of erbium upconversion lasers.
Properties of a self-locking, externally doubled diode laser incorporating a ring cavity
Authors: Dixon G. J.
Published: February 2023

Passive optical locking of an 865-nm diode laser to a standing wave, external resonator containing potassium niobate has resulted in efficient harmonic generation at 432 nm.1 Our most recent experiments have yielded 1.5 mW of unidirectional blue output (3.0 mW total) at an input power level of 15 mW. The measured intracavity infrared buildup factor of >100 in these experiments indicated that total absorption and scattering losses within the resonator were below 1% and that >90% of the incident power was matched into the fundamental mode of the resonator. As demonstrated by Kozlovsky,2 further improvements in unidirectional conversion efficiency are possible by using a traveling wave ring cavity in place of a standing wave resonator. The use of a ring cavity also eliminates the need for optical isolation between the external cavity and the source laser. In this paper we describe a SLEDD laser in which a ring resonator is used to build fundamental power. Passive locking is accomplished by imaging the cavity throughput back onto the diode laser.
Optical fiber fabrication from formed bodies of colloidal silica
Authors: MacChesney John B.
Published: February 2023

Fibers made by the now standard vapor deposition processes have achieved performance levels limited only by the intrinsic properties of the fused silica which comprise them. Yet there is active exploration of other means of preparing optical waveguides. These are directed toward achieving ultimate economy while maintaining adequate performance. The strategy is to form silica into a waveguide configuration at low temperatures starting with submicroscopic silica particles (10-100 nm). The porous silica body once formed is then vitrified by viscous sintering like the soot formed by OVD or MCVD.
Cascade high repetition rate femtosecond amplifier
Authors: Ulman M. , Schoenlein R. W. , Fujimoto J. G.
Published: February 2023

We describe a multistage, copper vapor laser pumped, high repetition rate amplifier for femtosecond optical pulses using a compact design. Increased efficiency compared with earlier designs allows two dye cell stages to be pumped by a single copper vapor laser. The pump beam is delivered to antireflection coated cells1 via multimode optical fiber. Small signal single pass gains of 20 are achieved. Using three passes in each stage, 70-fs pulses from a colliding pulse mode-locked dye laser are amplified to >20 µj at 8-kHz repetition rate. The output from the first stage of the amplifier may also be used to generate a white light continuum from which different wavelengths can be selected and amplified in the second stage, thus providing a tunable femtosecond source.2 Alternately a single-mode fiber may be used between the stages for chirped pulse amplification. Dispersion is compensated after the second stage with prisms. This permits the generation of high intensity compressed femtosecond optical pulses.3 A modular design which cascades gain and nonlinearity provides a versatile and compact system that can be used for a wide range of femtosecond time-resolved spectroscopic studies.
High resolution optical profiler
Authors: Creath Katherine
Published: February 2023

An interferometric optical profiler using a Videk Megaplus camera interfacing to an 80386 based computer has been developed for measuring surface roughness and 3-D profiles at high resolution. The camera has 1320×1035 square pixels which are 6.8 µm on a side. Fields of view over 1024×1024 pixels can range from 35 to 4642 µm with spatial sampling intervals of 0.034-4.5 µm. With this camera the system is limited by the optical resolution and not the detector spacing. The computer system consists of an 80386 microprocessor and a floating point coprocessor, 16MB of RAM, and a VGA monitor with software running under Xenix 386. Digital data from the camera are captured using a frame grabber and displayed as live video on a monitor at 7 frames/s. The rms repeatability of measurements made with this instrument is better than 0.7 nm and can be half of this value with averaging.
Superregenerative detection: experiment and theory
Authors: Littler Ian , Balle S. , Bergmann K. , Vemuri G. , Roy Rajarshi
Published: February 2023

The initiation time for the intensity of an injection seeded laser is measured as a function of the externally injected signal strength. The operation of a radiation detector based on the measurement of initiation times is demonstrated, with a dynamic range of 7 orders of magnitude, bandwidth resolution of 5 MHz, and tuning range limited by the active medium of the laser employed. Such a detector, based on the Q-switched operation of an oscillator, is known as a superregenerative receiver. We report experimental measurements of the characteristics of such a receiver and explain the behavior with reference to a simple stochastic model of a laser with an injected signal. The weakest signals that can be detected are shown to be comparable to the level of spontaneous emission into the cavity mode, which corresponds to an average photon number less than unity for our system.
Power-independent, low astigmatism high power single-quantum well ridge waveguide laser
Authors: Ovadia Shlomo , Lau Kam Y.
Published: February 2023

Low astigmatism in high power semiconductor lasers is desirable for optical storage use. In general, the relationship between astigmatism and the waveguiding mechanism is known. In fact, the amount of index and gain guiding in a given laser structure plays an important role in determining the astigmatism at a given power. We measured the astigmatic length and its optical power dependency for AlGaAs singlequantum well graded-index separate-confinement ridge waveguide lasers and for conventional laser structures such as channeled substrate planar (CSP). At low output power (2 mW), we observed 0.8-µm astigmatic length for the single-quantum well laser compared with 4.1 µm for the CSP laser. As the single-quantum well laser output power is increased, the astigmatic length is almost power-independent, shifting <0.5 µm at 40 mW. In contrast, the astigmatic length of the CSP laser shifts dramatically to –0.25 µm at 40 mW. In addition, calculations of astigmatic length using previously published models do not take into account optical power dependency and yield values much larger than the measured ones.
Lasing properties and excited state absorption of Nd:Ba2MgGe2O7
Authors: Ferry M. J. , Kliewer M. L. , Reeves R. J. , Powell R. C. , Allik T. H.
Published: February 2023

Site-selective spectroscopy of Nd:Ba2MgGe2O7 (BMAG) reveals fifteen crystal field sites for the Nd3+ ion. The 4F3/2 to 4I11/2 peak transition wavelength varies from 1050 to 1080 nm at 10 K, depending on the site. Energy transfer between sites is thermally activated and time independent for excitation into the 2P1/2 and 4G5/2 + 2G7/2 manifolds. Selectively pumping any site at room temperature results in complete energy transfer to a single site that has a laser wavelength of 1054 nm. Laser slope efficiency measurements were performed at room temperature on a BMAG sample using 1-J, 60-µs pulses from an alexandrite laser. The slope efficiency (laser output power vs absorbed power) is linear at low pump power, saturates and curves downward at high power. The slope efficiencies were found to be pump wavelength dependent and ranged from 0.7 to 3.5%. Excited state absorption (ESA) of the pump photons is thought to be responsible for the differences in slope efficiencies and the blue-green fluorescence observed while lasing at 1054 nm. Several ESA transitions are used to model the results, and excited state absorption cross sections are given.
Growth and characterization of the spectra of EuAlO3:Ti and GdAlO3:Ti
Authors: Merkle L. , Allik T. H. , Verdun H. , Brauch U. , De Lafuente G. , Thomas L. M. , Behrens E.
Published: February 2023

EuAlO3 and GdAlO3, Ti doped, have been grown by laser-heated pedestal growth as potential visible/near IR lasers. The absorption spectrum of EuAlO3:Ti shows a strong, very broad absorption band in the visible and near infrared which is not consistent with that expected for Ti3+ in an octahedral field, or with intraionic transitions in other charge states of Ti. Its strength is indicative of a charge transfer transition. In addition, a strong ultraviolet band is evident above the absorption edge of undoped EuAlO3. Both bands are attributed to Eu3+ + Ti3+ → Eu2+ + Ti4+ charge transfer transitions. The absorption spectrum of GdAlO3:Ti in the visible region is consistent with Ti3+. The fluorescence band position at 610 nm and its room temperature lifetime of ~5 µs are consistent with a transition from the lower 2eg state to the 2t2g state. The 9×1013-Hz fluorescence bandwidth and estimated 36% quantum efficiency imply a peak stimulated emission cross section of ≃4×10–20 cm2. This cross section is smaller than that in Ti:Al2O3 and comparable with that of typical Cr3+ based laser materials. The absence of the charge transfer transitions in the spectra of GdAlO3:Ti is due to the third ionization potential of Gd being ~4 eV smaller than that of Eu. This should shift the charge transfer absorption well into the ultraviolet.
Codoping of Er:YAG and its effect on lifetimes of 4I11/2 and 4I13/2 of the 3-µm lasing transition
Authors: Sengupta S. , Djeu N. , Chang R. S. F.
Published: February 2023

The lasing transition between the 4I11/2 and 4I13/2 of Er3+ in YAG is self-terminating due to the significantly longer lifetime of 4I13/2. This paper describes the codoping of Er:YAG with Eu and Tb as deactivator ions to reduce this undesirable effect by energy transfer between the 4I13/2 of Er3+ and the codopant ions. Unique grouping of levels in Eu3+ and Tb3+ provide numerous levels between 4I13/2 and the ground state for fast multiphonon relaxation but none near 4I11/2. Crystals of 16% Er:YAG codoped with different Eu and Tb concentrations were grown by the laser-heated pedestal growth technique for measuring lifetimes of excited Er3+ ions. We found that with both Eu and Tb, doping at ~1% increased the ratio of upper to lower state lifetimes from 0.02 to better than 0.3. Higher doping did not increase this ratio as quenching of 4I11/2 became significant. This is likely due to cross relaxation involving ground state ions of the codopant. Although unable to achieve a ratio of unity and above, codoping Er:YAG should still lead to improvement in pulsed laser performance.
Beam reshaping in unstable resonators with nonlinear components
Authors: Piche Michel , Chandonnet Alain
Published: February 2023

Unstable resonators are an elegant solution to the problem of generating single transverse mode beams of large size. The size of variable reflectivity mirrors1 as output couplers makes these resonators particularly attractive since these mirrors eliminate the diffraction lobes caused by hard apertures. In this paper we show that the insertion of nonlinear components in unstable resonators modifies significantly the spatial shape of the emitted laser beams. The nonlinear components under consideration are introduced to favor axial mode locking; typical examples are saturable absorbers and nonlinear interferometers.2 Our analysis of the problem has revealed that a component with a nonlinear transmittance, such as a saturable absorber, can produce an effective variable reflectivity mirror controlled by the laser beam itself; experimental results obtained with a pulsed Nd:YAG laser have demonstrated a reduction in mode diameter by a factor of 1.6.
Hybridly mode-locked dye lasers: theory and experiment
Authors: Macfarlane Duncan L. , Casperson Lee W. , James Steven M.
Published: February 2023

We present a semiclassical model for a synchronously pumped mode-locked dye laser with a saturable absorber included in the cavity, a hybridly mode-locked laser. The model is verified through comparison with experiments performed on a Quantronix Nd:YLF pumped hybridly mode-locked dye laser operating with separate jets of rhodamine 6G and DODCI/DQOCI solutions. The effects of varying the absorber molarity and the saturation pararmeter are mapped theoretically for the two different saturable absorber dyes, DODCI and DQOCI. The nonlinear model is general enough to allow the numerical study of semiclassical and dipole orientational effects, and we show that the inclusion of these points is important for quantitative agreement with experiment.
Near infrared continuum generation in an optical fiber
Authors: Carruthers T. F. , Askins C. G. , Friebele E. J. , Shaw C. M.
Published: February 2023

Continuum radiation from 650 to at least 1800 nm has been generated in silica optical fibers pumped by 100-ps pulses at 1064 nm from a mode-locked Q-switched Nd:YAG laser. The peak spectral power density of the ~100-ps pulses exiting the fiber, as high as 8 W/nm near 1250 nm, is easily sufficient to serve as a broadband source for characterizing fast photodetectors. The 5-m length of continuum generating singlemode silica fiber has a numerical aperture of 0.13. It contains 1.8% P2O5 in the cladding and 1.2% in its core; the 6.2-µm core also contains 10% GeO2. We have tested a number of fibers drawn from the same preform but with differing draw tensions and diameters. Efficient near infrared continuum generation and efficient second harmonic generation seem to be mutually exclusive characteristics of these fibers.
High-power, monolithic, phase-locked arrays of diode lasers
Authors: Botez D.
Published: February 2023

Work on monolithic phase-locked arrays of diode lasers has been carried out for the last decade. Gain-guided arrays have demonstrated some impressive results, but the devices are inherently unstable. Positive-index-guided devices, while stable with changes in the drive condition, have been plagued either by self-focusing due to gain spatial hole burning (i.e., evanescently coupled devices) or by weak overall interelement coupling (i.e., Y-junction coupled devices). Arrays of negative-index guides (antiguides) are both immune to gain spatial hole burning as well as strongly coupled. Resonant leaky wave coupling1 allows, for the first time, that each element equally couples to all others (i.e., parallel coupling). Such devices are called resonant optical waveguide (ROW) arrays.2 ROW arrays have two main characteristics: (1) they must include means of suppressing oscillation of antiresonant array modes (e.g., out-of-phase modes); and (2) they have strong discrimination against adjacent mode oscillation (Δα = 10-20 cm−1). Out-of-phase modes have been suppressed by diffraction coupling,3 the incorporation of a monolithic spatial filter based on the Talbot effect,4,5 and/or enhanced interelement loss.5 Best results to date are diffraction-limited beam operation to 10× threshold and 450 mW from ten-element devices.5 Fringe visibility measurements confirm full coherence across the array. Nonresonant twenty-element devices have demonstrated inphaselike operation to 325 mW in beams with 80% of the energy in the main lobe, and lobe widths twice the diffraction limit.
Effect of gain and index nonlinearities on single-mode dynamics in semiconductor lasers
Authors: Agrawal Govind P.
Published: February 2023

The dynamic response of semiconductor lasers is generally studied by solving the single-mode rate equations which assume an instantaneous relaxation of carriers within the conduction and the valence bands. Even though the intraband relaxation time is typically ~0.1 ps, it can affect laser dynamics significantly. To the first order, the effects of a finite intraband relaxation time are included in the rate equations by assuming that the modal gain g decreases linearly with the intracavity mode intensity I, i.e., g = g0 (1–εI). This functional form of the nonlinear gain becomes questionable at high intensities. A nonperturbative solution of the densitymatrix equations shows that the modal gain saturates with the intensity as g = g0(I + I/Is)−1/2, where Is is the intraband saturation intensity. At the same time, the modal refractive index also becomes intensity dependent. We include the intensity dependence of both the gain and the refractive index in the single-mode rate equations and study how their inclusion affects the dynamic response of semiconductor lasers by considering the laser parameters such as the modulation bandwidth and the damping time of relaxation oscillations.
Understanding leaky-mode arrays via 2-D coupled mode theory
Authors: Hadley G. Ronald
Published: February 2023

The first insights into index-guided diode laser array behavior were provided by using the effective index method to solve for the modes of the individual waveguides and coupled-mode theory to describe how these mix to form the array modes. Although this procedure has been fruitful for the study of conventional arrays, new devices now being fabricated employ large index steps for defining the channels, and thus are not amenable to the effective index approximation. Even worse, the modes of these devices, when calculated numerically, are very complex. This fact, together with the enormous parameter space resulting from the many possible shapes and spacings of the buried ridges, lead to serious difficulties in mode categorization and device design. Sidestepping the effective index approximation, we use coupledmode theory directly by considering the active region and the buried ridges as separate 2-D waveguides. The device modes are then expressed as linear combinations of these waveguide modes. This picture serves to unify our understanding of index-guided arrays in general, and offers a means of understanding leaky-mode array operation in particular by dramatically reducing the parameter space to consist only of the propagation constants of the lowest-order waveguide modes and their coupling coefficient.
Optical properties of strain-confinement-defined quantum wires and dots
Authors: Kash Kathleen , Worlock J. M. , Mahoney Derek D. , Gozdz A. S. , Van Der Gaag B. P. , Harbison J. P. , Lin P. S. D. , Florez L. T.
Published: February 2023

One may strongly modify the linear and nonlinear optical properties of semiconductor materials by quantum confinement of carriers to wires or dots. We describe a new method for achieving this confinement, based on inhomogeneous strain, that applies to a wide variety of semiconductors. We deposit a compressively stressed overlayer onto a semiconductor substrate that contains, for example, an epitaxial layer, or a quantum well. The stressed overlayer is then patterned and etched into arrays of wires and dots. The inhomogeneous strain pattern thus produced in the substrate lowers the bandgap under the center of the wire or dot, creating both lateral and vertical electron, and exciton, confinement. For example, we have patterned strain in a GaAs epitaxial layer by etching a compressed diamondlike carbon overlayer into dots and wires of 100-400 nm in width. Potential wells for excitons in those structures, shown in photoluminescence red shifts, are >50 MeV, with electron zeropoint energies of up to 4 MeV for the smaller structures. We compare our photoluminescence, excitation spectroscopy, and photoluminescence decay time measurements for a variety of strain-confining structures with band structure theory based on finite-element calculations of the strain patterns.
Guided electron waves in semiconductor quantum wells
Authors: Gaylord T. K. , Glytsis E. N. , Brennan K. F.
Published: February 2023

The quantum mechanical wave behavior of ballistic electrons allows them to be guided by a semiconductor quantum well. These structures act as optical slab waveguides for guiding directions normal to the confinement direction. The allowed modes in an asymmetric quantum well slab waveguide are described quantitatively. Electron waveguiding can occur for energies above one or both of the potential barriers. Due to dispersion, each electron waveguide mode has an upper-energy cutoff as well as a lower-energy cutoff. An example waveguide consisting of Ga0.85 Al0.15AS (substrate), GaAs (film), and Ga0.70 Al0.30AS (cover) is presented. For [100] GaAs layer thicknesses of from six through thirty-one monolayers this structure is a single-mode electron waveguide. These waveguides are potentially useful in high speed electronic circuitry and as a central component in future electron guided-wave integrated circuits.
Alignability of optical interconnects
Authors: Ghosh Anjan K.
Published: February 2023

A theory for analyzing the alignability, the degree of difficulty of aligning the devices and light beams, of a given optical interconnection system is developed. From various experiments it has been found that the alignment of a basic optical interconnect consisting of one beam and one device is a random process, N(0,σ). The standard deviation of alignment is inversely proportional to the overall cost measure modeled as a function of the time spent, the skill of operators, positioning devices, degrees of freedom in positioners, etc. In an incoherent interconnect, Pf = prob (alignment with at least f% power coupling) = 1 exp[−(d + s +fs/50)2/2σ2], where d and s are the radii of the device and the spot, respectively, and d ⪰s. The alignability of the interconnect, A = ∫ Pf df. Aincreases with the device size and the overall cost measure indicating that alignment gets better if more money is spent or larger devices are used. Practical optical systems consisting of many optical paths and devices, such as, optical crossbar switches, can be modeled as series and parallel combinations of many independent basic interconnects. Thus, overall alignability of a large interconnect = IIAi, of all the component interconnects. From this concept of alignability guidelines for the design and deployment of optical interconnects can be developed.
Polymeric electrooptic devices
Authors: Holland W. R. , Singer K. D. , Wolk G. L.
Published: February 2023

The development of polymer materials exhibiting large second-order nonlinearities is an area of intense research activity. The goal of producing low cost and high performance photonic switching devices using such materials is an attractive one for telecommunications manufacturing. Incorporating nonlinear compounds into a glassy polymer matrix yields a versatile material that may be electrically poled to produce electrooptic properties. This paper reports on recent progress in the synthesis, alignment, and characterization of nonlinear polymers suitable for device demonstration. Important for the design and operation of photonic devices are the linear and nonlinear optical properties of materials. Because EO polymers resemble a distribution of dipoles in an embedding polymer, refractive index data can yield valuable information about a given class of compounds. After electric field poling, the induced birefringence of the polymer indicates the efficiency of the alignment process.
Applications of a phase conjugate Fabry-Perot to real time moire interferometry, alignment, and motion detection
Authors: Lo K. P. , Indebetouw G.
Published: February 2023

The field inside a Fabry-Perot cavity made of a conventional mirror and a phase conjugate mirror (PCM) is the sum of two terms, one proportional to the input wavefront, the other to its complex conjugate. Near threshold, the amplitudes of the two waves are nearly equal, while their relative phase depends on the phase of the pumps. In steady state, the interference of the input wavefront and its conjugate forms a selfreferencing interferogram having double the resolution of a conventional interferogram and little sensitivity to variations of wavefront amplitude. To a good approximation, the device displays the real part of the input. When an abrupt change affects the input wavefront, the moire between the new and the old interferograms is momentarily displayed during the finite response time of the PCM. This moire can be used to measure wavefront perturbations or shifts in beam alignments. To measure the motion of an object against a stationary background, a transducer (LCTV, for example) captures the scene at time intervals larger than the PCM response time and inputs it to the interferometer as phase information. The device then acts as a real time speckle interferometer, showing object translations as Young fringes superposed to the scene spectrum.
Investigation of the origin of microroughness in multilayer coatings through variation in the degree of isotropy
Authors: Amra C. , Grezes-Besset C. , Pelletier E.
Published: February 2023

Scattering in optical multilayers is a current problem. These losses are essentially due to the interface roughnesses of the coatings, and it is therefore important, for minimization of scattering, to have a good knowledge of the origin of microroughness: substrate quality, effect of materials, conditions and techniques of production. Much work has been done but, because of the number of parameters, the conclusions are often difficult and always relative to integrated values (roughness, global scattering, mean section of angular scattering). We show that the use of a causal model together with isotropy degree variations permits a detailed investigation of microroughness and leads to clear conclusions.
Induced absorption in thin films
Authors: Balasubramanian K. , Macleod H. Angus
Published: February 2023

Induced transmission filters with buried metal layers have been developed in the past to produce interesting filter characteristics. We have employed similar concepts to design filters that would show total absorption in 5-10-nm thick metal and alloy film. We report typical examples of such designs with intermetallic and semiconductor films and discuss potential applications, particularly in relation to thin film media for optical data storage.
Optical properties of quantum semiconductor crystallites
Authors: Brus Louis
Published: February 2023

Quantum semiconductor crystallites (typically 20-60 A in diameter) show a 3-D quantum size effect. In zero order they exhibit discrete excited electronic states, even though their crystal structure is the same as that of bulk materials. In this paper I consider electronic structure, coupling of excited states to crystallite vibrations, and the picosecond time scale relaxation dynamics. The crystallites are made by chemical synthetic techniques that chemically terminate the surface with organic molecules. Such surface passivation prevents crystallite fusion into bulk material. Photophysical hole burning and resonance Raman experiments on 40-Å diam CdSe crystallites show that the coupling of the 1s—1s lowest excited state to LO phonons, via the Frohlich interaction, is far weaker than in bulk material. However, coupling to low frequency modes is stronger than in bulk material and is surface sensitive. The very large oscillator strengths present cause nonlinear optical phenomena—population saturation, AC stark effects, and χ3 coherent coupling of pump and probe beams—to be prominent in the picosecond transient optical spectra.
Laser vaporization of ceramics and superconductors and optical spectroscopic characterization of the plume
Authors: Froben F. W. , Kolenda J.
Published: February 2023

The plume visible where a high power laser strikes metals, ceramics, or other composite material (like a high Tc superconductor) is investigated spectroscopically. Signals from neutral and ionized atoms, molecules, and clusters are visible in the emission spectra which are recorded time and spatially resolved, to allow some inside view into the dynamics of the vaporization process and possibly optimize thin film production of exotic materials. The measurement is thought of as an additional tool to characterize clusters. For identification of cluster size distribution the beam is deposited on a carbon film for electron microscopy measurement.
Compact joint transform correlator with thick photorefractive crystal
Authors: WU Shudong , Rajan Sumati , Song Qiwang , Yu Francis T. S.
Published: February 2023

Photorefractive crystals are promising materials for real time correlation detection. However, the finite thickness of the crystal severely limits the size of the objects to be correlated and the shift invariance of the correlator due to Bragg diffraction. In this paper a beam compression technique in a joint transformation correlator is employed for reducing the writing angle. The technique greatly alleviates the Bragg diffraction limitation on the thickness of the crystal and increases the diffraction efficiency from the reading beam. On the other hand, the technique requires a proportional increase in the transverse size of the crystal to keep the correlation resolution constant.
Demonstration of a real time acoustooptic image correlator
Authors: Molley Perry A. , Stalker K. Terry
Published: February 2023

A demonstration of a real time image correlator based on acoustooptic and charge coupled devices is presented. This architecture utilizes 1-D devices to perform the desired 2-D correlation. The 2-D correlation is performed as a series of multichannel time integrating correlations between each input image line and the reference template which is stored in an electronic memory. The rows of the reference template are introduced into the processor in parallel using a 1-D light emitting diode array. The correlation in the vertical direction is performed using a modified charge coupled device operating in the shift and add mode. Modifications which dramatically improved both the dynamic range and light efficiency of the original architecture1 were previously reported.2 This demonstration shows the real time capability of this high dynamic range acoustooptic correlator implementation. The real time correlation of a 64 × 64 pixel reference template with a 256 × 232 pixel input image at video rates is presented.
Recent results in nonlinear matched filtering
Authors: Ersoy O. K. , Yoon Young R.
Published: February 2023

Nonlinear matched filtering involves the transformation of the signal spectrum and the filter transfer function through a nonlinearity before they are multiplied in the transform domain.1 The resulting filter structures can be considered to be analogous to three-layer neural nets. They have better performance in terms of signal discrimination and lack of false correlation signals and artifacts than previously known filters. Because of nonlinearities, the analysis of nonlinear matched filters requires new approaches different from techniques valid in linear systems. We present recent results on analysis of nonlinear matched filters as well as their generalization to incorporate maximum clustering within class and maximum separation between classes in pattern recognition applications.
White light joint transform correlator
Authors: Yu Francis T. S. , Cheng Y. S.
Published: February 2023

A joint transform correlator (JTC) was proposed to avoid the alignment problem of the spatial filter in the traditional VanderLugt system. By incorporating some spatial light modulators in the optical systems, the JTC was applied to real time pattern recognition. Due to the many advantages, including higher signal-to-noise ratio, associated with the white light optical processing system, we investigate the possibility of utilizing white light illumination in the JTC. The spatial spectra belonging to different wavelengths are dispersed by a phase grating. One of the first-order spectrum is recorded and the coherence requirements are estimated. In the read-out process, the incident light is predispersed by a pair of parallel gratings before shining on the joint transform hologram. Only the object with the line joining its center and that of the reference object parallel to the grating lines can produce correlation spots. This suggests that, when spatial light modulators are placed in position, a 2-D correlation pattern can be generated with correlation spots at their corresponding positions if the input plane and the output detector are rotated synchronously.
Acoustooptic Deflector Configurations in Optically Active Uniaxial Crystals
Authors: Hoffman A. J. , VAN Rooyen E.
Published: February 2023

Optically active uniaxial crystals like paratellurite (TeO2) are widely used as interaction media in acoustooptic devices. Paratellurite finds its most important application in low frequency deflectors that utilize the [110] propagating slow shear wave. Optical activity plays a significant role in the behavior of such devices. In this paper acoustooptic deflector configurations in optically active media with arbitrary interaction planes are investigated by using an exact formulation. Attention is focused on the so-called optically rotated configuration, where the optical axis is offset from the interaction plane. The acoustic frequency at which tangential phase matching occurs, and the associated Bragg angles, are solved analytically as a function of the offset angle. Exact expressions are derived for polarizations of incident and diffracted beams, as well as for the effective acoustooptic figure of merit. A formulation is presented that describes the behavior of phased array Bragg cells in uniaxial crystals. This formulation demonstrates for the first time that wideband deflection around a desired center frequency can be obtained while having freedom of choice regarding the acoustic mode and interaction plane. The design of phased array deflectors in optically active paratellurite which utilizes the [110] propagating slow shear wave is discussed. It is shown that the birefringent phased array design has distinct advantages over the more conventional optically rotated or acoustically rotated configurations.
Optical interconnects for high speed applications
Authors: Hutcheson Lynn D.
Published: February 2023

The entire communications industry has been revolutionized by the rapid advances made by fiber optic technology. The information age has increased the demand for higher speed communication channels and higher speed processors and computers. Because of this, we are seeing this technology being applied to expand the number of applications, enhance performance, and relieve limitations imposed by electrical interconnections. Some of the new applications include interconnecting high speed computers, boards and chips, as well as on-chip clock distribution. This tutorial provides a state-of-the-art review and status of this rapidly advancing technology, including applications, systems, packaging issues, devices, and materials.
Electronic interconnect technologies: a moving target
Authors: Smith Larry
Published: February 2023

The objective of this presentation is to highlight the advances being made in electronic packaging and interconnection technologies, so that researchers in the optical interconnect community may have a better understanding of the actual problems and of the technologies against which they are competing. Chip-on-board technology—which employs bare die on a high density substrate—is being developed to address the technology requirements of VLSI devices currently in production and of tomorrow's 0.5-µm semiconductor devices, which will pose even greater challenges due to increases in device speeds, power, die size, and level of integration. Packaging technology will be critical not only for large mainframe or supercomputers, but also for workstations, peripherals, avionics, telecommunications, and consumer electronics, where cost, reliability, physical size, and/or testability may be at least as important as raw performance. In particular, we discuss: high lead count TAB for achieving >400 IO/chip at 4-mil pitch, copper polymide (Cu-Pi) substrates for chip-on-board packaging, electrical propagation of signals on Cu-Pi transmission lines, potential impact of superconducting transmission lines, air cooling techniques for 25-70-W chips, power distribution challenges, and test technology.