bluuebunny/crossref_metadata_2025 huggingface dataset:
Lensless focusing with a fiber bundle
Authors: Josten G. , Weber H. P. , Luthy W.
Published: February 2023

The maximum transmitted power in an optical monomode fiber is limited to the 10-W range.1 Higher power can be transmitted if the beam is split and coupled into several fibers of a monomode fiber bundle. In this case dephasing of the beam components occurs due to different path lengths in the individual fibers. The coherence in the output of the fiber bundle can, however, be reestablishing by adjusting the phase in each individual fiber of the bundle by applying stress or strain.2 In our work we study theoretically the possibility of phase adjustment leading to a focus. Focusing is achieved with suitable phase differences between the individual fibers that lead to constructive interference at the focal point. The calculation shows that it is possible to concentrate more than 50% of the fiber output energy in the central peak of the resulting diffraction pattern, if a very dense packed array of fibers is used. Dense fiber bundles with core separation as low as 10 µm are already commercially available.3
Transition from Maxwell-Boltzmann to Bose-Einstein partition statistics by stochastic splitting of degenerate light
Authors: De Martini F. , Coluzzi R. , Di Fonzo S.
Published: February 2023

The first experimental verification of the Tersoff-Bayer interstatistics transition theory has been reported.1-3 A novel difference photon-counting method of detection, which reveals the field's particle fluctuations, is used to gain information on the beam splitting process of a highly Bose degenerate chaotic (and coherent) photon beam emitted from an ultrastable He-Ne laser. The splitting device consists of an electrooptic Pockels cell which acts as a stationary beam splitter or a chaotic beam splitter depending on the output properties of a driving noise generator. We found that the quantum Bose statistics in a splitting (i.e., scattering) process corresponds to a fully stochastic splitting process. This conclusion has been experimentally verified for two-way splitting and for general multichannel scattering induced by microscopic particles.
Photocount statistics of continuous wave squeezed light
Authors: Zhu Chang , Caves Carlton M.
Published: February 2023

Photon number probabilities for a single squeezed mode can exhibit sub-Poissonian or super-Poissonian statistics and can display sharp oscillations in appropriate conditions. We consider photocount probabilities for cw squeezed light occupying a single transverse mode. The squeezing spectrum is assumed to be flat over a bandwidth B, centered at optical frequency Ω/2π. We compute single-fold photoelectron probabilities for a counting interval of duration T. We proceed by decomposing the single transverse mode into a complete set of longitudinal wave-packet modes, defined in terms of prolate spheroidal wave functions. Each of these wave-packet modes is in a separate squeezed state. The Kelley-Kleiner photocount probability can then be written in terms of single-mode photon-number probabilities for these wave packet modes, which are detected with varying efficiencies. We need consider only a finite number of modes, because only about BT of the modes have high enough efficiency to contribute significantly to the photocount probability.
Implications of self-consistent QED to high voltage XUV free electron lasers
Authors: Chang C. S. , Fluhler Herbert U.
Published: February 2023

A self-consistent quantum electrodynamic (SCQED) theory of high voltage free electron lasers (FEL) is given. Spontaneous emission spectra and gain are predicted in the small signal gain regime and compared to FEL experimental data. The SCQED theory predicts line shift, spectral broadening, and gain reduction due to quantum mechanical effects which are not predicted by the classical/conventional FEL theories. Comparison with well accepted FEL experiments shows excellent agreement between the SCQED predictions and the data: much better than predicted by the classical/conventional FEL theories. Classical FEL theory is recovered as a subset of the SCQED theory in the classical limit of the SCQED FEL theory. It is found that the classical/conventional FEL theories are only valid in the limit of a short undulator containing a small number of periods. This limit results from the non-negligible finite electron state lifetime, and its associated spectral linewidth broadening (natural linewidth phenomena).
Subtracting incoherent optical neuron: experimental demonstration
Authors: Wang C. H. , Jenkins B. K.
Published: February 2023

The incoherent optical neuron (ION) model uses two separate incoherent optical device responses to subtract inhibitory inputs from excitatory inputs for general neural networks. The ION model comprises two elements: an inhibitory element and a nonlinear element. A Hughes liquid crystal light valve (LCLV) can implement both elements. Its nonmonotonic response can be used to approximate a linear, negative slope characteristic for inhibitory inputs as well as a nonlinear positive slope characteristic for the neuron hard-clipping or soft-threshold response. An optical beam is used to provide a suitable neuron threshold. Self-feedback is incorporated to increase the gain and nonlinearity.
Scaling laws for atmospheric thermal blooming
Authors: Krapchev Vladimir B.
Published: February 2023

The propagation of ground-based high energy lasers through the atmosphere leads to heating of the air due to molecular and aerosol absorption.1 This heating changes the local refractive index and the rays diverge from the center of the beam leading to thermal blooming. While the direct energy loss from absorption is small, the phase distortion from thermal blooming degrades the beam quality. To compensate for thermal blooming, one has to correct the beam phase at the transmitter. This is achieved by a closed loop adaptive optics. The conjugate of the incoming beacon phase is imposed on the outgoing laser beam. The phase compensation leads to instability, which places a limit on the critical HEL power (Pc) that can be transmitted through the atmosphere at a given beam diameter (D). The scaling law, Pc ~ Db is of critical importance for the design of a beam director. The role of the refractive index and velocity turbulence of the atmosphere have been studied. Without velocity turbulence the scaling law is Pc ~ D, and with velocity turbulence the result is Pc ~ D1.5.
Time-resolved studies of light scattering in random media
Authors: Yoo K. M. , Alfano R. R.
Published: February 2023

Angle- and time-resolved studies of the ultrashort pulses propagating through a slab of random media with a few transport mean free paths thick reveal that a coherent component coexists with an incoherent component in the forward direction. The coherent component behaves as an unscattered pulse propagating in the forward direction, whereas the incoherent part experiences multiple scatterings and exhibits a diffusive characteristic. In the intensity temporal profile of the scattered pulse in the forward direction, the peak intensity of the diffusive pulse displaces farther away from the ballistic pulse when the size of the scatterers decreases or the thickness of the random media increases. The smaller scatterers and thick medium also show a much larger distribution of arrival times. The theoretical results from the two-frequency coherent function in the Rytov approximation are in qualitative but not in quantitative agreement with the experimental results. The depolarization of light in the backward direction shows a significant dependence on the size and concentration of the scatterers. The parallel polarized backscattered pulse profile is significantly broader than its perpendicular counterpart in media with a particle size smaller than the wavelength of light.
Phase-to-amplitude noise conversion in heterodyne receivers and optical preamplifiers
Authors: Brown T. G. , Basch E. E.
Published: February 2023

A heterodyne optical receiver configuration employing incoherent demodulation consists of a bandpass intermediate frequency (IF) filter and an envelope detector followed by an integrator and data discriminator. A direct detection system employing a filtered optical preamplifier also employs these elements. It has been shown both theoretically and experimentally that such systems require careful optimization of both the IF filter bandwidth and the decision threshold to achieve optimum performance. While the theoretical treatments to date have been insightful, they have not allowed straightforward analytic treatments or simple computations of the output probability distributions. In this paper we apply the method of characteristic functions to a solution of the problem of phase noise in an incoherent linear envelope detection system. This approach was first used many years ago and has been described in well-known textbooks but has never, to our knowledge, been applied to the problem of phase noise. The method has the advantage over other approaches of analytically providing the eigenvalue equations for the poles of the characteristic function of the output probability distributions, allowing accurate system analysis without the necessity of long computation time.
Emission and laser-induced fluorescence spectroscopy in dynamic non-equilibrium plasmas
Authors: Maron Y.
Published: February 2023

Spectroscopic diagnostic methods have been developed to investigate plasmas in pulsed power systems. The plasma duration is ≃100 ns and the electron density and temperature are in the 1014-1016-cm–3 and 1-15-eV ranges, respectively. Because of the experimental irreproducibility and the hostile environment in high power discharges, the time histories of line intensities and their spectral profiles had to be observed in a single discharge. The temporal, spatial, and spectral resolutions required are a few nanoseconds, a fraction of 1 mm, and 0.01 Å, respectively. Using these techniques we determined the distribution of the strong electric field outside the plasma in intense ion diodes from line emission Stark shift,1 the magnetic field penetration into the plasma from Zeeman splitting,2 and the velocity distributions of ions and neutrals in various directions from Doppler broadening and shift.2 The determination of the electron temperature and the particle fluxes in the plasma, using the observed line intensities, necessitated detailed analysis of the time-dependent collisional radiative processes in the plasma.2 The ground state densities and atomic level deexcitation rates were observed using plasma-induced fluorescence.3 The frequencies and amplitudes of turbulent electric fields in the plasma were studied from plasma satellites and from anisotropy of Stark broadening.4 Evidence for a non-Maxwellian electron energy distribution was also obtained.
Multichannel phase conjugated multimode fiber interferometer
Authors: Jahoda F. C. , Forman P. R. , Netterfield C. B.
Published: February 2023

The motivation is replacement of numerous thermocouples on the vacuum liner of a magnetic confinement fusion experiment. Fiber optics′ advantages over hardwire include immunity from EMI, no voltage insulation requirements, and the capability of averaging temperature over finite lengths. Costs per channel are minimized by using a common light source, a common phase conjugator, and multimode fiber components. Phase conjugation permits multimode fibers to be used with single-mode fiber sensitivity. Each interferometer gets argonion laser light through a multimode star coupler arm spliced to the interferometer multimode 2×2 input coupler. One arm of each interferometer is piezoelectrically modulated at 20 kHz with amplitude of ~2.4-rad phase shift. Both arms have GRIN lens fiber terminations that project beams with 0.8-mm vertical displacements onto a self-pumped BaTiO3 phase conjugator. Three interferometer pairs can fit vertically on a 5-mm high crystal. An additional three pairs can be displaced horizontally on the same crystal with fiber lengths differing from the first three pairs by more than the laser coherence length to avoid phase crosstalk. Processing the detected light at two harmonics of the modulation frequency can give calibrated temperature-induced phase shifts unambiguously from fractional to multiple fringe counts.
Holographic optical system optimization using SUPER-OSLO
Authors: Li Q. X. , Tian R. S. , Ih C. S.
Published: February 2023

Holographic optical elements (HOEs), generated optically or by computer, are useful and are becoming common in complicated optical systems. We shall call these systems holographic optical systems. Most of the work reported in the literature concerns the optimization of the HOE itself, and many systems use specially written computer programs.
Progress in optical scanning holography
Authors: Duncan Bradley D. , Poon Ting-Chung
Published: February 2023

Hologram generation can be accomplished in the optics regime by scanning an object simultaneously with two optical beams of slightly different temporal frequencies.1 We present an overview and our progress in this area. Scanned holograms of simple objects, such as lines and points, are presented and discussed. For real-time reconstruction, techniques for using an electron-beam-addressed spatial light modulator (EBSLM)2 have been investigated. By using an EBSLM we are able to avoid the photographic development procedures encountered in more traditional holographic arrangements and are thus able to reconstruct the object field from the scanned hologram in real time. We also present a basic overview of EBSLM operation in the context of object reconstruction. The advantages and limitations of using an EBSLM for this purpose are also discussed.
Theoretical and experimental study of stratified volume holographic grating
Authors: Song Li , Lessard Roger A.
Published: February 2023

Stratified-volume holographie gratings (SVHGs) consist of sandwiched active and passive layers1. The thin gratings are recorded on active layers, and the passive layers are used to modify the diffraction properties for a given order. Theoretical analyses of such a system were carried out by using a convolution approach. The two-active-layer system was constructed, and a good agreement between the experimental results and the theoretical analyses was demonstrated. The stratified-volume holographic gratings show some particular properties that make them different from the thin and thick holographic gratings. For example, the diffraction intensity of a given diffraction order varies periodically with the incident angle of the light. From theoretical and experimental studies of the SVHG, we can obtain the desired diffraction properties (the diffraction efficiency, the angular selectivity, the wavelength selectivity, and the angular period) by selecting the system parameters (such as the modulation strength of the gratings, the grating spacing, the thickness and refraction index of the passive layers, and the transverse relative position of the gratings). Their particular properties make them useful for many applications.
Investigation of americium-doped fluorozirconate glass as a laser medium
Authors: Svatos Michelle M. , Brundage R. T. , Beitz James V.
Published: February 2023

Recent experiments have revealed longer fluorescence lifetimes for the excited states of tripositve americium in a fluorozirconate glass than have been previously reported in other hosts. In particular, the 5D11-to-7F11 transition was found to have a lifetime of 1 ms, indicating efficient fluorescence emission. This, together with the energy-level structure, suggests the possibility of using the 5D11-to-7F11 transition as a four-level laser. A possible complication is the presence of color centers arising from the radioactive decay of Am243. We will present results of optical gain measurements, and we will report on the variation of lasing threshold and efficiency with ion concentration and excitation configuration if laser action can be demonstrated. The samples will be excited by optical pumping into the 5L6 state at 501 nm; we expect laser action at 695 nm.
Free-electron lasers in China today and tomorrow
Authors: Hui Zhongxi
Published: February 2023

In this paper the free-electron laser (FEL) study in China is presented. The FEL theoretical research in China commenced in the mid- 1970s, but experiments commenced later. In 1987 Hefei National Synchrotron Radiation Laboratory, in cooperation with Shanghai Institute of Optics and Fine Mechanics, produced an experiment based on rf linac of 30 MeV, in which an infrared radiation at wavelength of 10 µm is observed. In 1988 China Academy of Engineering Physics (CAEP) had performed an experiment based on pulse line accelerator, in which 7.6 MW radiation has been observed at 36.9 GHz representing 5% efficiency. These early experiments were characterized by small efficiency due to poor quality of electron beams at the time. However, now the induction linac of 4 MeV and 1 kA with qualitative electron beam is projected for SG 1 FEL amplifier by CAEP and 30 MeV rf linac is being upgraded by Institute of High Energy Physics to provide electron beam for FEL oscillator in the infrared region. The FEL with electromagnet wave pump is projected by University of Electronics Science and Technology of China. The superconductor accelerator is by accelerator laboratory of Peking University and the Cherenkov FEL experiment is produced by Changsha Polytechnic Institute. Now the FEL research is intensely produced by theoretical and experimental aspect in China.
Squeezed soliton fiber gyro
Authors: Bergman K. , Haus H. A. , Lai Y. , Shirasaki M.
Published: February 2023

Experiments in our laboratory have arrived at squeezing in a pulse-excited fiber ring. The pulse excitation suppresses stimulated Brillouin scattering, and at the low frequencies used for the measurement, which are made possible by the pump separation, guided acoustic-wave Brillouin scattering is ineffective. It is likely that large squeezing can be realized by using solitons in the same fiber-ring configuration, because solitons assure better squeezing phase profiles. These findings encouraged us to "design" a fiber gyro that performs below the standard quantum limit. It consists of two fiber-ring reflectors; the first acts as a squeezer, and the second acts as a Sagnac interferometer. We use the theory of quantization of solitons.1 The pulse injected into the first fiber ring is a sech adjusted so as to travel as a pair of countertraveling solitons in the squeezing ring.2 The ring acts like a Mach-Zehnder interferometer folded back on itself. The pump is reflected from the pump port, and the vacuum port has squeezed vacuum emerging from it. The pump is recycled in the second Sagnac ring to act as the signal for the rotation sensing. The squeezed vacuum from the first ring is injected into the vacuum port of the Sagnac interferometer. Because practical design considerations dictate fiber lengths on the order of 100 m for both rings, the nonlinearity in the Sagnac ring cannot be neglected. It produces its own squeezing but in the wrong phase for detector noise reduction. However, by proper adjustment of the squeezing in both interferometers, it is possible to counteract the harmful squeezing effect in the Sagnac interferometer and still arrive at a net reduction of the detector noise.
Cat bistability in a coherently driven micromaser
Authors: Meystre Pierre , Slosser John J. , Wilkens Martin
Published: February 2023

We show that a coherently driven dissipative micromaser may display a new kind of bistability of the cavity field and/or the photon number. The bistability is a direct consequence of the presence of trapping states and the robustness of the related cotangent states against the perturbations caused by the cavity losses. The novel features of the bistability stem from the fact that even in the presence of cavity losses, the stationary state of the maser is close to a macroscopic superposition of number states (Schrödinger's cat) in the bistable regime. With increasing injection rate, the bistable regime of the micromaser is reached by means of a route that resembles a first-order phase transition. Hysteresis and other related irreversible effects are discussed.
Switching at a planar interface to a nonlinear medium
Authors: Lin L. , Leung K. M. , Tamir T.
Published: February 2023

When plane waves are incident on a nonlinear medium with a self-focusing nonlinearity, switching phenomena associated with abrupt changes in the reflection coefficient can occur near the critical angle and at specific threshold intensities.
Optical aberrations of the eye following radial keratotomy
Authors: Applegate Raymond A. , Howland Howard C.
Published: February 2023

The quality of the eye's in-focus optical image (and visual performance) is directly influenced by the magnitude of the eye's higher-order wave aberrations. Although there have been numerous studies of the normal eye, little is known about the optics of eyes after various clinical interventions (e.g., radial keratotomy, epikeratophakia, and corneal transplants). To help fill this information void, wave aberrations were measured with the Howland and Howland aberroscope on 15 radial-keratotomy (RK) patients and nine normal control subjects (no surgery) before and approximately 7, 120, 240, and 360 days after surgery. Although measurements could always be performed on the control subjects (88 of 88 times), RK patients' optics were usually sufficiently disrupted outside the surgery-free zone that quantification of the wave aberrations was impossible. In RK eyes where measurements were possible (12 of 96 times), the measured aberrations were, as a rule, larger than those of the control subjects and the optical consequences were more severe. These measurements are consistent with the frequent report from RK patients that their vision is excellent under high photopic conditions, where their pupil is small, but that it deteriorates under mesopic-to- scotopic conditions, where the pupil is relatively large.
Coherent guided-wave implementations for photonic control of phased-array radar systems
Authors: Surette Marc , Mickelson Alan Rolf
Published: February 2023

Coherent guided-wave optical systems offer significant advantages for use with phased-array drive and readout schemes. Single-side-band modulation of received fields allows for optical mixing of the microwave signals through Butler-matrix techniques with the attendant low loss and high resolution of optics. Such methods obviate the need for adaptive antenna processing and also produce a baseband signal at the output of an optical detector. Coherent drive allows for phase shifting to be performed at optical, rather than microwave, wavelengths, thereby allowing for compact, high- bandwidth true-time-delay, integrated-optical phase shifting. Heterodyning at the antenna-element end allows one to use a high-power locked local oscillator, thereby minimizing any need for at-element microwave amplification. Many of the elements necessary to realize such photonic array control schemes already exist and, in the near future, should become monolithically integrable, allowing for fiber-compatible modular construction of various drive schemes. We will discussion the performance of the various system elements, including those fabricated and tested in our laboratory as well as others for which data is available, and thereby project eventual system performance in terms of the standard phased-array performance measures.
Preconditioning of real-time optical Wiener filters for array processing
Authors: Ghosh Anjan , Paparao Palacharla
Published: February 2023

In adaptive array processors, a performance measure, such as mean-square error or signal-to-noise ratio, converges to the optimum Wiener solution after starting from an initial setting. The choice of adaptive algorithms to solve the Wiener-filtering problem is mainly guided by the desired processing time. An optical realization for direct calculation of the Wiener solution was discussed by Welstead.1 In this approach, the covariance matrix and vector for a Wiener filter are computed at high speed on acousto-optic processors. The resulting linear system of equations is solved on an iterative optical processor. The matrix and vector data are recomputed in every iteration. This introduces variations in their values owing to optical errors and noise. A nonstationary steepest-descent algorithm is a simple method that can be used for the optical calculation, and such an algorithm slowly converges to the Wiener solution.1 It has been shown that the size of the error in the solution introduced by perturbations in the sequence is governed by the condition number of the sequence; this is similar to the case of a single fixed system of linear equations.1
Detection of sharp absorption lines in very thin NdF3 films
Authors: Shortt D. W. , Jones M. L. , Schawlow A. L. , Macfarlane R. , Farrow R.
Published: February 2023

We report the use of a sensitive spectrometer system in studying sharp optical absorption lines of transitions between 4f3 crystal-field levels of Nd3+ in NdF3 films as thin as one monolayer. Epitaxial films as thin as 5 nm on GaAs substrates displayed absorption lines with asymmetric line shapes because of dispersion at the lines and surface reflections. Samples with an antireflecting buffer layer between the NdF3 layer and the substrate allowed observation of crystal-field lines in films as thin as 0.36 nm (0.5 unit cells). In the thinnest films, lines from the 4I9/2 → 4G5/2 transition were shifted to the blue by 6 cm-1 relative to their positions in thicker films, and the splitting of the two strongest components increased from 8.4 to 14.4 cm-1 as a result of lattice distortion by the buffer layer.
Conformational dynamics and barriers in free radicals
Authors: Lin T.-Y. David , Miller Terry A.
Published: February 2023

The laser-induced fluorescence spectra of a number of free radicals—o-, m-, and p-methyl benzyl, di- and triphenylmethyl, diphenylchloromethyl, and diphenylethyl—have been observed when these materials were cooled to near absolute zero in a free-jet expansion. Barriers to methyl rotation in both the ground and lowest excited electronic states have been determined for the substituted methyl radicals. Significant information is presented regarding these radicals' geometries and ring-twisting motions.
Low-frequency vibrations in highly vibrationally excited molecules
Authors: Crim F. F. , Likar M. D. , Ticich T. M. , Utz A. L.
Published: February 2023

Preparing highly vibrationally excited molecules by vibrational-overtone excitation and probing them or their reaction products spectroscopically provides new information on the interaction of high- and low-frequency modes. We use three different approaches in which we probe the excited molecule by laser-induced fluorescence, by monitoring unimolecular reaction products spectroscopically, or by photodissociating the vibrationally excited molecule and probing the dissociation products spectroscopically. The "action" spectra coming from these measurements show the influence that the excitation of stretching vibrations has on other vibrations in the molecule. One example is the interaction of the O-H stretching vibration with the torsion in hydroperoxides (HOOX), such as hydrogen peroxide (x = H) and t-butyl-hydroperoxide (x = t - bu). Our analysis shows that the height of the lower barrier in the double-minimum potential of these molecules increases with increasing-O-H stretching excitation.
Low-refractive-index quantum-well structures—an important element for low-threshold lasers
Authors: Ho S. T. , Slusher R. E. , McCall S. L.
Published: February 2023

An ideal laser with near-zero threshold can in principle be realized with microcavity resonators and quantum-well active layers.1 Recently, Yamamoto et al. proposed such a laser with two multilayer mirrors, each comprising 10 pairs of high-optical-index contrast (n1/n2 = 3.6/1.5) materials, forming a microcavity around a 70 Å quantum well between two half-wavelength thick (n ≈ 3.5) Al0.2Ga0.8As barriers. This high-index layer forms a slab waveguide with a thickness of one wavelength.
Laser-induced gratings in rare-earth-doped glasses
Authors: French Valentina , Behrens Edward G. , Powell Richard C. , Blackburn Douglas H. , Cranmer David C.
Published: February 2023

We have recently shown that superimposed permanent and transient laser-induced gratings can be established in rare-earth-doped glasses. The transient component is associated with a spatial distribution of the population of rare-earth ions in the metastable state. The permanent component is associated with a change in the local structure of the glass.
Optical and mechanical issues in free-space digital optical logic systems
Authors: McCormick F. B. , Tooley F. A. P. , Lentine A. L. , Morrison R. L. , Novotny R. A. , Cloonan T. J.
Published: February 2023

Three-dimensional array logic was recently demonstrated by optically interconnecting two-dimensional arrays of symmetric self-electro-optic effect devices (S-SEEDs).1 As in the operation of electronic logic, logiclevel uniformity of the optical input signals to each optical logic gate is necessary. As the outputs of one optical-logic-device array propagate to the next array, spatial variations of the logic levels can be introduced. Logic-level nonuniformity can accumulate from optical sources (spot-array generation, geometric aberration, and polarization effects), mechanical sources (misalignment and lack of stability), image plane sources (dust, damage, and device nonuniformity), and laser sources (wavelength and mode variations). We have found that although individual nonuniformity contributions can be kept to less than 10%, in complex systems the total accumulation of nonuniformity errors can exceed ±40%. These large variations of the input logic levels introduce errors in the desired logical decisions. The system constraints and limitations brought about by these nonuniformity contributions will be discussed, and experimental results will be presented.
Distributed feedback lasers with integrated electroabsorption modulator
Authors: Soda H.
Published: February 2023

Low-chirp and high-speed light sources are essential for the next long-haul multi-gigabit optical-fiber transmission systems. We realized a high-performance light source by the use of a photonic-integration technique. Our light source consists of a distributed-feedback laser and an electroabsorption modulator. For good performance, it is important to use two excellent solitary devices and to integrate devices without undesirable interactions. A buried heterostructure with semi-insulated InP layers and a butt joint are key points in our devices. The buried heterostructure with semi-insulating burying layers provides good electrical isolation with high-speed response and completely stabilized lasing characteristics. The butt-joint structure allowed us to attain a high optical-coupling efficiency, about 80%. The feedback of light from the joint to the laser was estimated to be negligible. We obtained a modulation bandwidth of 10 GHz and an output power of 17 mW in the fabricated devices. Also, clear eye opening with an extinction ratio of 13 dB was obtained under 10 Gb/s nonreturn-to-zero modulation. The wavelength broadening width (FWHM) was only 0.1 Å. Photonic-integration technology made this high performance possible.
Novel distributed-feedback surface-emitting laser design
Authors: Mahbobzadeh Mohammad , Osinski Marek
Published: February 2023

Surface-emitting semiconductor lasers are attractive for applications in optical computing, coherent communications, high-power two-dimensional arrays, and integrated opto-electronics. Vertical-cavity surface-emitting structures are especially promising because of their simplicity compared with other surface-emitting lasers, which use gratings or inclined mirrors to deflect the output from horizontal (in-plane) active region. Recently, the advantages of the resonant-periodic-gain (RPG) medium have been demonstrated in vertical-cavity structures.1,2 Here we describe a new concept for a vertical-cavity distributed-feedback (DFB) surface-emitting laser that incorporates an RPG active region. In this design, a vertical resonator with highly reflective mirrors is created by a stack of alternating high- and low-refractive-index quarter-wave layers that form a DFB medium. An RPG active region is interspersed within this medium. Each element of the RPG active region consists of a single or multiple quantum well whose position coincides with the antinodes of a standing wave at a design wavelength. The elements of the embedding DFB medium form spacers between active regions. This design eliminates the need for the end reflectors in previous RPG structures by interlacing the quarter-wave multilayer high reflectors with the gain medium, thus reducing the total thickness of the device. In the same instance, the characteristic features of the RPG medium (enhancement of gain in vertical direction, wavelength selectivity, and suppression of amplified spontaneous emission) are retained. Examples of cavity calculations for the novel structure will be given, and various configurations will be discussed.
Computer-aided design for optoelectronic computing
Authors: Kiamilev F. , Fan J. , Catanzaro B. , Esener S. , Lee S. H.
Published: February 2023

Over the last decade there has been a dramatic increase in the use of computer-aided design (CAD) tools in the design of very-large-scale integrated (VLSI) electronic systems. This increase has been driven by the increasing complexity and fabrication cost of VLSI systems. On the other hand, optoelectronic technology is emerging as a way to build high-performance computer systems. In optoelectronic computers, electronic circuitry is used for local processing, and free-space optical interconnects are used for global communication. Optoelectronic computers will be packaged by means of the same planar processing technology currently used for VLSI circuits. The design of optoelectronic computers requires detailed knowledge of optical interconnection technology, optical system setup, VLSI circuit layout and fabrication, computer architectures, computer algorithms, and packaging technology. To allow system builders to design optoelectronic computers with minimal knowledge of optoelectronic technolgy, an integrated set of CAD tools will be needed. An integrated optoelectronic CAD system will promote stronger interaction between optoelectronic device researchers and system designers, allow top-down and bottom-up design methods, provide interoperability, and, most important, reduce the design time and fabrication cost. In this paper we review existing electronic CAD systems and propose an optoelectronic CAD system that integrates architectures, algorithms, optics, and electronics under a common framework. The justification, requirements, and benefits of such a system will be discussed.
Optical waveguiding in Si/GexSi1−x/Si
Authors: Soref Richard A. , Namavar Fereydoon , Cortesi Elisabetta
Published: February 2023

We have observed low-loss waveguiding at λ = 1.3μm in a thin 0.15μm layer of GexSi1−x sandwiched between a 2μm Si capping layer and a silicon substrate (the first waveguide of its kind, to our knowledge). Germanium concentrations of 8–18% were tested, and the low-defect GeSi alloy film was grown by chemical vapor deposition at 1000°C. Electron diffraction microscopy revealed that the GeSi is coherently strained with 1% lattice dilation normal to the Si layer and that there is no distortion parallel to the layer. Previous waveguiding work1 focused on an air-clad GeSi layer that was several microns thick. The current experiments suggest that a submicron GeSi base layer will support infrared waveguiding within an elongated Si/GeSi/Si heterojunction bipolar transistor (HBT). The long-range goal of this work is to demonstrate that an npn or pnp channel-waveguided Si/GeSi/Si HBT will provide ultrafast optical intensity or phase modulation by carrier injection into the base.
Resonances of slanted dielectric diffraction gratings in asymmetric waveguide geometry
Authors: Magnusson R. , Wang S. S.
Published: February 2023

Wood's anomalies are natural phenomena associated with diffraction gratings. The term anomaly refers to a rapid variation of externally observable fields with respect to the wavelength or angle of incidence of the input wave. Hessel and Oliner1 pointed out that there are two types of anomalous effects, the Rayleigh type and the resonance type. Wang et al.2 treated the resonance-type anomaly associated with planar dielectric-layer diffraction gratings. There, the guided-mode resonance characteristics of the evanescent and propagating fields were studied by using the rigorous coupled-wave theory. The present contribution is an extension of the work by Wang et al. The resonances are studied for a generalized geometry allowing grating slant and arbitrary surrounding media (asymmetric waveguide case). For small modulation and thin gratings, the usual eigenvalue equation of the corresponding unmodulated waveguide accurately predicts the locations of the resonances. For increasing modulation or waveguide thickness, numerical results calculated with the rigorous coupled-wave theory are presented to quantify these phenomena with respect to the grating parameters.
Modulation of a Helium-neon Laser Using Cadmium Sulphide
Authors: Wickstead J.P. , John W.D. St. , Myers D. B.
Published: February 2023

Modulation of a helium-neon laser beam has been observed by exciting a CdS sample with the 514.5 nm output of an argon-ion laser. The strong absorption of this latter wavelength results in a surface effect which indicates the formation of a lens due to (i) the distortion of the front face of the sample, (ii) a transverse gradient in the index of refraction. In addition to the induced positive lens, a Fabry-Perot interferometer is recognized due to thermal expansion, yielding a temporal variation in the transmittivity of the HeNe beam. The above observations are modeled using a ray matrix method and dividing the sample into two sections. The front section is the one over which complete absorption of the exciting beam (514.5 nm) occurs. This results in the creation of a thin lens and a time varying interferometer. The rest of the sample is modeled as having a uniform index of refraction and a fixed thickness. The time dependent focal length of the thin lens results from two contributions: The bulging of the front face with radius of curvature R; the radial dependence of the thermally induced change in the index of refraction dn/dT.
Generation of ultrafast electromagnetic radiation
Authors: Hu B. B. , Darrow J. T. , Froberg N. M. , Zhang X.-C. , Auston D. H.
Published: February 2023

A large variety of semiconductor samples have been used to generate highly directional, optically steerable, and diffraction-limited ultrafast electromagnetic radiation. Samples have been selected from III-V, II-VI, and group IV semiconductors, including single-crystal, polycrystalline, and amorphous structures. We have compared the optically induced radiated fields from different semiconductor surfaces (interfaces), including an air/semiconductor interface, a metal/semiconductor interface, a p-n junction (solar cell), and a strain-induced piezoelectric layer. In addition, we report the temperature dependence of optically induced electromagnetic pulses. When the surface depletion width was tuned to cross the optical absorption length by varying the sample temperature, a dramatic change of the radiation waveform from the narrow-bandgap semiconductors was observed. We also observed a three-fold rotation symmetry of the radiation from (111)-oriented samples. Finally, we describe the conversion efficiency of optically induced electromagnetic radiation from these samples.
Transverse Optical Switching and Scanning in Twin-stripe Semiconductor Laser Amplifiers
Authors: Yuan Li , Salvi Theodore C. , McInerney John G.
Published: February 2023

Semiconductor lasers and laser amplifiers have intrinsic nonlinearities which, while often undesirable for optical communication and data storage, show considerable promise for optical switching, modulation and bistable logic. These functions will be limited by nanosecond carrier lifetimes whenever it is necessary to change the carrier numbers in the active region, as in switching of the gain or absorption. However, phenomena such as nonlinear waveguiding which redistribute existing carriers over short distances may be much faster [1]. Here we consider theoretically the use of nonlinear waveguiding based on the well-known dependence of the refractive index on local carrier density, to produce transverse mode switching and beam scanning in closely-coupled twin-stripe laser amplifiers. Light from an external source is coupled into one of the guides, and is propagated numerically through the structure without imposition of cavity end losses or boundary conditions. The paraxial wave equation is solved using a Crank-Nicholson algorithm with small variations in implicitness for numerical stability. The effects of carrier diffusion and amplifier gain saturation are included, and the results are checked by solving the scalar wave equation in the transverse plane. Several regimes of behavior, including bistable switching and gradual beam scanning, are identified.
Second-harmonic generation in atomic hydrogen at Lyman-α
Authors: Marmet L. , Hakuta K. , Stoicheff B. P.
Published: February 2023

We have observed second-harmonic generation in atomic hydrogen in the Lyman-α region, with and without an applied dc electric field, Ea. Experiments with a continuous atomic beam were carried out by using a pulsed dye laser tuned to 243 nm for excitation of the n = 2-1 transition. The laser beam was focused 0.4 mm below the H-source nozzle, where the H-atom density was estimated to be 5 × 1013 cm-3. Measurements of second-harmonic generation (SHG) at ~122 nm were made at various electric fields up to ~27 kV/cm. It was shown that the SH signal produced at E
a
= 0 occurs because of a charge-separation field of ~800 V/cm that is caused by three-photon ionization. Measurements of the SHG and ion current were found to vary as I2
L
and I3
L

,
respectively. At high fields (E
a
> 10 kV/cm) the SH signal at the center of the Stark-split components decreased somewhat from the peak value, but the ion current (corresponding to photon absorption) approached zero. This observation is a clear demonstration of a similar effect, predicted by Harris et al.1 of efficient nonlinear conversion with induced transparency, and it suggests the possibility of obtaining intense Lyman-a radiation.
Photorefractive time-intergrating correlator
Authors: Hong John H. , Chang Tallis Y.
Published: February 2023

The time-integrating property of photorefraction is a useful tool that has been used in conjunction with acousto-optic devices to produce correlation functions of rf signals.1That system consisted of two acousto-optic devices that were positioned so that the two diffracted beams (resulting from the two drive signals feeding the devices) wrote a grating within the photorefractive BiSiO12crystal. The two diffracted beams, with the spatiotemporal modulation impressed by the acousto-optic devices, intersect within the photorefractive BSO to create a holographic grating that forms with a time constant that is inversely proportional to the total intensity falling on the crystal. The time-integrated grating is then read out with a third beam, which then carries the correlation information. To obtain appreciably large amplitude gratings, the two beams must intersect with a large relative angle at the photorefractive crystal, resulting in a loss of resolution because of crosstalk within the thickness of the crystal; otherwise, a large eternal field must be applied, which also has some associated drawbacks.2We present a different architecture in which the acousto-optic devices are oriented orthogonally to the photorefractive grating direction, so that larger intersecting angles can be accommodated without a loss of resolution. Experimental results are given.
Optical MOSAIC processors
Authors: Athale Ravindra A. , Yu Jeffrey W.
Published: February 2023

The operation of 2-D Fourier tTransform has been the foundation of most of the coherent optical processors developed. Two prominent examples are the tilted-plane optical processor for SAR data and the Fourier plane image correlator/processor. In all of these cases the processing throughput of the system is limited by the space-bandwidth product and the frame-time of the input (spatial light modulators - SLMs) and output (detector arrays) transducers (typically 512 x 512 at 30 frames/s). The passive optical elements (e.g. lenses) are, however, capable of handling much larger size images (4000 × 4000). In this paper we describe a system design that uses several discrete SLMs and combines their outputs coherently to optically synthesize a much larger size image that can then be processed in parallel by the Fourier transform-based optical processor. The output (power spectrum or correlation) can be similarly split optical and detected by several separate detector arrays. We call this concept "Optical MOSAIC Processors" since it is similar to puting several individual tiles together to cover a large area. The combining of the subimages in the input and splitting them in the output can be accomplished using prisms/gratings and mirrors or via coherent fiber bundles. The randomization of the optical wavefront in the input can be corrected via a phase correcting hologram. In this paper we will describe several promising designs and outline potential applications.
Comparative study of a rough silver thin film by means of a microdensitometer analysis of shadowed surface replicas and a simulation of conventional techniques
Authors: Varnier Françoise , Llebaria Antoine , Rasigni Georges
Published: February 2023

The surface-structure characterization our team has developed for several years (based on microdensitometer analysis of shadowed surface replicas) makes it possible to determine the statistical parameters that describe any rough surface. However, this method is particularly devoted to microroughness studies because of its very high resolution and the small size of the studied samples. Therefore, it is sometimes difficult to compare the results obtained this way with those given by more conventional techniques (such as profilometry). After the relief of a silver thin film was restored and its roughness parameters were determined, it was possible to simulate, from this relief, some profiles that, depending on the stylus size, could be obtained when using a profilometer. Results concerning the statistical roughness parameters of the profiles obtained this way are compared with those deduced from the microdensitometer method. Moreover, by simulation of an optical method, other results are available that are connected with the relief deduced from the reconstructed one. Hence, it is possible to explain the discrepancies between results related to the same sample and obtained by these three methods.
Holographic measurement of surface roughness
Authors: Cantalupo John , Olson D. W.
Published: February 2023

A holographic optical element gathers light scattered from a planar surface and directs it through a pinhole. The sample is rotated in its own plane, and that part of the pinhole light arising from microscopic roughness disappears. This leaves only specular light associated with the average surface contour. A graph of light power through the pinhole versus sample orientation shows a narrow peak centered on the original sample position, surrounded by a specular-reflection plateau. The ratio of the microscopic-roughness-peak height to the specular-reflection-plateau height gives a measure of the sample roughness. Comparisons of sample roughness determined by this method and by traditional methods show a semilogarithmic relationship between AA roughness and this holographic measure of roughness over a certain roughness range. This method is an extension of the Honda1 holographic method of rapid surface inspection.