bluuebunny/crossref_metadata_2025 huggingface dataset:
Depth measurement of trenches by higher-order diffraction interferometry
Authors: Noguchi Minori , Ootsubo Tooru , Aiuchi Susumu
Published: February 2023

The etching of fine patterns on Si substrates is an important technique for the fabrication of solid capacitors and ultrahigh-density circuit integration. This technology requires high-precision depth control. Therefore, a nondestructive method of measurement suitable for mass production applications, in place of the conventional method of breakage sections with scanning electron microscopes, is an important desideratum. A nondestructive measurement technique using optical diffraction and interference was developed. When the wafer with the etched pattern is illuminated, the light reflected from the floor of a trench carries phase information that corresponds to the trench depth. Analysis demonstrated that the interference pattern formed by this light, reflected from the trench floor and from the surface of the wafer, can be observed as intensity variations in the diffracted light that correspond to the depth of the trench. From the above results, it was deduced that the zeroth-order diffracted light must be cut off by a spatial filter to permit detection of the higher-order diffraction pattern. The higher-order interferometric method was devised, which permits depth measurement of surface depressions with area dimensions 1×1µm2 and a depth of 5 µm to an accuracy of ±0.2 µm.
Instabilities and solitons in a hybridly mode-locked dye laser
Authors: MacFarlane Duncan L. , Wetsel Grover C. , Strozewski Kirk J.
Published: February 2023

Pulse-train instabilities occur in a large number of mode-locked laser systems and have been studied in actively mode-locked lasers, passively mode-locked lasers, and self-locked lasers. In this paper we will discuss our recent experiments on the instability in a hybridly mode-locked dye laser, and we will compare and contrast its behavior to that of the synchronously pumped mode-locked dye laser. Experimental parameters of interest include the pump threshold parameter, the cavity-length detuning, the cavity lifetime, the intracavity group-velocity dispersion compensation, the gain dye choice, and the saturable absorber dye choice and molarity. For some regimes of operation, the ultrashort pulses of this laser may be compared to solitons found in other optical systems, such as fibers and colliding-pulse mode-locked dye lasers. We model this instability theoretically by means of nonlinear dynamics, and we will present some numerical results and an analysis that qualitatively agrees with experiment.
Generation and applications of cw additive-pulse mode-locked Nd:YLF pulses at high average powers
Authors: Chee J. K. , Kong M. N. , Liu J. M.
Published: February 2023

Recently, it has been demonstrated that additive-pulse mode-locking is an effective method to further shorten actively mode-locked laser pulses. Furthermore, it has been demonstrated that the nonlinear external coupled cavity acts as a passive mode-locker to generate a stable train of pulses without synchronous pumping or active mode-locking.
Time-resolved circular dichroism studies of biological systems
Authors: Simon John D. , Xie Kiaoliang , Dunn Robert
Published: February 2023

This paper examines a new experimental approach for studying ultrafast relaxation processes in biological systems—picosecond time-resolved circular-dichroism spectroscopy. The technical details of the experimental apparatus will be discussed. Optical analysis using Jones matrix calculus shows how complications due to pump induced linear dichroism can be eliminated enabling the isolation and measurement of transient circular dichroism signals. Extension of this approach to femtosecond time resolution will also be examined.
Progress in gravity-wave detection based on optical interferometers
Authors: Weiss R.
Published: February 2023

Several interferometer systems will be discussed, with emphasis on the noise budget in detection and when it might make sense to use squeezed light.
Quantum noise of lasers with intracavity harmonic generation
Authors: Polzik E. S. , Gorbachev V. N.
Published: February 2023

We present new results for the quantum noise reduction owing to the mth harmonic generation (m = ½, 2, 3) in a laser. Study of such a source of light with noise beyond the quantum limit is now in progress.1,2 We consider this problem for both the laser with conventional random pumping and the laser with reduced pump noise, i.e., the sub-Poissonian laser. In the case of the laser with a conventional pump, the noise in the mth harmonic can be suppressed by m times as compared to the shot noise limit. For the sub-Poissonian laser, perfect noise reduction can be achieved in any harmonic mode. Cross correlations between the fundamental mode and the harmonic mode lead to the quantum noise interference effect, which manifests itself in the noise of the total photon flux from the laser. The system with two nonlinear crystals inside the laser cavity can serve as a source of two perfectly squeezed modes with divisible frequencies. The first crystal is driven by the external field of frequency 2ω. This field is downconverted in the crystal to the frequency ω, which is the frequency of lasing. In the second crystal the lasing field is again downconverted to the frequency ω/2. The lasing mode m and the subharmonic mode ω/2 from such a laser can be perfectly squeezed; that means complete quantumnoise reduction of these two modes if mixing with local oscillators of phase shifts π/2 is used.
Excited-state absorption cross sections for Er-doped glasses
Authors: Zemon S. , Lambert G. , Davies R. W. , Miniscalco W. J. , Hall B. T. , Folweiler R. C. , Wei T. , Andrews L. J.
Published: February 2023

Investigations of excited-state absorption on a variety of Er-doped glasses indicate that fluorophosphates have significant advantages over silica active fibers for 800-nm band pumping.
Polarization-selective laser tomography and ellipsometric measurements of the retina
Authors: Dreher Andreas W.
Published: February 2023

The background of the human eye consists of a variety of layers, which have slightly different optical properties. The change in refractive index between the aqueous humor and the internal limiting membrane of the retina results in a specular reflection of light incident on this layer. The state of polarization of this specularly reflected light is generally maintained. However, the polarization state of light being scattered from deeper retinal layers is changed. In general, the light detected in an ophthalmoscope is a combination of specularly and diffusely reflected light of several retinal layers.
Retinal light scattering
Authors: Elsner Ann E. , Schwarz Richard A. , Burns Stephen A. , Webb Robert H.
Published: February 2023

By using a five-laser scanning laser ophthalmoscope with a choice of apertures in the retinal plane, we measured the reflectances of several retinal and choroidal features, relative to other features from the same image or to a model eye. Light scattering and absorption in the layers of the retina change the contrast of features as a function of wavelength. The mode of imaging also changes the appearance of these features, but this effect is not the same for all wavelengths. All wavelengths (488–830 nm) give a crisp image of the surface features of a normal retina with a 200 fim aperture confocal to the retinal plane. For other apertures, the appearance of features, such as the optic disk rim or the choroidal blood vessels, changes with wavelength.
Scanning optical microscopy
Authors: Kino Gordon S.
Published: February 2023

A review of the principles of operation of confocal microscopes will be given. The confocal microscope images a pinhole on the sample, and the reflected image of this spot is reimaged on the pinhole. As only a focused beam can pass back through the pinhole, a defocused image disappears rather than blurs, as it would in a standard microscope. Thus, the device can be used for optical cross sectioning of transparent biological objects and for profiling integrated circuits.
Scanning slit confocal microscope in ophthalmology and ear research
Authors: Koester Charles J. , Khanna Shyam M. , Tackaberry Robert B.
Published: February 2023

For examination of cellular details within the cornea and in the cochlea, the microscope must strongly reject stray light reflected or scattered from out-of-focus planes. In the scanning-slit microscope this is accomplished by separating the illuminating and imaging light paths except in the vicinity of the object plane. This produces a theoretically higher rejection ratio than is possible with conventional pinhole confocal microscopes, in which the falloff follows the inverse-square law. Special dipping cones have been designed to permit examination at 0.53 N.A. with working distances as great as 6 mm, immersed. In the search phase of an experiment, the optical-section depth can be made large by means of the variable-width slits. Resolution is not sacrificed because image detail is preserved across the width of the slit. After the region of interest is located, the optical section can be reduced to the optimum value for best image contrast. The high efficiency of the slit system permits the use of a conventional white-light source.
Dynamic analysis strategies for real-time object recognition
Authors: Burt Peter J.
Published: February 2023

Real-time computer vision requires fast search strategies analogous to alerting and focal attention in humans. I describe a pyramid implementation of such techniques and illustrate its application to human face recognition.
Toward a theory of face recognition
Authors: Turk Matthew A. , Pentland Alex P.
Published: February 2023

We describe a system for face recognition that has achieved 97% accuracy (with 20% classified as unknown) for 16 people under a range of lighting and viewing conditions. Recognition occurs in < 1 s on a Sun 4 computer. Our system characterizes the variation in face images from a small initial training set by performing an eigenvector decomposition similar to that performed by a Kohonen neural network. In the current system seven imagelike face basis vectors (termed eigenfaces) are extracted and converted into a distributed spatial-frequency representation. This conversion is accomplished by a windowed Fourier transform equivalent to a Gabor-filter representation. To locate and recognize people in a scene, the input image is converted into the same Gabor-like representation and is matched against the learned face basis vectors. Areas of the image that are facelike produce characteristic matches, indicating the presence of a person. The precise values of the match are then used to determine the identities of the people being viewed or to learn to recognize an unfamiliar face.
Wavelength dependence of the backscattering cross section of a roughened sphere
Authors: Schertler Donald J. , George Nicholas
Published: February 2023

An analysis is presented for the backscattering from a roughened sphere. Particular emphasis is given to the wavelength-dependence of the backscattering cross section. We examine in detail the varying contributions to the radiation from annular regions of the sphere as a function of the rms roughness, σ, the surface correlation length, lc, and the wavelength, λ. By examining the luminosity of these different portions of the sphere, we find that the surface roughness tends to provide appreciable backscattering from extended regions, thereby enabling one, at least conceptually, to remotely sense object shape. On the other hand, when the ratio σ/λ is small, only the well-known specular point leads to a substantial return. In analyzing the effect of increasing surface roughness on the backscatter, we also include the notion of shadowing, which becomes important above angles of approximately Φ = tan-1lc/σ.
Novel method for computer-generated holography based on neural networks
Authors: Kobayashi Seiji , Psaltis Demetri
Published: February 2023

Since Lohman et al. invented computer-generated holography, several methods have been tried to improve its image quality. We describe a method based on a feedback neural network that is designed to find the optimum (minimum-error) hologram by minimizing its error function. In this network, neurons are allowed to take only two output states, which directly represent the binary states of the corresponding pixels on the hologram. Thus, the number of neurons in the network equals the number of pixels in the hologram. The threshold value for each neuron and the connection strength between the neurons are determined by the Fourier transform of the desired wave front and the window function, respectively. The window function specifies the area of interest on the reconstruction plane, inside of which the image quality is to be evaluated. We have calculated several holograms by this network; starting from a random initial state and a 512 × 512 pixel hologram, the neural network typically converged after 2 hours of calculation (on a Sun SPARC station). The resultant holograms were recorded on an optical disk. The optically reconstructed images have excellent signal-to-noise ratios inside the specified reconstruction area.
Selection and creation of starting points for optical design
Authors: Hilbert Robert S. , Ford Eric H. , Hayford Michael J.
Published: February 2023

Strategies for selecting or creating starting designs vary depending on the designer and the problem. We describe several such techniques and include the basic approach, its advantages, and its limitations. Starting from previous designs is the simplest approach if relevant designs can be found. Most designs come from the literature, including in-house lens design libraries. When the literature is inadequate, the design must be created from scratch. Scratch design approaches vary from "minimum spherical bending" for the simplest cases to application of the designer's knowledge of past designs to provide a "guess" as to the overall configuration type, number of elements, shape, and materials. Experienced designers are often able to sketch the general form they think may apply and then use paraxial ray tracing to adjust the design to the desired values for a starting point. The y–y bar diagram is a tool for synthesizing the first-order properties of optical configurations that are limited by packaging requirements (e.g., periscope and laser beam delivery systems). One can directly specify on the y–y bar diagram such parameters as pupil and image locations, magnification, minimum beam size, and lens barrel diameter constraints. The diagram may then be modified to satisfy these requirements and packaging constraints; detection of system infeasibility is also possible. The diagram does not directly provide aberration performance, but an experienced user can usually infer this and other similar attributes.
Measuring optical signals and noise in hoovers
Authors: Levenson M. D. , Shelby R. M. , Perlmutter S. H.
Published: February 2023

It is common practice to report measurements of weak optical signals in terms of the relative-intensity noise, which is essentially the ratio of the fluctuations to the average power. Typical experiments in which weak optical signals are measured include light scattering, squeezed-state generation, and characterization of the optical noise on a laser beam. Although these measurements would be more meaningful if made in absolute units, e.g., mks or cgs, it is difficult to appropriately calibrate optical detection systems for such units.
Time and frequency response of avalanche photodiodes with arbitrary structure
Authors: Kahraman G. , Sargeant W. , Hayat M. , Saleh B. E. A. , Teich M. C.
Published: February 2023

We solve the coupled transport equations of the electron and hole currents for an avalanche photodiode (APD) of arbitrary structure and determine its time and frequency response. The injection can be localized to one or both ends of the multiplication region, or it can be distributed throughout an extended region, as in the case of thermal or photoinduced generation. The electron and hole ionization rates can be arbitrary functions of position. Because the equations contain position-dependent coefficients, they are difficult to solve. Our approach is to divide the device into incremental layers within each of which the ionization rates are assumed constant. We solve the transport equations within each of these layers and obtain the overall solution by matching the results at the layer boundaries. The total current is then determined as a function of time by integrating the electron and hole current densities over the depletion-layer width. We apply the solutions to superlattice multiple-quantum-well and conventional multiple-layered APD's; we study the effects of the carrier ionization rates and the type and position of the injected carriers, as well as the layout of the device, on the gain and bandwidth.
Efficient coherent radiation discrimination by using super-étalons
Authors: Ning Xiaohui
Published: February 2023

In many applications, the discrimination of coherent radiation against an incoherent background is required. Several approaches based on various forms of Fabry-Perot étalons have been investigated.1 Because of the intrinsic physical limitations of the étalons, the discrimination achievable with those devices is typically low. In this paper a new approach for achieving much higher discrimination is presented. The new device consists of multiple layers of thick films, referred to as super-étalons. The average optical thickness of the layers is chosen to be greater than the coherence length of the background light but less than that of the coherent light of interest. Thus, the phenomenon of multiple-beam interference cannot take place for background light. The reflectance is determined by the incoherent super-position of various orders of reflected beams from the layer interfaces. However, multiple beam interference still takes place for coherent radiation because of the long coherence length. The reflectance is determined by the coherent superposition of various orders of reflected beams from the layer interfaces. As a result, the transmittance for coherent light is dramatically different from that for incoherent light. New laser sensor concepts based on the super-étalons are discussed.
Phase-conjugate shear interferometry
Authors: Siahmakoun Azad , Yang Guanglu , Khorana Brij M.
Published: February 2023

A versatile shear interferometer is developed by using degenerate four-wave mixing in photorefractive crystal (PRC) BaTiO3. A major advantage of this system is its ability to shear a wave front by a small amount (10μm) in any direction. In addition, the system can exhibit both lateral and radial shearing without any change in the experimental setup.
Parametric study of coherent beam coupling through pulsations and oscillations in self-pumped phase conjugation and transmission
Authors: Mago V. K. , Venkateswarlu Putcha , Miahnahri Ali , Sekhar P. Chandra , George M. C.
Published: February 2023

Two horizontally polarized coherent beams, A1 and A2, from a He:Ne laser cross in an electrically poled BaTiO3 crystal. The crystal is adjusted so that at least one of the two beams pulsates/oscillates in self-pumping. Energy transfer between the two beams is analyzed with the help of four simultaneously recorded signals, for both individual pumping and simultaneous pumping, under different conditions. A beam (e.g., A1) can be diffracted in the directions of (1) the transmit ted beam (A2T) of the second beam, (2) the phase conjugate (A*2) of the second beam, and (3) the phase conjugate (A*1) of its own beam. Also, the fanning of this beam (A1) may help the other (A2) in its self-pumping. The relative importance of these factors depends the angles of incidence, the point of entry with respect to the crystal edge, and the orientation of the optic axis of the crystal. A study of the energy transfer from these parametric changes and the influence of the pulsations/oscillations on this transfer will be presented.
High-speed photorefractive phase conjugator with large dynamic range and wide field of view
Authors: Chiou Arthur E. , Chang Tallis Y. , Khoshnevisan Monte
Published: February 2023

Photorefractive materials have been used for demonstrating optical phase conjugation in a wide variety of configurations.1 A self-pumped photorefractive phase conjugator using internal reflection at the crystal faces (the CAT conjugator)2 has exhibited high fidelity. In this paper we describe a simple technique that uses the CAT conjugator to achieve optical phase conjugation with high speed (a few milliseconds to a fraction of a second), wide dynamic range of probe-beam intensities (less than a microwatt to tens of milliwatts), wide field of view (larger than ±50°), and a nearly uniform phase-conjugate reflectivity of approximately 15% throughout the entire range of intensities and angles. The "blackbox" consists of a CAT conjugator with a stabilized self-pumping beam and its phase-conjugate (of uniform or Gaussian intensity profile) of tens of milliwatts or higher. When a probe beam (from the same laser as the pumped beam) is properly injected into this black box, a phase-conjugate replica of the probe beam with the properties described above is generated by means of photorefractive four-wave mixing. Characterizations of this technique using a barium titanate crystal and potential applications for real-time image processing will be discussed.
Tunable solid-state lasers
Authors: Moulton Peter F.
Published: February 2023

The tutorial will begin with a discussion of the underlying physics of tunable solid-state laser media. A review of specific systems, including alexandrite, Ti:sapphire, and Co:MgF2, will follow and will include information on modes of operation and operating characteristics of each mode. The conclusion of the talk will be a survey of new research areas.
Cr:LiSrAIF6 laser
Authors: Bass Michael , Chai Bruce H.T. , Stadler Martin
Published: February 2023

Cr:LiSrAIF6 (LiSAF) was first reported to lase in 1989 by a group at the Lawrence Livermore National Laboratory using Kr ion laser pumping.1 We grew the first boules of this material which could yield significant rods for flashlamp pumped laser studies using the Czochralski technique. The growth parameters and some of the relevant crystalline properties are shown.
Hybrid electro-optic shift-invariant associative memory
Authors: Mendlovic David , Deutsch Meir , Konforti Naim , Marom Emanuel
Published: February 2023

Holography-based associative memories are of interest because of their large storage capacities. These systems require a nonlinear element, such as a phase-conjugate mirror or a square-law detector.
330-mW operation of buried-heterostructure strained-layer InGaAs lasers
Authors: Chen T. R. , Eng L. , Zhuang Y. H. , Xu Y. J. , Zarem H. , Yariv A.
Published: February 2023

High-power operation of an InGaAs/GaAs laser emitting at 0.98 µm is of great interest in applications such as optical pumping of Er-doped fiber amplifiers. Power outputs of ~100 mW have been reported in a ridge waveguide structure.1,2 In this paper we report on the high-power and high-efficiency operation of buried-heterostructure (BH) graded-index separated-confinement-heterostructure (GRINSCH) strained-layer single-quantum-well (SSQW) lasers. A high-reflectivity (~0.9) coating was applied to the rear facet, and a low-reflectivity (~0.05) coating was applied to the front facet. The lasers were tested in a junction-up mounting configuration. Owing to the excellent two-dimensional carrier and optical confinement of the BH-GRINSCH structure, the lasers are able to deliver optical powers in excess of 250 mW at room temperature over the entire spectrum of 0.87µm ≤ λ ≤ 1 µm. The highest power measured was 330 mW with an external quantum efficiency of 84%. The lasers operate in a stable single transverse mode to as great as the maximum power level with a beam divergence of ~20° (parallel to the junction plane). The characteristic temperature of these lasers was found to be as high as ~120 K, and output powers of over 120 mW were measured at ~90°C.
Observation of low-order lateral modes in broad-area diode lasers
Authors: Cherng Chung-Pin , Osinski Marek
Published: February 2023

Recent progress in broad-area semiconductor lasers with nearly 5 W of reported cw output power demonstrates that they represent an attractive alternative to diode-laser arrays for high-power applications. Therefore, understanding the lateral-mode structure of these devices becomes increasingly important. This paper reports on experimental investigations of carrier-guided broadarea lasers with thin GaAs/AlGaAs double-heterostructure active regions. Two types of Sony devices with different active region widths but with otherwise similar structures are used: 50 µm wide SLD-302NT (maximum cw output power Pmax = 200 mW) and 200 µm wide SLD-304XT (Pmax = 1 W). These devices are known to operate in a superposition of many lateral modes with complicated near-field patterns and broad double-lobe far fields. However, observations of near- and far-field patterns indicate that much simpler modal configurations can exist near threshold. For a 50 µm device at room temperature (threshold current Ith = 119 mA), the near field evolves from three peaks at 121 mA, four at 131 mA, and so on, which corresponds to excitation of a combination of first three, four, etc. lateral modes. Combination of two lowest-order modes was observed at 5°C for a pumping level of 120 mA (Ith = 107 mA). Because of the very small modal gain differences, no single lateral mode can be observed alone. These measurements are interpreted assuming weighted combinations of individual modes, which yields a basis of unperturbed broad-area modes important for studies of perturbed systems, such as thermal effects and spatial redistribution of carriers in high-power broad-area lasers and carrier-guided arrays. Similar measurements performed on one particular 200 µm device reveal four stable peaks from very low current level ( mA) up to the threshold. At some level above the threshold, the device suddenly switches to a combination of many lateral modes.
Effects of array geometry on the supermodes of two-dimensional vertical-cavity semiconductor laser arrays
Authors: Orenstein M. , Kapon E. , Stoffel N. G. , Florez L. T. , Harbison J. P.
Published: February 2023

We report experimental measurements of two-dimensional spatial lasing supermodes in phase-locked vertical-cavity semiconductor laser arrays. The phase locking was achieved by patterning the reflectivity of the back laser mirror. The dependence of the modal behavior on the array geometry for varius configurations is presented.
Dynamic characteristics of vertical-cavity surface-emitting lasers
Authors: Chang-Hasnain C. J. , Zah C. E. , Hasnain G. , Harbison J. P. , Florez L. T. , Stoffel N. G. , Lee T. P.
Published: February 2023

The dynamic behavior of low-threshold vertical-cavity surface-emitting lasers is investigated by using time-resolved spectra for the first time. Significant chirp is observed in the lasing spectrum owing to heating caused by the relatively high operating voltage and resistance.
Novel continuous-wave tunable IR laser system
Authors: Sigrist M. W. , Curl R. F. , Tittel F. K.
Published: February 2023

Continuous-wave (cw), narrow-bandwidth lasers that are tunable in the mid-IR wavelength range are important for analytical applications. The primary laser sources for this wavelength region include color-center as well as lead-salt diode lasers. Wide tunability can be achieved by difference-frequency mixing. Good results are obtained with LiNbO3 by using collinear 90° phase matching and temperature tuning. We achieved tuning between 2.4 µm and 3.6 µm and cw output powers of several microwatts by mixing cw dye- (Rh 6G) and 488 nm Ar+-laser radiation in LiNbO3. The required range of temperature was 230–400°C. The wavelength region beyond 4 µm can be accessed with new solid-state lasers and optical nonlinear materials. The design options of novel tunable cw IR laser systems based on difference-frequency mixing at near-room temperature in AgGaS2 or AgGaSe2 will be discussed. For AgGaS2, the mixing of Ti:sapphire- and dye-laser radiation permits tunability of 3.85–9.1 µm with a higher figure of merit than for LiNbO3. Furthermore, the wavelength can be extended to 12 µm by using AgGaSe2, which exhibits the highest figure of merit in this region. This can be accomplished, for example, by mixing cw single-frequency 1.32 µm Nd:YAG and NaCl:OH color-center laser radiation. The temperature-stability and damage-threshold issues were examined carefully for both new nonlinear optical materials.
Optical frequency measurement and synthesis using optical parametric oscillators
Authors: Wong N. C.
Published: February 2023

An optical parametric oscillator (OPO) performs the high-efficiency conversion of an input signal into two intense, coherent subharmonic outputs whose frequencies are tunable and whose linewidths are essentially limited by the input pump linewidth. By phase locking their difference frequency to a microwave, millimeter wave, or infrared reference source, the output frequencies are precisely determined, and the OPO functions as an optical frequency divider. The OPO dividers can be operated in series or in parallel to measure, compare, and synthesize frequencies from microwave to optical with high precision and resolution. The wide tunability of OPO's makes possible custom-tailored OPO configurations to suit a particular application. An example of this is the use of four phase-locked OPO dividers for the measurement of the hydrogen 1S-2S two-photon transition frequency (2λ = 0.486 µm) relative to the 3.39 µm CH4-stabilized He:Ne laser. A special parallel OPO configuration permits the generation of a comb of precisely known frequency markers, which are particularly useful in areas of high-precision measurements and frequency standards. A line-narrowing effect for the generation of ultrastable radiation will be discussed.
One-step dry ion-exchange technique for single-mode waveguides
Authors: Honkanen S. , Pöyhönen P. , Tervonen A. , Tahkokorpi M. , Tammela S.
Published: February 2023

This paper describes a one-step solid-state ion-exchange process for fabrication of Ag+ ion-exchanged single-mode glass waveguides. Ag+ ions are released into the glass electrolytically from a sputtered Ag film and are diffused deeper with a postbake immediately after the field-assisted exchange. The amount of Ag+ ions in glass can be accurately controlled by the ion current, because no appreciable purely thermal diffusion from the Ag film into the glass occurs. The postbake is started by disconnecting the voltage after the desired charge is driven into the glass.
Self-organizing optical neural networks
Authors: Tu Taiwei , Yang Xiang Y. , Yu Francis T. S. , Gregory Don A.
Published: February 2023

Two of the important features in neural computing are adaptation to a changeable environment and recognition of unknown objects. Kohonon's model is one of the simplest self-organizing algorithms,1 which is capable of performing statistical pattern recognition and classification. This model is modified and combined with several matching criteria that we have developed.
Ultrahigh intensity lasers for tomorrow’s ultrahigh field experiments
Authors: Mourou G. A.
Published: February 2023

Ultrahigh peak power, up to 20 TW has been produced in a single beam. In the near future 100-1000 TW should be demonstrated. These pulses will have to be produced with excellent spatial (diffraction limit) and temporal (contrast 1010:1) qualities compatible to experiments at intensity level greater than 10+20 w/cm2. we will review the work done in this novel and active area of research.
Powerful, femtosecond tunable dye oscillator-amplifier chain
Authors: François V. , Guan X. , Chin S. L.
Published: February 2023

We have developed a powerful, femtosecond pulse amplification chain to study high-intensity laser-matter interactions. The linear-cavity oscillator (Quantronics 4500) is synchronously pumped by a frequency-doubled Nd:YAG laser (Quantronics 416). An R6G jet acts as the gain medium, and a saturable absorber jet mode-locks the pulses. The group-velocity dispersion is compensated for by a series of four quartz prisms, with a slit between the two central prisms that permits selection of the lasing wavelength. Thus, the main characteristic of this laser is that pulses can be tuned over a range of 565-620 nm with a pulse duration between ~1 ps and ~100 fs. The amplifiers are a series of four Bethune cells transversely pumped by a frequency-doubled Q-switched Nd: YAG laser (Lumonics YP1200). The amplified spontaneous emission is filtered by a series of pinholes and saturable-absorber jets to preferentially amplify the femtosecond pulses and to improve the contrast ratio and beam quality. At 575 nm the net amplification factor is 107, and 2 mJ subpicosecond pulses are produced at a repetition rate of 10 Hz, providing an intensity of 1017 W/cm2 per pulse at best focus.
Ultrashort pulse x rays from laser heated solids
Authors: Falcone R. W. , Mumane M. M. , Kapetyn H. C.
Published: February 2023

High intensity, sub-picosecond laser pulses can be used to heat electrons to very high energies in and near solid surfaces. These electrons can cause the emission of short duration x-ray pulses through inelastic collisional processes in the solid and through recombination of highly ionized plasma formed at the surface of the solid. These processes can be enhanced by structuring the surface and by nonlinear processes at high laser intensity. New technology is required, including both lasers (with energy on order of one Joule and duration on order of 100 femtoseconds) and x-ray detectors (with subpicosecond resolution). Applications for short pulse x-rays include time resolved x-ray scattering and pumping x-ray lasers.
Femtosecond plasma physics at high light intensity
Authors: Downer M. C.
Published: February 2023

Production of plasmas with intense femtosecond laser pulses has permitted experimental access to the physics of highly ionized matter at unprecedented density and has created the prospect of novel x-ray sources. We study strong-field ionization of atmospheric-density gases by using a new diagnostic: spectral blue shifting of transmitted femtosecond pump or probe pulses caused by asymmetric phase modulation induced by the rapidly growing plasma density. In simple cases, observed shifts are well explained by an ab initio theory of plasma growth based on a Keldysh theory of collisionless ionization. However, anomalously large shifts occur in selected heavier gases (argon and nitrogen) at sufficiently high light intensity and pressure that electron-quiver amplitude exceeds interatomic spacing, inducing electron-impact ionization. Time-resolved experiments reveal further dynamics, such as a small time-delayed red shift of the probe spectrum, which is believed attributable to modulation of the collisional damping rate in the strong light field and implies a low final plasma temperature.
Parametric generation of intense femtosecond pulses around 800 nm
Authors: Joosen W. , Noordam L. D. , Muller H. G. , van Linden van den Heuvell H. B.
Published: February 2023

Picojoule colliding-pulse mode-locked pulses are amplified in four Bethune dye cells (sulforhodamine 640) and dispersion precompensated in a grating-telescope pulse shaper, typically to 200 pulses of ~100 fs duration. They pump a two-stage parametric generator of beta-barium borate (BBO). The large effective nonlinearity and angular sensitivity of BBO permit efficient conversion to 800 nm pulses, conserving the bandwidth of the pump. A collimated beam (2 mm diameter) passes the BBO crystals (7 and 5 mm length), which is cut at 20.3° for type I phase matching. The spatial separation (30 cm) and anti parallel orientation of the optical axes suppress noncollinear phase matching. Signal and idler frequencies at 830 nm and 2450 nm are generated with an overall energy conversion efficiency of 10%. This value is expected to increase closer to degeneracy. The signal can be precompensated to 150 fs: We use it for white-light generation in a 1 cm water cell and short-pulse amplification in titanium-doped sapphire. The idler provides interesting perspectives for saturation molecular spectroscopy around 3 μm with increased time resolution.
Higher energies at several kilohertz from a cw Nd:YLF regenerative amplifier
Authors: Saeed Muhammad R. , DiMauro Louis F.
Published: February 2023

We report on the development and application of a high-repetition-rate (up to 5 kHz) Nd: YLF regenerative amplifier, which is used in a synchronously pumped short pulse (below 250 fs) dye laser system.
Continous acousto-optic tuning of an erbuim-doped fiber laser at 1.5 μm
Authors: Maeda M. W. , Smith D. A. , Johnson J. H. , Patel J. S. , Von Lehman A. , Saifi M. S. , Lin C.
Published: February 2023

We demonstrate the first acoustically-tuned fiber ring laser. A two-stage integrated acousto-optic filter is the wavelength-selective intracavity element, providing continuous tunability over 40 nm and frequency-shift-free filtering for narrow linewidth operation. Such a laser is compact, can be tuned in microseconds, and has the potential for multi-wavelength lasing when an appropriate gain medium is used.
Effect on dispersion of rare-earth dopants in an optical fiber
Authors: Duling Irl N.
Published: February 2023

The propagation of an optical pulse in a fiber depends on the chromatic dispersion of the fiber. Rare-earth dopants, with their associated strong absorption and gain resonances, are expected to modify the linear dispersion as well as enhance the nonlinear index. By using the technique of white-light interferometry, the dispersion of an optical fiber can be measured with a spectral resolution of 5 nm and a temporal resolution of 13 fs.
Rigorous approach to imaging in scanning microscopy
Authors: Aguilar J. F. , Mendez E. R.
Published: February 2023

The usual description of imaging in a microscope is based on scalar diffraction theory and on some approximations to model the interaction of the electromagnetic field and the object. The validity of this description becomes suspect when there is structure in the object comparable to the wavelength of light and when the object is optically thick (< λ/4). Unfortunately, these cases are of interest in many practical applications, such as the inspection and metrology of integrated circuits.
Recording action potential in cultured Aplysia neurons using intrinsic optical signals
Authors: La Porta A. , Stepnoski R. , Raccuia-Behling F. , Kleinfeld D. , Slusher R. E. , Blonder G.
Published: February 2023

We have investigated the intrinsic optical properties of cultured Aplysia neurons and the changes in these properties associated with an action potential. By using conventional dark-field optics to collect light scattered at angles >4.5°, we recorded single action potentials in real time. The relative change in the level of scattered light during an action potential was ~3 × 10-4, and the signal-to-noise ratio was typically 10. The optical signal followed the same time course as the electrical signal. Furthermore, optical techniques allowed signals to be measured from fine neuronal processes. The angular dependence of light scattered from a resting neuron was measured and compared with calculated scattering from a nonabsorbing dielectric cylinder with an inhomogeneous index distribution. The data were consistent with a cylinder in which the index distribution was significantly peaked within the cell—not necessarily at the center. The average value of the cell index with respect to water was 1.02. Comparison with the changes in the scattered light observed in the dark-field measurements implies that the change in scattering reflects a change in the index of refraction of the cell. Based on these estimates, we have built a phase-sensitive microscope that can directly measure this phase shift. This should increase both the signal-to-noise ratio and the spatial resolution.
Recovery of integrated image descriptions
Authors: Darrell Trevor J. , Pentland Alex P.
Published: February 2023

Recovering useful descriptions of images is a central problem for vision. The minimal-description-length (MDL) criterion is a practical way to evaluate the relative usefulness of a set of surface descriptions as an overall approximation of the image. This criterion minimizes the residual error of the approximation as well as the cost of specifying the individual descriptions. Each individual description ideally models a single process in the scene and may overlap with several other descriptions to account for a portion of the image. The MDL criterion is equivalent to the Bayesian MAP estimation or Gestalt notions of simplicity in perceptual organization.We present a system that uses the MDL formulation to recover the set of part-based shape descriptions that best describes a given image. Our implementation on a Connection Machine uses parallel robust estimation to generate hypotheses of part shape, followed by a Hopfield-Tank network optimization to find the subset that minimally encodes the image.
Hypothesis testing in the planning of saccadic eye movements
Authors: Zelinsky Greg J. , Bülthoff Heinrich H.
Published: February 2023

Recent computer models of active vision adopt the broad categories of stimulus-driven and concept-driven eye movements to control the saccade-like movements of their cameras. Although low-level target selection may be accomplished by assigning priorities to areas on a saliency map, the question of how high-level feedback might influence saccadic movements is open to debate. One widely recognized solution to this problem defines this top-down input in terms of a hypothesistesting strategy. According to this model, foveal information and peripheral information are combined to suggest various hypothetical descriptions of a scene. Saccadic movements would then be used to confirm or reject the most likely of these hypotheses. An experiment was conducted to determine under what conditions a hypothesis-testing scheme is used and at what stage such information becomes available to the human occulomotor system. Subjects were asked to discriminate among four similar patterns while their eye movements were recorded by a two-dimensional binocular eye-tracker. The initial saccade made to each pattern was then used to determine if and when targets providing disambiguating information were preferred over targets providing only redundant information. This paradigm permits a systematic study of these top-down influences on the planning of saccades and may have implications for models of active vision.
Visibility of surface curvature in moving patterns
Authors: Norman J. Farley , Lappin Joseph S.
Published: February 2023

We have evaluated human observers' sensitivity to surface curvature in kinetic depth patterns. Observers viewed random-dot patterns that rotated (75°) around a central vertical axis, and they discriminated between two alternative nearly spherical surfaces that differed slightly in their relative elongations along a depth axis perpendicular to the display screen. One alternative was compressed and the other was expanded relative to a sphere, thereby preserving a qualitative difference in the two shapes and minimizing the quantitative difference in their depth extensions. We found that discriminations of surface curvature are remarkably accurate. This performance persisted when the absolute size of the two ellipsoids was randomly varied, when the dynamic boundary of the rotating ellipsoids was removed, and when the perspectivity of the projective displays was randomly varied between trials. Although the random variations in size strongly affect the variations in absolute depth and the variations in perspective affect the internal-velocity field of the patterns, these random variations do not affect the discriminations of curvature. In contrast, similar cube-like surfaces, with the same depth differences but without curvature, could not be discriminated. We conclude that surface curvature is a fundamental optical property involved in the discriminations of solid shapes and that this property may be detected at the early levels of human visual processing.
Optical Aharanov-Bohm effect
Authors: Levenson Marc D.
Published: February 2023

The minimal-coupling Hamiltonian, which describes the quantum mechanical interaction of charged matter and electromagnetic fields, implies that the phase of the matter-wave wave function depends on the vector and scalar potentials, whether or not gradients of those potentials produce forces. For steady potentials the resulting phase shifts have been detected as the magnetic and electrostatic Aharanov-Bohm effects. A similar effect occurs for optical-frequency fields. Free electrons cannot absorb energy from or emit energy into a uniform steady optical field, but the quantum-mechanical phase is nonetheless shifted. That shift can be detected by electron interferometry. The interaction between light and electrons results from the pondermotive potential, which is proportional to the square of the vector-potential amplitude. Only moderate intensities are required to produce detectable phase shifts. Both electron and optical waveguides can be fabricated in high-mobility GaAs/AlGaAs technology. In a device containing an optical waveguide and an electron interferometer, the optical Aharanov-Bohm effect could be detected as a resistance change. The pondermotive potential is also capable of shifting the oscillation frequency of a Josephson junction when the light interacts with superconducting condensate on only one side of the junction.
Phase-sensitive tests of nonlocality of quantum-mechanical fields
Authors: Tan Sze N. , Holland Murray J. , Walls Daniel F.
Published: February 2023

Nonlocal effects associated with quantum-mechanical states have been theoretically interesting because they emphasize the differences between classical and quantum mechanics. We describe a system in which a single input photon causes a nonlocal influence on two spatially separated homodyne detectors, as evidenced by a violation of Bell's inequalities. Unlike previous configurations for violating Bell's inequalities, the correlation is not imposed on a pair of particles that spatially separate before being detected. Rather, the correlation arises from the interaction of the local oscillators at each homodyne detector with the spatially extended field of the input photon. The above effect is only one of a class of situations in which nonlocal correlations can be imposed on separated homodyne detectors by quantum mechanical fields. By deriving the conditions for violating Bell's inequality with phase-sensitive detectors, new tests can be developed that relate to squeezed light and other nonclassical fields.
Albedo, clouds, and climate
Authors: Chertock Beth
Published: February 2023

This tutorial will provide an introduction to climate research that stresses the role of clouds in the radiation budget of the atmosphere. Radiative transfer theory, atmospheric scattering and absorption, and cloud-radiation feedback mechanisms will be discussed. The presentation will focus on the application of satellite remote sensing to problems of climate monitoring and climatemodel validation.
Measurement of ocean-particle size distribution by small-angle light scattering
Authors: Wang Tom D. , Chang Enson , Patton Randy J.
Published: February 2023

Ocean-particle sizes range from submicrometers to tens of micrometers and roughly follow an inverse-power-law distribution. The abundance of small particles causes most light-scattering techniques for particle-size measurement to suffer from instability or inaccuracy. This paper describes a compact instrument that determines the number density of particles of 1-50μm in size by measuring near-forward scattering (< 3.5°). The intensity data is inverted by a modified Phillips-Twomey method to obtain the size distribution. This technique is stable, accurate, and efficient in the size range of interest. The instrument performs well when applied to samples of simulated sea water. The simple design of the instrument, in conjunction with the robustness of the inversion algorithm, makes it a strong candidate for development into an in situ, expendable instrument.