Showing papers in "Optics Letters in 1998"
TL;DR: In this paper, a self-referencing interferometric technique for measuring the amplitude and phase of ultrashort optical pulses is presented, which uses a collinear geometry that requires no moving components.
Abstract: We present a novel, self-referencing interferometric technique for measuring the amplitude and the phase of ultrashort optical pulses The apparatus uses a collinear geometry that requires no moving components The phase-retrieval procedure is noniterative and rapid and uses only two one-dimensional Fourier transforms We apply the technique to characterize ultrashort pulses from a mode-locked Ti:sapphire oscillator
1,183 citations
TL;DR: The idea of a subwavelength-sized light guide represented by a linear chain of spherical metal nanoparticles in which light is transmitted by electrodynamic interparticle coupling may be useful for subwa wavelength transmission lines within integrated optics circuits and for near-field optical microscopy.
Abstract: We propose the idea of a subwavelength-sized light guide represented by a linear chain of spherical metal nanoparticles in which light is transmitted by electrodynamic interparticle coupling. The light-transport properties of this system are investigated by use of model calculations based on generalized Mie theory. Considering Ag particles of 50-nm diameter, we find optimum guiding conditions for an interparticle spacing of 25 nm, and a corresponding 1/e signal-damping length of 900 nm is evaluated. The proposed principle of optical energy transport may be useful for subwavelength transmission lines within integrated optics circuits and for near-field optical microscopy.
1,024 citations
TL;DR: Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure-a dye-doped cholesteric liquid-crystal film and the mode closest to the edge has the lowest lasing threshold.
Abstract: Low-threshold lasing is observed at the edge of the stop band of a one-dimensional structure—a dye-doped cholesteric liquid-crystal film. The mode closest to the edge has the lowest lasing threshold. The rates of spontaneous and stimulated emission are suppressed within the stop band and enhanced at the band edge. The ratio of right to left circularly polarized spontaneous emission is in good agreement with calculated density of photon states.
713 citations
TL;DR: A model is provided for the essential mechanism of intensity shifts as first-order far-field interference between the outgoing laser beam and scattered light from the trapped particle, where the latter is phase advanced owing to the Gouy phase anomaly.
Abstract: The lateral position of an optically trapped object in a microscope can be monitored with a quadrant photodiode to within nanometers or better by measurement of intensity shifts in the back focal plane of the lens that is collimating the outgoing laser light. This detection is largely independent of the position of the trap in the field of view. We provide a model for the essential mechanism of this type of detection, giving a simple, closed-form analytic solution with simplifying assumptions. We identify intensity shifts as first-order far-field interference between the outgoing laser beam and scattered light from the trapped particle, where the latter is phase advanced owing to the Gouy phase anomaly. This interference also reflects momentum transfer to the particle, giving the spring constant of the trap. Our response formula is compared with the results of experiments.
659 citations
TL;DR: A new method for the extraction of quantitative phase data from microscopic phase samples by use of partially coherent illumination and an ordinary transmission microscope is presented, able to recover phase even in the presence of amplitude modulation.
Abstract: We present a new method for the extraction of quantitative phase data from microscopic phase samples by use of partially coherent illumination and an ordinary transmission microscope. The technique produces quantitative images of the phase profile of the sample without phase unwrapping. The technique is able to recover phase even in the presence of amplitude modulation, making it significantly more powerful than existing methods of phase microscopy. We demonstrate the technique by providing quantitatively correct phase images of well-characterized test samples and show that the results obtained for more-complex samples correlate with structures observed with Nomarski differential interference contrast techniques.
545 citations
TL;DR: The observed independence of Q from wavelength indicates that losses for the WGM's are dominated by a mechanism other than bulk absorption in fused silica in the near infrared.
Abstract: Measurements of the quality factor Q ~ 8 x 10^9 are reported for the whispering-gallery modes (WGM’s) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse F ~ 2.2 x 10^6. The observed independence of Q from wavelength indicates that losses for the WGM’s are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that Q can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared.
539 citations
TL;DR: In this paper, the authors applied phase-shifting digital holography to microscopy by deriving the complex amplitude of light scattered from microscopic 3D objects through a microscope objective by video camera recording, phase shifting analysis, and computer reconstruction.
Abstract: We applied phase-shifting digital holography to microscopy by deriving the complex amplitude of light scattered from microscopic three-dimensional objects through a microscope objective by video camera recording, phase-shifting analysis, and computer reconstruction. This method requires no mechanical movement and provides a flexible display and quantitative evaluation of the reconstructed images. A theory of image formation and experimental verification with specimens are described.
525 citations
TL;DR: It is demonstrated that one-dimensional photonic crystal structures (such as multilayer films) can exhibit complete reflection of radiation in a given frequency range for all incident angles and polarizations.
Abstract: We demonstrate that one-dimensional photonic crystal structures (such as multilayer films) can exhibit complete reflection of radiation in a given frequency range for all incident angles and polarizations. We derive a general criterion for this behavior that does not require materials with very large indices. We perform numerical studies that illustrate this effect.
507 citations
TL;DR: The sensitivity of the technique is demonstrated by photoacoustic detection of single red blood cells upon a glass plate byPhotoacoustics as a tissue tomography technique for the detection of blood concentrations, e.g., angiogenesis around tumors.
Abstract: We applied photoacoustics as a tissue tomography technique for the detection of blood concentrations, e.g., angiogenesis around tumors. We imaged blood vessels in highly scattering samples, using 532-nm light, to depths of ,1 cm. The samples were real tissue (chicken breast) or 10% dilutions of Intralipid-10%. The blood flowed through nylon capillaries. Polyvinylidene difluoride (PVdF) piezoelectric detectors were used in a surface-scanning mode. We demonstrate the sensitivity of the technique by photoacoustic detection of single red blood cells upon a glass plate. Lateral resolution is limited by the detector diameter s200 mmd. The depth resolution is ,10 mm.
473 citations
TL;DR: A real-time, direct-view multiphoton excitation fluorescence microscope that provides three-dimensional imaging at high resolution using a rotating microlens disk that permits the observation with the eye of high-resolution two-photon images of (live) samples.
Abstract: We present a real-time, direct-view multiphoton excitation fluorescence microscope that provides three-dimensional imaging at high resolution. Using a rotating microlens disk, we split the near-infrared light of a mode-locked titanium:sapphire laser into an array of beams that are transformed into an array of high-aperture foci at the object. We typically scan at 225 frames per second and image the fluorescence with a camera that reads out the images at video rate. For 1.4 aperture oil and 1.2 water immersion lenses at 780-nm excitation we obtained axial resolutions of 0.84 and 1.4 µm, respectively, which are similar to that of a single-beam two-photon microscope. Compared with the latter setup, our system represents a 40–100-fold increase in efficiency, or imaging speed. Moreover, it permits the observation with the eye of high-resolution two-photon images of (live) samples.
448 citations
TL;DR: A new microscopy system for imaging in turbid media that is based on the spatial coherence gate principle and generates in parallel a complete two-dimensional head-on image without scanning is presented.
Abstract: We present a new microscopy system for imaging in turbid media that is based on the spatial coherence gate principle and generates in parallel a complete two-dimensional head-on image without scanning. This system has been implemented in a commercial microscope and preserves the lateral resolution of the optics used. With a spatially incoherent source, speckle-free images with diffraction-limited resolution are recorded at successive depths with shot-noise-limited detection. The setup comprises a photoelastic modulator for path difference modulation and a two-dimensional CCD array and uses a multiplexed lock-in detection scheme.
TL;DR: Numerical simulations of nonlinear pulse propagation in air show an initial pulse formed, absorbed by plasma generation, and subsequently replenished by power from the trailing edge, implying that the length scale for filament propagation is relatively insensitive to the peak input power beyond the threshold for filament generation.
Abstract: We present numerical simulations of nonlinear pulse propagation in air whereby an initial pulse is formed, absorbed by plasma generation, and subsequently replenished by power from the trailing edge of the pulse. This process can occur more than once for high-power input pulses and produce the illusion of long-distance propagation of one self-guided pulse.
TL;DR: An improved polarization-sensitive optical coherence tomography (OCT) system is developed and used to measure birefringence in porcine myocardium tissue and produce two-dimensional bireFringence mapping of the tissue.
Abstract: An improved polarization-sensitive optical coherence tomography (OCT) system is developed and used to measure birefringence in porcine myocardium tissue and produce two-dimensional birefringence mapping of the tissue. Signal-to-noise issues that cause systematic measurement errors are analyzed to determine the regime in which such measurements are accurate. The advantage of polarization-sensitive OCT systems over standard OCT systems in avoiding image artifacts caused by birefringence is also demonstrated.
TL;DR: The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions to derive uniform dispersion values for single mode and double mode fibers.
Abstract: The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 µm when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 µm.
TL;DR: The main result applies to steady-state (dc) diffusion-based optical tomography, wherein it is demonstrated that simultaneous unique recovery of diffusion and absorption coefficients cannot be achieved.
Abstract: A condition on nonuniqueness in optical tomography is stated. The main result applies to steady-state (dc) diffusion-based optical tomography, wherein we demonstrate that simultaneous unique recovery of diffusion and absorption coefficients cannot be achieved. A specific example of two images that give identical dc data is presented. If the refractive index is considered an unknown, then nonuniqueness also occurs in frequency-domain and time-domain optical tomography, if the underlying model of the diffusion approximation is employed.
TL;DR: A novel approach to the realization of nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts to yield the desired spatial distribution of the refractive index is reported on.
Abstract: We report on a novel approach to the realization of nematic liquid-crystal (LC) phase correctors to form spherical and cylindrical wave fronts. A LC cell with a distributed reactive electrical impedance was driven by an ac voltage applied to the cell boundary to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into the light beam depends on the frequency of the ac control voltage, the geometry of the boundary electrode surrounding the LC cell, and the electrical parameters of the cell. We realized a cylindrical adaptive lens with a clear aperture of 15 mm x 4mm and a spherical adaptive lens with circular aperture of 6.5 mm. Both devices are capable of focusing collimated light in the range infinity...0.5 m.
TL;DR: Single-mode excitation of step-index multimode fibers with light sources with short temporal coherence lengths is demonstrated and designs with reduced microbending-induced mode coupling are described that allow the propagation of the fundamental mode over long lengths with negligible mode coupling even in the presence of tight fiber bends.
Abstract: Single-mode excitation of step-index multimode fibers with light sources with short temporal coherence lengths is demonstrated. Multimode fiber designs with reduced microbending-induced mode coupling are described that allow the propagation of the fundamental mode over long lengths with negligible mode coupling even in the presence of tight fiber bends. At a wavelength of 1.56microm a fiber with a core diameter of 45microm can preserve the fundamental mode for a propagation length of ~20m . Such fibers allow coiling with a coil diameter as small as 7cm.
TL;DR: A novel application of a wavelength-swept fiber laser to fiber Bragg grating sensor-array interrogation and static-dynamic strain measurements with a resolution of 0.47mu?
Abstract: We demonstrate a novel application of a wavelength-swept fiber laser to fiber Bragg grating sensor-array interrogation. The laser provides high signal powers of >3 mW with <0.1-nm spectral resolution over a 28-nm wavelength span. Using time-interval counting, we demonstrate static-dynamic strain measurements with a resolution of 0.47mu? rms at a sampling rate of 250 Hz.
TL;DR: This approach, spectrally encoded confocal microscopy (SECM), uses a quasi-monochromatic light source and a transmission diffraction grating to detect the reflectivity simultaneously at multiple points along a transverse line within the sample.
Abstract: An endoscope-compatible, submicrometer-resolution scanning confocal microscopy imaging system is presented. This approach, spectrally encoded confocal microscopy (SECM), uses a quasi-monochromatic light source and a transmission diffraction grating to detect the reflectivity simultaneously at multiple points along a transverse line within the sample. Since this method does not require fast spatial scanning within the probe, the equipment can be miniaturized and incorporated into a catheter or endoscope. Confocal images of an electron microscope grid were acquired with SECM to demonstrate the feasibility of this technique.
TL;DR: A new structural cutoff beyond which subwavelength gratings cease to behave as homogeneous media is introduced and its effects on the proper selection of the sampling periods of subwa wavelength diffractive elements are discussed.
Abstract: We introduce a new structural cutoff beyond which subwavelength gratings cease to behave as homogeneous media and discuss its effects on the proper selection of the sampling periods of subwavelength diffractive elements. According to this analysis, a 3?-period blazed binary grating composed of square pillars is designed for He–Ne operation and is fabricated by etching of a TiO2 layer deposited upon a glass substrate. Its first-order measured diffraction efficiency is 12% larger than the theoretical efficiency of an ideal blazed echelette grating in glass with the same period.
TL;DR: High-efficiency double-peak resonances occurred at nonnormal incidence, with the spectral locations of the maxima vayring with the incidence angle agreed well with the theoretically calculated reflectance curves.
Abstract: A high-efficiency guided-mode resonance reflection filter is reported. The device consists of a surface-relief photoresist grating and an underlying HfO (2) waveguide layer deposited on a fused-silica substrate. The spectral response measured with a dye-laser beam at normal incidence exhibited a peak reflectance of 98% at a wavelength of 860 nm with sideband reflectance below approximately 5% extending over the wavelength range provided by the dye (800-900 nm). At normal incidence the filter linewidth was 2.2 nm. High-efficiency double-peak resonances occurred at nonnormal incidence, with the spectral locations of the maxima vayring with the incidence angle. The filter response at various angles of incidence agreed well with the theoretically calculated reflectance curves.
TL;DR: In this paper, the authors present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broadbandwidth IR pulse is mixed with a narrowbandwidth visible pulse, and the resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector.
Abstract: We present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broad-bandwidth IR pulse is mixed with a narrow-bandwidth visible pulse. The resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector, permitting rapid and high signal-to-noise ratio data acquisition over a 400?cm-1 spectral region without scanning the IR frequency. Application to the study of a self-assembled monolayer of octadecanethiol is discussed.
TL;DR: The first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser using wavelength modulation spectroscopy detected N(2)O and CH(4) in the chemical fingerprint wavelength range near 8microm.
Abstract: We report what we believe are the first spectroscopic measurements to be made with a room-temperature quantum-cascade distributed-feedback laser. Using wavelength modulation spectroscopy, we detected N2O and CH4 in the chemical fingerprint wavelength range near 8 µm. The noise equivalent absorbance for our measurement was 5 parts in 105, limited by excess amplitude modulation on the laser output, which corresponds to a 1-Hz bandwidth detection limit of 250 parts N2O in 109 parts N2 in a 1-m path length.
TL;DR: A novel optical arrangement for heterodyne detection of laser-induced gratings based on the use of a phase mask for both excitation and probe beams provides phase stability and control without the need for an active stabilization scheme.
Abstract: A novel optical arrangement for heterodyne detection of laser-induced gratings based on the use of a phase mask for both excitation and probe beams provides phase stability and control without the need for an active stabilization scheme. The arrangement greatly simplifies the laser-induced grating experiment. The performance of the technique in both transmission and reflection geometries is illustrated through measurements of bulk and surface acoustic waves generated by picosecond laser pulses.
TL;DR: It is shown that optical Bloch oscillations can emerge in waveguide arrays with linearly varying propagation constants and the existence of localized modes (Wannier-Stark states) with equidistant wave-number spacing that do not undergo diffraction is analytically proved.
Abstract: We show that optical Bloch oscillations can emerge in waveguide arrays with linearly varying propagation constants. The existence of localized modes (Wannier-Stark states) with equidistant wave-number spacing (Wannier-Stark ladder) that do not undergo diffraction is analytically proved. The evolution of arbitrary initial excitations is described, and potential applications are suggested.
TL;DR: A novel optical path-modulation technique for a low-coherence interferometric imaging system based on transverse scanning of the target with a galvanometric scanning-mirror pair is introduced.
Abstract: We introduce a novel optical path-modulation technique for a low-coherence interferometric imaging system based on transverse scanning of the target with a galvanometric scanning-mirror pair. The path modulation arises when the beam that is incident upon one of the scanning mirrors does not fall on its axis of rotation. The method is demonstrated by the production of en-face low-coherence images of different objects such as a fiber-optic tip and a human retina in vivo.
TL;DR: The signal-gain characteristics of tellurite-based erbium-doped fiber amplifiers are clarified based on spectroscopic properties and signal- gain measurements.
Abstract: The signal-gain characteristics of tellurite-based erbium-doped fiber amplifiers are clarified based on spectroscopic properties and signal-gain measurements. The potential of tellurite-based erbium-doped fiber for use as a broadband light source is also described.
TL;DR: A noncollinear optical parametric amplifier is presented that generates transform-limited sub-10-fs pulses that are tunable in both the visible and the near infrared (NIR), believed to be the first tunable sub- 10-fs light source.
Abstract: A noncollinear optical parametric amplifier is presented that generates transform-limited sub-10-fs pulses that are tunable in both the visible and the near infrared (NIR).?The pulse-front-matched pump geometry realizes tilt-free signal amplification, and pulses as short as 6.1 fs can be obtained from 550 to 700 nm. The large angular dispersion of the idler specific to the group-velocity-matching interaction is effectively eliminated by a grating–telescope compensator, and 9-fs NIR pulses are also successfully obtained from 900 to 1300 nm. This is believed to be the first tunable sub-10-fs light source.
TL;DR: In this paper, a noncollinear optical parametric amplifier in the visible, pumped by the second harmonic of a Ti:sapphire laser, was used to generate a 180-THz bandwidth and 2?µJ energy.
Abstract: Pulses with 180-THz bandwidth and 2?µJ energy were generated by a noncollinear optical parametric amplifier in the visible, pumped by the second harmonic of a Ti:sapphire laser. A portion of the amplified pulse spectrum was compressed to 7.2 fs by use of a thin prism sequence.
TL;DR: Third-order spherical aberration and coma were significantly larger for the cornea than for the complete eye, indicating a significant role of the lens in compensating for corneal aberrations.
Abstract: The relative contributions of optical aberrations of the cornea and the crystalline lens to the final image quality of the human eye were studied. The aberrations of the entire eye were obtained from pairs of double-pass retinal images, and the aberrations of the cornea were obtained from videokeratographic data. Third-order spherical aberration and coma were significantly larger for the cornea than for the complete eye, indicating a significant role of the lens in compensating for corneal aberrations. In a second experiment retinal images were recorded in an eye before and after we neutralized the aberrations of the cornea by having the subjects wear swimming goggles filled with saline water, providing a direct estimate of the optical performance of the crystalline lens.