Showing papers in "Optics Letters in 1995"
TL;DR: A new optical encoding method of images for security applications is proposed and it is shown that the encoding converts the input signal to stationary white noise and that the reconstruction method is robust.
Abstract: We propose a new optical encoding method of images for security applications. The encoded image is obtained by random-phase encoding in both the input and the Fourier planes. We analyze the statistical properties of this technique and show that the encoding converts the input signal to stationary white noise and that the reconstruction method is robust.
2,361 citations
TL;DR: This work presents what is to their knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy, and demonstrates applications to package inspection and chemical content mapping in biological objects.
Abstract: We present what is to our knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy Terahertz time-domain waveforms are downconverted from the terahertz to the kilohertz frequency range, and the waveform for each pixel is frequency analyzed in real time with a digital signal processor to extract compositional information at that point We demonstrate applications to package inspection and chemical content mapping in biological objects
1,628 citations
TL;DR: The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated and a preliminary model is shown to explain these results.
Abstract: The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.
1,205 citations
TL;DR: Two new techniques, based on optical coherence tomography (OCT), for determining the refractive index of highly scattering human tissue are described, using OCT to track the focal length shift that results from translating the focus along the optic axis within the tissue.
Abstract: We describe two new techniques, based on optical coherence tomography (OCT), for determining the refractive index of highly scattering human tissue. We obtained refractive indices of in vitro human tissue, using OCT to measure the physical and optical path lengths of the sample. We obtained measurements of the refractive index of in vitro and in vivo human tissue, using OCT to track the focal length shift that results from translating the focus along the optic axis within the tissue. The refractive indices of human skin, adipose, and muscle were measured and compared with previously published estimates.
692 citations
TL;DR: To the knowledge, the images are the first experimental reconstruction based on data collected in the frequency domain, and it is shown that this method is sensitive to the optical properties of the heterogeneity.
Abstract: We present images of heterogeneous turbid media derived from measurements of diffuse photon-density waves traveling through highly scattering tissue phantoms. To our knowledge, the images are the first experimental reconstruction based on data collected in the frequency domain. We demonstrate images of both absorbing and scattering heterogeneities and show that this method is sensitive to the optical properties of the heterogeneity. The algorithm employs a differential measurement scheme that reduces the effect of errors resulting from incorrect estimation of the background optical properties. The relative advantages of sources with low and high modulation frequency are discussed within this context.
494 citations
TL;DR: The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated and the molecular two- photon absorption coefficients are presented.
Abstract: The optical-limiting behavior and two-photon absorption properties of four novel organic compound solutions in tetrahydrofuran have been investigated. An ultrashort laser source with 0.5-ps pulse width and 602-nm wavelength was employed. The transmissivities of the various 1-cm-thick solution samples have been measured as a function of the beam intensity as well as of the solute concentration. The measured results can be fitted on the assumption that two-photon absorption is the only predominant mechanism causing the observed opticallimiting behavior. Based on the intensity-dependent transmissivity measurements, the molecular two-photon absorption coefficients for the four compounds are presented.
457 citations
TL;DR: A Kerr-lens mode-locked Ti:Al(2)O(3) oscillator, optimized for minimal coherence length, is demonstrated as a high-power source for high-resolution optical coherence tomographic imaging.
Abstract: A Kerr-lens mode-locked Ti:Al(2)O(3) oscillator, optimized for minimal coherence length, is demonstrated as a high-power source for high-resolution optical coherence tomographic imaging. Dispersion compensation and heterodyne noise rejection are demonstrated to yield in situ images of biological tissues with 3.7-mum resolution and 93-dB dynamic range.
407 citations
TL;DR: Estimates of each suggest that near-field fluorescence excitation microscopy/spectroscopy with molecular sensitivity and spatial resolution is possible.
Abstract: We can resolve multiple discrete features within a focal region of m spatial dimensions by first isolating each on the basis of n ≥ 1 unique optical characteristics and then measuring their relative spatial coordinates. The minimum acceptable separation between features depends on the point-spread function in the (m + n)-dimensional space formed by the spatial coordinates and the optical parameters, whereas the absolute spatial resolution is determined by the accuracy to which the coordinates can be measured. Estimates of each suggest that near-field fluorescence excitation microscopy/spectroscopy with molecular sensitivity and spatial resolution is possible.
392 citations
TL;DR: It is shown that the effective index responsible for waveguiding is dependent only on lateral changes in the Fabry-Perot resonance frequency, which leads naturally to new design methods for these lasers that are expected to result in more effective devices with superior modal characteristics.
Abstract: A new effective index optical model is presented for the analysis of lateral waveguiding effects in verticalcavity surface-emitting lasers. In addition to providing a concise formalism for reducing the dimensionality of the Maxwell equations describing the lasing mode, this model also provides new insights into waveguiding phenomena in vertical-cavity lasers. In particular, it is shown that the effective index responsible for waveguiding is dependent only on lateral changes in the Fabry-Perot resonance frequency. This concept leads naturally to new design methods for these lasers that are expected to result in more eff icient devices with superior modal characteristics.
347 citations
TL;DR: Extensive T-matrix computations for randomly oriented spheroids demonstrate that, although both delta(L) and delta(C) are indicators of particle nonsphericity, they cannot be considered a universal measure of the departure of particle shape from that of a sphere and have no simple dependence on particle size and refractive index.
Abstract: We derive theoretically and validate numerically general relationships for the elements of the backscattering matrix and for the linear, delta(L), and circular, delta(C), backscattering depolarization ratios for nonspherical particles in random orientation. For the practically important case of randomly oriented particles with a plane of symmetry or particles and their mirror particles occurring in equal numbers and in random orientation, delta(C) = 2delta(L)/(1 - delta(L)). Extensive T-matrix computations for randomly oriented spheroids demonstrate that, although both delta(L) and delta(C) are indicators of particle nonsphericity, they cannot be considered a universal measure of the departure of particle shape from that of a sphere and have no simple dependence on particle size and refractive index.
340 citations
TL;DR: It is observed that very high-Q Mie resonances in silica microspheres are split into doublets, attributed to internal backscattering that couples the two degenerate whispering-gallery modes propagating in opposite directions along the sphere equator.
Abstract: We have observed that very high-Q Mie resonances in silica microspheres are split into doublets. This splitting is attributed to internal backscattering that couples the two degenerate whispering-gallery modes propagating in opposite directions along the sphere equator. We have studied this doublet structure by high-resolution spectroscopy. Time-decay measurements have also been performed and show a beat note corresponding to the coupling rate between the clockwise and counterclockwise modes. A simple model of coupled oscillators describes our data well, and the backscattering efficiency that we measure is consistent with what is observed in optical fibers.
TL;DR: A theoretical model is presented that supports observations of optical phase-conjugate gain in sodium vapor by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.
Abstract: We have observed optical phase-conjugate gain (>50) in sodium vapor, using low-intensity pump lasers (1 W/cm2), with a response time of the order of 1 μs. Coherent population trapping is experimentally identified as the phase-conjugate mechanism. A theoretical model is presented that supports these observations by showing that coherent population trapping can write large-amplitude nonlinear-optical gratings at laser intensities well below those needed to saturate the optical transitions.
TL;DR: The basic physics underlying the approach may be considered a harbinger for the coupling of active photonic microstructures such as microdisk lasers.
Abstract: Morphology-dependent resonances (MDR's) of solid microspheres are excited by using an optical fiber coupler The narrowest measured MDR linewidths are limited by the excitation laser linewidth (<0025 nm) Only MDR's, with an on-resonance to off-resonance intensity ratio of 10(4), contribute to scattering The intensity of various resonance orders is understood by the localization principle and the recently developed generalized Lorentz-Mie theory The microsphere fiber system has potential for becoming a building block in dispersive microphotonics The basic physics underlying our approach may be considered a harbinger for the coupling of active photonic microstructures such as microdisk lasers
TL;DR: A novel light-emitting probe for scanning near-field optical microscopy is investigated theoretically and the three-dimensional vectorial Helmholtz equation is solved for the new probe geometry by using the multiple multipole method.
Abstract: A novel light-emitting probe for scanning near-field optical microscopy is investigated theoretically. The three-dimensional vectorial Helmholtz equation is solved for the new probe geometry by using the multiple multipole method. The novel probe consists of a dielectric tip that is entirely metal coated. It provides a single near-field spot that can be smaller than 20 nm (FWHM). The dependence on tip radius, taper angle, and metal thickness in front of the tip is investigated for the power transmission through the probe as well as for the spot size.
TL;DR: It is demonstrated the generation of nearly bandwidth-limited 8-fs optical pulses near 0.8 microm from a self-mode-locked Ti:sapphire laser oscillator, using chirped dielectric mirrors for dispersion control.
Abstract: We demonstrate the generation of nearly bandwidth-limited 8-fs optical pulses near 0.8 microm from a self-mode-locked Ti:sapphire laser oscillator, using chirped dielectric mirrors for dispersion control. The mode-locking performance is described, and limitations are discussed.
TL;DR: Continuous-wave ultrasonic modulation of scattered laser light has been used to image objects in tissue-simulating turbid media for what is to the authors' knowledge the first time and has the potential to provide a noninvasive, nonionizing, inexpensive diagnostic tool for diseases such as breast cancer.
Abstract: Continuous-wave ultrasonic modulation of scattered laser light has been used to image objects in tissue-simulating turbid media for what is to our knowledge the first time. The ultrasound wave focused into the turbid media modulates the laser light passing through the ultrasonic focal zone. The modulated laser light collected by a photomultiplier tube reflects the local mechanical and optical properties in the focal zone. Buried objects are located with millimeter resolution by scanning and detecting alterations of the modulated optical signal. This technique has the potential to provide a noninvasive, nonionizing, inexpensive diagnostic tool for diseases such as breast cancer.
TL;DR: In this article, a quasi-phase-matched optical parametric oscillator, using bulk periodically poled LiNbO(3), was reported, which was pumped by a 1.064-microm Q-switched Nd:YAG laser, was temperature tuned over the wavelength range 1.66-2.95 microm.
Abstract: We report a quasi-phase-matched optical parametric oscillator, using bulk periodically poled LiNbO(3). The optical parametric oscillator, pumped by a 1.064-microm Q-switched Nd:YAG laser, was temperature tuned over the wavelength range 1.66-2.95 microm. The oscillation threshold of approximately 0.1 mJ was more than a factor of 10 below the damage limit. The LiNbO(3) crystal, fabricated by application of an electric field to a sample with liquid and metal surface electrodes, was 0.5 mm thick with a 5.2-mm interaction length and a quasi-phase-matched period of 31 microm.
TL;DR: In this article, the authors present a method of recording successive holograms in a recording medium, using at least a fan of M waves along at least one first axis with a separation angle between adjacent waves and directing the fan of the M waves as a reference beam along a reference path onto the recording medium.
Abstract: The invention is embodied in a method of recording successive holograms in a recording medium, using at least a fan of M waves along at least a first axis with a separation angle between adjacent waves and directing the fan of M waves as a reference beam along a reference beam path onto the recording medium, successively modulating a wave with a succession of images to produce a succession of signal beams along a signal beam path lying at a propagation angle relative to the reference beam path so that the signal and reference beams intersect at a beam intersection lying within the medium, the beam intersection having a size corresponding to beam areas of the reference and signal beams, producing a succession of relative displacements in a direction parallel to the first axis between the recording medium and the beam intersection of the signal and reference beam paths in synchronism with the succession of signal beams, each of the displacements being less than the size of the intersection whereby to record successive holograms partially overlapped along a direction of the displacements.
TL;DR: It is shown that a soliton of the nonlinear Schrödinger equation perturbed by filter losses and/or the finite gain bandwidth of amplifiers can be kept stable by saturable absorbers with a relaxation time much longer than the width of the soliton.
Abstract: We show that a soliton of the nonlinear Schrodinger equation perturbed by filter losses and/or the finite gain bandwidth of amplifiers can be kept stable by saturable absorbers with a relaxation time much longer than the width of the soliton. This provides for ultrashort pulse generation with a slow saturable absorber only and may have possible applications in the stabilization of soliton storage rings.
TL;DR: Optical Doppler tomography is an accurate method for the characterization of high-resolution fluid flow velocity in transparent glass and turbid collagen conduits and is in excellent agreement with experimental data and theoretical calculations.
Abstract: The spatial profiles of fluid flow velocity in transparent glass and turbid collagen conduits are measured by optical Doppler tomography (ODT). The flow velocity at a discrete user-specified spatial location in the conduit is determined by measurement of the Doppler shift of backscattered light from microspheres suspended in the flowing fluid. Experimental data and theoretical calculations are in excellent agreement. ODT is an accurate method for the characterization of high-resolution fluid flow velocity.
TL;DR: Low error rates were obtained for the quantum transmissions, and error-free secret keys were distilled from the raw data by use of secure error-correction and privacy-amplification protocols.
Abstract: We report the secure transmission of cryptographic keys over as long as 30 km of optical fiber, using an interferometric quantum cryptography scheme. Low error rates in the range 1.2–4% were obtained for the quantum transmissions, and error-free secret keys were distilled from the raw data by use of secure error-correction and privacy-amplification protocols.
TL;DR: In this article, a mode-locked laser beam is focused into the solution, thereby defining a two-photon excitation volume localized in 3D. Molecules diffusing into and out of this volume produce fluorescence bursts, which are detected with a high signal to background ratio.
Abstract: We present a technique for observing single fluorophore molecules in solution. A mode-locked laser beam is focused into the solution, thereby defining a two-photon excitation volume localized in three dimensions. Molecules diffusing into and out of this volume produce fluorescence bursts, which are detected with a high signal-to-background ratio. The theoretical foundations for the technique are laid out, including the attendant fluorescence rate distribution, and agree with experimental results obtained for Rhodamine B molecules in water.
TL;DR: Stable single-frequency and polarization operation of a traveling-wave, Er(3+)-doped fiber loop laser is demonstrated by incorporating an unpumped Er(+3)- doped fiber section butted against a narrow-band feedback reflector with saturable absorber that acts as a narrow bandpass filter that automatically tracks the lasing wavelength.
Abstract: We demonstrate stable single-frequency and polarization operation of a travelling-wave Er3+:Yb3+-doped fiber loop laser by incorporating an unpumped Er3+-doped fiber section butted against a narrowband feedback reflector. The saturable absorber acts as a narrow bandpass filter which automatically tracks the lasing wavelength, thus ensuring single-frequency operation. Output powers up to 6.2 mW at 1535 nm were obtained for launched pump powers of 175 mW at 1064 nm. At this output, the RIN was less than -112 dB/Hz at frequencies above 200 kHz and the laser linewidth less than 0.95 kHz whilst the lasing frequency was observed to drift slowly (~ 170 MHz/hr) due to environmental effects.
TL;DR: P pulses tunable in the 590-666-nm range are produced, with durations down to 13 fs, using an 82-MHz Ti:sapphire second-harmonic-pumped, high-bandwidth, beta-barium borate optical parametric oscillator in a fused-silica prism group-delay-dispersion-compensated, six-mirror folded ring cavity.
Abstract: We have produced pulses tunable in the 590-666-nm range, with durations down to 13 fs, using an 82-MHz Ti:sapphire second-harmonic-pumped, high-bandwidth, beta-barium borate optical parametric oscillator in a fused-silica prism group-delay-dispersion-compensated, six-mirror folded ring cavity.
TL;DR: In this article, the design and performance of a new type of high-efficiency diffraction grating for use in either transmission or reflection are described, which is produced in a multilayer dielectric coating deposited upon optically flat substrates.
Abstract: The design and performance of a new type of high-efficiency diffraction grating for use in either transmission or reflection are described. The gratings are produced in a multilayer dielectric coating deposited upon optically flat substrates. By proper design of the multilayer stack and grating structure, a diffraction efficiency in excess of 96% for polarized light in the m = -1 order in reflection has been achieved.
TL;DR: An all-fiber wavelength-division multiplexer based on the nonreciprocity of the birefringence to the polarization states is proposed and theoretical analysis shows that the output characteristics of this WDM are similar to those of a fiber taper-based device.
Abstract: An all-fiber wavelength-division multiplexer (WDM) based on the nonreciprocity of the birefringence to the polarization states is proposed. The transfer function of a Sagnac interferometer is wavelength dependent if the loop birefringence of the interferometer consists of both circular and linear parts. Theoretical analysis shows that the output characteristics of this WDM are similar to those of a fiber taper-based device. Both the bandwidth and the peak wavelength of the new WDM can be tuned by changing the loop birefringence. Experimental prototypes exhibit a channel isolation greater than 25 dB with peak passband insertion loss of less than 1 dB.
TL;DR: Experimental results obtained with a target in a highly scattering medium are presented, demonstrating that a manmade polarization-difference system can render readily visible surface features invisible to conventional imaging.
Abstract: Many animals have visual systems that exploit the polarization of light, and some of these systems are thought to compute difference signals in parallel from arrays of photoreceptors optimally tuned to orthogonal polarizations. We hypothesize that such polarization-difference systems can improve the visibility of objects in scattering media by serving as common-mode rejection amplifiers that reduce the effects of background scattering and amplify the signal from targets whose polarization-difference magnitude is distinct from the background. We present experimental results obtained with a target in a highly scattering medium, demonstrating that a manmade polarization-difference system can render readily visible surface features invisible to conventional imaging.
TL;DR: The crystal structure, refractive indices, and phase-matching conditions for a new nonlinear optical material, L-histidine tetrafluoroborate, are reported and the effective nonlinearity is comparable with that of beta-barium borate.
Abstract: The crystal structure, refractive indices, and phase-matching conditions for a new nonlinear optical material, L-histidine tetrafluoroborate (HFB), are reported. HFB grows readily, displays favorable mechanical characteristics, and has adequate birefringence to permit phase-matched parametric processes over much of its transparency range (250 nm to 1300 nm). The phase-matching loci and angular sensitivity for second-harmonic generation of 1064-nm light in single crystals of HFB were measured. The effective nonlinearity for HFB is comparable with that of beta-barium borate (~2 pm/V), and its angular sensitivity [delta(Deltak)/deltatheta] is somewhat smaller.
TL;DR: Two liquid-crystal spatial light modulators are employed in a novel arrangement to permit the generation of programmable ultrashort waveforms with exceptional versatility and fidelity and the time-dependent polarization profile is controlled.
Abstract: We report advances in the generation of ultrafast optical waveforms with programmable time-dependent amplitude and temporal phase profiles. Two liquid-crystal spatial light modulators are employed in a novel arrangement to permit the generation of programmable ultrashort waveforms with exceptional versatility and fidelity. We also demonstrate ultrafast waveform generation in which the time-dependent polarization profile is controlled.
TL;DR: This novel approach to soot sizing exhibits several theoretical and technical advantages compared with the established combination of elastic scattering and LII, especially as it yields absolute sizes of primary particles without requiring calibration.
Abstract: The evaluation of the temporal decay of the laser-induced incandescence (LII) signal from soot particles is introduced as a technique to obtain two-dimensional distributions of particle sizes and is applied to a laminar diffusion flame. This novel approach to soot sizing exhibits several theoretical and technical advantages compared with the established combination of elastic scattering and LII, especially as it yields absolute sizes of primary particles without requiring calibration.