Showing papers in "Optics Express in 2002"
TL;DR: The first scanning laser ophthalmoscope that uses adaptive optics to measure and correct the high order aberrations of the human eye is presented, permitting axial sectioning of retinal tissue in vivo.
Abstract: We present the first scanning laser ophthalmoscope that uses adaptive optics to measure and correct the high order aberrations of the human eye. Adaptive optics increases both lateral and axial resolution, permitting axial sectioning of retinal tissue in vivo. The instrument is used to visualize photoreceptors, nerve fibers and flow of white blood cells in retinal capillaries.
933 citations
TL;DR: A generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material is proposed and advantages in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.
Abstract: We propose a generally applicable velocity matching method for THz-pulse generation by optical rectification in the range below the phonon frequency of the nonlinear material. Velocity matching is based on pulse front tilting of the ultrashort excitation pulse and is able to produce a large-area THz beam. Tuning of the THz radiation by changing the tilt angle is experimentally demonstrated for a narrow line in the range between 0.8-0.97 times the phonon frequency. According to model calculations broadband THz radiation can be generated at lower frequencies. Advantages of the new velocity matching technique in comparison to the electro-optic Cherenkov effect and non-collinear beam mixing are discussed.
730 citations
TL;DR: A novel Monte Carlo code for photon migration through 3D media with spatially varying optical properties, known as 'tMCimg', is described and can serve as a resource for solving the forward problem for complex 3D structural data obtained by MRI or CT.
Abstract: We describe a novel Monte Carlo code for photon migration through 3D media with spatially varying optical properties. The code is validated against analytic solutions of the photon diffusion equation for semi-infinite homogeneous media. The code is also cross-validated for photon migration through a slab with an absorbing heterogeneity. A demonstration of the utility of the code is provided by showing time-resolved photon migration through a human head. This code, known as 'tMCimg', is available on the web and can serve as a resource for solving the forward problem for complex 3D structural data obtained by MRI or CT.
638 citations
TL;DR: A focus shaping technique using generalized cylindrical vector beams that can be tailored by appropriately adjusting the rotation angle to obtain peak-centered, donut and flattop focal shapes is reported.
Abstract: We report a focus shaping technique using generalized cylindrical vector beams. A generalized cylindrical vector beam can be decomposed into radially polarized and azimuthally polarized components. Such a generalized cylindrical beam can be generated from a radially polarized or an azimuthally polarized light using a two-half-wave-plate polarization rotator. The intensity pattern at the focus can be tailored by appropriately adjusting the rotation angle. Peak-centered, donut and flattop focal shapes can be obtained using this technique.
572 citations
TL;DR: This work demonstrates photonic crystal fibers with ultra-flattened, near zero dispersion with micro-structured fibers showing dispersion of 0 +/- 0.6 ps/nm from 1.24 microm-1.6 microm wavelength.
Abstract: We demonstrate photonic crystal fibers with ultra-flattened, near zero dispersion. Two micro-structured fibers showing dispersion of 0 ± 0.6 ps/nm.km from 1.24 μm-1.44 μm wavelength and 0 ± 1.2 ps/nm.km over 1 μm-1.6 μm wavelength have been measured.
523 citations
TL;DR: A new mode solver is described which uses Yee's 2-D mesh and an index averaging technique to provide a full-vectorial finite-difference analysis of microstructured optical fibers.
Abstract: In this paper we present a full-vectorial finite-difference analysis of microstructured optical fibers. A new mode solver is described which uses Yee's 2-D mesh and an index averaging technique. The modal characteristics are calculated for both conventional optical fibers and microstructured optical fibers. Comparison with previous finite difference mode solvers and other numerical methods is made and excellent agreement is achieved.
450 citations
TL;DR: Numerical investigations using the finite-difference time-domain (FDTD) method predict that radiation losses can be significantly suppressed through these methods, culminating with a graded square lattice design whose total Q approaches 10;5 with a mode volume of approximately 0.25 cubic half-wavelengths in vacuum.
Abstract: The design of high quality factor (Q) optical cavities in two dimensional photonic crystal (PC) slab waveguides based upon a momentum space picture is presented. The results of a symmetry analysis of defect modes in hexagonal and square host photonic lattices are used to determine cavity geometries that produce modes which by their very symmetry reduce the vertical radiation loss from the PC slab. Further improvements in the Q are achieved through tailoring of the defect geometry in Fourier space to limit coupling between the dominant momentum components of a given defect mode and those momentum components which are either not reflected by the PC mirror or which lie within the radiation cone of the cladding surrounding the PC slab. Numerical investigations using the finite-difference timedomain (FDTD) method predict that radiation losses can be significantly suppressed through these methods, culminating with a graded square lattice design whose total Q approaches 105 with a mode volume of approximately 0.25 cubic half-wavelengths in vacuum.
358 citations
TL;DR: The concept of effective area is used to calculate the "phase" boundary between the regimes with single-mode and multi-mode operation and the results can be scaled to a given pitch and thus provide a general map of the effective area.
Abstract: We consider the effective area Ae. of photonic crystal fibers (PCFs) with a triangular air-hole lattice in the cladding. It is first of all an important quantity in the context of non-linearities, but it also has connections to leakage loss, macro-bending loss, and numerical aperture. Single-mode versus multi-mode operation in PCFs can also be studied by comparing effective areas of the different modes. We report extensive numerical studies of PCFs with varying air hole size. Our results can be scaled to a given pitch and thus provide a general map of the effective area. We also use the concept of effective area to calculate the "phase" boundary between the regimes with single-mode and multi-mode operation.
346 citations
TL;DR: The fabrication and properties of soft glass photonic crystal fibers for supercontinuum generation have zero or anomalous group velocity dispersion at wavelengths around 1550 nm, and approximately an order of magnitude higher nonlinearity than attainable in comparable silica fibers.
Abstract: We report the fabrication and properties of soft glass photonic crystal fibers (PCF's) for supercontinuum generation. The fibers have zero or anomalous group velocity dispersion at wavelengths around 1550 nm, and approximately an order of magnitude higher nonlinearity than attainable in comparable silica fibers. We demonstrate the generation of an ultrabroad supercontinuum spanning at least 350 nm to 2200 nm using a 1550 nm ultrafast pump source.
338 citations
TL;DR: Using time-resolved imaging and scattering techniques, the breakdown dynamics induced in water by femtosecond laser pulses are monitored and a 20 ps delay is observed before the laser-produced plasma expands.
Abstract: Using time-resolved imaging and scattering techniques, we directly and indirectly monitor the breakdown dynamics induced in water by femtosecond laser pulses over eight orders of magnitude in time. We resolve, for the first time, the picosecond plasma dynamics and observe a 20 ps delay before the laser-produced plasma expands. We attribute this delay to the electron-ion energy transfer time.
318 citations
TL;DR: The simulations allow several important physical processes responsible for supercontinuum generation to be identified and illustrate how the XFROG trace provides an intuitive means of interpreting correlated temporal and spectral features of thesupercontinuum.
Abstract: Numerical simulations are used to study the temporal and spectral characteristics of broadband supercontinua generated in photonic crystal fiber. In particular, the simulations are used to follow the evolution with propagation distance of the temporal intensity, the spectrum, and the cross-correlation frequency resolved optical gating (XFROG) trace. The simulations allow several important physical processes responsible for supercontinuum generation to be identified and, moreover, illustrate how the XFROG trace provides an intuitive means of interpreting correlated temporal and spectral features of the supercontinuum. Good qualitative agreement with preliminary XFROG measurements is observed.
TL;DR: A methodology for quantitative image correction in OCT which includes procedures for correction of nonlinear axial scanning and non-telecentric scan patterns, as well as a novel approach for refraction correction in layered media based on Fermat's principle are described.
Abstract: We describe a methodology for quantitative image correction in OCT which includes procedures for correction of nonlinear axial scanning and non-telecentric scan patterns, as well as a novel approach for refraction correction in layered media based on Fermat’s principle. The residual spatial error obtained in layered media with a fan-beam hand-held probe was reduced from several hundred micrometers to near the diffraction and coherence-length limits.
TL;DR: It is shown that the constructive supermodes, each of which has a single output in a different port, build up automatically because of the dense longitudinal-mode, length-unbalanced laser array with unbalanced port loss.
Abstract: Coherent addition of fiber lasers coupled with an intracavity fiber coupler is reported. Almost a single output is obtained from one of the fiber ports, which one can switch simply by unbalancing the losses in the ports. We show that the constructive supermodes, each of which has a single output in a different port, build up automatically because of the dense longitudinal-mode, length-unbalanced laser array with unbalanced port loss. High addition efficiencies of 93.6% for two fiber lasers and 95.6% for four fiber lasers have been obtained.
TL;DR: It is reported on the generation of linearly chirped parabolic pulses with 17-W average power at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier.
Abstract: We report on the generation of linearly chirped parabolic pulses with 17-W average power at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. Highly efficient transmission gratings in fused silica are applied to recompress these pulses down to 80-fs with an efficiency of 60%, resulting in a peak power of 1.7 MW. Power scaling limitations given by the amplifier bandwidth are discussed.
TL;DR: The Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS) is a hyperspectral imager specifically designed for imaging the coastal ocean that uses a thinned, backsideilluminated CCD for high sensitivity and an all-reflective spectrograph with a convex grating in an Offner configuration to produce a nearly distortionfree image.
Abstract: The Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS) is a hyperspectral imager specifically designed for imaging the coastal ocean. It uses a thinned, backsideilluminated CCD for high sensitivity and an all-reflective spectrograph with a convex grating in an Offner configuration to produce a nearly distortionfree image. The sensor, which was constructed entirely from commercially available components, has been successfully deployed during several oceanographic experiments in 1999-2001. Here we describe the instrument design and present the results of laboratory characterization and calibration. We also present examples of remote-sensing reflectance data obtained from the LEO-15 site in New Jersey that agrees well with ground-truth measurements.
TL;DR: The influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers and Raman scattering and the wavelength dependence of the group delay of the fiber are reported on.
Abstract: We report on the influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers. Different nonlinear mechanisms are observed depending on whether the pump is located in the normal or anomalous dispersion region. Raman scattering and the wavelength dependence of the group delay of the fiber are found to play an important role in the process. We give an experimental and numerical analysis of the observed phenomena and find a good agreement between the two.
TL;DR: The first measurements of spontaneous Raman scattering from silicon waveguides are reported, suggesting that a silicon optical amplifier is within reach as it lowers the pump power required for the onset ofRaman scattering.
Abstract: We report the first measurements of spontaneous Raman scattering from silicon waveguides Using a 143 m pump, both forward and backward scattering were measured at 154 m from Silicon-On-Insulator (SOI) waveguides From the dependence of the Stokes power vs pump power, we extract a value of (41 +/- 25) x 10-7 cm-1 Sr-1 for the Raman scattering efficiency The results suggest that a silicon optical amplifier is within reach The strong optical confinement in silicon waveguides is an attractive property as it lowers the pump power required for the onset of Raman scattering The SiGe material system is also discussed
TL;DR: This straightforward process enables an adjustable number of traps and realtime control of the position, size, shape and intensity of each individual tweezer-beam in arbitrary arrays by encoding the appropriate phase pattern on the SLM.
Abstract: We demonstrate a technique for obtaining fully dynamic multiple-beam optical tweezers using the generalized phase contrast (GPC) method and a phase-only spatial light modulator (SLM). The GPC method facilitates the direct transformation of an input phase pattern to an array of high-intensity beams, which can function as efficient multiple optical traps. This straightforward process enables an adjustable number of traps and realtime control of the position, size, shape and intensity of each individual tweezer-beam in arbitrary arrays by encoding the appropriate phase pattern on the SLM. Experimental results show trapping and dynamic manipulation of multiple micro-spheres in a liquid solution.
TL;DR: In this article, a real-time phase-resolved optical coherence tomography (OCT) and optical Doppler tomography system using optical Hilbert transformation (ODT) was developed.
Abstract: We have developed a novel real-time phase-resolved optical coherence tomography (OCT) and optical Doppler tomography (ODT) system using optical Hilbert transformation. By combining circularly polarized reference and linearly polarized sample signals, in-phase and quadrature interference components are produced in separate channels and treated as the real and imaginary parts of a complex signal to obtain the phase information directly. Using a resonant scanner at an axial scanning speed of 4 kHz in the reference arm of the interferometer, both structure and blood flow velocity images with 200 axial scans can be acquired at 20 frames per second with high sensitivity and large dynamic range. Real-time videos of in vivo blood flow in the chick chorioallantoic membrane using this interferometer are presented.
TL;DR: In this paper, the transmission of light through a subwavelength hole drilled in a metallic thin film is calculated by numerically solving Maxwell's equations both for a simple hole and for a hole with additional structure.
Abstract: The transmission of light through a subwavelength hole drilled in a metallic thin film is calculated by numerically solving Maxwell’s equations both for a simple hole and for a hole with additional structure. A maximum in the transmission cross section is observed for hole diameters of the order of but smaller than the wavelength. Transmission cross sections well above the hole area are shown to be attainable by filling the hole with a high-index material. The effect of adding a small particle inside the hole is also analyzed.
TL;DR: A new Thin Light Sheet Microscope that permits the observation of interactions at the microscale and has potential to yield insights into how microbes structure pelagic ecosystems is developed.
Abstract: Despite a growing need, oceanographers are limited by existing technological constrains and are unable to observe aquatic microbes in their natural setting. In order to provide a simple and easy to implement solution for such studies, a new Thin Light Sheet Microscope (TLSM) has been developed. The TLSM utilizes a well-defined sheet of laser light, which has a narrow (23 micron) axial dimension over a 1 mm x 1 mm field of view. This light sheet is positioned precisely within the depth of field of the microscope's objective lens. The technique thus utilizes conventional microscope optics but replaces the illumination system. The advantages of the TLSM are two-fold: First, it concentrates light only where excitation is needed, thus maximizing the efficiency of the illumination source. Secondly, the TLSM maximizes image sharpness while at the same time minimizing the level of background noise. Particles that are not located within the objective's depth of field are not illuminated and therefore do not contribute to an out-of-focus image. Images from a prototype system that used SYBR Green I fluorescence stain in order to localize single bacteria are reported. The bacteria were in a relatively large and undisturbed volume of 4ml, which contained natural seawater. The TLSM can be used for fresh water studies of bacteria with no modification. The microscope permits the observation of interactions at the microscale and has potential to yield insights into how microbes structure pelagic ecosystems.
TL;DR: The guidance of dry micron-sized dielectric particles in hollow core photonic crystal fiber using an Argon ion laser beam operating at a wavelength of 514 nm is reported.
Abstract: We report the guidance of dry micron-sized dielectric particles in hollow core photonic crystal fiber The particles were levitated in air and then coupled to the air-core of the fiber using an Argon ion laser beam operating at a wavelength of 514 nm The diameter of the hollow core of the fiber is 20 m A laser power of 80 mW was sufficient to levitate a 5 m diameter polystyrene sphere and guide it through a ~150 mm long hollow-core crystal photonic fiber The speed of the guided particle was measured to be around 1 cm/s
TL;DR: It is shown that healthy, neoplastic and scar tissues of the esophagus have different cross-polarization backscattering properties, which improves the specificity of diagnostics of pathological changes occurring in tissues.
Abstract: An experimental standard optical coherence tomography (OCT) setup that can be easily modified for cross-polarization OCT (CP OCT) operation has been developed to perform differential diagnosis of pathological tissues. The complementary use of CP OCT, a technique that provides a map of cross-polarization backscattering properties of an object being studied by means of low-coherence interferometry, and standard OCT imaging improves the specificity of diagnostics of pathological changes occurring in tissues. It is shown that healthy, neoplastic and scar tissues of the esophagus have different cross-polarization backscattering properties. A comparative analysis of CP OCT, OCT and histological images from one and the same tissue area has been made. A close correlation between the location of collagen fibers in biological tissue and signal intensity in CP OCT images is found.
TL;DR: Using the direct output of an extended cavity femtosecond laser, without the need for a laser amplifier, single mode waveguides can be rapidly fabricated with well controlled parameters.
Abstract: Coupled mode devices are fabricated in transparent glasses by nonlinear materials processing with femtosecond laser pulses Using the direct output of an extended cavity femtosecond laser, without the need for a laser amplifier, single mode waveguides can be rapidly fabricated with well controlled parameters A variety of photonic waveguide devices are demonstrated Directional couplers with various interaction lengths and coupling coefficients are fabricated and their coupling properties are characterized Measurements demonstrate coupled mode behavior consistent with theory An unbalanced Mach-Zehnder interferometer is also fabricated and demonstrated as a spectral filter
TL;DR: The volume depolarization ratio of the molecular backscatter signal detected with polarization lidar varies by a factor of nearly 4 depending on whether the rotational Raman bands are included in the detected signals of the individual system or not.
Abstract: The volume depolarization ratio of the molecular backscatter signal detected with polarization lidar varies by a factor of nearly 4 depending on whether the rotational Raman bands are included in the detected signals of the individual system or not. If the rotational Raman spectrum is included partially in the signals, this calibration factor depends on the temperature of the atmosphere. This dependency is studied for different spectral widths of the receiving channels. In addition, the sensitivity to differences between the laser wavelength and the center wavelength of the receiver are discussed.
TL;DR: Numerical results show that by increasing the number of air hole rings the attenuation constant decreases, which is very strong for triangular and cobweb fibers, whereas it is very weak for the honeycomb one.
Abstract: An analysis of the confinement losses in photonic crystal fibers due to the finite numbers of air holes is performed by means of the finite element method. The high flexibility of the numerical method allows to consider fibers with regular lattices, like the triangular and the honeycomb ones, and circular holes, but also fibers with more complicated cross sections like the cobweb fiber. Numerical results show that by increasing the number of air hole rings the attenuation constant decreases. This dependence is very strong for triangular and cobweb fibers, whereas it is very weak for the honeycomb one.
TL;DR: A 3D full-vectorial Beam Propagation Method is successfully applied to compute both the propagation constants and the modal profiles in high-contrast silica-air index-guiding Photonic Crystal Fibers, showing that efficient polarization preserving coupling can be expected.
Abstract: A 3D full-vectorial Beam Propagation Method is successfully applied to compute both the propagation constants and the modal profiles in high-contrast silica-air index-guiding Photonic Crystal Fibers. The approach is intrinsically suited to investigate longitudinally varying structures or propagation and polarization effects, which are of practical interest for advanced optical applications. As an example we model a dual-core coupler, showing that efficient polarization preserving coupling can be expected.
TL;DR: In a novel application of light torques, the rotation of nanorods is manipulated and control in a single-beam optical trap and two new modes of behavior - rocking motion and saltatory motion are described and explained.
Abstract: In a novel application of light torques, we manipulate and control the rotation of nanorods. We apply light torques to 250 nm diameter glass nanorods in a single-beam optical trap. Light-torque operated nanomotors whir at moderate speeds that depend on several factors, including the magnitude of the light torque, the viscosity of the surrounding medium, and the rotation rate of the electric field vector of the linearly polarized trapping light. Two new modes of behavior - rocking motion and saltatory motion -are also described and explained.
TL;DR: In this paper, a scanning laser ophthalmoscope with an integrated retinal tracker (TSLO) was designed, constructed, and tested in human subjects without mydriasis.
Abstract: A scanning laser ophthalmoscope with an integrated retinal tracker (TSLO) was designed, constructed, and tested in human subjects without mydriasis The TSLO collected infrared images at a wavelength of 780 nm while compensating for all transverse eye movements An active, high-speed, hardware-based tracker was able to lock onto many common features in the fundus, including the optic nerve head, blood vessel junctions, hypopigmentation, and the foveal pit The TSLO has a system bandwidth of ∼1 kHz and robustly tracked rapid and large saccades of approximately 500 deg/sec with an accuracy of 005 deg Image stabilization with retinal tracking greatly improves the clinical potential of the scanning laser ophthalmoscope for imaging where fixation is difficult or impossible and for diagnostic applications that require long duration exposures to collect meaningful information
TL;DR: Histological examination of laser-exposed porcine eyes reveal a minimum cut size below 1 microm without destructive effects to surrounding tissues, and 1 nanojoule near infrared 80 MHz femtosecond laser pulses for highly precise intratissue processing was evaluated.
Abstract: The use of 1 nanojoule near infrared 80 MHz femtosecond laser pulses for highly precise intratissue processing, in particular for intraocular refractive surgery, was evaluated. Destructive optical breakdown at TW/cm2 light intensities in a subfemtoliter intrastromal volume was obtained by diffraction-limited focussing with an 40x objective (N.A. 1.3) and beam scanning 50 to 140 μm below the epithelial surface. Using the same system at GW/cm2 intensities two-photon excited autofluorescence imaging was used to determine the target of interest and to visualize intraocular laser effects. Histological examination of laser-exposed porcine eyes reveal a minimum cut size below 1 μm without destructive effects to surrounding tissues.