Showing papers in "Optics Letters in 2008"
TL;DR: Three new algorithms for 2D translation image registration to within a small fraction of a pixel that use nonlinear optimization and matrix-multiply discrete Fourier transforms are compared to evaluate a translation-invariant error metric.
Abstract: Three new algorithms for 2D translation image registration to within a small fraction of a pixel that use nonlinear optimization and matrix-multiply discrete Fourier transforms are compared. These algorithms can achieve registration with an accuracy equivalent to that of the conventional fast Fourier transform upsampling approach in a small fraction of the computation time and with greatly reduced memory requirements. Their accuracy and computation time are compared for the purpose of evaluating a translation-invariant error metric.
1,715 citations
TL;DR: In vivo imaging of the microvasculature including single capillaries in mice using optical-resolution photoacoustic microscopy (OR-PAM) developed in the laboratory is presented.
Abstract: Capillaries, the smallest blood vessels, are the distal end of the vasculature where oxygen and nutrients are exchanged between blood and tissue. Hence, noninvasive imaging of capillaries and function in vivo has long been desired as a window to studying fundamental physiology, such as neurovascular coupling. Existing imaging modalities cannot provide the required sensitivity and spatial resolution. We present in vivo imaging of the microvasculature including single capillaries in mice using optical-resolution photoacoustic microscopy (OR-PAM) developed in our laboratory. OR-PAM provides a lateral resolution of 5 µm and an imaging depth >0.7 mm.
737 citations
TL;DR: This technique can visualize vessel-size-dependent vascular shutdown and transient vascular occlusion during Visudyne photodynamic therapy and may provide opportunities for studying therapeutic effects of antivascular treatments without on exogenous contrast agent.
Abstract: We report on imaging of microcirculation by calculating the speckle variance of optical coherence tomography (OCT) structural images acquired using a Fourier domain mode-locked swept-wavelength laser. The algorithm calculates interframe speckle variance in two-dimensional and three-dimensional OCT data sets and shows little dependence to the Doppler angle ranging from 75 degrees to 90 degrees . We demonstrate in vivo detection of blood flow in vessels as small as 25 microm in diameter in a dorsal skinfold window chamber model with direct comparison with intravital fluorescence confocal microscopy. This technique can visualize vessel-size-dependent vascular shutdown and transient vascular occlusion during Visudyne photodynamic therapy and may provide opportunities for studying therapeutic effects of antivascular treatments without on exogenous contrast agent.
677 citations
TL;DR: A novel nanometeric plasmonic filter in a tooth-shaped metal-insulator-metal waveguide is proposed and demonstrated numerically and analytic and simulation results show that the wavelength of the trough of the transmission has linear and nonlinear relationships with the tooth depth and the tooth width.
Abstract: A novel nanometeric plasmonic filter in a tooth-shaped metal-insulator-metal waveguide is proposed and demonstrated numerically. An analytic model based on the scattering matrix method is given. The result reveals that the single tooth-shaped filter has a wavelength-filtering characteristic and an ultracompact size in the length of a few hundred nanometers, compared to gratinglike surface plasmon polariton (SPP) filters. Both analytic and simulation results show that the wavelength of the trough of the transmission has linear and nonlinear relationships with the tooth depth and the tooth width, respectively. The waveguide filter could be utilized to develop ultracompact photonic filters for high integration.
440 citations
TL;DR: It is demonstrated both theoretically and experimentally that optical Airy beams propagating in free space can perform ballistic dynamics akin to those of projectiles moving under the action of gravity.
Abstract: We demonstrate both theoretically and experimentally that optical Airy beams propagating in free space can perform ballistic dynamics akin to those of projectiles moving under the action of gravity. The parabolic trajectories of these beams as well as the motion of their center of gravity were observed in good agreement with theory. The possibility of circumventing an obstacle placed in the path of the Airy beam is discussed.
434 citations
TL;DR: Phase-locked terahertz transients with peak electric fields of 108 MV/cm and center frequencies continuously tunable from 10 to 72 THz are generated via difference-frequency mixing of two parametrically amplified pulse trains from a single white-light seed.
Abstract: Phase-locked terahertz transients with peak electric fields of 108 MV/cm and center frequencies continuously tunable from 10 to 72 THz are generated via difference-frequency mixing of two parametrically amplified pulse trains from a single white-light seed. Free space electro-optic sampling with 8 fs gating pulses from a two-branch Er:fiber laser allows us to monitor all transients directly in the time domain. We identify extreme terahertz nonlinearities in the detector crystal with subcycle resolution.
430 citations
TL;DR: In this article, a ring-cavity thulium fiber laser with a single-wall carbon nanotube absorber was used in transmission, achieving an average output power of 3.4 mW.
Abstract: We report a ring-cavity thulium fiber laser mode locked with a single-wall carbon nanotube absorber used in transmission. A carboxymethyl cellulose polymer film with incorporated carbon nanotubes synthesized by the arc discharge method has an absorption coinciding with in the amplification bandwidth of a Tm-doped fiber. This laser is pumped by an erbium fiber laser at 1.57 μm wavelength and produces a 37 MHz train of mode-locked 1.32 ps pulses at 1.93 μm wavelength with an average output power of 3.4 mW.
377 citations
TL;DR: A novel architecture for optical image encryption based on interference is proposed that does not need iterative encoding and the parameters of the configuration can also serve as additional keys for encryption.
Abstract: We proposed a novel architecture for optical image encryption based on interference. The encryption algorithm for this new method is quite simple and does not need iterative encoding. The parameters of the configuration can also serve as additional keys for encryption. Numerical simulation results demonstrate the flexibility of this new proposed method.
365 citations
TL;DR: A broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals) that manifests high diffraction efficiency and high extinction ratio in both theory and experiment.
Abstract: We demonstrate a broadband, thin-film, polarizing beam splitter based on an anisotropic diffraction grating composed of reactive mesogens (polymerizable liquid crystals). This achromatic polarization grating (PG) manifests high diffraction efficiency (~100%) and high extinction ratio (⩾1000:1) in both theory and experiment. We show an operational bandwidth Δλ/λ0~56% (roughly spanning visible wavelength range) that represents more than a fourfold increase of bandwidth over conventional PGs (and significantly larger than any other grating). The diffraction angle and operational region (visible, near-infrared, midwave infrared, and ultraviolet wavelengths) may be easily tailored during fabrication. The essence of the achromatic design is a stack of two chiral PGs with an opposite twist sense and employs the principle of retardation compensation. We fully characterize its optical properties and derive the theoretical diffraction behavior.
344 citations
TL;DR: A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated and shows that the thermal-optics effect of the cavity material is much more appreciable than its thermal expansion.
Abstract: A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000 degrees C with a step of 50 degrees C confirm that it could be applicable as a high-temperature sensor.
340 citations
TL;DR: A simple refractive index sensor based on a Michelson interferometer in a single-mode fiber is constructed and demonstrated, and its ease of fabrication offers a low-cost alternative to current sensing applications.
Abstract: A simple refractive index sensor based on a Michelson interferometer in a single-mode fiber is constructed and demonstrated. The sensor consists of a single symmetrically abrupt taper region in a short piece of single-mode fiber that is terminated by ~500 nm thick gold coating. The sensitivity of the new sensor is similar to that of a long-period-grating-type sensor, and its ease of fabrication offers a low-cost alternative to current sensing applications.
TL;DR: High nonlinearity in a highly nonlinear arsenic selenide chalcogenide nanowire with tailored dispersion enables low-threshold soliton fission leading to large spectral broadening at a dramatically reduced peak power of several watts, corresponding to picojoule energy.
Abstract: We demonstrate low-threshold supercontinuum generated in a highly nonlinear arsenic selenide chalcogenide nanowire with tailored dispersion. The tapered submicrometer chalcogenide fiber exhibits an ultrahigh nonlinearity, n2~1.1×10−17m2/W and an effective mode area of 0.48 μm2, yielding an effective nonlinearity of γ~93.4W/m, which is over 80,000 times larger than standard silica single-mode fiber at a wavelength of ~1550nm. This high nonlinearity, in conjunction with the engineered anomalous dispersion, enables low-threshold soliton fission leading to large spectral broadening at a dramatically reduced peak power of several watts, corresponding to picojoule energy.
TL;DR: Using a classical-optics formalism, it is shown that beam propagation may greatly affect both Goos-Hänchen and Imbert-Fedorov shifts when the incident beam is focused.
Abstract: We derive the polarization-dependent displacements parallel and perpendicular to the plane of incidence for a Gaussian light beam reflected from a planar interface, taking into account the propagation of the beam. Using a classical-optics formalism we show that beam propagation may greatly affect both Goos-Hanchen and Imbert-Fedorov shifts when the incident beam is focused.
TL;DR: It is shown that a multilayer nanostructure having a graded-index profile, as predicted by theory, can accomplish a near-perfect transmission of all-color of sunlight.
Abstract: To harness the full spectrum of solar energy, Fresnel reflection at the surface of a solar cell must be eliminated over the entire solar spectrum and at all angles. Here, we show that a multilayer nanostructure having a graded-index profile, as predicted by theory [J. Opt. Soc. Am. 66, 515 (1976); Appl. Opt. 46, 6533 (2007)], can accomplish a near-perfect transmission of all-color of sunlight. An ultralow total reflectance of 1%-6% has been achieved over a broad spectrum, λ=400to1600 nm, and a wide range of angles of incidence, θ=0°-60°. The measured angle- and wavelength-averaged total reflectance of 3.79% is the smallest ever reported in the literature, to our knowledge.
TL;DR: In this article, a moving acoustic grating is generated by stimulated Brillouin scattering between writing beams in one polarization and used to reflect an orthogonally polarized reading beam at different wavelengths.
Abstract: We report a novel kind of all-optical dynamic grating based on Brillouin scattering in a polarization maintaining fiber (PMF). A moving acoustic grating is generated by stimulated Brillouin scattering between writing beams in one polarization and used to reflect an orthogonally polarized reading beam at different wavelengths. The center wavelength of the grating is controllable by detuning the writing beams, and the 3 dB bandwidth of approximately 80 MHz is observed with the tunable reflectance of up to 4% in a 30 m PMF.
TL;DR: A miniaturized inline Fabry-Perot interferometer directly fabricated on a single-mode optical fiber with a femtosecond laser is reported, attractive for sensing applications in high-temperature harsh environments.
Abstract: We report a miniaturized inline Fabry-Perot interferometer directly fabricated on a single-mode optical fiber with a femtosecond laser. The device had a loss of 16 dB and an interference visibility exceeding 14 dB. The device was tested and survived in high temperatures up to 1100°C. With an accessible cavity and all-glass structure, the new device is attractive for sensing applications in high-temperature harsh environments.
TL;DR: A slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2 microm is presented.
Abstract: Switching light is one of the most fundamental functions of an optical circuit. As such, optical switches are a major research topic in photonics, and many types of switches have been realized. Most optical switches operate by imposing a phase shift between two sections of the device to direct light from one port to another, or to switch it on and off, the major constraint being that typical refractive index changes are very small. Conventional solutions address this issue by making long devices, thus increasing the footprint, or by using resonant enhancement, thus reducing the bandwidth. We present a slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2
TL;DR: Through a series of simulations and experiments, it is demonstrated that the frequently cited criterion of matching speckle size to detector element (pixel) size in laser Speckle contrast imaging (LSCI) has the detrimental effect of reducing the contrast and thereby decreasing the variation in the laser speckel contrast image.
Abstract: Through a series of simulations and experiments, we demonstrate that the frequently cited criterion of matching speckle size to detector element (pixel) size in laser speckle contrast imaging (LSCI) has the detrimental effect of reducing the contrast and thereby decreasing the variation in the laser speckle contrast image. Unlike quasi-elastic light scattering, where this matching condition has been shown to maximize the signal-to-noise ratio, in LSCI, the minimum speckle size must exceed the Nyquist criterion in order to maximize the contrast of the speckle patterns.
TL;DR: In this paper, the authors demonstrate stimulated emission depletion (STED) microscopy implemented in a laser scanning confocal microscope using excitation light derived from supercontinuum generation in a microstructured optical fiber.
Abstract: We demonstrate stimulated emission depletion (STED) microscopy implemented in a laser scanning confocal microscope using excitation light derived from supercontinuum generation in a microstructured optical fiber. Images with resolution improvement beyond the far-field diffraction limit in both the lateral and axial directions were acquired by scanning overlapped excitation and depletion beams in two dimensions using the flying spot scanner of a commercially available laser scanning confocal microscope. The spatial properties of the depletion beam were controlled holographically using a programmable spatial light modulator, which can rapidly change between different STED imaging modes and also compensate for aberrations in the optical path. STED fluorescence lifetime imaging microscopy is demonstrated through the use of time-correlated single photon counting.
TL;DR: In this paper, an integrated biosensor based on a Si3N4/SiO2 Slot-waveguide microring resonator was used for label-free molecule detection.
Abstract: We demonstrate label-free molecule detection by using an integrated biosensor based on a Si3N4/SiO2 Slot-waveguide microring resonator. Bovine serum albumin (BSA) and anti-BSA molecular binding eve ...
TL;DR: It is demonstrated theoretically and experimentally that, by using the guided-wave surface-plasmon sensor configuration with a top layer of dielectric thin film having a high value of the real part of the dielectrics function, it is possible to improve the sensitivity of the sensor up to 1 order of magnitude.
Abstract: It is demonstrated theoretically and experimentally that, by using the guided-wave surface-plasmon sensor configuration with a top layer of dielectric thin film (10-15 nm) having a high value of the real part of the dielectric function, it is possible to improve the sensitivity of the sensor up to 1 order of magnitude. The stability is improved because the thin nanolayer acts as a protection layer for the metal. The enhancement is due to the increase in the interaction volume and the evanescent field enhancement near the top layer-analyte interface.
TL;DR: Regenerated gratings seeded by type I gratings in boron-codoped germanosilicate optical fiber written with 193 nm are shown to withstand temperatures beyond 1000 degrees C.
Abstract: Regenerated gratings seeded by type I gratings in boron-codoped germanosilicate optical fiber written with 193 nm are shown to withstand temperatures beyond 1000°C.
TL;DR: High sensitivity refractometers based on a long-period grating in a large-mode-area photonic crystal fiber (PCF) with maximum sensitivity is 1500 nm/refractive index unit at a refractive index of 1.33, to date the highest reported for any fiber grating.
Abstract: We present highly sensitive refractometers based on a long-period grating in a large-mode-area photonic crystal fiber (PCF). The maximum sensitivity is 1500 nm/refractive index unit at a refractive index of 1.33, to our knowledge the highest reported for any fiber grating. The minimal detectable index change is 2 x 10(-5). The high sensitivity is obtained by infiltrating the sample into the holes of the PCF to give a strong interaction between the sample and the probing field.
TL;DR: A measured invariant resolution of approximately 8 microm is demonstrated across a 4 mm measured DOF using the micro-optic axicon while achieving an invariant sensitivity of approximately 80 dB with a 25 mW input power.
Abstract: Endoscopic imaging in tubular structures, such as the tracheobronchial tree, could benefit from imaging optics with an extended depth of focus (DOF) to accommodate the varying sizes of tubular structures across patients and along the tree within the same patient. Yet the extended DOF needs to be accomplished without sacrificing resolution while maintaining sufficient sensitivity and speed of imaging. In this Letter, we report on the measured resolution and sensitivity achieved with a custom-made micro-optic axicon lens designed to theoretically achieve an 8 mm DOF. A measured invariant resolution of ~8 μm is demonstrated across a 4 mm measured DOF using the micro-optic axicon while achieving an invariant sensitivity of ~80 dB with a 25 mW input power. Double-pass Bessel beam spectral-domain optical coherence tomography with an axicon micro-optic lens (i.e., <1 mm in diameter) is, for the first time to our knowledge, demonstrated in a biological sample demonstrating invariant resolution and signal-to-noise ratio across a 4 mm measured DOF, which is compared to Gaussian beam imaging.
TL;DR: Conceptual studies and numerical simulations are performed for imaging devices that transform a near-field pattern into magnified far-zone images and are based on high-order spatial transformation in cylindrical domains.
Abstract: Conceptual studies and numerical simulations are performed for imaging devices that transform a near-field pattern into magnified far-zone images and are based on high-order spatial transformation in cylindrical domains. A lens translating a near-field pattern from an almost circular input boundary onto a magnified far-field image at a flat output boundary is considered. The lens is made of a metamaterial with anisotropic permittivity and permeability both depending on a single "scaling" parameter of the transformation. Open designs of the lens with a truncated body (3/4-body and 1/4-body lenses) are suggested and analyzed. It is shown that the ideal full lens and the 3/4-body lens produce identical images. Numerical simulations of 1/4-body designs indicate that further truncation of the lens could limit its performance. A light concentrator "focusing" far-zone fields into a nanometer-scale area is also considered.
TL;DR: The proposed all-optical switching structure based on a subwavelength metallic grating structure containing nonlinear optical materials shows great advantages of smaller size, lower requirement of pump light intensity, and shorter switching time at approximately the picosecond level.
Abstract: All-optical switching based on a subwavelength metallic grating structure containing nonlinear optical materials has been proposed and numerically investigated. Metal-dielectric composite material is used in the switching for its larger third-order nonlinear susceptibility (approximately 10(-7)esu) and ultrafast response properties. The calculated dependence of the signal light intensity on the pump light intensity shows a bistable behavior, which results in a significant switch effect. It rests on a surface plasmon's enhanced intensity-dependent change of the effective dielectric constant of Kerr nonlinear media, corresponding to a transition of the far-field transmission from a low- to high-transmission state. The study of this switching structure shows great advantages of smaller size, lower requirement of pump light intensity, and shorter switching time at approximately the picosecond level.
TL;DR: A novel, high-speed pulsed terahertz Fourier imaging system based on compressed sensing (CS), a new signal processing theory, which allows image reconstruction with fewer samples than traditionally required, and has potential application in THz imaging with cw sources.
Abstract: We describe a novel, high-speed pulsed terahertz (THz) Fourier imaging system based on compressed sensing (CS), a new signal processing theory, which allows image reconstruction with fewer samples than traditionally required. Using CS, we successfully reconstruct a 64 x 64 image of an object with pixel size 1.4 mm using a randomly chosen subset of the 4096 pixels, which defines the image in the Fourier plane, and observe improved reconstruction quality when we apply phase correction. For our chosen image, only about 12% of the pixels are required for reassembling the image. In combination with phase retrieval, our system has the capability to reconstruct images with only a small subset of Fourier amplitude measurements and thus has potential application in THz imaging with cw sources.
TL;DR: The confocal detection principle is extended to a highly parallel optical system that continuously analyzes thousands of concurrent sample locations and is demonstrated to detect and identify single fluorescent molecules from each of several thousand independent confocal volumes in real time.
Abstract: The confocal detection principle is extended to a highly parallel optical system that continuously analyzes thousands of concurrent sample locations. This is achieved through the use of a holographic laser illumination multiplexer combined with a confocal pinhole array before a prism dispersive element used to provide spectroscopic information from each confocal volume. The system is demonstrated to detect and identify single fluorescent molecules from each of several thousand independent confocal volumes in real time.
TL;DR: This work demonstrates the use of a tunable acoustic gradient (TAG) index of refraction lens as a fast varifocal element and indicates that the TAG lens can successfully be employed to perform high-rate imaging at multiple locations.
Abstract: Fluidic lenses allow for varifocal optical elements, but current approaches are limited by the speed at which focal length can be changed. Here we demonstrate the use of a tunable acoustic gradient (TAG) index of refraction lens as a fast varifocal element. The optical power of the TAG lens varies continuously, allowing for rapid selection and modification of the effective focal length at time scales of 1 μs and shorter. The wavefront curvature applied to the incident light is experimentally quantified as a function of time, and single-frame imaging is demonstrated. Results indicate that the TAG lens can successfully be employed to perform high-rate imaging at multiple locations.
TL;DR: This Letter presents a dual-wavelength DHM that allows us to separately measure both the RI and the cellular thickness by exploiting an enhanced dispersion of the perfusion medium achieved by the utilization of an extracellular dye.
Abstract: Digital holographic microscopy (DHM) allows optical-path-difference (OPD) measurements with nanometric accuracy. OPD induced by transparent cells depends on both the refractive index (RI) of cells and their morphology. This Letter presents a dual-wavelength DHM that allows us to separately measure both the RI and the cellular thickness by exploiting an enhanced dispersion of the perfusion medium achieved by the utilization of an extracellular dye. The two wavelengths are chosen in the vicinity of the absorption peak of the dye, where the absorption is accompanied by a significant variation of the RI as a function of the wavelength.