Showing papers in "Optics Letters in 2003"
TL;DR: A signal-to-noise ratio (SNR) analysis is presented for optical coherence tomography (OCT) signals in which time-domain performance is compared with that of the spectral domain.
Abstract: A signal-to-noise ratio (SNR) analysis is presented for optical coherence tomography (OCT) signals in which time-domain performance is compared with that of the spectral domain. A significant SNR gain of several hundredfold is found for acquisition in the spectral domain. The SNR benefit is demonstrated experimentally in a hybrid time-domain-spectral-domain OCT system.
1,505 citations
TL;DR: It is shown that the micrometer-long silicon-on-insulator-based nanotaper coupler is able to efficiently convert both the mode field profile and the effective index, with a total length as short as 40 microm, during compact mode conversion between a fiber and a submicrometer waveguide.
Abstract: We propose and demonstrate an efficient coupler for compact mode conversion between a fiber and a submicrometer waveguide. The coupler is composed of high-index-contrast materials and is based on a short taper with a nanometer-sized tip. We show that the micrometer-long silicon-on-insulator-based nanotaper coupler is able to efficiently convert both the mode field profile and the effective index, with a total length as short as 40 microm. We measure an enhancement of the coupling efficiency between an optical fiber and a waveguide by 1 order of magnitude due to the coupler.
994 citations
TL;DR: The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsor adaptation in the most sensitive region varying as 1/R(5/2).
Abstract: Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to micorsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1R 52 . Specific parameters for single protein or virus particle detection are predicted. © 2003 Optical
770 citations
TL;DR: It is shown that two-photon fluorescence images can be obtained throughout almost the entire gray matter of the mouse neocortex by using optically amplified femtosecond pulses.
Abstract: It is shown that two-photon fluorescence images can be obtained throughout almost the entire gray matter of the mouse neocortex by using optically amplified femtosecond pulses The achieved imaging depth approaches the theoretical limit set by excitation of out-of-focus fluorescence
660 citations
TL;DR: First applications of the two-photon polymerization technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
Abstract: Investigations of two-photon polymerization of inorganic-organic hybrid materials initiated by femtosecond Ti:sapphire laser pulses are performed. First applications of this technique for the fabrication of three-dimensional microstructures and photonic crystals in inorganic-organic hybrid polymers with a structure size down to 200 nm and a periodicity of 450 nm are discussed.
550 citations
TL;DR: In this article, the authors described a method and apparatus for detecting photoacoustic signals in fluid media by using piezoelectric crystal quartz, which is preferably in the shape of a tuning fork.
Abstract: Methods and apparatus for detecting photoacoustic signals in fluid media are described. The present invention differs from conventional photoacoustic spectroscopy in that rather than accumulating the absorbed energy in the fluid of a sample cell, the absorbed energy is accumulated in an acoustic detector or sensitive element. In a preferred embodiment, the acoustic detector comprises piezoelectric crystal quartz. The quartz is preferably in the shape of a tuning fork.
548 citations
TL;DR: Ultrahigh-speed tuning of an extended-cavity semiconductor laser is demonstrated, which is more than an order of magnitude faster than previously demonstrated and is facilitated in part by self-frequency shifting in the semiconductor optical amplifier.
Abstract: Ultrahigh-speed tuning of an extended-cavity semiconductor laser is demonstrated. The laser resonator comprises a unidirectional fiber-optic ring, a semiconductor optical amplifier as the gain medium, and a novel scanning filter based on a polygonal scanner. Variable tuning rates up to 1150 nm/ms (15.7-kHz repetition frequency) are demonstrated over a 70-nm wavelength span centered at 1.32 microm. This tuning rate is more than an order of magnitude faster than previously demonstrated and is facilitated in part by self-frequency shifting in the semiconductor optical amplifier. The instantaneous linewidth of the source is <0.1 nm for 9-mW cw output power and a low spontaneous-emission background of -80 dB.
497 citations
TL;DR: A new technique based on a random shifting, or jigsaw, algorithm is proposed, which does not require the use of phase keys for decrypting data and shows comparable or superior robustness to blind decryption.
Abstract: A number of methods have recently been proposed in the literature for the encryption of two-dimensional information by use of optical systems based on the fractional Fourier transform. Typically, these methods require random phase screen keys for decrypting the data, which must be stored at the receiver and must be carefully aligned with the received encrypted data. A new technique based on a random shifting, or jigsaw, algorithm is proposed. This method does not require the use of phase keys. The image is encrypted by juxtaposition of sections of the image in fractional Fourier domains. The new method has been compared with existing methods and shows comparable or superior robustness to blind decryption. Optical implementation is discussed, and the sensitivity of the various encryption keys to blind decryption is examined.
434 citations
TL;DR: An improved spectral optical coherence tomography technique is used to obtain cross-sectional ophthalmic images at an exposure time of 64 micros per A-scan, which allows real-time images as well as static tomograms to be recorded in vivo.
Abstract: An improved spectral optical coherence tomography technique is used to obtain cross-sectional ophthalmic images at an exposure time of 64μs per A-scan. This method allows real-time images as well as static tomograms to be recorded in vivo.
402 citations
TL;DR: An algorithm is presented that solves the phase unwrapping problem, using a combination of Fourier techniques, that is equivalent to the computation time required for performing eight fast Fourier transforms and stable against noise and residues present in the wrapped phase.
Abstract: A wide range of interferometric techniques recover phase information that is
mathematically wrapped on the interval (-π,π] . Obtaining the
true unwrapped phase is a longstanding problem. We present an algorithm that solves
the phase unwrapping problem, using a combination of Fourier techniques. The
execution time for our algorithm is equivalent to the computation time required for
performing eight fast Fourier transforms and is stable against noise and residues
present in the wrapped phase. We have extended the algorithm to handle data of
arbitrary size. We expect the state of the art of existing interferometric
applications, including the possibility for real-time phase recovery, to benefit
from our algorithm.
389 citations
TL;DR: All-optical switching action in a nonlinear photonic crystal cross-waveguide geometry with instantaneous Kerr nonlinearity is demonstrated, in which the transmission of a signal can be reversibly switched on and off by a control input.
Abstract: We demonstrate all-optical switching action in a nonlinear photonic crystal cross-waveguide geometry with instantaneous Kerr nonlinearity, in which the transmission of a signal can be reversibly switched on and off by a control input. Our geometry accomplishes both spatial and spectral separation between the signal and the control in the nonlinear regime. The device occupies a small footprint of a few micrometers squared and requires only a few milliwatts of power at a 10-Gbit/s switching rate by use of Kerr nonlinearity in AlGaAs below half the electronic bandgap. We also show that the switching dynamics, as revealed by both coupled-mode theory and finite-difference time domain simulations, exhibits collective behavior that can be exploited to generate high-contrast logic levels and all-optical memory.
TL;DR: The ability to image blood flow and hemoglobin concentration changes simultaneously with high resolution will permit detailed quantitative analysis of the spatiotemporal hemodynamics of functional brain activation, including imaging of oxygen metabolism.
Abstract: A simple instrument is demonstrated for high-resolution simultaneous imaging of total hemoglobin concentration and oxygenation and blood flow in the brain by combining rapid multiwavelength imaging with laser speckle contrast imaging. The instrument was used to image changes in oxyhemoglobin and deoxyhemoglobin and blood flow during cortical spreading depression and single whisker stimulation in rats through a thinned skull. The ability to image blood flow and hemoglobin concentration changes simultaneously with high resolution will permit detailed quantitative analysis of the spatiotemporal hemodynamics of functional brain activation, including imaging of oxygen metabolism. This is of significance to the neuroscience community and will lead to a better understanding of the interrelationship of neural, metabolic, and hemodynamic processes in normal and diseased brains.
TL;DR: A phase-imaging method with an axial range that can in principle be arbitrarily large compared to the wavelength and does not involve the usual phase unwrapping by detection of phase discontinuity is presented.
Abstract: We present a phase-imaging method with an axial range that can in principle be arbitrarily large compared to the wavelength and does not involve the usual phase unwrapping by detection of phase discontinuity. The method consists of the generation and combination of two phase maps in a digital holography system by use of two separate wavelengths. For example, we reconstructed the surface of a spherical mirror with approximately 10-nm axial resolution and an axial range of approximately 3 microm.
TL;DR: The end-pumped rubidium laser performance was well described by use of literature spectroscopic and kinetic data in a model that takes into account ground-level depletion and a pump spectral bandwidth that is substantially larger than the collisionally broadened pump transition spectral width.
Abstract: Population inversion of the 2P 1/2 and 2S 1/2 levels and continuous-wave, three-level laser oscillation at 795 nm on the D1 transition of the rubidium atom has been demonstrated. Using a titanium sapphire laser as a pump source, we obtained a slope power efficiency of 54% relative to absorbed pump power, consistent with homogeneous broadening of the rubidium pump and laser transitions. The end-pumped rubidium laser performance was well described by use of literature spectroscopic and kinetic data in a model that takes into account ground-level depletion and a pump spectral bandwidth that is substantially larger than the collisionally broadened pump transition spectral width.
TL;DR: The generation of spatial bright solitons similar to those observed in arrays of coupled optical waveguides are demonstrated and pairs of out-of-phasesolitons, which resemble twisted localized states in nonlinear lattices are created.
Abstract: We study experimentally nonlinear localization effects in optically induced gratings created by interfering plane waves in a photorefractive crystal. We demonstrate the generation of spatial bright solitons similar to those observed in arrays of coupled optical waveguides. We also create pairs of out-of-phase solitons, which resemble twisted localized states in nonlinear lattices.
TL;DR: The HGBs provide a convenient and powerful way to describe and treat the propagation of DHBs and can be used conveniently to analyze atoms manipulated with a DHB.
Abstract: A new mathematical model, described as hollow Gaussian beams (HGBs), is proposed to describe a dark hollow laser beam (DHB). The area of the dark region across the HGBs can easily be controlled by proper choice of the beam parameters. Based on the Collins integral, an analytical propagation formula for the HGBs through a paraxial optical system is derived. The HGBs also can be expressed as a superposition of a series of Lagurerre-Gaussian modes by use of a polynomial expansion. As a numerical example, the propagation properties of a DHB in free space are illustrated graphically. The HGBs provide a convenient and powerful way to describe and treat the propagation of DHBs and can be used conveniently to analyze atoms manipulated with a DHB.
TL;DR: An ultrahigh-resolution optical coherence tomographic system in which broadband continuum generation from a photonic crystal fiber is used to produce high longitudinal resolution is developed and subcellular imaging is demonstrated.
Abstract: We have developed an ultrahigh-resolution optical coherence tomographic system in which broadband continuum generation from a photonic crystal fiber is used to produce high longitudinal resolution. Longitudinal resolution of 1.3-microm has been achieved in a biological tissue by use of continuum light from 800 to 1400 nm. The system employed a dynamic-focusing tracking method to maintain high lateral resolution over a large imaging depth. Subcellular imaging is demonstrated.
TL;DR: A bit-error probability (BEP) model is developed that takes into account both building sway and turbulence-induced log amplitude fluctuations and assumes that the receiver has knowledge about the marginal statistics of the signal fading and the instantaneous signal-fading state.
Abstract: Urban optical wireless communication (UOWC) systems are considered a last-mile technology. UOWC systems use the atmosphere as a propagation medium. To provide a line of sight the transceivers are placed on high-rise building. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways distort the alignment between transmitter and receiver, causing pointing errors, the outcome of which is fading of the received signal. Furthermore, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, resulting in impaired link performance. A bit-error probability (BEP) model is developed that takes into account both building sway and turbulence-induced log amplitude fluctuations (i.e., fading of signal intensity) in the regime in which the receiver aperture, D0, is smaller than the turbulence coherence diameter, d0. It is assumed that the receiver has knowledge about the marginal statistics of the signal fading and the instantaneous signal-fading state.
TL;DR: A four-port nonlinear photonic crystal system is discussed that exhibits optical bistability with negligible backscattering to the inputs, making it particularly suitable for integration with other active devices on the same chip.
Abstract: A four-port nonlinear photonic crystal system is discussed that exhibits optical bistability with negligible backscattering to the inputs, making it particularly suitable for integration with other active devices on the same chip. Devices based on this system can be made to be small Oλ3 in volume, have a nearly instantaneous response, and consume only a few milliwatts of power. Among many possible applications, we focus on an all-optical transistor and integrated optical isolation.
TL;DR: A novel technique is presented that shows all the features of complex FDOCT with only two recorded interferograms, enabling standard phase-retrieval algorithms to double the measurement range.
Abstract: Standard Fourier-domain optical coherence tomography (FDOCT) suffers from the presence of autocorrelation terms that obscure the object information and degrade the sensitivity and signal-to-noise ratio. By exploiting the phase information of the recorded interferograms, it is possible to remove those autocorrelation terms and to double the measurement range. However, standard phase-retrieval algorithms need three to five interferograms. We present a novel technique that shows all the features of complex FDOCT with only two recorded interferograms.
TL;DR: It is shown that the position of a fluorescent nanoparticle can be measured in three dimensions with subnanometer precision and 100-ms temporal resolution by use of standard epifluorescence video imaging in off-focus mode.
Abstract: We show that the position of a fluorescent nanoparticle can be measured in three dimensions with subnanometer precision and 100-ms temporal resolution by use of standard epifluorescence video imaging in off-focus mode. The particle can be tracked without feedback in a volume of at least 40 µm×60 µm×3 µm. With the technique presented, the structure–mobility relationship of 216-nm latex particle in a porous polymer network was studied in three dimensions.
TL;DR: Modulation instability at high frequencies has been demonstrated in the normal dispersion regime by use of a photonic crystal fiber that provides efficient conversion of red pump light into blue and near-infrared light.
Abstract: Modulation instability at high frequencies has been demonstrated in the normal dispersion regime by use of a photonic crystal fiber. This fiber-optic parametric generator provides efficient conversion of red pump light into blue and near-infrared light.
TL;DR: In this article, high quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm.
Abstract: High-quality retroreflecting fiber Bragg gratings were written in standard Ge-doped telecom fiber (Corning SMF-28) after a few minutes exposure with pulsed 800-nm, 120-fs laser radiation by use of a deep-etch silica zero-order nulled phase mask optimized for 800 nm. Induced index modulations of 1.9×10-3 were achieved with peak power intensities of 1.2×1013 W/cm2 without any fiber sensitization. The fiber gratings are stable and did not erase after 2 weeks at 300 °C. The primary mechanism of induced index change results from a structural modification to the fiber core.
TL;DR: It is shown that fundamental and the vortex solitons are stable against small perturbations in the strong localization regime and in the weak localization regime.
Abstract: Fundamental and vortex solitons in a two-dimensional optically induced waveguide array are reported. In the strong localization regime the fundamental soliton is largely confined to one lattice site, whereas the vortex state comprises four fundamental modes superimposed in a square configuration with a phase structure that is topologically equivalent to the conventional vortex. However, in the weak localization regime, both the fundamental and the vortex solitons spread over many lattice sites. We further show that fundamental and the vortex solitons are stable against small perturbations in the strong localization regime.
TL;DR: It is argued that the multimode cutoff occurs at a wavelength lambda* that satisfies VPCF(lambda*) = pi, and by comparing this approach with numerics and recent cutoff calculations, this result is confirmed.
Abstract: We address the long-standing unresolved problem concerning the V parameter in a photonic crystal fiber. In formulating the parameter appropriate for a core defect in a periodic structure, we argue that the multimode cutoff occurs at a wavelength lambda* that satisfies VPCF(lambda*) = pi. By comparing this approach with numerics and recent cutoff calculations we confirm this result.
TL;DR: The development of a photon-number-resolving detector based on a fiber-optical setup and a pair of standard avalanche photodiodes is reported, showing that there is a high probability of inferring the input photon number from a measurement of the number of detection events on a single run.
Abstract: We report the development of a photon-number-resolving detector based on a fiber-optical setup and a pair of standard avalanche photodiodes. The detector is capable of resolving individual photon numbers and operates on the well-known principle by which a single-mode input state is split into a large number (eight) of output modes. We reconstruct the photon statistics of weak coherent input light from experimental data and show that there is a high probability of inferring the input photon number from a measurement of the number of detection events on a single run.
TL;DR: This work describes what is to their knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement.
Abstract: We describe what is to our knowledge the first use of fiber Bragg gratings written into three separate cores of a multicore fiber for two-axis curvature measurement. The gratings act as independent, but isothermal, fiber strain gauges for which local curvature determines the difference in strain between cores, permitting temperature-independent bend measurement.
TL;DR: It is demonstrated that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser's scanning direction, before the focusing lens.
Abstract: Theoretical and experimental investigations have been made of the three-dimensional microchannel fabrication of photostructurable glass by use of a femtosecond (fs) laser. Generally, a microchannel fabricated inside glass by the scanning focal spot of a fs laser perpendicular to the direction of laser propagation assumes an elliptical shape with a cross section of large aspect ratio. We demonstrate that one can greatly reduce the aspect ratio merely by inserting a slit, which is oriented parallel to the laser’s scanning direction, before the focusing lens. Computer simulations show that a more symmetrical pattern is obtained in the vicinity of the focal point with the help of such a slit, owing essentially to a diffraction effect.
TL;DR: A Mueller polarimeter is demonstrated in which the polarization-state generator and analyzer are both composed of a linear polarizer and two liquid-crystal variable retarders to optimize the accuracy of the final results by minimization of the condition numbers of the modulation and analysis matrices.
Abstract: We demonstrate a Mueller polarimeter in which the polarization-state generator and analyzer are both composed of a linear polarizer and two liquid-crystal variable retarders. The polarimeter is designed to optimize the accuracy of the final results by minimization of the condition numbers of the modulation and analysis matrices. The polarimeter calibration, a difficult task by conventional procedures, is achieved easily by use of the eigenvalue method of Compain et al. [Appl. Opt. 38, 3490 (1999)]. The overall polarimeter performance is tested with a linear polarizer at various angles and a compensator at various retardations.
TL;DR: To the knowledge this result represents the highest peak power pulses yet produced in any Ti:sapphire chirped-pulse amplification system.
Abstract: We have successfully produced a laser pulse with a peak power of 0.85 PW for a pulse duration of 33 fs in a four-stage Ti:sapphire amplifier chain based on chirped-pulse amplification. To our knowledge this result represents the highest peak power pulses yet produced in any Ti:sapphire chirped-pulse amplification system.