Showing papers in "Optics Express in 2003"

Sensitivity advantage of swept source and Fourier domain optical coherence tomography

Abstract: We present theoretical and experimental results which demonstrate the superior sensitivity of swept source (SS) and Fourier domain (FD) optical coherence tomography (OCT) techniques over the conventional time domain (TD) approach. We show that SS- and FD-OCT have equivalent expressions for system signal-to-noise ratio which result in a typical sensitivity advantage of 20-30dB over TD-OCT. Experimental verification is provided using two novel spectral discrimination (SD) OCT systems: a differential fiber-based 800nm FD-OCT system which employs deep-well photodiode arrays, and a differential 1300nm SS-OCT system based on a swept laser with an 87nm tuning range.

Performance of fourier domain vs. time domain optical coherence tomography.

Abstract: In this article we present a detailed discussion of noise sources in Fourier Domain Optical Coherence Tomography (FDOCT) setups. The performance of FDOCT with charge coupled device (CCD) cameras is compared to current standard time domain OCT systems. We describe how to measure sensitivity in the case of FDOCT and confirm the theoretically obtained values. It is shown that FDOCT systems have a large sensitivity advantage and allow for sensitivities well above 80dB, even in situations with low light levels and high speed detection.

Non-destructive terahertz imaging of illicit drugs using spectral fingerprints.

Abstract: The absence of non-destructive inspection techniques for illicit drugs hidden in mail envelopes has resulted in such drugs being smuggled across international borders freely. We have developed a novel basic technology for terahertz imaging, which allows detection and identification of drugs concealed in envelopes, by introducing the component spatial pattern analysis. The spatial distributions of the targets are obtained from terahertz multispectral transillumination images, using absorption spectra measured with a tunable terahertz-wave source. The samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.

High-speed optical frequency-domain imaging

Abstract: We demonstrate high-speed, high-sensitivity, high-resolution optical imaging based on optical frequency-domain interferometry using a rapidly-tuned wavelength-swept laser. We derive and show experimentally that frequency-domain ranging provides a superior signal-to-noise ratio compared with conventional time-domain ranging as used in optical coherence tomography. A high sensitivity of -110 dB was obtained with a 6 mW source at an axial resolution of 13.5 microm and an A-line rate of 15.7 kHz, representing more than an order-of-magnitude improvement compared with previous OCT and interferometric imaging methods.

Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides

Abstract: We report the design and testing of an SOI-based photonic integrated circuit containing two-dimensional membrane-type photonic crystal waveguides. The circuit comprises spot-size converters to efficiently couple light from a fiber into single-mode strip waveguides and butt-couplers to couple from strip waveguides to photonic crystal waveguides. Each optical interface was optimized to minimize back-reflections and reduce the Fabry-Perot noise. The transmission characteristics of each component are measured and record low propagation losses in photonic crystal waveguides of 24dB/cm are reported. The combination of an efficient two-stage coupling scheme and utilization of ultra-long (up to 2mm) photonic crystal waveguides reduces the uncertainty in determining the loss figure to 3dB/cm.

Chromatic dispersion control in photonic crystal fibers: application to ultra-flattened dispersion

Abstract: In order to control dispersion and dispersion slope of indexguiding photonic crystal fibers (PCFs), a new controlling technique of chromatic dispersion in PCF is reported. Moreover, our technique is applied to design PCF with both ultra-low dispersion and ultra-flattened dispersion in wide wavelength range. A full-vector finite element method with anisotropic perfectly matched layers is used to analyze the dispersion properties and the confinement losses in a PCF with finite number of air holes. It is shown from numerical results that it is possible to design a fourring PCF with flattened dispersion of 0 +/- 0.5 ps/(km.nm) from 1.19 m to 1.69 m wavelength range and a five-ring PCF with flattened dispersion of 0 +/- 0.4 ps/(km.nm) from 1.23 m to 1.72 m wavelength range.

In vivo dynamic human retinal blood flow imaging using ultra-high-speed spectral domain optical Doppler tomography

Abstract: An ultra-high-speed spectral domain optical Doppler tomography (SD-ODT) system is used to acquire images of blood flow in a human retina in vivo, at 29,000 depth profiles (A-lines) per second and with data acquisition over 99% of the measurement time. The phase stability of the system is examined and image processing algorithms are presented that allow accurate determination of bi-directional Doppler shifts. Movies are presented of human retinal flow acquired at 29 frames per second with 1000 A-lines per frame over a time period of 3.28 seconds, showing accurate determination of vessel boundaries and time-dependent bi-directional flow dynamics in artery-vein pairs. The ultra-high-speed SD-ODT system allows visualization of the pulsatile nature of retinal blood flow, detects blood flow within the choroid and retinal capillaries, and provides information on the cardiac cycle. In summary, accurate video rate imaging of retinal blood flow dynamics is demonstrated at ocular exposure levels below 600 microW.

Real-time assessment of retinal blood flow with ultrafast acquisition by color Doppler Fourier domain optical coherence tomography.

Abstract: We interfaced color Doppler Fourier domain optical coherence tomography (CD-FDOCT) with a commercial OCT system to perform in vivo studies of human retinal blood flow in real time. FDOCT does not need reference arm scanning and records one full depth and Doppler profile in parallel. The system operates with an equivalent A-scan rate of 25 kHz and allows real time imaging of the color encoded Doppler information together with the tissue morphology at a rate of 2-4 tomograms (40 x 512 pixel) per second. The recording time of a single tomogram (160 x 512 data points) is only 6,4ms. Despite the high detection speed we achieve a system sensitivity of 86dB using a beam power of 500microW at the cornea. The fundus camera allows simultaneous view for selection of the region of interest. We observe bi-directional blood flow and pulsatility of blood velocity in retinal vessels with a Doppler detection bandwidth of 12.5 kHz and a longitudinal velocity sensitivity in tissue of 200microm/s.

Optical devices based on liquid crystal photonic bandgap fibres

Abstract: Photonic Crystal Fibers (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are microstructured waveguides, usually in silica, with a large number of air holes located in the cladding region of the fiber. The size and location of these air holes opens up for a large degree of design freedom within optical waveguide design. Further, the existence of air holes in the PCF gives access close to the fiber core and by introducing new materials into the air holes, a high interaction between light and hole material can be obtained, while maintaining the microstructure of the waveguide. In this paper, we describe what we call Liquid Crystal Photonic Bandgap Fibers, which are PCFs infiltrated with Liquid Crystals (LCs) in order to obtain increased fiber functionality. We describe a thermo-optic fiber switch with an extinction ratio of 60dB and tunable PBGs using thermo-optic tuning of the LC. These devices operate by the PBG effect and are therefore highly sensitive to the refractive index distributions in the holes.

High-speed spectral-domain optical coherence tomography at 1.3 mum wavelength.

Abstract: We demonstrate a high-speed spectral domain optical coherence tomography (SD-OCT) system capable of acquiring individual axial scans in 24.4 micros at a rate of 19,000 axial scans per second, using an InGaAs line scan camera and broadband light source centered at 1.31microm. Sensitivity of >105 dB over a 2-mm depth range was obtained with a free-space axial resolution of 12-14 microm, in agreement with our signal-to-noise ratio predictions. Images of human tissue obtained in vivo with SD-OCT show similar penetration depths to those obtained with state-of-the-art time domain OCT despite the ten-fold higher image acquisition speed. These results demonstrate the potential of 1.3 microm SD-OCT for high-speed and high-sensitivity imaging in patients.

Observation of stimulated Raman amplification in silicon waveguides

Abstract: We report the first observation of Stimulated Raman Scattering (SRS) in silicon waveguides. Amplification of the Stokes signal, at 1542.3 nm, of up to 0.25 dB has been observed in Silicon-on-Insulator (SOI) waveguides, using a 1427 nm pump laser with a CW power of 1.6 W, measured before the waveguide. Two-Photon-Absorption (TPA) measurements on these waveguides are also reported, and found to be negligible at the pump power where SRS was observed.

Tunable liquid-filled microlens array integrated with microfluidic network.

Abstract: An elastomer-based tunable liquid-filled microlens array integrated on top of a microfluidic network is fabricated using soft lithographic techniques. The simultaneous control of the focal length of all the microlenses composing the elastomeric array is accomplished by pneumatically regulating the pressure of the microfluidic network. A focal length tuning range of hundreds of microns to several millimeters is achieved. Such an array can be used potentially in dynamic imaging systems and adaptive optics.

High-power air-clad large-mode-area photonic crystal fiber laser.

Abstract: We report on a 2.3 m long air-clad ytterbium-doped large-mode-area photonic crystal fiber laser generating up to 80 W output power with a slope efficiency of 78%. Single transverse mode operation is achieved with a mode-field area of 350 µm2. No thermo-optical limitations are observed at the extracted ~35W/m, therefore such fibers allow scaling to even higher powers.

Degree of coherence for electromagnetic fields.

Abstract: The relationship between the visibility of fringes and the degree of spatial coherence in electromagnetic two-pinhole interference is assessed. It is demonstrated that the customary definition of the degree of coherence of an electromagnetic field is flawed and a new quantity, free of the formal drawbacks, is introduced. The new definition, which is shown to be consistent with known results for Gaussian statistics, has some unusual properties characteristic only for electromagnetic fields. The degree of coherence is measurable by a sequence of interference experiments.

X-ray phase-contrast microscopy and microtomography.

Abstract: In-line phase contrast enables weakly absorbing specimens to be imaged successfully with x-rays, and greatly enhances the visibility of fine scale structure in more strongly absorbing specimens. This type of phase contrast requires a spatially coherent beam, a condition that can be met by a microfocus x-ray source. We have developed an x-ray microscope, based on such a source, which is capable of high resolution phase-contrast imaging and tomography. Phase retrieval enables quantitative information to be recovered from phase-contrast microscope images of homogeneous samples of known composition and density, and improves the quality of tomographic reconstructions.

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part I): System design, signal processing, and performance

Abstract: Improvements in real-time Doppler optical coherence tomography (DOCT), acquiring up to 32 frames per second at 250×512 pixels per image, are reported using signal processing techniques commonly employed in Doppler ultrasound imaging. The ability to measure a wide range of flow velocities, ranging from less than 20 µm/s to more than 10 cm/s, is demonstrated using an 1.3 µm DOCT system with flow phantoms in steady and pulsatile flow conditions. Based on full implementation of a coherent demodulator, four different modes of flow visualization are demonstrated: color Doppler, velocity variance, Doppler spectrum, and power Doppler. The performance of the former two, which are computationally suitable for real-time imaging, are analyzed in detail under various signal-to-noise and frame-rate conditions. The results serve as a guideline for choosing appropriate imaging parameters for detecting in vivo blood flow.

Phase-sensitive imaging and phase tomography using X-ray interferometers

Abstract: X-ray interferometry for imaging applications is discussed with a review of X-ray interferometric imaging activities reported to date. Phase measurement and phase tomography based on X-ray interferometry are also presented. Finally the advantage of X-ray interferometric imaging in comparison with other phase-sensitive X-ray imaging methods is discussed.

Plasmon modes and negative refraction in metal nanowire composites

Abstract: Optical properties of metal nanowires and nanowire composite materials are studied. An incident electromagnetic wave can effectively couple to the propagating surface plasmon polariton (SPP) modes in metal nanowires resulting in very large local fields. The excited SPP modes depend on the structure of nanowires and their orientation with respect to incident radiation. A nanowire percolation composite is shown to have a broadband spectrum of localized plasmon modes. We also show that a composite of nanowires arranged into parallel pairs can act as a left-handed material with the effective magnetic permeability and dielectric permittivity both negative in the visible and near-infrared spectral ranges.

Practical implementation of adaptive optics in multiphoton microscopy

Abstract: A dedicated two-photon microscope incorporating adaptive-optic correction of specimen-induced aberrations is presented. Wavefront alteration of the scanning laser beam was achieved by use of a micromachined deformable mirror. Post scan head implementation produces a compact module compatible with the Bio-Rad MRC-600 scan head. Automatic aberration correction using feedback from the multiphoton fluorescence intensity allowed the adaptive optic to extend the imaging depth attainable in both artificial and biological refractive-index mismatched samples. With a 1.3-NA, x40, Nikon oil immersion objective, the imaging depth in water was extended from approximately 3.4 to 46.2 µm with a resolution defined by a FWHM axial point-spread function of 1.25 µm.

Real-time multi-functional optical coherence tomography

Abstract: We demonstrate real-time acquisition, processing, and display of tissue structure, birefringence, and blood flow in a multi-functional optical coherence tomography (MF-OCT) system. This is accomplished by efficient data processing of the phase-resolved inteference patterns without dedicated hardware or extensive modification to the high-speed fiber-based OCT system. The system acquires images of 2048 depth scans per second, covering an area of 5 mm in width×1.2 mm in depth with real-time display updating images in a rolling manner 32 times each second. We present a video of the system display as images from the proximal nail fold of a human volunteer are taken.

The ‘Precessions’ tooling for polishing and figuring flat, spherical and aspheric surfaces

Abstract: The PrecessionsTM process has been developed for the control of texture ('polishing'), preservation of form during polishing, and control of form ('figuring'), on flat, spherical and aspheric surfaces. In this first and introductory paper, we summarize the need for aspherics, review some aspheric technologies, and then distill a 'wish-list' of attributes for an aspheric process. Within this context, we focus on special properties of Precessions tools, their use in a family of 7-axis CNC polishing machines, and present experimental results.

Simple plane wave implementation for photonic crystal calculations

Abstract: A simple implementation of plane wave method is presented for modeling photonic crystals with arbitrary shaped ‘atoms’ The Fourier transform for a single ‘atom’ is first calculated either by analytical Fourier transform or numerical FFT, then the shift property is used to obtain the Fourier transform for any arbitrary supercell consisting of a finite number of ‘atoms’ To ensure accurate results, generally, two iterating processes including the plane wave iteration and grid resolution iteration must converge Analysis shows that using analytical Fourier transform when available can improve accuracy and avoid the grid resolution iteration It converges to the accurate results quickly using a small number of plane waves Coordinate conversion is used to treat non-orthogonal unit cell with non-regular ‘atom’ and then is treated by standard numerical FFT MATLAB source code for the implementation requires about less than 150 statements, and is freely available at http://wwwlionsoduedu/~sguox002

Leakage loss and group velocity dispersion in air-core photonic bandgap fibers

Abstract: The wavelength dependence and the structural dependence of leakage loss and group velocity dispersion (GVD) in air-core photonic bandgap fibers (PBGFs) are numerically investigated by using a full-vector finite element method. It is shown that at least seventeen rings of arrays of air holes are required in the cladding region to reduce the leakage losses to a level of 0.1 dB/km in 1.55-microm wavelength range even if using large air holes of the diameter to pitch ratio of 0.9 and that the leakage losses in air-core PBGFs decrease drastically with increasing the hole diameter to pitch ratio. Moreover, it is shown that the waveguide GVD and dispersion slope of air-core PBGFs are much larger than those of conventional silica fibers and that the shape of air-core region greatly affects the leakage losses and the dispersion properties.

Coupling characteristics of dual-core photonic crystal fiber couplers

Abstract: Coupling characteristics of dual-core photonic crystal fiber (PCF) couplers are evaluated by using a vector finite element method and their application to a multiplexer-demultiplexer (MUX-DEMUX) based on PCF is investigated. The PCF couplers for 1.48/1.55-m, 1.3/1.55-m, 0.98/1.55-m, and 0.85/1.55-m wavelength MUX-DEMUX are designed and the beam propagation analysis of the proposed PCF couplers is performed. It is shown from numerical results that it is possible to realize significantly shorter MUX-DEMUX PCFs, compared to conventional optical fiber couplers.

Dispersion flattened hybrid-core nonlinear photonic crystal fiber

Abstract: Photonic crystal fibers are highly attractive as nonlinear media as they combine a large nonlinear coefficient and a highly customizable zero dispersion wavelength - flexibility not found in any other medium. However, the high dispersion slope at the zero-dispersion wavelength demonstrated so far is very limiting to the useful bandwidth. We propose a new fiber design comprising a hybrid core-region with three-fold symmetry that enables unprecedented dispersion control while maintaining low loss and a high nonlinear coefficient. The lowest dispersion slope obtained is 1·10-3ps/(km·nm2) or one order of magnitude lower than for conventional slope reduced nonlinear fibers. The nonlinear coefficient is more than 11 (W·km)-1 and loss below 7.9 dB/km at 1.55 µm has been achieved.

Multiphoton plasmon-resonance microscopy

Abstract: A novel method for detection of noble-metal nanoparticles by their nonlinear optical properties is presented and applied for specific labeling of cellular organelles. When illuminated by laser light in resonance with their plasmon frequency these nanoparticles generate an enhanced multiphoton signal. This enhanced signal is measured to obtain a depth-resolved image in a laser scanning microscope setup. Plasmon-resonance images of both live and fixed cells, showing specific labeling of cellular organelles and membranes, either by two-photon autofluorescence or by third-harmonic generation, are presented.

Resonances in microstructured optical waveguides

Abstract: We propose a simple physical model that predicts the optical properties of a class of microstructured waveguides consisting of high-index inclusions that surround a low-index core. On the basis of this model, it is found that a large regime exists where transmission minima are determined by the geometry of the individual high-index inclusions. The locations of these minima are found to be largely unaffected by the relative position of the inclusions. As a result of this insight the difficult problem of analyzing the properties of complex structures can be reduced to the much simpler problem of analyzing the properties of an individual high-index inclusion in the structure.

Composite structured light pattern for three-dimensional video

Abstract: Based on recent discoveries, we introduce a method to project a single structured pattern onto an object and then reconstruct the three-dimensional range from the distortions in the reflected and captured image. Traditional structured light methods require several different patterns to recover the depth, without ambiguity or albedo sensitivity, and are corrupted by object movement during the projection/ capture process. Our method efficiently combines multiple patterns into a single composite pattern projection allowing for real-time implementations. Because structured light techniques require standard image capture and projection technology, unlike time of arrival techniques, they are relatively low cost.

Tellurite photonic crystal fiber.

Abstract: We report the fabrication of a Tellurite photonic crystal fiber, and demonstrate its waveguiding properties. The measured minimum loss is 2.3 dB/m at a wavelength of 1055 nm. The fiber supports several modes, but in practice just the fundamental mode can be used. We have observed strong stimulated Raman scattering in a fiber with an effective area Aeff=21.2µm2, using sub-ns, ~1 µJ pump pulses at 1064 nm.

Self-guiding in two-dimensional photonic crystals.

Abstract: Dielectric periodic media can possess a complex photonic band structure with allowed bands displaying strong dispersion and anisotropy. We show that for some frequencies the form of iso-frequency contours mimics the form of the first Brillouin zone of the crystal. A wide angular range of flat dispersion exists for such frequencies. The regions of iso-frequency contours with near-zero curvature cancel out diffraction of the light beam, leading to a self-guided beam.