Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “OPTICS”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 523 records · Page 29Linked to original sources
PubMed2003

Geometrical-optics code for computing the optical properties of large dielectric spheres.

Absorption of electromagnetic radiation by absorptive dielectric spheres such as snow grains in the near-infrared part of the solar spectrum cannot be neglected when radiative properties of snow are computed. Thus a new, to our knowledge, geometrical-optics code is developed to compute scattering and absorption cross sections of large dielectric particles of arbitrary complex refractive index. The number of internal reflections and transmissions are truncated on the basis of the ratio of the irradiance incident at the nth interface to the irradiance incident at the first interface for a specific optical ray. Thus the truncation number is a function of the angle of incidence. Phase functions for both near- and far-field absorption and scattering of electromagnetic radiation are calculated directly at any desired scattering angle by using a hybrid algorithm based on the bisection and Newton-Raphson methods. With these methods a large sphere's absorption and scattering properties of light can be calculated for any wavelength from the ultraviolet to the microwave regions. Assuming that large snow meltclusters (1-cm order), observed ubiquitously in the snow cover during summer, can be characterized as spheres, one may compute absorption and scattering efficiencies and the scattering phase function on the basis of this geometrical-optics method. A geometrical-optics method for sphere (GOMsphere) code is developed and tested against Wiscombe's Mie scattering code (MIE0) and a Monte Carlo code for a range of size parameters. GOMsphere can be combined with MIE0 to calculate the single-scattering properties of dielectric spheres of any size.

Journal Article↗
PubMed2004

Optical-gradient antireflective coatings for 157-nm optical lithography applications.

We demonstrate an optical-gradient bottom antireflective coating (BARC) film, which can be easily prepared by treating a silicon nitride film with oxygen plasma. The oxygen composition is gradually decreased inside the silicon nitride film. The optical constants of the silicon nitride film are also changed gradually. A reflectance of less than 1% for various highly reflective substrates with high thickness-controlled tolerance has been achieved. The optical-gradient film is also shown to have high thermal stability during the postexposure bake procedure. Results indicate that the optical-gradient-type BARC is suitable in both ArF and F2 excimer lasers for sub-70-nm lithography applications.

Journal Article↗
PubMed2004

Performance of a write-once multilayer optical disk that uses transparent recording material with an optical switching layer.

A volumetric optical disk that has multiple transparent films with optical switching layers is used as a recording medium to increase the number of recording layers. In the disk the optical switching layer is adapted to reduce decay of laser energy and increase reading and recording sensitivity. Well-defined marks of approximately 100-nm depth can be placed precisely on the transparent films by a focused laser beam. Writing and reading of a four-layer recordable disk, fabricated by molding and spin bonding, have been demonstrated experimentally. The volumetric disk can achieve a high recording capacity with conventional optical pickups.

Journal Article↗
PubMed2005

Fiber-optic tunable multiwavelength variable attenuator and routing module designs that use bulk acousto-optics.

A compact fiber-coupled bulk acousto-optical multiwavelength variable optical attenuator module design that uses a retroreflective double-pass geometry within a single bulk acousto-optic tunable filter device is presented. The proposed attenuator module demonstrates a high 17-dB notch dynamic range at a low 100-mW drive power and uses a single bulk collinear-interaction acousto-optic tunable-filter device. Experiments show a low (<1.8-dB) fiber-to-fiber insertion loss with a fast 34-micros speed within a wide 1520-1640-nm agile multinotch band. The basic broadband attenuator module design is extended to allow for efficient architectures for routing modules such as agile drop filters, analog hitless tap filters, and digital add-drop switches.

Journal Article↗
PubMed2005

Bidirectional optical coupler for plastic optical fibers.

We have developed a low-loss bidirectional optical coupler for high-speed optical communication with plastic optical fibers (POFs). The coupler, which is fabricated by an injection molding method that uses poly (methyl methacrylate), has an antisymmetric tapered shape. We show that the coupler has low insertion and branching losses. The tapered shape of the receiving branch reduces beam diameter and increases detection efficiency coupling to a photodetector, whose area is smaller than that of the plastic optical fiber. The possibility of more than 15-m bidirectional transmission with a signaling bit rate up to 500 Mbits/s for simplex step-index POFs is demonstrated.

Journal Article↗
PubMed2005

Combination of highly nonlinear fiber, an optical bandpass filter, and a Fabry-Perot filter to improve the signal-to-noise ratio of a supercontinuum continuous-wave optical source.

We present a theoretical study of a supercontinuum (SC) continuous-wave (cw) optical source generation in highly nonlinear fiber and its noise properties through numerical simulations based on the nonlinear Schrödinger equation. Fluctuations of pump pulses generate substructures between the longitudinal modes that result in the generation of white noise and then in degradation of coherence and in a decrease of the modulation depths and the signal-to-noise ratio (SNR). A scheme for improvement of the SNR of a multiwavelength cw optical source based on a SC by use of the combination of a highly nonlinear fiber (HNLF), an optical bandpass filter, and a Fabry-Perot (FP) filter is presented. Numerical simulations show that the improvement in modulation depth is relative to the HNLF's length, the 3-dB bandwidth of the optical bandpass filter, and the reflection ratio of the FP filter and that the average improvement in modulation depth is 13.7 dB under specified conditions.

Journal Article↗
PubMed2005

Acousto-optic tunable filter sidelobe analysis and reduction with telecentric confocal optics.

The acousto-optic tunable filter (AOTF) has optical sidelobes that are due to the acoustic field produced by the transducer. These sidelobes were analyzed by wave-vector phase matching between the optical and acoustic fields, which correlated with measurements made with a TeO2 AOTF. A white-light point source was filtered and imaged, showing reasonably large and slowly decreasing sidelobes covering a large spectral range. This effect reduces the image quality of an AOTF system by producing faint secondary images of bright objects. The image quality can be improved with a telecentric confocal optical arrangement in which the angular shift of the sidelobes is greatly reduced, producing a much sharper image. This effect was also demonstrated experimentally with the point source.

Journal Article↗
PubMed2006

Optical activity measurement by use of a balanced detector optical heterodyne interferometer.

What we believe to be a novel amplitude sensitive optical heterodyne polarimeter in which a Zeeman laser is associated with balanced detector detection was set up. The aim was to measure the optical activity of a quartz crystal with a Cornu depolarizer at high accuracy. The features of this novel polarimeter, which include the use of a two-frequency laser that ensures the accuracy of the measurement, are discussed. Furthermore, the detection sensitivity of the optical activity of a quartz crystal was measured as 8.5x10(-10). To our knowledge, this is the highest sensitivity obtained for optical activity measurement of a quartz crystal when the error of the measurement is also analyzed.

Journal Article↗
PubMed2006

Integrated three-dimensional optical multilayer using free-space optics.

An integrated three-dimensional optical multilayer system for optical data communications is presented. It is based on the use of free-space optical light propagation and combines two integration principles, namely, planar and stacked integration. The combination of both integration schemes aims at a maximal design flexibility for complex geometric layouts. On the other hand, packaging issues that stem from assembly and tolerance have to be considered. Here we describe the basic concept and demonstrate the implementation of an optical interface module in a processor-memory bus.

Journal Article↗
PubMed2005

Description of the third-order optical aberrations of near-circular pupil optical systems without symmetry.

Many authors, dating back to at least the 1950s, have presented mathematical expansions of the wave-front aberration function for optical systems without symmetry, typically based on limiting assumptions and simplifications, with some of the most recent work being done by Howard and Stone [Appl. Opt. 39, 3232 (2000)]. This paper reveals that in fact there are no new aberrations in imaging optical systems with near-circular aperture stops but otherwise without symmetry. What does occur is that the field dependence of an aberration often changes when symmetry is abandoned. Each aberration type develops a characteristic field behavior in a system without symmetry. Specifically, for example, astigmatism, develops a binodal field dependence; e.g., there are typically two points in the field with zero astigmatism, and typically neither point is on axis. This construct, nodal aberration theory, for understanding the aberrations in systems without symmetry becomes a direct extension of an optical designer's knowledge base. Through the use of real ray-based analysis methods, such as Zernike coefficients, it is possible to understand completely the aberrations of optical systems without symmetry in terms of rotationally symmetric aberration theory with the simple addition of the concept of field nodes.

Journal Article↗
PubMed2004

Generation of 40-GHz control signals from flag pulses for switching all-optical gates for use with optical packets.

We demonstrate an all-optical circuit capable of generating 40-GHz control signals from flag pulses that can be used to define the switching state of all-optical gates for use with optical packets. The circuit comprises a Fabry-Perot filter and a semiconductor optical amplifier, and with a single pulse it can generate 12 control pulses with 0.64-dB amplitude modulation. With two and three flag pulses the number of control pulses becomes 36 and 54, respectively.

Journal Article↗
PubMed2005

Optical phase shifting with acousto-optic devices.

A novel optical phase-shifting method based on a well-known acousto-optic interaction is proposed. By using a pair of acousto-optic modulators (AOMs) and properly aligning them, we construct an optical phase shifter that can directly control the phase of a collimated beam. The proposed phase shifter is insensitive to the polarization of the incident beam when polarization-insensitive AOMs are used, and no calibration is necessary. The proposed approach is confirmed with experimental results.

Journal Article↗
PubMed2005

All-optical add-drop node for optical packet-switched networks.

We have demonstrated all-optical packet add-drop for all-optical packet-switched networks. Intelligent all-optical add-drop of packets is performed, based on all-optical processing of packet headers. The header and payload rates are 5 and 10 Gbits/s, respectively.

Journal Article↗
PubMed2006

Optical signal-to-noise ratio measurement by optical homodyne tomography.

An all-fiber optical homodyne tomography setup is introduced that measures the optical signal-to-noise ratio through reconstruction of the photon statistics. The scheme described has been conceived for applications to optical communications. In particular, the signal-to-noise ratio has been evaluated at lambda= 1.55 microm as a function of the received power. From the experimental data, in the case of optically amplified signals, the amplifier noise figure can be estimated.

Journal Article↗
PubMed2006

Sweep optical frequency synthesizer with a distributed-Bragg-reflector laser injection locked by a single component of an optical frequency comb.

A sweep optical frequency synthesizer is demonstrated by using a frequency-stabilized optical frequency comb and injection-locked distributed-Bragg-reflector (DBR) laser diode. The injection-locked DBR laser acts as a single-frequency filter and, simultaneously, a high-gain amplifier of the optical frequency comb. The frequency instability of the heterodyne beat signal between two independently injection-locked DBR lasers is measured to be 2.3 x 10(-16) at 1 s averaging time. The output frequency of the sweep optical frequency synthesizer can be precisely tuned over 1 GHz, and a saturated absorption spectrum of the Cs D2 line at 852 nm is recorded by the injected DBR laser.

Journal Article↗
PubMed2006

Synthesis of voltage-sensitive optical signals: application to panoramic optical mapping.

Fluorescent photon scattering is known to distort optical recordings of cardiac transmembrane potentials; however, this process is not well quantified, hampering interpretation of experimental data. This study presents a novel model, which accurately synthesizes fluorescent recordings over the irregular geometry of the rabbit ventricles. Using the model, the study aims to provide quantification of fluorescent signal distortion for different optical characteristics of the preparation and of the surrounding medium. A bi-domain representation of electrical activity is combined with finite element solutions to the photon diffusion equation simulating both the excitation and emission processes, along with physically realistic boundary conditions at the epicardium, which allow simulation of different experimental setups. We demonstrate that distortion in the optical signal as a result of fluorescent photon scattering is truly a three-dimensional phenomenon and depends critically upon the geometry of the preparation, the scattering properties of the tissue, the direction of wavefront propagation, and the specifics of the experimental setup. Importantly, we show that in an anatomically accurate model of ventricular geometry and fiber orientation, the morphology of the optical signal does not provide reliable information regarding the intramural direction of wavefront propagation. These findings underscore the potential of the new model in interpreting experimental data.

Animals↗
PubMed1994

Visual field profile of optic neuritis. One-year follow-up in the Optic Neuritis Treatment Trial.

PURPOSE: The purpose of this present study was to evaluate longitudinal visual field information for 448 patients over their first year of follow-up in the Optic Neuritis Treatment Trial. METHODS: We reviewed 6536 automated static visual fields performed on a visual field analyzer (Humphrey Visual Field Analyzer) at nine visits within the 1-year period for each of the patients. RESULTS: The median values of the mean deviations for affected eyes were as follows: -22.88 dB at baseline, -1.94 dB at 6 months, and -1.62 dB at 1 year. At 6 months, 51% of affected eye visual fields were normal, and at 1 year 55.9% were normal. Approximately two thirds (68.8%) of the fellow eyes were classified as abnormal at baseline, although the defects were generally slight. One third (33.2%) were abnormal at 6 months, and approximately one third were still abnormal at 1 year. More than 87% of those abnormal at 6 months and at 1 year had been abnormal at baseline. Binocular analysis revealed that 13.2% of patients showed a chiasmal or retrochiasmal type of field defect at least once during the year (5.1% bitemporal; 8.9% homonymous). Of the patients who showed a retrochiasmal visual field defect, 75.7% had an abnormal magnetic resonance imaging scan at baseline compared with 46% of the rest of the patients in the Optic Neuritis Treatment Trial (chi 2 = 10.73, df = 1, P < .002). CONCLUSION: Over the first year of follow-up, the majority of patients with visual field defects from acute optic neuritis returned to normal, as measured by automated static perimetry. Many fields showed variation in the pattern and location of the sensitivity loss. Chiasmal and retrochiasmal defects occurred more commonly than previously reported.

Acute Disease↗
PubMed1996

Characteristics of patients with nonarteritic anterior ischemic optic neuropathy eligible for the Ischemic Optic Neuropathy Decompression Trial.

OBJECTIVE: To describe the baseline clinical characteristics of patients in the Ischemic Optic Neuropathy Decompression Trial (IONDT). DESIGN: The IONDT is a single-masked, multicenter, randomized clinical trial. SETTINGS: Twenty-five US clinical centers. PATIENTS: Eligibility criteria for randomization to either optic nerve sheath decompression surgery or careful follow-up included a diagnosis of acute unilateral nonarteritic anterior ischemic optic neuropathy (NAION), a visual acuity of 20/64 or worse and better than no light perception, and an age of 50 years or older. Patients who were eligible except for having visual acuity better than 20/64 were not randomized, but were followed up. METHODS: Each patient underwent a standardized history and examination by certified study personnel within 14 days of the onset of symptoms. Masked personnel performed outcome measurements. RESULTS: Of 420 patients with NAION, 258 were randomized, and 162 were not randomized and are being followed up. Sixty-two percent of the patients were men and 95% were white. The mean +/- SD age at onset was 66.0 +/- 8.7 years. Hypertension was reported in 47% of the patients in the IONDT, and 24% of the patients had diabetes mellitus. Forty-two percent of the patients recalled the onset of visual symptoms to be within 2 hours of awakening. Initial visual acuities in the study eye ranged from 20/20 or better to light perception, with 49% of the patients seeing better than 20/64 and 34% of the patients seeing 20/200 or worse. The mean Westergren sedimentation rate was 18.4 mm/h, with 9% of the patients having a rate greater than 40 mm/h. The nonrandomized patients (visual acuity better than 20/64) were younger, 72% were male, and they had a lower prevalence of hypertension and diabetes mellitus. CONCLUSION: Although our baseline findings are derived from a selected population of patients with NAION who were eligible for the IONDT, they provide the first description of NAION from a large prospective study that used a standard definition of NAION and only included patients who were identified within 2 weeks of the onset of symptoms.

Aged↗