Showing papers on "Wavelength published in 1998"

Extraordinary optical transmission through sub-wavelength hole arrays

Abstract: The desire to use and control photons in a manner analogous to the control of electrons in solids has inspired great interest in such topics as the localization of light, microcavity quantum electrodynamics and near-field optics1,2,3,4,5,6. A fundamental constraint in manipulating light is the extremely low transmittivity of apertures smaller than the wavelength of the incident photon. While exploring the optical properties of submicrometre cylindrical cavities in metallic films, we have found that arrays of such holes display highly unusual zero-order transmission spectra (where the incident and detected light are collinear) at wavelengths larger than the array period, beyond which no diffraction occurs. In particular, sharp peaks in transmission are observed at wavelengths as large as ten times the diameter of the cylinders. At these maxima the transmission efficiency can exceed unity (when normalized to the area of the holes), which is orders of magnitude greater than predicted by standard aperture theory. Our experiments provide evidence that these unusual optical properties are due to the coupling of light with plasmons — electronic excitations — on the surface of the periodically patterned metal film. Measurements of transmission as a function of the incident light angle result in a photonic band diagram. These findings may find application in novel photonic devices.

Experimental Demonstration of Guiding and Bending of Electromagnetic Waves in a Photonic Crystal

Abstract: The routing and interconnection of optical signals through narrow channels and around sharp corners are important for large-scale all-optical circuit applications. A recent computational result suggests that photonic crystals may offer a novel way of achieving this goal by providing a mechanism for guiding light that is fundamentally different from traditional index guiding. Waveguiding in a photonic crystal and near 100 percent transmission of electromagnetic waves around sharp 90 degree corners were observed experimentally. Bending radii were made smaller than one wavelength.

An approach for recording and readout beyond the diffraction limit with an Sb thin film

Abstract: A technique for recording and retrieving small marks beyond the optical diffraction limit was proposed. The basic experiment with this technique was also carried out at a constant linear velocity of 2.0 m/s, rotating a disk with a multi-layered structure of Sb and GeSbTe, which were separated by a thin film of SiN. By use of the optically nonlinear property of the Sb thin film, carrier to noise ratio of more than 10 dB was obtained from recorded marks of 90 nm, using an optical system with the laser wavelength of 686 nm and a numerical aperture of 0.6.

Sinc-sampled fiber Bragg gratings for identical multiple wavelength operation

Abstract: Through a periodic sine modulation of the refractive index-profile in fiber Bragg gratings (FBGs), we demonstrate gratings with multiple equally spaced and identical wavelength channels. We show 10-cm-long gratings with 4, 8, and 16 identical uniform wavelength channels separated by the ITU spacing of 100 GHz and a 22.5-cm-long grating with four identical dispersion compensating channels with a 200-GHz separation designed to dispersion compensate 80-km data transmission through standard fiber at 1.55 /spl mu/m.

An Evaluation of the Scale at which Ground-Surface Heat Flux Patchiness Affects the Convective Boundary Layer Using Large-Eddy Simulations

Abstract: The effects on the convective boundary layer (CBL) of surface heterogeneities produced by surface sensible heat flux waves with different means, amplitudes, and wavelengths were investigated here. The major objective of this study was to evaluate at which scale surface heterogeneity starts to significantly affect the heat fluxes in the CBL. The large-eddy simulation option of the Regional Atmospheric Modeling System developed at Colorado State University was used for that purpose. Avissar et al. evaluated this model against observations and demonstrated its reliability. It appears that the impact of amplitude and wavelength of a heat wave is nonlinearly dependent upon the mean heating rate. The circulations (or rolls) resulting from surface heterogeneity are strong when the amplitude and the wavelength of the heat wave are large, especially at low mean heating rate. In that case the profiles of horizontally averaged variables are quite strongly modified in the CBL. The potential temperature is no...

Method for non-invasive measurement of an analyte

Abstract: Disclosed is a method and apparatus for measuring an analyte in a tissue of a subject. The method comprises contacting the tissue with electromagnetic radiation having a first excitation wavelength, wherein the first excitation wavelength is substantially equal to an absorption wavelength of a temperature probe within the tissue. The temperature probe and the analyte are sufficiently proximate to one another that energy deposited into one by absorption of radiation is transferred to the other. The Raman spectra emitted by the tissue are collected and analyzed to determine a concentration of analyte present in the tissue. The analysis can comprise measuring the Raman spectra associated with the temperature probe. In addition, the method can include simultaneously contacting the tissue with electromagnetic radiation having the first excitation wavelength and with electromagnetic radiation having a second excitation wavelength, wherein the second excitation wavelength is substantially equal to an absorption wavelength of the analyte. The analysis comprises comparing the spectra emitted in response to the first excitation wavelength in the presence and in the absence of the second excitation wavelength. In another embodiment, the analysis comprises measuring the anti-Stokes component of the Raman spectra associated with the analyte. The method provides a non-invasive measurement of blood glucose, using hemoglobin as the temperature probe.

Long period fibre gratings for structural bend sensing

Abstract: The authors examine the changes in wavelength and attenuation of long period fibre gratings subjected to bends with curvatures from 0 to 4.4 m/sup -1/. The wavelength change with curvature is nonlinear, with a projected minimum detectable curvature change of 2/spl times/10/sup -3/ m/sup -1/. The magnitude of the bend-induced wavelength shift depends on the rotation of the cylindrical fibre relative to the bending plane.

Multilayer infrared reflecting optical body

Abstract: An optical body comprising (a) a dielectric multilayer film having a reflecting band positioned to reflect infrared radiation of at least one polarization at an incident angle normal to the film, said reflecting band having a short wavelength bandedge μa0 and long wavelength bandedge μb0 at a normal incident angle, and a short wavelength bandedge μaυ and long wavelength bandedge μbυ at a maximum usage angle υ, wherein μaυ is less than μa0 and μa0 is selectively positioned at a wavelength greater than about 700 nm; and (b) at least one component which at least partially absorbs or reflects radiation in the wavelength region between μaυ and μa0 at a normal angle of incidence.

Four-wave mixing in fibers with randomly varying zero-dispersion wavelength

Abstract: The effect of random fluctuations in the zero-dispersion wavelength is considered for fiber four-wave mixing. Theoretical expressions for average parametric gain, phase-conjugation conversion efficiency, and gain bandwidth are obtained and found to be in good agreement with experiments. Possible limitations on the noise figure in phase-sensitive amplifiers based on fiber four-wave mixing are also discussed.

Laboratory measurements of angular distributions of light scattered by phytoplankton and silt

Abstract: We present laboratory measurements of scattering properties of 15 different types of coastal and inland water phytoplankton species and two types of estuarine sediments. These properties are the scattering function as well as the angular distribution of a ratio of scattering matrix elements, which in practice equals the degree of linear polarization of the scattered light if the incident light is unpolarized. Laser light with a wavelength of 633 nm was used, and a scattering angle range from 20 degrees to 60 degrees was covered. The results can be used in the context of water-quality studies and to test results of theoretical models. The measured scattering functions are all strongly peaked in forward directions, but not equally so. For the covered angles, they vary significantly as a function of scattering angle. The measured angular distributions of the degree of linear polarization are mostly bell shaped, showing a maximum near 90 degrees, whose magnitude is clearly different for the phytoplankton compared to the silt particles. We find that the morphology and structural features of the particles studied play an important and complex role in their light-scattering behavior. In particular, internal cell structures such as gas vacuoles alter the scattering patterns of the phytoplankton species considerably. The external shape of the cells appears to have a much smaller influence. The experimental results are compared with results of Mie calculations and with the "standard scattering function" of San Diego Harbor water. In most cases, Mie calculations cannot provide an adequate approximation of the measured scattering behavior, which indicates that more sophisticated models are required. Only 3 of the 17 measured scattering functions resemble the San Diego Harbor standard scattering function. One of these pertains to small silt particles, showing that this function is representative for water dominated by these particles. [KEYWORDS: Refractive-index; optical-properties; mueller matrix; cells; particles; chlorella; growth; water; attenuation; backscatter]

Fiber Bragg Gratingを用いた水中音波の検出

Abstract: A fiber Bragg grating (FBG), which is formed in a fiber core by introducing a periodic variation of the refractive index reflects selectively a particular wavelength of light due to the Bragg reflection. The reflected optical power is modulated if sound pressure is applied to the FBG. Using the FBG which has such a feature we made the fundamental experiments on the detection of underwater sound. Good linearity is obtained under various conditions with the insertion of optical isolators that avoid the fluctuation of detected output due to the Fabry-Perot interference effect between the FBG and the various facets. The sensitivity of detection is maximized if the slope of the curve in the transmittance-wavelength characteristics is steepest at the operating wavelength. Simultaneous multiplex detection is made possible by wavelength division multiplexing with two FBGs in series connection.

Long-wavelength (lambda approximately 8-11.5 microm) semiconductor lasers with waveguides based on surface plasmons.

Abstract: Laser waveguides based on surface plasmons at a metal–semiconductor interface have been demonstrated by use of quantum cascade (QC) lasers emitting in the 8–115?µm wavelength range The guided modes are transverse magnetic polarized surface waves that propagate at the metal (Pd or Ti–Au)–semiconductor interface between the laser top contact and the active region without the necessity for waveguide cladding layers The resultant structure has the advantages of a strong decrease in the total layer thickness and a higher confinement factor of the laser-active region compared with those of a conventional layered semiconductor waveguide, and strong coupling to the active material, which could be used in devices such as distributed-feedback lasers These advantages have to be traded against the disadvantage of increased absorption losses A peak output power exceeding 25??mW at 90??K and a maximum operating temperature of 150??K were measured for a QC laser with an emission wavelength ??8 µm At ??115 µm the peak power levels are several milliwatts and the maximum operating temperature is 110??K

Analytical models for the responses of the mesospheric OH* and Na layers to atmospheric gravity waves

Abstract: Analytic models are developed to describe gravity wave induced perturbations in the high ν OH* Meinel Band emissions and in atomic Na density. The results are used to predict the fluctuations in OH* intensity and rotational temperature, Na abundance, and the centroid heights of the OH* and Na layers. The OH* model depends critically on the assumed form for the atomic oxygen profile. In this study the O profile is modeled as a Chapman layer, which is in excellent agreement with MSIS-90. We also neglect the wave-induced redistribution of O3 because the chemical lifetime of ozone in the mesopause region is short compared to most gravity wave periods. Under these conditions the OH* response is ΔV/Vu ≃ −3[1 - (z - zOH)hOH + (z − zOH)2/σ12]Δρ/ρu, where ΔV/Vu are the relative emission rate fluctuations, Δρ/ρu are the relative atmospheric density fluctuations, zOH ≃ 89 km is the layer centroid height, hOH ≃ 3.6 km, and σ1 ≃ 8.0 km. By using these results, we show that cancellation of the induced perturbations in emission intensity and rotational temperature is significant for short vertical wavelengths. The amplitude attenuation in both parameters is proportional to exp(−m2σ2OH/2), where m = 2π/λz and σOH ≃ 4.4 km is the rms thickness of the OH* layer. For example, at λz = 15 km, the predicted rotational temperature perturbation is only 20% of the atmospheric temperature perturbation. Because the most sensitive instruments are only capable of accuracies approaching ±2 K, there are few reported observations of waves with λz ≤ 15 km. The cancellation effects are not as limiting in OH intensity observations because the relative intensity perturbations are larger than the relative temperature perturbations, and intensities can be measured more accurately than temperature, especially with broadband instruments. Fluctuations in the emission rate are largest on the bottomside of the OH* layer, ∼ 3.75 km below the layer peak (∼89 km), where the effects due to the redistribution of atomic oxygen dominate. Fluctuations in rotational temperature are largest near the peak of the OH layer, where the volume emission rate is largest. The ∼3.75 km separation between the maxima of the intensity and rotational temperature perturbations is largely responsible for the phase differences observed in the fluctuations of these parameters. Rotational temperature and Krassovsky's ratio are found to be very sensitive to the form of the background temperature profile. Wave-induced OH* layer centroid height fluctuations coupled with the mean lapse rate of the background temperature profile can contribute significantly to the observed rotational temperature fluctuations, especially for the shorter wavelength waves λz ≤ 15 km. The OH* intensity fluctuations are relatively insensitive to the temperature profile as well as variations in atomic oxygen density and therefore appear to be excellent tracers of gravity wave dynamics. OH temperature observations are best suited for studying long-period waves, including tides, with λz ≥ 15 km.

Analytical method and apparatus

Abstract: A method of examining thin layer structures on a surface for differences in respect of optical thickness, which method comprises the steps of: irradiating the surface with light so that the light is internally or externally reflected at the surface; imaging the reflected light on a first two-dimensional detector; sequentially or continuously scanning the incident angle and/or wavelength of the light over an angular and/or wavelength range; measuring the intensities of light reflected from different parts of the surface and impinging on different parts of the detector, at at least a number of incident angles and/or wavelengths, the intensity of light reflected from each part of the surface for each angle and/or wavelength depending on the optical thickness of the thin layer structure thereon; and determining from the detected light intensities at the different light incident angles and/or wavelengths an optical thickness image of the thin layer structures on the surface. According to the invention, part of the light reflected at said surface is detected on a second detector to determine the incident angle or wavelength of the polarized light irradiating the surface. An apparatus for carrying out the method is also disclosed.

WDM optical communication systems with wavelength stabilized optical selectors

Abstract: The present invention provides a wavelength division multiplexed optical communication system with dynamically stabilized wavelength selectors. To accurately correlate the wavelength of a wavelength selector to the wavelength emitted by an optical transmission source, the present invention uses an optical detector and feedback loop to optimize the wavelength of an optical selection element in accordance with a wavelength of an incident optical channel. In one embodiment, the optical selection element is a Bragg grating associated with a grating wavelength controller, such as a temperature regulator or strain tuning system, to adjust the wavelength band of maximum reflectivity of the grating. The feedback loop communicates with the optical detector and the wavelength controller to modify the grating's reflection wavelength band in accordance with the wavelength of an incident optical channel.

Measuring P700 Absorbance Changes in the Near Infrared Spectral Region with a Dual Wavelength Pulse Modulation System

Abstract: Monitoring the oxidation state of the primary donor P700 of photosystem I (PS I) via near infra red absorption spectroscopy has been providing valuable information on PSI-driven photosynthetic electron transport in vivo (1, 2, 3). Redox changes of P700 are preferentially detected at a single wavelength band in the region of the P700+ cation radical absorption band peaking at 820 nm. NIR-measuring light > 800 nm is not absorbed by chlorophyll, does not generate chlorophyll fluorescence and has no actinic effect on photosynthesis of green plants. It can be applied at high intensity with the advantage of a high S/N ratio. In order to discriminate the measuring light from continuous background light the former usually is modulated. Modulation up to MHz frequencies can be easily achieved by infra-red emitting diodes (IRED). Such IREDs display bandwidths of 30–50 nm which is suitable for the broad P700+ absorption band. Single wavelength systems based on a modulated WED (e.g. ED-800T, Walz, Effeltrich) have been successfully used in monitoring light induced P700 redox changes in intact leaves (1, 2). Superimposed less wavelength-specific changes (as scattering changes) in most cases are slow and may be separated from P700+ by application of saturation pulses (4) or by switching off the actinic light from time to time and observing the rapid signal relaxation. However, the low selectivity of the single wavelength measurement becomes a major problem with suspensions of isolated chloroplasts or algae. Unspecific optical changes caused by shrinking, swelling and settling of the chloroplasts or cells lead to serious distortions of the much smaller P700+ signal, and artefacts caused by chemical additions and sample stirring preclude the observation of bathochromic absorption changes.

A B-spline Galerkin scheme for calculating the hydroelastic response of a very large floating structure in waves

Abstract: This paper presents an effective scheme for computing the wave-induced hydroelastic response of a very large floating structure, and a validation of its usefulness. The calculation scheme developed is based on the pressure-distribution method of expressing the disturbance caused by a structure, and on the mode-expansion method for hydroelastic deflection with the superposition of orthogonal mode functions. The scheme uses bi-cubic B-spline functions to represent unknown pressures, and the Galerkin method to satisfy the body boundary conditions. Various numerical checks confirm that the computed results are extremely accurate, require relatively little computational time, and contain few unknowns, even in the region of very short wavelengths. Measurements of the vertical deflections in both head and oblique waves of relatively long wavelength are in good agreement with the computed results. Numerical examples using shorter wavelengths reveal that the hydroelastic deflection does not necessarily become negligible as the wavelength of incident waves decreases. The effects of finite water depth and incident wave angle are also discussed.

Observation of Temporal Solitons in Second-Harmonic Generation with Tilted Pulses

Abstract: By eliminating the group-velocity mismatch and enhancing the group-velocity dispersion via pulse-front tilting, we obtained the formation of temporal solitary waves in phase-mismatched second-harmonic generation. Experimental and numerical results match well in a wide range of intensities and $\ensuremath{\Delta}k$'s. In a 7 mm BBO crystal, 58 fs, 527 nm pulses are obtained starting from 200 fs pump, at the same wavelength.

Magnetic resonance imaging of shear wave propagation in excised tissue.

Abstract: The propagation of shear waves in ex vivo tissue samples, agar/gel phantoms, and human volunteers was investigated. A moving coil apparatus was constructed to generate low acoustic frequency shear perturbations of 50 to 400 Hz. Oscillating gradients phase-locked with the shear stimulus were used to generate a series of phase contrast images of the shear waves at different time-points throughout the wave cycle. Quantitative measurements of wave velocity and attenuation were obtained to evaluate the effects of temperature, frequency, and tissue anisotropy. Results of these experiments demonstrate significant variation in shear wave behavior with tissue type, whereas frequency and anisotropic behavior was mixed. Temperature-dependent behavior related mainly to the presence of fat. Propagation velocities ranged from 1 to 5 m/sec, and attenuation coefficients of from 1 to 3 nepers/unit wavelength, depending on tissue type. These results confirm the potential of elastic imaging attributable to the intrinsic variability of elastic properties observed in normal tissue, although some difficulty may be experienced in clinical implementation because of viscous attenuation in fat.

High-accuracy absolute distance measurement by means of wavelength scanning heterodyne interferometry

Abstract: It is easy to measure length accurately by means of optical interferometry. However, the measurement range of a single-frequency interferometer is limited to less than half the wavelength without a moving carriage to count interference fringes. To resolve this problem, a frequency scanning method had been developed. However, its phase resolution is not so high that the integer part of the order of the interference fringe can be accurately determined. We proposed a method to measure any length absolutely and accurately, by combining a high-resolution phase measurement technique with a frequency scanning technique. In this paper, this method was investigated by using a frequency scanning heterodyne interferometer. With heterodyne phase measurement, we achieved high resolution, higher than quarter wavelength, using the wide-range frequency scanning method. This means that we can measure the absolute length with nanometre accuracy, since the integer part of the order of the interference fringe for a wavelength is determined with the frequency scanning. We measured distances up to about 4 mm with an accuracy of about 3 nm.

Determination of the wavelength dependence of the differential pathlength factor from near-infrared pulse signals

Abstract: For the calculation of changes in oxyhaemoglobin, deoxyhaemoglobin and the redox state of cytochrome-c-oxidase from attenuation data via a modified Beer-Lambert equation the wavelength dependence of the differential pathlength factor (DPF(lambda)) has to be taken into account. The DPF, i.e. the ratio of the mean optical pathlength and the physical light source-detector separation at each wavelength, determines the crosstalk between the different concentrations and is therefore essential for a sensitive detection of chromophore changes. Here a simple method is suggested to estimate the wavelength dependence of the DPF(lambda) from pulse-induced attenuation changes measured on the head of adult humans. The essence is that the DPF is the ratio of the attenuation changes over absorption coefficient changes, and that the spectral form of the pulse correlated absorption coefficient change can be assumed to be proportional to the extinction coefficient of blood. Indicators for the validity of the DPF(lambda) derived for wavelengths between 700 and 970 nm are the stability of the calculated haemoglobin and cytochrome signals with variations of the wavelength range included for their calculation and its overall agreement with the data available from the literature.

Excitation and breaking of internal gravity waves by parametric instability

Abstract: We study the dynamics of internal gravity waves excited by parametric instability in a stably stratified medium, either at the interface between a water and a kerosene layer, or in brine with a uniform gradient of salinity. The tank has a rectangular section, and is narrow to favour standing waves with motion in the vertical plane. The fluid container undergoes vertical oscillations, and the resulting modulation of the apparent gravity excites the internal waves by parametric instability.Each internal wave mode is amplified for an excitation frequency close to twice its natural frequency, when the excitation amplitude is sufficient to overcome viscous damping (these conditions define an ‘instability tongue’ in the parameter space frequency-amplitude). In the interfacial case, each mode is well separated from the others in frequency, and behaves like a simple pendulum. The case of a continuous stratification is more complex as different modes have overlapping instability tongues. In both cases, the growth rates and saturation amplitudes behave as predicted by the theory of parametric instability for an oscillator. However, complex friction effects are observed, probably owing to the development of boundary-layer instabilities.In the uniformly stratified case, the excited standing wave is unstable via a secondary parametric instability: a wave packet with small wavelength and half the primary wave frequency develops in the vertical plane. This energy transfer toward a smaller scale increases the maximum slope of the iso-density surfaces, leading to local turning and rapid growth of three-dimensional instabilities and wave breaking. These results illustrate earlier stability analyses and numerical studies. The combined effect of the primary excitation mechanism and wave breaking leads to a remarkable intermittent behaviour, with successive phases of growth and decay for the primary wave over long timescales.

Field Investigation of Transformation of the Wind Wave Frequency Spectrum with Fetch and the Stage of Development

Abstract: The variability of frequency spectra of waves is considered; for example, the dependencies of integral and spectral parameters of waves on wave-development factors and the interrelationships of the parameters are examined. Also studied is the transformation of the frequency spectrum shape in the course of its development, as well as the transition from the spectrum of developing waves to the spectrum of fully developed waves. Data were obtained in situ with common methods during a long-term program in the Black Sea. The variability of the spectrum of developing waves, as a function of the stage of wave development, is described on the basis of field data using estimates of parameters for the spectrum form of the JONSWAP type. A novel approximation of the equilibrium interval level dependence on the dimensionless peak frequency fm is obtained, which includes periods of stable and changeable behavior of the spectrum level. Transformation of the spectrum of wind-generated waves related to the devel...

Path-integral approach to the dynamic Casimir effect with fluctuating boundaries

Abstract: A path-integral formulation is developed for the dynamic Casimir effect. It allows us to study small deformations in space and time of the perfectly reflecting (conducting) boundaries of a cavity. The mechanical response of the intervening vacuum is calculated to linear order in the frequency\char21{}wave-vector plane, using which a plethora of interesting phenomena can be studied. For a single corrugated plate we find a correction to mass at low frequencies and an effective shear viscosity at high frequencies that are both anisotropic. The anisotropy is set by the wave vector of the corrugation. For two plates, the mass renormalization is modified by a function of the ratio between the separation of the plates and the wavelength of corrugations. The dissipation rate is not modified for frequencies below the lowest optical mode of the cavity and there is a resonant dissipation for all frequencies greater than that. In this regime, a divergence in the response function implies that such high-frequency deformation modes of the cavity cannot be excited by any macroscopic external forces. This phenomenon is intimately related to resonant particle creation. For particular examples of two corrugated plates that are stationary, or moving uniformly in the lateral directions, Josephson-like effects are observed. For capillary waves on the surface of mercury a renormalization to surface tension and sound velocity is obtained.

Dispersion measurements of water with white-light interferometry

Abstract: We measure the second- and third-order dispersion coefficients, d2k/ dω 2 and d3k/dω3, of water for wavelengths from 0.45 to 1.3 μm using a Michelson white-light interferometer. In this interval, the second-order dispersion ranges from 0.068 to -0.1 fs2/μm, and the third-order dispersion ranges from 0.048 to 1.18 fs3/μm. We observe an oscillation in d2k/dω2 near 1.1 μm that is due to water absorption features near that wavelength. From the dispersion coefficients, derivatives of the index of refraction of water are calculated and compared with available equations. These measured values of d2n/dλ2 and d3n/dλ3 should be useful in the evaluation and improvement of existing equations for n(λ) in water.

Method and apparatus for controlling the wavelength of a laser

Abstract: A method and apparatus for controlling the wavelength of a laser. Initially, the wavelength of the laser is coarsely tuned to within a predetermined window around a specified wavelength. After coarse adjustment, a wavelength control loop is activated to finely tune and lock the laser wavelength. In an embodiment, the control loop dithers the wavelength of the optical carrier signal from the laser. The optical carrier signal is then modulated (i.e., with data) in the normal manner. A portion of the modulated optical signal is filtered and detected. The amplitude and phase of the detected signal, which comprises the error signal, is processed and averaged. The averaged signal is then summed with a dither signal to provide a composite signal. A control signal corresponding to the composite signal is then generated and used to adjust the laser wavelength.