Arbitrary Wavelength Research Articles

Form and structure factors for impedance and reflection from periodic layers

In an exact treatment of the Maxwell equations, we derive form and structure factors for reflection from periodic layers, and we show that these factors are significantly different from their analogs in kinematic x-ray diffraction. Quite generally, we show that reflection and impedance can be written precisely as the sum of an additive form factor and the product of a structure factor and a second form factor. This additive form factor does not have an analog in kinematic x-ray diffraction. It is demonstrated that the form factors are found by analytic continuation to an arbitrary wavelength of expressions for the impedance both at long wavelengths and at quarter wavelengths. A correction to the Bragg law relating fringe spacing to the total structure thickness is derived. We go beyond previous numerical work by deriving simple analytic exact expressions for reflection and impedance of periodic layers for all frequencies within the reflection passband, and for an arbitrary number of periods in the structure, an arbitrary index profile within each period, arbitrary layer thicknesses (not just quarter-wave layers), and for arbitrary sizes of the refractive index differences.

Verification of Wavelength-Fixing Mechanism for Rail Corrugation Caused by Multiple-Wheel Interaction

In this paper the author proposes a hypothesis to explain a wavelength-fixing mechanism for rail corrugation. Corrugation growth was predicted using a theoretical model that took into account multiple-wheel interaction and the Doppler effect of rail vibration caused by wheel movement. The wheel-rail contact forces due to virtual sinusoidal irregularities on the rail head were calculated, and the corrugation growth at an arbitrary wavelength was predicted. To verify the hypothesis, the author conducted a precise investigation into the corrugation wavelengths on various track sections using vertical acceleration of the axle box. Even on a specific section of track, large-amplitude corrugation was not generally found at a specific wavelength but rather in a few wide-band ranges. Moreover, in each range where significant corrugation occurred, several sharp peaks were observed at constant frequency intervals. These peaks were found to appear at regular intervals approximately equal to fractions of a wheelbase length, and the theoretical prediction showed good agreement with measured corrugation characteristics. It was concluded that various kinds of corrugation are the result of rail vibration interference excited by multiple wheels.

Individual switching of multi-wavelength lasing outputs based on switchable FBG filters

We demonstrate an individually-switchable multi-wavelength fiber ring laser using semiconductor optical amplifier with novel switchable band-pass filters. The switchable band-pass filter is based on a fiber Sagnac loop interferometer incorporating both fiber Bragg gratings and polarization controller. The proposed multi-wavelength fiber ring laser shows a high output performance of independent switching at arbitrary lasing wavelength and high extinction ratio for wavelength-division multiplexing source.

Simulation of Inertia–Gravity Waves in a Poleward-Breaking Rossby Wave

Poleward-breaking Rossby waves often induce an upper-level jet streak over northern Europe. Dominant inertia–gravity wave packets are observed downstream of this jet. The physical processes of their generation and propagation, in such a configuration, are investigated with a mesoscale model. The study is focused on an observational campaign from 17 to 19 December 1999 over northern Germany. Different simulations with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) have been performed. For a high-resolution process study, three domains were set up that encompass the evolution of Rossby waves and that of inertia–gravity waves. To minimize the impact of model damping, the horizontal and vertical resolution has been adjusted appropriately. With a novel statistical approach, the properties of inertia–gravity wave packets have been estimated. This method uses the horizontal divergence field and takes into account the spatial extension of a wave packet. It avoids the explicit treatment of the background field and works for arbitrary wavelength. Two classes of inertia–gravity waves were found: subsynoptic waves with a horizontal wavelength of about 500 km and mesoscale waves with a horizontal wavelength of about 200 km. The subsynoptic structures were also detected in radiosonde observations during this campaign. The similarity between simulated and observed wavelengths and amplitudes suggests that the simulations can be considered as near realistic. Spontaneous radiation from unbalanced flow is an important process of inertia–gravity wave generation. Synoptic-scale imbalances in the exit region of the upper-tropospheric jet streak were identified with the smoothed cross-stream Lagrangian Rossby number. In a number of simulations with different physics, it was found that the inertia–gravity wave activity was related to the tropospheric jet, orography, and moist convection. The upward propagation of inertia–gravity waves was favored during this event of a poleward-breaking Rossby wave. The presence of the polar vortex induced background winds exceeding the critical line. Consequently, the activity of inertia–gravity waves in the lower stratosphere increased by an order of magnitude during the case study. The successful simulation of the complex processes of generation and propagation showed the important role of poleward Rossby wave breaking for the appearance of inertia–gravity waves in the midlatitudes.

A tunable wavelength-interchanging cross-connect scheme utilizing two periodically poled LiNbO3 waveguides with double-pass configuration

A tunable wavelength-interchanging cross-connect (WIXC) scheme is proposed by utilizing two periodically poled LiNbO3 waveguides. A fiber Brag grating is utilized to link the two waveguides so as to configurate the double-pass operation. In each waveguide, sum frequency generation (SFG) and difference frequency generation (DFG) take place during the forward propagation and backward propagation, respectively. The two tunable pumps, utilized in the processes of SFG and DFG respectively, are arranged to select an arbitrary channel from a WDM signal and convert it to an arbitrary wavelength. The information carried by two different wavelengths in two different WDM signals can be interchanged by tuning the frequencies and powers of the two pumps.

Long‐wavelength (λ ≥ 1.3 µm) InGaAlAs–InP vertical‐cavity surface‐emitting lasers for applications in optical communication and sensing

AbstractIn this paper we present an overview of the properties and applications of long‐wavelength vertical‐cavity surface‐emitting lasers (VCSELs) based on the InGaAlAs–InP material system. With respect to significant temperature sensitivity of active material gain as well as insufficient thermal conductivity of InP‐based epitaxial compound layers, the effective thermal heat management appears as a major issue for application suitable device performance. In this context, the incorporation of a buried tunnel junction (BTJ) in connection with improved heat sinking resembles a breakthrough for long‐wavelength VCSELs. With the utilization of n‐type spreading layers and consequently ultralow series resistances, BTJ‐VCSELs exhibit sharply reduced excess heat generation. Furthermore, the BTJ‐approach enables self‐aligned optical and current confinement. A hybrid dielectric stack with Au‐coating yields an improved thermal heatsinking. The current status of BTJ‐VCSELs encompasses a number of superior performance values. At 1.55 µm wavelength, this includes room temperature single‐ and multimode continuous wave (cw) output powers of more than 3 mW and 10 mW, respectively, laser operation for heat sink temperatures well exceeding 100 °C, and optical data transmission rates up to 10 Gbit/s. The versatility of compound layer composition enables arbitrary emission wavelengths within a broad range of 1.3 and 2 µm. With respect to sensing applications, BTJ‐VCSELs appear as ideal components for optical detection of infrared active gases. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Short wavelength effects on the collisionless neoclassical polarization and residual zonal flow level

Sheared zonal flow helps to reduce the turbulent transport caused by the ion temperature gradient mode. Rosenbluth and Hinton (R-H) calculated the residual zonal flow level for radial wavelengths that are much larger than the ion poloidal gyroradius. Their calculation is extended to treat arbitrary radial wavelengths. For the radial wavelengths that approach the ion poloidal gyroradius, but are much larger than the ion gyroradius, an analytical formula is obtained. For radial wavelengths that are comparable or shorter than the poloidal ion gyroradius and the ion gyroradius a numerical solution is provided. These small radial wavelength results are then extended into the electron temperature gradient regime, where the residual zonal flow level is large but ineffective in regulating the turbulence, indicating that the conventional R-H explanation that zonal flow regulates turbulence is incomplete.

Interfacial capillary waves in the presence of electric fields

Large amplitude capillary waves on an inviscid, incompressible fluid layer of density ρ 1 bounded by a second inviscid, incompressible fluid of density ρ 2 are computed in the presence of a uniform electric field acting in a direction parallel to the undisturbed configuration. Periodic travelling waves of arbitrary amplitudes and wavelengths are calculated and the effect of the electric field is studied. The solutions extend the results of Papageorgiou and Vanden-Broeck [D.T. Papageorgiou, J.-M. Vanden-Broeck, Large-amplitude capillary waves in electrified fluid sheets, J. Fluid Mech. 508 (2004) 71–88; D.T. Papageorgiou, J.-M. Vanden-Broeck, Antisymmetric capillary waves in electrified fluid sheets, Eur. J. Appl. Math. 16 (2004) 609–623] where the case of ρ 2 = 0 was treated. Fully nonlinear solutions are computed using boundary integral equation methods. It is shown that there are both symmetric and antisymmetric waves and their characteristics are explored and compared. When there is a jump in the undisturbed horizontal velocities, the flow is susceptible to Kelvin–Helmholtz instability. It is shown analytically in the linear regime, that even in the absence of surface tension, the flow is stabilized by sufficiently large electric fields. In such situations two wave speeds are possible for given electric fields and we construct these branches numerically when the amplitudes are not infinitesimal, both in the absence and presence of surface tension.

Optical transfer cavity stabilization using current-modulated injection-locked diode lasers

It is demonstrated that rf current modulation of a frequency stabilized injection-locked diode laser allows the stabilization of an optical cavity to adjustable lengths, by variation of the rf frequency. This transfer cavity may be used to stabilize another laser at an arbitrary wavelength, in the absence of atomic or molecular transitions suitable for stabilization. Implementation involves equipment and techniques commonly used in laser cooling and trapping laboratories and does not require electro- or acousto-optic modulators. With this technique we stabilize a transfer cavity using a rf current-modulated diode laser which is injection locked to a 780nm reference diode laser. The reference laser is stabilized using polarization spectroscopy in a Rb cell. A Ti:sapphire ring laser at 960nm is locked to this transfer cavity and may be precisely scanned by varying the rf modulation frequency. We demonstrate the suitability of this system for the excitation of laser cooled Rb atoms to Rydberg states.

Measurement of Stokes Parameters by Quarter-Wave Plate and Polarizer

Formulations of the theory of automated photoelasticity are expressed simply by use of the Stokes parameters. In the automated photoelasticity, the measurement of the total relative phase retardation must often be performed over a wide wavelength range. The Stokes parameters (S0, S1, S2 and S3) need to be measured over a wide wavelength range. The Stokes parameters of monochromatic light can be measured by the adjustable azimuth settings of a retarder and analyzer (ARA) method. When undertaking the measurement of the Stokes parameters of light of an arbitrary wavelength over a wide wavelength range, the measurement of S3 by the conventional ARA method is dependent on the phase difference error ρ i of a quarter-wave plate mismatch as well as Stokes parameter S2. The measurement of S3 by a judicious choice of azimuth settings of a quarter-wave plate and a polarizer (JCAQP) as in the method proposed can be obtained by considering ρ I . The JCAQP method is clarified by employing the Poincaré sphere. It is shown that application of the JCAQP method yields the principal axis and the relative phase retardation of the birefringent plate free from the ρ i of the quarter-wave plate for incident elliptically polarized light of an arbitrary wavelength.

Voigt profile fitting to quasar absorption lines: an analytic approximation to the Voigt-Hjerting function

The Voigt-Hjerting function is fundamental in order to correctly model the profiles of absorption lines imprinted on the spectra of bright background sources by intervening absorbing systems. In this work, we present a simple analytic approximation to this function in the context of absorption-line profiles of intergalactic H i absorbers. Using basic calculus tools, we derive an analytic expression for the Voigt-Hjerting function that contains only fourth-order polynomial and Gaussian functions. In connection with the absorption coefficient of intergalactic neutral hydrogen, this approximation is suitable for modelling Voigt profiles with an accuracy of 10 -4 or better for an arbitrary wavelength baseline, for column densities up to N HI = 10 22 cm -2 , and for damping parameters a? 10 -4 , that is, the entire range of parameters characteristic to all Lyman transitions arising in a variety of H I absorbing systems such as Lyman a (Lya) forest clouds, Lyman limit systems and damped Lya systems. We hence present an approximation to the Voigt-Hjerting function that is both accurate and flexible to implement in various types of programming languages and machines, and with which Voigt profiles can be calculated in a reliable and very simple manner.

Frequency-dependent polarizabilities of alkali-metal atoms from ultraviolet through infrared spectral regions

We present results of first-principles calculations of the frequency-dependent polarizabilities of all alkali-metal atoms for light in the wavelength range 300--1600 nm, with particular attention to wavelengths of common infrared lasers. We parametrize our results so that they can be extended accurately to arbitrary wavelengths above 800 nm. This work is motivated by recent experiments involving simultaneous optical trapping of two different alkali-metal species. Our data can be used to predict the oscillation frequencies of optically trapped atoms, and particularly the ratios of frequencies of different species held in the same trap. We identify wavelengths at which two different alkali-metal atoms have the same oscillation frequency.

Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer

We report on a displacement metrology setup that provides sub-pm resolution in air. The setup is based on a Fabry-Perot cavity. However, unlike current Fabry-Perot cavity based displacement setups we incorporate a novel fs-laser based arbitrary wavelength synthesizer that provides efficient suppression of atmospheric disturbances while providing very wide and precise tuning of the output wavelength. The wavelength synthesizer provides sub-10 attometer wavelength resolution. The setup provides subpm length stability for integration times of up to one minute and sub-10 pm for up to half an hour without airtight enclosure of the Fabry-Perot cavities.

Drift wave driven zonal flows in plasmas

The generation of large-scale zonal flows by small-scale electrostatic drift waves in a plasma is considered. The generation mechanism is based on the parametric excitation of convective cells by finite amplitude drift waves. To describe this process a generalized Hasegawa–Mima equation containing both vector and scalar nonlinearities is used. The drift waves are supposed to have arbitrary wavelengths (as compared with the ion Larmor radius). A set of coupled equations describing the nonlinear interaction of drift waves and zonal flows is deduced. The generation of zonal flows turns out to be due to Reynolds stresses produced by finite amplitude drift waves. It is found that the wave vector of the fastest growing mode is perpendicular to that of the drift pump wave. Explicit expressions for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are obtained. A comparison with previous results is carried out. The present theory can be used for interpretations of drift wave observations in laboratory plasmas.

Automatically-Controlled C-Band Wavelength Conversion with Constant Output Power Based on Four-Wave Mixing in SOA's

We demonstrate the C-band wavelength conversion unit having functions of automatic wavelength recognition, power equalization, and elimination of original signal and pumping light for the first time, which is based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOA's). The constructed unit automatically detects signal wavelength, sweeps wavelength of a pumping light, and adjusts center wavelengths of band pass filters arid gain values of erbium-doped fiber amplifiers (EDFA's), in order to convert the wavelength of the signal to the arbitrary wavelength we set, and eliminate the original signal and pumping light after conversion. Amplification of the signal, pumping, and wavelength-converted lights compensates the detuning dependence of conversion efficiency and its asymmetry in the quantum-well (QW) SOA, to keep the power of the wavelength-converted light constant within the whole C-band region. The switching time of wavelength conversion by the unit is about a second, which is dominated by mechanical movement of the tunable filters. Wavelength-converted 2.5 and 10Gb/s NRZ signals show clear eye-openings when the detuning is positive (ω p > ω s ), and a 2-ps pulse train is also successfully wavelength-converted. To overcome the problem of the asymmetric conversion efficiency in the QW-SOA, we adopted quantum-dot (QD) SOA's. Although the 1,5μm QD-SOA still shows its asymmetry, which will be improved by optimization of quantum dot structure, wavelength conversion of a 160Gb/s RZ signal is demonstrated by the QD-SOA's. More improvement of the performance of the wavelength conversion unit should be possible by making switching time faster and installing the optimized QD-SOA's.

Slow nonlinear waves in magnetic flux tubes

A theory of weakly nonlinear slow waves in magnetic flux tubes is developed in the ideal MHD approximation. Fairly simple approximate dispersion relations are derived that are valid for waves of arbitrary wavelength. These dispersion relations make it possible to obtain a number of new model evolutionary equations for body and surface slow waves in magnetic flux tubes. It is established that there are two families of exact analytic solutions to the equations for weakly nonlinear slow waves. It is found that both the body and surface solitary waves can be in the form of either contractions or bulges running along the tube. A model Korteweg-de Vries-Burgers equation is derived and generalized to waves of arbitrary wavelength. It is shown that exact analytic solutions to these equations correspond to shock waves and hydraulic jumps (or bores) with nonoscillating fronts.

A tunable wavelength conversion and wavelength add/drop scheme based on cascaded second-order nonlinearity with double-pass configuration

A selective and tunable wavelength conversion and wavelength add/drop scheme based on sum- and difference-frequency generation (SFG+DFG) is proposed, in which the concept of "double-pass" is introduced. An arbitrary channel can be dropped from a wavelength division multiplexing (WDM) signal and added to another WDM signal at arbitrary wavelength. The channel to be dropped is selected and depleted (dropped) by adjusting the pump 1 through sum frequency generation (SFG) during the forward propagation. Subsequently, the difference frequency generation (DFG), taking place during the backward propagation, is employed to convert (add) the dropped channel to another channel in another WDM signal by adjusting the pump 2. For the dropped and added channel, the phase matching of SFG and DFG are nearly perfect and the theoretical expressions are derived under the assumption that the two pumps are undepleted. The power of pump 1 is optimized to deplete the dropped channel completely, while that of pump 2 is chosen to maximize the output power of the added channel. Numerical calculations are performed to investigate the propagation of the other channels whose phase is mismatched. To suppress the crosstalk, the spacing of the WDM channels is chosen to be 0.2 nm (25 GHz). We have also compared our scheme with others (such as the single-pass scheme and the double waveguide scheme) and shown that ours possesses several distinct advantages.

Proposal of all-optical wavelength-selective switching using waveguide-type Raman amplifiers and 3-dB couplers

We propose a new optical switch that can enable wavelength-selective high-speed switching for applications to photonic wavelength-division multiplexing (WDM) Internet protocol (IP) packet routing. The switch utilizes characteristics of waveguide-type Raman amplifiers and 3-dB couplers, and so it operates all-optically at a few tens of picosecond or faster speed. The operation of the switch is studied by coupled-mode theory and is verified numerically using a beam-propagation method (BPM). As a result, we found that optical waves having arbitrary wavelengths among those of incident WDM waves are amplified and switched when the amplification rate of each waveguide-type Raman amplifier is set to the proper value by pump wave injection. Moreover, the dynamic range over 30 dB was also obtained from the simulation results of the switch.

Fast measurements of solar spectral UV irradiance—first performance results of two novel spectroradiometers

Short spectral scan times of preferably less than 1 min in the UV region are an important prerequisite for modern spectroradiometers to reflect short-term solar irradiance radiation variations that can occur as a result of fast changes in cloud cover and/or cloud optical depth. Two different types of fast measuring spectroradiometers for solar UV irradiance are compared in a first field campaign: (1) the UV spectroradiometer on filter model basis (UV-SPRAFIMO) and (2) the modified version of the spectroradiometer SPECTRO 320D by Instrument Systems. The UV-SPRAFIMO instrument combines a filter radiometer with five narrowband (FWHM2.0 to 2.7 nm) filters centered at fixed wavelengths in the UV-B and UV-A regions with an advanced neural network-based model. Up to 5 measurements/s can be taken concurrently in the five wavelength channels. After averaging the measurements over preselectable time intervals, the measured irradiances are converted by the neural network model into a full spectrum from 280 to 450 nm at arbitrary wavelength steps (0.05 nm). The SPECTRO 320D spectroradiometer consists of a grating double monochromator with a cooled photomultiplier tube (PMT) receiver. The instrument version run by Deutscher Wetterdienst (DWD) is thermostatted and equipped with a Schreder type cosine diffuser as the entrance optics. A spectral scan from 290 to 450 nm with a selected 0.2-nm wavelength step takes less than 30 s. The two spectroradiometers are used in a field campaign at Izana (Tenerife Island) at a height of 2440 m above sea level (ASL) to compare measured spectral and integral values of solar irradiance. Results of that comparison and the instruments' characteristics are discussed. This first field comparison shows that due to the fast measurements regime, cloud effects on the measured spectra can be appreciably reduced. The campaign shows an acceptable agreement between the spectra measured by both instruments. It also reveals some issues for further improvements of the instrument design.

Nanogold plasmon resonance-based glucose sensing. 2. Wavelength-ratiometric resonance light scattering.

Gold colloids are well known to display strong plasmon absorption bands due to electron oscillations induced by the incident light. When the colloids are in proximity, the plasmon absorption bands are often perturbed. This has enabled us recently to successfully develop a glucose sensing platform based on the disassociation of dextran-coated gold colloids, cross-linked with Con A, by glucose. However, a much more useful and simpler property of gold colloids, which has been ill explored with regard to sensing, is their ability to efficiently scatter excitation light. We have found that our nanogold sensing aggregates are indeed efficient light scatters around the nanogold plasmon absorption band. By measuring the ratio of scattered light intensities at two different arbitrary wavelengths, 560 and 680 nm, glucose concentrations can be readily determined from a few millimolar up to approximately 60 mM, using a simple white light LED and detection system. Further, by measuring the ratio of the scattered intensities, this sensing approach is independent of the total sensing aggregate concentration and the excitation and detection instrumentation fluctuations or drifts. This simplistic and low-cost approach to glucose sensing, coupled with the sensing aggreagates' ability to scatter red light, suggests the potential use of these aggregates for use in physiological transdermal glucose monitoring, either for implantable skin sensors or glucose sensing tattoos (discussed later).