Direct measurement of dispersion of the group refractive indices of quartz crystal by white-light spectral interferometry

We present a white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the dispersion of the ordinary and extraordinary group refractive indices of a quartz crystal over the wavelength range approximately from 480 to 860 nm. The technique utilizes a dispersive Michelson interferometer with the quartz crystal of known thickness to record a series of spectral interferograms and to measure the equalization wavelength as a function of the displacement of the interferometer mirror from the reference position, which corresponds to a balanced non-dispersive Michelson interferometer. We confirm that the measured group dispersion agrees well with that described by the dispersion equation proposed by Ghosh. We also show that the measured mirror displacement depends, in accordance with the theory, linearly on the theoretical group refractive index and that the slope of the corresponding straight line gives precisely the thickness of the quartz crystal.     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

Distributed measurement of chromatic dispersion by four-wave mixing and Brillouin optical-time-domain analysis

A new nondestructive method for measuring the spatial distribution of chromatic dispersion along an optical fiber is presented. It is based on using Brillouin optical time-domain analysis to probe the power distribution of the four-wave mixing generated by two continuous-wave lasers. The results obtained prove that this new method is capable of providing better performance than comparable techniques. Furthermore, sensing the variations of Brillouin gain maximum produces additional information about the fiber, such as presence of strain and concentration of GeO2.     

First measurement of a phonon dispersion curve by inelastic X-ray scattering

Inelastic scattering of 13.8 keV X-rays with very high energy resolution of E=55 meV was used to measure the phonon dispersion curves for theLA andLO modes in the [00] direction in Be. The results agree with inelastic neutron scattering data known from the literature. The X-ray scattering intensities of the phonon excitations for different momentum transfers are in very good agreement with the prediction from the scattering law.     

Direct ion-transport measurement by optical tagging

Optical pumping of long-lived quantum states is used to identify groups of ions in phase space and follow their transport. The principles and variants of a novel diagnostic method, as well as new physical aspects and applications, including space-dependent line narrowing, are demonstrated.     

Chromatic dispersion characterization by direct instantaneous frequency measurement

A new concept for the measurement of chromatic dispersion is presented. The group delay of the device under test is directly obtained from the instantaneous frequency of a short optical pulse after propagation in the device, itself obtained with temporal and spectral interferometry. Temporal and spectral fringe encoding provide excellent accuracy and resistance to noise, and a wide variety of dispersions can be characterized even at high insertion losses.     

Direct measurement of a photoconductive receiver's temporal response by dithered-edge sampling

We present the results of a direct measurement of the temporal response of a terahertz (THz) photoconductive receiver obtained by dithered-edge sampling. The receiver response has structure that accounts for the negative-going leading edge of the pulse shape that is often seen in measurements made with these receivers in a conventional sampling arrangement. We show that the THz pulse shape measured by conventional photoconductive sampling is indeed a cross correlation of the pulse with the measured receiver's response.     

Time-domain multimode dispersion measurement in a higher-order-mode fiber

We present a new multimode dispersion measurement technique based on the time-of-flight method. The modal delay and group velocity dispersion of all excited modes in a few-mode fiber can be measured simultaneously by a tunable pulsed laser and a high speed sampling oscilloscope. A newly designed higher-order-mode fiber with large anomalous dispersion in the LP(02) mode has been characterized using this method, and experimental results are in good agreement with the designed dispersion values. The demonstrated technique is significantly simpler to implement than the existing frequency-domain or interferometry-based methods.     

Direct measurement of dipole-dipole/CSA cross-correlated relaxation by a constant-time experiment

Relaxation rates in NMR are usually measured by intensity modulation as a function of a relaxation delay during which the relaxation mechanism of interest is effective. Other mechanisms are often suppressed during the relaxation delay by pulse sequences which eliminate their effects, or cancel their effects when two data sets with appropriate combinations of relaxation rate effects are added. Cross-correlated relaxation (CCR) involving dipole-dipole and CSA interactions differ from auto-correlated relaxation (ACR) in that the signs of contributions can be changed by inverting the state of one spin involved in the dipole-dipole interaction. This property has been exploited previously using CPMG sequences to refocus CCR while ACR evolves. Here we report a new pulse scheme that instead eliminates intensity modulation by ACR and thus allows direct measurement of CCR. The sequence uses a constant time relaxation period for which the contribution of ACR does not change. An inversion pulse is applied at various points in the sequence to effect a decay that depends on CCR only. A 2-D experiment is also described in which chemical shift evolution in the indirect dimension can share the same constant period. This improves sensitivity by avoiding the addition of a separate indirect dimension acquisition time. We illustrate the measurement of residue specific CCR rates on the non-myristoylated yeast ARF1 protein and compare the results to those obtained following the conventional method of measuring the decay rates of the slow and fast-relaxing (15)N doublets. The performances of the two methods are also quantitatively evaluated by simulation. The analysis shows that the shared constant-time CCR (SCT-CCR) method significantly improves sensitivity.     

Direct measurement of the barycentric velocity of a plasma by time of flight

A simple apparatus is described which, when coupled to a mass spectrometer, enables measurement of the barycentric velocity of a plasma by time of flight. The results are discussed from experimental and theoretical standpoints and the absence of perturbation is proved.     

Direct observation of optical precursors in a region of anomalous dispersion

We create optical precursors by propagating a step-modulated optical pulse through a linear resonant dielectric absorber. The field emerging from the dielectric consists of a several-nanosecond-long spike with near 100% transmission, which decays to a constant value expected from Beer's law. This high-transmission spike might be useful for imaging applications requiring penetrating optical radiation. We compare our observations to two different theories, revealing that the spike consists of both the Sommerfeld and Brillouin precursors.     

Direct measurement of entropic forces induced by rigid rods

We present the first direct depletion potential measurements for a single colloidal sphere close to a wall in a suspension of rigid colloidal rods. Since all particle interactions are as good as hard-core-like, the depletion potentials are of entirely entropic origin. We developed a density functional theory approach that accurately accounts for this experimental situation. The depletion potentials calculated for different rod number densities are in very good quantitative agreement with the experimental results.     

Direct measurement of valence-charge asymmetry by x-ray standing waves

By monitoring valence-photoelectron emission under condition of strong x-ray Bragg reflection, we have determined that a majority of GaAs valence charge resides on the anion sites of this heteropolar crystal, in quantitative agreement with the GaAs bond polarity as calculated from the Hartree-Fock term values. In contrast, the valence-charge distribution in Ge is found to be symmetric. In both cases, the valence emission is found to be closely coupled to the atomic cores.     

Direct mapping of phonon dispersion relations in copper by momentum-resolved x-ray calorimetry

We have developed a new method of mapping phonon dispersion relations based on momentum-resolved x-ray calorimetry. X-ray scattering intensities are measured at selected points in reciprocal space with suitably chosen polarization configurations; the thermal part of the scattering intensity is extracted by scanning the temperature of the sample. The intensity variations, governed by the phonon populations, are analyzed to yield the energies of the phonons. This method is applied to copper. With high-order effects under control, the results are in excellent agreement with the known phonon dispersion relations.     

Non-contact scanning measurement utilizing a space mapping method

In this study, a novel approach to a measuring methodology and calibration method for an optical non-contact scanning probe system is proposed and verified by experiments. The optical probe consists of a line laser diode and two charge-coupled device (CCD) cameras and is placed on a computer numerical control (CNC) machine to measure the workpiece profiles. A space mapping method using the least-squares algorithm is presented for the probe calibration and profile measurement. This method provides a simple and accurate calculation of the relationship between the real space plane and its related image space plane in a CCD camera. A transparent grid with regularly spaced nodal points is used to construct the space mapping function. The space coordinate of an object can be obtained from its image in the CCD camera via the mapping function. The measured profile data are smoothed by the B-spline blending function and can be transferred to a CAD/CAM package for industrial applications. Experimental results show that this technique can determine the 3-D profile of an object with an accuracy of 60 μm.     

Direct experimental determination of the dispersion of a final state energy band

We report the first absolute and continous experimental determination of the dispersion of an unoccupied electron energy band in copper. This has been achieved by monitoring the extremal behaviour of binding energies in angular resolved photoelectron energy distribution curves obtained with a tunable light source. By this method the absolute value of the wave vector for transitions originating from the symmetry line ∑ was determined. The experiments confirm that the final state band deviates strongly from a parabola.