Dispersion Measurement Technique Research Articles

Polarization-mode dispersion measurements in high-birefringence fibers by means of stimulated Raman scattering.

A convenient technique for polarization-mode dispersion measurements in short lengths of high-birefringence fibers is reported. The technique is based on spectral interferometry with a frequency-doubled Nd:YAG laser, which is frequency shifted and broadened by self-stimulated Raman scattering in an optical fiber. The different Raman Stokes beams permit accurate measurements over a 40-nm wavelength range in the visible spectrum.

Non-linear waveform inversion for surface waves with a neighbourhood algorithm-application to multimode dispersion measurements

SUMMARY A new technique for multimode dispersion measurement has been developed by employing fully non-linear waveform inversion for a path-specific 1-D profile using a neighbourhood algorithm (NA). One-dimensional models derived from waveform inversion are quite sensitive to the model parametrization and the reference model used to start the inversion. With different approaches to the parametrization of the shear wave speed profile, we can find models with significant differences in velocity variation with depth, which provide similar levels of fit to the observed waveforms. Although the models differ, the calculated phase dispersion for the first few modes of the surface waves are very close indeed. We therefore regard the 1-D models derived from the multimode waveform inversion as an implicit description of the path-specific dispersion for each of the modes. The inversion procedure using NA samples a substantial number of models and we select the 1-D wave speed model that achieves minimum misfit. Phase speeds for each mode branch are then calculated from the model parameters with the minimum misfit. Because we use the 1-D models as a summary of the average multimode dispersion along the path, we do not need to make assumptions concerning the nature of anisotropy. We can employ simple isotropic models and the method can be applied to both Rayleigh and Love waves independently. The new method is applied to sets of paths in the Australian region and provides stable measurements of multimode dispersion from a single seismogram. The multimode phase speeds measured using the NA inversion can be used to retrieve phase velocity maps as a function of frequency for higher-mode branches as well as the fundamental mode, which will be crucial constraints on 3-D Earth models.

Novel technique for group velocity dispersion measurements in optical fibers

A novel technique of measuring group velocity dispersion (GVD) in optical fibers is introduced. The technique utilizes stimulated Raman scattering to couple small modulations from a pump beam to a signal beam. By measuring the coupling efficiency, a numerical value for the group delay difference between both beams is derived. Furthermore, we calculate the GVD from the measured data and show the good agreement with a conventional measurement technique.

The impact of rigidity and water exchange on the relaxivity of a dendritic MRI contrast agent.

Variable-temperature, multiple magnetic field (17)O NMR, EPR and variable-temperature (1)H nuclear magnetic relaxation dispersion (NMRD) measurement techniques have been applied to Gadomer 17, a new dendritic contrast agent for magnetic resonance imaging. The macromolecule bears 24 Gd(dota)-monoamide chelates (dota=N,N',N",N"'-tetracarboxymethyl-1,4,7,10-tetraazacyclododecane) attached to a lysine-based dendrimer. (17)O NMR and (1)H NMRD data were analysed simultaneously by incorporating the Lipari-Szabó approach for the description of rotational dynamics. The water exchange rate k(298)(ex)was found to be (1.0 +/- 0.1) x 10(6) s(-1), a value similar to those measured for other Gd(dota)-monoamide complexes, and the activation parameters DeltaH++ =24.7 +/- 1.3 kJ mol(-1) and DeltaS++ = -47.4 +/- 0.2 JK(-1) mol(-1). The internal flexibility of the macromolecule is characterised by the Lipari-Szabó order parameter S(2)=0.5 and a local rotational correlation time tau(298)(l)= 760 ps, whereas the global rotational correlation time of the dendrimer is much longer, tau(298)(g)=3050 ps. The analysis of proton relaxivities reveals that, beside slow water exchange, internal flexibility is an important limiting factor for imaging magnetic fields. Electronic relaxation, though faster than in similar, but monomeric, Gd(III) chelates, does not limit proton relaxivity of this contrast agent (r(1)=16.5mM(-1)s(-1) at 298 K and 20 MHz). This analysis provides direct clues for the design of high-efficiency contrast agents.

New practical dispersion measurement technique for Bragg grating based MZI-OADM

A new technique is presented to accurately determine the mean dispersion of Bragg gratings written in Mach-Zehnder interferometer arms. It could be used to control the dispersion of a Bragg grating based optical add-drop multiplexer. The letter will illustrate the preliminary experimental results.

Effect of self-phase modulation on group-velocity dispersion measurement technique using PM-AM conversion

A report is made on the effects of self-phase modulation (SPM) on the group-velocity dispersion measurement technique using the conversion of phase-modulated signal into the amplitude-modulated signals (i.e. PM-AM conversion technique). The results show that the modulation frequency should be in the range 0.3–1 GHz to suppress the SPM-induced errors while maintaining reasonable PM-AM conversion efficiency.

Interferometric technique for measuring dispersion of high order modes in optical fibres

An interferometric technique for chromatic dispersion measurement of higher modes in short lengths of high order mode fibres is presented. The dispersion is measured using the wavelength dependence of the interference signal between the basic mode and the high order mode. Neither mode transformers nor RF circuitry are required.

Improving the accuracy of the wavelength-scanning technique for PMD measurements

An improvement of the wavelength-scanning polarization mode dispersion (PMD) measurement technique is presented. It consists in the evaluation of the level crossing density for three or more levels, instead of only one. Experimental results show that measurement uncertainty can be reduced more than 40%.

Measurement of second-order polarization-mode dispersion vectors in optical fibers

A polarization-mode dispersion (PMD) measurement technique is described that allows the determination of second- and higher order PMD vectors in optical fibers. The algorithm, based on Muller matrices, requires the launch of only two polarizations per wavelength and uses large rotation angles as well as interleaving to attain low-noise high-resolution PMD data. It has been applied to fibers ranging from 2 to 40 ps in mean PMD.

Spectrally resolved white-light interferometry for measurement of ocular dispersion.

Spectrally resolved white-light interferometry was used to measure the wavelength dependence of refractive index (i.e., dispersion) for various ocular components. Verification of the technique's efficacy was substantiated by accurate measurement of the dispersive properties of water and fused silica, which have both been well-characterized in the past by single-wavelength measurement of the refractive index. The dispersion of bovine and rabbit aqueous and vitreous humors was measured from 400 to 1100 nm. In addition, the dispersion was measured from 400 to 700 nm for aqueous and vitreous humors extracted from goat and rhesus monkey eyes. An unsuccessful attempt was also made to use the technique for dispersion measurement of bovine cornea and lens. The principles of white-light interferometry, including image analysis, measurement accuracy, and limitations of the technique, are discussed. In addition, alternate techniques and previous measurements of ocular dispersion are reviewed.

Fast Time-Resolved Magnetic Optical Rotatory Dispersion Measurements. 1. Mueller Analysis of Optical and Photoselection-Induced Artifacts

A simple and sensitive near-null technique for multichannel nanosecond time-resolved magnetic optical rotatory dispersion measurements is introduced with a theoretical analysis describing the basis of the technique and the effects of optical imperfections, photoselection-induced linear dichroism and birefringence, and the Faraday rotation of the solvent and cell windows. This treatment identifies a potential artifact in photolyzed samples that is associated with the coupling of photoselection-induced linear dichroism with the solvent−cell Faraday rotation. Excitation geometries that minimize this problem are described. Experimental applications of the technique to the ligand rebinding reaction of myoglobin−CO after ligand photolysis are presented in the following article.

Nanosecond optical rotatory dispersion spectroscopy: application to photolyzed hemoglobin-CO kinetics

A standard technique for static optical rotatory dispersion (ORD) measurements is adapted to the measurement of ORD changes on a nanosecond (ns) time scale, giving approximately a million-fold improvement in time-resolution over conventional instrumentation. The technique described here is similar in principle to a technique recently developed for ns time-resolved circular dichroism (TRCD) spectroscopy, although the time-resolved optical rotatory dispersion (TRORD) technique requires fewer optical components. As with static ORD, TRORD measurements may be interpreted by empirical comparisons or may be transformed, via the Kramers-Kronig relations, to more easily interpreted TRCD spectra. TRORD can offer experimental advantages over TRCD in studying kinetic processes effecting changes in the chiral structures of biological molecules. In particular, the wider dispersion of ORD bands compared with the corresponding CD bands means that ORD information may often be obtained outside of absorption bands, a signal-to-noise advantage for multichannel measurements. Demonstration of the technique by its application to ns TRORD and the transform-calculated TRCD of carboxy-hemoglobin (Hb-CO) after laser photolysis is presented.

Chirped-pulse amplification of 85-fs pulses at 250 kHz with third-order dispersion compensation by use of holographic transmission gratings

We demonstrate pulse stretching and compression in a high-repetition-rate chirped-pulse Ti:sapphire regenerative amplifier, using high-efficiency holographic transmission gratings. A quantitative dispersion measurement technique is developed to characterize dispersion of the system to the third order. After recompression with third-order dispersion compensation, 3.1-microJ 85-fs, nearly transform-limited pulses are obtained.

Simple dispersion measurement technique with high resolution

A new highly accurate dispersion measurement technique is presented, which uses a standard lightwave component analyser. By employing a tunable external laser source the dispersion is measured over a wavelength range of 70 nm with an accuracy of 0.02 ps/km/nm, which enables us to observe dispersion fluctuations and substructures on the dispersion curve.

Femtosecond pulse generation in a Ti:Al_2O_3 laser by using second- and third-order intracavity dispersion

We investigate the effects of third-order dispersion on pulse width by using a femtosecond Ti:A1(2)O(3) laser with independently adjustable second- and third-order intracavity dispersion compensation. A novel technique for compensating third-order dispersion is demonstrated using Gires-Tournois interferometers that are fabricated monolithically by using multilayer dielectric films. With the addition of intracavity third-order compensation, pulse-width reduction from 45 to 28 fs is achieved. The dispersion compensation effect produced by the Gires-Tournois interferometers is measured in situ using frequency-domain dispersion measurement techniques.

Waveguide dispersion measurement technique for single-mode fibers using wavelength dependence of mode field radius

A waveguide dispersion measurement technique using a simple fitting function for wavelength dependence of the root-mean-square width in the far field and its theoretical measurement error are discussed. Results indicate that waveguide dispersion can be evaluated with theoretical accuracy of 0.5 ps/km/nm using this technique. The waveguide dispersions for conventional and dispersion-shifted fibers have been measured using the far-field pattern method. Good agreement between measured and theoretical values of waveguide dispersion was obtained. Material dispersion and dispersion sensitivity evaluation methods that are applications of this technique are also described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A novel technique for polarisation mode dispersion measurements in optical fibres

A novel technique for determining the polarisation mode dispersion in optical fibres is described. The technique makes use of a sinusoidally frequency modulated source, and is applied to the measurement of the beat length of highly birefringent monomode fibre. The temporal delay between the two modes of the fibre is measured with a resolution of approximately ±0.6 ps.

Modulation frequency-shift technique for dispersion measurements in optical fibres using LEDs

Reliable phase-shift measurements in optical fibres were performed by downshifting the modulation frequency of the optical signal to the low-frequency range before detection. This technique maintains the full resolution of the high-frequency modulation and increases the sensitivity allowing accurate group delay measurements in single-mode fibres over a 300 nm spectral range using a single LED.

Optical fiber dispersion measurement technique using a streak camera

A novel optical fiber dispersion measurement technique using a streak camera is presented. The feature of the technique is that the relation of the group delay versus wavelength can be seen instantaneously without changing wavelength one after another. It is also presented that polarization mode dispersion and chromatic dispersion can be simultaneously obtained by using the technique.

Comparison of single-mode fiber dispersion measurement techniques

This paper describes state-of-the-art techniques for characterizing dispersion in single-mode fibers. Special emphasis is placed on achieving high measurement accuracy within the 1.1-1.7 \mu m wavelength region.