Phase Modulation Patterns Research Articles

The Roles of CPG Phase Modulation and Reflexive Muscular Patterns in Balance Recovery During Walking—A Simulation Study

Most walking assist systems reported are not available for real world environments where frequent perturbations are caused by slips, uneven terrain, slopes and obstacles. It is evident that humans are able to cope with such perturbations with reflexes that cause unconscious, relatively fixed muscular response patterns to perturbations within a short period of time. In our previous study, we showed that artificial reflexes could improve the perturbation resistance for simulated walkers, though the roles of different reflexive mechanisms were not quantitatively clarified. In this study, we focused on the different roles of reflexive muscle responses and the CPG phase modulation mechanism. By proposing and evaluating two stability criteria through a series of simulation experiments, we revealed different roles for two mechanisms in the simulated walkers. These will not only further increase the possibility of realising artificial reflexes for paralysed individuals, but also bring new insights into the field of motor control.

Femtosecond time-resolved zero kinetic energy photoelectron and photoionization spectroscopy studies of I2 wavepacket dynamics

We report the application of femtosecond zero kinetic energy (ZEKE) photoelectron spectroscopy and photoionization to the study of bound wavepacket motion in the electronic B state of iodine by a [1 + 2] multiphoton process. The experimental approach is described in detail. Comparisons of ZEKE photoelectron detection with ion detection are made. The pump wavelength was fixed at 580 nm whereas the probe wavelength was tuned from 350 to 300 nm. Wavepacket motion in the B state with the fundamental vibrational period of 340 fs was observed, as well as wavepacket dephasing and rephasing. Differences in the modulation depths and Fourier transform power spectra were observed. The phase shifts (time offset from Δt = 0) and modulation patterns varied as the location of the Condon point for the probe step changed upon tuning the probe wavelength. Higher-order wavepacket fractional revivals were observed in time-windowed Fourier transform power spectra.

Doped polymer films with high nonlinear refractiveindices

Nonlinear refractive indices n2 of two push-pull azobenzene doped polymer materials have been measured using the single beam Z-scan method. The value of sample B is as high as 1.7 × 10-6 cm2/W, and is nearly two orders of magnitude larger than that of sample A. The self phase modulation pattern of sample B has been observed using very low incident laser intensity. The origins of the large nonlinear optical response of the two doped polymer materials are discussed.

Alternating-coded multipulse codes for incoherent scatter experiments

A possibility of using strong alternating codes as a phase modulation pattern in multipulse incoherent scatter experiments is investigated. It is shown that such a modulation leads to a high range resolution much in the same way as the use of Barker codes does. The suggested modulation gives two types of data, one with a range resolution corresponding to the pulse length in the pulse code and the other with that corresponding to the bit length in the alternating code. Most of the efficiency of the experiment goes to high resolution data; when long alternating codes are used, the proportion of low resolution data is negligible. Practical examples using this idea are constructed and their speeds are compared with those obtained using Barker codes. The comparison is made using the GUISDAP package; hence this paper also demonstrates the capabilities of GUISDAP in planning the experiments. It is found that an alternating-coded pulse code is approximately as fast as a Barker-coded pulse code and its speed is about one third of that of a Barker-coded alternating code. Barker-coded pulse codes have small range ambiguities within a wide altitude range, which make the fit residual depart from the theoretical value of unity in the least squares parameter fits and put an upper limit to the working range of the experiment. These limitations can only partly be removed by the range ambiguity correction. The suggested modulation has the benefit that it has no range ambiguities. As a result, no systematic errors should occur and the experiment is expected to work at greater heights. Although Barker-coded alternating codes also have range ambiguities, they are always situated close to the main peak of the range ambiguity function and are therefore much less harmful. The suggested modulation is not expected to surpass Barker-coded alternating codes, except perhaps in the presence of intense sporadic E layers.

Additive–subtractive phase-modulated electronic speckle interferometry: analysis of fringe visibility

Fringe-visibility issues of additive-subtractive phase-modulated (ASPM) electronic speckle pattern interferometry (ESPI) are explored. ASPM ESPI is a three-step method in which additive-speckle images are acquired rapidly in an analog fashion in every frame of a video sequence, a speckle phase modulation is intentionally introduced between frames, and a digital subtraction of consecutive pairs of additive-speckle images is performed. We show that this scheme has the good high-frequency noise immunity associated with additive-ESPI techniques as well as the good fringe visibility associated with subtractive-ESPI techniques. The method has better fringe visibility than can be obtained with purely additive ESPI and also does not suffer from the fringe distortions that can occur with subtractive ESPI in the presence of high-frequency noise. We show that even if full speckle decorrelation were to occur between the two additive speckle images that are to be subtracted, the visibility of ASPM ESPI fringes can be made to approach unity by suitable adjustment of the reference-to-object beam-intensity ratio.

Phase modulation and far-field spatial patterns due to the transformational thermal-lens effect.

In earlier work we have reported a thermal-lens effect that is opposite in sign from and much stronger than the well-known effect in fluids and isotropic solids: Very near the transformation temperature in crystals undergoing structural phase transitions, certain indices of refraction have temperature derivatives that are large and positive (\ensuremath{\simeq}+${10}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$), in contrast to the negative values of dn/dT that arise from thermal expansion in glasses or fluids. We evaluated the steady-state and time-dependent behavior of these effects in ${\mathrm{Ba}}_{2}$${\mathrm{NaNb}}_{5}$${\mathrm{O}}_{15}$ near ${\mathit{T}}_{\mathit{c}}$=850 K. In the present paper we extend these studies to examine the far-field spatial pattern produced by self-induced phase modulation. We also utilize the thermal-lens effect to investigate the critical behavior near the tricritical point of ${\mathrm{Ba}}_{2}$${\mathrm{NaNb}}_{5}$${\mathrm{O}}_{15}$, and the critical exponent \ensuremath{\beta} is obtained as ${\mathrm{\ensuremath{\beta}}}_{\mathit{c}}$=0.28\ifmmode\pm\else\textpm\fi{}0.01 and ${\mathrm{\ensuremath{\beta}}}_{\mathit{a}}$=0.31\ifmmode\pm\else\textpm\fi{}0.01.

Automatic profilometry of 3-D diffuse objects by spatial phase detection

A method automatically processing a projected grating to profile a 3-D diffuse object is proposed. A deformed grating pattern projected on the object is regarded as a phase modulated pattern with a constant spatial carrier frequency. To retrieve phase modulation, acquired data in a microcomputer are sinusoidally fitted using a phase detection algorithm similar to that used in communication techniques. High sensitivity measurements of height distribution can be done using simple optical geometry. Random and systematic errors inherent in the method are discussed in detail.