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A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions between minimum and maximum are instantaneous. The square wave is a special case of a pulse wave which allows arbitrary durations at minimum and maximum amplitudes. The ratio of the high period to the total period of a pulse wave is called the duty cycle. A true square wave has a 50% duty cycle (equal high and low periods). Square waves are often encountered in electronics and signal processing, particularly digital electronics and digital signal processing. Its stochastic counterpart is a two-state trajectory. Origin and uses Square waves are universally encountered in digital switching circuits and are naturally generated by binary (two-level) logic devices. They are used as timing references or "clock signals", because their fast

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Automatic procedure for aberrations compensation in digital holographic microscopy

Nicolas Aspert, Florian Charrière, Tristan Colomb, Etienne Cuche, Christian Depeursinge, Jonas Kühn, Mihaela Marian, Pierre Marquet, Frédéric Montfort

Digital Holographic Microscopy (DHM) is a powerful imaging technique allowing, from a single amplitude image (amplitude and phase), reflected acquisition (hologram), the reconstruction of the entire complex wave front by or transmitted through an object. Because holography is an interferometric technique, the reconstructed phase leads to a sub-wavelength axial accuracy (below lambda/100). Nevertheless, this accuracy is difficult to obtain from a single hologram. Indeed, the reconstruction process consisting to process the hologram with a digital reference wave (similar to classical holographic reconstruction) seems to need a-priori knowledge about the physical values of the setup. Furthermore, the introduction of a microscope objective (MO), used to improve the lateral resolution, introduces a wave front curvature in the object wave front. Finally, the optics of the set-up can introduce different aberrations that decrease the quality and the accuracy of the phase images. We propose here an automatic procedure allowing the adjustment of the physical values and the compensation for the phase aberrations. The method is based on the extraction of reconstructed phase values, along line profiles, located on or around the sample, in assumed to be flat area, and which serve as reference surfaces. The phase reconstruction parameters are then automatically adjusted by applying curve-fitting procedures on the extracted phase profiles. An example of a mirror and a USAF test target recorded with high order aberrations (introduced by a thick tilted plate placed in the set- up) shows that our procedure reduces the phase standard deviation from 45 degrees to 5 degrees.

SPIE2006

DC-voltage and cycling induced recovery of switched polarisation in fatigued ferroelectric thin films

Enrico Colla, Nava Setter, Alexander Tagantsev

We report results on electric DC-field induced and self induced recovery of the switched remanent polarisation (P-r(s)) in fatigued ferroelectric thin films. The field induced recovery is characterised by a strong asymmetry and depends on the ratio between the fatiguing and DC-field magnitudes. It is also dependent on the period of application of the DC-field. The self recovery effect occurs on further cycling beyond the fatigued state. It is dependent on the fatiguing field magnitude and also, if the cycling is performed with a square wave function, on the switching frequency. For both effects we propose a tentative explanation based on the pinning of ferroelectric domain walls by negatively charged defects close to the top electrode and on the annihilation of those defects by positively charged defects, the nature of which remains to be disclosed.

1995

Overvoltages induced by lightning on overhead power and distribution lines with sag

José-Luis Bermudez, Michel Ianoz, Marcos Rubinstein

Researchers working on the problem of the coupling of lightning electromagnetic fields to overhead lines have suggested that, if the formulation of the coupling problem given by Agrawal et al. is adopted, only the horizontal component of the electric field drives current in the direction of the line and that the vertical component induces current only at the vertical terminations. All of these studies assume that the wires are perfectly horizontal. In practice, however, the wires exhibit sag. A theoretical study of the effect of the sag in overhead conductors has been presented by Rubinstein but no experimental validation is available in the literature. In this paper, we report on the experimental test of the theory presented by Rubinstein. To carry out the test, an EMP (electromagnetic pulse) simulator and a model line were used. Measured and calculated waveforms are in reasonable agreement in amplitude and in wave shape, although the wave shape agreement worsens for larger amounts of sag. It is shown that the curvature of the wires results in a reduction of lightning-induced voltage magnitudes. Therefore, one can treat the perfectly horizontal wire case as a worst case

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Sawtooth wave

The sawtooth wave (or saw wave) is a kind of non-sinusoidal waveform. It is so named based on its resemblance to the teeth of a plain-toothed saw with a zero rake angle. A single sawtooth, or an in

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A pulse wave or pulse train is a type of non-sinusoidal waveform that includes square waves (duty cycle of 50%) and similarly periodic but asymmetrical waves (duty cycles other than 50%). It is a t

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The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a referenc

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