Michel Aillerie

Band structure treatment of the influence of nonstoichiometric defects on optical properties in LiNbO3

We present a band structure approach in connection with a molecular dynamics cluster optimization that accounts for various structural modifications related to the nonstoichiometry of LiNbO3 crystals. Variation of optical properties with the deviation from the stoichiometric composition can be understood within this approach. The particular role of the electron-phonon contributions to the electro-optic coefficient is shown. Model calculations yield a large dependence of the electro-optic coefficient r22 on the crystal composition, in agreement with the experimental data. The observed minimum of the r22 coefficient versus nonstochiometry is explained within a microscopic approach. The r33 tensor component shows substantially different behavior versus the stoichiometry.

Influence of the temperature‐dependent spontaneous birefringence in the electro‐optic measurements of LiNbO3

In spite of the large utilization of LiNbO3 in optical devices, its electro‐optic coefficients are not perfectly known. We show in the present paper that not taking into account the temperature dependence of the linear spontaneous birefringence can lead to an erroneous determination of the electro‐optic coefficients. The electro‐optic coefficient rc of LiNbO3 has been determined by an accurate technique issued from the Senarmont method. At room temperature, rc=19.9×10−12 m/V, with an estimated error of ±5%. The measurements are performed for values of the dc applied electric field as a function of the temperature around room temperature. We point out the drastic influence on the electro‐optic response of the spontaneous birefringence variation due to the temperature. Accurate and reproducible values of the electro‐optic coefficients are obtained only if this dependence is taken into account.

Optical damage resistance in undoped LiNbO3 crystals

Abstract Photo-induced birefringence changes are investigated on several undoped LiNbO 3 crystals as a function of laser-irradiation time and intensity. Comparison points out that the changes are the smallest in the crystals grown from high temperature top seeded solution (TSS). It is concluded that the optical damage resistance in undoped LiNbO 3 crystals is increased with a decrease of the amount of intrinsic defects related to the non-stoichiometry. The necessity to carefully control the true crystal composition is stressed.

Electro-optic properties in Fe-doped LiNbO3 crystals as a function of composition

Abstract We report the electro-optic properties in iron-doped LiNbO 3 crystals. We investigated samples with various crystal compositions and the same doping concentration. The value of the electro-optic coefficient r c monotonously decreases with respect to the crystal composition. To separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of iron, we compared the results with data obtained in pure LiNbO 3 crystals. The effect of Fe ions on the composition of the crystal and the electro-optic properties is discussed from their site occupation in LiNbO 3 lattice.

A frequency doubling electro-optic modulation system for Pockels effect measurements: Application in LiNbO3

A novel system is presented which is capable of measuring with high accuracy the linear (Pockels) electro-optic effect by means of a new dynamic (ac) method. This method is based on the observation of the photodetected output obtained from a Senarmont-type ellipsometric system with an ac (modulating) voltage being applied onto the electro-optic sample under test. This observation is made on a high sensitivity oscilloscope and allows us to determine accurately the null point of the system by locating the position of the analyzer that produces in the output a characteristic and abrupt doubling of the (modulation) frequency. By locating this frequency doubling position without and with electric field one can finally determine the corresponding electro-optic coefficients. Theoretical analysis and considerations of practical interest are presented in the article and show that the system in question can ensure reduced errors and increased sensitivity. Also, experimental evidence in support of the expected perfo...

New material with strong electro‐optic effect: Rubidium hydrogen selenate (RbHSeO4)

A very large electro‐optic Pockels coefficient is reported in a crystal of rubidium hydrogen selenate. This new material is found to be promising for modulation applications because of its very low half‐wave voltage (∼27 V) which is determined.

Efficiency of magnetic coupled boost DC-DC converters mainly dedicated to renewable energy systems: influence of the coupling factor

Summary This paper presents a specific analysis of an individual basic magnetically coupled direct current-to-direct current (DC–DC) converter specially designed for integration in a distributed architecture of renewable energy generators for smart grid applications. In such distributed architecture dedicated for renewable energy, parallel high-voltage DC presents many advantages over the classical centralized one. We show that in such setup, high voltage can be advantageously produced using a specific magnetically coupled boost converter, and we point out the influence of the coupling factor, generally considered equal to one, on the overall performance of the converter and on the global energy efficiency of the installation. In this study, the generalized concepts of system energy parameters of DC–DC converters are introduced and applied to the transient analysis. Consequently, the operation of a magnetic coupled DC–DC converter with a recovery stage is modeled. The simulation results are compared with those of the behavioral study, deduced from the model pointing out the large influence of the coupling factor value on the global behavior and mainly on the value of the recovery voltage, in all the various parts of the switching cycle. The renewable energy generator operating parameters, such as current and voltage values, can then be predicted in a more useful way to compute new similar DC–DC converter systems. Copyright

Water density and polarizability deduced from the refractive index determined by interferometric measurements up to 250 MPa

The refractive index of water is precisely determined in the visible light range as a function of the pressure until 250 MPa by means of a new measurement device that uses a special pipe tee included in an interferometer set. This technique allows revisiting the Bradley-Tait and Sellmeier equations to make them dependent on the wavelength and the pressure, respectively. The Bradley-Tait equation for the pressure dependence of the water refractive index is completed by a wavelength-dependent factor. Also, in the considered pressure and wavelength ranges, it is shown that the Sellmeier coefficients can be straightforwardly linked to the pressure, allowing the determination of the refractive index of water for either any wavelength or pressure. A new simple model allows the determination of the density of water as a function of the measured refractive index. Finally, the polarizability of water as function of pressure and wavelength is calculated by means of the Lorentz-Lorenz equation.

Luminescence of in lithium niobate: influence of the chromium concentration and crystal composition

The luminescence spectrum of Cr 3C in LiNbO3 has been investigated, especially in the R-line range from 7200 up to 7500 ˚ A. For this, we have studied several chromium-doped LiNbO3 crystals with various Li/Nb ratios and different doping concentrations. In addition to the lines which were previously reported, several new peaks were detected in our investigations. In fact, the structure, the shape and the relative intensity of each line are shown to be strongly dependent on the content of doping and on the intrinsic defects related to the non-stoichiometry of the crystal. Among all peaks, we focus our attention on four main lines, which can be separated into two groups, according to their behaviours. The contribution of the first group to the whole spectrum diminishes with increasing Li/Nb ratio so that it vanishes for the stoichiometric composition. The second group of lines, occurring at higher wavelengths, is reported for the first time and is shown to be closely related to the Li/Nb ratio, especially when approaching the stoichiometric composition. These lines exhibit a similar behaviour with increasing Li/Nb ratio or Cr concentration. An explanation of the possible origins of these four peaks is given.

R dson behavior in various MOSFET families

MOSFET are widely used for energy conversion and are must appropriated to photovoltaic systems up to 200VDC. Efficiency of these systems is very critical point. Efficiency of converters is mainly related to losses of its components and the MOSFET appears as one the main original. for the conversion of energy, it is very important to insure the higher efficiency as possible. In a first approach we studied MOSFET used in a switching mode. The losses in the transistor are directly linked to the Rdson value. As anterior studies have shown a linear dependence between the Rdson and the voltage supported by the transistor, we can predict a linear increasing of the losses for high output voltage. Our investigations clearly show that the MOSFET losses are greater than they would have to be, especially for high voltages, and our investigations on a large sample of transistors families clearly shows that this law have to be revisited. Indeed simulations show an increase much faster than expected of losses when voltage Vds increases. After compiling data on many power MOSFETs family data sheets a new behavioral model, including the fast evolution of Rdson, for high voltage, has been purposed. This model includes two parameters to be adjusted to a given MOSFET family and which make possible the losses calculation of the concerned family.