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Limoges, France

Reveyrand T.,XLIM UMR 6172 | Ramos I.,University of Colorado at Boulder | Popovic Z.,University of Colorado at Boulder
Electronics Letters | Year: 2012

The similarity between RF power amplifiers and rectifiers is discussed. It is shown that the same high-efficiency harmonically-terminated power amplifier can be operated in a dual rectifier mode. Nonlinear simulations with a GaN HEMT transistor model show the time-reversal intrinsic voltage and current waveform relationship between a class-F amplifier and rectifier. Measurements on a class-F-1 amplifier and rectifier at 2.14 GHz demonstrate over 80% efficiency in both cases. © 2012 The Institution of Engineering and Technology. Source


Gilard O.,French National Center for Space Studies | Caussanel M.,University of Perpignan | Duval H.,University of Perpignan | Quadri G.,French National Center for Space Studies | Reynaud F.,XLIM UMR 6172
Journal of Applied Physics | Year: 2010

A new theoretical approach is proposed to explain the dose, dose rate and temperature sensitivity of the radiation-induced absorption (RIA) in glasses. In this paper, a βth -order dispersive kinetic model is used to simulate the growth of the density of color centers in irradiated glasses. This model yields an explanation for the power-law dependence on dose and dose rate usually observed for the RIA in optical fibers. It also leads to an Arrhenius-like relationship between the RIA and the glass temperature during irradiation. With a very limited number of adjustable parameters, the model succeeds in explaining, with a good agreement, the RIA growth of two different optical fiber references over wide ranges of dose, dose rate and temperature. © 2010 American Institute of Physics. Source


Ramadan A.,XLIM UMR 6172 | Reveyrand T.,XLIM UMR 6172 | Martin A.,XLIM UMR 6172 | Nebus J.M.,XLIM UMR 6172 | And 4 more authors.
Electronics Letters | Year: 2010

This presented study focuses on the impact of gate-source voltage waveforms on power added efficiency performances of GaN HEMTs for the design of class F and class F-1 amplifiers. It is shown that second-harmonic signal injection at the gate port of transistors can lead to efficiency improvements in the case of class F operation and efficiency deteriorations in the case of class F-1 operation. This work is applied to a 15 W GaN HEMT die from Cree at a fundamental frequency Fo equal to 2GHz. Calibrated on-wafer time domain measurements are reported. © 2010 The Institution of Engineering and Technology. Source


Schuster K.,Intstitute of Photonic Technology IPHT | Kobelke J.,Intstitute of Photonic Technology IPHT | Litzkendorf D.,Intstitute of Photonic Technology IPHT | Schwuchow A.,Intstitute of Photonic Technology IPHT | And 6 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2011

We report about preparation technique and characterization of structured fibers composed of HMO core glasses and silica cladding. Two processes as material preparation techniques have been developed based on glasses prepared by melting of SAL (e.g. 70SiO2-20Al2O3-10La 2O3) glasses and the reactive powder sintering (REPUSIL) method. The melted glasses have been characterized by dilatometrical methods to find Tg values of 827-875°C and expansion coefficients between 4.3 and 7.0×10-6 K-1. The latter is one order of magnitude higher than the expansion coefficient of pure silica glass. Structured fibers (SAL core, silica cladding) were fabricated following the Rod-in-Tube (RIT) and Granulate-in-Tube (GIT) process. The HMO glasses were chosen due du their high lanthanum content and the expected high nonlinearity, suitable for nonlinear applications (e.g. supercontinuum sources). The partial substitution of lanthanum by other rare earth elements (e.g. Ytterbium) allows the preparation of fibers with extremely high rare earth concentration up to 5 mol% Yb2O3. The concentration of alumina in the HMO glasses as "solubilizer" for lanthanide was adjusted to about 20 mol%. So we overcame the concentration limits of rare earth doping of MCVD (maximum ca. 2 mol% RE2O3). Nevertheless, the investigated HMO glasses show their limits by integration in structured silica based fibers: Optical losses are typically in the dB/m range, best value of this work is about 600 dB/km. The mechanical stability of fibers is influenced by mechanical strain caused by the high thermal expansion of the core material and the lower network bonding stability of the HMO glasses, but partially compensated by the silica cladding. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE). Source

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