78 research outputs found

    Phonon scattering by impurities in semiconductors

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    Thermal conductivity measurements have been used to study the low lying energy levels of Cr ions in GaAs. Strong resonant phonon scattering was observed in semi-insulating (SI) and p-type samples, which is attributed to Cr 2+ or Cr 3+ ions, while the scattering in the n-type samples additional to that in undoped material was very small. From the computer fits of the thermal conductivity, zero-field ground state splittings have been deduced. A splitting at ~ 23 cm -1 is attributed to Cr 3+ ions, others at ~ 0.7 cm and 4.9 cm are due to Cr 2+. The phonon scattering in the n-type samples did not show any magnetic field dependence while big effects were observed in SI and p-type ones. This seems consistent with the results of the zero-field work. The effect of uniaxial stress on the phonon scattering has been measured in the temperature range ~2-15 K. Again no effects were seen in n-type material. The results for SI and p-type material are interpreted in terms of a static Jahn-Tellereffect of Cr 2+ ions. A preliminary investigation was made of the effect on the phonon scattering of sub-band-gap illumination. In an-type sample, the decay in the increase in the thermal resistivity produced by photoexcitation showed two parts. The first part with a characteristic time of ~ 1 hour is attributed to electron-capture at Cr 2+ ions. The second decay was very slow (persistent) with a characteristic time T » 10 5 s. This effect has tentatively been attributed to the occurrence of large lattice relaxation. The phonon scattering by the Cr ions is found to be consistent with the 'double acceptor' model for Cr in GaAs. Another model where er can act as a hole trap is discussed. Finally the effect of high magnetic fields on the thermal conductivity of acceptors in Ge was measured. From this and previous results, the g-values describing the ra8 ground state were found to be much lower than the predicted ones. A Thermally Detected EPR apparatus was designed and constructed in an attempt to check on the ground state structure of p-Ge and also p-Si but no results were obtained. This is believed to be due to the very large line widths resulting from strain splitting of the ra8 ground state

    Phonon scattering by impurities in semiconductors

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    Thermal conductivity measurements have been used to study the low lying energy levels of Cr ions in GaAs. Strong resonant phonon scattering was observed in semi-insulating (SI) and p-type samples, which is attributed to Cr 2+ or Cr 3+ ions, while the scattering in the n-type samples additional to that in undoped material was very small. From the computer fits of the thermal conductivity, zero-field ground state splittings have been deduced. A splitting at ~ 23 cm -1 is attributed to Cr 3+ ions, others at ~ 0.7 cm and 4.9 cm are due to Cr 2+. The phonon scattering in the n-type samples did not show any magnetic field dependence while big effects were observed in SI and p-type ones. This seems consistent with the results of the zero-field work. The effect of uniaxial stress on the phonon scattering has been measured in the temperature range ~2-15 K. Again no effects were seen in n-type material. The results for SI and p-type material are interpreted in terms of a static Jahn-Tellereffect of Cr 2+ ions. A preliminary investigation was made of the effect on the phonon scattering of sub-band-gap illumination. In an-type sample, the decay in the increase in the thermal resistivity produced by photoexcitation showed two parts. The first part with a characteristic time of ~ 1 hour is attributed to electron-capture at Cr 2+ ions. The second decay was very slow (persistent) with a characteristic time T » 10 5 s. This effect has tentatively been attributed to the occurrence of large lattice relaxation. The phonon scattering by the Cr ions is found to be consistent with the 'double acceptor' model for Cr in GaAs. Another model where er can act as a hole trap is discussed. Finally the effect of high magnetic fields on the thermal conductivity of acceptors in Ge was measured. From this and previous results, the g-values describing the ra8 ground state were found to be much lower than the predicted ones. A Thermally Detected EPR apparatus was designed and constructed in an attempt to check on the ground state structure of p-Ge and also p-Si but no results were obtained. This is believed to be due to the very large line widths resulting from strain splitting of the ra8 ground state

    Étude et réalisation de sources photoniques intégrées sur InP pour les applications télécoms à hauts débits et à 1,55 m

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    Les formats de modulation avancés, codant l information sur la phase, la polarisation ou plusieurs niveaux d amplitude de la lumière reçoivent aujourd hui un intérêt croissant. En effet, ceux-ci permettent d atteindre une meilleure efficacité spectrale et par conséquent des débits plus élevés. Ces caractéristiques sont actuellement très recherchées dans les télécommunications pour répondre à la demande constante d augmentation de capacité des transmissions optiques fibrées. L essentiel du travail effectué porte sur la génération de tels signaux dans des sources photoniques monolithiques sur InP faisant appel à un concept nouveau de commutation de phases optiques préfixées avec des modulateurs électro-absorbants. Une comparaison de notre technologie intégrée avec la technologie actuelle de génération de formats de modulation avancés démontre des possibilités nouvelles de réduction de taille, de diminution de consommation énergétique et d évolution en vitesse de modulation jusqu à 56 GBauds. Suite à la validation, par simulations, d architectures de transmetteurs spécifiques pour la génération de formats de modulation avancés, nous réalisons en salle blanche les circuits photoniques intégrés d étude. Les caractérisations statiques confirment le fonctionnement de toutes les fonctions intégrées des circuits et soulignent l efficacité de la filière technologique. Pour une première démonstration de fonctionnalité nous choisissons un transmetteur BPSK capable de générer une modulation de phase à 12,4 GB. Ce résultat démontre la plus petite source intégrée BPSK à l heure actuelle. Un autre circuit capable de générer des formats de modulation plus complexes est aussi caractériséAdvanced modulation formats, encoding data on the phase, polarization or multi-level intensity of the light are currently a hot topic in the telecommunication domain. By using them, high spectral efficiency and therefore higher bit rate signals could be generated. Those characteristics are really attractive for the telecommunication systems manufacturers in order to answer to the constant need of increased bandwidth in fiber optic communications. The study of advanced modulation formats generation in Photonic Integrated Circuits (PICs) based on a new concept of preset phases switching by Electro-Absorption Modulators is the main task of the current work. Compared to the actual technology used for generate advanced modulations, our choice could allow a strong reduction of the dimensions and of the energy consumption of the transmitter as well as bit rate up to 56 GB. After validating specific transmitters architectures by simulations, we fabricated the studied photonic integrated circuits in clean room. Through static characterizations, we verify that all integrated functions of the transmitters are working and we show the efficiency of our technological choices. Using the available equipments at the lab, we prove the validity of our concept of EAM based phase switching by using a BPSK transmitter. A 12.4 GB BPSK modulation is obtained as well as a wide open eye diagram. This result demonstrates the smallest BPSK integrated photonic source at this time. Another photonic circuit able to generate more complex modulation formats is also measuredEVRY-INT (912282302) / SudocSudocFranceF

    Numerical Study on Shear Stress Variation of RC Wall with L Shaped Section

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    In recent years, and after the 2003 Boumerdes earthquake, a new type of building is being constructed in Algeria. The new concept is based on the concentration of reinforced concrete shear walls with L shape at the building corners. The seismic behavior of such configuration is not well known nowadays. Numerical investigation was carried out on reinforced concrete structure to evaluate the stress distribution at the base of the corner L shaped walls. Influence of number of stories, length of the shear wall as well as the thickness of the wall was considered in our investigation. In total, more than 200 numerical models were crated and analyzed. The analyses showed that, reinforced concrete wall with 15 cm, or less, in thickness should have a minimum length of 10 times the thickness; however, for RC walls with a thickness of more than 20 cm, the length of the wall should be greater than 7  times the thickness. In this paper the main results of this investigation are presented

    Restoring robust binary switching operation and exceptional point using long-period grating-assisted parity-time symmetric couplers

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    International audienceThe impact of imbalance in waveguides propagation constants among Parity-Time symmetric coupled waveguides and/or of a complex-valued coupling coefficient is assessed. The narrow tolerance found implies that attempts to tightly control waveguides parameters appear as elusive because of fabrication technology limitations, calling for more feasible mitigation avenues. It is shown that a grating-assisted Parity-Time symmetric coupler design restores both technologically robust binary switching operation and exceptional point. In addition the proposed design is compatible with birefringence compensation techniques providing polarization-independent operation as well as coupling and/or gain-loss profile modulation techniques that extend the control of switching operation in the Parity-Time symmetric phase. Using wavelength as an additional tuning parameter near exceptional points opens promising avenues for manipulating the trajectory on Riemann sheets in topological photonics applications

    Comb-based WDM transmission at 10 Tbit/s using a DC-driven quantum-dash mode-locked laser diode

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    Chip-scale frequency comb generators have the potential to become key building blocks of compact wavelength-division multiplexing (WDM) transceivers in future metropolitan or campus-area networks. Among the various comb generator concepts, quantum-dash (QD) mode-locked laser diodes (MLLD) stand out as a particularly promising option, combining small footprint with simple operation by a DC current and offering flat broadband comb spectra. However, the data transmission performance achieved with QD-MLLD was so far limited by strong phase noise of the individual comb tones, restricting experiments to rather simple modulation formats such as quadrature phase shift keying (QPSK) or requiring hard-ware-based compensation schemes. Here we demonstrate that these limitations can be over-come by digital symbol-wise phase tracking algorithms, avoiding any hardware-based phase-noise compensation. We demonstrate 16QAM dual-polarization WDM transmission on 38 channels at an aggregate net data rate of 10.68 Tbit/s over 75 km of standard single-mode fiber. To the best of our knowledge, this corresponds to the highest data rate achieved through a DC-driven chip-scale comb generator without any hardware-based phase-noise reduction schemes

    Noise-induced broadening of a quantum-dash laser optical frequency comb

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    Single-section quantum dash semiconductor lasers have attracted much attention as an integrated and simple platform for the generation of THz-wide and flat optical frequency combs in the telecom C-band. In this work, we present an experimental method allowing to increase the spectral width of the laser comb by the injection of a broadband optical noise from an external semiconductor optical amplifier that is spectrally overlapped with the quantum dash laser comb. The noise injection induces an amplification of the side modes of the laser comb which acquire a fixed phase relationship with the central modes of the comb. We demonstrate a broadening of the laser comb by a factor of two via this technique.Comment: 4 pages, 4 figure

    Ultra-fast optical ranging using quantum-dash mode-locked laser diodes

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    Laser-based light detection and ranging (LiDAR) is key to many applications in science and industry. For many use cases, compactness and power efficiency are key, especially in high-volume applications such as industrial sensing, navigation of autonomous objects, or digitization of 3D scenes using hand-held devices. In this context, comb-based ranging systems are of particular interest, combining high accuracy with high measurement speed. However, the technical complexity of miniaturized comb sources is still prohibitive for many applications, in particular when high optical output powers and high efficiency are required. Here we show that quantum-dash mode-locked laser diodes (QD-MLLD) offer a particularly attractive route towards high-performance chip-scale ranging systems. QD-MLLDs are compact, can be easily operated by a simple DC drive current, and provide spectrally flat frequency combs with bandwidths in excess of 2 THz, thus lending themselves to coherent dual-comb ranging. In our experiments, we show measurement rates of up to 500 MHz—the highest rate demonstrated with any ranging system so far. We attain reliable measurement results with optical return powers of only – 40 dBm, corresponding to a total loss of 49 dB in the ranging path, which corresponds to the highest loss tolerance demonstrated so far for dual-comb ranging with chip-scale comb sources. Combing QD-MLLDs with advanced silicon photonic receivers offers an attractive route towards robust and technically simple chip-scale LiDAR systems

    Towards a monolithic optical cavity for atom detection and manipulation

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    We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F ~ 30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C ~ 1
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