Bande X

Résumé

La bande X est une plage de fréquences d'onde radio dite 'SHF' (Supra-haute fréquence) située aux alentours de 10 gigahertz, utilisée pour les radars, les télécommunications et la radio. Elle est attribuée en France principalement à l'armée, mais on la retrouve également dans certains radars météorologiques, où la forte atténuation par les précipitations ne permet de l'utiliser que sur de courtes portées. Elle est également utilisée dans le domaine spatial, où sa très longue portée dans l'espace est notamment exploitée par la sonde américaine New Horizons. Fréquences et affectations en France vignette|Antenne parabolique 8 GHz au Centre spatial de Toulouse en 1985. La bande X s'étend de 8 à . Selon le Tableau national de répartition des bandes de fréquences (TNRBF) édité par l'Agence nationale française des fréquences (ANFR), les affectataires sont répartis conformément au tableau simplifié disponible sur le site internet de l'ANFR. On a essentiellement dans cette bande

Source officielle

https://fr.wikipedia.org/wiki/Bande_X

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An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

Arnaud Comment, Tim Rolf Eichhorn, Martin Haag, Sami Jannin, Jacques Van Der Klink

In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1 K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient He-4 flow cryostat that cools the sample to temperatures between 4 K and 300 K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7 mm inner diameter that can accommodate cubic samples of 5 mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3 T. (C) 2013 Elsevier Inc. All rights reserved.

Academic Press Inc Elsevier Science2013

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Gain-switched semiconductor laser driven soliton microcombs

Jijun He, Tobias Kippenberg, Junqiu Liu, Erwan Guillaume Albert Lucas, Wenle Weng

Using phase-engineered coherent laser pulses produced by gain-switched semiconductor lasers, we generate low-power-threshold soliton microcombs whose repetition frequencies are in the X-band and K-band microwave ranges. (c) 2021TheAuthor(s)

IEEE2021

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Dissipative Kerr soliton generation using self-injection-locked III-V lasers has enabled fully integrated hybrid microcombs that operate in turnkey mode and can access microwave repetition rates. Yet, continuous-wave-driven soliton microcombs exhibit low energy conversion efficiency and high optical power threshold, especially when the repetition frequencies are within the microwave range that is convenient for direct detection with off-the-shelf electronics. Here, by actively switching the bias current of injection-locked III-V semiconductor lasers with switching frequencies in the X-band and K-band microwave ranges, we pulse-pump both crystalline and integrated microresonators with picosecond laser pulses, generating soliton microcombs with stable repetition rates and lowering the required average pumping power by one order of magnitude to a record-setting level of a few milliwatts. In addition, we unveil the critical role of the phase profile of the pumping pulses, and implement phase engineering on the pulsed pumping scheme, which allows for the robust generation and the stable trapping of solitons on intracavity pulse pedestals. Our work leverages the advantages of the gain switching and the pulse pumping techniques, and establishes the merits of combining distinct compact comb platforms that enhance the potential of energy-efficient chipscale microcombs. Here, the authors generate dissipative Kerr solitons with stable repetition rates and low optical power threshold. They achieve this by actively switching the bias current of injection-locked III-V semiconductor lasers and pulse-pumping crystalline and integrated microresonators with picosecond laser pulses.

NATURE RESEARCH2021

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