Double-line-density gratings structure for compression and generation of double femtosecond laser pulses

Grating pairs are widely used for pulse compression and stretching. Normally, the two gratings are identical. We propose a very simple structure with double-line-density reflective gratings for pulse compression and generation of double pulses, which has the advantages of no material dispersion, compact in volume, simple in structure, etc. The use of reflective Dammann gratings fully demonstrated the principle of this structure. The output pulses are well verified by a standard frequency-resolved optical gating apparatus. This structure will be highly interesting in ultrashort pulse compression and other more practical applications of femtosecond laser pulses. (c) 2007 Optical Society of America.

Effects of atmosphere visibility on performances of non-line-of-sight ultraviolet communication systems

Dependence of performances of non-line-of-sight (NLOS) solar-blind ultraviolet (UV) communication systems on atmosphere visibility is investigated numerically by correlating the propagation of UV radiation with the visibility. A simplified solar-blind UV atmospheric propagation model is introduced, and the NLOS UV communication system model is constituted based on the single scattering assumption. Using the model, numerical simulation is conducted for two typical geometry configurations and different modulation formats. The results indicate that the performance of the NLOS UV communication system is insensitive to variation of visibility in quite a large range, and deteriorates significantly only in very low-visibility weather, and is also dependent on the geometry configuration of the system. The results also show that the pulse position modulation (PPM) is preferable due to its high-power efficiency to improve the system performance. (c) 2007 Elsevier GmbH. All rights reserved.

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications. (c) 2006 Elsevier GmbH. All rights reserved.

Split-aperture laser pulse compressor design tolerant to alignment and line-density differences

We introduce a four-pass laser pulse compressor design based on two grating apertures with two gratings per aperture that is tolerant to some alignment errors and, importantly, to grating-to-grating period variations. Each half-beam samples each grating in a diamond-shaped compressor that is symmetric about a central bisecting plane. For any given grating, the two half-beams impinge on opposite sides of its surface normal. It is shown that the two split beams have no pointing difference from paired gratings with different periods. Furthermore, no phase shift between half-beams is incurred as long as the planes containing a grating line and the surface normal for each grating of the pair are parallel. For grating pairs satisfying this condition, gratings surfaces need not be on the same plane, as changes in the gap between the two can compensate to bring the beams back in phase. © 2008 Optical Society of America.

O turismo solidário e a redescrição social no Vale do Jequitinhonha-MG

Esta tese estuda o processo da implantação do Programa de Turismo Solidário pelo governo de Minas Gerais no Vale do Jequitinhonha - MG, bem como as características e os significados que o mesmo adquiriu, tanto junto aos moradores das localidades alvo quanto junto aos chamados turistas solidários, e avalia o estado da arte de modos de turismo semelhantes ao que ocorre no Vale, com ênfase em alguns casos nas favelas cariocas. A tese focaliza a apropriação pela população das diretrizes e dos aspectos teórico-metodológicos da proposta desse Programa; a percepção dos moradores em relação ao turismo que ali ocorre; o enquadramento em que os encontros no turismo solidário ocorrem; os significados do turismo solidário; os indicadores do turismo solidário. Traz, de um lado, uma visão abrangente, discutindo questões relacionadas às políticas públicas no campo do turismo, e de outro lado, uma abordagem de caráter etnográfico, relativa ao encontro intersubjetivo entre ―turistas solidários‖ e a população local. A construção da linha argumentativa da tese fundamenta-se nos seguintes pressupostos: as propostas de descentralização por meio da ideia de governança apresentadas pelas políticas públicas que sustentam a ideologia do Programa de Turismo Solidário são ferramentas importantes para o desenvolvimento local; o encontro entre pessoas com diferentes formas de vida, com reconhecimento recíproco entre si, suscita em ambas as partes um alargamento da percepção de mundo e uma possibilidade para redescrição dos sujeitos; o enquadramento em que a interação ocorre e o nível de intimidade entre os participantes influenciam na possibilidade do individuo repensar valores e atitudes. O documentário ―Retrato Brasil‖ filmado durante as pesquisas de campo, foi elaborado na perspectiva de auxiliar na promoção do que é chamado de turismo solidário, entre outras possibilidades.

Pesca experimental de atum com "long line" na ZEE moçambicana

Results of the analysis of statistical data gathered aboard 2 crafts licensed for experimental commercial tuna fishing with long-line in the EEZ of Mozambique are presented.

Line-width reduction of a laser diode array using an external cavity with two feedback mirrors

A novel Littman-Metcalf external cavity laser diode array with two feedback mirrors is introduced. The line-width broadening effect caused by smile can be reduced by the novel external cavity. At the drive current of 16A, the line-width is narrowed to 0.1nm from free-running width of 1.6nm with output efficiency of 84%.