Littérature scientifique sur le sujet « Beam fanning »

We propose passive optical limiting based on the photorefractive effect-induced asymmetric deterministic beam fanning in BaTiO 3, and symmetric deterministic beam fanning due to a combination of photovoltaic and thermal effects in LiNbO 3.

La lumière lente est le domaine scientifique qui s’intéresse aux processus non linéaires quipeuvent réduire la vitesse du groupe d’une impulsion lumineuse lorsqu’elle se propage dansun milieu. Cette technologie a récemment suscité un grand intérêt pour ses larges domainesd’application tels que le router optique, la photonique non linéaire et la spectroscopie.L’efficacité des systèmes à lumière lente est généralement mesurée par deux paramètresclés : le retard ou la vitesse du groupe et la bande passante de l’impulsion lumineuse desortie. Ce dernier est défini par le retard dit fractionnel qui est le rapport entre le retardet la largeur de l’impulsion de sortie. Un système de lumière lente dit efficace lorsqu’ilest capable de ralentir les courtes impulsions lumineuses, tout en maintenant une valeurimportante du retard fractionnel (FD).Au cours des dernières années, de nombreuses études du ralentissement de la lumière ontété réalisées dans plusieurs matériaux dispersifs à différentes longueurs d’onde. En fait, desvitesses de groupe inférieures à 17 m/s[1] ont été mesurées expérimentalement par Hau etal. dans un gaz atomique en utilisant la Transparence Induite Electromagnétiquement.Plus récemment, la décélération de la vitesse de groupe a été également observée avec succèsdans des matériaux à l’état solide tels que les fibres optiques [2], les cristaux photoniques[3]. D’autre part, plusieurs études ont montré que les cristaux photoréfractifs peuventégalement réduire la vitesse de propagation de la lumière à température ambiante. En effet,la plus petite vitesse du groupe de 0, 025 cm/s a été obtenue en utilisant l’enregistrementdes réseaux d’indice de réfraction dans le cristal photoréfractif BaTiO3[4]. Cette méthodeconsiste à coupler un faisceau pompe continu et une sonde pour augmenter la dispersionde l’indice de réfraction et générer un gain photoréfractif ainsi le ralentissement de lasonde à la sortie du cristal. Cependant, cette petite vitesse est souvent accompagnéed’une distorsion de l’impulsion de sortie, ce qui réduit la valeur du retard fractionnel (parexemple, un FD de l’ordre de 0, 4 a été mesuré dans [4]).Cette thèse porte sur l’étude de différentes méthodes qui permettent, en plus du ralentissementde la lumière, de limiter la distorsion de l’impulsion dans les milieux photoréfractifs.Tout d’abord, en utilisant la méthode TWM, le cristal SPS avec un temps de réponse de10 ms peut ralentir les impulsions lumineuses ou sombres de l’ordre de ms. Il est démontréque la valeur du retard et la largeur de l’impulsion transmise peuvent être contrôlées parle gain photoréfractif et la durée de l’impulsion d’entrée. En améliorant la configurationdu TWM, nous mesurons un retard fractionnaire de 0, 79 et 1 respectivement pour lesimpulsions lumineuses et sombres de durées proches du temps de réponse du cristal. Lecoma photoréfractive ou le « beam fanning » a également été utilisé pour ralentir lalumière dans le cristal photoréfractif. Le couplage du fanning avec l’impulsion d’entréeentraîne à la fois la modulation des réseaux d’indice de réfraction et le ralentissement del’impulsion de sortie à différentes longueurs d’onde.La lumière lente avec le TWM et le fanning peut être observée pour des impulsions longues,typiquement pour des impulsions de l’ordre de la ms à la seconde. En d’autres termes,seules les impulsions dont les durées sont proches du temps de réponse du cristal qui sontralenties. Dans cette thèse, nous montrons pour la première fois que l’utilisation du TWMen régime impulsionnel et un laser à haute intensité peut réduire le temps de réponsephotoréfractif du cristal et le ralentissement d’une impulsion plus courte (d’une largeur dens). Les résultats obtenus dans un cristal PR d’épaisseur 1 cm sont comparables à ceuxréalisés dans un 1 km de fibre optique pour les mêmes durées d’impulsions
Slow light is the science domain that focuses on the physical nonlinear processes that canreduce the group velocity of a light pulse as it propagates in the medium. This discoveryhas been a great deal of recent interest for a wide range of applications such as opticalbuffering, nonlinear photonics and various types of spectroscopic.The slow light performance is typically measured through two key parameters: the valueof the delay or the group delay and the bandwidth of the output light pulse. This lastone is generally defined by the so-called fractional delay, which is the ratio between theoptical delay and the width of the output pulse. It is important to know that the opticaltelecommunication needs a slow light system that is able to slowdown short input lightpulses with therefore a large value of the fractional delay (FD).In the last years, numerous studies of slow light have been performed in several dispersivematerials at different wavelengths. In 1999, group velocities smaller than 17 m/s [1] havebeen experimentally measured by Hau et al. in an ultra-cold gas using ElectromagneticallyInduced Transparency. More recently, the deceleration of the light pulses has been alsosuccessfully observed in solid-state materials such as in optical fibers [2] and in photoniccrystals [3]. On the other hand, several studies have shown that photorefractive (PR)crystals can also be used to reduce the light propagation velocity at room temperature.As a matter of fact, the smallest group velocity of 0.025 cm/s has been achieved using therecording of refractive index gratings in a BaTiO3 photorefractive crystal [4]. This methodconsists of the coupling of a continuous pump beam and a probe signal to increase therefractive index dispersion and leads the generation of a photorefractive gain. However,this small group velocity is often accompanied by the output pulse distortion which reducesthe value of the fractional delay (FD of the order of 0.4 in [4]).This thesis focuses on the study of the methods which allow in addition to the decelerationgroup velocity, the limitation of the distortion of the pulse in a photorefractive (PR) media.First, using the two-wave mixing (TWM) method, the PR crystal with a response time of10 ms can slow down bright or dark pulses with duration of the order of ms. It is shownthat the value of the time delay and the width of the transmitted pulse can be controlledby the photorefractive gain and the input pulse duration. By improving the TWM setup,we measure a fractional delay of 0.79 and 1 respectively, for the bright and the dark pulseswith a duration close to the response time of the crystal. The beam fanning in a PRcrystal has also been used to slow down a single light pulse. The coupling between thebeam fanning and the input beam leads both to the modulation of the noisy refractiveindex gratings and to the slowing down of the output pulse. The use of beam fanning forlight slow down is new and significantly simplifies the slow light setup.Slow light with the TWM and the beam fanning can be observed for long pulses, typicallyfor a pulse of the order of the milliseconds and the seconds. In other words, only pulseswith durations around the crystal response time are slowed down. In this thesis, we showfor the first time that the use of the TWM at the nanosecond regime and a high laserintensity can reduce the photorefractive response time of the crystal and the slowdown ofa shorter pulse (with a width of ns). The results achieved in a PR crystal with a thicknessof 1 cm are similar to those achieved in slow light systems using a km-long optical fiberand for the same pulse durations

Abstract The Mansholt Plan and the attempt to define and promote the efficient farm were based on an interpretation of Art. 39.1(b) of the Rome Treaty which equated a ‘fair standard of living’ with the achievement of a labour income comparable to that in other sectors. However, the problems of the fanning sector were considerably more complex than this equation would suggest. Running through the agricultural Articles of the Rome Treaty, there is an awareness of the need to pay particular attention to the diversity of circumstances in which farming takes place. There was a requirement to bear in mind, when devising and implementing the CAP, ‘the structural and natural disparities between the various agricultural regions’ (Art. 39.2(a) ). In addition, under Article 42, provision was made for a derogation from the rules on competition so as to allow the granting of aid ‘for the protection of enterprises handicapped by structural and natural conditions’.

This chapter begins with the demise of Bear Stearns in March 2008, which was a dress rehearsal for events that would take place five months later, when the global financial system verged on a meltdown. Lehman Brothers went bankrupt; Bank of America had to rescue Merrill Lynch; Berkshire Hathaway did the same for Goldman Sachs; and Freddy Mac and Fannie Mae were put into receivership. The chapter reviews polls that showed that public confidence in Wall Street had reached its lowest levels in forty years. The chapter highlights the Great Recession, which was a disaster for working people and their retirement funds. A cause of many pension fund losses were collateralized debt obligations, which the funds first began to snap up in the early 2000s.

The Fed’s rescue of Bear Stearns and the Treasury Department’s nationalization of Fannie Mae and Freddie Mac in 2008 provoked widespread criticism. Consequently, the Fed and Treasury were very reluctant to approve further bailouts, and they allowed Lehman Brothers to fail in September 2008. Lehman’s collapse triggered a global panic and a meltdown of financial markets around the world. The Fed and Treasury quickly arranged a bailout of AIG, and Congress approved a $700 billion financial rescue bill. Treasury established the Troubled Asset Relief Program, which injected capital into large universal banks, while the Fed provided trillions of dollars of emergency loans and the FDIC established new guarantee programs for bank debts and deposits. In February 2009, federal regulators pledged to provide any further capital that the nineteen largest U.S. banks needed to survive, thereby cementing the “too big to fail” status of U.S. megabanks. The U.K. and other European nations arranged similar bailouts for their universal banks. Meanwhile, thousands of small banks and small businesses failed, millions of people lost their jobs, and millions of families lost their homes during the Great Recession.

A phenomenological study of beam fanning in BaTiO3:Cr was made using a collimated Ar-ion laser beam (2-mm diameter λ = 4880 Å). Four distinct cases were observed. When the beam was initially incident antiparallel to the +c axis it was split into two components which separated in the opposite directions within the plane of polarization. These beams became nearly perpendicular to the c axis within 5 mm of the entrance surface. In the case where the beam was initially incident along the +c axis strong, diffuse scattering was observed within the first 2 mm of the surface. In both cases no transmitted beam was observed. With the incident beam propagating and polarized perpendicular to the c axis, there was a transmitted beam as well as a weak beam that fanned toward the +c direction. However, with the polarization along the c axis two beams were observed, one bending in the +c direction and the other in the −c direction. The bending angle of the beam in the +c direction was greater than that of the beam that bent in the −c direction. The polarization dependence of this phenomenon suggests that beam fanning may be related to the photovoltaic effect.

Asymmetric self-defocusing in barium titanate (BaTiO3) was first reported by Feinberg.1 Similar effects have been observed in SBN and other photorefractive materials.2 Cronin-Golomb and Yariv have demonstrated the use of this effect as an optical limiter.3 We present measurements of beam fanning in BaTiO3 at a number of wavelengths using cw lasers and a frequency-doubled Nd:YAG laser. We observe that the time dependence of fanning is approximately I − 0.5 at the higher intensities of the pulsed laser. The efficiency of the fanning improved, particularly in the pulsed case, as the crystal was cooled to near 10°C.

Simple coupled-wave theory1,2 accounts for depletion of the pump beam due to energy transfer (i.e., amplification) to the probe beam. Ignoring absorption, the probe gain g can be expressed as1 where I1(0) is the incident probe intensity, Γ is the two-beam-coupling coefficient and rpp is the incident pump-to-probe beam ratio given by (2) with I2(0) being the incident pump intensity.

A low-noise, high-gain optical image amplifier is the ultimate goal of two-wave coupling in the photorefractive crystal with large values of two-wave coupling coefficients. However, the beam fanning is also very strong in the photorefractive crystal with high two-wave coupling coefficient, which corrupts greatly the optical quality of the amplified image. Recently many techniques were demonstrated to get a high-gain, low-noise amplifier, in all of which people were making an effort to get rid of the beam fanning. On contrary, here we reported a new scheme utilizing the beam fanning effect to get a noise-free, high-gain amplified image by two-wave coupling.