Dipole influence on Shack-Hartmann vortex detection in scintillated beams

Optical vortices can appear in an optical beam that propagates over a long distance through a turbulent atmosphere. A Shack-Hartmann wavefront sensor can be used to detect such vortices. However, the morphology of these vortices, which changes with beam propagation, and nearby oppositely charged vortices will affect this vortex detection. The influence of the morphology and the separation distance from oppositely charged vortices on the Shack-Hartmann vortex detection is studied. Numerical simulations for vortex detection under these turbulent atmospheric circumstances are also provided.     

Anomalous transient behavior from an inhomogeneous initial optical vortex density

Inhomogeneous optical vortex densities can be produced in stochastic optical fields by a combination of coherent and incoherent superposition of speckle fields. During subsequent propagation, the inhomogeneity in the vortex density decays away. However, the decay curves contain oscillatory features that are counterintuitive: for a short while, the inhomogeneity actually increases. We provide numerical simulations and analytic calculations to study the appearance of the anomalous features in the decay curves.     

Synergistic effect of oligochitosan and silicon on inhibition of Monilinia fructicola infections

BACKGROUND: Oligochitosan has broad‐spectrum antimicrobial activity and shows an obvious inhibitory effect on phytopathogens. In addition, as an exogenous elicitor, it can induce various defence responses, including affecting the activities of several defence‐related enzymes and substances in some plants. Owing to this dual function of oligochitosan, it can be used to control postharvest diseases of fruits. Silicon, like oligochitosan, also has a dual function. In this study the synergistic effect of oligochitosan and silicon on the decay control of apple fruit was investigated. RESULTS: In vitro, both oligochitosan and silicon significantly inhibited spore germination, germ tube elongation and mycelial growth of Monilinia fructicola, with higher concentrations having a greater effect. The synergistic effect of oligochitosan and silicon at half‐maximal inhibitory concentration on disease control at 25 °C was much better than the effect of oligochitosan or silicon alone, not only in vitro but also in vivo. CONCLUSION: The results showed that a combination of oligochitosan and silicon had a synergistic effect on the control of disease caused by M. fructicola in apple fruit at 25 °C. Copyright © 2010 Society of Chemical Industry     

Evolution of the scintillation index and the optical vortex density in speckle fields after removal of the least-squares phase

Knowledge of the behavior of stochastic optical fields can aid the understanding of the scintillation of light propagating through a turbulent medium. For this purpose, we perform a numerical investigation of the evolution of the scintillation index and the optical vortex density in a speckle field after removing its continuous phase. We find that both the scintillation index and the vortex density initially drop and then increase again to reach an equilibrium level. It is also found that the initial rate of decrease in both cases is 1 order of magnitude faster than the eventual rate of increase. Their detail shapes are however different. Therefore different empirical functions are used to fit the shapes of these curves.     

Accelerating the annihilation of an optical vortex dipole in a Gaussian beam

When a Gaussian beam with two oppositely charged vortices propagates in free space, these two vortices will move around on the transverse beam plane. They may either move toward each other and annihilate each other spontaneously or survive all the way depending on the conditions. Here, we investigate how to force vortex dipoles to annihilate. We find that the background phase function created by two oppositely charged vortices during beam propagation can cause the vortices to move together and annihilate each other. The background phase function on a transverse plane just beyond the point where a dipole annihilated is continuous and retains the potential that forces a dipole to annihilate. We use this background phase function to accelerate the annihilation of vortex dipoles. Numerical results are provided to show the acceleration of dipole annihilation in a Gaussian beam, using such a background phase function.     

Geometric phase lens

The design of a lens that modulates the geometric phase of an optical beam by manipulating its polarization is presented. To produce such a geometric phase element with a spatially varying phase function, one needs a wave plate with varying orientation. One can use subwavelength grooves to produce form birefringence, but the variation in orientation generally leads to branch points in the groove pattern. These branch points do not affect the phase of the traversing beam directly because the grooves are subwavelength. However, they do produce errors in the groove orientation, which indirectly leads to errors in the geometric phase function that is implemented. A design procedure is provided to compute the groove pattern for such a rotationally symmetric geometric phase element; and, with the aid of a numerical simulation, the effect of the branch points in the groove pattern on its performance is investigated.     

Detection of phase singularities with a Shack-Hartmann wavefront sensor

While adaptive optical systems are able to remove moderate wavefront distortions in scintillated optical beams, phase singularities that appear in strongly scintillated beams can severely degrade the performance of such an adaptive optical system. Therefore the detection of these phase singularities is an important aspect of strong-scintillation adaptive optics. We investigate the detection of phase singularities with the aid of a Shack-Hartmann wavefront sensor and show that, in spite of some systematic deficiencies inherent to the Shack-Hartmann wavefront sensor, it can be used for the reliable detection of phase singularities, irrespective of their morphologies. We provide full analytical results, together with numerical simulations of the detection process.