Mode-medium instability and its correction with a Gaussian reflectivity mirror

A high power CO2 laser beam is known to deteriorate after a few microseconds due to a mode-medium instability (MMI) which results from an intensity dependent heating rate related to the vibrational-to-translational decay of the upper and lower CO2 lasing levels. An iterative numerical technique is developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO2 resonator with a hard-edge output mirror for different parameters like the Fresnel number and the gas density. The results show that the mode of the hard edge unstable resonator deteriorates because of the diffraction ripples in the mode. A Gaussian-reflectivity mirror was used to correct the MMI. This mirror produces a smoother intensity profile which significantly reduces the effects of the MMI. Quantitative results on peak density variation and beam quality are presented

Stochastic Reinforcement Learning

In reinforcement learning episodes, the rewards and punishments are often non-deterministic, and there are invariably stochastic elements governing the underlying situation. Such stochastic elements are often numerous and cannot be known in advance, and they have a tendency to obscure the underlying rewards and punishments patterns. Indeed, if stochastic elements were absent, the same outcome would occur every time and the learning problems involved could be greatly simplified. In addition, in most practical situations, the cost of an observation to receive either a reward or punishment can be significant, and one would wish to arrive at the correct learning conclusion by incurring minimum cost. In this paper, we present a stochastic approach to reinforcement learning which explicitly models the variability present in the learning environment and the cost of observation. Criteria and rules for learning success are quantitatively analyzed, and probabilities of exceeding the observation cost bounds are also obtained.Comment: AIKE 201

Dynamics of tilt-based browsing on mobile devices

A tilt-controlled photo browsing method for small mobile devices is presented. The implementation uses continuous inputs from an accelerometer, and a multimodal (visual, audio and vibrotactile) display coupled with the states of this model. The model is based on a simple physical model, with its characteristics shaped to enhance usability. We show how the dynamics of the physical model can be shaped to make the handling qualities of the mobile device fit the browsing task. We implemented the proposed algorithm on Samsung MITs PDA with tri-axis accelerometer and a vibrotactile motor. The experiment used seven novice users browsing from 100 photos. We compare a tilt-based interaction method with a button-based browser and an iPod wheel. We discuss the usability performance and contrast this with subjective experience from the users. The iPod wheel has significantly poorer performance than button pushing or tilt interaction, despite its commercial popularity

Ecogeographical variations of chromosomal polymorphism in Hawaiian populations of Drosophila immigrans

Reports were scanned in black and white at a resolution of 600 dots per inch and were converted to text using Adobe Paper Capture Plug-in.Eighteen samples from twelve populations of Drosophila immigrans in the islands of Kauai, Oahu and Hawaii in the Hawaiian archipelago were analyzed for inversion polymorphism in 1125 females and 206 males. Three kinds of second chromosome inversions, which appear to be identical with those previously reported by other workers, were present in all of our populations; two other new inversions of the same autosome were detected from the Hawaii collections, but their origin, whether natural or laboratory, could not be assured. The average proportion of inversion heterozygosity per individual of the populations from Kauai, Oahu and Hawaii was about 34%, 32% and 65% respectively. The frequencies of heterozygous inversions Here similar between different populations within islands (with one notable exception on Hawaii). In contrast, the frequencies were significantly heterogeneous from one island to the next. The results of gene arrangement frequency analysis consolidated the above findings. It is suggested that the inter-island differentiations are due to natural selection and probably maintained by the isolation by oceanic channels. Two near-by localities on Hawaii were inhabited with significantly heterogeneous populations. Such a microgeographic differentiation has been interpreted as being due to the presence of highly localized, differential selection forces in the two localities, and the difference seems to be maintained due to isolation by the lava flows. Our data suggest that the breeding units of Hawaiian populations of D. immigrans are not so small as to allow for genetic drift to significantly affect the populations. Inversion polymorphism was similar between females and males taken at the same time in the same localities