Mode-locking by nanotubes of a Raman laser based on a highly doped GeO 2 fiber

A mode-locked Raman laser, using 25 m of a GeO2 doped fiber as the gain medium, is reported employing carbon nanotubes. The oscillator generates 850 ps chirped pulses, which are externally compressed to 185 ps. © 2012 OSA.

Multiply conditioned analyses of stratification in highly swirling methane/air flames

The effects of stratification on a series of highly swirling turbulent flames under globally lean conditions (φg=0.75) are investigated using a new high-spatial resolution multi-scalar dataset. This dataset features two key properties: high spatial resolution which approaches the 60 micron optical limit of the measurement system, and a wavelet oversampling methodology which significantly reduces the influence of noise. Furthermore, the very large number of realizations (30,000) acquired in the stratified cases permits statistically significant results to be obtained even after aggressive conditioning is applied. Data are doubly conditioned on equivalence ratio and the degree of stratification across the flame in each instantaneous realization. The influence of stoichiometry is limited by conditioning on the equivalence ratio at the location of peak CO mass fraction, which is shown to be a good surrogate for the location of peak heat release rate, while the stratification is quantified using a linear gradient in equivalence ratio across the instantaneous flame front. This advanced conditioning enables robust comparisons with the baseline lean premixed flame. Species mass fractions of both carbon monoxide and hydrogen are increased in temperature space under stratified conditions. Stratification is also shown to significantly increase thermal gradients, yet the derived three-dimensional flame surface density is shown to be relatively insensitive to stratification. Whilst the presence of instantaneous stratification broadens the curvature distribution relative to the premixed case, the degree of broadening is not significantly influenced by the range of global stratification ratios examined in this study. © 2012 The Combustion Institute.

A novel MPT noise prediction methodology for highly-integrated propulsion systems with inlet flow distortion

Embedded propulsion systems, such as for example used in advanced hybrid-wing body aircraft, can potentially offer major fuel burn and noise reduction benefits but introduce challenges in the aerodynamic and acoustic integration of the high-bypass ratio fan system. A novel approach is proposed to quantify the effects of non-uniform flow on the generation and propagation of multiple pure tone noise (MPTs). The new method is validated on a conventional inlet geometry first. The ultimate goal is to conduct a parametric study of S-duct inlets in order to quantify the effects of inlet design parameters on the acoustic signature. The key challenge is that the mechanism underlying the distortion transfer, noise source generation and propagation through the non-uniform flow field are inherently coupled such that a simultaneous computation of the aerodynamics and acoustics is required. The technical approach is based on a body force description of the fan blade row that is able to capture the distortion transfer and the MPT noise generation mechanisms while greatly reducing computational cost. A single, 3-D full-wheel unsteady CFD simulation, in which the Euler equations are solved to second-order spatial and temporal accuracy, simultaneously computes the MPT noise generation and its propagation in distorted mean flow. Several numerical tools were developed to enable the implementation of this new approach. Parametric studies were conducted to determine appropriate grid and time step sizes for the propagation of acoustic waves. The Ffowcs-Williams and Hawkings integral method is used to propagate the noise to far field receivers. Non-reflecting boundary conditions are implemented through the use of acoustic buffer zones. The body force modeling approach is validated and proof-of-concept studies demonstrate the generation of disturbances at both blade-passing and shaft-order frequencies using the perturbed body force method. The full methodology is currently being validated using NASA's Source Diagnostic Test (SDT) fan and inlet geometry. Copyright © 2009 by Jeff Defoe, Alex Narkaj & Zoltan Spakovszky.

GENERATION OF PICOSECOND PULSES IN AN INJECTION LASER WITH AN EXTERNAL SELECTIVE RESONATOR.

Pulses of 15 psec duration were generated by an injection laser with an external dispersive resonator operating in the active mode-locking regime. This regime was attained by subjecting the laser diode to a current of high frequency equal to the intermode interval in the external resonator. The duration of the pulses was determined by an autocorrelation method in which the second harmonic was generated in an LiIO//3 crystal.

A highly conductive and transparent solution processed AZO/MWCNT nanocomposite

A solution processed aluminum-doped zinc oxide (AZO)/multi-walled carbon nanotube (MWCNT) nanocomposite thin film has been developed offering simultaneously high optical transparency and low electrical resistivity, with a conductivity figure of merit (σDC/σopt) of ~75-better than PEDOT:PSS and many graphene derivatives. The reduction in sheet resistance of thin films of pristine MWCNTs is attributed to an increase in the conduction pathways within the sol-gel derived AZO matrix and reduced inter-MWCNT contact resistance. Films have been extensively characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), photoluminescence (PL), and ultraviolet-visible (UV-vis) spectroscopy. © 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Origin of transient crosstalk and its reduction in phase-only LCOS wavelength selective switches

The transient crosstalk in a phase-only liquid crystal on silicon (LCOS) based wavelength selective switch using a Fourier transform setup was investigated. Its origin was identified using an in situ test procedure and found to be related to the transient phase patterns displayed by the LCOS device during the switching. Two different methods were proposed to reduce the transient crosstalk without the need to modify the optics or electronics in use. Experimental results show both methods are able to reduce the worst-case transient crosstalk by at least 5 dB. © 1983-2013 IEEE.

Mode-locking by nanotubes of a raman laser based on a highly doped GeO2 fiber

A mode-locked Raman laser, using 25 m of a GeO2 doped fiber as the gain medium, is reported employing carbon nanotubes. The oscillator generates 850 ps chirped pulses, which are externally compressed to 185 ps. © OSA 2012.

Demonstration of multi-casting in a 1 × 9 LCOS wavelength selective switch

A multi-functional 1 × 9 wavelength selective switch based on liquid crystal on silicon (LCOS) spatial light modulator technology and anamorphic optics was tested at a channel spacing of 100 and 200 GHz, including dynamic data measurements on both single beam deflection and multi-casting to two ports. The multi-casting holograms were optimized using a modified Gerchberg-Saxton routine to design the core hologram, followed by a simulated annealing routine to reduce crosstalk at non-switched ports. The effect of clamping the magnitude of phase changes between neighboring pixels during optimization was investigated, with experimental results for multi-casting to two ports resulting in a signal insertion loss of-7.6 dB normalized to single port deflection, a uniformity of ±0.6%, and a worst case crosstalk of-19.4 dB, which can all be improved further by using a better anti-reflection coating on the LCOS SLM coverplate and other measures. © 2013 IEEE.

Distinct Evolutionary Patterns Between Two Duplicated Color Vision Genes Within Cyprinid Fishes

We investigated the molecular evolution of duplicated color vision genes (LWS-1 and SWS2) within cyprinid fish, focusing on the most cavefish-rich genus-Sinocyclocheilus. Maximum likelihood-based codon substitution approaches were used to analyze the evolution of vision genes. We found that the duplicated color vision genes had unequal evolutionary rates, which may lead to a related function divergence. Divergence of LWS-1 was strongly influenced by positive selection causing an accelerated rate of substitution in the proportion of pocket-forming residues. The SWS2 pigment experienced divergent selection between lineages, and no positively selected site was found. A duplicate copy of LWS-1 of some cyprinine species had become a pseudogene, but all SWS2 sequences remained intact in the regions examined in the cyprinid fishes examined in this study. The pseudogenization events did not occur randomly in the two copies of LWS-1 within Sinocyclocheilus species. Some cave species of Sinocyclocheilus with numerous morphological specializations that seem to be highly adapted for caves, retain both intact copies of color vision genes in their genome. We found some novel amino acid substitutions at key sites, which might represent interesting target sites for future mutagenesis experiments. Our data add to the increasing evidence that duplicate genes experience lower selective constraints and in some cases positive selection following gene duplication. Some of these observations are unexpected and may provide insights into the effect of caves on the evolution of color vision genes in fishes.