8,586 research outputs found
2D Saturable Absorbers for Fibre Lasers
© 2015.Two-dimensional (2D) nanomaterials are an emergent and promising platform for future photonic and optoelectronic applications. Here, we review recent progress demonstrating the application of 2D nanomaterials as versatile, wideband saturable absorbers for Q-switching and mode-locking fibre lasers. We focus specifically on the family of few-layer transition metal dichalcogenides, including MoS2, MoSe2 and WS2
Towards 'smart lasers': self-optimisation of an ultrafast pulse source using a genetic algorithm
Short-pulse fibre lasers are a complex dynamical system possessing a broad
space of operating states that can be accessed through control of cavity
parameters. Determination of target regimes is a multi-parameter global
optimisation problem. Here, we report the implementation of a genetic algorithm
to intelligently locate optimum parameters for stable single-pulse mode-locking
in a Figure-8 fibre laser, and fully automate the system turn-on procedure.
Stable ultrashort pulses are repeatably achieved by employing a compound
fitness function that monitors both temporal and spectral output properties of
the laser. Our method of encoding photonics expertise into an algorithm and
applying machine-learning principles paves the way to self-optimising `smart'
optical technologies
Genetic algorithm-based control of birefringent filtering for self-tuning, self-pulsing fiber lasers
Polarization-based filtering in fiber lasers is well-known to enable spectral
tunability and a wide range of dynamical operating states. This effect is
rarely exploited in practical systems, however, because optimization of cavity
parameters is non-trivial and evolves due to environmental sensitivity. Here,
we report a genetic algorithm-based approach, utilizing electronic control of
the cavity transfer function, to autonomously achieve broad wavelength tuning
and the generation of Q-switched pulses with variable repetition rate and
duration. The practicalities and limitations of simultaneous spectral and
temporal self-tuning from a simple fiber laser are discussed, paving the way to
on-demand laser properties through algorithmic control and machine learning
schemes.Comment: Accepted for Optics Letters, 12th June 201
Dark solitons in laser radiation build-up dynamics
We reveal the existence of slowly-decaying dark solitons in the radiation
build-up dynamics of bright pulses in all-normal dispersion mode-locked fiber
lasers, numerically modeled in the framework of a generalized nonlinear
Schr\"odinger equation. The evolution of noise perturbations to
quasi-stationary dark solitons is examined, and the significance of background
shape and soliton-soliton collisions on the eventual soliton decay is
established. We demonstrate the role of a restoring force in extending soliton
interactions in conservative systems to include the effects of dissipation, as
encountered in laser cavities, and generalize our observations to other
nonlinear systems
Knowledge source preferences as determinants of strategic entrepreneurial orientation
In the knowledge intensive context, firms’ capacity to integrate external and internal
sources of knowledge becomes an important competitive advantage and may
distinguish entrepreneurial from conservative firms. This paper explores the
proposition that differences in strategic entrepreneurial orientation (EO) across firms
may be significantly determined by differences in firms’ preferences regarding
knowledge sources. Our research is based on 208 firms operating in knowledge
intensive industries in six Central and East European countries (CEEC). We
identified three types of firms in terms of patterns of sources of knowledge: external
R&D knowledge based firms, in-house knowledge based firms and value chain
dependent firms. By using different proxies or different dimensions of EO, we have
found that the EO is strongest in firms based on external knowledge. Firms with inhouse
based knowledge have an intermediate strength of the EO, and firms dependent
on value chains are the least entrepreneurially oriented. We have also found moderate
support for grouping different proxies of EO into three dimensions identified in
literature – innovativeness, pro-activeness and risk-taking. Value chain firms are not
pro-active, have the lowest innovativeness, and are the most risk averse. External
knowledge based firms are the most active in all three dimensions of EO, while inhouse
knowledge based firms are in an intermediate position. Our results point to
strong systemic features of entrepreneurial activities; i.e., EO is inherently different in
different sub-populations of firms depending on their patterns of sources of
knowledge. It seems that these patterns operate as a moderating factor between
performance and the EO, which explains mixed results from the literature
Infrared Observations of novae in the SOFIA era
Classical novae inject chemically enriched gas and dust into the local
inter-stellar medium (ISM). Abundances in the ejecta can be deduced from
infrared (IR) forbidden line emission. IR spectroscopy can determine the
mineralogy of grains that grow in nova ejecta. We anticipate the impact that
NASA's new Stratospheric Observatory for Infrared Astronomy (SOFIA) will have
on future IR studies of novae.Comment: To appear in the proceedings of "Physics of Evolved Stars 2015 - A
conference dedicated to the memory of Olivier Chesneau
The circumstellar dust of "Born-Again" stars
We describe the evolution of the carbon dust shells around Very Late Thermal
Pulse (VLTP) objects as seen at infrared wavelengths. This includes a 20-year
overview of the evolution of the dust around Sakurai's object (to which Olivier
made a seminal contribution) and FG Sge. VLTPs may occur during the endpoint of
as many as 25% of solar mass stars, and may therefore provide a glimpse of the
possible fate of the Sun.Comment: To appear in the proceedings of "Physics of Evolved Stars 2015 - A
conference dedicated to the memory of Olivier Chesneau
Aerodynamic and acoustic effects of eliminating core swirl from a full scale 1.6 stage pressure ratio fan (QF-5A)
Fan QF-5A was a modification of fan QF-5 which had an additional core stator and adjusted support struts to turn the core exit flow from a 30 deg swirl to the axial direction. This modification was necessary to eliminate the impingement of the swirling core flow on the axial support pylon of the NASA-Lewis Quiet Fan Facility that caused aerodynamic, acoustic and structural problems with the original fan stage at fan speeds greater than 85 percent of design. The redesigned fan QF-5A did obtain the design bypass ratio with an increased core airflow suggesting that the flow problem was resolved. Acoustically, the redesigned stage showed a low frequency broadband noise reduction compared to the results for fan QF-5 at similar operating conditions
Noise of fan designed to reduce stator lift fluctuations
An existing fan stage was redesigned to reduce stator lift fluctuations and was acoustically tested at three nozzle sizes for reduced noise generation. The lift fluctuations on the stator were reduced by increasing the stator cord, adjusting incidence angles, and adjusting the rotor velocity diagrams. Broadband noise levels were signficantly reduced in the middle to high frequencies. Blade passage tone sound power was not lessened, but decreases in the harmonics were observed. Aerodynamic improvements in both performance and efficiency were obtained
Acoustic and aerodynamic performance of a 1.83 meter (6 foot) diameter 1.2 pressure ratio fan (QF-6)
A 1.2-pressure-ratio, 1.83-meter-(6-ft-) diameter experimental fan stage with characteristics suitable for use in STOL aircraft engines was tested for acoustic and aerodynamic performance. The design incorporated features for low noise, including absence of inlet guide vanes, low rotor-blade-tip speed, low aerodynamic blade loading, and long axial spacing between the rotor and stator rows. The stage was run with four nozzles of different area. The perceived noise along a 152.4 meter (500-ft) sideline was rear-quadrant dominated with a maximum design-point level of 103.9 PNdb. The acoustic 1/3-octave results were analytically separated into broadband and pure-tone components. It was found that the stage noise levels generally increase with a decrease in nozzle area, with this increase observed primarily in the broadband noise component. A stall condition was documented acoustically with a 90-percent-of-design-area nozzle
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