7,896 research outputs found
Scaling laws for soliton pulse compression by cascaded quadratic nonlinearities
We present a detailed study of soliton compression of ultra-short pulses
based on phase-mismatched second-harmonic generation (\textit{i.e.}, the
cascaded quadratic nonlinearity) in bulk quadratic nonlinear media. The
single-cycle propagation equations in the temporal domain including
higher-order nonlinear terms are presented. The balance between the quadratic
(SHG) and the cubic (Kerr) nonlinearity plays a crucial role: we define an
effective soliton number -- related to the difference between the SHG and the
Kerr soliton numbers -- and show that it has to be larger than unity for
successful pulse compression to take place. This requires that the phase
mismatch be below a critical level, which is high in a material where the
quadratic nonlinearity dominates over the cubic Kerr nonlinearity. Through
extensive numerical simulations we find dimensionless scaling laws, expressed
through the effective soliton number, which control the behaviour of the
compressed pulses. These laws hold in the stationary regime, in which
group-velocity mismatch effects are small, and they are similar to the ones
observed for fiber soliton compressors. The numerical simulations indicate that
clean compressed pulses below two optical cycles can be achieved in a
-barium borate crystal at appropriate wavelengths, even for picosecond
input pulses.Comment: 11 pages, 8 figures, resubmitted version, to appear in October issue
of J. Opt. Soc. Am. B. Substantially revised, updated mode
Nonlocal explanation of stationary and nonstationary regimes in cascaded soliton pulse compression
We study soliton pulse compression in materials with cascaded quadratic
nonlinearities, and show that the group-velocity mismatch creates two different
temporally nonlocal regimes. They correspond to what is known as the stationary
and nonstationary regimes. The theory accurately predicts the transition to the
stationary regime, where highly efficient pulse compression is possible.Comment: 3 pages, 2 figures, published verison in Optics Letters. Contains
revised equations, including an updated mode
Limits to compression with cascaded quadratic soliton compressors
We study cascaded quadratic soliton compressors and address the physical
mechanisms that limit the compression. A nonlocal model is derived, and the
nonlocal response is shown to have an additional oscillatory component in the
nonstationary regime when the group-velocity mismatch (GVM) is strong. This
inhibits efficient compression. Raman-like perturbations from the cascaded
nonlinearity, competing cubic nonlinearities, higher-order dispersion, and
soliton energy may also limit compression, and through realistic numerical
simulations we point out when each factor becomes important. We find that it is
theoretically possible to reach the single-cycle regime by compressing
high-energy fs pulses for wavelengths in a
-barium-borate crystal, and it requires that the system is in the
stationary regime, where the phase mismatch is large enough to overcome the
detrimental GVM effects. However, the simulations show that reaching
single-cycle duration is ultimately inhibited by competing cubic nonlinearities
as well as dispersive waves, that only show up when taking higher-order
dispersion into account.Comment: 16 pages, 5 figures, submitted to Optics Expres
KNEE POWER IN LOW BACK PAIN SUBJECTS DURING RUNNING
The purpose of this study was to examine lower extremity shock absorption between runners with and without low back pain. We compared data from three groups based on low back pain status: current low back pain, resolved pain after a single bout of low back pain and runners who never had low back pain (CTRL). All subjects ran at least 20 km per week and ran on a force treadmill at 3.8 m•s-1 while kinematic and kinetic data were collected. Work was determined from joint power histories during the shock attenuation portion of the stance phase. Individuals with a history of low back pain exhibited less peak knee negative power and negative work suggesting that they exhibited decreased eccentric muscle activity during foot-ground impact. The results of this study suggest that decreased eccentric activity of the muscles crossing the knee joint is associated with individuals who have low back pain and, to a lesser extent, with those who have residual low back pain. We suggest that the decreased eccentric activity can result in the footground impact shock wave moving through the lower extremity with little attenuation to the low back region
Operational modules for space station construction
Identification of an effective space construction concept is a current objective of NASA studies. One concept, described in this memorandum, consists of repetitive use of operational modules, which minimizes on-orbit stay time for the shuttle. A space station constructed of operational modules may benefit from fabrication and system checkout in ground-based facilities, and since the modules are the primary structure of the space station, a minimum of additional structure, and trips and on-orbit stay time of the shuttle are required
Effect of alkaline treatment on the sulfate content and quality of semi-refined carrageenan prepared from seaweed Kappaphycus alvarezii Doty (Doty) farmed in Indian waters
Seaweed Kappaphycus alvarezii previously known as Eucheuma cottonii is one of the best sources of kappa carrageenan and is cultivated in Philippines, Indonesia, Malaysia and other countries including India. In the present study, semi-refined carrageenan (SRC) was prepared from K. alvarezii with different concentrations of KOH (6, 12, 18 and 24%) at 80 ± 2°C for 2 h; its sulfate contents were 13.73±0.74, 13.66±0.27, 13.84±0.66 and 14.76±0.18%, respectively, and it was 14.10±0.34% for untreated clean weed (control). The sulfate removal with increasing concentration of KOH used for processing SRC is not statistically significant (p < 0.05). The KCl gel strength of untreated clean weed and 6, 12, 18 and 24% KOH treated weed was 97±10.27, 650±12.11, 637±10.25, 552±18.71 and 526±26.55 g cm-2, respectively. Contents of 3,6-anhydrogalactose were 28.30±0.52, 33.41±0.50, 32.97±0.42, 31.15±0.60 and 31.61±0.17%. FTIR spectroscopy showed that molecules in all four SRC samples are quite similar. Spectral band was at 1257 cm-1 which referred to ester sulfate of 930 cm-1 for 3,6 anhydrogalactose and 848 cm-1 assigned to galactose-4-sulfate. From the present investigation, it was observed that sulfate removal and quality improvement with increasing concentration of KOH for cooking seaweed is not statistically significant; therefore, seaweed can be subjected to lower concentration of KOH treatment (6 to 12%) to produce semi-refined carrageenan on commercial scale.Keywords: Seaweed, Kappaphycus alvarezii, KOH treatment, semi-refined carrageenan (SRC), sulfate content, FTIR spectra, gel strength.Abbreviation: SRC, semi-refined carrageenan; 3,6-AG, 3,6-anhydrogalacotse; FTIR, Fourier Transform Infrared Spectroscopy
Demand for programs for key populations in Africa from countries receiving international donor assistance
There has been increasing attention in recent years to the HIV prevention, treatment, and care needs of key populations in Africa, in particular men who have sex with men (MSM), injection drug users (IDU), and female sex workers (FSW). While several major donors have undertaken efforts to prioritize these groups, it remains unclear which African countries are actively seeking donor support for these programs. For this analysis, we reviewed publicly available proposal and budget documentation from the US PEPFAR for fiscal years 2007 through 2010 and Rounds 1 through 10 of the Global Fund to Fight AIDS, Tuberculosis and Malaria for 40 countries in sub-Saharan Africa. Of the 164 searchable documents retrieved, nearly two-thirds contained at least one program serving FSW (65%, 107 proposals), less than one-third contained at least one program serving MSM (29%, 47 proposals), and a minority proposed programming for IDU (13%, 21 proposals). Demand for these programs was highly concentrated in a subset of countries. Epidemiological data for at least one key population was included in a majority of these proposals (63%, 67 proposals), but in many cases these data were not linked to programs
Study of an attitude control system for the astronaut maneuvering unit final report, dec. 1963 - jul. 1964
Attitude control system for astronaut maneuvering unit
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