5,837 research outputs found
Dark state lasers
We propose a new type of laser resonator based on imaginary "energy-level
splitting" (imaginary coupling, or quality factor Q splitting) in a pair of
coupled microcavities. A particularly advantageous arrangement involves two
microring cavities with different free-spectral ranges (FSRs) in a
configuration wherein they are coupled by "far-field" interference in a shared
radiation channel. A novel Vernier-like effect for laser resonators is designed
where only one longitudinal resonant mode has a lower loss than the small
signal gain and can achieve lasing while all other modes are suppressed. This
configuration enables ultra-widely tunable single-frequency lasers based on
either homogeneously or inhomogeneously broadened gain media. The concept is an
alternative to the common external cavity configurations for achieving tunable
single-mode operation in a laser. The proposed laser concept builds on a high-Q
"dark state" that is established by radiative interference coupling and bears a
direct analogy to parity-time (PT) symmetric Hamiltonians in optical systems.
Variants of this concept should be extendable to parametric-gain based
oscillators, enabling use of ultrabroadband parametric gain for widely tunable
single-frequency light sources
Enhanced Trellis Coded Multiple Access (ETCMA)
We propose an enhanced version of trellis coded multiple access (TCMA), an
overloaded multiple access scheme that outperforms the original TCMA in terms
of achieved spectral efficiency. Enhanced TCMA (ETCMA) performs simultaneous
transmission of multiple data streams intended for users experiencing similar
signal-to-noise ratios and can be employed both in the uplink and in the
downlink of wireless systems, thus overcoming one of the main limitations of
TCMA. Thanks to a new receiver algorithm, ETCMA is capable of delivering a
significantly higher spectral efficiency. We show that ETCMA approaches the
capacity of the Additive White Gaussian Noise channel for a wide range of
signal-to-noise ratios.Comment: 5 pages, 5 figure
Glass Transition in a Two-Dimensional Electron System in Silicon in a Parallel Magnetic Field
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional electron system (2DES) in Si in the vicinity of the
metal-insulator transition (MIT) persists in parallel magnetic fields B of up
to 9 T. At low B, both the glass transition density and , the
critical density for the MIT, increase with B such that the width of the
metallic glass phase () increases with B. At higher B, where the
2DES is spin polarized, and no longer depend on B. Our results
demonstrate that charge, as opposed to spin, degrees of freedom are responsible
for glassy ordering of the 2DES near the MIT.Comment: 4 pages, 5 figure
Tunable coupled-mode dispersion compensation and its application to on-chip resonant four-wave mixing
We propose and demonstrate localized mode coupling as a viable dispersion
engineering technique for phase-matched resonant four-wave mixing (FWM). We
demonstrate a dual-cavity resonant structure that employs coupling-induced
frequency splitting at one of three resonances to compensate for cavity
dispersion, enabling phase-matching. Coupling strength is controlled by thermal
tuning of one cavity enabling active control of the resonant
frequency-matching. In a fabricated silicon microresonator, we show an 8 dB
enhancement of seeded FWM efficiency over the non-compensated state. The
measured four-wave mixing has a peak wavelength conversion efficiency of -37.9
dB across a free spectral range (FSR) of 3.334 THz (27 nm). Enabled by
strong counteraction of dispersion, this FSR is, to our knowledge, the largest
in silicon to demonstrate FWM to date. This form of mode-coupling-based, active
dispersion compensation can be beneficial for many FWM-based devices including
wavelength converters, parametric amplifiers, and widely detuned correlated
photon-pair sources. Apart from compensating intrinsic dispersion, the proposed
mechanism can alternatively be utilized in an otherwise dispersionless
resonator to counteract the detuning effect of self- and cross-phase modulation
on the pump resonance during FWM, thereby addressing a fundamental issue in the
performance of light sources such as broadband optical frequency combs
Validation of Individual Non-Linear Predictive Pharmacokinetic Parameters in a Rabbit Phenytoin Model
Purpose: To evaluate the predictive performance of phenytoin multiple dosing non-linear pharmacokinetic model in rabbits for possible application in therapy individualization in humans.Methods: Phenytoin was intravenously administered to 10 rabbits (2 – 3 kg). Plasma concentrations were measured by high pressure liquid chromatography (HPLC). Rabbits received 3 single phenytoin doses (11, 22 and 44 mg/kg) and plasma concentrations were fitted according to linear twocompartmental model. In all the rabbits, based on 3 different multiple doses (D1, D2, D3, range 9 – 15 mg/kg), 3 steady state plasma concentrations (Css1, Css2, Css3, range 20 - 56mg/l) were achieved. Formultiple dosage, the non-linear parameters, Km and Vm, were calculated according to the equations: Km = (D1-D2)/[(D2/Css2)-(D1/Css1)] and Vm = D2+KmD2/Css2, and individually used to calculate Css3 = D3Km/(Vm-D3). Predicted and measured Css3 values were compared.Results: The values for pharmacokinetic parameters after single doses were dose-dependent. The pronounced inter-individual variations in Km (extreme values 18 – 91 mg/l differed 5.5 times) and Vm (11 – 28 mg/kg/h) values were recorded. Significant correlation of predicted Css3 with the measured value for the same dose (D3) was found (r = 0.854, N = 10, p < 0.01). There was no statistical difference between predicted and measured concentrations (t-dependent test = 1.074, p < 0.05).Conclusion: Non-linear parameters, Km and Vm, obtained from only two steady-state concentration measurements can be successfully used to compute and achieve a particular steady-state plasma concentration and optimal dosage regimen.Keywords: Phenytoin, Rabbit, Pharmacokinetic model, Multiple dosing, Non-linear, Individualizatio
Metal-insulator transition and glassy behavior in two-dimensional electron systems
Studies of low-frequency resistance noise demonstrate that glassy freezing
occurs in a two-dimensional electron system in silicon in the vicinity of the
metal-insulator transition (MIT). The width of the metallic glass phase, which
separates the 2D metal and the (glassy) insulator, depends strongly on
disorder, becoming extremely small in high-mobility (low-disorder) samples. The
glass transition is manifested by a sudden and dramatic slowing down of the
electron dynamics, and by a very abrupt change to the sort of statistics
characteristic of complicated multistate systems. In particular, the behavior
of the second spectrum, an important fourth-order noise statistic, indicates
the presence of long-range correlations between fluctuators in the glassy
phase, consistent with the hierarchical picture of glassy dynamics.Comment: Contribution to conference on "Noise as a tool for studying
materials" (SPIE), Santa Fe, New Mexico, June 2003; 15 pages, 12 figs.
(includes some low-quality figs; send e-mail to get high-quality figs.
Influence of Intra-cell Traffic on the Output Power of Base Station in GSM
In this paper we analyze the influence of intracell traffic in a GSM cell on the base station output power. It is proved that intracell traffic increases this power. If offered traffic is small, the increase of output power is equal to the part of intracell traffic. When the offered traffic and, as the result, call loss increase, the increase of output power becomes less. The results of calculation are verified by the computer simulation of traffic process in the GSM cell. The calculation and the simulation consider the uniform distribution of mobile users in the cell, but the conclusions are of a general nature
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