5,308 research outputs found
Phase error statistics of a phase-locked loop synchronized direct detection optical PPM communication system
Receiver timing synchronization of an optical Pulse-Position Modulation (PPM) communication system can be achieved using a phased-locked loop (PLL), provided the photodetector output is suitably processed. The magnitude of the PLL phase error is a good indicator of the timing error at the receiver decoder. The statistics of the phase error are investigated while varying several key system parameters such as PPM order, signal and background strengths, and PPL bandwidth. A practical optical communication system utilizing a laser diode transmitter and an avalanche photodiode in the receiver is described, and the sampled phase error data are presented. A linear regression analysis is applied to the data to obtain estimates of the relational constants involving the phase error variance and incident signal power
Direct measurement of the fine-structure interval in alkali atoms using diode lasers
We demonstrate a technique for directly measuring the fine-structure interval
in alkali atoms using two frequency-stabilized diode lasers. Each laser has a
linewidth of order 1 MHz and precise tunability: one laser is tuned to a
hyperfine transition in the D_1 line, and the other laser to a hyperfine
transition in the D_2 line. The outputs of the lasers are fed into a scanning
Michelson interferometer that measures the ratio of their wavelengths
accurately. To illustrate the technique, we measure the fine-structure interval
in Rb, and obtain a value of 237.6000(3)(5) cm^-1 for the hyperfine-free
5P_{3/2} - 5P_{1/2} interval.Comment: 3 pages, 2 figures, to be published in Applied Physics Letters, 20
May 2002 editio
High-accuracy wavemeter based on a stabilized diode laser
We have built a high-accuracy wavelength meter for tunable lasers using a
scanning Michelson interferometer and a reference laser of known wavelength.
The reference laser is a frequency stabilized diode laser locked to an atomic
transition in Rb. The wavemeter has a statistical error per measurement of 5
parts in which can be reduced considerably by averaging. Using a second
stabilized diode laser, we have verified that systematic errors are below 4
parts in .Comment: 3 pages, 2 figure
Complexes of Mn(II), Fe(II), Co(II), Ni(II) & Cu(II) with Naphthalene-l-sulphinic Acid
1102-110
Evolutionary dynamics of adult stem cells: Comparison of random and immortal strand segregation mechanisms
This paper develops a point-mutation model describing the evolutionary
dynamics of a population of adult stem cells. Such a model may prove useful for
quantitative studies of tissue aging and the emergence of cancer. We consider
two modes of chromosome segregation: (1) Random segregation, where the daughter
chromosomes of a given parent chromosome segregate randomly into the stem cell
and its differentiating sister cell. (2) ``Immortal DNA strand''
co-segregation, for which the stem cell retains the daughter chromosomes with
the oldest parent strands. Immortal strand co-segregation is a mechanism,
originally proposed by Cairns (J. Cairns, {\it Nature} {\bf 255}, 197 (1975)),
by which stem cells preserve the integrity of their genomes. For random
segregation, we develop an ordered strand pair formulation of the dynamics,
analogous to the ordered strand pair formalism developed for quasispecies
dynamics involving semiconservative replication with imperfect lesion repair
(in this context, lesion repair is taken to mean repair of postreplication
base-pair mismatches). Interestingly, a similar formulation is possible with
immortal strand co-segregation, despite the fact that this segregation
mechanism is age-dependent. From our model we are able to mathematically show
that, when lesion repair is imperfect, then immortal strand co-segregation
leads to better preservation of the stem cell lineage than random chromosome
segregation. Furthermore, our model allows us to estimate the optimal lesion
repair efficiency for preserving an adult stem cell population for a given
period of time. For human stem cells, we obtain that mispaired bases still
present after replication and cell division should be left untouched, to avoid
potentially fixing a mutation in both DNA strands.Comment: 9 pages, 3 figure
Do long-duration GRBs follow star formation?
We compare the luminosity function and rate inferred from the BATSE long
bursts peak flux distribution with those inferred from the Swift peak flux
distribution. We find that both the BATSE and the Swift peak fluxes can be
fitted by the same luminosity function and the two samples are compatible with
a population that follows the star formation rate. The estimated local long GRB
rate (without beaming corrections) varies by a factor of five from 0.05
Gpc^(-3)yr^(-1) for a rate function that has a large fraction of high redshift
bursts to 0.27 Gpc^(-3)yr^(-1) for a rate function that has many local ones. We
then turn to compare the BeppoSax/HETE2 and the Swift observed redshift
distributions and compare them with the predictions of the luminosity function
found. We find that the discrepancy between the BeppoSax/HETE2 and Swift
observed redshift distributions is only partially explained by the different
thresholds of the detectors and it may indicate strong selection effects. After
trying different forms of the star formation rate (SFR) we find that the
observed Swift redshift distribution, with more observed high redshift bursts
than expected, is inconsistent with a GRB rate that simply follows current
models for the SFR. We show that this can be explained by GRB evolution beyond
the SFR (more high redshift bursts). Alternatively this can also arise if the
luminosity function evolves and earlier bursts were more luminous or if strong
selection effects affect the redshift determination.Comment: 15 pages, 8 figures, accepted for publication in JCA
Observation of persistent flow of a Bose-Einstein condensate in a toroidal trap
We have observed the persistent flow of Bose-condensed atoms in a toroidal
trap. The flow persists without decay for up to 10 s, limited only by
experimental factors such as drift and trap lifetime. The quantized rotation
was initiated by transferring one unit, , of the orbital angular
momentum from Laguerre-Gaussian photons to each atom. Stable flow was only
possible when the trap was multiply-connected, and was observed with a BEC
fraction as small as 15%. We also created flow with two units of angular
momentum, and observed its splitting into two singly-charged vortices when the
trap geometry was changed from multiply- to simply-connected.Comment: 1 file, 5 figure
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