196 research outputs found
Brillouin amplification in phase coherent transfer of optical frequencies over 480 km fiber
We describe the use of fiber Brillouin amplification (FBA) for the coherent
transmission of optical frequencies over a 480 km long optical fiber link. FBA
uses the transmission fiber itself for efficient, bi-directional coherent
amplification of weak signals with pump powers around 30 mW. In a test setup we
measured the gain and the achievable signal-to-noise ratio (SNR) of FBA and
compared it to that of the widely used uni-directional Erbium doped fiber
amplifiers (EDFA) and to our recently built bi-directional EDFA. We measured
also the phase noise introduced by the FBA and used a new and simple technique
to stabilize the frequency of the FBA pump laser. We then transferred a
stabilized laser frequency over a wide area network with a total fiber length
of 480 km using only one intermediate FBA station. After compensating the noise
induced by the fiber, the frequency is delivered to the user end with an
uncertainty below 2x10-18 and an instability sigma(tau) = 2x10-14/(tau/second)
Frequency comparisons and absolute frequency measurements of 171Yb+ single-ion optical frequency standards
We describe experiments with an optical frequency standard based on a laser
cooled Yb ion confined in a radiofrequency Paul trap. The
electric-quadrupole transition from the ground state to the
state at the wavelength of 436 nm is used as the reference
transition. In order to compare two Yb standards, separate
frequency servo systems are employed to stabilize two probe laser frequencies
to the reference transition line centers of two independently stored ions. The
experimental results indicate a relative instability (Allan standard deviation)
of the optical frequency difference between the two systems of only, so that shifts in the sub-hertz range can be
resolved. Shifts of several hertz are observed if a stationary electric field
gradient is superimposed on the radiofrequency trap field. The absolute optical
transition frequency of Yb at 688 THz was measured with a cesium atomic
clock at two times separated by 2.8 years. A temporal variation of this
frequency can be excluded within a relative uncertainty of yr. Combined with recently published values for the constancy
of other transition frequencies this measurement provides a limit on the
present variability of the fine structure constant at the level of
yr.Comment: 12 pages, 5 figures, Proceedings of MPLP'04, Novosibirsk, August
22.-27., 200
Interrogation of caesium atoms in a fountain clock by a femtosecond laser microwave oscillator
A caesium fountain clock is operated utilizing a microwave oscillator that
derives its frequency stability from a stable laser by means of a fiber-laser
femtosecond frequency comb. This oscillator is based on the technology
developed for optical clocks and replaces the quartz based microwave oscillator
commonly used in fountain clocks. As a result, a significant decrease of the
frequency instability of the fountain clock is obtained, reaching 0.74E-14 at
100 s averaging time. We could demonstrate that for a significant range of
detected atom numbers the instability is limited by quantum projection noise
only, and that for the current status of this fountain clock the new microwave
source poses no limit on the achievable frequency instability.Comment: 4 pages, 4 figure
The optical calcium frequency standards of PTB and NIST
We describe the current status of the Ca optical frequency standards with
laser-cooled neutral atoms realized in two different laboratories for the
purpose of developing a possible future optical atomic clock.
Frequency measurements performed at the Physikalisch-Technische Bundesanstalt
(PTB) and the National Institute of Standards and Technology (NIST) make the
frequency of the clock transition of 40Ca one of the best known optical
frequencies (relative uncertainty 1.2e-14) and the measurements of this
frequency in both laboratories agree to well within their respective
uncertainties.
Prospects for improvement by orders of magnitude in the relative uncertainty
of the standard look feasible.Comment: 13 pages, 11 figures, to appear in Comptes Rendus Physiqu
Interrogation laser for a strontium lattice clock
We report on the setup and characterization of a 698 nm master-slave diode
laser system to probe the 1S0-3P0 clock transition of strontium atoms confined
in a one-dimensional optical lattice. A linewidth in the order of around 100 Hz
of the laser system has been measured with respect to an ultrastable 657 nm
diode laser with 1 Hz linewidth using a femtosecond fiber comb as transfer
oscillator. The laser has been used to measure the magnetically induced 1S0-3P0
clock transition in 88Sr where a linewidth of 165 Hz has been observed. The
transfer oscillator method provides a virtual beat signal between the two diode
lasers that has been used to phase lock the 698 nm laser to the 1 Hz linewidth
laser at 657 nm, transferring its stability to the 698 nm laser system.Comment: 5 pages, 7 figures, to be published in "IEEE Transactions on
Instrumentation and Measurement, Special Issue CPEM 2008
Основні закономірності зародження і росту втомних тріщин в алюмінієвих пластинах із зміцненими отворами
The method of modeling stress-strain state for holes burnishing using FEM has been
analyzed. A series of fatigue tests were carried out using plates containing plain holes and cold
expanded holes in aluminium For various diameters of holes and cold expansion degree there exists
a certain correlation between the stress range or maximum stress on the edge of hole on the entrance
face of plate and lifetime of fatigue crack initiation
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