244 research outputs found
Frequency stability of a wavelength meter and applications to laser frequency stabilization
Interferometric wavelength meters have attained frequency resolutions down to
the MHz range. In particular, Fizeau interferometers, which have no moving
parts, are becoming a popular tool for laser characterization and
stabilization. In this article, we characterize such a wavelength meter using
an ultra-stable laser in terms of relative frequency instability
and demonstrate that it can achieve a short-term instability
and a frequency drift of order
MHz/day. We use this apparatus to demonstrate frequency control of a
near-infrared laser, where a frequency instability below
from 1 s to 2000 s is achieved. Such performance is for example adequate for
ions trapping and atoms cooling experiments.Comment: 5 pages, 4 figure
DC-powered Fe3+:sapphire Maser and its Sensitivity to Ultraviolet Light
The zero-field Fe3+:sapphire whispering-gallery-mode maser oscillator
exhibits several alluring features: Its output is many orders of magnitude
brighter than that of an active hydrogen maser and thus far less degraded by
spontaneous-emission (Schawlow-Townes) and/or receiving-amplifier noise. Its
oscillator loop is confined to a piece of mono-crystalline rock bolted into a
metal can. Its quiet amplification combined with high resonator Q provide the
ingredients for exceptionally low phase noise. We here concentrate on novelties
addressing the fundamental conundrums and technical challenges that impede
progress. (1) Roasting: The "mase-ability" of sapphire depends significantly on
the chemical conditions under which it is grown and heat-treated. We provide
some fresh details and nuances here. (2) Simplification: This paper obviates
the need for a Ka-band synthesizer: it describes how a 31.3 GHz loop
oscillator, operating on the preferred WG pump mode, incorporating Pound
locking, was built from low-cost components. (3) "Dark Matter": A Siegman-level
analysis of the experimental data determines the substitutional concentration
of Fe3+ in HEMEX to be less than a part per billion prior to roasting and up to
a few hundred ppb afterwards. Chemical assays, using different techniques
(incl. glow discharge mass spectra spectroscopy and neutron activation
analysis) consistently indicate, however, that HEMEX contains iron at
concentrations of a few parts per million. Drawing from several
forgotten-about/under-appreciated papers, this substantial discrepancy is
addressed. (4) Excitons: Towards providing a new means of controlling the
Fe3+:sapph. system, a cryogenic sapphire ring was illuminated, whilst masing,
with UV light at wavelengths corresponding to known electronic and
charge-transfer (thus valence-altering) transitions. Preliminary experiments
are reported.Comment: pdf only; submitted to the proceedings of the 24th European Frequency
and Time Forum, 13-15th April, 201
The Nonlinear Evolution of Instabilities Driven by Magnetic Buoyancy: A New Mechanism for the Formation of Coherent Magnetic Structures
Motivated by the problem of the formation of active regions from a
deep-seated solar magnetic field, we consider the nonlinear three-dimensional
evolution of magnetic buoyancy instabilities resulting from a smoothly
stratified horizontal magnetic field. By exploring the case for which the
instability is continuously driven we have identified a new mechanism for the
formation of concentrations of magnetic flux.Comment: Published in ApJL. Version with colour figure
Ultra-low phase noise all-optical microwave generation setup based on commercial devices
In this paper, we present a very simple design based on commercial devices
for the all-optical generation of ultra-low phase noise microwave signals. A
commercial, fibered femtosecond laser is locked to a laser that is stabilized
to a commercial ULE Fabry-Perot cavity. The 10 GHz microwave signal extracted
from the femtosecond laser output exhibits a single sideband phase noise
at 1 Hz Fourier frequency, at
the level of the best value obtained with such "microwave photonics" laboratory
experiments \cite{Fortier2011}. Close-to-the-carrier ultra-low phase noise
microwave signals will now be available in laboratories outside the frequency
metrology field, opening up new possibilities in various domains.Comment: 8 pages, 3 figures. To be published in Applied Optics, early posting
version available at
http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=23114
Viscous and inviscid strato-rotational instability
We examine the critical viscous mode of the TaylorâCouette strato-rotational instability, concentrating on cases where the buoyancy frequency N and the inner cylinder rotation rate âŠin are comparable, giving a detailed account for N = âŠin. The ratio of the outer to the inner cylinder rotation rates ” = âŠout/âŠin and the ratio of the inner to the outer cylinder radius η = rin/rout satisfy 0 < ” < 1 and 0 < η < 1. We find considerable variation in the structure of the mode, and the critical Reynolds number Rec at which the flow becomes unstable. For N = âŠin, we classify different regions of the η”-plane by the critical viscous mode of each region. We find that there is a triple point in the η”-plane where three different viscous modes all onset at the same Reynolds number. We also find a discontinuous change in Rec along a curve in the η”-plane, on one side of which exist closed unstable domains where the flow can restabilise when the Reynolds number is increased. A new form of viscous instability occurring for wide gaps has been detected. We show for the first time that there is a region of the parameter space for which the critical viscous mode at the onset of instability corresponds to the inviscid radiative instability of Le DizĂšs & Riedinger (J. Fluid Mech., vol. 660, 2010, pp. 147â161). Focusing on small-to-moderate wavenumbers, we demonstrate that the viscous and inviscid systems are not always correlated. We explore which viscous modes relate to inviscid modes and which do not. For asymptotically large vertical wavenumbers, we have extended the inviscid analysis of Park & Billant (J. Fluid Mech., vol. 725, 2013, pp. 262â280) to cover the cases where N and âŠin are comparable
Influence of the ESR saturation on the power sensitivity of cryogenic sapphire resonators
Here, we study the paramagnetic ions behavior in presence of a strong
microwave electromagnetic field sustained inside a cryogenic sapphire
whispering gallery mode resonator. The high frequency measurement resolution
that can be now achieved by comparing two CSOs permit for the first time to
observe clearly the non-linearity of the resonator power sensitivity. These
observations that in turn allow us to optimize the CSO operation, are well
explained by the Electron Spin Resonance (ESR) saturation of the paramagnetic
impurities contained in the sapphire crystal.Comment: 8 pages, 9 figure
Compact Yb optical atomic clock project: design principle and current status
We present the design of a compact optical clock based on the 435.5 nm transition in Yb. The ion trap will
be based on a micro-fabricated circuit, with surface electrodes generating a
trapping potential to localize a single Yb ion a few hundred m from the
electrodes. We present our trap design as well as simulations of the resulting
trapping pseudo-potential. We also present a compact, multi-channel wavelength
meter that will permit the frequency stabilization of the cooling, repumping
and clear-out lasers at 369.5 nm, 935.2 nm and 638.6 nm needed to cool the ion.
We use this wavelength meter to characterize and stabilize the frequency of
extended cavity diode lasers at 369.5 nm and 638.6 nm.Comment: 7 pages, 5 figures. Proc. of the 8th FSM 2015, Potsdam, Germany. To
be published in IOP Journal of Physics: Conference Serie
Pressure-driven Instabilities in Cylindrical Geometry: A New General Criterion
A new criterion for pressure-driven interchange instabilities in cylindrical
geometry is derived, based on an alternate use of the Energy Principle. This
criterion is inequivalent to Suydam's criterion and does not contain the
magnetic shear. In fact, it is shown that Suydam's criterion relates to the
instability of the slow magnetosonic branch, while the present criterion
relates to the Alfv\'enic one, which is the most dangerous of the two. These
findings explain why pressure-driven modes nearly always exist even if Suydam's
criterion is satisfied by a large margin.Comment: 4 pages. Submitted to Phys. Rev. Let
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