245 research outputs found
Dosimetry of 3 CBCT devices for oral and maxillofacial radiology: CB Mercuray, NewTom 3G and i-CAT
Cone beam computed tomography (CBCT), which provides a lower dose, lower cost alternative to conventional CT, is being used with increasing frequency in the practice of oral and maxillofacial radiology. This study provides comparative measurements of effective dose for three commercially available, large (12'') field-of-view (FOV), CBCT units: CB Mercuray, NewTom 3G and i-CAT. Thermoluminescent dosemeters (TLDs) were placed at 24 sites throughout the layers of the head and neck of a tissue-equivalent human skull RANDO phantom. Depending on availability, the 12'' FOV and smaller FOV scanning modes were used with similar phantom positioning geometry for each CBCT unit. Radiation weighted doses to individual organs were summed using 1990 (E(1990)) and proposed 2005 (E(2005 draft)) ICRP tissue weighting factors to calculate two measures of whole-body effective dose. Dose as a multiple of a representative panoramic radiography dose was also calculated. For repeated runs dosimetry was generally reproducible within 2.5%. Calculated doses in microSv [corrected] (E(1990), E(2005 draft)) were NewTom3G (45, 59), i-CAT (135, 193) and CB Mercuray (477, 558). These are 4 to 42 times greater than comparable panoramic examination doses (6.3 microSv [corrected] 13.3 mSv). Reductions in dose were seen with reduction in field size and mA and kV technique factors. CBCT dose varies substantially depending on the device, FOV and selected technique factors. Effective dose detriment is several to many times higher than conventional panoramic imaging and an order of magnitude or more less than reported doses for conventional CT
Accuracy Evaluation of an Optical Lattice Clock with Bosonic Atoms
We report the first accuracy evaluation of an optical lattice clock based on
the 1S0 - 3P0 transition of an alkaline earth boson, namely 88Sr atoms. This
transition has been enabled using a static coupling magnetic field. The clock
frequency is determined to be 429 228 066 418 009(32) Hz. The isotopic shift
between 87Sr and 88Sr is 62 188 135 Hz with fractional uncertainty 5.10^{-7}.
We discuss the conditions necessary to reach a clock accuracy of 10^{-17} or
less using this scheme.Comment: 3 pages, 4 figures, uses ol.sty fil
Ultra-Low Noise Microwave Extraction from Fiber-Based Optical Frequency Comb
In this letter, we report on all-optical fiber approach to the generation of
ultra-low noise microwave signals. We make use of two erbium fiber mode-locked
lasers phase locked to a common ultra-stable laser source to generate an 11.55
GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz
from the carrier.The residual frequency instability of the microwave signals
derived from the two optical frequency combs is below 2.3 10^(-16) at 1s and
about 4 10^(-19) at 6.5 10^(4)s (in 5 Hz bandwidth, three days continuous
operation).Comment: 12 pages, 3 figure
75%-efficiency blue generation from an intracavity PPKTP frequency doubler
We report on a high-efficiency 461 nm blue light conversion from an external
cavity-enhanced second-harmonic generation of a 922 nm diode laser with a
quasi-phase-matched KTP crystal (PPKTP). By choosing a long crystal (LC=20 mm)
and twice looser focusing (w0=43 m) than the "optimal" one, thermal
lensing effects due to the blue power absorption are minimized while still
maintaining near-optimal conversion efficiency. A stable blue power of 234 mW
with a net conversion efficiency of eta=75% at an input mode-matched power of
310 mW is obtained. The intra-cavity measurements of the conversion efficiency
and temperature tuning bandwidth yield an accurate value d33(461 nm)=15 pm/V
for KTP and provide a stringent validation of some recently published linear
and thermo-optic dispersion data of KTP
Testing ultrafast mode-locking at microhertz relative optical linewidth
We report new limits on the phase coherence of the ultrafast mode-locking
process in an octave-spanning Ti:sapphire comb. We find that the mode-locking
mechanism correlates optical phase across a full optical octave with less than
2.5 micro Hz relative linewidth. This result is at least two orders of
magnitude below recent predictions for quantum-limited individual comb-mode
linewidths, verifying that the mode-locking mechanism strongly correlates
quantum noise across the comb spectrum.Comment: Expanded discussion to include correlated noise terms, made minor
formatting changes, added a reference, and fixed typographical errors. 10
pages, 5 figure
An Optical Lattice Clock with Spin-polarized 87Sr Atoms
We present a new evaluation of an 87Sr optical lattice clock using spin
polarized atoms. The frequency of the 1S0-3P0 clock transition is found to be
429 228 004 229 873.6 Hz with a fractional accuracy of 2.6 10^{-15}, a value
that is comparable to the frequency difference between the various primary
standards throughout the world. This measurement is in excellent agreement with
a previous one of similar accuracy
Quantum State Engineering and Precision Metrology using State-Insensitive Light Traps
Precision metrology and quantum measurement often demand matter be prepared
in well defined quantum states for both internal and external degrees of
freedom. Laser-cooled neutral atoms localized in a deeply confining optical
potential satisfy this requirement. With an appropriate choice of wavelength
and polarization for the optical trap, two electronic states of an atom can
experience the same trapping potential, permitting coherent control of
electronic transitions independent of the atomic center-of-mass motion. We
review a number of recent experiments that use this approach to investigate
precision quantum metrology for optical atomic clocks and coherent control of
optical interactions of single atoms and photons within the context of cavity
quantum electrodynamics. We also provide a brief survey of promising prospects
for future work.Comment: 23 pages of main text, 4 figures, and 12 pages of Supporting Online
Material (SOM
Measuring the frequency of a Sr optical lattice clock using a 120-km coherent optical transfer
We demonstrate a precision frequency measurement using a phase-stabilized
120-km optical fiber link over a physical distance of 50 km. The transition
frequency of the 87Sr optical lattice clock at the University of Tokyo is
measured to be 429228004229874.1(2.4) Hz referenced to international atomic
time (TAI). The measured frequency agrees with results obtained in Boulder and
Paris at a 6*10^-16 fractional level, which matches the current best
evaluations of Cs primary frequency standards. The results demonstrate the
excellent functions of the intercity optical fibre link, and the great
potential of optical lattice clocks for use in the redefinition of the second.Comment: 14 pages, 3 figure
Amplitude to phase conversion of InGaAs pin photo-diodes for femtosecond lasers microwave signal generation
When a photo-diode is illuminated by a pulse train from a femtosecond laser,
it generates microwaves components at the harmonics of the repetition rate
within its bandwidth. The phase of these components (relative to the optical
pulse train) is known to be dependent on the optical energy per pulse. We
present an experimental study of this dependence in InGaAs pin photo-diodes
illuminated with ultra-short pulses generated by an Erbium-doped fiber based
femtosecond laser. The energy to phase dependence is measured over a large
range of impinging pulse energies near and above saturation for two typical
detectors, commonly used in optical frequency metrology with femtosecond laser
based optical frequency combs. When scanning the optical pulse energy, the
coefficient which relates phase variations to energy variations is found to
alternate between positive and negative values, with many (for high harmonics
of the repetition rate) vanishing points. By operating the system near one of
these vanishing points, the typical amplitude noise level of commercial-core
fiber-based femtosecond lasers is sufficiently low to generate state-of-the-art
ultra-low phase noise microwave signals, virtually immune to amplitude to phase
conversion related noise.Comment: 7 pages, 6 figures, submitted to Applied Physics
Laser locking to the 199Hg clock transition with 5.4x10^(-15)/sqrt(tau) fractional frequency instability
With Hg atoms confined in an optical lattice trap in the Lamb-Dicke regime,
we obtain a spectral line at 265.6 nm in which the full-width at half-maximum
is <15Hz. Here we lock an ultrastable laser to this ultranarrow clock
transition and achieve a fractional frequency stability of
5.4x10^(-15)/sqrt(tau) for tau<=400s. The highly stable laser light used for
the atom probing is derived from a 1062.6 nm fiber laser locked to an
ultrastable optical cavity that exhibits a mean drift rate of -6.0x10^(-17)
s^(-1) (or -16.9 mHz.s^(-1) at 282 THz) over a five month period. A comparison
between two such lasers locked to independent optical cavities shows a flicker
noise limited fractional frequency instability of 4x10^(-16) per cavity
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