A stable, continuous wave, single frequency ber ampli er large mode double clad ber cooled to liquid nitrogen temper ampli ed light is frequency quadrupled to 254 nm and used for mode, plane-polarized ytterbium-doped ber master oscillator power ampli er source with 264 W of output Erbium-Doped Fiber Ampli ers: Principles and Applicationoped large-core ber laser with 1.36 kW continuous-
Objective. Current determinations of diagnostic ultrasound exposure parameters (eg, peak rarefactional pressure and pulse intensity integral) are intended to correspond to propagation through soft tissue with a propagation speed of 1540 m/s and attenuation of 0.3 dB ⋅ cm . These current measurements are made in water, which has very little attenuation, and a linear derating factor is applied to approximate 0.3 dB ⋅ cm -1 ⋅ MHz -1 attenuation. The fact that propagation through water as well as through soft tissue involves nonlinear propagation is not directly addressed. A better way to determine exposure parameters would be to use a liquid that has the desired tissue-mimicking properties, including a value of the nonlinearity parameter B/A representative of soft tissue. To be of practical use in the laboratory, the ultrasonic properties of this liquid must remain stable and spatially uniform for many months or years without need for periodic mixing by the user. Methods. Fifty-two samples of fat-free milk that was concentrated to one third of its original volume by ultrafiltration were created. Each sample was preserved by a different method. The speed of sound, attenuation, and nonlinearity parameter B/A of each sample were periodically monitored by narrowband through-transmission techniques. Results. Six of the 52 samples remained liquid and retained acceptably stable acoustic properties over 22 months of storage at room temperature. Conclusions. Fat-free milk, concentrated via ultrafiltration and preserved in 1 of 6 different methods, has been found to be a stable tissue-mimicking liquid with acoustic properties appropriate for use in exposimetry. Key words: acoustic output; acoustic properties; bovine milk; exposimetry; liquid; tissue mimic. tissue-mimicking (TM) liquid with acoustic properties that are stable for many months or years could be useful in ultrasound exposimetry 1 and transducer characterization. To our knowledge, a TM liquid with acoustic properties that are stable for more than a few weeks without mechanical agitation to maintain uniformity has not yet been reported in the literature. Water is used as a propagation medium for exposimetry and beam characterization studies. The acoustic properties of water (speed of sound at room temperature, attenuation, and nonlinearity parameter) are considerably different from those of soft tissue. Several recent studies have shown that the use of water may lead to appreciable errors in values of exposimetry parameters due to a nonlinear propagation effect 2-7 or the speed of sound not being 1540 m/s. 8,9 The current method of quantifying acoustic output is described in detail in the Acoustic Output Measurement Standard for Diagnostic Ultrasound Equipment. Received September 30, 2004, from the Medical Physics (T.A.S., E.L.M., G.R.F.) and Food
We demonstrate a reliable continuous-wave (cw) laser source at the 1 S-2 P transition in (anti)hydrogen at 121.56 nm (Lyman-α) based on four-wave sum-frequency mixing in mercury. A two-photon resonance in the four-wave mixing scheme is essential for a powerful cw Lyman-α source and is well investigated.
Laser-controlled charge exchange is a promising method for producing cold antihydrogen. Caesium atoms in Rydberg states collide with positrons and create positronium. These positronium atoms then interact with antiprotons, forming antihydrogen. Laser excitation of the caesium atoms is essential to increase the cross section of the charge-exchange collisions. This method was demonstrated in 2004 by the ATRAP collaboration by using an available copper vapour laser. For a second generation of charge-exchange experiments we have designed a new semiconductor laser system that features several improvements compared to the copper vapour laser. We describe this new laser system and show the results from the excitation of caesium atoms to Rydberg states within the strong magnetic fields in the ATRAP apparatus.
Cooling antihydrogen atoms is important for future experiments both to test the fundamental CPT symmetry by high resolution laser spectroscopy and also to measure the gravitational acceleration of antimatter. Laser cooling of antihydrogen can be done on the strong 1S–2P transition at the wavelength of Lyman-alpha (121.6 nm). A continuous wave laser at the Lyman-alpha wavelength based on solid-state fundamental lasers is described. By using a two-photon and a near one-photon resonance a scan across the whole phase matching curve of the four-wave mixing process is possible. Furthermore the influence of the beam profile of one fundamental beam on the four-wave mixing process is studied.
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