Narrow-band pulses

Fig. 11. Fundamental and harmonic imaging In the fundamental imaging mode (a), a narrow-band pulse of ultrasound (US) centered at a given frequency (e.g., 2.5 MHz) is emitted the sound reflected by the organs is used to create the image, (b) Microbubbles, because they are extremely compressible in comparison to organ tissue, not only reflect sound more efficiently than tissues but also emit harmonics. In the harmonic mode, the signal from the tissues is filtered out, leaving only the harmonics, resulting in specific imaging of the bubbles [37].

Composite 90° pulses, which are optimized for high sensitivity towards Bi variation are called narrow-band pulses. They are also referred to as retrograde pulses. 

The application of two-photon spectroscopy to molecules has brought a wealth of new insight to excited molecular states. One example is the two-photon excitation of CO in the fourth positive system A TJ Ug and of N2 in the Lyman-Birge-Hopfield system with a narrow-band pulsed frequency-doubled dye laser. Doppler-free spectra of states with excitation energies between 8-12 eV can be measured with this technique [253]. 

The three-photon absorption can be used for the excitation of high-lying molecular levels with the same parity as accessible to one-photon transitions. However, for a one-photon absorption, lasers with a wavelength A/3 have to be available in order to reach the same excitation energy. An example of Doppler-limited collinear three-photon spectroscopy is the excitation of high-lying levels of xenon and CO with a narrow-band pulsed dye laser at X = 440 nm (Fig. 2.40). For one-photon transitions light sources at A. = 146.7 nm in the VUV would have been necessary. 

Lower-speed ADC and lower-level processor It transmits a series of bursts of narrowband pulses where each burst is a sequence consisting of many pulses shifted in frequency from pulse to pulse with a fixed frequency step. Each received narrow-band pulse is phase detected and then combined into the large effective bandwidth (sequentially over many pulses). Therefore, the hardware requirement is less stringent relative to that of UWB-IR. The detector bandwidth is smaller, resulting in lower noise bandwidth and higher signal-to-noise (SN) ratio when compared with UWB-IR. 

The stepped-FM UWB sensor can detect radio interference by monitoring the phase detector output in passive mode (transmitter power off/receiver on) and is then designed to have some spectrum hole (nonactivated within a portion of the wide radio spectrum) at the transmitter in order to prevent any conflict and to coexist with the other narrow-band wireless systems. Note that the transmit radio consists of independent narrow-band pulses with a different frequency. As such, if provides the flexibility to support... 

Because of the unique features of the x-ray radiation available at synchrotrons, many novel experiments ate being conducted at these sources. Some of these unique features are the very high intensity and the brightness (number of photons per unit area per second), the neatly parallel incident beam, the abihty to choose a narrow band of wavelengths from a broad spectmm, the pulsed nature of the radiation (the electrons or positrons travel in bunches), and the coherence of the beam (the x-ray photons in a pulse are in phase with one another). The appHcations are much more diverse than the appHcations described in this article. The reader may wish to read the articles in the Proceedings of the Materials Research Society Hsted in the bibhography. 

Figures Comparison of nuciear reactor and pulsed spaliation sources. For reactor sources (steady-state method), a narrow band of wavelengths is seiected with a monochromator crystal and the scattering angle (26,) Is varied to scan dspacings. Pulsed sources (time-of-flight method) use almost the entire avail-abie neutron spectrum, fix the scattering angie (26,), and simultaneousiy detect a neutron while determining its time of flight.

The continuous wave technique has a definite advantage over the other techniques a very narrow band of frequencies is needed to transmit the information. The pulse techniques, on the contrary, use a large band of frequencies, and the various noises, pump noises in particular, are more difficult to eliminate. 

A family of vacuum-tube MMW sources is based on the propagation of an electron beam through a so-called slow-wave or periodic structure. Radiation propagates on the slow-wave structure at the speed of the electron beam, allowing the beam and radiation field to interact. Devices in this category are the traveling-wave tube (TWT), the backward-wave oscillator (BWO) and the extended interaction oscillator (EIO) klystron. TWTs are characterized by wide bandwidths and intermediate power output. These devices operate well at frequencies up to 100 GHz. BWOs, so called because the radiation within the vacuum tube travels in a direction opposite to that of the electron beam, have very wide bandwidths and low output powers. These sources operate at frequencies up to 1.3 THz and are extensively used in THZ spectroscopic applications [10] [11] [12]. The EIO is a high-power, narrow band tube that has an output power of 1 kW at 95 GHz and about 100 W at 230 GHz. It is available in both oscillator and amplifier, CW and pulsed versions. This source has been extensively used in MMW radar applications with some success [13]. 

Lozovoy, V. V., Shane, J. C., Xu, B. W., and Dantus, M. 2005. Spectral phase optimization of femtosecond laser pulses for narrow-band, low-background nonlinear spectroscopy. Opt. Exp. 13(26) 10882-87. 

As far as the controls are concerned, we here consider time-continuous modulation of the system Hamiltonian, which allows for vastly more freedom compared to control that is restricted to stroboscopic pulses as in DD [42, 55, 91]. We do not rely on rapidly changing control fields that are required to approximate stroboscopic a -pulses. These features allow efficient optimization under energy constraint. On the other hand, the generation of a sequence of well-defined pulses may be preferable experimentally. We may choose the pulse timings and/or areas as continuous control parameters and optimize them with respect to a given bath spectrum. Hence, our approach encompasses both pulsed and continuous modulation as special cases. The same approach can also be applied to map out the bath spectrum by measuring the coherence decay rate for a narrow-band modulation centered at different frequencies [117]. 

We describe beamline ID09B at the European Synchrotron Radiation Facility (ESRF), a laboratory for optical pump and x-ray probe experiments to 100-picosecond resolution. The x-ray source is a narrow-band undulator, which can produce up to 1 x 1010 photons in one pulse. The 3% bandwidth of the undulator is sufficiently monochromatic for most diffraction experiments in liquids. A Ti sapphire femtosecond laser is used for reaction initiation. The laser mns at 896 Hz and the wavelength is tunable between 290-1160 nm. The doubled (400 nm) and tripled wavelength (267 nm) are also available. The x-ray repetition frequency from the synchrotron is reduced to 896 Hz by a chopper. The time delay can be varied from 0 ps to 1 ms, which makes it possible to follow structural processes occurring in a wide range of time scales in one experiment. 

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