Multiple scattering effects on the remote sensing of the speed of sound in the ocean by Brillouin scattering

Monte Carlo calculations have been performed to investigate the effect of multiple scattering on the frequency spectra caused by Brillouin scattering in the ocean. It is shown that the use of the frequency spectra to determine the speed of sound profile and the hydrosol backscattering probability is stable under multiple scattering because the problem is limited to single backscattering events.     

Accuracy limitations on Brillouin lidar measurements of temperature and sound speed in the ocean

There are five mutually dependent variables relevant to Brillouin lidar measurements of temperature and sound speed in the ocean; they are (1) the Brillouin shift, (2) the sound speed, (3) the index of refraction, (4) the temperature, and (5) the salinity. We use three well-known relations to analyze rigorously the interdependence of these five variables. Clearly, a Brillouin shift measurement does not provide a stand-alone determination of temperature or sound speed; one more variable or one more relation must be known. The use of mean values of salinity that have been obtained by an analysis of a large set of historical in situ data is considered for this additional relation.     

Filling in of Fraunhofer lines in the ocean by Brillouin scattering

We have investigated the relative contribution of Brillouin scattering to the filling in of both narrow and wide Fraunhofer lines in the ocean. The spectral behavior of the filling in was studied in two ways. First we studied Fraunhofer lines of variable width, such as the 455-nm Ba line with full-width at half-maximum (FWHM) = 0.02 nm, the 486-nm H(β) line (FWHM = 0.08 nm), and the 518-nm Mg line (FWHM = 0.11 nm). We then used the 455-nm Ba line as a narrow-line model to calculate the spectral dependence of the filling in. We found that Brillouin scattering can play a significant role in the filling in of narrow Fraunhofer lines in the ocean. We have also shown that, compared with the filling in caused by Raman scattering, the filling in caused by Brillouin scattering has less dependence on both the wavelength and ocean depth but is strongly dependent on the linewidth of the Fraunhofer line.     

Brillouin scattering measurements of the temperature dependence of sound velocity and acoustic absorption in simple alcohols

A scanning Fabry–Perot interferometer was used to measure Brillouin spectra of methanol, isopropanol and a 95% ethanol–water mixture for temperatures ranging between 285?K and 320?K. The Brillouin frequency shifts and linewidths were used to calculate the velocities and absorption coefficients of hypersonic acoustic waves in these liquids. The temperature dependence of sound speed and acoustic attenuation was determined. For all three materials, both sound velocity and absorption coefficient decreased with temperature.     

Measurement of the bulk viscosity of liquid by Brillouin scattering

The aim is to develop a rapid and direct method for measuring the bulk viscosity of a liquid as a function of temperature. Brillouin scattering of a laser beam in fresh water and salt water at different temperatures has been studied. The results show that there exists a close temperature-dependent relationship among the Brillouin frequency shift, the Brillouin linewidth, and the bulk viscosity of water. Thus the bulk viscosity of water can be determined directly from Brillouin-scattering measurements. The method has a high signal-to-noise ratio and high accuracy.     

Structural dependence of guided acoustic-wave Brillouin scattering spectra in hole-assisted fiber and its temperature dependence

We describe the guided acoustic-wave Brillouin scattering (GAWBS) characteristics of hole-assisted fiber (HAF). We clarify numerically and experimentally that the GAWBS spectrum corresponding to a particular acoustic mode is observed for HAF and that the efficiency for that mode can be controlled simply by designing the air-hole size and position. We also reveal the temperature dependence of the GAWBS characteristics in HAF.     

Distributed fiber temperature and strain sensor using coherent radio-frequency detection of spontaneous Brillouin scattering

A novel technique that enables coherent detection of spontaneous Brillouin scattering in the radio-frequency (<500 MHz) region with excellent long-term stability has been demonstrated for distributed measurements of temperature and strain in long fiber. An actively stabilized single-frequency Brillouin fiber laser with extremely low phase noise and intensity noise is used as a well-defined, frequency-shifted local oscillator for the heterodyne detection, yielding measurements of spontaneous Brillouin scattering with high frequency stability. Based on this approach, a highly stable real-time fiber sensor for distributed measurements of both temperature and strain over long fiber has been developed utilizing advanced digital signal processing techniques.     

Observation of induced longitudinal and shear acoustic phonons by Brillouin scattering

To improve the accuracy of velocity measurements in the Brillouin scattering technique using weak thermal phonons, we have used induced coherent phonons, which intensify the scattering. To induce phonons in the gigahertz range, we used a c-axis tilted ZnO film transducer that was developed in our laboratory. This allowed us to induce longitudinal and shear acoustic phonons effectively at hypersonic frequencies. As a result, we obtained scattered light in the silica glass sample that was much more intense than that obtained from the thermal phonons. Because the Brillouin scattering from induced phonons was measured, the shift frequency was that of the electric signal applied to the ZnO transducer. Strong peaks lead to a reduction of the measurement time. This is useful for two-dimensional mapping of thin film elasticity using Brillouin scattering. Additionally, Brillouin scattering enables the simultaneous measurement of longitudinal and shear phonon velocities in the sample plane. This opens up a potential new technique for non-destructive elasticity measurements of various materials.     

The elastic behaviour of polycarbonate in the glassy state determined by Brillouin scattering

Measurements of the temperature and hydrostatic pressure dependence of the elastic constants of amorphous polycarbonate in the glassy state using high-resolution Brillouin spectroscopy are reported. It was found that the temperature and the pressure dependence of the longitudinal and the transverse sound velocity are strictly linear, in contrast to the behaviour found in polymethylmethacrylate (PMMA). A simple equation for the behaviour of the elastic constants c ₁₁ and c ₄₄, and the shear, Young's and bulk modulus, can be given, which fits the experimental data well.     

Biomechanics of fibrous proteins of the extracellular matrix studied by Brillouin scattering

Brillouin light scattering (BLS) spectroscopy is a technique that is able to detect thermally excited phonons within a material. The speed of propagation of these phonons can be determined from the magnitude of the Brillouin frequency shift between incident and scattered light, thereby providing a measure of the mechanical properties of the material in the gigahertz range. The mechanical properties of the extracellular matrices of biological tissues and their constituent biopolymers are important for normal tissue function and disturbances in these properties are widely implicated in disease. BLS offers the prospect of measuring mechanical properties on a microscopic scale in living tissues, thereby providing insights into structure–function relationships under normal and pathological conditions. In this study, we investigated BLS in collagen and elastin—the fibrous proteins of the extracellular matrix (ECM). Measurements were made on type I collagen in rat tail tendon, type II collagen in articular cartilage and nuchal ligament elastin. The dependence of the BLS spectrum on fibre orientation was investigated in a backscattering geometry using a reflective substrate. Two peaks, a bulk mode arising from phonon propagation along a quasi-radial direction to the fibre axis and a mode parallel to the surface, depending on sample orientation relative to the fibre axis, could be distinguished. The latter peak was fitted to a model of wave propagation through a hexagonally symmetric elastic solid, and the five components of the elasticity tensor were combined to give axial and transverse Young''s, shear and bulk moduli of the fibres. These were 10.2, 8.3, 3.2 and 10.9 GPa, and 6.1, 5.3, 1.9 and 8 GPa for dehydrated type I collagen and elastin, respectively. The former values are close to those previously reported. A microfocused BLS approach was also applied providing selection of single fibres. The moduli of collagen and elastin are much higher than those measured at lower frequency using macroscopic strains, and the difference between them is much less. We therefore believe, like previous investigators, that molecular-scale viscoelastic effects are responsible for the frequency dependence of the fibre biomechanics. Combining BLS with larger-scale mechanical testing methods therefore should, in the future, provide a means of following the evolution of mechanical properties in the formation of the complex structures found in the ECM.     

Investigation of the influence of temperature on threshold value and pulse duration of stimulated Brillouin scattering in liquid water

The threshold value and pulse duration of stimulated Brillouin scattering (SBS) have been measured over the temperature range 10–45°C in liquid water. Also, a theoretical analysis based on the temperature dependence of SBS has been carried out. The results show that the threshold value of SBS decreases with an increase of temperature, whereas the measured pulse duration increases with an increase of temperature.     

Computation of Brillouin scattering cross-sections for multilayers

Acoustic Green's function allows the analysis of localisation of acoustic modes and of their possible confinement. Electromagnetic Green's function allows a consistent analysis of both scattering mechanisms (elasto-optic and ripple) and their interference. The computed cross-section is directly comparable with experimental spectra.     

Threshold of stimulated Brillouin scattering by use of a solar pumped laser

What is to our knowledge the first stimulated Brillouin scattering experiment using a high-power low-gain solar pumped laser is presented. A threshold reflectivity of 0.23% was reached when a peak power of 20.7 kW was used at 7.6 GHz. A cw solar pumped laser was Q-switched with an acousto-optic modulator, and the bandwidth was narrowed with an intracavity etalon. A high polarization ratio (>99.4%) was achieved by use of an intracavity configuration. Higher reflectivity values were limited because of the lack of availability of optical switches.     

Monte-Carlo simulation of a neutron Brillouin scattering spectrometer

A Monte-Carlo simulation was done to optimise the setup of a new type of time-of-flight spectrometer, a thermal neutron Brillouin scattering (NBS) spectrometer. After collimation of the incident white neutron beam five incident energies between 20 and 138 meV can be obtained from four different monochromator crystal faces. The monochromatic beam is split into nine pencil-like separate beams to improve the Q-resolution of the NBS-spectrometer. Including the time resolution of the Fermi chopper (after a background chopper) an energy resolution for elastic scattering between 2% and 7% and a momentum transfer resolution of 1.3–2.0% are obtained.     

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO(2) substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.     

Sound velocity and sound attenuation measurements by dynamic light scattering

Dynamic light scattering (photon correlation spectroscopy) has been applied to the determination of sound velocity and sound attenuation from the Brillouin component of the frequency spectrum scattered from a fluid sample transversed by a laser beam. In this paper the time-resolved determination of the Brillouin component is described. The measurement of the linewidth allows an accurate determination of the sound attenuation, while the central frequency is connected to the adiabatic sound velocity. Sound attenuation and sound velocity measurements are presented for the new refrigerant pentafluorethane (R125). The accuracy and possible systematic errors of this technique are discussed and compared to those obtained from other spectroscopic and acoustic techniques.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado. U.S.A.     

Techniques for coherent phonon excitation and stimulated Brillouin scattering

The basic principle of a rotating chisel-shaped phonon coupling and detection probe, believed to be novel, is presented for the measurement of coherent phonon propagation and interaction characteristics.     

Influence of bubbles on scattering of light in the ocean

The scattering and backscattering properties of bubble populations in the upper ocean are estimated with Mie theory and a generalized bubble size spectrum based on in situ observations. Optical properties of both clean bubbles and bubbles coated with an organic film are analyzed; the results are compared with the corresponding optical properties of micro-organisms of similar size. Given a bubble number density (from ~10(5) to ~10(7) m(-3)) frequently found at sea, the bubble populations significantly influence the scattering process in the ocean, especially in oligotrophic waters. Bubbles appear to make a large contribution to the missing terms in constructing the observed total backscattering coefficient of the ocean. This contribution to backscattering is strongly enhanced if the bubbles are coated with organic film. The injection of bubbles will shift ocean color toward the green, resembling phytoplankton blooms, and hence introducing error in ocean color remote sensing if its effect is not corrected.