Aircraft laser sensing of sound velocity in water: Brillouin scattering

Abstract

Nearfield acoustic hologr.aphy is used to reconstruct the velocity distribufion of an acoustic source located in a cylindrical space. Two reconstruction algorithms have been developed, both of which account for the boundary effects of the cylindrical walls. The first algorithm models the space as an infinite duct, while the second models the space as a finite duct. A known source velocity distribution is used to simulate a pressure field in the space. Pressures on the holography surface are processed by the holography algorithms to reconstruct the source velocity distribution. The results of the algorithms are compared, with particular attention to the sensitivity of reconstructions to the location, number, and spacing of sensors, aperture lengths, and measurement noise. tacting instrumentation. A system composed of a scanning laser vibrometer with computer-controlled optical scanner allows for noncontacting measurement of the amplitude and phase of the vibration velocity of a structural member. The laser vibrometer has a dynam!c range of 160 dB and broadband frequency response of up to 25 kHz. The vibrometer is held steady on a seismically isolated optical table and a mirror is positioned accurately by a computer-controlled stepper motor. The measurement of nearfield or farfield structural intensity at a point requires the measurement of the structural velocity at a minimum of four points for a beam and eight points for a plate about that point. Thus the amplitude and phase of the velocity field on the surface of a structure is measured at a fine mesh of discrete number of points. Error analysis is performed to assess the theoretical error introduced by the mesh size, number of points used for evaluating the structural intensity, random phase and amplitude errors, and errors introduced by nonsimultaneous measurements. 8:55 JJJ3. Measurement of flexural and longitudinal vibration intensity by a two-channel laser vibrometer. Timothy E. McDevitt, Courtney B. Burroughs (Appl. Res. Lab., Penn State Univ., State College, PA 16804), and Gary H. Koopman (Mech. Eng., Penn State Univ., State College, PA 16804)

Liquid water absorption and scattering effects in DOAS retrievals over oceans

“…The prominent increase (step) around 460 nm results from the relatively sharp increase in intensity of the solar spectrum shortly before 400 nm (after the strong K and H Fraunhofer lines from Ca + ). Another inelastic scattering effect is the so-called Brillouin scattering that is caused by density fluctuations (phonons) in water (e.g., Hickman et al, 1991;Dickey et al, 2011). In principle, Brillouin scattering can also produce a filling-in of Fraunhofer structures, but the spectral shift is only several 10 −3 nm in the visible wavelength range (Xu and Kattawar, 1994).…”

Optics in Remote Sensing

“…Insert these in Eq. (9.8), 14) and combine this result with Eq. (9.13) to obtain the interesting result, (9.15) that these two frequencies are identical, i.e., the Brillouin frequency shift of the incident light is identical to the frequency of the sound wave producing it.…”

“…Specifically, a telescope would be required to reduce the diameter of the Lidar return from 80 cm to the few cm diameter of a practical Fabry-Perot; leading to a further significant increase in the beam divergence. The problem has been examined from several aspects by Hickman et al [14].…”

“…This scattering process is called Brillouin scattering and the remote sensing application, called Brillouin Lidar, has been studied [13][14][15]. As before, the depth is determined from the travel time of the light pulse; the speed (of sound) is measured by observing the Brillouin frequency shift f B in the backscattered light.…”

Background clean-up in Brillouin microspectroscopy of scattering medium

“…Brillouin spectroscopy is a powerful technique for material characterization [1,2], providing unique information about the mechanical properties of a substance [1], and has found wide usage in remote sensing [3], material science [4,5] and biomedical applications [6,7]. Brillouin scattering originates from the inelastic interaction between the incident electromagnetic wave and the acoustic phonons within the material.…”