Showing papers by "James S. Gerber published in 2000"
TL;DR: In this paper, the authors simulated optical propagation through atmospheric turbulence in which the spectrum near the inner scale follows that of Hill and Clifford [J. Opt. Soc. 68, 892 (1978)] and the turbulence strength puts the propagation into the asymptotic strong-fluctuation regime.
Abstract: We have simulated optical propagation through atmospheric turbulence in which the spectrum near the inner scale follows that of Hill and Clifford [J. Opt. Soc. Am. 68, 892 (1978)] and the turbulence strength puts the propagation into the asymptotic strong-fluctuation regime. Analytic predictions for this regime have the form of power laws as a function of beta0(2), the irradiance variance predicted by weak-fluctuation (Rytov) theory, and l0, the inner scale. The simulations indeed show power laws for both spherical-wave and plane-wave initial conditions, but the power-law indices are dramatically different from the analytic predictions. Let sigmaI(2) - 1 = a(beta0(2)/betac(2))-b(l0/Rf)c, where we take the reference value of beta0(2) to be betac(2) = 60.6, because this is the center of our simulation region. For zero inner scale (for which c = 0), the analytic prediction is b = 0.4 and a = 0.17 (0.37) for a plane (spherical) wave. Our simulations for a plane wave give a = 0.234 +/- 0.007 and b = 0.50 +/- 0.07, and for a spherical wave they give a = 0.58 + /- 0.01 and b = 0.65 +/- 0.05. For finite inner scale the analytic prediction is b = 1/6, c = 7/18 and a = 0.76 (2.07) for a plane (spherical) wave. We find that to a reasonable approximation the behavior with beta0(2) and l0 indeed factorizes as predicted, and each part behaves like a power law. However, our simulations for a plane wave give a = 0.57 +/- 0.03, b = 0.33 +/- 0.03, and c = 0.45 +/- 0.06. For spherical waves we find a = 3.3 +/- 0.3, b = 0.45 +/- 0.05, and c = 0.8 +/- 0.1.
31 citations
01 Dec 2000
TL;DR: In this paper, a new focusing technique based on the calculation of the spatio-temporal inverse filter of the propagation was proposed to correct the phase and amplitude aberrations of the wavefront.
Abstract: Ultrasonic imaging system capabilities are strongly dependent on the focusing quality of the ultrasonic beam. The beam width and sidelobe level constrain respectively the resolution and contrast of the final image. In the case of brain imaging, it is well-known that the skull strongly degrades the ultrasonic focusing pattern by introducing substantial phase and amplitude aberrations of the wavefront. In previous work, this degradation of the beam focus had been partially corrected by coupling the time reversal focusing process to an amplitude compensation of the emitted signals. In that case, the optimal focus was reproduced down to -20 dB, but the sidelobe level remained at about -25 dB. We propose here a new focusing technique based on the calculation of the spatio-temporal inverse filter of the propagation. Experimental focusing through the skull is now comparable to the focusing in a homogeneous medium. In the transmit-receive mode, focusing through the skull could reach the optimal level obtained in water down to -70 dB (i.e. constrained only by experimental noise levels.).
1 citations