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In this paper, we present the design and testing of a radiofrequency prototype coil with good performances in terms of B 1 magnetic field homogeneity and can be utilized for Magnetic Resonance Imaging. It is constituted of four coaxial separately tuned rings of wire and symmetrically located on a spherical surface. Compared to standard Helmholtz pair, which has 2nd-order magnetic field homogeneity, it yields to improvement in field homogeneity, while preserving the simplicity of design. The four coils of proposed structure are tuned to the same frequency. The proposed structure gets at 4th-order magnetic field homogeneity by optimizing the distance between rings and the diameters of outer loops. An electrical modeling of the four-coil system taking into account the coupling effects between all rings permits to determine the resonance frequency in the homogenous mode. Measurements of B1 field homogeneity were introduced in free space. Compared to the Helmholtz coil, the proposed structure presents good performances in terms of B1 homogeneity, quality factor and sensitivity. The design of proposed coil has been optimized for best SNR performances. Globally, this work claims to be a contribution to the study of the four-coil RF systems derived from the Helmholtz pairs.

Background: Optical Coherence Tomography OCT is a noninvasive imaging technique that takes pictures of cross sections of human body tissues with a great resolution compared to other techniques. Fourier Domain OCT method provides significant improvement of imaging speed and detection sensitivity but suffers from autocorrelation noise arising as interference signals from reflections of sample layers that tends to obscure some of sample structure details.Methods: We present in this paper a new implementation of Common Path Optical Coherence Tomography, based on a resonant structure. The structure employs a semiconductor optical amplifier SOA and uses two mirrors, one coated fiber end and the other is the sample under test. Amplified multiple reflections between the laser cavity high reflection mirror and the sample layers along with SOA gain behavior results in the reduction of autocorrelation noise. Results: Autocorrelation noise is greatly reduced by a factor of 5 dB compared to an ordinary FDOCT system. Conclusion:This new structure, with the absence of autocorrelation noise that covers some of the details of the sample under test in OCT setups, is capable practically of attaining images with higher resolution.

Abstract-The far field divergence angle distribution of verticalcavity surface-emitting lasers is found to exhibit dependence on the data pattern driving the lasers. Two 50% duty cycle 1.25-Gb/s data patterns chosen to cause the same thermal conditions but with frequency content differing by a factor of 16 resulted in changes in the beam profile distributions with up to a 30% power variation in the central on-axis portion of the beam. Examination of the temporal waveforms as a function of far field angle revealed overshoot in the on-axis power and undershoot in the off-axis portion of the beam.Index Terms-Beam divergence, data pattern dependent, far field, laser diode, multimode fiber optic.

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