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Sivakumar V.,University of Reunion Island | Sivakumar V.,Council for Scientific and Industrial Research National Laser Center | Sivakumar V.,University of South Africa | Bencherif H.,University of Reunion Island | And 2 more authors.
Journal of Applied Meteorology and Climatology | Year: 2011

In this paper, tropopause characteristics observed from tropical to subtropical Southern Hemisphere stations using Southern Hemisphere Additional Ozonesonde (SHADOZ) data are presented for the 11-yr period of 1998-2008. Three different definitions of tropopause-cold-point tropopause (CPT), lapse-rate tropopause (LRT), and ozone tropopause (OT)-are determined, and their variability for nine different SHADOZ sites is studied for the purpose of evaluating their usefulness as indicators of possible tropopause trends. For each station, the OT is uniquely defined by the ozone gradient and is found to be more variable than either LRT or CPT. The OT roughly coincides with the upper boundary of the region of most active convective mixing over the western Pacific Ocean and with the lower boundary of the transition region from the troposphere to the lower stratosphere that is generally referred to as the tropical tropopause layer. The monthly and year-to-year variations in the tropopause are examined, and the annual cycle in OT, the dominant signal, is described. The distance of separation of the OT from the CPT or LRT is smaller for the tropics (stations at 0°-15°S) than for the subtropics (15°-25°S). The decadal trend in tropopause heights is measured using a statistical model that accounts for natural variations expressed in El Niño-Southern Oscillation, the quasi-biennial oscillation, and the Indian Ocean dipole. The decadal trend estimation shows no statistically significant trend for the CPT and LRT in the tropics, in contrast to other studies. Adecrease in altitude for the OT is significant. In the subtropics, the CPT and LRT decline significantly, by -240 and -190 m (10 yr)-1, respectively, but the OT increases. © 2011 American Meteorological Society. Source


Flamm D.,Friedrich - Schiller University of Jena | Schulze C.,Friedrich - Schiller University of Jena | Naidoo D.,Industrial Research National Laser Center | Schroter S.,Institute of Photonic Technology | And 3 more authors.
Journal of Lightwave Technology | Year: 2013

A procedure for the multiplexing and demultiplexing of modes in optical fibers with digital holograms is presented. By using a spatial light modulator (SLM) to encode a digital hologram, the desired complex field is shaped and injected into the fiber. The SLM's ability to rapidly refresh the encoded transmission function enables one to excite pure single modes, as well as arbitrary coherent mode superpositions, in real-time. The modes from the output of the fiber are subsequently demultipexed by applying a correlation filter for modal decomposition, thus allowing for an all-digital-hologram approach to modal analysis of fibers. The working principle is tested using conventional step-index large mode area fibers being excited with higher-order single modes and superpositions. © 1983-2012 IEEE. Source


Flamm D.,Friedrich - Schiller University of Jena | Schulze C.,Friedrich - Schiller University of Jena | Naidoo D.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,Council for Scientific and Industrial Research National Laser Center | And 2 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

We introduce the correlation filter method for measuring the modal power spectrum of multi-mode beams. The method is based on an optical filter performing the integral relation of correlation. This filter is realized as a computer-generated hologram with a specifically designed transmission function based on the spatial distribution of the set of modes under test. The beam that is illuminating the hologram is generating a diffraction pattern containing information about modal amplitudes and intermodal phase differences. We will show that a simple single-shot intensity measurement is sufficient to gain the information about modal amplitudes and phases from the diffraction pattern which result in the ability to reconstruct the optical field under test. Beside a detailed presentation of the measurement process, the setup and the design of the correlation filters, the major advantage of the method, the ability to perform real-time measurements is introduced. As a test system, we investigate the guided modes of a few mode multi-mode fiber and show fast changing modal coupling processes. Thereby, we show measurement results of online-monitoring the reconstructed optical field of the beam under test. © 2013 SPIE. Source


Flamm D.,Friedrich - Schiller University of Jena | Naidoo D.,Council for Scientific and Industrial Research National Laser Center | Schulze C.,Friedrich - Schiller University of Jena | Forbes A.,Council for Scientific and Industrial Research National Laser Center | And 2 more authors.
Optics Letters | Year: 2012

A procedure for the real-time analysis of laser modes using a phase-only spatial light modulator is outlined. The procedure involves encoding into digital holograms by complex amplitude modulation a set of orthonormal basis functions into which the initial field is decomposed. This approach allows any function to be encoded and refreshed in real time (60 Hz). We implement a decomposition of guided modes propagating in optical fibers and show that we can successfully reconstruct the observed field with very high fidelity. © 2012 Optical Society of America. Source


Schulze C.,Optics 1 | Naidoo D.,Council for Scientific and Industrial Research National Laser Center | Flamm D.,Optics 1 | Schmidt O.A.,Optics 1 | And 4 more authors.
Optics Express | Year: 2012

We propose a new method to determine the wavefront of a laser beam based on modal decomposition by computer-generated holograms. The hologram is encoded with a transmission function suitable for measuring the amplitudes and phases of the modes in real-time. This yields the complete information about the optical field, from which the Poynting vector and the wavefront are deduced. Two different wavefront reconstruction options are outlined: reconstruction from the phase for scalar beams, and reconstruction from the Poynting vector for inhomogeneously polarized beams. Results are compared to Shack-Hartmann measurements that serve as a reference and are shown to reproduce the wavefront and phase with very high fidelity. © 2012 Optical Society of America. Source

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