Time filter

Source Type

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.


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.


Schulze C.,Friedrich - Schiller University of Jena | Flamm D.,Friedrich - Schiller University of Jena | Duparre M.,Friedrich - Schiller University of Jena | Forbes A.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,University of KwaZulu - Natal
Optics Letters | Year: 2012

We present a fast and easy technique for measuring the beam propagation ratio, M2, of laser beams using a spatial light modulator. Our technique is based on digitally simulating the free-space propagation of light, thus eliminating the need for the traditional scan in the propagation direction. We illustrate two approaches to achieving this, neither of which requires any information of the laser beam under investigation nor necessitates any moving optical components. The comparison with theoretical predictions reveals excellent agreement and proves the accuracy of the technique. © 2012 Optical Society of America.


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.


Schulze C.,Friedrich - Schiller University of Jena | Ngcobo S.,University of KwaZulu - Natal | Ngcobo S.,Council for Scientific and Industrial Research National Laser Center | Duparre M.,Friedrich - Schiller University of Jena | And 2 more authors.
Optics Express | Year: 2012

The modal decomposition of an arbitrary optical field may be done without regard to the spatial scale of the chosen basis functions, but this generally leads to a large number of modes in the expansion. While this may be considered as mathematically correct, it is not efficient and not physically representative of the underlying field. Here we demonstrate a modal decomposition approach that requires no a priori knowledge of the spatial scale of the modes, but nevertheless leads to an optimised modal expansion. We illustrate the power of the method by successfully decomposing beams from a diode-pumped solid state laser resonator into an optimised Laguerre-Gaussian mode set. Our experimental results, which are in agreement with theory, illustrate the versatility of the approach. © 2012 Optical Society of America.


Mafusire C.,Council for Scientific and Industrial Research National Laser Center | Mafusire C.,University of KwaZulu - Natal | Forbes A.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,University of KwaZulu - Natal
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2011

We outline an approach for the calculation of the mean focal length of an aberrated lens and provide closed-form solutions that show that the focal length of the lens is dependent on the presence of defocus, x-astigmatism, and spherical aberration. The results are applicable to Gaussian beams in the presence of arbitrary-sized apertures. The theoretical results are confirmed experimentally, showing excellent agreement. As the final results are in algebraic form, the theorymay readilybeappliedinthe laboratoryif the aberration coefficientsof the lens are known. © 2011 Optical Society of America.


Mafusire C.,Council for Scientific and Industrial Research National Laser Center | Mafusire C.,University of KwaZulu - Natal | Forbes A.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,University of KwaZulu - Natal
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2011

We outline a theory for the calculation of the beam quality factor of an aberrated laser beam. We provide closedform equations that show that the beam quality factor of an aberrated Gaussian beam depends on all primary aberrations except tilt, defocus, and x-astigmatism. The model is verified experimentally by implementing aberrations as digital holograms in the laboratory. © 2011 Optical Society of America.


Dudley A.,Council for Scientific and Industrial Research National Laser Center | Dudley A.,University of KwaZulu - Natal | Litvin I.A.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,Council for Scientific and Industrial Research National Laser Center | Forbes A.,University of KwaZulu - Natal
Applied Optics | Year: 2012

In this work we derive expressions for the orbital angular momentum (OAM) density of light, for both symmetric and nonsymmetric optical fields, that allow a direct comparison between theory and experiment. We present a simple method for measuring the OAM density in optical fields and test the approach on superimposed nondiffracting higher-order Bessel beams. The measurement technique makes use of a single spatial light modulator and a Fourier transforming lens to measure the OAM spectrum of the optical field. Quantitative values for the OAM density as a function of the radial position in the optical field are obtained for both symmetric and nonsymmetric superpositions, illustrating good agreement with the theoretical prediction. © 2012 Optical Society of America.


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.


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.

Loading Council for Scientific and Industrial Research National Laser Center collaborators
Loading Council for Scientific and Industrial Research National Laser Center collaborators