Naven Chetty

Measurement of the electric quadrupole moment of N2O

Measurements of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous nitrous oxide are presented. Measurements span the temperature range 298.5-473.9 K, which allows for separation of the temperature-independent hyperpolarizability term from the temperature-dependent quadrupole term, yielding a quadrupole moment of Θ = (-11.03 ± 0.41) × 10(-40) C m(2), and a hyperpolarizability term of b = (-0.638 ± 0.063) × 10(-60) C(3) m(4) J(-2). For dipolar molecules, the quadrupole moment is origin dependent, and the value reported here is referred to an origin called the effective quadrupole center (EQC). Comparison of this value with the center of mass (CM) quadrupole moment obtained from MBER experiments yields information about the dynamic dipole-quadrupole and dipole-magnetic dipole polarizabilities. The temperature-independent term, previously assumed to contribute negligibly to the EFGIB, is found to contribute some (5.2 ± 0.6)% to the effect at room temperature and clearly needs to be accounted for if the quadrupole moment is to be definitively established.

Implementing digital holograms to create and measure complex-plane optical fields

The coherent superposition of a Gaussian beam with an optical vortex can be mathematically described to occupy the complex plane. We provide a simple analogy between the mathematics, in the form of the complex plane, and the visual representation of these two superimposed optical fields. We provide detailed instructions as to how one can experimentally produce, measure, and control these fields with the use of digital holograms encoded on a spatial light modulator.

EXPERIMENTAL VERIFICATION OF THE TURBULENT EFFECTS ON LASER BEAM PROPAGATION IN SPACE

In this work, we have modified an existing experimental setup to fully classify the thermal effects on a laser beam propagating in air. Improvements made to the setup include a new, more powerful laser, a precision designed turbulence delivery system, an imbedded pressure sensor, and a platform for height adjustability between the laser beam and the turbulence model. The setup was not only able to reproduce previous results exactly but also allowed new data for the turbulence strength C2n, the Rytov variance (scintillation) and the coherence diameter (Fried’s parameter) to be successfully measured. Analysis of the produced interferograms has been discussed using fast Fourier transforms. The results confirm, within the Kolmogorov regime, that phase and intensity fluctuations increase relative to temperature. The turbulent region exhibited very strong disturbances, in the range of 1.1 × 10–12 m–2/3 to 2.7 × 10–12 m–2/3. In spite of the strong turbulence strength, scintillation proved otherwise, since the condition for a weak turbulence environment was determined in the laboratory and a low scintillation index was to be expected. This is as a result of the relatively short propagation distances achieved in the laboratory. In the open atmosphere, path lengths extend over vast distances and in order for turbulent effects to be realized, the turbulence model must generate stronger turbulence. The model was, therefore, able to demonstrate its ability to fully quantify and determine the thermal turbulence effects on a propagating laser beam.

Analysis of the fluctuations of a laser beam due to thermal turbulence

A laser beam propagating in air and passing through a point diffraction interferometer (PDI) produces stable interferograms that can be used to extract wavefront data such as major atmospheric characteristics: turbulence strength, inner scale and outer scale of the refractive index. These parameters need to be taken into consideration when developing defense laser weapons since they can be affected by thermal fluctuations that are due to the changes in temperature in close proximity to the propagating beam and results in phase shifts that can be used to calculate the temperature which causes wavefront perturbations on a propagating beam.

Experimental determination of thermal turbulence effects on a propagating laser beam

Abstract The effect of turbulence on propagating laser beams has been a subject of interest since the evolution of lasers back in 1959. In this work, an inexpensive and reliable technique for producing interferograms using a point diffraction interferometer (PDI) was considered to experimentally study the turbulence effects on a laser beam propagating through air. The formed interferograms from a propagating beamwere observed and digitally processed to study the strength of atmospheric turbulence. This technique was found to be sensitive enough to detect changes in applied temperature with distance between the simulated turbulence and laser path. These preliminary findings indicated that we can use a PDI method to detect and localise atmospheric turbulence parameters. Such parameters are very important for use in the military (defence laser weapons) and this is vital for South Africa (SA) since it has natural resources, is involved in peace keeping and mediation for other countries, and hence must have a strong defence system that will be able to locate, detect and destroy incoming missiles and other threatening atmospheric systems in order to protect its environment and avoid the initiation of countermeasures on its land.

Optical Detectors for Integration into a Low Cost Radiometric Device for In-Water Applications: A Feasibility Study

Higher water temperatures and nutrient loads, along with forecasted climate changes are expected to result in an increase in the frequency and intensity of eutrophication-linked algal blooms (Bernard, 2010, unpublished). The destructive impact such phenomena have on marine and freshwater systems threaten aquaculture, agriculture and tourism industries on a global scale (Bernard, 2010, unpublished). An innovative research project, Safe Waters Earth Observation Systems (SWEOS) proposes the use of space-based remote sensing techniques, coupled with in-situ radiometric technology to offer a powerful and potentially cost effective method of addressing algal bloom related hazards. The work presented in this paper focuses on the decision making processes involved in the development of autonomous bio-optical sensors whose purpose includes, but is not limited to; water constituent monitoring, satellite calibration validation and ocean colour satellite product matchups. Several criteria including cost, optical throughput, linearity and spectral sensitivity were examined in an attempt to choose the detector best suited for its intended application. The CMOS based module tested in the laboratory experiments that was found to have produced the best performance-cost ratio was chosen for integration into the in-water radiometric device built and tested at the Council for Scientific and Industrial Research (CSIR) (Ramkilowan et al. 2012, unpublished). Mass production of this prototype technology will commence, pending data quality comparable to that of an already calibrated, in-water radiometer; to be tested at field trials in Elands Bay (32°17′45.82″S; 18°14′44.45″E), Loskop Dam (25°27′15.25″S; 29°17′28.21″E) and Saldanha Bay (33° 3′11.38″S; 17°59′54.29″E).

Radiation dose and cancer risk estimates in helical CT for pulmonary tuberculosis infections

Abstract The preference for computed tomography (CT) for the clinical assessment of pulmonary tuberculosis (PTB) infections has increased the concern about the potential risk of cancer in exposed patients. In this study, we investigated the correlation between cancer risk and radiation doses from different CT scanners, assuming an equivalent scan protocol. Radiation doses from three 16-slice units were estimated using the CT-Expo dosimetry software version 2.4 and standard CT scan protocol for patients with suspected PTB infections. The lifetime risk of cancer for each scanner was determined using the methodology outlined in the BEIR VII report. Organ doses were significantly different (P < 0.05) between the scanners. The calculated effective dose for scanner H2 is 34% and 37% higher than scanners H3 and H1 respectively. A high and statistically significant correlation was observed between estimated lifetime cancer risk for both male (r2 = 0.943, P < 0.05) and female patients (r2 = 0.989, P < 0.05). The risk variation between the scanners was slightly higher than 2% for all ages but was much smaller for specific ages for male and female patients (0.2% and 0.7%, respectively). These variations provide an indication that the use of a scanner optimizing protocol is imperative.

Wind tunnel simulations to detect and quantify the turbulent effects of a propagating He-Ne laser beam in air

In this paper, we ascertain the effectiveness of our experimental setup in detecting and quantifying the turbulent effects experienced by a He-Ne laser beam as it passes through a wind tunnel. The beam propagated through a series of optical components as well as the in-house designed and manufactured wind tunnel under controlled laboratory conditions. The wind tunnel was built to fit within an existing setup, which has previously proven to be successful in detecting the turbulent effects from other turbulence models. For various wind speeds and temperature settings, the setup has been successful as it was able to detect and measure the atmospheric conditions within the turbulent environment and fully quantify the characteristics of the laser beam. With the use of highly accurate measuring devices, we were able to successfully measure the refractive index structure function ( C n 2 ) and the coherence diameter (Fried’s parameter). Values for C n 2 ranged between 1.61 × 10 –16 m –2/3 and 6.77 × 10 –15 m –2/3 , which can be classified under the moderate to strong turbulence regime. These results tie in well with various published works for similar atmospheric scenarios hence this setup was successfully able to fully detect and quantify the thermal turbulence and wind velocity effects on the laser beam using a point diffraction interferometer.

Techniques to measure complex-plane fields

In this work we construct coherent superpositions of Gaussian and vortex modes which can be described to occupy the complex-plane. We demonstrate how these fields can be experimentally constructed in a digital, controllable manner with a spatial light modulator. Once these fields have been generated we illustrate, with three separate techniques, how the constituent components of these fields can be extracted, namely by measuring the intensity of the field at two adjacent points; performing a modal decomposition and a new digital Stokes measurement.

Optical detectors for integration into a low cost radiometric device for in-water applications: HyDROW performance test at Loskop Dam

Abstract South Africas fresh water resources are under threat by Harmful Algal Blooms (HABs). A comprehensive and cost effective method for wide area detection and monitoring of HABs is therefore needed to manage and where possible circumvent the negative impact HABs may have on the countrys aquatic ecosystems. Current commercial radiometers used for such applications are often too costly to purchase in numbers. This study focuses on the performance of a low cost, in-house developed prototype radiometer, Hyperspectral Device for Radiometric Observations in Water (HyDROW). HyDROWs performance has been evaluated against data registered with a commercially available Hyperspectral Tethered Spectral Radiometer Buoy (HyperTSRB) during a field campaign at Loskop Dam in South Africa. The Loskop Dam is at risk for HABs and has been selected given its diverse environments from an optical perspective. Measurements were made at five optically diverse test points. The maximum percentage difference between the HyperTSRB and HyDROW were ∼8% in the blue, ∼19% in the green and ∼24% in the red bands of the spectrum. The correlation coefficients between the radiometers range from 0.97 at the most turbid of test sites, to better than 0.99 in clearer waters.