This paper reports on the measurement and reconstruction using two algorithms of an Adaptive Tilt Mirror's (ATM) reflected wavefront using a Shack-Hartmann (SH) based wavefront sensor. The ATM consists of a deformable mirror mounted onto a fast steering mirror platform. Reconstruction of the wavefront was performed using Finite Difference and Finite Element reconstruction algorithms for comparison. The SH wavefront sensor with high frame-rate readout camera and the two types of software reconstructors provide a visualization of the ATM's surface while being moved physically on the fast steering mirror platform at a rate of 30 Hz.
We have recently shown the refractive index structure constant C<sup>2</sup><sub>n</sub> in the visible and near infrared to be a strong function of humidity in the absence of solar insolation effects, in stark contrast to the commonly held assumption that the humidity contribution can be ignored in that waveband. We expand our analysis of the effects of humidity on C<sup>2</sup><sub>n</sub> as measured across a 100-m long horizontal beam path to include temperature. Also we present a new technique for extracting information on changes in the parameter space of C<sup>2</sup><sub>n</sub> and local weather variables, which we term Hilbert Phase Analysis (HPA). This methodology, based on extracting the phase of the analytic signal via Hilbert transforms, reveals a wealth of detail that conventional analysis techniques cannot determine. The HPA provides additional confirmation that C<sup>2</sup><sub>n</sub> is strongly influenced by local humidity in the visible region. We have also found that HPA provides a clear demonstration that humidity competes with temperature in affecting the value of C<sup>2</sup><sub>n</sub>.
The Hilbert Huang Transform is a new technique for the analysis of non-stationary signals. It comprises two distinct parts: <i>Empirical Mode Decomposition</i> (EMD) and the <i>Hilbert Transform</i> of each of the modes found from the first step to produce a Hilbert Spectrum. The EMD is an adaptive decomposition of the data, which results in the extraction of Intrinsic Mode Functions (IMFs). We discuss the application of the EMD to the calibration of two optical scintillometers that have been used to measure C<sub>n</sub><sup>2</sup> over horizontal paths on a building rooftop, and discuss the advantage of using the Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.
We explore the effect of a newly discovered mechanism on the detection of infrared radiation using Apertureless Near Field Scanning Optical Microscopy (ANSOM). The passage of the ANSOM tip over a sample surface is modelled as a dipole moving over a halfspace. A boundary induced excitation is shown to occur, related to the retarded radiation reaction force of the dipole close to the surface. This excitation modifies the spontaneous emission characteristics of the dipole as it alters the near field. We suggest that this physical effect can cause emission at detectable levels in the infrared region thereby altering the expected signal in a typical ANSOM setup.
We present the current status and developments of a horizontal beam path laser propagation experiment over the sea performed off the coast of Puerto Rico. Atmospheric turbulence effects have been measured by a Shack-Hartmann wavefront sensor with a Dalsa CCD camera and by a scintillometer from Optical Scientific, Inc* (OSI). We present preliminary scintillation measurements for an approximate period of two days from the two optical systems during the month of July 2005, also suggestions for improvement in the software, data acquisition protocol and hardware are presented.
We report on the current status of a horizontal path length laser propagation campaign that is being performed over the sea, just off the coast of Puerto Rico. The effects of atmospheric turbulence in a tranquil marine environment have been measured with a video rate Shack--Hartmann wavefront sensor. The small perturbations in the wavefront phase and the degree of scintillation are presently being determined in a single pass from source to receiver, over a trial distance of 110 meters. Additional sites have been identified which allow for single pass measurements up to approximately 1 kilometer. Over 70 hours of data have been sampled to date, between December 2003 and April 2004.
We describe the status and initial results of a long term campaign to measure the effects of atmospheric turbulence upon a horizontally directed laser beam, whose altitude above the sea surface is 2 metres. The measurements are made with a video rate Shack-Hartmann wavefront sensor. Currently the source-receiver distance is 110 metres and additional sites have been identified to extend this single pass geometry to around 1 km. To date we have sampled over 70 hours of data, from December 2003 to April 2004. The preliminary scintillation power spectra and phase structure functions have been determined for some of the data sets, which we present here.