A database of spectral and temperature-dependent emissivities was created for painted Al-alloy laser-damage-testing targets for the purpose of improving the uncertainty to which temperature on the front and back target surfaces may be estimated during laser-damage testing. Previous temperature estimates had been made by fitting an assumed gray-body radiance curve to the calibrated spectral radiance data collected from the back surface using a Telops Imaging Fourier Transform Spectrometer (IFTS). In this work, temperature-dependent spectral emissivity measurements of the samples were made from room temperature to 500 °C using a Surface Optics Corp. SOC-100 Hemispherical Directional Reflectometer (HDR) with Nicolet FTS. Of particular interest was a high-temperature matte-black enamel paint used to coat the rear surfaces of the Al-alloy samples. The paint had been assumed to have a spectrally flat and temperatureinvariant emissivity. However, the data collected using the HDR showed both spectral variation and temperature dependence. The uncertainty in back-surface temperature estimation during laser-damage testing made using the measured emissivities was improved from greater than +10 °C to less than +5 °C for IFTS pixels away from the laser burn-through hole, where temperatures never exceeded those used in the SOC-100 HDR measurements. At beam center, where temperatures exceeded those used in the SOC-100 HDR, uncertainty in temperature estimates grew beyond those made assuming gray-body emissivity. Accurate temperature estimations during laser-damage testing are useful in informing a predictive model for future high-energy-laser weapon applications.
Laser damage experiments were performed on painted and unpainted aluminum coupons using a 1.07-μm fiber laser at irradiances ranging from 0.2 to 1.4 kW/cm2 in a wind tunnel operating at Mach 0.1 to 0.9. Coupon penetration times of ∼0.5 to 10 s were measured using a silicon photodiode viewing a Lambertian scatter plate placed behind the target. Despite the thin, 0.81 to 0.95 mm, samples and large laser spot diameters, 2 to 3 cm, the effects of radial heat conduction dominate for irradiances of <1 kW/cm2. The fluence required to melt the back surface scales linearly with paint absorbance and the effects of paint aging have been observed. Penetration times for gray-painted aluminum at 287 W/cm2 decrease by 45% as the airflow speed increases from M=0.1 to M=0.2, but remains constant for flow speeds up to M=0.7.
We present a study of composite vortices in light beams using component beams with no integral topological
charge. We observed the same general features that are seen in when the component beams have an integral
topological charge [E.J. Galvez, N. Smiley, and N. Fernandes, "Composite optical vortices formed by collinear
Laguerre-Gauss beams," Proc. SPIE 6131, pp. 19-26, 2006.]. These are: (1) that new vortices appear at
distances from the beam that depend on the ratio of the intensity of the component beams, and (2) that the
angular location of the vortices depends on the phase difference between them. We also observed that some of
the vortices associated with fractional charge that did not follow the same dynamics.
We study the optical fields produced by non-integral binary forked gratings. We analyze experimentally the phase structure of the light beams by interference. We were able to identify individual optical vortices directly by observing the dislocations in the fringe pattern. Our experimental results agree well with of all the generic features predicted by the theoretical model [M.V. Berry, "Optical vortices evolving from helicoidal and fractional phase steps," J. Opt. A 6, pp. 259-268, 2004.]. Our results underscore the conservation of orbital angular momentum of the light/optical-apparatus system.