Aperture masking is a technique to transform a filled-aperture telescope into an interferometer. One reason masks help boost sensitivity is that they facilitate the measurement of closure phases. Closure phases are insensitive to differential piston in the wavefront of light captured by each subaperture, so are a precise observable encoding the structure of the observed object. Spatial structure in the wavefront of light over each subaperture biases visibilities and closure phases. All extant aperture masking data sets show residual closure phase dispersion after calibration larger than estimates based on photon-noise alone, suggesting time variable substructure in the wavefront is playing a role in limiting closure phase precision. We are developing a technique harnessing the capabilities of holographic aperture masks to multiplex subapertures to provide for simultaneous focal-plane wavefront sensing of each subaperture. This device can be used to measure the spatial structure of the wavefront, facilitating self-calibrating closure phases. We will present the results of simulations demonstrating the concept and describe a prototype instrument design.
We present the results of an adaptive optics system used to observe an object through the water surface. To be compact the system was composed of two refractive wavefront modulators mounted in stack configuration. One of the devices is a novel concept of tip/tilt refractive wavefront corrector. The second one is a deformable lens for correcting aberrations up to the 4th order. The two devices were driven using a Shack Hartmann wavefront sensor and a far field camera. The system was designed to be compact (about 15cm x 30cm) and was mounted in a sealed container placed below the surface in a water tank at NRL lab.
The results show that the system was able to correct the wavefront with moderate waves amplitude. The simultaneous correction of tilt and high order aberrations demonstrated to be more efficient for the higher waves level.
We constructed x-ray tracking detectors by coupling a perovskite X-ray scintillating film to several commercially available cameras. Perovskite x-ray scintillators are an attractive alternative to traditional scintillators due to their compelling combination of high light output, impressive resolution, low afterglow, and ultrafast speed. We coupled the perovskite film to a high dynamic range event-based sensor as well as to two regular CMOS monochrome cameras. We compare results of the perovskite-based x-ray tracker using the event-based sensor with the regular imaging cameras. Furthermore, as an example application, we will show the use of the x-ray tracker as a beam finder for an x-ray beamline experiment.
Transparent magnesium aluminate spinel (MgAl2O4) ceramic has excellent transmission from the UV to mid-wave IR. It is rugged with strength that is 5x that of glass. Spinel is being developed as a sensor window for numerous military platforms. At the U.S. Naval Research Laboratory (NRL), we have focused on process developments to facilitate wider acceptance of spinel for various applications. These developments include purification of spinel to reduce the absorption and scattering losses for use as an exit aperture on High Energy Laser (HEL) systems and various cost-effective densification methods to reduce manufacturing costs. In this paper, we will provide an update on some of the ongoing spinel activities at NRL.
Mercurous halides; mercurous iodide (Hg2I2), mercurous bromide (Hg2Br2) and mercurous chloride (Hg2Cl2) are high figure of merit materials for fabricating Acousto-optic (AO) devices that operate in the visible and infrared regions. Single crystals of mercurous halides were grown by physical vapor transport (PVT) method. Thermal expansion as well as the effect of annealing on the material are discussed. Mercurous halides show positive thermal expansion along "a" axis whereas no significant thermal expansion along "c" axis. The coefficients of thermal expansion of Hg2Cl2, Hg2Br2 and Hg2I2 are 6.72 × 10-5 °C-1, 6.44× 10-5 °C-1 and 6.08 × 10-5 °C-1, respectively. The optical band gap of Hg2Cl2 was calculated using the transmission spectra as 2.9 eV.
An acousto-optic devices were designed and fabricated using high quality single crystals of mercurous halide (Hg2X2) that were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide (Hg2Br2) crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and acousto-optic tunable filter (AOTF) applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator as well as acousto-optic tunable filter device fabrications. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The diffraction efficiencies of the fabricated AOTF device with 10600 nm wavelength laser found to be 26%.
Various rare earth doped single crystal YAG and sesquioxide fibers have been drawn using a state-of-the-art Laser Heated Pedestal Growth system. All crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers have been successfully fabricated using various crystal growth and deposition methods. We report on the various fabrication methods, optical characterization of these clad fibers.
Single crystal fibers are currently being developed for high power single frequency lasers in 1-2 µm region. Crystal fibers offer several advantages over traditional glass fibers such as silica fiber due to their higher thermal conductivity and higher stimulated Brillouin scattering (SBS) thresholds, along with excellent environmental stability and higher doping concentrations. Yb3+ and Ho3+ doped single crystal YAG fibers with diameters down to 17 µm and lengths >1m long have been grown using our state-of-the-art Laser Heated Pedestal Growth system. Single and double cladded rare earth doped crystal fibers have been fabricated using glasses where optical and physical properties were precisely matched to the core single crystal fiber. We also show successful fabrication of all crystalline core/clad fibers where thermal and optical properties are superior over glass based fibers. Various fabrication methods, optical characterization and gain measurements on these clad fibers will be reported.
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