We describe the properties and construction of a polarization-independent Fresnel lens array using nematic liquid crystals in which the diffraction efficiency of lenses can be electrically controlled. A novel structure is used such that the principal axis of the liquid crystals in two adjacent zones of each Fresnel lens are orthogonal. This makes the device polarization independent. We have characterized these lenses at different applied fields and input polarizations by diffracting an argon-ion beam into focus.
Two-dimensional arrays of mutually coherent optical beams are needed in holographic interconnections for both recording and reading gratings. We analyze the use and limitations of binary phase Fresnel lenses to generate beams for these applications. Two known techniques of ion beam milling and thin film deposition are compared to fabricate such lens arrays in SiO(2) and Si(3)N(4). Each lens in the 8 x 8 arrays has a 1.2-mm square aperture with a focal length of 20 mm. Diffraction of a single argon-ion beam into an 8 x 8 array of highly uniform coherent focused beams (with 12-microm spot size) was achieved by the lenses with an efficiency of approximately 30% (41% theoretical limit).
VLF signals from the Siple Station, Antarctica, transmitter received on the DE 1 spacecraft provide new information on whistler mode signal propagation paths in the magnetosphere. In two case studies, the measured group delay in conjunction with in situ density measurements and ray tracing analysis are used to distinguish between direct nonducted propagation and a hybrid mode consisting of one‐hop propagation in a duct followed after ionospheric reflection by nonducted propagation. The extent of the observations both in space and time indicates that such a hybrid propagation mode may be an important means by which whistler mode signals generated or amplified in ducts can populate the magnetosphere. Computed group time delays based on a diffusive equilibrium model for the density distribution along the field lines provide good agreement with measurements over a wide range of magnetic latitudes (50°S–25°N). In one case, observed Siple signals are associated with sidebands of ±30 Hz spacing, and emissions are occasionally triggered. Sidebands are associated with signal components that are believed to have propagated on direct nonducted paths as well as in a hybrid (ducted/nonducted) mode with the sideband spacing being equal in both cases. The sideband spacing is found to be independent of the carrier frequency and amplitude and the satellite location with respect to the magnetic equator.
Storage capacity of a volume is derived based on the space-bandwidth products of angularly multiplexedholograms and by considering their diffraction efficiency and cross-talk limitations. An erasable medium of Fe:LiNbO(3) (10MM × 1mm × 2mm) is simulated thatyieldsacapacity of ˜ 26 Gbits for 95 holograms, each with an efficiency of ~ 10(-4), for a total of ~ -31-dB cross talk. The recording time is estimated at 0.7 s (assuming 10(3) W/m(2) and λ = 0.5 µm). A nonerasable dichromated gelatin medium 10 mm × 10 mm × 0.025 mm is also analyzed that yields a capacity of ~ 3.3 Gbits for 11 holograms with efficiency of 0.09 and ~ -25-dB cross talk.
An active-fiber star coupler that uses arrays of N x N microlenses and liquid-crystal modulators is presented. A simplified implementation of this device uses an 8 x 8 array of Fresnel microlenses fan out an incident beam into 64 focused spots (an 18 dB fan-out loss); on-off capability (270:1 extinction ratio) at each element is provided by liquid-crystal spatial light modulators. A row of focused spots is coupled into an array of 1 x 8 multimode fibers with a measured excess loss of 12 dB along each path (with an estimated overall loss of 30 dB in an 8 x 8 device). A modification of this device that is capable of wavelength selection at individual output fibers is proposed for wavelength division multiplexing applications.
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