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Interferometric position sensor samples standing waves; Centrifugally made plastic optical fiber has high bandwidth; Surface-plasmon waves couple to form widely tunable optical filter; MORE...

Sep 1st, 2003
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Interferometric position sensor samples standing waves

An interferometric position sensor developed by researchers at the Technische Universität Ilmenau (Ilmenau, Germany), the Research Center Jülich (Jülich, Germany), and the International University Bremen (Bremen, Germany) contains partially transparent photodetectors placed in a cavity; the detectors sample the standing wave that forms in the cavity. One end of the cavity is formed by a moving mirror attached to the object whose position is being monitored.

Two photodetectors, each an amorphous-silicon n-i-p photodiode less than 100 nm thick, are layered between three transparent contact layers (TCOs). The total transmission of the device is approximately 70% at 633 nm. The detectors have an active area of 10 mm2 and are located by the TCOs such that they have a relative phase shift of 35°, allowing the detectors to determine not only the position of the movable mirror, but the direction of its movement as well (the ideal relative phase shift of 90° would increase resolution but decrease transmission). A Lissajous figure reveals a nearly sinusoidal photocurrent shape for both photodiodes. Contact Helmut Stiebig at h.stiebig@fz-juelich.de.

Centrifugally made plastic optical fiber has high bandwidth

Plastic optical fiber (POF) offers inexpensive transport of information over short distances in vehicles and the home. Just as in glass optical fiber, a gradient-index profile greatly boosts data rates in POF over a step-index profile by reducing pulse broadening. Researchers at Kwangju Institute of Science and Technology (Kwangju, Korea) have developed a way to fabricate gradient-index POF precisely and simply by centrifugal deposition.

In the technique, a small amount of a liquid plastic with a relatively low refractive index is introduced into a 30-mm-diameter tube spun at 5400 rpm, within which the liquid polymerizes. The liquid is a combination of two materials with different refractive indexes. The ratio of liquids is continuously varied to raise the refractive index as the layer thickens inward, creating the desired parabolic refractive-index profile. Afterward, the preform is dried at 110°C to remove volatile materials. When drawn to a 1-mm diameter (with a diameter fluctuation of less than 60 µm), the resulting fiber has a loss of 120 dB/km and a bandwidth of 3.45 Gbit/s over a 100-m length at a 650-nm wavelength. Contact Jang-Joo Kim at jjk@kjist.ac.kr.

Surface-plasmon waves couple to form widely tunable optical filter

A type of optical filter with wide tunability across the visible and near-infrared spectrum and only a single transmission peak has been developed by Yu Wang of the Jet Propulsion Laboratory (Pasadena, CA). The filter is based on the coupling of surface-plasmon waves, which are oscillations in electron density at the interface of a dielectric and a metal. It is a different phenomenon from the well-known frustrated total internal reflection, which results in a cutoff rather than a passband effect.

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In one experimental filter, two prisms were each coated on their hypotenuses with a 50-nm silver film (along with a 40-nm magnesium fluoride protective overcoat). The hypotenuses were placed almost in contact; light entered one prism and struck its hypotenuse at greater than the critical angle. When the surfaces were close enough, surface plasmons from the two films coupled and light falling within a narrow passband was transmitted. Varying the gap varied the peak wavelength. Air gaps of 50, 160, 380, 550, and 1000 nm resulted in transmission peaks of 414, 498, 624, 702, and 852 nm, respectively, with bandwidths of about 30 nm (red, green, and blue transmission are shown here). Although the theoretical peak transmission ranges from 35% to 70%, experimental transmission peaks hovered at about 20%, a result of nonoptimal metal films. Contact Yu Wang at yu.wang@jpl.nasa.gov.

Cat's-eye quantum-well modulating retroreflector reaches 50 Mbit/s

Combining optical retroreflectors with electro-optic modulators, modulating retroreflectors (MRRs) are finding use in asymmetric free-space optical communications links. In these systems, only one end of the free-space link contains a laser; the hardware at the other end only modulates and retroreflects the beam from the first end. Such a system is simpler than a symmetric link. Existing MRRs contain large-area modulators covering a substantial portion of the retroreflector's aperture; the large electro-optics limit modulation rates—even a quantum-well (QW) version is limited to rates less than 10 MHz.

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Now, researchers at the Naval Research Laboratory (Washington, DC) and NASA Goddard Space Flight Center (Greenbelt, MD) have developed a cat's-eye MRR with very small QW modulators that lift the modulation rate to as much as 50 Mbit/s. An array of 1-mm QW pixels permits retroreflection and modulation in the 1-µm wavelength region over a 30° field of view short-range and 20° long-range. A 40-mm-focal-length telecentric lens (a diffraction-limited version is shown here) provides a flat focal plane; the experimental device has three pixels in a sparse linear arrangement with 2.5-mm spacing. Submillimeter pixels could boost data rates to hundreds of megabits per second over many kilometers. Contact William Rabinovich at rabinovich@nrl.navy.mil.

Beetles steer themselves by polarized scattered moonlight

Recently it was discovered that the African dung beetle (Scarabaeus zambesianus) has, in the back of its eye, receptors for analysis of polarized light. Now, researchers at the University of Lund (Lund, Sweden), the University of the Witwatersrand (Wits, South Africa) and the University of South Pretoria (Pretoria, South Africa) have determined a purpose for the beetle's receptors: the measurement of polarized light in the moonlit sky for use as a nocturnal compass.

Just as happens in the daylit sky with the Sun, scattering of moonlight at night creates a band of polarized light in the sky, with maximum polarization at 90° away from the moon. Many animals, including the African dung beetle, use the band of polarization arising from sunlight, but when the Sun is more than approximately 18° below the horizon, the light ceases to be a reliable guide. The dung beetle is the first animal discovered to use the million-times-weaker polarization of moonlight for orientation. When the researchers rotated the scattered polarized moonlight received by the beetles, the beetles veered in flight by 90° (either left or right). Contact Marie Dacke at marie.dack@cob.lu.se.

1300-nm-emitting indium gallium arsenide nitride laser operates at up to 100ºC

Quantum-well (QW) semiconductor lasers based on indium gallium arsenide nitride (InGaAsN) have the potential to produce higher 1300-nm optical outputs at higher temperatures than conventional lasers based on indium phosphide. Now, researchers at Lehigh University (Bethlehem, PA), Alfalight, and the University of Wisconsin-Madison (both of Madison, WI) have developed In0.4Ga0.6As0.995N0.005 lasers that emit at temperatures up to 100°C. The lasers were fabricated by organometallic vapor-phase epitaxy.

The emitters have a single-QW active layer bounded by GaAs layers; tensile-strained layers of gallium arsenide phosphide partially compensate for the highly strained QW layers. Threshold current density was as low as 210 A/cm2 for a 2-mm-long cavity operating under continuous-wave conditions at a temperature of 20°C. Near-threshold wavelengths were 1295.2 and 1331 nm at 20°C and 100°C, respectively. The large material-gain parameter and better electron confinement of the InGaAsN laser permits use of only one QW at elevated temperatures, as opposed to 4 to 14 QWs for other semiconductor-laser materials. The maximum optical output was 1.8 W at a 20°C heat-sink temperature. A broad-waveguide design should allow for even higher output powers in the future. Contact Nelson Tansu at tansu@lehigh.edu.

Optical monitor measures paper shrinkage

When newsprint paper is manufactured, excess water must be removed, which causes the paper to shrink in the direction perpendicular to the direction it moves in processing. Knowing the amount of shrinkage helps in control of the manufacturing equipment. Researchers at Halmstad University (Halmstad, Sweden) have developed an optical technique to monitor paper shrinkage during processing.

In the technique, which is based on backscattered fluorescence, a one-dimensional recording is made of imprints on the paper. An unavoidable part of the processing, the imprints have a well-defined period. A 630-nm-emitting laser causes the lignin in the paper to fluoresce; a photomultiplier tube measures the return signal over a 660- to 740-nm wavelength band. In a prototype testbed, drying paper is cycled repeatedly past the sensor; Fourier transforms are taken of successive time slots as the paper shrinks. Three types of frequency estimation calculations were evaluated, with a correlation-energy-maximization estimator winning out as a result of its good performance at a low signal-to-noise ratio. Because computations are performed along with the sampling, the system can potentially be used in real time. Contact Anders Kaestner at ak@ide.hh.se.

High-resolution x-ray and extreme-ultraviolet lens is achromatic

Researchers at Xradia (Concord, CA) and Stony Brook University (Stony Brook, NY) have invented a type of x-ray or extreme-ultraviolet (EUV) lens that could become the highest-resolution achromatic optic for use at these short wavelengths. Currently, only awkward grazing-incidence mirrors provide practical achromaticity at x-ray wavelengths; other types of x-ray optics all require close-to-monochromatic light.

As of now only a concept, the achromatic lens is a combination of a refractive element and a diffractive Fresnel zone plate. The refractive element is operated at a band of wavelengths near an absorption edge of the lens material, providing anomalous dispersion that cancels the dispersion of the Fresnel zone plate. In one configuration, the refractive element is made as a Fresnel lens (not a Fresnel zone plate) to reduce its thickness. A 25-nm resolution could be achieved over a 15-mm field at a 1.34-nm wavelength. For 13.4-nm EUV, however, the field would shrink to a few tenths of a millimeter. A single thin membrane could be made to hold a Fresnel lens on one side and a Fresnel zone plate on the other. Contact Wenbing Yun at wyun@xradia.com.

Laser cuts and bends foil

Researchers at the Chiba Institute of Technology (Chiba, Japan) are laser-forming thin metal foil into three-dimensional structures by first cutting them out and then causing them to fold by irradiating certain portions with lower-power light. The foil is held flat between two glass plates during the scanning; after release, the foil bends. Post-release scanning is sometimes necessary to fold the foil further.

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For 10-µm-thick stainless-steel foil scanned with an Nd:YAG laser, the beam power, beam diameter, and number of scans for a sharp bend are 0.3 W, 25 µm, and 5 respectively; for 20-µm foil, the values are 0.8 W, 50 µm, and 10. Broadly scanning an area on a ribbon curls the ribbon. When scanned, flat foil bends toward the scanned surface; however, prebent foil always bends in the direction of the bend. Cut and bent shapes such as coils, spirals, deformed squares, and microcubes (shown here in formation) can be fabricated. Metals such as copper and aluminum can be bent, and even glass can be caused to bend by a 12° angle. Contact Shunro Yoshioka at yoshioka@pf.it-chiba.ac.jp.

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