Newsbreaks

Various news articles.

Jun 1st, 2002
Th 93687
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Microgear laser contains wraparound Bragg grating
A photopumped microgear laser developed by researchers at Yokohama National University (Yokohama, Japan) takes its form because it is the combination of a microdisk and a rotationally symmetric Bragg grating. In one version, the microgear laser has 20 "teeth." The grating minimizes the radiation field at the edge of the disk, increasing the cavity's Q-factor for whispering-gallery modes (modes that reflect at a grazing-incidence angle along the periphery of the disk).

The laser was fabricated from gallium indium arsenide phosphide/indium phosphide, with 5-nm-thick compressively strained quantum wells. It was pumped from the top at room temperature with 980-nm light from a laser diode; lasing occurred within the 1.60- to 1.67-μm wavelength region at a 17-μW threshold power. The threshold power density of one microgear laser was 280 W/cm2, lower than that of a similarly sized microdisk laser (420 W/cm2). As a result of fabrication anisotropy, some microgear lasers had an elliptical shape; these lasers demonstrated modes expected to exhibit directional light output. The researchers aim next to exploit the properties of quasiperiodic photonic crystals as microcavities for tailoring laser modes. Contact Toshihiko Baba at baba@dnj.ynu.ac.jp.


Largest ring laser is in operation
The University of Canterbury (Christchurch, New Zealand) is a leading center for the development of large ring lasers, which are used to measure the Earth's rotation rate. The measurement accuracy of these lasers rises in proportion to the enclosed area of the ring; two New Zealand ring lasers with enclosed areas of 1 and 12.25 m2 can routinely detect seismic rotations from earthquakes occurring anywhere over a large fraction of the globe. Now, the University of Canterbury group, along with Ulrich Schreiber of the Technischen Universität München (Kötzting, Germany) have constructed the largest ring laser yet, with an enclosed area of 366.5 m2.

Located in a former World War II bunker in Christchurch, the laser has a rectangular ring geometry with corner angles of 90° ±1.5 arc min. A He-Ne plasma tube produces a homogeneous pressure-broadened linewidth of 200 MHz; the beam is enclosed in nitrogen-filled tubes. Counterpropagating beams exhibit a beat frequency resulting from rotation of the ring. Foundational tilts force readjustment of the ring mirrors every few hours. The ring will be enlarged and placed under vacuum with the intent to reduce the relative Allan deviation (a measure of instability) from 3 x 10-6 to 9 x 10-9. Contact Robert Dunn at dunn@mercury.hendrix.edu.


Superlattices and blocking barrier create multicolor infrared detector
By stacking two superlattices (SLs) and separating them with a blocking barrier, researchers at National Taiwan University (Taipei, Taiwan) and National Chiao Tung University (Hsinchu, Taiwan) have created a multicolor infrared detector that can be electrically switched between the 7.5 to 12 and 6 to 8.5 μm wavelength ranges. The SLs are both composed of gallium arsenide/aluminum gallium arsenide layers, with the top SL having 6-nm wells and 4-nm barriers, and the bottom SL having 4.5-nm wells and 6-nm barriers. The blocking barrier is of aluminum gallium arsenide with a spatially slowly varying gradient of aluminum versus gallium concentration.

Because electrons can tunnel through the entire SLs, the SLs by themselves have low electrical resistance. This characteristic allows the photoresponses of the SLs to be alternately switched on by the bias polarity. In addition, the spectral sensitivity is tunable by changing the magnitude of the applied voltage, with higher bias magnitude shifting the spectral responsivity to longer wavelengths. Adjusting the barrier heights allows responsivity to be altered. The responsivity of the device, which operates at temperatures of 20 to 80 K, is not susceptible to temperature variations. Contact Chieh-Hsiung Kuan at kuan@cc.ee.ntu.edu.tw.


Nanometer-thickness layers tailor thin-film refractive index
Thin-film optical coatings with tailorable refractive index add great flexibility to coating design but are difficult to fabricate. One way to make such coatings is to stack nanometer-thickness alternating layers of two materials with different refractive indices; because the layers are only a small fraction of a wavelength in thickness, their properties blend to create an average index. Changing the layer thicknesses can change the average refractive index. Fabricating nanometer-scale layers of predictable thickness is difficult, however. Researchers at Osaka University and the Institute for Laser Technology (both of Osaka, Japan) have taken a technique used to create nanometer-thickness thin films for mirrors reflecting in the soft x-ray region and applied it to the optical region.

Called atomic layer deposition, the technique is based on the reaction of vapors with substrate materials; as the reaction proceeds, it fizzles out at a precise thickness. Alternating layers of aluminum oxide (Al2O3) and titanium oxide (TiO2) were grown on silica and silicon substrates at 200°C. Keeping the Al2O3 layers at 0.55-nm thickness while varying the thickness of the TiO2 layers from 0.2 to 3.9 nm changed the average refractive index from 1.870 to 2.318. Contact Shin-ichi Zaitsu at zaitsu@ile.osaka-u.ac.jp.


Sandwiched dispersion-managed fiber reduces Raman noise
A dispersion-managed fiber (DMF) consists of a segment of positive-dispersion (+D) fiber combined with a segment of negative-dispersion (-D) fiber so that overall dispersion is largely canceled. Because -D fiber has a small effective area, it is prone to multipath interference when operated at high Raman gains, such that double Rayleigh backscattering of light degrades the signal. Sandwiching the -D fiber section between two large-effective-area +D sections can reduce this effect. Researchers at Corning Inc. (Somerset, NJ and Deeside, England) have for the first time quantified the improvement in Raman noise figure for such a DMF configuration.

The distributed-Raman-amplified fiber was pumped with four laser diodes, wavelength and polarization multiplexed, with pump power ratios optimized for flat output signal power across the C-band; signal and pump were counterpropagating. A {25 km +D, 50 km -D, 25 km +D} stretch of fiber was compared to a conventional {50 km +D, 50 km -D} stretch. The researchers found that Raman gain was achieved earlier in the span of the new than for the conventional configuration, improving the Raman noise figure by 2.5 dB. There was no noticeable degradation from multipath interference. Contact Michael Vasilyev at vasilyevm@corning.com.


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Birefringence is key to phase-shifting scatterplate interferometer
Common-path interferometers, which reduce effects of vibration and air turbulence by passing the reference and measurement beams down the same path, are more difficult to outfit with phase-shifting apparatus than the common Fizeau and Twyman-Green interferometers. Existing methods to phase-shift the scatterplate interferometer (a type of common-path interferometer) require optics to be placed near the test surface—an inconvenience for practical use. By making the scatterplate birefringent, researchers at the Optical Sciences Center of the University of Arizona (Tucson, AZ) have phase-shifted the scatterplate interferometer without adding optics near the test surface.

The scatterplate is made by photolithographically etching a precise pattern to a depth of a few microns into a birefringent material such as calcite, where a layer of photoresist is exposed by use of a photomask or speckle pattern (the scatterplate surface shown here was produced by photomask). In use, the ordinary refractive index of the calcite is matched to that of a glass slide with index-matching fluid; phase shifting is provided by an electronically controlled variable liquid-crystal retarder nested within polarizing optics. Measurement accuracy of the tool is comparable to that of a phase-shifted Fizeau interferometer. Contact James Wyant at jcwyant@optics.arizona.edu.


Double acousto-optics narrow bandwidth of FSF laser
The development of solid-state frequency-shifted feedback (FSF) tunable lasers has improved tuning range and tuning speed, but has not narrowed emission lines to enable practical application. The broad spectrum of an FSF laser results from the continuous frequency shift induced by Bragg diffraction in the acousto-optic tunable filter (AOTF). Use of two AOTFs in the cavity of a Ti:sapphire FSF laser has enabled a group at the Institute of Physical and Chemical Research (Saitama, Japan) to compress the bandwidth without employing a Fabry-Perot etalon. Two tellurium dioxide AOTFs in the cavity of the Ti:sapphire laser were operated at 112.86 and 112.941 MHz. With a laser threshold of 3.2 W, the bandwidth of the laser varied nonlinearly from 6.36 to 12.28 pm over the tuning range of 167 nm centered at about 794 nm (compared to a measured bandwidth of 140 pm with only one AOTF in the laser cavity). The laser wavelength was electronically tunable without mechanical adjustment or use of additional dispersive elements. Contact Yimin Wang at ywang@laser.bli.uci.edu.


OCT resolves tadpole eye to 1 μm in 3-D
Optical coherence tomography (OCT), a three-dimensional (3-D) imaging technique with particular use in biological tissue studies, has conventionally been limited in resolution. Researchers at the Centre National de la Recherche Scientifique (Paris, France) overcame this limitation using a Linnik-type interference microscope and a white-light thermal lamp to reach the highest OCT 3-D resolution ever achieved of 1 μm. The previous highest resolution was 1 x 3 μm (longitudinal x transverse). The setup of the interference microscope was based on a Michelson interferometer with identical water-immersion microscope objectives in both arms to avoid degradation of the axial resolution caused by glass. In lieu of a laser, an inexpensive 100-W tungsten-halogen lamp with an extremely short coherence length illuminated the sample. In axial steps of 1 μm, the team recorded 300 1-s-exposure x-y tomographic images of a Xenopus Laevis tadpole eye and then reconstructed them into a 3-D image. Although low brightness limits the sensitivity of the microscope to 80 dB, the intensity of the lamp is very stable and the spatial incoherence minimizes speckle formation in the images. Contact Laurent Vabre at vabre@optique.espci.fr.


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FEL oscillator works at 190 nm
Free-electron lasers (FELs) can be operated in two different cavity configurations: self-amplified spontaneous emission (SASE; no mirrors, one pass straight through), or the traditional laser oscillator (a cavity with mirrors). An FEL oscillator can provide light of high spatial and temporal coherence, unlike SASE. Creating FELs that emit at short wavelengths is easier for SASE, however, with the output from such lasers reaching to 100 nm. Now, the higher-beam-quality FEL oscillator has been pushed to 190 nm by a group of scientists from Sincrotrone Trieste (Trieste, Italy), ENEA (Franscati, Italy), the Fraunhofer Institut für Angewandte Optik und Feinmechanik (Jena, Germany), the Laser Zentrum Hannover (Hannover, Germany), CEA/DSM/DRECAM/SPAM and LURE (Gif-sur-Yvette, France), and the CLRC Daresbury Laboratory (Warrington, England).

The laser has a cavity length of 32.4 m and is part of a synchrotron light source in Trieste. Optimized mirrors boost the gain of the laser to 20%, with emission demonstrated between 350 and 190 nm and average powers reaching to between a few and 330 mW (see fluorescence image of beam). At a 189.95-nm wavelength, the FEL produces 7.7-ps pulses. Contact Marino Marsi at marino.marsi@elettra.triest.it.

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