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-m wavelength region
at a 17-W 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 m 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
200C. 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.
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 m 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 m. The
previous highest resolution was 1 x 3 m (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 m, 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.
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.