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 [email protected].
Superlattices and blocking barrier create multicolor infrared detectorBy 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 [email protected].
Nanometer-thickness layers tailor thin-film refractive indexThin-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 [email protected].
Sandwiched dispersion-managed fiber reduces Raman noiseA 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 [email protected].