Bow Ties May Make Good Sensors
TUCSON, Ariz. -- In the future, bow ties may be more than a fashion accessory for political pundits and used-car salesmen. Researchers at the University of Arizona's Optical Sciences Center have developed a 940-nm edge-emitting semiconductor laser featuring a bow-tie design that could be the basis of better gas sensors.
The bow-tie appellation derives from the device's symmetrically flared resonator, which resembles the neckwear, but the chip lases in a unique way: There are two beams, one from each half of the laser. "This laser is a grating-coupled surface-emitting laser, with coherent output -- after all, it is the same source -- from both facets," explained Robert Bedford, a member of the research team that described the device in the August 2000 issue of IEEE Photonics Technology Letters.
The laser consists of three sections. A 200-µm ridge section sits at the center of the device, like the knot of a bow tie. This widens on either side in gain-guided flares, which expand over 1 mm to a width of 125 µm. Beyond the flares are the diffraction-grating reflectors, a series of curved grooves that enable surface emission and that look like the ends of the tie.
The demonstration laser incorporated a strained single-quantum-well GaAs/InGaAs structure. It displayed 85-mW, single-mode output at 1 A, but the researchers believe that the device will work at higher drive currents.
A key feature of the bow-tie laser is that it emits its coherent light from two facets in the same direction, which could be useful, for example, in the construction of a gas sensor. Gas sensors now operate by taking a single beam, splitting it in two, exposing one part to the sample, then recombining the beams and measuring the resulting interference fringes.
"One can remove a large part of the complexity and cost of this system if one introduces a small gas flow via a microjet in one of the outputs of the bow-tie laser," Bedford explained; beamsplitters would therefore no longer be necessary.
Constructing the bow-tie laser with its curved diffraction grating requires electron-beam lithography, which, Bedford acknowledged, is neither cheap nor fast. In the future, advances in semiconductor processing may make the devices less expensive and easier to produce. In particular, improvements in deep-ultraviolet lithography will ease the manufacturing of the 300-nm grooves in the diffraction gratings.
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