Wavelength-combined diode-laser array is nearly diffraction limited
Combining the individual beams of many diode lasers at high beam quality could result in high-power diffraction-limited lasers that are more compact and efficient than any thus far. One approach is wavelength beam combining, in which beams of different wavelengths are overlaid on each other with the aid of a dispersive optical element. This approach has the potential for high power, but can be complicated. Researchers at the Massachusetts Institute of Technology’s Lincoln Laboratory (Lexington, MA) have developed a spectrally varying diode-laser array that allows for simple wavelength beam combining.
The 100-µm array contains 100 slab-coupled optical-waveguide laser elements with wavelengths that vary linearly over a 17-nm band centered on 915 nm (the first prototype is not continuous-wave, but is pulsed at a 1% duty cycle). Each element has a rear cavity mirror, an antireflection-coated front facet, and a microlens. A common collimating lens sends the many beams toward a diffraction grating spaced one focal length away from the lens, where the axial rays converge; the grating sends out a single, combined beam to a common output-coupling mirror. The laser emits 35-W peak power with a beam quality M2 of 1.35. Future devices emitting 4 k W with a 36-nm bandwidth are possible. Contact Tso Yee Fan at fan@ll.mit.edu.
Millimeter-wave imaging detector is based on silicon
Millimeter-wave imaging lies at the boundary of optical and radio technology. Quasi-optical techniques using plastic Fresnel zone plates to focus millimeter waves enable hidden weapons to be detected (see Laser Focus World, April 2003, p. S14). Imaging detectors are usually diode-based—for example, Schottky diodes, which must be biased, adding to complexity, noise, and drift; or zero-bias germanium (Ge), gallium arsenide, or III-V-based backward diodes, which either cannot be mass-produced or are incompatible with silicon (Si) readout circuitry.
Researchers at Ohio State University (Columbus, OH), the Naval Research Laboratory (Washington, D.C.) and the University of Notre Dame (Notre Dame, IN) have developed an alternative—an epitaxially grown and annealed Si-based zero-bias backward diode that is compatible with Si/SiGe hetero-junction bipolar transistors. Minimizing the forward tunneling current produced devices that had a desirable highly nonlinear current-voltage characteristic with a curvature coefficient of 31 V–1 and a potential cutoff frequency of more than 100 GHz, which is in the millimeter-wave region. Changing the annealing temperature varies the doping, allowing the device properties to be tailored. Contact Paul Berger at pberger@ieee.org.
Photonic properties of quasicrystals are measured
Quasicrystalline lattices, which have long-range order but are not periodic, can have high degrees of symmetry useful for photonic-band-gap structures. Potential fabrication methods for quasicrystalline structures to be used at optical wavelengths include optical particle trapping (see Laser Focus World,
(New York, NY) have fabricated and tested one instead.
Designed for microwave frequencies, an icosahedrally symmetric quasicrystalline polymer structure and a version of the best conventional crystalline photonic structure (diamond) were fabricated stereolithographically and experimentally compared. The 3-D quasicrystal (shown here) had a sizable stop gap and a well-defined and simple Brillouin zone, despite its complex quasiperiodicity. Its Brillouin zone was much closer to spherical than that of the diamond structure, which in practical terms means photonic properties that are less dependent on direction. The far-from-optimized quasicrystal structure pointed the way to improvements that would further enhance the stop gap. Contact Weining Man at wman@princeton.edu.
September 2005, p. 13) or optical writing techniques such as two-photon polymerization.
The performance of 3-D photonic quasicrystals is difficult to model, however, so scientists at
Princeton University (Princeton, NJ), Philips Research Laboratories (Eindhoven, The Netherlands), and New York University
PRINCE TON UNIVERSITY
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