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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.

Oct 1st, 2005
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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 kW 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 heterojunction 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-bandgap structures. Potential fabrication methods for quasicrystalline structures to be used at optical wavelengths include optical particle trapping (see Laser Focus World, 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 (New York, NY) have fabricated and tested one instead.

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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.

Lidar reveals micrometer-size meteoritic dust

Contrary to the theory that most of the material in large meteoroids is converted to nanometer-size particles through ablation upon entering earth’s atmosphere, scientists at the Australian Antarctic Division (Kingston, Australia), the University of Western Ontario (London, ON, Canada), the Aerospace Corporation (El Segundo, CA), and Sandia and Los Alamos National Laboratories (Albuquerque, NM, and Los Alamos, NM, respectively), used light detection and ranging (lidar) to suggest that the dominant contribution to the mass of the residual atmospheric aerosol was micrometer-size particles that remain in the atmosphere over long time periods.

Infrared and visible sensors detected a large meteor and its associated fireball on Sept. 3, 2004. More than seven hours after the fireball, an anomalous “cloud” was detected in the upper stratosphere by a polarization Rayleigh lidar instrument at Davis station in Antarctica. Lidar depolarization backscatter data showed that the cloud was dominated by micrometer-size nonspherical (solid) particles of olivine and pyroxene, consistent with chondritic meteorites. The dust can remain in the atmosphere for weeks or months, and its larger size (compared to nanometer-size particles) would have a greater impact on climate forcing and ozone depletion. Contact Andrew R. Klekociuk at andrew.klekociuk@aad.gov.au.

Free-space board-to-board optical interconnect aims to conquer vibrations

Free-space optical board-to-board interconnects in computers will ease data-flow bottlenecks, but must contend with vibrations in the printed-circuit boards they are mounted on. Researchers at the University of Cambridge (Cambridge, England) and the University of Oxford (Oxford, England) are developing a free-space adaptive-optical interconnect that uses dynamic gratings induced on a liquid-crystal (LC) spatial-light modulator (SLM) for beam steering; although not yet ready to compensate for vibrations, the interconnect can compensate for stepwise displacements.

A spatially filtered 650-nm-emitting laser diode, two 50‑mm-focal-length doublets in a 4ƒ geometry, and a 128 × 128-pixel ferroelectric SLM make up the device. A camera monitors the dot position in 2-D; displacements up to 1 mm can be corrected. Nonlinearity errors are measured and compensated for. The control loop updates at 4 Hz. Faster vibration-compensating versions could be created at up to 100 to 200 Hz (using a nematic LC) for computer boards and up to 10 kHz (using a ferroelectric LC) for aircraft. The tradeoff: a nematic LC offers multi-phase-level gratings with fewer unwanted higher orders but slow speed, while a ferroelectric LC offers high speed but only binary phase modulation. Contact Charley Henderson at cjdh2@cam.ac.uk.

Waveguides in automotive-WDM module are written by light

Inexpensive wavelength-division-multiplexing (WDM) schemes using two wavelengths and plastic optical fiber (POF) allow a single POF to be used for bidirectional communications in automotive and home applications. Engineers at Toyota Central R&D Labs (Aichi, Japan) have come up with an extremely simple way to fabricate polymer-waveguide WDM modules for such a use: light-induced self-writing (LISW).

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With volume production mandating low cost, the WDM module design itself is simple-a block of plastic with a 45º dichroic WDM filter (designed to split red and green wavelengths) at its heart, from which extend waveguides that couple with a photodiode, a light-emitting diode (LED), and the incoming or outgoing fiber. To make the module, the researchers position the WDM filter in a bath of photopolymerizing resin that is transparent to the LED wavelength, then shine the collimated beam from a small 5-mW blue (457-nm) solid-state laser through the resin to create a branching waveguide (the WDM filter splits the blue light into two beams of approximately equal intensity). After exposure, the unexposed resin is removed and the empty space filled with a lower-index resin that is then UV-cured. The WDM link performs well at a 250-Mbit/s data rate. Contact Yonemura Masatoshi at yonemura@mosk.tytlabs.co.jp.

Thermoplastics limit microholographic storage density

Microholographic data storage is anticipated to attain capacities beyond the 100-GByte-per-disc maximum for other surface technologies. Although scientists at General Electric Global Research (Niskayuna, NY) successfully recorded microholograms with reflectivity reaching 3% in a dye-doped thermoplastic, testing revealed limitations of this material in achieving large data densities.

Microholograms were recorded on 1.2-mm-thick injection-molded dye-doped polycarbonate discs using a 532‑nm laser. Diffraction efficiency profiles of the microholograms were influenced by the laser power levels and by nonuniform dye distribution in the thermoplastic material. Tests on microholograms recorded at four depths (240, 420, 600, and 780 µm) and laterally spaced by 10 µm showed a reduction in laser dynamic range-and a corresponding reduction in recording sensitivity-on the order of 1/N, where N is the total number of layers. A reduction in diffraction efficiency on the order of 1/N2 was also observed, indicating that high-density data storage systems need to consider these effects for successful design. Contact Marc Dubois atmarc.dubois@lmco.com.

Ceramic slab Nd:YAG laser emits 5 kW

In work sponsored by the Air Force Research Laboratory (Kirtland AFB, NM), researchers from Textron Systems (Wilmington, MA) have achieved what they believe to be record performance from a ceramic Nd:YAG laser. Their diode-pumped slab laser, which they dubbed “ThinZag” after its distinctive geometry, reached an average output power of 5 kW for a period of tens of seconds. Unlike crystalline Nd:YAG, the ceramic version can be straightforwardly fabricated into meter-size chunks suitable for single-aperture weapons-grade lasers; it is also three times more fracture-resistant and has a more uniform index of refraction. Transmission loss for the 1% doped ceramic material is comparable to that for crystalline Nd:YAG.

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In the ThinZag configuration, a thin slab of gain material is suspended between two fused-silica windows with cooling fluid flowing between in a two-sided laminar-flow configuration; the laser beam zig-zags through the fused silica (and fluid) as well as the Nd:YAG, resulting in a near-field output with an aspect ratio close to unity and independent of the thickness of the laser slab. The larger zig-zag path averages out pump nonuniformities. Contact Daniel W. Trainor at dtrainor@systems.textron.com.

Semiconductor-cylinder fiber amplifier has wide bandwidth

A semiconductor-cylinder fiber (SCF) has a thin layer of semiconductor material at its core/cladding boundary that, when pumped, turns the fiber into an amplifier. Researchers at Dove Photonics (Rome, NY), Syracuse University (Syracuse, NY), and the NASA Goddard Space Flight Center (Greenbelt, MD) have created a theoretical model of a glass fiber with a 5- to 25-nm-thick cadmium phosphide (Cd3P2) layer between the core and cladding and compared the results with experimental data.

The 1.8-mm-long experimental fiber had a 10-µm core and a 125-µm cladding diameter and was side-pumped with 100 mW of light at 832 nm (a convenient method, but with low coupling efficiency to the Cd3P2). White light coupled into the fiber was used to measure the fiber transmission over a 1000- to 2000‑nm wavelength region. Calculated and experimental gain matched well, although the experimental data were noisy. An SCF has a wider bandwidth than a conventional semiconductor light amplifier, which has a bandwidth limited at the short-wavelength end by the junction-barrier potential; in contrast, the shortest amplifiable wavelength of the SCF has an energy equal to the difference between the electron and hole Fermi levels. Contact Philipp Kornreich at pkornrei@syr.edu.

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