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Fiberoptic gyroscope incorporates digital signal processing; Shifting fringe pattern reveals three-dimensional position; MORE...

Jan 1st, 2003

Quantum dots find a stable home

Pumped by ultraviolet light, cadmium selenide (CdSe) nanocrystal quantum dots (NQDs) emit light at a wavelength that depends on the size of the NQD. The emitters are thus potentially useful for many purposes, including lasers and solid-state lighting. Quantum dots can either exist "bare," or can be coated with an inorganic shell to decouple the NQDs from the surrounding matrix. Research is ongoing to find the best matrix and method to stabilize NQDs. In results presented at the Materials Research Society fall meeting (Dec. 2–6, 2002; Boston, MA), scientists at Los Alamos National Laboratory outlined a NQD nanocomposite stable enough to be suitable for nonlinear-optical and optical-gain applications.

Colloidal CdSe NQDs are passivated with alcohol-terminated amines, which link the NQDs to a surrounding titania matrix. Such a matrix is transparent, and high packing densities (up to 20% volume fraction) of the NQDs within the matrix can be achieved. Incorporated into a glass tube (left), the NQD composite lases at room temperature at wavelengths in the red in whispering-gallery modes (right). Efficient dynamic gratings useful for optical switching were also fabricated from films of the nanocomposite. A mixture of different-sized NQDs pumped by ultraviolet light-emitting diodes may someday serve as an efficient white-light emitter. Contact Victor Klimov at klimov@lanl.gov.


Fiberoptic gyroscope incorporates digital signal processing

Researchers at KVH Industries (Middletown, RI) have developed and patented a fiberoptic gyroscope (FOG) that incorporates digital signal processing (DSP), lowering the cost of the device while maintaining performance comparable to that of ring-laser and closed-loop gyroscopes. In combination with polarization-maintaining fiber, the gyroscope's integrated DSP handles data input of up to 500°/s and provides time- and temperature-insensitive output.

The directional couplers and coil in the device are made of polarization-maintaining fiber to ensure a single-mode path. Counterpropagating light travels the same path, ensuring that almost all environmental effects, such as temperature and vibration, have the same effect on light passing in either direction and are canceled. The sinusoidal relationship between the input and output characteristics of a FOG is a well-known analytic function that is dealt with by DSP. The optical configuration permits measuring the difference in phase between the two signals to one part in 1016. The digital rotational rate information can be output to recipients such as precision-guided munitions or unmanned aerial vehicles—systems playing an increasing role in the U.S. military's ongoing counterterrorism operations. See www.kvh.com/FiberOpt/index.asp.


Shifting fringe pattern reveals three-dimensional position

A simple method for measuring distances interferometrically in all three dimensions at once has been developed at the Korea Advanced Institute of Science and Technology (Taejeon, Korea). The apparatus measures distances over a 120 × 120 × 120-mm volume with an uncertainty in position of less than 1 ×m over the entire volume. The hardware consists of only two parts: a target that moves in three dimensions and consists of two point-diffraction sources (optical fibers), and a stationary charge-coupled-device (CCD) detector approximately 300 mm away from the target.

The two interfering point sources create hyperboloidal phase fronts that, on the surface of the CCD, become hyperbolic fringes. A phase shifter precisely determines the phase across the pattern, which is fitted to a geometric model. Motion along the optical axis enlarges or reduces the pattern, while motion in the two perpendicular directions shifts the pattern's position. In all cases, calculations determine the actual movement of the target. The repeatability of the measurement is 0.05 and 0.1 µm for the perpendicular directions and 0.01 µm for the optical-axis direction. A larger-area CCD may reduce the volumetric uncertainty from 1 to 0.1 µm. Contact Seung-Woo Kim at swk@kaist.ac.kr.


Protective layers boost electrochromic reliability

Electrochromic (EC) materials change transmission when a voltage is applied, making them potentially useful for "smart" windows. But the different chemical layers in some high-performance EC devices tend to react with each other, reducing lifetime. Scientists at the Kwangju Institute of Science and technology (Kwangju, Korea) and the Korea Advanced Institute of Science and technology (Daejon, Korea) are fabricating EC devices containing protective layers of tantalum pentoxide that prevent this slow eating away from within.

The protective layers were inserted between films of tungsten trioxide and acidic electrolyte, as well as between those of nickel hydroxide and electrolyte (all layers were amorphous). Two types of EC devices were stored in atmosphere for two days after fabrication, then tested at a cyclic voltage while transmittance at 633 nm was measured. The best-performing devices showed a high transmittance modulation of 18% minimum to 74% maximum, as well as response times of 8.5 s for bleaching and 18 s for coloring—reasonable for EC materials. The cyclic test showed that the protective layers remained stable with a high coloration efficiency (a measure of electrochromic performance). Contact Yung-Eun Sung at ysung@kjist.ac.kr.


Volume-holographic lens images microturbines

A volume hologram is a depth-selective hologram that produces Bragg-diffracted images of objects in three spatial dimensions and one spectral dimension. Arnab Sinha and George Barbastathis of the Optical Engineering Group at Massachusetts Institute of Technology (Cambridge, MA) have developed a volume-holographic telescope capable of imaging in high resolution the complex surface topology of distant objects.

To obtain the highest resolution image at the farthest distance, an expanded numerical aperture of the volume hologram is necessary, but costly and impractical. A telescope was used in front of the 2-mm-thick iron-doped lithium niobate hologram to make a smaller secondary image that demagnified the image and enhanced the bandwidth. A miniature silicon turbine with surface features 225 µm deep was placed 16 cm in front of the telescope and mounted on a linear translation stage. As the stage was scanned, the volume-holographic telescope recorded the microturbine surface layer by layer. Using a telescope with an angular magnification of 1.5, the depth resolution in the z direction was 100 µm, better than that of an equivalent binocular triangulation system. Contact Arnab Sinha at arnab@mit.edu.


Freestanding photodetectors are small and fast

The subpicosecond relaxation time of low-temperature-grown gallium arsenide (GaAs) films makes the material a good choice for ultrafast photodetectors. Processes allow for the fabrication of freestanding GaAs devices that consist simply of the film itself, which can then be placed on any convenient substate. To integrate such a device into an electronic circuit, however, the detector itself must be small—preferably of microscopic dimensions. Researchers at the Research Center Jülich (Jülich, Germany) and the University of Rochester (Rochester, NY) have now developed low-temperature-grown GaAs photodetectors only microns in size.

Fabricated by molecular-beam epitaxy, the square devices range from 10 to 150 µm on a side and after fabrication are transferred on a metallic tip to the final resting places. Compared to ordinary "as-grown" GaAs devices, the freestanding detectors show a tenfold decrease of dark current (below 3 × 10-7 A at 100 V) and ohmic linear behavior over a twentyfold increase in voltage r (200 V, corresponding to a 200-kV/cm electric field). Transient response for 810-nm light was 0.55 ps and for 405 nm was 1.35 ps. Contact Roman Adam at r.adam@fz-juelich.de.


Record low Ti:sapphire threshold leads to inexpensive lasers

Femtosecond laser technology is too expensive for many practical applications. The development of lasers that generate femtosecond pulses and broad bandwidths at low cost would make possible widespread application in ultrafast research, materials processing, and biomedical imaging. Researchers at the Massachusetts Institute of Technology (Cambridge, MA) have achieved this by means of a Kerr-lens modelocked (KLM) Ti:sapphire laser with an extended-cavity design. This reduces the pump-power requirements to an ultralow modelocking threshold of 156 mW, the record-low threshold achieved to date for a KLM Ti:sapphire laser.

The pump source itself is a frequency-doubled diode-pumped Nd:vanadate laser with a Ti:sapphire crystal cavity. The laser generates a 91-nm bandwidth centered on 840 nm, with pulse duration of 14 fs and a 16-mW power output. With 200 mW of pump power, the two-armed extended cavity gives the laser a repetition rate of 50 MHz. The laser cavity is made of a straight arm measuring 130 cm and a prism arm of 162 cm in length. The long cavity creates a narrow mode size with an 8-µm radius.

Contact James Fujimoto at jgfuji@mit.edu.


Light makes MEMS mirror resonate

Microelectromechanical systems (MEMS) devices are sometimes used to steer light; now, scientists at Agere Systems (Murray Hill, NJ) are using light to steer MEMS—or, more precisely, to excite their angular motion. When a weakly focused infrared 3-mW optical beam chopped at the resonant frequency of a MEMS mirror is shone on the mirror at a position away from its rotational axis, the change in momentum of the beam is transferred to the mirror in a periodic way, causing the mirror to resonate. The researchers are using the phenomenon to test MEMS mirrors after fabrication to determine their resonant frequencies.

With an interferometer monitoring mirror position and motion, the chopping frequency of the beam is swept. In one example, a beam of 1550-nm wavelength shone on a circular MEMS mirror mounted on gimbals helped determine three different resonant modes of the mirror with frequencies of 341, 530, and 2680 Hz at interferometrically measured amplitudes of 26.0, 5.7, and 2.0 nm respectively. Using a lithium niobate modulator to chop the beam extends the excitation range to gigahertz frequencies, say the researchers. Contact John Graebner at jeg@agere.com.


Photonic-crystal laser hangs out in the heat

Researchers at the University of Southern California (Los Angeles, CA) have created a photonic-crystal laser that operates at temperatures as high as 50°C. The photonic crystal itself is a 224-nm-thick square membrane supported hammock-like along two opposite sides, and contains a defect at the center of the membrane. The membrane defect cavity is pumped with 865-nm light from a vertical-cavity surface-emitting laser. The pump power is about 7 mW and the pump spot is 3.5 µm in diameter.

In the experiment, a thermoelectric controller is used to vary the temperature of the sample's substrate. The threshold input powers at substrate temperatures of 20°C, 35°C, and 50°C are 3.2, 5.3, and 7.4 mW respectively. During pumping (20-ns pulses at 0.5 MHz, or a 1% duty cycle), the defect region itself can increase in temperature by as much as 112 K. The laser emits at around 1645 nm and has a wavelength temperature dependence of 0.05 nm/K, explainable entirely by the temperature dependence of the refractive index alone. Contact Po-Tsung Lee at potsung@usc.edu.

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