Aug. 1, 2001
Screen-printing technique produces OLE displays; Dye and microspheres create an optical diode; Dual fiber-ring depolarizer is insensitive to input conditions ...
Screen-printing technique produces OLE displaysThe growing importance of the graphic presentation of information has spurred Siemens AG and Universität Erlangen-Nürnberg (both Erlangen, Germany) researchers to investigate a screen-printing technique for fabricating organic light-emitting (OLE) displays. Two major disadvantages of the current polymer processing technique (spin coating) are material waste and difficulties involved in patterning monochrome or full-color displays. Focusing on a printing technique for the coating of polymers in the production of passive-matrix displays, the researchers discovered that screen-printed OLE diodes are already comparable to spin-coated ones.

High-efficiency organic light-emitting diodes were fabricated on the basis of conjugated polymers, in which both polymer layers were screened. Luminance of 10,000 cd/m2 at 8V and peak efficiencies exceeding 10 cd/A for green diodes was observed. The rectification ratio of printed devices is about one order of magnitude lower, report the researchers, and the homogeneity of the printed layers is sufficient, whereas lateral resolution is still improvable. This indicates that printed organic displays, upon further development, may replace more-conventional spin-coated devices. Contact Jan Birnstock at [email protected].

Dye and microspheres create an optical diodeAn electronic diode permits a current in one direction but not the other. Researchers at the Raman Research Institute (Bangalore, India) have created an optical analogue of the diode that prevents the propagation of a band of light wavelengths in one direction while allowing it in the opposite direction. The device is based on a combination of random scattering and a gradient of laser dye.

A 10-cm-long, 100-µm-diameter glass capillary is filled with ethanol and a suspension of 0.21-µm-diameter polystyrene microspheres. Dye is introduced in one end, creating an exponential gradient. The capillary is end-pumped with frequency-doubled pulsed Nd:YAG laser light. The two effects of self-absorption and stimulated emission, which usually compete with each other, are separated in the device. Light from the low-concentration end exits at 588 nm, while light exits the high-concentration end at 608 nm, both at a 10-nm linewidth; in other words, effective transmission in each direction is centered on a different wavelength. The gradient must exist for the effect to arise. A practical version made of solid polymer is a possibility, say the researchers. Contact Sushil Mujumdar at [email protected].

Dual fiber-ring depolarizer is insensitive to input conditionsRecently, passive devices based on structures of either one or ten incoherent fiber rings have been found useful for depolarization of single-mode fiber. Also called recirculating delay lines, incoherent fiber rings are built by cascading 2 x 2 single-mode directional couplers spliced together to form a ring. Such rings are immune to path compensation and do not require a broadband laser source, but suffer the disadvantages of input state-of-polarization (SOP) dependence or high insertion loss. Researchers at Arizona State University's Department of Electrical Engineering (Tempe, AZ) have now modeled and successfully tested a similar depolarization device, based on two fiber rings, that is insensitive to input SOP variations and can be used with a low-cost laser diode.

The experiment used a single-longitudinal-mode laser diode (emission wavelength of 1310 nm) with a coherence length of 2 cm, and two rings of lengths 2.5 m and 2.0 m. The dual-fiber device provided a very low output degree of polarization of -25.7 dB independent of the SOP, which was maintained for half an hour. The researchers claim that temperature control would enable longer-term stability. Contact Joseph Palais at [email protected].

Traveling-wave photodetector uses unicarrier traveling photodiodesNTT Photonics Laboratory (Atsugi-shi, Kanagawa, Japan) researchers have demonstrated a periodic traveling-wave photodetector based on unicarrier traveling photodiodes. The photodetector consisted of three unicarrier photodiodes periodically arranged along a coplanar waveguide transmission line. The photoresponse of the photodetector was found to be three times greater than that of a single unicarrier-traveling photodiode.

A 3-dB bandwidth of 115 GHz at 1.55 µm was observed in the photodetector, with a terminal resistor located at the input end. This value is almost the same as that observed in a single unicarrier traveling photodiode. In the photodetector without a terminal resistor, the 3-dB bandwidth degrades to 56 GHz as a result of the reflection of backward-propagating microwaves at the input end. The researchers believe that their combined periodic traveling-wave photodetector and unicarrier traveling photodiode effectively provides velocity matching between electrical and optical waves. Contact Yukihiro Hirota at [email protected].

Lithium fluoride color-center waveguide may become a laserDamage by light or electron beams to certain crystals can cause color-center defects that are able to fluoresce. While a disadvantage in most optical systems, color centers can be exploited as the medium for active devices such as lasers. Lithium fluoride (LiF) crystals can serve as the host for fluorine color centers that emit in the green and red portions of the visible spectrum. Researchers at Ente per le Nuove Tecnologie and the Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali de Frascati (both of Rome, Italy) are not merely making color-center active devices from LiF, but are fabricating them in waveguide form.

Electron-beam lithography is used to write a color-center channel 15 mm long and 140 µm wide on the surface of an optically polished LiF crystal. When pumped with 458-nm light from an argon-ion laser, the waveguide exhibits amplified spontaneous emission in a broad band centered on 650 nm and with a gain coefficient of 4.67 cm-1. The researchers are investigating the device's guiding and amplifying properties, and are positing low-threshold tunable amplifiers and lasers compatible with integrated optical circuits. Contact Rosa Maria Montereali at [email protected].

Fluorescence decay observed in plastic optical fiberErbium-doped fiber amplifiers (EDFAs) operate over a range of wavelengths from 1520 to 1630 nm, but the need for amplifiers for other data-communications windows has driven development of fiber amplifiers doped with praseodymium (Pr) and other chelates. Scientists at Keio University (Kanagawa, Japan) and Polytechnic University (Brooklyn, NY) have fabricated what may be a plastic alternative to EDFAs at other wavelengths.

The team successfully incorporated neodymium (Nd), Pr, and erbium ions into the core of deuterated polymer-based optical fiber, using an interfacial-gel polymerization technique. The dopants are compatible with existing graded-index plastic optical fiber (POF) for amplification at 1060, 1310, and 1550 nm. The doped POFs were found to have strong characteristic absorption peaks at visible and near-infrared wavelengths. A fluorescence lifetime of 6.24 µs was determined at 1060 nm in the Nd-doped POF, using a frequency-doubled, Q-switched Nd:YAG pumped dye laser at 580 nm. The use of deuterated materials reduces the possibility of nonradiative transitions, say the researchers, and is very promising for further telecommunications applications. Contact Ken Kuriki at [email protected].

1.55-µm VCSEL provides efficient CW operation at high temperaturesWhile 1.55-µm vertical-cavity surface-emitting lasers (VCSELs) are attractive sources for optical networks—in part because of their low power consumption and efficient fiber coupling—their performance usually degrades at higher temperatures. Now, researchers at the University of California-Santa Barbara (UCSB) have developed a wafer-bonded 1.55-µm VCSEL that provides continuous-wave (CW) electrically pumped operation at 105°C.

The best high-temperature results with such devices to date have been achieved using wafer-bonded gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) distributed Bragg reflectors in both electrically pumped and integrated optically pumped designs. The UCSB devices incorporate an indium phosphide/indium gallium arsenide phosphide active region that has been wafer-bonded to GaAs/AlGaAs mirrors. Also included is a superlattice barrier to reduce the number of nonradiative recombination centers in the bonded active region. The VCSELs, which were tested in a p-side-up configuration without any special heat sinking, emit a maximum of 0.65 mW at 20°C and 0.22 mW at 80°C. With a 6-µm device, threshold current reached 0.9 mA. Significant improvements are expected for devices with a more-favorable mode-gain offset, a lower turn-on voltage, and a reduction in series resistance. Contact Adil Karim at [email protected].

Ultraviolet light induces shape changes in solid materialResearchers at the University of Freiburg (Freiburg, Germany) and the University of Cambridge (Cambridge, England) have used optical stimulation to generate reversible shape changes in solid materials that normally deform reversibly with changes in temperature. Nematic elastomers have demonstrated shape changes up to 400% in relative magnitude when stimulated by temperature changes in the vicinity of their nematic isotropic transition temperature. These shape changes have been correlated with shifts in nematic order, and the European researchers have recently produced similar shifts in nematic order under optical as opposed to thermal stimulation. The experiment consisted initially of synthesizing monodomain elastomeric networks through two-stage cross-linking. Exposure to ultraviolet light at 365 nm was observed to induce photoisomerization of the AV2 photoisomerizable linker in the network, which caused a length contraction in the material on the order of 20%. The material relaxed to its original state in the dark over a period of hundreds of minutes. Due to similarity between optical and thermal processes, the researchers hope to optically induce the type of 400% deformations that have been achieved thermally. Contact Heino Finkelmann at [email protected].Photonic bandgap provides high-frequency gyrotron oscillatorResearchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have built a laser-like gyrotron oscillator with a resonant 25-kW peak at 140 GHz (2.1-mm wavelength) using a photonic-bandgap (PBG) structure. Conventional vacuum electron (or microwave-tube) devices cannot be made small enough to reach frequencies in the 100-GHz range. The MIT device, however, replaced the cylindrical outer wall in the traditional gyrotron cavity with a PBG structure consisting of a triangular lattice of 102 1.59-mm-diameter copper rods with 2.03-mm spacing between rods. The rods were placed parallel to the axis of the gyrotron and magnetic-field system and were held in place by a matching triangular lattice of 121 holes in each of two oxygen-free high-conductivity copper end plates. Nineteen rods were omitted from a cylindrical area in the center of the lattice structure to support a high-order transverse-electromagnetic (TE)-like waveguide mode with a resonant frequency within the bandgap of the PBG structure. To optimize efficiency, the cavity length of about 1.4 m equaled eight wavelengths of the 140-GHz operating frequency. The cavity was pumped with a hollow annular electron beam from a magnetron electron gun. While operating at its nominal operating voltage of 68 kV and current of 5 A within a magnetic-field range of 4.1 to 5.8 T, the device yielded a single strong TE041-like peak at 140.05 GHz. Contact Jadagdishwar Sirigiri at [email protected].

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