Newsbreaks

Sept. 1, 2001
Ceramic Nd:YAG laser reaches output of 72 W; Switch based on SOA achieves femtosecond switching; Buckyballs boost efficiency of thin-film organic solar cell ...
Ceramic Nd:YAG laser reaches output of 72 WPolycrystalline (ceramic) Nd:YAG is easier and less expensive to make than single-crystal versions of the material. For half a decade, researchers have been working on improving the quality of ceramic Nd:YAG for use as a laser gain medium, with increasing success. Collaboration between researchers at the University of Electro-communications (Tokyo, Japan), Komoshima Chemical Co. (Kagawa, Japan), and the Russian Academy of Sciences (Moscow, Russia) has resulted in the development of a ceramic continuous-wave Nd:YAG laser that emits 72 W.

In the device, 32 sets of 10-W laser diodes emitting at 807 nm pump a ceramic laser rod. Thirty-two sets of optics ensure a uniform intensity distribution on the rod. The rod itself is 3 mm in diameter and 104 mm long, with a 1% neodymium concentration; external cavity mirrors are used. Large improvements in homogeneity make the high laser output possible; the optical-to-optical efficiency reaches 24.8%. Some thermally induced birefringence is observed at higher powers. The laser performs as well as a single-crystal laser but is suitable for mass production, said the researchers. Contact Jianren Lu at [email protected].

Switch based on SOA achieves femtosecond switchingA team of researchers in NEC Corp.'s Photonic and Wireless Device Division (Ibaraki, Japan) has demonstrated the effect of intraband carrier dynamics on nonlinear refraction induced in a semiconductor optical amplifier (SOA), in terms of its applicability to the Symmetric Mach-Zehnder (SMZ) all-optical switch. Nonlinear phase shifts in an SOA and a passive semiconductor waveguide were compared under control-pulse durations ranging from 3.2 to 0.4 ps. The nonlinear phase-shift component (caused by carrier heating) was found to enhance the output inside the switching window, while hindering the complete extinction outside the switching window. Required control-pulse energy for the nonlinear phase shift (due to carrier depletion) increases when the control pulse is reduced. However, the nonlinear shift in the SOA still shows higher efficiency than that in the passive nonlinear waveguide—even when the 0.4-ps control pulse is used. The researchers experimentally achieved femtosecond (670 fs), femtojoule (140 fJ) switching with the SOA-based SMZ-type switch, and believe that femtosecond switching with higher efficiencies is possible by using the SOA. Contact Shigeru Nakamura at [email protected].Buckyballs boost efficiency of thin-film organic solar cellThin-film organic photovoltaic (PV) solar cells, though lower in efficiency than their inorganic cousins, are potentially also lower in cost to manufacture. Advances such as the introduction of donor-acceptor (DA) heterojunctions and exciton-blocking layers (EBLs) have improved the efficiency of such devices. Researchers at the Center for Photonics and Optoelectronic Materials at Princeton University (Princeton, NJ) have developed a thin-film organic PV cell that contains carbon (C) in the form of buckminsterfullerene (C60), or buckyballs. The cells, which contain a DA interface incorporating an EBL, exhibit an efficiency of 3.6% under one sun of simulated solar illumination, or an increase in efficiency of a factor of two over previous thin-film organic solar cells.

The double-heterostructure PV cells were made on glass coated with an indium tin oxide (ITO) anode. The ITO was then coated with organic films; among others, these included a 5- to 40-nm-thick layer of donorlike copper phthalocyanine and a 10- to 40-nm-thick layer of acceptorlike C60. A bathocuproine EBL transported electrons to an aluminum cathode while blocking excitons from recombining at the cathode. The researchers believe that light-trapping structures will boost efficiency to the 5% level. Contact Stephen Forrest at [email protected].

White LED demonstrates promising optical and electrical performanceJilin University (Changchum, China) scientists demonstrated efficient organic white light-emitting devices (LEDs) with tris-(8-hydrolyquinoline)aluminum (Alq) as the chromaticity-tuning layer and that reportedly offer good optical and electrical performance. In their demonstration, the scientists created a white LED using N,N'-diphenyl-N,N'-bis(1-naphthyl)-(1,1'-biphenyl)-4,4'-diamine (NPB) as the hole-transporting layer, 1,6-bis(2-hydroxyphenyl) pyridine boron complex [(dppy)BF)] as the emitting layer, and Alq as the electron-transporting and chromaticity-tuning layer. The white light comes from exciplex emission at the solid-state interface between (dppy)BF and NPB, while chromaticity of white emission was tuned by adjusting the thickness of the Alq layer. The scientists showed that white LEDs with an Alq thickness of 15 nm exhibited a maximum luminescence of 2000 cd/m2, an efficiency of 0.58 lm/W, and that the Commission Internationale De l'Eclairage (CIE) coordinates of resulting emission varied from (0.29,0.33) to (0.31,0.35) with increasing forward bias from 10 to 25 V. These results promise excellent optical and electrical performance, according to the scientists. Contact Yue Wang at [email protected].Single-cycle supercontinuum measures ultrafast thermal transientsResearchers at Johns Hopkins University (Baltimore, MD) and Lawrence Livermore National Laboratory (Livermore, CA) have demonstrated a method for detecting ultrafast thermal transients in a single excitation-relaxation cycle that they expect will prove useful in applications such as quality assurance in the manufacture of semiconductor integrated circuits or in-process testing of materials under heat treatment. The measurements were made by reflecting a linearly chirped, supercontinuum optical pulse from gold film, where surface temperature changes were induced by an ultrafast pump pulse. The subpicosecond pulses for generating the supercontinuum were provided by a mode-locked Ti:sapphire oscillator seeding a regenerative amplifier operating at a 1-kHz repetition rate. Most of the energy from the 1-mJ, 80-fs, 800-nm pump pulse produced by this system was converted to 400 nm and focused to a 3-mm spot size to pump the sample with a fluence of 27 µJ per cm2. Seven millijoules of the amplifier output were passed through a sapphire window and a chirped pulse generator to form a linearly chirped white-light probe pulse, before focusing down to a 400-µm spot size. Thermal transients were encoded on the spectrum of the probe pulse and mapped onto the time domain with subpicosecond time resolution. Contact Christopher Richardson at [email protected].Intersubband Raman laser penetrates mid- to far-infraredResearchers at the National Research Council (Ottawa, Canada) have demonstrated an optically pumped, intersubband laser based on the Raman process and emitting at infrared wavelengths. They expect the design to enable development of far-infrared sources operating at room temperature. The intersubband Raman laser consisted of 150 double quantum wells, each made of 7.6-nm gallium arsenide (GaAs), 1.1-nm aluminum gallium arsenide (Al0.35Ga0.65As), and 4.9-nm GaAs, with the Al0.35Ga0.65As providing a 20-nm-wide separation barrier between the two halves of each double quantum well. The device was pumped by 50-ns pulses from a CO2 laser in resonance with the 1-to-3 transition in the artificial three-level GaAs/AlGaAs structure. The 3-to-2 transition of the device provided laser emissions. The 1-to-2 transition spacing was designed to favor the Raman process, instead of the optically pumped population inversion typical of other quantum fountain lasers. Definite evidence of the Raman process was provided by constant differences observed between pump and emission frequencies for given samples at given temperatures, independent of pump power. Taking into account the various loss factors in the experimental device and measurement process, the researchers estimated a power-conversion efficiency on the order of 0.1%, and they projected a peak output power in excess of 1 W. Contact Hui Chun Liu at [email protected].Diode laser with FBGs produces near-transform-limited picosecond pulsesA research team from the University of St. Andrews (St. Andrews, Fife, Scotland) and Aston University (Birmingham, England) has generated near-transform-limited picosecond pulses from a gain-switched indium gallium arsenide (InGaAs) diode laser combined with periodic and chirped fiber Bragg gratings in an external cavity configuration. Compared to a laser without optical feedback, this arrangement reduced the spectral bandwidth from 11 to 0.08 nm and cut pulse duration from 30 to less than 18 ps. The system used a commercial InGaAs/GaAs single-mode ridge waveguide laser with an active stripe width of 3 µm and emission wavelength at 980 nm. For comparative purposes, the laser was first butt-coupled to a 50-cm-long unprocessed fiber of the type from which fiber gratings would subsequently be fabricated. The fibers were cleaved perpendicular to the core, and the ends were not specially treated. Further improvements to this system will include fusion-splicing of a fiber-pigtailed laser or lensed fiber to one of the optical fibers containing fiber Bragg gratings, with the goal being to further maximize optical coupling. Contact David Birkin at [email protected].Scientists base modulator on resonance excitation of surface plasmonsWhile micrometer-sized high-speed modulators are of interest to designers of integrated optics, it is impossible to achieve the modulation frequencies required by using semiconductor lasers. Another problem involves the requirement for light modulation within the waveguide regime of propagation. One possible solution comes from researchers at Yeravan State University (Yerevan, Armenia). They propose a method in which light is modulated at high speed during its input into a standard planar waveguide composed of a substrate, core, and conductive material layers. This involves the preliminary excitation of a surface plasmon polariton, which then transforms into the waveguide mode during propagation. The three-stage process begins when light radiation excites a surface plasmon polariton on the top surface of the conductor layer, with the light energy localizing in a small region of the interface separating the metal layer with a vacuum. In the second stage, the localized light energy transfers into the core region. This localized surface-plasmon polariton energy then transfers to the waveguide mode. The Armenian scientists stress that fabricating such an integrated surface plasmon modulator will demand precise control of both the refractive indices and length of specific material layers. Contact K. Nerkararyan at [email protected].First four-wave mixing in a microstructure fiber is observedMicrostructure fibers (MFs) are expected to exhibit the same nonlinear effects as standard optical fibers. Scientists at Northwestern University (Evanston, IL) and Lucent Technology's Bell Labs (Murray Hill, NJ) have reported the first demonstration of four-wave mixing (FWM) in MFs. The behavior is consistent with that predicted in FWM theory, according to the group. The experiment used a pump and input pulse of 150-fs-duration pulses from a modelocked Ti:sapphire laser at 75 MHz. Two wavelengths were separated and sent into two individual fibers, creating two synchronous beams about 4 nm apart, with a center wavelength tunable from 720 to 850 nm. The two beams were injected into 1.7-µm-diameter MF with a small silica core surrounded by a hexagonal array of air voids. Nondegenerate parametric gain of more than 13 dB was achieved in 6.1 m of fiber with a pump peak power of about 6 W, over a 30-nm range of pump wavelengths near the zero-dispersion wavelength (750 nm). The dispersion of MF can be adjusted during fabrication, making the fiber potentially useful in applications that include broadband parametric amplifiers and wavelength shifters. Contact Jay E. Sharping at [email protected].

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