Newsbreaks

Dec. 1, 2000
Positive lens pair produces uniform Gaussian beam
The Gaussian intensity distribution of a TEM00 laser beam is undesirable for applications that require uniform illumination.

Positive lens pair produces uniform Gaussian beam
The Gaussian intensity distribution of a TEM00 laser beam is undesirable for applications that require uniform illumination. One possible solution, intensity redistribution by a pair of aspheric lenses, has been known for more than 30 years and consists of one lens with a negative aspheric shape and another with a positive aspheric shape. The first lens redirects light from an incoming Gaussian collimated beam so that it is uniform as it strikes the second lens, which then recollimates the beam. One problem with this setup is the difficulty in fabrication of the negative aspheric lens; another is the unwanted diffraction occurring as a result of apodization.

Researchers at the IBM Almaden Research Center (San Jose, CA) have solved both these problems in a Gaussian-to-flat-top design containing two positive aspheres with an intermediate focus. The monotonic form of the lenses eases fabrication. The design allows for acceptance of 99.7% of the beam, virtually eliminating undesired diffraction. Using magnetorheological finishing, lens pairs were fabricated for both broadband visible and narrowband ultraviolet use. The visible pair produced a flat-top beam with less than 5% root-mean-square nonuniformity from a Gaussian beam emitted by an argon-ion laser. Contact John Hoffnagle at [email protected].

All-optical switch is based on photochromic-doped waveguides
Researchers at Kwangju Institute of Science and Technology (Kwangju, Korea) have developed an all-optical switch comprised of a photochromic-dye-doped Y-branch waveguide and a thick light-blocking metal film. After fabricating the waveguide using spin-coating and oxide-reactive-ion etching, the scientists deposited a 200-nm-thick gold layer on its Y-branch and patterned the gold using photolithography and a wet chemical etching process. A section of coating on one arm of the Y-branch was then removed to allow light exposure. The optical switching occurs because the photochromic dye undergoes a structural change when exposed to both ultraviolet (356 nm) and visible (514 nm) light, which in turn produces a reversible change in the refractive index of the dye-doped polymer film. One benefit of the all-optical switch is its low crosstalk of about -14 dB at 1.55 µm. The response time also decreases exponentially as the intensity of visible light is increased, which the scientists say indicates that a fast switching time is possible when the device is exposed to high-intensity light. Contact Jang-Joo Kim at [email protected].

Annual Marketplace Seminar to focus on optical communications
Laser Focus World and Strategies Unlimited (Mountain View, CA) will present the 15th annual Laser and Optoelectronics Marketplace seminar on Monday, Jan. 22, 2001, in San Jose, CA. The seminar provides business leaders, investors, and technology analysts with a close look at the changing laser and optoelectronics markets—this year with a focus on communications. Jacek Chrostowski, technical deputy of optics in the Office of Chief Technology Officer of Cisco Systems (Ottawa, Ont., Canada), will discuss the growing role of optoelectronics in communications. Tom Willis, director of optical communications investments at Intel Capital (Hillsboro, OR), will speak on private equity financing in optical communications. Market trends and product evolution of laser diodes in communications will be addressed by Tom Hausken, senior analyst of optical communications at Strategies Unlimited. Richard Mack, vice president and general manager at market-research firm KMI (Newport, RI) will look at worldwide investment trends in fiberoptic communications infrastructure.

The seminar will conclude with a dinner event featuring certified speaking professional and author Phillip Van Hooser as keynote speaker. He will speak about leading through change, addressing the specific roles proactive leaders must embrace to deal effectively with the realities of change in the optoelectronics marketplace.—For more information visit www.marketplaceseminar.com.

Projection microstereolithography creates 3-D microstructures
High-resolution projection microstereolithography (PµSL) has been successfully developed at the University of California Los Angeles. Incorporating a digital micromirror device made by Texas Instruments (Dallas, TX) as a dynamic spatial light modulator, the technology provides parallel fabrication of highly complex three-dimensional (3-D) microstructures for microelectromechanical systems (MEMS). In the technique, uniform ultraviolet (UV) light is modulated by switching individual micromirrors; then, through a 1:5 reduction lens, the reflected pattern is projected onto a UV-curable monomer resin surface. The illuminated area is solidified all at once during exposure, while the dark regions remain liquid. Complex 3-D microstructures are fabricated by stacking polymerized layers with confined thickness of a few microns. The fabrication of complex 3-D structures such as matrices and microspring arrays with feature sizes down to 5 µm has been achieved. Using a digital micromirror device as the dynamic mask eliminates the cost of fabricating a mask for each layer. In addition, the PµSL process reduces fabrication time. Contact Cheng Sun at [email protected].

Zinc selenide-based detectors show quality in the blue
Zinc selenide (ZnSe) is a promising material for photodetectors operating in the blue region. Now, researchers at the Universidad PolitÈcnica de Madrid (Madrid, Spain) and the Centre de Recherche sur l'Hétéro-Epitaxie et ses Applications (Valbonne, France), who have been developing ZnSe detectors, have extended their study to zinc magnesium beryllium selenide (ZnMgBeSe). They grew both ZnSe and ZnMgBeSe-based photodetectors on semi-insulating gallium arsenide by molecular-beam epitaxy. Both materials show sharp cutoffs in the visible wavelengths, with ZnSe dropping off at 475 nm and ZnMgBeSe at 450 nm. Time responses at a -5-V bias for both materials were 750 and 330 ns, respectively. The materials show a responsivity of 0.01 A/W and detectivities of up to 1.4 x 1012 cm Hz0.5 W-1 at a -3.5-V bias.

The researchers examined the materials' noise characteristics, finding that 1/f noise was dominant at low frequencies. Noise-equivalent power was measured to be less than 1 pW/Hz0.5 at a -3.5-V bias. Photocurrent decays showed a combination of a fast exponential decay with a time constant of 86 ns and a slower tail with a time constant of 2.5 ms, with the slower component becoming dominant for high load and reverse bias. Contact Eva María Monroy at [email protected].

Hot-electron optical modulator is compatible with silicon photodetectors
Scientists at Heriot-Watt University (Edinburgh, Scotland) and the University of York (York, England) have proposed a surface-normal optical modulator based on aluminum gallium arsenide material for use in optical interconnects that improves the efficiency of absorption modulation. The alternative modulator is based on the variation of interband optical absorption arising from current-induced carrier-electron heating in a bulk semiconductor. Redistribution of carriers between conduction band valleys contributes to a considerable absorption edge shift at relatively small electric fields (much less than 10 kV/cm). By using a weakly doped material on the semiconductor, the equilibrium absorption edge is shifted to shorter wavelengths due to band-filling. Heating of the carriers reduces the Fermi energy, spreading them to a wider range of states, thereby moving the absorption edge back toward the position at which it occurs in undoped material. The device is unipolar, simply requiring a carrier heating current, which can be supplied at low voltages. This provides a practical very-high-speed modulator with a switching time on the order of several picoseconds, predicted to be efficient in the red spectral region. Contact Boris Ryvkin at [email protected].

Microlens is deposited directly onto laser-diode facet
At Nanyang Technological University (Singapore) researchers have combined a beam-shaping microlens with a laser diode by depositing the lens directly on the output facet of the diode. Made of fused silica, the lens is built up on the facet of the laser diode by using focused-ion-beam direct deposition. Gallium ions are focused to a spot size of 7 nm on the facet surface; with the ion energy set at 50 keV, pressures of a siloxane source gas and molecular oxygen are adjusted to achieve best deposition. Because the form accuracy of a spherical or cylindrical shape was easiest to achieve for the first try, the researchers settled on a planoconvex microcylindrical lens. Its radius and numerical aperture were 15 µm and 0.08, respectively. The lens was 50 x 5 µm in area and 0.85 µm in thickness.

A lens shaped to couple the laser output directly into a single-mode fiber showed a coupling efficiency of 80.1%, as opposed to a coupling efficiency of 42% without the lens. The far-field angle of the laser emission was reduced from 31° to 2.1°. The researchers believe that aspheric and diffractive microlenses can be made using this technique. Contact Yongqi Fu at [email protected].

Blue microchip laser features two intracavity frequency doublers
Pumped by a single-stripe, 1-W 808-nm diode, typical designs of compact, blue-emitting diode-pumped solid-state laser microchips produce up to 20 mW of frequency-doubled power at 473 nm, but they also have problems maintaining stable continuous-wave (CW) emission. Now researchers at the Optical Institute of the Technical University of Berlin (Berlin, Germany), working with colleagues at the Chinese Academy of Sciences and the Research Institute of Synthetic Crystal (both in Beijing, China), have developed a microchip cavity that combines a half-monolithic Nd:YAG-KNbO3 (potassium niobate) chip with another KNbO3 crystal. This device, which outputs 30 mW at 473 nm when pumped by a single-stripe, 1-W 808-nm diode, can operate in either single or multiple longitudinal mode (by rotating the second frequency-doubling crystal 90° from the first). In addition, the crossed-doubler configuration can provide stabilization independent of the type of phase-matching or the cavity setup. In both cases, the CW output noise is less than 2%. Contact Volker Gaebler at [email protected].

Ultraviolet laser pulse writes holographic gratings in polymer films
Researchers at Shizuoka University (Hamamatsu, Japan) have used two-beam ultraviolet-laser interference to write periodically poled structures (X(2) gratings) in polymeric nonlinear optical (NLO) materials. The 355-nm light source was a single 5-ns pulse from the third-harmonic wave of a Nd:YAG laser. An optical system based on a straightforward beamsplitting configuration was used to fabricate gratings with periods ranging from lambda/2 to tens of microns, but a modified Michelson interferometer setup provided a slight deviation of the spatial overlap between the two beams for larger grating periods. Due to the insignificantly short nanosecond time scale of the laser pulses compared to the microsecond scale of mechanical vibrations, however, the fabrication apparatus did not require vibration isolation.

Lower-energy irradiation on the order of 280 mJ/cm2 was sufficient to erase second-order nonlinearity from polymer film and higher energy densities, while higher energies on the order of 400 mJ/cm2 ablated the film. Grating types fabricated included volume, ridge, and surface-relief with periods ranging from nanometers to millimeters. The maskless fabrication technique can be applied to most NLO polymer films including several polymers for waveguide devices, according to the researchers. Contact Okihiro Sugihara at [email protected].

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