Newsbreaks

Jan. 1, 2001
Silicon may become a laser; Green LEDs look good for plastic-fiber LAN transmission; Medical-laser market review highlights exciting developments; Gallium-doped germanium yields two-dimensional photoconductor ...

Silicon may become a laser
Although silicon is the mainstay of the semiconductor microelectronics industry, it is a poor emitter of light. The material's other properties are ideal for integrated optics: silicon dioxide (SiO2, an insulator easily produced on silicon) makes an excellent waveguide, and electronics can be monolithically integrated with waveguide optics. Now, researchers at the Università di Trento (Povo, Italy) and the Università di Catania (Catania, Italy) have made silicon act a lot like a laser. By dispersing silicon quantum dots in a SiO2 matrix coated on silicon, the researchers created a medium that amplified light.

Negative ion implantation created a layer of 3-nm-diameter quantum dots at a concentration of 2 x 1019 cm-3. Near-infrared light was coaxed from the quantum dots by pumping them with 390-nm light. More important, the emission showed characteristics of amplification: when a weak 800-nm probe beam was sent through the medium, the result was a one-pass gain of 10,000 ±3000 cm-1. Adding mirrors will likely lead to the creation of the first silicon-based laser. The ultimate result could be mass-produced high-capability devices combining electronics and optoelectronics. Contact Lorenzo Pavesi at [email protected].

Green LEDs look good for plastic-fiber LAN transmission
Use of plastic optical fiber (POF) shows promise for reducing cost and increasing performance of local-area networks, but absorption loss of the POF has been a major obstacle to its application. A team of researchers at NTT (Atsugi, Japan) and Tohoku University (Sendai, Japan) has reported the first POF-transmission experiment using green light-emitting diodes (LEDs) at a wavelength of 520 nm. The experiment used a polymethylmethacrylate (PMMA) core POF, which had previously only been tested with laser diodes and LEDs at 650, 670, or 785 nm. Transmission distance through POF was less than 100 m due to absorption loss at these wavelengths. The 500-nm region in PMMA core POF inherently displays less transmission loss, and results confirm the possibility of a longer-length signal transmission of 200 m or more. The experiment used 100 m of fiber with diameter of 980 µm and numerical aperture of 0.51. With the LED directly modulated at 30 Mbit/s, the transmission loss over the fiber was 10 dB, considerably lower than that for red or blue LEDs. Contact Takashi Matsuoka at [email protected].

Medical-laser market review highlights exciting developments
The 15th annual Laser and Optoelectronics Marketplace seminar on Monday, Jan. 22, at the Fairmont Hotel in San Jose, CA will feature speakers from several industries, including optical communications and medicine. The seminar continues a longstanding tradition of bringing together business leaders, investors, and technology analysts for a close look at the rapidly moving laser and optoelectronics markets (see p. 88).

On the medical front, Scott Baily, VP of research at Bluestone Capital (New York, NY), an investment banking and brokerage firm, will give a review of the medical laser market. Notes Baily, "There are some very exciting developments in laser technology which show promise in medical applications." One of the promising applications to be presented is photodynamic therapy, which was approved last year for use in treating macular degeneration.

The day-long agenda also includes the annual review and forecast of the laser marketplace, an overview of the global semiconductor-laser market, and a focus on the growing optical communications market. For more information about the seminar, contact Sharon MacLeod at (603) 891-9224 or visit www.markeplaceseminar.com.

Gallium-doped germanium yields two-dimensional photoconductor
A gallium-doped germanium (Ge:Ga) photoconductor has been designed at the Communications Research Laboratories (Tokyo, Japan) with a longitudinal dimension for two-dimensional far-IR detection in the 3-THz range and for applications such as astronomy, spectroscopy of molecules and solids, and plasma diagnosis. The researchers fabricated a transparent electrode by implanting and annealing boron ions on either side of a 0.5-mm-thick Ge:Ga wafer with a Ga concentration of 1.56 x 1014 ions/cm2. Patterned gold and chromium layers were then evaporated onto the top and bottom of the wafer to provide metal electrodes, and insulating-tape shields on the sides of the wafer with black velvet paint prevented stray light absorption.

A high responsivity of 16.2 A/W and a noise-equivalent power (NEP) of 2.6 x 10-17 W/Hz0.5 were observed in a longitudinal configuration and a bias field of 1.2 V/cm when the detector was cooled to 2.1 K. Responsivity in the longitudinal configuration was more than double that obtained in the transverse configuration. Based on the responsivity values, the researchers estimated that between 10% and 20% of the boron ions on each electrode were activated and contributed to the production of photocurrent. Contact M. Fujiwara at [email protected].

Fiber-coupled laser-diode system reaches brightness high
Researchers at Apollo Instruments Inc. (Irvine, CA) have built a fiber-coupled laser-diode system having a 15-W power output when used with fiber of 0.1-mm diameter and 0.2 numerical aperture (NA). Based on a beam-shaping technique built around a group of mirrors, the system showed a record-high brightness output of 1.2 MW/cm-2sr-1. The coupling efficiency was found to be 49% for a single optical fiber of 0.1-mm diameter and 0.2 NA, and 70% for fiber of 0.2-mm diameter and 0.2 NA. The US Air Force Research Laboratory sponsored the development effort. The researchers expect that when combined with multiple laser-diode bars, the structure could provide a fiber-coupled laser-diode system having 100-W power output when used with an optical fiber of 0.2-mm diameter and 0.2 NA. The high-brightness devices have a number of applications including diode-pumped solid-state lasers, materials processing, medical therapeutics, printing, and imaging. Contact Peter Wang at [email protected].

Hyperfine WDM attains 1-GHz channel spacing
A hyperfine wavelength-division-multiplexing device developed at Essex (Columbia, MD) has shown a 1-GHz channel spacing and a total capacity of 16 GHz. Researchers at the company aim to demonstrate a system with 50 channels early this year; such a system could take one of a set of channels spaced at 50 GHz and split it up into 50 1-Gbit/sec channels. The technology can attain 200-MHz channel spacings and is ultimately capable of handling 4000 channels, say the researchers. The equipment contains all-passive solid-state optics that include electro-optic modulators. Hyperfine technology encompasses a family of devices, including multiplexers, demultiplexers, add/drop multiplexers, and tunable receivers. Although the researchers claim higher fiber data-carrying capacity, the largest benefit of the technology may result from the high channel count, resulting in efficient data distribution for metro and local networks.

In addition, the lower-bandwidth channels could mean less-expensive signal-demodulation terminals and intermediate switching and routing layers using CMOS (complementary metal-oxide semiconductor) electronics. Contact Terry Turpin at [email protected].

Nonlinear crystals promise higher-power UV lasers
Borates are not the only nonlinear crystals capable of converting solid-state laser light to ultraviolet (UV) for lithography and micromachining applications. At the Materials Research Society's Fall 2000 meeting (Boston, MA), Douglas Keszler from Oregon State University (Corvallis) described his group's work on crystals without the BO3 groups that contribute to the nonlinear properties of BBO and CLBO. Their search moved away from boron to materials to the left of it on the periodic table, including beryllium and strontium.

Ideal nonlinear materials are transparent well into the UV, have good nonlinear absorption, a high damage threshold, good thermal conductivity, and can be grown without exotic solvents or growth conditions. Keszler described SrBe3O4, a material that is transparent into the UV, generates second harmonic light with about the same 0.35 pm/V efficiency of KDP (potassium dihydrogen phosphate), and has less absorption in the infrared than lithium niobate. The material is not new, but the possibility of using it in this application is. Within the next year and a half, Keszler predicts, researchers will demonstrate a laser that operates at less than 200 nm, producing femtosecond pulses at a megahertz repetition rate. Within five years or so, he expects that solid-state lasers may be able to drive x-ray systems that use xenon plasmas, which produce 13-nm output. Contact Douglas Keszler at [email protected].

Coupled-resonator vertical-cavity laser diode provides bistable output
Researchers at Sandia National Labs (Albuquerque, NM) have observed bistable behavior in the light-current curve of a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity. In the devices developed so far, which have yielded output powers as high as 7 mW, the scientists determined the mechanism of the observed bistability to be the nonlinear optical response of the top cavity combined with changes in the intracavity intensity in the bottom cavity due to thermal rollover. Devices have produced large bistability regions over current ranges as wide as 18 mA with on/off contrast ratios greater than 20 dB. In addition, high-contrast switching from full-power lasing to off-state luminescence has been demonstrated using very low electrical power. In other words, both the position and width of the bistability region can be varied by changing the bias current to the top cavity—even a difference as small as 250 µW is enough to switch between on and off stages. Potential applications for such vertical-cavity lasers include optical data encoding, which can benefit from high-contrast switching at very low electrical-switching powers. Contact A.J. Fischer at [email protected].

Metal-island films provide wavelength-selective 3-D optical storage
Researchers at Tohoku University (Sendai, Japan) have constructed experimental, three-dimensional (3-D), write-once, multiwavelength optical disks using 3:2 aluminum-silver (Al-Ag) and 3:1 aluminum-gold (Al-Au) compound metal-island films, in which the Al-Ag film responded to a 490-nm signal and the Al-Au responded to 810 nm. In the experimental setup, both films were irradiated with 490-nm and 810-nm laser light modulated with a mechanical chopper. Wavelength selectivity was confirmed by relative motion of the two films on a 90 mm/min pulse stage. Resonance characteristics from thermal deformation and oxidation during fabrication were practically removed from the films by heating, in which the spectral reflectance of the 14-nm-thick Al-Ag film dropped from about 35% to about 15% at 490 nm after heating to 400°C, and spectral reflectance of the 8-nm-thick Al-Au film dropped from about 35% to about 15% at 810 nm after heating at 200°C. The experiment supports the feasibility of using the resonance-type absorption characteristics in the visible and infrared of high-density-metal nanoclusters to increase the potential storage density of optical disks, while also potentially boosting durability of the storage media above what might be expected using photochromic polymers, according to the researchers. Contact Kazutaka Baba at [email protected].

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