March 1, 2000
Single 1-cm diode-laser bar emits 267 watts; Detector finds single photons in far-infrared; All-optical binary counter promises optical switching

Single 1-cm diode-laser bar emits 267 watts

Scientists from the Fraunhofer-Institut (Munich, Germany) have built a monolithic diode-laser array that produces a record continuous-wave 980-nm output power of 267 W at a current of 333 A from a single 1-cm bar. According to the group, whose results were announced at Photonics West 2000 (San Jose, CA) last January, wall-plug efficiency is 40%, and the average power density in the active area at the facet is 9.5 MW/cm2. The bar—which has a fill factor of 50% and comprises 25 broad-area lasers with 200-µm aperture and 2-mm resonator length—is mounted on a microchannel copper heat sink having a thermal resistance of less than 0.29 K/W.

At 150-W optical output power, wall-plug efficiency increases to 50%, with a corresponding slope efficiency of 0.9 W/A. Although reliability data are not yet available, such arrays will be used eventually for pumping solid-state lasers and for materials processing. Because fewer diode-laser bars are needed to achieve a desired output power level, significant cost reductions for mounting, optics, and complete laser systems are expected. Contact Jürgen Braunstein at [email protected].

Detector finds single photons in far-infrared

Detection of single photons in the far-infrared, from 10 µm to 1 mm, has not been achieved before, but the region is interesting to spectroscopists because it covers the rotational spectra of molecules and the vibrational spectra of solids, liquids, and gases. Now researchers at the University of Tokyo (Tokyo, Japan) have built a device to perform single-photon detection between 175 and 210 µm.

The researchers fabricated a 700 x 700-nm quantum dot on a gallium arsenide/aluminum gallium arsenide single-heterostructure crystal. They placed the dot in a high magnetic field that affected the state of the dot's electrons, so that when it absorbed a far-infrared photon an electron hole was created, changing the conductance of the quantum dot. Absorbing a single photon created a current of from 106 to 1012 electrons through the quantum dot. Researchers were able to detect an incident flux of 0.1 photon/s on an effective detector area of 0.1 mm2, with a time resolution of 1 ms. This sensitivity is more than 104 greater than previously reported values. Contact Susumu Komiyama at [email protected].

All-optical binary counter promises optical switching

The growth of high-speed optical networks is driving demand for replacing electronic components with all-optical devices. Researchers at BT Adastral Park (Ipswich, England) and Aston University (Birmingham, England) built an all-optical binary counter comsisting of four all-optical switching gates based on semiconductor optical amplifiers (SOAs). Such a binary counter, operating at the bit level, could perform several functions, including packet regeneration, parity checking, network-performance verification, header extraction, and payload processing.

In their proof-of-principle experiment, the researchers built the counter using two optical regenerative memories. Each memory was based on two SOA-based all-optical switching gates, one for wavelength conversion and one to act as an AND/OR gate. The system was designed so that optical pulses would cause the gate to output either a zero or a one, depending upon the time it took the pulse to travel the system. The device should be capable of gate-switching speeds of approximately 100 Gbit/s, the researchers said. They predicted that further research would allow them to build a less-complex device. Contact Keith Blow at [email protected].

SNOM measures spatial phase distribution

Researchers at the University of Bath (Bath, England) have built a scanning near-field optical microscope (SNOM) that measures not only the intensity distribution of an optical field, but its phase distribution as well. Such information is needed for a complete characterization of an optical field. The microscope contains a Mach-Zehnder interferometer in which the measurement signal is combined with a reference signal via a fiberoptic 50:50 coupler. The resulting signals from the two output ports of the coupler are sent to two detectors. During scanning, the sum of the intensities measured by the two detectors gives the near-field intensity plus a constant, while the difference in intensities gives phase information.

The SNOM probe is a chemically etched single-mode fiberoptic tip with an apex diameter of 50 nm, providing a resolution of 110 nm in the evanescent regime. Near-field phase was accurately measured near a prism surface for a wave traveling parallel to the prism face. In addition, phase and intensity profiles were taken across the face of an optical fiber supporting the LP11 mode, which has two lobes of opposite phase. Measurements of this type will aid in the development of waveguides, photonic crystals, and lasers. Contact Pepe Phillips at [email protected].

Light stores and retrieves quantum information

Scientists at the University of Michigan (Ann Arbor, MI) have stored and retrieved information through quantum phase in N-state Rydberg atoms in a single operation, storing numbers as large as 27. In the experiment, three optical pulses intersect a beam of cesium atoms. A 10-ns, 1080-nm pulse excites the atoms to the proper state. A second 150-fs, 785-nm chirped pulse with computer-controlled shape from a Ti:sapphire laser programs the quantum register of the atoms by setting the phase of each quantum state, while a third pulse similar to the second then searches for the information. The number is read out by applying a ramped electric field and measuring the field's value at the instant of ionization.

The readout pulse must arrive within several nanoseconds of the programming pulse. The minimum number of atoms required for storage of a single bit is on the order of 100. When a computer-generated random number of size 2N-1 is programmed and then read out, correct retrieval occurs 96% of the time for N = 6, and 80% for N = 8. It may be possible to store numbers as large as 2100, say the researchers. Contact Philip Bucksbaum at [email protected].

Carbon dioxide lasers plastically bend larger-diameter sapphire fibers

Single-crystal sapphire fibers have excellent physical and chemical properties that facilitate their use in infrared sensing and power-delivery applications. The crystals are extremely difficult to bend, however, which has often limited applications to smaller-diameter, less-efficient fibers. Using two 80-W carbon dioxide (CO2) lasers, researchers at Zhejiang University (Hangzhou, China) have plastically bent 325-850-µm-diameter sapphire fibers with typical bending radii as small as 2.8 mm—much smaller than their minimum elastic bending radii. During the experiments, one laser directly irradiated each fiber to its softening temperature, the other, expanded by a zinc selenide lens, was used as an auxiliary heater to control the temperature distribution along the fiber and help adjust the bending radius.

Original losses of test fibers ranged from 2.1 to 2.8 dB/m at a wavelength of 900 nm. Additional optical losses caused by the bending process were less than 0.1 dB at 900 nm.

The average strength reduction of bent fibers was roughly 10%, still high enough that fibers could resist mechanical vibrations as strong as 10 g. The damage threshold of bent fibers was higher than 150 MW/cm2 when a 1064-nm Nd:YAG laser pulse was used, which makes fibers applicable in medical applications. Contact Limin Tong at [email protected].

Aluminum gallium nitride photodiodes are blind above 275 nm

There is great interest in solar-blind ultraviolet photon detectors, especially in the military, which could use such detectors for tracing missile launches. The best materials for building solar-blind devices are compounds of aluminum gallium nitride (AlGaN). The difficulty with these materials, however, is that while the solar blindness improves with the addition of aluminum, higher aluminum content introduces other problems, such as higher resistance. Researchers at the Center for Quantum Devices in the engineering department at Northwestern University (Evanston, IL) have fabricated AlGaN photodiodes grown on sapphire by low-pressure metal-organic chemical-vapor deposition. The devices exhibit a peak responsivity for -5 v bias of 0.11 A/W at 232 nm, which corresponds to an internal quantum efficiency greater than 90%. Responsivity at zero bias was 0.05 A/W, a quantum efficiency of greater than 40%. The response of the device drops four orders of magnitude at 275 nm, just below the solar-blind cutoff of 280 nm, and stays low across the near-ultraviolet and visible spectrum. Researchers overcame the high resistance of the aluminum-heavy device by adding a semitransparent nickel/gold layer, which assisted in carrier collection. Contact Manijeh Razeghi at [email protected].

Mesostructured waveguides exhibit mirrorless lasing

Working with colleagues at Harvard University (Cambridge, MA), researchers at the University of California (Santa Barbara, CA) have fabricated mesostructured silica-waveguide arrays in which waveguiding was enabled using a low-refractive-index mesoporous-silica thin-film support. When each mesostructure was doped with a laser dye, it exhibited amplified spontaneous emission with a low pumping threshold of 10 kW/cm2. The researchers attributed this mirrorless lasing to the mesostructure's ability to prevent aggregation of the dye molecules, even at a relatively high loading within the organized high-surface-area mesochannels of the waveguides. They anticipate that many other dye molecules, rare-earth complexes, or nanocrystals can be incorporated into mesostructures to obtain different optical properties and functions via the tuning of the host architecture, the orientation and alignment of the guest species, and control of the host-guest interactions. In addition, waveguide building is done using a low-cost, one-step self-assembly process that combines acidic sol-gel block copolymer templating chemistry and micromolding, micromolding in capillaries, and microtransfer molding—soft lithographic techniques that have already proved themselves in the rapid, low-cost fabrication of liquid-core, polymeric, and inorganic waveguides. Contact Galen Stucky at [email protected].

Cooled atom vortex may swallow light

Through a phenomenon known as electromagnetically induced transparency, a properly prepared Bose-Einstein condensate (BEC; a cloud of atoms cooled to microkelvin temperatures so that they all take the same quantum state) can slow light to speeds of only meters per second.

Now, scientists at the Royal Institute of Technology (Stockholm, Sweden) and the University of St. Andrews (Fife, Scotland) predict that introducing a vortex into such a BEC will drag light along and produce an "optical black hole"—a region that will attract and swallow light of specific wavelengths.

The researchers calculate that as the vortex flow approaches the speed of light in the BEC, light at a resonant wavelength begins to spiral toward the center of the vortex. If the flow is fast enough, an event horizon appears within which the light is trapped, similar to that of an astronomical black hole. The radius of the open core of the vortex must not exceed the event horizon for the trapping to occur. The researchers say an optical black hole could be made soon and might one day be used to develop a testable optical analogue of Hawking radiation. Contact Ulf Leonhardt at [email protected].

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