Newsbreaks

July 1, 2001
Long-wavelength VCSEL operates at high temperatures; Quasi-phase-matched SHG generates sub-6-fs pulses; Quantum-dot photodetectors exhibit a wide detection window...

Long-wavelength VCSEL operates at high temperatures
Although 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 degrades at higher temperatures. Now, researchers at the University of California (UCSB; Santa Barbara, CA) have developed a wafer-bonded 1.55-µm-emitting VCSEL that provides continuous-wave electrically pumped operation at temperatures up to 105°C.

So far, the best high-temperature results with such devices 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, emitted a maximum of 0.65 mW at 20°C and 0.22 mW at 80°C. 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].

Quasi-phase-matched SHG generates sub-6-fs pulses
Researchers from the Swiss Federal Institute of Technology (Zürich, Switzerland), Stanford University (Stanford, CA) and Kaiserslautern University (Kaiserslautern, Germany) reported frequency doubling of sub-6-fs laser pulses using quasi-phase-matched second-harmonic generation (QPM-SHG). The heart of the experimental pulse compression device consisted of a 310-µm-long grating written in a 300-µm-thick lithium tantalate substrate using electric-field poling. The crystal was placed in a crosscorrelator behind a pair of glass wedges, which yielded an adjustable positive first harmonic prechirp. The input signal consisted of 8.6-fs, 810-nm-wavelength pulses from a Kerr-lens modelocked Ti:sapphire oscillator. A 10-µm-thick potassium dihydrogen phosphate crystal crosscorrelated the second harmonic output pulse with a nearly unchirped first harmonic pulse. Output pulse durations of 5.2 and 5.4 fs were achieved using two different algorithms. Contact Lukas Gallmann at [email protected].

Quantum-dot photodetectors exhibit a wide detection window
The quantum-dot infrared photodetector (QDIP) has attracted much attention recently because of its intrinsic sensitivity and long relaxation times. Now, researchers at the Department of Electrical Engineering, National Taiwan University (Taipei, Taiwan) have reported the investigation of a new, high-performance QDIP containing ten layers of indium arsenide (InAs) quantum dots and a 30-nm gallium arsenide (GaAs) barrier. The ten layers, each three atoms thick, are sandwiched between a 0.5-µm top layer and a 1-µm bottom GaAs contact layer, silicon-doped to 1 x 1018 cm-3. To achieve superior responsivity and suppress dark current, Si-Chen Lee and colleagues added single aluminum gallium arsenide blocking layers on either side of the quantum-dot structure. The blocking layers effectively depress dark current and thus increase operating voltage to enhance the photoelectron avalanche process. Peak responsivity of 214 mA/W and specific detectivity of 1.17 x 1010 cmHz1/2/W occurred at 6 µm. The devices exhibited two different infrared absorption regions from 2 to 6 µm and 6 to 10 µm, which indicates a wide detection window for the device. Contact Si-Chen Lee at [email protected].

Nitrogen-doped waveguides transmit ultraviolet light
Ultraviolet (UV) optochemical sensors for biochemical use could be fabricated in the form of integrated optical systems, but the scarcity of available UV waveguide materials hinders this effort. The germanium-doped silica glass waveguides common in infrared integrated optics do not transmit well at the 190- to 400-nm wavelengths required for UV absorption spectroscopy. Now, researchers at the Technical University of Denmark (Lyngby, Denmark) have fabricated UV-transmitting waveguides using nitrogen as the dopant. The absorption of a 24-µm-wide nitrogen-doped waveguide is only 1.0 dB/cm in the 220- to 550-nm wavelength range.

The optical bandgap of nitrogen-doped silica glass depends on the relative nitrogen and oxygen content: the silicon oxide limit is 160 nm, while the silicon nitride limit is 250 nm. Only a small amount of nitrogen is required for doping, which shifts the optical bandgap only slightly away from the 160-nm bound. A sample waveguide contained a doped core with refractive index of 1.483 and an undoped cladding with an index of 1.460. An integrated test system containing nitrogen-doped waveguides and fluid microchannels measured the concentration of propranolol, detecting the substance to a concentration of 13 µM. Contact Klaus Bo Mogensen at [email protected].

One-dimensional cuprate shows promise for all-optical switching
A research team from the University of Tokyo (Tokyo, Japan), Japan Science and Technology Corp. (Kanagawa, Japan), and the University of Arizona (Tucson, AZ) recently demonstrated that a one-dimensional (1-D) cuprate (Sr2CuO3) shows promise as a more effective optoelectronic material in all-optical switching applications than either inorganic or organic semiconductors.

The team investigated the feasibility of ultrafast all-optical switching using 1-D Sr2CuO3, which is operable within the optical fiber communication window at room temperature, with subpicosecond pump-probe spectroscopy and Z-scan measurements. They found that the strength of the interband two-photon absorption in Sr2CuO3 is much larger than that of conventional semiconductors and is comparable to the largest reported values in p-conjugated polymers. The intensity-dependent refractive index, however, is considerably larger than that of polymeric materials. Recovery of optical transparency after the photoinjection of carriers lies within the picosecond time scale. Large nonlinearity, ultrafast response, high damage threshold, and a melting point of approximately 950°C make the 1-D cuprate a potential material for multi-terabit/s rate all-optical switches, according to the researchers. Contact Makoto Kuwata-Gonokami at [email protected].

Network demonstrates 1500-km unregenerated transmission at 40 Gbits/s
In what is believed to be a record in optical transport speed and distance, PhotonEx Corp. (Atlanta, GA) has demonstrated a 1500-km (932-mi) unregenerated transmission span with 16 channels at 40 Gbit/s, using commercially available components and nonzero dispersion-shifted fiber on terrestrial fiber spans of 100 km (62 mi). Raman amplifiers were used only on every fourth span, and no signal conditioners were needed. A 100-GHz channel separation provided a spectral efficiency of 0.4 bit/s/Hz. Problems that needed to be solved to reach high spectral efficiency included optical signal-to-noise degradation and tight tolerances on chromatic-dispersion compensation. Another challenge was that transmission impairments such as polarization-mode dispersion scale nonlinearly with transmission distance. PhotonEx believes that its high-speed channel rates combined with high spectral efficiency will help to make possible flexible bandwidth services provisioned within and across wavelengths, while at the same time minimizing the amount of hardware, system footprint, and operational requirements involved in transmitting at 40-Gbit/s and 100-GHz spacing. Contact Tara Hamre at [email protected].

Gratings reduce layer count in transmission filters
Researchers at Genuity (Irving, TX) and the University of Texas at Arlington (Arlington, TX) have modeled narrowline bandpass transmission filters containing far fewer layers than ordinary filters by replacing a homogeneous thin-film layer in a dielectric filter stack with a grating layer. (Previous grating-containing filters, both modeled and fabricated, have worked in reflection.) The zero-order waveguide grating layers work via guided-mode resonance, an effect that depends on wavelength. Such a reduction in the number of layers reduces both bulk and surface interface scattering losses. The filters are designed with the aid of a program based on rigorous coupled-wave analysis and a genetic algorithm.

In one example, a two-layer filter contains two gratings of different periods but containing the same refractive-index difference within the gratings: a high refractive index of 2.35 and a low of 1.65. The filter has a Lorentzian linewidth of 0.2-nm full-width half maximum centered at 550 nm; however, the limited selection of available thin-film materials results in close sidebands. A more practical filter designed for the infrared contains a single grating layer and has a 12.7-nm linewidth at a center wavelength of 10.6 µm—a relative linewidth of 0.12%. Refractive indices of the grating are 4.0 and 2.65, both of which are feasible at 10.6 mm. The infrared filter should have a sideband transmittance below 1% over a 2.21-µm spectral range. Contact Robert Magnusson at [email protected].

Optically pumped lead-salt VCSEL yields mid-IR signal
At the Conference on the Lasers and Electro-Optics (Baltimore, MD) in May, researchers from the University of Oklahoma (Norman, OK) and the Naval Research Laboratory (Washington, DC) reported pulsed emission in the mid-infrared region from an optically pumped, lead-salt vertical cavity surface emitting laser (VCSEL). The device was fabricated on a barium fluoride substrate with an active region consisting of lead selenium/lead strontium selenium. It was pumped by a 2.098-µm beam through the bottom mirror and emitted a 4.445-µm signal with a 21-nm bandwidth at full-width half-maximum through the top mirror at a device resonance temperature of 260 K. A relatively low lasing threshold of about 10 kW/cm2 was also achieved in the temperature range of 260 to 280 K. As the temperature decreased to 200 K, the threshold lasing intensity increased linearly to almost 50 kW/cm2 but the emission wavelength varied by only several nanometers. Contact Christopher Felix at [email protected].

Researchers scrub out rubbing
A group of 27 IBM researchers at sites in Yorktown Heights, NY; San Jose, CA; Kanagawa-ken, Japan; and Shiga-ken, Japan have created a new noncontact alignment technique for large-scale manufacturing of liquid-crystal (LC) displays, which is intended to replace rubbing substrate surfaces as the most widely used technique for alignment of LC molecules. The new technique uses low-energy ion beams of 50 to 500 eV that impinge on amorphous inorganic films at a glancing angle. A direct-current Kaufman-type ion source with a tungsten filament was used to produce the ion beam. The team found that a variety of transparent insulating films can be used to replace the polyimide film used in existing rubbing processes. The team chose hydrogenated diamond-like carbon and silicon nitride films 3 to 4 nm thick, deposited on glass substrates by plasma-enhanced chemical vapor deposition. Modeling shows the alignment is a result of the ion beam selectively destroying rings of atoms oriented perpendicular to the beam. The technique was used to produce both laptop and desktop computer displays in a pilot manufacturing run, resulting in reliable, higher-quality displays that can be manufactured at a lower cost. Contact Praveen Chaudhari at [email protected].

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