Liquid-crystal optical harmonic equalizer dynamically flattens gain
Erbium-doped fiber amplifiers (EDFAs) for wavelength-division multiplexing (WDM) require a gain equalizer to keep all laser wavelengths at the same power levels. In a dynamically reconfigurable WDM network in which lasers are added and dropped, the gain profiles of EDFAs will vary with network reconfigurations. Therefore, a dynamic gain equalizer will become an important element of next-generation WDM networks. Researchers at Chorum Technologies (Richardson, TX) have developed their own version of such a device in the form of a liquid-crystal optical harmonic equalizer (OHE).
The equalizer has multiple stages, with each stage including a liquid-crystal harmonic filter to adjust the wavelength shift and a liquid-crystal attenuator to adjust the amplitude of the harmonic waveforms. A digital signal processor compares the actual with the desired gain profile; electronics then adjust the amplitude and wavelength shift of each harmonic element. The OHE flattens gain to within ±3 dB for various 40-nm ranges in the 1500-nm band. The device can also create a linearly tilted gain for use with other types of optical amplifiers. Insertion loss of the OHE in its through status is 4.5 dB. Contact Jung-Chih Chiao at [email protected].
Laser lift-off process integrates GaN-based optoelectronics with a copper substrate
Using a two-step laser lift-off (LLO) process, researchers at the Xerox Palo Alto Research Center (Palo Alto, CA) have transferred continuous-wave tomography indium gallium nitride (InGaN) multiple-quantum-well laser diodes (LDs) from a sapphire growth substrate onto a copper (Cu) substrate. The researchers believe that the enhanced device performance of the LD transferred from the sapphire substrate to the Cu substrate further establishes LLO as a tool for integrating GaN-based optoelectronics with virtually any substrate material.
The two-step LLO process involves a KrF pulsed excimer laser, followed by low-temperature (40°C) separation. Reduced threshold currents and increased differential quantum efficiencies were recorded for LDs on Cu as a result of a 50% reduction of the thermal impedance. The Cu substrate served as both an electrical backside contact and an efficient heat sink, allowing the researchers to increase the light output for LDs to a level three times greater than that for comparable LDs on sapphire. At the same time, the differential quantum efficiency was improved from 0.45 to 0.65 W/A. Contact William S. Wong at [email protected].
Far-IR detectors for astronomy respond out to 70 µm
Researchers at Georgia State University (Atlanta, GA) have tested the performance of a new type of far-infrared (IR) photodetector-the heterojunction interfacial workfunction internal photoemission (HEIWIP) detector. Unil Perera, associate professor of physics and astronomy, and colleagues report the devices are an exciting alternative to existing astronomical IR detectors up to 70 µm. The HEIWIP detectors have a strong response for wavelengths shorter than 50 µm, a cutoff wavelength λc of 70 ±2 µm, and a responsivity of 0.5 A/W at 40 µm, increasing to 2.8 at 30 µm for a 0.7 V bias. This is slightly higher than the responsivity obtained from homojunction interfacial workfunction internal photoemission (HIWIP) infrared photodetectors and a large improvement over quantum-well IR photodetectors at 30 µm. The HEIWIP devices use an aluminum gallium arsenide (AlGaAs) layer next to a doped GaAs emitter layer. By adjusting the doping concentration in the emitter layer, lc can be tailored to the desired range. The emitters are doped to a sufficiently high level so that detection is by free-carrier absorption just as in the case of HIWIPs. Contact Unil Perera at [email protected].
Achromatic diffuser scatters light uniformly and efficiently
Optical diffusers are important for many illumination systems-those used in microscopy and projection, for example. The ideal diffuser for many of these systems would scatter light uniformly within a specified angular range, direct all light into that range, and scatter light achromatically. Pre-existing diffusers made of diffractive or micro-optical elements achieve one or two of these requirements, but not all three. Now, researchers at the Centro de Investigación CientÍfica y de Educación Superior de Ensenada (Ensenada, Mexico), the University of California-Irvine, (Irvine, CA), the Russian Academy of Sciences (Troitsk, Russia), and the University of Rochester (Rochester, NY) have designed and made a nonabsorbing diffuser that satisfies all three requirements.
Guided by their theoretical explorations, the researchers fabricated the diffuser (in a one-dimensional version) in photoresist on glass, using a projected slit image to create trapezoidal or triangular grooves of a specific shape in the photoresist via exposing and developing; many laterally shifted grooves-each of random depth-were created. All light was directed uniformly and achromatically into a ±10° angular range except for a small amount of stray light resulting from imperfections in construction. The theory also allows for two-dimensional diffusers. Contact Eugenio Mendez at [email protected].
Planar waveguide yields continuous-wave broadband emitter
Researchers at the Swiss Federal Institute of Technology (Lausanne, Switzerland) and the University of Southampton (Southampton, England) have demonstrated a continuous-wave broadband emitter based on a Ti:sapphire planar waveguide. The 9-mm-long and 10-µm-thick waveguide was grown by pulsed laser deposition and contained about 0.1% by weight of titanium trioxide. It was fixed on a mount with heat-conductive paint and no active cooling and was pumped with an argon-ion laser operating continuous-wave on all lines. A 16X microscope objective focused the pumped light into the waveguide; a broadband luminescent output signal was obtained from the single-mode laser propagation. The output signal was centered on a 755-nm peak with a full-width half-maximum (FWHM) bandwidth on the order of 122 nm. A maximum output power of 235 µW was obtained for an absorbed pumped power of about 700 mW; the slope efficiency was 3.3 x 10-4. Addition of a single in-coupling mirror improved the luminescent efficiency by a factor of 1.4 through back reflection of counterpropagating luminescence. It also broadened the FWHM bandwidth to 132 nm. The method is expected to find use in improving spatial resolution and dynamic range of broadband fiber interferometers for use in applications such as optical coherence tomography. Contact Markus Pollnau at [email protected].
Elastomeric phase-shift mask enables near-field photolithography
Photolithography in the optical near field is a promising way to fabricate features beyond the diffraction limit-usually around 100 nm. University of Washington (Seattle, WA) researchers recently demonstrated that a method based on elastomeric phase-shift masks is convenient and versatile for generating sub-100-nm features in the form of simple patterns.
Using a rigorous electromagnetic theory in the near-field region, the scattering effect was found to be so strong that the scalar theory was inadequate for fully describing near-field phenomena. A new bright line was discovered beside the dark line predicted by scalar theory, and widths of both lines were insensitive to the refractive index of the photoresist. The researchers believe the simulation results are in agreement with experimental studies that indicate the bright and dark lines could be used to generate trenches and lines in a positive-tone photoresist by controlling the exposure time. These simulations also suggest that straight parallel lines as small as 50 nm are possible by adjusting the parameters of the mask. Contact Younan Xia at [email protected].
Persistent wrinkles may ground inflatable mirrors
A few small but persistent wrinkles may permanently ground the strategy of lofting very large (several meter) telescopes based on inflatable mirrors into space, according to a researcher at the University of Arkansas (Fayetteville, AR). The membranes currently being used for space-based inflatable structures, such as reflectors or communication antennae, are not likely to be usable for very large optical-quality mirrors as a result of tiny wrinkles that degrade optical performance and do not disappear even 24 hours after the membranes are inflated, according to Bob Reynolds, an assistant professor of mechanical engineering.
Reynolds tested 13-µm and 76-µm-thick polyimide-film membranes commonly used in inflatable space structures. After introducing small wrinkles of the type that might be produced by folding the membrane fabric for packaging into a launch vehicle, Reynolds subjected the membranes to tensile loading like that produced when the structure is inflated. The wrinkle size decreased rapidly during the first two hours, but the membranes did not flatten. After 24 hours, between 50% and 80% of the wrinkles remained. Contact Bob Reynolds at [email protected].
Beryllium replaces zinc in lattice-matched blue-green quantum-well emitter
The blue-green spectral region longer than 450 nm is unreachable by gallium nitride-based laser diodes. Covering this range in a semiconductor laser has been traditionally attempted using II-VI wide-gap heterostructure diodes made from zinc cadmium selenide (ZnCdSe), but such lasers degrade quickly as a result of defect-causing high stress accumulated in the quantum-well (QW) active region. Researchers at the Technical Institute of RAS (St. Petersburg, Russia) and the Physikalisches Institut der Universität Würzburg (Würzburg, Germany) have come up with a new material replacing the Zn with beryllium (Be), which has the potential to form lattice-matched QWs with a blue-green band gap.
Both 100-nm-thick bulk BeCdSe layers lattice-matched to a gallium arsenide (GaAs) substrate and BeCdSe/ZnSe QW structures were grown via molecular-beam epitaxy. The structures have demonstrated good structural quality and bright photoluminescence at room temperature. Optically pumped stimulated emission at 460 nm occurred in a multiple-QW structure at 80 K and a threshold pumping density of 40 kW/cm2. Not only does the Be allow lattice matching to GaAs, but it also induces a substantial lattice hardening, said the researchers. Contact Andreas Waag at [email protected].
Particle beam follows a nonlinear version of Snell's law
Researchers at the University of California-Los Angeles, the University of Southern California (UCLA and USC; Los Angeles, CA), and Stanford University (Palo Alto, CA) have observed a 28.5 x 109-eV electron beam deflecting at a plasma-gas interface in the same manner as a beam of light deflects when changing media in accordance with Snell's law. The observation was made as the research team was attempting to increase the acceleration of a stream of electrons by sending them through a plasma created by photoionization of lithium vapor with an argon fluoride laser; they had designed a 1.4-m-long and 4.6-mm-diameter tube to hold the plasma while a beam of electrons passed through it. "The beam kicked around and it tripped off all the safety systems," said Patric Muggli, a research associate professor of electrical engineering at USC. Upon analyzing the data, the researchers found that the beam had deflected by more than a milliradian away from the normal upon emerging from the plasma in the same manner as a beam of light shifting from a higher- to a lower-refractive-index material. Contact Thomas Katsouleas at [email protected].