Nov. 1, 1999
Phase-locked laser diodes pool their power; Germanium-on-silicon photodetector shows high response in the near-infrared ; Photonic crystal shows angle-independent bandgap; and more . . .

Phase-locked laser diodes pool their power

Researchers at the Universität Stuttgart and the Deutches Zentrum für Luft- und Raumfart (both Stuttgart, Germany) have phase-locked together 19 low-power single-transverse-mode laser diodes in a power-scalable manner. The lasers are injection-locked with a master laser diode, and their outputs are coupled into separate polarization-maintaining optical fibers, each of which contains a piezoceramic phase modulator; all fiber outputs are then collimated and the beams grouped in a hexagonal array. A fraction of the resulting combined beam is split off and interfered with light from the master laser to provide phase feedback information. A full phase-control loop is necessary for each laser.

Perfect coherent combination would result in a focused spot of 19 times the central intensity of incoherent superposition; the actual central intensity was up by a factor of 13.2, corresponding to an average degree of coherence of 0.7. The researchers are aiming to raise the fraction of power in the central peak. Preliminary experiments with injection-locking of vertical-cavity surface-emitting-laser arrays show promise for miniaturization. Phase-locking of groups of higher-power laser diodes should be possible, setting the stage for applications in materials processing. Contact Adolf Giesen at [email protected].

Germanium-on-silicon photodetector shows high response in the near-infrared

Germanium (Ge) photodetectors are of interest in optical communications because germanium can be used to integrate emitters, modulators, and detectors with silicon (Si) based devices. Researchers at the Massachusetts Institute of Technology (Cambridge, MA) and Terza University of Rome (Rome, Italy) have fabricated heterojunction Ge-on-Si photodetectors that they say exhibit the highest known near-infrared responsivity at normal incidence.

The researchers grew a 1-µm layer of Ge on p-type Si wafers, which they annealed at 900°C. At 1.32 and 1.55 µm, the researchers measured responsivities of 0.55 and 0.25 A/W, respectively, which compare favorably with guided-wave devices based on SiGe multiple quantum wells. The device had a relatively large reverse-bias-current density of 30 mA/cm2 at 1 V, which the researchers attributed to the low barrier at the heterojunction between the p-type Si and intrinsic Ge. Using a p-i-n structure should reduce the density, they said. Contact Lionel Kimerling at [email protected].

Photonic crystal shows angle-independent bandgap

The lattice of a quasiperiodic crystal—or quasicrystal—has a form of long-range order even though it is not fully periodic. Such a crystal can have symmetries unavailable to periodic crystals. Researchers at the Chinese Academy of Sciences (Beijing, China) have used the quasicrystal's unique properties to create a two-dimensional microwave photonic crystal with eightfold symmetry that has a bandgap independent of the angle of light incidence. This is in contrast to periodic photonic crystals, in which a complete bandgap results from the intersection of differing spectral gaps in differing directions.

For easy experimentation, the researchers constructed the quasicrystal from 6.12-mm-diameter alumina pegs embedded in styrofoam at a spacing of 9 mm and subjected it to radiation in the range of 10 GHz (30-mm wavelength). The resulting bandgap of 8.9-10.5 GHz had a transmittance lower than that of the passband by a factor of 1000. By removing some pegs, the researchers created high-transmission waveguides with sharp 90° bends. Calculations show that higher-order bandgaps in the quasicrystal should also be independent of angle. Using microlithography, it may be possible to construct such a quasi-crystal with bandgaps at optical wavelengths. Contact Daozhong Zhang at [email protected].

Small-aperture laser has potential for optical storage

Researchers at Lucent Technologies' Bell Laboratories (Murray Hill, NJ) have built a very-small-aperture laser (VSAL) that they say has the potential to increase storage densities on optical disks by up to 100 times those of current magnetic disks. The predominant light sources for near-field optics, such as those used in data storage, have used tapered, metal-coated fibers, which have experimentally obtained resolutions down to 12 nm, or lambda/43. The drawback has been that most of the power put into them is lost through heat dissipation in the fiber. The VSAL, however, does not lose energy and can have an output power of more than 104 times that of coated, tapered fibers. The researchers used a 980-nm laser diode with a metal-coated facet in which a 250-nm aperture was created. At 40-mA current, the VSAL emitted 13 mW. Typical VSALs can be operated at up to 80 mA with proportionally higher powers, the researchers said. Laser diodes at shorter wavelengths should provide higher output. With sufficient power, data densities of about 540 Gbit/in.2 should be achievable. Contact Afshin Partovi at [email protected].

Single interferometer demultiplexes 40-Gbit/s optical-time-division-multiplexed signal

Swiss and Italian researchers have converted a 40-Gbit/s optical-time-division-multiplexed (OTDM) signal to four 10-Gbit/s wavelength-division-multiplexed (WDM) signals using a monolithically integrated indium phosphide (InP) Mach-Zehnder interferometer (MZI) with semiconductor optical amplifiers (SOAs) in its arms. The active SOAs were formed by 500-µm-long quaternary indium gallium arsenide phosphide (InGaAsP) regions butt-coupled to passive InGaAsP waveguides, and multimode interference couplers concatenated the waveguides and MZI structure. The 8.7 x 1.5-mm2 chip was flip-chip mounted on a silicon motherboard and packaged into a module using V-groove self-alignment techniques for precise waveguide and fiber coupling.

The device was tested using a 10-GHz regenerative modelocked erbium ring laser whose output wavelength was varied between 1542 and 1557 nm. The 7-ps, full-width-at-half-maximum (FWHM) ring-laser output pulses were externally modulated and passively multiplexed to 40 Gbit/s. The WDM source channels came from four distributed-feedback lasers with gain-switched output pulses further compressed to 8, 12, 7, and 6 ps FWHM through dispersion-compensating fibers, amplification, and filtering. Center wavelengths were 1542, 1547, 1553, and 1557 nm, and an overall penalty of less than 1.5 dB at a bit-error rate of 10-9 was obtained for the full 40-Gbit/s OTDM to WDM conversion. Contact Stefan Fischer at [email protected].

Enhanced photodetector fabricated within VCSEL epitaxial structure

Researchers at the University of Ulm (Ulm, Germany), the Massachusetts Institute of Technology (MIT; Cambridge, MA), Yale University (New Haven, CT), and the University of Palermo (Palermo, Italy) have fabricated resonant-cavity-enhanced (RCE) photodetectors adjacent to single-mode vertical-cavity surface-emitting lasers (VCSELs) in a common epitaxial layer structure. The structure was grown using solid-source molecular-beam epitaxy. The inner cavity was lambda/2 thick with three 8-nm-thick gallium arsenide (GaAs) quantum wells.

By etching away eight of the 15.5 mirror pairs in the epitaxial structure, the researchers fabricated an RCE detector with 73% peak quantum efficiency and a spectral full width at half maximum (FWHM) of about 1.7 nm with a -1.5-V reverse bias applied. The unbiased device yielded a peak quantum efficiency of 69% and a spectral FWHM of 1.8 nm. Etching away 11 mirror pairs for a 4.5 top-mirror-pair RCE device yielded peak quantum efficiencies of 26% and 36% for reverse-biased and unbiased devices, respectively. Spectral FWHM for the 4.5 top-mirror device was on the order of 6.5 nm. The researchers also observed a 3-dB detector bandwidth increase up to 2.8 GHz upon decreasing the active device diameter from 90 to 20 µm. Contact Thomas Knoedl at [email protected].

High-performance long-period fiber gratings are based on arc-induced microbends

Long-period fiber gratings work well as band-rejection filters because of compactness, low insertion loss, and low backreflection. Although ultraviolet (UV) radiation is typically used to induce a periodic index change in the fiber core, the process can be complex. At the Korea Advanced Institute of Science and Technology (Taejon, Korea), researchers have demonstrated a simpler grating-fabrication method that can be applied to any optical-fiber type. The gratings are based on periodic microbends induced by electric arcs, and, unlike the UV method, simple apodization techniques control microbend filter properties. During grating fabrication, an unjacketed optical fiber is held straight by two holders 5.5 cm apart. One holder is then displaced by about 100 µm in the direction orthogonal to the fiber axis to induce a lateral stress in the fiber. Applying an electric arc to a local section deforms the fiber slightly, with the amplitude of the microbend (typically less than 1 µm) controlled by the arc duration. By translating the electrodes according to the grating period and applying arcs, the researchers create successive microbends without additional displacement of the fiber holder. The resulting grating has low insertion loss and high thermal strength. Contact Byoung Yoon Kim at [email protected].

Insulator cuts leakage current in MSM detector

When used as the base material for a metal-semiconductor-metal (MSM) photodetector, silicon permits the construction of low-cost visible light sensors. This is a result of the MSM structure itself—in which complementary-metal-oxide-semiconductor (CMOS) integration of the detector onto a single chip along with transistor preamplifiers is possible—and of the maturity of silicon semiconductor technology. But MSM photodetectors have a large leakage current, making them less than ideal for use in applications such as optical-data-storage systems. This problem has been eased greatly by researchers at Philips Research (Eindhoven, The Netherlands), who added a 1-nm-thick layer of silicon dioxide to the structure to create a metal-insulator-semiconductor-insulator-metal photodetector. At 780 nm, the resulting CMOS-compatible device has a 5.2-fold reduction in leakage-current density to 18 µA/cm2 at 5 V and a 3.5-fold increase in photoresponsivity to 0.39 A/W. Its -3-dB bandwidth is 460 MHz—high enough for the next several generations of optical-data-storage systems. Contact Myron Seto at [email protected].

Chaos-control algorithm targets laser-diode applications

Researchers at Science Applications International Corp., Dynetics Inc. (Huntsville, AL), and the US Army Aviation and Missile Command (Redstone Arsenal, AL) have developed a chaos-control algorithm to handle high-speed applications such as the laser diodes found in CD players, barcode scanners, and fiberoptic communication systems. The new algorithm is based on the occasional-proportional-feedback (OPF) method of chaos control, but it eliminates several OPF steps, such as the sample-and-hold and difference operations. Instead, the new algorithm introduces a fixed amplitude-control pulse whenever the chaotic system enters a predetermined window in phase space. The pulse duration depends on the transit time of the system through the predetermined window.

Chaos control based on OPF has been achieved at frequencies as high as 200 kHz with controller latencies on the order of 10 µs, say the researchers. A control system based on the new algorithm was tested successfully on a 19-MHz circuit. The experimental controller has a theoretical upper limit of 200 MHz, however, and the algorithm is expected to exceed operational frequencies of 1 GHz with less than 1 ns of latency when implemented in the form of an integrated circuit. Contact Krishna Myneni at [email protected].

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