Tunable multiwavelength fiber laser employing a comb filter based on a polarization-diversity loop configuration

A tunable multiwavelength fiber laser based on a semiconductor optical amplifier (SOA) is newly proposed. The proposed fiber laser employs a fiber comb filter based on a polarization-diversity loop configuration (PDLC) for the multiwavelength selection. The spectral characteristic of the comb filter is graphically explained by tracing the state of polarization (SOP) of the light propagating through the filter on the Poincare sphere. Particularly, the method of adjusting the filter characteristics is established with the help of the Poincare-sphere representation. The experimental results show that 18 laser lines oscillate with the signal to amplified spontaneous emission noise ratio over 25 dB. Particularly, all of the laser lines with a fixed channel spacing of /spl sim/ 0.8 nm can be continuously shifted by adjusting only a wave plate contained within the filter.     

Two-step laterally tapered spot-size converter 1.55-/spl mu/m DFB laser diode having a high slope efficiency

We demonstrate a two-step lateral tapered 1.55-/spl mu/m spot-size converter distributed feedback laser diode (SSC DFB LD) having slope efficiencies as high as 0.457 and 0.319 mW/mA measured at 25 /spl deg/C and 85 /spl deg/C, respectively. The SSC DFB LD fabricated by using a nonselective grating process has a double core waveguide structure including a planar buried heterostructure type active waveguide and a ridge type passive waveguide. The fabricated SSC DFB LD operates at 1.553-/spl mu/m wavelength and shows a far-field pattern in horizontal and vertical directions of 7.3/spl deg/ and 11.6/spl deg/, respectively.     

Excellent Switching Uniformity of Cu-Doped $hbox{MoO}_{x}/hbox{GdO}_{x}$ Bilayer for Nonvolatile Memory Applications

We have investigated a Cu-doped MoO_x/GdO_x bilayer film for nonvolatile memory applications. By adopting an ultrathin GdO_x layer, we obtained excellent device characteristics such as resistance ratio of three orders of magnitude, uniform distribution of set and reset voltages, switching endurance up to 10⁴ cycles, and ten years of data retention at 85degC. By adopting bilayer films of Cu-doped MoO_x/GdO_x, a local filament was formed by a two-step process. Improved memory characteristics can be explained by the formation of nanoscale local filament in the ultrathin GdO_x layer.     

Improvement of the cell performance in the ZnS/Cu(In,Ga)Se2 solar cells by the sputter deposition of a bilayer ZnO : Al film

ZnS is a candidate to replace CdS as the buffer layer in Cu(In,Ga)Se₂ (CIGS) solar cells for Cd‐free commercial product. However, the resistance of ZnS is too large, and the photoconductivity is too small. Therefore, the thickness of the ZnS should be as thin as possible. However, a CIGS solar cell with a very thin ZnS buffer layer is vulnerable to the sputtering power of the ZnO : Al window layer deposition because of plasma damage. To improve the efficiency of CIGS solar cells with a chemical‐bath‐deposited ZnS buffer layer, the effect of the plasma damage by the sputter deposition of the ZnO : Al window layer should be understood. We have found that the efficiency of a CIGS solar cell consistently decreases with an increase in the sputtering power for the ZnO : Al window layer deposition onto the ZnS buffer layer because of plasma damage. To protect the ZnS/CIGS interface, a bilayer ZnO : Al film was developed. It consists of a 50‐nm‐thick ZnO : Al plasma protection layer deposited at a sputtering power of 50 W and a 100‐nm‐thick ZnO : Al conducting layer deposited at a sputtering power of 200 W. The introduction of a 50‐nm‐thick ZnO : Al layer deposited at 50 W prevented plasma damage by sputtering, resulting in a high open‐circuit voltage, a large fill factor, and shunt resistance. The ZnS/CIGS solar cell with the bilayer ZnO : Al film yielded a cell efficiency of 14.68%. Therefore, the application of bilayer ZnO : Al film to the window layer is suitable for CIGS solar cells with a ZnS buffer layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis of the hemispheric asymmetry using fractal dimension of a skeletonized cerebral surface

We investigated hemispheric asymmetry using the fractal dimension (FD) of the skeletonized cerebral surface. Sixty-two T1-weighted magnetic resonance imaging volumes from normal Korean adults were used. The skeletonization of binary volume data, which corresponded to the union of the gray matter and cerebrospinal flow classified by fuzzy clustering, was performed slice by slice in the sagittal direction, and then skeletonized slices were integrated into the three-dimensional (3-D) hemisphere. Finally, the FD of the 3-D skeletonized cerebral surface was calculated using the box-counting method. We measured the FD of the skeletonized cerebral surface and the volumes of intracranial gray matter and white matter for the whole hemispheres and obtained the hemispheric asymmetries of each measurement. The FD, the gray matter, and the white matter volumes for the whole hemispheres decreased in the old group. The asymmetry of the FD revealed a significant right-greater-than-left asymmetry showed rightward, but did not change according to age and gender. None of the intracranial gray matter or white matter volumes showed any significant asymmetric changes. It could be said that the FD of the skeletonized cerebral surface is a novel measure of cerebral asymmetry.     

Engineering of charge transport materials for universal low optimum doping concentration in phosphorescent organic light-emitting diodes

A universal low optimum doping concentration of below 5% was demonstrated in phosphorescent organic light-emitting diodes (PHOLEDs) by managing the energy levels of charge transport materials. The device performances of PHOLEDs could be optimized at a low doping concentration of 3% irrespective of the host material in the emitting layer. The suppression of charge trapping and hopping by the dopant through charge transport layer engineering optimized the device performance at low doping concentration. In addition, it was revealed that PHOLEDs with low optimum doping concentration show better quantum efficiency, low efficiency roll-off and low doping concentration dependency of the device performance.     

Frequency domain post-processing technique based on POCS

A novel post-processing technique in the DCT domain, based on the theory of the projection onto convex sets (POCS), to reduce the blocking artifacts in the block discrete cosine transform (BDCT)-coded images, is proposed. Computer simulation results indicate that the proposed scheme performs better than conventional post-processing algorithms. Furthermore, the proposed algorithm can be implemented without DCT/IDCT operations. Estimates of computation savings vary between 41 and 64% depending on the task.     

Space-Frequency Precoding with Space–Tap Correlation Information at the Transmitter

In closed-loop methods for obtaining exact channel state information at the transmitter (CSI-Tx), the overhead associated with the feedback can be excessive for fast mobiles. Channel statistics-based CSI-Tx requires a much smaller overhead and is, therefore, attractive for use with fast mobiles. We study ways to exploit correlation-based CSI-Tx in a multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) system. We focus on a channel environment in which spatial and tap correlations are present. We propose a channel model for the case that spatial and tap correlations can be separated and show that in this case channel correlation decreases the ergodic capacity of an MIMO-OFDM system when no CSI-Tx is available. However, this decrease can be mitigated when correlation-based CSI-Tx is exploited. We introduce an optimal precoding approach to maximize capacity with spatial and tap correlation-based CSI-Tx. We also propose a statistical waterfilling scheme, which leads to almost optimal capacity performance without requiring computationally intensive numerical optimization. Based on these approaches, the impact of spatial and tap correlations is investigated.     

Integrated MIMO antenna with high isolation characteristic

A novel two-channel multiple input multiple output (MIMO) antenna for handset terminals is proposed. The antenna is basically composed of two folded monopoles and is designed to operate in WiBro (Wireless Broadband Internet; the Korean version of mobile WiMax) service band, 2.30-2.39 GHz. To decrease mutual coupling between the antennas, ground wall and connecting line are added. Good impedance matching and enhanced isolation performances are observed. Radiation patterns clearly show omnidirectional performance of the proposed antenna     

Chiral Stereoisomer Engineering of Electron Transporting Materials for Efficient and Stable Perovskite Solar Cells

A series of chiral stereoisomers of electron transporting materials with two chiral substituents is rationally designed and synthesized, and the influence of stereoisomerism on their physical and electronic properties is investigated to demonstrate highly efficient and stable perovskite solar cells (PSCs). Compared to mesomeric naphthalene diimide (NDI) derivatives, which have heterochiral side groups with centrosymmetric molecular packing of symmetric‐shaped conformers in the crystalline state, enantiomeric NDI derivatives have homochiral side groups that exhibit non‐centrosymmetric molecular packing of asymmetric‐shaped conformers in the crystalline state and exhibit better solution processability based on one order of magnitude higher solubility. A similar trend is observed in different rylene diimide stereoisomers based on larger semiconducting core perylene diimide. The PSCs based on NDI enantiomers with good film‐forming ability and a very high lowest phase transition temperature (T_lowest) of 321 °C exhibit a high and uniform average power conversion efficiency (PCE) of 19.067 ± 0.654%. These PSCs also have a high temporal device stability, with less than 10% degradation of the PCE at 100 °C for 1000 h without encapsulation. Therefore, chiral stereoisomer engineering of charge transporting materials is a potential approach to achieve high solution processability, excellent performance, and significant temporal stability in organic electronic devices.