Realization of true-time-delay using cascaded long-period fiber gratings: theory and applications to the optical pulse multiplication and temporal encoder/decoder

This paper proposes and demonstrates a technique for repetition-rate multiplication of an optical pulse train having an arbitrary period or pattern and the optical temporal encoding/decoding for the optical-code-division multiple-access (O-CDMA) system. The technique exploits the difference in the propagation speeds between the core and copropagating cladding modes of a fiber to obtain true-time-delay between the modes traveling in the core mode and the cladding mode, which can be used to achieve pulse multiplication. For the coupling to the cladding mode, long-period fiber gratings (LPGs) were used. A series of cascaded LPGs imprinted in a fiber with a specific separation has been employed to obtain a specific rate of pulse multiplication with a single input pulse. Second, by controlling the separations among the gratings, the temporal encoder/decoder for O-CDMA could be implemented. The principle and the applications of the proposed device are investigated in detail. The effect of the birefringence of fiber and fiber gratings on the system performance in the time and spectral domains is presented. The sensitivity of the cladding modes in a conventional fiber to the perturbations at the cladding has been overcome by replacing the conventional fiber with inner-cladding fiber. The properties and the benefits of using the inner-cladding mode are investigated.     

The influence of electron-deficient comonomer on chain alignment and OTFT characteristics of polythiophenes

The structure–property relations of polythiophene derivatives have been the subject of research for both materials and electronic device applications. We have designed and prepared two kinds of polythiophene derivatives with similar chemical structures but different electronic properties by Stille polymerization. One is composed of only electron-rich thiophenes (PQT2T), while the other is composed of both electron-rich thiophenes and electron-deficient thiazoles (PTZT2T). The number-average molecular weights of PQT2T and PTZT2T were 23,000 (PDI = 1.5) and 13,000 (PDI = 1.7), respectively. PTZT2T showed a large bathochromic shift of absorption maximum from solution to film by 57 nm. Further investigation by differential scanning calorimetry and X-ray diffraction revealed the formation of well-ordered interdigitated structure of PTZT2T in the solid state (d₁ = 20.7 Å). From absorption onset, optical band-gap of PQT2T and PTZT2T was calculated to be 2.22 eV and 1.92 eV, respectively. A mobility of 1.1 × 10^?3 cm²/Vs, an on/off ratio of 4.7 × 10⁴ have been obtained from organic thin-film transistors (OTFTs) using PTZT2T as a channel. By comparison with PQT2T, the mobility was 18 times and the on/off ratio was 52 times higher for PTZT2T, which shows the usefulness of electron-deficient thiazoles as comonomer units of polythiophene derivatives for OTFT applications.     

Thermo-mechano-chemical stability of ceramic materials during the electrowinning process using liquid metal electrodes in molten salts

Pyroprocessing, which results in proliferation resistance, shows promise as an alternative to wet processing in the recycling of transuranics. However, the ceramic crucible used in the electrowinning process poses an issue during pyroprocessing. The crucible is chemically unstable and prone to thermal fatigue. In this study, the thermodynamic simulation software HSC (enthalpy, entropy and heat capacity) Chemistry was employed to evaluate the chemical stabilities of different ceramic crucibles containing liquid cadmium as well as liquid bismuth cathodes, which also contained rare earth elements and lithium. The chemical stabilities were experimentally validated by measuring the contact angles between the liquid cathode (LC) materials and four ceramic materials (Al₂O₃, MgO, Y₂O₃, and BeO) in situ. The infiltration depths of the liquid bismuth cathode elements were measured using X-ray photoelectron spectroscopy. To determine the Weibull distributions of the investigated ceramics, thermal fatigue tests were performed using plates of the ceramics.     

Low polarization mode dispersion measurements in ad hoc drawn spun fibers

Polarization mode dispersion causes significant impairment in high bit-rate optical telecommunications systems. It is well accepted that polarization mode dispersion can be strongly reduced by spinning the fiber while drawing, even if a comparison between theoretical predictions and experimental evidence has never been reported. In this paper we focus on the polarization mode dispersion measurement of ad hoc drawn periodically spun fibers, providing a first although preliminary experimental analysis of the spin induced differential group delay reduction. When measuring very low differential group delays as in the case of spun fibers, standard techniques are affected by rather high uncertainties. This problem is usually solved repeating several times the measurements while randomly perturbing the fiber. We support experimentally this method by means of polarization sensitive reflectometric measurements.     

Design and fabrication of dispersion-managed fibres by periodicetching during the MCVD process

A new design and fabrication method is proposed for a dispersion-managed fibre from a preform prepared by a single session of the modified-chemical-vapour-deposition (MCVD) process. Periodic etching during the MCVD process is used to change the index structure of the preform periodically alone the axial direction. The dispersion measurement of the fabricated prototype fibre shows that desired dispersion management characteristics can be achieved     

Behavioral and neuropathologic changes induced by central injection of carboxyl-terminal fragment of beta-amyloid precursor protein in mice

Expression of the carboxyl-terminal fragment (CT) of the beta-amyloid precursor protein (APP) in transgenic animals has been linked with neurotoxicity. However, it remains to be clarified whether the neurotoxicity is caused by beta-amyloid proteins (A betas) derived from CT or by CT itself. To study the in vivo neurotoxicity of CT, mice were given a single intracerebroventricular injection of a recombinant 105-amino acid CT (CT105; 68.5-685 pmol, intracerebroventricularly), and changes in behavior and in brain histology were examined. Animals given CT105 (410 or 685 pmol, intracerebroventricularly) showed a dose-dependent impairment in the passive avoidance performance, whereas boiled CT105 had no effect. CT105 (685 pmol, intracerebroventricularly) induced reactive gliosis in neocortex and hippocampus and neurodegeneration in neocortex. These results indicate that centrally administered CT105 induces behavioral impairment and neuropathologic changes, suggesting a direct toxic effect of CT105 per se.     

Multiwavelength Raman fiber-ring laser based on tunable cascaded long-period fiber gratings

Multiwavelength Raman fiber lasers using intracavity tunable cascaded long-period fiber gratings (LPFGs) are discussed. The application of the cascaded LPFGs as a multichannel fiber filter for the multiple-wavelength generation is proposed and experimentally demonstrated. The characteristics of multiwavelength fiber-ring lasers can be controlled by changing the physical parameters of cascaded LPFGs such as the separation distance between the gratings, grating length, and number of gratings. The multiwavelength Raman fiber-ring laser with nine wavelength-division-multiplexing channels with 100-GHz spacing and 19 channels with 50-GHz spacing has been achieved by varying the physical parameters of cascaded LPFGs.     

Fabrication of Germanosilicate glass optical fibers containing Tm/sup 2+/ ions and their nonlinear optical properties

Germanosilicate glass optical fibers incorporated with the Tm/sup 2+/ ions were fabricated to enhance optical nonlinearity by providing a strong reduction environment based on the solution doping technique in the modified chemical vapor deposition (MCVD) process. The incorporation of the Tm/sup 2+/ ions into the fiber core was identified by the electron paramagnetic resonance (EPR) spectrum in the fiber preform, and the absorption and emission properties between 350 and 1600 nm of the Tm/sup 2+/ ions in optical fibers and the fiber preform. A strong broad absorption band due to the Tm/sup 2+/ ions appeared from 350 to /spl sim/900 nm, and a broad emission from /spl sim/600 to /spl sim/1050 nm and the other emission from /spl sim/1050 to /spl sim/1300 nm, which were not shown in the Tm/sup 3+/ ions, were found upon Ar-ion laser pumping at 515 nm. Both absorption and emission results confirm that the Tm/sup 2+/ ions in the germanosilicate glass have the 4f-5d energy band from 350 to /spl sim/900 nm and the 4f-4f energy level at /spl sim/1115 nm. Also, the resonant nonlinearity at /spl sim/1310 and /spl sim/1530 nm due to the Tm/sup 2+/ ions in the fiber was measured upon the 515 nm optical pumping by using a long-period fiber grating (LPG) pair method. The nonlinear refractive index n/sub 2/ at /spl sim/1310 and /spl sim/1530 nm was found to be /spl sim/4/spl times/10/sup -15/ m/sup 2//W, where 70% and 30% of the n/sub 2/ are attributed to the nonradiative transitions and the radiative transitions, respectively.     

Complementary computing: policies for transferring callers from dialog systems to human receptionists

We describe a study of the use of decision-theoretic policies for optimally joining human and automated problem-solving efforts. We focus specifically on the challenge of determining when it is best to transfer callers from an automated dialog system to human receptionists. We demonstrate the sensitivities of transfer actions to both the inferred competency of the spoken-dialog models and the current sensed load on human receptionists. The policies draw upon probabilistic models constructed via machine learning from cases that were logged by a call routing service deployed at our organization. We describe the learning of models that predict outcomes and interaction times and show how these models can be used to generate expected-utility policies that identify when it is best to transfer callers to human operators. We explore the behavior of the policies with simulations constructed from real-world call data. See D’Agostino (2005) for a reflection from the business community about the failure to date of automated speech recognition systems to penetrate widely.