Frequency-Offset PLL for Synchronous Optical Sampling of OTDM Signals

We demonstrate a novel synchronization scheme for optical sampling which is based on a nonstandard phase-locked loop (PLL). Phase comparison is performed at a 10-GHz optical time-division-multiplexing (OTDM) base rate, thus avoiding ultrafast detectors and electronics. The employed frequency-offset PLL allows synchronous sampling of OTDM signals (or any other signals with bit rates given by integer multiples of the base rate), which would be impossible using a standard PLL. This provides a higher degree of flexibility for problem-specific sweeping than asynchronous sampling     

Word-Synchronous Optical Sampling of Periodically Repeated OTDM Data Words for True Waveform Visualization

An improved phase-locked loop (PLL) for versatile synchronization of a sampling pulse train to an optical data stream is presented. It enables optical sampling of the true waveform of repetitive high bit-rate optical time division multiplexed (OTDM) data words such as pseudorandom bit sequences. Visualization of the true waveform can reveal details, which cause systematic bit errors. Such errors cannot be inferred from eye diagrams and require word-synchronous sampling. The programmable direct-digital-synthesis circuit used in our novel PLL approach allows flexible adaption of virtually any problem-specific synchronization scenario, including those required for waveform sampling, for jitter measurements by slope detection, and for classical eye-diagrams. Phase comparison of the PLL is performed at 10-GHz OTDM base clock rate, leading to a residual synchronization jitter of less than 70 fs.     

Morphology and oxygen vacancy investigation of strontium titanate-based photo electrochemical cells

In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO₃) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO₃ powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO₃ lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively. It can be shown that PECs have to possess a low amount of oxygen vacancies to reach high EQE values, but not too low to allow for sufficient conductivity. This point is shown through our finding that the samples with lower and higher concentrations exhibit very low photo activity. Oxygen vacancies can be considered as intrinsic donors and hence increase electrical conductivity which is necessary but also act as recombination centers. For SrTiO₃ nanopowder-based samples, which have been sintered at 1200 °C for 20 min with a cooling rate of 10 K/s in reducing gas (with 5 vol% H₂) and a low flow rate of 1.7 l/h, very high external quantum efficiencies of 64.2 % under 365 nm illumination can be achieved.     

Analysis of the accuracy of the numerical reflection coefficient of the finite-difference time-domain method at planar material interfaces

This paper presents a rigorous analysis of the numerical error of the reflection coefficient of the finite-difference time-domain (FDTD) algorithm at planar material boundaries. The derived expressions show that the numerical reflection depends on a large number of parameters, such as the grid resolution and the time step, the frequency, the angle, and the polarization of the incident wave. In conclusion, the FDTD algorithm does not accurately fulfil the boundary conditions for the continuity of the fields. The theoretical findings enable the detailed characterization of the field behavior in the grid at material interfaces. The numerical total reflection and the Brewster angle are studied as well as the discretization influences on the specific absorption rate (SAR).     

Mastering Conformal Meshing for Complex CAD-Based C-FDTD Simulations

A robust and automatic discretization algorithm for complex conformal finite-difference time-domain (C-FDTD) simulation is presented in this publication. The targeted application range is to enable C-FDTD simulations for real-word engineering problems. Based on computer-graphics methods, complex CAD models with thousands of distinct parts can be efficiently and robustly discretized. A versatile concept is introduced to avoid numerical inaccuracies while calculating intersections, and to lead to a symmetric discretization without the overhead of "virtual lines." In addition, a necessary three-dimensional consistency check/correction, as well as merging of conformal cells of different CAD parts, are explained. The conformal geometric information is incorporated into the conventional FDTD algorithm using the conventional updating coefficients, which are conformally enhanced. Due to the derived stability criterion, the conformal updating scheme is always stable. The robustness and performance of the discretization algorithm presented is demonstrated with CAD models of increasing complexity towards real-world benchmarks. A conformal FDTD simulation with 80 million computational cells and 229 distinguished parts, representing a complete mobile phone and a head with hand, demonstrates the capabilities of the versatile technique.     

Ferroelectric thin film fabrication by direct UV-lithography

Due to their use in the fields of sensors, energy harvesting, capacitors and FeRAMs the fabrication of microstructured ferroelectric thin films is an important research field. Therefore a modified sol–gel process chain has been developed to produce fine patterned ferroelectric PZT (PbZr_0.52Ti_0.48O₃) thin films by direct UV-lithography. A sol based on methacrylic acid was developed to provide a photosensitive metal organic PZT precursor. The sol was used to obtain photosensitive xerogel films by spin-coating, which were patterned using conventional UV-photolithography equipment. After development the patterned xerogel films were pyrolized and crystallized in air via rapid thermal annealing in order to obtain crystalline PZT thin films. The patterned PZT films were investigated by XRD technique and SEM-micrographs. Finely patterned, crack free, crystalline PZT thin films were obtained.