STATUS OF THE FIBER LINK STABILIZATION UNITS AT FLASH

Abstract State-of-the-art X-ray photon science with modern freeelectron lasers (FEL) like FLASH (free-electron laser in Hamburg) and the upcoming European X-ray Free-Electron Laser Facility (XFEL) requires timing with femtosecond accuracy. For this purpose a sophisticated pulsed optical synchronization system distributes precise timing via lengthstabilized fiber links throughout the entire FEL. Stations to be synchronized comprise bunch arrival time monitors (BAM's), RF stations and optical cross-correlators (OXC) for external lasers. The different requirements of all those stations have to be met by one optical link stabilization unit (LSU) design, compensating drifts and jitter in the distribution system down to a fs-level. Five years of LSU operation at FLASH have led to numerous enhancements resulting in an elaborate system. This paper presents these enhancements, their impact on synchronization performance and the latest state of the LSUs

STEM nanoanalysis of Au/Pt/Ti-Si3N4 interfacial defects and reactions during local stress of SiGe HBTs

A new insight on the behavior of metal contact-insulating interfaces in SiGe heterojunction bipolar transistor is given by high-performance aberration-corrected scanning transmission electron microscopy (STEM) analysis tools equipped with sub-nanometric probe size. It is demonstrated that the presence of initial defects introduced during technological processes play a major role in the acceleration of degradation mechanisms of the structure during stress. A combination of energy-filtered transmission electron microscopy analysis with high angle annular dark field STEM and energy dispersive spectroscopy provides strong evidence that migration of Au-Pt from the metal contacts to Ti/Si3N4 interface is one of the precursors to species interdiffusion and reactions. High current densities and related local heating effects induce the evolution of the pure Ti initial layer into mixture layer composed of Ti, O, and N. Local contamination of Ti layers by fluorine atoms is also pointed out, as well as rupture of TiN thin barrier layer

CW Laser Based Phase Reference Distribution for Particle Accelerators

We present a cost-effective solution for the synchronization of RF signal sources separated by tens of kilometers with the femtosecond accuracy. For the synchronization a phase reference distribution system (PRDS) is developed, which is comprised of a CW optical transmitter connected via single mode fiber-optic links to remote receivers. This technique enables to use only one transmitter for multiple receivers and removes the necessity of active stabilization units (e.g. piezo-driven fiber stretchers or laser wavelength tuning), which reduces considerably the system cost. The concept of the new RF reference distribution, parameters of crucial components, phase drift detection and correction techniques are introduced, which lead to low noise and long-term stable PRDS operation. Detrimental effects of various linear and nonlinear fiber impairments are discussed. One of the most important elements is the phase detector, which is based on a direct RF-sampling ADC and it features a femtosecond measurement precision over 2pi phase change. Finally, the long-term performance of the designed PRDS is shown, which was evaluated with a 500-m single-mode fiber and an RF signal of 1.3 GHz

Operation of resonant-tunnelling-diode oscillators beyond tunnel-lifetime limit at 564 GHz

We present resonant-tunnelling-diode (RTD) oscillators, which are operating at frequencies up to 564 GHz. Due to heavy doping of the collector side of our diodes, the oscillators are operating beyond the tunnel-lifetime (τ) and relaxation-time (τrel) limits of RTDs. At 564 GHz we achieve ωτ≈1.2 and ωτrel≈2.6, the highest previously reported value of ωτ at frequencies >150 GHz was ≈0.6. Our study indicates that operating frequencies of RTD oscillators could be significantly increased and RTDs should be capable of operating at frequencies of several THz

Spectral characterization of broadband THz antennas by photoconductive mixing: toward optimal antenna design

The spectral characterization of a broadband antenna using a pump-probe photomixing continuous-wave (CW) terahertz (THz) system is presented. The high dynamic range of the system, comparable to or better than that of similar pump-probe systems reported in the literature, provides an accurate means of antenna characterization. The planar antenna exhibits a log-periodic behavior at low frequencies, a bow-tie behavior at high frequencies, and a resonance characteristic in between, well in agreement with the antenna geometry. It is predicted that an improved geometry that extends the log-periodic behavior to higher frequencies would contribute significantly in enhancing the broadband performance of antenna-coupled photomixers

Operation of resonant-tunnelling oscillators beyond tunnel lifetime limit

The tunnel lifetime of the electrons in the quantum well of a resonant-tunnelling diode (RTD) is usually assumed to be imposing an inherent fundamental limitation on the operating frequencies of RTD oscillators. Here, we experimentally demonstrate that one can overcome the limitation by heavy doping of the RTD collector. We present RTD oscillators with the fundamental oscillation frequency up to a factor of 3 above the tunnel lifetime limitation. Our results indicate that the inherent frequency limitations of RTDs should lie far above the state-of-the-art frequency of the contemporary RTD oscillators

Mis-evaluating the future. Affective Disorder and Decision-Making Capacity for Treatment:a Temporal Understanding

Background: Within psychiatric practice and policy there is considerable controversy surrounding the nature and assessment of impairments of decision-making capacity (DMC) for treatment in persons diagnosed with affective disorders. We identify the problems of "cognitive bias" and "outcome bias" in assessment of DMC for treatment in affective disorder and aim to help resolve these problems with an analysis of how time is experienced in depression and mania. Sampling and Methods: We conducted purposeful sampling and a qualitative phenomenological analysis of interview data on patients with depression and mania, exploring temporal experience and decision-making regarding treatment. Results: In both severe depression and mania there is a distinctive experience of the future. Two consequences can follow: a loss of evaluative differentiation concerning future outcomes and, relatedly, inductive failure. This temporal inability can compromise an individual's ability to appreciate or "use or weigh" treatment information. Conclusions: The decision-making abilities required for self-determination involve an ability to evaluate alternative future outcomes. Our results show that, within severe depression or mania, anticipation of future outcomes is inflexibly fixed at one end of the value spectrum. We therefore propose a temporal model of decision-making abilities, which could be used to improve assessment of DMC in affective disorder