A pre-registered short-term forecasting study of COVID-19 in Germany and Poland during the second wave.

Disease modelling has had considerable policy impact during the ongoing COVID-19 pandemic, and it is increasingly acknowledged that combining multiple models can improve the reliability of outputs. Here we report insights from ten weeks of collaborative short-term forecasting of COVID-19 in Germany and Poland (12 October-19 December 2020). The study period covers the onset of the second wave in both countries, with tightening non-pharmaceutical interventions (NPIs) and subsequently a decay (Poland) or plateau and renewed increase (Germany) in reported cases. Thirteen independent teams provided probabilistic real-time forecasts of COVID-19 cases and deaths. These were reported for lead times of one to four weeks, with evaluation focused on one- and two-week horizons, which are less affected by changing NPIs. Heterogeneity between forecasts was considerable both in terms of point predictions and forecast spread. Ensemble forecasts showed good relative performance, in particular in terms of coverage, but did not clearly dominate single-model predictions. The study was preregistered and will be followed up in future phases of the pandemic.

Investigation of directional coupler near-end cross talk by laser-induced refractive-index shifts.

The near-end cross talk of InGaAsP-InP waveguide directional couplers is investigated. It is shown that the cross-talk level is strongly dependent on the optical path difference between the two link-end arms of the coupler. By locally heating one of these arms with an argon laser beam this path difference, and hence the couplers' near-end cross talk, can be tuned. The ultimate limit for the near-end cross talk is determined by stray light and has therefore approximately the same level as the far-end cross talk.