Imaging errors in arrayed waveguide gratings

A detailed assessment of imaging errors in arrayed waveguide gratings (AWG) is presented, focussing on possible design related imaging errors as well as on typical imaging errors mediated by the fabrication process. From a design point of view, special interest is drawn to the effect of coupling within the array of grating waveguides and to the impact of the paraxial approximation for the star couplers which is commonly used in the design of AWGs. Both become important design issues affecting the performance of AWG devices when moving to high numbers of narrowly spaced channels. For the technology related imaging errors we focus on one side on the impact of almost uncorrelated phase errors at high spatial frequencies. They can typically be attributed to fluctuations in the index profile and waveguide dimensions in the grating region. On the other side correlated long range phase errors are treated which occur in the grating region as well as within the star couplers and which are typical for stress induced index changes in silica-on-silicon waveguides. It is shown that only modelling tools which include all types of the above imaging effects can provide reliable boundary conditions for a comprehensive design of high end AWG concerning spectral shape and chromatic dispersion.     

Real time study of the change in surface composition of insulators under electron bombardment

Abstract

We present a new real time technique to study the change in surface composition of solids, particularly insulators, due to electron bombardment. Secondary Ion Mass Spectrometry is used to monitor the surface composition during erosion by an electron beam. The technique is illustrated for LiNbO₃ and NaCl targets. We observe a preferential loss of oxygen in the case of LiNbO₃ and an alkali-metal enriched surface layer for NaCl.