Polarization manipulation in femtosecond laser direct written waveguides in fused silica

Using ultrashort laser pulses to create refractive index modifications in transparent materials has become a popular method for fabricating integrated photonic circuits, since it allows three-dimensional structuring and rapid prototyping. While the precise control of the light propagation path in such laser direct written photonic circuits has been subject of numerous investigations, the control of the lights polarization has rarely been examined. At the same time, achieving full control over the lights polarization in a photonic circuit is crucial for using this degree of freedom to encode information. The goal of this theses is to demonstrate ways of achieving this polarization control in femtosecond laser direct written circuits in fused silica. To ensure that a waveguide does not alter the polarization state (and thereby the information content) of transmitted light, the waveguide should exhibit minimal birefringence and minimal losses. For this purpose, the influence of different beam shaping methods for the inscription laser on the waveguides optical properties are investigated. Waveguides with lowest losses at a low birefringence were achieved using an anamorphic zoom system. Furthermore, two concepts for embedding compact polarization manipulating elements into the waveguides are presented. Firstly, it is shown that the self-ordered material modification nanograting can be used to create embedded waveplates, where the optical properties are determined by the laser inscription parameters during the fabrication process. The obtained structures can be used not only for classical applications, but also as single qubit quantum gates, which is demonstrated using single photon experiments. The second concept for embedding polarization manipulating elements into waveguides is based on liquid crystals, which allow active reconfiguration of the optical circuit after inscription and a dynamic change of the polarization state in a waveguide

Where do migrants and natives belong in a community : a Twitter case study and privacy risk analysis

Today, many users are actively using Twitter to express their opinions and to share information. Thanks to the availability of the data, researchers have studied behaviours and social networks of these users. International migration studies have also benefited from this social media platform to improve migration statistics. Although diverse types of social networks have been studied so far on Twitter, social networks of migrants and natives have not been studied before. This paper aims to fill this gap by studying characteristics and behaviours of migrants and natives on Twitter. To do so, we perform a general assessment of features including profiles and tweets, and an extensive network analysis on the network. We find that migrants have more followers than friends. They have also tweeted more despite that both of the groups have similar account ages. More interestingly, the assortativity scores showed that users tend to connect based on nationality more than country of residence, and this is more the case for migrants than natives. Furthermore, both natives and migrants tend to connect mostly with natives. The homophilic behaviours of users are also well reflected in the communities that we detected. Our additional privacy risk analysis showed that Twitter data can be safely used without exposing sensitive information of the users, and minimise risk of re-identification, while respecting GDPR

Digital Footprints of International Migration on Twitter

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