The combination of fiber Bragg grating inscription with femtosecond laser sources and the usage of the Talbot interferometer setup not only gives access to the fabrication of Bragg gratings in new types of materials but also allows, at the same time, to keep the high flexibility of an interferometric setup in choosing the Bragg grating wavelength. Since the spatial and temporal coherence properties of the femtosecond laser source differ strongly from those of conventional laser sources, specific limits and tolerances in the interferometric setup have to be considered. Such limits are investigated on the basis of an analytical ray tracing model. The results are applied to tolerance measurements of fiber Bragg grating reflections recorded with a DUV sub-picosecond laser source at 262 nm. Additionally we demonstrate the wavelength versatility of the two-beam interferometer setup for femtosecond inscription over a 40 nm wavelength band. Inscription experiments in Al/Yb doped silica glasses are demonstrated as a prove for the access to non-photosensitive fibers.

A number of redundancy reduction techniques are used in a coder that is about eight times more efficient than simple PCM. The coder is capable of transmitting Picturephone® signals at an average rate of one bit per picture‐element (2 Megabits per second). When there is movement in the scene, most transmission time is devoted to the parts of the picture that change significantly. The data are generated irregularly but the data flow is smoothed prior to transmission in a buffer that holds about one frame of data. The redundancy reduction techniques used and the behavior of the coder are discussed both from an intuitive and from a statistical viewpoint. The positions of elements that change are signaled by addressing the first element of a run of changes and marking the end of the run with a special code word. The changes of luminance are transmitted as frame‐to‐frame differences using variable‐length code words. When rapid motion makes the buffer more than a quarter full, only differences for every second element are transmitted, the values of the intervening changed elements being set equal to the average of their neighbors. If the buffer continues to fill, the threshold that determines which changes are significant is raised from 4/256 to 7/256 of the maximum signal value. When violent motion causes the buffer to fill completely, replenishment is stopped for about one frame while the buffer empties. Subsampling and raising the threshold are not objectionable because viewers rarely detect the small impairments introduced in moving images. Observers are critical, however, of small impairments in stationary scenes. Thus, to maintain high quality in stationary areas, the entire picture is forcibly updated every three seconds by transmitting 8‐bit luminance values for three lines of every frame. A record of the coder's behavior is available as a 16‐millimeter movie film.

SummaryThe first photocycloadditions of aromatic and aliphatic aldehydes to methylated isoxazoles are reported. The reactions lead solely to the exo-adducts with high regio- and diastereoselectivities. Ring methylation of the isoxazole substrates is crucial for high conversions and product stability. The 6-arylated bicyclic oxetanes 9a–9c were characterized by X-ray structure analyses and showed the highest thermal stabilities. All oxetanes formed from isoxazoles were highly acid-sensitive and also thermally unstable. Cleavage to the original substrates is dominant and the isoxazole derived oxetanes show type T photochromism.

This paper provides a review of the literature on vision-based on-board obstacle detection and distance estimation in railways. Environment perception is crucial for autonomous detection of obstacles in a vehicle’s surroundings. The use of on-board sensors for road vehicles for this purpose is well established, and advances in Artificial Intelligence and sensing technologies have motivated significant research and development in obstacle detection in the automotive field. However, research and development on obstacle detection in railways has been less extensive. To the best of our knowledge, this is the first comprehensive review of on-board obstacle detection methods for railway applications. This paper reviews currently used sensors, with particular focus on vision sensors due to their dominant use in the field. It then discusses and categorizes the methods based on vision sensors into methods based on traditional Computer Vision and methods based on Artificial Intelligence.