We use the power coupled-mode theory to study the interplay between multipath interference (MPI) and differential mode attenuation (DMA) in quasi-single mode (QSM) fibers. The analytical expressions derived assuming two mode propagation in QSM fibers show that MPI scales differently as a function of the span length for low and high DMA. Furthermore, we derive analytical expressions for the performance improvement of long-haul coherent optical communication systems using QSM fibers, taking into account the impact of excess loss and MPI on system performance. From these expressions, we calculate the maximum allowable coupling coefficient for different values of the DMA. We show, for example, that a QSM fiber with an effective area of 250 m 2 , a coupling coefficient 6 Â 10 À4 km À1 , and DMA equal to 4 dB/km offers a 1-dB performance advantage over a reference pure silica core single-mode fiber for spans of 100 km.

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Ultrafast laser cutting of a glass substrate at an oblique angle is demonstrated using a phase-corrected Bessel beam. Simulations are used to predetermine the ideal phase of the incident Bessel beam such that an unaberrated Bessel beam is formed inside the tilted substrate. Additional corrections to the beam such as shortening, moving the intensity of the beam within the substrate, and the formation of an elliptical focal spot were necessary to ensure consistent chamfering of the substrate and are discussed herein. Three cuts are combined to create a damage tract in the glass substrate in the shape of a chamfer, and then the glass is separated using a C O 2 laser resulting in a chamfered edge.

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There is considerable interest in finding conditions under which the quantum key distribution (QKD) propagation distances over fiber and secure key rate (SKR) are maximized for a given acceptable quantum bit error rate. One way to increase the secure key rate is to increase quantum bit rate, i.e. use shorter pulses. Short pulses propagating in a fiber are subject to temporal broadening caused by chromatic dispersion (CD) which leads to inter-symbol-interference and quantum bit-error rate increase. Current commercial QKD systems employ 1 Gb/s quantum bit rate sources, and the transition to 10 Gb/s system is being researched. While not very important in the 1 Gb/s, the effect of CD cannot be neglected in 10 Gb/s or higher quantum bit rate systems.