Principles of optical fibre transmission

The second part of this description indicates the geometry of the optimum imaging onto the fibre to guarantee subcritical angle coupling a lens of diameter D and focal length/, whose numerical values match the numerical aperture equation, that is placed at distance 

Laser Chemistry Spectroscopy, Dynamics and Applications Hefmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

In most realistic cases the fibre is not completely straight in fact, it is exactly this flexibility of optical fibres that make them attractive for use in light delivery systems. Therefore, even when the total internal reflection condition is met initially, losses into the cladding may occur as soon as the fibre is bent excessively. 

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