Attenuation of an optical fibre is the loss of optical power as a result of absorption, scattering, bending, and other loss mechanisms as the light travels through the fibre.
The cutback method is a typical way used for measuring the total attenuation of an optical fibre. The cutback method involves comparing the optical power transmitted through a longer piece of fibre to the power transmitted through a shorter piece of the fibre.
The cutback method requires that a test fibre of known length ‘L’ be cut back to a shorter length. It needs access to both ends of the fibre. The cutback method begins by measuring the output power Py of the test fibre of known length ‘L’. Without disturbing the input conditions, the test fibre is cut back to a shorter length. The output power Px of the short test fiber is then measured and the fibre attenuation ‘A’ and the attenuation coefficient ‘a‘are calculated. 
http://ioptic.wordpress.com/2011/01/12/cut-back-method-for-testing-fibre-loss/ 
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