[IEEE 2015 12th International Conference on High-capacity Optical Networks and Enabling/Emerging Technologies (HONET) - Islamabad, Pakistan (2015.12.21-2015.12.23)] 2015 12th International Conference on High-capacity Optical Networks and Enabling/Emerging Technologies (HONET) - A method of OSNR calculation considering an incoherent coupling-noise and instability of optical filters transfer-function in ROADM cascade