Dear Aulia,
make sure that the data structure that you load into the FBG has the following structure.
“The input file is formatted containing three items per line — the wavelength in microns, a complex value transmission, and a complex value for the reflection.”
Also, you need to place a delay component in the reflected path to allow the calculation.
If you still have a problem, please send me the files as zip file to ahmad.atieh@optiwave.com
Regards,
Ahmad