Hello Alexander,
Please find a very basic FBG design set for reflection at 1.55um. It should be noted that the simulation does not include the full cladding in order to save on computational resources. Modelling fibers with FDTD can be tricky as you need to be running with a mesh size finer than 1/10th of the wavelength in the highest refractive index so you can quickly run into resource issues.
It should also be noted that the example is built in OptiFDTD 15.0.1 which means it will run in any version from 15.0+. If you do not have an active up to date license you can try this with the evaluation copy of the product.
Scott