Dear Nurettin,
There is a single design tab where you set the grating parameters in OptiGrating. It is just the grating period relative to the effective wavelength that determined the operation mode of the device.
Please make sure the “Auto” checkbox on the bottom-left corner of the “grating definition” tab is unchecked so that the “period” field becomes editable. Otherwise it would be automatically set based on the Bragg condition for the selected input modes.
Kind regards,
Amin