Thanks, Tech Support
As you said, I use a single higher order mode as an input mode, but OptiGratings still cannot get the ideal transmission spectrum when I simulate LPFG. However, it will show good spectrum if I use a set of modes as an input mode. But I still do not understand why I should choose those modes as an input mode. For a single mode fiber, it has only one mode propagating within the fiber core region. I do not know the reason why there are so many modes after recalculating modes on the step of Modes Parameters.