Thanks Steve Dods,
As you said, I have to look at the mode field pattern if I want to know whether a single mode is cladding mode or not. So if I want to calculate the refractive index of cladding modes of multimode optical fiber with hundreds of core modes, I should check out every single core mode to find the cladding modes, especially those modes which are close to cladding mode. Apparently this is not convenient.
I am thinking why OptiFiber does not add a new function which is capable of calculating and simulating the core modes and the cladding modes, respectively.