Attributed to the profile of the refractive index, namely there is no difference in refractive index between the core and the outer cladding, the effective index of the core mode is absolutely lower than the refractive index of the outer cladding. As a result, the propagating core mode is a leaky one. Light power transmitted in the fiber core leaks out and transfers to the outer cladding. The outer cladding sandwiched between the inner cladding and air can also guide the light power as the cladding modes.