请写出: 1、光纤的归一化频率V的物理含义。 2、光纤的截止波长的物理含义。 3、第一类、二类、三类Bessel函数及其变形函数。 Please write: 1. Physical meaning of normalized frequency V of optical fibers. 2. Physical meaning of cut-off wavelength of optical fibers. 3. Bessel functions and their deformable functions of the first, second and third categories.