This is another example of a double-clad fiber amplifier. Different from the above example, we consider all cladding modes and use the built-in mode solution method. According to the refractive index distribution, the numerical aperture of the core, and the conditions of the cladding pumping mode, the mode characteristics are calculated. For simplicity, set the power to be evenly distributed in all pumping modes. Do not consider amplified spontaneous emission.
Figure 5 shows the horizontal intensity distribution of the input and output pump light and the output signal. It can be seen that the remaining pump light is distributed in a ring shape around the core. This is because only when the pumping mode is in this distribution, it has a lower overlap with the core area, and it appears to be absorbing when passing through. This explains the common problems of this type of fiber. Even if a simple model is used and a long fiber is allowed to be effectively absorbed, most of the pump light power is not absorbed when the change in pump intensity is ignored. If the numerical aperture of the pump cladding increases, the unabsorbed pump light increases, because the core area is avoided, and the higher-order pump mode is more advantageous. (Of course, users may still shift the pump power to low-level mode, which will improve this problem.)
