What are "nonlinear effects"?

Why can laser power be increased indefinitely? Does light also

In fiber optics and laser technology, laser power cannot be increased without limit. When the laser power injected into the optical fiber exceeds a certain threshold, the interaction between light and the transmission medium (fused silica glass, \text{SiO}_2) deviates from the traditional linear optics regime and enters the realm of Nonlinear Optics.

The light