The higher intensity portions of an

The higher intensity portions of an optical pulse encounter a higher
refractive index of the fiber compared with the lower intensity portions
while it travels through the fiber. In fact time varying signal intensity
produces a time varying refractive index in a medium that has an
intensity-dependant refractive index. The leading edge will experience
a positive refractive index gradient (dn/dt) and trailing edge a negative
refractive index gradient (−dn/dt). This temporally varying index
change results in a temporally varying phase change, as shown in
Figure 5. The optical phase changes with time in exactly the same
way as the optical signal [7]. Since, this nonlinear phase modulation