Why is laser light used with fiber optics?
In brief, this is because laser light is monochromatic (all the same colour/wavelength) and coherent (all the same phase). This allows the signals to remain together.
Different wavelengths of light travel at different speeds in a medium like the glass in optic fibres. If we used white light, some of the colours would travel more slowly and others faster, so the signal would break up. Using laser light means the signal stays together throughout the optic fibre.
Laser diodes also turn on and off more quickly, use less energy and last longer than other light sources.