Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

It refer to the broadening of pulse as light travel through a fiber. It is measured in ns / km.

i. Intermodal dispersion: - A ray of light launched into fiber fallows a zigzag path . when many modes are propagating they travel with different net velocities and comes out at different times which cause broadening of pulse. Low N.A fiber exhibit smaller dispersion also narrow spectral width of source limit the dispersion.

ii. Waveguide dispersion: - the effective refractive index for any mode varies with wavelength which cause pulse spread in. This is a waveguide dispersion

iii. Material dispersion: - it is based on the wavelength of optical signal. Light waves of different wavelengths travel at different speeds in a medium. The short wavelength waves travel slower than long wavelength wave. Thus, material dispersion can be reduced. By using more and more monochromatic source.

In GRIN fibers, the ray making larger angles with the axis traverses a longer path but in the region of lower refractive index and larger speed. The longer path length is compensated by a greater average speed. So that all the rays take the same time in traversing the fiber.