Subject :- Applied Physics 2.

Topic :- Laser.

Difficulty :- Medium.

Depending upon refractive index optical fiber classified into two types

Step index: -

1. The R.I index changes abruptly at core cladding boundary because of which is called step index fiber.

2. Light in step index fiber fallows a zigzag path inside the fiber.

3. Refractive index of core has a constant value.

4. Band width of step index fiber is 50km.

5. Step index fiber has higher attenuation.

6. Numerical aperture for step index fiber is 0.2 to 0.35

7. Pulse dispersion is high. The rays entering at different angle of incidence travel different path length so that they come out at different times. That causes broadening of input pulse.

Graded index fibre (GRIN )

1. The GRIN fibers, R.I of core decreasing continuously in a parabolic manner from maximum value at center of a core to a constant value at interface. Since R.I decreases away from the center, that is the rays entering the fiber continuously ends towards the axis of fiber. It fallows from Snell`s law because a ray encounters continuously a medium of lower R.I and hence bend away from the normal. That is towards the axis. So, the ray travel in the sinusoidal form.

2. R.I of core is not constant but change in parabolic manner.

3. Bandwidth of GRIN fiber is 200 to 600 MHz km.

4. GRIN have a lower attenuation.

5. Numerical aperture is 0.16 to 0.2

6. Pulse dispersion is low because of focusing effect.

7. Index profile is shown below

   . Index profile is a plot of R.I on x axis and core cladding distance on vertical axis.