Why use graded index fiber?

1. Graded Index vs. Step Index:
Why use graded index fiber?

2. • There are mainly two types of multimode
fibers: Step Index and Graded Index fibers
• The core on a step index fiber has the same
refractive index throughout.
• The refractive index on a graded index fiber
gradually changes from the center of the
core, to where the core meets the cladding.
MM Fiber Profiles: Step Index vs Graded

3. Step Index vs Graded
Step Index Fiber
Shows a clear distinction between
the core and cladding.
Graded Index Fiber
Shows a gradual change from the
core to the cladding.

4. Issue with Step Index Fiber
• Light rays travel somewhat uncontrolled inside a step index fiber.
• The blue ray travels directly down the center of the fiber, meaning
it travels a shorter distance than the red, green or black ray.
• Rays that travel a longer distance will take more time to get to the
end of the fiber than rays that travel a shorter distance.
• The result is that some of the signal arrives before the rest of the
signal. The signal becomes smeared.
• This is bad for communication.

5. Advantage of Graded Index Fiber
• Light rays are controlled well within a graded
index fiber.
• Just as with the step index fiber, certain rays
will travel shorter distances than other rays.
• However, in a graded index profile fiber the
rays will reach the end of the fiber at the
same time…
• How does this happen?

6. The speed of light depends on the refractive index of whatever the light is traveling in.
- When traveling in space (air, refractive index is 1.0) light travels at 300,000km/s
- When traveling in glass, where the refractive index is 1.5, it travels at 200,000km/s
- Changing the refractive index changes the speed of light!!!
Since the refractive index of the core in a step index
fiber is the same, the light rays will all travel the same
speed. (Rays that travel a shorter distance will arrive
sooner than rays that travel a longer distance.)
However, the refractive index of the core in a graded
index fiber changes, which means light rays inside the
core will travel a different speeds.
Optics of Step and Graded Index Fiber

7. Graded Index Fiber
• In a graded index fiber, the refractive index at the center
of the fiber is the highest. This means that the light
traveling at the center of the fiber travels slower.
• Rays that don’t travel in the center will travel faster. As
the rays move farther from center, they will travel faster.
• The result is that all of the rays in the core will arrive at
the end of the fiber at the same time. This is ideal for
communication.

8. • Long distance applications
• Higher Cost
• Less prone to attenuation
• Over longer distances, the benefits of
the graded fiber are worth the added
cost
• Short distance applications
• Lower cost
• More prone to attenuation
• Over shorter distances, the disadvantages
of step fiber are outweighed by the lower
cost of deployment
Step vs Graded in the Industry
Step Graded

9. OP1021
Launch Condition Analyzer
ü Nearfield Scanner profiles the optical power
density of the fiber
ü Farfield Scanner produces plots to determine
numerical aperture (NA) and mode field
diameter (MFD)
ü Measure the profile of Step Index and Graded
Index fiber

10. OP940
Insertion Loss & Return Loss
ü Fastest IL & RL testing
§ 3 seconds, using the front panel in Dual ILRL mode
ü Simultaneous, dual wavelength IL & RL
ü Easy front panel maintenance
ü It can be expanded using an external optical
switch or the switch can be integrated for
multifiber connector testing.

11. www.OptoTest.com
1.805.987.1700